Effects of oxygen content on the optical properties oftantalum oxide films deposited byion-beam sputtering

H. Demiryont, James R. Sites, and Kent Geib

Ion-beam sputter deposition of tantalum oxide films was investigated for possible optical coating applica-tions. Optical properties of such films were found to be a sensitive function of oxygen-to-argon ratio in theion beam. Refractive index and absorption coefficient were determined in the 250-2000-nm wavelengthrange by spectrophotometric transmissivity. The different bonding states of the tantalum atoms were re-vealed by x-ray photoelectron spectroscopy. The visible wavelength refractive index was found to be 2.18and optical band gap 4.3 eV, so long as the films did not contain inclusions of metallic tantalum. Films withan admixture of oxygen deficient suboxide components had a low-energy tail of increasing magnitude in theabsorption spectrum.

1. Introduction

Tantalum oxide films have recently been used in in-tegrated optical devices as antireflecting coating forsilicon solar cells' and as waveguides2 for light. Theseapplications require a deposition technique that yieldsreproducible low-loss dielectric thin films. Tantalumpentoxide films have been formed by various methods:thermal oxidation3 ; dc sputtering4 5; rf sputtering6; re-active sputtering7; magnetron sputtering8; and anodicoxidation. 9 The optical properties 8 0 1 1 of these Ta 2O5films have been reported in some detail. However, thevalues of refractive index and extinction coefficient oftantalum pentoxide films grown by different methodsexhibit significant differences; i.e., the refractive indexvaries from 2.01 to 2.23 (at X = 550 nm).

In this study, we examine the chemical compositiondependent optical properties of tantalum oxide filmsprepared by the ion-beam sputter deposition technique.Refractive index, extinction coefficient, dispersion,absorption, optical energy gap, and longwave tail of theabsorption spectra are reported. Film composition waschanged incrementally from metallic Ta to stoichio-metric oxide, Ta2O5 , by varying the oxygen fraction inthe ion beam from 0 to 56%. The stoichiometric oxide

All authors are with Colorado State University, Fort Collins, Col-orado 80523; K. Geib is in the Department of Electrical Engineering;the other authors are in the Department of Physics.

Received 17 May 1984.0003-6935/85/040490-06$02.00/0.© 1985 Optical Society of America.

of tantalum was found to be high-quality optical filmmaterial. The optical properties showed a continuoustransition from stoichiometric oxide to suboxide tocermet to metal films with decreasing oxygen fraction.Bonding states of tantalum were deduced from thebinding energies of the Ta 4d and Ta 4f levels usingx-ray photoelectron spectroscopy (XPS).

11. Sample Preparation

Oxides of tantalum were prepared by ion-beamsputter deposition' 2 using a Kaufman-type ion source.' 3

A pure tantalum target (99.99%), 15 cm in diameter, 12cm from a rotating substrate, was bombarded by a beamcontaining a mixture of positive argon and oxygen ions,neutralized by an equal number of electrons. The ionenergy and current density were 1100 eV and 1.0 mA/cm2, respectively, and the background pressure was 102Pa (8 X 10-5 Torr) during deposition. The oxygenfraction of the background pressure was varied fromnear 0 to 56% to obtain films varying in compositionfrom Ta to Ta2O5 . The films were simultaneously de-posited onto fused silica and Corning 7059 glass foroptical measurements and onto polished silicon wafersfor XPS studies. In each case, the substrate tempera-ture was 50'C. Table I summarizes the depositionresults for eight different films. As can be seen from thetable, the deposition rate varied considerably withoxygen fraction. There exists a critical 02 ratio in thebeam, below which oxidation of the target is incomplete.Below this ratio, the sputter rate of target phase ishigher than its oxidation rate, and we find that Ta-TaOs cermets are deposited on the substrates. Abovethe critical partial oxygen pressure, where the oxide

490 APPLIED OPTICS / Vol. 24, No. 4 / 15 February 1985

Table 1. Summary of Samples Examined

Oxygenfraction Depositionin the rate Thickness n k

Sample beam (%) (nm/min) (nm) (X = 550 nm) (X = 550 nm) Description

PIIB 11 56 3.75 182 2.18 0PIIB 10 44 3.7 197 2.18 0 StoichiometricPIIB 9 37.5 3.8 415 2.18 0.0001 oxideP1IB 8 30 5.0 610 2.18 0.0037 SuboxidePuB 6 25 5.5 659 2.20 0.018PHB 7 17.5 10.0 606 3.42 0.06 CermetPIIB 5 12.5 12.5 830 11 0.1PIIB 2 -0 14.0 52 2.8 Metal

growth rate is positive on the target (oxidation rate >sputter rate of the target), we obtain stoichiometricoxide films. The existence of an oxide layer on thetarget reduces the deposition rate.

Ill. XPS Measurements and Results

X-ray photoelectron spectroscopy data were obtainedusing a Physical Electronics Industries model 548 sys-tem with a Mg anode. Gold and copper levels withbinding energies of 83.8 and 932.4 eV, respectively, wereused as the reference levels to calibrate the system.Each sample was examined before and after sputteringwith 1-keV Ar+ ions at 2 ,uA/cm2 for a time ranging from1 to 25 min using the Ta 4f, Ta 4d, and 0 is photoelec-tron peaks. In addition, the C is peak was monitoreduntil the hydrocarbon surface contamination was ef-fectively removed. The bonding states of tantalumwere deduced by observing the XPS data taken on ametallic tantalum film covered with its native oxide.These spectra are shown in Fig. 1 for both easily ob-servable doublets in the tantalum spectrum. A sig-nificant change in energy is observed between materialnear the surface of the film (Ta205) and that aftersputter etching has exposed bulk tantalum metal.Figure 1 shows two intermediate spectra as well. Asmall 0 Is peak remained after the Ta 2 05 was removed,centered at its original location of 531 eV, indicatingsome oxygen in the metallic film.

A superposition curve fitting procedure was used toanalyze the spectra obtained on our composite samples.The Ta+5 and Ta spectra were used as reference curvesto investigate the intermediate valence states. Asummary of the Ta 4/5/2, Ta 4f7/2, Ta 4d3/2, and Ta 4d5/2Gaussian components of the composite samples isgiven in Table II. As can be seen in this table, there aretwo valence states of tantalum with energies betweenthat of the stoichiometric oxide and the metallic tan-talum. These valence states correspond to suboxideswhich we label TaO, with corresponding valance statesof Ta+2 x. Values of x are speculative but in generalcorrespond to integral valence states. Table III sum-marizes the type of binding corresponding to samplesin the three general composition categories exam-ined.

IV. Optical Measurements and Results

The films deposited on transparent substrates wereexamined by transmission spectrophotometry in the

Ta' Ta°I I

-

-

0~Z

22 25

Sputteringtime

1 min

7 min

9 min

20 min

20

Fig. 1. XPS spectra of a metallic tantalum thin film with a nativeoxide overlayer.

Table 1. Binding Energy of Observed Tantalum States

Ta+ 2 x Ta+2 x' Ta+5Binding energy Ta (Ta in (Ta in (Ta in

(eV) (Ta in Ta) TaOx) TaOx) Ta 2O5)

4f doublet 4f 5/2 24.0 -26 -28 28.54f 7/2 22.2 -24 -26 26.7

4d doublet 4d 3/2 238.5 - - 242.24d 5/2 227.0 - - 231.0

0.25-2.0-Am wavelength range using a Beckman spec-trophotometer. Spectrophotometry allows the valuesof the index of refraction n and extinction coefficientk to be obtained over a wide spectral range. In thiswork, the optical parameters n, k, and thickness d of thesamples have been calculated from T-vs-X transmit-tance spectra. The method14 15 is based on analysis ofthe transmittance spectrum of a weakly absorbing film,nonabsorbing substrate system. Inverse transmittancecan be decomposed into two components:

15 February 1985 / Vol. 24, No. 4 / APPLIED OPTICS 491

Table I. Type of Binding and Samples

Type of sampleType of Stoichiometric Suboxide Cermetbinding oxide Ta2 O5 TaOs Ta-TaOs

Surface Ta+5 Ta+5 Ta+5Bulk Ta+5 Ta+5, Ta+2 x Ta+5, Ta+ 2 x, Ta

0

For T > 10 min, sputter After -10 min, sputter Reduction increased Ta0reduction occurs, then reduction occurs, then and Ta+2 x peakTa+2 x peak appears. Ta peak appears. intensities.

To …-------

Oxygen fraction in the beam = 8%

(d}

…~~~~~~~~~TO/ ~~~~~~Ath--/ -~~~~~~T

I-~~~~~~~~T

Oxygen fraction in the beam = 12.5%

/' (c)I

, 0.6 1.0 1.4Acf A(pm)

Fig. 2. Transmittance spectra of four typicalsamples: To, bare substrate; (a) stoichiometricoxide; (b) suboxide; (c), (d) cermet films; Xcf, cutoffwavelength [ < Xcf, T(X) = 0]; th, threshold

wavelength [ > th, T(X) = T(X)].1.8 2.2

1

T(X(1)

where u (X) and v (X) consist of exponential and sinus-oidal terms, respectively. u(X) and c(X) can be calcu-lated as follows:

T+ + T-u( = T) T-^ (2)

2T() (A)

c(X) = (3)2TA)- T)

T+ and T- are the experimentally traced envelopecurves of the transmission spectrum, and X is thewavelength of the light.

Optical parameters of a weakly absorbing film aregiven by

n(X) = 1/2[8n,,c(X) + (n + 1)2]1/2 + [8nSc(X) + (n - 1)2]1/21,

(4)

d = f4 [n(i - (Xi+1 - (5)

k(X) = X ln + 'u +a (6)47rd 2a+

where

Sns /J\ nl(n + 1)3(n + n2)

16nsn 2

(7)

(8)

for the first-order approximation. n is the index ofrefraction of the substrate, and Xi, Xi+1 are the wave-lengths corresponding to two adjacent extrema. Forhigher-order approximations

nh(X) = n(X) - vk2(X) [ii = f(nn 8 )]. (9)

These corrections to refractive index and film thickness,however, are only necessary for highly absorbing films.For example, if n = 2.5 and k = 0.1, the correction to nis only -0.001.

Figure 2 illustrates the transmittance spectra of fourtypical samples. Transmittance of the bare substrateis indicated by To. Figure 2(a) corresponds to thestoichiometric oxide sample, (b) a suboxide film, (c), (d)two cermet films. In this figure two critical wavelengthsare indicated as Xth and Xcf. The threshold wavelengthXth is the edge of the nonabsorbing region [T+(X) =To(X), or k (X) = 0 for X> XthI. Xcf is the effective cutoff

492 APPLIED OPTICS / Vol. 24, No. 4 / 15 February 1985

._

E5

I-

60

40k

20

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E

X 80

60

40

20

0

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80r

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0

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0.1 - (c,) (c;

(C2)

6.0

6.0

4.0

2.0

D . I\ Is20

0.3 0.5 0.7 0.9Wavelength (im)

Fig. 3. Dispersion curves of refractive index and extinction coeffi-cient: (a) stoichiometric oxide; (b) suboxide; (cl), (c2) cermet

films.

wavelength of the transmittance [T(X) = 0 for X <Xcf].

The optical parameters of each film were calculatedfrom their experimental transmittance spectra and aretabulated in Table I. Figure 3 illustrates n-vs-X andk-vs-X curves of the stoichiometric tantalum oxide (a),suboxide (b), and cermet films (c). Stoichiometricoxide films are found to be nondispersive and nonab-sorbing in the X > 365-nm region with refractive indexof 2.18 and Xcf = 252 nm. As can be seen, the extinctioncoefficient increases sharply for X < 0.3 ,um, althoughit does remain lower than previously reported values.Refractive index also increases sharply in this region(see Table IV). Suboxide films with progressively de-creasing oxygen ratios showed (1) an increase in Xth

values (Xth 1300 nm for the film with the largest su-boxide component), (2) an increase in Xcf values (Xcf

10,000 l

E 5000

(a) (b) (c)

0.4 0.8 1.2

A (m)

Fig. 4. Wavelength dependence of absorption coefficient: (a)stoichiometric oxide; (b), (c) suboxide films with different

compositions.

270 nm for the most suboxide film), (3) an increase inn, and (4) an increase in k. In the cermet film case,depending on the metallic Ta concentration in the TaOxmatrix, we observed (1) visually metallic luster, deepgray-brown tinted films, (2) no region where k = 0 (Xth

- ), (3) 300 < Xcf(nm) < 500, and (4) an additionalincrease in both n and k values.

The sharp increase in n and k in the short wave-lengths is associated with fundamental band gap ab-sorption in these films. Figure 4 shows the wavelengthdependence of the absorption coefficient, ca(=4irk/X).Curve (a) corresponds to stoichiometric oxide, while (b)and (c) depict suboxide films with different composi-tions. The stoichiometric Ta2O5 films exhibit a verysteep rise in absorption curve after a small longwave tail,while the oxygen deficient films shift slightly to higherwavelengths and have a larger longwave tail. Thelongwave tail region obeys Urbach's1 6 rule that the ab-sorption coefficient measured at temperature T is givenby

Table IV. Optical Properties of Ta205 Films

n k Deposition Method of(X 550 nm) (X 550 nm) E' (eV) ;th(nm) X(f(nm) technique measurement Ref.

2.18 <10-4 4.2 252 365 Ion-beam Transmission Presentsputtering results

2.01 5 X 10-3 4.2 290 410 Magnetron Reflectance 1reactivesputtering

1.92 Very small 4.07-4.5 290 390 Reactive Reflectance- 8sputtering transmittance

2.16 <3 X 10-3 4.15-4.51 - - Sputtering Reflectance- 10transmittance

2.12-2.21 High - - - Reactive Reflectance 11dc sputtering extrema-angle

of incidence2.10 S2 X 10-3 - - - Reactive Transmission 5

dc sputtering2.23 - - - - Anodic Reflectance 18

oxidation

15 February 1985 / Vol. 24, No. 4 / APPLIED OPTICS 493

600

400-

_200 9

1 2 3 4

ho (eV)

Fig. 5. (hVa)112

-vs-hv plots of tantalum oxide films with variouscompositions. Ion-beam oxygen fraction during deposition is indi-

cated on each curve.

R.qZ2:1

9S.XWCL

0

800

600

400

200

Stolchometric OxideSuboxide

Cermet

Ta film

5 10 15 20Sputtering time (minutes)

Fig. 6. Sputtering time dependence of the XPS 0 is intensity forvarious composite tantalum oxide films.

a = A exp[a(hv - Eo)/kTJ, (10)

where A, a, and Eo are empirical constants, and v is thefrequency corresponding to X. For most amorphousmaterials a longwave tail with approximately expo-nential form is found in the optical absorption spectra.This region is associated with electrons originating instates in the mobility gap.

For energies above the band gap ae should obey apower law of the form17

(hvE)r (1hv

from which we may define the optical energy gap E'. InFig. 5 (hvx) 1 /2 is plotted against hp for films depositedwith several ion-beam oxygen fractions. The straightlines imply r = 2. The optical energy gap is found to beslightly composition dependent for tantalum oxidefilms: 4.3 eV for Ta2O5 and 4.0 eV for the film with thelargest suboxide component. In the cermet film case,depending on the Ta particle concentration in tantalumoxide, it is also possible to describe an optical energy gapfrom 3 eV down to 0.

To evaluate the effective stoichiometric oxide fractionin the transparent samples, the real part of the complexrefractive index and the intensity of the 0 is peak areexamined. Figure 6 shows the sputtering time depen-dence of the 0 s peak intensity for four samples ofdifferent composition. As can be seen, the oxygen levelgradually decreases from stoichiometric to cermet film.The maxima near the air-film interface are due to an

Suboxide2.27 9-

CD

.5

2(aM

2.18

StoichiometricOxide

580 600 620 640

0 1 s peak intensityFig. 7. Real part of complex refractive index vs XPS 0 is peak in-tensity. The two limits of transparent samples corresponding to

suboxide and stoichiometric oxide are indicated on the top scale.

oxygen-rich surface layer formed through exposure toair. The falloff at the surface itself is due to hydro-carbon contamination. Figure 7 shows an approximatelinear dependence between the refractive index and theoxygen content of the film. An approximate stoichio-metric oxide fraction obtained from the optical mea-surements agrees well with the XPS data.

In the case of a cermet film, the imaginary part of thecomplex refractive index 2nk may be used as a measureof equivalent metal fraction in the film. This evalua-tion gave the Ta concentration of the cermet samplesgiven in Table I as -2 and 3%. The XPS data could notbe analyzed in terms of Ta0 intensity due to the verycomplex spectrum of these films.

V. Summary

In this work, optical properties of ion-beam sputterdeposited tantalum oxide films were studied as afunction of composition. Stoichiometric Ta2O5 filmswere found to be nonabsorbing and nondispersivehigh-quality optical coating material. Suboxide-composite films were absorbing and dispersive, havinga larger longwave tail and a lower energy band gap.Since the quality of the films depends strongly on theiroxygen content, ion-beam deposition and other tech-niques should be carefully monitored to produce thehighest quality oxide films.

We would like to thank David Kerwin for his assis-tance with the ion-beam sputter deposition. This workwas supported by the Air Force Weapons Laboratoryunder contract F-29601-83-K-0079.

This paper was presented at the Third TopicalMeeting on Optical Interference Coatings, Monterey,Calif., 17-19 Apr. 1984.References1. F. Rubio, J. Denis, J. M. Albella, and J. M. Martinez-Duart,

"Title," Thin Solid Films 90, 405 (1982).2. W. M. Paulson, F. S. Hickernell, and R. L. Davis, "Title," J. Vac.

Sci. Technol. 16, 307 (1979).3. P. K. Tien, "Light Waves in Thin Films and Integrated Optics,"

Appl. Opt. 10, 2395 (1971).4. Y. C. Cheng and W. D. Westwood, "Title," J. Electron. Mater.

3, 37 (1974).5. S. Schiller, U. Heisig, K. Steinfelder, and J. Strumpfel, "Title,"

Thin Solid Films 63, 363 (1979).

494 APPLIED OPTICS / Vol. 24, No. 4 / 15 February 1985

6. W. M. Paulson, F. S. Hickernell, and R. L. Davis, "Effects ofDeposition Parameters on Optical Loss for rf-Sputtered Ta2 O5

and Si3 N4 Waveguides," J. Vac. Sci. Technol. 16, 307 (1979).7. T. M. Reith and P. J. Ficalora, "The Reactive Sputtering of

Tantalum Oxide: Compositional Uniformity, Phases, andTransport Mechanism," J. Vac. Sci. Technol. A 1, 1362 (1983).

8. F. Rubio, J. M. Abella, J. Denis, and J. M. Martinez-Duart,"Optical Properties of Reactively Sputtered Ta2 O5 Films," J. Vac.Sci. Technol. 21, 1043 (1982).

9. S. Wilcox and W. D. Westwood, "Anodic Oxidation of Tantalum,"Can. J. Phys. 49, 1543 (1971).

10. E. E. Khawaja and S. G. Tomlin, "The Optical Properties of ThinFilms of Ta2 O5 and ZrO2 ," Thin Solid Films 30, 361 (1975).

11. W. D. Westwood, R. J. Boynton, and S. J. Ingrey, "Effect ofPressure on the Properties of Reactively Sputtered Ta2O5," J.Vac. Sci. Technol. 11, 381 (1974).

12. J. R. Sites, P. Gilstrap, and R. Rujkorakarn, "Ion-Beam SputterDeposition of Optical Coatings," Opt. Eng. 22, 447 (1983).

13. H. R. Kaufman, in Advances in Electronics and Electron Physics,Vol. 36, L. Merton, Ed. (Academic, New York, 1974), p. 265.

14. E. Aktulga, "Optical Parameters of a Weakly Absorbing Film ona Non-Absorbing Substrate by Spectrophotometric Transmis-sivity," Ph.D. Thesis, Department of Physics, Istanbul U., Is-tanbul, Turkey.

15. H. Deminyont, D. B. Kerwin, and J. R. Sites, "Optical Propertiesof Ion-Beam Sputtered TiO2 Films," Natl. Bur. Stand. U.S. Spec.Publ. to be published.

16. F. Urbach, "The Long-Wavelength Edge of Photographic Sen-sitivity and of the Electronic Absorption of Solids," Phys. Rev.92, 1324 (1953).

17. J. Tauc, Amorphous and Liquid Semiconductors (Plenum, NewYork, 1974).

18. L. Young, Anodic Oxide Films (Academic, London, 1961), p.81.

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15 February 1985 / Vol. 24, No. 4 / APPLIED OPTICS 495