Kenneth K. S. Lau

Hot-wire chemical vapor deposition (HWCVD) of fluorocarbon and organosilicon thin films

Abstract HWCVD affords the capability to synthesize fluorocarbon and organosilicon thin films. These two classes of materials are of interest for a wide range of applications, including low dielectric constant coatings for microelectronic interconnection, ‘dry’ photoresists, directly patternable dielectrics for lithographic production of integrated circuits, insulating biomaterials for implantable devices with complex topologies and small dimensions, low friction coatings, and semipermeable membranes. HWCVD from hexafluoropropylene oxide (C 3 F 6 O) dramatically reduces cross-link and defect concentrations in fluorocarbon coatings, producing films which are spectroscopically indistinguishable from bulk polytetrafluoroethylene (PTFE, Teflon™). Organosilicon films can be deposited from cyclic precursors such as octamethylcyclotetrasiloxane (D 4 ) at extremely high rates (>2 μm/min) by HWCVD. The bonding structure of HWCVD organosilicon films is substantially different from both their plasma enhanced CVD (PECVD) counterparts and bulk siloxane polymers, such as poly(dimethysiloxane) (PDMS).

Designing polymer surfaces via vapor deposition

Chemical Vapor Deposition (CVD) methods significantly augment the capabilities of traditional surface modification techniques for designing polymeric surfaces. In CVD polymerization, the monomer(s) are delivered to the surface through the vapor phase and then undergo simultaneous polymerization and thin film formation. By eliminating the need to dissolve macromolecules, CVD enables insoluble polymers to be coated and prevents solvent damage to the substrate. Since de-wetting and surface tension effects are absent, CVD coatings conform to the geometry of the underlying substrate. Hence, CVD polymers can be readily applied to virtually any substrate: organic, inorganic, rigid, flexible, planar, three-dimensional, dense, or porous. CVD methods integrate readily with other vacuum processes used to fabricate patterned surfaces and devices. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, thickness control, and the synthesis of films with graded composition. This article focuses on two CVD polymerization methods that closely translate solution chemistry to vapor deposition; initiated CVD and oxidative CVD. The basic concepts underlying these methods and the resultant advantages over other thin film coating techniques are described, along with selected applications where CVD polymers are an enabling technology.

Pore Filling of Nanostructured Electrodes in Dye Sensitized Solar Cells by Initiated Chemical Vapor Deposition

The dye sensitized solar cell (DSSC) operation depends on a liquid electrolyte. To achieve better performance, the liquid should be replaced with a solid or gel electrolyte, e.g., polymers. Here, we demonstrate initiated chemical vapor deposition as an effective liquid-free means for in situ polymerization and pore filling. We achieve complete pore filling of 12 μm thick titania resulting in enhanced cell performance that is attributed to reduced charge recombination at the electrolyte-electrode interface.

Molecular Design of Fluorocarbon Film Architecture by Pulsed Plasma Enhanced and Pyrolytic Chemical Vapor Deposition

Pulsed plasma enhanced chemical vapor deposition (pulsed PECVD) and pyrolytic chemical vapor deposition (pyrolyric CVD) of fluorocarbon films from hexafluoropropylene oxide (HFPO) have demonstrated the ability to molecularly design film architecture. Film structures ranging from highly amorphous crosslinked matrices to linear perfluoroalkyl chain crystallites can be established by reducing the modulation frequency of plasma discharge in plasma activated deposition and by eventually shifting mechanistically from an electrically activated to a thermally activated process. X-ray photoelectron spectroscopy (XPS) showed CF2 content increasing from 39–65 mol%. Fourier transform infrared spectroscopy (FTIR) showed an increasing resolution between the symmetric and asymmetric CF2 stretches, and a reduction in the intensity of the amorphous PTFE and CF3 bands. High-resolution solid-state 19F nuclear magnetic resonance spectroscopy (NMR) revealed an increasing CF2CF2CF2 character, with the pyrolytic CVD film much like bulk poly(tetrafluoroethylene) (PTFE). X-ray diffraction (XRD) patterns evidenced an increase in crystallinity, with the pyrolytic CVD film showing a characteristic peak at 2θ = 18° representing the (100) plane of the hexagonal structure of crystalline PTFE above 19°C.

Mechanical properties of ultrahigh molecular weight PHEMA hydrogels synthesized using initiated chemical vapor deposition.

In this work, poly(2-hydroxyethyl methacrylate) (PHEMA), a widely used hydrogel, is synthesized using initiated chemical vapor deposition (iCVD), a one-step surface polymerization that does not use any solvents. iCVD synthesis is capable of producing linear stoichiometric polymers that are free from entrained unreacted monomer or solvent and, thus, do not require additional purification steps. The resulting films, therefore, are found to be noncytotoxic and also have low nonspecific protein adsorption. The kinetics of iCVD polymerization are tuned so as to achieve rapid deposition rates ( approximately 1.5 microm/min), which in turn yield ultrahigh molecular weight polymer films that are mechanically robust with good water transport and swellability. The films have an extremely high degree of physical chain entanglement giving rise to high tensile modulus and storage modulus without the need for chemical cross-linking that compromises hydrophilicity.

Pulsed plasma enhanced and hot filament chemical vapor deposition of fluorocarbon films

Abstract Fluorocarbon films from pulsed plasma enhanced chemical vapor deposition (PPECVD) and hot filament chemical vapor deposition (HFCVD) show a greater range in composition and structure compared to films from conventional CVD processes. Films were deposited using hexafluoropropylene oxide (HFPO), 1,1,2,2-tetrafluoroethane (HFC-134) and difluoromethane (HFC-32) as the feed gases. Film characterization was performed through high resolution solid-state 19 F and 13 C nuclear magnetic resonance (NMR) techniques. Increasing pulse off-time during HFPO PPECVD resulted in films with more linear CF2 character and reduced the amount of cross-linking/branching, attributed to CF2 chain propagation dominating during the off-time. HFC PPECVD films contained significantly less fluorine and more of carbon unsaturation, attributed to plasma hydrogen scavenging of fluorine to form hydrogen fluoride. Switching from PPECVD to HFCVD with HFPO as the feed gas resulted in films resembling bulk poly(tetrafluoroethylene) (PTFE), as a result of clean thermal breakdown of HFPO to form polymerizing CF2 radicals. Isothermal annealing of PPECVD films revealed two different thermal decomposition pathways: one which involved CF3 loss in more cross-linked films, and one which involved oligomer desorption/chain unzipping in films with a substantial linear CF2 chain component.

Thickness-dependent crossover from charge- to strain-mediated magnetoelectric coupling in ferromagnetic/piezoelectric oxide heterostructures.

Magnetoelectric oxide heterostructures are proposed active layers for spintronic memory and logic devices, where information is conveyed through spin transport in the solid state. Incomplete theories of the coupling between local strain, charge, and magnetic order have limited their deployment into new information and communication technologies. In this study, we report direct, local measurements of strain- and charge-mediated magnetization changes in the La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 system using spatially resolved characterization techniques in both real and reciprocal space. Polarized neutron reflectometry reveals a graded magnetization that results from both local structural distortions and interfacial screening of bound surface charge from the adjacent ferroelectric. Density functional theory calculations support the experimental observation that strain locally suppresses the magnetization through a change in the Mn-eg orbital polarization. We suggest that this local coupling and magnetization suppression may be tuned by controlling the manganite and ferroelectric layer thicknesses, with direct implications for device applications.

Variable angle spectroscopic ellipsometry of fluorocarbon films from hot filament chemical vapor deposition

Hot filament chemical vapor deposition using hexafluoropropylene oxide as the precursor gas yielded two sets of fluorocarbon films, one with varying OH/COOH content and the other with varying grain aspect ratio, as revealed by Fourier transform infrared spectroscopy and atomic force microscopy, respectively. Variable angle spectroscopic ellipsometry was performed to derive film thickness and film optical constants. A uniaxial Cauchy–Urbach dispersion layer, with separate in-plane and out-of-plane dispersion parameters, was found to realistically describe the films. Derived film thickness agreed well with profilometry measurements. Anisotropy in index of refraction n and extinction coefficient k was on the order of 10−2 and 10−5 to 10−3, respectively. The relationship between the complex index of refraction and the dielectric function allowed the optical dielectric constant e1 to be calculated. The presence of OH did not affect the film optical dielectric constant significantly. Even though OH/COOH groups ...

Full-Field Dynamic Characterization of Superhydrophobic Condensation on Biotemplated Nanostructured Surfaces

While superhydrophobic nanostructured surfaces have been shown to promote condensation heat transfer, the successful implementation of these coatings relies on the development of scalable manufacturing strategies as well as continued research into the fundamental physical mechanisms of enhancement. This work demonstrates the fabrication and characterization of superhydrophobic coatings using a simple scalable nanofabrication technique based on self-assembly of the Tobacco mosaic virus (TMV) combined with initiated chemical vapor deposition. TMV biotemplating is compatible with a wide range of surface materials and applicable over large areas and complex geometries without the use of any power or heat. The virus-structured coatings fabricated here are macroscopically superhydrophobic (contact angle >170°) and have been characterized using environmental electron scanning microscopy showing sustained and robust coalescence-induced ejection of condensate droplets. Additionally, full-field dynamic characterization of these surfaces during condensation in the presence of noncondensable gases is reported. This technique uses optical microscopy combined with image processing algorithms to track the wetting and growth dynamics of 100s to 1000s of microscale condensate droplets simultaneously. Using this approach, over 3 million independent measurements of droplet size have been used to characterize global heat transfer performance as a function of nucleation site density, coalescence length, and the apparent wetted surface area during dynamic loading. Additionally, the history and behavior of individual nucleation sites, including coalescence events, has been characterized. This work elucidates the nature of superhydrophobic condensation and its enhancement, including the role of nucleation site density during transient operation.

Enhanced charge storage of ultrathin polythiophene films within porous nanostructures.

In a single step polymerization and coating, oxidative chemical vapor deposition (oCVD) has been used to synthesize unsubstituted polythiophene. Coatings have been conformally coated within porous nanostructures of anodized aluminum oxide, titanium dioxide, and activated carbon. Significant enhancement in charge capacity has been found with ultrathin polythiophene coatings that preserve the surface area and pore space of the nanostructures. Pseudocapacitors consisting of ultrathin polythiophene coated within activated carbon yielded increases of 50 and 250% in specific and volumetric capacitance compared with bare activated carbon. Devices were stable up to the 5000 cycles tested with only a 10% decrease in capacitance.