Laser processing device and control method thereof

Abstract

To provide a laser processing device capable of maintaining a distance of a light focusing point from a front surface side of a workpiece to a constant distance with high accuracy, and provide a control method of the same.SOLUTION: A laser processing device comprises: a first laser light source that emits a processing laser light; a second laser light source that emits a detection laser light; a first light focusing lens that focuses a processing laser light into an inner part of a workpiece, and focuses a detection laser light onto a back surface of the workpiece; a first light detection part that detects a reflection light of the detection laser light from the back surface of the workpiece; a height position detection part that detects a height position of the back surface of the workpiece on the basis of a detection result of the first light detection part; a profile acquisition part that acquires a thickness profile from a cracking detection device; and a light focusing point position adjustment part that adjusts a position of a first light focusing point along a light axial direction of the first light focusing lens on the basis of the detection result of the height position detection part and the thickness profile, and maintains the position of the first light focusing point in a distance that is constant from the front surface of the workpiece.SELECTED DRAWING: Figure 16

Description

[Technical Field]

[0001] The present invention relates to a laser processing apparatus for performing laser processing on a workpiece and a control method thereof. [Background technology]

[0002] Conventionally, laser processing devices have been known that align a focal point inside a workpiece such as a silicon wafer (hereinafter simply referred to as a wafer) and irradiate the workpiece with laser light along a processing line (also referred to as a planned cutting line), thereby forming a laser processing area (also referred to as a "modified area" or "modified layer") within the wafer along the processing line, which serves as a starting point for cutting. The wafer with the laser processing area formed therein is then cleaved along the processing line by a cleaving process such as expanding or breaking, and divided into individual chips. With such laser processing devices, a laser processing area is formed inside the wafer, and the wafer is cleaved along the processing line starting from the laser processing area. This has advantages over typical dicing devices that use a blade to cut and cleave the wafer, such as lower dust generation and a lower likelihood of dicing scratches, chipping, or cracks on the material surface.

[0003] In recent years, the semiconductor manufacturing industry has seen a trend toward larger wafers, and they are becoming thinner to further increase packaging density. For such wafers, a back-grinding machine is used to grind the back surface of the wafer (back-grind) to thin the wafer, and then a laser processing machine is used to perform laser processing on the wafer to form a laser processing area.

[0004] In laser processing using such a laser processing device, the formation position of the laser processing area (the position of the focal point of the laser light) from the wafer's front surface (device formation surface) must be maintained at a constant distance to prevent thermal damage to the device layer formed on the wafer's front surface (device formation surface). For this reason, the laser processing device described in Patent Document 1 uses an autofocus function to adjust the vertical position of a focusing lens that focuses the laser light inside the wafer to follow the irregularities on the wafer's back surface (the surface opposite the device formation surface). During this process, wafers whose back surfaces have been back-ground using a back grinding device have variations in thickness. Therefore, even if the autofocus function is used to maintain a constant distance between the wafer's back surface and the focal point of the laser light focused inside the wafer, the distance between the wafer's front surface and the focal point changes depending on the wafer's thickness variation.

[0005] Therefore, the laser processing device described in Patent Document 1 acquires a thickness profile that indicates the change in wafer thickness along each processing line of the wafer based on the height profile of the back surface of the wafer along at least one scan line and the height profile of the upper surface of the stage (table) that holds the front surface of the wafer.Then, this laser processing device maintains the position of the laser processing area (focus point) at a constant distance from the front surface of the wafer based on the thickness profile. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2020-142284 Summary of the Invention [Problem to be solved by the invention]

[0007] The laser processing apparatus described in Patent Document 1 acquires a thickness profile based on the height profile of the back surface of the wafer and the height profile of the top surface of the stage. In this case, the front surface of the wafer is generally covered with a protective sheet, so the protective sheet is sandwiched between the top surface of the stage and the front surface of the wafer. Furthermore, the surface of the wafer is uneven due to the device layer. Therefore, the method described in Patent Document 1 cannot accurately obtain the thickness profile of the wafer. As a result, it is difficult for the laser processing apparatus described in Patent Document 1 to maintain the position of the laser processing area (focus point) at a constant distance from the front surface of the wafer with high precision.

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a laser processing apparatus and a control method thereof that can maintain the distance of the focal point from the surface side of the workpiece at a constant distance with greater precision. [Means for solving the problem]

[0009] A laser processing apparatus for achieving the object of the present invention includes a processing head that irradiates the processing laser light from the back side of the workpiece along a processing line of the workpiece with a first focusing point of the processing laser light aligned with the inside of the workpiece, thereby forming a laser processing area inside the workpiece, and a crack detection device that detects cracks that extend from the laser processing area inside the workpiece, in which the crack detection device includes an interface position detection unit that detects interface positions indicating the front and back sides of the workpiece at a plurality of predetermined measurement points, and a thickness detection unit that detects a thickness profile that indicates changes in the thickness of the workpiece along the processing line based on the detection results of the interface position detection unit, and the processing head includes a first laser light source that emits the processing laser light and a second laser light source that emits the detection laser light. a first focusing lens that focuses the processing laser light emitted from the first laser light source inside the workpiece and the detection laser light emitted from the second laser light source on the back surface of the workpiece; a first light detection unit that detects reflected light of the detection laser light from the back surface of the workpiece; a height position detection unit that detects the height position of the back surface of the workpiece based on the detection result of the first light detection unit; a profile acquisition unit that acquires a thickness profile from the thickness detection unit; and a focus position adjustment unit that adjusts the position of the first focus along the optical axis direction of the first focusing lens based on the detection result of the height position detection unit and the thickness profile acquired by the profile acquisition unit, to maintain the position of the first focus at a predetermined constant distance from the surface of the workpiece.

[0010] With this laser processing device, the crack detection device can detect the thickness profile of the workpiece more accurately than before, and based on this thickness profile and the detection results of the height position of the back surface of the workpiece, the position of the first focal point can be maintained at a constant distance from the surface of the workpiece.

[0011] In another aspect of the present invention, a laser processing apparatus includes a first moving mechanism that moves a first focusing lens relative to a workpiece in a direction perpendicular to the optical axis direction of the first focusing lens to move the first focusing point along a processing line, and the first light detecting unit, the height position detecting unit, and the focusing point position adjusting unit are repeatedly operated each time the first focusing point is moved along the processing line by the first moving mechanism. This makes it possible to maintain the position of the first focusing point at a constant distance from the surface of the workpiece while laser processing is being performed along the processing line.

[0012] In another aspect of the present invention, a laser processing apparatus includes a crack detection device including a light source that emits detection light, a second condenser lens that focuses the detection light emitted from the light source on a workpiece, a condenser scanning unit that scans the second condenser of the detection light along the optical axis direction of the second condenser lens, a second light detection unit that detects reflected light of the detection light from the workpiece while the condenser scanning unit scans the second condenser, and a second movement mechanism that moves the second condenser lens relative to the workpiece in a direction perpendicular to the optical axis direction of the second condenser lens, wherein the condenser scanning unit and the second light detection unit are repeatedly operated each time the second movement mechanism moves the second condenser to a plurality of measurement points in sequence, and the interface position detection unit detects the interface position for each of the plurality of measurement points based on the detection results of the second light detection unit for each of the plurality of measurement points. This allows the interface position of the workpiece to be detected with high accuracy, thereby enabling the detection of a thickness profile with higher accuracy.

[0013] In a laser processing device according to another aspect of the present invention, the interface position detection unit detects the interface position at each of a plurality of measurement points along a scan line that passes through the center of the workpiece whose back surface has been ground by the grinding device. Based on the detection results of the thickness of the workpiece at each of the plurality of measurement points along the scan line, the thickness distribution within the surface of the workpiece can be determined (estimated), and the thickness profile can be detected in a short time.

[0014] In a laser processing apparatus according to another aspect of the present invention, a crack detection device is provided in a processing head.

[0015] A control method for a laser processing device for achieving the object of the present invention is a control method for a laser processing device including a processing head that irradiates the processing laser light from the back side of the workpiece along a processing line of the workpiece while aligning a first focusing point of the processing laser light with the inside of the workpiece using a first focusing lens, and forms a laser processing area inside the workpiece, and a crack detection device that detects cracks that have propagated from the laser processing area inside the workpiece, in which the crack detection device detects interface positions that indicate the front and back surfaces of the workpiece at a plurality of predetermined measurement points, based on the detection results, detects a thickness profile that indicates a change in thickness of the workpiece along the processing line, and the processing head A processing laser beam is emitted from the source, a detection laser beam is emitted from the second laser light source, the processing laser beam emitted from the first laser light source is focused inside the workpiece by a first focusing lens, and the detection laser beam emitted from the second laser light source is focused on the back surface of the workpiece, the reflected light of the detection laser beam from the back surface of the workpiece is detected, the height position of the back surface of the workpiece is detected based on the detection result of the reflected light of the detection laser beam, a thickness profile is obtained from the crack detection device, and the position of the first focusing point is adjusted along the optical axis direction of the first focusing lens based on the detection result of the height position and the thickness profile, to maintain the position of the first focusing point at a predetermined constant distance from the front surface side of the workpiece. [Effects of the Invention]

[0016] The present invention can maintain the distance of the focal point from the front surface side of the wafer at a constant distance with higher precision. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a block diagram showing the configuration of a laser processing system; [Figure 2] FIG. 1 is a block diagram showing a schematic configuration of a laser processing device. [Figure 3] FIG. 2 is a block diagram showing an example of a crack detection device mounted on a laser processing device. [Figure 4] 10 is an explanatory diagram showing a state in which a wafer is subjected to oblique illumination with detection light; FIG. [Figure 5] 10 is an explanatory diagram showing a state in which a wafer is subjected to oblique illumination with detection light; FIG. [Figure 6] 10 is an explanatory diagram showing a state in which a wafer is subjected to oblique illumination with detection light; FIG. [Figure 7] 10A and 10B are diagrams showing how reflected light is received on a light receiving surface of a photodetector. [Figure 8] 10A and 10B are diagrams showing how reflected light is received on a light receiving surface of a photodetector. [Figure 9] 10A and 10B are diagrams showing how reflected light is received on a light receiving surface of a photodetector. [Figure 10] 10 is a diagram for explaining the path of light reflected from a wafer reaching a condenser lens pupil. FIG. [Figure 11] FIG. 2 is a functional block diagram of a control unit of the crack detection device. [Figure 12] 10 is an explanatory diagram for explaining scanning of the focal point of the detection light in the Z direction by the drive control unit. FIG. [Figure 13] 10 is a graph showing the relationship between the movement amount (μm) of the focal point caused by the piezoelectric actuator and the detection signal (V) output from the detector body of the photodetector. [Figure 14] FIG. 2 is an explanatory diagram for explaining the arrangement of measurement points on a wafer. [Figure 15] FIG. 15 is a cross-sectional view of the wafer along the scan line in FIG. 14. [Figure 16] FIG. 2 is a functional block diagram of a control unit of the laser processing device. [Figure 17] 1 is an explanatory diagram showing a planned formation line of a laser processing area within a wafer. FIG. [Figure 18] 10 is an explanatory diagram for explaining the adjustment of the position of the light-condensing point in the Z direction by the light-condensing point position adjusting unit. FIG. [Figure 19] 10 is a flowchart showing the flow of a process for detecting a thickness profile for each processing line of a wafer by a crack detection device. [Figure 20] 10 is a flowchart showing the flow of laser processing for each processing line by the laser processing device. [Figure 21]1 is an explanatory diagram showing a line along which a laser processing area is to be formed within a wafer having a non-flat surface; [Figure 22] 21 is an explanatory diagram for explaining the adjustment of the position of the focal point in the Z direction by the focal point position adjusting unit when laser processing the wafer shown in FIG. 20 is performed. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] [Laser processing system] FIG. 1 is a block diagram showing the configuration of a laser processing system 100.

[0019] 1, a laser processing system 100 divides a wafer W, which is a workpiece, along a processing line DL through multiple processes to form multiple chips (not shown). The laser processing system 100 includes a backgrinding apparatus 200, a laser processing apparatus 10, and a dividing apparatus 300.

[0020] In the laser processing system 100, first, the wafer is transported to the back-grinding apparatus 200, where back-grinding (back-grinding process) is performed to grind the back surface of the wafer W. The wafer W whose back surface has been back-grinded by the back-grinding apparatus 200 is thinned to a thickness of, for example, 250 μm or less. Note that, when back-grinding is performed, a protective sheet (also called back-grinding tape) is attached in advance to the front surface side of the wafer W, and this protective sheet protects the devices (device layer) formed on the front surface of the wafer W. Herein, of the two surfaces of the wafer, the surface on which the devices are formed will be referred to as the "front surface" of the wafer, and the surface opposite to the front surface will be referred to as the "back surface."

[0021] The wafer W that has been back-ground by the back grinding device 200 is loaded into the laser processing device 10, where a laser processing area is formed inside the wafer W along the processing line DL by laser light irradiated from the back side of the wafer W.

[0022] After the wafer has been laser processed on all processing lines DL, its backside is mounted on a dicing frame via dicing tape. In this state, the protective sheet attached to the front side of the wafer is peeled off.

[0023] Next, the wafer is carried together with the dicing frame into the dividing device 300, where the dicing tape is stretched radially to divide the wafer into individual chips. This completes the flow of wafer processing by the laser processing system 100.

[0024] The back grinding device 200 and the dividing device 300 that constitute the laser processing system 100 are well-known technologies, so a detailed description thereof will be omitted here.

[0025] [Laser processing equipment] 2 is a block diagram showing a schematic configuration of the laser processing apparatus 10. Note that the X, Y and Z directions in the figure are perpendicular to each other, the X and Y directions are horizontal directions, and the Z direction is the up and down direction.

[0026] 2, the laser processing apparatus 10 forms a laser processing region R inside the wafer W, where the wafer back surface Wb, which is the back surface of the wafer W, has been back-ground by a back-grinding apparatus 200. The laser processing apparatus 10 includes a processing head 16, a suction stage 18, a processing head drive unit 19, a table drive unit 14, and a control unit 12. The laser processing apparatus 10 further includes a crack detection device 500 attached to one side of the processing head 16. The configuration of the crack detection device 500 will be described later.

[0027] The suction stage 18 has a holding surface 18a that suction-holds the wafer W. The wafer surface Wa, which is the surface of the wafer W, is partitioned into lattice-like regions by, for example, a plurality of processing lines DL extending in the X and Y directions, and devices such as electronic circuits [device layers (films)] are formed in each of these lattice-like regions. The wafer W is placed with a protective sheet 220 that covers the holding surface 18a of the suction stage 18 facing downward, and is suction-held by the suction stage 18 via the protective sheet 220. Note that the wafer W may also be suction-held directly without the protective sheet 220.

[0028] The table driving unit 14 corresponds to the first moving mechanism and the second moving mechanism of the present invention, and includes an XY driving mechanism (not shown) for moving the suction stage 18 in the X and Y directions, and a rotary driving mechanism (not shown) for rotating the suction stage 18 in the θ direction around the Z direction. Under the control of the control unit 12, the table driving unit 14 moves the suction stage 18 in the X and Y directions using the XY driving mechanism, and rotates the suction stage 18 in the θ direction around the Z direction using the rotary driving mechanism. This makes it possible to perform relative alignment between the wafer W suction-held on the suction stage 18 and the processing head 16 (described later), and to perform laser processing by the processing head 16 along multiple processing lines DL provided on the wafer W.

[0029] The processing head 16 is a processing optical system main body (also referred to as a "laser engine") that houses various optical systems for performing laser processing. The processing head 16 includes a laser light source 20, a dichroic mirror 21, a condenser lens 22 (also referred to as an objective lens), a piezo actuator 23, and an AF (Automatic Focus) sensor 24. The laser light source 20, the dichroic mirror 21, the condenser lens 22, and the piezo actuator 23 are provided from top to bottom in the Z direction, and the AF sensor 24 is provided on the side of the dichroic mirror 21.

[0030] The laser light source 20 corresponds to the first laser light source of the present invention, and together with a laser light source 25 described below, constitutes a laser light source unit of the present invention. The laser light source 20 emits processing laser light LP used to form a laser processing region R inside the wafer W. For example, a semiconductor laser pumped Nd:YAG (Yttrium Aluminum Garnet) laser is used as this laser light source 20. The processing laser light LP is conditioned, for example, with a wavelength of 1.1 μm and a laser light spot cross-sectional area of ​​3.14×10 -8 cm 2 The oscillation form is Q-switched pulse, the repetition frequency is 80 to 120 kHz, the pulse width is 180 to 280 ns, and the output is 8 W.

[0031] The dichroic mirror 21 transmits the processing laser light LP and reflects the detection laser light LD emitted from the AF sensor 24, which will be described later. As a result, the dichroic mirror 21 transmits the processing laser light LP incident from the laser light source 20 and emits it toward the condenser lens 22. In addition, the dichroic mirror 21 emits the detection laser light LD incident from the AF sensor 24 toward the condenser lens 22.

[0032] The condenser lens 22 corresponds to the first condenser lens of the present invention, and may be, for example, a condenser lens. This condenser lens 22 condenses the processing laser light LP incident from the dichroic mirror 21 into the interior of the wafer W. This forms a laser processing region R inside the wafer W. Here, the laser processing region R refers to a region where the physical properties of the interior of the wafer W, such as density, refractive index, and mechanical strength, become different from those of the surrounding area due to irradiation with the laser light L, resulting in a lower strength than the surrounding area. The laser processing region R includes, for example, a crack region.

[0033] Furthermore, the condenser lens 22 condenses the detection laser light LD incident from the dichroic mirror 21 onto the wafer back surface Wb. This allows the condenser lens 22 to simultaneously condense the processing laser light LP into the inside of the wafer W and the detection laser light LD onto the wafer back surface Wb.

[0034] The piezo actuator 23 expands and contracts in the Z direction (the optical axis direction of the condenser lens 22) as the voltage applied from the control unit 12 changes, and this expansion and contraction action moves the condenser lens 22 in the optical axis direction. This makes it possible to precisely adjust the position of the condenser lens 22 in the Z direction, and to precisely adjust (finely adjust) the position of the focal point FP1 (corresponding to the first focal point of the present invention) of the processing laser light LP that is focused inside the wafer W in the Z direction.

[0035] The relationship between the amount of movement (amount of expansion / contraction: μm) of the piezoelectric actuator 23 and the voltage applied to the piezoelectric actuator 23 is determined in advance by experiment, etc. The control unit 12 stores a table that defines this relationship, and changes the voltage applied to the piezoelectric actuator 23 in accordance with this relationship.

[0036] The AF sensor 24 is used to detect the height position of the wafer back surface Wb in the Z direction along the processing line DL. The AF sensor 24 includes a laser light source 25 and a photodetector 26.

[0037] The laser light source 25 corresponds to the second laser light source of the present invention, and emits detection laser light LD in a wavelength range that is different from the wavelength range of the processing laser light LP and that can be reflected by the wafer back surface Wb toward the dichroic mirror 21. This detection laser light LD is reflected by the dichroic mirror 21 toward the condenser lens 22, and then condensed by the condenser lens 22 on the wafer back surface Wb. Then, reflected light LR of the detection laser light LD reflected by the wafer back surface Wb travels back along the optical path of the detection laser light LD and enters the photodetector 26 of the AF sensor 24. Note that the symbol RP in the figure indicates the reflection point of the detection laser light LD reflected by the wafer back surface Wb.

[0038] The photodetector 26 corresponds to the first photodetector of the present invention, has a light receiving surface that receives the reflected light LR, and outputs a detection signal to the control unit 12 according to the height position in the Z direction of the wafer back surface Wb based on the received reflected light LR.

[0039] Here, the distribution and light intensity of the reflected light LR reflected by the wafer back surface Wb change depending on the distance from the condenser lens 22 to the wafer W, i.e., the uneven shape (surface displacement) of the wafer back surface Wb. Utilizing this property, the AF sensor 24 determines the height position in the Z direction at the laser processing position (XY position) where the focal point FP1 is formed within the surface of the wafer W, based on the distribution and light intensity changes of the reflected light LR reflected by the wafer back surface Wb. Note that the AF sensor 24 can use, for example, an astigmatism method or a knife-edge method to measure the height position of the wafer back surface Wb. These methods are well known, so detailed description thereof will be omitted here.

[0040] The above-described measurement of the height position of the wafer back surface Wb by the AF sensor 24 is continuously performed on the processing line DL of the wafer W. This makes it possible to control the Z direction position of the focal point FP1 of the processing laser beam LP in real time based on the measurement results of the AF sensor 24 when forming the laser processing region R inside the wafer W along the processing line DL.

[0041] In this embodiment, the AF sensor 24 is provided in the processing head 16, and the condenser lens 22 is configured to simultaneously condense the processing laser beam LP and the detection laser beam LD, but this configuration is not limiting. For example, the AF sensor 24 may be provided in a position adjacent to the processing head 16, independent of it.

[0042] The processing head drive unit 19 is equipped with a processing head Z drive mechanism (not shown) for moving the processing head 16 in the Z direction. The processing head Z drive mechanism is, for example, an actuator including a motor and gears. The processing head drive unit 19 moves the processing head 16 in the Z direction using the processing head Z drive mechanism under the control of the control unit 12. This allows for rough adjustment of the relative distance in the Z direction between the processing head 16 and the wafer W held by suction on the suction stage 18.

[0043] The control unit 12 is realized by, for example, a personal computer or a workstation, and includes a CPU (Central Processing Unit), memory (for example, a ROM (Read Only Memory), a RAM (Random Access Memory), etc.), storage (for example, an HDD (Hard Disk Drive), an SSD (Solid State Drive), etc.), and an input / output circuit unit.

[0044] The control unit 12 controls the operation of each unit of the laser processing device 10. Specifically, it controls the operation of the processing head 16 (emission control of the laser light sources 20 and 25, light reception control of the reflected light by the photodetector 26, position adjustment of the condenser lens 22 by the piezo actuator 23, etc.), the operation of the processing head drive unit 19 and the table drive unit 14, etc.

[0045] When forming a laser processing region R on the wafer W, the relative distance (working distance) between the processing head 16 and the wafer W is adjusted to a desired distance by the processing head drive unit 19, and then the above-mentioned AF sensor 24 measures the height position of the wafer back surface Wb and adjusts the position of the condenser lens 22 in the Z direction. Next, the processing laser light LP is emitted from the laser light source 20 and irradiated onto the inside of the wafer W via the condenser lens 22. At this time, as will be described in detail later, the Z direction position of the focal point FP1 of the processing laser light LP irradiated onto the inside of the wafer W is adjusted so as to maintain a constant distance from the wafer front surface Wa by adjusting the Z direction position of the condenser lens 22 by the piezo actuator 23.

[0046] In this state, the table drive unit 14 moves the suction stage 18 in the X direction, which is the dicing direction, i.e., in a direction perpendicular to the optical axis direction of the condenser lens 22. As a result, one layer of laser processing region R is formed along the processing line DL of the wafer W. Then, once one layer of laser processing region R has been formed along the processing line DL, the table drive unit 14 indexes and moves the suction stage 18 one pitch in the Y direction, and a laser processing region R is also formed on the next processing line DL. Next, once the laser processing regions R have been formed along all of the X-direction processing lines DL, the table drive unit 14 rotates the suction stage 18 90° in the θ direction around the Z direction, and laser processing regions R are similarly formed on the X-direction processing line DL after the rotation. Note that although FIG. 2 shows one layer of laser processing region R, multiple layers (e.g., two or three layers) of laser processing regions R may be formed within the wafer W depending on the thickness of the wafer W.

[0047] [Crack detection device] FIG. 3 is a block diagram showing an example of a crack detection device 500 mounted on the laser processing apparatus 10. As shown in FIG. 3, the crack detection device 500 not only has a basic function of detecting cracks generated inside the wafer W due to irradiation with the processing laser light LP, but also has a thickness detection function of detecting the thickness of the wafer W before irradiation with the processing laser light LP (i.e., after back grinding is performed by the back grinding apparatus 200). Below, the crack detection function, which is a basic function of the crack detection device 500, will be explained, and then the thickness detection function of the crack detection device 500 will be explained. The crack detection device 500 is attached to one side of the processing head 16 and can be moved in the Z direction together with the processing head 16 by the above-mentioned processing head drive unit 19.

[0048] [Crack detection function] When a laser processing region R is formed inside a wafer W by the laser processing device 10, a crack K extends from the laser processing region R in the thickness direction of the wafer W. In order to properly divide the wafer W into chips in the dividing device 300, which is a process subsequent to the laser processing device 10, it is important to properly form the laser processing region R, which serves as the starting point for dividing the wafer W, and to accurately detect the depth of the crack K formed inside the wafer W.

[0049] Therefore, the crack detection device 500 has a function (crack detection function) of detecting the crack depth of a crack K formed inside the wafer W by irradiating the wafer W with detection light L1 and detecting reflected light L2 from the wafer W. The crack detection device 500 is used in combination with the laser processing device 10, but in the following description, only the components related to the crack detection device 500 will be described, and the same components as those in the laser processing device 10 will be given the same reference numerals and may not be described again. In the following description, a three-dimensional Cartesian coordinate system will be used in which the suction stage 18 on which the wafer W is suctioned is defined as a plane parallel to the XY plane, and the Z direction is the thickness (depth) direction of the wafer W.

[0050] The crack detection device 500 includes a light source unit 550, an illumination optical system 600, an interface position detection optical system 650, a crack detection optical system 700, a control unit 750, a focal point position moving mechanism 752, a focusing lens 754, an operation unit 756, a display unit 758, and a communication IF 759 which is a communication interface (IF).

[0051] The light source unit 550 emits detection light L1. This detection light L1 is used to detect the interface position of the wafer W and to detect cracks K formed inside the wafer W. Here, if the wafer W is made of silicon, it is desirable to use infrared light with a wavelength of 1,000 nm or more as the detection light L1.

[0052] The light source unit 550 has light sources 552A, 552B, and 552C and a half mirror 554. The light sources 552A, 552B, and 552C and the half mirror 554 are arranged along a main optical axis AX that is coaxial with the optical axis of the condenser lens 754.

[0053] The light sources 552A, 552B, and 552C emit detection light L1 along the main optical axis AX. As the light sources 552A, 552B, and 552C, for example, a laser light source (infrared laser light source, laser diode) or an LED (Light Emitting Diode) light source can be used.

[0054] Light source 552A has a laser aperture capable of illuminating substantially the entire surface of condenser lens pupil 754a of condenser lens 754. This light source 552A is used to detect the interface position, which will be described later.

[0055] The light sources 552B and 552C each have a laser aperture capable of illuminating only a portion of the condenser lens pupil 754a of the condenser lens 754 that is decentered from the main optical axis AX (the optical axis of the condenser lens 754). These light sources 552B and 552C are used for crack detection, which will be described later.

[0056] The half mirror 554 reflects the detection light L1 emitted from the light source 552A for detecting the interface position and transmits the detection light L1 emitted from the light sources 552B and 552C for detecting cracks. Although not shown in the drawings, the detection light L1 emitted from the light sources 552A, 552B, and 552C will be described below as L1(A), L1(B), and L1(C), respectively.

[0057] The light sources 552A, 552B, and 552C are each connected to a control unit 750, and the control unit 750 controls the emission of the light sources 552A, 552B, and 552C.

[0058] The control unit 750 includes a CPU that controls the operation of each unit of the crack detection device 500, a ROM that stores a control program, and an SDRAM (Synchronous Dynamic Random Access Memory) that can be used as a work area for the CPU. The control unit 750 is connected to the control unit 12 of the laser processing device 10 via an interface (not shown), receives operation input from an operator via an operation unit 756, and transmits control signals according to the operation input to each unit of the crack detection device 500 to control the operation of each unit.

[0059] In this embodiment, the control unit 750 of the crack detection device 500 is shown separately from the control unit 12 of the laser processing device 10 (see Figure 3), but the processing performed by these control units 750, 12 may be performed by one control unit or multiple control units.

[0060] The operation unit 756 is a means for accepting operation input by an operator, and is, for example, a keyboard, a mouse, or a pointing device such as a touch panel.

[0061] The display unit 758 is a device that displays an operation GUI (Graphical User Interface) and images (for example, crack detection results) for operating the crack detection device 500. As the display unit 758, for example, a liquid crystal display can be used.

[0062] The operation unit 756 and the display unit 758 may be shared with the operation unit and the display unit (neither of which is shown) provided in the laser processing apparatus 10.

[0063] The communication IF 759 is connected by wire or wirelessly to a communication IF 28 (see FIG. 16) of the laser processing apparatus 10, which will be described later in detail. The communication IF 759, under the control of the control unit 750, outputs a thickness profile 42 (see FIG. 11), which is the thickness detection result of the wafer W detected by a thickness detection function, which will be described later, to the communication IF 28, i.e., the laser processing apparatus 10.

[0064] The illumination optical system 600 guides the detection light L1 emitted from the light source unit 550 to a condenser lens 754. The illumination optical system 600 has relay lenses 602 and 606 and a mirror (e.g., a total reflection mirror) 604. The detection light L1 emitted from the light source unit 550 passes through the relay lens 602 and is reflected by the mirror 604, causing its optical path to be bent. The detection light L1 reflected by the mirror 604 passes through the relay lens 606, and is then reflected sequentially by the half mirrors 654 and 652 of the interface position detection optical system 650, and is emitted toward the condenser lens 754. In addition, the observation light (return light from the wafer W) transmitted through the half mirror 652 can be observed using the observation optical system 760. If the observation optical system 760 is not used, a dichroic mirror or a total reflection mirror can be used instead of the half mirror 652.

[0065] The condenser lens 754 corresponds to the second condenser lens of the present invention, and condenses (focuses) the detection light L1 emitted from the illumination optical system 600 onto the wafer W. The condenser lens 754 is disposed at a position facing the wafer back surface Wb, and the optical axis of the condenser lens 754 is disposed coaxially with the main optical axis AX.

[0066] The focal point position moving mechanism 752 changes the position of the focal point FP2 (corresponding to the second focal point of the present invention, see FIG. 12) of the detection light L1 relative to the wafer W in the Z direction (the optical axis direction of the focusing lens 754). The focal point position moving mechanism 752 includes a piezo actuator 752a that moves the focusing lens 754 in the Z direction.

[0067] The piezo actuator 752a expands and contracts in the Z direction (the optical axis direction of the condenser lens 754) as the voltage applied from the control unit 750 changes, and this expansion and contraction action moves the condenser lens 754 in the optical axis direction. This changes the position of the condenser point FP2 (see FIG. 12) of the detection light L1 relative to the wafer W in the thickness direction of the wafer W (Z direction).

[0068] Furthermore, the relationship between the amount of movement (amount of expansion / contraction: μm) of piezoelectric actuator 752a and the voltage applied to piezoelectric actuator 752a is determined in advance through experiments, etc. Control unit 750 stores a table that defines this relationship, and changes the voltage applied to piezoelectric actuator 752a in accordance with this relationship.

[0069] The focal point position moving mechanism 752 moves the condenser lens 754 in the Z direction by driving the piezo actuator 752a under the control of the control unit 750. This changes the relative distance in the Z direction between the condenser lens 754 and the wafer W, and changes the position of the focal point FP2 of the detection light L1 relative to the wafer W, making it possible to scan the focal point FP2 of the detection light L1 in the thickness direction of the wafer W (Z direction).

[0070] Furthermore, the focal point position moving mechanism 752 may include, in addition to the piezoelectric actuator 752a, the processing head Z drive mechanism of the processing head drive unit 19 described above. By moving the processing head 16 in the Z direction using the processing head Z drive mechanism, the crack detection device 500 can be moved in the Z direction together with the processing head 16. This changes the relative distance between the condenser lens 754 and the wafer W, making it possible to change the position of the focal point FP2 (see FIG. 12) of the detection light L1 relative to the wafer W. Therefore, by combining the focal point position adjustment (coarse adjustment) using the processing head Z drive mechanism and the focal point position adjustment (fine adjustment) using the piezoelectric actuator 752a, the degree of freedom (adjustment range) in adjusting the position of the focal point FP2 of the detection light L1 relative to the wafer W is increased compared to when the focal point FP2 of the detection light L1 is scanned in the thickness direction of the wafer W using only the piezoelectric actuator 752a, thereby enabling crack and thickness detection for wafers W of various thicknesses.

[0071] The reflected light L2 of the detection light L1, which is collected by the collecting lens 754 and reflected by the wafer W, is guided to the interface position detection optical system 650 and the crack detection optical system 700, and is used to detect the interface position and cracks of the wafer W, respectively.

[0072] In the following example, an example will be described in which the interface position of the wafer surface Wa is detected, and then the crack depth is detected based on the interface position of the wafer surface Wa.

[0073] In this embodiment, the crack depth is detected based on the wafer front surface Wa, but this is not limiting, and for example, the crack depth may be detected based on the wafer back surface Wb. It is also possible to take an average value of the crack depths detected based on the interface positions of both the wafer front surface Wa and the wafer back surface Wb.

[0074] <Interface position detection optical system> The interface position detection optical system 650 is an optical system for detecting the interface of the wafer W (wafer front surface Wa or wafer back surface Wb), and has a half mirror 652, a half mirror 654, a relay lens 656, a half mirror 658, and a photodetector 660.

[0075] When detecting the wafer W's interface, for example, the wafer front surface Wa, the control unit 750 causes the light source 552A to emit light and irradiate the wafer W with detection light L1(A).

[0076] The detection light L1(A) from the light source 552A is a laser beam having an opening of approximately the same size as the condenser lens pupil 754a of the condenser lens 754, and is reflected successively by the half mirror 654 and the half mirror 652 and guided to the condenser lens 754. The detection light L1(A) is irradiated onto approximately the entire surface of the condenser lens pupil 754a of the condenser lens 754.

[0077] Here, the reflected light of the detection light L1(A) reflected by the wafer W is designated as L2(A). The reflected light L2(A) passes through the condenser lens 754, is reflected by the half mirror 652, passes through the half mirror 654, and is then guided to the relay lens 656. The reflected light L2(A) that passed through the relay lens 656 is reflected by the half mirror 658 and is then guided to the photodetector 660.

[0078] The photodetector 660 is a device for receiving reflected light L2(A) from the wafer W and detecting the wafer surface Wa of the wafer W, and includes a detector body 660A and a pinhole panel 660B.

[0079] As the detector main body 660A, a photodetector (for example, a photodiode) that converts received light into an electrical signal and outputs it to the control unit 750 can be used.

[0080] A pinhole is formed in the pinhole panel 660B to transmit a portion of the incident light. The pinhole panel 660B is disposed upstream of the light receiving surface of the detector main body 660A, and is disposed so that the pinhole in the pinhole panel 660B is positioned on the optical axis of the reflected light L2(A). The position of the pinhole in the pinhole panel 660B is optically conjugate with the focal point (front focal position) of the condenser lens 754 (confocal pinhole). The size of the pinhole in the pinhole panel 660B is adjusted to approximately the diffraction limit of the condenser lens 754.

[0081] The reflected light L2(A) reflected by the wafer W is focused at the pinhole position of the pinhole panel 660B, which is optically conjugate with the focusing point of the focusing lens 754. When the focusing point of the focusing lens 754 coincides with the wafer front surface Wa of the wafer W, which serves as the reflective surface, the light beam of the detection light L1(A) is reflected by the wafer front surface Wa of the wafer W, becomes a parallel light beam, and returns after passing through the focusing lens 754. Therefore, the detection signal output from the detector main body 660A has a sharp peak when the focusing point of the focusing lens 754 coincides with the wafer front surface Wa, which serves as the reflective surface. Note that the detection signal output from the detector main body 660A also has a sharp peak when the focusing point of the focusing lens 754 coincides with the wafer back surface Wb, which serves as the reflective surface.

[0082] The control unit 750 changes the relative distance between the condenser lens 754 and the wafer W using the condenser position moving mechanism 752 while irradiating the wafer W with the detection light L1(A) from the light source 552A, thereby moving the position of the condenser point of the detection light L1(A) relative to the wafer W (i.e., the front focal position of the condenser lens 754) in the Z direction. This causes the condenser point of the detection light L1(A) to scan in the thickness direction of the wafer W. The control unit 750 detects, with the photodetector 660, the reflected light L2(A) from the wafer W when the condenser point of the detection light L1(A) is scanned in the thickness direction (Z direction) of the wafer W, and by detecting the peak of the detection signal from this photodetector 660, can detect the interface position of the wafer surface Wa in the Z direction.

[0083] In this embodiment, the interface position of the wafer W is detected using the confocal method, but this is not limited to this, and other focus detection methods such as the astigmatism method and the white light interferometry can also be applied.

[0084] <Crack detection optical system> The crack detection optical system 700 includes a relay lens 702 and photodetectors 704 and 706 .

[0085] When detecting a crack K formed inside the wafer W, the control unit 750 causes the light sources 552B and 552C to emit light and irradiate the wafer W with detection light L1(B) and L1(C). The light sources 552B and 552C each have a laser aperture at a position offset from the main optical axis AX. As a result, the wafer W is irradiated with the detection light L1(B) and L1(C) that are eccentric with respect to the main optical axis AX.

[0086] The reflected light beams L2(B) and L2(C) of the detection light beams L1(B) and L1(C), respectively, reflected by the wafer W, pass through the condenser lens 754 and are reflected in turn by the half mirror 652 and the half mirror 654, and then pass in turn by the relay lens 656 and the half mirror 658, before entering the relay lens 702. The reflected light beams L2(B) and L2(C) that have passed through the relay lens 702 are received by the photodetectors 704 and 706.

[0087] The photodetectors 704 and 706 are devices that receive reflected light L2(B) and L2(C) from the wafer W and detect cracks K inside the wafer W. As the photodetectors 704 and 706, photodetectors (e.g., photodiodes) that convert the received light into electrical signals and output them to the control unit 750 can be used.

[0088] The photodetectors 704 and 706 are arranged at positions conjugate with the condenser lens pupil 754a, and are further arranged at positions offset from the optical axis of the condenser lens 754 so as to receive the detection light beams L1(B) and L1(C).

[0089] 4 to 6 are explanatory diagrams showing the state when the detection light L1 is obliquely illuminated onto the wafer W. Fig. 4 shows a case where a crack K is present at the focal point of the condenser lens 754, Fig. 5 shows a case where a crack K is not present at the focal point of the condenser lens 754, and Fig. 6 shows a case where the focal point of the condenser lens 754 coincides with the crack depth (position of the bottom end of the crack) of the crack K.

[0090] 7 to 9 are diagrams showing the state of reflected light L2 received by light receiving surfaces 704C and 706C of photodetectors 704 and 706, and correspond to the cases shown in FIGS. 4 to 6, respectively.

[0091] 10 is a diagram for explaining the path of reflected light L2 from the wafer W reaching the condenser lens pupil 754a. Note that here, a case will be described in which detection light L1 passes through a first region G1 on one side (the right side in FIG. 10) of the condenser lens pupil 754a, and oblique illumination is performed on the wafer W.

[0092] 4, if a crack K is present at the focal point of the condenser lens 754, the detection light L1 is totally reflected by the crack K and then reflected by the wafer surface Wa, and the reflected light L2 travels a path on the same side of the main optical axis AX as the optical path of the detection light L1 and becomes a component that reaches an area of ​​the condenser lens pupil 754a on the same side as the detection light L1. That is, as shown in FIG. 10, if the path of the detection light L1 when the detection light L1 from the light source unit 550 is irradiated onto the wafer W via the condenser lens 754 is R1, the reflected light L2 that is totally reflected by the crack K inside the wafer W travels a path R2 on the same side of the main optical axis AX as the path R1 of the detection light L1 (the right side in FIG. 10) and passes through a first area G1 of the condenser lens pupil 754a.

[0093] 5, when no crack K is present at the focal point of the condenser lens 754, the detection light L1 is reflected by the wafer surface Wa, and the reflected light L2 is a component that reaches a region of the condenser lens pupil 754a on the opposite side to the detection light L1. That is, as shown in Fig. 10, the reflected light L2 reflected by the wafer surface Wa follows a path R3 on the opposite side of the main optical axis AX (the left side in Fig. 10) from the path R1 of the detection light L1, and passes through a second region G2 of the condenser lens pupil 754a.

[0094] 6, when the focal point of the condenser lens 754 coincides with the position of the lower end of the crack K, the detection light L1 is split into a reflected light component L2a that is totally reflected by the crack K, then reflected by the wafer surface Wa, and reaches an area of ​​the condenser lens pupil 754a on the same side as the detection light L1, and a non-reflected light component L2b that is not totally reflected by the crack K, but is reflected by the wafer surface Wa of the wafer W, and reaches an area of ​​the condenser lens pupil 754a on the opposite side from the detection light L1. That is, as shown in FIG. 10, of the reflected light L2, the reflected light component L2a (see FIG. 6) that is totally reflected by the crack K inside the wafer W passes through a first region G1 of the condenser lens pupil 754a along a path R2, and the non-reflected light component L2b (see FIG. 6) that is not totally reflected by the crack K and is reflected by the wafer surface Wa of the wafer W passes through a second region G2 of the condenser lens pupil 754a along a path R3.

[0095] The photodetectors 704 and 706 are arranged in positions optically conjugate with the first region G1 and the second region G2 of the condenser lens pupil 754a, respectively, so that the photodetectors 704 and 706 can selectively receive light that has passed through the first region G1 and the second region G2 of the condenser lens pupil 754a, respectively.

[0096] 4 (where crack K exists at the focal point of condenser lens 754), reflected light L2 is incident on light-receiving surface 704C of photodetector 704, of photodetectors 704 and 706, as shown in FIG. 7. Therefore, the level of the detection signal output from light-receiving surface 704C of photodetector 704 becomes higher than the level of the detection signal output from light-receiving surface 706C of photodetector 706.

[0097] 5 (where crack K is not present at the focusing point of condenser lens 754), the reflected light is incident on light-receiving surface 706C of photodetector 706, of photodetectors 704 and 706, as shown in Fig. 8. Therefore, the level of the detection signal output from light-receiving surface 706C of photodetector 706 becomes higher than the level of the detection signal output from light-receiving surface 704C of photodetector 704.

[0098] In addition, in the example shown in Figure 6 (where the focal point of the focusing lens 754 coincides with the lower end position of the crack K), as shown in Figure 9, components L2a and L2b of the reflected light L2 are incident on the light receiving surfaces 704C and 706C of the photodetectors 704 and 706, respectively, so that the levels of the detection signals output from the light receiving surfaces 704C and 706C of the photodetectors 704 and 706 are approximately equal.

[0099] In this way, the amount of light received by the light receiving surfaces 704C and 706C of the photodetectors 704 and 706 varies depending on whether or not a crack K is present at the focal point of the condenser lens 754. In this embodiment, this property is utilized to detect the crack depth (the crack bottom end position or the crack top end position) of a crack K formed inside the wafer W.

[0100] Specifically, when the detection signal outputs from the light receiving surfaces 704C and 706C of the photodetectors 704 and 706 are D1 and D2, respectively, the evaluation value S for determining the presence of a crack K at the focusing point of the focusing lens 754 can be expressed by the following equation:

[0101] S = (D1 - D2) / (D1 + D2) ... (1) In equation (1), when the condition S=0 is satisfied, that is, when the amount of light received by the light receiving surfaces 704C and 706C of the photodetectors 704 and 706 is the same, the focal point of the focusing lens 754 and the bottom end position of the crack (or the top end position of the crack) are aligned.

[0102] The control unit 750 controls the focal point position moving mechanism 752 to move the position of the focal point FP2 (see FIG. 12) of the detection light L1 in the Z direction (the optical axis direction of the focusing lens 754). The control unit 750 also sequentially acquires detection signals output from the light receiving surfaces 704C and 706C of the photodetectors 704 and 706 while sequentially changing the position of the focal point FP2 from the interface position of the wafer surface Wa in the thickness direction (Z direction) of the wafer W. The control unit 750 then calculates an evaluation value S shown in equation (1) based on the detection signals, and can detect the crack depth (the crack bottom end position or the crack top end position) of the crack K by evaluating the evaluation value S and the focal point position information. The above is the crack detection function of the crack detection device 500.

[0103] [Crack detection device thickness detection function and external output function] Next, the function of the crack detection device 500 to detect the thickness of the wafer W and the function of externally outputting the thickness detection result of the wafer W (thickness profile 42, see FIG. 11) to the backside grinding device 200 will be described.

[0104] As described above, the crack detection device 500 has a crack detection function, as well as a thickness detection function and an external output function. The thickness detection function of the crack detection device 500 utilizes each component of the crack detection device 500 to detect the thickness of the wafer W (particularly the silicon layer) after backgrinding by the backgrinding device 200 and before the formation of the laser processing region R by the laser processing device 10. Furthermore, the external output function of the crack detection device 500 outputs the thickness detection result of the wafer W to the backgrinding device 200. The thickness detection function and the external output function will be described below.

[0105] The thickness detection function detects the thickness of the wafer W using the components of the crack detection device 500, namely, the table drive unit 14, the suction stage 18, the light source 552A, the illumination optical system 600, the interface position detection optical system 650, the control unit 750, the focal point position moving mechanism 752 (piezo actuator 752a), the focusing lens 754, the operation unit 756, and the display unit 758.

[0106] Fig. 11 is a functional block diagram of the control unit 750 of the crack detection device 500. Note that in Fig. 11, functions other than the thickness detection function of the crack detection device 500 are omitted as appropriate.

[0107] <Thickness detection function> As shown in FIG. 11 and the above-described FIG. 3, when the control unit 750 executes the thickness detection function, it executes a control program read from a storage unit (not shown) to function as a light source control unit 800, a drive control unit 802, an interface position detection unit 803, and a thickness detection unit 804.

[0108] When the thickness detection function is executed, the light source control unit 800 causes the light source 552A to emit detection light L1(A). As a result, the detection light L1(A) is irradiated onto the wafer W via the illumination optical system 600 and the condenser lens 754, and reflected light L2(A) from the wafer W is detected by the photodetector 660 (corresponding to the second photodetector of the present invention) via the condenser lens 754 and the interface position detection optical system 650.

[0109] The drive control unit 802 controls the table drive unit 14 to move the suction stage 18 (wafer W) relative to the condenser lens 754 in the X and Y directions (corresponding to the vertical direction of the present invention), thereby aligning the optical axis of the condenser lens 754 with a measurement point MP (see FIG. 12) for detecting the thickness of the wafer W. In this case, the drive control unit 802 functions as a second movement mechanism of the present invention together with the table drive unit 14.

[0110] A plurality of measurement points MP (see FIG. 12) are positioned along a scan line CL (see FIG. 14) that passes through the center of the wafer W. Note that a plurality of measurement points MP may be positioned on the processing line DL of the wafer W, and further, a plurality of measurement points MP may be positioned at predetermined intervals on the processing line DL (for example, at the intersections of the vertical and horizontal processing lines DL).

[0111] Fig. 12 is an explanatory diagram for explaining scanning of the focal point FP2 of the detection light L1(A) in the Z direction by the drive control unit 802. As shown in Fig. 12, after aligning the focusing lens 754 with the measurement point MP, the drive control unit 802 drives at least one of the piezo actuator 752a and the processing head Z drive mechanism to adjust the relative distance between the focusing lens 754 and the wafer W, and sets the position of the focal point FP2 of the focusing lens 754 [detection light L1(A)] to a position spaced above the wafer back surface Wb in the Z direction.

[0112] Next, the drive control unit 802 drives the piezo actuator 752a of the focal point position moving mechanism 752 to move the condenser lens 754 downward in the Z direction (i.e., in a direction approaching the wafer W), thereby changing the position of the focal point FP2 of the detection light L1(A) relative to the wafer W in the Z direction, i.e., scanning the focal point FP2 in the Z direction. As a result, the photodetector 660 of the interface position detecting optical system 650 detects the reflected light L2(A) from the wafer W for each position of the condenser lens 754 in the Z direction, and outputs a detection signal of the reflected light L2(A) to the thickness detection unit 804. In this case, the drive control unit 802 and the piezo actuator 752a correspond to the focal point scanning unit of the present invention.

[0113] The reflected light L2(A) is focused at the position of a pinhole in the pinhole panel 660B, which is located optically conjugate with the focusing point of the focusing lens 754. Then, when the focusing point FP2 of the focusing lens 754 coincides with the wafer back surface Wb due to downward movement of the focusing lens 754, the detection signal output from the detector main body 660A has a sharp peak. Then, when the downward movement of the focusing lens 754 by the piezo actuator 752a continues and the focusing point FP2 of the focusing lens 754 coincides with the position of the wafer front surface Wa, the detection signal output from the detector main body 660A also has a sharp peak.

[0114] FIG. 13 is a graph showing the relationship between the movement amount (μm) of the focal point FP2 caused by the piezoelectric actuator 752a and the detection signal (V) output from the detector main body 660A of the photodetector 660. As shown in FIG.

[0115] 13, the detection signal (V) has peaks P1 and P2 when the movement amount (μm) of the piezo actuator 752a, i.e., the movement amount of the condenser lens 754, is around 20 μm and 230 μm, with peak P1 indicating the interface position of the wafer back surface Wb and peak P2 indicating the interface position of the wafer front surface Wa. This makes it possible to detect the interface positions indicating the wafer front surface Wa and the wafer back surface Wb, and further makes it possible to detect the thickness of the wafer W based on the interface positions of both the wafer front surface Wa and the wafer back surface Wb.

[0116] Hereinafter, when there are multiple measurement points MP, the drive control unit 802 repeatedly operates for each measurement point MP, and repeatedly performs alignment of the condenser lens 754 with respect to the measurement point MP and scanning of the condenser point FP2 in the Z direction. This allows scanning of the condenser point FP2 for each measurement point MP while changing the measurement point MP multiple times.

[0117] 11, the interface position detection unit 803 detects the interface position of the wafer W at each measurement point MP based on the detection signal (see FIG. 13) output from the photodetector 660 while the focus point FP2 is being changed in the Z direction by the drive control unit 802. Specifically, the interface position detection unit 803 detects the interface positions indicating the wafer front surface Wa and the wafer back surface Wb based on the detection signal output from the photodetector 660 by detecting peaks P1 and P2 of the detection signal. This makes it possible to detect the interface position of the silicon layer of the wafer W.

[0118] The thickness detection unit 804 detects the thickness of the wafer W at the measurement point MP measured by the interface position detection unit 803. For example, when the interface position detection unit 803 detects the wafer front surface Wa and the wafer back surface Wb from the detection signals shown in FIG. 13, the thickness detection unit 804 subtracts 20 μm, which corresponds to the interface position of the wafer back surface Wb, from 230 μm, which corresponds to the interface position of the wafer front surface Wa. In other words, the thickness detection unit 804 detects the thickness of the wafer W based on the amount of movement of the condenser lens 754 when the interface positions indicating the wafer front surface Wa and the wafer back surface Wb are detected. This makes it possible to detect the thickness (210 μm) of the wafer W at the measurement point MP.

[0119] Fig. 14 is an explanatory diagram for explaining the arrangement of measurement points MP on a wafer W. Fig. 15 is a cross-sectional view of the wafer W along the scan line CL in Fig. 14. Note that the symbol Th in Fig. 15 indicates the thickness of each measurement point MP of the wafer W. Furthermore, the measurement points MP shown in Figs. 14 and 15 show only some of the measurement points MP arranged along the scan line CL, and the number and arrangement of the measurement points MP are not limited to the examples shown in Figs. 14 and 15.

[0120] 14 and 15, a plurality of measurement points MP are arranged along a scan line CL that passes through the center of the wafer W. Therefore, each time the optical axis of the condenser lens 754 is moved to each measurement point MP in order by the table driving unit 14, the following operations are repeatedly performed: scanning of the condenser point FP2 in the Z direction by the piezo actuator 752a, detection of the reflected light L2(A) by the photodetector 660, detection of the interface position by the interface position detecting unit 803, and thickness detection by the thickness detecting unit 804. In this way, the thickness of the wafer W is detected at each of the plurality of measurement points MP along the scan line CL.

[0121] Here, the shape and thickness of the wafer W back-ground by the back grinding apparatus 200 are nearly point-symmetrical with respect to its center (the center during grinding) (see Patent Document 1 above). In this case, the thickness of the wafer W changes concentrically from the center and smoothly in the surface direction. Therefore, the in-plane thickness distribution of the wafer W can be determined (estimated) based on the detection results of the thickness of the wafer W at each of the multiple measurement points MP along the scan line CL.

[0122] Returning to Figure 11, the thickness detection unit 804 detects a thickness profile 42 indicating the change (distribution) in the thickness of the wafer W along the processing line DL for each processing line DL of the wafer W based on the detection results of the thickness of the wafer W at each of multiple measurement points MP along the scan line CL.

[0123] The thickness profile 42 for each processing line DL detected by the thickness detection unit 804 is output to the external output unit 806 and is also displayed on the display unit 758. The thickness detection function of the crack detection device 500 has been described above.

[0124] <External output function> Next, a specific description will be given of the external output function of the crack detection device 500. This external output function uses the control unit 750 and the communication IF 759, which are components of the crack detection device 500, to externally output the thickness profile 42 for each processing line DL to the laser processing device 10.

[0125] When the control unit 750 executes the external output function, it functions as an external output unit 806 by executing a control program read from a storage unit (not shown).

[0126] The external output unit 806 outputs the thickness profile 42 for each processing line DL detected by the thickness detection unit 804 to the communication IF 28 (see FIG. 16) of the laser processing apparatus 10 via the communication IF 759. This allows the laser processing apparatus 10 to perform laser processing by referring to the thickness profile 42 for each processing line DL. The above is the external output function of the crack detection device 500.

[0127] [Laser processing using laser processing equipment] 16 is a functional block diagram of the control unit 12 of the laser processing apparatus 10. Based on the thickness profile 42 detected by the crack detection device 500 and the detection result of the height position of the wafer back surface Wb by the AF sensor 24, the laser processing apparatus 10 performs laser processing to form a laser processing region R within the wafer W along the processing line DL and at a position a certain distance from the wafer front surface Wa for each processing line DL.

[0128] 16, in addition to the table driving unit 14, laser light sources 20 and 25, and photodetector 26, a memory unit 27 and a communication IF 28 are connected to the control unit 12 of the laser processing apparatus 10. The communication IF 28 is connected by wire or wirelessly to the communication IF 759 of the crack detection device 500, and a thickness profile 42 is input from the communication IF 759. The memory unit 27 stores the control program (not shown) of the control unit 12 as well as the thickness profile 42 input from the crack detection device 500.

[0129] The control unit 12 executes a control unit program read from the storage unit 27, thereby functioning as a light source control unit 32, a drive control unit 34, a height position detection unit 36, a profile acquisition unit 38, and a focal point position adjustment unit 40.

[0130] The light source control unit 32 continuously emits the detection laser light LD from the laser light source 25 of the AF sensor 24. As a result, the detection laser light LD is condensed onto the wafer back surface Wb via the dichroic mirror 21 and the condenser lens 22, and the reflected light LR from the wafer back surface Wb is continuously incident on the photodetector 26 of the AF sensor 24 via the condenser lens 22 and the dichroic mirror 21. In addition, the light source control unit 32 continuously emits the processing laser light LP from the laser light source 20. As a result, the processing laser light LP is condensed at a focusing point FP1 inside the wafer W via the dichroic mirror 21 and the condenser lens 22.

[0131] The drive control unit 34 controls the table drive unit 14 to move the suction stage 18 (wafer W) relative to the condenser lens 22 in the X and Y directions (corresponding to the vertical directions of the present invention), thereby aligning the optical axis of the condenser lens 22 with the processing start position of the processing line DL. In this case, the drive control unit 34 functions as the first movement mechanism of the present invention together with the table drive unit 14. Then, when the laser processing is started, the drive control unit 34 controls the table drive unit 14 to perform the above-mentioned processing feed and one-pitch indexing feed.

[0132] The height position detection unit 36 ​​continuously detects the height position in the Z direction of the wafer back surface Wb at the laser processing position of the wafer W using known methods such as the astigmatism method and the knife edge method based on the detection signal of the reflected light LR continuously input from the photodetector 26, and continuously outputs the detection results of this height position to the focal point position adjustment unit 40.

[0133] The profile acquisition unit 38 acquires the thickness profile 42 for each processing line DL input from the communication IF 759 of the crack detection device 500 via the communication IF 28 and stores it in the storage unit 27.

[0134] Fig. 17 is an explanatory diagram showing a planned formation line RP of the laser processing region R on the wafer W. As shown in Fig. 17 and the already-described Fig. 16, the focal point position adjustment unit 40 drives the piezo actuator 23 to adjust the position of the focal point FP1 (condenser lens 22) in the Z direction so that the focal point FP1 moves along the planned formation line RP at a predetermined distance from the wafer surface Wa, i.e., so that the laser processing region R is formed along the planned formation line RP.

[0135] FIG. 18 is an explanatory diagram for explaining the position adjustment of the condensing point FP1 (condensing lens 22) in the Z direction by the condensing point position adjusting unit 40. As shown in FIG.

[0136] 18, the focal point position adjusting unit 40 determines the height position in the Z direction of the wafer back surface Wb at the laser processing position based on the height position detection result input from the height position detecting unit 36. Here, the focal point position adjusting unit 40 is preset with a processing depth design value, which is a design value of the Z direction distance from the wafer back surface Wb to the planned formation line RP, and a thickness design value, which is a design value (reference value) of the thickness of the wafer W.

[0137] The focal point position adjusting unit 40 detects the thickness of the wafer W at the laser processing position by referring to the thickness profile 42 corresponding to the processing line DL in the memory unit 27. Next, the focal point position adjusting unit 40 adds an offset Δh to the processing depth design value based on the thickness of the wafer W at the laser processing position.

[0138] For example, if the actual thickness of the wafer W at the laser processing position is greater than the design thickness, a positive offset Δh is added to the design processing depth value in accordance with the increase. Conversely, if the actual thickness of the wafer W is smaller than the design thickness, a negative offset Δh is added to the design processing depth value in accordance with the decrease. The offset Δh is determined so that the distance from the wafer front surface Wa to the planned formation line RP is constant. Then, the focal point position adjustment unit 40 drives the piezo actuator 23 to adjust the Z-direction position of the focal point FP1 based on the Z-direction height position of the wafer back surface Wb at the laser processing position previously determined and the offset design processing depth value, thereby aligning the Z-direction position of the focal point FP1 with the planned formation line RP. Note that the symbol SD denotes the position of the focal point of the detection laser beam LD after position adjustment by the focal point position adjustment unit 40.

[0139] Thereafter, while the drive control unit 34 controls the table drive unit 14 to perform processing feed, the detection of the reflected light LR by the photodetector 26, the detection of the height position by the height position detection unit 36, and the adjustment of the Z direction position of the focal point FP1 by the focal point position adjustment unit 40 are repeatedly performed. As a result, a laser processing region R is formed inside the wafer W along the planned formation line RP.

[0140] [Method for controlling laser processing equipment (action)] FIG. 19 is a flowchart showing the flow of the process of detecting the thickness profile 42 for each processing line DL of the wafer W by the crack detection device 500 having the above configuration.

[0141] As shown in FIG. 19, the wafer W whose back surface Wb has been back-ground by the back-grinding device 200 is set on the suction stage 18 of the crack detection device 500 (laser processing device 10) (step S1).

[0142] Next, when the operator inputs a measurement operation for the thickness profile 42 of the wafer W into the operation unit 756, the control unit 750 functions as a light source control unit 800, a drive control unit 802, an interface position detection unit 803, a thickness detection unit 804, and an external output unit 806.

[0143] Then, the light source control unit 800 causes the light source 552A to emit the detection light L1(A) (step S2). As a result, the detection light L1(A) is irradiated onto the wafer W via the illumination optical system 600 and the condenser lens 754.

[0144] Furthermore, the drive control unit 802 controls the table drive unit 14 to align the optical axis of the condenser lens 754 with the measurement point MP on the scan line CL of the wafer W. Next, the drive control unit 802 drives at least one of the piezo actuator 752a and the processing head Z drive mechanism to adjust the relative distance between the condenser lens 754 and the wafer W, and then drives the piezo actuator 752a to change the position of the condenser point FP2 of the detection light L1(A) in the Z direction (step S3). This causes the condenser point FP2 to scan in the Z direction.

[0145] While the focal point FP2 is scanned in the Z direction, the reflected light L2(A) from the wafer W is detected by the photodetector 660 via the focusing lens 754 and the interface position detection optical system 650, and the photodetector 660 repeatedly outputs a detection signal to the thickness detection unit 804 (NO in steps S4 and S5).

[0146] When scanning of the focal point FP2 in the Z direction is completed (YES in step S5), the interface position detection unit 803 detects the interface positions indicating the wafer front surface Wa and the wafer back surface Wb based on the detection signal input from the photodetector 660 during scanning of the focal point FP2, as shown in Fig. 13. Then, the thickness detection unit 804 detects the thickness of the wafer W at the measurement point MP based on the amount of movement of the condenser lens 754 when the interface positions indicating the wafer front surface Wa and the wafer back surface Wb are detected by the interface position detection unit 803 (step S6).

[0147] Thereafter, for each remaining measurement point MP along the scan line CL, the alignment of the optical axis of the condenser lens 754 with respect to the next measurement point MP and the processes from step S3 to step S6 are repeatedly executed (YES in step S7, step S8). As a result, the thickness detection unit 804 detects the thicknesses of all of the measurement points MP along the scan line CL, and a thickness profile 42 for each processing line DL is detected based on the thickness of each measurement point MP (NO in step S7, step S9). In this manner, in this embodiment, by utilizing the interface detection function of the crack detection device 500, the thickness of the silicon layer of the wafer W can be accurately detected even when the wafer surface Wa is covered with a protective sheet 220 or when unevenness is formed in the device layer of the wafer surface Wa. As a result, the thickness profile 42 can be accurately detected.

[0148] When the thickness detection unit 804 completes the thickness profile 42 for each processing line DL, the external output unit 806 outputs each thickness profile 42 to the communication IF 28 of the laser processing apparatus 10 via the communication IF 759 (step S10).

[0149] When the external output unit 806 has completed external output of each thickness profile 42, the laser processing device 10 starts laser processing.

[0150] Fig. 20 is a flowchart showing the flow of laser processing for each processing line DL by the laser processing apparatus 10 configured as described above. As shown in Fig. 20, after detecting the thickness profile 42, when the operator inputs an operation to start laser processing of the wafer W to an operation unit (not shown) of the laser processing apparatus 10 (which may also serve as the operation unit 756), the control unit 12 functions as a light source control unit 32, a drive control unit 34, a height position detection unit 36, a profile acquisition unit 38, and a focal point position adjustment unit 40.

[0151] The profile acquisition unit 38 acquires the thickness profile 42 for each processing line DL input from the crack detection device 500 via the communication IF 28, and stores these thickness profiles 42 in the storage unit 27 (step S11). Note that step S11 is not particularly limited as long as it is performed before step S15, which will be described later.

[0152] Furthermore, the drive control unit 802 controls the table driving unit 14 to align the optical axis of the condenser lens 22 with the processing start position of the processing line DL.

[0153] When this alignment is completed, the light source control unit 32 causes the laser light source 25 to continuously emit the detection laser light LD (step S12) and the laser light source 20 to continuously emit the processing laser light LP (step S12A). As a result, the detection laser light LD is focused on the wafer back surface Wb by the condenser lens 22, and the reflected light LR from the wafer back surface Wb is continuously incident on the photodetector 26, which then continuously outputs a detection signal of the reflected light LR to the height position detection unit 36 ​​(step S13). In addition, the processing laser light LP is focused at a focusing point FP1 inside the wafer W via the condenser lens 22.

[0154] When the detection of the reflected light LR by the photodetector 26 begins, the height position detection unit 36 ​​continuously detects the height position in the Z direction of the wafer back surface Wb at the laser processing position of the wafer W based on the detection signal of the reflected light LR continuously input from the photodetector 26, and continuously outputs the detection result of this height position to the focal point position adjustment unit 40 (step S14).

[0155] Next, the focal point position adjusting unit 40 determines the height position in the Z direction of the wafer back surface Wb at the laser processing position based on the height position detection results continuously input from the height position detecting unit 36. The focal point position adjusting unit 40 also detects the thickness of the wafer W at the laser processing position by referring to the thickness profile 42 corresponding to the processing line DL in the memory unit 27, and adds an offset Δh to the processing depth design value based on this detection result as shown in FIG.

[0156] Then, based on the height position in the Z direction of the wafer back surface Wb at the laser processing position and the offset processing depth design value, the focal point position adjusting unit 40 drives the piezo actuator 23 to adjust the position of the focal point FP1 in the Z direction, and aligns the Z direction position of the focal point FP1 with the planned formation line RP (step S16). As a result, the laser processing region R is formed on the planned formation line RP.

[0157] Thereafter, the drive control unit 34 controls the table drive unit 14 to perform processing feed to move the suction stage 18 (wafer W) in the X direction relative to the condenser lens 22 (NO in step S17, step S18), while the processing from step S12 to step S18 is being performed repeatedly. As a result, the laser processing region R is formed along the planned formation line RP, and the formation position of the laser processing region R is maintained at a constant distance from the wafer front surface Wa.

[0158] Then, when laser processing of one processing line DL is completed (YES in step S17), the drive control unit 34 controls the table drive unit 14 to move the suction stage 18 in the X and Y directions relative to the condenser lens 22, thereby aligning the optical axis of the condenser lens 22 with the processing start position of the next processing line DL (NO in step S19, step S20). When this alignment is completed, the processes from step S12 to step S18 described above are repeated again, and laser processing of the next processing line DL is performed. Laser processing of the remaining processing lines DL is performed in the same manner (YES in step S19).

[0159] As described above, in this embodiment, by utilizing the interface detection function of the crack detection device 500, the thickness profile 42 of the wafer W can be detected more accurately than in the past, and the distance of the focal point FP1 from the wafer surface Wa can be maintained at a constant distance with greater precision.

[0160] [others] 21 is an explanatory diagram showing a planned formation line RP of a laser processing region R in a wafer W having a non-flat wafer surface Wa. Fig. 21 is an explanatory diagram for explaining the adjustment of the position of the focal point FP1 in the Z direction by the focal point position adjusting unit 40 when laser processing the wafer W shown in Fig. 20 is performed.

[0161] In the above embodiment, the case where the wafer surface Wa is flat has been described as an example, but in reality, the wafer surface Wa is not flat, and the distance between the wafer surface Wa (holding surface 18a) and the condenser lens 22 fluctuates due to disturbances, as shown in Fig. 21. In this case, the planned formation line RP also has a non-linear shape (for example, a wavy shape).

[0162] 22 , the focal point position adjustment unit 40 drives the piezo actuator 23 to align the Z-direction position of the focal point FP1 with the planned formation line RP based on the Z-direction height position of the wafer back surface Wb at the laser processing position and the design processing depth offset based on the thickness of the wafer W at the laser processing position. Then, while the processing feed is being performed, similar to the above embodiment, the detection of the reflected light LR by the photodetector 26, the detection of the height position by the height position detection unit 36, and the adjustment of the Z-direction position of the focal point FP1 by the focal point position adjustment unit 40 are repeatedly performed. In this case, while the processing feed is being performed, the piezo actuator 23 expands and contracts in the Z direction to maintain a constant distance between the wafer front surface Wa and the focal point FP1. As a result, a laser processing region R is formed within the wafer W along the planned formation line RP.

[0163] The laser processing apparatus 10 and the crack detection apparatus 500 of the above embodiment employ a configuration in which the processing head 16 is movable in the Z direction and the suction stage 18 is movable (rotatable) in the X, Y, and θ directions, but the configuration is not limited to the embodiment as long as the relative positions of the processing head 16 and the suction stage 18 in the X, Y, and Zθ directions can be changed. For example, the suction stage 18 may be configured to be movable (rotatable) in the X, Y, and Zθ directions, or the processing head 16 may be configured to be movable in the Y and Z directions and the suction stage 18 may be configured to be movable (rotatable) in the Xθ direction.

[0164] The laser processing apparatus 10 and the crack detection apparatus 500 in the above embodiment employ a configuration in which the processing head 16 and the crack detection apparatus 500 are integrally movable in the Z direction, but this is not limitative, and a configuration in which the processing head 16 and the crack detection apparatus 500 are independently movable in the Z direction may also be employed. Furthermore, the direction in which the processing head 16 and the crack detection apparatus 500 are independently movable is not limited to the Z direction, and they may be movable in other directions (for example, the X direction or the Y direction).

[0165] In the above embodiment, the thickness profile 42 for each processing line DL is detected based on the detection results of the thickness of the wafer W at each of multiple measurement points MP along the scan line CL, but the thickness profile 42 for each processing line DL may also be actually measured.

[0166] In the above embodiment, a wafer W (silicon wafer) has been used as an example of the workpiece of the present invention, but the present invention can also be applied to laser processing of glass substrates, piezoelectric ceramic substrates, glass substrates, etc. [Explanation of symbols]

[0167] 10 Laser processing equipment 12 Control Unit 14 Table drive unit 16 Processing head 18 Adsorption stage 18a Holding surface 19 Processing head drive unit 20 Laser light source 21 Dichroic mirror 22 Condenser lens 23 Piezo Actuator 24 AF sensors 25 Laser light source 26 Photodetector 27 Memory section 32 Light source control unit 34 Drive control unit 36 Height position detection unit 38 Profile Acquisition Unit 40 Focusing point position adjustment section 42 Thickness Profile 100 Laser Processing System 200 Back grinding equipment 220 Protective Sheet 300 splitting device 500 Crack detection device 550 Light source section 552A light source 552B light source 552C light source 554 Half Mirror 600 Illumination optical system 602 relay lens 604 Mirror 606 Relay Lens 650 Interface position detection optical system 652 Half Mirror 654 Half Mirror 656 Relay Lens 658 Half Mirror 660 Photodetector 660A detector body 660B Pinhole Panel 700 Crack detection optical system 702 Relay Lens 704 Photodetector 704C Photosensitive surface 706 Photodetector 706C Photosensitive surface 750 control section 752 Focus point position movement mechanism 752a Piezo Actuator 754 Condenser Lens 754a Condenser pupil 756 Operation section 758 Display section 760 Observation Optical System 800 Light source control unit 802 Drive control unit 803 Interface position detection unit 804 Thickness detection unit 806 External Output Unit AX main optical axis CL scan line DL processing line FP1 Focus point FP2 focal point G1 1st area G2 2nd area GUI operation IF28 Communications IF759 Communications K crack L laser light L1 Detected light L2 reflected light L2a reflected light component L2b non-reflected light component LD detection laser light LP processing laser light LR reflected light MP measurement point Nd semiconductor laser pumping P1 peak P2 peak R Laser processing area R1 Route R2 route R3 Route RP planned formation line S rating W wafer Wa wafer surface Wb Wafer backside Z processing head Δh offset

Claims

1. A laser processing apparatus including: a processing head that irradiates a workpiece with a processing laser beam from a back surface side thereof along a processing line of the workpiece while aligning a first focusing point of the processing laser beam with the inside of the workpiece, thereby forming a laser processing area inside the workpiece; and a crack detection device that detects a crack extending from the laser processing area inside the workpiece, The crack detection device an interface position detection unit that detects interface positions indicating the front and back surfaces of the workpiece at a plurality of predetermined measurement points; a thickness detection unit that detects a thickness profile indicating a change in thickness of the workpiece along the processing line based on a detection result of the interface position detection unit; Equipped with The processing head a laser light source unit including a first laser light source that emits the processing laser light and a second laser light source that emits the detection laser light; a first focusing lens that focuses the processing laser light emitted from the first laser light source inside the workpiece and focuses the detection laser light emitted from the second laser light source on a rear surface of the workpiece; a first light detection unit that detects reflected light of the detection laser light from a rear surface of the workpiece; a height position detection unit that detects a height position of the rear surface of the workpiece based on a detection result of the first light detection unit; a profile acquisition unit that acquires the thickness profile from the thickness detection unit; a focal point position adjusting unit that adjusts the position of the first focal point along the optical axis direction of the first focusing lens based on the detection result of the height position detecting unit and the thickness profile acquired by the profile acquiring unit, and maintains the position of the first focal point at a predetermined constant distance from the surface of the workpiece; Equipped with the interface position detection unit detects the interface position for each of the plurality of measurement points only along a scan line passing through the center of the workpiece whose back surface has been ground by a grinding device; A laser processing device in which the thickness detection unit estimates an in-plane thickness distribution of the workpiece based on the detection result of the interface position detection unit, and detects the thickness profile based on the estimated result of the thickness distribution.

2. a first moving mechanism that moves the first condenser lens relative to the workpiece in a direction perpendicular to an optical axis direction of the first condenser lens to move the first condenser point along the processing line; 2. The laser processing apparatus according to claim 1, wherein the first light detection unit, the height position detection unit, and the focus position adjustment unit are repeatedly operated each time the first focus is moved along the processing line by the first moving mechanism.

3. The crack detection device a light source that emits detection light; a second condenser lens that condenses the detection light emitted from the light source onto the workpiece; a condenser scanning unit configured to scan a second condenser point of the detection light along an optical axis of the second condenser lens; a second light detection unit that detects reflected light of the detection light from the workpiece while the second light focus scanning unit is scanning the second light focus; a second movement mechanism that moves the second condenser lens relative to the workpiece in a direction perpendicular to an optical axis direction of the second condenser lens; Equipped with the second moving mechanism moves the second condensing point to the plurality of measurement points in order, and the condensing point scanning unit and the second light detecting unit are repeatedly operated; The laser processing device according to claim 1 or 2, wherein the interface position detection unit detects the interface position for each of the plurality of measurement points based on a detection result from the second light detection unit for each of the plurality of measurement points.

4. 4. The laser processing device according to claim 1, wherein the crack detection device is provided in the processing head.

5. A control method for a laser processing device including: a processing head that irradiates the processing laser light from a back surface side of the workpiece along a processing line of the workpiece while a first focusing lens is used to align a first focusing point of the processing laser light with the inside of the workpiece, thereby forming a laser processing area inside the workpiece; and a crack detection device that detects a crack extending from the laser processing area inside the workpiece, The crack detection device detects interface positions indicating the front and back surfaces of the workpiece at a plurality of measurement points along only a scan line passing through the center of the workpiece whose back surface has been ground by a grinding device, and based on the detection results, estimates an in-plane thickness distribution of the workpiece, and detects a thickness profile indicating a change in the thickness of the workpiece along the processing line based on the estimated thickness distribution results; The processing head The processing laser light is emitted from a first laser light source, A detection laser beam is emitted from a second laser light source; a first focusing lens for focusing the processing laser light emitted from the first laser light source inside the workpiece and focusing the detection laser light emitted from the second laser light source on the back surface of the workpiece; Detecting reflected light of the detection laser beam from a rear surface of the workpiece; Detecting a height position of the rear surface of the workpiece based on a detection result of the reflected light of the detection laser light; obtaining the thickness profile from the crack detection device; A control method for a laser processing device, which adjusts the position of the first focal point along the optical axis direction of the first focusing lens based on the height position detection result and the thickness profile, and maintains the position of the first focal point at a predetermined constant distance from the surface side of the workpiece.

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