Exploring Inherently Selective Plasma Processing of Nitrides and Oxides Via Hollow-Cathode Plasma-Assisted Atomic Layer Deposition
Authors: Mohammad Adnan , Ilhom Saidjafarzoda , Shukla Deepa , Willis Brian , Okyay Ali K. , Biyikli Necmi 
Published: December 2020

Area-selective atomic layer deposition (AS-ALD) is an emerging field of research attracting accelerating interest from the CMOS materials and device research community. The continuing down-scaling in logic and memory field-effect transistor devices along with the increasing three-dimensional structural complexity brings new patterning and processing challenges. With the long-awaited adoption of extreme-UV (EUV) lithography, the escalating stress on precision patterning has decreased, however, alternative self-aligned processing solutions are always highly needed to reduce the number of lithography/deposition/etching steps. AS-ALD, therefore, provides a unique opportunity towards simplifying critical process steps where surface-selective and/or conformal material deposition with sub-nanometer thickness precision is required.

Being introduced in the early years of the 21st century, during the early adoption of ALD-grown high-k dielectrics in the CMOS manufacturing industry, significant progress has been made in developing alternative routes for AS-ALD of various oxide and nitride compounds, as well as elemental metallic films. Nucleation/growth inhibition on various surfaces including Si, oxide, nitride, and metal substrates have been explored extensively. The most promising results have been achieved via various self-assembled monolayers (SAMs), which serves as the masking layer to protect the non-growth surface and can be removed with relative ease after ALD process has been completed. Despite successful results for a set of materials grown via conventional thermal-ALD, SAMs are not compatible with plasma-ALD processes, mainly because their soft organic structure which is easily etched under energetic plasma exposure. Therefore, there is a significant need to develop alternative AS-ALD strategies for materials which require the utilization of energetic plasmas for their deposition.

While certain nitride materials necessitate the usage of various nitrogen plasmas including III-nitrides for their self-limiting growth at low substrate temperatures, plasma-assistance helps oxides as well in lowering their deposition temperatures and improving physical properties of the films. Previously, we have reported extensively on the self-limiting plasma-assisted ALD (PA-ALD) of crystalline III-nitride films within a common substrate temperature window of lower than 250 °C. In this study, we explore the feasibility of developing area-selective deposition recipes for nitride and oxide compounds via inherently selective nucleation/growth behavior on different surfaces.

Our initial results belong to two oxide (Ga2O3 and VOx) and one nitride (BN) binary compound films, all deposited in the same PA-ALD system featuring a large-diameter hollow-cathode plasma source. Ga2O3, VOx, and BN films were deposited using triethylgallium (TEG), TEMAV, and triethylboron (TEB) as metal precursors, respectively, while Ar/O2 and Ar/N2 plasma were used as oxygen and nitrogen co-reactants, respectively. Growth experiments have been performed at 200 °C substrate temperature and within 50 – 100 W rf-power range. Real-time in-situ ellipsometry was used to closely monitor the individual growth reactions taking place on the surface, depicting metal precursor chemisorption and plasma-assisted ligand removal/exchange reactions. 30-cycle deposition runs on three different substrate surfaces (solvent-cleaned Si, BOE-cleaned Si, and solvent-cleaned Al2O3) were in-situ monitored for the detection of any nucleation inhibition or growth retardation.

Our initial results show that inherent surface selectivity in PA-ALD growth of VOx and BN films on Si vs. Al2O3 substrates can be achieved. Our in-situ ellipsometry results indicate nucleation inhibition behavior of up to 15 ALD cycles for BN on Al2O3 surfaces, when compared to almost no retardation on both Si substrates. Moreover, we also observed that pre-deposition surface plasma treatments result in significantly different growth rates on Si surfaces, with the fastest growth occurring on non-plasma treated surfaces. Pretty similar results were obtained for VOx films, showing a rather significant nucleation delay of at least 25 cycles on Al2O3 surfaces, while plasma-treated Si surfaces show differences in growth behavior. These initial results confirm indeed the feasibility of achieving selective plasma processing, despite the known rather unselective behavior of energetic plasmas. We attribute these initial promising results to the radical-rich content of our hollow-cathode plasma which most probably shows more surface chemistry dependence when compared to ion-rich plasmas. We’ll present ex-situ material characterization data of thicker films along with possible routes to obtain desired higher selectivity values.

Figure 1