JPS6357104B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ion source with a long life, excellent ionization efficiency, and increased brightness.
More specifically, the present invention relates to a surface-emitting multi-electrode ion source with a hot filament Freeman configuration.

Ion sources are utilized in particle accelerators and commercial ion implanters. Until now, a wide variety of ion sources have been used.
has been developed. (For example, “Ion beam with Applications to Ion” by RGWilson et al.
(See "Implantation (1973)" p. 45 et seq.) The most common ion sources include the Penning cold cathode ion source, the Sieba hot filament ion source, the Bernas ion source, and the Freeman hot cathode ion source. are characterized by ion current generation, ionization efficiency, and brightness. These characteristics vary with the type of ion source and are chosen to meet the requirements of a particular application.

In commercial ion implanters, it is highly desirable to have an ion source that is reliable, relatively energy efficient, and has a long lifetime. Long life can reduce downtime for an ion implanter with an attached ion source and also reduce the cost of replacing the ion source. A common ion source for the implantation described above is a hot cathode (filament type) Freeman arrangement ion source. A single filament is positioned within the ionization chamber approximately parallel to the exit aperture. Ions generated within the separation chamber are extracted through the exit aperture. The cathode filament is negatively biased and heated to an emission temperature to provide a sufficient supply of electrons for electron impact ionization of the gas introduced into the separation chamber. The surface area of the cathode filament constitutes the effective area for the emission of energetic electrons. Electron impact ionization of the gas increases the plasma. Plasma density is a function of separation chamber pressure, electron emission and cathode potential. Positive ions are extracted from the plasma by means of a high potential field and emerge through an exit aperture in the wall of the separation chamber, resulting in an ion beam to be used for implantation. Multiple (usually dual) filaments have been used in incandescent light bulbs. Multiple filaments are
It provides greater brightness without increasing filament diameter or current, and allows connection of low resistance filaments to high potential commercial lighting circuits.
Alternatively, double filaments connected in series and folded have been used to provide a compact lamp. In these applications, the filament is heated in a vacuum or inert gas, and the purpose is to produce visible wavelength radiation, and there is no need to maintain an ionization reaction within an enclosed chamber. In fact, the radiation itself is the objective, and secondary effects are not included. Dual filaments have been used in electron beam guns to generate high electron beam power, provide redundancy, and avoid filament destruction due to backflowing ions. For example, US Patent No. 3701915 "Electron Beam Gun" and US Pat.
See No. 3172007, Folded Filament Beam Generator. There, there is no need to confine the electrons to a separation chamber, but rather the electrons travel from the filament to the electron beam with a minimum residence time within the filament housing area. The filament spring and housing geometry are selected to achieve electron generation and rapid extraction.

In prior art hot cathode Freeman type linear ion sources, the filament was typically mounted inside the separation chamber and centered with respect to the extraction electrode. Most of the electrons generated by the filament remain inside the separation chamber and have a statistical chance of collision with source molecules on the gas, producing ionized atoms. These ionized atoms are then electrostatically extracted through an extraction slit in the separation chamber. These ions are directed (with or without analysis or scanning) to a target, such as a semiconductor wafer.

One object of the present invention is to provide a thermal filament surface separation ion source with increased lifetime.

Another object of the invention is to reduce the filament operating temperature in a thermal filament surface separation ion source.

Another object of the invention is to provide a multi-filament ion source that increases the filament surface area for a given power consumption.

SUMMARY OF THE INVENTION The present invention provides a multiple filament ion source of the electron impact ionization type. The ion source operates at a lower filament temperature but produces an ion beam equivalent to that of a single filament ion source. The reduction in filament operating temperature
As a result, filament life is increased. The emission at the filament, which is a function of surface area and temperature, remains constant despite the decrease in temperature. This is because the use of multiple filaments increases the surface area. Power consumption is kept constant by keeping the total cross-sectional area of the electrodes constant;
Alternatively, it may be reduced by reducing the total cross-sectional area. Even when the total cross-sectional area is reduced, the surface area is increased sufficiently to maintain electron emission strong enough to yield unchanged ion source performance. In a preferred embodiment, the drawer slit straddles multiple filaments to eliminate filament breakage and provide even longer life.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Electron impact ion sources are useful for practical applications. This is because it can produce a wide range of ion currents and a wide variety of ion species, and it can produce either broad or focused beams. In such devices, a conductive wire or rod electrode is connected to a separation chamber (sometimes an arc chamber or a reactive chamber).
(also called a chamber) and is heated by passing an electric current through it. Electrons are generated by thermionic emission around the electrode surface. An ionizable material is introduced into the separation chamber in vapor form, electrons collide with atoms or molecules of the material, and ions are formed. The electrode acts as a cathode and the wall of the separation chamber acts as an anode, between which ionization collisions occur. These ions are drawn through the aperture by means of a large electrostatic field gradient and directed outward to the target or substrate. This type of bombardment ionization source is generally divided into two classes. That is, a wide beam ion source and a focused beam ion source. The Kaufmann ion source is a broad beam ion source that is used, for example, in ion attitude control rockets for space propulsion vehicles, ion milling, and reactive ion etching applications. These ion sources have relatively large area extraction apertures and are typically circularly shaped with a single electrode and multiple field poles located within the separation chamber. This type of ion source may have two semicircular filaments mounted inside a cylindrical separation chamber. On the other hand, a hot filament type Freeman ion source (known as a Caltron ion source) is an ion source with a narrow or focused ion beam, which can be used for example for beam analysis and focusing to achieve a high degree of uniformity. It is used for ion implantation of semiconductor wafers, etc. A typical focused ion source has an elongated extraction slit and a ribbon shaped ion source.
beam). In both cases, the electrodes
It will be located within the separation chamber and will have characteristic dimensions, shape and material and will be oriented with respect to the drawer opening.

Electrode degradation effects in impact ionization sources are primarily due to the release of electrode material. Atomic or molecular components of the electrode material are expelled from the surface (surface evaporation) as the operating temperature increases. The loss of material causes the diameter of the electrode to decrease until the rod or wire is severed. Losses in rods and wires occur at high rates in vacuum and are still significant in plasma or water-based environments. This loss effect is at least a second order exponential function of temperature. Therefore, the loss rate increases significantly for small increases in temperature and decreases significantly for small drops in temperature. Therefore, reducing the operating temperature reduces the rate of material loss due to surface emission and increases the life of the electrode.

In order to optimize the operating characteristics (lifetime, ionization efficiency, brightness, reproducibility and robustness) of the impact ionization ion source, a novel arrangement for the separation chamber and electrodes is adopted, with the preferred arrangement of the electrodes inside the separation chamber. The position has been determined empirically. These studies were employed to create magnetic confinement within certain regions of the separation chamber. That is, the residence time of a particle within these regions must be increased. In particular, the electrons must be restricted from reaching the anode until they have spent most of their kinetic energy in inelastic ion-producing collisions. Neutral atoms must also be confined inside the separation chamber until they are ionized. That is, neutral atoms should not be allowed easy access to the extraction aperture or slit, lest they escape and mix with the ion beam. And the ions must be prevented from reaching the walls of the separation chamber. Otherwise they will be ionized again.

Magnetic confinement means, which restrict the movement of particles without physical contact, may be obtained by choosing a special design for the separation chamber and electrode arrangement. Physical changes that result in a more optimal magnetic field for confinement may necessarily produce an ion source with improved ion efficiency.

In FIG. 1, a conventional Freeman ion source is shown schematically. As mentioned above, this type of ion source is used in analyzers where uniformity is desired. A single rod electrode 12
It is centrally located within the ion source housing 11 in the axial direction with respect to the extraction slit 13 . Magnetic pole piece 10 is
A magnetic field with no fluctuation of about 5,000 to 15,000 Gauss is created along the axial direction of the rod electrode 12, depending on the type of ionization. This stable magnetic field and the magnetic field generated by the current flowing inside the rod electrode (when the current inside the electrode is several hundred amperes, the magnetic field is several hundred Gauss near the electrode surface, and as it goes outward ) affects the progress of electrons after being emitted from the rod electrode 12. A spiral or turbulent electron trajectory is created, which helps keep the high-energy electrons within the central region where ionizing collisions will occur. Superimposed without any fluctuation
The magnetic field is not essential to operation but increases ionization efficiency. The ionizable raw material is shown as a solid state ion source 16 surrounded by an evaporative heater 17 and positioned in close proximity to the rod electrode 12 . An ionizable gas is provided to the region of rod electrode 12 . Due to the electron bombardment, the gas is ionized and can be drawn through the slit 13 by means of the accelerating electrode 14. The drawn beam 15 has a ribbon shape.

The multiple filament electrode arrangement of the present invention is shown in FIG. Although an external magnet is not shown there, it is actually used in the same manner as in the single filament ion source of FIG. In the preferred embodiment shown, two parallel bar segments 2
4 form the electrodes, which are located oppositely within the ionization chamber 21 and straddle the extraction slit 19 (also shown in FIG. 3). An extraction slit 19 is formed at the end of the beveled surface 18 to allow the beam to diverge after extraction.

In some embodiments, the drawer slit has a width on the order of 100/1000 inch (approximately 0.254 cm) and the two parallel segments of the electrode have a diameter of 60/1000 inch (approximately 0.152 cm) with a spacing between them. is 240/1
000 inches (approximately 0.61 cm). In the prior art embodiment of FIG. 1, there is a slight solid angle through which electrons can pass directly to the outside (more importantly towards the substrate), whereas this is absent in the embodiment of FIG. In both cases, electrons are available for collisions with neutral atoms inside the separation chamber. The bar segment 24 is
It is tightened with a bolt 25 and firmly supported by a support member 23 in a press-fit state, and is provided into the separation chamber 21 through an insulator 22. In order to maximize the residence time of the desired species in the area adjacent to the drawer slit, the rod segments are preferably located within a distance of 1/5 to 1/3 from the slit towards the rear wall of the separation chamber. be done. In the illustrated preferred embodiment, the electrodes have equal diameters so as to have a balanced and symmetrical magnetic field. The electrodes are typically formed of tungsten. The power rating (which is proportional to the total cross-sectional area of the rod segment) can be increased because the dual filament can be used as a plug-in replacement for the single filament ion source. Power supply providing 150 amps for single filament ion source
A dual filament ion source with a combined equivalent cross section is driven with 75 amps flowing through each filament. Two equal small (double)
If the electrodes and power are equivalent, the diameter D of a single filament will be √2 times the diameter D of a double filament. Therefore, for dual electrodes of equal diameter and length, the total surface area increases by a factor of √2. Since the emission rate is directly proportional to both temperature and surface area,
Increased surface area results in either greater electron emission for equal power consumption or decreased power consumption for equal electron emission.

In a conventional Freeman ion source with a single filament centered axially, some of the electrons emitted from the electrode will leak directly through the line of sight through the extraction slit. This single electrode also prevents the ions created within the reaction chamber from passing through the extraction slit.
Such collisions with the electrodes with ions reduce the number of ions. The double filament straddling arrangement of the ion source of the present invention reduces the possibility of such collisions. In the cross-sectional view of FIG. 4, the magnetic field B→ generated by the current i is shown. The magnetic field lines (towards the page) of the superimposed fixed magnetic field are not shown. It is clear that these fields necessarily occupy the region between the filament and the slit. Unless the separation chamber itself is made of part of a larger magnetic circuit and the field lines are not continuous towards the walls of the separation chamber, the magnetic field lines must be compressed in the region between the filament and the slit, so that the separation chamber This makes it difficult to prevent particles from colliding with the walls. Therefore, the ions generated inside the separation chamber must cross these dense magnetic field lines and may be deflected into neutralizing collisions with the separation chamber wall adjacent to the extraction slit. Dew.

The magnetic field associated with the double filament arrangement is
As shown in the figure. Since the filament current flows in the same direction within each filament, equipotential magnetic field lines B→ surround each filament in the same direction. The magnetic field in the region sandwiched between the two filaments will be weakened by cancellation, so that neutralizing collisions will be minimized in this region where ions are extracted. This is in contrast to the strong magnetic field that exists within the exit aperture region of the conventional ion source of FIG.
Superimposed on this magnetic field is a constant magnetic field perpendicular to the plane of the drawing and along the electrodes. These two magnetic field components create complex spiral or turbulent trajectories for electrons that participate in ionizing collisions.

In electron impact ion sources, sputtering of the constituent metal atoms of the electrode is one mechanism of degradation. This sputtering is caused by the bombardment of energetic atomic species generated within the separation chamber and by energetic species flowing back into the separation chamber from the outside. Internal energetic species also include neutral species that are neutralized after ionization and acceleration within the separation chamber, as well as ions of a different type than the type being extracted. These internally generated energetic species do not have very high kinetic energy. This is because the potential at which they are accelerated is the bias potential of the electrode (several 100 volts).
This is because. It was found that electrode lifetime is optimal at the lowest bias potential, which is closely related to sustained release at a suitable electrode.
Externally generated energetic species may include electrons, anions, and even neutral particles that are accelerated in the opposite direction by an extraction field on the order of 10,000 volts. These externally generated particles can have high kinetic energy and can cause significant sputtering. Such reverse sputtering creates a phenomenon of "necking" in the electrode, whereby the electrode thins out in the central portion exposed to countercurrent ions. This backward sputtering depends on the density of the backward flowing ions and the electric field in which the backward flowing ions are accelerated. Both back sputtering and sputtering by extracted ions are reduced by the straddling arrangement of the electrode segments of the ion source of the present invention. In addition, the operating temperature of the electrode determines how easily surface atoms can be removed by impinging energetic species;
That is, the higher the operating temperature, the more sputtering. These combined effects are essentially linear (or low order) functions of temperature.
Therefore, the multi-electrode ion source of the present invention with a low effective operating temperature reduces sputtering effects as a linear (or low-order) function of temperature for both internally and externally generated energetic species. let In preferred embodiments where multiple segments of the electrode are not in line-of-sight with respect to the substrate, the backward sputtering component is eliminated and total sputtering is significantly reduced. In either case, however, the improvement is secondary to the improvement due to lower material loss due to evaporation loss at the surface. The major improvement concerns the electron gun. The difference lies in the fact that temperature-dependent material evaporation and internal plasma environment-dependent sputtering always result in more destruction than by backflowing ions.

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional diagram of a prior art Freeman arrangement surface-emitting ion source. FIG. 2 is a perspective view of an embodiment of a dual filament ion source incorporating features of the present invention. Figure 3 shows the second
FIG. 3 is a partially sectional side view of the separation chamber in the dual filament embodiment shown; FIG. 4 is a schematic diagram of the magnetic field lines of the magnetic field generated by the filament inside the separation chamber of a prior art Freeman configuration surface emitting ion source. 5 is a schematic diagram of the magnetic field lines of the magnetic field generated by the filaments inside the separation chamber of the dual filament embodiment of FIG. 2; FIG. Explanation of main symbols, 21... Separation chamber, 24... Rod electrode, 19... Drawer slit.

Claims (1)

[Scope of Claims] 1. An electrode mounted inside a separation chamber, a power source for supplying current to the electrode, means for supplying an ionizable material to the separation chamber, and an electrode formed on the wall of the separation chamber. Freeman hot filament type electron impact ion source with an ion extraction aperture: to produce a larger surface area for a given electrode cross-section to allow for greater emission for a given input power; An electron impact ion source characterized in that the electrode is composed of multiple segments electrically connected in parallel. 2. The electron impact ion source according to claim 1, wherein: the electrode is composed of two segments electrically connected in parallel. 3. An electron impact ion source as claimed in claim 2, wherein each of the segments has a linear shape. 4. An electron impact ion source as claimed in claim 3, wherein the segments are oriented with parallel axes. 5. An electron impact ion source as claimed in claim 4: An ion source with external magnetic means for superimposing magnetic fields inside the separation chamber. 6. Electron bombardment ion source as claimed in claim 5, wherein: said magnetic means are positioned to produce aligned magnetic fields along the axes of said two parallel segments. . 7. An electron impact ion source as claimed in claim 5, wherein: the two parallel segments are connected to the separation chamber such that there is no line of sight to the target for the electrons emitted from the electrodes. an ion source mounted internally and symmetrically along both edges of said extraction opening; 8. An electron impact ion source as claimed in claim 7, wherein the two parallel segments are made of tungsten and have equal diameters. 9. The electron impact ion source according to claim 8, wherein the two parallel segments extend from 1/5 to 1/2 from the inner side of the extraction opening toward the rear wall of the separation chamber. The ion source is located within a distance range of 3.