Phase Switching Circuit to Prevent Duty Cycle Distortion Caused by the Burn-In Process
Publication Date: 2010-Oct-26
The IP.com Prior Art Database
Disclosed is a phase switching circuit to prevent duty cycle distortion caused by the burn-in process.
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During burn-in of semiconductors, the pFet devices held in the ON state will suffer performance degradation due to negative bias temperature instability (
effects, but pFets in the OFF state will be unaffected. In the case of a delay path containing several series inverters, the input of which is held in steady state or driven with low duty cycle, every 2nd inverter in this chain will suffer pFet degradation during burn-in. The result is a signal path that has impaired performance for only one input transition, resulting in duty cycle distortion at path output. In extreme cases, the duty cycle distortion could become so bad that the path would not function properly.
The prior art [*] only relates to storage devices; in particular, storage devices that hold a certain state for extended periods of time and suffer from NBTI. This patent covers logic paths, particularly those paths with many stages. An example path is a clock distribution, which has many stages that all suffer from NBTI shift that is additive and which, therefore, leads to much larger overall impact.
The core idea of the invention is to divide the pFet "on-time" equally between the even and odd stages of these long inverter chains during burn-in. So instead of having the even devices or odd devices on for the entire burn-in process, the even devices and odd devices would each be on for half of burn-in. This would uniformly degrade all inverter stages, eliminating the duty cycle distortion across the network. This could be done to any such paths that have many stages of series inverters that would be susceptible to the duty cycle distortion problem. A specific example would be a clock distribution path.
This invention makes it possible to selectively switch the phase seen at the input of each inverter stage of a long chain. One possible implementation is shown in Figure 1. A mux is used to swap in or out an inverter stage at beginning of the chain. Another mux is used to do the opposite at the far end of the chain. The se...