Influence of the anti reflective coating on the electromigration resistance of 0.5 μm technology metal-2 line structures

applied surface science ELSEVIER

Applied Surface Science 91 (1995) 208-214

Influence of the anti reflective coating on the electromigration resistance of 0.5/zm technology metal-2 line structures R. Stevens a,*, A. Witvrouw a, Ph.J. Roussel a, K. Maex ", H. Meynen a, A. Cuthbertson b a Dept. ASP/VMT, IMEC, Kapeldreef75, B-3001 Leuven, Belgium b ALCATEL M1ETEC, Westerring 15, B-9700 Oudenaarde, Belgium

Received 20 March 1995; accepted for publication 25 April 1995

Abstract In order to optimise the 0.5 /zm CMOS technology, electromigration (EM) tests were performed at different temperature and current stresses on metal-2 line structures with 2 different anti-reflective coatings (ARC). The failure data were analysed with an in-house made software package "FAILURE", based on Black's equation MTFF = Aj - " exp ( E a / k T ) . Both cases fulfill the criterium of less than 0.01% failures within 25 years under use conditions, but the T i / T i N ARC had a better EM resistance than the TiN ARC. In the case of a T i / T i N ARC a "TiA13''-layer was present, possibly acting as an alternative current path. This layer is formed during the thermal process steps in the 0.5/zm triple layer metallisation (TLM) processing of the wafer. This could explain the better EM resistance for the T i / T i N ARC case.

1. Experimental set-up

etchback, o f the via barrier ( 2 0 / 6 0 nm T i / T i N -

250°0. The testmaterial was extracted from 5 inch wafers, processed in the I M E C pilot line, with the standard IMEC 0.5 /xm T L M technology [1], including a nitride passivation top layer. The metal-2 stack consisted of a 2 0 / 6 0 nm T i / T i N barrier layer (deposited at 250°C), a 700 nm A1SiCu layer (deposited at 300°C) and an A R C layer on top o f it. A split in the A R C layer was made: 80 nm TiN versus 2 0 / 6 0 nm T i / T i N (both deposited at 250°C). This metal-2 stack was deposited on the remainings, after tungsten

* Corresponding author. Tel.: +32 16 281343; Fax: +32 16 281214; E-mail: [email protected].

All metal layers of the stack were sputter-deposited from an A l - l w t % S i - 0 . 5 % C u and a pure Ti target in a cluster tool ( C L C 9 0 0 0 / G a l a x y 100). The TiN layer was reactively sputtered from the Ti target, by introducing N 2 gas in the process chamber. A vacuum break was introduced between the barrier layer and the AISiCu layer, but not between the A1SiCu layer and the A R C . The teststructure was a 3420 /xm long and 1 /xm wide 2-point resistor with a meander-fork structure on metal-2 level. The E M tests were performed in a Sienna ITS8000 system, with a capacity o f 80 DUTs (devices under test) divided over 3 Sun furnaces. The E M tests were performed at temperatures of 200, 220 and 240°C and at current densities o f 1.4, 2.1 and 2.8 E + 6 A / c m 2. The total test time was 1647 hours. F o r

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R. Stevens et aL /Applied Surface Science 91 (1995) 208-214 Table 1 Filling of the EM test system

TI = 200°C T2 = 220°C T3 = 2400C

209

Table 2 0.01% failures after x years for the case of a TiN ARC

11 1.4E+6 A / cm2

I2 2.1E+6A/ cm2

13 2.8E+6 A / cm2

10/-

10/14 12/10/14

10/-

x / y , with x = number of DUTs for case Ti/TiN ARC, y = number of DUTs for case TiN ARC.

some DUTs with an T i / T i N ARC an extrusion monitor was used to follow-up eventual shorts. The filling of the EM system is listed in Table 1. The data analysis was performed according to the m a x i m u m likelihood method with an in-house made software package " F A I L U R E " , developed by Ph.J. Roussel. The stress acceleration is based on Black's equation M T T F = A j -n exp ( E a / k T ) , with M T T F = median time to failure, A = constant, j = current, n = acceleration factor, E a = activation energy, k = Boltzmann constant and T = absolute temperature.

5% criterium 20% criterium 100% cfiterium

Ref. temp. = 90°C

Ref. temp. = 125°C

1067 years 5674 years 10199years

201 years 644 years 962 years

were detected for this case than for the T i / T i N ARC case under conditions of equal stress factors. As this split was not stressed at different current densities no values for the acceleration factor could be derived. Values for the activation energy of 0.594, 0.774 and 0.840 eV could be derived in the cases of a 5%, 20% and 100% resistance increase-failure criterium. Fig. 2 gives the log-normal plot for the two stress temperatures of 200 and 240°C for the case of 20% resistance increase-failure criterium. Independent of the failure criterium, the evaluation criterium of maximum 0.01% failures within 25 years was easily met (see Table 2), even for a reference temperature of 125°C. 3.2. Case Ti / TiN A R C

2. E v a l u a t i o n c r i t e r i u m

After extrapolation of the failure data to the reference conditions of a temperature of 90°C, a current of 0.7 m A and with the use of an acceleration factor ( n ) of 2 and an activation energy ( E a) of 0.7 eV, it was checked whether the lifetime criterium of less than 0.01% failures within 25 years was met. As failure criterium a certain percentage of increase of the m i n i m u m resistance (Rmin) was taken. The m i n i m u m resistance (Rmi n) is defined as the bottom value after the initial exponential resistance decrease, which is probably caused by A12Cu precipitation [2].

A typical behaviour of resistance versus time is given in Fig. 3. Typically the failures were more abrupt than with TiN ARC. For the DUTs with extrusion monitor no shorts were detected. Independent of the failure criterium, almost no failures were detected during the test time. Values for activation energy and acceleration factor could not be derived. Fig. 4 gives the log-normal plot of the failure data for a stress temperature of 220°C and current density of 2.1 E + 6 A / c m 2 (case of 20% resistance increase-failure criterium). Independent of the failure criterium, the evaluation criterium of m a x i m u m 0.01% failures within 25 years was easily met (see Table 3).

3. R e s u l t s

3.1. Case TiN A R C A typical behaviour of resistance versus time is given in Fig. 1. Failures occurred over a broad time interval and self-healing was detected. More failures

Table 3 0.01% failures after x years for the case of a Ti/TiN ARC 5% criterium 20% criterium 100% criterium

Ref. temp. = 90°C 3888 years 5203 years 6466years

Ref. temp. = 125°C 543 years 727 years 904 years

R. Stevens et al. /Applied Surface Science 91 (1995) 208-214

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4. . . .

Page

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R. Stevens et aL /Applied Surface Science 91 (1995) 208-214

211

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212

R. Stevens et al. /Applied Surface Science 91 (1995) 208-214

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Fig. 5. Comparison of the 2 cases at 240°C ((1) = TiN; (2) = Ti/TiN ARC); 20% resistance increase-failure criterium.

3.3. C o m p a r i s o n o f the 2 cases

Fig. 5 gives a comparison o f the behaviour for the 2 cases at a stress temperature o f 240°C and a current density o f 2.1 E + 6 A / c m 2 (1 = TiN ARC; 2 = T i / T i N ARC). Apparently the electromigration resistance of the stack with a T i / T i N A R C layer is much better than the stack with a TiN A R C layer. Figs. 6 and 7 give the cross-section S E M pictures for the metal-2 stack in the case o f the TiN A R C and

--

Fig. 6. SEM cross-section of metal-2 stack - case TiN ARC.

T i / T i N A R C case, respectively. In Fig. 7 a reaction layer can be easily distinguished. XRD, RBS and AES on metal-2 stacks directly deposited on an oxide layer, and annealed for 70 min at 450°C, confirm the presence of this layer. It is shown that this layer contains A1, Ti and Si (see Fig. 8), indicating that this layer is probably A13Ti, with Si dissolved in it [3-5]. This layer can act as an alternative current path and could possibly explain

Fig. 7. SEM cross-section of metal-2 stack - case of Ti/TiN ARC.

R. Stevens et al. /Applied Surface Science 91 (1995) 208-214 AES

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the better EM behaviour of the stack with T i / T i N ARC.

(4) Both cases fulfill the criterium of less than 0.01% failures within 25 years for reference conditions of T = 9 0 ° C , E a = 0 . 7 eV, I = 0 . 7 mA and n=2.

4. C o n c l u s i o n s

(1) The EM resistance of the case T i / T i N ARC is better, which is most probably due to the presence of a "TiA13" layer, possibly acting as an alternative current path. (2) In the case of T i / T i N ARC a more abrupt failure mechanism occurred and no shorts were detected. (3) In the case of TiN ARC some values for E, could be derived, from 0.594 to 0.840 eV for the failure criteria of 5% respectively 100% resistance increase.

Acknowledgements

We like to thank the group of AMSIMEC for the packaging, electrical measurements and support during the electromigration test, the IMEC pilot-line for processing of the test material, H. Bender for AES analysis, B. Brijs for RBS analysis and R. Verbeeck for SEM work. This work was performed under a contract with Alcatel Mietec and with the support of the Flemish Institute of Scientific and Technological Research.

214

R. Stevens et al. /Applied Surface Science 91 (1995) 208-214

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T. Yamashita and K. Yoneda, VMIC Conf. Proc. (1994) pp. 275-277. [4] K. Hewes, D. Yost, H.A. Le and J.W. McPherson, VMIC Conf. Proc. (1994) pp. 278-280. [5] P.R. Besser, J.E. Sanchez, Jr. and R. Alvis, Mater. Res. Soe. Syrup. Proc. (1994) pp. 471-476.