Wafer processing method

Abstract

To apply laser processing to a wafer on a proper processing condition corresponding to thickness variations of wafers.SOLUTION: A wafer processing method includes: a holding step of sucking and holding a wafer with a chuck table having transmissivity with respect to laser beams; and a laser beam radiation step of radiating laser beams along a plurality of predetermined dividing lines. The laser beam radiation step includes: a proper condition selecting step of selecting a processing condition in a case where a photodetection unit detects laser beams permeating the wafer as a proper processing condition by changing the processing condition step by step from a processing condition on which the laser beams do not permeate the wafer to the processing condition on which the laser beams permeate the wafer when processing the wafer along at least one predetermined dividing line; and a dividing step of dividing the wafer into a plurality of device chips by radiating the laser beams along the other predetermined dividing line by using the processing condition which is selected in the proper condition selecting step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wafer processing method for dividing a wafer into a plurality of device chips by irradiating the wafer with a pulsed laser beam having a wavelength that is absorbed by the wafer along each of the planned dividing lines. [Background technology]

[0002] In the manufacturing process of semiconductor device chips, in order to thin the semiconductor device chips, the back side of a semiconductor wafer (hereinafter simply referred to as wafer) having multiple devices formed on its front side may be ground using a grinding device before being diced into individual pieces (see, for example, Patent Document 1).

[0003] After grinding, the wafer is thinned to a predetermined finished thickness and then divided into multiple semiconductor device chips. For example, the wafer is divided using a laser processing machine. To divide the wafer, first, dicing tape is attached to the backside of the wafer, and then the backside is suction-held on a chuck table via the dicing tape.

[0004] Next, a laser beam having a wavelength that is absorbed by the wafer is irradiated along the planned dividing line of the wafer to form a groove that penetrates the wafer from the front to the back (i.e., a full cut), thereby dividing the wafer into multiple semiconductor device chips (see, for example, Patent Document 2).

[0005] When performing such laser processing, the processing conditions such as the laser beam output and processing feed rate are determined in advance according to the type of wafer through test cutting, and therefore, the same processing conditions are uniformly applied to wafers of the same type to process the wafers.

[0006] However, when wafers manufactured in different lots are compared, there may be slight variations in wafer thickness due to variations in device manufacturing, variations in finished thickness after grinding, and the like.

[0007] Therefore, when a laser beam is used for ablation processing under predetermined processing conditions, defects may occur due to variations in wafer thickness. Specifically, in a relatively thick wafer, unpenetrated areas, where the wafer is not completely cut, may occur throughout the wafer.

[0008] Furthermore, with relatively thin wafers, after the wafer is cut, the dicing tape may melt due to excessive heating by the laser beam, and the melted residue may stick to the chuck table, reducing the flatness of the holding surface of the chuck table and potentially blocking the suction holes in the holding surface. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-90389 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-125448 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made in consideration of the above problems, and aims to perform laser processing on a wafer under appropriate processing conditions according to variations in wafer thickness when cutting the wafer by ablation processing. [Means for solving the problem]

[0011] According to one aspect of the present invention, there is provided a wafer processing method for dividing a wafer into a plurality of device chips by irradiating a pulsed laser beam having a wavelength absorbed by the wafer along each of the division lines of the wafer, the division lines defining a plurality of regions on the wafer, the regions being partitioned by the division lines in a grid pattern, and dividing the wafer into a plurality of device chips, the method comprising the steps of: a tape-applying step of applying a central portion of a tape to a back surface of the wafer opposite the front surface, and applying one surface of an annular frame to an outer periphery of the tape; a holding step of suction-holding the wafer via the tape using a chuck table that is transparent to the laser beam; and a step of irradiating a pulsed laser beam having a wavelength absorbed by the wafer along the division lines while changing the processing position by relatively moving the focal point of the laser beam and the chuck table in a predetermined direction after the holding step. a laser beam irradiation step of irradiating the laser beam, wherein the laser beam irradiation step, when processing along at least one planned dividing line, includes an appropriate condition selection step of selecting, as appropriate processing conditions, processing conditions at the time when the optical detection unit detects the laser beam that has penetrated the wafer, by gradually changing processing conditions from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer until the optical detection unit, which is arranged on the opposite side of the chuck table from the wafer, detects the laser beam that has penetrated the wafer; and a dividing step of dividing the wafer into the plurality of device chips by irradiating the laser beam along other planned dividing lines different from the at least one planned dividing line using the processing conditions selected in the appropriate condition selection step.

[0012] Preferably, the appropriate condition selection step includes changing the processing conditions from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer by gradually increasing the average output as the relative movement in the predetermined direction occurs.

[0013] Also, preferably, the appropriate condition selection step includes gradually slowing down the processing feed rate in accordance with the relative movement in the predetermined direction, thereby changing the processing conditions from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer.

[0014] Preferably, the wafer processing method further includes a re-irradiation step of irradiating the laser beam again onto an unpenetrated area formed in the appropriate condition selection step until the optical detection unit detects the laser beam that has penetrated the wafer.

[0015] Preferably, the wafer processing method further comprises a tape expanding step of expanding the tape after the laser beam irradiation step. [Effects of the Invention]

[0016] In one embodiment of the wafer processing method of the present invention, when processing along at least one planned dividing line, the processing conditions are gradually changed from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer until the optical detection unit detects the laser beam that has penetrated the wafer, and the processing conditions when the optical detection unit detects the laser beam that has penetrated the wafer are selected as appropriate processing conditions (appropriate condition selection step).

[0017] This allows appropriate processing conditions to be selected according to the wafer thickness, and by performing laser processing under the selected processing conditions, it is possible to prevent the occurrence of non-penetrated areas across the entire wafer and the sticking of molten residue to the chuck table. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a flow diagram of a wafer processing method. [Figure 2] FIG. 2(A) is a perspective view of a wafer, and FIG. 2(B) is a perspective view of a wafer unit. [Figure 3] FIG. 1 is a perspective view of a laser processing device. [Figure 4] FIG. [Figure 5] FIG. 5(A) is a diagram showing laser processing at a first average power, FIG. 5(B) is a diagram showing laser processing at a second average power, and FIG. 5(C) is a diagram showing the laser beam penetrating the wafer. [Figure 6] FIG. 2 is a perspective view of a plurality of device chips and the like. [Figure 7] FIG. 7(A) is a flow diagram of a wafer processing method according to a first modified example, and FIG. 7(B) is a flow diagram of a wafer processing method according to a second modified example. [Figure 8] FIG. 8(A) is a partial cross-sectional side view of the expansion device and the like, and FIG. 8(B) is a diagram showing the tape expansion step. [Figure 9] FIG. 9(A) is a diagram showing how laser processing is performed at a first processing feed rate, FIG. 9(B) is a diagram showing how laser processing is performed at a second processing feed rate, and FIG. 9(C) is a diagram showing how the laser beam penetrates the wafer. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment according to one aspect of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a flow diagram of a processing method for a wafer 11 (see Fig. 2(A)) according to a first embodiment. In the first embodiment, a tape-attaching step S10, a holding step S20, an appropriate condition selection step S32, a division step S34, and a re-irradiation step S40 are sequentially performed.

[0020] In this specification, the appropriate condition selection step S32 and the division step S34 may be collectively referred to as the laser beam irradiation step S30. First, the wafer 11 to be subjected to laser processing will be described.

[0021] 2(A) is a perspective view of the wafer 11. The wafer 11 is formed mainly of a semiconductor material such as silicon and has a disk shape. On the front surface 11a side of the wafer 11, a circuit layer (not shown) including an insulating layer formed of a low-dielectric-constant interlayer insulating film material (so-called low-k material) and a metallization layer is formed.

[0022] A plurality of linear dividing lines (streets) 13 are set in a grid pattern on the front surface 11a of the wafer 11. A device 15 such as an IC (Integrated Circuit) is formed in each of a plurality of rectangular regions defined by the dividing lines 13.

[0023] The back surface 11b of the wafer 11, which is located opposite the front surface 11a, has already been ground, and the wafer 11 has been thinned to a substantially uniform predetermined finished thickness (for example, a predetermined thickness of 50 μm or less).

[0024] When laser processing is performed on the wafer 11, first, a wafer unit 21 is formed (see FIG. 2(B)). FIG. 2(B) is a perspective view of the wafer unit 21.

[0025] When creating the wafer unit 21, the central portion of a dicing tape (tape) 17 having a diameter larger than that of the wafer 11 is attached to the back surface 11b, and one surface of a ring-shaped frame 19 having an opening with a diameter larger than that of the wafer 11 is attached to the outer periphery of the dicing tape 17 (tape attachment step S10).

[0026] The dicing tape 17 is substantially transparent and has a laminated structure of a base layer formed of a resin such as polyolefin or polyester, and an adhesive layer (glue layer) made of an uncured adhesive resin such as a thermosetting resin or an ultraviolet-curing resin.

[0027] However, the dicing tape 17 may have only the base layer. In this case, for example, the dicing tape 17 is thermocompression bonded to the wafer 11 and the frame 19 to form the wafer unit 21.

[0028] Next, the laser processing device 2 used in the holding step S20 after the tape applying step S10 will be described with reference to FIGS.

[0029] Fig. 3 is a perspective view of the laser processing device 2. Note that the laser beam irradiation unit 30 and the light detection mechanism 34 (see Fig. 4) are omitted from Fig. 3. Fig. 4 is a side view of the chuck table 22 and the like. Note that hatching is omitted from Fig. 4 for the sake of convenience.

[0030] 3 and 4, the X-axis, Y-axis, and Z-axis directions are perpendicular to one another. For example, the X-Y plane corresponds to the horizontal plane, and the Z-axis direction corresponds to the vertical direction. Each component of the laser processing device 2 is supported on the base 4.

[0031] A pair of guide rails 6 are fixed along the Y-axis direction on the upper surface of the base 4. A Y-axis direction moving plate 8 is slidably attached to the pair of guide rails 6. A nut portion (not shown) is provided on the lower surface side of the Y-axis direction moving plate 8.

[0032] A screw shaft 10 arranged substantially parallel to the Y-axis direction is rotatably connected to the nut portion. The nut portion and the screw shaft 10 form a ball screw. A Y-axis direction drive source 12 such as a pulse motor is connected to one end of the screw shaft 10.

[0033] When the screw shaft 10 is rotated by the Y-axis direction drive source 12, the Y-axis direction moving plate 8 moves along the guide rail 6. The guide rail 6, the nut portion, the screw shaft 10, the Y-axis direction drive source 12, etc. constitute a Y-axis direction moving mechanism.

[0034] A pair of guide rails 14 are fixed along the X-axis direction to the upper surface of the Y-axis direction moving plate 8. A moving table 16 is slidably attached to the pair of guide rails 14. A nut portion (not shown) is provided on the lower surface side of the bottom plate 16c of the moving table 16.

[0035] A screw shaft 18, which is disposed substantially parallel to the X-axis direction, is rotatably connected to the nut portion. The nut portion and the screw shaft 18 form a ball screw. An X-axis direction drive source 20, such as a pulse motor, is connected to one end of the screw shaft 18.

[0036] When the screw shaft 18 is rotated by the X-axis direction drive source 20, the moving table 16 moves along the guide rail 14. The guide rail 14, the nut portion, the screw shaft 18, the X-axis direction drive source 20, etc. constitute an X-axis direction moving mechanism.

[0037] The movable table 16 has a rectangular top plate 16a, side plates 16b, and bottom plate 16c. An upper end of the side plate 16b is connected to one end of the top plate 16a in the X-axis direction, and an lower end of the side plate 16b is connected to one end of the bottom plate 16c in the X-axis direction.

[0038] The top plate 16a, the side plates 16b, and the bottom plate 16c form a space 16d that is open at the other end in the X-axis direction and at both ends in the Y-axis direction. A chuck table 22 is provided on the top plate 16a.

[0039] The chuck table 22 has a disk-shaped holding plate 24. The holding plate 24 is made of a transparent material such as quartz glass and is transparent to a laser beam L (see FIG. 4) described later. The holding plate 24 includes one substantially flat surface 24a and another surface 24b (see FIG. 4) located opposite the one surface 24a.

[0040] A plurality of gas flow paths are formed inside the holding plate 24. For example, when the holding plate 24 is viewed from above, a first suction path 24c1 and a second suction path 24c2, each of which is linear, are formed so as to intersect at right angles at the center 24c3 of one surface 24a.

[0041] The first suction path 24c1 and the second suction path 24c2 are formed at the same depth in the holding plate 24 and are connected to each other directly below the center 24c3. A plurality of openings 24d are formed on the outer periphery of the one surface 24a at approximately equal intervals in the circumferential direction of the one surface 24a.

[0042] The plurality of openings 24d are formed at both ends of the first suction path 24c1, at both ends of the second suction path 24c2, etc. Each opening 24d is formed from one surface 24a to a predetermined depth that does not reach the other surface 24b, and is connected to each other by a peripheral suction path 24e that is formed at a predetermined depth on the outer periphery of the holding plate 24.

[0043] A suction passage (not shown) is formed radially outward of the openings 24d of the holding plate 24, and a suction source (not shown) such as an ejector is connected to this suction passage. When the suction source is operated to generate negative pressure, the negative pressure is transmitted to each opening 24d. As a result, the surface 24a functions as a holding surface.

[0044] A circular frame suction plate (not shown) is provided on the outer periphery of the chuck table 22, with multiple suction ports formed discretely along the circumferential direction of the chuck table 22, and the frame 19 is held by suction on the frame suction plate.

[0045] However, a portion of the incident light is scattered or reflected in the flow paths of the holding plate 24, such as the first suction path 24c1, the second suction path 24c2, the opening 24d, and the outer peripheral suction path 24e. Therefore, the flow paths of the holding plate 24 are not completely transparent to the laser beam L, and may be translucent or opaque.

[0046] However, a predetermined region excluding these flow paths is transparent from one surface 24a to the other surface 24b. For example, the region divided into four by the first suction path 24c1 and the second suction path 24c2 and positioned inside the outer peripheral suction path 24e in the radial direction of the holding plate 24 is transparent to the laser beam L from one surface 24a to the other surface 24b.

[0047] A cylindrical frame 26 made of a metal such as stainless steel is provided on the outer periphery of the holding plate 24. The holding plate 24 is fixed to this frame 26 so as to close a through opening (not shown) of the frame 26.

[0048] The frame 26 is rotatably supported on the top plate 16a of the moving table 16. The top plate 16a is formed with a through-opening (not shown) having substantially the same diameter as the holding plate 24, and light irradiated onto the one surface 24a passes through the through-opening and enters the space 16d of the moving table 16.

[0049] The cylindrical side surface of the frame body 26 functions as a pulley portion 26a. When the frame body 26 is supported by the top plate 16a, the pulley portion 26a is located above the top plate 16a. A rotary drive source 28 such as a motor is provided on the side plate 16b. A pulley 28a is provided on the rotation shaft of the rotary drive source 28.

[0050] An endless belt 28b is wound around the pulley portion 26a and the pulley 28a. When the pulley 28a is rotated by the rotary drive source 28, the force transmitted via the endless belt 28b causes the frame 26 to rotate around a rotation axis that is generally parallel to the Z-axis direction.

[0051] In this way, by controlling the rotation of the pulley 28a, the chuck table 22 can be rotated by any angle around a rotation axis that is approximately parallel to the Z-axis direction. As shown in Figure 4, a laser beam irradiation unit 30 is disposed above the surface 24a.

[0052] The laser beam irradiation unit 30 has a laser oscillator (not shown) having a laser medium made of Nd:YAG, Nd:YVO4, etc. The pulsed laser beam L is emitted from the laser oscillator and then converted by a wavelength conversion unit (not shown) so that the main peak has a predetermined wavelength.

[0053] The condenser 32 has a condensing lens (not shown). The condenser 32 irradiates the laser beam L toward the one surface 24a so as to condense the laser beam L onto the wafer 11 held by suction on the one surface 24a. The main peak wavelength of the laser beam L is a wavelength (e.g., 355 nm) that is absorbed by the wafer 11.

[0054] A light detection mechanism 34 is provided in the space 16d. The light detection mechanism 34 has an arm 36 whose longitudinal portion is disposed along the X-axis direction. A light detection unit 38 is provided at the tip of the arm 36.

[0055] The light detection unit 38 includes, for example, a power sensor or a power meter each having a photodiode as a light receiving element, but may also be a camera with a neutral density filter that includes a light receiving element such as a photodiode and a neutral density filter that reduces the amount of light received by the light receiving element.

[0056] The optical detection unit 38 is disposed so as to be located directly below the condenser 32, with the chuck table 22, wafer 11, etc. sandwiched between the optical detection unit 38 and the condenser 32. Therefore, the laser beam L that penetrates the wafer 11 and is transmitted through the dicing tape 17 and the holding plate 24 is incident on the optical detection unit 38.

[0057] Each component of the laser processing device 2 is controlled by a control unit 40. The control unit 40 is configured by a computer including, for example, a processor (processing device) represented by a CPU (Central Processing Unit), a main storage device such as a DRAM (Dynamic Random Access Memory), and an auxiliary storage device such as a flash memory.

[0058] The auxiliary storage device stores software including a predetermined program. The functions of the control unit 40 are realized by operating the processing device and the like in accordance with this software.

[0059] 1 are performed using this laser processing device 2. First, in the holding step S20, the dicing tape 17 of the wafer unit 21 is placed on one surface 24a of the chuck table 22.

[0060] Next, negative pressure is transmitted to the opening 24d to suction-hold the back surface 11b of the wafer 11 via the dicing tape 17 (holding step S20). After the holding step S20, an image of the front surface 11a is acquired using an imaging unit (not shown), and alignment is performed.

[0061] Then, the orientation of the chuck table 22 is adjusted using the rotary drive source 28 or the like so that the planned dividing lines 13 along one direction become approximately parallel to the X-axis direction. Thereafter, a laser beam irradiation step S30 is performed in which a laser beam L is irradiated along each planned dividing line 13.

[0062] Processing conditions are usually determined in advance depending on the thickness, type, etc. of the wafer 11. However, if the determined processing conditions are used as they are, a relatively thick wafer 11 may have unpenetrated areas across the entire wafer 11, where the wafer 11 has not been completely cut.

[0063] Furthermore, in the case of a relatively thin wafer 11, after the wafer 11 is cut, the dicing tape 17 may melt, and the melted residue may stick to one surface 24a of the holding plate 24. Therefore, in the laser beam irradiation step S30, first, laser processing is performed on at least one planned division line 13, thereby performing the appropriate condition selection step S32.

[0064] In the first embodiment, in the optimum condition selection step S32, for example, the optimum condition selection step S32 is first performed on the first wafer 11 out of 25 wafers 11 processed in one lot, thereby selecting processing conditions that are more optimum than the predetermined processing conditions.

[0065] It should be noted that instead of selecting appropriate processing conditions using one of the multiple wafers 11 as in the first embodiment, the appropriate condition selection step S32 may be performed for each wafer 11 to be subjected to laser processing.

[0066] If the optimum condition selection step S32 is performed for each wafer 11, the processing time will be longer than if the optimum condition selection step S32 is performed for only one wafer 11 out of multiple wafers 11, but the processing conditions can be optimized for each wafer 11.

[0067] In the first embodiment, in the appropriate condition selection step S32, the processing feed rate is kept constant, and the average output of the laser beam L is gradually increased. As a result, the processing conditions are gradually changed from processing conditions that do not penetrate the wafer 11 to processing conditions that penetrate the wafer 11 until the light detection unit 38 detects the laser beam L. The processing conditions are, for example, as follows:

[0068] Laser medium: Nd:YAG Wavelength: 355nm Average output: 5.0 W (at the start of irradiation on wafer 11) Average power change: 0.1W increase every 0.1s (i.e., 1.0W / s) Repetition frequency: 20kHz Focused spot diameter: 5.0 μm Processing feed rate: 100 mm / sec (constant value)

[0069] 5(A) to 5(C) show how the appropriate condition selection step S32 is performed when machining along one planned division line 13. The direction of the arrow indicates the direction in which the chuck table 22 is moved, and the length of the arrow corresponds to the magnitude of the machining feed speed.

[0070] In the appropriate condition selection step S32, first, the focal point P of the laser beam L is positioned at one end in the X-axis direction of one of the planned division lines 13. Next, the moving table 16 is processed and fed to move the focal point P of the laser beam L and the chuck table 22 relatively in the X-axis direction (predetermined direction).

[0071] As a result, while changing the processing position, the laser beam L is irradiated along the one planned dividing line 13 to perform ablation processing. Fig. 5(A) is a diagram showing how laser processing is performed with a first average output (about 5.0 W).

[0072] 5(B) is a diagram showing the state where laser processing is performed at the second average output (approximately 5.1 W). The depth of the processed groove 11c formed in the wafer 11 by ablation processing gradually increases as the average output increases.

[0073] 5(C) is a diagram showing a state in which laser processing is performed with a third average output (approximately 5.3 W) and the laser beam L penetrates the wafer 11. When the laser beam L penetrates the wafer 11 and the light detection unit 38 detects the laser beam L, the control unit 40 stores the XY coordinates of the condenser 32 at that time and fixes the average output of the laser beam L.

[0074] Then, at the fixed average output, a processed groove 11c is formed up to the other end in the X-axis direction of one planned division line 13. The control unit 40 selects and stores the average output at the timing when the light detection unit 38 detects the laser beam L as the average output of the appropriate processing conditions.

[0075] In this specification, it is expressed that the light detection unit 38 has detected the laser beam L when the power (W) or amount of light of the laser beam L received by the light detection unit 38 exceeds a predetermined threshold.

[0076] The predetermined threshold value is appropriately set for the purpose of not selecting processing conditions at the time when the laser beam L accidentally penetrates a locally thin area of ​​the wafer 11 due to variations in thickness across the surface.

[0077] In addition, the appropriate condition selection step S32 may be performed for a relatively short planned dividing line 13 located on the outer periphery of the wafer 11 among the multiple planned dividing lines 13 along one direction, or for a relatively long planned dividing line 13 located on the central side of the wafer 11.

[0078] It is preferable that the appropriate condition selection step S32 is performed for one planned division line 13, but if the light detection unit 38 does not detect the laser beam L for the first planned division line 13, the appropriate condition selection step S32 may be performed for a second planned division line 13 that is different from the first.

[0079] In this way, the optimum condition selection step S32 may be performed for two or more planned division lines 13, but in the first embodiment, the optimum condition selection step S32 is performed for one planned division line 13.

[0080] After forming the processed groove 11c up to the other end in the X-axis direction in one of the division lines 13 for which the appropriate condition selection step S32 has been performed, the moving table 16 is indexed and fed by a predetermined length. Next, laser processing is performed under the selected processing conditions on another division line 13 adjacent to this division line 13 in the Y-axis direction.

[0081] By performing laser processing under the selected processing conditions, appropriate processing conditions can be selected according to the thickness of the wafer 11, thereby preventing the occurrence of non-penetrated areas across the entire wafer 11 and the sticking of molten residue to the chuck table 22.

[0082] After laser processing is performed under the selected processing conditions along all the planned dividing lines 13 that are along one direction different from the planned dividing lines 13 laser processed in the appropriate condition selection step S32, the chuck table 22 is rotated by 90 degrees.

[0083] Then, laser processing is performed under selected processing conditions along each of the dividing lines 13 extending in another direction perpendicular to the one direction, thereby dividing the wafer 11 into a plurality of device chips 23 (see FIG. 6) (dividing step S34).

[0084] 6 is a perspective view of the device chips 23 etc. after the dividing step S34. For convenience, one device chip 23 is shown enlarged in FIG.

[0085] In the first embodiment, after the dividing step S34, the laser beam L is again irradiated onto the non-penetrating region 11d (see FIG. 5(C)) of the intended dividing line 13 formed in the appropriate condition selecting step S32 (re-irradiation step S40).

[0086] The non-penetrated area 11d is formed on one of the planned dividing lines 13 for which the appropriate condition selection step S32 has been performed, between the time when the laser beam L starts to be irradiated and the time when the optical detection unit 38 detects the laser beam L that has penetrated the wafer 11.

[0087] In the non-penetrated region 11d, 90% to 98% of the wafer 11 has already been removed in the thickness direction, so in the re-irradiation step S40, the average output is adjusted so that it is high enough to enable ablation processing, but low enough not to melt the dicing tape 17.

[0088] In the first embodiment, since appropriate processing conditions can be selected according to the thickness of the wafer 11, the occurrence of non-penetrated regions 11d in the entire wafer 11 and the adhesion of molten residue to the chuck table 22 can be prevented.

[0089] Furthermore, by performing laser processing on the remaining 24 wafers 11 under the selected processing conditions, it is possible to prevent the occurrence of non-penetrated areas 11d throughout the entire wafer 11 and the adhesion of molten residue to one surface 24a for the remaining 24 wafers 11 as well.

[0090] Next, modifications of the first embodiment will be described. Fig. 7(A) is a flow diagram of a method for processing the wafer 11 according to a first modification, and Fig. 7(B) is a flow diagram of a method for processing the wafer 11 according to a second modification.

[0091] In the first modified example, the tape expanding step S50 is performed after the re-irradiation step S40 using the expanding device 42 (see FIG. 8(A)). In contrast, in the second modified example, the tape expanding step S50 is performed after the division step S34 without going through the re-irradiation step S40.

[0092] 8(A) is a partial cross-sectional side view of the expansion device 42 etc. The expansion device 42 has a cylindrical drum 44 having a diameter larger than that of the wafer 11. A plurality of rollers 46 are provided at the upper end of the drum 44 at approximately equal intervals along the circumferential direction of the drum 44.

[0093] An annular frame support base 48 is provided on the outer side of the drum 44 in the radial direction of the drum 44. A plurality of clamps 50 are provided on the upper surface side of the frame support base 48, each clamping the frame 19 of the wafer unit 21 placed on the frame support base 48.

[0094] The frame support base 48 is supported by a plurality of legs 52 arranged at approximately equal intervals along the circumferential direction of the frame support base 48. Each leg 52 can be raised and lowered by a lifting mechanism such as an air cylinder.

[0095] In the tape expansion step S50, as shown in Figure 8(A), the upper end of the drum 44 and the upper surface of the frame support base 48 are brought to approximately the same height position, and the wafer unit 21 after the laser beam irradiation step S30 is placed on the drum 44 and the frame support base 48.

[0096] Next, when the elevators are operated to lower the legs 52, the frame support base 48 is lowered relative to the drum 44. As a result, as shown in Fig. 8(B), the dicing tape 17 is expanded in the radial direction, and the spacing between the device chips 23 is increased. Fig. 8(B) is a diagram showing a tape expanding step S50.

[0097] If the non-penetrated region 11d is not completely penetrated even after the re-irradiation step S40 (first variant of Figure 7(A)), or if the re-irradiation step S40 is not performed after the division step S34 (second variant of Figure 7(B)), the non-penetrated region 11d can be divided in the tape expansion step S50.

[0098] In the tape expansion step S50, the undivided region in the wafer 11 that has undergone the division step S34 and / or the re-irradiation step S40 is divided, thereby dividing the wafer 11 into a plurality of device chips 23. In addition, by widening the spaces between the device chips 23, it becomes easier to pick up the device chips 23 compared to when the spaces are not widened.

[0099] Next, a second embodiment will be described with reference to Figures 9(A) to 9(C). The second embodiment differs from the first embodiment in the appropriate condition selection step S32. In the appropriate condition selection step S32 of the second embodiment, the processing feed rate is gradually reduced as the focal point P and the chuck table 22 are moved relatively along the X-axis direction.

[0100] The lower the processing feed rate, the higher the ratio of the overlapping area of ​​the focused spots of the pulsed laser beam L (also referred to as the overlap rate). Therefore, the processing conditions can be changed from a processing condition in which the laser beam L does not penetrate the wafer 11 to a processing condition in which the laser beam L penetrates the wafer 11. The processing conditions are, for example, as follows:

[0101] Laser medium: Nd:YAG Wavelength: 355nm Average power output: 5.0W (constant value) Repetition frequency: 20kHz Focused spot diameter: 5.0 μm Processing feed speed: 100 mm / sec (when irradiation of wafer 11 begins) Change in machining feed rate: Decrease by 0.2 mm / s every 0.1 seconds (i.e., -2.0 mm / s)

[0102] FIG. 9(A) is a diagram showing how laser processing is performed at a first processing feed rate (approximately 100 mm / s), and FIG. 9(B) is a diagram showing how laser processing is performed at a second processing feed rate (approximately 98 mm / s).

[0103] 9(C) is a diagram showing the state in which laser processing is performed at a third processing feed rate (approximately 96 mm / s) and the laser beam L penetrates the wafer 11. In FIGS. 9(A) to 9(C), the direction of the arrow indicates the direction in which the chuck table 22 is moved, and the length of the arrow corresponds to the magnitude of the processing feed rate.

[0104] When the laser beam L penetrates the wafer 11 and the light detection unit 38 detects the laser beam L, the control unit 40 stores the XY coordinates of the condenser 32 at that time and fixes the processing feed rate.

[0105] Then, at the fixed processing feed speed, a processing groove 11c is formed up to the other end in the X-axis direction of one planned division line 13. The control unit 40 selects and stores the processing feed speed at the timing when the light detection unit 38 detects the laser beam L as the processing feed speed for the appropriate processing conditions.

[0106] After the processing groove 11c is formed up to the other end in the X-axis direction in one of the planned division lines 13 for which the appropriate condition selection step S32 has been performed, the moving table 16 is indexed and fed by a predetermined length.

[0107] Next, laser processing is performed under the selected processing conditions on other planned dividing lines 13 adjacent to this planned dividing line 13 in the Y-axis direction. Similarly, the other planned dividing lines 13 are processed at the processing feed rate selected in the appropriate condition selection step S32.

[0108] In the second embodiment as well, since appropriate processing conditions can be selected according to the thickness of the wafer 11, the occurrence of non-penetrated regions 11d across the entire wafer 11 and the adhesion of molten residue to the chuck table 22 can be prevented.

[0109] In the second embodiment, too, the optimum condition selection step S32 may be performed first on the first wafer 11 out of the 25 wafers 11 processed in one lot, or the optimum condition selection step S32 may be performed on each wafer 11 to be laser processed.

[0110] In addition, in the second embodiment, the tape expansion step S50 may be performed as described in the first and second modified examples. In addition, the structures, methods, etc. according to the above-described embodiments can be appropriately modified and implemented without departing from the scope of the object of the present invention.

[0111] For example, if the thickness of the wafer 11 is 50 μm or less, the wafer 11 can be cut by irradiating the above-mentioned laser beam L once from one end to the other end of one planned division line 13 (i.e., by passing the focal point P once (i.e., one pass)).

[0112] However, when the thickness of the wafer 11 exceeds 50 μm, laser processing may be required in which the focal point P passes along one planned dividing line 13 two or more times (i.e., two or more passes) in order to cut the wafer 11. Even when two or more passes of laser processing are required, the above-described embodiments and modifications can be applied.

[0113] Specifically, when cutting the wafer 11 by laser processing in N passes (where N is a natural number greater than or equal to 2), laser processing is performed along each planned division line 13 from the first pass to the (N-1)th pass under processing conditions that are predetermined according to the thickness, type, etc. of the wafer 11.

[0114] Thereafter, when performing the Nth laser processing pass, the above-described optimum condition selection step S32 is performed for at least one planned division line 13. For example, the optimum condition selection step S32 is performed for one planned division line 13. As a result, more optimum processing conditions than the predetermined processing conditions are selected.

[0115] Thereafter, the movable table 16 is indexed and fed by a predetermined length, and the Nth pass of laser processing is performed on another planned division line 13 adjacent to the one planned division line 13 that has already been processed in the Y-axis direction under the processing conditions selected in the appropriate condition selection step S32.

[0116] Similarly, after the Nth pass of laser processing is performed along all remaining planned division lines 13 along one direction under the processing conditions selected in the appropriate condition selection step S32, the chuck table 22 is rotated 90 degrees.

[0117] Then, an Nth pass of laser processing is performed along each of the planned dividing lines 13 along the other directions perpendicular to the first direction, under the processing conditions selected in the appropriate condition selection step S32. This allows the wafer 11 to be divided into a plurality of device chips 23, and also prevents the occurrence of unpenetrated areas across the entire wafer 11 and the adhesion of molten residue to the chuck table 22.

[0118] Incidentally, for example, after the tape application step S10 and before the holding step S20, a protective film (not shown) containing a water-soluble resin may be uniformly formed on the surface 11a, and then the laser beam irradiation step S30 may be performed.

[0119] By performing the ablation process with the protective film formed, it is possible to prevent the molten material (debris) of the wafer 11 from adhering to the surface 11a, and after the ablation process, the protective film can be washed and removed together with the debris. [Explanation of symbols]

[0120] 2: Laser processing device, 4: Base, 6: Guide rail, 8: Y-axis direction moving plate 10: screw shaft, 12: Y-axis direction drive source, 14: guide rail 11: wafer, 11a: front surface, 11b: back surface, 11c: processed groove, 11d: unpenetrated area 13: Dividing line, 15: Device, 17: Dicing tape, 19: Frame 16: moving table, 16a: top plate, 16b: side plate, 16c: bottom plate, 16d: space 18: screw shaft, 20: X-axis direction drive source, 22: chuck table 21: wafer unit, 23: device chip 24: Holding plate, 24a: One side, 24b: Other side 24c1: 1st suction path, 24c2: 2nd suction path, 24c3: Center, 24d: Opening 24e: outer periphery suction passage, 26: frame body, 26a: pulley portion 28: Rotation drive source, 28a: Pulley, 28b: Endless belt 30: laser beam irradiation unit, 32: condenser, 34: light detection mechanism, 36: arm 38: Optical detection unit, 40: Control unit, 42: Expansion device 44: Drum, 46: Roller, 48: Frame support, 50: Clamp, 52: Leg L: laser beam, P: focal point S10: Tape application step, S20: Holding step S30: Laser beam irradiation step S32: Select appropriate conditions step, S34: Division step S40: Re-irradiation step, S50: Tape expansion step

Claims

1. A wafer processing method comprising: dividing a wafer into a plurality of device chips by irradiating a pulsed laser beam having a wavelength that is absorbed by the wafer along each of a plurality of division lines defined on a surface of the wafer in a grid pattern, the plurality of division lines defining a plurality of regions in which devices are formed; a tape attaching step of attaching a central portion of a tape to a back surface of the wafer opposite to the front surface, and attaching one surface of an annular frame to an outer periphery of the tape; a holding step of suction-holding the wafer via the tape on a chuck table that is transparent to the laser beam after the tape-attaching step; a laser beam irradiation step of irradiating the laser beam along the plurality of planned dividing lines while changing a processing position by relatively moving a focal point of the laser beam and the chuck table in a predetermined direction after the holding step, The laser beam irradiation step includes: an appropriate condition selection step of selecting, when processing along at least one planned dividing line, the processing conditions at the time when the optical detection unit detects the laser beam that has penetrated the wafer by gradually changing the processing conditions from the processing conditions that do not penetrate the wafer to the processing conditions that penetrate the wafer until the optical detection unit, which is arranged on the opposite side of the wafer with respect to the chuck table, detects the laser beam that has penetrated the wafer, as the appropriate processing conditions; a dividing step of dividing the wafer into the plurality of device chips by irradiating the laser beam along other planned dividing lines different from the at least one planned dividing line using the processing conditions selected in the appropriate condition selecting step; A wafer processing method comprising:

2. The appropriate condition selection step includes:

2. The wafer processing method according to claim 1, further comprising changing the processing conditions from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer by gradually increasing the average output as the relative movement in the predetermined direction occurs.

3. The appropriate condition selection step includes:

2. The wafer processing method according to claim 1, further comprising changing the processing conditions from processing conditions that do not penetrate the wafer to processing conditions that penetrate the wafer by gradually slowing down the processing feed speed during the relative movement in the predetermined direction.

4. 4. A wafer processing method according to claim 1, further comprising a re-irradiation step of irradiating the laser beam again onto an unpenetrated area formed before the optical detection unit detects the laser beam that has penetrated the wafer in the appropriate condition selection step.

5. 5. The wafer processing method according to claim 1, further comprising a tape expanding step of expanding the tape after the laser beam irradiation step.

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