Microwave Ion Source Research Articles

Microwave ion source for low charge state ion production

The Plasma and Ion Source Technology Group at LBNL have developed a microwave ion source. The source consists of a stainless-steel plasma chamber, a permanent-magnet dipole structure and a coaxial microwave feed. Measurements were carried out to characterize the plasma and the ion beam produced in the ion source. These measurements included current density, charge state distribution, gas efficiency and accelerated beam emittance measurements. Using a computer controlled data acquisition system a new method of determining the saturation ion current was developed. Current density of 3–6 mA/cm 2 was measured with the source operating in the over dense mode. The highest measured charge-states were Ar 5+, O 3+ and Xe 7+. Gas efficiency was measured using a calibrated argon leak. Depending on the source pressure and discharge power, more than 20% total gas efficiency was achieved. The emittance of the ion beam was measured by using a pepper-pot device. Certain spread was noticed in the beam emittance in the perpendicular direction to the source dipole field. For the parallel direction to the magnetic field, the normalized rr′ emittance of 0.032 π-mm-mrad at 13 kV of acceleration voltage and beam exit aperture of 3-mm-in-diameter was measured. This compares relatively well with the simulated value of 4 rms, normalized emittance value of 0.024 π-mm-mrad.

On the stability of electronic perturbations on a graphite surface produced by low-energy ion bombardment

The stability of the electronic perturbations (EPs) on a graphite surface produced by low-energy argon ion bombardment was investigated. The ion bombardment was performed using an ion irradiation system consisting of a compact microwave ion source directed to the observation site for scanning tunneling microscopy. We measured the change in the number of the EPs upon repetitive ion bombardment and found that the numbers saturated with the ion dose. Similar experiments were made for a carbon ion-bombarded sample, but the number increased in proportion to the ion dose. Long-time observations of the surface after ion bombardment revealed that the number of EPs gradually decreased for the case of argon ion-bombarded sample. The experimental results suggest that desorption of argon may contribute to the decrease of the EP.

低エネルギー小型マイクロ波イオン源を用いたイオンビームアシスト蒸着装置の開発

New ion beam assisted deposition (IBAD) system equipped with a Low voltage extraction microwave ion source (MWIS) was developed for the purpose of orientation control of thin films. This system has three ports for MWIS to provide ion beam with different incident angles of 45°, 60°, and 75°. Fundamental properties of the ion source was examined, and it was found that sufficient ion current for deposition was obtained at the substrate holder. Tantalum carbide thin films were deposited and relationship between deposition parameter and property of the thin films was measured.

Titanium oxy-nitride films deposited on stainless steel by an ion beam assisted deposition technique

Surfaces of stainless steel SUS304 were coated with titanium oxy-nitride (TiON) films at temperatures of 400–770°C using an ion-beam assisted deposition technique constructed from an electron beam evaporator for Ti evaporation and a microwave ion source for ionizing nitrogen gas. The N ions were accelerated at energies of 0.5–2.0 keV. Most of the deposited TiON films consisted of (60–80)% TiN and (40–20)% TiO2, and the fraction of TiO2 increased with increasing substrate temperature. Hardness of the TiNO films varied in the range from 160 GPa to 260 GPa with increasing substrate temperature. The titanium oxy-nitride film could be deposited on stainless steel without a significant deterioration surface layer at 600°C. However, when TiNO films were deposited at temperatures higher than 700°C, the thickness of the TiNO films were significantly thinner and a thick layer containing nitride such as Cr2N, CrFe, Fe2N and Fe4N was formed in a near surface region of stainless steel because more nitrogen diffused into stainless steel.

Preferential orientation of high permittivity TiO 2 deposited on Si wafers by an IBAD technique

Titanium oxide films were deposited on (1 0 0) silicon wafers using an ion beam assisted deposition technique consisting of an electron beam evaporator for Ti evaporation and a microwave ion source for ionizing oxygen gas. The O ions were accelerated to energies of 0.5–2.0 keV. The intensity of the O ion beam was 0.01 mA/cm 2 on the substrate. The deposited TiO 2 films were rutile O-deficient TiO 2 crystals preferentially oriented in the 〈1 1 0〉 direction with very high permittivity values approximately (100–160) ε 0. The permittivity increased with increasing 〈1 1 0〉 orientation texture.

Multilayer TiN/Ti films for high-temperature annealing of GaAs

Gallium arsenide wafers were encapsulated with multilayer films consisting of TiN and Ti layers using an ion beam-assisted deposition technique employing an electron beam evaporator for Ti evaporation and a microwave ion source for ionizing nitrogen gas. The encapsulated GaAs surfaces were heated to 1000 °C and held at that temperature for 15 min. The GaAs surfaces had a desirable surface morphology when covered with a TiN/Ti-rich TiN/TiN multilayer film. A small fraction of the S atom population implanted in the GaAs diffused into the encapsulant during annealing at 1000 °C, while S atoms remained in the GaAs was electrically active. The redistribution profile of S atoms for S-doped GaAs annealed with the desirable composite film of TiN/Ti-rich TiN/TiN is well approximated by a Gaussian distribution function with a concentration-independent diffusion constant.

Test of negative ion beams from a microwave ion source with a charge exchange canal for accelerator mass spectrometry applications

A test facility has been constructed to evaluate negative ion beams from small gaseous samples for accelerator mass spectrometry applications. The positive ion beams from the microwave ion source are passed into a charge-exchange canal (CXC) where the ions exchange electrons with magnesium vapor and become negatively charged. Positive molecular ions were either neutralized or broken up into atomic ions and neutral atoms and molecules by collision processes. Most of the resulting particles were suspected to be neutrals. In studies with injections of CO2 pulses, the resulting positive and negative C12 current peaks gave a 0.09% yield of C− ions from CO2 molecules, which includes a charge-exchange efficiency of 10%. Since nitrogen does not form a stable negative ion, N14 background is virtually eliminated after the beam goes through the CXC, which is necessary for radiocarbon measurements.

Power absorption and intense collimated beam production in the pulsed high-power microwave ion source at RIKEN

Microwave power absorption and intense collimated beam production is studied in the pulsed high-power microwave ion source at RIKEN by varying the radius of the plasma chamber (circular waveguide) in integer multiples of a quarter wavelength of the wave (nλ/4), where n is an integer (n=2–5) and λ is the vacuum wavelength of microwaves. Optimum wave coupling and power absorption due to a change in the microwave power density and the electric field distribution are investigated from measurements of the total achievable current density and the optical intensity, emitted by the pulsed argon plasma in the visible region at a wavelength of 404.3 nm. Results indicate a nonlinear absorption of microwave power. Depending upon the pressure, a decrease in the chamber radius leads to an enhancement of the electron temperature. Favorable beam conditions, i.e., highest density and temperature are obtained for the chamber with radius ≃λ.

Energy distribution of the compact microwave ion source for extremely low voltage ion extraction

The energy distribution of the ion beam extracted from the compact microwave ion source for extremely low voltage ion extraction was measured with Ar and CO as a discharge gas. The energy distribution was measured by the retarding field method, and changes with respect to the change in gas pressure were observed. The obtained data were arranged by the peak energy and the energy spread. For both gases, the peak energy and the energy spread decreased with an increase in the gas pressure. The energy spread of approximately 5 eV with the peak energy of 15 eV were obtained for Ar gas at the pressure of 10−2 Pa. For CO gas, the peak energy was higher than Ar and approximately 20 eV. The energy spread was 6 eV at the pressure of 10−2 Pa. These values agreed with the peak energy and energy spread that were estimated previously from the mass spectra analysis. Since the ion source was designed to be used in the researches of low energy ion-solid interaction, these characteristics satisfy the requirements for this purpose.

Magnetic-field configuration fitted to the microwave ion-source plasma

Microwave ion sources for implantation have many advantages, such as the long life of source operation and the cleanliness of extracted beam, over the traditional dc-arc-discharge-type ion sources. We reported that optimizing the plasma-chamber structure made the microwave source exceed most of traditional dc-arc-discharge sources in extracted-ion currents of most species that are used for fabricating modern ultralarge scale integrations. After studying the source magnetic field in detail by computer simulation, the relationship between the magnetic-field profile and the plasma-chamber structure is found to be essential. Two-dimensional analysis is carried out since the plasma is generated between parallel plate electrodes. The optimum chamber structure with which the maximum current was obtained in the last beam-extraction experiments is shown to contain the major magnetic field that is focused by the iron pole pieces. Most of the magnetic-field lines that are crossing the region where ions are first generated do not disappear into the sidewalls of the chamber. Since the lifetimes of ions become longer, the plasma density increases.

Status of the EXCYT facility at INFN-LNS

The EXCYT facility at the INFN-LNS aims to the production of radioactive ion beams to be post-accelerated by a 15 MV Tandem. The primary stable heavy-ion beam (up to 80 MeV/amu, 1 pμA) is supplied by a K-800 Superconducting Cyclotron, which has been operating in a stand-alone mode, by means of the new axial injection beam line, since December, 1999. The magnets of the primary beam line have been aligned and most of the components of the mass separator have been purchased. Different types of Heavy Ion Target–Source (HITS) systems have been built and are here described; in particular, a microwave ion source fully designed and manufactured at LNS has been assembled and successfully tested. Finally, low-intensity beam diagnostics is also ready and reliability tests are under way.

Tests of positive ion beams from a microwave ion source for AMS

A test facility has been constructed to evaluate high-current positive ion beams from small gaseous samples for AMS applications. The major components include a compact permanent magnet microwave ion source built at the AECL Chalk River Laboratory and now on loan from the University of Toronto, and a double-focusing spectrometer magnet on loan from Argonne National Laboratory. Samples are introduced by means of a silica capillary injection system. Loop injection into a carrier gas provides a stable feed for the microwave driven plasma. The magnetic analysis system is utilized to isolate carbon ions derived from CO 2 samples from other products of the plasma discharge, including argon ions of the carrier gas. With a smaller discharge chamber, we hope to exceed a conversion efficiency of 14% for carbon ions produced per atom, which we reported at AMS-7. The next step will be to construct an efficient charge-exchange cell, to produce negative ions for injection into the WHOI recombinator injector.

Unstable plasma characteristics in mirror field electron cyclotron resonance microwave ion source

Electron cyclotron plasma reactor are prone to instabilities in specific input power [3–7] region (150–450 watts). In this region power absorption by gas molecules in the cavity is very poor and enhanced input power gets reflected substantially without increasing ion density. There are abrupt changes in plasma characteristics when input power was decreased from maximum to minimum, it was observed that reflected power changed from <2% to ∼50%. Minimum two jumps in reflected power were noticed in this specific power region and these appear to be highly sensitive to three stub tuner position in the waveguide for this particular input power zone. Unstable plasma region of this source is found to be dependent upon the magnetic field strength. Some changes in reflected power are also noticed with pressure, flow and bias and they are random in nature.

Microwave ion sources for industrial applications (invited)

Microwave ion sources for industrial use are usually driven by 2.45 GHz microwaves and operated in a very wide range of magnetic field from zero to over the electron cyclotron resonance magnetic field. They are used mostly under off-resonant conditions. For ion implantation into usual semiconductor devices, the weak points of the microwave ion source against the conventional implanter sources, the Freeman and the Bernas, had been the lower B+ ion current and the slightly narrower dynamic range of the current. However, the optimization of the discharge-chamber shape and volume resolved the problems. Consequently, the microwave sources exceed the conventional sources in most principal performances for implantation into semiconductor devices. For the sophisticated separation by implanted oxygen devices, the microwave ion source is very suitable for stable production of high-current O+ ion beams. 100 mA class O+ ion implanters dedicated to silicon on insulator technology were developed. On the other hand, for application to surface modification of materials, mass separation is completely eliminated in some cases. Recently, a new ion source for the purpose was developed, in which 2.45 GHz microwaves are absorbed by 13.56 MHz inductively coupled plasma without static magnetic field. The alternate magnetic field induced by 13.56 MHz rf power is considered to help microwaves penetrate into the plasma. Since the volume of the source is not restricted by solenoids as a usual microwave source, it can be applied to three dimensional implantation or plasma source ion implantation.

Negative ion beam production by a microwave ion source equipped with a magnetically separated double plasma cell system

Filamentless negative ion beam production was investigated with a compact microwave ion source (2.45 GHz). One of the key points for negative ion production is the magnetic configuration. A magnetic filter field to lower electron temperature was generated in a negative ion production cell, which was shielded magnetically from a discharge cell with a magnetic field to couple microwave to plasma. Production of H− beam was studied with this source. H− was extracted through a grid slit (2×16 mm2) from plasma and accelerated to 20–40 keV. H− beam current was measured with a Faraday cup after magnetic mass separation. Continuous H− beam current of 73 μA (0.23 mA/cm2) was obtained with a magnetron power of 700 W. H− beam current was increased around 1.4 times by adding Xe gas to the H2 gas. Other negative ion species, which have a potential for applications to industrial ion beam processing with little charge-up problem, were also investigated. Carbon and hydrocarbon negative ion beams were produced using boron alkoxide (B(OCH3)3) and methane. C2H2− beams (22 μA) were obtained with the alkoxide. C2−(1.6 μA), C2H−(2.3 μA), C2H2−(0.6 μA), and H−(6.9 μA) beams were produced with methane. SiF4 and BF3 were used to generate F−, Si−, SiF3− and B− beams. Beam currents of these ion species were 17, 0.25, 1.5, and 0.03 μA, respectively.

Application of compact microwave ion source to low temperature growth of transition metal nitride thin films for vacuum microelectronics devices

A compact microwave ion source was applied to the low temperature growth of transition metal nitride thin films for the cathode of vacuum microelectronics devices. An ion beam assisted deposition system consisted of a compact microwave ion source and an electron beam evaporator was developed. Depositions of zirconium nitride and niobium nitride thin films were performed and the film properties were investigated. As a result, it was found that polycrystalline films of zirconium nitride and niobium nitride were prepared at the substrate temperature as low as 500 °C, which was almost 200 °C lower than the results shown in the literature. The reason for this reduction of substrate temperature might be attributed to low gas pressure during deposition, due to the use of a single aperture ion source. The control of film composition by controlling the ion-atom arrival rate ratio achieved the control of work function. It was concluded that the ion beam assisted deposition with microwave ion source provides a possible process of cathode deposition.

Improvement of microwave ion source for higher B+ ion current

Microwave ion sources have many advantages over dc-discharge ion sources, such as long lifetime and high current for most ion species. But B+ ion current has been the only disadvantage. Here we experimentally study how to get higher B+ ion current by optimizing the shape and volume of the source chamber as well as by increasing the secondary electron emissivity of the chamber wall. We carry out the experiment with regard to the maximum ion beam, lifetime, and dynamic range of the beam current for various ion species. Then, we compare the data with those of the Bernas-type ion source that has been widely used in semiconductor manufacturing factories. As a result of the improvement, the performance of all the above-mentioned aspects of the microwave ion source exceed those of the Bernas-type ion source.

Development of a steady-state microwave ion source with large spherical electrodes for geometrical focusing

A steady-state electron cyclotron resonance ion source with large spherical electrodes has been developed to study the plasma wall interaction, erosion mechanism, and hydrogen retention of first wall materials in a nuclear fusion reactor. Discharge characteristics of the ion source were studied in terms of microwave power, magnetic field configuration, and gas (H2) pressure. Ion saturation current density Jis of about 21 mA/cm2 near the electrodes (target value is 20 mA/cm2) was achieved with microwave power of 3.0 kW and magnetic field of 2.3 kG at an optimal gas pressure. The measurement of Jis was made by a Langmuir probe. It was found that the dependence of ion saturation current density on the magnetic field was different between high (&amp;gt;5 mTorr) and low (&amp;lt;5 mTorr) pressure. The change of the gas inlet position from the center of the plasma production chamber to near the microwave input window gave higher ion saturation current density.

High-current microwave ion source for wide-energy-range O+ ion implantation

A high-current microwave ion source which is used for O+ ion implantation in separation by implanted oxygen (SIMOX) wafer fabrication is presented. The source consists of a new transform waveguide which efficiently propagates a 2.45 GHz microwave power into the ion source, a cylindrical plasma chamber of 90 mm in diameter, and a multiaperture extraction electrode system. The extracted beams are mass separated and then postaccelerated up to 200 keV. Ion source operates stably for a long time and the microwave absorption efficiency is as high as 80%. A total extraction current of 240 mA is obtained at the extraction voltage of 50–60 kV and the mass-separated O+ current reaches about 100 mA at the same extraction voltage. The data show that the ion source has a good potential to provide 100 mA-class O+ ion beams stably in the wide energy range demanded for SIMOX ion implantation.

Holey-plate ion source

A low-pressure and high-density microwave ion source, created and sustained by evanescent waves emitted from a holey plate (HP) has been studied. This source is called a HP ion source. Microwave power at 2.45 GHz is supplied from a rectangular waveguide and then converted into an evanescent mode through the use of a HP placed on a H plane located at one end of the rectangular waveguide. The argon plasma density in the ion source is 1×1011 cm−3 with an electron temperature of 3 eV. A 150 mm×150 mm argon ion beam is extracted from the ion source. The ion current densities are in excess of 2 mA/cm2 at an extraction voltage of 1 kV.