EXECUTIVE OVERVIEW

Many emerging wafer bonding technologies are requiring higher applied force for successful wafer level encapsulations. Pneumatic and hydraulic systems have both been used to apply force in wafer bonding equipment, however, the pneumatic approach, otherwise known as “pressure column,” has the benefits of extreme uniformity, lower cost-of-ownership, and higher reliability compared to hydraulic systems. These benefits are necessary to realize 300mm metal based bonding and high modulus materials integration on the wafer scale.

Wafer-level bonding has its roots in glass frit and anodic hermetic packaging of accelerometers and pressure sensors. Glass frit is a paste-like substance and vitrifies at a relatively low temperature. A very low force is required to compress the softened frit line and fuse the glass beads together. Anodic bonding uses electric field assisted thermal diffusion processes to enable mass transport between the silicon and glass substrate. The bond initiates in any location of physical contact between the two substrates. As the bond proceeds, the mass transport is significant enough to “fill in” areas that are not in contact such as areas around small particles. In both cases, an applied force is used to maintain physical contact between wafers and is typically <10kN for wafers up to 6 in. dia.

Need for increased applied force

Several factors have driven the need for increased applied force in commercial wafer bonding equipment. The initial and obvious driver is the ongoing transition from 6-in. to 8-in. manufacturing in MEMS. Larger diameter wafers are thicker and less compliant, and require more force to establish contact between mating surfaces. From the process mechanisms viewpoint, however, it is obvious that force uniformity is also critical. If the bond force has a 25% variation within the wafer, then some areas of the substrate will only experience a fraction of the total force and may not bond properly, if at all. Viscous interfaces, such as the example of glass frit, will have varying thickness uniformity on individual die.

In an effort to reduce die size, the glass frit and anodic sealing methods are being replaced with more hermetic metal seals. The use of hermetic metal seals allows for significant reductions in sealing area, higher yields, better performance, and smaller chips. The metal layers can also be used for electrical connection and have facilitated vertical integration of hybrid systems, and more recently, 3D ICs. Metal bonds require both higher force and more uniform force than other types of bonds due to the diffusion mechanisms involved, and the stress that the metal layers and vias introduce to the substrates.

Finally, it can be said that wafer bonding is viewed as a “method to fool Mother Nature,” meaning that more exotic materials are also entering the market via wafer bonding. Most of these “new materials” can generally be said to have less than desirable materials properties for wafer bonding. Some are very high modulus materials that translate directly into less compliance, such as the new glass frit replacements, SiC, and thick glass substrates for displays. Others are extremely brittle and require very uniform force applications, which is the case for the compound semiconductor materials.

Solutions

High force bonding is, at first glance, a brute force approach to solving difficult bonding applications. The increased force can be used to flatten wafer bow and warp and achieve intimate contact of the surfaces to be bonded. The situation however, is never that simple. Force uniformity is equally critical to good bonding results.

Force is applied to a pair of substrates by placing aligned pairs between upper and lower pressure plates. The pressure plates are then forced against the wafers by either pneumatic or hydraulic actuators.

In a pneumatic approach, a column of pressurized air that is equal in size to the area of the wafer surface is used to press the wafers together. In a hydraulic approach, a fluid is introduced through a cylinder to the center of the pressure plate. The force is then translated across the plate from center to edge. Modeling of pressure profiles within the pressure plate will show that the pressure column (much like a waterbed) will achieve a uniform force across the entire area. In contrast, the dispersion of the force from a central point, as in the hydraulic cylinder, will lead to center-to-edge variation as the distance from the fluid source increased. Any nonuniformity in force within the plate directly translates to the wafers. Not only does the hydraulic force vary across the wafer, but it introduces bending moments to the pressure plates that lead to deformation of the plates over time at high temperatures, resulting in increased maintenance costs and lower yields for the bonder.

Figure 1. Acoustic micrograph of a eutectic bond on a MEMS 8-in. wafer. The dark area in the center is indicative of poor force uniformity associated with center-to-edge force variation.Click here to enlarge image

In production, the force profiles may not be immediately apparent and are often wrongly associated with some other process defect. Figure 1 shows a whole wafer acoustic micrograph for a MEMS wafer bonded with a eutectic alloy. The image contrast around the die is fairly uniform and an inexperienced technician might miss the shadow in the center of the image. This shadow is the fingerprint of pressure nonuniformity in the wafer bonder. The center is significantly darker than the outer region.

Figure 2 shows the telling evidence of incomplete bonding. In the upper half of the figure, the acoustic image shows die with incomplete seal rings. The dark regions are fully reflowed eutectic alloy seals, while the white areas are voids. In these areas, the force was insufficient to maintain substrate to substrate contact during heating and the two surfaces did not fuse together.

Figure 2. These close-up acoustic images show the die-to-die variation associated with poor force uniformity. The upper image shows a mixed white and black seal ring around the die; these die are not properly sealed. The lower image shows uniform contrast around each die, indicating proper hermetic sealing.Click here to enlarge image

In the lower half of the figure are die that are properly sealed as indicated by the solid dark regions around the die. In this case, the overall applied for force (10kN) should have been more than enough to maintain contact. It was the uneven force profile (up to 50%) that limited contact in the low force areas. With 50% uniformity differences, it is possible that the unbonded die experience <5kN applied force. Low force combined with surface topography will prevent wetting of the metal layers to the opposing surfaces.

Conclusion

Metal bonding technologies hold the promise of reduced die size, increased hermeticity, and the possibility for 3D integration of MEMS and CMOS. However, these bonds require improved bonder performance that traditional tooling has been incapable of providing. Both total force and force uniformity must be addressed. Tools such as the SUSS CB8 and its automated cluster embodiment offer such improvements through the use of pneumatically controlled pressure plates. Both theoretical arguments and practical demonstrations illustrate the benefits of pneumatic control over hydraulic control for high performance bonding applications.

Shari Farrens received her PhD and two MS degrees from the U. of Wisconsin-Madison and is chief scientist for the Wafer Bonding Division at SUSS MicroTec, 228 Suss Drive, Waterbury Center, VT 05677 United States; ph 802/244-5181, e-mail shari.farrens@suss.com.

Greg George received his EE degree from Champlain College in Burlington, VT, and is manager of core technologies and new product development at SUSS MicroTec.