Variable Temperature STM

The variable temperature STM head allows a range of temperatures to be accessed during scanning over the same atomically resolved area, at speeds from 1 msec to 100 sec per scan line; this can be carried out across the full temperature range in the same sample holder with a variety of different specimens including metals.
Aunique feature of this STM is that when the temperature is ramped, thermal expansion of the sample is restricted to a small unidirectional shift, dependent only on the sample material. Drifts during temperature ramping are much lower than with alternative designs and a specif ic area can thus be brought back into view over a large temperature range. An additional benefit of this is that the microscope is ultra stable and drift free at both fixed low and high temperatures to levels within piezo hysteresis.

The STM is supplied complete with a vacuum chamber and local sample and scanner handling. This includes a compact storage and transfer mechanism for sample holders and scanner assemblies. A wobble stick is used to transfer the scanners and sample holders to the STM, and also sample holders to additional transfer mechanisms when the STM is incorporated into a more complex vacuum system. (our SurfaceLab UHV systems).

Features

Atomic Resolution on Metals 50 to 1000K
Unique thermal drift compensation using finite element analysis means that the same atoms can be in view over > 200 K temperature ramp
Fast Scanning - multiple images per second displayed in real time
No sample holder or STM changes for full temperature range
Open design for easy access to the sample
Cooling option down to 50K

Specifications

Applications

The variable temperature STM opens up the opportunity for the exploration of a wide range of temperature dependent phenomena. Applications include:

Dynamic phenomena including: step, kink, adatom and vacancy diffusion
Creation and activation energy studies
High speed dynamic snapshots
Nucleation and growth
Segregation
Catalysis

Unique thermal drift compensation
The VTSTM 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. ,

Scanner and Tip assembly
Fast scanning necessitates an ultra light tip mount and thermal drift compensation requires accurate tip alignment with the sample. For ease of use, the tip is mounted into the scanner assembly outside the vacuum system and transferred and stored within the UHV system as a complete unit. This assembly is used for the full temperature range of the instrument. For tip conditioning the assembly is compatible with Argon ion sputtering and annealing. The high degree of electrical tip shielding with low cross talk/coupling combined with the low noise capability of the TOPSystem electronics means that non-intrusive low current STM can be carried out at levels as low as 2 pA.

Open design
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
Real time "Movie 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 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.

Cooling 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. The Ultrastat 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, using a diaphragm pump together with a gas flow controller to maintain the optimized flow of helium. The controller includes a needle valve to adjust the flow of gas to the pump and a vacuum gauge and flow meter for monitoring. The temperature of the sample is controlled using the thermocouple and heater on the sample holder, ensuring that the true temperature of the sample is measured and controlled. The continuous flow cryostat which provides cooling for the temperature programmable STM is also available separately for other sample cooling requirements in UHV. The continuous flow design provides variable temperature operation over a range from 3.8 to 500 K in a compact cryostat with low helium consumption. For further details and a data sheet please contact your local Oxford Instruments representative.

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.

Specifications of Variable Temperature STM

Temperature range standard Low temperature option	Room temperature to 1000K 50K 1000K
Maximum scanned area	3.5 x 3.5 x 1.2 um
3 axis coarse lateral positioning	2.5 mm in X an Y, 2mm in Z
Approach Mechanism	Piezo inertial drive
Sample size	5 x 5 x 3 mm (max)
Z resolution	0.003 nm
X, Y resolution	0.02 nm or better
Tunnel current range	1 pA to 100 nA
System noise	Better than 0.7 pA RMS at 3.0 kHz bandwidth, 6 dB per octave roll-off
Bias Voltage	±400mV, ±4 V, ± 1 0 V, ±200V each with 12 bit resolution
Drift	Better than 1 nm/min at 500 K
Unidirectional drift	Typical 1- nm/K dependent on thermal expansion coefficient of the sample
Heater Power	20 W, 60 W with e beam heating.
Max. sample temperature at preparation point	1550K
Base temperature with cooling option	50K (120K with LN2 cooling)
Temperature drift at sample	Below 0.1K/min after 4.5 hrs (3,5 with LN2)