A DRAM based Physical Unclonable Function (PUF) utilizing the location of weak retention cells is demonstrated in 65nm CMOS. A new authentication scheme is proposed for the DRAM PUF where a random pattern is written to a small section of the DRAM and then retention failures are induced. To further increase the number of Challenge Response Pairs (CPRs), the data pattern including retention failures is transferred to a different memory location where additional retention failures are induced. This scheme enables more than 1032 unique CRPs from a 1Kbit array. To improve the stability of the PUF response, a zero-overhead repetitive write-back technique along with bit-masking was utilized. Voltage and temperature induced instabilities were mitigated by adjusting the read reference voltage and refresh time before each authentication operation. The proposed DRAM PUF has a bit cell area of 0.68μm2.