Method for polishing a silicon carbide surface

Abstract

A method for polishing a silicon carbide surface. The silicon carbide surface is polished with abrasive particles while being exposed to a composition of water, an oxidant, and an electrophilic agent. This method provides a material removal rate (MRR) that is no less than or better than that of conventional methods without the use of caustic chemicals.

Description

Technical Field

[0001] This invention relates to a polishing method, and more particularly to a method for polishing silicon carbide surfaces. Prior Technology

[0002] The fabrication of hard, wide-bandgap materials is limited by the rate of silicon carbide (SiC) removal. To increase the removal rate, a soft but dense oxide layer must be formed on the surface of the silicon carbide material. This layer is removed by mechanical polishing (through the hard abrasive and roughness of the polishing pad surface). Such mechanical polishing processes often use very high operating pressures and sliding speeds. The formation of this oxide layer is generally achieved by adding a strong oxidizing agent, potassium permanganate (KMnO4), which weakens the surface forces on the silicon carbide substrate.

[0003] Due to the strong and corrosive properties of permanganates, and the fact that polishing equipment often suffers severe discoloration after prolonged exposure to permanganates, the industry strongly desires to activate silicon carbide surfaces under less corrosive conditions. This would also increase the removal rate and reduce the number of defects at the final substrate level. To date, a completely satisfactory solution has not yet been found.

[0004] The above discussion is provided for general background information only and is not intended to assist in determining the scope of the claimed subject matter. Summary of the Invention

[0005] Cross-referencing of related applications This application is a non-provisional application of U.S. Patent Application No. 63 / 188,305 (filed May 13, 2021) and claims priority thereto, the entire contents of which are incorporated herein by reference.

[0006] A method for polishing silicon carbide surfaces is provided. The silicon carbide surface is polished with abrasive particles while being exposed to a composition of water, an oxidant, and an electrophilic agent. This method provides a material removal rate (MRR) that is no less than or better than that of conventional methods without the use of caustic chemicals. One advantage that can be achieved when implementing some of the disclosed embodiments of this method is that it provides a material removal rate that is no less than (or better than) that of the conventional KMnO4 method, but without the use of such strong, discoloring, and corrosive agents. The residual material is also more environmentally friendly.

[0007] In a first embodiment, a method for polishing a silicon carbide surface is provided. The method includes: exposing the silicon carbide surface to a composition comprising (1) water, (2) an oxidant, (3) an electrophilic agent, and (4) abrasive particles; and polishing the silicon carbide surface while exposing it to the composition.

[0008] In a second embodiment, a method for polishing a silicon carbide surface is provided. The method comprises: exposing a silicon carbide surface to a composition comprising (1) water, (2) an oxidant, (3) a metal ion electrophile with a binding ligand, and (4) abrasive particles; and polishing the silicon carbide surface while exposing it to the composition.

[0009] This brief description of the invention is intended only to provide a brief overview of the subject matter disclosed herein according to one or more illustrative embodiments, and is not intended to guide the interpretation of the claims or to define or limit the scope of the invention, which is defined only by the appended claims. This brief description is provided to introduce illustrative selected concepts in a simplified form, which are further described in the following embodiments. This brief description is not intended to identify key or essential features of the claimed subject matter, nor is it intended to assist in determining the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that address any or all of the disadvantages pointed out in the background. Simple Explanation of the Diagram

[0010] Therefore, embodiments of the invention can be understood by referring to certain examples, some of which are illustrated in the accompanying drawings, in a manner that enables the understanding of the features of the invention. However, it should be noted that the drawings illustrate only certain embodiments of the invention and should not be construed as limiting its scope, which encompasses other equally effective embodiments. The drawings are not necessarily drawn to scale and generally emphasize features of certain embodiments of the invention. In the drawings, similar numerals are used to indicate similar components in various views. Therefore, for a further understanding of the invention, reference can be made to the following embodiments, read in conjunction with the drawings, wherein: [Figure 1] is an illustration of silicon carbide surface activation using the disclosed method; [Figure 2] illustrates examples of nonmetallic electrophilic agents used in conjunction with the disclosed method; [Figure 3] Examples of various ligands used in conjunction with metal ion electrophiles; [Figure 4] is a graph depicting the material removal rate (MRR) of two nonmetallic electrophilic agents used in conjunction with the disclosed method. The MRR of conventional iron KMnO4 is shown in gray; [Figure 5] is a graph depicting the material removal rate (MRR) of the copper electrophilic agent used in conjunction with the disclosed method. The MRR of conventional iron KMnO4 is shown in gray; and [Figure 6] is a graph depicting the material removal rate (MRR) of vanadium metal electrophiles used in combination with two different ligands. The MRR of conventional iron KMnO4 is shown in gray. Implementation

[0011] This invention provides a slurry formulation for silicon carbide polishing with adjustable performance, which, with the addition of certain additives, improves the removal rate in a less aggressive physicochemical environment. More specifically, this invention provides formulations and systems that enhance the development of silicon carbide polishing processes and chemical mechanical polishing (CMP) processes (especially including planarization).

[0012] Generally, the formulation comprises (1) water, (2) a water-soluble electrophile (E+), such as a metal ion or non-metal electrophile chelated with the formulation group via organometallic complex coordination group exchange (OMC-LE), (3) an oxidizing agent (Ox), and (4) abrasive particles. The abrasive particles (such as alumina) are used at a pH above the isoelectric point (e.g., >2, such as pH 4 to 5 or pH 8 to 9) while applying a mechanical polishing force (e.g., between 3 psi and 7 psi (0.21 bar to 0.48 bar), applied by a rotating pad or brush). The abrasive is typically present at a concentration of about 2% to about 5% (wt / wt) and is insoluble in water. In one embodiment, the abrasive is alumina nanoparticles having an average diameter of less than 100 nm. In one embodiment, the formulation consists of a water-soluble electrophile, an oxidizing agent, and water. In another embodiment, the abrasive is nanoparticles of silicon dioxide, zirconium oxide, titanium dioxide, diamond, or a metal oxide. Polishing methods are typically performed at room temperature (e.g., between 20°C and 25°C).

[0013] Without being bound by any particular theory, Figure 1 depicts a possible mechanism to aid in understanding the revealed method. Block A of Figure 1 shows the oxide surface of a silicon carbide substrate, misaligned with a water-soluble electrophile (E+). The surface hydroxyl groups within the misaligned layer at the additive-surface interface are deprotonated and highly anionic (i.e., they act as nucleophiles), leading to the weakening of the Si-C bonds. Block B shows the activated surface oxidized with an oxidizing agent (Ox). In Block C, an abrasive (e.g., alumina) releases the uppermost silicon layer to produce the polished surface shown in Block D.

[0014] Examples of suitable oxidants include hydrogen peroxide (H₂O₂), permanganates (e.g., KMnO₄ (KPS)), and persulfates (such as ammonium persulfate (APS, (NH₄)₂S₂O₈)). Without being bound by any particular theory, these oxidants are believed to generate hydroxyl groups in situ. Oxidants are typically present at concentrations between 1% and 10% by weight. In one embodiment, the oxidant is present at a concentration between 1% and 5% by weight. In yet another embodiment, the oxidant is present at a concentration between 2% and 4% by weight.

[0015] Referring to Figure 2, in one embodiment, the water-soluble electrophile is a nonmetallic electrophile, such as an aldehyde, a chlorohydrin (or a carboxylic acid derived therefrom), or a boron-based compound. Examples of boron-based compounds include borates, such as ethylboronic acid, cyclopentylboronic acid, isopropylboronic acid, cyclohexylboronic acid, cyclopentylboronic acid, p-tolylboronic acid, phenylboronic acid, borax, or boric acid. Other suitable boron-based compounds include boroglycine or borate esters, such as trimethyl borate or triethyl borate. Due to the electron-deficient nature of these supramolecular molecules, enhanced nucleophilic etching of the silicon carbide substrate can occur.

[0016] In the case of OMC-LE, the M+ center of the organometallic complex acts as an electrophile, nucleophilically etching the silicon carbide substrate. Furthermore, the organometallic complex promotes the in-situ formation of hydroxyl groups from the auto-oxidant.

[0017] Examples of suitable metal ions include Group II metals (such as Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) and divalent transition metals (such as Cu²⁺, Zn²⁺). Further examples of suitable metal ions include Fe³⁺, Co³⁺, Ti⁴⁺, V⁴⁺, V⁵⁺, Cr⁶⁺, Mo⁶⁺, and Mn⁷⁺. Generally, the metal is water-soluble, or its water solubility is achieved through incorporation with ligands or microcells. The metal is typically present at a concentration between 0.005 wt% and 0.05 wt%. In another embodiment, the metal is present at a concentration between 0.005 wt% and 0.015 wt%.

[0018] Referring to Figure 3, examples of suitable ligands include amino acids (e.g., glycine, serine, arginine, cystine, phenylalanine, etc.), monoprotic carboxylic acids (e.g., formic acid, acetic acid, glycolic acid, propionic acid, butyric acid, hexanoic acid, heptanoic acid, octanoic acid, decanoic acid, pyruvic acid, tricinnamic acid, etc.), diprotic carboxylic acids (e.g., carbonic acid, itconic acid, malonic acid, tartaric acid, etc.), carbamates (e.g., N-(benzyloxy)carbamate tributyl ester), hydroxamic acids and hydroxamic esters (e.g., suberohydroxamic acid, salicylic acid, ethyl acetohydroxamic acid), hydroxyurea, and aliphatic amides (e.g., butyramine). Generally, the ligands are water-soluble. The ligands are present in a weight ratio of approximately 1:5 to approximately 1:20 (metal:ligand). In another embodiment, the ligand system is present at a weight ratio of about 1:8 to about 1:12 (metal:ligand). In yet another embodiment, the ligand system is present at a weight ratio of 1:10 (metal:ligand).

[0019] The revealed composition is comparable to conventional iron KMnO4 polishing techniques and, in some cases, exhibits excellent material removal rate (MRR). Figures 4 to 6 depict typical MMR of conventional KMnO4, shown in gray.

[0020] Figure 4 depicts the control group (see Example 1), the boric acid example (see Example 11), and the borax example (see Example 12). Under the same conditions, both the Cu-boric acid and Cu-borax examples showed better MRR than the conventional KMnO4 (shown in gray).

[0021] Figure 5 depicts the control group (see Example 1), and three examples of copper-coordination groups (including glycine ligand (Example 2), serine ligand (Example 3), cystine (Example 5)) and salicylic acid (Example 6). Under the same conditions, Cu-serine is superior to the conventional KMnO4 MRR (shown in gray). Cu-glycine, Cu-cystine, and Cu-salicylic acid are comparable to the conventional KMnO4 technique but avoid the disadvantages associated with these conventional reagents.

[0022] Figure 6 depicts the control group (see Example 1) and two vanadium-coordination group examples (including serine coordination group (Example 7) and tartaric acid coordination group (Example 8)). The V-serine example is comparable to the conventional KMnO4 technique but avoids the drawbacks associated with these conventional reagents. Under the same conditions, the V-tartaric acid example exhibits superior MRR (shown in gray) compared to the conventional KMnO4, with a material removal rate (MRR) nearly twice that of the conventional KMnO4.

[0023] Detailed Examples All polishing tests were performed on 100 mm diameter and 350 mm thick 4H-SiC N-type wafers on an Allied METPREP™ polisher, followed by repolishing of the wafers. A DuPont SUBA® 800-II-12 XY grooved pad was used on a 200-mm rotary platform. During polishing, a 3 M (PB33A-1) bristle brush conditioning pad was used in in-situ conditioning mode, followed by 1 minute of conditioning in an out-of-situ pad after polishing. As described in the examples, the Si surface of the wafer was polished for 10 minutes using a slurry composed of α-Al₂O₃ nanoparticles (NP), water, hydrogen peroxide, and various additives (organometallic complexes or electrophilic additives). The program pressure ranged between 3 and 7 PSI. The relative slide speed ranged between 0.25 and 1.05 m / s. The slurry flow rate was kept constant at 25 cc / min. Examples are summarized in Table 1.

[0024] Table 1 Table 1 Example H2O2 (wt%) Al2O3 (wt%) E+ (wt%) E+ Coordinating group (wt%) Coordinating group 1 3 3 0 --- 0 --- 2 3 3 0.01 Cu2+ 0.1 Glycine 3 3 5 0.01 Cu2+ 0.1 serine 4 3 3 0.01 Cu2+ 0.1 serine 5 3 3 0.01 Cu2+ 0.1 Cystine 6 3 3 0.01 Cu2+ 0.1 Salicylic acid 7 3 3 0.01 V4+ 0.1 serine 8 3 3 0.01 V4+ 0.1 tartaric acid 9 3 3 0.02 V4+ 0.1 tartaric acid 10 3 3 0.005 V4+ 0.1 tartaric acid 11 3 3 1.0 Boric acid --- --- 12 3 3 1.0 Borax --- --- 13 5% ammonium persulfate 3 0.01 Cu2+ 0.1 serine

[0025] Example 1 - Control Group (No Electrophilic Agent) The experimental silicon carbide (SiC) slurry contained 3% α-Al₂O₃ nanoparticles (NP), water, and 3% hydrogen peroxide. A DuPont SUBA® 800-II-12 XY grooved pad was used on a 200-mm rotary platform. During polishing, a 3 M (PB33A-1) bristle brush conditioning pad was used in in-situ conditioning mode, followed by 1 minute of conditioning in an out-of-situ pad after polishing. The silicon surface of a 100 mm diameter and 350 µm thick 4H-SiC N-type wafer was polished. The program pressure ranged between 3 and 7 PSI. The slide speed was kept constant at 1.05 m / s. The slurry flow rate was kept constant at 25 cc / min.

[0026] The observed SiC removal rates ranged from 348 to 532 nm / h. At 3 PSI, the average SiC removal rate was 348 nm / h. A control at 7 PSI showed an average removal rate of 532 nm / h, representing a 35% increase compared to the lower downpressure.

[0027] Example 2 - Cu2+-glycine Example 2 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and Cu2+-glycine (0.01% metal, 0.1% ligand).

[0028] After polishing, and depending on the process conditions, the observed SiC removal rates ranged from 936 to 1,198 nm / h. At 3 PSI, the average SiC removal rate was 936 nm / h. A control at 7 PSI showed an average removal rate of 1,198 nm / h, representing a 22% increase compared to the lower downpressure. See Figure 5.

[0029] Example 3 - Cu2+-serine and 5% aluminum oxide Example 3 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 5% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and Cu2+-serine (0.01% metal, 0.1% ligand).

[0030] After polishing and depending on the process conditions, the observed SiC removal rate was 1563 nm / h at 7 PSI.

[0031] Example 4 - Cu2+-serine and 3% aluminum oxide Example 4 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and Cu2+-serine (0.01% metal, 0.1% ligand).

[0032] After polishing and depending on the process conditions, the observed SiC removal rates ranged from 1,371 to 1,709 nm / h. At 3 PSI, the average SiC removal rate was 1,371 nm / h. A control at 7 PSI showed an average removal rate of 1,709 nm / h, representing a 20% increase compared to the lower downpressure. See Figure 5.

[0033] Example 5 - Cu2+-cystamine Example 5 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and Cu2+-cystine (0.01% metal, 0.1% ligand).

[0034] After polishing and depending on the process conditions, the observed SiC removal rate was 883 nm / hr at 7 PSI, a sliding speed of 1.05, and a flow rate of 25 cc / min. See Figure 5.

[0035] Example 6 - Cu2+-Syringylhydroxamic acid Example 6 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and Cu2+-salicylic acid (0.01% metal, 0.1% ligand).

[0036] After polishing and depending on the process conditions, the observed SiC removal rate was 753 nm / hr at 7 PSI, a sliding speed of 1.05, and a flow rate of 25 cc / min. See Figure 5.

[0037] Example 7 - V4+-serine Example 7 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and V4+-serine (0.01% metal, 0.1% ligand).

[0038] After polishing and depending on the process conditions, the observed SiC removal rates ranged from 926 to 1,132 nm / h. At 3 PSI, the average SiC removal rate was 926 nm / h. A control at 7 PSI showed an average removal rate of 1,132 nm / h, representing an 18% increase compared to the lower downpressure. See Figure 6.

[0039] Example 8 - V4+-tartaric acid Example 8 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and V4+-tartaric acid (0.01% metal, 0.1% ligand).

[0040] After polishing and depending on the process conditions, the observed SiC removal rates ranged from 1,837 to 2,152 nm / h. At 3 PSI, the average SiC removal rate was 1,837 nm / h. A control at 7 PSI showed an average removal rate of 2,152 nm / h, representing a 15% increase compared to the lower downpressure. See Figure 6.

[0041] Example 9 - V4+-tartaric acid Example 9 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and V4+-tartaric acid (0.02% metal, 0.1% ligand).

[0042] After polishing and depending on the process conditions, the observed SiC removal rate was 999 nm / hr at 7 PSI, 1.05 slide speed, and 25 cc / min flow rate.

[0043] Example 10 - V4+-tartaric acid Example 10 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3% α-Al2O3 nanoparticles (NP), water, 3% hydrogen peroxide, and V4+-tartaric acid (0.05% metal, 0.1% ligand).

[0044] After polishing and depending on the process conditions, the observed SiC removal rate was 1442 nm / hr at 7 PSI, 1.05 slide speed, and 25 cc / min flow rate.

[0045] Example 11 - Boric acid Example 11 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3.0% α-Al₂O₃ nanoparticles (NP), water, 3.0% hydrogen peroxide, and 1.0% boric acid. See Figure 4.

[0046] After polishing and depending on the process conditions, the observed SiC removal rates ranged from 1,427 to 1,904 nm / h. At 3 PSI, the average SiC removal rate was 1,427 nm / h. A control at 7 PSI showed an average removal rate of 1,904 nm / h, representing a 25% increase compared to the lower downpressure.

[0047] Example 12 - Borax Example 12 is essentially the same as Example 1, except that the silicon carbide (SiC) slurry contains 3.0% α-Al2O3 nanoparticles (NP), water, 3.0% hydrogen peroxide, and 1.0% borax.

[0048] After polishing and depending on the process conditions, the observed SiC removal rate was 2446 nm / hr at 7 PSI, a sliding speed of 1.05, and a flow rate of 25 cc / min. See Figure 4.

[0049] Example 13 – Ammonium persulfate Example 13 is essentially the same as Example 1, except that 5% ammonium persulfate is used instead of H2O2 and the pH is 4.0. The slurry contains 3.0% α-Al2O3 nanoparticles (NP), water, the aforementioned 5% (m / m) ammonium persulfate, and Cu2+-serine (0.01% metal 0.1% ligand).

[0050] After polishing and depending on the process conditions, the observed SiC removal rates ranged from 1,162 to 1,408 nm / h. At 3 PSI, the average SiC removal rate was 1,162 nm / h. A control at 7 PSI showed an average removal rate of 1,408 nm / h.

[0051] This written description uses examples to disclose the invention, including the best mode, and also enables those skilled in the art to practice the invention, including making and using any apparatus or system and performing any combined methods. The patentable scope of the invention is defined by the claims and may include other examples conceived by those skilled in the art. Such other examples are intended to be included within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that are not significantly different from the literal language of the claims.

[0052] none

Claims

1. A method for polishing a silicon carbide surface, the method comprising: exposing the silicon carbide surface to a composition having a pH of 2-5, the composition comprising (1) water; (2) an oxidant selected from the group consisting of hydrogen peroxide, permanganate and persulfate; (3) a metal ion electrophile with a ligand bonded to a ligand, the metal ion electrophile being present at a concentration between 0.005 wt% and 0.05 wt%, and the ligand being present at a weight ratio of metal to ligand of about 1:5 to 1:20; and (4) a particulate abrasive selected from the group consisting of alumina, silicon dioxide, zirconium oxide, titanium dioxide, diamond and metal oxides; and polishing the silicon carbide surface while exposing the silicon carbide surface to the composition.

2. The method of claim 1, wherein the oxidant is hydrogen peroxide.

3. The method of claim 1, wherein the electrophilic agent of the metal ion is selected from the group consisting of divalent metal ions: Mg2+, Ca2+, Sr2+, Ba2+, Cu2+, Zn2+.

4. The method of claim 1, wherein the electrophilic agent of the metal ion is Cu2+.

5. The method of claim 1, wherein the metal ion electrophile is selected from the group consisting of: Fe3+, Co3+, Ti4+, V4+, V5+, Cr6+, Mo6+, and Mn7+.

6. The method of claim 1, wherein the metal ion electrophile is selected from the group consisting of: Co3+, Ti4+, V4+, V5+, Cr6+, Mo6+, and Mn7+.

7. The method of claim 1, wherein the electrophilic agent of the metal ion is V4+ or V5+.

8. The method of claim 1, wherein the ligand is an amino acid.

9. The method of claim 1, wherein the ligand system is selected from the group consisting of amino acids: glycine, serine, arginine, cystine, and phenylalanine.

10. The method of claim 1, wherein the ligand is a monoprotic carboxylic acid.

11. The method of claim 1, wherein the ligand system is selected from the group consisting of monoprotic carboxylic acids: formic acid, acetic acid, glycolic acid, propionic acid, butyric acid, hexanoic acid, heptanoic acid, octanoic acid, decanoic acid, pyruvic acid, and tertiary cinnamic acid.

12. The method of claim 1, wherein the ligand is a diprotic carboxylic acid.

13. The method of claim 1, wherein the ligand system is selected from the group consisting of diprotic carboxylic acids: carbonic acid, icosinic acid, malonic acid, and tartaric acid.

14. The method of claim 1, wherein the ligand is tartaric acid.

15. The method of claim 1, wherein the ligand is a carbamate.

16. The method of claim 1, wherein the ligand is a hydroxamic acid or hydroxamic ester.

17. The method of claim 1, wherein the ligand system is selected from the group consisting of hydroxamic acids or hydroxamic esters of: octyl hydroxamic acid, salicylic acid, and ethyl acetohydroxamic acid.

18. The method of claim 1, wherein the ligand is an aliphatic amide.

19. The method of claim 1, wherein the ligand is a hydroxyurea.

20. The method of claim 1, wherein the composition comprises the water, the oxidant, the metal ion electrophile that binds the ligand, and the abrasive particles.

21. The method of claim 1, wherein the oxidant is hydrogen peroxide, the electrophilic metal ion is V4+, and the coordinating group is tartaric acid.

Images