25 MONITORING AND CONTROL TOM RITZDORF

To obtaining quality deposits it is important to monitor and control electrochemical deposition (ECD) processes. This is also essential for an automatic plating system designed to minimize operator attention to production operations. A skilled and knowledgeable operator can maintain a reasonable plating quality with little supporting equipment. However, trends to more automation to reduce costs with higher processing speeds, application of plating to higher value products, and assurance of quality plating make it necessary to utilize many monitoring and control facilities. Electroplating process results are dependent on process stability and control and on the chemical composition of the plating bath. This makes it important to monitor the concentration of the various species in the bath. At a minimum, it is usually important to monitor the concentration of the metal ions and the supporting electrolyte. It may also be necessary to monitor constituents that interact in trace amounts or organic additives that modify the behavior of the process. Electroless plating, as described elsewhere, is an autocatalytic process initiated by a catalyst such as palladium and then catalyzed by the deposited metal itself [1]. Electrochemically, the system is quasi-stable, as it must be to work. This necessitates a closer control of bath chemistry than that required for electroplating solutions. Frequent electroless bath analysis and maintenance are necessary, especially when fast plating is involved, due to the speed with which the chemistry changes. Fast electroless metal deposition or poor bath control can result in spontaneous bath decomposition, causing metal particles to form and strip the solution of all metallic ions. Manual bath sampling and analysis are often not practical. Machines have been developed which automatically take samples and analyze and reconstitute the bath chemistry [2]. These will be discussed in Section 25.2.11.

This chapter has three sections: process monitoring, bath constituent concentration monitoring and replenishment, and product monitoring. The first section focuses on sensors and techniques for monitoring the plating and associated processes and how to use these to increase the automation level of plating equipment. Because the concentration monitoring and control of chemistries used in the plating processes are so important and may be handled by equipment separate from the electroplating equipment in some cases, these topics are given their own section. Finally, a short section on product monitoring and quality control is included, although the reader should understand that product monitoring should be dictated more by the product requirements than by the process used to produce it. This section is meant merely as a reference to some of the common monitoring methods in use and how they are applied to control plating processes.

25.1

PROCESS MONITORING

Plating processes, whether electrolytic or electroless, are utilized for many varied purposes. These may be for the manufacture of simple and cheap parts or for the precise manufacturing of extremely high-tech or one-of-a-kind components. In any case, decisions regarding how much effort to put in to controlling the process parameters and their impact on the process must be made based on economics, product volume, and expected results. An electroplating line consists of a complex set of equipment, including motors, tanks, water rinses, product transport equipment, filters, heaters, electrical power supplies, process monitoring devices, process control devices, and the like. Electroplating process conditions

Modern Electroplating, Fifth Edition Edited by Mordechay Schlesinger and Milan Paunovic Copyright Ó 2010 John Wiley & Sons, Inc.

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MONITORING AND CONTROL

to be monitored and controlled in a typical line may include: Solution level Solution temperature Solution flow and agitation Solution chemistry . Metal concentration(s) . pH . Specific gravity . Additive concentration(s) . Impurity levels Current at each electrochemical step Charge passed Cell voltages Line speed Blow-off air pressure Exhaust vacuum Methods of monitoring these parameters will be discussed in the following sections. The designer of modern electroplating equipment has an ever-growing array of devices for monitoring and controlling almost every aspect of the process available to him or her. Plating machines can run automatically at high speeds, make adjustments, and signal an operator if help is needed. The capital investment and maintenance cost must be justified by an increased rate of production while maintaining or improving product quality. With the level of control and automation available today, completely automated equipment can be designed to handle almost any ECD task. Semiconductor and Microelectronic Processing Equipment Equipment that is designed for semiconductor processing or for making devices such as MEMS (microelectromechanical systems) and heads for magnetic recording is usually subjected to more demanding requirements than other plating equipment. This is due to the stringent requirements for contamination control in the thin-film manufacturing technologies used to make these devices as well as the high value of the products being produced. Semiconductor equipment is highly automated, capable of processing expensive parts with little operator intervention, and is subject to industry-specific manufacturing specifications [3–7]. These specifications are the result of many years’ worth of learning regarding what materials are compatible with semiconductor manufacturing and what may lead to particle or chemical contamination, requirements from insurance companies, and what is required to interface the equipment to automated data and product handling in an automated semiconductor fabrication facility. Communication between the automated processing equipment control

computer and the fab computer network is usually handled automatically and provides for the transfer and storage of large amounts of data regarding the process flow and processing of the wafers used to manufacture microelectronic devices. These data-handling requirements necessitate the incorporation of relatively powerful computers and the ability to work with host fab automation and control networks. In addition, these systems may include high-energy particulate air (HEPA) or ultralow particulate air (ULPA) filters and air ionizers to condition the environment around the wafer-handling area. Some systems may also include vision systems for verification of proper equipment operation [8, 9]. This extensive level of control requires many components. Most of the components can be implemented as simple controls or as part of a complex and automated plating system. 25.1.1

Solution Level

The liquid level or volume in the tanks used for plating is important to control for several reasons. Mechanically, enough solution must be kept in the tank to be able to operate pumps, cover any components or sensors that are meant to be immersed, and perform the plating operations themselves. Maintaining a relatively constant solution level is also important as it relates to controlling the concentrations of the constituents of the solution. In addition, it is important to be able to sense low level or absence of liquid in cases where electrical immersion heaters are used in order to prevent the possibility of fires. Liquid-level sensors are usually used to maintain solution level with replenishment of water. The solution level in a tank can be monitored simply with a float containing a magnet that operates reed switches: one for each level in the tank to be detected. Other types of sensors that may be used to detect liquid levels include capacitive, pressure sensing, optical, and ultrasonic devices. Load cells may also be used in some cases to determine solution volume. Some of these may be used on the exterior of the plating tank (depending on materials of construction), making them less prone to attack by the typically harsh chemicals used in a plating process. There are also several types of level sensors which may be designed to provide an analog output representing the distance to the plating solution, as opposed to a simple detector that indicates that solution is present at a certain location. Plating tanks that have a high concentration of electrolyte can form salt deposits on a float causing it to become immobile. This may be avoided by directing the replenishment water at the float area. Crystallization interfering with sensor operation, reliability in harsh chemical environments, and signal-to-noise ratios under practical operating conditions are all factors that should be considered when choosing solution-level sensors. It is also important to consider the failure mode of normally open and normally closed sensors and the impact of a failure on the equipment and process.

PROCESS MONITORING

One common exception to using automatic solution-level control is for precious metal plating tanks. Platers can be concerned about the reliability of an automatic solution-level system for a precious metal plating tank for fear of overflow and losing gold or platinum down the drain. Instead of automatic addition of water to gold and other precious metal plating tanks, an audio alarm can sound so the operator can adjust the level manually. 25.1.2

Solution Temperature

Solution temperature can be monitored with a thermocouple, thermistor, RTD (resistance temperature detector), or silicon integrated circuit device. Many sensors for plating parameters are temperature sensitive and the output signal must be compensated with an accurate temperature measurement. This is often done with an on-board silicon integrated device that senses temperature and corrects the parameter measurement [10]. 25.1.3

Solution Flow

Rapid solution flow around the cathode is a common method of providing agitation to enable higher plating speeds. Large tanks for barrel or rack plating improve agitation at the cathode by moving the cathode or sparging solution or air from the bottom of the tank upward. In a strip plater that uses a line of small plating cells and reservoir tanks underneath, solution is pumped into the cells at rates sufficient to replenish the metal ion content but also, perhaps more importantly, to produce rapid agitation around the strip to enable highspeed plating. Semiconductor or microelectronic plating systems typically use some combination of solution flow, wafer rotation, and mechanical agitation systems to provide fluid agitation at the cathode surface. For rate of flow, a rotameter, paddle wheel magnetic device, pressure differential, ultrasonic, or electromagnetic sensor can be used. It is relatively simple to buy or make a rotameter that provides output(s) indicating the actual solution flow by incorporating magnetic, optical, or capacitive sensors. There are also numerous manufacturers who produce flow sensors using some of the other technologies mentioned that are useful for plating applications. In any case, it is useful to have some understanding of the technology utilized and how the results may be impacted by viscosity, specific gravity, solution conductivity, or the presence of bubbles entrained in the solution. It is also important to note how the various flow sensors are calibrated and that the calibration may need to be repeated if the solution properties are changed. 25.1.4

Plating Current and Cell Voltage

The plating current is typically controlled during an industrial plating process because it has a direct relationship with

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the deposition rate and because it is not as sensitive as the cell voltage to changes in the physical system. The current at each electrochemical step can be displayed on an ammeter. It is also easily monitored by measuring the voltage drop across a resistor in series with the plating cell. That voltage can be used in an electronically controlled power supply to maintain the current at a constant value. An open circuit or power supply failure in the system can be detected by a programmable logic controller (PLC) or computer to stop a plating line and sound an alarm. Many industrial plating power supplies, or rectifiers, include automatic monitoring and control of output currents and/or voltages, along with the ability to set tolerance limits that will trigger alarms if exceeded. These alarms can be used to stop processing or to prevent additional product from being started, depending on the severity of the condition encountered. Monitoring of electroplating cell voltages is necessary to detect cell variations such as electrical shorts or opens between the anode and cathode, contamination or polarization of the anode, or changes in solution concentrations. Shorting can occur in electropolishing cells of strip platers with close anode–cathode spacing when the strip is a copper alloy. Copper powder is produced; it collects at the cathode and eventually forms a conducting path between the anode and cathode. Methods of automatically removing the copper powder are discussed in Chapter 24. Solution changes that affect the conductivity will also impact the cell voltage, as will any extra resistance caused by corrosion of electrical components or passivation of the anode or cathode surface. 25.1.5

Timers, Ammeters, and Coulometers

Timers with ammeters were the first means of monitoring the amount of metal electroplated. In many shops they are the only instruments needed to produce an acceptable product. The first automatic control device was the ampere-hour (or ampere-minute) meter that stops plating when a preset number of coulombs has passed through the cell. Simple automatic plating machines rely on timers to advance product through processing steps. Coulometer control of plating steps is commonly included in power supplies and rectifiers designed for electroplating. 25.1.6

Sensors

A typical plating tank will include several different kinds of sensors (Fig. 25.1). It is important to choose sensors carefully with consideration given to the purpose of the sensor and its interaction with other components of the system. The impact of the chemistry (or chemistries) used on the operation of each sensor may also be an important consideration, and contact or noncontact versions of several sensing systems may be available. It is important to keep in mind that any time a device with electrical connections is added to a tank

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MONITORING AND CONTROL

FIGURE 25.1 Typical sensor assembly on a plating tank cover. Left to right: Fluid level, temperature, pH, and conductivity.

containing plating chemistry there is potential for electrical interactions that may impact the process results, at a minimum upon failure of the sensor. Advanced sensors coupled with modern control technology revolutionized automatic manufacturing [11–13]. Sensors play a vital role in providing information about plating machines and processes that is necessary for automatic monitoring and control [14]. There are three basic sensor types: on–off, analog, and digital. The on–off type can be considered a form of digital sensor without quantitative information. For many situations, an on–off sensor is sufficient. For example, when a take-up reel of strip becomes full or a payoff reel becomes empty, a simple on–off signal is adequate. Analog sensors provide quantitative information that can be read on a meter, but for automatic control it is usually necessary to convert the signal from analog to digital for electronic processing. Many sensors produce a digital signal directly. One example is a photoelectric sensor that generates pulses at a rate proportional to a plating line speed. Electronic processing then converts pulse rate to line speed which can be read on a display or fed into an electronic motor-speed controller. An integrated system of sensors tied to an electronic controller is required to free an operator from machine watching and fine tuning critical process variables. Sensor technology has developed rapidly in recent years, and we now have a vast array of commercially available sensors, many of which are useful for plating applications. In fact the number of sensors available for some applications is so large that one must study the features of each sensor in order to make the best choice. For example, a survey showed there were 144 manufacturers of level sensors making over 20 different types [15].

There are several things to consider when choosing a sensor to monitor plating variables. The important items are accuracy, reliability, sensor life, cost, signal conditioning, and maintenance. The accuracy required depends on the process or machine function. Solution temperatures may need to be controlled to
10 C or to
0.2 C depending on the process. Cumulative variables such as plating current, plating efficiency, and line speed need to be controlled so the overall accuracy is within acceptable limits. For example, if each of these three parameters is allowed to vary as much as 1%, then the plating thickness can vary as much as
3%. Plating machine sensors usually must operate in a hostile environment with corrosion, fumes, and electrical noise present. For high reliability, the sensor design and choice of construction materials must be considered carefully. The sensor should hold its calibration as long as possible to minimize recalibration. The only assurance that a sensor is working properly is to either check it frequently or build in a self-checking system utilizing a sensor-within-sensor together with special electronics. These are called smart sensors, although sometimes there are other features included such as having the sensor directly provide a control signal to the line. Smart sensors often can be reprogrammed by an external computer. A schematic of a smart sensor and how it may interact in an automatic plating system is shown in Figure 25.2 Sensor life depends not only on how well it is designed and made but also on how it is used and maintained. A glass pH electrode, for example, once put into use must be kept wet for reliable operation. If removed from plating solution, it should be stored in water or buffer solution until returning to the bath. A float-level sensor in a plating solution can become inoperative if plating salts crystallize out on the float, causing it to stick. The problem may be avoided if water added to maintain the level is put in at the float. This removes the tendency for salts to form at the float. This is just one example of how the environment in a plating system may impact the operation of sensors that are meant to control its operation. How much one should pay for a sensor depends on the value of the variable being sensed and the product being processed. Temperature sensors are relatively cheap and are used wherever necessary without much thought of expense. On-line plating thickness monitors are expensive, but they can save many times their cost annually by avoiding excessive gold plating or production of scrap due to underplating. Sensor signal conditioning involves converting the electrical signal into a form that can be displayed in engineering units such as grams per liter of metal in solution. A raw signal may be conditioned using electronic hardware or computer software or a combination of the two. If only a few sensor systems are involved, hardware instead of software is usually the most cost-effective choice. Software can be developed so that a computer can convert raw sensor signals to a form useful for process control, but this tends to be more costly.

PROCESS MONITORING

531

SMART SENSOR

CHECKING SENSORS

SENSOR

MICROPROCESSOR

CONTROL DEVICE PROCESS COMPUTER

FIGURE 25.2

25.1.7

Smart sensor concept for monitoring with local control.

Line Speed

Line speed determines the dwell time of the product in each processing step in a strip plating application. In plating cells it can determine the plating thickness. Big machines that electrogalvanize steel strip monitor zinc thickness with an X-ray gauge that provides a feedback signal to a computer for line speed adjustment to maintain a specified zinc thickness [16]. Punched parts that interrupt light periodically are used to control strip speed with a photo-optical system that produces an electrical pulse as each segment moves by. An electronic pulse counter is calibrated to display line speed and provide feedback to control line speed. 25.1.8

Rinse Quality

In some cases, it makes sense to measure the quality of the rinse processes after chemical steps. This can be done by measuring the resistivity of the water that is used to rinse the parts. As chemicals are removed and the rinse water becomes less contaminated, the resistivity of the rinse water will increase. This can be especially useful when deionized water is used for the rinse operation. It is important to pay close attention to the system design, though, to ensure that contaminated rinse water is not held in the system where it will produce a high reading even after the parts are thoroughly rinsed. This can be a little tricky, since the resistivity probes normally used must be kept immersed at all times. 25.1.9

Solution Blow-Off

Solution drag-out as parts leave a processing cell can be minimized by blowing air at the part in the right direction as parts leave the tank. Large mill plating machines for electrotinning steel strip can drag out 30 gallons of solution per hour unless special measures are taken to remove as much solution as possible [16]. Standard shop floor compressors should not

be used for blow-off air, since the air usually contains oil. A good roof air blower is preferred; its pressure should be monitored to ensure that adequate solution blow-off occurs. 25.1.10

Fume Exhaust

A reliable fume exhaust vacuum system is important to the health of plating shop workers and to provide a warning of faults that will lead to equipment and part corrosion. A sensor placed in the exhaust plenum detects the level of exhaust which is necessary to remove chemical fumes. One type of vacuum sensor placed in the exhaust plenum works by monitoring the cooling of a heated leg of a Wheatstone bridge. If the exhaust air stops or decreases to an unacceptable level, the sensor can trigger an alarm. 25.1.11 Programmable Logic Controllers and Computers Sensors are the “eyes and ears” of a modern automatic monitoring and control system but PLCs and computers are the “brains” that assimilate sensor information with programmed instructions that initiate the control function. The first automatic plating machines used timers to advance product through a line. Next a control system based on punched tape and photocells was developed that operated relays in the proper sequence to control a plating line. General Motors pioneered a wired relay logic system for machine control. Modification of that system by rewiring proved too time consuming and costly. Electronics came to the rescue when the PLC was developed in 1969. Since then PLCs have become more sophisticated with computerlike features to monitor and control many machine operations including electroplating. While PLCs are still superior for direct machine monitoring and control, computers are better at processing data

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and communicating with other plant operations. Specialty computers have been developed for direct interface with machines for monitoring and control. Fuzzy logic control is a new technique that mimics human reasoning [17]. 25.1.12

Robots and Handling Systems

Robots are used in many automated plating systems. These handling and automation systems require their own monitoring and control devices. These devices may include such components as position sensors and encoders and vision systems. Automation and robotics are involved subjects which are not covered in detail here. The interested reader may find any number of references to robotic systems. When automating electrochemical processing systems with robotics, it is extremely important to remember that the environment will be corrosive due to the chemicals involved. 25.2 BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT The composition of electroplating solutions has changed very little in recent years with the exception of high-speed plating where higher metal content and special additives are used and new solutions designed to be more compatible with health and environmental requirements. Analytical techniques have been continuously developed and improved to enable more precise or more automated control, however. Plating baths can be analyzed by evaluating their deposit properties under a specific set of conditions, by analyzing the concentrations of specific chemical species, or by analyzing the effect of a chemical species or group of species on the performance of the plating bath. Each of these methods has its own advantages and drawbacks, and oftentimes combinations of these analytical approaches are utilized as specific situations dictate. Analysis of deposit properties may take place using a special plating apparatus or by plating a special part that is used for material property analysis, which may be destructive. This is considered a “qualification” test, and some of the testing techniques considered useful are described in the third section of this chapter. Special plating cells have been developed that can be used for plating bath analysis. Sometimes these require the qualitative judgment of an experienced technician in order to understand the relationship of the deposit produced to the chemistry in the plating bath. Wet chemical methods of analysis have been developed to determine the composition of most plating solutions [18, 19]. Instrumentation for rapid chemical analysis of inorganic and organic species has become highly automated, providing accurate and rapid analysis. The advantage of on-line monitoring and control of plating solutions is that deviations from

optimum concentration ranges can be corrected as soon as set limits are exceeded. Of course, this level of automatic monitoring comes at a cost. Plating solution chemistry is usually monitored off-line by taking a solution sample and performing an analysis at the machine or in a chemical analysis laboratory. Solution adjustments are then made as necessary. Metal ion concentration, supporting electrolytes, additives, and pH are most often monitored. Metal ion concentrations are measured by colorimetry, polarography, or ion-selective electrodes. Automated versions of these methods have been developed for on-line analysis of many metals including nickel, copper, lead–tin, tin–silver, and gold [20–22]. On-line methods may involve extracting a sample from the bath and performing analysis or incorporating a sensor in the plating bath. There is a fundamental question, especially when measuring bath additives, about the preference to measure the specific concentration of each chemical species in the bath or whether to monitor the effect of the chemicals that influence the performance characteristics of the plating process. While most scientists would prefer to know the exact concentration of each chemical in the system, it is often extremely difficult to quantitate the concentrations of similar organic compounds that might have a range of molecular weights, for instance. This problem becomes aggravated as the bath is used and some of the additives become oxidized and/or reduced or simply react to produce other compounds. Once you take into account complexation with metal ions, equilibrium balances, and the buildup of contaminants as parts are processed, it is sometimes much easier and more advantageous to consider the effect of these chemicals on the electrochemical activity, for instance [23]. Most electrochemical analysis methods simply measure the impact of the additives on the bath activity as being representative of the additive concentration. The impact of these decisions becomes especially important as the bath chemistry ages and impurities or additive breakdown products build up in the bath. Solution impurity buildup sometimes can be ignored, such as when drag-out removes enough solution to keep the impurity level low or when the impurity has little or no effect on the deposit or bath analysis. 25.2.1

Plating Test Cells

Test cells have been used for evaluating plating chemistry for many years [24]. These cells can be used in a development environment to understand effects of varying operating conditions on deposit properties or they can be used in a production environment to understand how a plating bath is performing compared to expected results. Also, they can provide a convenient means to separate the performance of the plating chemistry from effects produced by the equipment that is used for the process. A sample of the plating bath can be removed, tested in the test cell, then discarded or

BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT

TABLE 25.1

Electrochemical Test Cells

Hull cell Rotating cylindrical Hull cell Haring–Blum cell Stress cells Rotating disk electrode (RDE) Rotating ring disk electrode (RRDE) Electrochemical quartz crystal microbalance (EQCM)

Additive concentration effects, varying CD and agitation Additive concentration effects, varying CD and agitation Throwing power Deposit stress as function of varying CD and thickness Polarization, limiting current density, etc. Impact of electrochemical reactions on bath chemistry Deposit efficiency, absorption vs. applied potential

added back to the plating tank. Table 25.1 lists some common types of electrochemical test cells and their typical uses. Hull Cell The Hull cell is a plating test cell that was designed to allow a quick analysis of the “health” of the plating chemistry and to allow the effect of additions to the bath to be easily scaled up to an industrial system [25–27]. An example is shown in Figure 25.3. The Hull cell is a trapezoidal plating cell that utilizes a cathode placed diagonal to the anode to cause a variation of current density across the cathode surface. The Hull cell is usually used with a magnetic stir bar on the bottom, which results in stronger agitation at the bottom of the cathode panel than at the top. This allows a panel to be plated that shows qualitatively the effects of current density and agitation variations on the deposit properties (usually deposit morphology or brightness). Small additions of additives can be made to the chemistry in the Hull cell, which usually contains 267 mL of plating bath. Once the desired performance is produced on the Hull cell

panel, the additions to the plating tank can be quickly calculated since each gram added to the 267-mL Hull cell is equivalent to 0.5 oz/gal. There are many variants of Hull cells with different agitation mechanisms or temperature control capability, such as air-agitated or hanging Hull cells and Gornall cells, but they all work in generally the same way [25–27]. Rotating Cylindrical Hull Cell The rotating cylindrical Hull cell is a modification to the Hull cell concept that is especially useful for development of new chemistry and processes, as it allows for precisely controlled uniform agitation with the current density variation typical of a standard Hull cell. This cell essentially makes use of a rotating-disk electrode rotator system to control the rotation of a shaft that incorporates a cylindrical cathode. This arrangement provides uniform and controllable mass transfer across the cathode surface. The placement of the rotator with respect to the anode provides for variations in current density across the surface of the cathode [28, 29]. Haring–Blum Cell The Haring–Blum cell is a test cell that is used to measure the throwing power of a plating process. It is a 1000-mL cell that is rectangular and contains two cathodes and one anode. The cathodes are placed at different distances from the anode. Usually, one cathode is five times farther from the anode than the other. The ratio of the thickness deposited on each cathode can be used to calculate the throwing power of the system [30–32]. See Figure 25.4. Stress Cells There are several types of stress analysis cells available. Usually, they utilize a beam-bending technique to calculate the stress of the film deposited on a cantilever cathode. This can be done by using two beams and measuring how far apart from each other they bend or by using a sort of modified Hull cell test with tabs that bend in one direction as metal is deposited on one side [33, 34].

Cathode Anode Bus bar

FIGURE 25.3

Hull cell.

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FIGURE 25.4

Haring throwing-power box.

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MONITORING AND CONTROL

Recently, several researchers have begun using in situ stress measurement during electroplating experiments by incorporating a laser deflection measurement system as part of an electroplating cell [35–37]. This is a powerful technique because it has the capability to measure the stress in situ as the film is being deposited. This technique provides information related to the incremental stress added at each stage of the deposition, and it can also measure changes associated with holding the sample at open-circuit potential. With the right apparatus, the impact of monolayer adsorption on the cathode surface can be seen [36]. Electrochemical Quartz Crystal Microbalance There are specialized electrochemical measurement systems called electrochemical quartz crystal microbalances, or EQCMs, that utilize a quartz crystal as a working electrode [38–40]. The quartz crystal is caused to vibrate due to the piezoelectric effect and its oscillation frequency is monitored, along with the plating current and potential as a function of time. The oscillation frequency is a function of the mass of the crystal and the deposits on it, and the instruments are typically sensitive enough to measure submonolayer adsorption of organic materials on the electrode surface or micrometerthick deposits of metals. The Sauerbrey equation can be used to calculate the mass of a deposit on the crystal, which can be converted to thickness if the density of the deposit is known:  2 2ff Df ¼  mF Zq

sample. Fiber optics has made small colorimeters practical for automatic analysis of many plating solution species [45]. A light source and absorbance detector can be placed remotely from the analysis cell via a bundle of flexible glass or plastic pipe fibers. The optimum wavelength of light to be used for a given species is best determined from a spectrographic scan over a wide range of wavelengths, Awavelength at or near a strong absorption peak is selected and a calibration curve based on Beer’s law is prepared. The unknown can then be measured and its absorbance compared to the calibration curve to determine the concentration of the constituent of interest. UV/visible photometric analysis works well for the transition metals. Some examples and the wavelengths that are useful for analysis are shown in Table 25.2. For example, Cu(II) in electroless plating baths is determined at 620 nm wavelength. Cobalt(II) in hard gold baths may be analyzed by first oxidizing it with hypochlorite to Co(III). Cobalt(III) forms a pink complex with ethylenediaminetetraacetic acid (EDTA) which can be analyzed at 520 nm wavelength [46]: A¼elc

where A ¼ absorbance e ¼ molar absorptivity l ¼ path length or cell length c ¼ concentration 25.2.3

where Df ¼ frequency shift of the loaded crystal ff ¼ fundamental frequency Zq ¼ acoustic impedance of the material (8.8  106 kg m2 s1 for AT-cut quartz) mF ¼ mass per unit area The EQCM is a powerful tool for understanding the reactions that occur on a working electrode during a potential sweep or for understanding the cathode efficiency of a deposition. These instruments are typically used in process development or troubleshooting of electrochemical processes [41, 42]. Other Cells There are several other kinds of electrochemical cells that can provide useful information in the proper circumstances. These include cells such as jiggle cells and bent-cathode test cells [43, 44]. 25.2.2

Colorimetry or Photometry

Photometric analysis techniques are fast, simple, and relatively cheap. Additionally, these techniques can be easily automated, potentially without involving extraction of a bath

Beer’s law

Electrochemical Analysis

Additives in plating baths are important to achieving the desired physical properties of many electrodeposits. Additive concentrations must be maintained within a specific range for best results. Techniques used for monitoring additive concentrations include cyclic voltametric stripping (CVS) [47–53], polarography [54], and other electrochemical methods. The method used must be able to detect the active species in the presence of degraded nonactive material and/or impurities. Polarography Polarography using a dropping mercury electrode (DME) has made considerable progress over the past 50 years as a laboratory technique for chemical analysis of plating solutions (Table 25.3), but it was not used in the manufacturing environment until about 30 years ago, mainly because of the sensitivity of the electrode to manual handling. When manual handling is eliminated by automation, the DME can be used as a reliable and reproducible sensor in automatic analyzers and controllers in industrial applications [20, 21]. Cyclic Voltammetric Stripping CVS is used to determine the concentration of organic additives such as suppressors,

BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT

TABLE 25.2

Absorbed and Perceived Colors

Absorbed Wavelength (nm)

Absorbed Color

Perceived (Transmitted) Color

Metal Complexes

310 314 400 437 450 480 500 512 530 548 570 600 650–730

Ultra-violet Violet Violet Blue Indigo Blue Blue-green Yellow/green Green Yellow Yellow-green Orange Red

Pale yellow Yellow Green-yellow Orange Yellow Orange Red Violet/red Purple Violet Dark blue Blue Green

[Co(CN)6]3 Pd-NH3-complex

brighteners, and levelers from the effect the additive exerts on the electrodeposition rate at a given potential. The potential of a rotating platinum electrode is cycled at a constant sweep rate in a bath sample so that a small amount of metal is deposited on the electrode and then stripped off by anodic dissolution (Fig. 25.5). The charge required to strip the metal is related to the additive concentration using predetermined calibration curves, as in Figure 25.6. By using this electrochemical approach coupled with titrations of certain bath components or other suppressors and accelerators, a reasonable correlation can be made to the additives in the electroplating bath. Com-

TABLE 25.3

535

Co-NH3-complex [Co(NH3)5H2O]3+ [Ti(H2O)6]2+ CoCl2 KMnO4, Cu(ED)22+ Cu-(ED)2-complex Cu2+ Ni2+

puterized instruments have been developed that automatically determine additive concentrations [23]. CVS has become a very common method due to its relative simplicity and the availability of automated equipment to perform this analysis. One aspect of CVS analysis that is both an advantage and a drawback is that this technique provides a single numeric value that is used to monitor the characteristics of the plating bath. All of the information contained in the current values as the potential is dynamically scanned is lumped into a single number that is affected by all the bath constituents and contaminants at that point in time.

Examples of Polarographic Analysis of Several Plating Baths Polarography Can Determine These:

In These Baths

Major Components

Trace Metals

Organic Additives

1. Zinc Sulfate

Zinc

o-Chlorobenzaldehyde

2. Palladium 3. Gold(I) cyanide

Palladium, chloride Gold, free cyanide

4. Watts’ nickel

Nickel, chloride, boric acid

Copper, cadmium, arsenic Tin Cadmium, cobalt, copper, zinc, iron, tin, chromium —

5. Electroless copper 6. Copper sulfate

Copper, formaldehyde Copper

—

7. Solder bath 8. Brass 9. Nickel–cobalt

Lead, Sn(II) Copper, zinc Nickel, cobalt

Sn(IV) Lead, arsenic —

—

Hydroquinone

Saccharin, o-Benzaldehyde sulfonic acid Mercaptobenzothiazole Thiourea and thiourea derivatives

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MONITORING AND CONTROL

TABLE 25.4

Manufacturers of Plating Bath Analysis Systems

Company

Technique

ECI Metrohm Ancosys Technic ATMI/Semitool Dionex Applikon Ebara-Udylite

CVS, photometry, titration CVS, titration CVS, photometry, HPLC, titration CVS, polarography Chronopotentiometry HPLC Titration

that utilizes a small electrode as part of a probe that can be placed in a plating tank has been developed by Technic [58–60]. This has the benefits of an electrode probe which can be placed in the plating bath and which can run multiple analysis cycles without the need for withdrawing a chemical sample from the tank. This analyzer usually uses harmonic analysis of the plating response, which has been shown to be indicative of certain components of the plating bath. The method development seems to be a bit more empirical and the operating window of these systems can be narrower than with other techniques, but it is an operator-friendly system and is simple to use. 25.2.4

Chronopotentiometry and Chronoamperometry These techniques have also been used to analyze the additive concentrations in plating baths [55, 56]. A version of a chronopotentiometric technique that has been commercialized is referred to as pulsed cyclic galvanostatic analysis (PCGA) [57]. Figure 25.7 shows an example current pulse train and a series of potential response curves that may be generated as the suppressor concentration in a copper plating bath is increased. A set of response curves such as this can be used to generate a calibration curve similar to the one in Figure 25.6, which is then used to determine the additive concentration of an unknown bath. Advantages of these techniques include that they gather more information that is related to the state of the chemical constituents and they separate the dynamic nature of the process from the steady-state bath behavior (electrochemical activity). This allows the operator to try to better differentiate between the effects of multiple components of the chemistry. Real-Time Analyzer, or RTATM An analysis technique based on combinations of these electrochemical techniques

FIGURE 25.5

Ion-Selective Electrodes

Ion-selective electrodes have been developed which can monitor the concentration of a number of anions and cations [61–64]. A list of commercially available ion-selective electrodes is given in Table 25.5. The membrane can be glass, as for a pH electrode which measures H þ concentrations, or liquid or solid material. Some ion-selective electrodes have limited lifetimes such as the CN electrode. Only periodic analysis is practical with a water soak in between applications. However, this problem can be minimized with a welldesigned sampling and flushing system. Care should be exercised in selecting the most appropriate ion-specific electrode or oxidation–reduction potential (ORP) electrode for a particular application. Flow-through cells using ionselective electrodes have been used for continuous on-line monitoring of copper in electrowinning solutions [65]. Solution pH A pH electrode monitors hydrogen ion activity. Therefore it directly measures the acidity or alkalinity of a plating bath. All plating baths work best over a specific, sometimes narrow, pH range. pH sensing and control systems

CVS plot from copper sulfate plating bath.

BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT

537

Of course, hydrogen ion concentration can also be measured by acid/base titration. This technique has been used in several automated bath analysis systems for plating bath analysis and control. 25.2.5

FIGURE 25.6

CVS calibration curve.

are commercially available. pH electrodes are also used to detect the concentration of certain compounds in plating baths by measuring solution before and after a reagent addition. For example, in electroless copper plating the amount of formaldehyde present can be determined by measuring the pH of a sample of solution before and after fixed amounts of H2SO4 and Na2SO3 are added. This method was used by Photocircuits and McDermid for automatic analysis and control instruments for electroless copper plating processes. The standard glass electrode is a reliable pH electrode as long as it is kept wet. It cannot be immersed in strong alkali, fluoride or fluoborate solutions for very long before the glass is etched away. However, it can be used to measure pH in those solutions for a reasonable period of time if the electrode is rinsed and stored in distilled water after each quick pH measurement.

FIGURE 25.7

High-Performance Liquid Chromatography

High-performance liquid chromatography (HPLC) is another method that has been used to monitor the concentrations of plating bath constituents, especially organic additives. HPLC uses a pump-driven eluent stream (usually acidic) which carries the sample into a separation column containing a resin material designed to adsorb components of the bath with varying affinity. The resin material is chosen to retain the components of the sample in the column for differing amounts of time, dependent on their molecular weight and chemical interactions with the resin, which results in their separation into discrete bands before entering the detector. These bands are then detected by UV/VIS absorption, by conductivity, or possibly using an electrochemical detector. The output from this separation and detection is a chromatogram which is recorded with respect to elapsed time. As with the other techniques already discussed, the concentration of the samples can be determined by comparing their chromatogram peak heights or areas with those generated during calibration runs. HPLC and ion chromatography techniques are most often used in off-line applications [66]. They involve relatively high capital expenditure, and the maintenance requirements tend to be intensive with these systems, which has limited their use in automatic on-line analysis [23]. These techniques provide a measurement of the concentrations of certain constituents of the plating bath, as opposed to the impact they have on the electrochemical activity. This makes the methods a more direct measurement of the chemistry, which makes it important to be able to separate the constituents and

Current and potential plots from PCCA analysis.

538

MONITORING AND CONTROL

FIGURE 25.8 Mass spectroscopic analysis of fresh and used accelerator in copper plating bath.

identify the electroactive species as well as to quantitate them appropriately. 25.2.6

Mass Spectroscopy

Mass spectroscopy (MS) methods are based on separating atoms or compounds based on their mass. Mass spectrometers usually require some level of ionization, which may involve an inductively coupled plasma source (ICP-MS) or an electrospray ionization system. These systems can provide an impressive amount of information related to the chemical species present and their amounts, but they tend to be expensive and require significant data interpretation and maintenance.

A wide strip electrotinning line built by British Steel in Wales incorporates a fully automatic analytical system to monitor solutions along the line [67]. Samples are taken automatically from cleaning, pickling, and plating tanks by taking a chemical sample from each tank and conveying it to a remote analytical station. There a robot prepares the samples for analysis by an ICP mass spectrometer. Results are fed back to the control station where the operator makes the necessary adjustments. Recently, an automated bath analysis system has been developed using mass spectroscopy (Fig. 25.8) [1, 68, 69]. This system was designed as a process control system for plating baths, but it has the additional capability of determining an entire spectrum of components in a plating bath, including

BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT

TABLE 25.5 Electrodes

Commercially Available Ion-Selective

Lower Detection Membrane Limit (M)

Ion Hþ Na þ Kþ Kþ BF 4 NO 3 Cl Ca2þ Water hardness F Cl Br I S CN Ag þ Cd2þ Pb2þ Cu2þ

Principal Interferences

Glass Glass Glass Liquid Liquid Liquid Liquid Liquid Liquid

1014 106 104 104 104 104 104 104 103

None below pH 13 Hþ , Agþ Hþ , Na þ , Ag þ NH4þ , H þ    NO 3 , Br , CIO4 , I   I , Br  I, NO 3 , Br 2þ 2þ Zn , Fe , Pb2þ , Cu2þ , I Zn2þ , Fe2þ , Pb2þ , Cu2þ

Solid Solid Solid Solid Solid Solid Solid Solid Solid Solid

106 5  101 5  101 5  101 1017 106 2017 107 107 108

OH at high pH S, I, Br, CN I, S, CN S None S, I Hg2þ Agþ , Cu2þ , Hg2þ Agþ , Cu2þ , Hg2þ Agþ , Hg2þ

separating species that are similar but have slightly different molecular weights. This system, from Metara, provides volatilization through an electrospray system which keeps most of the constituents relatively intact for measurement purposes. 25.2.7

Atomic Absorption Spectrometry

Atomic absorption (AA) spectrometry is similar to photometry, but it utilizes an atomized sample of the liquid being analyzed and usually requires dilution of a bath sample before measurement. Atomization may be provided using a flame, an ICP source, or a graphite furnace. A narrowwavelength spectrum source such as a hollow cathode lamp or a laser is used to excite the electrons of the atomized sample to higher energy orbitals characteristic of the metal being measured by the absorption of the light from the illumination source. The absorption is measured and used to determine the amount of that metal present Atomic absorption analysis is fairly reliable and cheap but requires quite a bit of maintenance. Therefore, AA techniques have been used in off-line analysis in chemical laboratories but have not been used much in automated plating bath control. 25.2.8

X-Ray Fluorescence

X-ray fluorescence (XRF) analysis is discussed in Section 25.3 with respect to product monitoring, but it can also be used as

539

a technique to monitor the metal concentrations in plating baths. It is a relatively straightforward technique as long as the fluid handling is managed and is a good way to measure the ratios of metal ion concentrations in alloy plating baths. XRF is an expensive technique, though, and care must be taken to avoid errors due to drift of the X-ray source or detector. The frequent calibration requirements and expense have prevented widespread use of XRF for solution analysis, although industrial systems for this purpose have been built [70]. 25.2.9

Total Organic Carbon Analysis

Total organic carbon (TOC) analysis is sometimes used to provide a measurement of all the organic materials added to a plating bath. This usually includes additives such as grain refiners, brighteners, and surfactants as well as any other organic materials that are carried into the bath on the substrates being plated. TOC values do not include dissolved CO2 or carbonates from inorganic sources. These are removed from the bath sample by purging prior to the TOC analysis. The TOC level is determined by oxidizing the sample using chemical, electrochemical, or photochemical means. The evolved carbon dioxide can then be measured with a CO2 detector. TOC is a good way to evaluate the total amount of organic material introduced into the bath over its life and may be used to monitor the end of life of a plating bath due to buildup of organic additives and their breakdown products or other materials such as masking agents or photoresist components that may be leached out of the samples being processed. Organic impurities can be removed by activated carbon treatment when necessary. However, this usually strips the bath of all organic components so additives need to be replenished afterward. Harmful impurities that are known to build up in a plating solution should be monitored routinely. Often the first sign of an excessive impurity level is off-color plating or a film roughness and morphology changes. Water for rinsing often needs to be free of organic and inorganic impurities as well [71]. 25.2.10

Other Analysis Methods

Specific Gravity Specific gravity may be monitored to maintain a concentration range for sufficient solution conductivity or to detect excessive buildup of salts. A simple manually operated hydrometer can be used. A more elaborate automatic device uses a radio-frequency oscillator with a loop probe exposed to the solution. The oscillator frequency shifts in proportion to the specific gravity. Conductivity A conductivity measurement is sometimes used to determine the presence or absence of inorganic impurities. A good strategy to minimize the amount of

540

MONITORING AND CONTROL WASTE DISPOSAL PUMP ELECTROPOLISH

RINSE

D.I. WATER

D.I. WATER

PUMP

PUMP

PUMP ACID DIP

RINSE

NICKEL PLATE

RINSE

GOLD STRIKE

HARD GOLD

RINSE

PUMP FINAL RINSE

RESERVOIR TANKS

FIGURE 25.9

Water management system for a strip plater.

high-quality rinse water required and volume of wastewater produced is to use the good water first where impurities can be most harmful, for example, after nickel plating. When the rinse water conductivity reaches a set level, that water can be transferred back to the preceding rinses that do not require water of such high quality. This is illustrated in Figure 25.9 for a strip plater involving electropolishing, nickel plating, and gold plating. High-conductivity water is used only to maintain water levels in reservoir tank rinses after nickel and hard gold plating. When the conductivity in the nickel water rinse drops below a preset limit, water is pumped out of the rinse after electropolishing to waste disposal. That water is replaced by water from the rinse after acid pickling. Next water from the nickel rinse replenishes the acid rinse, and finally new water restores the rinse after nickel. The process continues until the conductivity of the nickel rinse water reaches a preset value. A similar feedback system is used in the gold plating side. The gold strike solution is operated at about 65 C (150 F) and loses water rapidly through evaporation into the exhaust. A low solution level activates a pump to transfer rinse water after hard gold to the gold strike reservoir. The final rinse water replenishes the preceding rinse. Fresh deionized (DI) water restores the level in the final rinse. In addition to conserving water, this system keeps the gold in rinses moving back to the plating solutions. A gold reclaim plating cell in the rinse tank after hard gold plating can also be used to reclaim gold.

ALARMS

SENSORS

SIGNAL CONDITIONING INTERFACE

PLATING PROCESS

CONTROL DEVICES

25.2.11 Automatic Plating On-Line Monitoring and Control Automatic electroplating may utilize on-line monitoring and control. The technology has developed rapidly with the availability of modern equipment, including sensors and electrical and electronic devices, and better mechanical designs [72–75]. Continuous-flow analysis of electroplating solutions can be fully automated [65, 76, 77]. Palladium, copper, nickel, tin, silver, iron, chromium, persulfate, peroxide, and chlorate are among substances that have been determined automatically. Figure 25.10 illustrates how the various parts of an automatic electroplating process monitoring and control system interact. Sensors provide information about the plating process. After signal conditioning, information is sent to a PLC or a computer that is programmed to control the system. Actual conditions can be analyzed and stored and/or displayed. Deviations from the programmed limits will activate control devices such as motors, valves, and power supplies to bring the plating process back in control. If the operator must be involved, an audio alarm sounds. When there are multiple alarms, each condition may have a different audio sound so the operator can quickly recognize the type of problem or a status display will show each alarm condition. Completely automatic analysis and control systems have also been developed which do not require any operator intervention during normal operation. DISPLAY

PLC or COMPUTER

DATA PROCESSING & STORAGE

Motor Valves Power supplies

FIGURE 25.10 Schematic of an automated electroplating process monitoring and control system.

BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT

541

FIGURE 25.11 Automated chemical analysis and replenishment system to support semiconductor copper plating tools.

Since the mid-1990s completely automated copper analysis and replenishment systems have been used with semiconductor plating equipment for copper damascene interconnect applications (Fig. 25.11). The analysis systems usually use a titration cell for determining copper and acid concentration, titration or an ion selective electrode for ppmlevel chloride determination, and titration combined with CVS or chronopotentiometry for analysis of organic additives such as suppressor, accelerator, and leveler. These systems typically make use of slipstreams that are recirculated from the plating equipment through the analyzer. The analysis system utilizes a block of valves to periodically divert a small sample from the slipstream to the analysis vessel, where it is analyzed and then sent to waste recovery. As explained earlier, electroless plating chemistries are not very stable and require frequent bath analysis and monitoring. Electroless copper plating parameters that need monitoring and control are temperature, pH, copper concentration, formaldehyde (reducing agent), and cyanide ion. Accurate temperature monitors/controllers are readily available. The pH value is measured with a standard glass electrode. Hydroxide solution is metered in as required to maintain pH. Copper concentration can be monitored colorimetrically or by polarography. Formaldehyde may be measured by adding 0.045 M sulfuric acid to a bath sample to reduce the pH to about 9.5. When a 0.05 M sodium sulfite solution (also at pH 9.5) is added, the resulting pH is proportional to the formaldehyde content. Polarography can be used to measure both formaldehyde and cyanide bath concentrations [20]. Several on-line electroless copper plating analyzers/controllers have been developed for copper plating of printed circuit boards [2, 20, 21, 78]. They are used to monitor and control baths that deposit a thin layer of copper in through holes prior to electroplating the thicker copper. Automatic

analyzers or controllers are essential when the entire copper circuit is built up by electroless plating. In 1971 Photocircuits developed its Mark VI automatic electroless copper plating bath analyzer to monitor and control its CC-4 electroless copper plating bath. Later McDermid developed a similar automatic bath analyzer which is shown schematically in Figure 25.12 [78]. A single peristaltic pump moves a sample and the reagents through the analyzer. The pH is adjusted, then the copper ion concentration is measured spectrophotometrically. A formaldehyde reagent is added next. The resulting pH is proportional to the formaldehyde concentration. The controller activates replenishment pumps to restore the copper ion and the formaldehyde concentrations to their optimum levels. Bell Laboratories machines utilized polarography to analyze for Cu2þ , HCHO, and CN [20]. With the solution still, polarographic currents are measured at 1.0 V for Cu2þ , 1.8 V for formaldehyde, and 0.25 V for free cyanide. This is illustrated in Figure 25.13. The machine automatically averaged 10 measurements of each component for better accuracy. The block diagram in Figure 25.14 shows how the various parts interact. The actual machine (Fig. 25.15) was built in two parts: one for wet chemistry on the left and the electronics on the right. The two units could be separated by several hundred feet if necessary. One advantage of the separation was that the wet chemistry cabinet could be hosed down occasionally. Electroless copper solution flows through the flow gauge at the left continuously. Periodically a sample is taken into the analysis cell along with some NaOH reagent. The pH is measured, and dropping mercury polarograph measurements are made. After electronic processing the results of analysis are displayed. If chemical additions are required, replenishment pumps are turned on. A complete analysis can be made every four minutes.

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MONITORING AND CONTROL

FIGURE 25.12 Schematic of McDermid electroless copper plating bath controller.

FIGURE 25.13 Polarographic analysis of electroless copper plating bath controller.

FIGURE 25.14 Electroless copper plating bath monitoring and control system.

MONITORING AND CONTROL OF FINISHED PRODUCT

543

usually done on a statistical sample for economic reasons. The advantage of on-line inspection is that every part may be tested. If plating is out of specification, either plating conditions can be changed automatically or an operator can be alerted for a manual correction. 25.3.1

FIGURE 25.15 Electroless copper plating bath automatic analyzer and controller: (A) mercury reservoir, (B) analysis cell, (C) plating solution flowmeter, (D) motorized syringes, (E) chart recorder, (F) analysis display meters, (G) pH monitor, (H) set point controls, (I) process controller, (J) polarograph, (K) viscosity monitor, and (L) power source.

25.3 MONITORING AND CONTROL OF FINISHED PRODUCT There are at least as many ways to measure the results of a plating process as there are products produced using plating processes. Ultimately, the monitoring of a product produced using a plating process or of the film plated should be done based on an understanding of the properties of the deposit that are important to that product. Having said that, we will review several types of measurements that are typically used for measuring the deposit quality of plated films. These are meant to be examples of common measurements and should be adjusted as necessary to ensure that a particular deposit meets the specifications demanded of it for a particular application. There is extensive literature regarding inspection of manufactured parts and quality control and inspections. These can be used to set up a statistically sound monitoring program to monitor the quality of parts being produced and to improve the product quality or reduce the variability. Typical electrodeposit properties inspected are thickness, color, brightness, hardness, magnetic properties, and alloy composition. Automatic instruments are available for off-line and on-line measurements. Off-line inspection is

Thickness

Various wet chemical methods are available for measuring electrodeposit thickness: coulometric, dropping test, and spot test [79]. A disadvantage of these tests is that the part used generally becomes scrap. A nondestructive thickness test is usually preferred. There are several methods available, and most have been automated as stand-alone instruments or incorporated into on-line monitoring and control plating systems [80, 81]. The methods used include beta backscatter, X-ray fluorescence, sheet resistance (eddy current or four-point probe), and magnetic field measurements. Deposited metals tested by these methods are shown in Table 25.6. Profilometry can also be used to measure deposit thickness when selective deposition is used or when part of the deposit can be etched away. In general, there are extremely sensitive instruments that have been developed for the microelectronics industry for measuring the properties of deposits produced on semiconductor wafers or similar substrates. These instruments are commonly automated wafer handling systems that are completely computer controlled and are capable of intensive data manipulation. While these systems are far different from the typical thickness measurement tools described in the sections that follow, they are increasingly becoming important in applications involving electrochemical deposition for microelectronic applications. Some of these thickness measurement techniques are then described under Profilometry, Optical Methods, and Laser-Acoustic Methods. These are examples of some of the systems that have been developed for microelectronics applications, where the substrate is very well controlled and where extreme precision is required of the deposits on them. Other new analytical techniques are being developed at a rapid rate, specifically for the microelectronics industry. Coulometric Method The coulometric method is based on Faraday’s law: One gram equivalent weight of metal is stripped away from an anode for every 96,500 coulombs of

TABLE 25.6 Nondestructive Electroplated Deposit Thickness Tests Method

Metals Tested

Beta backscatter X-ray fluorescence Eddy current

Au, Ag, Cd, Cr, Cu, Ni, Zn, alloys Au, Ag, Cd, Cr, Cu, Ni, Zn Cu, Zn, Cd, Ni, Cr

544

MONITORING AND CONTROL

electricity passed through the cell. Four parameters must be controlled: surface area, current, time, and anode (deposit) dissolution efficiency. At 100% anode efficiency, the deposit thickness is calculated from the formula 

10 Thickness ¼ eit Ad



where e ¼ electrochemical equivalent, g/A-s1 i ¼ constant current, A t ¼ time, s A ¼ area, cm2 d ¼ metal density It is necessary to use a specific electrolyte for each metal deposit and substrate [82]. The electrolyte must not chemically attack the plated film. An automated instrument can detect the endpoint by sensing a voltage change when the substrate metal is exposed. The method is capable of an accuracy of
10% of the true value. One advantage of the method is its ability to measure combination deposits such as copper–nickel–chromium. Dropping Test The dropping test is very simple. It is performed by dropping chemical etch solution on a particular spot at a rate of about 100 drops per minute. The operator observes the time it takes to expose the base metal. The test is calibrated using a sample with a known thickness of plated metal. For consistent results, the drop size, temperature, and etch solution must be controlled. Operator skill is an important factor in achieving reproducible results with an accuracy of
15% [83]. Spot Test This simple test was developed as a rapid and inexpensive test for chromium deposits on nickel or stainless steel. A drop of hydrochloric acid is placed inside a ring of wax on the part to be tested. Hydrochloric acid attacks chromium with the evolution of hydrogen gas bubbles. Gassing stops when all the chromium is dissolved. The time of dissolution is proportional to the chromium thickness. The test is calibrated with a known chromium-plated thickness specimen. An accuracy of
20% is obtained for deposits up to 1.2 mm thick [84]. Beta Backscatter The radioactive decay of certain isotopes produces beta rays, which are fast-moving electrons. When these are directed at an electrodeposit, some are slowed and change direction out of the deposit. The number of these backscattered electrons is proportional to the number of atoms per unit area and their atomic number. The backscatter is measured with a Geiger–Muller tube counter. The isotope used is chosen based on its maximum beta-ray energy and half-life. As the deposit thickness increases, the energy

needed to penetrate the deposit increases. Metals with higher atomic numbers require higher energies for the same thickness. Instruments designed to measure metal deposit thickness utilize a Geiger–Muller tube electron counter and, with a built-in microprocessor, compute the thickness. The device is also capable of feedback current control of on-line plating systems [85]. X-RayFluorescenceandX-RayReflectometry This method is similar to beta backscatter in that a radiation is directed at the deposit and the energy emitted is measured. The method uses X rays produced by an X-ray tube that is specific to the deposit metal. The radiation emitted from the deposit is proportional to the deposit thickness. The system may be calibrated with a known thickness and composition standard for additional accuracy. The instrument provides the thickness measurement which can be used to control the plating process. An XRF thickness monitor is used to control the thickness of zinc plating on steel mill strip by a feedback signal to a computer that adjusts the line speed in order to maintain a specified zinc thickness [16]. Emissions are specific for each metal, so alloy compositions can be determined. Metal deposit thickness in the range of 0.25–10 mm can be measured depending on the metal. Measurement accuracy is
10% with proper calibration [86]. Precision XRF measurement equipment has been developed for the microelectronics industry that provides much better precision than the simple instruments described above. These systems are capable of measuring with greater accuracy and potentially utilizing spot sizes as small as 50 mm. They can be used to measure thickness or composition of an alloy deposit. In some cases, XRF has been combined with X-ray reflectance (XRR) in order to provide precision thickness measurement of extremely thin metal films on flat substrates. XRR uses glancing angles close to the critical angle at which total external reflection occurs for a given film in a q/2q scan mode [87–89]. The value of the critical angle can be used to determine the density of the film being measured. At angles above the critical angle, the X rays penetrate the film and are reflected from the top and bottom surfaces of the film, giving rise to interference fringes which can be used to determine the film thickness based on the angular separation between the fringes. These systems are capable of measuring metal film thickness as thin as 2 nm, with accuracy on the order of 1 A [90, 91]. XRR does not work well beyond about 200 nm, so XRF techniques are employed for thicker films. XRR techniques can also provide information related to film roughness. Eddy Current (Sheet Resistance) Eddy current thickness gauges are electromagnetic instruments designed to measure a change in impedance of a coil that induces an eddy current into the plated metal. The phenomenon is based on the

MONITORING AND CONTROL OF FINISHED PRODUCT

difference between the electrical conductivity of the basis metal and the deposit that produces the change in impedance. The thickness measurement is performed with a probe which is positioned perpendicular to and sometimes in contact with the surface at the point of measurement. Thickness gauge instruments are available with a digital display, memory, and computer-prompted calibration procedure. The measurement accuracy is
10% to the true thickness in the range of 5–50 mm. Factors that influence measurement accuracy are surface contours, surface roughness, and type of plating process that can influence deposit conductivity [92]. High-precision specialized sheet resistance measurement equipment based on eddy current probes or four-point electrical contact is also available for the semiconductor and microelectronics industry [89, 93]. These instruments provide a very common method of determining the thickness of metal films based on measuring the sheet resistance. Magnetic Method The magnetic method utilizes the magnetic influence of the electrodeposit for measurement. Two types of probes are in use: (1) a mechanical device that measures the influence of the plated metal on the attractive force between a magnet and the base material and (2) an electromagnetic probe that measures the influence of the plated metal on the reluctance of a magnetic flux through the deposit and base metal. Three types of deposits can be measured with the second probe: (1) nonmagnetic deposits on ferromagnetic base metals, (2) nickel deposits on ferromagnetic base metals, and (3) nickel deposits on nonmagnetic base materials. Both probe types require perpendicular positioning on the surface. The magnet probe works by measuring the force required to pull it from the surface. The electromagnetic probe measures the reluctance difference with and without the plated metal. The use of both probes requires a calibration, and averaging several measurements improves the accuracy. Commercial instruments utilizing the electromagnetic probe are available with microprocessors that can automate the system by averaging data, display results, store or transmit information, and provide on-line monitoring and control [94, 95]. Profilometry Profilometry is a convenient method for measuring the plated thickness of features deposited through an insulating mask such as photoresist on a flat substrate. It can also be utilized when a portion of the deposited film is etched away after the deposition. Typically, a profilometer makes use of a very small needlelike stylus that is moved across the sample as its vertical position is monitored. The height of the sample can then be plotted against the lateral dimension to provide an image of the sample surface. Once the image is leveled, it can be used to provide information such as maximum deposit thickness, average deposit thickness, or deposit roughness.

545

Variations of stylus profilometry have been developed using other measurement techniques. Systems for measuring semiconductor wafers and similar substrates have been developed based on optical profilometry and atomic force microscopy (AFM). Each technique has its own advantages regarding sample area or length, measurement time, and height variation that can be measured. In the end, though, each of these techniques provides a two- or three-dimensional image of the surface profile that can be used to provide thickness profiles and roughness information. The microprocessors that are used to control these systems usually have enough computing power to manipulate the information and provide statistical details or to identify features that do not fit the expected profile for quality control or yield management purposes. Optical Methods Optical profilometers have been developed, as mentioned above. These typically use an optical lever to measure changes in step height of a sample. In addition, interferometric techniques have been used to measure height variations on flat substrates. These can be monochromatic light measurements or broad-spectrum measurements using phase shift interferometry or vertical scanning interferometry, respectively. Optical profilers can be used to do the same job as stylus profilometers, although they more commonly produce a three-dimensional image of a surface by scanning the laser measurement point or by measuring a broad area of the sample as in interferometry. Interferometry systems are very useful for measuring large areas quickly, such as measuring the height of electroplated solder bumps on a wafer for flipchip attachment. These systems rely on intensive computing power and calculations and are very good at identifying small numbers (ppm level) of features out of many that do not have the expected height. Examples of optical measurement systems for wafer applications are Rudolph Technology’s NSX systems, Zygo’s NewView interferometers, KLA-Tencor’s MicroXam, and Veeco’s Wyko instruments. Laser-Acoustic Methods Laser-based optoacoustic methods have been developed recently for measuring film thickness of opaque films such as metals. There are two main techniques that have been commercialized as automated systems for measuring metal film thickness of semiconductor wafers or other flat substrates. Both techniques are capable of measuring film thickness with a high degree of precision using a small spot size and are capable of measuring the thickness of multiple films in a stack. The two techniques make use of probing and excitation lasers and mathematical models of the optical and/or acoustical properties of the film stacks in order to provide film thickness measurements. One method uses a sonar technique, and the other uses surface acoustic waves to determine the film thickness [89].

546

MONITORING AND CONTROL

The optoacoustic technique that is known as picosecond ultrasonic laser sonar uses a very short laser pulse to thermally excite the sample surface. This launches an acoustic wave down into the film stack, which is reflected back to the surface at each material interface. As the sonic wave bounces through the film stack producing acoustic reflections, a probe laser detects the vibrations at the sample surface. A model based on the sonic velocity in each material and the expected film thickness is used to determine the actual film thickness in the stack. This method was commercialized by Rudolph Technologies as its MetaPULSE metrology tools. The surface acoustic wave technique uses two probing lasers to produce an optical grating that launches an acoustic wave in the film stack. The time-dependent diffraction of light is then analyzed with a physical model of the acoustic wave to calculate the film thickness. This technique detects surface acoustic waves traveling in the plane of the film being measured. This technique has been commercialized by Philips Analytical and is available from Semilab AMS (Advanced Metrology Systems) in its AMS 3300 metrology system. Both the Rudolph and Philips metrology systems rely on mathematical models of the physical system being analyzed and its acoustical properties. This makes them sensitive to variations in roughness at the film interfaces and density variations as well as defects within the small spot size being measured. On the other hand, they are high-resolution techniques that can be very useful with the proper understanding of the sample being analyzed and its relationship to the measurement configuration. 25.3.2

FIGURE 25.16 Schematic of an on-line gold plating color monitor.

color. The result can be displayed on a meter and/or fed into an automatic process control system to correct off-color plated gold.

Color

The color of electrodeposits has an esthetic value particularly on jewelry, home appliances, autos, and other decorative objects. Even on parts that are hidden from view when assembled into an object, an attractive appearance is considered a quality factor. Off-color plating or variable plating color often is considered a sign of a process problem. Generally, monitoring and control of color are done by visual inspection by an operator but on-line automatic inspection of color is feasible for many plating lines. An optical fiber system was developed to monitor the color of gold plating on a moving strip, shown schematically in Figure 25.16 [96]. An optical fiber bundle is positioned over a gold-plated strip. The bundle is divided into three parts. Light is directed into one arm that illuminates the moving strip. Reflected light from the gold surface passes into two separate optical fiber bundles. The light from one passes 700–800 nm through an optical filter that serves as a standard. The other light path passes 450–575 nm through the optical filter. An electronic comparator determines any deviation from the optimum

25.3.3

Brightness

Reflectivity is a common method used to measure deposit brightness, especially in the semiconductor industry. Reflectivity measurements are simply quantitative measurements of the amount of light reflected from a given surface area. The instruments used to measure reflectivity must usually be calibrated to a reference standard, and the values reported are relative to the reference. In industrial applications, deposit brightness may be judged qualitatively by simply deciding if the deposit is bright and shiny and has the desired surface appearance. Bright plating for decorative applications usually requires closer monitoring and control of the bath chemistry. Additives that produce brightening, usually organic, can become depleted or degraded by oxidation at the anode or from atmospheric exposure. Additive depletion is easily corrected by adding more additives, and this can be done by metering it in on the basis of plating time (coulombs) and/or elapsed time. Brightener concentration also may be controlled by

MONITORING AND CONTROL OF FINISHED PRODUCT

547

chemical analysis, plating tests (Hull cell), or CVS, as described in Section 25.2 [47, 48].

be affected by the film thickness or the edges of a patterned structure that is being used for measurements.

25.3.4

25.3.6

Roughness

Roughness of the plated surface is a measure of the quality of the plating process that is also related to the deposit brightness. A deposit that has a rough morphology will tend to be dull or matte in finish, rather than bright. Roughness can be measured quantitatively, or it can be gauged qualitatively by looking at optical or scanning electron microscopy (SEM) photomicrographs. The roughness of a deposit will typically have several dimensional scales and may be related to the grain size or the crystal structure of the deposit, as seen in Figure 25.20. In some cases, the deposit roughness may increase, become nodular, or even produce dendritic structures as the plating rate approaches the limiting current density of the system. The deposit morphology may also be affected by other structures on the surface, such as underlying topography or masking materials such as photoresist. These effects can be used as indicators of changes in the process or chemistry or can be used to gauge how close a process is to the edges of its process window. There has also been some work done on correlating the roughness evolution as a function of thickness for plated films. These data provide information related to the nucleation size and density and the film growth.

Film stress can be a very important parameter to monitor due to the possibility of creating problems with the part after plating. Excessive film stress can create problems with adhesion which lead to delamination or it can cause cracking or lead to excessive etching of the film when it is exposed to chemistry. Plating test cells for monitoring stress were discussed in Section 25.2.1. Film stress can also be measured on the part after the electroplated deposit is produced. In the semiconductor industry, global film stress is measured by evaluating the wafer bow before and after plating a film on one side. The radius of curvature, or deflection at a given distance from the center, is measured and Stoney’s equation is used to determine the stress of the thin film that was added to the substrate [97]. The reaction of the relatively thick substrate to the deposited film thickness produces curvature changes, as shown in Figure 25.17. The film stress may also be estimated from XRD measurements. These measurements are essentially a measure of the strain of individual crystals of the deposit taken from the measurement of their lattice parameter, so they may not correlate to the global film stress measured using the wafer bow or bending beam methods. 25.3.7

25.3.5

Hardness

Electroplating hardness is an important property to control in applications where minimum wear is required. There are many test methods for hardness available, but all are done off-line. Most testing is done with indentation instruments of somewhat different designs. The various types are Brinell, Rockwell, Vickers, Solersoope, Knoop, and Tukon. An indenter is used to form an indentation in the deposit using a known force, and the size of the indentation is measured to determine the hardness of the material. Care must be taken to ensure that the size of the indentation is small enough not to

Film Stress

Grain Size

The grain size of an electrodeposited film is important in determining the mechanical properties of the material. Of course, grain size relates to the hardness of the material. It has also been shown that the grain size of electroplated copper films has a strong impact on the resistivity of these deposits [98]. There has been quite a volume of work presented on the characterization of grain size changes that occur in these films after deposition at room temperature or at elevated temperatures [99–109]. The grain size changes have been related to changes in resistivity, hardness, crystal texture, and in-film incorporation of organic additives.

FIGURE 25.17 Effects of film stress.

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FIGURE 25.18 Grains in electrodeposited copper film.

The grain size of an electrodeposited film may be estimated from XRD data or it may be measured directly from an image that highlights the individual grains. Software may be used to identify individual grains and create grain size distribution plots, or standard methods may be used to estimate average grain sizes based on counting the number of grain boundaries intersecting a line of a given length [110]. Recently, focused ion beam (FIB), systems have been developed that utilize a focused beam of gallium ions to cut through thin films. The gallium beam may also be used to image the sample, and the channeling depth variation of the gallium ions into grains with varying orientation provides excellent contrast, as seen in Figure 25.18. 25.3.8

X-Ray Diffraction and Crystal Texture

X-ray diffraction (XRD) analysis can be useful in understanding the characteristics of a plated film. XRD analysis

can provide estimates of grain size and lattice constant as well as information related to the crystallographic orientation or texture of the grains of a deposit. Theta-Two theta scans and rocking curves provide standard plots that are used for these purposes. Complete XRD pole figures may also be used to get a more complete understanding of the orientation of the crystal lattice. The texture of plated films can be important in that it may affect the magnetic properties, the interactions with other films in a stack, or the mechanical or electrical properties of the film. 25.3.9

Recrystallization Rate

There has been an enormous amount of information published in the last decade regarding “self-annealing” or recrystallization of electrodeposited metal films, especially copper [99–108]. These publications focus on grain growth,

FIGURE 25.19 Effect of electroplated copper film recrystallization of sheet resistance.

MONITORING AND CONTROL OF FINISHED PRODUCT

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reduction of resistivity, changes in film stress, and changes in crystallographic texture (see Fig. 25.19). There is also information relating to the evolution of organic contaminants that have been incorporated in the film and the coalescence of dislocations to form microvoids in the film. Grain recrystallization and grain growth in plated films are driven by an excess of grain boundary energy. This process is accompanied by the diffusion of contaminants that have been incorporated in the film during deposition and migration of dislocations as well as stress changes in the film. These four processes are all occurring together, so the behavior can be somewhat complicated. Understanding these processes and noting differences in the extent to which they occur and their rates in plated films can be useful in understanding differences in plated grain size, dislocation density, formation of voids, incorporated contaminants, and film stress. Some of the first work done on the “self-annealing” effect, or grain growth at room temperature, dealt with grain growth in mechanically worked sheets of copper [111–114]. The mechanical properties and XRD analysis of these metal sheets were used to characterize the effects of recrystallization and grain growth. Obviously, this effect has been known for a long time, although it has been studied most extensively in the last 10 years as electroplated copper has been used in semiconductor devices. The behavior of copper films has been shown to relate to the electromigration resistance and stress migration induced voiding that can cause reliability failures in these devices.

measurement of the surface roughness with a profilometer and calculating one of the many roughness parameters that are commonly used. There is even an example of an in-line automated reflectometer that monitors the color or the roughness of plated gold, as shown in Figure 25.16. Roughness is an important deposit parameter that represents how the process is performing. The roughness of a plated film is determined by the plating rate, the waveform used for plating, additives in the chemistry (brighteners, levelers, grain refiners, etc.), contaminants in the chemistry, and cleanliness of the substrate before plating. If the desired deposit is bright and shiny, the roughness should be minimized. This may require the use of grain refiners and leveling agents and sometimes requires leveling of roughness that may already exist on the surface. On the other hand, sometimes a “matte” finish is desired, so a controlled amount of roughness is used to produce this surface finish. It may even be desirable to produce a rough film in order to improve mechanical adhesion or maximize surface area for some other purpose. In any case, it is important to understand the process variables that impact deposit roughness and to control them in order to minimize variations over the long term. Morphological variations are sometimes associated with masked features on a plated part. These may be due to simple current crowding effects or they may be due to interactions of the masking with additives in the plating chemistry [115]. These effects can be seen in Figure 25.20.

25.3.10

25.3.11

Surface Morphology

The surface morphology of plated films can be important for several reasons. The morphology can include local surface roughness as well as effects that relate to plating within masked patterns. Measurement of the morphology of plated films will depend on the application and may range from an optical inspection, or imaging with a microscope or SEM, to

Surface and Compositional Analysis

Of course, the composition of the electroplated deposit is usually an important parameter to measure. There are many methods available for measuring the bulk composition or surface composition of a deposit, some of which are destructive to the sample being measured. The composition may also be inferred by measuring some other property of the deposit,

FIGURE 25.20 Morphology of through-mask electrodeposits.

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such as magnetic moment, magnetostriction, or melting temperature. These methods are particularly useful for alloy deposits. X-ray fluorescence is one of the most common methods of compositional analysis used for measuring electroplated alloy composition. It works well for measuring heavy elements like metals in quantities above 0.5% and has the additional benefit of providing thickness measurements for films less than 10 mm in thickness. XRF is a nondestructive method and can be adapted to work for many different plating applications. Equipment ranges from expensive equipment adapted for measuring films on semiconductor wafers or similar substrates, as described above, to simple hand-held instruments that are designed as portable thickness gauges [86]. Destructive methods for composition analysis usually involve dissolving the film in an acid and measuring the quantity of each metal in the solution. These methods work best for compositional analysis of alloy deposits. The most common methods in this category include AA analysis and ICP-MS. A method somewhat similar physically to beta backscatter metrology is Rutherford backscatter spectrometry RBS. This method uses positively charged ions to bombard the sample and measures the backscattered ions. By measuring the energy loss of the backscattered electrons, the composition and thickness of thin films can be measured. This is a more involved method than beta backscatter thickness measurements that is useful for surface analysis [116]. When it is important to measure very small quantities of contaminants in plated films or to measure light elements, it is usually necessary to use some sort of surface analysis technique. The drawback is that these techniques usually measure only very near the surface of the film although they can be very sensitive. In order to measure below the surface of the film or to remove surface contamination before measuring, it may be important to use argon sputtering to remove the surface layers or to sputter through the film and make measurements in a depth profile. The most common methods used in this category include Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis or X-ray photoelectron spectroscopy (ESCA or XPS), and secondary ion mass spectroscopy (SIMS). 25.3.12

plated feature from the surface. The force required before failure is monitored, and the failure mode can be inspected to differentiate between interfacial failures, brittle bulk material failure, and ductile bulk material failure. Wire pull testing can also be used to understand the mechanical strength of an interconnect feature with forces perpendicular to the substrate surface. In this method, a wire is bonded to the pad or feature to be tested and it is pulled straight away from the surface. Again, the force is measured and the failure mode or interface can be determined by optical inspection or compositional analysis of the failed interface. Variants of these tests can be devised to monitor the most important failure mode for a given application. 25.3.13

X-Ray Transmission Imaging

X-ray transmission imaging has been used, especially for detecting voids or similar defects, in quality control of electronic components such as plated wiring or solder bumps on boards or semiconductor wafers or chips. This technique is analogous to medical X rays in that the X rays are passed through the sample and the image shows the intensity of the X rays that pass through the sample. Voids are seen as light spots in the image. This can be an important technique for inspecting solder bumps after reflow, because it is possible that voids may be created during the reflow process, especially if the amount of organic materials incorporated into the film is high. See Figure 25.21. Similar to medical X-ray imaging, systems are now being made that use X rays for tomographic imaging. By utilizing the relative rotation of the sample and X-ray detector, threedimensional tomographic images may be created and used to visualize a sample. These can be useful for viewing multiple

Shear Testing or Pull Testing

Mechanical testing can be an important part of monitoring plated deposit performance. Shear strength testing is a method that can be used to measure the mechanical properties of a deposit or to monitor for interfacial adhesion problems. This is a common technique used for monitoring electronic components such as solder bumps or other electrical interconnect features. It is a destructive method (at least for the feature being tested) which involves shearing off a

FIGURE 25.21 Transmission X-ray image of solder bumps after reflow.

REFERENCES

FIGURE 25.22 Transmission X-ray image of through-silicon vias.

levels of electrical interconnections on an electronic part. With improved X-ray sources and detectors, the resolution of these systems continues to be improved. See Figure 25.22. 25.4

SUMMARY

The electroplating industry has at its disposal an extensive amount of information and technology for monitoring and controlling all aspects of the plating process. Sensors have an important role in monitoring the status of each important parameter that must be controlled for successful plating. The design and engineering of individual sensor devices have improved greatly in recent years. Many incorporate a microprocessor for signal conditioning, and some now can also carry out control of certain plating conditions directly without relying on a PLC or computer. As new phenomena and technology have been converted into practical cost-saving devices for monitoring and controlling electroplating parameters and systems, the industry has adopted them. Electronic devices, computers in particular, have revolutionized the conversion of sensor signals to a useful form for monitoring and control. Cheap computers have made it easy and cost effective to automate plating systems. Further, plating information can be gathered and analyzed quickly to ensure that quality is maintained at all times. Automation of electroplating utilizing monitoring and control has resulted in lower manufacturing costs while improving quality and reproducibility of processing. Constituent monitoring and control for solutions used for electrochemical deposition have improved dramatically as well. The emphasis has been on providing rapid analysis while reducing the chemistry usage and increasing the level of automation and control of the chemical control system. Electrochemical analysis techniques have been widely used,

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and other analytical techniques have been integrated into on-line analysis systems as well. The level of understanding of what is happening to the bath chemistry over time and how to control it has been continuing to improve. Finally, the number of choices for measurement and monitoring of the finished product has continued to grow. Our ability to measure material properties with more accuracy and to ever-finer levels of resolution allows us to improve our understanding of the materials we are producing. While there has been a large number of measurement techniques discussed, there are many other techniques that were glossed over or not discussed at all. The appropriate techniques to monitor deposits for a particular application must be explored, adapted, and sometimes invented in order to ensure that the important deposit properties are being monitored to the level required to ensure product performance. The more we learn, the more we find that there is a lot more to be learned in order to completely understand our plating processes and the deposits we are creating.

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