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Instrumentation

UHV-System

The complete UHV system is mounted on a table carried by pneumatic damping legs which in turn stand on a separate foundation for decoupling from building vibrations. Liquid helium dewar and crystat insert are located underneath the main chamber in the mniddl. It reaches down into a 1.5m deep pit in the floor and can therefore not be seen completely in the photo above. Tip and sample are inserted via a load lock in the back. The right chamber allows sample cleavage and contains a sputter gun and an evoprator cell for tip and sample preparation. Standard surface analysis can be performed with a LEED/Auger unit in the left chamber. The main chamber in the center houses the microscope itself and the chain mechanism for the vertical transfer into the cryostat. Tip and sample can be exchange in situ with a wobble stick.

Dewar and Cryostat

The UHV compatible (bakable up to 120 °C) double wall cryostat sits inside of a superinsulation dewar, which can be filled with more than 90 l of liquid helium. This volume allows for three day of contineous operation. The interior of the double wall may be filled with helium gas to regulate the thermal coupling of the helium bath to the inner wall. The design of the insert’s inner tube includes a conically shaped copper band. A heavy counter cone, made of copper, carries the microscope body. It fits exactly in the cryostat cone and thereby the microscope is cooled by thermal conductance down to 10 K. The heavy cone with the attached microscope is moved by a chain based vertical transfer mechanism between center chamber, where in situ tip and sample exchange is performned, and cryostat, where the low temperature experiments are carried out.

Microscope

The design of the microscope body was primarily guided by its use for high-resolution atomic-scale imaging in UHV and at low temperatures. Central points were UHV and low temperature comptibility, reliability of all components and a rigid design. To avoid significant misalignment of the optical detection system and high mechanical stress during temperature changes, all used materials have very similar thermal expansion coefficients over the whole temperature range (450 K down to 10 K). Coarse approach of the sample and coarse alignment of the optical detection system is done by piezoelectric stepper motors. Tip and sample can be exchanged in situ. The scan range at low emperatures is about 1 µm. Atomic resolution is routinely obtained with a vertical rms noise of about 2 pm in a 1 kHz bandwidth.

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