Method and system for ion beam containment in an ion beam guide

Details Method and system for ion beam containment in an ion beam guide Method and system for ion beam containment in an ion beam guide

2003-05-23

2004-03-09

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

Reexamination Certificate

active

06703628

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to ion implantation systems, and more specifically to an improved apparatus and systems for ion beam containment in an ion implantation system.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters or ion implantation systems are employed to treat silicon wafers with an ion beam, so as to produce n or p type doped regions or to form passivation layers during fabrication of integrated circuits. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers, whereas if p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted. Ion implantation systems typically include an ion source for generating positively charged ions from such ionizable source materials. The generated ions are extracted from the source and formed into an ion beam, which is directed along a predetermined beam path in a beamline assembly to an implantation station, sometimes referred to as an end station. The ion implantation system may include beam forming and shaping structures extending between the ion source and the end station, which maintain the ion beam and bound an elongated interior cavity or passageway through which the beam is transported en route to one or more wafers or workpieces supported in the end station. The ion beam transport passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path through collisions with air molecules.
The charge-to-mass ratio of an ion affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Ion implantation systems typically include a mass analyzer in the beamline assembly downstream of the ion source, having a mass analysis magnet creating a dipole magnetic field across the beam path in the passageway. This dipole field operates to deflect various ions in the ion beam via magnetic deflection in an arcuate section of the passageway, which effectively separates ions of different charge-to-mass ratios. The process of selectively separating ions of desired and undesired charge-to-mass ratios is referred to as mass analysis. In this manner, the beam imparted on the wafer can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam path and implantation of other than desired materials can be avoided.
High energy ion implantation is commonly employed for deeper implants in a semiconductor wafer. Conversely, high current, low energy ion beams are typically employed for shallow depth ion implantation, in which case the lower energy of the ions commonly causes difficulties in maintaining convergence of the ion beam. In particular, high current, low energy ion beams typically include a high concentration of similarly charged (positive) ions which tend to diverge due to mutual repulsion, a space charge effect sometimes referred to as beam blowup. Beam blowup is particularly troublesome 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, while the low propagation velocity (low energy) of the ions expose them to these mutually repulsive forces for longer times than in high energy applications. Space Charge Neutralization is a technique for reducing the space charge effect in an ion implanter through provision and/or creation of a beam plasma, comprising positively and negatively charged particles as well as neutral particles, wherein the charge density of the positively and negatively charged particles within the space occupied by the beam are generally equal. For example, a beam plasma may be created when the positively charged ion beam interacts with residual background gas atoms, thereby producing ion electron pairs through ionizing collisions during beam transport. As a result, the ion beam becomes partially neutralized through interaction with the background residual gas in the beam path.
In the case of high energy ion implantation, the ion beam typically propagates through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. This plasma tends to neutralize the space charge caused by the ion beam by providing negatively charged electrons along the beam path in the passageway, thereby largely eliminating transverse electric fields that would otherwise disperse or blow up the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is very low. Also, in the dipole magnetic field of a mass analyzer, plasma diffusion across magnetic field lines is greatly reduced while the diffusion along the direction of the field is unrestricted. Consequently, introduction of additional plasma to improve low energy beam containment in a mass analyzer is largely futile, since the introduced plasma is quickly diverted along the dipole magnetic field lines to the passageway sidewalls. Furthermore, low energy implantation systems typically suffer from electrons being lost to the sidewalls along the beamline assembly, which reduces the number of such electrons available for space charge neutralization. Thus, there is a need for improved ion implantation systems and apparatus for reducing electron loss to enhance space charge neutralization and prevent or reduce beam blowup.
SUMMARY OF THE INVENTION
The present invention is directed to Ion implantation systems and beamline assemblies, in which multi-cusped magnetic fields are provided in a beamguide and the beamguide is energized to provide microwave electric fields in a traveling wave along the beamguide passageway. The magnetic and electric fields interact to provide an electron-cyclotron resonance (ECR) condition for beam containment in the beamguide passageway. The invention may be employed in conjunction with the transport of ion beams of any energy and form factor, such as low or ultra-low energy ion beams having circular or elongated profiles (e.g., pencil beams and/or ribbon beams) or beams of other shapes.
The inventors have appreciated that ion beams propagating through a plasma, such as the beam plasma created by beam interactions with the residual or background gas, reach a steady state equilibrium wherein charges produced by ionization and charge exchange are lost to the beamguide in ion implanters. The remaining plasma density results from a balance between charge formation due to the probability of ionizing collisions, and losses from the beam volume due to repulsion of positive charges by the residual space charge and electron escape as a result of kinetic energy. Absent plasma enhancement through the introduction of externally generated plasma or enhancement of the beam plasma, the probability for ionizing collisions with the background gas at very low ion beam energies is relatively low. Electrons generated in such a manner are trapped in the beam's large potential well, orbiting around and through the beam, interacting with each other by Coulomb collisions, resulting in thermalization of the electron energy distribution. Those electrons in the distribution having an energy greater than the ionization potential of a residual gas molecule have a probability of ionizing such a molecule. The ionizing probability decreases as the electron energy decreases.
In a low energy beam plasma, the majority of the ionization is produced by the trapped electrons. These electrons derive their energy from the center-to-edge beam potential difference, which is the same parameter that causes beam “blow-up”. Thus, transportation of low energy ion beams is difficult absent externally generated plasma or enhancement of the beam plasma. Becau

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