Overview of the SECM Apparatus

Accurate and reproducible positioning of the probe in three dimensions is an important design element in SECM. An SECM will typically allow movement in three orthogonal directions x, y, and z. Ideally, the positioning elements for the SECM will allow a probe to move at desired scan rate (e.g., /Ain/s) over a given range. In addition, the positioner axes of motion are 

Most SECM experiments use tips with 1-25 /xm diameters. Thus, a positioner for these probes should be able to scan regions of 100-1000 /xm square at scan rates up to 50 /xm/s. For these conditions, motorized positioners are suitable. With smaller probes, the greater accuracy of piezoelectric tube or tripod positioners of the type found in STM and AFM instruments is required. 

Several SPM instrument manufacturers have corrected scanner nonideality by incorporating sensing devices in the piezoelectric elements. This modification allows real-time feedback control of the extension and eliminates many of the above problems. A number of different sensor types are 

Micrometers are also useful positioning elements. Although manually operated micrometers are not suitable for most imaging applications, they provide an inexpensive way to acquire current-distance curves and perform substrate modifications (17,18). Submicron positioning is available with differential micrometers. Motorized micrometers are widely available, and micrometers with DC servo motor drives in closed-loop operation (see below) can give excellent results. 

Controllers for piezoelectric tube or tripod scanners require high-stability, low-noise voltage amplifiers. Since piezoelectric materials have a large 

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