Abstract:
High temperature superconductive (HTS) integrated circuits can be fabricated in three ways according to the invention. First, a planar multiple layer HTS integrated circuit is fabricated using multiple HTS layers. The layers include altered regions which have been bombarded using ion implantation to destroy superconductivity of the altered regions without interrupting the lattice structure of the altered regions. Second, a planar multiple-layer HTS integrated circuit includes upper and lower HTS layers, each including central and opposing regions. A first implant energy is used to destroy superconducting properties of the opposing regions of the lower HTS layer without interrupting the lattice structure. A second implant energy is used to destroy superconducting properties of a top portion of the central region to define a contact. Third, a HTS integrated circuit is formed from a single HTS layer using three ion implantation steps and ions having first, second and third energies and range.

Description:
This is a divisional of U.S. patent application Ser. No. 08/183,097 filed Jan. 14, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to high temperature superconductive integrated circuits and, more particularly, to planar high temperature superconductive integrated circuits fabricated using ion implantation. 
     2. Discussion 
     High temperature superconductor (HTS) integrated circuits typically include a first HTS layer patterned and deposited on a substrate using photo-lithography. Unwanted portions of the first HTS layer are physically etched off using a variety of techniques, for example ion milling, reactive ion etching, plasma etching, and wet etching. An insulating dielectric layer is formed over the HTS layer. Then, a second HTS layer is patterned and deposited. Additional dielectric layers and HTS layers can be alternately formed on the second HTS layer. 
     To effectively grow the second HTS layer, the underlying dielectric layer has to be monocrystalline or highly oriented (in contrast to polycrystalline or amorphous). However, it is very difficult to grow the epitaxial dielectric layer on the patterned first HTS layer since a lattice match is required between the dielectric layer and two distinctly different surfaces, the substrate layer the first HTS layer. In addition to providing the lattice match, proper crystal growth must be maintained by an angled portion formed over edges of the first HTS layer. 
     Stress, thickness, uniformality and conformality of the dielectric layer must also be considered. A short circuit could occur between the first and second HTS layers through the dielectric layer near edges of the first layer. Furthermore, since the second HTS layer is deposited over the non-planar dielectric layer (primarily where the second HTS layer crosses over the first HTS layer), the second HTS layer could break, develop line discontinuity, and/or encounter significant reduction in supercurrent carrying ability (J c ) due to crystal orientation disruption and/or non-uniform crystal thickness (e.g. crystal too thin) near the edges of the first layer. The problems described above increase as additional dielectric and HTS layers are formed. 
     Therefore, a high temperature superconductive integrated circuit addressing the above-identified problems is desirable. 
     When the first HTS layer, the dielectric layer and the second HTS layer are deposited in separate steps, contact can be made between the first and second HTS layers through a contact hole in the dielectric layer. A top interface surface of first HTS layer is typically cleaned using chemical etching or ion-cleaning before the second HTS layer is patterned and deposited. Such cleaning can damage or alter the top interface surface of the first HTS layer and can create a thin non-superconducting layer resulting in decreased supercurrent carrying ability (J c ) or nonsuperconductivity. 
     SUMMARY OF THE INVENTION 
     A multi-layer planar high temperature superconducting integrated circuit formed on a substrate includes a first planar high temperature superconducting (HTS) layer deposited and patterned on the substrate and including a central region and two opposing regions abutting the central region. Ion implantation is used to destroy superconductivity in the opposing regions without interrupting the lattice structure of the opposing regions. A second planar HTS layer is deposited and patterned on the first HTS layer and includes a central region and two opposing regions abutting the central region. Ion implantation is used to destroy superconductivity in the opposing regions without interrupting the lattice structure of the opposing regions. A third planar HTS layer is deposited and patterned over the second HTS layer. 
     According to another embodiment of the invention, a multi-layer planar high temperature superconducting integrated circuit is formed on a substrate and includes a first planar high temperature superconducting (HTS) layer deposited and patterned on the substrate and including a central region and two opposing regions abutting the central region. Ion implantation at a first implant energy level is used to destroy superconductivity in the opposing regions without interrupting the lattice structure ion of the opposing regions. Ion implantation at a second energy level lower than the first implant energy level is used to destroy superconductivity of a top portion of the central region without destroying the lattice structure of the top portion and to define a contact. A second HTS layer is deposited and patterned over the first HTS layer and abuts the opposing regions of the first layer, the contact, and the top portion. 
     According to another embodiment of the invention, a planar high temperature superconducting integrated circuit is formed on a substrate and includes a first high temperature superconducting layer deposited and patterned on the substrate. The HTS layer includes a lower portion having opposing regions abutting a central region. The opposing regions have been bombarded using ion implantation with high-energy, deep-range ions to destroy superconductivity of the opposing region of the lower portion. A middle portion has opposing regions abutting a central region. The opposing regions have been bombarded using ion implantation with medium-energy, medium-range ions to destroy superconductivity of the opposing regions of the middle portion. An upper portion includes a central superconducting region. 
     Other objects, features and advantages will be readily apparent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to those skilled in the art after studying the following specification and by reference to the drawings in which: 
     FIG. 1 illustrates a cross-sectional view of a high temperature superconducting (HTS) integrated circuit according to the prior art and including a first HTS layer which contacts a second HTS layer in a contact area formed by a dielectric layer; 
     FIG. 2 illustrates a cross-sectional view of a first multi-layer planar HTS integrated circuit according to the invention; 
     FIG. 3 illustrates a cross-sectional view of a second multi-layer planar HTS integrated circuit according to the invention; 
     FIG. 4 illustrates a cross-sectional view of a high-temperature semiconducting integrated circuit according to the invention and formed from a single HTS layer; and 
     FIG. 5 illustrates ion energies of ions used in fabricating HTS integrated circuit of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a high temperature superconducting (HTS) integrated circuit  10  according to the prior art includes a first HTS layer  18  deposited and patterned on substrate  19 , for example, using photo-lithography. The HTS layers can be made of Yttrium-Barium-Copper-Oxygen (Y—Ba—Cu—O), Bismuth-Strontium-Calcium-Copper-Oxygen (Bi—Sr—Ca—Cu—O), and Thallium-Barium-Calcium-Copper-Oxygen (Tl—Ba—Ca—Cu—O). Other materials will be readily apparent. 
     Unwanted portions of HTS layer  18  are physically etched using a variety of techniques, for example ion milling, reactive ion etching, plasma etching, and wet etching. As can be appreciated, other etching and liftoff techniques are also used. An insulating dielectric layer  22  is deposited and patterned over first HTS layer  18 . Dielectric layer  22  can be made using SrTiO 3 , and LaAlO 3 . Other materials will be readily apparent. Then, a second HTS layer  26  is deposited and patterned over dielectric layer  22 . As can be seen in FIG. 1, second HTS layer  26  contacts first HTS layer  18  along a top surface of first HTS layer  18 . Additional dielectric layers and HTS layers can be alternately formed on second HTS layer  26 , as described above, if desired. 
     To effectively grow second HTS layer  26 , underlying dielectric layer  22  should be epitaxial, in other words monocrystalline or highly oriented (in contrast to polycrystalline or amorphous). However, it is very difficult to grow epitaxial dielectric layer  22  on first HTS layer  18  since a lattice match is required between dielectric layer  22  and two distinctly different surfaces, substrate  19  and first HTS layer  18 . In addition to providing the lattice match, proper crystal growth must be maintained over edges  30  and  32  of first HTS layer  18 . 
     Stress, thickness, uniformality and conformality of dielectric layer  22  must also be considered. A short could occur between first and second HTS layers  18  and  26  through dielectric layer  22  near edges  30  and  32  of first layer  18  decreasing performance of HTS integrated circuit  10 . Furthermore, since second HTS layer  26  is deposited and patterned over non-planar dielectric layer  22  (primarily where second HTS layer  26  crosses over first HTS layer  18 ), second HTS layer  26  could break, develop line discontinuity, and/or encounter significant reduction in supercurrent carrying ability (J c ) due to crystal orientation disruption and/or non-uniform crystal thickness (crystal thinning). The problems described above increase as additional dielectric and HTS layers are formed. 
     When first HTS layer  18 , dielectric layer  22 , and second HTS layer  26  are deposited and patterned in separate steps, contact between first and second HTS layers  18  and  26  is made through a contact hole in dielectric layer  22 . A top interface surface of first HTS layer  18  is typically cleaned using chemical etching or ion-cleaning before second HTS layer  26  is deposited and patterned. Such cleaning can damage or alter the top interface surface of first HTS layer  18  and can create a thin non-superconducting layer. The thin non-superconducting layer can result in a non-superconducting contact or a superconducting contact with a low supercurrent carrying capacity (J c ) 
     Referring to FIG. 2, a first HTS integrated circuit  50  according to the invention includes a substrate  54 , a first HTS layer  58  including an unaltered HTS region  60  and altered HTS regions  62  and  64 , a second HTS or dielectric/contact layer  68  including an unaltered HTS region  69  and altered HTS regions  70  and  71 , and a third HTS layer  72 . First HTS layer  58  can be initially deposited and patterned, for example, using photo-lithography techniques described above. Other techniques will be readily apparent. Regions  62  and  64  of first HTS layer  58  are exposed using ion implantation to alter superconductive properties of regions  62  and  64  and to create altered HTS regions  62  and  64 , while unaltered HTS region  60  retains superconducting properties. 
     Second HTS layer  68  can also be deposited and patterned using photo-lithography techniques. Regions  70  and  71  are exposed to ion implantation to alter superconducting properties of regions  70  and  71 , while unaltered region  69  retains superconducting properties. Unaltered region  69  operate as a contact while altered regions  70  and  71  operate as a dielectric. 
     Third HTS layer  72  is then deposited and patterned on planar dielectric/contact layer  68  to complete first HTS integrated circuit  50 . If additional layers are desired, additional dielectric layers and HTS layers can be deposited and patterned on planar dielectric layer  68 . Ion implantation can also be used on third HTS layer  72  to create altered regions  75  and  76  and unaltered region  78  if desired in a manner analogous to first HTS layer  58  previously described. Regions  75  and  76  can also be removed using a wet or dry etch step if desired (for example, reactive ion etching, ion milling, etc.) instead of implanting third HTS layer  72 . 
     As can be appreciated, a photoresist/mask can be used during ion implantation to delineate altered and unaltered regions. Other techniques can also be employed. Prior to depositing and patterning planar dielectric/contact layer  68  on planar HTS layer  58  or to depositing and patterning third HTS layer  72 , an annealing step can be performed on first HTS layer  58  and/or second HTS (or dielectric/contact) layer  68  to anneal out ion implant damage, to activate chemical bonding between the implant species and oxygen in the altered HTS regions, and to minimize out-diffusion of implanted dopant from the altered HTS regions to the unaltered HTS region. 
     First HTS integrated circuit  50  uses ion-implantation techniques to alter superconducting properties of planar HTS regions instead of physically removing the regions using etching or liftoff techniques. As a result, planar surfaces are provided for subsequent layers. The ion implant species are selected to change the chemical and electrical properties of the regions to be altered without interrupting the lattice structure. Such ion implantation without destroying the lattice structure is disclosed in U.S. Pat. No. 5,194,419 to Shiga et al which is hereby incorporated by reference. 
     For example, the implant species can be chosen to form stable chemical bonds with oxygen atoms in the Copper-Oxide plane of HTS Perovskite thereby reducing the number of oxygen atoms available to Perovskite unit cells to convert the conducting properties of the implanted regions to non-superconducting. Converting the altered regions to non-superconducting by using ion implantation which damages the crystal lattice is unacceptable since the crystal lattice structure of the altered regions must be maintained so that additional HTS layers can be deposited and patterned thereon. 
     Referring to FIG. 3, a second HTS integrated circuit  100  according to the invention is illustrated. A first HTS layer  104  is deposited and patterned on a substrate  108  using photo-lithography. Using a first mask, first and second regions  112  and  114  of the first HTS layer  104  are exposed to ion implantation (using a first implant energy level) to neutralize superconducting properties thereof. Using a second mask, a top portion of region  116  is implanted using a second implant energy lower than the first implant energy used for altered HTS regions  112  and  114  to define altered portions  118  and  120  and a contact  121 . Since the ions are implanted at the lower second implant energy, the ions penetrate only part of a thickness “D” of HTS layer  104 . Altered portions  118  and  120  are converted to a dielectric or a high-resistance non-superconducting layer. A second HTS layer  122  is deposited and patterned on planar first layer  104 . Annealing can be performed prior to forming second HTS layer  122 . 
     Altered regions  124  and  126  and unaltered region  128  can be defined in second HTS layer  122  as previously described in conjunction with HTS layer  72  in FIG.  2 . Alternately, regions  124  and  126  of HTS layer  122  can be removed using wet or dry etching. Implant species used for first and second HTS integrated circuits  50  and  100  should have a low diffusion length in the material chosen for the HTS layers since temperature cycling between 600-800 degrees Celsius occurs during formation of subsequent dielectric and/or HTS layers. As can be appreciated, planar HTS integrated circuits  50  and  100  reduce the likelihood of short circuits, line discontinuity, breakage, and reduction in superconducting carrying ability (J c ) over conventional HTS integrated circuits. 
     Referring to FIG. 4, a third HTS integrated rated circuit  250  according to the invention includes a single HTS film or layer  251  patterned and deposited on a substrate  252 . In a first implantation step, using a first photoresist/mask, high-energy deep-range ions are implanted into regions  253  and  254  to delineate a first HTS region  256  having a thickness “A” in a lower portion  258  of HTS film or layer  251  adjacent substrate  252 . Regions  253  and  254  are transformed during ion implantation into relatively high-resistance, non-superconducting regions or dielectric. 
     In a second implantation step, using a second photoresist/mask medium-energy, medium-range ions are implanted to create non-superconducting dielectric regions  260  and  262  in a middle portion  264  (having a thickness “B”) of single HTS film or layer  251 . A contact region  268  remains unaffected (superconducting) by the medium-energy, medium-range (or second) ion implantation step. 
     In a third implantation step, using a third photoresist/mask, low-energy, low-range ions are implanted in a top portion  272  (having a thickness “C”) to create dielectric or high-resistance, nonsuperconducting regions  274  and  276  and to delineate a second unaltered HTS region  280 . Alternatively, regions  274  and  276  can be removed using a wet or dry etch step if desired (for example reactive ion etching, ion milling, etc.) instead of the third implantation step. 
     As can be appreciated from FIG. 4, first HTS region  256  is typically wider than contact region  268 . Therefore, the high energy, deep-range ions used in the first implantation step to alter regions  253  and  254  do not damage contact region  268 . Some damage occurs to regions  260  and  262  of middle portion  264  due to the high energy, deep-range ions implanted in regions  253  and  254 . Similarly, some physical damage occurs to second HTS region  280  and regions  274  and  276  due to the high energy, deep-range ions and the medium-energy, medium-range ions. However, first HTS region  256  and contact  268  remain “virgin”, Annealing can be used to restore the superconducting characteristics of second HTS region  280  if required. 
     Referring to FIG. 5, ion concentrations of first, second and third implantation steps at  300 ,  302  and  304  respectively, are shown as a function of penetration depth. As can be appreciated, the implanted ions preferably have a low straggle or distribution (σ). A, B, and C correspond to the thickness of bottom, middle and top portions  258 ,  264  and  272 . 
     The species of the ions used to implant during the ion implantation steps  1 - 3  should be selected to minimize physical damage to upper portions of HTS film or layer  251 , specifically where second HTS region  280  is to be delineated. Minimizing physical damage reduces or eliminates annealing required to restore superconducting characteristics of second HTS region  280 . 
     As can be appreciated, multi-layer and single layer planar HTS integrated circuits according to the invention reduce the likelihood of short circuits, line discontinuity, breakage, and reduction in superconducting carrying ability (J c ) over conventional HTS integrated circuits. In addition, implanting a single HTS layer  251  as described above in conjunction with FIG. 4 eliminates problems associated with chemical etching and/or ion cleaning of the top interface surface of dielectric  22  and first HTS layer  18  of HTS integrated circuit  10  according to the prior art. 
     HTS integrated circuit  250  simplifies fabrication of HTS integrated circuits. Only a thick layer of HTS film  251  is deposited and patterned initially. As such, crystal lattice mismatch and crystal growth orientation/interface problems can be avoided. Contact  268  is superconductive and has the exact HTS film properties of first and second HTS regions  256  and  280 . Contact  268  is a “virgin area” with no artificially created interfaces. As such, contact  268  is superconductive and can carry high superconducting current density. 
     Since the single HTS layer  251  of HTS integrated circuit  250  can be deposited and patterned in one step, second HTS region  280  has almost identical characteristics as first HTS layer  256 . By uniformly implanting middle portion  264  to create contact region  268  and altered regions  260  and  262  instead of using thin film deposition techniques, the middle portion  264  (or dielectric and contact) is less susceptible to growth related problems such as pin holes and cracks which are defects in the dielectric film. As such, the HTS integrated circuits according to the invention provide simple fabrication, lower defect rates, lower cost and better device performance and characteristics. 
     The various advantages of the present invention will become apparent to those skilled in the art after a study of the foregoing specification and following claims.