Grating Interferometry Displacement Measurement Using Circular Polarization

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Solution Overview

Problem

Conventional grating interferometry-type displacement measurement devices face challenges in achieving high precision due to manufacturing errors in grating patterns, such as irregular line widths and thicknesses, leading to measurement errors that vary with the measurement position, caused by structural birefringence and changes in polarization components.

Innovation Solution

The device employs a polarizer to transform diffracted light into linearly polarized light with equal polarization components, and a wavelength plate to convert the light into circular polarized light, ensuring that the polarization components are equal and linear, thereby reducing measurement errors by avoiding phase difference shifts and changes in polarization components during diffraction.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If a grating pattern is manufactured using laser drawing method or etching method, then the scale can be created with fine grating pitches for high-precision measurement, but manufacturing errors occur resulting in irregular line widths and thicknesses

Engineering Contradiction:

Improvegrating pattern line width uniformityVSAvoiddisplacement measurement accuracy

Core Design Contradiction:

Manufacturing precisionVSMeasurement precision

Solution Approach 1:

The invention changes the polarization state parameter of the light beam from linear polarization to circular polarization by introducing a wavelength plate. This parameter change compensates for the phase difference variations caused by manufacturing errors in the grating pattern, thereby maintaining measurement precision despite irregular line widths and thicknesses.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention converts the harmful effect of manufacturing errors (irregular line widths and thicknesses) into a manageable condition by using circularly polarized light. The equal polarization components in circularly polarized light make the measurement less sensitive to geometric imperfections, effectively turning a disadvantage into an advantage.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

2Measurement precision

If linear polarizers are disposed to convert diffracted light into linearly polarized light with orthogonal polarization directions, then positive and negative 1st-order diffracted light can be distinguished, but measurement errors occur due to changes in polarization components

Engineering Contradiction:

Improvephase change detection accuracyVSAvoidoptical system configuration

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The invention changes the polarization state from linear to circular by adding a wavelength plate. This parameter change simplifies the optical system by eliminating the need for multiple linear polarizers with orthogonal orientations, while simultaneously improving measurement precision by maintaining equal polarization components.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If the polarization directions of linearly polarized light are set to be mutually orthogonal, then diffracted light can be distinguished, but phase difference shifts occur leading to measurement errors

Engineering Contradiction:

Improvemeasurement consistencyVSAvoiddisplacement measurement accuracy

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The invention changes the polarization parameter from orthogonal linear polarization to circular polarization. This ensures that the polarization components remain equal and do not shift phase differently, thereby simultaneously improving reliability and measurement precision.

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This configuration allows for high-precision displacement measurements by minimizing measurement errors resulting from manufacturing errors in the grating pattern, ensuring accurate displacement measurements regardless of the measurement position, and enhancing the linearity and precision of the displacement measurement process.

Implementation Method 1

a wavelength plate to transform a plurality of diffracted lights from the scale into circular polarized light, respectively

Methodology Applied

Scientific EffectCircular polarization transformation: Polarisation

Implementation Method 2

an optical element configured to superposition and cause interference of the plurality of diffracted lights

Methodology Applied

Scientific EffectOptical interference: Interference

Implementation Method 3

a scale on which a grating pattern is formed; light from the light source is input onto the scale, a plurality of diffracted lights are transformed

Methodology Applied

Scientific EffectDiffraction: Diffraction

Data Source

PatentUS8749761B2Displacement measurement device, exposure apparatus, and working device

Publication Date: 2014.06.10 CANON KK

AI summary

A device has a scale on which a grating pattern is formed, a light source to irradiate light on the scale, a wavelength plate to transform multiple diffracted lights from the light source into circular polarized light, respectively, an optical element to superposition and cause interference of the multiple diffracted lights, and a photodetector to receive the interfered light. Also, a generating unit to generate linearly polarized light by the light from the light source, so that the multiple diffracted lights input to the wavelength plate become linearly polarized light with a same mutual polarization direction.