Method for laser processing of a workpiece

Abstract

This invention provides a laser processing method for workpieces that can suppress the reduction of flexural strength in semiconductor devices. The laser processing method is implemented using a laser processing apparatus comprising: a pulsed laser beam irradiation unit including a laser oscillator emitting a pulsed laser beam, a focusing lens for focusing the pulsed laser beam, and a branching unit for branching the pulsed laser beam; a chuck stage for holding the workpiece; and a processing feed unit that moves the chuck stage and the focusing lens relative to each other in the processing feed direction. During laser processing, with the branching interval of the pulsed laser beam on the front side of the workpiece set to L, the value obtained by dividing the processing feed speed of the processing feed unit by the frequency of the pulsed laser beam at the processing point set to S, and n set to any integer, the branching interval of the pulsed laser beam, the processing feed speed, and the frequency are set in a manner that satisfies L ≠ n × S.

Description

Technical Field

[0001] This invention relates to a laser processing method for workpieces. Background Technology

[0002] As a method for chip-forming semiconductor wafers and other plate-shaped objects by dividing them along spacers (predetermined dividing lines), there are known cutting methods that divide the objects by rotating a cutting tool while cutting into them, and laser processing methods that divide the objects by irradiating a pulsed laser beam with a wavelength that is absorbent to the plate-shaped objects (see Patent Documents 1 and 2).

[0003] Laser processing has the following advantages: because it can reduce the groove width compared to the case of using cutting tools, it can narrow the spacing between plates, thereby increasing the number of chips that can be obtained.

[0004] Patent Document 1: Japanese Patent Application Publication No. 4-99607

[0005] Patent Document 2: Japanese Patent Application Publication No. 2004-188475

[0006] However, the following problem exists: the flexural strength of the diced semiconductor device may be reduced due to thermal damage caused by the pulsed laser beam generated during laser processing along the spacer. Summary of the Invention

[0007] Therefore, the object of the present invention is to provide a laser processing method for a workpiece that can suppress the reduction of flexural strength in semiconductor devices.

[0008] According to the present invention, a laser processing method for a workpiece is provided, wherein a laser processing apparatus is used to process the workpiece. The laser processing apparatus includes: a pulsed laser beam irradiation unit comprising a laser oscillator, a focusing lens, and a branching unit. The laser oscillator emits a pulsed laser beam with a wavelength that is absorbent to the workpiece. The focusing lens focuses the pulsed laser beam. The branching unit is disposed between the laser oscillator and the focusing lens to branch the pulsed laser beam into multiple pulsed laser beams. A chuck stage is used to hold the workpiece. A processing feed unit is used to move the chuck stage and the focusing lens relative to each other in a processing feed direction. The laser processing method for the workpiece includes the following laser processing steps: while feeding the chuck table, the pulsed laser beam is branched into multiple pulsed laser beams in the processing feed direction and irradiated onto the workpiece held on the chuck table via the focusing lens. In this laser processing step, the branching interval of the pulsed laser beam on the front side of the workpiece is set to L, the value obtained by dividing the processing feed speed of the processing feed unit by the frequency of the pulsed laser beam at the processing point is set to S, and n is set to any integer. The branching interval of the pulsed laser beam, the processing feed speed, and the frequency are set in a manner that satisfies L≠n×S.

[0009] This invention can suppress the reduction of flexural strength in semiconductor devices. Attached Figure Description

[0010] Figure 1 This is a perspective view of the workpiece being processed in a laser processing method that is an embodiment of the workpiece being processed.

[0011] Figure 2 This is a side view schematically showing an example of the structure of the laser processing apparatus used in the laser processing method for the workpiece of the embodiment.

[0012] Figure 3 It is shown schematically. Figure 2 An enlarged top view of the main front part of the workpiece.

[0013] Figure 4 This is a schematic diagram illustrating the processing line of a pulsed laser beam according to an embodiment.

[0014] Figure 5 It is Figure 4 The diagram schematically illustrates the machining line broken down in the indexing feed direction.

[0015] Figure 6 This is a schematic diagram showing the processing line of a pulsed laser beam in a comparative example.

[0016] Figure 7It is Figure 6 The diagram schematically illustrates the machining line broken down in the indexing feed direction.

[0017] Label Explanation

[0018] 1: Laser processing device; 10: Chuck table; 14: Processing feed unit; 20: Pulsed laser beam irradiation unit; 21: Pulsed laser beam; 22: Laser oscillator; 23: Reflector; 24: Branching unit; 25: Focusing lens; 30, 31, 32, 33, 34, 35: Focusing spot; 36: Branching interval; 37: Excitation interval; 38: Beam diameter; 39: Overlap width; 100: Workpiece. Detailed Implementation

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention is not limited to the contents described in the following embodiments. Furthermore, the constituent elements described below include elements readily conceived by those skilled in the art, and substantially the same elements. Moreover, the structures described below can be appropriately combined. Additionally, various omissions, substitutions, or modifications to the structure can be made without departing from the spirit of the present invention.

[0020] The processing method of the workpiece 100 according to an embodiment of the present invention will be described with reference to the accompanying drawings. First, the structure of the workpiece 100, which is the object of processing in the embodiment, will be described. Figure 1 This is a perspective view of a workpiece 100, the object to be processed in a laser processing method according to an embodiment. In the following description, the X-axis direction is a direction on the horizontal plane. The Y-axis direction is a direction on the horizontal plane perpendicular to the X-axis direction.

[0021] like Figure 1 As shown, the workpiece 100 is a wafer such as a disc-shaped semiconductor wafer or optical device wafer, with silicon (Si), sapphire (Al2O3), gallium arsenide (GaAs), or silicon carbide (SiC) as the substrate 101. The workpiece 100 has: pre-defined dividing lines 103 arranged in a grid pattern on the front side 102 of the substrate 101; and a device 104 formed in the area divided by the pre-defined dividing lines 103. The device 104 is, for example, an integrated circuit such as IC (Integrated Circuit) or LSI (Large Scale Integration), or an image sensor such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor).

[0022] The workpiece 100 is supported by an annular frame 110 and adhesive tape 111. The annular frame 110 has an opening larger than the outer diameter of the workpiece 100. The outer periphery of the adhesive tape 111 is adhered to the back side of the annular frame 110. The workpiece 100 is positioned at a predetermined position in the opening of the annular frame 110, and the back side 105 of the workpiece 100 is adhered to the front side of the adhesive tape 111, thereby fixing the workpiece 100 to the annular frame 110 and the adhesive tape 111.

[0023] Next, the structure of the laser processing apparatus 1 used in the laser processing method for the workpiece in the embodiment will be described. Figure 2 This is a side view schematically showing a structural example of the laser processing apparatus 1 used in the laser processing method for the workpiece according to the embodiment, in partial cross-section. In the following description, the Z-axis direction is the direction perpendicular to the X-axis and Y-axis directions. In the laser processing apparatus 1 of the embodiment, the processing feed direction is the X-axis direction, the indexing feed direction is the Y-axis direction, and the focusing point position adjustment direction is the Z-axis direction.

[0024] The laser processing apparatus 1 includes a chuck table 10, a processing feed unit 14, and a pulsed laser beam irradiation unit 20. The laser processing apparatus 1 is a device that processes a workpiece 100 by irradiating it with a pulsed laser beam 21.

[0025] The chuck stage 10 holds the workpiece 100 using a holding surface 11. The holding surface 11 is a disc shape formed of porous ceramic or the like. In this embodiment, the holding surface 11 is a plane parallel to the horizontal direction. The holding surface 11 is connected to a vacuum suction source, for example, via a vacuum suction path. The chuck stage 10 attracts and holds the workpiece 100 placed on the holding surface 11.

[0026] A plurality of clamping parts 12 are arranged around the chuck table 10 to hold the workpiece 100 in a ring-shaped frame 110. The chuck table 10 rotates about an axis parallel to the Z-axis direction via a rotating unit 13. By rotating the chuck table 10 using the rotating unit 13, the machining feed direction of the workpiece 100 can be changed. The rotating unit 13 is supported by a machining feed unit 14.

[0027] The machining feed unit 14 moves the chuck table 10 and the pulsed laser beam 21 irradiated by the pulsed laser beam irradiation unit 20 relative to each other in the machining feed direction. In this embodiment, the machining feed unit 14 moves the rotary unit 13 and the chuck table 10 in the X-axis direction (machining feed direction) and the Y-axis direction (indexing feed direction). Furthermore, Figure 2 Arrow 15 indicates the direction of movement of the chuck table 10 during machining feed.

[0028] The pulsed laser beam irradiation unit 20 is a unit that irradiates the workpiece 100 held on the chuck table 10 with a pulsed laser beam 21. For example... Figure 2 As shown, the pulsed laser beam irradiation unit 20 includes a laser oscillator 22, a reflector 23, a branching unit 24, and a focusing lens 25.

[0029] The laser oscillator 22 emits a pulsed laser beam 21 with a predetermined wavelength for processing the workpiece 100. The laser oscillator 22 emits a pulsed laser beam 21 with a wavelength (e.g., 355 nm) that is absorbed by the workpiece 100. The laser oscillator 22 emits a pulsed laser beam 21 at a preset frequency.

[0030] The reflector 23 reflects the pulsed laser beam 21 toward the workpiece 100 held on the holding surface 11 of the chuck stage 10. In one embodiment, the reflector 23 reflects the pulsed laser beam 21 emitted from the laser oscillator 22 toward the branching unit 24.

[0031] A pulsed laser beam 21, emitted from the laser oscillator 22 and reflected by the mirror 23, is incident on the branching unit 24. The branching unit 24 branches the incident pulsed laser beam 21 into at least two pulsed laser beams, which then pass through the focusing lens 25. The direction in which the pulsed laser beam 21 is branched is the processing feed direction. The branching unit 24 is, for example, a diffractive optical element. A diffractive optical element has the function of branching the incident pulsed laser beam 21 into multiple pulsed laser beams using the diffraction phenomenon.

[0032] The focusing lens 25 converges the pulsed laser beam 21 emitted from the laser oscillator 22 and irradiates the workpiece 100 held on the holding surface 11 of the chuck table 10. In this embodiment, the focusing lens 25 converges multiple pulsed laser beams 21 that branch in the processing feed direction via the branching unit 24 into a focusing spot 30.

[0033] Next, the laser processing method for the workpiece according to the embodiment will be described. Figure 3 It is shown schematically. Figure 2 A top view of the main part of the front 102 of the workpiece 100. Figure 4 This is a schematic diagram illustrating the processing line 3 of the pulsed laser beam 21 in an embodiment. Figure 5 It is Figure 4 The diagram schematically illustrates the machining line 3 broken down in the indexing feed direction.

[0034] like Figure 3As shown, the focused spot 30 of the pulsed laser beam 21, i.e., the processing point, is set on the predetermined dividing line 103 of the front surface 102 of the workpiece 100. When the multiple branched pulsed laser beams 21 irradiate the front surface 102 of the workpiece 100, the focused spot 30 is arranged linearly and at equal intervals on the front surface 102 of the workpiece 100 in the X-axis direction, which is the processing feed direction. Figures 3 to 5 In one example shown, the pulsed laser beam 21 branches into 5 pulsed laser beams. That is, as... Figure 3 As shown, the focusing spot 30 includes five focusing spots 31, 32, 33, 34, and 35 branching off in the processing feed direction, i.e., the X-axis direction.

[0035] As described above, the pulsed laser beam 21 irradiates in a pulsed manner according to a preset frequency. In the laser processing method for the workpiece in this embodiment, the single-pulse laser beam 21 is branched into five pulsed laser beams, such as... Figure 3 As shown, focused light spots 31, 32, 33, 34, and 35 are formed on the predetermined dividing line 103 of the front surface 102 of the workpiece 100. That is, the pulsed laser beam 21 of each pulse irradiates the predetermined dividing line 103 of the front surface 102 of the workpiece 100 as multiple (five in this embodiment) focused light spots 31, 32, 33, 34, and 35. In this way, the pulsed laser beam 21 of one pulse branches into multiple (five in this embodiment) pulsed laser beams, which irradiate the front surface 102 of the workpiece 100 as multiple focused light spots 30 (five focused light spots 31, 32, 33, 34, and 35 in this embodiment).

[0036] The branching intervals 36 of the adjacent focused laser spots 31, 32, 33, 34, and 35 are all equally spaced. The branching interval 36 divides the single-pulse laser beam 21 into five pulse laser beams. Figure 3 The distance between the centers of the focused laser spots 31, 32, 33, 34, and 35 formed on the predetermined dividing line 103 on the front surface 102 of the workpiece 100, as shown. Furthermore, in the following description, each of the branched pulsed laser beams 21 will be referred to as the first branch, the second branch, ..., and the fifth branch.

[0037] While irradiating the processing point with pulsed laser beam 21, at the same time... Figure 2 The chuck table 10 shown is used for machining feed, thereby forming Figure 4 The processing line 3 is shown. A pulsed laser beam 21 irradiates the machine at a preset frequency in a pulsed pattern. That is, the preset processing feed unit 14 (refer to...) Figure 2 The excitation interval 37 is obtained by dividing the processing feed rate by the frequency of the pulsed laser beam 21 (refer to...). Figure 5 The processing line 3 is formed by exciting multiple focused light spots 30 with a spacing of 37.

[0038] The excitation interval 37 is the distance between the centers of the focal spots 31, 32, 33, 34, and 35 formed on the predetermined dividing line 103 of the front surface 102 of the workpiece 100 by a predetermined pulse laser beam 21 of a specified number of pulses and the centers of the focal spots 31-1 formed on the predetermined dividing line 103 of the front surface 102 of the workpiece 100 by the first pulse laser beam 21 and the centers of the focal spots 31-2 formed on the predetermined dividing line 103 of the front surface 102 of the workpiece 100 by the second pulse laser beam 21.

[0039] In this embodiment, the excitation interval 37 is smaller than the beam diameter 38 of the pulsed laser beam 21 in the focused spot 30. That is, the first excited focused spots 31-1, 32-1, 33-1, 34-1, and 35-1 overlap with the subsequent second excited focused spots 31-2, 32-2, 33-2, 34-2, and 35-2. Furthermore, the second excitation and subsequent third excitation, the third excitation and subsequent fourth excitation, and the fourth excitation and subsequent fifth excitation have the same relationship as the first excitation and subsequent second excitation.

[0040] exist Figure 4 as well as Figure 5 In one example shown, regarding processing line 3, when the branch interval 36 is set to L, the excitation interval 37 is set to S, and n is set to any integer, L ≠ n × S is satisfied. Furthermore, when the processing feed speed of the processing feed unit 14 is set to V, and the frequency of the pulsed laser beam 21 at the processing point is set to f, the excitation interval 37 is expressed as S = V / f. Therefore, the branch interval 36 of the pulsed laser beam 21 on the front surface 102 of the workpiece 100, the processing feed speed of the processing feed unit 14, and the frequency of the pulsed laser beam 21 at the processing point are set in a manner that satisfies L ≠ n × V / f.

[0041] Therefore, in processing line 3, the focused spot 31-5 of the first branch in the fifth excitation overlaps with the focused spot 32-1 of the second branch in the first excitation with a specified overlap width 39. The overlap width 39 is the length of the overlapping portion of the focused spots 31, 32, 33, 34, 35 of the pulsed laser beams 21 of different pulses in the processing feed direction (indicated by arrow 15).

[0042] The overlap rate is preferably less than 80%. This improves the bending strength. Furthermore, regarding the overlap rate (%), given an overlap rate of P, a beam diameter 38 of each focusing spot 31, 32, 33, 34, 35 of the pulsed laser beam 21 at focusing spot 30 of D, and an overlap width 39 of W, the overlap rate (%) is calculated by P = W / D × 100. That is, in the laser processing method for the workpiece of the present invention, the branch spacing 36, processing feed rate, and frequency are preferably set such that 0 ≤ P < 80. When the overlap rate is 80% or higher (P ≥ 80), the bending strength of the device 104 after chip segmentation may decrease due to overheating by the pulsed laser beam 21.

[0043] In the laser processing method for the workpiece of the present invention, the overlap rate can be 0% (P = 0). An overlap rate of 0% (P = 0) indicates a state where the focused laser spots are in contact with each other but do not overlap. In this case, overheating by the pulsed laser beam 21 can be suppressed, thus suppressing the reduction in the bending strength of the device 104 after chip dicing. Conversely, an overlap rate of less than 0% (P < 0) indicates a state where the center-to-center distance between the focused laser spots is greater than the beam diameter 38, resulting in the focused laser spots being separated from each other. When the overlap rate is less than 0% (P < 0), the processing line 3 becomes discontinuous, leading to excessive stress concentration during chip dicing, which may reduce the bending strength of the device 104 after chip dicing. Therefore, in the laser processing method for the workpiece of the present invention, it is preferable to set the branch spacing 36, processing feed rate, and frequency with P ≥ 0.

[0044] Furthermore, the focused light spot 32-5 of the second branch and the focused light spot 33-1, the focused light spot 33-5 of the third branch and the focused light spot 34-1, the focused light spot 34-5 of the fourth branch and the focused light spot 35-1 of the fifth branch also have the same relationship as the focused light spot 31-5 of the first branch and the focused light spot 32-1 of the second branch.

[0045] The laser processing apparatus 1 used in the laser processing method for the workpiece in the embodiment includes, for example, a control unit that controls each component of the laser processing apparatus 1; and an input unit that can accept the operator's settings for laser processing conditions. When any two of the following parameters are set: the branch interval 36 of the pulsed laser beam 21, the processing feed speed of the processing feed unit 14, and the frequency of the pulsed laser beam 21 at the processing point, the control unit calculates and outputs the value of the remaining parameter that satisfies L = n × V / f. The operator can exclude the calculated value and set the remaining parameter.

[0046] The laser processing apparatus 1 used in the laser processing method for the workpiece in the embodiment may, for example, have a notification unit that notifies a specified warning message when the branch interval 36 of the set pulsed laser beam 21, the processing feed speed of the processing feed unit 14, and the frequency of the pulsed laser beam 21 at the processing point satisfy L = n × V / f.

[0047] Next, the processing line 4 of the pulsed laser beam 21 in the comparative example will be described. Figure 6 This is an explanatory diagram schematically showing the processing line 4 of the pulsed laser beam 21 in a comparative example. Figure 7 It is Figure 6 The illustration diagram schematically shows the machining line 4 broken down in the indexing feed direction.

[0048] exist Figure 6 and Figure 7 In the comparative example shown, the pulsed laser beam 21 is branched into five pulsed laser beams in the same manner as in the embodiment, with five focusing spots 41, 42, 43, 44, and 45 set in the X-axis direction, which is the processing feed direction. The branching interval 46 of adjacent focusing spots 41, 42, 43, 44, and 45 is all equally spaced. The pulsed laser beam 21 irradiates in a pulsed pattern at a preset frequency. The preset processing feed unit 14 (see reference) is used for this purpose. Figure 2 The excitation interval 47 is obtained by dividing the processing feed rate of the laser beam 21 by the frequency of the pulse laser beam 21. The processing line 4 is formed by multiple focusing spots 41, 42, 43, 44, and 45 of the excitation interval 47.

[0049] Regarding processing line 4, when the branch interval 46 is set to Lc, the excitation interval 47 is set to Sc, and n is set to any integer, Lc = n × Sc is satisfied. Furthermore, when the processing feed speed of the processing feed unit 14 is set to V, and the frequency of the pulsed laser beam 21 at the processing point is set to f, the excitation interval 47 is expressed as Sc = V / f. Therefore, the branch interval 46 of the pulsed laser beam 21 on the front surface 102 of the workpiece 100, the processing feed speed of the processing feed unit 14, and the frequency of the pulsed laser beam 21 at the processing point are set according to Lc = n × V / f.

[0050] Therefore, regarding processing line 4, the focused laser spot 41-5 of the first branch in the fifth excitation overlaps with the focused laser spot 42-1 of the second branch in the first excitation according to a specified overlap width 49. In the comparative example, the overlap rate is 100%. Furthermore, when the overlap rate is set to P, the beam diameter 48 of the pulsed laser beam 21 at focused laser spots 41, 42, 43, 44, and 45 is set to D, and the overlap width 49 is set to W, the overlap rate is calculated using P = W / D.

[0051] In addition, the focused light spot 42-5 of the second branch has the same relationship with the focused light spot 43-1 of the third branch, the focused light spot 43-5 of the third branch has the same relationship with the focused light spot 44-1 of the fourth branch, the focused light spot 44-5 of the fourth branch has the same relationship with the focused light spot 45-1 of the fifth branch, and the focused light spot 41-5 of the first branch has the same relationship with the focused light spot 42-1 of the second branch, with an overlap rate of 100%.

[0052] As explained above, in the laser processing method for the workpiece in the embodiment, the branch interval 36, processing feed rate, and frequency are set such that the relationship between the branch interval 36 and the excitation interval 37 of the pulsed laser beam 21 on the front side 102 of the workpiece 100 satisfies L≠n×S. Here, L represents the value of the branch interval 36, S represents the value of the excitation interval 37, and n represents any integer.

[0053] As shown in the comparative example, in laser processing using pulsed laser beams 21 branched in the processing feed direction, in the processing line 4 where L = n × S holds, the focused spot 41-5 formed by the fifth excitation in the focused spot 41 of the branched pulsed laser beam 21 completely overlaps with the focused spot 42-1 formed by the first excitation in the focused spot 42 of the adjacent pulsed laser beam 21. Thus, by setting the focused spot at the same location, the thermal damage of the pulsed laser beam 21 at that location increases, and the bending strength decreases.

[0054] In contrast, in the processing line 3 of the embodiment, the focused spot 31-5 formed by the fifth excitation in the focused spot 31 of the branched pulsed laser beam 21 partially overlaps with the focused spot 32-1 formed by the first excitation in the focused spot 32 of the adjacent pulsed laser beam 21, but not completely overlaps. That is, by setting the branching interval 36, processing feed speed, and frequency in the manner of L≠n×S, the situation where the focused spot 31-5 formed in the fifth excitation completely overlaps with the focused spot 32-1 formed in the first excitation can be avoided. By reducing the overlap width 39 of adjacent focused spots, the thermal damage caused by the pulsed laser beam 21 can be reduced, thereby suppressing the reduction of the bending strength in the device 104.

[0055] Furthermore, the present invention is not limited to the embodiments described above. That is, various modifications can be made without departing from the spirit of the present invention.

[0056] For example, the laser processing conditions of the laser processing method for the workpiece are not limited to the specific implementation. When the substrate 101 of the workpiece 100 is silicon and has a thickness of 50 μm, the laser processing conditions can be set as follows, for example.

[0057] Number of branches: 2 to 32

[0058] Branch spacing: 10μm~400μm

[0059] Spot diameter: 3μm~10μm

[0060] Wavelength: 355nm

[0061] Frequency: 100kHz

[0062] Power: 7.5kW

[0063] Number of passes: 30

[0064] Machining feed rate: 10mm / s~2000mm / s

Claims

1. A laser processing method for a workpiece, comprising processing the workpiece using a laser processing device. This laser processing device has the following features: A pulsed laser beam irradiation unit includes a laser oscillator, a focusing lens, and a branching unit. The laser oscillator emits a pulsed laser beam with a wavelength that is absorbed by the workpiece. The focusing lens focuses the pulsed laser beam. The branching unit is disposed between the laser oscillator and the focusing lens to branch the pulsed laser beam into multiple pulsed laser beams. A chuck table that holds the workpiece; and The machining feed unit moves the chuck table and the condenser lens relative to each other in the machining feed direction. in, The laser processing method for the workpiece includes the following laser processing steps: while feeding the chuck table, the pulsed laser beam is branched into multiple pulsed laser beams in the processing feed direction and irradiated onto the workpiece held on the chuck table via the focusing lens. In this laser processing step, the branching interval of the pulsed laser beam on the front side of the workpiece is set to L, the beam diameter of each focused spot of the branched laser beam is set to D, the value obtained by dividing the processing feed speed of the processing feed unit by the frequency of the pulsed laser beam at the processing point is set to S, and n is set to any integer. The branching interval of the pulsed laser beam, the processing feed speed, and the frequency are set in a manner that satisfies L>D>S and L≠n×S. Given an overlap rate of P, a beam diameter of D for each spot of the pulsed laser beam at the focal spot, and an overlap width of W, the overlap rate is calculated by P = W / D × 100. This overlap rate P is greater than 0% and less than 80%. The overlap width W is the length of the overlapping portion of the focal spots of the pulsed laser beams of different pulses in the processing feed direction.

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