Langmuir 1994,10, 4089-4094
4089
Adsorption of Nanometer-Sized Palladium Particles on Si(100) Surfaces E. P. Boonekamp* and J. J. Kelly Debye Institute, Department of Condensed Matter, Utrecht University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
L. G. J. Fokkink Philips Research Laboratories, P.O. Box 80.000, 5600 J A Eindhoven, The Netherlands Received March 29, 1994. In Final Form: August 22, 1994@ It is shown that polymer-stabilized Pd particles, adsorbed on Si, initiate the growth of electroless Ni. The influence of the polymer (poly(vinylpyrro1idone))on the particle size and on the surface coverage is described. While Pd particles adsorb stronglyon Si, almost no adsorptionis observed on SiO2. Implications of this adsorptionselectivityfor the production of high-resolutionmetal patterns on Si are briefly discussed.
Introduction The wet-chemical metalization of semiconductors is of considerable industrial importance. In practice, this is usually achieved by electroless plating. For electroless deposition of a metal layer (e.g. Nil, the substrate is immersed in a solution containing a reducing agent (hypophosphite) and metal ions (Ni2+). The reduction of Ni2+by hypophosphite (eq 1)has a high activation energy and occurs at a significant rate only in the presence of a catalyst. If hypophosphite is used as the reducing agent, a small amount (9.8 w t % at pH 4) of phosphorus is incorporated during the deposition of an electroless Ni layer. The phosphorus concentration in the layer depends on the temperature and the pH ofthe electroless solution.2 In order to confine reaction 1to the substrate to be plated, its surface has to be catalytically activated. Palladium has proven to be a good catalyst for the initiation of Ni3,4 and Cu5 deposition from an electroless solution.
+
Ni2+ H,P02
+ H,O - Nio + H3P03 + 2H'
(1)
One of the options for activating Si single crystal surfaces for electroless plating involves immersion of the substrate in a solution containing a palladium salt and hydrofluoric acid.6 In this system Pd particles form at the surface and an equivalent amount of Si dissolves (eq 2). HF is used to complex dissolving Si species and keep the surface free of oxide, which would otherwise inhibit the reaction.
Si
+ 2Pd2+- Si4++ 2Pd0
(2)
We have found that this Pd deposition process depends strongly on the crystal orientation and results in the formation of a relatively low particle density (-lo9
* Abstract published inAdvanceACSAbstracts, October 15,1994.
(1)Modern Electroplating, 2nd ed.; Lowenheim, Frederick A., Ed.; John Wiley & Sons, Inc.: New York, 1963. (2)Fernandez, M.; Martinez-Duart, J. M.; Albella, J. M. Electrochim. Acta 1986,31,55. (3)van der Putten, A. M. T.; de Bakker, J. W. G. J.Electrochem. SOC. 1993,140,2221. (4)van der Putten, A. M. T.; de Bakker, J. W. G. J.Electrochem. SOC. 1995,140,2229. (5)van der Putten, A. M. T. J.Electrochem. SOC.1993,140,2376. (6)van der Putten, A. M. T.; de Bakker, J . W. G. The Electrochemical SocietyExtendedAbstracts,Vol.89-2,Hollywood, FL,Oct. 15-20,1989; Abstract 467,p 681.
Darticles/cm2for a Si(100) surface). The Pd deDosition process is also expected to depend on the initiafsurface structure (steps, defects). The particles are able to grow as long as uncovered Si is available. The morphology of an electroless metal layer will be affected by the particle size and density of the applied catalyst. In order to obtain rapid initiation of smooth electroless metal layers, a thin layer of very small Pd particles of high density on the Si surface is desirable. In the present work, a different strategy was adopted: nanometer-sized Pd particles, prepared in aqueous solution by the reduction of Pd ions with hypophosphite (eq 3, ref 7), were adsorbed onto Si(100) surfaces and onto Si02 layers, thermally grown on Si single crystal wafers. Pd2+
+ H3P02+ H 2 0
--c
PdO
+ H3P03+ 2H'
(3)
To prevent aggregation, the particles were prepared in the presence of the water-soluble polymer poly(viny1pyrrolidone) (PW). The polymer chains adsorb onto the Pd surface and prevent intimate contact between particles, thus preventing flocculation (steric stabilization). We mainly used PVP with an average molecular weight of 10000. This low molecular weight PVP was chosen because no multiparticle complexes (i.e. more than one Pd particle adsorbed onto one polymer molecule) were expected for this system. In addition, higher average molecular weights of 40 000 and 360 000 were used to investigate the possible effects of such multiparticle complexes. An important issue in this study is the influence of the polymer concentration on the adsorption of these particles on Si and Si02 surfaces. The adsorption of Pd particles will be affected by the chemical nature ofthe surface. Silicon surfaces pretreated with an aqueous hydrofluoric acid solution become covered by a mixed monolayer (ML) of Si hydride (-0.9 ML) and Si fluorine (-0.1 ML) specie^.^^^ On rinsing such a surface with water the fluorine is rapidly replaced (within a few minutes) by oxygen-containing species.lOJ1 These predominantly H-terminated Si surfaces have a hydrophobic (7) van der Putten, A. T. M.; de Bakker, J. W. G.; Fokkink, L. G. J.
J.Electrochem. SOC.1992,139,3475. (8)Burrows, V. A.;Chabal, Y. J.; Higashi, G. S.;Raghavachari, K.; Christman, S.B. Appl. Phys. Lett. 1988,53, 998. (9)Higashi, G. S.;Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990,56, 656. (10)Grundner, M.;Schultz, R. AZP Conf. 1987,167,329. (11)Graf, D.; Grundner, M.; Schultz, R. J.Vac.Sci. Techno1.A 1989, 7,808.
0 1994 American Chemical Society 0743-7463/94/241Q-4089$04.50/0
4090 Langmuir, Vol. 10, No. 11, 1994 character and show a remarkable stability in HzO at both low and neutral pH.12 The H-terminated Si surface can be oxidized by irradiation with UV light in air. The low concentration of ozone and oxygen radicals formed is capable of oxidizing the Si surface (UV/ozone treatment). After this treatment the surface is very hydrophilic, due to a high coverage of Si-0 species. The adsorption of colloidal Pd particles on the Hterminated Si, the UV/ozone treated Si, and the Si02 surface was studied as a function of the PdPW concentration ratio in the prepared sol. This was done by measuring the Pd surface coverage with X-ray fluorescence spectroscopy (XRF). In addition, the influence of the PVP chain length on the adsorption of Pd particles was studied. The influence of the polymer concentration on the size of the Pd particles, adsorbed on H-terminated Si(100) surfaces, was studied with transmission electron microscopy (TEM). The catalytic activity for electroless Ni depositionwas tested for Si samples covered with adsorbed Pd particles.
Experimental Section All chemicals used were of analytical grade and the solutions were prepared with deionized water. A 0.056 M palladium chloride solution was prepared by dissolving 1.00 g of PdCl2 in 35 mL of concentrated hydrochloric acid (36.5-38.0%, 99.999% purity, Merck) and making up to 100 mL with water. Solutions of poly(vinylpyrro1idone)(PVP) were made by dissolving 911 mg of PVP in 100 mL ofwater. PVP samples with various (average) molecular weights (Mpvp)were used in this study (Mwp: 10 000, 40 000, Fluka; 360 000, Janssen). A 0.48 M solution of sodium hypophosphite (NaH2P02, Janssen) was used as the reducing agent. The silicon single-crystal wafers were p-type (100) oriented samples (boron doped, 25.5-34.5 Q cm) obtained from WackerChemitronic GmbH (Germany). The wafer was cut into pieces of 15 x 15 mm2. The silicon samples were ultrasonically cleaned in ethanol. Subsequently, the native oxide was removed by etching for 30 s in an aqueous NHIF-HF solution (pH 6), and the wafer was rinsed under running water. After etching, the surface was hydrophobic (water contact angle 78"). The water contact angle was measured with the sessile drop method.13In some experiments, the H-terminated Si(100) surface was subsequently oxidized in an UV/ozone reactor (UV/ozone photoreactor, PR-100, UVP inc.) for 15 min. After this treatment, the surface was hydrophilic (water contact angle about 0'). The silicon oxide samples were 100 nm SiOz, thermally grown at 1000 "C in oxygen, on p-type Si(100) wafers (20 Q cm). These wafers were also cut into pieces of 15 x 15mm2and ultrasonically cleaned in ethanol. Subsequently, an oxide layer of about 10 nm was etched off in the above-mentioned NH4F-NF solution and the wafer was rinsed under running water. A number of sols containing Pd, PVP, and hypophosphite in varying concentration ratios were studied. First, a solution containing the required amounts of Pd and PVP was prepared. While the (air-saturated) solution was stirred, a given volume of the hypophosphite solution was added. The pH of the resulting solutions was 1.2. For some experiments, the sols were diluted with an aqueous solution containing HC1 (6.34x 10-2 M) and NaHzPO2 (1.56 x M) to keep the ionic strength constant. In general, 10 min after the addition of the hypophosphite solution a cleaned and etched Si or Si02 sample was introduced into the Pd sol. Adsorption of the Pd particles was allowed to occur in general for a period of 5 min. Afterward, the surface was briefly rinsed under running water and dried in air. No desorption or corrosion of Pd was observed during the rinsing with water (Figure 2). In all cases, adsorption caused no noticeable depletion of Pd in the solution. All experiments were performed at room temperature. (12) Boonekamp, E.P.;Kelly, J. J.;van de Ven, J.;Sondag,A.H. M. J . Appl. Phys. 1994,75, 8121. (13)Hiemenz, P. C.Principles of colloid and surface chemistry, 2nd ed.; Marcel Dekker, Inc.: New York and Basel, 1986; p 329.
Boonekamp et al.
4t
0' 0
I
5
I
10
15
I
20
t, minutes
Figure 1. Pd coverage ( r P d ) of H-terminated Si as a function of the adsorption time ( t ) : sol preparation CPd = 8.46 x M, M; (a) cpvp = 22.8 mg/L; (b) cpvp = 57.0 mg&; (c) cwp = 228 m a .
C N ~ H ~ P= O ~1.56 x
The Pd surface coverage ( r p d ) of the Si and Si02 samples was measured by X-ray fluorescence spectroscopy (XRF) using a Philips PW 1404 ~pectr0meter.l~ The detection limit for the measurements of Pd on Si and Si02 is 0.02 x 10'5 Pd atoms/cm2. The error in the measurements of r p d is < 10%. The Pd particles adsorbed on Si surfaces were studied by transmission electron microscopy (TEM). The samples were thinned from the rough backside of the Si wafer by chemical jet-etching using a solution of 2/5HF (50%)and 3/5 HNO3 (65%). The samples were studied using a Philips CM30transmission electron microscope operating at 250 keV. The average particle diameter was determined by dividing the Pd surface coverage by the particle number density obtained from TEM images. The catalytic activity of the Pd particles, adsorbed on Si, was tested by immersing (-1 min) the activated substrate in an electroless Ni plating bath (Shipley Niposit 468, Shipley Co., Newton, MA) operated at 65 "C. (Dimethy1amine)boraneis used in this bath (pH 6.8-7.5) as the reducing agent. This procedure results in the formation of a Ni film (0.25%B)with a growth rate of 7.5 ,um/h.
Results The reduction of palladium ions with hypophosphite in an aqueous solution gives rise to the formation of colloidal Pd particles. After an "induction period" of3-5 min, the color of an air-saturated solution in the presence of PVP turned from yellow to brown, due to the formation of metallic Pd. The solution remained transparent after the completion of sol formation. During sol formation, gas evolution was observed. The sol was formed immediately when the solution was purged with nitrogen gas before addition of the reducing agent. No sol formation occurred when the solution was continuously flushed with oxygen. The Pd particles contain 14 atom % phosphorus as determined with XRF. The XRF analysis was done with Pd particles, prepared without PVP, adsorbed on a H-terminated Si surface. The Pd surface coverage (rPd) was measured after the adsorption of colloidal Pd on a H-terminated Si surface. Figure 1 shows r p d for this surface as a function of the adsorption time for three different PVP (Mpvp, 10 000) concentrations in the prepared sol. Within a minute, r p d increases rapidly and reaches an almost steady value after which a slight increase was found. The polymer concentration has a strong influence on the adsorption of Pd particles. An increase in the PVP concentration results in a pronounced decrease in r P d . In all experiments of Figure 1, the surface was rinsed with water after adsorption prior to the XRF determination. The influence ofthis rinsing step was studied (Figure (14)van de Weijer,P.; de Boer, D. K. G. Philips J . Res. 1993,47,247.
Langmuir, Vol.10,No.11, 1994 4091
Adsorption of Pd on Si
01I 0
I
20
I
40
I
60
I
80
I
100
t, minutes
Figure 2. Pd coverage ( r p d ) of H-terminated Si, covered with Pd particles, as a function of the rinsing time in water: sol preparation CPd = 8.46 x M, CNaHZPQ = 1.56 x M, adsorption time 5 min; (a) cpvp = 22.8 mg/L; (b) cpvp = 114 mg/L.
2). No decrease of r p d was found up to a rinsing time of 100 min. Figure 3 shows typical TEM images of Pd particles, prepared in the presence of different amounts of PVP (Mp~p,10 000), adsorbed on H-terminated Si(100)surfaces. A considerable influence of the polymer concentration on the particle shape and size is apparent. Adsorption from a PVP-free sol results in relatively large Pd structures on
the surface with a broad size distribution (5-25 nm diameter, see Figure 3a). The particles are of irregular shape and no crystallographic facets can be observed. During sol formation, the presence of PVP, in increasing concentrations,results in a substantial decrease in particle size. In Figure 3b-d, the average particle diameters are 5.2,4.7, and 3.8 nm, respectively, and there is a transition from irregularly shaped to more spherical particles. The steady decrease in particle size occurs up to a PVP concentration of about 50 mg/L. For PVP concentrations larger than 50 mgL, the average particle size remains constant at about 3.8 nm. The particle density on the samples, shown in Figures 3b-d, is about 8 x loll particles/cm2. In Figure 4, the Pd surface coverage is plotted against the Pd concentration in solution. For both Pd/PVP ratios an increase in r p d was observed with increasing Pd concentration (Mpvp, 10 000). A maximum Pd coverage (rpd") was observed in Figure 4 for cpd > 2 x M. The absorption of Pd particles was measured as a function of the PVP (Mp~p,10 000) concentration in the sol. Curve a in Figure 5 shows a plot of r p d versus the logarithm of the polymer concentration (log cpvp) for a H-terminated Si(100) surface. A continuous decrease of the Pd surface coveragewith increasing PVP concentration is found. Two roughly linear regions can be observed in curve a with a sharp transition a t around cpvp 50 mgL.
Figure 3. TEM images (250 x 250 nm2)of Pd particles adsorbed on H-terminated Si: sol preparation CPd = 8.46 x M, CNaHzPOz = 1.56 x M, adsorption time 5 min; (a, top left) cpvp = 0; (b, top right) cpvp = 11.4mg/L; (c, bottom left) cpvp = 22.8 mg/L; (d, bottom right) cpvp = 114 mgb.
Boonekamp et al.
4092 Langmuir, Vol.10,No. 11, 1994
pr O'A 0
I 5 Cpd x 10-4, in mol/l
1.o
Figure 4. Pd coverage ( r p d ) of H-terminated Si as a function of the Pd concentration: CNdf2pO&pd = 18.4(c in M), adsorption time 5 min; (a)CpVpkpd = 4.04 x lo4(mg of Pvp/mol of Pd); (b) CPVphpd = 1.35 x lo6 (mg of PVP/mol of Pd).
-
3
2
1
Log cpvp. c in mgll
Figure 6. Pd coverage ( r p d ) of H-terminated Si as a function of log cpvp: sol preparation c p d = 8.46 x M,C N ~ H ~ P=O 1.56 ~ x M,adsorption time 5 min; (a) Mpvp 10 000; (b) MPVP 40 000; (c) Mpvp 360 000. indicating that the steric layer surrounding the Pd particles did not reduce their catalytic activity for electroless deposition. Bright Ni films were obtained with a reasonably good adhesion to the Si substrate.
Log cpvp, c in mg/l
Figure 5. Pd coverage ( r P d ) as a function of log cpvp: sol preparation Cpd = 8.46 x M,C N ~ H ~=~ 1.56 O ~ x M, adsorption time 5 min; (a) H-terrmnated Si; (b) 100 nm Si02 on Si.
The Pd surface coverage drops to zero at cpvp GZ 1200 mgl L. When a H-terminated Si(100)surface was given a UV/ ozone treatment, about the same Pd surface coverage was found as for the H-terminated Si surface. This was checked for three PVP concentrations: 22.8, 57.0, and M, C N ~ ~ = P O 1.56 ~ X IO-' M). 228 mg/L ( c p d = 8.46 X A r p d of 7.3 x 1015 Pd atoms/cm2 was obtained &r adsorption from a PVP-free solution on a H-terminated Si surface. The adsorption of Pd particles was also studied on H-terminated p-Si(100) (
