Adhesion to very smooth substrates utilizing an optimized Semi-Additive Process

Mobile phones, tablets and other hand held devices are pushing technology to new heights. These devices continue to add greater functionality while decreasing in size. This miniaturization demands tighter pitch, finer lines and better signal integrity. Nowhere is this clearer than in the IC substrate market. Semi-Additive Processing or SAP has been widely used in the manufacture of tighter pitch and finer lines for many years, however with the growing need to maintain and improve signal integrity, different dielectric materials are being evaluated. One of these materials is polyimide (PI), and although not new to the industry, PI shows good chemical resistance, low moisture adsorption and good UV absorption. In addition, PI has excellent thermal stability, low dielectric constant and good processability. These unique chemical, mechanical and electronic properties make PI one of the best candidates for interlayer dielectric. MacDermid developed an optimized semi-additive process (SAP-PI), which was tailored for PI film in build up application. This SAP-PI combines low-roughness desmear and efficient adhesion promotion processes. After desmear, the PI surface showed Ra <60 nm, while achieving initial peel strengths between 900–1700 gf/cm on different commercial PI substrates. After 7-day baking test at 150 °C, the peel strengths can be maintained between 500–1100 gf/cm. This paper will discuss an optimized process flow and associated steps in detail to alleviate concerns on processing complexity. It will include characterization and analysis on PI and metal plating topography using scanning electron microscopy, their compositions using energy dispersive X-ray spectroscopy, and adhesion between them from 90° peel tests.