6 Thermally Stable Polymers for Electronic Applications D. J. DAWSON, W. W. F L E M I N G , J. R. LYERLA, and J. ECONOMY
Downloaded by STONY BROOK UNIV SUNY on May 20, 2018 | https://pubs.acs.org Publication Date: July 9, 1985 | doi: 10.1021/bk-1985-0282.ch006
IBM Research Laboratory, San Jose,CA95193
High-temperature polymers which can be obtained from processable reactive oligomers would offer many unique opportunities, particularly to the microelectronics industry. The ideal oligomer would have properties such as low dielectric constant and good planarization and would be both thermally and photochemically curable to heat stable polymers. Initial research efforts have focused on melt processable diacetylenic oligomers with the hope that such units would thermally cure to condensed polyaromatics. Poly(triethynylbenzene) (PTEB) was chosen as a model compound and was prepared by the oxidative coupling of 1,3,5-triethynylbenzene with phenylacetylene. The composition and molecular weight of PTEB has been varied by incorporating different levels of phenylacetylene as a capping agent. The planarization of the oligomer was outstanding while the thermal and oxidative stability of the cured polymer was consistent with what one might expect for a network of aromatic rings. In order to study the cure behavior of the PTEB system, C NMR of uncured and cured PTEB in the solid state was performed using cross-polarization magic-angle spinning techniques. The results show the polymerization to be via aromatization. The extent of cure versus cure temperature was determined quantitatively. It was found that the material was almost completely cured after one hour at 215°C. As the cure goes to completion, the ability to react decreases due to the corresponding rapid increase in T . Chemical shifts of the resonances in the cured material are consistent with a highly crosslinked condensed aromatic network. 13
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The development of functional oligomers that can be thermally cured to heat-stable polymers has been the subject of considerable study over the past 15 years. Poly (aromatic acetylenes) were initially considered to be good candidates for such uses because the ethynyl groups were thought to undergo a thermally induced cyclotrimerization to a fully aromatic cured polymer. However, in recent years it has become evident (1) that ethynyl terminated oligomers such as Thermid 600 did not thermally cyclotrimerize. Presumably cyclotrimerization of ethynyl groups would require the presence of transition metal catalysts; however, these catalysts would remain in the cured polymer and could act as possible catalytic sites for subsequent thermal decomposition. As a result, the use of ethynyl terminated oligomers as precursors to polymers with high temperature thermo-oxidative stability appears questionable. 0097-6156/85/0282-0063$06.00/0 © 1985 American Chemical Society
Harris and Spinelli; Reactive Oligomers ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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REACTIVE OLIGOMERS
Recently, the microelectronics industry has developed a need for thermally stable polymers for dielectric insulating layers. In addition to good thermal stability (>400°C), these cured polymer films must exhibit a low dielectric constant, low moisture uptake, a high glass transition temperature (T ), excellent planarization, and possibly lithographic sensitiv ity which would allow these films to be used as resists. Only a few of these properties can be achieved with polyimides. Other candidates for high temperature polymers also have draw backs. Inorganic, fluoro-, and phenolic polymers have inadequate thermal stability. Heterocyclic and rigid-rodlike polymers are insoluble in solvents suitable for spin-coating. A research program was initiated at IBM to design a model polymer that would meet all these requirements. The low dielectric constant and low moisture uptake requirements would best be achieved by a hydrocarbon structure. Optimal planarization would require a low-molecular-weight oligomer, and the required T would necessitate a crosslinked network of aromatic units in the cured polymer. It appears that diacetylenic oligomers could meet these specifications and at the same time obviate the potential oxidative instability inherent in the products of thermal crosslinking of terminal ethynyl resins. In 1969, Wegner (2) re ported that monomeric diacetylenic single crystals could be polymerized by ultraviolet radi ation. This finding suggested that polymers containing diacetylene units would have potential as resists. Somewhat earlier, Hay (3) prepared high molecular weight diacetylene polymers by the oxidative coupling of m-diethynylbenzene. These polymers underwent an apparent exothermic decomposition upon heating to 180°C and showed a weight retention of over 90% after heating to 800°C under inert conditions. Alternatively, the decomposition ob served at 180°C could have been the result of an uncontrolled exothermic curing process. By working with very thin films typically used for dielectric insulators it was expected that the exotherm could be controlled to afford a material of high thermal stability. Our initial work on diacetylenic polymers was directed to the synthesis of lowmolecular-weight poly (m-diethynylbenzene) (Scheme 1). A low-molecular-weight resin was selected to permit the melt flow behavior essential for planarization. The molecular weight was kept low by including phenylacetylene in the polymerization reaction. Being monofunctional, phenylacetylene acted as a capping agent, terminating the growing polymer branch with a phenylbutadiyne group. The resulting oligomer exhibited melt flow properties below the curing temperature but formed poor films because of its low solubility in spincoating solvents and its tendency to crack prior to curing. Both of these unfavorable proper ties were attributed to the crystalline nature of the oligomer. We found that these characteristics were eliminated by replacement of the linear oligomer with a branched 1,3,5-triethynylbenzene unit (III) in the oxidative coupling reaction (4-5) (Scheme 2). In this paper methods are described for preparing the 1,3,5-triethynylbenzene monomer and oligomers. Evidence is then presented to support the theory that these diacetylene oligomers tend to thermally cure by an aromatization reaction. Since the cured polymer is highly crosslinked and therefore insoluble, most character ization techniques, including solution N M R , are not suitable for studying curing mechanisms in glassy polymers. However, one can obtain substantial information at the molecular level using C solids N M R techniques. Briefly, the carbon resonance signal from a solid sample experiences a proton-carbon dipolar coupling on the order of 40 K H z (6), an anisotropic chemical shift distribution of ~l-2.5 Khz at carbon frequencies of 15 M H z (7), and very long carbon spin-lattice, T,, relaxation times that can be as long as 10 seconds (8). The signal re sulting from a normal pulse-acquisition experiment as with solution samples is very broad and featureless. However, the large dipolar decoupling can be eliminated by the use of dipolar decoupling, DD, (9); the chemical shift anisotropics can be reduced to their singular values by spinning the sample rapidly at an angle of 54.7° with respect to the magnetic field (the so-called magic angle spinning, MAS) (10); and the limitations of long carbon relaxation times can be avoided by using the technique of cross-polarization, C P ( Π ) . The resulting spectrum consists of peaks of reasonably narrow line width and chemical shifts which corre spond to the isotropic values observed h. solution. The solid state line widths and chemical shift differences observed relative to those observed in solution samples are a result of solid state effects due to structure, stereochemistry, and motional effects. The solid state carbon g
Downloaded by STONY BROOK UNIV SUNY on May 20, 2018 | https://pubs.acs.org Publication Date: July 9, 1985 | doi: 10.1021/bk-1985-0282.ch006
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Harris and Spinelli; Reactive Oligomers ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
6.
DAWSON E T A L .
Thermally Stable Polymers
C=CH
C Hr
