Solar Cells, 5 (1981 - 1982) 81 - 86 81

THERMAL OXIDATION OF SPUTTERED TiN DIFFUSION BARRIERS

DAG SIGURD*, ILKKA SUNI t , LESZEK WIELUI~ISKI$, MARC-A. NICOLET and HERMANN VON SEEFELD §

California Institute of Technology, Pasadena, CA 91125 (U.S.A.)

(Received May 14, 1981 ; accepted May 22, 1981)

Summary

The oxidation of thin (up to 3000 £ ) TiN layers was studied exper- imentally. It was found that the TiN layers transform into TiO 2 with a rutile structure during heat t reatment at temperatures around 500 °C in oxidizing ambient. The growth of the oxide layer has a dependence that follows the square root of time. Measurements of the optical properties of the oxide layers so formed indicate that they can be used as antireflecting coatings on solar cells.

1. Introduct ion

Widespread terrestrial application of photovoltaic technology requires low cost and long-term reliable solar cells. One area of concern in this context is solar cell metallization, where it is important to be able to use in- expensive metals and still to maintain a long-term reliability comparable with the well-known T i -Pd -Ag metallization scheme. The introduction of diffu- sion barriers separating layers at critical interfaces can be instrumental in designing such metallization systems [ 1 ] . TiN has been shown to act as an efficient barrier for interdiffusion between silicon and various metals [2 - 5] . TiN also has a low electrical resistivity and makes a good contact to silicon with a contact resistivity low enough for solar cell applications [6, 7] .

During solar cell processing, and to a lesser extent in service, the solar cell metaUization is subjected to elevated temperatures in a chemically active environment. The stability of TiN under such conditions is of importance in assessing the viability of diffusion barriers made of this material. TiN is

*Permanent address: Insti tute of Microwave Technology, 8-10044 Stockholm 70, Sweden.

tPermanent address: Semiconductor Laboratory, Technical Research Centre of Finland, Otakaari 5A, SF-02150 Espoo 15, Finland.

SPermanent address: Insti tute of Nuclear Research, Warsaw, Hoza 69, Poland. § Permanent address: Fachbereich Physik der Technischen Universit~t Miinchen,

8046 Garching bei Miinchen, F.R.G.

0379-6787/81/0000-0000/$02.50 © Elsevier Sequoia/Printed in The Netherlands

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stable against metallic compound formation [2 - 5]. However, it is unstable against oxidation, which can be understood from the values of heats of for- mation o f - - 8 0 kcal mo1-1 and --219 kcal mol -~ for TiN and TiO2 respec- tively [8]. Hence in this study we investigated the behavior of TiN subjected to heat t reatment in oxidizing ambients.

2. Experimental details

TiN films for the evaluation of oxidat ion kinetics were deposited onto single-crystal silicon ~100) substrates. In a few cases, films were prepared on oxidized silicon wafers in order to study the effect of the substrate on the oxidation behavior. A few samples with a thin substrate of fused quartz were prepared for optical measurements.

All films were deposited by reactive sputtering from a pure metallic t i tanium target in a premixed A r - N 2 ambient. The sputtering source was a planar magnetron-type r.f. source 3 in in diameter yielding a deposition rate for TiN of the order of 100 - 200 A min -1 . The electrical properties of TiN are known to be critically dependent on the conditions during sputtering [6]. All films used in this study were deposited in a 30%N2-70%Ar gas mix- ture at a total pressure of 8 mTorr. The r.f. power level varied between 250 and 500 W, resulting in a variation of the electrical resistivity between 300 p~2 cm (250 W) and 60 p~2 cm (500 W). The variation in resistivity is associated with a variation in the oxygen content of the TiN films [6]. Most films used in this study had an initial TiN thickness of 2500 •, yielding an oxidized layer thickness of approximately 4000 A after a complete oxida- tion. The films for the optical measurements were deposited to a thickness of about 300 £ in order to fit the first quarter-wavelength reflection min- imum to the visible wavelength region.

The TiN samples described above were heat treated in an open quartz tube furnace in a flow of dry or wet oxygen. For wet oxidation the dry oxygen flow was bubbled through hot deionized water prior to entering the furnace tube. Heat t reatment temperatures in the range 400 - 600 °C were used.

The samples were primarily analyzed by backscattering, spectrometry (BSS) [9]. With this method the composition and the thickness, expressed in the amount of material per unit area, of the oxide layers were determined. X-ray diffraction was used for phase identification. The optical reflectivity of the oxide layers was determined with standard equipment for this purpose.

3. Results and discussion

TiN transforms into t i tanium oxide during heat t reatment in oxidizing ambient. From BSS measurements it was found that the composition of the

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t i tanium oxide is Ti:O = 0.5 + 0.03. It was also observed that the nitrogen initially in the sample is absent in the oxide layer and it is concluded that the nitrogen atoms leave the sample during oxidation.

Information on the structure of the oxide was obtained from X-ray dif- fraction measurements. Table 1 gives the measured d spacings from the six strongest diffraction lines. The corresponding values from ASTM [10] are also given in the table. The close match between the measured d spacings and the corresponding values from the literature positively identifies the oxide layer to be TiO2 with a rutile structure. This is the TiO2 structure that has also been identified in oxidation experiments with metallic t i tanium [11, 12].

TABLE 1

X-ray diffraction measurements on t i tanium oxide layers compared with the corresponding values from ASTM

Measurements a, A S T M [ 10 ] d (A)

d (A) h k l

aThe accuracy in the measured d values is estimated to be +1%.

The BSS method was also applied to monitor the progressive growth of the oxide layer. The results of such measurements are presented in Fig. 1, where the oxide layer thickness is shown as a function of the square root of the oxidation time. Results for three different temperatures for dry oxida- t ion (500, 550 and 595 °C) and for one temperature (465 °C) for wet oxida- t ion are included in the figure. For the conversion of the BSS measurements from areal density to thickness, the oxide bulk density of 4.2 g cm -3 was used [ 8].

The results presented in Fig. 1 include samples of various initial thick- nesses and resistivities and also deposited onto both silicon and oxidized silicon wafers. It was a general observation that the oxidation kinetics did not depend significantly on any of these parameters.

At all temperatures studied, the data can be fit ted to the simple relation

W = K t 1/2

where W is the oxide thickness, t is the oxidation time and K is a coefficient dependent on temperature. For dry oxidation the coefficient K was extracted from the data in Fig. 1 and the resulting values are shown in Fig. 2. In this figure the logarithm of K is plotted as a function of 1000/T

3.23 3.25 110 2.48 2.49 101 2.17 2.19 111 1.67 1.69 211 1.47 1.48 002 1.34 1.35 112

84

,~w4

2OOO

"" L i.--

iz i I000[--

I

/ 595°C

/ /

/

550°C

/ i

x / I ~ ' 465°C, WET ,!

y ~ I

500°C

0 I0 20 (t) I/2 [rain 1/2]

Fig. 1. Titanium oxide layer thickness W as a function of the square root of the oxidation time: A, O, 0, oxidation in dry oxygen at the indicated temperatures; [2, oxidation in wet oxygen at 465 °C.

where T is the absolute oxidation temperature. The data points can be fitted to a straight line, indicating that K is thermally activated with an activation energy of 1.1 + 0.1 eV. The fact that the kinetics of oxidation follow a t 1/2 dependence indicates that the rate of oxidation is limited by diffusion through the growing layer. Under the assumption that one moving species dominates the transport it may be inferred that the activation energy for that diffusion process is exactly double the value given above, i.e. 2.2 eV [13] . This is in close agreement with what has been suggested for the diffu- sion of oxygen in TiOx during the oxidation of metallic titanium (51 kcal mo1-1 = 2.2 eV) [14] .

The oxidation of TiN in wet oxygen was not studied as extensively as the above-discussed growth of t i tanium oxide in dry oxygen. However, from the data shown in Fig. 1 it is clear that the kinetics for wet oxidation are faster than those for dry oxidation. Preliminary experiments indicate that the activation energy is approximately 0.7 eV.

Titanium oxide is used as an antireflection (AR) coating on solar cells and in optical systems very generally. The fact that TiN can readily be

85

500

400

300

-% 200 7c

E

x-" I00

50

20 I I I I.I 1.2 k3

I O00 [ K_ I] T

Fig. 2. The coef f ic ien t K in t he r e l a t ionsh ip W = K t 112 ( ex t r ac t ed f r o m Fig. 1) as a func- t i o n o f inverse abso lu t e o x i d a t i o n temperature. The data show t h a t E a = 1.1 eV.

transformed into TiO2 opens up the possibility of making a twofold use of a TiN layer in solar cell applications, according to the following scheme. TiN is first deposited everywhere on the bare cell. Next the contact grid area is covered by a mask that inhibits oxidation. Subsequently the TiN outside the masked area is converted to t i tanium oxide that serves as an AR coating. The metal grid is then deposited onto the remaining TiN which serves as a diffusion barrier there. Conceivably the metallization itself could serve as a self-aligning oxidation mask. Experiments aimed at assessing the viability of this concept have been initiated.

Of primary importance are the optical properties of the t i tanium oxide layer formed by oxidation of TiN. Measurements of the reflectivity of a TiO2 layer approximately 500 A thick prepared in dry oxygen show a reflec- tance minimum at a wavelength of 6200 A. The initial TiN thickness was cal- culated from bulk density values to be 300 A. The reflectivity at the min- imum was 2% which compares favorably with that of standard TiOx AR coatings.

4. Conclusion

We have shown that reactively sputtered TiN layers can be transformed into t i tanium oxide during heat t reatment at temperatures in the range 400 - 600 °C in an oxidizing ambient. The oxide so formed was found to be T i O 2 with a rutile structure.

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Titanium oxide can be used as an AR coating, so that in concept the oxidation of TiN offers a possibility of obtaining an AR coating and a contact barrier layer by selective oxidation from a single layer of TiN.

Acknowledgments

The authors wish to thank M. M~/enp~/~, California Institute of Technology, for performing X-ray diffraction experiments and P. Iles, Applied Solar Energy Corporation, for optical reflectivity measurements.

This research was supported in part by the Department of Energy and monitored by Sandia Laboratories, Albuquerque, NM (H.T. Weaver and M.B. Chamberlain). One of the authors (D. S.) wishes to acknowledge the support of the National Swedish Board for Energy Source Development.

References

1 M.-A. Nicolet, Thin Solid Films, 52 (1978) 415. 2 H. yon Seefeld, N. W. Cheung, M. Mi/enp~'~ and M.-A. Nicolet, IEEE Trans. Electron

Devices, 27 (1980) 873. 3 N.W. Cheung, H. yon Seefeld and M.-A. Nicolet, in J. E. E. Baglin and J. M. Poate

(eds.), Proc. Syrup. on Thin Film Interfaces and Interactions, Electrochemical Society, Princeton, NJ, 1980, p. 323.

4 M. Wittmer, Appl. Phys. Lett., 36 (1980) 456. 5 N. W. Cheung, H. yon Seefeld, M.-A. Nicolet, F. Ho and P. Iles, J. Appl. Phys., 52

(1981) 4297. 6 M. Mi/enp~, H. yon Seefeld, N. W. Cheung, M.-A. Nicolet and A. G. Cullis, in J. E. E

Baglin and J. M. Poate (eds.), Proc. Syrup. on Thin Film Interfaces and Interactions, Electrochemical Society, Princeton, NJ, 1980, p. 316.

7 M. M~/enl~/~, M.-A. Nicolet, I. Suni and E. G. Colgan, Sol. Energy, 27 (4) (1981). 8 H. J. Goldschmidt, Interstitial Alloys, Plenum, New York, 1967, pp. 219,357. 9 W. K. Chu, M.-A. Nicolet and J. W. Mayer, Backscattering Spectrometry, Academic

Press, New York, 1978. 10 ASTM File 21-1276. 11 K. Hauffe, Oxidation o f Metals, Plenum, New York, 1965, p. 211. 12 H.J. Goldschmidt, Interstitial Alloys, Plenum, New York, 1967, p. 369. 13 J. M. Poate, K. N. Tu and J. W. Mayer (eds.), Thin Films -- Interdiffusion and Reac-

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