Reducing the Impact of Local Load Variation on the DUT in a Process Detector Using a Supply Controlled Ring Oscillator

Abstract

The extraction of threshold voltage ( $V_{TH}$ ) of a transistor using process detector is important for device and process characterization. This paper proposes a fully digital on-chip $V_{TH}$ detector using a supply controlled ring oscillator (RO) to extract local random variation of $V_{TH}$ . As a single RO is employed to extract $V_{TH}$ variation from an array of independently selectable device under tests (DUTs), the local load variation observed by each DUT is almost constant and makes the proposed detector insensitive to local load variation. The supply noise due to the oscillation of RO can be suppressed using a decoupling capacitor. The measurement results of 300 DUTs in a test chip, occupying a silicon area of $44 ~\mu \mathrm {m} \times 126 ~\mu \text{m}$ in 130 nm technology node demonstrates the efficacy of the proposed detector.