Modular ultra-high vacuum millikelvin scanning tunneling microscopy

Our newest UHV, millikelvin microscope – the ultra-low temperature scanning tunneling microscope (ULT-STM) – came online March 2019. However, instrumentation development is an ongoing enterprise due to its modular design (D. Wong, et al. Rev. Sci. Instr. 91, 2020). The microscope head (see Figure 1), which includes two slots for samples, the tip, scanner, and walkers, has an electrical connector on its bottom that allows the head to be plugged into a matching 30-pin electrical feedthrough. The ULT-STM has two such electrical feedthroughs: one at room temperature for rapid diagnostic purposes and one inside the cryostat, thermally connected to a dilution refrigerator. A transfer manipulator and linear translation stage (see Figure 2) allows the microscope head to be moved between these two electrical feedthroughs, as well as to a microscope load lock, where the microscope head can be removed from vacuum for repairs and upgrades. 

Since we can move the microscope head in and out of vacuum at will, we are able to work on constructing a next-generation microscope head without interrupting ongoing experiments in the ULT-STM. Potential upgrades that may be incorporated in the next iteration of the microscope head include cryogenic filtering, four-terminal electrical transport capabilities, a slidable door on the microscope cover, and modifications to the physical structure for improved thermal and vibrational performance. We are also planning to add other scanning probe techniques, such as non-contact atomic force microscopy (nc-AFM). (Read about our new millikelvin atomic force microscopy.)

An additional exciting feature on the ULT-STM is the ability to change the magnet. The liquid helium dewar and superconducting magnet can be detached from the instrument without warming the dewar (i.e. while cryogens are still inside the dewar) or venting the UHV system. The ULT-STM is currently equipped with a 9-1-1 T vector magnet, but we are in the process of procuring a 16 T single-axis magnet. The development of this capability will allow us to quickly switch between modest in-plane fields and large out-of-plane magnetic fields as soon as it is needed by an experiment.

Finally, we plan to outfit the ULT-STM with the capability to perform atomic depositions and to grow epitaxial thin films. Thin films can be prepared in a dedicated vacuum chamber outfitted with appropriate atomic sources and directly connected to ULT-STM.

ULT; cross-sectional view of vacuum chambers.

Figure 2. Cross-sectional view of vacuum chambers. The “transfer chamber” contains a vertical transfer manipulator that is able to grab the microscope head and lower it into the “UHV tube,” which is surrounded by the cryostat.  The “microscope load lock” contains a similar vertical manipulator that is able to move the microscope head into the load lock.