Flash Lamp Stress Control for Photomask Blank Flatness
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Solution Overview
Problem
The reduction of exposure wavelength in lithography for semiconductor integrated circuits leads to a decrease in depth of focus, resulting in a narrower process margin and lower stability, which is detrimental to manufacturing yield, and existing phase shift masks suffer from warp due to film stress, causing positional shifts and focusing offsets during pattern exposure.
Innovation Solution
A method for preparing a photomask blank with a light-absorbing film on a transparent substrate, where the film stress is controlled by irradiating the light-absorbing film with a flash lamp at specific energy densities to minimize warp and optimize flatness, allowing for finer feature sizes and higher precision in pattern exposure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a light-absorbing film is deposited on a substrate by sputtering to form a phase shift mask blank, then the phase shift function is achieved, but film stress is introduced causing substrate warp and flatness change
Solution Approach 1:
The patent applies preliminary action by irradiating the light-absorbing film with flash lamp light immediately after deposition to adjust film stress before the substrate undergoes patterning. This preliminary stress adjustment prevents subsequent flatness changes that would occur during the patterning process, thereby maintaining substrate flatness while achieving the phase shift function.
Solution Approach 2:
The patent changes physical parameters of the light-absorbing film by irradiating it with flash lamp light at specific energy densities (3-40 J/cm²). This irradiation modifies the film's stress state, transforming it from a high-stress condition that causes warp to a low-stress condition that maintains flatness, while preserving the phase shift functionality.
2Manufacturing precision
If exposure wavelength is reduced to improve resolution, then the resolution is improved, but depth of focus decreases leading to narrower process margin and lower stability
Solution Approach 1:
The patent applies local quality by creating a halftone phase shift mask where the light-absorbing film has spatially varying thickness. This produces different optical path lengths in different regions, generating phase shifts that enhance image contrast and extend depth of focus locally at the pattern interfaces, thereby compensating for the reduced DOF caused by shorter exposure wavelengths.
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 controlled stress in the photomask blank reduces warpage to less than 0.1 μm, maintaining optimal flatness and improving chemical resistance, thereby enhancing the precision and stability of the photomask for finer feature sizes and improved manufacturing yield.
Implementation Method 1
irradiating the light-absorbing film with light from a flash lamp at a predetermined energy density
Implementation Method 2
the film stress of the photomask blank can be controlled by irradiating the light-absorbing film with light from a flash lamp
Data Source
AI summary
A photomask blank is prepared by forming a light-absorbing film on a transparent substrate, and irradiating the light-absorbing film with light from a flash lamp at an energy density of 3 to 40 J/cm2. A photomask is prepared by forming a resist pattern on the photomask blank by photolithography, etching away those portions of the light-absorbing film which are not covered with the resist pattern, and removing the resist.