Interferometer System Vibration Damping Reference Flat
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Solution Overview
Problem
Existing interferometry techniques for measuring semiconductor substrate shape and thickness are prone to vibration and thermal sensitivity, especially at low frequencies, and require large, expensive optical components, leading to inaccurate and costly measurements.
Innovation Solution
An improved interferometer system with two phase-shifting Fizeau interferometers and parallel reference flats close to the substrate surfaces, using a positioning mechanism to position damping plates within 3 millimeters of the substrate, minimizing non-common optical path length and enhancing vibration damping while allowing the use of short coherence light sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If grazing incidence interferometry is used to measure rough surfaces, then adaptability to various sample types is improved, but beam path length increases making the system sensitive to vibration and thermal variation
Solution Approach 1:
The patent inverts the traditional grazing incidence geometry by using normal incidence interferometry instead. This reversal allows the use of short coherence length light sources and dramatically reduces the optical path length, thereby eliminating sensitivity to vibration and thermal variation while maintaining measurement capability on rough surfaces through the use of a reference plate in close proximity
Solution Approach 2:
The patent introduces a reference plate as an intermediary element placed in close proximity to the sample. This reference plate creates a common optical path that serves as a stable reference, enabling measurements on rough surfaces without requiring long beam paths or grazing incidence geometry
2Measurement precision
If normal incidence interferometry is used with reference plates close to the wafer, then measurement accuracy is improved, but the system becomes sensitive to wafer vibration and requires long coherence length light sources
Solution Approach 1:
The patent replaces the mechanical damping arrangement with an optical solution by creating a common optical path through the use of a reference plate. This substitution eliminates the need for physical contact or mechanical damping while achieving vibration insensitivity through optical path commonality
Solution Approach 2:
The patent merges the measurement and reference paths into a single common optical path. By having both the sample and reference plate illuminated by the same light source and detected through the same optical channel, the system becomes insensitive to vibrations affecting the entire path equally
3Area of stationary object
If large diameter reference flats and collimators are used to match wafer size, then measurement coverage is improved, but system cost increases significantly
Solution Approach 1:
The patent segments the measurement process by using a reference plate that can be smaller than the wafer, measuring different regions in sequence. This allows the use of smaller, less expensive optical components while still achieving full wafer coverage through multiple measurements and data stitching
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 configuration improves measurement accuracy and repeatability, reduces system costs, and enables the use of short coherence light sources, providing precise thickness and shape information with reduced sensitivity to vibrations and thermal variations.
Implementation Method 1
Interference of the two beams due to different optical path lengths can be detected with a photosensitive detector
Implementation Method 2
at least one damping plate proximate the portion of the first or second surface of the substrate that does not overlap the first or second reference surface
Data Source
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AI summary
An interferometer system and method may be used to measure substrate thickness or shape. The system may include two spaced apart reference flats having that form an optical cavity between two parallel reference surfaces. A substrate holder may be configured to place the substrate in the cavity with first and second substrate surfaces substantially parallel with corresponding first and second reference surfaces such that a space between the first or second substrate surface is three millimeters or less from a corresponding one of the reference surfaces or a damping surface. Interferometer devices may be located on diametrically opposite sides of the cavity and optically coupled thereto. The interferometers can map variations in spacing between the substrate surfaces and the reference surfaces, respectively, through interference of light optically coupled to and from to the cavity via the interferometer devices.