Hyperacuity Image Sensors
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DAVID K. BIEGELSEN, WARREN B. JACKSON, RENI LUJAN, DAVID JARED, AND RICHARD L.WEISFIELD Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT
The human visual system perceives much smaller spatial steps in edges between high contrast regions than equivalently fine, periodic features. This characteristic is known as hyperacuity. We have designed, simulated, fabricated and characterized amorphous silicon sensors which provide hyperacuity information. The individual pixels are position sensitive detectors, the outputs of which provide the x and y first moments of the cell illumination pattern as well as the average gray level. In the simplest case the top electrode of a standard p-i-n diode sensor is replaced by four edge strip electrodes. Both quadrilateral cells (having all four lateral electrodes on the same side of the p-i-n diode) and duolateral cells (having x-electrodes on top and y-electrodes on bottom) have been tested. Results of probing the cells with rastered spots show that both types provide usable linearity and sensitivity. The duolateral structure provides greater orthogonality of the x and y information. One pm spatial resolution can be achieved with devices compatible with standard amorphous silicon sensor processing.
INTRODUCTION
The human visual system manifests a sensitivity characteristic named hyperacuity. Visual acuity to periodic structures is described by the perceived contrast as a function of the spatial frequency. Thus, a black/white ruling produces maximum contrast in the range of 4 line pairs per cm (when viewed at a normal reading distance of approximately 40 cm.)l At 60 line pairs per cm (a period of 20 thousandths of a centimeter), negligible periodic contrast is observable and the image is perceived as uniform gray. Resolution of two neighboring point sources is limited to similar separations due to the inter-sensor spacing in the fovea of the human retina. On the other hand, edge offsets of one thousandth of a centimeter can be perceived readily. 2 This 1OX enhanced sensitivity is called hyperacuity.
POSITION SENSITIVE DETECTORS
An example of a sensor which mimics the hyperacuity characteristic of the human visual system is the position sensitive detector (PSD).3 Such a detector utilizes the lateral photoeffect 4 wherein lateral currents flow to equilibrate a locally nonuniform photocharge population. 5, 6 Fig. 1 shows both a standard p-i-n photodiode and a version of an amorphous silicon PSD, the quadrilateral cell. (Duolateral cells have one of the electrode pairs in the lower plane.) Like the standard sensor the PSD's consist of a reverse-biased vertical p-i-n photodidode. The main difference is 833
Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society
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Standard p-i-n photodiode (left) and quadrilateral position sensitive detector (right)
that in the PSD's the electrodes are patterned into orthogonal pairs of edge contacts. Photogene
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