Abstract:
A demodulation image sensor, such as used in time of flight (TOF) cameras, performs the acquisition and readout of the pixels in parallel. This is used to avoid motion artifacts due to samplings performed separated in time.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/539,591, filed on Sep. 27, 2011, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Electronic imaging sensors usually have an array of m×n photo-sensitive pixels, with m&gt;=1 rows and n&gt;=1 columns. Each pixel of the array can individually be addressed by dedicated readout circuitry for column-wise and row-wise selection. Optionally a block for signal post-processing is integrated on the sensor. 
         [0003]    The pixels typically have four basic functions: photo detection, signal processing, information storage, and analog or digital conversion. Each of these functions consumes a certain area on the chip. 
         [0004]    A special group of smart pixels, called demodulation pixels, is well-known for the purpose of three dimensional (3D) time of flight (TOF) imaging. Other applications of such demodulation pixels include fluorescence life-time imaging (FLIM). The pixels of these demodulation imaging sensors typically demodulate the incoming light signal by means of synchronous sampling or correlating the signal. Hence, the signal processing function is substituted more specifically by a sampler or a correlator. The output of the sampling or correlation process is a number n of different charge packets or samples (A 0 , A 1 , A 3  . . . An−1) for each pixel. Thus, n storage sites are used for the information storage. The typical pixel output in the analog domain is accomplished by standard source follower amplification. However, analog to digital converters could also be integrated at the pixel-level. 
         [0005]    The image quality of demodulation sensors is defined by the per-pixel measurement uncertainty. Similar to standard 2D imaging sensors, a larger number of signal carriers improves the signal-to-noise ratio and thus the image quality. For 3D imaging sensors, more signal carriers mean lower distance uncertainty. In general, the distance measurement standard deviation σ shows an inverse proportionality either to the signal A or to the square root of the signal, depending whether the photon shot noise is dominant or not. 
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         [0006]    A common problem for all demodulation pixels used in demodulation sensors, such as for TOF imaging or FLIM, or otherwise, arises when trying to shrink the pixel size to realize arrays of higher pixel counts. Since the storage nodes require a certain area in the pixel in order to maintain adequate full well capacity and thus image quality, the pixel&#39;s fill factor suffers from the shrinking process associated with moving to these larger arrays. Thus, there is a trade-off between the storage area needed for obtaining a certain image quality and the pixel&#39;s photo-sensitivity expressed by the fill-factor parameter. In the case of a minimum achievable image quality, the minimum size of the pixel is given by the minimum size of the total storage area. 
         [0007]    In 3D imaging, typically a few hundreds of thousands up to several million charge carriers, i.e. typically electrons, need to be stored in order to achieve centimeter down to millimeter resolution. This performance requirement, in turn, means that the storage nodes typically cover areas of some hundreds of square micrometers in the pixel. Consequently, pixel pitches of 10 micrometers or less become almost impossible without compromises in terms of distance resolution and accuracy. 
         [0008]    The aforementioned trade-off problem becomes even more critical if additional post-processing logic is to be integrated on a per-pixel basis. Such post-processing could include for example analog-to-digital conversion, logic for a common signal subtraction, integrators, and differentiators, to list a few examples. 
         [0009]    Another challenge of the demodulation pixels is the number of samples required to unambiguously derive the characteristics of the impinging electromagnetic wave. Using a sine-modulated carrier signal, the characteristics of the wave are its amplitude A, the offset B and the phase P. Hence, in this case, at least three samples need to be acquired per period. However, for design and stability reasons, most common systems use four samples. Implementing a pixel capable of capturing and storing n=4 samples requires in general the four-fold duplication of electronics per pixel such as storage and readout electronics. The result is the further increase in the electronics per pixel and a further reduction in fill factor. 
         [0010]    In order to avoid this loss in sensitivity, most common approaches use so-called 2-tap or 2-channel pixels, which are demodulation pixels able to sample and store two samples within the same period. Such type of pixel architectures are ideal in terms of sensitivity, since all the photo-electrons are converted into a signal and no light is wasted, but on the other hand, it requires at least two consequent measurements to get the four samples. Due to sampling mismatches and other non-idealities, even four images might be required to cancel or at least to reduce pixel mismatches. Such an approach has been presented by Lustenberger, Oggier, Becker, and Lamesch, in U.S. Pat. No. 7,462,808, entitled Method and device for redundant distance measurement and mismatch cancellation in phase measurement systems, which is incorporated herein by this reference in its entirety. Having now several images taken and combined to deduce one depth image, motion in the scene or a moving camera renders artifacts in the measured depth map. The more those different samples are separated in time, the worse the motion artifacts are. 
         [0011]    A new architecture has been disclosed by Oggier and Buettgen in U.S. Pat. Pub. No. 2011/0164132A1. The architecture enables the shrinking of the pixel size without significantly reducing the pixel&#39;s fill factor and without compromising the image quality due to smaller storage nodes. The solution even provides the possibility for almost arbitrary integration of any additional post-processing circuitry for each pixel&#39;s signals individually. Furthermore, it can reduce the motion artifacts of time-of-flight cameras to a minimum. Specifically, this demodulation sensor comprises a pixel array comprising pixels that each produce at least two samples and a storage or proxel array comprising processing and/or storage elements, each of the storage elements receiving the at least two samples from a corresponding one of the pixels. The pixels comprise photosensitive regions in which incoming light generates charge carriers and demodulators/correlators that transfer the charge carriers among multiple storage sites in the pixels. A transfer system is provided that transfers the samples generated by the pixels to the corresponding storage elements of the proxel array. In example embodiments, the transfer system analog to digitally converts the samples received by the storage elements. The proxel array then accumulates multiple subframes in time until the entire frame is readout from the proxel array. 
       SUMMARY OF THE INVENTION 
       [0012]    One of the biggest challenges in TOF imaging concerns the reliable acquisition of fast moving objects. Since the most sensitive demodulation pixel is based on a so-called 2-tap pixel (two storage and output sites per pixel), several acquisitions have to be performed to get depth information. During these acquisitions, the object might move and, hence, the each pixel might not sample of the same object. The derived depth measurement result will, thus, be corrupted. 
         [0013]    The present invention concerns an approach for increasing the processing speed of these sensors by enabling acquisition and readout of the pixels in parallel or in a more parallel fashion. 
         [0014]    In general, according to another aspect, the invention features a demodulation sensor, which comprises a pixel array comprising pixels that each produce at least two signals for subframes and a storage array comprising storage elements, each of the storage elements receiving the at least two signals from a corresponding one of the pixels. The pixels each comprise storage sites and readout nodes in which charge carriers are integrated in the storage sites and then transferred to the readout nodes for readout to the storage array. 
         [0015]    In the preferred embodiment, storage sites integrate charge carriers while the charge carriers are being readout from the readout nodes. For example, storage sites integrate charge carriers for a current integration time while the charge carriers are being readout from the readout nodes from a previous integration time. 
         [0016]    In general, according to another aspect, the invention features a demodulation method, comprising: producing at least two samples with a pixel array in storage sites, transferring the samples to readout nodes within each of the pixels in the pixel array and reading out the samples from the readout nodes to a storage array. 
         [0017]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0019]      FIG. 1  is a schematic diagram showing an electronic imaging sensor including a photo-sensitive pixel array and a separate storage or proxel array, which provides final frame storage and readout capability for each pixel; 
           [0020]      FIG. 2  illustrates the basic principle of time-of-flight cameras; 
           [0021]      FIG. 3  is a block diagram showing the functions of a demodulation pixel according to the present invention; 
           [0022]      FIG. 4  is a block diagram showing an example of a pixel to proxel transfer or connection system; 
           [0023]      FIG. 5  is a block diagram showing the functions of a proxel; 
           [0024]      FIG. 6  shows a timing of a 3D image acquisition showing parallel integration and readout based system architecture with a pixel field array and a storage/accumulation proxel array based on a 2-tap or channel pixel architecture according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  shows a demodulation sensor  100  that uses a proxel array. 
         [0026]    The illustrated architecture extracts elements, which are typically integrated inside the pixel but not necessarily required for the photo detection, out of the pixel into physically separated elements that are basically storage and sometimes processing elements, termed storage elements or proxels. As a consequence, the sensor includes a pixel array  110  of x×y pixels and a storage or proxel array  150  of x×y of storage elements or proxels  310  that are used for further processing, storage of the information and readout. Usually x and y are greater than 100, and preferably greater than 200. In some examples x, y are greater than 1000. The two arrays are physically separated from each other in preferably discrete arrays that do not physically overlap with each other on the chip. 
         [0027]    Subframe accumulation is handled in this proxel array  150 . Thus, the sensor  100  includes the pixel array  110  and the proxel array  150 , where each proxel  310  is linked to and associated with preferably one particular pixel  210 . 
         [0028]    It is worth mentioning that the proxel array  150  does not have to be one contiguous array. In examples the proxel array  150  is split into two, three, or four matrices that surround the pixel array  110 . 
         [0029]    The data transfer of the pixel  210  to the proxel  310  is controlled by the pixel readout decoder  182  and transferred through the transfer or connection system  180 . The pixel readout decoder  182  selects the pixel  210  and establishes the connection  180  to the corresponding proxel  310 . Preferably, the readout of the pixel field  110  is done row-wise. This happens during the pixel array or field readout that occurs during each subframe. Hence, the readout decoder selects at least one row of the pixel field  110  which is then connected to the corresponding rows of proxels  310  in the proxel field  150 . In that case, the connection lines of the transfer or connection system  180  are shared by all pixels in a column. In order to further speed up the pixel readout, multiple rows could be selected and transferred as well. 
         [0030]    Additionally included in the sensor  100  is the proxel readout decoder  186  for controlling the readout of the proxels. An optional signal post processing block  184  is provided for calculating phase/depth information based on the n acquired samples, for example. Further the readout is typically provided off of then sensor chip. 
         [0031]    In one example, the transfer or connection system  180  between the pixel array  110  and the proxel array  150  includes analog to digital converters. The information arriving and processed at the proxel array is therefore digital. 
         [0032]      FIG. 2  shows the typical application of a 3D TOF camera that uses the inventive sensor  100 . 
         [0033]    In more detail, a light source or emitter  510  with a possible reflector or projection optics  512  produces light  514  that is intensity-modulated in time and directed at the 3-D scene  516  at range R from the camera. The returning light  518  from the scene  516  is collected by the objective lens system  520  and possibly bandpass filtered so that only light at the wavelength emitted by the light emitter  510  is transmitted. An image is formed on the pixel array  110  of the TOF sensor  100 . A control unit  522  coordinates the modulation of the light emitter  510  with the sampling of the TOF detector chip  100 . This results in synchronous demodulation. That is, the samples that are generated in each of the pixels  210  of the pixel array  110  are stored in the storage buckets or sites in the pixels and/or proxels  310  in the storage or proxel array  150  synchronously with the modulation of a light emitter  510 . The kind of modulation signal is not restricted to sine but for simplicity, sine wave modulation only is used for illustration. 
         [0034]    The information or samples are transferred to the storage or proxel array  150  and then readout by the control unit  522 , which then reconstructs the 3-D image representation using the samples generated by the chip  100  such that a range r to the scene is produced for each of the pixels of the chip  100 . 
         [0035]    In the case of sine wave modulation, using the n=4 samples A 0 , A 1 , A 2 , A 3  generated by each pixel/proxel, the three decisive modulation parameters amplitude A, offset B and phase shift P of the modulation signal are extracted by the equations: 
         [0000]        A =sqrt [( A 3 −A 1)̂2+( A 2 −A 1)̂2]/2
 
         [0000]        B=[A 0 +A 1 +A 2 +A 3]/4 
         [0000]        P =arctan [( A 3 −A 1)/( A 0 −A 2)] 
         [0036]    With each pixel  210  of the sensor  100  being capable of demodulating the optical signal at the same time, the controller unit  522  is able to deliver 3D images in real-time, i.e., frame rates of up to 30 Hertz (Hz), or even more, are possible. Continuous sine modulation delivers the phase delay (P) between the emitted signal and the received signal, which corresponds directly to the distance R: 
         [0000]        R =( P*c )/(4*pi* f mod), 
         [0000]    where fmod is the modulation frequency of the optical signal  514 . Typical state-of-the-art modulation frequencies range from a few MHz up to a few hundreds of MHz or even GHz. 
         [0037]      FIG. 3  illustrates a demodulation pixel  210  with its different functional blocks. The impinging light is converted in charge carriers in the photo-sensitive area  212  of the pixel  210 . Typically a lateral electrical drift field is provided in the photo-sensitive area  212  to sweep the charge carriers to a demodulator/correlator  218 , which transfers the photo-generated charges in an alternating fashion to the n different storage sites  220 A,  220 B to  220 N, which accumulate the charges generated in each subframe. The transfer to the different storage sites  220 A,  220 B to  220 N is typically performed synchronously with the modulation of the light source  510 . 
         [0038]    The illustrated example shows a pixel with N storage sites, but in most conventional pixels, N=2. 
         [0039]    The charge carriers are accumulated in the storage sites  220 A,  220 B to  220 N for the integration time set by for the sensor. Typically this time is between a few tens of microseconds up to a few hundreds of milliseconds. 
         [0040]    At the end of this pixel integration time, the charge carriers accumulated in each of the storage sites  220 A,  220 B to  220 N are transferred to corresponding readout nodes  221 A,  221 B to  221 N in an in-pixel charge transfer process. These hold the charge carriers until they can be read out on the readout lines out of the pixel array  110  in a pixel array or field readout process. Relevant here, however, is the fact that once the charge carriers are transferred to the readout nodes  221 A,  221 B to  221 N, the storage sites  220 A,  220 B to  220 N are immediately available to receive new charge carriers for the next pixel integration time. This integration cycle begins even before the charges have been read out from the readout nodes  221 A,  221 B to  221 N from the previous integration time. 
         [0041]    Before reading out the readout nodes  221 A,  221 B to  221 N with the n samples, many demodulation pixels include in-pixel processing  222  e.g. for common mode suppression. 
         [0042]    The pixels&#39; storage nodes and separate readout out nodes are usually designed as a storage MOS gate and an output diffusion, respectively. During integration, the electrons are stored in the storage sites  220 A- 220 N. At the end of the integration, the charges are shifted to the readout nodes  221 A- 221 N and the storage sites are emptied. 
         [0043]    The sensing  212  and demodulation  218  can be done using dynamic lateral drift fields as described in U.S. Pat. No. 7,498,621 B2, which is incorporated herein in its entirety, or static lateral drift fields as described in U.S. Pat. Appl. No. 2008/0239466 A1, which is incorporated herein in its entirety. Various approaches have been published based on the static lateral drift field principle B. Büttgen, F. Lustenberger and P. Seitz, Demodulation Pixel Based on Static Drift Fields, IEEE Transactions on Electron Devices, 53(11):2741-2747, November 2006, Cédric Tubert et al., High Speed Dual Port Pinned-photodiode for Time-Of-Flight Imaging, International Image Sensor Workshop Bergen 2009, and D. Durini, A. Spickermann, R. Mahdi, W. Brockherde, H. Vogt, A. Grabmaier, B. Hosticka, “Lateral drift-field photodiode for low noise, high-speed, large photoactive-area CMOS imaging applications”, Nuclear Instruments and Methods in Physics Research A, 2010. Other methods do not have the photosensitive area  212  and the demodulation  218  physically separated such as the photo-detection assisted by switching majority currents, see M. Kuijk, D. van Niewenhove, “Detector for electromagnetic radiation assisted by majority current”, September 2003, EP 1 513 202 A1, or the methods based on toggling large transfer gates, see U.S. Pat. No. 5,856,667, U.S. Pat. No. 6,825,455, and US 2002/0084430 A1. All of those sensing/demodulation methods can be implemented here. 
         [0044]      FIG. 4  shows the pixel to proxel transfer or connection system  180  converting the information into the digital domain, according to one embodiment. There is one analog to digital converter  280 A,  280 B per output line. The Pixel_readout signal, controlled by the pixel readout decoder  182 , provides the analog signals from readout nodes  221 A- 221 N of the pixels  210  to the converters  280 A,  280 B, which analog to digital convert the charges accumulated into digital values. In other embodiment, there is no analog to digital converters in the proxel transfer or connection system  180  and the charge carriers are stored in separate storage sites even in the proxel array  150 . 
         [0045]      FIG. 5  shows the functions of the storage elements or proxels  310 . An information storage unit  314  accumulates each of the subframes of generated information that are readout from the pixel array and provides typically four different storage locations for each of the four samples A 1 -A 4  that are taken at 0, 90, 180, and 270 degrees of phase shift that are generated within a subframe. A data readout unit  316  enables the information readout typically at the time of the depth calculation and off-chip readout process. In any event, instead of reading out the pixel matrix  110  directly, the proxel array  150  is readout. 
         [0046]      FIG. 6  illustrates the timing of the operation of the sensor. Integration of at the storage sites and data transfer from the pixel array  110  to the processing and storage (proxel) array  150  occurs in parallel. This eliminates any “dead time” caused by reading out the pixel field  110 . Shown is the readout where the pixel has a 2-tap pixel architecture in which there are two storage sites  220 A,  220 B. 
         [0047]    In case the pixel field readout time becomes dominant compared to the subframe integration time, the need of having integration and readout in parallel becomes important. 
         [0048]    In more detail, for each of channel 1, associated with storage site  220 A and channel 2, associated with storage site  220 B, charge subframe integration occurs at each of the storage sites for 0° and 180°, respectively. At the end of the integration time, the charge carriers that have been accumulated in the storage sites are transferred to the readout nodes  221 A,  221 B, respectively. This typically is done simultaneously for all pixels of the pixel array and is performed in a negligible short time compared to the integration time. 
         [0049]    At this moment, the longer pixel array or field readout is performed to the information storage sites  314  in the corresponding proxels of the proximal array  150 . In one example, this readout further includes analog to digital conversion of the charge carriers stored in the readout nodes during the transfer. 
         [0050]    During the pixel field readout, the subframe integration time at the pixel level has already begun again. In channel 1, associated with storage site  220 A, and channel 2, associated with storage site  220 B, charge integration occurs at each of the storage sites for 90° and 270°, respectively. The storage sites  220 A,  220 B are accumulating the charge carriers associated with the next subframe integration time. 
         [0051]    This process repeats to generate the four samples. In the given example, four sub-frames are acquired to derive one sub-depth-frame. In the most typical example, this process is repeated over, including many sub-depth frames. At the end of the full acquisition, the full frame is then read out from the proxel array  150 . Preferably the depth calculation and off-chip readout are then performed. 
         [0052]    In a more sophisticated timing control, the sub-frame or the sub-depth-frame integration time might even vary between different cycles with the purpose to cover a bigger dynamic range. 
         [0053]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.