SIC SUBSTRATE SEPARATION PROCESS

Abstract

A SiC substrate separation method for planarly separating a SiC substrate (31) into at least two parts, wherein the first SiC substrate (31B) has a first surface (31a), a second surface (31b) opposite the first surface, a c-axis extending from the first surface (31a) to the second surface (31b), and a c-plane perpendicular to the c-axis, the SiC substrate separation method comprising: an adhesive tape applying step comprising applying a transparent adhesive tape (41) to the first surface (31a) of the SiC substrate (31); a support member attaching step comprising attaching a support member (43) to the second surface (31b) of the SiC substrate (31); after performing the adhesive tape application step and the support member application step, a separation start point formation step comprising setting the focal point of a laser beam having a transmission wavelength for the SiC substrate (31) and the adhesive tape (41) to a predetermined depth inside the SiC substrate (31) from the adhesive tape (41), and next applying the laser beam to the adhesive tape (41) while relatively moving the focal point and the SiC substrate (31), thereby forming a modified layer (45) parallel to the first surface (31a) of the SiC substrate (31) and cracks (47) propagating from the modified layer (45) to form a separation start point; and a separating step of applying an external force to the SiC substrate (31) after performing the separation start point forming step, to thereby separate the SiC substrate (31) in the state at the separation start point into a first SiC substrate (31B) having the first surface (31a) and a second SiC substrate (31A) having the second surface (31b) in which the adhesive tape (41) is attached to the first SiC substrate (31B) and the support member (43) is attached to the second SiC substrate (31A); where the separation start point formation step includes: a modified layer (45) forming step comprising relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by a deviation angle with respect to a normal to the first surface of the SiC substrate (31), and the deviation angle is formed between the c-plane and the first surface (31a), thereby linearly forming the modified layer (45) extending in the first direction inside the SiC substrate (31), and a dividing step comprising relatively moving the focal point in the second direction to thereby further move the focal point by a predetermined amount in the second direction.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a separation method for a silicon carbide (SiC) substrate for planarly separating a SiC substrate into at least two parts. Description of the related prior art

[0002] Various devices, such as integrated circuits (ICs) and large-scale integrations (LSIs), are manufactured by forming a functional layer on the front surface of a wafer made of silicon or the like, and dividing this functional layer into a plurality of regions along a plurality of intersecting division lines. The division lines of the wafer are processed by a processing device, such as a dicing device and a laser processing device, to thereby separate the wafer into a plurality of individual device chips corresponding to the devices. The device chips thus obtained are widely used in a variety of electronic equipment, such as mobile phones and personal computers.

[0003] Furthermore, power devices or optical devices, such as light-emitting diodes (LEDs) and laser diodes (LDs), are formed by forming a functional layer on the front surface of a wafer made of a hexagonal single crystal, such as SiC and GaN, and dividing this functional layer into a plurality of regions along a plurality of intersecting dividing lines. Generally, the wafer on which the devices are to be formed is manufactured by cutting an ingot with a wire saw. Both sides of the resulting wafer are polished to a mirror finish (see, for example, Japanese Unexamined Patent Application Laid-Open No. 2000-94221A).

[0004] This wire saw is configured such that a single wire, such as piano wire with a diameter of approximately 100 µm to 300 µm, is wound around a plurality of grooves formed on typically two to four guide rollers to form a plurality of cutting sections that are parallel to each other and spaced at a given pitch. The wire is driven to run in one direction or opposite directions, thereby cutting the ingot into a plurality of wafers. However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, 70% to 80% of the ingot becomes scrap, which is economically unfavorable. In particular, a hexagonal single-crystal ingot made of SiC or GaN, for example, has a high Mohs hardness, and it is therefore difficult to cut this ingot with the wire saw.Accordingly, considerable time is required to cut the ingot, resulting in a decrease in productivity. This means that efficient wafer production is problematic in this state-of-the-art technology.

[0005] A technique for solving these problems is described in Japanese Patent Application Laid-Open No. 2013-49161. This technique includes the steps of setting the focal point of a laser beam having a transmission wavelength for SiC to the interior of a SiC substrate or ingot, next, applying the laser beam to the first SiC substrate or ingot while scanning the laser beam along the SiC substrate or ingot, thereby forming a modified layer and cracks in a parting plane inside the SiC substrate or ingot, and next, applying an external force to the SiC substrate or ingot, thereby breaking the SiC substrate or ingot along the parting plane where the modified layer and cracks are formed, thus separating a wafer from the SiC substrate or ingot.In this technique, the laser beam (pulsed laser beam) is scanned spirally or linearly along the parting plane, so that a first application point of the laser beam and a second application point of the laser beam, which is closest to the first application point, have a predetermined positional relationship to each other. As a result, the modified layer and the cracks are formed with a very high density in the parting plane of the SiC substrate or ingot.

[0006] Furthermore, Japanese Unexamined Patent Publication No. 2011-60862 A proposes a method for cutting substrates, comprising the following processing methods: disposing a laser light condenser on a substrate in a non-contact state, irradiating a surface of the substrate with laser light and concentrating the laser light into the substrate by the laser light condenser, moving the laser light condenser and the substrate relatively to form a deformed layer in the substrate, exposing the deformed layer to a sidewall of the substrate and etching the deformed layer, forming grooves in the deformed layer, and peeling the substrate in accordance with the grooves as base points. Japanese Unexamined Patent Publication No. 2015-115573 A relates to a laser cutting method in which a substrate is irradiated with a laser beam through a protective surface material on the back of the substrate during laser processing. SUMMARY OF THE INVENTION

[0007] However, in the SiC substrate cutting method or SiC ingot cutting method described in the above-mentioned Japanese Patent Application Laid-Open No. 2013-49161 A, the laser beam is scanned spirally or linearly along the SiC substrate or ingot. In the case of linear scanning of the laser beam, the scanning direction of the laser beam is not specified. In the SiC substrate cutting method or SiC ingot cutting method described in Japanese Patent Application Laid-Open No. 2013-49161 A, the pitch (distance) between the first application point and the second application point of the laser beam is set to 1 µm to 10 µm. This pitch corresponds to the pitch of the cracks extending from the modified layer along a c-plane defined in the SiC substrate or ingot.

[0008] As a result, the pitch of the laser beam application points to be applied to the SiC substrate or ingot is very small. Accordingly, regardless of whether the laser beam is scanned spirally or linearly, the laser beam must be applied at a very small pitch, so the corresponding productivity improvement is still insufficient. Furthermore, when separating a wafer from the SiC substrate or ingot along the separation plane where the modified layer and cracks are formed, there is a problem that the wafer may be damaged because of its relatively thin nature.

[0009] It is therefore an object of the present invention to provide a SiC substrate separation method that can planarly separate a SiC substrate into at least two parts without causing damage.

[0010] In accordance with one aspect of the present invention, there is provided a SiC substrate separating method for planarly separating a SiC substrate into at least two parts, the first SiC substrate having a first surface, a second surface opposite the first surface, a c-axis extending from the first surface to the second surface, and a c-plane perpendicular to the c-axis, the SiC substrate separating method comprising an adhesive tape applying step of applying a transparent adhesive tape to the first surface of the SiC substrate; a support member applying step of applying a support member to a second surface of the SiC substrate;after performing the adhesive tape attachment step and the support member attachment step, a separation start point formation step of setting the focal point of a laser beam having a transmission wavelength for the SiC substrate and the adhesive tape to a predetermined depth inside the SiC substrate from the adhesive tape, and next, applying the laser beam to the adhesive tape while relatively moving the focal point and the SiC substrate, thereby forming a modified layer parallel to the first surface of the SiC substrate and cracks propagating from the modified layer to form a separation start point;and after performing the separation start point forming step, includes a separation step of applying an external force to the SiC substrate to thereby separate the SiC substrate in the state at the separation start point into a first SiC substrate having the first surface and a second SiC substrate having the second surface in which the adhesive tape is attached to the first SiC substrate and the support member is attached to the second SiC substrate;wherein the separation start point forming step includes a modified layer forming step of relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by a deviation angle with respect to a normal to the first surface of the SiC substrate, and the deviation angle is formed between the c-plane and the first surface, to thereby linearly form the modified layer extending inside the SiC substrate in the first direction, and a dividing step of relatively moving the focal point in the second direction to thereby further move the focal point by a predetermined amount in the second direction.

[0011] Preferably, the refractive index of the adhesive tape is higher than the refractive index of air and lower than the refractive index of the SiC substrate.

[0012] Preferably, the SiC substrate separating method further includes, by using a grinding member, a flattening step of grinding a separating surface of the first SiC substrate separated from the second SiC substrate at the separation starting point and further grinding a separating surface of the second SiC substrate separated from the first SiC substrate at the separation starting point, to thereby flatten the separating surface of the first SiC substrate and the separating surface of the second SiC substrate.

[0013] According to the SiC substrate separation method of the aspect of the present invention, the first surface of the SiC substrate is reinforced by the adhesive tape. Accordingly, the first SiC substrate having the first surface can be separated from the second SiC substrate having the second surface without causing damage. Furthermore, the refractive index of the adhesive tape attached to the first surface of the SiC substrate is higher than that of air and lower than that of the SiC substrate. Accordingly, reflection of the laser beam can be suppressed by attaching the adhesive tape to the SiC substrate, so that the laser beam can be efficiently introduced into the SiC substrate.

[0014] According to the SiC substrate separation method of the aspect of the present invention, the cracks of each modified layer propagate in opposite directions along the c-plane, so that any adjacent ones of the plurality of modified layers are connected to each other by the cracks. Accordingly, the SiC substrate can be efficiently separated at the separation starting point. Also in the aspect of the present invention, the first surface of the SiC substrate is reinforced by the adhesive tape, so that the first SiC substrate having the first surface can be separated from the second SiC substrate having the second surface without causing damage.

[0015] The above and other objects, features and advantages of the present invention and the mode for carrying them out will become more apparent and the invention itself will be best understood by studying the following description and appended claims with reference to the attached drawings which show a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a laser processing apparatus suitable for use in carrying out a SiC substrate separation method of the present invention; Fig. 2 is a block diagram of a laser beam generating unit corresponding to the Fig. 1 shown laser processing device; Fig. 3A is a perspective view of a SiC ingot; Fig. 3B is a side view of the Fig. SiC ingots shown in Figure 3A; Fig. 4 is a perspective view illustrating an adhesive tape attaching step of attaching an adhesive tape to the upper surface of a SiC substrate and a support member attaching step of attaching a support member to the lower surface of the SiC substrate; Fig. 5A is a perspective view showing a step of holding the SiC substrate by the support member on a turntable, which is connected to the Fig. 1 shown laser processing device; Fig. 5B is a perspective view showing a state in which the SiC substrate is held on the chuck table by the support member under suction; Fig. 6 is a perspective view illustrating a separation start point forming step; Fig. 7 is a plan view of the SiC substrate seen from the adhesive tape side; Fig. 8 is a schematic sectional view illustrating a formation step for a modified layer; Fig. 9 is a schematic plan view illustrating the modified layer forming step; Fig. 10 is a schematic plan view illustrating a dividing step; Fig. 11A is a side view illustrating a Fresnel reflection intensity in the case where a laser beam is directly applied to the SiC substrate; Fig. 11B is a side view illustrating a Fresnel reflection intensity in the case where the laser beam is applied to the adhesive tape attached to the SiC substrate; the Fig. 12A and Fig. 12B are perspective views illustrating a separating step; Fig. 13 is a perspective view showing a state in which the SiC substrate has been separated into a first SiC substrate having the adhesive tape and a second SiC substrate having the support member by performing the separating step; Fig. 14 is a perspective view showing a flattening step of flattening the upper surface of the second SiC substrate containing the Fig. 13 shown support element; and Fig. 15 is a perspective view of the second in Fig. 14 in the state achieved by performing the flattening step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] A preferred embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 1 shows a perspective view of a laser processing apparatus 2 suitable for use in carrying out the SiC substrate separation method of the present invention. The laser processing apparatus 2 includes a stationary base 4 and a first guide block 6 mounted on the stationary base 4 so as to be movable in the X direction. The first guide block 6 is moved in a feed direction, or in the X direction, along a pair of guide rails 14 by a feed mechanism 12 composed of a ball screw 8 and a stepper motor 10.

[0017] A second guide block 16 is mounted on the first guide block 6 so as to be movable in the Y direction. The second guide block 16 is moved in a division direction or in the Y direction along a pair of guide rails 24 by a division mechanism 22 composed of a ball screw 18 and a stepping motor 20. A chuck table 26 having a suction holding portion 26a is mounted on the second guide block 16. The chuck table 26 is movable in the X direction and the Y direction by the feed mechanism 12 and the division mechanism 20, and is also rotatable by a motor housed in the second guide block 16.

[0018] A column 28 is provided on the stationary base 4 so as to protrude upward therefrom. A laser beam application mechanism (laser beam application means) 30 is mounted on the column 28. The laser beam application mechanism 30 is composed of a housing 32, a laser beam generation unit 34 (see Fig. 2) housed in the housing 32, and a focusing means (laser head) 36 mounted at the front end of the housing 32 so as to be movable in the Z direction. An imaging unit 38 with a microscope and a camera is also mounted at the front end of the housing 32 so as to be aligned with the focusing means 36 in the X direction.

[0019] As in Fig. 2, the laser beam generating unit 34 includes a laser oscillator 40, such as an yttrium aluminum garnet (YAG) laser or an yttrium orthovanadate (YVO4) laser, for generating a pulsed laser beam, repetition frequency adjusting means 42 for adjusting the repetition frequency of the pulsed laser beam to be generated by the laser oscillator 40, pulse width adjusting means 44 for adjusting the pulse width of the pulsed laser beam to be generated by the laser oscillator 40, and power adjusting means 46 for adjusting the power of the pulsed laser beam to be generated by the laser oscillator 40. Although not specifically shown, the laser oscillator 40 has a Brewster window so that the laser beam generated by 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 belonging to the focusing means 36 and next focused by a focusing lens 50 belonging to the focusing means 36. The focusing lens 50 is positioned so that the pulsed laser beam is focused onto the interior of a SiC substrate 31, as a workpiece to be described below, which is held on the chuck table 26.

[0020] Referring to Fig. 3A shows a perspective view of a SiC ingot 11, which will be referred to simply as ingot hereinafter. Fig. 3B is a side view of the Fig. 3A. The ingot 11 has a first surface (upper surface) 11a and a second surface (lower surface) 11b opposite the first surface 11a. The first surface 11a of the ingot 11 is first polished to a mirror finish by applying the laser beam to the first surface 11a. The ingot 11 has a first alignment plane 13 and a second alignment plane 15 perpendicular to the first alignment plane 13. The length of the first alignment plane 13 is set longer than the length of the second alignment plane 15.

[0021] The ingot 11 has a c-axis 19 inclined by an angle of deviation α toward the second alignment plane 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 angle of deviation α with respect to the upper surface 11a. Generally, in a hexagonal single-crystal ingot including the SiC ingot 11, the direction perpendicular to the extension direction of the shorter second alignment plane 15 is the inclination direction of the c-axis 19. The c-plane 21 is innumerably defined at the molecular level of the ingot 11 in the ingot 11. In this preferred embodiment, the angle of deviation α is set to 4°. However, the angle of deviation α is not limited to 4° in the present invention. For example, the deviation angle α can be freely set in a range of 1° to 6° during ingot production.

[0022] Again referring to Fig. 1, a column 52 is fixed to the left side of the stationary base 4. The column 52 is formed with a vertical elongated opening 53, and a pressure mechanism 54 is vertically movably mounted on the column 52 so as to protrude from the opening 53.

[0023] Referring to Fig. 4 is shown to illustrate an adhesive tape attachment step of attaching an adhesive tape 41 to the first surface (upper surface) 31a of the SiC substrate 31 and also to illustrate a support member attachment step of attaching a support member 43 to the second surface (lower surface) 31b of the SiC substrate 31. The adhesive tape 41 is transparent. For example, the adhesive tape 41 is composed of a rigid base film made of polyethylene terephthalate (PET) or the like and an adhesive layer formed on the base film. The support member 43 is not necessarily transparent. For example, the support member 43 is formed of a glass substrate or a silicon wafer. The support member 43 is attached to the lower surface 31b of the SiC substrate by using an adhesive. An adhesive tape may be used instead of the support member 43. The SiC substrate 31 is formed by cutting the Fig. 3A using a wire saw. For example, the SiC substrate 31 has a thickness of approximately 700 µm.

[0024] The SiC substrate 31 has a first alignment plane 37 and a second alignment plane 39 perpendicular to the first alignment plane 37. The length of the first alignment plane 37 is set longer than the length of the second alignment plane 39. Since the SiC substrate 31 is formed by cutting the Fig. 3A is obtained with a wire saw, the first alignment plane 37 corresponds to the first alignment plane 13 of the ingot 11 and the second alignment plane 39 corresponds to the second alignment plane 15 of the ingot 11.

[0025] The SiC substrate 31 has a c-axis 19 which is inclined with respect to a perpendicular 17 to the upper surface 31a by a deviation angle α in the direction of the second alignment plane 39, and further has a c-plane 21 which is perpendicular to the c-axis 19 (see Fig. 3B). The c-plane 21 is inclined by the deviation angle α with respect to the upper surface 31a of the SiC substrate 31. In the SiC substrate 31, the direction perpendicular to the extension direction of the shorter second alignment plane 39 is the inclination direction of the c-axis 19.

[0026] After attaching the adhesive tape 41 and the support member 43 to the SiC substrate 31, the SiC substrate 31 is placed on the chuck table 26 in the state in which the support member 43, as shown in Fig. 5A, is directed downwards. Thereafter, a negative pressure is applied to the suction holding section 26a of the chuck table 26 to hold the SiC substrate 31, as shown in Fig. 5B, to be held by suction on the chuck table 26 via the support member 43. In this state, the adhesive tape 41 attached to the SiC substrate 31 held on the chuck table 26 is exposed. Thereafter, the chuck table 26 holding the SiC substrate 31 is rotated so that the second alignment plane 39 of the SiC substrate 31, as shown in the Fig. 6 and Fig. 7, parallel to the X-direction.

[0027] In other words, as in Fig. 7, the formation direction of the deviation angle α is shown by the arrow Y1. That is, the direction of the arrow Y1 is the direction where the intersection 19a exists between the c-axis 19 and the upper surface 31a of the SiC substrate 31 with respect to the perpendicular 17 to the upper surface 31a. Further, the direction perpendicular to the direction of the arrow Y1 is shown by an arrow A. Then, the chuck table 26 holding the SiC substrate 31 is rotated so that the direction of the arrow A becomes parallel to the X direction, that is, the direction of the arrow A parallel to the second alignment plane 39 coincides with the X direction. Accordingly, the laser beam is scanned in the direction of the arrow A perpendicular to the direction of the arrow Y1 or perpendicular to the formation direction of the deviation angle α.In other words, the direction of arrow A perpendicular to the direction of arrow Y1 where the deviation angle α is formed is defined as the feeding direction of the clamping table 26.

[0028] In the SiC substrate separation method of the present invention, it is important that the scanning direction of the laser beam to be applied from the focusing means 36 be set to the direction of arrow A perpendicular to the direction of arrow Y1 where the deviation angle α of the SiC substrate 31 is formed. That is, it has been found that by setting the scanning direction of the laser beam to the direction of arrow A as described above, in the SiC substrate separation method of the present invention, cracks propagating from a layer formed inside the SiC substrate 31 by the laser beam extend very far along the c-plane 21.

[0029] In carrying out the SiC substrate separating method according to this preferred embodiment, a separating start point forming step is carried out in such a manner that the focal point of the laser beam having a transmission wavelength (for example, 1064 nm) for the SiC substrate 31 and the adhesive tape 41 is set to a predetermined depth D1 from the upper surface of the adhesive tape 41 to the inside of the SiC substrate 31 held on the chuck table 26 via the support member 43, and then the laser beam is applied to the adhesive tape 41 with a relative movement of the focal point and the SiC substrate 31, to thereby form a modified layer 45 parallel to the first surface (upper surface) 31a of the SiC substrate 31 and cracks 47 propagating from the modified layer 45 along the c-plane 21, thus forming a separating start point (see Fig. 8).

[0030] 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 arrow A, which is perpendicular to the direction of arrow Y1, where the c-axis 19 is inclined with respect to the perpendicular 17 to the first surface (top surface) 31a of the SiC substrate 31 by the deviation angle α, and the deviation angle α as shown in Fig. 7, is formed between the c-plane 21 and the first surface (upper surface) 31a, whereby the modified layer 45 is formed inside the SiC substrate 31 and the cracks 47 are linearly formed, which propagate from the modified layer 45 along the c-plane 21, and further includes a dividing step of relatively moving the focal point in the direction of formation of the deviation angle α, ie, in the Y direction, to thereby divide the focal point as shown in FIGS. Fig. 8 and Fig. 9 to move a predetermined amount.

[0031] As in the Fig. 8 and Fig. As shown in Fig. 9, the modified layer 45 is formed linearly to extend in the X direction so that the cracks 47 propagate from the modified layer 45 in opposite directions along the c-plane 21. In the SiC substrate dicing method according to this preferred embodiment, the dicing start point formation step further includes a pitch amount setting step of measuring the width of the cracks 47 formed on one side of the modified layer 45 along the c-plane 21 and then setting the pitch amount of the focal point according to the above width measurement. Let W1 indicate the width of the cracks 47 formed on one side of the modified layer 45 so as to propagate from the modified layer 45 along the c-plane 21, more specifically, the pitch amount W2 of the focal point is set to a range from W1 to 2W1.

[0032] The separation start point forming step in the preferred embodiment is carried out, for example, under the following laser processing conditions. Light source: pulsed Nd:YAG laser Wavelength: 1064 nm Repetition frequency: 80 kHz average power: 3.2 W Pulse width: 4 ns Spot diameter: 10 µm numerical aperture (NA) of the focusing lens: 0.45 graduation: 400 µm.

[0033] Under the above-mentioned laser processing conditions, the width W1 of the cracks 47 extending from the modified layer 45 along the c-plane 21 in a Fig. 8, the wavelength propagating in the direction seen is set to approximately 250 µm, and the graduation amount W2 is set to 400 µm. However, the average laser beam power is not limited to 3.2 W. Good results were obtained when the average laser beam power was set to 2 W to 4.5 W.

[0034] In the case where the average power was set to 2 W, the width W1 of the cracks 47 was approximately 100 µm. In the case where the average power was set to 4.5 W, the width W1 of the cracks 47 was approximately 350 µm. In the case where the average power is less than 2 W or more than 4.5 W, the modified layer 45 cannot be well formed inside the SiC substrate 31. Accordingly, the average power of the laser beam to be applied is preferably set in a range of 2 W to 4.5 W. For example, in this preferred embodiment, the average power of the laser beam to be applied to the SiC substrate 31 was set to 3.2 W.

[0035] Referring to Fig. 10 is a schematic plan view illustrating the scanning direction of the laser beam. The separation start point formation step is carried out as shown in Fig. 10, is carried out on a forward path X1 and a reverse path X2. That is, the modified layer 45 is formed on the forward path X1 in the SiC substrate 31. The focal point of the laser beam is then moved further by a predetermined amount. After that, the modified layer 45 is again formed on the reverse path X2 in the SiC substrate 31.

[0036] A Fresnel reflection intensity is now considered for the case that a laser beam LB, as in Fig. 11A, is applied directly to the SiC substrate 31, and in the case that the laser beam LB, as shown in Fig. 11B, is applied to the adhesive tape 41 attached to the SiC substrate 31. Assuming that the laser beam LB enters a body B from a body A, and that the refractive index of the body A is n1, the refractive index of the body B is N2, the intensity of the incident light is I0, and the intensity of the reflected light is I, the Fresnel reflection intensity is given as I = I0{(n2 - n1) / (n2 + n1)} 2 expressed.

[0037] In the Fig. In the case shown in Figure 11A, body A is air and therefore n1 = 1, whereas body B is the SiC substrate 31 and therefore n2 = 2.6. Accordingly, the intensity I1 of the reflected light R1 is I1 = 100 × {(2.6 - 1) / (2.6 + 1)} 2 = 19.8%. In other words, the intensity of the laser beam LB transmitted through the SiC substrate 31 is given as 100 - 19.8 = 80.2%.

[0038] On the other hand, in the Fig. In the case shown in Figure 11B, which corresponds to the embodiment in accordance with the preferred embodiment of the present invention, the transparent adhesive tape 41 is attached to the upper surface 31a of the SiC substrate 31. In this case, body A is air, and therefore n1 = 1, whereas body B is the adhesive tape 41, and therefore n2 = 1.5. Furthermore, body C is the SiC substrate 31, and therefore the refractive index n3 of body C is n3 = 2.6.

[0039] First, the Fresnel reflection intensity I2 of the reflected light R2 at the upper surface of the adhesive tape 41 is given as I2 = I0{(n2 - n1) / (n2 + n1)} 2 = 100 × {(1.5 - 1) / (1.5 + 1)} 2 = 4%. Accordingly, the intensity of the laser beam LB transmitted through the adhesive tape 41 is given as 96%.

[0040] Next, the Fresnel reflection intensity I3 of the reflected light R3 at the upper surface 31a of the SiC substrate 31 is given as I3 = I2 {(n3 - n2) / (n3 + n2)} 2 = 96 × {(2.6 - 1.5) / (2.6 + 1.5)} 2 = 6.9%. Accordingly, the intensity of the laser beam transmitted through the SiC substrate 31 is 89.1% of the intensity of the laser beam LB applied to the adhesive tape 41.

[0041] It is clear from the above considerations that if the transparent adhesive tape 41, as in Fig. 11B, is attached to the upper surface 31a of the SiC substrate 31, the intensity of the laser beam transmitted through the SiC substrate 31 is improved by 89.1 - 80.2 = 8.9% compared with the case where the laser beam LB, as shown in Fig. 11A, is applied directly to the SiC substrate 31. Consequently, when a laser beam is applied from a body M1 having a refractive index n1 through a body M2 having a refractive index n2 to a body M3 having a refractive index n3, where the refractive index n2 is higher than the refractive index n1 and lower than the refractive index n3, the intensity of the laser beam transmitted through the body M3 is improved compared with the case where the laser beam is applied from the body M1 directly to the body M3 whose refractive index is greatly different from that of the body M1.

[0042] In this way, the focal point of the laser beam is gradually moved to form a plurality of modified layers 45 and the cracks 47 extending from each modified layer 45 along the c-plane 21 inside the SiC substrate 31 over its entire area. Thereafter, a separation step is carried out in such a manner that an external force is applied to the SiC substrate 31, thereby planarly separating the SiC substrate 31 into two parts at the separation starting point formed from the modified layers 45 and the cracks 47. This separation step is carried out by using the Fig. 1 shown pressing mechanism 54. The structure of the pressing mechanism 54 is shown in the Fig. 12A and Fig. 12B. The pressing mechanism 54 includes a head 56 which is formed by a not shown, Fig. 1 shown column 52, and a pressing element 58 which is movable in the vertical direction with respect to the head 56 in the direction indicated by an arrow R in Fig. 12B is rotatable.

[0043] As in Fig. 12A, the pressing mechanism 54 is positioned relatively above the SiC substrate 31 held on the clamping table 26. Thereafter, the head 56, as shown in Fig. 12B, until the pressing member 58 comes into pressure contact with the adhesive tape 41 attached to the upper surface 31a of the SiC substrate 31. In the state where the pressing member 58 is in pressure contact with the adhesive tape 41 attached to the upper surface 31a of the SiC substrate 31, the pressing member 58 is rotated in the direction of arrow R to thereby generate a torsional stress in the SiC substrate 31. As a result, the SiC substrate 31 is broken at the separation starting point where the modified layers 45 and the cracks 47 are formed. Accordingly, the SiC substrate 31 can be produced as shown in Fig. 13, into a SiC substrate 31A held on the chuck table 26 and a SiC substrate 31B, wherein the support member 43 is attached to the SiC substrate 31A and the adhesive tape 41 is attached to the SiC substrate 31B.

[0044] While the SiC substrate 31 is planarly separated into two parts by using the pressing mechanism 54 in this preferred embodiment, the SiC substrate 31 can be planarly separated into two parts by pulling the adhesive tape 41 and the support member 43 in opposite directions because the adhesive tape 41 is attached to the first surface (upper surface) 31a of the SiC substrate 31, the support member 43 is attached to the second surface (lower surface) 31b of the SiC substrate 31, and the separation starting point composed of the modified layers 45 and the cracks 47 is formed inside the SiC substrate 31 over the entire area thereof.

[0045] As in Fig. As shown in Fig. 13, the SiC substrate 31A held on the chuck table 26 has a parting surface 49. The parting surface 49 is a slightly rough surface where the modified layers 45 and the cracks 47 partially remain. That is, microscopic unevenness is formed on the parting surface 49. Accordingly, it is preferable to perform a flattening step of grinding the parting surface 49 of the SiC substrate 31A to thereby flatten the parting surface 49. Similarly, the other SiC substrate 31B has a parting surface not shown. Accordingly, the parting surface of the SiC substrate 31B is also preferably ground to be flattened.

[0046] This flattening step is, as in Fig. 14, by using a grinding device including a chuck table 58 and a grinding unit 60. When performing the flattening step, the SiC substrate 31A is held on the chuck table 58 via the support member 43 under suction in the state in which the parting surface 49, as shown in Fig. 14, is exposed upward. The grinding unit 60 includes a spindle 62 configured to be rotationally driven by a motor (not shown), a wheel mount 64 fixed to the lower end of the spindle 62, and a grinding wheel 66 detachably mounted to the lower surface of the wheel mount 64 by a plurality of screws 68. The grinding wheel 66 is composed of an annular wheel base 70 and a plurality of grinding elements 72 fixed to the lower surface of the wheel base 70 so as to be arranged along the outer periphery thereof.

[0047] In the flattening step, the clamping table 58 is rotated, for example, at 300 revolutions per minute, as indicated by an arrow a in Fig. 14. At the same time, the grinding wheel 66 is rotated, for example, at 6000 revolutions per minute, in the direction indicated by an arrow b in Fig. 14. Further, a grinding unit feed mechanism (not shown) is driven to lower the grinding unit 60 until the grinding elements 72 of the grinding wheel 66 come into contact with the parting surface 49 of the SiC substrate 31A held on the chuck table 58 via the support member 43. Then, the grinding wheel 66 is fed downward by a predetermined amount at a predetermined feed speed (for example, 0.1 μm / s), thereby grinding the parting surface 49 of the SiC substrate 31A to flatten the parting surface 49. As a result, the modified layers 45 and the cracks 47 remaining on the parting surface 49 of the SiC substrate 31A can be removed, as shown in Fig. 15 to obtain a flat surface.

[0048] In the case of flattening the parting surface 49 of the SiC substrate 31A obtained by the above-mentioned parting step, it is only necessary to lightly grind the parting surface 49 of the SiC substrate 31A by an amount of approximately 1 µm to 5 µm, so that the amount of wear of the grinding elements 72 can be reduced to approximately 4 µm to 25 µm. Furthermore, the parting surface of the other Fig.13 is flattened in a similar manner by using the above-mentioned grinding device. Specifically, the SiC substrate 31B is held under suction via the adhesive tape 41 on the chuck table 58 in the state where the parting surface of the SiC substrate 31B is exposed upward. In this state, the parting surface of the SiC substrate 31B is ground by the grinding wheel 66 to thereby remove the modified layers 45 and the cracks 47 remaining on the parting surface of the SiC substrate 31B. Consequently, the parting surface of the SiC substrate 31B can be flattened.

[0049] In the above preferred embodiment, the separation starting point composed of the modified layers 45 and the cracks 47 is formed in the SiC substrate 31 along the c-plane 21. However, the SiC substrate separation method of the present invention is also applicable to the case where the separation starting point composed of the modified layers 45 and the cracks 47 is not formed along the c-plane 21 as in Japanese Patent Laid-Open No. 2013-49161, since the SiC substrate 31 is reinforced by the adhesive tape 41 and the support member 43 on opposite sides.

[0050] The present invention is not limited to the details of the preferred embodiment described above. The scope of the invention is defined by the appended claims, and all changes and modifications that fall within the equivalent scope of the claims are thus included by the invention.

Claims

[1] A SiC substrate separation method for planarly separating a SiC substrate (31) into at least two parts, wherein the first SiC substrate (31B) has a first surface (31a), a second surface (31b) opposite the first surface, a c-axis extending from the first surface (31a) to the second surface (31b), and a c-plane perpendicular to the c-axis, the SiC substrate separation method comprising: an adhesive tape applying step comprising applying a transparent adhesive tape (41) to the first surface (31a) of the SiC substrate (31); a support member attaching step comprising attaching a support member (43) to the second surface (31b) of the SiC substrate (31); after performing the adhesive tape application step and the support member application step, a separation start point formation step comprising setting the focal point of a laser beam having a transmission wavelength for the SiC substrate (31) and the adhesive tape (41) to a predetermined depth inside the SiC substrate (31) from the adhesive tape (41), and next applying the laser beam to the adhesive tape (41) while relatively moving the focal point and the SiC substrate (31), thereby forming a modified layer (45) parallel to the first surface (31a) of the SiC substrate (31) and cracks (47) propagating from the modified layer (45) to form a separation start point; and a separating step of applying an external force to the SiC substrate (31) after performing the separation start point forming step, to thereby separate the SiC substrate (31) in the state at the separation start point into a first SiC substrate (31B) having the first surface (31a) and a second SiC substrate (31A) having the second surface (31b) in which the adhesive tape (41) is attached to the first SiC substrate (31B) and the support member (43) is attached to the second SiC substrate (31A); where the separation start point formation step includes: a modified layer (45) forming step comprising relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by a deviation angle with respect to a normal to the first surface of the SiC substrate (31), and the deviation angle is formed between the c-plane and the first surface (31a), thereby linearly forming the modified layer (45) extending in the first direction inside the SiC substrate (31), and a dividing step comprising relatively moving the focal point in the second direction to thereby further move the focal point by a predetermined amount in the second direction. [2] The SiC substrate separating method according to claim 1, wherein the refractive index of the adhesive tape (41) is higher than the refractive index of air and lower than the refractive index of the SiC substrate (31). [3] The SiC substrate separation method according to claim 1 or 2, further comprising: a flattening step by using a grinding member (72) comprising grinding a parting surface of the first SiC substrate (31B) separated from the second SiC substrate (31A) at the parting start point and further grinding a parting surface (49) of the second SiC substrate (31A) separated from the first SiC substrate (31B) at the parting start point, thereby flattening the parting surface of the first SiC substrate (31B) and the parting surface of the second SiC substrate (31A).

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