In-Situ Friction and Pad Topography Measurements During CMP
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In-Situ Friction and Pad Topography Measurements During CMP Caprice Gray*, Daniel Apone*, Chris Barns+, Moinpour Monsour+, Sriram Anjur`, Vincent Manno*, and Chris Rogers* *
Tufts University, Medford, MA Intel Corporation, Santa Clara, CA `Cabot Microelectronics, Aurora, IL Materials Research Society, April 5, 2004 +
Abstract Duel Emission Laser Induced Fluorescence (DELIF) and friction measurements are taken in-situ during CMP to observe slurry flow beneath a model of an integrated circuit (IC) wafer. Friction measurements average around 7.5 lb and multiple frequencies are observed. Slurry film thicknesses on the order of a 10±3µm were observed during CMP of a flat wafer. The film thickness seems uncorrelated to friction measurements except when the pad and wafer rotation speeds are significantly slowed. DELIF has also accurately measured a 9µm etched step, with noise in the image equal to ±3 µm. Introduction Understanding the mechanism behind Chemical Mechanical Planarization (CMP) has become vital to the semiconductor industry as integrated circuit patterning moves from micron-scale to the nano-scale range. CMP is a complicated process due to the number of variables involved [1]; therefore the planarization mechanism is not well understood [2,3]. The research presented here employs a technique called Duel Emission Laser Induced Fluorescence (DELIF) to study slurry transport between the polishing pad and the wafer. DELIF is an optical technique that allows for the measurement of micron scale pad-wafer gap widths to be observed during the polishing process [4,5]. It is able to measure a relative difference to better than a micron. Due to measurement issues, however the absolute distance is only accurate to within 5µm. Friction measurements between the pad and wafer are made during this polishing in addition to DELIF measurements [6]. In post processing these data, friction and gap thickness are correlated in time leading to a better understanding of slurry transport and polishing efficiency. Experimental Setup Various polishing parameters are adjusted to account for laboratory research scaling and the optical measurements required for DELIF. It is necessary to replace the traditional opaque silicon wafer with a transparent BK-7 glass wafer to enable DELIF measurements. The slurry used during conditioning is a 9:1 water dilution of Cabot Microelectronics’ Cab-O-Sperse SC-1 3.1 wt% abrasive. The dilution of the slurry is necessary to maintain a consistent shape of the BK-7 wafer during the polishing run. The
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laboratory CMP equipment is scaled down from an industrial CMP system by a factor of 2. A summary of these adjustments are listed in Table 1. Table 1. Industrial to Laboratory Scaling Adjustments. Adjustment Parameter Held Constant Wafer Diameter (Pad area)/ (Wafer area) Pad and Wafer (Relative Industrial Linear Velocity)/ Rotation Velocity (Relative Laboratory Linear Velocity) Slurry Flow Rate Flow rate/[(Pad Area)*(wafer position)*(pad rotation velocity)]
Lab Scale Dimensions 3” 60 RPM 50cc
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