Rodrigo Martins

The role of the deposition pressure and hydrogen dilution in the production of p-type Si:H films by hot wire chemical vapor deposition, was investigated. The system used permits to obtain uniform and homogeneous films properties over a 10cm×10cm substrate area. As heated filament we used Ta, since Ta filaments have longer life period without deteriorating than the W filaments ones. In this work, we show that the electrical properties of the films produced are dependent on the process gas pressure. In the pressure range of 13.3 Pa (0.1 Torr) to 66.5 Pa (0.5Torr), the film's coplanar electrical conductivity at room temperature varies by more than two orders of magnitude, for films produced at same hydrogen dilution and filament temperature, reaching values of about 0.1 (Ωcm)-1, at deposition pressures of about 40-53Pa (0.3-0.4Torr). On the other hand, the increase in hydrogen dilution (from 87% to 96%) promotes the surface roughness due to an enlargement of grain sizes in the direction of the {220} diffraction planes as observed by SEM micrographs without changing the crystalline fraction (48-50%) obtained by micro-Raman analysis.

This work analyse mainly the dissociation mechanism of the gas during the hot wire (HW) and hot wire plasma-assisted (HWPA) processes used to produce hydrogenated carbon films and silicon-carbide, using ethylene and silane as gas sources. The results show that ethylene is better decomposed by plasma-enhanced chemical vapour deposition (PECVD) than HW process. The data also show that the HWPA leads to a better carbon © incorporation than HW processes, when silicon carbide alloys are produced and, that the presence of atomic hydrogen (H) is beneficial for all processes. That is, the presence of the plasma and H lead to the formation of higher C radicals such as methylsilane (CH3SiH3), ethylsilane (C2H5SiH3) and silorane (C2H4SiH2), whose contributions are enhanced as the fraction of ethylene (Feth) in the gas mixture increases. © 2001 Published by Elsevier Science B.V.

We report in this paper the influence of the rf power on the properties of p-type silicon thin films produced by hot wire plasma assisted chemical vapor deposition (HWPA-CVD) technique, using a gas mixture containing SiH4, B2H6, CH4 and H2. The influence of the rf power in the film morphology, its structure and its composition has been determined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and infrared spectroscopy. The electrical dark conductivity, activation energy, optical band gap and growth rate values for the different rf power was also evaluated. The data achieved show that rf power rules the surface morphology, the film structure and its electrical characteristics. © 2001 Elsevier Science B.V. All rights reserved.

The performances of amorphous and nano-crystalline porous silicon thin films as gas detector are pioneer reported in this work. The films were produced by the hot wire chemical vapour deposition (HW-CVD). These films present a porous like-structure, which is due to the uncompensated bonds and oxidise easily in the presence of air. This behaviour is a problem when the films are used for solar cells or thin film transistors. For as gas detectors, the oxidation is a benefit, since the CO, H2 or O2 molecules replace the OH adsorbed group. In the present study we observe the behaviour of amorphous and nano-crystalline porous silicon thin films under the presence of ethanol, at room temperature. The data obtained reveal a change in the current values recorded by more than three orders of magnitude, depending on the film preparation condition. This current behaviour is due to the adsorption of the OH chemical group by the Si uncompensated bonds as can be observed in the infrared spectra. Besides that, the current response and its recover time are done in few seconds.

Flexible large area thin film position sensitive detectors based on amorphous silicon technology were prepared on polyimide substrates using the conventional plasma enhanced chemical vapor deposition technique. The sensors were characterized by spectral response, illuminated I-V characteristics position detectability measurements and atomic force microscopy. The obtained one-dimensional position sensors, 5-mm wide and 60-mm long, presented a maximum spectral response at 600 nm, an open circuit voltage of 0.6 V and a position detectability with a correlation of 0.9989 associated to a S.D. of 1×10-2, comparable to those produced on glass substrates.

In this work, we present the properties of n-type silicon films obtained by hot wire plasma assisted technique produced at different rf power and gas flow rate. The films were produced at a filament temperature of 2000°C and the rf power was varied from 0 W to 200 W while gas flow rate was varied from 15 to 100 sccm keeping rf power at 50 W. In this flow rate range, the growth rate of the films varied from 5Å/s to 250Å/s and the corresponding electrical room dark conductivity varied from 10-2 to 10(Ωcm)-1. On the other hand, we observed that the electrical conductivity increased from 2 to 6(Ωcm)-1, and the Hall mobility from 0.1 to 2 cm2/V.s as rf power change from 0 W to 200 W. The infrared, EDS and XPS analyses revealed the existence of oxygen incorporation, which is not related to post-deposition oxidation. The X-ray diffraction and μRaman data show the presence of Si crystals in the films structure and the SEM micrographs reveal a granular surface morphology with grain sizes lower than 60 nm.

This paper reports results on the role of high hydrogen dilution (above 80%) on the electro-optical and structural properties of boron doped silicon films produced by hot wire chemical vapor deposition (HW-CVD) technique, keeping constant the filament temperature. The structural, compositional, morphological, electrical and optical properties achieved show that the films present excellent homogeneity over the entire 10 x 10 cm deposited area. These results were obtained for films produced at gas pressures below 66.5 Pa, in spite of the high flow rate used. © 2001 Elsevier Science B.V. All rights reserved.

In this work, we report the optical and compositional properties of hydrogenated amorphous silicon carbide (a-SiC:H) thin films produced by plasma-enhanced chemical vapor deposition (PE-CVD), hot wire CVD (HW-CVD) and hot wire plasma-assisted CVD (HWPA-CVD) processes. The optical band gap of a-SiC:H films was controlled from 1.85 to 3.5 eV by varying the percentage of ethylene in the silane gas mixture from 3 to 100%. Adding a rf plasma to the hot wire process the carbon gas source dissociation is implemented leading to an increase in bulk carbon incorporation. This evidence is proved by the enhancement of the peak ascribed to the SiC stretching vibration mode, the reduction of the peak related to the SiH wagging modes, the decrease in the refractive index and the increase of optical band gap. The influence of hydrogen gas dilution on the properties of the films obtained by the different methods is also reported. © 2001 Elsevier Science B.V. All rights reserved.

Highly textured transparent conducting ZnO:Al thin films have been prepared by r.f. magnetron sputtering. The films were deposited on polyester (Mylar type D, 100 μm thickness) and glass substrates at room temperature. Surface stylus profiling, X-ray diffraction, scanning electron microscopy, transmission electron microscope and Hall effect measurements as a function of temperature, using the van der Pauw technique have characterized the films. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface (columnar structure). The ZnO:Al thin films with a resistivity as low as 3.6×10-2 Ωcm have been obtained, as deposited. © 2001 Materials Research Society.

Highly textured transparent conducting ZnO:Al thin films have been prepared by r.f. magnetron sputtering. The films were deposited on polyester (Mylar type D, 100 μm thickness) and glass substrates at room temperature. Surface stylus profiling, X-ray diffraction, scanning electron microscopy, transmission electron microscope and Hall effect measurements as a function of temperature have been used to characterize the produced films. The samples are polycrystalline with a hexagonal wurtzke structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface (columnar structure). The ZnO:Al thin films with a resistivity as low 3.6×10-2 Ωcm have been obtained, as deposited.

Aluminium doped zinc oxide thin films (ZnO:Al) have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, morphological, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface. The ZnO:Al thin films with 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×102 Ωcm have been obtained, as deposited. The obtained results are comparable to those ones obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices. © 2001 Materials Research Society.

Large area thin film position sensitive detectors based on amorphous silicon technology have been prepared on polyimide substrates using the conventional plasma-enhanced chemical vapour deposition. The sensors have been characterized by spectral response, light intensity dependence and linearity measurements in a bent state in order to evaluate the properties in real working conditions. The obtained one-dimensional (1D) position sensors with 10 mm width and 20 mm length present a non-linearity of ±1% which are comparable to the ones produced on glass substrates.

Large area thin film position sensitive detectors based on amorphous silicon technology have been prepared on polyimide substrates using the conventional plasma enhanced chemical vapour deposition technique. The sensors have been characterised by spectral response, illuminated I-V characteristics and position detectability measurements. The obtained one dimensional position sensors with 5 mm wide and 60 mm long present a maximum spectral response at 600 nm, an open circuit voltage of 0.6 V° and a position detectability with a correlation of 0.9989 associated to a standard deviation of 1 × 10-2, comparable to those ones produced on glass substrates. The surface of the sensors at each stage of fabrication was investigated by Atomic Force Microscopy.

Results are presented concerning the morphological and structural characteristics exhibited by the Cu-Sn-Cu system to be used in electronic lead-free soldering processes, under different process temperatures and pressures. The results show that the Cu3Sn or Cu6Sn5 phases needed to supply the thermal, mechanical and electrical stability to the joints formed require Sn layers (either electrodeposited or by using preforms) whose thickness depends on the process temperature used. For process temperatures of 533 K the thickness of the Sn layer should be above 20 μm, while for process temperatures of 573 K, the Sn thickness required is reduced to 10 μm. The joints formed support shear stresses above 12 MPa, as required by electronic standards. Apart from that, microcracks start appearing if an excess of Sn is used during the soldering operation. The set of tests performed indicates that this new joint is quite promising to substitute the conventional solder process applied to power diodes.

The role of the deposition pressure (p) and the type of filaments (tungsten, W or tantalum, Ta) used to produce large area (10cm×10cm) n-type Si:H films by hot wire chemical vapour (HW-CVD) deposition technique was investigated. The data show that the electro-optical properties of the films produced are dependent on the gas pressure used. In the pressure range of 1×10-3 Torr to 1.0 Torr, the room dark conductivity (σd) varies from 1×10-8 to 2 S/cm for films produced at the same hydrogen dilution and filament temperature (Tfil). On the other hand, the hydrogen concentration (CH) decreases from 10% to 2%, while the growth rate (R) shows an exponential increase, from 1 to 9 Å/s. The SIMS analysis, within the detection limits, does not reveal the existence of any significant W or Ta contamination in the films produced.

In this work, we show results concerning electro-optical properties, composition and morphology of nanocrystalline hydrogenated undoped silicon (nc-Si:H) films produced by hot wire plasma assisted chemical vapour deposition process (HWPA-CVD) and exhibiting a compact granular structure, as revealed by SEM micrographs. This was also inferred by infrared spectra, which does not present the SiO vibration band located at 1050-1200 cm-1, even when samples have long atmospheric exposition. The photoconductivity measured at room temperature also does not change when samples have a long time exposition to the air or to the light irradiation. The influence of hydrogen dilution on the properties of the films was also investigated.

In this paper we report on large area one dimensional (1D) amorphous silicon position sensors deposited on flexible polymer foil substrate. The pin sensor structure was deposited by rf plasma enhanced chemical vapour deposition (PECVD). For the electrical and optical characterisation the sensors have been mounted on a convex holder with a 14-mm radius-of-curvature, since the main goal of this work is to develop a flexible position sensor to be incorporated in a micromotor in order to measure its angular velocity continuously. The obtained sensors present adequate performances concerning the position non-linearity (±1% in 20 mm length), comparable to those fabricated on glass substrates. © 2000 Elsevier Science B.V. All rights reserved.

The aim of this work is to determine the role of different metal contacts on the performances of one-dimensional thin-film position-sensitive detectors produced by plasma-enhanced chemical vapour deposition, to be used in optical rulers for alignment applications. The device consists on an indium tin oxide/p-i-n structure where the metal contacts used were based on Al, Al + Cu and Ag. The results achieved show that the contact mainly influences the final sensor range by limiting the magnitude of the analogue signals recorded. In spite of soldering problems the Al contact was the contact that lead to better discrimination of the sensor, with a nonlinearity of ±0.8% and a fall-off parameter of 3.2 × 10-3 cm-1. The Al + Cu contact also exhibits good performances (nonlinearity, of ±1.1%; fall-off parameter, 1.4 × 10-2 cm-1) and should be chosen since it is much easier to solder but requires protection against oxidation. The integration of these sensors on the optical ruler lead to the production of a system with a response time below 0.5 ms, an accuracy better than ±1% and a mechanical precision of better than 0.25 mm in 100 mm, with a full-scale noise below ±0.1%.

The aim of this paper is to present results concerning the morphology, structure, mechanical and electrical characteristics of the new proposed Cu-Sn metallurgical alloy, which may be used in electronic joins. By proper choice of process temperature and pressure, Cu coated surfaces are soldered using Sn as pre-form. The main results achieved indicate that the formation of Cu3Sn phase begins at a temperature of about 473 K and that the Sn thickness (dSn) needed is slightly above 7 μm. Due to join wettability, higher temperatures (between 523 and 573 K) and dSn above 35 μm are required to form joins within the specifications of the electronic industry.

The present paper describes the properties of a photochemical sensor based on amorphous silicon MIS (Metal-Insulator-Semiconductor) diodes. The structure of the sensors used in this work are based on glass/Cr/a-SiH(n +)/a-Si:H(i)SiOx/Pd, where the amorphous silicon layers have been deposited by conventional plasma r.f. techniques. The proposed photochemical sensors present a 2-3 orders of magnitude change in the saturation current and a decrease of up to 40% on the open circuit voltage when in the presence of 400 ppm of hydrogen. The overall performance of these sensors, associated with the low cost fabrication technology, suggests that, in the near future, it will be possible to use them in several industrial applications. © 1998 Elsevier Science Ltd. All rights reserved.

The increasing demand in automation processes in finishing techniques also calls for automatic measurement and inspection methods. These methods ought to be installed as close as possible to the production process and they ought to measure the values needed in a safe and fast way, without disturbing the process itself. Simultaneously they should be free of wear and insensitive against mechanical perturbations. This approach can be reached by proper combination of the laser triangulation technique with an array of linear position sensitive detectors, able to supply information about the surface finishing of an object. This is the aim of this paper that envisages to present experimental results of the performances exhibited by such an array constituting 128 elements. The analogue information supplied by this array is processed by an analogue/digital converter, directly coupled to the array and whose information is computer processed, concerning the recognition of patterns and the processing of information collected over the object to be inspected. © 1999 Elsevier Science S.A. All rights reserved.

This work deals with the role of hydrogen dilution and filament temperature on the morphology, structure and electrical properties of nanocrystalline boron doped silicon carbide thin films produced by hot-wire technique. The structural and morphological data obtained by XRD, SEM and micro-Raman show that for filament temperatures and hydrogen dilutions above 2100 °C and 90%, respectively, the surface morphology of the films is granular with a needle shape, while for lower filament temperatures and hydrogen dilutions the surface morphology gets honeycomb like. The SIMS analysis reveals that films produced with filament temperatures of about 2200 °C and hydrogen dilution of 99% present a higher hydrogen and carbon incorporation than the films produced at lower temperatures and hydrogen dilutions. These results agree with the electrical and optical characteristics recorded that show that the films produced exhibit optical gaps in the range from 1.8 to 2 eV and transverse conductivities ranging from 10-1 S/cm to 10-3 S/cm, consistent with the degree of films crystallinity and carbon incorporation recorded.

In this work we present data concerning the structure, composition and electro-optical performances of nanocrystalline silicon carbide doped films produced at the different filament temperatures and hydrogen dilution ratios. The XRD spectra reveal the presence of the typical Si peaks ascribed to (111) (220) and (311) diffraction planes, where no traces of the carbon peaks were found. The average grain sizes ranges from 10 nm to 30 nm, depending on the temperature of filament and hydrogen dilution used. We observed an enhancement of the peak ascribed to the (220) plane when high H dilution rates are used, meaning that the film starts being textured. The infrared data reveal the typical silicon carbide modes and a hydrogen content that varies from 3% to 1%, with the increase of the filament temperature. Besides that, the IR spectra show the typical SiO2 and SiO modes, associated to the oxide species that are mainly incorporated in the surface of the films and can be removed by proper wet etching. The planar conductivity is enhanced as the temperature of the filament is increased, being the highest conductivity achieved in the range of 0.2 (Ωcm)-1 and almost non activated. © 1998 Elsevier Science Ltd. All rights reserved.

This work reports on the performances of undoped and n doped amorphous/nano-crystalline silicon films grown by hot wire plasma assisted technique. The film's structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), the composition (oxygen and hydrogen content) and the transport properties are highly dependent on the filament temperature and on the hydrogen dilution. The undoped films grown under low r.f. power (≈4 mWcm-2) and with filament temperatures around 1850 °K have dark conductivities below 10-10 Scm-1, optical gaps of about 1.5 eV and photo-sensitivities above 105, (under AM1.5), with almost no traces of oxygen content. N-doped silicon films were also fabricated under the same conditions which attained conductivities of about 10-2 Scm-1.

The aim of this work is to present an analytical model able to interpret the role of the thin collecting resistive layer on the static performances exhibited by 2D amorphous silicon hydrogenated pin thin film position sensitive detectors. In addition, experimental results concerning the device linearity and spatial resolution are presented and checked against the predicted values of the analytical model proposed.