Dual Critical Dimension Patterning With SRAF Photoresist Thinning
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Solution Overview
Problem
Existing photolithography processes face challenges in manufacturing semiconductor devices with features having at least two differing critical dimensions using a single mask, as they are susceptible to photoresist peeling and increased production costs due to the need for multiple masks.
Innovation Solution
The use of a photomask with a sub-resolution assist feature (SRAF), such as a mask scattering bar or a designed area of local phase shift, allows for light and/or radiation leakage, resulting in a thinned photoresist layer that retains the desired critical dimension, thereby facilitating the fabrication of features with smaller critical dimensions using a single mask.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single photomask is used to manufacture features with at least two differing critical dimensions, then production costs and cycle times decrease, but the process becomes susceptible to photoresist peeling
Solution Approach 1:
The photomask incorporates sub-resolution assist features (SRAFs) with specific optical properties that create localized variations in light intensity during exposure. These SRAFs are positioned adjacent to main features and have dimensions below the resolution limit, causing them to scatter or absorb light in a controlled manner. This produces intermediate light exposure in target areas, resulting in thinned photoresist layers at specific locations while maintaining full thickness elsewhere, thereby enabling different critical dimensions from a single mask without causing peeling
Solution Approach 2:
The photomask design pre-establishes the conditions for creating features with different critical dimensions by incorporating SRAFs during the initial exposure step. The SRAFs are strategically placed to produce the desired light intensity distribution before development, allowing the photoresist to be thinned in advance at locations where smaller critical dimensions are required. This preliminary structuring of light exposure eliminates the need for subsequent trimming steps and prevents peeling by maintaining adequate photoresist thickness in non-thinned areas
2Reliability
If multiple photomasks are used to manufacture features with at least two differing critical dimensions, then photoresist peeling is avoided, but production costs and cycle times increase
Solution Approach 1:
The invention merges the functions of multiple photomasks into a single photomask by incorporating SRAFs that create intermediate light exposure zones. These SRAFs enable the single mask to simultaneously define both larger and smaller critical dimension features through differential photoresist thinning. The merging is achieved by combining the main feature patterns with auxiliary SRAF patterns on the same mask substrate, allowing both feature types to be exposed and developed in a single step while maintaining photoresist integrity
Solution Approach 2:
The photomask achieves multi-functionality by serving dual purposes: defining main features with larger critical dimensions and creating subsidiary features with smaller critical dimensions. The SRAFs act as universal elements that contribute to both functions - they scatter/absorb light to create thinned photoresist regions for small features while the overall mask pattern defines the larger features. This universal design eliminates the need for separate masks for different feature sizes
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 approach reduces the risk of photoresist peeling, allows for the fabrication of semiconductor devices with features of varying critical dimensions using a single mask, and decreases production costs and cycle times.
Implementation Method 1
light and/or radiation leakage from a sub-resolution assist feature (SRAF) incorporated in and/or on the photomask
Implementation Method 2
mask scattering bar
Data Source
AI summary
A patterning process is performed on a semiconductor wafer coated with a bottom layer, a middle layer and a photoresist layer having a starting thickness. The patterning process includes: performing an exposure step including exposing the semiconductor wafer using a mask that includes a feature which produces an intermediate light exposure in a target area followed by processing that creates openings in the photoresist layer in accordance with the mask and thins the photoresist in the target area due to the intermediate light exposure in the target area leaving thinned photoresist in the target area; performing middle layer etching to form openings in the middle layer aligned with the openings in the photoresist layer, wherein the middle layer etching does not remove the middle layer in the target area due to protection provided by the thinned photoresist; and performing trim etching to trim the middle layer in the target area.