Chemical Vapor Deposition of Conformal, Functional, and Responsive Polymer Films

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Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers
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  Chemical Vapor Deposition of Conformal, Functional,and Responsive Polymer Films By  Mahriah E. Alf, Ayse Asatekin, Miles C. Barr, Salmaan H. Baxamusa, Hitesh Chelawat,Gozde Ozaydin-Ince, Christy D. Petruczok, Ramaswamy Sreenivasan, Wyatt E. Tenhaeff,Nathan J. Trujillo, Sreeram Vaddiraju, Jingjing Xu,  and  Karen K. Gleason* 1. Introduction 1.1. Motivation for Polymer Synthesis by ChemicalVapor Deposition The chemical vapor deposition (CVD) of polymers represents thetranslation of the well-known mechanisms for organic synthesisin the liquid phase to heterogeneousprocesses for functionalizing solid surfaces.CVD polymers are synthesized by deliveringmonomers to a surface through the vaporphase. Thus, polymerization and formationof a thin solid film occur in a single all-dry process. The CVD alternative becomesincreasingly attractive as the prevalence of micro- and nanostructured surfaces andparticles drives the desire for uniformcoatings over surface topology. Such con-formal coverage is a characteristic that candifferentiate CVD polymerization fromsolution methods that can suffer fromnon-wetting and surface-tension effects(Fig. 1a–c).This review focuses on CVD polymeriza-tion methods for surface-modification layersexhibiting strong structural retention of theorganic functionalities srcinally present inthe monomers. The retention of organicfunctionalgroupsintheCVDpolymerlayersprovides specific chemical sites for thesurfaceattachmentofmoietiesrangingfrombioactive molecules to inorganic nanoparti-cles. Functional-group retention providessystematic control over surface propertiessuch as wettability, lubricity, and adhesion.In addition, some functional groups impart the capability tocreate responsive surfaces.Byextending the well-recognizedadvantages of inorganicCVDprocesses into the realm of organic materials, the mechanicalproperties of polymers can be exploited for achieving integrationinto flexible devices and low-cost production utilizing roll-to-rollprocessing. [1,2] The widespread utilization of CVD for inorganicmaterials in the semiconductor industry stems from the ability topurify precursors to a high degree in order to obtain high-purity deposits and to create high-quality interfaces using vacuumcluster tools. [3] Chemical purity of the thin films is of paramount importance for obtaining polymeric coatings with desirablecharacteristics, such as high electrical conductivity and fractureresistance in bioimplants. Indeed, the additives typically requiredto achieve uniform films by spin-casting and the impuritiescontained in the polymeric solutions, rather than the polymeritself, can be responsible for failures in biocompatibility testingand poor electrical characteristics of polymer films applied from RE VI  E W www.MaterialsViews.comwww.advmat.de Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric filmsdirectly on the surface of a substrate. CVD polymers are desirable asconformal surface modification layers exhibiting strong retention of organicfunctional groups, and, in some cases, are responsive to external stimuli.Traditional wet-chemical chain- and step-growth mechanisms guide thedevelopment of new heterogeneous CVD polymerization techniques.Commonality with inorganic CVDmethodsfacilitatesthe fabricationofhybriddevices. CVD polymers bridge microfabrication technology with chemical,biological, and nanoparticle systems and assembly. Robust interfaces can beachieved through covalent grafting enabling high-resolution (60nm)patterning,evenonflexiblesubstrates.Utilizingonlylow-energyinputtodriveselective chemistry, modest vacuum, and room-temperature substrates, CVDpolymerization is compatible with thermally sensitive substrates, such aspaper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductivepolymers,andcontrollablycrosslinked networks and for the potential to reduceenvironmental, health, and safety impacts associated with solvents.Quantitative models aid the development of large-area and roll-to-roll CVDpolymer reactors. Relevant background, fundamental principles, and selectedapplications are reviewed. [ * ] Prof. Karen K. Gleason, M. E. Alf, Dr. A. Asatekin, M. C. Barr,Dr. S. H. Baxamusa, H. Chelawat, Dr. G. Ozaydin-Ince,C. D. Petruczok, Dr. R. Sreenivasan, Dr. W. E. Tenhaeff, N. J. Trujillo,Dr. S. Vaddiraju, J. J. XuDepartment of Chemical EngineeringMassachusetts Institute of TechnologyCambridge, MA 02138 (USA)E-mail: kkg@mit.edu DOI: 10.1002/adma.200902765  Adv. Mater.  2010 ,  22,  1993–2027    2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  1993      R    E    V    I    E    W www.advmat.dewww.MaterialsViews.com solution. [4] Additionally, a large body of knowledge exists forsystematically tuning the properties of CVD inorganic films andfor producing uniform films over ever-larger-diameter wafers.Extending this knowledge to conformal, functional and responsivepolymeric surfaces is desirable for a diverse set of applicationsincluding biomedical implants, microelectrical and mechanicalsystems (MEMS) devices, and membrane separations.Utilizing common vacuum deposition tools facilitates theintegration of organic and inorganic materials into novelmicrofabricated devices and hybrid structures. While inorganiclayers typically display fixed properties, organic materials canexhibit responsive behavior, such as swelling upon exposure to ananalyte or switching surface energy in response to a change intemperature, external field, or pH. Thus, devices incorporatingorganic materials can be designed to transduce chemical andbiological events into electrical and/or optical responses.One niche for CVD polymer technology is the deposition of insoluble materials, such as fluoropolymers, electrically con-ductive polymers, and controllably crosslinked organic networks.Additionally, certain monomers can more readily be polymerizedthrough CVD methods. One example is a homopolymercontaining pendant functionalities that undergo side reactionsin solution. [5] Another example involves copolymers, where themonomers have no common solvent, such as a fluoromonomerpaired with a hydrophilic monomer. [6] Finally, because CVD‘‘builds’’ polymer films from the substrate up, in situ adhesionpromotion is possible, including creation of covalent graftingbetween the substrate and the deposited film.All-dry processes are desirable for surface modification of substrates thatwould degrade,swell,ordissolveuponexposuretoliquids (Fig. 1d). All-dry methods are also desirable as ‘‘green’’manufacturing processes, avoiding the environmental and healthand safety concerns associated with solvents as well as theeconomic costs associated with solvent disposal. 1.2. Overview of CVD Chemistry and Processing In pioneering work by Gorham, [7] [2.2]paracyclophane dimervapor was thermally cracked and the resultant monomersubsequently self-initiated polymerization on a cool substrateto produce an electrically insulating material. The resultingpoly(  p -xylylene) films and their various functionalized forms,commonly termed ‘‘parylenes’’, have since been widely commercialized.Many subsequently developed CVD polymer processes alsomake use of substrates held at or below ambient temperature.Indeed, the rates of many CVD polymer processes are limited by the rate of adsorption of precursors onto the substrates. In thissituation, film growth rates increase as the substrate temperatureis lowered. Using low substrate temperatures is compatible withthe coating of thermally sensitive substrates, such as paper,textiles, and plastics. The use of low substrate temperatures alsodifferentiates most techniques for CVD polymers from their less‘‘gentle’’ inorganic CVD cousins. While using high temperaturesand/or high energy excitation successfully drives depositionthrough the fragmentation of inorganic precursors, suchaggressive conditions typically induce undesirable degradationof organic functionalities. Thus, selective chemical strategies aredesirable for achieving low-energy,low-temperature processes forCVD polymers.Many methods developed for CVD polymerization draw uponthe vast knowledge of conventionial polymer growth, where themonomer units and their polymerization occur in the liquidphase. [8–10] After polymer synthesis, film formation requires asecond step such as spin-casting, dip-coating, or spray-and-bake,which may need to be followed by a third curing step. In contrast,the most common processes for CVD polymers convert monomers to pure polymer films in a single step.Knowledge of the chemical pathways and kinetics in theliquid-phase for a specific polymer is an excellent starting point for the design of the corresponding CVD process. Solutionpolymerization reactions are generally classified as either chain-or step-growth polymerization. In chain-growth polymerization,the polymer chain grows by the reaction of a monomer moleculewith a reactive group at the end of the active polymer chain. Thenature of this active site, whether it is a free radical, anionic orcationic, is a common basis of classification. Many monomers forchain polymerization contain vinyl groups, including acrylates, Professor Karen K. Gleason is the Alexander and I. Michael Kasser Professor of Chemical Engineering and Associate Dean of Engineering at MIT. The coauthors are members of her lab: Mahriah E. Alf, Miles C. Barr, Hitesh Chelawat, Christy D. Petruczok,Nathan J. Trujillo, and Jingjing Xu are currently doctoral candidates; Drs. Salmaan H. Baxamusa and Wyatt E. Tenhaeff received theirPh.Ds in 2009; and Drs. Ayse Asatekin, Gozde Ozaydin-Ince, Ramaswamy Sreenivasan, and Sreeram Vaddiraju, are postdoctoralassociates. From left to right: RS, AA, GO-I, SV, NJT, SHB, HC, JX, MCB, MEA, WET, CDP, and KKG. 1994    2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  Adv. Mater.  2010 ,  22,  1993–2027  RE VI  E W www.MaterialsViews.comwww.advmat.de methacrylates, and styrenes. Anionic and cationic systems canalso lead to the ring-opening polymerization of cyclic monomersto form poly(dimethyl siloxane) (PDMS) and poly(oxymethylene).In step-growth polymerization, the polymer chain grows throughreactions that can occur between any two molecular species. Thisresults in a slow and steady growth in chain length withincreasing conversion. Step-growth polymerization leads to theformation of polymers ranging from poly(esters), poly(amides),and poly(imides) to electrically conductive materials such aspoly(thiophenes). [8,9] 1.2.1. Chain-Growth CVD Polymers Similar to CVD of poly(  p -xylylenes), plasma-enhanced CVD(PECVD), [11] initiated CVD (iCVD), [1] and photoinitiated(piCVD) [1] are one-step film-growth methods which draw onthe chemistry of free-radical chain-growth polymerization. InPECVD, plasma excitation of the vapor phase creates the radicalspecies. [11] However, the degree to which organic functionality ispreserved often improves by decreasing the plasma powerthrough strategies such as pulsing the plasma excitation [12–20] orperforming the deposition downstream of the active plasmaregion. [11,21,22] Alternatively, an initiating species can beintroduced through the gas phase along with the monomers.The initiator is selectively decomposed to free radicals throughgas-phase heating (iCVD) or by photons (piCVD). [1] By avoidingthe need for nonselective plasma excitation, high-rate depositionoftruelinearfree-radicalpolymerchainscanbeachievedbyiCVDand piCVD with essentially 100% functional retention.While radicals are essential for the polymerization of vinylmonomers, if these species are not fully reacted during CVDgrowth, the resultant polymer film contains so-called dangling-bond defects. Once exposed to the air, these defects can furtherreact with oxygen and water, altering the film properties fromtheir as-deposited state. [23] Because of steric effects in the solidfilms, these aging reactions can occur over the course of severalweeks. [24–26] Electron spin resonance (ESR) studies revealed that compared to traditional PECVD, a desirable reduction indangling-bond defect concentration by one to two orders of magnitude down to   10 18 spins cm  3 results from eitherreducing the excitation time used during pulsed PECVD [27] or by utilizing thermal excitation of the gas phase by hot filamentsrather than plasma excitation. [28] Plasma environments create ionic species in the gas phase andthe importance of ionic mechanisms in polymer-like film growthhas been proposed. [26] In the absence of a plasma, the largeenergetic barrier to creating charged species in the gas phaseguarantees that heterogeneous processes will be the dominant pathways for reactions of ions. Explicit reports of ionic chainpolymerization in CVD are limited but include the cationicpolymerization of isobenzofuran. [29–31] Examples of ring-opening CVD polymerizations are alsolimited. Insoluble poly(oxymethylene) films have been synthe-sized from the cyclic monomer trioxane using hot-filament CVD. [32] Additionally, hot-filament CVD growth from the cycliccompound octamethylcyclotetrasiloxane, also known as D 4 , wasused to deposit films structurally similar to PDMS. [33] The D 4 monomer also undergoes ring opening in the deposition of organosilicon polymers by PECVD methods. [26,34] Other CVD chain-growth polymerization methods entail twosteps: the preapplication of an initiator to the substrate, followedby exposure to monomer vapor. These strategies allow initiatorsof limited volatility to be employed. As compared to thecontinuous introduction of volatile initiators by iCVD andpiCVD, preapplication of initiator fixes the available supply forfilm growth. Methods for CVD polymerization involving pre-application of the initiator include vapor-phase assisted surfacepolymerization (VASP), [35–38] living free-radical polymerization Figure 1.  In some applications, CVD polymers (right column) offer advan-tages over solution processing (left column). a) Solvent surface tensionleadstopoorstepcoverageinsolution-coatedmicrotrencheswhereasCVDleads conformal coverage of   300 thick polymer thin films along the top,bottom, and sidewalls of the trench. b) Particles agglomerate after solventevaporation in solution processing (left) but remain dispersed after CVDcoating (right). c) Fiber mats coated with electrically conducting polymerfilms by solution casting (left) showing non-wetting effects and aggregateformation while the oxidative CVD (oCVD) coating is conformal (right).d) Nylon fabric with dye leached out after solution coating (left) versus thedye retained in substrate after CVD polymer application (right). Repro-duced with permission from a) [1], b) [59] and [506], c) [64].  Adv. Mater.  2010 ,  22,  1993–2027    2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  1995      R    E    V    I    E    W www.advmat.dewww.MaterialsViews.com methods such as nitroxide-mediated polymerization (surfaceinitiated vapor deposition polymerization (SI-VDP)) [39] and atomtransferradicalpolymerization(ATRP)(alsocategorizedasaformof gas-phase assisted surface polymerization (GASP)), [40,41] anionic [42] and cationic [43–45] ring-opening polymerization, catio-nic polymerization of acetylene derivatives, [46] photoinitiatedcationic polymerization of vinyl monomers, [47] and ring-openingmetathesis polymerization (ROMP) (sometimes termed solvent-less polymerization). [46,48,49] 1.2.2. Step-Growth CVD Polymers Step-growth polymerizations have also been extensively trans-latedtovapordepositionmethodsutilizingtheintroductionofthemonomers into a vacuum chamber, either sequentially as inmolecular layer deposition (MLD) [50] or by simultaneous flow during vapor deposition polymerization (VDP). Historically, theterm VDP is sometimes used as a generic label for other CVDpolymer processes. In this review, unless otherwise noted, theterm VDP will be used only for step-growth polymerization. Onesuch exception is SI-VDP, which has been retained to matchcurrentliterature. Step-growth ofelectrically conductivepolymershas been demonstrated through vapor phase polymerization(VPP) and oxidative CVD (oCVD). [1] 2. Characteristics of CVD Polymers 2.1. Conformal Conformality describes the degree to which coating thickness ismaintained over topography in non-planar substrates, thusallowing all angles between intersecting curves to remainunchanged. Conformally coating 3D structures enables theaddition of novel surface functionalities to substrates of practicaluse in industrial, consumer, medical, pharmaceutical, andmicrofluidic applications. As feature sizes decrease and thedevice architectures become more complex, achieving goodconformality over high-aspect-ratio structures becomes increas-inglychallenging. Avariety ofCVDmethodsenablethecoating of thin polymer films with good conformality (Fig. 2), an outcomewhich is difficult to achieve with solution polymerizationtechniques (Fig. 1a–c).CVD of poly(  p -xylylene) and its derivatives have long beenprized for enabling conformal coating of high-aspect-ratiostructures (Fig. 2a). [51] For example, poly(monochloro-  p -xylylene)(‘‘parylene-C’’), is used to insulate wafer through-holes incomplementary metal-oxide semiconductor (CMOS) fabricationdue to its high quality as a dielectric material and nearperfect step coverage. [52] Functionalized and non-functionalizedpoly(  p -xylylenes) have been successfully deposited in confinedmicrogeometries. [53] Poly(  p -xylylene) films as thin as 3.5nmsuccessfully prevented metal penetration into a porous methylsilsesquioxane matrix. [54] Atomic layer deposition (ALD) processes for synthesizingconformal inorganic coatings on nanoscale devices, nanoporousmaterials, and nanoparticles have recently been reviewed. [55] ALDgrows alternating layers of atoms while the related methodof molecular layer deposition (MLD) utilizes alternation of bifunctional molecular precursors. Each is based on sequential,self-limiting surface reactions. Thus, their growth rates areconstrained by the limited number of surface sites on thesubstrate and time required to alternate between precursors inthe deposition chamber. Uniform conformal coatings withthicknesses as low as 5nm were achieved on large quantitiesof particles by MLD of hybrid organic-inorganic polymer films of aluminum alkoxide (alucone) onto BaTiO 3 , silica, and titaniaparticles (Fig. 2c). [56,57] The sequential reactions of trimethyla-luminum and ethylene glycol provide precise chemical control of the alcone composition. Similar studies of MLD ‘‘zincone’’demonstratedthegoodconformalityofthesehybridcoatingswiththicknesses of    20nm on microtubes. [58] The iCVD method conformally coats high-aspect-ratio struc-tures at high deposition rates, enabling conformal layers > 100nm in thickness (Fig. 1a, right) to be deposited using awide range of iCVD polymers. Microparticles (Fig. 1b, right) andnanotubes were conformally encapsulated with iCVD poly-(glycidyl methacrylate) (PGMA). [59] Retention of the functionalgroups from the monomer in the iCVD film enabled subsequent  Figure 2.  Cross-section microscopy images of conformal CVD polymerson micro- and nanostructures. Scanning electron micrographs (SEMs)a) of 300nm deep trench conformally coated with poly( p -xylylene) andb) iCVD poly(tetrafluoroethylene) (PTFE) conformally coats an overhangMEMS test structure. c) Transmission electron micrograph (TEM) of a 13nm thick conformal MLD poly(aluminum ethylene glycol) on aninorganic nanoparticle. d) SEM cross-section of silica microparticle con-formally coated with a hydrogel by piCVD. Reproduced with permissionfrom a) [51], c) [56], d) [60]. c,d) Copyright 2008, American ChemicalSociety. 1996    2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  Adv. Mater.  2010 ,  22,  1993–2027  RE VI  E W www.MaterialsViews.comwww.advmat.de binding of surface-active groups to the polymer. For example,fluorescentmarkerswereboundtothe glycidylgroupoftheiCVDPGMA film (Fig. 1b, right). The related method of piCVD,produced conformal hydrophilic poly(2-hydroxyethyl methacry-late) (PHEMA) coatings on microspheres (Fig. 2d). [60] Vaporintroduction of methylmethacrylate and divinylbenzene coupledwith preapplication of initiator to titania nanoparticles resulted insuccessful encapsulation varying from 2.5 to 40nm in thickness,with potential applications for drug delivery, biocatalysis andphotonics. [61] The degree to which conformality can be achieved by PECVDpolymerization is highly dependent on processing condi-tions. [15,62] The electric field of the plasma is an inherent sourceof directionality during film growth involving charged species,which leads to directional filling of features or differentialsputtering of surfaces. Moreover, the high sticking coefficients of the ionic species accelerated to the substrate surface also decreasethe step coverage. Therefore, using high deposition rates of PECVD obtained at high RF power densities generally lead topoor conformality when coating high-aspect-ratio structures. [63] However, by lowering the input power using pulsed PECVD,optimization of conformality through processing conditions ispossible.Conformal deposition of conducting polymers is desired for avariety of electronic applications. The oCVD of poly(3,4-ethylenedioxythiophene) (PEDOT) allowed conformal depositionon delicate paper and nanofiber electrospun mats (Fig. 1c,right). [64] The conformality of oCVD PEDOT films dependsstrongly on the oxidant species employed, with CuCl 2  providing amuch greater degree of conformality as compared to oCVD filmsgrown using FeCl 3 . [65] 2.2. Functional 2.2.1. Surface Energy Control  Surface energy can be readily tuned by using numerous CVDpolymer methods. For low-surface energy films, CVD of fluoropolymers is particularly advantageous for the synthesisof insoluble and conformal coatings on substrates havingtopographies spanning from the nanoscale to the macroscale.Only a thin CVD layer of an expensive fluoropolymer can impart desired surface properties to a substrate that is inexpensive and/or has superior bulk properties. Often used in combination withsurface roughness to create hydrophobic and oleophobicsurfaces, [66,67] low-surface-energy CVD films have found applica-tion in MEMS devices, [68] antifouling/biofouling and stain-resistant textiles, [69] antiwetting, antisnow, and ice adherence. [70] PECVD fluorocarbon films [17,26,71–75] often contain a variety of bonding environments such as CF 3 , CF 2 , CF, and quaternary carbon (Fig. 3a, top). Moving the substrate downstream of theplasma glow discharge slows deposition, but enhancesthe fraction of CF 2  in the film (Fig. 3a, middle), resulting inchemical composition similar to poly(tetrafluoroethylene) (PTFE,(CF 2 ) n ,Teflon) (Fig. 3a, bottom). The precise stochiometry of PTFE is achieved by hot-filament CVD [76–78] where the monomerhexafluoropropylene oxide (HFPO) is thermally decomposedto difluorocarbene (CF 2 :) (Fig. 3b). [79] Adding the initiator Figure 3.  a) X-ray photoelectron spectroscopy (XPS) reveals the distri-bution of C, CF, CF 2 , and CF 3  bonding environments in PECVD (top) anddownstreamPECVDfluoropolymer (middle)filmsascomparedtothepureCF 2  composition of a bulk PTFE standard. b) Solid-state  19 F magic anglespinning nuclear magnetic resonance (NMR) confirms the linear bondingstructure of the CF 2  units in iCVD PTFE. c) iCVD PTFE (filled circle) retains100% of the desirable CF 2  functional groups, even at high depositionrates.In contrast, for pulsed PECVD fluoropolymer (open circles) there is atradeoff between growth rate and degree of functional group retention.Reproducedwith permissionfroma) [74],b) [79], c)[80]. a) Copyright1993American Chemical Society. b) Copyright 2000 Elsevier. c) Copyright 2007Elsevier.  Adv. Mater.  2010 ,  22,  1993–2027    2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  1997
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