Cusp Magnetic Field Research Articles

Resist Etching with Parallel-Plate RF Plasmas Enhanced by a Cusp Magnetic Field

Ring-shaped permanent magnets are placed on the back of both the cathode and the anode electrodes in a parallel-plate RF plasma device and magnetron plasmas with axial symmetry are produced. Positive photoresist films are etched using the magnetron plasma device. The S pole of the magnets faces the plasmas. Since the same poles face each other, the axial magnetic field is canceled and the cusp field is formed around the axis. An Al plate is placed on the cathode and its thickness is varied. Then, the sheath in front of the cathode is formed around the region where the axial magnetic field is canceled by the two magnets. The etch rate of 1.1 µm/min can be obtained. However, the region in which the etch rate is uniform is not large. The measured etching characteristics are discussed based on the spatial distributions of the magnetic field and the ion saturation current. Line-and-space of 0.25 µm patterns are etched although somewhat undercut profiles are observed.

Melt‐Motion during the Czochralski Growth of Silicon Crystals with a Cusp Magnetic Field

The relative importance of the three melt‐motion components (thermocapillary convection, the buoyant convection, and centrifugal‐pumping flow) during the Czochralski growth of silicon crystals with a cusp magnetic field is investigated by combining the separate asymptotic solutions. The top boundary layer, which is adjacent to the crystal‐melt interface and to the free surface, is very important because the thermocapillary and centrifugal‐pumping flows are confined to this layer, and because the flow here determines the radial distributions of oxygen and dopants in the crystal, as well as the rate of oxygen evaporation from the free surface. The buoyant convection is not confined to the top layer, extends over the entire melt, and is the only component of the melt motion which affects the melt temperature for O(1) thermal Péclet numbers. The analysis of the thermal problem with convective heat transport is presented. The effects of varying the Hartmann number, the thermal Péclet number, and the angular velocity of the crystal on the balance between the three melt‐motion components in the top layer are investigated.

Essential points for precise etching processes in pulse-time-modulated ultrahigh-frequency plasma

Since ultrahigh-frequency (UHF) plasma has a low electron temperature (less than 2 eV) and uniform density, the effects of electron confinement due to the sheath potential barrier or plasma potential distributions are less prominent than with electron cyclotron resonance plasma or inductive coupled plasma. Consequently, multiple cusp magnetic fields are needed to maintain pulsed UHF plasma even at pulse intervals of a few tens of μs and they can significantly improve the etching rate and etching selectivity. Moreover, the characteristics of pulse-time-modulated plasma strongly depend on the substrate position from the generation region to the downstream region. Near the plasma generation region, pulsed plasma can be used to produce a higher rate and higher selectivity for polycrystalline silicon etching, compared with a continuous discharge. By transporting the plasma from the generation region to the downstream region, however, improvements in the etching characteristics of the pulsed plasma are gradually reduced.

“Hollow cathode” gun optics

The generation of an electron beam by a hollow cathode gun in a cusp magnetic field is discussed. Such a gun is proposed for an electron cooling device without toroids. In a section with a homogeneous magnetic field, this beam experiences a disturbance region near the axis where the electron temperature becomes higher. The main purpose of the article is to define conditions for generating the beam so as to restrict the extent of this region as much as possible. It is shown that a state with a virtual cathode in the vicinity of the zero magnetic field point is the most suitable for this aim. The experimental and essential analytical results are presented.

カスプ磁場を有する平行平板型高周波プラズマの特性

カスプ磁場を有する平行平板型高周波プラズマの特性

Recording Characteristics of Co-Cr Perpendicular Magnetic Recording Media Prepared by ECR Sputtering

A sputtering using an electron-cyclotron-resonance microwave plasma was used to prepare Co-Cr perpendicular magnetic recording hard disks. The Co-Cr layer deposited under cusp magnetic field had compositional separation within Co-Cr grains. The recording characteristics of this Co-Cr disk was measured in contact sliding recording using a metal-in-gap ring head with a gap length of about 0.2 microns. D50 of 162kFRPI and low-density reproduced voltage of 100nVp-p/[turn *micron*(m/s)] was obtained. Comparing with a commercial longitudinal recording hard disk designed for inductive head whose Br•t was 289 G•microns, the ECR sputtered Co-Cr disk in cusp field showed lower medium noise and higher D50.

Etch rate and plasma density radial uniformity measurements in a cusped field helicon plasma etcher

Experiments on the University of Wisconsin helicon etcher were conducted to study the effects on uniformity of applying a cusped magnetic field to the etching chamber. Also a comparison was made between long and short antenna/substrate separation distances. The plasma radial density uniformities, measured using a Langmuir probe over the inner 10 cm diameter of the chamber, gave maximum density variations of ±22% with the cusp off and ±1.28% at the zero field region of the cusp. The maximum SiO2 etch rate in CF4/Ar plasmas occurred at 50% Ar concentration in the long source configuration. Etch rate uniformity of SiO 2 etching in a CF4/Ar/O2 plasma improved from ±8.1% to ±2.6% for a 82.5 mm wafer by shortening the antenna–substrate separation distance from 108 to 47 cm. Azimuthal asymmetry of the plasma produced from the Nagoya type III antenna was observed in the short source configuration in which the SiO2 etch rate uniformity varied from ±1.5% across the vertical axis of a 150-mm-diam wafer to ±12% across the horizontal axis of the wafer.

An 18-cm-diam intense beam current broad beam ion source for surface modifications (abstract)

A large diameter intense beam current broad beam ion source used for surface modifications is described. It consists of a discharge chamber and a three-grid extraction system. The discharge chamber has two anodes (front and rear) and a specially designed cusped magnetic field is used for forming a large field free region in the chamber so that a very uniform and intense ion beam has been extracted from this source. Beam currents of more than 900 mA for hydrogen and 750 mA for argon at 2.5 kV extraction voltage have been obtained. The highest beam current density is over 2.5 mA/cm2, measured at a location 25 cm downstream of the source. The density uniformity changes from +5% to +15% over a 15-cm-diam circle. The feature and the performance of this source will be described in this paper.

A numerical simulation of radiative heat transfer coupled with Czochralski flow in cusp magnetic field

The characteristics of flow and oxygen concentration are numerically studied in Czochralski 8 silicon crystal growing process considering radiative heat transfer. The analysis of net radiative heat flux on all relevant surfaces shows growing crystal affects the heater power. Furthermore, the variation of the radiative heat flux along the crystal surface in the growing direction is confirmed and should be a cause of thermal stress and defect of the crystal. The calculated distributions of temperature and, heat flux along the wall boundaries including melt/crystal interface, free surface and crucible wall indicate that the frequently used assumption of the thermal boundary conditions of insulated crucible bottom and constant temperature at crucible side wall is not suitable to meet the real physical boundary conditions. It is necessary, therefore, to calculate radiative heat transfer simultaneously with the melt flow in order to simulate the real CZ crystal growth. If only natural convection is considered, the oxygen concentration on the melt/crystal interface decreases and becomes uniform by the application of a cusp magnetic filed. The heater power needed also increases with increasing the magnetic field. For the case of counter rotation of the crystal and crucible, the magnetic field suppresses azimutal flow produced by the crucible rotation, which results in the higher oxygen concentration near the interface.

変形カスプコイルの磁場強度分布

The cusp magnetic field for the confinement of plasma does not used widely due to the large plasma-loss from the line cusp. To improve such drawback, we design new coil which shows the nil magnetic field at the center. Although the symmetry of the magnetic field is not completely satisfied.

Improved design of cusp-type electron-cyclotron resonance ion sources

An ion source of electron-cyclotron resonance using cusp magnetic field is designed. The field is generated with two cylindrical magnets polarized parallel to the axis of symmetry. Lines of magnetic induction are in this configuration longer than in conventional cusp machines. The plasma electrons escaping from a line cusp are repelled by applying negative potential to an electrode repeller. The repeller of low work function emits additional electrons. A whistler wave is launched through a helical antenna at a point cusp to produce overdense plasmas. An intense ion beam is formed by extracting ions at the other point cusp where the magnetic flux is focused. The repeller materials are sputtered at the line cusp and ionized in the plasma. The atomic flux from the cylindrical surface is focused at the center of plasma to give abundant ions from the solid materials.

Plasma discharge current–beam current relation for different gases in large-volume magnetic multipole ion sources

Production of a certain extracted particle beam current requires for different gases different discharge currents in the ion source plasma generated by an arc discharge. Experimental data of hydrogen, deuterium, helium-3, and helium-4 particle beams extracted from large-volume ion sources with permanent magnet multipole confinement show that higher arc currents are required for the particles of higher atomic mass. These differences can be understood by considering the particle loss mechanisms in the ion source plasma. The differences in plasma leakage can be described by the particle loss area in the magnetic multipole cusp field. Such loss area is higher for the particles with higher mass. The size of loss channels is found to be proportional to the ion Larmor radius.

A general critique of inertial-electrostatic confinement fusion systems

The suitability of various implementations of inertial-electrostatic confinement (IEC) systems for use as D–T, D–D, D–3He, 3He–3He, p–11B, and p–6Li reactors has been examined, and several fundamental flaws in the concept have been discovered. Bremsstrahlung losses for all of these fuels have been calculated in a general fashion which applies not only to IEC systems but also to most other fusion schemes; these calculations indicate that bremsstrahlung losses will be prohibitively large for 3He–3He, p–11B, and p–6Li reactors and will be a considerable fraction of the fusion power for D–3He and D–D reactors. Further calculations show that it does not appear possible for the dense central region of a reactor-grade IEC device to maintain significantly non-Maxwellian ion distributions or to keep two different ion species at significantly different temperatures, in contradiction with earlier claims made about such systems. Since the ions form a Maxwellian distribution with a mean energy not very much smaller than the electrostatic well depth, ions in the energetic tail of the distribution will be lost at rates greatly in excess of the fusion rate. Even by using one of the best electron confinement systems proposed for such devices, a polyhedral cusp magnetic field, and by making exceedingly optimistic assumptions about the performance of that confinement system, the electron losses from the machine prove to be intolerable for all fuels except perhaps DT. In order for IEC systems to be used as fusion reactors, it will be necessary to find methods to circumvent these problems.

Numerical analysis of the trajectories of a single charged particle in a nonadiabatic cusp magnetic field configuration

The trajectories of a nonrelativistic single charged particle in a nonadiabatic cusped magnetic field configuration are numerically analyzed. Depth of penetration of the particle into the cusped field, multiple reflections and radial excursion of the particle transmitted by the cusped field are discussed. A significant observation was that a meticulous choice of particle and field parameters could lead to several number of reflections. For radial excursion a plateau region over Φ was exhibited.

The precision implant 9500 plasma flood system — the advanced solution to wafer charging

An advanced wafer charge neutralization system based on plasma flood technique has been developed to achieve an ideal voltage control on the wafer surface during implantation. The system makes use of the “self-regulating” characteristics of low energy plasma electrons that clamp the surface potential within a few volts. Low energy electrons are generated in an arc discharge plasma chamber combined with an accel/decel extraction mechanism and are transported to wafers through a guide tube that confines a dense plasma by using a combination of electrostatic suppression and a magnetic cusp field. The system has been equipped as standard on the Precision Implant 9500 and shown full yield on various charge sensitive devices.

Development of an intense negative hydrogen ion source with an external magnetic filter

An intense negative hydrogen ion source has been developed, which has a strong external magnetic filter field in the wide area of 35 cm×62 cm produced by a pair of permanent magnet rows located at 35.4 cm separation. The filter strength is 70 G in the center and the line-integrated filter strength is 850 G cm, which keeps the low electron temperature in the extraction region. Strong cusp magnetic field, 1.8 kG on the chamber surface, is generated for improvement of the plasma confinement. These resulted in the high arc efficiency at the low operational gas pressure. 16.2 A of H− ion current with the energy of 47 keV was obtained at the arc efficiency of 0.1 A/kW at the gas pressure of 3.8 mTorr in the cesium-mode operation. The magnetic field in the extraction gap is also strong, 450 G, for the electron suppression. The ratio of the extraction current to the negative ion current was less than 2.2 at the gas pressure of 3 mTorr. The two-stage acceleration was tested, and 13.6 A of H− ion beam was accelerated to 125 keV.

Plasma Generation and Beam Extraction on Reentrant-Cavity-Type Electron Cyclotron Resonance Ion Source

An electron cyclotron resonance (ECR) ion source 8 cm in diameter using a reentrant coaxial cavity and a ring cusp magnetic field was developed to improve plasma uniformity and operational reliability. The reentrant coaxial cavity is utilized to achieve electrical breakdown easily, to tune in variable electromagnetic fields and to introduce microwaves into plasma from an annular window of a discharge chamber sidewall. Microwaves are resonantly absorbed by the ECR magnetic field. Above 10 sccm flow rate for Ar, uniform and overdense plasmas were obtained in an area 4 cm in diameter from the center axis. The maximum plasma density of 1.26×1011 cm-3 was achieved on the center axis at 20 sccm. However, plasma densities for N2 and O2 were lower than the cutoff density. The maximum ion beam current density of 1.26 mA/cm2 was obtained at a forward power of 304 W for Ar.

Plasma sputter multicusp negative/positive ion source with ECR discharge

A plasma sputter type of negative and positive ion source with electron cyclotron resonance (ECR) discharge has been developed at KEK. The ECR discharge was produced by a 2.45 GHz microwave source. In this ion source, negative heavy ions are produced at the surface of the metal which is placed in a Xe gas plasma confined in a cusp magnetic field. Positive ions are generated by ionizing of sputtered neutral atoms in the ECR discharge. The microwave is introduced into the source through a sputter target using a waveguide type of microwave transformer.

Measurement of some parameter on MEVVA ion source

The charge density and the charge state distribution of ions extracted from metal vapor vacuum arc ion source have been measured by single, double probe, and time of flight systems, and the ion beam density distributions near the extracting electrode have been measured with and without a cusped magnetic field in order to improve the performance of ion beams. The charge state distributions as a function for many operational cathode materials were investigated.

High current H− ion production in cesium seeded large ion sources

The characteristics of a large negative-ion source with external filter is investigated for the neutral-beam injection system in a large helical device. The magnetic cusp field is set up at almost the same strength as that in our well optimized 1/3 ion source with rod-type magnetic filter. An H− ion current of 5.2 A is extracted from the ion source with Cs seeding. The H− current corresponds to 19.7 mA/cm2, and the value is comparable to the current density obtained using the 1/3 scaled ion source with a rod-type magnetic filter at the same extraction voltage. In low operating pressure, the ion source with the external filter has a better performance on the extraction of H− ions.