Abstract:
A demodulation pixel improves the charge transport speed and sensitivity by exploiting two effects of charge transport in silicon in order to achieve the before-mentioned optimization. The first one is a transport method based on the CCD gate principle. However, this is not limited to CCD technology, but can be realized also in CMOS technology. The charge transport in a surface or even a buried channel close to the surface is highly efficient in terms of speed, sensitivity and low trapping noise. In addition, by activating a majority carrier current flowing through the substrate, another drift field is generated below the depleted CCD channel. This drift field is located deeply in the substrate, acting as an efficient separator for deeply photo-generated electron-hole pairs. Thus, another large amount of minority carriers is transported to the diffusion nodes at high speed and detected.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 61/233,961, filed on Aug. 14, 2009, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Time resolving or demodulating pixels have different fields of applications. Two typical examples are three dimensional (3D) range imaging and fluorescence lifetime imaging. Basically, in both applications higher in-pixel charge transport speed and higher optical sensitivity lead to more accurate per-pixel measurements. 
         [0003]    The time-domain demodulation of modulated light signals at the pixel level requires in all approaches the switching of a photo-generated charge current. It is possible to handle electron as well as hole currents. The common methods, however, use the photo-generated electron currents due to the higher mobility of electrons in the semiconductor material. 
         [0004]    Some pixel architectures do the necessary signal processing based on the photo-current whereas other architectures work in the charge domain directly. 
         [0005]    Common to all pixels is the necessary transfer of charges through the photo-sensitive detection region to a subsequent storage area or to a subsequent processing unit. In the case of charge-domain based pixel architectures, the photo-charge is generally transferred to a storage node. In order to demodulate an optical signal, the pixel has to have implemented at least two integration nodes that are accumulating the photo-generated charges during certain time intervals. Another minimum requirement would be the implementation of at least one integration node and having typically at least one other node for dumping charge carriers as well. 
         [0006]    Different pixel concepts have been realized in the last few decades. Schwarte [SCH98] R. Schwarte, “Verfahren and Vorrichtung zur Bestimmung der Phasen-und/oder Amplitudeninformation einer elektromagnetischen Welle”, Mar. 12, 1998, DE 197 04 496 A1; introduced a demodulation pixel, which transfers the photo-generated charge below a certain number of adjacent poly-silicon gates to discrete accumulation capacitances. Spirig [SPI99] T. Spirig, “Apparatus and method for detection of an intensity-modulated radiation field”, Jan. 5, 1999, U.S. Pat. No. 5,856,667; disclosed a charge coupled device (CCD) lock-in concept that allows the in-pixel sampling of the impinging light signal with theoretically an arbitrary number of samples. Another very similar pixel concept has been demonstrated by Kawahito [KAW06] T. Ushinaga et al., “A QVGA-size CMOS time-of-flight range image sensor with background light charge draining structure”, Three-dimensional image capture and applications VII, Proceedings of SPIE, Vol. 6056, pp. 34-41, 2006; where a thick field-oxide layer is used to smear the potential distribution below the demodulation gates. 
         [0007]    A common problem of the afore-mentioned pixel approaches is the slowness of the charge transport through the semiconductor material, which decreases significantly the accuracy or quality of the in-pixel demodulation process. In all pixel structures, the limiting factor for the transport speed is the non-perfect linear potential distribution in the semiconductor substrate that is used to transport the charges through the semiconductor in lateral direction. 
         [0008]    The first 3-D cameras that are based on highly-integrated demodulation pixels used the CCD lock-in pixels [SPI99]. However, their limitations in speed and in achievable distance measurement accuracy led to a change of the pixel concepts used. Today&#39;s most advanced 3-D cameras implement drift field pixels in order to be able to operate at high frequencies so that 3-D imaging with sub-millimeter resolution becomes possible [BUE06A] B. Büttgen, “Extending Time-of-Flight Optical 3D-Imaging to Extreme Operating Conditions”, Ph.D. thesis, University of Neuchatel, 2006; [BUE06B] 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. Measurements with such cameras have proven the pixel concept and have shown that gigaHertz demodulation becomes possible with comparable pixel pitches and fill factors as achieved with the standard CCD lock-in pixels [BUE06B]. 
         [0009]    New concepts of pixels have been explored in the last years accelerating the in-pixel transport of the charges by improving the characteristics of the lateral electric drift fields. Seitz, [SEI02] P. Seitz, U.S. Pat. Appl. Publ. No. US 2006/0108611 A1, invented the first drift field demodulation device that is based on a very high-resistive poly-silicon gate electrode. It even allows very easily the design of pixels that can generate an arbitrary number of samples. The concept was proven by Buettgen [BUE05A] B. Büttgen, “Device and method for the demodulation of modulated electromagnetic fields”, Filing date: Oct. 19, 2005; who disclosed later another concept of demodulation pixels: the static drift field pixel [BUE04][BUE06B]. In contrast to the architectures mentioned before, the static drift field pixel clearly separates the detection from the demodulation regions within the pixel. It further shows lower power consumption compared to the drift field demodulation approach of Seitz and, at the same time, it supports fast in-pixel lateral charge transport. 
         [0010]    Another pixel concept was presented by Nieuwenhoven [NIE05] D. van Nieuwenhoven et al., Novel Standard CMOS Detector using Majority Current for guiding Photo-Generated Electrons towards Detecting Junctions”, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2005. In this pixel architecture a modulated lateral electric drift field is generated by the current of majority carriers within the semiconductor substrate. Minority carriers are generated by the photons and transported to the particular side of the pixel depending on the induced current, or applied drift field consequently. 
         [0011]    One major application of demodulation pixels is found in real-time 3-D imaging. By demodulating the optical signal and applying the discrete Fourier analysis on the samples acquired, parameters such as amplitude and phase can be extracted for the frequencies of interest. If the optical signal is sinusoidally modulated, the extraction based on at least three discrete samples will lead to the offset, amplitude and phase information. The phase value corresponds proportionally to the sought distance value. Such a harmonic modulation scheme is often used in real-time 3-D imaging systems having incorporated the demodulation pixels [BUE06A]. 
         [0012]    The precision of the pixel-wise distance measurement is determined by the speed of the in-pixel transfer of the electrons from the area, where they are generated, to the area, where they are accumulated or post-processed. Thus, the ability of the pixel to sample high modulation frequencies is of high importance to perform distance measurements with high accuracy. 
       SUMMARY OF THE INVENTION 
       [0013]    Each of the three major concepts of drift field pixels has its particular drawbacks restricting the 3-D imaging applications. 
         [0014]    The drift field demodulation pixel of Seitz [SEI02] generates the lateral drift field by a constant electronic current through the poly-silicon gate. In order to reduce the power consumption, the gate should be as resistive as possible. However, the creation of sensors with large pixel counts is not possible without increasing the sensor&#39;s power consumption. The high in-pixel power consumption has also a negative impact on the thermal heating of the sensor and hence, on its dark current noise. Enhanced drift field demodulation pixel concepts disclosed by Buettgen [BUE05B], B. Büttgen, F. Lustenberger, P. Seitz, “Large-area pixel for use in an image sensor”, Filing date: Jul. 18, 2005, WO 2006/012761 A1; make use of dendritic gate structures. However, these structures just moderate the problem of in-pixel power consumption, but they do not solve it completely. 
         [0015]    The drift field pixel of Nieuwenhoven [NIE05] generates the drift field in the substrate by the current flow of majority carriers. One major problem of this pixel concept is the self-heating of the pixel and the associated dark current noise. Furthermore, the quantum efficiency suffers from the fact that the same semiconductor region is used to create the drift field by a current of majority carriers and to separate the minority carriers. High recombination rates are the result—reducing the optical sensitivity. Another drawback is that the induced current needs to be modulated at high-speed throughout all the pixels on the imager and, therefore, the resistive and capacitive optimized routing becomes difficult. 
         [0016]    The demodulation pixel based on static drift fields [BUE05a] requires the creation of a large region having a lateral electric drift field that guides electrons in the direction of the demodulation region. The drift region is currently implemented as non-uniform doping profiles, so-called built-in drift fields [TUB09] Cédric Tubert, Laurent Simony, François Roy, Arnaud Tournier, Luc Pinzelli, Pierre Magnan. STMicroelectronics Grenoble, France;  2 STMicroelectronics Crolles, France; ISAE/CIMI, France, “High Speed Dual Port Pinned-photodiode for Time-Of-Flight Imaging”, International Image Sensor Workshop Bergen 2009; [DUR10] 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; or a successive CCD gate structure [BUE06B]. In the case of the gate structure, each gate has a minimum width and the gate voltages linearly increase in the direction of the demodulation region. Those voltages are all constant, meaning that the lateral electric drift field is constant, also. A major drawback is the complex layout; in particular the connection of the gates to the constant voltages is difficult to realize. Even more dramatically, if a pure CCD process is used, the routing rules are more restricting than in a complementary metal-oxide-semiconductor (CMOS) process with CCD option generally making such a design more impractical. 
         [0017]    Another drawback of the afore mentioned static drift field pixel structures is the fact that charge carriers generated deep in the semiconductor substrate cannot be demodulated or even detected. The collection depth inside the semiconductor is mainly determined by the depth of the shallow space charge region created by the implemented drift field structure. 
         [0018]    In general, the invention features the combination of the following three pixel enhancement mechanisms: a) the enhanced charge transport and photo-sensitivity through a buried channel, b) the enhanced charge transport and photo-sensitivity enforced deeply in the semiconductor by majority carrier current, and/or c) the enhanced charge transport and photo-sensitivity achieved by the exploitation of high-low junctions or graded/gradually doped wafer structures. This results in a new pixel structure that outperforms prior-art demodulation pixels in terms of charge transport speed and photo-sensitivity. 
         [0019]    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 
         [0020]    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: 
           [0021]      FIG. 1  is a schematic diagram of a demodulation pixel to which the present invention is applied in embodiments; 
           [0022]      FIG. 2  is a schematic diagram showing photo-sensitivity enhancement and fast in-pixel charge transport mechanisms; 
           [0023]      FIG. 3  shows an example for a demodulation pixel with separated detection region and demodulation region incorporating a buried channel and the n+ diffusion area; 
           [0024]      FIG. 4  shows a cross section through the detection region in which the drift field is generated by photogates; 
           [0025]      FIG. 5  shows a cross section through the detection region in which the drift field is generated by a high resistivity layer; 
           [0026]      FIG. 6  shows a cross section through the detection region in which the drift field is generated by continuous change of the doping concentration of the n implantation; so-called built-in drift fields; 
           [0027]      FIG. 7  shows a cross section through the detection region; 
           [0028]      FIG. 8  shows a cross section through the detection region in which the drift field is generated by capacitively coupled gates; and 
           [0029]      FIG. 9  is a schematic illustration showing the operation of a TOF camera using a demodulation pixel according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The following description is based on the semiconductor materials with a p-type silicon substrate such that the electrons rather than holes are collected for processing. It is also possible to use n-doped silicon so that all doping and voltage relationships are swapped accordingly. Thus, the description herein is not to be understood as a limitation for using just p-doped semiconductor substrates. The same is meant for the photo-currents, which can either be electron or hole current depending on the type of doping of the semiconductor material. 
         [0031]      FIG. 1  is a schematic diagram of a demodulation pixel  100 . Photons  10  impinge onto the sensitive area  110  and create electron-hole pairs  12 . In general the holes are drained by the substrate and the electrons are used as the charge carriers for the signal processing and extraction of the information. The sensitive region  110  transports the electrons as fast as possible to the demodulation region  112  in order to support high (de)modulation frequencies. The output of the demodulation region is a number of charge packets into storage nodes a, b, . . . n, which correspond to the samples of the optical signal. In general, the samples are amplified and readout in circuitry  14 . 
         [0032]    According to a preferred embodiment of the present invention, the photo-sensitive region is optimized in terms of sensitivity and charge transport speed. To achieve maximum photo-sensitivity and fast in-pixel charge transport at the same time, one or more of three aforementioned pixel enhancement mechanisms are exploited, which can arbitrarily be combined together. 
         [0033]      FIG. 2  is a schematic diagram showing photo-sensitivity and fast in-pixel charge transport mechanism for the sensitive region  110  of a demodulation pixel. 
         [0034]    A first mechanism incorporates a shallow doping implantation layer  210  in the substrate  101  that creates a depleted region that separates and collects photo-generated charges. Since the layer  210  is close to the semiconductor surface, it is particularly highly sensitive to optical wavelengths in the visible part of the electromagnetic spectrum but also certain fractions of charge carriers generated by long wavelength radiation, e.g. near infrared light, are collected by this layer. The lateral charge transport through this doping layer can efficiently be realized for example by photo-transparent gates  212 ,  214 ,  216 ,  218  on top of a silicon oxide insulating layer  220 , so that the potential distribution applied to the gates  212 ,  214 ,  216 ,  218  is mirrored into the doping layer  210 . This technique is well-known from CCD devices. Depending on the particular gate arrangement, the speed of the lateral charge transport can be optimized. Embodiments for highest transport speeds are disclosed in patent [BUE05A]. A successive gate arrangement is shown. The idea of this embodiment is that the gate widths are small and increasing voltages are applied to the gates from left to right. Due to the small gate sizes and the buried channel implantation, the voltage distribution mirrored to the channel is smeared. Finally, an almost linearly increasing potential distribution in the channel is obtained, which is essential for a fast lateral charge transport. The implementation of the buried channels in general renders the electrical field deeper in the silicon, but for sensing and demodulating light in the near-infrared wavelength, the penetration depth of the depletion region in the substrate is still shorter than desired generally. 
         [0035]    A second mechanism relies on a current  230  of majority carriers that is generated through the semiconductor substrate  101  by applying a voltage difference between nodes or p implantations  236  and  238  and thus across the pixel sensitive area  102 . The current  230  flowing from p implantation  236  to p implantation  238  generates an electric field, which forces photo-generated minority charge carriers to drift into lateral direction. Here, photo-generated charge carriers are affected that are generated deeper in the substrate than the depth of the depletion region  210  of the shallow doping layer. Thus, this additional lateral force becomes more effective for electromagnetic radiation of longer wavelengths. 
         [0036]    If the substrate  101  is p-doped as shown, the minority carriers are electrons. Since the n+ diffusion areas are set to even higher potential than the p+ regions, the electrons will be collected in the n+ diffusion nodes  232 ,  234 . The diffusion nodes are preferably used as a sense node for accumulation of all photo-charges. Alternatively, instead of using diffusion nodes, a gate set to high potential is used in other examples as well so that the storage of the charge or even subsequent signal processing is accomplished in gate regions. 
         [0037]    A third mechanism provides even higher photo-sensitivity and vertical transfer speed in some instances by exploiting graded or gradually doped wafer types. In the easiest case of the EPI layer on top of the higher doped bulk wafer  105 , a junction at the interface occurs. Such a junction is called high-low junction. The principle of high-low junctions is described in [SIN77] Amitabha Sinha and S. K. Chattopadhyaya, “Effect of back surface field on photocurrent in a semiconductor junction”, Solid-State Electronics, Vol. 21, pp. 943-951, 1977; [SIN78] Amitabha Sinha and S. K. Chattopadhyaya, “Effect of Heavy Doping on the Properties of High-Low Junction”, IEEE Transactions on Electron Devices, Vol. Ed-25, No. 12, December 1978. In this new pixel architecture, the high-low junction at the EPI-bulk interface  106  is exploited for collecting additional photo-generated charge carriers that are deeply generated in the semiconductor. Those charge carriers are directly fed toward the surface and into the region of lateral drift fields so that the photo-current signal originating from deeply inside the semiconductor contributes to the global signal detection as well. 
         [0038]    In the following the main benefits that can be obtained by embodiments of the invention are summarized: 
         [0039]    a) High sensitivity due to the collection of charge carriers deeply generated in the silicon substrate. 
         [0040]    b) High optical sensitivity due to the collection of charges generated deeply in the silicon and transported to the sense node by deep lateral drift fields. 
         [0041]    c) Fast charge transport through the whole device due to strong lateral drift fields at the Si—SiO2 interface, in the highly optically sensitive buried-channel region and deep in the semiconductor. 
         [0042]    The combination of all these three items enables the realization of highly-sensitive and fast pixel devices that allow for resolving optical time signals with an accuracy by far less than a nanosecond and with an optical sensitivity at the same time, which is beyond those of prior-art demodulation pixels. 
         [0043]    A few examples of the invention are discussed below. They, however, do not restrict the invention from the generality of the pixel concept, which comprises an optimized pixel architecture in terms of photo-sensitivity and charge transport speed obtained by the combination of one or all of the mechanisms: 1) the enhanced charge transport through a buried channel, 2) the lateral enhanced transport deeply through the semiconductor, and 3) the deep photo-charge collection by vertical drift fields. 
         [0044]      FIG. 3  shows an example for a demodulation pixel with separated detection region  110  and demodulation region  112 . A series of gates  110   a ,  110   b ,  110   c ,  110   d  are used to generate a static drift field in the detection region  110  to transport electrons to the demodulation region  112 . This general architecture is shown in [BUE04], which is incorporated herein by this reference in its entirety. 
         [0045]    The funnel-shaped area  210  shows the extent of the buried channel and the n+ diffusion area. The majority current flows from p implantation  236  to p implantation  238   
         [0046]      FIG. 4  shows a cross section through the detection region  110  of  FIG. 3 . 
         [0047]    The creation of the lateral drift field  108  close to the semiconductor interface is achieved by the buried channel  210  formed by the n implantation and the arrangement of a successive gate structure  110   a ,  110   b ,  110   c ,  110   d  on top of the semiconductor and the insulator layer  220 , which is typically silicon dioxide. By applying increasing voltages on the gates  110   a ,  110   b ,  110   c ,  110   d  in the direction of the demodulation region  112 , a monotonously increasing potential distribution inside the buried channel region  210  is obtained as well. 
         [0048]      FIG. 5  a cross section through the detection region  110  of  FIG. 3  showing another embodiment. The creation of the drift field in the n doped implantation layer of  FIG. 5  is accomplished by a single high resistive gate  110   r  on top of the insulator layer  220 . The current flowing through the gate  110   r  when a potential difference is applied to the two ends of it via contacts  240 ,  242  generates a constant drift field  108 , which is seen in the buried channel  210  due to the capacitive coupling between the gate and the semiconductor substrate. This concept of drift field generation has been published in [SEI02], but here we enhance the pixel&#39;s sensitivity and speed even more by combining the prior-art drift field technique with the additional drift field concepts. 
         [0049]      FIG. 6  shows another example for the creation of the drift field in a cross section through the detection region  110  of  FIG. 3 . It uses a continuous change of the doping concentration of the n implantation  210  in a lateral direction and in the direction of the demodulation region. This means a change of the built-in voltage, so that the lateral electric field does not necessarily require the application of external voltages. Such a variation of the doping concentration is achieved by designing for example several layers of step-wise increasing doping concentrations. The process-inherent temperature diffusion steps are exploited for smearing the rather step-wise doping profile to a profile of more constant gradient. This technique assumes a certain flexibility of the process meaning that several doping profiles are available. Another possibility for the creation of a constant doping profile&#39;s gradient is the exploitation of gray-scale mask lithography as described in [HEN94] W. Henke et al, “Simulation and experimental study of gray-tone lithography for the fabrication of arbitrarily shaped surfaces”, IEEE Workshop on Micro Electro Mechanical Systems, 1994, MEMS &#39;94, pp. 205-210. 
         [0050]      FIG. 7  shows an example where just two of the charge transport mechanisms are used within the detection region  110  of  FIG. 3 . There is no buried layer anymore. The charge transport is only realized by the static lateral drift field generated by the majority carrier current  230  and the deeply generated electrons are collected by the vertical drift field components around interface  106 . 
         [0051]      FIG. 8  generated the drift field  108  using the capacitive coupling between two gates  110   a ,  110   b  through the detection region  110  of  FIG. 3 . Just two gates  110   a ,  110   b  are formed on top of the insulating layer  220  and separated by an arbitrary distance, which just needs to fulfill at least the minimum design rules. Since the buried implantation  210  fully depletes, the capacitive coupling between the gates  110   a ,  110   b  is exploited for generating potential distributions that are advantageous for a fast charge transport. 
         [0052]      FIG. 9  illustrates the 3D TOF camera system using the invented pixel architecture. The light emitter  20  produces modulated light  11  illuminates the scene  30 . The returning light  12  from the scene  30  is collected by the camera lens  40 , which generally includes a bandpass filter so that only light at the wavelength emitted by the light emitter  20  is transmitted. An image is formed on the TOF detector chip  90  which is a two dimensional array of pixels  100 . Control electronics  60 , which might be integrated on the same chip as the imager  90 , coordinate the modulation of the light emitter  10  with the sampling of the TOF detector chip  90 . This results in synchronous demodulation. A data output interface  70  is then able to reconstruct the 3-D image representation using the samples generated by the chip  90  such that a range to the scene is produced for each of the pixels  100  of the detector chip  90 . 
         [0053]    Typically the demodulation pixel is aimed for being used in 1D or 2D pixel arrays. The invention itself does not set any limitation to the total pixel count of the sensor. Standard or special dedicated read out schemes in the analogue or digital domain can be applied. The invention concentrates on the in-pixel charge transport properties according to the descriptions above, and it is independent on the dedicated sensor topology. 
         [0054]    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.