Cyclic azasilanes as volatile and reactive precursors for atomic layer deposition of silicon dioxide

Abstract

A suite of four volatile aminosilanes, cyclic azasilanes, was used to deposit silicon dioxide (SiO₂) films by atomic layer deposition (ALD) over the temperature range 100–300 °C by reaction with O₃. The unstable Si–N bonding makes the cyclic azasilanes chemically reactive with hydroxyl surfaces through a ring-opening reaction. Subsequent oxidation with O₃ affords silanol groups, which are amenable to further reaction with cyclic azasilanes. The influence of azasilane and O₃ exposure times on the growth rate was examined in detail. The growth rates obtained by spectroscopic ellipsometry are 0.6–1.2 Å per cycle for various azasilanes under different ALD conditions, due to side chain structure variation of the precursors. Refractive indices (1.45–1.46) and band gaps (8.5–8.7 eV) are found to be similar to thermal oxide. X-Ray photoelectron spectroscopy (XPS) revealed 3–5 at% C and 0.2–0.4 at% N in the films and an O/Si ratio of ∼1.9 when deposited at 190 °C. The first silane pulse resulted in a surface coverage of ∼1.2 molecules per nm² as determined by microbalance measurements. The O₃ oxidation rate is faster for silanes with Si–OMe groups than those with Si–Me functionalities, and less effective at lower temperatures for some silane precursors. These cyclic azasilanes are promising precursors for ALD SiO₂ and surface functionalization, and the variation in the structures provides possibilities to study reaction mechanisms and control surface chemistry.