Achromatic Retarder System for Ellipsometer Beam Deviation

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Solution Overview

Problem

Existing ellipsometer and polarimeter systems face challenges in maintaining acceptable performance over a wide spectral range due to wavelength dependence of retardance in compensator elements, leading to angular deviation and lateral offset of the electromagnetic beam.

Innovation Solution

A multiple element retarder system comprising at least two sequential elements, such as triangular prisms or parallelogram shaped rhombs, oriented to ensure internal reflection without significant angular or lateral deviation of the beam, with optional coatings and mechanisms for alignment to minimize beam deviation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single waveplate is used as compensator element, then the device complexity is low, but the measurement precision deteriorates due to wavelength dependence of retardance

Engineering Contradiction:

Improvecompensator structureVSAvoidellipsometer performance

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent divides the compensator into multiple waveplate elements (typically three) with different optic axis orientations and retardances. Each waveplate is configured to compensate for the wavelength-dependent retardance of the others, creating a composite compensator that maintains consistent performance across a broad spectral range (190-1700 nm). This segmentation allows the system to achieve achromatic compensation without requiring a single complex element.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent employs composite optical structures by combining multiple waveplate materials (such as quartz and MgF2) with different birefringence characteristics. These composite waveplate assemblies create a compensator system where the wavelength-dependent retardance of individual materials compensates for each other, resulting in a broadband achromatic compensator that maintains measurement precision across the entire spectral range.

Inventive Principle:

Principle #40Composite materials

2Device complexity

If traditional compensator designs are used, then the device complexity remains low, but angular deviation and lateral offset of the beam occur

Engineering Contradiction:

Improvecompensator systemVSAvoidbeam position stability

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent introduces asymmetric wedge angles in the waveplate configurations to deliberately compensate for beam deviation. By carefully selecting different wedge angles for each waveplate element, the system creates an asymmetric optical path that results in net zero angular deviation and lateral offset. This asymmetric design allows the compensator to maintain beam position stability while providing the required retardance compensation.

Inventive Principle:

Principle #4Asymmetry

3Measurement precision

If a dual/multiple waveplate design is used, then the measurement precision is improved by minimizing wavelength dependence, but the device complexity increases

Engineering Contradiction:

Improvespectral range performanceVSAvoidcompensator structure

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent systematically varies key parameters of the waveplate assembly including optic axis orientations (typically at 0°, 45°, and 90°), thicknesses, and material compositions to achieve broadband achromatic compensation. By optimizing these parameters, the system minimizes the wavelength dependence of total retardance while keeping the physical size and structural complexity of the compensator manageable. The parameter optimization allows three waveplates to provide consistent performance across 190-1700 nm.

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

The system effectively introduces relative phase retardation between orthogonal components of the electromagnetic beam, maintaining beam position and angle consistency across translations and rotations, thereby improving the spectral range performance and reducing noise in ellipsometric parameters.

Implementation Method 1

said entered electromagnetic beam undergoes internal reflection at least once in each of the elements

Methodology Applied

Scientific EffectInternal reflection: Total Internal Reflection

Implementation Method 2

introducing a relative phase retardation into orthogonally polarized components of an electromagnetic beam

Methodology Applied

Scientific EffectPhase retardation: Birefringence

Data Source

PatentUS7907280B2Method of constructing a deviation angle self compensating substantially achromatic retarder to compensate beam traslation

Publication Date: 2011.03.15 J A WOOLLAM CO

AI summary

A method of configuring a system for introducing a relative phase retardation into orthogonally polarized components of an electromagnetic beam entered thereinto, wherein the system involves a substantially achromatic multiple element retarder system for use in wide spectral range (for example, 190-1700 nm) rotating compensator spectroscopic ellipsometer and/or polarimeter systems.