Journal of Non-Crystalline Solids 299–302 (2002) 575–578
www.elsevier.com/locate/jnoncrysol

In situ ESR observation of interface dangling bond formation
processes during amorphous SiO2 growth on Si
W. Futako a,*, T. Umeda a,1, M. Nishizawa b,c, T. Yasuda b,c,J. Isoya b, S. Yamasaki b,ca

Joint Research Center for Atom Technology, JRCAT-ATP, 1-1-1 Higashi, Tsukuba 305-0045, Japanb
JRCAT-National Institute for Advanced Industrial Science and Technology (NAIR),AIST Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japanc
Advanced Semiconductor Research Center – AIST, AIST Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, JapanAbstract
We report on the ﬁrst observation of the processes of formation of interface dangling bond (Pb centers) during
amorphous SiO2 thin-ﬁlm growth on a clean Si(1 1 1) substrate. After initial termination of the surface dangling bondson the clean surface, rapid formation of Pb centers was observed. Even for a few monolayers of a-SiO2 ﬁlm (<0:4 nm),the number of Pb centers reached the same order of magnitude as that in thick a-SiO2 ﬁlms. These results suggest thatthe interface defect density between Si and a-SiO2 originates statistically from the short-range chemical bonding
conﬁgurations and not from the long-range accumulation of the structural misﬁts between the two materials. Ó 2002
Elsevier Science B.V. All rights reserved.
PACS: 81.65.M; 07.30.K; 73.20; 61.72.H

1. Introduction
Oxidation of silicon wafer is one of the mostbasic processes of semiconductor engineering. Recent device technology requires the thickness ofthe gate insulating oxide to be reduced to 1 nm,
but under such conditions, small concentrations ofdangling bond defects have a fatal inﬂuence on
device performance. In order to control the properties of such a thin oxide layer and its interfaces,*

Corresponding author.
E-mail address: futako@jrcat.or.jp (W. Futako).
1
Present address: System Device and Fundamental Research, NEC, Tsukuba, Japan 305-8501.

a fundamental understanding of the process offormation of the oxide layer is necessary. Under
the usual conditions, silicon substrate has threefold co-ordination silicon atoms with a neutral
dangling bond, namely Pb center, at the Si=a-SiO2
interface. This Pb center is detrimental to electrically active interface defects [1]. There have beensome reports on studies which successfully reduced
the interface defect density (for example, [2]). Also,
there have been several works using scanningtunneling microscopy (STM) which revealed thechange of the electronic states on the top surface
during oxidation (for example, [3]). However, thebasic understanding of defect formation forvery thin layers remains inadequate. A method of0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 9 6 4 - 4

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detecting defects with atomic-scale resolution
needs to be developed.
Electron spin resonance (ESR) is one of themost powerful tools available for investigating
paramagnetic centers including neutral dangling
bonds. There have been many reports of Pb centers
using ESR, although they basically investigate theinterface between Si and thick a-SiO2 [2,4]. Umeda
et al. [5] developed a combination of an ESR
system and an ultra-high vacuum (UHV) chamber
to study the clean silicon surface and its chemical
reaction with oxygen molecules, and successfully
traced the change of the dangling bond states
during the ﬁrst-layer oxidation process of a clean
silicon surface with oxygen molecules at room
temperature; however, the subsequent oxidation ofa few monolayers was not suﬃciently investigated.
In this study, we focus on the change in the interface dangling bond defects at the subsequent
stage of the oxidation process on a clean siliconsurface.

2. ExperimentFig. 1 shows a schematic diagram of the UHV
ESR system used in this study. An X-band ESR
spectrometer is attached to a UHV chamber with asample cell made of a silica tube (10 mm diameter).
The UHV chamber is arranged so that the sample cell is located within the microwave cavityof the ESR system. Base pressure of the UHVchamber is approximately 1010 Torr. Si(1 1 1)
wafers (7  0:3  0:03 cm3 ) were used as a sample

Fig. 1. A schematic diagram of the UHV ESR system.

in our study. The resistivity of the crystalline silicon is higher than 1000 X cm. The samples weresupported with sample holders at the end of avacuum transfer rod after conventional wet treatment by so-called RCA cleaning, and pre-heatedat 600 °C in the UHV chamber [5]. The transfer
rod rotates so that the ESR spectrum can bemeasured as a function of the angle, h, between the[1 1 1] direction and the magnetic ﬁeld B (namely,when h ¼ 0° and 90°, i.e. when the magnetic ﬁeld isparallel to the [1 1 1] and [1 1 2] axes, respectively).
The Si(1 1 1)-7  7 surface was prepared in theUHV chamber by direct current heating above
1500 K. The surface structure was conﬁrmed
by reﬂection high-energy electron diﬀraction(RHEED).
For oxidation experiments, the sample was heated to 780 °C ( 1050 K) by direct current
heating. Oxygen molecules were introduced into
the chamber through a variable leak valve. Oxygen
doses, to which the clean silicon surface was exposed, were controlled by the pressure and theexposure time. Typical oxygen partial pressure was1  106 –1  104 Torr. All measurements (ESR,RHEED) were carried out at room temperature.
The total thickness of a-SiO2 layer was estimatedusing Auger electron spectroscopy (AES) in adiﬀerent vacuum system after the sample was exposed to the air.

3. Results
Fig. 2 shows ESR spectra of various steps of theoxidation. As shown in the ﬁgure, a dynamicchange of the spectra is successfully traced duringoxidation. Especially, atomic layer scale detectionof the defect signal is achieved because the total
a-SiO2 thickness grown in this study is approximately 0.4 nm. At ﬁrst stage of the process, conventional wet treated (RCA cleaning) Si wafer
before ﬂashing shows an ESR signal of approximately 4 G linewidth which originates from Pbcenters. This initial interface dangling bond disappears in the Si(1 1 1)-7  7 clean surface although this surface should have a large density ofdangling bonds. This was thought to be due to thebroadening eﬀect caused by the metallic nature of

W. Futako et al. / Journal of Non-Crystalline Solids 299–302 (2002) 575–578

Fig. 2. ESR spectra of various steps of the oxidation; (1) before
ﬂashing, (2) 7  7 surface, (3) 100 L oxygen exposure, (4) 1000
L oxygen exposure.

the 7  7 surface structure [5,6]. A detailed interpretation is given elsewhere [7].
Introduction of oxygen molecules of approximately 100 Langmuir (1 L: 1  106 Torr s) at
780 °C resulted in a very small change of the spectrum as shown. The previous study demonstrated
that even at room temperature, 50 L of molecularoxygen is enough to make most of the surface
dangling bond signal disappear [5]. Therefore it isreasonable to regard that the top surface oxidationis completed at this dose. Subsequent introduction577

of oxygen molecules ( 1000 L) resulted in theappearance of the ESR signal. Angular dependence of the g-value of the signal demonstrated
that this signal mainly originated from a spin
center whose axis was parallel to the [1 1 1] direction. These results indicate that this second step of
the oxidation process produces the dangling bonddefect, namely Pb center. The linewidth of approximately 3 G was slightly small compared tothat observed in the wet treated sample (top). Thismight originate from the small structural ﬂuctuation of the smoother interface structure.
In Fig. 3, the eﬀective spin density estimatedfrom double integration of the ESR signal isplotted as a function of the oxygen dose. The initial Pb center density before ﬂashing is also indicated as ‘before ﬂashing’ for comparison. The
dotted line shows the result of Umeda et al. [5]which traced the eﬀective surface dangling bonddensity at a very early stage of the oxygen exposure. In that case, spin density decreased due to theoxidation. Although the oxidation temperature isdiﬀerent in this study, the small spin density observed at 100 L exposure can be understood interms of monolayer oxidation observed by Umeda.
As clearly shown in the ﬁgure, introduction ofoxygen of 1  103 L resulted in the appearance ofthe signal again. The spin density reached as large
a number as that of Pb centers seen before ﬂashing.
Even after an exposure of 105 L oxygen molecules,
the spin density was not drastically changed. Thethickness of the surface amorphous SiO2 estimatedby AES was approximately 0.4 nm at this dose.

4. Discussion

Fig. 3. Spin density estimated from double integration plotted
as a function of oxygen dose. Spin density measured before
ﬂashing is indicated for comparison. The dotted line indicatesthe result of monolayer oxidation reported by Umeda et al. [5].

Many eﬀorts have been made to understand theinitial oxidation, and several good microscopic
models have already been proposed for oxygen
molecular adsorption [3,8,9]. However, in contrast
to these eﬀorts, understanding of the process offormation of the initial oxide layer on an atomic
scale remains insuﬃcient. Especially from an engineering point of view, investigation of interface
defects such as Pb centers is crucial. As shown inFig. 3, even at an early stage of oxidation (1000 L
oxygen exposure), the number of Pb centers

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reached the same order of magnitude as that observed in the thick a-SiO2 ﬁlms although thethickness of the a-SiO2 layer is less than 0.4 nm inthis case. As mentioned above, angular dependence showed that the signal axis was parallel tothe [1 1 1] direction. These results indicate that theobserved Pb center is located at the bond between
the second and third atoms from the rest layers,
because it is the only layer whose bonding direction is parallel to the [1 1 1] direction within 0.4 nm
from the top surface. The bonding density of thislayer is approximately 1  1015 cm2 , and the observed number, 1  1013 cm2 , corresponds to 1%of the total silicon bonds.
This defect density number is diﬃcult to attribute to any speciﬁc regular structure. Therefore,
it is more realistic to consider the thermodynamicstatistics. Stesmans [2] reported the relationshipbetween Pb generation and stress, through a temperature dependence study of oxide formation.
According to the study, the stress at the interface,which arises from lattice mismatch, is one of themost dominant factors determining the Pb density,and relaxation of this stress reduces the Pb defectdensity. They also reported that the ESR linewidthof Pb centers could be well explained by the twodimensional distribution of Pb centers [4]. The
present results are consistent with this twodimensional distribution. Moreover, the present
results surprisingly demonstrate that a thick layerof a-SiO2 is not necessary to produce the usual
number of Pb centers, indicating that the very thininterface structure rather than the bulk latticemismatch determines the number of such defects.
In other words, the long-range accumulation ofstructural misﬁts between Si and a-SiO2 does notplay an important role in determining the defectdensity. Further experiments should be carried outfor more precise discussions.

5. Conclusion
The ﬁrst observation of the processes of formation of the interface dangling bond (Pb centers)
during amorphous-SiO2 thin-ﬁlm growth on aclean Si(1 1 1) substrate was reported. Aftermonolayer coverage of the clean surface with oxygen, rapid formation of Pb centers within 0.4 nmof the oxide layer was observed. Even for a few
monolayers of a-SiO2 ﬁlm (< 0:4 nm), the numberof Pb centers reached the same order of magnitude
as that in thick a-SiO2 ﬁlms. These results indicatethat the interface defects between Si and a-SiO2
originate statistically mainly from the short-range
chemical bonding conﬁgurations, not from thelong-range accumulation of structural misﬁts between the two materials.
Acknowledgements
This work, partly supported by NEDO, wasperformed in the JRCAT under the joint research
agreement between AIST and ATP.
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