polyimide adhesion durability

Microelectronic Engineering 19 (1992) 575-578 Elsevier

575

Ta/polyimide adhesion durability A. Callegari, B. Furman, T. G r a h a m , I1. Clearfield, W. Price, and S. P u r u s h o t h a m a n IBM T..I. W a t s o n Research center, P.O. Box 218, Yorktown lteights, NY 10598

Abstract The adhesion durability of Ta to plasma-modified polyimide surfaces has been investigated under thermal exposure. The failure mechanism was correlated with Auger analysis. Resistance to oxidation at the Ta/polyimide interface was found necessary to obtain a durable metal/polymer adhesion strength.

1. Introduction tligh performance thin film wiring is gaining a more important role in c o m p u t e r systems since it affects in-chip and chip to chip signal propagation delays. Most schemes use polyimide as an insulator and Cu for wiring. Polyimide is used bccause o f its high temperature chemical stability at "-~400 °C and low dielectric constant r.---3. Cu is used because of its low resistivity. Still, a major reliability issue remains the integrity o f the metal/polymer interface after thermal stress. Previous work t has shown that the Cu/polyimide adhesion is poor but when a thin Cr adhesion layer "--20 nm thick was deposited first on a plasma-modified polyimide surface the reliability of the metal/polymer interface was greatly improved 1. l lowever, for some applications alternative adhesion layers may be nceded. In this work, we have investigated Ta as an alternative adhesion layer to Cr. Ta is a good candidate because it is a refractory metal and acts as a good diffusion barrier to CuL T a / C u films were sequentially deposited in-situ on plasma treated B P I ) A - P D A polyimide (Pl). The effect of PI surface treatments with Ar, O2, and O> Ar plasma on the T a / P l adhesion is discussed. The T a / C u adhesion to PI was measured before and after thermal stress. The failure mechanism was correlated with Auger analysis, and the results were compared to those obtained from C u / T a / P M I ) A - O D A interfaces.

2. Experimental Silicon wafers .57 m m in diameter were spin coated with 10 /~m of PI which was sequentially cured in N2 ambient to 400 °C. The Pl coated wafers were rf plasma sputter cleaned in At, 0 2, and sequential O2, Ar. A seed layer consisting of a 50 nm Ta fihn followed by a 500 nm Cu film was deposited in-situ by I)C magnetron sputtering at a pressure o f 0.7 Pa. A metallic shadow mask in contact with the Si substrate was used to define peel strips on the underlying Pl surface, exposed to the plasma treatment and metal deposition. A Cu film 8 /~m thick was plated on top of the seed layer to complete the peel test structure. Afterwards, the samples were annealed in N 2 at 400 °C simulating a P I curing cycle. The samples were annealed up 1o 10 PI cycles with air exposure between cycles. The T a / P I adhesion was measured by 90 ° peel testing.

0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.

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A. Callegarl et al, /7a/polyimide adhesion durability

3. Results and discussion Peel strengtlv; o[" Ta/Pl interface, sub,iected to difl'erent plasrna treatments, as-deposited after ttmrmal exposure are shown in Table 1. Table I "I'a/III~I)A-I~I)A peel strenghts Sample

Plasma Treat.

Peel as-dep.(g/mm")

11'I ~

IOPl

1

Ar 02 02 + Ar

85 80 90

83 c~o 70

64 82 55

2 3

400 °C polyimide cure cycle

b error ±5 g/mm

Note that all the as-deposited samples have high adhesion strength. Sample 2 which received only an 02 plasma cleaning maintains high peel strength after the 10Pl cycles with very little degradation. A significant degradation was found for samples Ar (25%) or O2+ Ar (39%) plasma cleaning.

71

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w

6

6

S

5

,! J

AS-DEPOSITED

Z

~3 AS-DEPOSITED

C

2OO

400

600

KINETIC

ENERGY

800

1000

(eV)

Fig. !, Auger surface survey of an as-deposited metal peel strip with high strength cohesive PI fracture,

I

I

200

i

I

I

I

400

600

800

KINETIC

ENERGY

1000

(eV)

Fig. 2. Auger surface survey of an as-deposited metal peel strip with high strength mixed mode PI fracture (sample 1).

Auger analysis oF the peeled metal strips identified two modes o1" fracture. The first is a cohesive PI fracture shown in Pig. 1. This mode of [hilure was observed for as-deposited films on BPDA -PI)A substrates exposed to the two step plasma process, 0 2 followed by At. In general this type of fi'acture indicates good adhcsion between the metal layer and the modified polyimide surface. The peel strength and depth of fracture (amount of polyimide separating with the peeled metal), 5 -30 nm, depends on both the extent of damage introduccd by the plasma prior to metal deposition as well as thermal historyL A similar cohesive P[ fracture was observed for all BPDA-PI)A substr~,tes after thermal exposure. The second mode of fracture observed on as-deposited peel strips was a high strength fracture at

577

A. Callegari et al. I Ta/polyimide adhesion durability I00

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o~,

I0

SPUTTER TIME (rain)

Fig. 3. Auger depth profile of the as-deposited metal peel strip of Fig. 2.

Fig. 4. Auger depth profile of a metal peel strip (sample 3) after I0 1'I cycles.

or near the metal/polyimide interface. In this type of failure both modified polyimide (carbidic C) and metal or metal oxide are observed on the fracture surface as shown in Fig. 2. No metal is detected on the substrate side of the fracture. This type of fracture is referred to as mixed mode. Auger depth profile (Fig. 3) also shows little oxidation (O <10 at.~) at the Ta/BPDA-PI)A interface. Fig. 4 shows an Auger depth profile of a peel strip from sample 3 subjected to the !0 PI cycles. After the thermal exposure, oxidation at the interface is still low
IO~)

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ANNEALED ANNEALED

80

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KINETIC ENERGY (eV)

Fig. 5. Auger surface survey of an annealed metal peel strip with low strength interracial failure (PMI)A-ODA).

SPUTTER TIME (rain)

Fig. 6. Auger depth profile of the annealed metal peel strip of Fig. 5.

578

A. Callegari et al. / Ta/polyimide adhesion durability

Tiffs is in contrast to thc bchaviour of T a / P M I ) A - O I ) A polyimide substrates ira previous experimenls a. In this case peel strengths degraded to 0-10 g/mm levels aftEr exposure to two 400 °C heat cycles. The appearance of the Ta peak in the Auger surIitce survey of l:ig. 5 shows that an interfhcial (adhesive) fi-acturc occurred, l:urthermorE, the Auger depth prolile of Vig, 6 indicates that the Ta film is oxidized. This results in adhesive fi'acture at the T a O x / P M I ) A - O I ) A interface. Tim 0 content at the interface ix >40 at.,,o, °/ which is much higher than the <10 at.% Iound al tile Ta/BI'I)A-PI)A polyimide. The cause ix attributed to water absorbed in the polyimide reacting with tile Ta during annealing resulting in metal oxidation and adhesion degradation. The same mechanisn was also observed in the C r / P M I ) A - O I ) A system ~. l h i s type of interf2qce failure has not been observed on BI~I)A-P1)A substrates. 'l'his is probably a result of decreased water absorption in BPI)A-PI)A polyimides as well as of improved diflhsion barrier properties of tile plasma modified BPI)A surface as shown by water absorption cxpcriment¢.

4. Conclusions The Ta/IlPI)A-PI)A adhesion durability under thermal exposure has been investigated and correlated with Auger failure locus analvsis. A l o w O content (<10 at./o) o/ at the Ta/PI interfhce was Found to be responsible for a high peel strength cohesive fracture. A high O content (>40 at.%) found aIier thermal exposure at the T a O x / P M I ) A - O I ) A gave a low peel strength interracial failure. The decreased water absorption of the IlI~I)A-PI)A compared to the I~MI)A-ODA is probably responsible for the lower oxygen content found in the B I q ) A - P D A interfaces.

Acknowledgment We would like to thank F'. Bailey for his help with the adhesion measurements.

References 1. 2. 3. 4. 5.

ILK. l:urman, S. Purustmthaman, 1!. Castellani, S. Renick, and I). Neugroshl, "Metallization of Polymers", ACS Symposium series, 297, 1990. K. llolloway and P.M. l:ricr, "Tantalum as a diffusion barrier between copper and silicon", Appl. Phys. 1.ett. 57, 1736. 1990. I).i~. l'appas, J.J. Cuomo, and K.(;. Sachdev, "Studies of adhesion of metal films to polyimide', .1. Vac. Sci. Tcchnol. A9, 2704, 1991. ILK. Furrnan, unpublished ll.M. Clearfield, ILK. l:urman, N. Sheth, I'. Bailey, and S. Purushothaman, "Water transport across modified polyimide sur[hces", Proceedings of Adhesion Soc. Symposium, editor l:.J. Boerio (Adhesion Society, 1992), p. 148.