Acceleration and analysis architecture for ion implanter

Details Acceleration and analysis architecture for ion implanter Acceleration and analysis architecture for ion implanter

2000-07-26

2001-11-06

Anderson, Bruce (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S42300F, C250S281000, C250S298000

Reexamination Certificate

active

06313475

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to systems and methods for ion implantation of semiconductor wafers and, more particularly, to a beamline architecture for generating a high purity energetic ion beam.
BACKGROUND OF THE INVENTION
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity materials is ionized in an ion source, the ions are accelerated to form a ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or a combination of beam scanning and target movement. Examples of prior art ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al; and U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. Device manufacturers need to critically control the depth distribution of implanted dopants. To achieve this, the ion implanter must critically control the energy of ions that impinge on the wafer surface. The requirement for energy control affects many requirements, such as power supply stability. However, the performance of ion implanters has been limited by less obvious causes of energy contamination, which is the presence in the ion beam of particles with energies that differ from the desired implant energy.
Energy contamination can result from interaction between ions in the beam and residual gas molecules in the system. Charge exchange reactions may change the charge state of beam ions when they interact with neutral molecules in the system. As might be expected, the probability of such an exchange occurring depends on the neutral gas density and therefore the system pressure. If, after such a reaction, the beam is accelerated by an electric field, then the ions that have changed charge state will, in the absence of further analysis, reach the target with the incorrect energy. This is because the energy gained by an ion in traversing an accelerating or decelerating electric field is proportional to the charge state of the ion.
The energy range of ion implanter is often extended by tuning the system to transport the multiply charged ions that are produced by the source. In this way, for example, instead of using a 200 KV accelerator to implant 200 keV singly charged ions, 400 keV doubly charged ions can be implanted with suitable tuning. This approach, however, has problems due to the molecular ions that are produced by the source. Consider, for example, that the required ion on target is P
++
. Although the source may be tuned to maximize production of P
++
ions, it will also generate other ions and in particular generates P
2
+
ions. This molecular ion is a well-known source of energy contamination, since it can break up to form P
+
ions at almost exactly one-quarter of the energy of the required P
++
ions. Magnetic analysis can not distinguish between P
++
ions and P
+
ions at one-quarter of the energy, and so ions at lower than required energy reach the target.
Along with ions of the required species, implanters often deposit contaminants onto the wafer surface. The contaminants may be in the form of particles or ions and molecules of another species. The contaminants can be produced by the ion source and transported through the beamline or, alternatively, may be generated by sputtering by energetic ions impinging on surfaces in the beamline.
Accordingly, there is a need for ion implanters wherein the ion beam that is implanted into the semiconductor wafer has low energy contamination and a low content of contaminants.
SUMMARY OF THE INVENTION
According to one aspect of the invention, an ion beam generator is provided. The ion beam generator comprises an ion beam source for generating an ion beam, an acceleration/deceleration column for selectably accelerating or decelerating ions in the ion beam to desired energies, a source filter positioned between the ion beam source and the acceleration/deceleration column for removing first undesired species from the ion beam, and a mass analyzer positioned downstream of the acceleration/deceleration column for removing second undesired species from the ion beam.
The source filter may comprise a first dipole magnet for deflecting desired ion species and a first resolving aperture for passing the desired ion species. The mass analyzer may comprise a second dipole magnet for deflecting the desired ion species and a second resolving aperture for passing the desired ion species. In a preferred embodiment, the first dipole magnet deflects desired ion species by about 25°, and the second dipole magnet deflects desired ion species by about 90°. Preferably, the source filter has relatively low resolution and the mass analyzer has relatively high resolution.
The source filter is preferably located in close proximity to the ion beam source and may be located in a source enclosure with the ion beam source. In a preferred embodiment, the source enclosure comprises a first compartment containing the ion beam source and a second compartment containing the source filter. A passage interconnects the first and second compartments. A first vacuum pump may be coupled to the first compartment and a second vacuum pump may be coupled to the second compartment, so that the first and second compartments are differentially vacuum pumped.
The ion beam source may comprise an ion source for generating ions and an extraction electrode for extracting the ions from the ion source to form the ion beam. An extraction power supply may be coupled between the ion source and the extraction electrode for biasing the extraction electrode negatively with respect to the ion source. For operation in an acceleration mode, an acceleration power supply may be coupled between the extraction electrode and ground for biasing the extraction electrode positively with respect to ground. For operation in a deceleration mode, a deceleration power supply may be coupled between the ion source and ground for biasing the ion source positively with respect to ground.
The acceleration/deceleration column may comprise a terminal electrode, a ground electrode and a focus electrode positioned between the terminal electrode and the ground electrode. A focus voltage is coupled to the focus electrode for focusing the ion beam. The focus voltage may be adjustable.
According to another aspect of the invention, an ion implanter is provided. The ion implanter comprises an ion beam generator for generating a beam of energetic ions, a scanning assembly for deflecting the beam of energetic ions to form a scanned ion beam and an end station for supporting a semiconductor wafer in the path of the scanned ion beam, so that ions in the scanned ion beam are implanted into the semiconductor wafer. The ion beam generator comprises an ion beam source for generating an ion beam, an acceleration/deceleration column for selectably accelerating or decelerating ions in the ion beam to desired energies, a source filter positioned between the ion beam source and the acceleration/deceleration column for removing first undesired species from the ion beam, and a mass analyzer positioned downstream of the acceleration/deceleration column for remo

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