1. A multi-bit-per-cell three-dimensional read-only memory (3D-OTP MB ), comprising: a semiconductor substrate including transistors thereon; a plurality of OTP cells stacked above said semiconductor substrate, each of said OTP cells comprising an antifuse layer, where said antifuse layer is irreversibly switched from a high-resistance state to a low-resistance state during programming; a plurality of contact vias coupling said OTP cells to said semiconductor substrate; wherein said OTP cells have more than two states, the OTP cell in different states being programmed by different programming currents.