attoAFM III in Dry Dilution Refrigerator
scanning probe microscopy goes dry mK
For this project, we have developed a mK tuning-fork AFM for a dry toploading dilution refrigerator (DR) in close collaboration with BlueFors. The DR has a cooling power of typically 300 μW at 100 mK and a base temperature of 8 mK. The cooling power at the sample location on the toploading insert is 100 μW at 100 mK. Thanks to the toploading probe, the turnaround time is typically 9-11 hours, and hence tip and sample exchange can be achieved within a reasonable time frame without warming up the whole DR including the superconducting magnet.
The customized attoAFM III has been carefully adapted for the mK environment in terms of cabling and thermalization, and the whole configuration has been tested and optimized thoroughly at mK temperatures. The microscope is intended to be used for scanning gate microscopy (SGM), where the wire-type AFM tip on the tuning fork serves as a mobile local gate. This allows to characterized electrical transport properties in mesoscopic samples on the nanoscale as a function of gate position and tip potential.
Previous implementations of such mK microscopes  have involved heavy spring isolation of the microscope inside the DR, which yields good results. However, it makes the design and practical use much more complicated, since any material shows a finite susceptibility and hence undesired motion in strong magnetic fields.
Despite the toploading probe design, and despite having no spring isolation on the microscope module, the tuning fork AFM system reaches 2.9 nm rms z noise (5.12 ms). This is roughly on the same order of magnitude as previously reported in literature  including a heavy 6 kg damping stage inside the DR.
At typical tip-sample distances of a few tens of nanometers, this is well suitable for scanning gate microscopy experiments.
 M. Pelliccione et al., Rev. Sci. Instrum. 84, 033703 (2013)
PRODUCT KEY FEATURES
- dry dilution refrigerator, BF-XLD250, with 80 mm toploading insert
- base temperature (DR): < 10 mK (typical 8 mK)
- cooling power (DR): >250 μW@100 mK (typical 300 μW@100 mK)
- cooling power (sample location in toploading probe): >100 μW @ 100 mK
- base temperature (microscope): < 20 mK
- turnaround time (empty toploading probe): 6-8 h (typical)
- turnaround time (toploading probe with AFM): 9-11 h (typical)
- magnet: 5 T/1 T/1 T
- customized attoAFM III for mK operation
- modular microscope also compatible with attoDRY1000/1100/2100
- sample holder for 20 pin leadless ceramic chip carriers (LCCC)
attoAFM I Mounted on Mixing Chamber
This scan above nicely demonstrates the stability of a complete system, an attoAFM I together with a LD400 cryo-cooler from BluFors Cryogenics. For this 25 x 25 μm² (800 x 800 pixel) scan, the pulse-tube cooler was enabled the whole time. The temperature was stable at ~40 mK measured at the mixing chamber – it was slightly higher due to the higher scan speed of close to 1 μm/s.
|(attocube applications in collaboration with BluFors Cryogenics, 2014)|
attoAFM III in Toploading Insert
This data was taken with a tuning fork attoAFM III specifically designed for mK operation. The extremely sensitive microscope was mounted on a toploading insert (see images and project description on the left page), which ensures a much higher usability in terms of turnaround times upon tip and sample exchange than if it were mounted directly on the mixing chamber. The sample temperature in the toploading DR was 55 mK during the scan at 100 nm/s. The images nicely demonstrates that the delicate microscope works reasonably well 2 μm even under these extreme conditions.