Method of making attenuating phase-shifting mask using different exposure doses

A method of forming a photomask, comprising the steps of:

• providing a transparent mask substrate having a first region and a second region;
• depositing a layer of attenuating phase shifting material on said transparent mask substrate;
• depositing a layer of opaque material on said layer of attenuating phase shifting material;
• depositing a layer of resist on said layer of opaque material;
• exposing a first pattern in that part of said layer of resist over said first region of said transparent mask substrate using a first exposure dose;
• exposing a second pattern in that part of said layer of resist over said second region of said transparent mask substrate using a second exposure dose, wherein said second exposure dose is different from said first exposure dose;
• developing said layer of resist thereby forming a resist mask having said first pattern made up of first pattern elements having a first thickness over said first region of said transparent mask substrate and said second pattern made up of second pattern elements having a second thickness over said second region of said transparent mask substrate wherein said first exposure dose and said second exposure dose are adjusted so that said second thickness is greater than said first thickness;
• etching said first pattern and said second pattern in said layer of opaque material using said resist mask;
• etching said first pattern and said second pattern in said layer of attenuating phase shifting material using said resist mask;
• etching away said first thickness of said resist mask thereby removing said first pattern elements and reducing the thickness of said second pattern elements to the difference between said second thickness and said first thickness thereby forming a modified resist mask;
• etching away said first pattern in said layer of opaque material over said first region of said mask substrate using said modified resist mask;
• and
• removing said modified resist mask.