Micromachined Shear Stress Sensors for Characterization of Surface Forces during Chemical Mechanical Polishing
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0991-C06-03
Micromachined Shear Stress Sensors for Characterization of Surface Forces during Chemical Mechanical Polishing Andrew Mueller1, Robert White1, Vincent Manno1, Chris Rogers1, Sriram Anjur2, and Mansour Moinpour3 1 Mechanical Engineering, Tufts University, 200 College Ave, Medford, MA, 02155 2 Cabot Microelectronics, Aurora, IL, 60504 3 Intel Corporation, Santa Clara, CA, 95052
ABSTRACT This paper describes the fabrication and calibration of micromachined shear stress sensors intended for characterization of the local pad-wafer contact forces present during chemical-mechanical polishing. Sensors consist of arrays of microfabricated poly-dimethylsiloxane (PDMS) posts and are able to measure forces ranging from 2 to 200 µN. The posts are 100 µm high and have diameters of 40-100 µm. Calibrated post deflection sensitivities are linear and lie between 0.2 µm/µN and 1.3 µm/µN. Sensor design, fabrication, and calibration are detailed. Feasibility is established for sensor integration into a CMP scale model test setup, including an optical viewing method for observing post deflection during polishing. Initial micrographs of post deflection during polishing do not yet have sufficient resolution to determine the microscale forces during polishing. INTRODUCTION As feature sizes in the semiconductor industry continue to shrink, the importance of planarizing substrates both globally and locally grows. Chemical mechanical planarization (CMP) is now the foremost nanomanufacturing process worldwide. It has an annual economic impact over $1 billion and individual CMP processes are well established within certain enterprises. Still, the ability to change CMP parameters and predict the effect this will have on material removal rates is not yet realized [1, 2]. The CMP process is widely used and removal rates for certain systems have been characterized experimentally in terms of many of the polishing parameters [3], but a comprehensive model involving all variables and their effects on material removal rate and non-uniformity is lacking. The interfacial shear force between polishing pad and substrate is one of the many variables in the CMP process that has previously been studied in order to gain further understanding of the polishing model [4, 5]. These shear forces are an important parameter in CMP as they have a direct influence on material removal rates, processing temperature variations, and failure modes. Investigating local (micro scale) and global (coefficient of friction) shear forces in situ captures stick-slip phenomena and other vibration modes that timeaveraging techniques or post polishing analysis cannot [6]. Sensors developed for determining these shear forces must remain unaffected by the chemical slurry present during CMP (as well as the mechanical polishing process itself) and be able to provide in situ data from the pad-substrate interface.
Previous studies of CMP shear forces focus on in situ global (friction) forces obtained experimentally [6, 7], ex situ local forces obtained experimentally [8], or
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