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High-K Dielectrics:
Extending Current Semiconductor
Manufacturing Techniques
by
Alexander Glavtchev
Introduction
 Since the 1960’s semiconductor industry has used polySilicon gate with a Silicon dioxide gate dielectric layer.
 Continuing miniaturization has thinned the SiO2 gate dielectric
layer to ~5 atomic layers, or less than 2nm in thickness.
 Electron tunneling is a major concern at this level as it
contributes to current leakage through the gate.
 Further scaling would increase the already-problematic gate
current leakage (IG) and lead to power loss, increased power
consumption, and generate excess heat.
 New materials with high dielectric constant, k, are needed to
avoid abrupt and costly technological and manufacturing
process changes, reduce leakage, and improve performance.
Introduction
Introduction
Physics
 Research by Intel has shown that a thicker high-k dielectric gate increases
overall capacitance while decreasing the leakage current by ~100X.
 Transistors are becoming horizontally smaller, therefore a vertically-thicker
dielectric gate is not a problem.
Why is Capacitance Important?
The drive current ID for a MOSFET can be approximated by the following
equation:
W is the transistor channel width
L is the transistor channel length
μ is carrier mobility (which can be treated as a constant in this approximation)
C is the capacitance density associated with the gate dielectric
VG is the applied gate voltage
VD is the applied drain voltage
VT is the threshold voltage
Why is Capacitance Important?
It can be seen that ID increases almost linearly with VD until a maximum ID is
reached when:
and
IDS is the saturated drain current, and it
results when VG ≥ VT and a carrier
channel has been established under
the gate.
Thus, it can be seen that decreasing
the channel length or increasing the
Capacitance will increase the drain
current IDS and establish a channel
beneath the gate (ON state).
Physics
The capacitance of the gate can be modeled as a parallelplate capacitor (ignoring quantum effects and depletion):
C=
ke0A
t
where A is the Area of the capacitor and t is the thickness.
Since the t is greater for the new dielectric gate material, it
requires an even larger dielectric constant k to increase the
overall capacitance – that’s where the new high-k dielectric
materials come into play. These materials are Hafniumbased and will have k > 3.9, the dielectric constant of SiO2.
Why high-k dielectric?
 Experimental data shows that a higher k has a greater effect on total
capacitance than a decreasing thickness.
 Furthermore, a thicker Hafnium-based dielectric gate with a metal gate
increases resistance and reduces the unwanted gate leakage current.
Why high-k dielectric?
 In addition, Intel has shown
that the gate delay between
ION/IOFF is less when using a
high-k/metal gate setup
instead of the traditional
poly-Silicon/SiO2 setup.
 This allows for faster signal
transmission and a better
ION/IOFF ratio, allowing for
more accurate reading of
the ones and zeros of
binary data.
Challenges and Issues
 The poly-Si gate is not electromagnetically
compatible with the new high-k dielectric.
 Phonon scattering and Fermi level pinning.
 New manufacturing methods for atomic level
deposition needed.
 Short-term solution - additional advances in
lithography and technology are needed to
improve performance in the long run.
Metal Gate vs. SiO2
• Phonon scattering is greater using a high-k/poly-Si dielectric than
the traditional SiO2/poly-Si.
• This leads to decreased channel mobility.
• Use of a metal gate can help increase the surface mobility.
Metal Gate vs. SiO2
The Future
 High-k dielectrics are vital for next-generation
low power-consumption, low leakage, high
performance logic devices.
 Non-silicon high-mobility materials may require
extensive use of high-k dielectrics (e.g. Ge, III-V
quantum wells, carbon nanotubes).
 Formation and compatibility of high-k dielectrics
better with non-silicon materials (non SiO2based).
The Future
 Use of non-silicon Ge
substrate with a highk and a metal gate
increases the total
capacitance. High-k
dielectrics are more
compatible with nonSi materials.
 Intel has shown that
formation of high-k
dielectric on Ge is of
greater quality than
forming SiO2 on Ge.
Sources




www.intel.com
www.wikipedia.com
www.computer.org
High-k gate dielectrics: Current status and materials properties
considerations. Journal of Applied Physics, Vol. 89, No. 10.