Electrochemical process for advanced package fabrication

Interconnections for high-end applications are essentially low-resistance transmission-line structures with precisely controlled cross-sectional shapes and dimensions. The relatively thick copper conductors-typically 6 mu m or more-combined with the stringent control required on the 10-20-mu m-wide cross sections stretches the capabilities of the subtractive etch and lift-off processes that are typically used in semiconductor fabrication to pattern evaporated and sputtered metal films. Electroplating through a photoresist mask, which has proven itself to be a highly effective, precision manufacturing process for thin-film magnetic recording heads, is, however, capable of meeting and far exceeding the requirements of package fabrication. This paper describes the fabrication of a package structure that integrates traditional dry-process technologies with electrolytic copper plating to form the conductors, polyimide backfill and planarization steps to form the dielectric, and electroless deposition to selectively clad the copper lines to prevent adverse reaction of the copper with water generated during the polyimide cure. The discussion highlights salient issues which are pertinent to the compatibility of the individual process steps and to the extension of the technology to more demanding packaging structures and to other applications.