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Thermal Stress Cleaving of Brittle Materials by Laser Beam
Thermal Stress Cleaving of Brittle Materials by Laser Beam T. Ueda(2), K. Yamada , K. Oiso, A. Hosokawa Department of Mechanical Systems Engineering, Faculty of Engineering, Kanazawa University, Kodatsuno 2-40-20, Kanazawa 920-8667, Japan
Abstract Thermal stress cleaving is a prospective technique for separating a wafer or thin plate from brittle materials such as glasses and ceramics. In this paper, the cleaving mechanism of a silicon wafer irradiated with Nd:YAG laser is investigated. A pulsed laser is used for the purpose of investigating the mechanism of crack propagation more precisely. The temperature at the area irradiated with the laser is measured using a two-color pyrometer with an optical fiber. The AE signal is also measured to examine the mechanism of the crack propagation. The AE signal makes it possible to monitor the crack behaviour. During one pulse of the laser, crack propagation begins some milliseconds after laser heating and ceases at about the end of irradiation. The temperature at the area irradiated with the laser is an important factor in the control of the propagation of the crack to achieve high cleaving accuracy and low thermal damage. Keywords: Laser beam machining, Fracture, Temperature
1 INTRODUCTION Thermal stress cleaving using a laser beam is a prospective technique for separating thin plates from brittle materials such as silicon, ceramic and glass. The diamond saw, which is usually used for slicing these brittle materials, has disadvantages, such as possible contamination from the coolant used and the chips produced in the sawing process. Laser cleaving has many advantages. There is no danger of pollution by coolant and chips, and it is possible to fix the workpiece on a table by a simple method because no external forces act on it. Moreover, this method has the capability of producing an extremely smooth surface by cleaving along the cleavage plane of the wafer. There have been attempts to determine the cleaving mechanism and the relationship between irradiating conditions of the laser beam and the cleaving performances of brittle materials, adopting both experimental and theoretical approaches [I-31. However, there has been little investigation of the cleaving mechanism which is focused on the temperature of the work surface irradiated with a laser beam. It is very difficult to measure this correctly because the area is very small and the temperature changes very rapidly. In the present paper, the influence of irradiating conditions of a laser beam on cleaving accuracy and thermal damage to work materials is investigated experimentally. The pulsed Nd:YAG laser is used as a heat source because the relation between heating and crack propagation can be investigated more precisely. The temperature at the small area irradiated with a laser is measured by using a two-color pyrometer
Figurel: Fundamentals of thermal stress cleaving with laser beam.
with an optical fiber developed by the authors [4]. The AE signal is also measured to examine the mechanism of crack propagation .
2
FUNDAMENTAL OF LASER CLEAVING
The fundamental dynamics of thermal stress cleaving of brittle materials with a laser beam are indicated in Fig.1. A small area is irradiated with the laser beam, resulting in thermal gradients which cause compressive stress at the laser spot and tensile stress outside the spot. Afracture is caused by the circumferential tensile stress and propagates toward the center of the laser spot. The fracture propagates along the path of the laser beam, resulting in separation of a fragment from the workpiece.
Figure 2: Schematic illustration of experimental arrangement.
3
EXPERIMENTAL METHOD
3.1
3.2 Pyrometer In this experiment, a two-color pyrometer is used which was developed by the authors [4]. The schematic illustration is given in Fig.2. Afiber is set at a distance of 5 mm from the laser spot and at an angle of 45" from the optical axis. The center of the laser spot is always on the center axis of the fiber. The infrared energy radiated from the object is accepted by the optical fiber and led to a two-color detector consisting of lnSb and MCT detectors. We can obtain the temperature of the laser spot by taking the ratio of the output signals from these two detectors. A chalcogenide optical fiber whose core diameter is 300 pm and whose acceptance angle is 48" is used (Table 2). This optical fiber can transmit infrared rays of wavelength longer than about 3 pm, so that it works as an optical filter to cut off the YAG laser beam of 1.06 p m wavelength which is reflected on the surface of work material. Therefore, the pyrometer can measure the temperature of the laser spot without being affected by the reflected beam. Figure 3 shows the calibration curve of the pyrometer,which is obtained experimentally by sighting on the silicon wafer surface heated to constant temperature electrically. The theoretical result indicated by the solid line coincides well with the experimental results indicated by circles. As can be seen, temperatures higher than approximately 150 "C can be measured by the pyrometer. The frequency characteristics of the pyrometer are crucially important. It has a flat response to about 100 kHz (not indicated here) which is sufficient for this experiment [6].
Experimental procedure
A schematic illustration of the experimental arrangement is shown in Fig.2 and the experimental conditions are summarized in Table 1. As work materials, a silicon wafer with the thickness of 0.5 mm and several kinds of ceramics are used. The crystal plane and the cleaving direction of the silicon wafer mainly used in the experiments are (100) and (000 respectively, and its surface is polished to the roughness of Ra=20 nm. In the experiments, the work table moves at the constant speed V, and the laser beam in relation scans the work material at the same speed V. The YAG laser used emits 10 pulses per second. The intensity distribution of the laser beam, which is measured experimentally [5], can be approximated by Gaussian profile and expressed by :
where, Q is power, r i s the radius from the beam center and "a" is the Gaussian beam radius at which the power intensity is l/e(=1/2.72) of q,.In this experiment, "a" is 640 p m on the work surface. Nd:YAG laser Wave length Peak power Pulse duration Pulse cycle Beam radius at wafer surface Workpiece Dimension Feed rate (Scanning speed)
h = 1064nm
- 809 W -
Q = 109
z = 1.0 3.0ms
3.3 AE signal measurement
f = 10Hz
I
The AE signal is measured to monitor the propagation of a crack. An AE sensor is fixed on the wafer surface at a distance of 5 mm from the crack. Details of AE signal measurement are summarized in Table 3.
a = 640pm Si wafer, ceramics 10 x 20 x 0.5 mm V= 1
4
- 36.7 mm/s
EXPERIM ENTAL RESULTS
4.1 Temperature of laser spot
Table 1: Experimental conditions.
Output wave Core material Core diameter NA(Numerica1 aperture)
The output wave of the two-color pyrometer is shown in Fig.4; (a) is the output wave from InSb. One pulse corresponds to one pulse of the laser beam. Taking the ratio of the output voltages from the MCT and lnSb cells and using the calibration curve in Fig. 3, we can obtain the temperature. The temperature, OA , at pointA is 410 "C; Os at point B is 370 "C; and Oc at point C is 400 "C. These
Chalcogenide glass 300 pm 0.22
Table2: Characteristics of chalcogenide optical fiber.
I
Sensor Size
$5~3.2 mm
Preamplifier Gain "
0
1
2
3
4
5
Table3: Measuring system of AE signal.
Output ratio (InSblMCT) Figure 3:Calibration curve.
>
Q=567W, ~=2.5ms,V=3.33mrnJs ~=2.5ms
1.5
L
0
z
1
al
o
+
K
f?! 2
l
l
I
I
I
I
I
I
c
.-
a
s
u
-
3
Em = 230 "C
200
al
I
.c=3ms .c=3ms
Em = 300 "C
3
c
z2 5 u 0.5 Pst 05
Succeeded: 0 .c=2ms, 0 .c=2.5ms, .c=2ms, .c=2.5ms, Failed :
200 V=6.66mmls
V=3.33mmls
-
0
LL
al
I-
0
400
800
0
400
800
Time (0.5sldiv.) Time (5msldiv.) PeakpowerQ W Peak power Q W (b)V=6.66 mm/s (a)V=3.33 mm/s (a)Output wave of lnSb cell (b)Detail picture of (a) Figure 5:Temperature of laser spot on silicon wafer in cleaving Figure 4: Output wave of two-color pyrometer in process. cleaving of silicon wafer.
values are peaks of each temperature pulse as shown in Fig.4(b). These results indicate that the temperatures at OA and Oc are higher than that at the center of the workpiece. In the cleaving process, the temperature Os at the center of the workpiece is measured. Figure 4(b) is a detailed picture of the typical output wave in Fig.4(a). The temperature increases with laser irradiation, reaches the maximum in 2.5 ms, and then decreases to the initial temperature in about 20 ms. The period between one laser pulse and the next is 100 ms and this period is long enough for the workpiece to cool to room temperature. Critical temperature for crack propagation Figure 5 shows the temperature of the laser spot on a silicon wafer. (a) is when the laser beam is set to a scanning speed of 3.33 mmlsec. In the figure, z is the duration of laser pulse. The white symbols indicate when the cleaving of the wafer is successful; the black symbols indicate when it fails. There is a difference in the temperatures measured, even though the laser beam is under the same conditions. A critical temperature Om is needed for crack propagation. The wafer is cleaved when the temperature is higher than Om, and in this case, Om is approximately230 "C. The separation of the wafer fails when the temperature is lower than Om. In Fig. 5(b), the scanning speed of the laser beam is set to 6.66 mmlsec. The critical temperature Ocr is equally applicable and is approximately 300 " C . The cleaving of the wafer is successful by a probability of 100% at a temperature higher than Om. There is difference between the critical temperatures in (a) and (b). This is because the distance Lcs between the edge of the crack and the laser spot becomes larger as the scanning speed V increases. As a result, it can been seen that the critical temperature Om is dependent on the laser scanning speed when other conditions are constant.
4.2 Thermal damages From the experimental results in the previous section, it is clear that the silicon wafer must be heated to a higher temperature than Om. However, overheating causes thermal damage to the surface of wafer and the deterioration of cleaving precision. Figures 6 and 7 show photographs of silicon wafers cleaved at different peak powers of the laser beam. The peak power of the laser beam in Fig.6 is smaller than that in Fig.7, so that the temperature measured is approximately 260 " C , which is a little higher than Om = 230 "C (obtained in Fig.5(a)). The temperature in Fig.7 is approximately 350 " C , which is much higher than Om. In both cases there was successful separation of a wafer. Figure 6 shows that in (a) the work surface suffered from thermal damage, but in (b) a stripe pattern on the fracture surface can be observed which was
made by the periodic propagation of the crack. The distance between the stripes is equal to the interval between laser pulses. The profile of the fracture surface in (c) has a periodic wave whose wavelength is equal to the intervals of the laser pulse. The height of the wave is very small. In Fig.7, we can observe the thermal trace of the laser pulse with a width of rDin (a), and thermal damage on the fracture surface in (b). The profile in (c) is not periodic and there is cons idera ble surface roughness . Numerous experiments produced results (not indicated here) which revealed that the width rDof damage on the work surface and the degree of roughness on the fracture surface increase as the temperature becomes higher. This shows that, it is very important for precision in cleaving that the temperature at the laser spot on the workpiece should be a little higher than the critical temperature of Om. 4.3 A€ signal Figure 8 shows AE signals measured when the silicon wafer is irradiated with a laser beam of pulse width 3 ms. The output is very small in (a) because the crack does not progress, but high output voltages can be observed in both (b) and (c) because the crack progresses. The scanning speed of the laser beam in (a) is higher than that in (b), since higher laser power is necessary to propagate the crack. In (b) and (c), large output waves are observed during irradiation by the laser and it can be assumed that these AE signals are emitted while cracks are propagating. Especially in (c), another output wave can be observed after irradiation by the laser beam. The cause of the second AE signal must be considered. As shown in Fig.8, TI indicates the time from the start of laser irradiation to the first AE signal and T, the time to the second AE signal. In Fig.9, the influence of scanning speed Vof the laser beam on times TI and T, is investigated. The distance Lss between adjacent laser spots on the wafer increases with the increase of the scanning speed V, and this increase of Lss corresponds to the increase of the distance Lcs between the edge of the crack and the laser spot. As Fig.9 shows, TI increases with the increase of Ls. This indicates that it takes a longer time for the temperature level to be sufficient to propagate the edge of crack as Lcs increases. On the other hand, T, is almost constant and is independent of the change of Lss. This indicates that the
(a) Work surface (b)Fracture surface (c)Profile of (b) Figure 6: Silicon wafer cleaved with Q=215 W (V=3.33mm/s, z=2ms).
Time (Imsldiv.)
(a) Work surface (b)Fracture surface (c)Profile of (b) Figure 7: Silicon wafer cleaved with Q=449 W (V=3.33mm/s, z=2ms).
Figure 8:AE signals measured during laser irradiation (Silicon wafer, z=3ms).
Interval between irradiated spots ,L mm
Figure 10: Thermal damage on surface of silicon wafer when AE signal is as in Fig.8 (c).
Figure 9: Relation between scanning speed Vand time when AE signal is observed (Silicon wafer, Q=567W, z=3ms). Work material
Silicon wafer
Thickness (mm) 0.5 Laser beam Nd:YAG(pulsed) Average power (W) ---Peak Dower (W) 328 Cleaving rate (mrds) 6.66 Width of damage rD (pm) Roughness of fracture surface
(pm)
Silicon wafer 0.5
Nd:YAG(CW) 60
0.5
Nd:YAG(pulsed)
____
Alumina
AI,O,-TIC
Zirconium
CO,( CW) 50
1.2 CO,( CW) 40
CO,( CW) 30
0.5
0.5
987 1
____
____
____
5
8
3
10
0
100
460
107
70
0
0.7
0.7
20
1.3
0.7
0.64
____
second AE signal is independent of the crack propagation. Figure 10 is a photograph of the silicon wafer when the AE signal in Fig.8(c) is measured. There is thermal damage on the surface, with chipping of the work material, which is produced by a lateral crack under the laser spot. The crack is made when the wafer is cooling after a pulse of the laser [7]. There is no chipping on the surface when the AE signal in Fig.8(b) is measured. Therefore, the second AE signal is emitted when the lateral crack is produced. This reveals that the AE signal is useful for monitoring the behavior of cracks produced in the work material. 4.4 Cleaving of ceramics Table 4 is a listing of the experimental results in the successful cleaving of brittle materials. Two different laser beams are used, depending on the spectral absorption of work materials. Silicon nitride cannot be cleaved with CW YAG laser by contrast with a silicon wafer. 5
Silicon nitride
CONCLUSIONS
In thermal stress cleaving, the influence of the irradiating conditions of a pulsed YAG laser on the separation of work material, on the accuracy of the surface fracture and on thermal damage to the work surface is investigated. The temperature at the laser spot and the AE signal emitted at crack propagation are measured. The main results obtained are as follows. 1. A silicon wafer is separated when the temperature of the laser spot is higher than the critical temperature Om. In the present study, Oc! is 230°C when the scanning speed V o f laser beam IS 3.33 mmls and Ocr is 300°C when Vis 6.66 mrds.
Cleaving with no thermal damage is performed under the condition that the temperature of the laser spot is a little higher than Om. The crack which separates the wafer propagates when it is heated by laser beam. The lateral crack which causes chipping of the wafer is produced in the process of cooling. REFERENCES
R.M.Lumley, 1969, Controlled separation of Brittle Materials Using a Laser, The American Ceramic Society Bulletin, Vo1.48/1:850-854. Saimoto A,, lmai Y., Motomura F., 1999, Simulation of Crack Growth in Thermal Stress Cleaving Using Line Heat Source, JSME Series A, Vo1.42/4:578-584. Kurobe T., Noguchi M., Matsumoto T., 1996, Precision Breaking of a Silicon Wafer by YAG Laser, (in Japanese), JSPE, Vo1.62/1:95-99. Ueda T., Sat0 M . , N a k a y a m a , K . , 1 9 9 8 , The Temperature of a Single crystal Diamond Tool in Turning, Annals of the CIRP, Vol.47/1:41-44. Ueda T., Yamada K., Nakayama K., 1997, Temperature of Work Materials Irradiated with CO, Laser,Annals of the CIRP, Vo1.46/1:117-122. Ueda T., Hosokawa A,, Oda K., Yamada K., 2001, Temperature on Flank Face of Cutting Tool in High Speed Milling, Annals of the CIRP, Vo1.50/1:37-40. H.P.Kirchner, R.M.Gruver, D.M.Richard, 1979, Fragmentation and Damage Penetration During Abrasive Machining of Ceramics, The Science of Ceramic Machining and Surface Finishing II, NBS S.P. 562:23-42.
Abstract Thermal stress cleaving is a prospective technique for separating a wafer or thin plate from brittle materials such as glasses and ceramics. In this paper, the cleaving mechanism of a silicon wafer irradiated with Nd:YAG laser is investigated. A pulsed laser is used for the purpose of investigating the mechanism of crack propagation more precisely. The temperature at the area irradiated with the laser is measured using a two-color pyrometer with an optical fiber. The AE signal is also measured to examine the mechanism of the crack propagation. The AE signal makes it possible to monitor the crack behaviour. During one pulse of the laser, crack propagation begins some milliseconds after laser heating and ceases at about the end of irradiation. The temperature at the area irradiated with the laser is an important factor in the control of the propagation of the crack to achieve high cleaving accuracy and low thermal damage. Keywords: Laser beam machining, Fracture, Temperature
1 INTRODUCTION Thermal stress cleaving using a laser beam is a prospective technique for separating thin plates from brittle materials such as silicon, ceramic and glass. The diamond saw, which is usually used for slicing these brittle materials, has disadvantages, such as possible contamination from the coolant used and the chips produced in the sawing process. Laser cleaving has many advantages. There is no danger of pollution by coolant and chips, and it is possible to fix the workpiece on a table by a simple method because no external forces act on it. Moreover, this method has the capability of producing an extremely smooth surface by cleaving along the cleavage plane of the wafer. There have been attempts to determine the cleaving mechanism and the relationship between irradiating conditions of the laser beam and the cleaving performances of brittle materials, adopting both experimental and theoretical approaches [I-31. However, there has been little investigation of the cleaving mechanism which is focused on the temperature of the work surface irradiated with a laser beam. It is very difficult to measure this correctly because the area is very small and the temperature changes very rapidly. In the present paper, the influence of irradiating conditions of a laser beam on cleaving accuracy and thermal damage to work materials is investigated experimentally. The pulsed Nd:YAG laser is used as a heat source because the relation between heating and crack propagation can be investigated more precisely. The temperature at the small area irradiated with a laser is measured by using a two-color pyrometer
Figurel: Fundamentals of thermal stress cleaving with laser beam.
with an optical fiber developed by the authors [4]. The AE signal is also measured to examine the mechanism of crack propagation .
2
FUNDAMENTAL OF LASER CLEAVING
The fundamental dynamics of thermal stress cleaving of brittle materials with a laser beam are indicated in Fig.1. A small area is irradiated with the laser beam, resulting in thermal gradients which cause compressive stress at the laser spot and tensile stress outside the spot. Afracture is caused by the circumferential tensile stress and propagates toward the center of the laser spot. The fracture propagates along the path of the laser beam, resulting in separation of a fragment from the workpiece.
Figure 2: Schematic illustration of experimental arrangement.
3
EXPERIMENTAL METHOD
3.1
3.2 Pyrometer In this experiment, a two-color pyrometer is used which was developed by the authors [4]. The schematic illustration is given in Fig.2. Afiber is set at a distance of 5 mm from the laser spot and at an angle of 45" from the optical axis. The center of the laser spot is always on the center axis of the fiber. The infrared energy radiated from the object is accepted by the optical fiber and led to a two-color detector consisting of lnSb and MCT detectors. We can obtain the temperature of the laser spot by taking the ratio of the output signals from these two detectors. A chalcogenide optical fiber whose core diameter is 300 pm and whose acceptance angle is 48" is used (Table 2). This optical fiber can transmit infrared rays of wavelength longer than about 3 pm, so that it works as an optical filter to cut off the YAG laser beam of 1.06 p m wavelength which is reflected on the surface of work material. Therefore, the pyrometer can measure the temperature of the laser spot without being affected by the reflected beam. Figure 3 shows the calibration curve of the pyrometer,which is obtained experimentally by sighting on the silicon wafer surface heated to constant temperature electrically. The theoretical result indicated by the solid line coincides well with the experimental results indicated by circles. As can be seen, temperatures higher than approximately 150 "C can be measured by the pyrometer. The frequency characteristics of the pyrometer are crucially important. It has a flat response to about 100 kHz (not indicated here) which is sufficient for this experiment [6].
Experimental procedure
A schematic illustration of the experimental arrangement is shown in Fig.2 and the experimental conditions are summarized in Table 1. As work materials, a silicon wafer with the thickness of 0.5 mm and several kinds of ceramics are used. The crystal plane and the cleaving direction of the silicon wafer mainly used in the experiments are (100) and (000 respectively, and its surface is polished to the roughness of Ra=20 nm. In the experiments, the work table moves at the constant speed V, and the laser beam in relation scans the work material at the same speed V. The YAG laser used emits 10 pulses per second. The intensity distribution of the laser beam, which is measured experimentally [5], can be approximated by Gaussian profile and expressed by :
where, Q is power, r i s the radius from the beam center and "a" is the Gaussian beam radius at which the power intensity is l/e(=1/2.72) of q,.In this experiment, "a" is 640 p m on the work surface. Nd:YAG laser Wave length Peak power Pulse duration Pulse cycle Beam radius at wafer surface Workpiece Dimension Feed rate (Scanning speed)
h = 1064nm
- 809 W -
Q = 109
z = 1.0 3.0ms
3.3 AE signal measurement
f = 10Hz
I
The AE signal is measured to monitor the propagation of a crack. An AE sensor is fixed on the wafer surface at a distance of 5 mm from the crack. Details of AE signal measurement are summarized in Table 3.
a = 640pm Si wafer, ceramics 10 x 20 x 0.5 mm V= 1
4
- 36.7 mm/s
EXPERIM ENTAL RESULTS
4.1 Temperature of laser spot
Table 1: Experimental conditions.
Output wave Core material Core diameter NA(Numerica1 aperture)
The output wave of the two-color pyrometer is shown in Fig.4; (a) is the output wave from InSb. One pulse corresponds to one pulse of the laser beam. Taking the ratio of the output voltages from the MCT and lnSb cells and using the calibration curve in Fig. 3, we can obtain the temperature. The temperature, OA , at pointA is 410 "C; Os at point B is 370 "C; and Oc at point C is 400 "C. These
Chalcogenide glass 300 pm 0.22
Table2: Characteristics of chalcogenide optical fiber.
I
Sensor Size
$5~3.2 mm
Preamplifier Gain "
0
1
2
3
4
5
Table3: Measuring system of AE signal.
Output ratio (InSblMCT) Figure 3:Calibration curve.
>
Q=567W, ~=2.5ms,V=3.33mrnJs ~=2.5ms
1.5
L
0
z
1
al
o
+
K
f?! 2
l
l
I
I
I
I
I
I
c
.-
a
s
u
-
3
Em = 230 "C
200
al
I
.c=3ms .c=3ms
Em = 300 "C
3
c
z2 5 u 0.5 Pst 05
Succeeded: 0 .c=2ms, 0 .c=2.5ms, .c=2ms, .c=2.5ms, Failed :
200 V=6.66mmls
V=3.33mmls
-
0
LL
al
I-
0
400
800
0
400
800
Time (0.5sldiv.) Time (5msldiv.) PeakpowerQ W Peak power Q W (b)V=6.66 mm/s (a)V=3.33 mm/s (a)Output wave of lnSb cell (b)Detail picture of (a) Figure 5:Temperature of laser spot on silicon wafer in cleaving Figure 4: Output wave of two-color pyrometer in process. cleaving of silicon wafer.
values are peaks of each temperature pulse as shown in Fig.4(b). These results indicate that the temperatures at OA and Oc are higher than that at the center of the workpiece. In the cleaving process, the temperature Os at the center of the workpiece is measured. Figure 4(b) is a detailed picture of the typical output wave in Fig.4(a). The temperature increases with laser irradiation, reaches the maximum in 2.5 ms, and then decreases to the initial temperature in about 20 ms. The period between one laser pulse and the next is 100 ms and this period is long enough for the workpiece to cool to room temperature. Critical temperature for crack propagation Figure 5 shows the temperature of the laser spot on a silicon wafer. (a) is when the laser beam is set to a scanning speed of 3.33 mmlsec. In the figure, z is the duration of laser pulse. The white symbols indicate when the cleaving of the wafer is successful; the black symbols indicate when it fails. There is a difference in the temperatures measured, even though the laser beam is under the same conditions. A critical temperature Om is needed for crack propagation. The wafer is cleaved when the temperature is higher than Om, and in this case, Om is approximately230 "C. The separation of the wafer fails when the temperature is lower than Om. In Fig. 5(b), the scanning speed of the laser beam is set to 6.66 mmlsec. The critical temperature Ocr is equally applicable and is approximately 300 " C . The cleaving of the wafer is successful by a probability of 100% at a temperature higher than Om. There is difference between the critical temperatures in (a) and (b). This is because the distance Lcs between the edge of the crack and the laser spot becomes larger as the scanning speed V increases. As a result, it can been seen that the critical temperature Om is dependent on the laser scanning speed when other conditions are constant.
4.2 Thermal damages From the experimental results in the previous section, it is clear that the silicon wafer must be heated to a higher temperature than Om. However, overheating causes thermal damage to the surface of wafer and the deterioration of cleaving precision. Figures 6 and 7 show photographs of silicon wafers cleaved at different peak powers of the laser beam. The peak power of the laser beam in Fig.6 is smaller than that in Fig.7, so that the temperature measured is approximately 260 " C , which is a little higher than Om = 230 "C (obtained in Fig.5(a)). The temperature in Fig.7 is approximately 350 " C , which is much higher than Om. In both cases there was successful separation of a wafer. Figure 6 shows that in (a) the work surface suffered from thermal damage, but in (b) a stripe pattern on the fracture surface can be observed which was
made by the periodic propagation of the crack. The distance between the stripes is equal to the interval between laser pulses. The profile of the fracture surface in (c) has a periodic wave whose wavelength is equal to the intervals of the laser pulse. The height of the wave is very small. In Fig.7, we can observe the thermal trace of the laser pulse with a width of rDin (a), and thermal damage on the fracture surface in (b). The profile in (c) is not periodic and there is cons idera ble surface roughness . Numerous experiments produced results (not indicated here) which revealed that the width rDof damage on the work surface and the degree of roughness on the fracture surface increase as the temperature becomes higher. This shows that, it is very important for precision in cleaving that the temperature at the laser spot on the workpiece should be a little higher than the critical temperature of Om. 4.3 A€ signal Figure 8 shows AE signals measured when the silicon wafer is irradiated with a laser beam of pulse width 3 ms. The output is very small in (a) because the crack does not progress, but high output voltages can be observed in both (b) and (c) because the crack progresses. The scanning speed of the laser beam in (a) is higher than that in (b), since higher laser power is necessary to propagate the crack. In (b) and (c), large output waves are observed during irradiation by the laser and it can be assumed that these AE signals are emitted while cracks are propagating. Especially in (c), another output wave can be observed after irradiation by the laser beam. The cause of the second AE signal must be considered. As shown in Fig.8, TI indicates the time from the start of laser irradiation to the first AE signal and T, the time to the second AE signal. In Fig.9, the influence of scanning speed Vof the laser beam on times TI and T, is investigated. The distance Lss between adjacent laser spots on the wafer increases with the increase of the scanning speed V, and this increase of Lss corresponds to the increase of the distance Lcs between the edge of the crack and the laser spot. As Fig.9 shows, TI increases with the increase of Ls. This indicates that it takes a longer time for the temperature level to be sufficient to propagate the edge of crack as Lcs increases. On the other hand, T, is almost constant and is independent of the change of Lss. This indicates that the
(a) Work surface (b)Fracture surface (c)Profile of (b) Figure 6: Silicon wafer cleaved with Q=215 W (V=3.33mm/s, z=2ms).
Time (Imsldiv.)
(a) Work surface (b)Fracture surface (c)Profile of (b) Figure 7: Silicon wafer cleaved with Q=449 W (V=3.33mm/s, z=2ms).
Figure 8:AE signals measured during laser irradiation (Silicon wafer, z=3ms).
Interval between irradiated spots ,L mm
Figure 10: Thermal damage on surface of silicon wafer when AE signal is as in Fig.8 (c).
Figure 9: Relation between scanning speed Vand time when AE signal is observed (Silicon wafer, Q=567W, z=3ms). Work material
Silicon wafer
Thickness (mm) 0.5 Laser beam Nd:YAG(pulsed) Average power (W) ---Peak Dower (W) 328 Cleaving rate (mrds) 6.66 Width of damage rD (pm) Roughness of fracture surface
(pm)
Silicon wafer 0.5
Nd:YAG(CW) 60
0.5
Nd:YAG(pulsed)
____
Alumina
AI,O,-TIC
Zirconium
CO,( CW) 50
1.2 CO,( CW) 40
CO,( CW) 30
0.5
0.5
987 1
____
____
____
5
8
3
10
0
100
460
107
70
0
0.7
0.7
20
1.3
0.7
0.64
____
second AE signal is independent of the crack propagation. Figure 10 is a photograph of the silicon wafer when the AE signal in Fig.8(c) is measured. There is thermal damage on the surface, with chipping of the work material, which is produced by a lateral crack under the laser spot. The crack is made when the wafer is cooling after a pulse of the laser [7]. There is no chipping on the surface when the AE signal in Fig.8(b) is measured. Therefore, the second AE signal is emitted when the lateral crack is produced. This reveals that the AE signal is useful for monitoring the behavior of cracks produced in the work material. 4.4 Cleaving of ceramics Table 4 is a listing of the experimental results in the successful cleaving of brittle materials. Two different laser beams are used, depending on the spectral absorption of work materials. Silicon nitride cannot be cleaved with CW YAG laser by contrast with a silicon wafer. 5
Silicon nitride
CONCLUSIONS
In thermal stress cleaving, the influence of the irradiating conditions of a pulsed YAG laser on the separation of work material, on the accuracy of the surface fracture and on thermal damage to the work surface is investigated. The temperature at the laser spot and the AE signal emitted at crack propagation are measured. The main results obtained are as follows. 1. A silicon wafer is separated when the temperature of the laser spot is higher than the critical temperature Om. In the present study, Oc! is 230°C when the scanning speed V o f laser beam IS 3.33 mmls and Ocr is 300°C when Vis 6.66 mrds.
Cleaving with no thermal damage is performed under the condition that the temperature of the laser spot is a little higher than Om. The crack which separates the wafer propagates when it is heated by laser beam. The lateral crack which causes chipping of the wafer is produced in the process of cooling. REFERENCES
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