A DIGITAL INTEGRATOR AND SCAN GENERATOR COUPLED WITH DYNAMIC SCANNING FOR SCANNING TUNNELING MICROSCOPY

A novel digital integrator circuit has been engineered for constant-current mode STM feedback control. In contrast to analog integrators that rely on charging or discharging of a capacitor in an amplifier feedback loop, the integrator uses a 16-bit digital counter that is rapidly incremented or decremented depending on whether the tunneling current is higher or lower than a reference value. The counter drives a digital-to-analog converter (DAC) whose output is amplified and used to control the STM Z-axis scanner. A computer's parallel input port records the DAC output word at each sampling point, directly giving the digital value of the tip position. The integrator is used in conjunction with a two-axis 16-bit scan generator. The Z-axis feedback reference circuit is used to dynamically control the STM's lateral scanning rate, and scan rates of 300-mu-m s-1 are demonstrated. The digital integrator and scan generator facilitate fast switching between different modes of STM operation, allowing glitch- and drift-free locking of tip position, interchange of axes, scanning or switching the sign of the tip bias, and flexible tailoring of the feedback response.