Scanning a metal probe over a surface while maintaining a separation of a few angstroms requires very strict design parameters. The tunneling current measured by the STM is also exceedingly small. Generally, this current varies from several picoamps to a few nanoamps. Measuring currents at this level requires amplifiers with very high gain and noise immunity. Advances in computer technology, data acquisition, and digital control systems enable the modern STM to be completely computer controlled.
Computer control creates certain advantages as well as additional design constraints.
The STM designed here operates under computer control in an air environment. Figure 5 shows a block diagram of the system. To drive the piezoelectric transducers in the STM head, a two stage amplification system provides a gain of approximately 10. A current amplifier reads the tunneling current from the STM head and then the DSP card digitizes this information. Three power supplies provide DC power for the piezoelectric amplifiers and the current amplifier.