Article pubs.acs.org/Macromolecules
Chemical Vapor Jet Deposition of Parylene Polymer Films in Air Shaurjo Biswas,†,§ Olga Shalev,† Kevin P. Pipe,‡ and Max Shtein*,† †
Department of Materials Science and Engineering and ‡Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States S Supporting Information *
ABSTRACT: Parylene films are commonly used as transparent, flexible coatings in electronic devices and biomedical applications, exhibiting barrier properties against corrosion, low dielectric constant, and moisture resistance. Reactive vapor deposition of parylene results in conformal coverage of features at room temperature, which is advantageous for passivating, for example, organic optoelectronic devices. Conventional parylene deposition methods, however, coat surfaces virtually indiscriminately and utilize separate chambers for vaporization, pyrolysis, and polymerization, resulting in a large footprint and limited processing integration ability, especially at a laboratory scale. Here, we demonstrate the vaporization and pyrolysis of the di-p-xylylene (parylene dimer) in a single compact nozzle, producing a jet of monomer that polymerizes into a film upon contact with the substrate at room temperature. A guard flow jet is employed to shield the reactive monomer molecules en route to the substrate, thereby enabling polymer deposition and patterning in ambient atmosphere. We present an analytical model predicting film growth rate as a function of process parameters (e.g., gas flow rate and source, pyrolysis & substrate temperatures). The effect of jet flow dynamics on film morphology is also discussed. A 100% increase in the lifetime of air-sensitive OLEDs is demonstrated upon encapsulation of the devices with parylene-N film deposited by this technique. Potential advantages of this approach include increased material utilization efficiency, localized conformal coating capabilities, and an apparatus that is compact, inexpensive, and does not require vacuum.
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INTRODUCTION Parylene and its functional derivatives have a variety of different applications, including surface passivation of biomedical and implantable devices, surface functionalization for biosensors, generation of microscale patterns for neural probes,1,2 as a dielectric in organic thin-film transistors (OTFTs),3 and as a selectively permeable membrane for OTFT-based chemical sensors.4 Parylene polymer films are transparent and block the diffusion of moisture, enabling their use to encapsulate airsensitive devices, such as OLEDs, OTFTs, and solar cells.5,6 While more permeable than inorganic films such as SiOxNy and Al2O3,7 parylene can be vapor deposited as a conformal and flexible coating at low substrate temperatures. Parylene (a trade name for poly(p-xylylene)-based polymers) is available as parylene-N, -C, -D, -AF4, and so forth, where all but the first are functionalized derivatives of poly(p-xylylene). Over the years, numerous scientific research studies have reported on paralyne films, including the morphology and crystal structure of films deposited at various substrate temperatures,8−11 thermal properties,12−14 poly(p-xylylene) properties as gas barriers,5,6,15−17 mechanical and electrical properties of the films,12,18,19 mechanism of the polymerization process,12,20−22 and various applications of the parylene polymers.1−6,23,24 The most popular approach for depositing parylene films1,25 is chemical vapor deposition (CVD) by the Gorham method, in which the dimer first evaporates and diffuses through a © XXXX American Chemical Society
pyrolysis zone or chamber, where it breaks up into monomers that then diffuse to a cooled substrate where polymerization occurs.12,26,27 Often, three separate reactor stages or chambers are employed with each independently pumped to enable pressure-driven vapor transport from the sublimation zone to the polymerization zone. A carrier gas may be employed to assist vapor transport, enhancing control of the deposition rate.13,20 Traditional parylene coating systems are often limited in many applications by relatively low materials utilization efficiency and parasitic coating of the apparatus, large footprint, and capital cost. Film patterning in such traditional coating systems requires the use of a shadow-mask that contacts the substrate, presenting additional challenges for processing. Attempting to circumvent the limitations of the Gorham method, other parylene growth methods have been developed, including lithographic patterning28,29 and micromolding.30 Halpern et al.31 developed “H atom-assisted jet vapor deposition of parylene”, which localized the deposition of parylene-C and -N films onto cryocooled substrates with the aid of a hydrogen plasma jet in vacuum. Here, we demonstrate a remarkably simple way to deposit high quality films in air with increased materials utilization efficiency and direct additive patterning capability in a single Received: March 11, 2015 Revised: July 22, 2015
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DOI: 10.1021/acs.macromol.5b00505 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (A) Schematic and (C) photograph of the ultra compact parylene CVJD apparatus with coaxial glass tubes for carrier gas and guard jet flow. (B) Temperature profile along the nozzle axis specifying the spatial distribution of evaporation, pyrolysis, and polymerization zones. A water chiller cools the substrate, and the evaporation and pyrolysis zones are independently heated to maintain the required temperatures (dashed lines). (D) Patterns of parylene drawn on Si- and Al-coated glass and glass substrates, without the use of shadow masks, shown under UV illumination. Each deposit is
