Ph.D. Dissertation Defense by Byungki Kim
Tuesday, June 15, 2004

(Drs. Thomas Kurfess and Levent Degertekin, co-advisors)

"Miniaturized Diffraction Based Interferometric Distance Measurement Sensor"


In this thesis, new metrology hardware is designed, fabricated, and tested to provide improvements over current MEMS metrology. The metrology system is a micromachined scanning interferometer (µSI) having a sub-nm resolution in a compact design. The proposed microinterferometer measures distance using a reflective diffraction grating on a transparent substrate and a microlens fabricated using a photoresist reflow technique on the other side. This structure forms a phase sensitive diffraction grating with interferometric sensitivity, while adding the capability of better lateral resolution by focusing the laser to a smaller spot size.
Depending on the distance between a diffraction grating and a target surface, the diffraction pattern at the detector plane is changed. By monitoring the intensity of a certain diffraction order with the detector, a profile change on the target surface can be mapped. A detailed diffraction model of the microinterferometer was developed to simulate the device performance and to suggest the location of photo detectors for integrated optoelectronics.

A particular device is fabricated on a fused silica substrate using aluminum to form the deformable diffraction grating fingers and AZ P4620 photo resist (PR) for the microlens. The details of the fabrication processes are presented. The structure also enables optoelectronics to be integrated so that the interferometer with photo detectors can fit in an area that is 1mm x 1mm.

The µSI has shown its capability to measure vibration profiles up to 1.25 MHz of 190 µm diameter capacitive Micromachined Ultrasonic Transducer (cMUT). Detail experimental results are presented.