Design, Dynamics and Active Control of Micro Interferometers for Low Noise Parallel Operation

Micromachined Scanning Grating Interferometer (μSGI) array offers a viable solution to the high resolution, large bandwidth, non-contact and high throughput metrology. Parallel active control of μSGIs is necessary to reduce the effect of positioning errors and ambient vibration noise. To achieve individual control of the μSGIs, the gratings in the μSGI are micromachined on Silicon membranes, which can be electrostatically actuated. These tunable gratings are designed to have sufficient range of motion (∼400nm) and sufficient bandwidth (∼50kHz) for effective noise reduction. The tunable gratings are fabricated successfully using Silicon on Insulator wafers with a two mask process. A novel recurrent calibration based control algorithm is designed to actively control the tunable gratings. The novel algorithm is implemented digitally using FPGA on an array of μSGIs simultaneously. The algorithm compensates for the non-linearities of the actuator and problem due to limited range of motion. A system model is built to design and analyze the control algorithm and is verified by experimental results. Experimental results show 100 times noise reduction at low frequencies and 6.5kHz noise reduction cutoff frequency. A resolution of 1×10−4 nmrms/√Hz is achieved by implementation of this algorithm on μSGI.