Materials Science and Engineering, B I O ( 1991 ) 165-169 165

Diffusion of 195Au in amorphous W-N diffusion barriers

W. D6rner* and H. Mehrer lnstitut fiir Metallforschung, Universitiit Miinster, Wilhelrn-Klemm-Str. 10, W-4400 Miinster (F.R.G.)

P. J. Pokela, E. Kolawa and M.-A. Nicolet California Institute of Technology, Applied Physics and Electrical Engineering 116-81, Pasadena, CA 91125 (U.S.A.)

(Received February 22, 1991 )

Abstract

The diffusion of gold in amorphous Ws0N20 deposited onto oxidized silicon substrates is investigated by the radiotracer method using ion beam sputtering as a serial sectioning technique with high depth resolution. The diffusivity in the investigated temperature range (733-775 K) is given by D(195Au) = 5.83 × 10 -9 exp( - 195 kJ mol-I/RT) m 2 s- J. A comparison with available data on diffusion in amorphous metallic alloys shows that gold diffusion in amorphous W-N is indeed an extremely slow process.

1. Introduction

With increasing miniaturization the thermal stability of microelectronic devices is limited by interdiffusion and reaction of adjoining layers. Metal overlayers such as aluminium, silver, pal- ladium or gold employed in practical applications as interconnections have a low reaction tempera- ture with silicon or silicides [1-3]. Thin film diffu- sion barriers are needed to form stable interlayers which prevent a degradation of the contact. Ideally, a diffusion barrier should be elec- tronically transparent but atomically opaque. A diffusion barrier does not eliminate the thermo- dynamic instability of the metal-semiconductor combination. Real barriers are kinetic barriers which minimize interdiffusion and hence the rate of a degradation process.

Amorphous metallic alloys are attractive as diffusion barriers because their resistivity is low enough and because they contain no grain bound- aries or dislocations which are known to produce diffusion short circuits in crystalline materials. Diffusion below about one-half of the melting temperature of a crystal is dominated by short circuit diffusion rather than by bulk diffusion.

W-N in its crystalline form is an interstitial compound. In the composition range 18-38 at.%

* Linde AG, W-8023 Hrllriegelskreuth, F.R.G.

N reactive sputtering produces an amorphous structure [4, 5]. An amorphous film of W100-xNx as a thin film diffusion barrier of about 100 nm between a gold overlayer and a silicon substrate prevents metallurgical reaction during an anneal- ing treatment above 973 K for 30 min [4]. By contrast the Au-Si eutectic temperature is 633K.

In this paper we report on a systematic diffu- sion study of 195Au in amorphous Ws0N20 layers deposited on oxidized silicon substrates. The experiments are carried out by the radiotracer method using 195Au as radioisotope and ion beam sputtering as serial sectioning technique. This technique permits the measurement of diffusion lengths over a few tens to about several hundreds of nanometres. This high depth resolution is necessary since diffusion in the barrier materials is expected to be rather slow and since the total thickness of the barrier film is only about 600 nm.

2. Preparation of amorphous W-N films

Substrates (0.8 cm in diameter and 500 /~m thick) were prepared at the University of Mfinster from an Si(111) wafer by ultrasonic drilling. The deposition surface was lapped with 15, 5 and 1/~m grade AI203 and then polished with colloidal silicic acid (Syton). The deposition of the amor-

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166

Fig. 1. Read pattern of an as-deposited amorphous Ws0N211 film of about 600 nm thickness on an oxidized silicon sub- strate.

p h o u s W80N20 films was performed at the Cali- fornia Institute of Technology. Prior to the deposition the substrates were thermally oxidized to an oxide layer thickness of about 400 nm. The deposition of the amorphous W-N film was car- ried out in an r.f. sputtering system using planar tungsten cathodes and a premixed Ar-N 2 gas ambient at a pressure of 1.33 Pa. Details of the deposition procedure can be found elsewhere [4, 5]. The deposited film thickness measured on a Dektak profilometer was about 600 nm.

Structural information was obtained from X-ray diffraction analysis in a Read camera using Cu Ka radiation. A typical Read pattern of an as- prepared sample is shown in Fig. 1. Bragg reflection points from the silicon substrate and diffraction halos due to the amorphous WsoN20 film can be seen in the picture.

3. Diffusion experiments

The diffusion experiments were performed at the University of Miinster. A very thin layer of the radioisotope 195Au was deposited onto the amor- phous W-N films in a vacuum chamber. The isotope was obtained from two different sources, as follows. In a first series of experiments 195Au was obtained from the mass 195 of the mercury beam produced by the ISOLDE facility at CERN in Geneva. 195Hg was implanted with 60 keV into a tungsten foil. Within a few hours the 195Hg decayed into its longqived daughter nuclide 195Au with a half-life of 183 days. Later on the tungsten foil was used as an evaporation boat from which an adequate amount of the J95Au implant could be evaporated onto the samples. In a second series of experiments 195Au was pur- chased as chloride (NEN Corporation). Several microlitres were dried on a tungsten foil which was then used as an evaporation boat. The results revealed no systematic differences between the two radioisotope deposition techniques.

The diffusion annealings were carried out in a resistance furnace with the samples at various temperatures between 726 and 775 K sealed in quartz ampoules under argon atmosphere.

The concentration profiles of ~95Au in diffused samples were determined by a sputter sectioning technique employing a sputtering rig and a pro- cedure described elsewhere [6, 7]. The total penetration distance was evaluated from the total weight loss during sputtering, the specimen area, and the density of the amorphous Ws0N20 which is 17.1 g cm 3 [5]. Typical weight losses during sputtering of about 70 ~tg were determined within + 1 ~g using a Sartorius microbalance. Typical sputtering rates for amorphous Ws0N20 a re between 0.39 nm min -~ for an acceleration voltage of 200 V and 1.6 nm min-~ for 500 V.

A constant fraction of the material removed by sputtering was collected on a Nalophan foil (Hoechst AG, Wiesbaden) which was advanced stepwise and subsequently cut into pieces corre- sponding to each section. The count rate of ~gSAu in each section was determined in a scintillation counter using Opti-Fluor O as scintillation liquid.

The non-crystallinity of several W-N films was checked after the sputtering process following the diffusion runs by X-ray diffraction analysis at the California Institute of Technology. From four samples investigated (diffused at 726, 742, 763 and 775 K) the Read patterns for the tempera- tures 726, 742 and 763 K disclosed no signs of crystallinity. Only the sample diffused at the highest temperature (775 K) was partly crystal- lized.

4. Results and discussion

Penetration profiles of 195Au in WsoN20 films are displayed in Fig. 2. Except for the first 10-20 nm the profiles match a gaussian concentration distribution

c(x, t)= co exp -

In eqn. (1) c o denotes tion distance and OA 2 the variance of the distri- bution. The deviations near the film surface are probably due to surface hold-up and/or to diffi- culties in dissolving gold.

Fits of eqn. (1) to the deeper parts of the measured profiles are shown as full lines in Fig. 2. The evaluation of the diffusivity from the fairly

(1)

a constant, x the penetra-

shallow penetration profiles (o A in Fig. 2 is between 20 and 30 nm) is slightly complicated by the inherent depth resolution of the sputtering process which is typically of the order of a few nanometres. The variance is then given by

O'A 2 = 4Dt+ oB 2 (2)

where of is the variance of the broadening due

x / nm

1.0 20 30 40 ~ r i i i i i - i i

50

U3

g

g

K, 1 1 0 6 5 m i n

ruu ~ : ~ 9 ~ 7 K , 1544Omen

0 5 10 15 20 25

x ~ / 10 ~ nm ~

Fig. 2. Concentration depth profiles of "~SAu in amorphous Ws0N20 alloys after various thermal annealings: curve 1, 775 K, 8495 min; curve 2, 763 K, 9675 rain; curve 3, 753 K, 19 714 rain; curve 4, 753 K, 11 065 rain; curve 5, 743 K, 15 440 min; curve 6, 726 K, 39 807 min. In order to show several penetration profiles in one plot they have been shifted in the direction of the ordinate by arbitrary amounts with respect to each other.

167

to the measuring process, D the diffusivity and t the duration of the diffusion annealing. We used a value of %2= 28 nm 2, which had been deter- mined by sputtering an undiffused 195Au-de- posited sample. The diffusion coefficients determined from eqns. (1) and (2) are listed in Table 1. The correction due to oB 2 amounts to not more than 3-6% in the D values.

The temperature dependence of the D values is displayed in the Arrhenius diagram of Fig. 3. The D value at the highest temperature inves- tigated is shown in brackets, because the Read pattern after the diffusion anneal showed that the

T / °C

-21 520 500 480 460 10 . . . . . . .

7

E

E3

I 0 -22

-23 10

12.5

i n ~8oN~o

1:]=195 k J m o l I

0o=5.83.10 ~9 m~s -~

13.0 !B.5 1A.O

T -1 / 10 ~4 K-I

Fig. 3. Arrhenius diagram for diffusion of L~SAu in amor- phous Ws0N20 films ( Q = 195 kJ mol l; D 0 = 5 . 8 3 × 10 ') m 2 S - I ) .

T A B L E 1

'9-~Au diffusion in amorphous WsoN2o diffusion barrier material

T t OA 2 OA 2 - oB 2 D 2(Dt) b'2 X-ray (K) (min) (nm 2) (nm:) (m 2 s - ' ) (nm) analysis

775 8495 837.9 809.9 (3.97 x 10- '2) 28.5 763 9675 671.1 643.1 2.77 x 10 -22 25.4 753 19714 780.7 752.7 1.59 x 10 -22 27.4 753 11065 555.0 527.0 1.98 × 10 -22 23.0 743 15440 537.3 509.3 1.37 × 10 -22 22.6 742 22915 714.9 686.9 1.25 x 10 -22 26.2 733 44070 861.0 833.0 7.88 x 10 23 28.9 726 39807 554.1 526.1 5.51 x l 0 23 22.9

Partly crystalline Amorphous

Amorphous

Amorphous

168

W-N film was no longer completely amorphous. Within the experimental accuracy the data exhibit an Arrhenius-type behaviour. A least-squares fit of the D values (omitting the highest one) yields

D(195Au) = 5.83 x 10 -'~

195kJmol - I / 2 -,

×exp R T } m s (3)

where T denotes absolute temperature and R the universal gas constant.

Unfortunately a comparison of the present result with diffusion data in tungsten-based amor- phous materials is not possible. According to our knowledge no such data are available. (For a very recent comprehensive collection of diffusion data in amorphous metallic alloys see, for example, ref. 8.)

On the other hand in recent years considerable efforts have been undertaken to study the diffu- sion behaviour of zirconium-containing amor- phous binary alloys. Two of the authors [7] have

T / oC

~5 5 0 0 4 0 0 3 0 0 2 0 0 10 ,-, T , , , , --

~ Fe in FeglZP 9

17 10

" X ~ C o in C%~Zr u Au i n ~

7 lO

E ~ A

IB ]0 21 u in Cea~ZP~ ~

in Niso

23 ~0

I • in F % ~ Z r 9

i0 ~5 ~ , _ ~ • ; , ,_ ~ ~ _ ~

1.2 14 16 i 8 20 22

T ~ // I 0 ~ K -~

Fig. 4. Comparison between the gold diffusion in amor- phous WsoN20 and various diffusion data for zirconium- containing amorphous alloys: solid lines represent the investigated temperature ranges for the data for cobalt and gold in CosgZr~ [8], iron and zirconium in Fe~nZr~ [1 2], gold in Cus~Zrs0 [1 1] and gold in Nis0Zr5~ , [9]; e, gold in Ws~N2. (this work).

studied diffusion of 195Au and SVCo in melt-spun amorphous Co-Zr alloys. Hahn et al. [9] and Hoshino et al. [10] have studied the diffusion of various elements including gold in coevaporated amorphous Nis0Zrs0. Stelter and Lazarus [11] have made measurements of gold diffusion in melt-spun amorphous CusoZrs0 using Rutherford backscattering spectroscopy. Horvath et al. [12] have investigated tracer diffusion of 59Fe and 95Zr

in melt-spun Fe-Zr alloys. In Fig. 4 we compare the values of the gold diffusion coefficients deter- mined in the present experiments with D values of various diffusors in zirconium-containing metallic glasses. The most striking feature of this comparison is that diffusion of gold in amorphous Ws0N2~ ~ is by far the slowest diffusion process displayed in Fig. 4.

Figure 5 shows a normalized Arrhenius graph of gold diffusion in various metal-metal and metal-metalloid amorphous alloys. In this graph the reciprocal temperature scale is normalized to the glass transition temperature where available or to the crystallization temperature, T x. The earlier data have been taken from Fig. 7 of the review paper by Cantor [13]. The more recent gold diffusion data by Hahn et aL [9], by Stelter and Lazarus [11] and by D6rner and Mehrer [71 have been included into this diagram. The pres- ent gold data for amorphous Ws0N2, normalized to a crystallization temperature Z, = 793 K are also displayed in Fig. 5. It is obvious from this comparison that gold diffusion in the barrier

u~ ~7

i0 I'

tO ~9

!C 2o

10 "

] } ;~s

1: 2,:

O :

- - 7 - - - - - - ~ " • r • ]

1 L,e

m n n ~

'%% ~ o ', J o •

o

J __J

1 ~ i~:

× T

Fig. 5. Gold diffusion coefficients in various amorphous alloys in a normalized Arrhenius diagram: u, gold in metal-metal glasses according to Fig. 7 from the review by Cantor [13]; tz, gold in metal-metalloid glasses according to Fig. 7 from the review by Cantor [13]; A, gold in Cus0Zrs~ ~ [ 11] ( T~ = 823 K); o, gold in Nis~,Zrs0 [9] ( T X = 893 K); o, gold in Co~gZrll [8] (Tx=828 K); e, gold in WsoN20 (this work) ( Tx = 793 K). Tx denotes crystallization temperatures.

material WsoN20 is indeed an extremely slow process.

5. Summary and conclusions

Thin films of amorphous W-N were deposited onto oxidized silicon substrates by reactive sput- tering. The diffusion of gold in this material was measured by a radiotracer method using ion beam sputtering as a serial sectioning technique with high depth resolution. A comparison of the diffusivities with literature data for various amor- phous alloys demonstrates that the gold diffusion in amorphous WsoN20 is one of the slowest diffu- sion processes ever observed for amorphous materials. The extremely low diffusivity in reac- tively r.f.-sputtered amorphous W-N alloys appears to be a very favourable property for their application as thin film diffusion barriers in metallization schemes of semiconductor devices.

Acknowledgments

This collaborative work was started when H. Mehrer was a Visiting Professor at the California Institute of Technology during a sabbatical leave from the University of MOnster. W. D6rner is grateful to the Deutsche Forschungsgemeinschaft for financial support. P. J. Pokela thanks the Academy of Finland for a fellowship. The U.S.

169

Army Research Office supported this work at the California Institute of Technology. The isotope 195Au w a s partly produced at the ISOLDE facility at CERN in Geneva.

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