Multicusp Magnetic Field Research Articles

Profile control of the large area ribbon beam ion source using 500 MHz ultrahigh frequency discharge

This article is concerned with the method of controlling the large area ribbon beam profile produced by the same extraction electrode size ion source. We used a bucket type large area ion source with 500 MHz ultrahigh frequency (UHF) discharge for the plasma production. The size of the ion source is comparable to one wavelength of 500 MHz rf. As the UHF wave propagates in the plasma, electrons are accelerated by the rf fields along the multicusp magnetic field produced by the peripheral permanent magnets set on the outer surface of the plasma chamber. We used several entrances of the UHF wave around the plasma chamber. We can control the ion beam profile by adjusting the size of the entrance hole. With this control, less than 2% of uniformity of the large area ion beam of 120 mm×220 mm is achieved for the source gases such as diborane, phosphine, and arsine.

Change of diborane ion species depending on the magnitude of magnetic field using 100 MHz very high frequency discharge

In order to fabricate next generation semiconductor devices, low energy and high current doping ions are required to be transported. In particular the energy of boron ions is required to be less than 1 keV at a current of several mA. When hydrogen-diluted diborane is used as the ion source gas, usually hydrogen ions are also generated as well as borone hydride ions. But if the generation of the hydrogen ions is suppressed, mass separation may be unnecessary. Then the distance between the ion source and the target wafer can be shortened. As a result it becomes easy to transport low energy, high current ion beams. This work is concerned with a new method of plasma production which can suppress the hydrogen ion ratio in the very high frequency (VHF) discharge. We have improved the magnitude of multicusp magnetic field by changing the permanent magnet arrangement at the periphery of the plasma chamber, because the ratio of hydrogen ions strongly depends on the amount of high energy electrons which are produced through a relation between the VHF field and the multicusp field. Optimizing the magnitude of the magnetic field, the ratio of hydrogen ions can be reduced to less than 1% of the total ions.

Production of microwave plasma in narrow cross sectional tubes: Effect of the shape of cross section

A microwave plasma is produced in a conducting tube with a cross section smaller than the cutoff value. Waves of 2.45 GHz are launched perpendicularly to the multicusp magnetic field formed by permanent magnets surrounding the tube. Circular and square cross sectional tubes are tested. Overdense plasmas with a density of (0.8–2.0)×1011 cm−3 are obtained in the range of 10−4 Torr for powers of 200–360 W. The electron temperature is 6–12 eV. Under the same experimental conditions, the plasma density and the electron temperature are higher for the circular cross section.

Electron cyclotron resonance plasma in multicusp magnets with axial magnetic plugging

An electron cyclotron resonance plasma source is built by arranging SmCo magnets to form a multicusp magnetic field with axial magnetic plugging. The source is tested in H2, N2, and Ar by the microwave of 2.45 GHz in the range from 10−2 to 10−3 Pa. The discharge sustaining power is measured as a function of pressure. The plasma density, the average electron energy, and the electron energy distribution are measured by using a Langmuir probe. Plasma is obtained at pressures as low as 10−3 Pa in Ar. The average electron energy and the ion density increase with the feed microwave power. The electron energy distribution shows depletion at the low-energy part. Ion spectra obtained in Ar show a multicharged state up to Ar8+.

Electron Cyclotron Resonance Plasma in Multicusp Magnets with a Checkered Pattern

Electron cyclotron resonance (ECR) plasmas are generated by microwaves of 2.45 GHz in the pressure range of 0.1 to 0.01 Pa in a multicusp magnetic field by arranging SmCo magnets in a checkered pattern. Observations were made on the discharge property and the sustaining power. The plasma density and the electron energy distribution are measured by a Langmuir probe, and the ion spectra are measured by using a deflecting magnet. The characteristics are compared with those in the case of linear multicusp configuration and a discussion on the effects of the pattern of magnet arrangement is presented.

Design features of a high-intensity, cesium-sputter/plasma-sputter negative ion sourcea)

A versatile, high-intensity, negative ion source has been designed and is now under construction which can be operated in either the cesium-sputter or plasma-sputter mode. The cesium-sputter mode can be effected by installation of a newly designed conical-geometry cesium-surface ionizer; for operation in the plasma-sputter mode, the surface ionizer is removed and either a hot filament or rf antenna plasma-discharge igniter is installed. A multicusp magnetic field is specifically provided confining the plasma in the radial direction when the plasma-sputter mode is selected. This arrangement allows comparison of the two modes of operation. Brief descriptions of the design features, ion optics, and anticipated performances of the two source geometries will be presented in this report.

Design features of a high-intensity, cesium-sputter/plasma-sputter negative ion source (abstract)a)

A versatile, high-intensity, negative ion source has been designed and is now under construction which can be operated in either the cesium-sputter or plasma-sputter mode. The cesium-sputter mode can be effected by installation of a newly designed conical-geometry cesium-surface ionizer; for operation in the plasma-sputter mode, the surface ionizer is removed and either a hot-filament or rf antenna plasma-discharge igniter is installed. A multicusp magnetic field is specifically provided for confining the plasma in the radial direction when the plasma-sputter mode is selected. This arrangement allows comparison of the two modes of operation. Brief descriptions of the design features, ion optics, and anticipated performances of the two source geometries will be presented in this report.

Design studies for an advanced ECR ion source (abstract)a)

Magnet design codes, plasma dispersion solvers, and particle-in-cell (PIC) simulation codes have been used to arrive at the first step in the design of an advanced ion source based on ECR technology. The advanced concept design uses a minimum-B magnetic mirror geometry, which consists of a multicusp magnetic field, to assist in confining the plasma radially, a flat central field for tuning to the ECR resonant condition, and specially tailored mirror fields in the end zones to confine the plasma in the axial direction. The magnetic field is designed to achieve an axially symmetric plasma volume with constant mode-B, which extends over the length of the central field region. The design permits separation of the ‘‘tune’’ and confinement magnetic fields, thus enabling the use of low-frequency, low-cost microwave power sources; the location and increased physical size of the ECR zone over those in conventional sources is commensurate with the generation of higher beam intensities, higher charge states and a higher degree of ionization. A summary of the results of these studies is presented in this report.

Design studies for an advanced ECR ion source

Magnet design codes, plasma dispersion solvers, and particle-in-cell (PIC) simulation codes have been used to arrive at the first step in the design of an advanced ion source based on electron cyclotron resonance (ECR) technology. The advanced concept design uses a minimum-B magnetic mirror geometry which consists of a multicusp magnetic field to assist in confining the plasma radially, a flat central field for tuning to the ECR resonant condition, and specially tailored mirror fields in the end zones to confine the plasma in the axial direction. The magnetic field is designed to achieve an axially symmetric plasma ‘‘volume’’ with constant mod-B, which extends over the length of the central field region. This design, which strongly contrasts with ‘‘surface’’ ECR zones characteristic of conventional ECR ion sources, results in dramatic increases in the absorption of rf power, thereby increasing the electron temperature and ‘‘hot’’ electron population within the ionization volume of the source. The creation of a volume rather than a surface ECR zone is, therefore, commensurate with the generation of higher beam intensities, higher charge states, and a higher degree of ionization. A summary of the results of these studies is presented in this report.

Primary electron drift in a volume hybrid multicusp H− ion source

A new, large, hybrid volume negative ion source is described. Observations of curvature and grad B primary electron drift in the multicusp magnetic field are reported. The direction of the electron drift changes from one filament to the neighboring one, because of the change in the direction of the magnetic field. For a given discharge current the extracted negative ion current and the plasma density at the center of the extraction region are affected by the direction of the primary electron drift of the active filaments. It was shown, however, that the negative ion current is controlled by the plasma density at the center of the extraction region. A trajectory calculation, effected in the guiding center approximation, allows to estimate the average drift velocity.

Performance of Electron Cycrotron Resonance Plasma Produced by a New Microwave Launching System in a Multicusp Magnetic Field with Permanent Magnets

A multi-ring-cusp-type surface magnetic field is tested for production of large-diameter electron cyclotron resonance (ECR) plasma. The density and its distribution are improved by means of a new launching system consisting of an adjustable antenna and a microwave suppressor inserted into the vacuum chamber, which forces the microwave to propagate into the plasma from the stronger side of the surface magnetic fields.

Development of a high current and high energy metal ion beam system

A high current metal ion accelerating system, which consists of a metal vapor plasma ion source with a multicusp magnetic field and a single gap acceleration column, has been developed. The following results were obtained experimentally: (1) a 110 mA aluminum ion beam and a 95 mA chromium ion beam were extracted from the ion source, (2) the impurities contained in the beams were less than 1%, and (3) the aluminum ion beam was accelerated up to 90 keV at 50 mA on the target.

Low Temperature Silicon Oxidation with Electron Cyclotron Resonance Oxygen Plasma

AbstractSilicon was oxidized at low temperature with an oxygen plasma generated by an electron cyclotron resonance (ECR) source. The ECR source utilized a multicusp magnetic field formed by permanent magnets. Microwave power at 2.45 GHz was applied to the source and rf power at 13.56 MHz was applied to the sample stage. Si oxidation was studied as a function of source distance, pressure, microwave power, and rf power. The oxide thickness increases with microwave and rf power but decreases with source distance. The oxidation rate increases with pressure up to 12 mTorr, then decreases at higher pressure. The relative emission intensities in the plasma monitored using optical emission spectroscopy showed similar dependence on the source distance and microwave power. Oxidation temperature was estimated to be <100°C. Using ellipsometry and X-ray photoelectron spectroscopy, the oxidized films were found to be close to that of thermal oxide with refractive index at 1.45 and oxygen to silicon ratio of 2. From the current-voltage and capacitance-voltage measurements, the breakdown fields of these oxide films were 6.3 MV/cm and the fixed charge densities were 7×1010 cm−2.

Low Temperature Silicon Oxidation With Electron Cyclotron Resonance Oxygen Plasma

ABSTRACTSilicon was oxidized at low temperature with an oxygen plasma generated by an electron cyclotron resonance (ECR) source. The ECR source utilized a multicusp magnetic field formed by permanent magnets. Microwave power at 2.45 GHz was applied to the source and if power at 13.56 MHz was applied to the sample stage. Si oxidation was studied as a function of source distance, pressure, microwave power, and rf power. The oxide thickness increases with microwave and rf power but decreases with source distance. The oxidation rate increases with pressure up to 12 mTorr, men decreases at higher pressure. The relative emission intensities in the plasma monitored using optical emission spectroscopy showed similar dependence on the source distance and microwave power. Oxidation temperature was estimated to be <100°C. Using ellipsometry and X-ray photoelectron spectroscopy, the oxidized films were found to be close to that of thermal oxide with refractive index at 1.45 and oxygen to silicon ratio of 2. From the current-voltage and capacitance-voltage measurements, the breakdown fields of these oxide films were 6.3 MV/cm and the fixed charge densities were 7×1010 cm−2.

A 915 MHz/2.45 GHz ECR plasma source for large area ion beam and plasma processing

The technology for producing uniform, high density (1011–1012/cm3) microwave discharges over cross sections of 50 cm2 is well established. The present challenge is to extend the high density, and electrodeless benefits of microwave discharges to produce uniform densities over an area of 300–700 cm2. Such discharges have important applications for 6 to 8-in. single wafer processing and as large surface, broad beam, high current density ion sources. The design principles for scaling the 18 cm diam MPDR ECR cavity applicator technology to 38–47 cm diam are reviewed. Microwave discharges with diameters of 20–30 cm can be created when these applicators are excited with either 2.45 GHz or 915 MHz. The design and construction of a prototype cavity applicator with a 20 cm diam discharge is described. The discharge is enclosed with a 12-pole multicusp static magnetic field produced by 2-in. by 2-in. by 1-in. rare-earth magnets. Each magnet has a pole face field strength of 3 kG. The experimental test of this plasma source in argon gas excited with 2.45 GHz energy is reviewed.

High-intensity plasma-sputter heavy negative-ion source (invited)

A multicusp magnetic field plasma-surface ion source, normally used for H−ion-beam formation, has been modified for the generation of high-intensity, pulsed, heavy-negative-ion beams suitable for a variety of uses. A brief description of the source and basic pulsed-mode operational data (e.g., intensity versus cesium oven temperature, sputter probe voltage, and discharge pressure) are given. In addition, illustrative examples of intensity versus time and the mass distributions of ion beams extracted from a number of samples, along with emittance data, are also presented. Preliminary results obtained during dc operation of the source under low-discharge-power conditions suggest that sources of this type may alo be used to produce high-intensity (mA) dc beams.

The experimental test of a microwave ion beam source in oxygen

Recent experiments have demonstrated a microwave (2.45 GHz) plasma disk ion source operating with inert gases both with and without an applied static magnetic field. Earlier results were reported using a preliminary, multicusp static magnetic field configuration produced by alternating pole, small, ∼2.5 kG rare earth magnets surrounding the microwave discharge zone. This electrodeless ion source has been rebuilt incorporating several improvements and has been experimentally tested with different discharge diffusion lengths. The improvements include removing the rare earth magnets from the inside of the microwave cavity and embedding them in the cavity walls. The placement and strength of the magnets allows electron cyclotron resonant surfaces (∼875 G) to lie entirely within the discharge zone. Discharges with different diffusion lengths were created by varying the thickness of the disk-shaped discharge zone. The experimental performance with and without a static magnetic field is discussed at the low flow rates of 2–10 sccm and using a 2.5 cm diam double grid set. Experimental test results are compared to similar ion beam sources in oxygen that employ a dc discharge. The expected lifetime, startup, and shutdown problems and maintenance are also reviewed.

Rf multipolar plasma for broad and reactive ion beams

Hot cathode dc multipolar plasma sources are very efficient but have lifetime and contamination problems when they are operated with chemically active gases. As an alternative solution the rf excitation of a triode structure immersed in a multicusp magnetic field has been developed. The structure has an internal cathode, an anode around which are the magnet lines, and a third electrode which is either the target electrode in the case of ‘plasma processing’ or the beam forming electrode in the case of ‘ion beam processing’. The source has been operated with oxygen and fluorocarbon gases without any lifetime problems. The discharge may be run down to 10 −4 torr (within source chamber) and creates a plasma which is homogeneous to ± 1.5% over a 175 mm diameter section and which delivers at the beam forming electrode a current density of about 1 mAcm −2 for 500 W rf power.

The performance of a microwave ion source immersed in a multicusp static magnetic field

Many applications requiring high beam currents and operation with chemically active gases impose lifetime and operational requirements on the discharge electrodes in conventional ion sources. Despite the development of specially designed hot filaments and hollow cathodes, the presence of these electrodes in the discharge zone still limits the practical application of ion and plasma sources. Thus, the development of an efficient, simple, electrodeless discharge would result in an important improvement in ion beam and plasma processing technology. Recently, an electrodeless microwave ion source and plasma source have been developed. An improvement of this ion source is discussed. This is the redesign to surround the discharge zone with many closely spaced rare earth magnets producing a confining and cyclotron resonant multicusp static magnetic field. The experimental performance of this ‘‘modified’’ ion source using argon gas is presented. Experimental measurements of extracted ion beam current versus accelerating voltage and discharge electron and ion densities, etc. are presented over a range of gas flow rates and operating pressures.

Negative ion production in hydrogen plasmas confined by a multicusp magnetic field

The density of negative hydrogen ions and the plasma characteristics are investigated as a function of the discharge current and the neutral gas pressure for several configurations of the magnetic multipole, and in the absence of magnetic containment fields. It is shown that the hybrid magnetic multipole configurations, characterized by a relatively low lifetime for wall losses of primary electrons (10−7 s) contain high relative densities (10%–50%) of negative ions. The transition from the low density to the high density regime has been observed.