Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

Thin Solid Films, 120 (1984) 249-256 ELECTRONICS AND OPTICS

ELLIPSOMETRIC ON NATURALLY FILMS

249

AND X-RAY SPECULAR REFLECTION STUDIES GROWN OVERLAYERS ON ALUMINIUM THIN

P. B. BARNA, Z. BODd AND G. GERGELY Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O. Box 76 (Hungary)

P. CROCE Institut d’optique,

Centre Universitaire d’Orsay Bdtiment 503. BP43,91406

Orsay Ckiex (France)

J. ADAM AND P. JAKAB* Tungsram Research Center, H-1340 Budapest. P.O. Box I (Hungary)

(Received March 12,1984; accepted June 28,1984)

The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin (d2 = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick (d3 = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Y and A values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for I = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72,6.40,4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n, = 1.77 (alumina) and n3 = 1.58 (hydrated oxide). Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the dz and d, values of the samples was found, owing to hydration.

1. INTRODUCTION

The chemical interaction mechanisms of metal surfaces with various gases and vapours and the structure and properties of the surface layer formed by the interaction have been studied for a long time. A very abundant literature exists in this field. In many cases this interaction produces corrosion which is of great practical importance in the application of metals. A very simple method for determining the thickness and refractive index of a * Present address: Entreprise for Microelectronics MEV, H-1325 Budapest, P.O. Box 21, Hungary. 004~6090/84/$3.00

0 Elsevier Sequoia/Printed in The Netherlands

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thin transparent surface layer is offered by ellipsometry, which requires a knowledge of the substrate’s optical constants. Ellipsometry enables temporal changes in the layer thickness and its refractive index to be followed. It has proved to be practical for the study of thin transparent layers covering highly reflecting metal or semiconductor substrates, e.g. silicon, germanium and nickel. In some cases, however, such as aluminium (literature data collected in ref. 1) the ellipsometric measurement of the optical constants for the Al,O,/Al system resulted in a wide scatter of the experimental data. Anomalies have been found in ellipsometric results requiring repeated measurements over a long time period. It is well known that the surface roughness has a marked effect on the apparent optical constants determined by ellipsometry2. The wide scatter of the optical constants reported by various researchers can presumably be explained by differences in the roughness and in the microstructure of the samples. Another problem in interpreting experimental ellipsometric results lies in the interface between the substrate and the surface overlayer. It is uncertain how far the assumption of an abrupt sharp transition between the substrate and its oxide, as generally assumed, is justified or whether a transition layer has to be considered. A transition layer, e.g. between a semiconductor such as silicon and its oxide or contacts, is well known in semiconductor physics, as has been revealed by Auger electron spectroscopy in depth profiling of silicon metal/oxide/semiconductor structures3. More recently the Si/SiO, system has been studied in detail by numerous investigators using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. In a recent paper Grundner and Halbritter4 have reported on the interface oxide in Nb,O,/Nb. We are not aware of such a transition layer between aluminium and its natural surface layer. In an early XPS study Farrell and Naybour’ referred to a surface layer structure related to oxide covered with contaminants. In our studies on naturally grown surface layers on aluminium films we had to consider the problem of which values from the widely scattered literature data’ (summarized in Figs. 2 and 3) should be used for the optical constants of aluminium. A further problem is whether the temporal changes in the ellipsometric data during the long period of observations can be explained merely by the increase in the oxide layer thickness or whether it is necessary to consider a more complex system. It is well known that on very active materials such as aluminium a thin oxide layer will be formed within a short time even in a vacuum. Ellipsometric results are affected by even 1 monolayer of oxide coverage. Controlled atomically clean as-deposited aluminium thin films should be prepared and studied with the ellipsometer in an ultrahigh vacuum of less than 1O-9 Pa immediately after deposition to determine the optical constants of aluminium. However, this is a very difficult and uncertain procedure. The investigations presented in this paper deal with very flat thin aluminium films of high perfection which were exposed to air (laboratory atmosphere) over a period of 2 years. Our aim was to detect the natural surface layer formed on the aluminium substrates. Another purpose of our work was to ascertain whether a uniform Al,O, layer or a complex structure was formed. To overcome the problems discussed above, the ellipsometric measurements were complemented with soft X-ray scattering studies. Soft X-ray specular reflection analysis6 supplies information on the layer structure as well as on the surface roughness.

STUDY OF NATURALLY GROWN OVERLAYERS ON

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2. PREPARATION OF THE SAMPLES Glass substrates with very flat surfaces were prepared and checked with grazing incidence X-ray reflection’ at the Optical Institute, University of Orsay. Thin films of aluminium were deposited onto these glass substrates at the Institute of Technical Physics, Budapest. Very smooth aluminium films of high perfection were prepared by evaporation at lo-’ Pa onto room temperature substrates with a deposition rate of 5 nm s -I. In this paper results are presented on three selected samples of thickness 59.55,139.3 and 173.5 nm (determined with high accuracy by soft X-ray specular reflection at Orsay). The samples were stored in covered glass cups over the 2 year period of the investigations. Thus the samples were exposed to atmospheric air containing some moisture. 3. X-RAY SPECULAR REFLECTION STUDIES The grazing incidence X-ray specular reflection method has been described in detail in refs. 6 and 7 and papers quoted therein. The accuracy of the method for the analysis of a thin film layer structure depends on the models used and fitted to the experimental scattering intensity curves. It is approximately 0.2 nm in thickness. The naturally grown surface layers formed on the aluminium thin film substrates as well as the aluminium thin films themselves were studied by X-ray specular reflection analysis at Orsay to determine the structure, thickness and roughness of the layers. These measurements were carried out on the samples 6 months after their preparation. A four-layer model, which is actually somewhat more complicated, gives a suitable fit to the experimental curves. The multilayer system revealed by the X-ray reflection analysis is presented in Fig. 1. From the glass substrate upwards the layers are in the following order. (1) An interface is present between the glass and the aluminium film. Its thickness is 0.6-0.9 nm depending probably on the cleaning process of the glass substrate.

rZ?///

gla8s mbstratc

j Fig. 1. Schematic diagram of the system of naturally grown surface layer, aluminium thin film and glass substrate.

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(2) The first layer is the main aluminium substrate film with a thickness of d,. (3) The second layer is most probably a compact alumina layer of thickness dz = 0.8 nm. (4) The upper surface layer is of thickness d3 = 3 nm with a very low electronic density and hence a low compactness (density). It is probably a hydrated oxide in contact with air**‘. The application of the X-ray specular reflection analysis proved to be successful for determining the thickness of the layer system and its component layers. 4.

ELLIPSOMETRIC

MEASUREMENTS

Ellipsometric measurements were made at the Tungsram Research Center using four intense mercury spectral lines and working mainly with an angle of incidence of 70”. Some measurements were carried out by varying the angle of incidence 4 in the range 45”-80”. The latter measurements were confined to J = 546 nm. The initial stages of oxygen interaction with aluminium have been studied in detail by ellipsometry in an ultrahigh vacuum as reported by Hayden et al.” who presented results for O2 exposures of less than 1000 langmuirs. Our ellipsometric studies were made on aluminium films exposed and stored in atmospheric ambient. Measurements were started with the as-deposited thin aluminium films after they had been removed from the preparation chamber and transferred to the ellipsometer. In practice, this means an exposure t = 1 day. The atmospheric exposure leads to the immediate formation of a thin natural surface layer. Later changes in this natural surface layer due to exposure (oxidation-hydration) were followed by ellipsometry over a 2 year period. Conventional ellipsometric measurements yield Y and A, two experimental parameters. They are sufficient for determining the thickness and refractive index of a thin uniform transparent layer with known optical constants covering a flat substrate. With regard to our four-layer system revealed by X-ray specular reflection, only the three upper layers are observed with the ellipsometer. The “thick” aluminium film behaves as a bulk substrate with optical constants n, and k,. The two dielectric layers covering the substrate are considered to be transparent according to Neal and Rehalg but they have different indices of refraction n, (compact alumina) and n3 (low density hydrated oxide). The ellipsometric measurements exhibited changes in !P and A during the 2 year period caused by the atmospheric influence on the multilayer system. Another difficult problem is met with the optical constants n, and k1 of the aluminium substrate. As shown in ref. 1 their apparent values are affected by the roughness and microstructure of the aluminium film. In Figs. 2 and 3 a summary of literature data cited in ref. 1 is presented for the wavelength range 365-600 nm. The full curves in the two figures correspond to the data of Hass and Waylonis”. The problem is to determine which optical constants should be applied for our system. To overcome these difficulties the following assumptions were used. (1) For the two dielectric layers, k, = k, = 0. (2) The aluminium substrate is “perfectly” flat, as verified by X-ray specular reflection analysis.

STUDY OF NATURALLY

GROWN

OVERLAYERS

ON

Al

253

FILMS

oL,----_, 300

. 400

500

600 A (nml

300

400

500

600 A hml

Fig. 2. Optical constants of aluminium compiled in ref. 1: refractive index n1 plotted against 1. New data (11) refer to our results presented in Table I. Fig. 3. Optical constants of aluminium compiled in ref. 1: extinction coefficient k, plotted against 1. New data(r) refer to our results presented in Table I.

(3) The two dielectric layers are homogeneous, isotropic and uniform in thickness (d2 and dJ. (4) The Fresnel equations are valid for the very thin (0.8-3 nm) films which consist of a few monolayers. The Fresnel equations relating to our two dielectric films and the metal substrate film have been presented by Azzam and Bashara”. A personal computer program has been elaborated for solving the:system of Fresnel equations13 with a KFKI model TPA 11 computer in the Institute of Technical Physics, Budapest. The main problem lies in the fact that, in general, two experimental parameters (Y and d) are available for computing the six unknown quantities nl, k,, n,, d2, n3 and d,. The problem was solved using iteration and computer optimization of the parameters, fitting them to 72 experimental data. In principle this is a type of inverse ellipsometry13. In the first step of the iteration the Fresnel equations were solved with the d2 and d3 data determined by X-ray specular reflection analysis of the samples that had been stored for 6 months. Ellipsometric measurements made on our three samples supply in principle six equations for the optical constants n, and k1 of the aluminium substrates. These equations, however, contain only two independent pieces of information, since the Y and A data observed on our three selected samples are very similar. With optimal fitting of calculated data to experimental Y and A results, average optical constants n, and k, were deduced for the three samples. This procedure was used for the four mercury spectral lines. With regard to the refractive indices n2 and n3, for the very compact alumina layer the value n2 = 1.77 was takeni4. This value has been obtained for cl-alumina and corresponds to A= 546 nm. For the strongly hydrated oxide layer the value

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n3 = 1.58 was taken from ref. 9. The dispersion of n2 and n3 was neglected. The dispersion in the Iz = 365-579 nm spectral range is 1.5% according to the data in ref. 14. The same can be assumed for n,. The Y and A values were checked with n3 = 1.8 and n, = 1.6 at 1 = 365 nm and resulted in a change of 0.2% in A. This is equivalent to a 0.2% deviation in d2 + da, which can be neglected. Similar orders of magnitude have been reported for the Si/Si02 system’ 5. Computer evaluations of experimental data are presented in Table I for the four mercury lines. From a comparison of the optical constants n, and k, of aluminium determined for our samples of high perfection with the literature data compiled in Figs. 2 and 3, it was found that our results are close to those of Hass and Waylonis’ I. However, evaluation of the data of Hass and Waylonis under the assumption of a similar surface layer structure results in the optical constants presented for comparison in Table I. When d, = 0.3 nm and d, = 1.1 nm are taken the agreement for the four wavelengths is striking. TABLE I OPTICAL CONSTANTS fl, AND k, OF THE ALUMINIUM SUBSTRATE DETERMINED AT FOUR MERCURY SPECTRAL LINES WITH COMPUTER OPTIMIZATION Wavelength

(nm)

365.0 435.8 546.1 579.0

OF EXPERIMENTAL DATA

n,

k,

Our samples

Samples of Hass and Waylonis

Our samples

Samples of Hass and Waylonis

0.390 0.535 0.95

0.370 0.536 0.92

4.23 4.96 6.40

4.20 4.93 6.36

1.09

1.05

6.72

6.74

For comparison, re-evaluated results of Hass and Waylonis are presented in the Table.

in the second step of the iteration computer optimization of d2 and d3 was achieved for experimental results obtained for the three samples at the four mercury lines over the 2 year period. In the computer optimization the optical constants presented in Table I were used. The d2 and d, values at t = 0.5 years were close to those determined by X-ray specular reflection. The results are summarized in Table II. In our computer optimization procedure the sum of the absolute values of the deviations was minimized, 72 experimental data being evaluated. The average deviation was 0.095” and the maximum deviation 0.29”. As shown in Table II, d, and TABLE II DETERMINATION

OF COMPONENT

COMPUTER OPl-IMIZATION

Sample

dz bm)

d, (nm)

t = 1 day 392 393 396

1.35 1.85 1.35

LAYER

THICKNESSES

d,, dz

AND

d,

AT TIME t OF OBSERVATION

WITH

OF EXPERIMENTAL DATA

d, @ml

dz (nm) d, (nm)

t = 0.5 years 0.76 0.56 1.64

173.5 139.3 59.55

0.95 0.80 0.75

2.70 3.10 3.05

d2 (nm) d, (nm)

d2 (nm) d, (nm)

t = lyear

t = 2 years

0.75 1.25 1.55

3.20 2.95 2.45

0.75 1.25 1.55

3.70 3.30 2.72

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d3 for the sample are very close to, but slightly different from, the average data (d2 = 0.8 and d, = 3 nm) taken to be identical for the three samples in the first step of the iteration (t = 0.5 years). Table II shows the changes in dz and d, over the 2 year period, exhibiting a pronounced tendency: d, and the sum dz + d, increase. The latter value is nearly identical for the three samples, within the dispersion of natural oxidation. During the first 0.5 years d3 increases at the expense of dz, whereas after 1 year d, becomes stabilized as shown in Table II. To check the validity of our model and results, ellipsometric measurements were made at t = 2 years on samples 392 and 396 by varying the angle of incidence 4 at I = 546 nm. YJand A were calculated for these 4 values using the same optical constants (n,, k,, n2 and n3) shown in Table I. Values of d, and d3 are taken from Table II at t = 2 years. In Table III the calculated Y and A values are compared with experimental results. The agreement is quite good and supports our model and the assumed optical constants. TABLE III COMPARISON THE ANGLE

#J(deg)

OF EXPERIMENTAL OF INCIDENCE

AT t =

VALUES

OF ‘y AND

A

FOR A=

546.1 nm

AND VARYING

2 years

Data for sample 392” YJ@es)

80 70 60 45

AND CALCULATED

f#J, MEASURED

Data for sample 396 b A (de&

y (deg)

A Cd-%)

exp.

talc.

exp.

talc.

exp.

talc.

exp.

talc.

41.53 42.33 43.23 44.20

41.56 42.33 43.22 44.10

88.32 129.42 139.10 165.00

88.30 129.33 149.17 165.15

41.62 42.33 42.23 44.03

41.56 42.32 32.33 44.10

88.45 129.32 149.67 164.73

88.44 129.33 149.23 165.18

* d, = 0.75 nm; d, = 3.70 nm. b d, = 1.55 nm; d, = 2.72 nm. Values ofd, and d, taken from Table II. Substrate optical constants: n, = 0.95; k, = 6.40.

5.

CONCLUSIONS

Ellipsometric and X-ray specular reflection studies revealed a naturally grown complex surface layer structure formed on aluminium thin films exposed to air. This structure can be interpreted as a composite system consisting of a compact very thin film (of thickness d,) presumably of Al,O, in contact with the aluminium substrate and a lower density thick upper layer (of thickness d3) presumably of hydrated A&O,. A new computer optimization program was developed to evaluate ellipsometric measurements. The method is based on achieving the best fit of a great number of experimental Y and A data to calculated values. The optical constants n1 and k1 of the aluminium substrate of high perfection were determined for samples kept for 0.5 years by “inverse” ellipsometry for four mercury lines in the wavelength range 365-579 nm. In the evaluation of the ellipsometric measurements the data dz = 0.8 nm and d, = 3 nm determined by X-ray specular reflection and the refractive indices n, = 1.77 and n3 = 1.58 which are valid for u-alumina and hydrated A1,03 respectively were employed. From a comparison of our optical constants n, and k, with the widely scattered literature

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I’. B. BARNA et d.

data for aluminium (owing to the roughness and microstructure of the various samples), a good agreement was found with the results of Hass and Waylonis for the four mercury lines on the assumption of a similar surface layer structure. The ellipsometric measurements carried out over a 2 year period of air exposure revealed that the structure of the naturally grown surface layer (i.e. d, and d3) exhibits a systematic change with time. Variation in the angle of incidence in the ellipsometer over the range 45”~80 fully confirmed the validity of the computer procedure, the assumed model and the optical constants. REFERENCES 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15

P. B. Bama, Z. Bode, G. Gergely, D. Szigethy, J. Adam and P. Jakab, Vacuum, 33 (1983) 93. T. Smith and G. Lindberg, Surj Technol., 8 (1979) 1. J. S. Johannessen, W. E. Spicer and Y. E. Strausser, J. Vat. Sci. Technol., 13 (1976) 849. M. Grundner and J. Halbritter, Surf. Sci., 136 (1984) 144. T. Farrell and R. D. Naybour, Nature (London), 244 (1973) 14. P. Croce, Acta Electron., 24 (1981) 247. L. Ntvot and P. Croce, Reo. Phys. Appl., 15 (1980) 761. J. B. Peri, J. Phys. Chem., 69 (1965) 211,220. W. E. J. Neal and A. S. Rehal, Surface Contamination, Vol. 1, Plenum, New York, 1979, p. 165. B. E. Hayden, W. Wyrobisch, W. Oppermann, S. Hachicha, P. Hofmann and B. A. Bradshaw, Surf. Sci., 109(1981)207. G. Hass and J. E. Waylonis, J. Opt. Sot. Am., 51(1961) 719. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, North-Holland, Amsterdam, 1977, p. 339. R. M. A. Azzam, J. Opt. Sot. Am., 73(1983) 1080. Lundolt-Biirnstein, Vol. 8, Optische Konstanten, Springer, Berlin, 1962, p. 2-99. G. Gergely, G. ForgLs, B. Sziics and D. van Phouc, Ellipsometric Tables of the Si-SiO, System, Hungarian Academy of Sciences, Budapest, 1971.