WO2002063653A1

WO2002063653A1 - Ion source for ion implantation

Info

Publication number: WO2002063653A1
Application number: PCT/US2002/003258
Authority: WO
Inventors: Thomas N. Horsky
Original assignee: Semequip, Inc.
Priority date: 2001-02-07
Filing date: 2002-02-05
Publication date: 2002-08-15

Abstract

An ionization source (42) for an ion implantation system (43) includes an ionization chamber (44) having a plurality electron guns (70).

Description

ION SOURCE FOR ION IMPLANTATION

This invention relates to ion implantation, and more particularly to ion sources and to ion implanter systems that incorporate such sources.

BACKGROUND

The following patent applications, herein incorporated by reference, describe the background of this invention: PCT Application Serial No. (not yet available), inventor Thomas N. Horsky, filed December 13, 2000, entitled Ion Implantation Ion Source, System and Method and U.S. Application Serial No. (not yet available), filed November 30, 2000 entitled Electron Beam Ion Source with Integral Low Temperature Vaporizer. The referenced applications, for U.S. purposes, have the benefit of the date of my U.S. Provisional Application Serial No. 60/170,473 filed December 13. 1999, also incorporated by reference.

SUMMARY I describe electron-impact ion sources useful for producing ions from gaseous feed materials such as those containing elements (e.g., B, As, P, Ge, Si, In, Sb) which are commonly used as electrically active dopants in semiconductors. These elements are commonly introduced into Si, Ge and GaAs wafers by ion implantation. The ion source designs are different from conventional ion implanter ion source designs in that the ions are produced by direct electron impact of a primary electron beam of a controllable and variable electron energy, rather than by sustaining an arc discharge. The electron energy is selected to provide the highest yield of the ions of interest. The ion source designs will be especially useful to produce molecular ions such as B10Hx+ from decaborane vapor (B10H14) which is vaporized from solid decaborane material by a vaporizer, and introduced into the ion source as decaborane vapor. Molecular species such as decaborane experience dissociation when ionized in an arc discharge source, such that the B10Hx+ ion is not generally preserved, but rather fragments of lower order boranes BxHy+ are the dominant species (e.g., B2H6+) produced by the arc. The electron-impact ion sources provided herein are suitable to be retrofit into the existing fleet of ion implanters which are currently used in chip manufacturing, e.g. in the manufacture of Complimentary Metal-Oxide-Semiconductor (CMOS) devices, in which the transistor structures are created by ion implantation, in conjunction with masking techniques. In fulfilling this retrofit feature, the electron impact ion source preserves the ion optical design of ion implanters which extract ions produced in the ion source from an approximately 3.5mm- wide by 50mm-long vertical slot (the ion extraction aperture) on the side of the ion source which faces the extraction electrode of the ion implanter. To accomplish this, all of my ion source designs which are described herein form a narrow, extended electron beam which roughly matches the profile of the extraction aperture.

DESCRIPTION OF DRAWINGS

Fig. 1 is a general schematic ion of an ion implanter;

Fig. 2 shows in cross-section an ion source embodiment having a part of a proposed election guns. Fig. 3 is a cross-section on an enlarged scale of an electron gun shown in Fig. 2.

Fig. 4A is a side cross-sectional view and Fig. 4B a top view of an ion source in which the electron gun emits in the same direction as the direction of extraction of the ions.

Fig. 5 is a view of the key components of a further design of the general kind illustrated in Figs. 4A and 4B, but with certain enhancements. Fig. 6A is a side cross-sectional view. Fig. 6B a top view and Fig. 6C and perspective view of another electron beam gun and ion source.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Fig. 1 is a general schematic of an ion implanter such as is used for conventional boron implantation. Process gas and power connections 48 are fed into the ion source 42, which is maintained at high voltage with respect to the source vacuum housing 49 by dielectric bushing 52. The source housing and beam line are evacuated by high vacuum pumps 50 and 51, respectively. The ion source 42 produces ions which are extracted from a one-dimensional aperture (i.e., an elongated slot) and accelerated to a transport energy which may be equal to the desired implantation energy (in the case of a conventional high current implanter), or may be accelerated to a transport energy significantly greater than the desired final implantation energy, by electrode 53. The extracted ions are injected into analyzer magnet 43 which disperses the beam laterally according to the mass-to-charge ratio of the ions. A mass resolving aperture (slot) 44 allows only the ion of interest (the ion having a preselected mass-to-charge ratio) to pass downstream to a moveable Faraday for measuring ion beam current, or (when the Faraday is retracted) to the wafers on substrate holder 55. A portion of the beam is sampled by Faraday 47. In the case of an accel-decel implanter, in which the ions are extracted at a higher energy than the desired final implantation energy, the ions pass through a deceleration electrode 57 which decelerates the ions to their desired final energy. In still other implanter designs, electrode 57 is an acceleration electrode which adds additional energy to the ions for higher energy implantation, and the implanter beam line upstream of electrode 57 is isolated above ground (above the substrate potential) by isolation bushing 59. The schematic of Fig. 1 illustrates a batch-style implanter with a mechanically rotating and scanning disk 45, but the general approach of decel can also be adopted in serial implanters, as can the non-decel approach. The ion source designs being disclosed here are also not limited to batch implanters, but are also suitable for serial implanters which process one wafer at a time. In Fig. 1, the implanter volume occupied by the ion source is indicated by dotted lines as retrofit volume 60; this is the implanter volume which can be occupied by the footprint of a retrofitted ion source, without other structural change of the implanter.

Fig. 2 shows an ion source similar in design to one described in the referenced PCT application. The principal difference in the design of Fig. 2 from that shown in the PCT application is the provision of two small electron guns 42 and 43, arranged antiparallel to each other, with their optical axes oriented parallel with, off-set from the rectangular extraction slot of the extraction aperture 46. For clarity, I will describe some of the individual elements of the ion source. The external vaporizer 28 is comprised of vaporizer body 30 and crucible 31 in which solid source feed material 29 such as decaborane resides. Heater 26 and cooling element 27 are in intimate contact with vaporizer body 30, and are used to provide a uniform operating temperature above room temperature to the crucible 31. Thermal continuity between the crucible 31 and the temperature-controlled vaporizer body 30 is provided by pressurized gas introduced by gas feed 20, while the temperature of the vaporizer is monitored through thermocouple 25. Vaporized decaborane or other vaporized material is fed into the ionization chamber 44 through conductance channel 32. The source mounting flange 36 and source block 35 are also temperature controlled to a temperature near or above the vaporizer temperature. Ionization chamber 44 is in good thermal contact with block 35 through pressurized gas conducted through conduit 34 into the interface between ionization chamber 44 and block 35. Gaseous materials, for example available in gas cylinders, can be fed into the ionization chamber 44 through gas feed line 33. Typically, the gas pressure within the ionization chamber 44 is in the range of 1x10-3 Torr, while the region external to the ionization chamber 44 is in the range of 1x10-5 Torr or less. The electron beams produced by the opposed electron guns 42 and 43 enter the ionization chamber 44 through respective electron entrance apertures 45, and transit the ionization chamber 44 parallel to and in close proximity to the extraction aperture slot contained within aperture plate 46. Typical dimensions for these structures are a 7.5mm diameter round aperture for the electron entrance apertures 45, 25mm diameter by 65mm long (dimension "B" ) electron gun assemblies 42 and 43, and a height of 67mm (dimension "A" ) for the ionization chamber 45. The overall length "C" of the ion source assembly from the base of the mounting flange to the face of the ion extraction aperture is such that it replicates the length of the ion source originally shipped with the particular ion implanter into which the ton source is adapted to be retrofit, so that the ion optics of the implanter are preserved. (This dimension varies depending upon the make and model of the pre-existing ion implanter.) Likewise, the overall height "D" of the assembly must fit within the retrofit volume 60 of the ion implanter. In this case, "D" includes a source shield 41, a cylindrical metal enclosure which protects the several internal components of the ion source assembly, such as electron guns 42 and 43 and also the electrical harnesses 39, 40 which lead to electrical feedthroughs 37, 38. Importantly, cutouts 48, 49 in the source shield 41 enable the portion of the electron gun assembly which contains the cathodes (shown in Fig. 3) to be exposed to the vacuum environment of the source housing, extending the lifetime of the cathodes. There is also a cutout in the front of the source shield 41 to accommodate the extraction aperture plate, which must be exposed to the extraction electrode 53 of the implanter. Some of the advantages of this dual e-gun ion source design, relative to the related single e-gun design disclosed in the above-referenced PCT application, include: 1) Improved uniformity of the electron charge density profile across the ion extraction aperture, resulting in improved uniformity of the ion current density along the ion extraction aperture; 2) Increased electron beam current injected into the ionization chamber (up to a factor of two), resulting in a commensurate increase in ion current produced; 3) A single gun can be operated while the second gun is kept as a spare, to increase the required interval between source maintenances. (In this mode, the unused electron gun can be biased to act as an electron repeller by biasing the outermost lens of the unused electron gun (see Fig 3) to a voltage near to the cathode potential.)

With the application of an external magnetic field oriented along the electron beam axis, a reflex operating mode of electron ionization can also be sustained, in accordance with the description of such reflex mode described in the above-referenced PCT application. If a reflex mode is not desired, then when using a single gun, the outermost lens of the unused electron gun can be biased to a voltage significantly more positive than the cathode potential, and the lens can be used to collect the electron beam of the first electron gun after it has transited the ionization chamber 44, and to measure the value of the electron current. By locating the pressure-sensitive cathodes away from close proximity to the ionization chamber, and exposing the cathode to the high vacuum environment of the source vacuum housing 49, a long cathode life can be maintained. By making the electron guns small, and yet incorporating well-designed electron optics to predictably control the spatial and momentum properties of the electron beam, a high-performance, high-current electron impact ion source is realized to provide the desired ionization and production of ion beams of easily dissociated molecular species. Fig. 3 shows in more detail the electron gun assembly 42. The cylindrically symmetric assembly 42 (43) is contained within a cylindrical housing 50, and is comprised of cathode assembly 51, whenelt/grid assembly 52, anode cylinder 53, focusing cylinder 54, exit cylinder 55, exit aperture 56, and electron entrance aperture 45. A large aperture 61 is cut into the housing 50 to expose the cathode to the vacuum environment of the source housing 49, to extend cathode life. The structure defining electron entrance aperture 45 is in contact with the wall of ionization chamber 44, located at the bottom of a counterbore machined into the chamber 44, and is at chamber potential. This allows the electron optics to penetrate slightly into the chamber wall, to reduce the overall dimension "D" and to reduce the distance the electron beam has to propagate at its desired final electron energy, during which it is susceptible to space charge forces. All of these components 51-56, 45 are held at appropriately different DC voltages, and constitute a lens system. As also disclosed in the above-referenced PCT application, the lens system is comprised by extraction stage 58 (elements 51, 52, 53), asymmetric einzel lens (AEL) 59 (elements 53, 54, 55), and double aperture lens (DAL) 60 (elements 56, 45). Note that aperture 56 is at the same potential as exit cylinder 55, being in direct contact with cylinder 55. Thermionic electrons are produced by a planar cathode emitter 51 , which is held negative with respect to the ionization chamber 44 by a voltage equal to the desired electron energy of the electron beam as it transits the ionization chamber 44. These thermal electrons are collected and accelerated by extraction stage 58 to an energy high enough to overcome space charge forces, on the order of several keN while the AEL 59 decelerates the electron beam to a somewhat lower energy, and also collimates the electron beam. Upon entering the DAL 60. the electron beam is decelerated further to the desired final electron energy required for the efficient production of ions, and the beam enters the ionization chamber 44. An electron gun design similar to that shown in Fig. 3 is also shown in Fig. 18 of the above-referenced PCT application. For example, a relatively collimated beam can be produced at the output of the AEL 59 with an electron energy of 1000 eV, so that lens element 55 would be at 1000V relative to the cathode potential. If the potential of lens element 45 were 100V relative to the cathode, the DAL would act as a 10:1 decelerating lens, injecting lOOeV electrons into the ionization chamber 44.

Geometries other than those of my pending applications and in Fig. 2 and Fig. 3 are possible, and have certain advantages.

Fig.4a shows a simple design for an ion source in which the electrons are injected into an ionization chamber along the same direction as the extracted ion beam. A long filament 70 is heated through filament leads 71 and DC power supply 72 to emit electrons 73 along the length of the filament. The filament may be a ribbon, or a thick tungsten wire, for example. The filament 70 is biased below the potential of the ionization chamber 75 by power supply 72 such that the electrons are accelerated through a rectangular entrance slot 4 centered in the rear of the ionization chamber 75, and aligned with ion extraction aperture 76. This placement of the entrance slot relative to the chamber 75 constitutes a diode arrangement. A top view of this geometry is shown in projection D-D, Fig. 4b. The extended electron beam will ionize the gas within the ionization chamber 75; the ions are extracted through ion extraction aperture 76 within the ion extraction aperture plate 77. Apart from its simplicity, an advantage of the design of Fig. 4 is that high electron currents can be generated by the long filament 70 and focused uniformly along the ion extraction aperture 76. The ion beam thus produced should be generally uniform, since the electron path length through the gas within the ionization chamber 75 is the same along the length of the ion extraction aperture 76. Also, since the electron beam is elongated in the vertical dimension, it is less susceptible to space charge blow-up, and thus higher total electron currents can be delivered into the ionization chamber 75 than can be delivered with a small, round electron beam.

In a construction capable of still better performance, a grid electrode with a long rectangular slot is inserted between filament 70 and chamber entrance aperture 74 to improve the focusing of the electron beam, the component constituting a triode configuration. To prevent transition metal contamination of the ionization chamber due to evaporation of the filament onto the entrance aperture 74 and eventual migration of tungsten or rhenium into the chamber 75, the filament can advantageously be constructed of carbon. Fig. 5 shows another embodiment with general features similar to Fig. 4. In Fig. 5 filament 70 is located at a position spaced from the ionization chamber 75, and the electron beam is propagated through a system of lenses comprised by a series of long, rectangular apertures. The schematic representation of Fig. 5 shows a tetrode arrangement in which filament 70, first electrode 78, second electrode 79, and ionization chamber entrance aperture 74 are all held at different potentials but this approach is not limited to a tetrode. For instance, one or more additional electrodes similar to electrodes 78 and 79 may be added. Among the advantages of these embodiments are: 1) the filament can be disposed at a lower pressure location to enhance filament life (in certain embodiments this remote region has a dedicated pump); 2) remote location of the filament can prevent contamination of the ionization chamber by the filament material; 3) the lens system facilitates accel-decel transport of the electron beam, enabling higher electron currents to be achieved within the ionization chamber.

Figs. 6a-6c illustrate a further preferred embodiment of the invention, in which an extended electron gun 80 is mounted along the axis of the ion source, being mounted external to the vacuum on the source mounting flange 36. The electron gun is contained in part within a housing 81 which features a pump port 82 for separate pumping of the cathode and extraction stage of electron gun 80. The design of the electron gun is similar to that of Fig. 3, but has been scaled to a larger diameter cathode and lens structures. It is comprised by thermionic cathode 51', whenelt/grid electrode 52', anode cylinder 53', focusing cylinder 54', exit cylinder 55', quadrupole lens 83, and drift section 84. The electron gun 80 produces a high current electron beam in a manner similar to the gun shown in Fig. 3, however at the exit of the AEL (elements 53', 54', 55') the electron beam is slightly diverging and is at its final electron energy (preferred energy for ionization of the gas species within ionization chamber 75'). The beam optics upstream of quadrupole lens 83 can be cylindrically symmetric or rectangular, but in general will be extended in both the x (lateral) and y (vertical) dimensions. The action of a quadrupole lens is such that it is focusing in one dimension only. Lens 83 is an x-focusing quadrupole; it is constructed and arranged to produce a focus in the x-direction at approximately the position of the ion extraction aperture 76. Thus, after passing through an elongated rectangular aperture 85 at the end of the source block 35' and the ionization chamber entrance aperture 74', the quadrupole produces^ uniform "line" of electron current which approximately matches the dimension of the roughly 50mm by 3.5mm ion extraction aperture 76. Since the electron beam has an elliptical profile between quadrupole lens 83 and the ion extraction aperture 76, rectangular aperture 85 is wider than entrance aperture 74' , which in turn is larger than extraction aperture 76.

The quadrupole lens can be either electrostatic or magnetostatic, each having its particular advantages. If necessary, the drift region 84 is provided with additional optics to correct aberrations introduced by the quadrupole lens 84. Fig. 6b shows a top view of Fig. 6a, while Fig. 6c shows a three-dimensional view of the ion source assembly. In summary, the embodiment of Fig. 6 has the following among its advantages: 1 )

The extended, large diameter electron gun 80 has a dedicated pump to prolong cathode life: 2) a conventional disk-shaped planar cathode can be used to provide high currents and long service life; 3) the planar cathode is remote and will not radiate heat to the ionization chamber or other heat-sensitive components; 4) use of a sophisticated lens system provides flexibility in producing the desired electron beam characteristics and control of the uniformity and spatial extent of the electron beam as projected onto the ion extraction aperture; 5) high electron beam currents and commensurately high ion currents can be achieved, since space charge is better controlled with an elongated electron beam charge density profile; space charge is also controlled upstream of the quadrupole lens by. decelerating the beam through the AEL while expanding the beam prior to injection into the quadrupole.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, part or all of the guns shown may be oriented at other angles and one or more electron mirrors may be employed to direct the final beam to the desired location. Accordingly, other embodiments are of course possible.

Claims

1. An electron gun configuration for an ionization system characterized in that the electron gun configuration comprises a plurality electron guns.