Pseudo-Wet Plasma Mechanism Enabling High-Throughput Dry Etching of SiO2 by Cryogenic-Assisted Surface Reactions.

Manufacturing semiconductor devices requires advanced patterning technologies, including reactive ion etching (RIE) based on the synergistic interactions between ions and etch gas. However, these interactions weaken as devices continuously scale down to sub-nanoscale, primarily attributed to the diminished transport of radicals and ions into the small features. This leads to a significant decrease in etch rate (ER). Here, a novel synergistic interaction involving ions, surface-adsorbed chemistries, and materials at cryogenic temperatures is found to exhibit a significant increase in the ER of SiO2 using CF4/H2 plasmas. The ER increases twofold when plasma with H2/(CF4 + H2) = 33% is used and the substrate temperature is lowered from 20 to -60 °C. The adsorption of HF and H2O on the SiO2 surface at cryogenic temperatures is confirmed using in situ Fourier transform infrared spectroscopy. The synergistic interactions of the surface-adsorbed HF/H2O as etching catalysts and plasma species result in the ER enhancement. Therefore, a mechanism called "pseudo-wet plasma etching" is proposed to explain the cryogenic etching process. This synergy demonstrates that the enhanced etch process is determined by the surface interactions between ions, surface-adsorbed chemistry, and the material being etched, rather than interactions between ion and gas phase, as observed in the conventional RIE.

Study of optical emission spectroscopy using modified Boltzmann plot in dual-frequency synchronized pulsed capacitively coupled discharges with DC bias at low-pressure in Ar/O2/C4F8 plasma etching process.

We consider the corona model and local thermal equilibrium approximations of a real plasma to measure the electron temperature (Te) and density (ne), respectively, using the optical emission spectroscopy (OES) method in dual-frequency pulsed capacitively coupled plasmas (CCPs) in a reactive mixture of Ar/O2/C4F8 at a low operating pressure. The operation conditions such as DC continuous and synchronized were used for the study and plasma characterization for the intended plasma application such as high aspect ratio etching (HARE). We show that the present plasma conditions are dominated by a corona balance rather than the supremacy of multi-step excitation. This fact has enabled us to utilize the modified Boltzmann plot technique to evaluate the Te values. In the second method, we simultaneously used the Boltzmann and Saha equations to determine the ne value using the line intensity ratio and the value of Te. Time-resolved measurements of Te and ne were performed for completeness, and the insight of the pulsed discharge was investigated. Time evolution of ne and Te using the OES method revealed a similar trend in the change of plasma parameters, indicating electron impact ionization during the pulse on phase. It was seen that ne in the afterglow speedily decreased within a short time of ∼5 µs. Analysis suggests the formation of afterglow plasmas, which are composed of positive and negative ions with very low electron density. The results revealed that the DC-synchronized operation could be useful for plasma application such as HARE due to different plasma characteristics. It also suggests the production of ion-ion plasmas by the effective utilization of negative ions in the afterglow phase. The corona balance condition was validated in our experiments, and the results were compared with the existing literature.

Formation of spherical Sn particles by reducing SnO2 film in floating wire-assisted H2/Ar plasma at atmospheric pressure.

A green method to synthesize spherical Sn particles by reducing SnO2 film in atmospheric-pressure H2/Ar plasma at low temperatures for various applications is presented. The floating wire-assisted remotely-generated plasma with a mixture of 0.05% H2/Ar gas formed spherical metallic Sn particles by reducing a SnO2 layer on glass substrate. During the reduction process, H radical density was measured by using vacuum ultraviolet absorption spectroscopy, and plasma properties including electron density and gas temperature were diagnosed by optical emission spectroscopy. The inductively coupled generated plasma with a high electron density of 1014 cm-3, a hydrogen atom density of 1014 cm-3, and a gas temperature of 940 K was obtained at a remote region distance of 150 mm where the SnO2/glass substrate was placed for plasma treatment. The process has been modeled on the spherical Sn formation based on the reduction of SnO2 films using H radicals. Depending on the treatment condition, the total reduction area, where spherical Sn particles formed, was enlarged and could reach 300 mm2 after 2 min. The substrate temperature affected the expansion rate of the total reduction area and the growth of the Sn spheres.

In Situ Monitoring of Surface Reactions during Atomic Layer Etching of Silicon Nitride Using Hydrogen Plasma and Fluorine Radicals.

The atomic layer etching (ALE) of silicon nitride (SiN) via a hydrogen plasma followed by exposure to fluorine radicals was investigated by using in situ spectroscopic ellipsometry and attenuated total reflectance Fourier transform infrared (FTIR) spectroscopy to examine the surface reactions and etching mechanism. FTIR spectra of the surface following exposure to the hydrogen plasma showed an increase in the concentration of Si-H and N-H bonds, although the N-H bond concentration plateaued more quickly. In contrast, during fluorine radical exposure, the Si-H bond concentration decreased more rapidly. Secondary ion mass spectrometry demonstrated that the nitrogen atom concentration was decreased to a depth of 4 nm from the surface after the hydrogen plasma treatment and indicated a structure consisting of N-H rich, Si-H rich, and mixed layers. This indicated that Si-H bonds were primarily present near the surface, while N-H bonds were mainly located deeper into the film. The formations of the N-H and Si-H rich layers are important phenomena associated with modification by hydrogen plasma and fluorine radical etching, respectively.

Gene Expression of Osteoblast-like Cells on Carbon-Nanowall as Scaffolds during Incubation with Electrical Stimulation.

Nanostructured cell-culture scaffolds of carbon nanowalls (CNWs) were prepared by changing average wall-to-wall distances either 132 or 220 nm. Osteoblast-like cells (Saos-2) proliferated during 4 day incubation on the wider (220 nm) CNW scaffolds in the presence of electrical stimulation (ES). Differentiation gene expression levels of Runt-related transcription factor 2 (Runx2) and osteocalcin (OC) were suppressed after 10 day incubation, which indicated that the average wall-to-wall distances of the CNW scaffolds affect suppression of Runx2 and OC gene expression. This technique holds promise for controlling the differentiation of osteoblast-like cells.

Wavelength dependence for silicon-wafer temperature measurement by autocorrelation-type frequency-domain low-coherence interferometry.

We investigate silicon wafer temperature measurement characteristics based on optical low-coherence interferometry by altering the light source wavelength. Variations in Si wafer optical thickness with temperature are expressed by thermal expansion and the refractive index. The optical characteristics determine the measurement precision and range. In this study, the measurement precision and the measurable temperature range were evaluated for three wavelengths: 1040, 1310, and 1550 nm. The maximum measurable temperature at 1040 nm was the lowest because of signal light absorption caused by fundamental interband absorption. The measurement precision at 1040 nm was the highest at 0.020°C because optical thickness changes per degree C increase with decreasing wavelength.