Fluorocarbon Plasma Etching Research Articles

Effect of nonsinusoidal bias waveforms on ion energy distributions and fluorocarbon plasma etch selectivity

The distributions of ion energies incident on the wafer significantly influence feature profiles and selectivity during plasma etching. Control of ion energies is typically obtained by varying the amplitude or frequency of a radio frequency sinusoidal bias voltage applied to the substrate. The resulting ion energy distribution (IED), though, is generally broad. Controlling the width and shape of the IED can potentially improve etch selectivity by distinguishing between threshold energies of surface processes. In this article, control of the IED was computationally investigated by applying a tailored, nonsinusoidal bias waveform to the substrate of an inductively coupled plasma. The waveform we investigated, a quasi-dc negative bias having a short positive pulse each cycle, produced a narrow IED whose width was controllable based on the length of the positive spike and frequency. We found that the selectivity between etching Si and SiO2 in fluorocarbon plasmas could be controlled by adjusting the width and energy of the IED. Control of the energy of a narrow IED enables etching recipes that transition between speed and selectivity without change of gas mixture.

In vacuo measurements of dangling bonds created during Ar-diluted fluorocarbon plasma etching of silicon dioxide films

Dangling bond creation processes during fluorocarbon plasma etching of silicon dioxide (SiO2) films were studied using an in vacuo electron spin resonance technique. In a range of about 10nm underneath the interface of the SiO2 films with an amorphous fluorinated carbon film that was top-covered, a Si dangling bond in the films (E′ center, g value 2.0003) was located. Density of the E′ center was sustained during etching processes created by the illumination of vacuum ultraviolet emissions, higher photon energy than the bandgap of SiO2. The etching mechanism in this system is discussed taking into account the experimental results.

Measurements of desorbed products by plasma beam irradiation on SiO2

A plasma beam irradiation apparatus with a controllable plasma source was constructed to investigate the plasma-surface interactions for reactive etching processes. The apparatus can control the plasma parameters, such as ion energy, radical∕ion composition, and the ratio of neutral to ion flux. The desorbed products of SiFx (x=1–3) were observed separately from COFx (x=1–3) by using a C13F4–Ar gas mixture plasma beam. The major desorbed product was not SiF4. Unsaturated fluorinated silicons [SiFx(x=1–3)] were major desorbed products under ion-rich fluorocarbon plasma etching condition. It was found that the composition of silicon containing desorbed products does not depend on the incident ion energy in the ion energy range of 300–700eV.

Integrated feature scale modeling of plasma processing of porous and solid SiO2. I. Fluorocarbon etching

Increases in RC delay times in interconnect wiring for microelectronics as feature sizes decrease have motivated investigations into the use of low-dielectric constant insulators, and in particular, porous silicon-dioxide (PS). Profile evolution and maintenance of critical dimensions during plasma etching of PS are problematic due to the exposure of open pores. To investigate these issues, reaction mechanisms for fluorocarbon plasma etching of SiO2 in C2F6, CHF3, and C4F8 chemistries have been developed and incorporated into the Monte Carlo Feature Profile Model which was modified to address these two-phase systems. The reaction mechanism was validated by comparison to experiments by others for etching of PS and solid SiO2 (SS). We found that the etch rates for PS are generally higher than that of SS due to the inherently lower mass fraction. Mass corrected etch rates of PS can be larger or smaller than those for SS depending on the degree of pore filling by polymer and the degree of ion activated chemical sputtering. Pore filling is particularly important for PS having open networks with large pores and high porosities. We found little dependence of the taper of high aspect ratio profiles on the average pore radius and porosity. However, the profile changes from tapered to bowed as the interconnectivity of the porous network increases. Scaling laws for profile shapes are otherwise similar for both SS and PS.

Effect of thin-film imaging on line edge roughness transfer to underlayers during etch processes

For the patterning of sub-100 nm features, a clear understanding of the origin and control of line edge roughness (LER) is extremely desirable, from a fundamental as well as a manufacturing perspective. With the migration to thin photoresists coupled with bottom antireflective coating (ARC)-hardmask underlayers, LER analysis of the developed resist structures is perhaps an inaccurate representation of the substrate roughness after the etch process, since those underlayers can play a significant role in increasing/decreasing linewidth variations during the image transfer process and hence can impact the device performance. In this article, atomic force microscopy is used to investigate the contribution of the imaging resist sidewall topography to the sidewall roughness of the final etched feature in thin photoresists, ARC, and hardmasks. Resist systems suitable for 248 and 193 nm lithography as well as fluorine-containing resists were processed using N2-H2 or fluorocarbon plasma etch. It is shown that the interaction of different etch chemistries with existing sidewall profiles can result in loss of the original morphological information and creation of new spatial frequency domains that act as physical templates for subsequent image transfer processes. Excessive roughness transfer into the hardmask layer due to insufficient resist thickness or inadequate etch resistance originates from striation propagation from the resist layer into the hardmask layer. In the case of fluorine-containing materials, a decreased etch resistance and reduced initial film thickness values give rise to critical underlayer roughening during plasma etch. Based on the results shown, it is predicted that advanced resist systems for 157 nm lithography and beyond will require the use of ARC layers with built-in hardmask properties in those particular cases in which patterning of deep trenches is needed, in order to maintain LER values within acceptable levels.

Ion bombardment energy control for selective fluorocarbon plasma etching of organosilicate glass

The role of ion bombardment in plasma etch selectivity of organosilicate glass to etch stop layers of silicon carbide and silicon nitride has been investigated in a C4F8/N2/Ar plasma using a method that produces a narrow ion energy distribution (IED) at the substrate surface. The effects of the narrow IED are compared with those of the broad, bimodal IED produced by the conventional sinusoidal bias voltage wave form (at 13.56 MHz). A comparison of etch rate versus average ion bombardment energy shows a higher ion energy threshold for etching, a larger gap between the thresholds for the two materials, and high selectivity over a wider range of bias voltage with the narrow IED. A physical explanation of the observed phenomena is proposed.

Patterning of Nanoporous Anodic Aluminum Oxide Arrays by Using Sol−Gel Processing, Photolithography, and Plasma Etching

Patterned anodic aluminum oxide (AAO) arrays were fabricated by using sol−gel processing, photolithography, plasma etching, and two-step anodization. The fabrication process included the following general steps. First, a layer of aluminum was evaporated onto silicon wafers. Second, a silica layer was deposited by spin-coating a sol. Next, a patterned layer of a photoresist was deposited by standard photolithographic procedures. Subsequently, the pattern of photoresist was transferred to the underlying layer of silica by fluorocarbon plasma etching. Finally, AAO arrays were formed by the two-step anodization of exposed aluminum (e.g., not covered by silica). Deposition and etching of the silica layer were monitored by spectroscopic ellipsometry. Electrical measurements were performed to investigate the ability of the silica layer to act as a barrier that prevents anodization of aluminum. The structure of patterned AAO arrays was studied by scanning electron microscopy. A clear boundary was observed between...

Submillimeter-wavelength plasma chemical diagnostics for semiconductor manufacturing

Submillimeter-wavelength linear-absorption spectroscopy has been applied to the chemical diagnostics of reactive-ion etching plasmas in a modified capacitively coupled gaseous electronics conference reactor. Approximately 1 mW of narrow-band (<10 kHz) submillimeter radiation between 450 and 750 GHz is produced using a backward-wave oscillator (BWO). The BWO is frequency stabilized to a harmonic of a 78–118 GHz frequency synthesizer. The submillimeter method offers high sensitivity for the ≈1 MHz linewidth, Doppler-broadened absorption lines typical of gas-phase molecules at a total pressure of less than 133 Pa (1 Torr). A large number of molecules can be detected, limited primarily by the need for a permanent electric dipole moment and for accurate line frequency predictions, the latter of which are often available in the literature. The capabilities of the diagnostic method have been demonstrated by the following three applications: (1) the measurement of water-vapor contamination in the reactor and in the precursor gas by monitoring a rotational transition of H2O in the reactor just prior to the initiation of the plasma; (2) the assessment of progress in the cleaning of the reactor by an O2/Ar plasma after a fluorocarbon plasma etch by monitoring the build up of the concentration of O3 and the depletion of the concentration of CF2O in the plasma; and (3) the determination of the endpoint in the etching of a SiO2 thin film on silicon by an octafluorocyclobutane/O2/Ar plasma by monitoring the decrease in the concentration of SiO in the plasma. The last observation is made possible by the large electric dipole moment for SiO of 1×10−29 C m (3.1 D), which gives a low minimum detectable number density for the radical of 2×107 cm−3 for an optical pathlength of 39 cm.

Fluorocarbon plasma etching and profile evolution of porous low-dielectric-constant silica

To achieve shorter RC-delay times in integrated circuits low-dielectric-constant (low-k) materials are being investigated for interconnect wiring. Porous silicon dioxide (PS) is one such material. To address scaling issues during fluorocarbon plasma etching of PS, a feature profile model has been integrated with a plasma equipment model. To focus on issues related to the morphology of porous materials, the PS was treated as stoichiometric SiO2. The model was validated by comparison to experiments for PS etching in CHF3 plasmas sustained in an inductively coupled reactor. We found that etch rates (ER) for PS are generally higher than for SiO2 due to the inherent smaller mass density, although ER do not necessarily scale linearly with pore size or porosity. Mass-corrected ER can be either larger or smaller than that of solid SiO2. For example, in polymerizing environments, at high porosities and large pore radii, there is a reduction in ER due to pore filling with polymer. Profile scaling parameters, such as for tapering, observed for solid SiO2, are generally applicable to PS.

Integrated equipment-feature modeling investigation of fluorocarbon plasma etching of SiO2 and photoresist

As the microelectronics industry continues to shrink feature size and increase feature density in the back-end of integrated circuits, the traditional empirical approach to plasma etch process development is becoming prohibitively expensive and time consuming. Fundamental physics based models can prove useful in driving down process development time and cost. In this article, an integrated equipment-feature scale modeling infrastructure for SiO2 and photoresist (PR) etching in fluorocarbon based plasma discharges is described. The model correlates process conditions with plasma properties, surface interactions, and etch results. A validated plasma chemistry for Ar/c–C4F8/CF4 and detailed plasma–surface reaction mechanisms for SiO2/PR etching have been incorporated in the model. Major surface reactions for SiO2 etching include neutral surface passivation, fluorocarbon radical polymerization, and ion assisted etching of volatile products. The mechanism for PR erosion includes energy/angle dependent ion sputtering, ion activation, F atom etching with ion assistance, and fluorocarbon radical deposition. Computed SiO2 and PR etch profiles and rates have been validated by comparing with experimental results in a commercial inductively coupled plasma (ICP) etch tool. The validated model is used for a detailed investigation of SiO2/PR etching in a representative 300 mm wafer ICP tool. It is found that SiO2 etch rate is a nonlinear function of Ar/c–C4F8 ratio, where the highest etch rate is obtained when sufficient neutral passivation takes place while polymer deposition is still small. Deviating from this condition reduces SiO2 etch rate by either excessive polymerization or insufficient passivation. PR etch rate and facet size, however, increase monotonically with Ar/c–C4F8 ratio due to reduced polymer deposition. The effect of CF4 ratio in the Ar/c–C4F8/CF4 source gas on SiO2 etching depends on the Ar fraction. When Ar fraction is large, replacing c-C4F8 with CF4 reduces surface passivation and thereby decreases SiO2 etch rate. However, at small Ar fractions, CF4 addition reduces polymer formation and increases the SiO2 etch rate. For the range of conditions explored, SiO2 etch characteristics are insensitive to bias frequency as the ion energies are well above the threshold energy for etching. The plasma zone height (PZH) impacts the fluxes of etchants to the wafer and consequently the SiO2/PR etch rates. PZH, however, does not influence etch uniformity noticeably as diffusion is dominant at low gas pressures.

Design, development, and testing of real-time feedback controllers for semiconductor etching processes using in situ spectroscopic ellipsometry sensing

Real-time feedback controllers for two semiconductor etching processes are developed. Both controllers rely upon in situ spectroscopic ellipsometry measurements of sample thickness for their feedback variables. Spectroscopic ellipsometry (SE) is a commonly used nondestructive, noninvasive in situ sensor for dry etching. The first etching process we consider is the thermal chlorine etching of gallium arsenide. An empirical/first principles physics-based model for the etching process is developed. A linear-quadratic controller based on the model is designed and tested. The second etching process is the electron cyclotron resonance freon-14/oxygen (CF/sub 4/O/sub 2/) plasma etching of silicon nitride thin films. An adaptive etch rate controller for the fluorocarbon plasma etching process is designed, implemented, and tested.

Ion bombardment energy and SiO2/Si fluorocarbon plasma etch selectivity

The role of ion bombardment in the plasma etch selectivity of silicon dioxide to silicon has been investigated in a CHF3/H2 plasma using a method that produces a narrow ion energy distribution (IED) at the substrate surface. The effects of the narrow IED is compared with the broad, bimodal IED produced by the conventional sinusoidal bias voltage wave form (at 5 and 20 MHz). A comparison of the etch rate versus the average ion bombardment energy shows a higher ion energy threshold for etching, a larger gap between the thresholds for the two materials, and a high selectivity over a wider range of bias voltage with the narrow IED. A physical explanation of the observed phenomena is proposed. Additionally, reported models of the reactive sputter etch rate as a function of ion energy are used to show that the etch rate in this regime is higher for a narrow IED than for a broad one.

Simulation of fluorocarbon plasma etching of SiO 2 structures

A surface model for open area etching of SiO 2 is coupled with a model to calculate the local values of etching rate on each elementary surface of the structure being etched. The surface model includes the surface chemistry for ion-enhanced etching or deposition. The local etching model (essentially a local flux calculation model) includes shadowing effects of ions/neutrals and re-emission, while charging effects are simulated only by an increased ion angular spread. Aspect ratio dependent and independent etching as well as transition from etching to deposition are predicted and studied as a function of plasma phase composition. Variations of etching yield versus aspect ratio can be graphically depicted as paths on the two dimensional plot of equal yield contours versus the normalised fluorine and carbonaceous radicals flux. Operation regimes of the plasma allowing minimisation of aspect ratio dependent phenomena can be easily identified by such graphical representation.

Equilibrium and relaxation of particulate charge in fluorocarbon plasmas

Charging of micron-size particulates, often appearing in fluorocarbon plasma etching experiments, is considered. It is shown that in inductively coupled and microwave slot-excited plasmas of C4F8 and Ar gas mixtures, the equilibrium particle charge and charge relaxation processes are controlled by a combination of microscopic electron, atomic (Ar+ and F+), and molecular ion (CF3+, CF2+, and CF+) currents. The impact of molecular ion currents on the particulate charging and charge relaxation processes is analyzed. It is revealed that in low-power (<0.5 kW) microwave slot-excited plasmas, the impact of the combined molecular ion current to the total positive microscopic current on the particle can be as high as 40%. The particulate charge relaxation rate in fluorocarbon plasmas appears to exceed 108 s−1, which is almost one order of magnitude higher than that from purely argon plasmas. This can be attributed to the impact of positive currents of fluorocarbon molecular ions, as well as to the electron density fluctuations with particle charge, associated with electron capture and release by the particulates.

Investigations of surface reactions during C2F6 plasma etching of SiO2 with equipment and feature scale models

During fluorocarbon plasma etching of SiO2, a polymer passivation layer is generally deposited on the surface of the wafer. The polymer layer regulates the etch by limiting the availability of activation energy and reactants, and providing the fuel for removal of oxygen. To investigate these processes, a surface reaction mechanism for fluorocarbon plasma etching of SiO2 has been developed. The mechanism describes the polymerization process as resulting from neutral sticking, ion sputtering, F atom etching, and low-energy ion assisted deposition. The etch mechanism is a multistep passivation process which results in consumption of both the polymer and the wafer. The surface mechanism was incorporated into an equipment scale simulator to investigate the properties of SiO2 etching in an inductively coupled C2F6 discharge, and predicts that the SiO2 etch rate saturates at high substrate biases due to the depletion of passivation. Experimental results for SiO2 etch rates and selectivity of SiO2 over Si as a function of substrate bias were well reproduced. The blanket reaction mechanism was also employed in a feature scale simulator to investigate high aspect ratio (HAR) trench topography. Results from the feature scale model showed that strong sidewall passivation leads to tapered profiles in HAR SiO2 etching. The incident ion energy and the ratio of the passivating neutral to ion fluxes largely determine the degree of the taper or bowing. Profile control can be obtained by regulating this ratio, with one such method being argon dilution.

Mechanisms for CF2 radical generation and loss on surfaces in fluorocarbon plasmas

During fluorocarbon plasma etching, plasma-surface reactions result in the surface acting as either a source or sink for reactive species, thereby impacting the properties of the bulk plasma. For example, experiments have shown that surfaces in radio frequency (rf) capacitively coupled discharges can be either sources or sinks of CF2 depending on, among other properties, the sheath potential. The coupling of rf bulk and surface reactions, and their combined effects on the CF2 density, were investigated using an integrated plasma equipment and surface kinetics model. While CF2 sticking on surfaces led to its loss, CF2 can be generated from surfaces by energetic ion bombardment resulting in sputtering of polymeric films, or neutralization and dissociation of ions. The net effect of a surface for CF2 production depends on the relative rates of these loss and generation processes. A surface can transform from a net CF2 sink at low incident ion energies to a CF2 source at high ion energies because the CF2 yield by ion–surface interactions typically increases with increasing ion energy. The sensitivity of the model to probabilities of major surface reactions was also investigated.

Atomistic simulation of silicon bombardment by energetic CF 3+: product distributions and energies

We present an analysis of results obtained from molecular dynamics simulations of continuous bombardment of the Si surface with CF 3 + ions at normal incidence in the energy ( E i) range of 25–200 eV. Our analysis is aimed at understanding how the distributions in products and their kinetic energies depend on E i . As E i increases, the product distribution is shifted toward a lower average molecular weight, and with atomic F and molecular CF becoming the most common product species at the higher incident energies. These findings agree well with recent experimental results. The kinetic energy distributions of the products are sensitive to E i only in that the high-energy tail of the distribution becomes more prevalent with increasing E i . Linear cascade theory predictions agree reasonably well with our kinetic energy distributions. Individual species product kinetic energy distributions are much more sensitive to E i , and primarily reflect that most high-energy products are of low molecular weight. The product kinetic energy as a function of ejection angle is also sensitive to E i, displaying an increasing maximum value with increasing E i. These results could potentially help in the guidance and interpretation of molecular beam experiments which seek to detect products in simulated fluorocarbon plasma etching of silicon.

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.

Optical and electrical diagnostics of fluorocarbon plasmaetching processes

This article reviews recent work concerning the role of CF and CF2 radicals in etching and polymerization processes occurring in capacitively coupled radio-frequency plasmas in fluorocarbon gases used for the selective etching of SiO2 layers in microelectronic device fabrication. Laser-induced fluorescence (LIF) was used to determine time-resolved axial concentration profiles of these species in continuous and pulse-modulated CF4 and C2F6 plasmas. Calibration techniques, including broad-band UV absorption spectroscopy, were developed to put the LIF measurements on an absolute scale. A novel technique was used to determine the ion flux to the reactor walls in these polymerizing environments. The mass distribution of the ions arriving at the reactor walls was determined using a quadrupole mass spectrometer. It was found that CFx radicals are produced predominantly by the reflection of neutralized and dissociated CFx+ ions at the powered electrode surface. When the fluorine atom concentration is high, the CFx radicals are destroyed effectively by recombination catalysed by the reactor walls. When the fluorine atom concentration is lowered, the CF2 concentration rises markedly, and it participates in gas-phase oligomerization processes, forming large CxFy molecules and, after ionization, large CxFy+ ions. These species appear to be the true polymer precursors. This mechanism explains the well known correlation between high CF2 concentrations, polymer deposition and SiO2 over Si etch selectivity.

The Optimization of the Cleaning to Remove Residual Bonds of Si-C and Si-F after Fluorocarbon Plasma Etch on the Silicon Surface

The Optimization of the Cleaning to Remove Residual Bonds of Si-C and Si-F after Fluorocarbon Plasma Etch on the Silicon Surface