Sample holder: thermo-mechanical stability

The VT STM incorporates a unique thermal drift compensation (the result of finite element analysis performed at the FOM Institute for Atomic and Molecular Physics in Amsterdam). The heart of this design is the variable temperature sample holder with integral heater and thermocouple, ensuring that the true sample temperature is measured. The design is such that thermal expansion is symmetric, so the only remaining drift is the unidirectional movement of the sample with respect to the holder. The full temperature range is accessible without the need to change sample holders. Details about the thermal specifications can be found here.

High temperature capability

Not only is it possible to ramp the temperature while scanning at low temperature, where thermal expansion coefficients tend to zero, but also at high temperatures where the thermal expansion coefficient is high. The microscope is highly stable at elevated temperatures with a drift of less than 1 nm/min at 500 K. As the power requirements are more demanding at higher temperatures, 20 W heating capability, or 60 W using electron beam heating, is provided in the sample holder together with a thermocouple for convenient and accurate temperature measurement.

For changing samples, the whole sample holder can be transferred out of the STM by means of a wobble stick. The sample-holder is a compact seized unit which can be flexibly be transferred as a whole. It contains the actual sample, the approach mechanism, the heater and the thermocouples for the temperature read-out.

Fast scanning necessitates an ultra light tip mount and thermal drift compensation requires accurate tip alignment with the sample. To omtimize this, and for ease of use the tip is mounted into the scanner assembly outside the vacuum chamber. This tip mounting can be performed outside the vacuum system. Once transferred into the vacuum system, the scanner with tip can be mounted on the STM, or stored as a complete unit on a parking station (for fast in-situ tip replacements). The scanner assembly can be manipulated with the same wobble stick as used for the sample holder transfer. A single scanner-assembly can be used for the full temperature range of the instrument.

For tip conditioning the assembly is compatible with Argon ion sputtering and annealing. The scanner can be equipped with a standard cilindrical piezo tube, or with our special high-speed conical scan piezo. This not only improves the signal-to-noise due to its stiffness, but also the maximum scan rate.

Optimal access to sample

The open design makes it possible to direct an ion gun, laser or deposition source onto the sample, and to collect light from the sample, with the scanner in place and while tunneling. This provides considerable additional flexibility to study complex dynamic processes. With the scanner assembly removed there is completely open access to the sample, which may also be heated or cooled in situ without the need to be removed to a remote station.

Vibration isolation

The rigid design of the STM mounted on a high mass platform combined with spring suspension and eddy current damping, provides not only good vibration isolation but also low sensitivity to noise. The excellence of the vibration isolation has been demonstrated by the ability of the variable temperature STM to produce atomic resolution images of metals without the need for additional isolation.

Fast scanning: video STM

Real time video rate imaging is possible with this microscope. Dynamic phenomena can now be slowed down or sped up by changing the temperature and recorded as a sequence of images displayed in real time.

New applications include for fast scanning include studies of mobility and assembly processes as a function of temperature. Scans as short as 1 msec and up to 100 sec over the same atoms can be taken across an extended temperature range providing information of mobility over 5 orders of magnitude.

Other applications combining the variable temperature capability with fast scanning include the atomic mechanisms underlying the thermal movement of steps. Step edges which appear fuzzy with a standard STM have now been shown to fluctuate with the diffusion of pre-existing kinks.

Low temperature option

By specifying the cooling option, temperatures down to 50 K are accessible in addition to the standard range of room temperature to 1000 K. A continuous flow cryostat provides efficient sample cooling and may be used with liquid helium or liquid nitrogen cooling depending on the experiment. The cryostat remains in place at all times without affecting the ability to reach high temperatures. The cryostat is provided with a low loss, flexible transfer tube for efficient transfer from the helium storage dewar. The thermocouple and heater on the sample holder are used for temperature control, ensuring that the true temperature of the sample is measured and controlled.

Find out more about possibilities by viewing the results obtained with the LPM UHV VT-STM in our image and movie gallery.

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