High-k dielectric

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The term high-κ dielectric refers to a material with a high dielectric constant (κ) (relative to silicon dioxide) used in semiconductor manufacturing processes which replaces the silicon dioxide gate dielectric. The implementation of high-κ gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components, colloquially referred to as extending Moore's Law.

1 Need for high-k materials
- 1.1 First principles
- 1.2 Gate capacitance impact on drive current
2 Materials and considerations
3 Use in industry
4 References

[edit] Need for high-k materials

Silicon dioxide has been used as a gate oxide material for decades. As transistors have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance and thereby drive current and device performance. As the thickness scales below 2 nm, leakage currents due to tunneling increase drastically, leading to unwieldy power consumption and reduced device reliability. Replacing the silicon dioxide gate dielectric with a high-κ material allows increased gate capacitance without the concommitant leakage effects.

[edit] First principles

The gate oxide in a MOSFET can be modeled as a parallel plate capacitor. Ignoring quantum mechanical and depletion effects from the Si substrate and gate, the capacitance C of this parallel plate capacitor is given by

$C=\frac{\kappa\varepsilon_{0}A}{t}$

Conventional silicon dioxide gate dielectric structure compared to a potential high-κ dielectric structure

Cross-section of an NMOS showing the gate oxide dielectric

Where

A is the capacitor area
$κ$ is the relative dielectric constant of the material (3.9 for silicon dioxide)
$ε 0$ is the permittivity of free space
t is the thickness of the capacitor oxide insulator

Since leakage limitation constrain further reduction of t, an alternative method to increase gate capacitance is alter $κ$ by replacing silicon dioxide with a high- $κ$ material. In such a scenario, a thicker gate layer might be used which can reduce the leakage current flowing through the structure as well as improving the gate dielectric reliability.

[edit] Gate capacitance impact on drive current

The drive current $I D$ for a MOSFET can be written (using the gradual channel approximation) as

$I_D = \frac{W}{L} \mu C_{inv}(V_{G}-V_{T}-\frac{V_{D}}{2})V_D$

Where

W is the width of the transistor channel
L is the channel length
$μ$ is the channel carrier mobility (assumed constant here)
$C i n v$ is the capacitance density associated with the gate dielectric when the underlying channel is in the inverted state
$V G$ is the voltage applied to the transistor gate
$V D$ is the voltage applied to the transistor drain
$V T$ is the threshold voltage

It can be seen that in this approximation the drain current is proportional to the average charge across the channel with a potential $\frac{V_{D}}{2}$ and the average electric field $\frac{V_{D}}{L}$ along the channel direction. Initially, $I D$ increases linearly with $V D$ and then eventually saturates to a maximum when $V D, s a t = V G - V T$ to yield

$I_{D,Sat} = \frac{W}{L} \mu C_{inv}\frac{(V_{G}-V_{T})^2}{2}$

The term $(V G - V T)$ is limited in range due to reliability and room temperature operation constraints, since too large a $V G$ would create an undesirable, high electric field across the oxide. Furthermore, $V T$ cannot easily be reduced below about 200 mV, because kT is approximately 25 mV at room temperature. Typical specification temperatures < 100 °C could therefore cause statistical fluctuations in thermal energy, which would adversely affect the desired the $V T$ value. Thus, even in this simplified approximation, a reduction in the channel length or an increase in the gate dielectric capacitance will result in an increased $I D, s a t$ .

[edit] Materials and considerations

Replacing the silicon dioxide gate dielectric with another material adds complexity to the manufacturing process. Silicon dioxide can be formed by oxidizing the underlying silicon, ensuring a uniform, conformal oxide and high interface quality. As a consequence, development efforts have focused on finding a material with a requisitely high dielectric constant that can be easily integrated into a manufacturing process. Other key considerations include band alignment to silicon (which may alter leakage current), film morphology, thermal stability, maintenance of a high mobility of charge carriers in the channel and minimization of electrical defects in the film/interface. Materials which have received considerable attention are hafnium and zirconium silicates and oxides, typically deposited using atomic layer deposition.

It is expected defect states in the high-k dielectric can influence its electrical properties. It is difficult to measure defect states in an ultrathin high-k dielectric film. However, Lau has pioneered on the application of zero-bias thermally stimulated current or zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to detect defect states in ultrathin high-k dielectric films. (Please see reference.) Other methods like "inelastic electron tunneling spectroscopy" (IETS) have also been reported.

[edit] Use in industry

The industry has employed oxynitride gate dielectrics since the 1990s, wherein a conventionally formed silicon oxide dielectric is infused with a small amount of nitrogen. The nitride content subtly raises the dielectric constant and is thought to offer other advantages, such as resistance against dopant diffusion through the gate dielectric.

In early 2007, Intel announced the deployment of hafnium-based high-κ dielectrics in conjunction with a metallic gate for components built on 45 nanometer technologies, expected to ship in 2007.[1] At the same time, IBM announced plans to transition to high-κ materials, also hafnium-based, for some products in 2008. While not identified, it is most likely the dielectrics used by these companies are some form of HfSiON. HfO2 and HfSiO are susceptible to crystallization during dopant activation annealing. NEC Electronics has also announced the use of a HfSiON dielectric in their 55 nm UltimateLowPower technology. [2] The 2006 ITRS roadmap predicts the implementation of high-κ materials to be commonplace in the industry by 2010.

[edit] References

Review article by Wilk et al in the Journal of Applied Physics
Houssa, M. (Ed.) (2003) High-κ Dielectrics Institute of Physics ISBN 0-7503-0906-7 [3]
Huff, H.R., Gilmer, D.C. (Ed.) (2005) High Dielectric Constant Materials : VLSI MOSFET applications Springer ISBN 3-540-21081-4
Demkov, A.A, Navrotsky, A., (Ed.) (2005) Materials Fundamentals of Gate Dielectrics Springer ISBN 1-4020-3077-0
"High dielectric constant gate oxides for metal oxide Si transistors" Robertson, J. (Rep. Prog. Phys. 69 327-396 2006) Institute Physics Publishing [4]
Media coverage of Intel/IBM announcements [5] [6]

W.S. Lau, L. Zhong, A. Lee, C.H. See, T. Han, N.P. Sandler and T.C. Chong, "Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy", Appl. Phys. Lett., vol. 71, no. 4, pp. 500-502, 28 July 1997.
W.S. Lau, K.F. Wong, T. Han and N.P. Sandler, "Application of zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to ultrathin high-dielectric-constant insulator film characterization", Appl. Phys. Lett., vol. 88, article number 172906, 2006.

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