The present invention relates to the field of metallurgy and specifically to a method for chemical-mechanical planarization (CMP) of copper-containing surfaces as well as a slurry composition useful for CMP.
In the manufacture of integrated circuits it is common to connect the various components with a network of thin planar wiring (also called interconnects) so as to reduce circuit volume and thus increase component density, for example, as is necessary in submicron multilevel microelectronics. Copper, due to its low resistivity and high electromigration resistance, is one of the best materials from which to manufacture planar interconnects.
The copper damascene process is the preferred method for the production of copper interconnects. In the copper damascene process the required pattern of interconnects is made on a dielectric substrate such as a silicon wafer, copper is deposited onto the pattern (e.g. by electroplating deposition) and thereafter planarized. Due to the fact that copper is a soft metal without a protective native oxide, mechanical planarization techniques give poor results, often resulting in uneven surfaces, generating internal stress or even cracking the interconnects.
Compared to purely mechanical planarization, significantly better results are obtained by planarization of copper interconnects using chemical-mechanical polishing (CMP). CMP is substantially abrasion of a copper surface in the presence of an oxidizing composition and fine particulate abrasives, together making a slurry. In CMP, a copper surface to be polished is held against a mechanical polishing device (often a rotating polishing wheel) while being immersed in the slurry. By a combination of mechanical abrasion and chemical oxidation, thin layers of copper are removed. A detailed background of CMP can be found in, for example, U.S. Pat. Nos. 4,671,851; 4,910,155; 4,944,836; 5,676,587, 5,840,629 and 6,126,853.
Conceptually, CMP can be considered a two-step process.
In a first step, a layer of bare copper metal is oxidized by oxidizing agents in the slurry. In a second step, the oxidized layer is removed by the mechanical action of the fine abrasive particles in the slurry, leading to a net removal of copper. An advantage of CMP is that since only an oxide layer is abraded, no mechanical stress is applied to the bulk of the copper. Further, areas that are not to be abraded are coated with a protective oxide layer.
An ideal CMP process is a process whereby copper metal is removed only through conversion to oxide and subsequent abrasion. As a result, the efficiency and efficacy of a CMP process are in a large part determined by the reactivity of the slurry. An ideal slurry is preferably fast-acting, quickly oxidizing newly exposed copper both to protect the surface and to generate a new oxide layer to be removed.
Ideally, copper is removed only by formation of an oxide, which in turn is removed only by abrasion. When either oxide or metal are soluble in the slurry, pitting and uneven planarization result. Therefore, a copper CMP slurry is preferably configured so that neither copper metal nor a produced copper oxide is soluble therein.
It is known that copper is soluble in acidic solutions. Despite this, acidic slurries are often used in CMP of copper based on the belief that the slurry passivates the copper, that is, oxidation occurs so quickly that dissolution of copper metal is not significant. Such slurries give less than ideal results.
It has been proposed to inhibit copper dissolution in acidic solutions by non-native passivation, that is the addition of reagents such as benzotriazole, that form a passivating film on exposed copper metal, see for example, U.S. Pat. Nos. 6,569,350 and 5,770,095. The rate of formation of such non-native passivation films has been found to be too slow to be effective during the rapid surface abrasion conditions of CMP.
A number of methods using non-acidic slurries have been proposed for the CMP of copper.
Neutral or mildly basic slurries having a pH of up to about 9 have been disclosed, including U.S. Pat. Nos. 5,770,095, 5,800,577 and 5,840,629.
U.S. Pat. Nos. 6,126,853, 6,309,560 and 6,569,350 teach compositions having a preferred pH of up to 8 and including a film-forming agent. U.S. Pat. No. 6,432,828 teaches a related composition having a preferred pH of up to 8 but devoid of a film-forming agent. Mentioned in passing in all these patents is that the pH of the slurries can be raised up to 9 or even 12, but such high pH ranges are not actually taught. Further, in these slurries the use of metal ions in the slurry composition is expressly forbidden.
A few compositions more basic than pH 9 have also been discussed in the art. An ammonium hydroxide containing slurry with a pH of between 11 and 12 has been proposed (Streigerwald et al., J. Electrochem. Soc. 142 (1995) p. 2379). A slurry for the CMP of copper having a pH greater than or equal to 7, a copper oxidizing agent and ammonia as a copper complexing agent is taught in U.S. Pat. No. 6,676,484. In these slurries, copper is oxidized to form soluble ammonium salts (e.g. Cu(NH3)2) which are then dissolved by the slurry, in a manner analogous to the removal of copper metal by acidic slurries with the same disadvantages.
It would be highly advantageous to have a slurry composition suitable for use in chemical-mechanical planarization of copper and copper alloys not having the disadvantages of prior art.