Mask is clearly a part of the design phase, and required BEFORE sent to fabrication,
and our chips have been sent to fab.  Unless Intel is wrong too, lol.
this may help you understand too... http://computer.howstuffworks.com/euvl1.htm
http://users.wfu.edu/ucerkb/Nan242/L15-Photolithography.pdf
The mask itself is used in the lithography process to actually print onto the silicon. It's like a transparent photo that radiation is passed thru to burn the chip design onto the silicon, making the chips. When they submitted the artwork, or "mask" to the manufacturer, that was "Tapeout".