Desorption Of Fluorine Research Articles

Electron induced reduction on AlF3 thin films

We studied the modifications introduced in the chemical structure of AlF3 films by electron irradiation using Auger electron spectroscopy (AES) and factor analysis (FA). We examined the effects of the current density and energy of the electrons on the film composition. We found that the irradiation produces lower aluminum oxidation states (AlFx with 0<x<3, and Al0), and that while this effect is independent of the electron density it presents a clear dependence on the primary electrons energy. After comparison of experiments on the dose dependence of AlF3 and AlFx reduction, for different energies, with Monte Carlo (MC) simulations, we propose possible mechanisms that lead to electron induced fluorine desorption.

Nitrogen doping of fluorinated amorphous carbon thin films: structural and optical properties evolution upon thermal annealing

The effects of nitrogen addition on the properties of fluorinated amorphous carbon (a-C:H:F) thin films produced by radiofrequency plasma enhanced chemical vapor deposition have been investigated. The modifications of the structural and optical properties of the material upon thermal annealing were analyzed by: Raman spectroscopy; X-ray photoelectron spectroscopy; ellipsometry; as well as measurements of the film thickness. It has been found that nitrogen addition can thermally stabilize, in terms of better dimensional stability, the as-deposited films. X-Ray photoelectron spectroscopy combined with Raman measurements revealed that for the films grown with nitrogen dilution, nitrogen is bonded to carbon within a graphitic framework and that a nitrogen loss occurs upon thermal treatment evolving in a more stable graphitic configuration. The variation of the refractive index after annealing has been correlated to structural changes due to the desorption of both fluorine and nitrogen from the film. The experimental findings suggest that carrying out a thermal treatment, the films grown with the highest nitrogen fraction undergo a reduction of the refractive index to a value that is comparable with the one obtained for the nitrogen-free a-C:H:F film.

Effect of X-ray Irradiation on the Chemical and Physical Properties of a Semifluorinated Self-Assembled Monolayer

The effect of X-ray irradiation on the chemical and physical properties of a semifluorinated self-assembled monolayer (CF-SAM) derived from 1H,1H,2H,2H-perfluorodecanethiol (CF3(CF2)7(CH2)2SH) adsorbed on gold and copper substrates was studied using X-ray photoelectron spectroscopy. During the initial period of irradiation, the effects of electron-stimulated C−F, C−C, and S−X (X = Au, Cu) bond breaking are responsible for changes in the chemical composition of the CF-SAM. Furthermore, the evolution of the C(1s) X-ray photoelectron spectral region indicates that C−F rather than C−C bond cleavage dominates desorption within the CF-SAM. Except for fluorine desorption, irradiation-induced changes to the chemical and structural properties of the CF-SAM were most pronounced during the initial stages of irradiation, prior to the development of a highly cross-linked carbonaceous overlayer. X-ray irradiation of CF-SAMs adsorbed on Au also resulted in the production of new irradiation-induced sulfur species. A comparison with experiments carried out on a n-hexanedecanethiol (CH3(CH2)15SH)-based SAM revealed that the concentration and distribution of these irradiation-induced sulfur species were both sensitive to the SAM's initial chemical composition. On Cu substrates, native CF-SAMs formed an effective barrier to oxygen diffusion although the film's permeability to oxygen increased for CF-SAMs pre-exposed to X-ray irradiation.

Shape transition of calcium islands formed by electron-stimulated desorption of fluorine from a CaF2(111) surface

Extremely low-energy electrons emitted from a heated filament have been used to deplete fluorine from a cleaved CaF2(111) surface. Calcium left behind on the surface reorganizes itself in islands. A shape transition for the islands from a compact shape to an elongated shape has been observed for islands larger than ∼20 nm in diameter. Some cleavage steps show preferential fluorine erosion, which leads to the formation of very long calcium nanowires of uniform width ∼15 nm parallel to the step edge. The observed island and wire morphology is explained by energy minimization of strained commensurate islands.

Effects of electron irradiation on the structure and morphology of CaF 2/Si(111)

With the use of X-ray Standing Waves (XSW) and Atomic Force Microscopy (AFM), we have studied the structure and morphology of CaF 2 films (thickness range 3–15 Å) grown on Si(111) before and after modification by electron irradiation. The Ca monolayer of the as-grown CaF 2 monolayer relaxes towards the Si substrate. The F bilayer embedding the Ca monolayer has bulk spacing. Low Energy (80–100 eV) Electron-Stimulated Desorption (ESD) of fluorine reduces mainly the occupation of the top F layer due to generation of surface color centers. The Ca layer is affected only marginally by the electron bombardment. For 15 Å, CaF 2 films the color centers nucleate and form Ca clusters embedded in the film. These clusters are removed under ambient conditions (pin hole formation). Inspection of the holes shows that the Ca clusters penetrate the entire CaF 2 film.

Atomic force microscopy study of the CaF 2(111) surface: from cleavage via island to evaporation topographies

The development of (111) cleavage faces of CaF 2 upon thermal treatment is studied by atomic force microscopy (AFM). Vacuum-cleaved surfaces show the atomic periodicity of the uppermost fluorine layer. Cleavage steps have heights of about 0.32 nm, corresponding to the spacing of neighboring F −–Ca 2+–F − triple layers, or integer multiples thereof. The steps exhibit ‘lightning’-shaped arrangements with tips pointing in the direction of cleavage. Steps run essentially parallel to the 〈110〉, 〈211〉, and 〈321〉 directions. At about 820 K the original cleavage topography transforms into a topography with trigonal and hexagonal islands. The islands develop from the cleavage tips by a melt-off process mediated by step and/or surface diffusion. During further heat treatment at even higher temperatures the surface flattens and the formation of triangular islands with one type of 〈110〉 step is observed. The atomic configuration for this type of step is discussed. Strongly vaporized surfaces studied by an electron microscopic shadowing technique exhibit hexagonal evaporation pits. The exposure to stray electrons from the heater filament at a temperature of 820 K results in a strong surface erosion with deep triangular etch pits with edges parallel to 〈110〉. The formation of these etch pits is probably caused by a combined action of electron-stimulated desorption of fluorine, vaporization of metallic calcium clusters and a partial healing of the surface via step and/or surface diffusion.

Influence of Gas Desorption from SiOF Film Prepared by High-Density-Plasma Chemical Vapor Deposition upon TiN/Ti Film

The influence of gas desorption from fluorine-doped silicon oxide (SiOF) film prepared by high-density-plasma chemical vapor deposition (CVD) upon TiN/Ti film is investigated. In this investigation, two types of SiOF films, containing 14% and 6% fluorine atoms, are compared with regard to gas desorption and diffusion of fluorine, hydrogen and oxygen atoms into TiN/Ti film on SiOF film, as-deposited on TiN/Ti/SiOF film and after 400°C annealing. In addition, W film was prepared on TiN/Ti/SiOF film in order to examine the practical effect of gas desorption upon adhesion between Ti film and SiOF film. Thermal desorption mass spectroscopy (TDS) and secondary-ion mass spectroscopy (SIMS) spectral studies clarified the following. (1) SiOF film containing 14% fluorine atoms has a high hydrogen-fluoride content due to moisture absorption after exposure to air, while that containing 6% fluorine atoms does not. (2) During deposition of TiN/Ti film at 200°C, many fluorine and hydrogen atoms diffuse from SiOF film containing 14% fluorine atoms and are trapped within the Ti film or the Ti/SiOF interface. (3) During 400°C annealing, more fluorine atoms diffuse from SiOF film containing 14% fluorine atoms and are also trapped within the Ti film. (4) Fluorine atoms trapped within the Ti film degrade the adhesion between Ti film and SiOF film.

Effects of potential energy on sputtering ofF+from theCaF2(111)surface by noble-gas ion bombardment

The mechanism of ion- and electron-stimulated desorption of ${\mathrm{F}}^{+}$ from the ${\mathrm{CaF}}_{2}(111)$ surface has been investigated. By ${\mathrm{He}}^{+}$ bombardment, a considerable amount of the ${\mathrm{F}}^{+}$ ions are sputtered due to the potential energy of ${\mathrm{He}}^{+}.$ The yield of ${\mathrm{F}}^{+}$ via the potential sputtering from the polycrystalline ${\mathrm{CaF}}_{2}$ film is three times as large as that from the ${\mathrm{CaF}}_{2}(111)$ surface, suggesting that the fluorine at the lower coordination sites or the defect sites preferentially desorbs as ${\mathrm{F}}^{+}.$ The probability for the potential sputtering increases relative to that for the kinetic sputtering with increase of the primary energy of ${\mathrm{He}}^{+},$ whereas most of the ${\mathrm{F}}^{+}$ ions by ${\mathrm{Ar}}^{+}$ are kinetically sputtered. On the basis of these experiments as well as ab initio molecular-orbital calculations, it is concluded that desorption of fluorine is initiated by creation of the F $2s$ hole as a result of the nonadiabatic charge exchange with the primary ion. The ionization of the core-excited fluorine occurs on the way out from the surface due to the intra-atomic Auger decay. The fluorine can be ionized positively during collision with He backscattered from the solid due to the electron promotion mechanism, that is responsible for the kinetic sputtering of ${\mathrm{F}}^{+}.$

Stability of amorphous carbynoid surface: a comparative study of photoemission intensity from the valence and core states

X-ray photoelectron and X-ray excited Auger electron spectra (XPS and XAES, respectively) of a partially dehydrofluorinated PVDF film were measured and compared to those recorded in 1990. The prolonged storage of the film in air resulted in accumulation of oxygen in the film surface. The partial desorption of both oxygen and fluorine occurred under X-ray irradiation during ca. 180 min in a high-vacuum chamber of a spectrometer. This led to a significant modification of the C 1s line shape. The increase in the intensity of both the line itself and its satellite was observed, while the valence band XPS intensity remained unchanged.

Effect of Post Plasma Treatment on Reliability of Electron Cyclotron Resonance Chemical Vapor Deposited SiOF Films

Fluorine doped silicon dioxide (SiOF) films have a low dielectric constant. Unfortunately, the drawback of regarding SiOF is its low resistance to moisture which causes the increment of dielectric constant with time. Also, fluorine desorption of SiOF film during the postmetallization process causes degradation of metal interconnections. In this study, oxygen postplasma treatments are applied to as-deposited SiOF films for improving their dielectric properties and thermal stabilities. It has been observed that the postplasma treatment of SiOF films is quite an efficient method for blocking moisture and for the stabilization of the dielectric constant. Furthermore, the O2 postplasma treatment of Cu/W–N/SiOF/Si stacks by suppressing interdiffusion improved the thermal stability between SiOF and metal interconnections.

Integration of a stack of two fluorine doped silicon oxide thin films with interconnect metallization for a sub-0.35 μm inter-metal dielectric application

Fluorine doped silicon oxide films were deposited on HDP-CVD system and on PECVD system to realize a stack to be integrated in metal lines' architecture. Resistance to moisture absorption of both films was investigated by film exposure in humid atmosphere for 1 week followed by an annealing. Physical properties of uncapped FSG films were measured before and after test in humid atmosphere and after outgassing. Moisture absorption is increasing with the fluorine content for both films, and this moisture absorption creates fluorine desorption, clearly visible after outgassing, for concentration above 6%at. fluorine. The mechanical stress, density and refractive index were also measured to follow the stability evolution. A very stable process was confirmed for both FSG HDP and PECVD layers for a fluorine concentration less than 6%at. Finally it was demonstrated the capability to reach a dielectric constant at 3.5±0.05 for FSG HDP-CVD films. In a second step, integration was evaluated. No problem occurs for chemical mechanical polishing of FSG films, via etching, metal barrier adhesion and W plug metallization leading to a partial integrated structure. These results are very promising for the integration of FSG films as intermetal dielectrics for devices.

The Influence of Fluorine Desorption from ECR-CVD SiOF Film

AbstractThe influence of fluorine desorption from SiOF films deposited by biased ECR-CVD was studied. It was found that desorbed F atoms from the SiOF film react with Ti suicide film resulting in forming SiF4 gas. The evolution of SiF4 gas caused the peel-off of the films.

Self-Developing Properties of an Inorganic Electron Beam Resist and Nanometer-Scale Patterning Using a Scanning Electron Beam

The self-developing properties of an AlF3-doped LiF inorganic resist are studied. Electron-stimulated desorption (ESD) from the resist film is characterized by Auger electron spectroscopy and X-ray photoelectron spectroscopy. A conventional scanning electron microscope is employed for nanometer-scale direct writing and for in situ surface observation. Results show that a metal-rich layer which is formed by rapid and preferential fluorine desorption due to ESD suppresses subsequent fluorine desorption. This indicates that the surface diffusion of this residual metal plays an important role in the self-developing reaction. Nanometer-scale lithography onto the LiF(AlF3) film is achieved using a scanning electron beam.

Atomic-hydrogen-induced desorption of fluorine from silicon surfaces

Extraction of fluorine adsorbates on silicon surfaces is induced by exposure to atomic hydrogen. The decay process of fluorine has been investigated with Auger electron microscopy and is discussed in terms of chemical kinetics. The desorption rate depends on the hydrogen exposure time, the substrate temperature, and the supply of atomic hydrogen. In the initial stage, the reaction proceeds with second-order kinetics with an activation energy of about 0.4 eV. Moreover, two hydrogen atoms seem to take part in the reaction at the same time, according to the rate dependence on the supply of atomic hydrogen. With the decrease of the fluorine coverage, a first-order process dominates the reaction, and its activation energy is about 0.56 eV. The fluorine adsorbates on silicon are completely removed by annealing above 200°C for 30 min, using the atomic hydrogen.

The interaction of XeF 2 with sapphire

The interaction of xenon difluoride (XeF 2) with the sapphire {0001} surface has been investigated using X-ray photoelectron spectroscopy. Exposure to XeF 2 etches off submonolayer amounts of carbonaceous residue present on the surface. The exposure leaves a surface of stoichiometry Al 2O 3F 3.6 with fluorine atoms bonded to both aluminum and oxygen atoms. Annealing of the fluorinated alumina surface to 100°C or above results in the slow desorption of fluorine in a variety of species. The interaction of XeF 2 with sapphire is similar to that of XeF 2 with silica (SiO 2), with the exception that fluorination of silica results in the preferential removal of oxygen to form SiOF 2. The greater resistance of the sapphire surface to room temperature etching of oxygen by XeF 2 may be associated with the bonding of oxygen atoms in bulk sapphire to four aluminum atoms, whereas in bulk silica each oxygen atom bridges only two silicon atoms. Based upon the saturation coverages of fluorine on silica and alumina at room temperature, and the subsequent rates of fluorine desorption from SiOF 2 and Al 2O 3F 3.6 at elevated temperatures, the fluorinated alumina is roughly as thermally stable as the fluorinated silica surface.

Inverse-photoemission spectroscopy of electron irradiated epitaxial CaF 2 on Si(111)

We have investigated the electronic properties of the empty states of epitaxial layers of CaF 2 on Si(111) substrate, for film thicknesses of a few monolayers. The changes induced in the inverse photoemission spectrum by the electron beam irradiation were measured at a fixed angle, and are discussed in terms of fluorine desorption. After stabilisation of the surface, k-resolved inverse photoemission spectroscopy (KRIPES) was performed and the dispersion of the four lowest empty states was measured. The lowest band has a metallic character and is attributed to a Ca + plane at the surface. Other features are tentatively assigned to interface states.

Halogenation of diamond (100) and (111) surfaces by atomic beams

Diamond (100) and (111) surfaces have been exposed to beams of atomic and molecular fluorine and chlorine in an ultrahigh-vacuum environment. X-ray photoelectron spectroscopy, low-energy electron diffraction, and thermal desorption techniques have been used to elucidate the chemistry involved. F atoms add to both the diamond (100)-(1×1) and (111)-(2×1) surfaces to form a carbon-monofluoride species which reaches a saturation level of approximately three-quarters of a monolayer at 300 K. In other aspects of their behavior, the diamond surfaces differ. On the (111) surface, the rate of fluorine atom uptake is, to first order, proportional to the open site concentration. Adsorption produces a dimming of the half-order electron-diffraction spots, suggesting the breaking of surface π-bonded chains to form regions of the bulk 1×1 reconstruction. The (100) surface uptake rate, though, is second order with respect to open site concentration and no electron-diffraction pattern is observed. This difference in behavior between the two surfaces is ascribed to the difference in bonding geometry, leading to severe steric hindrance to ordered adsorption on the (100) surface. The thermal desorption data show fluorine desorption over a wide temperature range (500–1200 K) on both surfaces indicating binding sites with a range of energies. Limited mass spectrometric data indicates that atomic fluorine is the major desorption product. These results imply that atomic fluorine will act in a fashion similar to hydrogen atoms in that they will break surface dimer bonds, desorb from the surface at an appropriate temperature without etching diamond, and abstract any surface hydrogen in deposition systems utilizing halocarbon species. The much larger chlorine atoms weakly chemisorb on the diamond (100) surface, producing a saturation coverage of less than half a monolayer at 300 K. The adlayer neither shows a distinct C-Cl peak in the x-ray photoelectron spectra nor exhibits any electron-diffraction pattern. In addition, thermal desorption studies indicate that the concentration of chlorine atoms monotonically decreases to virtually zero as the substrate is heated from 223 to 423 K. A small residual chlorine concentration remains up to 600 K, presumably due to binding at defect sites. This behavior implies that atomic chlorine will exhibit a less significant role in the surface chemistry of diamond deposition systems.

Influence of fluorine desorption on electronic structure of CaF 2—LCAO slab calculations

The electronic structure of a 12 molecular layer slab of CaF 2 with fluorine ions desorbed from the (111) surface was calculated using the LCAO method. During experimental investigations this surface very often undergoes either electron beam bombardment or uv radiation illumination. It has been already proved that such a treatment leads to fluorine desorption. Using the standard LCAO procedure with semiempirical parameters, proved to reproduce well the electronic structure of a bulk crystal, we obtained an additional band of states inside the energy gap, with valence and conduction bands almost unchanged. The position on the energy scale and the width of this new band correspond well with the electron energy loss spectra (EELS) for CaF 2.

Radiation damage in an adsorbed monolayer: PF 3 on Ru(0001)

The chemisorption of PF 3 on Ru(0001) has been studied using soft X-ray photoemission spectroscopy (SXPS) and photon stimulated desorption (PSD). The initial adsorption of PF 3 on ruthenium is predominantly molecular. PF 3 is found to be extremely sensitive to both electron and photon beam exposure, leading to the formation of adsorbed PF x fragmentation products and the desorption of fluorine. Annealing the beam-damaged overlayers results in the reformation of some adsorbed PF 3. PSD ion yields at photon energies near the P 2p absorption edge show that direct photoexcitations of the adsorbed PF 3 and PF 2 species lead to F + desorption. Photoexcitation of the the P 2p level of an adsorbed PF unit does not lead to direct F + emission, however.

7140. Electron-stimulated desorption of fluorine from barium fluoride films deposited on silicon substrates: JKN Sharman et al, J Appl Phys, 68, 1990, 2489–2492

7140. Electron-stimulated desorption of fluorine from barium fluoride films deposited on silicon substrates: JKN Sharman et al, J Appl Phys, 68, 1990, 2489–2492