Commonly titanium nitride (TiN) thickness and resistivity wafer fab in-line metrology is based on ellipsometry and 4-point probe resistivity mapping. Alternative and relatively slower or more complex methods are X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR) and X-ray fluorescence (XRFS). TiN thin films are highly conductive and lose transparency for thicker layers which can make it challenging to accurately measure the thickness by ellipsometry above 10-20 nm. At about 50 nm layer thickness TiN is non-transparent and has a bronze color changing to gold for even thicker layers. In the case of resistivity mapping, 4-point probe is a destructive method leaving scratches from the needles that penetrates the TiN layer and possibly also damages the underlying layers and devices.
Atomic Layer Deposition of TiN on 200 mm wafers
TiN is used as a metal gate in complementary metal-oxide-semiconductor (CMOS) technology as it has low resistivity and is compatible with gate dielectrics. TiN is also deposited as a wear resistant coating, and barrier layer for copper diffusion due to its chemical and thermal stability. Traditionally TiN was deposited using physical vapour deposition techniques which suffer from as poor step coverage in deep contacts and via trenches due to the shadowing effects especially in high aspect ratio structures.
Atomic layer deposition (ALD) is a thin film deposition technique which allows for Å-level control of the film thickness, excellent uniformity, and conformal coating of high aspect ratio features.
Therefore, non-destructive characterization of thickness and electrical uniformity across the entire surface covered by the deposition is critical to ensure the quality of the final film. Oxford Instruments demonstrate the deposition of conductive TiN by plasma enhanced ALD with excellent thickness uniformity and collaborate with das-nano to map the resistivity uniformity using THz spectroscopy on 200 mm wafers.
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