Autofocus Method for Scanning Electron Microscope Using Interlaced Scan
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Solution Overview
Problem
Conventional autofocus techniques in scanning electron microscopes are time-consuming, leading to reduced throughput and electrical charging of the specimen due to repeated electron beam irradiation, causing image distortion and non-uniform brightness.
Innovation Solution
The method involves generating thinned images of a pattern by shifting the electron beam's scanning position and changing the focal position, using deflector voltages to cover the entire field of view, and calculating sharpness levels to determine the optimum focal position, thereby preventing overlapping electron beam lines and reducing charging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple images are generated by slightly shifting the focal position, then focus accuracy is improved, but inspection time increases and throughput decreases
Solution Approach 1:
The patent divides the field of view into multiple regions and generates multiple thinned images by scanning different regions with shifted electron beam positions. This segmentation allows parallel processing of different areas, reducing total inspection time while maintaining focus accuracy through comparative sharpness evaluation of the segmented regions.
Solution Approach 2:
The patent generates thinned images with reduced pixel density compared to full-resolution images. By using partial action (thinned images instead of complete images), the system achieves sufficient focus evaluation capability with reduced scanning time and data processing requirements, thereby improving throughput without sacrificing essential focus measurement precision.
2Measurement precision
If the specimen is repeatedly irradiated with an electron beam for generating multiple images, then focus evaluation is improved, but electrical charging occurs causing image distortion
Solution Approach 1:
The patent segments the inspection process into multiple passes over different regions of the specimen. By dividing the field of view into multiple areas and scanning each region separately with shifted beam positions, the system evaluates focus across different portions of the specimen without repeatedly irradiating the same location, thereby preventing electrical charging accumulation.
Solution Approach 2:
The patent introduces a spatial dimension shift by changing both the focal position and the irradiation position (scanning position) when generating multiple thinned images. This dimensional change ensures that different regions of the specimen are evaluated for focus without overlapping electron beam exposure, eliminating the harmful effect of electrical charging while maintaining focus evaluation accuracy.
3Loss of time
If thinned images are generated by shifting scanning position by multiple pixels, then scanning time is reduced, but image detail may be lost
Solution Approach 1:
The patent generates thinned images by sampling at reduced pixel density (shifting scanning position by multiple pixels). This partial action approach captures sufficient structural information for focus evaluation while significantly reducing scanning time. The thinned images retain enough pattern detail to enable accurate sharpness calculation and focus determination without requiring full-resolution data.
Solution Approach 2:
The patent creates multiple thinned images as simplified copies of the full-resolution specimen image. These thinned image copies contain the essential pattern information needed for focus evaluation but require less scanning time and computational resources. The copying approach maintains sufficient fidelity for the intended purpose (focus measurement) while achieving the time reduction goal.
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 allows for faster focus adjustment and prevents electrical charging, resulting in more accurate and efficient image generation with reduced scanning time and improved image quality.
Implementation Method 1
A scanning electron microscope generates a plurality of images while slightly shifting a focal position
Implementation Method 2
said changing the focal position comprises changing a voltage applied to a deflector of the scanning electron microscope to change the focal position
Implementation Method 3
since a wafer is repeatedly irradiated with an electron beam for generating the plurality of images, a film (for example, a resist) forming a surface of the wafer is electrically charged
Data Source
AI summary
The present invention relates to an autofocus technique for a scanning electron microscope using interlaced scan. The autofocus method for a scanning electron microscope, includes: generating a thinned image of a pattern (160) formed on a surface of a specimen by repeatedly scanning the specimen with an electron beam while shifting a scanning position of the electron beam by predetermined plural pixels in a direction perpendicular to a scanning direction; performing said generating a thinned image of the pattern (160) plural times, while changing a focal position and an irradiation position of the electron beam, to generate thinned images of the pattern (160); calculating a plurality of sharpness levels of the respective thinned images; and determining an optimum focal position based on the sharpness levels.