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.
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!
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