Structural, optical and mechanical properties of alumina and silica thin films grown by plasma enhanced atomic layer deposition (PEALD) have been investigated. These properties can additionally be manipulated in plasma enhanced processes by applying an electric field to the substrate. The substrate biasing can (i) drastically change the energy of the ions in the plasma leading to various ion-induced effects like adatom migration, sub-surface implantation, desorption, sputtering or displacement of lattice atoms and (ii) might also influence the surface chemical reactions in PEALD. In ALD, thin films are deposited layer by layer, where each monolayer is formed in a self-terminating manner. Hence highly uniform and conformal coatings can be realized.
In this study, alumina and silica thin films were deposited using PEALD. Substrate RF biasing up to -300 V was applied during O2 plasma step. The oxygen ions bombarding on thin film surface might influence the surface chemical reactions along with physical effects. Hence to investigate only physical effects of ion bombardment on the film surface, an additional step after the O2 plasma step (no substrate bias) was carried out. In this step biasing of -100 V was applied in an Ar plasma. Thin films were investigated using spectrophotometry, ellipsometry, XPS, FT-IR, AFM, wafer-curvature measurement, XRR and XRD. Further to determine chemical stability, films were etched in 85% H3PO4 and 1M NaOH solutions.
The significant change in growth per cycle (GPC) of alumina and silica films was observed with increasing bias voltage. The residual stress in alumina films drastically changed from tensile to compressive by applying bias. Optical losses (in UV) in alumina films deposited with bias were high which correlates to C impurities determined from XPS. The FTIR measurement showed an increase in OH impurities in alumina films with an increase in bias. In contrast, the alumina film deposited with biasing during Ar plasma showed low optical losses and a shift in the peak corresponding to OH stretching vibration to higher wavenumber while the mechanical stress was compressive. Thus, biasing in Ar plasma favors different kind of bonding geometry than in O2 plasma. The silica films deposited with substrate bias had slightly higher compressive stress. The FTIR measurement showed lower OH impurities and higher density in the samples with bias. Silica films deposited with bias are also chemically more stable than without bias.
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