Ž .Applied Surface Science 130–132 1998 197–201

ž /Oxidation of spatially controlled atomically flat Si 001 surface

Atsushi Ando ), Kunihiro Sakamoto, Kazushi Miki, Kazuhiko Matsumoto, TsunenoriSakamoto

( )Electrotechnical Laboratory ETL , 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan

Received 3 September 1997; accepted 10 December 1997

Abstract

Ž .We have demonstrated oxidation of the spatially controlled atomically flat Si 001 surface by exposing O gas, and2Ž .investigated morphologies of the SiO surface and the SiO rSi 001 interface using atomic force microscopy. When the O2 2 2

Ž .pressure was low at high substrate temperatures )4008C , anisotropic surface etching occurred. Monoatomic-deep pitsŽ .elongated in the direction of the dimer rows were formed on the Si 001 terraces. Thus, the O exposure resulted in the2

Ž .roughening of the atomically flat Si 001 surface. When the O pressure was kept high at the high substrate temperatures on2

the other hand, the oxidized specimens were free from the etching and had good morphologies at the SiO surface and the2Ž .SiO rSi 001 interface, whose root-mean-square values of roughness are small compared with a monoatomic-step-height.2

q 1998 Elsevier Science B.V. All rights reserved.

PACS: 61.16.Ch; 68.35.yp; 81.05. Cy

Ž .Keywords: Silicon; Step-free-surface; Oxidation; Surface ; Interface; Atomic force microscopy AFM

1. Introduction

Ž .The interface between SiO and Si 001 is impor-2Ž .tant for metal-oxide-semiconductor MOS devices

w x1,2 . As the gate oxide films become thinner, theŽ .flatness at the SiO rSi 001 interface becomes more2

important. It has been reported that the morphologyof the interface is affected by the surface roughness

w xbefore the oxidation 2,3 . Thus, the atomic steps onthe initial surface seem to play an important role in

Ž .the formation of the very thin -4 nm SiO layers.2

However, the influence of the atomic steps on theelectrical performance of MOS devices is not under-

) Corresponding author. Tel.:q81-298-54-5515; fax: q81-298-54-5523; e-mail: e9204@etl.go.jp.

stood thoroughly, because the density of the atomicsteps in the gate region is not strictly controlled. An

Ž .atomic-step-free Si 001 surface is an ideal substrate,but it was difficult to obtain the surface whosestep-free area was large enough for device fabrica-tion.

Recently, we have proposed a new technique forthe spatially controlled formation of an atomically

Ž . w xflat Si 001 surface at the desired position 4,5 . Theobtained surface has an area as large as several mm2

and is an attractive platform for future MOS devices.Ž .Our next concern is how these atomically flat Si 001

surfaces are oxidized.In this study, we demonstrate the oxidation of the

Ž .spatially controlled atomically flat Si 001 surfaceand investigate morphologies of the SiO surface2

0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0169-4332 98 00050-6

( )A. Ando et al.rApplied Surface Science 130–132 1998 197–201198

Ž .and the SiO rSi 001 interface by atomic force mi-2Ž .croscopy AFM .

2. Experimental

Ž . ŽA well-oriented Si 001 substrate Shin-Etsu.Semiconductors was used for a specimen. Its sur-

w xface normal was off from the 001 orientation byX w x X0–7 along the 110 direction and by 3–7 along the

w x110 direction. For the spatial control of the atomi-Ž .cally flat Si 001 surface, artificial step bands with

depths of 20–500 nm were formed into square pat-Ž 2 2 .terns 6=6 mm or 12=12 mm by chemical

etching. After a wet chemical cleaning and formationw xof thin chemical oxide layers on the surface 4,5 , all

specimens were loaded into a UHV system, the basepressure of which was about 2=10y8 Pa, and de-gassed at about 6008C for 8 h. A clean surface wasprepared by flashing 3 times at around 12008C for 30s using a direct current through the substrate. To

Ž .obtain a large area of an atomic-step-free Si 001terrace, the specimen was annealed at 10008C bypassing a direct current for 2 h in the UHV system.The pressure during the annealing was maintained inthe order of 10y8 Pa. The oxidation was succes-sively performed by exposing the surface to 2.0=

10y4 Pa dry O gas at 400–6008C for 10 min. The2

O gas was introduced at 400 or 6008C. After the O2 2

pressure reached 2.0=10y4 Pa, the substrate washeated up to a chosen value. The substrate tempera-ture was monitored by a pyrometer.

Morphology of the SiO surface was observed2

with an atmospheric AFM just after unloading thespecimen from the UHV system. To evaluate the

Ž .morphology of the SiO rSi 001 interface, AFM2

observation was performed just after the removal ofthe SiO layers by dipping into a dilute HF solution2

and a subsequent rinse in ultrapure water. The AFMexperiment was performed using a commercial mi-

Ž .croscope SPA300, Seiko Instruments . Gold-coatedcantilevers with a microfabricated silicon nitride tipŽ .Olympus Opt. were used for the AFM observation.Typical values for the curvature of the tip apex andthe spring constant were 15 nm and about 0.1 Nrm,respectively. The reference force during the AFMobservation was maintained in the order of 10y9 N.

Ž .The root-mean-square rms values of roughness at

Ž .the SiO surface and the SiO rSi 001 interface2 2

were calculated from AFM topographies scanned2 Ž .over a 640=640 nm area 256=256 points .

3. Results and discussion

It is known that the behavior of the Si surfaceexposed to O gas is dependent on the O pressure2 2

and the substrate temperature. When the O pressure2

is low and the substrate temperature is high, surfacew xetching occurs 6–10 . We first investigated the in-

fluence of the etching on the surface morphologyafter oxidation. Fig. 1 shows a typical AFM image of

Ž . y4a Si 001 surface after oxidation in 2=10 Pa O2

at 6008C for 10 min. The O gas was introduced at a2

substrate temperature of 6008C. After the O expo-2

sure for 10 min, the O gas was quickly evacuated2

and then the substrate was cooled down in UHV.The scanning area of the image is 6.71=6.85 mm2.Three large terraces separated by S and S stepsA B

are observed. There are many pits on the terraces.The pits on the upper terrace of the S step areA

elongated in the direction parallel to the S stepA

edge, and the pits on the lower terrace of S step areA

elongated in the direction perpendicular to the SA

Ž 2 . Ž .Fig. 1. AFM image 6.71=6.85 mm of a Si 001 surfaceoxidized in 2=10y4 Pa O at 6008C for 10 min. The O gas was2 2

introduced at a substrate temperature of 6008C. The substrate wascooled down in UHV.

( )A. Ando et al.rApplied Surface Science 130–132 1998 197–201 199

step edge, that is, all pits are elongated in thedirection of the dimer rows. This anisotropic pitformation comes from the difference in the vacancydiffusion between parallel and perpendicular to the

w xdimer rows 11 .To measure the depth of the pits, we observed

Ž .magnified AFM images. Fig. 2 a shows a typicalŽ .AFM image of a Si 001 surface after the oxidation

Ž . Ž 2 . Ž .Fig. 2. a AFM image 1.96=2.17 mm of a Si 001 surfacey4 Ž .oxidized in 2=10 Pa O at 6008C for 10 min. b A cross-sec-2

Ž .tion along the line A–B in a . The O gas was introduced at a2

substrate temperature of 6008C. The substrate was cooled down inUHV.

Ž 2 . Ž .Fig. 3. AFM image 3.29=3.61 mm of a Si 001 surfaceoxidized in 2=10y4 Pa O at 6008C for 10 min. The O gas was2 2

introduced at a substrate temperature of 6008C. The substrate wascooled down in 2.0=10y4 Pa O . Arrows indicate atomic steps.2

at the same conditions. The scanning area of the2 Ž .image is 1.96=2.17 mm . Fig. 2 b shows a cross-

Ž .section along the line A–B in Fig. 2 a . The cross-section clearly shows that the depth of the pits of;0.14 nm coincides with the monoatomic-height ofŽ .Si 001 . If these pits were the unoxidized region, the

pit depth should be ;0.07 nm. This fact indicatesthat the whole surface was oxidized after the oxida-tion process but that the atomically flat surface wasroughened during the oxidation by etching.

Since the O pressure of 2=10y4 Pa is high2

enough to prevent the surface etching, there weretwo possible stages for the etching during this oxida-

Ž .tion process: 1 O gas introduction stage from2

UHV to the 2=10y4 Pa at the start of the oxidation;Ž .and 2 the substrate cooling in UHV after the

oxidation. Although the time of the first stage isshort, it is enough for the formation of the pits. Fig.

Ž .3 shows a typical AFM image of a Si 001 surfaceafter oxidation in 2=10y4 Pa O at 6008C for 102

min. The O gas was introduced at the substrate2

temperature of 6008C. The substrate was cooleddown in 2=10y4 Pa O . In this procedure, the2

surface would not be etched during the cooling stage.The scanning area of the image is 3.29=3.61 mm2.Monoatomic-deep pits are also observed on a single

( )A. Ando et al.rApplied Surface Science 130–132 1998 197–201200

Ž .atomic Si 001 terrace. These facts indicate that thesurface roughening occurred during the O gas intro-2

duction stage and that it is necessary to keep thesubstrate from the exposure of low pressure O at2

high substrate temperature in order to avoid thesurface etching. When O gas is introduced into the2

UHV system, the O pressure varies in the range2

from 0 to 2=10y4 Pa. Thus, the substrate tempera-ture at the start of the oxidation must be chosen so asthe surface etching not to occur in the above-men-tioned range of O pressure. Previous experiments2

showed that the surface etching does not occur at thesubstrate temperature below 4008C in any range of

w xthe O pressure 9 . Therefore, the substrate tempera-2

ture must be kept below 4008C when the O gas is2

introduced and the O pressure must be kept in2

2=10y4 Pa until the substrate temperature is cooledbelow 4008C.

The surface morphology of thus oxidized speci-mens free from the surface etching were similar to

Ž .that of the atomically flat Si 001 surface before theoxidation. Fig. 4 shows a typical AFM image of aŽ . y4Si 001 surface after oxidation in 2=10 Pa O at2

6008C for 10 min. The O gas was introduced at a2

substrate temperature of 4008C. Then the substratewas heated up to 6008C. The substrate was cooled

Ž 2 . Ž .Fig. 4. AFM image 6.42=6.40 mm of a Si 001 surfaceoxidized in 2=10y4 Pa O at 6008C for 10 min. The O gas was2 2

introduced at a substrate temperature of 4008C. Then the substratewas heated up to 6008C. The substrate was cooled down in2.0=10y4 Pa O . Arrows indicate atomic steps.2

Table 1Ž .Rms roughness of SiO surface and SiO rSi 001 interface2 2

y2Ž .Rms roughness 10 nm

Ž .Oxidized temperature SiO surface SiO rSi 001 interface2 2

4008C 5.4–5.7 7.95008C 4.2–5.5 6.96008C 4.2–5.4 10

down in 2=10y4 Pa O . The scanning area of the22 Ž .image is 6.42=6.40 mm . A single atomic Si 001

terrace of about 4 mm in diameter is observed. Thereis no monoatomic-deep pits on the terrace. The rmsvalue of roughness at the terrace is about 0.054 nm,which is much smaller than the monoatomic-step-

Ž .height 0.136 nm .The rms values of roughness at the SiO surface2

Ž .and the interface between SiO and Si 001 which2

were formed at substrate temperatures of 400, 500and 6008C, are summarized in Table 1. All valuesare smaller than a monoatomic-step-height. These

Ž .results suggest that an ideal SiO rSi 001 structure2

with large area can be obtained by the oxidation ofŽ .the spatially controlled atomically flat Si 001 sur-

face.

4. Conclusion

We have demonstrated oxidation of the spatiallyŽ .controlled atomically flat Si 001 surface by expos-

ing to 2.0=10y4 Pa O gas at 400–6008C for 102

min, and investigated morphologies of the SiO sur-2Ž .face and the interface between SiO and Si 0012

using AFM. When the O gas was introduced or2Ž .turned off at high substrate temperatures )4008C ,

the anisotropic surface etching occurred. A lot ofmonoatomic-deep pits elongated in the direction ofthe dimer rows were formed and the atomically flatŽ .Si 001 surface was roughened. When the O pres-2

sure was kept high at the high substrate tempera-tures, on the other hand, the oxidized specimenswere free from the etching and had good morpholo-gies at the SiO surface and the interface between2

Ž .SiO and Si 001 whose rms values of roughness2Ž .0.042–0.1 nm are small compared with a

Ž .monoatomic-step-height 0.136 nm .

( )A. Ando et al.rApplied Surface Science 130–132 1998 197–201 201

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