Atmospheric Pressure Chemical Vapor Deposition of Titanium Nitride from Titanium Bromide and Ammonia
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DESIRABLE FEATURES OF A CVD PROCESS An ideal CVD reaction should satisfy many requirements. Properties of the precursors:
"* The precursors should be fluids (gases or liquids), not solids, at room temperature, in order to facilitate handling and metering.
"* The precursors should be inexpensive to manufacture and to purify. * They should remain stable during storage, and not react with air. * They should be non-flammable, non-toxic, and non-corrosive. Vaporization of the precursors:
"* Their vapor pressure should be sufficiently high (e.g., 1 Torr) at a relatively low temperature (e. g., less than 200 'C).
"• They should be thermally stable at their vaporization temperature. "* They should vaporize rapidly and reproducibly, which is usually the case if it is a non-associated liquid (not a solid) at its vaporization temperature. CVD reaction:
"* The reaction should take place at a temperature low enough not to damage
substrates.
"* The reaction should produce a pure film, with high conductivity and good step coverage.
"* A high percentage of the precursor should be converted to film. Byproducts:
"* The byproducts of its CVD reaction should be stable and non-reactive. "* The reaction byproducts should be non-flammable, non-toxic and noncorrosive.
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SURVEY OF CVD REACTIONS USED FOR TiN BARRIERS Several different CVD reactions have been used to deposit titanium nitride. The most commonly used precursor is titanium tetrachloride, TiCI4. When reacted with molecular nitrogen, according to Eq. (1), 2 TiCl 4 + N2 +41H2 -> 2 TiN + 8 HC1
(1)
titanium nitride is deposited at appreciable rates only for temperatures over about 900 °C.' Although this process has been used commercially on refractory substrates such as tungsten carbide, the deposition temperatures are far too high for use in silicon-based microelectronics. The deposition temperature is reduced to about 500 to 700 'C by2 the use of ammonia as a nitrogen source in place of the molecular nitrogen:
6 TiCI4 + 8 NH 3 -> 6 TiN + 24 HC1 + N 2
(2)
Preheating the reactant gases to around 250 °C is necessary in order to avoid adduct formation. Films deposited by reaction (2) contain significant amounts of chlorine (over 2x10 21 cm-3 ) if the reaction is carried out at temperatures below about 600 'C.' This chlorine causes corrosion of 21 adjacent layers of aluminum, if the chlorine content exceeds about 3x10 cm 3 (about 3 atomic per cent); thus corrosion-resistant TiN layers can only be achieved by reaction (2) at substrate temperatures over about 600 'C.' The chlorine content may be reduced by the use of methylhydrazine as a nitrogen source in place of the ammonia. 4' 5 However, methylhydrazine is an extremely potent carcinogen (TLV < 10 ppb), and thus is unlikely to be
accepted in production. The chlorine can be eliminated altogether by using a metal-organic source, such as tetrakis(dimethylamido)titanium (TDMAT). Use of TDMAT alone, however, results in a film with high resistance and high carbon contamination.6 Addition of ammonia to the TDMAT eliminate
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