There are three key blocks in RFID passive tag design:
1. The rectifier that powers the circuit. It is the process that mainly defines its performance;
2. The antenna and the matching issue with the integrated circuit;
3. The backscattering modulator that is responsible for the communication tag-to-reader. Here, the impedance that you choose to modulate have a high impact.
A good starting point is the book from Finkenzeller " RFID Handbook".