Laser slicing method for wafers

Abstract

To provide a laser slicing method that is effective in improving the material yield, with a smooth split cross-section of a wafer and a low split load.SOLUTION: The laser slicing method is for moving and scanning a laser beam relative to a substrate while forming a focal point at a predetermined depth from the substrate surface with a predetermined off-angle between the central axis of the substrate and the crystallographic orientation (C-axis) of the substrate, forming a modified layer and cracks, and manufacturing by splitting a wafer. The scanning direction of the moving and scanning is a straight dot irradiation along an inclined direction of a crystal plane orthogonal to the C-axis.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Single crystal semiconductor wafers such as Si (silicon), SiC (silicon carbide), GaN (gallium nitride), and Al2O3 (sapphire) are used in various devices such as ICs and LSIs. Ga Substrates (ingots) for manufacturing these single crystal wafers are manufactured by an epitaxial growth method or the like, and many of them have a predetermined off-angle of 2 to 8° in the crystal orientation from the viewpoints of growth stability and reduction of internal defects. By irradiating and scanning a laser beam while aligning the condensing point at a predetermined depth from the surface of these substrates, a method for manufacturing a wafer by a laser slicing method that generates a modified layer and accompanying cracks to form a cut surface has been proposed.

[0003] For example, Patent Document 1 describes that a modified layer is formed inside a substrate by condensing a laser beam inside the substrate and relatively moving the laser condensing means and the substrate, but there is no description about the case having an off-angle in the crystal orientation. Patent Document 2 describes a laser slicing method for moving the condensing point of a laser beam in a direction orthogonal to the direction in which an off-angle is formed. However, when scanning in a direction orthogonal to the direction in which the off-angle is formed, the publication explains that cracks occur along the formation plane of the off-angle. However, according to the investigation by the present inventors, it has been found that there is a large disturbance in the crack generation direction and the crack size, and the cut surface becomes scaly. In this case, a smooth cut surface cannot be obtained, and the material loss is large. Moreover, there is also a problem that the load required for cutting becomes large and the wafer is likely to be damaged.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Patent No. 5509448 [Patent Document 2] Patent No. 6399913 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The present invention aims to provide a laser slicing method that results in a smooth wafer cross-section, a low fracture load, and is effective in improving material yield. [Means for solving the problem]

[0006] The laser slicing method according to the present invention is a laser slicing method for manufacturing a wafer by cutting it, in which a laser beam is moved and scanned 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), thereby forming a modified layer and cracks, wherein the scanning direction for the moving and scanning is to irradiate a dot linearly along the inclination direction of the crystal plane perpendicular to the C-axis.

[0007] Many of the substrates (ingots) used to manufacture wafers are single crystals with an off-angle in their crystal orientation. With respect to the central axis connecting the center of the top surface and the center of the bottom surface of the substrate, the crystal orientations

[0100] ,

[0001] , etc., have a predetermined off-angle. Therefore, the crystal planes have a step-terrace structure that forms a predetermined off-angle. Therefore, the inventors have found that a smooth cross-section along the step terrace surface can be obtained by scanning a laser beam along the slope forming the off-angle while irradiating it with a dot.

[0008] In the present invention, dot irradiation refers to irradiation in a dot pattern, which is determined by a combination of the pulse period of the laser irradiation and the relative movement speed of the substrate. The irradiation may be continuous at equally spaced intervals, but the pitch of the dot irradiation may also be intermittent dot irradiation, which has irradiated and unirradiated areas. When a laser beam with a focal point is irradiated into the interior of a substrate, a modified layer is formed by the irradiation, cracks occur, and these cracks propagate. Therefore, the pitch interval of the dot irradiation can be set considering the length of crack propagation, or the dot irradiation can be performed intermittently. Furthermore, in this invention, the direction in which dots are irradiated linearly along the inclined direction that forms the off-angle is expressed as the irradiation line, and the modified layer and cracks are formed by the irradiation lines moving parallel to each other in the same direction or back and forth at predetermined intervals, and the interval between these irradiation lines is expressed as the scanning line pitch.

[0009] In the present invention, a laser slicing method for manufacturing a wafer by cutting it is provided, in which a laser beam is moved and scanned 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), thereby forming a modified layer and cracks, wherein the scanning direction for the moving scan may be linear dot irradiation so as to form a predetermined scanning inclination angle with respect to the inclination direction of the crystal plane perpendicular to the C-axis. In dot irradiation, the irradiation lines are formed parallel to each other in the same direction, or alternately parallel in the reciprocating direction, at a predetermined scanning line pitch, resulting in a fractured surface with countless cracks. In this process, scanning at an angle with a predetermined scanning inclination angle of 4 to 20° relative to the direction of inclination makes it easier for cracks to connect in adjacent scanning lines, resulting in a smaller fracture load. [Effects of the Invention]

[0010] In the present invention, for a substrate (ingot) having an off-angle, an irradiation line that linearly irradiates dots along the inclination direction of the off-angle of the crystal plane or linearly irradiates dots so as to form a predetermined scanning inclination angle with respect to this inclination direction is scanned in parallel or reciprocated at a predetermined scanning line pitch, whereby the cut surface becomes smooth and the cutting load becomes smaller than before.

Brief Description of the Drawings

[0011] [Figure 1] (a) An example in which the irradiation line of dot irradiation according to the present invention is scanned in the inclination direction [11-20] forming the off-angle of the substrate, and (b) shows a schematic diagram of dot irradiation and a confocal laser microscope image of the surface after cutting. [Figure 2] In the present invention, a schematic diagram of an example in which dot irradiation is intermittently irradiated and a confocal laser microscope image of the surface after cutting are shown. [Figure 3] In the present invention, (a) shows the side where the irradiation line of dot irradiation is scanned so as to form a predetermined scanning inclination angle clockwise with respect to the [11-20] direction in FIG. 3, and (b) shows the dot schematic diagram and the confocal laser microscope image of the surface after cutting. [Figure 4] For comparison, a dot schematic diagram and a confocal laser microscope image of the surface after cutting when the irradiation line of dot irradiation is scanned in the [10-10] direction orthogonal to the [11-20] direction forming the off-angle of the substrate are shown. [Figure 5] The evaluation results are shown. [Figure 6] The relationship between the tested laser scanning inclination angle and the peeling force at the time of cutting is shown in a graph. [Figure 7] The relationship between the tested laser scanning inclination angle and the maximum height and arithmetic mean roughness of the cut surface is shown.

Embodiments for Carrying Out the Invention

[0012] Since the irradiation method of dot irradiation, the surface roughness, and the cutting load at the time of cutting after laser slicing were measured and compared, they will be described below. FIG. 1(a) shows an example of a hexagonal single crystal substrate (ingot). The substrate usually has an orientation flat in the [10-10] direction and an orientation flat in the direction [10-20] orthogonal to it. The crystal orientation

[0001] has an off-angle of 2 to 8° with respect to the central axis of the substrate (the direction perpendicular to the substrate surface). When taking this crystal orientation as the C-axis, an inclined surface that forms an off-angle is formed on the crystal plane orthogonal to this C-axis. For this test and evaluation, a Si substrate with an off-angle of 4° was used.

[0013] The formation surface of the off-angle orthogonal to the C-axis of the Si substrate is formed in a step-terrace structure with a pitch of 150 to 200 μm. As the laser light, the wavelength is 532 nm, The pulse width is set to 8 to 30 ns (pulse oscillation frequency 20 kHz z ). Numerical aperture NA 0.2 That's all. (NA: beam diameter / focal length × 2) Pulse energy: 3 to 20 μJ Irradiation dot pitch: 1 to 5 μm Scanning line pitch: 20 to 70 μm was investigated within the range.

[0014] The evaluation results are shown in the table of Fig. 5. The peel strength was measured using a universal testing machine conforming to JIS, applying a tensile load to both sides of a small piece sample of a certain size, and measuring the maximum load until severance. In addition, in order to facilitate peeling, it was applied at a position offset by 90 mm from the center of the test piece. The surface roughness of the cut surface was measured by a confocal laser microscope for the maximum height (R Z ) of the entire cut surface of a small piece sample of a certain size. Examples 1 to 6 are those with dot irradiation in the inclined direction [11-20] forming the off-angle, and Comparative Example 1 shows those with dot irradiation in the direction [10-10] orthogonal to the inclined direction forming the off-angle. The schematic diagram of the dot irradiation at that time and the confocal laser microscope image of the surface after severance are shown in Fig. 4. Examples 1-6, in which the illumination line of the dot irradiation was in the inclined direction [11-20] that formed an off-angle, showed less surface roughness on the fractured surface than the comparative example, which was scanned in the orthogonal direction [10-10]. In Examples 1-6, the surface polishing allowance after cleavage was 100 μm or less, while in the comparative example, it required 150 μm or more. Furthermore, comparing the contents of Examples 1 to 6, the peeling force is reduced at irradiation dot pitches of 1 to 3 μm. From Example 4, it can be seen that even with relatively large irradiation dot pitches of about 5 μm and scanning line pitches of 50 μm, increasing the pulse energy reduces the peeling force. From these observations, pulse energy 2~6μJ, Irradiation dot pitch: 0.6~3μm Scanning line pitch: A range of 10 to 30 μm is preferable. The confocal laser microscope image of the surface after fracturing shown in Figure 1(b) is from Example 1.

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

[0016] Figure 3 shows an example of dot irradiation with a pulse laser so that the scanning inclination angle is formed in a clockwise direction relative to the inclination direction [11-20] which forms the off-angle of the irradiation line. This example uses a wavelength of 1030 nm Pulse width: 1-20 picoseconds Irradiation dot pitch: 5~15μm Scanning line pitch: 40-80 μm Pulse energy: 5-15 μJ It was evaluated using [the method described]. Figure 6 shows the results of investigating the change in peeling force per 10 mm square when the laser scanning tilt angle (clockwise) of the irradiation line is changed, and Figure 7 shows the change in the maximum height Rz (● marks) and arithmetic mean roughness (○ marks) of the fracture surface with respect to the change in laser scanning tilt angle. The results in Figure 6 show that the peeling load is minimized when the laser scanning tilt angle is around 4-8°. This is presumed to be because the direction of the cracks is aligned by creating an incline in the irradiation line, causing the cracks in adjacent irradiation lines to connect continuously. Figure 3(b) shows a confocal laser microscope image of the surface after cleavage at an inclination angle of 4°. In contrast, Figure 7 reveals that there is no significant difference in the roughness of the cleavage surface with respect to the scanning inclination angle.

Claims

[Claim 1] A laser slicing method for manufacturing a wafer by cutting it, wherein the laser beam is moved and scanned relative to the substrate while forming a focal point of the laser beam 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), thereby forming a modified layer and cracks. A laser slicing method for a wafer, characterized in that the scanning direction for moving and scanning is linear, with dot irradiation or intermittent dot irradiation, so as to form a scanning inclination angle in the range of 4 to 20° with respect to the inclination direction of the crystal plane perpendicular to the C axis.

Images