Residual Gas Ionization Research Articles

Low temperature epitaxial growth by molecular beam epitaxy on hydrogen-plasma-cleaned silicon wafers

In a single step, the surfaces of silicon wafers were cleaned in an argon-hydrogen discharge plasma using the Balzers ultrahigh vacuum plasma source. The residual gas ions were monitored during the cleaning procedure to elucidate the process chemistry. Homoepitaxial growth by molecular beam epitaxy (MBE) at low temperatures could be achieved after the cleaning step, indicating a damage-free plasma procedure. Secondary ion mass spectrometry depth profiles of the substrate-epilayer interface show interface contamination levels of approximately 10 −2 monolayers for oxygen and less than 10 −3 monolayers for carbon. In additional experiments, patterned wafers were cleaned in the plasma. Local epitaxial growth by MBE at a temperature of 550°C avoiding high temperature steps could be achieved on these wafers. Furthermore, the influence of the plasma treatment on the wafer surface was investigated in detail for electron vs. bombardment and different bombardment energies using high-resolution X-ray rocking curve diffraction.

Dry Hydrogen Plasma Cleaning for Local Epitaxial Growth

An argon/hydrogen discharge plasma was created with a UHV compatible plasma source. This low energy plasma was utilized to remove – in a single step – the native oxide and the hydrocarbons from the wafer surface at substrate temperatures between 100°C and 400°C. During the plasma cleaning procedure, residual gas ions were monitored to elucidate the process chemistry. To optimize the procedure, the interfaces between the plasma cleaned wafer and the MBE grown epilayer were investigated by scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (XTEM). The cleaning process was also applied to patterned silicon substrates. Subsequent local epitaxial growth by MBE at 550°C without the typical high temperature annealing step was achieved, suggesting that the in–situ dry cleaning procedure caused no surface damage.

A noninterceptive heavy ion beam profile monitor based on residual gas ionization

At GANIL we have developed a beam profile monitor based on ionization of the residual gas of the beam transport lines under the impact of high energy heavy ion beams. The charged ions of the residual gas drift by means of an electrostatic field onto a microchannel plate (MCP), which yields signals collected on a multistrip anode, providing the spatial distribution of the beam. Calculations and results are presented as well as discussions about the different parameters of the device.

Influence of collector ions on operation of magnetically insulated diode

We studied the formation of the collector ion flow in magnetically insulated diodes (MID) with plate-, sphere-, and cone-shaped cathodes. The ions are formed as a result of the ionization of residual gas by electron bombardment and are focused into the cathode plasma region. This effect can negatively influence diode operation in the microsecond range of pulse duration.

A beam profile monitor using the ionization of residual gas in the beam pipe

A beam profile monitor for high energy beams, which has no intercepting parts in the beam pipe, is described. It makes use of the ionization of the residual gas, which is still present in the vacuum chamber of the beam guiding system. The detection of the ionization products is performed with microchannel plates.

On the dose of bombarding residual gas ions for influencing pre-breakdown field emission in a vacuum

Field electron emission from extended Cu electrodes after cleaning by electric arcs has been compared with field emission from gas-covered monocrystalline W points when subjected to ion bombardment from the residual gas. With both emission systems, exposure to argon or to hydrogen in the pressure range 10-5-10-2 Pa caused two subsequent stages: an emission growth explained as the removal of adsorbed gases (stage 1), and a noisy increase of the emission explained as sharpening due to surface migration of metal atoms (stage 2). With Cu there occurred a third stage: a decrease of the emission explained as reduction of the emitters by sputtering. The time constant tau 1 of the first stage was found to be proportional to the product p-1i0-1SFN12/, p being the gas pressure, i0 the initial electron-current, and SFN the apparent emitting surface determined from Fowler-Nordheim plots. The proportionality factor is given for the metals and gases used. The completely analogous behaviour of W points and Cu electrodes in stages 1 and 2 indicates an analogy between the emitting sites in both cases.

Energy contamination in ion implantation

Extending the effective energy range of an implanter is possible by implanting multiple-charged species. A problem when using these species is energy contamination. The energy contamination using P 3+, B 2+ and B + has been investigated on commercially available high-current implanters. The major cause of energy contamination is charge exchange reactions due to (the residual) gas inside an implanter. Careful control and design of the vacuum system are therefore important. Energy contamination can also be caused by sputtering/reflection from slits and lining of the implanter. Ionisation of residual gas is probably the origin of fluorine contamination when using BF 3 as source feed material. The fluorine contamination again stresses the need to control the amount and composition of the residual gas in an implanter.

Enhanced performance of the LBL/SNLL leaky EBIS: Evidence for cooling of trapped heavy ions

The LBL/SNLL EBIS has been operated as a leaky electron beam ion source (EBIS), i.e., with continuous gas leaks and continuous extraction of the ions. Under normal conditions the charge-state distributions are determined by the extraction from the trap; recombination with the residual gas is negligible. A simple rate equation model of the charge-state distributions shows clear evidence that heavy ions (Xe) reach higher charge states than expected, while light residual gas ions (C,N,O) reach lower charge states. This is taken as evidence that ion-ion collisional cooling and heating of these ions results in enhanced trapping of the heavy ions, due to the presence of the residual gas.

Fast calibrator for superfine helium leak standards and ion current sensors

A versatile calibrator has been developed for fast calibration of helium leak standards ranging from 1×10−6 to 10−12 atm cm3/s at 20 °C (5×10−11–5×10−17 mol/s). It can also be used for calibrating residual gas analyzers (RGA’s) and ionization gauges. The calibrator uses pressure, volume, temperature, time, and mass measurements that are all referenced to the National Institute of Standards and Technology. The overall accuracy of the system is calculated to be in the range of ±0.5%. Time required for calibrating a superfine helium leak standard (e.g., 1×10−12 atmospheric cm3/s at 20 °C) will be ∼1 h. The basic principle, which is being patented, is to create a dose of precisely measured, minute mass of helium (or other inert tracer gas like argon), and release it to a small vacuum chamber equipped with a sensor (e.g., RGA). The resulting current reading is correlated to the moles of the gas. A test leak is then calibrated by measuring the helium ion current rate of rise. The sensitivity of the system is measured by the dose. With this information the leak rate can be calculated. The method of creating and measuring the dosage of helium and subsequent steps of calibration, as well as the accuracy and precision of measurement, is discussed.

Afterpulsing in 130 mm diameter photomultiplier tubes

Measurements have been made of afterpulses induced by light from a blue LED in EMI 9823B, Hamamatsu R1250, Mullard XP2041 and RCA 8854 fast linear-focussed photomultiplier tubes. The RCA and Hamamatsu tubes were found to produce significantly fewer afterpulses than the EMI and Mullard tubes. More detailed measurements of the Hamamatsu R1250 indicated that the total number of afterpulses is proportional to the magnitude of the initial flash and consists of two components. The first component is independent of the wavelength of the initial light flash, arises from ionization of residual gas in the phototube and can be effectively suppressed by gating the photomultiplier. The second component is wavelength-dependent, dominates the first component at short wavelengths, and cannot be significantly reduced by gating. It is possibly due to fluorescence of the phototube glass envelope or excitation of a metastable state of the photocathode. Similar components were found to contribute to afterpulsing in the RCA 8854 PMT.

Performance of a new ion optics for quasisimultaneous secondary ion, secondary neutral, and residual gas mass spectrometry

We have constructed an electron impact postionizing optics for sputtered neutrals which can be used on quadrupole mass filters for combined secondary neutral mass spectrometry (SNMS), positive and negative secondary ion mass spectrometry (SIMS), and residual gas analysis (RGA). In the SNMS mode, residual gas and secondary ion suppression exceeds 106, and the signal-to-background ratio is better than 104. Detection limits better than 30 ppm are shown using primary ion beam (3 keV Ar+) doses of 80 μAs per amu scan. The sensitivity in the SNMS mode exceeds that of SIMS only if the secondary ion fraction of the sputtered particle flux is in the order of 10−5, which is excessively low. As a first application, we have quantitatively determined secondary neutral and ion fluxes as well as oxygen concentrations on oxygen loaded Si(100) surfaces by purely mass spectroscopic experiments.

Study of the space charge potential of ion beams

The energy distribution of slow residual gas ions created by an ion beam in collisions with the residual gas molecules inside an ion beam transport system has been measured with an electrostatic spectrometer. For this investigations Ar + ion beams (dc) were used, with energies between 15 and 45 keV. The beam current has been varied from 0.01 up to 1.6 mA. Further parameters were the residual gas pressure, the gas composition, the beam diameter and the potentials of several clearing electrodes in the beam transport system. The experimental results are compared with the calculated values of the space charge potentials of the ion beam. In operating the spectrometer in a gated mode the time could be measured which is necessary to build up the space charge compensation in a pulsed ion beam. Additional residual gas ions produced at different radial distances from the ion beam axis by an electron beam allowed to determine the radial potential distribution.

Measurements of space charge compensation of ion beams

Slow ‘residual gas ions’ (RGI) are repelled from the space charge of an ion beam which is passing through the residual gas in a beam transport system. To measure the energy distribution of these ions an electrostatic analyzer combined with a position sensitive channel plate detector has been used. Measurements have been performed with beams of Ar + ions in the energy range between 15 and 45 keV with beam currents ranging from 0.01 up to 1.6 mA under different experimental conditions (residual gas pressure, beam diameter, potentials of electrodes along the beamline, pulsed beams). The mean energy of the residual gas ions varies between 0 and 70 eV.

A Noninteractive Beam Position and Size Monitor for Heavy Ions

The Ion Beam Fusion development program at Argonne National Laboratory requires noninteractive size measurements of a pulsed, 30 mA, Xe+l particle beam. Pulses of 100 μs duration will be produced by the 1.5 MV preaccelerator; therefore, fast response diagnostics are required. Techniques of utilizing residual gas ionization to profile particle beams have been reported before. This paper discusses the development of vertical and horizontal beam profile monitors that are synchronously clocked to interface with oscilloscopes and computers. Modern integrated circuitry is utilized which boosts performance to a point where pulses as short as 20 μs can be analyzed. A small, simple ionization chamber is shown which provides sixteen channels of position resolution over 12 cm of aperture.

Trajectory analysis of ions formed in the field emitter inter-electrode region

Ion trajectories are three-dimensionally analyzed to determine the main source of ions striking the emitting area of a field emitter. Computer calculations of the ion trajectories are made for a field emitter in the shape of a sphere radius 1000 Å, mounted on an orthogonal cone, and anode radii of 3 cm and 1 cm centered at the emitter. Approximately 10 −10 to 10 −9fraction of the ions formed at the anode by electron impact desorption reach the emitter tip. Only those residual gas ions generated in the space of a few μm around the tip are able to impinge on the emitting area. Under typical operating conditions of 10 −9 Torr, the residual gas ions striking the tip are 10 to 10 2 times greater than those from the anode, assuming an ion ejection of one ion per 10 5 electrons, and an anode potential of about 3 kV.

Abstract: Surface research for controlled thermonuclear fusion

The ERDA surface science research program for controlled thermonuclear fusion is closely coordinated with the general schedule for fusion power development, including the Tokamak Fusion Test Reactor (TFTR) in 1980, an Experimental Power Reactor (EPR) in 1985, and a Demonstration Plant (DEMO) in 1998. Consistent with this schedule, the program is based on the following objectives:(1) To treat surface effects from the standpoints of plasma contamination, wall erosion, and device efficiency.(2) To contribute to the solution of problems in confinement experiments over the near term while developing the data base for dealing with possibly more severe problems in reactors.(3) To view surfaces as part of a completely integrated system consisting of the plasma and its perimeter, the external blanket, subsystems for vacuum pumping, fueling and ash removal, and possibly bumpers or divertors.(4) To screen a number of materials to identify those that merit long‐range development.(5) To provide for correlation of experimental results by including a wide variety of experimental techniques in the program.(6) To couple experimental observations with strong theoretical support seeking fundamental understanding of surface processes as they relate to CTR, rather than an exhaustive cataloging of data.With these general guidelines, surface work is carried out on the following individual processes:(1) Desorption by (a) photons, (b) electrons, (c) ions and neutrals, and (d) thermally.(2) Sputtering by (a) hydrogen ions, (b) residual gas ions (O+, N+, etc.), (c) helium, (d) metallic ions (self‐sputtering), and (e) neutrons.(3) Hydrogen–wall interaction by (a) absorption, (b) diffusion, (c) surface migration, and (d) reflection.(4) Blistering, flaking, and surface erosion by (a) incident hydrogen and (b) helium from the DT reaction, (n, α) reactions, and tritium decay.(5) Chemical reactions involving (a) hydrogen and carbon, (b) residual contaminants, (c) carbon and metals (NbC), and (d) decomposition, principally of insulators.(6) Electron emission. Studies of two of these processes occurring at one time are now beginning and will be extended to studies investigating the synergistic effects of three or more processes. These studies are designed to simulate the effects of a burning DT plasma since the latter will not be available for several years.

1198. Ionization of residual gas and accumulation of molecular ions in electron rings: A A Drozdovskiy,ITEF-100, Institute of Teor and Experim phys Moscow 1973, 20 ( in Russian)

1198. Ionization of residual gas and accumulation of molecular ions in electron rings: A A Drozdovskiy,ITEF-100, Institute of Teor and Experim phys Moscow 1973, 20 ( in Russian)

Dynamic measurements of transverse dimensions of a beam in proton synchrotron by ionization of residual gas: A A Kuzmin et al, Rep of 3rd All-Union Conf on Accel of Charged Part, Vol 2, Nauka Moscow 1973, 144–147 (in Russian)

Dynamic measurements of transverse dimensions of a beam in proton synchrotron by ionization of residual gas: A A Kuzmin et al, Rep of 3rd All-Union Conf on Accel of Charged Part, Vol 2, Nauka Moscow 1973, 144–147 (in Russian)

Production and Measurement of Dense Metal Ions for Physical Vapor Deposition by a Hollow Cathode Discharge

High rate production of metal ions by using an optimum voltage, high current plasma electron beam with metal vapor evaporated from a molten pool is investigated. A focussed electron beam of several ten volts and upto 130 amps strikes an evaporation source at the glancing angle of 45°. Ag is evaporated at a high rate between 0.2 to 2 mTorr of argon and deposited on a substrate. The metal ion density on the substrate is observed separately from residual gas ions and secondary electron emissions from the substrate due to bombardment by photons, ions and metastables. The ratio of the number of the metal ions to the metal atoms, both are striking on the substrate, is measured as a function of the electron beam current, the argon pressure and the applied negative voltage to the substrate. Fraction of the metal ions in the metal species which strikes on the substrate varies from 22 to 40% depending on experimental parameters. Comparison of microscopic structures of the two Ag films deposited by using this plasma evaporation source and by a resistance heating source in a clean high vacuum respectively are presented as a function of substrate temperature during the deposition.

Plasma losses in the ring gap of an electromagnetic trap

The effect of diocotron instability on plasma losses in the ring slit of an electromagnetic trap was examined. It was found that the instability growth disturbs the initial symmetry of the electron layer in the ring slit. Secondary plasma generated by ionization of residual gas in the trap increases the instability growth rate. The instability can be suppressed by shorting one of the surface