Ultrasonic Force Microscopic Characterization of Nanosized Copper Particles
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Center for Materials Diagnostics, University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0121 ** Research Institute & Graduate Materials Engineering, University of Dayton, 300 College Park, Dayton, Ohio 45469-0160 *
ABSTRACT An Ultrasonic Force Microscope capable of imaging elastic modulus variations with nanometer resolution has been developed by modifying a Scanning Probe Microscope. Images of ultrasonic properties have been simultaneously obtained with the topography images. The technique has been utilized to characterize nanoscale copper droplets and grains deposited on a quartz substrate by ionized cluster beam deposition. Images of the same region obtained with atomic force microscope, lateral force microscope, and ultrasonic force microscope are compared. The origin of image contrast in ultrasonic force microscopy and its utilization for quantitative elastic property measurement of nanometer particles are discussed.
INTRODUCTION The Atomic Force Microscope (AFM) has been extensively used for surface topographic measurements with nanometer resolution [1]. Such high-resolution images are obtained by scanning a fine tip with a tip radius of 30-50 nm attached to a cantilever and detecting the height variations with a laser beam reflected off the cantilever's top surface. Several modifications to the AFM have led to various microscope systems capable of measuring different material properties. One such modification has been the Lateral Force Microscope (LFM) which operates in contact mode AFM [2]. The LFM detects the amount of torsion on the cantilever and thus measures the localized frictional forces. The Ultrasonic Force Microscope (UFM) is another modification and has been developed to study the elastic properties at nanometer scales. In a UFM, a piezo-electric transducer is attached to one face of the sample, and an AFM tip is in contact with the other face. An acoustic wave propagates through the sample when the transducer is excited by an ultrasonic frequency signal. Surface displacements generated by the acoustic waves through the sample are detected by the AFM tip. Several displacement-detecting methodologies have been developed. Kolosov and Yamanaka detected the nonlinear response of the tip-sample interaction force by propagating an amplitude modulated longitudinal wave through the sample [3-4]. Rabe and Arnold utilized a beam splitter and a knife-edge detection system to investigate cantilever deflection by generating amplitude images using broadband needle impulse MHz frequencies [5-6]. Burnham, et.al. have investigated three different UFM modes of operation: contact, mechanical diode, and subharmonic [7-8]. In the contact mode, a small amplitude continuous longitudinal wave is propagated through the sample. The probing tip remains in constant contact with the sample due to the small amplitude of the excitation signal throughout the cycle. Larger amplitudes cause the 473 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
tip to sporadically lose contact an
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