AVS2001 Session PS1-TuM: Diagnostics II

Tuesday, October 30, 2001 8:20 AM in Room 103

Tuesday Morning

Start	Invited?	Item
8:20 AM		PS1-TuM-1 Radical Concentrations and Temperatures in a Dual-Frequency Capacitive Reactor Determined By Broad-Band UV-Visible Absorption Spectroscopy J. Luque, E.A. Hudson (Lam Research); J.P. Booth (Ecole Polytechnique, France) Broad-band UV-visible absorption spectroscopy is next in simplicity to optical emission spectroscopy. Unlike optical emission, it provides direct information about ground states of neutral species in plasma environments. However, it has rarely been implemented as diagnostic in industrial plasma reactors. In the present work, we report measurements in a Lam dielectric etch reactor (27/2 MHz dual frequency capacitive) during semiconductor processing using Ar/C_xF_y/O₂ feedstock gas mixtures. Using a deuterium lamp source and a photodiode array detector we can measure absorption across the 200-260 nm region, allowing simultaneous detection of CF, CF₂ and SiF₂ radicals. Other species that we have detected include C₂ and C₃. We use the CF radical as example of this technique, and to illustrate the wealth of information easily extracted from moderately high resolution absorption spectra: CF radical ground state number densities are determined with a detection limit of ~10¹⁰ cm^–3 using off-the-shelf instrumentation. Typical densities are in the order of 10¹³ cm^–3 for a standard oxide etch recipe. CF rotational and vibrational temperatures are measured aided by spectral computer simulation code. Typical rotational temperatures, which are normally equivalent to gas temperature in the reactor, are in the order of 425 K. Vibrational distributions, with temperatures of 800 K, show non-thermalization compared to rotational temperatures. We found a previously unreported vibrational band, assigned as CF A-X(3,0), and it is heavily predissociated. Intensities of the CF A-X(v,0) and B-X(v,0) bands are used to determine accurate transition probabilities, enabling precise determination of concentrations and temperatures in future spectroscopic experiments via the CF A-X and B-X bands.
8:40 AM		PS1-TuM-2 CF_x Kinetics, Gas Temperatures and Instabilities in a CF₄ Inductively-coupled Discharge J.P. Booth, H. Abada, P. Chabert (Ecole Polytechnique, France); G. Cunge (CNRS/LETI, France) The use of inductive discharges in fluorocarbon gases for SiO₂ etch applications has been hampered by narrow process windows and severe process drift problems, despite promising etch rate, selectivity and anisotropy results. We have extended our study of free radical kinetics in capacitively-coupled plasmas to these systems in order to investigate the origin of these problems. Laser-induced fluorescence was used to probe the axial concentration and temperature profiles and the kinetics of CF and CF₂ radicals in a pure CF₄ ICP. Rotationally-resolved LIF of CF shows that very high gas temperatures can occur (up to 1000K). Therefore, large gas temperature and density gradients exist within the reactor. The CF and CF₂ axial concentration profiles are hollow, showing that these species are produced at the reactor surfaces due to C_xF_y⁺ ion bombardment, and are destroyed in the gas phase. The nature of the gas-phase destruction processes will be discussed, in relation to the formation of heavier C_xF_y species. We also observed the occurrence of plasma instabilities over a wide range of gas pressure and injected RF power. This phenomena can mostly be explained in terms of relaxation oscillations between capacitive and inductive plasma modes as observed by previous workers in SF₆ plasmas, but with the added mechanism of the formation of heavy C_xF_y oligomers, which are much more electronegative than the parent gas, CF₄.
9:00 AM		PS1-TuM-3 Time-Resolved CF₂ Rotational Temperature Measurements in Inductively-Coupled Pulsed Plasmas X. Wu, T.M. Bauer, J.L. Cecchi (University of New Mexico) We have measured the time evolution of the rotational temperature of CF₂ in pulsed plasmas. These measurements were performed in an inductively coupled plasma (ICP) reactor with a CHF₃/Ar gas mixture. We have explored a range of ICP powers of 300 to 900 W and a range of total pressure from 10 to 30 mTorr. The rotational temperature of CF₂ was determined from absorbance measurements, using a wavelength-modulated diode laser spectroscopy system, modified to provide data with a time resolution of less than 0.1 ms. The ν1 band ^RQ₈ of CF₂ eo<->oe and ee<->oo rotational lines J=16, 18, 19, 21-23 are used for analysis. >From the dependence of the line intensities on the energy of the lower state, we are able to extract the rotational temperature. Pulse frequency was varied from 4 to 20 Hz at duty factors from 10 to 50%. The CF₂ rotational temperature time evolution is characterized by a first order rise upon plasma ignition to a temperature that coincides with the steady state rotational temperature, providing the plasma-on time exceeds the heating time constant, which is typically in the range of 2-5 ms. Once the plasma is extinguished, rotational temperature shows first order decay to about 290 ± 20K. The time constant for this decay is in the range of 5-10 ms.
9:20 AM		PS1-TuM-4 Measurements of H atom and CF_x Radical Densities in High-density CHF₃ Plasmas by Laser-induced Fluorescence K. Sasaki, M. Okamoto, K. Kadota (Nagoya University, Japan) CHF₃ plasmas are widely used for dry etching of SiO₂. There are many reports on the diagnostics of CF_x radicals in CHF₃ plasmas. However, reliable diagnostics of H atoms in CHF₃ plasmas have not ever been carried out. H atoms play an important role in scavenging F atoms which obstruct selective etching of SiO₂. In addition, excess H atoms may results in the damage of Si. Accordingly, the reliable diagnostics of H atoms in CHF₃ plasmas is an important issue. In the present work, we measured the absolute H atom density in high-density CHF₃ plasmas by (2+1)-photon laser-induced fluorescence spectroscopy.¹ In addition, the absolute densities of CF and CF₂ radicals were also measured by laser-induced fluorescence. The experiments were carried out in a linear machine with a uniform magnetic field of 1 kG. Helicon-wave discharges were obtained by applying various rf powers to a helical antenna wound around a glass tube of 3 cm diameter. Since the plasma was confined radially by the external magnetic field, we had a slender plasma column with a diameter of 3 cm at the center of the cylindrical vacuum chamber. The H atom density was mainly on the order of 10¹³ cm^-3 at a CHF₃ gas pressure of 5 mTorr. The H atom density increased with the electron density of the plasma. Contrary to the H atom density, the CF₂ radical density was a decreasing function of the electron density. The H atom density was higher than the CF₂ radical density in plasmas with electron densities higher than 10¹² cm^-3. The spatial distribution of the H atom density in low-density plasmas was roughly uniform, while in high-density plasmas, slightly hollow distributions (i.e., the H atom density in the plasma column was lower than that in the outside region) were observed in the H atom density. ¹K. Sasaki, M. Nakamoto, and K. Kadota, Rev. Sci. Instrum., in press.
10:20 AM		PS1-TuM-7 c-C₄F₈/Ar Inductively Coupled Plasma Characterization M.T. Radtke, J.W. Coburn, D.B. Graves (University of California, Berkeley) A study of the composition and structure of a c-C₄F₈/argon inductively coupled plasma is reported. In this study, we first measured discharge properties (plasma density, potential and electron energy distribution function) in a pure Ar discharge using a Langmuir probe. In subsequent experiments, small amounts of c-C₄F₈ were added to argon and the ion and neutral plasma composition was measured using separate quadrupole mass spectrometers. Neutral species number densities were measured using a calibrated, molecular beam sampled, appearance potential mass spectrometer. The dominant radicals were CF₂ and CF₃. Positive ions were measured in a separate mass spectrometer. The dominant positive ions were Ar⁺, CF⁺, CF₂⁺, CF₃⁺, C₂F₄⁺, and C₃F₅⁺. The neutral gas temperature was estimated using optical emission spectroscopy of the rotational temperature of trace N₂ in an argon discharge. Using the measurements, the total dissociation rate coefficients for c-C₄F₈ were estimated with a volume-averaged balance on c-C₄F₈ for each of the experimental conditions. From the measured EEDFs and the total dissociation rate coefficients, we tested a proposed total dissociation cross section. The proposed cross section was found to be consistent with the measured values to within the experimental uncertainty.
10:40 AM		PS1-TuM-8 Use and Limitations of In-situ FTIR Spectroscopy for Fluorocarbon Plasma Analysis B.A. Cruden, M.V.V.S. Rao (Eloret Corp., NASA Ames Research Center); S.P. Sharma, M. Meyyappan (NASA Ames Research Center) Fourier Transform Infrared (FTIR) Absorption Spectroscopy has been used in an inductively coupled GEC Reference Cell for analysis of CF₄ plasmas. It was possible to detect undissociated CF₄ fraction and quartz window etch products (SiF₄, CO, COF₂) with this technique. From knowledge of the rotational/vibrational structure of the various bands, it is also possible to extract a gas rotational and vibrational temperature from this data. In interpretation of this data, many of the oft-ignored non-idealities of quanitative FTIR analysis are addressed. Of particular concern is the fact that Doppler-broadened absorption lines are significantly smaller than the instrument resolution. The resulting data represents a non-linear averaging of closely spaced absorbing lines. This produces cross-sections that do not obey Beer's law, i.e. are not constant. By examining the theory behind FTIR, these variations in cross-section are predicted for CF4. These theoretical errors are also manifested in inaccuracies in analyzing overlapping spectra. In FTIR, (as well as other absorption techniques) there is an additional averaging of absorption spectra over spatial coordinates. As a significant portion of the absorption signal will typically lie outside of the plasma, this averaging results in measurements that are not representative of the plasma species in terms of both number density and temperatures. When combined with the resolution limitations, it is predicted that a maximum measurable temperature exists that can differ significantly from the true plasma temperature. The results of a more carefully controlled absorption pathlength will also be presented.
11:00 AM		PS1-TuM-9 Measurements and Models of Ion Energy Distributions in High-Density, Radio-Frequency-Biased CF₄/Ar Discharges M.A. Sobolewski, Y. Wang, A.N. Goyette (National Institute of Standards and Technology) Ion dynamics in the narrow sheaths of high-density plasmas, especially in sheaths biased by radio-frequency (rf) voltages, are complicated and nonlinear. Models of such high-density, rf sheaths are needed to predict ion bombardment energies in simulations of high-density plasma etching. To provide data to test these models, we have measured ion energy distributions (IEDs) in CF₄/argon discharges in a high-density, inductively coupled plasma reactor, using a mass spectrometer equipped with an ion energy analyzer. Energy distributions of CF⁺, CF₂⁺, CF₃⁺, and Ar⁺ ions were measured as a function of pressure, mixture, inductive source power, rf bias frequency and rf bias amplitude. Simultaneous measurements by a capacitive probe and a Faraday cup provide enough information to completely determine the input parameters of sheath models and allow direct comparison of calculated and measured IEDs. For conditions where the rf bias period is much smaller than, or much larger than, the time it takes ions to cross the sheath, very simple models are able to predict the features of the measured IEDs. When the rf bias period approaches the ion transit time, however, more complicated models are required. One recently developed model¹ which include a complete treatment of the time-dependent ion dynamics in the sheath, was found to accurately predict the behavior of measured IEDs over the entire range of rf bias frequency. ¹ ¹M. A. Sobolewski, Phys. Rev. E 62, 8540 (2000).
11:20 AM		PS1-TuM-10 Submillimeter Absorption Spectroscopy of an Inductively Coupled Plasma E.C. Benck, G. Golubiatnikov, G. Fraser (National Institute of Standards and Technology) Millimeter and submillimeter (100 GHz to 1 THz) absorption spectroscopy is being developed as a sensor for measuring radical densities and temperatures in processing plasmas for microelectronics. Most molecules, radicals, and ions have transitions suitable for detection with submillimeter waves and the necessary spectroscopic data is available in the literature for determining the absolute radical densities. In addition, the narrow linewidths of cw submillimeter sources are suitable for measuring rotational, vibrational and translational temperatures of radicals. Initial measurements are being conducted with a backward wave oscillator (BWO) source and a liquid-He-cooled bolometer detector. Radical densities have been measured in an inductively coupled Gaseous Electronics Conference (GEC) RF Reference Reactor. At frequencies around 100 GHz, large absorption signals can be observed for CHF₃. Significant amounts of molecular dissociation can be measured when the discharge is ignited. The plasma does not significantly increase the translational temperature, probably due to a large volume of cool gas surrounding the plasma. A 25 times increase in sensitivity would be expected for absorption measurements with frequencies around 500 GHz. Measurements of C_xF_y and SiF_x radical densities will also be presented.