Solder Self-assembly for MEMS Kevin F. Harsh, Ronda S. Irwin and Y. C. Lee NSF Center for Advanced Manufacturing and Packaging of Microwave, Optical and Digital Electronics, Department of Mechanical Engineering University of Colorado, Boulder, CO 80309-0427 Keywords MEMS, Micro-electromechanical Systems, Solder, Self-assembly, Assembly Abstract: A new technology is being developed that uses solder surface tension force to assemble MEMS threedimensional (3-D) structures. Using solder, a single batch reflow process can be used to accomplish hundreds or thousands of precision assemblies, and the cost/alignment can be reduced considerably. The solder self assembled MEMS to be reported are categorized as single-hinge and multiple-hinge structures. The study on the single-hinge MEMS demonstrates the self-assembly concept with controlled angles. The study on the multiple-hinge structures demonstrates more features for complex MEMS; these features include locking mechanisms, enclosure structures, and stacked hinged structures. 1. Introduction MEMS (Microelectromechanical systems) are projected to fuel many innovative applications. One of the most common methods for manufacturing MEMS is surface micro machining. The problem with this method is its inability to produce highly three-dimensional structures. A common solution is the fabrication of hinged components that can be lifted or popped-up into assembled structures. The major downside of these structures is that they need to be assembled after fabrication. This is often done manually, a method that is not practical and is rarely effective. Another method is to use additional MEMS mechanisms incorporated into the designs to perform the actual assembly 1, 2. This method is also insufficient because these MEMS mechanisms are often large and thus, negate the size advantage inherent in MEMS. A new method of assembling MEMS has been proposed that uses the surface tension of molten solder as the assembly mechanism. The method involves using a standard hinged plate with a specific area metallized as solder wettable pads. As shown in Figure 1, during solder reflow, the force produced by the surface energy minimization pulls the free plate away from the silicon substrate. Solder is a predominant technology for electronics assembly, and it is being developed for optoelectronic passive alignment 3. Using solder, hundreds or thousands of precision alignments can be accomplished with a single batch reflow process, and the cost/alignment can be reduced by orders of magnitude 3. Some
MEMS have already been assembled using solder. Green et al. demonstrated solder self-assembly for MEMS 4 and Singh et al. demonstrated the integration of different MEMS devices using solder 5. The present study improves the solder technologies for more precise alignments and for more complex MEMS structures. Figure 1: Solder self-assembly mechanism 2. Single-Hinge Structures The first solder assembled structure can be classified as a single-hinge structure. It is a basic building block of more complex MEMS pop-up structures. As shown in Figure 2, the assembly is simply composed of a single polysilicon rectangular plate attached to the substrate by two hinges. The structure studied was fabricated using the Multi-user MEMS process (MUMPS) at the Microelectronics Center of North Carolina (MCNC). The structure elements were made of 1.4 µm thick polysilicon with 300 by 300- µm gold pad for soldering. Opposite this pad was another pad of similar dimensions attached to the substrate. The pads were separated by a gap that was a result of the fabrication limitations and the position of the hinge. The process for assembling the MEMS started by placing the solder onto the gold pads on the MEMS. For the small number of tests done for this experiment, manual placement was adequate. The solder used in these experiments was eutectic 63Sn/37Pb, which came in spheres with diameters ranging from 0.004" to 0.016". The diameter could change by ± 0.001". Flux was not used because its residue was not desirable for MEMS. A fluxless soldering process developed for optoelectronic packaging was used for the assembly 6. The balls were placed using a vacuum nozzle and a micro-positioner. Once the solder was placed, the MEMS were placed in a chamber filled with nitrogen and formic acid gas. The formic acid gas removed surface solder oxide for effective solder reflow. The chamber was then heated to 200 o C to melt the solder. Once the solder was molten, the surface tension force lifted the hinged plate away from the substrate to form a 3-D structure. Three solder self-assembled MEMS are presented in Figure 3 with different angles. A major task for the soldered MEMS was to control these angles to meet different specifications. We conducted measurement and modeling studies for the angle control. The measurement technique used a SEM (Scanning Electron Microscope) photo of the assembled MEMS, and then expanded the photo to a larger size. Once
expanded, the angle could be calculated geometrically. Because the measurements were taken using very accurate digital calipers for large picture size, the measurement error was less than +/- 0.2 degree. If the MEMS were not lined up exactly perpendicular to the SEM, the perspective would have skewed the angle. Fortunately, because of the extremely thin nature of the MEMS plates, it was very easy to align the MEMS to the SEM within 0.5 degrees. The skew error was negligible when compared to the measurement error discussed above. The angles shown in Figure 3 are for 87.4, 65.8, and 40.5 degrees, respectively. Their corresponding solder volumes are 3.51x10-5 cm 3, 1.75x10-5 cm 3, and 0.878x10-5 cm 3, respectively. Gap Free Plate Solder Pad -Free Solder Pad -Fixed 300 µm 300 µm 300 µm Hinge Figure 2: Diagram of MEMS used in this experiment A model specifically designed for solder self-assembly MEMS has been developed for use as a design tool 8. This model was modified from the one developed for optoelectronics packaging 6,7. As a verification of this model, a spectrum of solder volumes with 15 test assemblies was used to assemble MEMS. Three MEMS were assembled at each of five different volumes: 4.39, 8.79, 13.18, 17.57, and 21.97 x10-6 cm 3. The results are show in Figure 4. The three data series each represent one test at each volume level. The solder balls used were manufactured from 0.004" to 0.016" with +/- 0.001" variation in diameter. The variation in volume was not negligible, so its effect was calculated. It was found that the experimental error resulting from solder volume variation was +/- 2 degrees 8, which is plotted as error bar in Figure 4. Even with this volume variation, the single-hinge structures can be designed and precisely controlled. The solder self-assembly mechanism can be easily implemented for different applications.
Figure 3: SEM photographs of solder assembled plates with angles of 87.4, 65.8, and 40.5 degrees 80 70 Equilibrium Angle (degrees) 60 50 40 30 Model Predictions Data Series 1 Data Series 2 Data Series 3 20 0 5 10 15 20 25 Solder Volume (x10-6 cm 3 ) Figure 4: Comparison between modeled and measured final angles
3. Multiple-Hinge Structures The second set of structures is for multiple-hinge structures. The driving force for these types of structures stems from the need for complex MEMS. Listed below are several multiple-hinge designs. These designs were all fabricated using the MUMPS process at MCNC. 1. Hinged plate locking mechanism: The angle of rotation can be controlled fairly precisely, as shown in Figure 4; however, there are undoubtedly applications that would require more precise control. Figure 5 demonstrates a new design utilizing a second hinged plate to either restrict to rotation of a hinged plate or to lock it into a single position. The central plate is locked in place by two plates that trap it from each side. Tabs on the side of the center plate fit into tapered slots for fine alignment. The layout of this design includes a spinning optical micro-mirror composed of two, hinged, solder self-assembly structures. The mirror pivots around a central bearing and is driven by thermal actuators. This angle control would be a function of the geometry of the structure, and therefore is much more precise due to the high precision of the micro-machining process. Mirror Locking mechanism Figure 5: Hinged plate locked by two side plates (left: SEM photo; right: design layout) 2: Enclosures: For applications that require a considerable number of structures, a simple solution is the use of enclosure type structures. Multiple-hinge plates can be arranged such that, when assembled, they form the surfaces of more substantial geometric structures such as boxes or pyramids. By using thin flexible pieces of polysilicon as hinges, plates can be stacked to form caps for enclosure structures. An example of a box is shown in Figure 6. Unfortunately, the flexible hinge was not strong enough to cover the box as shown in the SEM photo. A better hinge design is needed.
Flexible hinge Figure 6: SEM photo of lidless box enclosure structure (left) and its CAD design layout 3: Stacked Hinged Structures: For very complex structures, it is necessary to utilize the stacking method with much greater complexity. Stacking method demand two design features: a flexible hinge to connect plates, and a mechanism for angle control. The control mechanism is a structure best described as a kickstand. As the solder rotates the structure out of the plane of the substrate, the non-soldered plates would fall backwards due to gravity. A rigid bar attached with a spring could rotate with the plate until it slides into a hole or grove in the plate, thus stopping the backward movement of the plate. The final angle of rotation would be a function of geometry. If designed correctly, the bar would not only stop the plate rotation, but also lock it in position. This assembly concept could in theory be utilized to construct very large structures of very complicated geometry. Unfortunately, with the weak flexible hinge, we have not demonstrated this concept. Figure 7 shows the expected stacking structure and the layout of a design. Improved design is being fabricated. Solder Pads Solder Kickstand Kickstand Figure 7: Diagram of self-assembled stacked hinged structure (left) and the CAD design (right)
4. Summary Solder self-assembly for MEMS has been demonstrated to be a precise method of assembling hinged, popped-up structures. The single-hinge structures could be controlled with an angle of rotation within +/- 2 degrees by using solder balls with a +/- 0.001 variation in diameter. The precision of such a control can be improved by using multiple-hinge MEMS. Using a locking mechanism, the angle was controlled by MEMS micro-machining. Multiple-hinge MEMS also provide us with an opportunity to create very complex MEMS such as boxes or stacking structures. Solder self-assembly is able to manufacture a large number of complex, precise MEMS. 5. Acknowledgements This project is supported by the Department of Defense (MDA904-97-C-0320). The authors would also like to thank Ms. Michelle Carpenter at the University of Michigan at Ann Arbor for her design of the spinning optical mirror. 6. References 1. Akiyama, T., Collard, D., Fujita, H., "Scratch Drive Actuator with Mechanical Links for Self- Assembly of Three Dimensional MEMS," Journal of Micro-electromechanical Systems, Vol. 6, No. 1:10-17, 1997. 2. Fan, L., Ming, C., Choquette, K.D., "Self Assembled Micro-actuated XYZ Stages for Optical Scanning and Alignment," paper no. 0-7803-3829-4, Transducers 97, Chicago, June 16-19, 1997. 3. Lee, Y. C. and Tan, Q., Soldering for Optoelectronic Packaging, IEEE Electronic Components and Technology Conference, Orlando, FL, May 28-30, 1996 4. Green, P.W., Syms, R.R.A., Yeatman, E.M., "Demonstration of Three-dimensional Microstructure Self-Assembly," Journal of Micro-electromechanical Systems, vol. 4, no. 4, 170-176, 1995. 5. Singh, A., Horsley, D. A., Cohn, M. B., Pisano, A. P. and Howe, R. T., "Batch Transfer of Microstructures Using Flip-chip Solder Bump Bonding," Transducers 97, Chicago, 1D4.03, pp. 265-68, 1997. 6. Lin, W., Study of soldering technology for liquid-crystal-on-silicon (LCOS) modules, Ph.D. Thesis, University of Colorado, Boulder, CO, 1995. 7. Lin, W., Patra, S. K. and Lee, Y. C., Design of Solder Joints for Self-aligned Optoelectronic Assemblies," IEEE Trans. on Components, Packaging and Manufacturing Technology, Part A, August 1995, pp. 543-551. 8. Harsh, K. and Lee, Y.C., Modeling for solder self-assembled MEMS, paper no. 3289-26, Proceedings of SPIE Vol. 3289, San Jose, January 24-30, 1998.