Low temperature Al-Cu planarization by PVD Al-Cu-Ge alloys (Invited lecture)

Abstract

This paper describes a low temperature reflow of Al-CU alloys by incorporating Germanium in the film. Germanium is added in Al-Cu alloys by reacting GeH4 with the films. The Al-alloy reflow occurs due to formation of low melting point eutectic with Germanium. This technique results in a low cost process for filling high aspect ratio vias/lines with Al-based alloys with improved damascene capability. This is achieved at temperatures below 400°C by reacting Germane (GeH4) with Al-Cu alloys deposited by conventional techniques which result in voids, gaps and poor filling. Using such a process it is demonstrated that high aspect ratio vias with large undercuts can also be filled without voids. Also it is possible to partially fill the vias with low pressure sputtering followed by Germane reactions. This reaction eliminates overhangs and fills vias with Al-Cu-Ge. The low temperature provides the capability to form a multilevel homogeneous Al-alloy via/line structure without degrading the resistance of underlying interconnects. The reliability data shows that Al-Cu-Ge via/interconnect structure deposited by this method is at least "1.5 X " better electromigration life time (t50) to that of hot sputtered Al-Cu (deposited at 535°C) and almost "2 X " to that of conventionally used CVD W stud/Al-Cu interconnect structure. The improvement in the reliability may be attributed to filling without voids high aspect ratio sub-half micron vias with low resistivity metal such as Al-Cu-Ge at temperatures well below 400°C. Since this technique does not rely on the wetting layer or the diffusion barrier a lower sheet resistance of Al-Cu-Ge line is achieved compared to high temperature reflow processes. Also the reaction between Al and GeH4 takes place uniformly resulting in reproducible contact resistance. Most importantly it is possible to achieve "high open-short yields" of comb-serpentine structures, and via chains of difficult to polish materials like Al-Cu using a "damascene" process.