Purge Gas Flow Control for Ellipsometry Precision
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Solution Overview
Problem
Current broadband spectroscopic ellipsometry systems face precision issues in film thickness measurements due to interactions between nitrogen purging gas and airborne molecular contaminants, leading to changes in the effective thickness of contaminant layers on semiconductor wafers, especially in the ultraviolet-visible spectral region.
Innovation Solution
An optical system with a purge gas source and controller that independently adjusts nitrogen gas flow rates through different components of the system, allowing for precise control of gas flow to minimize interactions with contaminants, thereby stabilizing the contaminant layer and maintaining measurement precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If nitrogen gas is used to purge the measurement spot, then VUV optical measurements can be performed, but the interaction between N2 gas and airborne molecular contaminants causes changes in the effective thickness of contaminant layers, reducing measurement precision
Solution Approach 1:
The patent applies local quality by providing different purge gas flow rates to different regions of the system. Specifically, a first purge gas flow rate is provided to a first region (illumination and collection optics) and a second purge gas flow rate is provided to a second region (sample stage and detector), allowing each region to have optimized gas flow conditions for its specific function while minimizing overall gas consumption and contaminant interaction effects
Solution Approach 2:
The patent implements dynamics by making the purge gas flow rates adjustable and controllable. The system can dynamically adjust the first and second purge gas flow rates based on measurement requirements, allowing optimization between VUV transmission (requiring higher flow rates) and measurement precision (requiring lower flow rates to minimize contaminant layer changes)
2Reliability
If high nitrogen gas flow rates are used, then VUV optical path is maintained, but the interaction with contaminants increases, causing changes in effective thickness and reducing precision
Solution Approach 1:
The patent applies parameter changes by optimizing the purge gas flow rates to specific values or ranges that balance VUV transmission requirements with measurement precision. The system determines optimal first and second purge gas flow rates that are sufficient to maintain VUV optical path quality while being low enough to minimize interactions with airborne molecular contaminants during measurement
3Object-affected harmful factors
If nitrogen gas purging is applied, then ambient contaminants are removed, but the purging gas interacts with physisorbed materials causing changes in effective thickness
Solution Approach 1:
The patent applies local quality by providing different purge gas flow rates to different regions. The first region (optics) receives a first flow rate optimized for VUV transmission, while the second region (sample) receives a second flow rate optimized for minimizing contaminant interaction. This spatial differentiation allows the system to remove ambient contaminants effectively while minimizing the harmful interaction between purging gas and physisorbed materials on the sample surface
Solution Approach 2:
The patent applies partial action by providing purge gas flow at reduced rates in the second region compared to the first region. Instead of using high flow rates throughout the entire system, the system uses lower flow rates in the sample region where contaminant interaction is most problematic, thereby reducing the harmful effect while still maintaining adequate contaminant removal in critical areas
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 achieves higher precision and repeatability in spectroscopic ellipsometry measurements by stabilizing the contaminant layer, with optimal nitrogen flow rates balancing measurement precision and recovery time, allowing for accurate thickness measurements across the UV-Vis spectral range.
Implementation Method 1
VUV light is absorbed mainly by oxygen and water molecules generally present in an ambient environment. Therefore, nitrogen or inert gas purging or vacuum is required to extend optical measurement down to vacuum ultraviolet (VUV) range
Implementation Method 2
it is useful to determine the surface metrology of samples such as thickness of thin films, their refractive indices and the profile parameters of surface features
Implementation Method 3
spectroscopic reflectometers and spectroscopic ellipsometers
Implementation Method 4
there is an interaction between the purging N2 gas and physisorbed materials (hydrocarbons, contaminants, etc henceforth referred to as Airborne molecular Contaminants or AMC), which results typically in a change to the effective thickness of the AMC layer on wafer during film thickness measurements
Data Source
AI summary
An optical method and system for measuring characteristics of a sample using a broadband metrology tool in a purge gas flow environment are disclosed. In the method a beam path for the metrology tool is purged with purge gas at a first flow rate. A surface of the sample is illuminated by a beam of source radiation having at least one wavelength component in a vacuum ultraviolet (VUV) range and/or at least one wavelength component in an ultraviolet-visible (UV-Vis) range. A flow rate of a purge gas is adjusted between the first flow rate for metrology measurements made when the source radiation is in the VUV spectral region and a second flow rate for metrology measurements made when the source radiation is in the UV-Vis spectral region. The system includes a light source, illumination optics, collection optics, detector, a purge gas source and a controller. The purge gas source is configured to supply a flow of purge gas to a beam path in the light source and/or illumination optics and/or sample and/or collection optics and/or detector. The controller is configured to control a flow rate of the purged gas flow in response to an output signal from the detector.