Abstract:
TOF distance sensor for capturing the distance to an object by receiving radiation reflected by the object, said radiation emanating from a radiation source modulated by a modulation frequency, comprising a pixel matrix for recording a pixel image. The pixel matrix consists of demodulation pixels which are designed for rear-side reception of the radiation. The demodulation pixels comprise a conversion region for generating charge carriers from the received radiation, and a separating device for separating the charge carriers in accordance with the modulation frequency, and also a stop for partitioning-off the conversion region from the separating device in relation to the charge carriers, and also an aperture for passing the charge carriers from the conversion region into the separating device. The TOF distance sensor is embodied in such a way that in each case at least two demodulation pixels form a common aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/EP2016/051296 filed Jan. 22, 2016, which designated the United States, and claims the benefit under 35 USC §119(a)-(d) of European Application No. 15154379.0 filed Feb. 9, 2015, the entireties of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a TOF distance sensor comprising a pixel field. 
       BACKGROUND OF THE INVENTION 
       [0003]    The prior art has disclosed TOF (time of flight) distance sensors which detect the phase shift of modulated light, which was emitted toward an object and reflected by the latter, and derive the distance to the object therefrom. An example of one such sensor is disclosed in US Patent Application Publication No. 2014/0145281 A1. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide an improved distance sensor. 
         [0005]    The distance sensor according to the present invention is a TOF distance sensor for capturing the distance to an object by receiving radiation reflected by the object from a radiation source modulated by a modulation frequency. The sensor comprises a pixel matrix for recording a pixel image. The pixel matrix consists of demodulation pixels, which are designed for the rear-side reception of the radiation. The demodulation pixels comprise a conversion region for generating charge carriers from the received radiation, and a separating device for separating the charge carriers in accordance with the modulation frequency, and also a stop for partitioning-off the conversion region from the separating device in respect of the charge carriers and also an aperture for passing the charge carriers from the conversion region into the separating device. Preferably, the TOF distance sensor may be embodied in such a way that in each case at least two demodulation pixels form a common aperture. 
         [0006]    The common aperture may embody the advantage of increasing the sensitivity. Of the demodulation pixels and of making the distance sensor more effective. The common aperture may also have the advantage of allowing pixel dimensions to be designed to be smaller. The pixel matrix may embody the advantage of generating images, in particular, 3-D images. 
         [0007]    Embodying the demodulation pixels for the rear-side reception of the radiation means that the radiation incidence into the conversion region is effected from the side facing away from the separating device and the evaluation region. 
         [0008]    Preferably, to this end, the conversion region is formed by a thinned, e.g. 50 um thick, semiconductor layer which, on the front side thereof, embodies the separating device, for example, using CCD technology. 
         [0009]    Preferably, the common aperture forms a closed circumference. 
         [0010]    Preferably, the demodulation pixels each comprise an electronic evaluation region, wherein at least two demodulation pixels in each case form a spatially common evaluation region. 
         [0011]    This may embody the advantage that parts of the common evaluation region may be used together and hence that parts of the evaluation region may be reduced. 
         [0012]    Preferably, four demodulation pixels in each case form a common aperture. Preferably, four demodulation pixels each case form a spatially common evaluation region. Preferably, the apertures and evaluation regions form a checkerboard pattern. Preferably, the pixel matrix is substantially point symmetrical in each case in relation to the common apertures. Preferably, the pixel matrix is substantially point symmetrical in each case in relation to the common evaluation regions. 
         [0013]    This may embody the advantage that the charge carriers which were generated in the region below a demodulation pixel reach the separating device of precisely this demodulation pixel. This may embody the advantage that the charge carriers keep the spatial information thereof in relation to the individual demodulation pixel. This may increase the resolution of the pixel matrix. 
         [0014]    Preferably, the conversion region exhibits a doped substrate. Preferably, the conversion region exhibits a transparent rear-side electrode. Preferably, the substrate is a semiconductor substrate. Preferably, the substrate is weakly n-doped. 
         [0015]    This may embody the advantage that the conversion region may be depleted such that the photoelectric effect may be formed therein by radiation. 
         [0016]    Preferably, the separating device comprises a drift gate on the upper side of the substrate for attracting the charge carriers from the conversion region into the separating region. Preferably, the drift gate may also be formed by a plurality of drift gates which, in particular, have an ever-increasing potential toward the modulation gates for the purposes of attracting the charge carriers. Preferably, a drift gate may have such an embodiment that two modulation gates may be arranged at opposing positions. Optionally, the drift gate may be concomitantly embodied by the modulation gate or gates, for example, if an additional, constant potential, analogous to the drift gate, is applied to the modulation gate or gates. 
         [0017]    Preferably, the separating device comprises at least one, in particular, two, modulation gates on the upper side of the substrate, in particular, at opposing positions of the drift gate, for alternately guiding the charge carriers in accordance with the modulation frequency from the drift gate to the modulation gates. 
         [0018]    Preferably, the separating device comprises at least one, in particular, two, storage gates on the upper side of the substrate, in each case assigned to a modulation gate or the modulation gate, for collecting the charge carriers directed toward the assigned modulation gate. 
         [0019]    Preferably, the separating device comprises at least one, in particular, two, transfer gates on the upper side of the substrate, in each case assigned to a storage gate or the storage gate, for intermittent forwarding of the charge carriers collected at the storage gates to floating diffusions. 
         [0020]    Preferably, the separating device comprises at least one, in particular, two, floating diffusions in the upper side of the substrate, in particular, as n+ doped well, in each case assigned to a transfer gate or the transfer gate, for receiving the charge carriers forwarded by the transfer gates and for feeding same as voltage into the evaluation region. 
         [0021]    Preferably, the gates are separated from the substrate by a nonconductive layer. Preferably, the demodulation pixels between stop and gates, in particular, between stop and modulation gate, storage gate and transfer gate, form a conduction channel for the charge carriers, which is controllable by the gates. Preferably, the gates control the charge carriers in the conduction channel in the style of a CCD. The conduction channel may facilitate an expedient rear-side illumination, which increases the efficiency of the demodulation pixel. 
         [0022]    According to the present invention, the separating device comprises a drift gate for attracting the charge carriers from the conversion region into the separating region, wherein the demodulation pixels forming a common aperture in each case form of at least one common drift gate. Preferably, the common drift gate covers the region of the common aperture with substantially the same extent. According to the present invention, the common drift gate is complemented by a drift date individually assigned to the individual pixel. Preferably, the modulation gates are assigned to an individually assigned drift gate. Preferably, the common drift gate consists of a plurality of common drift gates. 
         [0023]    This may embody the advantage that the TOF distance sensor becomes even more efficient. 
         [0024]    Preferably, the stop comprises a buried layer in the substrate. Preferably, the buried layer is a p+ doped pSub layer. 
         [0025]    This may embody the advantage that the generated electrons are reliably blocked by the layer. 
         [0026]    Preferably, the aperture forms a closed recess. 
         [0027]    This may embody the advantage that the pixel dimension may be designed to be particularly small. 
         [0028]    The junction FET effect may lead to it not being possible to embody the aperture arbitrarily small in relation to the substrate thickness. By the combination to form a common aperture, the pixel dimension may have a smaller design than in the case of individual apertures for a given minimum opening. 
         [0029]    Preferably, the evaluation region comprises at least a source follower, reset switch and select transistor. 
         [0030]    Further features of the invention are specified in the drawings. 
         [0031]    The advantages specified in each case may also be realized for feature combinations in the context of which they have not been specified. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Exemplary embodiments of the present invention are depicted in the drawings and are explained in more detail below. Here, the same reference signs in the individual figures denote elements which correspond to one another. 
           [0033]      FIG. 1  shows a TOF distance sensor system with an object; 
           [0034]      FIG. 2  shows a schematic side section of a demodulation pixel; 
           [0035]      FIG. 3  shows a top view of a demodulation pixel; 
           [0036]      FIG. 4  shows four demodulation pixels with a common aperture; and 
           [0037]      FIG. 5  shows a pixel matrix made of  6  x  6  demodulation pixels. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]      FIG. 1  shows a TOF distance sensor system  10  with object  22 . 
         [0039]    The object  22  is at a distance from the TOF distance sensor system  10 . By way of example, the radiation source  20  is an LED or an arrangement of a plurality of LEDs. The radiation source is actuated by the electronics device  13  which operates the radiation source in an intensity modulated manner with a modulation frequency. By way of example, the radiation source emits monochromatic light  21 , which is diffusely reflected at the object and incident on the TOF distance sensor  40  as reflected radiation  23 . In the TOF distance sensor, the received radiation produces a value profile of induced photoelectrons, and hence a signal curve for the received radiation. 
         [0040]    The electronics device  13  and the TOF distance sensor  10  are integrated on a chip and embodied in a combined CMOS/CCD method. The chip and the radiation source  20  are arranged on a common carrier  11  and surrounded by a housing  12 . The radiation source and the receiving device each have an optics device not depicted in the drawings, which focuses in the direction of the space in which the distance of objects is intended to be determined. 
         [0041]    The emitted radiation  21  has a wavelength of 860 nm and is pulsed with a sinusoidal signal or rectangular signal of 20 MHz. The reflected radiation received by the TOF distance sensor  10  remains pulsed with a sinusoidal signal or rectangular signal of 20 MHz and is phase shifted in relation to the modulation signal of the radiation emitted by the radiation source  20  by the time of the light path. The phase shift between emitted rectangular signal and received rectangular signal corresponds to twice the distance between TOF distance sensor and object. 
         [0042]      FIG. 2  shows a demodulation pixel  50  in a schematic side section. 
         [0043]    The side section follows - not proportionately—the cut line  52  in the plan view of  FIG. 3 . 
         [0044]    The demodulation pixel  50  comprises an n-doped float zone silicon semiconductor substrate  61  with a thickness of approximately 50 micrometers and a specific electric sheet resistance of greater than or equal to 2000 ohm cm. Arranged on the surface of the semiconductor substrate above a nonconductive SiO separating layer  77  on the substrate are a drift gate  71  and, on both sides in symmetric arrangement and in each case spaced apart from one another, in each case a modulation gate  73 , a storage gate  74 , a transfer gate  75  and, within the substrate, a floating diffusion  76 . The layers and contacts expedient to this end are not depicted. A stop  80  is arranged between the gates and the transparent rear-side contact, and shadows the storage gates, transfer gates and the floating diffusion, including the semiconductor substrate lying below the respective gates, in relation to the incident reflected radiation  23 , with the stop comprising an aperture  81  in the region below the drift gate. The semiconductor substrate is depleted, at least under the drift gate, but in particular overall. A positive potential is applied to the drift gate and the latter forms a depletion region in the semiconductor substrate. 
         [0045]    The separating device  70  comprises the drift gate, the modulation gates, the storage gates, the transfer gates, the separating layer, the floating diffusions, the stop, the aperture and the substrate situated between the stop and the gates, said substrate being of the same type as the semiconductor substrate  61  in the conversion region  60 . The conversion region  60  comprises the semiconductor substrate  61 , the rear-side electrode  62  and the stop  80 . The substrate has a thickness of approximately  50  micrometers. 
         [0046]    The reflected IR radiation  23  penetrating into the semiconductor substrate  61  under the drift gate via the transparent rear-side electrode  62  induces electron-hole pairs  24  in the semiconductor substrate. The photoelectrons are attracted toward the drift gate by the depletion region which is formed by the drift gate  71 . The drift gate has the potential of approximately 4 V. The number of attracted photoelectrons is proportional to the received radiation intensity. 
         [0047]    A modulated potential, the maximum of which lies between the potentials of the drift gate  71  and at the storage gate  74  and the minimum of which lies below that of the drift gate, may be applied to the modulation gates  73 . The potential of the modulation gate  73 , for example, modulates between the values of 0 V and 5 V. 
         [0048]    The two modulation gates are operated with mutually inverted potentials  20 , i.e. the potential of the one modulation gate is 0 V when the potential of the other one is positive, and vice versa. Then, it is always the case that a potential of 0 V is applied to the one modulation gate and a potential of 5 V is applied to the other modulation gate. A potential minimum, i.e. 0 V in this case, leads to a potential barrier for the photoelectrons under the drift gate, and so no photoelectrons are able to reach the storage gate assigned to this modulation gate. A potential maximum, i.e. 5 V in this case, leads to draining of the photoelectrons under the drift gate, past this modulation gate and into its associated storage gate. 
         [0049]    The flow of the photoelectrons produced by the received radiation intensity is guided in a manner corresponding to a switch by applying in each case a potential to the two modulation gates which in each case corresponds to mutually inverted sinusoidal signals or rectangular signals. The flow of these photoelectrons under the modulation gates arising thus corresponds to multiplication, i.e. a correlation of the corresponding sinusoidal signals or rectangular signals with the received radiation signal. Here, the sinusoidal signals or rectangular signals have the property of a correlating signal and are denoted correlation signal here. 
         [0050]    A higher potential is applied to the storage gates  74  than to the drift gate  71  and said storage gates  74  alternately collect thereunder the photoelectrons  25  in accordance with the status of the modulation gates  73 . The storage gates  74  for example have a potential of  10  V. The charges collected under the storage gates by the photoelectrons correspond to the correlation values. Hence, the correlation values are present in the charge domain. The collection of the photoelectrons under the corresponding storage gates corresponds to a temporal integration of the aforementioned correlation of correlation signal and received radiation signal. 
         [0051]    For the purposes of detecting the photoelectrons  25  collected under the storage gates  74 , the potential of the modulation gates  73  is firstly set to 0 V in order to form a potential barrier for the potential elections in the direction of the drift gate  71 . Secondly, the potential of the transfer gates is raised to a middling value, for example 6 V, in order to facilitate a qualified drain of the photoelectrons in the direction of the floating diffusions  76 . 
         [0052]    Now, the positive potential of both storage gates  74  of approximately 10 V is lowered in parallel by means of a time ramp. The added potential of the dropping positive potential applied to the storage gates and the negative potential of the charge situated therebelow, which added potential changes in the process, determines whether charge can drain via the transfer gates  75 . Here, the lowering process is subdivided into three phases. In a first phase of the time ramp, the aforementioned added potential still is more positive for both storage gates than the constant and equally positive potential  25  of the transfer gates, and no charge drains. In a subsequent second phase of the time ramp, the aforementioned added potential is more positive for one storage gate and more negative for the other storage gate when compared to the constant and equal positive potential of the transfer gate. As a result, charge under the storage gate with the more positive added potential drains via the associated transfer gate into the associated floating diffusion such that the added potential once again equals the potential of the corresponding transfer gate. In a subsequent third phase of the time ramp, the aforementioned added potentials of both storage gates are higher than the constant equal potentials. As a result, charges drain from under both storage gates via the respectively associated transfer gate into the respectively associated floating diffusion. The time ramp is stopped immediately once the third phase starts, i.e. the potential of the storage gates is not lowered any further such that, substantially, only the charge drainage from the second phase is relevant. The amount of charge now present in a charged floating diffusion thus corresponds to the difference between the amount of charge from both storage gates. Thus, the time ramp carries out a subtraction of the amounts of charge under the two storage gates. 
         [0053]    After carrying out the above-described time ramp, the amount of charge in the one charged floating diffusion corresponds to a value of the phase difference between emitted radiation  21  and reflected radiation  23 . 
         [0054]    The amount of charge in the one charged floating diffusion is now converted into a corresponding voltage by means of a source follower and processed further. The source follower is part of the evaluation region of the demodulation pixel. In addition to the source follower, the evaluation region also comprises a reset switch and a select transistor. 
         [0055]    The distance to the object may be calculated from the corresponding voltage by way of a method. By way of example, such a method is described in EP 2 743 724 A1 by the applicant. 
         [0056]      FIG. 3  shows a top view of a demodulation pixel  50 . The aperture  81  is  20  covered by the first drift gate  72  with approximately the same shape. A second drift gate  71  intersects the first drift gate in an electrically separated manner. The second drift gate and first drift gate act like a single drift gate, with a higher potential being applied to the second drift gate than to the first drift gate such that photoelectrons are forwarded from the first drift gate to the second drift gate. The demodulation pixel comprises two modulation gates  73  at opposite places at one end of the second drift gate. Lying opposite to the second drift gate, a storage gate  74  is in each case arranged next to the modulation gates. A transfer gate  75  is in each case arranged at one edge of the storage gates. Lying opposite to the storage gates, a floating diffusion  76  is in each case arranged next to the transfer gates. The floating diffusions are connected to the evaluation region and the source follower thereof. 
         [0057]    The section line  52  shows the schematic, non-proportional profile of the side section in  FIG. 2 . 
         [0058]      FIG. 4  shows four demodulation pixels  50  with a common aperture  82 . The demodulation pixels  50  arranged around the common aperture  82  correspond exactly to the demodulation pixel  50  shown in  FIG. 3  and are rotated by 90° in each case. Overlaid on the common aperture  82  with the same shape is a common central drift gate  72 . Objects of the drawings which are geometrically the same correspond to the corresponding devices in  FIG. 3 . 
         [0059]      FIG. 5  shows a pixel matrix  41  made of 6×6 demodulation pixels  50 . In addition to the common aperture  82 , the arrangement also forms common evaluation regions  91 . A common central drift gate is in each case overlaid on the common apertures  82 . Objects of the drawings which are geometrically the same correspond to the corresponding devices in  FIG. 4 . 
         [0060]    A method for evaluating the TOF distance sensor is disclosed in EP 2 743 724 A1. 
         [0061]    Another method for evaluating the TOF distance sensor is disclosed in Robert Lange, Peter Seitz, Alice Biber, Stefan Lauxtermann: Demodulation Pixels in CCD and CMOS Technologies for Time-of-Flight Ranging, IST/SPIE International Symposium on Electronic Imaging, Conference on Sensors, Cameras, and Systems for Scientific/Industrial Applications II, Proc. SPIE, Vol. 3965A, San Jose, USA, 24 th -25 th  January 2000. 
         [0062]    The rear-side electrode may be contacted through the semiconductor substrate  61  by means of a potential tunnel. An apparatus for contacting the rear-side electrode  62  of the TOF distance sensor by means of a potential tunnel is disclosed in U.S. Pat. No. 8,901,690 B2. 
         [0063]    The demodulation pixels may be embodied in analog CCD technology and the evaluation regions may be embodied in digital CMOS technology. A method for common production of demodulation pixels (CCD) and evaluation region (CMOS) is disclosed in U.S. Pat. No. 8,802,566 B2. 
         [0064]    A method for the common embodiment of demodulation pixels (CCD, analog) and evaluation region (CMOS, digital) on a chip as a system-on-a-chip is disclosed in EP 2 618 180 B1. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           10  TOF distance sensor system 
           11  Carrier 
           12  Housing 
           13  Electronics device 
           20  Radiation source 
           21  Emitted radiation 
           22  Object 
           23  Reflected radiation 
           24  Electron-hole pairs 
           25  Photoelectrons 
           40  TOF distance sensor 
           41  Pixel matrix 
           50  Demodulation pixel 
           51  Boundary 
           52  Section line 
           60  Conversion region 
           61  Semiconductor substrate 
           62  Rear-side electrode 
           70  Separating device 
           71  Drift gate 
           72  Common drift gate 
           73  Modulation gate 
           74  Storage gate 
           75  Transfer gates 
           76  Floating diffusions 
           77  Separating layer 
           80  Stop 
           81  Aperture 
           82  Common aperture 
           90  Evaluation region 
           91  Common evaluation region