No matter what you do, you can't make copying totally impossible. You can, however, make it really difficult through your choice of packaging. If you use a BGA/CSP type package, access to the contacts of your MCU will be improbable if not totally impossible. That will keep people wishing to copy your product on the cheap from benefiting from your work.

If you use a small, cheap, MCU in a small, inexpensive package, you invite small, cheap, competitors. Another problem, of course, will occur if you choose to have your product manufactured in Asia, where the cultural attitude regarding intellectual property differs greatly from ours. If you let your product be totally manufactured in Asia, then you'll never be in business long enough to figure out which supplier is your competitor. Have your boards made in China, assembled in Korea, and programmed in the U.S. or Europe. Then you have, at least, a small chance.

RE