Image sensor method and apparatus having addressable pixels and non-destructive readout

[0001] The invention relates generally to image sensors and, more particularly, to image sensors constructed using CMOS technology.

[0002] In the field of imaging sensors, charge-coupled devices (CCDs) are the most common type of image sensors in use today for both consumer electronics and machine vision applications. A CCD utilizes charge generated by the photo-electric effect and collected in an individual pixel's potential well. At the end of a light collection period, charge is transferred from one potential well to the next, across a row of pixels. One column of charge is next transferred to a sense node capacitance, where each pixel's charge value is converted to a voltage, and can be read out to form pixel image data. This is a serial process that is dependent on an efficient means of charge transfer from pixel to pixel.

[0003] Typically, the pixel is built on 10-20 microns of epitaxial silicon, on top of about 500 microns of silicon substrate. The substrate is highly doped and kept at ground potential. To achieve a high level of pixel-to-pixel charge transfer efficiency, a separate buried channel is fabricated to provide a low-loss path for charge transfer. In such a buried channel CCD structure, light passes through the surface region of the CCD and generates charges in the n- and p-regions. The n-epitaxial region defines the buried channel which collects the electrons from photon induced electron-hole pairs. Electrons generated in the deeper p-region diffuse towards this buried channel, and are also collected there. Doped p-material at the edges of the n-channel is grounded and defines the channel stops between pixel regions.

[0004] Various CCD architectures exist: full frame, frame transfer, interline transfer and frame interline transfer. Full frame CCD combines the imaging array with a serial readout register for data transfer. It requires a very fast readout time, or shuttering, to avoid smear as the data is read out serially. It is still used for long-stare applications in astronomy. Frame transfer CCD requires two arrays, an imaging array of CCDs, and a light-shielded storage array to receive the image array data in high-speed parallel fashion. A serial readout register then transfers the data from the storage array. An interline transfer CCD has an imaging section made of alternating vertical columns of light detector pixels and readout registers. Image data is transferred immediately to the storage register for fast frame rate and reduced smear, and then the data is transferred to the serial readout register at the edge of the array. There is an inherent 2 loss of resolution in the horizontal direction for interline transfer. Frame interline transfer adds a light-shielded storage array to the image array of the interline transfer CCD, and basically provides an electronic shuttering capability to the imaging system.

[0005] A process optimized to fabricate CCD devices is not suitable to fabricate standard complementary metal-oxide semiconductor (CMOS) devices. This presents great difficulty in any attempt to combine CCD and CMOS circuitry on the same chip. Other pixel structures do exist, including charge injection devices (CID) and active pixel sensors (APS); however, like CCDs, the designs in use today for these other structures have their own inherent disadvantages which reduce their suitability for certain types of imaging applications.

[0006] As described in Burke and Michon, Charge-Injection Imaging: Optical Techniques and Performance Characteristics, IEEE Journal of Solid-State Circuits, Vol. SC-11, No. 1, pp. 121-127 (1976), CID imagers consist of pixels made of two photocapacitors, one connected to column circuitry and one connected to row circuitry. Row/column circuits select individual pixels and sense charge capacitively on either gate structure. Charge is collected under one photocapacitor, and moved to the other for signal readout. However, this CID technology has significant noise issues associated with its use; for example, large bus capacitance for charge sensing leads to poor charge-to-voltage signal gain, and low noise floors are difficult to achieve throughout the array.

[0007] APS imagers utilize pixels that contain at least one active transistor to drive the output lines and aid in charge-to-voltage conversion gain. Thus, APS does not transfer charge serially as does the CCD. See, for example, Mendis, Kemeny, & Fossum, CMOS Active Pixel Sensor, IEEE Transactions on Electron Devices, Vol. 41, No. 3, pp. 452-453, March 1994, and U.S. Pat. Nos. 5,471,515 and 6,021,172 issued to E. R. Fossum et al. One advantage of the APS design is that the imager can be formed from pixels constructed to some extent within the bounds of a CMOS process. This opens up the possibility of incorporating other CMOS circuitry into the sensor.

[0008] Most image processing techniques utilize software-based algorithms to process frames of image data outputted from the imaging sensor. The sensor data is converted to digital data and stored in memory for subsequent processing by a microprocessor executing the desired image processing software algorithms. Although suitable for many applications, this approach to image processing can be overly complex for special purpose machine vision applications, resulting in unnecessarily slow frame processing rates using cumbersome hardware and computationally complex software algorithms.

[0009] One approach to simplify the desired image processing is to implement at least some portion of it in hardware. However, there are difficulties with this approach as well including, in particular, combining the pixel circuits with other hardware processing circuits, as well as providing storage of pixel values that must be accessed more than once to carry out the hardware processing algorithm. In the case of CCD imagers, the inability to combine the sensor with CMOS devices on a single substrate make on-chip hardware processing difficult if not impossible to accomplish using standard CMOS techniques. Also, while APS imagers of the type noted above can be combined with other CMOS circuitry, and while they provide a pixel array that is independently addressable on a row and column basis, they do not provide a non-destructive readout of data, meaning that the data can only be read once. Since certain signal processing algorithms require multiple use of selected portions on the pixel array, the pixel data must be buffered or otherwise stored so it can be accessed as many times as required by the algorithm. This again increases the complexity and expense of the hardware.

[0010] Accordingly, it is a general object of the invention to provide a imaging sensor having individually-addressable pixels that is compatible with CMOS technology and that provides a non-destructive readout which can be read multiple times.

[0011] In accordance with the invention, there is provided a pixel element for sensing light impinging on the pixel element and providing a non-destructive readout representative of the amount of impinging light. The pixel element includes a substrate, an insulating layer formed on the substrate, a collection capacitor electrode, a transfer electrode, a readout capacitor electrode, and a readout transistor. The substrate can be a semiconducting material such as silicon that is capable of forming localized depletion regions in the presence of an applied voltage at those regions. All three electrodes are electrically isolated from the substrate and each other by the insulating layer. The collection capacitor electrode and insulating layer are transparent to light so that incident light can pass through the electrode and insulating layer and be absorbed by the substrate. The transfer electrode is located adjacent both the collection and readout capacitor electrodes. The readout transistor has an insulated gate connected to the readout capacitor electrode, with the transistor providing an output signal that is indicative of the quantity of charge stored in the substrate under the readout capacitor electrode.

[0012] With this structure, depletion regions can be formed under the collection and readout capacitor electrodes with the collection depletion region collecting charge generated as a result of the incident light being absorbed by the substrate underneath the collection capacitor electrode. The collected charge can then be transferred to the depletion region under the readout capacitor electrode using the transfer electrode to generate a depletion region that overlaps the collection depletion region and thus permits the charge transfer. Once transferred, this stored charge can be used to activate the readout transistor to generate the output signal representing the pixel data and, since the readout transistor's gate is insulated, this readout of pixel data is done in a non-destructive manner, thereby permitting multiple reads of the data without separate buffering.

[0013] In one of its broader aspects, the present invention provides a semiconductor structure that can be used as an analog memory element to store data represented by charge that is provided to or otherwise generated at the depletion region underlying the collection capacitor electrode. The charge need not be photoelectrically generated by incident light, but rather can originate in other ways, such as by being introduced electrically, or by static charging, or by pressure (using for example a piezoelectric element), or by electromagnetic radiation outside of the visible or near infrared ranges.

[0014] Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:

[0015] FIG. 1 is a block diagram of preferred embodiment of an image sensor constructed according to the present invention;

[0016] FIG. 2 is a partially diagrammatic and partially schematic cross-sectional view of a pixel element used in the image sensor of FIG. 1; and

[0017] FIG. 3 is a view as in FIG. 2 showing an alternative embodiment of a pixel element constructed in accordance with the invention.

[0018] Referring to FIG. 1, there is shown an image sensor 10 constructed in accordance with the invention. In general, image sensor 10 comprises an integrated circuit chip 12 that includes an array 14 of light sensitive pixels and a control circuit 16 that provides all of the control requirements for operation of the image sensor 10, including addressing, timing signal generation, and output multiplexing of the pixel data. As will be described below, pixel array 14 is constructed using a CMOS process and in a manner that permits individual addressability of the pixels, as well as a non-destructive readout of the pixel data. These features make the image sensor 10 particularly useful in constructing special purpose image sensors that include on-chip hardwired processing of the raw image data produced by the array 14. The CMOS technology makes it much easier than CCD imagers to incorporate the additional processing circuitry on the same chip. The addressability makes it easy to access individual pixel data without the need to buffer whole rows of data. The non-destructibility of the readout makes it possible to implement hardware image processing algorithms on the chip that require multiple reads of individual pixel without having to provide buffers to temporarily store that data.

[0019] The circuitry of FIG. 1 is not meant to represent any particular addressing and control strategy for operation and use of pixel array 14. Rather, various suitable designs for control circuit 16 are well known to those skilled in the art. Any modifications that would be needed to known control circuit designs in order to provide the addressing and control of the pixel elements of FIGS. 2 and 3 will be apparent to those skilled in the art. Thus, no further discussion of control circuit 16 is necessary or provided herein.

[0020] Turning now to FIGS. 2 and 3, there is shown two different embodiments of a single pixel element of the pixel array 14 of FIG. 1. These pixel elements can be fabricated using a standard CMOS two level polysilicon process. For both of these embodiments, the pixel element is fabricated with a crystalline silicon substrate, silicon dioxide insulating layers, a first level polycrystalline silicon collection electrode, a first level polycrystalline silicon readout electrode, a second level polycrystalline silicon transfer electrode, a second level polycrystalline dump electrode, and a readout transistor having an insulated gate connected to the readout electrode. Although the preferred embodiments disclosed herein use a polycrystalline silicon-oxide-semiconductor structure, other suitable material combinations will be apparent to those skilled in the art of microelectronics. Before discussing the specific features and operation of each of the two pixel designs, a general discussion of the common features of their operation will now be provided.

[0021] The collection and readout electrodes are located within the insulating layer in spaced relation to the silicon substrate so that they each thereby form a separate photocapacitor. Only the collection photocapacitor is light sensitive due to the provision of an upper level metal layer (not shown) that blocks light from striking the underlying semiconductor structure at any place but the collection electrode. A depletion region, or well, is formed in the silicon substrate underneath the collection capacitor electrode by application of a suitable bias voltage VC (e.g., 5 volts) to this collection electrode. Light energy is collected in this depletion region during an integration period. The light (visible and near infrared) passes through the polysilicon collection electrode and the silicon dioxide insulating layer, both of which are transparent to the light, meaning that at least a substantial portion of the light can pass through these elements. The light is absorbed in the underlying crystalline silicon layer, with the photons producing electron-hole pairs in the underlying depletion region. The electric field in the depletion region separates these pairs, with holes passing to the substrate, and electrons being trapped in the inversion layer which forms in response to the electrode's potential. The amount of charge collected in this inversion layer, or well, during the integration interval is indicative of the intensity of the light impinging on this particular pixel. U.S. Pat. No. 6,180,937 to J. R. Troxell et al. describes a means for increasing the amount of light that can pass through the upper capacitor electrode. The complete disclosure of that patent is hereby incorporated by reference.

[0022] Once the integration interval has ended, the collected charge is moved to the readout capacitor electrode. As mentioned above, this readout capacitor is not light sensitive; rather, it is used to store the collected charge for one or more subsequent readouts. The readout electrode acts as a floating gate with the charge on this gate being provided to the insulated gate of the readout transistor, where the charge is converted to an analog voltage and that signal is read out column by column for each row of the array. Since both the readout capacitor electrode and the gate of the readout transistor are insulated, the stored charge is not lost upon readout and can therefore be sampled multiple times using the readout transistor. Thus, the pixel element provides a non-destructive readout of pixel data. After the pixel voltage is read out as many times as needed, the charge is dumped to the positive supply voltage through the dump electrode. Following the dumping of charge, another readout of each pixel (now emptied of signal charge) in the row is performed. The two signals, collected signal and empty well, are available at the chip output for correlated double sampling, which is a common technique that is used to deal with fixed pattern noise caused by circuit variations at individual pixels. The subtraction of the two signals yields only the data value on a pixel by pixel basis over the entire array. For multiple readouts of the pixel data value before dumping of the charge, the previously-read empty well signal can be used for purposes of correlated double sampling.

[0023] Referring now more specifically to FIG. 2, the pixel element 20 shown therein includes a substrate 22, a two level insulating layer 24, a collection electrode 26, a transfer electrode 28, a readout electrode 30, a dump electrode 32, a readout transistor 34, a biasing transistor 36, and a output transistor 38. The collection electrode 26, transfer electrode 28, and readout electrode 30 are spaced from the substrate 22 so that, when a biasing voltage is applied to these electrodes, respective depletion regions 40, 42, and 44 are formed in the substrate 22 underneath these electrodes. Only depletion regions 40 and 44 are specifically shown in FIG. 2 for the purpose of clarity in describing the functioning of pixel element 20. As shown, the collection and readout electrodes are laterally spaced such that their depletion regions do not overlap each other. However, the transfer electrode 28 is located adjacent to each of these electrodes and, in particular, between these electrodes at the second polysilicon layer such that, when the biasing voltage is applied to the transfer electrode 28, the resulting depletion region 42 under this electrode overlaps both depletion regions 40 and 44. Thus, transfer electrode 28 can be used to join the collection and readout depletion regions 40, 44 so that charge stored under one can be transferred to, or at least shared with, the other. When no bias voltage is applied to the transfer electrode, the collection and readout depletion regions 40, 44 are isolated from each other and no significant charge transfer occurs between them.

[0024] As mentioned above, depletion region 40 is formed by applying a biasing voltage VC to the collection electrode 26. This begins the charge collection process based upon the amount of light impinging upon the pixel element at that electrode. The transfer depletion region 42 is formed by applying a biasing voltage VT to the transfer electrode 28. For the depletion region 44, the biasing voltage is gated through biasing transistor 36 which allows the readout electrode 30 to be isolated from the low impedance voltage supply during readout of the pixel data using the readout transistor 34. Biasing transistor 36 thus has its gate connected to receive a control signal VB, its drain connected as an input to receive a supply voltage VCC (e.g., 5 volts), and its source connected to the readout electrode 30 to supply the VCC bias to that electrode. The readout transistor also has its drain connected as an input to receive the VCC supply voltage, with its source connected to the drain input of the output transistor 38. The source of the output transistor is connected to an output line 46 which can be a common column line to which all output transistors 38 for the pixels in the same column are connected. Of course, output line 46 could also be dedicated to that particular pixel with a multiplexor or other arrangement used to route the pixel data to one of the data outputs of the image sensor. Such arrangements will be known to those skilled in the art. Output of the pixel data through transistor 38 is enabled using the pixel select signal Vout. Referring back briefly to FIG. 1, these signals, as well as the other enable and control signals used by the pixel elements, can be provided by control circuit 16.

[0025] Operation of the pixel element 20 will now be described. The necessary timing signals needed to implement the steps described below will be apparent to those skilled in the art. At the beginning of the integration interval, voltage VC is applied to the collection gate electrode 26 of the photosensitive capacitor to form a potential well 40 in the underlying silicon 22. That is, the voltage applied to the collector electrode is of such a magnitude as to dynamically form the depletion region 40 which is larger than that which can exist in thermal equilibrium for the given materials used in the CMOS fabrication process. In the present case, this voltage (5 volts) significantly exceeds the threshold voltage for formation of an inversion layer in the underlying silicon (which is of the order of one volt). Consequently, for time scales of seconds or less, the underlying silicon is essentially in an excited state; any free electrons which are introduced into the depletion region 40 will be immediately accelerated to the vicinity of the interface between the silicon substrate 22 and the overlying silicon dioxide insulator 24. These electrons will form an inversion layer, which is comparable to that formed in many types of MOS field effect transistors, with the singular exception that there are no diffused n-type regions in electrical contact with the inversion layer. Consequently, the inversion layer cannot attain thermodynamic equilibrium with the remainder of the integrated circuit; the charge remains essentially trapped underneath the biased gate electrode of the photocapacitor.

[0026] After a suitable integration period, during which charge continues to be collected into this inversion layer in proportion to the amount of light incident upon the photocapacitor, the readout electrode 30 is biased to form the readout well 44 by switching on biasing transistor 36 using signal VB. Also, the transfer electrode 28 is biased to 5 volts using signal VT, thereby forming the depletion region 42. Essentially, this has the effect of electrically connecting the inversion layer beneath the photocapacitor gate 26 and the depletion layer 44 beneath the readout capacitor gate 30. Consequently, charge will distribute proportionately between the two capacitors, and, when the depletion layer 40 of the photocapacitor is collapsed (by reducing the applied voltage VC to zero or some other voltage below the inversion threshold voltage), essentially all of the charge will migrate over underneath the readout gate electrode 30. At this point, the voltage VT on the transfer gate electrode 28 is also reduced (again, for simplicity, assume to 0V), effectively trapping all of the charge underneath the readout gate electrode 30. Under these conditions, the charge that is present underneath the readout electrode 30 can be sampled (i.e., essentially converted to a voltage) by readout transistor 34, and read out to the output circuitry of the imager array by actuating the output transistor 38. It will be appreciated that the act of reading out the charge stored under the readout gate electrode does not change the amount of charge stored, and that the pixel therefore provides a non-destructive readout of the pixel data. Consequently, the readout of charge stored under the readout capacitor electrode 30 can be performed multiple times, limited only by the gradual return to thermal equilibrium of the depletion region 44 of the readout capacitor and the parasitic flow of current through the biasing transistor 36.

[0027] After all readouts have been performed, the charge stored on the readout capacitor can be dumped by biasing the dump gate electrode 32 to five volts using signal VD. This effectively connects the readout depletion region 44 to a supply connecting region 48 that can be an n-type diffused junction region which is connected to the 5 volt supply (VCC). In this manner the charge is destroyed by removing it from the depletion region 44. Also, at this point, another readout operation can be performed to effectively sample the voltage associated with an empty readout capacitor. This step is useful in compensating for fixed pattern noise using the correlated double sampling technique noted above.

[0028] In the pixel design of FIG. 2, each pixel is comprised of three addressing transistors, two charge storage capacitors, and a total of seven input signals (i.e. input connections). The number of required transistors and input signals can be reduced using the pixel structure 50 of FIG. 3. This pixel circuit performs in a generally analogous manner to that of FIG. 2, but its simpler construction makes possible smaller pixels and thus a smaller overall array. Structurally, the major difference in this circuit from that of FIG. 2 is that the biasing transistor is eliminated with the transfer gate electrode 52 being extended up and over the readout gate electrode 30 so that the readout depletion region can be formed by applying the bias voltage to the transfer electrode 52 rather than to the readout electrode 30. This has the effect of maintaining isolation of the readout electrode during read out of pixel data, yet permits formation of the readout depletion region for purposes of charge transfer from collection depletion region. This also has the advantage that it simplifies the addressing scheme and results in a pixel circuit that requires only two transistors and five signal lines. The overlapping of the two polysilicon gate layers is made possible by the use of a standard two-polysilicon layer CMOS fabrication process.

[0029] It will be apparent to those skilled in the art of semiconductor device design that the depletion region formed by a bias applied to transfer gate electrode 52 will include regions 44a and 44b. The output signal measured by output transistor 54 is proportional to the charge within the portion of the region 44b. As will be appreciated, the output signal in this embodiment may be reduced, as compared to the output signal in the embodiment shown in FIG. 2, due to the possibly unequal sharing of charge between regions 44a and 44b. Such unequal charge sharing could result if the effective gate insulator thickness between region 44a and transfer electrode 52 is different than that between region 44b and electrode 52. To increase the output signal measured on transistor 54, the threshold voltage for inversion layer formation in region 44a can be increased, which serves to redistribute the charge in favor of region 44b. Such a change in threshold voltage can be achieved by an appropriate implantation or diffusion of additional charged species into the region 44a or into the gate insulator directly above region 44a.

[0030] In this embodiment, the readout transistor 54 is biased with signal VR being asserted high when charge transfer from the collection well 40 is desired. This biasing signal is maintained high during readout, with the output of the readout transistor 54 representing charge stored in the readout well that is superimposed on the VR signal. As shown, the dump electrode 32 is on the opposite side of the structure so that both signals VC and VR are asserted in order to dump the charge stored in the transfer/readout depletion regions 44a and 44b. This dump electrode 32, along with the n-type diffused junction region 48 could also be placed at the location shown in FIG. 2, if desired.

[0031] Although the size of the transfer electrode in the area between the collection and readout electrodes is illustrated as being on the same order as that of the collection and readout electrodes themselves, it will be appreciated that in actual practice, this lateral spacing of the collection and readout electrodes can be relatively much smaller so that the depletion region formed by the transfer electrode is substantially limited to the area directly underlying the readout electrode.

[0032] An image sensor constructed using the pixel structures described above has the advantages of individual addressability of the pixels and non-destructive readout of the data. It also provides the advantages of using standard CMOS fabrication processes, making the resulting image sensor an excellent candidate for the inclusion of on-chip image processing circuits for such things as edge detection and other algorithms that involve convolutions or other combinations of pixel data.

[0033] More generally, it will be appreciated that the pixel structure described above can be extended to non-imaging applications in which the charge stored under the collection electrode is supplied to, or generated in, the depletion region in other ways, such as by being introduced electrically, or by static charging, or by pressure (using for example a piezoelectric element), or by electromagnetic radiation outside of the visible or near infrared ranges. In such an application, the circuits of FIGS. 2 and 3 would comprise memory elements that provide the random access and non-destructibility of read out described above.

[0034] It will thus be apparent that there has been provided in accordance with the present invention an image sensor method and apparatus which achieves the aims and advantages specified herein. It will of course be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific embodiments shown. Various changes and modifications will become apparent to those skilled in the art and all such variations and modifications are intended to come within the scope of the appended claims.