On the Etching Mechanism of Highly Hydrogenated SiN Films by CF4/D2 Plasma: Comparison with CF4/H2

Abstract

With the increasing interest in dry etching of silicon nitride, utilization of hydrogen-contained 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).

Finding relative negative ions concentration in dc glow discharges during undergraduate laboratory work

“…Negative ions appear in many gases that are often used in plasma technologies. Fluorine-containing gases (Carbon tetrafluoride CF 4 , Sulfur hexafluoride SF 6 and others) are widely used not only for etching different materials [1][2][3][4][5], but also for different plasma treatments of polymers' surfaces with the aim of changing their wetting properties, surface cleaning and activation before metal or paint layer deposition, for controlling their biocompatibility, chemical resistance [6][7][8][9][10], polymer film deposition [11], etc. Therefore our university students who study elementary processes in plasmas and plasma technologies carry out a set of undergraduate laboratory works.…”

Etching Mechanism Based on Hydrogen Fluoride Interactions with Hydrogenated SiN Films Using HF/H2 and CF4/H2 Plasmas

“…On the other hand, the molecular emission bands, e.g. , CF and CF 2 , are mainly located in the UV range and are not typically visible due to the spectral range chosen in this work and the fact that HF bands are typically not observable in OES measurement in the HF-containing plasmas. A clear dominance of Ar peaks (even with the 4% Ar admixture used in this study) allowed us to construct multiple actinometric ratios for calculating the F atom density, as presented below.…”

“…Simultaneously, the Si material undergoes oxidation and becomes passive by converting into SiOF . It has also been reported that high SiN/SiO 2 selectivity can be achieved using gas mixtures such as NF 3 /O 2 or CF 4 /N 2 /O 2 plasmas, which is attributed to the influence of NO radicals. , The addition of hydrogen to fluorocarbon gas, leading to the generation of CH x F y ions, can be used to enhance the etch rate (ER) and/or selectivity of SiN/SiO 2 or SiN/Si. − Special hydrofluorocarbon gases have been used to achieve high etch selectivity for SiN over SiO 2 and Si. − Additionally, the dependence of selectivities between Si-based materials on substrate temperature has recently been investigated by various research groups. , In hydrogen-contained fluorocarbon plasmas, the formation of hydrogen fluoride (HF) is anticipated to have a detrimental effect on etching for Si-based materials, as it tends to scavenge F radicals . In the mechanism of SiN etching, the H also reacts with N and CF x species, resulting in the formation of HCN as a byproduct, which helps to reduce the FC polymerization and facilitates the progress of SiN etching. , Based on the experimental and simulation results, it has been reported that HF molecules appear to play an important role in the selective etching of SiN over SiO 2 in NF 3 /N 2 /O 2 /H 2 remote plasmas , On the other hand, recently, there have been investigations into the selective etching of SiO 2 over SiN or Si using an HF vaper, employing both computational simulations and experimental approaches. , …”