A New Approach to Atomic Force Microscopy
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ANEW APPROACH TO ATOMIC FORCE MICROSCOPY NABIL M.AMER IBM Thomas J. Watson Research Center Yorktown Heights, NY 10598
ABSTRACT: A highly sensitive and simple optical method for detecting the cantilever deflection in atomic force microscopy is described. The method was incorporated in an atomic force microscope and imaging and force measurements, in ultra-high vacuum and in air, were sucessfully performed.
In the past few years, a new family of microscopy, which might be generically designated as stylus microscopy, has been developing. It is based on the use of a sharp stylus as the stuctural probe of matter. The best known member of this family is scanning tunneling microscopy (STM) [1]. A recent addition to stylus microscopy is atomic force microscopy (AFM) [2], which enables the investigation of the structure of insulating surfaces, an area which is typically not accesible to tunneling microscopy [3]. AFM is based on sensing the forces between a sharp stylus and the surface of interest. The interatomic forces induce a minute displacement of the stylus, which in its original implementation, used a tunneling junction to detect the motion of a diamond stylus attached to an electrically conductive cantilever beam [2]. Subsequently, optical interferometry was used to detect the cantilever deflection [4,5]. Having demonstrated earlier, in the context of photothermal spectroscopy, that displacements on the order of 10-4 A can be readily and accurately Mat. Res. Soc. Symrp. Proc. Vol. 139. k1989 Materials Research Society
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detected with a simple optical scheme [6,71, the applicabilty of such a scheme for sensing the the AFM cantilever defelection becomes obvious. In this approach [8], the cantilever displacement is measured by detecting the deflection of a weak laser beam which is reflected off its backside. The deflection is sensed with a position-sensitive detector (PSD). Inaddition to its sensitivity and simplicity, this scheme has proven to be well suited for measurements in ultra-high vacuum (UHV) environment. In force microscopy, the force, F, is measured by detecting the static deflection of the cantilever as it approaches the sample surface. (F= Azk, where Az is the cantilever displacement from its equilibrium position and k is its force constant). Alternatively, one can measure the force gradient F "(6F/6z) which can be obtained by, e.g., vibrating the cantilever or the sample. Aschematic representation of our approach is shown in Fig. 1.The styluscantilever system was a 75 [I tungsten wire electrochemically etched to form a sharp tip at one end and was subsequently bent near that end. The cantilever length was I mm, which yields a k=10 3 N/m and a resonance frequency =_20 kHz. To facilitate the reflection of the laser beam off the cantilever, a minute mirror (300 1i x 300 1') was attached to its backside . The mass of the mirror had practically no effect on the resonant properties of the lever [91. The microscope itself was a modified scanning tunneling microscope, which can be operated either as an AFM
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