Atomic Force Microscope Designs and Applications
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ATWMC FORME MICBJXE EESIIQ8 AMDAPRJCATI(1W* P.J. Bryant, R.G. Miller, R.H. Deeken and M.A. Pederson; Univ. of MissouriKansas City, Physics Department; 1110 E. 48th St.; Kansas City, MO 64110 ABSTRACT Atomic force microscopes (AFM) have been designed and tested. A simple reliable version with high resolving power is described here. Materials of construction and operational parameters are reported. Applications to a range of materials has confirmed the general usefulness of AFN technology to materials research. INTRODUCTION The technique named atomic force microscopy (AFM) [1] was proposed recently [2]. When operated in contact with a sample an AFM is similar to a stylus profilometer, but with higher resolving power. The AFM was designed to extend the high resolution capability of the scanning tunneling microscope (STM) to nonconducting samples. Several possible arrangements were proposed [2]. The design which promised atomic resolution was the one which utilized the sensitive tunnel current control principle. The high resolving power of the STM is made possible by the exponential variation of tunnel current with the tip to sample separation. This high sensitivity to spacing is the principle upon which the success of the STM is based. It is also the principle which gave promise of atomic resolving power to the AFM. That resolution goal has been achieved [31 by utilizing tunnel current control. The AFM designs employed here are based upon the high resolution principle of the exponential variation of tunnel current with separation as originally proposed [1]. The two designs reported here were simplified by omitting the piezo drive which provided for direct adjustment of the tip to lever separation. Also the 3-D piezo-electric drive from the sample was transferred in the second design to the small AFN lever assembly. This design is simple and compact and is capable of reproducible AFM operation. ATCMIC FORCE MICROSCOPE DESIGNS The original AFM design concepts [2] included a high resolution version based upon the STM tunnel gap control principle. We have adapted that principle to several simple designs which have proved to be practical and reliable. They have also demonstrated high resolving powers. A series of simplified AFM designs have been developed. _he first utilized a normally closed tunnel gap [4]. A minimal force of 10 N from a repulsive sample contact was required to open the gap. Tunnel current control then maintained that constant force of contact to the sample. The normally closed position for the flexible lever and tunnel probe was set by simply mounting the two in gentle contact with the aid of natural attractive forces. By utilizing micromanipulators the initial mounting of a lever could be accomplished with reasonable uniformity i.e. the force required to open the tunnel gap was repetitive within the 102 N order of magnitude. This simple AFM design consists of a small flexible lever mounted over a second electrode or probe, see Fig 1. When the lever contacts a sample and bends slightly, a tunnel gap is o
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