JP6347714B2 - Wafer processing method - Google Patents

Description

  In the present invention, after a modified layer is formed inside a wafer by irradiating a pulse laser beam having a wavelength that is transmissive to the wafer, an external force is applied to the wafer, and the wafer is made into a plurality of devices starting from the modified layer. The present invention relates to a method for processing a wafer to be divided into chips.

  A silicon wafer (hereinafter, simply referred to as a wafer) formed by dividing a plurality of devices such as ICs and LSIs on a surface by dividing lines is divided into individual device chips by a processing apparatus. Chips are widely used in various electric devices such as mobile phones and personal computers.

  A dicing method using a cutting device called a dicing saw is widely used for dividing the wafer. In the dicing method, a wafer is cut by cutting a wafer into a wafer while rotating a cutting blade having a thickness of about 30 μm by solidifying abrasive grains such as diamond with a metal or a resin at a high speed of about 30000 rpm. Divide into chips.

  On the other hand, in recent years, a condensing point of a pulse laser beam having a wavelength that is transmissive to the wafer is positioned inside the wafer corresponding to the division line, and the pulse laser beam is irradiated along the division line to be irradiated to the wafer. There has been proposed a method in which a modified layer is formed inside and then an external force is applied to divide the wafer into individual device chips (see, for example, Japanese Patent No. 4402708).

  The modified layer is a region where the density, refractive index, mechanical strength and other physical properties are different from the surroundings. In addition to the melt resolidification region, refractive index change region, dielectric breakdown region, A crack region and a region where these are mixed are also included.

  The optical absorption edge of silicon is in the vicinity of a light wavelength of 1050 nm corresponding to the band gap (1.1 eV) of silicon, and light having a shorter wavelength is absorbed by bulk silicon.

  In the conventional modified layer forming method, a Nd: YAG pulse laser doped with neodymium (Nd) that oscillates a laser with a wavelength of 1064 nm close to the optical absorption edge is generally used (for example, JP-A-2005-95952). reference).

  However, since the wavelength of 1064 nm of the Nd: YAG pulse laser is close to the optical absorption edge of silicon, a part of the laser beam is absorbed in the region sandwiching the condensing point, so that a sufficient modified layer is not formed, and each wafer is individually separated. May not be divided into device chips.

  Therefore, when the present applicant forms a modified layer inside the wafer using a YAG pulse laser having a wavelength of 1300 to 1400 nm, for example, having a wavelength of 1342 to 1400 nm, the laser beam is emitted in a region sandwiching the condensing point. It has been found that the absorption can be reduced and a good modified layer can be formed, and the wafer can be smoothly divided into individual device chips (see Japanese Patent Application Laid-Open No. 2006-108459).

Japanese Patent No. 4402708 JP-A-2005-95952 Japanese Patent Laid-Open No. 2006-108459

  However, if the laser beam is irradiated with the focused point of the pulse laser beam positioned inside the wafer adjacent to the modified layer formed immediately before the division line, the pulsed laser beam is formed. It has been found that a laser beam is scattered on the surface opposite to the surface irradiated with the laser beam, that is, the surface of the wafer, causing a new problem of attacking and damaging the device formed on the surface.

  When this problem was verified, fine cracks propagated from the modified layer formed immediately before to the surface side of the wafer, and the cracks refracted or reflected the transmitted light of the pulse laser beam to be irradiated next. It is inferred that they will attack. It has been verified that such a problem also occurs in a pulsed laser beam with a wavelength of 1064 nm, although not as long as a wavelength of 1342 nm.

  The present invention has been made in view of these points, and an object of the present invention is to form a modified layer inside a wafer by irradiating a pulse laser beam having a wavelength that is transmissive to the silicon wafer. At the same time, it is an object of the present invention to provide a wafer processing method capable of suppressing transmitted light from damaging a device on the wafer surface.

  According to the present invention, the holding means for holding the workpiece, and the modified layer is formed inside the workpiece by irradiating the workpiece held by the holding means with a pulse laser beam having a wavelength that is transmissive to the workpiece. A plurality of devices on the surface by a plurality of scheduled division lines by a laser processing apparatus comprising: a laser beam irradiating means for forming a laser beam; A wafer processing method for processing a wafer made of silicon formed in a partitioned manner, wherein a condensing point of a pulsed laser beam having a wavelength that is transparent to the wafer is positioned inside the wafer, and the wafer is processed from the back surface of the wafer. A pulse laser beam is applied to the area corresponding to the planned division line, and the holding means and the laser beam irradiation means are relatively processed and fed. A modified layer forming step for forming a modified layer inside the wafer, and after performing the modified layer forming step, an external force is applied to the wafer so that the wafer is separated from the modified layer starting from the modified layer. A step of splitting along the line, and in the modified layer forming step, a part of the pulse laser beam is missing from the center of the pulse laser beam irradiated to the wafer to the outer periphery on the downstream side in the processing feed direction. Thus, there is provided a wafer processing method characterized by positioning a condensing point of a pulsed laser beam inside the wafer.

  According to the wafer processing method of the present invention, in the modified layer forming step, a part of the pulse laser beam is lost in the portion from the center of the pulse laser beam to the outer periphery on the downstream side in the processing feed direction, and the focal point of the wafer is changed. Since it is positioned inside, sufficient energy is secured to form the modified layer, and even if fine cracks propagated from the modified layer exist on the surface side of the wafer, the condensing point is set. The transmitted pulse laser beam is inverted symmetrically around the focal point, and the leaked light (transmitted light) lacking the pulse laser beam is positioned at the crack formed on the surface side, so the scattering of the pulse laser beam is The problem of damaging a device formed on the surface of the wafer is extremely small.

It is a perspective view of the laser processing apparatus suitable for implementing the processing method of the wafer of this invention. It is a block diagram of a laser beam generation unit. It is a surface side perspective view of a silicon wafer. It is a perspective view which shows a mode that the outer peripheral part sticks the surface side of a silicon wafer to the dicing tape stuck to the annular frame. It is a back surface side perspective view of the silicon wafer supported by the annular frame via the dicing tape. 6A is a partially sectional side view for explaining the modified layer forming step, and FIG. 6B is a left side view of the mask shown in FIG. 6A. It is a schematic diagram explaining the strength of the power of the pulse laser beam irradiated to the wafer. It is a perspective view explaining a modified layer formation step. It is a perspective view of a dividing device. It is sectional drawing which shows a division | segmentation step.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Referring to FIG. 1, there is shown a schematic perspective view of a laser processing apparatus 2 suitable for carrying out the wafer processing method of the present invention.

  The laser processing apparatus 2 includes a first slide block 6 mounted on a stationary base 4 so as to be movable in the X-axis direction. The first slide block 6 is moved along the pair of guide rails 14 in the machining feed direction, that is, the X-axis direction, by the machining feed means 12 including the ball screw 8 and the pulse motor 10.

  A second slide block 16 is mounted on the first slide block 6 so as to be movable in the Y-axis direction. That is, the second slide block 16 is moved along the pair of guide rails 24 in the index feed direction, that is, the Y-axis direction, by the index feed means 22 constituted by the ball screw 18 and the pulse motor 20.

  A chuck table 28 is mounted on the second slide block 16 via a cylindrical support member 26. The chuck table 28 can be rotated, and can be rotated in the X-axis direction and the Y-axis direction by the processing feed means 12 and the index feed means 22. Can be moved to. The chuck table 28 is provided with a clamp 30 that clamps an annular frame that supports the wafer sucked and held by the chuck table 28.

  A column 32 is erected on the stationary base 4, and a laser beam irradiation unit 34 is attached to the column 32. The laser beam irradiation unit 34 includes a laser beam generation unit 35 shown in FIG. 2 housed in the casing 33 and a condenser 37 attached to the tip of the casing 33.

  As shown in FIG. 2, the laser beam generating unit 35 includes a laser oscillator 62 that oscillates a YAG pulse laser, a repetition frequency setting unit 64, a pulse width adjusting unit 66, and a power adjusting unit 68. In this embodiment, a YAG pulse laser oscillator that oscillates a pulse laser having a wavelength of 1342 nm is employed as the laser oscillator 62.

  As shown in FIG. 1, an image pickup unit 39 that detects a processing region to be laser processed in alignment with the condenser 37 and the X-axis direction is disposed at the tip of the casing 33. The imaging unit 39 includes an imaging element such as a normal CCD that images the processing area of the semiconductor wafer 11 with visible light.

  The imaging unit 39 further includes an infrared irradiation unit that irradiates the workpiece with infrared rays, an optical system that captures the infrared rays irradiated by the infrared irradiation unit, and an infrared ray that outputs an electrical signal corresponding to the infrared rays captured by the optical system. An infrared imaging means including an infrared imaging element such as a CCD is included, and the captured image signal is transmitted to a controller (control means) 40.

  The controller 40 includes a central processing unit (CPU) 42 that performs arithmetic processing according to a control program, a read-only memory (ROM) 44 that stores a control program, and a random read / write that stores arithmetic results. An access memory (RAM) 46, a counter 48, an input interface 50, and an output interface 52 are provided.

  A machining feed amount detection unit 56 includes a linear scale 54 disposed along the guide rail 14 and a reading head (not shown) disposed on the first slide block 6. These detection signals are input to the input interface 50 of the controller 40.

  Reference numeral 60 denotes an index feed amount detection unit composed of a linear scale 58 disposed along the guide rail 24 and a read head (not shown) disposed on the second slide block 16. The detection signal is input to the input interface 50 of the controller 40.

  An image signal captured by the imaging unit 39 is also input to the input interface 50 of the controller 40. On the other hand, a control signal is output from the output interface 52 of the controller 40 to the pulse motor 10, the pulse motor 20, the laser beam generation unit 35, and the like.

  Referring to FIG. 3, there is shown a front side perspective view of a semiconductor wafer 11 to be processed by the processing method of the present invention. A semiconductor wafer 11 shown in FIG. 3 is composed of, for example, a silicon wafer having a thickness of 100 μm.

  The semiconductor wafer 11 has a plurality of first division planned lines (streets) 13a extending in the first direction on the surface 11a and a plurality of second division planned extending in a second direction orthogonal to the first direction. A line 13b is formed, and a device 15 such as an IC or LSI is formed in each region partitioned by the first scheduled division line 13a and the second scheduled division line 13b. A notch 17 serving as a mark indicating the crystal orientation of the silicon wafer is formed on the outer periphery of the semiconductor wafer 11.

  In the wafer processing method according to the embodiment of the present invention, a semiconductor wafer (hereinafter abbreviated as “wafer”) 11 has a surface 11a attached to a dicing tape T whose outer periphery is attached to an annular frame F as shown in FIG. Then, as shown in FIG. 5, the processing is performed with the back surface 11b of the wafer 11 exposed.

  In the wafer processing method of the present invention, first, the wavelength of a pulsed laser beam having transparency to the silicon wafer 11 is set in the range of 1300 nm to 1400 nm (wavelength setting step).

  In this embodiment, a YAG laser oscillator that oscillates a pulse laser having a wavelength of 1342 nm is employed as the laser oscillator 62 of the laser beam generation unit 35 shown in FIG. However, in the wafer processing method of the present invention, it is not essential that the wavelength of the laser beam be in the range of 1300 nm to 1400 nm, and a pulsed laser beam with a wavelength of 1064 nm may be used.

  Next, the wafer 11 is sucked and held by the chuck table 28 of the laser processing apparatus 2 via the dicing tape T, and the back surface 11b of the wafer 11 is exposed. Then, the wafer 11 is imaged from the back surface 11b side by the infrared imaging element of the imaging unit 39, and alignment is performed so that the region corresponding to the first scheduled division line 13a is aligned with the condenser 37 in the X-axis direction. For this alignment, well-known image processing such as pattern matching is used.

  After the alignment of the first scheduled division line 13a, after the chuck table 28 is rotated 90 degrees, the same alignment is performed for the second scheduled division line 13b extending in the direction orthogonal to the first scheduled division line 13a. To implement.

  After the alignment, a modified layer forming step for forming a modified layer inside the wafer 11 is performed using, for example, a pulsed laser beam having a wavelength of 1342 nm. In this modified layer forming step, a mask 70 as shown in FIG. 6 is used to irradiate the wafer 11 with a part of the pulse laser beam missing.

  FIG. 6B is a left side view of the mask 70 shown in FIG. 6A, and the mask 70 has a light shielding portion 70a and a transparent portion 70b. A part of the pulse laser beam in a portion from the center of the pulse laser beam to the outer periphery on the downstream side of the processing feed is lost at the light shielding portion 70a of the mask 70.

  That is, the pulse laser beam 75 condensed by the condenser lens 74 lacks a portion 77 extending from the center of the pulse laser beam 75 to the outer periphery on the downstream side in the processing feed direction. This missing portion is preferably in the range of 1/20 to 1/50 of the cross-sectional area of the pulse laser beam 75.

  After performing the alignment step, as shown in FIG. 8, the condensing point of the pulse laser beam in which a part of the beam cross-sectional area is missing by the concentrator 37 is positioned inside the wafer corresponding to the first division line 13a. The modified layer 19 is formed inside the wafer 11 by irradiating the pulse laser beam from the back surface 11b side of the wafer 11 and processing and feeding the chuck table 28 in the arrow X direction (modified layer forming step).

  In this modified layer forming step, as shown in the schematic diagram of FIG. 7, the pulse laser beam 75 irradiated to the wafer 11 has a high-power portion 75a and a low-power portion 75b. Since the pulsed laser beam that has passed through 19 is inverted symmetrically with respect to the focal point 19, leakage light (transmitted light) 75 b ′ lacking the laser beam is positioned at the crack 23 formed on the surface 11 a side. The diffusion of the leaked light 75b 'is extremely small, and the device 15 formed on the surface 11a of the wafer 11 is prevented from being damaged.

  While the chuck table 28 is indexed and fed in the Y-axis direction, the modified layer 19 is formed inside the wafer 11 corresponding to all the first scheduled division lines 13a. Next, after the chuck table 28 is rotated by 90 °, a similar modified layer 19 is formed along all the second scheduled division lines 13b orthogonal to the first scheduled division line 13a.

  The modified layer 19 refers to a region where the density, refractive index, mechanical strength, and other physical characteristics are different from the surroundings. For example, it includes a melt resolidification region, a crack region, a dielectric breakdown region, a refractive index change region, and the like, and also includes a region in which these regions are mixed.

  The processing conditions of the modified layer forming step are set as follows, for example.

Light source: YAG pulse laser Wavelength: 1342nm
Average output: 0.5W
Repetition frequency: 100 kHz
Spot diameter: φ2.5μm
Feeding speed: 300mm / s

After the modified layer forming step, the dividing step shown in FIG. 9 is used to apply an external force to the wafer 11 and divide the wafer 11 into individual device chips 21. 9 includes a frame holding unit 82 that holds the annular frame F, and a tape expansion unit 84 that extends the dicing tape T attached to the annular frame F held by the frame holding unit 82. Yes.

  The frame holding means 82 includes an annular frame holding member 86 and a plurality of clamps 88 as fixing means arranged on the outer periphery of the frame holding member 86. An upper surface of the frame holding member 86 forms a mounting surface 86a on which the annular frame F is mounted, and the annular frame F is mounted on the mounting surface 86a.

  The annular frame F placed on the placement surface 86 a is fixed to the frame holding means 86 by a clamp 88. The frame holding means 82 configured as described above is supported by the tape extending means 84 so as to be movable in the vertical direction.

  The tape expansion means 84 includes an expansion drum 90 disposed inside the annular frame holding means 86. The upper end of the expansion drum 90 is closed with a lid 92. The expansion drum 90 is smaller than the inner diameter of the annular frame F and has an outer diameter larger than the outer diameter of the wafer 11 attached to the dicing tape T attached to the annular frame F.

  The expansion drum 90 has a support flange 94 integrally formed at the lower end thereof. The tape expanding means 84 further includes driving means 96 for moving the annular frame holding member 86 in the vertical direction. The driving means 96 is composed of a plurality of air cylinders 98 disposed on a support flange 94, and the piston rod 100 is connected to the lower surface of the frame holding member 86.

  The driving means 96 composed of a plurality of air cylinders 98 includes an annular frame holding member 86, a reference position where the mounting surface 86a is substantially the same height as the surface of the lid 92 which is the upper end of the expansion drum 90, and an expansion. It moves up and down between the extended position below the upper end of the drum 90 by a predetermined amount.

  The dividing steps of the wafer 11 performed using the dividing apparatus 80 configured as described above will be described with reference to FIG. As shown in FIG. 10A, the annular frame F, in which the wafer 11 is supported via the dicing tape T, is placed on the placement surface 86 a of the frame holding member 86, and the frame holding member 86 is clamped by the clamp 88. Fix it. At this time, the frame holding member 86 is positioned at a reference position where the mounting surface 86a is substantially flush with the upper end of the expansion drum 90.

  Next, the air cylinder 98 is driven to lower the frame holding member 86 to the extended position shown in FIG. As a result, the annular frame F fixed on the mounting surface 86a of the frame holding member 86 is also lowered, so that the dicing tape T attached to the annular frame F comes into contact with the lid 92 with the upper end of the expansion drum 90 closed. Mainly expanded radially.

  As a result, a tensile force acts radially on the wafer 11 adhered to the dicing tape T. When a tensile force is applied to the wafer 11 in a radial manner in this way, the modified layer 19 formed along the first and second scheduled division lines 13a and 13b serves as a division starting point, and the wafer 11 becomes the first and second. Are divided along the scheduled division lines 13a and 13b, and divided into individual device chips 21.

  According to the above-described embodiment, in the modified layer forming step, the portion from the center of the pulse laser beam to the outer periphery on the downstream side in the processing feed direction is omitted so that the focal point of the pulse laser beam is positioned inside the wafer 11. Therefore, sufficient energy is secured to form the modified layer 19, and even if the fine crack 23 propagated from the modified layer 19 exists on the surface 11 a side of the wafer 11, the condensing point 19. Since the pulsed laser beam that has passed through is inverted symmetrically with respect to the focal point 19, the leaked light 75 b ′ from which part of the pulsed laser beam is missing is positioned in the crack 23 formed on the surface side. Is extremely small, and the device 15 formed on the surface 11a of the wafer 11 can be prevented from being damaged.

2 Laser processing apparatus 11 Silicon wafer 13a First scheduled division line 13b Second scheduled division line 15 Device 19 Modified layer 21 Device chip 23 Crack 28 Chuck table 34 Laser beam irradiation unit 35 Laser beam generation unit 37 Condenser 39 Imaging unit 62 Laser oscillator 64 Repetitive frequency setting means 70 Mask 70a Light-shielding portion 70b Transparent portion 74 Condensing lens 75 Beam cross-sectional area 75a Powerful portion 75b Powerful portion 75b 'Lightly leaked light 80 Dividing device T Dicing tape F Annular frame

Claims (1)

A holding means for holding a workpiece, and a laser beam for forming a modified layer inside the workpiece by irradiating a pulse laser beam having a wavelength that is transmissive to the workpiece held by the holding means A plurality of devices are defined on the surface by a plurality of division lines on a surface by a laser processing apparatus including an irradiation unit, and a processing feed unit that relatively processes and feeds the holding unit and the laser beam irradiation unit. A wafer processing method for processing a wafer made of pure silicon,
A condensing point of a pulsed laser beam having a wavelength that is transmissive to the wafer is positioned inside the wafer, the pulsed laser beam is irradiated from the back surface of the wafer to a region corresponding to the planned division line, and the holding means and the laser A modified layer forming step of forming a modified layer inside the wafer by relatively processing and feeding the beam irradiation means; and
A splitting step for applying an external force to the wafer after the reformed layer forming step and dividing the wafer along the planned split line with the reformed layer as a split starting point; and
In the modified layer forming step, a part of the pulse laser beam is lost from the center of the pulse laser beam irradiated to the wafer to the outer periphery on the downstream side in the processing feed direction, so that the focal point of the pulse laser beam is A wafer processing method characterized by being positioned inside.