Sinusoidal Overlay Metrology via Diffracted Order Interference
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Solution Overview
Problem
Conventional methods for target acquisition and overlay measurement in semiconductor manufacturing face challenges due to imprecise focus positioning and optical aberrations, leading to errors in alignment and overlay determination.
Innovation Solution
The use of a sinusoidal imaging system that captures only two coherent diffracted orders from a periodic target, allowing for flexible sensor placement and minimizing the impact of optical aberrations, thereby achieving high accuracy and reduced sensitivity to tool-induced shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional imaging systems capture multiple scattered light rays from different layers, then comprehensive target information is obtained, but optical aberrations cause placement errors that differ for scattered light from different layers, leading to overlay measurement errors
Solution Approach 1:
The patent extracts and isolates only the first and second scattered light beams from the periodic target, excluding other scattered light rays. By selecting specific diffracted orders and using spatial filtering, the system separates the useful information-carrying beams from the harmful scattered light that causes optical aberration-induced placement errors, thereby improving overlay measurement accuracy while minimizing aberration effects.
2Measurement precision
If conventional mechanical movement mechanisms are used to position the sensor, then the system structure is simple, but it is impossible to position the sensor at the absolute optimum focus position for every acquisition operation or overlay measurement
Solution Approach 1:
The patent replaces the conventional mechanical movement mechanism with an optical field-based focusing method. Instead of mechanically moving the sensor to achieve focus, the system uses the interference pattern of coherent scattered light beams to create a sinusoidal image that is inherently focused across a large depth of focus range, eliminating the need for precise mechanical positioning while maintaining or improving measurement accuracy.
3Illumination intensity
If all scattered light rays are captured to form an image, then complete target information is obtained, but DC background noise increases and contrast decreases
Solution Approach 1:
The patent extracts only the coherent first and second scattered light beams that contain the target information, excluding other scattered light rays that contribute to DC background noise. By using spatial filtering and selecting specific diffracted orders, the system removes the harmful DC component while preserving the useful interference pattern, thereby significantly improving image contrast without losing target information.
Solution Approach 2:
The patent introduces an interference pattern as an intermediary between the scattered light and the final image. The coherent first and second scattered light beams interfere to create a sinusoidal pattern that modulates the DC background, making the target information visible despite the presence of DC noise. This intermediary transformation converts the low-contrast scattered light into a high-contrast sinusoidal image.
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 provides improved contrast and accuracy in target acquisition and overlay measurement, with a large depth of focus and reduced sensitivity to optical aberrations, resulting in more precise alignment and overlay determination.
Implementation Method 1
illumination of the system is only directed at specific angles at a grating target and the imaging lens is set to capture corresponding pairs of diffracted orders
Implementation Method 2
diffracted orders that interfere with each other to form a sinusoidal image
Data Source
AI summary
In one embodiment, a system includes a beam generator for directing at least one incident beam having a wavelength λ towards a periodic target having structures with a specific pitch p. A plurality of output beams are scattered from the periodic target in response to the at least one incident beam. The system further includes an imaging lens system for passing only a first and a second output beam from the target. The imaging system is adapted such that the angular separation between the captured beams, λ, and the pitch are selected to cause the first and second output beams to form a sinusoidal image. The system also includes a sensor for imaging the sinusoidal image or images, and a controller for causing the beam generator to direct the at least one incident beam towards the periodic target or targets, and for analyzing the sinusoidal image or images.