ALD/ALE 2022 Session AS2-WeA: Area Selective Deposition II

Area-selective atomic layer deposition is a key technology for modern microelectronics as it eliminates alignment errors inherent to conventional approaches by enabling material deposition only in specific areas. Typically, the selectivity originates from surface modifications of the substrate that allow or block precursor adsorption. The control of the deposition process currently remains a major challenge as the selectivity of the no-growth areas is lost quickly.

Here, we show that surface modifications of the substrate strongly manipulate the surface diffusion. The selective deposition of TiO₂ on poly (methyl methacrylate) and SiO₂ yields localized nanostructures with tailored aspect ratios. Controlling the surface diffusion allows to tune such nanostructures as it boosts the growth rate at the interface of the growth and no-growth areas. Kinetic Monte-Carlo calculations reveal that species move from high to low diffusion areas. Further, we identify the catalytic activity of TiCl₄ during the formation of carboxylic acid on poly (methyl methacrylate) as the reaction mechanism responsible for the loss of selectivity, and show that process optimization leads to higher selectivity. Our work enables the precise control of area-selective atomic layer deposition on the nanoscale, and offers new strategies in area-selective deposition processes by exploiting surface diffusion effects.

Area-selective atomic layer deposition (AS-ALD) offers the advantage of exploiting surface chemistry to deposit a material in a targeted area. Therefore, it may allow a reduction in the number of lithography and etch steps, resulting in lowering of errors in the patterning process as well as a decrease in manufacturing costs. For example, a self-aligned hard mask fabricated by AS-ALD can guide etching of via holes and deposition of metal wires in the metallization process to avoid shorts between metal layers.

Several metal oxide systems, such as Al₂O₃, TiO₂, ZnO, and HfO₂, have been explored for AS-ALD processes. For a hard mask, Al₂O₃ possesses advantages over other metal oxides due to its high hardness as well as chemical inertness for etching selectivity. However, despite extensive studies on ALD Al₂O₃, there are few studies on AS-ALD of Al₂O₃. Furthermore, literature suggests that Al₂O₃ may be comparatively difficult to block; for example, the blocking selectivity of Al₂O₃ is limited to only ~6 nm whereas ZnO can be blocked for over ~30 nm. The difference in blocking highlights the importance of precursor chemistry for AS-ALD, which motivates the current study to elucidate the mechanism of Al₂O₃ AS-ALD based on a comparative study of Al precursors.

In this work, the recent development of AS-ALD is introduced. Furthermore, as an example, fundamental study on AS-ALD Al₂O₃ is presented. Four Al precursors; trichloroaluminum (AlCl₃), dimethylaluminum chloride (Al(CH₃)₂Cl), trimethylaluminum (Al(CH₃)₃), and triethylaluminum (Al(C₂H₅)₃) are used for comparison, offering a comparative study of precursor ligand properties (reactivity, polarity, and geometric factors) by changing both the number of methyl (Me) and chloride (Cl) group in AlMe_xCl_3-x (x=0, 1, and 3) and the chain length of alkyl ligands in AlC_nH_2n+1 (n=1 and 2). Results of quantum chemical calculations of the reaction pathways show product energetics that are strongly correlated with experimental observations. The blocking properties of the four Al precursors will be compared and the results discussed based on the growth mechanism. Finally, Al₂O₃ selective pattern is successfully fabricated, which would be useful for back-end-of-line (BEOL) semiconductor process. By pursuing first principles design of selective ALD processes, this work may enable new methods for additive nanoscale.

Self-assembled monolayers (SAMs) formed or modified by rapid “click” chemical reactions are receiving significant attention due to their demonstrated ability to promote or facilitate area-specific or area-selective deposition utilizing processes that occur on time scales compatible with high-throughput manufacture and cyclic deposition schemes. By precision tailoring of the chemical structure of the SAM layer, the selectivity and speed of the reactions with both the underlying substrate and subsequent deposition cycles can be enhanced in order to provide rapid, selective processes with few or no chemical byproducts. In this work we demonstrate that cyclic azasilanes and cyclic thiasilanes can rapidly bond to the surface of hydroxyl-covered substrates in the vapor phase, with saturated coverage being reached in several seconds in a manufacturing-worthy atomic layer deposition tool. The effects of processing parameters, including pulse time, temperature, substrate type, and pre-treatment on the deposition profile, as well as the subsequent reactivity of cyclic azasilanes to further modify the chemical nature of the surface are discussed, with an emphasis on rapid “click” chemical processes compatible with high throughput. It was observed that on hydroxyl (OH)-terminated surfaces such as silicon native or thermal oxide the cyclic silanes reach a saturation point in approximately five seconds under typical ALD conditions over the temperature range of 30 °C to 300 °C, as determined by water contact angle and ellipsometry. The reactivity and selectivity of cyclic azasilanes to other oxide surfaces as well as non-oxides such as copper, silicon nitride, and HF-etched silicon will be discussed, as will process to remove azasilane monolayers after deposition.