As the ion beam
is directed into contact with the wafer, the wafer charges as the positively-charged
ions strike the wafer surface. The charging is often nonuniform and can
create large electric fields at the wafer surface (which could be as much
as several hundred volts) and damage the wafer, making it unsuitable for
use as a semiconductor material. In order to neutralize the charge, an
apparatus is provided in which electric charge of opposite polarity to
that of the charged beam is generated near the wafer surface to neutralize
the charged beam or buildup of electrostatic charge on the wafer surface.
There are several different methods of accomplishing this.
RF Plasma Cell Charging Control An
apparatus for maintaining an ion beam along a beam path from an
ion source to an ion implantation station where work pieces are treated
with the ion beam. An ion beam neutralizer is positioned upstream from
the ion treatment station and includes confinement structure which bounds
the ion beam path. An electron source positioned within the confinement
structure emits electrons into the ion beam. An array of magnets supported
by the confinement structure creates a magnetic field which tends to confine
the electrons moving within the confinement structure. An interior magnetic
filter field is created inside the confinement structure by a plurality
of axially elongated filter rods having encapsulated magnets bounding the
ion beam and oriented generally parallel to the ion beam path. This interior
magnetic field confines higher energy electrons from leaving the ion beam
path and permits lower energy electrons to drift along the ion beam. United
States Patent 5,703,375
An ion beam neutralizer. High energy electrons
are directed through an ion beam neutralizing zone or region containing
an ionizable gas. As the high energy electrons collide with the gas molecules,
they ionize the gas molecules and produce low energy electrons which are
trapped by a positively charged ion beam. As high energy electrons pass
out of the neutralizing zone they are deflected back to the neutralizing
zone by a cylindrical conductor biased to deflect the high energy electrons
and an accelerating grid for accelerating the electrons back through the
beam neutralizing zone. United States Patent 4,804,837
Electron-beam charge neutralization system
The apparatus includes an electron source for generating an electron beam
and a magnetic assembly for generating a magnetic field for guiding the
electron beam to the work piece. The electron beam path preferably includes
a first section between the electron source and the ion beam and a second
section which is coincident with the ion beam. The magnetic assembly generates
an axial component of magnetic field along the electron beam path. The
magnetic assembly also generates a transverse component of the magnetic
field in an elbow region between the first and second sections of the electron
beam path. The electron source preferably includes a large area lanthanum
hexaboride cathode and an extraction grid positioned in close proximity
to the cathode. The apparatus provides a high current, low energy electron
beam for neutralizing charge buildup on the work piece. United States Patent 5,136,171
Plasma Shower The electric charge for neutralization
is generated by admitting electric charge from a plasma generation unit
to the vicinity of the sample surface, ionizing gas generated from the
sample surface by causing the charged beam to collide the gas or by irradiating
electrons from an electron source on the sample surface. Especially when
there is a possibility that impurities other than the electric charge for
neutralization affect the sample adversely, an impurity generation source
is blind folded with a cover so as not to be seen through from the sample
and charged beam so that the impurities may be prevented from impinging
upon the sample surface or intersecting the charged beam path.
Plasma Flood System
for use in the implantation of ions in a semiconductor substrate comprising
a plasma and low energy electron source for developing a plasma containing
low energy electrons for magnetic field enhanced transmission to a negatively
biased, magnetic field assisted electron confinement tube and into an ion
beam flowing axially through the tube to the semiconductor substrate for
self regulating and neutralizing positive charges on the surface of the
substrate without causing significant negative charging of the substrate.
United States Patent 5,399,871
Electron Shower An electron source adjacent
to the beam for providing primary electrons at about 350 eV strike a production
target, giving off secondary electrons. The secondary electrons have a
low energy and are susceptible to being entrapped within the volume of
the positively charged beam. The ion beam attracts these low energy electrons
until effective beam neutralization is achieved.
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Beam blowup Low energy
beams which propagate through a given beamline construction suffer from
a condition known as beam "blow-up", which refers to the tendency for like-charged
(positive) ions within the ion beam to mutually repel each other (also
known as the space charge effect). Such mutual repulsion causes a beam
of otherwise desired shape to diverge away from an intended beamline path.
Beam blow-up is particularly problematic in high current, low energy applications because the high
concentration of ions in the beam (high current) exaggerates the force of the mutual repulsion of the ions, and the minimal propagation velocity (low energy) of the ions expose them longer to these mutually repulsive forces.
Space Charge Neutralization A mechanism
by which the space charge effect is reduced in an ion implanter is the
creation of a beam plasma, comprised of positively charged, negatively
charged, and neutral particles, wherein the charge density of the positively
and negatively charged particles within the space occupied by the beam
are the same. A beam plasma is generally created when the positively charged
ion beam interacts with residual background gas atoms, producing ion electron
pairs by way of ionizing collisions. The ion beam is thereby partially
neutralized by means of its interaction with the background residual gas
in its flight path.
Control of Beam Blowup There are two key elements needed to control beam blowup. The fist is the introduction of low energy electrons into the beam, and the second is to reduce electrons leaving or being stripped form the beam.
Low Energy Electrons Low energy electrons are generated as the ion beam collides with residual gas molecules creating free electrons that can be used for the reduction of beam blowup due to space charge effects. It takes less than 10eV to ionize residual gas and may still be effective even a pressure as low as 10e-6 torr(1). It is also very effective to intentionally introduce an inert gas into the vacuum system, raising the probability of collisions.
Neutralizing an ion beam using water vapor
The neutralization system comprises a source
of water; a vaporizer connected to the source of water; a mass flow controller
connected to the vaporizer; and an inlet connected to the mass flow controller.
The vaporizer converts water from the source from a liquid state to a vapor
state. The mass flow controller receives water vapor from the vaporizer
and meters the volume of water vapor output by a mass flow controller outlet.
United States Patent 5,814,819
Electron Stripping Whenever
electron are striped or removed form the beam, the beam has a tendency
to expand. The beam will be stripped of elections anytime low energy electron
are attracted and accelerated out of The beam by a high positive potential
of the source or post accel assemblies. It may also occur due to the buildup
of electrostatic charge on the wafer surface. The lose of electrons
due to electrostatic charge on the wafer surface can be controlled by techniques
described at the top of this page. Loses due to positive
potentials are controlled by the suppression electrodes on both the source
and post accel assemblies. Electrons downstream from the source and post
accel suppression electrodes will be repelled by the electrodes if they
move up within it's fields.
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This page is sponsored by Case Technology Inc. 5/1/98.