Common Path Interferometer for Wafer Thickness Measurement
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Solution Overview
Problem
Low coherence interferometry systems face errors due to temperature fluctuations and path separation in measuring wafer thickness and high aspect ratio structures, particularly in semiconductor manufacturing, where precise optical thickness measurements are crucial.
Innovation Solution
A common path auto-correlation low coherence interferometer system is developed, utilizing a virtual interface and fixed or static mirrors to minimize errors, allowing for accurate measurement of feature dimensions and profiles by generating lower frequency fringes that are analyzed for precise depth and profile determination, and enabling robust time and frequency domain optical coherence tomography.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If auto-correlation approach is implemented with long optical fiber bundles, then measurement capability is extended to larger optical separations, but temperature fluctuations cause significant errors in thickness measurements
Solution Approach 1:
The patent merges the reference arm and measurement arm into a common optical path by using a single optical fiber bundle for both transmitting and receiving light. This eliminates the separate reference arm that caused temperature sensitivity, while maintaining the ability to measure large optical separations through the auto-correlation technique.
Solution Approach 2:
The patent uses a virtual interface created by the auto-correlation technique to replicate the reference function without requiring a physical reference arm. The virtual reference is generated through signal processing of the reflected light from the sample, eliminating the need for a separate physical reference path that would be susceptible to temperature fluctuations.
2Measurement precision
If movable reference mirror is used in time domain low coherence interferometer, then optical separation measurement is enabled, but system complexity and sensitivity to environmental factors increase
Solution Approach 1:
The patent extracts the movable reference mirror from the system and replaces it with a stationary optical fiber bundle configuration. The reference function is achieved through the auto-correlation of the reflected light signal itself, eliminating the need for mechanical movement and associated complexity.
Solution Approach 2:
The patent replaces the mechanical movable reference mirror system with an optical signal processing approach. Instead of mechanically moving a reference mirror to create path length differences, the system uses optical auto-correlation techniques to extract separation information from the interference pattern, substituting mechanical complexity with signal processing.
3Adaptability or versatility
If long optical fiber bundle is used for light transmission, then measurement range is extended, but path separation errors increase due to temperature fluctuations
Solution Approach 1:
The patent combines the reference and measurement functions into a single optical path using one optical fiber bundle. This unified approach allows the system to measure large optical separations while eliminating the path separation errors that would occur with separate reference and measurement paths of different lengths.
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 eliminates temperature-induced errors and path separation issues, providing accurate and robust measurements of wafer thickness and feature dimensions with a large working distance and high dynamic range, while using static mirrors for increased reliability.
Implementation Method 1
The reflected beams 101A′, 101A′′ and 101B′ are recombined at the beam splitter 500 and directed towards photo diode detector 400
Implementation Method 2
Light beam 101 from a broadband source 100 is split by a beam splitter 500 into a first portion 101A directed towards a sample 300 and into a second portion 101B directed towards the movable reference mirror 200
Implementation Method 3
the measurement beam from beam splitter 500 is directed to the sample 300 via an optical fiber
Data Source
AI summary
A system and method of measuring feature depth using a common path auto-correlation low coherence interferometer including a light source having an output directed toward a first beam splitter, the first beam splitter directing at least a portion of a light beam from the light source toward a sample having two reflective interfaces including a top surface reflective interface and a feature bottom reflective interface. The first beam splitter can also pass toward a second beam splitter each of a reference light beam reflected from the top surface interface and a measurement light beam reflected from the feature bottom reflective interface. The second beam splitter directs the reference light beam to a first mirror and the measurement light beam to a second mirror and combines a reflected measurement light beam from the second mirror and a reflected reference light beam from the first mirror to form an interference pattern.