JP5902529B2 - Laser processing method - Google Patents

Description

The present invention relates to a laser processing method for irradiating a thin plate-like workpiece such as a semiconductor wafer with a laser beam having a wavelength having transparency so as to form a modified layer inside the workpiece.
About.

  For example, in a semiconductor device, a large number of electronic circuits made of IC, LSI, etc. are formed on the surface of a wafer made of a semiconductor such as silicon or gallium arsenide, and then the back surface of the wafer is ground and thinned to a predetermined thickness. Is manufactured by a process of dividing the device into each device region in which an electronic circuit is formed. In order to divide a thin plate-like workpiece such as this kind of wafer, a method of cutting the wafer with a cutting blade has been common, but in recent years, a laser having a wavelength that is transparent to the workpiece. A laser processing method has also been adopted in which a modified layer is formed inside a workpiece by irradiation with a beam, and then an external force is applied to the wafer to cleave and divide the wafer starting from the modified layer (patent) Literature 1 etc.).

Japanese Patent No. 3408805

  By the way, in the said semiconductor wafer etc., the oxide film and the nitride film may remain on the back surface before performing back surface grinding. Further, in various wafers, a low dielectric constant insulator film (Low-k film) made of an organic film such as polyimide or parylene polymer was formed on the surface, or a metal film was formed on the back surface. There are things.

  When laser processing is performed by irradiating the film-coated wafer with laser from the film side as a workpiece, a part of the irradiated laser beam may be reflected on the film. The reflectivity varies depending on the type and thickness of the film, and the reflectivity of the surface may be different for each workpiece, or the reflectivity may vary within one workpiece. In the case where the reflectance is different for each workpiece, when a plurality of workpieces are subjected to laser processing under a single processing condition, variations occur in the modified layer formed by laser beam irradiation. Further, when the laser processing is performed under a single processing condition in the case where the reflectance varies within one workpiece, the modified layer varies depending on the region where the reflectance is different. Note that the variation of the modified layer referred to here means that the depth at which the modified layer is formed, the thickness of the modified layer, and the like are different from the irradiated surface irradiated with the laser beam.

  The present invention has been made in view of the above circumstances, and the main technical problem thereof is a uniform modified layer in the workpiece regardless of the state of the irradiated surface irradiated with the laser beam of the workpiece. It is an object to provide a laser processing method capable of forming a film.

  The laser processing method of the present invention is a laser processing method for forming a modified layer on a workpiece by irradiating the workpiece with a laser beam having a wavelength that is transparent to the workpiece. A reflectance detection step for detecting the reflectance of the irradiated laser beam on the irradiated surface to be irradiated, and after performing the reflectance detection step, on the irradiated surface of the workpiece based on the detected reflectance After performing the antireflection film forming step of forming an antireflection film and making the irradiated surface below a predetermined reflectance, and the antireflection film forming step, a laser beam with a wavelength having transparency is applied to the workpiece. A laser processing step of irradiating the irradiated surface to form a modified layer inside the workpiece.

  In the present invention, the reflectance of the irradiated surface of the workpiece is detected before the laser processing is performed, and an antireflection film is formed on the irradiated surface of the workpiece based on the detected reflectance to obtain a predetermined reflectance. Thereafter, a laser beam is irradiated into the workpiece from the antireflection film side. For this reason, the laser beam can efficiently reach the inside of the workpiece, and as a result, a uniform modified layer can be formed in the workpiece regardless of the irradiated surface state.

  According to the present invention, an effect is provided that a laser processing method capable of forming a uniform modified layer in a workpiece regardless of the state of the irradiated surface irradiated with the laser beam of the workpiece is provided. Play.

It is the (a) perspective view and the (b) sectional view showing the state where the work processed by the laser processing method concerning one embodiment of the present invention was supported by the frame via the adhesive tape. 1 is an overall perspective view of a laser processing apparatus that suitably implements a laser processing method according to an embodiment. It is a figure which shows the optical system of the laser irradiation means with which the same laser processing apparatus is provided. It is sectional drawing which shows the film forming apparatus used at the antireflection film formation step of the laser processing method concerning one embodiment. It is a side view which shows the state which is performing the laser processing step of the laser processing method which concerns on one Embodiment. It is sectional drawing which shows the detail of a laser processing step. It is a perspective view which shows the state which gives an external force and divides | segments a workpiece into a device simultaneously with grinding the back surface side of a workpiece by a grinding means after a laser processing step.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Reference numeral 1 in FIG. 1 indicates a workpiece to which laser processing is performed in the present embodiment, and FIG. 2 shows a laser processing apparatus 20 that performs laser processing on the workpiece 1.

(1) Workpiece A workpiece 1 shown in FIG. 1 is a disk-shaped semiconductor wafer or the like having a thickness of, for example, about several hundreds μm, and is supplied to the laser processing apparatus 20. The adhesive tape 11 is attached to the annular frame 10. A plurality of rectangular device regions 3 are set on the surface of the workpiece 1 by grid-shaped division lines 2, and electronic circuits made of, for example, ICs or LSIs are formed in these device regions 3. . As shown in FIG. 1B, a film 4 is formed on the entire back surface 1 b of the workpiece 1. The film 4 is, for example, an oxide film, a nitride film, a metal film, or the like.

  The surface 1a of the workpiece 1 is attached to the adhesive tape 11 and the back surface 1b is exposed. The frame 10 is made of a metal plate having rigidity such as stainless steel, and the workpiece 1 is conveyed and set to the laser processing apparatus 20 via the frame 10 and the adhesive tape 11.

  The laser processing of this embodiment is to form a modified layer by irradiating a laser beam along the division line 2 inside the workpiece 1, and hereinafter, a laser processing apparatus 20 and a laser processing method. The process will be described.

(2) Laser Processing Apparatus A laser processing apparatus 20 that performs laser processing on the workpiece 1 will be described with reference to FIG.
The laser processing apparatus 20 has a base 21, and an XY movement table 22 is provided on the base 21 so as to be movable in the horizontal X-axis direction and the Y-axis direction. The XY moving table 22 is provided with a chuck table 51 that holds the workpiece 1. Above the chuck table 51, an irradiation unit 62 of a laser irradiation unit 60 that irradiates a workpiece 1 held by the chuck table 51 with a laser beam is disposed so as to face the chuck table 51. .

  The XY movement table 22 includes an X-axis base 30 provided on the base 21 so as to be movable in the X-axis direction, and a Y-axis base 40 provided on the X-axis base 30 so as to be movable in the Y-axis direction. It consists of a combination. The X-axis base 30 is slidably attached to a pair of parallel guide rails 31 fixed on the base 21 and extending in the X-axis direction, and an X-axis drive mechanism 34 that operates a ball screw 33 by a motor 32. Is moved in the X-axis direction. On the other hand, the Y-axis base 40 is slidably attached to a pair of parallel guide rails 41 extending in the Y-axis direction fixed on the X-axis base 30, and the Y-axis that operates the ball screw 43 by the motor 42. It is moved in the Y-axis direction by the drive mechanism 44.

  A cylindrical chuck base 50 is supported on the upper surface of the Y-axis base 40 so as to be rotatable about the Z-axis direction (vertical direction) as a rotation axis. A chuck table 51 is concentrically fixed on the chuck base 50. Has been. The chuck table 51 is of a generally known vacuum chuck type that holds the workpiece 1 by suction by a vacuum suction action. The chuck table 51 is rotationally driven integrally with the chuck base 50 by a rotational driving means (not shown). Around the chuck table 51, a pair of clamps 52 for detachably holding the frame 10 are disposed at positions separated from each other by 180 °. These clamps 52 are attached to the chuck base 50 through stays 53 (see FIG. 5).

  In the XY moving table 22, when the X-axis base 30 moves in the X-axis direction, it is a processing feed that irradiates the laser beam along the planned division line 2 of the workpiece 1. Then, when the Y-axis base 40 moves in the Y-axis direction, indexing and feeding for switching the division-scheduled line 2 to be irradiated with the laser beam is performed. Note that the machining feed direction and the index feed direction may be set oppositely, that is, the Y-axis direction may be set as the machining feed direction, and the X-axis direction may be set as the index feed direction.

  The laser irradiation means 60 has a rectangular parallelepiped casing 61 extending in the Y-axis direction toward the upper side of the chuck table 51, and the irradiation section 62 is provided at the tip of the casing 61. The casing 61 is provided on the column 23 erected on the base 21 so as to be movable up and down along the vertical direction (Z-axis direction). The casing 61 is moved up and down by a vertical drive means (not shown) accommodated in the column 23. Be moved.

  An alignment means 70 for detecting the division line 2 of the workpiece 1 is fixed at the tip of the casing 61 and in the vicinity of the irradiation unit 62. The alignment unit 70 includes a camera 71 that images the workpiece 1, and the alignment unit 70 detects (aligns) the planned division line 2 based on the image acquired by the camera 71.

Hereinafter, the laser irradiation means 60 will be described.
As shown in FIG. 3, a casing 61 accommodates a laser oscillation unit 63 that oscillates a laser beam, an output adjustment unit 64, and a reflected light amount detector 65. The laser oscillation unit 63 includes a laser oscillator 631 that oscillates a pulse laser such as YAG or YVO, and a repetition frequency setting unit 632 that sets the frequency of the laser beam oscillated by the laser oscillator 631. The laser oscillator 631 oscillates a pulse laser having a wavelength having a frequency set by the repetition frequency setting unit 632, and the laser beam is emitted toward the irradiation unit 62.

  The output of the laser beam emitted from the laser oscillator 631 is adjusted by the output adjustment unit 64 and then irradiated to the reflection mirror 621 disposed in the upper part of the irradiation unit 62. In the irradiation unit 62, a reflection mirror 621, a half mirror 622, and a condenser lens 623 are accommodated from above to below. The laser beam applied to the reflection mirror 621 is reflected downward by the reflection mirror 621, enters the condensing lens 623, passes through, and is applied to the workpiece 1 held on the chuck table 51.

  Further, the laser beam irradiated to the workpiece 1 is reflected by the irradiated surface which is the upper surface of the workpiece 1 as shown by the broken line in FIG. 3, and the reflected light is reflected by the half mirror 622 to the reflected light amount detector 65. Led to. The reflected light amount detector 65 detects the reflected light amount of the irradiated surface. Then, the reflected light amount is compared with the reflected light amount of a reference workpiece such as a mirror wafer that is detected in advance, and the reflectance value is obtained as a relative value. As will be described later, the irradiation unit 62 emits a laser beam for reflectance detection and a laser beam for laser processing for forming a modified layer.

(3) Laser processing method The above is the structure of the laser processing apparatus 20, and the process of the laser processing method which forms a modified layer in the to-be-processed object 1 using this laser processing apparatus 20 is demonstrated.

  First, the workpiece 1 is placed on the chuck table 51 of the laser processing apparatus 20 via the adhesive tape 11, and is sucked and held on the chuck table 51 by a vacuum suction action. The workpiece 1 is held on the chuck table 51 with the back surface 1b side, that is, the film 4 exposed. Further, the frame 10 is held by the clamp 52.

Next, the X-axis base 30 and the Y-axis base 40 of the XY moving table 22 are appropriately moved to position the workpiece 1 below the irradiation unit 62 of the laser irradiation means 60, and the X-axis base 30 is moved in the X-axis direction. Reflecting the laser beam irradiated to the surface to be irradiated (surface of the film 4) of the workpiece 1 while irradiating the workpiece 1 with the laser beam for reflectance detection from the irradiation unit 62 while performing the processing feed to be moved. The rate is detected (reflectance detection step). A processing feed rate for moving the workpiece 1 together with the chuck table 51 is, for example, about 400 mm / s. The laser beam for detecting the reflectance is smaller than the output when forming the modified layer, and is, for example, a laser beam under the following conditions.
-Wavelength: 1064nm pulse laser-Repetition frequency: 100kHz
・ Average output: 0.1W

  As shown in FIG. 3, the reflectance is such that the reflected light of the laser beam reflected by the irradiated surface of the workpiece 1 (the surface of the film 4) is guided to the reflected light amount detector 65 by the half mirror 622, and the reflected light amount. It is obtained by comparing the amount of reflected light detected by the detector 65 with the amount of reflected light of the reference workpiece detected in advance.

  When the reflectance of the irradiated surface of the workpiece 1 is obtained, the workpiece 1 is unloaded from the laser processing apparatus 20, and an antireflection film is formed on the irradiated surface of the workpiece 1 based on the detected reflectance. Then, the irradiated surface is set to a predetermined reflectance or less (antireflection film forming step).

  FIG. 4 shows a film forming apparatus that suitably forms an antireflection film on the irradiated surface of the workpiece 1. This film forming apparatus 80 spins by dropping liquid resin P from a resin supply nozzle 83 onto the upper surface of the workpiece 1, that is, the surface of the film 4 held on a disk-shaped spinner table 82 in the apparatus case 81. It is of a type that applies a coat.

  The device case 81 includes a cylindrical case body 811 that opens upward and has a hole 811a formed at the center thereof, and a cover 812 that closes the hole 811a of the case body 811. The cover 812 includes a motor 84 from below. The drive shaft 85 penetrates. The center of the spinner table 82 is fixed to the upper end of a drive shaft 85 protruding into the apparatus case 81, and is supported so as to be horizontally rotatable by driving a motor 84.

  The workpiece 1 is placed on the upper surface of the spinner table 82 via the adhesive tape 11 and is adsorbed and held on the upper surface of the spinner table 82 by a vacuum suction action. A plurality of centrifugal clamps 86 are attached to the peripheral portion of the spinner table 82 to operate so as to press the frame 10 from above when centrifugal force is generated by the rotation of the spinner table 82. Retained.

  The resin supply nozzle 83 is pivotally supported at the bottom of the case body 811 so that the resin supply port 831 at the tip can be positioned right above the center of the spinner table 82 by the rotation.

  In the antireflection film forming step, the workpiece 1 is held on the spinner table 82 via the adhesive tape 11, the spinner table 82 is driven to rotate, and the frame 10 is held by the centrifugal clamp 86. Then, the resin P is dropped from the resin supply port 831 of the resin supply nozzle 83 onto the upper surface of the rotating workpiece 1, that is, the center of the surface of the film 4. The resin P dropped onto the film 4 is spin-coated on the entire surface of the film 4 by the action of centrifugal force, so that the antireflection film 5 (see FIG. 5) made of the resin P is uniformly formed.

  Examples of the resin P used for forming the antireflection film 5 include polyimide, optical plastic, and polyvinyl alcohol (PVA). Further, the refractive index of the antireflection film 5 and the film 4 are larger than the difference between the refractive index of air and the refractive index of the surface of the film 4 with respect to the wavelength of the laser beam irradiated to the workpiece 1 in the subsequent laser processing. A material having a refractive index such that the difference from the refractive index is smaller is selected. This is because the laser beam for laser processing is difficult to be reflected by the antireflection film 5 during laser processing so that the laser beam is efficiently transmitted into the workpiece 1.

  The thickness of the antireflection film 5 to be formed is determined according to the reflectance detected in the reflectance detection step. For example, if the reflectance is detected as 30%, the thickness of the antireflection film 5 is determined. The thickness may be set to 2 μm to 100 nm, and the reflectance of the irradiated surface of the workpiece 1 during laser processing, that is, the surface of the antireflection film 5 may be adjusted to about 0 to 18%.

  After the antireflection film 5 is formed on the irradiated surface of the workpiece 1, the workpiece 1 is taken out of the film forming apparatus 80 and set again in the laser processing apparatus 20, and a laser beam having a wavelength having transparency is scheduled to be split. Irradiation is performed along the line 2 to form a modified layer along the line to be divided inside the workpiece 1 (laser processing step).

  The workpiece 1 is held on the chuck table 51 via the adhesive tape 11 and the frame 10 is held by the clamp 52 as in the reflectance detection step. And after detecting the position of the division | segmentation scheduled line 2 by the alignment means 70, as shown in FIG.5 and FIG.6, for the laser processing which has the transparency with respect to the workpiece 1 from the irradiation part 62 of the laser irradiation means 60 The condensing point is positioned inside the workpiece 1 and the laser beam L is scanned along the detected division line 2.

The laser beam L for laser processing has a larger output than the laser beam at the time of reflectance detection, and is, for example, a laser beam under the following conditions.
-Wavelength: 1064nm pulse laser-Repetition frequency: 100kHz
・ Average output: 1.5W

  As shown in FIG. 6, the laser beam L at the time of laser processing is condensed in the workpiece 1 through the antireflection film 5 and the film 4. A modified layer 1c is formed. The modified layer 1c is formed at a certain depth from the surface 1a of the workpiece 1 at a certain depth. The modified layer 1 c has a characteristic that the strength is lower than that of other portions in the workpiece 1.

  Scanning of the laser beam along the division line 2 by the laser processing apparatus 20 is performed by rotating the chuck table 51 to set the division line 2 in parallel with the machining feed and moving the chuck table 51 in the X-axis direction. Made by machining feed. An arrow B in FIG. 5 is a moving direction of the chuck table 51 at the time of laser processing, and accordingly, the irradiation unit 62 is relatively processed and fed in the arrow A direction. In this case, the machining feed rate is, for example, about 400 mm / s. Further, the division scheduled line 2 for irradiating the laser beam is switched by index feed for moving the chuck table 51 in the Y-axis direction. As a result, the modified layer 1 c is formed in the workpiece 1 along all the planned division lines 2.

  The laser processing for the workpiece 1 is thus completed. After that, the workpiece 1 is divided into individual device regions 3 starting from the modified layer 1c having reduced strength by applying an external force to form a chip-like device. As a method of applying an external force to the workpiece 1, as shown in FIG. 7, a grinding means 90 of a type in which a plurality of grindstones 93 of a grinding wheel 92 that is rotationally driven by a spindle 91 is pressed against the workpiece and ground. A method of grinding the back surface 1b side of the workpiece 1 can be mentioned. The workpiece 1 is held on a rotatable holding means 94 with the back surface 1b side, that is, the antireflection film 5 side exposed.

  When the external force is applied by grinding as described above, the antireflection film 5 and the film 4 are removed, and the workpiece 1 can be thinned to a predetermined thickness and simultaneously divided into a large number of devices. Note that the method of applying an external force to the workpiece 1 is not limited to grinding, and for example, it is possible to extend the adhesive tape 11.

  In some cases, the back surface 1b side is not ground after forming the modified layer 1c. In this case, the liquid resin for forming the antireflection film 5 is selected from water-soluble resin, and after the modified layer 1c is formed. The antireflection film 5 may be removed by washing with water. In that case, the water-soluble antireflection film 5 can be removed by using the film forming apparatus 80 shown in FIG. 4 and supplying water instead of the liquid resin. In addition, as water-soluble liquid resin, water-soluble resists, such as the said polyvinyl alcohol (PVA), polyethyleneglycol (PEG), polyethylene oxide (PEO), are used preferably.

 In the laser processing step, the workpiece 1 is supported while being supported by the frame 10 via the adhesive tape 11. However, in some cases, the workpiece 1 is peeled from the adhesive tape 11 and the chuck table 51 is peeled off. Alternatively, the modified layer 1c may be formed in a state where the workpiece 1 is directly held.

  According to the laser processing method of the above embodiment, the reflectance of the irradiated surface (the surface of the film 4) of the workpiece 1 is detected before the laser processing is performed, and the workpiece 1 is processed based on the detected reflectance. An antireflection film 5 having a predetermined thickness is formed on the surface to be irradiated so as to have a predetermined reflectance or less. Thereafter, a laser beam is irradiated from the antireflection film 5 side to modify the modified layer 1c in the workpiece 1. Is forming. That is, the surface to be irradiated with the laser beam when forming the modified layer 1c is not the surface of the film 4 formed in advance on the back surface 1b of the workpiece 1, but below a predetermined reflectance having a predetermined thickness. The antireflection film 5 is changed. Therefore, the laser beam can efficiently reach the inside of the workpiece 1 without being affected by the state of the film 4. As a result, a uniform modified layer 1 c can be formed in the workpiece 1 regardless of the surface state of the film 4.

  When the film 4 formed in advance on the surface 1a of the workpiece 1 has a variation, the reflectance of the entire surface 1a having a variation is detected in the reflectance detection step to detect the reflectance. It is preferable to detect the variation state and set the thickness of the antireflection film 5 formed in the antireflection film formation step in accordance with the variation state.

DESCRIPTION OF SYMBOLS 1 ... Workpiece 1c ... Modified layer 5 ... Antireflection film L ... Laser beam

Claims (1)

A laser processing method for forming a modified layer on a workpiece by irradiating the workpiece with a laser beam having a wavelength having transparency.
A reflectance detecting step for detecting the reflectance of the laser beam irradiated on the surface to be irradiated with the laser beam of the workpiece;
After performing the reflectance detection step, an antireflection film forming step for forming an antireflection film on the irradiated surface of the workpiece based on the detected reflectance so that the irradiated surface is less than or equal to a predetermined reflectance. When,
After performing the antireflection film forming step, a laser processing step of irradiating the irradiated surface of the workpiece with a laser beam having a wavelength to form a modified layer inside the workpiece;
A laser processing method comprising:

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