CONTROLLED SURFACE CHEMISTRY FOR POLYTYPIC AND MICROSTRUCTURAL SELECTIVE GROWTH ON HEXAGONAL SiC SUBSTRATES

Abstract

A high-throughput method for identifying single crystal hexagonal-SiC off-axis surfaces that support surface chemistries and kinetics to selectively produce various epitaxial growth modes of the metastable 3C-SiC polytype is provided. In execution of the aforementioned method, the present invention also encompasses the use of a single crystal hexagonal-SiC domed substrate, and a method for manufacturing thereof. Said method for screening silicon carbide growth surfaces is comprised of: fabrication of a silicon carbide domed substrate; forming a step-terrace growth surface on the domed surface of said silicon carbide domed substrate by hydrogen etching; performing silicon carbide deposition upon said growth surface, thereby creating an silicon carbide epitaxial domed wafer; and characterization of said silicon carbide epitaxial domed wafer. Silicon carbide deposition upon a silicon carbide domed growth surface allows for the modulation of the supersaturation ratio under a single set of growth conditions. There is provided a method to select a specific off-cut angle and orientation for a silicon carbide substrate that can be used to selectively and homogeneously grow a targeted 3C-silicon carbide microstructure best suited for the intended application.

Claims

1. A method for high-throughput screening of silicon carbide surfaces that support surface chemistries and kinetics of selective polytypic and microstructural growth modes during silicon carbide deposition, comprising: a. fabricating a silicon carbide domed substrate; b. forming a step-terrace growth surface on the domed surface of said silicon carbide domed substrate by hydrogen etching; c. performing silicon carbide deposition upon said growth surface, thereby creating an silicon carbide epitaxial domed wafer; and d. performing characterization of said silicon carbide epitaxial domed wafer.

2. The method of claim 1, wherein the silicon carbide domed substrate is a silicon carbide single crystal with a polytype selected from the group consisting of the 3C, 4H, 6H, 2H, and 15R polytypes.

3. The method of claim 1, wherein the silicon carbide domed substrate contains an atomically smooth domed surface with either a 0001, 1120, or 1100 polar axis.

4. The method of claim 1, wherein the silicon carbide domed substrate has a diameter within the range of 20 mm to 200 mm.

5. The method of claim 1, wherein the silicon carbide domed substrate contains an atomically smooth domed surface defined by a continuous variation of an off-cut angle from a polar axis ranging from 0 degrees to some maximum off-cut angle, and 360 azimuth degrees.

6. The silicon carbide domed substrate according to claim 5, wherein the maximum off-cut angle is within the range of 4 degrees to 20 degrees.

7. The method of claim 1, wherein the fabrication of a silicon carbide domed substrate further comprises: a. core drilling an on-axis bulk single crystal silicon carbide boule producing a single crystal silicon carbide cylinder such that the top and bottom surface of said silicon carbide cylinder is an on-axis surface; b. forming a flat silicon carbide substrate from dicing said silicon carbide cylinder with cuts oriented parallel to the on-axis surface such that the top and bottom surface of said silicon carbide substrate is an on-axis surface; and c. forming a domed silicon carbide substrate from said flat silicon carbide substrate by grinding, polishing, and chemical mechanical polishing such that the domed silicon carbide substrate has an on-axis surface and an atomically smooth domed surface with a 0001 polar axis.

8. The method of claim 1, wherein the fabrication of a silicon carbide domed substrate further comprises: a. forming a flat silicon carbide substrate from dicing an on-axis bulk single crystal silicon carbide boule with cuts oriented parallel to the on-axis surface such that the top and bottom surface of said silicon carbide substrate is an on-axis surface; and b. forming a domed silicon carbide substrate from said flat silicon carbide substrate by grinding, polishing, and chemical mechanical polishing such that the domed silicon carbide substrate has an on-axis surface and an atomically smooth domed surface with a 0001 polar axis.

9. The method of claim 1, wherein the fabrication of a silicon carbide domed substrate further comprises: a. dicing an on-axis bulk single crystal silicon carbide boule producing a silicon carbide parallelpiped such that the surfaces are comprised of {0001}, {1120}, and {1100} surfaces; b. core drilling said silicon carbide parallelpiped producing a single crystal silicon carbide cylinder such that the top and bottom flat surfaces are either an {0001}, {1120}, or {1100} on-axis surface; c. forming a flat silicon carbide substrate from said silicon carbide cylinder by dicing with cuts oriented parallel to the on-axis surface such that the top and bottom surface of said silicon carbide substrate are either an {0001}, {1120}, or {1100} on-axis surface; and d. forming a domed silicon carbide substrate from said flat silicon carbide substrate by grinding, polishing, and chemical mechanical polishing such that the domed silicon carbide substrate has an on-axis surface and an atomically smooth domed surface with a 0001, 1120, or 1100 polar axis.

10. The method of claim 1, wherein the fabrication of a silicon carbide domed substrate further comprises: a. dicing an {0001} off-axis bulk single crystal silicon carbide boule with cuts parallel to the {0001} plane forming an on-axis bulk single crystal silicon carbide boule; and b. forming a flat silicon carbide substrate from dicing said on-axis bulk single crystal silicon carbide boule with cuts oriented parallel to the on-axis surface such that the top and bottom surface of said silicon carbide substrate is an on-axis surface; and c. forming a domed silicon carbide substrate from said flat silicon carbide substrate by grinding, polishing, and chemical mechanical polishing such that the domed silicon carbide substrate has an on-axis surface and an atomically smooth domed surface with a 0001 polar axis.

11. The method of claim 1, wherein the fabrication of a silicon carbide domed substrate further comprises: a. dicing an {0001} off-axis bulk single crystal silicon carbide boule producing a {0001} off-axis silicon carbide parallelpiped such that one pair of identical surfaces are comprised of a {0001} off-axis surface, and at least one other pair of identical surfaces are comprised of either a {1100} or {1120} surface; b. forming an {0001} on-axis silicon carbide parallelpiped from cutting said off-axis silicon carbide parallelpiped such that one pair of identical surfaces are comprised of a {0001} on-axis surface, and at least one other pair of identical surfaces are comprised of either a {1100} or {1120} surface; c. forming a flat silicon carbide substrate from dicing said on-axis silicon carbide parallelpiped with cuts oriented parallel to either a {0001}, {1120}, or {1100} on-axis surface such that at least two pairs of identical surfaces contain a combination of {0001}, {1120}, or {1100} surfaces; d. forming a circular silicon carbide substrate from core drilling said flat silicon carbide substrate such that the top and bottom flat surface is either a {0001}, {1120}, or {1100} on-axis surface; and e. forming a domed silicon carbide substrate from said circular silicon carbide substrate by grinding, polishing, and chemical mechanical polishing such that the domed silicon carbide substrate has an {0001}, {1120}, or {1100} on-axis surface and an atomically smooth domed surface with a 0001, 1120, or 1100 polar axis.

12. The method for manufacturing the silicon carbide epitaxial domed wafer in claim 1, wherein one of chemical vapor deposition (CVD), physical vapor transport (PVT), continuous feed-PVT, physical vapor deposition (PVD), liquid phase epitaxy (LPE), or vapor-liquid-solid methods is used for forming the epitaxial film on the step-terrace growth surface of the domed silicon carbide substrate.

13. The method for characterizing the silicon carbide epitaxial domed wafer in claim 1, wherein characterization is performed in either a plan-view or cross sectional configuration with one or more methods including optical microscopy, differential interference contrast microscopy, atomic force microscopy, scanning electron microscopy, scanning/transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-Vis spectroscopy, ellipsometry, (micro) X-ray diffraction, Synchrotron X-ray analysis, electron diffraction, and neutron scattering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

[0035] FIG. 1 shows a schematic perspective view of the crystal structure for hexagonal silicon carbide including the coordinates for off-angle orientations;

[0036] FIG. 2 shows a schematic perspective view of a silicon carbide single crystal boule;

[0037] FIG. 3 shows a schematic perspective view of a dicing sequence step of a silicon carbide single crystal boule;

[0038] FIG. 4 shows a schematic side view of the orientation correcting dicing sequence step of a block cut from a single crystal silicon carbide boule;

[0039] FIG. 5 shows a schematic side view of the wafer cutting dicing sequence step of an orientation corrected single crystal silicon carbide block;

[0040] FIG. 6 shows a schematic perspective view of a core drilling step on a square on-axis single crystal silicon carbide wafer;

[0041] FIG. 7 shows schematic side view of the grinding/lapping/polishing steps of a circular single crystal silicon carbide wafer to form a domed-wafer.

[0042] FIG. 8 shows a schematic of the side view and top view of the contours from a hexagonal-silicon carbide domed wafer;

[0043] FIG. 9 shows a schematic side view of a hydrogen etched domed wafer and a corresponding diagram of the position dependent supersaturation ratio;

[0044] FIG. 10 shows a schematic side view of a film deposition growth step on a etched hexagonal-silicon carbide domed wafer;

[0045] FIG. 11 shows a schematic perspective view of a non-destructive characterization step on a hexagonal-silicon carbide domed wafer with an epitaxial layer;

[0046] FIG. 12 shows schematic perspective view of an orientation specific dicing sequence step to prepare samples for cross-sectional characterization.

[0047] FIG. 13 shows a schematic perspective view of an orientation specific wafer cross section.

DESCRIPTION

[0048] In the Summary above and the Description, and the claims below, and in the accompany drawings, reference is made to particular features (including method steps) of the invention. It is to be understood that the disclosure of the invention in this specification includes all possible combinations of such particular features. For example, where a particular aspect or embodiment of the invention, or a particular claim, that feature can also be used, to the extent possible, in combination with and/or in the context of the other particular aspects and embodiments of the invention, and in the invention generally.

[0049] It is noted that for reasons of continuity and clarity, the same or corresponding parts are labeled with the same reference numerals in the following drawings, and descriptions thereof may not be repeated. Now referring to FIG. 1, a schematic of a hexagonal prism 10 approximating the 4H- and 6H-silicon carbide unit cell and crystal structure applicable to silicon carbide single crystals is shown. The <0001> axis is along the axial direction of hexagonal prism 10 comprised of a {0001} plane orthogonally surrounded by six equivalent {1100} planes. The [0001 ] direction on the <0001> axis is normal to the (0001) plane that is the Si surface of the hexagonal-silicon carbide crystal structure, and the [0001] direction on the <0001> axis is normal to the (0001) plane that is the C surface. The six vertices of hexagonal prism 10 lie in the direction of the six equivalent 1120 directions and the sides of hexagonal prism 10 defined by the six equivalent {1100} planes are normal to the six equivalent 1100 directions. The 1120 directions and the 1100 directions are all orthogonal to the <0001> axis. Furthermore, the equivalent 1120 directions are separated by 60-degree increments, and the equivalent 1100 directions are separated by 60-degree increments, where adjacent 1100 and 1120 directions are separated by 30-degree increments.

[0050] Additionally, the growth surfaces of engineered single crystal silicon carbide wafers may be tilted at an off-cut angle OA1 relative to a {0001} plane of the hexagonal-silicon carbide unit cell such that the direction normal DN to the growth surface is tilted by off-cut angle OA1 relative to a 0001 direction. The directional orientation of the tilt is defined in the direction D01 that may be equivalent to any direction within the {0001} plane. On hexagonal prism 10, direction D01 is defined by an off-angle OA2 relative to a 1120 direction. The resulting orientation of the tilt for an engineered growth surface is such that a direction parallel DP to the surface is orthogonal to direction normal DN, and defined by OA1 and OA2 from directions D01 and 1120 respectively. In other embodiments, off-angle OA1 and off-angle OA2 may be oriented in reference to either the 0001, 1120, or 1100 directions. Likewise, the direction D01 may lie on either the {0001}, {1120}, or {1100} planes. The inventors can deduce similar novel-oriented wafer products can be obtained for purpose-driven epitaxial growth to achieve desirable epitaxial properties not previously disclosed.

[0051] A method of manufacturing a domed single crystal silicon carbide substrate will now be described with reference to FIGS. 2 to 8. FIG. 2 shows a bulk silicon carbide boule 20 used as the starting material for the fabrication of the domed silicon carbide substrate. Silicon carbide boule 20 is a single crystal silicon carbide boule with height h, and main surface M20 surrounded by round surface BSR and flat surface BSF. The diameter of silicon carbide boule 20 is 50 to 200 mm. Most commercially available silicon carbide boules are either on-axis, or possess an off-cut OA1 towards the 1120 or 1100 directions. In silicon carbide boule 20, main surface M20 is defined by direction normal DN and direction parallel DP such that the off-cut angle OA1 ranges from 0 degrees for on-axis, up to 12 degrees from the <0001> direction. Off-cut OA1 is oriented towards the 1120 direction and flat surface BSF is oriented parallel to a {1100} plane as a reference such that the 1100 direction is normal to surface BSF and orthogonal to both normal direction DN and parallel direction DP. Other embodiments of the present invention may possess a reference flat surface BSF that is parallel to either a {0001} or {1120} plane. Additionally, silicon carbide boule 20 may also contain a secondary reference flat surface of shorter length than flat surface BSF and oriented orthogonal to flat surface BSF.

[0052] FIG. 3 shows the primary dicing sequence step of silicon carbide boule 20. The dicing sequence consists of orthogonal cuts 31 and parallel cuts 32 relative to flat surface BSF. Both orthogonal cuts 31 and parallel cuts 32 are equally spaced by length , producing single crystal silicon carbide parallelpiped 30 with dimensions of ??. The spacing of the cuts may range from 20 mm to 70 mm.

[0053] FIG. 4 shows the orientation correcting dicing sequence step of single crystal silicon carbide parallelpiped 30 with main surface M30 identically oriented to main surface M20 of silicon carbide boule 20. For on-axis silicon carbide boules, off-cut angle OA1 equals zero degrees where direction normal DN is equivalent to the <0001> or low index directions 1120 and 1100, and no orientation correcting dicing sequence is needed. For silicon carbide boules with a non-zero off-angle OA1, single crystal silicon carbide parallelpiped 30 is cut at an off-cut angle OA1 oriented towards the parallel direction DP to produce single crystal silicon carbide parallelpiped 40 with an on-axis surface.

[0054] FIG. 5 shows the wafer dicing sequence step of single crystal silicon carbide parallelpiped 40 with main surface M50 that is oriented with direction normal DN equivalent to the <0001> or low index directions 1120 and 1100. A set of parallelpiped single crystal hexagonal-silicon carbide substrates 50a-50f (also collectively referred to as 50) each having a main surface M50 are candidates for fabricating single crystal silicon carbide domed substrates. Each silicon carbide substrate 50 is in the shape of a parallelpiped with main surface M50 surrounded by four flat parallelogram shaped sides. Main surface M50 is an on-axis surface, consisting of either a {0001}, {1120}, or {1100} plane.

[0055] FIG. 6 shows a core drilling step of a parallelpiped single crystal hexagonal-silicon carbide substrate 50(a-f) to produce round single crystal hexagonal-silicon carbide substrate 60 with main surface M60 surrounded by round surface WSR. The diameter of silicon carbide wafer 60 is 20 to 70 mm. Main surface M60 is identically oriented to main surface M50 with an on-axis surface consisting of either a {0001}, {1120}, or {1100} plane.

[0056] FIGS. 7 and 8 show wafer grinding, lapping, and polishing steps of round single crystal hexagonal-silicon carbide substrate 60 to form single crystal hexagonal-silicon carbide domed substrate 70. A grind radius is chosen that produces a domed surface defined by a continuous range of off-angle OA1 from 0 to 10 degrees with respect to a 0001, 1120, or 1100 polar axis, and 0 to 360 degrees of an azimuthal off-angle OA2. Silicon carbide is an exceptionally hard material, registering a hardness value of 9 out 10 on the Mohs hardness scale. Therefore, a sequence of lapping and polishing steps utilizing decreasing abrasive sizes follows the dome-shaping process to remove preceding grinding, lapping, and polishing defects. An example of the sequence is as follows. Subsequent to dome-shaping of single crystal hexagonal-silicon carbide substrate 60, a series of cast iron lapping steps is performed with abrasive size decreasing from 15 ?m, to 6 ?m, and finally 3 ?m with each lapping sequence lasting for 30 minutes. Next, a polishing sequence is performed with abrasive size decreasing from 1 ?m to 0.25 ?m for a period of 8 hours and 2 hours respectively. A pitch lapping step follows the polishing sequence using a 0.1 ?m abrasive size for 2 hours. The grinding, lapping, and polishing sequence is then finished with a chemical mechanical polishing (CMP) step to bring the domed surface to atomic level smoothness. The resulting single crystal hexagonal-silicon carbide domed substrate 70 is comprised of domed surface 71 and back surface 73 surrounded by round side surface 72.

[0057] Including the previously described method for fabricating silicon carbide domed substrate 70, a method for identifying hexagonal-SiC off-axis surfaces that support surface chemistries and kinetics to selectively produce various epitaxial growth modes of the metastable 3C-SiC polytype is described in FIGS. 9-13. Following the CMP process to produce atomically smooth domed surface 71, a conventional wafer degreasing and cleaning process step is conducted to remove residual particulate and carbon contamination prior to loading in a growth chamber. The growth chamber is that of a vapor phase mediated growth technique that may include either sublimation type growth, chemical vapor deposition type growth, or a combination thereof. Next, an in situ vapor phase hydrogen etching step is conducted to remove residual native oxide, polishing-induced surface damage, and to produce a step-terrace structured growth surface. FIG. 9 shows a schematic side view of hydrogen etched single crystal hexagonal-silicon carbide domed substrate 80 with step-terrace structured growth surface 81, and a corresponding qualitative diagram of the radial position dependent supersaturation ratio ?. Silicon carbide domed substrate 80 is shown projected along an 1120 or 1100 direction with {0001} oriented terraces where straight steps are produced. It is shown for a domed substrate that as the distance increases radially from the polar axis (in the case of FIG. 9 a 0001 polar axis), the magnitude of off-angle OA1 increases and the terrace width decreases. Consequently, the supersaturation ratio ? versus radial distance y diagram shows that growth surface 81 has the effect of modulating the supersaturation ratio ? for any given set of growth conditions defined by the user. For any given set of growth conditions there will exists some critical value of the supersaturation ratio ?.sub.crit, whose effects will be empirically evident by the resulting nucleation and growth behavior across growth surface 81. Furthermore, by incorporating growth upon a structure with varying off-angle OA1 across all rotational orientations, the effects of non-low index oriented step-terrace structures on the critical supersaturation ratio ?.sub.crit can directly be observed and compared to conventional low-index oriented off-axis surfaces. Accordingly, FIG. 10 shows a film deposition step to form epitaxial domed wafer 90 comprised of single crystal hexagonal-silicon carbide domed substrate 80 and epitaxial layer 91. The deposition step can be conducted using sublimation type growth, chemical vapor deposition type growth, or a combination thereof.

[0058] Due to both the anisotropy and symmetry of the crystal structure for hexagonal silicon carbide substrates, step-terrace structured growth surface 81 of hexagonal silicon carbide domed substrate 80 will have a unique range of step-terrace structure geometries between and including a 1120 direction and adjacent 1010 direction that includes a span of 30 azimuthal degrees. The entirety of step-terraced growth surface 81 is thus a repetition of this region through symmetry operations. Likewise, for any given set of growth conditions enacted upon step-terrace structured growth surface 81, an epitaxial layer 91 may have a unique set of structural characteristics between and including a 1120 direction and adjacent 1010 direction. FIG. 11 shows a plan-view characterization step of epitaxial domed wafer 90. Region of interest 92 corresponds to a particular region of epitaxial layer 91 grown upon an off-axis surface of off-angle OA1 oriented in a particular direction of which a materials characterization instrument can be focused to extract physical, optical, or chemical information. Characterization techniques with the spatial resolution to extract information unique to a particular off-axis surface may include but is not limited to atomic force microscopy, Raman spectroscopy, scanning electron microscopy, ellipsometry, differential interference contrast microscopy, and micro X-ray diffraction.

[0059] Next, a method for conducting cross-sectional characterization is described in FIGS. 11-13. FIG. 11 also shows a dicing step of epitaxial domed wafer 90 along the 1120 and 2110 directions, and FIG. 12 shows epitaxial domed wafer section 93 cut from epitaxial domed wafer 90 along the 1120 and 2110 directions. In other embodiments, epitaxial domed wafer section 93 may be defined by cutting along any other combination of low index directions. Next, an orientation specific cross-section dicing step is performed on epitaxial domed wafer section 93 with a pair of cuts 94 that are parallel to the 19 7 26 0 direction as an example orientation of interest. The dicing sequence forms epitaxial domed wafer cross section 95. The rotational orientation of the 19 7 26 0 direction is approximately midway between adjacent low index 1120 and 1010 directions, measuring 14.9 degrees from the 1120 direction and 15.1 degrees from the 1010 direction. Two narrow spaced cuts are made oriented along the direction of interest that incorporate the polar axis of epitaxial domed wafer section 93. FIG. 13 shows epitaxial domed wafer cross section 95 with primary edge 96 oriented along the direction of interest and low index oriented edges 97 and 98 that are retained from the cut faces of epitaxial domed wafer section 93. With such a specimen, the role of off-angle OA1 (and by extension the role of the off-angle dependent supersaturation ratio) on nucleation behavior and observed growth mode can be evaluated by previously mentioned materials characterization techniques. In particular, the use of Raman spectroscopy depth profiling is enabled, allowing for the determination of silicon carbide polytype evolution as a function of both epitaxial film thickness and off-angle OA1 for a particular directional orientation from a single specimen.

[0060] The present invention can be applied towards a wide range of applicable fields that benefit from the development and fabrication of silicon carbide enabling technologies, specifically technologies based on the 3C-silicon carbide polytype. These technologies include large area graphene monolayer production, pressure sensors, biosensors, optics, nanophotonics, high electron mobility transistors, and CMOS high power electronics. In particular, across the group of aforementioned applications there is a requirement for a variety of high quality 3C-silicon carbide microstructures including <111> textured, <110> textured, and single crystal 3C-silicon carbide. Utilizing the present invention according to the disclosed embodiments can be used to select a hexagonal silicon carbide off-axis surface that best supports the growth of a particular 3C-silicon carbide microstructure best suited for a specific application. Especially unconventional off-axis surfaces that would provide an advantage over commercially available silicon carbide substrates. In some instances, shorter deposition times can be used to evaluate the nucleation behavior and initial polytype selectivity and growth modes. In other instances, longer deposition times can be used to evaluate resulting preferred orientation of polycrystalline regions of the epitaxial layer, defect character, and transitions between growth modes. All crucial components necessary to formulate crystal growth and production strategies needed to meet application performance requirements.

[0061] The previously described versions of the present invention have many advantages, including a high-throughput method for screening multiple off-axis surfaces, providing the ability to perform direct comparisons of growth behavior upon multiple off-axis surfaces under identical growth conditions, and providing an efficient use of resources towards exploratory and process development objectives.

[0062] While we have shown and described several embodiments in accordance with our invention, it should be understood that the same is susceptible to further changes and modifications without departing from the scope of our invention. Therefore, we do not want to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.