Fluorocarbon-based Plasma Research Articles

Etching characteristics of low-k SiCOH thin films under fluorocarbon-based plasmas

Low dielectric constant (low-k) SiCOH thin films were prepared on silicon (Si) wafers using plasma enhanced chemical vapor deposition (PECVD) of the precursor tetrakis(trimethylsilyloxy)silane (TTMSS). The deposition RF plasma power was 20 and 100 W. The k-values were 2.14 and 3.2 for the films deposited at 20 and 100 W, respectively. The deposited films then underwent an inductively coupled plasma-reactive ion etching (ICP-RIE) using fluorocarbon-based etching gas of CF4, CF4+O2, and CF4+Ar. After etching, the etch rate and refractive index for the films deposited at 100 W were higher than those at 20 W. The surface roughness decreased for the films at 20 W but increased for those at 100 W after etching. Contact angles decreased indicating a hydrophilic surface after etching. Prominent absorption bands from the Fourier transform infrared (FTIR) spectra were observed as C–Hx stretching, Si–CH3 bending, Si–O–Si stretching, and Si–(CH3)x bending vibration modes. X-ray photoelectron spectroscopy (XPS) showed a significant fluorine concentration on the film surface. From high resolution scans for C1s and F1s in the XPS, dominant peaks were observed as C–H/C–C and C–F3 after etching, respectively. After etching, the k-values and leakage current densities increased from those of as-deposited films.

Multiscale modeling of plasma–surface interaction—General picture and a case study of Si and SiO2 etching by fluorocarbon-based plasmas

The physics and chemistry of plasma–surface interaction is a broad domain relevant to various applications and several natural processes, including plasma etching for microelectronics fabrication, plasma deposition, surface functionalization, nanomaterial synthesis, fusion reactors, and some astrophysical and meteorological phenomena. Due to their complex nature, each of these processes is generally investigated in separate subdomains, which are considered to have their own theoretical, modeling, and experimental challenges. In this review, however, we want to emphasize the overarching nature of plasma–surface interaction physics and chemistry, by focusing on the general strategy for its computational simulation. In the first half of the review, we provide a menu card with standard and less standardized computational methods to be used for the multiscale modeling of the underlying processes. In the second half, we illustrate the benefits and potential of the multiscale modeling strategy with a case study of Si and SiO2 etching by fluorocarbon plasmas and identify the gaps in knowledge still present on this intensely investigated plasma–material combination, both on a qualitative and quantitative level. Remarkably, the dominant etching mechanisms remain the least understood. The resulting new insights are of general relevance, for all plasmas and materials, including their various applications. We therefore hope to motivate computational and experimental scientists and engineers to collaborate more intensely on filling the existing gaps in knowledge. In this way, we expect that research will overcome a bottleneck stage in the development and optimization of multiscale models, and thus the fundamental understanding of plasma–surface interaction.

On the relationship between SiF4 plasma species and sample properties in ultra low-k etching processes

The temporal behavior of the molecular etching product SiF4 in fluorocarbon-based plasmas used for the dry etching of ultra low-k (ULK) materials has been brought into connection with the polymer deposition on the surface during plasma treatment within the scope of this work. For this purpose, time-resolved measurements of the density of SiF4 have been performed by quantum cascade laser absorption spectroscopy. A quantification of the non-linear time dependence was achieved by its characterization via a time constant of the decreasing SiF4 density over the process time. The time constant predicts how fast the stationary SiF4 density is reached. The higher the time constant is, the thicker the polymer film on top of the treated ultra low-k surface. A correlation between the time constant and the ULK damage was also found. ULK damage and polymer deposition were proven by Variable Angle Spectroscopic Ellipsometry and X-ray Photoelectron Spectroscopy. In summary, the observed decay of the etching product concentration over process time is caused by the suppressed desorption of the SiF4 molecules due to a more dominant adsorption of polymers.

Electron behaviors in afterglow of synchronized dc-imposed pulsed fluorocarbon-based plasmas

Electron behaviors in a pulsed dual radio frequency (RF) capacitively coupled plasma of a mixture of C4F8, O2, and Ar gases, where the DC bias of −300 V in the RF-on period was imposed and synchronously increased to −1000 V in the RF-off period, were investigated. The synchronous DC bias prolongs the electron density (ne) decay and provides emission of Ar at a wavelength of 750.38 nm in early afterglow at 3 µs during the RF-off period of 10 kHz pulse modulation. The rapid ne decay occurred with the electron attachments to the electronegative fluorocarbons, and thus the plasma consisting of positive and negative ions was generated. The DC bias voltage seems to be applied between the electrodes and the positive ions accelerated to the top electrode, and enhanced the secondary electron generation at the top electrode surface in the RF-off period with the ion bombardments, concomitantly with the synchronous emissions.

Rapid electron density decay observed by surface-wave probe in afterglow of pulsed fluorocarbon-based plasma

To elucidate the pulsed fluorocarbon plasma behavior, a surface-wave probe with high time resolution was used to measure the electron density ne in the afterglow of plasma. In a dual-frequency capacitively coupled plasma of fluorocarbon chemistry, e.g., an O2-based C4F6 and Ar mixture, ne vanished rapidly in a short time (∼5 µs), whilst the dc current flowing onto the top electrode biased at −300 V decreased very slowly (decay time ∼70 µs). This observation is clear evidence of ion–ion plasma formation by electron attachment in the afterglow. We point out that the electron attachment rates for fluorocarbon radicals significantly affect the electrons and ion–ion plasma behaviors observed at the afterglow phase.

Controlling Asymmetric Photoresist Feature Dimensions during Plasma-Assisted Shrink

Plasma-assisted shrink (PAS) of features, involving plasma deposition onto sidewalls to shrink features has been described in the past for symmetric features. In this work we explore shrinkage of asymmetric features using fluorocarbon-based plasma deposition. Using top-down and cross-section scanning electron microscopy, we find the dependencies of this shrink process on top-down blanket deposition thickness, pressure, source power, and deposition plasma chemistry with the aim of achieving the best 1:1 length to width shrink (ΔL:ΔW) of asymmetric features.

Impact of low-k structure and porosity on etch processes

The fabrication of interconnects in integrated circuits requires the use of porous low dielectric constant materials that are unfortunately very sensitive to plasma processes. In this paper, the authors investigate the etch mechanism in fluorocarbon-based plasmas of oxycarbosilane (OCS) copolymer films with varying porosity and dielectric constants. They show that the etch behavior does not depend on the material structure that is disrupted by the ion bombardment during the etch process. The smaller pore size and increased carbon content of the OCS copolymer films minimize plasma-induced damage and prevent the etch stop phenomenon. These superior mechanical properties make OCS copolymer films promising candidates for replacing current low-k dielectric materials in future generation devices.

Durability of nitrided fluorocarbon polymer films for nanoimprint lithography

Hydrophobic fluorocarbon polymer films have been used as anti-sticking and releasing coatings for templates in nano-imprint lithography, but have poor durability against repeated high pressure pressing processes. The addition of nitrogen into fluorocarbon-based gas plasma can provide nitrided fluorocarbon polymer films, as confirmed by Fourier-transform infrared spectroscopy. Fluorocarbon and nitrided fluorocarbon films have almost the same 110.0° contact angle with water drops. The nitrided fluorocarbon films are harder and significantly enhance durability as anti-sticking layers. Durability enhancement has been confirmed by fabricating replicas with silicon templates using nitrided fluorocarbon anti-sticking layers.

Residue growth on metallic-hard mask after dielectric etching in fluorocarbon-based plasmas. I. Mechanisms

This work focuses on the formation of residues that grow on a metallic-hard mask after etching of porous low-k materials in fluorocarbon-based plasmas. The residue growth, which is dependent on the air exposure time after etching, causes line and via opens that strongly impact the yield performance. The different elements which could play a role in the chemical reactions have been clarified. The authors have demonstrated that in their experimental conditions, after fluorocarbon etching and air exposure, the oxidized titanium nitride reacts with fluorhydric acid to form metallic salts. This is a reaction between fluorine from the reactive layer formed on titanium nitride and hydrogen coming from the atmosphere. This reaction is all the more fast because the titanium nitride is oxidized.

Patterning of porous SiOCH using an organic mask: Comparison with a metallic masking strategy

The etching of sub-100-nm porous dielectric trenches has been investigated using an organic mask. The etching process that is performed in an oxide etcher is composed of three steps: a thin dielectric antireflective coating (DARC) layer (silicon containing layer) is etched in the first step, the organic mask [carbon-based layer (CL)] is opened in the second step, and the dielectric layer is etched in the last step. The DARC layer is open in a fluorocarbon-based plasma (CF4∕Ar∕CH2F2) and the main critical dimension issue is the critical dimension control of the trench, which can be adjusted by controlling the amount of polymer generated by the etching chemistry (% of CH2F2). The CL is etched using NH3 based plasmas, leading to straight trench profiles. For dielectric patterning, the etch process results from a delicate trade-off between passivation layer thickness and mask faceting. This is driven by the polymerizing rate of the plasma (% of CH2F2) which controls the trench width. Using an optimized etching process (CF4∕Ar∕2%CH2F2), p-SiOCH trenches can be patterned with straight etch profiles down to 75nm trench width. In this article, the authors have also compared the organic mask and TiN metal hard mask strategies in terms of patterning performances (profile control, porous SiOCH modification, and reactor wall cleaning processes).

Plasma fluorination of diamond-like carbon surfaces: mechanism and application tonanoimprint lithography

Diamond-like carbon (DLC) films, used as molds for nanoimprint lithography, weretreated with a fluorocarbon-based plasma in order to enhance their anti-adhesionproperties. While ellipsometry and atomic force microscope measurementsshowed negligible changes in thickness and surface roughness after plasmaprocessing, contact angle measurement found fluorine plasma-treated DLCsurfaces to be highly hydrophobic, with surface energy values reduced from∼45 mJ m−2 for untreatedfilms to ∼20–30 mJ m−2 after fluorination. X-ray photoelectron spectroscopy revealed a thin (from∼0.5 to∼3 nm) fluorocarbon layer on the DLC surface. Proposed mechanisms for the formation ofthis layer include two competing processes: etching of DLC and deposition offluorocarbon material, with one or the other mechanism dominant, depending on theplasma conditions. Fluorocarbon plasma-treated DLC molds for nanoimprintlithography were used to pattern sub-20 nm size features with a high degree ofrepeatability, demonstrating an extended lifetime of the anti-adhesion coating.

Energy distribution of bombarding ions in plasma etching of dielectrics

In plasma etching for microelectronics fabrication, a sheath electric field accelerates ions into the substrate so they strike at normal incidence, contributing energy and reactive species to enable anisotropic etch profiles. The energy of the bombarding ions is typically controlled coarsely with a sinusoidal bias voltage applied to the substrate electrode, producing a broad bimodal ion energy distribution (IED) at the substrate. By systematically tailoring the shape of the waveform, the energies and relative fluxes of the two IED peaks are varied independently over a 100–500 eV range in a fluorocarbon-based helicon plasma, while silicon dioxide and photoresist etch rates are monitored. A 100 eV IED peak was combined with a higher energy peak, with the energy and relative flux of the high energy group as variables in the experiment. A relatively small contribution of high energy ions is found to lead to considerable etch rate enhancement, higher than predicted by a linear combination of single-peak etch rates at the two energies. Because high energy ion bombardment is known to suppress fluorocarbon deposition, it seems likely that changes in surface chemical composition in their presence enhances the contribution of lower energy ions to etching reactions.

Fluorinated diamondlike carbon templates for high resolution nanoimprint lithography

Nanoimprint templates were fabricated from diamondlike carbon (DLC) films grown on Si, using negative-tone e-beam lithography and oxygen plasma etching. An antiadhesion coating was provided through fluorocarbon-based plasma treatment, which was found to form a Teflon-like thin layer on the treated DLC surface. The fluorinated templates were used to imprint arrays of dots with diameters down to 10nm in polymethyl-methacrylate. Application of the fluorocarbon plasma treatment was also demonstrated in the antiadhesion treatment of the nanoimprint resist and in elastomer molding.

Plasma fluorination of carbon-based materials for imprint and molding lithographic applications

Diamondlike carbon nanoimprint templates are modified by exposure to a fluorocarbon-based plasma, yielding an ultrathin layer of a fluorocarbon material on the surface which has a very low surface energy with excellent antiwear properties. We demonstrate the use of these plasma fluorinated templates to pattern features with dimensions approximately 20 nm and below. Furthermore, we show that this process is extendable to other carbon-based materials. Plasma fluorination can be applied directly to nanoimprint resists as well as to molds used to form elastomer stamps for microcontact printing and other applications requiring easy mold release.

Ion energy control at substrates during plasma etching of patterned structures

In fluorocarbon-based plasma etching of dielectrics, deposition of fluorocarbon on the substrate contributes to a complex surface chemical structure that strongly affects etch rate and etch selectivity. Results reported herein demonstrate that the energy distribution of bombarding ions (IED) has a significant effect on this polymer layer, subsequently affecting etch rate and selectivity in submicron patterned structures. Specifically, we have narrowed the IED while keeping other process conditions unchanged by tailoring the shape of the rf voltage wave form used for substrate bias. Significant improvements in etch selectivity for organosilicate glass (OSG) over silicon carbide in a C4F8∕N2∕Ar plasma have been obtained by using a narrow IED compared to the broad IED resulting from the typical sinusoidal bias wave form. Trenches etched in OSG with the tailored bias voltage wave form show good feature profiles and high selectivity at feature bottoms. Slight differences in feature profiles between tailored and sinusoidal wave forms, as well as variations in etch selectivity with feature depth, are consistent with an enhancement in polymerization at the substrate in the case of the tailored bias voltage wave form.

Study of the plasma and cleaning impact on a CoWPB material

Self-aligned barriers (SAB) are investigated for the 45 nm technology node and beyond to improve copper electromigration and stress-migration resistance. The impact of plasma and cleaning processes used in the integration were studied on CoWPB-SAB. Results extracted from complementary characterizations show that after fluorocarbon-based and/or oxidizing plasmas, CoWPB is slightly etched and a less dense zone is formed at the CoWPB surface. This zone is not protective during cleaning: CoWPB is alterated by diluted HF solution leading to pits at the surface and crystals for the samples treated by fluorocarbon-based plasma. Pitting corrosion can be caused by the dissolution of one of the two phases existing in the CoWPB: amorphous and crystalline. The solubility of these phases depends on their thermodynamic equilibrium in solution and their reaction kinetics with HF.

Investigation of varying C4F8/O2 gas ratios on the plasma etching of carbon doped ultra-low-k dielectric layers

This study investigates the fluorocarbon-based plasma etching for ultra-low-k carbon doped oxide (CDO) dielectric layers which have a k value of 2.4; the CDO film is made up of SiOCH. The thickness of fluorocarbon layers deposited on the surface of the underlying CDO was controlled by independently varying the flow of C4F8 and O2 feed gases into the chamber. X-ray photoelectron spectroscopy and spectroscopic ellipsometry have been used to analyse the post-etched low-k CDO films. These studies have identified the conditions that lead to the formation of the thinnest fluorocarbon layers compatible with efficient etching, but that do not result in carbon depletion from the CDO layer. The results indicate that once the plasma gas mixture results in the presence of an extremely thin fluorocarbon layer, the carbon depletion is significantly suppressed. The thickness of the fluorocarbon film can be controlled by the C4F8/O2 gas ratio and the presence of this film has a large impact on the etch rate.

Influence of plasma parameters on the chemical composition of steady-state fluorocarbon films deposited on carbon-doped low-k dielectric layers during etching

This study investigates the fluorocarbon-based plasma etching (FBPE) of low dielectric constant (ULK) carbon-doped oxide (CDO) films, which have a dielectric constant (k) value of 2.4. The effects of different ion density and ion energy power settings on the chemical composition of the fluorocarbon layer deposited during the etch process were investigated. X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical composition of the post-etched low-k CDO films while spectroscopic ellipsometry (SE) was used to determine the overall film thickness. XPS spectra of the C1s core levels reveal that the relative concentration of CFx species in the fluorocarbon films reduced as ion density source power and ion energy power levels were increased, and this can be correlated with a higher etch rate and thinner fluorocarbon layer. Plasma conditions which led to the deposition of a thick fluorocarbon film significantly inhibited the etch rate. This work demonstrates that the chemical composition and the thickness of the fluorocarbon film can be controlled by the plasma power parameters, and this has implications for the etching of ULK CDO layers.

Donor passivation in pseudomorphic-high electron mobility transistors due to plasma-incorporated fluorine impurities observed using x-ray photoemission spectroscopy

To investigate the effect of plasma-incorporated fluorine on Si donors in pseudomorphic-high electron mobility transistors (P-HEMTs), we used x-ray photoemission spectroscopy to analyze three layers near the Si δ-doped layer and the Si δ-doped layer itself, in which we previously found fluorine accumulation after post-thermal annealing following fluorocarbon-based plasma exposure. For this evaluation, we developed controllable and low-speed AlGaAs wet-chemical etching using citric-acid-based wet etchant. We used it to expose one of the layers to be analyzed: one 7.5 nm above the Si δ-doped layer, one 1.5 nm above it, the δ-doped layer itself, and one 1.5 nm below it. We found that the accumulated fluorine localized in the δ-doped layer and reacted with Si donors. This is apparently the main reason for the carrier passivation in the fluorocarbon-based plasma-exposed P-HEMTs.

Nanoscale layer etching by short-time exposure of substrates to gas discharges using moving patterned shutter

We use a moving shutter containing slits in close contact with a substrate to control the interaction time of gas discharges with the substrate for nanoscale layer/nanostructure processing. Both shutter and substrate are located on the rf-powered electrode and can be biased relative to the plasma. Once the plasma properties and electrode bias voltage are fully established, the shutter with slits begins to move at a controlled speed across the substrate to expose the substrate for the desired interaction time to the plasma. We show that for a fluorocarbon-based plasma etching process, steady-state etching conditions can be achieved within less than 2s. This approach enables precise removal or deposition of nanoscale layers employing gas discharges.