COMPOSITION AND METHOD FOR POLISHING A COMPOSITE

Abstract: A composition is provided for polishing a composite comprised of silica and silicon nitride comprising: an aqueous medium, abrasive particles, a surfactant, and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different.

Description: COMPOSITION AND METHOD FOR POLISHING A COMPOSITE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to compositions which are useful as slurries for the chemical-mechanical polishing of substrates, especially substrates compπsed of silica and silicon nitride. More specifically the slurries of the present invention include an aqueous medium, abrasive particles, a surfactant, and a compound which complexes with silica and silicon nitride Description of Related Art In integrated circuit technology, various active and passive elements generally must be isolated from one another within the integrated circuit structure This has often been accomplished by deep or shallow trench isolation techniques. These techniques typically use silicon dioxide (silica) as a dielectric material and silicon nitride as a stop layer, requiring chemical-mechanical polishing (planaπzation) of each circuit layer To achieve efficient polishing and adequate planaπzation, a polishing slurry is generally useful and should provide a high selectivity involving the removal rate of silica relative to silicon nitride. In Silvestri et al., U. S. Patent 4526631, a slurry of 6 weight percent colloidal silica adjusted to a pH of about 12 with KOH provides a polishing ratio of about 10 Sι0 2 to 1 Sι 3 N d Beyer et al , U.S. Patent 4671851, states that the polishing ratios between Sι0 2 and Sι 3 N 4 preferably should be between a lower limit of 4 to 1 and a higher limit of 40 to 1 Beyer describes obtaining a ratio of 6.2 to 1 using a colloidal silica m water with small amounts of a sodium salt of dichloroisocyanuπc acid and sodium carbonate. Even more recent patents such as Murase, U.S. Patent 5502007, also describe obtaining selectivities of about 10 Sι0 2 to 1 Sι 3 N removal rates using a colloidal silica slurry as a polishing agent. Kodera et al., U. S. Patent 5445996, use ceπa as well as silica for the abrasive particles m slurries, but they also report selectivities for Sι0 2 to Sι 3 N 4 removal rates in the range of2 to 3 SUMMARY OF THE INVENTION A composition is provided for polishing a composite compπsed of silica and silicon nitride comprising: an aqueous medium, abrasive particles, a surfactant and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different A further aspect of the invention is the method for polishing a composite comprised of silica and silicon nitride comprising applying a slurry at a polishing interface between a polishing pad and the composite compπsed of silica and silicon nitride, the slurry comprising, an aqueous medium, abrasive particles, a surfactant and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been found that the addition of a compound which complexes with silica and silicon nitride to polishing slurries used in the chemical-mechanical polishing of composites comprised of silica and silicon nitride can provide a very high selectivity of rate of removal of Sι0 2 to the rate of removal of Sι 3 N 4 when a surfactant is used in conjunction with the complexing agent and when the concentration of the complexing agent in the slurry is sufficient to block the removal of Sι 3 N while not greatly affecting the removal of Sι0 at the pH of the polishing slurry. Compounds which act as complexing agents or chelating agents for Sι0 2 and Sι 3 N 4 are descπbed in great detail in U. S. Patent 5391258 and U. S. Patent 5476606 which are made part of this specification by reference. These compounds must have at least two acid groups present in the structure which can affect complexation to the silica and silicon nitride surface. Acid species are defined as those functional groups having a dissociable proton. These include, but are not limited to, carboxylate, hydroxyl, sulfonic and phosphonic groups Carboxylate and hydroxyl groups are preferred as these are present in the widest variety of effective species Particularly effective are structures which possess two or more carboxylate groups with hydroxyl groups in an alpha position, such as straight chain mono- and di-carboxy c acids and salts including, for example, malic acid and malates, tartaπc acid and tartarates and gluconic acid and gluconates Also effective are tπ- and polycarboxylic acids and salts with secondary or tertiary hydroxyl groups in an alpha position relative to a carboxylic group such as citric acid and citrates. Also effective are compounds containing a benzene ring such as ortho di- and polyhydroxybenzoic acids and acid salts, phthalic acid and acid salts, pyrocatecol, pyrogallol, gallic acid and gallates and tannic acid and tannates. In the examples to follow, a salt of phthalic acid is used as the complexing agent and, therefore, such salts are preferred complexing agents for this invention. Potassium hydrogen phthalate, "KHP", was the phthalate salt used in the experiments described below. The surfactant used in conjunction with the complexing agent in this invention is not present to perform the usual function of surfactants in slurries of stabilizing the particulate dispersion. As shown in the examples which follow, the surfactant in combination with the complexing agent affects the rate of removal of Si 3 N from the composite surface. It is believed that any surfactant, whether it be an anionic, cationic, non-ionic or zwitter-ionic surfactant, might be effective in the compositions of this invention. Particularly useful would be fluorocarbons or hydrocarbons with phosphate end groups. In the following examples several different surfactants were shown to be effective. "ZFSP", ZONYL™ FSP Fluorosurfactant, manufactured by the DuPont Company, was shown to be a particularly effective surfactant additive to the slurries of this invention. It is a long straight chain hydrocarbon with phosphate groups at one end and a fluoride at the other end. In these examples, ceria was used for the abrasive particles in the slurry because it is an effective polishing abrasive for chemical -mechanical polishing at all pH conditions and is stable against gelation. Any other polishing abrasive, such as alumina, zirconia, silica, titania and barium carbonate could also be used. To adjust the slurries of this invention to the pH range in which the highest selectivities of Si0 2 removal to Si 3 N 4 removal are obtained any base or amine compound might be used. In the examples to follow KOH is used to adjust the pH of the slurry compositions. Potassium hydroxide, ammonium hydroxide, and all types of soluble amine compounds may be used to adjust the pH of chemical-mechanical polishing slurries. EXAMPLE 1 Table 1 shows the results of polishing silicon dioxide and silicon nitride wafers with slurries containing various amounts of complexing agent at selected pH levels. These experiments were carried out on a Strasbaugh 6DS SP Planarizer using an IClOOO/SubalV polishing pad stack under the conditions of 7 psi down pressure, 1.5 psi back pressure, 30 rpm carrier speed and 32 rpm table speed with a slurry flow rate of 125 ml/min. 6 inch wafers were used and the pad was conditioned after each wafer was polished. All slurries in this series of experiments contained 0.45% colloidal ceria and 0.2% ZFSP surfactant and the pH of the slurry was adjusted using potassium hydroxide. Xab l Sample %KHP* PH RR Si0 2 RR S13N4 Selectivity 1 0 4 1419 256 5.5 2 3.1 4 6 3 2 3 0.5 7 3019 189 16 4 1 7 3000 15 200 5 3.1 7 1185 4 296 6 1 10 3397 994 3.5 7 2 10 3246 920 3.5 *Potassιum hydrogen phthalate These results clearly show that selectivities of silicon dioxide to silicon nitride removal rates much higher than reported heretofore can be obtained by using slurries comprising a complexing agent and a surfactant at a pH level where these additives effectively shut down the removal rate of silicon nitride while not greatly affecting the removal rate of silicon dioxide As shown above, selectivities of 200 and greater can be obtained by this inventive method of polishing. EXAMPLE 2 The following experiments show the necessity of using a surfactant in the slurries of this invention. They were carried out on a Strasbaugh 6DS SP Planaπzer under the same conditions descπbed in Example I. Slurries were similar to the one used for Sample 4 on Table I m that the slurries were compπsed of 1%KHP, 0.45% colloidal ceria and the pH was adjusted to 7 using KOH Percent surfactant, ZFSP, was either 0.2% as in Sample 4 or 0 0% Removal rates for Sι 3 N 4 in Angstroms per mmute are shown in Table 2 below: Table 2 ZESE E&& 0 793 0 768 0 736 0.2 10 0.2 8 It is obvious from these data that the surfactant is critical to the shutting down of the Sι 3 N removal rate so that such extraordinary selectivities can be obtained. EXAMPLE 3 In the following expenments the abrasive in the slurπes was commercial opaline ceπa which was milled before use. Polishing of wafers was accomplished under the same conditions as in Example 1. Results are given in Table 3 below: Table 3 Sample %Abrasιve % HP %ZFSP pH RR SiO, RR S13. Selectivity 8 2 0 0 7 5804 3504 2 9 2 1 0.2 7 2642 22 120 10 3.5 1 0.2 6.5 3195 13 246 1 1 5 1 0.2 6.5 3705 33 112 12 3.5 2 0.4 7 2123 10 212 13 5 2 0.4 7.5 3609 1 105 3 These results show that a complexing agent and a surfactant effectively shut down the removal rate of Sι 3 N 4 when used in a slurry adjusted to a desired pH range. At a pH of 6.5 and 7 the removal rate of Sι 3 N is greatly shut down giving selectivities for the removal of silica over that of silicon nitride of over 100 (samples 9 - 12). However, at a pH of 7.5 (sample 13), the removal rate of silicon nitride is no longer depressed and selectivities are very poor. EXAMPLE 4 In these experiments several surfactants were found to be effective in reducing the removal rate of silicon nitπde at a pH of 6.5. The surfactants used were FC-93, Fluorad™ FC- 93, an aniomc fluorochemical surfactant available from the 3M Company; "PVS", the sodium salt of polyvmyl sulphonate available commercially; and "ZFSN", ZONYL™ FSN, a non-ionic surfactant available from the DuPont Company. The slurries in this Example all contained 1.5% KHP (potassium hydrogen phthalate) and 0.45% commercial opalene ceria as the abrasive Polishing of wafers was accomplished under the same conditions as in Example 1. Results are given in Table 4 below: Table 4 Sample Surfactant %Surfactant RR SιO : RR Sia a Selectivity 14 FC-93 0.2 2975 464 6 15 PVS 0.3 3406 35 98 1166 ZZFFSSNN 0.3 2678 39 68 These results show that several surfactants are effective in slurries made to reduce the removal rate of silicon nitride. A given surfactant might be even more effective should pH and slurry compositions be optimized. EXAMPLE 5 In this Example the abrasive used was WS2000 available from Rodel, Inc. WS2000 is an abrasive which contains both ceria and silica. The slurry used for this expeπment contained 3.5% abrasive, 1.5% KHP (potassium hydrogen phthalate), and 0.2% ZFSP (ZONYL™ FSP). The pH was about 6.5. Results from the polishing of wafers under the same conditions as in Example 1 are shown on Table 5 below: Table 5 Sample RB__SιΩ 2 EB_£ι 2 ι Selectivity 17 2209 9 244 The above Examples show many embodiments of this invention and are not meant to restrict the invention in any way. The scope of the invention is defined only by the claims which follow

Claims: CLAIMS 1 A method of polishing a composite comprised of silica and silicon nitride comprising applying a slurry at a polishing interface between a polishing pad and said composite comprised of silica and silicon nitride, said slurry compπsing: an aqueous medium, abrasive particles, a surfactant, and a compound which complexes with said silica and silicon nitride, wherein said compound has two or more functional groups each having a dissociable proton, the functional groups being the same or different. 2. A method according to claim 1 wherein said compound which complexes with silica and silicon nitride contains a benzene πng. 3 A method according to claim 1 wherein said compound which complexes with silica and silicon nitride is a straight chain mono- or di-carboxyhc acid or salt which has a secondary hydroxyl group in an alpha position relative to a carboxylate group 4 A method according to claim 1 wherein said compound which complexes with silica and silicon nitride is a tπ- or poly-carboxyhc acid or salt which has a secondary or tertiary hydroxyl group in an alpha position relative to a carboxlate group. 5 A method according to claim 1 wherein said abrasive particles comprise ceria 6. A method according to claim 1 wherein said surfactant comprises a fluorosurfactant 7. A method according to claim 2 wherein said compound which complexes with silica and silicon nitride is potassium hydrogen phthalate 8. A method according to claim 2 wherein said polishing composition comprises- water, about 0.2% to about 5% ceria, about 0.5% to about 3.5% potassium hydrogen phthalate, about 0 1% to about 0 5% fluorosurfactant, all percentages by weight, and wherein the pH of said polishing composition is adjusted from about 6 to about 7 by the addition of a base or an amine compound to said polishing composition. 9 A composition for polishing a composite compπsed of silica and silicon nitride compπsing an aqueous medium, abrasive particles, a surfactant, and a compound which complexes with said silica and silicon nitride, said compound having two or more functional groups each having a dissociable proton, said functional groups being the same or different 10 A composition according to claim 9 wherein said compound which complexes with silica and silicon nitride contains a benzene ring. 11. A composition according to claim 9 wherein said compound which complexes with silica and silicon nitride is a straight chain mono- or di-carboxyhc acid or salt which has a secondary hydroxyl group in an alpha position relative to a carboxylate group 12. A composition according to claim 9 wherein said compound which complexes with silica and silicon nitride is a tπ- or poly-carboxyhc acid or salt which has a secondary or tertiary hydroxyl group in an alpha position relative to a carboxlate group 13. A composition according to claim 9 wherein said abrasive particles comprise ceria 14. A composition according to claim 9 wherein said surfactant compnses a fluorosurfactant 15. A composition according to claim 10 wherein said compound which complexes with silica and silicon nitride is potassium hydrogen phthalate. 16. A composition according to claim 9 compπsing: water, about 0.2% to about 5% ceπa, about 0.5% to about 3.5% potassium hydrogen phthalate, about 0.1% to about 0 5% fluorosurfactant, all percentages by weight, and wherein the pH of said composition is adjusted from about 6 to about 7 by the addition of a base or an amine compound to said composition

Inventors: COOK LEE MELBOURE, HOSALI SHARATH D, SETHURAMAN ANANTHA R, WANG JIUN-FANG, COOK, LEE, MELBOURE, HOSALI, SHARATH, D., SETHURAMAN, ANANTHA, R., WANG, JIUN-FANG
Applicants: RODEL INC, RODEL, INC.
Application year: 1997
Main classification: B44C1
IPC codes: B24B37, C03C19, C09K3, H01L21