No abstract

A very simple and reproducible cobalt silicide process with Mo/Co or W /Co bilayer metallization to overcome the oxidizing liability of Co annealed in a normal flowing-nitrogen furnace has been developed. Cobalt is deposited on blank and patterned silicon wafers in an electron-beam evaporation system followed by Mo (or W) deposition without breaking the vacuum. The cobalt silicidation is carried out using a two-step annealing process. The first annealing is performed at a temperature ranging from 400 to 600 °C, during which the role of the upper layer ofMo (or W) is to protect the underlying Co layer from being oxidized while not disturbing the cobalt's silicidation process. Perfect selective etching of Mo (or W) can be accomplished by a NR 4 0H + Hz O 2 + (2-3) H2 0 solution. The second annealing is performed at a higher temperature of7S0"C to completely transform the CoSi obtained in the first annealing into CoSi z and induce grain growth of CoSi 2 , thus lowering the resistivity. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and sheet resistance measurement are used to characterize the silicide phase and microstructure.

A simple metallization process to form a shallow CoSi2 silicided contact and W–Ti diffusion barrier simultaneously has been developed. The process starts with depositions of a Co–Ti alloy layer and an overlying W layer on silicon wafers using a dual e-beam evaporation system; this is followed by a single-step annealing treatment in a normal flowing-nitrogen furnace. On the other hand, bilayer self-aligned shallow MoSi2/CoSi2/Si silicided contact can be derived from the W/Co–Mo/Si system by a two-step annealing treatment performed in the same environment. I–V characteristics for silicided p+n diodes with the diffusion barriers formed in situ were measured after they had been finished with Al deposition and post-Al annealing at temperatures from 350 to 600 °C in N2. For comparison, Al/CoSi2/p+n and Al/p+n structures were also investigated. It turns out that the integrity of Al/W–Ti/CoSi2/p+n and Al/MoSi2/CoSi2/p+n silicided contacts can be preserved up to 550 and 500 °C, respectively, for a 20 min annealing, while that of Al/CoSi2/p+n can be kept only up to 450 °C.

A structure of W-Ti/CoSi2/Si can be formed from an appropriate metallization system of W(400 A)/C0 63 Ti 37 (360 A)/Si at temperatures from 620 to 760'C in a normal flowing-nitrogen ambient. The W-Ti alloy layer is only slightly oxidized on the shallow surface and can be used as a diffusion barrier between the aluminum and silicide. Furthermore, silicide lateral growth does not occur in this silicidation scheme. However, an additional intermediate Ti-silicide and/or Ti-oxide as well as a Ti-oxide layer on the surface will be formed if the content of Ti in the Co-Ti alloy exceeds that required to saturate in the overlying W film, e.g., W(400 A)/C0 63 Ti 37 (840 A)/Si. The AI/w-TilCoSi 2 Ip+n diodes with metallization using this scheme are able to maintain the integrity of I-V characteristics with a post-AI annealing at 550'C for 20 min. In addition, epitaxy of CoSi 2 occurs in this metallization system.

W or Mo directly deposited on Si cold substrate by electron-beam gun at a base pressure of 10-6 torr is not able to form silicide even annealed at 900 9C in either N 2 or H 2 ambient. We present an easy way that Mo and W silicides can be formed on the same depositing and annealing conditions with the help of an intervened layer of cobalt or its alloy. Investigation was made on various metallizations of Mo (or W)/Co/Si, W/Co-Mo/Si, and Co/Mo/Si in normal flowingnitrogen or in H 2 ambient at various temperatures. In the systems of Mo (or W)/Co/Si and W/Co-Mo/Si, the overlying Mo (or W) can be transformed into silicide at 900 9C, while in the Co/Mo/Si system, where stable Co-Mo compounds are formed in advance, no silicide can be formed. Why silicide is formed in preference to metal oxide in N 2 environment at higher temperature is based on Ellingham diagram.