Film Formation in Hexafluoropropylene Plasmas - American Chemical

Chapter 27

Film Formation in Hexafluoropropylene Plasmas Downloaded by UNIV MASSACHUSETTS AMHERST on October 11, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch027

M. S. Silverstein and R. Chen Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32 000, Israel

The gas phase dominated plasma polymerization (PP) of hexafluoropropylene (HFP) produces 0.2 to 0.5 μm particles and 1 μm spherical particle agglomerates which are all incorporated into a transparent and highly adhering PPHFP film. A n energy ratio plasma parameter, E, related to the plasma energy and strongly dependent on flow rate (or residence time) was derived. The maximum and plateau in deposition were successfully described when polymerization and etching were exponentially related to E. The chemical structure of the amorphous, crosslinked PPHFP consists largely of similar amounts of C*-CF, CF, C F and C F units. Plasma etching was inhibited by fluorine scavenging and H F formation on the addition of hydrogen to the feed. The deposition and coalescence of smaller particles into a smoother and more uniform surface on adding nitrogen reflects the incorporation of nitrogen into the polymer, the increase in the polar component of surface energy and, perhaps, the decrease in dominance of gas phase versus surface polymerization. The low P P ( N / H F P ) electrical breakdown strength may be attributed to an alternate conduction mechanism in the relatively polar plasma fluoropolymer. 2

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Thin fluoropolymer films can have many advantages including a low coefficient of friction, a low surface energy, thermal stability, biocompatibility, and chemical resistance. Unfortunately, the techniques used to deposit thin films of conventional fluoropolymers can involve etchants, solvents and temperatures that limit their applicability. Plasma polymerization is an ambient temperature, solvent-free process that can be used to deposit highly adhering, pinhole-free and crosslinked thin fluoropolymer films from a variety of compounds including those not polymerizable by standard techniques (7-3). These films would have potential applications in the microelectronics, biomedical, membrane, aerospace and automotive fields (4-9).

0097-6156/96/0648-0451$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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FILM FORMATION IN WATERBORNE COATINGS

In plasma polymerization the neutral species entering the reactor are fragmented into reactive species as energy is transferred by the electrons in the low pressure plasma environment. The substrate molecules are activated by exposure to the plasma environment and chemical bonds are formed with the polymerizing reactive monomer fragments. The structure and properties of plasma polymer (PP) thin films depend on the plasma environment, specifically the pressure (P), monomer mass flow rate (F ), power (W ) and feed composition, all of which interact in a complex manner. The plasma polymerization of fluorocarbons is even more complex than that of hydrocarbons since the rate of the competing plasma etching reaction is more significant. Hexafluoropropylene (HFP) (CF =CF-CF ) is an unsaturated fluorocarbon which tends to polymerize in a plasma rather than etch (10-12 ). The objective of this research is to develop insight into the formation of PPHFP films synthesized under various plasma environments (W, P, F and feed composition).

Downloaded by UNIV MASSACHUSETTS AMHERST on October 11, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch027

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Experimental Plasma Polymerization. The central part of the plasma reactor system illustrated schematically in Figure 1 is a parallel-plate electrode radio frequency (13.56 MHz) plasma reactor (March Instruments Jupiter ΙΠ) with an automatic impedance matching network. The anodized aluminum parallel-plate electrodes were 3 cm apart. To increase anisotropy the bottom electrode (21 cm diameter) is smaller, producing a negative D C bias, and is encircled by a ceramic ring, concentrating the plasma. The reactor could be evacuated to 2.5 Pa with a vacuum pump (Alcatel AC-2012) and the temperature of the anodized aluminum parallelplate electrodes was maintained at 20°C with a circulating liquid cooler (Neslab RTE-100). One inlet rotameter was calibrated for HFP (Matheson) while the other inlet rotameter was calibrated for argon, nitrogen and hydrogen. Glass optical microscopy slides were used as substrates and were centered on the bottom electrode. A detailed description of the plasma polymerization methodology is found elsewhere (10-12 ). The molar flow rate (F ) is commonly expressed in units of ml/min at STP (seem). The basic plasma conditions were 100 W, 187 Pa and 18.3 seem HFP. Only one plasma parameter, the power, the HFP flow rate or the amount of additional gas, was varied in each set of experiments. The PP films based on HFP and an additional gas will be referred to as PP(gas/HFP). The molar flow rate ratio (F R), the ratio of the molar flow rates of gas added to HFP, is used to describe the PP(gas/HFP) feed composition. The rate of deposition (R ) was calculated both on a mass and thickness basis. The thickness was measured in several different places using a stylus apparatus with an accuracy of 0.05 μπι (Tencor α-step-100) yielding an average deposition rate in terms of thickness (μιη/πιίη). The mass gained was measured with an accuracy of 0.1 mg (Mettler AE-163 microbalance) yielding the deposition rate in terms of mass per area (g/(m min)). n

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Chemical Structure and Topography. The chemical structure of PPHFP was measured using two complementary methods. Electron spectroscopy for chemical analysis (ESCA) was performed on the PP coated slides using an ΑΙ K X-ray source, a 40° angle of incidence and an energy of 25 eV for high resolution (Perkin Elmer Physical Electronics 555 ESCA/Auger). The atomic concentrations were determined with an accuracy of 1% in a low resolution scan and the nature of the bonds with carbon in a high resolution C scan. The C spectra, described a

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In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


27. SILVERSTEIN & CHEN

453

Film Formation in HFP Plasmas

as the sum of several mixed Gaussian/Lorentzian peaks, were deconvoluted in order to more accurately describe the contributions of the different bonds to the overall spectrum (72 ). Prior to deconvolution the background was subtracted from the spectrum using one iteration of the Shirley method (13 ). Transmission Fourier transform infrared spectroscopy (FTIR) (Unicam Mattson 1000 FTIR) provided chemical information that reflected the bulk PPHFP more accurately than the surface specific ESCA. In general, the PPHFP films for FTIR characterization were separated from the substrates by soaking in acetone overnight and drying at room temperature for several days. The similarity of the resulting spectra to those from PPHFP deposited on K B r pellets demonstrated that artifacts were not introduced using this methodology. Properties. The influence of the feed flow rate ratio on the dispersive and polar components of surface energy (γ* and γρ, respectively) and their sum, the total surface energy (γ), was examined. A contact angle (Θ) apparatus with an accuracy of 1° (Kernco) and an advancing droplet technique were used (14). The relationship between the components of the surface energy and contact angle is expressed by:

(1) where s represents solid and Iv represents liquid in the presence of its vapor. Equation 1 has two unknowns, y and y , and can be solved by measuring the contact angles of two different liquids (11 ). Bromonaphthalene (yi = 44.6 mN/m, γ = 0 mN/m) and distilled water (y = 22.0 mN/m, γ = 50.2 mN/m) were used {14). The breakdown strengths of the PP films were determined using the capacitor circuit illustrated schematically in Figure 2. The capacitor consists of a PP film on a metallic substrate which serves as one electrode. The metallic substrate was a thin copper sheet that had been polished and then washed with hydrochloric acid to remove the oxide layer. The other electrode was formed by evaporating 0.1 μπι thick, 2 mm diameter gold dots on the PP film. Over forty such capacitors were tested for each film using the 'short time test', ASTM-D149. The voltage was ramped from zero at 3.6 V/s (Keithley-487 picoammeter voltage source) and the breakdown voltage was taken as the voltage at which the current jumped by two orders of magnitude (usually reaching 0.1 mA). The breakdown strength is the breakdown voltage divided by the film thickness. As breakdown strength tends to increase with decreasing film thickness the plasma polymerization time was adjusted to yield films of a similar thickness (approximately 2 μπι). The breakdown strength results were interpreted using Weibull statistics. The cumulative failure (F(x)) at a given breakdown strength (x) is: d

p

s

s

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v

ρ

1ν

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(2) where σ is the mean breakdown strength and b is the Weibull shape parameter associated with the distribution breadth.


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Gas


HFP

Vacuum Pump

Timer Power Meter

RF Power Supply

Pressure Gauge

Flow Meters

Heating Cooling

Figure 1. Schematic illustration of the parallel-plate plasma reactor.

Figure 2. Schematic illustration of the circuit for breakdown strength measurements.


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Results and Discussion Deposition. The basic PPHFP synthesis conditions were 100 W, 187 Pa and 18.3 seem HFP. Plasma exposure time has little effect on deposition rate, as seen from the linear variation of thickness and mass per unit area with deposition time in Figure 3. The deposition rates calculated from the slopes in Figure 3 are 0.35 μηι/min or 0.77 g/(m min). The ratio of the two deposition rates yields a PPHFP density of 2.2 g/cm . In spite of the differences in structure, the density of the crosslinked amorphous PPHFP is similar to the densities of commercial semicrystalline fluoropolymers such as polytetrafluoroethylene (PTFE) and the copolymer of TFE and HFP, fluorinated ethylene-propylene copolymer (FEP). The variation of the deposition rate with W/F (a parameter commonly used to describe the energy to mass ratio in plasma polymerization) is seen in Figure 4. In polymerizations at a constant flow rate the deposition rate increases rapidly at low powers, reaches a maximum and then decreases to a plateau at high powers. The transition from an energy-starved plasma to a monomer-starved plasma occurs near this maximum (10). In polymerizations at a constant power the deposition rate increases with increasing flow rate (decreasing W/F ) reflecting the effect of increasing monomer throughput to a monomer-starved polymerization. The strong dependence of the deposition rate on flow rate, even at a constant W/F , is seen in Figure 4. One convenient expression that can be used to take the strong dependence on flow rate into account is the kinetic energy associated with flow. The kinetic energy per unit mass of the flowing monomer (E , J/kg) (equation 3), calculated from the reactor cross-sectional area (Λ) and the volumetric flow rate (F ), is inversely proportional to the residence time squared (12 ). The ratio of W/F to E (equation 4) yields a dimensionless plasma parameter (E ) that is proportional to W/F (based on the flow rate relationships in equation 5 where M is the monomer molecular weight, Τ is the temperature and R is the gas constant). This energy ratio is more strongly dependent on flow rate than simply W/F and thus may provide a better means of normalizing the influences of power and flow rate (12 ). 2


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The deposition rate is simply the difference between the rates of polymerization (R ) and etching (/ζ). The dependence of the deposition rate on the plasma environment reflects the effects of the environment on both the polymerization and etching reactions. The ratios of the rates of polymerization, etching and deposition to the monomer flow rate can yield a measure of the polymerization, etching and deposition efficiencies (C C and Q , respectively). Equation 6 expresses an exponential relationship that can be used to describe the p

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Time, m i n

Figure 3. Variation of PPHFP film thickness and mass per unit area with deposition time.

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W/F , MJ/kg m

Figure 4. Dependence of PPHFP deposition rate on W/F . m


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dependence of the polymerization and etching efficiencies (where i=p for polymerization and i-e for etching) on the energy ratio plasma parameter (is, from equation 4) (72). The relationship in equation 6 is characterized by a plateau at high Ε (C ) and an activation energy ratio (E ). The deposition efficiency can therefore be expressed by equation 7. ioo

ai


(6)

The very different sets of data in Figure 4 are united in one master set for the variation of deposition efficiency with Ε in Figure 5. The plateau at high Ε observed in Figure 5 (C ) is therefore the difference between the polymerization efficiency and etching efficiency plateaux (equation 8). The curve in Figure 5 was generated from equations 7 and 8 with C = 3 n r taken from the experimental data and E , E , and C as unknowns. The validity of equation 7 is supported by the goodfitof the curve in Figure 5 to the experimental data. This attests to the suitability of Ε for describing the plasma environment. A suitable fit to the data results when E^ (7.0 X 10 ) is less than one half of Ε (1.6 X 10 ). doo

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(8) The sharp increase in deposition at low Ε indicates that polymerization increases more rapidly than etching (Ε

Film Formation in Hexafluoropropylene Plasmas - American Chemical

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