EP0360932A1 - Source d'ions à micro-ondes - Google Patents

Source d'ions à micro-ondes Download PDF

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Publication number
EP0360932A1
EP0360932A1 EP88308974A EP88308974A EP0360932A1 EP 0360932 A1 EP0360932 A1 EP 0360932A1 EP 88308974 A EP88308974 A EP 88308974A EP 88308974 A EP88308974 A EP 88308974A EP 0360932 A1 EP0360932 A1 EP 0360932A1
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EP
European Patent Office
Prior art keywords
waveguide
section
microwave
ion source
source according
Prior art date
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Application number
EP88308974A
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German (de)
English (en)
Inventor
Norman A. Bostrom
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Individual
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Individual
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Publication date
Priority to US06/944,026 priority Critical patent/US4797597A/en
Application filed by Individual filed Critical Individual
Priority to EP88308974A priority patent/EP0360932A1/fr
Publication of EP0360932A1 publication Critical patent/EP0360932A1/fr
Withdrawn legal-status Critical Current

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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
    • H01J27/16—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation

Definitions

  • This invention relates generally to ion sources; that is, devices in which a feed material (usually a gas) is ionized by forming as plasma, from which an ion beam can be extracted. More particularly, the invention is concerned with so-called “microwave ion sources” in which the plasma is formed by microwave discharge in a magnetic field.
  • Ion sources are used commercially in equipment for implanting selected ions in materials such as semi-conductor wafers in the manufacture of electronics components. Proposals have also been made to use ion implantation for changing the physical properties of materials. For example, it has been found possible to improve properties such as corrosion and wear resistance and to change the fritional characteristics of metals by ion implantation.
  • an ion beam is extracted from the plasma and is refined (classified) and accelerated towards a target material into which ions are to be implanted.
  • the ion source should preferably have a long life and be capable of producing a high, stable current output. A well defined and stable beam is also desirable.
  • Prior art ion sources in commercial use have commonly used a hot cathode to produce electrons required in the ionization process. These hot cathode sources produce a stable current over a relatively long life time at low outputs of about 1 mA. At higher outputs of about 10 mA, however, these sources are subject to high frequency perturbations or "hash" in the ion beam thus reducing the uniformity of ion implantation and leading to rapid filament burn-out with consequently reduced source life time.
  • Microwave ion sources have been found to exhibit certain advantages over the hot cathode ion source.
  • the use of microwave energy to form the plasma avoids the need for a cathode; consequently the life of the sourse is not depen­dant on a consumable component.
  • Microwave ion sources have also been found to produce less "hash" at high ion densi­ties. Lower feed gas pressures are also possible, which results in less contamination of the plasma chamber and conservation of the feed material.
  • commercially available microwave generators of the type that have been extensively developed for microwave ovens can be used to provide the required microwave discharge.
  • Ridged waveguides are said to produce good microwave uniformity in the discharge chamber.
  • problems also arise in propagating microwave energy into the discharge chamber due to the fact that the chamber must be sealed from the waveguide and maintained under vacuum.
  • the sharp edges within the waveguide cause spurious discharges and consequent instability of the generated plasma, as well as unreliable on-off characteristics.
  • Using a ridged waveguide also requires large, awkward coils for producing the magnetic field in the discharge chamber. This is because the ridges create a physical barrier between the discharge chamber and the coils, which forces the coils away from the discharge chamber and thus requires that the coils be fairly large to create the required instense magnetic field. Using large coils also requires a higher power input.
  • U.S. Patent No. 4,409,520 attempts to overcome the problem of coupling microwave energy from the waveguide into the discharge chamber through a vacuum seal but still requires a tapered ridge to guide the microwaves into the discharge chamber.
  • This ridge is not only difficult to construct but the taper is a relatively inefficient means of coupling microwave energy into the discharge chamber, and requires a large volume of dielectric material to fill the waveguide in the discharge portion of the waveguide between the vacuum seal and the discharge chamber itself.
  • the ridges in the waveguide also require the use of the large coils to create the required magnetic field since the ridges are a physical barrier between the coils and the discharge chamber.
  • An object of the present invention is to provide an improved microwave ion source which offers advantages over the prior art.
  • the ion source provided by the invention includes a microwave generator and a waveguide having a first section to which the microwave generator is coupled for generating microwave radiation in the waveguide, a second section downstream of the first section in the direction microwave propagation along the waveguide, and in which a discharge chamber is defined, and a transformer section between the first and second sections.
  • Each of the first and second sections has a uniform rectangular internal cross-sectional shape throughout the length of the section.
  • each shape has a first dimension which is equal for both sections and is selected to at least approximate one-half of the nominal wavelength of the microwaves produced at the rated operating frequency of the generator, and a lesser, second dimension which is smaller in the second section of the waveguide than in the first section.
  • the transformer section is dimensioned to provide for transmission of microwaves from the first section to the second section substantially without impedance losses.
  • Means is provided between the first and second waveguide sections for providing a vacuum seal without impeding propagation of microwaves along the waveguide.
  • a liner of dielectric material forms the discharge chamber within the second waveguide section.
  • the ion source also includes means for generating a magnetic field in the discharge chamber, means for maintaining a vacuum in the chamber, means for introducing a feed material into the chamber for forming a plasma and means for extracting an ion beam from the chamber.
  • the beam extraction means includes a rectangular slit through which the mean is extracted from the discharge chamber and which is aligned with the retangular section of the waveguide.
  • a microwave generator 1 is shown connected to a waveguide 2 which includes a discharge chamber 3.
  • Waveguide 2 includes a first section 2a to which the generator 1 is coupled, a second section 2b which houses the discharge chamber 3, and an intermediate transformer section 2c.
  • Section 2b is maintained at a low pressure by a vacuum pump 5.
  • Microwaves propagate along the waveguide 2 in the direction indicated by the arrows in Fig. 1 and are concentrated in section 2b after passing through a transformer 6 forming waveguide section 2c, and a choke 7.
  • a magnetic field is generated in the discharge chamber 3 by magnets generally denoted 8.
  • Feed material is introduced into the discharge chamber 3 through a gas inlet 9. Inter­action of the feed material, microwave field and magnetic field creates a plasma in section 2b of the waveguide 2.
  • the plamsa is confined within the discharge chamber 3 by a liner 10 of dielectric material.
  • An ion beam generated by the ion source is extracted through an extraction slit 11 by extraction electrodes 12 and 13.
  • the ion beam is then accelerated in a convention acceleration chamber 14, refined by a conven­tional particle classifier 15 and directed onto a target in a target chamber 16.
  • the microwave generator 1 is connected to the waveguide 2 at an appropriate distance from the end of the waveguide so that the microwave energy reflecting from the end of the waveguide adds to the waves propagating directly from the microwave source away from the end of the wave­guide.
  • the microwave generator 1 may be of the type used in a commercial microwave oven and, for example, generates a microwave frequency of 2.45 GHz using 600 watts power. If a microwave generator of the type used in a microwave oven is used then the generator may be connected to the waveguide simply by inserting the antenna or probe of the generator into an opening in one of the sides of the waveguide as best seen in Fig. 2.
  • the waveguide 2 must be large enough to avoid unnecessary ionization of air molecules by the microwave electric field in the region of the waveguide near the microwave source. Such ionization would interfere with the propagation of microwaves.
  • the waveguide 2 must also be long enough to avoid interference from the intense magnetic fields at the discharge section 2b of the waveguide 2.
  • the waveguide 2 may be made of any good conducting material, but in the example shown has been constructed from aluminum.
  • a rectangular waveguide has been found to propa­gate microwaves well and to be easy to construct.
  • a rectangular waveguide is free of interior sharp edges and therefore avoids spurious microwave discharges which interfere with the operation of the source.
  • one dimension of the waveguide cross-section must be at least half ⁇ w-g where ⁇ w-g is the wave length of the microwave in the waveguide.
  • ⁇ w-g is the wave length of the microwave in the waveguide.
  • a half ⁇ w-g is approxi­mately 3.4 inches.
  • the height h (Fig. 3) of waveguide is taken to be exactly a half ⁇ w-g, in which case only the E10 microwave propagates along the waveguide in accordance with known electromagnetic wave theory.
  • Dimension h is uniform throughout the length of the waveguide.
  • the other dimension (width) of the waveguide is not critical but, as discussed above, must be wide enough to avoid excessive ionization of air molecules.
  • the width of the waveguide must also be less than the height so that only the E10 microwave propagates along the waveguide. It would therefore be a relatively simple matter to change the size of the microwave ion source by using microwave sources with different frequencies. Thus, if microwaves with a frequency of 10 GHz were used, a waveguide of height 1.8 inches would propagate the microwaves.
  • the electrodes are the vertical sides of the waveguide as indicated by refer­ence numeral 17 (Fig. 4) in the discharge section 2b of the waveguide, and 18 in the case of section 2a.
  • the distance between the electrodes 17 be smaller than the distance between the electrodes 18 to ensure that the microwave field has sufficient strength to create a high density plasma.
  • the distance between the electrodes 17 is .315 inches.
  • the magnitude of the E field in the discharge section 2b of the waveguide 2 will then be equal to the magnitude of the E field in waveguide section 2a times the ratio of the distance between the electrodes 18 to the distance between the electrodes 17.
  • the intensity of the microwave electric field in the discharge section 2b of the waveguide 2 increases by a factor of or more than 5 times compared with the intensity in the waveguide near the microwave generator. This increase in the magnitude of the E field allows a high ion density plasma to be generated in the discharge portion of the wave guide with low input power.
  • a quarter wave-length transformer 6 is used to transmit the microwave energy from waveguide section 2a to the discharge section 2c of the waveguide.
  • the transformer 6 will transmit 100% of the microwave energy between the waveguides if it is constructed as follows:
  • the length 19 of the transformer section 2c of the waveguide is equal to 1 ⁇ 4 ⁇ w-g.
  • the transformer 6 is shown as circular and in the form of a metal block. However, this construction is a matter of convenience only, since the essential part of the transformer is the interior conducting surface. Hence, the transformer could equally be made of sheet aluminum with flanges at its ends for connection to adjacent components, such as the flange 21 at the end of waveguide section 2a.
  • any microwave ion source must be maintained at low pressure to avoid contamination of the plasma with unwanted ions. It is clear that the wave­guide may be vacuum sealed all the way back to the microwave ion source 1. However, if such a large volume is to be maintained at a low pressure, a comparatively large vacuum pump is required.
  • the choke 7 is of essentially similar form to transformer 6 and is placed between the transformer 6 and discharge section 2b of the waveguide 2 and effectively forms part of that section. Choke 7 provides a vacuum seal in section 2b by incorporating a quartz panel or window 23. Window 23 allows propagation of microwaves without trans­mission losses. It may of course be made of any dielectric material instead of quartz, with suitable changes in dimensions.
  • choke 7 has a rectangular recess 25 for receiving the quartz window 23; the recess is dimensioned so that the outer face of the window 23 lies flush with the surface 26 of the choke 7.
  • Choke 7 also has slots 27 at right angles to recess 25 for receiving rectangular quartz plates 28 which extend perpendicular to the quartz window 23 and in effect form side flanges of the windows in the assembled waveguide.
  • the quartz window 23 and quartz flanges 28 are designed so that lengths 29 and 30 (Fig. 6) are each 1 ⁇ 4 ⁇ w-g. Since a standing wave is formed in slots 27 by this design, no current flows at corners 31; hence there is no impedence to the microwave propagation across the choke 7 into the discharge section 2b of the waveguide.
  • the discharge section 2b has a rectangular cross-section with one dimension 1 ⁇ 2 w-g and other dimension .315 inches, and is made of aluminum.
  • This portion of the waveguide may be made as long as is convenient, but should be sufficiently long to allow place­ment of the magnets 8 closely adjacent the sides of the waveguide.
  • the waveguide may be made of any good conductor (e.g. copper) but aluminum is chosen for convenience.
  • Both waveguide sections 2a and 2b are fabricated from sheet aluminum and have rectangular box-shaped centre portions. In the case of section 2a, one end of the centre portion is closed by an end plate while the other end is fitted with flange 21.
  • the centre portion of section 2b has flanges 22, 34 at both ends.
  • the flanges are of circular shapes selected to match the circular shapes of transformer 6 and choke 7. The assembly forming the waveguide is held together by bolts through the flanges as best seen in Fig. 4.
  • vacuum sealing choke 7 may be located anywhere between discharge chamber 3 and trans­former 6, but is located adjacent transformer 6 for convenience of construction.
  • Gas inlet 9 permits introduction of feed material into the discharge section 2b of the waveguide 2.
  • gas inlet 9 takes the form of a needle valve.
  • a liner 10 of dielectric material 10 fits loosely inside the discharge section of the waveguide and is formed with a recess which defines the discharge chamber 3.
  • the dielectric material is of boron nitride.
  • inlet 9 is located so that feed material can be delivered directly into the discharge chamber 3.
  • Discharge chamber 3 has a rectangular shape in cross-section, which is desirable for extracting ribbon-­shaped ion beams.
  • the dimen­sions of the discharge chamber are 1.50 inches x 1.50 inches x 0.20 inches.
  • the dielectric insert 10 is constructed in two halves as shown in Fig. 5, with the plane of bisection between the two halves vertical and parallel to the direction of the propagation of micro­waves. The abutting faces of the two halves are stepped as shown to provide impedance to stray ions.
  • Insert 10 protects the electrodes 17 of the discharge section 2b of the waveguide from being bombarded with ions and electrons in the plasma formed in the discharge chamber 3.
  • the dielectric material also confines the plasma to the small volume of the discharge chamber 3.
  • the microwave electric field established between the electrodes of the discharge section 2b of the waveguide 2 is relatively uniform and therefore provides a relatively uniform plasma.
  • the concentration of microwave electric energy between the narrow electrodes 17 provides sufficient energy to create a high ion density plasma from a feed material introduced through gas inlet 9. Insert 10 also reduces the volume which must be maintained at a low pressure.
  • An end plate 33 is connected to flange 34 at the outer end of waveguide section 2b (Fig. 2).
  • An extraction slit 11 is provided in plate 33.
  • the extraction slit has a rectangular shape and is oriented with its edges parallel to the walls of discharge chamber 3.
  • the edges of the extrac­tion slit 11 are cut at an angle of 128° to the outer face of the plate 33 in the preferred embodiment (Fig. 6), although the exact angle is not critical.
  • Plate 33 is made from a good conducting material such as mild steel and acts as an extraction electrode. Like the rest of the ion source assembly it is maintained at a high electrical potential. Plate 33 fits vacuum tightly with the end of acceleration chamber 14.
  • the extraction slit has dimensions of 0.75 inches x 0.0312 inches, and depth approximately .020 inches so that a stable ion beam may be extracted from the plasma generated in the discharge chamber 3, as is known in the art.
  • extraction electrode 12 is negatively charged with respect to discharge chamber 3 while extraction electrode 13 is grounded.
  • the two electrodes are placed adjacent the extraction slit 11 to extract positive ions from the plasma generated in the discharge chamber 3.
  • the extracted ions are then accelerated through a known acceleration chamber 14, through known particle analyzer 15 towards a known target chamber 16.
  • Vacuum pump 5 is connected to the acceleration chamber 14 and evacuates the discharge section 2b of the waveguide through the slit 11. As is known, evacuation of a relatively large volume through a slit is inefficient; hence it is necessary to keep the discharge section 2b of the waveguide as small as may be allowed by the requirement of allowing the magnets 8 to be placed close to the discharge chamber 3.
  • the magnets 8 are placed adjacent the discharge section 2b of the waveguide so that the magnetic field lines indicated at 35 are Fig. 6, are perpendicular to the micro­wave electric field.
  • a non-uniform magnetic field in the discharge chamber of an ion source allows the efficient absorption of microwave energy by the plasma.
  • the magnetic field generated by the magnets 8 must have an intensity greater than 890 gauss, being the electron cyclotron resonant field for a 2.45 GH z electromagnetic field.
  • the magnetic field required for a miniaturized source, in which the frequency of the microwave field was higher, would have to have higher strength in accordance with known electromagnetic theory.
  • the field thus produced causes the electrons in the plasma to spiral along the magnetic field lines and this reduces the number of collisions of the electrons with the walls of the discharge chamber 3, besides increasing the density of the plasma. In this manner the dielectric insert 10 will last longer, as is well known.
  • Permanent magnets or electro-magnets may be used. In either case, the magnets can easily fit between the flanges 34 and 22 of the discharge section 2b of the waveguide 2 as shown in Fig. 4.
  • Magnets 8 are floated at the same potential as the ion source assembly so as to avoid spurious discharges between the magnets and the waveguide.
  • FIG. 6 illustrates a further, but optional feature of the invention.
  • a charged rod or wire 36 may be inserted through the discharge chamber to intensify the plasma near the extraction slit.
  • the rod 36 is located a few millimeters behind and parallel to the slit 11.
  • the rod 36 creates a strong magnetic field near the extraction slit which intensifies and homogenizes the plasma by confining the electrons in that region.
  • the rod 36 also creates a physical barrier to neutral ions migrating towards the extraction slit and therefore improves the vacuum in the vicinity of the slit.
  • the improvement in vacuum also aids in avoiding breakdown of the plasma near the extraction slit where voltages are high (in the order of 80,00 volts near the gap).
  • the charged rod 36 must be oriented parallel to the length of the slit; that is, perpendicular to the microwave electric field, otherwise the field of the wire will interfere with the microwave electric field thereby reducing the uniformity of the electric field and thus reducing the uniformity of the plasma.
  • a charged helical wire may be used in place of rod 36.
  • m2 has been obtained from the ion source as described using permanent magnets and using 200 watts power. Power consump­tion may rise to 1800 watts if magnetic coils are used.
  • Fig. 7 is similar to the lefthand end part of Fig. 3 but showing an embodiment of the invention in which the flange 34 (Fig. 3) at the outer end of the discharge section of the waveguide is omitted and the end plate in which the extraction slit 11 is formed is attached directly to the open outer end of the waveguide; also, the two bar magnets 8 shown in Fig. 3 are replaced by a single large annular magnet that encircles the discharge portion of the wave­guide. In Fig. 7, this magnet is denoted by reference numeral 40 while the discharge section of the waveguide is denoted 2b′.
  • the annular magnet itself is a commercially available magnet. In some cases, it may be desirable to use a magnet of this form because it is generally easier to obtain the relatively high magnetic field strength required within the discharge section 2b than with bar magnets.
  • the end plate in which the extraction slit 11′ is formed in this case takes the form of a rectangular steel plate 42 that is secured directly to the open outer end of the waveguide section 2b′ by screws, one of which is indicated at 44, extending through openings 46 in plate 42 and received in tapped holes in the outer end face of the waveguide discharge section 2b′. Those holes are indicated at 47.
  • This form of end plate and its method of attachment to the waveguide represent a simplification in manufacture as compared with the preceding embodiment.
  • Figs. 8 and 9 show a further alternative embodi­ment in which rare earth bar magnets are used.
  • the magnets may, for example, be europium/cobalt magnets available from commercial sources. These magnets provide high field strengths and it has been found possible to construct an ion source of reduced size and weight using magnets of this type as compared with the preceding embodiments.
  • the waveguide discharge section 2b ⁇ is essentially the same as previously described and is fitted with an end plate 42 ⁇ which is generally the same as the end plate 42 of Fig. 7.
  • Four rare earth bar magnets, individually denoted 48, are placed in pairs above and below the waveguide discharge section 2b ⁇ with faces of opposing polarity in contact as shown in Fig. 9. The magnets are in effect clamped against the end plate 42 ⁇ by a clamp plate 50 that embraces the waveguide discharge section rearwardly of the end plate.
  • Plate 50 effectively has a central rectangular opening through which the waveguide extends but in practice may be formed into pieces, for example, as a generally C-shaped piece that will fit against one side of the waveguide and the limbs of which will extend above and below the waveguide, and a bar disposed at the opposite side of the waveguide and secured to the outer ends of the limbs of the other part of the plate by bolts.
  • Plate 50 may be frictionally clamped about the discharge section 2b ⁇ of the waveguide or may be secured to the end plate 42′ by bolts either extending through the magnets or disposed just outwardly of the magnets (in which case the magnets will have to be somewhat shorter than the overall width of the plates).
  • adhesives may be used to secure together the "sandwich" comprising the two plates and the four magnets.
  • the magnets form a magnetic circuit through the two plates 42′ and 50.
  • the fact that plate 50 in effect has a central opening where the waveguide extends through the plate creates the required non-uniformity in the field.
  • the preceding description relates to a particular preferred embodiment and that many modifications are possible within the broad scope of the invention.
  • the particular materials referred to previously may of course vary.
  • certain of the dimensions of the waveguide may also change, and may particularly change in accordance with the frequency of microwave generator used.
  • certain dimensions of the interior of the waveguide are critical, such as at the microwave transformer, the exterior shape is largely a matter of choice.
  • the actual voltages and power used may be varied depending on the desired nature of the ion beam.
  • gases or vapours may be used in the ion source according to the form of ion beam required.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Combustion & Propulsion (AREA)
  • Electron Sources, Ion Sources (AREA)
EP88308974A 1986-12-22 1988-09-28 Source d'ions à micro-ondes Withdrawn EP0360932A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US06/944,026 US4797597A (en) 1986-12-22 1986-12-22 Microwave ion source
EP88308974A EP0360932A1 (fr) 1988-09-28 1988-09-28 Source d'ions à micro-ondes

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP88308974A EP0360932A1 (fr) 1988-09-28 1988-09-28 Source d'ions à micro-ondes

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EP0360932A1 true EP0360932A1 (fr) 1990-04-04

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EP88308974A Withdrawn EP0360932A1 (fr) 1986-12-22 1988-09-28 Source d'ions à micro-ondes

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112655068A (zh) * 2018-07-13 2021-04-13 Mks仪器有限公司 具有带有改进的等离子抗性的介电等离子室的等离子源

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4041420A (en) * 1976-06-30 1977-08-09 Riblet Henry J Shunted stepped waveguide transition
US4058748A (en) * 1976-05-13 1977-11-15 Hitachi, Ltd. Microwave discharge ion source
GB2053559A (en) * 1979-06-04 1981-02-04 Hitachi Ltd Microwave plasma ion source
US4393333A (en) * 1979-12-10 1983-07-12 Hitachi, Ltd. Microwave plasma ion source
EP0154824A2 (fr) * 1984-03-16 1985-09-18 Hitachi, Ltd. Source d'ions

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4058748A (en) * 1976-05-13 1977-11-15 Hitachi, Ltd. Microwave discharge ion source
US4041420A (en) * 1976-06-30 1977-08-09 Riblet Henry J Shunted stepped waveguide transition
GB2053559A (en) * 1979-06-04 1981-02-04 Hitachi Ltd Microwave plasma ion source
US4393333A (en) * 1979-12-10 1983-07-12 Hitachi, Ltd. Microwave plasma ion source
EP0154824A2 (fr) * 1984-03-16 1985-09-18 Hitachi, Ltd. Source d'ions

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112655068A (zh) * 2018-07-13 2021-04-13 Mks仪器有限公司 具有带有改进的等离子抗性的介电等离子室的等离子源
CN112655068B (zh) * 2018-07-13 2024-04-12 Mks仪器有限公司 具有带有改进的等离子抗性的介电等离子室的等离子源

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