Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B23/00—Single-crystal growth by condensing evaporated or sublimed materials
  • C30B23/02—Epitaxial-layer growth
  • C30B23/025—Epitaxial-layer growth characterised by the substrate
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B25/00—Single-crystal growth by chemical reaction of reactive gases, e.g. chemical vapour-deposition growth
  • C30B25/02—Epitaxial-layer growth
  • C30B25/18—Epitaxial-layer growth characterised by the substrate
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/16—Oxides
  • C30B29/20—Aluminium oxides
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
  • C30B29/403—AIII-nitrides
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
  • C30B29/403—AIII-nitrides
  • C30B29/406—Gallium nitride
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02367—Substrates
  • H01L21/02428—Structure
  • H01L21/0243—Surface structure
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02367—Substrates
  • H01L21/02433—Crystal orientation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02436—Intermediate layers between substrates and deposited layers
  • H01L21/02439—Materials
  • H01L21/02455—Group 13/15 materials
  • H01L21/02458—Nitrides
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/02104—Forming layers
  • H01L21/02365—Forming inorganic semiconducting materials on a substrate
  • H01L21/02518—Deposited layers
  • H01L21/02521—Materials
  • H01L21/02538—Group 13/15 materials
  • H01L21/0254—Nitrides

Definitions

• the invention relates to a substrate used to produce a thick layer or a bulk crystal of group III nitride semiconductor materials such as GaN and A1N.
• the invention also provides methods of producing a thick layer or a bulk crystal of group III nitride semiconductor materials.
• the thick layer or the bulk crystal of group III nitride are used to a produce wafer of group III nitride semiconductor, such as a GaN wafer.
• Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors.
• LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives.
• heterogeneous substrates such as sapphire and silicon carbide because GaN substrates are extremely expensive compared to these heteroepitaxial substrates.
• group III nitride causes highly defected or even cracked films, which hinder the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
• HVPE hydride vapor phase epitaxy
• group Ill-nitride wafers by HVPE a thick layer (> 500 microns) of group Ill-nitride is grown on a substrate, typically sapphire, silicon carbide, silicon or gallium arsenide. Then, the substrates are removed by mechanical grinding, laser-assisted separation or chemical etching. These methods, however, require additional process to remove the substrates.
• CMP chemical mechanical polishing
• the invention provides a substrate for growing a thick layer or a bulk crystal of group III nitride having a thickness of more than 0.5 mm.
• the substrate such as sapphire, silicon carbide, quartz, glass, or gallium nitride has grooves on the backside, the side where group III nitride crystal may or may not be grown.
• the invention also provides a method of growing a thick layer or a bulk crystal of group III nitride by using a substrate having grooves on the backside.
• FIG. 1 is a schematic drawing of the substrate with a set of grooves on the backside, viewed from the substrate's edge.
• FIG. 2 is a schematic drawing of the bottom view of a substrate, illustrating how multiple grooves are formed in the second side of the substrate.
• FIG. 3 is a microscope image of the back surface of a grooved sapphire In the figure each number represents the followings:
• FIG. 4 is a microscope image of the groove surface made on the backside of a substrate. In the figure each number represents the followings:
• FIG. 5 is a schematic flow of the method of production.
• FIG. 5(A) is a substrate with a first side (front side) prepared for growth of group III nitride.
• FIG. 5(B) is a substrate with grooves made on the second side (back side).
• FIG. 5(C) is a substrate with group III nitride grown on the first side with thickness larger than that of the substrate.
• FIG. 5(D) is after spontaneous separation of the group III nitride film from the substrate.
• FIG. 6 is a schematic drawing of bow of the group III nitride layer.
• the substrate of this invention in one instance enables thick growth of group III nitride such as GaN with reduced bow and optional spontaneous separation.
• Group III nitride is commonly used for optoelectronic devices and electronic devices although majority of devices utilizes heteroepitaxial substrates such as sapphire, silicon carbide, and silicon. This is due to lack of low-cost, high-quality free-standing group-Ill nitride wafers.
• GaN substrates have been produced with hydride vapor phase epitaxy (HVPE), ammonothermal method, and flux method, and AIN wafers have been produced with HVPE and physical vapor transport methods. Among these methods, HVPE is most commonly used.
• HVPE production of GaN substrates involves growth of a thick GaN layer on a substrate and removal of the substrate.
• the layer is highly stressed due to mismatches of lattice constant and thermal expansion coefficient. This stress causes bowing of the layer and the substrate. If the bowing exceeds the critical value, the layer and/or the substrate will crack. In addition, even homoepitaxial growth of bulk/thick GaN on GaN substrate sometimes causes bowing and cracking. Thus, it is important to reduce bowing of the thick layer and/or substrate on which it is grown by reducing stress.
• Another issue in the production process of a group III nitride such as GaN by HVPE is removal of the substrate.
• Several methods, such as mechanical grinding, laser lift-off and chemical etching, are currently used although these methods require an additional process to separate the substrate and new growth.
• Several cases of spontaneous separation of the GaN layer from the substrates have been reported.
• One method utilizes selective growth with a mask or trenches, nevertheless the selective growth results in gathering of dislocations in one region, leading to non-uniform distribution of dislocations.
• the termination points of these dislocations at the surface often cause pits during CMP process, therefore non-uniform distribution of the dislocations would cause macroscopic thickness variation over the wafer.
• This invention discloses a substrate for growth of a thick layer of group III nitride and which substrate may address one or more of the issues discussed above.
• the substrate has a major surface at a first or front side which is prepared for epitaxial growth of group III nitride, and the substrate has a second major surface or back side that is opposite to the first side and that has a plurality of grooves.
• the grooves reduce stress in the epitaxially-deposited group III nitride that is caused by epitaxial growth of group III nitride on a substrate, especially as compared to an otherwise identical substrate that has no grooves on the comparative substrate or as compared to an otherwise identical substrate that has grooves on the first or front side of a comparative substrate on which group III nitride is to be grown.
• the first side of the substrate which preferably has no grooves, can have a highly -polished surface and/or one or more buffer layers such as A1N or GaN applied to it to make the surface of the first side suitable for epitaxial deposition, whereas the second side may not be polished or as highly polished and/or may not have buffer layers applied to it.
• the second side may therefore not be suitable for epitaxial deposition, although in one variation of the invention the second side is also suitable for epitaxial deposition.
• the substrate can be amorphous, polycrystalline, or single crystalline and can be a heteroepitaxial material such as quartz, glass, sapphire, silicon carbide or silicon, or the substrate may be a homoepitaxial material such as GaN or A1N.
• the substrate can have wurtzite crystal structure, for instance.
• the substrate is single crystalline silicon, sapphire, GaN, or A1N.
• a substrate is typically at least 250 microns thick.
• a substrate may be at least 500 microns thick, or thickness may be between 250 and 500 microns, for instance.
• the depicted substrate 1 has a first side or major surface la, and this surface is polished to a roughness R a on the order of less than 1 nm for epitaxial growth of group III nitride.
• the second side or major surface lb has a plurality of grooves 2.
• FIG. 2 provides a schematic view of the grooves across a major face or surface of the substrate.
• the backside of the substrate lb has grooves 2 which, in this case, are along a crystallographic orientation of the substrate and/or of the group III nitride being formed, and preferably at least some of the grooves are located along a cleavage direction or plane in the substrate.
• This orientation of grooves with crystal cleavage direction or plane enables the substrate to flex more as group III nitride is deposited and as temperature changes than if identical grooves are placed in other directions along the substrate.
• the grooves are preferably made along m-planes of the sapphire.
• the grooves are preferably made symmetrically for all possible equivalent planes, i.e. (10-10) plane, (01-10) plane, and (1-100) plane. Depending on the situation, however, the grooves can be made along only one crystallographic plane or two crystallographic planes, although the stress in the grown group III nitride may become asymmetric in these cases.
• the substrate consequently has a plurality of grooves on its second side, where the grooves have a spatial relationship with one another and/or with the substrate that allows the substrate to reduce bow in the substrate more than a comparative substrate bows as group III nitride is deposited and/or as temperature changes from epitaxial growth conditions to ambient temperature.
• a comparative substrate in this instance may have no grooves in it but be otherwise identical to the substrate of this invention.
• the comparative substrate may have grooves on a front-side on which group III nitride is deposited but not a back-side of the comparative substrate while being otherwise identical to the substrate of this invention.
• the grooved substrate of this invention preferably has sufficient rigidity that the surface of the first side has about the same bowing under epitaxial deposition conditions when group III nitride is first being deposited on the substrate as that surface has at room temperature.
• the spatial relationship is determined by the size and placement of grooves on the substrate's second side.
• Parameters that can be used to characterize size and placement of grooves include groove width, groove depth, groove pitch, groove shape, groove orientation relative to crystallographic planes of the substrate (as explained above), scratches on the surface of the grooves, and substrate thickness, for instance. Combinations of each of these parameters can be used to provide the desired substrate flexibility.
• the width 3 of the grooves is preferably between 100 microns and 300 microns, and groove depth 5 is preferably between 50 microns and 75% of the thickness 6 of the substrate 1.
• Grooves are spaced from one another.
• the grooves may be parallel or intersecting. Parallel grooves may all be spaced the same distance from one another so that all parallel grooves have the same period.
• parallel grooves may be positioned on the substrate so that a first set of grooves has a first periodic spacing, and a second set of grooves has a second periodic spacing different from the first periodic spacing.
• the spacing between adjacent parallel grooves may not be periodic. Spacing between adjacent grooves may be smaller in the vicinity of the center of the second side of the substrate, where stress is greater, than the spacing between adjacent grooves in a peripheral region away from the center.
• the pitch 4 of individual or sets of grooves as discussed is preferably between 0.1 mm and 5 mm.
• the shapes formed by intersecting grooves may be identical, so that all grooves have one shape that is e.g. triangular, or the shapes may be a mixture of different shapes as depicted in FIG. 2, where some portions of the second side of the substrate are e.g. triangular and some are another shape such as hexagonal with different length or identical length sides.
• FIG. 3 is a microscope image of the actual grooved sapphire surface.
• the grooves may also have a curved bottom such as an arc-shaped bottom as shown in FIG. 1, or the bottom of the grooves may be flat or V-shaped depending on how the grooves are formed. It is preferable to have scratches along the grooves as shown in FIG. 4.
• One particular substrate having these grooves is a sapphire substrate, and a group III nitride such as gallium nitride is deposited on the first side of this substrate.
• the groove width, depth, and pitch in this example are significantly greater than the width, depth, and pitch of grooves formed in a comparative substrate's first side on which group III nitride is grown.
• the need for planarity of the comparative substrate's first side during epitaxial deposition of group III nitride limits the size and placement of grooves in the comparative substrate's first side.
• the substrate as provided herein is therefore different from a substrate that only has grooves on the face on which group III nitride is to be grown, since the groove positions, shapes, and/or sizes differ so that the substrate affects group III nitride growth differently.
• the substrate of this invention may have an un-damaged, planar first side of the substrate to ensure epitaxial growth of high-quality group III nitride.
• the grooves on the second (back) side reduce mechanical strength of the substrate.
• the stress caused by mismatch of crystal lattice and/or thermal expansion may be absorbed by these grooves by allowing portions of the substrate to move slightly by compressing and/or flexing or generating cracks initiated from the scratches formed in the grooves. This may reduce the stress in and/or bow of the group III nitride layer and also may induce spontaneous separation of the group III nitride layer from the substrate upon cooling.
• FIG. 5 presents a schematic process flow of this invention.
• a substrate 7 is prepared to have a suitable front surface for epitaxial growth of group III nitride (FIG. 5(A)).
• Plurality of grooves is made on the backside of the substrate to form substrate 1 (FIG. 5(B)).
• the groove width, depth, and pitch in the current example are significantly larger.
• a sapphire substrate can be etched by hot (>80°C) phosphoric acid, and silicon carbide and silicon can be etched in the mixture of hydrofluoric acid and nitric acid, or molten alkali-hydroxide (sodium hydroxide, potassium hydroxide, etc.).
• the grooves can be made using other mechanical way such as wafer dicers or by dry etching such as reactive ion etching. Grooves may also be laser-etched into a substrate surface.
• the first side may be flat. Alternatively, the first side may have a periodic S1O2 stripe mask on the first side as mentioned above for reference [7] and/or holes, cutouts, and/or grooves in the first side as described in e.g. reference [6].
• the first (front) side of the substrate may be prepared for epitaxial deposition before and/or after grooves are formed or during the process of forming grooves. If the process to make grooves contaminates the surface of the first side upon which group III nitride will be deposited, the substrate is preferably cleaned and/or polished to remove the contamination.
• Group III nitride 8 such as GaN, A1N, InN or one of their solid solutions is grown on the first side of the substrate as shown in FIG. 5(C).
• the growth method is preferably HVPE although other methods such as metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), ammonothermal growth, flux growth, high-temperature solution growth, epitaxial sputtering, can be used.
• MOCVD metalorganic chemical vapor deposition
• MBE molecular beam epitaxy
• ammonothermal growth flux growth
• high-temperature solution growth epitaxial sputtering
• the substrate and the layer start to bow. If the layer experiences tensile stress such as for a sapphire substrate, the substrate 1 and the layer 8 become concave as shown in FIG. 5(C).
• the layer thickness is preferably larger than 500 microns. Also, the layer thickness preferably exceeds the thickness of the substrate.
• the group III nitride layer on the substrate is cooled.
• the group III nitride layer may occasionally, fully or partially delaminate from the substrate as shown in FIG. 5(D).
• FIG. 5(D) Although detailed mechanism of this spontaneous separation is unknown, the spontaneous separation tends to occur when the group III nitride layer thickness is more than 500 microns or the layer thickness exceeds the substrate thickness.
• Thick layer of GaN was grown by HVPE on a sapphire substrate without any grooves.
• a single-side polished, c-plane sapphire substrate having miscut within 5 degree, having diameter of 2" was loaded into a HVPE reactor.
• the group III source was GaCl synthesized in the reactor chamber by flowing HC1 over molten Ga.
• the group V source was NH3.
• a buffer layer of GaN was grown at about 900 °C with HC1 flow rate of 20 seem and NH3 flow rate of 3.5 slm for 10 minutes.
• the GaN thick layer was grown at about 1030 °C with HC1 flow rate of 60 seem and NEb flow rate of 2 slm for 16 hours.
• the total layer thickness of GaN was approximately 2900 microns. After growth, the GaN layer on the substrate was cooled, but the GaN layer did not separate from the substrate. The crystal bow was 607 microns toward the growth direction (convex). The bow 11 was measured as a height difference between the edge and the center of the substrate 10 (FIG. 6). Production of grooved substrate - Example 2
• Grooves are made on the backside of a 2" c-plane sapphire substrate.
• the miscut of the major face or surface was within 5 degree from c-plane sapphire.
• the sapphire substrate was mounted face down on a metal block with wax. Then, the assembly was loaded to a multiple wire saw. The wire diameter was approximately 160 microns, and wire pitch was 670 microns. Diamond slurry was supplied while the wire runs back and forth on the backside of the sapphire substrate. First the wire was set along (10-10) plane and approximately 160 micron-deep grooves were made over the entire back surface. Then, the wire was set along (1-100) plane and approximately 160 micron-deep grooves were made over the entire back surface.
• the wire was set along (01-10) plane and approximately 160 micron-deep grooves were made over the entire back surface.
• a sapphire substrate with grooves having straight walls and arc-shaped bottoms in profile was produced on the backside of the substrate (schematic in FIG. 2).
• the groove depth was approximately 160 microns
• the thickness of the substrate was approximately 430 microns
• the groove width was approximately 160 microns
• the groove pitch was approximately 670 microns.
• the directions of the grooves are within reasonable angular errors (+/- 5 degree) from m-planes.
• a thick GaN layer was grown on a grooved sapphire substrate produced in Example 2.
• the GaN layer was grown on a smooth ungrooved top surface of the substrate, with no GaN grown on the grooved surface exposed on the bottom of the substrate.
• HVPE growth condition was same as Example 1. After growth, the total thickness of the GaN was approximately 3600 microns.
• the GaN layer spontaneously separated from the sapphire substrate upon cooling.
• the bow of the GaN layer was 138 microns towards the growth direction (convex), which is greatly reduced from the value in Example 1 (607 micron convex).
• the bow was measured as the height difference between the edge and the center of the GaN layer.
• the sapphire substrate was broken into several pieces along the grooves, which indicates that the grooves induced cracking in the sapphire substrate. Since the groove direction is along the cleavage direction of sapphire (i.e. m-plane), the grooves helped cleavage or cracking of sapphire. Scratches in the grooves in this particular instance may also have aided cleavage or cracking, although scratches are not essential. Substrate cracking and/or breakage may be the mechanism of spontaneous separation and reduced bow.
• the cracking along the grooves may be promoted by abrasive nature of the multiple wire saw.
• a multiple wire saw enables to make grooves with uniform depth, width and pitch. This may also affect the effective reduction of stress at the interface of the substrate and new growth as well as in the grown ingot due to a highly symmetrical configuration of grooves, especially grooves positioned along cleavage planes in the substrate.
• the groove width in this example was 160 microns, which was determined by the diameter of the wire. If wire with a different diameter is used, the groove width can be changed. However, to maintain certain wire strength, the wire diameter is typically larger than 100 microns. In addition, if the groove width is too small, the effect of reducing stress may be limited. On the contrary, if the width is too large, the substrate becomes too fragile. In one instance, the groove width is between 100 microns and 300 microns.
• the groove depth of this example was 160 microns.
• the groove depth can be easily changed by adjusting the wire height relative to the substrate. If the depth is too small, the effect of reducing stress would be limited. On the contrary, if the depth is too large, the substrate becomes too fragile.
• the grooves in one instance may be between 50 microns to 75% of the thickness of the substrate.
• the groove pitch of this example was 670 microns, which was determined by the wire pitch of the wire saw. It can be easily changed by using a wire roller with an appropriate groove pitch. If the groove pitch is too large the effect of reducing stress would be limited. On the contrary, if the groove pitch is too small, the substrate becomes too fragile.
• the groove pitch in one instance is between 0.1 mm to 5 mm.
• Example 1 The growth conditions including growth time was same for Example 1 and Example 3, nevertheless the GaN layer thickness was surprisingly increased by
• Reduced stress by the substrate grooves during growth may therefore promote crystal growth of GaN, resulting in a greater growth rate of GaN using a substrate of this invention over a comparative substrate that does not have grooves on the second side of the substrate but is otherwise identical.
• the front side of the substrate has the same properties as a standard sapphire substrate, no special growth step is needed to obtain high-quality GaN film as is required when grooves are cut into the front side of the substrate.
• selective growth required for grooves on a first side is not used, there is no dislocation gathering, which will cause a problem in CMP process.
• the obtained 3.6 mm-thick free-standing GaN was processed to fabricate a GaN substrate by grinding, lapping and CMP.
• the final thickness of the GaN substrate was 529 microns.
• the substrate with grooves on the backside in this invention can provide a group III nitride layer with reduced bow and optional spontaneous separation.
• a simple process using a multiple wire saw produces a substrate with backside grooves.
• the grooves on the backside of the substrate can reduce bow of the group III nitride layer through reduction of stress.
• the grooves may also induce spontaneous separation of the group III nitride layer from the substrate.
• the optionally smooth front surface of the substrate enables high-quality growth of group III nitride on the front side, without requiring special processing steps as are needed when the front side of the substrate is grooved. This feature may help in realizing smooth surface of GaN after CMP finishing. Possible modifications
• the preferred embodiment describes a sapphire substrate, other material such as silicon carbide, silicon, quartz, gallium arsenide, gallium phosphide, gallium nitride, aluminum nitride, lithium gallate, lithium aluminate, magnesium gallate, magnesium aluminate can be used.
• the substrate may be a hetero-substrate or a homo- substrate.
• HVPE HVPE
• MOCVD metal-organic chemical vapor deposition
• MBE metal-organic chemical vapor deposition
• flux method flux method
• high-pressure solution growth physical vapor transport
• physical vapor transport can be used to grow on one side (with, e.g. masking on the back side where the method would typically grow on both sides) or to grow on both sides of the substrate.
• the preferred embodiment describes a multiple wire saw to make grooves
• other mechanical, chemical, physical methods such as dicing, wet etching, dry etching can be used.

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