IPC TM-650 2.2.21

Abstract

The object of a planarity measurement is to determine the effectiveness of a polymer thin film in smoothing topological features created by underlying layers of circuitry or etch patterns. In this case, the polymer thin film is a dielectric material for use in High Density Interconnect (HDI) and microvia technologies.  Test Structure Schematics given in Figure 1 (end view) and Figure 2 (top view) depict the essential features of a recommended test structure for measuring the planarity as a function of feature size. In the finished test structure, ‘‘d'' is the step height over the circuit trace after the polymer has been deposited and cured. The planarity is dependent on the trace height, ‘‘b,'' the polymer coating thickness, ‘‘c,'' and the trace width, ‘‘a.'' Rather than fully characterizing this relationship for each polymer coating, it is best to use simplified standard procedures for measurement and for comparing different types of coatings. Planarity is also affected by the proximity of the line feature being used for measurement to any neighboring topological feature in the test structure. Planarity is generally much lower for isolated features. Therefore, in order to consider the ‘‘worst case'' conditions, isolated lines are preferred in the test structure. To satisfy this requirement, no neighboring features can be within fewer than 15 line widths of the line in question. The test pattern in Figure 2 is recommended as one that provides planarity characterization over a broad range of feature sizes. All lines are isolated in accord with the above guideline. It is recommended planarity be determined over the complete range of these widths (but the specific nominal values within that range are not important). Normally, one finds the planarity is high over narrow lines, but it progressively rolls off in proceeding toward wider ones. The roll-off rate is important, and it varies from polymer to polymer. Thus any report of planarity must include the line width measured and, preferably, a plot of planarity versus line width should be reported.