By Nidhi DhullReviewed by Lexie CornerDec 20 2024
In a recent study published in Nature Communications, researchers added a heavy inert gas co-flow to improve step coverage (SC) in the chemical vapor deposition (CVD) process. This approach was demonstrated using xenon (Xe) as a co-flow gas during the deposition of boron carbide (BxC) films.
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Background
Modern semiconductor devices require the deposition of thin films onto surfaces with increasingly complex structures characterized by high aspect ratios (AR) and small feature sizes. SC is a measure of how uniformly a film conforms to these features.
While atomic layer deposition (ALD) is suitable for conformal films, it is not widely applicable to all materials, such as carbides, due to the lack of self-terminating surface chemistries. Consequently, CVD is often used for such materials, but it typically produces sub-conformal films (SC < 1) because of less controlled surface chemistry.
This study explored how promoting precursor diffusion with a heavy gas co-flow, such as Xe, could improve SC without altering surface reactions, enabling better conformal deposition at higher temperatures.
Methods
CVD was conducted in a horizontal hot-wall reactor using triethylboron (TEB) as a single-source precursor for BxC. Palladium-membrane purified H2 was used as a carrier gas and co-reactant. To enhance diffusion, a co-flow of xenon (Xe), either continuous or pulsed, was introduced, with argon used as a control for comparison.
BxC films were deposited on silicon substrates and on silicon substrates with a 10:1 AR trench pattern (~6 μm wide, ~60 μm deep). The films, both inside and outside the high-AR structures, were characterized using X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM). For cross-sectional SEM imaging, the substrates were broken perpendicular to the trench openings.
SEM was used to measure film thickness across the trench depth, while an EDX detector provided elemental mapping and line scans of the pillar-hall structures. Elemental compositions of the films, with and without Xe co-flow, were analyzed using time-of-flight elastic recoil detection analysis (ToF-ERDA).
Substrate curvatures were assessed using rocking-curve measurements via X-ray diffraction (XRD). XPS was also utilized to examine the chemical environment of the films, and ultraviolet photoelectron spectroscopy (UPS) was performed in the XPS chamber to estimate the work function of the material. Film density was determined through X-ray reflectivity (XRR) measurements.
Results and Discussion
SC was evaluated for samples deposited over 60 minutes under three conditions: without xenon (Xe), with a 100-sccm Xe co-flow, and with a 100-sccm argon co-flow. The SC improved from 0.71 (no Xe) to 0.97 with Xe co-flow but remained nearly unchanged (0.69) with argon co-flow. This indicates that the atomic weight of Xe, rather than its inertness, enhanced SC, supporting the hypothesis of competitive diffusion.
XPS confirmed the deposited films to be BxC. ToF-ERDA revealed the stoichiometry of the film deposited at 550 °C in a hydrogen ambient as B4.7C. The Xe co-flow resulted in a 1 at.% increase in carbon content, likely due to a slight shift in the decomposition pathways of TEB, specifically the ratio of β-hydride elimination to hydrogen-assisted ligand elimination.
Xe co-flow facilitated a more uniform film deposition over a flat substrate surface at a faster deposition rate than that without Xe. Thus, the Xe co-flow enhanced the precursor diffusion on the vertical microscale in the trenches and also on a lateral macroscale in the CVD reactor.
Xe co-flow improved the uniformity of film deposition across flat substrate surfaces and increased the deposition rate compared to conditions without Xe. This enhancement was observed on the vertical microscale within trenches and lateral macroscale across the reactor.
For lateral high-AR structures with a 500 nm gap height, the penetration depth improved from approximately 15 µm (no Xe) to about 25 µm with Xe co-flow. As a result, an AR of 50:1 was achieved, with conformal growth maintained due to reduced precursor back diffusion.
Conclusion
The study demonstrated the effectiveness of competitive co-diffusion in improving SC for boron carbide films in CVD using Xe as a heavy diffusion additive. SC increased from 0.71 to 0.97 in a 10:1 AR feature without significant changes to elemental composition or film density.
Although the exact mechanism remains unclear, it is hypothesized that Xe atoms influence the diffusion of lighter precursor molecules via competitive co-diffusion and enhance the desorption of reactive intermediates through surface collisions. Further experimental and computational studies are recommended to fully understand the mechanism.
Journal Reference
Choolakkal, AH., Niiranen, P., Dorri, S., Birch, J., Pedersen, H. (2024). Competitive co-diffusion as a route to enhanced step coverage in chemical vapor deposition. Nature Communications. DOI: 10.1038/s41467-024-55007-1, https://www.nature.com/articles/s41467-024-55007-1
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