Evaluation of a silicon-containing benzocyclobutene thermoset resin as a plasma etch stop
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I. INTRODUCTION A. Application of benzocyclobutene polymers in the microelectronic industry
Commercial production of high density circuits prompts a number of manufacturing concerns, among them, proper choice of dielectric materials. These circuits may consist of several layers of circuit lines separated by dielectric materials, frequently polymers. At times during the manufacturing process, the circuit packages face extremes in temperature due to soldering. For this reason, high temperature polymers are particularly interesting to the microelectronic industry. A class of new polymers, now commercially available from the DOW Chemical Co., is based on the benzocylcobutene structure.1'2 The strained side ring is prone to open2a and react in a Diels-Alder fashion with a dienophile, or even itself to form a cross-linked network,215 as illustrated in Fig. 1. Because of its high thermal stability and ease of processing, this class of polymers has been investigated both for advanced composites28 and circuit packaging applications.3 An interesting feature about the UX-13005.02L product is that the R group contains silicon, which prompted us to investigate using the resin as a plasma etch stop. B. Etching chemistry for Si and polymers in O2/CF4 plasmas
O2/CF4 plasmas have been used extensively in the manufacture of integrated circuits, largely for etching Si and SiO2 from wafers.4 Etching may be done in a CF4rich plasma, where Si is removed preferentially to SiO2. The reactive species for Si, and to a lesser extent SiO2, is atomic fluorine.5 In fact, care must be taken to keep the Si surface from reacting with oxygen in the plasma to form SiO2.6 The major point to note is that silicon becomes very resistant to plasma etching in an oxygenrich environment. For circuit packaging applications, interest in O 2 /CF 4 plasmas is focused largely on etching polyJ. Mater. Res., Vol. 5, No. 8, Aug. 1990
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mers.7 Introduction of CF4 (or SF6) into an oxygen plasma, characterized quantitatively by Turban,8 greatly enhances the etch rate. For polymers, oxygen appears to be the etchant, with atomic fluorine creating radicals on the polymer surface for subsequent attack by oxygen.9 The important point here is that polymers etch in an oxygen-rich plasma, but become very resistant to etching in a fluorine-rich (CF4-rich) plasma.810 From the preceding paragraphs, it is evident that etch selectivity between polymers and silicon may be controlled by judicious choice of the plasma feed gas composition. Thus, organosilicon compounds have been investigated for use in dry process patterning of polymers.11 The etch chemistry of organosilicon polymers in an oxygen-rich plasma appears to be governed by a buildup of SiO2 on the surface, and a kinetic model for such a buildup has been proposed.12 We have recently studied a silicon containing benzocyclobutene thermoset resin, and have obtained similar results. Because there are varied applications for plasma etching in the microcircuit industry, two
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