E. Scheid

Optical and structural properties of SiOx and SiNx materials

Abstract SiOx and SiNx films with varied stoichiometries were obtained by low-pressure chemical vapor deposition at low temperature from Si 2 H 6 N 2 O and Si 2 H 6 NH 3 gas mixtures respectively. Using the Clausius-Mossoti theory and the Bruggeman expression, a relation between the refractive index and the stoichiometry of these two materials is established and corroborated by experiments thanks to X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electron energy loss spectroscopy or/and the differential thickness method. These data have also been correlated in order to calculate the electronic polarizability of silicon atoms into SiOx and SiNx materials.

Theoretical and experimental study of silicon micromachined microheater with dielectric stacked membranes

Abstract In this paper, we model first the heating performance of a silicon micromachined microheater. The pros and cons of the use of a stacked dielectric membranes instead of p ++ -Si membranes are discussed, either in terms of electrical consumption, or in terms of emperature homogeneity. Then, we realize and characterize two examples among the simulated structures. This allows some practical problems during the fabrication, essentially due to mechanical breakdown, to be demonstrated. A general agreement between results is found. This study shows significant promise in the development of stacked dielectric membranes.

Design of a low power SnO2 gas sensor integrated on silicon oxynitride membrane

Abstract Power consumption is one of the major challenges for SnO2 gas sensors. We propose a new solution using a silicon oxynitride membrane (SiOxNy), allowing a low power consumption (65 mW) at the operating temperature (450°C). Using electro-thermal simulation by finite element method (SESES™), a new design was determined with optimized size of heater and membrane area. The corresponding gas sensors were fabricated and their mechanical strength and thermal performances were correlated with FEM electro-thermal simulations.

Optimization of an integrated SnO2 gas sensor using a FEM simulator

Abstract The application of a commercially available coupled electrothermal simulation software (SESES™) to the design and performance of an SnO 2 chemical sensor is presented. The resulting temperature distribution over a thin silicon membrane and the heating power consumption have been obtained for realistic device structures and compared with experimental measurements. Starting from an existing design, the excellent correlation between the measured and simulated results allows us to arrive at an adjusted model using suitable coefficients. Using this model, a new design is proposed allowing better thermal performance and lower power consumption, as confirmed by the experimental measurements.

Energy study of buckled micromachined beams for thin-film stress measurements applied to SiO2

Measurements of maximal deflection amplitude were carried out on arrays of clamped-clamped SiO2 microfabricated beams buckling under the effect of thin-film built-in compressive stress. The experimental deflection determination of these deflections yields the residual stress value in the SiO2 film. This proposal is confirmed by appropriate theory that considers the issue of microfabricated beam buckling from an energy point of view. In this model, the total potential energy stored in a buckled microbeam is computed and the residual stress value is given by considering the measured buckling maximal deflection and by making approximations about the shape of the microbeam deflection curve. The SiO2 film residual stress evaluated with the help of the energy method is in good agreement with values reported in the literature.

Characterization and modelling of ageing failures on power MOSFET devices

A method based on the failure analysis of power MOSFET devices tested under extreme electrothermal fatigue is proposed. Failure modes are associated to several structural changes that have been investigated through acoustic, electron and ion microscopy. The main aging mode is related to the exponential increase in drain resistance due to delamination at the die attach. Earlier failures are observed when very local defects due to electrical over stresses (EOS) occur at the source metallization or at the wire bonding. Aging models were elaborated to account for the die attach delamination, but are still lacking to take in account the structural evolution of the Al metallization. This new methodology, based on accelerated tests and structural observations aims at designing a new generation of power components that will be more reliable.

Boron Diffusion and Activation during Heat Treatment in Heavily Doped Polysilicon Thin Films for P+ Metal-Oxide-Semiconductor Transistors Gates

In this work, we present a study of heavily doped polycrystalline silicon which is used as a transistor gate in standard complementary metal-oxide-semiconductor (CMOS) technology. Our aim is to study the redistribution and activation of boron during thermal annealing in thin silicon films implanted and deposited by low-pressure chemical vapor deposition (LPCVD) using a disilane (Si2H6) gaseous source. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). The resistivity measurements by the four-probe method show that the films become conductors after annealing. Carrier mobility and active doping fraction are obtained by Hall effect measurements. To investigate the SIMS profiles, we have proposed a model based on the one-dimensional numerical resolution of Ficks laws. This model takes into account phenomena due to effects of very heavy doping such as that of clusters. The boron diffusion coefficient and its activation percentage are deduced from the adjustment of simulated profiles with SIMS experimental ones. We have used SUPREM IV software in order to estimate the boron diffusion coefficients in these films and compare them to our results. A small gap between the profiles simulated by our model and by SUPREM IV has been observed, in particular in the region where the boron limit solubility is exceeded. Nevertheless, the diffusion coefficient values obtained by the two methods are of the same order of magnitude.

Optical study of undoped, B or P-doped polysilicon

Abstract The refractive index n (λ), and optical absorption coefficient α(λ), of thin polycrystalline silicon films (Si-poly), undoped or heavily doped in-situ with boron 8.10 20 cm −3 or phosphorus 6.10 20 cm −3 , deposited inside a new kind of reactor, called sector reactor (a reduced model of an annular reactor). The optical constants of undoped Si-poly are obtained by a simple procedure which is based on the use of the fringe pattern in the transmission spectrum. For heavily B- or P-doped polysilicon films, the optical absorption coefficient become not negligible at near infrared and we deduced the optical constants from both the measured transmission T (λ) and reflection R (λ) for film-substrate structures. In the infrared, the index of refraction decreases systematically and the absorption coefficient increases systematically with wavelength. This variation of the optical parameters is attributed to the presence of free carriers. Drudes theory was used in order to calculate the free-carrier concentration and the mobilities, and these results compared well with those obtained by Hall-effect measurements.

Super Large Grain Polycrystalline Silicon Obtained from Pyrolysis of Si2H6 and Annealing

Polycrystalline silicon films on SiO2 are formed by Low Pressure Chemical Vapor Deposition from disilane at 450°C and subsequent annealing at 600°C. Homogeneity performances of the deposition are given, which compares with those of Si deposition from SiH4. The structure of the polysilicon material obtained after annealing is shown. The resulting grain size is much larger than the one that could be obtained by Si deposition from SiH4 and annealing. An optical evaluation of the film is given, confirming the potential of Si2H6 for the fabrication of high field effect mobility thin film transistors.

Properties of nitrogen doped silicon films deposited by low pressure chemical vapour deposition from disilane and ammonia

Abstract Nitrogen doped silicon films have been deposited by low pressure chemical vapour deposition from disilane Si 2 H 6 and ammonia NH 3 . Deposition kinetics is investigated, pointing out the influences of the deposition temperature, the total pressure and the gas flow rates. According to the Bruggeman theory, variations of the NH 3 /Si 2 H 6 gaseous ratio allow for a wide range of the SiN x stoichiometry as well as a good control of the film nitrogen doping. The different behaviours of the nitrogen atom in silicon films are discussed and an overview of the nitrogen doped silicon physical properties (optical, mechanical and electrical) is proposed for the development of boron-doped polysilicon gates.