DFT study on TiN film deposition on aluminum oxide for 3D VNAND memory device

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DFT study on TiN film deposition on aluminum oxide for 3D VNAND memory device

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Woo Jin Choi

Nano Flexible Device Materials Lab,

Department of Materials Science and Engineering,

Seoul National University

Introduction

 Issues in 3D V-NAND

 TiN ALD process

Calculation models

Results

 1^st half cycle – TiCl₄ adsorption

 2^nd half cycle – NH₃ adsorption

Contents

Introduction

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3D V-NAND

• NAND : Non-volatile flash memory

• 2D NAND : evaluating level of integration by reducing cell size - limitation of reducing cell size(max 128G)

• 3D V-NAND : vertical integration (256G ~)

https://news.skhynix.co.kr/1257 https://www.westerndigital.com/

Jang. J. et al., Dig.Symp. VLSI Technol. 2009, 192-193

Technical issues of word-line W

• Formation of W voids due to very deep trench in 128-layer VNAND

• Surface termination of Al₂O₃ surface helps W deposition

• Very thin TiN layer (a few Å), island growth of TiN

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Haodong Zhang. Et al., 2D Mater. 2018, 5 035006

W

TiN

Al

O

Technical issues of word-line W in V-NAND

• TiN film deposition – ALD(atomic layer deposition)

• Self-limiting process

• Good step coverage & thickness control

TiCl₄ – 1^st precursor NH₃ – 2^nd precursor

H.S. Hong, Formation of Al2O3 and TiN thin films by atomic layer deposition, M. S. Thesis, Inha University, South Korea, 2004

TiN ALD(atomic layer deposition)

Calculation models

• Hexagonal structure

• 2 layers of Al /1 layer of O

• (0001) – most stable surface

• Al termination is more stable than O termination.

• Surface reconstruction occurs

– Top Al atoms become coplanar with O atoms.

• Two surfaces : bare / hydroxyl-terminated

: O : Al : Al on top

Kenneth C. Hass et al., Science 282, 265-268 (1998)

α-Al ₂ O ₃ structure

: O : Al

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Bare α-Al ₂ O ₃ structure

Kenneth C. Hass. et al., Science 282, 265-268 (1998)

: O : Al : Al on top

Z Łodziana. et al., J. Chem. Phys. 118, 11179(2003)

: O : Al : OH

Hydroxyl-terminated α-Al ₂ O ₃ structure

: O : Al : H

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Hydroxyl-terminated α-Al ₂ O ₃ structure

Results

1

half cycle - TiCl

adsorption

①

② ③

④

: O : Al : H : Ti : Cl

ⓢ ⓢ

ⓢ ⓢ

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Bare α-Al ₂ O ₃ structure

: O : Al : H : Ti : Cl

TiCl ₄ adsorption-bare α-Al ₂ O ₃

: O : Al : H : Ti : Cl 15

TiCl ₄ adsorption-bare α-Al ₂ O ₃

: O : Al : H : Ti : Cl

TiCl ₄ adsorption-hydroxyl terminated α-Al ₂ O ₃

: O : Al : H : Ti : Cl 17

E

of 0.76 eV to 1.57 eV

TiCl ₄ adsorption-hydroxyl terminated α-Al2O3

TiCl ₄ adsorption-hydroxyl terminated α-Al ₂ O ₃

19 : O : Al : H : Ti : Cl

Yellow : charge accumulation Blue : charge depletion Isosurface value : 1.5×10

e Bohr

TiCl ₄ adsorption-hydroxyl terminated α-Al ₂ O ₃

Results

2. 2

half cycle – NH

adsorption

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E

of 1.52 – 2.05 eV

: O : Al : H : Ti : Cl : N

NH ₃ adsorption-bare α-Al ₂ O ₃

NH ₃ adsorption-bare α-Al ₂ O ₃

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E

of 0.51 eV to 1.32 eV

: O : Al : H : Ti : Cl : N

NH ₃ adsorption-hydroxyl terminated α-Al ₂ O ₃

NH ₃ adsorption-hydroxyl terminated α-Al ₂ O ₃

Conclusion

 Both surfaces are energetically favorable for TiCl

adsorption (1

half cycle).

 Bare surface – Cl atoms are bonded with surface Al atom – hard to be removed, lack of adsorption site.

 Hydroxyl terminated surface – hydroxyl group work as adsorption site of TiCl

&helps removing Cl atom from surface.

 For NH

adsorption (2

half cycle), hydroxyl terminated alpha Al

O

surface reduces energy barrier.

- Bare surface shows higher activation energy due to strong bond of Al atom and Cl atom.

 Due to Al-Cl bonding, Bare surface shows higher activation energy during TiN ALD process.

Summary

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