Determining Pad-Wafer Contact using Dual Emission Laser Induced Fluorescence

Abstract

ABSTRACTIt is becoming increasingly clear that understanding the small scale polishing mechanisms operating during CMP requires knowledge of the nature of the pad-wafer contact. Dual Emission Laser Induced Fluorescence (DELIF) can be used to study the fluid layer profile between the polishing pad and the wafer during CMP. Interactions between the polishing pad surface and the wafer can then be deduced from the fluid layer profile. Previous investigations of pad-wafer interactions using DELIF include in-situ measurements of average fluid layer thickness and asperity layer compressibility, surface roughness measurements and polishing pad rebound into etched wells. In this paper, DELIF is used to determine pad-wafer contact, the point at the fluid film thickness goes to zero. We present a technique and some preliminary data for instantaneous measurement of in-situ pad-wafer contact using DELIF. The imaging area is 1.30×1.74 mm with a resolution of 2.5 μm/pixel. At this magnification, some regions imaged contain contact, whereas others do not. For the contact regions discussed in this paper, contact percentage varies from 0.07% to 0.27% on a Cabot Microelectronics D100 polishing pad. The asperity contact area increases with applied load, which was varied from 0.28 to 3.1 psi.