Impact of parasitic reactions on wafer-scale uniformity in water-based and ozone-based atomic layer deposition

Abstract

The exceptional thickness control and conformality of atomic layer deposition (ALD) has made it the process of choice for numerous applications from microelectronics to nanotechnology. Its benefits derive from the self-limiting character of surface chemical processes (adsorption and chemical reaction) that occur upon saturation of the active sites present on the surface. However, identifying a suitable process window in which ALD benefits are realized at the wafer scale can be a challenge, even in favorable cases. The authors explore here the prototypical case of ALD Al2O3 obtained from trimethyl aluminum (TMA), using a cross-flow ALD reactor design chosen to highlight deviations from expectations for true ALD behavior. Cross-wafer uniformity at the wafer scale (100 mm diameter) is investigated for both water and ozone used as oxidants, as a function of precursor dose and nitrogen purge conditions outside as well as inside the parameter space where ALD’s process benefits are realized. While nonuniformities lower than 1% were achievable for both oxidants, the ozone-based process offers significant benefits over the water-based process, namely, a broader process window and 3 × lower TMA dosages. The growth per cycle and uniformity are essentially unchanged with overexposures of either TMA or ozone. However, for overdosing of water growth rates are considerably higher than the nominal 0.95 Å/cycle and as well as degraded uniformity are observed. While underdosing of all precursor results in depletion of film growth in the flow direction across the wafer surface, these nonuniformities are more dramatic for water than ozone. These observations suggest the use of water as oxidant, if in excess dose condition, can introduce parasitic reactions in addition to the fundamental half-reactions for ALD Al2O3. Such reactions, e.g., associated with excess hydrogen-bonded water, lead to enhanced growth rates and degradation of uniformity.