TimeDomain CVD, Inc.

Titanium Nitride Chemical Vapor Deposition

Why TiN?

Titanium nitride is a hard, dense, refractory material with unusually high electrical conductivity. TiN is widely employed in semiconductor manufacturing as a "diffusion barrier" layer: that is, it is placed between two metal or semiconductor layers to prevent intermixing and undesired interactions, while still permitting electrical current to flow. TiN has unusual optical properties, including an attractive gold-tinged appearance when pure, and high infrared transmission. It is used as an inorganic antireflective coating for lithography on top of aluminum metal, and as a coating for window glass and for decorative applications. TiN is also used as a hard coating for machine tools, turbine blades, and other metallic surfaces.

CVD Precursors and Techniques

TiN is most commonly deposited by sputtering, often from a titanium metal target in a nitrogen-containing atmosphere. Sputtered films can be produced with good stoichiometry control and purity, and resistivity as low as 30-40 microhm-cm, but conformality is difficult to achieve. Sputtered barrier layers are particularly thin at the bottom corners of via holes, and failure may occur at this point. Thick enough depositions to protect these vulnerable edges result in excessive TiN at the top of the hole, preventing filling with subsequent metal. Improvements in sputtered coverage using perforated metal masks (collimators) or ionized metal plasma deposition have enabled use of sputtered metals in current IC production. Extensive exploration of CVD techniques has been carried out over the last 15 years to provide a means of conformal deposition of the barrier.

The oldest techniques employ TiCl4 as a precursor, with ammonia or nitrogen as oxidants. TiCl4 is a fairly inexpensive, volatile liquid at room temperature, easily delivered as a vapor by bubbler or direct liquid injection sources. The liquid decomposes readily upon exposure to moisture or air, generating HCl vapor, but does not explode or burn: it can be handled using the same precautions appropriate for concentrated aqueous acids. As we have noted previously in our discussion of thermodynamics in CVD, the formation of TiN from TiCl4 and ammonia is not thermodynamically favorable at room temperature. In fact, the gases will react readily to form a dusty yellow adduct TiCl4:NH3, which oxidizes readily upon exposure to air producing white TiO2 dust and HCl. At temperatures around 200-300 C, no products are produced at all, whereas above 350 C TiN can be deposited from TiCl4 and NH3; very high temperatures are needed if N2 is used. (It is therefore very useful to maintain all reactor parts except the wafer at a well-controlled temperature aroiund 250 C, preventing any deposition and thus obviating the need for reactor cleaning.) However, to obtain reasonable deposition rates and low Cl concentrations in the deposited films, temperatures > 600 C are normally employed. Thus these techniques are inappropriate for processes where metal or other temperature-sensitive layers are present on substrates.

The resulting films have good conformality, and can achieve reasonable resistivities of a few hundred microhm-cm. TiN layers from TiCl4 have been used in various applications, such as contact layer barrier prior to CVD W via metallization, where deposition temperatures of 600 C can be used. Some exploration of the use of more active oxidants such as hydrazine and dimethylhydrazine has been reported; plasma enhancement has also been explored but not commercialized.

For lower temperature deposition, the most common precursors are the organometallic compounds:

tetrakis-(dimethylamido)titanium TDMAT:

and tetrakis-(diethylamido)titanium TDEAT:

These materials are much more expensive than TiCl4. They are not unduly toxic and are stable in storage, but react slowly with moist air or rapidly with water to produce TiO2 and dimethyl or diethylamines, which have a very unpleasant odor and are moderately toxic. TDMAT is somewhat more volatile than TDEAT, but in both cases vapor pressure is low and temperatures of 80-120 C are needed to supply vapor to the chamber. The compounds slowly decompose at these temperatures, so bubblers are inappropriate for vapor delivery, and direct liquid injection and vaporization must be employed.

The use of TDMAT alone deposits a film that has excellent conformality, but is heavily contaminated with carbon and susceptible to oxidation when exposed to air. TDMAT and TDEAT can be used in conjunction with ammonia to deposit films with much improved purity and conductivity of a few hundred microhm-cm, typically at temperatures around 350 to 400 C and pressures of a few Torr. However, the precursors react rapidly with ammonia even at room temperature and low pressure, so it is necessary to use multi-port showerheads which prevent mixing of the gases prior to their introduction into the deposition chamber. Conformality is also degraded as more ammonia is added: there is a tradeoff between conformal deposition and purity.

The current solution to this problem is to alternate deposition from TDMAT alone with a plasma treatment in N2 or NH3, which removes most of the carbon from the surface of the film and converts it to TiN. Two or three cycles of deposition and treatment suffice to produce acceptable barrier layers for electronic applications. Exposure in situ of a freshly deposited TDMAT film to silane has also been used to strip carbon and stabilize the film to atmospheric exposure, at some cost in resistivity.

 

Films: Table of Contents