Focus Calibration Using Aberration-Induced Target Offsets
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Solution Overview
Problem
Current focus measurement methods in lithographic processes, such as diffraction-based focus measurement, face challenges in EUV lithography due to insufficient focus sensitivity and signal strength, and require complex target geometry selection, which can be time-consuming and impractical.
Innovation Solution
The method involves calibrating focus measurements using pairs of targets printed with aberration settings that induce a relative best focus offset, allowing for updated calibration information to account for performance drift and using an anchor point to adjust measurements, thereby improving accuracy and reducing the need for specialized techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diffraction-based focus measurement techniques are used, then focus measurement capability is provided, but focus sensitivity and signal strength are insufficient in EUV lithography
Solution Approach 1:
The patent employs asymmetric target structures where targets are intentionally printed with different focus offsets relative to the nominal best focus position. By measuring the asymmetry in diffraction signals from these deliberately asymmetric targets, the system achieves enhanced focus sensitivity and signal strength suitable for EUV lithography applications.
Solution Approach 2:
The patent changes the focus parameter during target printing by applying different focus offsets to create pairs of targets. This parameter variation allows the measurement system to detect focus drift by comparing measurements from targets printed at different focus conditions, thereby improving measurement reliability.
2Measurement precision
If asymmetry based focus measuring techniques are used, then focus measurement is enabled, but complex target geometry selection is required which is time-consuming
Solution Approach 1:
Instead of requiring complex target geometry selection, the patent varies the focus parameter (best focus offset) during target printing. This parameter-based approach simplifies target design while maintaining measurement capability, as the focus offset variation itself creates the necessary measurement asymmetry without requiring intricate geometric configurations.
Solution Approach 2:
The patent uses standard grating targets that can serve multiple purposes: they are suitable for both EUV lithography applications and can be measured by various scatterometer configurations. This universal target design eliminates the need for specialized target geometry selection and reduces setup time.
3Productivity
If calibration is performed without accounting for performance drift, then measurement process is simpler, but measurement accuracy decreases
Solution Approach 1:
The patent performs preliminary calibration measurements using pairs of targets printed with different focus offsets. By establishing calibration curves that account for focus drift before actual measurements, the system maintains measurement accuracy while keeping the ongoing measurement process relatively simple through the use of these pre-established calibration relationships.
Solution Approach 2:
The patent implements a feedback mechanism where calibration measurements are periodically updated to account for performance drift. The system uses measured focus offsets from calibration targets to adjust and update calibration curves, ensuring that subsequent measurements remain accurate despite drift in the lithographic apparatus performance.
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 and efficiency of focus measurements in lithographic processes, particularly in EUV lithography, by simplifying the measurement process and reducing the reliance on complex target geometries and specialized techniques.
Implementation Method 1
Diffraction-based focus measuring techniques are known which comprise measuring asymmetry in opposite higher (e.g., first) order radiation scattered by special, focus dependent, target structures
Implementation Method 2
These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction 'spectrum'
Data Source
AI summary
Focus performance of a lithographic apparatus is measured using pairs of targets that have been exposed (1110) with an aberration setting (e.g. astigmatism) that induces a relative best focus offset between them. A calibration curve (904) is obtained in advance by exposing similar targets on FEM wafers (1174, 1172). In a set-up phase, calibration curves are obtained using multiple aberration settings, and an anchor point (910) is recorded, where all the calibration curves intersect. When a new calibration curve is measured (1192), the anchor point is used to produce an adjusted updated calibration curve (1004′) to cancel focus drift and optionally to measure drift of astigmatism. Another aspect of the disclosure (FIGS. 13-15) uses two aberration settings (+AST, −AST) in each measurement, reducing sensitivity to astigmatism drift. Another aspect (FIGS. 16-17) uses pairs of targets printed with relative focus offsets, by double exposure in one resist layer.