Mask Blank Oxide Layer Thickness Uniformity

The patent applies forcible cooling immediately after heat treatment to prevent non-uniform oxide layer formation. By actively cooling the mask blank before it is exposed to atmospheric oxygen, the oxidation process is controlled to occur uniformly during the heat treatment stage rather than forming non-uniform oxide layers during natural cooling. This preliminary action (forcible cooling) prevents the harmful effect of non-uniform oxidation that would occur with simple natural cooling.

The patent combines heat treatment with forcible cooling in a continuous process. The mask blank undergoes heat treatment to achieve the desired chemical resistance and stress relief, followed immediately by forcible cooling to control oxide layer formation. This continuous process ensures that the beneficial effects of heat treatment (improved chemical resistance and reduced compressive stress) are maintained while simultaneously controlling the oxide layer thickness uniformity to ensure transmittance uniformity.

The patent introduces a cooling gas (inert or reactive gas) as an intermediary medium to achieve uniform cooling across all substrates in the batch. The cooling gas is introduced into the furnace and circulated to ensure consistent cooling conditions for all mask blanks being processed simultaneously. This intermediary cooling gas enables the vertical furnace to maintain both high productivity through batch processing and uniform cooling across all substrates, preventing non-uniform oxide layer formation.