Micromachined Lateral Force Sensors for Characterization of Microscale Surface Forces During Chemical Mechanical Polishi
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Micromachined Lateral Force Sensors for Characterization of Microscale Surface Forces During Chemical Mechanical Polishing Douglas Gauthier1, Andrew Mueller1, Robert David White1, Vincent Manno1, Chris Rogers1, Donald Hooper2, Sriram Anjur3, and Mansour Moinpour2 1 Tufts University, Medford, MA, 02155 2 Intel Corporation, Santa Clara, CA, 95052 3 Cabot Microelectronics, Aurora, IL, 60504 Mater. Res. Soc. Symp. Proc. Draft
ABSTRACT Micromachined structures with diameters ranging from 50 - 100 μm have been applied to the measurement of the microscale shearing forces present at the wafer-pad interface during chemical mechanical polishing (CMP). The structures are 80 μm high poly-dimethyl-siloxane posts with bending stiffnesses ranging from 1.6 to 14 μN/μm. The structures were polished using a stiff, ungrooved pad and 3 wt% fumed silica slurry at relative velocities of approximately 0.5 m/s and downforces of approximately 1 psi. Observed lateral forces on the structures were on the order of 5-500 μN, and highly variable in time. INTRODUCTION Chemical Mechanical Planarization (CMP) is a critical process for semiconductor manufacturing. As feature sizes continue to shrink, planarity continues to be an important consideration for successful lithography. The CMP process is widely used and, for certain systems, has been characterized experimentally in terms of many of the polishing parameters. Still, a comprehensive model involving the multitude of process variables and their effects on material removal rates, planarity, and defectivity remains elusive [1,2]. The development and validation of some aspects of this “total CMP model” is hindered by lack of knowledge of in situ shear forces present at the micro-scale [3]. Experimental data of shear forces from single asperities would provide a comparison point for further research at this scale. In a more direct sense, knowledge of the local shear forces may be important in designing fragile structures, such as low-k dielectrics, to withstand polish. In this paper, we detail the development of micromachined shear stress sensors intended for characterizing these in situ local contact forces during CMP. Other researchers have investigated the average global shear force in situ [4-6]. In addition, some groups have investigated micro- or nano-scale forces ex situ [7,8]. However, to the best of our knowledge, the work described in this paper represents the first attempt to measure local microscale polishing forces in situ. DESIGN AND FABRICATION The sensor structure is shown in Figure 1. The structures are 80 μm tall poly-dimethylsiloxane (PDMS) cylindrical posts. The post diameters vary from 50 μm to 100 μm. Each post is
recessed in a well which leaves a 50 μm wide empty region around the post. The structure is immersed into the polishing slurry and polished. As microscale features on the polishing pad come into contact with the post top, the post deflects. This deflection is observed through the back of the transparent structure using a high speed microscopy setup.
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