Authorship：Corresponding author   Publisher：Applied Physics Letters  

The surface conductivity influences the etched pattern profiles in the plasma process. In the dielectric film etching, it is vital to reduce the charging build-up, which bends the trajectory of incoming ions for highly anisotropic etching. A significant increase in surface electric conductivity of SiO2 films was observed when exposed to down-flow plasmas containing hydrogen fluoride (HF) at cryogenic temperature (−60 °C). This phenomenon can be attributed to two factors: (i) the absorption of HF and/or its compounds and (ii) the presence of H2O, which is likely originating from the etching by-product of SiO2 and/or within the reactor. Comparing the surface electric resistance of the samples treated with HF plasmas to that of CF4/H2 and C4F8/H2 plasmas, we found that HF plasma treatment enables to be approximately three and six orders of magnitude lower. By using in situ x-ray photoemission spectroscopy, it was revealed that the presence of HF and/or its compounds and H2O were absorbed on the sample surface at −60 °C. These results strongly suggest that the cryogenic plasma etching with HF-contained gases can be used to alleviate the charge build-up issues.

DOI： 10.1063/5.0173553

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Publisher：Japanese Journal of Applied Physics  

We measured the etching rates of SiO2 and Si in dual-frequency excited CF4 plasma and compared the etching rates with the radical species distribution calculated by plasma simulation. The etching rates of SiO2 and Si at the wafer edge became higher than those at the wafer center, and the etching rate of Si distributed uniformly without bias voltage. In the simulation, the calculation model was calibrated on the basis of the measured (electron density). Assuming that CF3+ contributes to SiO2 and Si etching, and F contributes to Si etching without bias voltage, the radical fluxes flowing into the wafer were calculated by plasma simulation. The fluxes of CF3+ and CF2 became higher at the edge, and the F flux distribution was uniform without the bias voltage. It was shown that the distributions of the etching rate in the experiment and radical flux that contributes to etching in the simulation were in agreement.

DOI： 10.35848/1347-4065/acec56

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Language：English  

DOI： 10.1021/acsami.3c04705

PubMed

Publisher：Journal of Applied Physics  

Atmospheric repetitive He discharge with 10 ns current peak width and 3 × 10 11 V/s voltage front rise working in jet geometry is studied. This part deals with the ionization waves, electron density, and electric field dynamics. The electron density (n e ) is measured by Stark broadening of the H Balmer β (H β ) and He emission lines, the electric field is analyzed using Stark polarization spectroscopy, and the ionization waves are studied by fast imaging. We found that the ionization fronts propagate in the quartz tube with a velocity of about 5 × 10 5 m/s; this velocity slowly decreases along the tube but may jump in the open air at some conditions. In the space between electrodes, n e increases rapidly at the beginning, reaching about 7 × 10 15 cm − 3 , which corresponds to electron avalanche defining the discharge current peak. In the tube, the electrons are concentrated in the ionization wavefronts having low density ( < 10 14 cm − 3 ). Before the avalanche, a macroscopic (electrode-induced) electric field dominates between the electrodes peaking at about 8 kV/cm as deduced from H β peak splitting, whereas during the avalanche, H β reveals a double-Lorentzian polarization-insensitive profile imposed by two electron populations. In the low-density electron group, n e does not exceed 10 14 cm − 3 , whereas the high-density group is responsible for the observed electron density peak formation. After a rapid decay of the electrode-induced field, the microscopic electric field (induced by space-charge) dominates, peaking at about 25 kV/cm after the electron density peak. Certain electric field anisotropy is also detected in the quartz tube, confirming the wavefront propagation.

DOI： 10.1063/5.0138931

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Publisher：Vacuum  

The effects of substrate temperature (Ts) on the etch rate (ER) of the PECVD-prepared SiN, SiO2 and amorphous carbon (a-C) films, and their selectivity were investigated with a CF4/H2 plasma. The ERs for the SiN at all Ts were higher than that for the SiO2 films. As Ts was decreased from 50 to −20 °C, the ER for the SiN decreased. Contrarily, the ER of the SiO2 films increased. The etching selectivity of SiN over SiO2 reached to near unity when the Ts was −20 °C. At the same time, the ER of for the a-C films was found to be around 0.1 nm/s and irrespective of Ts. The fluorocarbon (FC) thickness was greater for the SiO2 films than that of the SiN. The lower ER for the SiO2 was therefore attributed to the thicker FC layer and resultant etching mechanism. As the Ts was decreased, the FC thickness on the SiO2 films decreased, which led to the ER decrease. The decrease of ER for the SiN etching at the low temperature was likely due to the higher stability of the surface N–H modification layer, compared with that processed at 20 °C, which was confirmed by the in situ FTIR.

DOI： 10.1016/j.vacuum.2023.111863

Scopus

Publisher：Japanese Journal of Applied Physics  

In a dual-frequency capacitively coupled Ar plasma, the secondary electron emission (SEE) coefficients were estimated using a 2D fluid model and experiments. The electron density was measured in the plasma chamber with the upper and lower electrodes of Si. The electron density was calculated by changing the SEE coefficient. The SEE coefficient of the Si electrode was estimated to be 0.06 so that the electron density of the 2D fluid model was close to that of the experimental result. Next, the electron density was measured in the chamber of the lower electrode covered with SiO2 and polyimide, and the SEE coefficients were estimated by comparing experiments and the 2D fluid model. The SEE coefficients of the SiO2 and the polyimide were estimated to be 0.24 and 0.22, respectively. Moreover, The SEE coefficients of SiO2 were almost the same in the range between 300 and 500 W of the lower electrode power.

DOI： 10.35848/1347-4065/ac80ea

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Publisher：IEEE Transactions on Semiconductor Manufacturing  

As a method to optimize the power frequency of dual-frequency plasma, we propose an optimization method that combines genetic algorithm and plasma simulation. A two-dimensional plasma simulation model of Ar plasma was constructed with a fluid model. Combining this simulation model with a genetic algorithm, 300 cases of plasma conditions with high plasma density and small variation in electron density were calculated. As a result, the optimum conditions were 175 to 210 MHz for the high-frequency generator and 1.0 to 2.5 MHz for the low-frequency generator. Furthermore, two types of single objective functions that combine the electron density and the variation rate were proposed. The single objective functions with emphasis on electron density and with emphasis on variation rate were introduced, and each objective function was optimized.

DOI： 10.1109/TSM.2023.3282566

Scopus

Language：Japanese   Publisher：Plasma Processes and Polymers  

Reduction of SnO2 to form spherical Sn particles and Sn etching are obtained by floating wire (FW)-assisted medium-pressure H2/Ar plasma. High-density H2/Ar plasma (1014 cm−3) with a larger treatment area at medium pressure (10 kPa) produces a two-times higher removal rate of SnO2 (0.111 mg/min) than that at atmospheric pressure with the same treatment area of 300 mm2. SnO2 film is removed from the glass surface by a two-step process involving (1) reduction of SnO2 by FW-H2/Ar plasma to form spherical Sn particles and (2) removal of low-contact Sn particles by water-based cleaning. High surface smoothness (roughness of 0.488 nm) and high optical transmittance (>92%) of treated samples indicate no damage compared to that of pristine quartz glass.

DOI： 10.1002/ppap.202100209

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Publisher：IEEE International Symposium on Semiconductor Manufacturing Conference Proceedings  

Plasma diagnostics including electron density, temperature, neutral atomic densities of the CH4H2 plasmas were performed in a capacitively-coupled reactor using surface-wave probe, Langmuir probe and vacuum ultraviolet absorption spectroscopy. The plasma density increased monotonically with varying H2 content from 30 to 90 %. The electron temperature first decreased with H2 up to 50 % and then increased at higher H2 concentration. The HF concentration reached a maximum value at a H2 of approximately 50 %, which is probably due to balance between H and F radicals from the plasma. Increasing the H2 content resulted in a higher H concentration and a less cross-linked structure of the amorphous hydrofluorocarbon films, analyzed by using in situ Fourier transformation infrared spectroscopy.

DOI： 10.1109/ISSM55802.2022.10027112

Scopus

Publisher：IEEE International Symposium on Semiconductor Manufacturing Conference Proceedings  

As a method to optimize the power frequency of dual-frequency plasma, we propose an optimization method that combines genetic algorithm and plasma simulation. A two-dimensional plasma simulation model of Ar plasma was constructed with a fluid model. Combining this simulation model with a genetic algorithm, plasma conditions with high plasma density and small variations in electron density were calculated. As a result, the optimum conditions were 175 to 210 MHz for the high-frequency generator and 0.5 to 4.0 MHz for the low-frequency generator.

DOI： 10.1109/ISSM55802.2022.10026920

Scopus

Language：Japanese   Publisher：Coatings  

With the increasing interest in dry etching of silicon nitride, utilization of hydrogencontained fluorocarbon plasma has become one of the most important processes in manufacturing advanced semiconductor devices. The correlation between hydrogen-contained molecules from the plasmas and hydrogen atoms inside the SiN plays a crucial role in etching behavior. In this work, the influences of plasmas (CF4/D2 and CF4/H2 ) and substrate temperature (Ts, from −20 to 50◦ C) on etch rates (ERs) of the PECVD SiN films were investigated. The etch rate performed by CF4/D2 plasma was higher than one obtained by CF4/H2 plasma at substrate temperature of 20◦ C and higher. The optical emission spectra showed that the intensities of the fluorocarbon (FC), F, and Balmer emissions were stronger in the CF4/D2 plasma in comparison with CF4/H2 . From X-ray photoelectron spectra, a thinner FC layer with a lower F/C ratio was found in the surface of the sample etched by the CF4/H2 plasma. The plasma density, gas phase concentration and FC thickness were not responsible for the higher etch rate in the CF4/D2 plasma. The abstraction of H inside the SiN films by deuterium and, in turn, hydrogen dissociation from Si or N molecules, supported by the results of in situ monitoring of surface structure using attenuated total reflectance-Fourier transform infrared spectroscopy, resulted in the enhanced ER in the CF4/D2 plasma case. The findings imply that the hydrogen dissociation plays an important role in the etching of PECVD-prepared SiN films when the hydrogen concentration of SiN is higher. For the films etched with the CF4/H2 at −20◦ C, the increase in ER was attributed to a thinner FC layer and surface reactions. On the contrary, in the CF4/D2 case the dependence of ER on substrate temperature was the consequence of the factors which include the FC layer thickness (diffusion length) and the atomic mobility of the etchants (thermal activation reaction).

DOI： 10.3390/coatings11121535

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Language：Japanese   Publisher：Plasma Processes and Polymers  

The dependences of etching characteristics on substrate temperature (Ts, from –20 to 50°C) of the plasma-enhanced chemical vapor deposition (PECVD) SiN films (PE-SiN) and low-pressure chemical vapor deposition (LPCVD) SiN films (LP-SiN) with CF4/H2 plasma were investigated. The Fourier-transform infrared spectroscopy shows that both film types were N–H bond-rich films, but in different hydrogen contents (PE-SiN 22.7 at% and LP-SiN 3.8 at%) from the Rutherford backscattering spectroscopy analyses. A higher hydrogen content led to a thinner fluorocarbon thickness because of the reaction between hydrogen outflux and C and N to form an HCN byproduct. The etch rates (ER) for the PE-SiN were higher than that of the LP-SiN at all Ts, due to the different FC thickness and etching mechanisms proposed. The formation of the N−Hx layer on PE-SiN at low temperature caused the decrease in ER. For the LP-SiN, the weak dependences of Ts on surface structure and ER were observed.

DOI： 10.1002/ppap.202100078

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Publisher：Vacuum  

DOI： 10.1016/j.vacuum.2021.110153

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Publisher：Applied Surface Science  

DOI： 10.1016/j.apsusc.2020.148550

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Publisher：Applied Surface Science  

DOI： 10.1016/j.apsusc.2020.148439

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Publisher：Applied Surface Science  

DOI： 10.1016/j.apsusc.2020.145304

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Language：Japanese  

DOI： 10.1109/ISSM51728.2020.9377537

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