High surface quality GaN wafer and method of fabricating same

[0001] This is a continuation-in-part of U.S. patent application Ser. No. 09/877,437 filed Jun. 8, 2001, now U.S. Pat. No. 6,488,767, in the names of Xueping Xu and Robert P. Vaudo, now allowed.

[0002] The invention disclosed herein includes aspects that were involved in the performance of United States Contract No. DASG60-00-C-0036 issued by the U.S. Army Space and Missile Defense Command and United States Contract No. N00014-00-3-0013 issued by The Office of Naval Research. The government has certain rights in the invention.

[0003] 1. Field of the Invention

[0004] This invention relates to Al_xGa_yIn_zN (wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1) having superior surface quality, including in various embodiments, articles formed of such Al_xGa_yIn_zN material, e.g., in wafer form, including surfaces comprising crystallographic plane surfaces and offcuts of such crystallographic plane surfaces that are suitable for fabrication of microelectronic and optoelectronic device structures. The invention also relates to methods for fabricating such Al_xGa_yIn_zN articles and surfaces.

[0005] 2. Description of the Related Art

[0006] GaN and related GaN-like III-V nitride crystal films, represented by the general formula Al_xGa_yIn_zN, where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, are useful materials in various applications, such as high temperature electronics, power electronics, and optoelectronics (e.g., light emitting diodes (LEDs) and blue light laser diodes (LDs)). Blue light emitting diodes (LEDs) and lasers are an enabling technology, allowing much higher storage density in magneto-optic memories and CDROMs and the construction of full color light emitting displays. Blue light emitting diodes may replace currently employed incandescent light bulbs in road and railway signals etc., since in such applications blue light emitting diodes have the potential to achieve very substantial cost and energy savings.

[0007] Currently, Al_xGa_yIn_zN films are grown on non-native substrates such as sapphire or silicon carbide, due to unavailability of high quality Al_xGa_yIn_zN substrates. However, differences in thermal expansion and lattice constants between such foreign substrates and the Al_xGa_yIn_zN crystals epitaxially grown thereon cause significant thermal stress and internal stress in the grown Al_xGa_yIn_zN crystals. The thermal stress and internal stress cause micro-cracks, distortions, and other defects in the Al_xGa_yIn_zN crystals, and make such Al_xGa_yIn_zN crystals easy to break. Growing on lattice non-matched foreign substrates also causes high density of lattice defects, leading to poor device performance.

[0008] In order to reduce the deleterious thermal stress and high defect density in the grown Al_xGa_yIn_zN crystals, it is desirable to provide high quality freestanding Al_xGa_yIn_zN wafers as film-growing substrates, in place of the above-mentioned foreign substrates.

[0009] U.S. Pat. No. 5,679,152 entitled “Method for Making a Single Crystal Ga*N Article” and U.S. patent application Ser. No. 08/955,168 filed Oct. 21, 1997 entitled “Bulk Single Crystal Gallium Nitride and Method of Making Same” disclose hydride vapor phase epitaxy (HVPE) processes for fabricating freestanding Al_xGa_yIn_zN crystals as substrates for homoepitaxial growth of Al_xGa_yIn_zN single crystal material thereon.

[0010] Since quality of a subsequently grown Al_xGa_yIn_zN crystal is directly correlated to the quality of the substrate surface and near surface region on which the Al_xGa_yIn_zN crystal is grown, it is important to provide a highly smooth initial substrate surface without any surface or subsurface damage.

[0011] However, after mechanical polishing, Al_xGa_yIn_zN crystals typically have very poor surface quality, with substantial surface and subsurface damage and polishing scratches. Additional wafer finish processing therefore is necessary to further enhance the surface quality of the freestanding Al_xGa_yIn_zN crystal, so that it is suitable for high-quality epitaxial growth and device fabrication thereon.

[0012] Crystalline Al_xGa_yIn_zN generally exists in a chemically stable wurtzite structure. The most common crystallographic orientation of Al_xGa_yIn_zN compounds has two polar surfaces perpendicular to its c-axis: one side is N-terminated, and the other one is Ga-terminated (Ga hereinafter in the context of the Ga-side of the crystal structure being understood as generally illustrative and representative of alternative Group III (Al_xGa_yIn_zN) crystalline compositions, e.g., of a corresponding Ga_xIn_y-side in Ga_xIn_yN crystals, of a corresponding Al_xGa_yIn_z-side in Al_xGa_yIn_zN crystals, of a corresponding Al_xGa_y-side in Al_xGa_yN crystals, etc.).

[0013] Crystal polarity strongly influences the growth morphology and chemical stability of the crystal surface. It has been determined that the N-side of the Al_xGa_yIn_zN crystal is chemically reactive with KOH or NaOH-based solutions, whereas the Ga-side of such crystal is very stable and not reactive with most conventional chemical etchants. The N-side can therefore be easily polished, using an aqueous solution of KOH or NaOH, to remove surface damage and scratches left by the mechanical polishing process and to obtain a highly smooth surface.

[0014] The Ga-side (Al_xGa_yIn_z side) of the Al_xGa_yIn_zN crystal, on the other hand, remains substantially the same after contacting the KOH or NaOH solution, with its surface damage and scratches unaltered by such solution. See Weyher et al., “Chemical Polishing of Bulk and Epitaxial GaN”, J. Crystal Growth, vol. 182, pp. 17-22, 1997; also see Porowski et al. International Patent Application Publication No. WO 98/45511 entitled “Mechano-Chemical Polishing of Crystals and Epitaxial Layers of GaN and Ga_1−x−yAl_xIn_yN”.

[0015] However, it has been determined that the Ga-side of the Al_xGa_yIn_zN crystal is a better film-growing surface than the N-side. See Miskys et al., “MOCVD-Epitaxy on Free-Standing HVPE-GaN Substrates”, Phys. Stat. Sol. (A), vol. 176, pp. 443-46, 1999. It therefore is important to provide a wafer finish process that is particularly effective for preparing the Ga-side of the Al_xGa_yIn_zN crystal to make it suitable for subsequent crystal growth thereupon.

[0016] Reactive ion etching (RIE) recently has been used to remove a layer of surface material from the Ga-side of an Al_xGa_yIn_zN wafer to obtain smoother wafer surface. See Karouta et al., “Final Polishing of Ga-Polar GaN Substrates Using Reactive Ion Etching”, J. Electronic Materials, vol. 28, pp. 1448-51, 1999. However, such RIE process is unsatisfactory because it is ineffective for removing deeper scratches, and it introduces additional damage by ion bombardment and additional surface irregularities by concomitant contamination, which in turn requires additional cleaning of the GaN wafer in an O₂ plasma.

[0017] It is therefore advantageous to provide an Al_xGa_yIn_zN wafer with high surface quality on its Ga-side, with substantially no or little surface and subsurface damage or contamination. It is also desirable that such Al_xGa_yIn_zN wafer is prepared by a surface polishing process that is both economic and effective, and requires no cumbersome cleaning process during or after polishing.

[0018] More generally, even though there is a particular need in the art for a surface polishing process that produces high surface quality on the Al_xGa_yIn_z-terminated side of the Al_xGa_yIn_zN(0001) substrate, since such Al_xGa_yIn_z-terminated surface is the most chemically stable surface, there is also a continuing need in the art for Al_xGa_yIn_zN wafer articles with high surface quality on other crystallographic surfaces and offcuts of such surfaces, e.g., non-polar a-axis surfaces, N-terminated (0001) surfaces, A-plane surfaces, M-plane surfaces, R-plane surfaces, and offcuts of the foregoing surfaces.

[0019] The present invention generally relates to Al_xGa_yIn_zN (wherein 0≦y≦1 and x+y+z=1) having superior surface quality, including in various embodiments, device fabrication surfaces comprising crystallographic plane surfaces and offcuts of such crystallographic plane surfaces, and to methods of fabricating such Al_xGa_yIn_zN material in wafer form with surfaces suitable for microelectronic and/or optoelectronic device manufacture.

[0020] One aspect of the present invention relates to a high quality Al_xGa_yIn_zN wafer having a surface roughness characterized by a root means square (RMS) roughness of less than 1 nm in a 10×10 μm² area, e.g., at its Ga-side, at its N-side, at offcuts of (0001) surfaces, at A-plane surfaces, at M-plane surfaces, at R-plane surfaces, at offcuts of A-plane surfaces, at offcuts of M-plane surfaces, and/or at offcuts of R-plane surfaces.

[0021] Although described hereinafter with illustrative reference to Ga-side surfaces of Al_xGa_yIn_zN articles, it will be understood that the polished surface articles of the invention, and the chemical mechanical polishing compositions and their methods of use, broadly encompass and relate to surfaces of Al_xGa_yIn_zN other than the Al_xGa_yIn_z-terminated side of Al_xGa_yIn_zN(0001) articles, such as the N-terminated side of Al_xGa_yIn_zN(0001) articles, and offcuts of (0001) surfaces, as well as A-, M- and R-plane surfaces, and offcuts of such respective A-, M- and R-plane surfaces.

[0022] Accordingly, subsequent references to Ga-side surfaces of Al_xGa_yIn_zN articles in the disclosure hereof will be understood as being representative of and alternatively applicable to such other surfaces, e.g., N-terminated surfaces of Al_xGa_yIn_zN(0001) articles, offcuts of (0001) surfaces of Al_xGa_yIn_zN(0001) articles, as well as A-, M- and R-plane surfaces of Al_xGa_yIn_zN articles, and offcuts of such respective A-, M- and R-plane surfaces of Al_xGa_yIn_zN articles.

[0023] The invention provides Al_xGa_yIn_zN articles of superior RMS surface roughness characteristics, such as Al_xGa_yIn_zN wafers having a surface, e.g., the exemplary Ga-side surface, useful for fabrication of microelectronic devices, optoelectronic devices, or corresponding device precursor structures.

[0024] In ranges of progressively increasing preference, the RMS surface roughness of such wafer, e.g., at its Ga-side, is within the following ranges: (1) less than 0.7 nm in a 10×10 μm² area; (2) less than 0.5 nm in a 10×10 μm² area; (3) less than 0.4 nm in a 2×2 μm² area; (4) less than 0.2 nm in a 2×2 μm² area; and (5) less than 0.15 nm in a 2×2 μm² area.

[0025] Al_xGa_yIn_zN wafers according to the present invention preferably are characterized by a regular step structure at the Ga-side thereof when observed by atomic force microscope.

[0026] Al_xGa_yIn_zN wafers according to the present invention preferably are characterized by the crystal defects of the Al_xGa_yIn_zN wafer at its Ga-side constituting small pits with diameters of less than 1 μm. Small pits of such size are readily visible by both atomic force microscope (AFM) and scanning electron microscope (SEM) techniques, while at the same time these pits do not constitute significant damage of the Al_xGa_yIn_zN wafer surface and therefore do not impair quality of Al_xGa_yIn_zN crystals subsequently grown thereon.

[0027] Such high quality Al_xGa_yIn_zN crystal wafers are readily manufactured by chemically mechanically polishing (CMP) of Al_xGa_yIn_zN wafer blanks, using silica or alumina-containing CMP slurry compositions. The corresponding CMP process enables the crystal defects of the Al_xGa_yIn_zN wafer (evidenced by small pits of less than 1 μm in diameter) to be readily visualized.

[0028] Another aspect of the present invention relates to an epitaxial Al_xGa_yIn_zN crystal structure, comprising an epitaxial Al_x′Ga_y′In_z′N (wherein 0≦y′≦1 and x′+y′+z′=1) film grown on the above-described Al_xGa_yIn_zN wafer of the invention. Such epitaxial Al_x′Ga_y′In_z′N crystal structure preferably comprises a wurtzite crystalline thin film, but may be in any other suitable form or structure suitable for specific semiconductor, electronic, or optoelectronic applications. The composition of the epitaxial film may be or may not be the same as the composition of the wafer substrate. The epitaxial Al_x′Ga_y′In_z′N crystal structure may comprise several epitaxial Al_x′Ga_y′In_z′N films with different composition or doping, sequentially grown on the above-described Al_xGa_yIn_zN wafer of the invention. The epitaxial film may have graded composition, i.e., the composition of the epitaxial film varies with the distance from the interface between the substrate and epitaxial film. As used herein, the term “thin film” means a material layer having a thickness of less than about 100 μm.

[0029] Yet another aspect of the present invention relates to an optoelectronic device that comprises at least one such epitaxial Al_x′Ga_y′In_z′N crystal structure grown on the above-described Al_xGa_yIn_zN wafer of the invention.

[0030] A further aspect of the present invention relates to a microelectronic device that comprises at least one such epitaxial Al_x′Ga_y′In_z′N crystal structure grown on the above-described Al_xGa_yIn_zN wafer of the invention.

[0031] A further aspect of the present invention relates to an Al_xGa_yIn_zN boule that comprises epitaxial Al_xGa_yIn_zN crystal structure grown on the above-described Al_xGa_yIn_zN wafer of the invention. A boule is defined as a bulk crystal body that can be sliced into at least two wafers. An Al_xGa_yIn_zN boule can be grown with any suitable method such as hydride vapor phase epitaxy (HVPE), the metallorganic chloride (MOC) method, metallorganic chemical vapor deposition (MOCVD), sublimation, liquid phase growth, etc.

[0032] The invention in a further aspect contemplates a method of chemically mechanically polishing an Al_xGa_yIn_zN wafer, using a CMP slurry comprising:

• • [0033] abrasive amorphous silica particles, e.g., having an average particle size of less than 200 nm;
  • [0034] at least one acid; and
  • [0035] optionally, at least one oxidation agent;
    wherein the pH value of the CMP slurry may be of a suitable character for the polishing operation, e.g., an acidic pH value (0 to <7.0).

[0036] The abrasive amorphous silica particles in the CMP slurry may for example comprise fumed silica or colloidal silica. The amorphous silica particles in the CMP slurry in one embodiment of the present invention have an average particle size in the range from about 10 nm to about 100 nm. The CMP slurry of the invention in various embodiments thereof can include one or more oxidation agent(s), e.g., hydrogen peroxide, dichloroisocyanuric acid, or the like.

[0037] The pH value of such CMP slurry may be varied in differing embodiments of the invention, and in particular embodiments the CMP slurry may be formulated so that its pH is in a specific range of values, e.g., a range of 0≦pH<7, a range of from about 0.6 to about 3, a range of from about 0.5 to about 4, or a range of from about 0.8 to about 2.5, in various respective embodiments.

[0038] In one such illustrative embodiment, the CMP slurry comprises an acidic colloidal silica slurry.

[0039] A further aspect of the present invention relates to a method of chemically mechanically polishing an Al_xGa_yIn_zN wafer, using a CMP slurry comprising:

• • [0040] abrasive colloidal alumina particles, e.g., having an average particle size of less than 200 nm;
  • [0041] at least one acid; and
  • [0042] optionally, at least one oxidation agent;
    wherein the pH value of the CMP slurry is of a suitable character for the polishing operation, e.g., an acidic pH value (0 to <7.0).

[0043] In one illustrative embodiment of such aspect of the invention, the pH of the CMP slurry composition is in a range of from about 3 to about 5. In another illustrative embodiment, the pH of the CMP slurry composition is in a range of from about 3 to about 4.

[0044] The abrasive colloidal alumina particles in the CMP slurry may vary in particle size in differing embodiments of the invention. In one embodiment of the invention, the CMP slurry has an average particle size in a range of from about 10 nm to about 100 nm.

[0045] The above-described colloidal alumina CMP slurry of the invention can in various embodiments include one or more oxidation agent(s), e.g., hydrogen peroxide, dichloroisocyanuric acid, or the like.

[0046] A further aspect of the present invention relates to chemical mechanical polishing (CMP) of the Al_xGa_yIn_zN wafer, using a CMP slurry that comprises:

• • [0047] amorphous silica particles, e.g., having an average particle size of less than 200 nm;
  • [0048] at least one base; and
  • [0049] optionally, at least one oxidation agent,
    wherein the pH value of the CMP slurry is in a basic pH range (pH>7.0).

[0050] The amorphous silica particles in such CMP slurry in one embodiment of the invention comprise flumed silica particles having an average particle size in a range of from about 10 nm to about 100 nm, and in another embodiment the amorphous silica particles comprise colloidal silica particles having an average particle size in a range of from about 10 nm to about 100 nm.

[0051] Bases useful for the practice of such CMP compositional aspect of the present invention include, but are not limited to, ammonia, alkanolamines, and hydroxides, e.g., KOH or NaOH. Ammonia and alkanolamines are particularly preferred, since they also function to stabilize the CMP slurry.

[0052] Such CMP slurry may include one or more oxidation agent(s), e.g., hydrogen peroxide, dichloroisocyanuric acid or the like.

[0053] The pH value of such basic amorphous silica CMP slurry may be widely varied in the practice of the invention. In one illustrative embodiment, the pH is in a range of from about 7 to about 14. In another illustrative embodiment, the pH is in a range of from about 8 to about 13.5. In a further embodiment, the pH is in a range of from about 9 to about 13. In yet another embodiment, the pH is in a range of from about 10 to about 11.

[0054] A further aspect of the present invention relates to a method of highlighting crystal defects of an Al_xGa_yIn_zN wafer, e.g., at its Ga-side, to facilitate determination of crystal defect density of such wafer, comprising the steps of:

• • [0055] providing an Al_xGa_yIn_zN wafer;
  • [0056] chemically mechanically polishing the wafer, according to one of the above-described CMP methods of the invention;
  • [0057] cleaning and drying the polished Al_xGa_yIn_zN wafer; and
  • [0058] scanning the wafer with an atomic force microscope or a scanning electron microscope to determine defect density in the wafer.

[0059] In one aspect of the invention, the CMP process is conducted using an acidic silica slurry as described hereinabove.

[0060] Yet another aspect of the present invention relates to a method of fabricating high quality Al_xGa_yIn_zN wafers, comprising the steps of:

• • [0061] providing an Al_xGa_yIn_zN wafer blank having thickness in a range of from about 100 μm to about 1000 μm;
  • [0062] optionally reducing internal stresses of the Al_xGa_yIn_zN wafer blank;
  • [0063] optionally lapping the Al_xGa_yIn_zN wafer blank at a first side thereof, using a lapping slurry comprising abrasives having an average particle size in a range of from about 5 μm to about 15 μm;
  • [0064] optionally mechanically polishing the Al_xGa_yIn_zN wafer blank at such first side thereof, using a mechanical polishing slurry comprising abrasives having average particle size in a range of from about 0.1 μm to about 6 μm;
  • [0065] optionally lapping the Al_xGa_yIn_zN wafer blank at a second side thereof, using a lapping slurry comprising abrasives having an average particle size in a range of from about 5 μm to about 15 μm;
  • [0066] mechanically polishing the Al_xGa_yIn_zN wafer blank at such second side thereof, using a mechanical polishing slurry comprising abrasives having average particle size in a range of from about 0.1 μm to about 6 μm;
  • [0067] chemically mechanically polishing the Al_xGa_yIn_zN wafer blank at such second side thereof, using a CMP slurry comprising at least one chemical reactant and abrasive colloidal particles having an average particle size of less than 200 nm, to produce a corresponding Al_xGa_yIn_zN wafer; and
  • [0068] optionally mild etching to further reduce internal stresses of the Al_xGa_yIn_zN wafer and improve the surface quality,
    wherein the resultant Al_xGa_yIn_zN wafer has a root mean square (RMS) surface roughness of less than 1 nm in a 10×10 μm² area at such second side thereof.

[0069] In one embodiment of such fabrication method, the first side of the wafer is the N-side of a GaN wafer and the second side is the Ga-side of such GaN wafer.

[0070] The Al_xGa_yIn_zN wafer blank as used in such fabrication method may be produced in any suitable manner, as for example: (1) growing an Al_xGa_yIn_zN boule and then slicing it into wafer blanks; or (2) growing a thick Al_xGa_yIn_zN film on a foreign substrate and then separating such thick film from the substrate. The wafer blank may be oriented so that the c-axis is perpendicular to the wafer surface or it may be intentionally slightly misoriented (c-axis not perpendicular to the wafer surface) to facilitate subsequent epitaxial growth, device processing or device design.

[0071] The Al_xGa_yIn_zN wafer blank may be subjected to processing for reducing the internal stress caused, for example, by the disparity of thermal coefficients and lattice constants between such Al_xGa_yIn_zN wafer and the foreign substrate on which it is grown. Reduction of internal stress may be conducted by any suitable technique, e.g., by thermally annealing the Al_xGa_yIn_zN wafer or chemically etching the wafer.

[0072] Thermal annealing can be carried out at elevated temperature conditions, e.g., from about 700° C. to about 1600° C., at appropriate pressure (which in various embodiments may be atmospheric pressure, sub-atmospheric pressure, or superatmospheric pressure), in nitrogen, ammonia or any other suitable environment, for sufficient time to effect the desired nature and extent of thermal annealing, e.g., a time of from about 1 minute to about 1 hour in some embodiments, or greater periods in other embodiments, with the choice of specific annealing process conditions being readily determinable by simple experiment involving variation of process conditions and characterization of the annealed product.

[0073] Chemical etching of the Al_xGa_yIn_zN wafer functions to remove a layer of surface material from said wafer, thereby relaxing the internal stress of said wafer. It is preferred that the chemical etching process effect a removal of surface material of less than 100 μm thickness from the wafer, and more preferably less than 10 μm thickness.

[0074] The Al_xGa_yIn_zN wafer can be chemically etched at elevated temperature by a very strong acid, e.g., sulfuric acid, phosphoric acid, combinations thereof, etc., or by a very strong base, e.g., molten KOH or NaOH.

[0075] Lapping slurry compositions advantageously used in the practice of the present invention can comprise any suitable abrasives, including, but not limited to, diamond powders, silicon carbide powders, boron carbide powders, and alumina powders. Preferably, the lapping slurry comprises diamond powder having an average particle size in a range of from about 6 μm to about 10 μm. More preferably, two or more lapping slurries lap the Al_xGa_yIn_zN wafer blank, with each subsequent lapping slurry comprising abrasives of a progressively smaller average particle size. For example, the Al_xGa_yIn_zN wafer blank may be lapped by a first slurry comprising abrasives of an average particle size of from about 8 μm to about 10 μm, and then by a second slurry comprising abrasives of an average particle size of from about 5 μm to about 7 μm.

[0076] Similarly, mechanical polishing slurries useful in the practice of the present invention may comprise any suitable abrasives, including but not limited to diamond powders, silicon carbide powders, boron carbide powders, and alumina powders. Diamond powders with an average particle size in a range of from about 0.1 μm to about 3 μm are particularly preferred. The mechanical polishing step may also employ two or more mechanical polishing slurries, with each subsequent mechanical polishing slurry comprising abrasives of a progressively smaller average particle size. For example, a first mechanical polishing slurry comprising abrasives of an average particle size of from about 2.5 μm to about 3.5 μm can be used, followed by a second mechanical polishing slurry comprising abrasives of an average particle size of from about 0.75 μm to about 1.25 μm, followed by a third mechanical polishing slurry comprising abrasives of an average particle size of from about 0.35 μm to about 0.65 μm, followed by a fourth mechanical polishing slurry comprising abrasives of an average particle size of from about 0.2 μm to about 0.3 μm, and finally by a fifth mechanical polishing slurry comprising abrasives of an average particle size of from about 0.1 μm to about 0.2 μm.

[0077] The CMP slurry comprises at least one chemical reactant, e.g., an acid or a base in an amount yielding a pH in a range of 0≦pH<7 for acidic CMP slurry compositions, or a pH in a range of 7<pH≦14 for basic CMP slurry compositions.

[0078] After the CMP, the Al_xGa_yIn_zN wafer may be subjected to additional processing for further reducing the stress of the wafer and improving the surface quality. A mild etching may be effective for this purpose. The mild etching may for example remove some residual surface damage on a Ga-side surface resulting from final CMP polishing while not etching the undamaged surface of such Ga-side, thereby improving the surface quality of the wafer. The mild etching can also remove the damage on an N-side surface, thus reduce the stress on the wafer caused by surface damage. Mild etching can also be employed to produce a matte finish on the N-side surface. For example, the wafer can be slightly etched in an aqueous solution of base (for example, KOH or NaOH) or an aqueous solution of acid (for example, HF, H₂SO₄, or H₃PO₄) at a temperature below the boiling point of the aqueous solution, typically about 100° C.

[0079] In a further aspect, the invention relates to a method of fabricating a laser facet on an article formed of Al_xGa_yIn_zN wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1. The method comprises chemically mechanically polishing the article at a surface thereof using a chemical mechanical polishing slurry including silica and/or alumina abrasive particles, and an acid or base, wherein the chemically mechanically polishing step is carried out to impart to the surface a root mean square (RMS) surface roughness of less than 1 nm in a 10×10 μm² area thereof.

[0080] A still further aspect of the invention relates to chemically mechanically polishing Al_xGa_yIn_zN, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1, using a chemical mechanical polishing slurry including silica and/or alumina abrasive particles, and an acid or base, wherein the chemically mechanically polishing step is carried out to impart to the surface a root mean square (RMS) surface roughness of less than 1 nm in a 10×10 μm² area thereof. Such methodology may be employed for processing of Al_xGa_yIn_zN wafers, e.g., in the fabrication of such wafers in a wafer fab, for planarization of such wafers, or for replanarization of such wafers, reworking of wafers to remove undesired layers or material therefrom, etc.

[0081] The term “Al_xGa_yIn_zN” as used herein per se is intended to be broadly construed as including all compositions wherein 0≦x≦1, 0≦y≦1 and 0≦z≦1 and x+y+z=1, and therefore is inclusive, inter alia, of AlN, AlGaN, AlInN, AlGaInN, GaN, GaInN, and InN.

[0082] Other aspects, features, and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.

[0093] Fabrication of high quality Al_xGa_yIn_zN wafers in accordance with a preferred aspect of the present invention is readily achieved by processing steps as hereinafter more fully described, including fabrication of freestanding Al_xGa_yIn_zN wafer blanks, lapping, mechanical polishing, chemical mechanical polishing, and reduction of internal stress.

[0094] Freestanding Al_xGa_yIn_zN wafer blanks are obtained by any of various suitable methods. One method involves first growing an Al_xGa_yIn_zN boule and then slicing it into wafer blanks. Another method for producing Al_xGa_yIn_zN wafer blanks utilizes the steps of: (1) growing a thick Al_xGa_yIn_zN film on a foreign substrate, using a suitable method such as hydride vapor phase epitaxy (HVPE), the metallorganic chloride (MOC) method, metallorganic chemical vapor deposition (MOCVD), sublimation, etc.; and then (2) removing the foreign substrate from the thick Al_xGa_yIn_zN film, by polishing or etching the foreign substrate, by a laser-induced liftoff process, or other suitable technique.

[0095] By way of example, GaN films of about 400 μm thickness can be grown on sapphire substrates using HVPE process techniques.

[0096] Marks such as flats are made on the wafer to identify the crystal orientation of such wafer blank. The Al_xGa_yIn_zN wafer blank can be sized into a round shape by, for example, particle beams, to facilitate subsequent mounting or processing of the wafer blank.

[0097] Mounting of the freestanding Al_xGa_yIn_zN wafer blank to a fixture enables it to be readily lapped or polished as necessary. The wafer blank can be mounted on a template with recesses for holding the wafer blank. Alternatively, the wafer blank can be mounted on a flat template, by, for example, (1) heating such template on a hotplate, (2) applying wax onto such template, and (3) pressing the wafer blank against the waxed template. After the template cools down, the wax solidifies and functions to hold the wafer blank on the template.

[0098] When the Al_xGa_yIn_zN wafer blank is obtained from a Al_xGa_yIn_zN boule and is relatively thick and uniform, a recessed template can be used for mounting such wafer blank, which is advantageous over waxed templates in respect of shorter process times, easier demounting, and reduced contamination.

[0099] On the other hand, for Al_xGa_yIn_zN wafer blanks which may be more fragile, thinner, or less uniform in thickness, for example, wafer blanks obtained from HVPE processes, the use of recessed templates may be less preferred due to the associated risk of breaking the Al_xGa_yIn_zN wafer during the lapping and/or polishing process.

[0100] The fixture used for mounting the Al_xGa_yIn_zN wafer blank can be of any suitable type appropriate to, and compatible with, the respective lapping or polishing apparatus. For the purpose of improving thickness uniformity of the Al_xGa_yIn_zN wafer, a special lapping fixture comprising three adjustable diamond stops defining a plane can be utilized. The plane defined by the stops is parallel to the fixture surface, at a predetermined distance away from the surface. Such predetermined distance defines a minimum thickness of the lapped Al_xGa_yIn_zN wafer, because the three diamond stops function as stop points preventing further removal of surface material from the Al_xGa_yIn_zN wafer.

[0101] In case the Al_xGa_yIn_zN wafer blank is slightly bowed or otherwise distorted due to internal stress present therein, it is preferable to dispose a weight on the wafer blank during the wafer being wax-mounted on the template. The type and amount of weight for such purpose is readily determinable within the skill of the art.

[0102] After the Al_xGa_yIn_zN wafer blank is appropriately mounted, the wafer blank can be lapped by pressing it against a lapping plate, with abrasive particles embedded on surface of such lap plate, to produce a flat surface on the wafer. The pressure on the wafer may be adjusted to control the lapping process.

[0103] When using the same abrasives and lap plate rotation rates, the lapping rates of the Al_xGa_yIn_zN wafer blank increase with increasing particle size of the abrasive. Larger abrasive particles thus result in a higher lapping rate, but produce rougher lapped surfaces.

[0104] Lapping rates also depend on the hardness of abrasive material used. For example, diamond powders have higher lapping rates than silicon carbide powders, which in turn have higher lapping rates than alumina powders.

[0105] Lapping rates also depend on the type of lapping plates employed. For example, a copper lapping plate has a lower lapping rate than that of a cast iron plate, but the copper lapping plate yields a smoother lapped surface than that produced by the cast iron plate.

[0106] For an optimal lapping result, many factors, such as process time, surface finish, and manufacturing cost, have to be considered, and many combinations of abrasive material, particle size, lapping rate, and wafer pressure can be employed in the practice of the present invention. In order to reduce the probability of Al_xGa_yIn_zN wafer cracking, a pressure below 5 psi, preferably 2 psi, is preferred. In order to reduce process time, a lapping rate above 50 μm/hr is preferred for stock removal. Among various kinds of abrasive materials, such as diamond, silicon carbide, boron carbide, and alumina, diamond slurry is preferred due to its high material removal rate and its production of better surface finishes.

[0107] Lapping of the Al_xGa_yIn_zN wafer blank can be achieved either by a single step, or by multiple steps, with each subsequent lapping step using abrasives of progressively smaller particle sizes. After each lapping step, an optical microscope can be used to examine the surface to make sure that surface damage from previous steps has been substantially removed, before proceeding to next step.

[0108] In one illustrative embodiment of the invention, a single lapping slurry is used, comprising 9 μm diamond abrasive, for lapping an Al_xGa_yIn_zN wafer on a cast iron lapping plate under a pressure of 1 psi. The size of diamond abrasive particles is provided by the diamond slurry manufacturer, and is the average size of diamond particles in the slurry.

[0109] In another illustrative embodiment of the invention, two lapping slurries are used: the first lapping slurry comprises 9 μm diamond abrasive for lapping an Al_xGa_yIn_zN wafer on a cast iron lapping plate, and the second slurry comprises 6 μm diamond abrasive for lapping the same wafer on a copper plate to achieve the desired surface finish.

[0110] After the Al_xGa_yIn_zN wafer is lapped, it can be mechanically polished to achieve smooth surface morphology. During the mechanical polishing process, the Al_xGa_yIn_zN wafer is pressed against a polishing pad with abrasive particles. Polishing processes typically yield better surface finish than lapping, even with a same size diamond slurry. Polishing can be achieved either by a single step, or by multiple steps, with each subsequent polishing step using abrasives of progressively smaller particle sizes.

[0111] After the mechanical polishing process, the Al_xGa_yIn_zN wafer surface becomes relatively smooth. FIG. 1 shows a Nomarski optical micrograph (×100) of a GaN surface after being mechanically polished with 1 μm diamond slurry until mirror finish has been achieved. However, such Al_xGa_yIn_zN wafer is not suitable for homoepitaxial growth of Al_xGa_yIn_zN crystals, since it still has significant surface and subsurface damage. The surface damage is characterized by dense polishing scratches that are visible under the atomic force microscope (AFM), as shown in FIG. 2.

[0112] To remove such surface and subsurface damage and polishing scratches, chemical mechanical polishing (CMP) of the Al_xGa_yIn_zN wafer is carried out.

[0113] A first CMP slurry effective for chemically mechanically polishing the Ga-side of the Al_xGa_yIn_zN wafer comprises an acid and abrasive amorphous silica particles, such as fumed silica or colloidal silica, having particle sizes of less than 200 μm. The pH value of such CMP slurry may for example be in a range from about 0.5 to about 4. In some applications, it may be advantageous to employ a CMP slurry that includes an oxidization agent, such as hydrogen peroxide, dichloroisocyanuric acid or the like.

[0114] FIGS. 3 and 4 show a Nomarski optical micrograph and an AFM image, respectively, of a GaN wafer chemically mechanically polished using an acidic colloidal silica slurry having a pH value of 0.8 for about 1 hour. The GaN wafer was first polished with 1 μm diamond slurry before CMP. Besides a few defects from the substrate, the GaN surface is very smooth, with RMS surface roughness of about 0.15 nm in a 2×2 μm² area and about 0.5 nm in a 10×10 μm² area. Further, a previously unseen step structure is observed on the GaN surface under AFM. The presence of such step structure is an indication that the CMP process has been successful in removing polishing scratches from previous mechanical polishing. The CMP rate using such slurry can for example be on the order of about 2 μm/hr.

[0115] To further ascertain that the CMP process has also removed the subsurface damage on the surface, the wafer after CMP processing is etched with a strong etchant, H₃PO₄ at 180° C. for 5 minutes. At this etching condition, crystal defects as well as surface and subsurface damage on the Ga-side of GaN surface will be etched at a greater rate than good crystalline material, producing etching pits. The size and number of the pits can be studied with an atomic force microscope. After hot H₃PO₄ etching, the CMP polished wafers show some etching pits, but the density of the etching pits is the same as the density of pits evident in the CMP polished surface. The size of the pits has increased, however. For comparison, a wafer that is not completely polished with the CMP process (i.e., using a shorter CMP processing time resulting in some residual polishing damage) shows more etching pits after etching with H₃PO₄ at 180° C. for 5 minutes, and many of the pits follow a line, indicating that the surface damage and subsurface damage are not completely removed when the CMP process is not complete.

[0116] Oxidation agents may optionally and advantageously be employed in the acidic CMP slurry, as necessary or desired in specific applications of such CMP methodology of the invention. When hydrogen peroxide or dichloroisocyanuric acid is used as an oxidation agent, the polishing rate is above 2 μm/hr, with RMS surface roughness being below 0.2 in a 2×2 μm² area and below 0.5 nm in a 10×10 μm² area. The step structures on the Al_xGa_yIn_zN wafer surface are readily observed by AFM inspection.

[0117] A second CMP slurry effective for chemically mechanically polishing the Ga-side of the Al_xGa_yIn_zN wafer comprises an acid and abrasive colloidal alumina particles having particle sizes of less than 200 nm. The pH value of such CMP slurry in one preferred embodiment is in a range from about 3 to about 4. Optionally, such CMP slurry can also comprise an oxidization agent, such as hydrogen peroxide, dichloroisocyanuric acid or the like.

[0118] FIG. 5 shows an AFM image of a GaN surface after being chemically mechanically polished with acidic colloidal alumina CMP slurry (pH=3.6) comprising hydrogen peroxide as an oxidization agent, for 1 hour. The step structure is observed under AFM, demonstrating that acidic colloidal alumina slurry is effective for removing mechanical damage from the GaN surface. However, at the same polishing operation conditions, the colloidal alumina-based slurry has a much lower polishing rate (about 0.1 μm/hr) than the polishing rate of the silica-based slurries. Because of such slow polishing rate, many polishing scratches are still present after 1 hour of polishing with the acidic colloidal alumina CMP slurry. A much longer polishing time is needed to completely remove the surface/subsurface damage with the colloidal alumina-based slurry than with the silica-based slurries.

[0119] A third CMP slurry effective for chemically mechanically polishing the Ga-side of the Al_xGa_yIn_zN wafer comprises a base and amorphous silica particles, either fumed silica or colloidal silica, having particle sizes of less than 200 nm. The pH value of such illustrative CMP slurry is in a range of from about 8 to about 13.5.

[0120] FIGS. 6 and 7 show a Nomarski optical micrograph and an AFM image of a GaN wafer chemically mechanically polished using a basic colloidal silica slurry having a pH value of 11.2, for about 1 hour. The surface appears rougher and has significantly more scratches when polished, in comparison with the surface finish achieved with an acidic silica slurry. Moreover, the scratches are larger and deeper than those of the GaN surface after mechanical polishing with diamond slurry comprising 1 μm diamond powders, indicating that larger particles or particle agglomerations are present in the basic silica slurry. Interestingly, step structures are also observed. The presence of step structures indicates that surface damage from previous mechanical polishing has been removed, but the presence of larger particles in the slurry introduces new damage. It therefore is desirable to filter the basic silica slurry before polishing to remove large particles and to ensure that the abrasive particles in such slurry have particle sizes of less than 200 nm.

[0121] Besides using hydroxides for pH alteration, the pH of the basic silica slurry can be adjusted with ammonia or alkanolamine. Ammonia- or alkanolamine-stabilized slurries provide smoother polished surfaces and therefore are preferred over hydroxide-based slurries.

[0122] To improve the stability of the CMP process, it may be advantageous to control the ambient humidity and temperature during the CMP process.

[0123] After chemical mechanical polishing, the Al_xGa_yIn_zN wafer can be cleaned and dried, using techniques known in the art. A mild etching can also be used to remove any remaining surface and subsurface damage from the final polished wafer. The condition for the mild etching is chosen to remove some residual surface damage on the Ga-side surface from final polishing, while not etching or etching to a limited degree the undamaged surface of the Ga-side. The mild etching can also remove the damage on the N-side surface to reduce the stress on the wafer caused by damage on the N-surface. This mild etching can also produce a matte finish on the N-surface. For example, the wafer can be slightly etched in an aqueous solution of base (for example, KOH or NaOH) or an aqueous solution of acid (for example, HF, H₂SO₄, or H₃PO₄) at a temperature below 100° C.

[0124] Al_xGa_yIn_zN wafers may suffer from internal stresses, which cause the wafer to bow or to warp. Thermal annealing or chemical etching of the AlxGayInzN wafer, which can be performed before, after or between the steps of the wafer fabrication sequence, can relax such internal stresses.

[0125] In the circumstance where the Al_xGa_yIn_zN wafer has large pits on its surface and contaminants are trapped in the pits from the fabrication process, it is beneficial to employ a chemical etching and cleaning step to remove the contaminants from the pits between the steps of wafer fabrication.

[0126] In one embodiment of the present invention, the Al_xGa_yIn_zN wafer is subjected to thermal annealing at temperature up to 1000° C. in a nitrogen ambient. Preferably, the annealing temperature is in a range of from about 700° C. to about 1000° C., and the duration of the thermal annealing is in a range of from about 1 minute to about 1 hour.

[0127] In another embodiment of the invention, the Al_xGa_yIn_zN wafer is subjected to chemical etching, which preferentially removes damaged surface material from the Al_xGa_yIn_zN wafer and reduces wafer bow and warp caused by surface damage.

[0128] Chemical etching of the Al_xGa_yIn_zN wafer can be accomplished by immersing the wafer in very strong acids or bases at an elevated temperature. Sulfuric acid or phosphoric acid at a temperature above 150° C. can be utilized to etch the Al_xGa_yIn_zN wafer. Alternatively, molten potassium or sodium hydroxide can also be employed to etch the Al_xGa_yIn_zN wafer. The etching conditions, such as etching temperature and etching time, are preferably controlled to yield removal of surface material of less than 100 μm thickness, and preferably less than 10 μm thickness.

[0129] After chemical mechanical polishing of a GaN surface, for example, using acidic silica CMP slurry (pH=0.8) for about 1 hr., small pits are formed, which may originate from dislocations in the crystal lattice of the GaN wafer. The diameter of the pits is typically below 1 μm, and more typically below 0.5 μm. The pits appear round without clear edges when imaged with an atomic force microscope. When the wafer has been completely CMP polished and is subjected to etching, for example, with H₃PO₄ at 180° C. for 5 minutes, the size of the pits is increased, but the density of the pits remain the same, i.e., no more pits are produced. Furthermore, the pits formed from etching the CMP polished wafer appear hexagonal when imaged with an atomic force microscope.

[0130] FIG. 8 shows an AFM image of a GaN surface, with clearly visible pits. The GaN surface was chemical mechanically polished using an acidic colloidal silica CMP slurry (pH=0.8) for about 1 hour.

[0131] FIG. 9 also shows a scanning electron microscopic (SEM) image of a GaN wafer, polished by acidic colloidal silica CMP slurry (pH=0.8) for 1 hour, with visible pits that can be counted for determining the defect density of such GaN wafer. Without chemical mechanical polishing of the GaN wafer surface, such pits are not observed with AFM or with SEM.

[0132] In one aspect of the present invention, a CMP process is used to prepare an Al_xGa_yIn_zN wafer, to highlight crystal defects for subsequent determination of defect density by AFM or SEM techniques.

[0133] This defect highlight technique is superior to other techniques such as transmission electron microscope (TEM), wet-chemical etching, and photo electrochemical etching, which are generally conducted under harsh etching conditions, making the etched Al_xGa_yIn_zN wafer unsuitable for subsequent epitaxial growth of Al_xGa_yIn_zN crystalline material thereon.

[0134] By contrast, the use of a CMP process for highlighting crystal defects does not damage the crystal surface of the Al_xGa_yIn_zN wafer and therefore permits subsequent crystal growth.

[0135] From the foregoing, it will be apparent that the present invention provides a superior technique for chemically mechanically polishing Al_xGa_yIn_zN, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, using a chemical mechanical polishing slurry including silica and/or alumina abrasive particles, and an acid or base, wherein the chemically mechanically polishing step is carried out to impart to the surface a root mean square (RMS) surface roughness of less than 1 nm in a 10×10 μm² area thereof.

[0136] Such methodology may be employed for processing of Al_xGa_yIn_zN wafers, e.g., in the fabrication of such wafers in a wafer fab, for planarization of such wafers, or for replanarization of such wafers, reworking of wafers to remove undesired layers or material therefrom, etc., as well as in fabricating laser facets on Al_xGa_yIn_zN for device applications.

[0137] The method of the invention for chemically mechanically polishing Al_xGa_yIn_zN may also be employed for shaping of Al_xGa_yIn_zN articles to counteract bow or other deformation or mis-shaping, such as otherwise may be manifested in the processing of such Al_xGa_yIn_zN articles.

[0138] A GaN film several hundred microns thick was grown on a sapphire substrate by an HVPE process and then separated from the sapphire substrate. The resultantly formed freestanding GaN wafer blank exhibited a textured Ga-surface with a RMS roughness of about 4 nm in a 2×2 μm² area.

[0139] The GaN wafer blank was then polished at the Ga-side with an acidic silica slurry without undergoing a lapping process.

[0140] After polishing, it was observed that the surface morphology of such GaN wafer was greatly improved, with the textured surface being entirely removed. The RMS roughness was reduced to below 0.3 nm in a 2×2 μm² area.

[0141] Thick GaN films with thickness in the range from 200-500 microns were grown on 2″ sapphire substrates by an HVPE process. The GaN films then were separated from the sapphire substrate, to yield freestanding GaN wafer blanks.

[0142] Flats for the GaN films were marked as 30° off the sapphire substrate's flat. The GaN wafer blanks then were sized into wafer shapes with diameter of 30, 35, and 40 mm using a particle beam jet. To prevent wafer breakage during wafer sizing, it was preferable to mount the GaN wafer on a glass plate of at least 1 mm thickness, using wax.

[0143] Nine GaN wafers were mounted on a lap fixture with wax with the N-side facing the lap fixture. A steel block was placed on top of each wafer while the wax cooled. The GaN wafers were first lapped on the Ga-side with diamond slurry of 9μm in diameter on a cast iron lapping plate. Before lapping, a large thickness variation existed between the wafers and within each wafer. After lapping, uniformity of wafer thickness was greatly improved.

[0144] The wafers then were removed from the lapping fixture, and wax-mounted on a mechanical polishing fixture. Each wafer was polished with diamond slurry of 3 μm in diameter until mirror finish was achieved. Under optical microscope examination, all surface damage from the lapping process was removed.

[0145] After mechanical polishing, the wafers were chemically mechanically polished with acidic colloidal silica slurry. A Nomarski optical microscope was used to examine the polished surfaces, to verify that the CMP process removed all mechanical polishing scratches.

[0146] Three GaN wafer blanks were mounted on a lap fixture with wax with the Ga-side facing the lap fixture. A steel block was placed on top of each wafer while the wax cooled. The GaN wafers were first lapped on the N-side with diamond slurry of 9 μm in diameter on a Lapmaster 15 lapping machine with cast iron lapping plate until a uniform mat finish was achieved.

[0147] After the N-side was lapped, the GaN wafers were removed from the lap fixture by heating on a hot plate. The wafers were cleaned and mounted on a lap fixture with wax with the N-side facing the lap fixture. A steel block was placed on top of each wafer while the wax cooled. The GaN wafers were lapped on the Ga-side with diamond slurry of 9 μm in diameter on a cast iron lapping plate until a desirable wafer thickness was obtained. Subsequently, the GaN wafers were lapped with diamond slurry of 6 μm in diameter on a copper lapping plate until surface features from the previous lapping step were removed.

[0148] After lapping, the three wafers were mechanically polished with diamond slurry of 1 μm in diameter on a Buehler ECOMET polisher until the surface features from the previous lapping step were removed.

[0149] After mechanical polishing, the three wafers were chemical mechanically polished with an acidic colloidal silica slurry on a Beuhler ECOMET polisher. The acidic colloidal silica slurry was prepared by mixing 2 parts of 1 molar aqueous hydrochloric acid with 1 part of commercial silica slurry, Nalco 2350 polishing slurry. A Nomarski optical microscope was used to examine the polished surfaces, to verify that the CMP process removed all mechanical polishing scratches.

[0150] After CMP process, the wafers were removed from the polish fixture and cleaned. The wafers were also cleaned in diluted hydrofluoric acid to remove any residual colloidal silica particles on the wafer surface. The wafers were imaged with atomic force microscope (Digital Instruments NanoScope III) to determine the density of the pits and the smoothness of the surface. For one wafer, the RMS roughness was 0.11 nm in a 2×2 μm² area and 0.28 nm in a 10×10 μm² area. The pit density for the three wafers was about 10⁶-10⁷ pits/cm², and the pit size was about less than 0.4 μm in diameter.

[0151] A GaN boule was sliced vertically, to produce wafer samples with surfaces that were parallel to the c-axis.

[0152] The sliced samples were first lapped with 9 μm diamond slurry on a cast iron plate with a Lapmaster 15 lapping machine to remove the wire saw marks. Subsequently, the samples were polished with 3 μm diamond slurry on a Suba500 polishing pad to obtain a mirror finish. The surfaces still had scratches that were observed under an optical microscope.

[0153] The samples next were chemical mechanically polished with a chemical mechanical polishing (CMP) slurry consisting of a mixture of colloidal silica and hydrochloric acid, for 1 hour. Such CMP processing removed the surface scratches from the samples.

[0154] FIG. 10 is an AFM image of a CMP-polished GaN sample (50×50μm) at a surface parallel to the c-axis of the GaN, after such processing.

[0155] The white spots on the sample surface were residual CMP slurry particles. The CMP polished surface was very smooth, and was determined to have a RMS roughness of 0.8 nm over the 50 μm×50 μm area. Furthermore, no polish scratches were observed, indicating that the CMP slurry was effective in removing the surface damage of this GaN sample on the surface parallel to the c-axis.

[0156] The GaN wafers of the present invention can be used to construct optoelectronic devices such as light emitting diodes and blue light lasers. Such devices are important, as the blue light emitting diodes (LEDs) and lasers are an enabling technology, allowing much higher storage density in magneto-optic memories and CDROMs and the construction of full color light emitting displays. Such devices can replace today's incandescent light bulbs in road and railway signals, etc., where they promise very substantial cost and energy savings.

[0157] The invention has been described herein with reference to specific features, aspects, and embodiments. It will be appreciated that the applicability of the invention is not thus limited, but readily extends to and encompasses numerous variations, modifications, and other embodiments, as will readily suggest themselves to those of ordinary skill in the art. Accordingly, the invention is to be broadly construed, consistent with the claims hereafter set forth.