High k Gate Dielectrics for Transistors

High k gate dielectrics are required for the sub-65 nm MOS structure because the conventional SiO₂ film is too thin (e.g. 2 nm) to minimize the tunneling current and the out diffusion of boron from the gate. A thick layer can be used with the high k material to lower the parasitic capacitance. Many issues have to be solved before it is acceptable for the products. In general, there are three types of high k dielectrics:

1. those with 4 < k < 10 such as SiN_x;

2. those with 10 < k < 100 such as Ta₂O₅, Al₂O₃, ZrO₂, and HfO₂; and

3. those with 100 < k such as PZT.

The type 2 dielectric film has been routinely used in transistors, such as TFTs. A thick layer is used to prevent the top-to-bottom metal shortage, which is a killing factor for the yield. The high k dielectric material is usually used in combination with a high quality dielectric interface layer to lower the interface density of states. For example, in the a-Si:H TFT case, the N-rich SiN_x is exclusively used as the interface layer. Examples on TFTs with the dual gate dielectric structure (e.g. SiN_x/TaO_x and two different types of SiN_x/SiN_x) are shown as follows.

We are researching on new type of high k dielectric materials using elements throughout the whole Periodical Table. The goals are to find new high k thin films that have enhanced dielectric constants and low leakage currents. We are also working on high-k based new devices, such as the nonvolatile memories where nanocrystals are embedded in the dielectric layer. .

Nanocrystals Embedded High-k for Nonvolatile Memories

- nc-ITO Embedded Zr-doped HfO₂

A. Birge, C.-H. Lin, and Y. Kuo, “Memory Functions of Nanocrystalline Indium Tin Oxide

Embedded Zirconium-Doped Hafnium Oxide MOS Capacitors,” JES 154(10) H887 (2007)

- nc-ZnO Embedded Zr-doped HfO₂

J. Lu, C.-H. Lin, and Y. Kuo, “Nonvolatile Memories Based on Nanocrystalline Zinc Oxide Embedded Zirconium-doped

Hafnium Oxide Thin Films,” ECST 11(4), 509-518 (2007).

- nc-Si Embedded Zr-doped HfO₂

J. Lu, Y. Kuo, J. Yan, and C.-H. Lin, “Nanocrystalline Silicon Embedded Zirconium-Doped Hafnium Oxide High-k Memory Device,” JJAP 45(34) L901 (2006).

- nc-RuO_x Embedded Zr-doped HfO₂ and RuZrHfO

C.-H. Lin, Y. Kuo, and J. Lu, “Influence of Ru Dopant on the Dielectric Properties of Zr-doped HfO2 High-k Thin Film,” ECST 6(1) 121 (2007).

- nc-RuO_x Embedded Zr-doped HfO₂ Reliability

TiN / ZrHfO_x/RuO_x/ZrHfO_x/p-Si

R. Wan, C.-H. Lin, Y. Kuo, and Way Kuo, “Charge Trapping of Ultra-thin ZrHfO_x/RuO_x/ZrHfO_x High-k Stacks,”
IIRW 2007.

Doping of TaO_x and HfO₂ films

- Sub 1 nm EOT Zr-doped HfO₂

J. Yan, Y. Kuo, and J. Lu, “Zirconium-Doped Hafnium Oxide High-k Dielectrics with Subnanometer Equivalent Oxide Thickness by Reactive Sputtering,” ESL 10(7), H199 (2007).

- Effects on bulk material properties

- Suppression of crystal formation

JES Letters (Accepted to appear in 2004)

XRD Patterns of TaOx (a) undoped, (b) Al-doped, (c) Si-doped, (d) Ti-doped films, 700°C, 10 Minutes

- Effects on interface layers

- Interface formation problems and improvement

Lu, Kuo, Tewg, and Schueler, Vacuum, 539-547, 2004.

(Courtesy of B. Foran of SEMATECH)



J. Lu, Y. Kuo, and J.-Y. Tewg, JES 153(5) G410 (2006).

- Interface modification by inserted 5Å TaN_x

Tewg, Kuo, Lu, and Schueler, “Influence of a 5 Å Tantalum Nitride Interface Layer on Dielectric Properties of Zirconium-Doped Tantalum Oxide High-k Films,”

JES 2005.

Kuo, Lu, and Tewg, JJAP 42 (3), Pt. 2, No. 7A, 2003

ESCA of Ta chemical states of the deposited TaNx interface layer after 700ºC 10 min O2 annealing.

Kuo, Lu and Tewg, JJAP 42, L769, 2003

- Effects on electric properties

- Increase of k value

Kuo, Lu and Tewg, JJAP 42, L769, 2003.

Measured k as a Function of the Dopant Zr Co-Sputtering Power

AVS ICMI Meeting, 02/13/02.

k vs. Film Thickness

700°C 10 min annealed

Y. Kuo. SEMATECH, Stacked Gate Dielectric Meeting Procs. Austin, TX. 05/22/01

Y. Kuo, J. Y. Tewg, and J. Lee. AVS 3^rd Natl. Microelectronics and Interfaces, 133-135, 2002.

Y. Kuo, J. Y. Tewg J. Donnelly, and J. Lee. Proc. 1^st Natl. Conf. Semi-conductor Tech., ECS. Vol. 2001-17, 324-327, 2001.

- Decrease of leakage current

Kuo, Lu and Tewg, JJAP 42, L769, 2003.

Leakage vs. Dopant Target Sputtering Power

600°C 60 min annealed

AVS ICMI Meeting, 02/13/02.

J-E curves of the Zr-doped HfO_x films, including pure HfO_x.

J. Yan, Y. Kuo, S. Chatterjee, and J. Lu, “Electrical Characterization of Doped Hafnium Oxide High-k Films,” 2005 ECS Abstract.

W. Luo, et al., IEEE IRW 2.1.1, 2003

- Enhance breakdown strength


	J. Lu, Y. Kuo, and J.-Y. Tewg, JES 153(5) G410 (2006).

- Effects on charge trapping V_FB, and ∆V_FB

J. Lu, J. Yan, S. Chatterjee, H. C. Kim, and Y. Kuo, “Doping of Hafnium Oxide High-K Film with Zirconium,” 2005 AVS ICMI.

Tewg, Kuo, Lu, and Schueler, JJAP 151 (3), F59-F67, 2004

- Gate electrode effects

Tewg, Kuo, and Lu, “Zirconium-Doped Tantalum Oxide Gate Dielectric Films Integrated with Molybdenum, Molybdenum Nitride, and Tungsten Nitride Gate Electrodes,” JECS 2005.

Gate Electrode Material and Annealing Effects on ALD HfO₂

S. Chatterjee, Y. Kuo, J. Lu, Microelectron. Eng. (2007).

- Reliability issues

- SILC

Applied voltage generates traps in the oxide, which act as coulombic scattering centers and as pathways for increased local leakage currents.

Figure 2: (a) Diagram illustrating log(1-[Q_o(t)/Q_o(0)]) as a function t for different CVS.

(b)The time dependence of the current density increase observed during constant voltage stressing @ -2.5 V of 800^oC PMA MoN/5nmHfO₂/Si MOS.

Chatterjee, Kuo, etc., “Effects of Post Metallization Annealing on the Electrical Reliability of Ultra-thin HfO₂ Films with MoN and WN Gate Electrodes,” 2005 IRPS.

- Time dependent current leakage

Figure 3: Breakdown phenomena of WN/HfO₂ (5 nm)/Si MOS structures during CVS –2.5 V.

Chatterjee, Kuo, etc., “Effects of Post Metallization Annealing on the Electrical Reliability of Ultra-thin HfO₂ Films with MoN and WN Gate Electrodes,” 2005 IRPS

- Hysteresis disappear due to breakdown