A digital control system for scanning tunnelling microscopy and atomic force microscopy

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Abstract
The authors present a flexible, high performance digital feedback and scanning system for the control of both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) which they have shown is capable of taking atomically resolved images. The system is based on an AT&T DSP32C digital signal processor (DSP) which functions as a 'front-end' controller independently of the AT compatible host computer. A 'pseudo 18-bit' system is employed for the feedback loop which can control the Z-movement of the tip down to 0.003 nm whilst allowing a total excursion of up to 800 nm. The system samples at 60 kHz; this is equivalent to an analogue system with a bandwidth of about 20 kHz. Apart from the feedback algorithm, the hardware and software for both the STM and AFM operation are identical. An 'open architecture' philosophy allows the system to be reconfigured for different instruments.

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