Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (<50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept.

Kelvin probe force microscopy

http://nathan.instras.com/documentDB/paper-44.pdf

Surface photovoltage phenomena: theory, experiment, and applications

Work function of metals

One of the important physical-chemical properties that characterizes the interaction of solid surfaces with gases is the bond energy of the adsorbed species. The determination of the bond energy is usually performed indirectly by measuring the heat of adsorption (or heat of desorption) of the gas [1, 2], In order to define the heat of adsorption, let us consider the chemisorption of a diatomic molecule, X2, onto a site on a uniform solid surface, M. The molecule may adsorb without dissociation to form MX2. M represents the adsorption site where bonding occurs to a cluster of atoms or to a single atom. In this circumstance, the heat of adsorption, ΔHads, is defined as the energy needed to break the MX2 bond:

/pdf/heats-of-chemisorption-of-o2-h2-co-co2-and-n2-on-13df04u8zi.pdf

Heats of Chemisorption of O2, H2, CO, CO2, and N2 on Polycrystalline and Single Crystal Transition Metal Surfaces

A simultaneous combination of scanning Kelvin probe microscopy and scanning atomic force microscopy has been applied to the problem of profiling dopant concentrations in two dimensions in silicon microstructures. By measuring the electrochemical potential difference which minimizes the electrostatic force between probe tip and sample surface, the work‐function difference between the tip and surface is estimated. To the extent that this work‐function difference is a consequence of the dopant concentration at or near the sample surface, doping profiles are inferred from the measurement. Structures examined and presented here include contact holes, and the technologically significant lightly doped drain of a metal–oxide–silicon field‐effect transistor. Using this methodology, one can distinguish relative changes in dopant concentration with lateral resolution less than 100 nm. Sample preparation is minimal, and measurement time is fast compared to other techniques. The measurements have been compared to predictions based on two‐ and three‐dimensional process and device simulation tools. The comparisons show that the technique is sensitive to changes in dopant concentration, from ≊1015 to 1020 cm−3, of less than 10% at these size scales. Suggestions to resolve absolute dopant concentration are made.

Two‐dimensional surface dopant profiling in silicon using scanning Kelvin probe microscopy

Measurements of the influence of stress upon contact potential necessitate the observation of shifts in contact potential of the order of 100 μV or less. The ac Kelvin technique was selected to yield this sensitivity. The application of phase sensitive detection techniques to the ac Kelvin method is discussed, and the equations are solved. The success of the method depends upon the use of a guarded field effect transistor input stage located within the Kelvin probe. The effect of nonuniform contact potentials and nonuniform capacitor plate separation is analyzed, and conditions are given for the balance point to be independent of plate spacing. The stress dependent contact potential measurements may be continuously displayed using a new nonlinear second harmonic detection scheme. This method is discussed.

Stress Dependence of Contact Potential: The ac Kelvin Method

The effect of baseline restoration on signal‐to‐noise ratio in pulse‐amplitude measurements is determined theoretically and experimentally. It is found that the signal‐to‐noise ratio with baseline restoration normalized to that without restoration is ηr/η=[1+α− 2α½kx(τ)]−½, where kx(τ) is the noise correlation factor at the output of the filter (amplifier), τ is the measurement interval, and α is the attenuation of noise power due to integration by the restorer capacitance and switch resistance (α=1 for fast restorer). Methods of calculation of correlation functions are outlined. A method for measurement of correlation functions by oscilloscope is described. The results are presented for the three commonly used filters: CR‐RC filter, quasi‐Gaussian filter, and delay‐line filter. Baseline restoration is found to degrade the signal‐to‐noise ratio. The amount of this degradation can be reduced to negligible proportions in filters with long memory (such as CR‐RC) by a small degree of integration in the restor...

Veljko Radeka

Papers

Stress Dependence of Contact Potential: The ac Kelvin Method

Effect of baseline restoration on signal-to-noise ratio in pulse amplitude measurements.