SAPPHIRE SURFACES

This application claims priority from US Provisional Application No. 61/292,556, filed January 6, 2010.

FIELD OF THE INVENTION

[0001] The invention described herein is chemical compositions and a process for wet chemical modification of sapphire surfaces to improve the surface attributes, by selective etching of defects and dislocations in the surface of the sapphire substrate, of single crystalline sapphire prior to nitride metal organic chemical vapor deposition (MOCVD) in the fabrication of semiconductor devices, especially light-emitting diodes, UV photodetectors and laser diodes to improve the overall device performance.

BACKGROUND TO THE INVENTION

[0002] Sapphire (AI2O3) is a material that is gaining importance across the semiconductor industry. The applications for sapphire include, but are not limited to, light emitting diodes (LEDs) capable of emitting visible light and silicon on sapphire (SOS) integrated circuits. In the manufacture of LEDs, the sapphire is used as a substrate for the growth of bulk films for the production of nitride based LEDs. The sapphire substrate may also, in some cases, be patterned via inductively coupled plasma (ICP) after standard photolithography. This process leaves photoresist residue on the surface of the sapphire along with some quantity of etch residue. After this process, a nitride film is grown on the sapphire surface and the LED is manufactured.

[0003] Light-emitting diodes (LEDs) have found use in many applications, such as traffic lights, back lighting components in liquid crystal displays, in electronic components of various devices, and in other various illumination devices. Other high performance optical devices such as laser diodes have also found numerous applications. These optical devices include amber, green, blue, white and ultraviolet LEDs. These high performance devices have generally been developed and produced for emitting visible light using Group ΙΠ-V compounds comprising AlGalnP and GaN grown by epitaxial growth as single crystal films on sapphire (AI2O3) substrates. The AlGalnP is generally used in LEDs emitting red, orange and yellow light while the GaN is used if forming LEDs emitting green, blue and ultraviolet light. In most cases the GaN crystal formed on the sapphire substrate.

[0004] Particularly in the LED industry, the sapphire is patterned by ICP techniques after conventional lithography. The ability to clean the photoresist residue and prepare the surface of the sapphire substrate prior to nitride film deposition by metal organic chemical vapor deposition (MOCVD) is desired.

[0005] The consequence of using sapphire as a substrate in the manufacture of LEDs is that there is both a lattice mismatch and a thermal expansion mismatch between the sapphire substrate and the film that is grown (typically GaN) on the substrate. These mismatches have been known to result in stacking faults and a high density of threading dislocations of the GaN crystal in the devices. The dislocation density has been reported to be on the order of 10 ⁹ -10 ¹⁶ cm- ² . This results in a LED device with reduced efficiency because of reduced power and lifetime of the light and increased current leakage and thus decreased reliability of such devices. Therefore, it is desirable to reduce such defects. The reduction in defects is believed to result in increased light output and efficiency and reliability of the devices.

[0006] There exist several techniques to improve the quality and light output of the LEDs through the reduction of dislocations and these include epitaxial lateral overgrowth (ELOG), pseudo- epitaxy, and patterned sapphire substrates. The present invention is one to provide a wet chemical solution to achieve similar, but improved results. The wet process of subjecting a sapphire substrate to the chemical solutions is to operate to achieve more crystalline nitride films and films that possess fewer threading dislocation defects. This will ultimately result in LEDs and other high performance optical devices with increased and more efficient light output.

SUMMARY OF THE INVENTION

[0007] The mechanism of action resulting in higher quality nitride films is believed to occur in the present invention by selective etching of defects and dislocations in the surface of the sapphire substrate resulting in a surface that is more perfect. This allows for higher quality nitride films to be grown. The improvement in the light output of the finished electro optical devices, such as LEDs, will be realized by one of two, or both, reasons. The first is that the chemical modification of the sapphire surface leads to more preferred growth of the nitride film, and the second is that the chemical modification has prevented or significantly reduced the dislocations from occurring or, at least, prevented them from propagating to throughout the entire nitride film.

[0008] The present invention provides a wet chemical solution to serve two roles: first remove the residue that remains on the sapphire surface and then to condition the surface of the sapphire to make it more amenable to the growth of nitride based films. If the sapphire has not been exposed to photoresist, the invention will be applied to the sapphire substrate prior to metal organic chemical vapor deposition (MOCVD) of the nitride film.

[0009] The composition of this invention is an aqueous composition comprising, consisting essentially of, or consisting of water, hydrochloric acid (HC1), hydrogen fluoride (HF), phosphoric acid (H ₃ PO ₄ ), and optionally, but preferably at least one metal chelating agent that is a phosphonic acid compound capable of chelating aluminum. The composition is employed to treat the sapphire substrate to remove or etch some quantity of the surface material of the sapphire substrate. The result of the treatment of the sapphire substrate is to improve the overall device performance by eliminating surface irregularities, scratches from chemical mechanical processing (CMP) thereby permitting more perfect growth of bulk nitride films on the sapphire substrate resulting in optical electro devices of increased efficiency and reliability.

BRIEF DESCRIPTION OF THE DRAWING

[0010] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

Figures are the following.

Fig.l is an atomic force microscope (AFM) topographical image of an untreated and annealed surface of a c-plane sapphire after annealing at 1050° C prior to imaging.

Fig. 2 is an atomic force microscope (AFM) topographical image of an invention treated and annealed surface of a c-plane sapphire after annealing at 1050° C prior to imaging.

Fig.3 is an atomic force microscope (AFM) topographical image of an untreated and annealed surface of a c-plane sapphire after annealing at 1050° C prior to imaging.

Fig. 4 is an atomic force microscope (AFM) topographical image of an invention treated and annealed surface of a c-plane of a CMP treated sapphire after annealing at 1050° C prior to imaging.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The aqueous compositions of this invention for treatment of sapphire substrates comprise, consist essentially of or consist of water, hydrochloric acid, hydrogen fluoride, phosphoric acid, and optionally, but preferably a metal chelating agent that is a phosphonic acid compound.

[0012] The HCl in the composition of this invention will generally be present in an amount of from about 1% to about 20 % by weight, the HF in an amount of from about 0.1% to about 1 % by weight, the H3PO4 in an amount of from about 1% to about 50% by weight, and the phosphonic acid compound, if optionally present, will generally present in an amount of from about 0.1% to about 5% by weight, with the remainder being water, generally in an amount of up to about 97.8% by weight. The percents by weights of the components are based on the total weight of the composition.

[0013] While the HCl may be present in an amount of from 1% to 20% by weight it is preferably present in an amount of from about 5% to about 15% by weight, more preferably from about 8 % to about 12%, and still more preferably about 10% to about 12%. The HF component in the com position will generally be present in an amount of from about 0.1% to about 1% by weight, prefer ably from about 0.2% to about 1%, more preferably from about 0.2% to about 0.5%, and still mor e preferably about 0.25%. The H3PO4 component of the composition will generally be present i n an amount of from about 1 % to about 50% by weight, preferably from about 10% to about 50 %, more preferably from about 30% to about 50%, and still more preferably about 35% to abou 1 40%. The phosphonic acid metal chelator compounds are not required and thus may represen t 0% by weight of the composition. However, such optional phosphonic acid metal chelator co mpounds when present in the composition of this invention will generally be present in an amo unt of from about 0.1% to about 5% by weight, preferably from about 0.5% to about 3%, and st ill more preferably from about 1 % to about 2%. The water component will make up the remain der of the composition and will generally be present in an amount of from about 24.9 % to abo ut 97.8% by weight, preferably from about 40% to about 75%, more preferably from about 40 % to about 60% and still more preferably from about 45% to about 65%.

[0014] The optional chelating agent that is preferably present in a composition of this invention may be any suitable phosphonic acid compound acting as a chelating agent for aluminum. The presence of such chelating agents acts to prevent redeposition of aluminum etched from the sapphire substrate. Such phosphonic acid compounds suitable for use in the compositions of this invention include, but are not limited to, aminotrimethylene phosphonic acid (ATMP); diethylenetriaminepenta(methylenephosphonic acid) (DETPA); N, N, Ν', '- ethylenediaminetetra(methylenephosphonic acid) (EDTMP); 1 , 5, 9-triazacyclododecane-N, N\ N", N"'-tris(methylenephosphonic acid) (DOTRP); 1, 4, 7, 10-tetraazacyclododecane- , N', N", N"'-tetrakis(methylenephosphonic acid) (DOTP); nitrilotris(methylene) triphosphonic acid; diethylenetriaminepenta(methylenephosphonic acid) (DETAP); aminotri(methylenephosphonic acid); 1-hydroxyethylene -1 , 1 -diphosphonic acid; bis(hexamethylene)triamine phosphonic acid; and 1, 4, 7-triazacyclononane-N, N', N"- tris(methylenephosphonic acid) (NOTP). Preferably the phosphonic acid chelating agent is aminotrimethylene phosphonic acid (ATMP). Aminotrimethylene phosphonic acid is a commercially available, nearly pure (>95%) product, which may contain preparative reaction side products such as phosphoric acid and hydrochloric acid.

[0015] Preferred compositions of this invention are the following. Composition A consists of 60 parts by weight 37% HC1, 50 parts by weight 50: 1 dHF, 85 parts by weight 85% H ₃ P0 _4, and 5 parts by weight added water. Composition B consists of 1 1.2 wt % HC1, 0.3 wt % HF, 36.7 wt % H3PO4, 1.0 wt % aminotrimethylene phosphonic acid, and 50.8 wt % water. Composition C c onsists of 1 1.3 wt % HC1, 0.25 wt % HF, 36.1 1 wt % H3PO4, 1.04 wt % aminotrimethylene phos phonic acid, and 51.3 wt % water.

[0016] The time and temperature for the treatment of the sapphire substrate with a composition of this invention will generally be for a period of from about 1 to about 120 minutes, preferabl y from about 1 to about 60 minutes, and more preferably from about 10 to about 30 minutes an d at a temperature of from about room temperature to about 80° C, more preferably at a temper ature of from about 20° C to about 45° C, more preferably from about 20° C to about 35° C. Si nee the purpose of the invention is to etch some quantity of the surface material from the sapph ire substrate and since, in acidic solutions, the solubility of aluminum hydroxide and aluminum oxide decreases with increasing temperature it is desirable and preferable to conduct the treatm ent of the sapphire with a composition of this invention at a relatively low temperature. Follow ing treatment of the sapphire substrate with a composition of this invention the treated substrat e is generally annealed in any suitable processing atmosphere, such as in air, hydrogen, ammon ia, nitrogen, oxygen and the like, but preferably in air. Any suitable annealing temperature may be employed, but is generally about 1050° C.

[0017] The following example illustrates the A1 ₂ C<3 etch/dissolution effect of the compositions of this invention compared to compositions not containing all the required components of the compositions of this invention. The experiments for the A1 ₂ 0 ₃ powder etch/dissolution were carried out according to the following protocol. 100 g of each solution was placed into a pre- rinsed Teflon beaker. A pre-rinsed stir bar and a stir plate were used in order to provide agitation. Next, 0.250 g of A1 ₂ 0 ₃ powder was added to the solution which was stirring at 400 rpm. The AI2O3 powder was treated for 2 hours at room temperature. After 2 hours, the solution was filtered and the recovered liquid was analyzed for trace metals (aluminum concentration) by 1CP-MS. All solutions contained less than 10 ppb aluminum prior to the etch/dissolution experiment. The results are set forth in the following table wherein the amounts of the individual components are expressed as parts by weight.

Table

37% 85% 50:1 Added Total

Composition HC1 H3PO4 dHF ATMP Water Mass ppb Al

Comparative No. 1 60 0 0 0 140 200 480

Comparative No. 2 60 0 50 0 90 200 880 Comparative No. 3 60 85 0 0 55 200 490

Comparative No. 4 60 0 0 2 138 200 450

Inventive

Composition 60 85 50 0 5 200 1200

Inventive

Composition with

ATMP 60 85 50 2 3 200 1 1 10

Comparative No. 5 60 85 0 2 53 200 450

Comparative No.6 60. 0 50 2 88 200 930

From the data above it can be determined that an aqueous composition of HCI, H3PO4, and HF will etch A1 ₂ 0 ₃ most effectively (Inventive composition) compared to the comparative composition nos. 1 to 6 without all of these components present in the composition. The addition of AMTP is also desired to behave as a chelator for dissolved Al ³⁺ to prevent redeposition or contamination from such dissolved species .

[0018] In the following Examples 1 and 2, when the treated sapphire substrate was treated, the composition of this invention employed to treat the sapphire substrate was a composition consisting of: 50.8% water 1 1.2% HCI, 36.7% H3PO4, 0.3% HF and 1.0% aminotrimethylene phosphonic acid. Example 1 involved treating the sapphire in the inventive composition for 120 minutes at room temperature followed by an anneal step at 1050 °C in air. Example 2 involved treating the sapphire in the inventive composition for 10 minutes at 35 °C followed by an anneal step at 1050 °C.

[0019] Example 1

In this example a sapphire substrate was treated with the afore-mentioned composition of this invention was subsequently annealed in air at 1050° C. After annealing the surface of the c-plane sapphire was imaged employing atomic force microscopy (AFM). An untreated and annealed c- plane sapphire AFM topographical image is displayed in Fig. 1, and the invention treated and annealed c-plane sapphire surface AFM topographical image is displayed in Fig. 2. The difference between the two figures is seen in both the root mean square height and the topographical features. The treated sapphire is flatter and shows a more regular surface demonstrating the beneficial effect of the disclosed composition and the treatment of sapphire s ubstrates therewith. [0020] Example 2

In this example a CMP processed sapphire substrate was treated with the afore-mentioned composition of this invention was subsequently annealed in air at 1050° C. After annealing the surface of the c-plane sapphire was imaged employing atomic force microscopy (AFM). An untreated and annealed c-plane sapphire AFM image is displayed in Fig. 3, and the invention treated and annealed c-plane sapphire surface AFM image is displayed in Fig. 4. Fig. 3 shows the CMP scratches or cuts on the untreated CMP process sapphire substrate after annealing. Fig. 4 shows that the treated CMP processed sapphire substrate after annealing and demonstrating removal of the CMP scratches or cuts from the c-plane of the sapphire substrate. Also, the difference between the two figures is seen in both the root mean square height and the topographical features. The treated sapphire is flatter and the absence of scratches from the CMP processing demonstrates the composition of this invention's ability to remove an adequate amount of surface material to yield a flat surface without scratches.

[0021] Epitaxial growth of bulk nitride film experiments were carried out on sapphire substrates that were treated with the inventive composition. The results show that the defect density and the surface roughness of the nitride film were reduced when grown on a sapphire substrate that was treated in the inventive composition relative to a non-treated sapphire substrate on which an analogous nitride film was grown. The comparison of the untreated and treated experiment yields a root mean square height of the nitride film surface of 0.85 vs. 0.43 nm for the untreated and treated, respectively, and a defect pit density in the nitride film of 3 x 10 ⁸ cm ^"2 vs. 1.6 x 10 ⁸ cm ^"2 for the untreated and treated, respectively. The reduction in both the film surface roughness and defect pit density is expected to yield higher quality LEDs.

[0022] While the invention has been described herein with reference to the specific embodiments thereof, it will be appreciated that changes, modification and variations can be made without departing from the spirit and scope of the inventive concept disclosed herein. Accordingly, it is intended to embrace all such changes, modification and variations that fall with the spirit and scope of the appended claims.