Dissociation Of CF4 Research Articles

Theoretical modeling study of the reaction H + CF4 → HF + CF3

AbstractThe high‐temperature hydrogenation of CF4 in mixtures of CF4 and H2 is assumed to involve the reaction H + CF4 → HF + CF3. The hydrogen atoms here are either formed by the reaction of F and CF3 (i.e., the products of the thermal dissociation of CF4) with H2, or by the thermal dissociation of H2. In the former case, a complicated chain process is started, while the reaction proceeds in a more direct way in the latter. This article determines the rate constant of the reaction H + CF4 → HF + CF3, characterizing its transition state by quantum‐chemical methods. Over the temperature range 1000–3000 K, the most accurate results for the rate constant can be represented in the form 1.64 × 1014 (T/1000 K)1.95 exp(−178.8 kJ mol–1/RT) cm3 mol–1 s–1, based on coupled cluster theory extrapolated to the complete basis set limit, and incorporating vibrational anharmonicity, electron correlation through CCSDT(Q), and relativistic and non‐Born–Oppenheimer effects.

Photoionisation study of Xe.CF4 and Kr.CF4 van-der-Waals molecules

We report on photoionization studies of Xe.CF4 and Kr.CF4 van-der-Waals complexes produced in a supersonic expansion and detected using synchrotron radiation and photoelectron-photoion coincidence techniques. The ionization potential of CF4 is larger than those of the Xe and Kr atoms and the ground state of the Rg.CF4 (+) ion correlates with Rg(+) ((2)P3/2) + CF4. The onset of the Rg.CF4 (+) signals was found to be only ∼0.2 eV below the Rg ionization potential. In agreement with experiment, complementary ab initio calculations show that vertical transitions originating from the potential minimum of the ground state of Rg.CF4 terminate at a part of the potential energy surfaces of Rg.CF4 (+), which are approximately 0.05 eV below the Rg(+) ((2)P3/2) + CF4 dissociation limit. In contrast to the neutral complexes, which are most stable in the face geometry, for the Rg.CF4 (+) ions, the calculations show that the minimum of the potential energy surface is in the vertex geometry. Experiments which have been performed only with Xe.CF4 revealed no Xe.CF4 (+) signal above the first ionization threshold of Xe, suggesting that the Rg.CF4 (+) ions are not stable above the first dissociation limit.

Reactive high power impulse magnetron sputtering of CFx thin films in mixed Ar/CF4 and Ar/C4F8 discharges

The reactive high power impulse magnetron sputtering processes of carbon in argon/tetrafluoromethane (CF4) and argon/octafluorocyclobutane (c-C4F8) have been characterized. Amorphous carbon fluoride (CFx) films were synthesized at deposition pressure and substrate temperature of 400mPa and 110°C, respectively. The CFx film composition was controlled in the range of 0.15<x<0.35 by varying the partial pressure of the F-containing gases from 0mPa to 110mPa. The reactive plasma was studied employing time averaged positive ion mass spectrometry and the resulting thin films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by elastic recoil detection analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, and water droplet contact angle measurements, respectively. The experimental results were compared to results obtained by first-principles calculations based on density functional theory. The modeling of the most abundant precursor fragment from the dissociation of CF4 and C4F8 provided their relative stability, abundance, and reactivity, thus permitting to evaluate the role of each precursor during film growth. Positive ion mass spectrometry of both fluorine plasmas shows an abundance of CF+, C+, CF2+, and CF3+ (in this order) as corroborated by first-principles calculations. Only CF3+ exceeded the Ar+ signal in a CF4 plasma. Two deposition regimes are found depending on the partial pressure of the fluorine-containing reactive gas, where films with fluorine contents below 24at.% exhibit a graphitic nature, whereas a polymeric structure applies to films with fluorine contents exceeding 27at.%. Moreover, abundant precursors in the plasma are correlated to the mechanical response of the different CFx thin films. The decreasing hardness with increasing fluorine content can be attributed to the abundance of CF3+ precursor species, weakening the carbon matrix.

Dissociation limit and dissociation dynamic of CF4+: Application of threshold photoelectron-photoion coincidence velocity imaging

Dissociation of internal energy selected CF4(+) ions in an excitation energy range of 15.40-19.60 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Only CF3(+) fragment ions are observed in coincident mass spectra, indicating all the X(2)T1, A(2)T2, and B(2)E ionic states of CF4(+) are fully dissociative. Both kinetic energy released distribution (KERD) and angular distribution in dissociation of CF4(+) ions have been derived from three-dimensional TPEPICO time-sliced images. A parallel distribution of CF3(+) fragments along the polarization vector of photon is observed for dissociation of CF4(+) ions in all the low-lying electronic states. With the aid of F-loss potential energy curves, dissociation mechanisms of CF4(+) ions in these electronic states have been proposed. CF4(+) ions in both X(2)T1 and A(2)T2 states directly dissociate to CF3(+) and F fragments along the repulsive C-F coordinate, while a two-step dissociation mechanism is suggested for B(2)E state: CF4(+)(B(2)E) ion first converts to the lower A(2)T2 state via internal conversion, then dissociates to CF3(+) and F fragments along the steep A(2)T2 potential energy surface. In addition, an adiabatic appearance potential of AP0(CF3(+)∕CF4) has also been established to be 14.71 ± 0.02 eV, which is very consistent with the recent calculated values.

CFx thin solid films deposited by high power impulse magnetron sputtering: Synthesis and characterization

Fluorine containing amorphous carbon films (CFx, 0.16≤x≤0.35) have been synthesized by reactive high power impulse magnetron sputtering (HiPIMS) in an Ar/CF4 atmosphere. The fluorine content of the films was controlled by varying the CF4 partial pressure from 0mPa to 110mPa at a constant deposition pressure of 400mPa and a substrate temperature of 110°C. The films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by applying elastic recoil detection analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and nanoindentation. First-principles calculations were carried out to predict and explain F-containing carbon thin film synthesis and properties. By geometry optimizations and cohesive energy calculations the relative stability of precursor species including C2, F2 and radicals, resulting from dissociation of CF4, were established. Furthermore, structural defects, arising from the incorporation of F atoms in a graphene-like network, were evaluated. All as-deposited CFx films are amorphous. Results from X-ray photoelectron spectroscopy and Raman spectroscopy indicate a graphitic nature of CFx films with x≤0.23 and a polymeric structure for films with x≥0.26. Nanoindentation reveals hardnesses between ~1GPa and ~16GPa and an elastic recovery of up to 98%.

Neutral gas temperature estimate in CF4/O2/Ar inductively coupled plasmas

Neutral temperatures in inductively coupled CF4/O2/Ar mixture plasmas have been characterized by fitting the rotational envelope of emission spectra in a gaseous electronics conference reference cell. CF and CO are found to present different rotational temperatures with opposite trends with respect to pressure. It is proposed that CF is created as a rotationally and translationally hot molecule due to CF4 dissociation and the disparity in temperatures and temperature trend is indicative of incomplete equilibration of the CF with other neutral species in the plasma. This may have important consequences from the standpoint of understanding neutral reaction chemistries.

Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: I. Production of CF2 and CF via electron-impact dissociation

The kinetics of the production and loss of CF2 and CF radicals in a glow discharge in pure CF4 is investigated by the laser-induced fluorescence method. The effective rate constants for electron-impact dissociation of CF4 molecules along the pathways toward CF2 and CF radicals are determined within a wide range of the reduced electric field (80–250 Td). It is shown that, along with the direct electron-impact dissociation of CF4, the radicals are also produced via the dissociation of the CxFy polymer fluorocarbon particles that form in the plasma. A detailed analysis of the kinetics of the radical production and loss in a modulated discharge made it possible to evaluate the contribution of the electron-impact dissociation of CF4 to the production of radicals and, consequently, to determine the dissociation rate constants $$k_{CF_2 } $$ and k CF. A comparison of the obtained $$k_{CF_2 } $$ and k CF values with the results of calculations by the Monte Carlo method and the literature data on the cross sections for electron-impact dissociation of CF4 molecules enabled the normalization of these cross sections in the threshold region and the construction of the model cross sections for the electron-impact dissociation of CF4 into neutral products. The calculated cross sections allow a satisfactory description of the experimental results throughout the entire range of E/N under study. A significant scatter (up to 100%) in the experimental data on $$k_{CF_2 } $$ and k CF at low values of E/N is related to the considerable contribution of the CxFy polymer molecules (and, probably, CxF y + ions and fluorocarbon grains) to the production of CF2 and CF radicals both in the plasma volume and on the surface of a fluorocarbon film covering the discharge tube wall.

CF x radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase production of these species by direct dissociation of CF4, and accounts for the unexpectedly high concentrations observed. The difference in behavior between the powered and grounded electrode surfaces is explained by the difference in the incident ion energy and mass distributions.

A model-based technique for real-time estimation of absolute fluorine concentration in a CF4/Ar plasma

A technique for quantitative interpretation of actinometric data to deduce bulk plasma fluorine concentration in a CF4/Ar plasma has been developed and tested on a commercial reactive ion etcher. This static, in situ measurement is useful for monitoring fluorine in a manufacturing environment and, in particular, for application of real-time feedback control to plasma etching. Based upon a model of CF4 chemistry reaction pathways and products, it improves upon current fluorine estimation techniques by accounting for varying levels of argon dilution resulting from CF4 dissociation. A simple experiment was also developed in order to obtain an estimate of the actinometric scaling factor without an independent measurement of fluorine. Performance of this model-based fluorine estimation technique was compared to that of a standard technique by using time resolved etch rate measurements as an independent indicator of fluorine concentration, while a feedback control scheme decoupled the effects of physical etching by stabilizing the induced dc bias. The model-based estimator significantly reduced perturbations in the etch compared to those seen when using the standard estimator.

Absolute Cross Sections for the Electron‐Impact Dissociation of CF4 and CHF3 into the CFx (x = 1−3) Neutral Radicals

AbstractThe electron‐impact energy dependence of the dissociation cross sections of CF4 and CHF3 into the CFx (x = 1 − 3) free radicals was previously reported, in which the absolute values of the cross sections were tentatively estimated based on certain assumptions for the unknown ionization cross sections of the CF2 and CF radicals. These assumptions are no longer needed, since a complete set of absolute parent and dissociative ionization cross sections of CFx radicals has been reported recently by Becker and co‐workers. Based on these data, the absolute cross sections for the electron‐impact neutral dissociation of CF4 and CHF3 into CF3, CF2 and CF from the threshold up to 200–300 eV have now been accurately calibrated. Adjustments to the previously reported absolute cross sections of up to a factor of 16 were necessary.

Partial Cross Sections for Electron Impact Dissociation of CF4 into Neutral Radicals

The electron energy dependence of partial cross sections for dissociation of carbon tetrafluoride (CF4) into neutral radicals CF3, CF2 and CF has been measured. This was accomplished with threshold-ionization mass spectrometry in a differentially pumped dual-electron-beam device. The threshold energy for dissociation into each neutral radical was measured for the first time to be 12.5 eV, 15.0 eV and 20.0 eV for CF3, CF2 and CF radicals, respectively. The absolute cross sections for electron impact energy between 10 and 300 eV were determined through measurements of the radical sticking coefficient and electron-impact nitrogen dissociation. The magnitude of the cross section at its maximum is 0.39×10-20 m2 at 120 eV for CF3 radical, 0.22×10-20 m2 at 130 eV for CF2 radical, and 0.21×10-20 m2 at 110 eV for CF radical.

Metastable fragment production by electron-impact dissociation of CF4

Time-of-flight detection of neutral metastable fragments has been applied to the study of electron-impact dissociation of CF4 molecules. The time-of-flight distributions indicate that a number of distinct processes are involved in the production of metastable fragments. Excitation functions for these processes have been measured up to 400 eV. The results presented are compared with other studies of fragmentation of CF4 and indicate that fragmentation into an excited CF3+ ion and an excited F atom is a prominent process.

Electron kinetics in dc CF4 + O2 mixture discharge

The electron distribution function, transport coefficients and rate coefficients for the dissociation of CF4 and the excitation of a1 Δg and b1 ∑g+ states of oxygen have been calculated for a CF4 + O2 mixture discharge. The two-term approximation was used in solving the electron Boltzmann equation. The influence of the dissociation was ignored, so that the results are parametric in the density reduced electric fieldE/N and the ratio of the mixture components. Only slight dependence of the calculated quantities on the composition of the gas mixture has been observed. The critical electric field for the onset of the discharge was found to beE/N=1.1−1.25 × 10−15 Vcm2 and it is weakly decreasing with growth of the oxygen content. With a small amount of oxygen added to CF4 the electron distribution function and the mean kinetic energy remain practically unchanged.

Si and SiO2 Etching under Low Self-Bias Voltage

Si and SiO2 reactive ion etching and the associated radiation damage were investigated employing CF4 in the low self-bias voltage region (less than 100 V). The etching was carried out using the substrate tuning method. In the low self-bias voltage region, both Si etching and SiO2 etching proceed accompanied by the polymerization of CFx (x=0–3) radicals generated from the dissociation of CF4. The polymer deposited on the surface is decomposed by the subsequent ion bombardment to form F and CFx radicals, which take part in the etching and the polymer formation reaction again. The radiation damage induced in the Si by energetic fluorocarbons depends strongly on the magnitude of the self bias. However, the damage induced by tens of volts can easily be removed.

Reactant supply in reactive ion etching

In reactive ion etching, the etch rate may be reactant limited, so that a measurement of reactant supply is neccessary to understand the details of the etching process. A quantitative determination of reactant supply can be obtained by measuring the total dissociation of the etching gas. In particular, this paper reports the total dissociation of CF4 as a function of input power and pressure in a diode system. A quadrupole mass spectrometer measuring the CF3+ peak indicates the presence of CF4 as well as other molecules containing the (CF3) radical. A transient technique is used to separate the contributions of the other molecules from CF4 so that the total CF4 number density can be calculated. The production of reactant by the dissociation of Cf4 causes etching of silicon and SiO2 on the cathode of the system. Possible reaction sequences of these etching processes are modelled by using the known supply of reactant. The partial pressures of the etching products are calculated and correlated with the etch rates and the rise in total pressure.