Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2000-02-21
2002-08-06
Rose, Robert A. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S254000, C451S146000
Reexamination Certificate
active
06428397
ABSTRACT:
FIELD OF INVENTION
This invention concerns the polishing of the edges of semi-conductor wafers such as silicon wafers and apparatus therefor.
BACKGROUND OF THE INVENTION
When silicon has been crystallised into large boules, which are generally cylindrical over most of their length, the crystalline material is sliced into thin circular discs known as silicon wafers, the diameter of each of which can be anything within the range 25-300 mm. After mounting in a vacuum chuck in an edge grinding machine, the peripheral edges of the discs are accurately ground so as to produce an accurate circular shape centered on the centre of the disc and a precisely formed edge profile, and are often notched at one position around the circular periphery.
Whereas over the majority of the area of disc the thickness is uniform and the two faces are parallel, the periphery of such discs are ground to a triangular section in which the apex of the triangle defines the outermost diameter of the wafer and is normally located midway between the two parallel faces of the disc.
In order to optimise the manufacture of semi-conductor devices using such a disc, it is important that the surface which is to be exposed to photo-lithography is damage free and in view of the microscopic size of the microcircuits which are formed on the wafers, damage near to the circumferential line between which the flat circular surface merges with the inclined surface leading to the apex, can significantly reduce the number of devices which can be made from the wafer.
In machines which do not produce a good quality ground finish to the inclined surfaces of the triangular section leading to the apex, it is necessary to etch and then polish the edges of the wafer before it can be used in the construction of semi-conductor devices. Where a good quality ground finish is produced on the inclined surfaces leading to the apex, it is only necessary to etch and polish a small quantity of material in order to obtain an undamaged edge.
It is an object of the present invention to provide an improved method and apparatus for polishing a finely ground wafer edge.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a method of edge grinding and polishing circular workpieces which are transported in turn to a grinding station in which the edge of the workpiece is ground, and after grinding, the workpiece is transferred to a buffer store whilst the next workpiece is conveyed to and located in the grinding station for grinding, which further comprises the steps of rotating the workpiece in the buffer store and engaging the edge thereof with a polishing wheel, to polish its periphery whilst the next workpiece is being ground, and after polishing, the polished workpiece is removed and the said next wafer is transferred from the grinding station to the buffer store for polishing to allow a further workpiece to be located in the grinding station.
The workpieces awaiting grinding may be contained in a first store and the ground and polished workpieces may be contained in a second store. The first and second stores may be different sections of a single storage device.
The invention is particularly applicable to circular semi-conductor wafers.
The buffer store conveniently comprises a wash station and in accordance with a preferred aspect of the invention, a method of edge grinding and polishing wafers as aforesaid includes the step of washing the ground wafer before the polishing step.
The invention also lies in a method in which the wafer is washed after it has been polished.
The invention also lies in a method in which the wafer is washed both before and after the polishing step.
In accordance with a preferred feature of the invention, the method includes the step of forming and reforming a groove in the polishing wheel after one or more polishing steps have been performed.
The forming and reforming of the groove in the wheel may be effected by means of a groove-forming wheel having a triangular cross-section peripheral rim, defining in cross-section the two slanting frusto-conical edges which converge in an apex, and after polishing one or more wafers, relative movement is effected between the groove-forming wheel and the polishing wheel whilst both are rotating about their respective axes, thereby to form or reform the cross-sectional shape of the groove in the polishing wheel ready for polishing the edges of further wafers.
According to another aspect of the invention, apparatus for grinding the edges of circular wafers of semi-conductor material comprises a store within which disc-like wafers of semi-conductor material can be stored, a grinding station, means for conveying the wafers one at a time to the grinding station for effecting edge grinding thereof and after grinding conveying each wafer in turn to a buffer store and locating the wafer on a storage chuck therein, said conveying means selecting the next wafer from the store and conveying it to the grinding station to effect edge grinding thereof, a polishing wheel in the buffer store, means for rotating it and the wafer therein, means for effecting relative movement between the wafer and the polishing wheel to polish the edge of the wafer whilst the next wafer is being ground, and said conveying means serving to remove the polished wafer from the buffer store and selecting a further wafer from the store and conveying it to the grinding station for edge grinding thereof, before moving to the buffer store to recover the polished wafer from therein.
Preferably in methods and apparatus embodying the invention, a slurry is conveyed to the point of contact between the wafer and the polishing wheel so as to assist in the polishing action.
Typically the polishing wheel is compliant and is formed from a material such as urethane or serium oxide.
Typically the slurry comprises an alkali solution, preferably of pH11.
The method of the invention may preferably include the further step of effecting a rocking movement between the polishing wheel and the wafer edge by rocking either the wheel or the wafer about an axis, coincident with or parallel to a tangent which intersects the point of contact between the wafer and the polishing wheel.
In a preferred arrangement the wafer and the grooved polishing wheel are mounted for rotation about horizontally spaced apart parallel axes, the wafer is supported on a vacuum chuck and is rotated by a drive motor, the polishing wheel is driven in rotation by a second drive means, linear drive means is provided to advance and retract the polishing wheel relative to the wafer, and oscillatory drive means acts on the assembly of polishing wheel and associated drive so as to rock the wheel about a vertical axis so that the two side cheeks of the groove in the polishing wheel alternately engage the inclined edges of the edge region of the wafer.
In an alternative arrangement the wheel and its rotational drive remains stationary except for the rotation of the wheel about its axis, and linear translation means is provided for advancing the wafer and associated drive and support means towards and away from the grooved polishing wheel to engage the edge of the wafer in the groove, and oscillatory drive means serves to rock the wafer support and therefore the wafer thereon about a vertical axis so as to produce the alternating engagement between the external periphery of the wafer and the sides of the groove as the wafer and polishing wheel rotate.
References to oscillatory drive means include rocking drives and drives which rotate first in one direction and then the other about a single axis.
According to a further aspect of the invention, it is to be understood that the technique of rocking the wafer or the wheel whilst rotating so as to alternately engage the two inclined faces of the periphery of the wafer with the corresponding faces of the groove in the wheel, is not linked to an arrangement in which the wafer is mounted in a buffer store on, or associated with, an edge grinding machine. This further aspect of the invention can be
Lee Mann Smith McWilliams Sweeney & Ohlson
Rose Robert A.
Unova U.K. Limited
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