Scott J. Limb

Growth of fluorocarbon polymer thin films with high CF2 fractions and low dangling bond concentrations by thermal chemical vapor deposition

Thermal chemical vapor deposition was used to deposit fluorocarbon films with chemical resemblance to bulk polytetrafluoroethylene. X‐ray photoelectron spectroscopy revealed that the films deposited from thermal decomposition of hexafluoropropylene oxide had fluorine to carbon ratios of 2.0 and CF2 fractions of 90% along with 10% of CF3 and CF moieties. Electron spin resonance results found the dangling bond density to be 1.2×1018 spins/cm3, low compared to conventional plasma polymerized films. Low dangling bond densities were achieved by using a clean source of CF2 in the absence of plasma source.

FLEXIBLE FLUOROCARBON WIRE COATINGS BY PULSED PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

A method to coat thin wires, 25 μm in diameter, with fluorocarbon material that is flexible and conformal has been achieved using pulsed plasma enhanced chemical vapor deposition (PECVD). This process enables the chemical composition of the films to be tailored in order to achieve similar stoichiometry and chemical composition to bulk polytetrafluoroethylene [PTFE, (CF2)n, Teflon™]. The deposited film had a F/C ratio of 1.9 and a CF2 fraction of 65%. In comparison, a film deposited under continuously applied power had a F/C ratio of only 1.6 and a CF2 fraction of 32% and was found to form brittle wire coatings. The flexibility of the pulsed PECVD film was consistent with its connectivity number of 2.1 corresponding to an underconstrained, flexible material. In addition, the connectivity number for the brittle film was 2.5, consistent with an overconstrained brittle network.

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).

Pulsed plasma-enhanced chemical vapor deposition from hexafluoropropylene oxide: Film composition study

Films deposited using pulsed plasma-enhanced chemical vapor deposition (PECVD) from hexafluoropropylene oxide (HFPO) were investigated by X-ray photoelectron spectroscopy (XPS). As compared to continuous rf PECVD, pulsed excitation increases the CF2 fraction in the film. Film composition was determined as a function of plasma processing conditions including on-time, off-time, pressure, flow rate, substrate temperature, electrode spacing, substrate potential, and input power. Varying the on–off pulsing cycle resulted in compositional control of the deposited films. At a low duty cycle [ton/(ton + toff)], up to 70% CF2 could be incorporated into the film. The input gas, HFPO, may facilitate greater CF2 incorporation into the films as this gas thermally decomposes into a difluorocarbene. Both absolute on-time and off-time, rather than simply duty cycle, are important parameters for determining film composition. A simple model was developed to describe the experimentally determined variation %CF2 as a function of substrate temperature and off-time. This model accounts for changes in film composition due to plasma-surface modification and differences in gas-phase chemistry. The model suggests that surface modification by the plasma is the dominant factor only for long on-times or for low deposition rates. However, the gas-phase concentration of CF2 relative to other film-forming species is typically the controlling factor under conditions which achieve the high %CF2 in the film. The gas-phase composition will depend on both abslute on-time and off-time, rather than simply on the duty cycle.

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.

Electron spin resonance of pulsed plasma-enhanced chemical vapor deposited fluorocarbon films

Pulsed-rf excitation of hexafluoropropylene oxide has been used to deposit poly(tetrafluoroethylene)-like thin films. Films were deposited at pulse rates of 10/20, 10/50, 10/200, and 10/400u2009ms on/ms off and analyzed using electron spin resonance spectroscopy (ESR). All four films produced similar broad ESR spectra, with an average width at maximum slope of ∼60u2009G and a g value of 2.0045. The number of free electrons in a sample decreased with increasing pulse off time. This behavior can be modeled by the reaction of a free radical with a gas species, assuming that free radicals are generated only during the pulse on time.

Preliminary Electrical Characterization of Pulsed-Plasma Enhanced Chemical Vapor Deposited Teflon-like Thin Films

Pulsed-rf excitation of hexafluoropropylene oxide (HFPO) has been used to deposit films with chemical compositions similar to poly(tetrafluoroethylene) (PTFE). Films were deposited at pulse cycles of 10/20, 10/50, 10/200, and 10/400 ms on/ms off and analyzed using electron spin resonance spectroscopy (ESR). All four films produced similar broad ESR spectra, with an average width at maximum slope of ˜60 G and a g-value of 2.0045. The concentration of free electrons in a sample decreased with increasing pulse off time. This behavior can be modeled by the reaction of a free radical with a gas species, assuming that free radicals are generated only during the pulse on time. The films dielectric constants were found to decrease from 1.99 to 1.95 for pulse off times increasing from 20 to 400 ms.

Polyethylene Glycol Deposition Techniques for Antifouling Surfaces: Using Antistiction to Conserve Bioparticles for Recovery and Analysis

For bioparticle analysis within microfluidic devices, there is a risk of analytes adhering to surfaces, thereby compromising particle manipulation and recovery. To address this we have implemented polyethylene glycol (PEG)-type coatings by self-assembled monolayer (SAM) and plasma-polymerizing deposition techniques. Silane chemistry is used to deposit SAM films directly onto silicon dioxide surfaces, and in a special case for which a composite of metallic arrays and SiON within our MEMS device must be coated, an ultrathin layer of sputtered Si is added before the SAM protocol. We have also deposited PEG-like tetraglyme by plasma-polymerization onto substrates using an onsite-built reactor to provide antistiction treatment to all surfaces, including those that do not have chemical compatibility for the SAM technique. Three tests demonstrate the effectiveness of our surface treatments: static exposure to microbial suspensions, bioparticle transport across MEMS traveling-wave (TW) arrays, and bioparticle recovery from a circulating flow device. Our static microbial assays show significant reduction in B. thuringiensis adhesion to both the SAM and plasma-polymerized coatings and reduction in B. globigii adhesion to the plasma-polymerized coating. Our TW arrays, when coated with either the SAM or plasma-polymerized PEG film, are effective at reducing adhesion to polystyrene beads as well as both Bacillus species. Lastly, our bioparticle recovery, as gauged by spectrophotometry, improves by as much as one order of magnitude when we coat flow chambers with our plasma-polymerized film.