Optical Auto-Focus Unit for SEM Misfocus Compensation
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Solution Overview
Problem
Existing scanning electron microscopes (SEMs) face challenges in achieving fast and accurate auto-focus on samples with non-flat and non-horizontal surfaces, such as semiconductor wafers.
Innovation Solution
A charged particle evaluation system that incorporates an optical auto-focus unit, which scans the sample with an optical beam proximate to the electron beam, determines the focus status, and compensates for electron beam misfocus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an optical auto-focus unit is added to the SEM system, then the focus accuracy and speed are improved, but the device complexity increases
Solution Approach 1:
An optical beam is introduced as an intermediary to indirectly measure the focus status of the electron beam. The optical beam reflects off the sample surface and its focal status correlates with the electron beam focus, allowing optical methods to monitor electron beam focusing without directly measuring electron optics
Solution Approach 2:
The patent replaces direct electron optical measurement systems with an optical measurement system. Instead of using complex electron optics to determine focus status, an optical beam path is used to sense the sample surface position and infer electron beam focus condition
2Device complexity
If traditional auto-focus methods are used on non-flat surfaces, then the system remains simple, but the focus accuracy deteriorates
Solution Approach 1:
The optical beam is focused at a specific local region of the sample surface, and the auto-focus system measures focus status at this localized area. This allows accurate focus determination on non-flat surfaces by adapting to the local surface geometry rather than requiring global surface flatness
3Device complexity
If image-based auto-focus is used, then the system remains simple, but the focus speed is slow
Solution Approach 1:
The patent extracts the focus detection function from full image acquisition and processing. Instead of capturing and analyzing complete images to determine focus, the system uses a dedicated optical beam path that directly measures focus status through optical reflection, separating the measurement function from the imaging function
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 solution enables accurate and rapid auto-focus in SEMs, improving the precision of surface evaluations and overcoming the limitations of non-flat sample surfaces.
Implementation Method 1
receive a reflected optical beam from the sample
Data Source
AI summary
A charged particle evaluation system that may include a column that includes an opening; an illumination unit that is configured to scan an area of a sample with an electron beam that passes through the opening; and an optical auto-focus unit that is configured to (i) illuminate the sample with an optical beam that is proximate to the electron beam, during the scan of the area with the electron beam; (ii) receive a reflected optical beam from the sample, (iii) determine a focus status of the electron beam, and (iv) participate in a compensating of an electron beam misfocus.