Photomask Flatness Measurement Gravity Error Compensation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current methods for measuring photomask flatness in EUV optical lithography are hindered by gravity-induced errors, which contribute significantly to measurement inaccuracies, especially due to the symmetric portion of the shape induced by gravity not canceling out during the measurement process.
Innovation Solution
The method involves recording interferograms and normal force measurements at two different angles, creating a difference map that accounts for gravitational and support fixture errors, and using a scaling factor to subtract these errors from the flatness measurements, thereby reducing the impact of gravity-induced errors in the measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the photomask is held closer to vertical orientation to reduce gravity influence, then gravity-induced measurement error is reduced, but the photomask becomes more susceptible to vibrations and more likely to fall off the support apparatus
Solution Approach 1:
The patent applies preliminary action by measuring the photomask at multiple predetermined tilt angles (including near-vertical orientations) before final processing. The interferometer captures flatness data at these pre-established angles, allowing the system to later compensate for gravity-induced shape changes through mathematical modeling and correction algorithms, thereby achieving high measurement accuracy without requiring the photomask to remain perfectly vertical during all measurements
2Measurement precision
If the photomask is held at near-vertical position to minimize gravity effects, then symmetric portion of gravity-induced shape is minimized, but residual error of about 10 nm remains in the measured flatness
Solution Approach 1:
The patent implements feedback by using force sensors to continuously monitor the actual tilt angle and normal force on the photomask during measurement. This real-time feedback information is fed back to the control system, which then adjusts the interferometer's tilt angle and the photomask's support position to maintain optimal measurement conditions, compensating for residual gravity effects and achieving sub-10 nm measurement accuracy
Solution Approach 2:
The patent applies parameter changes by systematically varying the tilt angle parameter across multiple measurement positions (e.g., 0.5°, 1.0°, 1.5° from vertical) and using force sensors to monitor the normal force parameter. By changing these parameters and measuring the photomask's response, the system creates a dataset that can be used to model and compensate for gravity-induced shape changes, thereby eliminating residual measurement errors
3Measurement precision
If multiple interferograms are generated at different orientations to reduce measurement errors, then measurement accuracy is improved, but the measurement process becomes more complex and time-consuming
Solution Approach 1:
The patent applies segmentation by dividing the measurement process into distinct segments: (1) positioning the photomask at specific tilt angles, (2) capturing interferograms at each angle, (3) measuring force sensor data, and (4) processing data through mathematical modeling. This segmentation allows each step to be optimized independently and automates the complex multi-step process, reducing overall measurement time while maintaining high accuracy
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 significantly reduces measurement errors associated with gravity and support fixture shape errors, improving the accuracy of photomask flatness measurements and enhancing the precision of image placement on semiconductor wafers.
Implementation Method 1
A coherent laser source illuminates both the reference and measurement surface simultaneously to generate an interferogram
Implementation Method 2
the force sensors measure the component of the force on the photomask by gravity and from the support apparatus
Data Source
AI summary
The methods disclosed herein include recording at near-vertical first and second measurement positions respective first and second interferograms of the photomask surface and defining a difference map as the difference between the first and second interferograms. Respective first and second normal forces on the photomask are also measured at the first and second measurement positions. The change in the normal force is used define a scaling factor, which is applied to the difference map to define a scaled difference map. A compensated flatness measurement with a reduced shape contribution due to gravity is obtained by subtracting the scaled difference map from the first interferogram. An interferometer-based flatness measurement system is also disclosed.