Patent Publication Number: US-2020299830-A1

Title: Method and apparatus for deposition of metal nitrides

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Application Ser. No. 62/822,585, filed on Mar. 22, 2019, the disclosure of which is incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosure concerns a reactor for processing a workpiece to deposit a metal nitride, particularly a metal nitride that is suitable as a superconductive material. 
     Background Discussion 
     In the context of superconductivity, the critical temperature (Tc) 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 method of forming a structure including a metal nitride layer on a workpiece includes pre-conditioning the chamber by flowing nitrogen gas and an inert gas at a first flow rate ratio into the chamber and igniting a plasma in the chamber before placing the workpiece in a chamber that includes a metal target, evacuating the chamber after the preconditioning, placing the workpiece on a workpiece support in the chamber after the preconditioning, and performing physical vapor deposition of a metal nitride layer on the workpiece in the chamber by flowing nitrogen gas and the inert gas at a second flow rate ratio into the chamber and igniting a plasma in the chamber. The second flow rate ratio is less than the first flow rate ratio. 
     In another aspect, a physical vapor deposition system includes chamber walls forming a chamber, a support to hold a workpiece in the chamber, a vacuum pump to evacuate the chamber, a gas supply to deliver nitrogen gas and an inert gas to the chamber, an electrode to support a metal target, a power source to apply power to the electrode, and a controller. The controller is configured to, cause the gas source flow nitrogen gas and the inert gas at a first flow rate ratio into the chamber and cause the power source to apply power sufficient to ignite a plasma in the chamber to pre-conditioning a chamber before a workpiece on which a metal nitride layer is to be deposited is placed in the chamber, and to cause the gas source flow nitrogen gas and the inert gas at a second flow rate ratio into the chamber and cause the power source to apply power sufficient to ignite a plasma in the chamber to deposit a metal nitride layer on the workpiece by physical vapor deposition after the workpiece is placed in the chamber. The second flow rate ratio is less than the first flow rate ratio. 
     In another aspect, a cluster tool for fabrication of a device having a metal nitride layer includes a load lock chamber to receive a cassette holding workpieces, a central vacuum chamber, a plurality of deposition chambers arranged in a cluster formation around and coupled to the central vacuum chamber, a robot to carry a workpiece between the vacuum chamber and the load lock chamber and plurality of deposition chambers, and a controller. The plurality of deposition chambers including a first deposition chamber having a first target, a second deposition chamber having a second target, and a third deposition chamber having a third target. The controller is configured to cause the robot to carry the substrate to the first deposition chamber and to cause the first deposition chamber to deposit a buffer layer on the workpiece, to cause the robot to carry the substrate from the first deposition chamber to the second deposition chamber and to cause the second deposition chamber to deposit a metal nitride layer suitable for use as a superconductor at temperatures above 8° K on the buffer layer, and to cause the robot to carry the substrate from the second deposition chamber to the third deposition chamber and to cause the third deposition chamber to deposit a capping layer on the metal nitride layer. 
     In another aspect, a physical vapor deposition system includes chamber walls forming a chamber, a first target support to hold a first target, a second target support to hold a second target, and a third target support to hold a third target, a movable shield positioned in the chamber and having an opening therethrough, an actuator to move the shield, a workpiece support to hold a workpiece in a lower portion of the chamber, a vacuum pump to evacuate the chamber, a gas supply to deliver nitrogen gas and an inert gas to the chamber, a power source to selectively apply power to the first target, the second target or the third target, and a controller. The controller is configured to cause the actuator to move the shield to position the opening adjacent the first target, cause the gas source to flow a first gas into the chamber, and cause the power source to apply power sufficient to ignite a plasma in the chamber to cause deposition of a buffer layer of a first material on the workpiece on the workpiece support, cause the actuator to move the shield to position the opening adjacent the second target, cause the gas source to flow a second gas into the chamber, and cause the power source to apply power sufficient to ignite a plasma in the chamber to cause deposition of a device layer of a first material that is a metal nitride suitable for use as a superconductor at temperatures above 8° K on the buffer layer, the first material being different composition from the second material, cause the actuator to move the shield to position the opening adjacent the third target, cause the gas source to flow a third gas into the chamber, and cause the power source to apply power sufficient to ignite a plasma in the chamber to cause deposition of a capping layer of a third material on the device layer, the third material being different composition from the first and second materials. 
     These aspects can include one or more of the following features. 
     The metal target may include niobium or a niobium alloy. The metal nitride layer may include niobium nitride or a niobium alloy nitride. The metal target may be substantially pure niobium, and the metal nitride layer may be substantially pure niobium nitride. The metal nitride layer may be δ-phase NbN. The plasma may sputter the metal of the metal target. 
     The second flow ratio may be 2-30% less than the first flow rate ratio. The first flow rate ratio may be 4:100 to 1:1, and the second flow rate ratio may be 3:100 to 48:52. The chamber may be evacuated to a pressure lower than 10 −8  Torr. 
     Pre-conditioning may include placing a shutter disk on the substrate support. A robot configured to position a shutter disk into the chamber for the pre-condition of the chamber. Pre-conditioning may include heating the shutter disk to a temperature, and performing the physical vapor deposition may include heating the workpiece to the same temperature. The temperature may be 200-500° C. Igniting plasma in preconditioning and igniting plasma in deposition may use the same power level. 
     A nitrogen ion concentration in the plasma may be measured with an optical sensor. A flow rate of the nitrogen gas and/or the inert gas may be adjusted in response to nitrogen ion concentration measured by the sensor to bring the nitrogen ion concentration to a desired concentration. The sensor is positioned outside the chamber, and wherein the chamber walls include a window to provide optical access to the chamber for the sensor. 
     A sputter shield may be positioning in the chamber. The sputter shield may have an opening to provide a clear line of sight for the sensor to the plasma. 
     A buffer layer may be formed on the workpiece before forming the metal nitride layer. The metal nitride layer may be deposited directly on the buffer layer. The buffer layer may be a metal nitride of a metal different than the metal of the target. The buffer layer may be aluminum nitride. 
     A capping layer may be formed on the metal nitride layer. The capping layer may include carbon, silicon, a metal different than the metal of the target, or a nitride of a material different than the metal of the target. The capping layer may be carbon, silicon nitride or titanium nitride. 
     The first target may be a metal other than a metal of the second target. The first gas may include nitrogen gas. The second target may include niobium. The second gas may include nitrogen gas. The third target may include carbon, silicon, or a metal other than a metal of the second target. 
     The shield may be rotatable and the actuator may be configured to rotate the shield. 
     Some implementations may provide one or more of the following advantages. The process permits reliable or stable deposition of high quality NbN with a high critical temperature. This permits fabrication of devices, e.g., SNSPD, that operate at higher temperatures, thus making such devices more practical. Devices can be fabricated with higher quantum efficiency and low dark current. Devices can also be fabricated with reduced timing jitter and faster detection response. A buffer layer, a superconductive film, and a capping layer can be deposited by a single tool without removing the workpiece from vacuum. This can significantly improve process stability and manufacturability, and reduces risk of contamination, e.g., oxidation, which also helps preserve the high critical temperature. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other potential features, aspects, 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  is a schematic cross-sectional side view of a reactor to deposit metal nitrides. 
         FIG. 3  is a graph illustrating voltage on the target as a function of nitrogen flow and critical temperatures measured for various nitrogen flow values. 
         FIG. 4  is a flow chart of a process for depositing a metal nitride. 
         FIG. 5  is a schematic cross-sectional view of a device that includes a metal nitride layer for use as a superconductive material during operation. 
         FIG. 6  is a flow chart of a process for fabricating a device that includes a metal nitride layer for use as a superconductive material. 
         FIG. 7  is a schematic top view of a cluster tool to deposit a seed layer, a metal nitride, and a capping layer. 
         FIG. 8  is a schematic side view of a processing chamber to deposit multiple layers of different composition. 
         FIG. 9  is a schematic top view of the processing chamber of  FIG. 8 . 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     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. For example, it may need a high vacuum (10 −9  Torr or lower), as well as high mobility species (high temp, high peak power, and low duty cycle in pulsed DC). Although semiconductor grade deposition tools can provide good uniformity, they typically are configured for a lowest vacuum of about 10 −8  Torr. However, increased pump capacity and additional traps for smaller atomic weight gases, e.g., water vapor, can improve vacuum performance. 
     Another issue is that a buffer layer, e.g., an aluminum nitride (AlN) layer, below the (super)conductive layer can help improve the critical temperature of the metal nitride layer. 
     Similarly, a capping layer, e.g., a carbon layer or silicon nitride layer, above the (super)conductive layer can help protect the metal nitride layer, e.g., to prevent oxidation. The capping and buffer layer would typically be provided by separate deposition tools. Unfortunately, removing the workpiece from the tool used for deposition of the metal nitride can result in contamination or oxidation, thus reducing the critical temperature. However, a cluster tool can be configured to have multiple chambers, each of which can deposit a layer without removing the workpiece from the vacuum environment, or a single chamber can be configured to deposit each of the layers, thereby avoiding the need to remove the workpiece. 
     Yet another issue is that even under good deposition conditions, reliably depositing a film with as high a critical temperature as possible can be difficult. However, it has been discovered that the critical temperature of niobium nitride as a function of nitrogen content exhibits a hysteresis effect; different critical temperatures can be obtained depending on whether the nitrogen content has been ramped up or ramped down. By performing a preconditioning of the chamber using nitrogen gas before depositing the layer on the workpiece, the process can follow the more advantageous curve of this hysteresis effect. As a result, a higher critical temperature can be obtained more reliably. 
     Deposition System 
     Referring now to  FIG. 2 , a physical vapor deposition (PVD) reactor  100  includes a vacuum chamber  110 . The chamber  110  is enclosed by chamber walls, including a side wall  112 , a floor  114  and a ceiling  116 . A workpiece support  120 , e.g., a pedestal or susceptor, can be positioned inside the chamber  110 . The workpiece support  120  has a top surface  120   a  to support a workpiece  10  inside the chamber  110 . The support  120  can be elevated above the floor  114 . 
     In some implementations, a temperature control system can control the temperature of the support  120 . For example, the temperature control system can include a resistive heater embedded in or placed on the surface  120   a  of the workpiece support  120  and a power source electrically coupled to the heater. Alternatively or in addition, coolant channels can be formed in the workpiece support  120 , and coolant from a coolant supply can flow, e.g., be pumped, through the channels. 
     In some implementations, the vertical position of the workpiece support  120  is adjustable, e.g., by a vertical actuator. 
     An opening  118  (e.g., a slit valve) can be formed in a wall of the chamber  110 . An end effector (not shown) can extend through the opening  118  to place the substrate  10  onto lift pins (not shown) for lowering the substrate onto a support surface  120   a  of the workpiece support  120 . 
     In some implementations, the support  120 , or a conductive electrode  121  (see  FIG. 8 ) within the support, is grounded. Alternatively, an external power source  136  (see  FIG. 8 ), e.g., a DC or RF power source, can be used to apply a bias voltage or RF power to the support  120  or a conductive electrode  121  within the support  120 , and thus apply a bias voltage or RF power to the workpiece  10 . Optionally, the power source  136  can be coupled to the electrode  121  by an RF matching network  137  (see  FIG. 8 ). 
     A sputter shield  126  can be positioned inside the chamber  110  to prevent sputtering of material onto the chamber side walls  112 . 
     An electrode  130  forms a portion of the ceiling  116 , and a target  140  can be supported from the electrode  130 . The electrode  130  is electrically coupled to a power source  132 . The power source  132  can be configured to apply a pulsed DC voltage. The power source  132  can be coupled to the electrode  10  by an RF matching network  133 . The power applied can be 500 W to 20 kW, the voltage can be 200V to 600V, the frequency can be 50 kHz to 250 kHz, and the duty cycle can be 60-100%. 
     The target  140  is a body, e.g., a disk, formed of the metal from which the metal nitride is to be deposited. On installation, the target can be substantially pure metal, e.g., a body of substantially pure niobium. However, during processing nitrogen can react with the surface of the target, forming a surface layer of metal nitride. 
     A vacuum pump  150  is coupled to the chamber  110 , e.g., by a passage with an opening in a region below the workpiece support  120 , e.g., in the floor  112 , to evacuate the chamber  110 . Examples of vacuum pumps include an exhaust pump with throttle valve or isolation valve, cryogenic pump and turbo pump backed up by a mechanical pump. Although  FIG. 2  illustrates a single vacuum pump, in some implementations multiple pumps can be used to increase the vacuum level. The assembly of one or more vacuum pumps  150  can bring the chamber down to a vacuum of less than 8×10 −9  Torr. The vacuum pumps  150  can maintain a rate of rise of less than 50 nTorr/min at the elevated deposition temperature. 
     A condensation plate  152  can be located in the passage connecting the chamber  110  to the vacuum pump  150 . The condensation plate  152  provides a surface of or positioned in the passage that is cooled sufficiently for water to condense on the plate  152 . Thus, the condensation plate  152  acts to capture trap water vapor, and other lower-weight molecules that can condense, thus preventing such gas from forming part of the plasma and reducing the impurity in the metal nitride film. 
     A gas source  160  is fluidically coupled to the chamber  110 . The gas source  160  includes a source  162  of nitrogen gas (N 2 ) and a source  164  of an inert gas, e.g., argon or helium. Flow rates of the nitrogen gas and the inert gas, and thus the ratio of flow rates, can be controlled by independently controllable valves  166  and mass flow controllers. Although  FIG. 2  illustrates separate passages entering the chamber, the gasses could be mixed before entering the chamber  110 , and more complicated gas distributor apparatus, e.g., a gas distribution plate or showerhead, an array of radial passages through the side wall, etc., could be used to distribute gas into the chamber  110 . 
     Application of power at an appropriate frequency and power to the electrode  130  by the power source  132  can ignite a plasma  111  in the chamber  110 . In particular, a pulsed DC bias can be applied through the electrode  130  to the sputtering target  140 , and the workpiece support  120  can be electrically floating. The resultant electric field in the chamber  110  ionizes the sputtering gas to form a sputtering plasma  111  that sputters the target  140 , causing deposition of material on the workpiece  10 . The plasma  111  is typically generated by applying a DC power level between 100 Watts and 20 kWatts, e.g., 1-5 kWatts. The power source  132  can supply DC pulses at a frequency 50 kHz to 250 KHz, e.g., 200 kHz. The duty cycle of the pulses can be 50-100%, e.g., 50-70% or 60-100%. 
     In some implementations, a magnet assembly  170  is positioned outside the chamber  110 , e.g., above the ceiling  116 . The magnet assembly  170  can help confine the ions in the plasma and increase ion energy on the substrate  10 . 
     A controller  190 , e.g., a programmed general purpose computer having a processor, memory and non-transitory storage media to store a computer program, can be coupled to the various components to control the processing system  100 . 
     An optical emission sensor  180  can be used to monitors the plasma concentration and/or to monitor the gas composition during deposition. The sensor  180  can be positioned outside the chamber  110 , but be placed adjacent a window  182  through a wall of chamber, e.g., the side wall  112 , to have a view of the plasma  111 . If necessary, an aperture  184  can be formed in the shield  126  to provide the sensor  180  with a clear line of sight to the plasma  111 . The optical emission sensor  180  can measure the nitrogen ion concentration in the plasma  111 . In some implementations, the optical emission sensor  180  can also measure the inert gas ion concentration in the plasma  111 . The optical emission sensor  180  can provide these measurements to the controller  190 . 
     The controller  190  can be configured to control the gas source  160  to adjust the flow rate(s) of the nitrogen gas and/or the inert gas in response to the measured ion concentration(s). For example, the controller  190  can operate in a feedback loop to control the gas flow rate(s) to bring the nitrogen ion partial pressure to a desired partial pressure, or to bring the nitrogen ion concentration to a desired concentration. The controller  190  can also be configured to control gas flow rates of the gas source  160  to maintain a stable plasma and/or achieve a desired condition on the surface of the target  140 . 
     Deposition of Niobium Nitride 
     The workpiece processing tool  100  can be employed to perform deposition of niobium nitride, in particular s-phase NbN, on a workpiece. In one example, the workpiece  110  includes a buffered layer, e.g., aluminum nitride, onto the niobium nitride is to be deposited. 
       FIG. 3  illustrates voltage on the target as a function of nitrogen flow. A fixed pulsed DC power was applied to the target. The potential of the target was measured with respect to the ground. This potential will change depending on the sputtering yield of the target and ion concentration in the plasma. In general, the target voltage can be a stand-in value for the critical temperature, albeit not as a linear relationship. 
     In general, a higher quality film will exhibit both a higher critical temperature. As noted above, it has been discovered that the critical temperature of niobium nitride as a function of nitrogen content exhibits a hysteresis effect. Still referring to  FIG. 3 , where the nitrogen flow rate is being increased for successive workpieces, the target voltage follows curve  202 . It is believed that the surface of the niobium target goes from a metallic mode to a “poisoned” mode when sufficient N 2  is present in the chamber to form a thin layer of NbN on the target surface. In contrast, where the nitrogen flow rate is being decreased for successive workpieces, the target voltage follows curve  204 . Again, it is believed that as the N 2  partial pressure decreases, the target starts to become “de-poisoned” and switch back to the metallic mode. 
     For both curves  202 ,  204 , the target voltage has a maximum value just before a sudden drop. It is believed that this is because niobium-rich niobium nitride (NbN) film is typically formed if the target is in the metallic mode, whereas a NbN film with good stoichiometry and desired cubic phase is typically formed if the target is in the poisoned mode. 
     However, where the flow rate is being successively decreased (curve  204 ), the drop-off of the target voltage occurs at a lower nitrogen flow rate. This may indicate that the partial pressure where the niobium target is de-poisoned, is lower than the partial pressure at which the niobium target becomes poisoned. 
     In addition, where the flow rate is being successively decreased, the target voltage actually reaches a higher value (indicated at  206 ), as compared to where the flow rate is being successively increased (curve  202 ). Moreover, measurement of the critical temperatures of the niobium nitride deposited at these flow rates confirms that a higher critical temperature can be achieved if the nitrogen flow rate has been decreased rather than increased as compared to a prior process. 
     It is possible to take advantage of this hysteresis effect. In particular, by performing a preconditioning of the chamber using nitrogen gas before depositing the layer on the workpiece, the process can follow the more advantageous curve of this hysteresis effect. As a result, a higher critical temperature can be obtained more reliably. 
     Referring to  FIGS. 2 and 4 , a process  260  for fabricating the metal nitride layer begins by preconditioning the chamber  110  (step  262 ). The chamber  110  is evacuated. A shutter disk, e.g., a metal disk of about the same diameter but thicker than a workpiece, can be placed on the support  120  (the workpiece on which the layer is to be deposited is not present in the chamber  110 ). The gas distribution assembly  160  supplies nitrogen gas to the chamber  110 . 
     The gas distribution assembly  160  can also supply an inert gas, e.g., argon or helium, to the chamber  100 . The inert gas can be used to dilute the nitrogen gas; this can increase plasma density. The gas distribution assembly  146  can establish a total pressure (nitrogen and inert gas) of 2 to 20 mTorr. In this preconditioning step, the nitrogen gas is supplied at a first flow rate, e.g., 15 to 40 sccm, e.g., 20 sccm. The nitrogen gas and the inert gas can be supplied at a first ratio, 4:100 to 1:4, e.g., 4:100 to 1:1, e.g., 2:1 to 1:1, of nitrogen to inert gas (the ratio can be a ratio of flow rates in sccm). Power is applied to the electrode  130 , e.g., as described above, to induce the plasma  111 . 
     This conditioning process can be carried out with the shutter disk at a temperature of, e.g., 200-500° C. The preconditioning process can proceed, e.g., for 60-300 seconds. 
     After preconditioning, the camber is evacuated again, e.g., drawn down to 10 −9  Torr, the dummy substrate is removed, and the workpiece is placed into the chamber  110  and onto the support  120 . 
     Now the metal nitride can be formed on the workpiece by a physical vapor deposition process (step  264 ). The gas distribution assembly  160  supplies nitrogen gas to the chamber  110 , and power is applied to the electrode  130 , e.g., as described above, to induce a plasma  111 . The power source  132  can apply the same RF power, frequency and duty cycle during deposition as in preconditioning. 
     In the deposition step, the nitrogen gas is supplied at a second flow rate that is lower than the first flow rate, while the flow rate of the inert gas remains the same as in the preconditioning step. For example, the second flow rate of the nitrogen gas can be at least 2% lower, e.g., at least 10% lower. For example, the second flow rate can be 2-30% lower, e.g., 10-30% lower. For example, the second flow rate can be 15-18 sccm. 
     Alternatively, the flow rate of the nitrogen gas could be held constant, but the flow rate of the inert gas could be increased. 
     In the deposition step, the nitrogen gas and the inert gas can be supplied at a second ratio, e.g., 3:100 to 1:6, e.g., 3:100 to 45:52, e.g., 1.5:1 to 1:3, of nitrogen to inert gas (the ratio can be a ratio of flow rates in sccm). The second ratio can be less than the second ratio, e.g., at least 2% lower, e.g., at least 10% lower. For example, the second ratio can be 2-30% lower, e.g., 10-30% lower. 
     This physical vapor deposition process can be carried out with the workpiece at a temperature of, e.g., 200-500° C. The workpiece can be processed at the same temperature as the shutter disk in the preconditioning process. The deposition process can proceed, e.g., for 10-600 seconds. 
     Application of power to the electrode  130  at appropriate frequency and duty cycle will ignite plasma in the chamber  110 . The plasma will cause sputtering of material from the target  130  onto the workpiece  10 . Due to the presence of nitrogen in the plasma, a combination of nitrogen and the metal, e.g., niobium nitride, is deposited onto the workpiece. The preconditioning process can enable the deposition of NbN of the right stoichiometry and crystal quality from the beginning to end of the deposition process. 
     Appropriate processing conditions for the physical vapor deposition to form δ-phase NbN should be about in the ranges discussed above, although differences in process chamber configuration, etc., can cause variations. If necessary, appropriate process conditions can be determined empirically. 
     Finally, the chamber  110  is evacuated again and the workpiece is removed. 
     Although the discussion above focuses on niobium nitride, these techniques can be applicable for other metal nitrides, e.g., a nitride of a mixture of niobium with another metal, e.g., NbTiN. 
     Multilayer Device 
       FIG. 5  is a schematic illustration of some layers in a device  220  that includes a metal nitride layer  226  for use as a superconductive material. The device  220  could be superconducting nanowire single photon detectors (SNSPD), a superconducting quantum interference device (SQUID), a circuit in a quantum computer, etc.  FIG. 6  is a flowchart of a method of fabrication of the layers. 
     Initially, a buffer layer  224  can be deposited on a substrate  222  (step  250 ). The substrate can be, for example, a silicon wafer. Although illustrated as a single block, the substrate  222  could include multiple underlying layers. 
     The buffer layer  224  can be a material to help improve the critical temperature of the metal nitride, especially when the metal nitride layer is thin. Alternatively or in addition, the buffer layer  224  can improve adhesion between the metal nitride layer  226  and the substrate  222 . The buffer layer  224  can be dielectric or conductive but is not superconductive at the operating temperature of the device  200 . In some implementations, the buffer layer  224  is be formed of a different metal nitride than the metal nitride used for layer  226 . For example, the buffer layer  224  can be formed of aluminum nitride (AlN), hafnium nitride (HfN), gallium nitride (GaN), or indium nitride (InN). Alternatively, the buffer layer  224  could be formed of a carbide, e.g., silicon carbide. The buffer layer can have a ( 002 ) c-axis crystal orientation. The buffer layer  224  can be deposited by a standard chemical vapor deposition or physical vapor deposition process. 
     Next, the metal nitride layer  226  is deposited on the buffer layer (step  260 ). The metal nitride layer  226  can be deposited using the two-step process  260  and system  100  discussed above. 
     After the metal nitride layer  226  is deposited, a capping layer  228  can be deposited on the metal nitride layer  226  (step  270 ). The capping layer  228  serves as a protective layer, e.g., to prevent oxidation of the metal nitride layer  226  or other types of contamination or damage. The capping layer  228  can be dielectric or conductive but is not superconductive at the operating temperature of the device  200 . In some implementations, the capping layer  228  is a nitride of a different material from the metal of the metal nitride used for layer  226 . In some implementations, the capping layer  228  is a metal different from the metal of the metal nitride used for layer  226 . Examples of materials for the capping layer  228  include carbon, silicon, titanium nitride (TiN), and silicon nitride (SiN). The buffer layer  224  can be deposited by a standard chemical vapor deposition or physical vapor deposition process. 
     Etching can be used to form trenches  230  through at least the metal nitride layer  226  to form the conductive lines or other structures needed for the device (step  280 ). Although  FIG. 4  illustrates the trench as extending through the buffer layer  224 , metal nitride layer  226  and capping layer  228 , other configurations are possible. For example, if the buffer layer  224  and capping layer  228  are both dielectric, then the etching can extend through just the metal nitride layer  226 . In this case, the etching step  280  could be performed before the step  270  of depositing the capping layer. As a result, the capping layer  228  could directly contact the buffer layer  224  in regions between the metal nitride islands. For example, if the buffer layer  224  and capping layer  228  are both dielectric, then the etching can extend through just the metal nitride layer  226 . As another example, the etch can extend through the metal nitride layer  226  and the buffer layer  224  or the capping layer  228  (but not both). 
     Tool for Multilayer Fabrication 
     As noted above, removing the workpiece from a tool used for deposition can result in contamination or oxidation, thus reducing the critical temperature. 
     One technique to avoid this issue is to use a cluster tool with multiple chambers, each of which can deposit a layer without removing the workpiece from the vacuum environment.  FIG. 7  is a schematic top view of a cluster tool  300  to deposit the buffer layer, the metal nitride layer, and the capping layer. The cluster tool  300  includes one or more central vacuum chambers  310 , one or fab interface units  315  to receive cassettes that workpieces, and one or more robots  320  to transfer workpieces from the fab interface units  315  to other processing chambers, between the processing chambers, and from the processing chambers back to the fab interface units  315 . 
     The processing chamber of the cluster tool  300  includes one or more physical vapor deposition chambers  325  for deposition of the buffer layer, e.g., for deposition of aluminum nitride (AlN), one or more physical vapor deposition chambers, e.g., a physical vapor deposition chamber  100  as described above, for deposition of the metal nitride layer, and one or more physical vapor deposition chambers  330  for deposition of the capping layer, e.g., for deposition of a carbon layer. Chambers can be separated by appropriate slit valves. The cluster tool  300  can be controlled by a controller  350 , e.g., a general purpose programmable computer. 
     Another technique to avoid the need to remove the workpiece from vacuum is to deposit each of the layers in a single chamber.  FIG. 8  is a schematic side view of a physical vapor deposition reactor  400  to deposit multiple layers of different composition. For example, the physical vapor deposition reactor  400  can be used to deposit the buffer layer, the metal nitride layer, and the capping layer.  FIG. 9  is a top view of the physical vapor deposition reactor  400  ( FIG. 8  can be considered along section line  8 - 8  in  FIG. 9 ). 
     The physical vapor deposition reactor  400  is constructed in a manner similar to physical vapor deposition reactor  100 , but includes three separate targets  140   a ,  140   b ,  140   c  (additional targets can be present if needed for other layers). The targets can be supported on the ceiling  116  of the chamber  110  of the reactor  400 . Each target is supported on a respective electrode  130   a - 130   c . The different electrodes  130   a - 130   c  can be coupled to a common power source  132 , or to different power sources  132 . 
     A rotatable shield  410  is positioned inside the chamber  110  and is shared by all the electrodes  130 . The shield  410  can be suspended from the ceiling  116  by shaft  420 , and the shaft can be rotated (shown by arrow A) about a vertical axis  426  by an actuator  422 . In some implementations, the actuator  422  can also move the shield  410  vertically (shown by arrow B). 
     The rotatable shield  410  can have a hole  412  to expose a corresponding target. The shield  410  advantageously limits or eliminates cross-contamination between the plurality of targets  140   a - 140   c . The rotatable shield  410  can also have a pocket  414  for each target that is not being sputtered. For example, in some embodiments where three electrodes  130  are provided, the shield  410  can include a hole  412  to expose one target at a time, and two pockets  414  to house the targets that are not being sputtered. By rotating the shield  410 , a different target can be exposed and operated. 
     In some embodiments, the physical vapor deposition reactor  400  includes a plurality of grounding rings  430  to provide improved grounding of the shield  116  to the ceiling  116 , e.g., when the shield  410  is in the retracted position. 
     The three targets  140   a ,  140   b ,  140   c  are formed of different materials, e.g., the materials to be sputtered to form the buffer layer, metal nitride layer and capping layer, respectively. For example, a first target  140   a  can be composed of the non-nitrogen component, e.g., the metal, of the element or compound used for the buffer layer. For example, if the buffer layer is to be formed of aluminum nitride, the first target  140   a  can be aluminum. The second target  140   b  can be the non-nitrogen component, e.g., the metal, of the compound used for the superconductive layer. For example, if the superconducting layer is to be formed of niobium nitride, the second target  140   a  can be niobium. The third target  140   b  can be the non-nitrogen component of the element or compound used in the capping layer, e.g., carbon, silicon or titanium. 
     In operation, the actuator  422  rotates the shield  410  so that the aperture  412  is aligned with the first target  140   a  and the other targets are covered. The vacuum pump  150  evacuates the chamber  110 , the gas source  160  supplies a sputtering gas to the chamber  110 , and the power source  132  applies a power to the electrode  130   a  to generate a plasma in the chamber. The plasma can cause sputtering of the material of the first target  140   a , resulting in physical vapor deposition of the buffer layer onto the substrate  10 . If appropriate, the gas source can supply nitrogen or another gas that will form a compound with the material of the first target  140   a . For example, if aluminum nitride is to be deposited, the first target  140   a  can be aluminum and the gas source can supply both an inert gas, e.g., argon, and nitrogen. If the material of the first target  140   a  is to be deposited as a substantially pure element, then the gas can include only inert elements, e.g., argon or xenon. 
     When deposition of the buffer layer is complete, the chamber  110  is evacuated, and the actuator  422  rotates the shield  410  so that the opening  412  is aligned with the second target  140   b  and the other two targets are covered. The material of the metal nitride layer, e.g., niobium nitride, can be deposited according to the method described above. 
     When deposition of the metal nitride layer is complete, the chamber  110  is evacuated, and the actuator  422  rotates the shield  410  so that the opening  412  is aligned with the third target  140   c . The material of the capping layer can be deposited, in a manner similar to described above for the buffer layer. The chamber  110  can evacuated again, and the workpiece removed, e.g., by a robot. 
     Controller 
     A controller, e.g., the controller  190  and/or controller  150 , can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, or in combinations thereof. The controller can include one or more processors, e.g., a controller can be a distributed system. One or more computer program products, i.e., one or more computer programs tangibly embodied in a machine-readable storage media, can be executed by, or control the operation of, the controller, e.g., a programmable processor, a computer, or multiple processors or computers. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file. A program can be stored in a portion of a file that holds other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. 
     The operations of the controller described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The operations of the controller can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     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.