Phase-Shifted Target Segmentation for Lithography Polarization Monitoring
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Solution Overview
Problem
Existing lithographic projection systems face challenges in accurately monitoring focus and tracking polarization drift, particularly in high-NA immersion scanners, due to ghost images and the need for specialized reticles with multiple polarimeters and high illumination doses.
Innovation Solution
A method involving a phase-shifted target with specific polygonal opaque line segments and optical depth variations is used to characterize beam parameters, such as polarization, by determining registration slopes across focal distances, allowing for independent monitoring of focus and polarization without ghost images and across varying illumination doses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If prior art phase-shifting mask structures are used for focus monitoring, then focus can be monitored, but ghost images appear that confound overlay and defocus determination
Solution Approach 1:
The patent segments the phase-shift target into multiple discrete polygons with different orientations and phase shifts. Each polygon is independently designed to respond to specific polarization states, allowing the system to distinguish between ghost images and actual focus measurements by analyzing the differential responses of segmented elements.
Solution Approach 2:
The patent employs asymmetric polygonal structures with specific orientations (e.g., rectangular, triangular, pentagonal shapes at various angles) that create directionally-dependent diffraction patterns. This asymmetry allows the system to differentiate between ghost images (which appear symmetrically) and genuine focus-related overlay errors through polarization-sensitive detection.
2Measurement precision
If specialized reticles with multiple polarimeters are used to track polarization drift, then polarization state can be monitored, but device complexity and cost increase
Solution Approach 1:
The patent designs a universal phase-shift target that simultaneously performs focus monitoring, overlay measurement, and polarization tracking functions. The same polygonal structures used for focus monitoring also serve as polarization sensors, eliminating the need for separate polarimeter reticles and reducing overall system complexity.
Solution Approach 2:
The patent merges the focus monitoring target and polarization tracking target into a single integrated phase-shift reticle structure. By combining these functions into one reticle with appropriately designed polygonal elements, the system reduces the number of components, simplifies alignment procedures, and lowers costs while maintaining measurement capabilities.
3Measurement precision
If high illumination doses are used for polarization monitoring, then measurement accuracy improves, but productivity decreases due to increased exposure time and energy consumption
Solution Approach 1:
The patent optimizes the geometric parameters of the polygonal structures (size, orientation, phase shift values) to maximize polarization sensitivity at reduced illumination doses. By carefully designing the polygons to resonate with specific polarization states, the system achieves accurate measurements with lower energy input, improving throughput while maintaining precision.
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 enables accurate and robust focus monitoring and polarization state tracking, improving feature resolution and system stability in lithographic projection systems, while being cost-effective and compatible with existing tools.
Implementation Method 1
a first region having an optical depth different from a characteristic optical depth of the substrate by an amount corresponding to a fraction of a wavelength of the electromagnetic radiation
Implementation Method 2
a set of polygonal opaque line segments extending along a perimeter of the polygonal enclosed region
Implementation Method 3
determining an x-registration value and a y-registration value at each focal distance
Data Source
AI summary
Methods for characterizing a beam parameter associated with an electromagnetic beam of a light source. The light source exposes a phase-shifted target through a set of focal distances relative to a focal plane of a substrate. At each focal distance of the set, registration values are measured and used to determine one or more registration slopes as a function of focal distance. The registration slopes are compared with baseline registration slopes to characterize the current relative state of the beam parameter in question. Beam parameters that may be characterized in this manner include degree of polarization and polarization rotation relative to an initial polarization direction. Phase shift test patterns advantageously used for beam characterization are described.