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The facile surface modification of poly(p-xylylene) ultrathin films
Colloids and Surfaces A: Physicochem. Eng. Aspects 216 (2003) 167 /173 www.elsevier.com/locate/colsurfa
The facile surface modification of poly(p-xylylene) ultrathin films Jay J. Senkevich *, G.-R. Yang, T.-M. Lu Department of Physics SC 1C25, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA Received 12 June 2002; accepted 15 November 2002
Abstract The parylene polymers studied here are vacuum deposited by a common but unique method that is self-initiating and unterminating. The lack of chain termination allows the chain endings of the polymers to react with atmospheric oxygen and 50 wt.% aqueous ammonium sulfide. However, the latter reaction in particular can only take place at the polymer’s surface due to the excellent barrier properties of the parylenes, which leaves the surface of parylene hydroxyl terminated and consequently hydrophilic. High parylene polymer typically cannot be easily modified since few chain ends exist at its surface; however, this is in contrast to ultrathin oligomeric parylene films. The surface chemistry of the modified and annealed ultrathin parylene films was undertaken with X-ray photoelectron spectroscopy and contact ˚ ) react with atmospheric oxygen angle goniometry. Results show that as-deposited ultrathin parylene films (15 /32 A (from XPS) due to a rather large O1s peak and asymmetry in the C1s spectrum due to C /O bonding. The ultrathin film also exhibits a precipitous drop in its contact angle after exposure to 50 wt.% ammonium sulfide. However, after postdeposition annealing at 200 8C, little oxygen is observed by XPS and none after 250 8C. Further, films annealed at 350 8C in high vacuum show little reaction with aqueous ammonium sulfide much like the thick parylene films /2000 ˚ giving evidence that annealing forms a higher molecular weight polymer. The work here has implications for the A gentle surface modification of parylene and most other polymers, both for functionalizing their surfaces and leading to improved adhesion to other dielectrics and metals. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Parylene; Ultrathin; Surface; Modification; XPS
1. Introduction The area of interfacial and surface engineering is of interest in order to control condensed matter on
* Corresponding author. Tel.: /1-518-276-2974; fax: /1518-276-8761. E-mail address: [email protected] (J.J. Senkevich).
an atomic scale, which is finding importance in nano and biotechnology. Polymeric surfaces are notoriously hard to metallize and adhere to, since they are relatively inert, possessing few surface reactive groups and a low surface free energy. To improve polymer metallization and polymer/polymer adhesion, a myriad of processes have been developed to functionalize polymeric surfaces.
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These processes may be grouped as either dry, e.g. in a vacuum system, or solution-borne, as in with the use of wet chemistry. The dry methods typically use an arc discharge (capacitively coupled plasma) or a microwave plasma with various chemistries, i.e. H2O [1 /4], O2 [5], N2 [6], N2O [7], NH3 [2,8] and H2S [9]. Surface modification of polymer surfaces has also been undertaken in vacuum by flowing H2O over a hot filament [10,11] and by an Ar irradiation method [12,13]. Solution-borne processes typically use aggressive oxidizing reagents, such as H2CrO4/H2SO4 [14], KClO3/H2SO4 [15], H2CrO4 [16], fuming H2SO4 [17], solvated electrons, e.g. low temperature Mg/NH3 [18], KMnO4 [19], OsO4 [20], NH3/ AlCl3 [21] or HNO3/H2SO4 [22]. The primary drawback of the solution-borne and the dry methods is their tendency to modify, not only the polymer surface, but also the underlying polymer subsurface and often inducing bond cleavage. The one exception is the hot-filament method that modifies only the surface of the hydrocarbon or fluorocarbon polymer [10,11]. This communication investigates the surface reactivity of poly(p -xylylene) (PPXN) and poly(chloro-p-xylylene) (PPXC) ultrathin films ˚ ) versus thick films (2500 /4700 A ˚ ). The (15 /33 A surface of the polymer films will be investigated with X-ray photoelectron spectroscopy (XPS) and contact angle goniometry to determine the chemical composition and hydrophobicity of the modified polymer surface. The investigated method here relies on the ability to deposit ultrathin oligomeric films with a high concentration of chain endings onto ‘passivated’ polymeric surfaces, which are those polymeric surfaces whose chain-endings have already reacted with oxygen. The chain endings of the ultrathin deposit can then be functionalized with wet or dry chemistry. Subsequently, the oligomeric film can be converted to polymer and covalent links can be easily obtained between the ultrathin film and the base polymer with post-deposition annealing. Thus, a functionalized surface is left where the polymer subsurface is relatively undisturbed and with a robust inexpensive process that requires no additional deposition equipment.
2. Experimental The vacuum deposition system used to deposit poly(p -xylylene) (PPXN) and poly(chloro-p-xylylene) (PPXC) from their respective cyclophane precursors has been explained in detail before [23]. The Gorham process, shown in Fig. 1, outlines the basic mechanism behind the deposition of PPXN and PPXC [24]. The PPXC ultrathin films were deposited for 2.75 min onto RCA-1 cleaned (5:1:1 DI-H2O:NH4OH:H2O2 at 70 8C for 2 min) native oxide on Si (50 V-cm) or thick PPXN films deposited previously. The base pressure of the vacuum system was B/10 6 Torr and the pressure of deposition was 0.250 /0.270 mTorr. ˚ PPXC film with These conditions yielded a 15.1 A ˚ . The a native oxide thickness of 14.09/1.0 A PPXN thin films were deposited at 1.00 /1.09 ˚ thick mTorr for 2.75 min yielding 32.69/0.5 A films. The thick PPXN films were deposited at 5.0 mTorr for 20 and 35 min, yielding thicknesses of ˚ . The pyrolysis chamber 25199/10 and 47529/7 A for ‘cracking’ the cyclophane was kept at 650 8C and the sublimator temperatures were kept at 140 and 125 8C for the PPXN and PPXC dimers. The thicknesses of the ultrathin parylene films were determined using a variable angle spectroscopic ellipsometry (VASE) (M-44, J.A. Woollam Co., Lincoln, Nebraska) at three angles, 75, 76 and 778, to the sample normal. The ultrathin films grown on the native oxide were characterized by the same method as the native oxide itself. However, instead of just SiO2/Si (Jellison) another layer was added to the model to fit for the ultrathin films. The isotropic Cauchy model was used for the ultrathin PPXC and PPXN films assuming Cauchy parameters of An /1.62, Bn /0.01 and Cn /0, yielding an index of refraction of 1.645 at 634.1 nm. n(l) An Bn =l2 Cn =l4 (Isotropic Cauchy Model)
(1)
˚ , an anisotropic For the thicker films, /50 A Cauchy model was used since PPXN and PPXC exhibit optical birefringence [25] and three angles 65, 70 and 758 to the sample normal were used. The thick PPXN film used for XPS analysis was
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Fig. 1. The Gorham process for vacuum depositing poly(p -xylylene) (or poly(chloro-p -xylylene)) where no termination reactions take place.
˚ with indices of refraction of nin-plane / 47529/7 A 1.65699/0.0005 and nout-of-plane /1.59999/0.0005. The thick PPXN film used for contact angle ˚ with indices of measurements was 25199/10 A refraction of nin-plane /1.66829/0.0005 and nout-ofplane /1.60429/0.0003. The ultrathin PPXN films used for contact angle measurements were 32.3 / ˚ . After a 350 8C anneal for 60 min at B/ 33.1 A 6 10 Torr in a quartz furnace the films were 24.1 / ˚. 26.8 A A custom-built contact angle goniometer was used to measure the equilibrium contact angles of the hydroxyl functionalized PPXN surfaces (50 wt.% ammonium sulfide Aldrich, Milwaukee, WI). The polymer thin films were placed in borosilicate beakers and sealed with ParafilmTM. After 5, 15 or 30 min, the samples were removed and washed with DI-H2O (18 MV-cm) and blown dry with nitrogen. The equilibrium contact angles were measured by the method of Good using the sessile drop technique [26]. All the contact angles measured in this study were undertaken with DI-H2O with an accuracy of 9/28 and a drop volume of 5 ml. A Perkin/Elmer 5500 with a Mg Ka (1.253 keV) source was used for X-ray photoelectron spectroscopic characterization of the PPXC and PPXN films. The X-ray beam diameter was 1.5 mm and an electron take-off angle of 458 was used for analysis. Each sample was loaded into the UHV chamber and allowed to out-gas for a minimum of 12 h until a base pressure of B/1 /10 9 Torr was reached. The silicon substrate was used as an internal standard for XPS peak positions, since the electron escape depth of the X-ray photoelectrons ˚ (for carbon). A binding energy of 99.15 is :/70 A eV was used for elemental silicon [27].
3. Results and discussion The parylene polymers have certain unique characteristics, which has allowed the current work to be undertaken. Fig. 1 best illustrates these unique features. The parylene polymers via the Gorham process (Fig. 1), via p-dihalogenated precursors [28,29], or plasma deposition [30] are self-initiating generating the monomer diradical as the active polymerizing species. All three methods of depositing the parylene polymers can be undertaken in vacuum. However, most important is the lack of termination reactions and therefore, the parylene polymers contain unterminated chain ends as-deposited, that contain very active free radicals. Experimentally, it has been shown that PPXN has a molecular weight approaching 2.5 /105 g mol 1 by neutron activation analysis via a Wurtz reaction (solution polymerization) [31]. It was also shown that with increasing reaction time, the molecular weight of the polymer increases, a sensible observation and important for the present communication. Another study found a molecular weight approaching :/104 g mol 1 via the vacuum deposition of PPXC at /196 to 0 8C. Using electron paramagnetic spectroscopy, an electron spin concentration of 1.2 /1020 spins g 1 was calculated. In vacuum, the half live of these free radicals was assumed to follow second order kinetics and found to be 157 min. In air, the decay was much more complex and rapid and after several months exposure to air the spin concentration dropped to 5 /1015 spins g 1 [32]. Oxygen has a difficult time diffusing into the PPXN or PPXC polymers, since they are excellent barriers to oxygen and other gases and therefore the surface free radicals react rapidly compared to
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ones contained in the bulk. PPXN and PPXC have oxygen permeabilities of 33.6 /10 10 and 6.18 / 10 10 (cm3 cm 1/cm2 s 1 Pa) at 25 8C [33]. In comparison, poly(styrene) has a permeability of 2000 /10 10 (cm3 cm 1/cm2 s 1 Pa) at 25 8C towards oxygen [34]. Then the observations that are important to the current work are: 1) The parylene polymers are unterminated. 2) Since the permeability of oxygen into PPXC and PPXN is low, then the surface reaction with chain endings is rapid compared to bulk sites. This is especially true for aqueous solutions. 3) It is likely that as parylene’s film thickness increases its molecular weight correspondingly increases. Fig. 2 shows the XPS spectra of: thick PPXN ˚ ), as-deposited PPXC (15.1 A ˚ ) on thick (4752 A PPXN, the film stack annealed at 200 8C for 60 min at B/10 6 Torr and the film stack annealed at 250 8C. The most important observation of the communication is contained in Fig. 2, namely the ˚) O1s peak for the as-deposited PPXC film (15.1 A on PPXN. The rather large peak is associated with the reaction between atmospheric oxygen and the
PPXC ultrathin film. The PPXC ultrathin film was deposited onto the thick PPXN film only after vacuum broke, so that the thick PPXN film would be become ‘passivated’ with oxygen according to the proposed reactions below. If the vacuum is not broken than the peak is not present and the spectrum looks identical to the thick PPXN spectrum shown in Fig. 2. The reaction between the ultrathin PPXC film and oxygen is proposed as: Propagation of the free-radical C6 H4 CH2 + O2 0 C6 H4 CH2 OO+ (PPXN)
(2)
or C6 H3 (Cl)CH2 + O2 0 C6 H3 (Cl)CH2 OO+ (3) (PPXC) Reactions (2) or (3) were previously proposed by Gazicki et al. [32]. These surfaces are considered ‘passivated’ since they are significantly less reactive than without oxygen. Termination of the free-radical at room-temperature (sluggish) 2-C6 H3 (Cl)CH2 OO+ 2H2 O 0 2-C6 H3 (Cl)CH2 OOHHOOH 2-C6 H3 (Cl)CH2 OO+
(4)
0 C6 H3 (Cl)CH2 OCH2 (Cl)C6 H3 O2
(5)
Termination of the free-radical at elevated temperatures forming high polymer n-C6 H3 (Cl)CH2 OO+ 0 (CH2 C6 H3 (Cl)CH2 )n O2
Fig. 2. Full XPS spectra of (from bottom to top): (a) thick ˚ ; (b) ultrathin PPXC 15.1 A ˚ on thick PPXN as-deposited 4752 A PPXN as-deposited but vacuum broke between depositions; (c) same stack as (b) but annealed to 200 8C in high vacuum; and (d) same stack as (b) annealed to 250 8C.
(6)
˚ PPXC on thick PPXN) is When the stack (15.1 A annealed at 200 8C the oxygen signal is largely reduced and at 250 8C is eliminated entirely. Since both of these films originally contained much oxygen like the as-deposited stack than the postdeposition anneal eliminated oxygen from the PPXC surface. The oxygen then should be bonded weakly to the surface as in reactions (2) and (3). Once annealed, the surface is largely unreactive
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Fig. 3. Equilibrium contact angles (with respect to water) of ˚ ; (b) (from bottom to top): (a) unannealed PPXN 32.6 A ˚ ; and (c) unannealed thick PPXN 2519 annealed PPXN 24.1 A ˚ . The 50 wt.% (NH4)2S functionalizes the surface with /OH A groups where there is a reactive site available.
Fig. 4. The Cl 2p XPS spectra for the samples shown in Fig. 2. PPXC was grown on SiO2 as a reference (top spectrum). The lower integrated area of the annealed films reflects the loss of film thickness.
towards (NH4)2S(aq) (Fig. 3) suggesting that a loss of free radical species results and high polymer is formed as in reaction (6). Two sets of films were grown on a RCA-1 (5:1:1 DI-H2O:NH4OH:H2O2 at 70 8C for 2 min) cleaned native oxide on silicon. The ultrathin ˚ ) and thick (2519 A ˚ ) PPXN films were (32.6 A exposed to 50 wt.% (NH4)2S(aq) solution for 5, 15 and 30 min. Aqueous ammonium sulfide has been previously proposed as a buffered source of ammonium hydroxide at a pH /9.5 [35]. The thick film showed little change in its contact angle (with respect to DI-H2O), however, the unannealed ultrathin film exhibited a precipitous drop in its contact angle even after 5 min exposure. Another film of the same thickness was annealed for 350 8C for 60 min at B/10 6 Torr to ensure that the film did not debond from the native oxide surface and the effect was real. The annealed film showed a slight drop in its contact angle due to either its thickness loss or its changing surface roughness. The film’s thickness was 26% less than its original value, however, the contact angle only dropped slightly and gradually much like the thick parylene film. The reduced thickness can also be seen in Fig. 4 with the Cl2p XPS spectra. The Cl/C ratio for the 200 and 250 8C annealed films is
reduced by a similar degree as the PPXN ultrathin films on SiO2 annealed at 350 8C, indicating that some mass loss took place from the annealed ultrathin films. Also, the integrated area of the asdeposited PPXC film on PPXN and on SiO2 is the same, even though it is a less reliable method then calculating the ratios between different elements. The un-annealed PPXC ultrathin films exhibit a Cl 2p3/2 peak at 200.8 eV slightly shifted from 200.5 to 200.6 eV for the annealed films. The spin orbital splitting between the Cl 2p3/2 and Cl 2p1/2 peaks is 1.6 eV and represents literature value [27]. This slight shift maybe due to the presence of oxygen at the chain ends. Further, these binding energies are slightly higher then what has been previously reported for polyvinylchloride, at 200.0 ˚ PPXC eV for the Cl 2p3/2 peak [36]. The 15.1 A thin film on SiO2 exhibited two peaks in its C1s XPS spectrum, as shown in Fig. 5. The spectrum is calibrated with respect to Si at 99.15 eV. The primary peak at 285.1 eV is attributed to C /C bonding, whereas the peak at 286.6 eV is attributed to C /O bonding from the oxygen end-capped oligomer. The relative percent C /O bonding, based on the integrated areas, is 20% and is significantly more then what would be expected
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˚ PPXC grown on Fig. 5. The C 1s XPS spectrum of a 15.1 A SiO2. The peak at 286.6 eV is attributed to C /O bonding at the end of the oligomer chain. It is 20% of the main peak at 285.1 eV due to C /C bonding, which is much greater than bulk parylene, namely B/1%.
for adventitious carbon or the thick PPXN thin films, B/1%. From the data presented, it is proposed that ultrathin thin parylene films are oligomeric and therefore have a high percentage of end-groups compared to thick or high polymer films (:/105 g mol 1). The end groups at the surface are reactive not only towards atmospheric oxygen but to chemicals such as ammonium sulfide, that can functionalize the surface with hydroxyl groups. These groups in turn can lower the contact angle of the polymer surface with respect to water. Once the polymer surface is functionalized, then it is available for further reactions with, for example, self-assembled monolayers.
4. Conclusions The parylene polymers are commonly deposited in vacuum by a unique method that is selfinitiating and unterminating. The lack of termination reactions allows the polymers to possess unterminated chain-ends that are reactive towards atmospheric oxygen and common organic reagents. However, such reactions have not been
˚ , parylene films due observed with thick, /1000 A to their high molecular weight. As a result, few chain-ends reside at the polymer surface to be available for further reaction and therefore, no O1s XPS peak is observed. However, when ultra˚ , parylene films were deposited onto thin, 15 /32 A ˚ ) a rather large thick parylene films (2520 /4750 A O1s peak was observed. However, this peak was only observed when the vacuum broke between depositing the thick film and the ultrathin film. The same stack that was annealed in high vacuum (B/10 6 Torr) at 200 8C exhibited a reduced O1s peak that disappeared completely after a 250 8C anneal. Therefore, the oxygen bonded to the parylene chain-ends is thermally labile, indicative of perhaps peroxide bonding. The ultrathin parylene films were also exposed to 50 wt.% ammonium sulfide for 5, 15 and 30 min. The unannealed ultrathin parylene film exhibited a large reduction in its contact angle; however, this was unlike the same film that was annealed at 350 8C in high vacuum. That film, like the thick unannealed parylene film, showed only a modest decline in its contact angle as a function of exposure time. Most important, once the polymer is functionalized, it can further react with other organics, e.g. self-assembled monolayers, or more important, its surface chemistry can be easily controlled to improve adhesion to other dielectrics or metals.
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The facile surface modification of poly(p-xylylene) ultrathin films Jay J. Senkevich *, G.-R. Yang, T.-M. Lu Department of Physics SC 1C25, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA Received 12 June 2002; accepted 15 November 2002
Abstract The parylene polymers studied here are vacuum deposited by a common but unique method that is self-initiating and unterminating. The lack of chain termination allows the chain endings of the polymers to react with atmospheric oxygen and 50 wt.% aqueous ammonium sulfide. However, the latter reaction in particular can only take place at the polymer’s surface due to the excellent barrier properties of the parylenes, which leaves the surface of parylene hydroxyl terminated and consequently hydrophilic. High parylene polymer typically cannot be easily modified since few chain ends exist at its surface; however, this is in contrast to ultrathin oligomeric parylene films. The surface chemistry of the modified and annealed ultrathin parylene films was undertaken with X-ray photoelectron spectroscopy and contact ˚ ) react with atmospheric oxygen angle goniometry. Results show that as-deposited ultrathin parylene films (15 /32 A (from XPS) due to a rather large O1s peak and asymmetry in the C1s spectrum due to C /O bonding. The ultrathin film also exhibits a precipitous drop in its contact angle after exposure to 50 wt.% ammonium sulfide. However, after postdeposition annealing at 200 8C, little oxygen is observed by XPS and none after 250 8C. Further, films annealed at 350 8C in high vacuum show little reaction with aqueous ammonium sulfide much like the thick parylene films /2000 ˚ giving evidence that annealing forms a higher molecular weight polymer. The work here has implications for the A gentle surface modification of parylene and most other polymers, both for functionalizing their surfaces and leading to improved adhesion to other dielectrics and metals. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Parylene; Ultrathin; Surface; Modification; XPS
1. Introduction The area of interfacial and surface engineering is of interest in order to control condensed matter on
* Corresponding author. Tel.: /1-518-276-2974; fax: /1518-276-8761. E-mail address: [email protected] (J.J. Senkevich).
an atomic scale, which is finding importance in nano and biotechnology. Polymeric surfaces are notoriously hard to metallize and adhere to, since they are relatively inert, possessing few surface reactive groups and a low surface free energy. To improve polymer metallization and polymer/polymer adhesion, a myriad of processes have been developed to functionalize polymeric surfaces.
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These processes may be grouped as either dry, e.g. in a vacuum system, or solution-borne, as in with the use of wet chemistry. The dry methods typically use an arc discharge (capacitively coupled plasma) or a microwave plasma with various chemistries, i.e. H2O [1 /4], O2 [5], N2 [6], N2O [7], NH3 [2,8] and H2S [9]. Surface modification of polymer surfaces has also been undertaken in vacuum by flowing H2O over a hot filament [10,11] and by an Ar irradiation method [12,13]. Solution-borne processes typically use aggressive oxidizing reagents, such as H2CrO4/H2SO4 [14], KClO3/H2SO4 [15], H2CrO4 [16], fuming H2SO4 [17], solvated electrons, e.g. low temperature Mg/NH3 [18], KMnO4 [19], OsO4 [20], NH3/ AlCl3 [21] or HNO3/H2SO4 [22]. The primary drawback of the solution-borne and the dry methods is their tendency to modify, not only the polymer surface, but also the underlying polymer subsurface and often inducing bond cleavage. The one exception is the hot-filament method that modifies only the surface of the hydrocarbon or fluorocarbon polymer [10,11]. This communication investigates the surface reactivity of poly(p -xylylene) (PPXN) and poly(chloro-p-xylylene) (PPXC) ultrathin films ˚ ) versus thick films (2500 /4700 A ˚ ). The (15 /33 A surface of the polymer films will be investigated with X-ray photoelectron spectroscopy (XPS) and contact angle goniometry to determine the chemical composition and hydrophobicity of the modified polymer surface. The investigated method here relies on the ability to deposit ultrathin oligomeric films with a high concentration of chain endings onto ‘passivated’ polymeric surfaces, which are those polymeric surfaces whose chain-endings have already reacted with oxygen. The chain endings of the ultrathin deposit can then be functionalized with wet or dry chemistry. Subsequently, the oligomeric film can be converted to polymer and covalent links can be easily obtained between the ultrathin film and the base polymer with post-deposition annealing. Thus, a functionalized surface is left where the polymer subsurface is relatively undisturbed and with a robust inexpensive process that requires no additional deposition equipment.
2. Experimental The vacuum deposition system used to deposit poly(p -xylylene) (PPXN) and poly(chloro-p-xylylene) (PPXC) from their respective cyclophane precursors has been explained in detail before [23]. The Gorham process, shown in Fig. 1, outlines the basic mechanism behind the deposition of PPXN and PPXC [24]. The PPXC ultrathin films were deposited for 2.75 min onto RCA-1 cleaned (5:1:1 DI-H2O:NH4OH:H2O2 at 70 8C for 2 min) native oxide on Si (50 V-cm) or thick PPXN films deposited previously. The base pressure of the vacuum system was B/10 6 Torr and the pressure of deposition was 0.250 /0.270 mTorr. ˚ PPXC film with These conditions yielded a 15.1 A ˚ . The a native oxide thickness of 14.09/1.0 A PPXN thin films were deposited at 1.00 /1.09 ˚ thick mTorr for 2.75 min yielding 32.69/0.5 A films. The thick PPXN films were deposited at 5.0 mTorr for 20 and 35 min, yielding thicknesses of ˚ . The pyrolysis chamber 25199/10 and 47529/7 A for ‘cracking’ the cyclophane was kept at 650 8C and the sublimator temperatures were kept at 140 and 125 8C for the PPXN and PPXC dimers. The thicknesses of the ultrathin parylene films were determined using a variable angle spectroscopic ellipsometry (VASE) (M-44, J.A. Woollam Co., Lincoln, Nebraska) at three angles, 75, 76 and 778, to the sample normal. The ultrathin films grown on the native oxide were characterized by the same method as the native oxide itself. However, instead of just SiO2/Si (Jellison) another layer was added to the model to fit for the ultrathin films. The isotropic Cauchy model was used for the ultrathin PPXC and PPXN films assuming Cauchy parameters of An /1.62, Bn /0.01 and Cn /0, yielding an index of refraction of 1.645 at 634.1 nm. n(l) An Bn =l2 Cn =l4 (Isotropic Cauchy Model)
(1)
˚ , an anisotropic For the thicker films, /50 A Cauchy model was used since PPXN and PPXC exhibit optical birefringence [25] and three angles 65, 70 and 758 to the sample normal were used. The thick PPXN film used for XPS analysis was
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Fig. 1. The Gorham process for vacuum depositing poly(p -xylylene) (or poly(chloro-p -xylylene)) where no termination reactions take place.
˚ with indices of refraction of nin-plane / 47529/7 A 1.65699/0.0005 and nout-of-plane /1.59999/0.0005. The thick PPXN film used for contact angle ˚ with indices of measurements was 25199/10 A refraction of nin-plane /1.66829/0.0005 and nout-ofplane /1.60429/0.0003. The ultrathin PPXN films used for contact angle measurements were 32.3 / ˚ . After a 350 8C anneal for 60 min at B/ 33.1 A 6 10 Torr in a quartz furnace the films were 24.1 / ˚. 26.8 A A custom-built contact angle goniometer was used to measure the equilibrium contact angles of the hydroxyl functionalized PPXN surfaces (50 wt.% ammonium sulfide Aldrich, Milwaukee, WI). The polymer thin films were placed in borosilicate beakers and sealed with ParafilmTM. After 5, 15 or 30 min, the samples were removed and washed with DI-H2O (18 MV-cm) and blown dry with nitrogen. The equilibrium contact angles were measured by the method of Good using the sessile drop technique [26]. All the contact angles measured in this study were undertaken with DI-H2O with an accuracy of 9/28 and a drop volume of 5 ml. A Perkin/Elmer 5500 with a Mg Ka (1.253 keV) source was used for X-ray photoelectron spectroscopic characterization of the PPXC and PPXN films. The X-ray beam diameter was 1.5 mm and an electron take-off angle of 458 was used for analysis. Each sample was loaded into the UHV chamber and allowed to out-gas for a minimum of 12 h until a base pressure of B/1 /10 9 Torr was reached. The silicon substrate was used as an internal standard for XPS peak positions, since the electron escape depth of the X-ray photoelectrons ˚ (for carbon). A binding energy of 99.15 is :/70 A eV was used for elemental silicon [27].
3. Results and discussion The parylene polymers have certain unique characteristics, which has allowed the current work to be undertaken. Fig. 1 best illustrates these unique features. The parylene polymers via the Gorham process (Fig. 1), via p-dihalogenated precursors [28,29], or plasma deposition [30] are self-initiating generating the monomer diradical as the active polymerizing species. All three methods of depositing the parylene polymers can be undertaken in vacuum. However, most important is the lack of termination reactions and therefore, the parylene polymers contain unterminated chain ends as-deposited, that contain very active free radicals. Experimentally, it has been shown that PPXN has a molecular weight approaching 2.5 /105 g mol 1 by neutron activation analysis via a Wurtz reaction (solution polymerization) [31]. It was also shown that with increasing reaction time, the molecular weight of the polymer increases, a sensible observation and important for the present communication. Another study found a molecular weight approaching :/104 g mol 1 via the vacuum deposition of PPXC at /196 to 0 8C. Using electron paramagnetic spectroscopy, an electron spin concentration of 1.2 /1020 spins g 1 was calculated. In vacuum, the half live of these free radicals was assumed to follow second order kinetics and found to be 157 min. In air, the decay was much more complex and rapid and after several months exposure to air the spin concentration dropped to 5 /1015 spins g 1 [32]. Oxygen has a difficult time diffusing into the PPXN or PPXC polymers, since they are excellent barriers to oxygen and other gases and therefore the surface free radicals react rapidly compared to
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ones contained in the bulk. PPXN and PPXC have oxygen permeabilities of 33.6 /10 10 and 6.18 / 10 10 (cm3 cm 1/cm2 s 1 Pa) at 25 8C [33]. In comparison, poly(styrene) has a permeability of 2000 /10 10 (cm3 cm 1/cm2 s 1 Pa) at 25 8C towards oxygen [34]. Then the observations that are important to the current work are: 1) The parylene polymers are unterminated. 2) Since the permeability of oxygen into PPXC and PPXN is low, then the surface reaction with chain endings is rapid compared to bulk sites. This is especially true for aqueous solutions. 3) It is likely that as parylene’s film thickness increases its molecular weight correspondingly increases. Fig. 2 shows the XPS spectra of: thick PPXN ˚ ), as-deposited PPXC (15.1 A ˚ ) on thick (4752 A PPXN, the film stack annealed at 200 8C for 60 min at B/10 6 Torr and the film stack annealed at 250 8C. The most important observation of the communication is contained in Fig. 2, namely the ˚) O1s peak for the as-deposited PPXC film (15.1 A on PPXN. The rather large peak is associated with the reaction between atmospheric oxygen and the
PPXC ultrathin film. The PPXC ultrathin film was deposited onto the thick PPXN film only after vacuum broke, so that the thick PPXN film would be become ‘passivated’ with oxygen according to the proposed reactions below. If the vacuum is not broken than the peak is not present and the spectrum looks identical to the thick PPXN spectrum shown in Fig. 2. The reaction between the ultrathin PPXC film and oxygen is proposed as: Propagation of the free-radical C6 H4 CH2 + O2 0 C6 H4 CH2 OO+ (PPXN)
(2)
or C6 H3 (Cl)CH2 + O2 0 C6 H3 (Cl)CH2 OO+ (3) (PPXC) Reactions (2) or (3) were previously proposed by Gazicki et al. [32]. These surfaces are considered ‘passivated’ since they are significantly less reactive than without oxygen. Termination of the free-radical at room-temperature (sluggish) 2-C6 H3 (Cl)CH2 OO+ 2H2 O 0 2-C6 H3 (Cl)CH2 OOHHOOH 2-C6 H3 (Cl)CH2 OO+
(4)
0 C6 H3 (Cl)CH2 OCH2 (Cl)C6 H3 O2
(5)
Termination of the free-radical at elevated temperatures forming high polymer n-C6 H3 (Cl)CH2 OO+ 0 (CH2 C6 H3 (Cl)CH2 )n O2
Fig. 2. Full XPS spectra of (from bottom to top): (a) thick ˚ ; (b) ultrathin PPXC 15.1 A ˚ on thick PPXN as-deposited 4752 A PPXN as-deposited but vacuum broke between depositions; (c) same stack as (b) but annealed to 200 8C in high vacuum; and (d) same stack as (b) annealed to 250 8C.
(6)
˚ PPXC on thick PPXN) is When the stack (15.1 A annealed at 200 8C the oxygen signal is largely reduced and at 250 8C is eliminated entirely. Since both of these films originally contained much oxygen like the as-deposited stack than the postdeposition anneal eliminated oxygen from the PPXC surface. The oxygen then should be bonded weakly to the surface as in reactions (2) and (3). Once annealed, the surface is largely unreactive
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Fig. 3. Equilibrium contact angles (with respect to water) of ˚ ; (b) (from bottom to top): (a) unannealed PPXN 32.6 A ˚ ; and (c) unannealed thick PPXN 2519 annealed PPXN 24.1 A ˚ . The 50 wt.% (NH4)2S functionalizes the surface with /OH A groups where there is a reactive site available.
Fig. 4. The Cl 2p XPS spectra for the samples shown in Fig. 2. PPXC was grown on SiO2 as a reference (top spectrum). The lower integrated area of the annealed films reflects the loss of film thickness.
towards (NH4)2S(aq) (Fig. 3) suggesting that a loss of free radical species results and high polymer is formed as in reaction (6). Two sets of films were grown on a RCA-1 (5:1:1 DI-H2O:NH4OH:H2O2 at 70 8C for 2 min) cleaned native oxide on silicon. The ultrathin ˚ ) and thick (2519 A ˚ ) PPXN films were (32.6 A exposed to 50 wt.% (NH4)2S(aq) solution for 5, 15 and 30 min. Aqueous ammonium sulfide has been previously proposed as a buffered source of ammonium hydroxide at a pH /9.5 [35]. The thick film showed little change in its contact angle (with respect to DI-H2O), however, the unannealed ultrathin film exhibited a precipitous drop in its contact angle even after 5 min exposure. Another film of the same thickness was annealed for 350 8C for 60 min at B/10 6 Torr to ensure that the film did not debond from the native oxide surface and the effect was real. The annealed film showed a slight drop in its contact angle due to either its thickness loss or its changing surface roughness. The film’s thickness was 26% less than its original value, however, the contact angle only dropped slightly and gradually much like the thick parylene film. The reduced thickness can also be seen in Fig. 4 with the Cl2p XPS spectra. The Cl/C ratio for the 200 and 250 8C annealed films is
reduced by a similar degree as the PPXN ultrathin films on SiO2 annealed at 350 8C, indicating that some mass loss took place from the annealed ultrathin films. Also, the integrated area of the asdeposited PPXC film on PPXN and on SiO2 is the same, even though it is a less reliable method then calculating the ratios between different elements. The un-annealed PPXC ultrathin films exhibit a Cl 2p3/2 peak at 200.8 eV slightly shifted from 200.5 to 200.6 eV for the annealed films. The spin orbital splitting between the Cl 2p3/2 and Cl 2p1/2 peaks is 1.6 eV and represents literature value [27]. This slight shift maybe due to the presence of oxygen at the chain ends. Further, these binding energies are slightly higher then what has been previously reported for polyvinylchloride, at 200.0 ˚ PPXC eV for the Cl 2p3/2 peak [36]. The 15.1 A thin film on SiO2 exhibited two peaks in its C1s XPS spectrum, as shown in Fig. 5. The spectrum is calibrated with respect to Si at 99.15 eV. The primary peak at 285.1 eV is attributed to C /C bonding, whereas the peak at 286.6 eV is attributed to C /O bonding from the oxygen end-capped oligomer. The relative percent C /O bonding, based on the integrated areas, is 20% and is significantly more then what would be expected
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˚ PPXC grown on Fig. 5. The C 1s XPS spectrum of a 15.1 A SiO2. The peak at 286.6 eV is attributed to C /O bonding at the end of the oligomer chain. It is 20% of the main peak at 285.1 eV due to C /C bonding, which is much greater than bulk parylene, namely B/1%.
for adventitious carbon or the thick PPXN thin films, B/1%. From the data presented, it is proposed that ultrathin thin parylene films are oligomeric and therefore have a high percentage of end-groups compared to thick or high polymer films (:/105 g mol 1). The end groups at the surface are reactive not only towards atmospheric oxygen but to chemicals such as ammonium sulfide, that can functionalize the surface with hydroxyl groups. These groups in turn can lower the contact angle of the polymer surface with respect to water. Once the polymer surface is functionalized, then it is available for further reactions with, for example, self-assembled monolayers.
4. Conclusions The parylene polymers are commonly deposited in vacuum by a unique method that is selfinitiating and unterminating. The lack of termination reactions allows the polymers to possess unterminated chain-ends that are reactive towards atmospheric oxygen and common organic reagents. However, such reactions have not been
˚ , parylene films due observed with thick, /1000 A to their high molecular weight. As a result, few chain-ends reside at the polymer surface to be available for further reaction and therefore, no O1s XPS peak is observed. However, when ultra˚ , parylene films were deposited onto thin, 15 /32 A ˚ ) a rather large thick parylene films (2520 /4750 A O1s peak was observed. However, this peak was only observed when the vacuum broke between depositing the thick film and the ultrathin film. The same stack that was annealed in high vacuum (B/10 6 Torr) at 200 8C exhibited a reduced O1s peak that disappeared completely after a 250 8C anneal. Therefore, the oxygen bonded to the parylene chain-ends is thermally labile, indicative of perhaps peroxide bonding. The ultrathin parylene films were also exposed to 50 wt.% ammonium sulfide for 5, 15 and 30 min. The unannealed ultrathin parylene film exhibited a large reduction in its contact angle; however, this was unlike the same film that was annealed at 350 8C in high vacuum. That film, like the thick unannealed parylene film, showed only a modest decline in its contact angle as a function of exposure time. Most important, once the polymer is functionalized, it can further react with other organics, e.g. self-assembled monolayers, or more important, its surface chemistry can be easily controlled to improve adhesion to other dielectrics or metals.
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