Batch microassembly techniques for MEMS and related systems are described. As an alternative to monolithic integration, these processes address the need for low-cost MEMS packaging, MEMS / CMOS integration, and large, sparse arrays of microfabricated devices.
Two approaches are pursued. The first, self-assembly, utilizes random vibration and weak, static potentials to enable massively parallel manipulation of small objects. These may consist of singulated microelectronic devices or MEMS structures. The technique is modeled on natural thermodynamic processes such as protein folding and epitaxial film growth. In a typical implementation, an ultrasonic shaker table is used to break surface forces between a planar substrate and microstructures lying thereon. Friction is effectively eliminated, allowing structures to respond to extremely weak forces. Forces may, for example, be generated electrostatically, using planar electrodes fabricated in standard VLSI thin-film technology.
The second approach, termed constrained assembly, is targeted specifically at transferring a planar arrangement of microstructures from one substrate to another. Micromechanical structures such as breakaway tethers and self-aligning latching mechanisms are developed to facilitate transfer.
Using self-assembly, arrays of up to one thousand elements were assembled. The potential for assemblies of 10^6or more elements with the same apparatus is demonstrated, based on thermodynamic analysis. Microelectrode arrays are fabricated by an extremely simple, single-mask / single-metal process and used to demonstrate self-assembly of 100-micron scale chiplets. "Binding sites" are demons~ated to provide orientation selectivity.
As an alternative means of combating friction, a novel type of passively stable, electrostatic levitation mechanism was developed, applicable to dielectric structures in a liquid medium. Techniques for manipulating, trapping, and precisely orienting microstructures are demonstrated.
Constrained assembly techniques enabled batch transfer and sealing of vacuum micropackages, transfer of operable microresonators, and transfer of polysilicon microactuators onto CMOS control electronics.
A fundamental goal of this work has been to enable transfer of devices to and from standard commercial processes, such as MOSIS CMOS. To this end, emphasis is placed on room-temperature, nonaggressive processes, with a view toward broad-based compatibility. A modular approach, based on microassembly of standard components, would minimize redundant process development in future applications.
August 31, 1998
Cohn, M. B. (1997). Assembly Techniques for Microelectromechanical Systems. United States: University of California, Berkeley.