Wafer producing method

Abstract

A hexagonal single crystal wafer is produced from a hexagonal single crystal ingot. A wafer producing method includes a separation start point forming step of applying a laser beam to the ingot to form a modified layer parallel to the upper surface of the ingot and cracks extending from the modified layer, thus forming a separation start point. The focal point of the laser beam is relatively moved in a first direction perpendicular to a second direction where a c-axis in the ingot is inclined by an off angle with respect to a normal to the upper surface. The off angle is formed between the upper surface and a c-plane perpendicular to the c-axis, thereby linearly forming the modified layer extending in the first direction. The laser beam is applied to the ingot with the direction of the polarization plane of the laser beam set to the first direction.

Claims

1. A wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal ingot having a first surface, a second surface opposite to the first surface, a c-axis extending from the first surface to the second surface, and a c-plane perpendicular to the c-axis, where the c-axis is inclined by an off angle with respect to a normal to the first surface, wherein the c-axis and the normal to the first surface intersect each other, and thereby the c-plane is inclined by said off angle with respect to the first surface, the wafer producing method comprising: a separation start point forming step of setting a focal point of a laser beam inside the ingot at a predetermined depth from the first surface, which depth corresponds to a thickness of the wafer to be produced, and next applying the laser beam to the first surface as relatively moving the focal point and the ingot to thereby form a modified layer parallel to the first surface and cracks extending from the modified layer along the c-plane, thus forming a separation start paint; and a wafer separating step of separating a member having a thickness corresponding to the thickness of the wafer from the ingot at the separation start point after performing the separation start point forming step, thus producing the wafer from the ingot; the separation start point forming step including: a modified layer forming step of relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction, and further wherein said second direction is defined as a direction parallel to a line connecting a point where the c-axis intersects the first surface to a point where the normal intersects the first surface, thereby linearly forming the modified layer extending in the first direction, resulting in a modified line parallel to the first direction, and an indexing step of relatively moving the focal point, with respect to the ingot, in the second direction to thereby index the focal point by a predetermined amount; during the modified layer forming step, a direction of a polarization plane of the laser beam is set to coincide with the first direction.

2. The wafer producing method according to claim 1, wherein the hexagonal single crystal ingot is selected from a SiC single crystal ingot, GaN single crystal ingot, and MN single crystal ingot.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a laser processing apparatus suitable for use in performing a wafer producing method of the present invention;

(2) FIG. 2 is a block diagram of a laser beam generating unit;

(3) FIG. 3A is a perspective view of a hexagonal single crystal ingot;

(4) FIG. 3B is an elevational view of the ingot shown in FIG. 3A;

(5) FIG. 4 is a perspective view for illustrating a separation start point forming step;

(6) FIG. 5 is a plan view of the ingot shown in FIG. 3A;

(7) FIG. 6 is a schematic sectional view for illustrating a modified layer forming step;

(8) FIG. 7 is a schematic plan view for illustrating the modified layer forming step;

(9) FIG. 8A is a schematic plan view for illustrating an indexing step;

(10) FIG. 8B is a schematic plan view for illustrating an index amount;

(11) FIGS. 9A and 9B are perspective views for illustrating a wafer separating step;

(12) FIG. 10 is a perspective view of a hexagonal single crystal wafer produced from the ingot; and

(13) FIG. 11 is a schematic view showing the relation between the processing direction and the polarization plane of a laser beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(14) A preferred embodiment of the present invention will now be described in detail with reference to the drawings. Referring to FIG. 1, there is shown a perspective view of a laser processing apparatus 2 suitable for use in performing a wafer producing method of the present invention. The laser processing apparatus 2 includes a stationary base 4 and a first slide block 6 mounted on the stationary base 4 so as to be movable in the X direction. The first slide block 6 is moved in a feeding direction, or in the X direction along a pair of guide rails 14 by a feeding mechanism 12 composed of a ball screw 8 and a pulse motor 10.

(15) A second slide block 16 is mounted on the first slide block 6 so as to be movable in the Y direction. The second slide block 16 is moved in an indexing direction, or in the Y direction along a pair of guide rails 24 by an indexing mechanism 22 composed of a ball screw 18 and a pulse motor 20. A support table 26 is mounted on the second slide block 16. The support table 26 is movable in the X direction and the Y direction by the feeding mechanism 12 and the indexing mechanism 22 and also rotatable by a motor stored in the second slide block 16.

(16) A column 28 is provided on the stationary base 4 so as to project upward therefrom. A laser beam applying mechanism (laser beam applying means) 30 is mounted on the column 28. The laser beam applying mechanism 30 is composed of a casing 32, a laser beam generating unit 34 (see FIG. 2) stored in the casing 32, and focusing means (laser head) 36 mounted on the front end of the casing 32. An imaging unit 38 having a microscope and a camera is also mounted on the front end of the casing 32 so as to be aligned with the focusing means 36 along the X direction.

(17) As shown in FIG. 2, the laser beam generating unit 34 includes a laser oscillator 40 for generating a pulsed laser beam such as YAG laser and YVO4 laser, repetition frequency setting means 42 for setting the repetition frequency of the pulsed laser beam to be generated from the laser oscillator 40, pulse width adjusting means 44 for adjusting the pulse width of the pulsed laser beam to be generated from the laser oscillator 40, and power adjusting means 46 for adjusting the power of the pulsed laser generated from the laser oscillator 40. Although especially not shown, the laser oscillator 40 has a Brewster window, so that the laser beam generated from the laser oscillator 40 is a laser beam of linearly polarized light. After the power of the pulsed laser beam is adjusted to a predetermined power by the power adjusting means 46 of the laser beam generating unit 34, the pulsed laser beam is reflected by a mirror 48 included in the focusing means 36 and next focused by a focusing lens 50 included in the focusing means 36. The focusing lens 50 is positioned so that the pulsed laser beam is focused inside a hexagonal single crystal ingot 11 as a workpiece fixed to the support table 26.

(18) Referring to FIG. 3A, there is shown a perspective view of the hexagonal single crystal ingot 11 as a workpiece to be processed. FIG. 3B is an elevational view of the hexagonal single crystal ingot 11 shown in FIG. 3A. The hexagonal single crystal ingot (which will be hereinafter referred to also simply as the ingot) 11 is selected from a SiC single crystal ingot or a GaN single crystal ingot. The ingot 11 has a first surface (upper surface) 11a and a second surface (lower surface) 11b opposite to the first surface 11a. The first surface 11a of the ingot 11 is preliminarily polished to a mirror finish because the laser beam is applied to the first surface 11a.

(19) The ingot 11 has a first orientation flat 13 and a second orientation flat 15 perpendicular to the first orientation flat 13. The length of the first orientation flat 13 is set greater than the length of the second orientation flat 15. The ingot 11 has a c-axis 19 inclined by an off angle toward the second orientation flat 15 with respect to a normal 17 to the upper surface 11a and also has a c-plane 21 perpendicular to the c-axis 19. The c-plane 21 is inclined by the off angle with respect to the upper surface 11a. In general, in the hexagonal single crystal ingot 11, the direction perpendicular to the direction of extension of the shorter second orientation flat 15 is the direction of inclination of the c-axis 19. The c-plane 21 is set in the ingot 11 innumerably at the molecular level of the ingot 11. In this preferred embodiment, the off angle is set to 4. However, the off angle is not limited to 4 in the present invention. For example, the off angle may be freely set in the range of 1 to 6 in manufacturing the ingot 11.

(20) Referring again to FIG. 1, a column 52 is fixed to the left side of the stationary base 4. The column 52 is formed with a vertically elongated opening 53, and a pressing mechanism 54 is vertically movably mounted to the column 52 so as to project from the opening 53.

(21) As shown in FIG. 4, the ingot 11 is fixed to the upper surface of the support table 26 by using a wax or adhesive in the condition where the second orientation flat 15 of the ingot 11 becomes parallel to the X direction. In other words, as shown in FIG. 5, the direction of formation of the off angle is shown by an arrow Y1. That is, the direction of the arrow Y1 is the direction where the intersection 19a between the c-axis 19 and the upper surface 11a of the ingot 11 is present with respect to the normal 17 to the upper surface 11a. Further, the direction perpendicular to the direction of the arrow Y1 is shown by an arrow A. Then, the ingot 11 is fixed to the support table 26 in the condition where the direction of the arrow A becomes parallel to the X direction.

(22) Accordingly, the laser beam is scanned in the direction of the arrow A perpendicular to the direction of the arrow Y1, or the direction of formation of the off angle . In other words, the direction of the arrow A perpendicular to the direction of the arrow Y1 where the off angle is formed is defined as the feeding direction of the support table 26.

(23) In the wafer producing method of the present invention, it is important that the scanning direction of the laser beam to be applied from the focusing means 36 is set to the direction of the arrow A perpendicular to the direction of the arrow Y1 where the off angle of the ingot 11 is formed. That is, it was found that by setting the scanning direction of the laser beam to the direction of the arrow A as mentioned above in the wafer producing method of the present invention, cracks propagating from a modified layer formed inside the ingot 11 by the laser beam extend very long along the c-plane 21.

(24) In performing the wafer producing method according to this preferred embodiment, a separation start point forming step is performed in such a manner that the focal point of the laser beam having a transmission wavelength (e.g., 1064 nm) to the hexagonal single crystal ingot 11 fixed to the support table 26 is set inside the ingot 11 at a predetermined depth from the first surface (upper surface) 11a, which depth corresponds to the thickness of a wafer to be produced, and the laser beam is next applied to the upper surface 11a as relatively moving the focal point and the ingot 11 to thereby form a modified layer 23 parallel to the upper surface 11a and cracks 25 propagating from the modified layer 23 along the c-plane 21, thus forming a separation start point (separation plane) where the modified layer 23 and the cracks 25 are formed.

(25) This separation start point forming step includes a modified layer forming step of relatively moving the focal point of the laser beam in the direction of the arrow A perpendicular to the direction of the arrow Y1 where the c-axis 19 is inclined by the off angle with respect to the normal 17 to the upper surface 11a and the off angle is formed between the c-plane 21 and the upper surface 11a, thereby forming the modified layer 23 inside the ingot 11 and the cracks 25 propagating from the modified layer 23 along the c-plane 21, and also includes an indexing step of relatively moving the focal point in the direction of formation of the off angle , i.e., in the Y direction to thereby index the focal point by a predetermined amount as shown in FIG. 7 and FIGS. 8A and 8B.

(26) As shown in FIGS. 6 and 7, the modified layer 23 is linearly formed so as to extend in the X direction, so that the cracks 25 propagate from the modified layer 23 in opposite directions along the c-plane 21. In the wafer producing method according to this preferred embodiment, the separation start point forming step further includes an index amount setting step of measuring the width of the cracks 25 formed on one side of the modified layer 23 along the c-plane 21 and then setting the index amount of the focal point according to the width measured above. More specifically, letting W1 denote the width of the cracks 25 formed on one side of the modified layer 23 so as to propagate from the modified layer 23 along the c-plane 21, the index amount W2 of the focal point is set in the range of W1 to 2W1.

(27) For example, the separation start point forming step is performed under the following laser processing conditions.

(28) Light source: Nd:YAG pulsed laser

(29) Wavelength: 1064 nm

(30) Repetition frequency: 80 kHz

(31) Average power: 3.2 W

(32) Pulse width: 4 ns

(33) Spot diameter: 10 m

(34) Numerical aperture (NA) of the focusing lens: 0.45

(35) Index amount: 400 m

(36) In the laser processing conditions mentioned above, the width W1 of the cracks 25 propagating from the modified layer 23 along the C-plane 21 in one direction as viewed in FIG. 6 is set to about 250 m, and the index amount W2 is set to 400 m. However, the average power of the laser beam is not limited to 3.2 W. When the average power of the laser beam was set to 2 to 4.5 W, good results were obtained in the preferred embodiment. In the case that the average power was set to 2 W, the width W1 of the cracks 25 was about 100 m. In the case that the average power was set to 4.5 W, the width W1 of the cracks 25 was about 350 m.

(37) In the case that the average power is less than 2 W or greater than 4.5 W, the modified layer 23 cannot be well formed inside the ingot 11. Accordingly, the average power of the laser beam to be applied is preferably set in the range of 2 to 4.5 W. For example, the average power of the laser beam to be applied to the ingot 11 was set to 3.2 W in this preferred embodiment. As shown in

(38) FIG. 6, the depth D1 of the focal point from the upper surface 11a in forming the modified layer 23 was set to 500 m.

(39) Referring to FIG. 8A, there is shown a schematic plan view for illustrating the scanning direction of the laser beam. The separation start point forming step is performed on a forward path X1 and a backward path X2 as shown in FIG. 8A. That is, the modified layer 23 is formed in the hexagonal single crystal ingot 11 on the forward path X1. Thereafter, the focal point of the laser beam is indexed by the predetermined amount. Thereafter, the modified layer 23 is formed again in the ingot 11 on the backward path X2.

(40) Further, in the case that the index amount of the focal point of the laser beam is set in the range of W to 2 W where W is the width of the cracks 25 formed on one side of the modified layer 23 along the c-plane 21, the index amount of the focal point is preferably set to W or less until the modified layer 23 is first formed after setting the focal point of the laser beam inside the ingot 11.

(41) For example, in the case that the index amount of the focal point of the laser beam is 400 m, the index amount is set to 200 m until the modified layer 23 is first formed inside the ingot 11, and the laser beam is scanned plural times with this index amount of 200 m as shown in FIG. 8B. That is, a first part of the plural scanning paths of the laser beam is idle, and when it is determined that the modified layer 23 has been first formed inside the ingot 11, the index amount is set to 400 m and the modified layer 23 is then formed inside the ingot 11.

(42) In this manner, the focal point of the laser beam is sequentially indexed to form a plurality of modified layers 23 at the depth D1 in the whole area of the ingot 11 and also form the cracks 25 extending from each modified layer 23 along the c-plane 21. Thereafter, a wafer separating step is performed in such a manner that an external force is applied to the ingot 11 to thereby separate a platelike member having a thickness corresponding to the thickness of the wafer to be formed from the ingot 11 at the separation start point composed of the modified layers 23 and the cracks 25, thus producing a hexagonal single crystal wafer 27 shown in FIG. 10.

(43) This wafer separating step is performed by using the pressing mechanism 54 shown in FIG. 1. The configuration of the pressing mechanism 54 is shown in

(44) FIGS. 9A and 9B. The pressing mechanism 54 includes a head 56 vertically movable by a moving mechanism (not shown) incorporated in the column 52 shown in FIG. 1 and a pressing member 58 rotatable in the direction shown by an arrow R in FIG. 9B with respect to the head 56. As shown in FIG. 9A, the pressing mechanism 54 is relatively positioned above the ingot 11 fixed to the support table 26. Thereafter, as shown in FIG. 9B, the head 56 is lowered until the pressing member 58 comes into pressure contact with the upper surface 11a of the ingot 11.

(45) In the condition where the pressing member 58 is in pressure contact with the upper surface 11a of the ingot 11, the pressing member 58 is rotated in the direction of the arrow R to thereby generate a torsional stress in the ingot 11. As a result, the ingot 11 is broken at the separation start point where the modified layers 23 and the cracks 25 are formed. Accordingly, the hexagonal single crystal wafer 27 shown in FIG. 10 can be separated from the hexagonal single crystal ingot 11. After separating the wafer 27 from the ingot 11, the separation surface of the wafer 27 and the separation surface of the ingot 11 are preferably polished to a mirror finish.

(46) An experiment was made under the following conditions. A SiC ingot was adopted as the hexagonal single crystal ingot 11, and a laser beam was applied to the SiC ingot to form ten lines of modified layers. As shown in FIG. 11, the direction of the polarization plane of the laser beam was changed to the directions shown by arrows A, B, C, and D with respect to the processing direction (scanning direction) shown by an arrow +X, wherein the direction A coincides with the processing direction +X. Then, the lengths of a maximum crack and a minimum crack extending from each modified layer to the right side thereof were measured. In Experiments 1 to 4 to be described later, the direction of the polarization plane of the laser beam was changed with respect to the processing direction by rotating a half-wave plate.

(47) [Experiment 1]

(48) The direction of the polarization plane of the laser beam was set to the processing direction A (the direction perpendicular to the direction where the off angle is formed) to perform the modified layer forming step.

(49) TABLE-US-00001 Maximum crack (m) Minimum crack (m) First line 358 288 Second line 382 305 Third line 385 294 Fourth line 378 289 Fifth line 380 295 Sixth line 383 305 Seventh line 381 294 Eighth line 363 289 Ninth line 375 288 Tenth line 382 290 Sum 3767 2937 6704
[Experiment 2]

(50) The direction of the polarization plane of the laser beam was set to the direction B (the direction where the off angle is formed) perpendicular to the processing direction to perform the modified layer forming step.

(51) TABLE-US-00002 Maximum crack (m) Minimum crack (m) First line 357 278 Second line 432 276 Third line 345 264 Fourth line 342 253 Fifth line 445 295 Sixth line 352 263 Seventh line 382 255 Eighth line 452 289 Ninth line 375 263 Tenth line 353 260 Sum 3835 2696 6531
[Experiment 3]

(52) The direction of the polarization plane of the laser beam was set to the direction C inclined 45 degrees to the right with respect to the processing direction to perform the modified layer forming step.

(53) TABLE-US-00003 Maximum crack (m) Minimum crack (m) First line 366 299 Second line 319 232 Third line 282 189 Fourth line 365 283 Fifth line 321 222 Sixth line 283 186 Seventh line 355 276 Eighth line 332 251 Ninth line 273 165 Tenth line 342 263 Sum 3238 2366 5604
[Experiment 4]

(54) The direction of the polarization plane of the laser beam was set to the direction D inclined 45 degrees to the left with respect to the processing direction to perform the modified layer forming step.

(55) TABLE-US-00004 Maximum crack (m) Minimum crack (m) First line 387 313 Second line 350 257 Third line 322 265 Fourth line 334 276 Fifth line 390 308 Sixth line 342 243 Seventh line 369 261 Eighth line 380 302 Ninth line 313 272 Tenth line 356 264 Sum 3543 2761 6304

(56) Various considerations were made from the data obtained in Experiments 1 to 4.

(57) [Consideration 1]

(58) In Experiment 1, the difference between the maximum value and the minimum value for the maximum crack is 27 m, and the difference between the maximum value and the minimum value for the minimum crack is 6 m.

(59) In Experiment 2, the difference between the maximum value and the minimum value for the maximum crack is 87 m, and the difference between the maximum value and the minimum value for the minimum crack is 31 m. In Experiment 3, the difference between the maximum value and the minimum value for the maximum crack is 93 m, and the difference between the maximum value and the minimum value for the minimum crack is 134 m.

(60) In Experiment 4, the difference between the maximum value and the minimum value for the maximum crack is 79 m, and the difference between the maximum value and the minimum value for the minimum crack is 56 m.

(61) Accordingly, variations in length of the crack (both in the maximum crack and the minimum crack) in Experiment 1 is smallest.

(62) [Consideration 2]

(63) In Experiment 1, the difference between the maximum value for the maximum crack and the minimum value for the minimum crack is 94 m.

(64) In Experiment 2, the difference between the maximum value for the maximum crack and the minimum value for the minimum crack is 199 m.

(65) In Experiment 3, the difference between the maximum value for the maximum crack and the minimum value for the minimum crack is 201 m.

(66) In Experiment 4, the difference between the maximum value for the maximum crack and the minimum value for the minimum crack is 147 m.

(67) Accordingly, the difference in length between the maximum crack and the minimum crack in Experiment 1 is smallest.

(68) [Consideration 3]

(69) In Experiment 1, the total sum of all the values for the maximum crack and all the values for the minimum crack is 6704 m.

(70) In Experiment 2, the total sum of all the values for the maximum crack and all the values for the minimum crack is 6531 m.

(71) In Experiment 3, the total sum of all the values for the maximum crack and all the values for the minimum crack is 5604 m.

(72) In Experiment 4, the total sum of all the values for the maximum crack and all the values for the minimum crack is 6304 m.

(73) Accordingly, the cracks can be formed best in Experiment 1.

(74) As apparent from Considerations 1 to 3 mentioned above, the laser processing by Experiment 1 is most effective in producing a SiC wafer from a SiC ingot. That is, the SiC wafer can be produced from the SiC ingot most effectively by the processing method such that the laser beam is applied to the ingot to form the modified layers in the condition where the direction of the polarization plane of the laser beam is set to the processing direction, i.e., the +X direction (the direction perpendicular to the direction where the off angle is formed).

(75) The present invention is not limited to the details of the above described preferred embodiment. 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.