Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Electron or ion source
Reexamination Certificate
2001-11-07
2003-07-15
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Electron or ion source
C451S041000, C219S121410, C156S345420
Reexamination Certificate
active
06593699
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention relates generally to batch ion implantation systems, and more particularly to an improved system, apparatus, and method for reducing particle generation and surface adhesion forces in a batch ion implanter, while maintaining a high heat transfer coefficient.
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 material 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.
Ion implanters may be separated into two different categories. The first category includes serial ion implanters, in which semiconductor wafers or other workpieces are completely implanted with ions in serial fashion. This type of implanter includes a single workpiece pad adapted to hold or support the workpiece being implanted. The second category of ion implanters includes batch implanters, wherein a plurality of wafers or other workpieces may be implanted with ions in a single batch. The workpieces being implanted are mounted on individual workpiece pads in a rotatable process disk. The workpiece pads are typically located on individual pedestals extending outward from a center portion of the process disk at a slight angle so as to use centrifugal force to keep the workpieces seated in the pads as the process disk is rotated in a controlled fashion via a drive motor. The ion source is located so as to present ions along a beam path offset from the rotational axis of the process disk, and thereby to implant ions onto the workpieces as they rotate into the beam path. This method of ion implantation is sometimes referred to as spinning disk ion implantation.
As ions are implanted in the workpieces in a batch ion implantation process, heat is generated within each workpiece, which may cause workpiece damage or other deleterious effects if the heat is not removed from the workpiece. Conventional batch ion implantation systems and apparatus remove heat from the process disk and pedestals onto which the workpieces are mounted using internal passages through which cooling fluid such as water is circulated. Heat is removed from the workpieces to the process disk through workpiece pads comprising vulcanized rubber or RTV on which the workpieces are seated. Therefore, one function of the workpiece pads are to transfer heat from each workpiece to the process disk. Another function of the workpiece pads are to provide a tacky surface whereon each workpiece resides, whereby the workpiece can be sufficiently retained during the rotation of the process disk.
RTV workpiece pads are typically formed via a molding process, wherein liquid-state RTV material is cured after being applied to a surface of a mold which has been coated with a mold release agent. A conventional mold surface comprises a lapped metal plate, wherein a rough surface comprising substantially random peaks and valleys are formed as a result of lapping the metal plate with lapping compound. An imprint of the peaks and valleys are further transferred to the RTV workpiece pad via the molding process.
The peaks and valleys on a conventional workpiece pad formed via a lapped metal plate mold facilitate removal of the wafer from the workpiece pad after ion implantation. The rough surface on the workpiece pad is utilized to decrease a total contact area between the workpiece and the workpiece pad in order to reduce surface adhesion forces. During a conventional batch ion implantation process, for example, a workpiece such as a silicon wafer is placed on each RTV workpiece pad. The wafers are then rotated through an ion beam, whereby ions are implanted in the silicon wafers. Rotating the process disk results in an increased normal force on each workpiece pad caused by centrifugal force pushing the wafer onto the workpiece pad. This normal force compresses the RTV pad, thereby further increasing the surface contact area between the wafer and the RTV pad. This increase in surface contact area increases the heat transfer from the wafer to the pedestal, as well as increasing the surface adhesion of the wafer to the RTV pad.
One disadvantage of utilizing a lapped metal plate for the mold surface is that contaminants comprising both lapping compound and fine particles of metal created during the lapping process are typically present on the mold surface after the lapping process and cleaning thereafter. These contaminants can be transferred to the RTV pads during the molding process, and can further be transferred to the workpiece or wafer substrate when placed on the RTV pads for ion implantation. Furthermore, a mold release agent is typically utilized to prevent sticking of the RTV material to the mold surface. The mold release agent is transferred to the workpiece pad, and, if not thoroughly cleaned and removed from the workpiece pad after molding, can further be transferred to the workpiece or wafer substrate. Contaminants on the workpiece can cause many detrimental effects to the quality of the resulting ion implanted workpiece.
Another disadvantage of utilizing a workpiece pad formed via a lapped metal plate mold is poor dimensional uniformity and location of the peaks and valleys. Poor dimensional uniformity can create locations on the workpiece pad which have greater surface contact area than other locations on the workpiece pad, thus causing non-uniform surface adhesion and non-uniform heat transfer properties. Furthermore, structural dimensions of the peaks and valleys formed by utilizing the lapped metal plate mold are limited by the grain size of the lapping compound utilized during the formation of the mold. These disadvantages can cause areas of the workpiece pad which adhere non-uniformly to the wafer. Such non-uniformity of adhesion of the wafer to the workpiece pad is disadvantageous such that a removal of the wafer from the workpiece pad can cause either cracking of the wafer, or disintegration of part of the wafer or workpiece
Axcelis Technologies Inc.
Eschweiler & Associates LLC
Lee Wilson
Wong Don
LandOfFree
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