Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-03-03
2002-05-21
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S442110
Reexamination Certificate
active
06392245
ABSTRACT:
The present invention relates to improvements in scanning wheels used in ion implantation process chambers.
Ion implantation is used for doping semiconductor materials to achieve desired conductivity states. A beam of ions of a desired species for implantation is formed and directed at the surface of a semiconductor wafer for implantation therein. In batch processing ion implanters, it is a usual practice to arrange for a batch of semiconductor wafers to be processed simultaneously, and for these wafers to be repeatedly scanned through the ion beam to ensure a homogenous distribution of ions over the surfaces of the wafers.
U.S. Pat. No. 5,389,793 discloses a scan wheel used for scanning a batch of wafers in the process chamber of an ion implanter. The scan wheel disclosed in this U.S. patent specification comprises a central hub and a plurality of separate spoke arms mounted to the central hub, each having at its outer extremity a wafer support element for carrying a semiconductor wafer. The spoked scan wheel is rotated rapidly about an axis to achieve scanning of the wafers through the beam in one co-ordinate direction. At the same time, the axis of rotation of the scan wheel is reciprocated to and fro to achieve scanning of the wafers in a second co-ordinate direction. The combined scanning motions produce a raster scanning of the ion beam across each of the wafers held on the wafer support elements of the scan wheel.
The present invention is concerned with improvements to the design of the scan wheel as disclosed in the above U.S. patent specification. There is a need for increased processing speed, a reduction in the level of contamination in processing chambers, and the ability to handle larger wafers.
In one aspect the invention provides a scan wheel for an ion implanter adapted for carrying a plurality of semiconductor wafers, the scan wheel being rotatable about an axis for scanning wafers carried by the wheel across an ion beam generated in the implanter, said scan wheel comprising a central hub, a plurality of separate spoke arms mounted to said central hub and extending radially outward therefrom, and a plurality of wafer support elements formed on the outer ends of respective said spoke arms and each including a front face for mounting a semiconductor wafer thereon, said spoke arms each having a dimension in the direction of rotation which is substantially less than the corresponding dimension of the wafer support element, and said spoke arms each having a front face extending generally in said direction of rotation and side faces extending rearwardly from said front face, wherein, at least over a predetermined radial distance inwards from an innermost edge of the respective wafer support element, each of said side faces has a major part which does not extend outwards beyond a rearward projection of said front face which, in cross-section, tapers symmetrically inwards, preferably by 7°, on each side relative to said axis of rotation.
The design of scan wheel using relatively narrow spoke arms supporting the larger wafer support elements reduces the surface area of the scan wheel which is irradiated by the ion beam during that part of the scanning process, called maximum over scan, when the axis of rotation of the scan wheel is at its closest to the ion beam so that the ion beam is just clear of the innermost edges of the wafers mounted on the wafer support elements. This is desirable not only to reduce the thermal loading on the scan wheel resulting from the energy of the ion beam dissipated on the spoke arms, in addition to the energy dissipated on the semiconductor wafers supported by the support elements, but also to reduce the area of the scan wheel from which contaminants may be sputtered during irradiation by the ion beam. Any such sputtered contaminants may produce contamination on the surface of the semiconductor wafer being processed and increase unwanted material on the processed wafers.
It is established practice to arrange for the axis of rotation of the scan wheel to be adjustable relative to the direction of the ion beam, with the effect that the angle of implant of ions into the semiconductor wafers can be correspondingly adjusted. In implanters available currently, implant angle is typically adjustable between plus and minus 10° to the normal. An arrangement of this kind is shown in U.S. application Ser. No. 08/626,746.
In the design of process chamber illustrated, adjusting the axis of rotation of the scan wheel to provide a desired implant angle has the effect of adjusting the angle of the semiconductor wafers as they traverse the ion beam about an axis generally parallel to the spoke arm on which the semiconductor wafer is mounted. As a result, when the implant angle is set to be other than normal to the wafer surface, the spoke arms traverse the ion beam also with a corresponding longitudinal angle of rotation.
By arranging for the side faces of the spoke arms not substantially to extend beyond a tapered rearward projection of the front face of the spoke arm, the side faces can be shadowed by the front face of the respective arm for a range of implant angles on either side of normal. This is important since the total area of the spoke arms irradiated by the ion beam during this part of the scan is thereby reduced. Indeed, for a taper of 7° and implant angles of less than about 7°, the side faces of the spoke arms should scarcely be irradiated by the ion beam.
It is also known from U.S. Pat. No. 4,937,206 to coat areas of a scan wheel which are liable to be irradiated by the ion beam with a process compatible material, typically silicon. This minimises contamination problems from any of the material being sputtered from the scan wheel during processing.
By minimising the risk of the ion beam contacting the side faces of the spoke arms, it can be unnecessary to provide any such compatible coating on the side faces, so that only the front face of the spoke arms need be coated.
Preferably, each spoke arm comprises a first radial flange piece extending generally in the plane of rotation of the scan wheel and providing said spoke arm front face, and at least one radial web piece extending generally in an axial plane and connected to a rear face of the flange piece. In a particular embodiment, each spoke arm includes two said radial web pieces having a space between them in the direction of rotation.
This construction provides excellent strength and rigidity with reduced mass.
The radial web pieces have respective outwardly directed side faces which form said major parts of said spoke arm side faces.
Preferably, the major parts of the spoke arm side faces do not extend outwards beyond a rearward projection of said front face which, in cross-section, tapers symmetrically inwards by 10°, and more preferably by 12°-13°, on each side relative to said axis of rotation. This increased tapering effect allows a maximum implant angle of up to plus or minus 10° without the ion beam impinging on the side faces of the spoke arms.
In a preferred embodiment, each said wafer support element front face is adapted to mount a semiconductor wafer thereon in a wafer mounting plane defining a wafer axis normal to said wafer mounting plane, and, at least over said predetermined radial distance inwards from an innermost edge of the respective wafer support element, each spoke arm front face has no region facing towards the respective said wafer axis. This arrangement is preferably, though not necessarily, used with the tapered profile feature described above.
By ensuring that the spoke arm front faces do not face towards the wafer axis over the predetermined radial distance, the danger of contamination of the wafer during over scanning, i.e. when the ion beam impinges on the spoke arm front faces, is reduced. Material sputtered from the front faces of the spoke arms by the impinging ion beam have an angular distribution relative to the surface of the front face with a maximum at the normal to the front face. Therefore, if the front face of the spoke arm faces towards the a
Cooke Richard
Edwards Peter I. T.
Applied Materials Inc.
Boult Wade & Tennant
Nguyen Kiet T.
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