Ellipsometer Focusing with Filtered Signals for Moving Samples
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Solution Overview
Problem
Existing ellipsometers face challenges with imprecise focusing systems that require expensive and difficult alignment, suffer from inaccuracies due to stray light, and lack the precision needed for a small illuminating spot size, leading to systematic errors and reduced measurement accuracy.
Innovation Solution
A high-precision focusing system for ellipsometers that uses a beam splitter to sample the entire reflected light beam, coupled with a camera and a compensator to correct optical aberrations, and employs closed-loop control with real-time focal adjustment using filtered focus signals to maintain focus during sample movement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If independent focusing systems are used, then focusing capability is provided, but alignment precision requirements become very high and cost increases
Solution Approach 1:
The patent merges the focusing detection function into the main measurement optical path by using the same reflected light beam for both ellipsometry measurement and focus detection. The focus detector receives light that has already reflected from the sample through the same optics, eliminating the need for separate alignment of independent focusing systems.
Solution Approach 2:
The reflected light beam from the sample serves dual purposes: it carries the measurement information for ellipsometry and simultaneously provides the focus detection signal. This multi-functional use of the same optical path reduces alignment requirements and system complexity.
2Ease of operation
If position sensitive devices or quad cells are used as focus detectors, then focusing detection is enabled, but measurement accuracy deteriorates due to susceptibility to stray light
Solution Approach 1:
The patent introduces a beam splitter as an intermediary that separates the measurement beam path from the focus detection path. The beam splitter directs a portion of the reflected light to the focus detector while allowing the main beam to continue to the ellipsometry detector, enabling focus detection without exposing the detector directly to stray light.
Solution Approach 2:
The reflected light beam is segmented into two paths: one for measurement detection and one for focus detection. This segmentation allows the focus detector to receive a controlled portion of the light while the measurement detector receives the full beam, reducing stray light interference.
3Ease of operation
If only outer part of the beam is sampled by the focus detector, then focus detection is achieved, but systematic errors increase and precision decreases
Solution Approach 1:
The focus detector is designed to sample the entire reflected light beam rather than only the outer part. By making the focus detection universal to the full beam, the system eliminates systematic errors that arise from sampling only portions of the beam, improving measurement precision.
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 precise focusing with a small spot size, reducing errors and maintaining focus on the sample during movement, thereby improving measurement accuracy and throughput.
Implementation Method 1
A high-precision focusing system for ellipsometers that uses a beam splitter to sample the entire reflected light beam
Implementation Method 2
coupled with a camera and a compensator to correct optical aberrations
Implementation Method 3
coupled with a camera and a compensator to correct optical aberrations
Implementation Method 4
employs closed-loop control with real-time focal adjustment using filtered focus signals to maintain focus during sample movement
Data Source
AI summary
An optical metrology device, such as an ellipsometer, includes a focusing system that adjusts the focal position of the metrology device in real time so that focus may be maintained during movement of the measurement locations on the sample, e.g., using closed loop control. A filtered focus signal may be used to adjust the focal position while moving to a measurement location. Additionally, the focus signal may be coarsely filtered and finely filtered, where a coarse filtered focus signal is used to adjust the focal position while moving to a measurement location and a fine filtered focus signal is used to adjust the focal position when at the measurement location. An open loop control may be used in which once at the measurement location, a filtered focus signal is used to adjust the focal position when the filtered focus signal has no offset with respect to the focus signal.