Interface tailoring for adhesion using ion beams

764

INTERFACE

Nuclear

TAILORING

FOR ADHESION

Instruments

and Methods

in Physics Research B39 (1989) 764-768 North-Holland, Amsterdam

USING ION BEAMS

J.E.E. BAGLIN IBM Almaden Research Center, 650 Hary Road, San Jose, CA 95120-6099, USA

The adhesion performance of thin deposited films is of critical importance in many technological applications. These applications include metal/ceramic and metal/polymer structures for semiconductor packaging and contact fabrication, optical coatings on glass, and protective coatings on metals. In many such cases, film and substrate have no bulk chemical affinity. However, strong and stable adhesion has been obtained by means of ion beam treatment to tailor the interface atomic structures for the purpose. We shall review successful application of (i) ion beam mixing of existing interfaces, (ii) ion beam pre-treatment of substrate surfaces prior to deposition and (iii) implantation from various diagnostics including XPS, TEM, Mossbauer conversion electron spectrometry, correlated with adhesion tests, to show how optimum interface tailoring achieves chemical bonding across the interface, preferably with contaminant removal and crack toughening of the interface region.

1. Introduction

for adhesion enhancement, as an indication ising directions for future research.

Reliable adhesion of thin films to substrates is a matter of considerable significance for a variety of current technological applications including semiconductor packaging and metallization, corrosion-prevention coatings, optical coatings, and surface coatings for high power laser mirrors. In most situations, a bond is required between materials having little or no bulk chemical affinity for each other, such as copper and alumina. Nevertheless the bond should be strong enough for the system to withstand abrasion and peeling, and it should usually be able to survive reasonable thermal cycling, such as soldering or annealing. These requirements are apparently contradictory. In systems displaying bulk chemical affinity, interface linkage should be strong; yet thermal treatment would lead to continued interdiffusion and reaction, representing a non-equilibrium situation until the film itself were fully reacted. However, in the absence of interface chemical linkage, only Van der Waals forces or physically interlocking structures could join the film and the substrate, the former clearly representing a weak junction. In recent years, evidence has accumulated to show that the above problem can be solved by tailored chemistry of the interface atomic layers [l]. This may be achieved in a number of ways, which involve the strategic use of ion beam technology. However, not only the interface chemistry, but also a variety of factors related to the resistance of an interface to fracture may also be addressed with ion beam approaches. The matter of interface region toughening is now emerging as an important auxiliary field for study and development. This review is intended to present a little of the experimental history of the ion beam techniques applied 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

of prom-

2. Ion beam interactions Fig. 1 displays pictorially some important factors which influence adhesion performance of a thin film i.e. its ability to remain bonded in the face of some service disturbance such as scratching, peeling or pulling. The actual figure of merit for “adhesion” must always be established in terms of the kind of treatment the film is intended to withstand. However, in almost all cases, film detachment must be accomplished by the initiation and propagation of a physical fracture in the interface region. The representative example chosen for the figure is that of a film being peeled off its substrate. Ion beam processing may be used to tailor the properties of the interface region and perhaps of the film itself in order to suppress initiation and propagation of a delamination crack. 2.1. Interface chemistry The basic determinant of interface free energy will be the strength of electronic bonding possible between atoms of film and substrate at the interface, and the population of such bonds produced. In many compound substrates, the natural atomic configuration of the terminated surface will not have free bonds to offer an arriving deposited atom, and a poor as-deposited adhesion will be found. Examples would be the continuous chains of a polymer such as Teflon, or the equilibrium outer layer of oxygen atoms on clean Al,O,. A film of Cu deposited on either substrate has no chem-

J. E. E. Baglin / InterJace tailoring

for adhesion plastic deformation

differential

Fig. 1. Schematic

illustration

of some important

factors

to break up such surface structure, interface energy remains high, and adhesion is negligible. However, it was found that ion beam irradiation of the interface region, using inert ions, could greatly improve adhesion [2,3] as shown in fig. 2 (a) and (b). This effect has in fact been observed for a host of different metal/insulator systems, such as those tabulated in ref. [l], and is sometimes referred to as ion beam stitching. Analysis of peeling interfaces [3] shows that no ion beam mixing has occurred beyond the extent of the interface monolayers (consistent with the absence of a chemical driving force for bulk reaction). However, in the similar case of Fe on Al,O,, conversion electron Mossbauer spectrometry (CEMS) has shown [4] the formation of interface chemical bonds for Fe with both Al and 0, following ion beam stitching. In all cases, the bonding proved to be enhanced by heat treatment following irradiation. This implies the formation of thermodynamically stable configurations of interface-bonded atoms, and leads to the concept of ical ability

known

to influence

thin film adhesion

performance.

stability for interface-plane compounds or interface phases which can exist even when no bulk compound phase exists. The existence of such phases would be closely correlated with good adhesion. However, it can not be assumed that all ternary systems will have such phases. One such case seems to be Cu-Si-0 [l], where both ion stitching and substrate pretreatment have so far failed to bond Cu stably on SiO,. It is our suggestion (yet to be proved) that the mechanism of interface stitching is one where both ionizing (electronic) and ballistic deposition of ion energy serves to disorder all interface structures within one or two monolayers. Especially upon heat treatment, the disordered configurations will relax to enable random clustering in ternary groups of atoms. The completeness of interface conversion will depend on the ion fluence and on the mobility of the interface species. A more direct way to control interface chemistry is to use a low energy ion beam (typically 500 eV Ar+) from a Kaufman source to sputter the substrate surface

(b)

Cu on sapphire

Cu on Teflon

/i

Ne+(250keV)

I

substrate damaged

__-__-_____________-----0 1-l He+(200keV)

01 0

I

I

I

1

I

2

3

4

DOSE

(ions/cm’

Xl 015)

J

5

0

I

I

I

10

20

30

DOSE

(ions/cm2

xl

1

40

015)

Fig. 2. Adhesion (peel strength) following ion irradiation through the interface (stitching) and thermal annealing. Adhesion is plotted as a function of ion dose for (a) Cu on Teflon and (b) Cu on sapphire (Al,O,). Peel test stripes were Cu of thickness 10 pm. VIII. POLYMERS

J. E. E. Baglin / Interface tailoring for adhesion

766

, 0

ION

BEAM

EXPOSURE

5

ION

(min)

10

BEAM

15

EXPOSURE

_

20

25

(min)

Fig. 3. Adhesion (peel strength) produced by substrate sputtering with Ar+(SOO eV) in situ prior to deposition of Cu films, and subsequent thermal annealing. Adhesion is plotted as a function of sputtering beam exposure time at 50 PA/cm’. Substrates were (a)

Teflon and (b) sapphire(Al 203). Peel test stripes were Cu of thickness 10 pm.

in situ before film-deposition. This serves the multiple purpose of removing contaminant layers, leaving disorder, with dangling bonds, at the surface, and altering the surface atomic composition by preferential sputtering. A substrate can be so prepared as to receive a deposited material with the best possibility of forming an electronically bonded interface. Examples of adhesion enhancement by this process are shown in figs. 3 (a) and (b) [5,6], where for Cu on Teflon and Cu on Al,O, the peel strength of adhesion is plotted as a function of presputtering exposure. In each case, very strong adhesion is observed, especially after heat treatment for the sapphire substrate. In each case, too, studies by XPS of the interface electronic structure of strongly adhering films showed the formation of chemical bonding between Cu and the substrate elements. The creation of stable interface phases can be inferred. Since this process always produced much stronger adhesion than ion beam stitching, it seems likely that substrate pretreatment is simply a more efficient way of producing such interface chemical bonding. Not all systems can be expected to display favorable interface chemistry. So far, the Cu-SiO, system has not responded to this treatment, and in such a situation, it may be desirable to explore the process of exposing the sputtered substrate to a selected adsorbate capable of making a mutual bond. This idea is supported by the work of Pepper [7] demonstrating that the interface friction between clean metals and sapphire is considerably changed by exposure to O,, N, or Cl. There is obviously great scope for exploration in this area. 2.2. Contaminants Ion beam stitching has proved its ability to overcome the effect of thin contaminant layers at an interface. The adhesion of vapor deposited Cu on Cr provides an

example where a thin layer of native oxide (- 20 A) on the Cr can prevent Cu adhesion. However, as shown in table 1 [8], irradiation through the interface with 200 keV Ne+ or He+ can greatly improve adhesion. However, if the oxide layer is grown to a thickness of 80 A before adding Cu, the ion stitching is much less successful. The evidence implies that interface layer mixing by the beam has caused the formation of islands or precipitates of the contaminant oxide, between which Cr-Cu bonding occurs. (The formation of a discontinuous layer from thick oxides would be much less likely.) Further interesting examples of contaminant dispersion by ion beam stitching have been reported. These include the bonding of Fe or Cu on substrates of alumina or silica whose surface had been exposed to OH or hydrocarbon contamination during a washing procedure before metal deposition. In the case of Cu on Al,O,, adhesion enhancement was inhibited on a water-washed substrate and was even further inhibited where hydrocarbon residues remained [9]. In the case of Fe on carbon-contaminated silica or glass surfaces, studies by CEMS indicated [lo] that the ion beam dispersed the contaminant, Fe-C bonding present in the untreated sample was removed, and adhesion was simultaneously improved. Table 1 Adhesion enhancement for Cu deposited on oxidized sulting from interface irradiation with inert ions [El. Substrate surface

Ion beam

Cr with native

no ions

oxide ( - 20 A)

Cr with grown oxide ( - 80 A)

Peel strength (g/mm)

5x 5x 6x no 6

x

< 0.1

10’s Ne+/cm* 10” He+/cm2 lOI Het/cm2 ions lOI

Cr, re-

Net/cm2

z 25.0 0.4 2.0 i 0.1 0.5

767

J.E.E. Bag& / interfnfe tailoring for ~~~esio~ Table 2 Adhesion treatment

of Cu deposited on Al,O, at 300-400 o C [11]

Substrate

Sapphire Quartz

(Al 20,) (SiO,)

Sapphxe (Al,O,) Quartz (SiO,)

and SiO,,

as enhanced

by interface

implantation

Ion species

Peel strength not heated

heat treated

no ions Ti+ no ions Ti’ Cr+ Cr+

0.4 > 200.0

0.6 > 200.0

with Tit

(g/mm)

0.2

0.1

19.0 1.5 0.2

66.0 88.0 0.6

or Cr+ (5 x 10’6/cm2)

and thermal

Heat treatment 350°C2h 450 o C implant 350°C2h 350*C2h 450 o C implant 450 o C implant

A more dependable means of overcoming substrate contaminant layers is to remove such layers by sputtering in situ prior to film deposition. In view of the potential benefits of tailoring interface composition by preferential sputtering, the process of substrate pretreatment seems to be the method of choice. It is also eminently simple and compatible with conventional vapor deposition systems.

was poor and Cr+ seemed not to have much effect on SiO,). Subsequent TEM pictures of Ti+-implanted Cu/Al,O, interface region showed evidence of precipitates. believed to consist of products of the reaction of the active metal. The good adhesion was attributed to toughening by these precipitates, together with the promotion of interface chemical bonding by the implanted metal ions.

2.3. Interface roughness; toughening

2.4. Material properties

The resistance of an interface region to the initiation and propagation of fracture depends on the interface chemical bonding discussed above, and also on the configuration of that interface. The resistance to fracture will be enhanced when stress (normal or shear) applied by external sources, or interface stress developed within the film, can be distributed over a rough or ill-defined interface rather than appearing at an atomically flat interface plane. Fracture propagation will be inhibited by the presence of irregularities, voids or precipitates of appropriate size in the surface region. Low energy ion sputtering will often produce a rough surface on a polymer substrate, which must serve to assist adhesion of deposited layers, provided that its own integrity has not been weakened in the process. An example of such roughening of Teflon has been presented by Chang et al. [5]. The dramatic enhancement of adhesion shown in fig. 3(a) was found to be accompanied by a roughening of the substrate on a scale of approximately 1000 A. Since new chemical bonding was simultaneously produced, it is not known how much the roughening contributed to adhesion. However, in general, this must be of assistance. Ion implantation of an active metal (Ti or Cr) at the interface region was shown by Madakson and Baglin 1111 to produce adhesion enhancement of copper films deposited on both Al@, and SiOz substrates. Their results are summarized in table 2. Heat treatment at about 400°C was done in each case, either during implantation or afterwards. Excellent adhesion could be obtained (although in that experiment reproducibility

Intrinsic stress within a film can serve to cause delamination or weaken adhesion. Ion-beam assisted deposition of metal films [12,13] can often produce films of low intrinsic stress, with consequently superior adhesion performance.

3. Summary It is evident that beam processing can greatly expand the possibilities for creating strong, stable adhesion between materials having no bulk chemical affinity. Both chemistry and fracture mechanisms at the interface can be adjusted by ion beam treatment. Although high energy ion beam stitching has had remarkably widespread success in qualitatively improving adhesion, it seems that low energy substrate surface treatment in situ offers the greatest benefits for future study. Implantation of active ion species at the interface provides a further helpful tool. In conclusion, it is worth noting that interface toughness and cohesive strength can readily be made to exceed those of either substrate or film. In many practical situations, excellent attachment of a metal to a glass or ceramic substrate can therefore create a system with little tolerance to large differential thermal expansion, where heating can cause fracture within the substrate. An intriguing challenge now seems to be the construction of buffer interface regions to avoid the potentially disastrous consequences of achieving strong metal-ceramic adhesion at a well-defined interface plane. VIII. POLYMERS

768

J.

E.E. Baglin / Interface tailoring for adhesion

References [l] J.E.E. Baglin, Chapter 15 in Ion beam Modification of Insulators, eds. P. Mazzoldi and G. Arnold (Elsevier, Amsterdam, 1987) p. 585. [2] J.E.E. Baglin, G.J. Clark and J. Bottiger, Proc. Mater. Res. Sot. 25 (1984) 179. [3] J.E.E. Baglin and G.J. Clark, Nucl. Instr. and Meth. B7/8 (1985) 881. [4] S.B. Ogale, D.M. Phase, S.M. Chaudhari, S.V. Ghaisas, S.M. Kanetkar, P.P. Patil, V.G. Bhide and SK. Date, Phys. Rev. B35 (1987) 1593. [5] Chin-An Chang, J.E.E. Baglin, A.G. Schrott and K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. [6] J.E.E. Baglin, A.G. Schrott, R.D. Thompson, K.N. Tu and A. Segmuller, Nucl. Instr. and Meth. B19/20 (1987) 782.

[7] S.V. Pepper, J. Appl. Phys. 47 (1976) 2579. [8] J. Bottiger, J.E.E. Baglin, V. Brusic, G.J. Clark and D. Anfiteatro, Proc. Mater. Res. Sot. 25 (1984) 203. [9] J.E.E. Bagfin, Proc. Mater. Res. Sot. 47 (1985) 3. [lo] M. Carbucicchio, A. Valenti, G. Battaglin, P. Mazzoldi and R. Dal Maschio, Radiat. Eff. 98 (1986) 21. [ll] P.B. Madakson and J.E.E. Baglin, Proc. Mater. Res. Sot. 93 (1987) 41. [12] J.M.E. Harper, J.J. Cuomo, and H.R. Kaufman, J. Vat. Sci. Technol. 21 (1982) 737. [13] J.M.E. Harper, J.J. Cuomo, R.J. Gambino and H.R. Kaufman, in: Ion Bombardment Modification of Surfaces: Fundamentals and Applications, eds. 0. Auciello and R. Kelly (Elsevier, Amsterdam, 1984) chapter 4.