Afocal System for Reducing Beam Shear in Interferometry

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Solution Overview

Problem

Interferometers face non-cyclic non-linear errors due to beam shearing and wavefront errors, which affect the accuracy of displacement measurements, particularly in applications like lithography where precise positioning is critical.

Innovation Solution

Incorporating an afocal system in the interferometry system to reduce beam shearing by changing the diameter of the measurement beam as it propagates to and from the measurement object, thereby minimizing non-cyclic non-linear errors associated with phase measurements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If beam diameter is increased to reduce beam shearing effects, then measurement precision improves, but interferometer size increases

Engineering Contradiction:

Improvephase measurement accuracyVSAvoidinterferometer size

Core Design Contradiction:

Measurement precisionVSVolume of moving object

Solution Approach 1:

The patent places a first afocal system inside the measurement beam path within the interferometer, and a second afocal system inside the reference beam path. This nested configuration allows the beam diameter to be increased for reducing beam shearing effects while containing the optical components within the existing interferometer structure, thus improving measurement precision without proportionally increasing the overall interferometer size.

Inventive Principle:

Principle #7Nested doll (Nesting)

2Measurement precision

If beam diameter is increased to reduce beam shearing, then non-cyclic non-linear errors decrease, but thermal sensitivity increases

Engineering Contradiction:

Improvedisplacement measurement accuracyVSAvoidthermal sensitivity

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent replaces mechanical adjustments for beam diameter control with optical systems (afocal systems). By using optical magnification rather than mechanical expansion, the beam diameter can be increased to reduce beam shearing and non-cyclic non-linear errors while avoiding the thermal expansion issues associated with mechanical structures. The optical system allows precise control of beam parameters without the thermal sensitivity inherent in mechanical systems.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Measurement precision

If afocal system is added to reduce beam shearing, then measurement accuracy improves, but device complexity increases

Engineering Contradiction:

Improvephase measurement accuracyVSAvoidinterferometer structure

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent designs the afocal systems to serve multiple functions: they magnify the beam diameter to reduce beam shearing effects, they control the beam parameters to minimize non-cyclic non-linear errors, and they are integrated into the existing interferometer structure to maintain compactness. By making the afocal systems multi-functional, the patent improves measurement accuracy while minimizing the increase in device complexity.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS7528962B2Apparatus and methods for reducing non-cyclic non-linear errors in interferometry

Publication Date: 2009.05.05 ZYGO CORP

AI summary

In general, in one aspect, the invention features interferometry systems that include an interferometer configured to direct a first beam and a second beam derived from common light source along different paths and to combine the two beams to form an output beam including information related to an optical path difference between the different paths, wherein the path of the first beam contacts a measurement object. The interferometry systems also include an afocal system positioned in the path of the first beam and configured to increase a dimension of the first beam as it propagates from the interferometer towards the measurement object and reduces the dimension of the first beam as it returns from the measurement object propagating towards the interferometer.