Tungsten film prepared by a Low Pressure Chemical Vapor Deposition (LPCVD) has an advantage over a sputtered aluminum for its successful filling contacts or vias of high aspect ratio. Although selective deposition may simplify the metallization process, it has some problems, encroachment and tunneling which are induced by a preferable reaction of $WF_6$ with silicon instead of reducing agents ($H_2$ or $SiH_4$) at the initial stage of the deposition. Junction failure or poor contact property are closely associated with these problems in the selective deposition. On the other hand, blanket deposition covers the entire surface of a wafer coated with barrier layer. TiN or TiW has often used as a barrier metal for this process. In this case, a reliable contact property can be attained by the help of the barrier layer sandwitched with tunsten and silicon layers, though the process complexity including etchback is a drawback in the blanket tungsten process.
Barrier metal has played a role as diffusion barrier in between some metals and silicon in VLSI process. Besides this role the barrier metal also promotes the adhesion strength of a metal layer which has an inherently poor adhesion on the insulator layers such as $SiO_2$, $Si_3N_4$ and doped silicon oxides. Sandwitched layer used for this purpose is called as adhesion promoting or glue layer. TiN film has been proved to be a superior material to others in terms of electrical resistivity, contact with silicon and adhesion at present stage of developing ULSI devices. TiN has been also employed as a glue layer in blanket tungsten deposition. Unfortunately, although TiN is an excellent glue layer, it reacts with $WF_6$ very slowly. So a initial time delay is needed for a continuous tungsten film to be deposited on TiN surface. Initial stage reactions during the time delay will affect a W/TiN interfacial properties. There have been a few suggestions, experimental results and a thermodynamic modeling associated with tungsten nucleation on TiN. However, other reactions except for the nucleation have not been investigated. The surface of reactive sputtered TiN will be oxidized in a normal laboratory condition. This highly oxidized surface layer seems to have been treated lightly. However, although the reactions of this layer with $WF_6$ and/or reducing agents may have little effect on the nucleation reaction itself, they will affect the W/TiN interfacial properties.
In this study, the W/TiN interfacial property with various tungsten deposition temperatures was investigated from the viewpoint of the reactions between gas mixture and oxidized surface layer of TiN at the initial stage of the deposition. The reacting gas mixture was $WF_6-SiH_4-H_2$ and TiN was prepared by a reactively sputtering Ti in $N_2$ atmosphere. The adhesion strength was examined as a interface property. Residue at the interface was also investigated by Auger electron spectroscopy (AES) depth profiling. The remanent elements at the interface are probably oxgen and fluorine, which can be detected by AES analyses. However, ever if any impurity element exists at the interface, it is impossible to identify the exact chemical species. For this purpose, the advanced techniques such as mass spectrometer and in-situ XPS analyses under a ultra high vaccum should be used. So in this study, thermodynamic calculations using SOLGASMIX-PV program were performed to predict the chemical compounds at the W/TiN interface. The component of TiN surface layer was assumed to be $TiO_2$ in the calculations.
Initial time delay phenomenon is also observed in a tungsten deposition using $WF_6-SiH_4-H_2$ gas mixture on TiN glue layer. Reactively sputtered TiN surface is a kind of naturally oxidized layer, which of elemination takes place simultaneously with tungsten nucleation during the time delay. Below 300℃, the elimination reaction of the TiN surface layer seems to enhance a segregation of fluorine at the W/TiN interface. On the other hand, above 300℃, such a reaction seems to have little effect on the segregation of fluorine at the interface. The fluorine accumulation causes a poor adhesion between tungsten film and TiN glue layer. A fluorine-based species ascribed to the fluorine accumulation at the interface is predicted as $TiF_3$ through the thermodynamic modeling. However the exact chemical specie is not indentified in this experiment. The fluorine accumulation can be removed by Rapid Thermal Anneal at above 750℃ for 30 seconds and thus the adhesion strength is improved.
Some results of this study show that an optimum range of tungsten deposition temperature is from 350℃ to 450℃ in the $WF_6-SiH_4-H_2$ chemistry. The other results will provide some new idea to solve the practical problems associated with contact properties and junction characteristics in the application of the blanket tungsten as a metallization in ULSI.
