Optical properties of Zr films grown under ion bombardment

100

Thin Solid Films, 228 (1993) 100-104

Optical properties of Zr films grown under ion bombardment J. F . T r i g o , E . E l i z a l d e a n d J. M . S a n z Departamento Fisica Aplicada CXII, Universidad Autrnoma de Madrid, E-2g049 Madrid (Spain)

Abstract Zr films (300-600 nm) have been deposited on Si(100) in a double-ion-beam system (residual pressure, less than 10 -7 Torr) using Ar ÷ for both sputtering and irradiation of the growing film. The films have been optically characterized by spectroscopic phase-modulated ellipsometry in the range 1.5-4.5 eV as a function of the deposition variables. The results show significant changes in the real part n and in the imaginary part k of the complex refractive index ( ~ = n + ik). These changes mainly depend on the flux and energy of the Ar ÷ ions impinging the growing film. Furthermore, the stability of the films against the exposure to atmosphere and annealing in vacuum (10 -7 Torr) was also investigated. The results are discussed in terms of packing densities, damage and surface oxidation of the films.

1. Introduction The aim o f this paper was to study the optical properties and stability o f thin films (about 30006000 A) of zirconium grown under Ar ÷ bombardment in a dual-ion-beam-sputtering system. Low energy ion bombardment during growth of thin films to modify their properties is now a well-established method [1]. In fact, modification of intrinsic stress, refraction index and other properties have been extensively reported in the literature [1-5]. In the case of optical properties o f thin films, the main problem refers to the formation o f a columnar structure which adsorbs a large amount o f water vapour and causes large variations and stability problems. In the case of metal films, this problem becomes more serious because internal oxidation of the films can cause their rapid degradation. Furthermore, as thin films of high atomic number metals are being used in combination with carbon in multilayer structures for soft X-ray applications [6-9], where the films must be free of oxides and voids to obtain the maximum density, the present study appears well motivated.

2. Experiment Zr films were deposited in a dual-ion-beam-sputtering system at a base pressure of about 10 -7 Torr. A 3 cm Kaufmann-type ion source [10] was used for deposition. The angle of incidence of the beam was 45 ° from the target normal. The target was 99.9% pure Zr from Cerac Inc. The substrate was Si(100) wafers. Deposition rates were typically 1 - 3 A s -~. The thickness

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ranged between 3000 and 6000 A as measured with a quartz microbalance. An end Hall ion source [11] was used for Ar ÷ cleaning and bombardment o f the growing film. The angle of incidence was 60 ° from the substrate normal. The ion beam average energy was 30 eV and the ion flux was varied in the range 0.18-0.4 mA cm -2, so that the energy input into the growing film ranged between 30 and 120 eV atom -I. The substrate temperature increased to 70 °C owing to the ion beam, although it was cooled by water. The sample holder was rotated at 2 rev min -t in order to improve film uniformity. Substrates were degreased with detergents and alcohol, outgassed and bombarded with a low energy Ar ÷ beam. A shutter was used to sputter clean the target prior to deposition. The optical characterization o f the films was performed by spectroscopic ellipsometry between 1.5 and 4.5 eV so that n and k were determined for each sample. In order to observe the effect of the exposure to the atmosphere, the ellipsometric measurements were performed as rapidly as possible (usually less than ½h) after fabrication and also after several weeks or months. The X-ray photoelectron spectroscopy (XPS) was performed in a VG ESCALAB 21 instrument. The binding energies are refered to Eb(C Is) = 284.6 eV.

3. Results and discussion Figures l(a) and l(b) show n and k as functions of the photon energy for samples grown at the conditions given in Table 1, just after deposition (i.e. less than ½h

© 1993 -- Elsevier Sequoia. All rights reserved

J. F. Trigo et al. / Optical properties of Zr films grown under ion bombardment 3.5

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Fig. l. (a) n and (b) k as functions of the photon energy for Zr films prepared by double-ion-beam sputtering under different conditions (see Table I).

exposure to air. As expected [12], XPS measurements (Fig. 2) showed that in all the cases a ZrO2 layer (about 10-20 A thick) is formed at the surface. Both n and k were observed to depend on the deposition parameters, showing differences (Fig. 1), which usually are presumed to derive from structural effects,

since all the films were amorphous. The refractive index is very similar (about 3-2.5) for all the samples at low photon energies up to about 2 eV but diverges for higher energies except for samples Zr2 and Zr4 which behave similarly. Above 2 eV, sample Zr3 bombarded with 70 eV atom -t shows the largest refractive index,

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J. F. Trigo et al. / Optical properties of Zr films grown under ion bombardment

102

TABLE I. Growth conditions for the Zr films Sample

Deposition rate (A s-t)

Total' thickness (,~)

Reduced energy (eV atom-i)

Ion flux (mAcm -2)

Zrl Zr2 Zr3 Zr4

1.5 2.9 1.2 1.5

5000 5000 6000 3500

-30 70 120

-0.2 0.18 0.4

whereas samples Zr2 and Zr4, which were bombarded with very low (30 eV atom -1) and high (120 eV atom-i) energies show a lower index than the non-bombarded Zrl. The behaviour of k (Fig. l(b)) is complementary to that of n. According to this, sample Zr3 has the lowest k in the whole wavelength range. Interestingly, the results clearly indicate the existence of an optimum reduced energy (about 70eVatom -I) during the deposition at which the properties of the film improve significantly, whereas the use of higher and lower reduced energies during deposition causes lower n and higher k even than for the non-bombarded sample. In fact such behaviour has been observed in other systems [5]. The most significant effect of concurrent ion bombardment at low energies is the variation in the density of the film and the creation of defects. According to this, at low energies the cross-section for forward scattering (i.e. densification) and resputtering is low, whereas at high energies the momentum transfer per atom is high enough to create defects which do not recover during growth [5]. The effect of ion irradiation on the optical properties of the films is better observed by studying the effects of exposure to the atmosphere on n, k and the reflectivity. Therefore the samples were analysed after exposure to the atmosphere for some weeks or months. The changes were significant not only in the optical properties but also in the surface topography. Even simple visual observation showed very clearly that, after some days, samples Zrl and Zr2 (i.e. non-bombarded and bombarded at a very low energy), developed a rough surface with the typical compressive stress relief pattern. When wrinkles occur, rapid oxidation and film peeling will be produced. This initial state for sample Zrl is shown in Fig. 3. A similar effect was observed for sample Zr2 but not for samples Zr3 and Zr4. Obviously the reflectance of the film changed significantly as shown in Fig. 4 where, for comparison, samples Zr3 and Zr2 which were bombarded at 70 eV atom-I and 30 eV atomrespectively have been depicted together. In addition the reflectances deduced from the ellipsometric measurements of the respective fresh samples, assuming

Fig. 3. Compressive stress relief of sample Zrl deposited on Si.

R = ( R s +Rp)/2 have been included. The figure indicates very clearly the rapid degradation of the surface of the sample Zr2 compared with that of sample Zr3 even after a much shorter time of exposure to the atmosphere (2 weeks for sample Zr2 as against 4 months for sample Zr3). In general [2] such improvement in samples Zr3 and Zr4 is attributed to a significant reduction in the void density and consequently to a densification of the films which improve the stability of the films when they are exposed to the atmosphere, reducing the effect of adsorbed water and internal oxidation. The transfer of momentum to the growing film from the ion bombardment promotes a more densely packed structure and reduces the intrinsic stress so that a fiat surface is preserved. According to this the films deposited at 70 eV atom-1 would be the less porous (i.e. the largest n) which would explain the excellent stability of their properties, as shown in Fig. 5. Exposure to air of sample Zr3 for 4

J. F. Trigo et al./ Optical properties o f Zr films grown under ion bombardment

103

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(nm.) Fig. 4. Specular and integrated reflectance for sample Zr2 after exposure to air for 2 weeks and for sample Zr3 after exposure to air for 4 months. The reflectance was deduced from ellipsometric measurements of fresh samples, assuming that R = (R s + Rp)/2.

months caused only a small reduction in the refractive index which recovered after annealing at 210 °C in a poor vacuum (about 10-5 Torr) although some oxidation effect can be seen at low energies. The data were simulated from the Bruggeman effective-medium approximation [13] with respect to the original as reference, indicating that, after exposure to air for 4 months, the main effect was the absorption of some water. After annealing, this water was desorbed but the surface oxidized to form a layer about 700 A. thick containing 30 at.% ZrO2.

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Fig. 5. Optical properties of sample Zr3 after exposure to air for 4 months and posterior annealing at 210°C for 30min in a poor vacuum (about 10 -5 Torr).

Films bombarded with energies higher than 30 eV atom -t show a better stability against air exposure, a lower stress and smoother surfaces.

Acknowledgments Financial support from the Comisi6n Interministerial de Ciencia y Tecnologia (Spain) and Comunidad Aut6noma de Madrid (CAM) through Projects MAT 90/0513 and CO 32/91 are gratefully acknowledged.

4. Conclusions References Zr films deposited by double-ion-beam sputtering have shown enhanced optical properties. An optimum bombardment energy (70 eV a t o m - ' ) has been found for which the highest n and lowest k are obtained in the range 270-500 nm.

1 J. J. Cuomo, S. M. Rossnagel and H. R. Kaufman, Handbook o f lon Beam Processing Technology, Noyes, Park Ridge, N J, 1989. 2 E. Kay, F. Parmigiani and W. Parrish, J. Vac. Sci. Technol. A, 5 (1) (1987) 44.

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J. F. Trigo et al. / Optical properties of Zr films grown under ion bombardment

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9 J. Verhoeven, L. Chunguang, E. J. Puik and M. J. van der Wiel, Appl. Surf. Sci., 47 (1991) 63. 10 H. R. Kaufman, J. M. E. Harper and J. J. Cuomo, J. Vac. Sci. Technol., 16 (1979) 899. 11 H. R. Kaufman, R. S. Robinson and W. E. Huges, Characteristics, Capabilities, and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation, Alexandria, VA, 1987. 12 C. Morant, J. M. Sanz, L. Gal~n, L. Soriano and F. Rueda, Surf. Sci., 218 (1989) 331. 13 B. Drevillon, J. Perrin, R. Marbot, A. Violet and J. L. Dalby, Rev. Sci. Instrum., 53 (7) (1982) 969.