Oxidation and diffusion
Dangling bonds at the Si/SiO2 interface that were terminated with hydrogen by a forming gas anneal can turn back into interface states as a transistor is used over a long time. Which pair correctly gives the main cause and a countermeasure that has been studied and is used in some processes?
- AHydrogen in the oxide gradually oxidizes the silicon; raise the anneal temperature to thicken the oxide
- BEnergy from hot carriers and the like breaks Si-H bonds; anneal with deuterium instead of hydrogen
- CInterface states increase naturally at low temperature; always keep the chip at a high temperature during use
Show answer and explanation
Answer: B. Energy from hot carriers and the like breaks Si-H bonds; anneal with deuterium instead of hydrogen
In a forming gas anneal, hydrogen bonds to silicon dangling bonds (Si-H) and reduces interface states. However, when high-energy carriers accelerated in the channel (hot carriers) strike the interface, or when voltage is applied continuously at high temperature (such as NBTI in pMOS), Si-H bonds break, interface states reappear, and the threshold voltage drifts. Si-D bonds terminated with deuterium, an isotope heavier than hydrogen, are known to be harder to break with hot carriers, so annealing in a deuterium atmosphere has been studied and is used in some manufacturing processes.
- A. Hydrogen does not drive oxidation of the interface, and a thicker oxide does not solve the breaking of Si-H bonds.
- B. Correct. Si-D bonds are known to be harder to break than Si-H bonds.
- C. Interface states do not increase when the temperature is lowered; high temperature actually accelerates degradation.