Ž .Applied Surface Science 156 2000 26–38www.elsevier.nlrlocaterapsusc

Chemical surface preparation for metallization ofstereolithography polymers

B. Luan ), M. Yeung, W. Wells, X. LiuIntegrated Manufacturing Technologies Institute, National Research Council Canada, 800 Collip Circle, London, ON, Canada N6G 4X8

Received 20 March 1999; accepted 1 August 1999

Abstract

Ž .Chemical surface preparation specifically for stereolithography SLA polymers prior to its metallization is reported forthe first time in the present paper. Contact angle analysis was conducted to assess the surface hydrophilicity so as tooptimize the preparation process. The applicability of this technology was verified by subsequent metallization process. Thisintroduces an efficient method for metallization of SLA polymers and enhances its performance in applications. X-ray

Ž . Ž .diffraction XRD and atomic force microscopy AFM image analysis were applied to understand the phase compositionand the micromorphologies of the substrate. Surface profiling and optical microscopy were applied for surface analysis tounderstand the progression of surface modification during chemical preparation process. Crown copyright q 2000 Publishedby Elsevier Science B.V. All rights reserved.

Keywords: Etching; Polymer; Stereolithography

1. Introduction

Ž .Stereolithography SLA is a new technologylinking the power of computer graphics to the rapid

w xformation of a solid, shaped object 1,2 . This tech-nology uses UV laser to selectively solidify succes-sive thin layers of a photocurable resin and providesgreat economies for both the design laboratory and

w xthe modeling process 1,3 . Since Hull’s invention ofSLA in 1984, this novel technology has enabled a

Ž .new method for rapid prototyping RP , which ap-peared in 1988, that allows designers to verify theirproduct design at an early stage, by using 3D repre-

) Corresponding author. Tel.: q1-519-430-7043; e-mail:ben.luan@nrc.ca

sentations for design review with sales, marketingw xand production engineers 4–6 . However, many of

these systems produce only fragile parts, which canonly be used for form-and-fit analysis but cannot betested in an apparatus as though they were functionalparts.

w xSince 1995, Yeung and McKeen 7 , McKeen andw x w xYeung 8 and Yeung et al. 9 have developed a

novel technology to deposit metals onto the surfacesof non-metallic RP parts. Of all the available metal-lization processes such as electroless plating, physi-

Ž .cal vapor deposition PVD and chemical vapor de-Ž .position CVD , the former is the most applicable for

SLA parts due to its possession of combined charac-teristics such as uniformity, simple operation, lowcost and no damage to the substrate.

0169-4332r00r$ - see front matter Crown copyright q 2000 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 99 00339-6

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 27

In order to apply electroless plating to a non-cata-lytic surface, the surface must be subjected to aseries of pretreatment such as pickling, sensitizationand activation. Of all the three pretreatment pro-cesses, pickling is an essential process which pro-vides the substrate with cavities, and in some cases,a modified chemistry which improves the wettabilityof the surface and sometimes renders a hydrophobicsurface hydrophilic. Chemical etching is a commonlyknown pickling process that has been successfullyapplied to numerous types of polymers such as ABSw x10 . Using the same approach, the present communi-cation introduces a chemical etching process specificfor pickling of polymers fabricated by laser-inducedpolymerization using SLA apparatus.

2. Experimental

2.1. Sample fabrication

Simple SLA RP samples of 30 mm=30 mm=10mm were fabricated in house. SLA5195 resin pro-vided by Ciba Geigy was used for fabrication. It is ofnote that the resin is widely used in RP but thechemical composition of the final polymer is un-known to customers due to the commercial concerns.There are three types of commercially available SLApolymers which are coded as SLA5170, SLA5180and SLA5195. The only information revealed byCiba Geigy about the SLA5195 resin, used in ourcurrent research, is that it is a complicated six-com-ponent mixture containing cycloaliphatic epoxy resin,aliphatic glycidyl ether, polyols, modified acrylateester, acrylate ester and photocuring agent. The partswere designed on a Proengineer with Version 18.0software provided by Parametric Technologies. Carewas taken to place the resin, as supplied, in the vatof Model SLA5000 SLA apparatus where the resin isphotopolymerized during a laser-induced photocur-ing process and parts characterized of layered struc-ture are manufactured. The system is supplied by 3DSystems, equipped with Build-Station Version 4.1software. The parts produced are then drained andsubsequently rinsed in a RAMCO tank containingTPM solvent with mechanical agitation, and subse-quently thoroughly rinsed with water to remove liq-

uid resin, if it still remains, from the surface of theSLA parts. The cleaned parts are then subjected to a4-h post-curing process in a UV irradiation chamber,an attachment of the 3D SLA5000, to complete theentire photocuring process.

2.2. Chemical etching

Chemicals, CrO and H SO , of AR grade used3 2 4

for chemical etching of the parts were purchasedfrom Sigma. It can be reasonably expected thatfactors such as bath composition, temperature andthe duration of the etching process may affect theetching process and hence, the finishing of the sub-strate surface. Taguchi design, a simple and fastmathematical method to study the effects of variousfactors, was applied in the present research. Temper-ature was controlled using NESLAB GP200 waterbath.

2.3. Contact angle analysis

As is known, there exists an intimate link betweenw xcontact angle and wettability or hydrophilicity 11 .

To assess the hydrophilicity of the surfaces subjectedto different etching so as to optimize the operationconditions, contact angle was measured using aRame-Hart Model 100 Contact Angle Goniometerequipped with a microsyringe attachment. Deionized,18-MV water, which had been filtered to removehydrocarbons, was employed as the probe liquid.Four measurements were made on each sample, andthe data are presented as a mean with a standarddeviation.

2.4. Physical analysis

Ž .X-ray photoelectron spectroscopy XPS was ap-plied to understand more about the composition andchemistry of the SLA5195 part fabricated. ModelSSX-100 XPS machine was manufactured by Sur-face Science Laboratories.

Phase compositional analysis of the sample wascarried out using Philips X’PERT-MRD. Cu LFFX-ray tube was employed for measurements. ModelTopometrix Explorer Atomic Force Microscopy

( )B. Luan et al.rApplied Surface Science 156 2000 26–3828

Ž .AFM provided by Topometrix was used for topo-graphic image analysis. Morphological analysis wasconducted on the surfaces before and after etching,using Model Opympus PMG3 optical microscope.Surface roughness and profile were analyzed usingModel WYCO NT2000 Surface Profiler.

3. Results and discussion

3.1. XPS analysis

As was aforementioned, information provided byCiba Geigy revealed that the SLA5195 resin is acomplicated six-component mixture containing cy-cloaliphatic epoxy resin, aliphatic glycidyl ether,polyols, modified acrylate ester, acrylate ester andphotocuring agent. To further understand the chem-istry of the polymer formed by laser-induced poly-merization, survey scans for elemental compositionwere taken and the spectrum is presented in Fig. 1.As is manifested in Fig. 1, there exist two peaks atthe binding energies of 285 and 530 eV regions,corresponding to the 1S of C and O, respectively.The elemental composition of the sample is calcu-

lated as 78.7% carbon and 21.3% oxygen. It ought tobe mentioned that H is not detectable by XPS. Tofurther understand the nature of bonding, surveyscans for higher-resolution carbon spectra were takenof the cured polymer and the spectra are presented inFig. 2. The high-resolution carbon spectra provideinformation about the oxidation state of the carbon atthe surface of the polymer. Typically, the largestpeak is attributable to the C–C and C–H bonds andis assigned a binding energy of 281.75 eV. The peakfrom carbon in a higher oxidation state is shifted to ahigher binding energy so that the peaks from carbonsingly bonded to oxygen are shifted to 1.06 and 1.87eV. The co-existence of two C–O bonds is attributedto the environmental difference of singly bondedcarbon such as formations of H–C–O and O–C–O.Similarly, the peak from carbon double bond, C5O,is shifted 3.63 eV and the peak attributable to O–C5O is shifted to 4.16 eV. The above results re-vealed that carbon in the SLA5195 polymer hasdifferent energy states when bonded to oxygen,namely, different binding energy. This indicates anon-uniform surface energy state of the SLA5195polymer and therefore, a possible non-uniform phasestructure formed during polymerization process.

Fig. 1. The XPS spectrum of SLA5195 polymer showing the elemental composition.

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 29

Fig. 2. High-resolution carbon spectra of SLA5195 polymer showing different energy state of carbon.

3.2. Chemical etching

Chemical etching was carried out in an aqueoussolution containing CrO and H SO and deionized3 2 4

Žwater, following the Taguchi orthogonal array see.Table 1 . The Taguchi array consists of four factors,

which are temperature, duration of etching and CrO3

and H SO concentrations. Three levels were as-2 4

signed to each factor. The solution was kept stirredand CrO and H SO were added slowly during the3 2 4

entire dissolution process. Furthermore, due to itsexothermic nature of dissolution, the solution madewas cooled down to below each nominated tempera-ture in Table 1 and was then heated up to andmaintained at the nominated temperature using NES-LAB GP200 water bath. In addition, as the practicaletching process was also noticed of exothermic, agi-tation of the solution was necessary to ensure ahomogeneous temperature distribution. Temperaturewas monitored during the entire etching process anda fluctuation of "28C was observed. The etchedsamples were then thoroughly rinsed with distilledwater for subsequent analysis.

3.3. Contact angle analysis

As was aforementioned, the objective of chemicaletching was to improve the hydrophilicity of the

surface. The effects of chemical etching were there-fore assessed by contact angle measurements, shownin Table 2.

The data are presented in Table 2 as static, ad-vancing and receding contact angles. The static anglewas determined by applying approximately 0.004 mlof water on the surface and measuring the angle thatthe static drop makes with the surface. The advanc-ing angle is measured as water is added to the dropand the front advances. The receding angle is mea-sured as water is removed from the drop and thefront recedes. As shown in Table 2, the static andadvancing contact angles give identical hydrophilic-

Table 1w Ž 4 .xTaguchi orthogonal arrays L9 3 for chemical etching

Sample Temperature Etching CrO H SO3 2 4Ž . Ž . Ž . Ž .number 8C min grl grl

1 25 10 300 02 25 30 600 1003 25 60 900 1504 50 10 600 1505 50 30 900 06 50 60 300 1007 75 10 900 1008 75 30 300 1509 75 60 600 0

10 Sample not subjected to chemical etching

( )B. Luan et al.rApplied Surface Science 156 2000 26–3830

Table 2Contact angles of surfaces subjected to different chemical etching

Ž . Ž . Ž .Sample number Static angle 8 Advancing angle 8 Receding angle 8 Comments on surface

1 72.9 79.3 31.0 shiny, heterogeneous2 45.5 51.9 13.0 homogenous3 40.7 43.7 10.7 homogenous4 38.2 43.0 13.0 homogenous5 45.9 50.8 14.0 shiny, crazed6 59.8 66.3 14.5 heterogeneous, crazed7 52.4 58.5 15.9 shiny, crazed8 65.9 71.9 -10 heterogeneous, crazed9 62.0 66.6 13.8 heterogeneous, crazed

10 72.8 78.5 27.9 unetched homogenous sample

ity trends. The receding contact angles give verysimilar, low results for all samples, except the un-etched sample, no. 10, and the inadequately etchedsample, no. 1, which are higher. Sample no. 8 isanomalous, because it has relatively high static andadvancing contact angles and the lowest recedingcontact angle. This is probably attributable to the

Fig. 3. Optical microscopic image of polymer surface beforechemical etching.

heterogeneity of the surface. From the results, weŽ .can see that, the unetched sample no. 10 and the

Ž .sample no. 1 , which was subjected to an apparentinsufficient etching, exhibit the highest contact an-gles, namely, the lowest hydrophilicity. The rest ofthe samples subjected to different degrees of chemi-cal etching show a decrease in contact angle. This

Fig. 4. Optical microscopic image of polymer surface after chemi-cal etching.

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 31

clearly indicates a significant improvement of thesurface hydrophilicity by chemical etching. The ef-fect of chemical etching varies according to thevariation of operation conditions such as composi-tion of the oxidizing reagents, temperature and dura-tion of etching. The best operation condition yields adrastic decrease of static contact angle from 72.88 to38.28, corresponding to the unetched sample and thechemically etched sample, respectively. It is readilyapparent in Table 2 that chemical etching provides,in a wide operation range, a hydrophilic surface formetallization.

In addition, it ought to be mentioned that theabove contact angle data were not obtained fromideally smooth surfaces and are therefore not equilib-rium contact angles. The equilibrium contact angleu corresponds to the lowest energy state for ae

system. On an ideally smooth and compositionallyhomogeneous surface, the equilibrium contact angleis the Young’s angle u . However, many real sur-y

faces are rough or heterogeneous. A liquid dropresting on such a surface may reside in a metastable

Žequilibrium energy trough separated from neighbor-.ing states by energy barriers , exhibiting a metastable

contact angle. In this case, advancing and recedingangles are different, known as hysteresis. The differ-

ence, u yu , is the extent of hysteresis. The generala r

Young equation is then invalid for contact angleanalysis on a rough surface. Numerous works havebeen done to deal with contact angle measurementson a rough surface and the first model was proposed

w x w xby Wenzel 12 . Joanny and De Gennes 13 alsoproposed a model analyzing the surface roughnesseffect on the contact angle measurements. However,due to the fact that contact angle measurements areperformed on surfaces of different roughness, practi-cal contact angle analysis is normally based on ex-perimental practices. It is generally accepted thathysteresis will be negligible when the surface rough-

w xness is below 0.5 mm 14,15 . Surface profilingstudies revealed that the average microroughness of

Ž .our samples ranged from 0.31 X profiling to 0.43Ž .mm Y profiling , which are within the negligible

topographic limit. However, contact angle for eachsample was still measured at four different locationson the surface, and measurement at each locationwas repeated for three times.

3.4. Optical microscopy and surface profiling studies

In order to understand the significant improve-ment on the hydrophilicity of the substrate surface

Fig. 5. The 3D surface profiling of surface polymer before chemical etching.

( )B. Luan et al.rApplied Surface Science 156 2000 26–3832

Fig. 6. The 3D surface profiling of surface polymer after chemical etching.

during chemical etching, optical microscopy studieswere performed. The unetched surface is shown in

Fig. 3, observed at a magnification of 500= . It wasobserved that the unetched surface was not only very

Fig. 7. Optical microscopic image of polymer surface chemically etched in the presence of crazes.

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 33

smooth but also highly transparent, which made theoptical microscope observation at lower magnifica-tion very difficult. In fact, good focusing was notaccessible. A typical etched surface is presented inFig. 4, showing a regularly distributed patternedfeature. It is manifested that the surface is roughenedduring chemical etching process. Electroless nickelplating was subsequently carried out successfully ona chemically etched surface in a conventional bathbut the unetched polymer surface was not platable atall. These verified the applicability of the presentchemical etching process. The reason for this im-provement may be attributable to the improved sur-face hydrophilicity and therefore, a better adhesionof the coating to the polymer surface. However, thepossibility of a modified surface chemistry shouldnot be excluded. These then lead to a conventionaldebate between two theories on the nature of adhe-sion that have been co-existing for many years. Thefirst theory was ‘chemical bonding theory’ proposedbased on the belief that there exists a chemicalbonding between the coated metal and the plastic

w xsubstrate 10 . The other theory is called ‘mechanicalanchoring theory’ proposed based on the belief that abetter adhesion of coating on a microroughened sur-face is due to the availability of microcavities which

w xprovide metal atoms better ‘anchoring sites’ 16 .This was supported by another interesting study con-

w xducted by Ebneth and Klimaschewski 17 . In theirexperiments on determining whether or not chemicalbonding is a contributory factor to the adhesion,Ebneth and Klimaschewski plated different metalssuch as gold, silver and copper. Should chemicalbond between metal and plastic be an essential con-tributory factor for adhesion, it would be influencedby the type of conductive metal deposited. This,however, was not the case shown by his experimen-tal results. It is now generally believed that themicroroughness of the plastic surface dominates theadhesion of coating but possible contribution ofchemical bonding has never been excluded. To quan-titatively analyze the roughness change before andafter chemical etching, surface-profiling techniquewas applied. The 3D plots corresponding to sampleswithout and with chemical etching are given in Figs.5 and 6, respectively. It ought to be mentioned thatthe two figures are not of the same axis scales, whichare automatically determined by the software. The

measurements reveal that although the wavinessŽ .macroroughness, described by R of the samplesa

are almost identical, the microroughness, R , of thet

chemically etched sample surface is significantlyincreased. This verifies the optical microscopic ob-servation of the surface roughness change, whichmay contribute to the hydrophilicity of the surface.

It is readily apparent that microroughening of thesurface is well-indicative of a selective chemicaletching on the surface of the SLA part. To furtherunderstand the nature of this selective chemical etch-ing process, a sample was overetched until the sur-

Ž .face was severely crazed see Fig. 7 . As manifested,the etched surface shows four different features:

Ž .plain surface background , a grey strip area and twodark bands along both sides of the grey strip andspherical holes which may be the onset of the stripformation. Detailed observations were subsequentlycarefully conducted at a higher magnificationŽ .500= for further understanding of the surface

Fig. 8. Optical microscopic image of crazed polymer sample,focused at the top surface.

( )B. Luan et al.rApplied Surface Science 156 2000 26–3834

features. It was then found that different features didnot come in focus simultaneously, indicating thatthese features are of different heights or depths. Thesequence of focusing revealed that the plain area isthe top surface and the grey strip is a ridge at a lowerlevel and the two dark bands are of etched-in slots atthe lowest level along both sides of the ridge. Theimages obtained at the same spot are given in Figs.8–10, with focusing sequence from top of the sur-face, top of the ridge and the bottom of the slots. Thedial-meter readings showed that the slots are ofabout 8 mm in depth and the ridge is about 5 mm inheight as from the bottom of the slots. It ought to bementioned that the outer patterned lines, known as‘Newton’s Fringes’, generated from the interferenceof light at very thin edges between adjacent featuresare not characterized by the intrinsic feature of thesurface. The abovementioned observations were con-firmed by further surface profiling studies, shown inFig. 11. The depth of the slots and the height of the

Fig. 9. Optical microscopic image of crazed polymer sample,focused at the top of the ridge.

Fig. 10. Optical microscopic image of crazed polymer sample,focused at the bottom of the trench.

ridge are very close to those measured by opticalmicroscopy dial-meter. Fig. 12 provides a schematicof the above observations in favor of an easierunderstanding of the features of the chemically etchedSLA5195 surface.

The aforementioned optical microscopy and sur-face profiling studies clearly suggested that theSLA5195 part is not a single-phase material, whichprompted our XRD phase studies presented below.

( )3.5. X-ray diffraction XRD and AFM

The XRD measurements were conducted on afresh SLA5195 sample. The diffraction pattern ispresented in Fig. 13, showing two broad peaks at 2u

of 18.308 and 41.638. As is known, the most dra-matic difference among crystalline and amorphousmaterials is on how they scatter an X-ray beamw x18,19 . The curve of scattered intensity vs. 2u for acrystalline solid is almost zero everywhere except at

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 35

Fig. 11. The 2D surface profiling of the crazed polymer surface.

certain angles where high sharp maxima occur, cor-responding to diffracted beams from grains of differ-ent orientation. On the contrary, a ‘perfect amor-phous’ solid should yield no diffraction maxima dueto the diffused X-ray scattering from randomlypacked atoms in the atomic arrangement. However,

materials practically referred to as ‘amorphous’ havestructures characterized by an almost complete lackof periodicity and a tendency to ‘order’ only in thesense that the atoms are fairly tightly packed to-gether and show a statistical preference for a particu-lar inter-atomic distances. The diffraction result is an

Fig. 12. Schematic of a chemically etched trench.

( )B. Luan et al.rApplied Surface Science 156 2000 26–3836

Fig. 13. The XRD pattern of the fabricated polymer surface.

X-ray scattering curve showing nothing more thanw xone or two broad maxima 19 . This is consistent

with the XRD spectroscopy shown in Fig. 13, sug-gesting the existence of amorphous phase androramorphous-related phase in the SLA5195 sample.However, it ought to be mentioned the intensity ofX-ray scattering peak at 18.308 is much greater thanthat of at 41.638, indicating a certain degree of‘ordered’ structure, namely, crystallinity. To verifythis point, AFM imaging was subsequently con-ducted and is presented in Fig. 14. The shadedtopographic image shows the presence of two differ-ent structures in the SLA5195 sample within a 5-mmsquare region. In the region on the right-hand side ofthe topographic image, the polymer shows a typicalamorphous structure without ordering while the left-hand side of the image shows ordered and isolatedfine grains of about 200 nm in diameter. This clearlyrevealed the co-existence of two phases in theSLA5195 polymer: an amorphous phase and a crys-tal phase. The small grain size is indicative of a fastsolidification process during laser-induced curingprocess. However, difficulty arises in understandingthe nature of this phase, for the peak is apparentlytoo broad for single crystal and, on the other hand,the co-existence of other peaks should present if thisis a polycrystalline phase, and this is apparently notthe case shown by the XRD pattern. The laser-in-duced polymerization process for fabrication of the

SLA material is then described below to understandthis phenomenon. When an SLA part is made, laserbeam scans over the liquid resin mixture at a highrate of about 5000 mmrs to form an instantly solidi-fied thin layer of about 100 mm in thickness. Laserbeam is then applied to form another layer on top ofthe previous layer. The direction of solidification is

Fig. 14. The AFM topographic image of the polymer surface,showing the co-existence of amorphous and polycrystalline phases.

( )B. Luan et al.rApplied Surface Science 156 2000 26–38 37

Ž . Ž .Fig. 15. Schematic showing the progression of chemical etching in different areas of phase structure. a Phase structure. b Chemicallyetched trench.

always perpendicular to the direction of laser scan-ning. Liquid solidifies at a high rate from the top intothe bulk of each layer and is repeated for every layer.This fast and oriented solidification process thenyields a preferred orientation of crystallization dur-ing which very small grains are formed. It is nowclarified that our SLA5195 polymer fabricated usinglaser-curing technique consists of amorphous andpolycrystalline phases with preferred orientation.Based on the above discussion, the progression ofchemical etching may be proposed below. This pro-posal adopts the models used for metal corrosionsince chemical etching of materials, organic or inor-ganic, is essentially a corrosion process. The co-ex-istence of two phases inevitably yields the existenceof non-homogeneous phase boundaries, which aregenerally believed to have low ‘corrosion’ resis-tance. When the SLA part is immersed in the highlyoxidizing media, localized etching therefore prefer-ably starts and develops along the phase boundaries.In comparison to the phase boundary regions, thepolycrystalline phase and the amorphous phase are

homogeneous in nature with less defects, hence,suffer etching of a less degree. However, experimen-tal results shown in Figs. 8–11 suggested that thegrain boundaries in the polycrystalline phase maystill lead to a preferred intergranular etching, whichrenders the polycrystalline phase a relatively loweretching rate than the amorphous phase in which nograin boundary is present. A schematic is given toillustrate the progression of chemical etching at dif-ferent areas of the surface phase structure, presentedin Fig. 15.

4. Conclusions

Ø Chemical etching was proved capable of signif-icantly improving the hydrophilicity of the surface ofSLA parts, which is beneficial for the subsequentpretreatment for metallization. The performance ofmetallization for RP is enhanced due to the uniform-ity of chemical etching.

( )B. Luan et al.rApplied Surface Science 156 2000 26–3838

Ø The SLA part fabricated from laser-curingtechnique was found consisting of two phases: anamorphous phase and a polycrystalline phase withpreferred orientation due to oriented laser-inducedsolidification during photopolymerization process.

Ø The improvement to the surface hydrophilicitymay be related to the creation and formation ofgrooved microtrenches on the surface due to thephase composition and structure of the SLA part.This, however, does not exclude the possibility of amodified surface chemistry. Further research is cur-rently being conducted.

Acknowledgements

Assistance from Mike Meinert in surface profilingand optical microscopy measurements is gratefullyacknowledged. Thanks are also given to Mary JaneWalzak, from the University of Western Ontario, forassistance in the contact angle and AFM measure-ments.

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