Selection Criteria for Small-Molecule Inhibitors in Area-Selective Atomic Layer Deposition: Fundamental Surface Chemistry Considerations.

ConspectusAtomically precise and highly selective surface reactions are required for advancing microelectronics fabrication. Advanced atomic processing approaches make use of small molecule inhibitors (SMI) to enable selectivity between growth and nongrowth surfaces. The selectivity between growth and nongrowth substrates is eventually lost for any known combinations, because of defects, new defect formation, and simply because of a Boltzmann distribution of molecular reactivities on surfaces. The selectivity can then be restored by introducing etch-back correction steps. Most recent developments combine the design of highly selective combinations of growth and nongrowth substrates with atomically precise cycles of deposition and etching methods. At that point, a single additional step is often used to passivate the unwanted defects or selected surface chemical sites with SMI. This step is designed to chemically passivate the reactive groups and defects of the nongrowth substrates both before and/or during the deposition of material onto the growth substrate. This approach requires applications of the fundamental knowledge of surface chemistry and reactivity of small molecules to effectively block deposition on nongrowth substrates and to not substantially affect deposition on the growth surface. Thus, many of the concepts of classical surface chemistry that had been developed over several decades can be applied to design such small molecule inhibitors. This article will outline the approaches for such design.This is especially important now, since the ever-increasing number of applications of this concept still rely on trial-and-error approaches in selecting SMI. At the same time, there is a very substantial breadth of surface chemical reactivity analysis that can be put to use in this process that will relate the effectiveness of a potential SMI on any combination of surfaces with the following: selectivity; chemical stability of a molecule on a specific surface; volatility; steric hindrance, geometry, packing, and precursor of choice for material deposition; strength of adsorption as detailed by interdisplacement to determine the most stable SMI; fast attachment reaction kinetics; and minimal number of various binding modes.The down-selection of the SMI from the list of chemicals that satisfy the preliminary criteria will be decided based on optimal combinations of these requirements. Although the specifics of SMI selection are always affected by the complexity of the overall process and will depend drastically on the materials and devices that are or will be needed, this roadmap will assist in choosing the potential effective SMIs based on quite an exhaustive set of "SMI families" in connection with general types of target surfaces.

Unfolding an Elusive Area-Selective Deposition Process: Atomic Layer Deposition of TiO2 and TiON on SiN vs SiO2.

Area-selective atomic layer deposition (AS-ALD) processes for TiO2 and TiON on SiN as the growth area vs SiO2 as the nongrowth area are demonstrated on patterns created by state-of-the-art 300 mm semiconductor wafer fabrication. The processes consist of an in situ CF4/N2 plasma etching step that has the dual role of removing the SiN native oxide and passivating the SiO2 surface with fluorinated species, thus rendering the latter surface less reactive toward titanium tetrachloride (TiCl4) precursor. Additionally, (dimethylamino)trimethylsilane was employed as a small molecule inhibitor (SMI) to further enhance the selectivity. Virtually perfect selectivity was obtained when combining the deposition process with intermittent CF4/N2 plasma-based back-etching steps, as demonstrated by scanning and transmission electron microscopy inspections. Application-compatible thicknesses of ∼8 and ∼5 nm were obtained for thermal ALD of TiO2 and plasma ALD of TiON.

High-Throughput Area-Selective Spatial Atomic Layer Deposition of SiO2 with Interleaved Small Molecule Inhibitors and Integrated Back-Etch Correction for Low Defectivity.

A first-of-its-kind area-selective deposition process for SiO2  is developed consisting of film deposition with interleaved exposures to small molecule inhibitors (SMIs) and back-etch correction steps, within the same spatial atomic layer deposition (ALD) tool. The synergy of these aspects results in selective SiO2  deposition up to ~23 nm with high selectivity and throughput, with SiO2  growth area and ZnO nongrowth area. The selectivity is corroborated by both X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The selectivity conferred by two different SMIs, ethylbutyric acid, and pivalic acid has been compared experimentally and theoretically. Density Functional Theory (DFT) calculations reveal that selective surface functionalization using both SMIs is predominantly controlled thermodynamically, while the better selectivity achieved when using trimethylacetic acid can be explained by its higher packing density compared to ethylbutyric acid. By employing the trimethylacetic acid as SMI on other starting surfaces (Ta2 O5 , ZrO2 , etc.) and probing the selectivity, a broader use of carboxylic acid inhibitors for different substrates is demonstrated. It is believed that the current results highlight the subtleties in SMI properties such as size, geometry, and packing, as well as interleaved back-etch steps, which are key in developing ever more effective strategies for highly selective deposition processes.

Relation between Reactive Surface Sites and Precursor Choice for Area-Selective Atomic Layer Deposition Using Small Molecule Inhibitors.

Implementation of vapor/phase dosing of small molecule inhibitors (SMIs) in advanced atomic layer deposition (ALD) cycles is currently being considered for bottom-up fabrication by area-selective ALD. When SMIs are used, it can be challenging to completely block precursor adsorption due to the inhibitor size and the relatively short vapor/phase exposures. Two strategies for precursor blocking are explored: (i) physically covering precursor adsorption sites, i.e., steric shielding, and (ii) eliminating precursor adsorption sites from the surface, i.e., chemical passivation. In this work, it is determined whether steric shielding is enough for effective precursor blocking during area-selective ALD or whether chemical passivation is required as well. At the same time, we address why some ALD precursors are more difficult to block than others. To this end, the blocking of the Al precursor molecules trimethylaluminum (TMA), dimethylaluminum isopropoxide (DMAI), and tris(dimethylamino)aluminum (TDMAA) was studied by using acetylacetone (Hacac) as inhibitor. It was found that DMAI and TDMAA are more easily blocked than TMA because they adsorb on the same surface sites as Hacac, while TMA is also reactive with other surface sites. This work shows that chemical passivation plays a crucial role for precursor blocking in concert with steric shielding. Moreover, the reactivity of the precursor with the surface groups on the non-growth area dictates the effectiveness of blocking precursor adsorption.

Area-Selective Atomic Layer Deposition of SiO2 Using Acetylacetone as a Chemoselective Inhibitor in an ABC-Type Cycle.

Area-selective atomic layer deposition (ALD) is rapidly gaining interest because of its potential application in self-aligned fabrication schemes for next-generation nanoelectronics. Here, we introduce an approach for area-selective ALD that relies on the use of chemoselective inhibitor molecules in a three-step (ABC-type) ALD cycle. A process for area-selective ALD of SiO2 was developed comprising acetylacetone inhibitor (step A), bis(diethylamino)silane precursor (step B), and O2 plasma reactant (step C) pulses. Our results show that this process allows for selective deposition of SiO2 on GeO2, SiNx, SiO2, and WO3, in the presence of Al2O3, TiO2, and HfO2 surfaces. In situ Fourier transform infrared spectroscopy experiments and density functional theory calculations underline that the selectivity of the approach stems from the chemoselective adsorption of the inhibitor. The selectivity between different oxide starting surfaces and the compatibility with plasma-assisted or ozone-based ALD are distinct features of this approach. Furthermore, the approach offers the opportunity of tuning the substrate-selectivity by proper selection of inhibitor molecules.