Characterization of BCB Planarization of Isolated and Dense Features in a High-Topography HBT Process

Tom Dungan, Avago Technologies

Abstract
Spin-on organic dielectrics are useful as planarizing inter-level dielectric layers in compound semiconductor processes, particularly in processes with relatively high topography and multiple levels of interconnect metal. Such spin-on materials can coat over surfaces of composed of features of varying heights and partially smooth out the height variations to produce a flattened top surface more suitable for fine feature photolithography on following interconnect. However, the planarization is not perfect, so that the top of the dielectric after spin is taller over regions of relatively higher feature density. Two issues caused by such iso-dense planarization effects are variations in via depths through the spin on layer and insufficient coverage over isolated features.

This paper describes characterization of iso-dense BCB planarization effects through optical depth profiling and through electrical test of parts from a mask set designed to demonstrate the extremes of the thickness range possible in an RF HBT process. Methods are also described to predict layout-dependent thickness variation from density computations of various feature layers, weighted by feature thickness, with the resulting weighted-density computation implemented in a design rule checker.

Some figures from the work are included below. Figure 1 shows a comparison of surface height after BCB spin and cure, measured optically across example RF circuits versus a profile computed by a marching-squares weighted-density DRC check. Figure 2 shows SEM cross-section examples of shorted and separated metal lines over isolated features, and of well-formed and problematic plated vias through regions of low and high density. Figure 3 shows electrical data collected from two process splits across a set of test structures of varying density, producing a picture of where density variation begins to cause process failures for the different process options.

Paper 8a.4.pdf