Asymmetric fluid-structure dynamics in nanoscale imprint lithography

Chapter 4: Real-Time Gap Sensing Via Fast Fourier Transformsof Spectral ReflectivityAs part of an active stage system, a feedback control system is required toaccurately align the template and the wafer substrate. Several techniques wereconsidered to obtain real-time gap information at several locations between thetemplate and wafer. Based on specific design requirements, FFT-based spectralreflectometry is relatively low-cost method that promises to deliver real-time gapsensing for SFIL.4.1 INTRODUCTIONIn the multi-imprint stepper, the template calibration stages can align thetemplate and wafer to within a single interference fringe across a one-inchtemplate. However, the initial orientation of the template may becomemisaligned beyond the calibration specifications due to excessive forces, hardcontact with the wafer, etc. Correction requires that the template be removed andthe calibration to be repeated. A real-time gap sensing method implemented inan active stage system will help to solve the issues of calibration, base layerthickness, and wedged base layers. During the squeezing process, an in situspectral reflectometry technique based on Fast Fourier Transform (FFT) analysiswill provide the necessary gap and orientation information. Orientationinformation is obtained in the form of gap information between two flats at threepoints.Other optical methods to measure film thickness have been proposedwhich employ spectral or Fourier analysis of the reflectivity data from thin films.[Bouldin et al 2000], [Chalmers 2001], [Chason 2000], and [Sakurai and Iida1992]. However, these methods are either not commercially available or not52