In this research, we conducted a series of experiments to investigate the mechanisms of chemical mechanical polishing (CMP) of silicon. Experimental approaches include tribological tests of frictional and lubricating behavior, chemical analysis, and surface characterization. Specifically, the effects of pH in slurry, surface roughness of wafers, and nano-particle size on removal rate were studied. A transmission electron microscope (TEM), a scanning electron microscope (SEM), and x-ray characterization tools were used to study the change of surface structure and chemistry. Experimental results indicate that the removal rate and planarization are dominated by the surface chemistry.

Key words: Silicon CMP, tribology, surface analysis

INTRODUCTION

Chemical mechanical polishing (CMP) has been the primary planarization method for manufacturing integrated circuits for more than a decade.1-3 Chemical mechanical polishing is used to planarize the wafer surface and at a number of intermediate stages, and as a final step after deposition of various features, interconnects, and coatings. There is little room for error during CMP because insufficient planarity can result in difficulties with subsequent process steps.4 Planarization can be achieved by several methods other than CMP, for example, the etch-back method, essentially a "negative" of the original photolithographic deposition of the features on silicon. However, the CMP process has been proven to be the planarization process of choice because it produces better results for smaller length scales. It has also been used to polish rigid disks in the magnetic memory industry.

Materials removed in CMP such as W, Al, Cu, Si, and SiO^sub 2^, as well as silica's variant in forms of plasma-enhanced tetraethyl ortho silicate (PETEOS), boro phospho silicate glass (BPSG), and noble metals, are of industrial importance. There has been tremendous effort put into studying the CMP process for metals and dielectrics.5'6 For metal polishing, the dominant mechanisms are related to passivation and removal of tungsten,7 slurry viscosity,8 and electrochemical interaction.9 The contact between the pad and wafer10"15 and other systematic approaches have been reported. The polishing mechanisms of silica have been studied as has the formation of silica triggered by friction forces.16"19 Humidity causes the formation of a layer of amorphous SiO^sub 2^ on a Si^sub 3^N^sub 4^ surface under friction. Such SiO^sub 2^ molecules can be dissolved by the interaction of frictional stresses and chemical attack in water. Further experiments indicated that a competition between the oxidation and wear was possible. In this work, the authors revisited the polishing of silicon. A unique approach with the tribological investigation combined with surface analysis to explore the effects of particle and slurry chemistry on removal mechanisms and planarization was used. The objective of this work is to study the mechanisms of chemical mechanical removal of silicon using slurries containing either alpha or gamma nano-sized alumina particles. We investigated the performance of nano-alumina particles, and also the effects of pH, silicon dissolution and polishing, and silicon dioxide formation and polishing. Surface analysis was conducted using high-resolution transmission electron microscopy in order to detect the nature of nano-scale surface features after CMP.

EXPERIMENTAL PROCEDURE

A table top polisher was used for polishing. The wafers were mounted on a holder. A slurry container was held stationary. The polishing pad was soaked in the slurry during polishing. The pad diameter was 101.6 mm (4 in.). The wafer holder moved with a self-spinning motion.

The slurries contained de-ionized water and nano-alumina particles. The pH of the slurry was adjusted with HNO^sub 3^ or NaOH. The concentration of alumina particles is 5 wt.%. Two types of alumina particles were used: [alpha]-alumina and [gamma]-alumina particles. Their properties are shown in Table I. The hardness and density of [alpha]-alumina are higher than those of [gamma]-alumina. The hardness of silicon is 7 Mohs (640 Vickers). It had the lowest hardness of the two types of alumina particles. Slurries contain 50 g of alumina powder each and 450 g of de-ionized water. The pH was then adjusted with 1N HNO^sub 3^ or NaOH and measured with a pH-meter. Before polishing, the slurry was stirred to maintain suspension. The stir continued during polishing. The amount of slurry used was 300 mL. The slurry was not replenished during 20 min of polishing. New slurry and a polishing pad were used for each test.

The particle size range measurements were made with a small volume module of a particle analyzer (Beckman Coulter LS230 (Beckman Coulter, Fullerton, CA)). The alumina polishing slurry contains de-ionized water and alumina powder with a specified pH. The reservoir of the LS230 instrument was first filled with the de-ionized water at a desired pH. The reservoir was stirred for 60 sec to ensure it was stabilized before a background measurement was made. The slurry was then put into the test container using a micropipette. The sample was again stirred for 60 sec and then was ultrasonically vibrated for 2 min at 40 watts power. The refractive index of alumina was 1.7 and the refractive index of water was 1.33. The particle size was in the range of 0.04-2 µm at 50% total point of distribution.