Abstract:
A GaN high electron mobility transistor (HEMT) device having a silicon carbide substrate including a top surface and a bottom surface, where the substrate further includes a via formed through the bottom surface and into the substrate. The device includes a plurality of epitaxial layers provided on the top surface of the substrate, a plurality of device layers provided on the epitaxial layers, and a diamond layer provided within the via.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to a GaN semiconductor device and, more particularly, to a GaN high electron mobility transistor (HEMT) device fabricated on a silicon carbide (SiC) substrate, where the substrate includes a back-side via that includes diamond. 
     2. Discussion of the Related Art 
     Integrated circuits are typically fabricated by epitaxial fabrication processes that deposit or grow various semiconductor layers on a substrate to provide the circuit components for the device. Substrates for integrated circuits can include various materials, usually semiconductor materials, such as silicon, sapphire, SiC, InP, GaAs, etc. As integrated circuit fabrication techniques advance and become more complex, more circuit components are able to be fabricated on the substrate within the same area and be more closely spaced together. Further, these integrated circuit fabrication techniques allow the operating frequencies of the circuit to increase to very high frequencies, well into the GHz range. 
     HEMT devices are popular semiconductor devices that have many applications, especially high frequency and high power applications, for example, power amplifiers. GaN HEMT devices are typically epitaxial grown on a suitable substrate for these applications, where the substrate needs to be highly thermally conductive, electrically insulative, have a thermal expansion coefficient similar to GaN and provide lattice spacing matching for suitable epitaxial growth. Suitable materials that are both highly thermally conductive and electrically insulative are relatively unique. 
     A high thermally conductive substrate is necessary so that heat is removed from the device junction through the epitaxial layers and the substrate so that the device is able to operate at high power in a reliable manner. Particularly, as the temperature of the device increases above some threshold temperature, the electrical performance of the device is reduced, which reduces its high power capability. Further, too high of a temperature within the device reduces its reliability because its time to failure will be reduced. Also, these types of devices are typically high frequency devices, which become smaller in size as the frequency increases, which reduces their ability to withdraw heat. The conductive path for heat generated at the device junction layer in an HEMT device causes the heat to propagate through the epitaxial layers and the substrate and into the device packaging. Therefore, it necessary to provide a high thermally conductive substrate that does not impede the path of the heat exiting the device, and allows the heat to spread out over a larger area. The thickness of the substrate is optimized to provide a low resistance heat path into the packaging from the device and provide the ability to spread the heat out away from the device. 
     For GaN HEMT devices, silicon carbide (SiC) substrates are currently the industry standard for providing the desirable characteristics of electrically insulating, highly thermally conductive, a close lattice match to that of GaN and a similar thermal expansion coefficient to that of GaN. However, although SiC is a good thermal conductor, its thermal conductivity is still limited, and as the junction temperature rises in the device, the ability of the SiC substrate to remove the heat is limited, which limits the output power of GaN HEMT devices, and subsequently their reliability, as discussed above. 
     It is desirable to provide a suitable substrate for a GaN HEMT device that has a greater thermal conductivity than SiC. Diamond is electrically insulating and has the highest thermal conductivity of any bulk material. However, it is currently not possible to epitaxial grow GaN layers on large area single-crystal diamond substrates for many reasons, including availability, a large lattice spacing mismatch and different thermal expansion coefficients. Efforts have been made in the industry to overcome these problems so as to use diamond substrates in a semiconductor device, such as GaN HEMT devices. For example, it is known in the art to remove the SiC substrate, or other substrate, that the GaN layers can effectively be grown on, and then bond a diamond substrate to the device using a bonding layer. However, there is now a bonding layer of some thickness between the GaN device layers and the diamond substrate that does not have the proper thermal conductivity, and thus affects the ability of heat to be removed from the device through the diamond substrate. Further, because bulk diamond has a low thermal coefficient of expansion, there is still the problem that the difference between the thermal expansion coefficients of the device layers and the substrate causes wafer curvature and possibly epitaxial layer cracking. 
     It is also known in the art to grow diamond on the front-side of the device opposite to the substrate. However, it has been shown that these types of devices have limited improvement in thermal conductivity and heat flow out of the device because heat flow through the substrate is still highly important. Further, GaN layers may not survive the high temperature diamond deposition process, and thus may need to be protected using a thermally resistive layer, which again limits the thermal performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional profile view of a portion of a wafer including GaN HEMT devices each having a diamond via extending completely through a substrate of the wafer; 
         FIG. 2  is a cross-sectional profile view of a GaN HEMT device including a diamond via extending partially through a substrate of the device; 
         FIG. 3  is a cross-sectional profile view of a GaN HEMT device including a diamond via extending through a substrate and into one or more epitaxial layers of the device; and 
         FIG. 4  is a cross-sectional profile view of a GaN HEMT device including a diamond layer formed in a via in a substrate of the device. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a GaN HEMT device including a diamond filled via extending through a back-side of a substrate of the device is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a partial cross-sectional profile view of a semiconductor wafer  10  including HEMT devices  12  fabricated on a substrate  14 , where the various epitaxial and device layers of the HEMT devices  12  are deposited or grown using known epitaxial growth techniques. Although only two of the devices  12  are shown in  FIG. 1 , during the device fabrication process, many of the devices  12  are simultaneously fabricated on a single wafer in a manner that is well understood by those skilled in the art. The substrate  14  can be any substrate suitable for the purposes discussed herein, such as SiC, sapphire, GaN, AlN, silicon, etc. Once the device layers are grown and the devices  12  are further fabricated, certain processes are performed to separate the devices  12  on the wafer  10 , where the devices  12  are later packaged. 
     For the particular non-limiting device design being discussed, an AlN nucleation layer  16  is grown on the substrate  14  to provide a base layer for proper epitaxial growth of the device profile layers. Next, a GaN buffer layer  18  is grown on the nucleation layer  16 , and an AlGaN barrier layer  20  is grown on the buffer layer  18 . Other layers may be deposited on the barrier layer  20 . The piezoelectric/spontaneous polarization effect between the AlGaN barrier layer  20  and the GaN buffer layer  18  generates a 2-DEG layer  22 . Layer  26  represents all other device layers that may be present on top of the 2-DEG layer  22 , including the epitaxial contact layers, ohmic contacts, such as the drain and source terminals and the gate metal, etc. It is noted that this specific devices profile is a general representation for a GaN HEMT in that other HEMT device designs may have other layers. 
     Once the epitaxial layers for the devices  12  have been grown on the substrate  14 , the front side of the epitaxial layers is protected with a thermally stable layer (not shown), such as silicon nitride (SiN) or other refractory materials. The wafer  10  is flipped over and the back-side of the substrate  14  is patterned using a suitable mask (not shown) to provide thermal vias  28  and alignment marks (not shown). Particularly, the back-side of the substrate  14  is selectively etched through the substrate  14  to stop at the AlN nucleation layer  16  to form the vias. A diamond seed layer is dispersed on the back-side of the wafer  10  and in the vias  28 , and then a polycrystalline diamond is grown on the back-side of the wafer  10  to form a diamond layer  30  so that the vias  28  are filled with diamond. The back-side of the wafer  10  can then be polished to smooth the diamond layer  30  to make it more conforming to the device structure to provide a better thermal contact to the packaging. 
     Once the diamond layer  30  is grown on the back-side of the wafer  10  and contacts the AlN nucleation layer  16  through the via  28 , the wafer  10  is again flipped over to fabricate the remaining device layers, namely the layers  26 , on the front side of the wafer  10 . Suitable patterning and metal deposition steps are then performed to deposit the source, drain and gate terminals on the 2-DEG layer  22 . Suitable metallization lines and the like are formed to provide electrical connections, and those conducting layers that extend between the devices  12  on the wafer  10  are rendered insulative by suitable techniques, all well understood by those skilled in the art. Once the final fabrication steps are performed, the wafer  10  is diced such that the separate circuits or chips, possibly including many of the devices  12 , are separated and can be packaged for use. Thus, each device  12  in the circuit or chip includes a diamond area in the substrate  14  directly below the device layers  26  to provide greater thermal conductivity during operation of the device  12 . 
     Alternatively, it may be desirable to selectively seed only the areas in and around the vias  28  to reduce wafer curvature from mismatched thermal expansion coefficients. Also, the diamond can be deposited across the entire backside of the wafer  10 , and then selectively removed from those areas that are outside of the vias  28 . 
     As is known in the art, polycrystalline diamond can be grown in various ways so that the amount of impurities and the crystalline formation of the diamond during the growth process can have different qualities. The larger the polycrystalline structure of the diamond and the higher the purity level, the higher the thermal conductivity of the diamond. However, the higher the purity of the diamond, the longer the deposition process will take and the more costly it will be. Therefore, it may desirable that when growing the diamond layer  30  on the backside of the substrate  14  the growth process produces a purer higher thermally conductive diamond material closest to the AlN layer  16 , and then subsequently grow a lower quality diamond layer farther away from the AlN layer  16 , which would reduce the deposition time to fill the vias  28 . Alternately, it may be desirable to grade the diamond purity over time as the diamond layer is being deposited by first providing the higher thermally conductive pure diamond and then slowly reducing the quality of the diamond during the growth process over time. 
     Alternately, the substrate  14  can be etched so that not all of the substrate material is removed in the via  28  where there would be a thin layer of the substrate material between the diamond layer  30  and the AlN layer  16  after the diamond is deposited in the via  28 .  FIG. 2  is a cross-sectional profile view of a partial structure for an HEMT device  40  similar to the device  12 , where like elements are identified by the same reference numeral. In this embodiment, the substrate  14  is not completely etched to the AlN nucleation layer  16 , but is stopped short of the layer  16  so that a thin layer  42  of the substrate material remains between the via  28  and the layer  16 . Providing the thin layer  42  of the substrate material may have certain semiconductor properties for certain HEMT devices. Also, this embodiment shows diamond  44  only within the via  28 . 
     Further, it may be desirable to etch into the AlN nucleation layer  16  so that the diamond filled via  28  extends into the device epilayers.  FIG. 3  is a cross-sectional profile view of a partial structure of an HEMT device  50  similar to the devices  12  and  40 , where like elements and layers are identified by the same reference numeral. In this embodiment, the via  28  extends completely through the substrate  14  and into the AlN nucleation layer  16 . 
     In an alternate embodiment, the via  28  may only be partially filled with the diamond layer  30  so that part of the via  28  remains open to air where that remaining portion may then be filled with a metal to provide a background plane in closer proximity to the epitaxial layers.  FIG. 4  is a cross-sectional profile view of an HEMT device  60  similar to the HEMT devices  10  and  40 , where the same device layers are identified by the same reference numeral. In this embodiment, a diamond layer  62  only partially fills the via  28  so that an open area  64  remains. 
     Although the devices discussed herein are HEMT devices, other types of devices, such as laser diodes or light emitting diodes, that employ GaN device layers deposited on a substrate may benefit from the higher performance provided by the thermally conductive diamond vias discussed herein. Further, although the embodiments discussed herein are specifically for SiC substrates, other suitable substrates, such as those referred to above, may also include formed vias filled with diamond for the same purpose. 
     The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.