Plasma etching of III–V semiconductors in BCl3 chemistries: Part II: InP and related compounds

BCl₃/Ar and BCl₃/N₂ plasma chemistries were compared for patterning of InP, InAs, InSb, InGaAs, InGaAsP, and AlInAs. Under electron cyclotron resonance conditions etch rates in excess of 1 μm/min can be achieved at room temperature with low additional rf chuck power (150 W). The etch rates are similar for both chemistries, with smoother surface morphologies for BCl₃/Ar. However, the surfaces are still approximately an order of magnitude rougher (as quantified by atomic force microscopy) than those obtained under the same conditions with Cl₂/Ar. InP surfaces etched at high BCl₃-to-Ar ratios have measurable concentrations of boron-and chlorine-containing residues.     

Dry etching of InGaP and AlInP in CH4/H2/Ar

Electron cyclotron resonance (ECR) plasma etching with additional rf-biasing produces etch rates 2,500 A/min for InGaP and AlInP in CH₄/H₂/Ar. These rates are an order of magnitude or much higher than for reactive ion etching conditions (RIE) carried out in the same reactor. N₂ addition to CH₄/H₂/Ar can enhance the InGaP etch rates at low flow rates, while at higher concentrations it provides an etch-stop reaction. The InGaP and AlInP etched under ECR conditions have somewhat rougher morphologies and different stoichiometries up to 200 Å from the sur face relative to the RIE samples.     

Use of CF3,Br/Al,discharges for reactive ion etching of III-V semiconductors

The reactive ion etching of GaAs, InP, InGaAs, and InAlAs in CF₃Br/Ar discharges was investigated as a function of both plasma power density (0.56-1.3 W - cm^–2) and total pressure (10-40 mTorr) The etch rate of GaAs in 19CF₃Br:1Ar discharges at 10 m Torr increases linearly with power density, from 600 Å min^–1 at 0.56 W · cm^–2, to 1550 Å · min at 1.3 W · cm^–2. The in-based materials show linear increases in etch rates only for power densities above – 1.0 W · cm^–2. These etch rates are comparable to those obtained with CCI₂F₂:O₂ mixtures under the same conditions. Smooth surface morphologies and vertical sidewalls are obtained over a wide range of plasma parameters. Reductions in the near-surface carrier concentration in n-type GaAs are evident for etching with power densities of >0.8 W cm^–2, due to the introduction of deep level trapping centers. At 1.3 W· cm^–2, the Schottky barrier height of TiPtAu contacts on GaAs is reduced from 0.74 to 0.53 eV as a result of this damage, and the photoluminescent intensity from the material is degraded. Alter RIE, we detect the presence of both F and Br on the surface of all of the semiconductors. This contamination is worse than with CCl₂F₂-based mixtures. High-power etching with CF₃Br/Ar together with Al-containing electrodes can lead to the presence of a substantial layer of aluminum oxide on the samples if the moisture content in the reactor is appreciable.     

Low-temperature dry etching of tungsten,dielectric, and trilevel resist layers on GaAs

Dry etching of common masking materials used in GaAs device technology, was examined down to temperatures of –30°C. The etch rates of SiN_x, SiO₂, and W in SF₆/Ar are reduced below 0°C, but the anisotropy of the etching is improved at low temperature. Microwave enhancement of the SF₆/Ar discharges produces increases in etch rates of several times at 25°C, but much lower increases at –30°C substrate temperature. The underlying GaAs surface shows increased S and F coverage after low-temperature etching, but these species are readily removerd either by anex-situ wet chemical cleaning step or an in-situ H₂ plasma exposure. Photoresist etching is less sensitive to temperature, and anisotropic profiles are produced between –30 and +60°C in pure O₂ discharges.     

Plasma etching of refractory metals (W,Mo, Ta) and silicon in SF6 and SF6-O2. An analysis of the reaction products

The etching rates and reaction products of refractory metals (W, Mo, and Ta) and silicon have been studied in a SF₆-O₂ r.f. plasma at 0.2 torr. The relative concentrations of WF₆ and WOF₄ and the intensities of the WF _n ⁺ (n=3–5), WOF _m ⁺ (m=1–3), MoF _n ⁺ , and MoF _m ⁺ ions have been measured by mass spectroscopy. An analysis of the neutral composition of the plasma during etching of these metals and a comparison with the results obtained for silicon show that at least two species are involved for W and Mo etching: fluorine and oxygen atoms. A reaction scheme is proposed.     

Effect of Plasma and Control Parameters on SiC Etching in a C2F6 Plasma

Effects of radio frequency (RF) source power (plasma density) on silicon carbide etching are examined with variations in RF bias power, pressure, O₂ fraction, and gap spacing. The etching was conducted in a C₂F₆ inductively coupled plasma. Depending on parameters or plasma conditions, the etch rate varied quite differently. When the source power was varied, the bias power (ion energy) was strongly involved in determining the relative variation in the etch rate. Complex interactions between the parameters were ascertained by means of a predictive model. The model was obtained by using a neural network in conjunction with a 2⁵ full factorial experiment. Model behaviors were consistent with experimental ones. By correlating the etch rate to the DC bias, it was identified that the source power effect on the etch rate is significantly enhanced as the DC bias is maintained at relatively low values.     

Plasma etching of III–V semiconductors in BCl3 chemistries: Part I: GaAs and related compounds

BCl₃/Ar discharges provide rapid, smooth pattern transfer in GaAs, AlGaAs, GaP, and GaSb over a wide range of plasma conditions. At high BCl₃-to-Ar ratio there is significant surface roughening on GaSb, which is correlated with the presence of B- and Cl-containing residues detected by Auger electron spectroscopy. BCl₃/N₂ discharges provide similar etch rates to BCl₃/Ar, but when used with photoresist masks lead to rough morphologies on the semiconductor materials due to enhanced dissociation and redeposition of the resist. Etch rates with electron cyclotron resonance discharges are up to two orders of magnitude higher than for rf-only conditions.     

Development and Evaluation of Gold 3D Cylindrical Nanoelectrode Ensembles

Gold 3D cylindrical nanoelectrode ensembles （NEEs）, 100 nm in diameter and 500 nm in length were prepared by electroless template synthesis in polycarbonate filter membranes, followed by selective controlled chemical etching. The morphology of the nanowires and cylindrical NEEs was imaged by scanning electron microscopy. The protruding nanoelectrodes were in good parallel order. EDX study showed that the nanoelectrode elements consisted of pure gold. The electrochemical evaluation of the 3D electrodes was conducted using the well known [Fe（CN）6]^3-/[Fe（CN）6]^4- couple. Cyclic voltammgrams （CV） show a very low double layer charging current and a higher ratio of signal to background current than 2D disc NEEs. Electrochemical impedance spectroscopy （EIS） indicates that the 3D cylindrical NEEs effectively accelerate the charge transfer process, which is in consistent with the results of CV. The linear relationship with a slope of 0.5 between lg Ipc and lg v shows that linear diffusion is dominant on the 3D cylindrical NEEs at conventional scan rates.     

Some aspects of plasma polymerization of fluorine-containing organic compounds. II. Comparison of ethylene and tetrafluoroethylene

Plasma polymerizations of ethylene and tetrafluoroethylene are compared. In the plasma polymerization of ethylene and of tetrafluoroethylene, glow characteristics play an important role. Glow characteristic is dependent on a combined factor of W/F_m, where W is discharge power and F_m is monomer flow rate. At higher flow rates, higher wattages are required to maintain “full glow.” In the plasma polymerization of tetrafluoroethylene, simultaneous decomposition of the monomer competes with plasma polymerization. Above a certain value of W/F_m, decomposition becomes the predominant reaction, and the polymer deposition rate decreases with increasing discharge power. ESCA results indicate that the plasma polymer of tetrafluoroethylene that is formed in an incomplete glow region (low W/F_m) is a hybrid of polymers of plasma polymerization and of plasma-induced polymerization of the monomer. Polymers formed under conditions of high W/F_m to produce “full glow” are similar, regardless of the extent of decomposition of the monomer. They contain carbons with different numbers of F(CF₃, ? CF₂? , >CF? , >C<) and carbons bonded to other more electronegative substituents.     

Hybrid electron cyclotron resonance-radio-frequency plasma etching of III–V semiconductors in Cl2-based discharges. Part II: InP and related compounds

Electron Cyclotron Resonance (ECR) discharges of CCl₂F₂ or PCl₃ have been used to etch InP, InAs, InSb, InGaAs and AlInAs. The etch rates of these materials increase linearly with additional RF power level applied to the cathode and are in the range 50–180 Å · min^–1 for 50 W (DC bias 308 V), 10 mTorr, 38 CCl₂F₂/2 O₂ plasmas. The etch rates fall rapidly with increasing pressure or increasing O₂-to-CCl₂F₂ ratio. Polymeric surface residues up to 40 Å thick are found on all of these semiconductors when using Freon-based gas mixtures. Etching at practical rates is possible with only 100 V self-bias when using PCl₃ discharges, and the addition of microwave excitation under these conditions enhances the etch rates by factors of 2–9. At higher self-biases (300 V) etch rates of 3500–8000 Å · min^–1 are possible with PCl₃ although the surface morphologies are significantly rougher and the etching less anisotropic than with CCl₂F₂-based mixtures.     

Reactions of NO in a Positive Streamer Corona Plasma

The reaction of NO in a streamer corona plasma is studied systematically as a function of the composition of the gas mixture, the initial concentration of NO, and the discharge repetition rate. The experimental results show that the reactions of NO depend strongly on the composition of the gas mixture. Reduction is observed in the absence of oxidants such as oxygen and water, but at very high energy cost (>200 eV/NO). In the presence of both these oxidants, more than 90% of the NO conversion is oxidation. The lowest energy costs, 24 eV/NO for He mixtures and 45 eV/NO for N ₂ mixtures, are obtained at water and oxygen concentration above 3% and at low discharge repetition rates (<10 Hz). Chemical kinetics calculations of the production of radicals in the plasma show a good agreement with the value derived from the experiments.     

Optical Diagnostics of the Effect of Oxygen on Bi-Level Contact Etch

The effect of oxygen flow rate on bi-level contact etch was studied by observing uv-visible emission from the plasma, during CHF₃/CO/O₂ etching of di-electric layers consisting of SiO₂ and SiN_x. The emission intensity of CN at 387 nm drifted progressively from wafer to wafer during plasma etch. Such a phenomenon became more obvious when using low or high oxygen flow rate, whereas for intermediate flow rates, no significant drift of emission intensity was observed. The critical dimension (CD) bias of each wafer showed a strong correlation with CN emission intensity. Possible mechanisms for such an intensity drift phenomenon are proposed. The drift of emission intensity indicates that the contribution of chamber wall polymers in wafer etching is non-negligible. The CN emission intensity is an indication of the magnitude of etching rate. Our results suggest that the variation of plasma emission intensity might be used as an index for in-line monitoring of CD bias fluctuation.     

Hybrid electron cyclotron resonance-radio-frequency plasma etching of III–V semiconductors in Cl2-based discharges. Part I: GaAs and related compounds

A systematic study has been performed of the dry etching characteristics of GaAs, Al_0.3Ga_0.7As, and GaSb in chlorine-based electron cyclotron resonance (ECR) discharges. The gas mixtures investigated were CCl₂F₂/O₂, CHCl₂F/O₂, and PCl₃. The etching rates of all three materials increase rapidly with applied RF power, while the addition of the microwave power at moderate levels (150 W) increases the etch rates by 20–80%. In the microwave discharges, the etch rates decrease with increasing pressure, but at 1 m Torr it is possible to obtain usable rates for self-bias voltages 100 V. Of the Freon-based mixtures, CHCl₂F provides the least degradation of optical (photoluminescence) and electrical (diode ideality factors and Schottky barrier heights) properties of GaAs as a result of dry etching. Smooth surface morphologies are obtained on all three materials provided the microwave power is limited to 200 W. Above this power, there is surface roughening evident with all of the gas mixtures investigated.     

Relative Rates for Plasma Homo- and Copolymerizations of a Homologous Series of Fluorinated Ethylenes

The relative rates of plasma homo- and Copolymerizations of ethylene, vinyl fluoride, vinylidene fluoride, trifluoroethylene and tetrafluoroethylene (VF _x , x = 0–4, respectively) were determined in an rf, capacitively coupled, tubular reactor with external electrodes using identical plasma parameters. The deposition rates for VF _x (x = 1–3) and 20 different monomer blends, when plotted versus the F/C ratios of the monomers or monomer blends, followed a concave-downward curve situated above the straight-line joining the rates for VF₀ and VF₄. The deposition rates for VF _m /VF _n blends (m = 3 or 4; n = 0–2) likewise yielded concave-downward curves situated above the straight lines joining the rates for the respective monomers; the rates for VF _m /VF _n blends (m = 0 or 1; n = 1 or 2) yielded concave-upward curves situated below the straight lines joining the rates for the respective monomers; while the rates for VF₃/VF₄ blends fell along the straight line joining the rates for VF₃ and VF₄. The mechanisms for plasma (co) polymerizations of VF _x monomers responsible for the wide range of relative deposition rates remain to be elucidated.     

Characteristics of vinyl iodide microwave plasma etching of GaAs/AlGaAs and InP/InGaAs heterostructures

Vinyl iodide (C₂H₃I) microwave discharges with additions of H₂ and Ar are found to provide faster etch rates than conventional CH₄/H₂/Ar discharges for InP, InGaAs, GaAs, and AlGaAs. This is a result of the relatively high volatilities of indium, gallium, and aluminum iodide species. The etched features are smooth and anisotropic over a wide range of do self-biases (–150 to –350 V), process pressures (1–20mTorr), and microwave powers (150–500 W). The polymer that forms on the mask during the plasma exposure can be readily removed in O₂ discharges. Electron spectroscopy for chemical analysis (ESCA) showed that the etched surfaces are slightly deficient in the group V elements under most conditions, but changes to the optical properties of the semiconductors are minimal. No defects are visible by transmission electron microscopy (TEM) in GaAs or InP samples etched at dc biases –250 V.     

Reactive polymers incorporating silyl enol ether groups. I. synthesis by radical polymerization of 2-(trimethylsiloxy)-butadiene and 2-(tert)-butyldimethylsiloxy butadiene

2-(Trimethylsiloxy)butadiene (TMSBD) and 2-(tert-butyldimethylsiloxy)butadiene (TBMSBD) were copolymerized with styrene (St) and methyl methacrylate (MMA) under free-radical conditions. The obtained polymers were found to contain reactive silyl enol ether groups in a ratio identical to the TMSBD or TBMSBD molar fraction in the copolymer. All investigated samples displayed only 1,4- and 3,4-microstructures. The influence of several experimental factors on the yields, rates of polymerization, microstructures, and copolymer compositions were examined. Monomer reactivity ratios r₁ and r₂ at 60°C were determined from copolymer composition curves at low conversions. The homopolymerization of TBMSBD was also investigated and results were compared with those previously obtained for TMSBD. A slight increase in rates was observed and was rationalized on the basis of the higher viscosity resulting from the structural change in the monomer. Thermal stabilities of the synthesized polymers were investigated by TGA and their glass transition temperatures were determined by DSC. All measurements are compatible with a possible use of TMSBD and TBMSBD copolymers as reactive polymers. © 1996 John Wiley & Sons, Inc.     

Effect of Aging on the Nature of the Wichterle-Marek-Trekoval Catalyst

The effect of aging on the Wichterle-Marek-Trekoval (WMT) solid cationic catalyst for isobutene polymerization has been investigated. It was confirmed that this complex catalyst gives high molecular weight polyisobutenes at high rates at relatively high temperatures. Aging for various times and temperatures the Al(O-s-C₄H₉)₃/BF₃ component and the Al(O-s-C₄H₉)₃/BF₃/TiCl₄ final catalyst strongly depresses rates of polymerization but only slightly affects the molecular weights produced. Fastest rates and molecular weights are obtained with freshly prepared catalyst whose Ti/Al ratio is ～ 0.3 at about 30% conversions. Rates and polyisobutene molecular weights obtained by the present authors were higher than those reported by the Czechoslovakian workers. These results are discussed in terms of cationic polymerization mechanism, and an explanation for the reduced rates and molecular weights produced with aged WMT catalysts is proposed.     

Laser Doppler anemometry under plasma conditions. Part II. Measurements in an inductively coupled r.f. plasma

Laser Doppler anemometry is used for the measurements of the plasma and particle velocity profiles in the coil region of an inductively coupled r.f. plasma. Results are reported for a 50 mm i.d. induction plasma torch operated at atmospheric pressure with argon as the plasma gas. The oscillator frequency is 3 MHz and the plate power is varied between 4.6 and 10.5 kW. Plasma velocity measurements are obtained using a fine carbon powder as a tracer. Measurements are also given for larger silicon particles ( ) centrally injected into the discharge under different operating conditions.Nomenclature d _p particle diameter - P ₀ plasma power - Q ₁ powder carrier gas flow rate - Q ₂ plasma gas flow rate - Q ₃ sheath gas flow rate - r distance in the radial direction - V axial plasma velocity - V _p axial particle velocity - Z distance in the axial direction - standard deviation     

DC plasma polymerization of hexamethyldisiloxane

Hexamethyldisiloxane (HMDS) was polymerized onto metallic and insulating substrates in a parallel-plate DC reactor. The limits of the DC reactor with respect to pressure and power were determined for deposition of PP-HMDS films. In all conditions ranging from 5 Pa/0.3 W to 100 Pa/50 W, solid films were deposited. No powders or oily films were obtained under any condition in this operating range. The films were polymeric in nature,i.e., they were neither carbon-like nor SiO _x -like films. The structures and crosslink densities of the plasma films dependend strongly on the deposition conditions. The highest deposition rates, up to 2 μm per minute (or0.3 mg/cm² min), were obtained at high power, pressure, and flow rate conditions. An efficiency ɛ is introduced, defined as the fraction of the monomer that is retained in the form of a polymer deposited on the substrate. Efficiencies as high as 25% could be obtained in certain conditions. Pulsing the discharge power increased the conversion efficiency markedly, but the effect depended strongly on the monomer used. In addition to HMDS, plasma polymers were also deposited from pyrrole in pulsed conditions for comparison. A method is described for depositing films on insulators from a DC glow discharge using two wire meshes held at a negative potential.     

Evolution of Etching Kinetics and Directional Transition of Nanowires Formed on Pyramidal Microtextures

Two‐scale roughened silicon (Si) textures are considered promising architectures for versatile applications because of their excellent self‐cleaning, light‐trapping, and biosensing capacities. In this study, we explore the directional control of nanowires formed on pyramidal microtextures through a single‐step metal‐assisted chemical etching (MACE). The measured current density of Si dissolution at catalytic etching enables quantitative monitoring of the etching kinetics of nanowire formation. The preferential orientation of fabricated nanowires on {111}‐plane pyramidal textures was found to positively correlate with the molar ratio of [AgNO₃] to ([AgNO₃]+[HF]), referred to as ρ. A distinct transition from <100> to <111> axial directions at ρ≥0.2 and ρ=0.07, respectively, was revealed. The <111>‐oriented nanowires on the pyramidal microtextures exhibited an excellent antireflection performance, with a reflectivity as low as 1.2 % at 600 nm. The results of this study may aid the design for the development of high‐performance Si‐based optoelectronic devices.