Ž .Applied Surface Science 162–163 2000 401–405www.elsevier.nlrlocaterapsusc

Conducting atomic force microscopy studies on local electricalproperties of ultrathin SiO films2

Atsushi Ando a,), Ryu Hasunuma b, Tatsuro Maeda a, Kunihiro Sakamoto a,Kazushi Miki a, Yasushiro Nishioka b, Tsunenori Sakamoto a

a ( )Electrotechnical Laboratory ETL , 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japanb Texas Instruments Tsukuba R&D Center Limited, 17 Miyukigaoka, Tsukuba, Ibaraki 305-0841, Japan

Abstract

Ž . Ž .We have demonstrated the characterizations of the local electrical properties of ultrathin 1–4 nm SiO rSi 0012Ž y5 .structures using a conducting atomic force microscopy with a nanometer-scale resolution in a vacuum 1=10 Pa . The

measurement in a vacuum enables to reduce the influence of adsorbed water on quantitative current measurements, whilethere is a problem at the measurement in air of 60% humidity. Fitting to the Fowler–Nordheim equation is good at thickerSiO films more than ;3 nm. We have also demonstrated the results of continuous current–voltage measurements during2

the breakdown process by charge injection through a conducting probe. q 2000 Elsevier Science B.V. All rights reserved.

PACS: 61.16.Ch; 71.55.Ch; 73.40.Qv; 77.22.JPŽ .Keywords: Silicon oxide; Electron tunneling; Dielectric breakdown; Atomic force microscopy AFM

1. Introduction

As the gate oxide films of metal-oxide-semicon-Ž .ductor MOS devices become thin, it is increasingly

Ž .requisite to obtain the ultrathin -5 nm SiO films2

with good insulation. In order to form such films, theŽ .following subjects are important: 1 the surface

flatness of Si substrates prior to form the SiO films,2Ž . Ž .2 methods of oxidation, and 3 studies on theelectrical properties of the ultrathin SiO films on a2

nanometer scale.

) Corresponding author. Tel.: q81-298-61-5515; fax: q81-298-61-5523.

Ž .E-mail address: e9204@etl.go.jp A. Ando .

Ž .Recently, atomic force microscopy AFM hasbeen developed to measure the local electrical prop-erties with conducting probes. This method has beenapplied to studies on the local electrical properties of

Ž . w xSiO rSi 001 structures by several groups 1–9 .2

However, studies on ultrathin SiO films with the2

thickness ranging from 2 to 5 nm are rare. To obtaina goal, further studies are necessary.

In this paper, we demonstrate quantitative charac-terizations of the local electrical properties of ultra-

Ž .thin 1–4 nm SiO films using a conducting AFM2Ž y5 .in a vacuum 1=10 Pa . We also show the results

of continuous current–voltage measurements duringthe breakdown process by charge injection through aconducting probe.

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 00 00223-3

( )A. Ando et al.rApplied Surface Science 162–163 2000 401–405402

2. Experimental

Two types of SiO films were used as specimens.2

One was the SiO film formed on a spatially con-2Ž .trolled atomically flat n-type Si 001 substrate by

thermal oxidation under 2=10y4 Pa O at 6008C.2

The details of the methods to form the spatiallyŽ .controlled atomically flat Si 001 surface and to

w xoxidize the surface are described elsewhere 9,10 .Another was the SiO film formed on a p-type2Ž .Si 001 substrate by thermal oxidation in dry O rN2 2

ambient at 9008C. Thickness of the SiO films was2

measured by an ellipsometer.The conducting AFM measurement was per-

Ž y5 .formed in a vacuum 1=10 Pa using a commer-Ž .cial microscope SPA300HV, Seiko Instruments in

a contact-mode. To examine the influence of theadsorbed water on quantitative current measure-ments, the measurement in a humidity-controlled

Ž .atmosphere ;60% was also performed. The refer-ence force during all AFM experiments was main-tained to the order of 10y9 N. For a conductingprobe, we used a 30-nm-thick Pt-coated cantilever.The Pt-coated probe presents high-resolution topo-graphic images while it is weak in local heating byexcessive current through itself. To prevent the catas-trophic damage to the probe, the current through theprobe was limited to less than 1 nA. The currentlimitation enables us to measure the current quantita-tively without degradation of the Pt-coated probe.

3. Results and discussion

3.1. Influence of adsorbed water on quantitatiÕecurrent measurements

To use a conducting AFM for quantitative analy-sis, it is important to study the influence of measure-ment conditions on current measurements. When thecurrent is measured in a humid atmosphere, there isadsorbed water on the surfaces of the SiO films and2

the probes. Fig. 1 shows current–voltage character-Ž . Žistics of 1-nm-thick SiO rn-Si 001 measured in A,2

. Ž . y5B air of 60% humidity and in C 1=10 Pa. Thecurrent observed at a sample bias voltage of y5.5 Vwas varied from y0.326 to y0.1 nA at differentspots when the measurements were performed in air

Fig. 1. Current–voltage characteristics of 1-nm-thick SiO rn-2Ž . Ž . Ž .Si 001 measured in A,B air of 60% humidity and in C

1=10y5 Pa.

of 60% humidity. On the other hand, the currentdecreased to y0.03 nA in 1=10y5 Pa.

Fig. 2 shows the topographic AFM images afterŽ .the current–voltage measurements in a air of 60%

Ž . y5humidity and in b 1=10 Pa. The scanning areaof the images is 761.8=540.8 nm2. The positions

Ž . Ž .labeled ‘A’ and ‘B’ in Fig. 2 a and ‘C’ in Fig. 2 bindicate the spots measured current–voltage charac-

Ž . Ž .teristics A – C in Fig. 1, respectively. Protrusionswere observed only in the AFM image after themeasurements in air of 60% humidity. Their diame-ter and height are 30 nm and ;0.3 nm, respectively.The origin of these protrusions is not clear but seemsdue to the existence of adsorbed water.

These results indicate that the current measure-ment in the existence of adsorbed water is not suit-able for quantitative analysis. For this reason, thefollowing experiments were performed in a vacuumŽ y5 .1=10 Pa to reduce the adsorbed water.

3.2. Fitting to the Fowler–Nordheim equation

Since the emission area of the conducting probe isvaried for different probes, it is difficult to analyzethe current–voltage characteristics strictly. To solvethis problem, the Fowler–Nordheim equation has

w xbeen used when the SiO film is thick 2–7 .2

( )A. Ando et al.rApplied Surface Science 162–163 2000 401–405 403

Ž 2 .Fig. 2. Topographic AFM images 761.8=540.8 nm of theŽ .surfaces after current–voltage measurements in a air of 60%

Ž . y5humidity and in b 1=10 Pa. The positions labeled ‘A’ andŽ . Ž .‘B’ in a and ‘C’ in b indicate the spots measured current–volt-

Ž . Ž .age characteristics A – C in Fig. 1, respectively. Arrows indi-cate atomic steps.

The Fowler–Nordheim equation is expressed as

V 2 tI sAa exp ybFN 2 ž /Vt

where A is the effective emission area of the con-ducting probe, V is applied voltage, t is thickness ofthe SiO film, respectively. a and b are given by2

1r23 ) 3r2q m 4 2m fŽ .as , bs

)8p hm f 3"q

where q is the electron charge, m is the free-spaceelectron mass, h is Planck’s constant, m) is the

Fig. 3. The Fowler–Nordheim plots for current–voltage character-Ž . Ž .istics measured on three different samples A – C . The average

Ž . Ž .thicknesses of the SiO films were A 2.96 nm, B 3.36 nm, and2Ž .C 3.68 nm, respectively.

( )A. Ando et al.rApplied Surface Science 162–163 2000 401–405404

Table 1Thickness of the SiO films and fitted parameters of Fowler–Nordheim equation2

2Ž . Ž . Ž . Ž .SiO thickness nm Average thickness nm Barrier height eV Effective emission area nm2

Sample A 2.62–3.18 2.96 4.3 1600Sample B 3.08–3.74 3.36 4.0 100Sample C 3.49–3.87 3.68 3.8 100

Ž )effective electron mass in the SiO film m rms2.0.42 , and f is barrier height, respectively. When a

Ž 2Fowler–Nordheim plot ln JrE versus 1rE, Js.I rA and EsVrt is performed, the plot producesFN

Ž .a straight line whose slope yb determines barrierheight.

Fig. 3 shows the Fowler–Nordheim plots for cur-rent–voltage characteristics measured on three dif-

Ž . Ž .ferent SiO films A – C whose average thick-2Ž . Ž . Ž .nesses were A 2.96 nm, B 3.36 nm, and C 3.68

nm, respectively. The values of electric field werecalculated with the average thickness of the SiO2

film. Thickness of the SiO films and fitted parame-2

ters of Fowler–Nordheim plots in Fig. 3 are summa-rized in Table 1. The Fowler–Nordheim plots of

Ž . Ž .samples B and C show good linearity, but the plotŽ .of sample A is different from a straight line. These

results indicate that the current mechanism throughthe SiO films can be explained by the Fowler–2

Nordheim equation when the thickness of the filmsis more than ;3 nm. Our result on the thickness atwhich the current mechanism changes fromFowler–Nordheim tunneling is consistent with previ-ous conventional measurement with MOS capacitorw x11 .

Fig. 4. Continuous current–voltage characteristics of 2.66-nm-thickŽ .SiO rp-Si 001 at one point.2

3.3. Continuous current–Õoltage measurements dur-ing the breakdown process

When continuous current–voltage measurementsare performed at one point, a conducting AFM canobserve the breakdown process by charge injection

w xthrough a conducting probe 5 . Fig. 4 shows contin-uous current–voltage characteristics measured at onepoint. The average thickness of the SiO film was2

2.66 nm. Sample bias voltage was scanned from 0 toq6.6 V. Scanning speed of the voltage was q3.3Vrs and the interval of each current–voltage mea-surement was 5 s. A significant change was notobserved in current–voltage characteristics until the15th measurement, and these characteristics were

ŽFig. 5. Continuous two-dimensional current images 184.3=205.82 .nm after the ‘breakdown’. The sample bias voltage was q5.0

V. The contrast from white to black corresponds to the variationof current from 0 to q0.001 nA.

( )A. Ando et al.rApplied Surface Science 162–163 2000 401–405 405

Ž .available to quantitative analysis a5 . However, atthe 20th measurement, threshold voltage of currentdetecting decreased and the current became unstableŽ .a25 . As a result of more continuous measure-ments, the voltage shift and the current instability

Ž .grew larger a40 , and finally, the SiO film was2Ž .broken down at the 50th measurement a50 . How-

ever, this ‘breakdown’ is not an irreversible one, thearea of the ‘breakdown’ recovered when applying ofhigh electric stress was stopped. Fig. 5 shows thetwo-dimensional current images after the ‘break-down’. The scanning area of the images is 184.3=

205.8 nm2 and a sample bias voltage was q5.0 V atwhich leakage current is not detected in a freshsample. The contrast from white to black in Fig. 5corresponds to the variation of leakage current from0 to q0.001 nA. The ‘breakdown’ area became lessconductive. The current instability before the break-

w xdown was also reported on MOS capacitors 12 .These phenomena are important because they are akey to understand breakdown mechanism, and aconducting AFM has possibility to investigate thebreakdown phenomenon on a nanometer resolution.

4. Conclusion

We have demonstrated the characterizations of theŽ .local electrical properties of ultrathin 1–4 nm

Ž .SiO rSi 001 structures using a conducting AFM2Žwith a nanometer-scale resolution in a vacuum 1=

y5 .10 Pa . The measurement in a vacuum is useful to

reduce the influence of adsorbed water on quantita-tive current measurements. The current mechanismthrough the SiO films can be explained by the2

Fowler–Nordheim equation when the thickness ofthe films is more than ;3 nm. A conducting AFMhas possibility to investigate the breakdown phe-nomenon, such as the current instability before thebreakdown, on a nanometer resolution.

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