A Modular Design for Ultra-High Vacuum Millikelvin STM Systems

We describe the unique modular design of an ultra-high vacuum (UHV) STM system, capable of reaching dilution-refrigerator temperatures (10 mK) and equipped with a vector magnetic field. The goals of our design are to separate the UHV needs of STM instrumentation from the typical non-UHV construction of a dilution fridge system, and to make the STM head module interchangeable. To accomplish this, a custom-built dilution refrigerator surrounds a cooled UHV tube chamber, which houses our STM, with specially designed electrical connections at low temperatures to connect to an interchangeable STM head module. For diagnostic purposes, the microscope can also be used at room temperature in the UHV preparation chambers that are integrated with the low-temperature UHV tube system. The microscope exchange can be done at any time without compromising cryogenics or vacuum. The system works with a 240L liquid He cryostat, with a vector magnet (9T-1T-1T), that has a 10-day hold time. Our current microscope head design features multiple contacts to perform STM on gated structures and simultaneous low-temperature transport measurements on devices. Microscope designs, instrument construction, and preliminary data will be discussed.