Expanded Cold Mirror UV-DUV Reflectance
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Solution Overview
Problem
Conventional cold mirrors lack high reflectance in the deep ultraviolet (DUV) range and are opaque in the infrared (IR) range, making them unsuitable for DUV lithography applications, particularly with high power light sources like laser-produced plasma systems, due to their low damage threshold and limited reflective band.
Innovation Solution
An expanded cold mirror design featuring a substrate with multiple coating stacks, including low and high refractive index metal oxide and fluoride layers, providing high reflectance across a broad wavelength band from UV to DUV regions while being IR transparent, formed using specific deposition techniques to ensure density and smoothness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional cold mirrors are used, then they can reflect visible light effectively, but they lack high reflectance in the DUV range and are opaque in the IR range
Solution Approach 1:
The patent applies composite materials by combining multiple coating layers with different optical properties (alternating high and low refractive index materials such as TiO2/SiO2, Ta2O5/SiO2) to create a multilayer dielectric coating system. This composite structure enables the mirror to achieve high reflectance across the UV-DUV range while maintaining IR transparency, resolving the contradiction between wavelength range coverage and reflectance performance.
Solution Approach 2:
The patent changes the optical parameters of the coating by carefully selecting and controlling the thickness of each layer (optical thickness of quarter-wave or half-wave multiples), the refractive index contrast between adjacent layers, and the number of periods in the multilayer stack. These parameter optimizations enable broadening the high-reflectance band into the DUV region while preserving IR transmission characteristics.
2Adaptability or versatility
If aluminum-based mirrors are used to achieve IR transparency, then they allow IR laser passage, but they have low damage threshold and require center hole design
Solution Approach 1:
The patent replaces the single-material aluminum-based mirror with a composite multilayer dielectric coating system consisting of alternating high and low refractive index layers. This composite structure provides both IR transparency and high damage threshold, eliminating the need for center hole designs and improving compatibility with high power laser sources.
Solution Approach 2:
The patent avoids using aluminum-based mirrors that require frequent replacement due to low damage threshold. The multilayer dielectric coating provides a durable, long-lasting solution that can withstand high power laser irradiation without degrading, reducing operational costs and maintenance requirements.
3Reliability
If conventional cold mirror coatings are used, then they provide high reflectance in VIS-IR range, but the high reflective band in DUV range is limited to about 10-15 nm
Solution Approach 1:
The patent segments the coating into multiple distinct coating stacks, each optimized for different wavelength regions. The first coating stack (TiO2/SiO2 or Ta2O5/SiO2) provides high reflectance in the VIS-NIR range, while the second coating stack (additional dielectric layers) extends the high-reflectance band into the UV-DUV region. This segmentation allows the mirror to maintain VIS-IR performance while significantly broadening the DUV reflective band width.
Solution Approach 2:
The patent creates a universal coating design that performs multiple functions: it maintains high reflectance in the VIS-IR range (original cold mirror function) while simultaneously providing broad high-reflectance coverage in the UV-DUV range (new extended function). The multilayer dielectric structure achieves this multi-functionality through careful design of layer thicknesses and refractive index contrasts.
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
The expanded cold mirror achieves high reflectance across the UV-DUV range, enhances laser damage resistance, and reduces design complexity and operational costs compared to conventional aluminum-based mirrors, making it suitable for high power light sources and extending the lifetime of laser-driven light sources.
Implementation Method 1
The coating includes a first coating stack comprising at least one period of a low refractive index metal oxide coating layer and a high refractive index metal oxide coating layer, a second coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal oxide layer, and a third coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal fluoride coating layer
Data Source
AI summary
An expanded cold mirror is provided. The mirror includes a substrate and a coating deposited on the substrate. The coating includes a first coating stack comprising at least one period of a low refractive index metal oxide coating layer and a high refractive index metal oxide coating layer, a second coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal oxide layer, and a third coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal fluoride coating layer.