Microelectronics Technology - ACS Publications - American Chemical

Chapter 24

Fluoropolymers with Low Dielectric Constants: Triallyl Ether—Hydrosiloxane Resins 1

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Henry S.-W. Hu , James R. Griffith , Leonard J. Buckley , and Arthur W. Snow Downloaded by NANYANG TECH UNIV LIB on November 1, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch024

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Geo-Center, Inc., 10903 Indian Head Highway, Fort Washington, M D 20744 Naval Research Laboratory, 4555 Overlook Avenue, S.W., Building 207, Washington, DC 20375 The preparation of a class of processable heavily fluorinated triallyl ether homo- and co-polymers is carried out to elucidate the structure-property relationships in comparison with the acrylic analogs. The triallyl ether monomer was prepared in good yield through the condensation of the triol, 1,3,5-tris(2-hydroxy-hexafluoro-2propyl)benzene, in dry acetone with allyl bromide in the presence of potassium carbonate. Homopolymers were obtained and cured through a slow free radical polymerization, while copolymers were obtained through a fast hydrosilylation with catalyst. The dielectric constant of the copolymer of the triallyl ether and an equivalence of polymethylhydrosiloxane (PMHS) catalyzed by a trace of dicyclopentadienylplatinum(II) chloride is 2.33 at 13.2 GHZ with a dissipation factor of 0.004. The factors which affect the dielectric constant and thermal stability are the fluorine content, the polymer type and the molecular architecture.

With recent trends toward microminiaturization of electronic systems and utilization of very thin conductor lines, close spacings, and very thin insulation, greater demands are being placed on the insulating layer. Reductions in such parasitic capacitance can be achieved in a number of ways through the proper selection of materials and the design of circuit geometry. In 1988 St. Clair et al. reported a reduction of dielectric constant to 2.39 by modifying a polyimide to reduce the interchain interactions and by the incorporation of carbon-fluorine bonds. In 1991 Kane et al. reported a reduction of dielectric constant to 2.32 for the hexafluoroisopropylidene-containing polyacrylates and copolyacrylates. In 1992 Snow et al. reported that the thermally induced trimerization of a perfluorohexamethylene linked aromatic cyanate resin to a cyanurate linked network gave a dielectric constant between 2.3 and 2.4. In 1993 Babb et al. stated that the la

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0097-6156/95/0614-0369$12.00/0

© 1995 American Chemical Society

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

MICROELECTRONICS TECHNOLOGY

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thermally induced cyclodimerization of a trifluorovinyl aryl ether to a perfluorocyclobutane aromatic ether polymer gave a dielectric constant of 2.40. Recently we reported the preparation of a series of processable heavily fiuorinated acrylic and methacrylic homo- and co-polymers which exhibit dielectric constants as low as 2.06, very close to the minimum known values of 2.0-2.08 for Teflon® and 1.89-1.93 for Teflon AF*. However, these measurements were made using a coaxial transmission line method with samples in cylindrical donut molds that are not optimized for high accuracy. The factors which affect the reduction of dielectric constant from structure-property relationships have been elucidated from our experimental findings. 2

Downloaded by NANYANG TECH UNIV LIB on November 1, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch024

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The polar nature of the C-F bond has been used to provide some high-performance characteristics in comparison with their hydrogen-containing or other halogencontaining analogues. Fluorine-containing epoxies or acrylics generally exhibit resistance to water penetration, to chemical reaction, and to environmental degradation; they also show unusual values for surface tension, friction coefficient, optical clarity, refractive index, vapor transmission rate, and electromagnetic radiation resistance. Fiuorinated polyethers are the key intermediates for new types of practical organic coatings and plastics which have fluorocarbon properties including high thermal stability in some cases. Polysiloxanes offer other unusual properties such as very low glass transition temperature, high permeability to gas, oxidative stability and the ability to be fabricated into useful products. It is this versatility that has established the reputation of siloxane containing polymers in many applications. Polysiloxane elastomers capped with functional groups have been used to increase flexibility, to improve thermal stability, and to lower internal stress of cured epoxy resins. 4

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The hydrosilylation, an addition reaction of Si-Η compounds to unsaturated organic molecules with the aid of a platinum or rhodium complex catalyst, has been used widely for Si-C bond formation. Siloxane-containing polymers obtained from hydrosilation normally have several advantages in properties and processing. The curing proceeds in the presence of oxygen, with little shrinking, and with good dimensional stability. In addition, the hydrosilation occurred with exclusive antiMarkovnikov addition. 6

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In this paper we report a class of processable heavily fiuorinated triallylic ether homo- and co-polymers which are readily prepared in rectangular block molds to elucidate the structure-property relationships in comparison with the acrylic analogs. EXPERIMENTAL Only the general preparations are procedures will be reported else where.

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24. HU ET AL.

Fîuoropolymers with Low Dielectric Constants

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Materials Allyl bromide was obtained from Aldrich Chemical Co. and was distilled to collect the fraction with bp 71-72°C/1 atm as a clear liquid (the fraction before that is cloudy). Acetone was dried over potassium carbonate, decanted, and distilled from potassium carbonate before use. Dicyclopentadienylplatium(II) chloride was prepared according to the procedure of Apfel et al. Alumina (neutral, Brockman activity 1, 80-200 mesh) was obtained from Fisher Scientific Co. Polymethylhydrosiloxane (PMHS) was obtainedfromAldrich (#17620-6) and a Si-Η equivalent formula weight of 63.13 g/mole is used for calculation. All other reagents were used as received or purified by standard procedures.


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Preparation of triallyl ether 2 from triol 1. l,3,5-Tris(2-hydroxy-hexafluoro-2propyl)benzene I was prepared by a multistep route according to the procedure of Soulen and Griffith. * This compound was hygroscopic as observed by Griffith and 0'Rear. 9

9b

Over the course of 30 min, allyl bromide (31.6 g, 261 mmol) was added dropwise to a solution of triol 1 (40.0 g, 69.4 mmol) in dry acetone (500 mL) in an ice-water bath under nitroge. After 10 min, potassium carbonate (32.0 g, 231 mmol) was added in portions in 3 min and stired 1/2 hr at 0 °C followed by 1 hr at room temperature. The mixture was then slowly heated to reflux over 1 hr and kept refluxing for 12hr. The reaction was worked up by filtering through Celite, and evaporated at reduced pressure and in vacuo at 30 °C for 3 hr to yield a liquid (48.4 g). The liquid was dissolved in hexanes (200 mL), percolated through a column of neutral alumina (80 g) twice and each time washed with hexanes (150 ml). It was evaporated at aspirator pressure and then in vacuo at room temperature for 4 hr to give a colorless liquid (40.9 g) of triallyl ether 2, yield 85%. Rf 0.81, Hexane/CH C1 (2:1) (v/v), 0.41 (Hexane); IR(neat film) 3104, 2952, 2891, 1652, 1609, 1457, 1429, 1414, 1373, 1350-1100 (C-F), 1051, 1012, 979, 933, 889, 734, 709 cm ; H NMR(CDCyTMS) δ 7.99 (s, 3H, Ar-H), 5.92 (d,d,d, J=17.2, 10.5, 5.1 Hz, 3H, CH =CH- ), 5.43 (d, d, J=17.2, 1.3 Hz, 3H, cisoid CH =CH-), 5.32 (d,d, J=10.5, 1.3 Hz, 3H, transoid CH =CH-), 4.09 (d, J=5.1 Hz, 6H, -CH 0- ); F NMRiCDCWCFCLs) -71.51. 2

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Preparation of Molded Samples Semitransparent homopolymers and 50/50 equimolar copolymers were obtained as follows. Hydrosilylation of triallyl ether 2 with PMHS: Triallyl ether 2 (1.99 g, 2.86 mmol) and PMHS 4 (0.55 g, 8.71 mmol) were mixed with [(Cp^PtJC^ (0.8 mg) at room temperature and transferred into a rectangular block mold (16.02x8.15x8.94mm, lxwxh) madefromGeneral Electric RTV 11 silicone molding compound. To effect cure, the temperature was slowly raised to 150 °C over 2 days and kept at 150 °C for 1 hr. Homopolymerization of triallyl ethers 2: Triallyl ether 2 was mixed with a trace amount of solid azobisisobutyronitrile (AIBN) or liquid


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1 Scheme 1

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contains siloxane linkages in the structure, and the substituted alkyl groups are nonpolar hydrocarbon which lower the DE below that of glass. This effect was reported in the case of polydimethylsiloxane (PDMS) by Bass and Kauppi, the type500 fluid with viscousity 3 cS has a DE of 2.41-2.39, while the type-200 fluid with higher viscosity 250 cS has a DE of 2.74-2.70. 15

Lower dielectric constants are obtained as fluorine content on the polymer backbone or sidechain increases, when acrylate is replaced by methacrylate, when ether linkages are present in thefluorocarbonand when aromatic structure is symmetrically meta-substituted. The wide range of frequency independence of these low e polymers and the processability of these monomers suggest many potential applications. 2

Lower dielectric constants. Dielectric constants of these materials can be further lowered by known means such as by incorporating voids in the materials. A difference of a couple of hundredths in the dielectric constant value may be important when one is at the low extremes thereof. Singh et al. calculated the dielectric constants of polyimide films from the measured free volume fraction and found that the calculated values for the dielectric constants are close to the experimental results. 16


24. HU ET AL.

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Fluoropolymers with Low Dielectric Constants

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In 1991 Groh and Zimmermann estimated the theoretical lower limit of the refractive index of amorphous organic polymers by using the Lorenz-Lorentz equation and reported the lower limit to be very close to 1.29. Using the Maxwell Relation of DE = η for the case of nonmagnetic materials withoutfreecharges this refractive index translates to a dielectric constant of 1.664. They reported that functional groups with a high fluorine content, like CF and CF , have the lowest refractive index contribution. The value for the ether group is also remarkably low, while the values for the carbonyl and carboxyl groups are high. Using this theoretical guide, synthetic modifications are continuing with hopes of achieving lower dielectric constants. 18

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CONCLUSIONS In this work we have demonstrated that a new class of heavily fiuorinated triallyl ether resins can be efficiently synthesized and then cured to solid form with a catalyst at elevated temperatures. Homopolymers were obtained and cured through a slow free radical polymerization, while copolymers were obtained through a fast hydrosilylation. The factors which affect the dielectric constant and thermal stability are thefluorinecontent, the polymer type and the molecular architecture. ACKNOWLEDGEMENT Partial funding support from the Office of Naval Research is gratefully acknowledged. REFERENCES AND NOTES (1) (a) St. Clair, A. K.; St. Clair, T. L.; Winfree, W. P. Polym. Mater. Sci. Eng. 1988, 59, 28. (b) Kane, Κ. M.; Wells, L. Α.; Cassidy, P. E. High Perform. Polym. 1991, 3(3), 191. (c) Snow, A. W.; Griffith,J.R.; Soulen, R. L.; Greathouse,J.Α.; Lodge, J. K. Polym. Mater. Sci. Eng. 1992, 66, 466. (d) Babb, D. Α.; Ezzell, B. R.; Clement, K. S.; Richey, W. R.; Kennedy, A. P. Polym. Prepr. 1993, 34(1), 413.; J. Polym. Sci. Part A: Polym. Chem. Ed. 1993, 31, 3465. (2) (a) Hu, H. S.-W.; Griffith,J.R. Polym. Mater. Sci. Eng. 1992, 66, 261. (b) Hu, H. S.-W.; Griffith,J.R. In Polymers for Microelectronics, Willson, G.; Thompson, L. F.; Tagawa, S. Eds., ACS Symposium Series 1994, 537, 507. (c) Hu, H. S.-W.; Griffith, J. R. Polym. Prepr. 1993, 34(1), 401.(d) Griffith,J.R.; Hu,H.S.-W. U. S. Patent 5,292,927. March 8, 1994. (e) Griffith,J.R.;Hu.H.S.-W. U. S. Patent 5,405,677. April 11, 1995. (3) (a) Licari,J.J.;Hughes, L. A. Handbook of Polymer Coating for Electronics; Noyes Publications: Park Ridge, NJ, 1990. see p. 378, Table A-13: Dielectric Constants for Polymer Coatings (at 25°C). (b) Resnick, P. R. Polym. Prepr. 1990, 31(1), 312. (4) Iwahara, Y.; Kusakabe, M.; Chiba, M.; Yonezawa, K. J. Appl. Polym. Sci. 1993, 50, 825. (5) Tong,J.;Bai, R; Zou, Y.; Pan,C.;Ichimura, S.J.Appl. Polym. Sci. 1994, 52, 1373.



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(6) Crivello,J.V.; Bi, D.J.Polym. Sci. Part A: Polym. Chem. Ed. 1993, 31, 2729. (7) Mathias, L.J.;Lewis, C. M. Macromolecules 1993, 26, 4070. (8) Apfel, M. A.;Finklemann,H.;Janini, G. M.; Laub, R. J.; Lühmann, B. -H.; Price, Α.; Roberts, W. L.; Shaw, T.J.;Smith, C. A. Anal. Chem. 1985, 57, 651. (9) (a) Soulen, R. L.; Griffith,J.R.J.Fluorine Chem. 1989, 44, 210. (b) Griffith, J. R.; O'Rear, J. G. Polym. Mater. Sci. Eng. 1985, 53, 766. (10) (a) Nicolson, Α.; Ross, G. IEEE Trans. Instrumentation and Measurement 1970, IM-19, 377. (b) Von Hippel, A. R. Dielectric Materials and Applications; M.I.T. Press: Cambridge, MA, 1954. (11) Odian, G. Principles of Polymerization; 3rd ed., Wiley: New York, NY, 1991. see p. 266. (12) Pouchert, C. J. The Aldrich Library of FT-IR Spectra; Aldrich Chemical: Milwaukee, WI, 1985. (13) (a) Hu, H. S.-W. et al. unpublished results, (b) Carey, F. Α.; Sundberg, R. J. Advanced Organic Chemistry; Plenum: New York, NY, 1984. see p. 635 and 653. (14) In Handbook of Chemistry and Physics; Chemical Rubber Co.: Cleveland, OH. see p. E-66. (15) Clarson, S. J.; Semlyen, J. A. Siloxane Polymers; PTR Prentice Hall: Englewood Cliffs, NJ, 1993. see p. 420 for ref. 26. (16) (a) Singh,J.J.;Eftekhari, Α.; St. Clare, T. L. NASA Memorandum 102625, 1990. (b) Eftekhari, Α.; St. Clare, A. K.; Stoakley, D. M.; Kuppa,S.;Singh,J.J.Polym. Mater. Sci. Eng. 1992, 66, 279. (17) Groh, W.; Zimmermann, A. Macromolecules 1991, 24, 6660. (18) Lin, L.; Bidstrup, S. A.J.Appl. Polym. Sci., 1994, 54, 553. RECEIVED July 17, 1995


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