Wafer processing method

A water processing method for providing a gettering sink effect to a wafer having a plurality of streets which are formed in a lattice pattern on the front surface of a substrate and devices which are formed in a plurality of areas sectioned by the plurality of streets, comprising the steps of removing distortion produced on the rear surface of the substrate of the wafer whose rear surface of the substrate has been ground to a predetermined thickness; forming a gettering sink effect layer by applying a laser beam of a wavelength having permeability for the substrate of the wafer which has undergone the distortion removing step, with its focal point set to the inside of the substrate to form a deteriorated layer in the inside of the substrate; and dividing the wafer which has undergone the gettering sink effect layer forming step, into individual chips along the streets.

FIELD OF THE INVENTION

The present invention relates to a wafer processing method for providing a gettering sink effect to a wafer such as a semiconductor wafer having a plurality of devices on the front surface.

DESCRIPTION OF THE PRIOR ART

In the production process of a semiconductor device, a large number of rectangular areas are sectioned by cutting lines called “streets” arranged in a lattice pattern on the front surface of a substantially disk-like semiconductor wafer, and a device such as IC or LSI is formed in each of the rectangular areas. Individual semiconductor chips are manufactured by cutting this semiconductor wafer having a large number of devices along the streets. To reduce the size and weight of each semiconductor chip, the rear surface of the semiconductor wafer is generally ground to a predetermined thickness before it is cut into the individual rectangular areas along the streets.

The rear surface of the semiconductor wafer is generally ground by pressing grinding stones formed by bonding diamond abrasive grains with a suitable bond such as a resin bond against the rear surface of the semiconductor wafer while it is rotated at a high speed. When the rear surface of the semiconductor wafer is ground by this grinding method, a grinding distortion layer composed of about 1 μm micro-cracks is formed on the rear surface of the semiconductor wafer. It is known that this grinding distortion layer has the function of providing a gettering sink effect for suppressing the bad influence upon memory function of a metal atom such as copper, which is contained in the semiconductor wafer in the production process of the semiconductor wafer and moves freely in proximity to a device, as disclosed, for example, by JP-A 2006-41258 A.

Meanwhile, especially when the thickness of the semiconductor wafer is reduced to 100 μm or less by grinding, the grinding distortion layer formed by grinding the rear surface of the above semiconductor wafer greatly reduces the deflective strength of the semiconductor chips. To remove the grinding distortion layer formed on the ground rear surface of the semiconductor wafer, as disclosed, for example, by JP-A 2002-283211 and JP-A 2001-85385, the ground rear surface of the semiconductor wafer is polished, or wet etched or dry etched.

When the grinding distortion layer formed by grinding is removed by polishing or etching after the rear surface of the wafer is ground, a problem arises in that the deflective strength of each chip becomes stable but the gettering sink effect of the grinding distortion layer is lost and the function of each device is lowered.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wafer processing method capable of retaining a gettering sink effect and ensuring deflective strength.

To attain the above object, according to the present invention, there is provided a water processing method for providing a gettering sink effect to a wafer having a plurality of streets which are formed in a lattice pattern on the front surface of a substrate and devices which are formed in a plurality of areas sectioned by the plurality of streets, comprising:

a grinding distortion layer removing step for removing grinding distortion produced on the rear surface of the substrate of the wafer whose rear surface has been ground to a predetermined thickness;

a gettering sink effect layer forming step for forming a gettering sink effect layer by applying a laser beam of a wavelength having permeability for the substrate of the wafer which has undergone the grinding distortion layer removing step, with its focal point set to the inside of the substrate to form a deteriorated layer in the inside of the substrate; and

a dividing step for dividing the wafer which has undergone the gettering sink effect layer forming step, into individual chips along the streets.

According to the present invention, there is also provided a water processing method for providing a gettering sink effect to a wafer having a plurality of streets which are formed in a lattice pattern on the front surface of a substrate and devices which are formed in a plurality of areas sectioned by the plurality of streets, comprising:

a grinding distortion layer removing step for removing grinding distortion produced on the rear surface of the substrate of the wafer whose rear surface has been ground to a predetermined thickness;

a dividing step for dividing the wafer which has undergone the grinding distortion layer removing step, into individual chips along the streets; and

a gettering sink effect layer forming step for forming a gettering sink effect layer by applying a laser beam of a wavelength having permeability for the substrate of the wafer which has been divided into individual chips by the dividing step, with its focal point set to the inside of the substrate of each chip to form a deteriorated layer in the inside of the substrate.

According to the present invention, since the grinding distortion layer formed by grinding is removed from the rear surface of the substrate by carrying out the grinding distortion layer removing step, deflective strength becomes stable. Further, since a deteriorated layer is formed in the intermediate portion in the thickness direction of the substrate of each chip by carrying out the gettering sink effect layer forming step, this deteriorated layer functions as a gettering sink effect layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in more detail hereinunder with reference to the accompanying drawings.

FIG. 1is a perspective view of a semiconductor wafer as a wafer to be processed according to the present invention. In the semiconductor wafer2shown inFIG. 1, a plurality of streets21are formed in a lattice pattern on the front surface20aof a substrate20made of silicon and a device22is formed in a plurality of areas sectioned by the plurality of streets21. A protective member3is affixed to the front surface20aof the substrate20of the semiconductor wafer2constituted as described above, as shown inFIG. 2(protective member affixing step).

After the protective member3is affixed to the front surface20aof the substrate20of the semiconductor wafer2by carrying out the protective member affixing step, next comes the step of grinding the rear surface20bof the substrate20of the semiconductor wafer2to a predetermined thickness. This grinding step is carried out by using a grinding machine4shown inFIG. 3. The grinding machine4shown inFIG. 3comprises a chuck table41for holding a workpiece and a grinding wheel43having grinding stones42for grinding the workpiece held on the chuck table41. To carry out the above grinding step by using the above grinding machine4, the protective member3side of the semiconductor wafer2is placed on the chuck table41(therefore, the rear surface20bof the substrate20of the semiconductor wafer2faces up), and the semiconductor wafer2is suction-held on the chuck table41by a suction means that is not shown. The grinding wheel43having grinding stones42is rotated at 6,000 rpm and brought into contact with the rear surface20bof the substrate20while the chuck table41is rotated at, for example, 300 rpm to grind the rear surface to a predetermined thickness, for example, 100 μm. When this grinding step is carried out as described above, a grinding distortion layer having a thickness of about 1 μm is formed on the rear surface20bof the substrate20of the semiconductor wafer2by the above grinding.

The above grinding step is followed by the step of removing the grinding distortion layer formed on the rear surface20bof the substrate20of the semiconductor wafer2. This grinding distortion layer removing step is carried out by using a polishing machine5shown inFIG. 4. The polishing machine5shown inFIG. 4comprises a chuck table51for holding a workpiece and a polishing tool53for polishing the workpiece held on the chuck table51, which has a polishing stone52manufactured by dispersing zirconia oxide abrasive grains into a soft member such as felt and fixing them with a suitable bonding agent. To carry out the above grinding distortion layer removing step by using this polishing machine5, the protective member3side of the semiconductor wafer2which has undergone the above grinding step is placed on the chuck table51(therefore, the rear surface20bof the substrate20of the semiconductor wafer2faces up), and the semiconductor wafer2is suction-held on the chuck table51by the suction means that is not shown. The polishing tool53having a polishing stone52is rotated at, for example, 6,000 rpm and brought into contact with the rear surface20bof the substrate20while the chuck table51is rotated at, for example, 300 rpm to polish the rear surface20bof the substrate20. As a result, the grinding distortion layer formed on the rear surface20bof the substrate20of the semiconductor wafer2is removed by carrying out the above polishing step. The grinding distortion layer removing step may be carried out by polishing or wet etching, dry etching and others.

The grinding distortion layer removing step is followed by the step of forming a gettering sink effect layer by applying a laser beam of a wavelength having permeability for the substrate20of the semiconductor wafer2from which the grinding distortion layer has been removed, with its focal point set to the inside of the substrate20to form a deteriorated layer in the substrate20. The gettering sink effect layer forming step is carried out by using a laser beam processing machine6shown inFIG. 5. The laser beam processing machine6shown inFIG. 5comprises a chuck table61for holding a workpiece and a laser beam application means62for applying a laser beam to the workpiece held on the chuck table61. The chuck table61is designed to suction-hold the workpiece and to be moved in a processing-feed direction indicated by an arrow X inFIG. 5by a processing-feed mechanism that is not shown and an indexing-feed direction indicated by an arrow Y by an indexing-feed mechanism that is not shown. The above laser beam application means62applies a pulse laser beam from a condenser622mounted on the end of a cylindrical casing621arranged substantially horizontally. The illustrated laser beam processing machine6has an image pick-up means63attached to the end portion of the casing621constituting the above laser beam application means62. This image pick-up means63is constituted by an infrared illuminating means for applying infrared radiation to the workpiece, an optical system for capturing the infrared radiation applied by the infrared illuminating means and an image pick-up device (infrared CCD) for outputting an electric signal corresponding to the infrared radiation captured by the optical system in addition to an ordinary image pick-up device (CCD) for picking up an image with visible radiation. An image signal is supplied to a control means that is not shown.

To carry out the gettering sink effect layer forming step by using the above laser beam processing machine6shown inFIG. 5, the protective member3side of the semiconductor wafer2which has undergone the above polishing step is first placed on the chuck table61(therefore, the rear surface20bof the substrate20of the semiconductor wafer2faces up), and the semiconductor wafer2is suction-held on the chuck table61by the suction means that is not shown. The chuck table61suction-holding the semiconductor wafer2is brought to a position right below the image pick-up means63by a moving mechanism that is not shown.

After the chuck table61is positioned right below the image pick-up means63, alignment work for detecting the area to be processed of the semiconductor wafer2is carried out by the image pick-up means63and the control means that is not shown. That is, the image pick-up means63and the control means (not shown) detect the area where the plurality of devices22are formed on the front surface20aof the substrate20of the semiconductor wafer2to obtain the XY coordinate values of the area to be processed. Although the front surface20a, on which the device22is formed, of the substrate20of the semiconductor wafer2faces down at this point, as the image pick-up means63comprises an infrared illuminating means, an optical system for capturing infrared radiation and an image pick-up device (infrared CCD) for outputting an electric signal corresponding to the infrared radiation as described above, it can pick up an image of the devices22through the rear surface20b.

The chuck table61is then moved to a laser beam application area where the condenser622of the laser beam application means62for applying a laser beam is located, to bring the processing start position to a position right below the condenser622of the laser beam application means62, as shown inFIG. 6(a). The chuck table61is then moved at a predetermined speed in a direction indicated by an arrow X1inFIG. 6(a) while a pulse laser beam of a wavelength having permeability for the substrate20is applied from the condenser622(gettering sink effect layer forming step). When the application position of the condenser622of the laser beam application means62reaches the right end inFIG. 6(b) of the area where the devices are formed as shown inFIG. 6(b), the application of a pulse laser beam is suspended and the movement of the chuck table61is stopped. In this gettering sink effect layer forming step, a deteriorated layer210is formed in an intermediate portion in the thickness direction of the semiconductor wafer2by setting the focal point P of the pulse laser beam to the intermediate portion in the thickness direction of the semiconductor wafer2.

The processing conditions in the above gettering sink effect layer forming step are set as follows, for example.Light source: LD excited Q switch Nd:YVO4 pulse laserWavelength: pulse laser beam having a wavelength of 1,064 nmRepetition frequency: 100 kHzPeak power density of focal point: 1×108W/cm2Focal spot diameter: 1 μmProcessing-feed rate: 100 mm/sec

A deteriorated layer210having a thickness of 1 to 2 μm is formed in the inside of the substrate20of the semiconductor wafer2by carrying out the gettering sink effect layer forming step under the above processing conditions. The above gettering sink effect layer forming step is carried out on all the areas corresponding to the plurality of devices formed on the front surface20aof the substrate20of the semiconductor wafer2. The interval between the deteriorated layers210should be about 1 μm. The deteriorated layers210formed in the inside of the substrate20serve as gettering sink effect layers.

After the gettering sink effect layer forming step is carried out as described above, the rear surface20bof the substrate20of the semiconductor wafer2is put on the front surface of a dicing tape T mounted on an annular frame F, as shown inFIG. 7(wafer supporting step). Then, the protective member3is removed from the front surface20aof the substrate20of the semiconductor wafer2.

After the wafer supporting step, next comes the step of dividing the semiconductor wafer2, which has undergone the gettering sink effect layer forming step, into individual chips along the streets21. This dividing step is carried out by using a cutting machine7shown inFIG. 8. The cutting machine7shown inFIG. 8comprises a chuck table71for holding a workpiece, a cutting means72having a cutting blade721for cutting the workpiece held on the chuck table71and an image pick-up means73for picking up an image of the workpiece held on the chuck table71. The chuck table71is designed to suction-hold the workpiece and to be moved in a processing-feed direction indicated by an arrow X and an indexing-feed direction indicated by an arrow Y inFIG. 8, by a moving mechanism that is not shown. The cutting blade721comprises a disk-like base and an annular cutting edge which is mounted on the side surface outer periphery of the base and formed by fixing diamond abrasive grains having a diameter of about 3 μm to the base by electroforming. The above image pick-up means73comprises an ordinary image pick-up device (CCD) for picking up an image with visible radiation and supplies an image signal to the control means (not shown) in the illustrated embodiment.

To carry out the dividing step by using the cutting machine7constituted as described above, the dicing tape T affixed to the semiconductor wafer2in the above wafer supporting step is placed on the chuck table71. By activating the suction means (not shown), the semiconductor wafer2is held on the chuck table71through the dicing tape T. InFIG. 8, the annular frame F holding the dicing tape T is not shown but the annular frame F is held by a suitable frame holding means provided on the chuck table71. The chuck table71suction-holding the semiconductor wafer2is brought to a position right below the image pick-up means73by the cutting-feed mechanism that is not shown.

After the chuck table71is positioned right below the image pick-up means73, the alignment step for detecting the area to be cut of the semiconductor wafer2is carried out by the image pick-up means73and the control means that is not shown. That is, the image pick-up means73and the control means (not shown) carry out image processing such as pattern matching, etc. to align a street21formed in a predetermined direction of the semiconductor wafer2with the cutting blade721, thereby performing the alignment of the area to be cut (aligning step). The alignment of the area to be cut is also carried out on streets21formed on the semiconductor wafer2in a direction perpendicular to the above predetermined direction.

After the alignment of the area to be cut is carried out by detecting the street21formed on the semiconductor wafer2held on the chuck table71as described above, the chuck table71holding the semiconductor wafer2is moved to the cutting start position of the area to be cut. At this point, the semiconductor wafer2is positioned such that one end (left end inFIG. 9) of the street21to be cut is located on the right side a predetermined distance from a position right below the cutting blade721, as shown inFIG. 9. The cutting blade721is then rotated at a predetermined speed in a direction indicated by an arrow721ainFIG. 9and moved down (cutting-in fed) until the lower end of the cutting blade721reaches the dicing tape T, as shown by a solid line inFIG. 9, from a stand-by position shown by a long dashed double-short dashed line by a cutting-in feed mechanism.

After the cutting blade721is moved down as described above, the chuck table71is moved in the direction indicated by the arrow X1inFIG. 9at a predetermined cutting-feed rate while the cutting blade721is rotated at the predetermined revolution in the direction indicated by the arrow721ainFIG. 9. When the right end of the semiconductor wafer2held on the chuck table71passes a position right below the cutting blade721, the movement of the chuck table71is stopped. As a result, the semiconductor wafer2is cut along the street21.

The above dividing step is carried out under the following processing conditions, for example.Cutting blade: outer diameter of 52 mm, thickness of 30 μmRevolution of cutting blade: 40,000 rpmCutting-feed rate: 50 mm/sec

The above cutting step is carried out on all the streets21formed on the semiconductor wafer2. As a result, the semiconductor wafer2is cut along the streets21and divided into semiconductor chips200corresponding to the devices22as shown inFIG. 10(dividing step). Since the grinding distortion layer formed in the grinding step is removed from the rear surface20bof the substrate20of each semiconductor chip200by carrying out the above grinding distortion layer removing step, the deflective strength becomes stable. Further, since the deteriorated layer210is formed in the intermediate portion in the thickness direction of the substrate20of each semiconductor chip200by carrying out the above gettering sink effect layer forming step, the deteriorated layer210serves as a gettering sink effect layer.

A description is subsequently given of another embodiment of the wafer processing method of the present invention.

In this embodiment, the semiconductor wafer2is first divided into individual semiconductor chips by carrying out the above grinding step, grinding distortion layer removing step and dividing step and then, the gettering sink effect layer forming step is carried out. The gettering sink effect layer forming step which is carried out after the semiconductor wafer2is divided into individual semiconductor chips will be described with reference toFIGS. 11 to 14. The front surface20aof the substrate20of the semiconductor wafer2that has been divided into individual semiconductor chips200is first put on the front surface of the dicing tape T mounted on the annular frame F, as shown inFIG. 11. Therefore, the rear surface20bof the substrate20of the semiconductor wafer2divided into individual semiconductor chips200faces up. Then, the gettering sink effect layer forming step is carried out by subjecting each semiconductor chip200to a trepanning processing by using the above laser beam processing machine6shown inFIG. 5.

The laser beam application means for carrying out the trepanning processing will be described hereinbelow with reference toFIG. 12.

The laser beam application means62of the laser beam processing machine6shown inFIG. 5comprises a pulse laser beam oscillation means81, an output adjustment means82, a first acousto-optic deflection means83for deflecting the optical axis of a laser beam oscillated from the pulse laser beam oscillation means81to the processing-feed direction (X direction) and a second acousto-optic deflection means84for deflecting the optical axis of a laser beam oscillated from the pulse laser beam oscillation means81to the indexing-feed direction (Y direction), all of which are installed in the above casing621. The above condenser622comprise a direction changing mirror622afor changing the direction of a pulse laser beam passing through the first acousto-optic deflection means83and the second acousto-optic deflection means84to a downward direction, and a condenser lens622bfor converging the laser beam whose direction has been changed by the direction changing mirror622a.

The above pulse laser beam oscillation means81is constituted by a pulse laser beam oscillator811and a repetition frequency setting means812connected to the pulse laser beam oscillator811. The above output adjustment means82adjusts the output of a pulse laser beam oscillated from the pulse laser beam oscillation means81.

The above first acousto-optic deflection means83comprises a first acousto-optic device831for deflecting the optical axis of a laser beam oscillated from the pulse laser beam oscillation means81to the processing-feed direction (X direction), a first RF oscillator832for generating RF (radio frequency) to be applied to the first acousto-optic device831, a first RF amplifier833for amplifying the power of RF generated by the first RF oscillator832to apply it to the first acousto-optic device831, a first deflection angle adjustment means834for adjusting the frequency of RF generated by the first RF oscillator832, and a first output adjustment means835for adjusting the amplitude of RF generated by the first RF oscillator832. The above first acousto-optic device831can adjust the deflection angle of the optical axis of a laser beam according to the frequency of the applied RF and adjust the output of a laser beam according to the amplitude of the applied RF. The above first deflection angle adjustment means834and the above first output adjustment means835are controlled by a control means that is not shown.

The above second acousto-optic deflection means84comprises a second acousto-optic device841for deflecting the optical axis of a laser beam oscillated from the pulse laser beam oscillation means81in the indexing-feed direction perpendicular to the processing-feed direction (X direction), a second RF oscillator842for generating RF to be applied to the second acousto-optic device841, a second RF amplifier843for amplifying the power of RF generated by the second RF oscillator842to apply it to the second acousto-optic device841, a second deflection angle adjustment means844for adjusting the frequency of RF generated by the second RF oscillator842, and a second output adjustment means845for adjusting the amplitude of RF generated by the second RF oscillator842. The above second acousto-optic device841can adjust the deflection angle of the optical axis of a laser beam according to the frequency of the applied RF and adjust the output of a laser beam according to the amplitude of the applied RF. The above second deflection angle adjustment means844and the above second output adjustment means845are controlled by the control means that is not shown.

The laser beam application means62in the illustrated embodiment comprises a laser beam absorbing means85for absorbing a laser beam not deflected by the first acousto-optic device831as shown by the dashed dotted line inFIG. 12when RF is not applied to the above first acousto-optic device831.

The laser beam application means62in the illustrated embodiment is constituted as described above. When RF is not applied to the first acousto-optic device831and the second acousto-optic device841, a pulse laser beam oscillated from the pulse laser beam oscillation means81is guided to the laser beam absorbing means85through the output adjustment means82, first acousto-optic device831and second acousto-optic device841, as shown by the dashed dotted line inFIG. 12. When RF having a frequency of, for example, 10 kHz is applied to the first acousto-optic device831, the optical axis of a pulse laser beam oscillated from the pulse laser beam oscillation means81is deflected and focused at a focal point Pa as shown by the solid line inFIG. 12. When RF having a frequency of, for example, 20 kHz is applied to the first acousto-optic device831, the optical axis of a pulse laser beam oscillated from the pulse laser beam oscillation means81is deflected and focused at a focal point Pb which shifts from the above focal point Pa by a predetermined distance in the processing-feed direction (X direction) as shown by the broken line inFIG. 12. When RF having a predetermined frequency is applied to the second acousto-optic device841, the optical axis of a pulse laser beam oscillated from the pulse laser beam oscillation means81is focused at a focal point which shifts from the above focal point Pa by a predetermined distance in the indexing-feed direction (Y direction: perpendicular to the sheet inFIG. 12) perpendicular to the processing-feed direction (X direction).

Therefore, trepanning processing for moving the spot S of the pulse laser beam spirally as shown inFIG. 13can be carried out by activating the first acousto-optic deflection means83and the second acousto-optic deflection means84to deflect the optical axis of the pulse laser beam in the X direction and Y direction sequentially.

To carry out the gettering sink effect layer forming step on each semiconductor chip200by trepanning using the laser beam processing machine6comprising the laser beam application means62, the semiconductor wafer2whose front surface20aof the substrate20is affixed to the front surface of the dicing tape T mounted on the annular frame F as shown inFIG. 11is held on the above chuck table61. Then, a predetermined semiconductor chip200of the semiconductor wafer2held on the chuck table61is brought to a position right below the condenser622of the laser beam application means62. Then, the above trepanning processing is carried out as shown inFIG. 14while a pulse laser beam of a wavelength having permeability for the substrate20is applied from the condenser622by setting the focal point of the pulse laser beam to the intermediate portion in the thickness direction of the semiconductor wafer2. As a result, a deteriorated layer which serves as a gettering sink effect layer can be formed in the inside of the semiconductor chip200.