Integrated Circuit "Astrolabe" Angular Displacement Sensor Using On-Chip Pinhole Optics
Wijesinghe, Udumbara C
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An integrated circuit sensor capable of tracking the angular displacement of an object tagged with a quasi-point source of light, such as a light emitting diode (LED), is designed, developed, experimentally characterized and physically modeled. The sensing element consists of four photocathodes enclosed inside an integrated circuit metal box with a pinhole aperture, which eliminates the need for external focusing optics. The angular displacement of an LED along both orthogonal latitudinal and longitudinal arcs is encoded as normalized photo-cathode current imbalances. A set prototype sensor including variations in aperture shape, aperture dimension, cathode separation, surface gratings, and blocking structures were fabricated using industrially standard “0.18 µm technology node” silicon complementary metal-oxide semiconductor (CMOS) technology. In these prototype sensors, the sensor signal is found to be linearly proportional to LED angular position across an approximately ± 50° field-of-view. A simple one-dimensional model of sensor response is developed, and the fundamental performance characteristics of prototype sensors are presented. A figure-of-merit is introduced that helps determine the uncertainty in angular measurement for a given measurement bandwidth and incident optical power. In these prototype astrolabes, the amplified signal figure-of-merit roughly a factor of 10 worse than needed to be practically useful. Based on the results of the prototype sensors, a wide range of improved second-generation sensor layout variations was designed, fabricated, and experimentally tested. Second-generation astrolabe variations included, anode gratings, guard rings, aperture area variations, cathode separation variations, cathode type variations, unit cell dimension variations and some sensors had integrated on-chip preamplifiers. The improved features and their impact on sensor characteristics are presented. More advanced physics-based 1-d and 2-d theoretical models have been derived in order to understand the operating principles of the sensor thoroughly. Numerical technology computeraided design (TCAD) models of the type used widely in the semiconductor industry have been used to simulate the device physics of the sensors. The figure-of-merit obtained from the unamplified signals of the second generation astrolabes is three times better than that of the prototype sensors. However, second generation sensors show only 30% improvement with signal amplification compared to the 85% improvement resulted from the sensor first generation. Possible noise sources that could affect the sensor performance have been studied, modeled and a new measurement setup is proposed to track the angular position of a moving object in real time.