Determining Pad-Wafer Contact using Dual Emission Laser Induced Fluorescence
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0991-C01-04
Determining Pad-Wafer Contact using Dual Emission Laser Induced Fluorescence Caprice Gray1, Chris Rogers1, Vincent P. Manno1, Robert White1, Mansour Moinpour2, and Sriram Anjur3 1 Tufts University, Medford, MA, 02155 2 Intel Corporation, Santa Clara, CA, 95052 3 Cabot Microelectronics, Aurora, IL, 60504
ABSTRACT It 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. We present a technique and some preliminary data for instantaneous measurement of in-situ pad-wafer contact, defined as the point at which the fluid film thickness goes to zero, using DELIF. The imaging area is 1.30mmx1.74 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% using a Cabot Microelectronics D100 polishing pad. The asperity contact area increases with applied load, which was varied from 0.28 to 3.1 psi.
INTRODUCTION It is beneficial to understand polishing pad-wafer contact during chemical mechanical polishing (CMP) so we can gain insight into the material removal rate (MRR) mechanism during polishing1. Integrated circuit manufacturers expend significant effort trying to control MRR during CMP. Manufacturing processes are often determined by trial and error due to incomplete understanding of the polishing mechanisms. One removal mechanism proposed by Cook2 suggests that the mechanical action of slurry particles can provide additional energy required to break and re-order the chemical bonds on the glass substrate surface so as to significantly increase the MRR. More recently, it has been suggested that the primary mode of transport of slurry particles to the glass surface is via polishing pad asperities that come into contact with the substrate3. The first step in determining pad-wafer contact is to accurately examine the 3D polishing pad topography. There are several standard techniques for attaining 3D topography, some of which include profilometer scanning, laser interferometry and confocal microscopy.
Profilometry provides excellent 3D images, but is not a viable option for observing pad-wafer contact because it the technique requires contact of a stylus with the surface being characterized. Optical techniques are more promising for determining contact between a polishing pad and an optical glass disk. Interferometry has been used in tribology to make thin fluid film measurements between an optical glass flat and a steel ball bearing4. However interferometry is not a practical technique for measuring contact or fluid films in CMP because the technique requires that the pad be specularly reflectiv
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