For decades polysilicon has been the gate material of choice for CMOS devices. Easy processing and tuning of the work function by doping are the advantages of polysilicon gate, while the disadvantage is the formation of depletion layer that leads to unwanted increase of EOT (equivalent oxide thickness). Metal gate materials do not form any notable depletion layer, which enables further EOT down scaling. These must, however, have a suitable work function to meet either the requirements of NMOS or PMOS devices. Also good thermal stability at high temperature is preferred. Tantalum carbide (TaC) has shown to be one promising candidate for n-type gate metal electrode material. In this paper we report a low temperature TaC deposition process in which two metal compound precursors are alternatively pulsed to the surface in an ALD like mode.

Tantalum carbide films were deposited in ASM Pulsar^®2000 ALCVD™ reactor at 325 °C from tantalum pentachloride (TaCl₅) and trimethylaluminum, TMA. Substrates were 200mm silicon wafers either with 20 nm thermal oxide or 3 nm ALD HfO₂. Film thickness was determined by spectroscopic ellipsometry and X-ray reflectometry (XRR). Crystallinity and composition were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Rutherford Backscattering Spectrometry (RBS). Sheet resistance was measured with four-point probe. For thermal stability study, the films were annealed at 400-1000 °C in ASM Levitor^®4200.

TaC growth rate was as high as 2.3Å/c, which could be a sign of some other growth mode than only pure ALD. The films were smooth, XRR roughness was < 1 nm, and only slightly crystalline which did not change during high temperature annealing. Density of as deposited film was ~7 g/cm³ and resistivity was ~1200 μΩcm, while thermal annealing in He atmosphere 30s at 1000°C decreased the resistivity to ~600 μΩcm.In this paper the effect of deposition parameters on growth of TaC and on the film properties will be presented.

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1. Investigating New Precursors for Silicon Dioxide