SF6 O2 Research Articles

Investigation of etching optimization in capacitively coupled SF6–O2 plasma

The selective etching process is widely used for achieving the desired etch rate in semiconductor fabrication. Parameters such as input power, operating pressure, gas mixture, chamber geometry, and amplitude of the radio-frequency voltage govern the etch rate and etch quality in plasma. In this work, we experimentally investigated the optimum plasma etching conditions required to achieve an anisotropic etch profile and analyzed how the optimum etching can be carried out using an appropriate operating pressure and oxygen concentration. Optical emission spectroscopy was used to measure the concentrations of oxygen and fluorine, and Langmuir probe was used to measure the electron density in the plasma. The oxygen concentration was varied from zero to 100 vol. % for pressures in the range of 20–600 mTorr. The optimum etch conditions are used to study the ion energy distribution given by Kawamura et al., Plasma Sources Sci. Technol. 8, R45 (1999). The results suggest that in addition to O2% and pressure, the DC bias is another crucial parameter for achieving the optimum etch conditions.

Experimental investigation of SF6–O2 plasma for advancement of the anisotropic Si etch process

This study examines the impact of varying the internal process parameters, such as the concentrations of oxygen and fluorine in a SF6–O2 plasma, in two capacitively coupled plasma etch chambers with different geometries. Silicon wafers were used to investigate the anisotropic nature of etch profiles. The oxygen and fluorine concentrations were measured via optical emission spectroscopy using the actinometry technique, which requires the electron energy distribution function to remain unchanged under the different plasma conditions employed in this work. A Langmuir probe was used to investigate the electron energy distribution function, where the chamber pressure, power, and process duration were kept constant and the oxygen concentration was varied from 0 to 60 vol. %. The results showed that in both the chambers, the atomic concentrations of oxygen and fluorine increased rapidly when the fraction of oxygen in the SF6 plasma was increased to 20 vol. % and decreased with further addition of oxygen. Scanning electron microscopy showed an etch feature with a minimal lateral run-out at an O2 concentration of 20 vol. % in both the chambers. The distribution of electron energy and the concentrations of oxygen and fluorine exhibited similar patterns as functions of the oxygen concentration in the SF6 plasma in the two chambers, but the values were different because of the different chamber geometries, which also affected the silicon etch rate and lateral run-out.

Investigation of surface features using reactive ion etching method for the enhanced performance of multi-crystalline silicon solar cells

Maskless, random RIE texturing with a gas mixture of SF6/O2 (SO) and SF6/O2/Cl2 (SOC) was investigated to achieve higher efficiency for mc-Si solar cells. Cone structure with aspect ratio of 3.7 was highly effective for reducing surface reflectance particularly for wavelength from 310 to 700nm and triangular pyramid structure with aspect ratio of 2 revealed rather uniform reduction of reflectance with respect to the optimized wet texturing for whole wavelength range of 310–1100nm. On the contrary to results of reflectance, performances of fabricated cell were much better for triangular pyramid structure than cone structure. The degradation of cell performances for the cone structure was assigned both to the poor e-h pair generation efficiency relevant to the formation of highly defective surface layer with high density recombination centers, and ohmic shunt. The efficiency enhancement by 0.6% in mc-Si solar cells with triangular pyramid structures was assigned also to the slightly enhanced e-h pair generation efficiency with respect to the reference cell.

Dry micro-electro-discharge machining of carbon-nanotube forests using sulphur-hexafluoride

The effect of using sulphur hexafluoride (SF6), a high-dielectric-strength gas, for dry micro-electro-discharge machining (μEDM) of carbon-nanotube (CNT) forests is investigated. It is found that SF6 enables μEDM of CNTs without O2, which is known to be essential for CNT machining in N2. The process in the SF6 ambient at a discharge voltage of 25V is found to lead to a smaller discharge gap, i.e., tighter tolerance as well as higher machining quality compared with the N2 case at the same voltage. The N2 environment produces smaller discharge gap when 10V is used; however, both the quality and rate of machining are somewhat lower in this case. The mixture with 20% O2 in SF6 is revealed to be an optimum condition for machining tolerance and quality. CNT forests are used as the cathode in the process, as opposed to conventional μEDM where the workpiece forms the anode. This configuration in the SF6–O2 mixture is observed to generate higher discharge currents at low voltages, presumably due to effective field-emission by the CNTs, leading to finer and cleaner machining. Energy-dispersive X-ray analysis reveals that the optimal conditions result in less contamination by the electrode element on the processed forest surfaces.

Cryogenic etching of nano-scale silicon trenches with resist masks

Cryogenic silicon etching using SF6–O2 at the sub-50 nm scale has been developed for nano-electromechanical systems (NEMS) and nano-photonics systems where high aspect ratio trenches are desired. It was found that the SF6–O2 chemistry at cryogenic temperatures (−100 to −130°C) provides the best combination of etch rate, selectivity, and profile control for the smallest trenches etched. The profile can be well controlled with aspect ratios on the order of 8:1 for 20nm wide trenches. The various etch parameter trends will be discussed along with methods to achieve the optimal profiles and etch rates.

Transport properties of in SF6–Ne, SF6–N2 and SF6–O2 mixtures

The mobility of has been calculated for the gas mixtures SF6–Ne, SF6–N2 and SF6–O2 using an optimized Monte Carlo code for the ion transport simulation in a drift tube. The elastic momentum transfer collision cross sections needed for the calculation were determined from a semi-classical JWKB approximation, while the inelastic ones (detachment, dissociative charge transfer and conversion to and F−) were taken from the literature for the case of the collision system. The resulting sets of collision cross sections were validated by comparing the calculated mobilities with those measured in the above mixtures with a time-resolved pulsed Townsend technique. The longitudinal and transverse density-normalized diffusion coefficients were calculated for these mixtures for the case where the share of SF6 in the mixture was 50%. Finally, the validity of Blanc's law was discussed at low and high electric fields, whereby we show that it fails at high fields, where inelastic processes are dominant.

Cold Plasma Processing and Plasma Chemistry of Metallic Cobalt Surface

Plasma processing of metallic cobalt was experimentally investigated with three fluorine-containing gases, CF4–O2, SF6–O2, and NF3 to determine the surface decontamination rate and to examine the reaction mechanism. Results show that the maximum etching rate reaches 17.12 μm/min with NF3 gas at 420°C, while the rates are 2.56 μm/min and 1.14 μm/min with CF4–O2 and SF6–O2 gas, respectively, at the same temperature. AES analysis identified the constituent elements of the reaction products to be oxygen, fluorine, and cobalt, and XPS analysis reveals that the reaction product with all three plasma gases is very likely to be CoF2.

Measurement of electron transport and effective ionization in SF6–air and SF6–O2 mixtures

The electron drift velocity ve, the density-normalized longitudinal diffusion coefficient NDL, the density-normalized effective ionization coefficient (α − η)/N and the limiting field strength E/Nlim have been measured for the binary mixtures SF6–air and SF6–O2 for SF6 concentrations ranging from 1% to 50%. The overall reduced electric field strength E/N was varied between 1.2 and 500 Td (1 Townsend = 10−17 V cm2). A pulsed Townsend technique was used to perform these measurements, which turned out to be sensitive enough to distinguish relatively small differences in the electron drift velocities and longitudinal diffusion coefficients over the mixture range 1 to 50% SF6 in SF6–air and SF6–O2. On the other hand, the effective ionization coefficients show a well-defined variation with the SF6 content in the mixture. The limiting value of E/N at which the effective ionization coefficient is zero, that is, the critical field strength, turned out to be smaller than that for the SF6–N2 mixture. However, the difference in behaviour between SF6–air and SF6–N2 is small. To the best of our knowledge, we are not aware of any previously published measurements of these parameters for these mixtures.

Modeling of fluorine-based high-density plasma etching of anisotropic silicon trenches with oxygen sidewall passivation

The kinetics of high aspect ratio, anisotropic silicon etching in a SF6–O2 plasma is investigated with a combination of Monte Carlo simulations and inductively coupled plasma etching experiments. The spontaneous reaction of atomic fluorine is dominant at room temperature and Knudsen transport of the radicals is the only limitation in narrow structures. At low temperatures (typically between −125 and −95 °C) oxygen passivation becomes effective and anisotropic profiles are obtained because the oxygen passivation can only be removed by the directional ion bombardment. The input parameter settings for the Monte Carlo model are based on measurements with plasma diagnostics. Simulations show that anisotropy is controlled by the oxygen sidewall passivation which depends on the oxygen flux, the oxygen adsorption coefficient, and the aspect ratio. The simulated trench profiles and the aspect ratio dependent etch rate are consistent with the experimental results. Experimentally the etch rate behavior can be tuned from aspect ratio dependent to aspect ratio independent by decreasing the ion flux. This effect can be described well by the recently developed chemically enhanced ion-neutral synergy model. It turns out that aspect ratio independent etching is obtained if the downwards depletion of fluorine radicals due to Knudsen transport is compensated by an increase of the available reaction sites.

Isotropic Plasma Etching of SiO2 Films

The isotropic etching of SiO2 with an SF6–O2 plasma is studied experimentally. It is shown that the key factors in the process are the total and partial pressures of SF6 and O2 and the RF power. A smoothed-down edge profile of contact windows is obtained if SF6 and O2 are mixed in a ratio of 1 : 10 to 1 : 5. The maximum etch rate is achieved at total pressures of 250 to 450 Pa.

Comparison of etching processes of silicon and germanium in SF6–O2 radio-frequency plasma

Reactive ion etching of Si and Ge in SF6–O2 is investigated. Etch rate shows that Si etching is selective with respect to Ge in SF6–O2 (O2<50%); the reverse is observed in SF6–O2 (O2≳50%). In agreement with the compared evolution of the F and O concentration in the plasma and of the etch product formation rate obtained respectively, by optical emission spectroscopy and mass spectrometry, the x-ray photon spectroscopy surface analysis reveals that the growth of a SiOxFy layer quenches the Si etching whereas the formation of GeOxFy does not inhibit Ge etching. Using a simple model, an experimental surface reactivity is defined and expressed in function of the experimental data. Results suggest a different behavior for sulfur and oxygen on both materials.

Surface characterization of GaAs after the reactive ion etching of GeMoW ohmic contact in radio frequency SF6–O2 plasma

A characterization of monocrystalline GaAs surfaces after reactive ion etching of a GeMoW contact in SF6–O2 is presented. Scanning electron microscopy (SEM), monochromatized x‐ray photoelectron spectroscopy (XPS), and Auger spectroscopy are used as means of investigation. Irregular 0.1 μm structures are observed by SEM on the surface while the XPS analysis reveals the existence of a gallium oxyfluoride superficial layer which composition is typically GaOF. Contributions in the As 2p and As 3d spectra at 1.8 eV with respect to As (GaAs) are identified as arsenic sulfides. Moreover, they have been localized on top of the GaAs substrate, and in the lowest part of the gallium oxyfluoride layer. An annealing at 800 °C for 10 s partly removes the SEM observed structures which can be assigned to the AsxSy and As2O3 species. The XPS analysis reveals the decomposition of the interface As–species when the temperature increases and the concomitant Ga–S bonds formation. Concurrently, the GaOF layer could be degraded ...

Planarization techniques for multilevel metallization : A. N. Saxena and D. Pramanik. Solid St. Technol., 95 (October 1986)

This paper describes a procedure based on reactive ion etching developed to reduce the surface roughness of CVD diamond thin films. The technique involves etching a bilayer made up of diamond and a planarizing SiO2 layer in an SF6O2 plasma mixture. Etching conditions have been determined which yield equal rates for both diamond and the SiO2 coverlayer and favour the removal of diamond peaks. Smoothed surfaces have been characterized by SEM, AFM and XPS: the results exhibit a significant decrease in roughness. This novel technique offers promising prospects for polishing thin diamond membranes for X-ray lithography applications without the removal of significant amounts of diamond material. The optical transmittance of diamond membranes, mainly affected by light scattering due to surface roughness, is significantly improved using this method of planarization.