AVS1996 Session PS-WeP: Diagnostics, Fundamentals, Modeling, and ICF Poster Session

Wednesday, October 16, 1996 5:00 PM in Ballroom A
Wednesday Afternoon

Time Period WeP Sessions | Topic PS Sessions | Time Periods | Topics | AVS1996 Schedule

PS-WeP-1 Investigations of High Density Plasma Tool Process Uniformity using Mini-Ion Energy Analysis
S. Radovanov, H. Anderson (University of New Mexico); D. Rasmussen, V. Resta (Sematech)
A miniature retarding grid ion energy analyzer (mini-IEA) was built and used to determine radial profiles of the ion saturation current and ion energy distribution (IED) in a low pressure, high density inductively coupled plasma tool. The mini-IEA design was based on previous work reported by [1]. Radial profiles were collected from the high density tool using a convenient reference gas (N\sub 2\) and a typical reactive processes gas mixture (BCl\sub 3\/Cl\sub 2\). The influence of the coil grounding on the radial uniformity of the plasma ion current and the shape of the IED was studied by intentionally shifting the coil ground location. In the normal, or virtual ground mode (ground near the outer radial edge of the coil) the plasma had a relatively uniform radial profile in the ion current and a sharp, narrow IED. With the ground location imposed at three turns out from the center of the coil, a large dip in the ion current formed nearly coincident with the imposed ground location on the coil. The IED also significantly broadened, suggesting a larger electrostatic coupling component to the discharge. The nonuniformity of the ion current and the change in the IED both have significant implications for plasma processing. [1] G.W. Gibson, Jr, H.H. Sawin, I. Tepermeister, D.E. Ibbotson, and J.T.C. Lee, J. Vac. Sci. Technol. B, 12, 2333 (1994).
PS-WeP-2 Effect of ICP Tool Rf Ground Location on Plasma Density and Ion Energy
G. Bell (Oak Ridge National Laboratory); S. Radovanov (University of New Mexico); V. Resta, D. Rasmussen (Sematech); H. Zhang, D. Hoffman (Oak Ridge National Laboratory)
Inductively coupled plasma (ICP) reactors are used extensively in the microelectronics industry to etch semiconductor materials. Uniformity of the bulk plasma density and ion energies arriving at the wafer surface contribute to etch rate uniformity across the wafer. Measurements of the electromagnetic (EM) fields produced by a high density ICP etch tool show that a virtual radio frequency (RF) ground exists on the current strap. A set of experiments were performed to investigate the impact of altering the location of this RF ground on the EM fields produced and on the uniformity of the plasma density and ion energy distribution. Vacuum RF field measurements for "virtual" and "imposed" ground configurations show a dramatic effect on the electric and magnetic fields. For example, in the standard "virtual" ground case (ground located near the outer edge of the coil), the electric field magnitude is center peaked, while in the "imposed" ground mode (ground shifted toward the coil center), the electric field magnitude has an annular shape. Multi-point optical emission and ion energy analyzer measurements reflect similarly dramatic alterations in the plasma density and ion energies. These experiments will be described and changes in the fields and plasma properties will be discussed. ORNL is managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract no. DE-AC05-96OR22464.
PS-WeP-3 New Cross Beam Ionizer Design for Plasma Monitoring
R. Pedder (ABB Extrel, Quadrupole Mass Spectrometry)
In order to accurately characterize a plasma , you need to obtain a representative sample. Sampling the plasma as a molecular beam through rapid pressure reduction immediately after ions, radicals and neutrals leave the plasma, allows for an unadulterated snapshot of the plasma chemistry. With molecular beam sampling, you are looking directly at the chemical soup which comprises the plasma. This includes noise sources such as photons, electrons, metastable neutrals, and even particulates in addition to the ions, neutrals and radicals to be monitored. A cross beam ionizer allows for the separation of the analyte signal, which is comprised of plasma ions and ions created in the ionizer from plasma molecules and neutrals, from these sources of noise. We have developed a new high sensitivity cross beam ionizer with integral energy analyzer which offers a dramatic increase in signal-to-noise over axial conventional ionizers, while protecting the quadrupole analyzer and detector from direct bombardment with the reactive species common to plasmas used in semiconductor applications. This presentation will include the results of our experimental characterization of this new ionizer.
PS-WeP-4 Lifetime Measurements of Fluorine Atoms in Afterglow CF\sub 4\ Plasmas by Vacuum Ultraviolet Absorption Spectroscopy
K. Sasaki, Y. Kawai, C. Suzuki, K. Kadota (Nagoya University, Japan)
Investigation about the kinetics of F atoms in high- density CF\sub 4\ plasmas generated under low gas pressures is of great importance to optimize discharge conditions for obtaining high etching selectivity for SiO\sub 2\ over Si. This paper reports lifetime measurements of ground-state F atoms in afterglow CF\sub 4\ plasmas excited by helicon-wave discharges. Vacuum ultraviolet (VUV) absorption spectroscopy was adopted for the lifetime measurements. This method employed an ECR CF\sub 4\ plasma device as a VUV light source to construct a windowless spectroscopy system. A high electron density greater than 10\super 12\ cm\super -3\ was obtained for a CF\sub 4\ gas pressure of 2.5 mTorr and an rf power of 1 kW. The F atom density in the discharge plasma was also on the order of 10\super 12\ cm\super -3\. The temporal variation of the F atom density in the afterglow had two decay time constants. After the rf power was terminated, the F atom density decreased to 70 % of the peak value within a time of approximately 2 ms. After that, the F atom density decreased exponentially with a long time constant of approximately 20 ms. In the same helicon plasma, the lifetimes of CF and CF\sub 2\ radicals measured by LIF were approximately 0.3 and 4 ms, respectively. Hence a considerable amount of F atoms remained in the CF\sub 4\ gas after the CF\sub x\ radicals disappeared. The time of the first density decay of the F atoms was of the same order as the lifetime of CF\sub 2\. The kinetics of F atoms in the afterglow will be discussed in reference to the rf power and pressure dependences of the decay time constants.
PS-WeP-5 Temperature Dependence of Dissociative Electron Attachment to Halogenated Hydrocarbons
Y. Wang, L. Christophorou, J. Olthoff, R. Van Brunt (National Institute of Standards & Technology)
Most of the gas mixtures currently in use for plasma processing of semiconductors involve halogenated hydrocarbons such as the strongly electronegative gases CCl\sub 4\ and CFCl\sub 3\, the weakly electronegative gas CF\sub 2\Cl\sub 2\ and the very weakly electronegative gases CHF\sub 3\ and CF\sub 4\. In plasmas containing these compounds, molecular dissociation can be regarded as the first step of the etching reactions leading to the formation of volatile etched material. Many dissociation processes are known to occur for these molecules\super 1\. One of these dissociation reactions which is particularly effective for the strongly electronegative hydrocarbons is dissociative electron attachment. Even for weakly electron attaching gases, molecular dissociation via dissociative electron attachment at low energies can be an efficient dissociation process if the gas temperature is higher than ambient. Dissociative electron attachment is known to increase with increasing temperature above room temperature for many such compounds\super 1\. In an effort to provide quantitative information on dissociative attachment for plasma modeling, we have undertaken a systematic study of the effect of temperature on dissociative attachment of etching gases which are weakly electron attaching at room temperature. In this paper, we report our measurements on the increases of the total electron attachment rate constant for CF\sub 2\Cl\sub 2\ with increasing gas temperature from room temperature to about 600K. Total dissociative electron attachment cross sections as a function of electron energy have been obtained at various temperatures and are reported also.1. L. G. Christophorou, "Electron Molecule Interactions and Their Applications", Academic Press, New York, Vol. 1, 1984. Research sponsored in part by the U.S. Air Force Wright Laboratory under contract F33615-96-C-2600 with the University of Tennessee, Knoxville, TN
PS-WeP-6 Ion Temperatures in Inductively Coupled Plasmas
G. Hebner (Sandia National Laboratories)
Atomic chlorine and argon ion temperatures have been measured in inductively coupled plasmas using laser induced fluorescence. Argon ion temperatures were determined by exciting the 3d\super 4\F\sub 7/2\ - 4p\super 4\P\super 0\\sub 5/2\ metastable ion transition at 811.2 nm and monitoring the fluorescence of the 4p\super 4\P\super 0\\sub 5/2\ - 3d\super 4\D\sub 7/2\ transition at 440.1 nm. Chlorine ion temperatures were determined by exciting the \super 4\S\super 0\4p\super 5\P\sub 3\ - 3d\super 5\D\super 0\\sub 4\ metastable ion transition at 542.3 nm and monitoring the fluorescence of the \super 4\S\super 0\4p\super 5\P\sub 3\ - 4s\super 5\S\super 0\\sub 2\ transition at 479.5 nm. Cl ion temperatures were measured in both pure chlorine and argon / chlorine mixtures while the Ar ion temperatures were measured in pure argon discharges. In the center of the plasma, the Ar ion temperature was between 550 and 1000 K for plasma powers between 30 and 240 W and pressures between 4 and 50 mTorr. Over this same operational parameter space, the Cl ion temperature was between 1400 and 3500 K. At the edge of the plasma, the Ar ion temperature increased to between 3000 and 9000 K. Ar ion drift velocity in the radial direction was between 1 x 10\super 5\ and 2 x 10\super 5\ cm/s at the edge of the plasma. Ion temperatures as a function of plasma power, pressure, argon / chlorine ratio and spatial location will be presented. Implications of these measurements on the Ar/Cl\sub 2\ chemistry, plasma electric fields, and the energy transport in the plasma will be discussed. This work was supported by SEMATECH and the United States Department of Energy (DE-AC04-94AL85000).
PS-WeP-7 Diagnostic Measurements of Cl\sub2\ Discharges in an Inductively Coupled GEC Reference Cell Reactor
R. Forrister, S. Radovanov, H. Anderson (University of New Mexico)
Measurements were made using single-pass, FM diode laser absorption spectroscopy and miniature retarding grid ion energy analysis (mini-IEA) to study Cl\sub 2\ dissociation, ion current uniformity and ion energy distributions (IED's) in inductively coupled Cl\sub 2\ discharges. Both bulk and spatially resolved diode laser measurements were performed. The Cl\sub2\ discharge exhibited two characteristic modes. At 20 mTorr and low power (less than 65 W calculated power) the discharge was in a "dim" mode (probably capacitive), whereas for powers greater than 65 W, the discharge exhibited a "bright" inductive mode. In the bright mode, the magnitude of the absorption signal indicates substantial dissociation (>75 %) of Cl\sub 2\ at even the lowest sustaining power of the inductive mode. Spatially resolved diode laser measurements show this response is nearly flat across the entire diameter of the coil diameter, indicating Cl\sub 2\ is largely dissociated throughout the chamber at any inductive powers. This was also verified by actinometry measurements. The mini-IEA measurements showed a strongly nonuniform ion flux at various radial positions across the plasma and a complex IED shape indicative of both light Cl\super +\ and heavy Cl\sub 2\\super+\ ions. The complex shape appears to be related to the different sheath transit times for ions of different mass.
PS-WeP-8 Determination of Electron Density and Temperature using Optical Emission Spectroscopy (OES) and Self-Consistent Modeling in a Nonequilibrium Microwave Plasma
U. Kelkar, M. Gordon (University of Arkansas)
Self-consistent numerical solutions for a microwave CVD reactor are generated by coupling solutions of the Boltzmann equation to a collisional-radiative model (CRM). The modeling procedure emphasizes the importance of considering neutral and charged particle transport processes in a self-consistent manner. The various transport processes considered include one dimensional ground and excited state diffusion and convection. Energy balance calculations were performed to validate the results. Experimentally, bound-excited and free electrons are measured by collecting absolute line and continuum emission, respectively, from the plasma. The numerical simulations are combined with experimental OES measurements to demonstrate the potential of OES as a quantitative diagnostic tool for microwave CVD reactors. The self-consistent procedure is applied to both a pure argon plasma (300 sccm, 5 Torr, 800 W) used for diagnostic purposes and a H\sub 2\/CH\sub 4\ plasma (300 sccm H\sub 2\, 3 sccm CH\sub 4\, 40-70 Torr, 1600-3200 W) used for diamond deposition. For the pure argon plasmas studied, we can infer the electron density and temperature by self-consistently combining the OES measurements with the numerical results. For our operating conditions, it is found that argon excited state diffusion is insignificant. For the H\sub 2\/CH\sub 4\ plasma, it is observed that H diffusion losses significantly affect the H mole fraction predictions. Also, diffusion was found to be an important loss mechanism for atomic hydrogen's first excited state. Of greater significance, no self-consistent predictions of electron density and temperature were obtained. A meaningful interpretation, however, is possible if the electron-H\sub 2\ cross-section, used for calculating electron-neutral free-free continuum emission, is increased by a factor of about five above the momentum cross-section value. We believe that such an increase is justified because of enhanced energy exchange in electron-molecule interactions.
PS-WeP-9 Development and Measurement of Hyperthermal Neutral Beams
Z. Wang, M. Goeckner, S. Cohen (Princeton Plasma Physics Lab)
There are many new applications for high intensity hyperthermal neutral beams which have energies in the 5 to 100 eV range. These include low-Earth-orbit simulation, charge- and UV-free material processing, beam-enhanced surface catalysis studies and deposition of thin film and coatings. Our way of producing the beam is through the wall neutralization of ions generated in a plasma by the absorption of microwave power at the electron cyclotron resonance. A numerical code that calculates the neutral flux was developed and used to study the size and geometrical effects of the cavity on the beam flux. Neutral beam flux is inversely proportional to the distance between the target and the neutralization wall. Total neutral flux of about 1% of the plasma current can be obtained with a variation of \+-\5% across the 10-cm-diameter target. Microwave power absorption in the plasma was shown to have direct effect on the beam intensity. Studies for different gases under different pressures showed that under high pressures (greater than 20 mT), the microwave absorption is fundamentally different from the absorption at low pressures (order of 1 mT, dependent on gas species). More than 80% of wave input power can be absorbed in the low pressure regime. At lower pressure, the discharge quenches. Measurement of the beam energy and density is critical to the research. The beam atoms are excited by electron beam with variable electron energies, 10 to 100 eV. Optical emission have been detected for different gas pressure as well as electron beam current. Thus far the experiment of the beam detection has been operated at high gas pressure (10 mT).
PS-WeP-10 Formation of a Steep Electron Temperature Gradient and Radiation Zone in a High Density Linear Plasma Device
J. Park, T. Bennett, M. Goeckner, S. Cohen (Princeton Plasma Physics Lab)
Plasma-neutral interaction is studied in a linear plasma device utilizing lower-hybrid-wave heating at 2.45GHz at a magnetic field strength of 4kG. The plasma column is about 2 cm diam. and extends 25-40 cm following a uniform magnetic field before being terminated at a material target. Input power coupled to the plasma can be varied between 200W to 2kW with neutral pressure between 1 mT and 500 mT, depending on the gas species. It is found that the axial plasma particle and energy transport to the target changes dramatically with the neutral pressure. In a 60mT He discharge, the Te profile is close to isothermal, 3-5 eV, along the magnetic field. When the neutral pressure exceeds 120 mT, T\sub e\ exhibits a steep drop, from 3 eV to 0.5 eV over 2-3 cm, well before the target. The density profile is relatively flat for 3-5 cm after the temperature drop but then decreases toward the target by factor of about 5, from 3x10\super 13\ cm\super -3\ to 6x10\super 12\ cm\super -3\. When this "detachment" occurs, particle and heat flux to the target drops exponentially and an intense radiation zone (an increase of 10-100 in emission) appears in the cold region. It is also found that the location of this detachment is rather independent of the target position. Similar results have been obtained with other noble gases, like Ne and Ar. This result shows the complexity of plasma-neutral interactions. Attempts to simulate this result with current 2D fluid codes have been unsatisfactory. This indicates a need for a better understanding of these processes.
PS-WeP-11 DSMC-PIC Simulation of Chlorine Plasma Etch Reactor
G. Font, I. Boyd (Cornell University)
While simulating plasma etch reactors, it is often necessary to make approximations about surface chemistry processes due to the lack of definitive experimental measurements. One such process, surface recombination of atoms into molecules, is explored in this study. The value of the surface recombination probability is varied and the effect on the plasma flow inside a helicon type reactor is explored. The simulation is carried out for chlorine (Cl\sub 2\) flow using direct-simulation- Monte-Carlo (DSMC) and Particle-in-Cell (PIC) methods. Computations for the gas discharge are carried out modeling the ions and neutrals as particles and imposing the electrons as a background conforming to experimental measurements. The resulting plasma is found to be composed primarily of chlorine atoms (Cl) and ions (Cl\super +\). The recombination coefficient affects the electronegativity of the plasma by making the walls act as a source of chlorine molecules (Cl\sub 2\) which can then produce more negative ions (Cl\super -\). In the study, an effort is also made to quantify the sensitivity of the flow field to the wafer etch chemistry approximations.
PS-WeP-12 A Two Dimensional Modeling of Capacitively Coupled Plasma
N. Nakano, T. Makabe (Keio University, Japan)
2D modeling of nonequilibrium plasma is highly demanded to design the processings. We have developed a two dimensional modeling based on the relaxation continuum(RCT) model for capacitively coupled plasma(CCP). In present work, 2D time-dependent plasma structures with an asymmetric electrode geometry, ion flux distributions, and effects of selfbias are investigated under a pressure range from 50m - 1Torr with Ar. 2D profiles of the net excitation rate are compared with the results by the tomography using optical emission spectroscopy[1], and both results are in preferable agreement. Non uniform plasma structures including a radial peak, are caused by the existence of radial electric field at the electrode gap.A 2D-ion flux distribution provides an important information for the surface uniformity control of plasma processings. The ion distribution of the dependence on the supplied voltage between 30 to 150V at 0.5Torr, is discussed. At the minimum sustaining condition of 30V, plasma distribution has no radial peak controlled by the diffusion. Hence, the flux is uniform inside the electrode. With increasing the voltage, the radial peak of ion flux appears and is strengthened. Over 100V, the axial field component is much higher than the radial one, even near the rf electrode edge, and the uniformity of the ion flux is recovered again. 2D-modeling is one of the powerful tool to understand and design dry processes using a nonequilibrium plasma. --- [1] T. Kitajima, M. Izawa, R. Hashido, N. Nakano and T. Makabe, Appl. Phys. Lett. (accepted)
PS-WeP-13 Modeling the Chemical Downstream Etch Reactor
B. Lane (Plasma Dynamics); E. Hyman, A. Drobot (Science Applications International Corporation)
The chemical downstream etch (CDE) reactor is a commonly used plasma etching tool with applications to etching resist, using oxygen as the etchant, and to etching silicon, using fluorine as the etchant, among others. An important design issue for such reactors is maximizing the flux of radicals while minimizing thermal stresses to the dielectric tube containing the plasma. These stresses often originate from localized, high density plasma concentrations and their geometry is determined by reactor geometry, microwave power, and pressure. Because current reactor designs are not axisymmetric, a full 3D model is required. We have been developing a 3D model that will simulate the microwave generated electromagnetic field and plasma, the hydrodynamics and heat transfer, and the chemistry in a realistically configured CDE reactor. The goal is to come to an understanding as to which parameters are critical in the etching process, as an aid in the design of new reactors needed as substrate dimension is increased, and to understand the 3D plasma structures that form in current reactors. The code uses a structured grid and incorporates a block structure with separate blocks for the plasma and process chambers. We will present a description of the code and our most recent results.
PS-WeP-14 Neutral and Ion Temperature in the Vicinity of a Tungsten Limiter in a Plasma Confinement Machine
S. Sekine, Y. Hirano, T. Shimada, Y. Yagi (Electrotechnical Laboratory, Japan)
Research of the wall materials of plasma confinement machines is important to develop a future nuclear fusion reactor. In order to sustain a long discharge period of high temperature plasma, the possibility of the use of high Z materials (i.e. metals) should be examined instead of widely-used low Z materials. In our TPE-1RM20 reversed-field-pinch machine, molybdenum limiters are used to protect stainless-steel vacuum vessel. The strong interaction between plasma and limiters is apparent from the observation of molybdenum emission lines. For the evaluation of plasma-facing materials, measurements of plasma parameters in the edge region are necessary in addition to those in core plasma. In this study, polycrystalline tungsten was used for a limiter. Tungsten is a promising candidate for plasma-facing parts such as limiter and diverter because of its high melting point. We observed neutral and ion temperatures in the vicinity of the tungsten limiter by varying the heat load to the tungsten surface. Those temperatures were derived from atomic and ionic emission lines of discharge gas. A 1m double monochrometer was used to observe the Doppler broadening of the spectral lines. To avoid the effect of the Stark broadening, helium was used as filling gas in this special experimental campaign. Surface temperature of tungsten was also measured using a far-infrared temperature monitor. The results were compared with those for molybdenum.
PS-WeP-16 Characterization of Wall Conditions in DIII-D
K. Holtrop, G. Jackson, A. Kellman, R. Lee, W. West (General Atomics); R. Wood (Lawrence Livermore National Laboratory)
Wall conditioning in DIII-D is one of the most important factors in achieving reproducible high confinement discharges. For example, the very high confinement mode (VH-mode)was only discovered in DIII-D after initiating boronization, a CVD technique to deposit a thin boron film over the entire surface of the tokamak. In order to evaluate wall conditions and provide a database to correlate these wall conditions with tokamak discharge performance, a series of nominally identical reference VH-mode discharges (1.6 MA, 2.1T, double-null diverted) were taken at various times during a series of experimental run periods with evolving wall conditions. These reference discharges have allowed a quantitative determination of how the wall conditions have evolved. For instance, core carbon and oxygen levels on the VH-mode phase remained at historically low levels during the last run year and there was also a steady decrease in the oxygen levels at plasma initiation during this period. We will discuss the long term changes in low Z impurities and the effect of wall conditioning techniques such as boronization and baking on these impurities. In addition, the evolution of the deuterium recycling rate will be presented.
PS-WeP-17 Ion Energy and Angular Distribution Measurements in Inductively Driven Discharges Containing Mixtures of Argon and Chlorine
J. Woodworth, M. Riley, B. Aragon (Sandia National Laboratories); T. Hamilton (Applied Physics International)
We are measuring fluxes, energies and angular distributions of positive ions striking the grounded electrode in discharges containing mixtures of argon and chlorine. The measurements are being made in a Gaseous Electronics Conference Reference Cell which has been modified to produce inductively driven discharges. Ion fluxes as well as energy and angular distributions are measured with a hemispherical gridded energy analyzer which is located behind a differentially-pumped pinhole in the center of the 6"-diam. grounded electrode. Total ion fluxes, mean ion energies and the FWHM of the ion energy distributions all decrease as chlorine is added to the discharge. For 20 millitorr total pressure and 160 Watts of rf power dissipated in the plasma, we see ion fluxes in pure Ar, 50-50 Ar-Cl\sub 2\ mixes, and pure Cl\sub2\ of 20, 7, and 7 mAcm\super -2\ respectively. Mean ion energies for these mixtures are 13 eV, 10.5 eV and 10.0 eV respectively. Fluxes, energies and angular distributions also become less dependent on pressure as the Cl\sub2\ fraction is increased. At 20 mTorr and 160 W, the ion angular distributions are essentially independent of Ar-Cl\sub2\ mix ratio, with 63% of the ions being contained within a cone with a half-angle of 6.5 degrees. Details of the apparatus and results will be presented.
PS-WeP-18 Fabrication of Cross-linked Polymer Shells for ICF Experiments
U. Kubo, H. Nakano (Kinki University, Japan); H. Kim (Kwangju Institute of Science and Technology, Korea)
Polymer shells having good fuel retention capability are desirable in the ICF experiments. Polyvinyl alcohol (PVA) has excellent barrier property to gas permeation but it is easily degraded by beta-decay of tritium, a comp onent of the ICF fuel gas. We fabricated plastic shells consisting of PVA an d acrylonitrile (AS) which retain barrier property of PVA and resistance to the beta-decay. Such shells were fabricated by the standard emulsion method. The first water phase contained 3wt% of PVA and 1wt% surfactant. The oil pha se was made of isophthaloyl chloride and trichloroethane. The second water p hase had the identical composition as that of the first water phase. The pre viously made AS shells were immersed in the first water phase. This mixture was poured into the oil phase while stirring. This system was then poured in to the second water phase to make water/oil/water emulsion. The interfacial polycondensation reaction took place between water and oil phases. The resul ting AS shells were covered with cross-linked PVA. Upon drying, the cross-li nked PVA overcoated AS shells were produced. Permeation properties of the sh ells and detail of the fabrication method will be presented.
Time Period WeP Sessions | Topic PS Sessions | Time Periods | Topics | AVS1996 Schedule