Abstract:
A Single-Photon Avalanche Diode (SPAD) is disclosed. The SPAD may include an active region for detection of incident radiation, and a cover configured to shield part of the active region from the incident radiation. An array is also disclosed and includes SPADs arranged in rows and columns. A method for making the SPAD is also disclosed.

Description:
RELATED APPLICATION 
       [0001]    This application is based upon prior filed Application No. PCT/GB2014/051895, filed Jun. 20, 2014, the entire subject matter of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to Single-Photon Avalanche Diodes (SPADs), and, in particular to, SPADs usable in ambient light sensing and ranging applications. 
       BACKGROUND 
       [0003]    A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionization. The avalanche is quenched, either actively or passively to allow the device to be “reset” to detect further photons. The initiating charge carrier can be photo-electrically generated by way of a single incident photon striking the high field region. It is this feature which gives rise to the name “Single Photon Avalanche Diode”. This single photon detection mode of operation is often referred to as “Geiger Mode”. 
         [0004]    Single photon counting devices output response to incident light intensity becomes significantly non-linear as intensity increases. This may be problematic for many applications, such as ambient light sensing and ranging. It may be desirable to provide a SPAD and/or SPAD array with a better linear output response to illumination levels. 
       SUMMARY 
       [0005]    A SPAD is disclosed. The SPAD may include an active region for detection of incident radiation, and a cover configured to shield part of the active region from the incident radiation. An array is also disclosed and includes SPADs arranged in rows and columns. A method for making the SPAD is also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments of the present disclosure will now be described, by way of example only, by reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  shows a schematic top plan view of an array of SPADs, each SPAD having a different attenuation profile, according to the present disclosure. 
           [0008]      FIG. 2   a  is a diagram of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (dashed lines), against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with no attenuation, according to the present disclosure. 
           [0009]      FIG. 2   b  is a diagram of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (dashed lines), against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with 1/1.73x attenuation, according to the present disclosure. 
           [0010]      FIG. 2   c  is a diagram of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (dashed lines), against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with 1/117x attenuation, according to the present disclosure. 
           [0011]      FIG. 3   a  is a diagram of the non-linearity error against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with no attenuation, according to the present disclosure. 
           [0012]      FIG. 3   b  is a diagram of the non-linearity error against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with 1/1.73x attenuation, according to the present disclosure. 
           [0013]      FIG. 3   c  is a diagram of the non-linearity error against light intensity for unfiltered SPADs, and for SPADs having red, green and blue filters with 1/117x attenuation, according to the present disclosure. 
           [0014]      FIG. 4  is a diagram of count rate (minus dark rate count) against light intensity for SPADs attenuated with varying aperture sizes, according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An avalanche is triggered when reverse biasing a PN-junction to around the breakdown voltage. This effect can be used in two modes of operation. Commonly, the avalanche photodiodes are biased just below the breakdown voltage, the photocurrent remaining proportional to the incoming light intensity. Gain values of a few hundred are obtained in III-V semiconductors as well as in silicon. 
         [0016]    SPADs are solid-state photo detectors which utilize the fact that p-n diodes can be stable for a finite time above their breakdown voltage. When an incident photon with sufficient energy to liberate an electron arrives, avalanche multiplication of the photo-generated electron occurs due to the high electric field. This produces a measurable current pulse signaling the arrival of the photon which negates the need for amplification due to the internal gain of the device. 
         [0017]    Essentially SPADs are photodiodes that are biased above the breakdown voltage in the so-called Geiger mode. This mode of operation requires the introduction of a quenching mechanism to stop the avalanche process. Each incoming photon results in a strong current pulse of few nanoseconds duration. The device works as an optical Geiger counter. 
         [0018]    Quenching is required to stop the avalanche process, which is done by reducing the SPAD&#39;s reverse bias below its breakdown voltage. The simplest quenching circuit is commonly referred to as passive quenching. Usually, passive quenching is simply performed by providing a resistance in series to the SPAD. The avalanche current self-quenches simply because it develops a voltage drop across the resistance (a high-value ballast load), reducing the voltage across the SPAD to below its breakdown voltage. After the quenching of the avalanche current, the SPAD&#39;s bias slowly recovers to at or above the breakdown voltage and the detector is ready to be triggered again. 
         [0019]    An alternative to passive quenching is active quenching. There are a number of different active quenching arrangements, although in general active quenching refers to detection of a breakdown event by some subsequent digital logic connected to the SPAD output, and actively pulling the SPAD moving node to a voltage below breakdown, quenching the avalanche. Active quenching is desirable for several reasons, including a reduction in dead time, and improved photon counting rate at high light levels enabling a dynamic range extension. Active quenching is essential in many applications of SPAD technology. 
         [0020]    Dead time is the time interval after detection of a photon, during which it is quenched and returned to its active state. During this time, no photons are detected by the SPAD. While actively quenching the SPAD will bring it to its active state more quickly after a detection event than with passive quenching, under high light levels (with many detection events), the SPAD can still spend a large fraction of its time inactive. This can lead to saturation and reduction in the number of photons detected by the SPAD. The result of this is that the SPAD output does not track the light level linearly, which causes problems in some applications. By the nature of their design, SPADs have no gain control mechanism to address this problem. It is proposed therefore, to attenuate the SPAD to improve output linearity. 
         [0021]      FIG. 1  shows a SPAD array  100  coupled to logic circuitry  180 . Shown are nine SPADs  110   a - 110   i.  Each SPAD comprises a guard ring region  120 , a breakdown region  130 , an anode  140 , and a track area  150  for connection to the anode. Only photons incident on the breakdown region are detected. The breakdown region is shielded by the anode and the track area reducing its active area. Taking one specific, non-limiting, example, the SPAD may have an effective active area of 48.05 μm 2 . 
         [0022]    Attenuation may be achieved by further reducing the SPAD&#39;s active area. This may be done by covering more of the breakdown region with a non-transmissive cover, which acts to block any photons incident on the cover surface. One way of doing this is to cover part of the breakdown region with a metal layer when forming the metal track and anode. The metal layer may completely cover the active area, except for one or more apertures  160 . In this way, the only photons detected by the SPAD are those which pass through the aperture and onto the SPAD&#39;s active area. 
         [0023]    In  FIG. 1 , one SPAD  110   e  is shown uncovered as is conventional. The remaining SPADs are shown with their active areas covered, the covers having various aperture  160  configurations, thereby providing varying degrees of attenuation. The cover of SPAD  110   a  has two apertures  160  providing 1/1.73 area attenuation. SPADs  110   b,    110   c,    110   d ,  110   f,    110   g  all have covers with aperture  160  arrangements providing 1/7.3 attenuation. SPADs  110   b,    110   c,    110   d,    110   f  all have a single aperture located in a different quadrant of the active area of the SPAD, while SPAD  110   g  has four apertures that are each quarter of the size than that of SPADs  110   b,    110   c ,  110   d,    110   f.  SPAD  110   h  has four apertures, each of which provides an attenuation of 1/117, thereby providing a total attenuation of 1/29.3. SPAD  110   i  has only one aperture, which provides an attenuation of 1/117. This particular SPAD array arrangement is shown to illustrate the effect of different attenuation levels. 
         [0024]    The effect of the attenuation is to reduce the number of photons detected by the SPAD. In doing this, the SPAD does not need to be reset so often, and therefore the likelihood is reduced, of a photon arriving during a SPAD reset following a previous detection event. Consequently, fewer photons are missed by the SPAD, and linearity is maintained at higher incident light levels. This results in a more linear output response to light intensity. 
         [0025]      FIG. 2   a  is a plot of the number of photon arrivals in 50 ms (solid lines) and the number of photon counts recorded in 50 ms (dashed lines), against light intensity for clear c, red r, green g and blue b SPADs with no attenuation. The counts are given for a 50 ms worst case for flicker immunity. The plot shown is for 6500K CCT illumination. The plot shows significant non-linearity with a large number of missed photons even at 500 lux for the clear SPAD. Green, red and blue are all significantly affected above 1000 lux. In this example clear SPADs have no optical filter, whereas red, green and blue SPADs have optical filters which pass red, green and blue light respectively. Attenuation resultant from the optical filters is discounted when describing a SPAD as having no attenuation. 
         [0026]      FIG. 2   b  shows a similar graph to that of  FIG. 2   a , but for SPADs having covers providing 1/1.73x attenuation. It shows significantly reduced non-linearity compared to SPADs having no attenuation. The output is essentially linear up to 1000 lux for clear SPADs and up to 2500 lux for red/green/blue SPADs.  FIG. 2   c  shows that same graph for 1/117x attenuated SPADs. Non-linearity is greatly reduced for all channels in this case, with the plots for photon arrivals and photon counts largely overlapping, illustrating a linear response. 
         [0027]      FIGS. 3   a - 3   c  show a plot of the non-linearity error against light intensity for clear c, red r, green g and blue b SPADs, with no attenuation, 1/1.73x attenuation, and 1/117x attenuation respectively. It can be seen that 1/117x attenuation results in a non-linearity error of less than 1% (up to 5 klux) for red, green and blue SPADs; and a non-linearity error of less than 2% (up to 5 klux) for clear SPADs.  FIG. 4  shows a graph of count rate (corrected for the dark rate count) against light intensity for SPADs attenuated with varying aperture sizes. It can be seen that the area aperture attenuations match actual area reductions. 
         [0028]    In practical applications, the SPADs disclosed herein will be arranged in arrays. The attenuation levels chosen for particular arrays of SPADs will depend on the specific application, the likely illumination conditions and the acceptable level of non-linearity. For example, if the SPAD array is always likely to be used in situations where illumination levels are high, then an array of SPADs all having high attenuation (such as 1/117x attenuation) may be suitable. However, should illumination levels then fall, the detection rate may fall close to zero. 
         [0029]    Consequently, for many practical applications, an array of SPADs with different attenuation levels may be advantageous. This allows an acceptably linear output to be achievable in a wide range of light conditions. In one embodiment, an array may comprise banks of SPADs, one or more having no attenuation, one or more having lesser attenuation (for example 1/1.73x attenuation) and one or more having greater attenuation (for example 1/117x attenuation). In very low light levels (less than 500 lux as illustrated in  FIG. 2   a ), the unattenuated SPADs will provide a suitably linear response. At medium light levels, the output from the 1/1.73x attenuated SPADs should be selected. At the highest light levels, the outputs of the 1/117x attenuated SPADs should be selected. 
         [0030]    Suitable logic  180  may be provided to select the appropriate SPAD outputs depending on illumination conditions. There are a number of different ways such logic  180  may be implemented. For example, the outputs of particular banks (having particular attenuation levels) may be selected if their count rate is measured to be over a certain lower threshold level, or between upper and lower threshold levels. 
         [0031]    It should be appreciated that the above description is for illustration only and other embodiments and variations may be envisaged without departing from the spirit and scope of the present disclosure. In particular, the specific levels of attenuation may be varied from those illustrated. SPADs according to the concepts described herein may have, depending on the embodiment, over 40% over 50%, over 75%, over 85%, over 95% or over 99% of the active area covered/shielded. Arrays may comprise any combination of such SPADs.