Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

Microwave plasma deposition of diamond like carbon coatings

The promising applications of the microwave plasmas have been appearing in the fields of chemical processes and semiconductor manufacturing. Applications include surface deposition of all types including diamond/diamond like carbon (DLC) coatings, etching of semiconductors, promotion of organic reactions, etching of polymers to improve bonding of the other materials etc. With a 2.45 GHz. 700 W, microwave induced plasma chemical vapor deposition (CVD) system set up in our laboratory we have deposited diamond like carbon coatings. The microwave plasma generation was effected using a wave guide single mode applicator. We have deposited DLC coatings on the substrates like stainless steel, Cu-Be, Cu and Si. The deposited coatings have been characterized by FTIR, Raman spectroscopy and ellipsometric techniques. The results show that we have achieved depositing ∼95% sp³ bonded carbon in the films. The films are unform with golden yellow color. The films are found to be excellent insulators. The ellipsometric measurements of optical constant on silicon substrates indicate that the films are transparent above 900 nm.     

Deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode

Amorphous hydrogenated carbon doped with silicon oxide (a-C:H:Si:O), which is referred to as silicon–carbon coatings in this work, consists of thin amorphous films, which are used as commercial solid lubricants due to their higher stability under extreme environmental conditions as compared to amorphous hydrogenated carbon. The deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode is considered. Silicon–carbon coatings are deposited using polyphenul methylsiloxane as a precursor at a flow rate of 0.05 mL/min in an argon atmosphere at a pressure of 0.1 Pa. A high-frequency power supply is used to apply a high-frequency bias voltage to a substrate during deposition. After deposition, the mechanical properties of the coatings are studied. The maximum hardness of the coating is 20 GPa at a minimum friction coefficient of 0.16 and a wear rate of 1.3 × 10^–5 mm³ N^–1 m^–1. Energy dispersive analysis shows that the coatings contain a significant content of carbon and oxygen (about 80 and 15%, respectively) and a low content of silicon (about 5%).     

Influence of electron saturation of Tamm levels on the field-emission properties of silicon crystals

It has been shown that the plasma-chemical modification of the morphology and composition of the surface phase influences the emissivity of silicon crystals. It has been found that the saturation of Tamm states with electrons during the preparation of atomically clean silicon surfaces, along with stabilizing passivation of surface atoms in a highly ionized microwave plasma using Halon 14, decreases a threshold electric field at which field emission begins more than twofold and increases the maximal density of the field emission current by more than an order of magnitude compared with wafers covered by native oxide or subjected to ion physical etching in argon. Physicochemical mechanisms responsible for the modification of the silicon surface and the field-emission properties of silicon have been considered.     

Analysis of the composition of Ti-based thin films deposited on silicon by means of self-ion assisted deposition

The composition of Ti-based thin films deposited on silicon using a self-ion assisted deposition (SIAD) method was investigated by utilising the Rutherford backscattering spectrometry technique and RUMP simulation code. The hydrogen affinity of the coatings produced by means of SIAD was investigated using the ¹H(¹⁵N, αγ)¹²C nuclear resonance reaction. The titanium–based films on silicon were found to have a high content of oxygen, carbon, hydrogen and substantial concentration of the substrate. Near 10% H content enrichment was found at the surface of coatings but no hydrogen enrichment at the coating–substrate interfaces was observed.     

The evolution of roughness of supersmooth surfaces by ion-beam etching

The behavior of roughness on surfaces of Cr/Sc multilayer structure, crystalline silicon [100], and fused quartz upon ion beam etching in the region of middle and high frequencies of the spatial spectrum (10⁻²–10² μm⁻¹) is studied. The possibility of keeping surface roughness at level σ ∼ 0.3 nm upon etching by Ar atoms/ions to depths of up to 10 μm is demonstrated.     

Formation of silicon carbide and diamond nanoparticles in the surface layer of a silicon target during short-pulse carbon ion implantation

Synthesis of silicon carbide and diamond nanoparticles is studied during short-pulse implantation of carbon ions and protons into a silicon target. The experiments are carried out using a TEMP source of pulsed powerful ion beams based on a magnetically insulated diode with radial magnetic field B _r . The beam parameters are as follows: the ion energy is 300 keV, the pulse duration is 80 ns, the beam consists of carbon ions and protons, and the ion current density is 30 A/cm². Single-crystal silicon wafers serve as a target. SiC nanoparticles and nanodiamonds form in the surface layer of silicon subjected to more than 100 pulses. The average coherent domain sizes in the SiC particles and nanodiamonds are 12–16 and 8–9 nm, respectively.     

Further investigation on the formation mechanisms of (NH4)2SiF6 synthesized by dry etching technique

The validity of two formation mechanisms of ammonium silicofluoride (ASF), which are proposed to take place when a silicon surface is exposed to the vapor of HNO₃/HF acid mixture is investigated. Of the two proposed mechanisms regarding the synthesis of ASF on silicon surface, validity of the first predicting the release of hydrofluosilicic acid (H₂SiF₆) at the intermediate stage is examined by FTIR spectroscopy and the second mechanism suggesting O₂ release is investigated using the Winkler technique. IR absorbance bands of SiF₆²⁺ are observed on the fresh samples prepared at low (1/100) HNO₃/HF volume fractions. No significant amount of oxygen is detected during the synthesis of ASF films on silicon surface by dry etching technique. These two observations together provide firmer support for the validity of the second mechanism.     

Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

CO oxidation on a platinum foil was studied in a high pressure flow cell (10²−10² Pa) and an UHV chamber (10⁻⁸ −5 × 10⁴ Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (∼ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Auger spectroscopy performed in the UHV chamber showed that surface carbon quantitatively accounted for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and adsorbed CO can block carbon diffusion. Although silicon oxide was always detected on the surface with infrared spectroscopy, no periodic variation in it could be observed during the reaction-rate oscillations. Auger studies confirmed that the maximum and the variations in surface concentration of silicon oxide could not account for the variations in the number of active sites. A mechanism is therefore proposed in which carbon is driving the long-period self-sustained oscillations in the rate of CO oxidation on Pt.     

Etching of silicon in fluoride solutions

The development and status of what is commonly called the Gerischer mechanism of silicon etching in fluoride solutions is reviewed. The two most widely used and studied wet etchants of silicon are F^? and OH^?. Their mechanisms of atom removal share many things in common; in particular, chemical passivation by a hydrogen-terminated surface plays an important role in both. Crucially, however, their initiation steps are different, and this leads to important differences in the structures of the materials produced by the etchants. The initiation of etching by F^? is electrochemical in nature, responding to the electronic structure of the Si, and is, therefore, a self-limiting reaction that can produce nanocrystalline porous silicon. Hydroxide etching destroys porous silicon because its initiation step is a catalytic chemical reaction and not a self-limiting process. A number of unanswered questions regarding the dynamics of fluoride etching are highlighted.     

Formation mechanism of Si(1 0 0) surface morphology in alkaline fluoride solutions

Formation mechanism of Si(1 0 0) surface morphology in alkaline fluoride solutions was investigated both theoretically and experimentally. By analysis of Raman spectra of silicon wafer surfaces and three kinds of etching solutions (NaOH, NaOH/NH₄F, and NaOH/NH₄F/Na₂CO₃) with and without addition of Na₂SiO₃·9H₂O, no Si-F bond is formed, F⁻ and CO₃²⁻ ions accelerate the condensation of Si-OH groups. Based on experimental results, it is proposed that bare silicon and silicon oxide coexist at the wafer surface during etching process and silicon oxide of different structure, size, and site at the surface manufacture different surface morphology in alkaline fluoride solution.     

Diamond deposition on siliconized stainless steel

Silicon diffusion layers in AISI 304 and AISI 316 type stainless steels were investigated as an alternative to surface barrier coatings for diamond film growth. Uniform 2 μm thick silicon rich interlayers were obtained by coating the surface of the steels with silicon and performing diffusion treatments at 800 °C. Adherent diamond films with low sp² carbon content were deposited on the diffused silicon layers by a modified hot filament assisted chemical vapor deposition (HFCVD) method. Characterization of as-siliconized layers and diamond coatings was performed by energy dispersive X-ray analysis, scanning electron microscopy, X-ray diffraction and Raman spectroscopy.     

X-ray spectroscopic examination of thin HfO<Subscript>2</Subscript> films ALD- and MOCVD-grown on the Si(100) surface

HfO₂ films 5 nm thick grown on Si(100) substrates by the methods of MOCVD hydride epitaxy and atomic layer deposition (ALD) are studied using X-ray photoelectron spectroscopy combined with Ar⁺ ion etching and X-ray reflectometry. It is found that (i) the ALD-grown HfO₂ films are amorphous, while the MOCVD-grown films show signs of a crystal structure; (ii) the surface of the ALD-grown films is more prone to contamination and/or is more reactive; and (iii) the amount of interfacial silicon dioxide in the case of the MOCVD-grown film is greater than in the case of the films synthesized by ALD. It is also shown that the argon ion etching of the HfO₂ film results in the formation of a metallic hafnium layer at the interface. This indicates that HfO₂ can be used not only as a gate dielectric but also as a material suitable for fabricating nanodimensional conductors by direct decomposition.     

Electrochemical synthesis and STM-STS studies of thin diamond-like films on the surface of oxidized aluminum

Thin carbon films on the surface of aluminum containing native oxide were prepared at room temperature by electrochemical deposition from a solution of lithium acetylenide in dimethylsulfoxide. The structure of the coatings obtained was studied by scanning tunnel microscopy and spectroscopy. The mechanical characteristics of carbon coatings were found to depend strongly on the main parameters determining deposition conditions. The presence of a considerable amount of sp ³ carbon (diamond-like phases) in the films was substantiated.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Peculiarities of the capacitance-voltage characteristic of a photoelectric solar energy convertor based on a silicon p-n junction with a porous silicon antireflection coating

Experimental results on the high-frequency capacitance-voltage characteristic of a photoelectric solar energy converter based on the n ⁺-p junction with a thin porous silicon film on the frontal surface are considered. It is shown that the capacitance-voltage characteristic is determined by the surface metal-insulator-semiconductor (MIS) structure formed as a result of growing of a porous silicon layer by electrochemical anode etching. The effective thickness of the insulator layer of the MIS structure, the impurity concentration in its semiconductor region, and the density of surface states are determined.     

Projection-patterned etching of silicon in chlorine atmosphere with a KrF excimer laser

The formation of relief features in silicon by a one-step process that avoids resist patterning has been achieved by laser-projection-patterned etching in a chlorine atmosphere. Etching is performed with a pulsed KrF excimer laser (λ=248 nm, τ=15 ns) and deep UV projection optics having an optical resolution of 2 μm. Etching takes place in two steps. Between laser pulses, the silicon surface is covered with a monolayer of chemisorbed chlorine atoms (one Cl per Si). During the laser pulse, surface transient heating at temperatures in excess of 1250 K results in the desorption of the reaction products (mainly SiCl₂). At laser energy densities that induce surface melting, this desorption results in a saturated etch. rate of 0.06 nm per pulse, corresponding to the removal of about 0.5 Si monolayer per pulse. At densities below the melting threshold, reduced thermal and possibly a small amount of photochemical etching result in lower etch rates. Projection of a resolution test photomask onto the silicon surface shows that the size of etched features differs from the size of the projected features and strongly depends on the laser energy density. As a result of the heat spread in silicon and of the highly nonlinear character of the etching reaction, etched features smaller than the irradiated area are obtained at all fluences in the range 350–700 mJ/cm². Etched lines having a width down to about 1.3 μm were produced. Proximity effects due to heat spread were also evidenced for small projected features (<4 μm). The characteristics of the etched patterns are compared with those obtained for GaAs etching in chlorinated gases with the same experimental set-up. Significant differences in pattern resolution for Si and GaAs etching are observed. This variation in resolution is believed to result from the fact that Si has a greater thermal diffusivity than GaAs.     

Epitaxial growth of cadmium sulfide films on silicon

A 300-nm-thick cadmium sulfide epitaxial layer on silicon was grown for the first time. The grown was performed by the method of evaporation and condensation in a quasi-closed volume at a substrate temperature of 650°C and a growth time of 4 s. In order to avoid a chemical reaction between silicon and cadmium sulfide (at this temperature, the rate constant of the reaction is ~10³) and to prevent etching of silicon by sulfur, a high-quality silicon carbide buffer layer ~100 nm thick was preliminarily synthesized by the substitution of atoms on the silicon surface. The ellipsometric, Raman, electron diffraction, and trace element analyses showed a high structural perfection of the CdS layer and the absence of a polycrystalline phase.     

Effect of a magnetic field on the rate of etching of silicon dioxide in a CF4 + O2 plasma

A near-electrode nonuniform magnetic field crossed with an electric field is found to strongly affect the rate of etching of silicon dioxide on glass substrates in a CF₄ + O₂ plasma when the Larmor frequency (≈10⁹ s⁻¹) is much higher than the frequency of collisions of an electron with surrounding plasma particles (≈10⁶ s⁻¹) and the frequency of the applied rf electric field (≈10⁷ s⁻¹). The confinement of electrons by the magnetic field in the immediate vicinity of the substrate surface to be treated increases the rate of generation of chemically active particles, which increases the etching rate of silicon dioxide.     

Possibilities of C 1s XPS and N(E) C KVV Auger spectroscopy for identification of inherent peculiarities of diamond growth

The interaction of C-atoms and CH_n-radicals with uncleaned and argon cleaned silicon substrate and with diamond surface after H-treatment have been studied in situ by XPS and Auger spectroscopy. It was found the formation of a new chemical surface state of carbon atoms in the case of carbon atoms and radicals interaction with cleaned silicon. The same chemical state was revealed on the H-treated diamond surface. Graphite-like structure of carbon atoms was observed on the surface of unlearned silicon and H-treated diamond after interaction with carbon atoms and radicals. N(E) C KVV Auger spectrum for the new chemical state of carbon atoms significantly differs from typical spectra for sp²- and sp³-bonded carbon materials. The high energy part of this spectrum was interpreted under the hypothesis of sp³-bonded carbon atoms but with shifted fermi level position.