Tantalum nitride

Tantalum nitride (TaN) is a chemical compound, a nitride of tantalum. There are multiple phases of compounds, stoichimetrically from Ta₂N to Ta₃N₅, including TaN.

As a thin film TaN find use as a diffusion barrier and insulating layer between copper interconnects in the back end of line of computer chips. Tantalum nitrides are also used in thin film resistors.

The tantalum - nitrogen system includes several states including a nitrogen solid solution in Tantalum, as well as several nitride phases, which can vary from expected stoichiometry due to lattice vacancies.^[2] Annealing of nitrogen rich "TaN" can result in conversion to a two phase mixture of TaN and Ta₅N₆.^[2]

Ta₅N₆ is thought to be the more thermally stable compound - though it decomposes in vacuum at 2500 °C to Ta₂N.^[2] It was reported the decomposition in vacuum from Ta₃N₅ via Ta₄N₅, Ta₅N₆, ε-TaN, to Ta₂N.^[3]

θ-TaN has the highest high thermal conductivity of any metal, 1100 W/mK, nearly 3 times that of copper.^[4]

TaN is often prepared as thin films. Methods of depositing the films include RF-magnetron-reactive sputtering,^[5][6] Direct current (DC) sputtering,^[7]Self-propagating high-temperature synthesis (SHS) via 'combustion' of tantalum powder in nitrogen,^[2] low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD),^[8]ion beam assisted deposition (IBAD),^[9] and by electron beam evaporation of tantalum in concert with high energy nitrogen ions.^[10]

Depending on the relative amount of N₂, the deposited film can vary from (fcc) TaN to (hexagonal) Ta₂N as nitrogen decreases.^[6] A variety of other phases have also been reported from deposition including bcc and hexagonal TaN; hexagonal Ta₅N₆; tetragonal Ta₄N₅; orthorhombic Ta₆N_2.5, Ta₄N, or Ta₃N₅.^[6] The electrical properties of TaN films vary from metallic conductor to insulator depending on the relative nitrogen ratio, with N rich films being more resistive.^[11]

It is sometimes used in integrated circuit manufacture to create a diffusion barrier or "glue" layers between copper, or other electrically conductive metals. In the case of BEOL processing (at c. 20 nm), copper is first coated with tantalum, then with TaN using physical vapour deposition (PVD); this barrier coated copper is then coated with more copper by PVD, and infilled with electrolytically coated copper, before being mechanically processed (grind/polishing).^[12]

It also has application in thin film resistors.^[5] It has the advantage over nichrome resistors of forming a passivating oxide film which is resistant to moisture.^[13]