Asymmetric Diffraction Focus Targets for Sensitive Metrology
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Solution Overview
Problem
Current focus and dose measurement methods in lithographic printing tools face challenges in achieving high sensitivity and accuracy, particularly with tight focus variation tolerances of ±10nm, and often require expensive test masks or complex calibration procedures.
Innovation Solution
The use of diffraction-based focus targets with asymmetric elements arranged at fine and coarse pitches, designed to provide different diffraction order amplitudes and rotational symmetry, enabling sensitive focus measurements through scatterometry tools.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional focus measurement methods (FEM wafers, line end shortening, phase shift masks) are used, then focus measurements can be obtained, but measurement sensitivity is insufficient and/or device complexity increases
Solution Approach 1:
The patent employs asymmetric diffraction grating patterns where the grating lines are intentionally designed with asymmetric profiles (e.g., different widths on opposite sides of the line center). This asymmetry creates distinctive diffraction patterns that are highly sensitive to focus variations, enabling precise focus measurement without requiring complex test structures or calibration procedures
Solution Approach 2:
The patent transitions from measuring focus through direct dimensional changes (line width variations) to measuring focus through diffraction pattern analysis in the spatial frequency domain. By analyzing the intensity distribution and phase information of diffracted orders, the system achieves higher measurement sensitivity and can separate focus and dose effects independently
2Measurement precision
If FEM wafers with isolated lines and assist features are used, then focus measurement is enabled, but the measurement accuracy is insufficient for tight focus tolerances (±10nm)
Solution Approach 1:
The patent systematically varies multiple parameters of the diffraction grating structure including line width, pitch, grating depth, and asymmetric profile ratios to optimize the diffraction pattern's sensitivity to focus changes. By tuning these parameters, the measurement system achieves sufficient signal contrast and sensitivity to detect focus deviations within the tight ±10nm tolerance range reliably
3Adaptability or versatility
If multiple measurement methods (isolated lines, line end shortening, phase shift masks) are employed, then comprehensive focus/dose measurement is achieved, but device complexity and cost increase
Solution Approach 1:
The patent designs a universal diffraction-based measurement target that can simultaneously provide focus measurement, dose measurement, and process monitoring capabilities. The asymmetric diffraction grating structure responds to both focus and dose variations in its diffraction pattern, allowing a single target design to replace multiple specialized test masks and measurement methods
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 provides accurate focus measurements with high sensitivity and simplicity, allowing separation of focus and dose parameters, and reduces the need for costly test masks, while maintaining measurement accuracy comparable to overlay budgets.
Implementation Method 1
a diffraction-based focus target cell (110) having asymmetric elements arranged at a fine pitch
Data Source
Figure 1
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AI summary
Diffraction-based focus target cells, targets and design and measurement methods are provided, which enable sensitive focus measurements to be carried out by overlay measurement tools. Cells comprise a periodic structure having a coarse pitch and multiple elements arranged at a fine pitch. The coarse pitch is an integer multiple of the fine pitch, with the fine pitch being between one and two design rule pitches and smaller than a measurement resolution and the coarse pitch being larger than the measurement resolution. The elements are asymmetric to provide different amplitudes in +1st and -1st diffraction orders of scattered illumination, and a subset of the elements has a CD (critical dimension) larger than a printability threshold and the other elements have a CD smaller than the printability threshold.