High Resolution Tem Studies of Solid-Phase Epitaxial Si at Contact Holes Through Al-Si Alloy
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HIGH RESOLUTION TEM STUDIES OF SOLID-PHASE EPITAXIAL Si AT CONTACT HOLES THROUGH Al-Si ALLOY Norio Hirashita, Masako Kinoshita, and Tsuneo Ajioka Oki Electric Industry Co., Ltd., VLSI Research & Development Laboratory, 550-1 Higashi-asakawa, Hachioji, Tokyo 193, Japan ABSTRACT Microstructural studies of Si precipitated on contact holes through Al-Si alloy has been carried out for 1-Mbit DRAMs using cross-sectional transmission electron microscopy and X-ray microanalysis. Lattice images reveal the perfect epitaxial growth of Si on Si substrates at contacts through Al-Si alloy at temperatues • 420°C. At the interface between substrates and the solid-phase epitaxial Si, ellipsoidal micro-defects, associated with amorphous oxygen-rich materials containing Al, are observed. In the epitaxial Si evidence of dislocations and twins is found. INTRODUCTION Al-Si alloy has been widely used as a metallization material because of its low resistivity, strong adhesion to silicon dioxide, and manufacturing feasibilities. In Al-Si alloy, Si is contained beyond the solid solubility limit to prevent Al spikes at contacts. As device dimensions are miniaturized, the excess Si causes serious problems on reliability. The Si precipitates as Si nodules in interconnections, which results in degraded electromigration and stress induced migration characteristics. The Si also precipitates on Si substrates at contact holes, which causes an increase in the contact resistance. The Si precipitated on contact holes is believed to be the solid-phase epitaxial (SPE) growth, since it had been reported that the dissolved Si and Ge in a transport media such as Al (1) or thin palladium silicide films (2) grow epitaxially onto the substrates at temperatures less than one-half of the melting point of transport media. This report first presents direct observation of the Si SPE growth at contacts of VLSIs through Al-Si alloy using lattice imaging electron microscopy of cross-sectional specimens. Observations of defects at the interface and in the SPE are also reported. EXPERIMENTAL CMOS 1-Mbit DRAMs were used in this study. After a reflow of dielectric interlayers, Al-iSi (700nm) was deposited by a DC magnetron sputtering method. The Al-Si alloy was sintered in furnace at 400'C for 30 min. in H2 atmosphere following pattern delineation of Al-Si metallization. Final passivation films consisted of phosphosilicate glass (200nm) and silicon nitride (80Onm) layers, where the maximum process temperature was 42 0 'C. Then passivation anneal was performed at 350'C for 60 min. in N2 ambient. Specimens for a cross-sectional transmission electron microscopy (XTEM) were prepared using the standard technique, (3) where the final thining process was done by Gatan model 600 ion miller with 4 kV Ar . High resolution TEM images were obtained using JEOL JEM-2000EX electron microscope operating at 200 kV. Energy-dispersive X-ray (EDX) spectra for chemical analysis and electron diffraction patterns of local regions were obtained using a micro-electron beam in a VG Microscope HB5
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