Microstructure of thick polycrystalline silicon ®lms for MEMS application H.-W. Zhou a , B.G. Kharas b , P.I. Gouma a,* a Department of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275, USA b Sarcon Microsystems Inc., Sarnoff Corporation, 201 Washington Road, Princeton, NJ 08543, USA Received 9 July 2002; received in revised form 11 October 2002; accepted 2 November 2002 Abstract Thick polysilicon ®lms (>0.5 mm) become increasingly attractive for MEMS applications. However, thicker ®lms can experience greater variability in morphology and in surface roughness due to the longer deposition time. In this study, 3.0 mm thick polysilicon ®lms were deposited on silicon wafers by the LPCVD process. Asperities were observed on the surface of the polysilicon ®lms by SEM and AFM. TEM studies showed the co-existence of equiaxed ®ne grains and some discrete long columnar crystallites containing microtwins (®ne laths) in the materials microstructure. The asperities on the surface are found to be the caps of the columnar grains that grow continuously out of the ¯at surface of the equiaxed grains. The effect of the ®lm thickness on these asperities and the surface quality was discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface asperities; Columnar grains; Film thickness; Non-uniform conditions 1. Introduction Polycrystalline silicon ®lms are of great use in micro- electronics and micro-electro-mechanical systems (MEMS). Recently, thick polysilicon ®lms with thickness over 0.5 mm become increasingly attractive for use as MEMS structure materials [1,2]. Increased ®lm thickness improves mecha- nical rigidity in applications such as pressure sensors, provides increased surface area in comb-drive actuators, or increased mass in accelerometer applications. The per- formance of the polysilicon ®lm in MEMS is strongly dependent on its residual stress and stress gradient, which are determined by its microstructure. The effect of deposi- tion parameters, such as the substrate temperature, gas ¯ow andpressure,onthemicrostructureandpropertiesofthethin ®lm is signi®cant [3±9]. It was suggested that the micro- structure evolution from columnar grains to equiaxed grains in polysilicon ®lms during the high temperature rapid thermal annealing process would result in the lower residual stress [10]. It has been recognized that the surface morphology of the polysilicon ®lms has a great impact on the tribological characteristics of MEMS [11]. The rough surface can lead to serious adhesion, friction and wear problems in MEMS devices, and subsequently on contributing to failure [12]. Various techniques have been used to characterize the sur- face morphology of the polysilicon ®lms. Previous works by other authors have revealed small surface grains (30±50 nm) as observed in SEM and AFM images [7,13±15]. In the present work, large asperities (100±1000 nm) were observed on the surface of LPCVD polysilicon ®lms. The spatial density of these asperities on the surface was found to vary with the ®lms deposited at different positions in the furnace. The structure analysis has been performed and possible mechanisms of their formation are discussed. 2. Experimental Polysilicon ®lms were deposited on the p-type h100i silicon wafers covered with a 50-nm-thick silicon dioxide bytheLPCVDmethod.The®lmsweregrownbythethermal decompositionofsilaneinahorizontalhotwalledfurnaceat 590 8C. The temperature is selected to yield an amorphous as-deposited structure, which is crystallized to an equiaxed ®ne grain structure later during the continuous deposition of the®lm.A®lmwithsuchastructureissupposedtogivelow tensile stresses [15], which are preferred by MEMS fabrica- tion.Asimpli®edschematicofthereactorisshownin Fig.1. There are two injectors for the silane, one at the front and one in the middle of the tube to compensate for depletion. Sensors and Actuators A 104 (2003) 1±5 * Corresponding author. Tel.: 1-631-632-4537; fax: 1-631-632-8052. E-mail address: pgouma@notes.cc.sunysb.edu (P.I. Gouma). 0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0924-4247(02)00433-8