Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1998-05-22
2001-08-07
Anderson, Bruce C. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S251000, C250S42300F, C315S111810
Reexamination Certificate
active
06271529
ABSTRACT:
BACKGROUND
This invention relates to ion implantation.
As is well known, ion implantation is a process of generating an ion beam, focusing that beam, and directing it toward a wafer to implant ions into the wafer.
As fast moving particles in an ion implanter collide with the residual gas and the walls of the implanter, they generate low energy ions and free electrons. A positively charged ion beam traps these electrons and simultaneously rejects the positive ions. The positive ion beam has an inherent potential that is typically distributed nonuniformly across the beam cross-section. (The inherent potential of the beam is also known as the space charge of the beam.)
When the ion beam strikes the wafer surface, low energy electrons are emitted and the wafer tends to become positively charged. Generally, the net amount of positive charge delivered to the wafer is directly proportional to the beam current. In the case of a high current beam, the positive charge on the wafer tends to become quite high - in tens of volts.
When the wafer surface is well grounded to the vacuum enclosure and free of dielectric layers, the charge mainly flows to ground. However, ions are typically implanted after one or more dielectric layers have already been formed on the surface. These layers act as isolated islands on which the ion beam creates electrostatic charge.
The charge build up creates problems. The electrostatic charge interacts with the beam and causes it to lose density, which is a disadvantage because variations in ion beam density results in a nonuniform implantation process. Also, electrostatic charge may discharge and destroy the already formed dielectric layers. With smaller size integrated circuits, the susceptibility of dielectric layers to destruction by such discharge increases. Hence, there is low tolerance for surface charge buildup during ion implantation process.
A solution to these problems is to introduce a neutralizing charge, e.g electrons, to the surface of the wafer and/or to the beam before it contacts the wafer. One implementation of this method is to use a so-called electron shower to supply the neutralizing charge. Another is to use plasma-generating sources to supply low energy electrons and positive ions. Both of these methods typically apply the neutralizing charge near where the beam contacts the wafer.
Electron showers, however, typically supply a large number of high energy electrons which themselves contribute to charging of the wafer surface.
Plasma sources, which typically supply a higher proportion of low energy electrons and ions than do electron showers, not only better neutralize the beam and the surface charge but also contribute less to negative charge buildup on the wafer. When using a plasma source, however, a large plasma density is required to neutralize the beam. The required density can increase the pressure in the vacuum enclosure and degrade the efficiency of the implantation process. Moreover, a uniform and dense plasma is necessary. This is a particularly stringent requirement in respect of scanned beams, because of the wide scanning path. In the case of high current, magnetically scanned beams, the scanning area can be quite large, e.g. approximately 400 mm ×100 mm.
SUMMARY
In one general aspect, the invention features an ion implanter for implanting ions in a workpiece. The ion implanter includes an apparatus for generating an ion beam and directing it toward a surface of a work piece. The ion implanter also includes a plasma generator for generating plasma to neutralize the ion beam and the work piece surface, where the plasma generator has a plasma generator chamber defined by walls, a relatively narrow outlet aperture for plasma produced in the chamber to leave the chamber to neutralize the beam and work piece surface, at least one cathode, and at least one anode spaced from the cathode and from the walls of the chamber. The plasma generator also has magnets arranged within the plasma generator chamber, adjacent the chamber walls to generate a magnetic field to deflect primary electrons emitted from the cathode from directly reaching the anode. The plasma generator also features a conductive shield, positioned within the chamber between the anode and the magnets, the shield having an electric potential selected to deflect electrons, the magnetic field and the conductive shield effective during operation to cause electrons from the cathode to trace extended paths to ionize gas within the chamber to generate plasma before reaching the anode.
Preferred embodiments of this aspect of the invention may include one or more of the following features.
The ion implanter can be for use with a scanned ion beam, and its chamber and its outlet aperture being elongated and arranged across a scanned path of the beam.
The apparatus for generating and directing the ion beam includes a magnetic scanner and the ion beam is a high energy beam.
The cathode is a spiral coil and the anode is in a generally surrounding relationship with the cathode. The implanter also has a second anode and cathode. The magnets of the plasma generator generate a magnetic cusp field that generally contains the plasma along an axis connecting the cathodes.
The chamber comprises a wall onto which the cathode is mounted and the wall has substantially the same potential as the cathode so as to deflect electrons from the wall.
Other magnets surround the relatively narrow outlet aperture and the other magnets create a magnetic field for preventing primary electrons from leaving the chamber.
The chamber has a generally trapezoidal cross-section and the relatively narrow outlet aperture is located at a narrow end of the trapezoid. The relatively narrow outlet aperture is a discontinuous slit. Two magnets are located at either side of the relatively narrow outlet aperture and the two magnets either create a magnetic field for preventing primary electrons from leaving the chamber.
The chamber includes a wall opposing the cathode, and the magnets comprise a series of linear magnets disposed in parallel and inside of the chamber walls, a side facing inside the chamber and an opposing side facing outside having opposite polarity. The adjacent magnets are of opposite polarity and are sized and arranged in a pattern along the tube to create a magnetic field that generally contains the plasma along an axis between the cathode and the opposing wall.
A second anode and cathode are positioned on the opposing wall. The chamber walls are cooled. The chamber walls are water-cooled.
A drift tube defined by walls through which the ion beam passes before reaching the workpiece is opened into by the aperture opens into the tube. A series of parallel, linear magnets are positioned perpendicular to the general path of the ion beam. The magnets have a side facing the beam and an opposing side facing away from the beam, the two sides of the magnets having opposite polarity. The adjacent poles of adjacent magnets are of opposite polarity and the magnets are sized and arranged in a pattern along the tube to create a magnetic field that prevents the plasma from reaching the walls of the drift tube and to enable any deflection of the ion beam produced by one magnet pole to be substantially neutralized by the successive magnet pole of the next adjacent downstream magnets.
The drift tube has generally a rectangular cross-section and allows for a beam scanning path in order of 40 cm. The conductive shield is constructed from graphite.
The plasma generator is tilted relative to a general path of the beam. The plasma generator is characterized by a discharge axis defining an axis of discharge of plasma from the plasma generator and the discharge axis is either perpendicular to a general path of a beam or at an angle to a general path of a beam. The discharge axis may be tilted toward the wafer.
The ion implanter may be for use with a scanned ion beam and the chamber and its outlet aperture are arranged perpendicular to a scanned path of the beam. A drift tube defined by walls through which the ion beam passes
Dudnikov Vadim G.
Farley Marvin
Nasser-Ghodsi Mehran
Anderson Bruce C.
Ebara Corporation
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