C. Julian Chen

Electromechanical deflections of piezoelectric tubes with quartered electrodes

The deflection of a piezoelectric tube, with the outer (or inner) metal coating sectioned into four quadrants, is analyzed. We show that by applying a voltage V on one of the quadrants, the electromechanical deflection is (√2d31VL2/πDh), where d31 is the piezoelectric coefficient, L is the length, D the diameter, and h the wall thickness of the tube. The deflections calculated with it agree well with the results of finite‐element calculations and direct experimental measurements. The formula can be used in the design and application of tube scanners in scanning tunneling microscopes and scanning force microscopes.

Theory of scanning tunneling spectroscopy

A new three‐dimensional tunneling theory is introduced for interpreting scanning tunneling spectroscopy (STS) images. By expanding the asymptotic wave function of the acting atom in terms of complete sets of eigenfunctions in spherical coordinates and parabolic coordinates, a derivative rule is derived, which can be applied to interpret scanning tunneling microscopy and STS images immediately. The relation between the observed dynamic conductance and the density of states of the sample is shown in conjunction with a linear bias‐distortion correction.

Microscopic view of scanning tunneling microscopy

We present a theory of the atomic resolution in scanning tunneling microscopy (STM) in terms of localized surface states on the tip. The tunneling matrix elements arising from these tip states are evaluated with the derivative rule. For example, a pz surface state on the tip generates a tunneling matrix element proportional to [∂ψ/∂z] at the nucleus of the apex atom, and a d3z2−r 2 tip state generates a tunneling matrix element proportional to [3∂2ψ/∂z2−κ2ψ], (ψ is the sample wave function, κ is the decay constant of surface wave function, κ=(2meφ)1/2/ℏ ). To obtain analytic results of theoretical STM images, we further developed a simple independent‐orbital model to describe the wave functions of the sample surface. With this model, we present qualitative and quantitative explanations of the observed atomic resolution on metals and semiconductors, the spontaneous switching of instrument resolution during imaging, and various tip‐sharpening procedures.

In situ testing and calibration of tube piezoelectric scanners

Abstract The tube scanner, a piezoelectric ceramic tube with the outer metal coating sectioned into four quadrants, is widely used in scanning probe microscopes. Based on stress-and-strain analysis, we obtain analytic expressions for the deflections of tube scanners, which are found to be in good agreement with the results of finite-element analyses and experimental measurements. Also, we show that by applying an AC voltage on one of the quadrants, an AC current of well-defined intensity and phase is generated from each of the other three quadrants to the ground (inner metal coating). This current can be used to measure the piezoelectric coefficient and to inspect the uniformity and geometrical accuracy of tube scanners. A combination of those who results provides an easy and accurate method for testing and calibrating tube scanners in situ, for example, in a vacuum chamber or in a cryostat.

High‐conductance customized copper interconnections produced by laser seeding and selective electrodeposition

We report a two‐step process for producing high‐conductance customized copper interconnections utilizing a localized electrodeposition process induced by Joule heat at a constriction. An initial metal interconnection is made by localized decomposition of an organometallic film using a focused laser beam. The conductance of such an initial interconnection can be low, but is enough to induce localized copper deposition by passing an ac current through the entire line in a copper‐containing electrolyte. The interconnections produced by this process are solid, continuous, and highly conducting.

Localized electrodeposition induced by Joule heat at a constriction

We report a novel localized electrodeposition process based on localized Joule heating at a constriction and the temperature dependence of the equilibrium potential at a metal‐electrolyte interface. Assuming a local temperature rise of 50 K, a deposition rate as high as 2 μm per minute of copper is theoretically predicted in acidified copper sulfate solution, which is verified by a series of experiments. Scanning electron microscopy micrographs show that the deposited copper is dense and crystalline. As an immediate application of this novel phenomenon, a method of self‐induced repair for incipient opens, i.e., a self‐locating and self‐terminating process to treat constrictions in circuits, is established.

In-situ characterization of tip electronic structure in scanning tunneling microscopy

Abstract The problem of determining and understanding the role of tip electronic states is one of the central and critical scietific problems in scanning tunneling microscopy (STM) and spectroscopy (STS). In this paper, we described a theory and a practical method for in-situ characterization of the tip electronic states. The method consists of a deconvolution procedure, from which the density of states (DOS) of the tip is recovered from the measured tunneling spectra. The tip DOS, obtained from deconvolving an experimental tunneling spectrum, can be compared with predictions of first-principle numerical calculations of various realistic tip structures, or independent measurements, for example, field emission spectroscopy and photoemission spectroscopy. The reference tunneling spectrum of the sample can be obtained using a flat-DOS blunt tip, for example, following the tip-treatment process described by Feenstra et al. Using the deconvolution method, we analyzed the STM and STS data published by Pelz. The results indicates that the W tip picked up a Si cluster in the middle of a scan on a Si sample. To further clarify the role of tip electronic states in STM imaging process, a set of new experiments is proposed.

Constriction‐Induced Local Electrodeposition The Principle of Self‐Induced Repair

A fundamental study of the self-induced repair (SIR) of constrictions is presented. SIR is a local electrodeposition process induced by the Joule heating at a constriction, together with the temperature dependence of the equilibrium potential at a metal-electrolyte interface