The present invention relates to a method of thinning semiconductor wafers by applying a protection tape to the front of a semiconductor wafer and conducting grinding/etching of the back thereof. The present invention also relates to a back grinding device for grinding the back of a semiconductor wafer with a protection tape attached to the front thereof.
In recent years, thinner semiconductor packages have been in demand. To realize the thinner semiconductor packages, a step of thinning wafers is introduced after completion of a wafer process in a semiconductor assembling operation. In this step, generally, a protection tape is attached to the front of a wafer, and then the back thereof is abraded by grinding.
Further, for the purpose of further thinning the wafers and increasing transverse strength of semiconductor chips by removing a wafer damage layer generated in a grinding process, the back of ground wafers is more and more often subjected to wet etching (hereinafter just referred to as etching).
Generally, when a protection tape is attached to the front of the wafer to grind the back thereof, the size of the protection tape is set to be equal to or larger than the size of the wafer. This is because in the grinding process, by thus setting the size of the protection tape which is disposed between a wafer and a grinding table, the protection tape is allowed to be uniformly present between the wafer and the grinding table. As a result, vibration of the wafer is suppressed and the stable grinding of the back is realized.
In a generally adopted method of attaching the protection tape to the front of the wafer, which is disclosed in Japanese Patent Laid-Open Publication No. HEI 6-310480, a long tape is applied to the front of a wafer, and then the tape is cut along the peripheral edge of the wafer. Another method in practical use is a method of attaching a sheet which has preparatorily been cut to be round to the front of a wafer.
FIGS. 5A-5C are schematic explanatory views showing a method of thinning a wafer in which the front of the wafer is first ground and then etched.
In this thinning method, as shown in FIG. 5A, a protection tape 2 is first applied to the front of a wafer 1, and then a portion of the protection tape 2 protruded from the wafer 1 is cut away along the peripheral edge of the wafer with a cutter 3. Next, as shown in FIG. 5B, the wafer 1 is fixed to a vacuum chuck table 4 for the grinding via the protect ion tape 2. Then, the vacuum table 4 is rotated, and the back of the wafer 1 is brought into contact with a rotating grinding wheel 5 to conduct grinding. After that, as shown in FIG. 5C, the ground wafer 1 is fixed to a vacuum chuck table 6 for the etching via the protection tape 2. The vacuum chuck table 6 is then rotated, and chemical liquid 7 is applied to the rotating wafer 1 from an upper position to conduct etching of the back of the wafer 1.
However, the conventional thinning method in which the back of wafers are ground and then etched has the following problems. First, in the case where the diameter W of the wafer 1 is larger than the diameter L of the protection tape 2, as shown in FIGS. 6A and 6B, the chemical liquid 7 tends to be accumulated on the protection tape 2 in an area around the periphery of the wafer 1 during the etching process. Consequently, part 7xe2x80x2 of the accumulated chemical liquid 7 infiltrates toward the front side of the wafer 1, and thereby contaminates the front of the wafer.
On the other hand, in the case where the diameter W of the wafer 1 is equal to the diameter L of the protection tape 2, the peripheral edge of the wafer 1 is etched by chemical liquid 7, which gradually decreases the diameter W of the wafer 1. As a result, the diameter L of the protection tape 2 becomes larger than the diameter W, which generates the same effect as stated above, resulting in contamination of the front of the wafer 1. Therefore, in the case of Wxe2x89xa6L, deterioration of quality and reliability of devices is inevitable.
Accordingly, it is an object of the present invention to provide a method of thinning a semiconductor wafer which can eliminate contamination of the front of the wafer to thereby provide good quality and reliability of semiconductor devices. It is also an object of the present invention to provide a back grinding device for grinding the back of a semiconductor wafer with a protection tape attached to the front thereof to implement the method of thinning the semiconductor wafers.
In order to accomplish the above object, the present invention provides a method of thinning a semiconductor wafer, comprising:
applying a protection tape to a front of the semiconductor wafer, and
etching a back of the semiconductor wafer to thereby thin the semiconductor wafer,
wherein the protection tape is smaller in size than the semiconductor wafer.
According to the above constitution, when a semiconductor wafer is fixed to a vacuum chuck stage via a protection tape and the back of the semiconductor wafer is etched, chemical liquid applied to the wafer falls down without remaining on the protection tape in the peripheral portion of the semiconductor wafer. This prevents the chemical liquid from infiltrating into a front surface of the semiconductor wafer and causing contamination of the surface.
In one embodiment, the method further comprises, between applying a protection tape to a front of the semiconductor wafer and etching a back of the semiconductor wafer,
grinding the back of the semiconductor wafer.
In one embodiment, a diameter W of the semiconductor wafer and a diameter L of the protection tape have the following relationship:
L=Wxe2x88x92a(0 mm less than axe2x89xa63 mm).
According to the above constitution, in the process of grinding the back, a stable grinding operation is performed without vibration generated by a grinding force of a grinding wheel which in turn would cause breakage of the periphery of the semiconductor wafer In addition, in the process of etching the back, the diameter of the protection tape disposed between the semiconductor wafer and the vacuum chuck stage will not become larger than the diameter of the semiconductor wafer, which ensures a stable etching.
Further, in one embodiment, the semiconductor wafer has a chamfered portion along an edge thereof, and no peripheral portion of the protection tape overlaps the chamfered portion of the edge of the semiconductor wafer.
According to the above constitution, because the peripheral portion of the protection tape attached to the semiconductor wafer does not overlap the chamfered portion of the wafer edge, the attached protection tape is not detached at its edge from the wafer.
The present invention also provides a back grinding device for semiconductor wafers for grinding a back of a semiconductor wafer with a protection tape attached to a front thereof, the protection tape being smaller in size than the semiconductor wafer, comprising:
a first vacuum chuck stage for vacuum-fixing a first area of the front of the semiconductor wafer thereto via the protection tape, the first area being an area in which the protection tape being attached to the front of the semiconductor wafer, and
a second vacuum chuck stage for vacuum-fixing a second area of the front of the semiconductor wafer thereto, the second area being a peripheral area of the front of the semiconductor wafer in which no protection tape is attached thereto.
With the above constitution, the first area attached with the protection tape is the front vacuum-fixed to the first vacuum chuck stage. Further, the second area attached with no protection tape is vacuum-fixed to the second vacuum chuck stage. Thus, the entire front of the semiconductor wafer is vacuum fixed. Consequently, vibration will hardly be generated when the peripheral portion of the back of the semiconductor wafer is ground, and therefore a stable grinding operation for the back of the semiconductor wafer is achieved without necessity of increasing particle size of the grinding wheel or decreasing the back grinding seed.
In one embodiment, the back grinding device further comprises:
a distance detector for detecting a distance between the second area of the semiconductor wafer and a surface of the first vacuum chuck stage to which surface the semiconductor wafer is fixed; and
a second vacuum chuck stage displacement means for raising the second vacuum chuck stage by a distance obtained from la detection result by the distance detector to bring the second vacuum chuck stage into contact with the second area of the semiconductor wafer and, upon completion of grinding the back of the wafer, lowering the second vacuum chuck stage to a level equal to the first vacuum chuck stage.
With the above constitution, when the second area of the semiconductor wafer is vacuum-fixed to the second vacuum chuck stage, the distance detector detects a distance between the surface of the first vacuum chuck stage to which the semiconductor wafer is fixed and the second area of the semiconductor wafer. Then, the second vacuum chuck stage displacement means raises the second vacuum chuck stage by a distance obtained on the basis of a detection result of the distance detector until the second vacuum chuck stage is brought into contact with the second area of the semiconductor wafer. Consequently, even if the thickness of the protection tape varies, the second area of the semiconductor wafer is not dragged nor thrust by the second vacuum chuck stage and thus, the semiconductor wafer is vacuum-fixed in a stable condition without bending.
Further, upon completion of grinding of the back of the wafer, the second vacuum chuck stage displacement means lowers the second vacuum chuck stage to a level equal to the first vacuum chuck stage. This enables simultaneous cleaning of the upper surfaces of the first vacuum chuck stage 23 and the second vacuum chuck stage 24, thereby facilitating removal of grinding dust and the like generated in the back grinding process.
Further, in one embodiment, the first vacuum chuck stage has a diameter slightly larger than or equal to a diameter of the protect tape.
According to the above constitution, when the second vacuum chuck stage is raised to be brought into contact with the second area of the semiconductor wafer, the second vacuum chuck stage is allowed to be raised quickly and smoothly to the contact position without touching the protection tape. In addition, the second vacuum chuck stage implements uniform vacuum-fixation of almost the entire second field.