Diffractive Optical Element Calibration for Free-Form Surface Measurement
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Solution Overview
Problem
Current interferometric measurement techniques for free-form surfaces, such as those in microlithographic optical elements, face challenges in achieving high accuracy due to insufficient calibration of diffractive optical elements, particularly for non-rotationally symmetric surfaces, where manufacturing errors affect the measurement precision.
Innovation Solution
A measurement apparatus and method utilizing a multiply-encoded diffractive optical element that produces a test wave with a free-form surface wavefront and a calibration wave with a non-rotationally symmetric wavefront, allowing for improved calibration accuracy by minimizing the difference between the test and calibration wavefronts, thereby predicting and mitigating manufacturing errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional diffractive optical element is used for measuring free-form surfaces, then the measurement can be performed, but the measurement precision is insufficient due to manufacturing errors in the CGH
Solution Approach 1:
The patent applies preliminary action by performing calibration measurements before actual shape measurements. The CGH is calibrated by measuring a calibration object with known geometry, and the measured CGH errors are stored for subsequent correction during actual measurements, thereby compensating for manufacturing errors in advance
Solution Approach 2:
The patent implements feedback by using the measured CGH errors from calibration objects to correct the CGH data before performing actual shape measurements. The system continuously refines the CGH accuracy by comparing measured data with known reference geometries and applying corrections to subsequent measurements
2Ease of manufacture
If the CGH is calibrated using rotationally symmetric calibration objects, then calibration is simplified, but calibration accuracy for free-form surfaces is insufficient
Solution Approach 1:
The patent applies asymmetry by introducing calibration objects with non-rotationally symmetric geometries that match the free-form nature of the surfaces to be measured. This allows the calibration process to capture and correct for the specific error patterns that affect free-form surface measurements, rather than assuming rotational symmetry
3Measurement precision
If multiple calibration objects are used to calibrate the CGH, then calibration accuracy improves, but the measurement process becomes more complex
Solution Approach 1:
The patent applies segmentation by dividing the calibration process into multiple independent calibration objects, each targeting specific error patterns. By segmenting the calibration into manageable parts with different geometric characteristics, the system can systematically correct different types of CGH errors while maintaining organizational control over the complexity
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 approach enhances the precision of shape measurement for free-form surfaces by accurately accounting for manufacturing errors in the diffractive optical element, leading to improved calibration and reduced root mean square deviation of the actual shape from the desired shape in microlithographic exposure apparatuses.
Implementation Method 1
a diffractive optical element, which is configured to produce by diffraction from the input wave a test wave
Implementation Method 2
the accuracy of the shape measurement depends on the accuracy of the CGH... deviations from the desired shape can be determined by the superposition of a reference wave on the reflected test wave
Data Source
AI summary
A measurement apparatus (10) for interferometrically determining a surface shape of a test object (14). A radiation source provides an input wave (42), a multiply-encoded diffractive optical element (60), which is configured to produce by diffraction from the input wave a test wave (66) that is directed at the test object and has a wavefront in the form of a free-form surface and at least one calibration wave (70), and a capture device (46). The calibration wave has a wavefront with a non-rotationally symmetric shape (68f), wherein cross sections through the wavefront of the calibration wave along cross-sectional surfaces each aligned transversely to one another have a curved shape. The curved shapes in the different cross-sectional surfaces differ in terms of an opening parameter. The capture device (46) captures a calibration interferogram formed by superimposing a reference wave (40) with the calibration wave after interaction with a calibration object (74).