Six Degree-of-Freedom Displacement Measuring Interferometry

A six degree-of-freedom displacement measuring interferometer would allow for a complete calibration to be performed directly at the measurement point of interest.
A six degree-of-freedom displacement measuring interferometer would allow for a complete calibration to be performed directly at the measurement point of interest.

Heterodyne Displacement Measuring Interferometers (DMIs) are routinely used to track the motion of lithography equipment, metrology tools and precision machinery. Due to their large dynamic range and high sensitivity, DMIs are also used in the calibration of a wide range of precision transducers. Regardless of the specific application, inevitable errors in translational motion present themselves due to sources such as manufacturing tolerances, stresses, thermal effects and vibrations.

Errors in pitch, yaw, roll, and straightness are typically measured using up to six separate DMI measurements to account for all degrees of freedom (DOFs). Our aim with this research is to calibrate all six DOFs simultaneously using a single measurement beam. Tangential research within the Precision Instrumentation Group has already qualified the use of a 3-DOF heterodyne interferometer to simultaneously measure longitudinal displacement simultaneously with pitch and yaw errors.

We are investigating methods to amplify the sensitivity in the roll and straightness DOFs in order to achieve unprecedented resolution limits on all 6 axes. Furthermore, the use of a single measurement beam allows the user to calibrate directly at the measurement point of interest, mitigating the effects of Abbé error.