Journal of Electronic Materials, Vol. 19, No. 12, 1990 Benzocyclobutene (BCB) Dielectrics for the High Density, Thin Film Multichip Modules Fabrication of DAVID BURDEAUX, PAUL TOWNSEND and JOSEPH CARR The Dow Chemical Company, Central Research, Midland, MI PHILIP GARROU The Dow Chemical Company, Central Research, Charlotte, NC 28210 A new class of organic dielectrics, benzocyclobutenes, 1, are described and their appli- cation to the fabrication of thin film multichip modules is detailed. Key properties for 3, a siloxy containing BCB derivative include low dielectric constant (2.7), low loss (0.008 at 1 MHz), low water absorption (0.25% after 24 h water boil) and high degree of plan- arization (>90% from one layer coverage). All other properties meet the requirements necessary for fabrication of thin film MCM structures. Key words: Benzocyclobutene, BCB, multichip module, dielectric INTRODUCTION Performance advantages inherent in advanced VLSI semiconductor devices will not be realized at the system level, during the 1990's, with conven- tional electronic packaging and interconnect. 1-3 In- dividually packaged chips have typically been in- terconnected on a multilayer circuit board. Although this approach has served the industry well for many years, it has become clear that new technology will be necessary to meet the performance demands that will be required in the '90s and beyond. One high density interconnect technology that has evolved to meet these needs has been dubbed the "Multichip Module" (MCM). As shown in Fig. 1, it has blended IC, hybrid and PWB technologies into a new inter- connect methodology. Described early on by IBM, 4 MCM technology has 5 7 8-9 seen intense recent activity in the US, - Japan and Europe. TM In addition to achieving far superior electrical performance, equally important gains are made in size and weight and significant advances are ex- 11 12 1 pected in reliability. - It is also becoming clear that MCM will show significant cost performance benefits over other technologies once available in a full production mode. 13-14 Thus, if MCM lives up to its billing, it will find applications not only in high performance mainframe computers but also in the workstation, laptop, military, telecom, automotive and consumer market segments. Each active research group has its own modifi- cations to the overall fabrication process, such as, substrate material (silicon or ceramic), conductor (aluminum vs copper), via interconnection tech- niques (unfilled vs filled vs plated up posts) or methodology to attach the bare IC's (wire bonding vs TAB vs flip chip), to name just a few. However, one commonality among nearly all the current vari- ations is the use of polymeric insulators to separate the conductor traces. Polymer dielectrics allow the fabrication of the thick interlayer insulators nec- (Received June 22, 1990; revised August 15, 1990) essary to produce matched impedance transmission lines with acceptable losses. Typical inorganic di- electrics used in conventional integrated circuits cannot exceed ca 1 micron thickness without seri- ous fabrication problems. 15 Material requirements for the ideal polymeric thin film dielectric can be described as follows: LOW DIELECTRIC CONSTANT THERMAL STABILITY IN EXCESS OF SUBSEQUENT PROCESSING AND REPAIR STEPS GOOD ADHESION TO SUBSTRATE, CONDUCTOR AND SELF LOW STRESS FORMATION LOW WATER ABSORPTION GOOD PLANARIZATION EASY TO PROCESS SPIN COATABLE SPRAY COATABLE PIN HOLE FREE ETCHABLE Benzocyclobutenes, BCB, a new class of polymeric dielectric have been developed to meet these re- quirements for MCM fabrication. BENZOCYCLOBUTENE POLYMER FAMILY Benzocyclobutene polymers are derived from monomers of the generic form 1.16 The polymeri- zation of such molecules is based on thermal rear- rangement of the cyclobutyl functionality to give the highly reactive intermediate o-quinodimethane, 2, which can polymerize with similar molecules or react with a variety of other unsaturated functional groups. R R STRUCTURE 1 STRUCTURE 2 1 2 The polymerization is a purely thermal process. No catalysts are required and no volatiles or pro- duced, in contrast to polyimides which produce water 0361-5235/1990/1401-135755.00 9 AIME 1357