Charge stability of pulsed-laser deposited polytetrafluoroethylene ...

APPLIED PHYSICS LETTERS

VOLUME 73, NUMBER 20

16 NOVEMBER 1998

Charge stability of pulsed-laser deposited polytetrafluoroethylene film electrets Reinhard Schwo¨diauer, Simona Bauer-Gogonea,a) Siegfried Bauer,a) Johannes Heitz, Enno Arenholz, and Dieter Ba¨uerle Angewandte Physik, Johannes Kepler Universita¨t Linz, A-4040 Linz, Austria

~Received 8 July 1998; accepted for publication 17 September 1998! Pulsed-laser deposited ~PLD! polytetrafluoroethylene ~Teflon-PTFE! films from press-sintered powder targets are found to be highly crystalline, with spherulite sizes adjustable over more than one order of magnitude by suitable thermal annealing. Films with large spherulites show an excellent charge stability, comparable and even superior to commercially available Teflon-PTFE foils. PLD-PTFE enlarges the family of Teflon materials and may thus become interesting for potential miniaturized electret devices. © 1998 American Institute of Physics. @S0003-6951~98!04646-4#

Polytetrafluorethylene ~Teflon-PTFE! has been largely considered for high-performance electronic1 and electret2 applications due to its excellent thermal and chemical resistance, low dielectric constant, small dielectric loss, and excellent charge-storage capability. However, the increasing demand for miniaturized device structures raises new problems, for example, the preparation and micromachining of thin Teflon-PTFE films on substrates. The deposition of high-quality Teflon-PTFE films is a technologically challenging task, as the remarkable processing difficulties of PTFE make conventional preparation techniques such as spin coating not feasible.3 Several techniques have been reported for the deposition of Teflon or Teflon-like fluoropolymer films. In backplate electret microphones, for example, Teflon PTFE or copolymers of tetrafluorethylene and hexafluoropropylene ~Teflon-FEP! films with thicknesses on the order of 10 mm are stuck on the substrate by a heat sealing technique,4,5 which is difficult to implement in an automatic process. Other techniques, more common to industry, like RF- and ion-beam sputtering,6,7 electron-beam8 and ionization-assisted evaporation,9 plasma polymerization,10 and standard chemical-vapor deposition,11 have been reported for the preparation of Teflon-like fluoropolymer films. Although high-quality films with smooth surfaces could be demonstrated on relatively large areas, the dielectric loss was often found to be only moderate in comparison to Teflon PTFE. One reason for the poor electrical properties reported seems to be the more or less gradual deviations of the chemical structure of the resulting fluorocarbon films from Teflon PTFE. These deviations are, for example, due to impurities contamination, an extensive lowering of the molecular weight, oxidation, fluorine deficiency, etc. In plasma polymerization ~PP!, for example, the significant amount of dangling bonds makes the films extremely sensitive to oxidation.12,13 Even PP films with a high fluorine concentration show only moderate charge stability as compared to Teflon PTFE.14 Pulsed-laser deposition ~PLD! is an established tool for thin-film fabrication of a wide variety of compounds and

alloys.15 One of the questions is whether PLD films can be produced without possessing a structure significantly different from that of the target material. Since the plasma plume observed with PTFE targets contains radicals, which were identified as the monomers also present in the PP repolymerization process16,17 ~where the highly cross-linked PP films show severe structural differences from PTFE!, one may also expect in PTFE-PLD films structural deviations from the target material. For example, films prepared from polished bulk PTFE pellet targets consist mainly of polymer chain segments in amorphous regions, which only partly recrystallize at elevated substrate temperatures.17 The surface of such films is rough and contains many particulates. Both dielectric loss18 and charge stability are poor compared to PTFE. Recently, it has been demonstrated that highly crystalline PTFE films can be prepared by PLD from a suitably prepared press-sintered PTFE powder target.19 In this case, it has been suggested that PTFE grains are laser-transferred from the target to the substrate with subsequent melting and crystallization to a continuous film. Dielectric and IR investigations have confirmed that such PLD films exhibit the same structural phase transitions of the helical chain as commercially available Teflon PTFE.18 However, the most important feature to be compared is the charge stability, which has made Teflon PTFE interesting for many electret applications. In this letter, the charge stability of PLD films will be investigated in comparison to commercially available Teflon-PTFE foils. Most notably, we have found a surprisingly strong influence of the film morphology on the chargestorage properties. Figure 1 shows optical polarization microscope and scanning electron microscope ~SEM! micrographs of the surface morphology of the laser-deposited PTFE samples investigated. For the preparation of the samples, KrF-laser light has been used ~l5248 nm, F50.5–6 J/cm2 , t l525 ns, and repetition rate from 1 to 10 Hz!. The target was prepared from PTFE powder ~supplied by Goodfellow Corporation, molecular weight of 53104 – 43105 , grain sizes of 6–9mm!, which was pressured at 3.83108 N/m2 and subsequently annealed at 275 °C for 24 h. PLD was performed at substrate temperatures of 355 °C within an Ar atmosphere of 0.3 mbar.

a!

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FIG. 1. SEM ~a!, ~c! and polarization optical micrographs ~b!, ~d! of two PLD films after different postannealing procedures. ~a!, ~b!: film quenched from 340 °C to room temperature within 10 s. Only small spherulites are observed, with sizes comparable to the grain size of the powder target. ~c!, ~d!: film cooled at a rate of 1 °C/min through the crystallization region. The SEM micrograph shows part of a single spherulite, the polarization optical micrograph demonstrates spherulite sizes up to 0.5 mm.

The substrate mainly employed was ~100! Si, which was partly metallized with a 100 nm thick Al electrode for the charge stability investigations. Smooth, pinhole-free films with typical thicknesses of 5–15 mm were obtained from the powder target by annealing the film at temperatures up to 500 °C and subsequent cooling at a rate of 10 °C per minute. Crystallization occurs within a temperature interval between 310 and 280 °C. In this temperature region different cooling rates were used in order to vary the size and the amount of spherulites. Figures 1~a! and 1~b! show the surface morphology of a film quenched from 340 °C to room temperature within 10 s. Only small spherulites with dimensions comparable to the grain size of the powder are observed in the SEM picture @Fig. 1~a!# and the polarization optical micrograph @Fig. 1~b!#. Figure 1~c! shows a SEM micrograph of a highly crystalline polymer film, prepared by cooling at 1 °C/min through the crystallization region. Only a part of a single spherulite is observed in a micrograph recorded with the same magnification as in Fig. 1~a!. The distribution of spherulite sizes is shown in Fig. 1~d!. The largest spherulites have diameters up to 0.5 mm. Thus, postannealing subsequent to film deposition is an important factor for the control of the surface morphology of PLD films. The film thickness remains almost unchanged during the postannealing process, in contrast to films deposited from bulk targets. This indicates that these films have not incorporated large amounts of low molecular weight polymer fragments. Annealing at 500 °C is really necessary in order to obtain large spherulites. Experiments with annealing temperatures in the range of 350–500 °C, with the same cooling rates as described before, showed that the spherulite dimensions increase with increasing annealing temperatures. The suitability of deposited films as electret materials has been investigated by measuring the thermally stimulated voltage decay of corona charged PLD samples. For comparison, commercially available 25 mm thick PTFE foils supplied by Goodfellow served as a reference. Corona charging was performed with a corona-triode grid charger under normal atmospheric conditions. The corona voltage was 67 kV

Schwo¨diauer et al.

FIG. 2. Thermally stimulated surface potential decay for PTFE foils ~s, d! and the different PLD-PTFE films of Fig. 1. ~L,l!: quenched film @Figs. 1~a! and 1~b!#, and ~n, m!: slowly cooled film @Figs. 1~c! and 1~d!#; closed symbols denote negative, open symbols positive surface voltages, respectively. Note the excellent thermal stability of surface charges in PLD PTFE with large spherulites.

for positive and negative charges, respectively. A grid voltage of up to 6300 V was used in order to limit the initial surface potential, the charging was performed for all samples for 5 min at room temperature. The 10 mm thick PLD samples and the PTFE foil were charged to an initial surface charge density of 3.831028 and 1.431028 C/cm2 , respectively, for both positive and negative charging. The thermally stimulated voltage decay was measured in an arrangement consisting of the electrostatic probe, an air gap, and the electret films ~one side metallized PTFE foil or PLD films deposited on metallized Si substrates!. The samples were mounted on a small heating stage. The temperature-dependent surface voltage has been recorded with a vibrating electrode method with field compensation ~Isoprobe 244 electrostatic voltmeter from Monroe!. Figure 2 shows the surface voltage decay at a heating rate of 4 °C/min for the different electret samples. Most notable is the excellent temperature-dependent charge stability of the PLD electret film with large spherulites, which is comparable or even better than that of the PTFE foil. A large shift at half of the initial surface voltage of up to 100 °C for the negative and 25 °C for the positive charging is observed on the two different PLD films, the quenched film without spherulites being least stable. It is important to note the more pronounced influence of the spherulite size on the negative charge stability in PLD PTFE. The positive surface charge decays in the PTFE foils more rapidly than the negative charge, as known from early investigations.20 The same observation stands for the PLD electrets, thereby suggesting that the surface trap distribution of PLD PTFE is similar to that of PTFE foils. Obviously, it is advantageous to have large spherulite sizes for good charge stability in PLD electrets. Similar observations of the dependence of charge stability versus crystallinity have been reported on conventional PTFE .21

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main to be solved on this class of electrets; for example, a detailed study of surface and bulk trap distributions, systematic investigations of the observed relationship between film morphology and charge stability, and long-term investigations of the charge stability at room temperature. This may shed new light on the still controversial discussion of the mechanisms responsible for the excellent charge-storage capabilities of Teflon-like materials. An even improved thermal stability of both negative and positive charging may be expected by corona charging at high temperatures ~as already demonstrated for conventional Teflon foils23,24! or by suitable chemical surface modification. Financial support by the Fonds zur Fo¨rderung der wis¨ sterreich ~FWF! ~Project Nos. senschaftlichen Forschung in O P12898-NAW and P11981-PHY! is gratefully acknowledged. C. R. Davis and F. D. Egitto, CHEMTECH March ~1995!. R. Kressman, G. M. Sessler, and P. Gu¨nter, IEEE Trans. Dielectr. Electr. Insul. 3, 607 ~1996!. 3 S. G. Hansen and T. E. Robitaille, Appl. Phys. Lett. 52, 81 ~1988!. 4 C. W. Reedyk, IEEE Trans. Audio Electroacoust. AU-19, 1 ~1971!. 5 J. A. Voorthuyzen, A. J. Sprenkels, W. Olthuis, and P. Bergveld, IEEE Trans. Electr. Insul. 24, 255 ~1989!. 6 D. T. Morrison and T. Robertson, Thin Solid Films 15, 87 ~1973!. 7 F. Quaranta, A. Valentini, P. Faria, R. Lamendola, and R. d’Agostino, Appl. Phys. Lett. 63, 10 ~1993!. 8 W. De Wilde, Thin Solid Films 24, 101 ~1974!. 9 H. Usui, H. Koshikawa, and K. Tanaka, J. Vac. Sci. Technol. A 13, 2318 ~1995!. 10 R. d’Agostino, F. Cramarossa, F. Fracassi, and F. Iluzzi, Plasma Deposition, Treatment and Etching of Polymers, edited by R. d’Agostino ~Academic Press, New York, 1990!, Chap. 2. 11 S. J. Limb, C. B. Labelle, K. K. Gleason, D. J. Edell, and E. F. Gleason, Appl. Phys. Lett. 68, 2810 ~1996!. 12 J. M. Tibbitt, A. T. Bell, and M. Shen, J. Macromol. Sci. Chem. 10, 519 ~1976!. 13 U. Hetzler and E. Kay, J. Appl. Phys. 49, 5617 ~1978!. 14 N. Amyot, J. E. Klemberg-Sapieha, M. R. Wertheimer, Y. Segui, and M. Moisan, IEEE Trans. Electr. Insul. 27, 1101 ~1992!. 15 D. Ba¨uerle, Laser Processing and Chemistry ~Springer, Berlin, 1996!. 16 G. B. Blanchet, C. R. Fincher, Jr., C. L. Jackson, S. I. Shah, and K. H. Gardner, Science 262, 719 ~1993!. 17 W. Jiang, M. G. Norton, L. Tsung, and J. T. Dickinson, J. Mater. Res. 10, 1038 ~1995!. 18 R. Schwo¨diauer, J. Heitz, E. Arenholz, S. Bauer-Gogonea, S. Bauer, and W. Wirges ~unpublished!. 19 S. T. Li, E. Arenholz, J. Heitz, and D. Ba¨uerle, Appl. Surf. Sci. 125, 17 ~1998!. 20 G. M. Sessler and J. E. West, J. Appl. Phys. 43, 922 ~1972!. 21 V. V. Kochervinskij, N. N. Kuzmina, and I. M. Sokolova, Proceedings of the 7th International Symposium on Electrets, ISE7, 117 ~1991!. 22 H. von Seggern, J. Appl. Phys. 50, 7039 ~1979!. 23 S. S. Bamji, K. J. Kao, and M. M. Perlman, J. Electrost. 6, 373 ~1979!. 24 H. von Seggern and J. E. West, J. Appl. Phys. 55, 2754 ~1984!. 1 2

FIG. 3. Isothermal surface potential decay of PLD-PTFE films at 145 °C. The symbols have the same meaning as in Fig. 2. The charge stability of the PLD film with large spherulites ~n, m! is superior to that of the quenched ~L,l! PLD film.

The results presented for the PLD electrets are supported by isothermal surface potential decay measurements at elevated temperatures ~Fig. 3!. The films were again charged at room temperature, followed by heating to 145 °C at a rate of 20 °C/min, in order to ensure that the decrease of the initial surface potential was small during heating. The surface potential decay is highly nonexponential. However, for a rough estimation, the decay time can be defined at 1/e of the initial surface potential. At 145 °C, the decay time for the most stable negatively charged film has been extrapolated to be on the order of 104 min. The decay rates vary only by a factor of 6 for the positively charged films, but by more than a factor of 150 for the negatively charged films, in agreement with the different temperature shifts observed in thermally stimulated decay experiments. The isothermal surface charge decay of the negatively charged PLD-PTFE at 145 °C favorably compares with previously obtained data on negatively charged Teflon FEP.22 In summary, we can say that Teflon-like films with excellent charge-storage capability have been fabricated on Si substrates by pulsed-laser deposition. They add a promising member to the family of Teflon-like materials, interesting for miniaturized electret device applications. Many problems re-