<title>Measurement of thermal expansion coefficient of poly-Si using microgauge sensors</title>

Abstract

ABSTRACT Thermal expansion coefficient of heavily doped LPCVD polycrystalline (poly-Si) thin film was extracted by microgaugesensors. When electrical power was applied to the microgauge, it was heated up and thermal expansion occurred. From therelation between applied current and measured displacement at the microgauge, thermal expansion coefficient of thin filmwas extracted. The results revealed a value of 2.9x106 /K of thermal expansion coefficient of highly doped poly-Si thinfilms with standard deviation ofO.24x106 /K.Keywords: thermal expansion coefficient, microgauge, MEMS, thin film INTRODUCTION Poly-Si is frequently used for active layers in many applications in the field ofmicroelectromechanical systems (MEMS)based on Si micromaching. Especially, the proper design ofthennally driven microactuators requires a sound understandingofthe process dependence ofbasic material properties like thermal expansion coefficient13. Also, optical measurement ofintrinsic stress of thin film originated from deposition process requires exact values of thermal expansion coefficient ofdeposited thin film45. Therefore thermal expansion coefficient for phosphorus doped poly-Si thin film was studied usingmicrogauge method. In this paper, a micromachined poly-Si microgauge structure for measurement of thermal expansioncoefficient ofthin film is presented. Microgauge structures have been used for measuring intrinsic stress ofthe thin film6. Inthis work, thermal expansion of microgauges due to Joule heating were used to extract thermal expansion coefficient ofpoly-Si thin film. Thermal expansion coefficient ofpoly-Si thin films were extracted from the relation between calculatedaverage temperature and measured displacement at the microgauge. In microgauge equation, thermal expansion coefficientof thin film is represented as a function of its average temperature, design parameters and measured displacement at theVernier site. Therefore, by measuring the displacement and calculating average temperature ofthe microgauge, thermalexpansion coefficient ofthin film can be calculated. The average temperature ofthe microgauge, when current was applied,was calculated from heat conduction equation with simple modeling. Microgauge method has some advantages comparedwith other reported optical techniques to measure thermal expansion coefficient ofthin films. Mechanical properties ofsubstrate like Elastic modulus, thermal expansion coefficient is not needed in microgauge method because all measuredparts ofthe thin film are free standing above substrate.