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Tuning the SCM Sensor

You may tune the SCM sensor for maximum sensitivity using Sweep commands:

  1.  Click the SCM Tune icon in the Workflow Toolbar to open the SCM Tune window, shown in Figure 1.

Figure 1: SCM Sensor Tuning Curve Parameter Settings, Tip Engaged

 
  1. Select DC Capacitance for Channel 1 > Data Type. Observe the SCM sensor resonance curve. See Figure 1.
  2. Use Sweep > Deflection Setpoint or Cursor Mode > Offset to set the Capacitance Sensor Frequency operating point.
NOTE: Sensitivity is optimized when the Capacitance Sensor Frequency is set to the frequency corresponding to the highest slope of the tuning (sensor resonance) curve.
  1. Click the Offset radio button in the Cursor Mode panel.
  2. Drag the dashed red cursor to the point of maximum response and click Execute to set the Capacitance Sensor Frequency. See Figure 1.

Tuning the SCM sensor frequency is complicated by the change in resonant frequency with engage. In a sweep width of approximately 890 MHz to 1050 MHz, the peak may shift 50–100 MHz, due to increasing capacitance as the probe tip approaches the sample. Two tuning curves are displayed in Figure 2 (withdrawn) and Figure 3 (engaged), illustrating the resonance shift with tip/sample proximity.

Figure 2: Capacitive Sensor Frequency tune (tip withdrawn)

Figure 3: Setting the Capacitance Sensor Frequency (Tip Engaged)

  A better way to perform this tune is to Engage then start the Generic Sweep by raising the probe tip 100–150 nm above the sample surface (see Figure 4) and then adjusting the VCO (Voltage Controlled Oscillator) on the side of the SCM Application Module using a non-metallic screwdriver (supplied) to maximize the slope before setting the center frequency. The probe tip will return to the surface when you Exit the Generic Sweep function.

Figure 4: Raise the Probe Tip 150 nm when starting the Generic Sweep

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