Surface Film Formation by Chemical Vapor Deposition of Di-p

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Surface Film Formation by Chemical Vapor Deposition of Di-p-xylylene: Ellipsometrical, Atomic Force Microscopy, and X-ray Studies Ulrich Go¨schel* Eindhoven Polymer Laboratories (EPL), Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands

Harald Walter† Max-Planck-Institute for Polymer Research, P.O. Box 3148, D-55021 Mainz, Germany Received July 5, 1999. In Final Form: November 24, 1999 Thin and transparent poly(p-xylylene) (PPX) layers in the range 2-1550 nm have been prepared by means of chemical vapor deposition (CVD) via vapor-phase pyrolysis of the cyclic di-p-xylylene dimer. Ellipsometrical measurements are employed on those materials to study in detail the dependence of layer formation on sublimation temperature (80-140 °C) and deposition time (5-360 min). The layer growth rate on silicon wafers is found to be 0.3, 1.0, 6.3, and 11.8 mm/min at sublimation temperatures of 80, 100, 120, and 140 °C, respectively. The PPX layers are optically transparent in the investigated range of wavelengths from 190 to 1700 nm. Atomic force microscopy (AFM) studies show a surface roughness of 2-4 nm and larger values, 5-9 nm, at a high sublimation temperature of 140 °C or large deposition times tdc > 120 min. Wide angle X-ray diffraction (WAXD) experiments describe a monoclinic crystalline R-form with reflections (020) and (110) at 2θCu ) 17.7 and 22.8°, respectively. Small angle X-ray scattering (SAXS) studies reveal a lamellar structure with a long period of 8.3 nm.

Introduction Structure formation in ultrathin polymeric layers with a thickness on the nanometer scale is the subject of numerous scientific studies. Those layers are often of high purity and molecular order, which is fundamental to designing structures with well-defined and new properties. Chemical vapor deposition (CVD) via vapor-phase pyrolysis is one of the possible methods to prepare polymeric films in a large variety of molecular topologies. The CVD process of poly(p-xylylene) (PPX) (CH2C6H4CH2)1 via a vapor-phase pyrolysis was already established in the 60s by Gorham.2 This requires several preparation stages under vacuum conditions, such as the evaporation of the powdered di-p-xylylene dimer, the monomer formation by pyrolysis, and the subsequent PPX layer growth with simultaneous or successive polymerization and crystallization.2-5 Important process parameters are the sublimation6 and deposition temperatures,7,8 the inert gas pressure, the pyrolysis conditions, and the substrate * Corresponding author. Present address: Department Polymer Physics and Plastography, Institute for Polymer Testing and Polymer Science (IKP), University of Stuttgart, P.O. Box 801140, 70511 Stuttgart, Germany. † November AG, Ulrich-Schalk-Strasse 3, 91056 Erlangen, Germany. (1) Encyclopedia Polymer Science and Engineering, John Wiley & Sons Inc.: New York, 1989; Vol. 17, p 990. (2) Gorham, W. F. J. Polym. Sci. 1966, A1-4, 3027. (3) Isoda, S. Polymer 1984, 25, 615. (4) Zhang, W.; Thomas, E. L. J. Polym. Sci., Polym. Phys. 1992, 30, 1285. (5) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1978; Vol. 2. (6) Surendran, G.; Gazicki, M.; James, W. J. Polym. Sci., Polym. Chem. 1987, 25, 1481. (7) Kubo, S.; Wunderlich, B. J. Polym. Sci., Polym. Phys. 1972, 10, 1949. (8) Gazicki, M.; Surendan, G.; James, W.; Yasuda, H. J. Polym. Sci., Polym. Chem. 1986, 24, 215.

surface morphology. This underlines the complexity of the processing. Since then, PPX has often been used as an example to describe the CVD process on polymeric materials. Nowadays, a variety of polymer classes can by synthesized by CVD, which composes PPX derivatives,9 poly(p-phenylene vinylene) (PPV) derivatives,10 polydiacetylenes (PDA),11 poly(vinylidene fluorides),12 polysiloxanes,13 phthalocyanine derivatives,14 poly(imides),15 and nitrogene-containing heterophanes.16 Milestones of the PPX synthesis include Gorham’s process (1965),2 polymorphism (Niegisch, 1966;17 Iwamoto and Wunderlich, 1973;18 Isoda and co-workers 198319), calorimetrical studies (Wunderlich, 197520), and the CVD process and parameters (Kubo and Wunderlich, 1972;7 Yasuda and co-workers, 1984-87;6,8,21,22 Mailyan et al., (9) Scha¨fer, O.; Greiner, A. Macromolecules 1996, 29, 6074. (10) Iwatsuki, S.; Kubo, M.; Kumeuchi, T. Chem. Lett. 1991, 1071. (11) Tatsuo, K.; Ishikawa, K.; Koda, T.; Tokura, Y.; Takeda, K. Appl. Phys. Lett. 1987, 51, 1957. (12) Kubono, A.; Kitoh, T.; Kajikawa, K.; Umemoto, S.; Takezoe, H.; Fukuda, A.; Okui, N. Jpn. J. Appl. Phys. 1992, 31, 1195. (13) Shirai, M.; Kinoshita, H.; Sumino, T.; Miwa, T.; Tsunooka, M. Chem. Mater. 1993, 5, 98. (14) Sekiguchi, A.; Pasztor, K.; Masuhara, H. J. Vac. Sci. Technol. 1992, A10, 1508. (15) Salem, J. R.; Sequeda, F. O.; Duran, J.; Lee, W. Y.; Yang, R. M. J. Vac. Sci Techn., 1986, A4, 369. (16) Itoh, T.; Iwasaki, T.; Kubo, M.; Iwatsuki, S. Polym. Bull. 1995, 35, 307. (17) Niegisch, W. D. J. Appl. Phys. 1966, 37, 4041. (18) Iwamoto, R.; Wunderlich, B. J. Polym. Sci., Polym. Phys. 1973, 11, 2403. (19) Isoda, S.; Tsuji, M.; Ohara, M.; Kawaguchi, A.; Katayama, K. Polymer 1983, 24, 1155. (20) Iwamoto, R.; Bopp, R. C.; Wunderlich, B. J. Polym. Sci., Polym. Phys. 1975, 13, 1925. (21) Kramer, P.; Sharma, A. K.; Hennecke, E. E.; Yasuda, H. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 475. (22) Gazicki, M.; Surendran, G.; James, W. J.; Yasuda, H. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2255.

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199223). The epitaxial character (lattice matching) of the PPX layer formation synthesized from gaseous monomers on alkali halide (001) surfaces was proven by Isoda (1984).3 However, the fundamentals of the polymerization mechanism and deposition kinetics are not well understood,21,23,24 even for a simple molecule like that of PPX. The crystalline structure and polymorphism of PPX were discovered after several refinements and using both gasphase-deposited and solution (R-chloronaphthalene)grown materials. Known are the monoclinic R-crystals7 with unit cell parameters a ) 0.592 nm, b ) 1.064 nm, c ) 0.655 nm (chain axis), and β ) 134.7° as well as a hexagonal unit cell19 with the dimensions a ) 2.052 nm, c ) 0.655 nm, and γ ) 120°. Both phases are quite stable at certain temperatures. A vacuum deposition at room temperature yields a crystalline R-modification preferentially, whereas low polymerization temperatures ( 80 °C lead to a mixture of R- and β-modifications.20 It is widely accepted that the deposition temperature has a large effect on the crystal modification and supermolecular structure. However, the role of the sublimation temperature is less explored. Discussed is an increased partial pressure of the monomer with sublimation temperature, resulting in an increased deposition rate,6,23 whereas the crystallization rate remained unchanged.23 As a consequence, the disorder, associated with lower crystallinity and texture, should increase with the difference between deposition and crystallization rate.23 PPX is a material with outstanding properties. It can be used as an insulating and protective coating material, based on properties such as optical transparency, a low dielectric constant of 2.65 almost unchanged in a large frequency range from 60 Hz to 1 MHz, a high crystalline melting temperature of 420 °C, a tensile modulus of 2.4 GPa, a yield stress of 45 MPa, and a yield strain of 2.5%.1 Gorham’s technique2 for expitaxial synthesis of PPX represents a solvent-free process applicable to many substrate geometries and shapes, which may open up new application fields. New surface sensitive techniques offer the opportunity to gain new insights into the layer formation process during CVD processing. In the first series of our CVD investigations we would like to describe the effects of the di-p-xylylene sublimation temperature on the surface film formation and morphology. Experimental Section Epitaxial Synthesis. The deposition experiments (Figure 1) were carried out according to Gorham’s method.2 For that purpose, a deposition apparatus25 was constructed consisting of three quartz glass chambers: for evaporation (length 200 mm, diameter 30 mm), pyrolysis (length 600 mm, diameter 30 mm), and deposition (length 120 mm, diameter 30 mm). The temperature of each chamber can be controlled independently with a precision of (0.5 °C. Using low pressure (on the order of 0.02 mbar, obtained by pre-evacuation and subsequent flushing with gaseous N2), di-p-xylylene powder (dimer) from Aldrich was sublimed at a temperature (Tsu) in the range from 80 to 140 °C for a given period of time. Next, the dimer was transformed into gaseous monomers under pyrolysis conditions at Tpy ) 630 °C. The deposition of the PPX polymer on (100) and (111) silicon (Si) wafer substrates took place at about Tde ) 25 °C, which involves polymerization and crystallization. Prior to use, the substrates (23) Mailyan, K. A.; Chvalun, S. N.; Pebalk, A. V.; Kardash, I. E. J. Polym. Sci. 1992, 34, 761. (24) Greiner, A.; Mang, S.; Scha¨fer, O.; Simon, P. Acta Polym. 1997, 48, 1. (25) Go¨schel, U.; Bastiaansen, C.; Lemstra, P. J. EPS Conf. Abstract 1997, 21B, 153.

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Figure 1. Poly(p-xylylene) deposition process consisting of sublimation, pyrolysis, and deposition stages.

Figure 2. Angle of incidence and layered structures used in ellipsometrical studies. were cleaned by a procedure comprising ultrasonic treatment in ethanol, followed by a flushing in distilled H2O, ultrasonic treatment in Hellmanex (Hellma GmbH & Co., Mu¨hlheim, 2 wt %), flushing in distilled H2O, ultrasonic treatment in ethanol, and finally, drying under gaseous N2. Vapor Pressure. Using an apparatus developed and built by TNO, Eindhoven,26 the vapor pressure and compound decomposition could be determined. The differential pressure was measured by means of a MKS pressure gauge which was placed between two valves of a vacuum loop of about 0.013 mbar. To avoid evaporation, the sample holder next to the pressure gauge was cooled to -80 °C before heating. During the measurement, the two vacuum valves were closed, whereas the valve between the sample and the pressure gauge was opened. Differential pressure and temperature were recorded automatically and displayed in real time. Ellipsometry. The measurements were performed with a computer-controlled null ellipsometer in a vertical polarizercompensator-sample-analyzer (PCSA) arrangement.27 The angle of incidence R was set to 70° (Figure 2) to obtain the best sensitivity for our system. A He-Ne laser (λ ) 632.8 nm) was used as the light source. The thickness dPPX and the index of refraction nPPX of the polymer layer were obtained from the ellipsometric angles ψ and ∆, measured at null intensity of reflected light, assuming a multilayer composed of a homogeneous isotropic PPX film on a silicon (Si) wafer with a natural SiO2layer of about 1.7 nm in contact with air.28 The thickness dSiO2 of the substrate oxide layer was measured before coating. The refractive index nSiO2 of the oxide layer was set to 1.462 and that of Si to nSi ) 3.882 - i0.019.29 Additional ellipsometry measurements were performed with a variable-angle spectroscopic ellipsometer (VASE) from Woolam Co. Inc. over the spectral range 190-1700 nm at an incidence angle of 70° to study whether there is an effect of the wavelength on the refractive index. We have modeled and calculated the (26) van Mol, A. M. B.; Driessen, J. P. A. M.; Spee, C. I. M. A. J. Chem. Vap. Deposition, submitted. (27) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (28) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland Publications: Amsterdam, 1987. (29) Edward, D. P., Ed. Handbook of Optical Constants of Solids; Academic Press: London, 1985. (30) Lautenschlager, P.; Garriga, M.; Vina, L.; Cardona, M. Phys. Rev. B 1987, 36, 4821.

Surface Film Formation by CVD of Di-p-xylylene

Figure 3. Vapor pressure p of di-p-xylylene versus reciprocal temperature, illustrating a small vapor pressure, which does not change with the decomposition time. refractive index of the PPX material using the thickness values of the PPX layers and the dielectric function of the Si and SiO2 substrate.28,29 AFM Measurements. The atomic force microscopic data were obtained on a Nanoscope IIIa Digital Instruments. The experiments were performed in tapping mode in ambient air at 275.13 kHz with a damping amplitude of approximately 95% with respect to vibration at free resonance, as described in detail elsewhere.31 A rectangular silicon cantilever 120 µm in length with a free resonance frequency of 275.43 kHz from Nanosensors, Germany, was used. The radius of curvature of the tip was estimated to be 120 min). Under those extreme conditions a roughness on the order of 5-9 nm is found. Moreover, AFM images reveal the existence of a globular structure of different particle distribution with an in-plane extension ranging from 40 to 100 nm and exceptionally up to 380 nm in the case of a high sublimation temperature of 140 °C. In order to achieve a well-defined layer formation, sublimation temperatures between 80 and 120 °C are preferred, as illustrated in Figures 4-6. The layer formation with a thickness range from the monolayer up to about 1.5 nm has not been studied. The existence of aggregates even at very thin layers on the order of several nanometers may lead to the dewetting phenomenon; however, we do not have any experimental proof of its occurrence from ellipsometrical, IR absorption, and AFM measurements. Morphology. The refractive indices from ellipsometrical investigations are used to obtain information about the regularity of the molecular order. According to the Lorenz-Lorentz formalism33

Rmol )

L n2 - 1 M ) R 30 n2 + 2 F

(1)

with the molar refraction Rmol as the sum of concentrations kj and refractive increments Rj (32) Go¨schel, U.; Ho¨hne, G. W. H. J. Therm. Anal. Calorim., in press. (33) Groh, W.; Zimmermann, A. Macromolecules 1991, 24, 6660.

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a

a

b b

Figure 5. (a) AFM image of a PPX thin film deposited on a silicon wafer using a sublimation temperature of 80 °C and a deposition time of 60 min. Total scan size: 2000 × 2000 nm2. (b) Layer roughness along a line through the scanned area. m

Rmol )

kjRj ∑ j)1

Figure 6. (a) AFM image of a PPX thin film deposited on a silicon wafer using a sublimation temperature of 140 °C and a deposition time of 15 min. Total scan size: 2000 × 2000 nm2. (b) Layer roughness along a line through the scanned area.

(2)

the experimental refractive index n depends on the molecular weight of the repeating units M, the density F, and the electrostatic polarization of the molecule (R). In eq 1 0 denotes the dielectric constant in a vacuum and N the Loschmidt number L ) NVmol, which is the product of the number of molecules N and the molecular volume Vmol. Finally, the experimental refractive index of a given chemical structure is strongly affected by the spatial order of the individual groups of the polymer repeating units and their refractive index increments. Consequently, changes in the interference as well as absorption may occur depending on CVD conditions. Figure 7 shows the experimental refractive indices at λ ) 632.8 nm, which have been obtained in the PPX layer thickness range from 0 to 160 nm using sublimation temperatures of 80, 100, 120, and 140 °C. At a very small PPX layer thickness on the order of 90 nm, the refractive indices vary with the CVD conditions. Moreover, the variation increases with the layer thickness, which may arise from the layer growth speed or an increase in inhomogeneities or the number of ordering defects. In this sense, probably a mismatching of polymerization and crystallization

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Figure 8. Effect of the wavelength on the refractive index for two samples prepared at the sublimation temperature Tsu ) 100 °C using the deposition times tde ) 20 min to obtain a layer thickness of dPPX ) 7.9 nm (9-9 line) and tde ) 120 min for dPPX ) 142.6 nm (solid line).

conditions can play a larger role. It is known that variations in the refractive index are related to the local electrostatic polarization (or dipole moment). Consequently, the assumption of an isotropic and homogeneous layer as used in the data analysis may not hold for the thickness range above. Additional ellipsometrical measurements in a large range of wavelengths from 190 to 1700 nm have shown almost no absorption across the entire range. Consequently, the PPX layers are optically transparent. Figure 8 represents the changes of the refractive index in the wavelength range from 500 to 725 nm for two different layer thicknesses (7.9 and 142.6 nm) using the same sublimation temperature of 100 °C. In both cases, the refractive index decreases with the wavelength, whereas the gradient is larger for the thinner layer. The difference in the gradient is due to differences in the morphologies between both samples. Probably, the larger interfacial effect in the case of the smaller layer thickness will contribute to the larger gradient. The intersection of both curves at about 630 nm is accidental, and in that particular case, only a minor effect of the layer thickness on the refractive index at the experimental wavelength of 632.8 nm is shown. The refractive indices determined at 632.8 nm using a different setup (see Figures 7 and 8) are in good agreement. Consequently, the additional ellipsometrical measurements at a large wavelength range allowed new insights into the effect of morphology on the refractive index. The PPX layer on silicon is isotropic within the film plane, as represented by WAXD studies on a free-standing film using a sublimation temperature of 140 °C and a deposition time of 45 min (Figure 9). The degree of crystallinity is determined to be about 33%. From the WAXD reflections (020) and (110) at 2θCu ) 17.7 and 22.8°, respectively, the existence of a monoclinic R-crystal form is concluded (Figure 10). The d-spacings of d020 ) 0.50 nm and d110 ) 0.39 nm are in agreement with those reported by Iwamoto and Wunderlich.18 SAXS measurements show a lamellar structure with a long period of about L ) 8.3 nm according to L ) 2π/qmax, where qmax is the scattering angle at the intensity (I) maximum in the Lorentz plot Iq2 ) f(q). Such a d-spacing is in accordance with refs 23 and 34. (34) Isoda, S.; Kawaguchi, A.; Katayama, K. I. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 669.

Figure 9. 2D WAXD pattern of PPX in transmission.

Figure 10. WAXD intensity (integrated along the Debye ring) versus diffracting angle.

Conclusion The chemical vapor deposition (CVD) technique has been known for decades. However, the application on polymers is not well understood. Poly(p-xylylene) (PPX) is one of the classical polymeric materials to study the CVD process. Certainly, basic knowledge of CVD on PPX can be applied to other polymer systems. Among them are the preparation of ultrathin conjugated poly(1,4-phenylene vinylene) (PPV) films for electronic and optical applications such as light-emitting diodes (LED). CVD from gaseous monomers has the advantages of solvent-free processing, applicability to many substrate geometries and shapes, and the possibility of obtaining a large variety of molecular topologies. However, the surface film formation during CVD is very complex. The epitaxial mechanism, the role of the substrate, and the local architecture remain to be explored. The present study describes the formation of thin PPX layers on silicon wafer substrates using the CVD technique. Maintaining the conditions for pyrolysis (Tpy ) 630 °C) and deposition (Tde ) 25 °C, p ) 0.02 mbar) constant, the sublimation temperature has been changed in the range between 80 and 140 °C. Ellipsometrical studies are performed to investigate in detail the layer growth dependence on CVD processing

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conditions. For all sublimation temperatures, we find a linear growth with time. The morphology has been studied by ellipsometry, AFM, and X-ray techniques. Ellipsometrical studies in a large wavelength range show differences in the gradient of refractive index versus wavelength, depending on the processing conditions. This relates to the local electrostatic polarization, originating from inhomogenities, local order, and surface roughness. State-of-the-art hybrid techniques. such as ellipsometry in combination with scanning near field microscopy (SNOM), are expected to give new insights in exploring the mesoscopic structure with lateral dimensions < 100 nm. AFM studies describe differences in the PPX surface morphology characterized by the layer roughness and the existence of nanoscale aggregates. The (RMS) roughness ranges from 2 to 4 nm and reaches values on the order of 5-9 nm at high sublimation temperatures (e.g. Tsu ) 140 °C) or large deposition times tde > 120 min. These results are related to an increase in the partial pressure of the monomers with sublimation temperature, leading to an increase in the deposition rate. Furthermore, globules with

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an in-plane extension from 40 to 100 nm and exceptionally up to 380 nm (at a high sublimation temperature of 140 °C) are found. WAXD and SAXS measurements on free-standing films are important to characterize the polymorphism and lamellar structure, respectively. For the investigated PPX samples, we find a crystalline monoclinic R-form in a lamellar structure with a long period of 8.3 nm. Acknowledgment. The financial support from PTN Project No. 37 PTN4B-3-DP176 is gratefully acknowledged. Technical support was kindly provided by Mr. F. A. W. Kuypers, GTD, TU Eindhoven, The Netherlands. The authors thank Dr. K. Spee from TNO-TPD, Eindhoven, for the vapor pressure measurements and Dr. K. Flipse, Physics Dept., TU Eindhoven, for ellipsometrical measurements at different wavelengths and Dr. J. Pickering, University Twente, The Netherlands, and Dr. G. M. Kim, TU Eindhoven, The Netherlands, for AFM studies. LA9908743