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Solvent-Dependent Adhesion Strength of Electroless Deposited Ni− P Layer on an Amino-Terminated Silane Compound-Modified Si Wafer Wei-Yen Wang, Kannankutty Kala, and Tzu-Chien Wei* Department of Chemical Engineering, National Tsing-Hua University, 300 Hsinchu, Taiwan
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S Supporting Information *
ABSTRACT: Amino-terminated silane compound modification was wet-processed on a silicon wafer using four different solvents to investigate the property of the self-assembled monolayer (SAM) and its influence on the adhesion of electroless deposited nickel−phosphorus (Ni−P) films. Analyzed by various tools including dynamic light scattering, the atomic force microscope, X-ray photoelectron spectroscopy, inductively coupled plasma with mass spectroscopy, a proper link between the processing solvent and SAM quality is established. It is found that at least the chemical compatibility, the polarity, and the acidity of solvents can affect the final morphology of the resultant SAM. Unlike toluene and ethanol that are most frequently chosen in literature, we conclude that isopropyl alcohol (IPA) is a superior solvent for aminoterminated silane compounds. Owing to the good SAM quality formed in IPA, the adhesion of electroless deposited Ni−P films is largely strengthened, even as high as the bulk strength of silicon wafers. in ultra large scale integration (ULSI) metallization.29 They silanized the substrate surface by immersion of the substrate into a toluene (TOL) solution containing 1 wt % of aminosilane compound and concluded that the uniform distribution of Pd catalysts, which are chemisorbed on the amino-silane compounds, is the key for forming a good nickel−boron barrier layer. On the other hand, Kovač et al. studied the influence of silanization solvents on the morphology of SAM and found that using TOL as the solvent for silane compound modification could lead to a rough SAM, which is attributable to the formation of polymerized silane compound islands because of bad compatibility of the silane compound and TOL.20,21 Consequently, how differently SAM morphologies forming in different solvents affect the performance of further applications, such as the adsorption of Pd catalyst and the properties of ELD film, has not been explored systematically. Motivated by this, in this study we investigated the properties of SAMs formed by 3-2-(2-aminoethylamino)ethylamino propyl trimethoxysilane (ETAS) in four different solvents. For the first time, we clearly identify that the chemical compatibility, the polarity, and the acidity of solvents are crucial factors in determining the final morphology of SAM and the resultant adhesion of the ELD Ni−P film.
1. INTRODUCTION Amino-terminated alkylsilanes with the general formula of (RO)3−Si−X, where OR is a hydrolysable head group and X is an alkylchain tail decorated with one or multiple amino (NHx) moieties, are widely used in surface modification. It is generally accepted that silane compound modification is achieved by forming a self-assembled monolayer (SAM) between the head group and the substrate, whereas the tail is capable of chemisorbing a variety of materials such as nanoparticles,1−3 metal ions,4−6 polymers,7,8 and so on. Silane compound modification can act as an adhesion promotor for coatings,9−11 surface functionalization for biochemical characterizations,12,13 or copper diffusion barrier layer in semiconductor manufacturing.14−17 Generally, the formation of SAM involves four steps including silanol formation, hydrogen bond formation, condensation of water, and formation of the O−Si−O covalent bond.18 Although the abovementioned principle is easy to understand, the operation of silane compound modification is, however, tricky and sensitive to process conditions. For instance, the number of amino groups,19,20 the solvent used in dissolving silane compounds,21,22 the immersion time of silanization,23−25 and the baking condition during covalent bond formation26 are reported to affect the final configuration of SAM considerably. Assisting palladium (Pd) catalyst adsorption for electroless deposition (ELD) is one of the important applications of amino-silane compound modification.21,27,28 Osaka and Yoshino investigated the effectiveness of an ELD nickel− boron barrier layer on different silane compound underlayers © XXXX American Chemical Society
Received: June 8, 2018 Revised: September 28, 2018 Published: October 16, 2018 A
DOI: 10.1021/acs.langmuir.8b01927 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Schematic representation of ETAS modification and ELD Ni−P metallization on a texturized Si wafer.
2. EXPERIMENTAL SECTION
dispersed in ETL and IPA, over 98.5% ETAS clusters are sized less than 1 nm and only less than 1.5% are oligomers of approximately 10 nm. In the case of ACT, the average sizes of the ETAS cluster and oligomers increase to 2.51 ± 0.24 and 41.31 ± 51.26 nm, respectively. For TOL, the DLS test reported a monotonic average size distribution of >800 nm, indicating the occurrence of intense aggregation. Although the particle size reported in DLS is not precisely correct, it still provides valuable information regarding the configuration of ETAS in the solution state. It turns out that the ETAS configuration is highly dependent on the nature of solvent. As ETAS itself contains dipole moments31 and could undergo selfpolymerization to form ETAS oligomers, swelling in the solvent is less favorable when ETAS is dispersed in a nonpolar solvent such as TOL. As a result, ETAS molecules tend to polymerize to form large clusters in TOL.21 On the other hand, in polar protic solvents such as IPA and ETL, the majority of ETAS molecules tend to swell in the solvent so that the size distribution reported in DLS is small (2.5 nm, accompanied by the change of color of the solution from transparent to yellowish within 1 h (shown in Figure S1 in the Supporting Information), which is a result of Schiff base formation32,33 illustrated in Figure 2. Owing to this irreversible chemical reaction, ACT is an improper solvent of ETAS, so further evaluations of using ACT is excluded.
The Ni−P metallization process on SiO2-covered Si wafers including cleaning, silanization, Pd activation, and ELD is cartooned in Figure 1. In detail, a KOH-texturized n-type Si wafer (62.4 Ω/square, Gintech Corporation, Taiwan) was used as the substrate. The substrate was pretreated by the standard radio corporation of america clean (RCA clean) process to remove surface organic species and form a thin SiO2 skin. Then, the substrate was silanized in a solution containing 1 vol % ETAS (>95%, Acros) for 30 min. Four organic solvents including nonpolar TOL (99.5%, J.T. Baker), polar aprotic acetone (ACT, 99.5%, Macron), polar protic ethanol (ETL, 99.8%, Honeywell), and another polar protic isopropyl alcohol (IPA, 99.5%, Macron) were tested. After ETAS grafting, the substrate was sonicated in pure solvents for 5 min to remove weakly physisorbed ETAS. Finally, the ETAS grafting process was completed by baking the wafer in an oven at 120 °C for 30 min for the purpose of forming the Si−O−Si covalent bond. A home-made aqueous polyvinyl alcohol-capped palladium nanoparticles solution (PVA-Pd, 50 ppm) was synthesized using a procedure from previous reports.30 Pd activation was conducted by immersing the ETAS-grafted Si wafer into the PVA-Pd solution for 5 min at 40 °C, rinsing with deionized water, and drying at ambient temperature sequentially. A 200 nm Ni−P film was ELD onto the PVA-Pd activated Si wafer using a commercial Ni−P solution (9026M, OMG, USA) at 80 °C for 60 s. Dynamic light scattering (DLS, Malvern, Nano ZS, USA) was used to examine the size of ETAS clusters in the solvents quantitatively. The surface conditions of the Si wafer after baking as in Figure 1 were examined using various tools. Specifically, the topography and surface roughness were investigated by atomic force microscopy (AFM, Nanosurf, C3000, Switzerland); the surface chemical state of the Si wafer was determined by X-ray photoelectron spectroscopy (XPS, VGS, Thermo K-Alpha, USA). In addition, the microimages of the ELD Ni−P film were observed by field-emission scanning electron microscopy (FESEM, SU1000, Hitachi, Japan). Pd loading on the Si wafer was determined by immersion of the Pd-adsorbed Si wafer in aqua regia to dissolve the Pd and then applying inductively coupled plasma with mass spectroscopy (ICP−MS, Agilent 7500ce, Japan) to obtain the Pd loading. The adhesion of the ELD Ni−P film on Si wafer was quantitatively determined using a pull-off adhesion tester (PosiTest AT-M, DeFelsko, USA), which compiles ASTMD4541/ D7234. Details of the pull-off adhesion test can be found from our previous reports.10,25
3. RESULTS AND DISCUSSION Table 1 summarizes the DLS result of ETAS clusters in different solvents. It can be seen that when ETAS was
Figure 2. Schiff base formation between ETAS and ACT.
Table 1. DLS Result of the Size of ETAS Clusters and Oligomers in Different Solvents samples
primary size (nm) (percentage)
oligomer size (nm) (percentage)
TOL−ETAS ACT−ETAS ETL−ETAS IPA−ETAS
807 ± 27 nm (100%) 2.51 ± 0.24 nm (98.28%) 1.00 ± 0.06 nm (98.74%) 0.90 ± 0.19 nm (99.01%)
no oligomer detected 41.31 ± 51.26 nm (1.72%) 14.03 ± 9.29 nm (1.26%) 8.06 ± 5.86 nm (0.99%)
AFM topographies of an ETAS-grafted Si wafer are summarized in Figure 3. Once again, AFM topographies of TOL−ETAS, ETL−ETAS, and IPA−ETAS were taken three times to ensure the repeatability. Here, only representative images are shown in Figure 3 and the rest are shown in Figure S2 in the Supporting Information. The images echo DLS results. The bare Si wafer (Figure 3a) exhibits an extremely smooth surface with a surface roughness (Ra) of merely 0.106 B
DOI: 10.1021/acs.langmuir.8b01927 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. AFM topographies of a (a) bare Si wafer (without ETAS modification), (b) TOL−ETAS, (c) ETL−ETAS and IPA−ETAS and the imaginary schematic configurations are shown on the right of AFM topographies.
few white spots seen in ETL−ETAS. As DLS reports the same results in both solvents, the formation of white spots must relate to certain properties of solvents during grafting. For the first time, we try to explain the morphology difference of SAM from the acidity of alcohols. It is known that the amino moieties (NH2) can be protonated by alcohols to form NH3+; this NH3+ can further interact with alkoxy groups (OR) of ETAS, either intermolecularly or intramolecularly through
nm. When ETAS grafting was processed in TOL (TOL− ETAS), the Si surface became rugged with an increased Ra of 0.601 nm (Figure 3b). Considering the mean size of the ETAS cluster reported by DLS approaches of 800 nm, the topography must be composed of large ETAS clusters stacking each other, which is far from an ideal SAM. In the case of ETAS grafting in ETL (ETL−ETAS) and IPA (IPA−ETAS), AFM images are similarly smooth (Figure 3c,d) except for a C
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Langmuir electrostatic interaction.34 This “alcohol-induced protonation” not only consumes NH2 moieties but also induces ETAS aggregation during grafting. This effect is particularly profound in the case of ETL because primary alcohol (ETL) is more acidic than secondary alcohol (IPA).35 Consequently, the white spots, which are believed to be the aggregated ETAS molecules resulting from the ETL-induced aggregated ETAS, were observed in the topography as Figure 3c. For the case of IPA, this alcohol-induced ETAS aggregation is alleviated because of the weak acidity of IPA. In other words, IPA instead of ETL is more suitable to form an ideal ETAS SAM. Imaginary configurations of ETAS SAM formed in different solvents are illustrated underneath their AFM images in Figure 3. To further evidence this viewpoint, XPS was conducted to study the amount of ETAS and the types of ETAS SAM on the substrate. XPS on each sample was conducted three times to ensure the repeatability and only one of them has been shown here; complete data are summarized in Table S1 in the Supporting Information. Because ETAS is the only source of nitrogen atoms in the sample (ETAS-modified Si wafer), we consequently use the nitrogen content as an index to represent the amount of ETAS on the Si surface. XPS wide scans are shown in Figure 4 and the nitrogen contents of the samples are
Figure 5. N 1s spectra of TOL−ETAS, ETL−ETAS, and IPA−ETAS in XPS (from top to bottom).
treated as the effect of protonation by ambient moisture. On the basis of this assumption, the effect of alcohol-induced protonation in ETL−ETAS SAM is 0.91, which is doubled when compared with that of IPA−ETAS SAM (0.49). This result echoes the hypothesis of alcohol-induced ETAS aggregation in previous discussions. Figure 6 depicts the time-dependent Pd amount in TOL− ETAS, ETL−ETAS, and IPA−ETAS after PVA-Pd uptake; it
Figure 6. ICP−MS results of Pd loading in TOL−ETAS, ETL− ETAS, and IPA−ETAS.
Figure 4. XPS wide spectrum of bare and ETAS-grafted Si wafers processed with different solvents; the average percentage of nitrogen content in each sample is marked below the curves.
can be seen that IPA−ETAS adsorbs considerably more Pd than ETL−ETAS and TOL−ETAS, primarily because of it owns most exposed NH2 moieties (as illustrated in Figure 3). In the case of ETL−ETAS, Pd loading is only 30% over the entire duration when compared with IPA−ETAS. This is a result of the strong alcohol-induced aggregation in ETL− ETAS because this effect consumes NH2, which is designed to adsorb the PVA-Pd catalyst. As Pd offers catalytic sites for ELD, the amount of Pd on the substrate can certainly affect the property and quality of the ELD film. For the case of TOL− ETAS, Pd loading is higher than that of ETL−ETAS but lower than that of IPA−ETAS. The lower Pd loading found in TOL−ETAS is another evidence of chaotic stacking in the SAM processed by TOL, although it owns the most abundant total nitrogen atoms among all samples. It is worth noting that ICP−MS was repeated on five samples from different batches and complete data are shown in Table S2 in the Supporting Information. After PVA-Pd activation, ELD was successfully processed in all samples and the coverage of ELD Ni−P films is visually identical. We further checked their FESEM images (as shown
shown in the inset. The nitrogen content in TOL−ETAS SAM is more than double when compared with ETL−ETAS SAM and IPA−ETAS SAM, indicating its nature of multilayer stacking. The N 1s XPS spectra of TOL−ETAS, ETL−ETAS, and IPA−ETAS SAM are zoomed-in in Figure 5a−c, respectively. They all show an asymmetric broadband at 400 eV, which can be deconvoluted into three bands at 400.9, 399.9, and 398.8 eV. The binding energy peak at 400.9 eV is assigned to the protonated amine groups (NH3+), the binding energy peak at 399.9 eV is assigned to the hydrogen-bonded amine groups (NH), and the binding energy peak at 398.8 eV is assigned to pristine NH2 groups.36−38 The existence of NH3+ here can be ascribed to two reasons: one is the protonation of NH2 by ambient moisture during sample transportation and the other is the protonation by alcohol as described above. The ratio of deconvoluted peak area of NH3+ to NH2 is 1.24, 2.15, and 1.73 for TOL−ETAS, ETL−ETAS, and IPA−ETAS SAM, respectively. Because there is no alcohol-induced protonation in TOL−ETAS SAM, its NH3+/NH2 ratio of 1.24 can be D
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to nonproductive consumption of NH2 moieties and thus insufficient Pd adsorption. This further results in dramatically different adhesion of ELD Ni−P films on a Pd-activated SAM surface. Through this study, we suggest that IPA is a suitable processing solvent relative to TOL and ETL, both in SAM formation and adhesion promotion of an ELD metal film.
in Figure S3) and proved again that the morphologies of the ELD Ni−P film were no different. However, the different SAM structures underneath the ELD Ni−P film show great impact on the adhesion of the ELD Ni−P film. Figure 7 summarizes
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01927. Photo images of pure ACT and ACT-ETAS; replicated AFM topographies of TOL-ETAS, ETL-ETAS, and IPAETAS; raw data of N1s content from XPS of ETASgrafted Si wafers; raw ICP-MS data of Pd loading; FESEM images of ELD Ni-P; raw data of adhesion strength of an ELD Ni-P film; and photo images of fracture result between Ni-P and the Si wafer after adhesion values >10 MPa (PDF)
Figure 7. Adhesion strength of the ELD Ni−P film on the ETASgrafted Si wafer processed with different solvents.
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the pull-off adhesion strength of ELD Ni−P films on the ETAS-grafted substrate. Each condition in Figure 7 contains at least five samples (complete data are shown in Table S3) and the average adhesion strength was determined to be 5.30 ± 1.88, 3.08 ± 0.27, and 10.83 ± 1.08 MPa for the case of TOL− ETAS, ETL−ETAS, and IPA−ETAS, respectively. First, in a separate experiment the adhesion of the ELD Ni−P film catalyzed using a commercial Sn/Pd colloid is only 4.63 ± 1.52 MPa, evidencing the merit of applying SAM as the adhesion promoter. Second, it is clear that the processing solvent for ETAS grafting plays a key role in determining the Ni−P film adhesion despite the macroscopic appearance seeming to be unaffected. Note that in this study the pull-off adhesion test was done on the samples without any post annealing; the fact of >10 MPa Ni−P adhesion in the case of IPA−ETAS implies the fracture could be in the Si wafer itself, not in the interface of the Ni−P film and Si wafer (see Figure S4). From the above discussion, ETAS modification processed by the IPA approaches the ideal SAM configuration and owns the most exposed NH2; it thus absorbs most PVA-Pd catalysts, which facilitates the formation of a tight and fully covered ELD Ni−P film in a short time because ELD occurs on the catalyst surface initially. In the case of ETL−ETAS and TOL−ETAS, PVA-Pd adsorption is hampered, either by chaotic stacking or insufficient NH2 moieties; rendering ELD occurs on limited sites initially. Although the coverage of the Ni−P film can be satisfactory sooner or later because ELD is autocatalytic,39 it is the idea of using SAM to strengthen the interface that has been depreciated.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 886-35715131. Fax: 886-35715408. ORCID
Tzu-Chien Wei: 0000-0002-9608-8275 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the General Research Fund of the Ministry of Science and Technology of ROC (Taiwan) under contract no. MOST-106-2218-E-002-038 and MOST-1052628-E-007-012-MY3.
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4. CONCLUSIONS Four organic solvents were applied as the processing solvents for ETAS SAM formation on a Si wafer. It was found that ACT is chemically incompatible with ETAS because of the irreversible formation of the Schiff base. The SAM formed using TOL as the solvent is far from ideal because of intense ETAS aggregation in TOL prior to SAM formation. Polar aprotic solvents such as ETL and IPA can form quality SAM relative to TOL because of good swelling of ETAS. However, because the acidity of ETL is stronger than that of IPA, the protonation of NH2 tails in ETAS is profound in ETL, leading E
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