Hybrid Plasma Equipment Model Research Articles

Ion motion above a biased wafer in a plasma etching reactor

The behavior of ions in the plasma is an essential component in the process of industrial etching. We studied the motions and energy distribution of argon ions in a inductively coupled plasma (ICP) etching tool, by the method of laser induced fluorescence (LIF). The silicon wafer clamped to a chuck at the bottom of the chamber was biased with a 1 MHz 1–1.2 kV peak-to-peak sinusoidal voltage. The plasma is formed with a 2 MHz ICP coil pulsed at 10 Hz. Sheath thickness was measured at different phases of the bias waveform. The experiment also compared the ion motions with and without wafer bias, as well as different switch-on time of wafer bias. For all cases, ion energy distribution functions and the two-dimensional flow pattern were studied near the center and edge of the wafer. Significant vortex flows were observed near the wafer edge. Experiments in which the wafer was biased in the plasma afterglow resulted in a narrow distribution of ion energy close to the bias voltage at the vicinity of the wafer, and the ion incident angle on the wafer was the smallest. The results were compared to simulations using the Hybrid Plasma Equipment Model code.

Low-pressure inductively coupled plasmas in hydrogen: impact of gas heating on the spatial distribution of atomic hydrogen and vibrationally excited states

Non-equilibrium inductively coupled plasmas (ICPs) operating in hydrogen are of significant interest for applications including large-area materials processing. Increasing control of spatial gas heating, which drives the formation of neutral species density gradients and the rate of gas-temperature-dependent reactions, is critical. In this study, we use 2D fluid-kinetic simulations with the Hybrid Plasma Equipment Model to investigate the spatially resolved production of atomic hydrogen in a low-pressure planar ICP operating in pure hydrogen (10–20 Pa or 0.075–0.15 Torr, 300 W). The reaction set incorporates self-consistent calculation of the spatially resolved gas temperature and 14 vibrationally excited states. We find that the formation of neutral-gas density gradients, which result from spatially non-uniform electrical power deposition at constant pressure, can drive significant variations in the vibrational distribution function and density of atomic hydrogen when gas heating is spatially resolved. This highlights the significance of spatial gas heating on the production of reactive species in relatively high-power-density plasma processing sources.

Computational approach for plasma process optimization combined with deep learning model

As semiconductor device structures become more complex and sophisticated, the formation of finer and deeper patterns is required. To achieve a higher yield for mass production as the number of process steps increases and process variables become more diverse, process optimization requires extensive engineering effort to meet the target process requirements, such as uniformity. In this study, we propose an efficient process design framework that can efficiently search for optimal process conditions by combining deep learning (DL) with plasma simulations. To establish the DL model, a dataset was created using a two-dimensional (2D) hybrid plasma equipment model code for an argon inductively coupled plasma system under a given process window. The DL model was implemented and trained using the dataset to learn the functional relationship between the process conditions and their consequential plasma states, which was characterized by 2D field data. The performance of the DL model was confirmed by comparison of the output with the ground truth, validating its high consistency. Moreover, the DL results provide a reasonable interpretation of the fundamental features of plasmas and show a good correlation with the experimental observations in terms of the measured etch rate characteristics. Using the designed DL, an extensive exploration of process variables was conducted to find the optimal processing condition using the multi-objective particle swarm optimization algorithm for the given objective functions of high etch rate and its uniform distribution. The obtained optimal candidates were evaluated and compared to other process conditions experimentally, demonstrating a fairly enhanced etch rate and uniformity at the same time. The proposed computational framework substantially reduced trial-and-error repetitions in tailoring process conditions from a practical perspective. Moreover, it will serve as an effective tool to narrow the processing window, particularly in the early stages of development for advanced equipment and processes.

Modelling radio-frequency plasma cleaning of fusion optics

Metallic mirrors are to be used extensively within ITER for diagnostics and real time control. Erosion of the first wall within ITER will cause particles to be redeposited around the machine, including on these first mirrors, which will cause a reduction in reflectivity and a degradation in quality of signal received by the detectors. Powering these mirrors to form capacitively-coupled plasmas (CCPs) with an induced self bias, and using the ions within the plasmas to bombard and remove the deposits, has shown some experimental success in recovering mirror reflectivity. In this work the ion energy distribution functions (IEDFs) from an Ar CCP formed on a 5 cm radius metallic mirror are modelled and investigated using the hybrid plasma equipment model. Initially a geometry variation is done showing that a simple increase in reactor volume can significantly impact the spatial distribution of the ion flux to the mirror surface leading to non-uniform etch rates across the surface, even after the maximum bias has been achieved. The ion energies need to be sufficient to remove depositions (focussing on the first wall material of Be which forms a surface oxide BeO) but not subsequently damage the underlying mirror. In order to achieve this both the voltage (50–1000 V) and the frequency (13.56–60 MHz) have been varied within the model showing trends that may lead towards IEDF optimisation. The increase in voltage increases the self bias linearly and the plasma density super-linearly, whereas increasing the frequency barely effects the self bias while increasing the plasma density sub-linearly. Both increases cause an increase in ion flux for these reasons but both also decrease the homogeneity of the ion flux across the mirror surface which will be required should the energies be above the threshold for the mirror. These results are also unique to the geometry being investigated and thus the conclusion is that it would be prudent to model individual mirror geometries to find optimal parameters. This becomes especially clear with the introduction of a perpendicular magnetic field into the simulation that significantly reduces electron transport within the plasma.

Spatio-temporal plasma heating mechanisms in a radio frequency electrothermal microthruster

Low-power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest since they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimization of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio frequency (RF) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric RF (13.56 MHz) electrothermal microthruster operating in argon between 186–226 Pa (1.4–1.7 Torr) plenum pressure, and between 130–450 V (0.2–5 W). Three distinct peaks in the phase-resolved Ar(2p1) electron impact excitation rate were observed, arising from sheath collapse heating, sheath expansion heating, and heating via secondary electron collisions. These experimental findings were corroborated with the results of two-dimensional fluid/Monte Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to the position within the plasma source during an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, this is distinct from the center of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimization of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms.

Investigation of feature orientation and consequences of ion tilting during plasma etching with a three-dimensional feature profile simulator

Pattern transfer in microelectronics fabrication using plasma-assisted etching processes is being challenged by the three-dimensional (3d) structures of devices such as fin field effect transistors. Etching of 3d structures typically requires a longer over-etch time to clear material in corners, introducing additional selectivity challenges to maintain feature scale critical dimensions. Feature open area, orientation, aspect ratio, and proximity to other nearby structures can influence the outcome of the etch process. In this paper, the authors report on the development and application of a 3d profile simulator, the Monte Carlo feature profile model in the investigation of aspect ratio, and feature orientation dependent etching. In these studies, energy and angularly resolved reactant fluxes were provided by the hybrid plasma equipment model. Results from the model were validated with trends from experimental data. Using reactant fluxes from He/Cl2 and Ar/Cl2 inductively coupled plasmas, etching of two dimensional (2d) and 3d structures in the context of ion tilting and orientation of the feature was investigated.

Field reversals in electrically asymmetric capacitively coupled radio-frequency discharges in hydrogen

In this paper, we present a simulation study of electrically asymmetric capacitively coupled radio-frequency hydrogen discharges using the hybrid plasma equipment model operated at the combined frequencies of 10 and 20 MHz. We find that, in such discharges, field reversals cause ionization near the electrodes during the sheath collapse. In the case of the investigated asymmetric voltage waveforms, the field reversals are asymmetrically distributed over the sheaths, which causes asymmetric ionization and density profiles. The asymmetry of these profiles can be controlled by the phase angle between the two frequencies. As a result, the possibility to control the ion energy independently from the ion flux via the electrical asymmetry effect (EAE) is reduced in discharges displaying strong field reversals, as the asymmetric field reversals compensate the electrically induced asymmetry. The reason for this is understood by an analytical model. Furthermore, we demonstrate, that the EAE can be restored by the addition of specific gases to a pure hydrogen discharge.

The effect of F2 attachment by low-energy electrons on the electron behaviour in an Ar/CF4 inductively coupled plasma

The electron behaviour in an Ar/CF4 inductively coupled plasma is investigated by a Langmuir probe and a hybrid model. The simulated and measured results include electron density, temperature and electron energy distribution function for different values of Ar/CF4 ratio, coil power and gas pressure. The hybrid plasma equipment model simulations show qualitative agreement with experiment. The effect of F2 electron attachment on the electron behaviour is explored by comparing two sets of data based on different F atom boundary conditions. It is demonstrated that electron attachment at F2 molecules is responsible for the depletion of low-energy electrons, causing a density decrease as well as a temperature increase when CF4 is added to an Ar plasma.

Numerical study of the plasma chemistry in inductively coupled SF6 and SF6/Ar plasmas used for deep silicon etching applications

A hybrid model, called the hybrid plasma equipment model, was used to study inductively coupled SF6 plasmas used for Si etching applications. The plasma properties such as number densities of electrons, positive and negative ions, and neutrals are calculated under typical etching conditions. The electron kinetics is analysed by means of the electron energy probability function. The plasma chemistry taking place in pure SF6 and in an Ar/SF6 mixture is also discussed, and finally the effect of the argon fraction on the plasma properties is investigated.

Plasma chemistry modeling for an inductively coupled plasma used for the growth of carbon nanotubes

A hybrid model, called the hybrid plasma equipment model (HPEM), is used to describe the plasma chemistry in an inductively coupled plasma, operating in a gas mixture of C2H2 with either H2 or NH3, as typically used for carbon nanotube (CNT) growth. Two-dimensional profiles of power density, electron temperature and density, gas temperature, and densities of some plasma species are plotted and analyzed. Besides, the fluxes of the various plasma species towards the substrate (where the CNTs can be grown), as well as the decomposition rates of the feedstock gases (C2H2, NH3 and H2), are calculated as a function of the C2H2 fraction in both gas mixtures.

Investigating the plasma chemistry for the synthesis of carbon nanotubes/nanofibres in an inductively coupled plasma-enhanced CVD system: the effect of processing parameters

A parameter study is carried out for an inductively coupled plasma used for the synthesis of carbon nanotubes or carbon nanofibres (CNTs/CNFs), by means of the Hybrid Plasma Equipment Model. The influence of processing parameters including gas ratio for four different gas mixtures typically used for CNT/CNF growth (i.e. CH4/H2, CH4/NH3, C2H2/H2 and C2H2/NH3), inductively coupled plasma (ICP) power (50–1000 W), operating pressure (10 mTorr–1 Torr), bias power (0–1000 W) and temperature of the substrate (0–1000 °C) on the plasma chemistry is investigated and the optimized conditions for CNT/CNF growth are analysed. Summarized, our calculations suggest that a lower fraction of hydrocarbon gases (CH4 or C2H2, i.e. below 20%) and hence a higher fraction of etchant gases (H2 or NH3) in the gas mixture result in more ‘clean’ conditions for controlled CNT/CNF growth. The same applies to a higher ICP power, a moderate ICP gas pressure above 100 mTorr (at least for single-walled carbon nanotubes), a high bias power (for aligned CNTs) and an intermediate substrate temperature.

Investigating the plasma chemistry for the synthesis of carbon nanotubes/nanofibres in an inductively coupled plasma enhanced CVD system: the effect of different gas mixtures

A hybrid model, called the hybrid plasma equipment model (HPEM), was used to study an inductively coupled plasma in gas mixtures of H2 or NH3 with CH4 or C2H2 used for the synthesis of carbon nanotubes or carbon nanofibres (CNTs/CNFs). The plasma properties are discussed for different gas mixtures at low and moderate pressures, and the growth precursors for CNTs/CNFs are analysed. It is found that C2H2, C2H4 and C2H6 are the predominant molecules in CH4 containing plasmas besides the feedstock gas, and serve as carbon sources for CNT/CNF formation. On the other hand, long-chain hydrocarbons are observed in C2H2-containing plasmas. Furthermore, the background gases CH4 and C2H2 show a different decomposition rate with H2 or NH3 addition at moderate pressures.

Inductively Coupled Pulsed Plasmas in the Presence of Synchronous Pulsed Substrate Bias for Robust, Reliable, and Fine Conductor Etching

Inductively coupled pulsed plasmas in the presence of synchronous pulsed substrate bias are characterized in a commercial plasma etching reactor for conductor etching. The synchronous pulsed plasma characteristics are evaluated through the following: 1) Ar-based Langmuir probe diagnostics; 2) Ar/Cl2 plasma modeling utilizing the hybrid plasma equipment model and the Monte Carlo feature model for the investigation of feature profile evolutions; 3) basic etching characteristics such as average etch rate and uniformity; 4) sub-50-nm Dynamic Random Access Memory (DRAM) basic etching performance and profile control; and 5) charge damage evaluation. It is demonstrated that one can control the etching uniformity and profile in advanced gate etching, and reduce the leakage current by varying the synchronous pulsed plasma parameters. Moreover, it is shown that synchronous pulsing has the promise of significantly reducing the electron shading effects compared with source pulsing mode and continuous-wave mode. The synchronous pulsed plasma paves the way to a wider window of operating conditions, which allows new plasma etching processes to address the large number of challenges emerging in the 45-nm and below technologies.

Synchronous Pulse Plasma Operation upon Source and Bias Radio Frequencys for Inductively Coupled Plasma for Highly Reliable Gate Etching Technology

Synchronous pulse operation upon both source and bias RFs for inductively coupled plasma (ICP) etching system, having both dynamic matching networks and RF frequency-sweeping to ensure the lowest RF reflected power, is introduced for the first time. A superior performance of synchronous pulse operation to conventional continuous wave (cw) as well as source pulse operations is confirmed through plasma diagnostics by using Langmuir probe, plasma simulation by using hybrid plasma equipment model (HPEM) and etching performance. Significant reduction of RF power reflection during pulse operation as well as improvement of 35 nm gate critical dimension (CD) uniformity for sub-50 nm dynamic random access memory (DRAM) are achieved by adapting synchronous pulse plasma etching technology. It is definitely expected that synchronous pulse plasma system would have a great ability from a perspective of robustness on fabrication site, excellent gate CD controllability and minimization of plasma induced damage (PID) related device performance degradation.

Investigation of etching and deposition processes of Cl2/O2/Ar inductively coupled plasmas on silicon by means of plasma–surface simulations and experiments

In this paper, a simulation method is described to predict the etching behaviour of Cl2/O2/Ar inductively coupled plasmas on a Si substrate, as used in shallow trench isolation for the production of electronic devices. The hybrid plasma equipment model (HPEM) developed by Kushner et al is applied to calculate the plasma characteristics in the reactor chamber and two additional Monte Carlo simulations are performed to predict the fluxes, angles and energy of the plasma species bombarding the Si substrate, as well as the resulting surface processes such as etching and deposition. The simulations are performed for a wide variety of operating conditions such as gas composition, chamber pressure, power deposition and substrate bias. It is predicted by the simulations that when the fraction of oxygen in the gas mixture is too high, the oxidation of the Si substrate is superior to the etching of Si by chlorine species, resulting in an etch rate close to zero as is also observed in the experiments.

Simulation of an Ar/Cl2 inductively coupled plasma: study of the effect of bias, power and pressure and comparison with experiments

A hybrid model, called the hybrid plasma equipment model, was used to study Ar/Cl2 inductively coupled plasmas used for the etching of Si. The effects of substrate bias, source power and gas pressure on the plasma characteristics and on the fluxes and energies of plasma species bombarding the substrate were observed. A comparison with experimentally measured etch rates was made to investigate how the etch process is influenced and which plasma species mainly account for the etch process. First, the general plasma characteristics are investigated at the following operating conditions: 10% Ar 90% Cl2 gas mixture, 5 mTorr total gas pressure, 100 sccm gas flow rate, 250 W source power, −200 V dc bias at the substrate electrode and an operating frequency of 13.56 MHz applied to the coil and to the substrate electrode. Subsequently, the pressure is varied from 5 to 80 mTorr, the substrate bias from −100 to −300 V and the source power from 250 to 1000 W. Increasing the total gas pressure results in a decrease of the etch rate and a less anisotropic flux to the substrate due to more collisions of the ions in the sheath. Increasing the substrate bias has an effect on the energy of the ions bombarding the substrate and to a lesser extent on the magnitude of the ion flux. When source power is increased, it was found that, not the energy, but the magnitude of the ion flux is increased. The etch rate was more influenced by a variation of the substrate bias than by a variation of the source power, at these operating conditions. These results suggest that the etch process is mainly affected by the energy of the ions bombarding the substrate and the magnitude of the ion flux, and to a lesser extent by the magnitude of the radical flux.

A New Fabrication Method of Aluminum Nanotube Using Anodic Porous Alumina Film as a Template

Aluminum nanotube has been fabricated by a physical vapor deposition/atmospheric pressure injection using an anodic porous alumina film as a template. The pore external-, and inside diameters and the length of the aluminum nanotubes fabricated by this method are 60 nm, 35 nm and 2 <TEX>$\mu$</TEX>m, respectively. The structure of the fabricated aluminum nanotubes was examined by a kind of chemical treatment as extraction of copper on the cross-sectional area of these aluminum tubes in a mixed solution of <TEX>$CuCl_2$</TEX> and HCl by difference of ionization tendency between aluminum and copper. The composition of the aluminum nanotube was identified by the two dimensional Hybrid Plasma Equipment Model (HPEM) employing the inductively coupled plasma.

Investigation and modeling of plasma-wall interactions in inductively coupled fluorocarbon plasmas

Plasma-wall interactions in fluorocarbon based feedgas chemistries, namely CF4, are examined in a standard inductively coupled Gaseous Electronics Conference reference cell using in situ Fourier-transform infrared spectroscopy and microwave interferometry. Measurements show the dissociation of the CF4 feedgas into radical CFx species, as has been observed elsewhere [M. J. Goeckner and R. A. Breun, J. Vac. Sci. Technol. A 11, 3 (1993)], and qualitatively reveal a decrease in plasma-wall interactions as wall temperature is increased. Experimental results such as plasma density, 1011 cm−3, and CF4 density 1013 cm−3, are further compared to results from the hybrid plasma equipment model [R. Kinder and M. J. Kushner, J. Vac. Sci. Technol. A 19, 76 (2001)] to better elucidate the influence of wall temperature on plasma exposed surfaces and sticking coefficients. Last, CF4 vibrational temperatures were also measured, revealing that the line-averaged vibrational temperature remains at a constant 40–60 K above the chamber wall temperature while the vibrational temperature in the center of the discharge is significantly higher. Moreover, the vibrational temperatures are further compared to results from a global thermal model and are in good agreement.

Model for photoresist trim etch in inductively coupled CF4/O2 plasma

A two-dimensional model for photoresist (PR) trim etching in an inductively coupled CF4/O2 plasma is described. The model couples a Hybrid Plasma Equipment Model based simulation of the plasma reactor with a string-based model for PR feature surface evolution. The etch mechanism in the feature profile simulation considers chemical etching and physical sputtering of PR, fluorocarbon polymer deposition on PR, polymer etching, and etching and redeposition of bottom antireflecting coating (ARC). O atoms are the primary PR trim etch precursor in the mechanism and F atoms play a major supporting role. Modeling results show that polymer deposition impacts feature profile and trim etch rate under conditions where large amount of CFx radicals are generated in the plasma. Since trim etching is primarily driven by neutral atoms who have a broad angular distribution, closely spaced feature sidewalls can become tapered during trimming and dense features trim at a slower rate than isolated structures. PR trim etch rate exhibits nonmonotonic dependence on CF4 fraction in the CF4/O2 gas mixture. F extraction of H from PR enhances O atom reactivity with PR at low CF4 fractions, enhancing PR trim etch rate with increasing CF4 fraction. When CF4 fraction increases beyond 10%, trim etch rate decreases due to O depletion and polymer deposition. Trim etch rate increases monotonically with coil power and saturates at large coil powers. This dependence has been linked to O2 dissociation characteristics and polymer deposition. Increase in rf bias voltage enhances ion energy, which results in ARC layer etching and PR sputtering. Consequent ARC redeposition on PR sidewalls slows down trim etching of PR.

Plasma and process characterization of high power magnetron physical vapor deposition with integrated plasma equipment—feature profile model

High power magnetron physical vapor deposition (HPM-PVD) has recently emerged for metal deposition into deep submicron features in state of the art integrated circuit fabrication. However, the plasma characteristics and process mechanism are not well known. An integrated plasma equipment-feature profile modeling infrastructure has therefore been developed for HPM-PVD deposition, and it has been applied to simulating copper seed deposition with an Ar background gas for damascene metalization. The equipment scale model is based on the hybrid plasma equipment model [M. Grapperhaus et al., J. Appl. Phys. 83, 35 (1998); J. Lu and M. J. Kushner, ibid., 89, 878 (2001)], which couples a three-dimensional Monte Carlo sputtering module within a two-dimensional fluid model. The plasma kinetics of thermalized, athermal, and ionized metals and the contributions of these species in feature deposition are resolved. A Monte Carlo technique is used to derive the angular distribution of athermal metals. Simulations show that in typical HPM-PVD processing, Ar+ is the dominant ionized species driving sputtering. Athermal metal neutrals are the dominant deposition precursors due to the operation at high target power and low pressure. The angular distribution of athermals is off axis and more focused than thermal neutrals. The athermal characteristics favor sufficient and uniform deposition on the sidewall of the feature, which is the critical area in small feature filling. In addition, athermals lead to a thick bottom coverage. An appreciable fraction (∼10%) of the metals incident to the wafer are ionized. The ionized metals also contribute to bottom deposition in the absence of sputtering. We have studied the impact of process and equipment parameters on HPM-PVD. Simulations show that target power impacts both plasma ionization and target sputtering. The Ar+ ion density increases nearly linearly with target power, different from the behavior of typical ionized PVD processing. The total metal flux to the wafer increases with target power due to enhanced target sputtering. However, the ionization fraction of the total flux decreases due in part to the increased diffusion loss of charged species. Wafer bias power controls ion energy, and it has a negligible impact on plasma ionization and deposition flux composition. Feature simulations show the redistribution of deposited metals within a feature when wafer resputtering is promoted at sufficient bias power. Target-wafer spacing (TWS) impacts the total ionization and metal flux to the wafer. The Ar+ density and deposition rate decrease with increasing TWS due to increased surface loss. Simulations suggest that reducing the TWS results in more efficient usage of target source.