6.1 Introduction

The development of very complex ICs with extremely high I/O counts and steadily increasing clock rates has forced electronics manufacturers to develop new packaging and assembly techniques. In addition, more and more electronic devices have to be portable and, consequently, system integration, volume, and weight are rapidly gaining importance. At the same time, the pressure to eliminate any environmental impact and to reduce cost is exponentially increasing. As a result of these considerations, the future substrates have to be high-tech, low-cost interconnects characterized by very fine lines and spaces and the use of microvias, as both through-vias and blind vias as well as combinations thereof. In addition, the increasing demand for 3D packaging calls for flexible and rigid-flexible designs and highlights the importance of new interconnect substrate technologies able to cope with these recent and emerging requirements in modern electronics.1

Microvias can be formed by various etching techniques. Plasma etching and chemical etching can be extremely cost effective for generating high volumes of small holes in dielectric layers. In either case, the process cost is derived from the number of holes in a given working area. The basic principle of via etching is to create a mask that defines the positions and sizes of the holes. This may be achieved by using dry film to image, then etching a hole pattern in a copper layer, or simply by using the dry film as the etch mask by imaging and developing. Both methods are isotropic such that they etch...