MEMS Applications for Scanning Acoustic Microscopy The need for acoustic inspection of mems is far-reaching BY JOHN GOINGS Microelectromechanical systems (MEMS) are finding applications in products or systems that require reliable operation over extended periods of time. The reliability requirements for the final product encompass both the mechanical behavior and the electrical characteristics of the overall system. One critical element in many MEMS applications is chip-to-chip bonding (component bonding), for which long-term operation and storage reliability needs to be understood. MEMS packages are likely to have a large number of bond layers because of multiple interfaces inside the package. The integrity of these bond layers frequently has stringent quality requirements. A primary indicator of failure (or impending failure) in a chip-to-chip bonded system is delamination between the chip and the material used to bond the chips together. Also, the bond layers in MEMS devices often must maintain precise component or chip alignment. In addition, the bond layers may have to withstand loading from both the macro-environment and loading within the package. In spite of its importance in MEMS packaging, previous work on bonding in MEMS structures is limited. Little MEMS-specific work on the reliability of the chip-to-chip bonds exists, let alone nondestructive methods for determining the reliability of chip-to-chip bonded MEMS. Scanning acoustic microscopes (SAMs) are just now being realized as a potential solution to this inspection need. Scanning Acoustic Microscopy Acoustic microscopes use high-frequency ultrasound (typically 15 to 260 MHz) to view internal bond and device integrity (Figure 1). The higher the frequency ultrasound, the better the resolution — but the poorer the penetration through the sample. Frequencies in this range do not travel through air; therefore, a coupling medium is used. Isopropyl alcohol (IPA) solutions (less than 20 percent concentration) and deionized water are the most common mediums due to convenience, cost and availability, although other mediums are possible. It is required that the inspection sample be immersed in the coupling medium, although transmitting the ultrasound through a liquid column using a “squirter” is possible in an effort to minimize exposure. Figure 1. C-Mode SAM capable of inspecting bonded wafers or packaged MEMS devices.Click here to enlarge image null Acoustic images are generated by raster scanning across the sample, continually sending and receiving ultrasonic signals resulting in an amplitude plot of the inspection interface. Interface specific images are called C-scans. Air gaps, such as delaminations and voiding within the bond region, cause a complete reflection of ultrasonic energy. These defects can be seen as bright areas in the resultant C-scan image. SAM's ability to identify slight density variations and air gaps makes it the preferred inspection method when the suspected defect results in a thin, planar delamination. Bonding defects in MEMS typically result in such a delamination. SAM's inspection capability is in contrast to X-ray inspection, which requires a more significant density variation to contrast a defective area from a well-adhered bond. SAMs also have the benefit of being 100 percent nondestructive and capable of producing images at a depth-specific interface. Although the defect resolution is dependent on material specifics of the inspection sample, the minimum detectable defect size for SAM in the case of bonded or SOI wafers can be as low as 5 µm in an x-y direction in the optimal scenario. Some SAM systems can obtain as many as 100 million data points per image (10,000 x 10,000 points) Air gaps as thin as 0.01 µm can be found in depth, (z) direction. Figure 2. Shown at left is an infrared (IR) image of a fusion-bonded wafer pair. Dark areas indicate the presence of defects. The image on the right is a SAM image of the same wafer. Red areas signify the location of defects. The same defects in the IR image can be seen, however, many small defects were previously undetected.Click here to enlarge image null Until recently, all SAM systems were primarily used in a laboratory setting for failure analysis or low-volume reliability inspections. However, recent software and hardware developments have allowed automatic SAM inspection in a production or lot qualification role. Throughputs of hundreds of MEMS units per hour are now capable on fully automated systems. The variables affecting throughput include desired resolution, size and nature of the inspection sample. Automated wafer inspection is also possible with inspection rates of 2-3 minutes per wafer pair, leading to inspection rates as great as 10,000 wafer pairs per month. Additionally, advanced software tools are available to provide automated statistical analysis of an inspection sample including defect counting, sizing and depth measurement. Acoustic Inspection of Bonded Wafers Silicon-on-glass, SOI and other types of bonded wafers are becoming commonplace in the MEMS field. These processes present unique inspection concerns for nondestructive inspection tools because of the small defect sizes involved. In the mid to late 1990s, ultrahigh-frequency (greater than 200 MHz) ultrasonic transducers were developed that provide the resolution necessary to detect very small defects (approximately 5 µm) in wafers. Traditionally, infrared (IR) technology was used for wafer bond inspection. However, SAM provides better spatial resolution. Figure 2 provides an example of an IR image of a bonded wafer compared to a SAM image of the same wafer. All of the defects in the IR image can also be seen in the SAM image, although numerous smaller defects can be seen in the acoustic image that are not detected by IR. Figure 3 shows a high-resolution C-scan image of a bonded silicon wafer. This slice of the x-y plane was taken at the user-defined depth of the dioxide layer of a SOI wafer pair. Red areas in Figure 3 represent disbonding between the two wafers. In this case, destructive cross-sectioning later revealed the disbond was caused by a poor cleaning process. Die cut from the voided areas had a high rate of failure. Figure 3. This C-scan of a fusion-bonded wafer shows defects, in red, caused by contamination trapped during the bonding process.Click here to enlarge image null Technological advances in SAM systems have added the capability for automatic wafer handling, scanning, drying and separation of good wafers from bad, based on the acoustic data. Defective wafers can be culled from the lot and rebonded, if necessary. Acoustic Inspection of MEMS Packages Unlike the packaging of microelectronics, such as BGAs and flip chip packages, where the package-board interface needs to be completely underfilled with no voids, with MEMS devices it is often useful and necessary to have a void in the center of the surrounding underfill or dam material, because the underfill material locks in place any moving part in the MEMS. Knowing the distribution and location of the underfill is a common application task for the acoustic inspection of MEMS packages. Likewise, other MEMS packaging analysis needs fit well with the capabilities of SAM systems. Figure 4. An acoustic C-scan image of a packaged MEMS device shows the interface between a die and the package.Click here to enlarge image null For example, accelerometers require unique packaging techniques. The device seen in Figure 4 is an accelerometer used for seismic detection that failed functionality tests. The manufacturer wanted to discuss the root cause of the failure and find a method to control the newly developed manufacturing process. The production of the unit involves placement of a die precisely in the center of the package, which needs to be capable of floating with the center circular cavity. A red arrow notes the location of excess solder that has spilled into the cavity and now “locks” the die in place. It was later determined that an inadequately controlled reflow temperature was to blame. The need for acoustic inspection of MEMS devices is far-reaching. Future applications include the imaging of closed MEMS containing microfluidic structures, i.e. plumbing voids and in-system MEMS components. Conclusion By the nature of SAM's abilities to detect extremely small and thin air gaps, SAM systems can be used for a wide range of MEMS inspection uses leading to improved processes, failure analysis and more reliable products. Acoustic inspection can occur early in the process, at the bonded wafer stage or after the MEMS has been completely packaged and used in the field. Acknowledgment The author wishes to acknowledge Robert Dean, Ph.D., of Auburn University for his insight into acoustic inspection applications for MEMS. References For a complete list of references, please contact the author. JOHN GOINGS, consultant, may be contacted at Sonix Inc., 8700 Morrissette Dr., Springfield, Va. 22152; (703) 440-0222; e-mail: email@example.com.