Dc Bias Potential Research Articles

Design and characterization of a radio-frequency-powered glow discharge source for double-focusing mass spectrometers

A radio-frequency- (rf-) powered glow discharge has been interfaced to a double-focusing mass spectrometer. This type of discharge allows direct analysis of nonconducting, as well as conducting, solids. The rf discharge source and electrical system overcome several problems which have inhibited success in prior efforts. Problems of inadequate rf shielding, maintaining the necessary dc bias potential on the sample surface, preventing rf modulation of ion energies, and coupling of the accelerating potential to the discharge are resolved. Representative spectra of glass, soil, and brass matrices are presented. Preliminary relative sensitivity factors for conducting and non-conducting matrices show relatively small differences in ion yields across the periodic table. 31 refs., 9 figs., 2 tabs.

Mass-selected ion angular impact energy distributions at the powered electrode in CF4 reactive-ion etching

The ion energy distributions and ion angular distributions of CF+, CF+2, and CF+3 currents were measured at the powered electrode of a capacitively coupled 13.56 MHz discharge parallel-plate reactor running on CF4. The pressure range is varied between 5 and 50 mTorr, and by adjusting the rf power dissipated in the plasma the dc bias potential at the cathode is varied between −150 and −450 V. Ions are sampled through a small orifice in an aluminum wafer bonded to the cathode. Downstream the orifice ions are selected in a quadrupole mass spectrometer equipped with an energy filter. Tilting this detection system with the vertex lying in the orifice ion angular distributions are measured. The ion energy distributions were found to exhibit structures which appear as continua. The structures result from collision processes of CF+x (x=1–3) with the CF4 parent gas in the sheath. Charge-exchange collisions could be identified neither in the ion energy distributions nor in the ion angular distributions. The ion angular distributions of CF+3 at the powered electrode are characterized by angular widths between 3° and 4° for the entire energy range. Angular distributions of CF+2 have angular widths of about 5° for high-energy ions while low-energy ions show angular widths of about 15°. Similar observations are made for CF+ ions with angular widths of about 6° for high-energy ions and about 16° for low-energy ions. In general, experiments indicate a slight increase in angular width for increasing CF4 pressure. The existence of continua in the energy distributions and the broad angular distributions are interpreted as being caused by the combined effect of elastic scattering and dissociative collisions in the sheath. Additionally, for increasing CF4 pressure multiple scattering becomes important giving rise to further broadening of impact angles and intensifying low-energy continua. Some consequences for the interpretation of profile developments in submicrometer dry etching are discussed.

An electron-beam plasma source and geometry for plasma processing

The operation of an electron-beam plasma source and its application as a secondary source of ionization in a radio frequency- (RF) powered reactive-ion-etching (RIE) reactor is investigated. RF-powered RIE is an important tool in the manufacture of semiconductor devices. One aspect of RF RIE is its ability to produce a DC substrate potential on insulating as well as conducting substrates. Substrate potential is an important determinant of etch anisotropy. However, traditional RF RIE reactors can only be operated over a limited range of parameter space. Ion flux, neutral radical flux and ion energy must all be controlled with only two independent variables: RF power and pressure. Expanded and precise control over process parameters is becoming more important as device structures are made smaller and less damage tolerant. Process control may be enhanced by the addition of an alternative source of power. The experimental arrangement is designed to investigate these possibilities. As a test case, the system is utilized to etch patterned silicon wafers with SF6 gas. Etch rate, etch uniformity and DC bias potential versus RF power and beam current are presented. Electron-gun generated contamination is addressed and several solutions are reported.

Effect of directed (radial) gas flow on sputtering and emission characteristics in radio frequency powered glow discharge devices

An assembly has been constructed for the radial introduction of Ar support gas toward the cathode surface of a radio frequency glow discharge atomic emission device. Studies of directed gas flow suggest two counteracting processes. At lower flow rates (< 300 ml min ), the gas has a sweeping effect to remove sputtered material from the region directly above the cathode surface, presumably inhibiting the redeposition process. At higher flow rates, a localized high-pressure region is established with a consequent lowering of the dc bias potential and resultant decrease in sputtering rates. Use of directed gas flow is found to increase the extent of self-absorption for low-lying atomic transitions, while ionic emission is found to be enhanced. The use of continuous, radial gas introduction allows for retention of desired sputter crater shapes.

Energy distributions of argon neutrals at the rf-powered electrode of a parallel-plate reactor

In a parallel-plate 13.56 MHz rf reactor energetic argon neutrals are created by charge-exchange collisions in the sheath adjacent to the powered electrode. Energetic argon neutrals are detected by sampling neutrals through a 100 μm orifice in the cathode. Downstream the orifice neutrals are analyzed parallel to the cathode normal with a quadrupole mass spectrometer equipped with an energy filter having an angular acceptance of about 2.5°. The pressure range considered was between 5 and 50 mTorr with dc bias potentials between −100 and −400 V. The measured neutral energy distributions for low pressures and low dc bias potentials have a maximum intensity at low energies slowly decreasing toward higher energies. For higher pressures and higher dc bias potentials the energy distributions have minimal intensities at low energies, exhibit a maximum at roughly 40 eV, subsequently decreasing toward higher energies. This behavior is explained as being caused by multiple scattering of energetic neutrals in the sheath. Experimental neutral energy distributions are compared with Monte Carlo simulations. In the calculations charge exchange collisions, elastic scattering, and multiple collisions are taken into account. Experimental and theoretical neutral energy distributions are in fairly good agreement clearly indicating that multiple scattering of neutrals in the sheath is an important mechanism determining their energy distributions.

Growth of epitaxial TiN films on MgO(100) by high current discharge plasma

High current discharge plasma produced by the Ta-LaB6 cathode was used for ion plating of TiN films. The dependence of the deposition rate of TiN on the substrate dc bias potential was measured using the Rutherford backscattering spectrometry (RBS) method. The results indicate that the particles impinging on the substrate, i.e., vaporized particles (Ti) and reactive gases (N2), could be ionized at high efficiency of 70%. Epitaxial TiN films were grown at temperatures between 400 and 700 °C on cleaved MgO(100) substrate supplied with negative dc anf rf potential. Structures of the epitaxial film were investigated by RBS and ion channeling, and scanning tunneling microscope (STM). The minimum channeling yields for (100) and (110) axes obtained at a substrate temperature of 550 °C with the growth rate of 40 Å/s were found to be 1.4% and 2.0%, respectively, which were almost the same as that of the MgO substrate. Atomically resolved tunneling images of (100) surface of the epitaxial TiN film were observed by STM.

Energy resolved angular distribution of argon ions at the substrate plane of a radio frequency plasma reactor

The energy resolved angular distributions of argon ions of a 13.56 MHz rf discharge bombarding the powered electrode in a parallel plate reactor are investigated. Argon ions from the plasma are sampled through a small orifice in the powered cathode and are analyzed using a quadrupole mass spectrometer equipped with an energy filter. The angular resolution of the instrument is slightly better than 1°. The process pressure ranges from 5 to 50 mTorr and the rf power is varied to yield dc bias potentials between −100 and −450 V. The measured energy distributions of argon ions extend from 0 eV up to a maximal energy corresponding to the dc bias potential and exhibit significant structures which are explained by the effect of collisions in the sheath combined with rf modulations of the sheath potential. The principle structures in the ion energy distributions are nearly independent from the angle of ion trajectories with respect to the electrode normal. The energy resolved ion angular distributions are investigated in the range between ±3° and show structures which may be resulting from charge exchange collisions and elastic scattering of ions in the sheath. Angular distributions centered around the surface normal are found with angular widths between 1° and 2°. Furthermore, extrema at angles between 1° and 2° are detected for distinct energies and plasma parameters.

The Effects of Biasing on the Optical and Electrical Properties of RF Sputtered VO2

ABSTRACTVanadium dioxide was deposited on sapphire substrates by RF reactive sputtering. Biasing of substrates during deposition was used to alter both resistivity and infrared transmission of VO2 thin films. It has been found that as the negative dc bias potential is increased, films in the metallic state show an increased transmittance of IR wavelengths between 2.5 and 6 μm. Also, the resistivity ratio of the semiconductor to metal phase as a function of bias potential is found to reach a maximum value after which further increases in bias cause degradation of electrical properties. X-ray diffraction analysis was used to study film composition.

Implantation and replacement of hydrogen isotopes in 304 stainless steel vacuum wall under RF discharge and RF-DC glow discharge

The saturation and replacement of hydrogen isotopes implanted into a 304 stainless steel vacuum wall at 300 K under RF discharge with and without DC bias potential on an RF antenna have been studied using partial pressure measurements of desorbed isotopes. The energy of ions hitting the wall was estimated to be 30–90 eV from electrostatic probe measurement for RF discharge plasmas. Nearly 90% of the desorbed gas in the isotopic exchange process was HD molecules. The amount of desorbed gas showed a tendency to saturate at a fluence of 2.3 × 10 16 cm −2. The desorption rate can be expressed by an inverse-exponential function with time; a two-component function is normally observed. The time constant of each component is evaluated by introducing an effective trapping coefficient.

Low‐frequency sheath admittance of a sphere in a collisionless plasma

A systematic theoretical study of the low‐frequency sheath admittance of a spherical electrode immersed in a collisionless, nonflowing, Maxwellian plasma with no magnetic field is done for the following ranges of plasma parameters: nondimensional electrode dc bias potential −25 ≤ eϕp/kTe ≤ 25, ion to electron temperature ratio 0 ≤ Ti/Te ≤ 1, electrode radius to Debye length ratio 0 ≤ rp/λD ≤ 200. A surprising feature of the results is the existence of double and triple peaks in the sheath capacitance as a function of dc bias potential at certain values of rp/λD. The same capacitance results also show discontinuous behavior at space potential. Various plasma‐diagnostics applications are suggested.

Generation of ultrasound at metal-electrolyte interfaces

When a radio frequency potential difference is applied across a metal-electrolyte interface, ultrasound of the same frequency should be generated at the interface. This effect is expected on the basis of uncompensated volume changes in the electrolyte adjacent to the metal surface. The effect has been detected for the first time at ∼ 500 kHz and preliminary results are reported. The measurements have been made in a specially designed electroacoustic cell with gold electrodes in acid and alkaline electrolytes. The sound generated within the cell propagates into a large tank where it is detected with a ceramic hydrophone, calibrated separately against radiation pressure measurements. Pulse modulation has been used to discriminate against any electromagnetic crosstalk. The sound intensities although small (∼10−9 W/cm2) are easily detected and depend on the dc bias potential applied to the interface and the electrolyte composition. The effect is very sensitive to impurities, especially organics. This effect is a promising tool for the study of metal-electrolyte interfaces. Methods for greatly enhancing the effect will be discussed. [Research sponsored by ONR.]

A system of electrostatic recording

A new system of electrostatic recording in which electrical charges corresponding to the signal are injected into a thin insulating tape record has been investigated. The recorded charge pattern is produced by the application of ac or dc bias potentials to the recording electrodes, in addition to the signal voltage, and subsequently stabilized by treating the tape surface with ions to neutralize excess charge. Electrostatic recordings with extrapolated lifetimes in excess of several hundred years have been obtained.