Towards implementation of a nickel silicide process for CMOS technologies

In this paper, we review some of the advantages and disadvantages of nickel silicide as a material for the electrical contacts to the source, drain and gate of current and future CMOS devices. We first present some of the limitations imposed on the current cobalt silicide process because of the constant scaling, of the introduction of new substrate geometries (i.e. thin silicon on insulator) and of the modifications to the substrate material (i.e. SiGe). We then discuss the advantages of NiSi and for each of the CoSi₂ limitations, we point out why Ni is believed to be superior from the point of view of material properties, miscibility of phases and formation mechanisms. Discussion follows on the expected limitations of NiSi and some of the possible solutions to palliate these limitations.     

Electrotransport in copper alloy films and the defect mechanism in grain boundary diffusion

Among the polycrystalline thin film conductors of Al, Cu, Ag, Au and their alloys, conductors made of Cu alloyed with Be exhibit the longest lifetimes under similar conditons of electromigration testing. Data have been analyzed for a Cu-Al sample tested at 2×10¹⁶ A cm^-2 and 265°C for 1400 h and for a Cu-Be sample tested at 3×10⁶ A cm^-2 and 291°C for 13 000 h. In both cases the transport of Cu occured from the negative to the positive terminal. The rate of grain boundary diffusion of Cu in Cu-Be is more than fifteen times smaller than that in Cu-Al.Observations of mass transport effects due both to electromigration and to compressive stresses in Cu-Al samples indicate that the atomic flux in grain boundary diffusion is accompanied by a counter flux of defects, presumably vacancies.     

Mass transport during electromigration in aluminum-magnesium thin films

Single-crystal and polycrystalline aluminum thin film conductors, containing up to 6 at.% magnesium, were subjected to electromigration experiments at 175° and 225°C with current densities up to 4 × 10⁶ A/cm² and for times up to 50 000 h. These conductors were subsequently examined in order to obtain quantitative data on mass transport during electromigration.The results indicate that in an alloy containing about 6 at.% magnesium the lattice diffusivity of aluminum is larger by an order of magnitude than the corresponding lattice diffusivity in pure aluminum, whereas the grain boundary diffusivity of aluminum in this alloy is two orders of magnitude smaller than the corresponding value in pure aluminum. Further, the lattice and grain boundary diffusivities of magnesium in this alloy are of the same order of magnitude as the aluminum diffusivities, and appear to be slightly smaller in both cases.     

2.22 The deposition by evaporation of CuAl alloy films

3hin films of CuAl alloys from pure Cu to pure Al were deposited by vacuum evaporation from single alloyed sources heated by electron bombardment. Information about the composition of the films obtained by electron microprobe was completed by lattice parameter measurements and structure information obtained by X-ray diffraction, as well as electrical resistivity measurements. The anticipated compositions of the films were calculated from the vapor pressures of the pure elements and thermodynamic data for liquid CuAl alloys at 1373 K. The agreement between the calculated values and the experimentally observed compositions is good only for alloys with low Al concentrations: less than 20 at.% Al in the source material. For the remainder of the CuAl system with more than 20 at.% Al in the source, the films were found to contain less Al than was calculated. This discrepancy might be due to too high values for the reported vapor pressures of Al.     

Effects of alloying elements on cobalt silicide formation

Alloying elements can substantially affect the formation of cobalt silicide. A comprehensive study of phase formation was performed on 23 Co alloys with alloying element concentrations ranging from 1 at.% up to 20 at.%. Using in-situ characterization techniques in which x-ray diffraction (XRD) and elastic-light scattering are monitored simultaneously, we follow the formation of the silicide phases and the associated variation in surface roughness in real time during rapid thermal annealing. For pure Co silicide, we detect the formation of all stable silicide phases (Co₂Si, CoSi, and CoSi₂) as well as abnormal grain growth in the Co film and thermal degradation of the silicide layer at high temperatures. The effect of the various additives on phase formation was determined. The roughness of the interface was also measured using grazing incidence x-ray reflectivity (GIXR). We show that by selecting an alloy with a specific composition, we can change the phase-formation temperatures and modify the final CoSi₂ film texture and roughness.     

Effects of Mg additions on the electromigration behavior of Al thin film conductors

Aluminum thin film conductors containing Mg alloying additions have been tested for electromigration failure by formation of electrically open circuits. The test conditions were either 2 × 10⁶ A/cm² or 4 × 10⁶ A/cm² for the current density, and either 175 or 225°C for the temperature. The median lifetimes were found to increase with increasing Mg concentrations up to the highest concentration tested, about 6%. With polycrystalline films the maximum increase in lifetime resulting from Mg additions corresponds to a factor of about 100, as compared to pure Al films. This is about equal to previously reported results obtained with Cu additions. The increase in lifetime has been shown to result from a decrease in the rate of grain boundary diffusion for the Al atoms. Magnesium atoms diffuse at approximately the same rate as Al atoms. Thus the mechanism of failure formation in Al films containing Mg is thought to be different than in Al-Cu films, where Cu atoms diffuse faster than Al atoms and failure ensues upon local Cu depletion.     

Aluminum films deposited by rf sputtering

The feasibility of depositing aluminum thin films by means of rf sputtering has been investigated. Film characteristics compatible with the requirements of conductive stripes in integrated circuits have been obtained. The rate of film deposition has been related to rf power dissipation, argon pressure, geometry, and magnetic field. Film characteristics such as electrical resistivity, stress, grain size, and orientation have been studied and compared to the characteristics of films deposited by means of evaporation. The effects of annealing on some of these characteristics have been determined.     

Silicide interfaces in silicon technology

This paper does not constitute an overall review of the role of interfaces in the use of silicides in current microelectronic technology. However, for the sake of some degree of completeness, most of the uses of silicides, be it infrared detection or thermoelectric generation, are mentioned and some recent references listed. Emphasis is given to four different topics. The first is the use of PtSi for ohmic and rectifying contacts; this provides the opportunity a) to look at the development of planar device metallization, starting with aluminum, and b) to survey briefly elements of the theory of Schottky and ohmic contacts to semiconductors. A second topic is the epitaxy of silicides on silicon, with a brief discussion of β-FeSi₂, recently shown to be electroluminescent. What is known about grain boundary diffusion in silicides and other intermetallic compounds is reviewed with some speculation about the structure of grain boundaries and the consequences for silicide growth via solid state reactions. Finally, the role of epitaxy between various morphological variations of TiSi₂ is examined because of the important place of this silicide in metal oxide semiconductor field effect transistor technology.     

The formation of silicides in Mo-W Bilayer films on si substrates: A marker experiment

Use is made of the metallurgical similarity between Mo and W, and of their different atomic weights to study the diffusive formation of MoSi₂ and WSi₂. Thin film bilayers of Mo and W were deposited via e-beam evaporation upon Si single crystal substrates and annealed in an inert atmosphere at temperatures up to 1000‡C. The annealed samples were analysed by means of Rutherford backscattering and x-ray diffraction. The results indicate that Si is the dominant diffusing species. Observations on the formation of WSi₂ are consistent with a process dominated by grain boundary diffusion through the W film.