Characterization of a colloidal Pd(II)-based catalyst dispersion for electroless metal deposition

COLLOIDS A AND Colloids and Surfaces A: Physicochernicaland Engineering Aspects 108 (1996) 101 11!

ELSEVIER

SURFACES

Characterization of a colloidal Pd(II)-based catalyst dispersion for electroless metal deposition Walter J. Dressick a,,, Lynne M. Kondracki a, Mu-San Chen b Susan L. Brandow a, Egon Matijevi6 c, Jeffrey M. Calvert a aCenterfor Bio/Molecular Science and Engineering, Naval Research Laboratory, Code 6900, Washington, DC 20375-5348, USA bGeo-Centers, Inc., 10903 Indian Head Highway, Fort Washington, MD 20744, USA cCenterfor Advanced Materials Processing, Clarkson University, Box 5814, Potsdam, NY13699-5814, USA

Received 6 April 1995; accepted 24 August 1995

Abstract

An aqueous Pd(II) dispersion, useful as a catalyst for the selective electroless deposition of nickel metal at ligandbearing surfaces, is prepared by the hydrolysis of PdC14 2 - a t pH 5 in an approximately 0.01 mol dm 3 NaC1 solution. The catalyst dispersion is characterized by UV-visible absorption spectroscopy, electroless metallization, ultracentrifugation, and electrophoresis. The dispersion is found to consist of a distribution of anionic and uncharged Pd(II) species ranging in type from monomeric to colloidal. The species responsible for the initiation of electroless metal deposition at the ligand surface are identified as colloidal. Atomic force microscopy indicates that the colloidal catalysts are bound at the surface and range in diameter from approximately 4 to 53 nm with an average size of 30 + 12 nm. The behavior of the catalyst dispersion is consistent with a model in which colloid formation is initiated by polymerization of monomeric precursors generated by successive hydrolytic C1- loss from PdC142 , and deprotonation of the corresponding aquo-Pd(II) complex(es). Keywords: Colloids; Dispersions; Electroless plating; Metallization; Nickel deposition; Palladium catalysts

1. Introduction Electroless metal deposition is an autocatalytic redox process in which chelated metal ions in solution are chemically reduced to the metal at a substrate surface [ 1]. The process is widely used in the automotive and electronics industries for the metallization of nonconducting substrates, such as ceramics and plastics [2]. In the electronics industry, a variety of applications, including the production of ohmic contacts [ 3 - 6 J , shielded

* Corresponding

author,

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materials, chip-level interconnects [ 7 ], and printed circuit boards [ 8 - 1 0 ] have been described. Palladium/tin colloids are used as the traditional catalysts for electroless metallization, which involves a multistep procedure [1,2]. First, surface cleaning and roughening are commonly performed to permit adhesion of the palladium/tin species. Subsequent chemical treatments ("accelerations") of the surface-bound palladium/tin colloids are required to prepare the catalyst for metal deposition. An alternative approach, described elsewhere [ 11-13], utilizes a direct, covalent selective binding interaction between Pd(II) particles and a surface-

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W.J. Dressick et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 101-111

bound ligand to anchor the catalyst to the surface site. The Pd(II) catalyst may then be used for surface metallization using either electroless [11-13] or chemical vapor deposition (CVD) [14,15] processes, Functionalized organosilanes, which are formed as monolayer films on a broad range of substrates [ 16], provide appropriate surface ligands for the Pd(II) species. Direct patterned irradiation of these films using a variety of energy sources such as X-rays [17,18], UV [19,20], ion beams [18], or electron beams [21] can selectively destroy their ligating ability, thereby permitting the fabrication of patterned metal deposits (positive tone imaging) at the substrate surface [22]. Copper CVD features with resolutions to 1 gm [ 153 and nanometer-scale electroless nickel patterns [213 have been successfully fabricated using this approach. Photochemically inert ligand functionalized organosilane films used in conjunction with a conventional polymer photoresist overlayer can also be utilized [23]. Following photoresist patterning and development, the catalyst is selectively bound at the exposed underlying ligand. In this manner, high aspect ratio metal structures can be fashioned, because lateral metal growth is constrained by the photoresist channel walls. Electroless metal patterns exhibit extremely high plasma etching resistance and have been used as masks for pattern transfer into the underlying substrate [23-26]. A key aspect of the fabrication of patterned metal deposits is the nature of the catalyst which initiates the autocatalytic metal deposition reaction. Various aspects of the Pd(II) catalyst, including a model for its formation and the characterization of its surface binding mechanism, have been reported earlier [11-13]. Recently, it was shown that the size of the catalyst bound to the surface ligand directly influences the particle size of a subsequent electroless nickel deposit [27], which is essential in the fabrication of highresolution patterns, Because of the utility of these catalyst systems, it is important to understand in detail their solution properties and morphology, as well as their behavior at the substrate surface. In the previous studies [11-13], spectrophotometric techniques were

essentially used for all characterizations of the catalyst system. In order to better identify the colloidal nature of the palladium catalyst, additional studies are presented here, including ultracentrifugation, electrophoresis, and UV-visible absorption spectroscopy. Atomic force microscopy (AFM) is used to visualize the catalyst bound at a ligand-functionalized surface. The results are interpreted using the previously developed model to describe the formation of the catalyst species [11].

2. Experimental 2.1. Reagents

Deionized water of 18 M£2 resistivity was used for all experiments. All reagents were ACS reagent grade or equivalent and were used as received, unless otherwise noted. Phenothiazine was purified by vacuum sublimation and 2-(trimethoxysilyl)ethyl-2-pyridine (PYR) was vacuum-distilled (105°C; 0.3 mm Hg) immediately prior to use. GIBCO-BRL #55100B ultra pure agarose was employed in the electrophoresis experiments. 2.2. Catalyst preparation

The catalyst, hereafter referred to as PD1, was prepared as follows. An l l . S m g amount of Na2PdC14'3H20 was placed in a 100 cm 3 volumetric flask to which was added 1 cm 3 of 1.0 mol dm -3 aqueous NaC1 solution, and the solid was completely dissolved. A 10 cm 3 portion of 0.1 mol dm -3 2-morpholinoethanesulfonic acid (MES) buffer solution (pH 5) was then added and the solution was diluted to the mark with H20. The flask was incubated in a water bath at 25.0 __ 0.1 °C for 20 h. A 10 cm 3 aliquot of the catalyst system was then removed and replaced by 10 cm 3 of the same NaC1 solution, The resulting PD1 dispersion, which corresponded to "solution IV" described previously [11], contained approximately 3.5 x 10 4 mol dm -3 Pd(II) in a pH 5 solution of approximately 9 x 10 .3 mol dm -3 MES and approximately 0.11 mol dm-3 NaC1. PD1 dispersions were stored in a water bath at 25.0 + 0.1°C

w.31 Dressicket al./ColloidsSurfaces A: Physicochem. Eng. Aspects 108 (1996) 101-111 and used within 9 days after preparation. However, the PD1 dispersions could still be used to catalyze selective electroless nickel deposition up to at least 1 month after the preparation. 2.3. Substratepreparation Native oxide Si(100) wafers were n-type, of 2 in diameter (Wafernet, Inc). Fused-silica slides (1 in square; 1 mm thick) were from Dell Optics, Inc. All substrates were handled in a cleanroom facility, cleaned as described elsewhere [11], and kept in boiling water until used in the experiments. To chemisorb N-2-aminoethyl-3-aminopropyltrimethoxysilane (EDA) on the substrate, the latter was immersed in a freshly prepared solution of EDA in aqueous 1 x 10 .3 mol dm -3 acetic acid (1% (v/v)) for 20 min at room temperature (22 _+ 1 °C), then rinsed twice in water, and baked for 4 min at 120°C on a programmable hotplate, The cis-PdClz(PYR)2 silane films were prepared from cis-PdC12(benzonitrile)2 and PYR using the literature procedure [-11]. 2.4. Substrate electroless metallization For metallization the substrates were contacted with the PD1 catalyst for 30 min, then rinsed twice with water, and finally immersed in a 10% N I P O S I T ® nickel electroless bath at room temperature for 20 min. After removal from the electroless nickel bath, the substrates were rinsed twice with water, dried in a stream of filtered nitrogen gas, and examined in order to determine the extent and quality of the nickel metal deposit, 2.5. Centrifugation experiments Separations of colloidal materials from the PD1 catalyst were carried out at 25.0_+ 0.1°C at speeds ranging from 19 to 50 krev min -x (i.e. 65 000-175 000g). Following centrifugation from 1 h to 4 days, the supernatant solution was carefully removed from the tube, leaving behind a solid pellet. The UV-visible spectrum of the supernatant solution was recorded using a CARY 2400 UV-Vis spectrophotometer. The ability of this solution to catalyze electroless nickel deposition was tested

103

using EDA-coated silicon native oxide wafers as described above, and compared with that of the corresponding dispersion before it was centrifuged. The solid pellet remaining in the tube, following removal of the supernatant solution, was washed twice with the NaC1-MES solution, and then placed in about 1 cm a of water to which was added 1 cm 3 of the 10% N I P O S I T ® nickel bath. The sample was observed for about 5 rain and any formation of hydrogen bubbles or deposition of electroless nickel at the solid surfaces was noted. 2.6. Electrophoresis The electrophoresis experiments were carried out with a Hoefer Scientific model HE33 Minnie Submarine agarose electrophoresis gel unit which was filled with propylene glycol, cooled and stored overnight at - 2 0 ° C as directed by the manufacturer. A 0.75 wt.% agarose gel in aqueous pH 7 MES-NaC1 buffer was cast while hot in a mold at a thickness of approximately 0.8-0.9 cm and solidifled in a refrigerator for about 15 rain. After removing the "gel unit" from the freezer and connecting it to a GIBCO-BRL model 200 electrophoresis power supply, the agarose gel was carefully transferred into it and the sample wells were loaded with the PD1 catalyst dispersion (0.3 cma). Each well was capped with an agarose plug to prevent sample leakage and pH 7 MES-NaC1 buffer solution, which had been chilled to about 4°C, was added until the gel was entirely submerged. The measurements were carried out at 100 V for 10 min, after which time the electrophoresis was interrupted and the buffer solution drained. Fresh, cold buffer solution was added and the electrophoresis was resumed for 10 min. The buffer solution was again drained and the intact gel was removed for analysis. 2.7. Interactions in the agarosegel To test if the PD1 catalyst reacted with the agarose gel and the buffers used in the electrophoresis experiments, 0.75 g of agarose was dissolved in 100 cm 3 of boiling buffer solution (MES-NaC1 either at pH 5 or 7) and the mixture was immediately poured into a casting mold containing a clean

104

w.J. Dressicket al./Colloids Surfaces A. Physicochem. Eng. Aspects 108 (1996) 101 111

fused-silica slide, one face of which was covered approximately 1 mm deep by the mixture. The assembly was then allowed to cool undisturbed for 30 rain at room temperature while the gel solidified, and a spectrum was obtained over the 190-600 nm wavelength region. The gel was then treated with a few drops of the catalyst dispersion. Following adsorption of the catalyst, the gel was kept undisturbed for 20-30 min (the time corresponding to that necessary to perform the electrophoresis experiment). A UV-visible spectrum of the catalyst in the gel (corrected for the gel absorbance) was then obtained and compared to that of the PD1 catalyst dispersion, 2.8. Detection of P d ( I I ) in the gel

To determine the ability of various reagents to detect Pd(II) in the gel, a cube (1 x 1 x 1 cm 3) of Pd(II)-doped MES-NaC1 pH 7 agarose gel was placed in a test tube, and about 1 cm 3 of approximately 0.05 0.10 mol dm -3 aqueous reagent solution (unless noted otherwise) was then added. The reagents tested included ethidium bromide (10 lag cm-3), phenothiazine (in dimethylsulfoxide), N-(n-hexyl)4Br, N-(n-butyl)4Br, Na2S, and N I P O S I T ® nickel electroless bath (10 and 100%). After 5 min, the gel surface was inspected for any changes indicative of a chemical reaction. The sample was then heated to dissolve the gel and the contents were again examined for evidence of color change or other signs of reaction. Gel blanks without the catalyst were similarly evaluated as controls. The following procedure was adopted to detect the Pd(II) catalyst in the gel using the N I P O S I T @ (100%) nickel bath following electrophoresis. The gel containing the catalyst was sliced into 1 × 1 × 0.8 cm 3 pieces, which were placed in test tubes coded for distance and direction from the sample well. A 1 cm 3 aliquot of warm (65-70°C) N I P O S I T ® nickel bath was added to each test tube, and the metallization of the sample was observed for 5 min. The nickel bath dissolved approximately 50% of the gel volume before cooling to room temperature, so areas of the gel containing larger amounts of palladium could be immediately identified. To free any Pd(II) trapped

in the undissolved gel, all samples were briefly heated to dissolve the remaining gel and observed for an additional 5 min. All sample tubes exhibiting nickel metallization were then plunged into an ice salt bath ( - 5 ° C ) to resolidify the gelatin and quench the metal deposition reaction. The relative quantities of nickel metal present in each sample were estimated visually and recorded. Gel metallization observations were confirmed using a second gel track from the same experiment. The track was sliced in half along its length parallel to the migration direction during the electrophoresis experiment. Treatment with the N I P O S I T ® nickel bath initiated metallization simultaneously on all areas of the exposed gel interior bearing Pd(II) complexes. The relative rates and amounts of metallization corresponded within experimental error to those observed above for the process involving gel cubes. 2.9. Atomic force microscopy ( A F M )

AFM images were collected on a Nanoscope III instrument with a median filter in both the tapping and error signal modes [28]. Data were corrected for sample tilt.

3. Results and discussion A general scheme for the hydrolysis and oligomerization of PdC142- in aqueous solution, as first presented elsewhere [11], is shown in Scheme 1. The loss of C1- from the PdC142- initially occurs with the formation of PdC13(H/O )- according to Eq. (1) [29-32], followed by the loss of a second C1 ligand to generate eis- and/or transPdC1/(H20) 2 (Eq. (2)). In solutions containing little or no added C1- ion, hydration may proceed by the substitution of additional C1- by U20. Deprotonation of the coordinated H2O molecules in PdC13(H20) and PdC1/(H/O)2 can occur (Eqs. (3) and (4)), for which the corresponding pKa values are approximately 7 and 4.3, respectively [33]; significant quantities of PdC13(OH) 2- and PdClz(OH)(H20 )- may therefore be present even in slightly acidic solutions. Polycondensations of these latter species ultimately lead to the formation

W.J. Dressick et al./Colloids Surfaces A." Physicochem. Eng. Aspects 108 (1996) 101-111

PROPOSED Pd(II) OLIGOMER FORMATION PATHS IN AQUEOUS SOLUTION PdCI;-+ H20 ~ C l - + OdCr~(H20)-

ity on the EDA-modified silicon oxide substrate with the 10% N I P O S I T ® nickel bath for 15 min. (I)

PdCI~(Ha0)-+ H20 ~ CI-+ cis/lrons-PdCl2(HsO)a (2) PdCI3(Hz0)-~ H++ PdCl3(0H)2(5) PdCI2(HzO)~ ~

H++ PdCI2(OH)(H20)-

(4)

CI\ CI- CI /OHa~ CI CI\ /OH 2Pd + Pd ~ Pd Pd + cl-+ H20 ~ ~

c( "0H2H0" "Cl -~--~

C( b~ "cl

[\ OH OH /CI OH OH Cl7 y /pd" "p~" "pd "Pd" "Pd" "Pd i L" "c~" "o~ 6d "od "od "0H~ ×

105

(5)

where x,y ore integers with ×>2 ond -2.2and-2<~y<~2,

Only in one case (a PD1 dispersion aged 9 days and centrifuged for 1 h at 19000 rev rain -1) did metallization occur. All other supernatant solutions w e r e p r o v e n inactive. The UV-visible spectra of the PD1 catalyst dispersion aged 1 day before and after exhaustive centrifugation (50 000 rev min 1 for 114 h) are shown in Fig. 1. As discussed previously [11], absorption bands at 2 < ~ 2 6 0 n m are due primarily to H(C1-)---,dcr*(Pd(II)) ligand-to-metal

charge-transfer (LMCT). The absorption tail

at

2 > ~ 300 nm is assigned to unknown transitions involving Pd(II) oligomers. Although the absorption maximum at about 280nm is assigned to

hydrolyzed chloropalladium(II)complexes [34].

the PdC142- species, contributions to the solution absorbance in this region from the LMCT and oligomer-based bands are also possible. Following centrifugation, the absorption intensities of the remaining supernatant solution are signifcantly diminished as compared to the original PD1 dispersion. Indeed, at 2 > 3 0 0 n m , the

3.1. Colloidal nature of the catalyst species

absorbance is essentially eliminated. Only those weak ligand-field bands at about 320 n m (1Alg

From a catalysis point of view, it is of particular interest and importance to establish the presence or the absence of colloidal palladium species in the reaction sequence shown in Scheme 1. The formation of palladium-containing colloids has been considered in earlier studies of the hydrolysis of Pd(II) salt solutions [34,35], and the appearance of UV absorption bands at 2>300 nm in hydrolyzed Pd(II)solutions has been interpreted, at least in part, to be due to colloidal Pd(II) species [35]. In the case of the Pd/Sn-based catalysts, it was indeed demonstrated that activity for electroless plating was due to the colloidal component of the dispersion [36]. Thus, if Pd(II)-conraining colloids are present in the PD1 system, the question of their role, if any, as catalysts for electroless metal deposition needs to be established, For this purpose, PD1 dispersions were aged from 1 to 9 days and centrifuged at 19 000 and 50 000 rev rain-1 for times ranging from 1 to 114 h. In all cases, a pellet separated out, and the supernatant solution was tested for catalytic activ-

"4"IA2g) a n d about 440 n m (1Alg--~IEg) [29], generally associated with monomeric Pd(II) complexes, are observed in the supernatant solution spectrum. Analogous results are observed with the PD1 dispersion aged 9 days. The influence of centrifugation on the metallization behavior of the 1- and 9-day-aged PD1 dispersions is readily understood. The latter system is expected to contain a larger concentration and wider distribution of colloidal particles than the former, given the age difference. Thus, the longer centrifugation times are required to completely eliminate the catalyst. The settling of a solid material, which is reactive with the electroless nickel bath following centrifugation, clearly establishes Pd(II)-containing colloids as the active catalyst for electroless metal deposition in the PD1 systems. Furthermore, the decrease in the absorbance of the supernatant solution, especially at 2 > 300 nm, lends support to the original assignment of these transitions being due to the presence of colloids [35].

of oligomers of Pd(II) as shown in Eq. (5), which can further grow to eventually precipitate complex hydroxo-oxo- bridged Pd(II) solids. The formation of o!igomers via reactions analogous to Eq. (5) may occur using other combinations of

106

W.J. Dressick et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 101-111

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200

300

400

Wave,eng, ,nm,

500

600

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0.0 200

300

400

500

600

Wavelength(nm) Fig. 1. UV-visible absorption spectra of a PD1 dispersion aged for 1 day before (upper spectrum) and after (lower spectrum) ultracentrifugation at 50 krev min -1 for 4 days after subtraction of the background absorption of the 0.11 M NaCI 9 mM MES (pH 5) aqueous buffer solution. The optical pathlength was 0.1 cm. Insert: UV-visibleabsorption spectrum of the PD1 dispersion in 0.75 wt.% agar gel after the agar background spectrum was subtracted. The agar buffer solution was 0.11 M NaC!-9 mM MES (pH 7) and the gel path length was approximately 0.1 cm.

3.2. Electrophoresis Once the colloidal nature of the PD1 catalyst was established, it was of interest to further characterize its properties. For this purpose, gel electrophoresis was used to determine the particle charge, which is dependent on the pH. A buffer was used to maintain a constant pH. The buffer may react with different components of the PD1 dispersion (solute and solid) and its high ionic strength could cause colloid aggregation. Thus, it was necessary to choose an appropriate system, which was found to consist of 9 x 10 -3 mol dm -3 MES and 1.1 x 10 -1 mol dm -3 NaC1 at p H 7. The composition of this buffer corresponded most closely to that of the catalyst without the Pd(II)-containing species. This buffer avoided the hydrogen gas evolution during the electrophoresis experiments noted with buffers of lower pH. The UV spectrum of the gel containing the above-described buffer following equilibration with the PD1 dispersion for about 30 min (note Fig. 1,

insert) showed the same general features as the PD1 dispersion, indicating that no reaction between the PD1 dispersion and the gel took place. To further avoid the possibility of any undesirable reactions within the gel at p H 7, the electrophoresis experiments were carried out at about 4°C. Control experiments have shown that processes in the PD1 catalyst systems are very slow at low temperatures. For example, a PD1 formulation prepared at about 15°C showed no catalytic activity even after the 20 h incubation (note experimental section). In the electrophoresis experiments, it was found that a certain gel thickness was necessary for adequate mechanical integrity of the sample, and to allow for the use of sufficient PD1 dispersion for the detection of Pd(II) following the experiment. Eventually, 0.8-0.9 cm thick gel samples were prepared and loaded with 0.3 cm 3 of the PD1 dispersion. Originally, it was attempted to follow the migration of the catalyst in the gel by UV spectroscopy. However, this procedure was pre-

W.J. Dressick et al./ Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 101-111

cluded by the large UV background absorbance of the gel, making it necessary to detect Pd(II) in the gel by chemical means. To do so, different PD1 concentrations in the MES-NaC1 buffer were used to prepare gels. A number of reagents (phenothiazine, ethidium bromide, N-(n-hexyl)4Br, N-(nbutyl)4Br and NazS ) were then used to assess their Pd(II) detection limits in the gel, but none proved effective. Finally, it was established that the NIPOSIT ® electroless nickel bath made it possible to identify palladium-containing zones in the gel, following electrophoresis, because it initiated the nickel deposition at the palladium sites at concentrations of the catalyst as low as approximately 5 x 10 - 6 tool dm -3. Once initiated, the growth of nickel metal could be quenched by freezing the mixture. Regions of the gel containing low levels of palladium species after the electrophoresis could, therefore, be readily identified by allowing the metallization to proceed until observable amounts of nickel metal were generated, The results of the electrophoresis experiments are summarized in Table 1, which gives the relative amounts of metallization of each of the gel samples normalized to the maximum amount of nickel observed as a function of distance from the edge of the gel sample. Thus, sample " - 2 " includes all PD1 (if any) which migrated a distance of at least 1 cm but less than 2 cm from the sample well in the direction of the anode. An examination of Table 1 Electroless Ni Metallization of the Electrophoresis Gel Samples Distance a Relative Ni A m o u n t s b

--2

-1

0

0

well 15

1

2

3

4

5

15

100

30

0

0

a Gel interval sample measured from the nearest edge of the sample well (1 cm width). N u m b e r s represent the distances in cm from the well edge at which gel tracks were sliced to create samples for analysis. Negative values refer to migration in the direction of the anode and positive values represent migration in the direction of the cathode, b Approximate a m o u n t s of Ni metal deposited during the analysis of the sample as described in the Experimental Section relative to the sample exhibiting the largest a m o u n t of metallization (set to 100%). Values are estimated visually and uncertainties are _+ 15% of the stated value.

107

Table 1 indicates that there is no nickel metallization of the samples which would contain positively charged PD1 catalyst components. In contrast, negatively charged and neutral (sample well) catalyst components were active in metallizing the gel substrate with nickel (These results are supported by simple coagulation experiments conducted using the PD1 dispersion. Reagents containing the N-(n-butyl)4 +, N-(n-hexyl)4 +, or AsPh4 + (where Ph is phenyl) cations, which can interact with anionic species in the dispersion, led to the settling of solid materials when added to the dispersion. In contrast, the use of cation-specific reagents, such as those containing C104- , PF 6 , Fe(CN)64-, or C 0 ( C 2 0 4 ) 3 2 - gave no separation of solid materials. This behavior is consistent with the presence of anionic rather than cationic components in the PD1 dispersion, in agreement with the electrophoresis findings. Consequently, the range of permissable charges for Pd(II) oligomers described in Scheme 1 is found to be integers y ~<0.). The electrophoresis experiments indicate that the components of the dispersion differ in the magnitude of the surface potential, but do not allow for quantification of this parameter and the elucidation of surface sites responsible for the charge. 3.3. A F M experiments

In order to gain further insight into the nature of the PD1 colloids responsible for the initiation of electroless metal deposition, their interaction with an EDA-modified surface was examined by the atomic force microscope. This technique is convenient for mapping the distribution of surfacebound colloidal Pd(II) species [27]. Fig. 2 shows an AFM scan of an EDA-modified silicon substrate following treatment by the PD1 dispersion for 20 min, which corresponded to a surface coverage of approximately 35% by the colloidal catalyst. The particle size distributions in the same system are

given in Fig. 3, as well as for treatments for 10 rain and 60 rain. Surface coverages for the latter times a r e approximately 25% and approximately 65%, respectively. Particle diameters ranging from about 4 to 53 nm with an average size of 30 _+ 12 nm are essentially independent of treatment times. T w o questions naturally occur in the interpreta-

108

W..L Dressick et al./Colloids Surfaces A. Physicochem. Eng. Aspects 108 (1996) 101-111

Fig. 2. A F M scan of the EDA-modified native oxide silicon substrate following treatment with PD1 dispersion (aged 1 day) for 20 min.

Catalyst Particle Distribution vs Contact Time 35 30

Series 1: Contact Time 10 rain Series 2: Contact Time 20 min Series 3: Contact Time 60 min

l

N25

~o

m m

" i

m

°

,

,7 Particle Diameter (nm) Fig. 3. Surface Pd(II) colloid particle size distributions for the substrate from Fig. 2, as a function of PD1 dispersion treatment times in descending order: black, 1-60 min; gray, 2-20 rain; white, 3 10 min. Identical particle size distributions with an average particle size of 30 _ 12 n m were obtained for all treatment times.

W.Z Dressick et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 101-111

tion of Fig. 2. The first involves the relationship, if any, between the observed surface particle size distribution and that of the dispersed Pd(II) catalyst. It was shown elsewhere that the observed surface size distributions depend on factors such as the density and the type of surface ligands bound to the substrate [-27]. Therefore, it is not possible to resolve this problem using the AFM results, One must also consider the possibility of further reaction of the surface-bound catalyst particle with solution species during the deposition process, which may result in the continuous growth of the particles. This possibility can be examined by treating a surface, bearing only hydrolyzable Pd C1 bonds, with the PD1 dispersion and noting any further binding of Pd(II). Fig. 4 shows a UV absorption spectrum for a cis-PdC12(PYR)2 film before and after treatment for 30 min with a PD 1 dispersion aged for 9 days. Binding of the catalyst clearly occurs despite the absence of free PYR ligand sites in the cis-PdC12(PYR)2 complex, Unfortunately, the extent of this reaction cannot be quantitated from Fig. 4 in the absence of UV

0.08

.

.

.

.

-

I

,

109

absorptivity data for the distribution of bound colloids. In Fig. 3, however, surface colloid particle size distributions on the EDA-modified substrate are clearly unaffected by a range of treatment times with the PD1 dispersion. This behavior indicates that the extent of the growth reaction of the catalyst on the surface during the course of the PD1 dispersion treatment is not sufficient to be observed in terms of the surface particle distribution. Consequently, the AFM scan shown in Fig. 2 does represent, within the limits of our imaging abilities, an accurate picture of those colloids which promote electroless metal deposition at the surface.

4. Conclusions A detailed characterization of a tin-free Pd(II) dispersion, useful as a catalyst for electroless metal deposition at ligand surfaces E l l ] , has been described. The catalyst is prepared by the hydrolysis and oligomerization of PdC142 at fixed solution pH and [C1- ]. From the described experiments emerges a clearer picture of the nature of

,

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300

400

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W.J. Dressick et al./Colloids Surfaces A: Physicochem. Eng. Aspects 108 (1996) 101 111

110

the dispersion and of the species responsible for the initiation of the electroless metal deposition process at a catalyzed surface. These results indicare that the P d ( I I ) catalyst is composed of a distribution of principally anionic and some neutral metal complexes and that Pd(II)-containing

References [1] C.R. Shipley, Jr., Plating Surf. Finish., 71 (1984) 92. [,23 J. Colaruotolo and D. Tramontana, in G.O. Mallory and J.B. Hadju (Eds.), Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Orlando, FL, 1990, Chapter 8.

colloids function as the catalytic species for electroless metal deposition. The UV-visible absorption signature of the colloidal materials in the dispersion is definitively established at 2 > 3 0 0 nm, and provides a convenient method of gauging the catalytic activity of the dispersion. Although the distri-

[-33 G. Stremdoerfer, Y. Wang, D. Nguyen, P. Clechet and J.R. Martin, J. Electrochem. Soc., 140 (1993) 2022. E4] G. Stremdoerfer, C. Calais, J.R. Martin, P. Clechet and D. Nguyen, J. Electrochem. Soc., 137 (1990)835. E5] G. Stremdoerfer, D. Nguyen, J.R. Martin and P. Clechet, J. Electrochem. Soc., 137 (1990) 256.

bution of specific P d ( I I ) species in the system has not been established, surface-bound colloidal particles ranging in size from approximately 4 to 53 nm have been observed by AFM. The particle size distribution remains invariant with increased PD1 catalyst treatment times, consistent with direct deposition and binding of the colloids by the surface ligand, The present results lend further support for a model developed previously to explain the behavior of the Pd(II) catalyst dispersion [,11]. The

J. Electrochem. Soc., 135 (1988) 2881. [,73 C.H. Ting, in M. Paunovi and I. Ohno (Eds.), Electroless Deposition of Metals and Alloys, ElectrochemicalSociety Proc. Ser., Pennington, NJ, 1988, PV 88-12, p. 223. [83 . A. Viehbeck, C.A. Kovac, S.L. Buchwalter,

understanding of the catalyst chemistry gained here provides a basis for the further exploration of the catalyst and its interaction with the ligand surface. Of special interest, of course, are factors affecting the performance of the catalyst in lithographic applications [22], which include the complex questions of nuclei formation and growth of the colloidal catalyst in the dispersed state and their influence on the corresponding properties of the electroless metal films. Integration of the results obtained here with ongoing investigations of the morphologies of the Pd(II) colloidal particles bound to the ligand-modified surface will further our

understanding of the mechanisms of catalysis

and electroless metal deposition processes [--273. In addition, these solution and surface characterization studies provide a paradigm for investigation of the selective surface attachment of other types of particles, such as gold colloids [-37]. Acknowledgments The authors gratefully acknowledge financial support for this work from Shipley C o m p a n y and the Office of Naval Research.

[6] G. Stremdoerfer, H. Perrot, J.R. Martin and P. Clechet,

M.J. Goldberg and S.L. Tisdale, in E. Sacher, J.-J. Pireaux and S.P. Kowalczyk (Eds.), Metallization of Polymers, ACS Symp. Set. 44, Washington, DC, 1990, Chapter 29,

p. 394. E9] s.P. Murarka and M.C. Peckerar, Electronic Materials Science and Technology, Academic Press, New York, 1989, Chapter 6.

[,10] D.P. Seraphim, J. IBM Res. Dev., 26 (1982) 37. [-11] W.J. Dressick, C.S. Dulcey, J.H. Georger, Jr., G.S.

Calabrese and J.M. Calvert, J. Electrochem. Soc., 141 (1994)210. [12] W.J. Dressick, C.S. Dulcey, J.H. Georger, Jr. and J.M. Calvert, Chem. Mater., 5 (1993) 148. [13] W.J. Dressick, C.S. Dulcey, J.M. Calvert, J.H. Georger, Jr., G.S. Calabrese, M.E. Thomas and H.A. Stever, Mater. Res. Soc. Symp. Proc., 260 (1992) 659. [14] S.E. Potochnik, D.S.Y. Hsu, J.M. Calvert and P.E. Pehrsson, Mater. Res. Soc. Symp. Proc., 337 (1994) 429. El5] S.E. Potochnik, P.E. Pehrsson, D.S.Y. Hsu and J.M. Calvert, Langmuir, in press. El6] J.M. Calvert, in A. Ulman (Ed.), Organic Thin Films and Surfaces, Vol. 1, Academic Press, Boston, 1994. [,,173 C.R.K. Martian, F.K. Perkins, S.L Brandow, T.S. Koloski, E.A. Dobisz and J.M. Calvert, Appl. Phys. Lett., 64(3)(1994) 390. [,18] J.M. Calvert, T.S. Koloski, W.J. Dressick, C.S. Dulcey,

M.C. Peckerar, F. Cerrina, J.W. Taylor, D. Suh, O.R. Wood, A.A. MacDowell and R. D'Souza, Opt. Eng., 32(10) (1993)2437. [19] C.S. Dulcey, T.S. Koloski, W.J. Dressick, M.-S. Chen, J.H. Georger, Jr. and J.M. Calvert, Proc. SPIE, 1925 (1993) 657. [203 c.s. Dulcey, J.H. Georger, Jr., V. Krauthamer, D.A. Stenger, T.L. Fare and J.M. Calvert, Science, 252 (1991) 551. [213 F.K. Perkins, E.A. Dobisz, S.L. Brandow, T.S. Koloski, J.M. Calvert, K.W. Rhee, J. Kosakowski and C.R.K. Marrian, J. Vac. Sci. Technol. B, 12(6) (1994) 3725.

W.J. Dressick et al./Colloids Surfaces A: Physicoehem. Eng. Aspects 108 (1996) 10I 111 [22] W.J. Dressick and J.M. Calvert, Jpn. J. Appl. Phys., 32 (1993) 5829. [23] J.M. Calvert, G.S. Calabrese, J.F. Bohland, M.-S. Chen, W.J. Dressick, C.S. Dulcey, J.H. Georger, Jr., J. Kosakowski, E.K. Pavelcheck, K.W. Rhee and L.M. Shirey, J. Vac. Sci. Technol. B, 12(6) (1994) 3884. [24] J.M. Calvert, J.H. Georger, Jr., M.C. Peckerar, P.E. Pehrsson, J.M. Schnur and P.E. Schoen, Thin Solid Films, 210/211 (1992) 359. [25] J.M. Calvert, C.S. Dulcey, M.C. Peckerar, J.M. Schnur, J.H. Georger, Jr., G.S. Calabrese and P. Sricharoenchaikit, Solid State Technol., 34(10) (1991) 77. [26] J.M. Calvert, M.-S. Chen, C.S. Dulcey, J.H. Georger, Jr., M.C. Peckerar, J.M. Schnur and P.E. Schoen, J. Vac. Sci. Technol. B, 9 (1991) 3447. [27] S.L. Brandow, W.J. Dressick, G.-M. Chow, C.R.K. Marrian and J.M. Calvert, J. Electrochem. Soc., in press.

111

[28] C.A.J. Putman, D.O. van der Weft, B.G. de Grooth, N.F. van Hulst, J. Greve and P.K. Hansma, Proc. SPIE, 1639 (1992) 198. [29] L.I. Elding and L.F. Olsson, J. Phys. Chem., 82 (1978) 69. [30] P. van Z. Bekker and W. Robb, J. Inorg. Nucl. Chem., 37 (1975) 829. [31] L.I. Elding, Inorg. Chim. Acta, 6 (1972) 683. [32] L.I. Elding, Inorg. Chim. Acta, 6 (1972) 647. [33] J.V. Rund, Inorg. Chem., 9 (1970) 1211. [34] R. Wyatt, Chem. Weekbl., 62 (1966) 310. [35] V.I. Kazakova and B.V. Ptitsyn, Russ. J. Inorg. Chem., 12 (1967) 323. [36] E. Matijevi6, A.M. Poskanzer and P. Zuman, Plating Surf. Finish., 62 (1975) 958. [37] J. Schmitt, W.J. Dressick, S.L. Brandow, R.E. Geer, G. Decher, R. Shashidhar and J.M. Calvert, in preparation.