1. Field of the Invention
The present invention relates to an ion implanter for use in semiconductor manufacturing, and more particularly to an ion implanter equipped with an interceptor which intercepts undesired impurities in an ion beam.
2. Background of the Related Art
An ion implantation process, which facilitates control of impurity concentration and junction depth, is widely used to form an impurity layer in a semiconductor device, e.g., a source/drain area of a transistor. The ion implantation process involves ionizing a gas containing desired impurities, accelerating the ionized impurities, and implanting the result into a predetermined area of a semiconductor wafer.
Referring to FIG. 1, in a conventional ion implanter, an ion beam emitted from an ion source 10 passes through a pre-analyzing magnet 12 to remove undesired types of ions. Since ions having identical energies in a magnetic field exhibit different bending effects due to their mass, only those beams of the desired ions pass through the pre-analyzing magnet 12.
After passing through the pre-analyzing magnet 12, the ion beam is accelerated to a desired energy by an accelerator 14. Meanwhile, negative ions accelerated by the accelerator 14 reach a stripper canal (not shown). Here, the negative ions are changed into positive ions by a charge exchange process involving collisions with a gas such as N.sub.2 gas. The positive ions pass through a post-analyzing magnet 16 and finally reach a wafer 18.
However, during the ion implantation process, undesired impurities can be generated at a main acceleration portion of the accelerator 14. The impurities generally have a different radius of curvature as compared to the high energy ion beam in the post-analyzing magnet 16, so that they are prevented from reaching the wafer 18. However, on occasion, the product of the mass M and the energy E of some impurities can be the same as that of the ion beam. In such cases, since the impurities have the same radius of curvature as the ion beams, they are likely to pass through the post-analyzing magnet 16 and reach the wafer 18. For example, impurities accelerated to several tens of kilo electron volts (KeV) can have the same radius of curvature as ions accelerated to hundreds of KeV, depending on the type of impurities. Thus, undesired impurities can pass through the post-analyzing magnet 16.
Even if the impurities reach the wafer 18, it is not easy to verify the arrival of the impurities. Also, even if only a very small amount of undesired impurities reach the wafer 18 and are present on the surface of the wafer 18, the undesired impurities can degrade the quality of a gate oxide film formed thereon and cause the gate oxide film to grow to an undesired thickness. As a result, the reliability of the semiconductor device is deteriorated, and the yield is reduced.