On-Chip Threshold Voltage Variability Detector Targeting Supply of Ring Oscillator for Characterizing Local Device Mismatch

In this work, an all-digital on-chip threshold voltage variability detector is proposed to detect the local random threshold voltage variation from an array of device under test (DUTs). The use of single ring oscillator (RO) results in the compact structure of the proposed design. Any mismatch in the DUT is reflected as the change in the voltage of the sense node which is connected to the supply voltage of the RO. The difference of RO periods due to change in voltage level from $V_{DD}$ to $(V_{DD}-V_{TH})$ at the sense node actually represents the impact of threshold voltage of a DUT. The mismatch in threshold voltage of the DUTs can be modeled exponentially from the RO period difference. The difference nature of estimation technique enables to mitigate the impact of local supply voltage variation and error due to systematic variations in the path of the RO. The potency of the proposed technique is demonstrated using measurement results from a fabricated test chip designed in a $\pmb{0.13}\mu m$ process technology node. Measurement results from a test chip indicate that the test structure can estimate local random threshold voltage variations and is also useful in improving the yield as well as in process optimization.