Titanium Nitride Diffusion Barrier

A typical semiconductor device (found in the back-end of the line or the interconnects) consists of a layer of glass followed by a sputtered layer of titanium which is thermally treated to form a titanium silicide. Next a layer of titanium nitride is deposited on top of the silicide and on the sidewall of the contacts by sputtering or by MOCVD (Fig. 16.3).This layer of TiN acts as a difiusion barrier and an adhesion promoter. It is followed by the main interconnect which is an aluminum-copper alloy, in turn followed by another layer of TiN which acts as adhesion and antireflecting layer. 

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Many CVD reactions are being investigated for the deposition of carbides and nitrides, particularly for titanium nitride for semiconductor applications, such as diffusion barrier. The following is a summary of the metallo-organic precursors and deposition condition presently used in development or production of these materials. 

CVD titanium nitride (TiN) is the most important interstitial-nitride coating from an application standpoint. It is used extensively to provide wear resistance and as a diffusion barrier and antireflection coating in semiconductor devices. 1 °]... 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

CVD plays an increasingly important part in the design and processing of advanced electronic conductors and insulators as well as related structures, such as diffusion barriers and high thermal-conductivity substrates (heat-sinks). In these areas, materials such as titanium nitride, silicon nitride, silicon oxide, diamond, and aluminum nitride are of particular importance. These compounds are all produced by CVD. 1 1 PI... 

Copper is intrinsically a better metal than aluminum for the metallization of IC s. Latest developments in MOCVD show that it can be readily deposited without major changes in existing processing equipment. Diffusion problems are minimized and it appears that present barrier materials, such as titanium nitride or titanium-tungsten alloys, should provide adequate diffusion barriers for the copper-silicon couple, certainly up to the highest temperatures presently used in IC s processing (see Ch. 6). The development of CVD copper for semiconductor metallization is on a considerable scale at this time.Clt ]... 

Researchers in Japan have determined that copper interconnects deposited by metallo-organic chemical vapor deposition (MOCVD), then followed by chemical mechanical polishing, provides sub-quarter-micron interconnects and can be achieved on a production basis. Titanium nitride and borophosphosilicate glass provide effective barriers against copper diffusion.PL[H]... 

Titanium Nitride. TiN is chemically stable. TiN forms an excellent diffusion barrier and has a low coefficient of friction. As such, it is well suited for reducing corrosion, erosion, and galling. It is used extensively as a coating for gear components and tube- and wire-drawing dies. 

If a diffusion barrier is required, then a titanium/nitride (Ti/TiN) is used. This counteracts the tendency of most metals to diffuse through a given structure, particularly if the layers are composed of several different t rpes of metals, connections. 

The behavior of these metals, in fact, must be carefully checked in the actual device configurations on account of the simultaneous presence of two different metals — conductor plus diffusion barrier—which induces different electrochemical potentials. As an example, the difference of behavior of copper in HF-based chemistry (pH = 7) with or without titanium nitride is illustrated in Fig. 4. According to the Pourbaix diagram, at this pH the copper is theoretically passivated by CuO as verified by the slight increase of copper layer thickness. But in presence of titanium nitride copper is severely corroded. 

All experiments were performed on 200mm wafers using Semitool s plating tool. Trenches with various geometries and aspect-ratios were patterned in silicon oxide coated wafers. Titanium Nitride (TiN) or Tantalum (Ta) diffusion barriers with nominal thickness of 300 A were deposited on the trenches by vacuum techniques such as PVD or CVD. Unless specified differently, a PVD copper adhesion layer with a nominal thickness of 200A was deposited on top of the barrier by PVD techniques. This thin PVD copper adhesion layer was electrochemically enhanced in Semitool s proprietary ECD seed plating solution prior to the full deposition from an acid copper sulfate bath. 

First, a thin barrier layer of titanium is deposited by PVD on the wafer to line the bottom and the inside walls of the via-1 holes. The titanium serves to improve the adhesion of the tungsten plug to the ILD-1 oxide. Next, titanium nitride (TiN) is immediately deposited over the titanium as a diffusion barrier for the tungsten metal. Then, tungsten metal is deposited by CVD to fill the via-1 holes and coat the entire wafer. Finally, the tungsten is polished down to the upper surface of... 

Reaction-bonded titanium nitride (RBTN) ceramics are like RBSN made from a porous green shape of titanium powder that is reacted with nitrogen to titanium nitride (TiN) at temperatures up to 1000°C. Here the titanium hardly increases in molar volume when nitrided and the initial porosity remains the same but the gas permeability of a pressed titanium tablet is increased after it has been converted to titanium nitride. If the titanium powder particles are too large, the reaction stops after passivation of the metal surfaces the TiN formed at the surface is a diffusion barrier that stops the reaction. A fractal powder morphology of the starting metal (such as can be obtained from gas-phase preparation) is a very suitable reactant for complete reaction at modest temperatures. 

Titanium Nitride. TiN a special refractory material m.p. approx. 3000°C thermal expansion (25-1400°C) 9 x lO- . It can readily be produced from TiC and NH3. TiN coatings on metal cutting tools act as a diffusion barrier to impurities. They have higher electrical conductivity and better adhesion than alumina, and are used as an undercoat for AI2O3 coatings on tungsten carbide. Thin TiN films show iridescent interference effects, and are used as decorative hard surfacing for watches and jewelry. 

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