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 the influence of the DC grid bias on the plasma impedance and the I-V behaviour of silane plasmas used to grow undoped amorphous silicon films by plasma enhanced chemical vapour deposition technique using a triode configuration at or close to the powder regime is studied. The aim is to determine the correlation between the r.f. power and the DC grid voltage with the plasma parameters, under isothermal gas conditions. The results should lead to the production of nanostructured films, with the required optoelectronic characteristics for photovoltaic applications. The results achieved show the existence of a boundary region close to the γ-regime (powder formed) where nanoparticles can be formed by moderated ion bombardment of the growing surface. This is characterised by the plasma resistance of the same order of magnitude of the plasma reactance. Under this condition, it is possible to grow amorphous silicon films that can incorporate nanoparticles, exhibiting photosensitivities of about 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with densities of states determined by the constant photocurrent method below 3 × 1015 cm3. Apart from that, the growth of the films is less affected by light soaking than the conventional films grown by standard techniques. © 2001 Elsevier Science Ltd. All rights reserved.

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.

Nitrous oxide (N(2)O) is a greenhouse gas, the third most significant contributor to global warming. As a key process for N(2)O elimination from the biosphere, N(2)O reductases catalyze the two-electron reduction of N(2)O to N(2). These 2 x 65 kDa copper enzymes are thought to contain a CuA electron entry site, similar to that of cytochrome c oxidase, and a CuZ catalytic center. The copper anomalous signal was used to solve the crystal structure of N(2)O reductase from Pseudomonas nautica by multiwavelength anomalous dispersion, to a resolution of 2.4 Angstrom. The structure reveals that the CuZ center belongs to a new type of metal cluster, in which four copper ions are liganded by seven histidine residues. N(2)O binds to this center via a single copper ion. The remaining copper ions might act as an electron reservoir, assuring a fast electron transfer and avoiding the formation of dead-end products.

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%.

This work aims to study the role of the r.f. electrode configuration on the plasma characteristics of a PECVD asymmetric reactor. The configurations used are the usual diode configuration, the triode configuration and a new configuration that we named short-circuited grid electrode (SGE). The plasma generated was characterized with the use of a Langmuir probe and an impedance probe. We demonstrate that the plasma parameters are highly dependent on the reactor geometry. The results achieved show that by changing the r.f. electrode configuration the DC self-bias varies from about 100 to close to 0 V. This variation causes changes in the ion bombardment of the reactor surfaces, which can affect the growing of the films deposited. We also demonstrate that for the SGE configuration the area seen by the plasma does not correspond to the exposed physical area of the electrode, and we suggest a model to explain this phenomenon.

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

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 aerobic purification of Pseudomonas nautica 617 nitrous oxide reductase yielded two forms of the enzyme exhibiting different chromatographic behaviors. The protein contains six copper atoms per monomer, arranged in two centers named CUA and Cut. Cut could be neither oxidized nor further reduced under our experimental conditions, and exhibits a 4-line EPR spectrum (g(x)= 2.015, A(x) = 1.5 mT, g(y) = 2.071, A(y) = 2 mT, g(z) = 2.138, A(z) = 7 mT) and a strong absorption at similar to 640 nm. Cu-A can be stabilized in a reduced EPR-silent state and in an oxidized state with a typical 7-line EPR spectrum (g(x) g(y) = 2.021, A(x) = A(y) = 0 T, g(z) =0.178, A(z) = 4 mT) and absorption bands at 480, 540, and similar to 800 nm. The difference between the two purified forms of nitrous oxide reductase is interpreted as a difference in the oxidation state of the CuA center. In form A, CUA is predominantly oxidized (S = 1/2, Cu1.5+-Cu1.5+), while in form B it is mostly in the one-electron reduced state (S = 0, Cu1+-Cu1+). In both forms, Cu-Z remains reduced (S = 1/2). Complete crystallographic data at 2.4 Angstrom indicate that Cu-A is a binuclear site (similar to the site found in cytochrome c oxidase) and Cu-Z is a novel tetracopper cluster [Brown, K., et ai. (2000) Nat. Struct. Biol. (in press)]. The complete amino acid sequence of the enzyme was determined and comparisons made with sequences of other nitrous oxide reductases, emphasizing the coordination of the centers. A 10.3 kDa peptide copurified with both forms of nitrous oxide reductase shows strong homology with proteins of the heat-shock GroES chaperonin family.

Mycobacterium tuberculosis KatG is a multifunctional heme enzyme responsible for activation of the antibiotic isoniazid. A KatG(S315T) point mutation is found in >50% of isoniazid-resistant clinical isolates. Since isoniazid activation is thought to involve an oxidation reaction, the redox potential of KatG was determined using cyclic voltammetry, square wave voltammetry, and spectroelectrochemical titrations. Isoniazid activation may proceed via a cytochrome P450-like mechanism. Therefore, the possibility that substrate binding by KatG leads to an increase in the heme redox potential and the possibility that KatG(S315T) confers isoniazid resistance by altering the redox potential were examined. Effects of the heme spin state on the reduction potentials of KatG and KatG(S315T) were also determined. Assessment of the Fe3+/Fe2+ couple gave a midpoint potential of ca. -50 mV for both KatG and KatG(S315T). In contrast to cytochrome P450s, addition of substrate had no significant effect on either the KatG or KatG(S315T) redox potential. Conversion of the heme to a low-spin configuration resulted in a -150 to -200 mV shift of the KatG and KatG(S315T) redox potentials. These results suggest that isoniazid resistance conferred by KatG(S315T) is not mediated through changes in the heme redox potential. The redox potentials of isoniazid were also determined using cyclic and square wave voltammetry, and the results provide evidence that the ferric KatG and KatG(S315T) midpoint potentials are too low to promote isoniazid oxidation without formation of a high-valent enzyme intermediate such as compounds I and IT or oxyferrous KatG.

Radiationless transition rates to L1 vacancy states have been calculated ab initio in the Dirac-Fock approximation. The calculations include quantum-electrodynamic corrections. Results in the jj coupling scheme for all possible L1 transitions are tabulated for elements Yb and Hg.

In this work we present a study of influence of ion bombardment on the optical, electrical and compositional properties of the intrinsic amorphous silicon films deposited in a modified triode plasma-enhanced chemical vapour deposition (PECVD) reactor. The application of a DC voltage to a grid placed in front of the r.f. electrode allowed us to control the energy and polarity of the ions striking the substrate during film growth. The results show a variation by two orders of magnitude in the dark conductivity from 10-10 to 6.2 × 10-12 (Ω cm)-1, while the photosensitivity varied from 2 × 105 to 2 × 107. A process plasma that takes place in the γ-type regime was associated with the use of a negative bias, while a process plasma like the one of the α-type regime was associated with the use of a positive bias. The films deposited with a bias ≈ 38 V are highly intrinsic and the abrupt change in the conductivity properties observed at this bias is attributed to a change in the density of the states ascribed to the position of the Fermi level. That is, a precise control of the energy of the ions striking the substrate during the film growth leads to improved film optoelectronic properties, very important for device applications. © 2001 Elsevier Science Ltd. All rights reserved.