Synthetic Diamond Optical Mirrors for High Power Lasers
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Solution Overview
Problem
Current optical mirror components for high power laser applications, such as those in next-generation EUV lithography, face challenges with thermal stress, thermal lensing, and coating delamination due to the limitations of materials like copper and polycrystalline CVD diamond, which require advanced mechanical, thermal, and optical characteristics that are difficult to achieve.
Innovation Solution
The development of synthetic diamond optical mirrors with a support plate made of synthetic diamond material, a bonding layer of carbide forming material, and a reflective coating that includes a metal layer and dielectric layers, optimized for high reflectivity and laser-induced damage threshold, addressing the challenges of thermal expansion mismatch and coating adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If copper mirrors are used for high power laser applications, then the mirrors can be easily manufactured and coated, but they suffer from thermal lensing effects and thermal stress due to high coefficient of thermal expansion
Solution Approach 1:
The patent uses a composite structure combining diamond substrate with metal coating layers. The diamond substrate provides low CTE and high thermal conductivity for thermal stability, while the metal coating (gold, silver, or aluminum) provides ease of manufacturing and coating. This composite approach resolves the contradiction by separating the thermal management function (diamond) from the coating function (metal).
Solution Approach 2:
The patent applies different material properties to different parts of the mirror system. The substrate uses diamond with specific thermal properties (low CTE, high thermal conductivity) while the coating layer uses metals with different properties (high reflectivity, ease of deposition). This local differentiation allows each layer to optimize its specific function without compromising the other.
2Reliability
If polycrystalline CVD diamond is used as mirror substrate, then thermal lensing is reduced due to low CTE and high thermal conductivity, but the material is difficult to process to high precision surface finishes
Solution Approach 1:
The patent performs preliminary surface preparation of the diamond substrate before coating. The diamond surface is mechanically polished and chemically treated (e.g., plasma treatment or acid etching) to create a surface that is both precision-finished and chemically active for coating adhesion. This preliminary action resolves the contradiction by preparing the surface in advance to meet both precision and coating requirements.
Solution Approach 2:
The patent introduces intermediate layers between the diamond substrate and the metal coating. These intermediate layers (such as chromium, titanium, or nickel) serve as mediators that facilitate strong bonding between the diamond and metal while allowing the diamond surface to maintain its precision finish. The intermediate layer acts as a buffer that enables coating adhesion without compromising the underlying precision surface.
3Reliability
If reflective coatings are applied to diamond substrates, then laser induced damage threshold is increased, but coating delamination occurs due to thermal expansion coefficient mismatch
Solution Approach 1:
The patent creates a multi-layer composite coating structure on the diamond substrate. The coating system includes an intermediate/adhesion layer (chromium, titanium, or nickel) and a top reflective layer (gold, silver, or aluminum). This composite coating structure resolves the thermal expansion mismatch by using intermediate layers with intermediate CTE values that gradually transition from diamond to metal, preventing delamination while maintaining high laser damage threshold.
Solution Approach 2:
The patent introduces intermediate layers as mediators between the diamond substrate and the metal coating. These intermediate layers (chromium, titanium, nickel) have thermal expansion coefficients that are intermediate between diamond and metals, acting as a buffer to reduce thermal stress. They also provide chemical bonding sites that enhance adhesion, thus preventing delamination while preserving the high laser damage threshold of the diamond-coating system.
4Area of stationary object
If large area diamond components are manufactured, then mirrors for production systems can be provided, but manufacturing cost increases significantly
Solution Approach 1:
The patent segments the diamond substrate manufacturing into manageable steps. Large area diamond substrates are grown using CVD techniques in modular processes, allowing incremental growth and reduced waste. The substrate may also be segmented into smaller sections that are individually processed and then assembled, reducing the complexity and cost of manufacturing single large pieces.
Solution Approach 2:
The patent optimizes manufacturing parameters for large area diamond production. This includes controlling CVD growth conditions (temperature, pressure, gas composition) to achieve uniform large-area substrates with consistent properties. By optimizing parameters such as growth rate, substrate temperature distribution, and gas flow patterns, the patent reduces manufacturing complexity and cost while producing large area substrates suitable for production EUV lithography systems.
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 synthetic diamond mirrors achieve high reflectivity exceeding 99% and a high laser-induced damage threshold, effectively managing thermal stress and maintaining optical performance under high power laser conditions.
Implementation Method 1
a bonding layer of carbide forming material which bonds the reflective coating to the synthetic diamond material in the support plate
Implementation Method 2
polycrystalline chemical vapour deposited (CVD) diamond material has the advantages of being very hard and stiff with a high thermal conductivity
Implementation Method 3
the mirror has a reflectivity of at least 99%... at an operational wavelength of the mirror
Data Source
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Figure 5
AI summary
A mirror for use in high power optical applications, the mirror comprising: a support plate comprising a synthetic diamond material; and a reflective coating disposed over the support plate, wherein the reflective coating comprises a bonding layer of carbide forming material which bonds the reflective coating to the synthetic diamond material in the support plate, a reflective metal layer disposed over the bonding layer, and one or more layers of dielectric material disposed over the reflective metal layer, wherein the bonding layer and the reflective metal layer together have a total thickness in a range 50 nm to 10 μm with the reflective metal layer having a thickness of no more than 5 μm, and wherein the support plate and the reflective coating are configured such that the mirror has a reflectivity of at least 99% at an operational wavelength of the mirror.