Wafer bonding is a critical part of most MEMS process flows. Especially with the increased importance of wafer stacking and 3D technologies, selecting the best wafer bonding process will likely determine the cost structure, and therefore, the ultimate market success of many MEMS devices. We recently spoke with Eric Pabo, MEMS Business Development Manager of EV Group, about trends and developments with bonding techniques for MEMS applications. In this interview, Eric discusses the history of wafer bonding, emerging MEMS bonding techniques, equipment providers, and bonding process cost drivers.
MEMS Investor Journal: Let’s start from the very beginning. What product needs drove the development of initial wafer bonding technologies?
Eric Pabo: Although the practice of joining two surfaces such as two metal layers in jewelry goes back to antiquity, the application of these fundamentals to the bonding of two wafers in the electronics industry began in the 1960s. This initial work was focused on the construction of substrates with specific electronic properties by direct bonding techniques. Some of the companies that did research in this area were NEC, Philips, RCA, IBM, and Toshiba. This work led to the techniques of direct bonding that are currently applied to the manufacture of SOI (silicon on insulator) wafers.
In the 1960s and 1970s, work was done on developing wafer to wafer bonding processes that could be applied to enable the wafer level capping of MEMS devices to avoid the cost of capping each MEMS device individually. Direct bonding, glass frit and anodic bonding techniques are a result of these efforts.
MEMS Investor Journal: Aside from MEMS, where else are wafer to wafer bonding techniques being currently applied in electronics manufacturing?
Eric Pabo: SOI (silicon on insulator) wafers are typically manufactured using direct bonding techniques. Thick SOI and cavity SOI wafers as starting material for MEMS and other products are also manufactured by direct bonding techniques.
Back side illuminated CMOS image sensors are manufactured using direct bonding and adhesive bonding, with direct bonding predominating.
Extensive development work is being done on bonding techniques for the 3D integration of ICs. Thermo-compression bonding of Cu is thought to be the leading process but many other options such as hybrids of direct and thermo-compression have been considered. For example, wafer level optics may be aligned and bonded to form lens stacks. This is most commonly done with adhesive bonding.
MEMS Investor Journal: What are the major pros and cons of the wafer bonding techniques currently used for MEMS manufacturing?
Eric Pabo: The following table compares the major bonding methods used for making MEMS devices and structures. (Please click on the table to enlarge it).
MEMS Investor Journal: Who are currently the main providers of MEMS bonding equipment?
Eric Pabo: Per Yole Development, EV Group was recently reported to have 69% market share, Suss MicroTec had 17%, AML had 4%, and the remaining 10% is split between AST, Ayumi, TEL, others and homemade equipment.
MEMS Investor Journal: In general, what are the main challenges and drawbacks with metal based bonds?
Eric Pabo: The main challenge is selecting the proper metal stack, which usually includes an adhesion layer as well as a barrier layer, the processes for depositing and patterning these layers and oxide management if not using noble metals.
MEMS Investor Journal: What have been the major challenges in aligned bonding for MEMS during the past decade?
Eric Pabo: The primary challenge in aligned wafer bonding has been transitioning from low volume “craft” processes to robust high volume manufacturing. These improvements were mostly due to the development of a thorough understanding of the major bonding processes including the incoming variables, the process variables, and the output variables. Development of robust design rules based on understanding of the process was also important as was the development of reliable fully automated alignment and bonding equipment, such as the EV Group Gemini, with cassette to cassette process and the ability to fully integrate with automated process management, shop floor control systems, and fab automation. Additionally, metrology tools now play a key role – they allow the automated measurement of post bond alignment and SAM tools which provide feedback on bonding defects. With these and other tools, thorough process control is possible.
MEMS Investor Journal: What are the key issues when setting up a bonding process for high-volume manufacturing?
Eric Pabo: In addition to the points already mentioned, one must select the equipment supplier carefully and not just consider the initial purchase price but the long term cost of ownership and the level of support that this vendor will provide. Also, for HVM, applications data collection is very important because it enables tight process control.
MEMS Investor Journal: What are the main cost drivers in long term cost of ownership considerations?
Eric Pabo: The main driver for the cost of ownership of bonding is the bonding process that is selected. For a given process, the three main factors are the cost of the upstream process needed for bonding, the cycle time of the bonding process and the yield of the bonding process. The cost of the upstream processes vary depending on the process selected, the main drivers for the process cycle time is the maximum temperature required and the hold time at this temperature, and the yield is dependent on many factors.
MEMS Investor Journal: What are typical values for bonding yield?
Eric Pabo: Bonding yields are typically considered proprietary information but for mature products running in high volume, they are typically in the high 90s. For high volume, anything less would be unacceptable. This yield is driven by a combination of the design rules used, the alignment and bonding processes selected, the quality of the incoming wafers, how the bonding process is executed, the bonding equipment used, and the metrology and control plan. Of course, yields are lower for bonding steps using new processes, aggressive or inappropriate design rules, or poorly controlled processes.
MEMS Investor Journal: What are the latest wafer bonding processes that are being incorporated into MEMS fabrication flows?
Eric Pabo: Metal based bonds such as solder, eutectic, TLP (transient liquid phase), and thermo-compression are being incorporated into more MEMS flows because they do not have the real estate consumption issue of glass frit or the sodium issue of anodic bonding, provide a good degree of hermeticity, are electrically conductive, and can be compatible with CMOS devices.
Also, direct bonding is being used in the manufacture of specialty SOI substrates such as SOI cavity wafers by companies such as Okmetic which are used in some MEMS fabrication flows.
MEMS Investor Journal: How does one select a wafer bonding process?
Eric Pabo: One has to consider the post bond performance of the selected bonding technique, the processing conditions required for the bonding technique, the effect of these processing conditions on the devices present on the wafers, and the upstream process requirements for forming the bonding layers. This process is documented in a BKP (Best Known Practice) which is available by contacting EV Group.
For example, if one needs a conductive bond with good hermeticity, the choice is between solder/eutectic and thermo-compression. If conductivity and hermeticity are not a concern, then an adhesive bond may appropriate and it will minimize the cost of the bonding step.
MEMS Investor Journal: Wafer bonding is highly specialized and the knowledge of these processes is not widespread. How does one avoid the cost and time of developing these processes on their own?
Eric Pabo: There are several ways to minimize the cost and time of developing this expertise. The cost of purchasing equipment for developing the process can be avoided by accessing the application laboratory of the equipment vendor and when the correct vendor is selected, one can leverage the process knowledge of the application engineers and bonding scientists of that vendor. Another possibility is to hire a consultant that specialized in wafer bonding.
This article is a part of MEMS Investor Journal's ongoing market research project in the area of MEMS bonding techniques. If you would like to receive our comprehensive market research report on this topic, please contact Dr. Mike Pinelis at [email protected] for more information about rates and report contents.
Copyright 2011 MEMS Investor Journal, Inc.