Materials Science and Engineering, Al16 (1989) 135-142 135

Effects of N z + Ion Implantation on the Oxidation of Polycrystalline Copper*

D. C. KOTHARI, L. GUZMAN, S. GIRARDI and A. TOMASI

LR.S. T., 1-38050, Povo, Trento (Italy)

S. GIALANELLA

Department of Engineering, University ofTrento, 1-38100, Trento (Italy)

R M. RAOLE and R D. PRABHAWALKAR

R.S.L C., 1.L T., Powai, Bombay400076 (India)

(Received September 16, 1988)

Abstract tion is a promising technique to introduce

30 ke V N, + ions are implanted at doses ranging nitrogen athermally into copper. Prabhawalkar et from 8 ×10 ~ to 4 ×1017 N atoms cm -2 on poly- al. [1] reported X-ray photoelectron spectroscopy

(XPS) studies of nitrogen-implanted copper crystalline and oxygen-free high conductivity copper. Oxidation is carried out at 200 and 600 °C. showing that the implanted nitrogen chemically

combines with copper and may thus form Cu3N. Thermogravimetric measurements are performed for the specimens oxidized at 600 °C. Oxygen The oxidation behaviour of nitrogen-implanted intake is also estimated from the depth profiles copper was studied by Naguib et al. [2]. They

observed the increase in oxidation rate for low obtained from Auger electron spectroscopy mea- dose and very high dose specimens and the surements. Scanning electron microscopy, X-ray decrease in oxidation rate for medium dose speci- photoelectron spectroscopy, X-ray diffraction and mens. They concluded that in general nitrogen optical microscopy measurements are carried ions are not effective in reducing the oxidation out to characterize the oxide layer. It was estab- lished that doses less than 5 x 1016 N atoms cm-: rate of copper, in contradiction with the Wagner- deteriorate whereas doses from 5 × 1016 to 2 × 1017 Hauff valency rule [3].

The present study was undertaken to (i) deter- N atoms cm -z improve the oxidation resistance mine whether nitrogen ion implantation can be of copper. For doses higher than 2×1017 N atoms used to improve the oxidation resistance of cm-: bubbles are observed in a scanning electron micrograph which are harmful for high-tempera- copper, (ii) understand the mechanisms of

changes in oxidation behaviour at various doses ture oxidation resistance. The ideal dose for and (iii) determine the optimum dose for obtain- obtaining better oxidation resistance is concluded ing oxidation-resistant copper. Thermogravi- to be equal to 8 × 1016 N atoms cm -2 for 30 keV metric measurements were performed at 600 °C. N2 ÷ ions. The mechanisms of changes in oxidation The oxidation rate is estimated from the depth behaviour for different doses are briefly discussed, profiles obtained from Auger electron spectros-

copy (AES). Specimens are characterized by 1. Introduction scanning electron microscopy (SEM), X-ray

photoelectron spectroscopy (XPS), X-ray diffrac- The effects of the introduction of nitrogen in tion (XRD)and optical microscopy.

copper on oxidation behaviour have not been studied extensively, probably because of the insolubility of nitrogen in copper. Ion implanta- 2. Experimental details

Two sets of specimens were prepared from

*Paper presented at the Sixth International Conference on commercially available copper and oxygen-free Surface Modification of Metals by Ion Beams, Riva del high-conductivity (OFHC) copper. (Results Garda, Italy, September 12-16, 1988. shown in Figs. 1-12 are for commercial copper

0921-5093/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands

136

specimens and others are for OFHC copper seems to affect the initial growth of oxide film. It specimens.) The specimens were subjected to is well known that the initial oxide film formation normal cleaning and metallographic polishing follows the mechanism of seeding and growth, in procedures. Implantation was carried out using which the oxide first nucleates at a few centres an ion-beam-enhanced deposition machine, and then grows until the continuous film is details of which are reported elsewhere [4]. formed [5]. The number of centres remain con- A 30 keV N2 + ion beam was used for implanta- stant during this period. Thus the implantation tion. Implantation doses ranged from 8 x 10 t5 to increases the number of centres for low dose 4 X 1 0 t7 N atoms cm -2. XRD studies were specimens and decreases them for high dose carried out using an Italstructures XRD machine specimens. employing a Seeman-Bohlin geometry. XPS Figure 2 shows the depth profiles of oxygen studies were performed using a VG ESCA Mark atoms taken using the AES technique for various II model employing M g K a radiation (1253.6 specimens implanted at different doses and then eV). The specimens used for thermogravimetric oxidized at 200 °C. It can be seen that oxygen is measurements were 1 mm thick and were present up to greater depths for specimens implanted on both sides. The specimens were cut implanted at low doses. This result confirms the into pieces 1 cm x 0.5 cm and suspended in a results of Fig. 1 and is similar to the result of recrystallized alumina reaction tube, which was a Naguib et al. [2]. The curves of Fig. 1 are para- part of a Mettler Thermobalance. The oxidation bolic in nature, which is a characteristic of the was carried out in air for 2 h. Specimens attained oxide film formed by cation-anion diffusion the test temperature of 600 °C at the rate of 10 °C through an oxide film. Diffusion through the grain min- 1. The specimens were allowed to cool at the boundary and extended defects also play an same rate. Mass changes during oxidation were important role. They may act as nuclei for oxide monitored continuously with an accuracy of 0.01 rag. Table 1 shows the descriptions of specimens which were extensively analyzed.

1,4

3. Results and discussions

Figure 1 shows a plot of mass gain vs. time for ~ '= various specimens. It can be seen that for the r specimens implanted at 8 x 1015 and 2.4 x 1016 N ~ 0,

atoms cm -2, the mass gain is higher than the unimplanted specimen. The lowest mass gain is for the specimen implanted at the dose of 6 x 1016 °'

atoms cm-2. Also, it can be seen that the oxida- tion rate is different for different doses only during initial period. Once the saturation is '2° ',0 '0 '° ;00

r i v e ~mm)

reached almost all the curves are parallel. The initial .mass gain curves correspond to a discon- Fig. 1. Plot of mass gain vs. time, during oxidation in air of

N2+-implanted copper at 600 °C. Implantation doses (in N tinuous oxide film which does not protect the atoms em-2): 1, unimplanted; 2, 8x10~5; 3, 2.4x10J6; 4, host metal from oxidation. Thus the implantation 1.6 × 10 i7, 5, 6 × 10 ]6.

TABLE 1 Description of specimens and their identification

Specimen Quality of copper Implantation dose Treatment after implantation (N atoms cm 2)

a OFHC 2.4 × 10 I~' oxidation in air (600 °C, 2 h) b OFHC 8 x 10 j6 oxidation in air (600 °C, 2 h) c OFHC 2.4 x 10 j~ oxidation in air (200 °C, 2 h) d OFHC 8 × 10 I~' oxidation in air (200 °C, 2 h) e commercial 2 x 10" lab. ambient for 4 months f commercial 6 x 10" lab. ambient for 4 months

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e_A~,,A COPPER Dens.: 8.96

,,~ 4.gl~ 4.71

N -- ' t~ , . X X I "~ X ~ o~,~\ "\ a-o . . . . 0 "

E ~'~1 I~A " 0 . !~11~ 2,~ :-~- " - . . " . . . . . .

' ~! ~i~ %,. "o_o,o ,~ . . . . . -~...:'-,",,, T \/ ~. . " - ' - "o o ¢ ~ S ~ "

0 i o-o-o-o-.-.-m-n-n-n-n-n-~-n-n-m-n-~-m-n-n-n-q ,00 1Q0 200 3~ 488 5~ DEP[H 5 10 15 20 25 30

SPUTTER. TIME (MIN.) (]~}

Fig. 2. Oxygen depth profiles of specimens oxidized in air at Fig. 4. Computed depth profiles for nitrogen-implanted 200 °C, taken using the AES technique. Implantation doses copper. Implantation doses of 15 keV nitrogen (in atoms (in atoms cm 2): n, 8 x 1()'5; A, 2 x 1()'~'; A, unimplanted; ", cm-2): A, 8 x 10~5; B, 2 x 10~6; C, 4 x 1016; D, 6 x 10~; E, 4 x 10'~; e, 1.85 x 10t7; o, 1.6 x 1017. 8 x 10~6; F, 1.6 x 1017; G, 1.85 x 1017.

observation that the oxide growth rate on some copper planes (e.g. (111 )) is much higher than on other planes (e.g. (311 )) [5]. Thus different grains may exhibit different oxidation rates. After the seeding and growth process a continuous film is formed, as shown in Fig. 3(b). Note that this oxide film is porous in nature, thus allowing diffusion to take place through pores. The porosity of the film can be understood if one considers molar volumes of Cu20 and copper, which are 23.4370 cm 3 and 7.1128 cm 3 respectively. Since the molar volume of Cu20 is greater than that of copper, the oxide

........ film is stressed and later broken, forming pores. Thus, to reduce oxidation, it is necessary to have a continuous oxide film of low thickness. A thin- ner oxide film has less stress, and thus does not get broken, and protects the metal from further oxidation. Therefore, it is believed that the speci- mens exhibiting better oxidation properties have such an optimum thickness of oxide film.

To understand further the effects of different doses, the depth profiles of implanted nitrogen atoms can be considered (Fig. 4). It can be seen that the atomic ratios of nitrogen to copper atoms for the dose of 8 x 10 ~6 atoms cm- 2 is about 0.33, i.e. that required to form Cu3N. Thus, it is

Fig. 3. SEM micrograph of specimens oxidized at 200 °C for expected that the specimen implanted at this dose 2 h: (a) implanted at a dose of 1.6 x 1017 atoms cm 2 and will exhibit better oxidation resistance, which is (b)unimplanted. indeed the case. Since nitrogen is expected to

remain unbonded for higher concentration, it may accumulate at defect sites to form bubbles at

growth; this is observed in the SEM micrograph high doses. Further implantation may burst the in Fig. 3(a). It can be seen that there are more bubbles, exposing the native copper. Also, the oxide nuclei at the grain boundary and it is also bubbles burst during the high-temperature oxida- evident that some grains have more oxide nuclei tion process to expose more native copper and than others. This result correlates well with the increase the oxidation rate. Thus high dose

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implantation greater than 2 × 1 0 1 7 atoms c m -2 The XPS analysis of the above specimens would deteriorate the oxidation properties, which shows the presence of CuO and Cu20 on the was observed by Naguib et al. [2]. To check this surface of the low dose specimen. This can be hypothesis an SEM micrograph of the specimen seen in Fig. 9 in which the copper 2p3/2 spectrum implanted at the dose of 6 x 1017 N atoms cm -2 taken before argon etching shows a peak at was taken (shown in Fig. 5), which exhibits 933.5 eV and a shake up satellite which are both bubbles. Figure 6 shows the same specimen after characteristics of CuO. In Fig. 10 it can be seen argon bombardment showing the burst bubbles, that the X-ray-induced Auger line C u ( L 3 V V ) ,

To compare the oxidation behaviour of low before argon etching, has a peak at 916.5 eV and high dose specimens, two representative which is characteristic of Cu20. On the other specimens were chosen for further experimenta- hand, the high dose specimen shows only Cu20 tion. Specimens implanted at 2 x 1016 N and on the surface as can be seen from Figs. 11 and 6 x 1016 N atoms cm -2 were exposed to a labora- 12. Table 2 shows the characteristic peaks tory ambient for about 4 months. The low dose observed in the above specimens and their specimen showed atmospheric tarnishing where- possible interpretation as compared with the as the other specimen retained its metallic shine, results of ref. 7. The SEM micrographs of these specimens in The XPS spectra after argon ion bombardment Figs. 7 and 8 show a thick oxide layer for the low were taken to check the hypothesis stated earlier dose specimen and no such oxide film for the that low dose ion implantation creates more high dose specimen, centres from which oxide grows. As can be seen

in Figs. 9-12, argon ion bombardment reduces CuO to Cu20 and finally to copper. Reduction of

Fig. 5. SEM micrograph of 30 keV N 2 ÷-implanted copper at a dose of 6 x 10 ~ 7 N atoms cm- :.

Fig. 7. SEM micrograph of specimen (e).

Fig. 6. SEMmicrograph of 30 keV N2+-implanted copper at a dose of 6x l017 N atoms cm -2 and then Ar + ion bombarded. Fig. 8. SEM micrograph of specimen (f).

139

~- =

Ti T; 4oA m 40A

Binding Energy (eV) Binding Energy (eV)

Fig. 9. Copper2p XPS spectra of specimen (e) at various Fig. I I. Cu2p XPS spectra of specimen (f) at various depths, depths.

Cu (L 3 VV) Cu (L 3 VV)

_= ~ _=

I I I I I I I I " 924 .6 91 9.6 91 4.6 909 .6 924 .6 91 9.6 91 4.6 909 .6

9 K inet ic Ene rgy (eV) K inet ic Ene rgy (eV)

Fig. 10. X-ray-induced Auger line Cu(L~VV) of specimen Fig. 12. X-ray-induced Auger line Cu(L~VV) of specimen (e) at various depths. (f) at various depths.

140

TABLE 2 X-ray photoelectron spectroscopy binding energy and kinetic energy observed and identification of phases [6]

Speci- Depth Cu 2p;/_, Cu(L ~VV) Phase present men ( A ) Binding energy Kinetic energy

(eV) (eV)

e 0 933.5 916.5 CuO and CuzO 10 932.5 916.5-918.5 Cu2Oand Cu 40 932.5 918.5 Cu

f 0 932.5 916.5 Cu,O 10 932.5 918.5 Cu 40 932.5 918.5 Cu

Fig. 14. Optical micrograph of specimen (d) showing both implanted (right) and unimplanted (left) regions.

Fig. 13. Optical micrograph of specimen (c) showing both implanted (left) and unimplanted (right) regions.

Fig. 15. SEM micrograph of specimen (c).

the oxides was also observed by Panzner et al. [7]. Similarly, the low dose ion implantation can have the effect of reduction on the already present partially protective oxide layer. This brings native copper to the surface, which is more prone to oxidation, and thus increases the number of centres for the growth of oxide. The defects created due to ion bombardment also help to create new centres of oxide growth.

Similar experiments were carried out on OFHC copper implanted at representative doses of 2.4 x 1016 and 8 x 1016 N atoms cm -2. Optical micrographs of implanted and unimplanted regions of OFHC specimens oxidized at 200 °C are shown in Figs. 13 and 14. For the lower dose Fig. 16. SEM micrograph of specimen (d).

specimen the implanted portion is darker, whereas for the high dose specimen the un- shows more scales and porous oxide film. Figure implanted portion is darker. The high dose speci- 17 shows the oxygen depth profile taken for the men appeared to shine, while the low dose same specimens. The depth up to which oxygen is specimen showed more tarnishing than the un- present is higher for the low dose specimen and implanted specimen. Figures 15 and 16 show the lower for the high dose specimen as compared SEM micrographs of the same specimens taken with that of the unimplanted specimen. Small- in the implanted region. The low dose specimen angle XRD spectra of OFHC copper implanted

141

- - - play a useful role even after the continuous oxide ...... - - - - ~ ' ~ o film is formed, which can be seen using the

i -o ,O.o Wagner-Hauff theory [3]. Within the framework , \ \ \ of this theory, C u 2 0 can be considered as a TN o\\ X"X ° \ p-type semiconductor having defects as a result E o \ ~ of the presence of cupric Cu 2+ ions. The oxida-

\ " " x ) tion takes place because of the cation-anion ' ~ " ~ diffusion through these defects. The number of

~ ' , °"-X defects can be reduced by adding either a cation <a.o~ % \ ' -" -° \ . O-o of lower valency or an anion of higher valency.

0 ~ ~ , , h . - . ~ Since nitrogen has a valency of - 3 in Cu~N and 1 0 2 0 3 0 4 0 5 0 6 0

SPUTTER. TIME (MIN.) oxygen has a valency of - 2 in CuO, nitrogen is expected to be helpful in reducing the oxidation

Fig. 17. Oxygen depth profiles of OFHC copper oxidized in air for 2 h at 200 °C. Implantation doses (in N atoms cm-2): ra te . However, Fig. 1 does not provide enough o, 8 x 10~*,; m, unimplanted; Ez, 2.4 x 10 l'. evidence for this prediction, since all c u r v e s a r e

almost parallel in the later period of oxidation. The reason for the almost parallel nature could

37 49 6 ~ 73 be due to the fact that more diffusion could occur I I I I through pores rather than through the oxide

, layer. Another reason could be the instability of Cu3 N at the temperatures reached during the oxidation process. However, during the room- temperature oxidation process, nitrogen can play an important role in reducing the diffusion

"-- through the oxide layers.

"rn

4. Conclusions

2 It was shown that nitrogen affects the initial oxidation rate corresponding to the nucleation

1 ~ 2 , , and growth process. Once a continuous oxide 2 2 1

film is developed, nitrogen helps by reducing the I I I I number of defects in p-type semiconducting 37 49 61 73 Cu20, thereby reducing the diffusion through the

28° defects and thus reducing the growth of oxide Fig. 18. Small-angle XRD spectra for specimens (a) and (b). film. It was also shown that the grain boundary Phases: 1. Cu20: 2. CuO. acts prominently as a nucleus for oxide growth

and some grains have a higher number of growth centres than others. Further, it was shown that on

at doses of 2.4 x 1016 and 8 x 1016 N atoms cm -2 a thick oxide film scales and pores are produced and then oxidized at 600 °C are shown in Fig. 18. as a result of the different molar volumes of Cu~O The low dose specimen shows the presence of and copper. both CuO and Cu20. However, the XRD spec- It has been established that 30 keV N2 + ions trum of the high dose specimen shows an amor- implanted at doses between 5 × 10 L6 and 2 x 10 17

phons surface having one peak corresponding to N atoms cm 2 improve the oxidation resistance Cu20. Thus it is well established that the opti- of copper. At low doses oxidation properties mum dose for 30 keV N2 + ions, necessary for deteriorate because of the bombardment-induced better oxidation properties of copper, is about reduction of the initially present partially protec- 8 × 1016 N a t o m s c m -2 tive oxide film and because of the creation of

Up to now only the effects of ion implantation more defects, both of which lead to an increase in during the seeding and growth process have been the number of centres for nucleation and growth considered. However, nitrogen is expected to process of the oxide. Higher dose specimens

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show bubbles which are harmful for high-tem- References perature oxidation. Finally, it is concluded that 1 P. D. Prabhawalakar, D. C. Kothari, M. R. Nair and P. M. the ideal dose to obtain better oxidation resis- Raole, Nucl. lnstrum. Methods B, 7/8(1985)147. tance is about 8 x 1016 S a toms cm -2 for 30 keV 2 H. M. Naguib, R. J. Kriegler, J. A. Davies and J. B. N2 + ions. Mitchell, J. Vac. Sei. Technol., 13 (1976) 396.

3 O. Kubaschewski and B. E. Hopkins, Oxidation of Metals and Alloys, Butterworths, Guildford, 1962.

4 B. Margesin, E Giacomozzi, L. Guzman, G. Lazzari and V. Zanini, Nucl. Instrum. Methods B, 21 (1987) 566.

Acknowledgments 5 J. Oudar, Physics and Chemistry of Surfaces, Blackie,

One of the authors (D.C.K.) has carried out this Glasgow, 1975. 6 CRC Handbook of Physics and Chemistry, 59th edn.,

work with the support of the "ICTP Programme CRC, Boca Raton, Florida, 1979. for Training and Research in Italian Laboratories, 7 G. Panzner, B. Egert and H. P. Schmidt, Surf. Sci., 151 Trieste, Italy". ( 1985) 400.