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Langmuir 2000, 16, 1078-1082
Thermally Induced Intramolecular Surface Reactions on a Photoimageable Dielectric Anastasios P. Angelopoulos* Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts 01854
Luis J. Matienzo and Gerald W. Jones IBM Microelectronics, Endicott, New York 13760 Received December 8, 1998. In Final Form: October 22, 1999 The effect of bake temperature on the surface chemistry of a photoimageable dielectric is investigated. The dielectric contains mixtures of multifunctional bisphenol A based epoxies which are polymerized using cationic aryl sulfonium hexafluoroantimonate. Surface characterization via X-ray photoelectron spectroscopy and contact-angle titration reveals that baking the dielectric yields nucleophilic sulfur substitution of hydroxylated surface species to produce a diphenyl sulfoxide reaction product. Such reactivity provides a convenient means of altering the dielectric surface acidity. Surface acidity is shown to be well correlated to the adsorption of a high molecular weight cationic polyacrylamide deposited from solution during microelectronics fabrication.
I. Introduction Epoxy-based photoimageable dielectrics are finding increased use in the microelectronics industry for the sequential buildup of multilayer boards with high circuit line density.1 Such boards are produced by applying a photoimageable dielectric onto a previously circuitized substrate, forming vias with UV light, baking to fully cure the dielectric, and metallizing the dielectric surface to form the desired circuit pattern. Prior to metallization and after cure, the dielectric surface must be textured with a series of mechanical abrasion and/or wet-chemical etch steps to promote adhesion to the copper metallization layer which is subsequently deposited. The overall process may then be repeated as many times as desired. In what is known as a “fully additive” process, copper metallization is performed from the liquid phase by first depositing a noble metal colloidal catalyst from suspension onto the dielectric surface.2 A photoresist is then applied directly onto the catalyzed dielectric surface, areas are masked where metallization is not desired, and the circuit pattern formed by placing the board in a solution of copper sulfate and formaldehyde. Careful control of catalyst deposition is required to produce a high-quality metal deposit from such a process sequence. Too little catalyst will result in voiding of the metal deposit and the presence of “opens” in the final circuit pattern. On the other hand, too much catalyst will result in separation of the plating resist from the dielectric surface, leaking of plating solution beneath the photoresist, and the creation of shorts in the circuit pattern. One method of ensuring sufficient catalyst deposition onto the surface of bisphenol A based epoxy resins such as FR-4 is to adsorb a cationic polyacrylamide from solution onto the resin surface immediately prior to immersion into the catalyst suspension.3,4 However, excess catalyst deposition and poor circuit line quality occur when the polyacrylamide is adsorbed onto photoimageable dielec* To whom correspondence is to be addressed. (1) Crum, S. Electron. Packag. Prod. 1996, 33. (2) Seraphim, D. P., Lasky, R. C., Li, C.-Y., Eds. Principles of Electronic Packaging; McGraw-Hill: New York, 1988.
trics. Baking the fully cured dielectrics at various temperatures after surface texturing but prior to polyacrylamide adsorption, while having no impact on surface texture, causes substantial variation in catalyst deposition. The amount of polyacrylamide adsorbed onto epoxy resin substrates depends on both the pH of the polyacrylamide solution as well as subtle changes in the acid/base character of the substrate surface.4,5 Consequently, to optimize the catalyst deposition process onto photoimageable dielectrics, changes in surface chemistry due to such a bake step were closely investigated. II. Experimental Section II.A. Materials. The photoimageable dielectric investigated in this study is known as ASM (advanced solder mask) and was obtained fully cured from the IBM Corporation in Endicott, NY. The dielectric surface had been previously textured using physical abrasion and chemical etching in hot alkaline permanganate solution. ASM contains mixtures of multifunctional bisphenol A based epoxies which are polymerized using cationic aryl sulfonium hexafluoroantimonate. Sulfonium salts are commercially available as UVI from Union Carbide and have well-known UV absorbance curves. Irradiation of these materials with light from a mercury arc lamp generates a strong acid which initiates the ring opening of epoxies. The effect of baking as-received ASM samples was investigated by employing a vacuum oven which permitted sample exposure to various ambient conditions, including air, N2, and O2. Bake time in every case was 1 h at the desired bake temperature. Surface properties, as characterized using wetting, X-ray photoelectron spectroscopy (XPS), and catalyst deposition, did not change appreciably with longer bake times. All samples of the as-received dielectric were cleaned for 5 min in a hot alkaline solution containing sodium silicate to remove fingerprints and other potential contaminants prior to catalyst (3) (a) Alpaugh, W. A.; Amelio, W. J.; Markovich, V.; Sambucetti, C. J. U.S. Patent 4,525,390, 1985. (b) Amelio, W. J.; Lemon, G. K.; Markovich, V.; Panasik, T.; Sambucetti, C. J.; Trevitt, D. J. U.S. Patent 4,448,804, 1984. (c) Angelopoulos, A. P.; Jones, G. W.; Malek, R. W.; Marcello, H.; McKeveny, J. U.S. Patent 5,866,237, 1999. (4) Angelopoulos, A. P.; Benziger, J. B.; Wesson, S. P. J. Colloid Interface Sci. 1997, 185, 147. (5) Angelopoulos, A. P.; Benziger, J. B.; Matienzo, L. J. J. Colloid Interface Sci. 1999, 212, 419.
10.1021/la981692w CCC: $19.00 © 2000 American Chemical Society Published on Web 11/24/1999
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Table 1. Surface Tension Components and Wetting Behavior of Various Liquids surface tension components (dyn/cm)
work of adhesion from advancing wetting mode (dyn/cm)
contact angle (deg) from advancing wetting mode
liquid
γ
γd
γ+
γ-
no bake
bake at 200 °C
no bake
bake at 200 °C
water ethylene glycol diiodomethane
72.8 48 50.8
21.8 29 50.8
25.5 1.9 ∼0
25.5 47 ∼0
42.0 ( 1.8 89.9 ( 2.4 98.1 ( 0.1
29.9 ( 2.0 78.5 ( 2.7 94.8 ( 0.5
115.0 ( 1.6 29.2 ( 6.0 21.4 ( 0.3
126.1 ( 2.0 50.5 ( 4.2 30.0 ( 1.2
deposition. Samples were then thoroughly rinsed in hot deionized (DI) water for 2 min. Following cleaning, catalyst deposition was immediately performed on the wet dielectric surface at room temperature from a suspension containing 2 g of PdCl2, 100 g of SnCl2, 175 g of NaCl, 200 mL of 37% HCl, and 0.1 g of a fluorocarbon surfactant in 1 L of water.4 Immersion time was 3 min followed by rinsing in DI water for 2 min. Removal of excess Sn (but little Pd) from the surfaces following catalyst deposition was accomplished by a 2 min immersion in 0.5 N NaOH, followed by a 2 min rinse in DI water. This step is typically used to accelerate the initial rate of metallization by exposing more of the catalytic Pd. Samples were thoroughly dried after catalyst deposition and then metallized with copper by immersion into a standard electroless plating bath containing copper sulfate and formaldehyde.2 The process described above typically leads to a catalyst deposition of about 2 µg/cm2 Pd on the as-received samples. This amount is found to be inadequate and typically leads to voids in the metallization layer. To enhance catalyst deposition, the dielectrics were also exposed to a polyelectrolyte solution immediately prior to immersion into the catalyst suspension but after cleaning. The polyelectrolyte has been previously characterized4 and consists of a cationic polyacrylamide containing a weakly acidic fraction of prehydrolyzed amide groups. Cationic functionality is achieved with pendant quarternary amine groups along the backbone. Polyelectrolyte concentration in solution was 0.6 g/L. The pH was adjusted to between 0 and 1 with H2SO4 (Baker Chemical). We have previously shown that the polyelectrolyte possesses only positive charges along the backbone under such acidic conditions.4 Immersion time in the polyelectrolyte solution was followed by a 2 min rinse in DI water. The surfaces were not dried prior to immersion into the catalyst suspension. Catalyst deposition on the as-received samples treated with the polyacrylamide exhibited almost an order of magnitude increase in catalyst deposition to about 20 µg/cm2 Pd. Such excess catalyst was found to yield poor circuit line definition, and a method for reducing catalyst deposition was clearly required. II.B. Methods. Wetting is a powerful tool for characterizing the various intermolecular bonding components of surfaces.4-9 Semiempirical techniques introduced by Fowkes and Good decompose the solid surface tension, γ, into its acidic, γ+, basic, γ-, and van der Waals, γd, bonding components.6 The key limitation of such a quantitative approach is that solid surfaces to be evaluated must be sufficiently smooth to isolate the effects of surface chemistry on wetting. For rough surfaces, as employed in this study, one is unable to separate the effects of surface chemistry from those of surface texture. Hence, only qualitative comparisons are possible for surfaces of comparable roughness. Table 1 lists the fundamental surface tension components of the three liquids employed to probe changes in the acid, base, and van der Waals bonding character of the textured surfaces. Diiodomethane (Aldrich) exhibits little acid/base character, unbuffered deionized (DI) water (pH ∼ 6) is bipolar, exhibiting equivalent acidic and basic character, and ethylene glycol (Aldrich) exhibits primarily monopolar basic character. Another useful technique which employs the surface sensitivity of wetting permits titration of ionizable surface groups.4,7 Ions are more readily solvated than neutral species. Hence, dissociation of surface groups enhances the work of adhesion between solid and liquid. Two plateaus in the work of adhesion occur as (6) Good, R. J.; van Oss, C. J. In Modern Approaches to Wettability; Schrader, M. E., Loeb, G. I., Eds.; Plenum Press: New York, 1992. (7) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725. (8) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (9) Liu, F. P.; Gardner, D. J.; Wolcott, M. P. Langmuir 1995, 11, 2674.
the pH of the wetting solution is altered. The first occurs when the surface is completely neutral and the second when all available surface groups have ionized. The inflection point between these two plateaus represents the continuing variation of the degree of dissociation. One may apply the Cassie relationship8 within this region to determine the degree of dissociation, K, at a given pH value
W ) (1 - R)W1 + RW2
(1)
where W1 is the work of adhesion for the neutral surface and W2 is the work of adhesion for the completely ionized surface. W is the observed work of adhesion at a given pH. This equation may be derived by treating wetting as a reversible, energy activated process.9 NaOH (Baker) and H2SO4 (Baker) in water were used to carry out the titrations. The liquid-vapor interfacial tension of test water prepared at different pH values remained to within 3% of the mean value obtained for pure DI water (71.9 dyn/cm). Wetting analyses for this study were performed via the Wilhelmy plate method.4 Work of adhesion measurements were obtained using a Sigma 70 dynamic wetting balance from KSV Instruments with a retractable stage velocity of 0.2 mm/min. All samples examined were completely wetted in the receding mode due to surface texture regardless of the wetting liquid (one may visually observe the liquid clinging to the solid as it is being retracted). Hence only data obtained as the wetting meniscus advanced across the solid surface are reported. Once the degree of dissociation has been obtained as a function of pH, one may proceed to evaluate the dissociation constant. For first-order dissociation of pendant groups on a polyacid, the dissociation constant may be obtained from the extended Henderson-Hasselbalch equation10
(1 -R R)
pH ) pKa + n log
(2)
where n accounts for the proximity of ionized species on a polyacid and is found to vary between -1 (the value for monoacids) and -2. The pKa may be obtained directly from a titration curve by realizing the pH ) pKa when R ) 0.5 or, in the case of contact angle titration curves, when (W - W1)/(W2 - W1) ) 0.5. XPS of the polymer surfaces was performed with a PHI 5550 multiprobe spectrometer using Al KR radiation. To enhance the low signals of the S2p regions, spectra were collected for a minimum of 10 h in the high-resolution mode using a pass energy of 11.95 eV. The reported binding energies were referenced to the C1s signal present at 284.6 eV. In all cases, charge neutralization was applied in order to compensate for charging effects. The absence of nitrogen in the spectrum for the dielectric suggested the possibility that the nitrogen signal could be followed to determine variation in polyacrylamide adsorption. However, direct determination of the polyacrylamide via XPS was not possible on the highly textured industrial surfaces. Instead, changes in polyacrylamide surface coverage were inferred from changes in catalyst deposition. Scanning electron microscopy (SEM) of the samples were obtained with a JEOL microscope. Samples were sputter coated with 300 Å of platinum and imaged at 10 or 15 keV accelerating voltage. Analysis of the Pd catalyst deposits was performed with a JEOL 733 superprobe with a Tracor Northern TN-5500 X-ray analyzer.
III. Results III.A. Catalyst Deposition and Surface Texture. Baking the as-received samples had a substantial effect (10) Mandel, M. In Encyclopedia of Polymer Science and Engineering; Wiley: New York, 1988; Vol 11.
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Figure 1. Palladium catalyst levels on photoimageable dielectric after cationic polyacrylamide adsorption versus bake temperature.
on catalyst deposition in the presence of the cationic polyacrylamide. Varying the temperature of the bake between 25 and 180 °C resulted in catalyst deposition with a range falling roughly between that on the unbaked samples with the polyacrylamide (20 µg/cm2 Pd) and that on the unbaked samples without polyacrylamide (2 µg/ cm2 Pd). The effect of bake temperature on the amount of catalyst deposited is shown in Figure 1. A linear variation in catalyst deposition with bake temperature is observed with a correlation coefficient of 0.93. SEM images of the ASM surface before and after baking in air at 180 °C are shown in panels a and b of Figure 2, respectively. Baking is found to have little effect on the ASM surface texture. The surface roughness in both figures is about 5 µm and is comparable to other epoxy resin substrates such as FR-4, which are metallized using a similar process.4 Variation in catalyst deposition with bake temperature, as shown in Figure 1, is thus attributed primarily to alteration of the ASM surface chemical composition. Such a result is consistent with the observation that catalyst deposition is insensitive to bake temperature in the absence of cationic polyacrylamide. Alteration of the surface texture would be expected to impact catalyst deposition whether or not the polyacrylamide is present. III.B.Wetting. The presence of ionizable groups on the surface of the dielectric was detected using contact angle titration. Figure 3 shows the effect of the pH of the wetting solution (water) on the measured work of adhesion on the as-received (unbaked) dielectric surface. Whereas the surface is almost completely wetted by strongly alkaline solutions, the work of adhesion declines with the pH of the wetting solution, approaching a plateau at a value of about 40 dyn/cm for pH values below about 9. The inflection point occurs at a pH value of about 10, suggesting that this is the pKa of the weakly acidic surface groups. The surface of the dielectric is therefore negatively charged under sufficiently alkaline conditions. The source of these ionizable groups can be either hydroxyl end groups from the epoxy polymerization reaction or hydroxyl byproducts of epoxy resin hydrolysis in the hot alkaline permanganate etch process used to texture the surface. The precise
Figure 2. Scanning electron microscopy image of the photoimageable dielectric at 2000×: (a) unbaked; (b) baked at 180 °C.
identity of these groups could not be established when the X-ray spectra in the oxygen and carbon binding energy regions were examined. A regression line has been fitted to the experimental data for the unbaked ASM substrate in Figure 3 as the alkaline wetting plateau is approached. The effect of bake temperature on contact angle titration behavior is also depicted in Figure 3. When the film is baked at 160 °C, wetting by 1 N NaOH solution is substantially reduced relative to the unbaked surface. At 200 °C, wetting by 1 N NaOH solution approaches that of unbuffered DI water (pH ∼ 6). A regression line has been fitted to the wetting data for the ASM substrate baked at 200 °C in Figure 3. Applying the Cassie relationship to the 1 N NaOH wetting data suggests that the fraction of ionizable surface groups has an inverse linear relationship with temperature with a correlation coefficient of 0.95. Finally, the effect of baking the dielectric surface at 200 °C in pure oxygen and pure nitrogen is also shown in Figure 3 and indicates that the ionizable functionality is eliminated at this temperature regardless of the ambient gas. The effect of baking the dielectric at 200 °C on wetting by three probe liquids is shown in Table 1. From the average work of adhesion and contact angle values shown,
Surface Reactions on a Photoimageable Dielectric
Langmuir, Vol. 16, No. 3, 2000 1081 Table 2. Percentage of Total Atomic Sulfur Obtained from X-ray Photoelectron Spectroscopy % binding energy at peak emission (relative to C1s peak at 284.6 eV) sample treatment
164 eV
169 eV
unbaked (as-received) baked at 140 °C for 1 h baked at 200 °C for 1 h
74 48 30
26 52 70
Scheme 1
Figure 3. Contact angle titration of photoimageable dielectric surfaces.
Figure 4. X-ray photoelectron spectrum for the S2p binding energy region of the “as-received” (unbaked) photoimageable dielectric.
baking inhibits wetting by ethylene glycol and water. The ASM surface is essentially completely wetted by diiodomethane whether or not it is baked. III.C. XPS. The surface concentration of sulfur on the dielectric was determined via XPS to be between 0.2 and 0.3 atom %. This value is comparable to the bulk sulfur concentration typically found in photoimageable dielectrics due to the cationic photoinitiator. The majority of the sulfur detected by XPS occurs at a peak with a S2p binding energy of 164 eV (relative to C1s signal at 284.6 eV). However, two local sulfur environments may be clearly distinguished in the X-ray spectra, as shown in Figure 4, for the asreceived (“unbaked”) sample. The second sulfur peak occurred at the higher energy of 169 eV, which indicated a somewhat higher oxidation state. Curve fitting determined that about 74% of the total sulfur signal may be attributed to the lower oxidation state. The source of the lower sulfur oxidation state is likely to be the diphenyl sulfide irradiation product of the cationic sulfonium photoinitiator. One possible source for the higher oxidation state may be unreacted photoinitiator. However, one cannot preclude oxidation of the diphenyl sulfide during subsequent processing, such as baking to fully cure the dielectric as well as etching in hot alkaline permanganate solution to texture the surface. The primary change in the X-ray photoelectron spectra of the ASM surface which occurred with baking was
observed in the sulfur binding energy region. Table 2 shows the percentage of total sulfur obtained through curve fitting which may be assigned to each of two sulfur oxidation states after various bake treatments. When the surface is baked, a distinct shift in the relative amount of oxidized sulfur species is observed. As shown in Table 2 for a surface baked at 200 °C, the percent of total sulfur with the lower oxidation state becomes 30%, a complete reversal in the relative amounts of the two sulfur species as compared to the unbaked surface. Temperatures below 200 °C exhibit intermediate shifts in the relative amounts of sulfur oxidation states, as shown in Table 2 by the dielectric sample baked at 140 °C. To determine whether ambient oxygen participates in this oxidation process, samples which were baked in pure nitrogen and pure oxygen were also examined. The same shift in the relative amounts of the two sulfur oxidation states was observed regardless of the ambient gas employed. IV. Discussion Thermal modification of titratable surface groups on the industrial photoimageable substrates investigated in the present study without alteration of surface texture permits qualitative evaluation of the effect of surface acidity on the adsorption behavior of the cationic polyacrylamide employed to enhance catalyst deposition. Figure 3 demonstrates that baking the photoimageable dielectric at a sufficiently high temperatures eliminates weakly acidic groups from the surface of the dielectric. Comparison of this result with Figure 1 indicates that elimination of acidic surface groups substantially reduces polyelectrolyte adsorption and the associated catalyst deposition. Such positive correlation between substrate acidity and the amount of polyelectrolyte adsorbed suggests specific interaction between acidic substrate sites and basic amide groups on the polyacrylamide (which constitute roughly 80% of the total pendant groups4). Examination of the substrate surfaces with XPS provides detailed information as to how thermally activated reduction of surface acidity occurs. The X-ray data shown in Table 2 indicate that sulfur oxidation is occurring simultaneously with the elimination of the ionizable surface groups. This behavior is apparent regardless of the ambient gas employed (air, nitrogen, or oxygen). An energy-activated intramolecular surface reaction scheme which would account these observations envisions nucleophilic substitution by sulfur according to Scheme 1. The sulfur-based reactant in this scheme is the diphenyl sulfide irradiation product of the cationic sulfonium salt.
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The hydroxyl reactant group can be either an end group from epoxy polymerization or the hydrolysis product of epoxy resin etching to texture the surface of the photoimageable dielectric. The thermally activated product of Scheme 1 when these reactant groups are in close proximity is diphenyl sulfoxide. The oxidation of sulfides via nucleophilic substitution, to produce dimethyl sulfoxide (DMSO) for example, is well-known. The fact that such thermally activated reaction occurs on previously fully cured samples suggests that surface texturing in permanganate solution after cure plays a major role in the excess colloidal catalyst deposition observed on the as-received (unbaked) samples. Resin hydrolysis in the hot alkaline permanganate solution, which substantially increases the surface concentration of hydroxyl groups, is a likely mechanism for this. A reduction in the hydrogen bonding capacity of the ASM surface with baking is also suggested by the decline in the water and ethylene glycol work of adhesion as shown in Table 1. Wetting by diiodomethane, which interacts primarily via dispersion interactions, is not appreciably affected by baking. This observation is consistent with the elimination of surface hydroxyl groups, as shown in Scheme 1, to produce low hydrogen-bonding sulfoxides
Angelopoulos et al.
(e.g., the acid-base surface tension component of DMSO has a value of 8 dyn/cm, compared to a value of 25.5 dyn/ cm for water8 and 19 dyn/cm for ethylene glycol6). V. Conclusions 1. Evidence is presented for a diphenyl sulfoxide reaction product on the surface of a textured photoimageable dielectric via thermally activated intramolecular nucleophilic sulfur substitution. This reaction provides a convenient means for reducing the amount of colloidal metal catalyst deposited during microelectronics fabrication without alteration of the substrate surface texture. 2. Dielectric surface modification is preferred over other methods of reducing catalyst deposition (e.g., the use of alkaline polyelectrolyte solutions instead of acidic polyelectrolyte solutions4) due to the fact that the same catalyst deposition process may be employed regardless of substrate. 3. A positive correlation is found between substrate surface acidity, as quantified via contact angle titration, and the adsorption of cationic polyacrylamides, as inferred from microprobe data of Pd catalyst deposition. LA981692W
