Microelectromechanical systems (MEMS) can be realized in many fashions. To date, however, pressure sensors and accelerometers have been the majority of high-volume applications. In fact, a number of these devices are currently produced in quantities greater than a million devices per month. As time goes on, other MEMS devices, including rate sensors/gyros, optical switches, RF MEMS, and chem/bio MEMS, will experience widespread application.
The early focus on the development of MEMS has been on device design. Now that MEMS are being commercialized at an ever increasing rate, the focus is on delivering a robust and cost-sensitive product. Packaging and testing have become major ways to differentiate between products. Furthermore, the cost of undertaking these activities is receiving great attention by manufacturers. Typically, the cost of testing can be as much as 33%, with packaging from 33% to 45%, of the solution's total cost.
Any company that wishes to be a market leader must pay attention to this fact. No longer will it be acceptable to design the test strategy after the design of the device and package. Testing issues must instead be included in the overall design of the device/package in the early phase of development.
Testing of MEMS devices can occur at various stages during the development of a MEMS solution. Figure 1 is a flow chart illustrating a standard MEMS development cycle. From early on in the determination of material properties by real life testing to final acceptance testing, MEMS are a major candidate for extensive testing. This article will focus on front end, or materials testing, and the back end, the reliability and acceptance testing.
The current rapid pace of materials development for MEMS rivals the similar development experienced by the steel industry about a century ago. New MEMS materials are continually developed to exploit specific material properties and process compatibilities. A decade from now, the structural material of choice for MEMS might not be any of the materials utilized in today's devices. A sample list of materials presently used in MEMS devices appears in Table 1. The exciting pace of material development in the MEMS industry requires the development of material testing techniques that can provide the properties of these materials as they evolve.
The unknown reliability of many MEMS devices limits their incorporation into commercial products. The long-term stability of these devices can only be ensured with greater knowledge of the basic material properties and failure mechanisms of the materials employed in MEMS designs. The motivation for this knowledge arises for numerous reasons.
Some "new" materials actually aren't new but rather just being employed for the first time in MEMS. These materials include thin films for actuation, such as shape-memory alloys and multilayers for optical reflectance. Other materials are truly new "alloys" like the new class of silicon-germanium (SiGe) materials. Those provide special processing advantages over the more mature polysilicon comprising the majority of MEMS devices. The obvious advantages provided by SiGe materials, including CMOS compatibility, ensures their continued development and incorporation into commercial products.
Still, there's only limited data available on the properties and long-term performance of these SiGe alloys, including elastic stiffness, corrosion resistance, fracture toughness, and creep resistance. But, opportunities for developing additional "alloys" haven't ended with SiGe. There's no question that more MEMS materials will arise, and their properties will need to be quantified.
There are substantial challenges in measuring the properties of MEMS materials—whether those materials are new or old. The materials making up MEMS are deposited as thin films. Although the electrical characterization of thin films is well established, the mechanical characterization of the same films is difficult. There are numerous properties that need to be measured for each material, including elastic modules, yield strength, fracture toughness, fatigue resistance, corrosion resistance, creep behavior, and residual stress. No single standard of testing for any of these properties exists. Numerous tests have been proposed and are being carried out by different manufacturers and institutions. But, the rigorous comparison and benchmarking of these test techniques have yet to be performed.
The challenge of testing materials is increased by the variation in material between manufacturing sites and even between manufacturing batches. Material properties of a common MEMS material, polysilicon for example, deposited by one manufacturer can vary substantially from that deposited by another. Manufacturers even see variations in film properties between wafer batches. Furthermore, Exponent has documented variations in properties across single wafers. A map of the variation in residual stress, measured by Exponent, in a silicon nitride film across a single silicon wafer is shown in Figure 2. The variation is sufficiently large to change device performance if the design depends on a given value of residual stress.
Therefore, the question of appropriate, validated testing remains an open issue. An important effort to coordinate MEMS testing is currently taking place within the American Society of Testing and Materials Task Group (ASTM) on Structural Films and Electronic Materials, E.08.05.03. This task group is meeting to compare the various test techniques used to evaluate MEMS materials with the final objective of setting qualified, tested standards through which MEMS designers, manufacturers, and end users can communicate. The next meeting of the task group is at the Fall ASTM meeting in Orlando, Florida (Nov. 10-12).
MEMS material testing also is essential because numerous MEMS devices operate under conditions unknown in the macro world. Certain characteristics of two classes of devices, RF and optical MEMS switches, can be examined in Table 2.
The actuation frequencies provided in the table result in a number of total accumulated actuation cycles that extends far beyond what has been required in "macro" applications. Both of these classes of devices can experience fatigue and wear from contacting surfaces during individual actuation cycles. There's little or no information about fatigue or wear for virtually any engineering material under these conditions—for neither macro nor MEMS devices. As a result, lifetime predictions are device specific. Plus, given the lack of fundamental material testing, predictions aren't always largely validated by statistics.
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