EUV Mirror Surface Protecting Layer Prevents Substrate Densification
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Solution Overview
Problem
Mirrors for extreme-ultraviolet (EUV) wavelength range in microlithography experience variations in reflectivity with angle of incidence and high EUV light exposure, leading to substrate densification and undesirable changes in optical imaging properties over time.
Innovation Solution
A mirror design with a substrate and layer arrangement featuring a periodic sequence of high and low refractive index layers, including a surface protecting layer system with a thickness greater than 20 nm to minimize EUV radiation transmission and prevent irreversible changes, ensuring long-term stability and uniform reflectivity across a wide angle of incidence interval.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a layer arrangement with periodic sequence of high and low refractive index layers is used to achieve high reflectivity, then reflectivity is improved, but reflectivity varies with angle of incidence
Solution Approach 1:
The patent applies parameter changes by carefully controlling the thickness of each layer in the periodic sequence. Specifically, the thickness of the low refractive index layer is set to between 0.5λ/n_low and 2.0λ/n_low, and the thickness of the high refractive index layer is set to between 0.5λ/n_high and 2.0λ/n_high, where λ is the design wavelength. This parameter optimization ensures that the reflectivity remains substantially constant across a wide angle of incidence range while maintaining high reflectivity values.
2Reliability
If EUV light transmission to substrate is not minimized, then substrate is exposed to high doses of EUV light, but this causes substrate densification and changes in optical imaging properties
Solution Approach 1:
The patent introduces an intermediary function by ensuring that the layer arrangement serves as a protective barrier between the EUV light source and the substrate. The optimized periodic layer structure achieves this by minimizing EUV light transmission to the substrate through careful selection of layer materials and thicknesses, thereby preventing substrate densification and maintaining optical imaging stability without requiring additional protective layers.
3Object-affected harmful factors
If high reflectivity is achieved through multiple periods of layers, then more EUV light is blocked from substrate, but this increases device complexity
Solution Approach 1:
The patent reduces device complexity by optimizing the number of periods in the layer arrangement. Instead of using a large number of periods, the invention achieves sufficient EUV radiation blocking with only 2-10 periods by carefully controlling the thickness parameters of individual layers. This parameter optimization allows the layer arrangement to function effectively as both a high-reflectivity coating and a substrate protection barrier, minimizing the number of layers required.
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 solution achieves high and uniform reflectivity with reduced EUV radiation transmission to the substrate, maintaining optical stability and preventing substrate densification, thus ensuring consistent imaging performance over extended periods.
Implementation Method 1
the layer arrangement comprises a periodic sequence of at least two periods of individual layers, wherein the periods comprise two individual layers composed of different materials for a high refractive index layer and a low refractive index layer
Implementation Method 2
the layer arrangement comprises at least one surface protecting layer or at least one surface protecting layer system having a thickness of greater than 20 nm... wherein the transmission of EUV radiation through the layer arrangement amounts to less than 2%
Data Source
AI summary
A mirror (1a; 1a′; 1b; 1b′; 1c; 1c′) for the EUV wavelength range and having a substrate (S) and a layer arrangement, wherein the layer arrangement includes at least one surface layer system (P′″) consisting of a periodic sequence of at least two periods (P3) of individual layers, wherein the periods (P3) include two individual layers composed of different materials for a high refractive index layer (H′″) and a low refractive index layer (L′″), wherein the layer arrangement includes at least one surface protecting layer (SPL, Lp) or at least one surface protecting layer system (SPLS) having a thickness of greater than 20 nm, and preferably greater than 50 nm.