Papers

AVS1996 Session PS-MoA: Oxide Etch I - Mechanisms

Monday, October 14, 1996 1:30 PM in Room 201C

Monday Afternoon

Start	Invited?	Item
1:30 PM		PS-MoA-1 Mechanistic Studies of SiO\sub 2\ Etching Processes in an Inductively-Coupled High-Density Plasma Reactor G. Oehrlein, M. Doemling, J. Mirza, N. Rueger, M. Schaepkens, T. Standaert (State University of New York, Albany) Mechanistic aspects of SiO2 fluorocarbon etching processes in an inductively- coupled high-density plasma reactor with independent rf biasing of the substrate have been examined for several fluorocarbon gases, including CHF3 and C3F6. As is the case for high-density plasma reactors based on other plasma source designs, three distinct process regimes are observed as a function of rf bias voltage: Fluorocarbon film deposition for no or low rf bias voltages, a fluorocarbon suppression regime at intermediate rf voltage and an oxide sputtering regime for higher rf bias voltages. The role of source power, gas flow and pressure on discharge composition, deposition rate and properties of fluorocarbon films for no rf bias will be described. The surface chemistry of the transition from fluorocarbon deposition to oxide etching is examined as the bias voltage is increased. Finally, mechanisms of selective etching of SiO2 over silicon, resist and Si3N4 films will be reviewed, and the factors that limit the etch rate ratios will be highlighted.
2:10 PM		PS-MoA-3 Kinetics of Polymer Film Growth during Selective Oxide Etching in a Low-Pressure High-Density Plasma Reactor J. Cecchi, M. Mark, J. Pender (University of New Mexico) Selective oxide etching of contacts and vias relies upon the deposition of protective polymer on non-oxidized (e.g., Si, Si3N4) surfaces. In the low-pressure high-density regime, suitable for etching submicron features, it has proven difficult to achieve a stable, reproducible chemistry for the passivating polymer film, a condition which sometimes results in "etch stop." We have measured and modeled the kinetics of the polymer film deposition during oxide etching in an ICP reactor for a variety of HFC feedstocks. We have explored the pressure range of 5-15mTorr, power range of 300-2000W, and residence times from 0.5s to 1.0s. Polymer growth rate on a Si wafer is measured using a laser interferometer. Fluorocarbon precursors, including CFx, (x=1-3) are measured with wavelength-modulated diode laser spectroscopy. Ion current is measured with a Langmuir probe. We have analyzed our measurements in terms of a polymer growth model that includes ion-assisted deposition, and are able to infer the associated kinetic parameters. Data fit the model well over the large ranges of pressure, power, and residence time. We find good agreement among kinetic parameters for the different fluorocarbon feedstocks, although the concentrations of particular CFx species differ greatly. A particularly important result is that in the low-pressure high-density regime, both CF\sub 2\ and CF are significant precursors.
2:30 PM		PS-MoA-4 Fluorocarbon Species Measurement and Modeling for Low Pressure, High Density SiO\sub 2\ Plasma Etching M. Sowa, J. Pender, M. Littau, J. Cecchi (University of New Mexico) Plasma etching of SiO2 is highly dependent upon the fluorocarbon concentrations present in the discharge. Understanding the production and loss mechanisms of the fluorocarbon species will aid in the development of new low-pressure, high-density oxide etching systems. We report on fluorocarbon species models for measurements made in high density C2F6/H2 plasmas produced with an inductively coupled plasma(ICP) and a low magnetic field(5-22G) helicon sources. CF2 and CF models are derived by performing production and loss balances. These first order mechanisms include dissociation through electron impact, recombination reactions with F and H, and wall losses(e.g. polymer growth). Measurements of the fluorocarbon species (CF3, CF2, CF) were performed with wavelength modulated IR diode laser absorption spectroscopy, atomic species (F, H) with optical emission spectroscopy, and electron density with a Langmuir probe. Collecting data over a wide range of power (300-800W), pressure (5-15mT), residence time (0.5-1s), and magnetic field (0-22G), the models were fit to determine the significant terms. CF2 production is dominated by electron impact dissociation of CF3 and F recombination with CF while losses come from CF2 dissociation and wall losses. CF production is dominated by CF2 dissociation with losses coming from dissociation, F recombination, and wall losses.
2:50 PM		PS-MoA-5 Residence Time Effect on SiO\sub 2\/Si Selective Etching in High Density Fluorocarbon Plasma T. Ichiki, Y. Chinzei, Y. Horiike (Toyo University, Japan); H. Shindo (Tokai University, Japan); N. Ikegami (Oki Electric Industry Co., Ltd., Japan); M. Sekine (Toshiba Corporation, Japan) An effect of residence time on SiO\sub 2\/Si selective etching employing ICP with C\sub 4\F\sub 8\. Experiments were carried out in a 200 degrees C heated tube set in a SUS reactor of 30 cm in diameter. The residence time was varied in the range of 5 to 300 msec under 30 mTorr. The etch rate of SiO\sub 2\ showed two local maxima for the residence time. The ion current measured by Langmuir probe, the CF\sub 1\ \super +\ ion and radical densities by AMS(appearance mass spectroscopy) also demonstrated two maxima at the exactly same residence times, but with the minima in the electron temperature there. The two local mode of ionization may be explained by the relaxation times of the ionization and recombination of radicals. For low residence time, a direct process for CF\sub 1\ \super +\ production and destruction is dominant, because its ionization and recombination times are small. For a high residence time, multi-process for CF\sub 1\ \super +\ may be dominant, because its relaxation times are long. With decreasing residence time, the etch selectivity becomes high, but the SiO\sub 2\ etch rate fell down rather rapidly due to the polymer deposition which resulted from decrease in CF\sub 1\ \super +\ ion density caused by reduced net RF power in high exhaust rate. Then Ar gas was added to C\sub 4\F\sub 8\ under the condition of low residence time of 10 msec. The SiO\sub 2\ etch rate was quite improved with Ar addition and the Si etch rate showed only slight increase. The AMS measurement indicated that from about 60% Ar concentration in C\sub 4\F\sub 8\, flux of C\super +\, CF\sub 1\ \super +\ and Ar\super +\ ions was enhanced greatly and CF\sub 1\ radical density was also significantly decreased. The former C\super +\ enhancement was found to be due to the collisional dissociation of CF\sub 1\ radical with the argon metastable atom Ar\super \. Indeed, a self-absorption measurement of ArI emission light showed a strong quench of Ar\super \ in that condition. Consequently, the role of Ar addition in fluorocarbon plasma under the short residence time is considered to surpress excess polymer deposition and supply carbon species to hole bottoms, leading to finer resolution etching.
3:10 PM		PS-MoA-6 Investigation of Plasma Precursors in Selective HDP Etching M. Passow, M. Armacost, M. Powers, T. Cotler (IBM Microelectronics Division) High density plasma (HDP) etch systems are being used for highly selective etch processes that are difficult or impossible to implement in conventional tools. The process window is distinctly different for various gas chemistries (e.g. C4F8, C3F8, C2F6). Thus the need exists to better characterize the relationship between etch performance and discharge parameters. Various plasma diagnostic techniques were used to establish a correlation between process conditions in the discharge and polymer formation on the wafer surfaces. Detailed analysis of mass spectral data under a variety of detection conditions was conducted. Results from line-of-sight, neutral, and charged particle analyses contributed to an understanding of the chemical decomposition of feed gases in the discharge. These results, when correlated to optical emission spectroscopy (OES) and actinometry under controlled conditions, allow the extension of these techniques to more complex etch conditions and chemistries.
3:30 PM		PS-MoA-7 Effects of the Magnetic Field on the Reduction of the Dielectric Window Damage Due to Capacitive Coupling in the Inductively Coupled Plasma J. Kim, H. Lee, Y. Kim (Seoul National University, Korea); J. Joo (Kunsan National University, Korea); K. Whang (Seoul National University, Korea) In the RF inductively coupled RF plasma, the dielectric window(quartz) damage due to the capacitive coupling could become very severe to affect the plasma chemistry. Especially in the etch process plasma, oxygen atoms released from quartz by the reaction with fluorine atoms or ions change the plasma chemisty. Faraday shield is often used to prevent the damage, but it has some problems of low power efficiency and cooling of shield. In this studty, we used quadrupole mass spectrometer(QMS) to monitor the dielectric window damage owing to the capacitive coupling in the C\sub \F\sub 8\ etch plasma. We observed that signals of the main etch products from quartz such as O\super +\, SiF\sub 2\\super+\, etc. were as high as those of species released from the feed gas (CF\super +\, CF\sub 2\\super+\, etc.). Using a modified high voltage probe, we measured the capcitive coupling directly. When we applied a weak, volumic, axial magnetic field (~20 gauss), the electric potential at the inner side of the quartz window decreased from -80V to -30V and the main quartz etch products almost disappears from the plasma which demonstrates that the application of a magnetic field can be a useful method to prevent the quartz window damage due to the capacitive coupling in the in the RF inductively coupled plasma.
3:50 PM		PS-MoA-8 CF\sub X\(X=1-3) Radical Densities during SiO\sub 2\, Si\sub 3\N\sub 4\ and Si Etching Employing ECR-CHF\sub 3\ Plasma K. Miyata, T. Kuno, M. Hori, T. Goto (Nagoya University, Japan) High-density fluorocarbon plasmas have been applied to the selective etching of SiO\sub 2\/Si and SiO\sub 2\/Si\sub 3\N\sub 4\. The chemistry of CF\sub X\(X=1-3) radicals in the etching plasmas has attracted much attention. The roles of the radicals, however, have not been well understood. This is an important problem to be solved for further development of the etching technology.In this study, CF\sub X\ radical densities in ECR-CHF\sub 3\ plasma were measured during etching of SiO\sub 2\, Si\sub 3\N\sub 4\ and Si. The radical densities were measured as functions of SiO\sub 2\, Si\sub 3\N\sub 4\ and Si etch rates. The etch rates were ranged by varying bias power applied to the substrate holder. Infrared diode laser absorption spectroscopy was used for measuring CF\sub X\ radical densities.During SiO\sub 2\ and Si etching, all of the CF\sub X\ radical densities decreased with increasing etch rates. During Si\sub 3\N\sub 4\ etching, only CF\sub 3\ radical density decreased with increasing etch rate while CF\sub 2\ and CF radical densities were constant against the etch rate. The fact indicates the etching reactions consume CF\sub X\ radicals, that is, the CF\sub X\ radicals contribute the ion-assisted etching. Especially, the CF\sub 3\ radical contributes the Si\sub 3\N\sub 4\ etching.When the substrate with resist film was placed on the holder, the CF\sub X\ radical densities were constant against the bias powers. This result shows the radical densities are influenced only by the etching reactions, not by the change in bulk-plasma caused by biases.The roles of CF\sub X\ radicals in the etching processes will be discussed on the basis of these experimental results.
4:10 PM		PS-MoA-9 Remote Plasma Etching of Silicon Nitride and Silicon Dioxide using NF\sub 3\/O\sub 2\ Gas Mixtures B. Kastenmeier, P. Matsuo, G. Oehrlein (State University of New York, Albany); J. Langan (Air Products & Chemicals, Inc.) The etching of silicon nitride (Si\sub 3\N\sub 4\) and silicon dioxide (SiO\sub 2\) in the afterglow of a NF\sub 3\/O\sub 2\ microwave discharge has been characterized. The etch rates of both materials increase roughly linearly with the flow of NF\sub 3\ due to the increased availability of F atoms. The etch rate of Si\sub 3\N\sub 4\ is enhanced significantly upon O\sub 2\ injection into the NF\sub 3\ discharge, whereas the SiO\sub 2\ etch rate is less influenced. X-ray photoemission spectroscopy of processed Si\sub 3\N\sub 4\ samples shows that the fluorination of the reactive layer, which forms on the bulk Si\sub 3\N\sub 4\ during the etching, decreases with the flow of O\sub 2\. The oxidation of the reactive layer follows the same dependence on the flow of O\sub 2\ as the etch rate. Quadrupole mass spectrometry is used to identify reactive species in the etching of both materials. The atomic fluorine density in the reaction chamber decreases as O\sub 2\ is added to the discharge due to dilution. The NO density shows the same dependence on the O\sub 2\ flow as the Si\sub 3\N\sub 4\ etch rate and the surface oxidation. Based on this observation, we propose that the etch rate enhancement for Si\sub 3\N\sub 4\ is due to the splitting of the NO on the Si\sub 3\N\sub 4\ surface, followed by the formation of N\sub 2\ with an N atom from the surface. The O atom can then adsorb on the surface, contributing to the oxidation.
4:30 PM		PS-MoA-10 Electron and Ion Kinetics in SiF\sub 4\ R. Nagpal, A. Garscadden, P. Haaland (Wright Laboratory); J. Clark (Wright State University) The electron energy degradation in SiF\sub 4\ divides into two (low- and high-energy) components. The low-energy component consists of momentum transfer (q\sub m\) and vibrational excitation, and the high-energy component constitutes excitation, attachment, dissociation and ionization.Starting point of the low-energy analysis is the derivation of three vibrational excitation cross sections from electron drift velocity measurements in highly dilute mixtures of SiF\sub 4\ in Ar. The q\sub m\ is then derived from the electron drift velocity data in relatively high (> 0.5%) concentrations of SiF\sub 4\ in Ar, in conjunction with the data in pure SiF\sub 4\. It is found that the q\sub m\ of SiF\sub 4\ has a Ramsauer-Townsend minimum at electron energy around 0.7 eV. The vibrational excitation cross section magnitudes are found to be larger than those of q\sub m\ at electron energies around the Ramsauer minimum. Therefore a Monte-Carlo description of the electron kinetics is used to achieve consistency of the derived cross sections with the transport properties.The high-energy analysis compares the experimental ionization and attachment coefficients with the calculated data using cross section measurements from Fourier Transform Ion Cyclotron Mass Spectrometry. Since the thresholds of dissociation processes in SiF\sub 4\ lie between those of attachment and ionization, the swarm analyses of breakdown in SiF\sub 4\ is very sensitive to both the shape, and the magnitude of the dissociation cross sections. The converged cross section set permits a complete description of electron kinetics in SiF\sub 4\, which is the major by-product in flourine based Si etching plasmas.