Improvements of the EpoxyâCopper Adhesion for Microelectronic

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Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Improvements of the Epoxy−Copper Adhesion for Microelectronic Applications Leŕ ys Granado,*,†,‡,§ Stefan Kempa,*,† Laurence John Gregoriades,† Frank Brüning,† Tobias Bernhard,† Valeŕ ie Flaud,§ Eric Anglaret,‡ and Nicole Fret́ y§ †

Atotech Deutschland GmbH, Erasmusstraβe 20, Berlin 10553, Germany L2C, CNRS, and §ICGM, CNRS, Univ. Montpellier, Montpellier, France

‡

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S Supporting Information *

ABSTRACT: Upcoming electronic devices are required to be further miniaturized. The microelectronics industry and packaging technology focus efforts on optimizing adhesion between plated copper and printed circuit board (PCB) substrate (here epoxy/glass buildup composite), while keeping smooth surfaces for high-frequency application. We propose herein to review and deepen the basic understanding of the sequential buildup process employed worldwide for the past several decades for multilayered PCB manufacturing. This multiscale and interdisciplinary study aims to establish the relationships between degrees of precuring (α), oxidative etching (permanganate desmear wet treatment), and copper adhesion. The epoxy curing states on industrial coupons were evaluated by diffuse-reflectance infrared Fourier transform spectroscopy. Then, atomic force microscopy (AFM) described the desmear performances through topography evolution with α between the different sequence steps. Finally, polymer−copper adhesions were investigated by using peel strength tests, AFM, and X-ray photoemission spectroscopy. We remark that high adhesion strengths were obtained for very smooth surfaces. This study outlines the contribution of the polymer network viscoelasticity (relaxation dynamic) on the polymer−copper adhesion. We observed that faster polymer relaxation rates tended to increase polymer−copper adhesions. KEYWORDS: microelectronics, curing, wet treatment, polymer−metal adhesion, surface analyses

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INTRODUCTION In the course of electronic device miniaturization,1 the microelectronics industry concentrates efforts on the integration design and packaging; this is sometimes called the “More than Moore” approach.2 Especially, integrated-circuit (IC) substrates consisting of multilayer printed circuit boards (PCB) are required to be further miniaturized with highly dense interconnections.3 To manufacture very fine copper circuitry with line and space width lower than 1 μm (e.g., fanout level packaging), the adhesion between the copper and dielectric substrate needs to be increased. Additionally, highfrequency applications require low surface roughness. Silicon and glass interposers are relevant materials, but they are expensive.4 Still, a more cost-effective option relies on the improvement of organic buildup (BU) layer performances and their processing optimizations. Buildup layers mainly consist of organic polymers or organic−matrix composites. Among the wide range of BU available,5 epoxy−phenol/silica filler composites are being massively used for PCB manufacturing.6 Epoxy matrices present serious advantages such as adhesion, wear resistance, good thermomechanical properties, and ease of processing (i.e., out-of-autoclave curing process).7−9 The use of phenolic © XXXX American Chemical Society

hardener is justified by its cost efficiency, good dielectric, and thermomechanical and intrinsic flame retardance behaviors.10 Glass fibers or fillers bring thermomechanical stability of the final piece,11 reducing the thermal expansion mismatch between the buildup layer and the overlying materials (copper, IC) and ensuring additional heat dissipation. The adhesion between BU and copper can be effectively improved by surface modification approaches, such as the etching wet treatments12 and plasma ablation13−15 and more recently selective electroless metallization with the use of ligands,16−20 UV-ozone treatments,21 and so on. These latter approaches yield excellent and promising results, but they still remain poorly scaled at the industrial level. For example, the plasma ablation enables notable adhesion enhancement thanks to high surface oxidation.13−15 However, plasma treatment presents difficulties for industrial applications due to timeconsuming batch-to-batch treatment as well as excessive costs. Still, most of the PCB found in current smartphones, ultrabooks, and other electronic devices of our daily life are Received: May 10, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acsaelm.9b00290 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Electronic Materials

Scheme 1. Three Steps of the Permanganate Desmear Sequence: (1) Sweller Molecules Diffuse through the Epoxy Polymer Network; (2) Permanganate Ions Attack the Polymers from the Surface Leading to Cavities at the Polymer Surface (Dotted Line); (3) MnO2 Is Reactively Dissolved by Reduction with Hydroxylammonium (or Peroxide), Yielding Aqueous Manganese Ions and Gaseous Nitrogen (or Water)

oxidative etching bath, the permanganate ions preferentially attack the polymer chains in the previously formed clusters because of electrostatic interactions. Indeed, it has been determined that the permanganate ions attack on both carbons in the α-position of the alcohol and ether groups, the inductive effect from the oxygen leading to a weakening of the vicinal C−C bonds.35,36 The conditions of oxidation are strong, leading to an alteration of the resin, removing up to a few micrometers thickness from the surface. Previous studies of desmear treatments concluded that the permanganate etching has a double action on epoxy polymers: creation of a roughness, ensuring the phase interlocking (i.e., anchoring points for mechanical adhesion) and oxidation of the polymer, allowing the enhancement of substrate wettability and further interaction with copper (chemical adhesion).13 The performances of desmear and the final adhesion of the copper onto BU vary strongly with the material state. Especially, Lee and co-workers showed that the key factor to control to achieve a maximum reliability is the upstream precuring stage (partial cross-linking) of the thermosetting BU.37,38 Furthermore, Zhang et al. showed that the copper adhesion on nondesmeared BU varies strongly with the degree of curing achieved during the precuring stage.39 Thus, precuring and desmear were suggested to be two crucial steps for enhancement of epoxy to copper adhesion. However, there is still an important lack in the mechanistic comprehension of the relationships between precuring, desmear, and polymer−copper adhesion. Therefore, we previously reported a series of investigations focused on the measurement and the control of the curing kinetics in an industrially relevant epoxy buildup. Diffusereflectance infrared Fourier transform (DRIFT) absorption

manufactured by using sequential buildup processes with wet treatment, which consists of alternating organic buildup, desmear wet treatment, and electroless and then electrolytic copper plating yielding multilayer PCB.22−26 We propose herein to review and deepen the basic understanding of this industrial process employed worldwide for the past several decades. The desmear is a key process prior to copper plating to ensure proper adhesion. Two desmear strategies are available: plasma ablation and wet treatment. Yet again, wet treatment is the preferred approach for most manufacturers. Among the wet treatments, alkaline permanganate etching was developed during the 1980s;12 it is efficient and cheap and continues to be the most widely used desmear. Permanganate etching presents the advantages of being more environmental friendly than chromic acid etching, and it is readily applicable to a wide class of BU polymers and composites.6,27−29 The permanganate desmear sequence consists of three successive baths in which the PCB are immersed:30 (1) sweller, (2) permanganate etching, and (3) reduction of manganese oxide, as shown in Scheme 1. The first treatment causes the swelling of the polymer BU matrix. Sweller molecules, for example 2-(2-butoxyethoxy)ethanol, diffuse into the epoxy matrix, creating hydrogen bonds in between these polymer chains.30,31 As observed earlier by Schröer et al., the created hydrogen bonding leads to a spacing of the polymer chains and thus to the macroscopic swelling of the matrix.31 Siau and co-workers proposed that the sweller leads to bending of polymer segments at the surface,32,33 yielding accumulation of dense polar group clusters, in agreement with atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) results.34 Then, during the B


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Sequential Buildup Process. All experiments were done in triplicates (4 × 7 cm2 sized cut coupons). After lamination and precuring, the ABF surface was desmeared. AFM topography measurements were performed after precuring (before desmearing), after swelling, and after desmearing (prior to copper plating). Then, ABF surfaces were chemically cleaned during pretreatments (removal of dust, fingerprints, etc.). The subsequent activation stage consisted of a predip step (protecting the next bath from drag-in), the activation step (palladium ions), and then reduction to metallic palladium. The adsorbed palladium clusters act as heterogeneous catalyst for electroless copper deposition. After electroless plating, the coupons were annealed for copper recrystallization at 140 °C for 1 h. Then, the galvanic copper layer, 35 μm thick, was electroplated on the electroless copper seed layer. Finally, the galvanic copper was recrystallized at 190 °C for 1 h, by that means also achieving the final curing of the ABF layers (postcuring). The peel strength tests were subsequently performed. The XPS analysis was performed on peeled fracture surfaces on both copper strip and ABF sides. Experimental procedures as well as characterization techniques are further detailed in the Supporting Information (Table S1). The data throughout the article are presented along with the standard uncertainty (±) involved in the measurements based on one standard deviation (1σ).

spectroscopy has been proven to be a reliable and accurate technique to measure the degree of curing on industrial panels, in agreement with calorimetric results.40 Furthermore, a deep knowledge of curing behavior and kinetics was provided for the control of the curing process.41,42 With these methods at hand, the present investigation aims to establish the relationships between the precuring step, the desmear performances, and polymer−copper adhesion, which currently remain to be poorly known. Thus, the chemical and rheological states of the epoxy matrix are hereafter proposed to be some of the central parameters. Finally, we propose an adhesion mechanism and how the adhesion can be further improved, whereas decreasing the surface roughness by 1 order of magnitude.

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MATERIALS AND METHODS

Materials. The buildup layer studied was an Ajinomoto buildup film (ABF) by Ajinomoto Fine Techno., Co. (currently in use for PCB mass production4,15,43,44 and especially in smartphones assembly). This buildup sheet consists of an epoxy−phenol matrix and spherical glass fillers. The amount of the fillers was as high as 63 wt % (from thermogravimetric analysis). The filler diameters range from a few tens of nanometers to a few micrometers. The buildup layer is received as b-stage films (i.e., considered uncured resin, stored at −18 °C), 35 μm thick, stacked between a supporter poly(ethylene terephthalate) (PET) film and a cover film. PCB coupons studied herein (Figure 1) consist of one double-sided FR-4 core (1 mm

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RESULTS AND DISCUSSION

Topography Evolution. The evolution of the root-meansquare roughness RMS as a function of the precuring degree after precuring, swelling, and the whole desmear sequence is depicted in Figure 2a (with according AFM micrographs in Figure 2b−e). The precuring step leads to a smoothing of the ABF surface: RMS decreases by a factor 2 (double arrow in Figures 2a and 2b,c). This is explained by the formation of a resin-rich (filler-poor) surface layer, i.e., “butter layer”. The butter layer leads to a significantly lower roughness, from 35 nm for α = 0 to 15 nm for α > 0. The interface ABF/PET would preferentially comprise similar organic phases (epoxy polymer) rather than an organic/inorganic composite (glass fillers), following the principle of interface energy minimization. Hence, the butter layer is mainly composed of the resin, with a layer thickness of a hundred nanometers to a few micrometers. This effect is desirable for high-frequency applications, providing desirable low roughness surfaces. On the other hand, the sweller treatment tends to slightly increase the surface roughness for αgel < α.The uncured ABF is completely dissolved by the sweller solution. For α = 0.25, the sweller solution partially dissolves the epoxy matrix, leading to more exposed fillers at the surface and thus drastically increasing the roughness. After gelation, the swelling of the matrix leads to a small increase of the roughness because of the presence of the percolated and cross-linked polymer network that limits drastic diffusion of the sweller. The slight roughness increase is assigned to the formation of polymer clusters at the surface, as previously theorized by Siau et al.32 After the subsequent permanganate etching, a large number of cavities are observed at the ABF surface (Figure 2d,e). The weight loss during this step ranges from 0.1 to 0.3 mg/cm2. These cavities come from severe breakdown of polymer segments by permanganate oxidation and are relatively wide (few micrometers) and deep (down to one micrometer). Thus, the roughness dramatically increases by a factor of 10−20, as compared to untreated surfaces, reaching a few hundreds of nanometers. As expected, the lower the degree of precuring, the weaker the chemical resistance and the higher the roughness after desmearing.

Figure 1. SEM micrographs of printed circuit board coupon cross sections. (1) FR-4 epoxy/glass fiber composite (note the out-of-plane glass fibers in light gray), (2) copper clad, (3) ABF epoxy/glass filler composite, and (4) embedding resin for cross-section imaging. Inset is a SEM micrograph with higher magnification. The secondary electron detection mode was employed.

thick), copper clads (36 μm thick), and ABF layers (35 μm thick) laminated on top of each side. The lamination was performed with a pilot-scale hot pressing apparatus with 50 × 50 cm2 sized panels, at 100 °C for 2 min, with a pressure of 6 × 105 Pa. Curing Behavior. The precuring of the epoxy matrix was performed at 180 °C for 5−90 min. The partial degrees of curing achieved during this precuring stage (i.e., degrees of precuring, α) were controlled by DRIFT, with the methodology previously reported.40 The glassy and rubbery states of the ABF layers were identified in an enhanced temperature−time transformation diagram which was published elsewhere.42 The ultimate glass transition temperature, Tg, of the fully cured ABF was as high as 155 °C (DSC). The gel point, i.e., the degree of precuring at which the percolated cross-linking network was formed, was αgel = 0.58, as determined by dynamic mechanic analysis (DMA). C


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Figure 2. (a) RMS roughness of uncured, nondesmeared, swollen, and desmeared ABF as a function of the degree of precuring. Dotted lines highlight the smoothing occurring during curing. The dotted line represents the gel point. Corresponding AFM micrographs b (α = 0, before desmear), c (α = 0.58, after sweller), d (α = 0.59, after desmear), and e (α = 1, after desmear). Error bars represent standard deviations over five measurements.

Overall, one notes that the ungelled ABF (α < αgel) is completely etched away in the permanganate bath. The fact that the matrix is ungelled explains the very poor chemical resistance to oxidation. Interestingly, the roughness strongly decreases for increasing degrees of curing up to α < 0.8 and then reaches a plateau. RMS values are similar for all samples with α ≥ 0.8, suggesting equivalent chemical resistances for the highly precured samples. These results outline the need of a precise control of the curing kinetics to increase the microelectronics reliability. Adhesion on Desmeared Epoxy Composites. The peel strength of copper-plated ABF was evaluated after the whole buildup sequence (Table S1). It is important to stress that all the samples were fully cured during the phase of copper annealing (140 and 190 °C, after electroless and galvanic copper, respectively). Figure 3 depicts the peel strengths and

several spontaneous delaminations were observed for about 80% of the samples area (in these cases the peel strength values were considered null), leading to low peel strength averages with large error bars. The peel strength follows the same trends with respect to the degree of precuring as the roughness. This is confirmed by the good linearity between peel strength and RMS values after desmearing (inset of Figure 3). A one-to-one relationship between the peel strength and the roughness after desmearing is suggested a priori, in agreement with the literature.46,48 Thus, for desmeared epoxy surfaces, phase interlocking is assumed to have a significant role in the adhesion (mechanical adhesion). Furthermore, the peeled fracture surfaces of both ABF and copper strip were analyzed by XPS to investigate the chemical interactions involved in the adhesion. Bearing in mind that this analysis cannot provide the actual polymer−copper interface (sampling a thickness of a few nanometers), it is interesting to compare the evolution of the fracture compositions with the precuring and peel strength data. Typical survey spectra are displayed in Figure S1, with the peak assignments of the signals of interest: Cu 2p3/2, O 1s, C 1s, and Si 2p. Atomic compositions of the peeled fracture surfaces (both copper strip and ABF) were determined by using high-resolution acquisitions and are detailed in Figure 4a. Considering the peeled ABF surface, carbon and oxygen from the polymer matrix are mainly observed as well as silicon from glass fillers. The relative amount of carbon decreases at the expense of oxygen and silicon amounts increasing. This is explained by the transfer of glass fillers from ABF to the copper layer, while peeling on lower precured ABF surfaces (on which more fillers are exposed than highly precured samples). Considering the copper strip, surface analysis displayed on the lower plot metallic copper with some carbon and oxygen coming from polymer are mainly detected. For lower degrees of precuring, silicon is observed as a consequence of filler dragin. Overall, a large amount of carbon is quantified (larger carbon content for lower the degree of precuring). This large amount, varying from 40 to 60 at. %, shows that extensive polymer residues remain on the copper surface after peeling, suggesting a cohesive failure within the ABF especially for relatively low degrees of precuring. The carbon spectra assessed on copper strip fracture interfaces were deconvoluted with a peak fitting method (cf. the Supporting Information). Examples of deconvolution of the C 1s peak are displayed in Figure S2. Three contributions

Figure 3. Peel strength as a function of the degree of precuring (black scatter line, primary axis) and roughness after desmearing and prior to electroless copper plating (blue scatter line, secondary axis). All plated samples were fully cured prior to being analyzed with the peel strength test. Inset: the suggested linear dependence of the peel strength on the roughness. Error bars represent one standard deviation. Lines guide the eye.

the corresponding RMS values (prior to electroless copper plating) against the degree of precuring (only for α > αgel since the whole ABF were dissolved for lower α). At low degrees of precuring, the peel strength values are comparable to the literature.15,20,44−47 The peel strength gradually decreases with increasing degree of precuring to reach a value smaller than 1 N/cm for α > 0.8. At higher degree of curing (α > 0.85), D


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Figure 4. (a) Atomic composition of the fracture interfaces as a function of the degree of precuring, as determined using high-resolution XPS. (b) Diagram summarizing results of the C 1s contribution relative areas as a function of the precuring degree, as the results of the deconvolution (pseudo-Voigt function fitting, adjusted R2 > 0.990) of the C 1s peak measured on the copper strip fracture interface.

underlay the carbon signals: C−C (centered on 285.0 ± 0.1 eV), C−O (286.5 ± 0.1 eV), and CO (289.5 ± 0.1 eV), in agreement with the literature.13,34 The diagram in Figure 4b depicts the C−O contribution decreasing with increasing degree of precuring, at the expense of the C−C signal increasing. Thus, the oxidation state of the polymer residues, at the fracture interface, decreases with increasing degree of precuring. Two regions can be outlined: the oxygen-bonded carbon (C−O + CO) content is above 40 at. % at a relatively low degree of precuring (α ≤ 0.85) and below 40 at. % for α > 0.85. Interestingly, spontaneous delaminations were observed for samples with α = 0.93 and 1.00 whereas the samples displayed similar roughness values, RMS ∼ 120 nm, than samples with α = 0.80 and 0.85. Thus, this points out that adhesion is not solely driven by surface topography. As reported earlier by Ge et al.,13 a certain level of oxidation is required to ensure proper adhesion (here >40 at. %). An increased oxidation state of the epoxy matrix allows the oxygen moiety to be involved in coordination with copper, bringing additional chemical forces to the interlocking phase mechanical adhesion. Copper Adhesion on Nondesmeared Epoxy Buildup. To better appreciate the relationships between the degree of precuring and adhesion, the copper was plated on nondesmeared ABF surfaces, keeping the same surface topography prior to copper plating for all samples. Omitting the rather aggressive desmearing, the adhesion can now be evaluated for an extended range of degrees of precuring, even well before the gelation. After lamination, precuring, AFM, and DRIFT measurements, the samples were copper plated according to the sequence shown in Table S1, omitting the desmearing steps. Following copper plating and epoxy full curing, the peel strength, SEM, and XPS measurements were used to characterize the adhesion. The roughness of the nondesmeared ABF is extremely low for all precured samples, RMS < 20 nm (Figure 5). The ABF surface consists mainly of epoxy polymer, and hardly any filler is exposed (unetched butter layer, inset in Figure 5). The peel strength values ranged from 0 to 3.5 N/ cm. The peel strength increases with the degree of precuring, reaching a maximum for α ∼ αgel and then dropping down to zero rather sharply for α > 0.8. For α < αgel, SEM evidences that the peeling fractures are not located at the epoxy/copper interface (Figure S3). Rather, many blisters are observed in the side of the ABF layer, which constitutes fragile points, ultimately leading to cohesive fracture in the ABF. The mechanism of fracture here does

Figure 5. Peel strength as a function of the degree of precuring (black scatter line, primary axis) and RMS roughness measured after precuring and prior to electroless copper plating (blue scatter line, secondary axis). Lines are drawn as guides to the eye. All plated samples are fully cured prior to be analyzed with the peel strength test. Four measurements × 1−3 samples per degree of precuring. Error bars represent one standard deviation over at least four measurements. The star indicates samples with many blisters. The dotted lines represent the gel point. Inset: AFM image of the precured ABF surface at α = 0.54 before copper plating.

not involve epoxy/copper delamination. Therefore, here the peel strength test does not measure the adhesion forces between epoxy and copper, the actual adhesion force supposedly being higher. The blisters are supposed to arise from degassing during postcuring. Two reasons can explain the blister formation: degassing of residual solvent from epoxy formulation and/or degassing water uptake during the plating process. We will further discuss the blistering mechanisms elsewhere. As for α < αgel the polymer is still ungelled, and the matrix behaves like a viscous liquid, with poor chemical resistance (large amount of solvent uptake). At α ∼ αgel (here α = 0.54), the polymer network is percolated; thus, the chemical resistance is fair enough to ensure proper chemical resistance during the plating process. The plated surfaces were smooth and clear, as can be seen at the fracture surface on copper strip shown in Figure S3. Nevertheless, some blisters were occasionally observed for some samples, probably due to punctual degassing during copper annealing/postcuring. Interestingly, the peel strength maximum value is obtained at this degree of precuring (Figure 5). The adhesion value of 3.5 N/cm is remarkably high, keeping in mind the very low roughness of the buildup layer (RMS < 20 nm). As point of comparison, the same peel strength E


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Figure 6. (a) Atomic composition of the fracture interfaces as a function of the degree of precuring, as determined by using XPS. (b) Diagram summarizing results of the C 1s contribution relative areas as a function of the precuring degree, as a result of the deconvolution (pseudo-Voigt function fitting, adjusted R2 > 0.990) of the C 1s peak measured on the copper strip fracture interface.

Figure 7 displays the relaxation time, τ, of the α-relaxation (relaxation rate) as a function of the temperature. The

value was recorded for desmeared samples with a roughness of 200 nm for α = 0.68 (Figure 3). This decrease of 1 order of magnitude of roughness, for equivalent peel strengths, points out that the huge contribution of the degree of precuring on the adhesion. For α ≥ αgel, the peel strength decreases dramatically with the degree of precuring. And finally, for the highly cured samples α > 0.8, delaminations were recorded for all samples. Again, this decrease cannot be explained by surface topography (constant RMS vs α). The XPS results, assessed on the ABF and copper surfaces, are displayed in Figure 6. No silicon atoms are detected on both fracture surfaces, showing that the fracture occurs within the resin-rich top ABF layer (Figure 6a). The fracture surface composition of ABF is composed of 80 at. % of carbon and 20 at. % of oxygen which corresponds to the polymer matrix, independently of the degree of precuring. More interestingly, the composition of the copper fracture surfaces varies with the degree of precuring. For ungelled matrices, the copper fracture surface is mainly composed of polymer residues (C and O) showing that the fracture occurs within the ABF (i.e., cohesive fracture). Considering the gelled matrices, the growing significant amount of copper (20−30 at. %) on copper strip surface shows that the fracture involves more and more delaminations between the matrix and copper. Notably, different oxidation states of the polymer at the copper strip fractures are revealed by the deconvolution of the carbon XPS signal (cf. composition evolution in Figure 6b). For α < αgel, the amount of polar groups (C−O + CO) is around 60 at. % and drops at ca. 50 at. % for α > αgel. The presence of fewer interacting polar groups at the interfaces for the gelled matrices suggests lesser copper−polymer interactions. This explains the observed lower adhesion strengths and severe delaminations. Contribution of Viscoelastic Polymer Dynamics to Adhesion. The previous results suggest that more polar groups are present at the ABF/copper interface for lower degrees of precuring. However, for the nondesmeared sample series, no further oxidative treatment was performed to the polymer matrix. The presence of a larger number of polar groups at the interface can be rather explained by the higher polymer relaxation rate for lower cross-linking degrees during the copper annealing (at 140 °C for 1 h, after the electroless plating step). Therefore, we investigated the α-relaxation, i.e., cooperative network relaxation, of partially cured buildup layers using frequency scanning isothermal DMA (Figure S4).

Figure 7. Temperature dependency of the cooperative relaxation times of partially cured epoxy polymers in the ABF. The degrees of precuring are reported next to the curves. Symbols are experimental data, and solid lines are WLF fits (adjusted R2 > 0.998). The dotted line highlights the electroless copper annealing temperature (vertical) and the suggested highest relaxation time allowing good adhesion (horizontal).

experimental data are well fit using the WLF (Williams− Landel−Ferry) model49 (the viscoelastic coefficients are reported in Table S2). For a given sample, the relaxation time decreases as a function of the temperature (activation of the glass transition). For a given temperature (e.g., 140 °C), the relaxation time increases with the degree of precuring. The rising glass transition temperature together with the gelation and restriction of the motion by cross-linked chains explains this drastic increase of relaxation time with the degree of precuring.50 These viscoelasticity results explain that the adhesion strength is higher when the copper was plated and annealed on polymer with a faster relaxation rate (lower τ at lower α). A twofold mechanism can therefore be considered in agreement with the literature.51,52 First, the faster relaxation rate allows polar groups borne on dangling chains and un-cross-linked modules to bend rapidly to the surface and interact (i.e., coordinate) with the copper (higher C−O bonds content for low α, Figure 6b).51 Second, the faster relaxation rate yields lower internal stresses (the epoxy can relax stress); therefore, the adhesion strengths increase.51,52 The lower internal stresses allow polar groups to remain interacting with copper F



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ACKNOWLEDGMENTS The authors acknowledge Atotech Deutschland GmbH for financial support. Grégoire Dietrich and Hélène Garcin are thanked for AFM measurements. Florian Gaul and Stefanie Bremmert are acknowledged for assistance in copper plating. The XPS analyses have been carried at the University of Montpellier.

compounds during the concomitant postcuring and copper annealing, tending to enhance adhesion. In summary, plating the copper on loosely cross-linked epoxy is beneficial due to the higher chain mobility. We noticed, in the previous section, a maximum copper adhesion on samples for α ∼ αgel. On the basis of Figure 5, for α = 0.6 ∼ αgel at 140 °C, we can define a critical relaxation time of 10−5 s that should not be exceeded. Higher τ values would not allow sufficient reorganization to yield good adhesion. This critical relaxation time is obviously dependent not only on the polymer matrix and the temperature but also on the sequence conditions.

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CONCLUSIONS In the course of electronic devices miniaturization, this paper gives interesting insights into the relationships between polymer cross-linking state, oxidative etching (permanganate desmear wet treatment) properties, and buildup composite/ copper adhesion. The degree of precuring has been clearly demonstrated to play a crucial role to enhance the adhesion between copper and epoxy substrate. Remarkably, we demonstrated that copper plating on ungelled and nonetched epoxy composite can lead to comparable peel strengths than gelled and etched buildups, with an outstanding diminution of the surface roughness, decreased by 1 order of magnitude. We note that further optimizations are required to avoid the blistering caused by degassing and hence to achieve a maximum reliability. Nevertheless, this proof-of-concept is very promising in the target of manufacturing finer and finer copper circuitry without any use of further adhesion promotion. In agreement with AFM, XPS, and polymer relaxation data, we propose a twofold mechanism: loosely cross-linked epoxy networks, with faster relaxation rates, allow (i) polar moieties (high oxygen content) diffusing toward the polymer−copper interface, enabling further coordination interactions, which are expected to remain (ii) during postcuring due to lower internal stresses. This implies that the mechanisms of adhesion are shifting from mechanical anchoring (in the case of high degrees of precuring and roughened surfaces) to chemical interactions (low degrees of precuring and smooth surfaces). ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00290. PCB manufacturing test sequence, fitting methods, atomic force microscopy method, weight loss during desmearing, peel strength tests, X-ray photoemission spectroscopy, SEM images, dynamic mechanical analysis (PDF)

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REFERENCES

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