Abstract:
A high-speed, high-sensitivity demodulation sensor usable for e.g. time-of-flight application uses a back side illuminated (BSI) image sensor chip, in which the photo-generated charges are first transferred to a demodulation area, from which the charges are then sampled and stored on at least one specific storage node. The storage node is electrically isolated from the sensitive area. Such a pixel might find its use specifically in 3D time-of-flight imaging given its improvements in sensitivity because the presented invention allows to design pixel with up to 100% fill factor and enables charge detection even if the charge generation by the photon occurs deep in the silicon substrate.

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 61/228,198 filed on Jul. 24, 2009, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Three Dimensional (3D) time-of-flight (TOF) cameras are capable of getting range information for all pixels of the TOF detector in real-time. TOF cameras typically include a light source for illuminating the scene with modulated light, usually light emitting diodes (LEDs) or laser diodes, an optical lens system to form an image of the scene on an image sensor, and the image sensor comprising an array of high-speed demodulation pixels. 
     Other TOF approaches might use single photon avalanche diodes (SPAD) to detect the time of flight. SPADs measure the travel time of the light from the emitter to the object and back to the camera directly, whereas demodulation pixels are measuring the back-reflected phase information or correlation information representing a mean of the measured distance. Hence, the demodulation pixels measure the distance indirectly. 
     The light sources of TOF cameras usually operate in the near-infra red light spectra, most commonly between 800 and 900 nanometers (nm), which is outside the visible range. Operating in this range is mainly to avoid bright, visible light bothering people in the surroundings of the camera. 
     General system setups based on demodulation pixels and full 3D TOF camera implementation has been demonstrated in [LAN01] and [OGG04] as well as a description of several applications in [OGG05]. Further, the SPAD-based system approach has been presented in [NIC03]. 
     Demodulation pixels in image sensors have the function to sample and accumulate the charge carriers generated by the incoming photons. During or after this integration time, the collected charge is then converted into a voltage signal, which is then read out. 
     Demodulation pixels allow sorting the arriving photons or the respective generated charge according to their arrival time. For that they need at least one fast switching element, and, connected to it, at least one dedicated storage or integration region or integration node, called tap. In these storage nodes, the charge corresponding to the time interval of the open switch is stored and is then read out. Based on these samples, the phase or distance is deduced. 
     One of the main challenges of 3D TOF pixels is to collect and store quickly as many photo-generated electrons as possible in the storage nodes. A common issue is that near-infra red (NIR) photons penetrate deeper in the substrate compared to photons in the visible spectra. Nevertheless, the electrons that are generated in the substrate by the NIR photons still need to be detected and collected and transferred quickly to corresponding storage nodes. 
     In recent years, the pixel pitch of standard pixel has been shrinking while the stack of layers on top of the photo-sensitive area have been increasing. This creates the challenge to bring the photons impinging on the pixel through all the stack layers into the photo-sensitive area. The efficiency of converting the photon falling onto a pixel into electrons is drastically lowered by adding more layers on top of the substrate. 
     A state-of-the-art approach to reduce the problem is the use of micro-lenses to improve the conversion rate from photons impinging on the pixel to electrons. However, the use of micro-lenses has limitations as soon as the layer stack gets too big in relation to the pixel pitch. 
     Micro-lenses combined with in-pixel lightguides further improve the efficiency. The so-called lightpipes direct all photons through the stack onto the sensitive area [GAM09], [AGA09]. 
     Recent developments propose to backthin the substrate and to illuminate the pixel from the backside [RHO09]. This approach results in most designs having a fill factor of up to 100%, since no electronics is shadowing the photo-sensitive are from the impinging photons. The layer stack on top of the substrate can be increased in that case without losing sensitivity. 
     As in all solid-state image sensors, the incoming photons generate charge carriers, meaning electron-hole pairs within the substrate. These pairs have to be separated by an electric field, e.g. a p+/n+ doped junction. After the separation one type of charge carriers (normally the electron) is stored in a storage element, e.g. a capacitance, whereas the other type of charge carriers is absorbed by the substrate. In image sensors providing a visual representation of the surrounding, the separation is often slow, as the information is stored just in the amount of the generated charge carriers, giving a gray scale or, with the help of color filters, a color image. 
     In demodulation pixels, e.g. used for time-of-flight applications, the amount of generated charge carriers is not the main information one is interested in. Instead, the arriving time has to be measured. For this, the separation stage must distinguish the charge carriers depending on their arrival time. Therefore the collection of the charge carriers has to be fast. For this, drift fields are necessary to transfer the carriers to the point through the demodulation region into the corresponding storage node. 
     The first demodulation pixels, which are mainly dominated by electron diffusion transfer, have been described in [SPI99]. Based on this approach, several prototypes have been built [LAN01], [KAW06], [KAU04]. A similar demodulation principle has been described in [SCH99]. 
     However, in order to improve 3D TOF imaging based on demodulation pixels, a higher electron transfer speed from the sensitive area to the storage nodes is essential if not even required. High speed electron transfer is mainly achieved by applying electric fields across the sensitive area to the storage node. Demodulation pixels based on pure drift transfer have been described in [SEI02], [BUE05B], [NIE05]. 
     All these methods perform demodulation directly in the photo-sensitive area. A new approach separating the sensitive area from the demodulation area but keeping a drift-dominated electron transfer is described in [BUE05A]. 
     SUMMARY OF THE INVENTION 
     The novel 3D TOF technology presented herein addresses several performance issues from which state-of-the-art 3D TOF pixel suffer from, namely the pixel fill factor and the quantum efficiency, both contributing to a higher pixel sensitivity. 
     The invention disclosed herein provides an enhancement of prior-art demodulation pixels in terms of sensitivity and depending on the implementation, in terms of speed. 
     The sensitivity is increased by the fill factor, which can reach up to 100%, and the capability of capturing photo-generated charges deep in the silicon, hence increasing quantum efficiency (QE) for long wavelengths. 
     In general according to one aspect, the invention features a demodulation sensor with back side illumination comprising an array of pixels, usually a two dimensional array. Each of the pixels comprises a photo-sensitive area for receiving photons which are transferred to a demodulation area, a switching element that directs said electrons from said demodulation area to alternative storage areas, and a charge barrier to prevent said electrons from flowing from said sensitive area to said storage areas. 
     In general according to one aspect, the invention features a demodulation sensor with back side illumination comprising an array of pixels, such as a two dimensional array. Each of the pixels comprises a photo-sensitive area for receiving photons which are transferred to a demodulation area, a switching element that directs said electrons from said demodulation area to alternative storage areas, and a lens to direct said photons toward the demodulation area. 
     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 
       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: 
         FIG. 1  is a diagram illustrating a front side illuminated pixel with a low fill factor. 
         FIG. 2  is a diagram illustrating a back side illuminated pixel with an increased fill factor. 
         FIG. 3  is a diagram that illustrates an embodiment of the invention based on two demodulation pixels. 
         FIG. 4  is a diagram illustrating an embodiment of the invention for a back side illuminated demodulation pixel. 
         FIG. 5  is a diagram illustrating an embodiment of the invention for a back side illuminated demodulation pixel without integration gates. 
         FIG. 6  is a diagram illustrating an embodiment that includes micro lenses to better focus the light on the sensitive area. 
         FIG. 7  is a diagram illustrating the back side illuminated pixel with post-processing performed on the sample values. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A drawback of state-of-the-art TOF pixels is that the pixels  101 ,  102  suffer from quite a low fill factor. This is mainly due to the electrical circuits required in the TOF pixel. A front side illuminated (FSI) sensor  100  with such a low fill factor is illustrated in  FIG. 1 . The metal layers  104  cover part of the pixel area and hinder the arriving photons  110  to reach the sensitive areas  101 ,  102 . 
       FIG. 2  illustrates a back side illuminated (BSI) senor  200 , which has increased fill factor since the effective are of the sensitive areas  101  and  102  is larger. 
     A second drawback of 3D TOF pixel is the aforementioned near-infra red (NIR) photon penetration in the silicon. While the demodulation generally takes place close to the front side of the silicon surface, the electrons need to be first of all detected deeply in the silicon and secondly transferred quickly to the corresponding storage node. 
     In FSI imager sensors, the depletion region usually reaches depths into the silicon of a few microns, depending on applied voltages and used implants. In BSI pixels however, the substrate  103  is preferably backthinned to e.g. less than 100 micrometers, and preferably to less than 50 micrometers and less than about 20 micrometers in some embodiments. Therefore even electrons generated deeply in the substrate  103  can be detected. Therefore, a backthinned BSI pixel becomes ideal for 3D TOF system working with NIR illumination. 
     As shown in  FIG. 3 , an issue using BSI process for 3D TOF represents the storage of the electrons. While in standard intensity the imager sensitivity node is directly used as storage node, and 3D TOF pixels require clear physical separation of the photo-sensitive area and the storage nodes. On a BSI process, different implants are used to generate built-in drift fields that transfer the electrons generated in the substrate  103  towards the demodulation area  112  and only from there to the storage nodes  114 . So, all photo-generated electrons have to pass the demodulation stage to finally get transferred into the appropriate storage node  114 . Therefore, a barrier  116  has to be built to avoid leakage of electrons from the sensitive area  108  to the storage nodes  114 . 
       FIG. 3  illustrates an exemplary embodiment of the inventive sensor  200  showing two pixels  101 ,  102 . The device is illuminated from the back side. In the sensitive area  108 , the impinging photons  110  generate electron-hole pairs. The electrons are transferred from the sensitive area  108  into the demodulation region  112 . From there, the electrons are then directed to one of the storage nodes  114 . The storage nodes  114  themselves are isolated from the sensitive area  108  by a charge barrier  116 . That is, no charge carrier can flow from the sensitive area  108  into the storage sites  114 . 
     The storage areas  114  have to be designed carefully. By the electron barrier  116 , one can almost perfectly avoid the electron diffusion from the sensitive area  108  into the storage node  114 , however, the generation of photo-electrons directly in the storage  114  node has to be minimized by design. 
     One option to reduce photo-electron generation in the storage node  114  is to use micro lenses  120 ,  122 . The micro-lenses  120 ,  122  focus the light beam on the central sensitive area  108  of the pixel  102 . Hence, photons getting into the storage area  114  are minimized or even completely avoided. 
       FIG. 4  illustrates is an embodiment of the present invention based on electrons as charge carriers. However, the invention is not restricted to that. An embodiment illustrated in  FIG. 4  shows built-in drift fields that transfer the electrons into the demodulation region  118 , which is above transfer gate  1  (TG 1 ) and transfer gate  2  (TG 2 ). The barrier is created from a deep n-well  210  and shallow p-wells  212  over the storage nodes under integration gates Intg 1 , Intg 2 . This prevents charges from flowing from the sensitive area directly into the storage nodes, which are integration gate  1  (Intg 1 ) and integration gate  2  (Intg 2 ). The out-gates (Outg 1 , Outg 2 ) serve as a barrier between the sense node diffusions (Sens 1 , Sens 2 ) and the integration gates Intg 1 , Intg 2 . In the illustrated diagram, switching TG 1  between 0V and 5V and TG 2  inversely from 5V and 0V, the electrons arriving in the demodulation region  118  are steered into either the storage node  1  (Intg 1 ) or the storage node  2  (Intg 2 ). Therefore, switching TG 1  and TG 2  enables high speed sampling of the photo-generated electrons. 
       FIG. 5  illustrates an alternative embodiment, where the charges are directly transferred from the demodulation region into two diffusion nodes Sens 1 , Sens 2 . The demodulation is still done on transfer gates TG 1 , TG 2 . As before, the deep n-well  210  and p-wells  212  prevent charge from directly entering the diffusion nodes Sens 1 , Sens 2 . 
       FIG. 6  illustrates an alternative embodiment that includes micro lenses  120 ,  122  attached or formed on the backside of the substrate of the sensor  200 . As illustrated in  FIG. 6 , the light rays are now better deflected to the sensitive area  108 . This reduces the probability that “photo-charges” are directly generated in the storage sites  114 , before passing through the demodulation stage  112 . 
       FIG. 7  illustrates an alternative embodiment wherein the samples of the pixel are post-processed before read out. Typically post-processing  124  includes subtracting the charges/voltages from two nodes or amplifying the output to increase the dynamic range, i.e. reduction of noise or increase of saturation level. However, in alternative embodiments other methods of post processing are performed as well. 
     The invention described above shows a demodulation pixel implemented on a back side illumination (BSI) technology. The pixel is processed in a BSI technology and includes, a photo-sensitive area, where electron hole pairs are generated, electrical drift fields driving the electrons from the sensitive area to the demodulation region, a demodulation region, where the electrons are sampled and transferred to at least one storage and/or dump node, at least one storage node, which is electrically isolated from the sensitive area and will store the sampled electrons. 
     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. 
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