Improving the performance of interferometers in metrological scanning probe microscopes

The traceability of metrological scanning probe microscopes (MSPMs) is achieved in most cases by laser interferometers. Different means have been adopted to account for the nonlinearity of those interferometers. The thorough investigation of an existing MSPM shows the necessity of interferometrical position measurement with real time full-bandwidth nonlinearity correction. The paper demonstrates that the ellipse parameters of Heydemann nonlinearity correction are sufficiently stable and position independent. This is used in reducing the signal processing time by calculating the ellipse parameters in advance and fixing them during real time nonlinearity correction. As a result, a real time signal processing system with the ability of executing Heydemann correction in 0.32??s and a complete demodulation in 2.2??s is designed and implemented. It reduces the residual nonlinearity of interferometers from about 3.5 to <0.3?nm. Some measurement results of a flatness standard illustrate the effectiveness of this new method.