Frequency Modulation Detection Method Research Articles

Surface potential measurement of n-type organic semiconductor thin films by mist deposition via Kelvin probe microscopy

We partially deposited fullerene (C60) and phenyl-C61-butyric acid methyl ester thin films that are typical n-type semiconductor materials on indium–tin oxide by mist deposition at various substrate temperatures. The topographic and surface potential images were observed via dynamic force microscopy/Kelvin probe force microscopy with the frequency modulation detection method. We proved that the area where a thin film is deposited depends on the substrate temperature during deposition from the topographic images. It was also found that the surface potential depends on the substrate temperature from the surface potential images.

Development of Noncontact Atomic Force Microscopy Operating at Low Temperatures

Noncontact atomic force microscopy (NC-AFM) using frequency modulation detection method has been widely used to investigate the various surfaces with atomic resolution. In this paper, we introduce the measurement technique of NC-AFM operating at low temperatures (LTs). First, we theoretically discuss the enhancement of the force sensitivity in NC-AFM operating at LTs. Then, we present the design and performance of LT-NC-AFM using fiber optic interferometer with quick sample and cantilever exchange mechanism. We also show the present status of the LT-NC-AFM imaging. In detail, we show the experimental results to investigate the influence of the surface stress around an SA step of Si(001) surface onto the buckled dimer at 5 K. We demonstrate that the LT-NC-AFM has a capability to detect the surface stress with atomic resolution.

Dynamic force spectroscopy using cantilever higher flexural modes

By means of force spectroscopy measurements performed with the cantilever first and second flexural modes under the frequency modulation detection method, the authors corroborate the validity of the relation between tip-surface interaction force and frequency shift for force spectroscopy acquisition using higher cantilever eigenmodes. They estimate a cantilever effective stiffness for the second eigenmode 73 times larger than the static stiffness. This large effective stiffness enables them to perform force spectroscopy with a cantilever oscillation amplitude (A0) as small as 3.6Å. The authors provide experimental evidence that, at such small A0 values, normalized frequency shift curves deviate from a A03∕2 scaling and the signal-to-noise ratio is considerably enhanced.

Noncontact-mode scanning capacitance force microscopy towards quantitative two-dimensional carrier profiling on semiconductor devices

Scanning capacitance force microscopy (SCFM) is a promising tool for investigation of two-dimensional carrier density distribution on semiconducting devices. Its sensitivity is strongly dependent on the Q factor of the mechanical resonance mode of the cantilever. Therefore, measurement in vacuum is more appropriate for increasing the sensitivity. In this letter, the authors describe noncontact-mode (NC) SCFM which is combined with the frequency modulation detection method and its signal characteristics. The authors derived a quasiquantitative calibration curve which correlates to the amplitude signal in NC-SCFM to the dopant density. Using the calibration curve, the authors obtained a quasiquantitative two-dimensional dopant density distribution map on a cross-sectional transistor device.

Investigations of Organic Materials by Kelvin Probe Force Microscopy using Frequency Modulation Method

Kelvin probe force microscopy (KFM), which has been a common method for studying electrical properties of nanometer-scale structures, is a dynamic-mode atomic force microscopy (AFM) technique combined with a traditional Kelvin probe method used for measuring macroscopic contact potential differences. It allows us to map electron affinity or work function on a nanometer scale. In this article KFM using a frequency modulation detection method (FM-KFM) is first explained briefly. The frequency modulation method plays an essential role in high-sensitive interaction force detection. We also describe applications of FM-KFM to the nanometer-scale surface potential investigations of organic materials including phase-separated self-assembled monolayer films (PS-SAM films) and single polymer crystal surfaces. Furthermore, recent results on the local surface potential study of carbon nanotube field effect transistors using FM-KFM is presented.

Capacitive resonant mass sensor with frequency demodulation detection based on resonant circuit

In this letter, capacitive mass sensing with a 250-nm-thick single-crystalline silicon cantilever is investigated. The mass sensor employs the frequency modulation detection method using an electrical LC oscillator, in which the capacitance of the sensor serves as the component of the oscillator. The displacement noise of the demonstrated capacitive detection is 0.05nm∕(Hz)0.5, which is equivalent to the capacitance change of 2.4×10−21F. It is experimentally shown that the capacitive detection is less affected to temperature fluctuation noise than optical detection. The detectable minimum mass of 1×10−14g is achieved using capacitive detection in ambient atmosphere.

Scanning probe microscopy with quartz crystal cantilever

This paper reports a quartz crystal cantilever for piezoelectric vibration sensing in scanning probe microscopy (SPM). SPM imaging by a frequency modulation detection method is demonstrated in ambient atmosphere using a 22.5-μm-thick cantilevered AT-cut quartz crystal with metal electrodes on both sides. The spring constant of the cantilever is calculated to be 10N∕m. Despite the low electromechanical coupling between the piezoelectric effect and the flexural vibration, a high sensitivity of 0.07nm∕(Hz)0.5 to vibration is achieved in the second flexural mode. The cantilever self-oscillates on the basis of piezoelectric detection, and offers short-term stability of within approximately 0.5Hz in ambient atmosphere at room temperature. The force curve of the self-oscillating cantilever shows that the self-oscillation can be sustained even when in contact with a sample.

Dynamic force microscopy using FM detection in various environments

High resolution imaging of the organic monolayer film in an ambient condition by dynamic force microscopy (DFM) utilizing the frequency modulation (FM) detection method was demonstrated. Although the mechanical Q-factor of the cantilever used was about 1000, we could image molecular rows in the monolayer film whose pitch was about 2 nm. The image was not clear probably due to the adsorbed water layer on the film. Furthermore, we also demonstrated the operation of the FM detection method in liquid with mechanical excitation of the cantilever by using a PLL circuit with a narrow operating range.

Analog frequency modulation detector for dynamic force microscopy

A new analog frequency modulation (FM) detector (demodulator) for dynamic force microscopy (DFM) is presented. The detector is designed for DFM by utilizing the FM detection method where the resonance frequency shift of the force sensor is kept constant to regulate the distance between a tip and a sample surface. The FM detector employs a phase-locked loop (PLL) circuit using a voltage-controlled crystal oscillator (VCXO) so that the thermal drift of the output signal is negligibly reduced. The PLL is used together with a frequency conversion (heterodyne) circuit allowing the FM detector to be used for a wide variety of force sensors with the resonance frequency ranging from 10 kHz to 10 MHz. The minimum detectable frequency shift was as small as 0.1 Hz at the detection bandwidth of 1 kHz. The detector can track a resonance frequency shift as large as 1 kHz. We also present some experimental results including the observations of the Si(111)-7×7 reconstructed surface and fullerene molecules deposited on the surface by DFM using this FM detector.

Surface Potential Measurement of Oligothiophene Ultrathin Films by Kelvin Probe Force Microscopy

Surface structures and local surface potential of oligothiophene single molecular films deposited on metal substrates were investigated by Kelvin probe force microscopy using the frequency modulation detection method. Two-dimensional growth of the dimethylquinquethiophene (DM5T) films with the molecular axes perpendicular to the metal substrates was observed. Furthermore, the obtained surface potential of the DM5T film was 170 mV higher than that of Pt substrate and 200 mV lower than that of Ta substrate.

Surface potential measurement on organic ultrathin film by Kelvin probe force microscopy using a piezoelectric cantilever

We measured surface potential (SP) on a π-conju-gated thiophene oligomer monolayer film deposited on a metallic substrate by Kelvin probe force microscopy using a piezoelectric cantilever. Since the cantilever has a relatively large spring constant (calculated as about 150 N/m), the frequency modulation detection method was used for tip–sample distance regulation in order to achieve high-sensitivity SP measurement. A contact potential difference between monolayer regions and the metallic substrate was clearly observed in an SP image. Furthermore, an apparent change in a contrast of the SP image was observed while the sample was irradiated with ultraviolet light.

Comparative Surface Studies at Atomic Resolution with Ultrahigh Vacuum Variable-Temperature Atomic Force and Scanning Tunneling Microscopes.

: With the ultrahigh vacuum variable-temperature scanning tunneling microscope (UHV-VT-STM), atomic-level observation has been achieved. An ultrahigh vacuum atomic force microscope (UHV-AFM) has also been developed, with success in obtaining atom images where observation in noncontact (NC) mode with a frequency modulation (FM) detection method was attempted. Using the FM detection method in the constant oscillation amplitude of the cantilever excitation mode, we have obtained atomic-resolution images of Si(111) 7 x 7 structures and Si(100) 2 x 1 structures and other structures together with STM images in an ultrahigh vacuum environment. Also shown here are contact potential difference (CPD) images using the NC-AFM method.

True atomic resolution imaging of surface structure and surface charge on the GaAs(110)

We demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously on an atomic scale, which is based on frequency modulation detection method. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs(110) surface have been simultaneously resolved with true atomic resolution under ultra-high vacuum condition. From the bias voltage dependence of the image contrast, we can verify that the sign of the atomically resolved surface charge at the point defect was positive.

Detection mechanism of an optical evanescent field using a noncontact mode atomic force microscope with a frequency modulation detection method

By using the noncontact atomic force microscope with a frequency modulation detection method, the force gradient induced by the optical evanescent field was detected in a high vacuum. We succeeded in measuring the exponential distance dependence of the force gradient induced by the optical evanescent field. Furthermore, we investigated the incident beam intensity and bias voltage dependence of the force gradient induced by the optical evanescent field. We confirmed that the detection mechanism is not photothermal effect but the surface photovoltage effect.

Measurement of the evanescent field using noncontact mode atomic force microscope

Using the noncontact mode atomic force microscope (AFM) with frequency modulation detection method, force gradient acting on the AFM tip induced by the evanescent field was measured in a high vacuum. Exponential distance dependence of the force gradient by the evanescent field was successfully measured for the first time. Decay lengths of the force gradient were estimated to be 40±3 nm and 43±3 nm for Ar and He-Ne lasers, respectively, and independent of wavelength within the experimental error. The minimum detectable force was estimated to be about 0.1 pN. There was a tendency for the measured decay length to become shorter at a distance less than z=10 nm in many cases. The force gradient induced by the evanescent field inp-polarization was larger than that ins-polarization.