Complementary Waveplate Rotating Compensators for Deep UV Ellipsometry
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Solution Overview
Problem
Rotating compensator ellipsometers face limitations in broadband operation due to wavelength-dependent sensitivity, particularly at short and long wavelengths, leading to reduced sensitivity and a dead zone in the deep UV spectrum, making it challenging to accurately measure properties of surfaces and thin films across a wide spectral range.
Innovation Solution
The introduction of a second waveplate with complementary quarter wavelength retardation, optimized for specific spectral ranges, extends the useful spectral range and increases sensitivity, allowing operation from 150 nm to 1000 nm while maintaining high sensitivity in the conventional range, and can be implemented with existing hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single rotating compensator is used in a normal incidence ellipsometer, then the device structure is simple and easy to manufacture, but the spectral range is limited and sensitivity is reduced at short and long wavelengths
Solution Approach 1:
The single compensator is segmented into two separate compensators with different waveplate orientations. The first compensator has its fast axis parallel to the plane of incidence, while the second compensator has its fast axis perpendicular to the plane of incidence. This segmentation allows each compensator to be optimized for different spectral ranges, extending the overall useful spectral range from 190-820 nm to 150-1000 nm while maintaining simple construction using existing hardware
Solution Approach 2:
The two compensators work together to provide multi-functional coverage across the electromagnetic spectrum. Each compensator handles specific wavelength ranges where the other has reduced effectiveness, creating a universal system that maintains high sensitivity across the entire 150-1000 nm range. The system can operate with both compensators rotating simultaneously or with one stopped, providing operational flexibility
2Measurement precision
If the waveplate retardation is optimized for conventional spectral range, then sensitivity is high in that range, but the deep UV spectrum becomes a dead zone with reduced sensitivity
Solution Approach 1:
Each compensator is designed with local quality optimization for specific spectral ranges. The first compensator (fast axis parallel to plane of incidence) is optimized for the conventional range (190-820 nm), while the second compensator (fast axis perpendicular to plane of incidence) is optimized for the deep UV range (150-370 nm). This local optimization ensures high sensitivity in each respective range without creating dead zones
Solution Approach 2:
The system uses composite functionality by combining two compensators with different waveplate orientations and material optimizations. The MgF2 waveplates in each compensator have different thicknesses and orientations, creating a composite system where the combined retardation characteristics cover the entire 150-1000 nm spectrum effectively, with each compensator contributing its strength in specific wavelength regions
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 enhances diagnostic power by extending the spectral range into the deep UV, increases sensitivity across the conventional range, and simplifies construction by using existing hardware, avoiding the need for expensive custom components.
Implementation Method 1
The rotating compensator introduces a relative phase delay ξ (phase retardation) between a pair of mutually orthogonally polarized components of the probe beam. The rotating compensator includes a rotating optical component, such as a waveplate, that delays the light polarized parallel to its slow axis relative to light polarized parallel to its fast axis by an amount proportional to the refractive index difference along the two directions and the thickness of the plate
Implementation Method 2
A polarizer is optically coupled to the probe beam to impart a known polarization state to the probe beam
Implementation Method 3
The sample reflects or scatters the probe beam back through the compensator and the polarizer, which acts as an analyzer for the beam returned from the sample
Data Source
AI summary
In embodiments of the present invention a second, different waveplate is introduced into a single rotating compensator normal incidence ellipsometer. The second waveplate provides a quarter wavelength retardation that is different from and complementary to that of the first waveplate in order to increase the spectral range for which useful retardation is available, especially towards the deep UV spectrum. The sensitivity for the system may also be increased in the conventional spectral range, since each of the two waveplates may be optimized for its own, somewhat more narrow spectral range of operation. With the proper choice of two waveplates of different retardation, the useful spectral range may be extended from typically 190-820 nm to 150-1000 nm, and beyond if necessary, while increasing the sensitivity within the conventional wavelength range at the same time.