Lithographic Overlay Measurement with Biased Gratings
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Solution Overview
Problem
Current overlay measurement methods in lithographic apparatuses do not provide a clear indication of the quality or reliability of the measurement, limiting the accuracy and confidence in the quantitative overlay values.
Innovation Solution
The implementation of a lithographic apparatus with two reference gratings and two measurement gratings, where the measurement gratings are oppositely biased relative to the reference gratings, allows for the measurement of asymmetry in pixel data to determine both overlay values and process-dependent values, along with a quality indicator for these measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional overlay measurement methods are used, then overlay values can be obtained, but there is no indication of measurement quality or reliability
Solution Approach 1:
The invention introduces a feedback mechanism where the measurement system not only determines overlay values but also generates quality indicators that feedback about the reliability of these measurements. The system uses multiple measurement gratings with known biases to assess measurement quality and provides this information back to the user, enabling informed decisions about measurement reliability.
Solution Approach 2:
The invention introduces measurement gratings with known biases as intermediaries between the reference gratings and the measurement process. These intermediary gratings serve dual purposes: they enable overlay measurement while simultaneously providing reference data for assessing measurement quality and generating quality indicators.
2Reliability
If multiple measurement gratings with opposite biases are used, then measurement quality assessment is enabled, but device complexity increases
Solution Approach 1:
The invention deliberately introduces asymmetry by using measurement gratings with known opposite biases (positive and negative). This asymmetric configuration allows the system to compare measurements and assess quality by analyzing differences and consistencies, thereby improving reliability through controlled asymmetry rather than symmetric redundancy.
Solution Approach 2:
The measurement system is segmented into multiple independent measurement gratings, each with specific known biases. This segmentation allows parallel measurement channels that can be independently evaluated and combined to generate quality indicators, distributing the complexity across modular components rather than a monolithic system.
3Measurement precision
If pixel asymmetry measurements are performed to determine overlay values, then measurement accuracy is improved, but processing complexity increases
Solution Approach 1:
The invention performs preliminary actions by pre-defining the biases of measurement gratings during system setup. These known biases are established beforehand, allowing the measurement process to focus on detecting asymmetries and comparing results against predetermined references, thereby simplifying real-time processing while maintaining high accuracy.
Solution Approach 2:
The system exploits parameter changes in pixel asymmetry measurements as overlay conditions change. By monitoring how asymmetry parameters vary with known grating biases, the system can determine overlay values and quality indicators through parameter analysis rather than complex image processing, reducing computational 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 accuracy of overlay measurements by providing a quality assessment, ensuring reliable and precise overlay determination, which is critical for the manufacturing process.
Implementation Method 1
an image sensor for obtaining pixel data of a measurement spot in each of the two measurement gratings
Data Source
AI summary
A lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate includes two reference gratings provided in the substrate and two measurement gratings on top of the reference gratings, the measurement gratings being similar to the reference gratings, and oppositely biased in a single direction relative to the respective reference gratings. An overlay measurement device with an image sensor is used for obtaining pixel data of a measurement spot in each of the two measurement gratings. Asymmetry for each pixel in the measurement spot is measured, and from the pixel asymmetry measurements in associated pixels of each of the two measurement gratings an overlay value and a process dependent value is determined, as well as a quality indicator for the overlay value and the process dependent value.