FIELD OF THE INVENTION
The present invention relates to a method of grinding a wafer such as a semiconductor wafer to a predetermined thickness.
DESCRIPTION OF THE PRIOR ART
In the production process of a semiconductor device, a large number of rectangular areas are sectioned by dividing lines called “streets” formed in a lattice pattern on the front surface of a substantially disk-like semiconductor wafer, and a circuit is formed in each of the rectangular areas. Individual semiconductor chips are manufactured by dividing this semiconductor wafer having a large number of circuits along the dividing lines. In order to reduce the size and weight of each semiconductor chip, the back surface of the semiconductor wafer is generally ground to a predetermined thickness before the semiconductor wafer is cut along the dividing lines to separate individual rectangular areas from one another. To reduce the size and weight of the semiconductor chip, the semiconductor wafer is nowadays formed as thin as 100 μm or less.
To prevent inconvenience with that chippings are produced while the semiconductor wafer is transferred between steps before it is divided into semiconductor chips, the outer peripheral surface of the semiconductor wafer is chamfered arcuatedly. When the back surface of the semiconductor wafer having a chamfered portion at an outer periphery is ground to reduce the thickness of the wafer to half or less, a sharp knife-edge is formed in the arcuatedly chamfered portion. Therefore, there is a problem that the semiconductor wafer may be cracked during the grinding or transportation of the semiconductor wafer. Further, another problem arises that when the back surface of the semiconductor wafer is polished with a polishing cloth to remove a grinding mark or micro-cracks formed on the back surface of the semiconductor wafer, the polishing cloth is caught by the above knife-edge, whereby the semiconductor wafer is broken during polishing.
To solve the above problems, JP-A2003-273053 discloses a technology for cutting the arcuatedly chamfered portion at right angles to a top surface of the wafer by holding the semiconductor wafer on the chuck table of a cutting machine, positioning a cutting blade on the top surface of the outer peripheral portion of the semiconductor wafer, and turning the chuck table while the cutting blade is rotated.
When the wafer is cut along the outer periphery with the cutting blade, however, there exists a problem that since it is cut arcuatedly in defiance of the linear movement of the cutting blade, stress remains at the outer periphery of the wafer, thereby damaging the wafer during grinding. Further, it takes a long time to cut the wafer along the outer periphery with the cutting blade. For instance, when a silicon wafer having a diameter of 200 mm is cut along the outer periphery, it takes more than 30 minutes, thereby reducing productivity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a wafer grinding method capable of grinding a wafer to a predetermined thickness without damaging the wafer during grinding and without forming a sharp knife-edge at the outer periphery.
To solve the above main technical problems, according to the present invention, there is provided a wafer grinding method for grinding the surface to be ground of a wafer having an arcuatedly chamfered outer peripheral surface, comprising:
- an outer peripheral portion removal step for removing the outer peripheral portion of the wafer by applying a laser beam from one surface side of the wafer along the outer periphery at a location on the inside of the outer periphery by a predetermined distance; and
- a grinding step for grinding the surface to be ground of the wafer whose outer peripheral portion has been removed, to a predetermined finish thickness.
A plurality of function elements are formed on the front surface of the wafer, and the surface to be ground of the wafer is the back surface. The above outer peripheral portion removal step comprises applying a laser beam of a wavelength capable of passing through the wafer along the outer periphery to form an annular deteriorated layer along the outer periphery in the inside of the wafer and dividing the wafer along the deteriorated layer. Further, the above outer peripheral portion removal step is to form an annular groove which reaches the other surface side from one surface side along the outer periphery of the wafer by applying a laser beam of a wavelength having absorptivity for the wafer.
Further, according to the present invention, there is also provided a wafer grinding method for grinding the back surface of a wafer having a plurality of function elements on the front surface and an arcuatedly chamfered outer peripheral surface, comprising:
- a groove forming step for forming an annular groove deeper than at least the finish thickness of the wafer from the front surface of the wafer by applying a laser beam of a wavelength having absorptivity for the wafer from the front surface side of the wafer along the outer periphery at a location on the inside of the outer periphery by a predetermined distance; and
- a grinding step for grinding the back surface of the wafer having the groove formed thereon, to a predetermined finish thickness.
In the wafer grinding method of the present invention, since the outer peripheral portion removal step for removing the outer peripheral portion of the wafer by applying a laser beam along the outer periphery at a location on the inside of the outer periphery of the wafer by a predetermined distance is carried out before the grinding step for grinding the surface to be ground of the wafer, a sharp knife-edge is not formed at the outer periphery by grinding, even when the outer peripheral surface of the wafer is chamfered arcuatedly. Since the outer peripheral portion removal step is carried out by laser processing, stress, which is generated by cutting with a cutting blade, does not remain. Therefore, it is possible to prevent the wafer from being damaged by the residual stress during grinding. Further, since the outer peripheral portion removal step in the present invention is carried out by laser processing, its operation time can be greatly shortened as compared with the operation of cutting with the cutting blade.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a semiconductor wafer to be ground by the wafer grinding method of the present invention;
FIG. 2 is a perspective view of the principal section of a laser beam processing machine for carrying out the outer peripheral portion removal step or the groove forming step in the wafer grinding method of the present invention;
FIG. 3 is a block diagram schematically showing the constitution of a laser beam application means provided in the laser beam processing machine shown in FIG. 2;
FIG. 4 is a schematic diagram showing the focusing spot diameter of a pulse laser beam;
FIG. 5 is an explanatory diagram showing an embodiment of the outer peripheral portion removal step in the wafer grinding method of the present invention;
FIG. 6 is an explanatory diagram showing another embodiment of the outer peripheral portion removal step in the wafer grinding method of the present invention;
FIG. 7 is an explanatory diagram showing still another embodiment of the outer peripheral portion removal step in the wafer grinding method of the present invention;
FIG. 8 is a perspective view showing a state where a wafer subjected to the outer peripheral portion removal step in the wafer grinding method of the present invention has a protective member affixed to the front surface;
FIG. 9 is an explanatory diagram showing the step of grinding the back surface of the wafer that has been subjected to the outer peripheral portion removal step in the wafer grinding method of the present invention;
FIG. 10 is an explanatory diagram showing the groove forming step in the wafer grinding method of the present invention;
FIG. 11 is a perspective view showing a state where a wafer subjected to the groove forming step in the wafer grinding method of the present invention has a protective member affixed to the front surface; and
FIG. 12 is an explanatory diagram showing the step of grinding the back surface of the wafer that has been subjected to the groove forming step in the wafer grinding method of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the wafer grinding method of the present invention will be described in detail hereinunder with reference to the accompanying drawings.
FIG. 1 is a perspective view of a semiconductor wafer as a wafer to be ground according to the present invention. The semiconductor wafer 2 shown in FIG. 1 is, for example, a silicon wafer having a diameter of 200 mm and a thickness of 500 μm, and a plurality of dividing lines 21 are formed in a lattice pattern on the front surface 2a. And, a circuit 22 as a function element is formed in a plurality of areas sectioned by a plurality of dividing lines 21. The outer peripheral surface 2c of the outer peripheral portion of the semiconductor wafer 2 is chamfered arcuatedly. A description is subsequently given of a first embodiment of the method of grinding this semiconductor wafer 2 to a predetermined thickness.
In the first embodiment of the wafer grinding method of the present invention, the step of removing the outer peripheral portion of the wafer by applying a laser beam from one side of the wafer along the outer periphery of the wafer at a position on the inside of the outer periphery by a predetermined distance is first carried out. This outer peripheral portion removal step is carried out by using a laser beam processing machine 3 shown in FIGS. 2 to 4. The laser beam processing machine 3 shown in FIGS. 2 to 4 comprises a chuck table 31 for holding a workpiece and a laser beam application means 32 for applying a laser beam to the workpiece held on the chuck table 31. The chuck table 31 is so constituted as to suction-hold the workpiece on the top surface and is designed to be turned in the direction indicated by an arrow in FIG. 2 by a turning mechanism that is not shown.
The above laser beam application means 32 comprises a cylindrical casing 321 arranged substantially horizontally. In the casing 321, there are installed a pulse laser beam oscillation means 322 and a transmission optical system 323 as shown in FIG. 3. The pulse laser beam oscillation means 322 is constituted by a pulse laser beam oscillator 322a composed of a YAG laser oscillator or YVO4 laser oscillator and a repetition frequency setting means 322b connected to the pulse laser beam oscillator 322a. The transmission optical system 323 has suitable optical elements such as a beam splitter, etc. A condenser 324 housing condensing lenses (not shown) constituted by a combination of lenses that may be formation known per se is attached to the end of the above casing 321. A laser beam oscillated from the above pulse laser beam oscillation means 322 reaches the condenser 324 through the transmission optical system 323 and is applied from the condenser 324 to the workpiece held on the above chuck table 31 at a predetermined focusing spot diameter D. This focusing spot diameter D is defined by the expression D (μm)=4×λ×f/(π×W) (wherein λ is the wavelength (μm) of the pulse laser beam, W is the diameter (mm) of the pulse laser beam applied to an objective lens 324a, and f is the focusing distance (mm) of the objective lens 324a) when the pulse laser beam showing a Gaussian distribution is applied through the objective lens 324a of the condenser 324, as shown in FIG. 4.
The outer peripheral portion removal step that is carried out by using the above laser beam processing machine 3 will be described with reference to FIG. 5.
In this outer peripheral portion removal step, the semiconductor wafer 2 is first placed on the chuck table 31 of the above laser beam processing machine 3 in such a manner that the front surface 2a faces up, and suction-held on the chuck table 31. The chuck table 31 suction-holding the semiconductor wafer 2 is moved to a processing area where the condenser 324 is located, by a moving mechanism (not shown) to bring a location on the inside of the outer periphery of the semiconductor wafer 2 by a predetermined distance to a position right below the condenser 324, as shown in FIG. 5. Then, the chuck table 31, that is, the semiconductor wafer 2 is turned in the direction indicated by the arrow in FIG. 5 while a pulse laser beam of a wavelength capable of passing through the semiconductor wafer 2 is applied from the condenser 324. At this point, the focusing point P of the pulse laser beam applied from the condenser 324 is set to a position near the back surface 2b (undersurface) of the semiconductor wafer 2. As a result, an annular deteriorated layer 210 is exposed to the back surface 2b (undersurface) at the location on the inside of the outer periphery by a predetermined distance and formed from the back surface 2b (undersurface) toward the inside of the semiconductor wafer 2. This deteriorated layer 210 is formed as a molten and re-solidified layer (that is, as a layer that has been molten when the pulse laser beam is converged and then, re-solidified after the convergence of the pulse laser beam) and has greatly reduced strength. Therefore, by exerting external force to the outer peripheral portion of the semiconductor wafer 2, the outer peripheral portion of the semiconductor wafer 2 is fractured along the deteriorated layer 210 to be removed. Stress, which is generated when the outer peripheral portion is cut with a cutting blade, does not remain in the semiconductor wafer 2 whose outer peripheral portion has been thus removed by laser processing. Although the deteriorated layer 210 may be formed only in the inside without being exposed to the front surface 2a and the back surface 2b of the semiconductor wafer 2, it is desirable that a plurality of deteriorated layers 210 are formed by carrying out the above laser processing a plurality of times by changing the above focusing point P stepwise so that they extend from the front surface 2a to the back surface 2b of the semiconductor wafer 2.
The processing conditions in the above outer peripheral portion removal step are set as follows, for example.
- Light source: LD excited Q switch Nd:YVO4 laser
- Wavelength: pulse laser beam having a wavelength of 1,064 nm
- Pulse output: 10 μJ
- Focusing spot diameter: 1.0 μm
- Repetition frequency: 100 kHz
- Revolution of chuck table: 1 rpm
Since the deteriorated layer which is formed at a time under the above processing conditions is as thick as about 50 μm, when the semiconductor wafer 2 has a thickness of 500 μm, ten deteriorated layers are formed in the inside of the semiconductor wafer 2 so that they can extend from the front surface 2a up to the back surface 2b of the semiconductor wafer 2. As the revolution of the chuck table under the above processing conditions is 1 rpm, one deteriorated layer can be formed in minute, and therefore, even when 10 deteriorated layers are formed in the inside of the semiconductor wafer 2 from the front surface 2a up to the back surface 2b, the operation time of the outer peripheral portion removal step is 10 minutes which is much shorter than the operation time of cutting with the cutting blade.
A description will be subsequently given of another embodiment of the outer peripheral portion removal step with reference to FIG. 6.
In the embodiment shown in FIG. 6, a pulse laser beam of a wavelength having absorptivity for the semiconductor wafer 2 is applied from the condenser 324 to remove the outer peripheral portion of the wafer. That is, as shown in FIG. 6, the location on the inside of the outer periphery by a predetermined distance of the semiconductor wafer 2 held on the chuck table 31 is so brought as to be a position right below the condenser 324. The chuck table 31, that is, the semiconductor wafer 2 is turned in the direction indicated by the arrow in FIG. 6 while a pulse laser beam of a wavelength having absorptivity for the semiconductor wafer 2 is applied from the condenser 324. At this point, the focusing point P of the pulse laser beam applied from the condenser 324 is set to a position near the front surface 2a (top surface) of the semiconductor wafer 2. As a result, an annular groove 220 reaching the back surface 2b from the front surface 2a is formed at the location on the inside of the outer periphery by a predetermined distance of the semiconductor wafer 2 as shown in FIG. 6, whereby the outer peripheral portion of the semiconductor wafer 2 is removed.
The processing conditions in the above outer peripheral portion removal step are set as follows, for example.
- Light source: LD excited Q switch Nd:YVO4 laser
- Wavelength: pulse laser beam having a wavelength of 355 nm
- Average output: 1.35 W
- Focusing spot diameter: 13 μm
- Repetition frequency: 100 kHz
- Revolution of chuck table: 0.1 rpm
Since the revolution of the chuck table under the above processing conditions is 0.1 rpm, the operation time of the outer peripheral portion removal step is 10 minutes, which is much shorter than the operation time of cutting with the above cutting blade.
A description will be subsequently given of still another embodiment of the outer peripheral portion removal step with reference to FIG. 7.
In the embodiment shown in FIG. 7, the outer peripheral portion of one of the wafers of an SOI wafer 20 having a double-layer structure manufactured by joining two wafers 20A and 20B through an oxide film is removed. That is, as shown in FIG. 7, the SOI wafer 20 is first placed on the chuck table 31 in such a manner that the wafer 20A, which is one of the two wafers, faces up, and suction-held on the chuck table 31. Laser processing is then carried out in the same manner as in the embodiment shown in FIG. 6 to form an annular groove 220 in the wafer 20A at the location on the inside of the outer periphery by a predetermined distance, whereby the outer peripheral portion of the wafer 20A is removed. In the outer peripheral portion removal step for removing the outer peripheral portion of one wafer of the two wafers constituting the SOI wafer 20, a deteriorated layer may be formed in the inside of the wafer 20A along the outer periphery at the location on the inside of the outer periphery by a predetermined distance, in the same manner as the above embodiment in FIG. 5.
After the above outer peripheral portion removal step is carried out as described above, a protective member 4 is affixed to the front surface 2a of the semiconductor wafer 2 whose outer peripheral portion has been removed, as shown in FIG. 8 (protective member affixing step). Since the other wafer 20B functions as a protective member in the case of the above SOI wafer 20, the protective member may not be affixed to the undersurface of the wafer 20B.
After the protective member 4 is affixed to the front surface 2a of the semiconductor wafer 2 by carrying out the protective member affixing step, next comes the grinding step for grinding the back surface to be ground of the wafer whose outer peripheral portion has been removed, to a predetermined finish thickness. This grinding step is carried out by using a grinding machine 5 in the embodiment shown in FIG. 9. That is, in the grinding step, the protective member 4 side of the semiconductor wafer 2 is first placed on the chuck table 51 of the grinding machine 5 (therefore, the back surface 2b of the semiconductor wafer 2 faces up), and the semiconductor wafer 2 is suction-held on the chuck table 51 by a suction means that is not shown. A grinding wheel 53 having a grindstone 52 is rotated at 6,000 rpm and brought into contact with the back surface 2b of the semiconductor wafer 2 while the chuck table 51 is turned at 300 rpm, for example, to grind the back surface 2b until its thickness becomes a predetermined finish thickness, for example, 100 μm. Even when the semiconductor wafer 2 is thus ground until it becomes thin, as the outer peripheral portion of the semiconductor wafer 2 has been removed and the outer peripheral surface of the semiconductor wafer 2 is not arcuate, a sharp knife-edge is not formed. Further, since stress does not remain in the outer peripheral portion of the semiconductor wafer 2 whose outer peripheral portion has been removed as described above, the semiconductor wafer 2 is not damaged during grinding.
Next, a description will be given of a second embodiment of the wafer grinding method of the present invention.
In this grinding method, the step of forming an annular groove deeper than at least the finish thickness of the wafer from the front surface by applying a laser beam of a wavelength having absorptivity for the wafer along the outer periphery at a location on the inside of the outer periphery of the wafer by a predetermined distance, from the front surface side of the wafer is first carried out. In this groove forming step, the laser beam processing machine 3 shown in FIG. 2 is used, and a pulse laser beam having absorptivity for the semiconductor wafer 2 is applied from the condenser 324 as in the embodiment shown in FIG. 6 to form an annular groove deeper than the finish thickness of the wafer from the front surface in the outer peripheral portion of the wafer. In this embodiment, the focusing point of the laser beam is set at a position higher than the focusing point in the embodiment shown in FIG. 6. And, as shown in FIG. 10, the location on the inside of the outer periphery by a predetermined distance of the semiconductor wafer 2 held on the chuck table 31 is brought to a position right below the condenser 324. Then, the chuck table 31, that is, the semiconductor wafer 2 is turned in the direction indicated by the arrow in FIG. 10 while a pulse laser beam having absorptivity for the semiconductor wafer 2 is applied from the condenser 324. At this point, the focusing point P of the pulse laser beam applied from the condenser 324 is set to a position slightly higher than that of the embodiment shown in FIG. 6 near the front surface 2a (top surface) of the semiconductor wafer 2. As a result, an annular groove 230 having a predetermined depth from the front surface 2a is formed in the semiconductor wafer 2 at the location on the inside of the outer periphery by a predetermined distance, as shown in FIG. 10. Stress, which is generated by cutting with the cutting blade does not remain in the semiconductor wafer 2 having the groove 230 that has been thus formed in the outer peripheral portion by this laser processing.
The processing conditions in the above groove forming step are set as follows, for example.
- Light source: LD excited Q switch Nd:YVO4 laser
- Wavelength: pulse laser beam having a wavelength of 355 nm
- Average output: 1.35 W
- Focusing spot diameter: 13 μm
- Repetition frequency: 100 kHz
- Revolution of chuck table: 0.1 rpm
Since the revolution of the chuck table is 0.1 rpm in this groove forming step, the operation time of the groove forming step is 10 minutes, which is much shorter than the operation time of cutting with the cutting blade.
After the above groove forming step is carried out as described above, the protective member 4 is affixed to the front surface 2a of the semiconductor wafer 2 having the groove 230 formed in the outer peripheral portion, as shown in FIG. 11 (protective member affixing step).
After the protective member 3 is affixed to the front surface 2a of the semiconductor wafer 2 by carrying out the protective member affixing step, next comes the grinding step for grinding the back surface of the wafer having the groove in the outer peripheral portion to a predetermined finish thickness. This grinding step is carried out by using the grinding machine 5 shown in FIG. 9 in accordance with the above-described procedure. When the semiconductor wafer 2 is ground until its thickness becomes a predetermined finish thickness, for example, 100 μm, as shown in FIG. 12, the groove 230 formed in the outer peripheral portion of the semiconductor wafer 2 is exposed to the back surface 2b (top surface). As a result, the outer peripheral portion having an arcuate outer peripheral surface of the semiconductor wafer 2 is removed. Therefore, the outer peripheral portion of the semiconductor wafer becomes not knife-edged. Since the above-mentioned stress does not remain in the outer peripheral portion of the semiconductor wafer 2 having the groove 230 in the outer peripheral portion, the semiconductor wafer 2 is not damaged during grinding.
Patent Application
20060079155
