Patent Publication Number: US-2023154959-A1

Title: Microlens structures for semiconductor device with single-photon avalanche diode pixels

Description:
This application is a continuation of U.S. patent application Ser. No. 16/684,033, filed Nov. 14, 2019, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This relates generally to imaging systems and, more particularly, to imaging systems that include single-photon avalanche diodes (SPADs) for single photon detection. 
     Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Image sensors (sometimes referred to as imagers) may be formed from a two-dimensional array of image sensing pixels. Each pixel typically includes a photosensitive element (such as a photodiode) that receives incident photons (light) and converts the photons into electrical signals. Each pixel may also include a microlens that overlaps and focuses light onto the photosensitive element. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format. 
     Conventional image sensors with backside-illuminated pixels may suffer from limited functionality in a variety of ways. For example, some conventional image sensors may not be able to determine the distance from the image sensor to the objects that are being imaged. Conventional image sensors may also have lower than desired image quality and resolution. 
     To improve sensitivity to incident light, single-photon avalanche diodes (SPADs) may sometimes be used in imaging systems. However, SPADs may require larger photosensitive regions than conventional image sensors and therefore may require thicker microlenses to focus light on the photosensitive elements within the SPADs. In order to apply microlenses thick enough to focus light in a desired manner, high viscosity material may be required. It may be difficult to control uniformity, patterning, and reflow characteristics when using high viscosity materials. 
     It would therefore be desirable to be able to provide improved microlens structures for single-photon avalanche diode pixels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a circuit diagram showing an illustrative single-photon avalanche diode pixel in accordance with an embodiment. 
         FIG.  2    is a diagram of an illustrative silicon photomultiplier in accordance with an embodiment. 
         FIG.  3    is a schematic diagram of an illustrative imaging system with a SPAD-based semiconductor device in accordance with an embodiment. 
         FIG.  4    is a diagram of an illustrative pixel array and associated readout circuitry for reading out image signals in a SPAD-based semiconductor device in accordance with an embodiment. 
         FIG.  5    is a cross-sectional diagram of illustrative SPAD pixels covered with microlenses in accordance with an embodiment. 
         FIG.  6    is a cross-sectional diagram of illustrative SPAD pixels covered with microlenses that are separated by a containment grid in accordance with an embodiment. 
         FIG.  7    is a process flow diagram of an illustrative method of forming a containment grid and interspersed microlenses in accordance with an embodiment. 
         FIG.  8    is a process flow diagram of an illustrative method of forming a containment grid and interspersed microlenses using phobic material over the containment grid in accordance with an embodiment. 
         FIG.  9    is a process flow diagram of an illustrative method of forming a containment grid and interspersed microlenses using phobic material that surrounds portions of the containment grid in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention relate to imaging systems that include single-photon avalanche diodes (SPADs). 
     Some imaging systems include image sensors that sense light by converting impinging photons into electrons or holes that are integrated (collected) in pixel photodiodes within the sensor array. After completion of an integration cycle, collected charge is converted into a voltage, which is supplied to the output terminals of the sensor. In complementary metal-oxide semiconductor (CMOS) image sensors, the charge to voltage conversion is accomplished directly in the pixels themselves and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The analog pixel voltage can also be later converted on-chip to a digital equivalent and processed in various ways in the digital domain. 
     In single-photon avalanche diode (SPAD) devices (such as the ones described in connection with  FIGS.  1 - 4   ), on the other hand, the photon detection principle is different. The light sensing diode is biased slightly above its breakdown point and when an incident photon generates an electron or hole, this carrier initiates an avalanche breakdown with additional carriers being generated. The avalanche multiplication may produce a current signal that can be easily detected by readout circuitry associated with the SPAD. The avalanche process needs to be stopped (quenched) by lowering the diode bias below its breakdown point. Each SPAD may therefore include a passive and/or active quenching circuit for quenching the avalanche. 
     This concept can be used in two ways. First, the arriving photons may simply be counted (e.g., in low light level applications). Second, the SPAD pixels may be used to measure photon time-of-flight (ToF) from a synchronized light source to a scene object point and back to the sensor, which can be used to obtain a 3-dimensional image of the scene. 
       FIG.  1    is a circuit diagram of an illustrative SPAD device  202 . As shown in  FIG.  1   , SPAD device  202  includes a SPAD  204  that is coupled in series with quenching circuitry  206  between a first supply voltage terminal  208  (e.g., a ground power supply voltage terminal) and a second supply voltage terminal  210  (e.g., a positive power supply voltage terminal). During operation of SPAD device  202 , supply voltage terminals  208  and  210  may be used to bias SPAD  204  to a voltage that is higher than the breakdown voltage. Breakdown voltage is the largest reverse voltage that can be applied without causing an exponential increase in the leakage current in the diode. When SPAD  204  is biased above the breakdown voltage in this manner, absorption of a single-photon can trigger a short-duration but relatively large avalanche current through impact ionization. 
     Quenching circuitry  206  (sometimes referred to as quenching element  206 ) may be used to lower the bias voltage of SPAD  204  below the level of the breakdown voltage. Lowering the bias voltage of SPAD  204  below the breakdown voltage stops the avalanche process and corresponding avalanche current. There are numerous ways to form quenching circuitry  206 . Quenching circuitry  206  may be passive quenching circuitry or active quenching circuitry. Passive quenching circuitry may, without external control or monitoring, automatically quench the avalanche current once initiated. For example,  FIG.  1    shows an example where a resistor is used to form quenching circuitry  206 . This is an example of passive quenching circuitry. After the avalanche is initiated, the resulting current rapidly discharges the capacity of the device, lowering the voltage at the SPAD to near to the breakdown voltage. The resistance associated with the resistor in quenching circuitry  206  may result in the final current being lower than required to sustain itself. The SPAD may then be reset to above the breakdown voltage to enable detection of another photon. 
     This example of passive quenching circuitry is merely illustrative. Active quenching circuitry may also be used in SPAD device  202 . Active quenching circuitry may reduce the time it takes for SPAD device  202  to be reset. This may allow SPAD device  202  to detect incident light at a faster rate than when passive quenching circuitry is used, improving the dynamic range of the SPAD device. Active quenching circuitry may modulate the SPAD quench resistance. For example, before a photon is detected, quench resistance is set high and then once a photon is detected and the avalanche is quenched, quench resistance is minimized to reduce recovery time. 
     SPAD device  202  may also include readout circuitry  212 . There are numerous ways to form readout circuitry  212  to obtain information from SPAD device  202 . Readout circuitry  212  may include a pulse counting circuit that counts arriving photons. Alternatively or in addition, readout circuitry  212  may include time-of-flight circuitry that is used to measure photon time-of-flight (ToF). The photon time-of-flight information may be used to perform depth sensing. 
     In one example, photons may be counted by an analog counter to form the light intensity signal as a corresponding pixel voltage. The ToF signal may be obtained by also converting the time of photon flight to a voltage. The example of an analog pulse counting circuit being included in readout circuitry  212  is merely illustrative. If desired, readout circuitry  212  may include digital pulse counting circuits. Readout circuitry  212  may also include amplification circuitry if desired. 
     The example in  FIG.  1    of readout circuitry  212  being coupled to a node between diode  204  and quenching circuitry  206  is merely illustrative. Readout circuitry  212  may be coupled to any desired portion of the SPAD device. In some cases, quenching circuitry  206  may be considered integral with readout circuitry  212 . 
     Because SPAD devices can detect a single incident photon, the SPAD devices are effective at imaging scenes with low light levels. Each SPAD may detect how many photons are received within a given period of time (e.g., using readout circuitry that includes a counting circuit). However, as discussed above, each time a photon is received and an avalanche current initiated, the SPAD device must be quenched and reset before being ready to detect another photon. As incident light levels increase, the reset time becomes limiting to the dynamic range of the SPAD device (e.g., once incident light levels exceed a given level, the SPAD device is triggered immediately upon being reset). 
     Multiple SPAD devices may be grouped together to increase dynamic range.  FIG.  2    is a circuit diagram of an illustrative group  220  of SPAD devices  202 . The group of SPAD devices may be referred to as a silicon photomultiplier (SiPM). As shown in  FIG.  2   , silicon photomultiplier  220  may include multiple SPAD devices that are coupled in parallel between first supply voltage terminal  208  and second supply voltage terminal  210 .  FIG.  2    shows N SPAD devices  202  coupled in parallel (e.g., SPAD device  202 - 1 , SPAD device  202 - 2 , SPAD device  202 - 3 , SPAD device  202 - 4 , . . . , SPAD device  202 -N). More than two SPAD devices, more than ten SPAD devices, more than one hundred SPAD devices, more than one thousand SPAD devices, etc. may be included in a given silicon photomultiplier. 
     Herein, each SPAD device may be referred to as a SPAD pixel  202 . Although not shown explicitly in  FIG.  2   , readout circuitry for the silicon photomultiplier  5  may measure the combined output current from all of SPAD pixels in the silicon photomultiplier. In this way, the dynamic range of an imaging system including the SPAD pixels may be increased. Each SPAD pixel is not guaranteed to have an avalanche current triggered when an incident photon is received. The SPAD pixels may have an associated probability of an avalanche current being triggered when an incident photon is received. There is a first probability of an electron being created when a photon reaches the diode and then a second probability of the electron triggering an avalanche current. The total probability of a photon triggering an avalanche current may be referred to as the SPAD&#39;s photon-detection efficiency (PDE). Grouping multiple SPAD pixels together in the silicon photomultiplier therefore allows for a more accurate measurement of the incoming incident light. For example, if a single SPAD pixel has a PDE of 50% and receives one photon during a time period, there is a 50% chance the photon will not be detected. With the silicon photomultiplier  220  of  FIG.  2   , chances are that two of the four SPAD pixels will detect the photon, thus improving the provided image data for the time period. 
     The example of a plurality of SPAD pixels having a common output in a silicon photomultiplier is merely illustrative. In the case of an imaging system including a silicon photomultiplier having a common output for all of the SPAD pixels, the imaging system may not have any resolution in imaging a scene (e.g., the silicon photomultiplier can just detect photon flux at a single point). It may be desirable to use SPAD pixels to obtain image data across an array to allow a higher resolution reproduction of the imaged scene. In cases such as these, SPAD pixels in a single imaging system may have per-pixel readout capabilities. Alternatively, an array of silicon photomultipliers (each including more than one SPAD pixel) may be included in the imaging system. The outputs from each pixel or from each silicon photomultiplier may be used to generate image data for an imaged scene. The array may be capable of independent detection (whether using a single SPAD pixel or a plurality of SPAD pixels in a silicon photomultiplier) in a line array (e.g., an array having a single row and multiple columns or a single column and multiple rows) or an array having more than ten, more than one hundred, or more than one thousand rows and/or columns. 
     While there are a number of possible use cases for SPAD pixels as discussed above, the underlying technology used to detect incident light is the same. All of the aforementioned examples of devices that use SPAD pixels may collectively be referred to as SPAD-based semiconductor devices. A silicon photomultiplier with a plurality of SPAD pixels having a common output may be referred to as a SPAD-based semiconductor device. An array of SPAD pixels with per-pixel readout capabilities may be referred to as a SPAD-based semiconductor device. An array of silicon photomultipliers with per-silicon-photomultiplier readout capabilities may be referred to as a SPAD-based semiconductor device. 
     An imaging system  10  with a SPAD-based semiconductor device is shown in  FIG.  3   . Imaging system  10  may be an electronic device such as a digital camera, a computer, a cellular telephone, a medical device, or other electronic device. Imaging system  10  may be an imaging system on a vehicle (sometimes referred to as vehicular imaging system). Imaging system may be used for LIDAR applications. 
     Imaging system  14  may include one or more SPAD-based semiconductor devices  14  (sometimes referred to as semiconductor devices  14 , devices  14 , SPAD-based image sensors  14 , or image sensors  14 ). One or more lenses  28  may optionally cover each semiconductor device  14 . During operation, lenses  28  (sometimes referred to as optics  28 ) may focus light onto SPAD-based semiconductor device  14 . SPAD-based semiconductor device  14  may include SPAD pixels that convert the light into digital data. The SPAD-based semiconductor device may have any number of SPAD pixels (e.g., hundreds, thousands, millions, or more). 
     The SPAD-based semiconductor device  14  may optionally include additional circuitry such as bias circuitry (e.g., source follower load circuits), sample and hold circuitry, correlated double sampling (CDS) circuitry, amplifier circuitry, analog-to-digital (ADC) converter circuitry, data output circuitry, memory (e.g., buffer circuitry), address circuitry, etc. 
     Image data from SPAD-based semiconductor device  14  may be provided to image processing circuitry  16 . Image processing circuitry  16  may be used to perform image processing functions such as automatic focusing functions, depth sensing, data formatting, adjusting white balance and exposure, implementing video image stabilization, face detection, etc. For example, during automatic focusing operations, image processing circuitry  16  may process data gathered by the SPAD pixels to determine the magnitude and direction of lens movement (e.g., movement of lens  28 ) needed to bring an object of interest into focus. Image processing circuitry  16  may process data gathered by the SPAD pixels to determine a depth map of the scene. 
     Imaging system  10  may provide a user with numerous high-level functions. In a computer or advanced cellular telephone, for example, a user may be provided with the ability to run user applications. To implement these functions, the imaging system may include input-output devices  22  such as keypads, buttons, input-output ports, joysticks, and displays. Additional storage and processing circuitry such as volatile and nonvolatile memory (e.g., random-access memory, flash memory, hard drives, solid state drives, etc.), microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, and/or other processing circuits may also be included in the imaging system. 
     Input-output devices  22  may include output devices that work in combination with the SPAD-based semiconductor device. For example, a light-emitting component may be included in the imaging system to emit light (e.g., infrared light or light of any other desired type). Semiconductor device  14  may measure the reflection of the light off of an object to measure distance to the object in a LIDAR (light detection and ranging) scheme. 
       FIG.  4    shows one example for a semiconductor device  14  that includes an array  120  of SPAD pixels  202  (sometimes referred to herein as image pixels or pixels) arranged in rows and columns. Array  120  may contain, for example, hundreds or thousands of rows and columns of SPAD pixels  202 . Each SPAD pixel may be coupled to an analog pulse counter that generates a corresponding pixel voltage based on received photons. Each SPAD pixel may be additionally or instead be coupled to a time-of-flight to voltage converter circuit. In both types of readout circuits, voltages may be stored on pixel capacitors and may later be scanned in a row-by-row fashion. Control circuitry  124  may be coupled to row control circuitry  126  and image readout circuitry  128  (sometimes referred to as column control circuitry, readout circuitry, processing circuitry, or column decoder circuitry). Row control circuitry  126  may receive row addresses from control circuitry  124  and supply corresponding row control signals to SPAD pixels  202  over row control paths  130 . One or more conductive lines such as column lines  132  may be coupled to each column of pixels  202  in array  120 . Column lines  132  may be used for reading out image signals from pixels  202  and for supplying bias signals (e.g., bias currents or bias voltages) to pixels  202 . If desired, during pixel readout operations, a pixel row in array  120  may be selected using row control circuitry  126  and image signals generated by image pixels  202  in that pixel row can be read out along column lines  132 . 
     Image readout circuitry  128  may receive image signals (e.g., analog or digital signals from the SPAD pixels) over column lines  132 . Image readout circuitry  128  may include sample-and-hold circuitry for sampling and temporarily storing image signals read out from array  120 , amplifier circuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry, column memory, latch circuitry for selectively enabling or disabling the column circuitry, or other circuitry that is coupled to one or more columns of pixels in array  120  for operating pixels  202  and for reading out signals from pixels  122 . ADC circuitry in readout circuitry  128  may convert analog pixel values received from array  120  into corresponding digital pixel values (sometimes referred to as digital image data or digital pixel data). Alternatively, ADC circuitry may be incorporated into each SPAD pixel  202 . Image readout circuitry  128  may supply digital pixel data to control and processing circuitry  124  and/or image processing and data formatting circuitry  16  ( FIG.  1   ) over path  125  for pixels in one or more pixel columns. 
     The example of image sensor  14  having readout circuitry to read out signals from the SPAD pixels in a row-by-row manner is merely illustrative. In other embodiments, the readout circuitry in the image sensor may simply include digital pulse counting circuits coupled to each SPAD pixel. Any other desired readout circuitry arrangement may be used. 
     If desired, array  120  may be part of a stacked-die arrangement in which pixels  202  of array  120  are split between two or more stacked substrates. Alternatively, pixels  202  may be formed in a first substrate and some or all of the corresponding control and readout circuitry may be formed in a second substrate. Each of the pixels  202  in the array  120  may be split between the two dies at any desired node within pixel. 
     It should be understood that instead of having an array of SPAD pixels as in  FIG.  4   , SPAD-based semiconductor device  14  may instead have an array of silicon photomultipliers (each of which includes multiple SPAD pixels with a common output). 
     As shown in  FIG.  5   , each SPAD pixel  202  in a group  220  of SPAD devices (see  FIG.  2   ) may be covered by a microlens  502  (alternatively, each SPAD pixel  202  in the array of SPAD pixels shown in  FIG.  4    may be covered by a microlens  502 ). In particular, each microlens  502  may focus incident light on an associated one of SPAD pixels  202 . In general, microlenses  502  may be formed by any desired method. As an example, microlens material may be applied over SPAD pixels  202  and the reflowed to form microlenses  502 . However, this merely illustrative. Any desired method may be used to form microlenses  502 . 
     Regardless of the method used to form the microlenses, they may be too thin to focus light properly on the photosensitive regions when formed using conventional manufacturing methods and equipment. In particular, the SiPM devices may have SPAD pixels with pitches that are approximately between 20 microns and 35 microns wide. To focus light incident on image sensor onto the array of SPAD pixels, spherical microlenses with thicknesses of approximately 20 microns may be required. This is thicker than traditional microlenses, which may have thicknesses of approximately 5 microns, and standard equipment may therefore not be capable of forming microlenses for SiPM devices. To form microlenses with higher thicknesses, microlens material with a higher viscosity may be used. However, this poses additional issues, as uniformity, photo patterning, and reflow characteristics may be challenging when using high-viscosity material. For example, adjacent microlenses may merge together, as it may be difficult to maintain the shape and size of the microlenses, such as microlenses  502 , during reflow operations. Therefore, it may be desired to use additional structures to control the formation of microlenses. 
     As shown in  FIG.  6   , microlenses  602  may be formed on substrate  604 . Substrate  604  may be a silicon substrate or may be formed from any desired material. SPAD pixels, such as SPAD pixels  202 , may be formed in substrate  604 , or substrate  604  may overlap SPAD pixels  202 . In any case, microlenses  602  may overlap SPAD pixels (or other desired type of pixels) and direct light into the pixels. 
     As shown in  FIG.  6   , SPAD pixels  202  may optionally be formed in substrate  604 . Each SPAD pixel  202  may have an active area  608  and an inactive area  610 . Inactive area  610  may contain circuitry and space the SPAD pixels apart for more accurate photon detection (e.g., the space between pixels may reduce crosstalk between the pixels). Active area  608  may be sensitive to photons of incoming light. As a result, microlenses  602  may be formed over active areas  608 , and containment grid  606  may be formed over inactive areas  610 . If desired, however, portions of containment grid  606  may extend at least partially into active area  608 . As an example, a tapered containment grid portion, such as grid portion  602 - 1  may overlap only inactive region  610  at substrate  604 , but may flare up to additionally overlap a portion of active region  608 . However, this is merely illustrative. Containment grid  606  may be confined to inactive region  610  or may extend partially into active region  608 . 
     Microlenses  602  may be separated by containment grid  606 . Containment grid  606  may help contain microlenses  602  within openings within the containment grid during reflow operations. In other words, the containment grid may prevent adjacent microlenses from merging. As shown in  FIG.  6   , containment grid portions  606 - 1  and  606 - 2  may help contain microlens  602 - 1 , containment grid portions  606 - 2  and  606 - 3  may help contain microlens  602 - 2 , and containment grid portions  606 - 3  and  606 - 4  may help contain microlens  602 - 3 . The containment grid portions may have tapered shapes, as shown by containment grid portions  606 - 1 ,  606 - 2 , and  606 - 3 , may have flat walls, as shown by containment grid portion  606 - 4 , or may have any other desired shape. In some embodiments, containment grid portions may have tapered shapes to help contain microlens material during the formation of microlenses  602 . In general, containment grid portions  606  may all have substantially the same shape over the array of SPAD pixels  202 , or may vary in shape across the array of pixels. 
     In some embodiments, the containment grid  606  may include material with a lower index of refraction than the microlens material (e.g., the material used to form microlenses  602 ). This may allow containment grid portions  606  to absorb high-angle light during operation of the SPAD pixels, thereby improving the accuracy of detection by the underlying SPAD pixels. Containment grid  606  may be formed from black material, metal, metal oxide, or dielectric materials, as examples. In general, any desired material may be used to form containment grid  606 . In some cases, black or metal material may be used to absorb off-angle light. By blocking off-angle light, containment grid  606  may reduce crosstalk between adjacent SPAD pixels  202 . In particular, crosstalk in SiPM/SPAD devices often occurs due to the emission of light of one pixel moving to an adjacent pixel and being absorbed. This is known as secondary photon generation crosstalk. By forming containment grid  606  from black material, metal material, or other absorptive material, the containment grid may absorb the generated photons and reduce crosstalk detected by neighboring pixels. In some embodiments, containment grid  606  may be extended at least partially into substrate  604  to provide absorption for light at higher angles. 
     Although containment grid portions  606  are shown as being much thinner than microlenses  602 , this is merely illustrative. In general, containment grid portions  606  may extend to any desired height from substrate  604 . For example, the containment grid may be made thicker to help converge normal incident light within silicon substrate  604 . Alternatively, the containment grid may be made thinner to better focus off-angle light. In some embodiments, the microlenses may be at least two times thicker than the containment grid, at least three times thicker than the containment grid, at least ten times thicker than the containment grid, less than 15 times thicker than the containment grid, or at least five times thicker than the containment grid, as examples. Microlenses  602  may have thicknesses of less than 25 microns, less than 20 microns, less than 10 microns, less than 5 microns, greater than 3 microns, or less than 4 microns. However, the thickness of containment grid portions  606  and microlenses  602  may be adjusted as desired. 
     Moreover, although microlenses  602  are shown in  FIG.  6    as being entirely between containment grid portions  606 , this is merely illustrative. In some embodiments, microlenses  602  may at least partially overlap containment grid portions  606 . For example, edge portions of microlenses  602  may overlap containment grid portions  606 . This arrangement may allow microlenses  602  to have an increased standoff height relative to substrate  604 , which in turn may improve the focusing ability of microlenses  602 . 
     Although the example of  FIG.  6    shows the microlenses applied directly over SPAD pixels  602  (e.g., a backside illuminated arrangement), a containment grid can also be used when forming microlenses over frontside illuminated image sensors, as well. 
     Microlenses  602  and containment grid  606  may be formed using any desired method. However, as previously discussed, it may be desirable to form microlenses  602  using a reflow process, so that microlens material may be applied across an array of SPAD pixels and then reflowed to shape the lenses. An illustrative process by which containment grid  606  and microlenses  602  may be formed is shown in  FIG.  7   . 
     As shown in  FIG.  7   , containment grid material  702  may be deposited on substrate  604 . As discussed, substrate  604  may be any desired material, such as silicon. Containment grid material  702  may be metal, metal oxide, black material, or any other desired material. In some embodiments, containment grid material  702  may be configured to absorb light. 
     After containment grid material  702  has been deposited, the process flow may proceed along arrow  704 , and the containment grid may be patterned to form openings in which microlenses will be formed. Any desired method may be used to pattern the containment grid, such as photolithography or etching. As shown in  FIG.  7   , containment grid portions  706 - 1  and  706 - 2  may at least partially surround opening  708 - 1 , and containment grid portions  706 - 2  and  706 - 3  may at least partially surround opening  708 - 2 . These openings may extend in two dimensions across the array of SPAD pixels  14 . 
     Although containment grid portions  706  are shown as having flat walls, some or all of containment grid portions  706  may have tapered shapes, like containment grid portions  606 - 1 ,  606 - 2 , and  606 - 3  of  FIG.  6   . This may help contain microlens material during formation of the microlenses. However, this shape is merely illustrative. In general, containment grid portions  706  may have any desired shapes. 
     The process may then proceed along arrow  710 , and microlens material  712  may be deposited over containment grid  706  and substrate  604 . Microlens material  712  may be formed from acrylic, silicon, any other desired material, or any desired combinations of materials. If desired, microlens material  712  may have a higher index of refraction than containment grid material  702 . In this way, high-angle light may be redirected by microlens material  712  and be detected by the underlying SPAD pixels. However, this is merely illustrative. In general, any desired material may be used for microlens material  712 . 
     After depositing microlens material  712 , the process may proceed along arrow  714 , and microlens material  712  may be patterned to form an array of patterned portions  716 , which includes patterned portions  716 - 1  and  716 - 2 . Microlens material  712  may be patterned using any desired technique, such as photolithography or etching. Although gaps are shown between patterned portions  716  and containment grid  706 , this is merely illustrative. In some examples, it may be desirable to have less of a gap or no gap between patterned portions  716  and containment grid  706 , as doing so may allow microlenses to overlap containment grid  706  and create additional standoff height from substrate  604 . 
     The process may then proceed along arrow  718 , and the patterned portions  716  may be reflowed to form an array of microlenses  720 . All of the patterned portions  716  may be reflowed simultaneously, or some of the patterned portions  716  may be reflowed before other patterned portions  716 . Patterned portions  716  may be reflowed in any desired manner to form microlenses  720 . Microlenses  720  may have a spherical shape or any other desired shape. Moreover, microlens  720 - 1  may have the same shape as microlens  720 - 2  or the microlenses may have different shapes. In general, the reflow  5  processes may be adjusted to create any desired shapes for microlenses  720  across the array of microlenses. 
     As shown, containment grid portions  706 - 1 ,  706 - 2 , and  706 - 3  may help prevent microlenses  720 - 1  and  720 - 2  from merging during reflow operations. In particular, the containment grid portions may act as barriers to contain the microlens material as it is being reflowed. After formation of the microlenses, containment grid  706  may absorb off-axis secondary photons generated within the avalanche region of the SPAD, which could otherwise reflect back into adjacent SPADs, creating cross-talk. For example, containment grid  706  may be formed from black material, metal, or other light absorptive material to help prevent cross talk between SPAD pixels. 
     Although the method of  FIG.  7    allows for the formation of microlenses within a containment grid, it may be desirable to use phobic material to prevent neighboring microlenses from merging above the containment grid. A process diagram illustrating the use of phobic material is shown in  FIG.  8   . 
     As shown in  FIG.  8   , containment grid material  802  may be deposited on substrate  604 . As discussed, substrate  604  may be any desired material, such as silicon. Containment grid material  802  may be metal, metal oxide, black material, or any other desired material. In particular, containment grid material  802  may be phyllic with respect to microlens material. Therefore, the containment grid material may be silicon, oxides, or any other material that promotes the attachment of microlens material. In some embodiments, containment grid material  802  may also be configured to absorb light. 
     Phobic material  803  may be deposited on containment grid material  802 . In particular, phobic material  803  may adhere poorly to microlens material. In general, phobic material  803  may be any desired material, such as a fluoropolymer. 
     After containment grid material  802  and phobic material  803  have been deposited, the process flow may proceed along arrow  804 , and the containment grid and phobic material may be patterned to form openings in which microlenses will be formed. Any desired method may be used to pattern the containment grid and phobic material, such as photolithography or etching. As shown in  FIG.  8   , containment grid portions  806 - 1  and  806 - 2  and phobic portions  807 - 1  and  807 - 2  may at least partially surround opening  808 - 1 , and containment grid portions  806 - 2  and  806 - 3  and phobic portions  807 - 2  and  807 - 3  may at least partially surround opening  808 - 2 . These openings may extend in two dimensions across the array of SPAD pixels  14 . 
     Although containment grid portions  806  are shown as having tapered shapes, some or all of containment grid portions  806  may have flat walls, like containment grid portion  606 - 4  of  FIG.  6   . However, this shape is merely illustrative. In general, containment grid portions  806  may have any desired shapes. 
     The process may then proceed along arrow  810 , and microlens material  812  may be deposited over containment grid  806 , phobic material  807 , and substrate  604 . Microlens material  812  may be formed from acrylic, silicon, any other desired material, or any desired combinations of materials. If desired, microlens material  812  may have a higher index of refraction than containment grid material  802 . In this way, high-angle light may be redirected by microlens material  812  and be detected by the underlying SPAD pixels. However, this is merely illustrative. In general, any desired material may be used for microlens material  812 . 
     After depositing microlens material  812 , the process may proceed along arrow  814 , and microlens material  812  may be patterned to form an array of patterned portions  816 , which includes patterned portions  816 - 1  and  816 - 2 . Microlens material  812  may be patterned using any desired technique, such as photolithography or etching. Although gaps are shown between patterned portions  816  and containment grid  806 , this is merely illustrative. In some examples, it may be desirable to have less of a gap or no gap between patterned portions  816  and containment grid  806 , as doing so may allow microlenses to overlap containment grid  806  and create additional standoff height from substrate  604 . 
     The process may then proceed along arrow  818 , and the patterned portions  816  may be reflowed to form an array of microlenses  820 . All of the patterned portions  816  may be reflowed simultaneously, or some of the patterned portions  816  may be reflowed before other patterned portions  816 . Patterned portions  816  may be reflowed in any desired manner to form microlenses  820 . Microlenses  820  may have a spherical shape or any other desired shape. Moreover, microlens  820 - 1  may have the same shape as microlens  820 - 2  or they may have different shapes. In general, the reflow processes may be adjusted to create any desired shapes for microlenses  820  across the array of microlenses. 
     As shown, containment grid portions  806 - 1 ,  806 - 2 , and  806 - 3  may help prevent microlenses  820 - 1  and  820 - 2  from merging during reflow operations. In particular, the containment grid portions may act as barriers to contain the microlens material as it is being reflowed. Additionally, phobic portions  822  may help prevent adjacent microlenses  820  from merging because the phobic material may be resistant to the microlens material. After formation of the microlenses, containment grid  806  may absorb off-axis light secondary photons generated within the avalanche region of the SPAD, which could otherwise reflect back into adjacent SPADs, creating cross-talk. For example, containment grid  806  may be formed from black material, metal, or other light absorptive material to help prevent cross talk between SPAD pixels. 
     In some cases, it may be desirable to use containment grid material that is phobic to microlens material. A process diagram illustrating the use of phobic containment grid material is shown in  FIG.  9   . 
     As shown in  FIG.  9   , containment grid material  902  may be deposited on substrate  604 . As discussed, substrate  604  may be any desired material, such as silicon. Containment grid material  902  may be metal, metal oxide, black material, or any other desired material. In particular, containment grid material  902  may be phobic with respect to microlens material. Therefore, the containment grid material may be metal, oxides, or any other material that resists the attachment of microlens material. In some embodiments, containment grid material  902  may also be configured to absorb light. 
     After containment grid material  902  has been deposited, the process flow may proceed along arrow  904 , and the containment grid may be patterned to form openings in which microlenses will be formed. Any desired method may be used to pattern the containment grid and phobic material, such as photolithography or etching. Additionally, material that is phyllic to microlens material may be applied over containment grid portions  906 . Phyllic material  907  may cover the top and sides of each of containment grid portions  906 - 1 ,  906 - 2 , and  906 - 3 . Alternatively, phyllic material  907  may cover only part of the containment grid portions. Phyllic material  907  may be acrylic, silicon, resin, oxides, or any other desired material that promotes adhesion to microlens material. In general, phyllic material  907  may cover any desired portion of the underlying containment portions and may be formed from any desired material. 
     As shown in  FIG.  9   , containment grid portions  906 - 1  and  906 - 2  and phyllic portions  907 - 1  and  907 - 2  may at least partially surround opening  908 - 1 , and containment grid portions  906 - 2  and  906 - 3  and phobic portions  907 - 2  and  907 - 3  may at least partially surround opening  908 - 2 . These openings may extend in two dimensions across the array of SPAD pixels  14 . 
     Although containment grid portions  906  are shown as having tapered shapes, some or all of containment grid portions  906  may have flat walls, like containment grid portion  606 - 4  of  FIG.  6   . However, this shape is merely illustrative. In general, containment grid portions  906  may have any desired shapes. 
     The process may then proceed along arrow  910 , and top portions of the phyllic material may be removed from each of the containment grid portions. As shown, this may expose upper surface  909  of each containment grid portion, and leave phyllic portions on the edge surfaces (as illustrated by phyllic portions  907 - 3 A and  907 - 3 B). The phyllic material may be removed using any desired process, such as etching. 
     Although  FIG.  9    illustrates removing only a top portion of the phyllic material over each containment grid portion, this is merely illustrative. Any portion or portions of the phyllic material may be removed, as desired. 
     After removing the desired portions of the phyllic material, the process may proceed along arrow  911 , and microlens material  912  may be deposited over containment grid  906 , phyllic portions  907 , and substrate  604 . Microlens material  912  may be formed from acrylic, silicon, any other desired material, or any desired combinations of materials. If desired, microlens material  912  may have a higher index of refraction than containment grid material  902 . In this way, high-angle light may be redirected by microlens material  812  and be detected by the underlying SPAD pixels. In some embodiments, microlens material  912  may be the same material as the material used to form phyllic portions  907 , thereby promoting adhesion between the microlens material and the phyllic portions. However, this is merely illustrative. In general, any desired material may be used for microlens material  912 . 
     After depositing microlens material  912 , the process may proceed along arrow  914 , and microlens material  912  may be patterned to form an array of patterned portions  916 , which includes patterned portions  916 - 1  and  916 - 2 . Microlens material  912  may be patterned using any desired technique, such as photolithography or etching. Although gaps are shown between patterned portions  916  and containment grid  906 , this is merely illustrative. In some examples, it may be desirable to have less of a gap or no gap between patterned portions  916  and containment grid  906 , as doing so may allow microlenses to overlap containment grid  906  and create additional standoff height from substrate  604 . 
     The process may then proceed along arrow  918 , and the patterned portions  916  may be reflowed to form an array of microlenses  920 . All of the patterned portions  916  may be reflowed simultaneously, or some of the patterned portions  916  may be reflowed before other patterned portions  916 . Patterned portions  916  may be reflowed in any desired manner to form microlenses  920 . Microlenses  920  may have a spherical shape or any other desired shape. Moreover, microlens  920 - 1  may have the same shape as microlens  920 - 2  or they may have different shapes. In general, the reflow processes may be adjusted to create any desired shapes for microlenses  920  across the array of microlenses. 
     As shown, containment grid portions  906 - 1 ,  906 - 2 , and  906 - 3  may help prevent microlenses  920 - 1  and  920 - 2  from merging during reflow operations. In particular, the containment grid portions may act as barriers to contain the microlens material as it is being reflowed. Due to the presence of phyllic portions  907  along the sides of the containment grid portions, the microlens material may be attracted to the phyllic portions during reflow operations. Additionally, since the phyllic material was removed from the top surfaces of the containment grid portions and the containment grid portions are formed from phobic material, the microlens material may not adhere well to these surfaces. This may help prevent merging between adjacent microlenses. After formation of the microlenses, containment grid  906  may absorb off-axis secondary photons generated within the avalanche region of the SPAD, which could otherwise reflect back into adjacent SPADs, creating cross-talk. For example, containment grid  906  may be formed from black material, metal, or other light absorptive material to help prevent cross talk between SPAD pixels. 
     In any of the aforementioned embodiments, it should be understood that a silicon photomultiplier (with multiple SPAD pixels having a common output) may be used in place of a single SPAD pixel. Each SPAD pixel in the silicon multiplier may be covered by a microlens, or multiple SPAD pixels within the silicon multiplier may be covered by a single microlens, if desired. 
     Although each of the aforementioned embodiments have been described as applying a microlens over SPAD pixels, the microlenses may be formed over any desired pixel type. For example, the foregoing microlenses may be applied over pixels in conventional CMOS imagers. 
     In accordance with an embodiment, a semiconductor device may include a plurality of single-photon avalanche diode pixels. Each of the single-photon avalanche diode pixels may have an active region and an inactive region. The semiconductor device may also include a plurality of microlenses, each of which covers the active region of a respective one of the single-photon avalanche diode pixels, and a containment grid that covers the inactive regions of the single-photon avalanche diode pixels. Portions of the containment grid may be interposed between adjacent microlenses of the plurality of microlenses. 
     In accordance with various embodiments, the portions of the containment grid may each have a tapered shape between the microlenses. 
     In accordance with various embodiments, the portions of the containment grid may each have flat sidewalls between the microlenses. 
     In accordance with various embodiments, the containment grid may include containment grid material with a first index of refraction, the microlenses may include microlens material with a second index of refraction, and the second index of refraction may be higher than the first index of refraction. 
     In accordance with various embodiments, the containment grid may include a material selected from the group consisting of: metal material, metal oxide material, silicon material and black material. 
     In accordance with various embodiments, each of the containment grid portions may have a top surface, and the semiconductor device may further include material that is phobic to the microlens material on at least some of the top surfaces. 
     In accordance with various embodiments, the containment grid may include material that is phobic to the microlens material. 
     In accordance with various embodiments, the semiconductor device may further include material that is phyllic to the microlens material interposed between at least some of the containment grid portions and the microlenses. 
     In accordance with various embodiments, each of the microlenses may have a first height, the containment grid may have a second height, and the first height may be greater than the second height. 
     In accordance with various embodiments, the first height may be at least ten times greater than the second height. 
     In accordance with an embodiment, a method of forming microlenses over a plurality of single-photon avalanche diodes may include depositing containment grid material on a semiconductor substrate, patterning the containment grid material to form an array of openings, depositing microlens material over the containment grid material and the semiconductor substrate, and patterning and reflowing the microlens material to form microlenses in the openings of the containment grid material. 
     In accordance with various embodiments, the method may further include depositing phobic material over the containment grid material, and patterning the containment grid material to form an array of openings may include patterning the containment grid material and the phobic material. 
     In accordance with various embodiments, the containment grid may be phobic to the microlens material, and the method may further include before depositing the microlens material, depositing phyllic material on the patterned containment grid material, and etching a surface of the phyllic material to expose a portion of the containment grid material. 
     In accordance with various embodiments, depositing the containment grid material may include depositing the containment grid material to a first height from semiconductor substrate, and depositing the microlens material may include depositing the microlens material to a second height from the substrate that is at least ten times greater than the first height. 
     In accordance with various embodiments, patterning the containment grid material may include forming containment grid portions having a shape selected from the group consisting of: a tapered shape and a flat-walled shape. 
     In accordance with an embodiment, a semiconductor device may include a single-photon avalanche diode pixel having an active region and an inactive region, a containment grid having portions that cover the inactive region and having an opening that overlap the active region, and a microlens in the opening of the containment grid that overlaps the active region. 
     In accordance with various embodiments, the single-photon avalanche diode pixel may be a pixel in an array of single-photon avalanche diode pixels, and the containment grid may include containment grid material that absorbs stray light and prevents cross talk between adjacent pixels in the array of pixels. 
     In accordance with various embodiments, the containment grid material may be selected from the group of material consisting of: black material and metal oxide material. 
     In accordance with various embodiments, the microlens may include microlens material, and the containment grid material may be phobic to the microlens. The semiconductor device may further include additional material that is phyllic to the microlens material interposed between the containment grid material and the microlens material. 
     In accordance with various embodiments, the containment grid material may have a first index of refraction and the microlens may be formed from microlens material that has a second index of refraction that is greater than the first index of refraction. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art. The foregoing embodiments may be implemented individually or in any combination.