Multi-Spot Matrix Autofocus for Dense Pattern Samples
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Solution Overview
Problem
Current autofocus techniques, such as beam displacement and confocal depth sensing, are inefficient in dense pattern areas due to signal corruption by edges and corners, leading to ambiguous focal plane determination and increased sample damage with higher laser power.
Innovation Solution
A multi-spot focus system using an M×N matrix of identical spots is projected onto the sample, generating V(z) curves for each spot to determine a robust focus, eliminating the need for location searching and enhancing signal-to-noise ratio through normalization and averaging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single beam/spot is used for focusing, then the system is simple and easy to operate, but the V(z) curve is corrupted by edges in dense patterns, reducing signal quality and focus determination accuracy
Solution Approach 1:
The single beam is segmented into multiple beams (e.g., 3x3 matrix) that are spatially separated and focused onto different locations on the sample. Each beam independently measures local focus conditions, avoiding corruption from dense pattern edges at any single location. The multiple measurements are then combined to determine the overall best focus position.
2Measurement precision
If laser power is increased to improve signal-to-noise ratio, then measurement sensitivity improves, but sample damage increases and detector saturation occurs
Solution Approach 1:
The total laser power is segmented and distributed across multiple beams. Each beam operates at lower power levels that avoid sample damage and detector saturation, while the combined signal from all beams maintains a high signal-to-noise ratio. This distributes the energy load and prevents localized overheating or saturation.
3Measurement precision
If the spot is positioned on an edge or corner of a feature, then the V(z) curve becomes non-conformal and ambiguous, but searching for ideal locations in dense patterns is highly inefficient
Solution Approach 1:
Instead of searching for a single ideal location, the system segments the measurement across multiple locations simultaneously. The matrix of beams covers a broader area, increasing the probability that at least some beams will land on suitable measurement points. The system can then select or average results from valid measurements, eliminating the need for time-consuming sequential location searching.
4Reliability
If a single spot is used, then the device complexity is low, but the probability of obtaining unambiguous V(z) curves in dense pattern areas is low
Solution Approach 1:
The single spot measurement is segmented into multiple spots arranged in a matrix. This segmentation increases the statistical probability that at least some spots will fall on locations suitable for generating valid V(z) curves, even in densely patterned areas. The multiple independent measurements provide redundancy and improve reliability.
Solution Approach 2:
The system transitions from a one-dimensional single spot measurement to a two-dimensional matrix of spots. This dimensional expansion allows simultaneous sampling across a broader spatial area, increasing the likelihood of capturing valid focus information from multiple locations and improving measurement reliability.
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 multi-spot focus system increases the probability of obtaining unambiguous V(z) curves, providing a robust definition of best focus and improving accuracy by a factor of M×N, while reducing sample damage and detector saturation.
Implementation Method 1
a laser generates a collimated beam that is deflected by beam splitter and then focused by an objective lens onto the sample as a single point
Implementation Method 2
focused by an objective lens onto the sample as a single point
Implementation Method 3
the reflected light from the sample is recollimated into a confocal beam
Implementation Method 4
focused by another lens through a pin hole
Implementation Method 5
A detector can use this optical image from the confocal beam to generate a V(z) curve of that particular position on the sample, wherein a V(z) graph plots voltage as a function of height z
Data Source
AI summary
A plurality of spots forming an M×N matrix can be used in a focus system. Specifically, a plurality of identical spots can be simultaneously projected onto the sample. A V(z) curve can be generated for each spot. A robust focus can be determined based on the generated V(z) curves. Using the spot matrix significantly increases the probability that at least one of the plurality of spots in the matrix can provide an unambiguous V(z) curve. Thus, the spot matrix eliminates the need to search for an appropriate site because the spot matrix increases the probability of landing on a “good” location by a factor of M×N.