6.1 First Report of NBTI

The effect of negative bias temperature instability has first been published by Miura and Matukura in 1966 [83]. The authors investigated a 300nm , thermally grown in a dry oxygen atmosphere, on an n-type silicon substrate. The metal contact was formed of aluminum. This MOS structure was stressed at a temperature of 300^C at different gate voltages. The stress at each voltage was retained until the degradation saturated, which was then measured.

Figure 6.1: First report of the NBTI effect by Miura and Matukura in 1966 [83]. The graph shows the saturated value of the accumulated electron density at flat-band conditions for different stress voltages. The stress temperature was 300 $^\circ$C.

Figure 6.1 shows the measured figure of merit, . It represents the accumulated electron density in the silicon bulk close to the interface at flat-band conditions. is extracted from the flat-band voltage which is obtained by capacitance-voltage measurements (Section 4.3).

At that time the effect was remarkable, as a major concern was the migration of mobile ions due to the electric field. For negative gate bias stress this would suggest positive ions from the oxide to be attracted by the negative gate electrode leading to a ``more negative'' dielectric and thus to a decrease of for p-channel MOSFETs. This effect can be seen in Figure 6.1 for gate voltages down to $-4$V. At more negative stress voltages another degradation mechanism prevails, leading to positive charging of the dielectric and/or the interface and therefore to an increase of .

Miura and Matukura proposed an electrochemical reaction under the influence of the strong electric field at the interface. This reaction leads to positively charged oxygen vacancies in the film. As this mechanism proceeds at higher electric fields it dominates the ion migration process which saturates at large biases.