Workpiece processing method

Abstract

To provide a workpiece processing method for suppressing the occurrence of warping of a workpiece due to grinding from a back surface side.SOLUTION: A processing method of a wafer 11 which is a plate-shaped workpiece in which a plurality of division lines are set, comprises: a step of holding the wafer on a chuck table; a step of irradiating a laser beam along division lines in a state in which a focal point of the laser beam having a wavelength having transparency with respect to the wafer held on the chuck table is located closer to a side of a first surface 11a than a center 11c in a thickness direction of the wafer, thereby forming a first modified layer 17 which is a starting point of division; a step of forming a second modified layer 19 that suppresses warping of the wafer by irradiating the wafer in a state in which a converging point of the laser beam of a wavelength having transparency with respect to the wafer is positioned at a side closer to the second surface 11b than the center in the thickness direction of the wafer; and a step of forming a third modified layer 21 for maintaining suppression of warping of the wafer in the center region in the thickness direction of the wafer.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a technique for grinding a workpiece from its back surface, and more particularly to a technique for suppressing the occurrence of warping of the workpiece due to grinding. [Background technology]

[0002] Conventionally, a silicon wafer (hereinafter referred to as a workpiece) having a plurality of devices such as ICs and LSIs formed on its surface has its back surface ground to a predetermined thickness, and then is divided into individual devices, for example, by a laser processing device, and used in electrical equipment such as mobile phones and personal computers.

[0003] One known processing method for dividing a workpiece into individual devices using a laser processing device involves irradiating the workpiece with a laser beam that is transparent to wafers along a planned division line closer to the surface than the center in the thickness direction of the workpiece, forming a modified layer inside the workpiece in the direction of travel of the laser beam to form a modified region that serves as the starting point for division, and then applying an external force to the workpiece to divide it into individual devices (see, for example, Patent Document 1).

[0004] It is also known that after forming a modified layer, the back side of the workpiece is ground by grinding to a predetermined thickness, and then the workpiece is divided (see, for example, Patent Document 2). Patent Document 2 discloses a method for solving the problem that when a modified layer is formed along a planned division line closer to the front side than the center in the thickness direction of the workpiece, the front side expands, causing the workpiece to warp. Specifically, this method involves forming a modified layer on the front side of the workpiece, and then forming a modified layer on the back side of the workpiece. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-179302 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-099522 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even in the method disclosed in Patent Document 2, the modified layer on the back side is removed by grinding from the back side, which may cause the workpiece to warp again after grinding.

[0007] In view of the above problems, an object of the present invention is to propose a new technique for suppressing the occurrence of warpage in a workpiece due to grinding from the back surface side. [Means for solving the problem]

[0008] The problem to be solved by the present invention is as described above, and the means for solving this problem will now be described.

[0009] According to one aspect of the present invention, there is provided a method for processing a plate-shaped workpiece having a plurality of planned dividing lines set thereon, the method comprising: a holding step of holding the workpiece on a chuck table; a first modified layer forming step of irradiating a laser beam having a wavelength that is transparent to the workpiece held on the chuck table with the focal point positioned closer to the first surface than the center of the thickness of the workpiece, with the laser beam irradiated along the planned dividing lines to form a first modified layer that serves as a starting point for dividing; a second modified layer forming step of irradiating a laser beam having a wavelength that is transparent to the workpiece held on the chuck table with the focal point positioned closer to the second surface than the center of the thickness of the workpiece, with the laser beam irradiated along the planned dividing lines to form a second modified layer that suppresses warping of the workpiece; and a third modified layer forming step of forming a third modified layer in a central region of the workpiece in the thickness direction to maintain suppression of warping of the workpiece.

[0010] Furthermore, according to one aspect of the present invention, the first modified layer and the third modified layer are formed at positions offset from each other, and the second modified layer and the third modified layer are formed at positions offset from each other.

[0011] Furthermore, according to one aspect of the present invention, the first modified layer formation step and the second modified layer formation step are performed alternately as a set, and the third modified layer formation step is performed before or after the second modified layer formation step in one set.

[0012] Furthermore, according to one aspect of the present invention, the third modified layer and the cracks generated thereby overlap the first modified layer and the cracks generated thereby in the thickness direction of the workpiece, and the third modified layer and the cracks generated thereby overlap the second modified layer and the cracks generated thereby in the thickness direction of the workpiece.

[0013] According to one aspect of the present invention, after the first to third modified layer forming steps are completed, The method further comprises a backside grinding step in which grinding is performed from the second surface side of the workpiece to thin it. [Effects of the Invention]

[0014] The present invention provides the following effects. In other words, according to one aspect of the present invention, when a grinding process is performed, even if the first modified layer and crack are removed during the process of thinning the wafer by the grinding process, a third modified layer and crack can be present on the opposite side of the workpiece thickness direction as a counter (opposing element) to the first modified layer and crack during the process of thinning the workpiece, and warping can be suppressed until the grinding process is completed. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a perspective view of the front surface side of a wafer that is a workpiece. [Figure 2]10 is an exploded perspective view showing how the back surface of the wafer is attached to an adhesive tape whose outer periphery is attached to an annular frame. FIG. [Figure 3] FIG. 10 is a partial cross-sectional side view showing a holding step. [Figure 4] FIG. 2 is a perspective view showing a first modified layer forming step. [Figure 5] FIG. 2 is a block diagram of a laser beam generating unit. [Figure 6] 10 is a flowchart illustrating an example of a processing method. [Figure 7] FIG. 4 is a partial cross-sectional side view showing a first modified layer forming step. [Figure 8] FIG. 10 is a partial cross-sectional side view showing a second modified layer forming step. [Figure 9] 3 is a cross-sectional view of a wafer showing how first and second modified layers are formed in sequence. FIG. [Figure 10] FIG. 2 is a cross-sectional view of a wafer on which all first and second modified layers have been formed. [Figure 11] FIG. 10 is a cross-sectional view of a wafer on which a third modified layer is formed. [Figure 12] FIG. 10 is a cross-sectional view of a wafer on which another type of third modified layer is formed. [Figure 13] FIG. [Figure 14] 1A to 1C are diagrams illustrating the thinning process by grinding. DETAILED DESCRIPTION OF THE INVENTION

[0016] 1 is a perspective view of a semiconductor wafer according to an embodiment of the present invention, showing a semiconductor wafer according to the present invention;

[0017] 1, the planned dividing lines 13 are set in a first direction and a second direction that are perpendicular to each other within the wafer surface (horizontal plane). In the following description, the directions that are perpendicular to each other within the horizontal plane are referred to as the X-axis direction and the Y-axis direction, and the vertical direction that is perpendicular to the X-axis direction and the Y-axis direction is referred to as the Z-axis direction. The Z-axis direction corresponds to the thickness direction of the wafer.

[0018] The plate-like object to be processed by the processing method of the present invention is not limited to the semiconductor wafer 11 shown in FIG. 1, but includes other wafers such as optical device wafers, wafers with no pattern formed on the surface, glass substrates, and other plate-like objects.

[0019] 2, the surface 11a of the wafer 11 is adhered to an adhesive tape T. An annular frame F is also adhered to the adhesive tape T, and the wafer 11 is disposed on the annular frame F via the adhesive tape T, thereby forming a wafer unit in which the wafer 11 and the annular frame F are integrated.

[0020] 3 is a diagram showing an example of the chuck table 10 of the laser processing device used in the processing method of the present invention. The table is connected to a suction source (not shown), and is configured to hold the wafer 11 by suction with the adhesive tape T sandwiched between them, with the annular frame F clamped by the clamps 12.

[0021] Figure 4 shows an example of a laser processing apparatus used in the processing method of the present invention. In Figure 4, 14 is a laser beam irradiation unit of the laser processing apparatus, and the laser beam irradiation unit 14 has a cylindrical casing 18 arranged substantially horizontally. A laser beam generating unit 20 as shown in Figure 5 is housed inside the casing 18.

[0022] The laser beam generating unit 20 includes a pulse laser oscillator 24 such as a YAG laser oscillator or a YVO4 laser oscillator, a repetition frequency setting means 26, a pulse width adjusting means 28, and a power adjusting means 30.

[0023] The laser beam irradiation unit 14 is configured to include a condenser (laser head) 22 attached to the tip of a casing 18 shown in FIG. 4, and a laser beam generating unit 20 shown in FIG.

[0024] 4, reference numeral 16 denotes an imaging unit that images the wafer 11 held on the chuck table 10, and includes an imaging element such as a microscope and a normal CCD that captures images using visible light. The imaging unit 16 further includes an infrared irradiation means that irradiates the wafer 11 with infrared rays, and an infrared imaging element (infrared CCD) that outputs an electrical signal corresponding to the infrared rays, and the captured image signal is sent to a controller of the laser processing device (not shown).

[0025] Next, an example of the processing method of the present invention will be described for each step constituting the flowchart shown in FIG.

[0026] <Holding step> The holding step is a step in which a wafer 11, which is a workpiece, is held on a chuck table 10, as shown in FIG.

[0027] Specifically, the front surface 11a of the wafer 11 is suction-held by a chuck table 10 of the laser processing device via an adhesive tape T, and the annular frame F is clamped and fixed by a clamp 12.

[0028] <Alignment step> After the holding step is performed, alignment is performed to align the planned dividing lines, the planned dividing lines 13 (FIG. 1), and the light collector 22 (FIG. 4) in the X-axis direction.

[0029] Specifically, as shown in FIG. 4, the infrared imaging element of the imaging unit 16 images the wafer 11 from the back surface 11b side, and the planned dividing line extending in the first direction of the wafer 11 and the collector 22 are aligned in the X-axis direction.

[0030] Next, the laser beam irradiation position is similarly aligned with respect to the dividing lines formed on the semiconductor wafer 11 and extending in a direction perpendicular to the first direction.

[0031] <First modified layer forming step> This is a step in which the focal point of a laser beam having a wavelength that is transparent to the workpiece held on the chuck table is positioned closer to the first surface (surface 11a) than the center of the thickness of the workpiece, and the laser beam is irradiated along the intended dividing line to form a first modified layer that serves as the starting point for dividing.

[0032] Specifically, as shown in FIG. 7, after the alignment step, the chuck table 10 is moved to the laser beam irradiation area where the condenser 22 that irradiates the laser beam is located, and one end of the planned division line 13 extending in the first direction is positioned directly below the condenser 22.

[0033] 7 at a predetermined processing feed rate while irradiating the wafer 11 with a pulsed laser beam from the condenser 22. When the laser beam irradiation position of the condenser 22 reaches the other end of the planned division line 13, irradiation of the pulsed laser beam is stopped and movement of the chuck table 10 is also stopped.

[0034] 9, in the first modified layer forming step, the focal point of the pulsed laser beam is aligned closer to the surface 11a of the wafer 11 than the center in the thickness direction of the wafer 11, thereby forming a first modified layer 17 inside the wafer 11 near the surface. Preferably, the position where this first modified layer 17 is formed is within a range from the surface 11a of the wafer 11 to the finished thickness t1 of the wafer 11 plus a predetermined thickness t2. Here, the finished thickness t1 is, for example, 80 μm, and t2 is 10 to 20 μm.

[0035] The first modified layer 17 refers to a region where the density, refractive index, mechanical strength, and other physical properties are different from those of the surrounding area, and is formed as a melted and re-hardened layer.

[0036] The processing conditions in the first modified layer forming step are set, for example, as follows.

[0037] Light source: LD pumped Q-switched Nd:YVO4 pulsed laser Wavelength: 1064nm Repetition frequency: 90kHz Average power output: 1.4~1.9KW Processing feed rate: 700mm / s

[0038] As shown in FIG. 9, when the first modified layer 17 is formed near the surface 11a of the wafer 11 in the first modified layer forming step, cracks 17a occur from the first modified layer 17 to the surface 11a of the wafer 11.

[0039] <Second modified layer forming step> This is a step in which the focal point of a laser beam having a wavelength that is transparent to the workpiece held on the chuck table is positioned closer to the second surface (back surface 11b) than the center of the thickness of the workpiece, and the laser beam is irradiated along a line parallel to the intended dividing line to form a second modified layer that suppresses warping of the workpiece.

[0040] Specifically, as described above, when the first modified layer formation step is completed for one planned dividing line 13, the chuck table 10 is moved a predetermined distance in the Y-axis direction, and as shown in Figure 8, the focal point of the pulsed laser beam is aligned near the back surface 11b of the wafer 11 and the pulsed laser beam is irradiated while the chuck table 10 is processed and fed in the direction of arrow X2, thereby forming a second modified layer 19 closer to the back surface 11b than the center of the wafer 11 in the thickness direction.

[0041] Here, as shown in the YZ-axis cross section in Fig. 9, the movement amount Ym of the chuck table 10 (Fig. 8) in the Y-axis direction when performing the second modified layer forming step after completing the first modified layer forming step is preferably half the distance between adjacent dividing lines 13, 13. As a result, each second modified layer 19 is disposed at a position equidistant (approximately the middle position) from adjacent first modified layers 17 in the second direction (the Y-axis direction in Fig. 9), and the modified layers 17, 19 are disposed alternately and evenly in the Y-axis direction. Note that this arrangement is not limited to this. For example, the second modified layer 19 may be formed at the same position as the first modified layer 17 in the Y-axis direction, i.e., the second modified layer 19 may also be formed at the position of the dividing line 13. In this case, the movement amount Ym is zero.

[0042] Furthermore, as shown in the YZ-axis cross section of Figure 9, preferably, the distance Z2 from the center 11c in the thickness direction of the wafer 11 to each second modified layer 19 is set to be the same as the distance Z1 from the center 11c in the thickness direction of the wafer 11 to the first modified layer 17, but is not limited to this.

[0043] The processing conditions for the second modified layer forming step may be the same as or different from the processing conditions for the first modified layer forming step described above.

[0044] As shown in FIG. 9, when the second modified layer 19 is formed near the back surface 11b of the wafer 11 in the second modified layer forming step, cracks 19a occur from the second modified layer 19 to the back surface 11b of the wafer 11.

[0045] Then, as shown in Figure 9, by sequentially repeating a set of a first modified layer formation step of forming a first modified layer 17, moving the chuck table 10 (Figure 8) in the Y-axis direction by a movement amount Ym, a second modified layer formation step of forming a second modified layer 19, and moving the chuck table 10 (Figure 8) in the Y-axis direction by a movement amount Ym, first modified layers 17 and second modified layers 19 are formed alternately in the Y-axis direction, and as shown in Figure 10, a first modified layer 17 is formed on the first surface side (front surface 11a side) of the wafer at the position of the planned dividing line 13, and a second modified layer 19 is formed on the second surface side (back surface 11b side) of the wafer at a position offset from the planned dividing line 13.

[0046] Next, the chuck table 10 is rotated 90 degrees, and then the first modified layer formation step, movement of the chuck table 10 (FIG. 8), second modified layer formation step, and movement of the chuck table 10 (FIG. 8) are repeated sequentially in the second direction (the Y-axis direction in FIGS. 7 and 8) to form modified layers 17 and 19.

[0047] In the embodiment described above, after the first modified layer formation step is performed for a certain planned dividing line 13, the second modified layer formation step is performed before the first modified layer formation step is performed for the next planned dividing line 13.Therefore, the warping caused by the modified layer 17 on the front surface 11a side each time each planned dividing line 13 is processed is eliminated by forming the modified layer 19 on the opposite back surface 11b side, thereby reducing problems such as suction retention during grinding and damage to the wafer 11 caused by the warping.

[0048] In the above-described embodiment, as shown in FIG. 7, the first modified layer formation step is performed while the chuck table 10 is processed and fed in the direction of the arrow X1, and then, as shown in FIG. 8, the second modified layer formation step is performed while the chuck table 10 is processed and fed in the direction X2 opposite to the X1 direction. However, the processing and feeding directions for the first modified layer formation step and the second modified layer formation step may be the same.

[0049] <Third modified layer forming step> This is a step of forming a third modified layer in the central region of the wafer 11 in the thickness direction, before or after the second modified layer forming step, in order to keep the wafer 11 from warping.

[0050] 11, in addition to the first modified layer formation step and the second modified layer formation step, a third modified layer 21 is formed. This third modified layer 21 is formed in the central region in the thickness direction of the wafer 11, which is the workpiece.

[0051] In the example shown in Figure 11, the focal point of the third modified layer 21 is set at a position that is approximately the center 11c in the thickness direction of the wafer 11, and is set at a position offset from the first modified layer 17 and the second modified layer 19 in the in-plane direction of the wafer 11 (horizontal direction, Y-axis direction in Figure 11).

[0052] The processing conditions for forming the third modified layer 21 may be the same as or different from the processing conditions in the first modified layer forming step described above. The processing feed of the chuck table and the like are the same as those in the first modified layer forming step described above, and therefore will not be described here.

[0053] The third modified layer 21, like the first modified layer 17 and the second modified layer 19, is formed in the X-axis direction and the Y-axis direction, and is formed so as to extend parallel to each of the planned division lines set in a grid pattern.

[0054] The third modified layer 21, like the first modified layer 17 and the second modified layer 19, may be formed continuously in the X-axis direction and the Y-axis direction, or may be formed intermittently.

[0055] The third modified layer forming step can be carried out, for example, after all of the first modified layer forming steps and all of the second modified layer forming steps have been completed. Specifically, this can be performed after the formation of the first modified layer 17 and the second modified layer 19 on the wafer 11 is completed.

[0056] Alternatively, the third modified layer forming step can be carried out, for example, before or after each second modified layer forming step. Specifically, the first modified layer forming step and the second modified layer forming step are performed alternately as a set, and the third modified layer forming step is performed before or after the second modified layer forming step in the set. The chuck table is moved between each step.

[0057] Alternatively, a set may be formed by performing a first modified layer formation step for a certain planned division line, moving the chuck table, performing a second modified layer formation step, moving the chuck table, performing a third modified layer formation step, and moving the chuck table, and this set may be repeated.

[0058] As shown in FIG. 11, when the third modified layer 21 is formed near the center 11c of the wafer 11 in the thickness direction, cracks 21a occur from the modified layer 21 toward the front surface 11a or back surface 11b of the wafer 11.

[0059] 11, it is preferable that the third modified layer 21 and the cracks 21a generated thereby are not continuous with the first modified layer 17 and the cracks 17a generated thereby, or the second modified layer 19 and the cracks 19a generated thereby, because if they were continuous, there is a concern that the entire wafer 11 would break at the continuous location.

[0060] In other words, it is preferable that the third modified layer 21 and the cracks 21a generated thereby are offset in the in-plane direction of the wafer 11 (horizontal direction, Y-axis direction in Figure 11) from the first modified layer 17 and the cracks 17a generated thereby, and the second modified layer 19 and the cracks 19a generated thereby.

[0061] In this way, the first modified layer 17 and the third modified layer 21 are formed at positions offset from each other, and the second modified layer 19 and the third modified layer 21 are formed at positions offset from each other, so that the modified layers are not continuous with each other. This prevents the entire wafer 11 from being broken.

[0062] 11, the third modified layer 21 and the cracks 21a generated thereby preferably overlap the first modified layer 17 and the cracks 17a generated thereby in the thickness direction of the wafer 11. Similarly, the third modified layer 21 and the cracks 21a generated thereby preferably overlap the second modified layer 19 and the cracks 19a generated thereby in the thickness direction of the wafer 11.

[0063] As a result, as will be described in detail later, even if the second modified layer 19 and crack 19a are removed during the process of thinning the wafer by grinding, the third modified layer 21 and crack 21a can be present on the opposite side of the wafer thickness direction to the first modified layer 17 and crack 17a as a counter (opposing element), and warping can be suppressed until the grinding process is completed.

[0064] 11, in the third modified layer forming step, the focal point of the third modified layer 21 may be set at a plurality of positions shifted in the thickness direction near the center 11c in the thickness direction of the wafer 11, as in the example shown in Fig. 12. By forming a plurality of modified layers 21 in the thickness direction in this manner, it is possible to make it easier to form cracks 21a in the thickness direction.

[0065] <Back grinding step> After the first to third modified layer forming steps are performed, a backside grinding step is performed to grind the backside 11b of the wafer 11. This backside grinding step is performed using a grinding unit 32 of a grinding device, the main part of which is shown in FIG.

[0066] The grinding unit 32 is configured to include a spindle 34 that is rotationally driven by a motor, a wheel mount 36 fixed to the tip of the spindle 34, and a grinding wheel 38 that is detachably attached to the wheel mount 36. The grinding wheel 38 is configured to include an annular base 42 and a plurality of grinding stones 44 fixed to the outer periphery of the lower surface of the annular base 42.

[0067] In the backside grinding step, the front side 11a of the wafer 11 having the first to third modified layers formed therein is suction-held by the chuck table 46 of the grinding device, and the backside 11b of the wafer 11 is exposed.

[0068] Then, while the chuck table 46 is rotated in the direction of arrow a at, for example, 300 rpm and the grinding wheel 38 is rotated in the direction of arrow b at, for example, 6000 rpm, a grinding unit feed mechanism (not shown) is driven to grind and feed the grinding wheel 38 downward at a predetermined grinding feed speed (for example, 3 μm / s), while grinding the back surface 11 b of the wafer 11 with the grinding wheel 44.

[0069] Figure 14 shows the process in which the back surface 11b of the wafer 11 is ground and thinned by the grinding wheel 44. At the start shown in Figure 14(A), the presence of the second modified layer 19 and crack 19a makes it possible to suppress the occurrence of warping caused by the first modified layer 17 and crack 17a.

[0070] 14(B), even after the thinning progresses and the second modified layer 19 and cracks 19a are removed, the presence of the third modified layer 21 and cracks 21a makes it possible to continue to suppress the occurrence of warpage caused by the first modified layer 17 and cracks 17a. If the third modified layer 21 and cracks 21a were not present, there is a concern that warpage would occur in this situation.

[0071] Then, as shown in Figure 14(C), when the thinning progresses to the finishing thickness, the first modified layer 17 is removed and the substrate is split using the crack 17a as the splitting starting point, and when the grinding process is completed, individual device chips 15C are formed as shown in Figure 14(D).

[0072] In this way, even if the second modified layer 19 and crack 19a are removed during the process of thinning the wafer by grinding, the third modified layer 21 and crack 21a can be present as a counter (opposing element) on the opposite side of the wafer thickness direction to the first modified layer 17 and crack 17a, and warping can be suppressed until the grinding process is completed. [Explanation of symbols]

[0073] 10 Chuck table 11 wafers 11a surface 11b Back side 11c center 13 Planned division line 15C device chip 16 Imaging unit 17 First modified layer 17a Crack 19 Second modified layer 19a Crack 21 Third modified layer 21a Crack 22 Concentrator 32 Grinding unit 44 Grinding Wheel 46 Chuck table

Claims

1. A method for processing a plate-shaped workpiece on which a plurality of planned dividing lines are set, comprising: a holding step of holding the workpiece on a chuck table; a first modified layer forming step of irradiating the workpiece held on the chuck table with a laser beam having a wavelength that is transparent to the workpiece at a focal point positioned closer to the first surface than the center of the workpiece in the thickness direction, along the planned dividing line, to form a first modified layer that will serve as a dividing starting point; a second modified layer forming step of irradiating the workpiece held on the chuck table with a laser beam having a wavelength that is transparent to the workpiece at a focal point positioned closer to the second surface than the center of the workpiece in the thickness direction, along the planned dividing line, to form a second modified layer that suppresses warping of the workpiece; a third modified layer forming step of forming a third modified layer in a central region in the thickness direction of the workpiece to maintain suppression of warpage of the workpiece; A method for processing a workpiece, comprising:

2. the first modified layer and the third modified layer are formed at positions offset from each other; 2. The method for processing a workpiece according to claim 1, wherein the second modified layer and the third modified layer are formed at positions offset from each other.

3. A method for processing a workpiece as described in claim 1 or claim 2, characterized in that the first modified layer formation step and the second modified layer formation step are performed alternately as a set, and the third modified layer formation step is performed before or after the second modified layer formation step in one set.

4. the third modified layer and the cracks generated thereby overlap the first modified layer and the cracks generated thereby in the thickness direction of the workpiece; the third modified layer and the cracks generated thereby overlap the second modified layer and the cracks generated thereby in the thickness direction of the workpiece; 3. The method for processing a workpiece according to claim 1 or 2.

5. After the first to third modified layer forming steps are completed, a backside grinding step of grinding the workpiece from the second surface side to thin it; Further comprising:

3. The method for processing a workpiece according to claim 1 or 2.

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