Linnik Interferometry Focusing for Fast Semiconductor Inspection
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Solution Overview
Problem
Existing semiconductor inspection systems face challenges in achieving fast, accurate, and repeatable focus due to limitations in current focus systems, such as OTL and TTL systems, which affect speed, accuracy, and repeatability, particularly in optical metrology and inspection tools.
Innovation Solution
A system incorporating an imaging subsystem with a light source, camera, and spectrometer, and a focusing subsystem with a reference mirror and processor that uses Fourier transforms and phase delay to determine defocus conditions, enabling rapid adjustment of focus using actuators and a shutter to improve focus accuracy and speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If mechanical moving parts are used to detect defocusing by scanning the sample along a vertical axis, then focus measurement can be achieved, but the speed, accuracy, and repeatability of the focus measurement are limited
Solution Approach 1:
The patent replaces mechanical scanning systems with an optical interferometry-based focus measurement system. The Linnik interferometer uses light interference patterns to detect focus conditions without mechanical movement, thereby eliminating the speed and accuracy limitations imposed by mechanical scanning while maintaining precise focus measurement capability
Solution Approach 2:
The patent introduces an interferometer as an intermediary device between the imaging system and the focus measurement process. The interferometer converts focus information into optical interference signals that can be analyzed to determine defocus conditions, providing a non-mechanical means of achieving accurate and rapid focus measurement
2Measurement precision
If TTL focus systems are used to measure focus right on the spot where measurement takes place, then accuracy and correlation with optical system real focus are improved, but the system complexity increases
Solution Approach 1:
The patent integrates the Linnik interferometer into the existing imaging system such that the same optical path and components serve both imaging and focus measurement functions. This multi-functionality approach allows TTL focus measurement with high accuracy while minimizing additional system complexity by reusing existing optical infrastructure
3Ease of operation
If independent illumination source is used for focusing, then the probing beam can be easier to shape, manipulate, and control power, but using a different wavelength band for imaging and focusing can interact differently with the sample and produce inaccurate results
Solution Approach 1:
The patent uses the same wavelength band for both imaging and focus measurement, ensuring that light interacts with the sample in the same manner for both operations. This parameter consistency (wavelength) eliminates measurement inaccuracies while the independent illumination source still provides beam shaping and power control capabilities through optical elements
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 system achieves fast, accurate, and repeatable focus by determining defocus conditions through frequency-domain analysis, allowing for quick adjustment and improved imaging throughput in semiconductor inspection.
Implementation Method 1
a spectrometer configured to generate an interference signal based on the collocated light reflected by the sample and the reference mirror
Implementation Method 2
The processor may be configured to transform the interference signal to obtain a frequency-domain signal. In some embodiments, the processor may be configured to transform the interference signal by applying a Fourier transform to the interference signal
Data Source
AI summary
The system may include an imaging subsystem, a focusing subsystem, and a processor. The imaging subsystem includes a light source configured to emit light, a main objective lens configured to focus the light onto a sample, and a camera configured to generate one or more images of the sample based on the light emitted from the light source reflected by the sample. The focusing subsystem includes a reference objective lens configured to focus a portion of the light onto a reference mirror that is reflected and collocated with the light reflected by the sample, and a spectrometer configured to generate an interference signal based on the collocated light reflected by the sample and the reference mirror. The processor is configured to transform the interference signal from a time-domain signal to a frequency-domain signal and determine a defocus condition of the imaging subsystem based on the frequency-domain signal.