Trace Rare Gases Optical Emission Research Articles

Probing suprathermal electrons by trace rare gases optical emission spectroscopy in low pressure dipolar microwave plasmas excited at the electron cyclotron resonance

In microwave plasmas with the presence of a magnetic field, fast electrons could result from collisionless energy absorption under electron cyclotron resonance (ECR) conditions. In this case, electrons are trapped between the two poles of the magnetic field and rotate at the cyclotron frequency ωce. When crossing a zone where the cyclotron frequency equals the microwave frequency (ωce=ω), electrons see a steady electric field in their reference frame and are constantly accelerated by the right handed polarized (RHP) wave. When the plasma density reaches the so-called critical density nc at which ωpe2=ω2±ωωce, where ωpe is the plasma electron frequency, the left handed polarized (LHP) electromagnetic wave can excite electrostatic waves that can produce collisionless electron heating and fast electron generation by Landau damping. In this study, a combination of the Langmuir probe and trace rare gas optical emission spectroscopy (TRG-OES) is used to analyze the electron energy probability function (EEPF) in microwave (2.45 GHz) low-pressure argon plasmas excited at ECR in a dipolar magnetic field. While both TRG-OES and Langmuir probe measurements agree on the effective electron temperature (TeAll) from 1.6 to 50 mTorr, TRG-OES, which is more sensitive to high energy electrons, shows that the EEPF is the sum of two Maxwellian populations: one described by TeAll and a high energy tail characterized by a temperature TeTail. Spatially resolved-TRG-OES measurements show that the high-energy tail (TeTail) in the EEPF is spatially localized near the magnet, while the effective electron temperature (TeAll) stays constant. The ratio between the high energy tail and the effective temperatures is found to increase with the absorbed microwave power and decrease with increasing pressure. The former phenomenon is ascribed to a rise in ECR heating due to a stronger RHP wave electric field and to an enhanced absorption of the LHP waves. On the other hand, the decrease in the ratio is attributed to a smaller magnetic confinement of the electrons (increase in the collision frequency), which lessens ECR heating and to a decrease in the LHP field intensity at the resonant position, which impedes the conversion into electrostatic waves.

Spatially resolved electron density and electron energy distribution function in Ar magnetron plasmas used for sputter-deposition of ZnO-based thin films

Langmuir probe and trace rare gases optical emission spectroscopy were used to analyze the spatial structure of the electron density and electron energy distribution function (EEDF) in a cylindrical Ar magnetron plasma reactor used for sputter-deposition of ZnO-based thin films. While a typical Bessel (zero order) diffusion profile was observed along the radial direction for the number density of charged particles at 21 cm from the ZnO target, a significant rise of these populations with respect to the Bessel function was seen in the center of the reactor at 4 cm from the magnetron surface. As for the EEDF, it was found to transform from a more or less Maxwellian far from the target to a two-temperature Maxwellian with a depletion of high-energy electrons where magnetic field confinement effects become important. No significant change in the behavior of the electron density and EEDF across a wide range of pressures (5–100 mTorr) and self-bias voltages (115–300 V) was observed during magnetron sputtering of Zn, ZnO, and In2O3 targets. This indicates that sputtering of Zn, In, and O atoms do not play a very significant role on the electron particle balance and electron heating dynamics, at least over the range of experimental conditions investigated.

Investigation of 50 Hz Pulsed DC Nitrogen Plasma with Active Screen Cage by Trace Rare Gas Optical Emission Spectroscopy

Optical emission spectroscopy is used to investigate the nitrogen-hydrogen with trace rare gas (4% Ar) plasma generated by 50 Hz pulsed DC discharges. The filling pressure varies from 1 mbar to 5 mbar and the current density ranges from 1 mA·cm−2 to 4 mA·cm−2. The hydrogen concentration in the mixture plasma varies from 0% to 80%, with the objective of identifying the optimum pressure, current density and hydrogen concentration for active species ([N] and [N2]) generation. It is observed that in an N2-H2 gas mixture, the concentration of N atom density decreases with filling pressure and increases with current density, with other parameters of the discharge kept unchanged. The maximum concentrations of active species were found for 40% H2 in the mixture at 3 mbar pressure and current density of 4 mA·cm−2.

Determination of Plasma Parameters in a Dual-Frequency Capacitively Coupled CF4 Plasma Using Optical Emission Spectroscopy

Optical emission spectroscopy measurements of dual-frequency capacitively coupled CF4 plasmas were carried out. The gas temperature (Tg) was acquired by fitting the optical emission spectra of a CF B—X system in 201∼206 nm. The atomic fluorine concentration and the electron temperature (Te) were obtained by trace rare gas optical emission spectroscopy and a modified Boltzmann plot technique, respectively. It was found that the gas temperature was about 620±30 K at 50 mTorr and the atomic fluorine concentration increased while the electron temperature decreased with increasing gas pressure and power of high frequency (60 MHz). With increasing low frequency (2 MHz) power, the electron temperature also increased, but the atomic fluorine concentration was insensitive to this change. The generation and disappearance mechanisms of F atoms are discussed.

Effect of helium mixing on excitation temperature and nitrogen dissociation in inductively coupled plasma

Trace rare gas optical emission spectroscopy (TRG-OES) is carried out to investigate the excitation temperature, relative densities of active species (N, N2+) and nitrogen dissociation in inductively coupled helium admixed nitrogen plasma for different rf power (50, 100, 150 W), pressure (0.2–0.5 mbar) and helium percentage (10–90%) using Ar as an actinometer (4%). The excitation temperature is obtained from Boltzmann plot method using emission intensity of several argon lines. The dissociation of nitrogen has been investigated by both the actinometry method and the ratio (IN/IN2) of the atomic nitrogen line emission intensity at (746.83 nm) to the vibrational band (0–0) of the N2 second positive system at 337.1 nm. The excitation temperature increases with the increase in power and helium percentage and decreases with increase in fill pressure. The nitrogen dissociation as well as the relative densities of [N] and [N2+] increases with the increase in helium percentage.

Nonlocal effect of plasma resonances on the electron energy-distribution function in microwave plasma columns

Spatially resolved trace rare gases optical emission spectroscopy was used to analyze the electron energy-distribution function (EEDF) in low-pressure argon plasma columns sustained by surface waves. At frequencies >1 GHz, in the microwave-sustained region, the EEDF departs from a Maxwellian, characterized by a depletion of low-energy electrons and a high-energy tail, whereas in the field-free zone, the EEDF is Maxwellian. Abnormal behavior of the EEDF results from the acceleration of low-energy electrons due to the conversion of surface waves into volume plasmons at the resonance point where the plasma frequency equals the wave frequency and their absorption by either collisional or Landau damping.

Measurement of electron temperatures and electron energy distribution functions in dual frequency capacitively coupled CF4/O2 plasmas using trace rare gases optical emission spectroscopy

Measurements of electron temperatures (Te) and electron energy distribution functions (EEDFs) in a dual frequency capacitively coupled etcher were performed by using trace rare gas optical emission spectroscopy (TRG-OES). The parallel plate etcher was powered by a high frequency (60 MHz) “source” top electrode and a low frequency (13.56 MHz) “substrate” bottom electrode. Te first increased with pressure up to ∼20 mTorr and then decreased at higher pressures. Increasing the bottom rf power resulted in higher electron temperatures. Electron temperatures in 90% CF4+10% O2 plasmas were similar to those in 80% CF4+20% O2 plasmas. EEDF exhibited bi-Maxwellian characteristics with enhanced high energy tail, especially at pressures >20 mTorr.

Characterization of microwave plasmas for deposition of polyparylene

Polyparylene, a noncritical, nontoxic layer material well suited for long-term applications in the human body, has been deposited by plasma-enhanced chemical vapor deposition of the monomeric species. For that end, a microwave discharge in a pulsed mode has been applied. Important plasma parameters have been evaluated by simultaneous application of Langmuir probe and trace rare gas optical emission spectroscopy. Plasma densities and electron temperature have been found to cover values from an almost Langmuir plasma up to some 1010∕cm3 and between 1 and 3.5eV, respectively. The differences in electron temperature between the two methods clearly show the efficiency of microwave fields to excite the high-energy tail of the electron energy distribution function. Due to diffusion loss, the plasma is spatially inhomogenous which has been taken care of by measuring at four different radial positions and different pressures with the Langmuir probe. This holds true for both ambients: argon and parylene-C. However, the plasma density in parylene is lower by a factor of almost 10, indicating that this molecule and/or its fragments exhibit a strong power for electronic attachment or that the process of ionization must compete with other, parasitic reaction paths.

Studies of chlorine-oxygen plasmas and evidence for heterogeneous formation of ClO and ClO2

Plasma and surface diagnostics of Cl2/O2 mixed-gas inductively coupled plasmas are reported. Using trace rare gas optical emission spectroscopy and Langmuir probe analysis, electron temperatures (Te) and number densities for Cl atoms (nCl), electrons (ne), and positive ions were measured as a function of percent O2 in the feed gas and position in the plasma chamber. Adsorbates on and products desorbing from a rotating anodized aluminum substrate exposed to the plasma were detected with an Auger electron spectrometer and a quadrupole mass spectrometer. Te and ne increased with increasing percent O2 in the plasma, while nCl fell off with O2 addition in a manner reflecting simple dilution. Cl atom recombination probabilities (γCl) were measured and were found to be a nearly constant 0.036±0.007 over the range of Cl2/O2 mixing ratios and Cl coverage. Large yields of ClO and ClO2 were found to desorb from the surface during exposure to the plasma, ascribed predominantly to Langmuir–Hinshelwood reactions between adsorbed O and Cl.

Spatially resolved electron temperatures, species concentrations, and electron energy distributions in inductively coupled chlorine plasmas, measured by trace-rare gases optical emission spectroscopy

Determining the spatial dependence of charged and neutral species concentrations and energies in inductively coupled plasmas (ICP) is important for understanding basic plasma chemistry and physics, as well as for optimizing the placement of the wafer with respect to the ICP source to maximize properties such as etching rate uniformity, while minimizing charging-induced damage and feature profile anomalies. We have determined the line-integrated electron temperature (Te) and Cl-atom number density (nCl) as a function of the axial distance (z) from the wafer in a chlorine ICP, using trace rare gases optical emission spectroscopy (TRG-OES). By selecting rare gas lines that are either (a) excited mostly from the ground states, or (b) excited mainly from the metastable states we were also able to obtain approximate electron energy distributions functions (EEDFs). The gap between the wafer and the window adjacent to the flat coil inductive source was fixed at 15 cm. The pressure was 2, 10, or 20 mTorr (95% Cl2, 1% each of He, Ne, Ar, Kr, Xe) and the inductive mode source power was 340 or 900 W. Te measured by TRG-OES, mostly characteristic of the high-energy (>10 eV) part of the EEDF, peaked near the source under all conditions except 2 mTorr and 900 W, where a maximum Te of 5.5 eV was observed at midgap. The falloff in this high-electron-energy Te away from the source is mainly due to a preferential loss of high-energy electrons, which can be explained by an increasingly depleted (with increasing energy) EEDF, combined with the nonlocal effect: electrons lose kinetic energy as they approach the higher potential energy regions of lower electron density near the wafer. At 20 mTorr and 340 W, the mean free path for inelastic scattering by high-energy electrons becomes comparable to the reactor dimensions, causing added cooling of the EEDF near the wafer. TRG-OES EEDFs measured at a distance of 3 cm from the wafer and 900 W are in excellent agreement with previous Langmuir probe measurements. nCl increased with power and was highest at 900 W in the region between midgap and the ICP window, reaching a level corresponding to a high degree of dissociation of Cl2.

Determination of electron temperature, atomic fluorine concentration, and gas temperature in inductively coupled fluorocarbon/rare gas plasmas using optical emission spectroscopy

Recent advances in the interpretation of optical emission spectra from plasmas have made it possible to measure parameters such as electron temperature (Te), relative electron density, and gas temperature (Tg) with this nonintrusive technique. Here we discuss the application of trace rare gas optical emission spectroscopy (TRG-OES), optical actinometry, and N2 rotational spectroscopy to determine Te, relative electron density, fluorine atom concentration, and Tg for fluorocarbon/Ar plasmas in an inductively coupled reactor. Various etch processes, containing mixtures of a carrier gas, C2F6, and C4F8, were evaluated as a function of pressure and flowrate. Ar, Kr, and Ne were used individually or were mixed to comprise the carrier gas. In the case of TRG-OES and optical emission actinometry, a mixture containing equal parts of He, Ne, Ar, Kr, and Xe (∼1% ea.) was added. A method for correcting excitation cross sections is introduced for cases when radiation trapping affects the emission of a rare gas (Ar) that is present at high concentrations. Experiments revealed that Te can be controlled through the choice of carrier gas: Ne tends to increase Te and Kr tends to decrease Te relative to Ar. This phenomenon was verified qualitatively with a simple zero-dimensional energy balance model. Additional measurements revealed that the absolute atomic fluorine concentration, determined from calibrated F-to-Ar actinometry ratios, is roughly 20% of the total gas at 5 mTorr, and decreases to 5% at 60 mTorr. The gas temperature in the Ar-carrier plasma was measured to be ∼1200 K and was found to be insensitive to pressure whereas Tg in Kr and Ne carrier gas plasmas increased from 1500–1900 K and 700–1500 K, respectively between 5 and 30 mTorr.

Electron temperatures of inductively coupled Cl2–Ar plasmas

Trace rare gases optical emission spectroscopy has been used to measure the electron temperature, Te, in a high-density inductively coupled Cl2–Ar plasma at 18 mTorr as function of the 13.56 MHz radio frequency power and Ar fraction. Only the Kr and Xe emission lines were used to determine Te, because of evidence of radiation trapping when the Ar emission lines were also used for larger Ar fractions. At 600 W (10.6 W cm−2), Te increases from ∼4.0±0.5 eV to ∼6.0±2.0 eV as the Ar fraction increases from 1% to 96%. In the H (inductive, bright) mode, Te, for a “neat” chlorine plasma (including 1% of each He/Ne/Ar/Kr/Xe) increases only slightly from ∼3.8 to 4.0 eV as power increases from 450 to 750 W. This increase is much larger for larger Ar fractions, such as from ∼4.0 to 7.3 eV for 78% Ar. Most of these effects can be understood using the fundamental particle balance equation.

Diagnostics of chlorine inductively coupled plasmas. Measurement of electron temperatures and electron energy distribution functions

The goal of the study that begins with this paper is the creation of a base set of parameters (densities and temperatures or energy distributions of all charged and neutral species) in a chlorine transformer-coupled plasma, measured with the same commercial plasma source over an extensive range of pressure and power. Electron temperatures Te and electron energy distribution functions (EEDFs) are reported as a function of pressure (1–20 mTorr) and power (10–1000 W) during slow etching of SiO2-covered Si wafers. Te values are obtained both by trace rare gases optical emission spectroscopy and Langmuir probe methods. EEDFs are obtained with the Langmuir probe. A zero-dimensional (global) model with revised rate coefficients is used to compute Te from both minimum (pressure, total plasma density, gas temperature, and wall recombination coefficient) and maximum (all experimentally measured parameters necessary to calculate Te) sets of input parameters.

Trace rare gases optical emission spectroscopy: nonintrusive method for measuring electron temperatures in low-pressure, low-temperature plasmas.

Trace rare gases optical emission spectroscopy (TRG-OES) is a new, nonintrusive method for determining electron temperatures (T(e)) and, under some conditions, estimating electron densities (n(e)) in low-temperature, low-pressure plasmas. The method is based on a comparison of atomic emission intensities from trace amounts of rare gases (an equimixture of He, Ne, Ar, Kr, and Xe) added to the plasma, with intensities calculated from a model. For Maxwellian electron energy distribution functions (EEDFs), T(e) is determined from the best fit of theory to the experimental measurements. For non-Maxwellian EEDFs, T(e) derived from the best fit describes the high-energy tail of the EEDF. This method was reported previously, and was further developed and successfully applied to several laboratory and commercial plasma reactors. It has also been used in investigations of correlations between high-T(e) and plasma-induced damage to thin gate oxide layers. In this paper, we provide a refined mechanism for the method and include a detailed description of the generation of emission from the Paschen 2p manifold of rare gases both from the ground state and through metastable states, a theoretical model to calculate the number density of metastables (n(m)) of the rare gases, a practical procedure to compute T(e) from the ratios of experimental-to-theoretical intensity ratios, a way to determine the electron density (n(e)), a discussion of the range of sensitivity of TRG-OES to the EEDF, and an estimate of the accuracy of T(e). The values of T(e) obtained by TRG-OES in a transformer-coupled plasma reactor are compared with those obtained with a Langmuir probe for a wide range of pressures and powers. The differences in T(e) from the two methods are explained in terms of the EEDF dependence on pressure.

Ultrahigh frequency versus inductively coupled chlorine plasmas: Comparisons of Cl and Cl2 concentrations and electron temperatures measured by trace rare gases optical emission spectroscopy

Using trace rare gases optical emission spectroscopy, Cl and Cl2 number densities (nCl and nCl2) and electron temperatures (Te) were measured for two source configurations of high-density chlorine plasmas. In one configuration, the reactor was outfitted with a spoke antenna, operated at a resonant ultrahigh frequency (UHF) of 500 MHz. Alternatively, the same reactor was configured with a single loop, inductively coupled plasma (ICP) source operated at a radio frequency of 13.56 MHz. Optical emission from trace amounts (1% each) of rare gases added to the main Cl2 feed gas were recorded as a function of power and pressure. Modeling was used to derive Te from these data. Additional emission from Cl2 (at 3050 Å) and Cl (numerous lines between 7000 and 9000 Å), normalized to the appropriate emission from the rare gases (i.e., actinometry) was used to obtain nCl2 and nCl. In the ICP plasma, Te decreased monotonically from 5.5 to 1.2 eV as a function of increasing pressure between 1 and 20 mTorr. Conversely, with the UHF configuration, Te was 3.3 eV, independent of pressure between 1 and ∼7 mTorr, and then decreased to 1.7 eV as pressure was increased to 27 mTorr. At the same input power (1000 W), both sources resulted in electron densities of 1×1011 cm−3 at 3.5 mTorr, yet the UHF plasma was much less dissociated (30%) than the ICP plasma (70%). This can be attributed to differences in the electron energy distribution functions in the UHF and ICP plasmas, especially at low pressure.

Trace Rare Gases Optical Emission Spectroscopy for Determination of Electron Temperatures and Species Concentrations in Chlorine-Containing Plasmas

Trace rare gas optical emission spectroscopy has been used to obtain electron temperatures (Te) and percent dissociation of Cl2 in chlorine-containing, high-density inductively (transformer) coupled plasmas. In this method, a small amount of an equal mixture of the rare gases is added to the plasma and emission spectra that include lines from the rare gases are recorded. Modeling of the dependence of the rare gas emission intensities on Te allows Te to be derived from the best match between the observed and computed intensities. Te in Cl2 plasmas increases from 1.7 eV at 20 mTorr to 3.4 eV at 0.5 mTorr. These values are 1.3 to 1.8 times lower than those recorded with a Langmuir probe. This discrepancy is due at least in part to artifacts associated with using a single Langmuir probe in a reactor with semiinsulating walls. Absolute percent dissociations of Cl2 were determined by monitoring the intensity of an emission band of Cl2, normalized to emission from Ar and Xe. Cl2 percent dissociation increased with power and decreased with pressure. At 1.0 mTorr, Cl2 is about 85% dissociated at a power density of 0.1 W/cm3 and 95% dissociated at 0.3 W/cm3 in a Cl2 plasma. Addition of BCl3 to Cl2 increases the dissociation of Cl2.