Electrical Extraction of the In-plane Dielectric Constant of Fluorinated Polyimide

Fluorinated polyimide can potentially replace TEOS as an interlevel dielectric in future ULSI interconnect technologies because its lower dielectric constant offers reduced crosstalk, signal propagation delay, and dynamic power dissipation. One issue associated with polyimides is the anisotropy in dielectric constant, where the smaller out-of-plane dielectric constant, typically measured using parallel-plate capacitors, can misleadingly exaggerate the advantage in reducing crosstalk. In this paper, we present a novel electrical technique to estimate the in-plane dielectric constant of DuPont FPI-136M fluorinated polyimide without requiring dielectric gapfill. A blanket FPI-136M film is deposited over interdigitated inlaid Al(0.5%Cu) structures and the crosstalk capacitance is measured. Identical inlaid structures with air and TEOS passivations are also measured for capacitance calibration. Differences in measured capacitances reflect electric fields fringing in the various passivation dielectrics above the inlaid metal. With the known dielectric constants of air and TEOS, the effective dielectric constant of FPI-136M is interpolated to be 2.8. Interconnect simulations confirm that the effective dielectric constant extraction technique is valid and accurate provided that the passivation layer is sufficiently thick to contain the fringing fields. To estimate the in-plane dielectric constant, we use simulations to determine the combination of in-plane and out-of-plane dielectric constants that is equivalent to the extracted effective dielectric constant. With an out-of-plane dielectric constant of 2.6, the in-plane dielectric constant of FPI-136M is estimated to be 3.0. This technique is applicable to other dieletrics. INTRODUCTION In the ongoing search of low dielectric constant (lowK ) interlevel dielectrics for high-performance interconnects, fluorinated polyimide [1] emerges as a potential candidate, offering advantages of reduced crosstalk [2], faster signal propagation [3], and lower dynamic power dissipation [4] over conventional TEOS. An important consideration when comparing polyimides against other lowK contenders is the anisotropy in dielectric constant, where the in-plane dielectric constant, K in-plane , can exceed the typically advertised out-of-plane value, K out-of-plane , by as much as 10 to 20%. This leads to an optimistic prediction of crosstalk should K out-of-plane be used in estimating lateral capacitance. The anisotropy is conventionally extracted from the relationships K out-of-plane = n TM 2 and K in-plane = n TE 2 where n TM and n TE are the out-of-plane and in-plane refractive indices respectively, measured using a prism coupler [1]. Optical estimates of dielectric constant, however, are optimistic since among dielectric polarization components consisting of ionic, dipolar, atomic, and electronic contributions, fewer are activated at optical frequencies than at lower electrical fre1997 MRS Spring Meeting, Symposium N, Paper N3.4, San Francisco, CA, April 3, 1997 2 quencies [5]. K in-plane can be estimated electrically by gapfilling spaces between metal lines with dielectric and measuring lateral capacitance [6]. However, this approach is compromised in accuracy by the filling and confinement of polymer chains in narrow gaps, and is not feasible for dielectrics having poor gapfill ability yet possessing attractive qualities for inlaid metallization architectures. In this paper, we present an electrical technique to extract K in-plane of a blanket film of DuPont FPI-136M fluorinated polyimide.