Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-01-05
2001-12-04
Nelms, David (Department: 2898)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S073000
Reexamination Certificate
active
06326230
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to image sensors.
BACKGROUND
Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. Imaging circuits often include a two-dimensional array of photosensors each of which forms one picture element (pixel) of the image. Light energy emitted or reflected from an object impinges upon the array of photosensors and is converted by the photosensors to electrical signals. The individual photosensors can be scanned to read out and process the electrical signals.
One class of solid-state image sensors includes an array of active pixel sensors (APS). An APS is a light sensing device with sensing circuitry inside each pixel. Each active pixel includes a sensing element formed in a semiconductor substrate and capable of converting optical signals into electronic signals. As photons strike the surface of a photoactive region of the solid-state image sensors, free charge carriers are generated and collected. Once collected, the charge carriers, often referred to as a charge packet, are transferred to output circuitry for processing.
An active pixel also includes one or more active transistors within the pixel itself. The active transistors can amplify and buffer the signals generated by the light sensing element. Thus, in contrast to charge coupled devices (CCDs) and metal oxide semiconductor (MOS) diode arrays, an APS can convert the photocharge to an electronic signal prior to transferring the signal to a common conductor that conducts the signals to an output node.
APS devices can be fabricated in a manner compatible with complementary metal oxide semiconductor (CMOS) processes. Compatibility with CMOS processes allows many signal processing functions and operation controls to be integrated on an APS chip. Use of CMOS circuitry with APS devices also reduces the costs of manufacturing. CMOS circuitry also allows simple power supplies to be used and can result in reduced power consumption. Moreover, the active pixels of APS devices allow non-destructive readout and random access.
In an exemplary CMOS APS, charge carriers are collected in the photosite via a photogate. The charge packet is stored in spatially defined depletion regions of the semiconductor, also known as potential wells, in the semiconductor substrate beneath the photosite. The charge packet then is transferred to an isolated diffusion region via a transfer gate. The diffusion region receives the charge from the photogate well and sends a corresponding electrical signal to the pixel amplifier for further processing.
The near-surface potential within the semiconductor can be controlled by the potential of an electrode near the semiconductor surface. If closely-spaced electrodes are at different voltages, they will form potential wells of different depths. Free positive charges (e.g., holes) move from a region of higher potential to a region of lower potential. Similarly, free negative charges (e.g., electrons) move from the region of lower potential to the region of higher potential.
Typically, a CMOS active pixel array is operated in a rolling shutter mode in which each row of the array is exposed at different instants of time. The non-simultaneous exposure of the pixels can lead to image distortion, for example, when there is relative motion between the imager and the image that is to be captured. Furthermore, although the exposure time generally is defined by the duration for which the photogate is turned on, floating diffusion regions can continue to collect photocharges even after the photogate is turned off. Transfer of such unwanted charges into the sense node can result in image distortion and excess noise. Furthermore, the distortions tend to become more pronounced as the exposure time is reduced.
SUMMARY
An image sensor includes pixels formed on a semiconductor substrate. Each pixel includes a photoactive region in the semiconductor substrate, a sense node, and a power supply. A first electrode is disposed near a surface of the semiconductor substrate. A bias signal on the first electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the sense node. A second electrode is disposed near the surface of the semiconductor substrate. A bias signal on the second electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the power supply node. The image sensor includes a controller that causes bias signals to be provided to the electrodes so that photocharges generated in the photoactive region are accumulated in the photoactive region during a pixel integration period, the accumulated photocharges are transferred to the sense node during a charge transfer period, and additional photocharges generated in the photoactive region are transferred to the power supply node during a third period without passing through the sense node.
According to another aspect, an image sensor includes an array of pixels formed in a semiconductor substrate. Each pixel includes a photoactive region in the semiconductor substrate, a sense node, a power supply node, and first and second transfer gates disposed in proximity to the surface of the semiconductor substrate. The image sensor also includes a controller that causes bias signals to be provided to the electrodes to operate the pixels in one of at least three modes. In a first mode, photocharges generated in the photoactive region of a pixel are accumulated in the pixel's photoactive region. In a second mode, the accumulated photocharges are transferred to the pixel's sense node via the pixel's first transfer gate. In a third mode, photocharges generated in the pixel's photoactive region are transferred to the pixel's power supply node via the pixel's second transfer gate without passing through the pixel's sense node.
In another aspect, a method of operating a photosensitive pixel includes biasing first and second transfer gates disposed in a vicinity of a photoactive region of a semiconductor substrate to accumulate photocharges in the photoactive region during a pixel integration period. The first and second transfer gates are biased to transfer the accumulated photocharges to a sense node via a region of the semiconductor substrate disposed below the first transfer gate. Subsequently, the first and second transfer gates are biased to transfer additional photocharges in the photoactive region to a power supply node via a region of the semiconductor substrate disposed below the second transfer gate without passing through the sense node.
In some implementations, one or more of the following features may be present. During the pixel integration period, the second transfer gate can be biased with a voltage higher than a bias voltage on the first transfer gate. For example, the second gate can be biased to provide an anti-blooming function, in other words, to allow excess photocharges in the photoactive region to be transferred to the power supply node via the region of the semiconductor substrate below the second transfer gate during the integration period.
To transfer the accumulated photocharges to the sense node, a bias voltage on a photogate above the photoactive region may be lowered. To transfer photocharges in the photoactive region to the power supply node via the second transfer gate prior to commencing a subsequent pixel integration period, the second transfer gate can be biased with a voltage higher than that of the first transfer gate.
Preferably, the first and second transfer gates are biased to transfer the additional photocharges in the photoactive region to the power supply node prior to commencing a subsequent pixel integration period.
The integration period of the pixels can take place at substantially the same time to achieve snap-shot imaging. Similarly, photocharges stored by the pixels in an array can be transferred to respective sense nodes at substantially the same time. An imager wit
Pain Bedabrata
Wrigley Chris
Yadid-Pecht Orly
Yang Guang
California Institute of Technology
Fish & Richardson P.C.
Nelms David
Nhu David
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