Specs Zurich Gmbh

Specs Surface Nano Analysis Gmbh

Jörg Rychen

Andreas Christian Stemmer

Dominik Ziegler

The piezoelectric response of a sample () is measured by applying a scanning probe microscope, whose probe () is in contact with the sample (). The probe is mounted to a cantilever () and an actuator () is driven by a feedback loop () to oscillate at a resonance frequency f. An AC voltage with principally the same frequency f but with a phase (with respect to the oscillation) and/or amplitude varying periodically with a frequency fmod is applied to the probe for generating a piezoelectric response of the sample (). A lock-in detector () measures the spectral components at the frequency fmod of the control signals (K, f) of the feedback loop. These components describe the piezoelectric response and can be recorded as output signals of the device. The method allows a reliable operation of the detector oscillator resonator () at its resonance frequency and provides a high sensitivity.

Method for measuring a piezoelectric response by means of a scanning probe microscope

The scanning probe microscope has a primary control loop () for keeping the phase and/or amplitude of deflection at constant values as well as a secondary control loop () that e.g. keeps the frequency of the cantilever oscillation constant by applying a suitable DC voltage to the probe while, at the same time, a conservative AC excitation is applied thereto. By actively controlling the frequency with the first control loop () and subsequently controlling the DC voltage in order to keep the frequency constant, a fast system is created that allows to determine the contact potential difference or a related property of the sample () quickly.

Scanning probe microscope and method for operating the same

The piezo-electric actuator () to oscillate the probe of a scanning probe microscope is arranged in the feedback branch () of an analog amplifier (). A current source () is provided for feeding a defined alternating current to the input of the amplifier (). The amplifier () strives to adjust the voltage over the actuator () such that the current from the current source () flows through the actuator (). As the current through the actuator () is proportional to its deflection, this design allows to run the actuator at constant amplitude without the need of complex feedback loops.

Scanning probe microscope with current controlled actuator

The scanning probe microscope applies a sum of an AC voltage (Uac) and a DC voltage (Udc) to its probe. The frequency of the AC voltage (Uac) substantially corresponds to the mechanical oscillation frequency of the probe, but its phase in respect to the mechanical oscillation varies periodically. The phase modulation has a frequency fmod. The microscope measures the frequency (f) or the amplitude (K) of a master signal (S) applied to the probe's actuator, or it measures the phase of the mechanical oscillation of the cantilever in respect to the master signal (S). The spectral component at frequency fmod of the measured signal is fed to a feedback loop controller, which strives to keep it zero by adjusting the DC voltage (Udc), thereby keeping the DC voltage at the contact voltage potential.

Scanning probe microscope with periodically phase-shifted AC excitation

To increase the accuracy and resolution of an m bit digital analog converter, n bit input values with n&#x3e;m are fed to a control circuit and converted to a series of control values for the digital analog converter using dithering techniques. When the series of control values straddles a major transition where a large number of bits are switched between 1 and 0, a corrected series of control values is retrieved from a calibration table. The corrected series takes into account the glitch effects observed at the output of digital analog converter at a major transition.

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Zurich, Switzerland

Jörg Rychen