A.F. Beloto

Morphological and optical characteristics of porous silicon produced by anodization process in HF-acetonitrile and HF-ethanol solutions

Porous silicon (PS) samples were obtained by anodization etching process of n-type silicon wafer phosphorus-doped. Electrochemical oxidation of PS was investigated in aqueous hydrofluoric acid (HF) containing additive such as ethanol or acetonitrile. Pore formation was studied with the variation of type and resistivity of the silicon wafer, taking into account the most important anodization process parameters such as: acid concentration, current density and anodization time. Scanning Electron Microscopy (SEM) and Raman Scattering Spectroscopy measurements were used to characterize the macropore morphology changes and sample photoluminescense responses, respectively. PS layer formed in HF-acetonitrile solution showed more uniform and homogeneous macropore distributions with different shapes and sizes. Behavior may be explained because acetonitrile surface tension is greater than that of ethanol. Therefore, acetonitrile molecules might passivate the silicon surface dissolved during the anodization process.

Plasma immersion ion implantation experiments at the Instituto Nacional de Pesquisas Espaciais (INPE), Brazil

Abstract Historical perspective of the development of PIII devices at the Instituto Nacional de Pesquisas Espaciais (INPE) is given, together with the description of the present system under operation and our overall results on this three-dimensional implantation research. Starting with an ignitron switched pulser (1 pulse per 3 min) and an intermittent microwave plasma, we improved our PIII system developing a pulse forming network (PFN) based pulser (20 Hz), 2 years later. We also improved our plasma source towards a DC, highly stable, medium density glow discharge system. A much faster hard tube pulser was recently incorporated to our PIII system (670 Hz) allowing us to achieve good implantation results in different materials. Presently, we are testing a recently purchased RUP-4 commercial pulser to obtain arc prevented, 1.1 kHz, square pulses for new experiments in this active field of PIII research.

Plasma immersion ion implantation of nitrogen in Si : formation of SiO2, Si3N4 and stressed layers under thermal and sputtering effects

Abstract Plasma immersion ion implantation (PIII) of nitrogen in silicon (Si) wafers was carried out using a dc glow discharge plasma source and a hard tube pulser. Ion irradiation times ranging from 3 to 60 min were used to accumulate different doses. Surface analysis of these samples was carried out by Auger electron spectroscopy (AES), revealing a high atomic concentration of nitrogen (up to 60%) in the as-implanted Si wafer, besides the presence of different impurities as oxygen and carbon in significant quantities. Depth profiles of these elements were obtained as well as of compound species as SiO2 and Si3N4, using this high-energy resolution AES. Comparing the concentration profiles of implanted nitrogen in Si and the corresponding retained doses in these samples, it was possible to understand the thermal and sputtering effects in our present PIII experiment. High-resolution XRD results corroborate the formation of highly stressed layers in the as-implanted substrates. These experimental results are compared to simulations obtained by TRIDYN code.

Magnesium plasma immersion ion implantation in a large straight magnetic duct

Magnesium ions were implanted on silicon wafers using a vacuum arc plasma system with a straight 1 m long magnetic duct, 0.22 m in diameter. Good macroparticle filtering was obtained in samples positioned facing the plasma stream and complete filtering was achieved in samples with surfaces parallel to the plasma stream and magnetic field. Deposition is also minimized by placing sample surfaces parallel to the plasma stream. High resolution x-ray diffraction rocking curves of implanted samples show that the changes in lattice constant are due to compressive strain, and the distortion is larger for higher voltages. Without magnetic field the implantation was a few hundred angstroms deep, as expected, but with magnetic field the depth profile was surprisingly extended to over 0.1 μm, a fact for which we do not yet have a convincing explanation, but could be related to radiation enhanced segregation. The presence of a magnetic field increases substantially the retained implantation dose due to the increase in plasma density by two orders of magnitude.

Plasma immersion ion implantation using a glow discharge source with controlled plasma potential

Abstract A DC glow discharge plasma source was used in a plasma immersion ion implantation (PIII) experiment providing nitrogen plasmas with densities of 1–3 ×10 10 cm −3 and temperatures of 5–10 eV. Nitrogen ions were extracted from these plasmas and implanted in a variety of immersed samples (Al 5040, SS 304, Si) using repetitive high voltage pulses from two types of sources: PFN pulser and a hard tube pulser. Due to the high potential present in our plasma (350 V), a significant sputter etching of the samples surface occurred at long irradiation times. An electron shower source was used to lower this potential allowing its control from 0 to 350 V. Operating the plasma source at potentials below 70 V reduced the sputtering to negligible levels and a retained dose of 1.5×10 17 cm −2 was achieved in a silicon surface, after irradiation of 1500 min. For plasma with potential of 350 V (no electron shower), the retained doses in Al 5040 and SS 304 samples were smaller than 5×10 16 cm −2 , for same plasma and pulser conditions (but 2500 min irradiation), confirming the deleterious effects of sputtering measured in Si samples. Upon using the higher repetition rate pulser, the treatment time was reduced by a factor of 700, thus easing considerably the sputtering problem.

Aluminum Implantation in Kapton® for Space Applications: Magnetic Field Effects on Implantation in Vacuum Arcs

Aluminum was implanted in samples of Kapton®, a polyimide commonly used in spacecrafts, in order to form a protective layer against degradation by atomic oxygen, abundant in space. Implantation was carried out in a vacuum-arc-generated aluminum plasma, with and without the presence of a confining magnetic field. The main effect of the magnetic field is to increase plasma density by two orders of magnitude and, as a result, the dielectric Kapton® sample should charge much faster than in the unmagnetized case. Implantation depths should therefore be larger in the unmagnetized case. Results of X-ray Photoelectron Spectroscopy depth profile analysis, however, showed similiar implantation depths in both cases, with magnetized samples having a slightly deeper and larger mixing layer. Possible mechanisms to explain this result are discussed. Both treatments resulted in an excellent protective layer as demonstrated by samples exposed to oxygen plasmas, adhesion, thermal cycling, transmission and reflectance tests.

Annealing Effects on Silicon Oxynitride Layer Synthesized by N Plasma Immersion Ion Implantation

A silicon oxynitride layer was obtained on a polished silicon wafer surface by nitrogen plasma immersion ion implantation. Oxygen is provided by the residual gas in the implantation chamber (base pressure of 3times10-5 mbar) and is also implanted as the main impurity. As-implanted Si samples were analyzed by high-resolution Auger electron spectroscopy (AES), which indicated the formation of a SiOxNy layer of about 30 nm with varying x and y, along the depth of the treatment layer. AES also provided concentration profiles of the implanted elements at the as-implanted stage. Annealing of samples from a batch of such oxynitrided Si samples was carried out at different temperatures ranging from 200 degC to 1060 degC. The AES analysis of these annealed samples indicated a significant escape of the implanted nitrogen atoms (starting already at 200 degC), but even at 1060 degC, there was a very thin (about 12 nm) remaining layer of the silicon oxynitride, which is probably in crystalline form. Results from high-resolution X-ray diffraction measurements also corroborate the aforementioned results

Photoluminescence and reflectance measurements on annealed porous silicon implanted with nitrogen by plasma immersion ion implantation (PIII)

Porous Silicon (PS), approximately 1 lm thick, was obtained from n-type (1 0 0) monocrystalline silicon wafers using two different anodization conditions. The PS samples were implanted with nitrogen by PIII and annealed at temperatures between 100 and 1000 C. Photoluminescence (PL) and reflectance measurements on the implanted samples for wavelengths between 220 and 800 nm were carried out before and after annealing. Two peaks of PL intensity in the visible region, (640 and 730 nm) were observed. Increasing the annealing temperature reduced the PL intensity and changed the relative intensity between the peaks. A reduction in the ultraviolet reflectance was observed for polished implanted Si samples and for both types of implanted PS samples before annealing. After annealing, the reflectance decreased in the UV region up to 600 C. Above 800 C, there was an increase in reflectance, indicating occurrence of recrystallization.

Metal-arc plasma ion implantation in a straight magnetic filter

Ion implantation is a well-known technique used in surface treatment of materials. During the 1980s, ion implantation using a sample immersed in an ionized gaseous medium (known as plasma immersion ion implantation) revealed to be an alternative to the conventional ion beam implantation. Development of this nonconventional surface processing using metallic ion species increased significantly when metal vacuum arcs began to be used as ion sources. In this paper, we present the experimental results obtained on Si wafers used as test targets, using metallic arc ion implantation. High-density plasmas (up to 10/sup 19/ m/sup -3/) made of aluminum vacuum arcs were used for the metallic implantation process. Arc currents of 200-700 A and 16 ms duration were pulsed every 30 s, and samples were biased from 2 kV to 10 kV with 50 /spl mu/s pulses at 700 Hz, using a hard tube pulser. A straight magnetic filter was used to clean the plasma from macroparticle contaminants, with complete filtering being achieved in samples oriented so that their surfaces were parallel to the plasma stream. This is an interesting alternative to 90/spl deg/ curved magnetic duct filters usually used for this purpose. High-resolution X-ray diffraction analysis shows that aluminum was successfully implanted.

Boron Doped Ultrananocrystalline Diamond Films on Porous Silicon: Morphological, Structural and Electrochemical Characterizations

Boron doped ultrananocrystalline diamond (BDUND) films were grown and characterized on porous silicon (PS) substrates. PS samples were prepared from n-type monocrystalline silicon wafers (100) with 1-20 Ω.cm of resistivity, by electrochemical etching, using HF-acetonitrile solution as electrolyte. BDUND films were grown by Hot Filament Chemical Vapor Deposition using CH4, H2 and Ar. The doping process consisted of an additional hydrogen line, passing through a bubbler containing B2O3 dissolved in methanol, with boron/carbon ratio of 20000 ppm in solution. Raman spectroscopy and X-Ray diffraction were used to evaluate the quality of the films. Scanning electron microscopy was used for morphological characterization, and confirmed that the films covered the pores without filling them. Electrochemical response and capacitance behavior of the electrodes were explored, by cyclic voltammetry. Samples presented high capacitance, confirming that BDUND/PS electrodes are promising for application as electrochemical capacitors.