AVS1999 Session PS-WeA: Dielectric Etching
Wednesday, October 27, 1999 2:00 PM in Room 609
Wednesday Afternoon
Time Period WeA Sessions | Abstract Timeline | Topic PS Sessions | Time Periods | Topics | AVS1999 Schedule
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| 2:00 PM |
PS-WeA-1 Dielectric Etching : From Oxide to Low k Dielectrics
P. Berruyer (LETI (CEA-Grenoble), France); O. Joubert, D. Fuard (CNRS-LTM Grenoble, France); C. Verove (ST-Microelectronics, France); M. Assous (LETI (CEA-Grenoble), France); R. Blanc, H. Feldis (ST-Microelectronics, France); E. Tabouret (LETI (CEA-Grenoble), France); Y. Morand (ST-Microelectronics, France) Dielectric etching for interconnection is one of the most critical processes of the ULSI technology. Up to now oxide has been used as inter-metal dielectric with an aluminum based metallisation. The well known issue of contact and vias etch process is the etch-stop phenomenon occurring in high aspect ratio structures, if highly selective process is required. The introduction of copper and thus dual damascene architectures, has increased the number of dielectric etch processes required in the fabrication of ICs. Moreover the level of difficulty of these processes has increased : aspect ratio can be higher than it is in contact and vias, holes but also lines can be etched and high selectivity to nitride is required. If the introduction of copper has led to a more critical oxide etch process, the introduction of low k material dielectrics will lead to a break off in dielectric etch processes. These low k materials can be either mineral or organic with various porosity. This paper deals with the different etching processes and plasma source required for these different dielectrics. First of all, process performances such as profile, selectivity, CD, trenching, µloading, etch stop, yield, plasma induced damage will be studied as a function of process parameters. We will point out that, if etch stop in high aspect ratio structures is the main issue in oxide etch, profile control is the main issue of the etching of low k polymer materials (SiLK). Mechanisms related with these 2 issues will be proposed. Process conditions required for the etching of aerogel materials will also be discussed. Then, taking into account the process performances and limitations obtained previously, different schemes of dual damascene structures with copper metallisation will be compared.
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| 2:40 PM |
PS-WeA-3 Surface Kinetics Study of Silicon Dioxide Etching with Fluorocarbons in Inductively-coupled Plasmas
H. Chae, H.H. Sawin (Massachusetts Institute of Technology) High-density fluorocarbon plasma for silicon dioxide etching has various ion and neutral compounds. Depending upon the plasma condition, many difficulties arise such as RIE lag, inverse RIE lag, etch stop, and low selectivity of photoresist. Profile evolution modeling can provide understanding of these difficulties in etching as well as trenching, bowing, and faceting. In this research we have measured etching and deposition rates as function of ion bombardment energy, ion impinging angle, ion-to-neutral flux ratio, which are necessary for profile evolution modeling of silicon dioxide etching in inductively coupled plasma. In this work, ions and neutrals are extracted directly from plasma to differentially pumped side chambers. Surface reaction is studied by measuring etching and deposition rate with quartz crystal microbalance (QCM). At the same time, ion and neutral composition of the plasma is determined with mass spectrometer. Etching or deposition rate is measured with QCM as function of ion acceleration energy, ion-impinging angle, ion-to-neutral flux ratio, with various fluorocarbon plasmas. Ion flux is characterized by measuring physical sputtering rate of oxide with Ar plasma and neutral flux is characterized by measuring fluorocarbon deposition rate with CHF3 plasma. Three different fluorocarbon plasmas-C2HF5, C2HF5 + C2H4F2, C4F8- are studied for oxide and photoresist etching. Hydrogen rich chemistry - C2HF5 + C2H4F2 (20%)- has high deposition rate at low ion bombardment energy less than 100V. That hydrogen rich chemistry has the smallest neutral flux and the largest ion flux among them while C4F8 has the largest neutral flux and the smallest ion flux at the same given power and flowrate. |
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| 3:00 PM |
PS-WeA-4 Chemical Bonding Arrangement Approach for Selective Radical Generation in High-density, Low-pressure Fluorocarbon Plasma
S. Samukawa, T. Mukai (NEC Corporation, Japan) Generally, SiO2 etching is done with fluorocarbon gases so that a fluoropolymer layer is deposited on the underlying silicon to enhance the etching selectivity for SiO2 over silicon and silicon nitride. CF2 radicals have been used as the main gas precursor for polymer deposition, and CF3+ ions have been the dominant etchant for SiO2 films. The CF3+ ions are mainly generated from CF3 radicals. Thus, to realize high-performance SiO2 etching, precise control of CF2 and CF3 radicals in the fluorocarbon gas plasmas is indispensable. In this paper, we discuss how the efficiency of radical generation is affected by the chemical bonding of gas molecules in the fluorocarbon gas plasma. We found that dissociation of the C=C and C-I bonds are 5 times and 6 times higher than that of the C-C bond in a fluorocarbon gas plasma. As a result, a C2F4 plasma could generate a higher density of CF2 radicals than a C4F8 plasma. The CF3I is also efficient source of CF3 radicals (CF3+ ions). By changing the gas-flow ratio of the CF3I and C2F4 mixture, the density ratios of CF2 and CF3 (CF3+) could be independently controlled and high performance SiO2 etching could be obtained. The appropriate choice of chemical bonding in the fluorocarbon gases is a useful way to control the generation of radicals and ions for SiO2 etching. |
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| 3:20 PM |
PS-WeA-5 Flux Control for High-Aspect-Ratio Contact Hole Etching
T. Tatsumi, M. Matsui, Y. Hikosaka, M. Sekine (Association of Super-Advanced Electronics Technologies (ASET), Japan) The relationship between SiO2 etch rates and incident fluxes of reactive species in a dual-frequency (27/0.8 MHz) parallel plate system1 was evaluated by using various in-situ measurements tools, such as infrared laser absorption spectroscopy (IRLAS), quadruple mass spectroscopy (QMS), and optical emission spectroscopy (OES). The thickness and composition of a fluorocarbon (C-F) polymer layer on the etched SiO2 surface was also measured by XPS. The SiO2 etch rate in a large area depends on both the total amount of F (ΓF-total) in the C-F reactive species and the energy at the SiO2 surface. ΓF-total could be estimated from the net fluxes calculated by using F, CF, CF2, and CF3 radical densities.2 Reaction energy depends on the total amount of ion fluxes (Γion) which is a function of the plasma density (ne) at the sheath edge, the acceleration energy of ions (assumed to be peak to peak voltage Vpp), and the energy loss in the C-F polymer layer. The thickness of C-F polymer layer (TC-F) could be varied by the amount of both the CFx species and the oxygen radical in the incident fluxes, the oxygen out-flux from the SiO2, Γion, and Vpp. Excess CFx reactive species induced the thicker polymer layer (>1 nm). The thick polymer layer of 5 nm corresponded to the energy loss of about 1 kV. When using ΓF-total (CFx), Γion (ne), Vpp, and TC-F, we can conduct in-situ monitoring of the SiO2 etch rate. The etch rate at the bottom of contact hole was also evaluated. The decrease in SiO2 etch rate in the fine holes can be similarly explained by either the lack of etchant or the lack of reaction energy. In order to obtain the high aspect ratio contact holes, it is important to suppress the excess formation of the C-F polymer layer on the etched surface. Increasing the oxygen flux is one way to do this, however it decreases the selectivity to the resist mask and the SiN. Therefore, higher ion flux is needed to obtain an etching process that enables us to achieve the deeper contact holes with higher selectivity.
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| 3:40 PM |
PS-WeA-6 Etching Chemistry and Kinetics of BCB Low-k Dielectric Films
S.A. Vitale, H.H. Sawin (Massachusetts Institute of Technology) Etching of BCB has been performed in a high density inductively coupled plasma reactor using O2 + hydrofluorocarbons, O2 + F2, and O2 + Cl2 chemistries. The etch rates of the films and the selectivities over oxide are correlated to the flux of ions and reactive radicals to the wafer. Species identification and fluxes to the wafer are determined by mass spectrometry, two gridded ion energy and current analyzers, and a Langmuir probe. Etch rates at many points on the wafer are simultaneously measured using Full Wafer Interferometry. Etching yields as a function of ion bombardment energy, neutral/ion flux ratio, and ion impingement angle are quantitatively determined using a novel plasma beam / QCM system. It is proposed that in high density, low pressure plasmas, the etching rate can be limited by the radical flux and by the ion flux to the wafer under different conditions. The selectivity of BCB etching over oxide etching is greatest for etchant gas compositions of approximately 20-40% halogenated gas in oxygen. Selectivity over oxide greater than 20 has been realized with BCB etch rates over 1 um/min . The implications of these results for the integration of BCB as a low-K dielectric into a copper dual damascene architecture are discussed. |
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| 4:00 PM |
PS-WeA-7 The Effect of Capacitive Coupling on Inductively Coupled Fluorocarbon Plasma Processing
M. Schaepkens, N.R. Rueger (State University of New York at Albany); J.J. Beulens (ASM International, The Netherlands); I. Martini, E.A. Sanjuan, X. Li, T.E.F.M. Standaert, P.J. Matsuo, G.S. Oehrlein (State University of New York at Albany) Different inductively coupled plasma reactors differ in the amount of capacitive coupling, which may influence the plasma process in a non-obvious fashion. We performed a study of inductively coupled fluorocarbon plasmas in which the amount of capacitive coupling was systematically varied. It is found that the plasma density decreases while the electron temperature increases as the amount of capacitive coupling is increased at a constant inductive power level. The rate at which the dielectric (quartz) coupling window is eroded is found to scale with both the peak-to-peak RF voltage and the ion current density, and the dielectric window erosion is found to influence the resulting plasma gas-phase chemistry. The changes in these plasma electrical and chemical characteristics can, on their turn, have a large impact on the surface processes occurring in inductively coupled fluorocarbon plasmas, such as fluorocarbon deposition, fluorocarbon etching, SiO2 etching and Si etching. An important result obtained in this study is that certain plasma etch processes, such as selective SiO2-to-Si etching, can benefit to a certain extent from capacitive coupling effects. |
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| 4:20 PM |
PS-WeA-8 High Density Plasmas Etching of Low Dielectric Constant Materials
D. Fuard (CNRS, France); O. Joubert, L. Vallier (France Telecom-CNET and CNRS) The etching step remains one of the key technological issue for low K integration in advanced CMOS technologies. We have studied the etching mechanisms of SiO2 masked fluorine free aromatic hydrocarbon polymer. Experiments are performed in a high density plasma helicon source operated at low pressure. Previous work has shown that the SO2/O2 chemistry, even if suitable for a good profile control, induces the formation of sulphur-based acids which may generate corrosion latter in the process. In this paper, we present results using sulphur free chemistries based on N2, H2 and O2. First, we have used X-Ray Photoelectron Spectroscopy (XPS) analyses to understand the etch mechanisms of the polymer. XPS analyses reveal that under high energy ion bombardment conditions, the polymer structure is strongly graphitized : C1s binding energy originating from the polymer shifts from 285 to 283.5 eV. The graphitization phenomenon is also dependent on the chemistry used. Without changing the plasma operating conditions, very reactive chemistries, such as pure O2, prevent the polymer graphitization whereas adding N2 to O2 may lead to a severe graphitization by decreasing the chemical component and favoring the physical component of the etch. Using appropriate gas mixture and plasma operating conditions, high aspect ratio contact holes are etched with a good profile control. XPS analyses of the etched structures reveal that the passivation layer formed on the polymer sidewalls is also strongly graphitized, suggesting that the passivation layer originates from the etch products. Some results obtained in other high density plasma tools using identical chemistries will be presented and general conclusion on polymer etching in high density plasmas will be drawn.
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| 4:40 PM |
PS-WeA-9 A Mechanism of Oxide to Nitride Selective RIE
T. Sakai, T. Ohiwa (Toshiba Corporation Semiconductor Company, Japan) In highly integrated ULSIs, selective etching of oxide to nitride has been widely used for Self-Aligned Contact (SAC) etching to increase the packing factor. It is known that the CFx polymer formed selectively on the nitride surface suppresses Si3N4 etching. However the origin of selective CFx polymer formation is not understood well. We studied the mechanism of oxide to nitride selective etching with focus on selective polymer formation. In CHF3-based chemistry, the oxide etch rate decreased slightly from 540 nm/min to 470 nm/min when the cathode temperature was increased from RT to 120 °C. On the contrary, the nitride etch rate decreased abruptly from 720 nm/min at RT to 220 nm/min at 60 °C. XPS analysis showed CFx polymer formation on nitride at 60 °C, but no CFx polymer at RT. Increase of temperature increases the C/F ratio of the adsorbed species on the surface, therefore CFx polymer formation is considered to be enhanced on nitride. Following this result, the temperature in actual SAC etching using C4F8/CO/Ar chemistry was increased from 20 °C to 70 °C, and the selectivity at the corner of nitride increased from 10 to 18. Further surface analysis of the nitride surface etched in C4F8/CO/Ar chemistry at 70 °C revealed that the etched nitride surface has C-N bonds. At low temperature, the nitride etching reaction forms volatile etching products of SiF4, CFN, C2N2 and etc., leading to no CFx polymer formation similar to oxide. However, at the higher temperature, the higher C concentration of adsorbed species on the nitride surface suppresses the formation of volatile CFN, resulting in remaining of CN compounds, and forms CFx polymer. CO, which is the etching product in oxide etching, has a much higher vapor pressure compared to CN compounds. So a difference of CFx polymer formation arises between the oxide surface and the nitride surface, resulting in selective etching of oxide to nitride. |
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| 5:00 PM |
PS-WeA-10 SiON SAC Etching Technique Using C4F8/CH2F2/Ar Plasma for 0.18µm Technology and Beyond
J.H. Kim, J.S. Yu, J.S. Na, J.W. Kim, Y.S. Seol, J.C. Ku, C.K. Ryu, S.J. Oh, S.B. Kim, S.D. Kim, I.H. Choi (Hyundai Electronics Industries Co. Ltd., Korea) A SAC technique using an oxynitride (SiON) layer as a contact oxide etch barrier has been developed for 0.18µm technology and beyond. Generally, a SAC which uses a SiN etch barrier for 0.25µm technology may exhibit some disadvantages such as wafer warpage, film lifting, transistor reliability degradation, large contact junction leakage, needs for additional anti-reflection coating (ARC) layer, and large parasitic capacitance due to its high dielectric constant. These demerits can be eliminated or improved when the SiON SAC technique is applied. But it is not easy to obtain an oxide etching process with a high selectivity to the SiON etch barrier because of oxygen component within the SiON layer. To overcome this problem, we intentionally introduced excessive Si during the SiON film deposition in order to increase the selectivity to SiON. The developed SiON layer plays the roles of ARC for wordline and bitline photo resist patterning, and side-wall spacer to build a MOS transistor as well as SAC oxide etch barrier. The contact oxide etch was done using C4F8/CH2F2/Ar in a dipole ring magnet (DRM) plasma. As the C4F8 flow rate increases, the oxide etch selectivity to the SiON increases but etch-stop tends to happen. In highly selective SAC oxide etching, it is very important to avoid etch-stop for a wide process window. It was reported that CH2F2 chemistry helps to widen the process window through its hydrogen effects.1 Our optimized contact oxide etch process showed the high selectivity to SiON larger than 25 and a wide process window (≥ 4 sccm) for the C4F8 flow rate. When the SiON SAC process was applied to a giga-bit DRAM of cell array, there was no short failure between conductive layers.
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