Integrated Circuit "Astrolabe" Angular Displacement Sensor Using On-Chip Pinhole Optics
Abstract
Abstract
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.