Double chamber ion source

Abstract

The ion source is comprised of two discharge chambers one of which is provided with a filament and an aperture leading into the other chamber which in turn has an extraction orifice. A low voltage arc discharge is operated in an inert gas atmosphere in the filament chamber while an arc of higher voltage is operated in the second ionization chamber which contains a vapor which will give the desired dopant ion species. The entire source is immersed in an axial magnetic field parallel to a line connecting the filament, the aperture between the two chambers and the extraction orifice.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention is directed to an ion source and more specifically to a double chamber ion source wherein the filament in one of the chambers is operated in an inert atmosphere and the electrons from the filament chamber are used to sustain the discharge in the ionization chamber.

2. PRIOR ART

Ion sources of the hot cathode, arc type have been used for many years but the utility of these sources is often limited by the relatively short lifetimes of their heated filaments. Processes which limit the filament lifetime are sputtering, the vaporizing of tungsten, the reactive evaporation of volatile molecular species such as tungsten oxide and the incorporation foreign elements such as boron into the tungsten lattice. The latter two processes involved the interactions of chemically active gases with the filament. currents

Several attempts have been made to isolate the filament from the chemically active gases in an ion source. One of these prior art devices utilizes a post-ionization chamber which is connected to the filament chamber by means of an aperture. The filament operates in an inert gas environment and delivers a plasma jet into the post-ionization chamber wherein the plasma interacts with a dopant gas or vapors from a solid to provide atomic ion currents of the order of 25 .mu.A for most ions. Thus, while it is possible to produce ions of chemically active elements in the post-ionization chamber which will not affect the filament life this type of prior art ion source only contains a single plasma and does not provide a separate arc supply for the second chamber. Therefore, the single plasma from the first chamber is merely drawn into the second chamber to ionize the dopant gas or vapor.

SUMMARY OF THE INVENTION

The present invention provides an ion source having a filament chamber and an ionization chamber in which two uncoupled discharges are maintained whose characteristics can be controlled independently and where electrons from the filament chamber are used to sustain the discharge in the ionization chamber.

The present invention provides an ion source having a filament chamber and an ionization chamber wherein the filament chamber is provided with an inert gas atmosphere and a dopant gas or vapor is supplied to the ionization chamber so that the chemically active ion will not interact with the filament thereby substantially increasing the filament life span. By maintaining a low voltage arc discharge in the inert gas atmosphere of the filament chamber sputtering is minimized thus greatly increasing the source lifetime.

The present invention provides a double chamber ion source comprised of a filament chamber and an ionization chamber connected by means of a small aperature in alignment with a filament in the filament chamber and an extraction orifice in the ionization chamber, means for supplying an inert gas to said filament chamber, means for supplying a dopant gas or vapor to the ionization chamber, means for maintaining independent uncoupled discharges in each of said chambers and means for providing an axially directed magnetic field parallel to the line connecting said filament, said aperture and said extraction orifice.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of the double chamber ion source according to the present invention.

FIG. 2 is a schematic view of a modified form of double chamber ion source according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The double chamber ion source according to the present invention is comprised of a cylindrical filament chamber 10 which is substantially closed at one end by a plate 12 having an aperture 14 centrally located therein. The cylindrical walls of the filament chamber 10 are provided with cooling passages 16 to provide for the circulation of a suitable cooling medium therethrough. The passages 16 have been shown somewhat schematically in FIG. 1 and numerous variations in the actual construction of the cooling passages can be utilized. For example, the cylindrical filament chamber 10 could be a double-wall chamber defining an annular space for the circulation of the cooling medium. The cooling medium may be supplied to the space 16 by means of one or more conduits 18 which may be connected to any suitable source of cooling medium.

A filament 20 of tungsten or the like is disposed centrally of the chamber 10 and is supported by an electrically conductive paid of leads 22, the ends of which are stablized by a cross brace 24 of insulative material.

The ionization chamber 26 is comprised of a cylindrical, open-ended member which is co-axially disposed with respect to the filament chamber 10. A plate 28 having an aperture 30 is mounted on the end of the filament chamber 10 and the ionization chamber 26 is located by means of an insulating ring 32 supported by the plate 28. At the opposite end of the ionization chamber 26 an outlet plate 34 is located relative to the ionization chamber by means of another insulating ring 36. The outlet plate 34 is provided with an extraction orifice 38 on the face thereof adjacent the ionization chamber 26 and which communicates with a conical extraction orifice 40 extending in from the opposite surface of the plate 34. A passage 42 extends through the plate 34 in communication with the interior of the ionization chamber 26. A dopant gas may be supplied to the interior of the ionization chamber 26 through the passage 42. Suitable connecting means may be provided at the outer end of the passage 42 for communicating with the gas supply.

A cylindrical housing 44 completely surrounds the double chamber ion source and is provided with a circumferential annular groove 46 in which an electro-magnetic coil 48 is disposed. The filament chamber 10 is located concentrically relative to the cylindrical housing by means of an insulating ring 50 having a plurality of passages 52 extending therethrough. Likewise, the ionization chamber 26 is located concentrically with respect to the cylindrical housing 44 by means of an insulating ring 54 having a plurality of passages 56 extending therethrough. The outlet plate 34 having the extrusion orifice therein is located relative to the cylindrical housing 44 by means of an annular ring 58 having one or more apertures 60 extending therethrough for connection to a suitable vacuum pump means. An insulating ring 62 is secured to a flange 64 on the end of the cylindrical housing 44 by any suitable means to provide a mounting means for locating the double chamber ion source within a standard ion implantation device. The ion implantation device and the extraction electrodes thereof which will be disposed adjacent the extraction orifice 38, 40 have not been shown since the details thereof do not form a portion of the present invention.

The opposite end of the cylindrical housing 44 is closed by means of a circular plate 66 which is secured to the housing 44 by any suitable means. An O-ring 68 or other suitable sealing means may be disposed between the plate 66 and the housing 44 to provide vacuum integrity within the housing.

The cylindrical filament chamber 10 and the cylindrical ionization chamber 26 are fabricated from a non-magnetic electrically conductive material. The filament chamber 10 is connected to a suitable source of voltage through a lead 70 and the ionization chamber 26 is electrically connected to a suitable source of voltage by the lead 72. The electrical lead 70 extends through an aperture in the end plate 66 and is insulated therefrom by means of an insulating sleeve 74. Likewise, the electrical lead 72 extends through an aperture in the end plate 66 and is insulated therefrom by means of sleeve 76. The lead 72 also extends through an aperture in the positioning flange 78 on the filament chamber 10 and is insulated therefrom by means of a sleeve 80. The electrical leads 22 are also insulated from the end plate 66 by means of insulating sleeve 82. The sleeves 74, 76 and 82 also provide an airtight seal so that the interior of the housing 44 can be maintained at a reduced pressure. The coolant conduits 18 may be of a soft resilient material which is force fitted through the apertures in the end plate 66 to provide a tight air seal. Finally the end plate 66 is provided with an aperture 84 for the admission of an inert gas into the interior of the housing 44. A suitable fitting may be provided on the outer end of the passage 84 for connection to a suitable supply source.

In the operation of the double chamber ion source an inert gas such as argon, helium or hydrogen may be supplied to the interior of the housing 44 through the passage 84. Since the filament chamber 10 is open at one end the inert gas will permeate into the filament chamber 10. A low voltage arc discharge is maintained in the filament chamber while an arc of higher voltage is operated in the ionization chamber in which a dopant gas or vapor has been introduced capable of providing ions of boron, phosphorus, arsenic, antimony or the like. The entire source is immersed in an axial magnetic field parallel to the line connecting the filament 20, the apertures 14 and 30 between the two chambers and the extraction orifice 38. Electron beams from the filament chamber are used to sustain the discharge in the ionization chamber. To produce maximum ionization of the dopant vapor or gas the electrons in the ionization chamber should have energies determined by the voltage applied to the ionization chamber which are in the range of 100-150 eV since it is at these energies that the ionization probabalities for most gases have their maximum values.

The inert gas used in the filament chamber should have a low sputtering yield under typical operating conditions and should not increase the thermionic work function of the filament. The inert gas should not react chemically with the hot filament thereby creating volatile compounds, should not cause filament failure by becoming incorporated into the lattice and should not adversely affect the source characteristics if a small amount escapes through the aperture into the ionization chamber. The arc voltage in the filament chamber should be as small as possible to minimize sputtering and yet large enough to provide sufficient electrons to the ionization chamber. One reason that a discharge is created in the filament chamber rather than, for example, using a high vacuum electron gun to supply electrons to the ionization chamber is that the discharge permits the filament to operate in a temperature-limited regime at a very low arc voltage. Under most conditions the energy of the ions bombarding the filament is determined by the arc voltage. The apertures 14 and 30 between the two chambers should be small enough so that the two plasmas are not coupled.

The active gas in the ionization chamber can be isolated from the filament by keeping the pressure of the inert gas in the filament chamber high enough so that as it passes through the orifiice into the ionization chamber it inhibits the diffusion of the chemically active gas back into the filament chamber. A vacuum pump can also be connected to the apertures 60 so that active gas molecules that do diffuse through the aperture can be quickly pumped away. The pumping speed required for this type of operation is determined by the aperture size. It is estimated that for a one mm aperature a pumping speed of 1000 liters/sec would be desirable thus maintaining a partial pressure of active gas of less than 10.sup.-.sup.5 torr in the filament chamber. By cooling the walls of the filament chamber 10 by a cooling medium supplied through the conduit 18 any dopant gas which migrates through the apertures 14 and 30 will condense on the walls of the cooled chamber thereby preventing the active gas from adversely interacting with the filament.

FIG. 2 shows a modified arrangement in a schematic form wherein the filament chamber is represented by the cylindrical wall 110 and the ionization chamber is represented by the cylindrical wall 126. A filament 120 is disposed in the filament chamber and is electrically connected to a voltage source V.sub.fil. The apertured plate 112 between the ionization chamber and the filament chamber is connected to a voltage source V.sub.arc and maintained at the same voltage as the chamber wall 110. The apertured wall 112 is separated from the cylindrical wall 110 and by maintaining both of these walls at the same potential an oscillating electron discharge is obtained. The cylindrical wall 126 of the ionization chamber is connected to a voltage source V.sub.i and the outlet plate 134 having the extraction orifice 138 therein is connected to voltage source V.sub.v. The electrical connection for the chambers in FIG. 1 would be similar to that shown in FIG. 2 with the exception of the voltage source V.sub.b which is eliminated. Thus, by providing the ionization chamber with its own arc supply a separate plasma independent from the plasma in the filament chamber is created and the electrons from the filament chamber which are drawn into the ionization chamber will sustain the seperate plasma and ionize the dopant gas or vapor therein. The ions can then be extracted through the extraction orifice in the conventional manner.

While the invention has been particularly shown and described with reference to preferred embodiments thereof it will be understood by those in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. An ion source comprising a first cylindrical chamber, filament means disposed in said first chamber, a second cylindrical chamber axially aligned with said first cylindrical chamber, wall means separating said first and second chambers and having aperture means therein, end closure wall means disposed at the opposite end of said second chamber and having extraction orifice means therein, said filament means, said aperture means and said orifice means being axially aligned with each other, voltage supply means connected to said filament means and said first and second cylindrical chambers to maintain a separate arc discharge in each of said chambers and magnetic field generating means surrounding said chambers to provide an axially directed magnetic field.

2. An ion source as set forth in claim 1 further comprising a cylindrical housing surrounding said first and second chambers is spaced relation thereto, means for supplying an inert gas to the interior of said housing, said end of said first chamber remote from said apertured wall between said first and second chambers being in open communication with the interior of said housing, means for supplying a chemically active gas to said second chamber and means for cooling the cylindrical walls of said first chamber.

3. A method of generating ions comprising maintaining of first arc discharge in a first chamber having a filament disposed therein in an inert gaseous atmosphere, maintaining a second arc discharge in a second chamber having a chemically active gaseous atmosphere therein, directing a stream of electrons from said first discharge into said second chamber to ionize the gas therein and extracting the ions from said second chamber.