CONTROL-CIRCUIT FOR A SCANNING TUNNELING MICROSCOPE

We have successfully built and tested a circuit designed to control a piezoelectric tube scanner having the standard single inner-electrode quartered outer-electrode configuration, using digital-to-analog (D/A) converters commercially available. To avoid noise associated with the PC, the signals transmitted by the D/A channels to the control electr onics are received by instrumentation amplifiers WA 105 at the control circuit, providing 86 dB common mode rejection, thereby over four ord ers of magnitude of immunity to common mode noise. To prevent ground l oops in the communication between the control electronics and the anal og-to-digital (A/D) converters, a novel approach was used. The signals sent by the control electronics to the A/D converters were transmitte d via isolation amplifiers ISO 122 followed by a 10 kHz Sallen-Key low pass filter incorporated at each output of the control circuit, provi ding galvanic isolation between the control electronics and the PC, th ereby eliminating ground loops. The control circuit was designed to al low analog as well as digital feedback, selectable via a toggle switch . The design also incorporates the possibility of using two independen t external signals to modulate the polarization of the sample and two independent external signals to modulate the piezoelectric transducer drive along the Z direction. It also incorporates the possibility of e lectronically canceling the slope that might occur while scanning due to the sample being tilted along the X axis (fast scan direction) and/ or along the Y axis (slow scan direction),. The circuit was rested usi ng two 12 bit A/D-D/A converters DAS 1602 to control the scanner of a scanning tunneling microscope, with a home-built scanning head, electr ometer, and preamplifier. With the complete system in operation but in the absence of tunneling current, the electrometer exhibits a current noise under 3 pA rms and a response time of 30 mu s to a step input c urrent, a performance that compares well with;that of bulkier and more expensive commercial low noise current amplifiers. To calibrate the i nstrument and verify proper control operation, we obtained images of a commercial holographic grating covered with gold running the instrume nt in the digital feedback mode, using the algorithm described by Pine r and Reifenberger [Rev. Sci. Instrum. 60, 3123 (1989)]. The control c ircuit and the electrometer turn out to be about one order of magnitud e less expensive than commercially available control circuits and low noise current amplifiers of similar performance. (C) 1998 American Ins titute of Physics. [S0034-6748(98)00509-7].