Patent Publication Number: US-2021167102-A1

Title: Ultraviolet light image sensor

Description:
TECHNICAL FIELD 
     The disclosure herein relates to an ultraviolet (UV) light image sensor, particularly relates to a UV light image sensor comprising avalanche photodiodes (APD). 
     BACKGROUND 
     An image sensor or imaging sensor is a sensor that can detect a spatial intensity distribution of a radiation. An image sensor usually represents the detected image by electrical signals. Image sensors based on semiconductor devices may be classified into several types, including semiconductor charge-coupled devices (CCD), complementary metal-oxide-semiconductor (CMOS), N-type metal-oxide-semiconductor (NMOS). A CMOS image sensor is a type of active pixel sensor made using the CMOS semiconductor process. Light incident on a pixel in the CMOS image sensor is converted into an electric voltage. The electric voltage is digitized into a discrete value that represents the intensity of the light incident on that pixel. An active-pixel sensor (APS) is an image sensor that includes pixels with a photodetector and an active amplifier. A CCD image sensor includes a capacitor in a pixel. When light incidents on the pixel, the light generates electrical charges and the charges are stored on the capacitor. The stored charges are converted to an electric voltage and the electrical voltage is digitized into a discrete value that represents the intensity of the light incident on that pixel. 
     UV light is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, between X-rays and visible light. UV image sensors may be useful in a wide range of applications, including fire detection, industrial manufacturing, biochemical research, light sources, and environmental and structural health monitoring. 
     SUMMARY 
     Disclosed herein is an apparatus, comprising: an array of avalanche photodiodes (APDs) configured to detect UV light; a bandpass optical filter that blocks visible light and passes UV light incident on the array of APDs. 
     According to an embodiment, each of the APDs comprises an absorption region and an amplification region. 
     According to an embodiment, the absorption region is configured to generate charge carriers from a UV photon absorbed by the absorption region. 
     According to an embodiment, the amplification region comprises a junction with an electric field in the junction. 
     According to an embodiment, the electric field is at a value sufficient to cause an avalanche of charge carriers entering the amplification region, but not sufficient to make the avalanche self-sustaining. 
     According to an embodiment, the junctions of the APDs are discrete. 
     According to an embodiment, the absorption region has an absorptance of at least 80% for UV light. 
     According to an embodiment, the absorption region has a thickness of 10 microns or above. 
     According to an embodiment, the absorption region comprises silicon. 
     According to an embodiment, an electric field in the absorption region is not high enough to cause avalanche effect in the absorption region. 
     According to an embodiment, the absorption region is an intrinsic semiconductor or a semiconductor with a doping level less than 10 12  dopants/cm 3 . 
     According to an embodiment, the absorption regions of at least some of the APDs are joined together. 
     According to an embodiment, the apparatus further comprises two amplification regions on opposite sides of the absorption region. 
     According to an embodiment, the amplification regions of the APDs are discrete. 
     According to an embodiment, the junction is a p-n junction or a heterojunction. 
     According to an embodiment, the junction comprises a first layer and a second layer, wherein the first layer is a doped semiconductor and the second layer is a heavily doped semiconductor. 
     According to an embodiment, the first layer has a doping level of 10 13  to 10 17  dopants/cm 3 . 
     According to an embodiment, the first layers of at least some of the APDs are joined together. 
     According to an embodiment, the apparatus further comprises electric contacts respectively in electrical contact with the second layers of the APDs. 
     According to an embodiment, the apparatus further comprises a passivation material configured to passivate a surface of the absorption region. 
     According to an embodiment, the apparatus further comprises a common electrode electrically connected to the absorption region. 
     According to an embodiment, the junction is separated from a junction of a neighbor junction by a material of the absorption region, a material of the first or second layer, an insulator material, or a guard ring of a doped semiconductor. 
     According to an embodiment, the junction further comprises a third layer sandwiched between the first and second layers; wherein the third layer comprises an intrinsic semiconductor. 
     Disclosed herein is a system comprising an apparatus described above, wherein the system is configured to scan along a high voltage transmission line, to capture images of the high voltage transmission line using the apparatus, and to detect a location of damage on the high voltage transmission line based on the images. 
     The system may further comprise an unmanned aerial vehicle (UAV), wherein the apparatus is mounted to the UAV. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  schematically shows a UV image sensor, according to an embodiment. 
         FIG. 2  schematically shows electric currents in an avalanche photodiode (APD) as functions of an intensity of UV light incident on the APD, according to an embodiment. 
         FIG. 3A ,  FIG. 3B  and  FIG. 3C  schematically show the operation of the APD, according to an embodiment. 
         FIG. 4A - FIG. 4D  each schematically shows a cross section of a portion of an APD layer of a UV image sensor, according to an embodiment. 
         FIG. 5A  and  FIG. 5B  each schematically shows a system comprising the UV image sensor described herein, for corona discharge detection for high voltage transmission lines. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a UV image sensor  100 , according to an embodiment. The UV image sensor  100  has an array of avalanche photodiodes (APDs)  110  and a bandpass optical filter  130 . The APDs  110  may detect UV light. The bandpass optical filter  130  blocks visible light and passes UV light. The bandpass optical filter  130  does not necessarily passes all UV light. Instead, the bandpass optical filter  130  may pass UV light of certain wavelengths. For example, the bandpass optical filter  130  may pass UV light with a wavelength between 250 nm and 320 nm and block UV light of other wavelengths and visible light. The bandpass optical filter  130  may also block infrared light. The bandpass optical filter  130  may include crystalline alkali metals such as nickel sulfate hexahydrate (NSH), potassium nickel sulfate hexahydrate (KNSH), cesium nickel sulfate hexahydrate (CNSH), or a combination thereof. The bandpass optical filter  130  may have a stack structure of multiple layers of dielectric materials, or a metal nanometer-scale square grid structure. 
     An APD (e.g., one of the APDs  110 ) is a photodiode that uses the avalanche effect to generate an electric current upon exposure to light. The avalanche effect is a chain process where free charge carriers in a material are strongly accelerated by an electric field, subsequently collide with atoms of the material, and eject additional charge carriers from the atoms by impact ionization. Impact ionization is a process by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in a semiconductor, an electron (or hole) with enough kinetic energy can free a bound electron from its bound state (e.g., excite the electron from the valance band to the conduction band). 
     An APD (e.g., one of the APDs  110 ) may work in the Geiger mode or the linear mode. When the APD works in the Geiger mode, it may be called a single-photon avalanche diode (SPAD) (also called a Geiger-mode APD or G-APD). A SPAD is an APD working under a reverse bias above the breakdown voltage. Here the word “above” means that absolute value of the reverse bias is greater than the absolute value of the breakdown voltage. A SPAD may be used to detect low intensity light (e.g., down to a single photon) and to signal the arrival times of the photons with a jitter of a few tens of picoseconds. A SPAD may be in a form of a p-n junction under a reverse bias (i.e., the p-type region of the p-n junction is biased at a lower electric potential than the n-type region) above the breakdown voltage of the p-n junction. The breakdown voltage of a p-n junction is a reverse bias, above which exponential increase in the electric current in the p-n junction occurs. An APD working at a reverse bias below the breakdown voltage is operating in the linear mode because the electric current in the APD is proportional to the intensity of the light incident on the APD. 
       FIG. 2  schematically shows electric currents in an APD (e.g., one of the APDs  110 ) as functions of an intensity of light incident on the APD, according to an embodiment. The APD may work in the Geiger mode or the linear mode. A function  112  is the function of intensity of light incident on the APD when the APD is in the linear mode, and a function  111  is the function of the intensity of light incident on the APD when the APD is in the Geiger mode. In the Geiger mode, the current shows a very sharp increase with the intensity of the light and then saturation. In the linear mode, the current is essentially proportional to the intensity of the light. 
       FIG. 3A ,  FIG. 3B  and  FIG. 3C  schematically show the operation of an APD (e.g., one of the APDs  110 ), according to an embodiment.  FIG. 3A  shows that when a photon (e.g., a UV photon) is absorbed by an absorption region  210  of the APD, multiple electron-hole pairs maybe generated. The absorption region  210  has a sufficient thickness and thus a sufficient absorptance (e.g., &gt;80% or &gt;90%) for the photon. For UV photons, the absorption region  210  may be a layer of silicon or another suitable semiconductor material with a sufficient thickness (e.g., 10 microns or above). The electric field in the absorption region  210  is not high enough to cause the avalanche effect in the absorption region  210 .  FIG. 3B  shows that the electrons and holes drift in opposite directions in the absorption region  210 .  FIG. 3C  shows that the avalanche effect occurs in an amplification region  220  of the APD when the electrons (or the holes) enter that amplification region  220 , thereby generating more electrons and holes. The electric field in the amplification region  220  is high enough to cause an avalanche of charge carriers entering the amplification region  220  but may or may not be high enough to make the avalanche effect self-sustaining. A self-sustaining avalanche is an avalanche that persists after the external triggers disappear, such as photons incident on the APD or charge carriers drifted into the APD. The electric field in the amplification region  220  may be a result of a doping profile in the amplification region  220 . For example, the amplification region  220  may include a p-n junction or a heterojunction that has an electric field in its depletion zone. The threshold electric field for the avalanche effect (i.e., the electric field above which the avalanche effect occurs and below which the avalanche effect does not occur) is a property of the material of the amplification region  220 . The amplification region  220  may be on one side or two opposite sides of the absorption region  210 . 
       FIG. 4A  schematically shows a cross section of the APDs  110  of the UV image sensor  100 , according to an embodiment. Each of the APDs  110  may have an absorption region  310  and an amplification region  320  as the example shown in  FIG. 3A ,  FIG. 3B  and  FIG. 3C . At least some, or all, of the APDs  110  in the UV image sensor  100  may have their absorption regions  310  joined together. Namely, the UV image sensor  100  may have joined absorption regions  310  in a form of an absorption layer  311  that is shared among at least some or all of the APDs  110 . The amplification regions  320  of the APDs  110  are discrete regions. Namely the amplification regions  320  of the APDs  110  are not joined together. In an embodiment, the absorption layer  311  may be in form of a semiconductor wafer such as a silicon wafer. The absorption regions  310  may be an intrinsic semiconductor or very lightly doped semiconductor (e.g., &lt;10 12  dopants/cm 3 , &lt;10 11  dopants/cm 3 , &lt;10 10  dopants/cm 3 , &lt;10 9  dopants/cm 3 ), with a sufficient thickness and thus a sufficient absorptance (e.g., &gt;80% or &gt;90%) for incident photons of interest (e.g., UV photons). The amplification regions  320  may have a junction  315  formed by at least two layers  312  and  313 . The junction  315  may be a heterojunction of a p-n junction. In an embodiment, the layer  312  is a p-type semiconductor (e.g., silicon) and the layer  313  is a heavily doped n-type layer (e.g., silicon). The phrase “heavily doped” is not a term of degree. A heavily doped semiconductor has its electrical conductivity comparable to metals and exhibits essentially linear positive thermal coefficient. In a heavily doped semiconductor, the dopant energy levels are merged into an energy band. A heavily doped semiconductor is also called degenerate semiconductor. The layer  312  may have a doping level of 10 13  to 10 17  dopants/cm 3 . The layer  313  may have a doping level of 10 18  dopants/cm 3  or above. The layers  312  and  313  may be formed by epitaxy growth, dopant implantation or dopant diffusion. The band structures and doping levels of the layers  312  and  313  can be selected such that the depletion zone electric field of the junction  315  is greater than the threshold electric field for the avalanche effect for electrons (or for holes) in the materials of the layers  312  and  313 , but is not too high to cause self-sustaining avalanche. Namely, the depletion zone electric field of the junction  315  should cause avalanche when there are incident photons in the absorption region  310  but the avalanche should cease without further incident photons in the absorption region  310 . 
     The UV image sensor  100  may further include electric contacts  304  respectively in electrical contact with the layer  313  of the APDs  110 . The electric contacts  304  are configured to collect electric current flowing through the APDs  110 . 
     The UV image sensor  100  may further include a passivation material  303  configured to passivate surfaces of the absorption regions  310  and the layer  313  of the APDs  110  to reduce recombination at these surfaces. 
     The UV image sensor  100  may further include a heavily doped layer  302  disposed on the absorption regions  310  opposite to the amplification region  320 , and a common electrode  301  on the heavily doped layer  302 . The common electrode  301  of at least some or all of the APDs  110  may be joined together. The heavily doped layer  302  of at least some or all of the APDs  110  may be joined together. 
     When a UV photon passes the bandpass optical filter  130  and incidents on the APDs  110 , it may be absorbed by the absorption region  310  of one of the APDs  110 , and charge carriers may be generated in the absorption region  310  as a result. One type (electrons or holes) of the charge carriers drift toward the amplification region  320  of that one APD. When the charge carriers enter the amplification region  320 , the avalanche effect occurs and causes amplification of the charge carriers. The amplified charge carriers can be collected through the electric contact  304  of that one APD, as an electric current. When that one APD is in the linear mode, the electric current is proportional to the number of incident photons in the absorption region  310  per unit time (i.e., proportional to the light intensity at that one APD). The electric currents at the APDs may be compiled to represent a spatial intensity distribution of light, i.e., an image. The amplified charge carriers may alternatively be collected through the electric contact  304  of that one APD, and the number of photons may be determined from the charge carriers (e.g., by using the temporal characteristics of the electric current). 
     The junctions  315  of the APDs  110  should be discrete, i.e., the junction  315  of one of the APDs should not be joined with the junction  315  of another one of the APDs. Charge carriers amplified at one of the junctions  315  of the APDs  110  should not be shared with another of the junctions  315 . The junction  315  of one of the APDs may be separated from the junction  315  of the neighboring APDs by the material of the absorption region wrapping around the junction, by the material of the layer  312  or  313  wrapping around the junction, by an insulator material wrapping around the junction, or by a guard ring of a doped semiconductor. As shown in  FIG. 4A , the layer  312  of each of the APDs  110  may be discrete, i.e., not joined with the layer  312  of another one of the APDs; the layer  313  of each of the APDs  110  may be discrete, i.e., not joined with the layer  313  of another one of the APDs.  FIG. 4B  shows a variant of the UV image sensor  100 , where the layers  312  of some or all of the APDs are joined together.  FIG. 4C  shows a variant of the UV image sensor  100 , where the junction  315  is surrounded by a guard ring  316 . The guard ring  316  may be an insulator material or a doped semiconductor. For example, when the layer  313  is heavily doped n-type semiconductor, the guard ring  316  may be n-type semiconductor of the same material as the layer  313  but not heavily doped. The guard ring  316  may be present in the UV image sensor  100  shown in  FIG. 4A  or  FIG. 4B .  FIG. 4D  shows a variant of the UV image sensor  100 , where the junction  315  has an intrinsic semiconductor layer  317  sandwiched between the layer  312  and  313 . The intrinsic semiconductor layer  317  in each of the APDs  110  may be discrete, i.e., not joined with other intrinsic semiconductor layer  317  of another APD. The intrinsic semiconductor layers  317  of some or all of the APDs  110  may be joined together. 
       FIG. 5A  schematically shows a system comprising the UV image sensor  100 . The system may scan along a high voltage transmission line  1002 , capture images of the high voltage transmission line  1002  with UV light using the UV image sensor  100 , and detect locations of damages on the high voltage transmission line  1002 . UV light may be emitted from the damages due to corona discharge. The system may include an unmanned aerial vehicle (UAV)  1102  with the UV image sensor  100  mounted thereto, as schematically shown  FIG. 5B . The UAV may fly along the high voltage transmission line  1002 . 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.