Optical Thickness Measurement Device Substrate Refraction Correction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing optical thickness measurement devices for intransparent layers on substrates face inaccuracies due to the refractive properties of the substrate, which can lead to significant errors in thickness measurement, especially when the substrate's thickness varies.
Innovation Solution
A device using a combination of chromatic-confocal and interferometric distance measurement methods to measure distances between reference planes and the intransparent layer's surfaces, with an additional measurement of the substrate's surface, accounting for the refractive index and thickness of the substrate to correct for optical effects and improve accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical distance measurement is performed through the substrate to measure the intransparent layer thickness, then the measurement can be carried out, but the refraction properties of the substrate corrupt the measurement result
Solution Approach 1:
The system measures the substrate thickness and uses this information to calculate and correct the optical path length error caused by refraction. The correction value is derived from the measured substrate thickness and the known refractive index, and this correction is applied to the distance measurement through the substrate to obtain the accurate second distance value.
Solution Approach 2:
The system changes the measurement approach by measuring the substrate thickness (third distance) separately and using this parameter to correct the refraction effect. Instead of directly measuring through the substrate with a single method, the system uses multiple measurement values (first distance, second distance, third distance) and applies parameter-based correction using the substrate thickness and refractive index.
2Adaptability or versatility
If the substrate thickness varies, then the measurement can be performed on different substrates, but the measurement accuracy deteriorates due to unknown or varying substrate thickness
Solution Approach 1:
The system performs a preliminary measurement of the substrate thickness (third distance) before using the substrate as a reference for the layer thickness measurement. This preliminary action of measuring the substrate thickness allows the system to prepare the correction value in advance, ensuring accurate measurements even when substrate thickness varies.
Solution Approach 2:
The system continuously measures the substrate thickness and uses this feedback to dynamically adjust the correction applied to the layer thickness measurement. This feedback mechanism ensures that variations in substrate thickness are automatically compensated for, maintaining measurement accuracy across different substrates.
3Reliability
If interferometric distance measurement is used to measure through the substrate, then distance values can be obtained, but the optical path length change due to substrate refraction causes apparent distance changes
Solution Approach 1:
The system introduces an intermediary correction step that accounts for the optical path length change caused by the substrate. The substrate thickness measurement and refractive index information serve as intermediary parameters that mediate between the raw interferometric measurement and the corrected distance value, eliminating the apparent distance changes caused by refraction.
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
Significantly reduces measurement inaccuracies caused by the substrate's refractive properties, allowing for precise determination of the intransparent layer's thickness even with varying substrate thickness, achieving the required accuracy of 10 μm or better.
Implementation Method 1
The first optical means for distance measurement is preferably based on the chromatic-confocal measurement principle. In chromatic-confocal distance measurement, a measuring head is used which contains a chromatically uncorrected optical unit, which focuses the measurement light on the surface of the measurement object.
Implementation Method 2
The second optical means for distance measurement is preferably based on the interferometric measuring method. In distance measurement using the interferometric measuring method, measurement light which is reflected by a measurement object interferes with measurement light which was reflected in a reference arm by at least one first reference mirror.
Implementation Method 3
The optical effect of the substrate originates from the fact that the substrate has an index of refraction n>1. The propagation of the light in the substrate therefore differs from the propagation in air and/or vacuum.
Data Source
AI summary
The invention relates to a device for optical measurement of a thickness of an intransparent layer on a substrate, comprising first means for optical distance measurement configured to measure a first distance between a first reference plane and a first surface of the intransparent layer, and second means for optical distance measurement configured to measure a second distance between a second reference plane and a second surface of the intransparent layer. The second means measures a third distance between the second reference plane and a surface of the substrate. The thickness of the intransparent layer is computed from the first distance and the second distance. The measurement of the third distance is used to take into account the influence of the optical effect of the substrate on the distance measurement of the second distance. The invention also relates to a method for optical measurement of a thickness of an intransparent layer on a substrate.