To get rid of the "Feathering" in your masks, turn off the "anti-alias". There is an icon on the Property Bar for this, it looks like a stair step, and there is an icon the looks like an Eye next to it.
To test this, create an elliptical mask with the Anti-alias turned on, then make sure that the "Plus" [+] sign on the mask properties is selected (Additive mode). Click on the Anti-alias icon again to turn it off, now create a similar elliptical mask near it, select the Paint Brush tool and fill this with a solid color. Remove the masks and you will see the difference right away! B-)