Precision positioning and measurement require an accurate “ruler” and an equally accurate method to read the ruler. Until recently, the state-of-the-art for positioning and metrology with nanometer level accuracy over relatively long distances has been the displacement measuring interferometer (DMI). A DMI is based on a Michelson two-beam interferometer and monitors displacement and change in orientation of a stage or part over an appreciable distance using heterodyne detection. The ultimate accuracy of DMI based systems is limited by the uncertainty of the refractive index of air resulting from turbulence and thermal fluctuations.

An emerging alternative to DMIs is the precision heterodyne optical encoder system in which a two-dimensional (2D) grating, sometimes called a “grid plate,” replaces DMI stage mirrors as the reference reflection, thus enabling the interferometric sensor to remain in close proximity to the reference (see figure below). As a result, the air path length is minimized and the accuracy and stability of the measurement system are greatly improved. The improved noise performance also enables much higher stage movement or measurement speeds for a given accuracy. And using multiple beams diffracted off of the same reference grating, these systems can be configured to monitor six degrees of freedom with little increase in complexity.

Enabled applications

Grid plate encoder technology can be used in any system that requires nanometer-level motion control. Photolithography steppers and metrology instruments for production of integrated circuits and solid-state memory have the most demanding requirements on positioning accuracy and stability. Positioning noise in systems that use grid plate encoders has been demonstrated to improve by more than a factor of three compared to those based on DMIs. The resulting overlay precision in these systems is improved by more than a factor of two, leading to dramatic improvements in density and speed.

This same technology can be applied to other stepping or scanning nanoscale photolithography systems, ranging from two- and three-dimensional nanoprinting systems to the patterning of the very gratings PGL manufactures, whether for laser pulse compression or precision metrology applications! There are also emerging applications related to precision machine tools, coordinate measurement machines (CMMs), and even data storage.

Gratings matter

Gratings for grid plate encoder systems must maintain excellent diffraction efficiency (DE) uniformity over the entire grating surface, which can be up to 0.5 meters and larger. This requirement must be met for both dimensions and polarizations. The wavefront error must be extremely small, demanding tiny surface figure imperfections and grating period and phase variations. In principle any imperfections can be measured and incorporated into the system calibration, but since even the calibration procedure itself has limitations, better performance before calibration results in better ultimate performance.