Abstract:
Methods of forming compound semiconductor layers include the steps of forming a plurality of selective growth regions at spaced locations on a first substrate and then forming a plurality of semiconductor layers at spaced locations on the first substrate by growing a respective semiconductor layer on each of the selective growth regions. The first substrate is then divided into a plurality of second smaller substrates that contain only a respective one of the plurality of semiconductor layers. This dividing step is preferably performed by partitioning (e.g., dicing) the first substrate at the spaces between the selective growth regions. The step of forming a plurality of semiconductor layers preferably comprises growing a respective compound semiconductor layer (e.g., gallium nitride layer) on each of the selective growth regions. The growing step may comprise pendeoepitaxially growing a respective gallium nitride layer on each of the selective growth regions. Each of the selective growth regions is also preferably formed as a respective plurality of trenches that have sidewalls which expose compound semiconductor seeds from which epitaxial growth can take place.

Description:
FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under Office of Naval Research Contract No. N00014-98-1-0384. The Government may have certain rights to this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods of forming semiconductor substrates and substrates formed thereby, and more particularly to methods of forming compound semiconductor substrates and compound semiconductor substrates formed thereby. 
     BACKGROUND OF THE INVENTION 
     Compound semiconductor materials such as gallium nitride (GaN) have been widely investigated as suitable substrate materials for microelectronic devices including but not limited to transistors, field emitters and optoelectronic devices. It will be understood that, as used herein, compound semiconductor materials may include III-V and II-VI alloys, for example. Reference to specific compound semiconductors such as gallium nitride will also be understood to include a family of gallium nitride alloys such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride. 
     A major problem in fabricating gallium nitride-based microelectronic devices is the fabrication of gallium nitride semiconductor layers having low defect densities. It is known that one contributor to defect density is lattice mismatch with the substrate on which the gallium nitride layer is grown. Thus, although gallium nitride layers have been grown on sapphire substrates, it is known to reduce defect density by growing gallium nitride layers on aluminum nitride buffer layers which are themselves formed on silicon carbide substrates. Notwithstanding these advances, continued reduction in defect density is desirable. 
     It also is known to produce low defect density gallium nitride layers by forming a mask on a layer of gallium nitride, the mask including at least one opening that exposes the underlying layer of gallium nitride, and laterally growing the underlying layer of gallium nitride through the at least one opening and onto the mask. This technique often is referred to as “Epitaxial Lateral Overgrowth” (ELO). The layer of gallium nitride may be laterally grown until the gallium nitride coalesces on the mask to form a single layer on the mask. In order to form a continuous layer of gallium nitride with relatively low defect density, a second mask may be formed on the laterally overgrown gallium nitride layer, that includes at least one opening that is offset from the underlying mask. ELO then again is performed through the openings in the second mask to thereby overgrow a second low defect density continuous gallium nitride layer. Microelectronic devices then may be formed in this second overgrown layer. ELO of gallium nitride is described, for example, in the publications entitled Lateral Epitaxy of Low Defect Density GaN Layers Via Organometallic Vapor Phase Epitaxy to Nam et al., Appl. Phys. Lett. Vol. 71, No. 18, Nov. 3, 1997, pp. 2638-2640; and Dislocation Density Reduction Via Lateral Epitaxy in Selectively Grown GaN Structures to Zheleva et al, Appl. Phys. 
     Lett., Vol. 71, No. 17, Oct. 27, 1997, pp.2472-2474, the disclosures of which are hereby incorporated herein by reference. 
     It also is known to produce a layer of gallium nitride with low defect density by forming at least one trench or post in an underlying layer of gallium nitride to define at least one sidewall therein. A layer of gallium nitride is then laterally grown from the at least one sidewall which acts as a “seed”. Lateral growth preferably takes place until the laterally grown layers coalesce within the trenches. Lateral growth also preferably continues until the gallium nitride layer that is grown from the sidewalls laterally overgrows onto the tops of the posts. In order to facilitate lateral growth and produce nucleation of gallium nitride and growth in the vertical direction, the top of the posts and/or the trench floors may be masked. 
     Lateral growth from the sidewalls of trenches and/or posts also is referred to as “pendeoepitaxy” and is described, for example, in publications by Zheleva et al, entitled “Pendeo-Epitaxy: A New Approach for Lateral Growth of Gallium Nitride Films”, Journal of Electronic Materials, Vol. 28, No. 4, pp. L5-L8, February (1999) and Linthicum et al, entitled “Pendeoepitaxy of Gallium Nitride Thin Films” Applied Physics Letters, Vol. 75, No. 2, pp. 196-198, July (1999), the disclosures of which are hereby incorporated herein by reference. Pendeoepitaxy has also been shown to be successful at reducing threading dislocations and cracks caused by lattice mismatch by about three to four orders of magnitude relative to other conventional heteroepitaxy techniques. Nonetheless, because pendeoepitaxy may not always be successful in preventing the formation of cracks and bowing when large compound semiconductor layers such as gallium nitride are cooled to room temperature during back-end processing steps, there continues to be a need for improved methods of forming compound semiconductor layers with reduced susceptibility to cracking and bowing. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide improved methods of forming compound semiconductor substrates and substrates formed thereby. 
     It is another object of the present invention to provide methods of forming compound semiconductor substrates that can be less susceptible to cracking and bowing during back-end processing steps. 
     These and other objects, advantages and features of the present invention may be provided by methods of forming semiconductor substrates which include the steps of forming a plurality of selective growth regions at spaced locations on a first substrate and then forming a plurality of semiconductor layers at spaced locations on the first substrate by growing a respective semiconductor layer on each of the selective growth regions. The first substrate is then divided into a plurality of second smaller substrates that contain only a respective one of the plurality of semiconductor layers. This dividing step is preferably performed by partitioning (e.g., dicing) the first substrate at the spaces between the selective growth regions. Because this step of dividing the first substrate need not take place until all microelectronic processing steps have taken place, the first substrate with the selective growth regions may be processed on a wafer scale, as with conventional semiconductor wafers. 
     According to one preferred embodiment of the present invention, the step of forming a plurality of semiconductor layers preferably comprises growing a respective compound semiconductor layer (e.g., gallium nitride layer) on each of the selective growth regions. In particular, the growing step may comprise pendeoepitaxially growing a respective gallium nitride layer on each of the selective growth regions. Each of the selective growth regions is also preferably formed as a respective plurality of trenches that have sidewalls which expose compound semiconductor seeds from which epitaxial growth can take place. In particular, the step of forming a plurality of semiconductor layers may comprises epitaxially growing a first continuous semiconductor layer from sidewalls of a first plurality of trenches (within a first selective growth region) and epitaxially growing a second continuous semiconductor layer from sidewalls of a second plurality of trenches (within a second selective growth region). According to a preferred aspect of this embodiment of the present invention, when the growth steps are completed, the first continuous semiconductor layer and the second continuous semiconductor layer will have opposing edges that are spaced from each other by a wide trench that extends between adjacent edges of the first selective growth region and the second selective growth region. 
     Advantageously, because each selective growth region is separated from a next adjacent selective growth region by a wide trench the preferably has a width at least about ten (10) times greater than the widths of the narrower trenches within a respective selective growth region, adjacent monocrystalline compound semiconductor layers preferably do not coalesce with each other. Nonetheless, because each monocrystalline compound semiconductor layer is of sufficiently large dimension to ultimately serve as a separate compound semiconductor substrate or “platform” once the supporting substrate has been diced along the dicing streets defined by each wide trench, the failure of the adjacent compound semiconductor layers to coalesce into a single wafer-sized layer need not limit the suitability of the preferred methods for large scale integrated circuit and optoelectronic applications. Instead, by preventing adjacent compound semiconductor layers from coalescing with each other, the likelihood that the compound semiconductor layers will bow or crack upon cooling, as a result of lattice mismatch and thermal expansion coefficient differences, is substantially reduced. In other words, by growing separate compound semiconductor “platforms” separated by wide trenches, the lattice stress and concomitant build-up in strain within each “platform”, caused by differences in the thermal expansion coefficients of the compound semiconductor material and underlying substrate is greatly reduced relative to a large fully coalesced pendeoepitaxially formed compound semiconductor layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan layout view of a semiconductor wafer having a plurality of selective growth regions at spaced locations therein, according to an embodiment of the present invention. 
     FIG. 2 is a cross-section view of the wafer of FIG. 1 at a first intermediate stage of processing, taken along line  2 - 2 ′. 
     FIG. 3 is a plan layout view of a semiconductor wafer having a plurality of compound semiconductor layers thereon, according to an embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of the wafer of FIG. 3 at a second intermediate stage of processing, taken along line  4 - 4 ′. 
     FIGS. 5A-5J are cross-sectional views of intermediate structures that illustrate a method of forming a single semiconductor layer of FIG. 4 using a pendeoepitaxial growth technique. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     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. 
     Referring now to FIGS. 1-4, preferred methods of forming a plurality of compound semiconductor substrates will be described. In particular, FIGS. 1-2 illustrate preferred steps of forming a plurality of selective growth regions  12  at spaced locations across a composite semiconductor substrate  10 . These selective growth regions  12  may be rectangular in shape and have an area greater than about 1×10 4  μm 2 . According to one aspect of the present invention, the substrate  10  may comprise a (111) silicon wafer having a plurality of semiconductor layers stacked thereon. In particular, the plurality of semiconductor layers may include an underlying silicon carbide (SiC) layer, an intermediate aluminum nitride (AIN) layer on the silicon carbide layer and a gallium nitride (GaN) seed layer on the intermediate aluminum nitride layer, as described more fully hereinbelow with respect to FIGS. 5A-5J. Alternative exemplary substrates are also described in U.S. application Ser. No. 09/441,754, entitled “Methods of Fabricating Gallium Nitride Microelectronic Layers on Silicon Layers and Gallium Nitride Microelectronic Structures Formed Thereby”, filed Nov. 17, 1999, the disclosure of which is hereby incorporated herein by reference. Other substrates including those containing sapphire and/or compound semiconductor seed layers besides gallium nitride may also be used. 
     Referring now to FIG. 2, each of the selective growth regions  12  is preferably formed by selectively etching the substrate  10  to define a plurality of narrow parallel stripe-shaped trenches  14  and a plurality of parallel stripe-shaped semiconductor mesas  16  extending between the narrow trenches  14 . The depths of these trenches  14  are preferably selected so that the sidewalls thereof expose portions of an underlying compound semiconductor seed layer. As illustrated, the semiconductor mesas  16  may be formed to have a uniform width “W m ” and the narrow trenches  14  may formed to have a uniform width “W t ”. These widths may also be nonuniform. Typical values of W m  and W t  may be about 1.0 μm and 4.0 μm, respectively. Each of the selective growth regions  12  may also have a width “W sg ” and may be spaced from a next adjacent selective growth region  12  by a respective wide trench  18  having a width “W ds ”. Typical values for W sg  and W ds  may be 500 μm and 50 μm, respectively. The value of W ds  is also preferably set at a level greater than about ten (10) times W t . As will be understood by those skilled in the art, the narrow trenches  14  and wide trenches  18  may be formed simultaneously by anisotropically etching the substrate  10  using a photolithographically patterned mask having openings therein where the trenches are to be formed. Typical etching techniques include reactive ion etching (RI E) and inductively coupled plasma (ICP). Selective growth techniques may also be used to define the mesas  16 . 
     Referring now specifically to FIGS. 3-4, pendeoepitaxial growth techniques are preferably used to form a separate monocrystalline compound semiconductor layer  20  on each of the selective growth regions  12 . In particular, pendeoepitaxial growth techniques may be used to grow a respective monocrystalline gallium nitride layer from gallium nitride “seeds” that are exposed by the sidewalls of the mesas  16  within each selective growth region  12 . Exemplary pendeoepitaxial growth techniques are more fully described in U.S. application Ser. No. 09/198,784, entitled “Pendeoepitaxial Methods of Fabricating Gallium Nitride Semiconductor Layers on Silicon Carbide Substrates by Lateral Growth from Sidewalls of Masked Posts, and Gallium Nitride Semiconductor Structures Fabricated Thereby, filed Nov. 24, 1998, now U.S. Pat. No. 6,177,688, and in U.S. application Ser. No. 09/468,995, entitled “Pendeoepitaxial Methods of Fabricating Gallium Nitride Semiconductor Layers on Weak Posts, and Gallium Nitride Semiconductor Structures Fabricated Thereby, filed Dec. 21, 1999, the disclosures of which are hereby incorporated herein by reference. 
     Advantageously, because each selective growth region  12  is separated from a next adjacent selective growth region  12  by a wide trench that preferably has a width at least about ten (10) times greater than the widths of the narrow trenches within a respective selective growth region  12 , adjacent monocrystalline compound semiconductor layers  20  preferably do not coalesce with each other. However, because each monocrystalline compound semiconductor layer  20  is of sufficiently large dimension to ultimately serve as a separate compound semiconductor substrate or “platform” once the supporting substrate  10  has been diced along the dicing streets defined by each wide trench  18 , the failure of the adjacent compound semiconductor layers  20  to coalesce into a single wafer-sized layer need not limit the suitability of the preferred methods for large scale integrated circuit and optoelectronic applications. Instead, by preventing adjacent compound semiconductors  20  from coalescing with each other, the likelihood that the compound semiconductor layers  20  will bow or crack upon cooling, as a result of lattice mismatch and thermal expansion coefficient differences, is substantially reduced. In other words, by growing separate compound semiconductor “platforms” separated by wide trenches, the lattice stress and concomitant build-up in strain within each “platform” caused by differences in the thermal expansion coefficients of the compound semiconductor material and underlying substrate is greatly reduced relative to a large fully coalesced pendeoepitaxially formed compound semiconductor layer. 
     Referring again to FIG. 3, conventional techniques can then be used to dice the substrate  10  into a plurality of separate compound semiconductor substrates. Here, the wide trenches that extend between the grid of adjacent selective growth regions can be treated as dicing streets for aligning the directional sawing of the substrate  10  in the x and y directions. The width of the wide trenches is preferably selected so that the substrate  10  can be sawed without damaging the compound semiconductor layers  20 . According to another aspect of the present invention, the arrangement of each of the compound semiconductor layers  20  may be designed to preserve usable active regions within the underlying substrate  10 . For example, the arrangement of compound semiconductor layers  20  may be designed so that after dicing each separate substrate (e.g., semiconductor chip) includes one or more compound semiconductor layers  20  thereon and one or more active regions therein in which semiconductor devices can be formed. Thus, if each compound semiconductor layer  20  comprised gallium nitride and the original substrate  10  comprised a silicon carbide layer, both gallium nitride and silicon carbide devices could be integrated within a single semiconductor chip. 
     Referring now to FIGS. 5A-5J, an exemplary method of forming a respective gallium nitride semiconductor layer on a central portion of a selective growth region  12  will be described. According to this exemplary method, a (111) silicon SIMOX supporting substrate  202  is provided. As illustrated by FIG. 5A, this substrate  202  includes a buried layer of silicon dioxide  202   b  therein that defines a (111) silicon surface layer  202   c  on an underlying (111) silicon substrate  202   a . The buried layer of silicon dioxide may be fabricated by implanting oxygen into a (111) silicon substrate to define a (111) silicon surface layer on the (111) silicon substrate. This process generally is referred to as SIMOX and is described for example in an article by Chen, entitled “Silicon-on-Insulator Why, How, and When”, AIP Conference Proceedings, Vol. 167, No. 1, pp. 310-319, Sep. 15, (1988). 
     Then, referring to FIG. 5B, at least a portion of the (111) silicon surface layer  202   c  is preferably converted to 3C-silicon carbide. As illustrated by FIG. 5C, a layer of 3C-silicon carbide  202   d  is then epitaxially grown on the converted (111) silicon surface layer  202   c ′. As shown in FIG. 5D, the epitaxially grown layer of 3C-silicon carbide  202   d  optionally is thinned to produce a thinned epitaxial layer of 3C-silicon carbide  202   d ′. A 2H-aluminum nitride layer and/or gallium nitride buffer layer  202   e  then is grown on the thinned epitaxially grown layer of 3C-silicon carbide  202   d ′, as illustrated by FIG.  5 E. Then, as shown by FIG. 5F, an underlying layer of 2H-gallium nitride  204  is grown on the buffer layer  202   e  using, for example, a heteroepitaxial growth technique. 
     FIGS. 5G-5I now will show the use of pendeoepitaxy to laterally grow the underlying seed layer of 2H-gallium nitride  204  into a monocrystalline gallium nitride semiconductor layer. As described in the aforementioned U.S. application Ser. No. 09/441,754, pendeoepitaxial techniques may be used advantageously to grow a monocrystalline gallium nitride semiconductor layer having an average defect density level therein of less than about 10 5  cm −3 . Referring to FIG. 5G, the underlying gallium nitride layer  204  may be formed to have a plurality of sidewalls  205  therein. It will be understood by those having skill in the art that the sidewalls  205  may be thought of as being defined by a plurality of spaced apart posts  206 , that also may be referred to as “mesas”, “pedestals” or “columns”. The sidewalls  205  also may be thought of as being defined by a plurality of trenches  207 , also referred to as “wells” in the underlying gallium nitride layer  204 . The sidewalls  205  may also be thought of as being defined by a series of alternating trenches  207  and posts  206 . It will be understood that the posts  206  and the trenches  207  that define the sidewalls  205  may be fabricated by selective etching, selective epitaxial growth and/or other conventional techniques. Moreover, it also will be understood that the sidewalls need not be orthogonal to the substrate  202 , but rather may be oblique thereto. Finally, it also will be understood that although the sidewalls  205  are shown in cross-section in FIG. 5G, the posts  206  and trenches  207  may define elongated regions that are straight, V-shaped or have other shapes. The trenches  207  may also extend into the buffer layer  202   e  and into the underlying silicon carbide layer  202   c ′/ 202   d ′, so that subsequent gallium nitride growth occurs preferentially on the sidewalls  205  rather than on the trench floors. In other embodiments, the trenches may not extend into the silicon carbide layer  202   c ′/ 202   d ′, and also may not extend into buffer layer  202   e , depending, for example, on the trench geometry and the lateral versus vertical growth rates of the gallium nitride seed layer. 
     Referring now to FIG. 5H, the sidewalls  205  of the underlying gallium nitride layer  204  are laterally grown to form a lateral gallium nitride layer  208   a  in the trenches  207 . Lateral growth of gallium nitride may be obtained at 1000-1100° C. and 45 Torr. The precursors TEG at 13-39 μmol/min and NH 3  at 1500 sccm may be used in combination with a 3000 sccm H 2  diluent. If gallium nitride alloys are formed, additional conventional precursors of aluminum or indium, for example, may also be used. As used herein, the term “lateral” means a direction that is orthogonal to the sidewalls  205 . It will also be understood that some vertical growth on the posts  206  may also take place during the lateral growth from sidewalls  205 . As used herein, the term “vertical” denotes a directional parallel to the sidewalls  205 . 
     Referring now to FIG. 51, continued growth of the lateral gallium nitride layer  208   a  causes vertical growth onto the underlying gallium nitride layer  204 , specifically onto the posts  206 , to form a vertical gallium nitride layer  208   b . Growth conditions for vertical growth may be maintained as was described in connection with FIG.  5 H. As also shown in FIG. 5I, continued vertical growth into trenches  207  may take place at the bottom of the trenches. Referring now to FIG. 5J, growth is allowed to continue until the lateral growth fronts coalesce in the trenches  207  at the interfaces  208   c , to form a continuous gallium nitride semiconductor layer in the trenches. The total growth time may be approximately 60 minutes. As illustrated by FIG. 4, each of the steps described above with respect to FIGS. 5A-5J may be performed simultaneously with respect to each of the selective growth regions  12  so that a plurality of compound semiconductor substrates  20  can be formed at spaced locations across a substrate  10 . 
     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.