Indirect hot cathode (IHC) ion source

Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Electron or ion source

Reexamination Certificate

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C250S42300F

Reexamination Certificate

active

06348764

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to an ion source used in an ion implanter and more particularly, relates to an indirect hot cathode (IHC) ion source used in an ion implanter that does not have arcing problems caused by ion deposition at near the electrode.
BACKGROUND OF THE INVENTION
Ion implantation method has been used for placing impurity, or doping ions in a semiconductor material such as in a silicon substrate at precisely controlled depths and with accurate control of dopant ion concentration. One of the major benefits of the method is its capability to precisely place ions at preselected locations and at predetermined dosage. It is a very reproducible process that enables a high level of dopant uniformity. For instance, a typical variation of less than 1% can be obtained across a wafer.
An ion implanter operates by providing an ion source wherein collisions of electrons and neutral atoms result in a large number of various ions being produced. The ions required for doping are then selected out by an analyzing magnet and sent through an acceleration tube. The accelerated ions are then bombarded directly onto the portion of a silicon wafer where doping is required. The bombardment of the ion beam is usually conducted by scanning the beam or by rotating the wafer in order to achieve uniformity. A heavy layer of silicon dioxide or a heavy coating of a positive photoresist image is used as the implantation mask. The depth of the dopant ions implanted can be determined by the energy possessed by the dopant ions, which is normally adjustable by changing the acceleration chamber voltage. The dosage level of the implantation, i.e. the number of dopant ions that enters into the wafer, is determined by monitoring the number of ions passing through a detector. As a result, a precise control of the junction depth planted in a silicon substrate can be achieved by adjusting the implantation energy, while a precise control of the dopant concentration can be achieved by adjusting the dosage level.
A schematic of a conventional high energy ion implantation apparatus
10
is shown in FIG.
1
. In the ion implanter
10
, an ion source
20
is utilized in which collisions of electrons and neutral atoms result in a large quantity of various ions. The ions required for doping are then selected out by an analyzing magnet
12
and sent through an acceleration tube
14
that are then accelerated again by a high energy accelerator
16
equipped with an electron stripper and a magnet
18
to bombard a wafer
22
mounted on a mechanically scanned coned disc
24
. The coned disc
24
has a capacity of
17
wafers for mounting on its surface and for scanning each wafer upon rotation of the disc
24
. The high energy accelerator
16
operates at a high voltage, i.e. normally in a range between about 150 kV and about 750 kV. The coned disc
24
can be preprogrammed to tilt the wafers
22
mounted thereon at an implant angle between −100 to +10°. A usual implantation time required for each wafer is about 20 min.
A detailed cross-sectional view of the ion source
20
of
FIG. 1
is shown in FIG.
2
. The ion source
20
is constructed by a chamber
26
which includes a gas inlet
28
for feeding a reactive gas into the chamber cavity
30
. A small quantity of a gas is passed through a vaporizer oven and then into the ion source chamber
30
which includes a cathode
40
and an anti-cathode
42
. The cathode
40
further includes a heated filament
44
and a filament shield
46
. The filament
44
is heated by a filament power supply
48
while the filament shield
46
is connected to a bias power supply
50
. The ion source chamber
20
is further powered by an arch power supply
52
and a pre-acceleration power supply
54
.
The ion source
20
can be operated in the following manner. First, the filament
44
is heated by passing electric current through it, derived from the power supply
48
. The heating of the filament causes thermionic emission of electrons from the surface of the filament. An electric field, typically of a magnitude between 30 and 150 volts is applied between the filament and the chamber walls using the arc power supply
52
. The field accelerates the electrons in the filament area to the chamber wall. A magnetic field is then introduced that is perpendicular to the electric field and causes the electrons to spiral outward, increasing the path length and chances for collisions with the gas molecules. The collisions break apart many of the molecules and ionize the resultant atoms and molecules by knocking outer shell electrons out of place. As charged particles, these atomic or molecular ions can now be controlled by magnetic and/or electric fields. Source magnets are employed to change the ion path from a straight path to a helicoid path. With one or more electrons missing, the particles carry a net positive charge. An extraction electrode, i.e. the anti-electrode, is placed in proximity to a slit and held at a negative potential attracts and accelerates the charged particles out of the chamber through the slit opening
32
provided in a sidewall
34
of the chamber
26
. Ions
36
existing the chamber cavity
30
are passed through an acceleration tube
14
(
FIG. 1
) where they are accelerated and through the high energy accelerator
16
to the implantation energy as they move from high voltage to ground. The accelerated ions form a beam that is collimated by a set of apertures (not shown). The ion beam is then scattered over the surface of a wafer
22
mounted on the coned disc
24
.
In the conventional ion source
20
shown in
FIG. 2
, after operation over a period of time, the processing of gases in the chamber cavity
30
results in the accumulation of materials
38
deposited from the gases. The material accumulation is especially severe at vicinities
56
that is close to the filament shield
46
. Since the gap
58
provided in-between the filament shield
46
and the endwall
60
is usually very small, i.e. in the range of 1 mm to avoid the escape of plasma ions, the gap
58
is easily filled with the deposited materials and causing either arcing or electrical shorting between the chamber wall
60
and the filament shield
46
. The arcing or electrical shorting around the filament shield
46
can cause serious machine malfunction by stopping the generation of plasma ions inside the ion source cavity.
It is therefore an object of the present invention to provide an indirect hot cathode ion source for an ion implanter that does not have the drawbacks or shortcomings of the conventional ion sources.
It is another object of the present invention to provide an indirect hot cathode ion source that does not have arcing or electrical shorting problems between a cathode and a chamber wall.
It is a further object of the present invention to provide an indirect hot cathode ion source that utilizes a cathode including a filament shield that does not have shorting or arcing problems with the chamber wall to which the shield is in close proximity.
It is another further object of the present invention to provide an indirect hot cathode ion source that is equipped with a cathode including a filament shield mounted in close proximity to an inner periphery of an opening in an endwall equipped with a torroidal-shaped recess adjacent to the filament shield for avoiding shorting or arcing.
It is still another object of the present invention to provide an indirect hot cathode ion source that is equipped with a filament shield spaced apart from an inner periphery of an opening in an endwall by a distance of at least 2 mm.
SUMMARY OF THE INVENTION
In accordance with the present invention, an indirect hot cathode ion source equipped with a filament shield spaced apart from an inner periphery of an opening in a chamber wall by a distance of at least 2 mm such that arcing or electrical shorting does not occur is provided.
In a preferred embodiment, an indirect hot cathode ion source is provided which includes a chamber formed by two endwalls, two sidewalls,

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