Applied Surface Science 162–163 Ž2000. 299–303
www.elsevier.nlrlocaterapsusc

In situ electron spin resonance of initial oxidation processesof Si surfaces
Takahide Umeda a,b,),1, Satoshi Yamasaki a , Masayasu Nishizawa a , Tetsuji Yasuda a ,Kazunobu Tanaka a,baJoint Research Center for Atom Technology (JRCAT), National Institute for AdÕanced Interdisciplinary Research (NAIR), 1-1-4 Higashi,
Tsukuba, 305-8562, JapanbUniÕersity of Tsukuba, Tsukuba, 305-8513, Japan

Abstract
We have made for the first time electron spin resonance ŽESR. measurements on SiŽ111.7 = 7 and SiŽ100.2 = 1 surfaces
during the initial oxidation processes at room temperature. The present results clearly show that, at a very initial stage ofoxidation of Si surfaces where only a few surface layers were oxidized, a variety of defects are formed that have not been
seen in ex situ ESR studies of SiO 2rSi structures. q 2000 Elsevier Science B.V. All rights reserved.
PACS: 73.20.y r; 76.30.y v
Keywords: Electron spin resonance; Silicon; Surface; Oxidation; Dangling bond

1. Introduction
Electron spin resonance ŽESR. detects defects andradicals with unpaired electrons such as dangling
bonds ŽDBs. or hydrogen atoms. It is one of the best
tools to reveal the microscopic and electronic structures of the point defects in semiconductors andinsulators. This technique has been applied to investigate defects at the hetero-interface between Si and
SiO 2 , which is the most important structural part ofsilicon microelectronic devices. The nature of this)
Corresponding author. Tel.: q81-298-54-2634; fax: q81-29854-2642.
E-mail address: umeda@jrcat.or.jp ŽT. Umeda..
1
Research fellow of the Japan Society for the Promotion of
Science.

interface is particularly important due to the decreasing thickness of the SiO 2 layer in future devices.
Previous studies have found that: Ži. the SiO 2rSi
interfaces generate three sorts of Si DBs, so-called
Pb w1,2x, Pb0 and Pb1 w3–6x; Žii. such DBs are theorigin of interface electrical traps and thus seriously
degrade a device quality w7–10x.
However, all previous ESR works were made on
SiO 2rSi structures with an oxide layer over 0.5-nm
thick. Recently, we have developed a novel ESR
setup that enables ESR observations on clean Si
surfaces. This setup allows us to explore the defectformation on the Si surfaces during exposure ofoxygen gas. This paper describes results of our ESR
observation on SiŽ111.7 = 7 and SiŽ100.2 = 1 surfaces when the oxide layers grew from 0 Ži.e., theclean surface. to around 0.5 nm in thickness.

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 2 0 7 - 5

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T. Umeda et al.r Applied Surface Science 162–163 (2000) 299–303

2. Experimental: UHV–ESR system
To observe the oxidation of clean and well-defined Si surfaces, it is necessary to prepare ultrahigh-vacuum ŽUHV. environments inside a cylindrical space Ž1 cm diameter= 1 cm height. for ESR
measurements. Thus, we developed a novel UHV–ESR system, which is shown in Fig. 1. In the gap of
the ESR magnet, a microwave cavity was combinedwith a high-purity vitreous silica tube of a small
UHV chamber with a base pressure of 1 = 10y8 Pa.
Two Si wafers Ždouble-sided mirror finish, 8 = 0.3= 0.03 cm in dimensions. with a 0.05-cm separation
between the two were supported on the tip of atransfer rod and were transferred from end-to-end in
the chamber. Both ends of the rectangular wafers
were attached to electrical contacts to allow direct
current heating.
We used SiŽ111. and SiŽ100. wafers with a very
high resistivity Žover 1000 V cm. to reduce themicrowave losses. The wafers were subjected to theconventional RCA cleaning and to a pre-heating at
600–7008C under UHV. Finally, they were cleaned
by flash heating with direct current at 1000–11008C
for about 30 s. During the flash heating, the chamberpressure was maintained at about 10y7 Pa. The
surface was oxidized at room temperature by exposure to pure molecular oxygen without any hot filaments. After flash heating and oxidation, the surface
structures were confirmed by reflection high energy
electron diffraction ŽRHEED., and then the surfaces
were subjected to the ESR measurements.
The ESR spectra were measured by a BRUKER
ELEXSYS X-band spectrometer at room temperature
with a microwave frequency of 9.6 GHz, power ofFig. 1. Schematic view of UHV–ESR system.

1–200 mW and a magnetic field modulation of
0.4–1.0 mT at 100 kHz. The spectrometer parameters for each spectrum were adjusted within theabove range to get the highest signal-to-noise ratio
and to prevent an artificial distortion of the spectrum.
The number of defects observed here was between
1 = 10 12 and 2 = 10 13 within an ESR-active area of
1.2 cm2 , which is close to the detection limit of our
ESR instrument Žapproximately 10 11 spinsrmT..
Hence, it was necessary to accumulate each ESR
spectrum for 6–12 h, and for 1 day in the longest
cases. During the accumulation, the chamber was
evacuated to the base pressure, which prevents thesurfaces being changed by contaminants. We observed ESR spectra for different orientation angles,u , between the wafer axes Žeither w111x or w100x. andthe magnetic field Ž B . direction.

3. Oxidation of Si(111)7 = 7 surface
Fig. 2Ža. shows the ESR spectra of SiŽ111. wafers
just before flash heating, when u s 08 Ž Brrw111x.
and 908 Ž Brrw110x.. These spectra show the ESR
signal of the Pb center w1,2x, whose principal gvalues are estimated to be g rrs 2.002 and g H s
2.009 for u s 08 and 908, respectively. Since theorigin of the Pb center has been identified as a
w111x-oriented Si DB at the SiO 2rSiŽ111. interface
w1,2x, it is reasonable to observe the Pb center in our
sample with an oxide layer formed by the RCA
treatment. In addition to the Pb , this sample showeda small ESR signal, which is tentatively attributed to
the 198 Pb0 centers w6x. The 198 Pb0 centers are
essentially identical to the Pb0 observed in SiO 2r
SiŽ100. interfaces and they are positioned towards
w111x, w111x, and w111x. As is our case, these centerswere observed as a minority in SiO 2rSiŽ111. structures w6x. In the figure, the resonant positions of the
Pb and 198 Pb0 Žvertical dashed lines. are calculated
by referring to the previous g data w1,2,6x. Naturally,
the Pb signal observed here showed a good agreement with the calculation, and the 198 Pb0 case islikely to be consistent, too.
After the wafers were cleaned by the flash heating, the Pb and 198 Pb0 signals disappeared, as shownin Fig. 2Žb., simply because the oxide layer was

T. Umeda et al.r Applied Surface Science 162–163 (2000) 299–303

Fig. 2. ESR observation of SiŽ111. wafers during room temperature oxidation processes. ESR spectra Ža. before flash heating, Žb.
for 7=7 surface after flashing and its RHEED pattern in the 0th
Laue zone Žarrows indicate fundamental reflections., and Žc.
during oxidation processes. Solid and broken lines were obtained
for u s 08 Ž Brrw111x. and 908 Ž Brrw110x., respectively. Vertical dashed lines indicate the resonant positions of Pb and 198 Pb0
for u s 08 and 908 and their thickness represents a relative intensity of each signal.

evaporated completely. RHEED measurements reveal a clear 7 = 7 reconstructed structure on thissurface ŽFig. 2Žb... Therefore, we expected to see an
ESR signal due to the large number ŽG 10 13 cmy2 .
of surface DBs on the 7 = 7 surface w11,12x. However, room temperature ESR measurements could notreveal any DB signals. It is speculated that we did
not see the surface DBs is due to the broadening of
the ESR line, which causes a large loss of signal
intensity. This issue will be discussed in a forthcoming paper.
Although the clean surface showed no appreciable
ESR signals, the exposure of the surface to oxygen

301

molecules caused drastic changes in the ESR spectrum. Fig. 2Žc. shows the evolution of ESR spectra
as a pressure of oxygen was increased from 10y6 Pa
to 1 atm. After the exposure to 1 atm oxygen, the Pb
signal appears again, simply because the oxide layer
was formed again on the surface. At that time, theoxide layer thickness was measured to be around 1
bilayer Žapproximately 0.3 nm. by ex situ Auger
electron spectroscopy ŽAES.. The 198 Pb0 signal also
seems to appear again, although its intensity is lessthan in the spectrum Ža..
On the other hand, before the 1-bilayer-thick oxidation was completed, various transient ESR spectrawith an appreciable u-dependence were observed, as
is seen in Fig. 2Žc., that are obviously different fromthe Pb spectra observed either before flashing or after
the 1-bilayer-thick oxidation. Furthermore, in the
10y4 -Pa oxidation, the transient ESR spectrum involves two or more signals. Judging from the simulated resonant positions Žvertical dashed lines., these
new transient signals are not due to 198 Pb0 . We havealso confirmed that the Pb1 center is not consistent
with the transient signals. Therefore, it is suggested
that surroundings of the transient defects andror
their microscopic and electronic structures are different from those of the Pb-type defects. We emphasizethat they are seen transiently only during the period
for the 1-bilayer-thick oxidation and disappear afterwards; eventually, we can only see the Pb-type defects. Thus, the transient defects have not been detected previous ESR studies of SiO 2rSi structures.
We also note that our results shown here are quite
different from previous results of Haneman et al.
w13–16x who observed Si surface states by ESR.
They detected only an isotropic resonance of g ;2.0055 in their Si samples that is quite similar to a
DB signal in amorphous Si w17x.

4. Oxidation of Si(100)2 = 1 surface
We also made ESR studies of SiŽ100. surfaces
during room temperature oxidation processes assummarized in Fig. 3. For the case of SiŽ100., thePb0 and Pb1 centers were seen in the SiO 2rSiŽ100.
interface of our initial sample ŽFig. 3Ža.., which isconsistent with the previous ESR works w3–5x. Afterthe flash heating, these interfacial defects vanished302

T. Umeda et al.r Applied Surface Science 162–163 (2000) 299–303

have not been detected in previous ESR studies
w3–5x. Those transient spectra showed significant
angular dependence and a complex overlap of two or
more signals. We also found that the transient defects exhibit different resonant positions with respect
to the Pb0 and Pb1 Žsee vertical dashed lines in Fig.
3..
When an oxide layer was getting thicker up to 1
bilayer Žapproximately 0.4 nm in thickness measured
by ex situ AES and consistent with a photoemission
data w18x. by 1 atm oxygen exposure, the transient
signals disappeared, and instead, the Pb0 and Pb1
signals dominated again. Detailed analyses of the
ESR spectra will be given in the future, which will
give us a novel information on the microscopic
mechanism of the oxidation involving the defectformation.

5. Conclusion

Fig. 3. ESR spectra of SiŽ100. wafers Ža. before flash heating, Žb.
after flashing and a 2=1 RHEED pattern of the surface Žcf. Fig.
2Žb.., and Žc. during room temperature oxidation processes, whichwere measured for u s 08 Ž Brrw100x, solid lines. and 908
Ž Brrw011x, broken lines.. We also indicate the resonant positionsof Pb0 and Pb1 by vertical dashed lines, as is the same as Fig. 2.

completely due to the removal of the oxide. The new
surface showed the 2 = 1 reconstruction in a RHEED
pattern ŽFig. 3Žb.. and also a broad resonance in the
ESR spectrum which corresponds to the spin densityof an order of 10 12 cmy2 . This resonance has thesame positions as the Pb1 center, and it was tentatively assigned to a Si DB at one end of a strained
Si–Si dimer at the SiO 2rSiŽ100. interface w4,5x.
Therefore, the novel surface signal observed here
may be related to defected dimers on the SiŽ100.2 = 1
surface. To be conclusive, we need more data suchas a precise u-dependence of this resonance.
As in the case of SiŽ111., the SiŽ100. surface wasalso greatly affected by oxygen exposure as shownin Fig. 3Žc.. Before the exposure to 1 atm oxygen, avariety of transient ESR spectra were observed which

Using a novel UHV–ESR setup, we have observed in situ defect formation on SiŽ111.7 = 7 andSiŽ100.2 = 1 surfaces during room temperature oxidation. Our ESR measurements revealed a variety ofdefects not previously detected in ex situ ESR studies w1–10x at the initial stage of oxidation of Si
surfaces.

Acknowledgements
This work, partly supported by NEDO, was performed in the Joint Research Center for Atom Technology ŽJRCAT. under the joint research agreement
between the National Institute for Advanced Interdisciplinary Research ŽNAIR. and the Angstrom Technology Partnership ŽATP..

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