Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/24—Ion sources; Ion guns using photo-ionisation, e.g. using laser beam
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
  • H01J37/08—Ion sources; Ion guns
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/30—Electron-beam or ion-beam tubes for localised treatment of objects
  • H01J37/317—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation
  • H01J37/3171—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation for ion implantation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/08—Ion sources
  • H01J2237/0812—Ionized cluster beam [ICB] sources
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/06—Sources
  • H01J2237/08—Ion sources
  • H01J2237/0815—Methods of ionisation

Definitions

• Yamada et al. An important suggestion to avoid this problem was made in 1993 by Yamada et al., and reported in the journal Nuclear Instruments and Methods, volume 79, page 223. Yamada' s suggestion was that if singly-charged molecular clusters of the wanted ions were substituted for individual atoms of the wanted atomic species the problems of low-energy implantation would be substantially alleviated.
• One such molecular substitution described by Jacobson et al. in the IEEE Conference Report "DT 2000” used ionized molecular decaborane (BioH 14 + ) that was extracted from a suitable ion source and accelerating to a kinetic energy approximately eleven times greater than would be used were the implantation ion beam composed solely of individual B + atoms.
• the parent decaborane ion would be accelerated to an energy of 5.5 keV; an energy that allows the molecular ions to be readily transported through a modern implanter.
• the measured implantation dose, using charge collection is magnified ten-fold, compared to that using conventional boron implantation, causing space-charge forces to be correspondingly reduced.
• an alternative highly-efficient ionization process for production of singly-charged molecular decaborane ions.
• the process employs the well-known phenomenon of resonant charge exchange between molecular or atomic ions where an incoming primary ion beam is directed through a region containing cluster molecules or atoms that are to be ionized and accelerated. While an immediate application is the production of high currents of singly-charged decaborane ions, applications involving other atom and molecular species are also expected to become of importance as requirements grow for other beams needed for low-energy implantation.
• AMS has been described in detail by a number of authors who have presented methods by which AMS techniques can be applied to the detection of rare stable and radioactive isotopes, such as l ⁇ Be, ⁇ C, 26A1, 36Q 5 an( j 129j.
• Such descriptions include U.S. Patent No. 4.037.100 to Purser; an article by Purser, K.H., Litherland, A.E. and Gove, H.E. in the journal Nuclear Instruments and Methods volume 162, page 637, (1979) entitled "Ultra-sensitive particle identification systems based upon electrostatic accelerators".
• a central problem for detecting radioactive atoms at such low abundance is that high beam currents of the parent element are essential. For example, for single atom detection of radioactive atom concentrations below 10 "16 (compared to the number of atoms of the parent element) milliampere beams of the parent element are desirable. In addition, such beams should be as pure as possible with low levels of molecular and isobaric contaminants.
• the present invention can exploit the differences between resonant and non-resonant processes allowing enhancement or attenuation of a particular elemental species.
• a beam of Cs" ions traversing a cell containing cesium atoms would transfer charge to the Cs atoms much more efficiently than to non-identical atoms.
• ⁇ is a constant related to that of said Norskov and Lundquivst.
• ⁇ is the work function of the sputtered surface.
• A is the electron affinity of the sputtered species.
• M is the mass of the sputtered species.
• the above sputtering impediment is no longer a limitation to the formation of beams of weakly-bound negative ions.
• precisely equal electron affinities of the two partners are not essential for efficient charge transfer, as will be discussed later.
• a class of implants that have become important involves the ions having energies close to a million electron Volts.
• One reason for this growth of high-energy implantation has been transistor miniaturization. Over time, individual transistors have become smaller and closer together and also operate at much lower voltages. These changes have led to increases in electrical capacitance between elements that may cause parasitic current-coupling between individual circuits that can cause circuit ⁇ nstaBi ⁇ f ⁇ es. Tb avoid these undesirable effects it is often useful to introduce barriers that electrically isolate the transistor circuits one from another and from the underlying substrate through which parasitic currents might flow. Such substrate isolation can be produced by implanting a low-resistivity layer below the active circuit. Generally, the energies needed for such a process are in the range 0.8 to 3.0MeV and require the use of acceleration voltages that are high compared to those employed during conventional implantation or alternatively the installation of complicated radio-frequency radio frequency accelerators.
• One class of commercial high-energy implanters uses d.c. voltages and the tandem acceleration principle for the production of ions having energies in the million electron-volt range. During this process, negative ions are accelerated from ground to a positive terminal where the charge of the incoming negatively charged ion are converted to positive polarity by stripping electrons from the negative ions. A following stage of acceleration returns these positive ions to ground potential.
• the cross section for the necessary negative ion production tends to be small and it is this limitation of the tandem-type acceleration system that often restricts beam-current intensity.
• One method of avoiding this problem is to employ a neutral beam of the wanted ions that is drifted without acceleration through the first stage of a million- volt dc tandem configuration.
• the neutral beam delivers neutral particles to a high voltage terminal where they are charge changed to positive polarity by removing one or more electrons as the ions pass through a suitable gas cell or foil. Effectively, this process creates positive ions within a positive polarity terminal from whence they are accelerated back to ground potential, gaining energy on the way.
• the importance of such a scheme is that beam intensities of ions having ⁇ MeV energies can be substantially increased - often by as much as an order of magnitude.
• Cross sections were measured for three materials, water (ionization potential 12.612 eV), meffio ⁇ al" ⁇ 10" ' 85 ' eV) and anisole (8.21 eV).
• the measured cross sections for B + to B 0 at lOkeV, in units of 10 ⁇ ls cm 2 were Water ⁇ 2.1+/- 0.2; Methonal -.2.7+/- 0.2; Anisole ⁇ .5.9+/- 0.6 indicating a near-resonant effect.
• both X and Y particles can be either atoms or molecules.
• X + refers to the incoming projectile atom or molecule initiating the reaction and Y 0 is the target atom or molecule.
• Y 0 is the target atom or molecule.
• the incoming positive ions, X + can also be converted to neutral charge allowing previously focused directed fast ions to be converted into directed neutral-particle beams.
• the same type of resonant interaction can be used for producing resonant charge exchange between negative ions and neutral particles.
• the dominant controlling parameter for estimating cross sections for such interactions is the energy difference, ⁇ E, between the ionization potential or electron affinity of the neutral target atom, Y 0 , and those of the uncharged projectile atom, X 0 .
• ⁇ E the energy difference between the ionization potential or electron affinity of the neutral target atom, Y 0 , and those of the uncharged projectile atom, X 0 .
• an important example of the usefulness of such resonance processes is the ionization of decaborane by a slow beam of arsenic atoms.
• the key to efficient transfer is that the ionization potential of decaborane must be close to that of the exciting beam.
• the ionization potential OfB 1O H 14 has been accurately measured to be 9.88 +/- 0.03eV.
• the closest atomic particle that matches this ionization potential is arsenic, As, which has an almost identical ionization potential - 9.815 eV.
• the first excited state of As is at 131.9 meV (milli electron Volts) or only 30 me V away from resonance with the measured ionization potential of the decaborane molecule.
• the difference in ionization potential between the two ground states, ⁇ E, is only - 80 meV so a strong resonance can be expected at the lowest energies.
• ⁇ E milli electron Volts
• the calculated cross section for interactions with decaborane is 2.77 x 10 '15 cm 2 ; for As + ions having energy of lkeV the resonance effect is even more pronounced with the calculated exchange cross sections being - 3.94 x 10 "15 cm 2 - a substantial area on an atomic scale.
• Figure 1 illustrates operational details of a geometry where the wanted extracted beam is directed along the direction of the incident primary ion beam.
• Figure 2 illustrates the operational details of an extraction geometry where the emission direction of the extracted ions is a right angle to the direction of the incident primary ion beam.
• a primary ion beam, 101 entering a cell, 102, and passing through a gas or vapor, 103, contained within the cell.
• Either negative or positive ions can be used as the primary ion beam, 101, which can be either atomic or molecular.
• the sample, 103 is maintained in a gaseous or vapor form at an appropriate vapor pressure. If necessary, temperature control may be needed using suitable heaters or refrigerators.
• the primary ion beam species, 101 is chosen so that the electron affinity or ionization potential of the ion species, 101, is approximately equal to the electron affinity or ionization potential of the atom or molecule comprising the gas or vapor, 103, that is to be converted to negative or positive ions.
• the resulting ions, 106 are extracted longitudinally from the opposite side of the cell from that where the primary ion beam enters and is formed into a suitable ion beam, 107, using extraction optics, the design and operation of which are well known to those skilled in the art.
• the cross sections are anticipated to be approximately 3.5 xlO "15 cm 2 . If the integrated thickness of the decaborane is adjusted to be ⁇ 10 15 /cm 2 , there will be about three resonant charge exchange collisions of As+ ions with decaborane molecules as the arsenic ions pass through the cell. Extraction of the decaborane ions formed within the source box is achieved by an electric field that penetrates into the source box through the extraction slot, 108. Extraction can also be enhanced by the introduction of a small electric field within the source box or by using ion motion induced by crossed electric and magnetic fields to produce E x B drifts, well known to those skilled in the art.
• the vapor or gas sample in the center of the cell, 202 which is elevated to a positive potential, is bombarded by the incoming primary particle beam. Ions that are produced within the gas move away from the incoming primary ion beam by a suitable electric field that directs the wanted charged ions to the region of the extraction optics.
• a graded extraction field of a few volts/cm extending along the width of the cell or a crossed E x B field arrangement should permit efficient extraction of the ions.
• the ions reach the extraction aperture, 204, they are accelerated and formed into a directed ion beam. If necessary a suitable electric field can be introduced within the cell, 202, by a series of equipotential planes or from the fringes of the acceleration fields which percolate through the aperture, 204.
• an ionized primary beam 101 enters the cell 102 and passes through gas or vapor 103 contained within the cell. Electrons from the gas or vapor are transferred to the primary ion beam 101, thereby converting the ionized beam into a neutral beam. The resulting neutral beam 106 which then exits the cell 102 through extraction slot 108.
• a ionized beam of boron is used, in conjunction with a target molecule of anisole.

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