WAFER MANUFACTURING PROCESS

Abstract

A wafer manufacturing method for producing a hexagonal single-crystal wafer (27) from a hexagonal single-crystal ingot (11) having a first surface (11a), a second surface (11b) opposite the first surface (11a), a c-axis extending from the first surface (11a) to the second surface (11b), and a c-plane perpendicular to the c-axis, the wafer manufacturing method comprising: a separation start point formation step comprising setting the focal point of a laser beam having a transmission wavelength for the ingot (11) to a predetermined depth inside the ingot (11) from the first surface (11a), the depth corresponding to the thickness of the wafer (27) to be produced, and next applying the laser beam to the first surface (11a) while relatively moving the focal point and the ingot (11), thereby forming a modified layer (23) parallel to the first surface (11a) and cracks (25) extending from the modified layer (23) along the c-plane, thus forming a separation start point; and a wafer separating step of separating a plate-shaped member at the separation start point from the ingot (11) with a thickness corresponding to the thickness of the wafer (27) after performing the separation start point forming step, thereby manufacturing the wafer (27) from the ingot (11); wherein the separation start point formation step comprises: a forming step of a modified layer (23) 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 (α) with respect to a normal to the first surface (11a) and the deviation angle (α) is formed between the first surface (11a) and the c-plane, whereby the modified layer (23) extending in the first direction is formed linearly, and a dividing step comprising a relative movement of the focal point in the second direction to thereby move the focal point by a predetermined amount, wherein in the modified layer (23) forming step, the laser beam comprises a plurality of laser beams to be applied simultaneously to form a plurality of linear modified layers (23), the focal points of the laser beams being arranged at a predetermined pitch in the second direction.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a wafer manufacturing method for cutting a hexagonal single crystal ingot to produce a wafer. Description of the related prior art

[0002] Various devices, such as integrated circuits and 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 divide the wafer into a plurality of individual device chips corresponding to the devices. The resulting device chips are widely used in a variety of electronic devices, such as mobile phones and personal computers.Furthermore, power devices or optical devices such as LEDs and 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 division lines.

[0003] Generally, the wafer on which the components are to be formed is manufactured by cutting an ingot with a wire saw. Both sides of the wafer thus obtained are polished to a mirror finish, or high gloss, see, for example, Japanese Unexamined Patent Application No. 2000-94221A. This wire saw is configured such that a single wire, like a piano wire, with a diameter of approximately 100 to 300 µm is wound around many grooves, usually formed on two to four guide rollers, to form a plurality of cutting sections spaced parallel to each other at a predetermined pitch. The wire is actuated to run in one direction or opposite directions, thereby cutting the ingot into a plurality of wafers.

[0004] However, when the ingot is cut by a wire saw and both sides of each wafer are polished to obtain the product, 70 to 80% of the ingot becomes scrap, which raises the problem of poor economics. In particular, a hexagonal single-crystal ingot made of SiC or GaN, for example, has a high Mohs hardness, making it difficult to cut this ingot with a wire saw. Accordingly, cutting the ingot requires considerable time, resulting in a decrease in productivity. This means that efficient wafer production using the current technology is problematic.

[0005] A technique for solving this problem 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 inside of a hexagonal single-crystal ingot, next, applying the laser beam to the ingot while scanning the laser beam on the ingot to thereby form a modified layer and cracks in a parting plane inside the ingot, and next, applying an external force to the ingot to thereby break the ingot along the parting plane where the modified layer and the cracks are formed, thus separating a wafer from the 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 cracks are formed with a very high density in the parting plane of the ingot.

[0006] Furthermore, the published patent application JP 2016 - 111 145 A discloses a wafer manufacturing method for manufacturing a wafer from a hexagonal single crystal ingot, comprising a separation starting point forming step and a wafer peeling step. SUMMARY OF THE INVENTION

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

[0008] Thus, the pitch of the laser beam applied to the ingot is very small. Regardless of whether the laser beam is scanned spirally or linearly, the laser beam must be applied with a very small pitch, and the productivity improvement is still insufficient.

[0009] It is therefore an object of the present invention to provide a wafer manufacturing method that can efficiently produce a wafer from an ingot.

[0010] In accordance with one aspect of the present invention, there is provided a wafer manufacturing method for manufacturing 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, the wafer manufacturing method including: a separation start point forming step of setting a focal point of a laser beam having a transmission wavelength for the ingot to a predetermined depth from the first surface within the ingot, the depth corresponding to a thickness of the wafer to be manufactured, and next, applying the laser beam to the first surface while relatively moving the focal point and the ingot, thereby forming a modified layer parallel to the first surface and cracks,extending from the modified layer along the c-plane, thereby forming a separation start point; and a wafer separation step of separating a plate-shaped 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 formation step, thereby producing the wafer from the ingot, wherein the separation start point formation step comprises a modified layer formation 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 and the deviation angle is formed between the first surface and the c-plane,whereby the modified layer extending in the first direction is formed linearly; and a dividing step for relatively moving the focal point in the second direction to thereby advance the focal point by a predetermined amount; wherein in the modified layer forming step, the laser beam comprises a plurality of laser beams to be simultaneously applied to form a plurality of modified layers, and the focal points of the laser beams are arranged in the second direction at a predetermined pitch.

[0011] Preferably, in the modified layer forming step, the predetermined distance between any two adjacent ones of the focal points is set so that the upper limit of the predetermined distance becomes almost equal to a distance defined when the leading ends of the cracks of the adjacent modified layers overlap each other in the second direction.

[0012] Preferably, in the dividing step, the dividing amount is given as L = H × M, where H is the predetermined distance and M is the number of focal points.

[0013] According to the wafer manufacturing method of the present invention, the focal point of the laser beam is relatively moved in the first direction perpendicular to the second direction, forming the deviation angle between the first surface and the c-plane of the ingot, thereby linearly forming the modified layer extending in the first direction. Thereafter, the focal point of the laser beam is further moved by the predetermined amount in the second direction. After that, the focal point of the laser beam is again relatively moved in the first direction, thereby linearly forming the modified layer extending in the first direction.Such a sequence of steps is repeated to form a plurality of modified layers extending in the first direction, each modified layer being formed to a predetermined depth from the first surface of the ingot, and the cracks being formed on both sides of each modified layer to propagate along the c-plane. Accordingly, any adjacent ones of the modified layers are connected to each other by the cracks formed therebetween, so that the plate-shaped member having the thickness corresponding to the thickness of the wafer can be easily separated from the ingot at the separation starting point, thereby producing the hexagonal single-crystal wafer from the ingot.

[0014] Furthermore, a plurality of laser beams are simultaneously applied to form a plurality of linearly modified layers parallel to each other in the state where the cracks formed between the adjacent modified layers are interconnected. As a result, the separation starting point can be efficiently formed, thereby sufficiently improving productivity. Furthermore, the amount of ingot that becomes scrap can be sufficiently reduced by approximately 30%.

[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 the wafer manufacturing method of the present invention; Fig. 2 is a block diagram of a laser beam generating unit; Fig. 3 is a schematic view of a radiating diffractive optical element; Fig. 4A is a perspective view of a hexagonal single crystal ingot; Fig. 4B is a view of the Fig. Ingots shown in Figure 4A; Fig. 5 is a perspective view illustrating a separation start point forming step; Fig. 6 is a plan view of the Fig. Ingots shown in Figure 4A; Fig. 7 is a schematic sectional view illustrating a formation step of a modified layer; Fig. Fig. 8 is a schematic plan view illustrating the formation step of a modified layer; Fig. 9 is a schematic plan view illustrating a dividing step; The Fig. 10A and Fig. 10B are perspective views illustrating a wafer separation step; and Fig. 11 is a perspective view of a hexagonal single crystal wafer produced from the ingot. 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 wafer manufacturing 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 feed direction, or X direction, along a pair of guide rails 14 by a feed mechanism 12 comprising a ball screw 8 and a stepper motor 10.

[0017] 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 a dividing direction or in the Y direction along a pair of guide rails 24 by a dividing mechanism 22 comprising a ball screw 18 and a stepping 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 feed mechanism 12 and the dividing mechanism 22, and is also rotatable by a motor housed in the second slide 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 includes a housing 32, a laser beam generating unit 34 (see Fig. 2) and a focusing means (laser head) 36 mounted at the front end of the housing 32. An imaging unit 38, which includes a microscope and a camera, is also mounted at the front end of the housing 32 so that it is aligned with the focusing means 36 in the X direction.

[0019] As in Fig. As shown in Fig. 2, the laser beam generating unit 34 includes a laser oscillator 40, such as a YAG laser and a 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 included in the focusing means 36 and then branched into three laser beams by a diffractive optical element (DOE) 50 included in the focusing means 36. These three laser beams obtained by the DOE are then focused by a focusing lens 51 included in the focusing means 36. The focusing lens 51 is positioned to focus the pulsed laser beam within a hexagonal single-crystal ingot 11 as a workpiece fixed to the support table 26.

[0020] For example, the DOE 50, as in Fig. 3, is provided as a blaze DOE. The blaze DOE includes a transparent plate 62 and a sawtooth section structure 64 finely milled on one side (the one shown in Fig. 3) of the transparent plate 62. The size d1 of each sawtooth in the structure 64 is set to several decades to several hundreds of micrometers. The laser beam incident on the blazed DOE 50 is branched into zero-order light, first-order light, and second-order light. The zero-order light, first-order light, and second-order light next emerge from the blazed DOE as three separate laser beams.

[0021] Referring to Fig. 4A shows a perspective view of the hexagonal single crystal ingot 11 as a workpiece to be machined. Fig. 4B is a view of the Fig. 4A. The hexagonal single-crystal ingot 11, hereinafter referred to simply as an ingot, is selected from a SiC single-crystal ingot and a GaN single-crystal ingot. 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 previously polished to a mirror finish because the laser beam is applied to the first surface 11A.

[0022] 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 longer than the length of the second alignment plane 15. The ingot 11 has a c-axis 19 inclined by an angle of deviation α with respect to a normal 17 to the upper surface 11a in the direction of the second alignment plane 15, 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 the hexagonal single-crystal ingot 11, the direction perpendicular to the extension direction of the shorter second alignment plane 15 is the inclination direction of the c-axis. The c-plane 21 is innumerably defined on the molecular plane of the ingot 11. In this preferred embodiment, the deviation angle α is set to 4°.However, the deviation angle α is not limited to the 4° of the present invention. For example, the deviation angle α can be freely adjusted within a range of 1° to 6° during the production of the ingot 11.

[0023] 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 vertically extending elongated opening 53, and a pressing mechanism 54 is vertically movably mounted on the column 52 so as to protrude from the opening 53.

[0024] As in Fig. 5, the ingot 11 is fixed to the upper surface of the support table 26 using a wax or an adhesive in the state where the second alignment plane 15 of the ingot 11 is parallel to the X direction. In other words, the formation direction of the deviation angle α, as shown in Fig. 6, is indicated by an arrow Y1. That is, the direction of the arrow Y1 is the direction in which the intersection 19a exists between the c-axis 19 and the upper surface 11a of the ingot 11 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 state where the direction of the arrow A becomes parallel to the X direction.

[0025] Accordingly, the laser beam is scanned in the direction of arrow A perpendicular to the direction of arrow Y1 or the direction of formation 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 support table 26.

[0026] In the wafer manufacturing 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 ingot 11 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 mentioned above in the wafer manufacturing 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.

[0027] In carrying out the wafer manufacturing method according to this preferred embodiment, a division start point forming step is carried out in such a manner that the focal point of the laser beam having a transmission wavelength such as 1064 nm for the hexagonal single-crystal ingot 11 fixed to the support table 26 is set to a predetermined depth inside the ingot 11 from the first surface 11a (upper surface) corresponding to the thickness of a wafer to be manufactured, and next, the laser beam is applied to the upper surface 11a with relative movement of the focal point and the ingot 11, to thereby form a modified layer 23 parallel to the upper surface 11a and cracks 25 extending from the modified layer 23 along the c-plane 21, thus forming a division start point.

[0028] 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 normal 17 to the upper surface 11a by the deviation angle α and the deviation angle α 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 extending from the modified layer 23 along the c-plane 21, and further includes a dividing step of relatively moving the focal point in the formation direction of the deviation angle α, that is, in the Y-direction, to thereby form the focal point as shown in FIGS. Fig. 8 and Fig. 9 to advance a predetermined amount.

[0029] As in the Fig. 7 and Fig. 8, the modified layer 23 is formed linearly to extend in the X-direction so that the cracks 25 extend from the modified layer 23 in opposite directions along the c-plane 21. In the wafer manufacturing method according to this preferred embodiment, the division start point formation step further includes a pitch amount adjustment 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 adjusting the pitch amount of the focal point according to the above-mentioned width.Letting W1 denote the width of the cracks 25 formed on one side of the modified layer 23 so as to extend from the modified layer 23 along the c-plane 21, the predetermined distance H between any two adjacent ones of the two or more focal points is set so that the upper limit of the distance is almost equal to 2W1, which is defined as the front ends of the cracks 25 extending from adjacent modified layers 23 in the Y direction overlapping each other.

[0030] The pitch amount L set in the case where a plurality of laser beams are applied simultaneously is given as L = H × M, where M is the number of focal points. In this preferred embodiment, the number of focal points is three, so L = 3H. When H = 400 µm, L = 1200 µm.

[0031] For example, the separation start point formation step is performed with 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: 3 µm numerical aperture (NA) of the focusing lens: 0.43 Graduation amount: (250 to 400 µm) × (number of focal points) Feed speed: 120 to 260 nm / s

[0032] Under the above-mentioned laser processing conditions, the width W1 of the cracks 25 extending from the modified layer 23 along the c-plane 21 on a plane in Fig. 7, the width W1 of the cracks 25 is set to approximately 250 µm, and the pitch amount L is set to 1200 µm. 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 W to 4.5 W, good results were achieved in the preferred embodiment. When the average power was set to 2 W, the width W1 of the cracks 25 was approximately 100 µm. When the average power was set to 4.5 W, the width W1 of the cracks 25 was approximately 350 µm.

[0033] In the case where the average power is less than 2 W or greater than 4.5 W, the modified layer 23 cannot be well formed within the ingot 11. 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 ingot 11 was set to 3.2 W. As shown in Fig. 7, the depth D1 of each focal point for forming the modified layer 23 from the upper surface 11a was set to 500µm.

[0034] Referring to Fig. 9 is a schematic plan view illustrating the scanning direction of the laser beams. The separation start point formation step is carried out as shown in Fig. 9, the laser beam etching is performed on a forward path X1 and a reverse path X2. That is, the modified layers 23 are formed in the hexagonal single-crystal ingot 11 on the forward path X1. Thereafter, the focal points of the laser beams are moved by the predetermined amount. After that, the modified layers 23 are again formed in the ingot 11 on the reverse path X2.

[0035] In this way, the focal points of the laser beams are sequentially moved to form a plurality of modified layers 23 at the depth D1 in the entire area of the ingot 11 and the cracks 25 extending from each modified layer 23 along the c-plane 21. Thereafter, a wafer separation step is carried out in such a manner that an external force is applied to the ingot 11 to thereby separate, at the separation start point including the modified layers 23 and the cracks 25, a plate-shaped member having a thickness corresponding to the thickness of the wafer to be produced from the ingot 11, thus forming a Fig. 11 to produce the hexagonal single crystal wafer 27.

[0036] This wafer separation step is performed using the Fig. 1. The structure of the printing mechanism 54 is shown in the Fig. 10A and Fig. 10B. The printing mechanism 54 includes a head 56 which is moved by a moving mechanism not shown, which is shown in Fig. 1, is included, is movable in the vertical direction, and a pressure element 58 which is movable with respect to the head 56 in the direction indicated by an arrow R in Fig. 10B shown direction. As shown in Fig. 10A, the pressure mechanism 54 is positioned relatively above the ingot 11, which is fixed to the support table 26. Thereafter, the head 56, as shown in Fig. 10B, until the pressure element 58 comes into pressure contact with the upper surface 11A of the ingot 11.

[0037] In the state where the pressure member 58 is in pressure contact with the upper surface 11a of the ingot 11, the pressure member is rotated in the direction of arrow R to thereby generate a torsional stress in the ingot 11. As a result, the ingot 11 is broken at the separation starting point where the modified layers 23 and the cracks 25 are formed. Accordingly, the Fig. 11 is 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.

Claims

[1] A wafer manufacturing method for manufacturing a hexagonal single-crystal wafer (27) from a hexagonal single-crystal ingot (11) having a first surface (11a), a second surface (11b) opposite the first surface (11a), a c-axis extending from the first surface (11a) to the second surface (11b), and a c-plane perpendicular to the c-axis, the wafer manufacturing method comprising: a separation start point formation step comprising setting the focal point of a laser beam having a transmission wavelength for the ingot (11) to a predetermined depth inside the ingot (11) from the first surface (11a), the depth corresponding to the thickness of the wafer (27) to be produced, and next applying the laser beam to the first surface (11a) while relatively moving the focal point and the ingot (11), thereby forming a modified layer (23) parallel to the first surface (11a) and cracks (25) extending from the modified layer (23) along the c-plane, thus forming a separation start point; and a wafer separating step of separating a plate-shaped member at the separation start point from the ingot (11) with a thickness corresponding to the thickness of the wafer (27) after performing the separation start point forming step, thereby manufacturing the wafer (27) from the ingot (11); wherein the separation start point formation step comprises: a forming step of a modified layer (23) 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 (α) with respect to a normal to the first surface (11a) and the deviation angle (α) is formed between the first surface (11a) and the c-plane, whereby the modified layer (23) extending in the first direction is formed linearly, and a dividing step comprising a relative movement of the focal point in the second direction to thereby move the focal point by a predetermined amount, wherein in the modified layer (23) forming step, the laser beam comprises a plurality of laser beams to be applied simultaneously to form a plurality of linear modified layers (23), the focal points of the laser beams being arranged at a predetermined pitch in the second direction. [2] The wafer manufacturing method according to claim 1, wherein in the modified layer (23) forming step, the predetermined distance between any two adjacent ones of the focal points is set so that the upper limit of the predetermined distance becomes almost equal to a distance defined when the leading ends of the cracks (25) extending from the adjacent modified layers (23) in the second direction overlap each other. [3] The wafer manufacturing method according to claim 1 or 2, wherein in the dividing step, the dividing amount L is given as L = H × M, where H is the predetermined pitch and M is the number of focal points. [4] A wafer manufacturing method according to any preceding claim, wherein the hexagonal single crystal ingot (11) is selected from a SiC single crystal ingot and a GaN single crystal ingot.

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