UV Lasers Enable Smart Phones and Other Microelectronic Devices The smaller package sizes and increased functionality of today’s smart phones create numerous manufacturing challenges, many of which are now being solved both with high performance UV DPSS and excimer lasers. by Larry Shi, DPSS Product Manager and Ralph Delmdahl, Excimer Product Marketing Manager, Coherent Inc, [email protected] Laser technology's role in semiconductor and microelectronics fabrication is growing dramatically as manufacturers seek to economically produce smaller and more energy efficient devices having ever greater functionality. Nowhere is this trend more clearly evident than in the current generation of smart phones, which combine impressive processing horsepower with high quality displays in a handheld package. Increasingly, component manufacturers have turned to ultraviolet (UV) and deep UV (DUV) DPSS and excimer lasers for high resolution, high energy, and low damage processes to enable smart phone production. This article reviews a sample of the key laser-based applications used in smart phone manufacturing, as well as the laser technology that has been developed to service them with an outlook toward the future. Wafer Singulation The combination of small physical size and high functionality required for smart phones creates a need for thinner memory wafers (for advanced packaging), as well as wafers incorporating low-κ dielectrics to improve power consumption. Both these wafer types present significant challenges for traditional die singulation using saws. In particular, the low-κ dielectrics exhibit high porosity, physical softness and poor adhesion making traditional saw cutting problematic. As a result, laser scribing, often called “half-cutting,” has already become the most common method for singulating low-κ dielectrics. In this process, the laser is used to cut through the soft epi layers, thereby isolating them on the die and leaving their edges layers relatively clean and undamaged. This allows the wafer to then be mechanically sawed without the blade ever coming into contact with these layers.

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Four Laser Beams Thin Saw Blade

Street Circuitry

Low-k Layers

Silicon Tape

Figure 1. Schematic of one type of laser half-cutting, in which both a UV laser and a conventional saw are used for singulation. For memory wafers, mechanical sawing is still predominant in the industry; but, as wafers become thinner, mechanical sawing create cracks or breakage, and so must be performed more slowly, thus reducing throughput. These problems can be avoided by using the laser to cut completely through the wafer – referred to as laser dicing. Overall cost parity between lasers and sawing can be expected for wafer thickness below 100 microns, with the advantage to laser processing at 50 microns and less. Currently, q-switched, DPSS UV (355 nm) lasers like the Coherent AVIA 355-23-250, which has been specifically designed and optimized for wafer scribing, are the source of choice for singulation applications. The short wavelength produces a small heat affected zone (HAZ), and the short pulsewidth of q-switched lasers (tens of nanoseconds) means that the thermal energy from each pulse is further minimized and can dissipate by conduction before the next pulse arrives. The high rep rate and high power also reduce cost of ownership (COO), thus driving cost of manufacturing down.

Figure 2. The AVIA 355-23-250 is optimized for wafer scribing.

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Future trend There are benefits (processing simplicity and precision) to be gained in going to higher precision cutting. The industrial picosecond laser has recently emerged as a solution. This is because pulses in the picosecond regime remove material mainly through an optical process called multiphoton absorption. This is a relatively cold (non-thermal) process which can deliver superior edge quality over nanosecond lasers, thus improving yields and potentially eliminating the need for post processing. The Coherent Talisker laser, which provides 4W of output at 355 nm with a 15 ps pulse width, exemplifies this new breed of industrial pico-second lasers which will offer unprecedented precision, speed and reliability. Quality/Defect control – Chip Inspection The purpose of wafer inspection is to find yield limiting or performance reducing defects during chip manufacturing. As critical circuit dimensions shrink, manufacturers are tasked with finding smaller and smaller killer defects. Current state-of-the-art for integrated circuit fabrication is 65 nm critical dimensions. This is expected to transition to 45 nm, and then 32 nm within the next five years. Most laser-based wafer inspection utilizes scattering and absorption phenomena to locate defects or contamination. There are different configurations for how laser inspection is implemented, depending upon the type of surface being examined (pattern or unpatterned wafer), and the type of defect being sought (pattern defects, contaminants, voids, etc.). The laser is typically transformed into a line or scanned across the surface of the wafer under test, and scattered or returned light is detected by a photomultiplier or array detector. UV Laser Beam

Focusing Optics

Array Detector Collection Optics

Scattered Sc Light

Wafer Under Test

Figure 3. Schematic of patterned wafer inspection.

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Meeting the need for higher sensitivity and decreased feature sizes has pushed tool builders to transition from visible wavelengths (600 – 800 nm) to deep into the UV, in particular to 355 nm and 266 nm. This trend to DUV wavelengths mirrors the lithography trends; smaller defects are generally detected with shorter wavelengths. The demand for throughput is met by increasing output power. While early inspection tools operated in the 10 to 50 mW range, today’s high throughput tools require watts of power. A typical laser source for wafer inspection is the mode locked DPSS laser. Mode locking enables efficient frequency tripling to 355 nm, while delivering the desirable, near CW, lower peak UV powers. Use of mode-locking architecture also enables scalable high average power. At the high end of performance is the Coherent Azure, which is a continuous wave (CW) laser that delivers 200 mW at 266 nm. The shorter wavelength enables the highest sensitivity and the combination of power, beam quality and CW operation enables the surface of the wafer to be scanned relatively rapidly, thus supporting high throughput. Future Trend The requirements for reliable systems with higher power at shorter wavelengths will continue as manufacturers seek lower and lower per part inspection costs and higher sensitivity to meet 32 nm nodes and below. Laser Direct Imaging The demand for smart phones that fit more circuitry into a smaller space has also led to increased use of high-density interconnect (HDI) circuit boards, and Laser Direct Imaging (LDI) has become a key technology in the production of these. In LDI, a modelocked UV laser is used to image a pattern directly onto a photoresist-coated panel, completely eliminating the use of a traditional photo tool, i.e. film. The most obvious benefits of LDI are the time and costs savings associated with the creation, use, handling and storage of these photo tools. In addition, LDI avoids any quality problems associated with film-related defects. LDI also delivers significantly better registration than traditional contact printing fabrication methods. This improvement can increase process yields, especially when dealing with the tight tolerances encountered in HDI boards.

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Modulator Scan Optics

Paladin Laser Direction of Motion Panel

Figure 4. Schematic of LDI operation. The Coherent Paladin laser was developed to meet the needs of LDI and other applications that need a reliable, high-power UV laser source with reduced operating costs. The Paladin is a modelocked, diode-pumped, solid-state laser with frequency-tripled output at 355 nm. Output powers of up to 16W are available. Paladin’s all-solid-state construction produces ruggedness, high reliability and long lifetime, and yields an output beam with excellent mode quality and extremely good pointing, power stability and noise characteristics. The 16W output level makes it possible to use less expensive dry films while still maintaining adequate process throughput rates.

Figure 5. With 16W of output, the Coherent Paladin 355-16000 increases LDI throughput over earlier lasers.

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Low Temperature Polysilicon Annealing Display screens for high end smart phones such as the iPhone are based on poly-silicon rather than the amorphous silicon used for most flat panel displays. Polysilicon possesses substantially higher electron mobility than amorphous silicon. As a result, liquid crystal displays (LCDs) based on polysilicon technology can deliver higher resolution and brightness, greater angle of view, and higher pixel refresh rates. The use of polysilicon also offers the possibility of display driver circuitry integration on the panel for the next step in the ongoing miniaturization process. Excimer laser based low temperature polysilicon (LTPS) annealing with a wavelength of 308 nm is now the preferred approach for producing the critical polysilicon backplane layer during display fabrication. This is because it can be performed at temperatures as low as 200°C, eliminating the need for expensive quartz or thermal glass substrates. At present, the most widely used LTPS annealing technique is called excimer laser annealing (ELA). In ELA, the rectangular beam from a 308 nm excimer laser is optically homogenized and reshaped to form a long narrow line (typically 465 mm x 0.4 mm) that has a high degree of energy uniformity throughout its profile. This line profile is directed at the silicon coated substrate which is then scanned relative to the beam. Excimer laser line beam a-Si

poly-Si

Substrate Translation

Figure 6. Schematic of ELA. Silicon efficiently absorbs 308 nm radiation making it possible to achieve near complete melt with each individual pulse. This leads to efficient crystal formation due to crystal growth in the vertical direction, starting at the interface between the molten and residual un-molten silicon. ELA requires an excimer laser that combines high pulse energy (1 joule) and repetition rates of several hundred hertz at very high energy stability. High pulse energy enables a wider area to be processed with each pulse, while maintaining fluence levels in the process window. High repetition rate is necessary to achieve the required throughput. Traditional excimer lasers

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delivered either high pulse energy or high repetition rate, but not both. Coherent has met the needs of ELA with the LSX series, which can deliver 1 joule pulses and repetition rates of up to 600 Hz.

Figure 7. The LAMBDA SX offers the high pulse energy and repetition rate necessary for ELA.

Touch Screen ITO Patterning The cost of touch screens has been steadily declining over the past few years, and, as a result, they are becoming increasingly common on smart phones. For example, the global touch screen market was about $1.2 billion in 2007, and is expected to reach over $5 billion in 2012. Currently, there are three different technologies employed to construct touch screens; these are resistive, capacitive, and surface acoustic wave. Resistive and capacitive are most commonly used for the small to medium sized screens found in smart phones. While resistive technology dominates the market, the iPhone uses capacitive technology to allow sensing of multiple touches simultaneously, and the popularity of this technology is expected to increase over the next few years. A touch screen panel typically consists of a top protective cover; a bonding layer, a patterned transparent conducting oxide (typically ITO) layer, and a glass substrate. Some manufacturers use UV DPSS lasers to scribe a series of lines of about 25 µm to 50 µm in width through the TCO layer. In some cases, they are also used to remove TCO in order to create a more linear response across the face of the device.

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Excimer and DPSSL UV laser processing offers several advantages over conventional ITO patterning technologies, such as lithography. In particular, large area excimer laser patterning offers higher throughput, the elimination of wet chemicals and all their safety and environmental concerns, greater process flexibility and the ability to produce smaller features. The laser typically employed for touch screen scribing is a UV DPSS laser that delivers over 20W at 355 nm, such as the Coherent AVIA 355-20. Not only can this type of laser easily deliver the required feature size for touch screen scribing, but, more importantly, the short wavelength doesn’t penetrate far into the substrate, meaning a minimal heat load on what is often a delicate thin glass or plastic substrate. Conclusion A broad range of UV excimer and DPSSL lasers have become an important tool in a wide variety of microelectronics production applications because they support the trend towards smaller circuit geometries, and often enable a greener and more economical process than other technologies, especially those based on wet chemistry. As miniaturization and increase in functionality of personal electronic devices continues, we expect that new and exciting applications requiring lasers generally, and short wavelength UV lasers in particular, will develop as manufacturers seek enabling, low cost technology to produce the next generations of personal microelectronic devices Figure Captions 1. Schematic of one type of laser half-cutting, in which both a UV laser and a conventional saw are used for singulation. 2. The AVIA 355-23-250 is optimized for wafer scribing. 3. Schematic of patterned wafer inspection. 4. Schematic of LDI operation. 5. With 16W of output, the Coherent Paladin 355-16000 increases LDI throughput over earlier lasers. 6. Schematic of excimer laser polysilicon annealing (ELA) processes.

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7. The LAMBDA SX excimer laser offers the high pulse energy and repetition rate necessary for ELA.

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