Optical properties of PECVD dielectric thin films: thickness and deposition method dependence

Low-k dielectric carbon doped silicon dioxide films 105-1255 nm in thickness, prepared by plasma-enhanced chemical vapor deposition (PECVD) in a six-station sequential deposition system and in a single deposition station, have been investigated for their optical properties using an optical spectrometer coupled with a hot stage. A decrease in refractive index, n, for films with six sub-layers compared with films with a single layer of similar thickness has been observed. This decreased refractive index is thought to be caused by the different effect of crystallinity of the substrate, as a film interface effect is introduced due to the different deposition methods. Both types of PECVD thin films show an increasing refractive index with increasing thickness, which could be attributed to the increased effective density with the increased thickness indicated from Fourier transform infrared spectroscopy microstructure analysis. Cauchy dispersion function is found to be valid for films within all the thickness range and with different deposition methods from visible spectrum to IR spectrum. The refractive index is found to decrease as the temperature increases from 25 to 450 °C at a fixed wavelength for all the films.     

Thickness dependent glass transition temperature of PECVD low-k dielectric thin films: effect of deposition methods

Low-k dielectric carbon-doped silicon dioxide films created by Plasma Enhanced Chemical Vapor Deposition (PECVD) using a six-station sequential deposition system exhibit different glass transition behavior from films created by PECVD in a single deposition station. The enhanced glass transition temperature (T_g) for the PECVD thin films of a layer consisting of six sub-layer deposited in a six-station sequential deposition system to the T_g for films of a single layer deposited in a single deposition system is traced back to the introduced film interface effect inherent to the different deposition methods. Both types of PECVD thin films range in thickness from 50 to 1255 nm and show an increasing T_g with decreasing film thickness. The observed glass transition behavior for films with six sub-layers can be well explained by a theoretical model of thickness dependent T_g for multiple sub-layers obtained by modifying the currently existing theoretical model for the single layer thickness dependent T_g behavior, which explains the observed thickness dependent T_g for single layer PECVD thin films.     

Characteristics of thin film transistors fabricated in polysilicon films deposited by plasma enhanced chemical vapor deposition

Thin-film transistors (TFTs) fabricated in polysilicon films deposited by plasma enhanced chemical vapor deposition (PECVD) were characterized. The transistors were fabricated using a low temperature process (i.e., <- 700° C). The characteristics of the devices were found to improve as the deposition temperature of the polysilicon film increased. The best characteristics (μ _FE of 15 cm²/V _s andV _TH of 2.2V) were measured in the devices fabricated in the film deposited at 700° C. The devices fabricated in the PECVD polysilicon films were compared to those fabricated in polysilicon films deposited by thermal CVD in the same reactor in order to decouple the effect of the plasma. A coplanar electrode structure TFT with adequate characteristics (μ _FE of 8 cm²/V _s) was also demonstrated in the PECVD polysilicon films.     

Variation of the parameters and composition of thin films of porous silicon as a result of oxidation: Ellipsometric studies

The variation of the optical characteristics of thin films of oxidized porous silicon as a function of the preparation regime and subsequent heat treatment is investigated by ellipsometry. It is shown that the refractive index, optical thickness, and extinction coefficient of porous silicon films decrease monotonically, but the film thickness increases as the degree of oxidation of the silicon base layer increases. An analysis of the film thickness as a function of the degree of oxidation shows that it differs very little from the same dependence for the nonporous film. The composition of the films is determined from the measured refractive index at a wavelength λ=632.8 nm by means of curves calculated on the basis of the three-component Bruggeman model of the effective medium for layers with different initial porosities. Fiz. Tekh. Poluprovodn. 33, 1264–1270 (October 1999)     

Polarization characteristics of surface plasmon resonance in SnO<Subscript>2</Subscript> nanocluster films

Internal reflection features caused by the surface plasmon resonance in nanoscale films containing defect tin dioxide clusters in the stoichiometric dielectric matrix are studied by the method of polarization modulation of electromagnetic radiation. The angular and spectral characteristics of reflectances R _s ² and R _p ² of s- and p-polarized radiation and their polarization difference ρ = R _s ² − R _p ² are measured in the wavelength range λ = 400–1600 nm. The experimental characteristics ρ(ϑ, λ) (ϑ is the radiation incidence angle) obtained represent the optical property features associated with the film structure and morphology. Surface plasmon polaritons and local plasmons excited by s- and p-polarized radiation are detected; their frequency and relaxation properties are determined. The structural sensitivity of the technique for studying the surface plasmon resonance for tin dioxide films is shown.     

ZnSe/ITO thin films:candidate for CdTe solar cell window layer

The crystal structure, electrical and optical properties of ZnSe thin films deposited on an In₂O₃:Sn (ITO) substrate are evaluated for their suitability as the window layer of CdTe thin film solar cells. ZnSe thin films of 80, 90, and 100 nm thickness were deposited by a physical vapor deposition method on Indium tin oxide coated glass substrates. The lattice parameters are increased to 5.834 Å when the film thickness was 100 nm, which is close to that of CdS. The crystallite size is decreased with the increase of film thickness. The optical transmission analysis shows that the energy gap for the sample with the highest thickness has also increased and is very close to 2.7 eV. The photo decay is also studied as a function of ZnSe film thickness.     

Electrical properties of sputtered (Ba, Sr)TiO3 thin films prepared by two-step deposition method

The influence of two-step deposition on the electrical properties of sputtered (Ba,Sr)TiO₃ thin films was investigated. BST thin films with thickness 40 nm were deposited by a simple two-step radio frequency-magnetron sputtering technique, where the BST thin film consisted of a seed layer and a main layer. The dielectric constant was strongly dependent on the thickness of seed layer, but there was no dependence on deposition temperature of the seed layer. For a 2 nm seed layer, the dielectric constants were higher by about 29% than those of single-step BST thin films due to higher crystallinity and the leakage current was nearly the same as that of a single-step sample in bias voltage from −2 to 2.5 V. However, an improvement of the dielectric constant was not observed for samples having above 4 nm thick seed layers. A 40 nm thick BST film with 2 nm thick seed layer deposited by a two-step method exhibited a SiO₂ equivalent thickness of 0.385 nm and a leakage current density of 2.74 × 10⁻⁸A/cm²at+1.5V after post-annealing under an atmosphere of flowing N₂ for 30 min at 750°C.     

Optical properties of nanostructured lead sulfide films with a <Emphasis Type="Italic">D</Emphasis>0<Subscript>3</Subscript> cubic structure

The optical transmittance of nanostructured cubic PbS (lead sulfide) films with a D0₃ structure is measured in the wavelength range 300 to 3200 nm. The film thicknesses are in the range ∼120 to ∼400 nm. Electron microscopy of the microstructure shows that about half of the films’ particles are 60 nm or smaller in size. As the average particle size is decreased, the band gap E _g increases from 0.85 to 1.5 eV, being noticeably larger than the band gap of coarse-grained PbS, 0.41 eV. This is indicative of a blue shift of the optical absorption band in nanostructured PbS films.     

Effect of deposition methods on dielectric breakdown strength of PECVD low-k carbon doped silicon dioxide dielectric thin films

The effect of deposition methods on dielectric breakdown strength of PECVD low-k dielectric carbon doped silicon dioxide films is investigated. I-V measurements were performed using metal-insulator semiconductor structures for carbon doped silicon dioxide thin films with various thicknesses by single deposition station and six sequential deposition systems. I-t measurements are also performed for films with the thickness of 32 nm prepared using both deposition methods. Comparison studies have been carried out for the thickness dependence, temperature dependence, conduction mechanism and time dependence of dielectric breakdown for carbon doped silicon dioxide with single layer and six sub-layers. Results demonstrated that both films follow the newly obtained relationship between dielectric strength E_B and thickness d, i.e. E_B∝(d−d_c)⁻ⁿ, but with a lower exponential factor n and a larger thickness limit d_c for films with six sub-layers. It is also demonstrated that films with six sub-layers have a higher dielectric strength in all the thickness and temperature ranges, a thickness independent thermal behavior and a longer lifetime under constant voltage stressing. This indicates that by tuning the deposition methods smaller thickness with desired dielectric properties can be achieved.     

High-quality conformal silicon oxide films prepared by multi-step sputtering PECVD and chemical mechanical polishing

High-quality conformal oxide films were obtained by using multi-step sputtering (MSSP) plasma enhanced chemical vapor deposition (PECVD) process with argon ion sputtering and chemical mechanical polishing (CMP). The repeated deposition by plasma enhanced chemical vapor deposition (PECVD) and anisotropic etching of oxide films by multi-step sputtering PECVD improve the step coverage and gap filling capability significantly. The argon plasma treatment enhances the binding energy of Si-O in the SiO₂ network, and the temperature dependence of stress for MSSP oxide film showed no hysteresis after the heating cycle up to 440 °C. The stress-temperature slope of MSSP oxide film was found to be much less than that of conventional PECVD oxide film. The slope for 1.1 μm thick film is about 5.8×10⁵ dynes/cm²/°C which is smaller than that of thermally grown oxide film. It seems that MSSP oxide film reduces stress-temperature hysteresis and becomes more dense and void-free in the narrow gaps with inter-metal spacing of 0.5 μm. After filling of the narrow gap, we adopted the CMP process for global planarization and obtained good planarization performance. The uniformity of the film thickness was about 4% and the degree of the planarization was over 95% after CMP process.     

Optical properties of thin GaSe/<Emphasis Type="Italic">n</Emphasis>-Si(111) films

Morphological and optical studies (ellipsometry and reflectance spectroscopy in the ranges 400–750 nm and 1.4–25 μm) of thin GaSe films fabricated by thermal evaporation on the n-Si (111) single-crystal substrates are reported. The film thickness was 15–60 nm. It is established that, in the initial stage of growth, the growth of GaSe on the n-Si (111) substrates occurs via formation of islands (three-dimensional growth). It is shown that, as the thickness increases, the physical parameters of the film change and the films approach single crystals in crystalline and energy band structure. For films with a thickness of 60 nm, the reflectance band peak is attributed to indirect optical transitions enhanced by reflection from the film-substrate interface. From the results of optical studies, quantum effects in the surface region of the thin films are conjectured.     

Growth-Temperature-Controlled Optical Properties of Textured MgxZn1?xO Thin Films

Pulsed laser deposition was used to grow magnesium zinc oxide thin films on amorphous fused silica substrates at several temperatures between room temperature and 750°C. In this study, the effect of growth temperature on the optical properties of textured Mg _x Zn_1−x O thin films was examined. The optical properties of the films were measured using absorption and photoluminescence spectrometry. Absorption spectra revealed that the bandgap values of textured Mg _x Zn_1−x O thin films were enhanced in films grown at higher temperatures. The absorption spectra near the absorption edge were fitted using the Urbach equation in order to investigate the effects of growth temperature on exponential band tail and bandgap. The photoluminescence spectra were measured for magnesium zinc oxide thin films deposited at 250°C, 350°C, 450°C, 550°C, and 650°C. The film grown at 350°C provided the highest excitonic peak intensity. On the other hand, the film grown at 250°C exhibited the lowest excitonic peak intensity. The excitonic peak intensity was considerably reduced in magnesium zinc oxide thin films grown at temperatures greater than 350°C. The ability to perform substrate-temperature-dependent bandgap engineering of Mg _x Zn_1−x O will enable use of this material in next-generation optical and optoelectronic devices.     

The Influence of Film Thickness on the Transparency and Conductivity of Al-Doped ZnO Thin Films Fabricated by Ion-Beam Sputtering

To evaluate the influence of film thickness on the structural, electrical, and optical properties of Al-doped ZnO (AZO) films, a set of polycrystalline AZO samples with different thickness were deposited on glass substrates by ion-beam sputtering deposition (IBSD). X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive x-ray spectroscopy (EDS), four-point probe measurements, and spectrophotometry were used to characterize the films. XRD showed that all the AZO films had preferred c-axis orientation. The ZnO (110) peak appeared, and the intensity increased, with increasing thickness. All the samples exhibited compressive intrinsic stresses. AFM showed that the grain size along with the root-mean-square (RMS) roughness increased with increasing thickness. The decrease of resistivity is due to the corresponding change in grain size, surface morphology, and chemical composition. The average optical transmittance of the AZO films was over 80%, and a sharp fundamental absorption edge with red-shifting was observed in the visible region. The optical band gap decreased from 3.95 eV to 3.80 eV when the AZO film thickness increased from 100 nm to 500 nm.     

Photo CVD system for silicon nitride film

An ultraviolet light excitation photo CVD system for silicon nitride film deposition, in which the use of mercury photo-sensitizer and the undesirable wall deposition onto the optical window inside are eliminated, has been developed. The elimination of the use of mercury sensitizer is achieved by employing direct photolysis of SiH₄/NH₃ gas mixture, using 185 nm light emitted from a low pressure mercury lamp. The wall deposition prevention is achieved by inserting an optically transparent “separator plate” with a number of through-holes on its plane area underneath the optical window and by draining inert gas into the reaction chamber through this “separator plate.” With this system, silicon nitride films have been deposited without marked degradation of deposition rate, keeping a reasonable deposition rate of about 40 A/minute. The inert gas used for wall deposition prevention has no influence on the properties of deposited films.     

Complex Refractive Indices of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O Thin Films Grown by Molecular Beam Epitaxy

The complex refractive indices of Cd _x Zn_1−x O thin films were determined by transmission spectrophotometry. Transmission spectra were modeled from 375 nm to 800 nm for samples having cadmium concentrations ranging from 2% to 77%. The transparent and absorptive regimes were fitted separately by Sellmeier and Forouhi–Bloomer models, respectively. Real refractive indices of Cd _x Zn_1−x O shift to higher values in the transparent region and the optical absorption edge shifts to longer wavelengths with increasing cadmium concentration. Spectroscopic ellipsometry was carried out on one sample from λ = 190 nm to 1.8 μm. Comparison between the two methods shows that the results are in general agreement.     

The properties of Si/Si1-xGex films grown on Si substrates by chemical vapor deposition

A growth parameter study was made to determine the proper of a SiGe superlattice-type configuration grown on Si substrates by chemical vapor deposition (CVD). The study included such variables as growth temperature, layer composition, layer thickness, total film thickness, doping concentrations, and film orientation. Si and SiGe layers were grown using SiH₄ as the Si source and GeH₄ as the Ge source. When intentional doping was desired, diluted diborane for p-type films and phosphine for n-type films were used. The study led to films grown at ∼1000°C with mobilities from ∼20 to 40 percent higher than that of epitaxial Si layers and ∼100 percent higher than that of epitaxial SiGe layers grown on (100) Si in the same deposition system for net carrier concentrations of ∼8x10¹⁵ cm^-3 to ∼2x10¹⁷ cm^-3. Enhanced mobilities were found in multilayer (100)-oriented Si/Si_1-xGe_x films for layer thicknesses ≥400A, for film thicknesses >2μm, and for layers with x = 0.15. No enhanced mobility was found for (111)-oriented films and for B-doped multilayered (100)-orlented films. Supported in part by NASA-Langley Research Center, Hampton, VA, Contract NAS1-16102 (R. Stermer & A. Fripp, Contr. Mon.)     

Field Emission and Electric Discharge of Nanocrystalline Diamond Films

Nanocrystalline diamond (NCD) films were produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using gas mixtures of Ar, H₂, and CH₄. The structural properties, electron emission, and electric discharge behaviors of the NCD films varied with H₂ flow rates during MPECVD. The turn-on field for electron emission at a pressure of 2.66 × 10⁻⁴ Pa increased from 4.2 V μm⁻¹ for the NCD films that were deposited using a H₂ flow rate of 10 cm³ min⁻¹ to 7 V μm⁻¹ for films deposited at a H₂ flow rate of 20 cm³ min⁻¹. The NCD film with a low turn-on field also induced low breakdown voltages in N₂. The grain size and roughness of the NCD films may influence both the electron emission and the electric discharge behaviors of the NCD cathodes.     

Ultra Thin Film Encapsulation of Organic Light Emitting Diode on a Plastic Substrate

We have carried out the fabrications of a barrier layer on a polyethersulfon (PES) film and organic light emitting diode (OLED) based on a plastic substrate by means of atomic layer deposition (ALD). Simultaneous deposition of 30 nm AlO_x film on both sides of the PES film gave a water vapor transition rate (WVTR) of 0.062 g/m²/day (@38°C, 100% R.H.). Further, the double layer of 200 nm SiNx film deposited by plasma enhanced chemical vapor deposition (PECVD) and 20 nm AlOx film by ALD resulted in a WVTR value lower than the detection limit of MOCON. We have investigated the OLED encapsulation performance of the double layer using the OLED structure of ITO / MTDATA (20 nm) / NPD (40 nm) / AlQ (60 nm) / LiF (1 nm) / Al (75 nm) on a plastic substrate. The preliminary life time to reach 91% of the initial luminance (1300 cd/m²) was 260 hours for the OLED encapsulated with 100 nm of PECVD‐deposited SiNx and 30 nm of ALD‐deposited AlO_x.     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of the film thickness on the optical constants and dispersion parameters of chalcogenide Ge20Se70Ag10 thin film

Chalcogenide thin films of Ge₂₀Se₇₀Ag₁₀ of thicknesses 150, 300 and 450 nm are prepared by a thermal evaporation technique. The crystalline phases of the deposited film are identified by X-ray diffraction. The transmittance and reflectance of the films are measured in the wavelength range 200–2500 nm. The optical band gap decreases while the width of the localized states tail increases with increasing film thickness. Variation of refractive index and extinction coefficient with the film thickness is studied to analyze the optical efficiency of these films. Application of the single oscillator model to the films reveals that the oscillator energy decreases while the dispersion energy increases with increasing thickness. The variation of the optical constants suggests that the thickness change is a good choice to control the optical properties of Ge₂₀Se₇₀Ag₁₀ film.