Closed Loop TUNA Imaging

TUNA can be operated in both open and closed loop modes. However, due to the strong capacitance between the tip and sample (see (C) in Figure 1), closed loop TUNA measurements may not yield the desired results.

Figure 1: Feedback Loop in Closed Loop Mode

The tip-sample capacitance is on the order of a few pF and yields a current (I).

where:

• I = current
• C = probe/sample capacitance
• dV/dt = time rate of change of voltage

As the voltage changes rapidly (due to the feedback loop), it is possible for the feedback current (see (A), Figure 1) to become greater than the current being measured. Therefore, lower gain settings (e.g.,1 nA/V and 100 nA/V) are recommended when operating in closed loop mode.

Closed Loop Procedure

Prior to engaging in closed loop mode, follow the procedure in Open Loop TUNA Imaging. Be sure to adhere to the control parameters set forth in Table 1 in Software Selection. After finishing the procedure and while still in open loop mode, complete the following:

1. In the TUNA panel:
   1. Select the Bias Lower and Upper Limits. Start with a low setting (+/– 2V) and slowly increase the voltage while observing the current at the selected feedback point.
   2. Select the Current Setpoint. This value is dependent on the gain setting.
   3. Set the Feedback Mode to Enabled (you are now operating in closed loop mode).

In the appropriate channel, set the Data Type to Feedback Bias to view the applied bias.

In the appropriate channel, set Real-Time Plane Fit and Offline Plane Fit to None.

The feedback current and bias limits will depend upon the sample being imaged and the gain settings for the TUNA sensor.

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