Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1998-03-16
2001-02-27
Saadat, Mahshid (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S448000, C250S2140RC, C250S214100
Reexamination Certificate
active
06194770
ABSTRACT:
This invention relates generally to photo receptors, and more particularly to a photo receptor for imaging applications that exhibits a higher signal to noise ratio than photo receptors heretofore known.
Semiconductor photo detectors (photo gates) for quantifying the charge created by incident radiation generally are well known.
Photogate structures include: Active Pixels, Charge Couple Devices (CCD), Charge Injection Devices (CID) and their variants that include virtual phase, buried channel and other variations that utilize selective dopants. The selective dopants are used to control charge collection and transfer underneath and between the photogate(s) and the sense node.
Such photo detectors utilize the phenomenon by which free electrons are generated by the interaction of photons with semiconductor materials, such as silicon. In known imaging photo detectors, a variety of techniques have been employed to form a potential well for accumulating a charge created by photons impinging on the surface of the semiconductor device above the well. In a particularly useful device, a photon transmissive electrode is formed on an insulating layer on the surface of the device, so that when a potential is applied to the electrode, a potential well is formed in the semiconductor layer beneath the electrode by the depletion of majority or minority charge carriers within the region. Free electrons generated by photons impinging upon the surface of the device, and passing through the electrode, are accumulated in the well. The magnitude of the accumulated charge can subsequently be sensed either directly or by transferring the charge to a sensor region where the magnitude of the charge can be measured.
Another form of photo detector uses an implanted region of opposite conductivity type from the substrate to form a potential well without the need for an electrode overlying the photoreceptive area. This type of construction, referred to as photo diode construction, is more sensitive to certain wavelengths than the photo gate construction just described, but may exhibit lower signal to noise ratios.
One of the primary characteristics of importance for an imager is the dynamic range. The dynamic range is typically defined as the charge in a full well of the pixel to the root means square (rms) of the noise. Full well of a pixel is the total amount of carriers, electrons or holes, that a pixel will hold. In the case of a photogate, it historically has been directly related to the top surface area of the MOS capacitor that forms the photogate and to the applied bias that creates the potential well to collect the carriers.
Standardization of imager processing and shrinking feature sizes have resulted in lowered operating biases. Since, photogate structures are Metal Oxide Semiconductor (MOS) capacitors, the amount of charge a photogate can hold is dependent on the area of the gate electrode, referred to as the top plate, that is typically made of polysilicon, and the total bias applied across the insulator. The lowered operating biases for photogates have lowered their full well correspondingly. Also, the drive toward smaller and smaller pixels for higher resolution has further reduced the surface area available and therefore the amount of charge that can be collected.
It is a problem of both photo diodes and photo gates that the signal to noise ratio is often less than optimum, the dynamic range of the device may be less than required for a particular application, and the size of the device required to obtain the necessary sensitivity may reduce the resolution of the device, that is the number of pixels that can be formed in a given area, to a lower than required number.
There is a need for photoreceptors that are, at the same time, smaller than existing constructions, exhibit greater dynamic range, and exhibit higher signal to noise ratios than those heretofore known.
The location (depth) at which free electron hole pairs are generated by photons impinging on the surface of a photoreceptor depends on the wavelength of the photons. Longer wavelengths tend to generate charge further from the surface of the device than shorter wavelengths. Therefore, it is difficult to design a photoreceptor that exhibits an even approximately linear sensitivity to light over a useful color range. Shorter wavelengths are attenuated more strongly as they pass through even largely transparent materials, such as polysilicone materials, while longer wavelengths generate free electrons at distances from the surface that may be beyond the bottom of the potential well and which therefore will usually recombine before they can be sensed.
While higher voltages can be used to form deeper potential wells, there is a demand for semiconductor devices including imagers that operate at lower rather than higher voltages and therefore it is not feasible to increase the voltage on the device in many applications.
The signal to noise ratio of a photoreceptor is generally proportional to the ratio of the surface area of the photoreceptor, that is the area of the surface of the layer on which the photoreceptor is formed that is occupied by the photoreceptor, to the total volume of the photoreceptor. Since the volume of a conventional photoreceptor increases with the cube of the linear dimension thereof, while the surface area increases with the square thereof, large photosensors exhibit a higher signal to noise ratio than small sensors. However, small sensors are desirable for achieving high resolutions. Heretofore, signal to noise ratio has placed a limit on the degree to which the size of photosensors can be reduced that is far more significant than any limitations created by process geometries or the like.
It is an object of this invention to provide a construction for a photoreceptor that overcomes one or more of the problems just mentioned.
It is another object of this invention to provide a photoreceptor that can create a potential well for accumulating a charge generated by photons impinging on the detector, which well is deeper than has heretofore been possible without the need for applying higher voltages to create the well.
It is another object of this invention to provide a photoreceptor construction that simultaneously reduces the surface area of the photoreceptor and increases the volume thereof to produce a higher signal to noise ratio.
It is another object of this invention to provide a photosensitive imager that permits a substantially higher density of picture elements (pixels) to be formed in a given area than has heretofore been possible, while at the same time providing higher signal to noise ratios and greater dynamic range than are achievable with known constructions.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an improved low voltage, small surface area, high signal-to-noise ratio photo gate includes a layer of photoreceptive semiconductor material having an impurity concentration selected to enhance the formation of hole electron pairs in response to photons impinging on a surface of the substrate, an electrode extending from the surface of the substrate into the substrate a substantial distance; an insulating layer disposed between the electrode and the substrate for electrically insulating the electrode from the substrate; so that upon the application of an electrical potential to the electrode, a potential well is formed in the substrate surrounding the electrode for accumulating charge generated when photons impinge on the surface of the substrate surrounding the electrode.
In accordance with another aspect of this invention, a photo gate in accordance with the invention comprises a charge sensor disposed in the substrate adjacent the electrode.
In accordance with another aspect of the invention, the electrode comprises a photo opaque metal electrode.
In accordance with another aspect of the invention, the electrode comprises a phototransmissive electrode.
In accordance with still another aspect of the invention, a photo gate comprises a plurality of electrodes
Pace Matthew A.
Zarnowski Jeffrey J.
Eckert II George C.
Harter Secrest & Emery LLP
Photon Vision Systems LLC
Saadat Mahshid
Salai Esq. Stephen B.
LandOfFree
Photo receptor with reduced noise does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photo receptor with reduced noise, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photo receptor with reduced noise will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2607669