Thin Solid Films 518 (2010) 4252–4255

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Template-based and template-free preparation of nanostructured parylene viaoblique angle polymerization

Gökhan Demirel a,b, Niranjan Malvadkar a,b, Melik C. Demirel a,b,c,⁎a Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, United Statesb Department of Engineering Science, The Pennsylvania State University, University Park, PA 16802, United Statesc Huck Institutes of Life Sciences, The Pennsylvania State University, University Park, PA 16802, United States

⁎ Corresponding author. Materials Research InstitUniversity, University Park, PA 16802, United States. Te814 865 9974.

E-mail address: mdemirel@engr.psu.edu (M.C. Demi

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.12.096

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 May 2009Received in revised form 16 December 2009Accepted 19 December 2009Available online 4 January 2010

Keywords:NanostructuresPolymersChemical vapor depositionStructural propertiesGrowth mechanismSurface morphology

Parylene is an important class of polymer thin film that has wide-ranging applications due to its well-recognized properties, such as low-dielectric constant, biocompatibility, and exceptional barrier properties.Here, we studied the deposition of parylene nanofibers by template-based and template-free methods,which combine both vapor deposition polymerization and oblique angle deposition. A comparative study oftwo approaches based on surface characterization techniques is presented. These nanofibers, deposited byboth approaches, have potential applications in catalysis, biodetection, and biomedical coatings.

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1. Introduction

Nanoporousmaterials with controlled architecture (i.e., well-orderedarrays of nanotubes or nanofibers) have potential applications inbiomedical, optical and microelectronic industries. Structurally designedorganic nanoporous films, such as vertically aligned carbon nanotubes[1], carbon nanofibers [2], peptides [3] or polymer fibers [4], can beprepared using various methods, including chemical vapor deposition[1], glancing angle deposition [5,6], oblique angle polymerization (OAP)[4], solid-phase self organization [3] and template-based methods [7].

Template-based approaches have been widely used for thepreparation of polymeric nanomaterials [7,8]. In general, polymericnanotubes can be prepared in templates using polymermelts or liquidphase polymerization. Steinhart et al. [7] developed a template-wetting method for the fabrication of polymeric nanostructures thatshow monodisperse size distribution of nanofibers with uniformorientation. However, in template-wetting, it is difficult to controlwallthickness and the length of polymeric structures because of capillarycondensation and strong interfacial tension between the polymer andthe templatematerial (e.g. anodized aluminumoxide, AAO). Jang et al.[9] have proposed another method based on vapor depositionpolymerization in order to overcome the drawbacks of the template-

wetting technique. In this method, the outer walls of the template aremodified with a radical initiator in solution phase, and polymerizationis carried out in the template by vaporizing the monomer. The wallthickness of the polymeric nanotube can be tuned by adjusting thevolume ratio of the monomer to the solvent. The limitation of thisapproach is the formation of a thick bulk polymeric layer on thetemplate induced by radical initiatormodification prior to vapor phasepolymerization. The formation of this bulk polymer layer can hinderthe diffusion of themonomer vapor into the template. Dickey et al. [10]suggested an oblique angle gas phase sputtering method fordeposition of inorganic fibers (i.e., gold, platinum and indium-tinoxide) inside templated materials (i.e. AAO) using a collimatedelectron beam evaporation source. The advantage of a template-based gas phase method compared to a solution phase method is theabsolute control of fiber height and fiber diameter by the incidentangle of evaporation and the AAO pore diameter, respectively.

Gas phase has advantages compared to solvent-based polymeri-zation techniques. For example, conformal coating of non-planarsubstrate geometries can be obtained, and impurities associated withthe use of solvents and initiators can be eliminated [11–13]. Parylenecan be easily prepared using a solvent free and environmental friendlygas phase deposition via radical polymerization [14]. Parylene is animportant class of polymer thin film that has wide-ranging applica-tions due to its well-recognized properties, such as low-dielectricconstant [15], biocompatibility [16], and exceptional barrier proper-ties [17]. Here, we studied the deposition of parylene nanofibers bytemplate-based and template-free methods, which combine both

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vapor deposition polymerization and oblique angle deposition. Acomparative study of two approaches based on surface characteriza-tion techniques is presented.

2. Experimental details

2.1. Template-free parylene

Film adhesion to the substrate surface was enhanced by an allyl-functionalized self-assembled monolayer. Native oxide p-type Si(100) wafers were first sonicated in acetone. Afterwards, the waferswere washed in water and dried under nitrogen gas flow. The waferswere then transferred to a 1/1 (v/v) solution of HCl (36.5–38% w/w)and methanol (98.5% w/w). After 30 min, the wafers were removed,washed with water, and dried with nitrogen gas. The wafers werethen kept in concentrated H2SO4 (95–98% w/w) for another 30 min,after which they were washed and sonicated in water for 10 min. Thewafers were thoroughly dried under nitrogen gas. A self-assembledmonolayer (SAM) solution was prepared by adding 1% (v/v)allyltrimethoxysilane (Gelest, PA) in toluene containing 0.1% (v/v)acetic acid. The cleaned wafers were transferred to this solution andleft for SAM formation for 60 min at 25 °C. The wafers were removedafter 60 min and sonicated in anhydrous toluene for 10 min. Thewafers were then dried on a hot plate at 140 °C for 5 min. Template-free parylene (a.k.a. poly(chloro-p-xylylene)) films were depositedusing [dichloro-[2,2]paracyclophane as the precursor using a modi-fied vapor deposition technique called Oblique Angle Polymerization(OAP) developed by our research group [4]. A detailed description ofthe experimental methods used for preparing parylene nanowiresusing OAP is reported in our previous works [4]. Briefly, the 0.4 g ofthe precursor was first sublimed at 175 °C under a pressure of∼100 Pa. Afterwards, the precursor vapor was pyrolyzed at 690 °C.

Fig. 1. (A) Descriptions of template-based OAP technique. (B–C) Top side and cross-section Snanofibers from the template, and (E–F) SEM and TEM images of removed parylene nanofi

The pyrolysis of dichloro-[2,2]paracyclophane results in the cleavageof the alkyl bridges to form quinodimethane radicals. These radicalsundergo polymerization on the allyl-functionalized Si wafer whichwas kept at ambient temperature. The monomeric radical flux wasdirected onto the substrates through a nozzle held at an angle α=25°(Fig. 1A). The substrates were axially rotated with an angular speed ofω=5 rpm. Deposition of parylene structures using OAP was alsocarried out on lithographically patterned Si substrates prepared usingthe methodology explained below.

2.2. Template-based parylene

AAOmembranes having 100 nm of pore diameters and a thickness of60 µm (Whatman, Inc.) were placed into the deposition chamber. OAPwas carried out by using 0.4 g of dichloro-[2,2]paracyclophane as theprecursor. The polymer deposition conditions were same as in the case oftemplate-free films. After polymer deposition, AAO membranes wereremoved fromthedeposition chamber and immersed in1 MNaOHfor1 hdissolving the aluminum oxide template resulting in an open nanowirestructure connected by a continuous backing of a bulk parylene film. Inorder tominimize collapse of the nanowires prior to drying, sampleswererinsed with a low surface-tension solvent such as methanol and dried atroom temperature. To obtain individual parylene nanowires after thepolymer deposition, excess polymer was removed from the back side ofAAO membrane using a scalpel, followed by polishing with a nail file.Parylene nanowires were freed by soaking the AAO in 1 MNaOH for 24 hat 70 °C. Individual parylene nanowires were extracted from the mixtureby three cycles of centrifugation (10,000 rpm, 90 s), supernatant removal,and redispersion in 1 MNaOH. This procedurewas followedby two cyclesof centrifugation in water. Individual parylene nanowires were thendispersed in ethanol by sonication (30 s) for TEM characterization.

EM images of parylene film prepared by template-based OAP, (D) removing of parylenebers, respectively.

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2.3. Lithographic pattern

To prepare the patterned Si, wafers were first cleaned with EKC830 for 10–15 min. They were then rinsed with acetone, isopropylalcohol and deionized water for 1–2 min each and dehydrated at100 °C for 1 min. Wafers were then spin coated with PR S1837, bakedat 110 °C for 1 min and air cooled.Wafers were then exposed to UV for10.5 s using MA6 mass aligner and developed in CD-26 for 2 min andrinsed with deionized water. Patterns were prepared on the Sisubstrate using deep reactive ion etching. To remove the photoresist,wafers were treated with EKC 830 for 30 min and rinsed in acetone,isopropyl alcohol and deionized water for 1–2 min each.

2.4. Characterization

A JEOL 6700F field-emission scanning electron microscope (FESEM)operated at an accelerating voltage of 3 kVwas used to characterize themorphology of the nanostructured parylene films. The FT-IR (ThermoNicolet IR) operated under attenuated total reflection (diamond crystal)mode and a Si wafer reference was used to collect the IR spectra of theparylene films. The IR spectra were recorded using Norton-Beerapodization with 4 cm−1 resolution. For each spectrum, 100 scanswere co-added. TEM images were obtained using a Philips-420tungsten-based electron microscope operated at 120 kV acceleratingvoltage.

3. Results and discussion

Fig. 1A shows a schematic of the preparation of template-basedparylene structures using OAP. Fig. 1B shows the highly ordered poresof a pristine AAO template. During OAP, the monomeric radicalsdiffuse into the AAO pores and initiate the polymerization on the porewalls to form parylene nanofibers. SEM images of parylene nanofibersafter removing AAO (Fig. 1C) show well-aligned, high aspect ratio,closely-packed parylene nanofibers with 100 nm diameter with anarrow diameter distribution (±25 nm). The vapor flux directed at anangle α to the surface of the AAO template can control the nanofiberheight similar to the method suggested by Dickey et al. [10]. Afterprolonged treatment with 1 M NaOH and mechanical etching of theparylene bulk layer (to remove the excess polymer deposition outsidethe AAO pores), individual nanofibers were obtained, which are

Fig. 2. (A–C) Schematic representations of template-free and template-free patterned parylimage of helical fibers on a patterned Si wafer.

evident in the SEM (Fig. 1E) and the TEM (Fig. 1F) characterizations.The TEM image clearly demonstrates a nanotube structure (Fig. 1F),but we should note that nanowires (not shown) are also observed, inaddition to nanotubes. This can be attributed to polymer aggregationin the AAO pores characterized by the irregularities of the pore-widthin the length direction.

A well-ordered array of parylene fibers can also be grown by atemplate-freemethod. This method, whichwas recently developed byour group, is a bottom-up approach to prepare parallel arrays ofnanofibers [18,19]. During the deposition of parylene films, a flatsubstrate (e.g. glass or Si) is tilted with respect to the incident vaporflux. When the radical flux was directed at an angle of lower than 25ºto the substrate, the resulting film was a porous low-density polymerwith unique nanostructured morphology. Due to the directionalcharacter of the flux, the surface instabilities of the first thin layer ofparylene are enhanced by the self-shadowing effect [4]. As the surfaceinstabilities grow, they take the form of nanofibers that grow in analigned manner. The substrate can also be axially rotated to createvariousmorphologies, such as helices and chevrons. A cross-section ofthe template-free parylene film deposition using OAP was character-ized by SEM. The SEM image in Fig. 2B shows the helical nanofibers ofparylene deposited at an oblique angle by rotating the substrateaxially at 5 rpm. The nanostructure consists of parallel assemblies ofnanofibers, each having a diameter of approximately 150–200 nm.The template-free approach of depositing parylene nanowires offers alimited control over the nanowire dimensions by engineering thedeposition parameters and/or the monomer side-group functionality.

In order to precisely control the separation between adjacentnanofibers, we deposited parylene via OAP on lithographicallypatterned Si substrates. Fig. 2C shows the schematic of the fabricationof ordered nanofibers of parylene by template-free OAP on alithographically patterned Si surface. The fibers are approximately1 μm in diameter. There is a geometric relationship between patternseparation, d, and the deposition angle (α),

h= tanðαÞ = d ð1Þ

where h is the height of the pattern. The pattern separation (d)determines the fiber separation, assuming a uniform vapor flux.Larger separation lengths (or lower deposition angle) should lead toan increased distance between nucleating columns on a substrate. The

ene fibers, (B) SEM cross-section image of parylene helices on a Si wafer, and (D) SEM

Table 1Comparison of template-based and template-free parylene deposition.

Parameter Template-based OAP Template-free OAP

Deposition 2 steps 1 stepFiber height control Yes YesFiber dia. control Yes DifficultFiber morphology Rods/tubes RodsSubstrate topology Planar only Planar/non-planar

Fig. 3. FTIR spectra of parylene nanofibers prepared by template-based and template-free OAP, and bulk parylene film deposited by a conventional Gorham process [14].

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SEM image in Fig. 2D shows individual helical parylene fibers grownon a patterned Si substrate having 2 μm line shapeswith 2 μmspacing.

A comparative study of the two OAP approaches based on FTIR ispresented in Fig. 3. The chemical composition of template-free andtemplate-based parylene, characterized by FTIR, showed identicalpeaks. Characteristic features for CH stretching (2800–3000 cm−1),aromatic CH stretching (3018 cm−1), CH deformation (1344 cm−1), Cdeformation (1404 cm−1), and benzene breathing (964 cm−1) wereobserved for all deposition conditions.

A comparison of template-based and template-free parylene filmsis given in Table 1. The template-free OAP method is a one-stepgrowth technique for preparing polymer nanostructures without anypost-processing step. The control of deposition parameters, simply byrotating the substrate or the choice of substrate topology, can createfilms with varying parylene morphology. However, it is difficult tocontrol the diameter of polymeric nanofibers using this approachlargely due to unregulated growth of the nanofiber structures via theself-shadowing effect. On the other hand, the primary advantage ofthe template-based OAPmethod is the absolute control of fiber heightand fiber diameter by the incident angle of evaporation and the AAOpore diameter, respectively. Similar to Eq. (1), there is a geometricrelationship between the height of the tubes, h, and the diameter, d, ofthe AAO pores [10]. Furthermore, both methods are scalable and canproduce large-area polymeric nanofiber arrays (e.g. 1 cm×1 cm).

4. Conclusion

In summary, OAP provides a vapor phase deposition process forcreating well-ordered arrays of parylene films. We have shown that

parylene nanofibers can be produced using template-free andtemplate-based methods. Well-ordered nanofibers of parylene canbe fabricated by template-based OAP into 100±25 nm pore diameterAAO membranes. Parylene nanofibers can also be fabricated byfacilitating self-shadowing and nucleation approaches using atemplate-free OAP approach. These nanofibers, deposited by bothapproaches, have potential applications in catalysis [20], biodetection[21], and biomedical coatings [22].

Acknowledgments

We thank the Office of Naval Research for financial support for thiswork through a Young Investigator's Program Award, and the Bectonand Dickinson Technology for financial support.

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