thermal oxidation of ion implanted copper

P. J. RATCLIFFE and R. A. COLLINS: Thermal Oxidation of Ion Implanted Copper 537

phys. stat. sol. (a) 108, 537 (1988)

Subject classification: 61.70; 73.40; S1.2

Department of Physics, University of Lancasterl)

Thermal Oxidation of Ion Implanted Copper

BY P. J. RATCLIFFE and R. A. COLLINS

Twenty two chemical species arc implanted in copper and the subsequent thermal oxidation behaviour is investigated. Ion dose and implantation energy dependences are observed but it is apparent tha t the implant location and profile are of equivalent importance in the determination of the oxidation kinetics. A variety of electrical measurements are made on the oxide layers in a n attempt to verify the p-n junction model proposed by Morris e t al. on the basis of a much smaller range of samples. The present findings yield only limited support to this model and suggest that the oxidation behaviour for a particular implant will be determined by a range of inter-related parameters, in particular, impurity concentration and distribution with depth, implantation in- duced radiation damage and stress, and the chemical nature of the implanted species.

Es werden 22 chemische Elemente in Cu implantiert. Nachfolgend wird das thermische Oxyda- tionsverhalten untersucht. Es wcrden Abhangigkeiten von Ionendosis und Implantationsenergie beobachtet, jedoch die raumliche Tiefe und das Profil sind von ebenso groBer Bedeutung bei der Oxidationskinetik. An einer kleineren Gruppe von Proben werden eine Reihe elektrischer Unter- suchungen a n den Oxydschichten durchgefiihrt, um das Modell des p-n-tfbergangs von Morris e t al. zu verifizieren. Die Ergebnisse stiitzen das Modell jedoch nur in begrenztem MaBe und legen die Vermutung nahe, daIJ das Oxydationsverhalten fur ein bestimmtes implantiertes Element durch eine Reihe wechselseitig abhangiger Parameter bestimmt wird. Hierzu gehoren insbesondere die Konzentration und das Tiefenprofil der implantierten Atomsorte, Strahlungs-induzierte Schaden und mechanische Spannungen sowie die chemische Natur des implantierten Elements.

1. Introduction

A variety of studies have been made on the effect of ion implantation on the oxidation of copper [l to 141. These studies have included a report by the present authors [lo] which indicated the effects of 31 different implants on the oxidation of polycrystalline copper and 9 implants on the oxidation of (110) single crystal copper. One aspect of this work was to determine which ion species were appropriate for more detailed investigation with regard to such variables as ion dose and implant energy and also in order to test the validity of the model proposed by Morris e t al. [ 5 ] t o explain the effect of impurities on oxidation rate. Morris et al. had made electrical measurements on oxidised copper samples which had been implanted with ten different ion species. Normally thermally grown copper oxide acts as an irreversible switch [15 to 171 and Morris et al. found a correlation between the switching voltage and the effect of the particular implant on oxidation. This led to the tentative proposal of a model whereby the observed behaviour was ascribed to the creation of an n-type region in the predominantly p-type oxide such that inward hole diffusion is inhibited during oxide growth. However, there were exceptions to this behaviour, particularly with Xe and Cs implants and it was clear that the model needed testing with a wider range of implant species.

1) Lancaster, LA1 4YB, Great Britain.

538 P. J. RATCLIFFE and R. A. COLLINS

In the present work we have examined the dose and energy dependence of the ion species on oxidation for Cr implantation, Cr being the species which had the greatest effect on oxidation in our previous study. Additionally, we have extended the switching measurements to a range of 22 ion species in order to test the model of Morris et al.

2. Experimental

Experimental details were as described previously [5, 101. Samples were implanted using the Harwell 500 kV Cockcroft-Walton accelerator [ 181. Samples were oxidised for periods from 15 min to 9 h in dry oxygen. Oxygen uptake was measured using the W ( d , p)17*0 reaction [19] using the Harwell 6 MV Van de Graaff accelerator. The oxygen uptake was obtained by calibration against a Ta,O, standard [20]. Elec- trical measurements were as described by Morris et al.

3. llesults and Discussion

3.1 Dose dependence

Relatively little work has been done on the effect of implant dose on copper oxidation. Naguib et al. [3] found that for boron implants, higher doses gave better oxidation resistance. They also reported that Ne implantation reduced oxidation by 20% for doses in the range 5 x 1016 to 1 x 1017 but that the effect of N implantation was more complex, enhancing oxidation for doses less than 1 x 1016 N+ and reducing oxidation for higher doses. For A1 implantation, Rickards and Dearnaley [2] reported reduced oxidation with increasing dose.

In the present work dose dependence was investigated using Cr, which was the impurity that had the greatest effect on oxidation in our earlier work, and also Cd. Cd was implanted a t various doses in the range 2 x 1015 to 1 x l0l7 Cd+ cme2. The samples were oxidised for one hour a t 200 "C. Fig. l a shows that all doses reduced oxidation with the largest effect associated with the highest dose. Similar results were obtained for Cr implantation as shown in Fig. l b . Also shown in Fig. 1 are the maximum concentrations of impurity, expressed as percentages, calculated assuming the implant distributions to be as predicted by LSS theory [21]. Deviations from calculated projected ranges for implanted ions in copper have been reported by Sood and Dearnaley [22]. I n the present work we have determined the implant profile of an oxidised chromium implanted sample using the CAMECA ion microprobe a t

implanted concentration(% - I0 20 30 -

8 1 I I

5 implanted dose (loT6 crr-j) -

a b

Fig. 1. Oxide ratio (ratio of oxide thicknesses on the implanted and unimplanted regions) as a function of ion dose for thermally oxidised a ) cadmium and b) chromium implanted copper

Thermal Oxidation of Ion Implanted Copper 539

L 100

deaih'nm

AERE, Harwell and this is shown in Fig. 2 . The microprobe trace confirms that the Cr is entering the oxide. The metal-oxide interface is located by a sharp change in the Cu trace. The depth and concentration scale were determined on the basis of LSS theory. These will be approximate as the different profiling rates and sputter yields in oxide and metal were not taken into account. Additionally, the implant shape shows a slightly greater spread than predicted by theory. However, Fig. 2 confirms that the basic distribution is approximately Gaussian as required by the LSS model.

The overall effect of greater oxidation resistance with higher implant concentration may be due to stress-related effects or to a change in the oxide conductivity. The former seems unlikely as other systems where implant associated stress is proposed [23, 241 exhibit greatly enhanced oxidation. Conductivity modification may be associated with agglomeration of the implant above a certain dose or may relate simply to the formation of an increasingly n-type region by dispersed material as according to the Morris et al. model.

3.2 Ion energy dependence

Fig. 3 shows the ratio of oxide thicknesses for implanted and unimplanted samples for chromium doses of 2 x 10l6 om-2 for a range of ion energies. Samples were oxidised for 1 h a t 200 "C. An approximately linear correlation is found, with higher energies leading to increased oxidation. Two factors arc relevant here, changes in the effective dose and radiation damage. The greater implantation depth at higher energies would lead to a lower average implant concentration and in view of the dose dependence described above this would be expected to yield less oxidation resistance. Additio- nally, for lower energies, the maximum local implant concentration is not only higher but is nearer the surface and will have a more immediate effect on the oxidation behaviour. In this case the relatively rapid initial oxidation will slow when the oxide- metal interface reaches the chromium-rich region when a mixture of Cu,O and Cr,O, or the spinel CuCr,O, may be formed, resisting further oxidation.

Increased implant energy also leads to increased radiation damage and the importance of this in oxidation studies has been discussed recently by Chenakin [14].

It is clear from these results that neither the total dose nor the ion energy can be viewed in isolation. The implant location and concentration profile resulting from

540 P. J. RATCLIF'BE and R. A. COLLINS

I I I 1 Fig. 3. Oxide ratio as a function of ion energy for chromium implanted copper t l5

._ c e

ion energyiheV1-

the combination of implant parameters employed will determine the subsequent oxidation behaviour.

3.3 Electrical switching measurements

Thermally grown copper oxide acts as an irreversible switch [15 to 171. When a voltage greater than a threshold value is applied across the oxide film there is an instantaneous change in the electrical resistance from, typically, a few megohms to a few ohms. The decrease in resistance has been attributed to Joule heating, resulting in the breakdown of the Cu,O and the formation of a copper monofilament of about 0.5 nm in diameter [15, 161. Morgan and Howes [15] found, by scanning electron microscope examination, that a circular area around the upper current probe (contact wire) had undergone violent heating. Morris e t al. made switching measurements on a variety of oxidised implanted copper samples and observed a correlation between switching behaviour and the effect of the implant on oxidation rate. This led to the proposal of a model involving the creation of a p-n junction in the oxide layer and the inhibition of inward hole diffusion. However, this model was only tentative as a number of factors complicate the issue. Firstly, only ten different implant species were studied. Secondly, the samples were polycrystalline and there was a wide variation in oxide thickness between neighbouring grains of different orientation. Thirdly, the various chemical effects of differing implant species will modify the electrical behaviour. The present work sought to clarify some of these problems.

The experimental method was similar t o that employed by Morris et al. A positive voltage was applied to a gold wire probe touching the top surface of the oxide layer. The switching voltage was measured a t ten points on the unimplanted region and ten points on the implanted region of each sample and the average switching voltage plotted against oxide ratio (ratio of the oxide thicknesses on the implanted and unimplanted regions of a sample). 22 implant species were studied and the results are shown in Fig. 4. It is apparent that the results do not lend support t o the proposals of Morris et al. The results for unimplanted copper and those implants which enhance oxidation lie on the line UU, through the origin, as found by Morris e t al. For implants which were good inhibitors of oxidation they had found that the points lay on a line 11, parallel to UU but displaced by 1.7 V along the voltage axis. In the present work the results for inhibiting implants do not show good correspondence with this line but lie closer to the line UU.

It is tempting to suggest that the limited range of implants studied by Morris et al. fortuitously yielded results close to line 11. Extension of the work to the larger range of implants indicates that the situation is more complex than the simple p-n junction model would suggest. Chemical effects and radiation damage are likely to be relevant


O O 8,

switching voltage (V)-

Fig. 4. Oxide ratio as a function of average switching voltage for a range of implant, species. The horizontal bars indicate the range of switching voltages observed (0 present work, o Morris e t al. [ 5 ] )

in determination of the oxidation behaviour. Additionally, it is clear that oxidation will proceed non-uniformly with depth by virtue of the implant concentration variation with depth, as shown in Fig. 2. Some of these points are illustrated by consideration of those points in Fig. 4 which consti- tute “anomalies” in terms of the result of Morris e t al. [5]. For example, the result for boron lies on the line UU and not on I1 as would be expected. How- ever, the implant depth for this sam-

ple (330 nm), was very much greater than for the other implants and therefore the concentration in the oxide would be both lower and more uniform, so that the con- ditions for formation of a p-n junction would not be present. In the case of lead, the electrical behaviour of the oxide, exhibiting a very high switching voltage, con- trasts with its limited effect on oxidation. However, the oxide had a milky appearance, in contrast to the clear interference colours observed with other implanted species. A similar milky appearance for vanadium implanted copper [ 101 was associated with drastic changes in the physical morphology of the oxide, which enhanced oxidation.

It is clear that verification or otherwise of the p-n junction model is complicated by other factors, and in particular the differing implant distributions with depth. A study involving control of the implant energies, in order to ensure that the projected range was the same for all of the implants would not solve this problem. Firstly the radiation damage would vary and attempts to ensure uniformity for different implants by using varying doses would lead to greatly differing tendencies to form n-type regions, if in fact, this is happening. Secondly, for isochronal annealing experiments, the varying effects of the different implants on oxidation rate will still result in the various oxide interfaces lying in different regions relative to the peak implant concentrations.

A number of measurements were made on Cr implanted samples to observe the change in switching behaviour as the oxide front approached the peak in the implant distribution. Six samples were implanted with 2 x 1 0 1 6 Cr ions cm-2 a t 140 keV but were oxidised for varying times. The sxvitching voltages for both the implanted and unimplanted regions were then measured as shown in Fig. 5. It can be seen that the switching values for the implanted samples lie on or below the line AB for thin oxides but as the oxide thickness approaches the peak in the implant distribution the

542 P. J. RATCLIFFE and R. A. COLLINS

j- In the work of Morris e t al., support for the p-n junction model was also provided by measurements of the electrical charac- teristics of the oxide in the sub-switching voltage range. It was found that unimplanted copper oxide gave a symme- trical characteristic but that samples im-

-20

u,cv, - Fig. 6. Current voltage characteristic of a thulium implanted sample


change along path 2-3 after which a stable characteristic 3-4 was established. I n some cases the change in the resistance of the “device” was as much as los. This phenomenon is similar to the time dependent forming process which has been reported with silicon monoxide and magnesium fluoride thin film sandwich devices [251. We did not investigate this behaviour extensively in the present study but the observa- tions that we did make of this effect serve to emphasise further the complex nature of this type of system and the difficulties of drawing unambiguous conclusions from simple electrical measurements.

To summarise, i t would appear from the present results that the rather small range of implants studied by Morris et al. fortuitously yielded results consistent with a model bared on the establishment of a p-n junction in the oxide layer and the subsequent inhibition of inward hole diffusion. The present results, covering a much larger range of implant species, and including dose and ion energy variation do not rule out the p-n junction concept but indicate that this model is too simplistic to be used in isolation and that the type, depth, and distribution of the implant species are equally important in determination of the oxidation behaviour.

The authors would like to acknowledge stimulating discussions with Dr. G. Dearnaley Aclmowledgement

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[24] S. P. S P E A K D I ~ , R . A. COLLINS, and G. DEARNALEY, Appl. Phys. A35, 99 (1984). [ 2 3 ] R. A. COLLIKS, S. C. EDWARDS, F. J. JOHNSON, A. G. JONES, and I,. D. &ICMAIION, phys. stat.

(Rtceived April 27, 1988)

No. 1 (1963).

Pergamon Press, Oxford 1982 (p. 87).

sol. (a) $6, 353 (1986).

35 pliysica (a) l08j2

thermal oxidation of ion implanted copper

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