Dielectric properties of plasma-polymerized ferrocene films

Thin Solid Films, 48 (1978) 319- 324 © Elsevier Sequoia S,A., Lausanne Printed in the Netherlands

319

D I E L E C T R I C P R O P E R T I E S OF P L A S M A - P O L Y M E R I Z E D F E R R O C E N E FILMS SHIREESH D. PHADKE

Armament Research and Development Establishment, Pashan, Poona-411021 (India) (Received July 18, 1977; accepted July 29, 1977)

The variations of the dielectric constant e and of tan 6 for Al-polyferrocene-A1 sandwich structures were studied over the frequency range 40 kHz-50 MHz and the temperature range 30-400 °C. Heating in dry air led to a permanent change in the value of e by approximately 15 % while the change in tan 6 was of a temporary nature. From IR studies a possible structure for polyferrocene is suggested.

1. INTRODUCTION The synthesis of polymers in a low pressure electrical discharge is referred to as plasma polymerization. By using this technique, highly cross-linked pinhole-free polymeric films have been prepared from a large number of organic and organometallic monomers 1. Because of their good dielectric properties such films have been found to be useful as thin film insulators and capacitors 1-5 in integrated microelectronics. In addition, knowledge of their dielectric behaviour throws light on the molecular structure of the polymers. The dielectric properties of plasmapolymerized styrene 6'7, acrylonitrile 4 etc. have already been published. In this article, the dielectric properties of plasma-polymerized ferrocene thin films are reported. An attempt has also been made, on the basis o f l R studies, to correlate (a) the increase in the dielectric constant e and (b) the increase in the loss factor tan with the structure of the polymer. 2. EXPERIMENTAL

The following deposition sequence was used. (1) The polymerization system was evacuated to 10-5 Torr. (2) Ferrocene Fe(CsHs) 2 was heated in a glass ampoule that was evacuated to approximately 10 -4 Torr. (3) Ferrocene vapour was fed through a needle valve into the polymerization system and the operating pressure was kept at 10-1 Torr. (4) The ferrocene vapour was then discharged at 400-500 V a.c. and 2-3 mA c m -2.

Under these conditions, polymerization and deposition onto clean glass substrates with evaporated aluminium electrodes took place simultaneously. Suitable masks were used to obtain the desired geometry of the polymer films.

320

s.D. PHADKE

Aluminium counterelectrodes were deposited by vacuum evaporation to obtain sandwich structures. The thicknesses of the top and bottom electrodes were approximately 1500 A. The thickness of the polymer film was measured by the Fizeau fringe method. The capacitance of the sandwich structure was measured using a Marconi circuit magnification Q-meter (TF 1248) with an oscillator (TF 1246) which operated between 40 kHz and 50 MHz. From the capacitance and Q measurements, the dielectric constant e and the loss factor tan 6 were calculated. The samples were kept in a vacuum cell ( ~ 10-5 Torr) and the measurements were carried out at room temperature as well as at elevated temperatures. The measurements of these samples were repeated after introduction of dry air into the cell. The IR spectra were obtained with a Perkin-Elmer model 457 spectrometer by the KBr disc technique. Solubility tests with acids, alkalis and polar and non-polar organic solvents were all negative, which confirmed that the m o n o m e r was completely converted to polymer. The polymer was observed to soften between 350 and 400 °C. The original colour of the polymer films was brown and it was observed to have changed to a darker brown above 300 °C; the original brown colour was regained when the films were cooled to room temperature. The estimated rate of polymer deposition was 40 A rain- 1 3. RESULTS 3.1. Dielectric measurements

The characteristic dependences of e and tan 6 on temperature in vacuum and after introduction of dry air are shown in Figs. 1 and 2. The variations in both e and tan 6 are greater in dry air than in vacuum. The dielectric constant e shows peaks at 6.0

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321

PLASMA-POLYMERIZED FERROCENE FILMS

higher temperatures but becomes steady after a certain temperature. The cooling curves do not follow the heating paths, which implies that some permanent change in the polymer structure causes the increase in e on heating. The dependences of e and tan 6 on frequency at room temperature for different polymer film thicknesses is shown in Fig. 3. The ~ v e r s u s frequency plot shows a steady increase between 60 kHz 0,2

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and 3 MHz and peaks at higher frequencies. The change in e with thickness is not appreciable, which demonstrates the layer-by-layer and pinhole-free formation of the films. The tan 6 versus frequency plot shows characteristic maxima and minima, and the loss peak shifts towards lower frequency as the thickness decreases. 3.2. I R spectra

The spectra of ferrocene and polyferrocene taken under identical conditions by the KBr disc method are presented in Fig. 4 for the region 2000-400 c m - ~ ; the spectrum of deuterated ferrocene reported by Lippincott and Nelson 8 is also given. The two bands at 1104 cm -~ and 814 cm -1 which are absent in the polyferrocene spectrum can be assigned to the C H modes as they are shifted in the deuterated ferrocene. All the bands belonging to the five-membered ring are present in the spectra of ferrocene and polyferrocene. In addition, the metal carbon stretching frequency which is assigned at 478 c m - l is present in the spectra of ferrocene and polyferrocene. Thus we can confirm that the structure of the new compound consists of ferrocene monomers linked together, possibly by C - C bonding as in the case of diphenyl. The absence of a few C - H modes supports this structure.

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4. DISCUSSION It is well known 9-14 that the polymerization of an organic monomer by glow discharge is preceded by free radical formation either due to the opening of the double bond or by hydrogen abstraction. The I R spectrum clearly indicates that hydrogen abstraction is the mechanism for polymerization. The salient features of the polyferrocene spectra are (1) the disappearance of C - H bands at 1104 c m - 1 and 814 c m - ~, (2) the existence of the metal ring frequencies and (3) the appearance of a strong broad and intense band at 630 c m The characteristic frequencies ofdiphenyl are assigned at 780 c m - ~ by Cannon and Sutherland i s. However, a clear assignment of the C - C bond connecting the two

PLASMA-POLYMER1ZED FERROCENE FILMS

323

rings is not made by Sutherland. This is probably due to the fact that there are always some ring C - H bending frequencies in this region for the six-membered ring. Fortunately, for the five-membered ring of ferrocene there is no band between 800 and 600 cm-1. The new broad and intense band at 630 cm-1 can therefore be assigned to the bond connecting the two rings. The large intensity of the band suggests that there is a large dipole associated with this mode. Thus it may further be inferred that the polymer is a coiled one. There is a tendency for e and tan fi to increase, although not appreciably, with temperature, and the thickness of the polymer film remains unchanged after heating. No simple linear dependence of E and tan 6 on temperature exists. The relation between the dielectric properties and temperature depends on the structure of the polymer, i.e. the nature and size of the molecule, its polarity, the stability of the molecular bonds etc. 16 The tan 6 versus temperature plot at constant frequency shows characteristic maxima and minima. The loss of energy is associatdd not only with the degree of orientation of the molecules and sections but also with the degree of internal friction of the polymer, as confirmed by the above observations. Further, the increase in tan 6 is expected as a result of the introduction of polar impurities leading to changes in the C-C bonds in the polymer. As explained in IR studies, plasma-polymerized 17.18 films contain a considerable concentration of free radicals which on introduction of air must result in a great deal of oxidative reaction. This assumption is supported by the facts that (a) there is a marked increase in e after heating and (b) the original value ofe in vacuum is not obtained when the sandwich structure is subjected to a heating cycle in dry air. From the foregoing results and discussion we may conclude that (a) the polymerization process is through free radical formation/hydrogen abstraction, (b) the dielectric properties depend on the structure of the polymer and the linking of the ferrocene monomer and (c) the loss peaks are due to the large dipole associated with the C-C bond. Thus it may further be inferred that the polymer consisfs of ferrocene molecules connected through C C bonds and that the cyclopentene rings joining the two molecules do not lie in the same plane; thus ferrocene molecules form a coiled structure in the polymer product. ACKNOWLEDGMENTS

The author is greatly indebted to Professor Dr. K. Sathianandan, University of Cochin, and Dr. R. N. Karekar, University of Poona, for discussion of the results and to Professor M. R. Bhide, Head of the Physics Department, for allowing him to carry out experimental work in the Physics Department, University of Poona. REFERENCES I 2 3 4 5 6 7

A. Bradley and J. P. Hammes, J. Electrochem. Soc., 110 (1963) 15,543. R. Christy, J. Appl. Phys., 31 (1960) 1680. P.R. Emtage and W. Tantraparn, Phys. Rev. Left., 8 (1962) 267. T. Hirai and O. Nakada, Jpn. J. Appl. Phys., 7 (1968) 112. O.H. LeBlanc, Jr.,J. Chem. Phys.,33(1960)626. G. Sawa, O. Ito, S. Morita and M. Ieda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 1231. D. Sanchez, M. Carchano and A. Bui, J. Appl. Phys., 45 (1974) 1233.

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