High-Speed Scanning Interferometer Using CMOS Image Sensor and FPGA Based on Multifrequency Phase- Tracking Detection

Applications include LCD/plasma display inspection and semiconductor wafer process characterization.

A sub-aperture stitching optical interferometer can provide a cost-effective solution for an in situ metrology tool for large optics; however, the currently available technologies are not suitable for high-speed and real-time continuous scan. NanoWave’s SPPE (Scanning Probe Position Encoder) has been proven to exhibit excellent stability and sub-nanometer precision with a large dynamic range. This same technology can transform many optical interferometers into real-time subnanometer precision tools with only minor modification. The proposed field-programmable gate array (FPGA) signal processing concept, coupled with a new-generation, highspeed, mega-pixel CMOS (complementary metal-oxide semiconductor) image sensor, enables high speed (>1 m/s) and real-time continuous surface profiling that is insensitive to variation of pixel sensitivity and/or optical trans mission/ reflection. This is especially useful for large optics surface profiling.

Due to the patented phase synchronous tracking detection scheme in the time domain, the new method has already demonstrated better than 65 pm rms measurement noise (from single-pixel information alone) using an experimental setup, while up to 60 pm dynamic phase accuracy over the entire measurement range is predicted. Simulation shows that the measurement noise level could reach 1–2 pm.

It also correctly maps the phase even when high-density fringe is present under faint light condition (similar to lock-in amplifier). The real-time scanning also provides sub-pixel spatial resolution. This new technology is capable of measuring steep wall objects or aspheres with more than a few hundred waves of aspheric departure without the use of dedicated null lenses or computer-generated holograms. Thus, a compact, scalable, low-cost and low-energy consumption system can be achieved.

This work was done by Tetsuo Ohara of Goddard Space Flight Center. GSC-15942-1

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