Microassembly promises to extend MEMS beyond the confines of silicon micromachining. This paper surveys research in both serial and parallel microassembly. The former extends conventional "pick and place" assembly into the micro-domain, where surface forces play a dominant role. Parallel assembly involves the simultaneous precise organization of an ensemble of micro components. This can be achieved by microstructure transfer between aligned wafers or arrays of binding sites that trap an initially random collection of parts. Binding sites can be micromachined cavities or electrostatic traps; short-range attractive forces and random agitation of the parts serve to fill the sites. Microassembly strategies should furnish reliable mechanical bonds and electrical interconnection between the micropart and the target substrate or subassembly.

This paper describes essential components of a prototype system for teleoperated microassembly. High aspect ratio molded polysilicon (hexsil) [ 11 tweezers with integrated in-situ phosphorous doped thermal expansion actuator beams and piezoresistive polysilicon strain gages for tactile feedback are described. The tweezers are normally closed, and require 75 mW to open 35 pm. Piezoresistor performance remains to be measured. Metal lines on surface polysilicon flexible electrical interconnects bridge between rotating rigid hexsil beams. Surface polysilicon tweezer tips provide compliance suitable for force-controlled micromanipulation. The task demonstrated is the pick and placement of a 1 pm x 4 pm x 40 pm peg in a 4 pm x 4 pm hole.The test parts used were surface micromachined SiO, and polysilicon beams held in organized arrays on the silicon wafer by break-away photoresist tethers.

It is hoped that each advance in axon repair technology will spur additional research to provide us with a comprehensive understanding on how best to pursue neurosurgical intervention at the microscale.

We report the first demonstration of a molded micromachine consisting of a high adpect-ratio CVD polysilicon shell filled with electroless nickel for the conductive regions, in situ P-doped polysilicon for the resistive regions, and undoped polysilicon for the insulating regions of the machine. One mask defines all regions simultaneously on the basis of mold trench width. Electroless Ni enables high current-canying capacity with minimal power dissipation. Electroless plating proceeds conformally over the entire polysilicon surface so that a deep trench can be filled by deposits growing from the sidewalls toward the middle of the trench, a distance of only a few pm. In these first prototypes, tip displacement of 25 pm was observed with a current of 300 mA at 3 V. Wire actuated, normally closed, and normally open, tweezers with 200 pm tip motion are also demonstrated. INTRODUCTIONThe essence of this work is the use of trench width in the mold to control beam composition in the structure.

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