Recent progress in semiconductor technology includes the utilization of advanced metallization schemes and low k dielectrics in integrated circuit (IC) design. 1,2 As critical dimensions decrease, intermetal dielectrics (IMD) with low dielectric constant (k < 3) play an increasingly important role in developing and producing high speed, high performance ICs. Low k dielectrics reduce interconnect RC [resistance (R) ϫ capacitance (C)] delay, power consumption, and offer many advantages such as reduced line-to-line capacitance and interconnnect cross-talk noise.Hydrogen silsesquioxane (HSQ), commercially available in solution as FOx ® flowable oxide, is one of the most intensively studied low k spin-on dielectrics (SOD) concerning both its basic properties as well as integration-related issues. 3-12 Used as gap-filling dielectric in advanced metallization, HSQs built-in microporosity results in reduced dielectric constant and thus lowers coupling capacitance between tightly pitched metal lines. The successful integration of HSQ into IC interconnnects requires a careful choice of materials together with heat-treatment optimization. However, properties of HSQ films, particularly dielectric properties are strong function of curing parameters, i.e., time, temperature, etc. It is therefore of current research interest to carry out an in-depth study of the curing characteristics of this SOD, to achieve a better understanding of how thermal curing affects the properties of HSQ.HSQ is a subset of polyhedral oligomeric silsequioxane (POSS) macromers, a family of ordered three-dimensional polymers whose structure resembles a cage (Fig. 1a). 3 With the empirical formula of (HSiO 3/2 ) 2n (n ϭ 2, 3, 4, etc.), there are one and one-half (sesqui) stoichiometry of oxygen bound to silicon in a caged structure of HSQ. The structure of HSQ resin, however, is composed of mixture of Si-O network and cage-like network (Fig. 1b), i.e., a random structure of partially formed cages of various sizes as the chemical reactions which produce the resin do not produce completely formed cages. HSQ has no terminal Si-OH or Si-OR groups and it polymerizes after bake and cure by opening of the cages. Therefore, HSQ is cured from a near cage-like structure to a three-dimensional network which provides mechanical integrity to the film for subsequent wafer processing. 4
ExperimentalThin film characterization.-HSQ films (about 4,000 Å) were prepared by spin-coating Dow Corning FOx ® solution onto 8 in. Si wafers followed by heating on three hot plates at 150, 200, and 335ЊC for 1 min each. After that, these films were cured in a furnace at different curing temperature for different durations. The curing conditions used for this study are listed in Table I. The thickness of HSQ films was measured using ellipsometer prior to and after thermal curing. Recent studies reported the importance of an inert environment during the HSQ cure at around 400ЊC in order to prevent oxidation. 4,5 Hence, in this study the HSQ cure employs double ramp rates to prevent tem...
