Capacitive Approach provides a fast and reliable SPM pre-approach procedure based on tip-sample capacitance.

The measurement of the absolute tip-sample capacitance provides an ideal solution for a fast and reliable pre-approach as the absolute tip-sample capacitance shows a generic behavior as a function of the distance, even though measured on several completely different setups [Voogd, de et al.].

Key aspects are:

After this pre-aproach procedure, the conventional approach facility is used to complete the approach procedure.

The tip-sample capacitance is measured by modulating the bias voltage (u-sample) with a sine wave of ca. 10 kHz, 1Vp and determining it from the amplitude of the sine on the tunnel current measured via the first ADC.

The timing generator and u-sample (in y-generator mode) are configured to generate a 100 sample, 10 kHz sine. Amplitude samples are obtained at maximum (hardware-bus determined) frequency in a separate thread. On the WinXP ELDPC2 computer at hand the frequency of sampling and amplitude detection is about 165 kHz.

This configuration gives a 3-digit stable result with a low-pass filter cut off frequency of 0.5 Hz.

Setup for amplitude measurement:

--------------------------------------------+
                                            |   Amplitude detector
   +-----+   +--------+  C   +---+   +---+  |  +------------------+
   |T-Gen|---|U-sample|--||--|IO |---|ADC|-----| HPF...abs...LPF  |--o Upeak
   +-----+   +--------+      +---+   +---+  |  +------------------+
                  Fmod               Fs-adc |
                                            |
                                SPM Rack    |   Software
                                            +--------------------------------