Detection of Voids in Tungsten Interconnect Vias Using Laser-Induced Surface Acoustic Waves
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E3.4.1
Detection of Voids in Tungsten Interconnect Vias Using Laser-Induced Surface Acoustic Waves Joshua Tower1, Michael Gostein1, Koichi Otsubo2, and Atsushi Kawasaki3 1 Philips Advanced Metrology Systems, Inc., 12 Michigan Drive, Natick, MA 01760 USA 2 Philips Advanced Metrology Systems, Inc., Kohnan, Minato-ku, Tokyo 108-8507 Japan 3 NEC Electronics, Process Technology Division, Sagamihara, Japan
ABSTRACT Test wafers were prepared under different process conditions of barrier and seed film deposition, in order to study the effects on tungsten plug filling. Tungsten fill percentage was measured using laser-induced surface acoustic wave metrology on plug arrays of varying plug diameter and the results were verified by SEM cross-section. The data show that, for via diameters of 0.12 micron or less, the seed deposition process is a critical factor in effective (void free) plug filling. Seed deposition by ALD and low-temperature CVD were found to be far more effective than conventional CVD. Changing the thickness of the TiN barrier layer was found to have minimal effect on the occurrence of voids. For via diameters greater than 0.12 microns, all of the seed processes led to relatively little voiding. INTRODUCTION Interconnect technology is steadily advancing to narrower lines and vias with increased aspect ratio, in order to accommodate the push to higher densities and smaller features. This trend presents challenges for deposition of metals into the damascene trenches and vias without leaving un-filled areas (voids). These voids result in increased resistivity and can cause open circuits and device failure. The goal of void-free metal deposition for future technology nodes is being pursued through intensive development efforts aimed at both copper and tungsten filling processes. As trenches and vias become narrower, one of the keys to effective filling is making the barrier and nucleation (seed) layers thinner and more conformal. In this work, we compare how several barrier and seed processes lead to different degrees of voiding. In order to optimize and monitor metal-fill processes, novel metrology techniques are needed to provide timely and non-destructive feedback. Conventional techniques for characterization of voids, such as cross-sectional SEM, take a long time and require the wafers to be sacrificed. Electrical testing does not require destruction of the wafers, but it does require contact to the wafer, and in the case of via chains, several metal levels must be fabricated before the electrical test can be completed. Optoacoustic technology based on laser-induced surface acoustic waves [1,2], which has previously been implemented for measurement of metal film thickness, can provide the capability for detection of process-induced voids. This enables realtime feedback for improved process control and also facilitates process development.
E3.4.2
EXPERIMENTAL Test wafers were fabricated with tungsten via (plug) arrays of various sizes, ranging from 0.08 micron to 0.50 micron diameter. These wafers were speci
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