EUV Mask Blank Dual-Layer Absorber Phase Control
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Solution Overview
Problem
Conventional EUV mask blanks with absorber layers containing Ta as the main component have a limited minimum film thickness, and achieving a thinner absorber layer while maintaining sufficient phase difference and contrast is challenging due to the refractive index and extinction coefficient constraints, which complicates the manufacturing process and affects the precision of transferred patterns.
Innovation Solution
The EUV mask blank incorporates a dual-layer absorber structure with a Cr film containing oxygen or nitrogen as the upper layer and a TaPd film containing oxygen or nitrogen as the lower layer, optimized to achieve a phase difference of 180° ± 10° with a small gradient, allowing for a thinner absorber layer with improved precision and reduced manufacturing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional single-layer absorber containing Ta is used, then the absorption coefficient to EUV light is high, but the minimum film thickness is limited to 50-60 nm
Solution Approach 1:
The patent employs a composite absorber layer structure consisting of two different materials: a first absorber layer containing Ta (tantalum) and a second absorber layer containing Mo (molybdenum). This composite structure allows the total absorber layer thickness to be reduced below the conventional 50-60 nm limit while maintaining the necessary optical properties for EUV lithography, specifically achieving the required phase difference and contrast ratios.
2Manufacturing precision
If the absorber layer is made thinner to improve pattern precision, then shadow formation is reduced, but the absorption of EUV light becomes insufficient
Solution Approach 1:
The composite structure of Ta and Mo absorber layers provides superior EUV light absorption efficiency compared to a single-layer structure of equivalent thickness. The specific combination of Ta and Mo layers achieves both reduced shadow formation (enabling thinner overall absorber) and sufficient light absorption (maintaining contrast), resolving the contradiction between thickness reduction and absorption efficiency.
Solution Approach 2:
The patent optimizes the thickness parameters of the individual Ta and Mo layers within the composite absorber structure. By carefully controlling the thickness of each layer (with the Ta layer being thinner and the Mo layer providing the bulk absorption), the design achieves the minimum necessary absorption while minimizing shadow effects, enabling pattern precision improvement.
3Reliability
If a thicker absorber layer is used to ensure sufficient light absorption, then contrast is maintained, but shadow formation impairs pattern precision
Solution Approach 1:
The Ta/Mo composite absorber layer structure achieves sufficient EUV light absorption and contrast with a total thickness significantly less than conventional single-layer designs. The Mo layer provides high absorption coefficient to maintain contrast, while the optimized structure minimizes shadow formation, thereby preserving mask pattern shape precision without requiring excessive thickness.
4Manufacturing precision
If the absorber layer thickness is reduced to below 50 nm, then shadow formation is minimized, but the phase difference becomes difficult to control
Solution Approach 1:
The patent controls the phase difference by precisely adjusting the thickness parameters of the Ta and Mo layers. The Ta layer thickness is optimized to be in the range of 5-30 nm, and the Mo layer thickness is set to provide the necessary phase shift. This parameter optimization ensures the phase difference remains within the required range (170°-190°) even at total absorber thicknesses below 50 nm.
Solution Approach 2:
The composite Ta/Mo structure enables precise phase difference control at thin total thicknesses because the Mo layer has a different optical constant (refractive index and extinction coefficient) than Ta. This allows the Mo layer to provide the necessary phase shift contribution even when the overall absorber thickness is minimized, maintaining phase difference control below the conventional thickness limit.
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 enables the absorber layer to be made thinner than conventional layers, enhancing the shape and dimensional precision of transferred patterns while broadening the acceptable film thickness range, thus improving the overall EUV lithography process.
Implementation Method 1
a reflective layer to reflect EUV light and an absorber layer to absorb EUV light
Implementation Method 2
an absorber layer to absorb EUV light
Data Source
AI summary
To provide an EUV mask blank which has an absorber layer having such a structure that the phase difference between a reflective layer and the absorber layer is in the vicinity of 180°, and the change of the phase difference between the reflective layer and the absorber layer is small to the film thick change of the absorber layer, and of which the absorber layer can be expected to be further thinner than a conventional absorber layer.A reflective mask blank for EUV lithography, comprising a substrate, and a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light formed in this order on the substrate,wherein the absorber layer consists of a layer (upper layer) on the surface side and a layer (lower layer) on the substrate side,one of the upper layer and the lower layer of the absorber layer is a Cr film containing chromium (Cr) as the main component and containing at least one of oxygen (O) and nitrogen (N),the other is a TaPd film containing tantalum (Ta) and palladium (Pd) as the main components and containing at least one of oxygen (O) and nitrogen (N), andthe absorber layer satisfies the following conditions (1) to (5):(1) the total film thickness (L) of the absorber layer is at least 30 nm and at most 45 nm;(2) the film thickness of the TaPd film is at least 8 nm and at most 36 nm;(3) the peak reflectivity of the surface of the absorber layer to EUV light is at least 5% and at most 12%;(4) the phase difference (φ) between EUV reflected light on the surface of the reflective layer and EUV reflected light on the surface of the absorber layer, is within a range of 180°±10°; and(5) the gradient (Δφ/ΔL) of the change (Δφ) of the above phase difference (φ) to the change (ΔL) of the total film thickness (L) of the absorber layer is at most 15 deg/nm.