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

2001-05-24

2004-07-06

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S3960ML

Reexamination Certificate

active

06759665

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to ion implantation systems, and more specifically to an improved method and system for ion beam containment in an ion beam guide.
BACKGROUND OF THE INVENTION
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters are used to treat silicon wafers with an ion beam, in order to produce n or p type extrinsic materials doping or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductors, the ion beam implanter 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.
Typical ion beam implanters include an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
For shallow depth ion implantation, high current, low energy ion beams are desirable. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions which tend to diverge due to mutual repulsion. To maintain low energy, high current ion beam integrity at low pressures, a plasma may be created to surround the ion beam. High energy ion implantation beams typically propagate 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, thereby largely eliminating transverse electric fields that would otherwise disperse the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is very low. Moreover, 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 chamber walls.
In ion implantation systems, there remains a need for a beam containment apparatus and methodologies for use with high current, low energy ion beams which may be operated at low pressures, and which provides uniform beam containment along the entire length of a mass analyzer beam guide.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for providing a low energy, high current ion beam for ion implantation applications. The invention provides ion beam containment without the introduction of auxiliary plasma and instead enhances beam plasma associated with the ion beam by utilizing the background gas in the beam guide to create the additional electrons required for adequate beam containment. This is accomplished by providing a multi-cusped magnetic field in a beam guide passageway in order to create a magnetic mirror effect in a controlled fashion, as illustrated and described in greater detail hereinafter.
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 beam guide. 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 center, 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 the 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. Because mass analyzers inherently involve magnetic fields, externally generated plasma fails to diffuse adequately along the arcuate length of a mass analyzer beam guide, instead diffusing quickly along the direction of the magnetic field lines. The use of a multi-cusped magnetic field in accordance with the present invention provides for enhancement of the beam plasma in a low pressure, low energy, high current ion beam system through the controlled creation of a magnetic mirror effect in the passageway.
Additional plasma may also be generated within the ion beam space by electric fields at RF or microwave frequencies. This RF or microwave energy is transferred efficiently to plasma electrons, when a proper magnetic field is present, at a magnitude that yields the ECR condition. The RF or microwave energy may be introduced into the passageway at an appropriate port in the beam guide via any number of coupling methods (e.g., windows, antennas, and the like). Although the dipole magnetic field alone might be employed for the creation of an ECR condition, the selection of the dipole magnetic field strength for a mass analysis magnet is dictated by the momentum of the particle selected for implantation. Consequently, the RF or microwave power source frequency would need to be tuned to that which provides the ECR condition according to the dipole magnetic field strength.
For example, for very low energy Boron beams, the dipole magnetic field is well below the ECR condition at the common 2.45 GHz microwave frequency. Lower frequency energy sources (or variable frequency sources) are available, but are costly. In addition

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