High-density Fluorocarbon Plasmas Research Articles

Porous SiOCH, SiCH and SiO2 Etching in High Density Fluorocarbon Plasma with a Pulsed Bias

AbstractThe effect of radio frequency bias pulsing on porous SiOCH, SiCH and SiO2 etching using inductively coupled fluorocarbon plasmas was investigated. It was found that pulse frequency had only a small influence on material etch rates. However, pulse duty cycle, defined as the time during which a bias is applied over the total period, clearly modified etch rates and selectivities. Indeed, etch selectivities between porous SiOCH and SiCH or SiO2 were considerably improved when the duty cycle was decreased. This enhancement was associated with relatively high porous SiOCH etch rates, and pattern transfers under low duty cycle conditions proved to be successful. To better understand the pulse process, surface analysis was also realised. According to XPS analysis, the material surface structure was found to be similar after etching in continuous or in pulsed mode. However, fluorocarbon species on material surfaces were fluorine richer after etching in the pulsed mode.magnified image

High density fluorocarbon plasma etching of methylsilsesquioxane SiOC(H) low-k material and SiC(H) etch stop layer: surface analyses and investigation of etch mechanisms

This paper focuses on plasma etching and XPS surface analyses of new dielectric materials used in integrated circuits. We investigate by XPS surface modifications of methylsilsesquioxane low-k polymer (SiOC(H)) and amorphous hydrogenated silicon carbide (SiC(H)) when exposed to Ar, SF6 and C2F6-based high density plasmas. Ar and SF6 plasmas remove the carbonaceous groups from the surface leading to the formation of fluorinated top layers (SiOF and SiF-like layers) in SF6 plasma. In the case of C2F6-based mixtures, the surface structure fits well with a two-layer model, consisting of a fluorocarbon top layer above a fluorinated interaction layer (SiOF or SiF) on the bulk materials (SiOC(H) or SiC(H)). Determination of top layer thicknesses from XPS data is discussed. We show that material etch rate is not correlated with the total modified thickness, in contrast to the top fluorocarbon layer thickness which is in good correlation with etch rates. Etch yields (etch rates divided by the ion flux) are calculated and are studied in order to draw material etch mechanisms in C2F6 based mixtures. We conclude that for both materials, etching mechanisms in C2F6/H2 mixtures, and to a lesser extent in C2F6/Ar mixtures, are close to those of silicon in fluorocarbon plasmas and different from those of the conventional interlevel dielectric material SiO2. On the contrary etching mechanisms in C2F6/O2 mixtures are similar to those of silicon dioxide in fluorocarbon plasmas.

Angular dependence of silicon oxide etching yield in fluorocarbon chemistries

High density fluorocarbon plasma for silicon oxide etching has various ion and neutral species. Profile evolution modeling can provide understanding of many difficulties caused by the complexity of the plasma in etching. In this research we have measured etching and deposition rates as functions of ion impinging angle, sample temperature, which are necessary for profile evolution modeling of silicon oxide etching in inductively coupled plasma. Angular dependence of etching yield of oxide in fluorocarbon plasma shows very unique behavior unlike typical ion-induced chemical etching or physical sputtering. Ion-induced deposition model is suggested and tested.

Etching of low-k materials in high density fluorocarbon plasma

Low dielectric constant materials (low-k) are used as interlevel dielectrics in integrated circuits. This paper concerns the etching process of these materials in high density plasma with the aim to provide some insights concerning the etch mechanisms. Materials studied are methylsilsesquioxane (MSQ) polymers, either dense (SiOC) or containing 40% of porosity (porous SiOC). Amorphous hydrogenated silicon carbide (SiC) material, used as hard mask and/or etch stop layer, is also investigated. Etch is performed in an inductively coupled reactor using fluorocarbon gases, which have proven to be very successful in the etch of conventional SiO 2 . First, etching with hexafluoroethane (C 2 F 6 ) is performed. Although etch rates are high, etch selectivities with respect to SiC are weak. So, oxygen, argon, and hydrogen are added to C 2 F 6 with the aim of improving selectivities. The best selectivity is obtained for the C 2 F 6 /H 2 (10%-90%) mixture. To understand etch rate and selectivity variations, plasma analyses by optical emission spectroscopy are correlated to surface analysis using X-Ray Photoelectron Spectroscopy (XPS). In general, atomic fluorine concentration in the plasma explains the etch rate, while the presence of a fluorocarbon layer on the surface is well correlated to the selectivity. To ensure that the etch process does not affect materials properties, and particularly their dielectric constant, bulk analysis by Fourier Transformed Infra-Red spectroscopy and images by Scanning Electron Microscopy have also been carried out.

Monitoring of CF and CF2 Number Density by Optical Emission Intensity Corrected using Probe-Measured Plasma Parameters

The use of the CF and CF2 optical emission (OE) intensity corrected by the probe-measured electron density and temperature is proposed for the estimation of CF and CF2 relative number density in low-pressure, high-density fluorocarbon plasmas. The estimation of the CF and CF2 density by the corrected OE is mainly based on an assumption that the electron energy distribution function near the threshold energy of these OE is approximated by a Maxwellian using the probe-measured electron temperature. The comparison of the corrected OE intensity with CF and CF2 laser-induced fluorescence (LIF) intensity, which represents the CF and CF2 number density, confirms good proportionality between the LIF and the corrected OE intensity. Thus, the proposed procedure is expected to be useful for the monitoring of CF and CF2 radicals.

Correlation between Metastable and Ground-State Fluorine Atom Densities Measured by Laser-Induced Fluorescence and Vacuum Ultraviolet Absorption Spectroscopies

The correlation between F atom densities at the ground state and the metastable state was determined in high-density fluorocarbon plasmas by laser-induced fluorescence (LIF) and vacuum ultraviolet absorption spectroscopies. A proportional relationship has been determined between the metastable and ground-state F atom densities, which can be understood reasonably well by considering the production and loss processes of the metastable state. The metastable state is produced from the ground state by electron impact excitation. According to the results of lifetime measurements, the loss of the metastable state is dominated by electron collision in high-density plasmas (≥2×1012 cm-3). Since both the production and the loss of the metastable state are dominated by electron impact processes, the density of the metastable state is proportional to the ground-state F atom density. The present experimental results suggest the possibility of monitoring the spatial distribution of the ground-state F atom density using LIF detection of the metastable state.

Etching of xerogel in high-density fluorocarbon plasmas

The etching of various xerogel films has been studied in high-density fluorocarbon plasmas. The xerogel etch rate is in part enhanced by the porosity. In discharges resulting in low surface polymerization, such as CF4 or oxygen-rich fluorocarbon plasmas, an additional enhancement up to 60% is observed. When the polymerization of the discharge is increased, this additional enhancement disappears and the xerogel etch rate becomes more suppressed. The suppression is more pronounced for xerogel films with a higher porosity and a larger pore size. X-ray photoelectron spectroscopy analysis on partially etched samples shows that the suppression in etch rate is accompanied by an increasing amount of fluorocarbon material at the xerogel surface, especially in the pores of the xerogel structure. Finally, a 30% porous xerogel film was patterned using CHF3 as an etching gas. Slight bowing of the sidewalls was observed.

Vacuum beam studies of photoresist etching kinetics

One factor limiting the development of reliable models of high density, low pressure oxide etch plasmas is the relatively poor understanding of the plasma-photoresist surface interactions. In particular, the relatively high rates of photoresist (PR) loss experienced in high density fluorocarbon plasmas is a significant problem. It has long been accepted that fluorine plays a key role in controlling the oxide to PR etch rate selectivity. The addition of hydrogen has been shown to improve this selectivity, presumably by scavenging fluorine from the tool by forming HF. By reducing the fluorine to carbon ratio in the plasma and more specifically at the PR surface itself, the rate of polymer deposition increases causing the net PR etch rate to decrease. In this work, the complex surface chemistry of fluorocarbon plasmas is simplified to facilitate the study of the interaction of fluorine atoms and hydrogen atoms on the PR surface. This chemistry is modeled in vacuum beam experiments with argon ions and independent fluxes of neutral deuterium and fluorine atoms intersecting at the surface of photoresist samples. We present experimental evidence that the etch yield of photoresist (carbon atoms removed per incident argon ion) under these conditions is high compared to that of silicon and silicon dioxide. The presence of a simultaneous flux of deuterium atoms on the photoresist surface does not affect the etch yield despite the fact that DF is formed during the etching process.

Roughness and chemistry of silicon and polysilicon surfaces etched in high-density plasma: XPS, AFM and ellipsometry analysis

Surface chemistry and morphology of polysilicon thin films etched in a high-density fluorocarbon plasma under various conditions are studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry. XPS reveals the presence of a fluorocarbon layer, which composition and thickness depend on the plasma conditions. Ellipsometry measurements show the need to consider the superficial roughness. Surface roughness and morphology obtained by AFM are used to define geometric models suitable to represent the top layer when processing the ellipsometry data. Results are discussed and compared to that given by the Bruggeman effective medium approximation (BEMA). The BEMA model always agrees with the geometric model, which is the closest to the observed surface morphology. However, if a good agreement is obtained between surface roughness and top layer thickness for unetched or weakly damaged samples, a discrepancy is observed for the etched samples. Formation of a non-transparent fluorocarbon layer on these sample is put forward to explain this behaviour.

High density fluorocarbon plasma etching of new resists suitable for nano-imprint lithography

In this work, we studied etching of resists suitable for nano-imprint lithography. First, various resists have been tested in a SiO 2 process under low pressure and high plasma density conditions in order to get the best SiO 2/resist selectivity. Second, to understand resist etching mechanism and thus optimize the process, we focused our study on polymer etching behavior in different plasma conditions. Finally, transfer of imprinted features in SiO 2 has been successfully achieved.

Study on polymeric neutral species in high-density fluorocarbon plasmas

Production and extinction processes of polymeric neutral species (CmFn;m⩾2) in electron cyclotron resonance C4F8 and CF4 plasmas have been studied by using a quadrupole mass spectrometer (QMS) employing low-energy electron attachment technique. This technique allows the detection of electronegative CmFn species as negative ions by scanning the attaching electron energy in the QMS typically in the range of 0–10 eV. In addition to the most abundant F− and CF3− signals resulting from dissociative attachment to various fluorocarbon species, pronounced attachment resonances of negative ions corresponding to the series of CmF2m±1− such as C3F7−, C4F9−, and C5F9− were primarily observed especially at low microwave powers and high pressures. The C4F8 plasma contained a large amount of polymeric species and a high fraction of reactive F-stripped species as compared to the CF4 plasma, providing evidence of a high potential of gas phase and surface polymerization in a low F/C ratio plasma. The amount and composition of polymeric species were examined by varying gas residence time and diluted hydrogen or argon concentration. At 20 mTorr, the overall amount of polymeric species was suppressed by enhanced gas flow with decreasing residence time, while a fraction of F-stripped species was increased. The amount of polymeric species was also suppressed with increasing diluted hydrogen, and the different behavior in the two plasmas was interpreted as the result of interactions between H atoms and polymeric species. The results provide insights into the kinetics and chemical activity of polymeric species in a high-density plasma as a practical etching source.

Differences in radical generation due to chemical bonding of gas molecules in a high-density fluorocarbon plasma: Effects of the C=C bond in fluorocarbon gases

We investigated the differences in radical generation due to chemical bonding of fluorocarbon gas molecules in the plasma. We found that dissociation of the C=C bond is five times easier than that of the C–C bond in a fluorocarbon gas plasma. As a result, a C2F4 plasma could generate a higher density of CF2 radicals than a C4F8 plasma. Additionally, the same dissociation processes occurred in the C3F6 and C5F8 plasma, which both have the C=C bond and C–C bond in their molecules. In the C3F6 plasma, the density of generated CF2 radicals was 3.5 times higher than that for CF or CF3 radicals, whose radical densities were the same. The C5F8 gas plasma mainly produced CF2 and CF radicals, and the CF radical density was higher in comparison to other fluorocarbon gas plasmas.

Patterning of fluorine-, hydrogen-, and carbon-containing SiO2-like low dielectric constant materials in high-density fluorocarbon plasmas: Comparison with SiO2

Successful pattern transfer of 0.36–0.62 μm features into fluorinated silicon dioxide, hydrogen silsesquioxane (HSQ), and methyl silsesquioxane (MSQ) has been demonstrated in a transformer coupled plasma (TCP) source using fluorocarbon feedgas chemistries. These films have a lower dielectric constant than conventional SiO2. It is this property that makes them attractive for implementation in future integrated circuit technology. The etching of these novel dielectrics was compared to conventional SiO2. We have observed that the different chemical makeup of these SiO2-like dielectrics does not affect the etching when weakly polymerizing gases are used, such as CF4. In this case, the etch rate is primarily dependent on the ion energy. For more polymerizing chemistries, like CHF3 or C3F6/H2 gas mixtures, x-ray photoelectron spectroscopy analysis showed that an increasing steady state fluorocarbon film thickness limits the ion and neutral flux at the interface of the various dielectrics. It is suggested that, as the fluorocarbon film thickness increases, the etching becomes more dependent on neutral species from the gas phase. In this case, hydrogen and carbon impurities in HSQ and MSQ, respectively, limit the etch rate. On the other hand, fluorine in the fluorinated SiO2 film enhances the etch rate as compared with the etch rate of conventional SiO2. In line with these observations, we conclude that fluorine from the gas phase is most likely the controlling etchant as the fluorocarbon film increases beyond the ion penetration depth.

Selective SiO2-to-Si3N4 etching in inductively coupled fluorocarbon plasmas: Angular dependence of SiO2 and Si3N4 etching rates

In the fabrication of microstructures in SiO2, etch selectivity of SiO2 to masking, etch stop, and underlayer materials need to be maintained at corners and inclined surfaces. The angular dependence of the SiO2-to-Si3N4 etch selectivity mechanism in a high density fluorocarbon plasma has been studied using V-groove structures. The SiO2 etch rate on 54.7° inclined surfaces is lower than on flat surfaces, while the SiO2 etch yield (atoms/ion) is a factor of 1.33 higher. The results are consistent with a chemical sputtering mechanism. The Si3N4 etch yield is greater by a factor of 2.8 for 54.7° inclined surfaces than for flat surfaces. This large enhancement is explained by a fluorocarbon surface passivation mechanism that controls Si3N4 etching. The fluorocarbon deposition is decreased at 54.7° whereas the fluorocarbon etching rate is increased at 54.7°. This produces a thinner steady-state fluorocarbon film on the inclined Si3N4 surface, and results in a large enhancement of the Si3N4 etch yield.

Surface productions of CF and CF2 radicals in high-density fluorocarbon plasmas

Spatial distributions of CF and CF2 radical densities in high-density fluorocarbon plasmas were measured by laser-induced fluorescence spectroscopy. In both pulsed and continuous-wave (cw) C4F8 discharges, the radical densities were lower in the center of the discharge and higher near the walls. Namely, hollow-shape profiles of the radical densities were maintained in the C4F8 discharges. This indicates the presence of surface productions of the radicals on the chamber wall. The rf power dependences of the radical fluxes from the wall, which were estimated from the density gradients, showed similar trends to the gas-phase radical densities. This result revealed that the surface productions predominantly determine the gas-phase CF and CF2 radical densities in high-density C4F8 plasmas. In contrast to C4F8, almost uniform profiles of the radical densities were always observed in cw CF4 discharges, while hollow profiles were observed in pulsed CF4 discharges. The CF2 flux from the wall in the pulsed CF4 discharge was one or two orders smaller than that in the C4F8 discharge, and the rf power dependence of the CF2 flux showed a dissimilar trend to the gas phase CF2 density. The large difference in the radical flux from the wall observed in the C4F8 and CF4 discharges suggests that heavy neutral species (CxFy, x⩾2) in the C4F8 plasma make great contributions to the film deposition on the wall where the deposited films enhance the surface productions of CF and CF2 radicals.

Influence of reactor wall conditions on etch processes in inductively coupled fluorocarbon plasmas

The influence of reactor wall conditions on the characteristics of high density fluorocarbon plasma etch processes has been studied. Results obtained during the etching of oxide, nitride, and silicon in an inductively coupled plasma source fed with various feedgases, such as CHF3, C3F6, and C3F6/H2, indicate that the reactor wall temperature is an important parameter in the etch process. Adequate temperature control can increase oxide etch selectivity over nitride and silicon. The loss of fluorocarbon species from the plasma to the walls is reduced as the wall temperature increased. The fluorocarbon deposition on a cooled substrate surface increases concomitantly, resulting in a more efficient suppression of silicon and nitride etch rates, whereas oxide etch rates remain nearly constant.

Asymmetric microtrenching during inductively coupled plasma oxide etching in the presence of a weak magnetic field

When fabricating microscopic features in SiO2 layers using low pressure, high-density fluorocarbon plasmas, microtrenching has commonly been observed. Microtrenching has been explained either as due to ion scattering from sloped sidewalls or negative charging of the sidewalls by electrons, and the influence of the associated electric field on ion trajectories. In this work, we show that a weak magnetic field produces a significant asymmetry in microtrenching. Our results demonstrate unambiguously that electron-based sidewall charging is to a significant extent responsible for microtrenching, and, more generally, that differential charging is an important effect in microstructure fabrication using high-density plasmas.

Measurements of electron density in helicon-wave excited high-density reactive plasmas by vacuum ultraviolet emission spectroscopy

This paper reports vacuum ultraviolet (VUV) emission spectroscopy for non-contact measurements of the electron density in reactive plasmas. The advantages of the VUV emission spectroscopy compared with the visible one are (1) the applicability to high-density plasmas close to and (2) the insensitivity against the variation of the velocity distribution function of electrons. In this paper, the VUV emission spectroscopy is adapted to helicon-wave excited high-density fluorocarbon plasmas. As a result, a reasonable agreement has been obtained between the result of the present method and the electron density measured by a microwave interferometer. The accuracy of the present method is discussed in detail considering several sources of error.

SiO2 to Si selectivity mechanisms in high density fluorocarbon plasma etching

This study examines the influence of plasma chemistry on SiO2 to Si etch selectivity in high density C2H2F4(1,1,1,2-tetrafluoroethane)/O 2 and CHF3/H2 discharges. Etch rate measurements of Si and SiO2 have been combined with chemical characterizations of the discharge using optical diagnostics and an in-line quadrupole mass spectrometer. The gas phase concentrations of CF2 and F as well as the mass spectrum of ions incident at the substrate have been measured for conditions producing SiO2/Si selectivities from 1 to over 30. For low density sources with high neutral to ion fluxes, the conventional theory is that selectivity is governed by the relative content of fluorine versus carbon in the fluorine and fluorocarbon radicals incident on the substrate. However, for the high density plasma discharges studied, the conventional theory may apply only when the contributions of both neutral radicals and fluorocarbon ions, CF+x, are considered. For the case of oxygen addition to C2H2F4 discharges, reactions between the fluorocarbon surface film and oxygen in the gas phase may also be important in controlling selectivity.