role of chemical surface heterogeneity in the adhesion of photocured resins to ceramic substrates

SURFACE AND INTERFACE ANALYSIS, VOL. 21, 697-702 (1994)

Role of Chemical Surface Heterogeneity in the Adhesion of Photocured Resins to Ceramic Substrates?

A. M. Taylor,' J. F. Watts,' J. Bromley-Barratt' and G. Beamson* 'Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey GU2 SXH, UK 'Research Unit for Surfaces, Transforms and Interfaces, SERC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK

The surface composition of various ceramic substrates used for electronics applications is heterogeneous. In the case of debased alumina, this may result from the diffusion of sintering aids such as silica, added during the manufacturing process, which produce a complex bonding surface. In this study, the durability of a visible light curing resin applied to a variety of ceramic substrates of differing aciditybasicity was investigated. Butt-joints consisting of photocured resin adhered to quartz, silicon wafers and polished alumina were produced. Following immersion in water at 50°C for periods of several weeks, joints were fractured (in sifn in the VG ESCALAB Mk I1 and just prior to loading into the Scienta ESCA300). Angle-resolved XPS analysis was carried out on both sides of the failure. Failure occurred cohesively, with 1-2 nm of polymer remaining on the ceramic side of the failure. Orientation of the oxygenated functional groups of the polymer was observed within this overlayer, with methoxy groups being aggregated immediately adjacent to the inorganic surface; subtle differences were observed between the polished alumina and other substrates. An absence of the aromatic groups from the photocured ploymer was also noted on the ceramic side of the failure. These results are explained in terms of acid-base interactions, and a model for adhesion and subsequent failure of the system is proposed.

INTRODUCTION

The increased use of ceramic substrates, notably alumina, within the electronics industry has stimulated interest from adhesives manufacturers in formulating products suitable for applications such as screen print- ing and device encapsulation. Electronics manufacturers stipulate that all adhesives/encapsulants used in their components must be able to withstand hostile environ- ments, i.e. exposure to moisture and/or thermal shock.' Visible light curing resins (e.g. LuxtrakTM2 resins from ZENECA Ltd.) provide an example of one class of products available for use in this area. The particular LuxtrakTM2 products studied here are derived from aromatic methacrylate oligomers and are therefore based on non-epoxy chemistry. The properties that make them of particular interest for such applications include their rapid cure times (a matter of seconds at room temperature), cure initiated by a blue light source (wavelength -470 nm) and the ability to cure through a variety of translucent materials, including alumina.2

The surface composition of many of the substrates used for electronics applications is inhomogeneous. In the case of Coors AD96 alumina (which is a 96% debased alumina), this is believed to arise by the diffusion of silica to the grain boundaries (added during the manufacturing process to form the glassy phase during liquid-phase sintering), and results in the production of a chemically heterogeneous bonding ~ u r f a c e . ~ . ~ Resin formulators need to be aware of the existence of regions of such heterogeneity, particularly, as in this case, when

Paper presented at ECASIA 93, Cataria, Sicily, 4-8 October 1993.

CCC 0142-242 1/94/100697-06 8 1994 by John Wiley & Sons, Ltd.

the acid/base properties of those regions vary so widely. Alpha alumina has an isoelectric point of between 6.6 and 9.2, and can thus be described as an amphoteric oxide. The isoelectric point of silica is between 1.8 and 2.2, tending to give it more acidic properties.' In addi- tion to regions of chemical heterogeneity, SEM and CSLM (confocal scanning laser microscope) studies carried out on the surface of as-received Coors AD96 alumina revealed a variation of 3-4 pm in height. The aim of the work described in this paper was to employ angle-resolved XPS (ARXPS) to investigate the manner in which a typical photocured resin interacts with model inorganic substrates. In order to achieve this aim it is necessary to design a system which is both flat and extremely smooth, hence a requirement to work with polished rather than as-received alumina. To study the separate effects of substrate heterogeneity on the durability of the system, other substrates were included in the study; these substrates were quartz and single- crystal silicon (plus its native oxide), which, like alumina, were polished to a 1 pm finish. By polishing the alumina, it was possible to remove surface- segregated silica to produce a flat alumina surface. Thus, the three types of specimen provided an opportunity to study the different chemical effects present on the as-received alumina surface with the increased depth resolution offered by ARXPS.

EXPERIMENTAL

Materials

Quartz discs, 10 mm in diameter and 1.5 mm thick, were obtained from UQG Ltd., Cambridge. Silicon

Received 4 October 1993 Accepted 12 December 1993

698 A. M. TAYLOR ET AL.

wafers (( 100) direction) were sectioned into squares of -10 mm x 10 mm using a diamond scribe. Coors AD96 alumina (used routinely by the electronics industry) was also supplied in the form of 10 mm diameter discs by the Laser Cutting Company. The as- received quartz and silicon samples had a mirror finish to them and required no further surface preparation for angle-resolved XPS (ARXPS); however, the surface roughness of the alumina sample made polishing essen- tial. The alumina discs were polished to a 1 pm finish and cleaned ultrasonically in isopropanol before being heated overnight in an oven at 120°C to remove any remaining polishing residue. The LuxtrakTMZ photo- curable resins were supplied prepackaged in syringes by ZENECA Ltd.

The as-received materials were characterized using XPS before starting the durability studies. The XPS spectra were obtained using a VG Scientific ESCALAB Mk I1 interfaced to a VGS-5000s data system based on a DEC PDP 11/73 computer. Both A1 Kcr and Mg Kcr were employed at a power of 340 W, the analyser was operated in the CAE mode with pass energies of 50 eV for survey spectra and 20 eV for high-resolution spectra and the slit width was 6 mm. Quantification and curve fitting were achieved using the VGS software.

Durability studies

Butt-joints had previously been chosen as the most suitable specimen geometry for carrying out in situ fracture experiments to study interaction at the resin/ceramic interface.6 The various substrates (quartz, silicon and polished alumina) were sandwiched together using LuxtrakTMZ resin and cured with visible blue light. The samples were then immersed in Milli-Q water at 50°C for varying lengths of time (up to 5 weeks). On removal from water, ESCALAB stubs (used for mounting samples in XPS spectrometers) were glued to either side of the sample using Araldite, to form simple butt-joints.

Angle-resolved XPS

Joints were fractured in the ultra high vacuum (UHV) preparation chamber of the ESCALAB using a modified VG Scientific T-peel stage.6 The use of a sample manipulator modified to provide 360" rotation enabled both halves of the sample to be retrieved for XPS analysis (using Mg Kcr radiation).

Analyses of both halves of the joint were carried out at a series of take-off anlges (TOAs) (15, 25, 35, 45, 60, 75 and 90') relative to the sample surface; the transfer lens iris assembly was set at a reduced value to provide an acceptance angle of 6", which allows adequate counting statistics within a reasonable time scale. A survey spectrum was recorded initially at 45" TOA and repeated after analysis at the seven angles was complete, to check for sample degradation. High-resolution spectra for the C Is, 0 1s and either Si 2p or A1 2p regions (depending on the substrate) were recorded at each angle. In order to minimize the effects of thermal damage from the heat generated by the X-ray gun, the

gun was partially withdrawn to give a gun-to-sample distance of -2.5 cm. No sample degradation was observed during this procedure; a typical analysis time was around 60-70 min for a complete set of ARXPS data. The C 1s spectra were peak fitted using a procedure established by comparison of ESCALAB spectra of the resin with those obtained at highest resolution using a Scienta ESCA300 spectrometer.6

The completed experimental angular profile (i.e. TOA versus at.%) was used in conjunction with the #ANGULAR computer program, which has been fully described elsewhere in the l i terat~re,~~' to generate a compositional depth profile. In essence, # ANGULAR is a forward transform routine that calculates the expected angular depth profile from a hypothetical compositional depth profile [i.e. depth (nm) versus at.?/o]. Comparison between this calculated angular profile enables the operator to assess whether the hypothetical depth profile is a realistic option. The criterion used is to obtain a mean difference of <lo% between the two angular profile data sets. In practice, the absence of a unique solution to the experimental angular profile means that some prior knowledge concerning the sample is required. In the current work it is sufficient to know that the polymer is present as an overlayer on the inorganic substrate. All ARXPS rou- tines implicitly assume lateral homogeneity unless some coverage factors are included in the calc~lation;~ steps were taken, using a Beer-Lambert model, to ensure validity of this assumption for these specimens.

A recent innovation in ARXPS depth profiling is the application of maximum entropy methods as described by Smith and Livsey." Such methods provide an iter- ative route (using the MaxEnt program) to generate a compositional depth profile from the angular profile when provided with a very general starting point. The merit of this approach is not lost on the current authors, but in the absence of a generally available ARXPS depth profiling routine based on MaxEnt it is felt that the forward transform approach can be as applicable in cases where there is substantial prior knowledge concerning the samples. This is clearly the case for this work.

Analyses of the fracture surfaces of polished alumina joints and related quartz and silicon samples, following 5 weeks of aqueous exposure, were also carried out using a Scienta ESCA300 X-ray photoelectron spectrometer." It had been noted during previous studies that alumina samples appeared to be particularly sus- ceptible to charging during analysis. By utilizing the Scienta's monochromated X-ray source (to provide superior spectral resolution), coupled with an electron flood gun to neutralize charge,' additional valuable information was obtainable. An extended degradation study was also carried out on a quartz sample from a fractured test specimen, with ten spectra being acquired over a period of -5 h in order to determine the time scale over which degradation of the sample occurs for these particular analysis conditions (i.e. monochromated A1 Ka source operated at 14 kV and 200 mA, spectrometer pass energy of 150 eV and slit width of 0.5 mm). The degradation index" of the resin, as assessed by the O/C ratio, was 25. As with the ESCALAB data, all spectra were recorded within a time window where no degradation could be observed.

CHEMICAL SURFACE HETEROGENEITY

7000 C l S (c> ;::::FA; t o 3000 5000

S 2000

1000

0

699

8000 L ' C I S

7000 - c 6000 - 0 u 5000 -

4000 - t S 3000 -

2000 -

294 292 290 288 286 284 282 280 Bind ing Energy / eV

Figure 1. The C 1 s spectra obtained from: (a) ceramic side of the failure, ESCALAB data; (b) ceramic side of the failure, Scienta ESCA300 data; (c) polymer side of the failure, ESCALAB data; (d) polymer side of the failure, Scienta ESCA3OO data.

RESULTS

Quartz samples immersed in water at 50 "C for 3 weeks and fractured in situ failed cohesively, with - 1-2 nm of polymer remaining on the ceramic side of the fracture. The C 1s spectra obtained from varying TOAs indicate that the oxygen-containing functional groups are

oriented closer to the resin/ceramic interface. A similar result was observed for silicon wafer samples fractured under the same conditions. In particular, the CO groups appeared to be even more strongly orientated towards the silicon oxide/resin interface than with the quartz samples.

Analysis of the fracture surface of a polished alumina joint on the ESCALAB produced C 1s spectra that were

795 290 205 260 Binding Energy [eVI

Figure 2. Angle-resolved XPS C 1s spectra obtained from the silicon side of a sample previously immersed in water for 3 weeks and fractured ex situ.


10

0 0 5 1'0 1'5 20 25 30 35 40 45 50

901 \ Depth (A)

100 R

(4

.,. 0 6 I0 1'5 20 25 30 35 40 45 do

Depth (A)

Figure 3. Compositional depth profiles obtained using the #ANGULAR routine on ARXPS data from the ceramic sides of failures: (a) quartz; (b) silicon wafer; (c) polished alumina.

spectrometer (conditions : spectrometer pass energy of 150 eV, 1.1 mm slit width, X-ray source operated at 14 kV and 200 mA). With the flood gun settings optimized, it became clear from the shape of the C 1s peak that immersion in water brought about cohesive failure in joints comprised of polished alumina, and not interfacial failure as suggested by the data from the ESCALAB. A comparison of the C 1s spectra obtained from the two failure surfaces using the ESCALAB and ESCA300 spectrometers is shown in Fig. 1. Analysis of polished alumina at varying TOAs using an ESCA300 also indicated that oxygen groups were attracted towards the ceramic interface, although the effect did not appear to be as pronounced as with quartz and silicon samples. Data obtained from quartz and silicon samples on an ESCA300 were consistent with the ESCALAB data. This result tends to suggest that certain samples (in this case alumina) are more prone to VDC than others, possibly owing to differences in their electrical properties.

Figure 2 shows the ARXPS C 1s spectra (ESCA300) obtained from the silicon side of a failed specimen that had undergone aqueous exposure for 3 weeks. The enhancement of the methoxy component at high TOAs is readily apparent, indicating that these species tend to be adjacent to the inorganic surface. A consistent feature of these and data sets from other substrates is the lack of an aromatic component of the C 1s peak, although this feature is still clearly visible on the polymer side of the failure, as shown in Figs l(c) and

The compositional depth profiles obtained using the #ANGULAR routine for the quartz, silicon and alumina substrates are presented in Fig. 3. It should be noted that because of the problems described above concerning the C 1s spectrum from the alumina substrate acquired on the ESCALAB system, this profile is merely expressed in terms of elemental concentration. For the other substrates, the C 1s spectra have been peak fitted in the manner previously described6 in order to indicate the distribution of the various carbon species within the interphase region.

W).

~~~ ~

DISCUSSION

dissimilar to those obtained from both the quartz and silicon samples. The shape of the peak was broad, yet fairly symmetrical, as might be expected from hydrocarbon contamination [see Figs l(a) and l(b)]. None of the characteristic oxygen-containing functional groups from the polymer were apparent on the ceramic side of the failure, tending to support the idea of failure occurring interfacially or within a pre-existent adventitious layer. Owing to the susceptibility of the alumina to differential charging, it was at this stage unclear whether the carbon detected on the ceramic side of the failure was hydrocarbon contamination or whether vertical differential charging'' (VDC) was in fact obscuring the true shape of the C 1s peak. A common observation for all substrates was the absence of any aromatic character (as deduced from the IL + IL* satellite). The polymer side, however, showed clear evidence of this feature.

The two fracture surfaces of polished alumina samples were also analysed using a Scienta ESCA300

There are two main observations that arise from the depth profiles constructed from the angle-resolved XPS data. Firstly, there is preferred orientation of the oxygenated functional groups within the overlayer of polymer remaining on the ceramic fracture surfaces. In the case of silicon (the most acidic of the substrates investigated), it is the methoxy (CO) component that is most strongly attracted to the interface. This is in con- trast to work carried out by Chehimi et ~ 1 . ' ~ on poly- methylmethacrylate (PMMA) films on glass, where the CO component was found to be most strongly orientated towards the glass. The LuxtrakTM2 resin studied here is based on aromatic methacrylate-type oligomers; more precise structure and formulation details are not accessible because it is a commercial product. It can, therefore only be hypothesized that the differences in orientation observed in this system are the result of steric hindrance effects. Alternatively, the CO groups may in some way be involved in the cross-linking

CHEMICAL SURFACE HETEROGENEITY 70 1

mechanism of the polymer. When dealing with a com- mercially available system of complex formulation, it is more difficult to comment on the acid/base nature of the polymer itself. The fact that certain groups (i.e. C-0 and C=O) are attracted towards very acidic substrates tends to suggest a basic character. Results obtained from polished alumina (amphoteric in nature) also suggest partial orientation of oxygen groups towards the interface (a fact that may have been over- looked without access to a Scienta ESCA300). This apparent anomaly may be explained by the fact that many polymers display mixed character and it depends very much on the material with which they are inter- acting as to whether or not they behave in an acidic or a basic manner (e.g. PMMA, which is regarded as a basic polymer and interacts strongly via the oxygen of the carbonyl group with acidic probes such as trichloro- methane, will exhibit acidic character when probed with a classical base such as ~yridine'~). Analysis of the polymer used in this work by inverse gas chromatog- raphy (IGC)" indicated that this was in fact the case; the polymer displayed mixed character, with a bias towards basic properties.

The next significant observation from the data is the fact that, regardless of the substrate used, the aromatic component of the polymer (the n+n* shake-up satellite) is absent in the polymer overlayer but present on the polymer side of the failure. Such an observation has been reported in the past by Pignataro et ~ 1 . ' ~ ~ ' ~ for markedly different systems. The reason for such behaviour is not clear at present but there would seem to be three potential explanations: (1) sterochemistry effects at the interface prevent the

intimate association of the aromatic component of the resin and the inorganic substrate;

(2) the lack of any specific interactions between these two moieties ensures that interactions between the substrate and the methoxy and carbonyl groups dominate the interphase region;

(3) there are interpolymer interactions resulting from surface free energy considerations that leave the interphase region depleted in aromatic character.

Whatever the source of such a phenomenon, the result tends to suggest a 'weak link' within the polymer through which failure can occur, which is characterized by the presence of aliphatic rather than the aromatic components of the resin. This probably results from the segregation of non-aromatic low-molecular-weight additives of unspecified components in the resin.

Turning now to the depth profiles of Fig. 3, a consistent observation is the aggregation of the polar carbon species adjacent to the inorganic substrate for the acidic materials. A potential difficulty in the analysis of these data concerns the lateral uniformity of the residual polymeric layer. As a test of such uniformity, graphs of In Isubstratc versus l/sin 8 were constructed for the three substrate groups, a straight line indicating the uniformity of the organic overlayer. Such a plot is shown in Fig. 4 for the alumina substrate and the good match achieved (correlation coefficient of 0.999) indicates the uniformity of the overlayer. Values of R = 0.994 and 0.965 were obtained for the quartz and silicon substrates, respectively. The thickness of the overlayer can also be estimated (d = - slope x A) and a value of 1-2 nm was obtained for all substrates, consistent with the

3.5/

0.5 '1 1.5 2 2.5 3 3.5

0 1 4

0 0.5 1

1 I sine 0

Figure 4. Graph of In lsubstrate versus l/sin 0 for a polished alumina sample (R = 0.999'. gradient = -0.43).

data from the depth profiles in Fig. 3. The fact that the silicon data do not fit as well as the other two substrates is assumed to be a result of the forward scat- tering of the Si 2p photoelectrons from the single-crystal substrate. Although buried below some 3 nm of SiO, and polymer overlayer, it is clear that this is sufficient to distort the ARXPS profile. The alternative possibility, that of local variations in the overlayer thickness, was examined by scanning probe microscopy. Atomic force microscopy of the silicon surface indicated a slight undulation of the surface profile with a periodicity of - 100 nm; such a phenomenon appears to replicate that of the polished substrate. These results will be described fully elsewhere.I8

Combining the information provided above, it is possible to postulate a failure mechanism for the polymer/ inorganic surface previously immersed in water at elevated temperature. In the intact state it is envisaged that the carbonyl and the methoxy groups interact with the substrate. Donor-acceptor (or acid-base) interactions are thought to play an important role in this p r o c e s ~ ' ~ * ~ ~ and this means that the interaction is more intense for the more acidic substrates. The methoxy group at the plane of the interface is in greater concentration than the carbonyl-like species, and aromatic species are generally ,absent from this region. This is illustrated in Fig. 5(a), where the cooperative interaction results in a nanoscale interphase region of the type reported for PMMA on acidic s~bstrates. '~

On exposure to water, lateral diffusion from the joint edge brings about failure at the edge of the interphase zone. The interfacial polymer surface is spectro- scopically indistinguishable from the standard XPS spectrum of the bulk polymer.6 Interfacial failure is pre- cluded as a result of the discrete interfacial interactions described above. The organic characteristics of the two (failure) zones of the polymer are described in Fig. 5(b). Although chemically similar, there is a clear distinction between the interphase composition of the polymer applied to an alumina system and to the quartz and silicon systems. This is ascribed to the extent of acid- base interactions achieved between the polymer and the three substrates, and such possibilities should be con- sidered when the as-received alumina substrate is employed. Work is under way using multi-source XPS


I I I I

(4 , 0 o/'o 0 o y b 0 o/\o 0 /-

.... 9 d . L .......... \,IL .... ..... 1 . L L .......... LLP Si Si Si Si

Figure 5. Schematic of the polymer/ceramic substrate nanoscale interphase region: (a) before failure; (b) after failure.

to establish which model is most applicable for this substrate. A complete understanding of interphase chemistry provides a potential route to the design of a system with enhanced durability. The next stage of the work will be to develop a correlation between interphase chemistry, durability and mechanical properties for the systems described in this paper.

CONCLUSION

From this study it can be concluded, firstly, that extreme care must be taken when attempting to establish the precise locus of failure within an adhesive joint comprised of two insulating materials. By elimination of differential charging, the observation of fine structure within the C 1s peak becomes possible.

The LuxtakTM resins exhibit slightly basic properties, especially in the presence of a strongly acidic substrate such as silicon. This is evident from the orientation of the methoxy and, to a lesser extent, the carbonyl groups adjacent to the inorganic substrate. The absence of any aromatic component in the C 1s spectrum from the ceramic side of the failure (for all substrates investigated) indicates a depletion of this species within a nanoscale interphase region; this is the result of the interface segregation of minor components of the resin.

Acknowledgement

The financial support of both I C I Chemicals & Polymers Ltd. and SERC i s gratefully acknowledged by A.M.T. for funding this case award. Special thanks also go to S. J. Greaves for technical advice and informative discussions.

REFERENCES

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4. D. R. Clarke, J. Am. Ceram. Soc. MaylJune, 339 (1 980). 5. L. H. Lee (ed.), Fundamentals of Adhesion, p. 106 Plenum

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7. R. W. Paynter, Surf. Interface Anal. 3, 186 (1 981 ). 8. J. F. Watts, J. E. Castle and S. J. Ludlam, J. Mater. Sci. 21,

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15. M. M. Chehirni and J. F. Watts, unpublished results. 16. A. Torrisi, G. Marletta, 0. Puglisi and S. Pignataro, Surf. Inter-

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role of chemical surface heterogeneity in the adhesion of photocured resins to ceramic substrates

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