Silicon Electromechanical Microgrippers: Design, Fabrication, and Testing

Microgrippers with jaw spans of l0um have been made from thin films (2um thick) of polycrystalline silicon (polysilicon) by using micromachining technology derived from IC-manufacturing methods.  A 500um-long, on-wafer gripper, electrostatically driven by flexible-comb actuators, has been designed, fabricated, and tested successfully. The gripper deformation have been analyzed and modeled, and the testing results showed agreement with the modeling. This on-wafer prototype has been followed by the development of a more practical, overhanging microgripper (400um long), which has successfully seized several microscopic objects in laboratory experiments with typically less than 30V for operation. Among these has been a Euglena, a single cell protozoan measuring 7um in diameter and 40um in length. Microfabricated vernier gauges have also been designed and built to measure the residual strain in the thin films of n+-polysilicon and p+-silicon used for the fabrication of the overhanging microgrippers.

The evolution of microelectromechanical systems (MEMS) has been briefly reviewed with an emphasis on micromachining and microactuators. Scaling issues and similitude considerations for dimensional analysis have been discussed as an underlying physics in micro domain. Several actuation.schemes for IC-technology-based microgrippers have been considered; parallel-pIate electrostatic, piezoelectric, thermal expansion, and shape-memory-alloys actuation have been studied, and the corresponding microgripper designs utilizing each have been presented. Difficulties in fabrication and results of preliminary testing have indicated that, at the present time, it has been most feasible to adopt a flexible-comb actuator driven by fringe-field electrostatic force.

Electrostatic actuation via rigid, interdigitated comb structures, previously used for microactuators that resonate, has been implemented in a new comb design in which the interdigitated finger overlap has been reduced to zero and comb structures themselves made flexible. These comb structures have now served dual purposes: as both the actuation means and the structures from which the gripper arms were fashioned. Three different electromechanical models have been developed to describe the gripper-arm deformation, and despite differences in the intrinsic elastic model, the deflections predicted by all three models agreed to within 3%. Among them, the lumped-beam model has been used to estimate the gripper-jaw dsplacement and the gripping force as functions of applied voltages, and showed that a maximum gripper force of O.1uN is achievable at 40V.

A 500um-long, prototype microgripper made by surface micromachining of polysilicon has been designed, fabricated, and tested on a silicon wafer. The main features are a cantilever drive-arm with flexible combs, a bidirectional actuation scheme, and an over-range protector. Fabrication sequences and testing results have been presented. Experiments have demonstrated that a gripping range of l0um can be effected with an applied potential of 20V. The motion dependence on drive voltage has been measured and compared with the prediction from the lumped-beam model; gripper motion has been observed to be smooth, stable, and controllable.  

Based on the success of the flexible-comb drive, on-wafer microgripper, an overhanging microgripper (similar to on-wafer version but now designed to be suitable for mounting on a micropositioner) has been designed and fabricated by combining surface and bulk micromachining. This microgripper consists of a silicon die (7x5mm), a 1.5mm-long support cantilever, made from boron-doped silicon substrate material (protruding from the die), and a 400um-long overhanging polysilicon gripper extending from the end of the support cantilever. The microgripper has significantly smaller feature sizes than have been reported by other investigators for overhanging microstructures. Problems addressed successfully in the microgripper fabrication included the protection of surface-micromachined fine structures during bulk-silicon etching and rinsing. This microgripper seized several microscopic objects successfully in laboratory experiments.

The design and application of microfabricated vernier gauges have been described for easy in-situ measurements of the average residual strain of thin films on a silicon wafer. The films of particular interest are the structural layers of micromechanical elements. Limitations to precision of the vernier gauges have been considered, and compensation of the vernier reading in the presence of film warpage has been discussed. The devised residual strain gauges have vernier gauges mounted on a double-cantilever microstructures. These strain gauges have been used to measure the residual strains in phosphorus-doped polycrystalline and boron-doped single crystal silicon films.