Ion Angular Distribution Functions Research Articles

Ion Trajectory Control in Processing Plasmas for Nano-Fabrication

To realize ion trajectory control in processing plasmas for nano-fabrication, we applied amplitude modulation (AM) discharges to control of ion trajectory in high aspect trenches. We investigated behavior of incident ions in AR25 (aspect ratio = 25) trench structure in AM discharges using data of Ar+ ion with ion energy and ion angular distribution functions (IEDF and IADF) on the substrate obtained by the PIC-MCC model. AM discharges have higher ion flux onto the trench sidewalls than the continuous waveform (CW) discharges, whereas AM discharges have almost the same ion energy as CW ones. SRIM simulation results suggest that AM discharges can desorb more hydrogen atoms from TEOS-PECVD SiO2 films on the trench sidewall than CW ones, which explains the previous results of improved SiO2 film quality on trench sidewall by AM discharges.

Effects of amplitude modulated capacitively coupled discharge Ar plasma on kinetic energy and angular distribution function of ions impinging on electrodes: particle-in-cell/Monte Carlo collision model simulation

We investigated the effects of amplitude modulated (AM) capacitively coupled Ar discharge plasma on the ion energy distribution function (IEDF) and the ion angular distribution function (IADF) incident on electrodes using the particle-in-cell/Monte Carlo collision model. For AM discharge, the electron density and electron temperature and the kinetic energy and angle of ions incident on the ground electrode change periodically with AM frequency, whereas ones for continuous wave discharge are almost constant. For AM discharge, the plasma had hysteresis characteristics. The peak energy of IEDF varies from 53 to 135 eV and the FWHM of IADF varies from 1.82 to 3.34 degrees for gas pressure 10mTorr, the peak-to-peak input voltage 400 V and AM level of 50%. The variation width of the peak energy of IEDF and FWHM of IADF increases with the AM level. These effects of AM method discharge are more noticeable at lower pressures. Thus, the AM discharge offers a way to control simultaneously IEDF and IADF, which opens a new avenue for plasma processes such as an ALD-like PECVD.

Dependence of electric potentials at trench surfaces on ion angular distribution in plasma etching processes

Ion-stimulated etching of dielectrics in radio frequency plasma results in positive charging of a trench bottom because of the significant difference in the angular distribution functions of ions and electrons. They are anisotropic for ions and quasi-isotropic for electrons. The charging leads to a decrease in the energy of the ions bombarding the trench bottom and to undesirable sputtering of the walls near the trench bottom because of the curving of the ion trajectories. This process is normally investigated by Monte Carlo methods in the absence of experimental data. In this paper the analytical dependence of the ion flux bombarding the trench bottom on a trench aspect ratio and ion angular distribution function is obtained. Numerical calculations of the electric potential on the trench bottom for a set of trench aspect ratios and angles of the ion angular distribution function were performed based on a Monte Carlo method to demonstrate the ion flux and electric potential correlated well with each other. The proposed formula for an ion flux is suggested to be helpful for analyzing charging the trenches with different aspect ratios in plasma with an arbitrary angular ion distribution function.

Mechanism study of sidewall damage in deep silicon etch

The formation and etch mechanism of sidewall damage in deep silicon etch using standard Bosch process have been investigated in this work for the first time. The sidewall damage occurs at a certain depth where the sidewall is not sufficiently protected from lateral etch during long-time ion bombardment, and further, we demonstrate that the formation of sidewall damage is not only related to passivation film on the trench sidewall, but also closely relies on ion-enhanced etch mechanism. In addition, it is found that the starting depth of sidewall damage is almost inversely proportional to etch pressure and ion incidence angle, which can be attributed to a broader ion angular distribution (IAD) at higher chamber pressure. Then, a quantitative etch model based on IAD function has been also proposed to investigate the dependence of the starting depth of sidewall damage on ion incidence angle. Finally, by optimizing process parameters, high-quality etch profile without any observable sidewall damage has been successfully achieved at etch pressure of 60 mTorr.

Process Parameters Effect on Deep Silicon Etching for High Density Capacitor Structure

In deep silicon etching process, there have been many investigations on etching rate. However, only few published papers mentioned the sidewall roughness, which is a critical issue for NMES/MEMS application. In this work, we encountered a special phenomenon: sidewall damage, which was firstly observed during deep silicon etching. Various process parameters such as chamber pressure, bias power, passivation gas flow and etch time have been investigated using a conventional two steps Bosch process for high density Metal-insulator-metal (MIM) capacitor application. During the etching, obvious sidewall damage can be observed in the etched trench sidewall. It can be noted its starting depth has a close dependence on process parameters such as chamber pressure and bias power, which play a significant role influencing etching profile and trench depth. At a lower pressure, a smooth sidewall without any damage can be achieved. With pressure increasing above a certain value, sidewall damage has almost occurred in the full trench, which will lead a series of severe issues such as poor conformal deposition of high-k insulation film, discontinuity of the electrode layer, and reliability failures in MIM capacitor structure.On the other hand, at a given pressure, bias power has a direct effect on etching profile and sidewall damage. As bias power increases, etching profile becomes more negative from vertical one. In particular, sidewall damage is not located in certain depth range but distributed in almost whole trench sidewall and, multi-bowing profile can be seen with increasing etching depth.According to the experimental results, an etching mechanism has been presented to explain the etching phenomena by the ion angular distribution function (IADF) fundamental. Finally, by optimizing process parameters, a perfectly smooth profile without any sidewall damage can be obtained having an aspect ratio as high as 30 with 5μm width.

A semi-analytic collisionless sheath model for multicomponent plasmas and ion energy and angular distributions at rf-biased electrodes

The ion energy and angular distributions (IEADs) arriving at substrates strongly affect the etching rates in plasma etching processes. In order to determine the IEADs accurately, it is important to obtain the characteristics of radio frequency (rf) sheaths with multicomponent plasmas. However, very few studies have been conducted on an rf sheath model for multiple ion species including negative ions over the past few decades. Therefore, in this work, we extended previous semi-analytic collisionless rf sheath models for electronegative plasmas. The extended model was based on the previously developed models, and an equivalent circuit model was used to determine the sheath characteristics. Also, we obtained the IEADs using the rf sheath model and an analytic model for evaluation of the ion angular distribution functions. We observed that the developed model was in good agreement with the experimental results and the one-dimensional dynamics model. Also, we found that negative ion species could affect the characteristics of rf sheaths, hence negative ion species should be considered to obtain more accurate IEADs.

Global Model of <formula formulatype="inline"><tex Notation="TeX">$\hbox{Cl}_{2}\hbox{/Ar}$</tex> </formula> High-Density Plasma Discharge and 2-D Monte-Carlo Etching Model of InP

A multiscale approach has been developed in order to simulate the etch process of InP in an inductive coupled plasma (ICP) Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ar plasma discharge. The model consists of three modules: a global kinetic model of the Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ar plasma discharge, a sheath model, and a 2-D Monte-Carlo etching model. The densities and the fluxes of all neutral and charged species considered in the reaction scheme as well as the electron temperature are calculated from the global model. The angular and energy distribution functions of ions are computed from the sheath model. The output parameters of both the global kinetic model and the sheath model in terms of particle fluxes, ion angular distribution function and ion energy distribution function are used as input parameters in the 2-D etching model. The latter allows tracking in time the evolution of the etched surface. The ultimate goal of the multiscale approach is to predict the etch rate, the etched surface chemical composition, and the etch profile as a function of the operating conditions (power, pressure, gas flow rates, etc). In this paper, the results from the global model are first compared to the measurements carried out in the ICP etching tool, showing a satisfactory agreement. The etching model is then used to simulate the etching of narrow trench and small-diameter hole in InP. The mechanisms involved in the development of undercut below the mask and in the bowing effect are analyzed. The comparison between simulated and experimental etch profiles evidences the important role of Cl adsorption probability on the development of the undercut, and the significant impact of the redeposition of the etched species on the etch rate variation and on the narrowing in the bottom of the etched hole/trench.

Effect of gas pressure on ion flux in 2MHz capacitive discharge

We have studied the ion energy and angular distribution functions in a 2MHz capacitive discharge using combined particle in cell Monte Carlo collision simulations. We found that secondary electrons produced by ion bombardment from the electrodes make important contribution to the ionization at higher gas pressure. Calculations showed that the ion flux on the electrode is very sensitive to the plasma parameters. However, the ion angular distribution function weakly responds to a change of the gas pressure.

Effect of radical species density and ion bombardment during ashing of extreme ultralow-κ interlevel dielectric materials

The significance of ion impact and radical species density on ash-induced modification of an extreme ultralow-κ interlevel dielectric (ILD) material (κ&amp;lt;2.0) in a patterned single damascene structure exposed to Ar∕O2 and Ar∕N2 dual frequency capacitive discharges is determined by combining plasma diagnostics, modeling of the ion angular distribution function, and material characterization such as angle resolved x-ray photoelectron spectroscopy. Radical species density was determined by optical emission actinometry under the same conditions and in the same reactor in a previous study by the present authors. ILD modification is observed and correlated with changes in the plasma for a range of pressures (5–60mTorr), bias powers (0–350W), and percent Ar in the source gas (0%, 85%). For the Ar∕O2 discharge, extensive modification of the ILD sidewall was observed for significant ion scattering conditions, whereas minimal modification of the ILD sidewall was observed under conditions of minimal or no ion scattering. Further, for an identical increase in the O-radical density (∼ an order of magnitude), a different degree of modification was induced at the ILD trench bottom surface depending on whether pressure or percent Ar was used to increase the radical density. The different degrees of modification seemingly correlated with the relative changes in the ion current for increasing pressure or percent Ar. For the Ar∕N2 discharge, reduced damage of the ILD sidewall and trench bottom surfaces was observed for increasing pressure (increasing N-radical density) and decreasing ion current to both surfaces. It is, thus, proposed that the mechanism for modification of the porous ILD is dominated by the creation of reactive sites by ion impact under the present conditions. A detailed discussion of the results which support this proposal is presented.

Analytical model for ion angular distribution functions at rf biased surfaces with collisionless plasma sheaths

The article presents an analytical model for evaluation of ion angular distribution functions (IADFs) at a radio frequency (rf)-biased surface in a high-density plasma reactor. The model couples a unified rf sheath model to an assumed ion velocity distribution function-based formulation for determining the IADF under any general rf-bias condition. Under direct-current (dc) bias conditions the IADF profile shape shows a strong dependence on the bias voltage and the ion temperature is relatively independent of the plasma electron temperature, ion density, and the ion mass. The model establishes the importance of rf-bias frequency in determining the IADF. For conditions where the sheath current wave form is sinusoidal, low bias frequencies result in a large-angle tail contribution to the IADF which can potentially lead to poor anisotropic plasma etching behavior. The large-angle tail is absent at higher bias frequencies. An increase in bias power leads to a general narrowing of the IADF, but the large-angle tail for the IADF at low frequencies persists despite increasing bias powers. Therefore, plasma etch anisotropy can be improved by increasing bias powers only if the bias frequency is sufficiently high. Tangential ion drift velocities introduce azimuthal angle dependence on the IADF and a shift in the peak IADF to off-normal polar angles. While the location of the peak IADF in the azimuthal direction is dictated purely by the direction of the drift velocity, the shift in peak IADF in the polar angle depends on both the drift velocity as well as the bias frequency.

Ion transport anisotropy in low pressure, high density plasmas

Controlling ion transport is critical for controlling line profiles during the plasma etching of submicron structures for ultra-large-scale integrated circuits. Fortunately, the newest generation of plasma reactors are operated at low enough pressures that the sheath region separating the plasma from the wafer becomes collisionless. Under these circumstances, the ion angular distribution function at the wafer surface can be calculated from the sheath voltage and the ion velocity distribution function at the sheath edge. In this work, several limiting cases for the ion angular distribution function are considered. A good approximation for the angular distribution at the wafer surface is derived by assuming that the perpendicular velocity distribution is Gaussian, in agreement with experiment, while the parallel velocity distribution is a delta function with the magnitude of the parallel velocity simply given by the magnitude of the sheath voltage. Parallel velocity distributions with finite width at the sheath edge are shown to have only a small effect on the ion angular distribution function at the wafer surface. With these simple assumptions, the ion angular distribution function at the wafer surface is a function of only one parameter: the ratio of the sheath voltage to the perpendicular ion temperature.