New variable low-temperature scanning tunneling microscope for use in ultrahigh vacuum

We describe the design and characterization of a new variable low-temperature scanning tunneling microscope @TM) which has been proven to give atomic resolution at temperatures between 77 K and room temperature but which is also capable of performing experiments as low as 4 K. The STM “head” itself consists of a unique cold dewar made up of an upper and lower reservoir connected together by two tubes which pass through an 8 in. conflat flange. The STM stage is suspended in between the two reservoirs by three long springs which pass through holes in the upper reservoir and also attach to the top flange. An adjustable cold shroud surrounds the STM stage for radiation shielding while allowing sample and tip transfer when raised. It has the additional advantage of providing a controllable heat leak. By raising the shroud, the temperature can be increased: by lowering it, the temperature can be decreased. The cold reservoirs can be filled with either liquid nitrogen or liquid helium. In the case of liquid helium, an additional liquid nitrogen “jacket” surrounds the STM head. Everything is operated inside a customized ultrahigh vacuum chamber containing low-energy electron diffraction and various sample preparation facilities. A detailed description of the STM is presented together with performance results.