Low Thermal Expansion Glass Substrate Cleaning for EUV Masks
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Solution Overview
Problem
Conventional cleaning methods for reflective mask blank substrates with low thermal expansion glass substrates, containing titanium oxide, result in the generation of numerous convex defects due to substrate dissolution and redeposition during hydrofluoric acid treatment and alkaline cleaning, which are not adequately addressed, leading to suboptimal surface quality for EUV lithography applications.
Innovation Solution
A method involving polishing a low thermal expansion glass substrate with titanium oxide, followed by treatment with a hydrofluoric acid solution and subsequent cleaning with an acidic solution at a pH of 4 or less to remove dissolved components before alkaline cleaning, which suppresses condensation reactions and reduces convex defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional cleaning method (hydrofluoric acid treatment followed by alkaline cleaning) is used on low thermal expansion glass substrates containing titanium oxide, then cleaning effectiveness is achieved, but convex defects are generated due to substrate dissolution and redeposition
Solution Approach 1:
The cleaning process is divided into multiple sequential steps: hydrofluoric acid treatment, acidic cleaning with pH 4 or less, and alkaline cleaning. This segmentation allows each step to perform a specific function - the hydrofluoric acid removes polishing residues, the acidic step prevents titanium oxide dissolution, and the alkaline step completes the cleaning, thereby achieving effective cleaning without generating convex defects
Solution Approach 2:
The acidic cleaning step with pH 4 or less is introduced as a preliminary action before the alkaline cleaning step. This preliminary acidic treatment creates a protective environment that prevents titanium oxide from dissolving and redepositing during the subsequent alkaline cleaning, thus preventing convex defect formation while maintaining cleaning effectiveness
2Stability of the object's composition
If low thermal expansion glass substrate containing titanium oxide is used for reflective mask blank, then thermal stability is improved, but convex defects are generated during cleaning due to titanium oxide dissolution and redeposition
Solution Approach 1:
The pH parameter of the cleaning solution is precisely controlled to be 4 or less during the acidic cleaning step. This parameter change creates an acidic environment that prevents titanium oxide dissolution, thereby maintaining the thermal stability benefits of the low thermal expansion glass substrate while avoiding convex defect generation during cleaning
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 significantly reduces the number of convex defects on the substrate surface, achieving a high-quality mask blank substrate with improved flatness and surface roughness suitable for EUV reflective mask blanks, ensuring high reflectance and transfer accuracy.
Implementation Method 1
treating the polished glass substrate using an aqueous solution containing hydrofluoric acid
Implementation Method 2
cleaning the treated glass substrate using an acidic solution with a pH of 4 or less, thereby removing dissolved substrate components
Implementation Method 3
further cleaning the cleaned glass substrate using an alkaline solution
Data Source
AI summary
Provided is a mask blank substrate manufacturing method in which a low thermal expansion glass substrate containing titanium (Ti) oxide is polished using a polishing agent, then treated using an aqueous solution containing hydrofluoric acid, then cleaned using an acidic solution with a pH of 4 or less, and then further cleaned using an alkaline solution.