Papers

AVS1996 Session PS1-MoM: High Density Plasmas I

Monday, October 14, 1996 8:20 AM in Room 201C

Monday Morning

Start	Invited?	Item
8:20 AM		PS1-MoM-1 Langmuir Probe Measurements in a Planar Inductive Coupled RF Discharge T. Ni, W. Collison, M. Barnes, J. Holland (Lam Research Corporation) An rf compensated cylindrical Langmuir probe was used to characterize the Transformer Coupled Plasma (TCP) in an etch chamber which is capable of processing substrates up to 300 mm in diameter. The Langmuir probe measurements reveal the spatial profiles of several plasma parameters including electron density, plasma potential, floating potential and electron temperature. In this study, we investigated the effects of gas pressure, TCP rf power and chamber geometry on plasma properties. The results show that high density plasma (10\super 10\ - 10\super 12\/cm\super 3\) with excellent uniformity near the wafer surface can be achieved in this chamber using TCP source. The electron energy distribution functions (EEDFs) from the analysis of the voltage-current traces using the Druyvesteyn theory were also obtained for rf discharge in Ar, N\sub 2\, Cl\sub 2\ and HBr. The EEDFs for Cl\sub 2\ and HBr are found to be near Maxwellian. Ar plasma and N\sub 2\ plasma exhibit dual temperature behavior. In order to gain insight of the rf discharge in these gases, we have performed 2-dimensional simulations using the Hybrid Plasma Equipment Model (HPEM). A general agreement between the 2-D HPEM simulation and the results from the Langmuir probe measurements was obtained.
8:40 AM		PS1-MoM-2 Plasma Uniformity Study in a 300mm Inductively Coupled Oxide Etch Tool with Diagnostics and Simulations V. Vahedi, A. Lamm, A. Perry, T. Wicker (Lam Research Corporation) Simulations are used along with Langmuir probe measurements to investigate the plasma uniformity across a 300mm wafer in an inductively coupled oxide etch tool. The ion saturation measurements are collected with cylindrical and planer probes in Ar and CHF\sub 3\ plasmas over 200mm and 300mm wafers. Laframboise's analysis is used to determine the magnitude of the ion flux and the approximate plasma potential. In both Ar and CHF\sub 3\ plasmas, the ion flux profile over 200mm and 300mm wafers quantitatively compare very well with results from the simulation code HPEM from the university of Illinois. Both simulation and measurements show ion fluxes on the order of 10-20 mA/cm\super 2\ and better than 6% 3\sigma\ uniformity in CHF\sub 3\ over a 200mm wafer. The simulation results suggest that voltage and cuurent distributions along the the antenna significantly affect the power deposition profile and hence ion flux uniformity over the wafer. We will present simulation results and measured data on the effect of liner materials, aspect ratio, gas flow and antenna configurations on ion flux uniformity, plasma composition, and plasma potential.
9:00 AM		PS1-MoM-3 Assessment of Coil Design and Internal Structures on Ion Flux Uniformity using a 3-Dimensional Model of Inductively Coupled Plasma Tools M. Kushner (University of Illinois, Urbana-Champaign); M. Grapperhaus (University of Illinois, Urbana); W. Collison, J. Holland, M. Barnes (Lam Research Corp.) During development of Inductively Coupled Plasma (ICP) etching tools it is not uncommon to observe side-to-side or azimuthally asymmetric etch properties. These observations have been attributed to asymmetries in pumping, reactor structure and coil properties. A 3-dimensional computer model of ICP tools has been developed to investigate these issues. The model is a 3-d extension of the previously described 2-d Hybrid Plasma Equipment Model (HPEM). In HPEM-3D, coil currents are resolved using a transmission line model embedded within solution of Maxwell's equations for the inductive field. Electron impact rates are resolved using an electron energy equation and Boltzmann solution. In the fluid module, continuity and momentum equations are solved for neutrals and ions. Results of the model with comparison to experiments for Cl\sub 2\ plasmas show that azimuthal asymmetries in the ion flux and etch rates can be attributed to both the shape and termination of the coil which produce azimuthal hot-spots in the inductive electric field. This produces local maxima in the ionization rate which persists to the plane of the wafer. The effect of internal structures such as gas nozzles on the uniformity of the ion flux to the wafer will also be assessed. We find that these structures can produce shadows in the ion flux to the wafer if they are placed between the ion source and the substrate.\super *\Work supported by SRC, NSF, LAM Research, SNLA/Sematech and U of Wisconsin ERC.
9:40 AM		PS1-MoM-5 Numerical Modeling of Neutral Gases in High Density Plasmas P. Canupp, R. MacCormack (Stanford University); R. Brinkmann (Siemens AG, Germany); J. McVittie (Stanford University) High density, low pressure plasmas have been proposed for large (300mm) wafer processing due to low sheath voltages and highly directional ion flux to the wafer surface. However, under these conditions, the neutral gas component of the plasma can exhibit significant variations through the reactor. Since neutral gas radicals play a critical role in etching, neutral gas dynamics can affect process uniformity and should be studied when designing new high density plasma reactors. This paper reports results of a numerical simulation of chemically reacting neutral gases in an inductively coupled plasma etch reactor. An efficient, implicit numerical method is developed that simulates the neutral gas flow using a fluid approach. As a result of the enhanced numerical stability of the scheme, large time steps can be used to advance the solution from initial conditions to a final steady state in fewer iterations than simpler explicit methods. This implicit technique effectively overcomes temporal stiffness that arises due to the highly diffusive flow associated with low reactor pressures. Results in this paper are shown for the case of a constant background plasma. In the future, this neutral gas model will be implemented in a large scale plasma simulation tool for complete, self-consistent reactor simulation.
10:00 AM		PS1-MoM-6 Robot Assisted Three Dimensional Optical Emission Tomography of Inductively Coupled Plasma A. Okigawa, M. Tadokoro, N. Nakano, A. Itoh, T. Makabe (Keio University, Japan) Inductively coupled plasma(ICP) in low pressures has been widely developed as a source of a high density, low damage, high uniformity required for industrial applications in microelectronics.We have developed a robot assisted optical emission tomography system [1] to measure a 3-D profile of the production rate in an ICP reactor and to predict the plasma structure by an optical sensor attached to four-axis industrial robot capable of measuring the line integral of the emission. In this work, a typical ICP is maintained in a coaxial quartz cylinder 10 cm in diameter and 20 cm in height by a one-turn current coil driven at 13.56 MHz in Ar. Short lived-excited states Ar(3p\sub 5\) with radiative lifetime, 90 ns, is used as the probe of electrons with energy greater than 14.57 eV. The profile of excitation rate to Ar(3p\sub 5\) is reconstructed from a series of measurements of line integrals by utilizing the algebraic reconstruction techniques. The diagnostics are performed for pressure of 10 mTorr - 1 Torr, power of 50 W - 500 W, and flow rate of 10 sccm - 100 sccm.An azimuthal asymmetry of the net excitation is found from the present sliced-images. The result is caused by the lack of the azimuthal field between the current feed terminals. With increasing the power and pressure, a ratio of the net excitation rate in the vicinity of the quartz wall increase and the profile become more asymmetry.[1]A.Okigawa, T.Makabe, T.Shibagaki, N.Nakano, Z.Lj.Petrovi\aa c\, T.Kogawa, A.Itoh, Jpn.J.Appl.Phys. Vol.35 (1996) pp.1890-1893
10:20 AM		PS1-MoM-7 Determination of Plasma Electron Temperatures by Trace Multiple Rare Gas Optical Emission, and Applications of Advanced Actinometry in High-Density Plasmas V. Donnelly (Bell Laboratories) Small amounts (1% each) of He, Ne, Ar, Kr, and Xe were added to a high-density, helical resonator Cl\sub 2\ plasma, and optical emission spectra were recorded in the 700 - 900 nm region. Emission intensities for roughly 20 levels of Ar, Kr and Xe were determined. This emission effectively samples the high-energy tail of the electron energy distribution between 10 and 20 eV. The observed emission data were then compared with predictions from a model that includes both direct electron impact excitation of the Paschen 2p\sub x\ (x=1-10) levels from the ground state, and step-wise electron impact excitation of the 2p\sub x\ levels from metastable levels that are populated by electron impact excitation from the ground state. For an electron density of 1 x 10\sup 11\cm\sup -3\, the predicted metastable number densities are only 10\sup -4\ to 10\sup -5\ that of the ground state. The reported peak cross sections for excitation from the metastable levels are typically several hundred times those for excitation from the ground state, however, and maximize at lower energy. Consequently, two-step excitation of emission through the intermediate metastable levels contributes a substantial portion of the total emission. The electron temperature was determined by comparing observed and computed intensities at various assumed electron temperatures. Reasonable agreement was found between the observed and calculated emission intensities at an electron temperature of ~2 eV at 10 mTorr. The use of electron temperatures determined in this manner, and the role of the two-step excitation process in advanced actinometry will be discussed.
10:40 AM		PS1-MoM-8 Fine Pattern Etching by Parallel Plate Type Reactor in Low Operating Pressure using Plasma Transport from Gas Puff Plasma Source T. Oomori, M. Taki, K. Nishikawa, H. Ootera (Mitsubishi Electric Corporation, Japan) Recent studies of low-pressure, high density plasma sources such as an ECR or an ICP source for fabrication of fine patterns of ULSI have revealed serious problems, e.g., pattern distortion, reactive ion etching lag and unstable process properties, because of charge-up effect and/or plasma-wall interaction. Moreover, in the case of large wafer processing, these sources sometimes induced severe electrical damages to the ULSI devices by lack of uniformity of plasma properties over a wafer such as nonuniformity of RF biasing to the wafer, probably due to no parallel plate electrode opposite to the wafer. In this paper, we present results of our investigation on a parallel plate type reactor with low-pressure, high-density plasmas using plasma transport from a gas puff plasma source\super 1)\ (generation method of pulsed plasma flow employing a nozzle beam system with a plasma source and a high-speed gas puffing valve). In the reactor, chlorine plasmas periodically (normally 2Hz) flowed from a plasma chamber into the region between the parallel plate electrodes (specimen chamber) through a set of 3-30 mm diameter apertures of the upper electrode. The time-average pressure of the plasma chamber was more than 10 mTorr, and that of the specimen chamber was less than 1 mTorr. We also discuss the experimental results of plasma parameters and its etching properties for sub-quarter micrometer patterns. 1) T. Oomori, et. al., Jpn. J. Appl. Phys. 34, 2101(1995).
11:00 AM		PS1-MoM-9 Electron Temperature Control using Mesh Grid Bias and its Application in CF\sub4 \/Ar Plasma J. Hong (Korea Advanced Institute of Science and Technology, Korea); S. Seo, H. Chang (Korea Advanced Institute of Science and Technology) The inductively coupled plasma(ICP) has been used in place of the capacitively coupled plasma(CCP) because of the efficiency in plasma generation. However in ICP, electron temperature(T\sub e\) is higher than that in CCP, and fluorine density(n\sub F\) increases rapidly in CF\sub 4\ and Ar plasma due to more dissociation of CF\sub x\. Too high n\sub F\ causes low slectivity in oxide etching. The T\sub e\ is controlled by biasing mesh grid which is set up between source region and reaction region to controll n\sub F\ in CF\sub 4\ and Ar plasma. T\sub e\ and plasma density(n\sub e\) are measured in Ar plasma by changing external parameters such as mesh grid bias, mesh grid size, Ar pressure, RF power, RF frequency, and pyrex tube diameter. At lower range than -30V in grid bias, 1 mm in grid size, 800 W in RF power, 1 mTorr in pressure, and 18 MHz in RF frequency, T/sub/e decreases from 2.5 eV to 0.6 eV and n\sub e\ also decreases from 310\super 9\ cm\super -3\ to 210/sup/8 cm\super -3\ in the reaction region. The pyrex tube diameter of ICP chamber is increased in order to compensate the decrease of plasma density. Its increasement is very effective for n/sub/F increasement with keeping lower T/sub/e (less than 1 eV). The n/sub/F is decreased about 50% in CF\sub 4\ and Ar plasma with lower electron temperature compared to that of normal ICP without grid. The mechanism of controlling T\sub e\ and its application to decreasing n/sub/F are discussed.
11:20 AM		PS1-MoM-10 Fast Dry Etching with New High-Density Plasma Source F. Shimokawa, Y. Nishida (NTT Interdisciplinary Research Laboratories, Japan) We developed a fast dry etching system with a new high-density plasma source. This system is indispensable for practical micromachining. To generate higher density plasma (which yields a higher etching rate), we designed a magnetically enhanced plasma source by modifying a conventional inductively coupled plasma (ICP) source. This etching system fabricates high aspect ratio microstructures at a high etching rate. Our plasma source consists of three separate electromagnetic coils and two electron shielding electrodes, as well as quartz vessel chamber with a one-turn RF antenna coil, which is a main component of the conventional ICP source. This source produces a high plasma density of 10\super 16\m\super -3\, which is 10 times higher than that of the conventional ICP source under lower pressure conditions (<0.1Pa). Using this system, we etched Si, SiO\sub 2\, and polymeric materials. Etching rates up to several micrometers per minute were obtained by utilizing the higher density plasma, and were ten times faster than with conventional dry etching, such as reactive ion etching. We also achieved highly directional etching with a depth of more than several tens of micrometers in SiO\sub 2\, and of more than several hundred micrometers in polymeric materials. Due to the uniform plasma production by adjusting the electromagnetic intensity, the uniformity of these etch depths was less than \+-\3% over a 5-in. wafer. Thus our proposed etching system has the potential for fabricating three-dimensional microstructures with a high etching rate. These results indicate that our system can be used to fabricate practical microelectromechanical systems.
11:40 AM		PS1-MoM-11 Full Wave Helical Resonator: The First Capacitively Compensated Inductive Plasma Source G. Vinogradov, S. Yoneyama (MC Electronics Co. Ltd, Japan) Some inductive plasma sources are candidates for 300 mm wafer processing. They represent an alternative to capacitively coupled and microwave plasma sources. However, all known inductive plasma sources have a distributed RF voltage along the inductor. Hence, they are inevitably coupled capacitively to plasma. It develops a high RF voltage plasma sheath producing several well-known deleterious effects, such as sputtering, electric breakdowns, arcing, and charge up damage. This feature of inductive sources gives them back to the problems of conventional capacitive reactors. The capacitive currents has been long supposed to be an attribute of any inductor. The only one straightforward approach was known long ago to decrease the capacitive coupling: to shield plasma electrically from the inductor. However, it decreases drastically the efficiency of plasma sources. Besides, it is cumbersome and degrades inductive characteristics as well. Full Lambda Helical Resonator (\lambda\-HR) is proposed and tested as a first inductive plasma source with an intrinsic capacitive compensation. It produces up to three high density plasma toroids strictly confined at their positions inside the inductor. It does not interact capacitively with outside grounded surfaces as any other inductive source. Plasma RF potential does not fluctuate at fundamental RF excitation frequency. It can be easily scaled up to 300 mm wafer. The \lambda\-HR seems to be one of the most efficient free radical source. A record ashing rate was achieved for normal positive resist: 20.6 micron/min at 270o C with an apparent activation energy of about 0.5 eV typical for atomic oxygen chemical etching. The \lambda\-HR plasma source can be utilized for damage free rapid resist stripping, cleaning, etching, deposition, oxidation etc. It is also very well defined source for any study or demonstration of inductive plasmas.