Abrasive-Free Polishing for Copper Damascene Interconnection

Abstract

The copper damascene process is one of the most promising technologies to fabricate Cu interconnection for high-speed logic LSIs (large-scale integrated circuits). 1,2 Chemical mechanical polishing (CMP) of Cu and barrier metals is one of the most important techniques for fabricating Cu damascene interconnects. Since conventional CMP slurries contain alumina abrasives, which are mechanically very hard, many scratches are generated not only on Cu but also on a SiO 2 film surface. The SiO 2 surface damage produced after CMP of the barrier metal degrades dielectric breakdown reliability. SiO 2 erosion and Cu dishing have been also serious problems, especially in the area of high pattern density. 3 This erosion is defined as oxide loss around the patterned Cu area and dishing is defined as Cu thickness loss inside the Cu pattern. Erosion and dishing degrade the interconnect planarity and make it difficult to fabricate high-density multilevel interconnects at a high yield. 4 To minimize thickness loss, a slurryless polishing, which is a combination of a fixed abrasive pad and aqueous chemical solution, has been developed. 5 However , defectivity and post-CMP cleaning are still problems with this method. 6 Post-CMP cleaning technologies to remove abrasive particles on polished wafers have been developed. For example, PVA (polyvinyl alcohol) sponge-brush cleaning and megasonic cleaning have been used with chemical additives. 7,8 Since these additives often produce Cu surface damage, electrolyzed waters (anode water and cathode water) have been proposed. 9 These cleaning systems, however, raise the total cost of the CMP process. Although a CMP machine uses large amounts of slurry, which generates particles (abrasives), it must be operated in a clean room. These particles decrease chip yield in the metallization process. To suppress the particles, a special room must be set up for CMP machines in order to maintain the degree of cleanliness. As explained previously, the metal CMP process has many problems that must be solved. It is obvious that these problems originate from abrasives included in the slurry, but the solid abrasives have been previously thought indispensable in order to increase removal rate of the metal. In polishing without abrasives, the removal rate was quite low (less than 10 nm/min); thus, abrasive-free polishing could not be put to practical use. Accordingly, we have developed a complete abrasive-free process for Cu damascene interconnection by using a combination of new Cu-AFP (abrasive-free polishing) and barrier-metal dry etching. 10 Experimental AFP conditions.-The new polishing solution comprises only chemical agents such as oxidizer, etchant, and corrosion inhibitor, but it does not contain abrasives. It is, therefore, transparent as shown in Fig. 1a. In this experiment, the oxidizer was mixed with the other agent just before polishing in order to suppress the decomposition of the oxidizer. We used hydrogen peroxide aqueous solution (30 wt %) as the oxidizer. A popular, commercially available, alu-mina-abrasive-type CMP slurry for polishing Cu was also evaluated for comparison (Fig. 1b). The size of the abrasives (3.5 wt %) is about 230 nm and pH of the slurry is about 3.8. A CMP machine (Lapmaster SFT, LGP-552XJ-2) with two 24 in. diam platens was used. A foamed-polyurethane-type, hard polishing pad (Rodel, IC1000) was used as a standard pad. A lattice groove (XY groove) with a 15 mm pitch was formed (Rodel, A21 type). A concentric-circle groove (K groove) pad, a stacked pad (Rodel, IC1400), a softer pad (Rodel, XHGM1158), a harder pad (Universal, ESM-S), and a fixed abrasive pad (Universal, LP99) were also used for comparison. Ex situ pad dressing was carried out using an acid-proof diamond dresser. The distance between the center of the plat-en and the center of the wafer holder is 150 mm. The down force was varied from 5 to 40 kPa (50-400 g/cm 2) and the standard force for polishing patterned wafers was 22 kPa. The platen speed was varied from 15 to 60 rpm and the standard speed was 30 rpm. The AFP solution was supplied onto the polishing pad at a flow rate of 200 mL/min. A complete abrasive-free process for fabricating copper damascene interconnection has been developed. The process is a combination of newly developed abrasive-free polishing (AFP) of Cu and dry etching of a barrier metal layer. A new aqueous chemical polishing solution and a polyurethane polishing pad produce complete stop-on-barrier characteristics of Cu polishing. The AFP provides a very clean, scratch-free, anticorrosive polished surface, and the total depth of erosion and dishing is reduced to less than one fifth of that produced by conventional slurries, even after 100% overpolishing. The barrier metal is successfully dry etched by using SF 6 gas at a high selectivity ratio (more than 10) of barrier metal to SiO 2. It was found that the developed AFP significantly reduces both Cu line resistance and its deviation. Moreover, AFP can also contribute to cost reduction of chemical mechanical polishing and help solve environmental problems related to waste slurries. Figure 1. Comparison of (a) the new AFP solution and (b) a conventional alumina slurry. The picture behind is a bird's eye view of multilevel Cu inter-connection fabricated by AFP.