Multi-Column Microscope Auto-Focus for Non-Planar Substrates
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Solution Overview
Problem
Multi-column scanning electron microscopes face challenges in maintaining proper focus across multiple columns due to mechanical tolerances and substrate non-planarity, with existing auto-focus systems being inadequate for multi-column configurations and sensitive to substrate charging.
Innovation Solution
A system comprising an array of auto-focus sensors positioned in-line with the microscope columns, a controller for implementing feedback control loops to adjust column focal distances and working distances, and a stage for holding the imaging sample and allowing for multi-directional swathing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple microscope columns are arrayed for parallel imaging to increase scan speed and inspection throughput, then productivity is improved, but maintaining proper focus across multiple columns becomes difficult due to mechanical tolerances and substrate non-planarity
Solution Approach 1:
The patent divides the focus measurement function into multiple independent auto-focus sensors, each assigned to a specific microscope column. This segmentation allows each column to have its own dedicated sensor for independent focus control, resolving the contradiction by enabling parallel imaging while maintaining individual focus accuracy for each column.
Solution Approach 2:
The patent implements feedback control loops where auto-focus sensors continuously measure the working distance between each microscope column and the substrate, and the controller adjusts the column positions or focal lengths based on these measurements. This feedback mechanism maintains focus accuracy across multiple columns simultaneously, enabling high productivity without sacrificing precision.
2Device complexity
If existing auto-focus systems are used in multi-column SEMs, then device complexity is reduced, but the systems are inadequate for multi-column configurations and sensitive to substrate charging
Solution Approach 1:
The patent employs multiple independent auto-focus sensors instead of a single shared sensor system. Each sensor is dedicated to a specific column, providing reliable focus measurement for that column independently. This segmentation approach handles multi-column configurations effectively and reduces sensitivity to substrate charging by distributing the measurement function across multiple sensors.
Solution Approach 2:
The patent places auto-focus sensors at specific locations near each microscope column to measure local working distances. This local measurement approach provides accurate focus information for each column's specific position and conditions, improving reliability by accounting for local variations in substrate position and charging effects rather than using a single global measurement.
3Device complexity
If substrate charging is not corrected for in the auto-focus system, then device complexity is lowered, but discrepancy between effective focal length and physical working distance occurs
Solution Approach 1:
The patent uses feedback control loops where auto-focus sensors measure the actual working distance and the controller adjusts column positions or focal lengths to compensate for substrate charging effects. This feedback mechanism maintains measurement precision by continuously correcting for charging-induced discrepancies between physical and effective working distances.
Solution Approach 2:
The patent dynamically adjusts focus parameters (column position or focal length) based on real-time measurements from auto-focus sensors. By changing these parameters in response to measured working distance variations caused by substrate charging, the system maintains accurate focus without requiring complex predictive models or pre-correction mechanisms.
Data Source
AI summary
An array of localized auto-focus sensors provides direct measurement of the working distance between each microscope column in the array and the substrate being imaged below. The auto-focus sensors measure the working distance between each column and the imaging substrate as it passes over a point on the substrate to be imaged. The working distance measurement from the sensors is input into a control system, which in turn outputs the required working distance adjustment to the microscope column. The control system independently adjusts microscope working distance and/or physical distance of an individual microscope column in a multi-column microscope based on auto-focus sensor input. The individual microscope columns in the multi-column microscope can also be used as the auto-focus sensor itself.