Solder self-assembly for three-dimensional microelectromechanical systems

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Abstract

A solder technology has been developed that utilizes molten solder surface tension forces to self-assemble MEMS 3-D structures. Using solder, a single batch reflow process can be used to accomplish hundreds or thousands of precision assemblies, and the cost per assembly can be reduced considerably. A model, based on surface energy minimization of molten liquids, has been developed for predicting assembly motion. The modeling, combined with experimental studies, have demonstrated ±2° assembly angle control is possible when the MEMS structures are assembled by solder alone. To improve the self-assembly angle precision, a self-locking mechanism can be added, which reduces the assembly angle variation down to ±0.3°.

Introduction

Microelectromechanical systems (MEMS) have been projected to fuel many innovative applications. One of the most common methods for manufacturing MEMS is surface micro-machining. A major problem with this method is its inability to produce highly three-dimensional (3-D) structures. A common solution is the fabrication of hinged components that can be lifted or “popped-up” into assembled structures. Such structures have been widely used in many microelectronics fields such as micro-optics, and biomedical applications 1, 2. The major downside of these structures is that they need to be manually assembled after fabrication. Manual assembly usually consists of rotating the plates by hand using micro-manipulators. This form of assembly is not practical for mass assembly and is rarely effective. Another method is to use additional MEMS mechanisms incorporated into the designs to perform the actual assembly 3, 4. This method is also insufficient because these MEMS mechanisms are often large and complex, and thus negate many advantages inherent in MEMS.

A new method of assembling MEMS has been proposed that uses the surface tension properties of molten solder or glass as the assembly mechanism 5, 6. The solder method involves using a standard hinged plate with a specific area metalized as solder wettable pads. Once the solder is in place, it is heated to its melting point, and the force produced by the natural tendency of liquids to minimize their surface energy pulls the free plate away from the silicon substrate (Fig. 1). Solder is a predominant technology for electronics assembly and packaging. It is not only used for electrical connections, but also for sub-micron accuracy alignment in many packaging applications such as optoelectronic passive alignment [7]. 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. In addition, solder provides high quality mechanical, thermal and electrical connections. Soldered 3-D MEMS are expected to have a major impact on MEMS advancement. However, many issues related to soldering MEMS need to be addressed.

The studies carried out by Syms et al. 5, 6focused on fabrication techniques assisted by a simple model [8]. Another solder study integrated MEMS with another MEMS through thermocompression bonding [9]. The feasibility of the use of solder for MEMS assembly has been demonstrated by these publications; however, there is a need to conduct further studies for reliable knowledge and technology advancement. We report an in-depth characterization of solder self-alignment for 3-D MEMS through modeling and experimental studies. In addition, a self-locking concept integrated with soldering is presented to enhance angle control for precision alignment.

Section snippets

Modeling technique

An accurate model is vital if solder self-assembly is to be precisely characterized. The modeling work on planar structure solder's self-alignment, used in microelectronics and optoelectronics, is well established 7, 10. But, there is a lack of models for three-dimensional MEMS. The only existing model for a solder assembled MEMS device was developed by Syms [8], which was based on a geometric method of calculating the force on the hinged plate. This model calculated the solder profile as two

Self-locking mechanisms

Self-locking mechanisms are an established method for controlling the angle of manually assembled hinged structures 14, 15. Although locks are not absolutely necessary when using solder self-assembly, they are useful as an additional form of angle control. There are many commonly used variations. Fig. 10 presents one basic type, referred to as a kickstand lock, which was utilized in all experiments presented below. As the solder rotates the structure out of the plane of the substrate, a rigid

Summary

Solder self-assembly technology is important for the manufacturing of surface micro-machined 3-D MEMS structures. Modeling and experimental studies have been conducted. Precise assembly of hinged structures can be achieved using controlled solder volumes, and utilizing the surface energy minimization principle. Precise solder volume control is necessary to achieve accurate angle control. Using solder spheres with 0.001 in. diameter variation results in an angle variation of ±1° to 3°. The

Acknowledgements

This project is supported by the Department of Defense (MDA904-97-C-0320), the Defense Advanced Research Projects Agency (DARPA), the Air Force Research Laboratory, Air Force Materiel Command, USAF, under agreement number F30602-98-1-0219.

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