Dolores Rivero Fernández

Photo-induced chemical vapour deposition of silicon oxide thin films

A review of the photo-induced chemical vapour deposition (photo-CVD) processes yielding silicon oxide thin films that have emerged in the last decade is presented. Both lasers and UV lamps as photon sources are included. The basic principles, processing conditions, precursors, geometries, advantages and limitations of the various types of photo-CVD processes are described and compared. Their technological applicability and potential for industrial large-scale installation are discussed.

Gas mixture dependence of the LCVD of SiO2 films using an ArF laser

Abstract Large-area silica films have been deposited on silicon wafers using silane, nitrous oxide and argon gas mixtures and an ArF excimer laser in parallel configuration. An exhaustive study has been carried out on the role of the total and partial pressure of the various components of the gas mixture on the growth and the properties of silica films. Films are characterized by FT-IR spectroscopy and ellipsometry.

Ceramic coating for high-temperature corrosion protection by laser-CVD processes

Abstract In this paper we report the potentials of laser chemical vapour deposition (laser-CVD) processes in the high-temperature corrosion protection field. Amorphous silica coatings are produced on metallic substrates by using an ArF excimer laser in parallel configuration to photolytically induce the reaction between SiH 4 and N 2 O, using Ar as a buffer gas. It is shown that by tuning the laser beam to the substrate distance stoichiometric SiO 2 films are deposited with different physico-chemical properties, namely refractive index, density and elastic modulus as determined by ellipsometry, FTIR, mass-volume measurements, nanoindentation, SEM, EDAX and EMPA. Corrosion tests of coated and uncoated Incoloy 800H samples have demonstrated that these LCVD silica coatings significantly improve the resistance of the metallic substrates to corrosive attack in aggressive atmospheres, even when submitted to thermal cycling. No sign of coating spalling or cracking was observed after nearly 2000 hours of exposure to the high-temperature corrosion test.

Modification of silicon nitride films to oxynitrides by ArF excimer laser irradiation

Abstract An intensive study of the effects induced by the ArF excimer laser irradiation of silicon nitride films has been carried out. Amorphous silicon nitride films were deposited on silicon substrates by CO 2 laser-induced chemical vapour deposition (LCVD) in parallel configuration using a SiH 4 /NH 3 /Ar gas mixture. Post-deposition UV photon irradiation of the silicon nitride films was performed at room temperature and in an inert gas atmosphere. Changes in composition and refractive index were systematically followed by Fourier transform infrared spectroscopy (FTIR), and single-wavelength ellipsometry. The FTIR studies show that the initial ArF laser pulses provoke the elimination of bonded hydrogen (Si-H, N-H) incorporated in the as-deposited silicon nitride films. By increasing the UV irradiation time, incorporation of oxygen in the film is observed allowing the film composition to be tailored from silicon nitride to silicon oxide. This tendency is confirmed by a gradual decrease in the film refractive index. The proposed film modification mechanism is basically the breaking of Si-H, N-H and Si-N bonds, followed by a reaction with the adsorbed water in the films.

Study of the gas-phase parameters affecting the silicon-oxide film deposition induced by an ArF laser

A study of the gas-phase parameters involved in ArF laser induced chemical vapour deposition of silicon-oxide thin films is presented. A complete set of experiments has been performed showing the influence of the concentration of the precursor gases, N2O and SiH4, and their influence on total and partial pressures on film growth and properties. In this paper we demonstrate the ability of this LCVD method to deposit silicon oxide films of different compositions and densities by appropriate control of gas composition and total pressure. Moreover, a material specific calibration plot comprising data obtained using different preparation techniques is presented, allowing determination of the stoichiometry of SiOxfilms by using FTIR spectroscopy independently of the deposition method. For the range of processing conditions examined, the experimental results suggest that chemical processes governing deposition take place mainly in the gas phase.

Low-temperature silicon oxide films deposited using a CO2 laser

Abstract Continuing our previous work on hydrogenated amorphous silicon deposition with a CW CO 2 laser, our group has performed a systematic study of low-temperature silicon oxide film deposition for passivating purposes. Thus, the parallel configuration and silicon wafers as substrates have been used. Starting from silane-argon mixtures, we have added nitrous oxide as oxidizing agent, resulting in the formation of silicon oxide films on the substrate. Therefore, the growth rate and properties of the films can be tuned widely by changing the SiH 4 /N 2 O ratio at a constant partial pressure of silane. Sample characterization was carried out by ellipsometry, FT-IR spectroscopy and etch rate tests.

The effect of coating density on the strain tolerance of corrosion resistant amorphous silica

Abstract Amorphous silica coatings were deposited on Incology 800H by laser chemical vapour deposition (LCVD) using an ArF excimer laser to photolytically induce reaction of SiH 4 and N 2 O at 250°C. By varying the process parameters, particularly the beam substrate distance, coating density was varied and three different films were obtained. The coating density was directly determined from mass and volume measurements and also indirectly by using ellipsometry and Fourier transform infra-red spectroscopy (FTIR), and good agreement was found in the data produced by these methods. During corrosion testing coatings were subjected to tensile strain by heating to temperatures in excess of the deposition temperature. Fully dense silica, 1 μm in thickness, cracked when heated to 450°C whereas lower density films, 2 μm thick did not fracture even when heated to 900°C. Fracture of the scale was determined by examination in the scanning electron microscope at ambient temperature, but sulphide decoration of the cracks was also carried out at temperature to verify the existence of crack formation. Modulus values were determined using nanoindentation techniques. The fracture behaviour of the films is interpreted in terms of the mechanical and thermal properties of the silica layer.

Deposition of amorphous silicon nitride thin films by CO2 laser-induced chemical vapour deposition

Abstract Hydrogenated amorphous silicon nitride films have been prepared by laser-induced chemical vapour deposition from NH3SiH4Ar gas mixtures, heated by the gas-phase absorption of CO2 laser radiation. Samples have been characterized by ellipsometry (λ = 632 nm) and Fourier transform infrared spectroscopy to determine refractive index, thickness and bond composition. With regard to the power density effect, there is a non-linear dependence of the reaction process leading to film growth, opposite to what has been found for the SiO2 deposition from silane and nitrous oxide using the same experimental set-up. Starting from certain conditions, by only increasing the laser power from 24 to 30 W a dramatic change in growth rate is obtained from 10 to 200 A/min. Moreover by holding constant all other processing parameters and increasing the total pressure the deposition rate could be further enhanced, the upper limit being when the formation of powdery deposits began. The study of the dependence of growth rate on total gas pressure for a NH 3 SiH 4 = 20 gas flow rate ratio has demonstrated that the peak gas temperature controls the deposition rate, revealing that the process is driven by gas phase reactions. A strong influence of the NH 3 SiH 4 gas flow ratio on the film properties has been observed. An increment of this ratio produces a drop in the refractive index, indicating that the film incorporates more nitrogen. This behaviour is corroborated by infrared spectra. The SiN stretching peak shifts from 844 to 835 cm−1 and a diminution of the full width at half-maximum is observed. These changes are probably related to the incorporation of bonded hydrogen.

High-temperature corrosion-resistant ceramic coatings obtained by laser chemical vapour deposition

An ArF excimer laser-induced CVD method is described in which amorphous silicon and silica layers were deposited on Incoloy 800H and 214Cr1Mo steel substrates from a gas mixture of SiH4, (N2O) and Ar for corrosion protection at high temperatures. A parallel configuration was used, which would allow easy scale-up of the method for large area deposition. All process parameters (SiH4/N2O ratio, total and partial pressures, substrate temperature, laser repetition rate, energy density and beam-substrate distance) had been previously optimized for SiO2 deposition on Si wafers. Based on this experience, SiO2 and a-Si films have been deposited on both metallic substrates with and without a sputter ion-plated TiN interlayer. Samples were characterized by SEM, EDAX and AES. Analyses taken before and after corrosion testing in an aggressive environment at 450°C have demonstrated that these LCVD SiO2 films perform well as diffusion barriers and were resistant to spalling.

Excimer laser CVD of silicon oxide on GaAs: a comparison with deposition on c-Si

Abstract Conventional low temperature techniques for deposition of silicon oxide on GaAs can encounter problems in achieving adherent films. We have deposited uniform, non-spalling and adherent silicon oxide films on polished GaAs substrates by photo-CVD using an ArF excimer laser (193 nm) in parallel configuration. The source gases were silane and nitrous oxide diluted in argon (purge gas). A study of the growth rate and refractive index, measured by ellipsometry, as well as FTIR spectroscopic analyses of the simultaneously deposited SiO x films on both GaAs and Si substrates is reported. Film properties on GaAs substrates are very similar to those obtained on silicon substrates. The flexibility of this LCVD approach has allowed the growth of silica films on both substrates with properties very close to those of thermal silicon oxide.