Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-01-07
2004-04-13
Dickey, Thomas L (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S292000, C257S293000, C250S208100, C348S302000
Reexamination Certificate
active
06720594
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to imager systems, and in particular imager systems utilizing high fill-factor image sensor arrays.
BACKGROUND OF THE INVENTION
Recent developments in the field of image sensing technology have focused on the switch from relatively low fill-factor image sensor arrays, which utilize an array of isolated sensors to detect light, to relatively high fill-factor image sensor arrays that utilize a continuous layer of sensor material formed over an array of pixel circuits. Each pixel circuit of these high fill-factor image sensor arrays includes an access transistor and a contact (i.e., a metal pad) that is connected to the lower surface of the sensor material layer. A continuous transparent bias layer (e.g., indium tin oxide (ITO)) is typically formed on an upper surface of the continuous sensor material layer. Each sensor operates on the principal of integrating a charge representative of the quantities of radiation incident on the sensor. When an image is to be captured by the image sensor array, radiation (e.g., light or X-rays) conveying the image strikes the sensor material layer, which responds by freeing electrons and holes that generate a local current in the sensor material layer between the pixel contacts and the continuous bias layer. These local currents change the potentials on the underlying pixel contacts according to the amount of light incident thereon. The potential on each pixel contact is periodically “read” by sequentially turning on the access transistors to couple the pixel contacts to a series of charge-sensing amplifiers. The differences between the various potentials read from the pixel contacts are then used to reconstruct the captured image.
One well-known type of high fill-factor image sensor array utilizes hydrogenated amorphous silicon (a-Si H) sensor material for real time imaging (see R. A. Street et al., “Large Area Image Sensor Arrays”, in Technology and Applications of Amorphous Silicon, Editor R. A. Street, Springer Series in Materials Science 37, Springer-Verlag, Berlin, 2000, chapter 4, p 147, for a general description of the structure of the arrays). Such a-Si H sensor arrays are particularly advantageous for X-ray imaging because they present a relatively large size image sensor array. In the direct detection approach, incident high-energy radiation (e.g., X-ray photons) is directly converted to a charge by the sensor. In the indirect detection approach, a phosphor converter absorbs high-energy radiation (e.g., X-ray photons) and generates a proportional amount of visible light that is then converted to a charge by the sensor.
An obvious problem associated with the use of continuous sensor material layers is crosstalk between adjacent pixels, which occurs when the continuous sensor layer allows conduction between pixel contacts. This form of crosstalk directly reduces the resolution of the image sensor array because a sharp feature will be blurred into neighboring pixels. As mentioned above, as the sensor material located over one pixel is illuminated, the charge from the illumination builds up on that pixel's contact. This shifts the voltage on that contact towards the bias voltage level applied to the continuous bias layer. If the sensor layer allows lateral conduction, then the potential difference between adjacent pixels will result in conduction from one pixel to the next. Experimentally, in image sensor arrays utilizing continuous a-Si H sensor material layers, this form of crosstalk has been observed with varying magnitude, but primarily is a problem as the pixel reaches saturation (i.e., approaches forward bias). See Rahn J. T. et al. “High-Resolution High Fill Factor a-Si H Sensor Arrays for Medical Imaging,” Proc. of SPIE, Vol. 3659, pp. 510-517, 1999.
Another problem associated with high fill-factor image sensor arrays, which is also a problem with all pixilated structures, is the rejection of high spatial frequency signals. Because the pixilation of an image sensor array acts as a sampling function, high spatial frequency signals are aliased into lower frequencies. High fill-factor image sensor arrays (described above) reduce the impact of aliasing, but do not eliminate it. In many imaging systems, the image source can be designed to reject high spatial frequencies, for example, by designing the focus of the optical system to blur the image and reject high spatial frequencies. In addition, indirect x-ray detection typically does not have much of a problem with aliasing, since the phosphor screen rejects high spatial frequencies. However, in direct detection imagers that do not include optical blurring, the effects of aliasing can be clearly seen. Even if the imager can be designed so that high spatial frequencies are filtered on the imager, the noise will also be aliased and the total noise power increased, which reduces the Detector Quantum Efficiency (DQE) of the imager.
Accordingly, what is needed is a high fill-factor image sensor array that significantly reduces crosstalk between adjacent pixels. What is also needed is a high fill-factor image sensor array that filters high spatial frequency signals prior to imaging.
SUMMARY OF THE INVENTION
The present invention is directed to a high fill-factor image sensor arrays in which the image resolution is improved by reducing crosstalk between adjacent pixels. This crosstalk reduction is achieved by the various embodiments of the present invention by clamping the sensor voltage (e.g., the voltage across the photodiode of each pixel) to prevent saturation, and/or by maintaining the pixel contact at a fixed voltage.
In accordance with a first embodiment, a high fill-factor imager system includes a scanning control circuit for generating gate voltage signals on a plurality of gate lines, a bias voltage source, and an imager including a plurality of pixels arranged in an array. Each pixel of the array includes a sensor (e.g., a photodiode) for generating a charge, a storage capacitor for storing the charge, and an access transistor connected between the storage capacitor and an associated data line of the array. The sensor includes a first terminal (e.g., an anode) that is maintained at a predetermined bias voltage by the bias voltage source, and a second terminal (e.g., a cathode) connected to a first terminal of the storage capacitor. A second terminal of the storage capacitor is connected to a system voltage source. At the beginning of an imaging cycle, the second terminal (cathode) of the sensor is reset such that a predetermined voltage exists across the sensor. Light (or other radiation) striking the sensor generates a proportional charge therein. This charge is stored by the storage capacitor, and is passed to the associated data line during a subsequent readout operation.
Pixel clamping in the first embodiment is achieved either by maintaining the bias voltage well below the gate off voltage of the access transistors, or by periodically pulsing the gate voltage to drain excess charge during exposure and between readout cycles. Note that the description of this invention assumes n-type transistors and a sensor biased negative with respect to the data line. This invention is not limited, however, to this polarity. According to the first approach, the scanning control circuit generates either a gate on voltage, which turns on the access transistors of a column of pixels during readout/reset, or a gate off voltage that turns off the access transistors. By maintaining the bias voltage at least one threshold voltage of the access transistors below the gate off voltage, excess charge is drained from the storage capacitor onto the data line through the turned-off access transistor when the cathode voltage gets too close to the bias voltage, thereby preventing the photodiode (sensor) from reaching saturation. A potential problem with this approach is that draining charge onto the data line during the readout cycle of another pixel connected to that data line can result in unwanted crosstalk. Therefore, according to the second approach,
Lu Jeng Ping
Rahn Jeffrey T.
Van Schuylenbergh Koenraad F.
Bever Patrick T.
Bever Hoffman & Harms LLP
Dickey Thomas L
Xerox Corporation
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