Ion beam Etching and Reactive Ion beam etching

Ion beam Etching and Reactive Ion beam etching By Tim Jolly Introduction Ion beam etching is a versatile etch process in which the substrate to be etc...
Author: Shon McKenzie
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Ion beam Etching and Reactive Ion beam etching By Tim Jolly Introduction Ion beam etching is a versatile etch process in which the substrate to be etched is placed in a vacuum chamber in front of the broad-beam ion source (see Figure 1). Ions (typically argon) are generated inside the ion source and are accelerated into a broad parallel beam, and to a defined energy, by the extraction grids on the front of the source. As the ion beam etches the surface, the substrate is tilted to an angle in the beam and continuously rotated in order to optimize the smoothness of the etch. If a pattern is being etched by the use of a photomask, the use of tilt and rotation allows the user adjust the wall angles in the resulting etch. If one uses an inert gas such as argon, the process is relatively slow, (typically 50 - 100nm/minute,) and the heat that is generated must be removed with care.

Figure 1 source

Oxford Instruments Ionfab300Plus with optional sputter

Ion beam processing has been transformed in recent years by the introduction by OIPT of a range of inductively-coupled ion sources that in the case of the 35cm ion source allows the etch of substrates up to 8” diameter (see Figure 2). The 35cm ion source uses a 2MHz RF generator (see Figure 3) and the 15cm ion source (see Figure 4) uses 13.56MHz. The ion source is virtually maintenance-free. This is in stark contrast to the earlier “Kaufman” ion sources that were first used in the 1970s and that had filaments that lasted as little as 10 hours.

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Figure 2

Typical etch uniformity with 30cm ion beam

Figure 3

Oxford Instruments 35cm ion source

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Figure 4

Oxford Instruments 15cm ion source

Ionfab300Plus The Oxford Instruments Ionfab300Plus represents the state of the art for ion beam etching. It takes the 15cm or 35cm ion source, and has a loadlock so that throughput is greatly increased, and processing can be cassette-to-cassette if necessary, see Figure 5. The loadlock is to standard “MESC” specification, so that this system can be built into clusters with other process chambers from Oxford Instruments or other vendors.

Figure 5

Oxford Instruments Ionfab300Plus with loadlock

As an argon ion beam etching system, the 300Plus is used for etching materials such as gold or platinum, cadmium mercury telluride (CMT), indium phosphide (InP) or aluminium gallium arsenide (AlGaAs) The chief advantage of the process is that it will etch through all materials. A typical example of its advantage is when etching a multilayer stack of alternate layers of GaAs and AlGaAs. The ion beam process will etch through all of the layers with minimum deviation in the wall angle.

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In an argon ion beam etch, it is inevitable that heat will be generated in the substrate by the ion beam, as a high energy of ion is needed. In fact, in many materials, the minimum heat is generated by the use of ions with an energy of around 600V. The 300Plus uses helium cooling of the wafer backside to remove the heat. Reactive Ion Beam Etching A major advantage of the RF ion source, is that you can perform reactive etches by feeding gases such as Cl2, F2, CF4, O2 directly into the ion source. This process is known as Reactive Ion Beam Etching (RIBE). The gases are cracked in the ion source, and the positive species are extracted into the ion beam, see Figure 6. Again, unlike the older Kaufman ion source, source reliability is not affected by the use of such gases. The process that results will be partially chemical, as the ions react with the surface, and partially physical, as the reaction products are sputtered away by the energy of the ion flux. An evident advantage of RIBE is that the relative importance of these processes can be changed by adjusting the ion energy. Typical RIBE processes include CMT, GaAs, InP, see Figure 7.

Figure 6 Cracking pattern of SF6 gas in Oxford Instruments ion source courtesy of Stefan Schneider

Figure 7

Ion beam etching a substrate

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Flexibility Ion beam etching systems are uniquely capable of developing new processes, since all of the parameters are directly and independently controlled: ion energy; ion flux density; angle of incidence; process chamber pressure; substrate temperature. It is fair to note that for high volume production, the process will generally need to be transferred to another technology - generally a plasma system -- in order to reduce the production costs. Hence, for example, almost no mainstream silicon production steps are performed with ion beam. However, for a number of complex processes, where it is too difficult or simply not costeffective to attempt to transfer the process to another technology, ion beam etching or RIBE will remain the process of choice. OIPT supply both ion beam systems and plasma etch systems. Our ICP plasma etch systems have similar capabilities. In the ICP380, for example, a plasma is generated in a chamber above the substrate that is to be etched, and ions are accelerated down onto the substrate by applying RF power onto the table that holds the substrate. The process is known as Reactive Ion Etching (RIE) although in fact it is possible to use argon for an inert etch. The following table highlights some of the differences between ion beam etching, RIBE and plasma etching.

Process Research

Advantages of Ion Beam Etching

Ion Beam Etch and RIBE Fundamental reseach into process physics and chemistry in ion beam systems helps us to understand ICP as well as ion beam processes. Ion beam remains an important production process in its own right. All parameters can be set independently for rapid process optimization.

Inert etching with variable incidence to optimize the etch process. Ion enegy can be adjusted 20eV – 1500eV. Etch process is face-down, which can improve process cleanliness. Advantages of ICP Etch

Applications

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Mainstream production of specialist IIIV devicies. Mainstream etching of materials such as CMT and V2O5. Production etching of Au and Pt structures and multilayer devices.

ICP Etch Process development proceeds by empirical research and process modelling Process parameters are adjustable but cannot be set independently. For example, increasing gas flow will affect chamber pressure, plasma density, bias voltage etc. Inert etching possible, but results limited by the use of normal incidence. Etch process is face-up Reactive processing can etch most materials at lower energy and (sometimes) with lower damage. Faster speeds At great expense, certain ICP processes have been developed that allow unmatchable results; for example in the case of the “Bosh” process, deep structures with vertical walls. Mainstream processing of Si for semiconductor and MEMS markets. Mainstream processing of GaAs etc

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