Fabrication and assembly of 3D MEMS devices
07/01/2005
The ability to integrate mechanical elements with supporting electronics on a micro scale has bolstered microelectromechanical systems (MEMS) as an enabling technology, sparking interest in an impressive range of applications. The fabrication of MEMS has certainly benefited from the same equipment and standard processes utilized in the semiconductor industry.
Unfortunately, every semiconductor manufacturing technique places restrictions on materials, dimensions, and processing parameters, and these in turn place strict design boundaries onto the interdisciplinary nature of MEMS devices. To get around these limitations, components can be constructed independently by combining traditional processes and novel technologies. The components can then be assembled or hybridized to make a more sophisticated device.
This scheme, however, presents a number of challenges for handling, maneuvering, aligning, and attaching each component. By addressing these issues, flip-chip bonding can enable more complicated MEMS assembly [1].
Machine capabilities and limitations
The Süss MicroTec FC150 flip-chip bonder has been used to attach microfabricated planar arrays of infrared detectors to its complementary read-out circuitry. Analogous to 3D MEMS assembly, characteristics intrinsic to infrared detectors forced producers to fabricate them separately from the circuitry used to route the electronic signals.
The delicate nature of the detectors and fine-pitched features used for attachment requires equipment with specific handling, maneuvering, aligning, and bonding attributes. Advances in machine performance can now be extended toward MEMS assembly as described in the next sections.
Bonding methods for MEMS assembly
A variety of bonding methods exist that can cope with the spectrum of mechanical, electrical, and thermal requirements for MEMS assembly. The majority can be categorized under intermediate-layer bonding due to an additional material used to attach the components being bonded. The most common intermediate materials include metal, glass, and polymers.
Bonding methods that use metal as an intermediate layer can be divided into either a eutectic or thermocompression-type bond. In eutectic bonding, two or more metals are combined to create a solder with a lower melting temperature than its constituents. It is important to consider the appropriate foundation layers (also referred to as the under-bump metallization, or UBM) for optimum adhesion. The table shows the melting range and shear strengths of common solders. In thermocompression bonding, soft homogeneous metals such as gold are pressed together using high force and heat. The metal is not melted in the process, only softened. Both eutectic and thermocompression bonds provide good electrical and thermal conductance. Many metals can be deposited onto components by sputtering, evaporation, electroplating, or preforms.
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Glass frit is a low-temperature melting glass that is typically used for hermetically packaging MEMS devices. The material is commonly screen-printed onto the components, making fine lines and accurate patterning difficult.
Most polymer bonds incorporate organic compounds that harden or cure with the application of heat and/or ultraviolet radiation. Thermoplastic polymers also exist, which soften when heated.
Direct bonds such as silicon fusion bonding typically occur in two steps: prebond followed by annealing. Direct bonds require extremely clean and smooth (<5Å rms roughness) mating surfaces to be successful. After the prebond is finished, the annealing step is typically performed in an annealing oven because temperatures up to 1100°C are used. However, research is currently being done to lower the anneal temperature to <250°C.
One other type of direct bond known as anodic bonding (or field-assisted direct bond) requires temperatures >350°C and high voltage of ~800V.
Examples
Rotary engine. One example of a microfabricated device assembled using the FC-150 is a MEMS-based internal combustion engine currently being developed at the Berkeley Sensor and Actuator Center. This device, along with a small-scale electrical generator, is part of an effort to develop a portable, autonomous power-generation system capable of vast improvement in energy density over conventional batteries [2].
The engine uses a Wankel-type design that can be constructed with a minimal number of moving parts. It comprises three basic components: housing, rotor, and shaft. Each part can be mass-produced using a timed deep reactive-ion etch process. Several challenges were addressed during the assembly of this device. Because the rotor was very small (~2.5mm), a 50mm vacuum tool with a tungsten carbide insert was used to handle it. The other two components were square, flat pieces - ~8mm on a side and 300μm thick - and were secured using a flat silicon carbide tool.
The first step of the assembly involved aligning and positioning the housing onto the shaft component using patterned alignment marks. For proper operation, the housing sleeve (epitrochoid) had to be positioned within 2μm of the shaft gear for proper clearance of the rotor (Fig. 1).
 Figure 1. Image of spur gear and housing subassembly. |
Once in position, the housing is attached to the shaft component using a direct silicon fusion bond. Although no intermediate material is required, it is important for the mating surfaces to be very smooth and clean. No heat and only a slight pressure are needed to initiate the prebond. Later, the parts are annealed to increase bond strength.
 Figure 2. Aligning the annular gear of the rotor to the spur gear located on the rear plate; images show a) before gears are meshed and b) after gears are meshed. |
The next step was to place the rotor into the housing. The challenge here was to align the rotor so that its edges would not contact the walls of the housing sleeve; the teeth of the rotor gear were meshed with the spur gear located on the rear plate (Fig. 2). The final step was to bond a cover plate to the whole assembly to keep the rotor from falling out.
Optical switch. The packaging of a MEMS optical switch developed by Analog Devices Inc. is another application using the flip-chip bonder. In this device, a MEMS mirror array was already attached and wire-bonded to the bottom of a ceramic chip carrier. This application involves attaching a cover plate to the chip carrier using solder as a bond material. The cover plate has an array of lenses in its center, while the chip carrier houses an array of actively controlled mirrors. Processing issues include special tooling for the carrier, aligning the center of each lens to the mirrors, ensuring proper leveling of the cover to the MEMS device, and achieving a strong hermetic solder bond.
The quartz lid was a flat, square piece ~12mm on a side and 0.5mm thick. It also had an adhesion layer along its perimeter. The preform was a square frame cut to fit around the opening of the ceramic chip carrier and made from a gold-tin eutectic alloy.
Before assembling the device, appropriate tooling was constructed for each component. Because the quartz lid contained protruding lenses on its handling surface, a special silicon carbide tooling was made with an accommodating recess on its surface. The preform needed tooling that redirected the vacuum from the center to the perimeter of the preform frame. The chip carrier required tooling with two slots cut in it so the two lines of pins could pass through. This allowed direct contact between the backside of the ceramic package and the tooling surface that optimized the amount of heat transfer needed to reflow the preform.
The first step in the assembly process was to align and tack the preform onto the opening of the ceramic package using low heat (180°C for the chuck and 215°C for the arm) and slight force (~350 gram-force).
The next step was to align and bond the lid to the chip carrier. Because the distance between each lens and mirror was critical, it was important to level the lid with respect to the MEMS device during alignment. Sufficient reflective area was available for leveling to be performed with the autocollimator. The x, y, and θ alignment was accomplished by aligning features and not alignment marks. Two sets of lenses and mirrors were used; the lid was bonded using 300°C for 30 sec.
The addition of alignment marks and laser-leveling targets would greatly improve the efficiency and accuracy of the alignment process. It was recommended to place the targets along the four corners of the lens array. Alignment marks should be placed far enough apart as possible to be able to distinguish rotational error more easily.
Adaptable optics. Iris AO has developed an advanced adaptive-optics technology that is lower in cost, which makes it an attractive choice in cutting-edge optical applications such as LASIK surgery, custom contact lenses, and early detection of eye diseases [3]. The technology relies on the implementation of tiny MEMS mirrors arranged in a 2D array.
Fabrication of the device involves bonding the mirrors, which are mounted on a sacrificial piece of silicon, onto a substrate containing the driving circuitry. Bonding of this nature requires precise alignment, a high degree of parallelism, and flexible control of the bonding parameters. The substrates contained the circuitry and Pb/Sn solder pads, while the chips had the mirrors and posts. The parts were ~10mm2.
The bonder was configured to use the solder reflow arm for low force and high accuracy. The chuck was fitted with a gas confinement cover and a pedestal tool was used to hold the chip. An automatic cycle was written to facilitate processing. Parallelism between the chip and substrate was done using laser leveling because the curvature of the features on the chip caused the light crosses associated with the autocollimator to be blurry and unusable.
Once the laser leveling achieved good parallelism (threshold set to 0.4μm), the chuck was purged with forming gas (5% H2, 95% N2) and the parts were heated to an alignment temperature of 130°C. The x and y alignments were then performed using the post and solder pads as alignment features.
Bonding was initiated automatically after aligning. The machine was programmed to bond at a temperature of 190°C (both arm and chuck) for 20 sec. Once the bonding cycle was completed, active cooling was switched on to bring the samples down to 100°C before turning off the forming gas. At this point, the samples were swapped out and the process started over again.
Initial attempts without scrubbing yielded weak solder bonds. It is possible that a thick layer of oxides was present before the bond process started, in which forming gas would have limited effect. Therefore, for subsequent attempts, a mechanical scrubbing was employed to break through the oxide barrier. Once scrubbing was activated, the parts did not separate easily when a razor blade was used to pry them apart. Inspection of the posts revealed that the bond was strong enough to remove the solder from the bond pads. Some suggestions for improving this process include the possible use of alignment marks for automatic alignment. Additionally, device-specific cassettes would facilitate material handling.
Conclusion
A device bonder machine enables construction of MEMS components such as rotary engines, optical switches, and adaptive optics devices. Processing these types of components requires specialized tooling to ensure proper handling; in particular, positioning must be accomplished along all degrees of motion including leveling.
Incorporating a sufficient amount of handling area, using alignment marks, and depositing laser-leveling targets are some examples of how tool design can work in concert with MEMS device design. The knowledge of how the machine works provides insight during the design phase, which will allow more efficient production.
References
- K. Boustedt, K. Persson, D. Stranneby, “Flip Chip as an Enabler for MEMS Packaging,” Proc. IEEE Electron. Components and Technol. Conf., 2002.
- K. Fu, A.J. Knobloch, F.C. Martinez, D.C. Walther, A.C. Fernandez-Pello, et al., “Design & Fabrication of a Silicon-based MEMS Rotary Engine,” IMECE/MEMS-23925, Proc. ASME 2001 Intl. Mech. Engin. Congress & Expo., Nov. 2001.
- M.A. Helmbrecht, U. Srinivasan, C. Rembe, R.T. Howe, R.S. Muller, “Micromirrors for Adaptive-optics Arrays,” Transducers ’01, June 2001.
Daniel N. Pascual received his MS in mechanical engineering at Boston U. and is an applications engineer at Süss MicroTec, 228 Suss Dr., Waterbury Center, VT 05677; ph 802/244-5181, fax 802/244-5103, e-mail [email protected].