Under Bump Metal Etching - A Little Known Key Process in Wafer Level Packaging
Author: Ross Kulzer Company: Semsysco GmbH Date Published: 2/11/2014
Pan Pacific Symposium
Abstract: Increasing die complexities required by next generation mobile communications are driving the development of flip chip bump and assembly technologies. Semsysco has been focused on developing Under Bump Metal (UBM) etching, a key process in the Back End of Line (BEOL) wafer flow, for next generation Wafer Level Packaging (WLP) technologies. Over the last several years, with the increasing penetration of flip chip technology into the 300mm Wafer-Level Packaging market, requirements for process performance of solder and copper pillar bumping technology have become more stringent. The largest challenges that must be addressed are uniformity, complete film removal, control of undercut and chemical attack of the bump. The performance of each of these challenges is largely dependent on the chamber chemical dispense technique and the wet etch chemistry selected to etch each UBM layer. In the past, conventional processes for UBM etching use batch processing, either wet bench or automated batch systems. These conventional approaches are subject to issues of non-uniformity that can be related to poor control of chemical distribution across the wafer surface and between parallel wafer surfaces. Although successful at demonstrating UBM etching on multiple metal stacks, in this paper we chronicle the results of copper pillar microbumps using a single wafer chamber. Due to the possibility of tighter control in single wafer processing, etch times in a single wafer tool are typically less than in batch processing. Orientation of the wafer, spray pattern and the individual care offered to each wafer in a single wafer chamber can also provide process advantages over a less versatile bath processor. As a results of these advantages, Semsysco’s Triton UBM etch single wafer platform can provide a very uniform complete film removal, while maintaining stringent undercut control of less than 0.5um at a high throughput. We have successfully demonstrated UBM etch processes for Ti/Cu and TiW/Cu based bumps for 28 and 20nm technology nodes.