Characterization of Interfacial Oxide Layers in Heterostructures of Hafnium Oxides Formed on NH3-nitrided Si(100)

Hiroshi Nakagawa, Akio Ohta, Fumito Takeno, Satoru Nagamachi, Hideki Murakami Seiichiro Higashi and Seiichi Miyazaki

Department of Electrical Engineering, Graduate School of Advanced Sciences of Matter, Hiroshima University1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan

E-mail: semicon@hiroshima-u.ac.jp

1. Introduction Control of interfacial oxidation is one of key

issues to implement high-k gate dielectrics in the sub-100nm CMOS generations where the SiO2 equivalent thickness of the high-k dielectrics stack structures below 1.2nm is required imperatively [1]. From the requirement for the gate dielectric application such as high dielectric constant (~25) and favorable band offset energy(>1.2eV), HfO2 and ZrO2 are promising candidates for SiO2 substitution [2]. However, since such transition oxides are good conductors for oxygen ions, the oxide deposition on Si(100) and post-deposition anneal are always accompanied with the interfacial oxidation [3]. In this work, the interfacial oxidation in the heterostructures ultrathin HfO2 formed on thermally-nitrided Si(100) has been studied by x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared attenuated total reflection (FT-IR ATR) measurements. 2. Experiment 2.5nm-thick HfO2 layers are prepared on thermally grown SiNx(x=~1.3, ~1.0nm in thickness) /Si(100) at room temperature by HfO2 evaporation in ambient O2 at ~1x10-4Pa. The thermal nitridation of Si(100) was carried out at 700ºC in ambient NH3 at 54Pa. After the HfO2 evaporation, the samples were annealed at 300~500ºC in ambient O2 at 34Pa for 5min. 3. Results and Discussion

The stability of ultrathin silicon nitride so prepared against oxidation was first examined. No changes in Si2p and N1s spectra between the samples before and after O2 anneal at 500ºC were observed. The result indicates that the nitrided surface is stable enough against the O2 anneal. In contrast to this, when the stack structure of HfO2 on nitrided Si(100) was annealed in the same condition, the interfacial oxidation proceeds markedly. As shown in Fig. 1, an increase in the chemically-shifted Si2p signals by the O2 anneal was observed, indicating the growth of interfacial oxide. From the intensity ratio of the chemically-shifted Si2p signals to the signals from the Si substrate, it is found that the interfacial layer is grown up to 2.2nm in thickness from 1.0nm. By the complete removal of the top HfO2 layer from the O2 anneal sample with a dilute HF etching, the signals in the lower binding energy side of the

Si4+ peak are reduced by amount of two monolayers at most. Considering the 2nd nearest neighbor effect on the Si2p chemical shift, the reduced signals can be attributed to Si4+ states at the interface between the top HfO2 layer and the newly grown interfacial oxide because there exist less-electronegative Hf atoms as the 2nd nearest neighbors of Si at the interface. The observed change in the N1s spectrum by the HfO2 deposition on nitrided Si(100) is interpreted in terms that the N-H bonds on the nitride surface are changed partly into Hf-N bonds during the HfO2 deposition. The O2 anneal of HfO2/nitided Si(100) causes a new component in the N1s spectrum at the higher binding energy side, which is attributable to the oxidation of the nitride surface. The partially oxidized component in the N1s spectra was examined by subtracting the reference N1s spectrum of nitrided Si(100) from the N1s spectra measured at each thinning step in a dilute HF solution as show in Fig. 2. Taking into account the fact that such a wet etching introduces a oxidized component on the surface with a monolayer level, the oxidized component detected in the case of 0.3nm in remaining layer thickness is thought to be caused by the wet etching. The result of Fig. 2 indicates that the silicon oxide layer is formed on the pre-grown nitrided layer and interestingly the nitride layer thickness almost remains unchanged. Namely, we suggest that the oxidation of SiNx is accompanied with the movement of N atoms towards the substrate side resulting in the nitridation of the Si surface. This was also confirmed by p-polarized FT-IR-ATR measurements using a Ge prism as shown in Fig. 3. In the case that HfO2 was directly formed on HF-last Si(100), an absorption band peaked around 1230cm-1 due to Si-O-Si LO phonons is remarkably increased by the O2 anneal. In fact, the change in the spectrum between the samples before and after the O2 anneal is almost identical to the ATR spectrum of ultrathin SiO2, where the interfacial silicon oxide layer is grown up to 2.2nm form ~0.6nm (in the as-evaporated state) as obtained from the XPS analysis. In contrast, for the stack structure of HfO2 on nitrided Si(100), an increase in the absorption band originating from the silicon oxide layer by the O2 anneal with the same condition is suppressed significantly. Notice that there is no significant change in the absorption band around ~1100cm-1 due to the Si-N network., indicating that

the chemical bonding features of N atoms is almost unchanged by the O2 anneal although the silicon oxide layer is formed on the nitride layer. When the O2 anneal temperature is decreased down to 300ºC, no interfacial oxidation proceeds for the stack structure of HfO2 on nitrided Si(100) as indicated in Fig. 4. Consequently, even in the case with a use of ultrathin SiNx as an oxidation barrier layer, the control of O2 partial pressure is required to avoid the interfacial oxidation during the thermal anneal higher than 350ºC.

4. Conclusions

For the stack structure of HfO2 on nitrided Si(100), the formation of the interfacial oxide layer is not completely suppressed with a 1.0nm-thick

SiNx layer pregrown by direct nitridation at 700ºC in ambient NH3. This result is attributed to the fact that the surface oxidation of the SiNx layer induces the movement of N atoms towards the substrate side and results in the nitridation of the Si surface. Acknowledgements This work was partly supported by the NEDO/MIRAI project. References [1] G. D. Wilk et al., J. Appl Phys. 89 (2001) 2057. [2] S. Miyazaki et al., Microelec. Eng. 59 (2001) 373. [3] H. Watanabe, Appl. Phys. Lett. 81 (2002) 22.

Fig. 1. Si2p and N1s spectra taken after annealed of the HfO2/SiNx/Si(100)stacked structure. The Si2p spectrum for the annealed sample after completeremoval of the HfO2 layer by dipping in a 0.1% HF solution and the N1sspectrum obtained from nitrided Si(100) are also shown. All the spectra weretaken at a photoelectron take-off angle of 90º.

Fig. 3. FT-IR-ATR spectra taken for (a) HfO2 formed on HF-last Si(100) and (b) on nitrided Si(100) before and after O2-anneal at 500ºC. In each of the cases the change in the spectrum was also so show.

395401 399 397

O2-annealed

As-evaporated

N1s

SiNx

HfO2/SiNx(1nm)/Si

As-evaporated

After O2-annealedat 500°C

0.1%HF-Treatment

Si2p

98100102104

(B)HfO2/SiNx/Si

A/BSi-O-Si LO PHONON1194.4cm-1

O2-annealed: A

As-evaporate: B

WAVENUMBER(cm-1)7008009001000110012001300

0.25

0.20

0.15

0.10

0.05

0

(A)HfO2/SiA/BSi-O-Si LO PHONON1234.1cm-1

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0.25

0.20

0.15

0.10

0.05

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Fig. 4. P-polarized ATR spectra for HfO2formed on nitrided Si(100) taken afterO2-anneal at 300~500ºC. The spectrumfor HfO2 formed on HF-last Si(100) after O2anneal at 300ºC was also shown as areference.

Fig. 2. Changes in non-oxidized andpartially-oxidized nitrogen bonding units as afunction of the interfacial layer thickness,which were evaluated by the spectraldeconvolution of N1s spectra measured ateach thinning step.

2.52.01.51.00.50INTERFACIAL LAYER THICKNESS(nm)

120

100

80

60

40

20

0

OSi

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N

SiN3

N3Si SiN3

N 398.0eV

398.3eV

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300°CHfO2/Si

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300°C

400°C

500°C

350°C

1. Motivation & Background

2. Sample Preparation & Experimental Procedure

3. Characterization

Chemical Bonding Features in the Interfacial Oxide

Blocking Properties of Pregrown SiNx against Oxidation

4. Summary

OUTLINE

Characterization of Interfacial Oxide Layers in Heterostructuresof Hafnium Oxides Formed on NH3-nitrided Si(100) surfaces

H. Nakagawa, A. Ohta, F. Takeno, S. Nagamachi, H. Murakami

S. Higashi and S. Miyazaki

Department of Electrical EngineeringGraduate School of Advanced Sciences of Matter

Hiroshima University

Control of the interfacial layer between high-k materials and Si(100)

Suppression of undesirable interfacial oxidation & interface defect generation

One of the major research issues for thehigh-k gate dielectric technology

Aggressive scaling of gate dielectric thickness below 1.5nm in EOT

Exponential increase in direct tunneling currentwith decreasing SiO2 thickness

Intense efforts in the replacement of conventional SiO2-based

gate dielectrics with physically-thicker high-k dielectrics

Sub-100nm Technology Generation of CMOS Devices

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O2 Annealed at 500° C

The intentional growth of an ultrathinoxidation barrier before high-k deposition

The reduction in the thermal budget ofthe post-deposition anneal (PDA)

Approaches to the Interface Control

Impact of NH3-nitridation of Si(100) on the interfacial oxidation

Characterization of chemical structures in the interfacial oxide layer grown by O2 anneal

This Work

Candidates for Oxidation Barrier

Ultrathin Silicon Nitrides (SiNx)or Oxynitrides

Previous Work

0.6

0.8

1.0

1.2

1.4

1.6

0 2 4 6 8 10 12NITROGEN CONTENT

AT THE INTERFACE (at.%)

AnnealingTime

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NE

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AF

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O2-

AN

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5min

As-evaporated

1.0nm-thick ZrO2

Impact of Ultrathin Oxynitrides on the Interfacial Oxidation

in 0.1% HF

ZrO2 Evaporation

850°C, 2.5 Torr

500°C; 10 s & 5 min

Oxidation : SiO2 (2.5 nm)850°C in Dry O2

NH3 Anneal : SiO2:N

SiO2-thinning to ~0.6 nm

in O2 2x10-5 TorrO2 Anneal

Hf4f, Si2p, O1s & N1s Core-Line SpectraX-Ray Photoelectron Spectroscopy (XPS)

SAMPLE PREPARATION & EXPERIMENTAL PROCEDURE

NH3 Nitridation : 650, 700°C,SiNx (0.8, 1.0 nm)

HfO2 Evaporation (2.5~3.6nm)in O2 1.0x10-4 Pa

Precleaning

300~600°C5min, 33Pa

O2 Anneal

Pure water Rinse

0.5%HF (1min30sec)

NH4OH:H2O2:H2O=0.15:3:7(80°C, 10 min)

Substrates: p-Si(100)

0.1% HF Treatmentfor HfO2 removal

Fourier Transform InfraredAttenuated Total Reflection (FT-IR-ATR)

Transmission Electron Microscope(TEM)Si-O-Si, Si-N vibration

Si(100)

SiNx

Si(100)

SiNx HfO2

Si(100)

Interfacial Layer

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SiNx(0.8nm)/Si As-nitrided

After O2 -annealed at 500°CN1s Take-off Angle : 90°

Si

Si SiN

SiNx/Si(100) Si2p,N1sSiNx/Si(100) Si2p,N1sSi2p & N1s Spectra for SiNx/Si(100) Before & After O2 Annealing

500°C anneal in Dry O2

Si(100)

SiNx(0.8nm)

Si(100)

SiNx(0.8nm)

Si(100)

SiNx(0.8nm)

500°C anneal in Dry O2 500°C anneal in Dry O2

As-nitrided

Si(100)

SiNx(0.8nm)

Si(100)

SiNx(0.8nm)

Si(100)

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Si2p HfO2/SiNx(1nm)/Si

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O1s

530532534

As-evaporated

O2-annealed

0.1% HF-Treatment

Si2p, O1s & Hf4f Spectra Before & After O2-anneal of the

HfO2/SiNx/Si(100) Stacked Structure at 500°C

1921 17 15

0.1% HF-TreatmentHf4f

As-evaporated

O2-annealed

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Si(100)

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N1s Spectra Before & After Evaporation of HfO2 and followed

by O2 anneal at 500°C

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Si2p

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0.1%HF TreatmentHfO2/SiNx/Si

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P-polarized ATR Spectra taken Before & After O2 Anneal at 500°C

A/BSi-O-Si LO PHONON1234.1cm-1

A/BSi-O-Si LO PHONON1194.4cm-1

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As-evaporate : B

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0.05

0.10

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P-polarized ATR Spectra taken After O2 Annealing at 300~600°C

SiNx

500°C O2-anneal

Cross Sectional TEM Image for HfO2 Evaporated on Nitrided Si(100)

After O2 Annealing at 500&600°C

600°C O2-anneal

5nm 5nm

Si(100)

HfO2HfO2

Si(100)

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3.5nm-Thick HfO2 Evaporated on NH3-nitrided Si(100)Before & After O2 Annealing

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n-Si(100)

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103

105

ND : 7x1014cm-3

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H-terminatedP+- Si(100)

500°C

600°C

300°C

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103

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Photoelectron Yield Spectra

Filled Gap States

The Electronic-Active Defect States in HfO2 Films

Acknowledgment

This work was partly supported by the NEDO/MIRAI project

Summary

The formation of the interfacial oxide layer is not completely sup-pressed with a 1.0nm SiNx layer prepared by 700°C NH3-nitridation

SiNx surface oxidation induces the movement of N atoms towardsthe substrate interface and promotes the nitridation of Si surface

The control of O2 partial pressure is required to avoid the interfa-cial oxidation during the thermal anneal higher than 350°C