The Physics and Technology of Ion Sources, Second Edition
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The European Physical Journal D. The influence of ion-beam plasma on ion extraction efficiency in a single-grid ICP ion source is researched. The single-grid ion source is considered as a system with two plasmas, ion-beam plasma and the source plasma, separated by an extraction grid. Results of experimental measurements of the potentials of the two plasmas and the ion beam current dependence on these potentials are presented.
It is shown that the ion extraction efficiency depends equally on both the acceleration potential and on the potential of the ion-beam plasma. The obtained results demonstrate the key role of the ion-beam plasma in the ion source operation, which is important in technological applications and space thrusters. Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide. Influence of ion-beam plasma on ion extraction efficiency in a single-grid ion source.
Authors Authors and affiliations S. Dudin D. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Rius, J.
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Llobet, M. Esplandiu, L. The gas from which the plasma is produced must be brought from ground potential to the plasma potential along the path between the tank and the plasma.
In a preferred embodiment, most of the voltage change occurs where the gas pressure is relatively high and resistant to arcing. Gas is provided to plasma chamber from a gas source, such as a tank Tank is typically maintained at ground potential and contains the gas at a high pressure. A regulator reduces the pressure of the gas leaving the tank entering a conduit An optional adjustable valve further reduces the pressure in the gas line or closes the conduit completely when the source is not in use.
A flow restrictor, such as a capillary , further reduces the gas pressure before the gas reaches plasma chamber Restrictor provides a desired gas conductance between the gas line and the interior of plasma chamber Restrictor is preferably in electrical contact with plasma and so is at the plasma potential. In other embodiments, the flow restriction can have an electrical bias applied from a voltage source other than the plasma. An insulating shield surrounds capillary and a grounded metallic collar at the end of insulating shield ensures that the electrical potential of the gas is zero at that position.
Thus, the entire electrical potential change from ground to the plasma voltage occurs within insulating shield in which the gas is at a relatively high pressure and therefore resistant to arcing. In one example embodiment without a valve , regulator reduces the pressure of the gas leaving the supply tank from psig to 5 psig. The gas pressure remains at 5 psig until the gas reaches capillary , and which point the gas pressure drops to the plasma chamber pressure of, for example, 0.
Insulating shield preferably has sufficient length to keep the field sufficiently low to prevent a damaging discharge. Insulating shield is typically about at least about 5 mm long, and more typically between about 30 mm and 60 mm. Skilled persons will understand that the local electric field will be a function of the geometry and that initial low current discharges may occur to reach a static charge equilibrium within insulating shield In some embodiments, valve may reduce the gas pressure further before the gas reaches the final restrictor before the plasma.
Instead of a capillary, the flow restrictor could be a valve, such as a leak valve.
Any type of gas source could be used. For example, the gas source may comprise a liquid or solid material that is heated to produce gas at a sufficient rate to supply the plasma. The different output pressures of the different gas sources may require different components to reduce the pressure to that required in the plasma chamber. The Faraday shield is positioned against the ridges , defining passages for the cooling fluid to flow between the valleys and the shield In the embodiment shown in FIG. A portion of the metal sleeve is then bent outward at the bottom to form grounding tab FIG.
Cooling fluid flows through the space which is bounded by the plasma chamber outer wall and the shell In an alternative embodiment, the outer wall may be smooth and the Faraday shield formed with corrugations. Alternatively, neither the wall nor the faraday shield may be corrugated. At the top of the ion column, an inductively-coupled plasma ICP ion source is mounted, comprising an electromagnetic enclosure , a source chamber , and an induction coil , which includes one or more windings of a conductive material. The coolant then flows back to coolant reservoir and chiller through a return conduit In an alternative embodiment, the coolant region around the source chamber contains a static liquid for high voltage isolation.
In yet another embodiment, liquid in the plasma source is cooled by one or more heat pipes, as described in more detail below. An RF power supply is connected to a match box by an RF coaxial cable The match box is connected to the induction coil by coil leg extensions The induction coil is mounted coaxially with the source chamber To reduce capacitive coupling between the induction coil and the plasma generated within the source chamber , a split Faraday shield not shown may optionally be mounted coaxially with the source chamber and inside the induction coil When a split Faraday shield is used in the ICP ion source , the high voltage typically several hundred to a few thousand volts across the induction coil will have minimal effect on the energies of the ions extracted from the bottom of the ICP ion source into the ion column.
This will result in smaller beam energy spreads, reducing the chromatic aberration in the focused charged particle beam at or near the substrate surface. The presence of a plasma within the source chamber may be detected using the light emitted by the plasma and collected by the source-facing end of optic fiber , and transmitted through optic fiber to a plasma light detection unit An electrical signal generated by the plasma light detection unit is conducted through cable to a programmable logic controller PLC Signals from the plasma source controller may then pass through cable or data bus to the focused ion beam FIB system controller The FIB system controller may communicate via the Internet to a remote server These details of the interconnections of the various components of the FIB system control are for exemplary purposes only.
Other control configurations are possible as is familiar to those skilled in the art. Gas is provided to the source chamber by inlet gas line which leads to inlet restrictor , which leads to the interior of the source chamber Restrictor is maintained at an electrical potential closer to the potential of the plasma in chamber than to the potential of the gas source and regulator so that the voltage drop occurs primarily across gas of higher pressure. Insulating shield insulates the gas line upstream of restrictor and is terminated with a grounded collar A gas supply system for the ICP source comprises a gas supply , a high purity gas regulator , and a needle regulating valve The gas supply may comprise a standard gas bottle with one or more stages of flow regulation, as would be the case for helium, oxygen, xenon or argon feed gases, for example.
Alternatively, for gases derived from compounds which are solid or liquid at room temperature, gas supply may comprise a heated reservoir. Other types of gas supplies are also possible. The particular choice of gas supply configuration is a function of the type of gas to be supplied to the ICP source. Gas from supply passes through high purity gas regulator , which may comprise one or more stages of purification and pressure reduction. The purified gas emerging from high purity gas regulator passes through an optional needle valve Gas emerging from optional needle valve passes through a hose to an optional second needle valve , mounted in close proximity to the ICP source.
Gases emerging from needle valve pass through inlet gas line , which connects through restriction to the top of the source chamber At the bottom of the ICP source , a source electrode serves as part of the ion beam extraction optics, working in conjunction with the extractor electrode and the condenser A plasma igniter is connected to a source electrode not shown , enabling the starting of the plasma in the source enclosure Other known means of igniting the plasma can also be used. Details of the operation of the ICP source are provided in U. The source electrode is biased through the igniter to a high voltage by beam voltage power supply PS The voltage on the source electrode determines potential of the plasma and therefore the energy of the charged particles reaching the substrate surface in the case of singly-ionized atomic or molecular ion species or electrons.
Doubly-ionized ion species will have twice the kinetic energy. The extractor electrode is biased by extractor power supply , while the condenser is biased by condenser power supply The combined operation of the source electrode , the extractor , and the condenser serves to extract and focus ions emerging from the ICP source into a beam which passes to the beam acceptance aperture The beam acceptance aperture is mechanically positioned within the ion column by the beam acceptance aperture actuator , under control of the FIB system controller The ion column illustrated in FIG.
The lens 1 power supply is controlled by the FIB system controller Between the first einzel lens and the second einzel lens in the ion column, a beam defining aperture assembly is mounted, comprising one or more beam defining apertures three apertures are shown in FIG. Typically, the beam defining aperture assembly would comprise a number of circular apertures with differing diameter openings, where any one of which could be positioned on the optical axis to enable control of the beam current and half-angle at the substrate surface.
Alternatively, two or more of the apertures in the beam defining aperture assembly may be the same, thereby providing redundancy to enable the time between aperture maintenance cycles to be extended. By controlling the beam half-angle, together with corresponding adjustments of the lenses, the beam current and diameter of the focused ion beam at or near the substrate surface may be selected, based on the spatial resolution requirements of the milling or imaging operations to be performed.
The particular aperture to be used and thus the beam half-angle at the substrate is determined by mechanical positioning of the desired aperture in the beam defining aperture assembly on the optical axis of the column by means of the beam defining aperture BDA actuator , controlled by the FIB system controller The first and third electrodes are typically grounded 0 V , while the voltage of the center electrode is controlled by lens 2 L2 power supply PS The lens 2 power supply is controlled by the FIB system controller Isolation valve enables the vacuum in the ion column vacuum chamber to be maintained at high levels, even if the vacuum level in the sample chamber is adversely affected by sample outgassing, during sample introduction and removal, or for some other reason.
Turbopump also pumps the ion column enclosure through pumping line For example, the ion column illustrated in FIG. The ion column may alternatively be implemented using a single electrostatic einzel lens, or more than two electrostatic lenses. Other embodiments might include magnetic lenses or combinations of two or more electrostatic or magnetic quadrupoles in strong-focusing configurations. For the purposes of this embodiment of the present invention, it is preferred that the ion column forms a highly demagnified image of the virtual source in the ICP source at or near the surface of the substrate Details of these possible demagnification methods are familiar to those skilled in the art.
Static fluid may comprise a liquid, such as Fluorinert or distilled water, or gas, such as sulfur hexafluoride. A split Faraday shield is also positioned between shell and outer wall Faraday shield can be positioned against shell as shown, against outer wall , or away from both walls and immersed in the static fluid When positioned between a grounded split Faraday shield and the outer wall , fluid provides part of the high voltage isolation of the plasma chamber. Static fluid is preferably not circulated outside of the source by an external pump, although static fluid may move internally by convection within.
One or more optional cooling devices assist in cooling the plasma chamber Cooling devices may comprise cooling loops that encircle the plasma chamber and through which a fluid circulates.
Ion Implantation Science and Technology - 2nd Edition
Because cooling devices are positioned outside of the Faraday shield, which is at ground potential, these devices do not perform any voltage isolation and hence any type of cooling fluid may be used in cooling devices Alternatively, cooling devices may comprise one or more thermoelectric coolers, such as Peltier effect coolers. The RF coils may be hollow and cooled by flow of a coolant through the internal passages in the coils.
An optional gap between the dielectric media and outer wall defines a fluid cavity , which can be filled with a fluid, such as Fluorinert, distilled water, or sulfur hexafluoride. Non-encapsulated portions of the Faraday shield are available to form grounding connections. In some embodiments, fluid is pumped through the fluid cavity and then through a cooler, using a system similar to the system shown in FIG. In other embodiments, fluid is not pumped outside the source and remains within fluid cavity In some embodiments, dielectric media can be positioned against outer wall without an intervening fluid.
To avoid an air gap in such embodiments, dielectric media should fit tightly against outer wall Air gaps can also be avoided by providing a flowable material to fill displace any air between outer wall and dielectric media The flowable media can be, for example, a high dielectric constant grease or gel.
The flowable material can remain liquid or may solidify after positioning the dielectric media relative to the plasma chamber. In some embodiments, the dielectric media can comprise a flowable medium that hardens or remains liquid. The flowable medium may also coat the Faraday shield on the side opposite to outer wall , thereby preventing contact between any cooling fluid and the Faraday shield.
In some embodiments, the Faraday shield can be molded into the wall of plasma chamber In this embodiment, a static fluid coolant surrounds the plasma chamber having one or more heat pipes integrated into the upper portion of the coolant jacket Charge-exchange ionization also known as charge-transfer ionization is a gas phase reaction between an ion and an atom or molecule in which the charge of the ion is transferred to the neutral species.
Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or molecule with an atom or molecule in an excited state. Associative ionization is a gas phase reaction in which two atoms or molecules interact to form a single product ion. Penning ionization is a form of chemi-ionization involving reactions between neutral atoms or molecules. Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom or molecule.
Surface Penning ionization also known as Auger deexcitation refers to the interaction of the excited-state gas with a bulk surface S, resulting in the release of an electron according to. Ion-attachment ionization is similar to chemical ionization in which a cation is attached to the analyte molecule in a reactive collision:. In a radioactive ion source, a small piece of radioactive material, for instance 63 Ni or Am , is used to ionize a gas. These ion sources use a plasma source or electric discharge to create ions. Ions can be created in an inductively coupled plasma , which is a plasma source in which the energy is supplied by electrical currents which are produced by electromagnetic induction , that is, by time-varying magnetic fields.
Microwave induced plasma ion sources are capable of exciting electrodeless gas discharges to create ions for trace element mass spectrometry. It is capable of exciting electrodeless gas discharges. If applied in surface-wave-sustained mode , they are especially well suited to generate large-area plasmas of high plasma density.
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If they are both in surface-wave and resonator mode , they can exhibit a high degree of spatial localization. This allows to spatially separate the location of plasma generations from the location of surface processing. Such a separation together with an appropriate gas-flow scheme may help reduce the negative effect, that particles released from a processed substrate may have on the plasma chemistry of the gas phase. The ECR ion source makes use of the electron cyclotron resonance to ionize a plasma.
Microwaves are injected into a volume at the frequency corresponding to the electron cyclotron resonance, defined by the magnetic field applied to a region inside the volume. The volume contains a low pressure gas. Ions can be created in an electric glow discharge. A glow discharge is a plasma formed by the passage of electric current through a low-pressure gas. It is created by applying a voltage between two metal electrodes in an evacuated chamber containing gas. When the voltage exceeds a certain value, called the striking voltage , the gas forms a plasma.
A duoplasmatron is a type of glow discharge ion source that consists of a cathode hot cathode or cold cathode that produces a plasma that is used to ionize a gas. In a flowing afterglow , ions are formed in a flow of inert gas, typically helium or argon. Flowing-afterglow mass spectrometry is used for trace gas analysis  for organic compounds.
Electric spark ionization is used to produce gas phase ions from a solid sample. When incorporated with a mass spectrometer the complete instrument is referred to as a spark ionization mass spectrometer or as a spark source mass spectrometer SSMS. A closed drift ion source uses a radial magnetic field in an annular cavity in order to confine electrons for ionizing a gas. They are used for ion implantation and for space propulsion Hall effect thrusters.
Photoionization is the ionization process in which an ion is formed from the interaction of a photon with an atom or molecule. In multi-photon ionization MPI , several photons of energy below the ionization threshold may actually combine their energies to ionize an atom.
Resonance-enhanced multiphoton ionization REMPI is a form of MPI in which one or more of the photons accesses a bound-bound transition that is resonant in the atom or molecule being ionized. Atmospheric pressure photoionization uses a source of photons, usually a vacuum UV VUV lamp, to ionize the analyte with single photon ionization process. Analogous to other atmospheric pressure ion sources, a spray of solvent is heated to relatively high temperatures above degrees Celsius and sprayed with high flow rates of nitrogen for desolvation.
The resulting aerosol is subjected to UV radiation to create ions. Field desorption refers to an ion source in which a high-potential electric field is applied to an emitter with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have formed. Mass spectra produced by FI have little or no fragmentation. Particle bombardment with atoms is called fast atom bombardment FAB and bombardment with atomic or molecular ions is called secondary ion mass spectrometry SIMS. In FAB the analytes is mixed with a non-volatile chemical protection environment called a matrix and is bombarded under vacuum with a high energy to 10, electron volts beam of atoms.
Common matrices include glycerol , thioglycerol , 3-nitrobenzyl alcohol 3-NBA , crown-6 ether, 2-nitrophenyloctyl ether , sulfolane , diethanolamine , and triethanolamine. This technique is similar to secondary ion mass spectrometry and plasma desorption mass spectrometry. Secondary ion mass spectrometry SIMS is used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions.
In a liquid metal ion source LMIS , a metal typically gallium is heated to the liquid state and provided at the end of a capillary or a needle. Then a Taylor cone is formed under the application of a strong electric field. As the cone's tip get sharper, the electric field becomes stronger, until ions are produced by field evaporation.
These ion sources are particularly used in ion implantation or in focused ion beam instruments.
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Plasma desorption ionization mass spectrometry PDMS , also called fission fragment ionization, is a mass spectrometry technique in which ionization of material in a solid sample is accomplished by bombarding it with ionic or neutral atoms formed as a result of the nuclear fission of a suitable nuclide , typically the californium isotope Cf. The sample is mixed with a matrix material. Upon receiving a laser pulse, the matrix absorbs the laser energy and it is thought that primarily the matrix is desorbed and ionized by addition of a proton by this event.
The analyte molecules are also desorbed. The matrix is then thought to transfer proton to the analyte molecules e. A laser vaporization cluster source produces ions using a combination of laser desorption ionization and supersonic expansion. In aerosol mass spectrometry with time-of-flight analysis, micrometer sized solid aerosol particles extracted from the atmosphere are simultaneously desorbed and ionized by a precisely timed laser pulse as they pass through the center of a time-of-flight ion extractor.
Spray ionization methods involve the formation of aerosol particles from a liquid solution and the formation of bare ions after solvent evaporation. Solvent-assisted ionization SAI is a method in which charged droplets are produced by introducing a solution containing analyte into a heated inlet tube of an atmospheric pressure ionization mass spectrometer.
Just as in Electrospray Ionization ESI , desolvation of the charged droplets produces multiply charged analyte ions. Volatile and nonvolatile compounds are analyzed by SAI, and high voltage is not required to achieve sensitivity comparable to ESI. In MAI, analyte ions have charge states similar to electrospray ionization but obtained from a solid matrix rather than a solvent.
No voltage or laser is required, but a laser can be used to obtain spatial resolution for imaging. Matrix-analyte samples are ionized in the vacuum of a mass spectrometer and can be inserted into the vacuum through an atmospheric pressure inlet. Less volatile matrices such as 2,5-dihydroxybenzoic acid require a hot inlet tube to produce analyte ions by MAI, but more volatile matrices such as 3-nitrobenzonitrile require no heat, voltage, or laser. Simply introducing the matrix:analyte sample to the inlet aperture of an atmospheric pressure ionization mass spectrometer produces abundant ions.
Compounds at least as large as bovine serum albumin [66 kDa] can be ionized with this method. Atmospheric pressure chemical ionization is a form of chemical ionization using a solvent spray at atmospheric pressure. Thermospray ionization is a form of atmospheric pressure ionization in mass spectrometry.
It transfers ions from the liquid phase to the gas phase for analysis. It is particularly useful in liquid chromatography-mass spectrometry. In electrospray ionization , a liquid is pushed through a very small, charged and usually metal , capillary. Volatile acids , bases or buffers are often added to this solution too. The analyte exists as an ion in solution either in its anion or cation form.
The aerosol is at least partially produced by a process involving the formation of a Taylor cone and a jet from the tip of this cone. An uncharged carrier gas such as nitrogen is sometimes used to help nebulize the liquid and to help evaporate the neutral solvent in the droplets. As the solvent evaporates, the analyte molecules are forced closer together, repel each other and break up the droplets. This process is called Coulombic fission because it is driven by repulsive Coulombic forces between charged molecules. The process repeats until the analyte is free of solvent and is a bare ion. Probe electrospray ionization PESI is a modified version of electrospray, where the capillary for sample solution transferring is replaced by a sharp-tipped solid needle with periodical motion.
Contactless atmospheric pressure ionization is a technique used for analysis of liquid and solid samples by mass spectrometry. Thus, the technique provides a facile means for analyzing chemical compounds by mass spectrometry at atmospheric pressure. Sonic spray ionization is method for creating ions from a liquid solution , for example, a mixture of methanol and water.
Ions are formed when the solvent evaporates and the statistically unbalanced charge distribution on the droplets leads to a net charge and complete desolvation results in the formation of ions. Sonic spray ionization is used to analyze small organic molecules and drugs and can analyze large molecules when an electric field is applied to the capillary to help increase the charge density and generate multiple charged ions of proteins.
Sonic spray ionization has been coupled with high performance liquid chromatography for the analysis of drugs. Ultrasonication-assisted spray ionization UASI involves ionization through the application of ultrasound. Thermal ionization also known as surface ionization, or contact ionization involves spraying vaporized, neutral atoms onto a hot surface, from which the atoms re-evaporate in ionic form.