Metal silicide films

Metal and polysilicon films are formed by a chemical-vapor deposition process using organometallic gases that react at the surface of the IC structure. Various metal silicide films may also be deposited in this manner by reaction with the surface of the silicon wafer to form metal silicides. Glass and pol3uner films are deposited or spin cast or both, as are photoresist films (those of a photosensitive material). This process is accomplished by applying a liquid polymer onto a rapidly rotating wafer. The exact method used varies from manufacturer to manufacturer and usually remains proprietary. 

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

Aylett, B. J., and Tannahill, A. A., Chemical Vapour Deposition of Metal Silicides from Organometallic Compounds with Silicon-Metal Bonds, SIRA Int. Seminar on Thin Film Preparation and Processing Technolgy, Brighton, UK (March 1985)... 

These examples illustrate the important general point that the metal silicon ratio of the silicide film is always the same as that in the volatile precursor. 

In the present chapter, we will review the nature of plasma-enhanced CVD (PECVD) films for a variety of applications. We will look at dielectrics (silicon nitride, silicon dioxide), semiconductors (polysilicon, epi silicon) and metals (refractory metals, refractory metal silicides, aluminum). There are many other important films (i.e., amorphous silicon for solar cells and TiN for tool harden-... 

The next three chapters review the deposition of thermally-induced dielectric films (Chapter 3) and metallic conducting films (Chapter 4), as well as plasma-enhanced films of either type (Chapter 5). The many chemical systems employed to create these films are considered, and the nature of the resulting films is presented. Films studied are silicon dioxide, silicon nitride, polysilicon, epitaxial silicon, the refractory metal silicides, tungsten and aluminum. 

Cohalt Silicides. The interest in the study of metal silicides is growing at much faster rate because of their use as interconnects and contacts in semiconductor and VLSI technology. The silicides in general have lower resistivity than polysilicon and are able to withstand high annealing temperatures than most pure metal interconnects. In the development of the metal-silicide studies the most important quantities of interest are metal/Si ratio as a function of depth, the silicide film thickness and the identification and the quantification of any contaminants present. The conventional surface analysis techniques... 

Tungsten hexafluoride is readily reduced to the metal by CVD as seen from Eqn. (27). Accordingly, it has been widely used for the formation of tungsten metal and tungsten silicide films. Besides Eqn. (27), the basic chemical reactions for... 

Improvements in the performance of integrated circuits and the trend towards VLSI-technology require the replacement of polycrystalline silicon by materials with a lower resistivity for use as gate electrodes. Transition metal silicides appear to be valuable possibilities for these applications. Timgsten-silicon compounds could be suitable precursors for the precipitation of tungsten-silicide thin films. Moreover tungsten-silicon compounds are nearly unknown and of scientific interest. 

In the last years transition metal-silyl complexes have received special attention for several reasons [1, 2], On the one hand, they are assumed to be important intermediates in catalytic processes [2] (transition metal-catalyzed hydrosilylation reaction, dehydrogenative coupling of silanes to polysilanes, etc.), on the other metal-substituted silanes show special properties, which can be tuned systematically by judicious choice of the metal and its ligands [3] Furthermore, silylenes (silanediyls) are stabilized by unsaturated transition metal fragments leading to metal-silicon double-bonds [4]. In the light of a possible application in MOCVD processes some of these complexes are of interest as potential single-source precursors for the manufacture of thin silicide films [5]. 

Silicon is highly unstable in aqueous electrolytes due to the formation of an insulating oxide film which prevents the use of n-Si as photoanode. On the other hand, the silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its use as a photocathode. Many methods have been explored to stabilize Si electrodes in aqueous solutions for possible applications as photochemical cells. They include coating the surface with noble metals, metal oxides, metal silicides, or organic materials as shown in Table 6.6. Also, some redox species, the reduction of which can favorably compete with the oxidation of silicon, can be used to stabilize silicon anodes... 

Five stages were resolved during interface formation in Yb/Si(lll) system by AES, EELS data and in situ Hall measurements. Some amplitude oscillations have been observed in sheet conductivity, hole mobility and surface hole concentration within the Yb coverage range below 6 ML. The conductivity oscillations are explained by transition from semieonductor-type conductivity at the first two-dimensional Yb growth stages to metal-like conductivity of 2D and 3D Yb silicide films. 

The increase of the substrate temperature up to 100 °C results in an appearance of the new surface phase ((2/3) 3x(2/3) 3)-R30° (Fig. lb). This surface phase looks to be a thin epitaxial magnesium silicide film with the misfit 1.9 % with a silicon lattice [5]. According to our EELS data this surface phase is characterized by surface (hux = 9.8 eV) and bulk (hux, = 13.6 eV) plasmons, while for thick magnesium silicide films the surface (hoy = 10.3 eV) and bulk (htav= 14.6 eV) plasmons are typical [8]. The observed difference could be caused by tension of surface phase lattice. At further adsorption the metallic magnesium grows atop the silicide surface phase. 

If an alloy or compound can be created in the UHV environment, surface contamination can be often reduced below one percent. Coevaporation has been used in some instances, as well as thermal reaction of a metal deposited film on silicon to form a silicide... 

Metal silicides are widely used in microelectronic devices for the increase of the potential barrier height in metal-sUicon Schottky contact. Titanium silicide is favored for its low resistivity and thermal stability. IR spectroscopy is not typically used for investigation of metal silicides, because they do not have a characteristic and selective absorption in the IR range. Nevertheless, FTIR spectroscopy has been used to measure the thickness of Ti and titanium silicide films [274] as well as to monitor titanium silicide formation during the reaction of Ti films on a Si wafer [275]. 

Most alloys for metal film resistors are based on nickel-chromium. For lower values, alloys based on copper-nickel also are used. For very high values, a higher resistivity is needed, and some silicides can be used. For resistors with a high-power rating, special alloys or metal oxide films maybe employed. 

For high electric and thermal conductivity, metals such as gold (Au), copper (Cu), and aluminum (Al) are used widely. Magnetic metals such as nickel (Ni) and iron (Fe) are utilized to form magnetic actuators. Some metal thin films such as chromium (Cr) and titanium (Ti) are applied to enhance the adhesion of other metal thin films to a substrate. Doped polycrystalline silicon and metal silicides [12] have electric conductivities slightly inferior to metals but much better than insulators. They have also become integral materials for microelectronics. 

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