Reticle Autofocus via Topographic Mapping
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Solution Overview
Problem
Current reticle inspection tools face challenges in achieving accurate autofocus due to plate topography and pattern effects, leading to errors in detecting defects on integrated circuit devices during manufacturing.
Innovation Solution
An auto focus system that maps the topographic surface of the reticle, using triangulation and confocal methods to determine the objective-lens to reticle distance, and corrects for errors by generating three-dimensional maps and applying algorithms to estimate focus height, thereby reducing plate topography and pattern effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional autofocus methods are used for reticle inspection, then the inspection process is simpler, but measurement precision deteriorates due to plate topography and pattern effects
Solution Approach 1:
The autofocus system is divided into multiple independent modules: a topographic mapping module that maps the reticle surface topology, a confocal microscopy module that measures focus position, and a control module that coordinates between them. This segmentation allows each module to specialize in one function, improving overall measurement precision while managing system complexity through modular design
Solution Approach 2:
The patent introduces an intermediary topographic map that mediates between the reticle surface and the autofocus system. The map serves as a reference that corrects for plate topography effects, allowing the confocal microscopy to achieve accurate focus measurements despite variations in reticle surface height and complex patterns
2Productivity
If manual focus adjustment is used, then device complexity is reduced, but productivity decreases due to time-consuming focus adjustment
Solution Approach 1:
The system performs preliminary topographic mapping of the reticle surface before the actual inspection process. This pre-acquired map is stored and used to guide the autofocus system during inspection, eliminating the need for real-time manual focus adjustment and significantly improving inspection speed while managing complexity through offline preparation
Solution Approach 2:
The confocal microscopy provides real-time feedback on focus position by measuring the intensity of reflected light at different depths. The control module uses this feedback to automatically adjust the objective lens position, creating a closed-loop system that maintains optimal focus throughout the inspection process without manual intervention
3Measurement precision
If simple focus measurement is used, then device complexity is lower, but measurement precision worsens due to plate topography effects
Solution Approach 1:
The system applies local quality by using the topographic map to determine the specific focus position needed for each location on the reticle. Instead of using a single global focus setting, the control module adjusts focus locally based on the measured surface height at each inspection point, correcting for plate topography effects and improving measurement precision
Solution Approach 2:
The patent transitions from two-dimensional reticle inspection to three-dimensional focus measurement by using confocal microscopy to measure the Z-height of the reticle surface. This addition of the third dimension allows the system to account for topographic variations and achieve accurate focus measurement across the entire reticle surface
4Measurement precision
If autofocus corrects for all variations, then measurement precision improves, but device complexity increases due to multi-material and complex pattern handling
Solution Approach 1:
The system performs preliminary characterization of the reticle, including topographic mapping and identification of multi-material regions and complex patterns, before the actual defect inspection. This pre-characterization data is stored and used to guide the autofocus system, allowing accurate defect detection without requiring complex real-time corrections during inspection
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
The system provides more accurate and efficient autofocus, reducing errors and ensuring precise detection of defects on reticles, even in complex patterns and multi-material surfaces, by continuously tracking and correcting for changes during the inspection process.
Implementation Method 1
a triangulation-approach autofocus system may be used... in which the objective-lens to reticle-surface distance is determined by 'triangulation'
Implementation Method 2
Their reflected images may be directed onto two split-diode detectors
Implementation Method 3
Confocal drill-down (which may be referred to herein as 'confocal microscopy')... allows an absolute elevation check even in dense pattern areas
Data Source
AI summary
Methods and apparatus relating to the inspection of photomasks are described. In an embodiment, an inspection tool may be automatically focused on a reticle utilizing various topographic mapping techniques. Other embodiments are also described.