Cleaning Method and Laminate of Aluminum Nitride Single-Crystal Substrate

Abstract

A method for effectively removing minute impurities of 1 μm or less in size that are present on a surface of an aluminum nitride single-crystal substrate without etching the surface includes scrubbing a surface of an aluminum nitride single-crystal substrate using a polymer compound material having lower hardness than an aluminum nitride single crystal, and an alkali aqueous solution having 0.01-1 mass % concentration of potassium hydroxide or sodium hydroxide, the alkali aqueous solution being absorbed in the polymer compound material.

Claims

1. A cleaning method of an aluminum nitride single crystal substrate, comprising: a scrub cleaning step wherein a surface of the aluminum nitride single crystal substrate is scrubbed by moving a polymer compound material in a parallel direction of the surface of said substrate while said polymer compound material contacts the surface of said substrate, and wherein said polymer compound material has a lower hardness than the aluminum nitride single crystal and absorbed with an alkaline aqueous solution having concentration of 0.01 to 1 wt % of an alkaline which is selected from the group consisting of potassium hydroxide and sodium hydroxide.

2. The method as set forth in claim 1, wherein said alkaline aqueous solution includes citric acid in said scrub cleaning step.

3. The method as set forth in claim 1, wherein said polymer compound material in said scrub cleaning step comprises a melamine foam resin, a porous polyvinyl alcohol resin, a fibrous polyester resin, or a nylon resin.

4. The method as set forth in claim 1, wherein a surface of said aluminum nitride single crystal substrate is polished by a chemical mechanical polishing method using colloidal silica.

5. A production method of a laminated body, comprising: removing an oxide film of a surface of the substrate by immersing the aluminum nitride single crystal substrate cleaned by the cleaning method as set forth in claim 1 into a mixture solution of phosphoric acid and sulfuric acid, and depositing an aluminum gallium nitride layer Al.sub.xGa.sub.1-xN, 0≦x≦1 by MOCVD method on the substrate obtained from said oxide film removing.

6. The production method of the laminated body as set forth in claim 5, wherein a film thickness of the aluminum gallium nitride layer Al.sub.xGa.sub.1-xN, 0≦x≦1 on the aluminum nitride single crystal substrate is thicker than 1 μm.

7. An aluminum nitride single crystal substrate, wherein: a number of foreign matters larger than 1 μm per 400 μm.sup.2 of a surface of the substrate is less than 1, a number of the foreign matters of 1 μm or less is less than 1, and a root mean square roughness per 4 μm.sup.2 of the surface of the substrate is 0.06 to 0.30 nm.

8. A laminated body comprising the aluminum gallium nitride layer Al.sub.xGa.sub.1-xN, 0≦x≦1 on the substrate as set forth in claim 7.

9. The laminated body as set forth in claim 8, wherein the film thickness of the aluminum gallium nitride layer Al.sub.xGa.sub.1-xN, 0≦x≦1 is thicker than 1 μm.

10. A polymer compound material used for cleaning an aluminum nitride single crystal substrate comprising a polymer compound material absorbed with an alkaline aqueous solution having concentration of 0.01 to 1 wt % of an alkaline which is selected from the group consisting of potassium hydroxide and sodium hydroxide.

11. A cleaning method of an aluminum nitride single crystal substrate, comprising: scrubbing a surface of the aluminum nitride single crystal substrate by moving a polymer compound material in a parallel direction of the surface of said substrate while said polymer compound material contacts the surface of said substrate; and cleaning said substrate, wherein said polymer compound material has a lower hardness than the aluminum nitride single crystal and is absorbed with an alkaline aqueous solution having concentration of 0.01 to 1 wt % of potassium hydroxide or sodium hydroxide.

Description

EXAMPLE

[0036] Hereinafter, the present invention will be described based on the examples; however the present invention is not to be limited thereto. Note that, in below, % refers to wt % unless mentioned otherwise.

[0037] The polymer compound material and ultrasonic cleaning apparatus used for the below examples and comparative examples will be described.

Polymer Compound Material

[0038] Melamine foam resin: made by LEC, Inc (a porous material, the water retention rate of about 2900%)

[0039] Porous polyvinyl alcohol resin: AION Clean Room Sponge D-3 made by AION Co., Ltd. (a porous material, the water retention rate of about 650%).

[0040] Mixture of fibrous polyester resin and nylon resin: Savina MX made by KB SEIREN, LTD (a fibrous material, the water retention rate of about 200%).

Measurement of the Root Mean Square Roughness (RMS) of the Substrate Surface

[0041] The root mean square roughness was calculated by scanning 512 points×512 points of the visual field of 4 μm.sup.2 (2 μm×2 μm) using an atomic force microscope (AFM). Note that, in the below examples and comparative examples, the measurement of the root mean square roughness (RMS) was carried out to arbitrary one range.

Measurement of the Number of the Foreign Matters on the Substrate Surface

[0042] The number of foreign matters which can be identified was measured by scanning 256 points×256 points of the visual field of 400 μm.sup.2 (20 μm×20 μm) using an atomic force microscope (AFM). Due to the relation between the visual field and the points, the foreign matters of about 80 nm or less cannot be measured. Note that, in the below examples and comparative examples, the measurement of number of foreign matters was carried out to arbitrary one range.

Production Example 1

[0043] The substrate used in the below examples and the comparative examples was the C plane aluminum nitride single crystal substrate of which the crystal growth was carried out by HVPE method, and the Al polarity plane side was processed to have ultraflat surface by CMP method using colloidal silica abrasive (COMPOL80 made by Fujimi Incorporated). After polishing, it was rinsed off by flowing water using pure water (flow amount: 1.8 L/min) for 5 minutes, then 1% hydrofluoric acid aqueous solution was added to Teflon (registered trademark) beaker, then the substrate was placed in, and immersed for 10 minutes. The obtained substrate was rinsed off by flowing water using pure water (flow amount: 1.8 L/min) for 1 minute, then immersed in isopropanol for 1 minute, and spin dried at 6000 rpm for 30 seconds. The size of the obtained substrate had an outer diameter of 20 mm, and the thickness of 600 μm. As a result of analysis of the substrate surface by an atomic force microscope (AFM), the root mean square roughness

[0044] (RMS) per visual field of 4 μm.sup.2 was 0.15 nm or less, and the number of the foreign matters in the visual field of 400 μm.sup.2 was 20 or more. In the below examples and comparative examples, the substrate obtained as such was used and the examination was carried out.

Example 1

[0045] The aqueous solution as the alkaline aqueous solution including 0.1% of citric acid and 0.1% of potassium hydroxide was prepared. The melamine foam resin cut into a shape of 30 mm×30 mm square was immersed in the alkaline aqueous solution to absorb, then while contacting the substrate surface it was scrubbed by moving for 60 times in a direction parallel with the substrate. Note that, per every 15 movements, the melamine foam resin was immersed in the alkaline aqueous solution for absorption. The obtained substrate was rinsed off by flowing water using pure water (ultrapure water having the electric resistivity of 18 MΩ.Math.cm or more, same applies hereinbelow) for 1 minute, then immersed in isopropanol for 1 minute, then spin dried at 6000 rpm for 30 seconds. As a result of the analysis of the surface of the obtained substrate by an atomic force microscope (AFM), the number of the foreign matters in the visual field of 400 μm.sup.2 was 0, and the root mean square roughness (RMS) per visual field of 4 μm.sup.2 was 0.11 nm.

Examples 2 to 12

[0046] The procedure as same as the example 1 was carried out except for changing the alkaline aqueous solution and the polymer compound material as shown in Table 1. The result is shown in Table 1.

Example 13

[0047] The procedure as same as the example 1 was carried out except for using, as the alkaline aqueous solution, 20 fold dilution of SUNWASH MD-3041 made by Lion Specialty Chemicals Co., Ltd using pure water. The result is shown in Table 1. Note that, the stock solution of MD-3041 had the sodium hydroxide concentration of 1 to 5%, and the citric acid concentration of about 3%. Therefore, 20 fold dilution of the alkaline aqueous solution has the sodium hydroxide concentration of 0.05 to 0.25%, and the citric acid concentration of about 0.15%.

Examples 14 and 15

[0048] The procedure as same as the example 13 was carried out except for changing the polymer compound material to that shown in Table 1.

Comparative Examples 1 to 3

[0049] The procedure as same as the example 1 was carried out except for changing the alkaline aqueous solution to pure water, and changing the polymer compound material as shown in Table 1. The results are shown in Table 1.

Comparative Example 4

[0050] The cleaning solution wherein a 20 fold dilution of SUNWASH MD-3041 (made by Lion Specialty Chemicals Co., Ltd) using pure water was added to a quartz beaker then heated to 50° C., then the substrate was placed in, followed by applying an ultrasonic wave having the frequency of 100 kHz for 10 minutes. Then, the obtained substrate was rinsed off by flowing water (the flow amount: 1.8 L/min) using pure water for 1 minute, and immersed in isopropanol for 1 minute, and spin dried at 6000 rpm for 30 seconds. The result is shown in Table 1.

Comparative Example 5

[0051] The procedure as same as the comparative example 4 was carried out except for changing the frequency of the ultrasonic wave to 45 kHz. The result is shown in Table 1.

Comparative Example 6 (DHF Cleaning)

[0052] 1% hydrofluoric acid aqueous solution was added to Teflon (registered trademark) beaker, then the substrate was placed in, and immersed for 10 minutes. The obtained substrate was rinsed off by flowing water (flow amount: 1.8 L/min) using pure water for 1 minute, then immersed in isopropanol for 1 minute, and spin dried at 6000 rpm for 30 seconds. The result is shown in Table 1.

Comparative Example 7 (SPM Cleaning)

[0053] 1% hydrofluoric acid aqueous solution was added to Teflon (registered trademark) beaker, then the substrate was placed in, and immersed for 10 minutes. Then, the substrate was taken out, and rinsed off by flowing water using pure water (flow amount: 1.8 L/min) for 1 minute. The mixture solution of 96% sulfuric acid and hydrogen peroxide (96% sulfuric acid:hydrogen peroxide=3:1 (volume ratio)) was added to Teflon (registered trademark) beaker and heated to 120° C., then the substrate which was rinsed off by a flowing water was introduced, and immersed for 10 minutes. The obtained substrate was rinsed off by flowing water using pure water (flow amount: 1.8 L/min) for 1 minute, then immersed in isopropanol for 1 minute, and spin dried at 6000 rpm for 30 seconds. The result is shown in Table 1.

Comparative Example 8

[0054] The procedure as same as the example 1 was carried out except for changing the alkaline aqueous solution and the polymer compound material as shown in Table 1. As a result of the analysis of the surface of the obtained substrate by an atomic force microscope (AFM), the number of the foreign matters in the visual field of 400 μm.sup.2 was 0, and the root mean square roughness (RMS) per visual field of 4 μm.sup.2 was 0.70 nm.

TABLE-US-00001 TABLE 1 Number of foreign Alkaline aqueous solution matters *1 Concentration of Concentration larger than 1 μm or Type of alkaline alkaine of citric acid Polymer compound material 1 μm less Example 1 pottasium 0.1% 0.1% Melamine foam resin 0 0 hydroxide Example 2 pottasium 0.3% 0.3% Melamine foam resin 0 0 hydroxide Example 3 pottasium 1.0% 1.0% Melamine foam resin 0 0 hydroxide Example 4 pottasium 0.1% 0.1% Porous polyvinyl alcohol 0 0 hydroxide resin Example 5 pottasium 0.3% 0.3% Porous polyvinyl alcohol 0 0 hydroxide resin Example 6 pottasium 1.0% 1.0% Porous polyvinyl alcohol 0 0 hydroxide resin Example 7 pottasium 0.1% 0.1% Mixture of fibrous polyester 0 0 hydroxide resin and nylon resin Example 8 pottasium 0.3% 0.3% Mixture of fibrous polyester 0 0 hydroxide resin and nylon resin Example 9 pottasium 1.0% 1.0% Mixture of fibrous polyester 0 0 hydroxide resin and nylon resin Example 10 pottasium 0.3% — Melamine foam resin 0 3 hydroxide Example 11 pottasium 0.3% — Porous polyvinyl alcohol 0 8 hydroxide resin Example 12 pottasium 0.3% — Mixture of fibrous polyester 0 5 hydroxide resin and nylon resin Example 13 *2 Melamine foam resin 0 0 Example 14 *2 Porous polyvinyl alcohol 0 0 resin Example 15 *2 Mixture of fibrous polyester 0 0 resin and nylon resin Comparative example 1 *3 Melamine foam resin 0 14 Comparative example 2 *3 Porous polyvinyl alcohol 0 32 resin Comparative example 3 *3 Mixture of fibrous polyester 0 20 resin and nylon resin Comparative example 4 *2 *4 0 19 Comparative example 5 *2 *4 0 25 Comparative example 6 *5 — 0 18 Comparative example 7 *6 — 0 21 Comparative example 8 pottasium 2.0% 0.1% Melamine foam resin 0 0 hydroxide *1 The number per visual field of 400 μm.sup.2 by atomic force microscope analysis is shown *2 20 fold dilution of SUNWASH MD-3041 using pure water was used as alkaline aqueous solution *3 Pure water was used instead of alkaline aqueous solution *4 Ultrasonic wave cleaning was carried out instead of scrub cleaning *5 1% hydrofluoric acid aqueous solution was used instead of alkaline aqueous solution *6 1% hydrofluoric acid aqueous solution and the mixture solution of 96% sulfuric acid and hydrogen peroxide was used insead of aline aqueous solution

Example 16

Oxide Film Removing Step

[0055] The mixture solution of phosphoric acid and sulfuric acid (phosphoric acid:sulfuric acid=1:3 (volume ratio)) was added to Teflon (registered trademark) beaker and heated to 90° C., then the substrate obtained in example 2 was introduced and immersed for 10 minutes. The obtained substrate was rinsed off by flowing water using pure water (flow amount: 1.8 L/min) for 1 minute, then immersed in isopropanol for 1 minute, and spin dried at 6000 rpm for 30 seconds. The result is shown in Table 1.

Depositing Step

[0056] The substrate was set on a susceptor of MOCVD reactor so that the polished face of the substrate was the outer most face, then while the mixed gas of hydrogen gas and nitrogen gas was being introduced; the pressure inside the reactor was reduced to 100 mbar in 3 minutes. The mixing ratio of the mixed gas under the standard condition was 76 vol % of hydrogen gas and 24 vol % of nitrogen gas; and the total flow amount of the mixed gas was 8.5 slm. After the pressure is reduced, the temperature inside the reactor was increased to 1210° C. in 8 minutes, and maintained at 1210° C. for 10 minutes, and then the temperature was changed to 1070° C. and the pressure to 50 mbar in 1 minute. Then, the nitrogen gas was blocked in 1 minute so that the hydrogen gas was the only gas flowing inside the reactor. Then, 80 sccm of trimethyl aluminum, 5 sccm of trimethyl gallium and 1.2 sccm of tetraethyl silane, and 1500 sccm of ammonia were introduced to the reactor, then the n-type Al.sub.0.7Ga.sub.0.3N was grown at the pressure for 50 mbar and the reactor temperature of 1070° C. By going through the above mentioned steps, the laminated body was obtained wherein the n-type Al.sub.0.7Ga.sub.0.3N layer (Si concentration of 1.0×10.sup.19 cm.sup.−3 or so) having the film thickness of 1.2 μm was provided on the aluminum nitride single crystal substrate.

[0057] Using the differential interference contrast microscope LV150 made by Nikon Corporation, from the randomly selected 5 points on the n-type Al.sub.0.7Ga.sub.0.3N layer surface, the number of micropipes observed within the range of 640 μm×480 μμm were counted, and the average value thereof was divided by the area of the observed range, and the obtained value was defined as the micropipe density. In the present step, the micropipe density of the n-type Al.sub.0.7Ga.sub.0.3N layer of the obtained laminated body was 0 per mm.sup.2.

Comparative Example 9

[0058] For the obtained substrate according to the comparative example 7, the oxide film removing step and the depositing step as same as the example 16 were carried out. In the present step, the micropipe density of the n-type Al.sub.0.7Ga.sub.0.3 layer of the obtained laminated body was 248 per mm.sup.2.