On Design of Temperature Invariant Physically Unclonable Functions Based on Ring Oscillators

CMOS implementation of Physically Unclonable Functions (PUFs) facilitates a number of applications ranging from digital rights management, device authentication, counterfeit detection/prevention and cryptographic key generation. Key expectations from such PUF circuits are high (i) uniqueness and (ii) reliability. Uniqueness refers to differentiated responses over challenge-response pairs. Reliability demands unvarying responses under varying environmental conditions such as temperature, supply voltage and noise. This paper describes two methods for achieving the above goals in a ring oscillator based PUF. The first method exploits the negative temperature resistance property of n+ and p+ polysilicon placed as source feedback resistors to de-sensitize ring oscillators to temperature variations. The second method uses an optimized supply voltage (V´DD) to reduce the temperature sensitivity of delay based ring oscillator PUFs. We report an improvement in reliability of 16% by combining these methods. Further, we propose a temperature-invariant ring oscillator PUF architecture based on Serial-Input Serial-Output (SISO) topology. In the proposed design, the relative phase difference between two ring oscillators is translated to a digital response bit. We show that this phase difference based response generation is superior to frequency based response generation in terms of area and power.