Amorphous Ta-Si-N diffusion barriers in Si/Al and Si/Cu metallizations

: . . : . . . . . ,: ,,) ,jj ,,:,j .:,:i ::..:., i:.j;v:.:. ‘... .: :..

;,; ::.; ..:j.... “. “.’ . . . . :: ij,j:-,:j:,,::,b:j::

Applied Surface Science 53 (1991) 373-376

North-Holland applied surface science

Amorphous Ta-Si-N diffusion barriers in Si/Al and Si/Cu metallizations

E. Kolawa, P.J. Pokcla, J.S. Reid, J.S. Chen and M.-A. Nicolct C‘rrlifiv?rirr Iturirrrte of T~~.l~nolo~~, Pusurlrnrr, CA ‘)I 125, USA

Rcccivcd LS March 1991; accepted for publication 30 April 1991

Thin films of amorphous Ta-Si-N alloys were deposited by reactive RF sputterrng of a Ta5Si, target in an Ar/N, amhient.

These alloy films were tested as diffusion barriers between Al and Si, as well as between Cu and Si.

Electrical meahurementt, on Schottky diode5 and on shallow n+p junction diodes were used to evaluate the thermal stability of

the (Si)/W,,SiZ,,N,2(20 nm)/Ta,,Si ,,N,,,(RO nm)/Al(1000 nm) metallization. The amorphous W,,Si,,,N,, contacting layer was

added to raise the Schottky barrier height of the metallization on n-type Si. Both the shallow junctions and the Schottky diodes are

rtable up to 700 ‘C for 20 min (above the Al melting point of 660 “C) which makes this material the best thin-film diffusion barrier

on record. Furthermore. the same Ta,,Si ,.rN,,, amorphous film maintains the integrity of the I-V characteristics of the shallow

II +p junctions with the (Si)/TiSi,(30 nm)/Ta.~,,Si,,N,,,(XO nm)/Cu(SOO nm) mctallization up to 900°C for 30 min annealing in

vucuum. The TiSi, contacting layer was added ttr assure an ohmic characteristic of the contact. For comparison, the same shallow

,junctionb with (Si)/Cu metalli~ations were shorted after annealing at 300°Cc.

1. Introduction

Reliable metallization schemes on shallow junction devices are of great importance in semiconductor technology. Aluminum is commonly used for contacts and interconnections in silicon integrated circuits. Since aluminum reacts easily with silicon and with contacting layers of transition metals or silicides, diffusion barriers are em- ployed in VLSI metallizations [I-S].

Copper, which has a lower electrical resistivity and a higher resistance to electromigration than Al is currently being evaluated for VLSI applications [6]. Copper does not form any compounds with transition metals of the V, Cr and Mn group, but is very mobile in metals as well as in silicon. Copper can destroy devices by interdiffusion into silicon during thermal post-metallization treat- ments; therefore, diffusion barriers are necessary to secure the stability of devices.

It has been demonstrated recently that an amorphous Ta,,Si,,N,,, thin film is an effective liffusion barrier between Al and Si up to 675 o C,

which is above the Al melting point [71. In this study, we report on the electrical stability and properties of contact structures employing such a barrier on (Si)/W,,Sizr,N,,/Ta,,Si,,N,,,/AI Schottky diodes and (Si)/TiSi,/Ta,,Si,,N5,,/Cu n+p shallow junctions.

2. Experimental procedures

n-Type Si( 100) substrates with 30-60 Q. cm resistivity were thermally oxidized to form a 240 nm layer of SiO, in which the active regions of the Schottky diodes were delineated with a posi- tive photoresist mask. A buffered HF etchant and ammonium persulfate photoresist strip were then used to open rectangular contact windows of 6.75 X IO-’ cm’ in the silicon oxide. n+p shallow

junctions in p-type Si( 100) wafers were also used to test a stability of metallizations. The junction depth was about 300 nm, with an arsenic surface concentration of about 10” cm-’ and contact areas of about 230 x 230 pm2. Prior to loading

llh9-3332,/91/$03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved

them into the sputtering chamber, both types of wafers were quickly etched in a 1 : 10 HF : H ?O solution and blown dry with N,.

Deposition of the films was performed in a RF sputtering system equipped with a ctyopump and a cryogenic baftle. The sputtering chamber was evacuated to a base pressure of about S x lo-’ Tot-r before deposition. A magnetron-type circu- lar target, 7.5 cm in diameter, was used as a sputtering source. The substrate holder was placed about 7 cm below the target and was neither cooled nor heated externally.

The TiSi, layer was deposited from the TiSi, target by RF sputtering in an Ar ambient. The thickness of this film was about 30 nm. The W-Si-N and Ta-Si-N films were reactively sputter-deposited from W,Si, and a Ta,Si, target, respectively, in an Ar/N, gas mixture. The flow ratios of N2 to Ar (O.OSS) and a total gas pressure (10 mTorr) were adjusted by mass flow con- trollers and monitored with a capacitive manome- ter in a feedback loop. Both films were deposited with 300 W forward sputtering power. The nomi- nal compositions of the films derived from ‘He backscattering spectra arc W,,Si2,,N3? and Ta,,,Si,,N,,,, and their thicknesses measured by stylus instrument are 20 and 80 nm. respectively. The aluminum (1000 nm) or copper (SO0 nm) were sputter-deposited in pure argon (5 mTorr) with 300 W forward power on top of the Ta,,,Si ,,N,,, films without breaking the vacuum. The contact metallizations were dclincated by the lift-off technique for the shallow junction diodes and for the Schottky diodes by photoresist mask- ing. etching away the unmasked Al layer with a standard Al etchant, and the ternary thin-film layers with a 10 : 5 : 1 H ?O : HNO, : HF solution. The Schottky diodes were annealed for 20 min in vacuum at a pressure of less than 1 X 10 -” Torr. at temperatures in the 400-700 a C range.

The shallow junction diodes wcrc annealed in the 700-YSO o C range for 30 min in vacuum. The shallow junction diodes with Si/ W,,Si,,,N,J Ta,,,Si,,N,,,/Al metallization were also annealed in argon for 700 h at 350 o C in order to measure long-term stability of the metallization. After the vacuum annealings, the I-V characteristics of all devices were evaluated. The barrier height was

n Barner (Ta-SI-N)

ing Layer (TE$)

5

-5 -5 -4 -3 -2 -1 0 1

<Si>lTiSi21 Ta3&$&u

0 as&p l 933"C/30min

n 95O"C/3Omin

BIAS VOLTAGE 0’)

Fig. I. I-C’ characteristics of the n ’ p shallow junction d~odc\ with the (Si)/TiSi 430 nm)/Ta ,,,Si ,,N,,,(r(O nm)/Cu(500 nm) mrtallization before and after annealing at 000 ” C’ for 3tl min.

derived from the saturation current using the simple Schottky model.

3. Results and discussion

The properties of the Tajc,SiIJN,,, films arc described elsewhere [7]. The as-deposited film is amorphous and it crystallizes after annealing in vacuum at 1100°C for 2 h as determined by X-ray diffraction. The crystallization products in- clude hexagonal Ta,N, tetragonal Ta,Si_, and hexagonal Ta, (Ta,, &Si ,I,72). The resistivity of the 100 nm thick. as-deposited Ta,,Si,,N5,, film is about 625 ,LLQ. cm and the film contains below 3 at% 0, and about 3 at!% Ar.

Fig.-1 shows the I-V characteristics of the shallow n ’ p junction diodes with the (Si)/ TiSi,GO nm)/Ta,,Si,,N,,,(80 nm)/Cu(SOO nm) metallization before and after annealing at 900 and ‘$50 o C. The reverse current does not change after annealing at 900 o C, which means that the

E. Kowala et al. / Amorphous Ta-S-N diffusion harriers itI Si /Al and Si / Cu metaliizations 17s

ing Layer (W-Si-N)

N-

N+

II ’ I-’ ’ ’ ’ ” ” ” ” I

5201’ 0 100 200 300 400 500 600 700

ANNEALING TEMPERATURE (“C)

Fig. 2. Schottky barrier height of diodes with the W,,Si2,,N,,(20

nm)/Ta,,Si,,N,,,(80 nm)/A1(1000 nm) metallization as a

function of annealing temperature.

metallization is still stable. After annealing at 950” C, the leakage current at - 1 V increases from about 0.4 to 2.5 nA. A failure of the metallization at 9.50 o C can be explained by the prema- ture crystallization of the Ta,,Si,,N,,, alloy. It has been shown using a cross-sectional transmis- sion electron microscopy that the Ta,,Si,,N,,, layer in a direct contact with a polycrystalline copper layer crystallizes at 900 ’ C, which is 200 o C below its crystallization temperature on a sap- phire substrate [S]. Once Ta,,Si,,N,,, is fully crystallized, the rapid diffusion of copper into a silicon substrate through the grain boundaries srematurely takes over and destroys the junction. The leakage current of the same diodes with the :Si)/TiSi,(30 nm)/Cu(SOO nm) metallization is tbout 0.4 PA at - 1 V for the as-deposited ample. After annealing of these diodes at 300 o C or 30 min all of them were shorted. An addition If the Ta,,Si,,N,, diffusion barrier to this metal- zation raises its stability by 600 ’ C.

Fig. 2 shows typical barrier heights of (Si)/

W,,Si20N32/Ta26Si,4N50/AI diodes measured after 20 min annealing in vacuum. In this metallization, the W,,Si,,,N32 layer delivers the required Schottky barrier height whereas the Ta,,Si,,N,,, layer guarantees its high stability. The barrier height increases from 530 meV for as-deposited samples to 630 meV for samples annealed at 700 o C. A similar behavior has been reported for sputter-deposited TiN Schottky diodes on n-type Si [93, and it decreases on p-type Si upon thermal annealing. An interpretation of these data as- sumes that donor-like traps are created in the Si adjoining the silicon/ metal interface during the sputtering process. As a consequence, the electric field in the Si near the silicon surface is enhanced and an effective barrier height value determined from the I-V characteristic on n-type Si, is lower than it would be obtained in the absence of traps. An annealing process partially anneals out the traps, reduces the strength of the electric field, and raises the barrier height toward its trap-free value. The rise by 100 meV is equal to that observed for sputtered TIN Schottky barrier [9].

The Schottky diodes fabricated without the

Ta36Si,,Ns,, diffusion barrier but with the

W,,Si ro NX layer fail at 600 “C. The Schottky diodes fabricated without the W,,Sizr,N,, layer but with the Ta,,Si,,N,,, directly in contact with Si, have a barrier height in the range of 400 meV as-deposited. After annealing at 600 o C the barrier height increases to about 540 meV. The metallization is stable, but the Schottky barrier height is too low for practical applications. The barrier height of both Ta,,Si,,N,,, and W,,Si,,,N,, appears to correlate with that of the correspond- ing metal silicide 1101.

Fig. 3 shows the electrical characteristics of the shallow nip junction diodes with the (Si)/ W,,SizoN12/Ta3hSi,4Ns0/Al contact scheme. The reverse current density prior to any heat treat- ment is about 13 X lo-’ A/cm*. After annealing the diodes for 30 min at 7OO”C, which is above the melting point of Al, the leakage current re- mains unaltered. The aluminum film coalesces into islands but does not react with the Ta,,Si,,N,,, barrier.

The (%)/AI diodes tested for a reference were all shorted after annealing at 500 o C for 30

D,,,us,on Barner (1&31-N)

Layer (W-S-NJ

thermal stability in Si/AI and Si/Cu mctalliza- tions, the Ta,,,Si ,JN,,, alloy is superior to any

diffusion barrier reported in the litcraturc.

4

3

a 5 2

E w 1

2 3 0 u

-1

<Si

I- 0 ASDEPOSITED

x 7afC/30m*n

-2’ ’ ’ ’ I ’ ’ ’ -6 -5 -4 -3 -2 -1 0 1

BIAS(V)

Fig. 3. I- 1’ characteristics of the n* p shallow junction diodes

with the (Si)W,,Si,,,N,,(20 nm)/Ta ;,,Si,,N,,,(XO nm)/

Al(1000 nm) metattization hefore and after annealing at

700 DC for .?O min.

min. To check their long-term stability, the (Si)/W,,Si,,,N,z/Ta,,Si,,N,,,/Al shallow junction diodes were annealed at 350 ’ C for 700 h in argon. No change in the I-V characteristics of the diodes was observed after this annealing.

4. Conclusions

The results presented here convincingly show that an amorphous Ta,,Si,,N,,, film is an excep- tionally effective diffusion barrier. In terms of its

Acknowledgements

The authors wish to thank Rob Gorris and Bart Stevens for their technical assistance and to Rachacl Samplcy for help in the manuscript preparation. Financial support for this project was provided hy the Army Research Office undcl Contract No. DAALO3-Xc)-K-004’. P.J. Pokela ac- knowledges a fellowship by the Academy of Fin- land. J.S. Reid gratefully acknowlcdgcs a fellowship by AMOCO Foundation.

References

[II M-A. Nicotet. Thin Solid Films 52 (lY7S) 511

[2] P.S. Ho. Thin Solid Films 96 C lYX3-) 301. R.S. Nowicki and M.-A. Nicoler. Thin Solid Films YO

(14X2) 317.

M. Wittmer. J. Vat. Sci. Technot. A ? (IY)xJ) 172.

I1.P. Kattrlus and M-A. Nicotet, Diffusion harrier\ in

semiconductor contact metaltizations. in: I>ittu5ion Phc-

nomena in Thin Film< and Microrlec~rcmic Mxtcrials.

Eds. D. Gupta and P.S. tlo (Noyeb. Park Ridge. N.t.

1’188) pp. 13?-10x.

P.J. Pan and C.11. Ting. IEEE Electron t>wicc Lett.

EDL-to (19X’)) 323.

E. Kolawa. J.M. Motariur. (‘.W. Nieh and M.-A Nicotct.

.I. Vat. Sci. Technol. A X (1990) 30110.

E. Kolawa. J.S. (‘hen. P.J. Pokela. J.S. Reid and M.-A.

Nicolrt. J. Appt. Phy\.. submitted.

M. Finetti. 1. Suni, M. Barter. 7‘. Banucll and M.-:I

Nicolet, Solid State Electron. 27 (1984) hl7.

S.P. Murarka. Silicide\ for VLSI Application\ (Academic

PI-~\. New York. lY8.1).

Documents

Amorphous Ta-Si-N diffusion barriers in Si/Al and Si/Cu metallizations