Investigation of the oxidation behaviour of thin film and bulk copper

......:.:.~::~:::~:::i~ii~i~:.'.5!!~i~i~ii~i~:~:~:::::::::::::.:.: .....

applied surface science ELSEVIER

Applied Surface Science 91 (1995) 152-156

Investigation of the oxidation behaviour of thin film and bulk copper M. O'Reilly *, X. Jiang, J.T. Beechinor, S. Lynch, C. NI Dheasuna, J.C. Patterson, G.M. Crean National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland

Received 19 March 1995;accepted for publication 17 May 1995

Abstract

Thermogravimetry (TGA) and spectroscopic ellipsometry (SE) were employed to examine the oxidation behaviour of Cu. Thermogravimetry was used to studythe oxidation behaviour of bulk Cu (99.9% purity) in the range 250 to 500°C. Within this temperature range the predominant oxidation product was cupric oxide (Cut) and the reaction followed a cubic rate law. The activation energy was calculated to be 71 kJ/mol. At lower temperatures ( < 150°C) spectroscopic ellipsometry was employed to characterise the oxidation kinetics of as-deposited electroless and sputtered Cu films. Analysis of the experimental spectra indicated that the rate at which the oxide grew followed an inverse logarithmic rate law. The oxidation product, which had the typical ruby red hue of cuprous oxide (CuzO), was confirmed as the predominant oxidation product with X-ray diffraction (XRD). The oxide scale which formed had very poor adhesion to the base material. Field emission electron microscopy (FEM) was used to examine the oxide scale topography. The low temperature product was seen to be composed of small, tightly packed grains approximately 60 nm in diameter whilst at temperatures above 300°C whisker-like crystals were also present.

1. I n t r o d u c t i o n

Copper (Cu) is a promising candidate for use in electronic packaging and ultra-large scale integrated (ULSI) devices due to its low resistivity and high electro-migration resistance [1]. Deposition of thinfilm Cu by a variety of routes for microelectronic applications are being investigated, for example sputtering, chemical vapour deposition (CVD) and electroless deposition. Cu films deposited by CVD and sputtering tend to be microcrystalline whilst electroless deposition on activated surfaces produces fine-

* Corresponding author. Tel.: +353-21-904024; Fax: +35321-270271.

grained copper initially, followed by the growth of a columnar structure [2]. Considerable research is now focused on electroless deposition due to the potential for a highly selective, low thermal budget metallisation process. However, one obstacle in the way o f widespread application of Cu technology is that it oxidises at a significant rate at temperatures as low as 150°C, forming a non-protective surface scale. No single theory has yet been formulated to explain the phenomenon o f Cu oxidation but, in general the results of a number of studies indicate that low temperature oxidation ( < 250°C) follows an inverse logarithmic rate law, at intermediate temperatures a mix of both parabolic and cubic behaviour is noted and as the temperature increases the parabolic rate

0169-4332/95//$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00111-5

M. O'Reilly et al. /Applied Surface Science 91 (1995) 152-156

dominates [3]. The activation energy of Cu oxidation below 500°C has been reported to be approximately 85 k J / m o l for bulk Cu [4] and 75.5 k J / m o l for thin films oxidised under low oxygen partial pressures

[1]. The objective of this paper is to examine the oxidation behaviour of both as-deposited electroless and sputtered Cu films. Thermogravimetric analysis (TGA) was the method applied in the study of oxidation kinetics of the pure (99.9%) bulk copper in a dry synthetic air environment for the temperature range 250 to 500°C. The advantage of this method was that the weight change associated with oxidation under various isothermal test conditions was monitored continuously. This reduced the errors introduced by having to repeatedly cool, weigh and reheat the specimen so that enough data points could be accumulated from which a rate constant (Kp) and an activation energy ( E a) could be calculated. Also complementary studies have shown the value of spectroscopic ellipsometry as a method for monitoring oxide layer growth at lower temperatures [5]. This optical method was applied in the present work to the study of the oxidation of sputtered and electroless deposited Cu films at temperatures of 50 to 150°C, where the weight changes due to oxidation were below the lower detection limit of the TGA balance (i.e. 5 /xg).

2. Experimental For the oxidation study of bulk Cu, coupons of 3 × 3 × 2 mm dimensions were used; the surfaces were ground (SiC #4000) and the coupons were degreased and dried. For each test a single sample was suspended from the TGA micro-balance in a platinum crucible. The system was continuously flushed (20 ml/min) with dry synthetic air and in order to reach the isothermal test condition in the minimum time, heating rates of 50°C/min were used. Cu was deposited by both sputtering and electroless deposition onto titanium nitride (TIN) coated Si(100) wafers. The exact structure was TIN(1 /~m)/SiO2(1 brm)/Si. The electroless deposition route used was discussed in Ref. [6] and standard sputtering conditions were applied. The Cu layer

153

thicknesses ranged from 800 ,~ for electroless Cu to 5000 ,~ for sputtered Cu. Both scanning electron microscopy (Hitachi S-4000), to which an energy dispersive X-ray spectroscope was attached (EDX) and X-ray diffractometry (Philips PW 3710 MPD) were used to characterise both the as-deposited films and their oxidation products. A Cu X-ray source was utilised ( K a line where h = 1.54 ,~). Samples were held at a fixed 5 ° in order to maximise sample thickness. For the SE analysis, the as-deposited Cu film samples were heated in an oven at 5°C/min in a nitrogen atmosphere until the pre-set isothermal temperature was reached, at which point the system was flushed with dry synthetic air for the duration of the testing period. Specimens were also cooled in a nitrogen atmosphere. The slower heating rate was used to prevent the Cu films from debonding from the underlying substrate due to thermal shock and the mismatch in their coefficient of thermal expansion ( a ) , i.e. 17 and 8 ppm/°C, respectively. SE spectra were recorded using a phase modulated spectroscopic ellipsometer over the energy range 1.8 to 4.2 eV at a measurement interval of 0.02 eV. The incidence angle employed was 75 °,

3. Results and discussion

Table 1 details the experimental oxidation conditions for the Cu coupons and the as-deposited sputtered and electroless Cu films, respectively. The condition and composition of the oxide scale which developed was found to be temperature and time dependent. Fig. 1 is a SEM micrograph, showing a typical example of the low temperature oxidation Table 1 Experimental test conditions to which bulk copper and as-deposited thin copper films were exposed in dry air environments Specimen

Time (min)

Temperature

Cu coupons

300

250-500

As-deposited sputtered Cu film

25, 50, 75, 100, 300

50, 100, 150

As-deposited electroless Cu film

25, 50, 75, 100

50, 100

(°c)

M. O'Reilly et al. /Applied Surface Science 91 (1995) 152-156

154

*2

i

i

i

i

i

i

-2.S

-3

~

-3.15

-4

-4J

~

.$

1.2

I

I

I

1.3

1.4

1.$

I

I

I

I

1.6

1.7

1.11

1.9

2

I000tT (I/K)

Fig. 1. Topography of a C u 2 0 layer which developed on an as-deposited electroless copper film.

product of the as-deposited Cu films. The scale was composed of tightly packed, fine grains which were identified as C u 2 0 by XRD. In contrast, the high temperature oxidation of the Cu coupons resulted in the development of both C u 2 0 and CuO. The scale was friable and non-adherent. At temperatures above 350°C, whisker-like oxide crystals developed. Fig. 2 shows the results of weight change associated with the oxidation of copper coupons heat treated from 250 to 500°C, Analysis of the curves indicated that the reaction kinetics followed a cubic rate law, M 3 = kpt + C,

where M is the weight gain, kp the rate constant, t time and C a constant. Numerous experimental studies of oxidation reactions have shown the tempera-

Fig. 3. Plot of log kp versus 1 / T for the bulk Cu, where the slope of the line is equal to the activation energy (Ea).

ture dependence of the rate c o n s t a n t , kp, obeys an Arrhenius-type relationship, kp = k 0

exp(-Ea//ez),

where E a is the activation energy, R the gas constant and T the absolute temperature. Fig. 3 is a plot of log kp versus 1 / T for the series of bulk Cu exposures. The extracted activation energy for the oxidation of Cu to CuO was calculated to be 71 k J / m o l , this value appears low in comparison to previously published data. SE was employed to characterise the low temperature oxidation kinetics of both the sputtered and electroless thin films. Fig. 4 shows the experimental SE spectra for an electroless Cu film oxidised at 100°C as a function of oxidation time. A consistent decrease in the measured ellipsometric angle delta, indicative of an overlayer which increases in thick75

1,4 ¸

°:t &50°C

0.4

300°C

i

i

i

~,

,~\

..... 5o m ~ .

~'~ 65 _~

~,~,\ ',% ~,~.-~..~....~ ,~,~,....

----. 75 mlns. -~oo~. -Ref.

6C

0.6

i

"i5(~

-. <-.=..-<

- ~ ~--<..

.

0

50

100

150

200

250

300

350

Time (rain)

Fig. 2. Plot of the weight change gain of Cu coupons associated with their oxidation, from 250 to 500°C, in a dry synthetic air environment under isothermal test conditions.

4'

'

213

'2!8'31.4 Energy (eV)

'

31.9

'

4.4

Fig. 4. Experimental spectroscopic ellipsometry spectra for the electroless copper film oxidised at 100°C as a function of the oxidation time.

155

M. O'Reilly et al./Applied Surface Science 91 (1995) 152-156

0.71]

i

i

I

'

0.08

I

0.58

i

~

i

t

i

i

•

0.07

50"C

----o--- 100"C

0.47

~.

0.06

~,~ 0.35

~-

0.05

~0.23

~

0.04

/

0.12

0.0,

0.03

..... v.xp~i~

,213

0.02

'218'314'319'4.4

1.2

I

I

I

1.4

1.6

1.8

Energy(eV)

Log

Fig. 5• Experimental (dashed line) and best fit (solid line) SE spectra for a sputtered copper film oxidised for 100 min in dry air at 150 and 100°C.

ness with oxidation time was observed for all electroless and sputtered thin-film Cu samples. Using theoretical modelling o f the experimental SE spectra and least squares fitting, the thickness of the overlayer was extracted [7]. Two distinct models were used in the study. For all the as-deposited Cu films with the exception o f the electroless Cu film exposed at 150°C, the model comprised of a C u 2 0 layer on a Cu substrate consistent with XRD analysis. The model used for the as-deposited Cu film oxidised at 150°C was C u 2 0 on Cu on TiN. This was necessary because the film oxidised to such an extent that the underlying TiN has to be taken into account. Fig. 5 shows the experimental and best fit SE spectra for sputtered Cu films oxidised for 100 min in dry air at 150°C. The extracted C u 2 0 layer thickness was 202 ,~,. Fig. 6 shows the change in C u z O layer thickness as a function of time at both 50 and 100°C for as-deposited sputtered Cu. As expected, the higher oxidation temperature produced

1

35

r

lo0"c z

--o--

30

75

I

j

50"C

70

~ @

65

m" 2 0

.~ 60 I 80

I 160 Time (~m)

I 240

55

320

Fig. 6. Extracted Cu20 layer thicknesses for a sputtered copper thin film as a function of time for oxidation at 50 and 100°C.

I

2.4

2.6

the thicker C u 2 0 layer. Numerical fitting of the data in Fig. 6 showed the reaction kinetics to follow an inverse rate law, which is shown in Fig. 7. The regression statistics showed a goodness of fit of 99.2% and 99.3% for the 50 and 100°C oxidations, respectively. In contrast to this, analysis of the corresponding curve for the higher temperature (150°C) produced a linear oxide growth rate. This change in the reaction kinetics on going from 100 to 150°C may be ascribed to the fact that at the higher temperature the oxide scale which formed had flaked off the surface and there was continuous access o f the oxidant to the base substrate. SE analysis of the electroless Cu thin films was confined to the higher temperatures (100 and 150°C) as oxidation of the 50°C sample set proved too slow for times up to 300 min. This phenomenon has previously been reported and was attributed to the fact that electroless Cu films tend to be preferentially oriented along the (111) axis which are more oxidation resistant than either the (100) or the (110) crystal planes [1]. Fig. 8 shows the oxidation be-

25

0

Time

~

Fig• 7. Rate of oxide growth for 50 and 100°C oxidation of sputtered copper.

,tt

10

I

2, 2.2 (mira)

I

i

I

j I 30

I

I

60 T i m e (rains)

90

120

Fig. 8. Extracted Cu20 layer thickness for electroless Cu oxidised at 100°C as a function of oxidation time•

156

M. O'Reilly et al. /Applied Surface Science 91 (1995) 152-156

haviour at 100°C. As can be seen, the Cu film before oxidation had a C u 2 0 layer of 55 ,~. Analysis of the oxidation curve showed that oxidation followed an inverse logarithmic rate consistent with that o f sputtered Cu samples. Extraction of meaningful information from the ellipsometry data for the electroless Cu film exposed at 150°C was limited due to the near complete removal of the Cu layer upon oxidation. Analysis showed that, as the oxidation proceeded the incident light penetrated through to the TiN layer, hence the need for a three-layered ellipsometric model. Calculations showed that complete oxidation had occurred after 50 min. This was confirmed by E D X analysis which identified Ti and Si as the primary elements with a small Cu concentration. SEM analysis showed the Cu to be present as a series of dispersed nodules. This result reiterated the friable nature of the oxide scale.

composed of whisker-like crystals. The scale was composed of both ruby red C u 2 0 and black opaque CuO. Bulk Cu oxidised according to a cubic rate law between 250 and 500°C and an activation energy of 71 k J / m o l was calculated. SE analysis showed that the low temperature oxidation ( < 100°C) of as-deposited sputtered Cu films followed an inverse logarithmic law and at 150°C a linear rate law. It was observed for the as-deposited electroless films that they too oxidised according to an inverse logarithmic rate law at 100°C, the rate being apparently more rapid than that of as-deposited sputtered Cu. This difference was attributed in part to the initial roughness of the electroless film as compared to the sputtered Cu. However, this does not account for the apparent oxidation resistance of the electroless Cu films at 50°C. Further work is in progress to address this issue.

4. Conclusions

References

The oxidation behaviour, chemical state and morphology of bulk, sputtered and electroless Cu have been characterised using thermogravimetry, SE, XRD and S E M / E D X . The oxidation product at 150°C and below was initially a tightly packed fine-grained C u 2 0 layer but its friable nature increased with increasing thickness. At temperatures above 300°C, again the scale showed very poor adhesion to the base material. Examination of the structure showed two layers, a compact inner layer and an outer layer

[1] J. Li, Y. Shaeham-Diamandand J.W. Mayer, Mater. Sci. Rep. 9 (1992) 1. [2] S. Nakhara, C.Y. Mak and Y. Okinara, J. Electrochem. Soc. 140 (1993) 533. [3] A. Ronnquist and H. Fisher, J. Inst. Met. 89 (1960-61) 65. [4] W.H.J. Vernon, J. Chem. Soc. 129 (1926) 2273. [5] M. Rauh, P. Wissman and M. Wolfel, Thin Solid Films 233 (1993) 289. [6] J.C. Patterson, C. Ni Dheasuna, J. Barrett, T.R. Spalding, M. O'Reilly, X. Jiang and G. Crean, Appl. Surf. Sci. 91 (1995) 124, [7] E.A. Irene, Thin Solid Films 233 (1993) 96.