Z Control

Software is required for controlling the feedback control electronics, see Section 2.3. There are two functions that are controlled; the set point voltage and the PID parameters.

• Set-point voltage: This is the voltage that goes into the differential amplifier, so this voltage is compared with the force sensor output voltage and an error signal is generated. The set point voltage controls the “relative” force. A calibration of the specific cantilever is required to convert the set-point voltage to a force, (see Section 2.5).
• PID parameter: These parameters control the “responsiveness” of the feedback control electronics. These parameters must be adjusted such that the probe tracks the surface while scanning. Section 3.4 provides a description of optimizing the feedback control parameters.

F/D Curves

Force distance (F/D) curves are used to measure the forces experienced by the probe as a function of distance from the surface. In F/D measurements, the probe is moved toward the sample surface to a pre-selected position, and then retracted. The extent of cantilever deflection over the course of this movement is expressed by the Z(ERR) signal which is used to generate a force-distance curve. Software for making F/D curves, illustrated in Figure 2-34, has several variable parameters including:

• Start and end position for probe
• Rate of probe approach motion
• Number of F/D curves to signal average
• Location on image for F/D curve