thiol based chemical treatment as adhesion promoter for...

2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008) 978-1-4244-2740-6/08/$25.00 © 2008 IEEE

Thiol based chemical treatment as adhesion promoter for Cu-epoxy interface

Cell K Y Wong and Matthew M F Yuen Department of Mechanical Engineering

Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China [email protected], [email protected]

(852) 2358 8814

Abstract

The paper focuses on the use of thiol as adhesion promoter in Cu-epoxy interface. A parametric study on the thiol deposition procedure for Cu-epoxy system was conducted. Thiol concentration and deposition time was found to be important parameter in adhesion. Low concentrated solution leads to insufficient morphological modification while prolong treatment time results in fracture happened along weak boundary layer. The optimum condition for surface treatment with the current thiol was 5mM solution for 10 hours. With this thiol treatment, adhesion which was given as fracture toughness has increased from 4.8Jm-2 for the untreated sample to 159Jm-2. The 30 times adhesion improvement was related to presence of nanostructure upon thiol modification. Both the chemical bonding and morphological changes was found contributes to adhesion.

Introduction

A general trend in electronic industry requires small package size, higher integration and increased system speeds. To align with this trend, the heat dissipation requirement for high density integrated circuit (IC) packages becomes increasingly important during these years [1]. Copper (Cu)-based leadframes is a popular choice for new generation leadframe material for its excellent thermal conductivity. It, however, have a weak point in term of package reliability [2, 3]. The Cu leadframe generally exhibits poor adhesion with polymer encapsulant as compare to Alloy 42 leadframe [4]. In a plastic packages, epoxy compound are usually employed as encapsulating materials for their excellent insulating and mechanical properties. Nevertheless, the poor bondability to epoxy makes the copper leadframe package fails during its service life or even in its manufacturing process. The likelihood of package delamination and cracking hinders the usage of Cu-based leadframes.

Several approaches [5-7] have been taken in an effort to obtain higher adhesion strength between Cu and EMC. The most common approach was copper oxidation. Chemical oxidation through acidic or alkaline solution is preferred over thermal method because the morphological modification obtained from chemical method gives better adhesion [8]. The chemical modification enhances surface roughness of the substrate. It provides more site of attachment which can interlock mechanically with the polymer. Several groups reported the influence of oxide thickness on adhesion [4, 5, 9-11]. They pointed out that there exists an optimum oxide thickness that exhibits the strongest adhesion. The proposed optimum oxide thickness was 20-40nm from Tankano et al [12] and 20-30nm from Cho et al [9]. Choi et al [4] showed

that when CuO/ Cu2O ratio fell between 0.2 to 0.3, the maximum adhesion strength was achieved, regardless of oxidation time. Lee et al [13] produced the oxide in a hot alkaline solution and reported the maximum adhesion was reached within 10-20min of immersion. They reported the optimum thickness for Cu2O and CuO were 200nm and 1300nm respectively. The discrepancy among these studies came from the variation in the oxide treatment and more importantly the microstructure obtained from different treatment process. According to Cho et al [9], with formation of CuO, the pull out strength of Cu leadframe from EMC increased due to formation of needle like CuO which interlock the epoxy material. Despite this, under excessive oxidation, the adhesion strength dropped. They explained the phenomenon by formation of low density copper oxide. As fracture happened along the weak copper oxide, the interfacial adhesion deteriorated. The results show that copper oxide which result in characteristic morphology plays a dominant role in copper/ EMC interfacial strength. Strictly control of oxide growth is thus essential for optimal adhesion. However, given that frequent thermal process is likely to be encountered by copper leadframe before molding (die bonding: 150-250°C for a few hours, wire bonding: 200-300°C), oxide thickness control is difficult if not impossible. Cu oxide growth is therefore not an applicable solution for the adhesion problem.

Another method for adhesion enhancement adopted in metal-polymer system is the formation of chemical bonding. In a metal-polymer system, covalent bonds play predominate role in interfacial bonding phenomenon. As reviewed by Pauling [14-16], the covalent bond energy is about ten times stronger than energy of the secondary forces. Thus, it is beneficial to enhance the adhesion between epoxy and Cu through chemical bonds formation. Song et al. [6, 17] promote the adhesion between Cu and epoxy resin with the used of azole and triazole compounds. They demonstrated an improvement from 54N/m to 408N/m with polybenzotriazole treatment in a peel test. Eight times enhancement was obtained with the polybenzotriazole treatment. Nevertheless, the study also revealed a decline in adhesion strength with prolongs surface treatment time. The peel strength of Cu/epoxy joint has been dropped from 353N/m to 40N/m as treatment time increased from 15s to 10min. Due to the formation of porous Cu-azole complex layer at prolong treatment, failure happen inside the weak complex layer and therefore dwindle the adhesion strength [6]. The study indicates the importance of promoter candidate selection and the treatment procedure.

The use of self assembly monolayer as promoter for Cu-epoxy adhesion has been introduced by Muller et al. [7]. Through the affiliation of the self assembly coupling agent to

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2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008)

Cu substrate, chemical bonding can easily realize between the Cu and epoxy. Hydroxylated thiol, disulfide, ethylene diamine and phthalocyaines were used as surface modifiers in Muller’s study. About 20% increase in adhesion strength of epoxy molded button sample has been measured when the Cu was treated with disulfide and ethylene diamine. With the use of amino disulfide treatment, our group [18] has reported a 60% increase in button shear strength. Although the adhesion enhancement with the self assembly coupling agent is not as significant as compare to Song’s result in the previous study, sulphur containing molecules like thiol and disulfide is advantageous in forming a chemical linkage between the Cu and epoxy compound. Given the low bonding energy in Cu-S bond, copper reacts readily with thiol/disulfide. By choosing a correct functional group in the thiol/ disulfide coupling agent, it is capable to form strong covalent linkage (C-S-Cu) between the Cu and epoxy. Unlike the Cu-azole complex, this Cu-S covalent bond is strong which can prevent failure happening within the coupling agent.

In addition to the treatment method and deposition time, the interfacial adhesion is also affected by concentration of the treatment solution [6, 19, 20]. From Song’s study, they reported a change of adhesion from 408N/m to 327N/m to 187N/m as mole ratio of triazole compound decrease from 0.33 to 0.1 to 0.003. These studies disclosed the essence of research on deposition procedure for a surface treatment. Rather than strengthening the interface, prolonged surface treatment can fail the interface by producing a weak boundary layer.

This paper discusses the influence of solution concentration and treatment time to the adhesion of thiol modified copper substrate and epoxy encapsulate. Copper substrate was modified by thiol solution with concentration ranged from 0.1mM to 10mM for 1 to 7 days. The substrate was then bonded with epoxy based underfill encapsulate. The interfacial adhesion was evaluated by measuring fracture toughness of the interfaces form tapered double cantilever beam (TDCB) test. The fracture toughness results were then related to morphological study of the treated surface. To understand the failure mechanism, failure locus of the fractured sample was examined by elemental mapping in an energy dispersive X-ray analysis (EDS).

Experimental

A. Material To investigate the surface composition and the adhesion of

the modified substrate, two types of substrate were used in this study. C194 Cu based leadframe was employed as substrate for surface composition analysis and the morphological study. In the interfacial adhesion study, fracture resistance experiment was conducted using tapered double cantilever jig machined form red brass. The nominal composition of the Cu substrates was evaluated by X-ray fluorescence spectrometer (XRF) model JSX-3201Z from JOEL. The reported composition for C194 leadframe and the red brass jig was Cu-2.2Fe-0.08Si-0.48Zn-0.06Sn (wt%) and Cu-0.05Si-0.38Zn-0.02Sn (wt%) respectively. A proprietary thiol of 125gmol-1 molecular weight were purchased from

Aldrich and used as adhesion promoter in the study. Thiol was applied to Cu substrate through solvent adsorption. Absolute ethanol (>96% purity, Aldrich) was chosen as solvent in dissolving the thiol. Epoxy adhesive adopted in fracture test was Hysol® FP4526, a commercially available underfill obtained from Henkel Loctite®. It is a low viscosity, fast flow epoxy based material designed for capillary underfill on flip chip application.

B. Surface preparation The thiol was used as received. It was dissolved into

absolute ethanol and diluted to 0.1mM-10mM solution. All types of the substrate were cleaned with anti-grease solvent followed by Fry 90 flux (Fry Technology) to reduce surface oxide. To further eliminate carbonate and hydroxide on the Cu surface, the samples were immersed into acetic acid for 10min before adsorption. Thiol was adsorbed onto the pre-cleaned Cu surface from a pre-mixed solvent for 10-168 hours. The treated substrate was then removed from the solution and rinsed thoroughly with an ethanol before blow drying with nitrogen.

C. Surface characterization Upon deposition, the existence and quality of the SAM

modified surface has been characterized by numerous of surface analysis techniques. Detailed descriptions are written below. Scanning Electron Microscope (SEM)

The surface topography of the SAM treated sample was evaluated by a SEM 6700F from JEOL Ltd. The electron microscope was operated under 5-15kV u in secondary electron (SE) mode. Images under 50,000x magnifications were taken to reveal the microstructure, feather size and coverage of the modified layer. 20,000 x magnifications were adopted to investigate morphology in large area. X-ray Photoelectron Spectroscopy (XPS)

To investigate the surface composition and the bonding of the treated surface, XPS, model 5600 multi-technique system bought from Physical Electronics, was utilized. A monochromatic AlKα X-ray source was run at 1486.6eV. The spot size was kept at 600μm for all sample studied in this paper. Time-of-flight secondary ion mass spectrometer (ToF-SIMS)

ToF-SIMS is a dedicated technique for chemical bonding identification for thin molecular layer. Since the mass for bonded and non-bonded molecules are different, it can classify a free adsorbed molecule from a chemically bonded one. In our chemical bonding inspection, a mass spectrometer model TOF SIMS V from ION-TOF GmbH was used. The machine was equipped with a Bi3+ ion gun and a reflectron ToF mass analyzer as detector. The spot size of the ion gun was controlled in 200x200μm2 with less than 2nm penetration depth. Both the thiol treated samples and a control sample prepared from neat ethanol was put under investigation. A fragment having characteristic peaks should be observed in the positive ion spectrum if the thiol molecules were bonded to Cu substrate.

D. Tapered Double Cantilever Beam (TDCB) Fracture test



To determine the fracture toughness (GIC) of the modified Cu-epoxy joint, TDCB fracture test was conducted according to ASTM D3433. The specimen was prepared from epoxy underfill sandwiched between a pair of tapered Cu jig. Tapered geometry guarantees a constant moment during the crack growth. It is advantageous in a stable crack experiment. In order to eliminate the roughness effect, the Cu jigs was polished with P4000 sandpaper before surface treatment. To ensure that crack opened at the right interface, a 60mm precrack was prepared in the front end of the testing jig. 0.5mm thick epoxy underfill was cured between the two jigs at 80oC for 6 hours. The width and total length of the jig was 7mm and 102mm respectively. Fig. 1 showed a schematic diagram for the TDCB configuration.

Fig. 1 Schematic diagram for TDCB specimen configuration

The prepared joint was tested with a MTS 858 Universal

Testing Machine equipped with a 25kN±5N axial force load cell. Crosshead displacement rate was kept constant as 30μm/min under control displacement mode. It was assumed that the crack developed under linear fracture mechanics condition. The GIC was calculated according to the following equation:

2 2 2

2 3

4 [3 ( ) ]( )IC

P a h aG

EB h a+

= Eq. 1

where P is the critical load for stable crack opening, B is the width of the specimen, a is the crack length, h is the correcsponding thickness of the jig at the point of crack front.

2 2

3

3 ( )( )

a h am

h a+

= Eq. 2

where m is a constant for the tapered jig which is determined by the jig geometry To compare the adhesion of a modified substrate to that of

untreated, normalized adhesion was calculated. It was defined as the average GIC for a modified surface divided by that for the control sample. Equation for the calculation is shown below:

normalized adhesion IC sample

IC control

G

G= Eq. 3

E. Failure locus determination Failure locus of a joint is critical in understanding the

adhesion phenomenon. To determine the failure location of the specimen, X-ray microanalysis (EDS) was adopted. The EDS information was obtained from QUANTAX 4010 (manufactured by Bruker AXS) which was installed inside a JEOL JSM-6390 SEM. The EDS system, operated with a LN2-free XFlash® Silicon Drift Detector (SDD), was tailored-

made for low energy element detection. This enables sensitive polymer material detection. For our fracture locus analysis, the carbon content measurement is crucial. An accurate analysis can help to locate the failure locus and evaluate the weakest point in a joint. In this EDS experiment, spectra and 2D mapping signals were acquired at 15kV with 1mm2 beam spot shone on the Cu jig. Acquisition range was set at 0-10keV with at least 10 kc/s detection rate.

Result

A. Surface analysis of thiol adsorbed substrate Scanning Electron Microscope (SEM)

Fig. 2 showed the surface morphology of the thiol treated Cu surface in 50,000x magnification. Thiols modified the Cu by forming string-like nano-structures on the substrate. The tiny strings were bundled together and covered almost the whole substrate. From the side view picture, the thiol molecules constructed a thin nano scale network layer.

Fig. 2 SEM microscopy of thiol treated Cu: top view (left);

side view (right)

X-ray Photoelectron Spectroscopy (XPS) Fig. 3 illustrated the XPS spectra for a control substrate

and a substrate treated with 1mM thiol solution. Table 1 reported the atomic composition of the two samples. In comparing with the control spectrum, significantly higher C, N, S content has been measured from the treated sample. The detection of these element implies thiol molecules were presence on the Cu surface.

Fig. 3 XPS spectra for a control (left) and a thiol treated

sample (right) Table 1 Atomic percentage of elements on sample surface

Element control 1mM thiol treated C1s 40.43 63.70 N1s 1.40 7.86 O1s 11.77 12.03 S2p 2.76 8.26 Cl2p 5.50 0.11

Cu2p3 38.14 8.04



High resolution XPS spectra for Cu2p and S2p peaks are shown in Fig. 4. From the Cu2p spectrum, a ‘shake-up satellite’ peaks were found in 944eV. This suggested that some Cu (II) compound existed on the sample. In the S2p spectrum, a board peak was observed at ~163.8eV. As reviewed in literature [21, 22], chemisorbed S should give S2p peak at ~162eV. The slightly downward shift of 163.8eV may suggest formation of free unbonded S species on top of the bonded thiol layer.

Fig. 4 High resolution spectra for Cu2p (left) and S2p (right)

peak on a thiol treated sample Although XPS is a sensitive tool for surface analysis,

identification of Cu-thiol bonding in XPS is not trivial. Since the binding energy for Cu2S, Cu2O, CuS and CuO was very close [23] and the oxide formation on Cu was so ready, the differentiation of Cu-S bond from Cu-O bond was almost not possible especially when unknown species might involve. The existence of Cu-S bond should thus be evaluated by other surface technique: ToF-SIMS. Time-of-flight secondary ion mass spectrometer (ToF-SIMS)

Fig. 5 showed the positive (+ve) ToF-SIMS spectra for a thiol treated sample prepared from 1mM thiol solution and a control sample prepared from ethanol. Both samples were treated with solvent for 2 days before the analysis. From the +ve SIMS spectrum of the control sample, the major mass/charge (m/z) peaks found were 143, 207 and 223. These numbers represent the presence of Cu hydroxide for 143 (Cu2OH) and Cu oxide for 207 (Cu3O) and 223 (Cu3O2) on the control sample. The fragments indicate that the oxygen dissolved in ethanol can oxidize the Cu substrate during the adsorption process. In comparison to the thiol samples, two distinct peaks were identified. The m/z 124 and 187 were observed on the thiol treated surface only. Table 2 summarized the major peaks obtained from the two samples. As mentioned, the molecular weight of the thiol molecule was 125gmol-1. A characteristic fragment at 124 represents the existence of free unbonded thiol (SR+). While reacting with Cu surface, 187 peak should be spotted (CuSR+). The identification of the 187 peak revealed strong evidence that the thiol molecules were chemically bonded with the substrate. Another crucial proof for 187 contained Cu was the presence of an isotopic peak at 189. The isotopic peaks originated from 63Cu+ and 65Cu+ isotopes whose relative abundance was 1:0.45. The area ratio for the 187 and 189 peaks was 1:0.5 which matched the expected value. The surface analysis proves that Cu-S covalent bonds have been realized on a thiol modified copper.

Fig. 5 Positive ToF-SIMS spectra for control (prepared by

dipping in ethanol, left) and thiol treated sample (right) Table 2 The major peaks of treated and control samples

obtained from positive ToF-SIMS spectra Sample Peak1 (m/z) Peak2 (m/z) Peak3 (m/z) control in ethanol, 2days

124 187 and 189 221, 223, 225

1mM thiol, 2days 143, 145,147 205, 207, 209 221, 223, 225

B. Tapered Double Cantilever Beam (TDCB) Fracture test Concentration effect

Fracture toughness (GIC) of the surface modified Cu jig was examined. A control Cu specimen without any treatment was used as benchmark. The fracture toughness was regarded as critical load for stable crack propagation along the sample interface. The average critical load for stable crack propagation for control and 1mM, 2 days specimen was 39±3N and 182±13N respectively. Typical force curves for the control and treated sample were given in Fig. 6.

-50

0

50

100

150

200

0 0.4 0.8 1.2 1.6

Load

(N)

Displacement (mm)

-50

0

50

100

150

200

0 0.5 1 1.5 2 2.5

Load

(N)

Displacement (mm) Fig. 6 Typical loading curve for TDCB test on a control (left)

and thiol treated specimen (right) GIC was calculated from the critical load needed for

propagating a stable crack according to Eq. 1. Fig. 7 summarizes the GIC results of the control and the thiol modified Cu-epoxy interface in different concentration. The deposition time was fixed at 2 days. The results demonstrate that with sufficient thiol treatment, the interfacial adhesion for Cu-epoxy improves significantly. Fracture toughness of Cu-epoxy is boosted from 4.8±0.8Jm-2 to more than 80Jm-2 when treated with 0.5mM thiol solution. The trend continues to climb to 104±14Jm-2 to 140±17Jm-2 for 1mM and 5mM treatment respectively. Despite a positive trend has been revealed by higher concentrated solution, the adhesion decreases as substrate being treated with 10mM thiol. The GIC dropped to 101±7Jm-2 with 10mM solution.



Fig. 7 Fracture toughness of Cu-epoxy interface treated with

thiol solution in various concentrations Deposition time effect

Fig. 8 shows the time effect on GIC for 1mM and 5mM modified interface. With 1mM solution, the recorded GIC is104±14 Jm-2 with 48 hours deposition and it reduces to 73±14 Jm-2 after 168 hours incubation. A similar trend has been discovered from the 5mM specimen. Maximum GIC is measured as 159±33 Jm-2 from 10 hours treated sample. The fracture toughness then drops to 140±17 Jm-2 as deposition time increase to 48 hours.

0

50

100

150

200

0hr (control) 48hr 168 hr

GIC-1mM

4.80

104

73.0

Gic

(Jm

-2)

Treatment

0

50

100

150

200

0hr (control) 10 hr 48 hr

GIC-5mM

4.80

159

140

Gic

(Jm

-2)

Treatment Fig. 8 Fracture toughness (GIC) of Cu-epoxy interface with

1mM thiol (right) and 5mM (left) treatment in treatment time

C. Fracture surface analysis An optical picture showing the fracture surface for a

control and 5mM treated sample is given in Fig. 9. A clean copper surface is observed in the control sample while blue underfill epoxy is found on the 5mM sample. Epoxy residue is found around the jig edge. Some white spot are observed in the center of the thiol treated jig.

Fig. 9 Optical images of the Cu jig after fractured in an

control (left) and 5mM treated (right) specimen To study the failure mechanism, SEM-EDX was used.

200x magnification SEM picture was taken on the fracture jig (Cu side). The elemental analysis on the Cu jig could roughly determine the failure location. Fig. 10 illustrates a 2D mapping for C, O, Cu and S on a fracture jig from 5mM solution. 2D mapping inside the red square are examined.

Obviously, the white spot (black feature under SEM) consisted of carbon and oxygen mainly. It contains mainly copper in the grey area. This implies that cohesive failure happened at some location in the 5mM specimen. The analysis explains the high fracture resistance in a thiol treated samples.

Fig. 10 2D mapping image on a Cu jig after fractured:

analysed element included C, O, Cu and S from the left.

Discussion

A. Normalized adhesion The effect of i) concentration and ii) deposition time was

evaluated by the normalized adhesion as calculated from eq. 3. The normalized results are summarized in Table 3 and Table 4 for the i) concentration and ii) deposition time effect respectively.

Table 3 Effect of concentration on interfacial adhesion treatment Normalized adhesion control 1 0.1mM, 48hr 0.6 0.5mM, 48hr 18 1mM, 48hr 22 5mM, 48hr 29 10mM, 48hr 21

Table 4 Effect of deposition time on interfacial adhesion treatment Normalized adhesion control 1 1mM, 48hr 22 1mM, 168hr 15 5mM, 10hr 33 5mM, 48hr 29

B. Morphological analysis To understand the adhesion phenomenon on the modified

surface, surface morphology of the treated specimen were examined under SEM. Pictures were taken under 20,000 x magnifications in order to give topography information in large area. Concentration effect

Surface morphology and the corresponding normalized adhesion from samples in different concentration are shown in Fig. 11. While comparing with the control sample (time 0 hour), the treated surface is smoother overall. The rolled marks, which were introduced during leadframe manufacturing process, are covered by string like structures upon treatment. Topography on the 1mM, 5mM and 10mM surfaces looked quite similar. Nanoscale strings covered the rolled marks over large area. Nevertheless, little topographical feather was identified on the 0.1mM sample. The rolled marks on the 0.1mM treatment sample are still visible. It seems that, with 2 days dipping, sufficient modification has not been achieved with 0.1mM solution. This explains the extremely low adhesion (0.6x of the control) recorded by the 0.1mM specimen as compared to larger than 20x with high concentrated treatment.



Fig. 11 Morphological image of Cu surface treated with thiol

solution in different concentration as taken by SEM, magnification being fixed at 20,000x. Corresponding

normalized adhesion were written inside bracket. Deposition time effect

Fig. 12 and Fig. 13 show the surface morphology of Cu with 1mM and 5mM thiol treatment for different time respectively. String-like features are observed on all these samples, except the 1mM, 168 hours sample. Large holes appear in the sample after 168 hours treatment. The surface appears to be more porous after prolong treatment. This may explain the decline of normalized adhesion from 22 for 48 hours sample to 15 for 168 hours treatment. Interfacial adhesion may deteriorate as failure path shifted to the weak porous layer.

Fig. 12 Morphological image of Cu surface treated with 1mM

thiol solution for 48 hours (left) and 168 hours (right), magnification being fixed at 20,000x. Corresponding

normalized adhesion were written inside bracket.

Fig. 13 Morphological image of Cu surface treated with 5mM thiol solution for 10 hours (left) and 48 hours (right), magnification being fixed at 20,000x. Corresponding

normalized adhesion were written inside bracket. The above results reveal that there exists an optimum

condition for adhesion. As reported by Bell et al [19] and Plueddemann [24] who studied the coupling effect of silane in metal/glass-polymer system, they illustrated the effect of coupling agent thickness to adhesion. Although the thickness value might have changed with different system, the optimum thickness can also be found in different cases.

C. Failure mechanism study The 2D mapping data suggested that surface modification

can shift the failure path of a joint. Without thiol treatment, failure occurs inside the Cu-epoxy interface. With adequate

solution treatment, more than 10 times improvement has been demonstrated. Failure mode has changed from interfacial failure to cohesive failure inside bulk epoxy. It demonstrates the effect of chemical bonding in interfacial adhesion.

Conclusions

This paper focuses on the use thiol treatment for Cu-epoxy interfacial adhesion. Parametric study on the effect of solution concentration and deposition time has been discussed. The findings are summarized in the following: 1. With thiol modification, the interfacial adhesion for Cu-

epoxy joint has been improvement for more than 30 times as demonstrated in the currents study. Thiol is thus an effective adhesion promoter for a Cu-epoxy joint.

2. Solution concentration and deposition time are crucial parameters affecting the formation of covalent bonds for adhesion improvement .

3. For prolong time treatment, adhesion may be worsen due to formation of weak structure.

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