Abstract:
An iridium interfacial stack (“IrIS”) and a method for producing the same are provided. The IrIS may include ordered layers of TaSi 2 , platinum, iridium, and platinum, and may be placed on top of a titanium layer and a silicon carbide layer. The IrIS may prevent, reduce, or mitigate against diffusion of elements such as oxygen, platinum, and gold through at least some of its layers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of, and claims priority to, U.S. Provisional Patent Application Ser. No. 61/566,310, filed Dec. 2, 2011, the subject matter of which is hereby incorporated by reference in its entirety. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefore. 
    
    
     FIELD 
     The present invention generally pertains to reducing diffusion in integrated circuits, and more specifically, to using an iridium interfacial stack to reduce, mitigate against, or prevent diffusion of certain elements at high temperatures. 
     BACKGROUND 
     Electronics used for applications such as deep oil well drilling, jet engine testing, and Venus exploration should generally be capable of operating effectively at high temperatures. For instance, in order to be effective, such electronics should be able to signal condition, amplify, and transmit sensor information at temperatures that may exceed 500° C. In order to achieve viable high temperature silicon carbide (SiC) electronics, the contacts, dielectrics, die attach, wire bond, and packaging should all be capable of operating at high temperatures. Recently, simple SiC circuits have been demonstrated that operate over a temperature range of −150° C. to over 500° C. and for over 10,000 hours at 500° C. in air ambient. 
     However, In order to make more complicated integrated circuits using SiC, high temperature interconnect metals and processes are generally required. These processes should work in concert with one another. Previous work has produced ohmic contacts that survived 600° C. in air ambient for over 1,000 hours. However, the 600° C. nitrogen anneal for 30 minutes that is necessary to form the ohmic contact leaves Pt 2 Si on the surface, which makes gold wire bonding to the metal stack difficult. A second metallization stack could be deposited to make an easy wire bond, but this adds extra masking and deposition steps. Accordingly, a simpler, faster, and more cost-effective process for making integrated circuits for high temperature operation may be beneficial. 
     SUMMARY 
     Certain embodiments of the present invention may be implemented and provide solutions to the problems and needs in the art that have not yet been fully solved by conventional high temperature electronics. For example, certain embodiments of the present invention pertain to an iridium interfacial stack (“IrIS”) that prevents or retards diffusion of certain elements into monolithically integrated circuits and silicon carbide (SiC) above certain temperatures. 
     In one embodiment of the present invention, an apparatus includes an IrIS. The IrIS is configured to prevent, reduce, or mitigate against diffusion of at least one element through at least some layers of the IrIS. 
     In another embodiment of the present invention, an apparatus includes an IrIS including an iridium layer above a TaSi2 layer. The apparatus also includes a contact. The IrIS is configured to prevent, reduce, or mitigate against diffusion of gold, platinum, oxygen, and silicon through at least some layers of the IrIS. 
     In yet another embodiment of the present invention, a method includes depositing metals of an IrIS to a silicon carbide substrate. The method also includes annealing the IrIS and the silicon carbide substrate at a predetermined temperature for a predetermined period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of certain embodiments of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. While it should be understood that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a flowchart  100  illustrating a method for creating an SiC integrated circuit with an IrIS metallization stack, according to an embodiment of the present invention. 
         FIG. 2  illustrates an AES depth profile of Ti/TaSi 2 /Pt/Ir/Pt as deposited on 6H SiC, according to an embodiment of the present invention. 
         FIG. 3  illustrates an AES depth profile after the metal stack is annealed for 600° C. in N 2  for 30 minutes, 500° C. in air for two hours, and 10 minutes at 180° C. on a hot plate, according to an embodiment of the present invention. 
         FIG. 4  illustrates a SEM image of a gold wire bond after a pull test, according to an embodiment of the present invention. 
         FIG. 5  illustrates an AES depth profile of the metal stack plus 1 μm of gold after 100 hours at 500° C. in air ambient, according to an embodiment of the present invention. 
         FIG. 6  illustrates an AES depth profile of the metal stack plus 1 μm of gold after 1,000 hours at 500° C. in air ambient, according to an embodiment of the present invention. 
         FIG. 7  illustrates a cross-sectional image taken by a FIB-FESEM of an IrIS on a contact, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Some embodiments of the present invention pertain to an iridium interfacial stack (“IrIS”) that acts as a diffusion barrier, and a method of making the same. More specifically, the IrIS is a bondable metal that, in some embodiments, prevents diffusion of certain elements, such as oxygen and gold, into monolithically integrated circuits above 400° C. and SiC integrated circuits above 500° C. Relatively complex high temperature operation integrated circuits (for instance, 100 to 1,000 transistors in some embodiments) may be developed using IrIS, but any number of transistors may be used as a matter of design choice, accounting for functionality and space limitations. High temperature transistors are generally larger than those found in typical silicon electronics, which are generally designed for operation at room temperature to 100° C., making typical systems unsuitable for high temperature applications. 
     Embodiments of the present invention may be well suited to various practical applications. For instance, some embodiments may be used in electronics that are part of drilling equipment since the temperature generally gets hotter the deeper the rig drills. Conventional technology involves stuffing equipment with dry ice and hoping the temperature remains sufficiently low for operation. Also, embodiments may be used for jet engine electronics such as in the combustion section, coal power plants, high temperature space applications, and any other application where high temperature operation is desired. 
     The IrIS may include (from bottom to top) layers of TaSi 2 /Pt/Ir/Pt. In some embodiments, the TaSi 2  layer may be 400 nm thick and the two platinum layers and the iridium layer may each be 200 nm thick. In many embodiments, the IrIS metallization is easily bonded for electrical connection to off-chip circuitry and does not require extra anneals or masking steps. The IrIS may be used directly on ohmic contact metals, dielectric insulating layers, or interconnect metal since the IrIS readily adheres to silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and titanium. 
     Example Method for Creating a Silicon Carbide Integrated Circuit with an Iris Metallization Stack 
       FIG. 1  is a flowchart  100  illustrating a method for creating a SiC integrated circuit with an IrIS metallization stack, according to an embodiment of the present invention. The method begins with cutting and cleaning a SiC wafer at  110 . The SiC wafer may be any suitable commercially available wafer in some embodiments. For example, the SiC wafer may be a 50 mm diameter 6H n-type SiC wafer cut at 3.91° off the c-axis with resistivity of 0.087Ω per cm. The samples may be cut into any suitable size and shape, such as 1 cm×1 cm squares, but may have any desired shape and/or different shapes with respect to one another. The SiC wafer may be cleaned with any suitable chemical, such as acetone and isopropanol. 
     The SiC squares are then etched, rinsed, and dried at  120 . More specifically, in some embodiments, the SiC squares may be etched in buffered hydrofluoric acid for 5 minutes, rinsed in deionized water, and nitrogen dried. The SiC squares may then be etched in a 1:1 mixture of sulfuric acid:hydrogen peroxide for 15 minutes, rinsed in deionized water, and nitrogen dried. 
     The SiC squares are then baked in a vacuum and cooled at  130 . In many embodiments, the SiC squares are immediately loaded into a vacuum system for metal deposition. The deposition system may be load-locked and may be pumped to a low pressure (e.g., 8×10 −9  Torr) before the run. The samples may be baked in these vacuum conditions at 300° C. for one hour before cooling for one additional hour, for example. However, any suitable temperatures and bake times may be used. 
     Metals are then deposited at  140 . For instance, in some embodiments, Ti/TaSi 2 /Pt/Ir/Pt layers (in order) are each deposited at respective thicknesses of 100 nm/400 nm/200 nm/200 nm/200 nm without breaking vacuum. All targets may be presputtered for a predetermined time (e.g., 5 minutes) prior to each deposition. 
       FIG. 2  illustrates an Auger electron spectroscopy (“AES”) depth profile  200  of Ti/TaSi 2 /Pt/Ir/Pt as deposited on 6H SiC, according to an embodiment of the present invention. Although the titanium, platinum, and iridium are all pure metal layers and should be at 100% in the profiles, the only Auger peaks for platinum and iridium that had no other elemental spectral interference were small high-energy lines with high noise, which contributed signals to all other elements scanned, thereby reducing the intensity of the platinum and iridium. 
     Returning to  FIG. 1 , the SiC squares with the deposited metals are then subjected to annealing at  150 . Each anneal is performed in a desired gas environment at a predetermined temperature for a predetermined period of time. However, the temperature and/or gas environment may be varied during an anneal, if desired. For example, in some embodiments, an anneal may be performed at one atmosphere in pure N 2  in a cleanroom tube furnace at 600° C. for 30 minutes. However, these conditions may be varied as a matter of design choice. Additional thermal processes may also occur during die attach and wire bonding. A gold wire bond is then applied at  160  via an e-beam. The process then ends. Such embodiments can be fabricated in a single processing step, unlike conventional approaches, where multiple processing steps and alignments are required. 
       FIG. 3  illustrates an AES depth profile  300  after the metal stack is annealed for 600° C. in N 2  for 30 minutes, 500° C. in air for two hours, and 10 minutes at 180° C. on a hot plate, according to an embodiment of the present invention.  FIG. 3  simulates a bond metal layer in a device after processing and packaging is complete, but before any duration testing at 500° C., for example. This sample was first annealed at 600° C. for 30 minutes in N 2 , allowing the titanium to react with the SiC and the TaSi 2  to form TiC and Ti 5 Si 3 , which form the ohmic contact at the SiC interface. 
     The platinum layer that is sandwiched between the iridium and TaSi 2  layers also reacts during this anneal to form a layer of Pt 2 Si near the Ir—Pt interface. This has been found to be very effective as a gold and oxygen diffusion barrier. Interestingly, while there is some migration of iridium into the Pt 2 Si layer, there is no evidence for either platinum or silicon in the iridium layer for the samples annealed in air. Since the iridium layer does not readily form a silicide, it prevents the silicon from migrating into the topmost platinum layer during further annealing or high-temperature integrated circuit operation. This leaves a pure platinum layer at the surface, ideal for gold wire bonding. 
     This pure platinum surface was used for wire bond testing. Ten gold wire bonds were successfully made to this surface out of ten attempts. All of the wire bonds were then subjugated to a pull test. The gold wires broke from the necking of the wire and not from the wire bond itself. The gold wire used has yield strength of 10 g to 12 g, which means the bond strength is sufficient, and is also greater than that of the wire. A SEM image  400  of one of the gold wire bonds after the pull test is shown in  FIG. 4 . 
       FIG. 5  illustrates an AES depth profile  500  of the metal stack plus 1 μm of gold after 100 hours at 500° C. in air ambient, according to an embodiment of the present invention. The Pt 2 Si layer that had formed below the iridium during the 600° C. 30 minute N 2  contact anneal is still present with no silicon migration through the iridium layer. However, the topmost platinum is beginning to diffuse into the gold at the Au—Pt interface, as well as partially into the iridium. The metal layers below the iridium layer remain mostly unchanged, except for some broadening of the layers due to a tailing effect in the AES elemental depth profile caused by differing degrees of surface roughness that occur in the gold layer after heating. Both of the AES depth profiles of  FIGS. 3 and 5  in air show some mixing of the top platinum and gold, but no platinum at or near the surface. The ohmic contact region at the SiC interface remains pristine and unchanged in all of the samples. 
       FIG. 6  illustrates an AES depth profile  600  of the metal stack plus 1 μm of gold after 1,000 hours at 500° C. in air ambient, according to an embodiment of the present invention. The gold in this sample has nearly completely dissolved the top platinum. However, no gold or oxygen has penetrated the iridium layer. There is about 10% to 15% platinum in the gold layer, extending all the way to the surface. 
       FIG. 7  illustrates a cross-sectional image  700  from a focused ion beam field-emission scanning electron microscope (“FIB-FESEM”) of an IrIS  720  on a contact  710 , according to an embodiment of the present invention. The cross-sectional image is taken after the metal stack plus 1 μm of gold was at 500° C. for 1,000 hours in air ambient, as in  FIG. 6 . IrIS  720  includes (in deposition order) TaSi 2 /Pt/Ir/Pt layers  722 ,  724 ,  726 ,  728  with TaSi 2  layer  722  resting on top of a contact  710  including a SiC layer  712  and a titanium layer  714 . 
     Per the above, these layers were annealed at 600° C. for 30 minutes in nitrogen (N 2 ) ambient, but air or other suitable gases may be used in some embodiments. The annealing allows TaSi 2  layer  722  to react with lower platinum layer  724  to form a Pt 2 Si diffusion barrier at the interface between lower platinum layer  724  and iridium layer  726 . Iridium layer  726  does not readily form a silicide, and thus keeps upper platinum layer  728  free of silicon. Thus leaves pure platinum on the surface of IrIS  720 , making it ideal for gold wire bonding. 
     1 μm of gold was then e-beamed to mimic a gold wire bond for a 1,000 hour anneal in air, forming gold layer  730 . While gold or platinum wire bonds may be preferred, other suitable materials may be used for wire bonds in some embodiments. Such a material should preferably be ductile, soft, a good conductor, and noble (i.e., it doesn&#39;t oxidize). Gold has all of these characteristics. 
     The result of this process, from bottom to top, is a SiC layer  712  under a titanium layer  714  (layer separation indicated by a dashed white line). Above the titanium layer is IrIS  720 . IrIS  720  includes a TaSi 2  layer  722 , a lower platinum layer  724 , an iridium layer  726 , and an upper platinum layer  728 . Lower platinum layer  724  includes a platinum rich silicon layer on the bottom and a Pt 2 Si layer on the top. The platinum mixes with both tantalum and silicon. Iridium layer  726  prevents the silicon from migrating between lower platinum layer  724  and upper platinum layer  728 . Thus, upper platinum layer  728  remains pure platinum. Gold layer  730  is above IrIS  720 , simulating a wire bond. 
     IrIS  720  prevents diffusion in two ways. First, the Pt 2 Si layer within lower platinum layer  724  of IrIS  720  prevents gold from gold layer  730 , platinum from upper platinum layer  728 , or oxygen from diffusing downward towards SiC layer  712 . As can be seen upon careful inspection of iridium layer  726  and gold layer  730  of  FIG. 7 , iridium and gold have column-like structures when deposited due to nanometer cracks. Pt 2 Si acts as a good diffusion barrier by “stuffing” the cracks between the iridium grains. This keeps the ohmic contact interface that transports signals in and out of the semiconductor of the SiC substrate functioning properly and increases the life of the circuit since gold and oxygen reduce circuit life if they get through the cracks. 
     The dark line  725  on the bottom of iridium later  726  is believed to be extra silicon, which provides an extra layer of protection. If gold, platinum, or oxygen were to reach titanium layer  714 , a voltage drain may be caused, changing the resistance, or the circuit may become semi-rectifying like back-to-back diodes. This results in an undesired change in behavior of the ohmic contact, which is no longer stable. The second way diffusion is prevented is via iridium layer  726  of IrIS  720  preventing silicon from diffusing upward to reach gold layer  730 . 
     Some embodiments of the present invention pertain to an IrIS and a method of making the same. In some embodiments, TaSi 2 /Pt/Ir/Pt acts as a gold and oxygen diffusion barrier, while at the same time leaving pure platinum on the surface for easy gold ball bonding. The metal layers can be deposited in a single deposition run while having the two platinum layers perform very different tasks. This simplifies and expedites fabrication of circuits designed for operation at 500° C. or more in air ambient. Iridium, which does not readily form a silicide at temperatures greater than 600° C., acts as a good silicon diffusion barrier. The platinum underneath the iridium reacts with TaSi 2  to from Pt 2 Si, which acts as a gold and oxygen diffusion barrier. 
     It will be readily understood that the components of various embodiments of the present invention, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the detailed description of the embodiments of the present invention, as represented in the attached figures, is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention. 
     The features, structures, or characteristics of the invention described throughout this specification may be combined in any suitable manner in one or more embodiments. For example, reference throughout this specification to “certain embodiments,” “some embodiments,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in certain embodiments,” “in some embodiment,” “in other embodiments,” or similar language throughout this specification do not necessarily all refer to the same group of embodiments and the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     It should be noted that reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and/or with hardware elements in configurations which are different than those which are disclosed. Therefore, although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.