# Patent

## Title
Laser processing method for semiconductor wafer

## Abstract

A laser processing method for a semiconductor wafer including a groove forming step of applying a pulsed laser beam having an absorption wavelength to the semiconductor wafer along a division line formed on the semiconductor wafer to thereby form a laser processed groove along the division line on the semiconductor wafer, wherein the pulse width of the pulsed laser beam to be applied in the groove forming step is set to 2 ns or less, and the peak energy density per pulse of the pulsed laser beam is set less than or equal to an inflection point where the depth of the laser processed groove steeply increases with an increase in the peak energy density.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser processing method for a semiconductor wafer including a groove forming step of applying a laser beam to the semiconductor wafer along a plurality of division lines to thereby form a plurality of laser processed grooves along these division lines on the semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer such as a silicon wafer and a gallium arsenide wafer to partition a plurality of regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer is divided into the individual devices along the division lines by using a cutting apparatus or a laser processing apparatus, and these devices are widely used in various electrical equipment such as mobile phones and personal computers.
In general, a dicing apparatus is used as the cutting apparatus. The dicing apparatus includes a cutting blade having a thickness of about 30 to 300 pm. The cutting blade is obtained by bonding super abrasive grains such as diamond and CBN (Cubic Baron Nitride) with metal or resin. Cutting is performed by rotating the cutting blade at a high speed of about 30000 rpm and feeding the cutting blade into a semiconductor wafer.
On the other hand, the laser processing apparatus essentially includes a chuck table for holding a semiconductor wafer, laser beam applying means for applying a pulsed laser beam to the semiconductor wafer held on the chuck table, and feeding means for relatively feeding the chuck table and the laser beam applying means. The pulsed laser beam has an absorption wavelength to the semiconductor wafer, and it is applied to the semiconductor wafer along the division lines formed on the front side of the semiconductor wafer to thereby form a plurality of laser processed grooves along these division lines. After forming the laser processed grooves, an external force is applied to the semiconductor wafer to break the semiconductor wafer along the laser processed grooves, thereby dividing the semiconductor wafer into the individual devices (see Japanese Patent Laid-open No. 2007-19252, for example).
SUMMARY OF THE INVENTION
In the case of cutting the semiconductor wafer by using the dicing apparatus having the cutting blade as mentioned above, each device divided from the semiconductor wafer has a die strength of 800 MPa. To the contrary, in the case of dividing the semiconductor wafer by performing a conventional laser processing method, each device divided from the semiconductor wafer has a die strength of 400 MPa. Such a low die strength causes a degradation in quality of electrical equipment.
It is therefore an object of the present invention to provide a laser processing method for a semiconductor wafer which can manufacture a device having a high die strength.
In accordance with an aspect of the present invention, there is provided a laser processing method for a semiconductor wafer including a groove forming step of applying a pulsed laser beam having an absorption wavelength to the semiconductor wafer along a division line formed on the semiconductor wafer to thereby form a laser processed groove along the division line on the semiconductor wafer, wherein the pulse width of the pulsed laser beam to be applied in the groove forming step is set to 2 ns or less, and the peak energy density per pulse of the pulsed laser beam is set less than or equal to an inflection point where the depth of the laser processed groove steeply increases with an increase in the peak energy density.
According to the present invention, the pulse width of the pulsed laser beam is set to 2 ns or less, and the peak energy density per pulse is set less than or equal to an inflection point where the depth of the laser processed groove steeply increases with an increase in the peak energy density. Thus, the die strength of each device can be improved to 800 MPa or more.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic perspective view of a laser processing apparatus for performing the laser processing method according to the present invention;
FIG. 2 is a perspective view of a semiconductor wafer supported through an adhesive tape to an annular frame;
FIG. 3 is a block diagram of a laser beam applying unit;
FIG. 4 is a perspective view showing a groove forming step in the laser processing method according to the present invention;
FIG. 5A is a view for illustrating the groove forming step;
FIG. 5B is an enlarged sectional view of the semiconductor wafer at a position where a laser processed groove is formed by the groove forming step shown in FIG. 5A;
FIG. 6 is a schematic diagram for illustrating the amount of overlap between adjacent beam spots;
FIG. 7 is a perspective view of a dividing apparatus; and
FIGS. 8A and 8B are sectional side views for illustrating a semiconductor wafer dividing step.
FIG. 9 is a graph showing the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 1064 nm is applied by one pulse to a silicon wafer;
FIG. 10 is a graph showing the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 532 nm is applied by one pulse to a silicon wafer;
FIG. 11 is a graph showing the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 355 nm is applied by one pulse to a silicon wafer; and
FIG. 12 is a graph showing the relation between the peak energy density and the depth of a laser processed groove in the case that laser beams having a pulse width of 10 ps for each and different wavelengths of 1064 nm and 532 nm are applied by one pulse for each to a GaAs wafer.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a schematic perspective view of a laser processing apparatus 2 for performing the laser processing method according to the present invention. The laser processing apparatus 2 includes a stationary base 4 and a first slide block 6 supported to the stationary base 4 so as to be movable in an X direction in FIG. 1. The first slide block 6 is movable in a feeding direction, i.e., in the X direction along a pair of guide rails 14 by feeding means 12 including a ball screw 8 and a pulse motor 10.
A second slide block 16 is supported to the first slide block 6 so as to be movable in a Y direction in FIG. 1. The second slide block 16 is movable in an indexing direction, i.e., in the Y direction along a pair of guide rails 24 by indexing means 22 including a ball screw 18 and a pulse motor 20. A chuck table 28 is supported through a cylindrical support member 26 to the second slide block 16. Accordingly, the chuck table 28 is movable both in the X direction and in the Y direction by the feeding means 12 and the indexing means 22. The chuck table 28 is provided with a pair of clamps 30 for clamping a semiconductor wafer (which will be hereinafter described) held on the chuck table 28 under suction.
A column 32 is provided on the stationary base 4, and a casing 35 for accommodating a laser beam applying unit 34 is mounted on the column 32. As shown in FIG. 3, the laser beam applying unit 34 includes a laser oscillator 62 such as a YAG laser oscillator or a YVO4 laser oscillator, repetition frequency setting means 64, pulse width adjusting means 66, and power adjusting means 68. A pulsed laser beam is oscillated by the laser oscillator 62, and the power of the pulsed laser beam is adjusted by the power adjusting means 68. Focusing means 36 is mounted at the front end of the casing 35 and includes a mirror 70 and a focusing objective lens 72. The pulsed laser beam from the laser beam applying unit 34 is reflected by the mirror 70 and next focused by the objective lens 72 in the focusing means 36 so as to form a laser beam spot on the front side (upper surface) of a semiconductor wafer W held on the chuck table 28.
Referring back to FIG. 1, imaging means 38 for detecting a processing area of the semiconductor wafer W to be laser-processed is also provided at the front end of the casing 35 so as to be juxtaposed to the focusing means 36 in the X direction. The imaging means 38 includes an ordinary imaging device such as a CCD for imaging the processing area of the semiconductor wafer W by using visible light. The imaging means 38 further includes infrared imaging means composed of infrared light applying means for applying infrared light to the semiconductor wafer W, an optical system for capturing the infrared light applied to the semiconductor wafer W by the infrared light applying means, and an infrared imaging device such as an infrared CCD for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means 38 is transmitted to a controller (control means) 40.
The controller 40 is configured by a computer, and it includes a central processing unit (CPU) 42 for performing operational processing according to a control program, a read only memory (ROM) 44 for storing the control program and so on, a random access memory (RAM) 46 for storing the results of computation, etc., a counter 48, an input interface 50, and an output interface 52.
Reference numeral 56 denotes feed amount detecting means including a linear scale 54 provided along one of the guide rails 14 and a read head (not shown) provided on the first slide block 6. A detection signal from the feed amount detecting means 56 is input into the input interface 50 of the controller 40. Reference numeral 60 denotes index amount detecting means including a linear scale 58 provided along one of the guide rails 24 and a read head (not shown) provided on the second slide block 16. A detection signal from the index amount detecting means 60 is input into the input interface 50 of the controller 40. An image signal from the imaging means 38 is also input into the input interface 50 of the controller 40. On the other hand, control signals are output from the output interface 52 of the controller 40 to the pulse motor 10, the pulse motor 20, and the laser beam applying unit 34.
As shown in FIG. 2, the front side of the semiconductor wafer W as a workpiece to be processed by the laser processing apparatus 2 is formed with a plurality of first streets S1 and a plurality of second streets S2 perpendicular to the first streets S1, thereby partitioning a plurality of rectangular regions where a plurality of devices D are respectively formed. The semiconductor wafer W is attached to a dicing tape T as an adhesive tape, and the dicing tape T is supported at its outer circumferential portion to an annular frame F. Accordingly, the semiconductor wafer W is supported through the dicing tape T to the annular frame F. The semiconductor wafer W is held on the chuck table 28 shown in FIG. 1 under suction in the condition where the annular frame F is clamped by the clamps 30.
A laser processing method for the semiconductor wafer W will now be described with reference to FIGS. 4 to 12. As shown in FIGS. 4 and 5A, a pulsed laser beam 37 having an absorption wavelength to the semiconductor wafer W is focused by the focusing means 36 onto the front side of the semiconductor wafer W. At the same time, the chuck table 28 is moved in a direction shown by an arrow X1 in FIG. 5A at a predetermined feed speed from one end (left end as viewed in FIG. 5A) of a predetermined one of the first streets S1.
When the other end (right end as viewed in FIG. 5A) of this predetermined first street S1 reaches the laser beam applying position of the focusing means 36, the application of the pulsed laser beam is stopped and the movement of the chuck table 28 is also stopped. As a result, a laser processed groove 74 is formed on the front side of the semiconductor wafer W along this predetermined first street S1 as shown in FIG. 5B. Such a groove forming step of forming the laser processed groove 74 is performed similarly along the other first streets S1. After thus performing the groove forming step along all of the first streets S1, the chuck table 28 is rotated 90° to similarly perform the groove forming step along all of the second streets S2 perpendicular to the first streets S1. As a result, the laser processed grooves 74 are formed along all of the first and second streets S1 and S2 on the front side of the semiconductor wafer W.
In the laser processing method according to the present invention, as shown in FIG. 6, the overlap amount OL of the adjacent spots S of the pulsed laser beam in the feeding direction is preferably adjusted to fall within the range of 16D1/20≦OL≦19D1/20, where D1 is the spot diameter of the pulsed laser beam, by optimally setting the repetition frequency, pulse width, spot diameter D1, and feed speed of the pulsed laser beam applied from the focusing means 36.
After forming the laser processed grooves 74 along all of the first and second streets S1 and S2 as mentioned above, a wafer dividing step is performed by using a dividing apparatus 80 shown in FIG. 7 in such a manner that the semiconductor wafer W is divided into the individual devices (chips) D along all of the laser processed grooves 74. The dividing apparatus 80 shown in FIG. 7 includes frame holding means 82 for holding the annular frame F and tape expanding means 84 for expanding the dicing tape T supported to the annular frame F held by the frame holding means 82.
The frame holding means 82 includes an annular frame holding member 86 and a plurality of clamps 88 as fixing means provided on the outer circumference of the frame holding member 86. The upper surface of the frame holding member 86 functions as a mounting surface 86a for mounting the annular frame F thereon. The annular frame F mounted on the mounting surface 86a is fixed to the frame holding member 86 by the clamps 88. The frame holding means 82 is supported by the tape expanding means 84 so as to be vertically movable.
The tape expanding means 84 includes an expanding drum 90 provided inside of the annular frame holding member 86. The expanding drum 90 has an outer diameter smaller than the inner diameter of the annular frame F and an inner diameter larger than the outer diameter of the semiconductor wafer W attached to the dicing tape T supported to the annular frame F. The expanding drum 90 has a supporting flange 92 integrally formed at the lower end of the drum 90. The tape expanding means 84 further includes driving means 94 for vertically moving the annular frame holding member 86. The driving means 94 is composed of a plurality of air cylinders 96 provided on the supporting flange 92. Each air cylinder 96 is provided with a piston rod 98 connected to the lower surface of the frame holding member 86. The driving means 94 composed of the plural air cylinders 96 functions to vertically move the annular frame holding member 86 so as to selectively take a reference position where the mounting surface 86a is substantially equal in height to the upper end of the expanding drum 90 and an expansion position where the mounting surface 86a is lower in height than the upper end of the expanding drum 90 by a predetermined amount.
The wafer dividing step using the dividing apparatus 80 will now be described with reference to FIGS. 8A and 8B. As shown in FIG. 8A, the annular frame F supporting the semiconductor wafer W through the dicing tape T is mounted on the mounting surface 86a of the frame holding member 86 and fixed to the frame holding member 86 by the clamps 88. At this time, the frame holding member 86 is set at the reference position where the height of the mounting surface 86a is substantially the same as that of the upper end of the expanding drum 90.
Thereafter, the air cylinders 96 are driven to lower the frame holding member 86 to the expansion position shown in FIG. 8B. Accordingly, the annular frame F fixed to the mounting surface 86a of the frame holding member 86 is also lowered, so that the dicing tape T supported to the annular frame F comes into abutment against the upper end of the expanding drum 90 and is expanded mainly in the radial direction of the expanding drum 90 as shown in FIG. 8B. As a result, a tensile force is radially applied to the semiconductor wafer W attached to the dicing tape T. When a tensile force is radially applied to the semiconductor wafer W, the semiconductor wafer W is broken along the laser processed grooves 74, thereby dividing the semiconductor wafer W into the individual semiconductor chips (devices) D.
The die strength of each device formed by a conventional laser processing method is as low as 400 MPa, causing a degradation in quality of electrical equipment. To cope with this problem, attention has been paid to a peak energy density per pulse in laser processing, and the relation between the peak energy density and the depth of a laser processed groove has been examined.
FIG. 9 shows the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 1064 nm is applied by one pulse to a silicon wafer. In FIG. 9, the horizontal axis represents the peak energy density (GW/cm2) graduated logarithmically, and the vertical axis represents the depth (nm) of the laser processed groove. As apparent from FIG. 9, when the peak energy density is small, the depth of the laser processed groove gradually increases with an increase in the peak energy density, whereas when the peak energy density becomes a point A or more, the depth of the laser processed groove steeply increases with an increase in the peak energy density. In the present invention, this point A is defined as an inflection point. The peak energy density at the point A is about 200 GW/cm2. According to the present invention, it has been found that the die strength of each device can be improved to 800 MPa or more by setting the pulse width to a predetermined value or less and also setting the peak energy density per pulse to the inflection point A or less to thereby control the depth of the laser processed groove per pulse.
FIG. 10 shows the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 532 nm is applied by one pulse to a silicon wafer. As in the case of using the laser beam having a wavelength of 1064 nm shown in FIG. 9, there is an inflection point B in the case of using the laser beam having a wavelength of 532 nm as shown in FIG. 10. By applying this laser beam to the silicon wafer with the peak energy density less than or equal to the inflection point B, the die strength of each device can be improved to 800 MPa or more. The inflection point B is about 200 GW/cm2.
FIG. 11 shows the relation between the peak energy density and the depth of a laser processed groove in the case that a laser beam having a pulse width of 10 ps and a wavelength of 355 nm is applied by one pulse to a silicon wafer. As in the cases of using the laser beam having a wavelength of 1064 nm shown in FIG. 9 and the laser beam having a wavelength of 532 nm shown in FIG. 10, there is an inflection point C in the case of using the laser beam having a wavelength of 355 nm as shown in FIG. 11. By applying this laser beam to the silicon wafer with the peak energy density less than or equal to the inflection point C, the die strength of each device can be improved to 800 MPa or more. The inflection point C is about 200 GW/cm2.
FIG. 12 shows the relation between the peak energy density and the depth of a laser processed groove in the case that laser beams having a pulse width of 10 ps for each and different wavelengths are applied by one pulse for each to a GaAs wafer. In FIG. 12, reference numeral 100 denotes the laser beam having a wavelength of 1064 nm, and reference numeral 102 denotes the laser beam having a wavelength of 532 nm. As apparent from FIG. 12, in the case of using the GaAs wafer, there are a remarkable inflection point D (wavelength: 1064 nm) and a remarkable inflection point E (wavelength: 532 nm) where the depth of the laser processed groove steeply increases. Thus, also in the case of using the GaAs wafer, the die strength of each device can be improved by setting the peak energy density per pulse to the inflection point or less. In FIG. 12, the inflection point D is about 800 GW/cm2, and the inflection point E is about 70 GW/cm2.
The following test was carried out to confirm the above findings by the present invention, determine the preferable pulse width of the pulsed laser beam, and examine the processing conditions for attaining a die strength of 800 MPa or more.
Pulsed laser beams having wavelengths of 1064 nm, 532 nm, and 355 nm were used and the pulse width of each pulsed laser beam was changed to 30 ns, 10 ns, 5 ns, 3 ns, 2 ns, 1 ns, 100 ps, 50 ps, and 10 ps. In each pulse width, the power was changed to experimentally obtain an energy per pulse for attaining desired laser processing. This energy was divided by the pulse width and the spot area to thereby calculate a peak energy density. Then, the relation between the pulse width, the peak energy density, and the die strength was examined.
The peak energy density is given by the following equation.

Peak energy density (W/cm2)=Average power (W)/(Repetition frequency (Hz)×Spot area (cm2)×Pulse width (s))

As a result, the following results were similarly obtained for all of the pulsed laser beams having the wavelengths of 1064 nm, 532 nm, and 355 nm.
Test 1
The semiconductor wafer was processed to form the laser processed grooves under the following conditions.
Repetition frequency: 10 kHz
Average power: 0.1 W
Pulse width: 2 ns
Spot diameter: 10 μm
Feed speed: 10 mm/s
Peak energy density: 6.35 GW/cm2 
The semiconductor wafer was next divided along the laser processed grooves to obtain the individual devices. The die strength of each device was measured to attain 800 MPa.
Test 2
The semiconductor wafer was processed to form the laser processed grooves under the following conditions.
Repetition frequency: 100 kHz
Average power: 0.1 W
Pulse width: 10 ps
Spot diameter: 10 μm
Feed speed: 100 mm/s
Peak energy density: 63.66 GW/cm2 
The semiconductor wafer was next divided along the laser processed grooves to obtain the individual devices. The die strength of each device was measured to attain 1800 MPa.
Test 3
The semiconductor wafer was processed to form the laser processed grooves under the following conditions.
Repetition frequency: 100 kHz
Average power: 0.3 W
Pulse width: 10 ps
Spot diameter: 10 μm
Feed speed: 100 mm/s
Peak energy density: 190.9 GW/cm2 
The semiconductor wafer was next divided along the laser processed grooves to obtain the individual devices. The die strength of each device was measured to attain 1000 MPa.
Test 4
The semiconductor wafer was processed to form the laser processed grooves under the following conditions.
Repetition frequency: 100 kHz
Average power: 0.4 W
Pulse width: 10 ps
Spot diameter: 10 μm
Feed speed: 100 mm/s
Peak energy density: 254.6 GW/cm2 
The semiconductor wafer was next divided along the laser processed grooves to obtain the individual devices. The die strength of each device was measured to attain 500 MPa.
Test 5
The semiconductor wafer was processed to form the laser processed grooves under the following conditions.
Repetition frequency: 10 kHz
Average power: 0.2 W
Pulse width: 3 ns
Spot diameter: 10 μm
Feed speed: 10 mm/s
Peak energy density: 8.2 GW/cm2 
The semiconductor wafer was next divided along the laser processed grooves to obtain the individual devices. The die strength of each device was measured to attain 500 MPa.
As apparent from the results of Tests 1 to 3, it was confirmed that a die strength of 800 MPa or more can be obtained in the case that the pulse width is set to 2 ns or less and the peak energy density is set to 200 GW/cm2 or less. This value of 200 GW/cm2 for the peak energy density substantially corresponds to each of the inflection points A, B, and C respectively shown in FIGS. 9, 10, and 11.
However, as apparent from the result of Test 5, although the peak energy density is less than 200 GW/cm2, the die strength of each device is 500 MPa in the case that the pulse width is set to 3 ns. That is, when the pulse width is greater than 2 ns, it was confirmed that the improvement in the die strength of each device is insufficient. It is therefore necessary to set the pulse width to 2 ns or less. Further, as apparent from the result of Test 4, the peak energy density is greater than each of the inflection points A, B, and C. In this case, although the pulse width is less than 2 ns, it was confirmed that the improvement in the die strength of each device is insufficient.
Further as described above with reference to FIG. 6, the overlap rate of the adjacent spots S in the feeding direction is preferably set in the range of 16/20 to 19/20 with respect to spot diameter. Further, the spot diameter of the laser beam focused on the semiconductor wafer W is preferably set in the range of 5 to 15 μm.
The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.

## Claims

1. A laser processing method for a semiconductor wafer comprising a groove forming step of applying a pulsed laser beam having an absorption wavelength to said semiconductor wafer along a division line formed on said semiconductor wafer to thereby form a laser processed groove along said division line on said semiconductor wafer, wherein
the pulse width of said pulsed laser beam to be applied in said groove forming step is set to 2 ns or less, and the peak energy density per pulse of said pulsed laser beam is set less than or equal to an inflection point where the depth of the laser processed groove steeply increases with an increase in said peak energy density,
wherein the laser processed groove is formed on an upper surface of the semiconductor wafer and extends into the wafer.

2. The laser processing method of claim 1, wherein said pulsed laser beam is applied to the semiconductor wafer as a series of overlapping spots having a diameter D , and wherein an overlap amount OL of adjacent spots is set within a range of 0.8D≦OL≦0.95D.

3. The laser processing method of claim 1, wherein said peak energy density is within a range of about 6.35 GW/cm2 to about 200 GW/cm2.

4. The laser processing method of claim 3, wherein said peak energy density is approximately 200 GW/cm2.

5. The laser processing method of claim 1, wherein said pulsed laser beam has a wavelength of 532 nm.

6. The laser processing method of claim 1, wherein said pulsed laser beam has a wavelength of 355 nm.