K.F. Harsh

Solder self-assembly for three-dimensional microelectromechanical systems

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°.

Design and modeling of RF MEMS tunable capacitors using electro-thermal actuators

A series mounted MEMS tunable capacitor in a CPW line is reported. An electro-thermal actuator has been used for driving the top plate of the parallel plate capacitor. The MEMS structure is bonded on an alumina substrate using flip-chip technology so that the silicon on the backside of the MEMS can be removed to reduce the RF losses. The lumped-element model of the capacitor up to 40 GHz has been developed based on Y-parameters, which are derived from measured S-parameters. The measured Q-factor is 256 at 1 GHz for a 0.102 pF capacitor and C/sub max//C/sub min/ ratio of the capacitor is about 2:1.

The realization and design considerations of a flip-chip integrated MEMS tunable capacitor

Abstract Microelectromechanical systems (MEMS)-based radio frequency (RF) components are being developed for various microwave and millimeter-wave applications. Using standard foundry processes, it is possible to create very complex MEMS devices. However, most RF MEMS need to be fabricated using GaAs, ceramics, high resistivity silicon or other RF-compatible materials. Such fabrication techniques are not commonly used by the mainstream silicon-based MEMS manufacturing infrastructure. As a result, the complexities of these MEMS devices are very limited. What is needed is a way to utilize the existing cost effective foundry processes, but not sacrifice RF performance. Utilizing a flip-chip transfer process, a complex, foundry fabricated, MEMS tunable capacitor has been demonstrated that yields high quality RF performance ( Q ∼100 at 10 GHz, 1050 at 1 GHz). The transfer process is described, and its performance (control, success rate, etc.) is presented. Several major design considerations for implementing the tunable capacitor using flip-chip technology are presented, including warpage, actuator design, and structural rigidity. Using the transfer process and design considerations, there is an opportunity to integrate complex MEMS onto any RF compatible substrate without the silicon semiconductor effects. Thus, it is possible to manufacture complex MEMS cost-effectively for a new generation of RF MEMS with superior functionality.

Flip-chip assembly for Si-based RF MEMS

MEMS-based RF components are being developed for various microwave and millimeter-wave applications. However, most RF MEMS have to be fabricated using GaAs, ceramics, high-resistivity silicon or other RF-compatible materials; such fabrication techniques are not commonly used by mainstream silicon-based MEMS manufacturing infrastructure. As a result, the complexity of these MEMS is limited. Using flip-chip assembly and silicon removal techniques, there is an opportunity to integrate MEMS onto any RF compatible substrate without the silicon semiconductor effects. Thus, it is possible to manufacture complex MEMS cost-effectively for a new generation of RF MEMS with superior functionality, e.g. tunable capacitors, multi-way switches and arrays of hundreds of these or other RF components. This new technology is described with an emphasis on four issues: warpage, actuators, release and flip-chip bonding.

Prototype microrobots for micro positioning in a manufacturing process and micro unmanned vehicles

The design and performance of two prototype microrobots are presented in this paper. The microrobots were implemented by surface micromachining arrays of 270 /spl mu/m long, polycrystalline silicon legs across the surface of a silicon chip. The method of motion of the microrobots is designed to mimic the way six legged insects walk. One microrobot leg design has two degrees-of-freedom motion, and the other leg design has one degree-of-freedom motion. Both microrobot designs are able to transport objects across their bellies while lying on their backs. The microrobot with one degree-of-freedom motion is able to support several times its own weight, making available the option to carry an autonomous power supply (such as a solar cell), microprocessor, control circuitry, test equipment, and sensing or surveillance devices.

MEMS designed for tunable capacitors

A new tunable capacitor based on a standard microelectromechanical systems (MEMS) technology has been demonstrated. Its unique feature was the use of thermal actuators as indirect drives to change air gap from 2 to 0.2 /spl mu/m for high-Q MM-wave capacitors. Such a drive scheme achieved a sub-/spl mu/m controllability. The insertion loss of a polysilicon MEMS capacitor was measured to be -4dB at 40 GHz. Such a loss would have been better than -1 dB if the polysilicon were coated with metal.

Quick prototyping of flip chip assembly with MEMS

This paper describes a process to transfer microelectromechanical systems (MEMS) devices to a secondary substrate using flip-chip thermosonic bonding. A standard wire-bonding machine was used to place /spl sim/100-/spl mu/m bumps on unreleased MEMS chiplets. The bumped chiplet was then flip-chip bonded to a secondary substrate containing a microwave coplanar waveguide (CPW). After bonding, the entire assembly was run through the MEMS release process, after which the MEMS host substrate was removed. The thermosonic bonding was a very reliable prototyping tool with a 100% bonding yield. The transfer process can be used with any MEMS that can be wire bonded. The process can also be applied to a variety of applications.

Micromirror arrays fabricated by flip-chip assembly

This paper presents the design, fabrication, modeling, and testing of several Flexure-Beam Micromirror Device (FBMD) arrays fabricated using flip-chip assembly. These arrays were prefabricated using a silicon surface-micromachining technology and then transferred to a ceramic receiving substrate using a thin layer of indium placed between the bond pads of the two chips. Device characterization was completed using an interferometric microscope in which micromirror deflection was determined as a function of address potential. The arrays demonstrate reasonable micromirror performance with optically flat surfaces measuring only 5 - 6 nm of variance across as much as a 150 micrometer mirror surface.

Study of micro-scale limits of solder self-assembly for MEMS

Solder self-assembly of MEMS has been shown to be an excellent approach for assembling three-dimensional MEMS structures by allowing more precise alignments and vastly increased complexity. This paper investigates if solder self-assembly could be applied at the sub-micron scale. Experiments studying the surface energy minimization properties of molten solder demonstrate that surface energy minimized solder spheres as small as 5 nm are possible. However, the intermetallic formation may limit our ability to reach the nano-scales. Nevertheless, even with intermetallic formation, it should be possible to use solder self-assembly at sub micron scale.

Modeling for solder self-assembled MEMS

A new method of assembling MEMS is being developed that uses solder surface tension force to manipulate and assemble MEMS 3D structures. Modeling is critical to design solder joints for precision assembly. An accurate model has been developed based on the principle of surface energy minimization. Using surface evolver software, this model considers 3D MEMS configurations with different pad dimensions, geometries, and volumes of the solder joint. The software calculates solder shapes with local minimum surface energies and identifies the final shape with the global minimum energy. A two-plate popped-up MEMS structure was modeled and experimentally measured. The experiment confirmed the model could predict the final, equilibrium angle to within +/- 2 degrees. This accuracy level is actually limited by the experimental error bar of +/- 2 degrees, which was caused by the volume variation of the solder spheres used. The models accuracy is expected to be much better. Nevertheless, the present model, with the verified accuracy, can help MEMS researchers design innovative 3D MEMS assembled using solder.