Abstract:
The present disclosure describes quantum dot film based demodulation structures and optical ranging systems including an array of QDF demodulation structures.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of priority U.S. Application No. 62/289,726, filed on Feb. 1, 2016 and U.S. Application No. 62/300,294, filed on Feb. 26, 2016. The disclosure of the prior applications is incorporated herein by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to quantum dot film based demodulation structures. 
       BACKGROUND 
       [0003]    Optical ranging systems can include an illumination module and an array of  3 D pixels. For example, an optical ranging system can be implemented as a time-of-flight-based system. Accordingly, an illumination module in such a system can be operable to generate modulated light. The modulated light can be incident on a scene including one or more objects. Some of the modulated light directed onto the object or objects can be reflected back to an array of demodulation pixels. In such instances, a comparison of the phase of the light first generated by (emitted from) the illumination module and the phase of the light reflected back from an object to the array of demodulation pixels can be used to determine the distance to the object. 
       SUMMARY 
       [0004]    Time of flight (TOF) pixels and other demodulation structures that include quantum dot film (QDF) elements are described. In some implementations, the demodulation structures facilitate detection of long wavelength photons with relatively high efficiency (i.e., better than silicon). Further, in some cases, the structures can facilitate charge transport at relatively high speeds (e.g., 1 ns). 
         [0005]    In general, the structures can be implemented as one or more QDF photo detection elements on a CMOS semiconductor wafer. The QDF functions to convert photons into photo-generated charge (electrons and holes). The demodulation area, including high-speed switches, and charge storage regions, as well as read-out and reset circuitry, can be implemented in the CMOS wafer. The QDF can be embedded between top and bottom transparent metal contacts (e.g., ITO) that serve as electrodes. The electrodes provide electrical contact between the QDF and the CMOS wafer, and also create an electric field within the QDF to separate the photo-generated electrons and holes when a voltage is applied across the QDF. In some implementations, providing a 1-3 voltage across the QDF can result in a high speed (&gt;1 ns) charge transport. 
         [0006]    The disclosure also describes optical ranging systems that include an illumination module and an array of QDF demodulation structures. The illumination module is operable to generate modulated light to be incident on a scene including one or more objects such that some of the modulated light incident on the one or more objects is reflected back to the array of QDF demodulation structures. 
         [0007]    The disclosure also describes a pinned photodiode structure that can be used, for example, in combination with a QFD photo detection element. 
         [0008]    Other aspects, features and advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates an example of a QDF-based demodulation structure. 
           [0010]      FIG. 2  illustrates another example of a QDF-based demodulation structure using a pinned photodiode. 
           [0011]      FIGS. 3A, 3B and 3C  illustrate a structure for a pinned photodiode. 
           [0012]      FIG. 4  illustrates an example of a 2-tap QDF-based demodulation structure using a charge amplifier. 
           [0013]      FIG. 5  illustrates an example of a 4-tap QDF-based demodulation structure using charge amplifiers. 
           [0014]      FIG. 6  illustrates an example of a QDF-based demodulation structure for detecting and sampling photo generated electrons and holes. 
           [0015]      FIG. 7  illustrates an example of a QDF-based demodulation structure using a single ended charge amplifier. 
           [0016]      FIG. 8  illustrates an example of a 4-tap QDF-based demodulation structure. 
           [0017]      FIG. 9  illustrates an example of a QDF-based demodulation structure including multiple QDF photo detection elements. 
           [0018]      FIG. 10  illustrates a side view of the grid structure for the QDF photo detection elements. 
           [0019]      FIG. 11  illustrates another example of a QDF-based demodulation structure including multiple QDF photo detection elements. 
           [0020]      FIG. 12  illustrates a further example of a QDF-based demodulation structure including multiple QDF photo detection elements. 
           [0021]      FIGS. 13A and 13B  depict, respectively, plane and side views of a grid structure for a QDF. 
           [0022]      FIGS. 13C, 13D, 13E and 13F  depict a demodulation pixel structure operable as a two-tap demodulation pixel or as a four-tap demodulation pixel. 
           [0023]      FIG. 13G  is a timing diagram associated with  FIGS. 13C-13F . 
           [0024]      FIGS. 14A and 14B  illustrate examples of QDF-based demodulation structures including multiple QDF photo detection elements arranged one over the other. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  illustrates an example of a demodulation pixel  100 A that includes a QDF  102  having a top electrode  104  and a bottom electrode  106 . The QDF  102  serves as a photo detecting element. The bottom electrode  106  is connected to a capacitor (implemented, e.g., as a MIM transistor)  111  as well as a reset switch  108  and serves as a charge collection node. In some implementations, the top electrode  104  is set to a specified voltage (e.g., 5 V) and the bottom side of the capacitor  111  is modulated with an external drive signal (e.g., 0-5 V). Applying the modulation signal modulates the node  106 , thereby activating and deactivating the QDF  102 . The modulation frequency can depend on the implementation; frequencies of 1 MHz or higher can be used to enable high-speed operation. In some cases, modulation frequencies (and switching frequencies) of hundreds of MHZ can be used. Photo charge generated by the QDF  102  is stored by the capacitor  111 . Readout circuitry can be coupled to node  110 . Such readout circuitry can include, for example, a source follower, a select transistor and an off-pixel column-wise current source. Multiple pixels can be arranged in an array having row and column select circuitry for readout. 
         [0026]      FIG. 2  illustrates another example that includes a pinned photodiode (PPD)  300  that creates a fixed potential well to which photo generated charges are fed from the QDF  102  via the QDF&#39;s bottom electrode  106 . Switching is performed with respect to the charges stored by the PPD  300 . In particular, the charges are transferred, alternately, to one of two (or more) storage nodes  114 ,  118  associated with respective capacitors  111  by applying a modulation signal to close/open first high-speed switches  113 . The circuitry includes reset switches  116 ,  120 . Readout circuitry can be coupled to nodes  117 ,  122 . Although  FIG. 1B  shows two storage nodes  114 ,  118  and associated storage capacitors  111 , other implementations can include more than two storage nodes and associated storage capacitors so as to provide an n-tap structure (i.e., n≧3). 
         [0027]      FIGS. 3A, 3B and 3C  illustrate an example of a structure for the PPD  300 , which can be used for charge transfer and which provides connection to the QDF. In this example, a semiconductor region  202  having a first-type dopant (e.g., P+ doped) is disposed over a region  208  having second-type dopant (e.g., N− doped) opposite that of the first-type dopant. Storage nodes  206  of the second-type dopant (e.g., N+) also are provided in the bulk polysilicon  204 . Polysilicon gates  210  correspond to the switches  113  in  FIG. 2 . An additional node  212  of the second-type dopant (e.g., N+) in the bulk polysilicon  204  below the region  208  provides direct electrical connection to the bottom electrode  106  of the QDF  102  (see  FIG. 2 ). 
         [0028]    As shown in the illustrated example of  FIGS. 3A-3C , the N− doped region  208  can extend laterally, along a first axis, beyond the boundary of the P+ region  202  toward the N+ storage nodes  206 . The N− doped region  208  also can extend laterally somewhat, along a second axis perpendicular to the first axis, beyond the boundary of the P+ region  202 . As further shown in the example of  FIGS. 3A-3C , the node  212  can extend laterally, along the second axis, beyond the boundary of the N region  208 . In other implementations, the N and P doped regions can be reversed for the PPD structure of  FIGS. 3A-3C . 
         [0029]    In some implementations, the PPD  300  includes a drift field and/or a dump gate. The dump gate can be useful so that photo charge generated during a readout period is not collected. In some cases, a MOS gate can be used instead of the PPD  300 . 
         [0030]    One issue that can arise in the implementation of  FIG. 1  is that as node  106  is driven, it accumulates charge, thereby causing the potential across the QDF to vary.  FIG. 4  illustrates an example that can help hold the potential at the bottom electrode of the QDF substantially constant. As shown in  FIG. 4 , the bottom electrode of the QDF  402  is connected, by way of switches  404 ,  406 , to positive (+) and negative (−) terminals of an charge amplifier  420 . The input terminals of the charge amplifier  420  are held at the same potential, and modulation signals are applied to the switches  404 ,  406  such that when one switch is open, the other switch is closed. Charges are accumulated and stored by the respective capacitors depending on which switch  404 ,  406  is closed. Differential output of the two samples (2-tap) are provided at nodes  412 ,  414 . Reset switches are indicated by  408 ,  410 . The pixel is reset by closing both switches  408 ,  410 . 
         [0031]    The structure of  FIG. 4  can be expanded to provided, for example, a 4-tap pixel by adding a second charge amplifier  420  and associated circuitry as shown in  FIG. 5 . The signal from the QDF photo detector  402  is demodulated by 0 and 90 degrees, respectively, such that two times the charge amplifiers are provided in one pixel. Thus, one pixel has two differential outputs. 
         [0032]      FIG. 6  illustrates an example in which the both photo generated electrons and holes can be detected and sampled. This structure allows information to be acquired at close to 100% of the time, rather than only 50% of the time (e.g., as in the structure of  FIG. 1A  where the QDF is activated only 50 of the time). For example, the modulation signal can be applied to the top electrode  104  and is toggled between Vlow and the voltage (V 106 ) at node  106 , where Vlow&lt;V 106 . Such operation results in 1-tap pixel operation in which only electrons are collected. Instead, however, the modulation signal can be toggled between Vlow and Vhigh, where Vlow&lt;V 106  and where Vhigh&gt;V 106 . Such operation results in differential pixel operation in which the signal on the capacitor represents the difference between two samples that are separated by 180 degrees (assuming the duty cycle of the modulation signal is 50%). Collection of electrons and holes on the capacitor leads to charge-domain subtraction of unmodulated light (e.g., background light or ambient light). If the potential on the top and bottom electrodes  104 ,  106  is the same, then the QDF is in an inactive state. 
         [0033]      FIGS. 7 and 8  illustrate examples of single ended charge amplifiers. In particular, the 2-tap structure of  FIG. 7  delivers a sample that represents the difference between the electron and hole photocurrents. The bottom electrode of the QDF  402  is coupled to the positive (+) terminal of the charge amplifier, and thus the potential of the bottom electrode is well controlled (i.e., is relatively stable). The modulation signal is applied to the negative terminal (−) of the charge amplifier. The modulation signal toggles between two voltages: a lower level that is below the QDF top electrode voltage, and a higher level that is greater than QDF top electrode voltage. This structure can provide inherent background suppression by collecting both electrons and holes. The amplifier should support high speed of the modulation signal. 
         [0034]    The structure of  FIG. 7  can be expanded, as shown in  FIG. 8 , to provide a 4-tap implementation. In this case, two QDF detectors  402  are disposed in close proximity to each other so that the QE modulation becomes effective. The pixel has two QDF photo detectors and two charge amplifiers. One section receives 0-degree modulation, whereas the other section receives 180-degree modulation. 
         [0035]    In any of the foregoing examples, each QDF can be implemented as a single QDF photo detection element, or as a grid or array of QDF photo detection elements which allows at least one QDF element to be active at all times during operation. An example is shown in  FIG. 9 , in which the bottom electrodes of a first group of QDF photo detection elements  502 A are coupled together and connected to a first capacitor. The bottom electrodes of a second group of QDF photo detection elements  502 B are coupled together and connected to a second capacitor. Modulation signals separated by 180 degrees are applied, respectively, to switches  504  such that each group of QDF photo detection elements is activated about 50% of the time.  FIG. 10  illustrates a side view of the grid structure for the QDF. 
         [0036]      FIG. 11  illustrates an example incorporating a grid of QDF photo detection elements  502  each of which is coupled, respectively, to one of two PPDs  300 . The bottom electrodes of a first group of QDF photo detection elements are coupled together and connected to a first one of the PPDs  300 . The bottom electrodes of a second group of QDF photo detection elements are coupled together and connected to a second one of the PPDs  300 . Modulation signals separated by 180 degrees are applied, respectively, to switches  504  such that each group of QDF photo detection elements is activated about 50% of the time. 
         [0037]    In some implementations, instead of PPDs  300 , transistors  620  having their respective gates at the same bias potential as one another can be used, as shown in  FIG. 12 . Modulation signals separated by 180 degrees are applied, respectively, to switches  504 ,  506  such that each group of QDF photo detection elements is activated about 50% of the time. When the switches are closed, charge is stored on the storage nodes associated with the capacitors. The circuitry includes reset switches (e.g.,  616 ,  620 ). Readout circuitry can be coupled to nodes  612 ,  614 . 
         [0038]      FIGS. 13A and 13B  depict, respectively, plane and side views of a grid structure for the QDF. Each of  FIGS. 13C, 13D, 13E and 13F  depicts a single demodulation pixel that has two taps and can be operated as a two-tap demodulation pixel or as a four-tap demodulation pixel. The latter, however, is a virtual 4-tap pixel; that is, there are physically 2-taps, but it is operated as a 4-tap pixel. This implementation is described in greater detail below. The virtual 4-tap implementation is possible because both electrons and holes contribute to the signal (from the quantum dot film).  FIG. 13B  shows capacitors  807  attached to the bottom electrodes  806 . Other electronic components suitable for collecting charges/converting charges to signals (e.g., charge amplifiers) can be used instead of the capacitors  807 . 
         [0039]      FIGS. 13C-13F  depict the virtual 4-tap demodulation pixel at four instances in time over an integration time (e.g., corresponding to 0°, 90°, 180°, and 270° phase collection) with example voltages applied to the top, bottom left, and bottom right electrodes. Modulated light is incident on the pixel in  FIGS. 13C-13F . A timing diagram corresponding to  FIGS. 13C-13F  is shown in  FIG. 13G . 
         [0040]    In  FIG. 13C , the applied voltages direct electrons to the bottom right electrode where charge accumulates in a corresponding node (e.g., a capacitor or charge amplifier) where the electron charge is associated with the 0° phase. 
         [0041]    In  FIG. 13D , the applied voltages direct electrons to the bottom left electrode where charge accumulates in a corresponding node (e.g., a capacitor or charge amplifier) where the electron charge is associated with the 90° phase. 
         [0042]    In  FIG. 13E , the applied voltages direct holes to the bottom right electrode where charge accumulates in a corresponding node (e.g., a capacitor or charge amplifier) where the hole charge is associated with the 180° phase. 
         [0043]    In  FIG. 13F , the applied voltages direct holes to the bottom left electrode where charge accumulates in a corresponding node (e.g., a capacitor or charge amplifier) where the hole charge is associated with the 270° phase. 
         [0044]    At the end of the integration time, charge (signal) from each node is read out. Since holes have the inverse electric charge of electrons, and since electrons associated with the 0° phase and holes associated with 180° phase are collected at the same node, the two charges effectively cancel each other. The charge that remains (i.e., the signal amplitude (“A”)) at that node is equal to 0°-180°. This is the numerator in the TOF equation below: 
         [0000]    
       
         
           
             ϕ 
             = 
             
               arctan 
                
               
                 ( 
                 
                   
                     A0 
                     - 
                     A160 
                   
                   
                     A90 
                     - 
                     A270 
                   
                 
                 ) 
               
             
           
         
       
     
         [0045]    The foregoing also holds for the 90° and 270° phases. Accordingly, a 2-tap demodulation pixel can be operated as a virtual 4-tap demodulation pixel. 
         [0046]    In some implementations (e.g., when the hole mobility and electron mobility are not equal), the QDF demodulation structure can include additional switches and capacitors such that the QDF demodulation structure can be operable to suppress a common mode offset (e.g., background light). Examples of the arrangement of switches and capacitors are described, for example, in US Published Patent Application No. 2010/0308209, which is incorporated herein by reference. 
         [0047]      FIG. 14A  depicts a combined quantum dot film and TOF structure. In this example, a first quantum dot film  902 A is optimized for a particular wavelength, or range of wavelengths for an application other than TOF (e.g., to collect RGB information of a scene). The quantum dot film  902 A is isolated from a second quantum dot film  903 A that can be optimized for TOF applications (e.g., for infrared and/or near infrared light). The first quantum dot film  902 A need not have its own contacts (i.e., other than the contacts or nodes or electrodes connected to the second quantum dot film  903 A). In this example, both functions (e.g., TOF and collection of RGB data) can be performed simultaneously. 
         [0048]      FIG. 14B  depicts a single quantum dot film optimized for both TOF and another application (e.g., collection of RGB information of a scene). For example, the quantum dot film  902 B can include a multi-modal (e.g., bi-modal) distribution of quantum dot diameters. In this example, one size of quantum dot can be optimized for the collection of light having a first particular wavelength, and a second size quantum dot can be optimized for the collection of light having a second particular wavelength. The first particular wavelength can correspond, for example, to modulated light in the scene and incident on the pixel; accordingly, modulated light can be demodulated and converted into a signal, for example, as described above. After demodulation of the incident light having the first particular wavelength, incident light of a second particular wavelength (or range of wavelengths) can generate a signal. For example, the second particular wavelength can correspond to infrared wavelengths emitted by humans. Accordingly, such a pixel can be used for both TOF (distance measurements) and for passive infrared detection of human subjects. 
         [0049]    In some implementations, the quantum dot film includes quantum dots of different composition (e.g., a QDF composed of HgTe and PbSe). 
         [0050]    Any of the foregoing TOF pixels and other demodulation structures can be included as part of an optical ranging system that includes an illumination module. In particular, the system can include an array of QDF demodulation structures as described above. The illumination module in such a system is operable to generate modulated light that can be incident on a scene including one or more objects. The illumination module can be implemented, for example, one or more light emitting elements such as light emitting diodes (LEDs), infra-red (IR) LEDs, organic LEDs (OLEDs), infra-red (IR) lasers or vertical cavity surface emitting lasers (VCSELs)). Some of the modulated light directed onto the object(s) can be reflected back to the array of QDF demodulation structures. A comparison of the phase of the light first generated by (emitted from) the illumination module and the phase of the light reflected back from an object to the array of QDF demodulation structures can be used to determine the distance to the object. The comparison and distance determination can be performed, for example, in a microprocessor chip coupled to the read-out circuits for the demodulation and reference pixels. 
         [0051]    Various modifications can be made within the spirit of this disclosure and, in some cases, features described in different implementations can be combined in a single implementation. Accordingly, other implementations are within the scope of the claims.