Abstract:
A Superconducting Quantum Interference Device (SQUID) is disclosed comprising a pair of resistively shunted Josephson junctions connected in parallel within a superconducting loop and biased by an external direct current (dc) source. The SQUID comprises a semiconductor substrate and at least one superconducting layer. The metal layer(s) are separated by or covered with a semiconductor material layer having the properties of a conductor at room temperature and the properties of an insulator at operating temperatures (generally less than 100 Kelvins). The properties of the semiconductor material layer greatly reduces the risk of electrostatic discharge that can damage the device during normal handling of the device at room temperature, while still providing the insulating properties desired to allow normal functioning of the device at its operating temperature. A method of manufacturing the SQUID device is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a non-provisional patent application of currently pending U.S. provisional patent application Ser. No. 60/513,747, filed Oct. 23, 2003, by Cantor et al., titled “Charge Dissipative Dielectric for Cryogenic Devices,” the entirety of which application is incorporated herein by reference. 
     
    
     GOVERNMENT LICENSE RIGHTS  
       [0002]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contracts NAS5-00236 and NAS5-00237, awarded by NASA. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates to Superconducting Quantum Interference Devices (SQUIDs) in general and more particularly to a process for the fabrication of thin film cryogenic devices which include a semiconductor layer that is conductive at room temperature and prevents electrostatic charge accumulation which can damage the device, but which is insulating at the operating temperature of the device.  
       BACKGROUND OF THE INVENTION  
       [0004]     Thin film devices typically comprise a multiplicity of metal wiring layers separated by insulating, or dielectric, material layers to provide electrical isolation. If the dielectric used to electrically isolate the wiring layers of a thin film circuit is a perfect insulator, normal handling of the chips cut from fabricated wafers can lead to the accumulation of electrostatic charge and the formation of very large potential differences between the thin-film layers of the device. An electrostatic discharge (ESD) occurs if the potential difference becomes too large, which can damage or destroy the circuit. In order to minimize device failure owing to electrostatic discharge, special ESD-safe workstations and tools are required, and all personnel handling the circuits must wear grounded wrist straps and heel straps. Anti-static lotions and smocks also are commonly used when handling sensitive circuits.  
         [0005]     Thus, it would be desirable to provide a device which minimizes the potential difference between adjacent metal wiring layers during normal handling, but which also provides the desired isolation between such layers during operation, when such isolation is functionally required. Specifically, for superconducting devices it would be desirable to provide a transition layer which provides conductance between adjacent metal layers during handling at room temperature, when the potential for ESD is greatest, but which acts as an insulator between the metal layers during operation, at temperatures below about 100 Kelvins (K).  
         [0006]     Devices incorporating layers of material capable of undergoing an insulator to metal transition are known. For example, in U.S. Pat. No. 6,365,913 to Misewich et al., a Field effect transistor semiconductor switch is disclosed in which the channel is made from materials having an electrical conductivity which can undergo an insulator-metal transition (i.e., Mott transition) upon application of an electric field. Likewise, U.S. Pat. No. 5,721,197 to Downar et al. discloses a microelectronic thin-film device having a thin superconducting layer in contact with a thin quasi-insulating conversion layer. The critical current of the superconducting layer is controlled by application of a voltage to the conversion layer by means of a gate electrode, causing the conversion layer to undergo an insulator-metal transition. Both references require the application of a voltage or electric field in order to change the conductivity of the transition material.  
         [0007]     Thus, there remains a need for a device that allows for conductance between metal layers during normal handling at room temperatures, thus minimizing the potential for destructive ESD, but which provides the desired electrical isolation between metal layers during normal operation at the low operating temperatures associated with superconducting devices.  
       SUMMARY OF THE INVENTION  
       [0008]     This invention discloses a whole-wafer process for the fabrication of SQUID circuits which includes a semiconductor layer that is conductive at room temperature and prevents electrostatic charge accumulation, but which is insulating at the operating temperature of the SQUID (typically well below 100 K).  
         [0009]     The superconducting metal wiring layers used to fabricate the SQUID circuits are separated by a conductive doped semiconductor film that prevents charge accumulation at room temperature, thereby greatly reducing the risk of electrostatic discharge that may damage the SQUID circuits during normal handling at room temperature. When the SQUID is cooled to its normal operating temperature, the semiconductor film undergoes a metal-insulator transition (MIT) and becomes an insulating dielectric. The semiconductor film may therefore be used in place of or in conjunction with conventional dielectric films typically used to fabricate integrated circuit devices for cryogenic applications. Alternatively, the semiconductor film may be deposited as the final passivation layer on top of the completed SQUID circuit, or as the first film of a multiple film passivation layer.  
         [0010]     The metal-insulator transition (also referred to as the metal-nonmetal transition) is characterized by a sudden change in the electrical conductivity due to a reversible change from localized to itinerant behavior of the electrons. In order for the room-temperature semiconductor film to become an insulating dielectric at cryogenic temperatures, the doping level of the semiconductor must be below the MIT critical concentration N c , above which the doped semiconductor film remains metallic even as the temperature T approaches 0 K. When using crystalline silicon, for example, N c ˜3.5×10 18 /cm 3 , with only a weak dependence on the specific dopant used. For other forms of doped silicon films, for example amorphous, polycrystalline, or compensated silicon films, N c  can be much higher. The controlled deposition of doped semiconductor films with a doping level below the critical threshold is straightforward. In addition to doped semiconductors, amorphous metal-semiconductor mixtures also undergo metal-insulator phase transitions, depending on the concentration of the constituent metal. Such amorphous metal-semiconductor films provide an alternative to the use of doped semiconductor films in the disclosed process.  
         [0011]     A thin film device is disclosed comprising a substrate, a conductive material layer overlying the substrate, and a layer of metal-insulator transition material overlying the conductive material layer. The transition material layer acts as a conductor between features, structures or circuit elements patterned in the conductive material layer when the device is at a first temperature. The transition material layer further acts as an insulator between the features, structures or circuit elements patterned in the conductive material layer when the device is at its operating temperature.  
         [0012]     A method of forming a thin film device is also disclosed comprising: providing a substrate; depositing a first conductive material layer on at least a portion of the substrate; and depositing a metal-insulator transition material layer over at least a portion of the first conductive material layer. The metal-insulator transition material acts as a conductor when the device is at a first temperature. The metal-insulator transition material layer further acts as an insulator when the device is at a second temperature.  
         [0013]     A superconducting device is disclosed comprising a substrate, a first superconducting wiring layer, and a metal-insulator transition material layer overlying at least a portion of said first superconducting wiring layer. The metal-insulator transition material layer acts as a conductor when the device is at a first temperature and acts as an insulator when the device is at its operating temperature. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a plan view of an integrated SQUID layout;  
         [0015]      FIG. 2  is plan view of the Josephson junction area of the SQUID layout of  FIG. 1 ;  
         [0016]     FIGS.  3 A-E are cross-sectional views taken along line A-A′ of  FIG. 2 , showing the steps of fabrication of a Josephson junction, shunt resistor, and contact via to the base electrode in accordance with one embodiment of the present invention;  
         [0017]     FIGS.  4 A-F are cross-sectional views taken along line B-B′ in  FIG. 2 , showing the steps of fabrication of a Josephson junction and the two outer turns of the input coil, constructed in accordance with one embodiment of the present invention;  
         [0018]     FIGS.  5 A-F are cross-sectional views taken along line C-C′ of  FIG. 1  showing the steps of fabrication of the two innermost turns of the input coil and the contact via to a portion of the input coil that is electrically isolated from and passes underneath the SQUID washer, constructed in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Throughout the following description, specific process details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. Accordingly, the specifications and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0020]     A direct current (dc) SQUID consists of two resistively-shunted Josephson junctions which are connected in parallel within a superconducting loop and biased by an external dc source. The SQUID may be current biased or voltage biased depending on the type of readout electronics used. Dc SQUIDs are typically multilayer thin-film devices, incorporating polycrystalline films of niobium for the Josephson junction electrodes. The element niobium is a borderline Type II superconductor with a relatively high critical temperature T c =9.2 K and energy gap Δ≈1.5 millivolts (mV) which provide a reasonable temperature margin for junction operation at liquid helium temperatures. High-quality thin films of niobium can be fabricated using electron-beam or ion-beam evaporation or dc sputtering techniques. The latter is more common for Josephson junction fabrication.  
         [0021]     Although niobium films can easily be oxidized to form the thin, insulating tunnel barrier required for Josephson junction fabrication, the oxide formed on niobium includes not only the stable pentoxide Nb 2 O 5 , but also the sub-oxides of niobium NbO 2  and NbO, the latter of which is a superconductor. The presence of these sub-oxides degrades the performance of Josephson junctions fabricated using oxidized niobium barriers. A very thin film (a few monolayers) of aluminum will “wet” and completely cover the surface of a clean niobium film. Thus, high-quality junctions can be fabricated on niobium thin-film electrodes by first depositing a very thin aluminum film  2  nanometers (nm) to 4 nm thick onto the niobium surface and then oxidizing the aluminum film. The junction is completed with the deposition of the top niobium electrode.  
         [0022]     Whole-wafer processes can be applied for the fabrication of Josephson junctions. With these processes, Josephson junction devices with reproducible parameters and narrow parameter spreads can be fabricated on entire silicon wafers. These processes start with the deposition of a superconductor-barrier-superconductor trilayer on a silicon wafer in a single deposition sequence without breaking vacuum. For niobium tunnel junctions fabricated with oxidized aluminum barriers, these trilayers are usually denoted as Nb/Al—AlO x /Nb structures. Individual Josephson junctions are then defined using a photoresist stencil and the exposed top superconductor film is either completely anodized down to the barrier layer, or the exposed top superconductor film is removed down to the barrier layer using a reactive ion etch (RIE) process, and the exposed areas are then lightly anodized. The Josephson junctions are then completed by depositing and patterning a superconducting wiring layer with or without an intermediate insulating layer. More complex structures may require the deposition of a second insulator layer and second superconducting wiring layer.  
         [0023]     The insulating layers used to separate the superconducting wiring layers typically are oxides with low dielectric constants. Commonly used dielectrics include SiO, SiO 2  and Si 3 N 4 , but other dielectrics including films of polycrystalline silicon have been used as well. For most wafer-scale SQUID processes, however, thin films of SiO 2  are preferred owing to the low dielectric constant of SiO 2 . High-quality SiO 2  films may be RF sputter deposited using a quartz target, reactively sputtered using a silicon target and Ar—O 2  plasma, or, more commonly, using Plasma-Enhanced Chemical Vapor Deposition (PECVD) via the chemical decomposition of silane.  
         [0024]     Wafer-scale processes are important for cost-effective production of SQUIDs and large-area integrated SQUID sensors, such as thin-film SQUID magnetometers and planar SQUID gradiometers.  
         [0025]     A process suitable for the fabrication of SQUIDs and integrated SQUID sensors consists of six mask levels. To begin, a Nb/Al—AlO x /Nb trilayer is deposited over an entire oxidized silicon wafer in a single vacuum sequence. Then, the trilayer is patterned using a wet or dry etch process to define the base electrode, contact pads, and interconnects. The Josephson junctions are defined next using a photoresist stencil, and the top niobium layer is anodized slightly past the bottom Nb/Al interface. A niobium wiring layer is deposited and patterned using a photoresist stencil and reactive ion etch process to define the SQUID washer, Josephson junction top electrode, and interconnects. The Josephson junction shunt resistors are deposited next and patterned using a photoresist stencil and either a lift-off process or dry etch, such as an argon ion mill process. Next, an insulating dielectric layer, typically SiO 2 , is deposited over the entire wafer, and contact vias are opened to the first wiring layer or to the base electrode of the trilayer using a photoresist stencil and a wet etch or RIE process. To complete the SQUIDs, the top niobium wiring layer is deposited and patterned using a photoresist stencil and lift-off process or RIE process to define the input and feedback coils and remaining interconnects. In many cases, it is desirable to deposit a final dielectric layer to passivate and protect the SQUID circuit from moisture or mechanical abrasion. Typically, SiO 2  or Si 3 N 4  films are used for this purpose.  
         [0026]     The following is a description of the invention as used to fabricate integrated SQUIDs intended for operation at about 4K, although SQUIDs having operating temperatures of greater or less than 4K can be fabricated using the principles of the invention. Additionally, it will be appreciated that although the invention will be described in relation to constructing Josephson junctions having a “stack” arrangement, it is equally applicable to “weak link” type junctions, in which a narrow constriction is formed in a conductive material wire to provide the same type of functionality as the “stack” type junction. Similarly, the invention could be applied to a “step-edge” junction, in which the metal film layer is formed over a step-like feature in the substrate to form a similar weak link in the wire.  
         [0027]     The integrated SQUID layout depicted in  FIG. 1  as a plan view comprises two Josephson junctions  102 ,  104  connected within a superconducting loop by the washer-shaped SQUID inductance  106  and base electrode  108 , along with two shunt resistors  110 ,  112  between the top electrode of the Josephson junctions  102 ,  104  and two contact vias  114 ,  116  to the base electrode  108 . An input coil  118  is integrated on top of the SQUID washer  106 . A contact via  117  is used to connect the input coil  118  to a stripline  119  that is electrically isolated from and located underneath the SQUID washer  106  which is used as the current return path for the input coil  118 .  
         [0028]     The partial plan view in  FIG. 2  of the Josephson junction area of the SQUID layout in  FIG. 1  references cross-sectional views A-A′ and B-B′ which are depicted in FIGS.  3 A-E and  4 A-E, respectively. These cross-sectional views, constructed in accordance with one embodiment of the present invention, illustrate the layer sequence described below that is used to fabricate the SQUID layout depicted in  FIG. 1 .  
         [0029]     The fabrication process will be described in relation to  FIGS. 3A through 5F , which detail how the individual deposition, patterning and etching steps are used to create the individual elements of the SQUID device. As such, FIGS.  3 A-E detail the fabrication steps in relation to the formation of Josephson junction  104 , shunt resistor  112  and contact via  116 ; FIGS.  4 A-F illustrate the fabrication steps in relation to the formation of Josephson junction  104  and the two outer turns of the input coil  118 ; and FIGS.  5 A-F illustrate the fabrication steps in relation to the formation of the two innermost turns of the input coil  118  and the contact via  117 . Thus, referring to  FIGS. 3A through 5F  together, to start the process, a 250 nm niobium film  120 , 70 nm aluminum film  122 , and 30 nm niobium film  124  are deposited over an entire oxidized silicon wafer substrate  126 . Although specific film layer thicknesses are disclosed, it is noted that such thicknesses are merely exemplary. Thus, niobium film  120  can be from about 150 nm to about 350 nm; aluminum film  122  can be from about 20 nm to about 120 nm; and niobium film  124  can be from about 20 nm to about 200 nm. These three films deposited in a single vacuum sequence are referred to as a trilayer. Before depositing the top, 30 nm niobium film  124  of the trilayer, oxygen is bled into the chamber to oxidize the exposed aluminum film  122 . The oxidation pressure and time are chosen depending on the desired Josephson junction critical current density. Typically, the oxidation time is 30 minutes and the oxidation pressure is around 1 to 100 Torr, depending on the critical current density required. After the oxidation step is completed, the oxygen is pumped away and the top niobium film  124  is deposited to complete the trilayer. The trilayer is patterned using a photoresist stencil  128  and either a wet or reactive ion etch process. After etching, the photoresist stencil  128  is removed.  
         [0030]     The etched trilayer film is patterned again using a photoresist stencil  130  to define the Josephson junctions  102 ,  104  and contact vias  114 ,  116 . The exposed top niobium film  124  and aluminum film  122  are anodized to form an insulating oxide film  132 . After anodization, the photoresist stencil  130  is removed.  
         [0031]     The first niobium wiring layer  134 , which can be from about 150 nm to about 500 nm thick, but preferably is about 250 nm thick, is deposited onto the anodized trilayer and patterned to define the SQUID washer  106  using a photoresist stencil  136  and a reactive ion etch process. After etching, the photoresist stencil  136  is removed.  
         [0032]     A palladium or other resistive film  138  that is non-superconducting at the operating temperature of the device, which can be from about 10 nm to about 100 nm thick, but preferably is about 50 nm thick, is deposited onto the entire wafer, followed by the deposition of a silicon film or other sacrificial mask material  140 , which can be from about 10 nm to about 500 nm thick, but preferably is about 110 nm thick, in the same vacuum sequence. The Josephson junction shunt resistors  110 ,  112  are patterned using a mask formed from the silicon film  140  on top of the palladium film  138 . The silicon film  140  is patterned using a photoresist stencil  142  and reactive ion etch step, for which the palladium film serves as etch stop. Then, the photoresist stencil  142  is removed, and the palladium is etched using an argon ion mill process. After the palladium film  138  has been patterned, the silicon film mask  140  is removed using a reactive ion etch.  
         [0033]     A doped polycrystalline or amorphous silicon semiconductor dielectric film  144 , 390 nm thick, with a boron concentration below the MIT critical concentration Nc, is sputter-deposited using a boron-doped silicon target with a nominal room-temperature resistivity in the range from 0.020 to 0.005 Ω-cm over the entire wafer. It is noted that although a thickness of about 390 nm is disclosed, film thicknesses of from about 100 to about 600 nm may be used. Likewise, although the use of silicon films and the use of boron doping are disclosed, other semiconductor materials, for example germanium and gallium arsenide, and other dopants, for example phosphorus and arsenic, are also appropriate.  
         [0034]     In order to improve step coverage over the underlying first niobium wiring layer  134 , the doped silicon film  144  is deposited using a series of deposition/etch steps. First, 60 nm of doped silicon is deposited and immediately followed by argon ion milling 20 nm of the freshly deposited silicon away. This deposition/etch cycle is repeated three times, then the wafer is removed and the exposed silicon is partially oxidized in an oxygen plasma. This process is repeated two times, then 150 nm of doped silicon is deposited over the entire wafer, resulting in a final doped silicon film  144  with a thickness of about 390 nm. It is noted that although specific deposition and etch thicknesses are disclosed, other deposition and etch thicknesses may be used, such that the ratio of the deposited film thickness to the etch thickness is around three to one. It is also noted that the above described application procedure would be appropriate when applying materials using a sputter deposition technique, in order to provide good step coverage and to avoid a “bread loafing” effect. If a PECVD process is used, the doped silicon layer can be deposited in a single step.  
         [0035]     The contact via  117  is opened in the doped silicon film  144  as follows. First, a 40 nm aluminum film  146  is deposited over the entire wafer. Then, the contact via  117  is opened in the silicon film  144  using a photoresist stencil  148  and reactive ion etch process. When the photoresist stencil is exposed and developed, the developer also etches away the exposed areas of the aluminum film  146 , exposing the silicon where the via  117  will be formed. Since the reactive ion etch process attacks oxidized silicon much more slowly, the doped silicon film  144  deposited with intermediate plasma oxidations will have a “stair-cased” edge profile after via etch. This improves step coverage by the subsequent layers. After etching the via  117 , the photoresist stencil  148  and aluminum film  146  are removed.  
         [0036]     The second niobium wiring layer  150 , 500 nm thick, is deposited and patterned to define the input coil  118  using a bilayer photoresist  154  and 200 nm aluminum film  152  lift-off stencil. It should be noted that the second niobium wiring layer  150  can be provided in a thickness of from about 200 nm to about 700 nm, and that the aluminum film  152  thickness can be from about 150 nm to about 500 nm. After exposing and developing the photoresist  154 , the exposed aluminum areas are overetched with an aluminum wet etchant in order to undercut the aluminum film  152  underneath the photoresist. Then, the top niobium film  150  is deposited and the combined aluminum/photoresist stencil  152 ,  154  is lifted off by soaking the wafer in aluminum etchant. Alternatively, a reactive ion etch process may be used to pattern the top niobium film  150 . In this case, the aluminum film  146  is not removed after the via  117  is opened in the doped silicon film  144 . The top niobium wiring layer  150  is deposited after the via  117  is opened, and patterned using a reactive ion etch process with the aluminum film  152  serving as a sacrificial etch stop layer that can easily be removed from the wafer following patterning.  
         [0037]     It is noted that although the substrate material has been described as being oxidized silicon, any substrate commonly used for thin film devices (e.g. sapphire, gallium arsenide) can also be used. Likewise, although resistive film layer  138  has been described as being palladium, other non-superconducting materials can also be used. Further, wiring layers  134 ,  150  made from materials other than niobium can also be used (e.g., tantalum, aluminum, magnesium diboride, niobium nitride, high-temperature superconductors such as YBa 2 Cu 3 O 7 , etc.)  
         [0038]     Furthermore, as an alternative to using doped semiconductors for dielectric layer  144 , amorphous metal-semiconductor mixtures having the desired metal-insulator-transition properties can also be used. Such amorphous metal-semiconductor mixtures undergo the desired metal-insulator phase transitions, depending on the concentration of the constituent metal. Examples of such mixtures are chromium-silicon mixtures and niobium-silicon mixtures.  
         [0039]     In one embodiment of the invention, the doped silicon film  144  is replaced with a conventional dielectric film such as SiO 2 , and the doped silicon film is deposited instead as a final conductive passivation layer, or as the first film of a multiple film passivation layer, on top of the entire wafer. The above process description remains unchanged, with the exception that, if the top niobium film  150  is patterned using a reactive ion etch process, the aluminum film  146  used to pattern the via  117  is not needed as a sacrificial etch stop and can be removed after etch of the via  117 . When used as the final passivation layer only, the doped silicon film can contact all of the device contact pads, and thus the desired charge distribution between metal layers is still achieved (at room temperature). This embodiment can simplify device construction compared to the previous embodiments because it allows the manufacturer to use standard materials and processes to build all but the final layer of the device.  
         [0040]     The invention has been described in relation to the construction of Josephson tunnel junctions in a SQUID device, to protect the sensitive Josephson junctions with tunnel barriers on the order of several Angstroms thickness as well as wiring layers from the detrimental effects of ESD. It will be appreciated, however, that in addition to this specific implementation, other applications are envisioned. In one non-limiting example, the invention could be implemented to build superconducting tunnel junction (STJ) detectors, such as those commonly used in photon (e.g., visible or x-ray) detection devices. In short, the invention has application to any of a wide variety of cryogenic devices that are susceptible to static damage at room temperatures, as will be appreciated by one of ordinary skill in the art.  
         [0041]     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.