Silicided Shallow Junction Formation Using Ion Implantation and Thermal Annealing
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SILICIDED SHALLOW JUNCTION FORMATION USING ION IMPLANTATION AND THERMAL ANNEALING LEONARD M. RUBIN*, N. HERBOTS*, D. HOFFMAN**, AND D. MA** *Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 "**StandardMicrosystems Corp., 35 Marcus Boulevard, Hauppauge, NY 11788 ABSTRACT The combination of arsenic and boron implantation with rapid thermal annealing (RTA) has been investigated to form shallow p-n junctions under a titanium silicide (TiSi 2 ) metallization. The use of TiSi2 as a connection material can lead to the destruction of the junction if the kinetics of silicidation and doping are not well controlled. The purpose of this study is to better understand and control these kinetics, using far-from equilibrium processing such as ion implantation and RTA. The structures were characterized by Rutherford Backscattering Spectometry (RBS) for arsenic and silicide profiling, Secondary Ion Mass Spectometry (SIMS) for boron profiling, Scanning Electron Microscopy (SEM), and electrical sheet resistance measurements. Two procedures were investigated. Both involved the thermal reaction of Ti thin films, sputter-deposited with thicknesses ranging between 40 and 80 nm. In the first experiment, the as-deposited films were implanted with either 115 keV arsenic or 28 keV boron to form the junction, disperse the native oxide, and ion beam mix the Ti and Si. The films were then subjected to an RTA at 750'C for 15 to 60 seconds, which leads to TiSi2 formation in unimplanted films. Implantation was found to actually prevent TiSi 2 formation. Ion transport calculations indicated that dopant pile-up at the interface might inhibit silicidation while higher energies and larger implant doses can more effectively ion beam mix Ti and Si. A more attractive solution consists of first forming TiSi 2 from the as-deposited Ti by RTA, and then implanting to form the junction. This resulted in better control of the junction thickness. A sharp increase in the TiSi 2 resistivity was found after implantation but the original value could be restored by a second RTA. This RTA also electrically activated the dopants and recrystallized the junction. The material properties ofTi/Si and TiSi 2/Si under ion bombardment, RTA, doping, and conventional furnace annealing will be discussed. INTRODUCTION Silicides have come into widespread use in recent years as low resistivity interconnect films for integrated circuits. Titanium silicide is especially suited for this purpose, because of its low resistivity (about 15 paf-cm) [ 1,21 and its ease of formation by thermal reaction of Si and a deposited Ti film. This step is often performed in a rapid thermal annealing machine, because of the tight process control afforded by RTA and the rapid rate at which the reaction proceeds in the temperature range 650-850°C [3,4]. Sheet resistivities of 1-2 W/square have been achieved over the diffused regions of MOS circuits by forming TiSi2, as compared to 50W1square for unsilicided regions [5]. I
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