A Model of Chemical Mechanical Planarization of Patterned Wafers with Fixed Abrasives

Abstract

Chemical mechanical polishing ~CMP! has been established as an important enabling technology in the semiconductor industry. It is used routinely for planarizing a variety of films. A review of modeling efforts in CMP in the semiconductor field up to approximately 1995 is given by Nanz and Camilletti, 1 and a review of CMP in the context of glass polishing is provided by Cook. 2 In a typical CMP tool, a wafer with an uneven film deposited on it is held on a polishing head, and relative motion occurs between the surface of the wafer and the surface of a polishing pad in the presence of a liquid containing colloidal abrasive particles. The liquid also contains chemicals that soften the surface of the wafer for removal by the abrasives. After a period of polishing in this manner, the features on the wafer surface are planarized. A variety of polishing tools exist for performing CMP. In recent years, a new type of polishing process has come into use in which the abrasive is embedded in pillars on a polymer pad surface. In this case, the liquid is simply a chemical solution and contains no added particles. This fixed abrasive process is currently being used for polishing oxide patterns in shallow trench isolation ~STI! as noted by Gagliardi and Vo 3 and Lee et al. 4 Whereas conventional CMP of patterned wafers has been modeled in Ref. 5-8, no physically based models are currently available for predicting the rate of step height reduction when a fixed abrasive pad is used. As noted by Lee et al., 4 conventional models are unable to predict the large reduction in polish rate when the pattern density becomes unity, i.e., when the situation becomes one of polishing a blanket wafer; also such models do not predict the observed steep dependence of the polish rate on the pattern density at low pattern densities. Our objective is to present a model of fixed abrasive CMP based on physical principles that correctly predicts the behavior in both limiting situations in a natural way. The model permits the user to calculate the rate of step height reduction on a patterned wafer, given certain measurable geometrical input parameters, and two additional parameters based on physical and chemical features of the system that can be fitted to experimental data in the absence of fundamental information about these features. We restrict the model to the planarization that occurs when a patterned wafer is polished by a fixed abrasive pad or roll containing pillars with embedded abrasives. From hereon, we generically refer to the fixed abrasive system as a pad, even though in STI applications, a roll-to-roll format is used. 3,4 We assume that a chemical solution free of suspended abrasives is used for polishing. This is typically an alkali in the case of oxide polishing. For example, Gagliardi and Vo 3 use a solution of potassium hydroxide at a pH of 12.0.