AFM control system

Atomic Force Microscopes (AFMs) are widely used for the investigation of samples at the nanometer scale. The metrological AFM used at the Dutch Metrology Institute (VSL) is developed for the calibration of transfer standards, which can be used to calibrate commercial AFMs. In this AFM the movements can be measured such that they are traceable to the official Dutch standard of length.

The setup uses a 3 degrees-of-freedom (DOFs) stage driven by piezo-stack actuators for sample manipulation in combination with a fixed cantilever. The piezo-stack actuators suffer from hysteresis, which acts as a nonlinear disturbance on the system and/or can change the system dynamics.

Since the metrological AFM does not operate within the desired specifications due to coupling between the different movements and external disturbances, the Control Systems Technology group of the TU/e has investigated the application of feedback control to all 3 DOFs of the metrological AFM and the design and application of a hysteresis feedforward for the asymmetric hysteresis present in the system.

To enable the implementation of this control scheme a real-time data-acquisition computer system was required with the ability to read: three 32 bit laser interferrometer signals, the AFM tip deflection and to drive the piezo stage with 20 bit accuracy.

IME Technologies has realized a hardware platform based on an EPIC single board computer and data-acquisition over a pc/104 bus. The control applications can be designed and compiled directly in Matlab/Simulink and executed on the real-time Linux platform on the EPIC board. The system is capable of performing its real-time task at a 10 kHz sample frequency and has a ultra low power consumption for using it in the cleanroom environment.

The combination of feedback control for all DOFs and the asymmetric hysteresis feedforward enables the AFM to track scanning profiles within the sensor bound of 5 nm. Real-time imaging of the sample is possible with an accuracy of 2 nm.