Using TipProtect to extend AFM tip life expectancy

Using a Zurich Instruments Lock-In amplifier, you are enabled to perform a multitude of AFM measurement modes. Particularly interesting is NC-AFM operation which makes use of the built-in PLL capability. Earlier, a user could easily crash his tip into a substrate by providing a wrong z-feedback gain or by getting into the region of inverted resonance frequency shift. To prevent that situation, TipProtect can be activated to detect an operation case that is potentially harmful for the AFM tip. Any user who is equipped with the “AFM bundle” consisting of the HF2LI-PLL and HF2LI-PID options can make use of this feature for PLL-based NC-AFM scans.

Hardware Requirements

Optional Software Requirements

  • National Instruments LabVIEW
  • The Zurich Instruments Q-Control Example VI which can be found in the Zurich Instruments LabVIEW API’s example folder
  • PuTTY Telnet Client

TipProtect Principle

TipProtect acts on an auxiliary output channel by applying a default offset voltage to it. This is either the z-feedback signal or the Δf signal used for the z-feedback loop. As TP builds on top of the PID option, any other output accessible with the HF2’s PID controllers can be selected as well.

The trigger to enable TP is derived from the PLL’s phase error signal. The phase error is read at a high speed (about 100kS/s), squared, averaged and compared to a threshold level. If the error exceeds this limit for a specified amount of time, TipProtect will set an “Active” flag which is accessible using the API. Furthermore, PID operation is interrupted and the above-mentioned “default” value is written to the PID output. This behavior corresponds to enabling “output default” and turning off the PID.

TipProtect Block Diagram

TipProtect’s Role in the Z-Feedback Loop

In the following block diagram, the role of TipProtect in the AFM setup is depicted. As previously explained, PID operation is interrupted when TipProtect turns active. In this state, the “output default” value is applied to the PID’s output. Therefore, the z-piezo is retracted to a default position that is safe for the tip.

Tip Protect Globa

Note: Depending on the way the HF2PLL is connected to the AFM setup, the output signal applied to the auxiliary port of the HF2 might also be the Δf-feedback signal instead of z. In this case, the PID would offset the output Δf signal by a large amount leading to a large error signal at the z-controller which – in turn – will retract the tip as well.

TipProtect example LabVIEW VI

To allow an easy usage of the TipProtect functionality, a LabVIEW base graphical user interface is available. It visualizes the most relevant parameters and enables to set up TipProtect for any of the four PID controllers.

TipProtect Screenshot

Setting up TipProtect

  • Open the TipProtect example VI
  • Select a PID controller that you want to use to protect your tip
  • Select the PLL to be kept an eye on
  • Setup the time constants to activate / deactivate TipProtect
  • Select the PID’s output to be the auxiliary output that you use for the delta f signal (to Z-controller)
  • Activate “default output” and specify the value that you’d like to add on top of the df signal in case TipProtect is active (PLL unlocked)
  • Set the output center and all gain parameters to zero to ensure that no offset is added to the delta f signal in case TipProtect is inactive
  • Enable TipProtect by clicking the example VI’s enable button

TipProtect Underneath

TipProtect can be set up using the text-based interface of the HF2LI. Please read the chapter “Programming and Connectivity” of the instrument’s user manual to learn about how to set up PuTTY and browse through the settings.

HF2LI Textbased Interface

  • PIDS/*/TIPPROTECT/PLLSelects which PLL’s error is supposed to be tracked
  • PIDS/*/TIPPROTECT/INACTIVETIMECONSTANTTime constant for activating TipProtect
  • PIDS/*/TIPPROTECT/ACTIVETIMECONSTANTTime constant for deactivating TipProtect
  • PIDS/*/TIPPROTECT/INACTIVETHRESHOLDError square thresholdlevel for activating TipProtect
  • PIDS/*/TIPPROTECT/ACTIVETHRESHOLDError square thresholdlevel for deactivating TipProtect
  • PIDS/*/TIPPROTECT/ACTIVEIndicates that TipProtect is active
  • PIDS/*/TIPPROTECT/ENABLEEnables TipProtect and thus automatically interferes with the PID operation

Theoretically, up to four PID controllers can be set up with TipProtect. In practice, only two PID controllers working in parallel seem to be reasonable due to the availability of only two PLLs. However, an additional controller working on the same PLL’s error can be used to output a digital or analog “trigger” signal, highlighting the regions of the scan where TipProtect interfered with the scan.

Appendix: Protection based on Dissipation

TipProtection relies on the PLL lock condition. It does not take into account an output amplitude corresponding to the energy dissipated in the substrate. If the PLL locks on a contact resonance, TipProtect does not activate. There are two ways to circumvent this. The easies is restricting the PLL frequency range to force an unlock if the resonance frequency drifts off too far. Another way would be to limit the drive amplitude or compare the drive amplitude to some threshold using a PID controller. By setting a high gain for this controller and restricting the its output range, an offset can be applied to any auxiliary output.

Acknowledgements

I’d like to acknowledge the contributions of Dragan Lesic and Daniel Wright to this blog entry as well as Tino Wagner for field testing.