The effects of hydrogen on interdiffusion and reaction in titanium, titanium/copper and titanium/copper/gold thin film

The requirement for low electrical resistance in integrated circuit interconnections makes copper an attractive material for this application. Copper requires an adhesion layer such as titanium and sometimes a protective layer such as gold. During process heat treatment, these layers can interdiffuse and react increasing interconnection resistance. The presence of hydrogen in the heat treatment ambient can affect these reactions and minimize resistance increases. In order to study these effects, thin films of titanium, titanium/copper, and titanium/copper/gold were deposited on oxidized silicon wafers. Samples were heat treated in UHV and pure hydrogen in the temperature range 275-450$spcirc$C for times up to four hours. Sample analysis included sheet resistance measurements, RBS, X-ray diffraction, TEM and AES. It was found that heat treatment of titanium/copper in hydrogen resulted in no increase in metallization sheet resistance. This was due to the reaction of hydrogen with titanium to form titanium hydride which precluded the formation of titanium-copper intermetallic compounds. Heat treatment in vacuum resulted in significant increases in metallization sheet resistance due to titanium-copper intermetallic compound formation. Three compounds, TiCu, TiCu$sb2$ and TiCu$sb3$ grew as nonplanar layers in vacuum in a manner which was consistent with linear, interface reaction rate controlled kinetics.;The titanium films showed a slight decrease in sheet resistance followed by an increase during heat treatment in both vacuum and hydrogen. This was due to grain growth and redistribution of oxygen introduced during deposition. Titanium films heat treated in hydrogen were completely transformed into TiH$sb2$ which has a lower sheet resistance than as-deposited titanium.;Titanium/copper/gold films heat treated in both vacuum and hydrogen showed a moderate increase in sheet resistance. The resistance increase was due to the formation of copper-gold solid solution. Titanium-copper compound formation was not observed. A gold rich layer formed at the titanium interface due to the formation of TiAu$sb4$. This layer grew more slowly than titanium-copper compound layers.