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High-K Dielectrics: Extending Current Semiconductor Manufacturing Techniques. by Alexander Glavtchev. Introduction. Since the 1960’s semiconductor industry has used poly-Silicon gate with a Silicon dioxide gate dielectric layer.

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High-K Dielectrics:Extending Current Semiconductor Manufacturing Techniques

by

Alexander Glavtchev


Introduction l.jpg
Introduction

  • Since the 1960’s semiconductor industry has used poly-Silicon gate with a Silicon dioxide gate dielectric layer.

  • Continuing miniaturization has thinned the SiO2 gate dielectric layerto ~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.


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Introduction

Introduction


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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.


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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


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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).


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Physics

The capacitance of the gate can be modeled as a parallel-plate capacitor (ignoring quantum effects and depletion):

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 Hafnium-based and will have k > 3.9, the dielectric constant of SiO2.


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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.


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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.


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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.


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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.



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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 SiO2-based).


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The Future

  • Use of non-silicon Ge substrate with a high-k and a metal gate increases the total capacitance. High-k dielectrics are more compatible with non-Si materials.

  • Intel has shown that formation of high-k dielectric on Ge is of greater quality than forming SiO2 on Ge.


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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.


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