Using a new technique, researchers from imec,
TU Vienna, and Infineon have shown that both electron and hole capture and emission
times in SiO2 and HfO2 dielectrics are strongly thermally activated. The new
technique, Time Dependent Defect Spectroscopy (TDDS), can be used to study the
capture and emission times of single oxide defects over a very wide time range.
This knowledge will help to better understand the time variability of future
deeply scaled devices, as well as the operation of charge-storage memories.
Negative Bias Temperature Instability (NBTI) is a critical reliability issue
in modern CMOS technologies. Despite several decades of research, there is still
no consensus about the exact mechanisms involved in NBTI. In 2009, researchers
from imec and TU Vienna have argued that the phenomenon called NBTI relaxation
in pFET devices is another facet of the well-known low-frequency noise in these
devices. While the low-frequency noise corresponds to the channel/gate dielectrics
system being in the state of dynamic equilibrium, NBTI relaxation corresponds
to the perturbed system returning to this equilibrium. This finding could signify
a fundamental advance in the understanding of NBTI.
Continued research has now yielded new insights into the trapping phenomena
in gate oxides in deeply scaled FETs. In such devices, the relaxation is observed
to proceed in discrete voltage steps, with each step corresponding to the discharge
of a single oxide defect. Upon repeated perturbation, each defect shows up in
the relaxation trace with a characteristic fingerprint consisting of its discharge,
or emission time, and its voltage step. It is possible to construct a spectrum
mapping the properties of up to a dozen of individual defects. The capture time
of each defect is obtained by varying the perturbation time. Contrary to techniques
for the analysis of random telegraph noise (RTN), which only allow monitoring
the defect behavior in a rather narrow time window, the new TDDS technique can
be used to study the capture and emission times of the defects over an extremely
wide time range.
Using this new technique, researchers from imec, TU Vienna, and Infineon have
shown that both electron and hole capture and emission times in SiO2 and HfO2
dielectrics are strongly thermally activated. Such behavior is incompatible
with elastic tunneling, but is consistent with the non-radiative multiphonon
theory previously applied to the analysis of RTN. A quantitative model based
on this theory explains the bias as well as the temperature dependence of the
characteristic time constants. Apart from the reliability issues, the model
is also applicable to charge-storage memories.
The knowledge generated from the research is moreover directly useful for understanding
time variability of future deeply scaled devices. Imec researchers observe that
in 70 x 90nm2 pFETs, a single-carrier discharge event can cause a voltage shift
exceeding 30mV, which is the failure criterion typically used to determine the
device lifetime. The overall device-to-device threshold voltage shift distribution
after NBTI stress is argued to be a convolution of exponential voltage distributions
of uncorrelated individual charged defects Poisson-distributed in number. An
analytical description then allows, among other things, to calculate NBTI threshold
voltage shifts in an unlimited population of devices, a feat impossible through
device simulations. These findings, presented at the 2010 International Reliability
Physics Symposium, shed new light on our understanding of NBTI and can lead
to a more accurate prediction of the NBTI lifetime.