Microwave Interferometry Research Articles

Measurement of plasma density using wavelet analysis of microwave reflectometer signal

A new method of plasma density profile reconstruction in microwave reflectometry is proposed and implemented on an X-mode broadband reflectometer of the GAMMA 10 mirror device with an ultrafast sweep rate of 10–20 μs. The proposed method makes use of the wavelet transform of the detected signal. Excellent resolution in the time-frequency domain, inherent to wavelet analysis, allows one to obtain a radial electron density profile for every frequency sweep. The electron density reconstruction algorithm, besides the wavelet transform of the reflectometer signal, also includes the calibration, profile initialization, and the solution of an integral equation, ultimately yielding the local values of the electron density. Calibration of the measured signal phase and profile initialization is performed using the independent results of microwave interferometry. Inversion of the integral equation is implemented utilizing the gradient method, numerically stable even for plasma regions with steep density gradients and density profile plateaus. A wavelet-based profile reconstruction algorithm is especially advantageous for monitoring transient plasma phenomena and fast processes, such as in pellet injection, ultrafast swept reflectometry, and short pulsed reflectometry.

Contactless microwave short-range inclinometer radar

A two dimensional short-range inclinometer radar, based on microwave interferometry, is presented. It is able to detect the angles of deviation of a reflecting plane with respect to a reference area with an accuracy greater than /spl plusmn/2/spl deg/ over a dynamic range of /spl plusmn/25/spl deg/. Theoretical and experimental results are presented for a metallic reflecting panel at 10 GHz.

A method for improved microwave‐interferometer remote sensing of convective boundary layer turbulence using water vapor as a passive tracer

Water vapor column irregularities within the lower troposphere's convective boundary layer constitute a nearly passive tracer of turbulent air motions and eddy convection. Remote sensing of turbulence in this layer from the ground is done with microwave radars and with lidars. Microwave interferometry is also sensitive to vapor column fluctuations via the known relative refractive delay of water vapor compared to dry air. However, interferometric measurements are intrinsically nonlocal, yielding only the baseline difference (the difference of measurements at two ends of an interferometer baseline) of vapor column, and thus are not amenable to classical data‐reduction methods for fluid turbulence, other than the equal‐time structure function. In order to remove this limitation of interferometry we have developed a method to estimate the time‐lagged cross covariance of vapor‐column fluctuations over two sites using interferometry data. This effectively circumvents the limitation to baseline‐differenced vapor column data at least in statistical second moments. The method is illustrated with 15‐GHz data taken by the very large array.

Ion-ion neutralization of iodine in radio-frequency inductive discharges of Xe and I2 mixtures

Xe/I 2 low-pressure electric discharges are being developed as efficient, long-lived ultraviolet lighting sources. In this work the kinetics of low pressure, 0.5–5 Torr, radio-frequency inductively excited discharges sustained in Xe and I2 were investigated to determine the source of radiating states. The diagnostics applied in this study include optical absorption and emission spectroscopy, microwave interferometry, and microwave absorption. We found that in time modulated discharges, the emissions from excited states of atomic iodine decays with time constants of hundreds of microseconds. These observations are consistent with those states being populated by ion-ion neutralization. Electron-ion recombination leading to excited states appears not to be an important source of emission.

Phenomenology of a dual-mode microwave/RF discharge used for the deposition of silicon oxide thin layers

A remote plasma enhanced chemical vapour deposition (RPECVD) reactor has been developed to deposit silicon oxide films. It consists of a microwave discharge (created by surface waves) along a quartz tube and a capacitively coupled radiofrequency (RF) discharge on a planar electrode (substrate holder) perpendicular to the tube and facing the gas flow. Plasma diagnostics have been performed in Ar, , Ar - and Ar - He - discharges, at two different positions of the reactor. The densities of electrons (a few ), argon metastables (a few ) and oxygen atoms ( - ) have been determined as a function of different plasma parameters, using microwave interferometry and optical emission spectroscopy (self-absorption and actinometry) respectively. In the case of , the gas flow has little effect on the local equilibrium along the discharge tube due to fast quenching on the walls and quenching in the plasma bulk by slow electrons and oxygen molecules. In contrast, the O atom density profile is governed by the gas flow velocity due to a slow recombination probability on the walls. However it clearly appears that the O atom recombination probability is much larger in the microwave discharge region (due to ion bombardment and plasma heating of the walls) than in the afterglow region. We have also shown that the fraction of O atoms with respect to molecules is enhanced by using helium and/or argon dilution due to the production of O atoms by the quenching reactions of or metastables with . Comparing the densities of electrons, argon metastables and oxygen atoms and their respective rate constants for the reaction with silane , we have deduced that the plasma chemical kinetics leading to silicon oxide deposition can be summarized in a simple scheme: electrons dissociate into O atoms and produce which subsequently react with to enhance the O atom density. In the flowing afterglow, is almost entirely decomposed by O atoms, the direct electron impact dissociation being negligible except when applying an RF discharge in the substrate region.

Effects of O2 Addition on BCl3/Cl2 Plasma Chemistry for Al Etching

Effects of low level (0.5–20%) O2 addition on BCl3/Cl2 plasma chemistry have been investigated using several diagnostic tools: optical emission spectroscopy, microwave interferometry, and mass spectrometry. Experiments were performed using a magnetically enhanced planar 13.56-MHz rf plasma reactor, where aluminum etching was also performed using samples masked with a photoresist pattern of lines and spaces. As O2 was added into a BCl3/Cl2 plasma, the Al etch rate first increased and then dropped above ≈3% O2 added; a transition from reactive-ion-etching (RIE) lag to inverse RIE lag occurred at an O2 percentage of ∼8%. Optical and mass spectrometric measurements indicated that the Cl concentration increases as O2 is added into a BCl3/Cl2 plasma, and that above a critical O2 percentage (∼6% O2) Bx Oy species are formed in the plasma through a reaction between boron chlorides and oxygen and then deposit onto the wafer surface during etching. The Al etching characteristics obtained in BCl3/Cl2/O2 plasmas are interpreted in terms of competitive effects of increased concentrations of Cl and Bx Oy .

Plasma Diagnostics in rf Discharges Using Nonlinear and Resonance Effects

In modelling rf discharges, nonlinear phenomena usually are treated as inconvenient effects. A method based on a nonlinear phenomenon is the self excited electron plasma resonance spectroscopy (SEERS). This new method for plasma monitoring allows to determine density and collision rate of the electrons and the power dissipated in the plasma body. Microwave interferometry (MWI) and Langmuir probe (LP) measurements were used to verify this method. This is shown for inert (He, Ar) and electronegative gases ( O2, CF4). Examples of in situ control in plasma etching process are described. The high sensitivity and the capability of endpoint detection are shown.

Quenching of electron temperature and electron density in ionized physical vapor deposition

The addition of sputtered metal atoms to a high density plasma results in the ionization of the metal species. The ionized physical vapor deposition technique allows the deposition of metal films from ionic species and is a potential method for the fabrication of integrated circuits with high aspect ratio interconnect features. Maximizing the temperature and density of the plasma electrons is important to optimizing the ionization of metal. The present work reports that the electron temperature and electron density of the plasma decrease with increasing metal density. Direct measurement of electron temperature quenching using a rf compensated Langmuir probe demonstrates a ∼20% decrease in electron temperature due to metal additions of ∼1012 atoms/cm3. The electron density is determined using microwave interferometry and decreases by ∼15% as metal atoms are added to the discharge. A fixed temperature global model, however, indicates that thermalized metal atoms should increase the electron density. Gas heating due to energetic sputtered neutrals is shown to be responsible for the observed decrease in electron density.

A microwave plasma source of neutral nitrogen atoms

A microwave source of nitrogen atoms is described. Atom production in the discharge bulk ( mixtures) and atom losses in the flowing afterglow are analysed. The rotational temperature is measured by emission spectroscopy along the discharge (0 - 0 band of the second positive system) and the afterglow (first positive or first negative system depending on the afterglow type). Depending on the experimental conditions ( concentration), two types of flowing afterglow can exist. At high percentage, a `pink' afterglow occurs upstream of the classical `Lewis - Rayleigh' afterglow. An NO titration method provides the atom concentration downstream of the discharge. Then, the N atom transport is explained from a simple model accounting for the energy characteristics of the plasma (obtained by the microwave interferometry methods) and the kinetics.

Diagnostics for plasma processing (etching plasmas) (invited)

Plasma processing diagnostics play two different roles—characterization and control. The goal of plasma characterization is to establish connections of data with external parameters and to verify models. The goal of control diagnostics is to make noninvasive in situ measurements of relevant processing parameters. Diagnostics used in semiconductor etching are considered. These include Langmuir probes, laser induced fluorescence, optical emission spectroscopy, infrared and Fourier transform infrared absorption spectroscopy, mass spectrometry, microwave interferometry, and radio frequency diagnostics. An example is given of the use of many diagnostics in characterizing SiO2 and Si etching by fluorocarbons.

The accuracy of Langmuir probe ion density measurements in low-frequency RF discharges

The ion densities of low-frequency inductively coupled discharges are estimated from Langmuir probe ion saturation currents over a wide range () of plasma densities. The ion densities are compared to the corresponding electron densities obtained from microwave interferometry. For low plasma densities, the cylindrical probes tend to overestimate the ion densities by factors of up to three. These discrepancies correlate well with RF probe currents and can be reduced with RF compensation or with a symmetrical double probe. For high plasma densities, the ion densities from simple probes without RF compensation agree within 50% with the electron densities from the interferometer.

Cl2/Ar and CH4/H2/Ar dry etching of III–V nitrides

Electron-cyclotron-resonance (ECR) and reactive ion etching (RIE) rates for GaN, AlN, InN, and InGaN were measured using the same reactor and plasma parameters in Cl2/Ar or CH4/H2/Ar plasmas. The etch rates of all four materials were found to be significantly faster for ECR relative to RIE conditions in both chemistries, indicating that a high ion density is an important factor in the etch. The ion density under ECR conditions is ∼3×1011 cm−3 as measured by microwave interferometry, compared to ∼2×109 cm−3 for RIE conditions, and optical emission intensities are at least an order of magnitude higher in the ECR discharges. It appears that the nitride etch rates are largely determined by the initial bond breaking that must precede etch product formation, since the etch products are as volatile as those of conventional III–V materials such as GaAs, but the etch rates are typically a factor of about 5 lower for the nitrides. Cl2/Ar plasmas were found to etch GaN, InN, and InGaN faster than CH4/H2/Ar under ECR conditions, while AlN was etched slightly faster in CH4/H2/Ar plasmas. The surface morphology of InN was found to be the most sensitive to changes in plasma parameters and was a strong function of both rf power and etch chemistry for ECR etching.

Partial-depth modulation study of anions and neutrals in low-pressure silane plasmas

Partial-depth modulation of the rf power in a capacitive discharge is used to investigate the relative importance of negative ions and neutral radicals for particle formation in low-power low-pressure silane plasmas. For less than 85% modulation depth, anions are trapped indefinitely in the plasma and particle formation ensues, whereas the polymerized neutral flux magnitudes and dynamics are independent of the modulation depth and the powder formation. These observations suggest that negative ions could be the particle precursors in plasma conditions where powder appears many seconds after plasma ignition. Microwave interferometry and mass spectrometry were combined to infer a rough estimate of anion density of which is approximately twice the free electron densit in these modulated plasmas.

Characterization of a low-frequency inductively coupled plasma source

A hemispherical inductively coupled plasma source is characterized with Langmuir probes, microwave interferometry, voltage, and current monitors, optical emission, and mass spectrometry. The plasma source is operated with up to 2 kW of 0.46 MHz rf power and with a 5 mTorr mixture of argon, oxygen, and hydrogen gases. The rf power is efficiently coupled to the plasma. The ion and neutral compositions show substantial dissociation and water vapor formation. Maps of ion density, electron temperature, and plasma potential are obtained that are consistent with diffusive particle transport and edge power deposition. The latter contributes to an excellent plasma radial uniformity near the bottom electrode.

Characterization of plasma in an inductively coupled high-dense plasma source

An inductively coupled radio-frequency plasma source was characterized by optical emission spectroscopy, microwave interferometry and electrical measurements. The plasma source works like a transformer. The plasma acts as a single-turn secondary winding of a transformer. The primary current produces a magnetic flow in the transformer core resulting in a closed-path axial electrical field in the toroidal discharge tube. The source was found to be capable of generating high plasma densities. Electron densities were measured by microwave interferometry. Maximal mean electron densities of 4.2 × 10 18 m −3 in helium at 600 mTorr and 3.6 × 10 18 m −3 in oxygen discharges at 400 mTorr were detected. Mean collision frequencies of electrons were determined by electrical measurements. Optical emission spectra showed strong radial variations in electron density in both discharges. In oxygen discharges the radial variation of the extent of dissociation was small compared to the variation of the electron density.

New calibration method for the determination of the absolute density of CH radicals through laser-induced fluorescence

Laser-induced fluorescence (LIF) was applied at the B-X transition of the CH radical to measure the absolute densities of CH radicals in an electron-cyclotron resonance methane plasma. The absolute experimental uncertainty is only approximately 30% as a result of a new calibration procedure. The experimental setup was calibrated through the comparison of the LIF signal of N(2)(+) with that of CH. The absolute N(2)(+) density was derived from the spatially resolved N(2)(+) LIF signal and the line-averaged electron density as measured with microwave interferometry.

Microwave Diagnostic Results from the Gaseous Electronics Conference RF Reference Cell.

Electron density measurements with even the simplest microwave interferometry techniques can range over three to four orders of magnitude, can be responsive on time scales as fast as 50 ns, and are simple to obtain and interpret. Three groups have published electron density data taken in the Gaseous Electronics Conference (GEC) reference reactor using microwave interferometry. The agreement in the data from these groups at higher pressures is excellent especially when one considers that the GEC reactors involved have some key differences. These may have been the cause of some differences between the results obtained at low pressures, although, the manner in which the measurements were interpreted may also have contributed. The electron densities compare favorably in argon, helium, and nitrogen above 33.3 Pa (250 mTorr); but, the measurements tend to diverge some at 13.3 Pa (100 mTorr) and in 133 Pa helium above approximately 200 mA. It is speculated that the latter difference occurs as the discharges change from a bulk ionization or α-mode to a secondary electron emission or γ-mode, and that this transition occurs at lower voltages and currents for reactors with aluminum electrodes than it does for those with stainless steel electrodes. In addition, time resolved electron densities are presented. There is agreement between time resolved measurements in the two reactors, in particular, the electron density in helium discharges is found to rise dramatically after the rf excitation is turned off while the electron densities in argon and nitrogen glows exhibit only slight increases.

The TEXT neutral lithium beam edge density diagnostic

A fast neutral lithium beam has been installed on the TEXT tokamak for beam emission spectroscopy studies of the edge plasma electron-density profile. The diagnostic was recently upgraded from ten to twenty spatial channels, each of which has two detectors, one to measure lithium beam signal and one to monitor plasma background light. The spatial resolution is 6 mm, and the temporal resolution is designed to be as high as 10 ms for studies of transient events including plasma density fluctuations. Initial results are presented from the ten-channel system: Edge electron densities unfolded from the LiI (2 s 2S–2 p 2P) 670.8 nm emission profile have the same general time dependence as the line-averaged density measured by microwave interferometry.

High Density Plasma Diagnostics For Predictive Model Development

AbstractHigh density plasmas are being increasingly employed in both the deposition of low temperature, high quality passivant films and for highly anisotropic, nanometer-scale pattern transfer. The high degree of control necessary in these processes emphasizes the need for a more complete understanding of the basic physical and chemical mechanisms involved. To this end, a wide range of diagnostic techniques have been employed to characterize electron cyclotron resonance microwave plasmas applied to the etching of semiconductors in both chlorine- and methane-based chemistries. In particular, vacuum ultraviolet spectroscopy has been employed to identify important reactants and, where feasible, measure plasma constituent temperatures (e.g., neutral temperatures range from 0.1–0.3 eV in these plasmas). Langmuir probes, microwave interferometry and microwave electric field probes are used to monitor plasma parameters as a function of process conditions and provide understanding of microwave power deposition. Mass spectroscopy is utilized to characterize the plasma flux incident on the substrate, in terms of mass signature and charge state distribution. This flux is largely ionic with a considerable degree of dissociation, highly appropriate for directional etching. Results from each of these diagnostics are incorporated into an evolving predictive model for highly anisotropic, nanometer-scale etching of semiconductors.

Telemetric sensors by microwave interferometry

Abstract This paper presents some possibilities offered by a specific microwave sensor called an I-Q demodulator or complex correlator. It is shown that this narrow-band complex correlator is able to provide information about the velocity profile and the position of a monochromatic transmitter which is inside a predefined area; this situation is qualified as cooperative configuration. Nevertheless, this complex correlator can also be used in the situation where only reflected signals are considered — non cooperative configuration — such as in anticollision and level measurement processes. Such sensors can naturally be used in many industrial application fields like robotics, tank gauging and berthing aid. Therefore, to answer industrial requirements, we show in this paper that the necessary compromise between good performance and low cost can be reached.