Changes of Contact Potential Difference Induced by Frictional Damage in Ultrahigh Vacuum
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161 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
EXPERIMENT Expeimental Apparatus An ultrahigh vacuum scanning Kelvin probe force microscope (SKPM), which was developed based on the noncontact atomic force microscope (NC-AFM), was used to observe the contact potential difference between the tip and the surface [9-12]. An advantage of this experimental system is that surface topography and contact potential difference observations can be performed simultaneously without surface destruction. In the NC-AFM, the tip of an AFM cantilever is vibrated in close proximity to a surface, and optical heterodyne detection is used to accurately measure the vibration of the tip. Since the force between the tip and the surface is an attractive force, a cantilever having a large spring constant is
used to protect against sudden attraction to the surface and to obtain high-resolution images. The cantilever used in this experiment is a commercial type of conductive Si cantilever, which has a spring constant of -69 N/m and a resonance frequency of -400 kHz. Using such a cantilever, the force generated between the tip and surface can be more efficiently and accurately measured by detecting the force gradient rather than by directly measuring the cantilever deflection. Therefore, the cantilever is driven at a resonance frequency, and force gradients are detected as shifts in the resonant frequency of the vibration. Based on the NC-AFM, the SKPM was developed. When there is a potential difference U between the tip and sample, the electrostatic force between them is expressed as: 1
F = - l U 2 a _fC( 2 az
where C is the effective capacitance between the tip and the sample, and z is the distance between them. Therefore, when an ac voltage VXsin(wOt) and dc voltage V. are applied between the tip and sample, the electrostatic force between them can be expressed as:
F = --Z Vd, -
2+2(Vd, e
-
VK.sin(a)+Vac2
.sin 2(4)
J-az
(2)
where A 4 is the work function difference between the tip and the sample surface. The work function of the Si tip used in the experiment is 4.6 eV. The potential information (A,,= - (Vc A d4/e)V. 0 C/d z) that is contained in the W component is derived from the lock-in detection. If the voltage feedback control works such that Vdc= - Aý 44/e, the surface potentials Vd can be obtained directly regardless of the tip-sample capacitance. Since the electrostatic force gradient d F/0 z is also proportional to the square of the potential difference, the force gradient enables a higher resolution of the CPD images to be achieved in the same manner as the NC-AFM. Fig. 1 shows a block diagram of the system used in this experiment. The cantilever is kept oscillating at its resonance frequency by applying feedback from the optical deflection detection system to the piezo through the preamplifier and phase & amplitude adjuster. The frequency of the oscillating cantilever is detected by an FM demodulator. Feedback is applied to the Z piezo to
162
AM=0
Fig. 1 Block diagram of NC-AFM and SKPM. mai
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