The application of electrodeposition processes to advanced package fabrication

Abstract

Conductors for advanced packaging have thicknesses of the order of 6 microns and aspect ratios that are approaching 1:1. These requirements are well within the capabilities of electrodeposition technology. The experience of the last decade in using electrodeposition to build thin-film recording heads, which have similar and, in some respects, even more demanding specifications than packaging structures, is directly applicable to the needs of packaging. This paper will show the application of resist-pattern plating to fabricating conductors for packaging, will discuss the capabilities and limitations of resist-pattern plating plating, and will indicate the parameters that need to be understood and controlled for the successful application of electrodeposition technology to microelectronic structures. A multi-level package structure can be considered as a repetition of several conductor and via levels. Each conductor/via level is made by first sputter depositing a seed layer of Cr/Cu in which the Cu is of the order of 2000 A thick. A layer of photoresist is then applied over the seed layer, and openings are patterned in the resist to define the conductor pattern. Electrical contact is now made to the seed layer, and the part is immersed in an electroplating solution to deposit Cu in the openings defined by the resist pattern. The thickness of the deposit is determined by the time and current density of plating; the thickness of the photoresist must slightly exceed the desired conductor thickness. A new layer of photoresist is now applied and patterned to provide openings on top of the conductors where required for the vias, and the electroplating process is repeated to form these vias. Finally, the resist is stripped, and the seed layer is removed by sputter etching, leaving the desired conductor pattern. The conductor/via level is completed by applying a planarized polyimide layer. Additional levels are built by depositing a new seed layer and repeating the process sequence. Electroplating processes can easily produce 6-micron (and even much thicker) conductors with near bulk resistivity and very low internal stress. Because plating is done through a resist pattern, the lateral dimensions of the features are defined by the lithography. Conductor formation is virtually an atom-by-atom molding process in which Cu ions in the electrolyte, which initially fills the pattern, are reduced to form Cu atoms in the solid phase. The lithographic cross-section is thus precisely replicated in the copper down to atomic dimensions. Thin-film heads are routinely manufactured with copper coils whose cross-section is about 4 microns high by 3.5 microns wide on 6 micron centers. Structures of submicron dimensions and with aspect ratios greater than 10:1 are also attainable. The requirements of even advanced packages are thus seen to lie comfortably within the capabilities of electrodeposition processes. Achieving a uniform deposit across the entire pattern requires an understanding of how the current distribution is affected both by the tool design and by the pattern. The mathematical modeling that has been done to assist in meeting the stringent requirements of the thin-film head provides the guidance needed to obtain ± 5-percent thickness uniformity across a 125-mm substrate.