JP6998149B2 - Laser processing method - Google Patents

Description

The present invention relates to a laser machining method capable of reliably forming a modified layer on a workpiece.

A wafer in which a plurality of devices such as ICs and LSIs are partitioned by a scheduled division line and formed on the surface is divided into individual devices by a dicing device, a laser processing device, or the like and used for electric devices such as mobile phones and personal computers.

A laser processing device is a type in which a condensing point of a laser beam having a wavelength that is transparent to a work piece is positioned inside the work piece and irradiated to form a modified layer to perform internal processing (for example, Patent Document). 1) and a type in which a focusing point of a laser beam having a wavelength that is absorbent to the workpiece is positioned on the upper surface of the workpiece and irradiated to perform ablation processing (see, for example, Patent Document 2). It is roughly divided into.

Japanese Patent No. 3408805 Japanese Unexamined Patent Publication No. 10-305420

In the above-mentioned type of laser processing in which a focused point of a laser beam having a wavelength having a transmissive wavelength is positioned inside the workpiece and irradiated to form a modified layer to perform internal processing, for example, silicon ( Si) Laser processing is performed using the wafer as the workpiece. However, the silicon wafer is subjected to so-called doping in which a small amount of impurities are added in order to change the physical characteristics of the crystal when the silicon wafer is formed. The type of substance to be doped or the amount of the substance to be doped varies depending on the manufacturer forming the substrate of this silicon wafer or the type of device formed on the silicon wafer, and is permeable to silicon. Even if a laser beam set as a wavelength having a property is used, the irradiated laser beam does not sufficiently pass through the workpiece, and even if laser processing is performed under the processing conditions set in advance, processing failure occurs. In some cases.

Further, not only when the degree of permeability changes due to the difference in doping described above, for example, an oxide film or the like is formed on the surface even when a lapse of time has passed since the silicon wafer was manufactured. Changes in transmittance can occur and similar problems can occur. Such a problem is a problem that can occur not only in a silicon wafer but also in a workpiece made of another material.

The present invention has been made in view of the above facts, and its main technical problem is whether or not it is a work piece capable of forming a modified layer inside under the processing conditions before performing laser processing. It is an object of the present invention to provide a laser processing method that can be easily determined.

In order to solve the above-mentioned main technical problem, according to the present invention, a holding means for holding a work piece and a condensing point of a laser beam having a wavelength that is transparent to the work piece held by the holding means are provided. At least a laser beam irradiating means having a condenser for irradiating and irradiating the inside of the workpiece to form a modified layer, and a machining feeding means for relatively machining and feeding the holding means and the laser beam irradiating means. It is a laser processing method using a provided laser processing device, and is a first detection step of irradiating a laser beam by confronting a concentrator of the laser beam irradiating means with a power meter to detect a first power, and a collection thereof. A second detection step of locating a work piece between a light device and the power meter and irradiating it with a laser beam to detect a second power, and a work piece from the first power and the second power. A transmission rate calculation step for calculating an index representing the transmittance of light, a modified layer formation determination step for determining whether or not a modified layer can be formed inside the workpiece from the index representing the transmittance, and the modification. It is composed of at least a modified layer forming step of forming a modified layer by irradiating a work piece determined to be able to form a modified layer by the quality layer formation determination step by positioning a light collecting point of a laser beam inside. The power meter is arranged adjacent to the chuck table disposed in the holding means, and the light collector and the holding means are relatively moved to perform the first detection step. A laser processing method is provided in which the workpiece is held in the chuck table so as to protrude from the chuck table of the holding means and reach the power meter, and the second detection step is carried out.

The workpiece is a silicon wafer, and the wavelength of the laser beam can be near infrared rays.

In the laser processing method of the present invention, a holding means for holding the workpiece and a condensing point of a laser beam having a wavelength that is transparent to the workpiece held by the holding means are positioned inside the workpiece. A laser processing apparatus equipped with at least a laser beam irradiating means provided with a light collector for irradiating and forming a modified layer and a processing feeding means for relatively processing and feeding the holding means and the laser beam irradiating means is used. The first detection step of irradiating the laser beam with the concentrator of the laser beam irradiating means facing the power meter to detect the first power, and the concentrator and the power meter. A second detection step in which a work piece is positioned between the two and irradiated with a laser beam to detect a second power, and an index representing the transmission rate of the work piece from the first power and the second power. It is modified by the calculated permeability calculation step, the modified layer formation determination step for determining whether or not the modified layer can be formed inside the workpiece from the index representing the permeability, and the modified layer formation determination step. The power meter comprises at least a modified layer forming step of irradiating a work piece determined to be able to form a quality layer with a condensing point of a laser beam positioned inside to form a modified layer. It is arranged adjacent to the chuck table disposed in the holding means, and the light collector and the holding means are relatively moved to carry out the first detection step, and the holding means of the holding means is performed. By holding the workpiece on the chuck table so as to protrude from the chuck table and reach the power meter and carry out the second detection step , the workpiece can form a modified layer. It is possible to easily determine whether or not the light is formed, and perform laser processing to ensure that the modified layer is formed.

It is a perspective view which shows the whole of the laser processing apparatus used for carrying out this invention, and is the perspective view which shows the silicon wafer of a work piece. It is the schematic (a) explaining the operation of the 1st detection step of this invention, and the schematic (b) explaining the operation of the 2nd detection step. It is a flowchart which shows the procedure of the laser processing method carried out based on this invention. It is a schematic diagram for demonstrating the modified layer formation step of this invention.

Hereinafter, the laser processing method configured based on the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an overall perspective view of a laser processing apparatus 2 for carrying out a laser processing method configured based on the present invention, and a perspective view of a silicon wafer (100, 110) as a workpiece. There is. The workpiece whose transmittance is detected by the present invention may be a silicon wafer 100 (see (a) in the figure, hereinafter referred to as "dummy wafer") before a device or the like is formed on the surface. It may be a silicon wafer 110 ((b) in the figure) in which a device 114 is formed in a region partitioned by a planned division line 112 on the surface 110a of the dummy wafer 100.

The laser processing apparatus 2 shown in FIG. 1 includes a holding means 22 for holding an workpiece, a moving means 23 arranged on a stationary base 2a for moving the holding means 22, and a workpiece held by the holding means 22. From the laser beam irradiating means 24 that irradiates an object with a laser beam, the vertical wall portion 51 erected in the Z direction indicated by the arrow Z on the side of the moving means 23 on the stationary base 2a, and the upper end portion of the vertical wall portion 51. It includes a frame body 50 composed of a horizontal wall portion 52 extending in the horizontal direction. The optical system of the laser beam irradiating means 24 constituting the main part of the laser processing apparatus 2 of the present invention is built in the horizontal wall portion 52 of the frame body 50, and the lower surface side of the tip portion of the horizontal wall portion 52 has a built-in optical system. The condenser 241 constituting the laser beam irradiating means 24 is arranged, and the imaging means 26 is arranged at a position adjacent to the condenser 241 in the direction indicated by the arrow X in the figure. The image pickup means 26 includes a normal image pickup element (CCD) that images with visible light, an infrared ray irradiation means that irradiates an workpiece with infrared rays, an optical system that captures infrared rays irradiated by the infrared ray irradiation means, and the optical system. Includes an image pickup element (infrared CCD) that outputs an electric signal corresponding to the infrared rays captured by the camera.

The holding means 22 has a rectangular X-direction movable plate 30 mounted on the base 2a so as to be movable in the X direction indicated by the arrow X in the drawing, and the holding means 22 to be movable in the X direction indicated by the arrow Y in the drawing. A rectangular Y-direction movable plate 31 mounted on the movable plate 30, a cylindrical strut 32 fixed to the upper surface of the Y-direction movable plate 31, and a rectangular cover plate 33 fixed to the upper end of the strut 32. including. The cover plate 33 holds a circular workpiece extending upward through an elongated hole formed on the cover plate 33, and is provided with a chuck table 34 rotatably configured by a rotation driving means (not shown). Has been done. On the upper surface of the chuck table 34, a suction holding means composed of a circular suction chuck 35 formed of a porous material and extending substantially horizontally is arranged. The suction chuck 35 is connected to a suction means (not shown) by a flow path passing through the support column 32. A power meter 36 for detecting the power (output) of the laser beam emitted from the laser beam irradiating means 24 is arranged at a position adjacent to the chuck table 34 in the X direction on the cover plate 33. The power meter 36 is connected to a control device 20 described later by a cable (not shown), and is composed of a plurality of light receiving elements arranged in an area capable of receiving the total amount of the laser beam to be measured, and controls the power of the received laser beam. Output to device 20. The X direction is the direction indicated by the arrow X in FIG. 1, and the Y direction is the direction indicated by the arrow Y and is orthogonal to the X direction. The planes defined in the X and Y directions are substantially horizontal.

The control device 20 is configured by a computer and temporarily stores a central processing unit (CPU) that performs arithmetic processing according to a control program, a read-only memory (ROM) that stores a control program and the like, detected detection values, arithmetic results, and the like. It has a readable and writable random access memory (RAM) for storing in, an input interface, and an output interface (details are not shown).

The moving means 23 is controlled by the control device 20, and includes an X-direction moving means 40 and a Y-direction moving means 42. The X-direction moving means 40 converts the rotational motion of the motor into a linear motion via a ball screw and transmits it to the X-direction movable plate 30, and X-direction movable plate 30 is X along the guide rail on the base 2a. Move forward and backward in the direction. The Y-direction moving means 42 converts the rotational motion of the motor into a linear motion via the ball screw, transmits it to the Y-direction movable plate 31, and transmits the Y-direction movable plate 31 along the guide rail on the X-direction movable plate 30. Is advanced and retreated in the Y direction. Although not shown, the X-direction moving means 40 and the Y-direction moving means 42 are provided with position detecting means, respectively, in the X-direction position, the Y-direction position, and the circumferential direction of the chuck table 34. The rotation position is accurately detected, and the X-direction moving means 40, the Y-direction moving means 42, and the rotation driving means (not shown) are driven based on the signal instructed from the control device 20, and the chuck table 34 is driven to an arbitrary position and angle. Can be accurately positioned. In the normal processing state, the entire laser processing apparatus 2 and the moving means 23 and the like described above are covered with a cover, a bellows, etc. (not shown) omitted for convenience of explanation, and dust, dust, etc. are contained inside. It is configured not to.

The laser processing apparatus 2 for carrying out the present invention is generally configured as described above, and the laser processing method of the present invention will be described below.

FIG. 2 is a flowchart showing the procedure of the laser processing method carried out by the present invention. The laser processing method of the present invention will be described with reference to this flowchart.

(First detection step)
In carrying out the laser processing method of the present invention, first, the first detection step (S1) is carried out. In order to carry out the first detection step, first, the power meter 36 arranged on the cover plate 33 and the concentrator 241 of the laser beam irradiating means 24 are aligned. In this alignment, the center position of the power meter 36 is imaged by the image pickup means 26 by controlling the X direction moving means 40 and the Y direction moving means 42 that move the holding means 22 in the X direction and the Y direction, and the position is determined. By detecting and moving the concentrator 241 and the power meter 36 relatively, the concentrator 241 faces the power meter. The alignment between the condenser 241 and the power meter 36 is not necessarily limited to the use of the image pickup means 26, and the operator visually confirms the irradiation position of the laser beam emitted from the condenser 241. You may go while doing it.

After aligning the condenser 241 and the power meter 36, as shown in FIG. 3A, the laser beam LB is oscillated from a laser oscillator (not shown) of the laser beam irradiating means 24, and the power is generated from the condenser 241. The meter 36 is irradiated with light, and the power of the laser beam LB is output to the control device 20. At this time, the condensing position P of the laser beam LB is not adjusted to the height of the light receiving element of the power meter 36, but is positioned upward by a predetermined distance (defocus). As a result, the power density of the condensing spot measured by the power meter 36 is prevented from becoming excessively large, and deterioration of the power meter 36 is suppressed. Further, the defocus amount of the laser beam LB is such that the total amount of light of the laser beam LB irradiated to the power meter 36 is received by the power meter 36. The power of the laser beam LB irradiated at this time is preferably lower than the power when actually processing the workpiece.

The irradiation conditions of the laser beam irradiated in the first detection step described above can be set as follows, for example.
Wavelength: 1342nm
Repeat frequency: 90kHz
Average output: 1000mW

As can be understood from FIG. 3A, the power of the laser beam detected in the first detection step is the power of the laser beam LB irradiated from the laser beam irradiating means 24, and in the present embodiment, 1000 mW (1 W). Is detected. The detected power is stored in the memory of the control device 20 as "first power (P1)". The power of the laser oscillator (not shown) mounted on the laser beam irradiating means 24 may decrease due to aging or the like, or may change due to the quality variation of the laser oscillator or the like, and this first detection step should be carried out. More accurately, the first power (P1) of the laser beam LB is detected.

(Second detection step)
As described above, when the first detection step is carried out and the first power (P1) is stored in the memory of the control device 20, the second detection step (S2) shown in FIG. 2 is carried out. Specifically, the dummy wafer 100 shown in FIG. 1 is prepared, and as shown in FIG. 3B, the dummy wafer 100 is positioned so as to cover the light receiving element of the power meter 36. At this time, the dummy wafer 100 is placed so as to protrude from the chuck table 34 and reach the power meter 36, and a suction means (not shown) connected to the suction chuck 35 is operated to hold the dummy wafer 100 so as not to shift. Is preferable. When the dummy wafer 100 is positioned in this way, the power meter 36 is irradiated with the laser beam LB under the same irradiation conditions as in the first detection step described above. The above-mentioned dummy wafer 100 is a substrate of a silicon wafer 110 to be processed as a workpiece, and is produced from the same ingot as the substrate of the silicon wafer 110 through the same manufacturing process. .. Therefore, by knowing the transmittance of the dummy wafer 100, it is possible to know the transmittance of the substrate of the silicon wafer 110.

When the laser beam LB is irradiated toward the power meter 36 while holding the dummy wafer 100 as described above, the laser beam LB transmitted through without being absorbed by the dummy wafer 100 is received by the power meter 36. In the present embodiment, 600 mW is detected, and the detected power is stored in the memory of the control device 20 as "second power (P2)".

(Transmittance calculation step)
As described above, if the first detection step (S1) and the second detection step (S2) are executed, the transmittance calculation step (S3) shown in FIG. 2 is executed. In this transmittance calculation step (S3), an index representing the transmittance of the workpiece is calculated from the first power (P1 = 1000 mW) and the second power (P2 = 600 mW) stored in the control device 20. do. Specifically, the following operations are performed.
Transmittance (R) = (second power (P2) / first power (P1)) × 100
= (600 (mW) / 1000 (mW)) x 100 = 60 (%)

By executing the above-mentioned transmittance calculation step (S3), the transmittance (R = 60%) of the dummy wafer 100 of the present embodiment is calculated and stored in the memory of the control device 20. The transmittance (R) calculated in the transmittance calculation step (S3) may be an index representing the transmittance, and is not limited to being calculated by the above calculation. For example, the absorption rate at which the laser beam LB is absorbed by the dummy wafer 100 may be calculated. When calculating the absorption rate, the numerator is the value (P1-P2) obtained by subtracting the second power (P2) from the first power (P1), and the first power (P1) is used as the denominator for absorption. The rate can be calculated. The higher the transmittance, the lower the absorption rate, and the lower the transmittance, the higher the value, and the absorption rate can be used in the present invention as an index substantially representing the transmittance.

(Modified layer formation determination step)
If the above-mentioned transmittance calculation step (S3) is executed, the modified layer formation determination step (S4) for determining whether or not the modified layer can be formed is carried out. Specifically, it is a step of determining whether or not the transmittance is 30% or more with respect to a predetermined determination criterion regarding the transmittance, for example, the wavelength (1342 nm) of the laser beam LB irradiated from the laser beam irradiating means 24. Since the transmittance measured in this embodiment is R = 60% (≧ 30%), it satisfies this determination criterion, that is, the modified layer can be formed (Yes) in the modified layer formation determination step (S4). It is judged. When the absorption rate is used as an index representing the transmittance, whether or not the absorption rate is 70% or less may be used as a determination criterion. Further, this determination criterion may be appropriately determined in consideration of the processing conditions of the laser processing apparatus to be used, the physical properties of the workpiece, the thickness, and the like.

(Modified layer formation step)
If it is determined to be Yes in the above-mentioned modified layer formation determination step (S4), then the modified layer formation step (S5) is carried out. As described above, the transmittance (R) is calculated based on the dummy wafer 100, but the actual formation of the modified layer is performed on the silicon wafer 110 on which the device 14 is formed. .. Specifically, laser processing is performed on the silicon wafer 110 which is conveyed from a cassette (not shown) containing a plurality of silicon wafers 110 and placed on the chuck table 34. The laser beam LB'irradiated at this time is a laser beam having the same wavelength as the laser beam LB irradiated in the first detection step (S1) and the second detection step (S2), but is actually a modified layer. Is set to a higher output than the laser beam LB when calculating the transmittance.

In this modified layer forming step (S5), first, the silicon wafer 110 is placed on the chuck table 34 of the laser processing apparatus 2 shown in FIG. 1 with the back surface 110b side facing up. Then, as shown in FIG. 4, the silicon wafer 110 is suction-held on the suction chuck 35 of the chuck table 34 by operating a suction means (not shown). A protective tape may be attached to the surface 110a side of the silicon wafer 110 and sucked onto the suction chuck 35 via the protective tape. In this way, the chuck table 34 that sucks and holds the silicon wafer 110 is positioned directly under the image pickup means 26 by the moving means 23.

When the chuck table 34 in which the silicon wafer 110 is held is positioned directly under the image pickup means 26, the image pickup means 26 and the control device 20 execute an alignment operation for detecting a machining area of the silicon wafer 110 to be laser-machined. That is, the image pickup means 26 and the control device 20 include a scheduled division line 112 formed in a predetermined direction of the semiconductor wafer 2 and a condenser 241 of the laser beam irradiating means 24 that irradiates a laser beam along the scheduled division line 112. Image processing such as pattern matching for alignment is performed to align the laser beam irradiation position. Further, the alignment of the laser beam irradiation position is similarly performed on the planned division line 112 formed on the silicon wafer 110 and extending in the direction orthogonal to the predetermined direction. At this time, the surface 110a on which the scheduled division line 112 of the silicon wafer 110 is formed is located on the lower side, but as described above, the image pickup means 26 is used for the infrared illumination means, the optical system for capturing the infrared rays, and the infrared rays. Since it is composed of an image pickup element (infrared CCD) or the like that outputs a corresponding electric signal, it is possible to take an image of the scheduled division line 112 on the front surface 110a side through the back surface 110b side.

When the planned division line 112 formed on the silicon wafer 110 held on the chuck table 34 is detected and the laser beam irradiation position is aligned as described above, the chuck table 34 is as shown in FIG. Is moved to the laser beam irradiation region where the condenser 241 is located, and one end of the predetermined division scheduled line 112 is positioned directly under the condenser 241 of the laser beam irradiation means 24. Next, the focusing point of the laser beam LB'irradiated from the condenser 241 is positioned at a predetermined depth position from the surface 110b of the semiconductor wafer 2. Then, the light collector 241 irradiates the silicon wafer 110 with a laser beam LB'having the same wavelength as the laser beam LB irradiated in the first detection step (S1) and the second detection step (S2) and having a large output. While moving the chuck table 34 in the direction indicated by the arrow X in FIG. 4, the chuck table 34 is moved at a predetermined machining feed rate. Then, when the other end of the scheduled division line 112 reaches the irradiation position of the condenser 241, the irradiation of the laser beam LB'is stopped and the movement of the chuck table 34 is stopped. The modified layer 120 as shown in FIG. 4 is formed inside the silicon wafer 110 while the chuck table 34 is rotated and moved by the moving means 22 in the laser processing for forming the modified layer 120 in this way. It is formed, and finally, the modified layer 120 is formed along all the planned division lines 112.

The laser processing conditions carried out in the modified layer forming step (S5) are set as follows, for example.
Wavelength: 1342 nm pulsed laser Repeat frequency: 90 kHz
Average output: 1.7W
Machining feed rate: 700 mm / sec

In the first detection step (S1), the second detection step (S2), and the modified layer formation step (S5) described above, the laser beams LB and LB'with a wavelength of 1342 nm were irradiated. The wavelength is not limited to a laser beam of 1342 nm, and an arbitrary wavelength can be selected from a near-infrared wavelength range, for example, a laser beam having a wavelength of 1000 nm to 2500 nm, depending on the physical characteristics of the workpiece and the selected laser beam irradiation means 24. You can choose.

(Laser processing stop step)
Returning to FIG. 3 and continuing the explanation, if the calculated transmittance (R) does not satisfy the predetermined condition (30% or more) in the modified layer formation determination step (S4) and is determined to be No, it is revised. Instead of proceeding to the layer forming step (S5), the process proceeds to the laser processing discontinuation step (S6). Since the transmittance of the silicon wafer 110 having such a transmittance (R) is too low, even if laser processing is performed based on the set processing conditions, the inside of the silicon wafer 110 is satisfactorily modified. It is determined that the layer 120 cannot be formed. Therefore, the laser processing set after that is stopped. Even if the laser processing is stopped by this laser processing stop step (S6), if the laser processing conditions can be changed, the laser processing conditions are reset (change of laser beam wavelength, output, etc.). ), And then the modified layer forming step of forming the modified layer 120 may be executed.

The present invention is not limited to the above-described embodiment, and various modifications can be assumed as long as it belongs to the technical scope of the present invention.

For example, in the above-described embodiment, the transmittance is calculated using the dummy wafer 100 constituting the silicon wafer 110 as the workpiece, and it is determined whether or not the silicon wafer 110 is suitable for forming the modified layer. The present invention is not limited to this. The silicon wafer 110 has an outer peripheral region 110c on which the device 114 is not formed, and in the second detection step described above, the silicon wafer 110 is placed in place of the dummy wafer 100, and the outer peripheral region 110c is powered at that time. It is arranged and held so as to cover the light receiving element of the meter 36. Then, the transmittance of the silicon wafer 110 that is actually processed is determined by receiving the laser beam transmitted by irradiating the outer peripheral region 110c of the silicon wafer 110 with the laser beam LB with the power meter 36 and detecting the second power. It may be calculated. By doing so, the transmittance will be obtained in consideration of the change in the transmittance that occurs in the process of actually forming the device 114 on the substrate, and the transmittance can be grasped more accurately and reflected in the modification layer formation determination. can.

2: Laser processing device 20: Control device 22: Holding means 23: Moving means 33: Cover plate 34: Chuck table 35: Suction chuck 36: Power meter 40: X direction moving means 42: Y direction moving means 100: Dummy wafer 110 : Silicon wafer 110a: Front surface 110b: Back surface 110c: Outer peripheral surplus area 112: Scheduled division line 114: Device

Claims (2)

A holding means for holding the work piece and a condensing point of a laser beam having a wavelength that is transparent to the work piece held by the holding means are positioned inside the work piece and irradiated to form a modified layer. It is a laser processing method using a laser processing apparatus provided with at least a laser beam irradiating means provided with a light collector and a processing feeding means for relatively processing and feeding the holding means and the laser beam irradiating means.
The first detection step of irradiating the laser beam with the concentrator of the laser beam irradiating means facing the power meter and detecting the first power,
A second detection step of locating the workpiece between the concentrator and the power meter, irradiating it with a laser beam, and detecting the second power.
A transmittance calculation step for calculating an index representing the transmittance of the workpiece from the first power and the second power,
A modified layer formation determination step for determining whether or not a modified layer can be formed inside the workpiece from the index representing the transmittance, and
The modified layer forming step of forming the modified layer by irradiating the workpiece determined to be able to form the modified layer by the modified layer formation determination step with the condensing point of the laser beam positioned inside.
Consists of at least from
The power meter is arranged adjacent to the chuck table arranged in the holding means, and the concentrator and the holding means are relatively moved to carry out the first detection step. ,
A laser machining method in which a workpiece is held on the chuck table so as to protrude from the chuck table of the holding means and reach the power meter, and the second detection step is carried out . The laser processing method according to claim 1, wherein the workpiece is a silicon wafer and the wavelength of the laser beam is near infrared rays .

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