METHOD OF FABRICATING AN OXIDE LAYER ON A SILICON CARBIDE LAYER UTILIZING N2O

Abstract: Methods for fabricating a layer of oxide on a silicon carbide layer are provided by forming the oxide layer by at least one of oxidizing the silicon carbide layer in an N2O environment or annealing an oxide layer on the silicon carbide layer in an N2O environment. Preferably, a predetermined temperature profile and a predetermined flow rate profile of N2O are providing during the oxidation or the anneal. The predetermined temperature profile and/or predetermined flow rate profile may be constant or variable and may include ramps to steady state conditions. The predetermined temperature profile and/or the predetermined flow rate profile are selected so as to reduce interface states of the oxide/silicon carbide interface with energies near the conduction band of SiC.

Description: METHOD OF FABRICATING AN OXIDE LAYER ON A SILICON CARBIDE LAYER UTILIZING N 2 O Related Applications The present application claims priority from United States Provisional Application Serial No. 60/294,307 entitled "METHOD OF N2O GROWTH OF AN OXIDE LAYER ON A SILICON CARBIDE LAYER" filed May 30, 2001 and claims priority from, and is a continuation-in-part of, United States Patent Application Serial No. 09/834,283 filed April 12, 2001 entitled "METHOD OF N2O ANNEALING AN OXIDE LAYER ON A SILICON CARBIDE LAYER" which claims priority from United States Provisional Application Serial No. 60/237,822, entitled "Method of Improving an Interface Between a Silicon Carbide Layer and an Oxide Layer" and United States Provisional Application Serial No. 60/237,426 entitled "SiC Power MOSFET and Method of Fabrication" which were filed October 3, 2000. The disclosures of each of the above-cited applications are incorporated herein by reference as if set forth fully herein. Field of the Invention The present invention relates to the fabrication of semiconductor devices and more particularly, to the fabrication of oxide layers on silicon carbide (SiC). Background of the Invention Devices fabricated from silicon carbide are typically passivated with an oxide layer, such as SiO 2 , to protect the exposed SiC surfaces of the device and/or for other reasons. However, the interface between SiC and SiO 2 maybe insufficient to obtain a high surface mobility of electrons. More specifically, the interface between SiC and SiO conventionally exhibits a high density of interface states, which may reduce surface electron mobility. Recently, annealing of a thermal oxide in a nitric oxide (NO) ambient has shown promise in a planar 4H-SiC MOSFET structure not requiring a p-well implant. See M. K. Das, L. A. Lip in, J. W. Palmour, G. Y. Chung, J. R. Williams, K. McDonald, and L. C. Feldman, "High Mobility 4H-SiC Inversion Mode MOSFETs Using Thermally Grown, NO Annealed SiO 2 ," IEEE Device Research Conference, Denver, CO, June 19-21, 2000 and G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, R. A. Weller, S. T. Pantelides, L. C. Feldman, M. K. Das, and J. W. Palmour, "Improved Inversion Channel Mobility for 4H-SiC MOSFETs Following High Temperature Anneals in Nitric Oxide," IEEE Electron Device Letters accepted for publication. This anneal is shown to significantly reduce the interface state density near the conduction band edge. G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, M. Di Ventra, S. T. Pantelides, L. C. Feldman, and R. A. Weller, "Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide," Applied Physics Letters, Vol. 76, No. 13, pp. 1713-1715, March 2000. High electron mobility (35-95 cm 2 /Vs) is obtained in the surface inversion layer due to the improved MOS interface. Unfortunately, NO is a health hazard having a National Fire Protection Association (NFPA) health danger rating of 3, and the equipment in which post- oxidation anneals are typically performed is open to the atmosphere.of the cleanroom. They are often exhausted, but the danger of exceeding a safe level of NO contamination in the room is not negligible. Growing the oxide in N 2 O is possible as described in J. P. Xu, P. T. Lai, C. L. Chan, B. Li, and Y. C. Cheng, "Improved Performance and Reliability of N 2 O-Grown Oxynitride on 6H-SiC," IEEE Electron Device Letters, Vol. 21, No. 6, pp. 298-300, June 2000. Xu et al. describe oxidizing SiC at 1100 °C for 360 minutes in a pure N O ambient and annealing in N 2 for 1 hour at 1100 °C. Post-growth nitridation of the oxide on 6H-SiC in N 2 O at a temperature of 1100 °C has also been investigated by Lai et al. P. T. Lai, Supratic Chakraborty, C. L. Chan, and Y. C. Cheng, "Effects of nitridation and annealing on interface properties of thermally oxidized SiO 2 /SiC metal-oxide-semiconductor system," Applied Physics Letters, Vol. 76, No. 25, pp. 3744-3746, June 2000. However, Lai et al. concluded that such treatment deteriorates the interface quality which may be improved with a subsequent wet or dry anneal in O 2 which may repair the damage induced by nitridation in N 2 O. Moreover, even with a subsequent O 2 anneal, Lai et al. did not see any significant reduction in interface state density as compared to the case without nitridation in N 2 O. Summary of the Invention Embodiments of the present invention provide methods for fabricating a layer of oxide on a silicon carbide layer by at least one of oxidizing the silicon carbide layer in an N 2 O environment or annealing an existing oxide layer on the silicon carbide layer in an N 2 O environment. Preferably, a predetermined temperature profile and a predetermined flow rate profile of N 2 O are provided during the oxidation or the anneal. The predetermined temperature profile and/or predetermined flow rate profile may be constant or variable and may include ramps to steady state conditions. The predetermined temperature profile and/or the predetermined flow rate profile are selected so as to reduce interface states of the oxide/silicon carbide interface with energies near the conduction band of SiC. In particular embodiments of the present invention where an anneal is performed, the predetermined temperature profile may result in an anneal temperature of greater than about 1100 °C. In embodiments of the present invention where an anneal is performed, the anneal temperature may be greater than about 1175 °C. In a particular embodiment, the anneal temperature is about 1200 °C. In further embodiments of the present invention, the anneal may be from about 1.5 hours to about 3 hours or longer. In embodiments of the present invention where oxidation is performed, the predetermined temperature profile may result in an oxidation temperature of at least about 1200 °C. In particular embodiments, the oxidation temperature is about 1300 °C. In further embodiments of the present invention, the duration of the oxidation may vary depending on the thickness of the oxide layer desired. Thus, oxidation may be carried out for from about 15 minutes to about 3 hours or longer. In additional anneal embodiments of the present invention, the predetermined flow rate profile includes one or more flow rates of from about 2 Standard Liters per Minute (SLM) to about 8 SLM. In particular embodiments, the flow rates are from about 3 to about 5 Standard Liters per Minute. In additional oxidation embodiments of the present invention, the predetermined flow rate profile includes one or more flow rates of from about 2 Standard Liters per Minute (SLM) to about 6 SLM. In particular embodiments, the flow rates are from about 3.5 to about 4 Standard Liters per Minute. In further embodiments, the anneal of the oxide layer or formation of the oxide layer may be followed by annealing the oxide layer in Ar or N 2 . This anneal may also be carried out in a hydrogen containing environment, such as H 2 or combinations of H 2 and one or more inert gases such as Ar or N 2 . Such an annealing operation may be carried out, for example, for about one hour. In still further anneal embodiments of the present invention, the predetermined flow rate profile provides a velocity or velocities of the N 2 O of from about 0.37 cm s to about 1.46 cm/s. In particular embodiments, the predetermined flow rate profile provides a velocity or velocities of the N 2 O of from about 0.5 cm/s to about 1 cm/s. In still further oxidation embodiments of the present invention, the predetermined flow rate profile provides a velocity or velocities of the N 2 O of from about 0.37 cm/s to about 1.11 cm/s. In particular embodiments, the predetermined flow rate profile provides a velocity or velocities of the N 2 O of from about 0.65 cm/s to about 0.74 cm/s. Additionally, in anneal embodiments of the present invention, the oxide layer may be formed by depositing the oxide layer and/or by thermally growing the oxide layer. In still further embodiments of the present invention, a wet reoxidation of the oxide layer may also be performed and/or the N 2 O oxidation may be carried out in an environment with a fraction or partial pressure of steam. In further anneal embodiments, methods for fabricating a layer of oxide on a silicon carbide layer include forming the oxide layer on the silicon carbide layer and annealing the oxide layer in an N 2 O environment at a predetermined temperature profile which includes an anneal temperature of greater than about 1100 °C and at a predetermined flow rate profile for the N 2 O. The predetermined flow rate profile may be selected to provide an initial residence time of the N O of at least 11 seconds. In particular anneal embodiments of the present invention, the initial residence time may be from about 11 seconds to about 45 seconds. In still further embodiments of the present invention, the initial residence time is from about 16 seconds to about 31 seconds. Additionally, a total residence time of the N 2 O may be from about 28 seconds to about 112 seconds. In such anneal embodiments of the present invention, the total residence time may also be from about 41 seconds to about 73 seconds. In further oxidation embodiments, methods for fabricating a layer of oxide on a silicon carbide layer include foraiing the oxide layer on the silicon carbide layer in an N 2 O environment at a predetermined temperature profile which includes an oxidation temperature of at least about 1200 °C and at a predetermined flow rate profile for the N 2 O. The predetermined flow rate profile may be selected to provide an initial residence time of the N 2 O of at least about 11 seconds. In particular oxidation embodiments of the present invention, the initial residence time may be from about 11 seconds to about 33 seconds. In still further embodiments of the present invention, the initial residence time is from about 19 seconds to about 22 seconds. Additionally, a total residence time of the N 2 O may be from about 28 seconds to about 84 seconds. In such oxidation embodiments of the present invention, the total residence time may also be from about 48 seconds to about 56 seconds. Brief Description of the Drawings Figure 1 is a schematic illustration of a furnace tube suitable for use in embodiments of the present invention; Figure 2A is a flowchart illustrating processing steps according to anneal embodiments of the present invention; Figure 2B is a flowchart illustrating processing steps according to oxidation embodiments of the present invention; Figure 3 is a graph illustrating the interface trap density versus energy level from the conduction band (Ec-E) for various flow rates of N 2 O at 1175°C; Figure 4 is a graph of interface trap density (Du) versus energy level from the conduction band for various flow rates at 1200°C; Figure 5 is a graph of D lτ versus energy level from the conduction band for various anneal temperatures; Figure 6 is a graph of Du versus energy level from the conduction band at 1175°C for anneals of various different durations; Figure 7 is a graph of Drr versus energy level from the conduction band for a post-treatment anneal in Ar and N 2 ; Figure 8 is a graph of Dι T versus energy level from the conduction band for an initial thermal oxide and an initial LPCVD oxide; Figure 9 is a graph of Drr versus energy level from the conduction band for oxide layers formed with and without a wet reoxidation; and Figure 10 is a graph of Drr versus energy level from the conduction band at 1175°C for anneals of various different durations. Figure 11 is a graph illustrating the interface trap density (Dit) versus energy level from the conduction band (EQ-E) for various anneal temperatures; Figure 12 is a graph of Drr versus energy level from the conduction band for various thermal oxidation, post-growth N 2 O anneals and N 2 O oxidation; Figure 13 is a graph of effective surface channel mobility versus gate voltage for 4H-SiC planar MOSFETs with and without N 2 O processing; Figure 14 is a graph of interface states for the oxides of Figure 13; Figures 15A and 15B are graphs of the effective Surface Channel Mobility for horizontal channel buffered gate devices fabricated with N 2 O grown oxides according to embodiments of the present invention; Figures 16A and 16B are graphs of the effective Surface Channel Mobility of lateral metal-oxide-semiconductor field effect transistors fabricated with N 2 O grown oxides according to embodiments of the present invention; and Figures 17 A, 17B and 17C are graphs of effective channel mobility at different gate biases for devices having oxide layers fabricated according to embodiments of the present invention. Detailed Description of the Invention The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present. Embodiments of the present invention provide methods which may improve the interface between an oxide layer and SiC in any device which includes such an interface. These methods are especially advantageous in the fabrication of Metal- Oxide-Semiconductor (MOS) devices created on SiC. Using embodiments of the present invention, interface states with energy levels near the conduction band of SiC may be dramatically reduced. Reduction of such defects may be advantageous, because these defects may limit a MOSFET's effective surface channel mobility. Embodiments of the present invention will now be described with reference to Figures 1, 2 A and 2B which are a schematic illustration of a furnace tube suitable for use in embodiments of the present invention and flow charts illustrating operations according to particular embodiments of the present invention. As seen in Figure 1, the furnace tube 10 has a plurality of wafers 12 of SiC either with an oxide layer, such as SiO 2 , formed thereon or on which an oxide layer is to be formed. Preferably, the SiC wafer is 4H-SiC. The wafers 12 are placed on a carrier 14 such that the wafers will, typically have a fixed position in the furnace tube 10. The carrier 14 is positioned so that the wafers are a distance L1+L2 from an inlet of the furnace tube 10 and extend for a distance L3 within the furnace tube 10. Input gases 16, include N 2 O which provides an N 2 O environment as described herein, are passed into the furnace tube 10 and are heated as they traverse the distance LI based on a predetermined temperature profile so as to provide the heated gases 18. The heated gases 18 are maintained at temperatures based on the predetermined temperature profile and traverse the distance L2 to reach the first of the wafers 12. The heated gases 18 continue to pass through the furnace tube 10 until they leave the furnace tube 10 through an outlet port as exhaust gases 20. Thus, the heated gases 18 traverse the distance L3. The heated gases 18 are preferably maintained at a substantially constant temperature for the distances L2 and L3, however, as will be appreciated by those of skill in the art in light of the present disclosure, various temperature profiles may also be utilized. Such profiles may include variations in temperature over time or distance. However, the predetermined temperature profile should include either an anneal temperature of greater than about 1100 °C or an oxidation temperature of at least about 1200 °C. As is seen in Figure 1, the heated gases 18 may reach a temperature at which the N 2 O begins to break down into its constituents at the end of the LI distance. This distance may depend on the physical characteristics of the furnace tube 10, the predetermined temperature profile and the flow rate profile. After reaching the temperature at which the N 2 O begins to break down, the heated gases 18 traverse the distance L2 before reaching the wafers 12. The amount of time that it takes the heated gases to traverse the distance L2 is referred to herein as an "initial residence time." Preferably, the heated gasses are maintained at a substantially constant temperature corresponding to an anneal temperature of greater than about 1100 °C or an oxidation temperature of at least about 1200 °C for the initial residence time. However, as will be appreciated by those of skill in the art, differing heating profiles could be utilized which increase or decrease the initial residence time. It is preferred, however, that the heating profile be rapid such that the initial residence time is substantially the same as the time that the heated gases 18 are maintained at an anneal temperature of greater than about 1100 °C or an oxidation temperature of at least about 1200 °C before traversing the L3 distance. The total amount of time that it takes the heated gases 18 to traverse the distance L2+L3 is referred to herein as the "total residence time." As will be appreciated by those of skill in the art in light of the present disclosure, these residence times depend on the velocity of the heated gases 18 through the furnace tube 10 which may be determined based on the flow rates of the heated gases 18 and the cross-sectional area of the furnace tube 10. Such velocities may be average velocities, for example, if turbulent flow is achieved, or may be actual velocities, for example, in laminar flow systems. Thus, the term velocity is used herein to refer to both average and actual velocities. Figure 2 A illustrates operations according to embodiments of the present invention and will be described with reference to Figure 1. However, as will be appreciated by those of skill in the art in light of the present disclosure, embodiments of the present invention are not limited to the furnace tube embodiment illustrated in Figure 1 but may be carried out in any system capable of providing the conditions described herein. Turning to Figure 2A, operations may begin by forming an oxide layer on SiC layer (block 30). The SiC layer may be an epitaxial layer and/or a substrate. Furthermore, the oxide layer may be formed by deposition, such as Low Pressure Chemical Vapor Deposition (LPCND), thermally grown through a thermal oxidation process and/or formed using other techniques. Preferably, the oxide layer is formed utilizing a wet reoxidation process as described in United States Patent No. 5,972,801, the disclosure of which is incorporated herein by reference as if set forth fully herein. Furthermore, the oxide layer may be formed in situ with the subsequent N 2 O anneal and in situ with the SiC layer and/or it may be formed in a separate chamber. The oxide layer is then annealed in an N 2 O environment at a predetermined temperature and a predetermined flow rate (block 32). Preferably, the oxide is annealed using a predetermined temperature profile which includes an anneal temperature of greater than about 1100 °C in a chamber in which N 2 O is supplied at a flow rate profile within predetermined flow rate limits. In further embodiments, the temperature of the anneal is about 1175 °C or higher. In particular embodiments, an anneal temperature of about 1200 °C may be utilized. The flow rate limits of N 2 O may be selected based on the particular equipment in which the process is used. However, in particular embodiments the flow rate limits of N 2 O may be as low as about 2 Standard Liters per Minute (SLM) or as high as about 8 SLM. In further embodiments, flow rate limits of from about 3 to about 5 SLM may be preferred. For a 6 inch diameter furnace tube, flow rates of from 2 SLM to 8 SLM result in gas velocities as low as about 0.37 cm/sec or as high as about 1.46 cm/sec or, and flow rates of from 3 to 5 SLM result in velocities of from about 0.55 cm/s to about 0.95 cm/s. In particular, for an L2 distance of about 12 inches (about 30.48 cm) and an L3 distance of about 18 inches (about 45.72 cm), such velocities result in an initial residence time of from about 11 seconds to about 45 seconds and a total residence of from about 28 seconds to about 112 seconds. In particular prefereed embodiments, the initial residence time is from about 16 seconds to about 31 seconds and a total residence time of from about 41 to about 73 seconds. The N 2 O anneal may be carried out for about 3 hours, however, anneals of from about 30 minutes to about 6 hours may also be utilized although longer times may also be utilized. As is further illustrated in Figure 2A, the N 2 O anneal may be followed by an optional anneal (block 34) in inert gas or gases, such as argon and/or N 2 or combinations thereof. The optional anneal may also be carried out in a hydrogen containing environment, such as H 2 or H 2 in combination with one or more inert gases, such as such as argon and/or N 2 or combinations thereof. Such an anneal may be carried out for about 1 hour, however, anneals of up to about 3 hours or longer may also be utilized. Figure 2B illustrates operations according to further embodiments of the present invention and will be described with reference to Figure 1. However, as will be appreciated by those of skill in the art in light of the present disclosure, embodiments of the present invention are not limited to the furnace tube embodiment illustrated in Figure 1 but may be carried out in any system capable of providing the conditions described herein. Turning to Figure 2B, operations begin by providing a SiC layer (block 60). The SiC layer may be an epitaxial layer and/or a substrate. The oxide layer is then formed in an N 2 O environment at a predetermined temperature and/or a predetermined flow rate (block 62). The oxide layer is formed by oxidizing the SiC wafers 12 using a predetermined temperature profile which includes an oxidation temperature of greater than about 1200 °C in a chamber in which N O is supplied at a flow rate profile within predetermined flow rate limits. In further embodiments, the temperature of the , oxidation is about 1300 °C. The flow rate limits of N 2 O may be selected based on the particular equipment in which the process is used. However, in particular embodiments, the flow rate limits of N 2 O may be as low as about 2 Standard Liters per Minute (SLM) or as high as about 6 SLM or higher. In further embodiments, flow rate limits of from about 3.5 SLM to about 4 SLM may be preferred. As used herein, N 2 O refers to pure N 2 O or N 2 O in combination with other oxidizing agents, such as steam, O 2 , and/or inert gases. For a 6 inch diameter furnace tube, flow rates of from about 2 SLM to about 6 SLM result in gas velocities as low as about 0.37 cm/sec or as high as about 1.11 cm/sec. Similarly, for a 6 inch diameter furnace tube, flow rates of from 3.5 SLM to 4 SLM result in velocities of from about 0.65 cm/s to about 0.74 cm/s. In particular, for an L2 distance of about 12 inches (about 30.48 cm) and an L3 distance of about 18 inches (about 45.72 cm), such velocities result in an initial residence time of from about 11 seconds to about 33 seconds and a total residence time of from about 28 seconds to about 84 seconds. In particular prefened embodiments, the initial residence time is from about 19 second to about 22 seconds and the total residence , time is from about 49 to about 56 seconds. The N 2 O oxidation may be carried out for an amount of time dependent on the desired thickness of the oxide layer. For example, oxidation times of about 3 hours or greater may be utilized. As is further illustrated in Figure 2B, the N 2 O oxidation may be followed by an optional anneal (block 34") in an inert gas, such as argon or N . Optionally, the anneal may be carried out in a hydrogen containing environment, such as H 2 or H 2 in combination with one or more inert gases, such as argon and/or N 2 or combinations thereof. Such an anneal may be carried out for about up to 1 hour, however, anneals of up to about 3 hours or longer may also be utilized. As seen in Figures 3 through 14, it has been found that, by appropriately controlling the anneal and/or oxidation temperature and N 2 O flow rate in accordance with the present invention, the SiC/SiO 2 interface quality may be improved, rather than damaged as taught by Lai et al. While not wishing to be bound by any theory of operation, it appears that at high temperatures (above 800°C), a fraction of N 2 O will break down into N 2 , O and NO. The fraction of NO is determined by the temperature and the amount of time the gas remains at elevated temperatures, which is determined by the flow rate of the gas, the cross-sectional area of the furnace tube and the distances in the tube. Table 1 shows the effect of the flow rate of N 2 O on the maximum interface state density for an anneal of 3 hours at 1175°C, followed by a 1 hour Ar anneal after the N O anneal. Table 1. Effect of Flow Rate on N 2 O Anneal. As shown in Table 1, the anneal with 4 SLM of N 2 O has the lowest interface state densities, and the most negative flat-band voltage. Accordingly, in particular embodiments of the present invention flow rates of from about 4 to about 6 SLM may be utilized. Figures 3 through 10 illustrate the profile of interface trap density (Dit) throughout the bandgap at the SiC/SiO 2 interface for various embodiments of the present invention. Drr may be measured using any technique known to those of skill in the art. Figure 3 illustrates the interface trap density versus energy level for various velocities of N 2 O for the flow rates in Table 1 with an anneal temperature of 1175 °C. As seen in Figure 3, while each of the flow rates results in a reduced trap density as compared to no N 2 O anneal, the greatest reduction in trap density is provided by flow rates yielding velocities of 0.7 cm/s and 1.1 cm/s. Figure 3 illustrates that the optimal flow rate is approximately 0.7 cm/s (or approximately 4 SLM). Figure 4 is a graph of Drr versus energy level for various velocities with an anneal temperature of 1200°C. Figure 4 likewise indicates that for a 1200°C anneal, the greatest reduction in trap density is achieved with a velocity of approximately 0.7 cm/s (or approximately 4 SLM). Thus, from Figures 3 and 4, initial residence times of about 22 seconds may provide the greatest reduction in trap density. Figure 5 is a graph of Drr versus energy level for various anneal temperatures. Figure 5 illustrates that the temperature should be above 1100°C to obtain a reduction in Dit, and preferably above 1175°C. Figure 6 is a graph of Drr versus energy level at 1175°C for anneals of different durations, namely one minute and three hours. As seen in Figure 6 a reduction in trap density is achieved by a longer duration anneal (3 hours) over a short duration anneal (1 minute). Figure 7 is a graph of Drr versus energy level for a post-treatment anneal in Ar and N 2 . Figure 7 indicates that both atmospheres are suitable for purposes of the present invention, since they produce substantially similar results. Figure 8 is a graph of Drr versus energy level for two different types of oxides, a thermal oxide and an LPCND oxide. Figure 8 illustrates that trap densities may be reduced utilizing embodiments of the present invention for both types of oxides as similar results are achieved for both types of oxide. Figure 9 is a graph of Drr versus energy level for anneal times of 3 hours where the oxide layer included a wet reoxidation as described in United States Patent No. 5,972,801, and for an anneal which did not utilize a wet reoxidation process. As can be seen from Figure 9, decreased interface densities were achieved when a wet re-oxidation process was utilized. Figure 10 is a graph of D r versus energy level for durations of 1.5 and 3 hours. As can be seen from Figure 10, it appears that durations as long as 3 hours may be no more effective, and possible less effective, than durations of about 1.5 hours. However, either duration appears to provide acceptable results. As illustrated in Figure 11, annealing in an N 2 O environment at higher temperatures results in better interface characteristics over lower temperature anneals, as it appears that the chemistry yields more of the desired NO, by breaking down the N 2 O during the process. This is seen in Figure 11, where N 2 O processing at 1100°C, 1200°C and 1300°C are compared. Furthermore, at higher temperatures, the oxidation rate increases significantly. At these higher rates, growing the oxide (as opposed to annealing an existing oxide) in the N 2 O ambient would be expected to be feasible. Oxides grown in N 2 O at 1300°C have interface characteristics as good or better than SiO 2 /SiC interface characteristics reported to date and may significantly reduce the processing time required to separately grow the oxide and anneal it. As seen in Figure 12, the "1300 grown" line illustrates the interface characteristics for an N 2 O oxidation process. Such an N 2 O oxidation may save several hours of processing time by eliminating the need for both growth and anneal steps. As described above, SiC devices having an oxide-SiC interface, such as SiC Metal Oxide Semiconductor (MOS) devices may be severely impacted by the large density of interface states present at the SiC/SiO 2 interface. Interface states near the conduction band-edge may be particularly effective at inhibiting SiC device performance. Early improvements in oxidation processes typically reduced interface states only from the valence band to mid-gap. More recent progress has been accomplished using an NO anneal, which may lower the interface state densities near the conduction band-edge. While these improvements using NO annealing may be important, use of this gas in traditional furnaces may not be desirable with the health risks associated with pure NO. As described herein, the use of N 2 O has been pursued and effectively developed as an alternative to NO. As shown in Figure 13, the temperature of the N 2 O processing may be important. At lower temperatures (1100°C), exposing an existing oxide to N 2 O increases the interface state density, as shown by comparing the heavy solid line representing a thermal oxide to the data for the same thermal oxide exposed to an 1100°C N 2 O anneal. At 1200°C, the thermal oxide is significantly improved with the N 2 O anneal. Thermal oxides processed in a wet ambient may be further improved by the 1200°C N 2 O anneal, as seen by comparing the solid circle to the open circle data. Further improved results were obtained using a 1300°C N 2 O process. At this temperature, the oxidation of SiC is significant. So, in addition to annealing existing oxides in N 2 O, some oxides were grown in the N 2 O ambient (500 A was grown in 3 hours). Growing the oxide in N 2 O may save about 9 hours of processing time over annealing an existing oxide in N 2 O, by eliminating the oxidation step. Substantially the same results were obtained regardless of whether the oxide was grown prior to N 2 O processing or grown in N 2 O. MOSFETs processed with a 1200°C N 2 O annealed oxide had higher effective surface channel mobility than devices that did not receive the N 2 O anneal, as shown in Figure 13. The interface state densities measured on nearby p-type capacitor structures and corresponding n-type capacitors are shown in Figure 14. The reduction in interface state density appears to directly conelate with an improvement in effective surface channel mobility. In further examples of embodiments of the present invention, two n-type 4H wafers and two p-type 4H wafers were obtained. These four wafers were further divided, such that one wafer from each pair received an implant layer, and epitaxial re-growth to simulate a horizontal channel buffered gate FET (HCBGFET) device while MOSFETs were fabricated on the other wafers. The horizontal channel buffered gate FET devices were fabricated on the selected wafers by a blanket Al implant and an implant anneal. An additional n-type epi layer was grown on this implanted layer. The Source/Drain regions were implanted and annealed. These wafers were isolated by etching the n-type epi layer in non-device regions. A field oxide was deposited and densified. Active device areas were opened in the field oxide. The gate insulator was grown in N 2 O at 1300°C. The gate metal was deposited and patterned. Source/Drain contacts were deposited. Backside Pt was deposited. The contacts were annealed to make them ohmic. Effective surface channel mobilities and threshold voltages were measured for the MOSFETs. Additionally, "fatFET" devices, which had a gate length of 200 μm and width of 200 μm, were utilized for these measurements as the device characteristics will be dominated by the channel resistance. Table 2 summarizes this fatFET data. Table 2. Parameters for MOSFETs with gate oxides thermally grown in N O. In Table 2, the averages are given, with the highest effective surface channel mobility being given in parentheses. Ten columns and seven rows (less four corners) were probed = 66 total. The horizontal channel buffered gate devices exhibited high effective surface channel mobilities, but the highest mobilities corresponded with normally-on devices, which is undesirable. More appropriate statistics are shown in the last two rows where all normally-on devices have been excluded from the calculations. Even eliminating the normally-on devices, the HCBGFET results indicate improvement over conventional oxide processes. One wafer exhibited an average effective surface channel mobility of 55.5 cm 2 /V-s, with a maximum of 99 cm 2 /V-s, while the other wafer has an average effective surface channel mobility of 41 cm 2 /N- s, with a maximum of 75 cm 2 /N-s. The variation is likely due to the variation in the doping of the epitaxial layer. This variation is shown in Figures 15A and 15B. Figures 15A and 15B illustrate the effective Surface Channel Mobility across Wafers #3 (Figure 15 A) and #4 (Figure 15B). The doping was higher on the edge of the wafer, turning the device normally-on, and producing very high mobilities. The lateral MOSFETs did not show the same degree of variation in mobility as the HCBGFETs as is shown in Figures 16A and 16B. Figures 16A and 16B are graphs of the effective Surface Channel Mobility across Wafers #1 (Figure 16A) and #2 (Figure 16B). As seen in Figures 16A and 16B, the mobility was very uniform across the wafer, except for the occasional non-yielding device. Typically, 4H-SiC MOSFETs with thermal or deposited oxides without NO or N 2 O processing typically had mobilities in the single digits. The reduction of interface states obtained by growing the oxide in N 2 O has effectively increased by a factor of tenthe surface channel mobility. The mobility of these lateral MOSFETs may be important because the HCBGFET mobility is believed to be limited by this surface mobility at high fields. The mobility at different gate biases is shown in Figures 17A, 17B and 17C. Notice that at high gate biases, there is very little difference between the MOSFETs of wafers #1 and #2, in Figure 17A, and the HCBGFETs of wafers #3 (Figure 17B) and #4 (Figure 17C). As is illustrated in the above example, improved effective surface channel mobility may be provided in MOSFETs with gate oxides grown in N 2 O. Standard lateral devices have mobilities around 20 cm 2 /v-s. HCBGFET mobilities as high as 240 cm 2 /v-s have been obtained on normally-on devices and 99 cm 2 /v-s for normally- off devices. Furthermore, as is illustrated by Figures 3-17C above, through use of embodiments of the present invention, interface trap densities for oxide layers formed on silicon carbide may be reduced utilizing an N 2 O oxidation and/or anneal without the need for a subsequent wet O 2 anneal. Additionally, the N 2 O oxidation may be carried out with other gases in the ambient as described above. Also, N 2 O oxidation may be followed by N 2 O anneal as described in commonly assigned United States Patent Application Serial No. 09/834,283, entitled "METHOD OF N 2 O ANNEALING AN OXIDE LAYER ON A SILICON CARBIDE LAYER", filed April 12, 2001. In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims: That which is claimed is: 1. A method of fabricating a silicon carbide structure having a silicon carbide layer and an oxide layer on the silicon carbide layer, the method being characterized by at least one of: annealing an existing oxide layer in an environment comprising N 2 O using a predetermined temperature profile which includes an anneal temperature of greater than about 1100 °C and a flow rate profile which includes a flow rate which provides an initial residence time of the N 2 O of at least about 11 seconds; or oxidizing a layer of silicon carbide in an N 2 O environment using a predetermined temperature profile which includes an oxidation temperature of at least about 1200 °C. 2. A method according to Claim 1, wherein the method is characterized by annealing an existing oxide layer and wherein the initial residence time is from about 11 seconds to about 45 seconds. 3. A method according to Claim 2, wherein the initial residence time is from about 26 seconds to about 31 seconds. 4. A method according to Claim 2, wherein a total residence time of the N 2 O is from about 28 seconds to about 112 seconds. 5. A method according to Claim 3, wherein a total residence time of the N 2 O is from about 41 seconds to about 73 seconds. 6. A method according to Claim 1, wherein the method is characterized by annealing an existing oxide layer and wherein the anneal temperature is at least about 1175 °C. 7. A method according to Claim 6, wherein the method is characterized by annealing an existing oxide layer and wherein the anneal temperature is about 1200 °C. 8. A method according to Claim 1, wherein the method is characterized by annealing an existing oxide layer and wherein the flow rate profile provides a flow rate of from about 2 Standard Liters per Minute (SLM) to about 8 SLM. 9. A method according to Claim 1, wherein the method is characterized by annealing an existing oxide layer and wherein the flow rate profile provides a flow rate of from about 3 to about 5 Standard Liters per Minute. 10. A method according to Claim 1 , wherein the step of annealing an existing oxide layer is carried out for about 3 hours. 11. A method according to Claim 1 , wherein the step of annealing an existing oxide layer is carried out for about 1.5 hours. 12. A method according to Claim 1 , wherein the step of annealing an existing oxide layer or the step of oxidizing is followed by the step of annealing in an inert gas environment. 13. A method according to Claim 1, wherein the step of annealing an existing oxide layer or the step of oxidizing is followed by the step of annealing in at least one of an environment comprising Ar, an environment comprising N 2 and an environment comprising H 2 . 14. A method according to Claim 12, wherein the step of annealing in an inert gas environment is carried out for about one hour. 15. A method according to Claim 1, wherein the method is characterized by annealing an existing oxide layer and wherein the predetermined flow rate provides a velocity of the N 2 O of from about 0.37 cm/s to about 1.46 cm/s. 16. A method according to Claim 15, wherein the method is characterized by annealing an existing oxide layer and wherein the predetermined flow rate provides a velocity of the N 2 O of from about 0.5 cm/s to about 1 cm/s. 17. A method according to Claim 1 , wherein the existing oxide layer is formed by depositing the oxide layer. 18. A method according to Claim 1, the existing oxide layer is formed by thermally growing the oxide layer. 19. A method according to Claim 1, wherein the existing oxide layer is formed by performing a wet reoxidation of the oxide layer. 20. A method according to Claim 1, wherein the silicon carbide layer comprises 4H polytype silicon carbide. 21. The method of Claim 1 , wherein the method is characterized by the step of oxidizing the silicon carbide layer and wherein the step of oxidizing a layer of silicon carbide in an environment comprising N 2 O, further comprises oxidizing the layer of silicon carbide utilizing a flow rate profile of N 2 O which includes a flow rate which provides an initial residence time of the N 2 O of at least about 11 seconds. 22. The method according to Claim 21, wherein the initial residence time is from about 11 seconds to about 33 seconds. 23. The method according to Claim 22, wherein the initial residence time is from about 19 seconds to about 22 seconds. 24. The method according to Claim 22, wherein a total residence time of the N 2 O is from about 28 seconds to about 84 seconds. 25. The method according to Claim 22, wherein a total residence time of the N 2 O is from about 49 seconds to about 56 seconds. 26. The method according to Claim 1 , wherein the oxidation temperature is about 1300 °C. 27. The method according to Claim 22, wherein the flow rate profile provides a flow rate of from about 2 Standard Liters per Minute (SLM) to about 6 SLM. 28. The method according to Claim 22, wherein the flow rate profile provides a flow rate of from about 3.5 to about 4 Standard Liters per Minute. 29. The method according to Claim 1, wherein the method is characterized by the step of oxidizing the silicon carbide layer and wherein the step of oxidizing the silicon carbide layer is followed by the step of annealing in an inert gas environment. 30. The method according to Claim 1 , wherein the method is characterized by the step of oxidizing the silicon carbide layer and wherein the step of oxidizing the silicon carbide layer is followed by the step of annealing in at least one of an environment comprising Ar, an environment comprising N 2 and an environment comprising H 2 . 31. The method according to Claim 29, wherein the step of annealing in an inert gas environment is carried out for about one hour. 32. The method according to Claim 22, wherein the predetermined flow rate provides a velocity of the N 2 O of from about 0.37 cm/s to about 1.11 cm/s. 33. The method according to Claim 22, wherein the predetermined flow rate provides a velocity of the N 2 O of from about 0.65 cm/s to about 0.74 cm/s. 34. The method according to Claim 1 , wherein the method is characterized by the step of oxidizing the silicon carbide layer and wherein the step of oxidizing the silicon carbide layer comprises oxidizing the silicon carbide layer in an environment comprising N 2 O and including at least one of oxidizing agents and inert gases. 35. The method of Claim 34, wherein the oxidizing agents comprise at least one of steam and O 2 . 36. The method according to Claim 1, wherein the method is characterized by the step of oxidizing the silicon carbide layer and wherein the step of oxidizing the silicon carbide layer is followed by the step of annealing in an environment comprising N 2 O at an anneal temperature of greater than about 1100 °C. 37. A method of fabricating a silicon carbide structure, comprising; forming the oxide layer on the silicon carbide layer; and annealing the oxide layer in an N 2 O environment using a predetermined temperature profile and at a predetermined flow rate profile of N O, wherein the predetermined temperature profile and the predetermined flow rate profile are selected so as to reduce interface states of the oxide/silicon carbide interface with energies near the conduction band of SiC. 38. A method according to Claim 37, wherein the predetermined temperature profile includes an anneal temperature greater than about 1100 °C. 39. A method according to Claim 38, wherein the anneal temperature is greater than about 1175 °C. 40. A method according to Claim 38, wherein the anneal temperature is about 1200 °C. 41. A method according to Claim 37, wherein the predetermined flow rate profile provides a flow rate of from about 2 Standard Liters per Minute (SLM) to about 8 SLM. 42. A method according to Claim 41, wherein the flow rate is from about 3 to about 5 Standard Liters per Minute. 43. A method according to Claim 37, wherein the step of annealing the oxide layer is carried out for about 3 hours. 44. A method according to Claim 37, wherein the step of annealing the oxide layer is carried out for about 1.5 hours. 45. A method according to Claim 37, wherein the step of annealing the oxide layer is followed by the step of annealing the oxide layer in an inert gas environment. 46. A method according to Claim 37, wherein the step of annealing the oxide layer is followed by the step or annealing the oxide layer in at least one of an environment comprising Ar, and environment comprising N 2 and an environment comprising H 2 . 47. A method according to Claim 45, wherein the step of annealing the oxide layer in an inert gas environment is carried out for about one hour. 48. A method according to Claim 37, wherein the predetermined flow rate profile provides a velocity of the N 2 O of from about 0.37 cm/s to about 1.46 cm/s. 48. A method according to Claim 47, wherein the predetermined flow rate profile provides a velocity of the N 2 O of from about 0.5 cm/s to about 1 cm/s. 49. A method according to Claim 37, wherein the step of forming the oxide layer comprises the step of depositing the oxide layer. 50. A method according to Claim 37, wherein the step of forming the oxide layer comprises the step of thermally growing the oxide layer. 51. A method according to Claim 37, wherein the step of forming the oxide layer further comprises performing a wet reoxidation of the oxide layer. 52. The method according to Claim 1 or Claim 37, wherein the silicon carbide layer comprises a silicon carbide layer on a non-silicon carbide substrate. 53. The method according to Claim 1 or Claim 37, wherein the silicon carbide layer comprises a portion of a silicon carbide substrate.

Inventors: DAS MRINAL KANTI, LIPKIN LORI, PALMOUR JOHN W, DAS, MRINAL, KANTI, LIPKIN, LORI, PALMOUR, JOHN, W.
Applicants: CREE INC, CREE, INC.
Application year: 2001
Main classification: H01L21
IPC codes: H01L21, H01L29