Papers

AVS1997 Session PS1-TuM: Novel Plasma Sources & Systems

Tuesday, October 21, 1997 8:20 AM in Room A7/8

Tuesday Morning

Start	Invited?	Item
8:20 AM		PS1-TuM-1 Magnetically Neutral Loop Discharged Plasma Sources and Systems T. Uchida (ULVAC Japan, Ltd.) Up until 1993, no one had tried to generate a doughnut- shaped plasma along a magnetically neutral closed loop, nor utilized such a plasma source in an application. The magnetically neutral loop discharged (NLD) plasma was applied first for a processing use with a machine composed of three steady current coils and an rf induction coil. This demonstrated the NLD plasma is much more useful than ICP for Si/SiO₂ etch processes using Ar,C₃F₈,C₄F₈ and H₂ gases. A remarkable feature of the NLD plasma method is that a high density plasma with a doughnut-shaped distribution in any size on both radii of the major and minor axes can be generated at any distance apart from wafer. Second, the NLD high density plasma is generated in at rather low gas pressure range of 10^-2Pa with the characteristic that when a stronger rf induction power is supplied, thermalization of the electrons is easier. Sufficiently high etch rates with high selectivity were realized in an uniform manner by controlling the characteristics of the NLD plasma. Furthermore, really vertical etching was also achieved. The NLD is based on an unusual mechanism of power absorption that is effective in low pressure regimes. In the NL region, an electron shows so called meandering motion. When an RF electric field is applied, the meandering motion becomes chaotic, and effective heating occurs. The theory predicts that the effective resistance becomes 10-10² times larger than the classical collisional resistance. Under an appropriately selected set of parameters, the effective collision induced by the chaos is comparable to that in a gas of 0.1Pa. These estimates are consistent with experimental data. In conclusion, the NLD system opens a new region of plasma processing because of its versatile capability for being used to advantage in the age of 12"φ substrates.
9:00 AM		PS1-TuM-3 Diagnostic and Simulation Study on the Relation between Electron Energy Distribution and Etching Characteristics of Ultrahigh-Frequency Plasmas T. Nakano, H. Akashi (National Defense Academy, Japan); S. Samukawa (NEC Corporation, Japan) Ultrahigh-frequency (UHF) plasmas have drawn attention in fabrication of ultra-large-scale-integration (ULSI) devices in terms of precise patterning capability of fine-line features and high selectivity. Recently, it has also been found that the UHF plasmas possess a wide process window for highly selective etching of poly-Si, which is attributable to pressure-independent ion saturation current in the UHF plasmas. Since discharge frequency (500 MHz) is even higher than electron collision frequency in the UHF plasmas, electron energy distribution function (EEDF) is expected to be insensitive to pressure. Monte Carlo simulation of EEDF's in homogeneous infinite plasmas through Ar and Cl₂ confirms the pressure-independent feature of the EEDF's in the plasmas produced by 500-MHz UHF power. In contrast, the high energy tails of the EEDF's in RF (13.56 MHz) plasmas diminish significantly with an increase in pressure. The simulation also shows that the high electric field strength is needed to sustain the UHF plasmas. Electrons oscillate by the high field and gradually gain energy through elastic collisions after a number of UHF periods. Thus, the electrons which cause ionization are expected to spend a long time where the field strength is high. This suggests that UHF power deposition takes place in the high field region which is believed to appear off the symmetry axis of the plasma source, judging from the structure of the antenna used for UHF power introduction. Laser-induced fluorescence measurement indicates that metastable chlorine ion density becomes higher off axis than in the center of the source. Since production of the metastable ions from the ground-state ions requires higher energy than ionization potential of chlorine, the radial density distribution of the metastable ions indicates that the power deposition has a strong correlation with the UHF field distribution and occurs more off axis for the plasma source used in this work.
9:20 AM		PS1-TuM-4 ECR Plasma Source for 12-inch Wafer Etching Using Slant Slot Antenna H. Tamura, M. Furuse, M. Sumiya (Hitachi Ltd., Japan); S. Watanabe (Hitachi, Ltd., Japan) In ULSI manufacturing, the production process becomes more complicated and the chip size becomes larger as the degree of integration becomes higher. To avoid a reduction in productivity, the wafer size has become larger. In next-generation ULSI manufacturing systems, 12-inch wafers will be used to improve productivity. We have developed a new ECR plasma source for etching 12-inch wafers. The main characteristics of the plasma source are (1)microwave distribution stabilization by cavity resonator and (2) microwave distribution control by a slant slot antenna. We achieved 6.2mA/cm2$B!^(J 5.8% (Cl2,5mTorr, on the electrode) over the 12-inch wafer region using this plasma source. Microwaves with a frequency of 2.45 GHz are introduced into a cavity resonator through a coaxial waveguide. The cavity resonator is resonated at the height of half a wavelength of the TM01 mode, which is the lowest mode of the axially symmetric modes in a cylindrical cavity. The cavity resonator is loaded by the slant slot antenna. Adjusting the slot pattern, the electromagnetic field can be controlled because the electric field is perpendicular to the slot direction. Ring shaped power distribution like the TE01 mode in circular waveguide is advantageous to achieve high uniformity. Slanting the slot antenna, azimuthal electric field component can be generated. As a result, TE01 mode can be generated because the TE01 mode has only an azimuthal electric field component. This plasma source can control plasma distribution near the wafer from mild convex to mild concave distribution- by adjusting the slot antenna pattern and the static magnetic field. And the distribution is maintained when pressure and microwave power change.
9:40 AM		PS1-TuM-5 Large Area High Density Plasma Excitation Using Standing Surface Waves I.P. Ghanashev, M. Nagatsu, H. Sugai (Nagoya University, Japan) Plasma processing of large flat surfaces requires low pressure high density (n_e=10¹¹ - 10¹²cm^-3) plasmas with uniform plasma density distribution near to the processed surface. Microwave discharges may provide a valuable alternative to the inductively coupled plasmas widely applied now for this purpose. Recently we proposed a new plasma source in which the plasma is sustained by a standing surface wave propagating radially and azimuthally along the interface between the plasma and a dielectric plate located at the top wall of a large-diameter cylindrical metal chamber ¹. The wave is launched by a pair of slot antennas cut in the top chamber wall above the dielectric plate. Here we present new experimental results at lower pressures (down to 5 mTorr) and in a non-noble reactive gas (CF₄) demonstrating the applicability of the new source for dry etching. The plasma density was 3-10 times lower than the one observed by previous experiments at 0.2-1 Torr in Ar. We present an eigen-mode analysis suggesting that this lower density should cause the appearance of eigen-modes with an axial mode number s=1, which, in contrast to the case s=0 observed previously ² at higher plasma densities, are evanescent along the chamber axis only in the plasma but not in the dielectric region. ¹M. Nagatsu, G. Xu, M. Yamage, M. Kanoh and H. Sugai, Jpn. J. Appl. Phys., vol. 35 (1996) p. L341. ²I. Ghanashev, M. Nagatsu and H. Sugai, Jpn. J. Appl. Phys., vol. 36 (1997) p. 337.
10:00 AM		PS1-TuM-6 Investigations of 3D Asymmetries in rf Biased Inductively Coupled Plasma Tools Using a New Ambipolar Acceleration Technique¹ E.R. Keiter, M.J. Kushner (University of Illinois, Urbana) Proposals have recently been made to scale plasma equipment to larger wafers by having multiple plasma sources. Long time scale phenomena, such as neutral transport, in plasma equipment having such designs, has important implications in determining processing uniformity, particularly when radical production and pumping are azimuthally asymmetic. The application of 3-dimensional plasma equipment models to investigating these issues has been limited due to lack of sufficient computing resources. In this paper, we present results from a 3-d version of the Hybrid Plasma Equipment Model (HPEM)² with which we investigate the consequences of multiple plasma sources and asymmetric gas pumping on the uniformity of reactants in rf biased inductively coupled plasma tools. To address long time scale phenomena, a new acceleration technique has been developed to quickly approximate the steady state neutral flows in the reactor under plasma conditions. In this method, the implicit solution of Poisson's equation is temporarily replaced with a solution for the electrostatic potential based on ambipolarity coupled to 1-d sheath model which lines the inside surfaces of the tool. Results from HPEM-3D will be used to develop scaling laws for reactant uniformity to the wafer with asymmetric radical sources and pumping. ¹Work supported by SRC, NSF, DARPA and the University of Wisconsin ERC for Plasma Aided Manufacturing. ²M.J. Kushner, et. al. J. Appl. Phys. 80, 1337 (1996).
10:20 AM		PS1-TuM-7 Simulations of Particle Trapping Produced by 3-Dimensional Structures in Plasma Tools¹ M.J. Kushner, E.R. Keiter (University of Illinois, Urbana); H.H. Hwang (Thermosciences Institute, NASA Ames Research Center) Particle traps in plasma equipment most commonly result from the ion drag and electrostatic forces being in near equilibrium. The large ion drag forces in high plasma density reactors typically exceed the electrostatic forces, thereby preventing particle trapping. To the degree that 3-dimensional structures in plasma equipment perturb the plasma potential and ion fluxes, these structures can produce particle traps which would not otherwise exist. To investigate these processes a 3-dimensional particle tracking model, the Dust Transport Simulator (DTS-3D), has been developed. DTS-3D obtains plasma properties (electric fields, charged and neutral particle fluxes and temperatures) from a 3-dimensional version of the Hybrid Plasma Equipment Model (HPEM-3D). DTS-3D then integrates the trajectories of dust particles while considering ion drag, electrostatic, thermophoretic, and fluid drag forces. The propensity to form particle traps in RIE and ICP tools as produced by 3-dimensional structures such as gas injection nozzles and load-lock bays will be discussed. We found, for example, that perturbations of the plasma potential by nozzles can extend many centimeters thereby generating particle traps far from the structure. ¹Work supported by SRC, NSF, DARPA and the University of Wisconsin ERC for Plasma Aided Manufacturing.
10:40 AM		PS1-TuM-8 Control of Capacitive Currents in Inductively Coupled Plasma Sources for Uniformity Optimization L.A. Berry, R.H. Goulding, J.B.O. Caughman (Oak Ridge National Laboratory) Processing of 300 mm wafers requires larger plasma sources that operate at higher power levels. This change results in increased capacitive currents and thus more current variation in antennas due to transmission line effects. While the current variations are 3-D, transmission line parameters can be estimated via 2-D calculations of mutual inductances and capacitances¹ and used to calculate the self-consistent radial and azimuthal variations in coil currents. In addition to an understanding of the detrimental effects of capacitive coupling on uniformity, this approach also provides the means for evaluating a variety of techniques to improve plasma uniformity. With respect to azimuthal uniformity, capacitive terminations have been previously used to modify current variations along a coil as well as the accompanying azimuthal current asymmetries². However, there are limits to the improvements that can be made with a reactive termination. We show that a resistive termination results in significantly improved azimuthal uniformity, although with drastically reduced rf efficiency. The efficiency can be restored by using a unidirectional resonant system. Similar estimates of azimuthal uniformity show the benefits of parallel coils, as has been seen experimentally³. Using the transmission line model, we have also explored the extent to which radial unformity can be affected. With a shorted termination, currents peak towards the shorted end of the coil, while with an open, they peak at the beginning. With azimuthal uniformity held at acceptable levels by using two or more parallel antennas, the termination can be varied from near shorted to near open with a variable capacitor termination resulting in a variation of the radial power deposition. In some cases, capacitive loading of individual turns is needed to produce the desired transmission line parameters. With suitable sensors, real time control of plasma uniformity is possible. Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U. S. Department of Energy under Contract No. DE-AC05-96OR22464. ¹E. F. Jaeger, et al., Phys. Plasmas 2(6), 2597 (1995). ²M, J. Kushner, et al., J. Appl. Phys. 80(3), 1337 (1996) and references therein. ³M. S. Barnes, private communication.
11:00 AM		PS1-TuM-9 Coupling Between a Planar Geometry ICP Coil and an rf Bias Platten A.R. Ellingboe, R. Benjamin (Lawrence Livermore National Laboratory) A recent developpment in plasma processing of materials is the use of a plasma source in conjunction with rf substrate bias to give independent control of ion flux and an ion's energy arriving at the substrate. This work investigates the coupling between a planar inductive source and an rf powered substrate platten. With the power to both the source and the platten fixed, and changing only the rf phase between the source and platten power supplies a 25% modulation in plasma density is measured. The rf magnetic field in the plasma from the source coil is found to modulate in amplitude by 25%, in agreement with the measured density variation. The electron energy distribution function is also found to vary considerably with the relative phase. These results prove that the ICP coil and bias platten are coupled, and not independant. How this coupling affects process optimization and field service reliability will be discussed.
11:20 AM		PS1-TuM-10 Characterization of a Novel Lambda Balanced Inductive Plasma Source G.K. Vinogradov, V.M. Menagarishvili, S. Yoneyama (MC Electronics Co., Japan) Inductive plasma sources are widely spreading in the last decade as high plasma density etching/deposition tools. They have some drawbacks which couldn't be overcome until recently. The main of them are: capacitive coupling inductor-plasma-wafer, azimuthal discharge asymmetry, and pressure/power instabilities. A novel internally balanced inductive plasma source has been developed. It operates in a Full Wave Helical Resonator (λ-R) mode and represent the first RF plasma source utilizing standing wave patterns in order to overcome well known drawbacks and extend the applicability of inductive plasma sources for a damage free wide area, especially 300 mm wafers, processing. This source has not yet been well characterized. We used several diagnostic tools to characterize the plasma source and diagnose discharge characteristics in the source area, in the vicinity of the processing wafers and on the surface. Several kinds of single Langmuir RF compensated probes, flat wall probes, optical emission spectroscopy, and thermal probes were used to characterize the spatial discharge structure and diagnose plasma characteristics. Argon and oxygen discharges at 0.1-2 Torr pressure were studied. A plasma potential in 1-4.5 kW oxygen discharges is about 15 V. There is only a decaying downstream plasma near the wafer which is separated from the bottom of the plasma source by only about 100 mm distance. The wafer surface floating potential is about 3-6 V and does not depend on the radial coordinate within about ± 0.5 V. Process data on the damage free highly uniform super fast normal and heavily implanted resist ashing on 8 and 12 wafers will be also presented for 9 and 13 ID plasma sources.
11:40 AM		PS1-TuM-11 Two Dimensional Modeling of Two-Frequency-Operated-Plasma in Ar by Relaxation Continuum Model N. Nakano, T. Makabe (Keio University, Japan) A capacitively coupled plasma (CCP) by two-frequency operation have been developed for various plasma processings. This reactor has the advantage of new function to control the processes, even though it has a simple geometrical structure with conventional parallel plate electrodes. The specific feature is that a different frequency is applied to each of electrodes. Radio frequencies from high frequency(HF) to very high frequency(VHF) are used for plasma source. VHF is appropriate for high density plasma production at low pressure. On the other hand, low frequency(LF) source is used for ion acceleration to the wafer set on the biased electrode for processings. The incident ion energy is effectively controlled by LF with period longer than an ion transit time, and the amplitude of LF is required to be higher than that of VHF to make the sheath for the acceleration of ions. In this work CCP by two frequency operation is two dimensionally modeled using the relaxation continuum(RCT) model. The structure and function of the plasma driven by two frequency is discussed. VHF plasma make the high density plasma asymmetry with the peak near the driving electrodes. LF affects not only the ion flux to the bias electrode but also the plasma density. When LF has a sin waveform, at the first half period a high sheath potential appeared in front of VHF electrode and cause large production rate. At the second half period, high field in front of LF and high energy ion flux to the wafer is realized. This type of plasma reactor is effective to sputter deposition and etching processes.