Low-pressure Microwave Research Articles

Dry etching and implantation characteristics of Al0.5Ga0.5P

Highly anisotropic pattern transfer into AlGaP was achieved using low pressure (1 mTorr) microwave BCl3/Ar or CH4/H2/Ar discharges. Much faster etch rates were obtained with the chlorine-based discharges and etching was initiated at lower dc biases. The electrical activation of implanted Si+ and Be+ ions was investigated at fixed dose (5×1014 cm−2) as a function of annealing temperature (500–1000 °C). Activation efficiencies of ∼50% were obtained for Be+ implantation after annealing at ≥750 °C. Activation of Si+ was less efficient and required higher annealing temperatures than for Be+. Thermally stable high resistance (≳108 Ω/⧠) regions were formed in initially n+ AlGaP by O+ implantation at doses ≳1014 cm−2 while normal damage-induced compensation was observed for lower O+ doses. No thermally stable isolation was found in O+-implanted p+ AlGaP.

Temperature Distributions Across the Plasma Layer of Planar Low Pressure Microwave Plasmas — A Comparative Investigation by Optical Emission Spectroscopy

AbstractNowadays low temperature non‐equilibrium plasmas received considerable attention in very different fields of plasma processing.The subject of the present paper is the comparative measurement of neutral gas temperature and optical excitation temperature to analyze the temperature distributions across the plasma layer of H2 non‐equilibrium plasmas (p = 0.2 – 1.5 kPa) with small admixtures of hydrocarbons in a novel planar microwave plasma source (2.45 GHz) used for plasmachemical deposition purposes by means of optical emission spectroscopy. Typical microwave power flux densities into the plasma lie within a range of 2 W cm−2 to 20 W cm−2. Results of neutral gas temperature measurements derived from Hα line Doppler profiles are compared with rotational temperatures of H2 and N2 molecules. The neutral gas temperature (800–1700 K) corresponds to the rotational temperature of the H2 molecules (Fulcher band, R 0–0 branch) but shows a more distinct spatial gradient. The rotational temperature of admixtured N2 molecules (2000–3000 K) is much more higher although Boltzmann distribution was ensured. The spatially resolved measured excitation temperature (1–3 eV) determined with the help of line intensity ratios of admixtured Ar well agrees with Langmuir probe measurements.The reported measurements as a whole demonstrate the feasibility of comparative investigations of different optically determined temperatures for expressive characterization of low pressure microwave plasmas.

Optical Deep-Level Transient Conductance Characterisation of Semi-Insulating Gallium Arsenide Treated with Hydrogen Plasma

Optical deep-level transient conductance spectroscopy (ODLTCS) has been employed to characterise Schottky devices fabricated on liquid encapsulation Czochralski (LEC) semi-insulating (SI) GaAs material which has been exposed to low-pressure microwave (2.42 GHz) H2 plasma. A range of substrate temperatures and exposure times to the plasma were tested. An increase in device conductance spectra amplitudes were observed after 60 min of H2 plasma treatment at 100° C, 300° C, and 400° C. In devices exposed to H2 plasma at 200° C, the conductance spectra decreased in magnitude with time and after 60 min and 120 min treatments, the spectra were identical to that of the control device. The change in device conductance under the above conditions suggests that there is (i) an optimal plasma condition for the passivation of defects by atomic H with the formation of neutral defect-H (DH) complexes, (ii) the reaction between defects and atomic H is time dependent, and (iii) the LEC SI GaAs materials are contaminated with H during the growth process. In contrast, the increase in magnitude of ODLTC spectrum after 200° C treatment with molecular H for 60 min confirms the role of passivation by the atomic H.

High efficiency a-Si:H alloy cell deposited at high deposition rate

We have made an a-Si:H alloy triple cell, which has intrinsic layers, prepared by low pressure microwave plasma CVD of SiH4 or SiH4 mixed with GeH4. The deposition rate is about one hundred angstrom/sec. The efficiency of the triple cell has reached 11.6% under AM1.5 100mW/cm2. Using the intrinsic layer deposited at 7.5 angstrom/sec in the top cell, the efficiency of the triple cell has reached 12.3% under AM1.5 100mW/cm2. The Urbach energy of the intrinsic a-Si:H layer deposited at 75 angstrom/sec is about 50meV, and the Urbach energy of the intrinsic a-SixGe1−x:H layer which has 100 angstrom/sec deposition rate is about 50meV.

Quantitative measurements of oxygen atom density using LIF

A pulsed atomic oxygen source, used to simulate oxidization mechanisms of spacecraft surface protective material (CASOAR), is probed by LIF. LIF measurements of oxygen atom density were first conducted in the post-discharge of an O2 low-pressure microwave excited plasma. It appears that the fluorescence yield depends strongly on the laser beam focusing conditions. This dependence must be taken into account for proper analysis of the experiment. Thus, LIF measurements appear to be quantitatively correct and consistent with the results of NO2 titration.

Growth and morphological changes of chemically vapour deposited diamond in the presence of argon

Diamond films were produced on Si(100) substrates by decomposing gaseous mixtures of methane and hydrogen in a low pressure microwave discharge. The added argon gas can be regarded as an efficient dilution gas although hydrogen is the main dilution gas reacting actively in the diamond deposition. Different ratios of argon gas to methane and hydrogen concentrations have been studied. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron energy loss spectroscopy and Raman spectroscopy were used to characterize the morphology, microstructure, composition and crystallinity of the deposited diamond films. Results indicated that the introduction of argon gas effectively influenced the growth rate, surface morphology and crystalline perfection of the diamond deposits at a higher starting concentration of methane which normally yielded high ratios of non-diamond carbon phases.

Dry and wet etching characteristics of InN, AlN, and GaN deposited by electron cyclotron resonance metalorganic molecular beam epitaxy

The dry etching characteristics of thin film InN, AlN, and GaN epitaxial layers deposited using metalorganic group III sources (trimethylindium, trimethylamine alane, and triethylgallium) and electron cyclotron resonance (ECR) plasma excited atomic nitrogen in a metal organic molecular beam epitaxy (MO-MBE) chamber have been examined using CCl2F2/Ar, BCl3/Ar, and CH4/H2/Ar discharges. In a hybrid ECR-rf system, the Cl-based chemistries readily etch GaN and AlN, while addition of F to the discharges dramatically reduces the etch rate of AlN. Slow, smooth etching of InN is achieved with the CH4/H2/Ar chemistry. Controlled rates of ∼200 Å min−1 are obtained for all three materials at moderate dc biases (≤−200 V), low pressure (1 mTorr), and low microwave power (200 W). The dc bias and pressure dependence of the etching have been systematically studied. GaN films can be wet chemically etched in 30–50% NaOH solutions, AlN in HF and InN in HCl:HNO3 solutions.

Formation and tribological application of CVD diamond films on steels

It is well known that the mutual alloying effect of carbon and iron hinders the direct coating of diamond films on steels. One of the promising ways to achieve direct coating of diamond film on tool steel is to interpose a buffer layer which impedes the interdiffusion of carbon and iron. This layer should be compatible both thermally and structurally with the substrate material as well as the subsequent diamond film. Electroless plating was utilized to deposit a layer of nickel on the tool steel SKD 61. Subsequent deposition of molybdenum was conducted in an electron beam evaporator. This bi-buffer layer of Mo/Ni deposited on SKD 61 served as a substrate for synthesizing diamond film in a low temperature (about 500 °C), low pressure (about 5.3 KPa or 40 Torr) microwave plasma chemical vapour deposition system using hydrocarbons and hydrogen as reactant gases. The results showed that the adhesive strength was much improved through the application of a bi-buffer layer, e.g. Mo/Ni or Mo/TiN.

Comparison of optical emission spectrometric measurements of the concentration and energy of species in low-pressure microwave and radiofrequency plasma sources

Understanding of specific plasmachemical reactions occurring in plasma sources with the addition of complicated molecules requires knowledge of the particular plasma conditions. The subject of the present paper is a comparison of the spectrometrically measured relative concentration distributions and energies of species in low-pressure argon discharges containing an organosilicon compound, hexamethyldisiloxane (HMDSO), which has a large molecular size. The optical diagnostics were performed with three different plasma sources: a special planar microwave plasma source (v= 2.45 GHz), an r.f. planar reactor (v= 13.56 MHz) and a capacitively coupled r.f. model discharge tube (v= 460 kHz). The energy distribution functions of each plasma are not the same, but their general forms are similar to a Maxwell–Boltzmann distribution. The results of comprehensive, spatially resolved measurements of the relative concentrations of atoms and radicals (H, Si, CH and C2), the neutral species gas temperature, rotational temperature and optical excitation temperature are reported. The use of the same gas mixture in three plasma sources, each of distinct construction, excited by different frequencies, once again clearly indicates that the comparison and interpretation of optical diagnostic results has to be done very carefully, taking into consideration the discharge conditions. Results obtained by actinometry show considerably different particle density gradients in the plasmas. Gradients of the excitation temperatures (Texc= 0.45–0.7 eV) are less, but the neutral gas temperatures also exhibit large spatial gradients (Tg= ambient temperature, about 2000 K). This clearly indicates the absolute necessity of spatially resolved optical emission measurements for the purpose of comparisons between different plasma sources.

Measurement of gas temperatures and metastable state densities in a microwave boosted glow discharge using a diode laser

The radiation of a diode laser was used for studying the plasma in a microwave boosted glow discharge source. In this source sample atoms are produced by cathode sputtering and excited in a low pressure microwave induced plasma (MIP) in argon. The plasma was investigated by laser absorption directing the beam of a semiconductor diode laser through the plasma perpendicular to the axis of the source at two different places. Two transitions in neutral argon with wavelengths in the near infrared spectral region (810.60 nm and 811.76 nm) have been studied. From the recorded absorption line profiles of Gaussian shape, the gas temperatures and the concentration of excited argon atoms in the microwave boosted glow discharge, as well as in the non-boosted discharge, have been calculated. The gas temperatures were found to be in the range 320–520 K, depending on the conditions; the metastable state densities in the range 1 × 10 11–1.4 × 10 12cm −3.

Low-pressure microwave plasma nucleation and deposition of diamond films

Low-pressure microwave plasma nucleation and deposition of diamond films were investigated in the pressure range 10-mtorr to 10 torr, at substrate temperatures 400-750 C and with CH4 and O2 concentrations in H2 plasma of 2-15 percent and 2-10 percent, respectively. The experiments were performed in a microwave plasma system consisting of a microwave plasma chamber, a downstream deposition chamber, and an RF induction heated sample stage. Scanning electron microscopy of diamond films deposited at 600 C with 5 percent CH4 and 5 percent O2 in H2 plasmas showed high-quality well faceted crystallites of 1/2 micron size. Cathodoluminescence measurements of these films showed very few nitrogen impurities and no detectable silicon impurities.

Microstructure and characterization of diamond film grown on various substrates

Abstract High purity diamond films were successfully grown on various substrate materials, including silicon, SiC, fused silica and a number of refractory metals, by low pressure microwave plasma chemical vapour deposition from CH4:H2 gas mixtures. The morphology and microstructures were studied using scanning electron microscopy and transmission electron microscopy, and characterized by x-ray diffraction, Auger electron spectroscopy, energy electron loss spectroscopy, electron dispersive spectroscopy and Raman spectroscopy. The results indicate that cubo-octahedra are the equilibrium shapes of crystalline diamond. Both {100} and {111} facets were observed in all the substrates studied. SiC, SiO, amorphous carbon and carbides of refractory metals are the dominant interphases in this study. The composition and microstructure of the interphase appear to have significant effects on the adhesion strength, as shown in the diamond/molybdenum and diamond/hafnium systems. Structural defects such as twins were frequently observed inside the diamond grains, while line defects of dislocations were mainly constrained to the grain boundaries. The surface of the substrate was modified using a process that ultrasonically stimulated cavitation-erosion in an aqueous suspension of diamond dust. The diamond crystallites were of a more uniform size than was achieved after the more conventional lapping process (mechanical polishing).

Mechanism of substrate heating in diamond-forming low-pressure microwave discharges

Experimental results concerning basic heat transport processes to and from substrates in diamond-forming low-pressure microwave discharges are presented. Special properties of the used planar microwave plasma source allow to distinguish clearly between different transport processes. As a rule, at pressures of few kPa substrate heating is dominated by surface recombination of atomic hydrogen. Substrate cooling is dominated by thermal conductivity of neutral gas. Detailed neutral gas temperature measurements demonstrated the completely non-isothermal character of the plasma with gas temperatures down to ambient temperature. Therefore, a basic problem of achieving very low deposition temperatures is to get detailed information about the concentration of atomic hydrogen that is absolutely necessary for high-quality diamond growth and its effect on substrate temperature.

The effect of noble gases on the formation of CVD diamond film

Diamond thin films were deposited by decomposing gaseous mixtures of methane and hydrogen in a low pressure microwave discharge. The addition of noble gases such as helium, neon, argon, krypton and xenon can be regarded as providing efficient dilution, though hydrogen is the main dilution gas reacting actively in the diamond deposition. Different concentration ratios of noble gases to methane and hydrogen have been studied. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron energy loss spectroscopy, energy-dispersive spectrometry and Raman spectroscopy were used to characterize the morphology, microstructure, composition and crystallinity of the deposited films. Results indicated that the introduction of the noble gases effectively influenced the nucleation, surface morphology and crystalline perfection of the diamond deposits at higher methane concentrations. Proper selection of the noble gases and their concentrations in hydrogen effectively suppressed the formation of nondiamond carbon phases and thus improved the crystal quality significantly.

Reduction of Gap State Density in a-SiGe:H Alloys

The reduction of the gap states in a-SiGe:H alloys down to the level in a-Si:H was realized by the low-pressure microwave plasma chemical vapour deposition (CVD) which utilizes hydrogen plasma stream. It was found that the densities of defect states and tail states in the alloy vary with the flow rate of hydrogen introduced into the deposition chamber, and a-SiGe:H alloy with low defect density was attained with an optimized flow rate of hydrogen when the flow rate of source gas (SiH4 and GeH4) was reduced. The Urbach energy also decreased when the flow rate of hydrogen was optimized, and a-SiGe:H alloy with an Urbach energy of 48 meV was obtained. It was also found that the photoconductive property of a-SiGe:H alloys is not sensitive to the density of gap states.

Modeling of low-pressure microwave discharges in Ar, He, and O/sub 2/: similarity laws for the maintenance field and mean power transfer

A kinetic study of low-pressure microwave discharges in Ar, He, and O/sub 2/ is carried out using electron-transport parameters and rate coefficients derived from solutions to the Boltzmann equation, together with the continuity and transport equations for the charged particles, taking into account stepwise-ionization processes. The Boltzmann equation is solved over a wide range of the applied frequency, omega /2 pi , but assuming that the angular frequency omega > tau /sub e//sup -1/, with tau /sub e/, denoting the characteristic time for electron-energy relaxation by collisions. The formulation provides discharge characteristics for the maintenance field and for mean absorbed power per electron in the three gases, which are shown to agree satisfactorily with experimental data obtained from surface-wave discharges. It is shown that such an agreement would not always be obtained without consideration of the role played by stepwise-ionization processes in sustaining the discharge. >

Characterization of a new electron cyclotron resonance source working with permanent magnets

Most electron cyclotron resonance (ECR) sources available today are working with electromagnets. A new source, developed for etch applications, is supplied with permanent magnets. This leads to strong short-range magnetic fields and the ECR condition is fulfilled on a ring near the chamber wall. Having the maximum electric field strength of the 2.45 GHz along this ring in the microwave radiator leads to efficient plasma production. The principle source characteristics are demonstrated with oxygen plasmas. At low pressure (some 10−4—some 10−3 mbar) and moderate microwave power (100–500 W) the plasma burns as a ring plasma. Increasing pressure and/or microwave power over the above mentioned values leads to a ‘‘bulk’’-type plasma. The change from ‘‘ring’’-type to bulk-type plasma occurs discontinuously and is connected (a) with a discontinuity in the run of the reflected power and (b) with a sudden change in the relative ion flux densities of different ionic fragments measured by mass spectrometry. Absolute ion densities up to 5×1010 cm−3 have been found by Langmuir probe measurements in a bulk plasma at a pressure of 1×10−3 mbar. Anisotropic etching of polysilicon over SiO2 was performed in the ring mode with pure Cl2 as feed gas and applying a radio-frequency (rf) extraction to the ECR plasma. Low damage etching was achieved due to the low direct-current (dc)-bias voltage of 150–200 V with very good anisotropy. The poly-Si etchrate was 85 nm/min with a selectivity of 18:1 to SiO2 and a uniformity of ±7.5% over 200 mm wafers.

I n s i t u emission and mass spectroscopic measurement of chemical species responsible for diamond growth in a microwave plasma jet

Chemical species responsible for diamond growth were examined by in situ methods using a microwave plasma jet from Ar-H2-CH4/C2H2. Irrespective of the reactants, diamond was successfully deposited, and morphology was likely to depend only upon the carbon equivalent concentration in the gas phase. Emission spectroscopy revealed that the C2 radical was the main emissive species in the plasma, and that its vibration temperature was estimated to be about 5000 K with little dependence on the axial position. Moreover, emission from just above the substrate showed a considerably lower concentration ratio of H atom to C, including radicals, than the case of low-pressure microwave plasma. On the other hand, mass spectroscopy mainly detected CH4 and C2H2 in the species impinging on a substrate. The importance of kinetic processes in a boundary layer was strongly emphasized.

Action of a static magnetic field on an argon discharge produced by a traveling wave

This paper deals with a low-pressure argon microwave discharge (5–100 mTorr), created and sustained by a traveling wave (390 MHz), in the presence of an axial static magnetic field (up to 550 G). First, the propagation characteristics of the wave (wave number, wave collisional damping, radial distribution of the electromagnetic energy) have been computed from the numerical resolution of the dispersion equation. It has been emphasized that the plasma is sustained either by a surface wave, or by a volume wave, according to the electron density and magnetic field values. Second, the electron density, the effective electron-neutral collision frequency ν for momentum transfer, the mean power θ required to maintain an electron in the discharge, and the effective electric field Eeff for the discharge maintaining have been experimentally determined, versus the pressure and the magnetic field. At a fixed pressure, the power required to maintain the discharge decreases as the magnetic field increases. It has been shown that this experimental result is consistent with simple theoretical modeling, describing the decrease of the electron loss by diffusion and recombination on the discharge tube walls.

Treatment of polymer surfaces by low pressure microwave plasmas

Treatment of polymer surfaces by low pressure microwave plasmas