Closed loop controller and method for fast scanning probe microscopy
A method of operating a metrology instrument includes generating relative motion between a probe and a sample at a scan frequency using an actuator. The method also includes detecting motion of the actuator using a position sensor that exhibits noise in the detected motion, and controlling the position of the actuator using a feedback loop and a feed forward algorithm. In this embodiment, the controlling step attenuates noise in the actuator position compared to noise exhibited by the position sensor in a bandwidth of about seven times the scan frequency. Scan frequencies up to a third of the first scanner resonance frequency or greater than 300 Hz are possible.
1. A method of operating a metrology instrument, comprising:
generating, with an actuator, relative motion between a probe and a sample at a scan frequency greater than ⅓ rd the fundamental resonance of the actuator over a selected scan size;
controlling the XY position of the actuator using both a feedback loop and a feed forward algorithm, wherein the bandwidth of the feedback loop is different than a bandwidth associated with operation of the feedforward algorithm; and
repeating the generating and the controlling steps to zoom to a second scan size smaller than the selected scan size while maintaining XY position error at less than about 1% of the selected scan size.
2. The method of claim 1 , wherein the feed forward algorithm includes using an inversion-based control algorithm.
3. The method of claim 2 , wherein the inversion-based control algorithm uses at least one transfer function associated with the actuator.
4. The method of claim 2 , wherein the inversion-based control algorithm adaptively produces a correction that contributes to a control signal that compensates for at least one of the non-linearities and the dynamics of the actuator.
5. The method of claim 4 , wherein the control signal produces a peak position error of less than about 1% of the total scan range after no more than about 10 iterations of 10 scan lines per iteration.
6. The method of claim 5 , wherein the control signal produces a peak position error of less than about 1% of the total scan range after no more than about 5 seconds.
7. The method of claim 1 , wherein the resonant frequency of the actuator is greater than about 100 Hz and the scan frequency is at least about 50 Hz.
8. The method of claim 7 , wherein the scan frequency is at least about 300 Hz.
9. The method of claim 1 , wherein the bandwidth of the feedback loop is less than the scan frequency.
10. The method of claim 1 , wherein the controlling step attenuates the noise in the actuator position to less than about 1 Angstrom RMS within a noise bandwidth equal to about seven times the scan frequency.
11. The method of claim 10 , wherein the controlling step includes using a PI controller.
12. The method of claim 1 , wherein the feed forward algorithm is an adaptive feed forward algorithm that estimates a transfer function of the actuator in response to the position error and adjusts the generating step based at least in part on the transfer function.
13. The method of claim 12 , wherein the response of the actuator is dependent on an operating condition.
14. The method of claim 13 , wherein the operation condition is at least one of scan frequency, size, angle, and offset.
15. The method of claim 1 , wherein the feed forward algorithm includes using an inversion-based control algorithm and iteratively produces a correction that contributes to a control signal that compensates for at least one of the non-linearities and the dynamics of the actuator.
16. The method of claim 15 , wherein the control signal produces a peak position error of less than about 1% of the total scan range after no more than about 5 seconds.
17. The method of claim 15 , wherein the correction is a waveform.