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.

Updated 6/6/99.