A Solar-Powered, Milligram Prototype Robot from a Three-Chip Process

A simple 5-mask process, the Iolanthe process, was devised for the robot’s frame, inchworm motors, and legs. The Iolanthe process combined bulk SOI micromachiningwith polysilicon surface micromachining to achieve high-force in-plane electrostaticmotors with compliant out-of-plane structures. The enabling step of the process was planarization of the SOI device layer using thick glass planarization technology. Out-of-plane leg motion was demonstrated using electrostatic inchworm motors with polysiliconpin hinges. In addition to the one-DOF legs for the robot, a two-DOF leg and a bidirectional inchworm motor were also fabricated and tested. Inchworm motors demonstrated 400 μm of travel at speeds up to 6.8 mm/s. The two-DOF leg demonstrated angular deflections over 90o, and the one-DOF leg output forces up to 60 μN.

In the second process, high-voltage solar cell arrays were created to drive the electrostatic actuators of the inchworm motors. In order to generate the high voltages, individual solar cells were electrically isolated with isolation trenches on the SOI device layer.Solar cell arrays as small as 0.6 mm2 generated over 30 V. NMOS and PMOS transistorswith breakdown voltages ranging from 42 V to 100 V were also integrated into the process. With these devices, a 50 V solar-powered buffer was demonstrated in an area of 3.6x 1.8 mm^2.

The third and final chip of the robot was fabricated in a commercial CMOSfoundry. The chip provided the logic signals to drive the legs in an open-loop walking gait.