Maskless, Direct Deposition of Copper onto Aluminum Bond Pads for Flip Chip Applications
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ABSTRACT Flip chip interconnection of integrated circuits (IC) for packaging applications such as direct chip attachment use Pb-Sn solders as the connection between the die and the substrate. Underbump metallization is typically used to transition from the non-solderable Al bond pad on the IC to a solderable surface such as copper using traditional blanket metal deposition, photolithography and etching procedures. In this study, we report for the first time the use of a novel process for selectively depositing adherent copper directly onto aluminum thin films, eliminating the need for adhesion promoting transition layers and additional patterning steps. Utilizing copper bearing organic solutions and standard electroless and electrolytic copper plating baths, as-deposited and annealed sputter deposited AI-x%Cu (x = 0 to 2) thin films were coated with metallic copper. An increase in the organically deposited copper nucleation site density was observed with increasing copper concentration in the sputtered aluminum/copper thin films. Preliminary results using focused ion beam microscopy indicated that dissolution of the aluminum oxide surface and subsequent deposition of copper by cementation occurs in the non-conducting organic solution at sub-micron reaction lengths. Qualitative adhesion testing of samples resulted in the majority of films passing the tape test. Demonstration of the process using 50 micron diameter vias in BCB coated flip chip test vehicles from MCNC will be presented.
Introduction The increasing demands for smaller, lighter weight, low profile, portable electronics lead to the increase in the density of packaging and interconnection of semiconductor devices. In order to increase packaging density, advanced packaging techniques need to be developed to replace current packaging process such as wire bonded and perimeter leaded die molded packagings. As interconnection technologies such as chip scale packaging and flip chip solder attachment emerge as the new paradigm for advanced packaging applications, there are increasing demands on reducing the complexity and cost of fabricating electronic assemblies. In many instances, the ability to solder bump individual die has a number of technical and financial advantages when compared to conventional wafer bumping processes. The development of novel direct write deposition techniques, such as solder jet deposition, has addressed the issue of individually solder bumping bond pads. However, in order for reliable, adherent solder interconnection to the bond pad to occur, a solderable underbump metallization must be deposited onto the aluminum bond pad prior to solder deposition. While copper is the underbump metal of choice, adhesion promoters such as Cr and Ti are often deposited onto the aluminum bump pad prior to copper deposition. Typically the deposition of the underbump metallization is done in wafer form using blanket deposition, photolithography patterning and etching to remove unwanted materials. To date, attempts to directly deposit copper onto alumin
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