EP0000843B1 - Plasma discharge ion source - Google Patents

Plasma discharge ion source Download PDF

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Publication number
EP0000843B1
EP0000843B1 EP78300268A EP78300268A EP0000843B1 EP 0000843 B1 EP0000843 B1 EP 0000843B1 EP 78300268 A EP78300268 A EP 78300268A EP 78300268 A EP78300268 A EP 78300268A EP 0000843 B1 EP0000843 B1 EP 0000843B1
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EP
European Patent Office
Prior art keywords
plasma
anode
ions
magnetic field
species
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP78300268A
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German (de)
English (en)
French (fr)
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EP0000843A1 (en
Inventor
Norman Williams
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
AT&T Corp
Original Assignee
Western Electric Co Inc
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Filing date
Publication date
Application filed by Western Electric Co Inc filed Critical Western Electric Co Inc
Publication of EP0000843A1 publication Critical patent/EP0000843A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/08—Ion sources; Ion guns using arc discharge
    • H01J27/14—Other arc discharge ion sources using an applied magnetic field

Definitions

  • This invention relates to ion sources, and particularly to the type of ion source in which a compound of the material of a desired ion is dissociated in a plasma discharge process to provide a beam of charged particles.
  • the beam includes the desired ions, which are generally subsequently separated from the beam by mass-charge separation techniques.
  • Plasma ion sources are well known, see, for example, Fig. 2 of U.S. Patent 2 373151, issued April 10, 1945.
  • One problem with prior art plasma dissociation ion sources is that it has not been known how to control fully the dissociation process, whereby the proportion of the desired ion in the output current is generally significantly less than what, at least, would appear to be possible.
  • singly charged boron ions are desired from a source gas of a compound of boron
  • the total quantity of boron in the desired ionic form has, heretofore been significantly less than the total quantity of boron present in the gas. That is, because it has not been known how to control fully the extent and completeness of the dissociation process, most of the boron present in the gas remains tied-up in non-useful molecular and electrically neutral forms.
  • the source comprises an anode in the form of a chamber through which a cathode extends.
  • an electrical discharge is maintained between the cathode and the anode to produce a plasma and a magnetic field is applied to the plasma generally parallel to the direction of the cathode.
  • the paper is concerned with reducing sputter erosion of the cathode and to achieve this end a number of recommendations are made of which one is to provide electron mirrors (e.g. of tantalum) at each end of the chamber to inhibit electrons from the plasma from reaching the ends of the chamber.
  • Other recommendations include reducing the magnitude of the magnetic field so that electrons reach the side wall of the chamber before they reach the end.
  • the present inventor has discovered that the proportions of the various ions in the ionic output current depends on the ion source plasma temperature and that the lack of control over the dissociation process in known ion sources is due to the limited maximum plasma temperature which could be obtained. Furthermore a significant mechanism whereby the plasma loses energy and which therefore limits its maximum temperature is axial drift of electrons to the ends of the anode chamber. Accordingly, by providing suitable means for restraining the drift of electrons to the ends of the anode chamber and by suitably selecting the magnitudes of the discharge voltage and the magnetic field, a higher plasma temperature, and thus a higher proportion of one species of ion relative to the others, can be obtained.
  • Ion sources which rely upon the plasma dissociation of a gaseous source material are well known.
  • a known source 10 is shown as comprising a generally closed cylindrical anode 12 of, for example, graphite or tantalum, having disposed therein (see also Fig. 4) an axially extending electrical resistance heated filamentary cathode 14.
  • the source 10 is contained in an evacuated chamber (not shown), and a gaseous compound of the desired ionic material is passed through the anode between an input tubing 15 and an exit slit- like opening 16.
  • a steady voltage differential is established between the anode and the cathode, the voltage being of sufficient amplitude to cause an electric discharge through the gas between the cathode and the anode.
  • the electric discharge causes a dissociation of the gas into various neutral and charged particles.
  • the neutral particles exit as part of the gas flow through the slit 16, and the charged particles, both positive and negative, fill the space within the anode 12.
  • Positively charged particles which drift close to the slit 16 are extracted from the anode 12 and are accelerated by an electric field external to the source 10 to provide the beam of charged particles.
  • the desired particles are separated from this beam using known mass-charge separation techniques.
  • a magnet 18 is used to provide an axial magnetic field (represented by the dashed lines 19) about and within the anode 12.
  • Such axial field tends to increase the path length of the plasma electrons, and thus the plasma density, by inducing the electrons to circle about the cathode rather than proceeding relatively directly from the cathode towards the anode.
  • an additional magnetic field is present which causes the electrons to drift axially along the length of the anode towards the anode axial ends 30 where the electrons are collected. The importance of this electron axial drift is discussed hereinafter.
  • boron trifluoride boron trifluoride
  • Mass spectrographic analysis of the ionic beam produced using this source material reveals the presence of the desired boron ions, but also such ions as BF + and BF 2 + , with the proportion of the desired singly charged boron ions to the total beam current (depending upon the particular ion source used) being generally less than 15 percent. That is, although the ion current contains much boron, much of it is tied up with fluorine atoms in non-useful forms.
  • the present inventor has discovered that the proportion of the various ions in the ion beam is a function of the temperature of the ion source plasma, and that the proportion of a selected ion of the beam current can be optimised to an extent not heretofore possible by control and selection of the plasma temperature. This is explained as follows.
  • the output beam from the ion source contains all the different positive ions produced in the dissociation process.
  • the present inventor has demonstrated, however, that the proportion of these different ions in the beam depends upon the statistical probability or rate of occurrence of the different types of possible collisions, that is, upon the probability that certain fragments will be produced in the dissociation process, and upon the probability that these fragments will collide with electrons of sufficient energy to cause ionisation thereof.
  • Such probabilities are a function of the dissociation and ionisation energies of the impacted particles and a function of the energy of the impacting electrons.
  • Figs. 2 and 3 show the proportional composition of the ion beam from an ions source of the type shown in Fig. 1 plotted against the plasma temperature in electron volts.
  • Fig. 2 is for a source material of boron trifluoride
  • Fig. 3 is for boron trichloride.
  • the data for these graphs were derived mathematically, and owing to certain assumptions made to simply the calculations, it is expected that certain inaccuracies exist. Experimental data do exist, however, which support the general validity of the relationships shown.
  • a desired proportion of any ion in the ion beam can be obtained, within the possible range of proportions of the ion, by adjusting the temperature of the plasma to the corresponding plasma temperature indicated on the graph.
  • the maximum proportion of singly charged chlorine ions (CI + ) in an ion beam produced from a source gas of boron trichloride is obtained at a plasma temperature of about 1.0 eV.
  • the curves representing the proportions of singly charged boron ions (B + ) begin peaking at a plasma temperature of about 1.5 eV for both source gases (Figs. 2 and 3).
  • the plasma temperature can be adjusted by varying the axial magnetic field strength and/or the anode to cathode discharge voltage. Because the plasma temperature is not strictly an independent variable, being a function of the plasma density and the particular source gas material used, a trial and error plasma temperature varying process can be used.
  • the plasma electrons tend to drift axially along the length of the anode 12. Those electrons which reach the anode axial ends 30 are collected by the anode and are thus removed from the plasma. Because the electrons of highest energy and thus of highest velocity drift the fastest, the higher energy electrons are removed more quickly from the plasma than the lower energy electrons. The result of this is that a disproportionately large number of higher energy electrons is removed from the plasma by collection at the anode axial ends. This tends to reduce the energy distribution of the electrons of the plasma and thus reduce the plasma temperature. Accordingly, one means for increasing the plasma temperature is to reduce the collection of electrons at the anode axial ends.
  • this is accomplished by modifying the shape of the magnetic field to improve the magnetic "bottle" characteristics of the field.
  • Fig. 4 which shows a magnetic field (indicated by the dashed lines 32) which is more concentrated or constricted at the axial ends 30 of the anode 12 than at the centre thereof.
  • the effect of such a magnetic field shape is to turn back or "reflect" electrons which are drifting from the central, lower strength regions of the field towards the higher strength axial ends of the field.
  • the end constricted magnetic field tends to reduce the drift of electrons towards the axial ends of the anode 12 and to thus reduce the collection of electrons thereat. As aforenoted, such reduction of electron collection causes an increase in the temperature of the plasma.
  • One means for providing the desired constricted magnetic field of the shape shown in Fig. 4 is by the use of two discs 34 (Fig. 5) of magnetic material, such as steel, disposed closely adjacent to each axial end 30 of the anode 12.
  • the constricting effect of the discs 34 on the magnetic field produced by the magnet 18 is evident by comparison of the arrangement shown in Fig. 5 with the prior art arrangement shown in Fig. 1.
  • the mirror ratio of the magnetic field in the arrangement shown in Fig. 5 is 1.35, whereas the mirror ratio of the prior art arrangement shown in Fig. 1 is 1.17.
  • the maximum content of the singly charged boron ion in the output beam heretofore obtainable is about 15 percent with a source gas of boron trifluoride, and about 6 percent with a source gas of boron trichloride.
  • These boron contents correspond to a plasma temperature of about 1.0 eV with the boron trifluoride source gas (Fig. 2), and about 0.85 eV (Fig. 3) with the boron trichloride source gas.
  • the proportion of singly charged boron ions in the output beam is increased to about 25 percent for the boron trifluoride source gas and to about 10 percent for the boron trichloride source gas.
  • These increases in the proportion of the boron ions in the two output currents correspond to an increase of plasma temperature of about 0.1 eV.
  • a means for further improving the mirror ratio of magnetic fields for increasing the plasma temperature in ion sources of the type herein described is the substitution of two disc-like permanent magnets (not illustrated) for the steel discs 34 shown in Fig. 5. By proper spacing of such permanent magnets (which would also replace the external magnet 18), a mirror ratio of about 15 is considered possible. An example of such proper spacing is provided hereinafter.
  • a difficulty with the disc permanent magnet arrangement is that by disposing the permanent magnets close to the anode 12, in order to obtain the necessary magnetic field shaping, the magnets are subject to being heated by radiation from the anode which operates at a quite high temperature. Thus, unless special precautions are taken, such as water cooling of the permanent magnets, overheating of the magnets and destruction of the magnetic properties thereof can occur.
  • refractory metal shields 36 for example, of tantalum
  • the shields 36 in use, the shields 36, at filament potential, electrostatically shield the anode axial ends 30 from the plasma and thus reduce the collection of electrons by these portions of the anode. Accordingly, for the same reasons previously described in connection with the description of the embodiment of the invention shown in Fig. 4, the plasma temperature is increased.
  • Each of the aforedescribed embodiments of the invention is effective to increase the maximum attainable plasma temperature.
  • Such maximum plasma temperatures are obtained at an optimum setting, determined by a trial and error process, of the magnetic field strength and the anode to cathode discharge voltage. Adjustment of the plasma temperature to less than the maximum possible temperature is possible by adjustments away from the optimum settings of the magnetic field strength and/or the discharge voltage.
  • the ion source is identical to the prior art ion source 10 shown in Fig. 1.
  • the anode 12 has a length of about 7.5 cm and a diameter of about 2.54 cm.
  • the magnets 18 have a diameter of about four inches (10 cm), and are spaced about 7.5 cm from the axial ends 30 of the anode 12.
  • the discs 34 have a thickness of about 0.62 cm a diameter of about 3.75 cm, and are spaced about 1.8 cm from the anode.
  • the permanent magnets can be of identical dimensions and spacings from the anode 12 as aforedescribed for the discs 34.
  • the maximum plasma temperature heretofore obtainable is about 1.0 eV with a source gas of boron trifluoride and about 0.85 eV with a source gas of boron trichloride.
  • increases in the plasma temperature, and corresponding increases of the boron ion content of the output beam are obtained, according to one aspect of this invention, by the use of magnetic fields having a mirror ratio in excess of 1.2.
  • increases in the boron ion proportions are obtained by the use of plasma temperatures in excess of 1.0 eV with a source gas of boron trifluoride and in excess of 0.85 eV with a source gas of boron trichloride.
  • the invention thus finds an application in the production of singly charged boron ions for ion implantation processes such as are employed in the manufacture of semiconductor devices.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Combustion & Propulsion (AREA)
  • Electron Sources, Ion Sources (AREA)
EP78300268A 1977-08-08 1978-08-08 Plasma discharge ion source Expired EP0000843B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US05/822,866 US4139772A (en) 1977-08-08 1977-08-08 Plasma discharge ion source
US822866 1977-08-08

Publications (2)

Publication Number Publication Date
EP0000843A1 EP0000843A1 (en) 1979-02-21
EP0000843B1 true EP0000843B1 (en) 1981-03-11

Family

ID=25237181

Family Applications (1)

Application Number Title Priority Date Filing Date
EP78300268A Expired EP0000843B1 (en) 1977-08-08 1978-08-08 Plasma discharge ion source

Country Status (6)

Country Link
US (1) US4139772A (enrdf_load_stackoverflow)
EP (1) EP0000843B1 (enrdf_load_stackoverflow)
JP (1) JPS5429970A (enrdf_load_stackoverflow)
CA (1) CA1102931A (enrdf_load_stackoverflow)
DE (1) DE2860523D1 (enrdf_load_stackoverflow)
IT (1) IT1121501B (enrdf_load_stackoverflow)

Families Citing this family (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4542321A (en) * 1982-07-12 1985-09-17 Denton Vacuum Inc Inverted magnetron ion source
JPH0746593B2 (ja) * 1983-08-15 1995-05-17 アプライド マテリアルズ インコーポレーテッド イオン打込み用大電流イオンビーム発生方法及びイオン打込み装置
EP0154824B1 (en) * 1984-03-16 1991-09-18 Hitachi, Ltd. Ion source
US4774437A (en) * 1986-02-28 1988-09-27 Varian Associates, Inc. Inverted re-entrant magnetron ion source
US4760262A (en) * 1987-05-12 1988-07-26 Eaton Corporation Ion source
US4891525A (en) * 1988-11-14 1990-01-02 Eaton Corporation SKM ion source
US5449920A (en) * 1994-04-20 1995-09-12 Northeastern University Large area ion implantation process and apparatus

Family Cites Families (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2373151A (en) * 1942-07-29 1945-04-10 Cons Eng Corp Analytical system
US2427484A (en) * 1943-10-22 1947-09-16 Stanolind Oil & Gas Co Ionic gas analysis
US2829259A (en) * 1954-08-13 1958-04-01 Samuel N Foner Mass spectrometer
US2826708A (en) * 1955-06-02 1958-03-11 Jr John S Foster Plasma generator
US2831996A (en) * 1956-09-19 1958-04-22 Eugene F Martina Ion source
NL266057A (enrdf_load_stackoverflow) * 1960-06-21 1964-03-10
FR1346091A (fr) * 1962-01-30 1963-12-13 Ass Elect Ind Nouveau spectromètre de masse
FR1459469A (fr) * 1965-11-29 1966-04-29 Atomic Energy Commission Procédé et appareil pour la production d'un plasma complètement ionisé
US3500077A (en) * 1967-12-19 1970-03-10 Atomic Energy Commission Method and apparatus for accelerating ions out of a hot plasma region
FR1598559A (enrdf_load_stackoverflow) * 1968-12-20 1970-07-06
GB1414626A (en) * 1971-11-24 1975-11-19 Franks J Ion sources
US3900585A (en) * 1972-02-12 1975-08-19 Agency Ind Science Techn Method for control of ionization electrostatic plating
JPS5148097A (en) * 1974-10-23 1976-04-24 Osaka Koon Denki Kk Iongen

Also Published As

Publication number Publication date
JPS6130372B2 (enrdf_load_stackoverflow) 1986-07-12
CA1102931A (en) 1981-06-09
IT7826513A0 (it) 1978-08-04
DE2860523D1 (en) 1981-04-09
IT1121501B (it) 1986-04-02
JPS5429970A (en) 1979-03-06
US4139772A (en) 1979-02-13
EP0000843A1 (en) 1979-02-21

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