Novel Wafer-Scale Uniform Layer-by-Layer Etching Technology for Line Edge Roughness Reduction and

Surface Flattening of 3D Ge Channels

Y. Morita, T. Maeda, H. Ota, W. Mizubayashi, S. O’uchi, M. Masahara, T. Matsukawa, K. Endo

Nanoelectronics Research Institute (NeRI) AIST Japan

What's oxygen etching?

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Oxide Growth Si + O2 SiO2

Etching 2Si + O2 2SiO

What's oxygen etching?

3

Si

O O

Si

O O O

Si

O

O Si

or Low O2 Pressure�

O O O

Si Si SiO2�

High O2 Pressure�

Etching�

Island Formation�

J. R. Engstrom and T. Engel, Phys. Rev. B 41 (1990) 1038.�

What's oxygen etching?

4

Si�

J. J. Lander and J. Morrison, J. Appl. Phys. 33 (1962) 2089.�

What's oxygen etching?

5

Oxygen Etching of Si 2Si + O2 2SiO

Oxygen Etching of Ge 2Ge + O2 2GeO

Outline •  What's oxygen etching? •  Background – Why 3D Ge FET?

•  Objective •  Measurement of O2 Etching •  Electrical Characterization •  Discussion •  Summary

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Background

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Why 3D Ge FET?�

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S D

Buried oxide Si

G

3-Dimensional transistor (Fin or nanowire-FET)�

3D channel�

Thin Channel Off-Current Reduction�

3D Si FET to 3D Ge FET

Issues in 3D Channel�

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BOX

Ge

HM�

Resist / Hard Mask �RIE

BOX

PR�

BOX

Ge�

Damage Removal Flattening Slimming�

�  Plasma Damage Free �  Roughness Reduction �  Channel Slimming

Objective •  Can O2 etching be applicable for

nano-device fabrication?

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Measurement of O2 Etching

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Surface Morphology •  Ge(001) AFM 2 × 2 μm

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PO2 = 10−6 torr 640oC �

RMS 0.28 nm� RMS 0.12 nm�

O2 Etching �HF Treatment (Initial)�

Rough Step Edge Shape Etching of Step Edge

Etch Depth Measurement •  Mask Rebate Technique�

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Mask Patterning & Clean Surface Formation

O2 etching Mask light etch & AFM

Wet-cleaned Ge or Si surface SiO2 SiO2 SiO2 AFM

Ge or Si Ge or Si

Etch depth�

Ge or Si

Y. Morita, et al., Surface Science 604 (2010) 1432.�

Visualization of Etch Depth

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Etch depth� SiO2 mask�

Ge surface�

AFM after Mask Light Etch�Air View

0.5 × 0.5 μm�

Visualization of Etch Depth�

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AFM after Mask Light Etch�

Ge(001)

SiO2�Etch Depth�

AFM Error Image 5 × 5 μm�

Visualization of Etch Depth •  Surface Cross-Sectional Profile�

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0� 100� 200� 300�

Distance (nm)�

Depth

(nm)�

0�

1�

2�Etch depth�

Mask rebate length�

Ge(001)�

Etch Depth ~1 nm�

Summary of Etch Rate

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Ge�Si�

10−5�

10−6�

10−7�

10−5�

10−6�

PO2 (Torr)�

500�(°C)�640�720�800�840�900�950�

~1 nm/min�

Summary of Etch Rate •  Weak Temperature Dependence •  Etch Rate O2 Pressure

–  Supply of O2 Bottle-Neck Step

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Ge

O O

Ge

O O O

Ge

O

O Ge

or Low O2 Pressure�

Etching�

∞�

Comparison of Etch Rate Variation�

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Ge� Variation of Temp. ±5%� Ge�640oC ±5%�EA ~ 0.3 eV�

O2 etch�

HCl etch�Variation of Etch Rate (HCl)�

Variation of Etch Rate (O2)�

EA ~ 0 eV�

+12.2%�

-11.6%�

± ~ 0%�

O2 etch�HCl etch�

HCl Etch: Y. Bogumilowicz, et al., Semicond. Sci. Tech. 20 (2005) 127.

Uniform SOI Thinning�

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3.6 nm

SOI

BOX 500 nm

1000 nm

SOI

Buried oxide

Si sub.

Glue

•  Uniform Etching for Large Area

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O2 Etching of 3D Channel�

Nanowire-FET by O2 slimming�

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SOI and hard mask formation -- (a) EB lithography & RIE -- (b) Cleaning O2 Slimming -- (c) ALD HfO2 & poly-Si gate NiSi2 source/drain Dopant implantation & activation

BOX BOX BOX SOI SiNW

SiO2 Hard Mask HM HM

(a) (b) (c)

SiNW

SiNW direction: [110]

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(b)

3.9 nm BOX

SiNW

HfO2 HM

(a)

50 nm BOX

SOI

HfO2 HM

Without O2 Slimming O2 Slimming

50 nm-width SOI is etched from both sides.

PO2 = 1 × 10−5 torr 900oC 3.9 × 9.0 nm�

Nanowire-FET by O2 slimming�Channel Cross-Section�

LER Reduction

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w/o O2 etch O2 etch Ge O2 etch Si�

Ra (nm)� 3σ (nm)�2.26� 7.84 w/o O2 etch�1.1� 3.31�O2 etch Ge�

0.52 1.94�O2 etch Si�

I-V of O2-Slimmed Si NW FET

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S D

Gate

NiSi2 Source NiSi2

Drain

360 nm

Lc ~360 nm WNW ~8 nm

Discussion

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Roughness Reduction •  Higher Temperature Smaller Roughness�

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N2 Anneal�

O2 Etch�

RT�

w/o Anneal�Ge(001)�

Etch Depth ~1 nm�

Roughness Reduction •  O2 etch can selectively reduce roughness. –  AFM 2 × 2 μm�

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Ge(001) w/o anneal� PO2 = 10−6 torr, 500oC ~ 1 nm Etching �

RMS 0.14 nm

O2 Etching �

N2 Anneal�

PN2 = 0.1 torr, 500oC �

RMS 0.18 nm�RMS 0.28 nm�

Summary •  Novel O2 Etching Technology for 3D Ge

Channel –  Uniform Etch Rate for Large Size Wafer –  Atomically Flattened Surface –  No Plasma Damage –  Slimming and Smoothing for 3D Channels

•  Enhancement of Device Performance

•  Applicable for Future Channels –  Vertical Nanowire –  V-Groove etc..

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