SF6 Gas Mixtures Research Articles

The role of sulfur during Mo etching using SF6 and Cl2 gas chemistries

As the design rule of integrated circuits (IC) has decreased in size, the requirements for making interconnections of these circuits have become more stringent. When fabricating ®ne pattern interconnections, the resistance and contact reliability of interconnection materials have to improve in quality. In the past, ®ne pattern interconnection has been conducted with the use of polysilicon; however, resistance and contact reliability have now called for the use of refractory metal silicide [1, 2]. The use of refractory metal has brought some valuable characteristics into question. Among the variety of refractory metals, Mo has an extremely high melting point (2620 8C), good thermal stability and a low resistivity. Also, the thermal expansion coef®cient is similar to that of silicon (Mo ˆ 5:0 3 10y6 8C, Si ˆ 3:0 3 10y6 8C), so when it is heated, Mo experiences little stress. Due to these favorable qualities, Mo has been used in many other applications such as hard coatings, superconductors, Al diffusion barriers, gate materials and high dielectric electrode materials [3, 4]. Thus, we will continue to see Mo compounds used in many more applications. When Mo compounds are to be used in semiconductor processing, the dry etching of Mo is an essential process. Meanwhile, the etching of Mo compounds has been generally carried out with SF6 or SF6 mixed gas plasma. This is due to the high vapor pressure of MoF6 compounds. At the same time, the role of S in the etching of Si or W with SF6 plasma has recently been reported. Ninomiya et al. [5] concluded that S promotes the etching of Si. Moreover, Turban et al. [6] explained that in the case of an SiO2 cathode, S reacted with oxygen generated from cathode materials to form SO2, and SO2 was removed on the W surface. During the etching process of Mo compounds, however, the role of S has not been investigated thoroughly. To improve the etching characteristics, the role of plasma species such as F and S in the etch reaction has to be examined closely. Therefore, the role of S on the surface reaction during Mo etching with SF6 gas plasma was investigated in this work. In this study, the surface reaction of Mo etching was investigated with SF6=Cl2 gas plasma by using X-ray photoelectron spectroscopy (XPS). To examine the role of S during the etching of Mo with SF6 gas plasma, pure Mo was used in this process. The role of S and the relationship between the etch rate and gas-mixing ratio will be further discussed in this letter. The 5-inch Si substrates used for this study were doped with B (0.85±1.15 U-cm), oriented (1 0 0) and chemically etched for 60 s using 1% HF : H2O prior to chemical vapor deposition (CVD) growth. The substrates were coated with a 600-nm-thick layer of SiO2 grown by low-pressure-CVD (SiH4 ‡ O2, 420 8C, 240 mTorr). Deposition of the Mo ®lms was performed using a Varian 3180 d.c. sputtering system equipped with a 7-inch conical magnetron sputtering source. The Mo sputtering target was speci®ed at 99.999% purity. Sputtering was performed in research-grade Ar at a pressure of 8 mTorr, and the distance from source to substrate was ,3.3 inch. Typical sputtering power was 1 kW. During deposition, the substrate was grounded, and the substrate temperature was held at 100 8C using gas conduction heating. The ®nal thickness of the sputtered Mo ®lm was ,300 nm and was ,95% uniform across the surface of the 5-inch wafers. Plasma etching of the Mo ®lms was performed using an Applied Materials Model P-5000 etching system. Wafers were placed on a bottom electrode. Substrate holder temperature during the Mo etch was held at ,45 8C using the circulation of cooling water. A turbo pump reduced residual gas pressure to below 5 3 10y5 Torr before the processing step. The Cl2=SF6 gas mixture was used for Mo etch, and the total gas ow was 24 sccm. The Cl2=(SF6 ‡ Cl2) mixing ratio was changed from 0 to 1. RF power, chamber pressure and magnetic ®eld were constant at 250 watts, 100 mTorr, and 30 Gauss, respectively. The etch rate was measured using a four-point probe. After removing the plasma-etched samples from the etching system, the samples were exposed to the atmospheric environment for approximately 10 min prior to XPS analysis. Compositional analysis of the Mo surface was performed using a VG Scienti®c

Stress Control and Etching Study of W-Re as X-Ray Mask Absorber

Stress control and etching of W-Re as X-ray mask absorber was studied. The W-Re film was deposited using an rf magnetron sputtering apparatus with two material targets. The substrates were revolved by themselves and over the W and Re targets to achieve uniform film thickness and stress. When the content ratio of W to Re was 1:1, non-columnar film structure was obtained. The stress repeatability, when the deposited film stress is near zero, was ±5×108 dyn/cm2. To further control the stress, implantation of Ar ions was carried out with an acceleration voltage of 180 kV. The stress change was 1×109 dyn/cm2 compressive at the ion dose of about 6×1015 ions/cm2. The etching of W-Re was carried out using SF6 and CHF3 gas mixture and a Cr etching mask by applying magnetron reactive ion etching. The etching selectivity between W-Re and Cr was over 250 in the CHF3 content below 60%. We applied two-step etching to remove etch-obstructing films on the W-Re surface, and fabricated 0.1 µ m lines and spaces of 0.5-µ m-thick W-Re film.

Ionization and attachment coefficients measured in SF6/Ar and SF6/Kr gas mixtures

The Townsend first ionization coefficient alpha and the electron attachment coefficient eta were measured in SF6-Ar and SF6-Kr gas mixtures using the steady-state Townsend method over the range 15<or=E/P<or=94 V mm-1 kPa-1 with mixture ratios 10:90, 25:75, 50:50, 75:25 and 90:10. The limiting E/P values of these two gas mixtures were derived from the pre-breakdown current growth measurements. The dielectric strengths of both gas mixtures are found to be almost the same, and they vary linearly with SF6 concentration in the mixture, which is different from the curve pattern for most SF6 gas mixtures.

Intrapulmonary gas mixing and pulmonary gas exchange in artificially ventilated dogs

To investigate the effect of incomplete gas mixing between tidal air and residual gas on pulmonary gas exchange, anaesthetized dogs were ventilated artificially with breathing patterns with different durations of the post-inspiratory apnoea (ta = 0, 0.5, 1.0 and 2.0 s), where tidal volume, breathing frequency, inspiratory and expiratory flow patterns were kept constant. We determined the alveolar ventilations (VA) of He and SF6 from the product of end-expiratory lung volume (VL,E') and specific ventilation (VA/VL,E'). VL,E' was determined by the dilution technique and the specific ventilations of the two gases were obtained from their multiple-breath washout. Further, tracer amounts of acetone, ether and enflurane were infused continuously into a peripheral vein and a bolus of a gas mixture of krypton, Freon12 and SF6 was introduced into the peritoneal cavity. We determined the Excretion (E) and Retention (R) of these six gases according to the multiple-inert-gas-elimination technique (MIGET). VA increased with increasing ta, where VA,He was about 14% larger than VA,SF6. For both gases, however, the increase in VA relative to control (VA for ta = 0) was virtually the same: 9, 11 and 19% (mean values) for ta = 0.5, 1.0 and 2.0 s respectively. For all dogs the E/R curve shifted to larger E values with increasing ta. E for the most soluble tracer gas (acetone) increased by 11, 21 and 25% for ta = 0.5, 1.0 and 2.0 s respectively. VA, determined with MIGET from the ventilation/perfusion distribution, increased by almost the same percentages.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on metal/benzocyclobutene (BCB) interface and adhesion

Interfacial characteristics such as chemical reaction, metal diffusion, and morphology were investigated for Cu/BCB, Cr/BCB and Ti/BCB structures. Using Auger and XPS depth profiling, the formation of titanium carbide and chromium oxide was confirmed at the metal/BCB interface. Annealing at 250°C for extended periods resulted in the diffusion of Cu, Cr and Ti into the BCB and subsequent formation of Cu-Si, CrSi2 and Ti-Si compound precipitates. The reaction is a thermal diffusion controlled process which is dependent on time and temperature. Ar sputtering treatment of BCB film before metallization was found to roughen the surface, resulting in metal spikes which penetrate into the roughened BCB film. However, the peel strength of metals on BCB was only about 177 g cm_1presumably due to the brittleness of the BCB film. The etch rates of the BCB film in a reactive ion etcher (RIE) and a plasma etcher were measured using Ar, O2, O2 + CF4, and O2 + SF6 gas mixtures. Faster etch rates were obtained when CF4 and SF6 were added to oxygen, since the presence of atomic fluorine enhances the etch rate of organics, while also etching Si and SiO2 formed by exposure of Si-containing BCB film to oxygen gas. Surface compositional changes on the BCB film were observed by XPS after plasma modification. Pure O2 and O2 + CF4 plasmas oxidized the carbo-siloxane linkage (C—Si—O) of the BCB, resulting in the formation of SiO2 on the surface. The O2 + SF6 plasma, however, did not produce the surface SiO2, because of its faster Si and SiO2 etch rates.

X-Ray Mask Fabrication Process Using Cr Mask and ITO Stopper in the Dry Etching of W Absorber

An X-ray mask fabrication technology using a tungsten (W) absorber with a chromium (Cr) mask and indium titanium oxide (ITO) stopper was developed. When SF6 was used as the dry etching gas, substantial side etching occurred because the F radical reacts with W on the sidewall. In order to prevent side etching, a SF6 and CHF3 gas mixture was applied; however, the ratio of dry etching rate of W to that of resist is low. Furthermore, the underlying layer such as that of silicon dioxide (SiO2), which was used as the etching stopper, was easily damaged. Instead of a resist mask and SiO2 stopper, a Cr layer as the etching mask and ITO layer as the stopper layer were applied. By the use of these structures and etching procedures, high aspect ratio W patterns with vertical sidewalls have been successfully fabricated.

Electron transport coefficients in SF6 and c-C4F8 mixtures

The ionization alpha , electron attachment eta and other electron transport coefficients for SF6 and perfluorocyclobutane (c-C4F8) gas mixtures have been calculated by a three-term Boltzmann equation method over the E/N range 283-707 Td. A set of electron collision cross-sections that gives transport parameters in agreement with measurements for the pure gases is used. The calculation is performed for important experimental conditions, i.e. the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) methods. It is found that the greater the reduction in E/N the more pronounced is the peak of eta values of mixtures, e.g. at E/N=283 Td the peak appears at the SF6 partial pressure k around 20%, though alpha values show monotonical variation with k, at a fixed E/N for the E/N range studied here. As a result, the E/N value at which alpha = eta , i.e. the limiting E/N of 20% SF6 and 80% c-C4F8, can exceed the values for the respective pure gases. This fact suggests that this combination may be one desirable synergistic mixture since the limiting E/N is an important parameter for predicting the dielectric characteristics in electronegative gases.

Studies on the Surface Modification of Benzocyclobutene(BCB) Film By Plasma Ions

ABSTRACTThe etch rates of BCB film in a reactive ion etcher(RIE) were measured using Ar, O2, O2+CF4, and O2+SF6 gas mixtures. Faster etch rates were obtained when CF4 and SF6 were added to oxygen, since the presence of atomic fluorine enhances the etch rate of organics, while also etching Si and SiO2 formed by exposure to oxygen gas. Surface compositional changes on the BCB film were observed by XPS after plasma modification. Pure O2 and O2+CF4 plasma oxidized the carbo-siloxane linkage (C-Si-O) of the BCB, resulting in the formation of SiO2 on the surface. The O2 +SF6 plasma, however, did not produce the surface SiO2, because of its faster Si and SiO2 etch rates. Ar ion sputtering following the plasma modification, restored the surface chemical composition to a state similar to the initial BCB surface.

Reactive ion-etching-induced damage in silicon using SF6 gas mixtures

Damage introduced in silicon during reactive‐ion etching (RIE) in SI6 gas containing 10 Vol % of H2, He, N2, O2, or Ar has been investigated using Schottky barrier measurements and Rutherford backscattering. RIE was performed at 0.1 Watt cm−2 power density with the radio frequancy (rf) cathode covered with a quartz plate. The annealing effect on damage was studied at various temperatures up to 800 °C in dry N2. The results indicate that the damage has two components namely, a shallow near‐surface region with lattice damage and a deep layer containing discrete defect centers which act as donors. The extent of the deep damage layer is the net balance between two competing processes: (a) the etch rate, and (b) the rate of damage migration into silicon. However, the damage has been found to reach a saturation level after RIE for 1 min in all the gas mixtures studied. The results also suggest that RIE in SF6+10% H2 gas mixture causes a deep damage layer which could, however, be annealed at 150 °C aided signifi...

Boltzmann-equation analysis of electron transport properties in CCl2F2/SF6/N2 gas mixtures

A Boltzmann-equation analysis has been used to study electron swarm phenomena in ternary gas mixtures of N2, SF6 and CCl2F2. Electron energy distribution functions, collision frequencies and transport coefficients have been calculated for a number of cases with percentage mixture ratios ranging from 0 to 100%, for either of the electronegative components and, from 0 to 66.7%, for N2. The distribution function is presented for selected compositions with 0 or 50% N2 content and several percentage mixture ratios of SF6 and CCl2F2; its variation with partial contents of the gas components is discussed in terms of collisional cross sections. Ionisation ( alpha ) and attachment ( eta ) coefficients, drift velocity (V) and the diffusion coefficient to mobility ratio (D/ mu ) are shown as a function of the reduced electric field E/p20 for the particular percentage mixture ratios 40:10:50(%), 10:40:50(%) and 16.7:16.7:66.6(%) of CCl2F2, SF6 and N2, respectively. The theoretical limit-field values, defined by the condition alpha = eta , range from 75 to 122.5 V cm-1 Torr-1 and are found to agree well with breakdown-potential measurements reported in the literature.

Annealing of Si damage caused by reactive ion etching in SF6 gas mixtures

Damage introduced in silicon during reactive ion etching (RIE) in SF6 gas mixtures has been studied using x-ray photoelectron spectroscopy, Schottky barrier measurements, and Rutherford backscattering. RIE was done at 0.1 W cm−2 power density with the rf cathode covered with a quartz plate, and the annealing behavior of damage was studied at various temperatures up to 800 °C in N2 ambient. The results obtained indicate the following: (a) contamination by F, S, and metal ions is insignificant; (b) lighter atoms such as hydrogen cause higher electrical damage and lower lattice damage, both of which can be annealed at 400 °C; (c) heavier atoms such as argon cause lower electrical damage but higher lattice damage which requires a temperature ≥800 °C for recovery.

Pressure-dependent electron attachment and breakdown strengths of unary gases and synergism of binary gas mixtures: A relationship

The relationship between the pressure-dependent electron attachment rate constants which have been observed in 1-C3F6 and in several perfluoroalkanes and the uniform field breakdown strengths (E/N)lim in these gases is discussed. Also discussed are the various types of synergistic behavior in (E/N)lim which have been observed in binary dielectric gas mixtures. For the latter, a new mechanism is outlined which explains the synergism observed in several gas mixtures where the (E/N)lim values of the mixtures are greater than those of the individual gas constituents, which we call positive synergism. Model calculations are presented which support this mechanism and can be used to explain the pressure-dependent synergistic effects which have been reported in 1-C3F6/SF6 and other gas mixtures. Experimentally observed ion–molecule reaction processes for several gases are discussed which support the proposed mechanism. Based on this mechanism, we outline several conditions which must be fulfilled in order to observe positive synergistic behavior in dielectric gas mixtures.

Spacer Flashover in Compressed Gases

Surface flashover studies of cylindrical spacers are carried out in SF6-N2 and other SF6-gas mixtures in a uniform field. In the presence of the spacer, the value of the surface flashover voltage is typically found to be lower than the corresponding breakdown own voltage of the gas gap without the spacer. It is experimentally found that this reduction in the breakdown voltage depends to a large extent on the dielectric constant of the spacer material, the type of end-contact between the spacer and the electrode surface, the surface condition of the spacer, the content of SF6 in the gas mixture, and the gas pressure. re. From these experimental results, an empirical expression is developed for the first time to predict in compressed gases. The calculated values of the flashover voltage show a good agreement with the measured values of the author as well as with those previously published in the literature.

Dc Voltage Breakdown of SF6-Air and SF6-CO2 Mixtures in Rod-Plane Gaps

dc breakdown and corona inception voltages are reported for SF6-air and SF6-CO2 mixtures in positive and negative rod-plane gaps. These voltages are compared with values obtained for SF6-N2 mixtures. The results show that the breakdown voltages of SF6-air mixtures exhibit corona stabilization over a pressure range which depends upon SF6 content of these mixtures, and is higher than the corresponding values for SF6-N2 and SF6-CO2 mixtures. Moreover, a simple method to estimate the corona-stabilized breakdown voltages of SF6-gas mixtures is discussed. It is shown that at low pressures, this method gives estimated voltages which are generally in reasonable agreement with the experimental measurements of several authors.

Reactive ion etching for submicron structures of refractory metal silicides and polycides

Refractory metal silicides and n+ polysilicon/silicides are of particular interest for VLSI gate and interconnection technology. Reactive ion etching experiments have been conducted on sputter-deposited films of MoSi2, CVD n+ polysilicon, and thermal SiO2 using admixtures of SF6 with O2. The influence of partial pressure and total pressure on profile control and etch selectivity has been determined. Highly anisotropic etching of n+ polysilicon and MoSi2 was obtained using a gas mixture of SF6 plus 50% oxygen. Examples of 0.2μ anisotropically etched structures of silicides and n+ polysilicon/silicide stacked films have been realized using electron beam lithography and one step dry etching. Mechanisms of profile control involved in the SF6/O2 system are discussed. In addition, exploratory reactive ion etching studies using SiF4/Cl2 gas mixtures for these materials and TaSi2 have been carried out.

Calculation of Sparking Potentials of SF6 and SF6-GAS Mixtures in Uniform and Non-Uniform Electric Fields

A simple formula is developed to predict the sparking potentials of SF6 and SF6-gas mixture in uniform and non-uniform fields. The formula has been shown to be valid over a very wide range from 1 to 1800 kPa·cm of pressure and electrode gap separation for mixtures containing 5 to 100% SF6. The calculated values are found to be in good agreement with the previously reported measurements in the literature. The formula should aid design engineers in estimating electrode-spacings and clearances in power apparatus and systems.

Relative influence of hydrogen/deuterium donors and driving circuit parameters on the performance of a uv preionized pulsed HF/DF laser

The influence of the preionizer and initiating discharge circuit parameters on the performance of a UV preionized pulsed HF/DF chemical laser with relatively low active volume (about 20 cm3) has been investigated. The laser has been operated with gas mixtures of SF6 and H2, C2H6, and D2 at pressures ?90 Torr, giving laser output energies up to ∼70 mJ with a maximum efficiency of about 2.2%. We observed that the insertion of a peaking capacitor bank across the main discharge gap strongly affected the laser output characteristics. The laser output energy was seen to vary with the capacitance of the peaking capacitor bank, whose influence is a function of the SF6 partner in the laser mixture, the gas pressure, and the input energy density into the discharge.

Boltzmann equation analysis of the electron swarm development in SF6 and nitrogen mixtures

The ionisation alpha and electron attachment eta coefficients and other electron swarm parameters are calculated for SF6 and nitrogen gas mixtures for E/p0=75-200 V cm-1 Torr-1 by a Boltzmann equation method, in which the effect of generation and loss of free electrons due to ionisation and attachment are fully considered, for the steady-state Townsend (SST) experiment. The results show that the deduced values of the effective ionisation coefficient alpha (= alpha - eta ) are in good to fair agreement with those measured by Itoh et al. (1979) by a SST method, but agreement is less satisfactory with the experimental values of Aschwanden (1979) by a pulse method. The limiting E/p0 value, at which alpha = eta , is also calculated to give slightly higher values than previous experiments. The variation of the electron energy distribution, electron drift velocity, diffusion coefficient and mean energy, and the excitation frequency to C3 Pi u of nitrogen, as a function of the partial SF6 pressure, are calculated and discussed.

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

The outstanding electrical and arc-quenching characteristics of sulphur-hexafluoride (SF6) have already set new trends and standards for the construction of high voltage equipment. Intensive research is being carried out on the electrical properties of SF6, other electronegative gases and their mixtures with more common gases. The aims are to obtain an economical insulation with optimal electrical properties and to acquire a better understanding of their prebreakdown and breakdown behavior. Recently [1] we reviewed the breakdwon mechanisms operating in compressed gases and discussed the theoretical results in the light of experimental measurement in SF6. This paper deals with the breakdown and prebreakdown behavior of SF6 mixtures with other electron attaching and non-attaching gases. Uniform and non-uniform field breakdown under the applications of direct, alternating and impulse voltages is discussed. Improvement in properties such as higher breakdown voltages in non-uniform field gaps, less sensitivity to the electrode surface roughness and to the presence of contaminants and improved arc interruption characteristics of SF6-gas mixtures are discussed.

Laser Induced Infrared Fluorescence: Thermal Heating, Mass Diffusion, and Collisional Relaxation in SF6

Experimental results of the study of 10-μ and 16-μ laser induced fluorescence in SF6 and SF6-rare gas mixtures are presented. The fluorescence at both wavelengths is found to exhibit a fast and a slow decay process. The major features of the experimental fluorescence decay curves are well described by a simple model with only two energy levels. The rate equations for this model system include terms which couple collisional relaxation with heat and mass transport. The fast fluorescence decay process at each wavelength is found to be characteristic of binary collisional deactivation. The rate of decay of the slow process, however, is controlled by transport of both heat and mass to the wall of the fluorescence cell with heat transport being the dominant effect. The experimental results and the analysis based on a two level model system are consistent with a two step vibrational deactivation mechanism. The first step is a rapid equilibration of the vibrational modes. The second step is a somewhat slower equilibration of vibrational and translational degrees of freedom.