AVS2004 Session PS-WeA: Plasma Diagnostics

Wednesday, November 17, 2004 2:00 PM in Room 213A

Wednesday Afternoon

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2:00 PM		PS-WeA-1 Excitation Mechanisms in Low Pressure Capacitively Coupled Discharges G.F. Franz (University of Applied Sciences, Germany); M.K. Klick (Advanced Semiconductor Instruments, Germany) The dominating excitation mechanisms in capacitively coupled discharges are ohmic heating (mainly to electrons in the bulk, and, at high powers, also to the ions in the sheath) and stochastic heating (thermalization of the electric energy of the sheath). The latter process will gradually enhance its importance when the discharge pressure is reduced below a threshold value of about 100 mTorr. That means that the electron energy distribution function (EEDF) in this regime will be governed by stochastic heating. This is also the pressure range in which reactive ion etching takes place. Measurements with self-excited electron resonance spectroscopy (SEERS) in various plasmas reveal that stochastic heating will strongly depend on the nature of the gas (atomic or molecular, electronegative or electropositive). Since ohmic heating scales with discharge pressure whereas stochastic heating is nearly pressure independent, both the heating mechanisms can be separated. New results are presented which have simultaneously measured with VI- and Langmuir probes as well as with optical emission spectroscopy (OES) with traces of rare gases and are eventually compared with models created by Lieberman, Godyak, and Klick.
2:20 PM		PS-WeA-2 Influence of Frequency on the Characteristics of UHF Capacitively Coupled Plasmas in a 300 mm Chamber G.A. Hebner, E.V. Barnat, P.A. Miller (Sandia National Laboratories); A.M. Paterson, J.P. Holland, T. Lill (Applied Materials, Inc.) We have investigated the characteristics of UHF capacitively coupled plasmas produced in a modified Applied Materials chamber. The chamber had a 14-inch diameter upper electrode (source) that was driven at 10 to 160 MHz and a 300 mm diameter electrostatic chuck with a ceramic process kit that was driven at 13.56 MHz (bias). Diagnostics employed include a microwave interferometer to measure the line-integrated electron density, a hairpin microwave resonator to measure the spatially resolved electron density, absorption spectroscopy to determine the argon metastable temperature and density, laser induced fluorescence (LIF) to determine the spatial distribution of the excited species, and spatially resolved optical emission. We found that for constant source rf power, the electron density increased with rf frequency. The argon 1s5 metastable temperature was slightly above room temperature (300 â?" 400K), significantly cooler than our previous measurements in inductively coupled plasmas. The metastable density was not a strong function of source frequency or rf power. The metastable spatial distribution was always peaked in the center of the chamber and had a weak dependence on frequency. Scaling of the plasma parameters with frequency, power and pressure, and implications to energy deposition models will be discussed. This work was supported by Applied Materials and Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.
2:40 PM		PS-WeA-3 Monitoring Electron Density of Processing Plasmas Using a Transmission-line Type Microwave Sensor C.H. Hseih, K.C. Leou, C. Lin (National Tsing Hua University, Taiwan) The purpose of this study¹, was to develop an electron density sensor for applications in process real-time feedback control of plasma based semiconductor fabrication tools, such plasma etchers or PECVDs. The sensor was a dielectric waveguide based transmission-line where microwave propagates at a phase velocity determined by the structure and the plasma density (electron density) surrounding the structure. Thus the variation of plasma density can be estimated from the phase shift of the transmitted microwave from one to the other end of the transmission-line. For the proof-of-principle study, a coaxial type transmission-line was adopted with a Teflon outer dielectric and a copper inner conductor operated at a freqeuncy of 2-3 GHz. Analytical analysis of dispersion characteristics of the transmission line structure was carried and the resulting propagation constants were in good agreement with results from calculation using a commercial high frequency structure simulation code (HFSS by ANSOFT). Experimental demonstration have been performed with an inductively-coupled plasma. The sensor was mounted on the inner wall of plasma chamber with a coaxial line length of 8 cm and a distance of 5 cm between input and output ports. Measurement results show that the dependence of electron density of plasma source RF power predicted by the sensor agrees well with the Langmuir probe measurements. Compared to conventional microwave interferometers where line-averaged plasma density is measured, the trainsmission-line type microwave sensor will be less susceptive to the interference caused by multi-passes reflection/refraction effect resulting from nonuniformity of plasma density profiles. Therefore, it provides a measurement of higher sensitivity and wider dynamic range. Work supported by the grant from the National Science Concil of R.O.C. under contract #92-2218-E-007-018.
3:00 PM		PS-WeA-4 Real-time Monitoring of Charge Accumulated during SiO₂ Etching using Pulse-Time-Modulated-Plasma Y. Suzuki, T. Shimmura, S. Samukawa (Tohoku University, Japan) High aspect ratio SiO₂ contact hole etching is crucial for ULSI device fabrication. However, serious problems, such as charge-build-up damage, etching stops and microloading effect, mainly caused by charge accumulated in contact holes are not clearly solved. Therefore, it is very important to measure the amount of charges and to control such charge accumulated. In this paper, we monitored the amount of charge accumulated in real-time during the continuous wave (CW) and pulse-time-modulated (TM) plasma etching for a number of contact holes by using on-wafer monitoring device. This sensor consists of Poly-Si (300nm) / SiO₂ (1.7 µmm) / Poly-Si (300nm) layered structure on Si substrate. The diameter of contact holes was 300nm, the numbers of that were 6,400,000 and aspect ratio was 5.7. The potential differences between the top and bottom Poly-Si electrodes were measured during plasma discharge. In the case of TM plasma, the potential differences drastically reduced, compared to the CW plasma. The time-resolved measurement showed the potential differences were increased during a few tens of micro-seconds of pulse-on-time and was reduced during a few tens of micro-seconds of pulse-off-time. As the charge accumulated were saturated at the time constant of milli-seconds on the substrate surface, theresult suggests that a few tens micro-seconds TM plasma can drastically reduce the charge accumulated on the surface. The proposed on-wafer monitoring sensor can realize the real-time measurement of charge accumulated during the plasma etching processes.
3:20 PM		PS-WeA-5 The Use of the Radio Frequency Matching Network As a Diagnostic for Plasma Processing Chambers J. Caughman, G.L. Bell (Oak Ridge National Laboratory); V. Resta (Hewlett-Packard) Radio frequency (RF) power is commonly used in the plasma processing of semiconductors. As part of the RF system, many of the plasma processing chambers use RF matching networks with two variable tuning elements to transform the impedance at the chamber interface to be 50 ohms at the input of the matching network. The matching network is a highly tuned circuit, and the positions of the tuning elements are directly related to the RF impedance of the chamber. Thus, the positions of the tuning elements can be used as a diagnostic to determine processing sensitive parameters. Matching networks have been characterized to relate tuning element positions to the RF impedance and the power efficiency of the network. After characterization, the impedance and efficiency can be determined as a function of processing parameters simply by measuring the tuning element positions during processing. The technique has been demonstrated on both inductively coupled plasmas (ICP) and capacitively coupled plasmas (CCP). It has been found that the impedance is sensitive to changes in power, pressure, gas composition, and wall conditions. For example, an increase in ICP source power will cause an increase in the plasma density, which can be seen as an increase in the real part of the source impedance and a decrease in the real part of the bias impedance. The technique has been used on several different chambers and has been demonstrated to be helpful in terms of troubleshooting and chamber matching. Details of the characterization technique and the sensitivity of the impedance to processing conditions will be presented.
3:40 PM		PS-WeA-6 Measurements and Consequences of Non-Uniform RF Plasma Potential due to Surface Asymmetry in Large Area RF Capacitive Reactors L. Sansonnens, L. Derendinger, C. Hollenstein, A.A. Howling, H. Schmidt (Ecole Polytechnique Fédérale de Lausanne, Switzerland); J.P.M. Schmitt, E. Sakanaka (UNAXIS-France SA, France) In small area capacitive reactors, the RF and DC plasma potential can be assumed to be uniform over the reactor area, and asymmetry between the grounded and powered electrodes leads to the well-known area law for determining the uniform DC and RF voltage amplitude across both plasma sheaths. In large area reactors, however, the RF plasma potential can vary over a long range across the reactor area due to the finite plasma conductivity. A local asymmetry of electrode area due, for example, to the lateral grounded walls for plasma confinement, causes a local RF plasma potential perturbation which propagates along the resistive plasma between capacitive sheaths. This propagation can be described by a telegraph equation for which a typical damping length can be determined. In this way, for a non-symmetric reactor wider than the damping length, the RF sheath voltage amplitudes which are unequal close to the reactor edges tend to be the same in the centre as for a symmetric reactor. A predicted consequence of this non-uniform RF plasma potential is the presence of non-ambipolar current circulating through the plasma and along conducting electrodes. In this work, we present measurements of the RF plasma potential and DC net current distribution over the grounded electrode of a large area reactor (57 x 47 cm²) using an array of surface probes, for various reactor geometry configurations. These experimental results are compared with a two-dimensional solution of the telegraph propagation model. Finally, we present some effects of the non-uniform RF plasma potential such as non-uniform power dissipation in the plasma which have important consequences for plasma processing.
4:00 PM		PS-WeA-7 Radiofrequency Discharge and Sheath Structure Around Dissimilar Surfaces E.V. Barnat, G.A. Hebner (Sandia National Laboratories) Discontinuities present on a surface introduce perturbations to both the field and excitation around the discontinuity. Spatially resolved electric fields in an argon sheath near the discontinuous surfaces were measured using laser-induced fluorescence-dip spectroscopy (LIF-dip) while excitation is measured by LIF, optical emission and a Langmuir probe. Spatial maps of the electric field and excitation were obtained in the regions near these discontinuities as functions of power and pressure of the discharge. These measurements demonstrate the degree and extent of perturbation introduced into the plasma. Surface discontinuities investigated included metal-dielectric, metal-metal and metal-step junctions. This work was supported by the Division of Material Sciences, BES, Office of Science, U. S. Department of Energy and Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.
4:20 PM		PS-WeA-8 Real-time, Nonintrusive Monitoring of Drifting Ion Energy and Flux in a High-Density, Inductively Coupled Plasma Reactor M.A. Sobolewski (National Institute of Standards and Technology) Measurements of the radio-frequency (rf) current and voltage applied to a plasma reactor, interpreted by plasma sheath models, provide an ability to monitor the total ion flux and ion energy distribution at surfaces inside the reactor. Such measurements are useful for monitoring drift in manufacturing or laboratory reactors when direct measurements of ion flux or energy are impossible or impractical. In this study rf measurements were used to monitor drift in Ar and Ar/CF₄ discharges in an inductively coupled, high-density plasma reactor. One source of drift in such reactors is the deposition of a conductive surface layer on the dielectric window of the inductive source. As this layer grows, a greater fraction of the source power excites currents in the layer, rather than in the plasma, resulting in less efficient operation and a reduction in plasma density and ion flux. These changes in turn affect the coupling of rf bias power into the discharge, producing changes in delivered rf bias power or voltage, sheath voltages, and ion energy distributions. Using rf measurements, the resulting changes in ion flux and energy were monitored in real time, as a surface layer was deposited. Changes in ion energies as large as 100 eV were observed. Increases as well as decreases in ion energies were observed, depending on rf bias conditions. Three different mechanisms that explain the changes in ion energies were identified. The application of the technique to monitoring process drift or irreproducibility caused by factors other than deposition on the dielectric window will also be discussed.
4:40 PM		PS-WeA-9 Investigation of Correlation between Etching Profiles and Neutral Density in Cl₂/O₂ Plasmas M. Mori (Hitachi Ltd., Japan); G. Cunge, M. Kogelschatz (CNRS/LSP, France); L. Vallier (CNRS/LTM, France) Cl₂/O₂ plasmas are basically used in Shallow Trench Isolation (STI) etching of ULSI device fabrication. In STI etching, precise taper angle control is one of the issues for good coverage without stress damage to Si substrate at following deposition process. The taper angle is determined by the sidewall passivation layer during etching. Hence etching by-products (SiCl_x) are thought to play an important role in taper angle control, because they are believed to be the precursors to the deposition of sidewall passivation layers. In this study, we have investigated the correlation between SiCl_x density and etching profiles in Cl₂/O₂ plasmas. The SiCl_x(X=0-2) absolute densities are measured by UV broad band absorption spectroscopy. The thickness of the sidewall passivation layer is measured by comparing the etched profiles before and after removal of the passivation layer. The SEM results firstly confirm that good correlation between the taper angles and the thickness of the passivation layer on feature sidewalls over a wide range of plasma operating conditions (pressure, source power, O₂ flow rate and total gas flow rate). By contrast, there is no correlation between the thickness of sidewall passivation layers and the SiCl_x densities in the plasma. For example, the passivation layer thickness increases rapidly while the SiCl_x density decrease with increasing O₂ gas flow. Indeed, our data suggests that the growth rate of sidewall passivation layers is limited only by the O density in the plasma, which thus controls the etching profile. By roughly calculating O atom density as a function of electron temperature and density, we are then able to explain the trend of passivation layer thickness with pressure and source power. The role of O is to oxidize SiCl_x species chemisorbed on the surfaces exposed to the plasma, which, without oxidation are etched back into the plasma by Cl atoms.
5:00 PM		PS-WeA-10 2D-Imaging Measurements of Sputtered Atom Velocities in dc Magnetron Discharges by Doppler-Shifted LIF K. Sasaki (Nagoya University, Japan); K. Shibagaki (Suzuka National College of Technology, Japan); N. Nafarizal, H. Toyoda, T. Kato, S. Iwata, S. Tsunashima, H. Sugai (Nagoya University, Japan) In the deposition of multilayer magnetic thin films such as Co/Pt and Fe/Pt using magnetron sputtering discharges, a key issue is how to obtain very flat interfaces between the multilayers. Bombardment of energetic ions and neutrals during deposition may roughen the interface. Understanding on energies of particles supplied to the substrate is helpful to optimize the apparatus for the deposition of multilayer magnetic films. In the present work, we measured the spatial distribution of velocity distribution of Fe atoms in a conventional dc magnetron discharges. We employed laser-induced fluorescence imaging spectroscopy. By recording pictures of laser-induced fluorescence at various wavelengths of a tunable optical parametric oscillator, we obtained many Doppler spectra that represent the velocity distributions of Fe atoms in a r-z plane of the cylindrically symmetric magnetron discharge. In a low-pressure discharge at 3 mTorr, Fe atoms near the target had broad velocity distribution, and they contained energetic component with velocity faster than 10 km/s (29 eV). At a distance of 60 mm from the target, Fe atoms had a thermalized distribution having two temperatures. By integrating the velocity distribution, we obtained two-dimensional maps of the mean velocity of Fe atoms. In a low gas pressure of 3 mTorr, Fe atoms had a mean velocity of 3.6 km/s in the region adjacent to the target surface. In a gas pressure of 20 mTorr, the mean velocity of Fe adjacent to the target was slower than 1 km/s. This work was supported by the 21st Century COE Program by the Ministry of Education, Culture, Sports, Science and Technology of Japan.