Wafer laser slicing method

Abstract

[Problem] The purpose of the present invention is to provide a laser slicing method that makes a cutting cross-section on a wafer smooth, reduces a cutting load, and is effective for the improvement of a material yield. [Solution] The laser slicing method is used for cutting and manufacturing a wafer by performing movement scanning with laser light relative to a base material while forming a light-focused point at a prescribed depth from the base material surface having a prescribed off-angle between the central axis and the crystal orientation (C-axis) of the base material, thereby forming a modified layer and a crack, the method being characterized in that, in the scanning direction in which the movement scanning is performed, dot irradiation is performed linearly in the inclination direction of a crystal plane orthogonal to the C-axis.

Description

Wafer laser slicing method

[0001] The present invention relates to a slicing method using a laser beam to cut a wafer from a substrate.

[0002] Single crystal Si (silicon), SiC (silicon carbide), GaN (potassium nitride), Al ２ O ３ Semiconductor wafers such as Sapphire are used in various devices such as ICs and LSIs. The substrates (ingots) used to manufacture these single crystal wafers are produced by epitaxial growth methods, but many of them have a predetermined off-angle of 2 to 8 degrees in the crystal orientation from the viewpoints of growth stability, reduction of internal defects, etc.

[0003] Wafer manufacturing methods have been proposed using laser slicing, in which a laser beam is irradiated and scanned while the focal point is aligned at a predetermined depth from the surface of the substrate, generating a modified layer and associated cracks, thereby forming a fractured surface. For example, Patent Document 1 describes a method of forming a modified layer within a substrate by focusing laser light within the substrate and moving the laser focusing means relative to the substrate, but does not describe a case in which the crystal orientation has an off-angle. Patent Document 2 describes a laser slicing method in which the focal point of the laser beam is moved in a direction perpendicular to the direction in which the off-angle is formed. However, the same publication explains that scanning in a direction perpendicular to the direction in which the off-angle is formed generates cracks along the off-angle plane. However, research by the present inventors revealed that significant variations in the direction and size of crack generation resulted in scale-like fractured surfaces. This resulted in an inability to obtain smooth fractured surfaces, resulting in significant material loss. Furthermore, the load required for cleaving was large, making the wafer susceptible to breakage.

[0004] Japanese Patent No. 5509448 Japanese Patent No. 6399913

[0005] An object of the present invention is to provide a laser slicing method that produces smooth cut surfaces of wafers, reduces the cutting load, and is also effective in improving material yield.

[0006] The laser slicing method of the present invention is a laser slicing method for manufacturing a wafer by moving and scanning the laser light relative to the substrate while forming a focal point at a predetermined depth from the surface of the substrate, which has a predetermined off-angle between the central axis of the substrate and the crystal orientation (C-axis), thereby forming a modified layer and cracks, and cutting the wafer, characterized in that the scanning direction for the moving and scanning is to irradiate dots in a straight line along the inclination direction of the crystal plane perpendicular to the C-axis.

[0007] Many substrates (ingots) used to manufacture wafers are single crystals with an off-angle in their crystal orientation. The

[100] ,

[0001] , and other crystal orientations have a predetermined off-angle relative to the central axis connecting the centers of the top and bottom surfaces of the substrate. Therefore, the crystal planes have a step-and-terrace structure with a predetermined off-angle. The inventors discovered that smooth fractured surfaces along the step-and-terrace surfaces can be obtained by irradiating the surface with dots of laser light while scanning it along the slope forming the off-angle.

[0008] In the present invention, dot irradiation refers to irradiation in a dot pattern achieved by combining the pulse period of laser irradiation and the relative movement speed of the substrate. Irradiation may be continuous at an even pitch, but the dot irradiation pitch may also be intermittent dot irradiation, with irradiated and non-irradiated areas. When laser light having a focal point is irradiated into the interior of the substrate, a modified layer is formed by the irradiation, cracks occur, and these cracks propagate. Therefore, the pitch of dot irradiation may be set or dot irradiation may be intermittent, taking into account the length of crack propagation. In addition, in the present invention, the direction of linear dot irradiation along the tilt direction forming the off-angle is referred to as the irradiation line. The irradiation line is moved in the same direction or back and forth in parallel at a predetermined interval to form a modified layer and cracks, and the interval between these irradiation lines is referred to as the scanning line pitch.

[0009] In the present invention, a laser slicing method for cleaving a wafer involves moving the laser light relative to the substrate while forming a focal point at a predetermined depth from the substrate surface, which has a predetermined off-angle between the substrate's central axis and crystal orientation (C-axis), to form a modified layer and cracks. The scanning direction of the moving scan may be linear dot irradiation so as to form a predetermined scan tilt angle with respect to the inclination direction of the crystal plane perpendicular to the C-axis. The dot irradiation lines are formed parallel to the same direction at a predetermined scan line pitch, or alternately parallel in the reciprocating direction, resulting in a fractured surface with countless cracks. In this case, if the scan is performed obliquely at a predetermined scan tilt angle of 4 to 20° with respect to the inclination direction, cracks generated on adjacent scan lines are more likely to connect, reducing the cleaving load.

[0010] In the present invention, a substrate (ingot) having an off-angle is subjected to parallel scanning or reciprocal scanning at a predetermined scanning line pitch, in which dots are irradiated linearly along the tilt direction of the off-angle of the crystal plane, or linearly so as to form a predetermined scanning tilt angle with respect to this tilt direction, thereby smoothing the fracture surface and reducing the fracture load compared to conventional methods.

[0011] (a) An example in which the irradiation line of the dot irradiation according to the present invention was scanned in the tilt direction [11-20] that formed the off-angle of the substrate, (b) shows a schematic diagram of the dot irradiation and a confocal laser microscope image of the surface after cleavage. In the present invention, a schematic diagram of an example in which the dot irradiation was performed intermittently and a confocal laser microscope image of the surface after cleavage are shown. In the present invention, (a) shows the side in which the irradiation line of the dot irradiation is scanned so as to form a predetermined scanning tilt angle clockwise with respect to the [11-20] direction in FIG. 3, and (b) shows the schematic diagram of the dot and a confocal laser microscope image of the surface after cleavage. For comparison, a schematic diagram of the dots and a confocal laser microscope image of the surface after cleavage are shown when the irradiation line of the dot irradiation is scanned in the [10-10] direction perpendicular to the [11-20] direction that forms the off-angle of the substrate. Evaluation results are shown. The graph shows the relationship between the tested laser scanning tilt angle and the peel force during cleavage. The relationship between the tested laser scanning tilt angle and the maximum height and arithmetic mean roughness of the cleaved surface is shown.

[0012] The surface roughness and cutting load of the dot irradiation method and the cutting force after laser slicing were measured and compared, and are described below. Figure 1(a) shows an example of a hexagonal single crystal substrate (ingot). The substrate typically has an orientation flat in the [10-10] direction and an orientation flat in the [10-20] direction perpendicular to the [10-10] direction. The

[0001] crystal orientation forms an off-angle of 2 to 8 degrees with respect to the central axis of the substrate (perpendicular to the substrate surface). If this crystal orientation is the C-axis, then an inclined plane forming an off-angle is formed on the crystal plane perpendicular to this C-axis. A Si substrate with an off-angle of 4 degrees was used for this test and evaluation.

[0013] The off-angle formation surface perpendicular to the C-axis of the Si substrate is formed in a step-terrace structure with a pitch of 150 to 200 μm. The laser light has a wavelength of 532 nm and a pulse width of 8 to 30 ns (pulse oscillation frequency 20 kHz). ｚ The numerical aperture NA was set to 0.2 or more (NA: beam diameter / focal length x 2), pulse energy: 3 to 20 μJ, irradiation dot pitch: 1 to 5 μm, scanning line pitch: 20 to 70 μm.

[0014] The evaluation results are shown in the table in Figure 5. The peel force was measured using a universal testing machine conforming to JIS, where a tensile load was applied to both sides of a small sample piece of a certain size, and the maximum load until fracture was measured. Note that the load was applied at a position offset by 90 mm from the center of the test piece to facilitate peeling. The surface roughness of the fractured surface was measured using a confocal laser microscope, measuring the maximum height (R Ｚ) was measured. In Examples 1 to 6, dot irradiation was performed in the tilt direction [11-20] that formed the off angle, while in Comparative Example 1, dot irradiation was performed in the direction [10-10] perpendicular to the tilt direction that formed the off angle. A schematic diagram of the dot irradiation and a confocal laser microscope image of the surface after cleavage are shown in Figure 4. In Examples 1 to 6, where the dot irradiation target line was set to the tilt direction [11-20] that formed the off angle, the surface roughness of the cleaved surface was lower than in the Comparative Example, where scanning was performed in the perpendicular direction [10-10]. The polishing allowance of the surface after cleavage was 100 μm or less in Examples 1 to 6, while 150 μm or more was required in the Comparative Example. Furthermore, comparing the contents of Examples 1 to 6, the peel force was reduced when the irradiation dot pitch was 1 to 3 μm. Example 4 shows that even when the irradiation dot pitch was relatively large, such as approximately 5 μm and the scanning line pitch was relatively large, such as 50 μm, increasing the pulse energy reduced the peel force. From these findings, it is preferable to set the pulse energy to 2 to 6 μJ, the irradiation dot pitch to 0.6 to 3 μm, and the scanning line pitch to 10 to 30 μm. The confocal laser microscope image of the surface after cleavage shown in FIG. 1(b) is that of Example 1.

[0015] Figure 2 shows an example of intermittent dot irradiation. Since cracks are generated starting from the focal point of the laser light and the length of these cracks propagates, intermittent dot irradiation may be performed taking into account the length of crack propagation. Intermittent irradiation may be performed by directly controlling the intermittency of a pulse oscillator, or by controlling light blocking using a chopper or shutter.

[0016] Figure 3 shows an example of pulsed laser dot irradiation to form a scan tilt angle clockwise relative to the tilt direction [11-20] that forms the off-angle of the irradiation line. In this example, evaluation was performed using a wavelength of 1030 nm, a pulse width of 1-20 picoseconds, an irradiation dot pitch of 5-15 μm, a scan line pitch of 40-80 μm, and pulse energy of 5-15 μJ. Figure 6 shows the results of an investigation into the change in peel force per 10 mm square when the laser scan tilt angle (clockwise) of the irradiation line was changed, and Figure 7 shows the change in the maximum fracture surface height Rz (●) and arithmetic mean roughness (○) with respect to the change in laser scan tilt angle. The results in Figure 6 indicate that the peel load is minimized when the laser scan tilt angle is around 4-8°. This is presumably because the tilt of the irradiation line aligns the crack directions, resulting in continuous cracks between adjacent irradiation lines. Fig. 3(b) shows a confocal laser microscope image of the surface after cutting when the tilt angle was 4°. In contrast, Fig. 7 reveals that there is no significant difference in the roughness of the cut surface depending on the scanning tilt angle.

[0017] The present invention can be widely used in dividing wafers using laser light.

Claims

1. A laser slicing method for manufacturing a wafer by cutting the wafer, in which a laser beam and the substrate are moved and scanned relative to each other while forming a focal point at a predetermined depth from the substrate surface having a predetermined off angle between the substrate's central axis and crystal orientation (C-axis), forming a modified layer and cracks, wherein the scanning direction of the moving and scanning is linear dot irradiation along the inclination direction of the crystal plane perpendicular to the C-axis.

2. A method for laser slicing a wafer according to claim 1, characterized in that the pitch of the dot irradiation is intermittent dot irradiation having irradiated areas and non-irradiated areas.

3. A laser slicing method for manufacturing a wafer by cutting the wafer, by scanning the laser light relative to the substrate while forming a focal point at a predetermined depth from the substrate surface having a predetermined off-angle between the central axis of the substrate and the crystal orientation (C-axis), forming a modified layer and cracks, characterized in that the scanning direction of the scanning movement is linear dot irradiation so as to form a predetermined scanning tilt angle with respect to the tilt direction of the crystal plane perpendicular to the C-axis.

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