Ž .Applied Surface Science 144–145 1999 589–592

Characterization of ultrathin SiO layers formed on a spatiallyx

ž /controlled atomic-step-free Si 001 surface

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

( )Electrotechnical Laboratory ETL , 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan

Abstract

Ž .We have demonstrated the characterizations of the morphologies and local electrical properties of ultrathin -5 nmŽ . Ž .SiO rSi 001 structures that were formed by thermal oxidation of a spatially controlled atomic-step-free Si 001 surface.x

Ž .Both the SiO surface and the SiO rSi 001 interface had good morphology, with root-mean-square values of roughness,x x

less than 0.12 nm. In contrast, spatial differences were observed in the local electrical properties measured using an atomicŽ .force microscope AFM with nanometer scale resolution. q 1999 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

As the gate oxide films of MOS devices becomethin, it is increasingly requisite to obtain the ultrathinŽ .-5 nm SiO layers with good insulation. In such a2

case, the height of the monoatomic step is no longerconsidered negligible when compared with the totaloxide thickness. Thus, the flat and wide area formed

Ž .on the Si 001 substrate without atomic steps isideally important prior to form the SiO layers.2

Recently, we proposed a new technique for thespatially controlled formation of an atomic-step-free

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

Ž . 2Si 001 surface with an area of ;13 mm at thew xdesired position 1 . This surface is an attractive

platform for atomic-step-free MOS devices. Previ-ously, we reported that good morphologies at the

Ž .SiO surface and the SiO rp-Si 001 interface werex xŽ .obtained when very thin -0.7 nm SiO layersx

w xwere formed in an UHV system 2 . To obtain a goalof atomic-step-free MOS devices, further studies,such as a study on thicker oxide layers, are neces-sary.

In this paper, we investigate the morphologies ofŽ .ultrathin -5 nm SiO layers thermally formed onx

Ž .an atomic-step-free Si 001 surface using an atomicŽ .force microscope AFM . To evaluate the local elec-

Ž .trical properties of SiO rSi 001 structure, two-di-x

mensional current images and current–voltage char-

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00871-X

( )A. Ando et al.rApplied Surface Science 144–145 1999 589–592590

Ž .acteristics of the SiO rn-Si 001 structure were alsox

measured by AFM with a conductive probe.

2. Experimental

Ž 15 3.A p-type B-dope: 1=10 rcm and an n-typeŽ 15 3. Ž .P-dope: 5=10 rcm Si 001 substrates were usedas specimens. The surface was tilted by 0–7X along

Xw x w xthe 110 direction, and by 3–10 along the 110direction. For spatial control of the atomic-step-freeŽ .Si 001 surface, artificial step bands with depths of

20–500 nm were fabricated into square patterns byw xchemical etching 1 . Fig. 1 shows schematic struc-Ž .tural views of a Si 001 substrate with the artificial

step bands. After a wet chemical cleaning and forma-w xtion of protective oxide layers 1 , all specimens

were loaded into an UHV system, the base pressureof which was approximately 2=10y8 Pa, and de-gassed at 6008C for 8 h. To remove the protectivelayers, the specimen was flashed 3 times at 12008C

Ž . Ž .Fig. 1. a Schematic structural view of a Si 001 substrate withŽ .artificial step bands for spatial control of atomic-step-free Si 001

Ž . X Ž .surfaces. b A cross-section along the line A–A in a .

Ž . Ž 2 . Ž .Fig. 2. a AFM image 3.29=3.16 mm of the oxidized Si 001with surface etching by introduction of O gas at a substrate2

Ž . Ž 2 .temperature of 6008C. b AFM image 6.48=7.35 mm of theŽ .oxidized Si 001 free from surface etching. Arrows indicate atomic

steps.

for 30 s. To obtain a large area of atomic-step-freeŽ .Si 001 surface, the specimen was annealed at 10008C

by passing a direct current for 2 h. The pressure ofthe UHV system was maintained to the order of10y8 Pa during the annealing.

The ultrathin SiO layers were successivelyx

formed by thermal oxidation under 2=10y4 Pa O2Ž . Žfor 10 min at 4008C condition A , 5008C condition

. Ž .B , and 6008C condition C . To form thicker oxidelayers, the specimen oxidized under ‘condition C’,was additionally oxidized in dry O ambient at2

Ž .10008C for 3 min condition D .The morphology at the SiO surface was observedx

by AFM just after unloading the specimen from theUHV system. The AFM experiment was performed

Žusing commercial microscopes SPA300 and.SPA300HV, Seiko Instruments in a contact-mode.

Ž .To evaluate the roughness at the SiO rSi 001 inter-x

face, the AFM observation was performed after re-moving the SiO layers with a dilute HF solutionx

( )A. Ando et al.rApplied Surface Science 144–145 1999 589–592 591

Table 1Ž .rms roughness of SiO surface and SiO rSi 001 interfacex x

y2Ž . Ž .Oxidized condition SiO thickness nm rms roughness 10 nmx

Ž .SiO surface SiO rSi 001 interfacex x

y4A 2=10 Pa, 4008C, 10 min -0.6 5.4–6.7 7.9–8.1y4B 2=10 Pa, 5008C, 10 min -0.6 5.0–6.5 7.3–10.7y4C 2=10 Pa, 6008C, 10 min 0.68–0.73 3.4–7.4 7.1–11.7

D Cq1 atm O , 10008C, 3 min 4.08 9.1 10.72

and subsequently rinsing with ultrapure water andŽ .drying with dry N gas. The root-mean-square rms2

values of roughness at the SiO surface and thexŽ .SiO rSi 001 interface were calculated from thex

AFM topographies scanned over an area of 640=6402 Ž .nm 256=256 points without atomic steps.To characterize the local electrical properties of

Ž .the SiO rn-Si 001 structure, two-dimensional cur-x

rent images and current–voltage characteristics werew xmeasured by the contact-mode AFM 3,4 with a

30-nm-thick Pt coated cantilever. We roughly esti-mated an effective contact area at ;100 nm2. Toprevent the degradation of the SiO layers and addi-x

tional oxide growth, all measurements were per-Ž .formed in a humidity controlled atmosphere -30%

Ž y5 .or vacuum ;10 Pa . No modifications wereobserved in AFM topographies after the measure-ments. The reference force during all AFM experi-ments was maintained to the order of 10y9 N.

3. Results and discussion

The morphology of the oxidized Si surface isdependent on the oxidation condition. When the O2

pressure is low and the substrate temperature is high,w xsurface etching occurs 2,5–7 . Fig. 2a shows a

typical AFM image of the oxidized surface thatsurface etching occurred during oxidation process.The O gas was introduced at the substrate tempera-2

ture of 6008C and oxidation was performed under‘condition C’. The scanning area of the image is3.29=3.16 mm2. There are many pits on a largeterrace. The depth of them is ;0.14 nm coincided

Ž .with the height of monoatomic step of Si 001 . Sincethe formation of the pits results in the surface rough-

Ž .ening of an atomic-step-free Si 001 surface, it isnecessary to keep the surface from the exposure of

low pressure O at high substrate temperature during2

oxidation process.The surface morphology of thus oxidized speci-

mens free from the surface etching were similar toŽ .that of the atomic-step-free Si 001 surface before

the oxidation. Fig. 2b shows a typical AFM imagewhose scanning area was 6.48=7.35 mm2. The O2

gas was introduced at a substrate temperature of4008C. Then the specimen was heated up to 6008Cand oxidation was performed under ‘condition C’.An atomic-step-free SiO surface of approximately 5x

mm in diameter without pits, was observed.The rms values of roughness at the SiO surfacex

Ž .and the SiO rSi 001 interface which were oxidizedx

under ‘conditions A–D’ without the surface etching,are summarized in Table 1. All the values are lessthan 0.12 nm. This result indicates that an ideal

Ž .SiO rSi 001 structure can be obtained by the oxida-x

tion of the spatially controlled atomic-step-freeŽ .Si 001 surface when the thickness of the SiO lay-x

ers is less than ;4 nm. Our result is consistent with

Ž 2 .Fig. 3. Two-dimensional current image 167.8=182.0 nm of theŽ . y4SiO rn-Si 001 structure formed at 6008C for 10 min in 2=10x

Pa O . The sample bias voltage is y3.0 V.2

( )A. Ando et al.rApplied Surface Science 144–145 1999 589–592592

Ž .Fig. 4. Current–voltage characteristics of the SiO rn-Si 001x

structure formed at 6008C for 10 min in 2=10y4 Pa O .2

previous transmission electron microscopy observa-Ž .tion of SiO rSi 001 formed by oxidation of atomi-2

Ž . w xcally flat Si 001 y2=1 surface 8 .The quality of the SiO layers, such as the uni-x

formity of thickness, is also important for evaluationŽ .of the SiO rSi 001 structure. A two-dimensionalx

current image can examine the uniformity of theSiO layers, because the tunneling current throughx

the SiO layers is strongly affected by the quality ofx

the SiO layers. Fig. 3 shows a two-dimensionalx

current image obtained by scanning the conductiveŽ .probe on the SiO rn-Si 001 structure. The SiOx x

layers were formed under ‘condition C’ without thesurface etching. The average thickness was 0.68 nm.The scanning area of the image was 167.8=182.0nm2 and the sample bias voltage was y3.0 V. Thecontrast from white to black in Fig. 3 corresponds tothe variation of current from 0 to y0.015 nA. Thisimage was reproducible, but the influence of ab-sorbed water and the dependence of scanning condi-tion were not observed. The image presents thefinding that there is a spatial difference in the qualityof the SiO layers on a nanometer scale.x

The spatial difference in conductivity was moreclearly observed in the current–voltage character-istics as shown in Fig. 4. The current at more

Ž .conductive area ‘A’ indicated by an arrow in Fig. 3became larger at more negative bias voltage thany2.6 V. There are two possibilities to explain the

spatial difference in conductivity. One is the spatialdifference in the oxide composition, and another isthe fluctuation of the oxide thickness. It is difficult todiscuss these possibilities separately, since there isinsufficiency of resolution of current measurementsin a range below 7=10y12 A. However, taking intoaccount the fact that the conductivity difference of

Ž .ultrathin SiO rp-Si 001 depends slightly on the av-xw xerage SiO thickness 4 , the spatial differences inx

conductivity observed in Figs. 3 and 4, seems to bepartly due to the fluctuation of the thickness of theSiO layers.x

4. Conclusion

Ž .We have characterized ultrathin -5 nm SiOx

layers formed on a spatially controlled atomic-step-Ž .free Si 001 surface by AFM with a conductive

Ž .probe. The SiO rSi 001 structure has good mor-xŽ .phologies rms-0.12 nm at both the surface and

the interface. In contrast, there are spatial differencesin the local electrical properties of the SiO rn-xŽ .Si 001 structure, which indicates spatial differences

in the quality of the SiO layers, such as the thick-x

ness or the composition.

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