Patent Publication Number: US-11653576-B2

Title: SNSPD with integrated aluminum nitride seed or waveguide layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Application Ser. No. 62/969,637, filed on Feb. 3, 2020, the disclosure of which is incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosure concerns a superconducting nanowire single photon detector (SNSPD) that includes a seed layer below the metal nitride that provides the superconductive material. 
     Background Discussion 
     In the context of superconductivity, the critical temperature (T C ) refers to the temperature below which a material becomes superconductive. Niobium nitride (NbN) is a material that can be used for superconducting applications, e.g., superconducting nanowire single photon detectors (SNSPD) for use in quantum information processing, defect analysis in CMOS, LIDAR, etc. The critical temperature of niobium nitride depends on the crystalline structure and atomic ratio of the material. For example, referring to  FIG.  1   , cubic δ-phase NbN has some advantages due to its relatively “high” critical temperature, e.g., 9.7-16.5° K. 
     Niobium nitride can be deposited on a workpiece by physical vapor deposition (PVD). For example, a sputtering operation can be performed using a niobium target in the presence of nitrogen gas. The sputtering can be performed by inducing a plasma in the reactor chamber that contains the target and the workpiece. 
     SUMMARY 
     In one aspect, a superconducting nanowire single photon detector (SNSPD) device includes a substrate having a top surface, an optical waveguide on the top surface of the substrate to receive light propagating substantially parallel to the top surface of the substrate, a seed layer of metal nitride on the optical waveguide, and a superconductive wire on the seed layer. The superconductive wire is a metal nitride different from the metal nitride of the seed layer and is optically coupled to the optical waveguide. 
     In another aspect, a superconducting nanowire single photon detector (SNSPD) device includes a substrate having a top surface, a metal nitride optical waveguide on the top surface of the substrate to receive light propagating substantially parallel to the top surface of the substrate, and a superconductive wire on the optical waveguide. The superconductive wire is a metal nitride selected from the group consisting of niobium nitride, titanium nitride, and niobium titanium nitride. The metal nitride of the optical waveguide is different from the metal nitride of the superconductive wire. 
     Implementations may provide, but are not limited to, one or more of the following advantages. A device based on absorption of photons by a superconductive material, e.g., an SNSPD, can have high photon absorption efficiency while also achieving high material quality for the superconductive layer, e.g. the niobium nitride, and thus a higher critical temperature. This permits fabrication of devices, e.g., SNSPD, with superconductive wires with a higher critical temperature. The larger difference between the operating temperature (2-3° K) and the critical temperature provides superior detection efficiency, lower dark count, and possibly faster temporal response. 
     It should be noted that “superconductive” indicates that the material becomes superconducting at the operating temperature of the device, e.g., 2-3° K. The material is not actually superconducting during fabrication of the device at or above room temperature or when the device is not being cooled for operation. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other potential aspects, features, and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    diagram illustrating phase of niobium nitride as a function of processing temperature and atomic percentage nitrogen. 
         FIG.  2 A  is a schematic top view of a SNSPD that includes a distributed Bragg reflector. 
         FIG.  2 B  is a schematic cross-sectional side view of the device of  FIG.  2 A . 
         FIG.  3    is a schematic illustration of the operation of a SNSPD. 
         FIG.  4    is a schematic cross-sectional side view of a SNSPD that includes a distributed Bragg reflector and an aluminum nitride seed layer. 
         FIG.  5    is a graph of reflectance as a function of wavelength for two SNSPD designs. 
         FIG.  6    is a graph of critical temperature as a function of thickness of a NbN layer, with and without an aluminum nitride seed layer. 
         FIG.  7    is a flow chart of a method of fabricating a SNSPD. 
         FIG.  8 A  is a schematic top view of a SNSPD that includes a waveguide. 
         FIG.  8 B  is a schematic cross-sectional side view of the device of  FIG.  8 A . 
         FIG.  9    is a schematic cross-sectional side view of a SNSPD that includes a waveguide and an aluminum nitride seed layer. 
         FIG.  10    is a schematic cross-sectional side view of a SNSPD that includes a waveguide formed of aluminum nitride. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIGS.  2 A and  2 B  illustrate top and side views, respectively, of a conventional superconducting nanowire single photon detector (SNSPD) device  10 . The SNSPD device  10  can include at least one superconductive wire  12  disposed on a support structure  20 . The superconductive wire  12  can be connected between conductive electrodes  14 . The superconductive wire  12  can be arranged in a meandering pattern, e.g., a back-and-forth parallel lines, on the supporting structure  20 . In some implementations, multiple wires  12  are connected in parallel between the electrodes  14 , with each wire  12  covering a separate area  16 , but there could be just a single wire  12  covering the entire detection area of the device  10 . In addition, many other patterns are possible, e.g., zigzag or double spiral. The superconductive wire can be considered a nanowire, e.g., can have a width of about 30 nm and a thickness of about 10 nm. 
     The support structure  20  can include a substrate  22 , e.g., a silicon substrate, and a mirror structure  24  disposed on the substrate  22 . As an example, the mirror structure  24  can be a distributed Bragg reflector (DBR) that includes multiple pairs of layers formed of high refractive index and low refractive index materials. 
     A conventional SNSPD is operated with a photon (illustrated by light beam  30 ) approaching from the top of the device  10 , e.g., with normal incidence relative to the substrate  20 . A simple device would be have the NbN nanowires disposed directly on the silicon substrate (without the mirror structure). Because the NbN nanowires are typically very thin in a SNSPD device, most of the light is not absorbed by the NbN nanowire. To boost the light absorption efficiency, the mirror structure  24 , e.g., the distributed Bragg reflector, is incorporated in the device  10  between the substrate  20  and the wires  12 . In this case, incident photons that are not initially absorbed can be absorbed on reflection, so the photons have a higher probability to be captured by the NbN nanowires. 
     Referring to  FIG.  3   , the working principle of the SNSPD device is that the to-be-detected photon comes from top and shines on the SNPSD. Absorption of the photon creates a hot spot on the NbN nanowire which raises the temperature of the NbN above critical temperature so that a portion of the wire is no longer in the superconductive state. A region around the hot spot can experience current crowding, resulting in a higher current density than the critical current density, which can disrupt the superconductive state for the entire wire. The change in the NbN wire from the superconducting state to the normal resistive state can be electrically detected by flowing a current through the device and monitoring voltage differences between the electrodes. 
     NbN based SNSPDs are mainly used for time-correlated single-photon counting (TCPSC) related applications in the visible and infrared wavelength. For example, SNSPDs are used in quantum metrology (quantum key generation, quantum emitter) and optical quantum computing (detection module) due to their high efficiency, low dark count, low timing jitter, and fast recover time. They can be also used as detectors in classical space-to-ground communications and time-of-flight LIDAR system. 
     In the visible wavelength range, Si avalanche photodiodes (APDs) are typically used. The system detection efficiency is not ideal, e.g., is about 70%, and these devices are hard to integrate with chip-scale devices. 
     In the infrared wavelength range, InGaAs APDs are a candidate for many applications. But these devices usually suffer from high dark count rates and even lower system detection efficiency (&lt;30%) with limited detection speed. Compared to APDs, SNSPDs, have superior performance which includes low timing jitter (&lt;20 ps), fast recover time, high detection efficiency (&gt;85%), and low dark count rates (˜a few Hz). 
     As noted above, niobium nitride, particularly δ-phase NbN, has some advantages as a superconductive material. However, δ-phase NbN can be difficult to deposit at a satisfactory quality. A seed layer, e.g., an aluminum nitride (AlN) layer, below the (super)conductive layer can help improve the critical temperature of the NbN layer. The aluminum nitride (AlN) seed layer can also improve the critical temperature of TiN and NbTiN layers, and may also be helpful for other metal nitride layers. In particular, the aluminum nitride layer can be integrated into a SNSPD device, and in particular be integrated into the mirror structure or waveguide of a SNSPD device. This permits a higher crystal quality metal nitride detector (thus higher critical temperature, and thus better device performance) to be achieved while also enabling high light absorption efficiency. 
       FIG.  4    illustrates a cross-sectional side view of a superconducting nanowire single photon detector (SNSPD) device  100 . The SNSPD device  100  can be similar to the device described above with respect to  FIGS.  2 A and  2 B , except as described below. 
     The SNSPD includes a substrate  22 , which can be a dielectric material, e.g., sapphire, SiO 2 , fused silica, or quartz, or a semiconductor material, e.g., silicon, gallium nitride (GaN) or gallium arsenide (GaAs). 
     A distributed Bragg reflector (DBR)  24  is fabricated on top of the substrate  22 . The DBR  24  includes multiple bi-layers  26 , e.g., two to eight bi-layers, e.g., seven bi-layers. Each bi-layer  26  includes a lower layer  26   a  of a first material having a first index of refraction (the “low index of refraction”) and an upper layer  26   b  of a second material having a second index of refraction (the “high index of refraction”) that is greater than the first index refraction. The thicknesses and materials (and thus indices of refraction) in the bilayers  26  are selected to increase reflection in a selected wavelength or wavelength band. For example, the DBR may be optimized for reflection of light of about 1500-1600 nm, as 1550 nm is a widely used wavelength in optical communication systems. 
     The first material and the second material can both be selected from Table 1, subject to the restriction that the second material has a higher index of refraction than the first material. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Refractive index 
               
               
                   
                 Material 
                 (for 1550 nm light) 
               
               
                   
                   
               
             
            
               
                   
                 a-Si 
                 ~3.4-3.5 
               
               
                   
                 TiO 2   
                 ~2.2-2.3 
               
               
                   
                 Nb 2 O 5   
                 ~2.1-2.2 
               
               
                   
                 Ta 2 O 5   
                 ~2.05-2.15 
               
               
                   
                 AlN 
                 ~1.95-2.05 
               
               
                   
                 Si 3 N 4   
                 ~1.9-2.0 
               
               
                   
                 SiO 2   
                 ~1.4-1.5 
               
               
                   
                   
               
            
           
         
       
     
     Covering the upper layer  26   b  of the topmost bilayer  26  of the distributed Bragg reflector, e.g., in direct contact with the upper layer  26   b , is a metal nitride seed layer  102 . The seed layer  102  and the superconductive wires  12  are nitrides of different metals. In particular, the seed layer  102  can be aluminum nitride (AlN), as this improves the critical temperature of NbN. However, hafnium nitride (HfN), gallium nitride (GaN) might also be suitable. The metal nitride seed layer  102  can have a thickness of about 4 to 50 nm, e.g., about 5 nm or about 10 nm or about 20 nm thickness. The seed layer  102  can have a (002) c-axis crystal orientation. The seed layer  102  is not superconducting at the operating temperature of the device  100 . The seed layer  102  can be deposited by a standard chemical vapor deposition or physical vapor deposition process. 
     In some implementations, the high index of refraction material of the upper layer  26   b  is Ta 2 O 5 , e.g., of about 182 nm thickness, and the low index of refraction material of the lower layer  26   a  is SiO 2 , e.g., of about 263 nm thickness. The reflectance, as simulated by optical modelling software, of a stack of seven such bilayers with an AlN seed layer of 20 nm thickness is shown by curve  120  in  FIG.  5   . 
     In an embodiment of particular interest, the seed layer  102  is aluminum nitride, and the low-index material, i.e., the material of each lower layer  26   a , is also aluminum nitride. This permits the seed layer  102  to be fabricated using the same processing conditions as the lower-layers in the distributed Bragg reflector  24 , and thus simplifies processing requirements. In some implementations, the high index of refraction material is amorphous silicon (a-Si), e.g., of about 111 nm thickness, and the low index of refraction material is AlN, e.g., of about 197 nm thickness. The reflectance, as simulated by optical modelling software, of a stack of seven such bilayers with an AlN seed layer of 20 nm thickness is shown by curve  122  in  FIG.  5   . 
     The superconductive wires  12  are formed on, e.g., in direct contact with, the seed layer  102 . The wires are formed of niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-X N). The wires  12  can have a width of about 25 to 250 nm, e.g., about 60 nm, and a thickness of 4 to 50 nm, e.g., about 5 nm or about 10 nm or about 20 nm. 
     The seed layer  102  helps improve the critical temperature of the aluminum nitride, especially when the aluminum nitride layer is thin. For example,  FIG.  6    illustrates the measured critical temperature T C  (in Kelvin) as a function of thickness of a NbN layer. Curve  130  illustrates the critical temperature without an aluminum nitride seed layer, and curve  132  illustrates the critical temperature with an aluminum nitride seed layer (for a simplified stack of a silicon wafer, AlN seed layer, and NbN layer). Alternatively or in addition, the seed layer  102  can improve adhesion between the aluminum nitride layer  102  and the upper layer  26   b  of the distributed Bragg reflector  24 . 
       FIG.  7    is a flowchart of a method  200  of fabrication of the device  100  of  FIG.  4     
     Initially, the distributed Bragg reflector (DBR)  24  is deposited on a substrate  100  (step  202 ). The substrate can be, for example, a silicon wafer. Although illustrated as a single block, the substrate  22  could include multiple underlying layers. The DBR  24  can be deposited by alternating deposition of the high and low index materials using a standard chemical vapor deposition or physical vapor deposition process. 
     Next, the seed layer  102  is deposited on the DBR  24  (step  204 ). As mentioned above, the seed layer  102  can be aluminum nitride. The seed layer can be deposited using a standard chemical vapor deposition or physical vapor deposition process. Exemplary processing parameters are a power applied to the sputtering target of 1-5 kW, a total pressure (nitrogen and inert gas) of 2 to 20 mTorr with nitrogen gas and inert gas supplied at a ratio between 3:100 and 1:6, a wafer temperature of 200-500° C., and no bias voltage applied to the wafer. In some implementations, the seed layer  102  is deposited in the same processing chamber that is used to deposit the DBR  24 , e.g., by switching in a new target. This permits higher throughput manufacturing. Alternatively, the substrate can be transported to a different deposition chamber without breaking vacuum. Either case permits the seed layer to be deposited without exposure of the DBR to atmosphere and with lower risk of contamination. 
     Next, the metal nitride layer  12 , e.g., the niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-X N), is deposited on the seed layer (step  206 ). The metal nitride layer  12  can be deposited using a standard chemical vapor deposition or physical vapor deposition process. Exemplary processing parameters are a base pressure of 1e-8 Torr, a power applied to the target of 1-3 kW, a total pressure during processing of 5-7 mTorr, a wafer temperature of 400 C, no bias voltage applied to the wafer, and a percentage of the gas as N 2  sufficient to achieve cubic δ-phase NbN. In some implementations, the metal nitride layer  12  is deposited in the same processing chamber that is used to deposit the seed layer  102 , e.g., by switching in a new target. This permits higher throughput manufacturing. Alternatively, the substrate can be transported to a different deposition chamber without breaking vacuum. This permits the metal nitride layer to be deposited without exposure of the seed layer to atmosphere and with lower risk of contamination. 
     After the metal nitride layer  12  is deposited, a capping layer  104  can be deposited on the metal nitride layer  12  (step  208 ). The capping layer  104  serves as a protective layer, e.g., to prevent oxidation of the metal nitride layer  12  or other types of contamination or damage. The capping layer  104  can be dielectric or conductive but is not superconductive at the operating temperature of the device  100 . The capping layer  104  can be amorphous silicon (a-Si). In some implementations, the capping layer  104  is a nitride of a different material from the metal of the metal nitride used for the superconductive layer  12 . Examples of materials for the capping layer  104  include AlN, Al 2 O 3 , SiO 2 , and SiN. The capping layer  104  can be deposited by a standard chemical vapor deposition or physical vapor deposition process. 
     Etching can be used to form trenches  108  through at least the metal nitride layer  12  to form the conductive wires  12  or other structures needed for the device  100  (step  210 ). Although  FIG.  4    illustrates the trench as extending through the metal nitride layer  12  and capping layer  104 , other configurations are possible. As an example, the trenches can extend partially into or entirely through the seed layer  102 . However, the trenches should not extend into the mirror structure  24 . 
     Another form of superconducting nanowire single photon detector (SNSPD) device includes a waveguide to input photons into the detector along an axis generally parallel to the surface of the substrate.  FIGS.  8 A and  8 B  illustrate a conventional SNSPD  50  having such a waveguide. The SNSPD  50  can include ate least one superconductive wire  52  disposed on a support structure  60 . The support structure can include a substrate  62 , e.g., a silicon substrate, a dielectric layer  64  on the substrate  62 , and a waveguide  66  disposed on the dielectric layer  64 . The dielectric layer  64  is a first material having a first refractive index, and the waveguide  66  is a second material having a second refractive index that is higher than the first refractive index. 
     The superconductive wire  52  can be considered a nanowire, e.g., can have a width of about 30 nm and a thickness of about 10 nm. The superconductive wire(s)  52  can be arranged to form a plurality of parallel lines, with adjacent lines connected at alternating ends. Although  FIG.  8 A  illustrates four parallel lines, the device could have just two parallel lines, e.g., a U-shaped wire, or a greater number of lines. 
     Photons, shown by light beam  70 , are injected into the device from the side, e.g., generally parallel to the top surface of the substrate  62 , through the waveguide layer  66 . In particular, the photons can enter along an axis (shown by arrow A) generally parallel to the parallel lines of the wire  52 . In addition, along the axis transverse to the direction of light propagation, the wire  52  can be located near the center of the waveguide. For example, on each side of device, there can be a gap  58  between the outer edge of the wire  52  and the outer edge of the waveguide  66 . This gap  58  can have a width of about 25-30% of the total width of the waveguide. 
     In general, because the dielectric layer  64  below the waveguide  66  and the empty space or air above the waveguide  66  both have a lower refractive index than the waveguide  66 , the photons in the waveguide  66  are trapped by total internal reflection. However, due to the optical coupling between the waveguide  66  and the nanowire  52 , the photons can escape into the nanowires  52  and thus be absorbed by the nanowire  62 . The light coupling efficiency can be very high in this type of device. 
       FIG.  9    illustrates a cross-sectional side view of a waveguide-configuration of a superconducting nanowire single photon detector (SNSPD) device  150 . The SNSPD device  150  can be similar to the devices described above with respect to  FIGS.  4  and  8   , except as described below. 
     The SNSPD device  150  includes a substrate  62 , such as a silicon substrate. 
     Covering the top surface of the substrate  62  is the dielectric layer  64 . The dielectric layer  64  can be silicon oxide (SiO 2 ), although other materials having a refractive index less than that of the waveguide  66  are possible. The dielectric layer  64  can have a thickness of at least 100 nm, e.g., 200 nm to 2 um. 
     The waveguide  66  is disposed on the dielectric layer  64 . The waveguide  66  can be silicon nitride (Si 3 N 4 ), although other materials having a refractive index greater than that of the dielectric layer  64  are possible. The particular thickness and width of the waveguide can be selected based on the wavelength of light to be captured and detected. The waveguide  66  can have a thickness of 400 to 500 nm, e.g., 450 nm, for 1550 nm light. The width of the waveguide  66 , i.e., perpendicular to the direction of propagation of the light entering the waveguide  66 , can be 1.1 to 1.3 um, e.g., 1.2 um for 1550 nm light. 
     On the top surface of the waveguide  66 , e.g., in direct contact with the waveguide  66 , is a metal nitride seed layer  152 . The seed layer  152  and the superconductive wires  52  are nitrides of different metals. In particular, the metal nitride of the seed layer  152  can be aluminum nitride (AlN), as this improves the critical temperature of NbN. However, gallium nitride (GaN) might also be suitable. The seed layer  152  can have a thickness of about bout 4 to 50 nm, e.g., about 5 nm or about 10 nm or about 20 nm thickness. The seed layer  152  can have a (002) c-axis crystal orientation. The seed layer  152  is not superconducting at the operating temperature of the device  150 . 
     The superconductive wires  52  are formed on, e.g., in direct contact with, the seed layer  152 . The wires are formed of niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-X N). The wires  52  can have a width of about 25 to 250 nm, e.g., about 60 nm, and a thickness of 4 to 50 nm, e.g., about 5 nm or about 10 nm or about 20 nm. 
     A capping layer  154  can cover the superconductive wires  52 . The capping layer  154  serves as a protective layer, e.g., to prevent oxidation of the metal nitride of the superconductive wires  52  or other types of contamination or damage. The capping layer  154  can be dielectric or conductive but is not superconductive at the operating temperature of the device  150 . The capping layer  154  can be amorphous silicon (a-Si). In some implementations, the capping layer  154  is a nitride of a different material from the metal of the metal nitride used for the superconductive layer  52 . Examples of materials for the capping layer  104  include AlN, Al 2 O 3 , SiO 2 , and SiN. The capping layer  104  can be deposited by a standard chemical vapor deposition or physical vapor deposition process. 
     Trenches that separate the wires  52  can extend through the capping layer  154 , the superconductive layer that provides the wires  52 , and the seed layer  152 . The trenches need not extend into the waveguide. 
       FIG.  10    illustrates a cross-sectional side view of another embodiment of a waveguide-configuration of a superconducting nanowire single photon detector (SNSPD) device  150 ′. The SNSPD device  150 ′ can be similar to the devices described above with respect to  FIG.  9   , except as described below. 
     In the embodiment shown in  FIG.  10   , the waveguide  66 ′ is formed of aluminum nitride (AlN). Thus a separate seed layer is unnecessary, and the waveguide  66 ′ itself acts as the seed layer for the NbN. 
     The dielectric layer  64  can be silicon oxide (SiO 2 ), although other materials having a refractive index less than that of the aluminum nitride of the waveguide  66 ′ are possible, e.g., silicon nitride (Si 3 N 4 ). As noted above, the particular thickness and width of the waveguide can be selected based on the wavelength of light to be captured and detected. The waveguide  66 ′ can have a thickness of 400 to 500 nm, e.g., 450 nm, for 1550 nm light. The width of the waveguide  66 ′, i.e., perpendicular to the direction of propagation of the light entering the waveguide  66 ′, can be 1.1 to 1.3 um, e.g., 1.2 um for 1550 nm light. 
     The superconductive wires  52  are formed on, e.g., in direct contact with, the waveguide  66 ′. 
     While particular implementations have been described, other and further implementations may be devised without departing from the basic scope of this disclosure. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the drawings illustrate only exemplary embodiments. The scope of the invention is determined by the claims that follow.