ELECTRONIC SHUTTER FOR IMAGE SENSOR USING PHOTODIODES

Abstract: An image sensor has a plurality of photodiodes, each of which integrates light from a scene and accumulates charge in a potential well. A shallow diode is disposed above each such photodiode and is connected to a source of reference potential substantially clamping the top portion of the shallow diode at a fixed reference potential. A potential is selectively applied which removes the potential well and empties the diode of charge.

Description: ELECTRONIC SHUTTER FOR IMAGE SENSOR USING PHOTODIODES Technical Field This invention relates to image sensors having electronic shutters which empty photodiodes of accumulated charge. Background Art Image sensors include a number of different types of photosensing elements which, in response to incident light from a scene, collect charge. Photodiodes are often advantageously used as the sensing elements. Photodiodes have high quantum efficiency, produce low dark current and can be manufactured by simple processes. A major difficulty with photodiodes is that they have limited charge storage capacity. In many applications, the integration period (exposure time) is too long and some of the photodiodes overflow with charge. Excess charge spills out and causes a problem known as "blooming". To shorten the integration period, light must be interrupted such as, for example, either by a mechanical shutter or by resetting the photodiodes emptying them of charge. A photodiode, as is well— nown, is provided at the junction of p and n—type materials. A potential well is created in the depletion region at the interface between these two materials. Incident illumination causes electron—hole pairs to be formed. The electrons (the charge of interest) migrate to and are trapped in the potential well. Two ways have been devised for eliminating blooming. They are known as "lateral overflow drains" and "vertical overflow drains". Fig. 1 shows a cross—section of a typical prior art structure illustrating a lateral drain. The sensor 10 includes a p—type substrate 12 and three spaced n— ype regions. An n region has the reference numeral 14. This n region and the p—type substrate 12 combine to form a photodiode. The reference numeral 14 will also be considered to call out the photodiode. Similarly, region 16 is part of a buried channel structure of an interline CCD (see Fig. 3), and region 18 is part of a lateral drain for removing excess charge. On top of substrate 12 is thin oxide layer 20 which is transparent to incident illumination. Two polysilicon electrodes 22 and 24 are provided. Electrode 22 is adapted to be connected to a source of potential φ,. Electrode 22 is known as a transfer electrode. It is also the reset electrode. Electrode 24 is adapted to be connected to a source of potential φ~ and is known as an overflow drain electrode. Another oxide layer is provided over these electrodes 22 and 24. On top of the oxide layer is an aluminum layer 26 which prevents incident light from entering substrate 12 except in regions of the photodiodes 14. To isolate adjacent photodiodes, channel stops 28 are provided. Only one such channel stop is shown. The channel stop 28 comprises a thicker oxide portion and a p boron implant. In operation, after a photon of light illuminates a diode, an electrode—hole pair is formed. The electrons are accumulated in a potential well located in the depletion region in the n region 14. To transfer charge to the buried channel, a positive potential is applied to φ,. Electrons flow along a surface channel and then down into the potential well in the buried channel 16 of the CCD. At this point, potential on φ, is removed. Unwanted charge will now continue to accumulate in the photodiode. To prevent this from happening, a positive potential is applied to ~ > This causes charge accumulating in the photodiode to be transferred to n region 18 of the drain where it is removed in a well—known manner. All of electrodes 24 for each photodiode are connected together. Similarly, all the electrodes 22 are connected together. Unfortunately, variations in the device electrical properties inevitably result in diode to diode variations of the potential wells provided by the reset Φ 2 and readout φ, voltages. This results in sensor special pattern noise which can be explained by the following equation: Noise = C x (V sl ~ V g2 ) (1) q wherein: Noise is calculated in number of electrons, C is the capacitance of the photodiode 14, and Vg, and V g2 are RMS values of the surface potentials under the φ, and φ ? electrodes respectively. Such a spatial pattern noise can in certain applications severely reduce the sensitivity of the sensor. A prior—art vertical drain arrangement is shown in Fig. 2. Where the elements are the same as in Fig. 1, they have the same reference numbers. In this device, there is provided an n—type substrate 30. On top of the n—type substrate 30 is a p—type well 32. The n—type substrate 30 is connected to a source of potential V, and the p-well 32 is grounded. Just under the central portion of the photodiode 14, the n—type substrate region 30 extends upward to be relatively close to the photodiode 14. With this arrangement, if V, has a positive potential applied to it and if the potential well of the photodiode begins to overflow, electrons will punch or flow through the narrow portion of the p-well 32 into the n-type substrate 30. This arrangement is known as a vertical overflow drain. It prevents the lateral overflow of excess charge into the CCD. Although lateral and vertical overflow drains are effective for minimizing blooming problems, they do not eliminate unwanted charge from being stored in the potential well of a photodiode during nonintegration periods. The object of this invention is to provide an electronic shutter which will eliminate charge from accumulating in photodiodes during nonintegration periods. Disclosure of the Invention This object is achieved in an image sensor having a plurality of photodiodes which integrate light from a scene and accumulating charge in potential wells, and means associated with each photodiode for transferring accumulated charge from such photodiode, an electronic shutter for each photodiode, comprising (a) a shallow diode disposed above each photodiode and connected to a source of reference potential which clamps the top portion of the shallow diode at a fixed voltage; and (b) means for selectively applying potential which removes the potential well from the photodiode and empties the photodiode of charge. Description of the Drawings Fig. 1 is a cross—section of a prior art image sensor employing photodiodes using lateral drains; Fig. 2 is a cross—section of a prior art image sensor using photodiodes using vertical drains; Fig. 3 is a plan view of a portion of an interline image sensor which can be used in accordance with the invention; Fig. 4 is a cross—section taken along the lines 4—4 of Fig. 3 of a vertical drain image sensor in accordance with the invention; Figs. 5a and 5b illustrate potential diagrams for the vertical drain image sensors in Figs. 2 and 4, respectively; and Fig. 6 is a cross—section of a lateral drain image sensor in accordance with the invention. Modes of Carrying Out the Invention Turning now to Fig. 3 where the general organization of an interline image sensor 10 is shown. Sensor 10 includes an array (plurality) of photodiodes 14. In response to incident light from a scene, each photodiode accumulates electrons in a potential well. The holes are not of interest. The electrons are transferred via a surface channel to buried channel CCD 16. There is a vertical CCD buried channel 16 on either side of adjacent columns of photodiodes 14. This architecture is called "interline". The entire surface of the sensor except for the photodiodes 14 is covered with an aluminum layer 26 (see Fig. 4). Charge is transferred from the vertical CCD's 16 to a horizontal CCD 37. In a similar fashion, charge is transferred from the horizontal CCD to floating diffusion 38 which produces an output voltage ( v out )« The details of this particular organization is well—understood by those skilled in the art and need not be discussed further here. Turning now to Fig. 4, there is shown in cross—section a portion of a vertical drain image sensor, taken along the lines 4-4 of Fig. 3. The construction of this image sensor is similar to that shown in Fig. 2 and, where the elements correspond, the same numerals are used. For example, there is an n—type substrate 30, a p— ell 32, an n region (photodiode) 14 and an n buried channel CCD 16. In addition to which there are the polysilicon transfer electrodes 22 connected to potential source φ-. Also, conventional channel stops 28 are provided. In accordance with the invention, on top of n region 14 is a shallow p —type region 40. Region 40 in combination with region 14 provides a shallow diode. At this point it should be noted that, although the sensor is shown as having an n— ype substrate, reversed impurity polarities are possible (replace n—type materials with p—type materials throughout). Layer 40 is shallow to provide for short— avelength quantum efficiency and does not interfere with incoming photons of light from a scene. As shown, left channel stop 28 has its p boron implant grounded. This p implant is directly connected to p layer 40 and so it grounds this layer. In addition, p— ell 32 is also electrically grounded. By grounding layer 40, it clamps the top position of layer 40 of the shallow photodiode at a reference potential irrespective of the voltage applied to the substrate (V, ). This is illustrated with reference to Figs. 5a and 5b. Fig. 5a shows a potential diagram (in schematic form) for the vertical drain shown in Fig. 2. In the topmost situation, let us assume that approximately 5 volts are applied to V.. A potential well is formed in the photodiode and collects electrons (e—). If the potential well should become overloaded, electrons will flow across p—well 32 into substrate 30. This is shown by the electron e— with an arrow. Let us assume that the voltage on V. is increased to approximately 15 volts. This is shown with the dotted—line situation in Fig. 5a. The top portion of the shallow photodiode (V ) will now be approximately 7.5 volts. The term V actually represents the surface potential of the shallow photodiode. A potential well is again formed. In both cases (V,=5V and , excess charge can accumulate in the potential well. This excess charge causes noise. The present invention has recognized this problem and, as illustrated in Fig. 5b, eliminates the potential well by selectively applying a high enough voltage to V.. As shown in the solid portion of Fig. 5b, let us assume that the potential applied to V. is about 5 volts. The potential of shallow diode layer 40 near the silicon surface (V 3) is maintained at a fixed reference potential. Charge will be collected in a potential well, and accumulated excess charge can punch through down into substrate 30 through p—well 32. Now let us assume that a high potential of approximately 15 volts is applied to V.. As shown in this case, the potential well is substantially eliminated and all electrons generated will flow "downhill" and punch through region 32 into substrate 30 and be dissipated. In the absence of this top diode as illustrated in Fig. 5a, the voltage V can float. This is actually due to a capacitive dividing action due to lateral capacitances to adjacent field regions and the capacitance to the adjacent transfer gate. These capacitances are generally much smaller than the capacitance of the photodiode and are ineffective in controlling the photodiode surface potential. Turning now to Fig. 6, we see a cross—section of a portion of a lateral drain image sensor. This figure is similar to that shown in Fig. 1. Here, as in Fig. 4, a shallow p region 40 of a shallow photodiode is formed in the top of the photodiode. This shallow region 40 is also connected to a source of reference potential through p—type substrate 12. The elements in this device that are the same as those shown in Fig. 1 carry the same reference numerals. When a positive potential is applied to Φ 2 , the potential of layer 40 (excluding its depletion region) is clamped at a fixed reference potential and in a manner similar to that shown in the dotted—line portion of Fig. 5b, the potential well is completely eliminated and charge flows into drain 18. Of course, in a similar fashion when charge is to be transferred to buried channel 16 and positive potential is applied to φ, , the potential well of the photodiode is also completely eliminated and charge flows through the surface channel into buried channel 16. Industrial Applicability and Advantages Image sensors of the present invention are useful in electronic cameras. A feature of this invention is that photodiodes may be completely emptied of charge and that pattern noise explained by equation 1 above can be eliminated. An advantage of the invention is that it is possible to employ an electronic shutter to eliminate the need for mechanical shutters.

Claims: Claims : 1. Process to remove a photodiode's well which is used to accumulate charge and which is associated with an overflow drain and a readout device, unwanted charges being transferred to the overflow drain by a first level of potential, process characterized in that the potential is used in order to remove the potential well and cause charge in the photodiode to flow into the drain. 2. Process according to claim 1, characterized in that the reference potential is ground. 3. Image sensor having a plurality of photodiodes, each of which integrates light from a scene and accumulates charge in potential wells, means associated with each photodiode for transferring accumulated charge representative of scene brightness from such photodiode potential well, and a drain which removes excess charge from the well, characterized by: (a) a shallow diode disposed above such photodiode and connected to a source of reference potential for clamping the top portion of the shallow diode at a fixed voltage; and (b) means for selectively applying a potential which removes the potential well from the photodiode and causes charge in the photodiode to flow into the drain. 4. Image sensor as defined in claim 3, characterized in that the shallow diode is grounded. 5. Image sensor as defined in claims 3 or 4, characterized in that the drain is a vertical overflow drain. 6. Image sensor as defined in claim 5, characterized in it comprises means for selectively applying a first potential level to the susbtrate to cause it to act as a vertical drain and a second potential level to remove the photodiode's potential well and empty charge from such well. 7. Image sensor as defined in claim 3 or 4, characterized in that it comprises: " (a) readout means responsive to a first voltage φ. for transferring accumulated charge for further processing; and (b) reset means response to a second voltage φ- to remove the potential well and empty the photodiode of charge.

Inventors: NELSON EDWARD T, STANCAMPIANO CHARLES V, NELSON, EDWARD, T., STANCAMPIANO, CHARLES, V.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1987
Main classification: H01L27
IPC codes: H01L27, H04N3, H04N5