Silicon nitride (SiNx) is being used extensively as material in industrial applications, such as in solar cells and integrated circuits. However, the downscaling of integrated circuits, in particular of transistors, imposes new, strict requirements on the material. Not only the quality, but also the conformality, uniformity and thickness of the material should be controlled on nanometer scale. Atomic layer deposition (ALD) is a candidate to fulfill these requirements. For the development of suitable ALD processes, the factors that rule film quality and growth should be known. Therefore, fundamental understanding of the surface chemistry and reaction mechanisms is crucial. In this work, the plasma-assisted ALD process of SiNx employing BTBAS (SiH2[NH(C4H9)]2) as precursor and N2 plasma as co-reactant, has been studied with in situ infrared spectroscopy and a reaction mechanism for the ALD film growth is proposed. The ALD process, developed in the PMP research group in 2013, was first transferred to a home-built ALD setup. This setup was extended with a sample manipulator and a novel resistive heating method, to enable infrared spectroscopy in transmission mode over the full ALD temperature window of the SiNx process. The material properties of the grown films were determined to be similar to the previously developed process, where, despite slightly elevated growth rates, the films contained comparable C, O and H levels. The infrared measurements revealed the reaction products of the precursor step, the change in surface groups during both half-cycles and the composition of the deposited films. In the ALD process, part of the ligands were liberated as t-Butylamine(NH2C(CH3)3) during precursor adsorption, as confirmed by gas phase infrared measurements. Surface measurements revealed that CH groups were present in the precursor half-cycle, indicating that part of the ligands of the precursor molecule remained attached to the Si atom upon precursor adsorption. These ligands were removed by the N2 plasma, of which fragments, such as CN, could redeposit on the surface. At low deposition temperatures, these fragments were incorporated into the film, resulting in films with a high carbon content. At high deposition temperatures, fewer fragments were incorporated in the film. Hence, FTIR spectroscopy yielded important insight in how the carbon is incorporated in the film in this ALD process, a problem which may be resolved by the choice of a precursor molecule with a different molecular structure.
|Date of Award||31 Aug 2015|
|Supervisor||R.H.E.C. Bosch (Supervisor 1) & W.M.M. (Erwin) Kessels (Supervisor 2)|