Black Silicon formation using dry etching for solar cells applications

A study on the formation of Black Silicon on crystalline silicon surface using SF₆/O₂ and SF₆/O₂/CH₄ based plasmas in a reactive ion etching (RIE) system is presented. The effect of the RF power, chamber pressure, process time, gas flow rates, and gas mixtures on the texture of silicon surface has been analyzed. Completely Black Silicon surfaces containing pyramid like structures have been obtained, using an optimized mask-free plasma process. Moreover, the Black Silicon surfaces have demonstrated average values of 1% and 4% for specular and diffuse reflectance respectively, feature that is suitable for the fabrication of low cost solar cells.     

Effects of argon and oxygen flow rate on water vapor barrier properties of silicon oxide coatings deposited on polyethylene terephthalate by plasma enhanced chemical vapor deposition

Plasma polymer coatings were deposited from hexamethyldisiloxane on polyethylene terephthalate (PET) substrates while varying the operating conditions, such as the Ar and O₂ flow rates, at a fixed radio frequency power of 300 W. The water vapor transmission rate (WVTR) of the untreated PET was 54.56 g/m²/day and was decreased after depositing the silicon oxide (SiO_x) coatings. The minimum WVTR, 0.47 g/m²/day, was observed at Ar and O₂ flow rates of 4 and 20 sccm, respectively, with a coating thickness of 415.44 nm. The intensity of the peaks for the Si-O-Si bending at 800-820 cm^− 1 and Si-O-Si stretching at 1000-1150 cm^− 1 varied depending on the Ar and O₂ flow rates. The contact angle of the SiO_x coated PET increased as the Ar flow rate was increased from 2 to 8 sccm at a fixed O₂ flow rate of 20 sccm. It decreased gradually as the oxygen flow rate increased from 12 to 28 sccm at a fixed Ar carrier gas flow rate. The examination by atomic force microscopy revealed a correlation of the SiO_x morphology and the water vapor barrier performance with the Ar and O₂ flow rates. The roughness of the deposited coatings increased when either the O₂ or Ar flow rate was increased.     

Hydrogen etching of Si3N4 layers with plasma assisted hot wire CVD

In order to develop sustainable processes for clean manufacturing environment for thin film or other solar cell production, we studied the hydrogen etching of silicon nitride (Si₃N₄) films on flat crystalline silicon (c-Silicon) substrates. With an arrangement primarily constructed for hot wire CVD (HWCVD) deposition of thin silicon films also cleaning processes with atomic hydrogen were studied with a simplified three wire assembly. The three filaments could be biased independently by different potential. A variation of hydrogen pressure and flow was performed to find out conditions of high etching rates for the Si₃N₄ layers. The etching rate was simply determined by measuring the time for total removal of the film, since this could be easily detected by the change of the anti-reflection property. Etching rates of 0.1 nm/s have been obtained under 15 Pa and a flow of 50 sccm. An intensive study was carried out of the direct current (DC) plasma hot wire CVD conditions.     

Dry etching of AlN films using the plasma generated by fluoride

The plasma produced by the mixture of fluoride and argon (SF₆/Ar) was applied for the dry etching of AlN films. Very high etching rate up to 140 nm/min have been observed. The effects of the bias voltage and the plasma component on the etching results were investigated. It shows that AlN can be effectively etched by the plasma with the moderate SF₆ concentration and the etching rate varies linearly with the bias voltage. The FTIR spectra confirm that AlF₃ is formed due to the chemical reaction of Al and F atoms. The mechanism of AlN etching in F-based plasma is probably a combination between physical sputtering and chemical etching and can be briefly outlined: (i) F⁻ ions reacts with Al atoms to form low volatile product AlF₃ and passivate the surface, and (ii) at the same time the Ar⁺ ions sputter the reaction product from the surface and keep it fluoride free to initiate further reaction. AlF₃ formed on the patterned sidewall play a passivation role during the etching process. The etching process is highly anisotropic with quite smooth surface and vertical sidewalls.     

Optical constants of silicone-like (Si:Ox:Cy:Hz) thin films deposited on quartz using hexamethyldisiloxane in a remote RF hollow cathode discharge plasma

Deposition of amorphous silicone-like (Si:O_x:C_y:H_z) thin films in a remote RF hollow cathode discharge plasma using hexamethyldisoloxane as monomer and Ar as feed gas has been investigated for films optical constants and plasma diagnostic as a function of RF power (100-300 W) and precursor flow rate (1-10 sccm). Plasma diagnostic has been performed using Optical Emission Spectroscopy (OES). The optical constants (refractive index, extinction coefficient and dielectric constant) have been obtained by reflection/transmission measurements in the range 300-700 nm. It is found that the refractive index increases from 1.92 to 1.97 with increasing power from 100 to 300 W, and from 1.70 to 1.92 with increasing precursor flow rate from 1 to 10 sccm. The optical energy band gap E_g and the optical-absorption tail ΔE have been estimated from optical absorption spectra, it is found that E_g decreases from 3.28 to 3.14 eV with power increase from 100 to 300 W, and from 3.54 to 3.28 eV with precursor flow rate increase from 1 to 10 sccm. ΔE is found to increase with applied RF power and precursor flow rate increase. The dependence of optical constants on deposition parameters has been correlated to plasma OES.     

Texturization of ZnO:Al surface by reactive ion etching in SF<Subscript>6</Subscript>/Ar,CHF<Subscript>3</Subscript>/Ar plasma for application in thin film silicon solar cells

As-deposited sputtered ZnO:Al (AZO) thin films having high transparency (T?≥?85% at 550 nm of wavelength) and good electrical properties (ρ?=?2.59?×?10^?04 Ω cm) are etched to get suitable light trapping in thin film solar cells, using reactive ion etching method in sulfur hexafluoride–argon (SF₆/Ar) plasma and trifluoromethane–argon (CHF₃/Ar) plasma to texture their surface. Though the electrical properties of the films are not affected much by the etching process but significant increment in the average haze values in the wave length range of 350–1100 nm in the etched AZO films (19.21% for SF₆/Ar and 22.07% for CHF₃/Ar plasma etched) are found compared to as-deposited AZO films (5.61%). Increment in haze value is due to more scattering of light from the textured surface. These textured substrates are used as front transparent conducting oxide electrode for the fabrication of amorphous silicon solar cells. Solar cells fabricated on etched AZO substrates show 7.76% increase in conversion efficiency compared to as-deposited AZO substrates.     

Characteristics of hydrogenated silicon thin film deposited by RF-PECVD using He-SiH4 mixture

Nanocrystalline silicon thin films (nc-Si:H) were deposited using He as the dilution gas instead of H₂ and the effect of the operating pressure and rf power on their characteristics was investigated. Especially, operating pressures higher than 4 Torr and a low SiH₄ containing gas mixture, that is, SiH₄(3 sccm)/He(500 sccm) were used to induce high pressure depletion (HPD) conditions. Increasing the operating pressure decreased the deposition rate, however at pressures higher than 6 Torr, crystallized silicon thin films could be obtained at an rf power of 100 W. The deposition of highly crystallized nc-Si:H thin film was related to the HPD conditions, where the damage is decreased through the decrease in the bombardment energy at the high pressure and the crystallization of the deposited silicon thin film is increased through the increased hydrogen content in the plasma caused by the depletion of SiH₄. When the rf power was set at a fixed operating pressure of 6 Torr, HPD conditions were obtained in the rf power range from 80 to 100 W, which was high enough to dissociate SiH₄ fully, but meantime low enough not to damage the surface by ion bombardment. At 6 Torr of operating pressure and 100 W of rf power, the nc-Si:H having the crystallization volume fraction of 67% could be obtained with the deposition rate of 0.28 nm/s.     

Characteristics of hydrogenated silicon thin film deposited by RF-PECVD using He–SiH4 mixture

Nanocrystalline silicon thin films (nc-Si:H) were deposited using He as the dilution gas instead of H₂ and the effect of the operating pressure and rf power on their characteristics was investigated. Especially, operating pressures higher than 4 Torr and a low SiH₄ containing gas mixture, that is, SiH₄(3 sccm)/He(500 sccm) were used to induce high pressure depletion (HPD) conditions. Increasing the operating pressure decreased the deposition rate, however at pressures higher than 6 Torr, crystallized silicon thin films could be obtained at an rf power of 100 W. The deposition of highly crystallized nc-Si:H thin film was related to the HPD conditions, where the damage is decreased through the decrease in the bombardment energy at the high pressure and the crystallization of the deposited silicon thin film is increased through the increased hydrogen content in the plasma caused by the depletion of SiH₄. When the rf power was set at a fixed operating pressure of 6 Torr, HPD conditions were obtained in the rf power range from 80 to 100 W, which was high enough to dissociate SiH₄ fully, but meantime low enough not to damage the surface by ion bombardment. At 6 Torr of operating pressure and 100 W of rf power, the nc-Si:H having the crystallization volume fraction of 67% could be obtained with the deposition rate of 0.28 nm/s.     

Anisotropic etching of silicon in SF6 plasma

The reactive ion etching of silicon in SF₆ plasma is considered. During the experiment, silicon substrates are etched in SF₆ plasma at different pressures and energies of incident ions. High etching anisotropy is achieved decreasing the pressure in the reactor and increasing the energy of the bombarding ions. The obtained experimental measurements are compared with theoretical calculations. It is determined that the temperature of the sidewalls decreases with the decrease of concentration of F atoms due to suppressed plasmochemical etching of silicon. The etching anisotropy increases with the decrease of concentration of F atoms due to decreased desorption of SiF₄ molecules.     

The etching characteristics of Al2O3 thin films in an inductively coupled plasma

In this study, the etching characteristics of ALD deposited Al₂O₃ thin film in a BCl₃/N₂ plasma were investigated. The experiments were performed by comparing the etch rates and the selectivity of Al₂O₃ over SiO₂ as functions of the input plasma parameters, such as the gas mixing ratio, the DC-bias voltage, the RF power, and the process pressure. The maximum etch rate was obtained at 155.8 nm/min under a 15 mTorr process pressure, 700 W of RF power, and a BCl₃ (6 sccm)/N₂ (14 sccm) plasma. The highest etch selectivity was 1.9. We used X-ray photoelectron spectroscopy (XPS) to investigate the chemical reactions on the etched surface. Auger electron spectroscopy (AES) was used for the elemental analysis of the etched surfaces.     

Influence of N2 flow rate on the mechanical properties of SiNx films deposited by microwave electron cyclotron resonance magnetron sputtering

Hydrogen-free amorphous silicon nitride (SiN_x) films were deposited at room temperature by microwave electron cyclotron resonance plasma-enhanced unbalance magnetron sputtering. Varying the N₂ flow rate, SiN_x films with different properties were obtained. Characterization by Fourier-transform infrared spectrometry revealed the presence of Si-N and Si-O bonds in the films. Growth rates from 1.0 to 4.8 nm/min were determined by surface profiler. Optical emission spectroscopy showed the N element in plasma mainly existed as N⁺ species and N₂⁺ species with 2 and 20 sccm N₂ flow rate, respectively. With these results, the chemical composition and the mechanical properties of SiN_x films strongly depended on the state of N element in plasma, which in turn was controlled by N₂ flow rate. Finally, the film deposited with 2 sccm N₂ flow rate showed no visible marks after immersed in etchant [6.7% Ce(NH₄)₂(NO₃)₆ and 93.3% H₂O by weight] for 22 h and wear test for 20 min, respectively.     

Influence of nitrogen gas flow rate on the structural, morphological and electrical properties of sputtered TiN films

In this work, nanocrystalline titanium nitride (TiN) films have been deposited by reactive DC magnetron sputtering technique on the Si/SiO₂ (100) substrates. The influence of nitrogen gas flow rate [0, 3, 5, 7 and 9 sccm (standard cubic centimeter per minute)] on the structural, morphological and electrical properties of the nanocrystalline TiN films has been studied. As-deposited TiN films have been characterized by using X-ray diffraction (XRD), XPS (X-ray photoelectron spectroscopy), FESEM (field emission scanning electron microscopy) and four point probe resistivity measurement, respectively. The XRD patterns revealed the HCP symmetry for pure Ti (N₂ = 0 sccm) with (002) preferred orientations, and the FCC symmetry for TiN (N₂ = 3, 5, 7 and 9 sccm) films having (111) preferred orientations. The lattice parameters were found to be a = 2.950 ?, c = 4.681? for the Ti (N₂ = 0 sccm) film and a = 4.250Å for the TiN films. The presence of different phases such as TiN and TiO₂ were confirmed by XPS analysis. The FESEM images showed a smooth morphology of the film with columnar grain structures. The grain size of the TiN films was found to decrease from 22 to 15 nm as the nitrogen flow rate is increased from 0 to 9 sccm. The electrical resistivity measurement showed that the resistivity of the film increased from 11 × 10^?6 to 17 × 10^?6 Ohm cm on increasing nitrogen flow rate from 3 to 9 sccm, having the lowest resistivity of 11 × 10^?6 Ohm cm for the film deposited at 3 sccm nitrogen flow.     

Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF₆ and a mixture of SF₆ and O₂ followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that E_to = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm², and maximum current density J = 60.4 μA/cm² at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.     

Prediction of silicon dry etching using a piecewise linear algorithm

A piecewise linear algorithm for predicting silicon etch rates in fluorine-based plasmas is shown. Discrete experimental data of pressure and RF power in reactive ion etching are used to construct a set of local two-dimensional etching functions that serve as a basis for computing numerical solutions (pressure and power values for a specific predicted silicon etch rate). It must be pointed out that, although the algorithm scans the entire data domain, a testing procedure is applied to ensure that the computing task will be invoked only when a solution exists, and otherwise it will be discarded (this avoids brute force methods). In the last step of the algorithm, all solutions are collected and interpolated to construct a solution path. In order to verify the match between the experimental etching results and numerical predictions, the algorithm has been coded and tested using Maple® Release 13.0, showing a successful validation with a difference between experimental data and computed numerical solutions as low as 1% for SF₆, and 4% for SF₆/O₂ in the best case and a root-mean squared error of 0.03.     

Planar plasma etching of chromium

We have investigated the plasma etching characteristics of chromium thin films in an rf planar (parallel plate) reactor. The experimental work was performed using a commercial reactor operating at 13.56 MHz with power variable up to 500 W. The etch rate of the Cr films deposited on glass substrates by e-beam evaporation was measured as a function of the concentration of O₂ in a CCl₄/O₂ gas mixture, the total flow rate of input gases and the rf power density. Using a total gas flow of 15 sccm and an input power density of 0.4 W cm^?2, the maximum etch rate was obtained in CCl₄ plasma containing 40% O₂. It was found that doubling the number of the substrates in the reactor decreased the etch rate by 20%. Also, the etch rate at the back of the reactor was twice that at the front. Methods to alliviate non-uniformity and loading effects are discussed and the mechanism of plasma etching of Cr is examined through the effect of various processing parameters on the etching characteristics.     

Structure and mechanical properties of ZrO2 films deposited by gas flow sputtering

ZrO₂ films were deposited by reactive gas flow sputtering (GFS) where voltage is applied to a cyindrical hollow-cathode target from a DC source, the discharge being produced at relatively high sputtering pressure. In this system, secondary electrons form a major component of the total current flow and lead to heating of the substrate which in turn has an effect on the properties of deposited films. The present experiments were carried out under the following conditions: Ar gas flow rate of 200 sccm, O₂ flow rate FO₂ in the range between 0.003 and 1 sccm, and sputtering power (P_S) in the range of 50-800 W. The reults showed that the crystal structure of the films deposited for P_S below 200 W was monoclinic but for P_S above 400 W, the films included tetragonal cystals of stable structure formed at high temperature by the electron bombardment. The films were formed with grains of 20-100 nm in diameter in a porous structure. The mechanical properties of the films were determined by a nanoindentation technique. Martens hardness (H_M) of the porous films was found to be in the range between 220 and 330 MPa which is substantially less than that of films typically deposited by rf magnetron sputtering.     

Amorphous hydrogenated carbon films used as masks for silicon microtips fabrication in a reactive ion etching with SF6 plasma

A room temperature fabrication process for silicon microtips has been developed using amorphous hydrogenated carbon films as masks for silicon etching. Reactive ion etching using an SF₆ plasma has been employed to sharpen the microtips without any thermal oxidation technique.     

Structure and mechanical properties of Mo-N/Cu films produced by inductively coupled plasma reactive sputtering

Mo-N/Cu films were deposited on silicon and steel substrates by inductively coupled plasma sputtering using Mo-Cu alloy targets having different Cu concentrations. The structure and mechanical properties of the Mo-N/Cu films were investigated. The (200)-oriented γ-Mo₂N-Cu films were obtained at a nitrogen flow rate higher than 1.5 sccm, whereas metallic films were deposited at a nitrogen flow rate lower than 1 sccm. Cu contents in the films increased from 3 at.% to 8 at.% with corresponding increase in Cu concentrations in the target from 5 at.% to 20 at.%. Hardness of the films varied between 19 and 35 GPa as the Cu concentration and nitrogen flow rate were changed. HRTEM and XPS analyses showed that Mo-N/Cu films consisted of γ-Mo₂N grains lower than 10 nm in size and a copper amorphous phase. The minimum value of coefficient of friction (CoF) of the films was 0.17 when the film tested by alumina ball.     

GaN reactive ion etching using SiCl<Subscript>4</Subscript>:Ar:SF<Subscript>6</Subscript> chemistry

Reactive ion etching with SiCl₄:Ar:SF₆ mixtures of gallium nitride epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) has been studied. The effects of several factors such as gas mixture, chamber pressure, and drive power on the etch rate and etched profile have been investigated. A strong dependence of both properties with the amount of SF₆ present in the mixture has been found. High etch rates (> 50 nm/min) and controllable sidewall angles and smoothness have been achieved.     

Microstructural size effects on the hardness of nanocrystalline TiN/amorphous-SiNx coatings prepared by magnetron sputtering

It has been postulated that equiaxed nanocrystalline (<10 nm) TiN grains embedded in a thin amorphous silicon nitride (a-SiN_x) phase are a prerequisite to obtain ultrahard TiN/a-SiN_x coatings. The present study correlates hardness and microstructure of TiN/a-SiN_x coatings with Si contents between 0 and 17 at.%. The coatings have been deposited by magnetron sputtering in industrial-scale physical vapour deposition systems. Transmission electron microscopy studies revealed that increasing the silicon content causes the TiN grain size to decrease. This is accompanied by a change in grain morphology: At Si contents lower than 1 at.% TiN grains become columnar, while at Si contents higher than 6 at.% equiaxed grains with diameters of 6 nm form. For silicon contents between 1 and 6 at.%, a transition region with nanocrystalline columnar grains exists. This nanocrystalline columnar microstructure causes maximum hardness values of more than 45 GPa for TiN/a-SiN_x coatings as determined by nanoindentation. The elongated and equiaxed nanocrystalline TiN grains exhibit almost theoretical strength as dislocation-based deformation mechanisms are constrained.