Tip-sample Capacitance Research Articles

Dielectric-constant measurement of thin insulating films at low frequency by nanoscale capacitance microscopy

We demonstrate a method for quantitatively probing the local low-frequency dielectric constant of thin insulating films by nanoscale capacitance microscopy. The calibrated capacitance-distance curves are measured on the dielectric film and analyzed by using a tip-sample capacitance model here proposed. Applied to SiO2 films as small as 1×1μm2 area and 20–30nm thickness, the method gives a dielectric constant on the submicron scale in agreement with the value determined on the large scale. The observed precision is set by the capacitance noise level of the instrument and the tip radius.

Ferroelectric domain inversion: The role of humidity

The authors report on the effect of ambient humidity on domain inversion in ferroelectrics using atomic force microscopy. It is shown that the size of single domains inverted under low humidity in stoichiometric lithium tantalate single crystals is much smaller relative to ambient conditions. These differences are due to the much smaller tip-sample capacitance under low humidity. This phenomenon paves the way for the use of atomic force microscopy to tailor various nanodomain configurations for nonlinear optical applications.

Alternating Current of Scanning Tunneling Microscope for Organic Molecules Adsorbed on Metal in Terms of Equivalent Circuit of Scanning Tunneling Microscope

We have recorded the alternating current of the tunneling gap in scanning tunneling microscope (STM) for molecule adsorbed Au(111) as a function of tunneling gap distance using a lock-in amplifier for the oscillating bias voltage of STM at a DC bias voltage of 1 V, and measured the admittance between the STM tip and the sample in the tunneling region. The measured tip–sample capacitance steeply increased with decreasing gap distance in the range of 0 to 0.7 nm in correlation to the increase in tunneling current (conductance of admittance). The capacitance did not show a decrease with decreasing tunneling gap distance as expected for capacitance in the quantum regime in the experimental conditions studied.

Imaging of oxide charges and contact potential difference fluctuations in atomic layer deposited Al2O3 on Si

Ultrathin 2.5nm high-k aluminum oxide (Al2O3) films on p-type silicon (001) deposited by atomic layer deposition (ALD) were investigated with noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum, using a conductive tip. Constant force gradient images revealed the presence of oxide charges and experimental observations at different tip–sample potentials were compared with calculations of the electric force gradient based on a spherical tip model. This model could be substantially improved by the incorporation of the image of the tip in the semiconductor substrate. Based on the signals of different oxide charges observed, a homogenous depth distribution of those charges was derived. Application of a potential difference between sample and tip was found to result in a net electric force depending on the contact potential difference (CPD) and effective tip–sample capacitance, which depends on the depletion or accumulation layer that is induced by the bias voltage. CPD images could be constructed from height–voltage spectra with active feedback. Apart from oxide charges large-scale (150–300nm lateral size) and small-scale (50–100nm) CPD fluctuations were observed, the latter showing a high degree of correlation with topography features. This correlation might be a result from the surface-inhibited growth mode of the investigated layers.

A scanning capacitance microscope based on a coaxial resonator: High sensitivity and cheap and compact design without the use of lock-in detection systems

We describe a scanning capacitance microscope based on an oscillator whose frequency is controlled by a coaxial line λ/2 resonator tuned at the terminal ends by two capacitances, one obtained by a varicap, the other due to the tip-sample junction. Both the values of the tuning capacitances can be changed by a biasing voltage applied, respectively, to the varicap or to a piezoceramic which controls the tip-sample distance. The two input voltage controlled oscillator is part of a phase locked loop and, depending on the input used, it is possible to perform constant height or constant capacitance analysis. In the first case, the instrument gives the profile of the tip sample capacitance while, in the second case, constant capacitance profiles are obtained. Theoretical analysis and experimental results are reported. Capacitance–voltage metal–oxide–semiconductor curves and the images of a calibration grating obtained with our instrument are also reported.

Instrumentation for direct, low frequency scanning capacitance microscopy, and analysis of position dependent stray capacitance

We describe instrumentation for scanning capacitance microscopy (SCM), using an atomic force microscope, that is designed to make calibrated, low frequency measurements of tip–sample capacitance and spreading resistance of thin dielectric films. We also characterize spatial variations in stray capacitance Cstray that must be understood before such measurements can be interpreted. Existing SCM circuits are generally optimized for measuring dC/dV, and not for calibrated low frequency measurements of absolute capacitance. Our circuit uses a commercially available current amplifier and low frequency (∼5 kHz) lock-in detection. This circuit adds an inverted, constant amplitude current to suppress the stray displacement current from the large (∼0.5 pF) stray capacitance Cstray between the sample and the mm-sized cantilever–probe assembly. The capacitance noise spectrum is ∼0.35 aF/Hz and is flat down to 1 Hz measurement frequency, with an integrated noise <5 aF integrated over a 1–160 Hz bandwidth. We have also used this instrumentation to identify and characterize significant (>1 fF) variations in Cstray that must be understood in order to accurately measure aF-level variations in the nm-scale tip–sample capacitance. We find that Cstray varies with lateral probe position. This is due to tilting of the probe assembly as the piezoelectric scanner tube bends during scanning. We also find that Cstray varies significantly with probe–assembly height. This causes topography related artifacts in capacitance images of rough surfaces. However, we show that stray capacitance artifacts can be mostly eliminated by properly characterizing position and height dependent variations in Cstray and subtracting them from measured capacitance data.

Highly resolved electric force microscopy of metal-filled anodic alumina

A nanowire-filled membrane of porous anodic alumina is resolved by noncontact electric force microscopy. On nearly completely filled matrix areas, hillocks of about 50 nm diameter are observed. From the imaging of the tip-sample capacitance they could be identified as the endings of protruding Ag wires. Some are covered with matrix residues of about 10 nm diameter which could be resolved in the capacitance image. Sample parts with smaller filling amounts show that phosphoric acid enlarges the pore diameters. We conclude that the wire diameters appear to be smaller than the inner pore diameters.

Tip–sample distance feedback control in a scanning evanescent microwave probe for nonlinear dielectric imaging

We implemented tip–sample distance control in a scanning evanescent microwave probe for nonlinear dielectric microscopy. With the analytic expression of the tip–sample capacitance as a function of tip–sample distance, we can quantitatively regulate the tip–sample separation and independently measure the dielectric nonlinearity by application of an ac bias voltage. Simultaneous imaging of topography and ferroelectric domains has been demonstrated on periodically poled LiNbO3 single crystals.

Surface potential measurement of self-assembled InAs dots by scanning Maxwell stress microscopy

We have imaged surface potential of InAs self-assembled quantum dots on GaAs (001) surface by scanning Maxwell stress microscopy. Simultaneously obtained constant tip–sample capacitance image and surface potential image were compared with atomic force microscope (AFM) image, and was found that the surface potential on large dots of about 100-nm diameter is ∼610mV which is 30–40 mV lower than that on regular size dots. This result well coincides with previously reported Schottky barrier heights which were estimated from current versus voltage measurements with the same type of samples using conductive probe AFM.

Quantitative microwave evanescent microscopy

We have developed a scanning evanescent microwave microscope with shielded tip geometry allowing quantitative characterization of the electrical impedance of insulating and conducting materials. By modeling the tip–sample capacitance, quantitative estimates of the sample (both dielectric and conducting) electrical impedance (real and imaginary) and tip–sample separation can be made. Measurements of the tip–sample capacitance versus tip–sample separation have been made and agree with estimated values. Also, the slope of the tip–sample capacitance with respect to the tip–sample distance is calculated to implement tip–sample distance regulation for dielectric materials.

Nanometer-scale patterning and individual current-controlled lithography using multiple scanning probes

Scanning probe lithography (SPL) is capable of sub-30-nm-patterning resolution and nanometer-scale alignment registration, suggesting it might provide a solution to the semiconductor industry’s lithography challenges. However, SPL throughput is significantly lower than conventional lithography techniques. Low throughput most limits the widespread use of SPL for high resolution patterning applications. This article addresses the speed constraints for reliable patterning of organic resists. Electrons field emitted from a sharp probe tip are used to expose the resist. Finite tip-sample capacitance limits the bandwidth of current-controlled lithography in which the tip-sample voltage bias is varied to maintain a fixed emission current during exposure. We have introduced a capacitance compensation scheme to ensure continuous resist exposure of SAL601 polymer resist at scan speeds up to 1 mm/s. We also demonstrate parallel resist exposure with two tips, where the emission current from each tip is individually controlled. Simultaneous patterning with multiple tips may make SPL a viable technology for high resolution lithography.

Tip–sample distance feedback control in a scanning evanescent microwave microscope

We have developed a means of tip–sample distance regulation for a scanning evanescent microwave microscope over conductive samples. Changes in resonant frequency and quality factor are measured, where changes in resonant frequency are related to the tip–sample capacitance and changes in quality factor are related to microwave absorption. With the analytical expression of the tip–sample capacitance as a function of tip–sample distance, we can quantitatively regulate the tip–sample separation. We demonstrated simultaneous noncontact imaging of topography and surface resistance with high spatial resolution.

LocalC(U) spectroscopy on chemically bounded Au55 clusters

Ligand-stabilized Au 55 clusters were prepared on Au(111) surfaces and investigated by electrical force microscopy (EFM) in dynamic non-contact mode, which allows the detection of d.c. potential differences between tip and sample as well as the tip-sample capacitance. This work explains the measurement procedure and presents the observed C(U) characteristics of the special sample system.

Tip–sample capacitance in capacitance microscopy of dielectric films

The tip–sample capacitance in the scanning capacitance microscopy (SCM) of dielectric films is described through theoretical calculations based on the method of images. The results are explained with the charge density distribution in the tip–sample system. Furthermore, capacitance signals in the tapping mode SCM of dielectric films are experimentally investigated and found to be in good agreement with the simulation results.

Gap dependence of the tip-sample capacitance

The tip-sample capacitance has been studied in the nontunneling regime and the capacitance-distance characteristics and its dependence on the tip geometry have been determined for the gap distance 1<s<600 nm. Measurements were carried out in ultrahigh vacuum on a capacitor formed between a metal tip (W or Pt–Ir) and a clean Au(111) surface. Tips of different tip radius R=30∼4000 nm were used to investigate the influence of tip geometry on the capacitance. When the gap distance is reduced, the capacitance increases while its gap sensitivity |∂C/∂s| decreases with the gap distance. The capacitance therefore shows no 1/s divergence. The magnitude of the capacitance change is found to depend on the tip geometry: blunt tips (R>1000 nm) show larger capacitance increase than that for sharp tips (R⩽100 nm). The effective tip radius Reff estimated from the C−s characteristics agrees with the real tip radius in a limited distance range which varies with the tip geometry. At small distances (s<30 nm), Reff≃R for sharp tips but Reff<R for blunt tips. On the other hand at large distances (s>200 nm), the relation is reversed, Reff>R for sharp tips while Reff∼R for blunt tips. These results on Reff can be explained by the field concentration to the tip apex and the change of capacitance-contributing tip area with the gap distance. Capacitance calculations indicate that the capacitance of the “truncated cone + half sphere” tip well reproduces the observed C−s characteristics and its dependence on the tip geometry.

Circuit analysis of an ultrafast junction mixing scanning tunneling microscope

A lumped element circuit model for the operation of a junction mixing scanning tunneling microscope (JM-STM) is presented. Fits from this model show excellent agreement with experimental results in the picosecond time regime. The tip sample capacitance employed in the model was calculated to be 33 fF, using the method of images. By varying the capacitance, various tip/sample geometries can be investigated. Testing the response of the model tunnel junction, for faster electrical pulses, suggests how the JM-STM can be pushed into the femtosecond time regime.

Modeling electrostatic scanning force microscopy of semiconductors

A model is presented for the electrostatic interaction between the tip of a scanning force microscope and the surface of a semiconductor in condition of depletion. By representing the tip as a conducting sphere and suitably approximating the semiconductor space-charge region, analytical expressions are derived for the boundary of this region and the tip-sample capacitance and force. Operation in an immersion medium with high dielectric constant is found to enhance the doping sensitivity of the electrostatic force. As a function of the tip-sample distance, this force exhibits a zero point, if the permittivity of the immersion medium exceeds that of the semiconductor.

Measurement of the tip-sample capacitance for Si surfaces

We have measured the tip-sample capacitance C and its dependence on the gap distance s for the first time on the clean Si(001)2 × 1 and Si(111)7 × 7 surfaces. The capacitance is found to vary linearly with log s for s > 100 nm while its divergence at smaller distances is markedly suppressed. No qualitative differences are found for C-log s curves obtained on the Si(001)2 × 1 and Si(111)7 × 7 surfaces, showing little sensitivity of the capacitance to surface properties. This suggests that the tip geometry plays an important role in the C- s characteristics. We found that the capacitance formula for a conical tip gives an excellent agreement with the observed C-log s curves.

Geometrical capacitance of the tip–semiconductor junction

Capacitance–distance (C–s) and capacitance–voltage (C–V) characteristics have been measured for the tip–sample capacitance on Si(001)2×1 and Si(111)7×7 surfaces. On both surfaces, the C–s characteristics are found to be little affected by the types of sample doping and also show no appreciable bias dependence. These observations suggest that the space-charge region of the semiconductor gives no contribution to the tip–sample capacitance because of the high density of surface states on clean Si surfaces.

Scanning surface harmonic microscopy: Application to silicon and Langmuir-Blodgett films on silicon

The scanning surface harmonic microscope is a local probe method in which a microwave signal is applied across a tip-sample tunneling gap generating higher harmonics, which can then be detected. Thus this microscope is sensitive to nonlinearities in the tip-sample junction. In Si and other semiconductors it is sensitive to nonlinearities in the tip-sample capacitance voltage characteristics (C/V). Such a local C/V has been used to delineate regions of different dopant type and concentration in n + /p silicon gratings. On Langmuir-Blodgett films of dielectrics, such as docosanoic acid on Si, the scanning surface harmonic microscope is sensitive to changes in film thickness which cause shifts in the C/V characteristics of the underlying Si