Ring oscillator design for mosfet device reliability investigations and its use for in-line monitoring

Abstract

A method of determining the effect of the degradation of MOSFET on the frequency of a Ring Oscillator (RO) consisting of an odd prime number of inverter stages, each of the inverters stages having an NMOS and a PMOS field-effect transistor is described. The method includes the steps of: a) selecting one inverter from the inverter stages of the RO, the selected inverter having testable nodes, the testable nodes being connected to inputs and outputs of the NMOS and a PMOS field-effect transistors (FET) forming the selected inverter; b) simultaneously stressing under a set of stress conditions 1) all of the NMOS FETs of each of the inverter stages, 2) all of the PMOS FETs, and 3) all of the NMOS FETs and PMOS FETs in the RO; c) measuring a shift in selected device parameters in the selected inverter; d) measuring a frequency degradation of the entire RO; and e) establishing a relationship between the shift in the device parameters and the frequency degradation and relating the relationship to a known degradation mechanism Furthermore, on-chip pass gates controlled by appropriate off-chip DC voltage signals, allow parallel DC stressing, as well as forcing an off-chip AC voltage waveform to a given MOSFET type device (either PMOSFET or NMOSFET) on every inverter stage of the RO. The RO circuit makes it possible to investigate the effect on the RO frequency degradation, caused by any DC MOSFET degradation mechanism as well as by any external AC voltage waveform known to be representative of a critical circuit operation. Thus, the dependence of the RO frequency on device degradation mechanisms activated during a critical circuit operation can be carefully investigated and quantified.