High k dielectric

A

Term Paper

on

Achievements & Challenges of

MOSFET’s with High-K Gate

Dielectrics

Course instructor : Dr.Dipanjan Basu

Presented by :Anubhav Srivastava

Bandarupalli Jayadeepthi

Noor Mohamed EV

Why this is required…….? Scaling of transistor to drive Moore’s Law

SiO₂ is running out of atoms for further scaling

but still scaling continues.Thickness of SiO₂ layer required in 45nm technology is

about 1.2nm (4 atomic layers deep!!)

Quantum Mechanical phenomenon of electron

tunneling results in Gate Leakage Current….!

Gate Leakage Current

Quantum mechanical tunneling

Tunneling current increases exponentially

with decrease in oxide thickness

Choice of High-K oxide

High-K oxide should satisfy the following properties:

1. High Dielectric constant and Barrier Height

2. Thermodynamic stability

3. Interface Quality

Volume expansion

caused by cubic to

tetragonal to monoclinic

transformation induces

large stress in ZrO2

The Challenges for High-K

Dielectric Development

High-K and Poly-Si Incompatibility due

to Fermi Level PinningDefect formation at the polySi and

high-K interface is most likely the

cause of the Fermi level pinning in the

upper part of the band gap which

causes high threshold voltages in

MOSFET (M=Zr or Hf)

Results in:

1. High threshold voltage

2. Low drive current

So the need to replace poly-Si

gate by a suitable metal

Mobility Degradation

Coulombic scattering :

Dominant at low field

Caused due to high interface trapped charge

Higher trap density near conduction band

More severe for nMOSFET

Surface phonon scattering

The Metal Gate Solution

Use of Metal Gates

As a conductor metal can pack in hundred of times more

electrons than poly-Si

Metal gate electrodes (Co,Ni,Mo,W) are able to decrease

scattering and reduce the mobility degradation problem

Types of Metal Gates

Requires metal gate electrodes with “CORRECT” work functions on High-K for both nMOS and pMOS transistors for high performance.

Metal Gate/High-K Transistor

When SiO2 is replaced with High-K material it was found

that poly-Si and High –K material were not compatible.

So poly-Si is being replaced by a metal to make it

compatible with High-k material.

Mobility Improvement by using TiN

High-K/Metal-gate reduces leakage

Yeah…Nobody knows for sure……….!!!

Intel achieved 20 percent improvement in transistor

switching speed by using metal Gate /high-K transistor

with HfO2 as dielectric.

Intel 45nm Transistor – performance compared to 65nm

2x improvement in transistor density

30% reduction in switching power

20% improvement in switching speed

10x reduction in gate oxide leakage power

Reference

Achievements and Challenges for the Electrical Performance Of MOSFET’s

with High-k Gate Dielectrics by G. Groeseneken , L. Pantisano and M. Heyns

0-7803-8454-7/04/$20.00 2004 IEEE.