Mr. Patrick Baxter Profile

Patrick Baxter

at STMicroelectronics Ltd

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Proceedings Article | 27 September 2008 Paper

A low light level sensor with dark current compensating pixels

Mitchell Perley, Patrick Baxter, Jeffrey Raynor, David Renshaw

Proceedings Volume 7100, 71002E (2008) https://doi.org/10.1117/12.797268

KEYWORDS: Photodiodes, Capacitors, Capacitance, Sensors, Transistors, Cesium, Diodes, Switches, Amplifiers, Calibration

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In ultra-low light conditions the presence of dark current becomes a major source of noise for a CMOS sensor. Standard dark current compensation techniques, such as using a dark reference frame, bring significant improvements to dark noise in typical applications. However, applications requiring long integration times mean that such techniques cannot always be used. This paper presents a differential dark current compensating pixel. The pixel is made up of a differential amplifier and two photodiodes: one light shielded photodiode connected to the non-inverting input of the opamp and a light detecting photodiode connected to the inverting input of the opamp. An integrating capacitor is used in the feedback loop to convert photocurrent to voltage, and a switched capacitor network is present in parallel with the light shielded pixel, which is used to satisfy the output equation to compensate the dark current. The pixel uses 150 μm x 150 μm photodiodes and is fabricated in a standard 0.18 μm, 6M1P, CMOS process. The results show that the pixel is light sensitive and has a linear output as expected. However, the dark current is not predictably controlled. Further work will be carried out on the pixel design, and particularly the switched capacitor circuit, to determine the cause of the non-predictability of the pixel output.

Proceedings Article | 21 February 2007 Paper

Low cost optical polarization sensor

D. Patrick Baxter, Jeffrey Raynor, Jean-Luc Jaffard, David Renshaw

Proceedings Volume 6501, 65010R (2007) https://doi.org/10.1117/12.700502

KEYWORDS: Polarization, Sensors, Computer programming, Metals, Photodiodes, CMOS sensors, Standards development, Polarizers, Capacitors, Photodetectors

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Polarized light is a naturally occurring phenomenon that human vision does not discern, yet it can provide useful supplementary information from an image or optical system. Polarization detection can be implemented using hybrid sensors where additional polarizing material is mounted onto a standard sensor. However these types of sensor tend to be expensive, requiring extra manufacturing and materials. Presented is a low cost polarization sensor which is implemented using standard CMOS technology and manufacturing techniques, without the need for supplementary implants or optical layers. The polarization sensor is realised using a polarization grating, formed from a standard metal layer, above a CMOS sensor. To compensate for the loss of photons due to the polarization grating, a high dynamic range sensor is implemented using large, 110 micron photodiodes. The photosensor is used in a "light to frequency conversion pixel" where the photocurrent is converted to a digital square wave output with a frequency proportional to the photon flux density. A modulation depth of 10% is achieved. A rotary encoder application implementing the polarization sensor is discussed.

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