Minimization of mask undercut on deep silicon etch

A method for forming features in a silicon layer, comprising:

• forming a mask with a plurality of mask openings over the silicon layer;
• placing the silicon layer in a plasma process chamber;
• depositing a polymer layer over the mask, comprising:
• flowing a hydrogen free deposition gas consisting essentially of C4F8 into the plasma process chamber;
• forming a plasma from the deposition gas;
• depositing a polymer from the plasma for at least 20 seconds to form a layer at least 200 nm thick;
• and
• stopping the depositing the polymer after the at least 20 seconds;
• opening the deposited polymer layer, comprising:
• flowing an opening gas into the plasma process chamber;
• forming a plasma from the opening gas which selectively removes the deposited polymer on bottoms of the plurality of mask openings with respect to deposited polymer on sides of the plurality of mask openings;
• and
• stopping the opening when at least some of the plurality of mask features is opened;
• etching the silicon layer through the mask and deposited polymer layer, comprising:
• flowing an etching gas different from the opening gas;
• and
• forming a plasma from the etching gas, which etches the silicon layer, wherein the deposited polymer layer prevents undercutting of the silicon layer under the photoresist mask, and the deposited polymer layer is completely etched away by etching the silicon layer;
• and
• removing the silicon layer from the plasma process chamber.