You don't have to add any logic for the backdoor. Instead, you just intentionally leave a sutble zero day exploid "bug"unpatched in your chip. It is almost impossible to catch by scanning. You essentially need to have a program that can detect all the security vulnarabilities. If you can create it, I am pretty sure Microsoft are willing to pay you millions for the technology. Even if your scanner can detect it, it is just a "bug" right? And anybody has bugs.