A plasma-enhanced atomic layer deposition (ALD) process was developed for the growth of SiNx thin films using Si2Cl6 and NH3 plasma. At substrate temperatures ≤400 °C, we show that this ALD process leads to films with >95% conformality over high aspect ratio nanostructures with a growth per cycle of ∼1.2 Å. The film growth mechanism was studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. Our data show that on the SiNx growth surface, Si2Cl6 reacts with surface -NH2 groups to form surface -NH species, which are incorporated into the growing film. In the subsequent half cycle, radicals generated in the NH3 plasma abstract surface Cl atoms, and restore an NHx (x = 1,2)-terminated surface. Surface Si-N-Si bonds are also primarily formed during the NH3 plasma half-cycle. The infrared data and Rutherford backscattering combined with hydrogen forward scattering shows that the films contain ∼23% H atoms primarily incorporated as -NH groups.

The fabrication of next-generation semiconductor devices has created a need for low-temperature (≤400 °C) deposition of highly-conformal (>95%) SiO2, SiNx, and SiC films on high-aspect-ratio nanostructures. To enable the growth of these Si-based dielectric films, semiconductor manufacturers are transitioning from chemical vapor deposition to atomic layer deposition (ALD). Currently, SiO2 films deposited using ALD are already being integrated into semiconductor device manufacturing. However, substantial processing challenges remain for the complete integration of SiNx films deposited by ALD, and there are no known processes for ALD of SiC at temperatures that are compatible with semiconductor device manufacturing. In this focused review, the authors look at the status of thermal and plasma-assisted ALD of these three Si-based dielectric films. For SiO2 ALD, since low-temperature processes that deposit high-quality films are known, the authors focus primarily on the identification of surface reaction mechanisms using chlorosilane and aminosilane precursors, as this provides a foundation for the ALD of SiNx and SiC, two material systems where substantial processing challenges still exist. Using an understanding of the surface reaction mechanisms, the authors describe the underlying reasons for the processing challenges during ALD of SiNx and SiC and suggest methodologies for process improvement. While both thermal and plasma-assisted SiNx ALD processes have been reported in the literature, the thermal NH3-based ALD processes require processing temperatures >500 °C and large NH3 doses. On the other hand, plasma-assisted SiNx ALD processes suffer from nonuniform film properties or low conformality when deposited on high-aspect-ratio nanostructures. In the SiNx section, the authors provide a broad overview of the currently known thermal and plasma-assisted SiNx ALD processes using chlorosilane, trisilylamine, and aminosilane precursors, describe the process shortcomings, and review the literature on precursor reaction pathways. The authors close this section with suggestions for improving the film properties and conformality. In the case of SiC, the authors first outline the limitations of previously reported SiC ALD processes and highlight that unlike SiO2 and SiNx plasma-assisted ALD, no straightforward pathway for low-temperature plasma-assisted growth is currently apparent. The authors speculate that low-temperature ALD of SiC may require the design of completely new precursors. Finally, they summarize the progress made in the ALD of C-containing SiNx and SiO2 films, which may provide many of the benefits of SiC ALD in semiconductor manufacturing. In closing, through this review, the authors hope to provide the readers with a comprehensive knowledge of the surface reactions mechanisms during ALD of Si-based dielectrics, which would provide a foundation for future precursor and process development.

We developed a novel process for the atomic layer deposition (ALD) of SiC x N y films using a Si 2 Cl 6 and a CH 3 NH 2 plasma. Under self-limiting growth conditions, this ALD process led to SiC x N y films with up to 9 atomic percent carbon with a conformality >95% in 5:1 aspect ratio nanostructures. The surface reactions during ALD, and in particular the carbon incorporation mechanism, were studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. Similar to the Si 2 Cl 6 and NH 3 plasmabased process, we show that on the SiC x N y growth surface, Si 2 Cl 6 reacts primarily with surface −NH 2 species that were created after the CH 3 NH 2 plasma cycle. During the subsequent CH 3 NH 2 half cycle, the surface chlorine was liberated, creating −NH x (x = 1 or 2) groups, while carbon was incorporated primarily as −NCN− species. In situ ellipsometry showed that the growth per cycle and the refractive index were ∼1 Å and ∼1.85, respectively. Elemental depth profiling with secondary ion mass spectrometry showed that, as the plasma power was increased from 50 to 100 W, the carbon atomic fraction increased from ∼4 to ∼9%. At higher plasma powers, the CH 3 NH 2 plasma half cycle was not self-limiting and led to continuous carbon nitride growth.

We report a novel three-step SiN atomic layer deposition (ALD) process using SiCl, CHNH, and N plasma. In a two-step process, nonhydrogenated chlorosilanes such as SiCl with N plasmas lead to poor-quality SiN films that oxidize rapidly. The intermediate CHNH step was therefore introduced in the ALD cycle to replace the NH plasma step with a N plasma, while using SiCl as the Si precursor. This three-step process lowers the atomic H content and improves the film conformality on high-aspect-ratio nanostructures as Si-N-Si bonds are formed during a thermal CHNH step in addition to the N plasma step. During ALD, the reactive surface sites were monitored using in situ surface infrared spectroscopy. Our infrared spectra show that, on the post-N plasma-treated SiN surface, SiCl reacts primarily with the surface -NH species to form surface -SiCl ( x = 1, 2, or 3) bonds, which are the reactive sites during the CHNH cycle. In the N plasma step, reactive -NH surface species are created because of the surface H available from the -CH groups. At 400 °C, the SiN films have a growth per cycle of ∼0.9 Å with ∼12 atomic percent H. The films grown on high-aspect-ratio nanostructures have a conformality of ∼90%.

The authors have designed experiments to test three different approaches for the incorporation of carbon atoms into amorphous SiNx or SiO2 films grown using atomic layer deposition (ALD). In each approach, the surface reactions of the precursors were monitored in situ using attenuated total reflection Fourier transform infrared spectroscopy. In the first approach, for depositing carbon-containing SiNx films using ALD, carbon was introduced into the process through a silicon precursor, SiCl2(CH3)2, followed by NH3 plasma exposure. While our infrared data show that SiCl2(CH3)2 reacts with an –NHx (x = 1, 2) terminated surface created after NH3 plasma exposure, –CH3 groups are eliminated in the precursor adsorption step leading to no significant carbon in the films. In the second approach, the authors hypothesized a three-step ALD process, which would involve Si-, C-, and N-containing precursors, and tested the reactivity of two carbon-containing precursors, CH3I and Al(CH3)3, with H- and Cl-terminated silicon surfaces, respectively. The authors show that both precursors readily react with the silicon surfaces, but neither one provides the appropriate surface termination. CH3I reacts with surface –SiHx (x = 1, 2, 3) to create surface –SiIx (x = 1, 2, 3) species with CH3 as the leaving groups. While Al(CH3)3 reacts with the Cl-terminated Si surface to form a surface –SiCH3 group, residual aluminum remains on the surface as –Al(CH3)x (x = 1, 2) groups that are not completely removed as volatile Al(CH3)xCl3−x (x = 1, 2). Finally, in the third approach for ALD of carbon-containing SiO2 films, the authors used Si2Cl6 with CO and CO/O2 plasmas. A pure CO plasma led to amorphous carbon growth, and a CO plasma diluted with of O2 led to no detectable carbon incorporation in the SiO2 film.