Article pubs.acs.org/JPCC
pH-Dependent Selective Thickness Control of Polyelectrolyte Nanolayers on SiO2 and Cu Surfaces Yasuhisa Kayaba,* Hirofumi Tanaka, and Shoko Sugiyama Ono R&D Center, Mitsui Chemicals, Incorporated, 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan ABSTRACT: A strategy for selective formation of an organic film on SiO2 surface while minimizing its deposition on Cu surface is proposed. The organic film was deposited by spin coating a polyamine solution and an aromatic carboxylic acid solution in a sequential fashion. Then the ionically bonded film that consisted of the polyamine and aromatic carboxylic acid was formed. It was found that the thickness of the film was controlled by the simple pH adjustment of the polyamine solution. Under basic pH conditions, the thickness of the film was almost the same on both substrates. However, under acidic pH, the Cu layer was thinner and the thickness was constant on the SiO2. After baking the organic film, a single 10 nm scale thick covalently bonded organic film was obtained on the SiO2, while a ∼0.5 nm thick film was obtained on the Cu.
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INTRODUCTION In modern science, thin organic layers are studied across various research areas because of their uniform and conformal nature as well as their tailored functionalities. In advanced electronic devices, thin organic layers such as self-assembling monolayers (SAMs), molecular layer deposition films (MLDs),1 and polyelectrolyte layer-by-layer films (LbLs)2−4 attract a lot of attention for the compensation of the interfacial degradation at the Cu line and at inorganic dielectrics when studied for adhesion promotion, corrosion inhibition,5,6 Cu diffusion barrier,7−10 etc. Of equal significance against these organic films’ performance is the selective deposition of the film on the targeted surface area.11 A nonphotolithographic method for selective deposition is favorable because photolithographic patterning needs multiple processes and exact alignment control (the targeted area has to be covered exactly with the organic layer), and it is possible to cause process damage on the organic layer. In past papers, selective formation of organic film on metal−dielectric pattern has been reported on. For example, polyurea MLD1 on a dielectric surface when covering a Cu surface by a SAM resist layer11 and parylene film deposition on a SiO2 surface with suppressed growth in an air-exposed Cu surface12,13 were examined. These papers were based on the vacuum deposition process of volatile small precursors. In our approach, however, a new strategy for the selective formation of a thin organic layer on a SiO2 surface while minimizing its thickness on a Cu surface by a solution-phase process is examined. The organic layer is formed by sequential coating of a polyamine solution and an aromatic carboxylic acid solution based on a layer-by-layer self-assembly method. Selective formation of polyelectrolyte LbL was pioneered by Hammond’s group.14,15 In their method, polyanion−polycation inonically bonded LbL is selectively deposited onto the chemically patterned −COOH-functionalized area and the oligo(ethylene glycol) (EG)-terminated resistive region.16,17 Critical factors to control the adsorption onto the −COOH © 2015 American Chemical Society
and EG regions are the electrostatic force and the secondary interactions including hydrogen bonding and hydrophobic interaction among the polyelectrolytes and the solid surface in aqueous media. Their intensities and balance are controlled by optimizing the solution pH and ionic strength. For example, a LbL film derived from linear poly(ethylenimine) (LPEI) and poly(acrylacid) both in pH 4.8 solutions shows good selectivity on a COOH-EG patterned surface.18,19 The LPEI is repelled from the EG surface because of strong hydrations, but it adsorbs onto SiO2 via an electrostatic attraction force. Typically, the COOH−EG chemical pattern is formed by microcontact printing (μ-CP)20−22 on Au or on underlying LbL polymer surface. The selective deposition was performed on various kinds of substrates such as SiO2, polystyrene, and patterned LbL.23 To apply their method on the patterned metal−dielectric surface, the metal surface has to be covered with EGfunctionalized oligomers. Meanwhile, ideally, the metal surface should be exposed to ensure that the metallic electrical contact with metal wiring is to be formed on it. In addition, the selectivity can be realized using a specific type of polyelectrolytes. In the case of weak polyelectrolytes, branched PEI and poly(arylamine) show poor selectivity on an EG-COOH patterned surface even under the best pH conditions. This is due to their hydrophobic backbone structures (ethylene and tertial amine with their neighboring ethylene) which interact with the EG surface via a hydrophobic interaction going on between them. A more versatile method is preferred for designing organic layer molecular structures. To the best of our knowledge, our report is the first to examine the selectivity of polyelectrolyte nanolayer deposition on dielectric and metal surfaces as well as on positively and negatively charged surfaces. Received: May 23, 2015 Revised: September 16, 2015 Published: September 16, 2015 22882
DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
Article
The Journal of Physical Chemistry C In our study, organic film is made from branched PEI and pyromellitic acid (PMA) in a one-cycle deposition process. The PMA is selected instead of polyanions such as poly(acrylacid) so as to toughen the resulting organic film against the dry cleaning process such as plasma treatment, which is frequently used at the electronic device fabrication process. The PEI and PMA solutions are directly deposited on the SiO2 and Cu substrates without any surface chemical modification. Then the PEI solution pH dependences of the thickness of the ionically bonded PEI/PMA films on the SiO2 and Cu surfaces are investigated. After investigation of the detailed deposition conditions which maximize the selectivity, minimization of the thickness on the Cu is confirmed by X-ray photoelectron spectrometric (XPS) surface analysis. For the area-selective formation of thin organic films on a patterned SiO2−Cu surface, its edge resolution at the border of the substrate pattern is quite important as well as its thickness contrast between it on the targeted area to be deposited and on the nontargeted area. An evaluation is underway, and it will be reported in a future article.
solutions were coated immediately. All of these experiments were performed in clean room conditions. The PEI film, which was obtained after coating the PEI solution with pH 4.0, is referred to as “PEI(4.0)”. The same film after PMA coating and water rinse is referred to as “PEI(4.0)/PMA”. Without any specification, the PEI(4.0)/PMA film is not baked at 350 °C. In the same manner, the films which were obtained from the PEI solution with pH 9.5 and the PMA solution are referred to as “PEI(9.5)” and “PEI(9.5)/PMA”, respectively. The thickness of the organic film was measured by a UV−vis spectroscopic ellipsometer (Semilab, PS1100). The thickness on SiO2 was determined by fitting the ellipsometric parameters in 1.26−4.5 eV to a SiO2/(natural SiO2)/Si optical model using a WinElli-II program (Semilab). The refractive index of organic film was always assumed to be the same as that of SiO2, because its exact value was not known, while organic films have a similar refractive index to SiO2.1 For the organic film on Cu, ellipsometric parameters in 2.2−5.0 eV was fitted to a SiO2/ Cu model. The optical parameters of the Cu substrate were calculated from the measured ellipsometric parameters of the reference bare wafer which had the same heat treatment profile. Surface atomic composition was investigated by an X-ray photoelectron spectrometer (KRATOS Analytical, Inc., ESCA3400, Mg Kα source (200 W), hν = 1,253.6 eV). Emitted photoelectron from a sample was collected at the normal takeoff angle (0°). Samples were kept in a sealed box purged with N2 to avoid adsorption of contaminant organics and to suppress the oxidization of Cu samples before XPS analysis. Binding energy scale was corrected to the metallic copper (Cu0) peak position at 932.6 eV for Cu samples and correctted to the Si0 peak at 99.5 eV for the SiO2 samples. Spectrum peak intensity was quantified after Shirley background subtraction. The chemical composition of the bulk structure was measured by a Fourier transform infrared (FTIR) spectrometer in transmission mode. The incidence angle of the probe beam was 72° (near the Brewstar angle of Si) in order to improve the signal-to-noise ratio. A two-sides polished Si wafer was used in order to avoid diffusive scattering of IR light at the backside surface of the Si substrate. Ambient conditions were purged by N2 for 5 min to suppress the signals from air (i.e., CO2, H2O, and others). To compare the FTIR spectrum intensity of the films with each other, each spectrum was subtracted with a straight line (baseline correction); then each intensity was normalized to the thickness.
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EXPERIMENTAL PROCEDURE Thin organic films on SiO2 and Cu surfaces were formed based on the spin-coating procedure for the layer-by-layer method.24,25 By applying this method, a high-quality film with uniform thickness and pinhole-free structure on at least 4 × 4 cm2 Si substrate was obtained in short time duration (within 10 min) by the single-cycle process described below. A branched PEI with Mw 50 000−150 000 was diluted in secondary deionized water solution (18.2 ohm·cm, Millipore water). Its pH was controlled by adding formic acid. The pH was measured by a pH meter (As One Corp., KR5E) after correcting the electrode with standard buffer solutions of pH 6.86 and 4.01. Formic acid was selected because of its low boiling point (101 °C) to be evaporated simultaneously with the water solvent at the following semidrying process and because it does not reside on the organic film after baking at 350 °C. A single-crystalline Si wafer (300 mm diameter, p-type, low resistive (1−12 ohm·cm)) was cut into a 2 × 2 cm2 coupon, and it was treated by UV−O3 irradiation for 5 min in order to expose the surface SiOH groups of natural SiO2 (2 nm thick). Its water contact angle was always around 3°. The PEI film was deposited on the wafer using a benchtop manual spin coater. After putting the PEI solution on the SiO2 surface it was held for 20 s as is and rotated with 2000−4000 rpm. It was “semidried” on a hot plate at 100 °C for 1 min in order to evaporate the water solvent. A PMA solution (pH 3.4) was added dropwise in 30 s when rotating the wafer. The wafer was rinsed with DI water for 30 s by the same procedure as the PMA coat in order to remove the excess amount of PMA. Then it was baked under purified N2 (10 kPa) at 350 °C for 2 min after being pumped down the chamber to 0.7 Pa. Organic film on a Cu wafer was prepared employing the same procedure as on SiO2. A Cu film (100 nm thick) on a 300 mm diameter Si substrate was prepared by electroplating, and its surface was polished by chemical mechanical polishing. The wafer was cut into a 2 × 2 cm2 coupon. The cut Cu wafers were kept in a N2 box in order to avoid further oxidization. Its surface was cleaned by dipping in a 1 N H2SO4 solution for 5 min in order to remove the surface oxidization protection layer (i.e., benzotriazole (BTA)) and carefully rinsed with DI water for 5 min before use. After drying it by N2 blow, the PEI and PMA
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RESULTS AND DISCUSSION Covalently Bonded Film on SiO2. Before discussing the selective deposition, the structure of the covalently bonded film on the SiO2 surface was investigated. Figure 1 shows a crosssectional transmission electron microscopic (TEM) image of the PEI(4.0)/PMA film with baking at 350 °C. It is quite uniform, and defect-free structures were observed clearly. A similar high-quality film was also obtained for thinner films. Figure 2 shows the FTIR spectra of PEI(4.0) after semidrying, PEI(4.0)/PMA, PEI(4.0)/PMA after baking, and PEI(9.5)/PMA after baking. After PMA coating, the enhanced peak at 1572 cm−1 which corresponds to −COO− stretching26 and the broad peak at 2200−3200 cm−1 which corresponds to the stretching of the protonated amine in amine-COOH salt26 were measured. Therefore, the PMA is ionically adsorbed on the protonated amine group of PEI so as to compensate for both charge in the same fashion at the polyamine-poly(carboxylic acid) LbL film self-assembly. After baking the 22883
DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
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The Journal of Physical Chemistry C
Figure 3. Possible chemical structure of the PEI/PMA film after baking. Figure 1. Cross-sectional TEM image of the PEI(4.0)/PMA film after baking (∼13 nm) on the SiO2 (100 nm)/Si substrate. The brightest area between the dark line (sputtered Pt) and the bright area (plasma deposited SiO2) is the PEI(4.0)/PMA film after baking at 350 °C.
result indicates that the formic acid which was included in the PEI solution with pH 4.0 did not disturb the adsorption of PMA to the semidried PEI(4.0) film. Selectivity and Thickness Controllability. In this section, critical factors for the control of thickness of PEI/ PMA films on SiO2 and Cu are shown. PEI is a skeletal material of the PEI/PMA film, so the PEI thickness on each substrate determines the PEI/PMA thickness. In this section, 8−10 nm thick PEI was spin coated on both substrates from PEI solutions with different pH conditions (3−9.5). All of those thicknesses were not changed significantly by semidrying. After the post-PMA coat, the thickness stayed the same or became thinner (part of the PEI or all of the PEI was rinsed off), which depended on the PEI solution pH, semidrying temperature, and substrate type. PEI Solution pH Dependence of Thickness. In order to deposit the PEI/PMA film on the SiO2 surface selectively while keeping the Cu surface exposed (i.e., no film deposition), we tried to control the electrical force between the PEI and the substrate surfaces by pH adjustment of the PEI solution. As is well known, in aqueous media, PEI is charged positive with protonated amine groups in almost all pH conditions (pH ≈ 3−10).36,37 Its zeta potential increases linearly with the decrease of pH in the basic area and is saturated at the acidic area.36,37 SiO2 and Cu surfaces are also ionically charged depending on the pH condition in aqueous media. The point of zero charge (PZC) of the SiO2 surface is 2−4,38−40 so its surface is negatively charged at the higher pH condition. Thus, the PEI and SiO2 surface is always attractive above pH 2−4. The PZC of Cu−oxides are 7.6−9.5.38,39 At the lower pH conditions, the surface is always positively charged. Around the ZPC, Cu is neutral or weakly charged, so the PEI with free amine groups is possible to be adsorbed onto the Cu surface via coordination linkage by providing amine’s lone pair to Cu. When the pH goes away from the PZC to lower pH, the positive charge density of the Cu increases and the PEI can be released from the Cu surface via electrostatic repulsion force. To validate this concept, PEI solution pH dependence of PEI thickness which formed through PEI deposition (8−10 nm thick) and water rinse without the semidrying and PMA coating was measured as shown in Figure 4. The films on the SiO2 and Cu were deposited under the same condition. PEI on SiO2 was constant, but that on the Cu was decreased at acidic pH and vanished at pH 4.0. The thickness difference between them at pH 4.0 was small, but selectivity was obtained at pH 4.0. The thickness of PEI on the Cu was always lager than the PEI on SiO2 at pH ≥ 7. Concerning this, a speculation was considered as follows. It is well known that the polyelectrolytes take a different conformational arrangement depending on their own
Figure 2. FTIR spectra of PEI(4.0) with semidrying, PEI(4.0)/PMA, PEI(4.0)/PMA with baking, and PEI(9.5)/PMA with baking. Each intensity was normalized to the thickness (10−22 nm).
PEI(4.0)/PMA, the −COO− and the protonated amine peaks were lost, so the new specific three peaks at 1772, 1719, and 1393 cm−1 appeared. The spectrum shape resembles that of polyimide film. The three peaks correspond to the imide CO symmetric stretch, imide CO asymmetric stretch, and cyclic imide C−N−C stretch of polyimide, respectively.27 As is well known in previous works, the amine groups of PEI and the carboxylic groups of the anion make covalent bonds by converting the ionic bond between them through heat treatment.28−32 In our case, the specific peak of amide bond at 1654 cm−1 31,33,34 was absent. The CO stretch split peaks of carboxylic anhydride in 1750−1860 cm−1,26,31,35 which can be produced by anhydration of PMA at the 350 °C baking, were also not measured. Therefore, the two pairs of the neighboring two −COOH in PMA were thought to react with two primary amine groups to produce two imide bonds as depicted in Figure 3. The FTIR spectrum intensity of baked PEI(4.0)/PMA, which normalized to the thickness, was the same for a few nanometer to a 10 nm thick film. Therefore, the PMA was infiltrated into the PEI film up to the PEI/SiO2 interface, and the imide bond network was formed uniformly within the film in a depth direction. Interestingly, the spectral shape and normalized intensity were almost the same between the baked PEI(9.5)/PMA and PEI(4.0)/PMA as plotted in Figure 2. This 22884
DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
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The Journal of Physical Chemistry C
PMA coating. The deposited PEI(4.0) after semidrying was removed from the Cu surface even after the shorter PMA coat time (5 s). The PEI(4.0) was also removed by H2O rinse without PMA. Therefore, the PMA is not necessary to remove the PEI. The two different behaviors of PEI(4.0) and PEI(9.5) on the Cu surface indicate that the PEI must be protonated prior to adsorption on the Cu surface because the PEI(9.5) was difficult to be removed from the Cu surface by post-PMA coating as mentioned above. Drying Temperature Dependence. The drying temperature after the PEI deposition is important for maximization of selectivity. After 8−10 nm thick PEI(4.0) was coated on SiO2 and Cu surfaces, they were dried at different temperatures, and PMA was coated onto them. The PEI(4.0)/PMA thickness on SiO2 became larger gradually when the drying temperature increased from room temperature (no drying), and it was saturated at over 90 °C to become ∼10 nm. If the PEI film was not dried, almost all of the PEI was rinsed off at the post-PMA coating process and became thin up to 1 nm. The PEI(4.0)/ PMA thickness on the Cu surface also depended on the drying temperature. Up to 110 °C it was always a few nanometers, but it started to increase at higher temperatures. When the PEI was dried at 200 °C in ambient N2, 6.8 nm PEI remained on the Cu. Therefore, selectivity can be maximized at the narrow baking temperature range between 90 and 110 °C. This temperature of ∼100 °C matches the boiling point of water solvent as well as that of formic acid. The presence of formic acid in PEI film after drying at 100 °C was measured by FTIR spectroscopy. Figure 6 shows the
Figure 4. pH dependence of PEI thickness on SiO2 and Cu surfaces. The film was made through PEI coating and water rinse. Error at the numerical fitting of the measured ellipsometric parameters of the sample to an optical model is also shown.
charge density and that of the underlying surface. The conformational change results in different layer thickness.41 In our case, the weakly positively charged PEI on the Cu extended away from the weakly positively charged or neutral Cu surface to air, but the PEI on SiO2 adsorbed on the strongly negatively charged SiO2 surface like a blanket to cover the SiO2 surface. Figure 5 shows the PEI solution pH dependence of ionically bonded PEI/PMA film thickness, which was formed through
Figure 5. PEI solution pH dependence of thickness of the PEI/PMA films on SiO2 and Cu surfaces. Numerical fitting error is also shown.
PEI coating, semidrying at 100 °C, PMA coating, and water rinse. The films on the SiO2 and Cu were deposited under the same conditions. Before the PMA coating, the PEI thickness after semidrying was always 8−10 nm on both substrates. On the SiO2 surface, the thickness of PEI/PMA was 8−10 nm and was constant independent of solution pH. On the Cu surface, a totally different behavior was observed as shown in Figure 5. Around the basic pH condition, PEI/PMA had a similar thickness (6−8 nm) as that on the SiO2 surface. The Cu surface charge is supposed to be neutral or weakly charged to positive at this region, so the repulsion force between the PEI and the Cu surface was thought not to be enough to release the PEI. The PEI(9.5) on the Cu after semidrying did not decrease at all even after twice PMA coating. In this case, PEI might be adsorbed strongly on the Cu surface via coordination linkage. In the acidic pH area of the PEI solution, as the pH decreased the PEI thickness decreased steeply and vanished at pH 4.0. On the other hand, that on the SiO2 surface was kept constant (8− 10 nm) even at the acidic region. Therefore, selectivity was obtained at the acidic pH region, and it was maximized below pH 4.0. At this acidic pH area, it was assumed that both PEI and the Cu surface were highly charged to positive with protonation. The electrostatic repulsion force between them helped the removal of PEI from the Cu surface at the post-
Figure 6. FTIR spectra of PEI(4.0) after coating, PEI(4.0) after semidrying, PEI(9.5) after coating, and PEI(9.5) after semidrying. Each spectrum intensity was normalized by each thickness after a baseline correction. Dotted straight lines are a guide for the eye.
FTIR spectra of the PEI films on SiO2 after coating and the same after semidrying at 100 °C. In the PEI(4.0) films three specific peaks at 1593, 2170 (small hump), and 2300−3200 cm−1 (broad) were measured. These were absent in the PEI(pH 9.5) films. Each peak was assigned to the stretching vibrations of −COO−, protonated primary amine (NH3+), and protonated amines in the amine-COOH salt, respectively.26 All three peaks indicate the presence of amines−formic acid salt. The three peaks were explicitly observed in the PEI(4.0) after coating. They became small but were still observed in the PEI(4.0) after the semidrying at 100 °C. Therefore, with the small portions of protonated amine groups and formic acid, PEI was dissolved into aqueous solution from the Cu surface at the 22885
DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
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The Journal of Physical Chemistry C
thickness contrast after baking at 350 °C for 2 min in ambient N2. PEI(4.0)/PMA on the Cu surface was always thinner than 2−3 nm, so it was reduced to ∼1 nm after baking. The film on the SiO2 surface also shrunk to 55−60% after baking. This is due to the formation of an imide bond between PEI and PMA (Figure 3). PEI(9.5)/PMA film did not show any contrast for thick film (∼10 nm) on SiO2 even after baking. This is due to the consumption of Cu−oxide.44 XPS Analysis of the Cu Surface. To confirm the selective deposition of PEI(4.0)/PMA on the SiO2 surface while minimizing its thickness on the Cu surface, XPS surface analysis was performed. Figure 8a is the N 1s spectra of
post-PMA coating process. It has to be noticed that the solution pH plotted in Figure 5 could be different from the pH condition at the post-PMA coating process. However, it was difficult to measure it. Effect of Cu Surface Protection. The effect of the Cu surface pretreatment prior to PEI deposition was investigated. In the Pourbaix diagram42 for the Cu in water solution exposed under air, Cu dissolves at the acidic pH area to become Cu ion. In order to avoid this dissolution, the Cu surface was protected by a corrosion inhibitor. After precleaning of the Cu substrate by H2SO4 solution, it was covered by BTA. The same BTA treatment was also performed for the SiO2 substrate after UV− O3 cleaning. PEI/PMA film was deposited on both substrates with the same condition as the experiment for Figure 5. Then the selectivity which resembled the result in Figure 5 was measured. PEI(pH 9.5), 9.8 nm, was formed on the Cu surface, but the PEI(pH 4.0) was repelled immediately from the Cu surface after its coating. This repelling was thought to be due to the repulsion of the PEI solution against the semihydrophobic BTA-treated Cu surface (water contact angle 65°) as well as the electrostatic repulsion between the PEI and the Cu surface. As a result, the Cu surface protection promoted the selectivity. Limitations of Thickness Controllability. Figure 7a shows the thickness contrast between the PEI/PMA film on
Figure 8. N 1s XPS spectra of the PEI(9.5)/PMA (13.3 nm), PEI(4.0)/PMA (19.3 nm), and PEI(4.0)/PMA with baking (10.8 nm) on the (a) SiO2 and (b) Cu surfaces.
PEI(9.5)/PMA (13.3 nm), PEI(4.0)/PMA (19.3 nm), and PEI(4.0)/PMA with baking (10.8 nm) on the SiO2 surface. The spectrum shapes and intensities of the PEI(4.0)/PMA and PEI(9.5)/PMA were the same. The peak at 400 eV corresponds to the amine group of PEI,31 and the peak at 402 eV is thought to correspond to the amine group which is ionically bonded to PMA. After baking the PEI/PMA films, the two peaks gathered to the single peak at 400.8 eV associated with the formation of an imide bond. Figure 8b is the N 1s spectrum of the films deposited on the Cu surface. In this case, a single peak at 399.5 eV was measured on both PEI/PMA films with the absence of the peak at 402 eV. The N 1s peak intensity of PEI(4.0)/PMA was 60% that of PEI(9.5)/PMA. This reduction of PEI(4.0)/ PMA on the Cu was consistent with the thickness reduction as discussed above (Figure 5 and 7). After baking the PEI(4.0)/ PMA, the N 1s intensity almost vanished with the help of Cu− oxide-catalyzed pyrolysis.44 In Si 2p spectrum negligible small signal intensities compared to the bare SiO2/Si sample were measured for both PEI(9.5)/ PMA and PEI(4.0)/PMA (not shown), because both thicknesses were much larger than the attenuation length of photoelectrons. Figure 9a shows the Cu 2p spectrum. The Cu 2p3/2 peak at 932.6 eV corresponds to metallic Cu (Cu0) and Cu2O. The intensity of PEI(4.0)/PMA was 2.7 times larger than that of PEI(9.5)/PMA and 1/3 compared to that of the reference bare Cu sample. The inelastic mean free path (IMFP) of organic polymers at this energy are 1.3−1.4 nm,45,46 so the
Figure 7. Thickness contrast between the PEI(4.0)/PMA on SiO2 and that on Cu (a) without baking and (b) with baking. The same data for PEI(9.5)/PMA are also plotted. Equal thickness line (solid) is plotted as a guide for the eye. Numerical fitting errors in both axes are also shown.
the SiO2 surface and that on the Cu surface in various thickness ranges on SiO2. The thickness of organic films can be controlled from subnanometers to 20−30 nm on SiO2.43 Its surface was always optically flat and had no pit. PEI(9.5)/PMA was deposited on both substrates in a similar thickness, but the PEI(4.0)/PMA was deposited selectively on SiO2 in various thickness ranges (a few nanometers to 32 nm) while minimizing its thickness on the Cu surface to less 2−3 nm (Figure 7a). As reported in our other work,44 if the organic film on the Cu surface is thin enough (≤5 nm), it can be decomposed by Cu−oxide-assisted pyrolysis while keeping it on the SiO2 surface thermally stable. Figure 7b shows the 22886
DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
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Figure 9. (a) Cu 2p and (b) Cu 3p XPS spectra and (c) Cu L3VV Auger spectra of the PEI(9.5)/PMA, PEI(4.0)/PMA, and PEI(4.0)/PMA with baking on the Cu surface and those of the bare Cu with baking.
thickness of PEI(4.0)/PMA was calculated as ∼1.4 nm.47 The Cu 2p3/2 intensity of PEI(9.5)/PMA was 1/8 that of the reference bare Cu. This means its thickness was much larger than the IMFP. Both results were consistent with the thickness difference between them, which can be seen in Figure 7a. After baking the PEI(4.0)/PMA, the Cu 2p3/2 peak intensity became larger and comparable (67%) to that of reference bare Cu. From IMFP, the thickness was calculated as ∼0.5 nm.47 The same tendency was also measured in the Cu 3p spectrum (Figure 9b). In this energy scale the IMFP of common organic polymers is 3.6−3.7 nm.45,46 From the spectrum peak intensity of Cu 3p3/2 at ∼75 eV, the thickness of PEI(9.5)/PMA was sufficiently larger than the IMFP. A thickness of PEI(4.0)/PMA was ∼1.7 nm,47,50 and it became much smaller (∼0.5 nm) after baking.47,50 Figure 9c shows the Cu L3VV Auger spectrum. In the reference bare Cu surface, the peak at 916.6 eV corresponds to Cu2O, and the shoulder peak at 918.6 eV corresponds to Cu0. After baking the PEI(4.0)/PMA, the relative intensity of the Cu0 peak against the Cu2O peak was increased compared to that of PEI(4.0)/PMA. The Cu2 O was consumed to decompose the residual PEI(4.0)/PMA.44 As a result, pHdependent-selective thickness control of PEI/PMA on SiO2 and the Cu surfaces is shown.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Institute of Advanced Industrial Science and Technology (AIST) for providing the Cu samples. We also thank Professor Takamaro Kikkawa of Hiroshima University for his useful discussions.
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CONCLUSIONS Thickness contrast between the organic film on SiO2 which was made through PEI coating, semidrying at 100 °C, and PMA coating and the same film on Cu was investigated for selective formation of the film on the SiO2 surface. By simple pH adjustment of the PEI solution, the electrostatic forces between the PEI and the Cu substrate as well as the PEI and the SiO2 substrate were tried to be controlled. By adjusting the PEI solution pH to 4.0, the PEI on the Cu surface was stripped at the PMA coating process to become less than 2−3 nm while keeping its thickness on the SiO2 surface. The thickness of the ionically bonded PEI/PMA film on the SiO2 surface was able to be controlled from a few nanometers to 32 nm. After baking it at 350 °C under N2 ambient, a covalently bonded film (from a few nm to 18 nm) was obtained with uniform and defect-free structure on SiO2 with minimized thickness (∼0.5 nm) on Cu. Our newly developed method for the selective formation of the covalently bonded organic film will open new possibilities regarding a broad range of applications in electronic devices.
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DOI: 10.1021/acs.jpcc.5b04939 J. Phys. Chem. C 2015, 119, 22882−22888
Article
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