OPTICAL SCAN-CORRECTION SYSTEM APPLIED TO ATOMIC FORCE MICROSCOPY

Nonlinearities inherent in the piezoelectric actuators used in high-resolution scanning probe microscopies limit the usefulness of the instruments for precision dimensional measurements of submicrometer to micrometer scale structures. These nonlinearities can result in images where the scale changes by over 40% from one region of the image to another. This paper describes a simple two-axis optical beam displacement sensor that is used to accurately measure the (x,y) position of a piezoelectric tube scanner used in an atomic force microscope. This sensor has a noise level of 6.1 angstrom(rms) over a frequency range of 0.5 Hz- 1 kHz and a stability of about 200 angstrom over a 30-min period. Two different methods were used to correct the scans: postimaging software image correction and real-time feedback scan correction. The software method allows fast imaging and does not alter the control of the microscope, but requires postimaging image processing. It also loses some image information because of interpolation errors and the necessity of cropping the image to recover a rectangular image area. Feedback correction uses the sensor and a control system to accurately position the scanner for each data point of the image. This method eliminates the postprocessing of the images as well as interpolation errors, but limits the achievable scan speed and also adds the noise of the sensor and control system to the scan itself. Finally, both scan-correction systems reduce imaging errors dramatically over conventional open-loop piezoelectric scanners. Both linear and quadratic scan distortion are reduced to about 1%-2% of the image size.