Photomask Optimization for ArF Immersion Lithography Contrast
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Solution Overview
Problem
In photolithography with half pitches of 60 nm or less, the imaging performance is degraded due to the polarization dependency of high-NA optical systems, leading to lower optical image contrast and unresolved fine patterns in the photoresist on the wafer, which is challenging to improve with conventional photomasks.
Innovation Solution
A photomask with a mask pattern of a light shielding film or semi-transparent film on a transparent substrate, optimized for ArF excimer laser immersion exposure using quadrupole-polarized illumination, with specific ranges for film thickness, refractive index, extinction factor, and bias of the space part, enhancing optical image contrast through three-dimensional electromagnetic field simulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If high-NA optical system is used for photolithography with half pitch of 60 nm or less, then resolution is improved, but optical image contrast within resist is lowered due to polarization dependency
Solution Approach 1:
The patent changes the polarization state parameter of the illumination light from conventional linear polarization to circular polarization. This parameter change resolves the vector effect problem in high-NA photolithography by making the illumination less sensitive to polarization dependency, thereby maintaining high optical image contrast within resist while achieving the required resolution for half pitch of 60 nm or less
Solution Approach 2:
The patent introduces a circular polarization converter as an intermediary component between the light source and the photomask. This intermediary device transforms the linearly polarized light into circularly polarized light, enabling the high-NA optical system to achieve both high resolution and high optical image contrast by eliminating the adverse polarization effects
2Ease of manufacture
If conventional photomask structure is used, then manufacturing is simple, but imaging performance is degraded and fine patterns are not resolved
Solution Approach 1:
The patent optimizes the photomask pattern parameters including duty ratio, line width, and space width to specific ranges that are compatible with circular polarization illumination. These parameter changes enable the photomask to achieve superior imaging performance and resolve fine patterns while maintaining manufacturability through standard photomask fabrication processes
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 photomask achieves improved imaging performance and optical image contrast within the resist on the wafer, exceeding conventional limits, by optimizing the balance of diffracted light and adjusting the photomask parameters, effectively addressing the polarization dependency issues.
Implementation Method 1
phase shifting masks such as a Levenson type (also referred to as Levenson-Shibuya type) phase shifting mask intended for improvement in resolution by phase shifting effect utilizing light interference
Implementation Method 2
a photomask used for a leading-edge photolithography technology using high-NA exposure equipment for reducing and transferring a mask pattern nearly equal in size to the exposure wavelength on a wafer
Data Source
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Figure 3A~4B
Figure 5A~7C
AI summary
The present invention provides a photomask which improves the imaging performance that the photomask has and forming a good micro image on a wafer in photolithography with a half pitch of 60 nm or less. Provided is a photomask used for photolithography using an ArF excimer laser as an exposing source for immersion exposure by quadrupole-polarized illumination with a high-NA lens, which is characterized in that the photomask comprises a mask pattern of a light shielding film or semi-transparent film on a transparent substrate, and further characterized in that, given that a thickness of the light shielding film or semi-transparent film is "t" nm, a refractive index is "n", an extinction factor is "k", and a bias of a space part of the mask pattern is "d" nm, when "t", "d", "n" and "k" are adjusted and the photomask is used for the photolithography, optical image contrast takes a value exceeding 0.580.