Oxygen Recombination Probability Data for Plasma-Assisted Atomic Layer Deposition of SiO₂ and TiO₂

Abstract

Atomic layer deposition (ALD) can provide nanometer-thin films with excellent conformality on demanding three-dimensional (3D) substrates. This also holds for plasma-assisted ALD, provided that the loss of reactive radicals through surface recombination is sufficiently low. In this work, we determine the surface recombination probability r of oxygen radicals during plasma ALD of SiO 2 and TiO 2 for substrate temperatures from 100 to ∼240 °C and plasma pressures from 12 to 130 mTorr (for SiO 2 ). For both processes, the determined values of r are very low, i.e., ∼10 –4 or lower, and decrease with temperature and pressure down to ∼10 –5 within the studied ranges. Accordingly, deposition on trench structures with aspect ratios (ARs) of <200 is typically not significantly limited by recombination and obtaining excellent film conformality is relatively facile. For higher AR values, e.g., approaching 1000, the plasma time needed to reach saturation increases exponentially and becomes increasingly dependent on the process conditions and the corresponding value of r . Similar dependence on process conditions can be present for plasma ALD of other materials as well, where, in certain cases, film growth is already recombination-limited for AR values of ∼10. Radical recombination data and trends as provided by this work are valuable for optimizing plasma ALD throughput and feasibility for high-AR applications and can also serve as input for modeling of radical recombination mechanisms.

Impact of Ions on Film Conformality and Crystallinity during Plasma-Assisted Atomic Layer Deposition of TiO2

“…This confirms that excellent film conformality can be achieved for both materials, as reported in our earlier work. 14,15 Yet, Figure 3 also demonstrates that for both processes, the film growth is significantly influenced by ions. This is seen, for instance, in the top-view optical microscopy images (panel B), where the surface areas in the ion-exposed regions have a different darkness compared to the shielded cavity regions (i.e., darker for TiO 2 and lighter for SiO 2 ).…”

“…Another factor that often limits film conformality during plasma ALD is the loss of reactive radicals through recombination at surfaces. 14,15,37 This loss mechanism typically limits the AR up to which film growth by plasma ALD is feasible. 14,15,37 Compared to plasma ALD of TiO 2 and SiO 2 , surface recombination of oxygen radicals is much more significant in the case of plasma ALD of Al 2 O 3 .…”

“…14,15,37 This loss mechanism typically limits the AR up to which film growth by plasma ALD is feasible. 14,15,37 Compared to plasma ALD of TiO 2 and SiO 2 , surface recombination of oxygen radicals is much more significant in the case of plasma ALD of Al 2 O 3 . 14 Nevertheless, for the fairly low ARs used, panels A and B reveal that also for plasma ALD of Al 2 O 3 , a highly conformal film is obtained even in the narrowest trench, indicating that the impact of radical recombination was negligible.…”

“…13 In previous work, we demonstrated that plasma ALD of TiO 2 can fully penetrate into horizontally oriented trench structures with extremely high aspect ratio (AR) values of ∼800. 14,15 This indicates that excellent TiO 2 film conformality can be achieved, which is enabled by a low loss of reactive radicals on the TiO 2 surface. 14,15 However, it is possible that the film conformality is still reduced by any influence of ions, which were not taken into account in our previous study employing the horizontal trench structures.…”

“…14,15 This indicates that excellent TiO 2 film conformality can be achieved, which is enabled by a low loss of reactive radicals on the TiO 2 surface. 14,15 However, it is possible that the film conformality is still reduced by any influence of ions, which were not taken into account in our previous study employing the horizontal trench structures. 16 Even though ions often have a beneficial effect, 17−20 the flux of ions on surfaces is inherently anisotropic and can therefore induce nonuniform growth behavior on a three-dimensional (3D) surface topography.…”