Nader Sadeghi

CFx radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase...

Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

Ion transport in an electron cyclotron resonance plasma

Electron cyclotron resonance (ECR) plasma reactors are being developed for etching and deposition of thin films during integrated circuit fabrication. To control critical parameters such as the flux and energy distribution of ions impacting surfaces, it is necessary to understand how these parameters are influenced by physical construction, electromagnetic design, and chemical kinetics. In this work, we report detailed measurements of ion velocity distributions in both the source and reactor regions of an ECR system using mixtures of Ar and He. Using Doppler‐shifted laser‐induced fluorescence spectroscopy, we measure metastable Ar‐ion velocity distributions parallel and perpendicular to the magnetic field direction as a function of magnetic field amplitude, pressure, rf bias voltage, and microwave power. The measurements, in turn, are used to estimate the magnitude of electrostatic potentials and fields parallel and perpendicular to the magnetic field. Indicative of ion trapping, we find nearly isotropic ...

The role of helium metastable states in radio-frequency driven helium–oxygen atmospheric pressure plasma jets: measurement and numerical simulation

Absolute densities of metastable He(23S1) atoms were measured line-of-sight integrated along the discharge channel of a capacitively coupled radio-frequency driven atmospheric pressure plasma jet operated in technologically relevant helium?oxygen mixtures by tunable diode-laser absorption spectroscopy. The dependences of the He(23S1) density in the homogeneous-glow-like ?-mode plasma with oxygen admixtures up to 1% were investigated. The results are compared with a one-dimensional numerical simulation, which includes a semi-kinetical treatment of the pronounced electron dynamics and the complex plasma chemistry (in total 20 species and 184 reactions). Very good agreement between measurement and simulation is found. The main formation mechanisms for metastable helium atoms are identified and analyzed, including their pronounced spatio-temporal dynamics. Penning ionization through helium metastables is found to be significant for plasma sustainment, while it is revealed that helium metastables are not an important energy carrying species into the jet effluent and therefore will not play a direct role in remote surface treatments.

Ion and neutral temperatures in electron cyclotron resonance plasma reactors

Ion and neutral temperatures are measured by high‐resolution laser‐induced fluorescence spectroscopy both in the source and downstream of an electron cyclotron resonance discharge through mixtures of Ar, Ar/Ne, and Ar/He. Contrary to previous reports, both ions and neutrals are found to be cold. In the source, ion temperatures perpendicular to the magnetic field are ≤0.5 eV; downstream they are ∼0.25 eV. Neutral temperatures in the source and downstream are 0.068 and 0.030 eV, respectively.

Ion flux composition in HBr/Cl2/O2 and HBr/Cl2/O2/CF4 chemistries during silicon etching in industrial high-density plasmas

Anisotropic etching of silicon gates is a key step in today’s integrated circuit fabrication. For sub-100 nm gate dimensions, one of the main issues is to precisely control the shape of the etched feature. This requires a detailed knowledge of the various physicochemical mechanisms involved in anisotropic plasma etching. Since silicon etching in high-density plasmas is strongly ion assisted, the identities of the ions bombarding the wafer is a key parameter that governs the etch rates and the etched profiles. In the present article, mass spectrometry has been used to investigate the chemical composition of the ion flux bombarding the reactor walls of an industrial inductively coupled plasma used for 200-mm-diam silicon wafer processing. The plasma chemistries investigated are HBr/Cl2/O2 and HBr/Cl2/O2/CF4 mixtures optimized for sub-100 nm gate processes. Quantitative ion mass spectra show that under those conditions the ion flux contains up to 50% of SiClXBrY+ (X,Y=0–2) ions, although Cl+, Cl2+, and Br+ i...

Measurement of electron temperature and density in an argon microdischarge by laser Thomson scattering

Laser Thomson scattering in a novel, backscattered configuration was employed to measure the electron temperature (Te) and electron density (ne) in argon dc microdischarges, with an interelectrode gap of 600μm. Measurements were performed at the center of the gap that corresponds to the positive column. For 50mA microdischarge current and over the pressure range of 300–700Torr, the plasma parameters were found to be Te=0.9±0.3eV and ne=(6±3)×1013cm−3, in reasonable agreement with the predictions of a mathematical model.

Absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure He–H2O RF glow discharges

The measurement of radical densities in atmospheric-pressure plasmas has gained increasing importance in recent years in view of their crucial role in many applications. In this paper we present absolute OH density measurements by broadband UV absorption in diffuse atmospheric-pressure RF glow discharges in mixtures of He and H2O. The use of a 310 nm light-emitting diode as a light source and a very high resolution spectrometer (2.6 pm resolution) made the estimation of the total OH density possible by simultaneously measuring the absorption rates of different spectrally resolved rotational lines of the OH(A–X) transition. For different RF powers and water concentrations, OH densities and gas temperatures ranging between 6 × 1019and 4 × 1020 m−3 and 345 and 410 K, respectively, were obtained. The gas temperature Tg was also measured by three different methods. Tg deduced from the rotational temperature of N2(C–B) emission, nitrogen being present as a trace impurity, provided the most reliable value. The rotational temperature Tr of the ground state OH(X) presented values with a maximum deviation of 25 K compared with Tg. To obtain the gas temperature from the emission intensities of OH(A–X) rotational lines, the recorded intensities of different lines must be corrected for the effect of self-absorption inside the plasma.

Neutral gas depletion mechanisms in dense low-temperature argon plasmas

Neutral gas depletion mechanisms are investigated in a dense low-temperature argon plasma—an inductively coupled magnetic neutral loop (NL) discharge. Gas temperatures are deduced from the Doppler profile of the 772.38 nm line absorbed by argon metastable atoms. Electron density and temperature measurements reveal that at pressures below 0.1 Pa, relatively high degrees of ionization (exceeding 1%) result in electron pressures, pe = kTene, exceeding the neutral gas pressure. In this regime, neutral dynamics has to be taken into account and depletion through comparatively high ionization rates becomes important. This additional depletion mechanism can be spatially separated due to non-uniform electron temperature and density profiles (non-uniform ionization rate), while the gas temperature is rather uniform within the discharge region. Spatial profiles of the depletion of metastable argon atoms in the NL region are observed by laser induced fluorescence spectroscopy. In this region, the depletion of ground state argon atoms is expected to be even more pronounced since in the investigated high electron density regime the ratio of metastable and ground state argon atom densities is governed by the electron temperature, which peaks in the NL region. This neutral gas depletion is attributed to a high ionization rate in the NL zone and fast ion loss through ambipolar diffusion along the magnetic field lines. This is totally different from what is observed at pressures above 10 Pa where the degree of ionization is relatively low (<10 −3 ) and neutral gas depletion is dominated by gas heating.

Metastable chlorine ion transport in a diverging field electron cyclotron resonance plasma

For applications in ultralarge scale integration, low pressure, high density plasmas are being developed for etching and deposition of thin films. To control critical parameters such as the flux and energy distribution of ions impacting surfaces, it is necessary to understand how these parameters are influenced by physical and electromagnetic design. In this work, we extend previous measurements of ion velocity distributions in Ar/He electron cyclotron resonance plasmas to Cl2/He plasmas. Using Doppler‐shifted laser‐induced fluorescence spectroscopy, we measure metastable Cl ion velocity distributions parallel and perpendicular to the magnetic field as a function of magnetic field amplitude, pressure, and microwave power. We also examine the effects of the wafer platen on the distribution functions by repeating the measurements after removing the platen. Surprisingly, little qualitative difference is seen when chlorine and argon discharges are compared; this is most likely a result of the low pressures em...