Interactions between molecules and surfaces

Lloyd, Neil Stuart (1998) Interactions between molecules and surfaces. University of Southampton, Doctoral Thesis.

Abstract

Part 1

The processing of semiconductor layer systems relies upon controlled plasma etching to achieve the desired device definition. The physics and chemistry of plasma etching are not completely understood, and many process parameters are still chosen empirically. In this thesis, in-situ ellipsometry, quadrupole mass spectrometry and optical emission spectrometry have been used to monitor the etching of silicon (100), germanium (100) and a heterostructure system comprising of Si (100) grown on Si_1-xGe_x (100), where x=0.18. Information has been obtained on the etch rate, the layer thickness and the refractive index of the Si_1-xGe_x layer using the in-situ diagnostic tools. Etching conditions were identified which gave uniform, smooth etching of the Si/Si_1-xGe_x wafer system.

The selective etching of Si over Si_1-xGe_x was investigated, using CHF₃ and SF₆ to form etch inhibiting fluorocarbon polymer films. Si_1-xGe_x was found to etch faster than Si in all of the experiments carried out. The best selectivity obtained for the etching of Si over Si_1-xGe_x was 1:1.12.

Nomarski microscopy was used to study the surface quality of the Si/Si_1-xGe_x/Si wafers prior to etching. Surface roughness was attributed to relaxation of the Si_1-xGe_x layer, leading to misfit dislocation defects. The presence of defects within the structure were confirmed using TEM and SEM.

Semi-insulating polycrystalline silicon (SIPOS) thin films have been characterised using in-situ ellipsometry with plasma etching with CF₄ as a process gas.

The anisotropy of an etch is believed to be associated with the energy of ions impacting the wafer surface, and is therefore an area of particular interest. Experimental ion energy distributions (IED) have been recorded at the powered electrode. Theoretical IED were calculated using a FORTRAN program, and were in good agreement with the experimental IED.

Part 2

The rates of adsorption and desorption of methyl salicylate on various COLPRO wall coverings were measured. The adsorption and desorption curves have been fitted to an expression that can be derived for a simple adsorption/desorption model.

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