Closed loop vx open loop scanning
optional items for low temperature microscopy
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Scanner operation normally relies on the assumption that the relation between applied voltage and displacement is linear. While some scanners (mostly small range scanners) indeed behave relatively linear at low temperature, most scanners show large non-linear behaviour and hysteresis. Creep, i.e. drift in position after approaching a certain location, is a further phenomenon which is common to all piezo scanners. For experiments, where repeatably locating a single feature on a surface is crucial,
or where hysteresis and non-linearity are not acceptable, attocube now offers closed-loop functionality for all ANP101/ANS100 positioning units. Choose from low-temperature compatible interferometric or capacitive position encoders depending on application type and neccessary resolution.

Principle.
For closed-loop scanning functionality, the true position of a scanner is measured using highly sensitive position encoders. By comparing the true value with the position setpoint for every pixel of a scan frame and by running a feedback loop on the position, the scanner can be linearized. The ASC500 Scanning Probe Microscope controller stores recorded data (e.g. AFM topography) as function of true and setpoint position.

Interferometric encoders.
If highest precision and stability is required, the interferometric encoder (attoFPSensor) is the one and only choice. This encoder allows to maintain sample position with better than 5 Angstrom rms noise while simultaneously achieving 250 pm position resolution over the full travel and scan range of an ANP101 positioner/scanner assembly (5 mm).

Capacitive encoders.
Capacitive encoders provide a slightly higher measurement noise than the interferometric encoder, but are more cost efficient. With the capacitive encoder, the full 5 mm combined travel and scan range of ANP101/ANS100 stack can be accessed. In the quasi-static case when the sample is kept at a certain position for e.g. spectroscopic measurements, the capacitive encoded solution yields a position stability of typically 5 nm rms.

Closed-loop option for scanning probe microscopes.
Both encoder types can be integrated into any 2” version of attocube’s low temperature compatible SPM modules. With this design, attocube genuinely combines true closed-loop scanning and closed-loop positioning over the entire travel range (typ. 5mm).