Closed Loop SCM Imaging

1. Complete Step 1 – Step 6 of Open Loop SCM Imaging, to obtain a good open loop SCM image.
2. Set SCM Feedback to Enabled to automatically feedback on the amplitude signal from the lock-in amplifier.
3. Set the Channel 2 Data Type to Feedback Bias.

Figure 1: Feedback Bias SCM Image in Closed-Loop Mode

4. Set the SCM > Feedback Setpoint to an initial value of 0.2 V.
5. The proportional and integral gain controls of the capacitance feedback loop are in the SCM panel (see SCM Igain and SCM Pgain in Figure 2 in Software Selection). Adjust the gains as indicated in Table 1, in combination with Feedback Setpoint within its range (see Table 1 in Open Loop SCM Imaging) to optimize image quality.

Table 1: Feedback Settings for Closed Loop SCM in the SCM panel

An example of closed loop SCM operation is shown in Figure 1. The less doped areas result in a low signal, while the highly doped areas result in a high signal. In closed loop mode, N-type and P-type semiconductor material result in the same polarity of output signal.

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