Digital Scanning Tunneling Microscope Based On DSP

With its atomic scale resolution scanning tunneling microscope (STM) has found wide application in lots of fields, such as nano-mesurement, nano-electronics, nano-materials, etc. Incorporation of digital signal processor (DSP) to make fully digitized STM has become an important approach in STM instrumentation. It provides more simplicity for instrument structure, more stability of performance and more flexibility of functionality.The thesis developed a fully digitized STM employing a DSP chip, TMS320VC5416, which takes charge of all the logical control of the operation such as X, Y scan movement and feedback adjustment of the tip height along Z direction, the acquisition of the tunneling current, etc. The image data were transmitted to a computer via serial interface for post processing, display and storage, which greatly decreased the burden of the computer. In above digitized STM 14 bit D/A converters AD7840 with a conversion time of 21ns and 12 bit D/A converters AD565 with a conversion time of 21ns were used for high precision X,Y and Z movement control of the STM piez-electro tube respectively. Meanwhile a 12 bit A/D converter MAX120 with a conversion time of 1.6us was used for tunneling current acquisition. The whole image scan process was under the control of a CPLD. As a result, a X,Y movement precision of 0.1nm, a Z movement precision of 0.01nm and a feedback control speed of 500KHz were realized, which meet well the requirements for STM instrument.For software design the control program for TMS320VC5416 was composed under CCS environment and CPLD logic was realized using VHDL language. Correct X,Y and Z control signal were obtained based on these software. As the experiment shown, the above fully digitized STM based on DSP has a feedback control speed five times faster than that of a STM based on ISA bus, which make possible of more stable operation.