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A precision DIY Scanning Tunneling Microscope

Last weekend I paid a visit to Kris at the University of Bath. She is currently finishing-up with her doctoral studies and invited me to pay a visit to her lab and the materials group's DIY scanning tunneling microscope. During that weekend we scanned a few Silicon 111 wafer samples and injected Toulene molecules on the wafer.

I was quite impressed that the tunneling current noise floor of their microscope was in the order of a couple of hundred femto amperes, which is quite impressive for a measurement system operating at room temperature. It seems like they were using a commercial low-noise programmable gain amplifier produced by the German company Femto. It was quite tempting to open the amplifier box and see what's inside. I doubt that there is a custom OPA in it, but I really wonder which commercial chips did they use and what was this low-noise PGA feedback configuration used. Anyway, here are some fancy pictures of my visit and this has now inspired me to run an investigation on the OP and low-noise needle amplifiers used.

Outer look of the STM, the long pipes are manipulating shafts used inside the chamber; the turbopump as well as the piston pumps can be seen on the left side of the machine
Different angle, the turbopump as well as the head chamber are seen in the centre of the image
Here is the scanning head, together with a sample attached. The head is using piezo elements for micropositioning and scanning in both ways
The low-noise programmable gain amplifier manufactured by Femto Gmbh, note that the gain is changed by turning the rotary switch and not electronically
An STM image of the Si 111 sample, we can see the individual atoms as well as some impurities. Seems like the Czochralski method is indeed a brilliant casting technique

Besides the immense pressure/vaccuum required for this machine to work, the electronics behind it is fascinating. A PLL and all other sorts of regulating loops to precisely control the needle. The piezo elements have a natural vibrating frquency for which it should be corrected. A needle with passing fA currents though the sample, very low-noise programmable gain amplifier with filters and a 16-bit analog to digital converter. Moreover some digital image acquisition and needle vibration correction algorithms are used to reconstruct the images. Simply a symphony of virtuoso circuits!


Date:Tue May 09 15:00:23 CET 2015


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