Autofocus Step Size Control for SEM Observation
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Solution Overview
Problem
Existing scanning electron microscope (SEM) systems for semiconductor device fabrication face challenges in achieving sufficient autofocus precision due to fixed step sizes that do not account for pattern sparseness/density and contrast, leading to fitting errors and decreased throughput.
Innovation Solution
The method involves calculating a correction value for the step size based on image features from low-magnification images, optimizing the step size to match the spread of the focal point measure distribution, allowing for adaptive autofocusing that adjusts to the specific pattern characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a fine step size is used for autofocusing, then autofocus precision is improved, but the time required for autofocusing increases
Solution Approach 1:
The patent applies dynamics by making the step size adjustable rather than fixed. The control unit dynamically selects between a first step size and a second step size based on the observed image characteristics. When image contrast is sufficient, a larger second step size is used to reduce measurement points and shorten autofocus time. When image contrast is insufficient, a smaller first step size is used to ensure adequate measurement precision. This dynamic adaptation resolves the contradiction between precision and time.
2Device complexity
If a fixed step size is used for autofocusing, then the autofocusing process is simple, but fitting errors increase when image contrast is insufficient
Solution Approach 1:
The system dynamically adjusts the step size based on image contrast evaluation. The control unit automatically determines whether the image contrast is sufficient and selects the appropriate step size accordingly. This eliminates the need for operators to manually adjust settings through trial and error, maintaining simplicity while improving precision by adapting to actual image conditions.
Solution Approach 2:
The system performs self-adjustment by automatically evaluating image contrast and selecting the appropriate step size without operator intervention. The control unit monitors image characteristics and autonomously optimizes the autofocus parameters, eliminating fitting errors caused by inappropriate fixed step sizes while maintaining process simplicity.
3Measurement precision
If a large number of images are obtained by varying focal position with fine step sizes, then autofocus precision is improved, but productivity decreases
Solution Approach 1:
The system dynamically controls the number of images obtained during autofocusing by adjusting the step size based on image contrast. When contrast is sufficient, a larger step size reduces the number of measurement points, thereby increasing productivity without sacrificing precision. When contrast is insufficient, a smaller step size ensures adequate precision even if it requires more images. This dynamic control resolves the contradiction between precision and productivity.
Data Source
AI summary
With respect to a charged particle beam device, the step size of focal point measure for executing autofocusing is optimized to a value that is optimal with respect to the spread of an approximation curve for a focal point measure distribution. The step size of focal point measure for executing autofocusing is corrected using an image feature obtained based on a layout image derived from an image obtained at a first magnification or from design data. Autofocusing is executed based on the obtained step size to carry out observation, measurement, or to image the sample under inspection.