Characterization and Control of Copper CMP with Optoacoustic Metrology
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Characterization and Control of Copper CMP with Optoacoustic Metrology Michael Gostein, Paul Lefevre1, Alex A. Maznev, and Michael Joffe Philips Analytical, 12 Michigan Drive, Natick, MA 01760, U.S.A. 1 International SEMATECH, 2706 Montopolis Drive, Austin, TX 78741, U.S.A. ABSTRACT We discuss applications of optoacoustic film thickness metrology for characterization of copper chemical-mechanical polishing (CMP). We highlight areas where the use of optoacoustics for CMP characterization provides data complementary to that obtained by other techniques because of its ability to directly measure film thickness with high spatial resolution in a rapid, non-destructive manner. Examples considered include determination of planarization length, measurement of film thickness at intermediate stages of polish, and measurement of arrays of metal lines. INTRODUCTION The silicon integrated circuit industry is rapidly moving to adopt copper as the material of choice for circuit interconnects. Because of the lack of etching processes to remove copper, these interconnects are patterned with a Damascene process that uses chemical-mechanical polishing (CMP) to remove excess metal. (1,2) The challenge for developers of copper CMP processes is to obtain a process that planarizes uniformly across the wafer, minimizes copper dishing and dielectric erosion, and polishes uniformly on structures with widely varying density of features. Optoacoustic metrology (3,4) provides a valuable tool for CMP process development and control, by enabling rapid, non-destructive measurement of copper film thickness on both blanket and patterned films. Optoacoustics is based upon the initiation and detection of ultrasonic waves using laser light, and has been introduced in commercial tools for semiconductor metrology. (5) In contrast to profilometry, which measures surface topography, the optoacoustic technique directly measures metal thickness. It can be used at all stages of polishing, from measuring the thickness uniformity of films prior to polish, measuring removal rates at intermediate polishing stages, and measuring final post-polish thickness. In this paper, we highlight applications of optoacoustic metrology that provide unique CMP process characterization data complementary to that provided by other techniques. Applications discussed include determination of planarization length, measurement of remaining film between stages of polishing, and measurement of post-CMP thickness of arrays of submicron features. EXPERIMENTAL DETAILS Silicon wafers, 200 mm diameter, were used as substrates, and were covered with thermally grown SiO2 layers of typical thickness 4000-5000 Å. For unpatterned films, metal was deposited directly on the thermally grown oxide. For patterned films, a silicon nitride layer, ~1000 Å, was first deposited on the thermal oxide, ~5000 Å of additional oxide (TEOS) was deposited by chemical vapor deposition, and the oxide was etched back to the nitride to open the windows for the metal pattern. For the metal deposition, a tantalum liner
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