SEMICONDUCTOR MANUFACTURING: Excimer laser reflows copper in chip production

Sept. 1, 1996
Developers at XMR Inc. (Santa Clara, CA) have demonstrated a low-temperature excimer-laser-based method for filling prepatterned integrated-circuit silicon-wafer trenches with copper.

Developers at XMR Inc. (Santa Clara, CA) have demonstrated a low-temperature excimer-laser-based method for filling prepatterned integrated-circuit silicon-wafer trenches with copper. The advent of ultralarge-scale integration (ULSI) devices necessitates higher circuit densities with smaller design features. While aluminum alloys have been used to metallize circuit elements, with the smaller ULSI applications these metals are susceptible to electromigration (atomic movement due to voltage application) and stress effects. Copper has a lower resistivity and longer electromigration lifetime.

Pulsed lasers have been shown to cause melt and flow of sputter-deposited thin films of aluminum and silver on wafers. The molten metal fills contacts, vias, and interconnect trenches producing a planar surface for subsequent processing steps. These results, however, have been limited to surface features with aspect ratios (depth-to-width) of less than two. The excimer laser allows reflowing of sputter-deposited copper into submicron trenches with aspect ratios greater than four. The process, which takes place at a relatively low 400°C, results in good side-wall and bottom coverage and anneals the metal.

In reflowing copper, 308-nm light from a xenon chloride laser delivers 45-ns pulses of up to 667 mJ at a repetition rate of 300 Hz. In the system, a beam homogenizer converts the quasi-Gaussian beam into a rectangular top-hat profile with a uniformity across the beam. The homogenizer also governs beam dimensions from 2 to 50 mm in the x and y directions to vary fluence. A wafer is processed by stepping the laser beam over the surface to produce a grid of overlapping spots. Time for processing a 200-mm wafer is less than 40 s, which is a high throughput compared to the processing time of more than 5 min for current surface-diffusion methods for copper.

Laser energy density is a critical factor in reflowing copper. To fill high-aspect-ratio trenches the energy density must be greater than the threshold energy to completely melt the copper and fill the trenches, or a void in the trench volume may remain. A higher aspect ratio requires increased energy density. Sputtered-copper thickness affects the processif it is not thick enough there may not be enough copper to fill the trenches. But if the film is too thick, then a higher laser energy is needed for complete melting, and the laser could ablate some copper on the surface before the features are filled. The optimum thickness of the copper is on the order of the trench depth.

Yale Sun, XMR product manager, says other advantages include a larger resulting grain size of copper, which improves smoothness over the trenches. Such smoothness facilitates the next chemical-mechanical processing step to remove the copper layer over the trenches, leaving the completed metal circuit features. According to the researchers, excimer-laser reflow of copper results in increased overall circuit yields and potential processing costs on the order of $0.50 per 200-mm wafer.

About the Author

Rick DeMeis | Associate Editor, Technology

Rick DeMeis was Associate Editor, Technology for Laser Focus World from March 1995 through March 1997.

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