LASER PROCESSING PROCESS

Abstract

Laser processing method for processing a substrate (10) using a pulsed laser beam (LB), wherein the substrate (10) has a front (10a) and a back (10b), wherein a component (12) with an electrode pad (12a) is formed on the front (10a) of the substrate, wherein the pulsed laser beam is applied to the back (10b) of the substrate to form a fine hole (16) extending to the electrode pad (12a), wherein the laser processing method comprises: a laser beam application step to apply the pulsed laser beam (LB) to the back (10b) of the substrate at a position corresponding to the electrode pad (12a), thereby forming the fine hole (16) in the substrate; a detection step for detecting a first plasma light generated by the substrate (10) by applying the pulsed laser beam (LB) to the substrate (10), and also for detecting a second plasma light generated by the electrode pad (12a) by applying the pulsed laser beam (LB) to the electrode pad (12a); and a laser application termination step to terminate the application of the pulsed laser beam (LB) when the second plasma light is detected in the detection step; wherein Time intervals of the pulsed laser beam (LB) to be repeatedly applied to the same fine hole (16) in the laser beam application step are set to 0.1 ms or more, so that after the first plasma light generated by applying the pulsed laser beam (LB) to the same fine hole (16) disappears, the next pulsed laser beam (LB) generating the second plasma light is applied to the same fine hole (16).

Description

BACKGROUND OF THE INVENTION Technical field

[0001] The present invention relates to a laser processing method for applying a laser beam to the back of a substrate in order to form a fine hole extending to an electrode pad. Description of the related prior art

[0002] A multitude of electronic components, such as integrated circuits (ICs) and large-area integrated circuits (LSIs), are formed on the front face of a wafer, separated by numerous intersecting division lines. The wafer, thus displaying a multitude of components on its front face, is divided along these division lines using a cutting or laser processing device to obtain a multitude of individual component chips. These component chips, extracted from the wafer, are used in various types of electronic devices, such as mobile phones and personal computers.

[0003] In recent years, the functionality of each component has been enhanced by the following method. Each component is formed on the front side of a substrate, and an electrode pad is formed on the front side of the component. A fine hole is created through the substrate, extending from the back side of the substrate to the back side of the electrode pad. This fine hole is filled with a conductive material such as aluminum, thus forming a contact point. Another component is formed on the back side of the substrate such that it connects to the component formed on the front side of the substrate through this contact point.

[0004] The present applicant has proposed a technique for forming the aforementioned fine hole, in which a laser beam is applied to the back side of the substrate at a position corresponding to the electrode pad of each component (see JP 6 034 030 B2). In this technique described in JP 6 034 030 B2, a first plasma light is generated by the substrate when the laser beam is applied to the back side of the substrate, which has the components on its front side. A second plasma light is also generated by the electrode pad when the laser beam reaches it. The first and second plasma lights are detected to determine whether the laser beam has reached the electrode pad. Accordingly, the application of the laser beam to the substrate is stopped without the laser beam forming a hole in the electrode pad.

[0005] The publications JP 2009 - 125 756 A and JP 2008 - 212 999 A represent further state of the art. PRESENTATION OF THE INVENTION

[0006] According to the conventional technique described above, the second plasma light, inherent in the material forming the electrode pad, is generated when the pulsed laser beam applied to the back of the substrate reaches the electrode pad. Accordingly, once the second plasma light is detected, the laser beam application can be stopped. However, if the pulsed laser beam is applied repeatedly at short intervals (at a high frequency), time is required for the first plasma light, inherent in the substrate material, to completely dissipate after the laser beam reaches the electrode pad. Consequently, the second plasma light generated by the electrode pad cannot be detected quickly. This results in excessive laser beam application to the electrode pad, causing the formation of a hole in it.So there is a problem: it is difficult to form the fine hole correctly (or appropriately or suitablely).

[0007] It is therefore an object of the present invention to provide a laser processing method that can correctly form a fine hole in a substrate by applying a laser beam to the back of the substrate at the position corresponding to the electrode pad of each component.

[0008] The present invention is defined by the laser processing method according to the features of independent claim 1. The dependent claims relate to preferred embodiments.

[0009] According to one aspect of the present invention, a laser processing method for processing a substrate using a pulsed laser beam is provided, wherein the substrate has a front and a back, wherein a component with an electrode pad is formed on the front of the substrate, wherein the pulsed laser beam is applied to the back of the substrate, so that a fine hole is formed which extends to the electrode pad.The laser processing method comprises a laser beam application step for applying the pulsed laser beam to the back of the substrate at a position corresponding to the electrode pad, thereby forming the fine hole in the substrate; a detection step for detecting a first plasma light generated by the substrate through the application of the pulsed laser beam to the substrate, and also for detecting a second plasma light generated by the electrode pad through the application of the pulsed laser beam to the electrode pad; and a laser application termination step for terminating the application of the pulsed laser beam when the second plasma light is detected in the detection step; and in the laser beam application step, the time intervals of the pulsed laser beam being repeatedly applied to the same fine hole are set to 0.1 ms or more.

[0010] Preferably, the time intervals in the laser beam application step are set to 0.15 ms or more.

[0011] According to the laser processing method of the present invention, in the laser beam application step, the time intervals of the pulsed laser beam, which is repeatedly applied to the same fine hole, are set to 0.1 ms or more. With this configuration, the second plasma light can be sufficiently detected at the time when the fine hole extends to (or has reached) the electrode pad, thus solving the problem of a hole forming in the electrode pad.

[0012] The aforementioned and other problems, features and advantages of the present invention and the manner of its realization will become clearer, and the invention itself will be best understood, from a study of the following description and the attached claims with reference to the attached drawings, which show a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a perspective view showing a substrate as a workpiece and also showing a way of holding the substrate on a ring frame in a preferred embodiment of the present invention; Fig. Figure 2 is a perspective overall view of a laser processing device for carrying out laser processing on the in Fig. 1 substrate shown; Fig. Figure 3 is a block diagram representing a laser beam application unit and a plasma light detection device, which are shown in the Fig. The laser processing device shown in section 2 is included; Fig. Figure 4 is an enlarged sectional view of a substantial part of the substrate, illustrating a way of applying a laser beam to the back of the substrate to form fine holes extending to electrode pads formed on the front of a device in a laser beam application step. Fig. Figure 5 is a graph representing a change in the output (voltage value) of a first photodetector and a second photodetector, shown in Fig. 3 are shown; and Fig. Figures 6A to 6H are waveform diagrams representing the pulses of a laser beam applied in tests carried out by the present inventors. DETAILED DESCRIPTION OF THE PREFERRED VERSION

[0013] A laser processing method according to a preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Fig. Figure 1 is a perspective view of a disk-shaped substrate 10, which in this preferred embodiment is prepared as a workpiece to be laser-machined. The in Fig. The substrate 10 shown in Figure 1 is, for example, formed of lithium tantalate (LT) and has a thickness of 300 µm. The substrate 10 has a front side 10a and a back side 10b. A plurality of intersecting division lines 14 are formed on the front side 10a of the substrate 10, thus defining a plurality of separate regions, each containing a plurality of components 12. As shown in an enlarged section A on the right in Figure 10, the components 12 are formed in a multiplicity of separate regions. Fig. As shown in Figure 1, a plurality of (ten) electrode pads 12a are formed on the front face of each component 12. Each electrode pad 12a has a substantially rectangular shape. In this preferred embodiment, five electrode pads 12a are arranged in a line along one side edge of each component 12, and five electrode pads 12a are arranged in a line along another side edge of each component 12, parallel to the first side edge. Each electrode pad 12a has a size of about 500 × 600 µm. Each electrode pad 12a is, for example, made of copper (Cu). As shown in Fig. As shown in Figure 1, the prepared substrate 10 is inverted and the front side 10a of the substrate 10 is attached to a protective tape (adhesive tape) T, which is held in its circumferential section on a ring frame F. That is, the substrate 10 is held by the protective tape T on the ring frame F in the state in which the back side 10b of the substrate 10 is facing upwards or exposed upwards.

[0014] Fig. Figure 2 is a perspective overall view of a laser processing device 1 for performing laser processing on the substrate 10 to form a fine hole corresponding to each electrode pad 12a, in the present embodiment. The in Fig. 2 The laser processing device 1 shown comprises a holding unit 20 for holding the substrate 10, which is held by the protective band T on the ring frame F, a movement mechanism 30 for moving the holding unit 20, a laser beam application unit 50 for applying a laser beam to the substrate 10 held by the holding unit 20, an imaging unit 60 for imaging the substrate 10 held by the holding unit 20 and a plasma light detection means 70 for detecting plasma light generated by the substrate 10.

[0015] The laser processing device 1 has a base 2. The holding unit 20 comprises a rectangular, X-movable plate 21, which is mounted on the base 2 such that it is oriented in the direction indicated by an arrow X. Fig. 2 is movable in the X direction, a rectangular plate 22 movable in the Y direction, which is mounted on the plate 21 movable in the X direction, such that it is in the direction indicated by an arrow Y in Fig. The assembly, which is movable in the Y direction as shown in Figure 2, comprises a cylindrical support column 24, which is attached to a top surface of the Y-movable plate 22, and a rectangular cover plate 26, which is attached to an upper end of the support column 24. The X and Y directions are perpendicular to each other in a substantially horizontal plane. The cover plate 26 has an elongated hole 26a, and a circular receiving table 28 is provided on the cover plate 26, extending upward through the elongated hole 26a of the cover plate 26. The receiving table 28 has a top surface for holding the substrate 10. The top surface of the receiving table 28 is provided with a circular vacuum chuck 40, which is formed from a porous material. The vacuum chuck 40 has a substantially horizontal top surface for holding the substrate 10 under suction.This means that the vacuum chuck 40 is connected via a suction channel (not shown) to suction means (not shown) for generating a vacuum. The suction channel is formed within the support column 24. The receiving table 28 is provided with a plurality of clamps 42 for attaching the ring frame F, which supports the substrate 10.

[0016] The motion mechanism 30 is provided at the base 2 and serves as a means for the relative movement of the holding unit 20 and the laser beam application unit 50. The motion mechanism 30 comprises an X-motion mechanism 31 for moving the holding unit 20 in the X direction as a feed direction and a Y-motion mechanism 32 for moving the holding unit 20 in the Y direction as an indexing direction. The X-motion mechanism 31 includes a stepper motor 31a and a ball screw 31b, which is configured to be rotated by the stepper motor 31a, whereby a rotary motion by the stepper motor 31a is converted into a linear motion by the ball screw 31b and then transmitted to the X-moving unit 21.Accordingly, the X-movable plate 21 can be moved back and forth in the X direction along a pair of parallel guide rails 2a provided at the base 2, the guide rails 2a slidingly engaging with a pair of parallel grooves formed on the underside of the X-movable plate 21. Similarly, the Y-movement mechanism 32 comprises a stepper motor 32a and a ball screw 32b configured to be rotated by the stepper motor 32a, wherein a rotary motion by the stepper motor 32a is converted into a linear motion by the ball screw 32b and then transmitted to the Y-movable plate 22.Accordingly, the Y-movable plate 22 can be moved back and forth in the Y direction along a pair of parallel guide rails 21a provided on the upper side of the X-movable plate 21, the guide rails 21a slidably engaging with a pair of parallel grooves formed on the underside of the Y-movable plate 22. Furthermore, a rotary drive (not shown) for rotating the receiving table 28 is provided within the support column 24. Accordingly, the receiving table 28 can be rotated by a specific angle using this rotary drive.Although not shown, the X-movement mechanism 31, the Y-movement mechanism 32, and the rotary drive are each equipped with X-position sensing means for detecting the X-position of the receiving table 28 in the X-direction, Y-position sensing means for detecting the Y-position of the receiving table 28 in the Y-direction, and a rotary position sensing means for detecting the rotary position of the receiving table 28. Thus, the X-position, Y-position, and rotary position of the receiving table 28 at the base 2 can be accurately detected by these position sensing means. Detection signals from these position sensing means are transmitted to a control unit 100 (see figure). Fig. 3) transmitted, which is described below. According to a control signal transmitted by the control unit 100, the X-movement mechanism 31, the Y-movement mechanism 32 and the rotary drive are operated to control the receiving table 28 in order to obtain a desired X-coordinate position, a desired Y-coordinate position and a desired rotation angle θ.

[0017] An inverted L-shaped support element 4 is provided at the base 2 on a rear section behind the movement mechanism 30. The support element 4 comprises a vertical section 4a extending vertically upward from a top surface of the base 2 and a horizontal section 4b extending horizontally from an upper end of the vertical section 4a. The laser beam delivery unit 50 includes an optical system (not shown) located in the horizontal section 4b of the support element 4. The laser beam delivery unit 50 further includes a focusing means 52 located on the underside of a front end section of the horizontal section 4b.

[0018] As in Fig. As shown in Figure 3, the optical system of the laser beam application unit 50 comprises a pulsed laser oscillator 51 for generating a pulsed laser and emitting a pulsed laser beam LB, an attenuator 53 for adjusting the power of the laser beam LB emitted by the pulsed laser oscillator 51, a first acousto-optic deflection device 54, which essentially comprises an acousto-optic device as an optical deflection device for deflecting the optical path of the laser beam LB in a given feed direction (X-direction), a second acousto-optic deflection device 55, which essentially comprises an acousto-optic device as an optical deflection device for deflecting the optical path of the laser beam LB in a given indexing direction (Y-direction), and a reflection mirror 56 for changing the direction of the optical path of the laser beam emerging from the second acousto-optic deflection device 55. LB.The optical path of the laser beam LB reflected by the reflection mirror 56 is directed into the focusing device or a condenser 52 with an fθ lens 521. The pulse laser oscillator 51, the attenuator 53, the first acousto-optic deflection device 54, and the second acousto-optic deflection device 55 are connected to the control unit 100, and the operation of these components is controlled by control signals transmitted by the control unit 100.

[0019] In the case where, for example, a voltage of 5 V is applied to the first acousto-optic deflection device 54 by the control unit 100 and a frequency corresponding to 5 V is applied to the acousto-optic device contained therein (not shown), the laser beam LB generated by the pulse laser oscillator 51 is deflected in its optical path, as defined by LBa in Fig. 3 is shown, deflected and the laser beam LB is focused onto a point Pa. Furthermore, in the case where, for example, a voltage of 10 V is applied from the control unit 100 to the first acousto-optic deflection device 54 and a frequency corresponding to 10 V is applied to the acousto-optic device contained therein (not shown), the laser beam LB generated by the pulse laser oscillator 51 is deflected in its optical path, as defined by LBb in Fig. 3 is shown, deflected and the laser beam LB is focused on a point Pb which, when viewed from Fig. 3 is shifted to the right from point Pa by a predetermined amount. Furthermore, in the case where, for example, a voltage of 15 V is applied from the control unit 100 to the first acousto-optic deflection device 54 and a frequency corresponding to 15 V is applied to the acousto-optic device contained therein (not shown), the laser beam LB generated by the pulse laser oscillator 51 is deflected in its optical path as defined by LBc in Fig. As shown in Figure 3, the laser beam LB is deflected and focused onto a point Pc, which, when viewed from Fig. 3 is shifted to the right from point Pb by a predetermined amount. The first acousto-optic deflection device 54 deflects the optical path of the laser beam LB in the feed direction (X-direction parallel to the sheet of Fig. 3) off.

[0020] In contrast, the second acousto-optic deflection device 55 directs the optical path of the laser beam LB in the indexing direction (Y-direction perpendicular to the sheet of Fig. 3) The other configuration is similar to that of the first acousto-optic deflection device 54. Thus, the optical path of the laser beam LB in the feed direction (X-direction) can be deflected by the first acousto-optic deflection device 54 to a given position in a predetermined area according to a voltage applied to it. Likewise, the optical path of the laser beam LB can also be deflected in the indexing direction (Y-direction) to a given position in a predetermined area by the second acousto-optic deflection device 55 according to a voltage applied to it. As in Fig. As shown in Figure 3, the laser beam application unit 50 further comprises a laser beam absorption means 57 for absorbing a laser beam LB' (represented by a dashed line) which is deflected by the second acousto-optic deflection means 55 if a predetermined voltage is applied to it.

[0021] The control unit 100 is configured by a computer that includes a processor (CPU) for calculations according to a control program, a read-only memory (ROM) that previously stored, for example, the control program, a random access memory (RAM) for storing, for example, acquisition values ​​and calculation results, an input interface, and an output interface. Not only the laser beam application unit 50 is connected to the control unit 100, but also the motion mechanism 30, the imaging unit 60, the plasma light detection device 70, and the like. These components can be controlled by control signals transmitted by the control unit 100.

[0022] With reference to Fig. 2 The imaging unit 60 is provided on the underside of the front end section of the horizontal section 4b at a position adjacent to the focusing means 52 in the X direction. The imaging unit 60 includes a conventional image acquisition device for imaging the workpiece using visible light, an illumination means for illuminating the workpiece, an infrared image acquisition device for imaging the workpiece using infrared light, an infrared light application means for applying infrared light to the workpiece, and the like. Information about an image obtained by the imaging unit 60 is transmitted to the control unit 100. The imaging unit 60 is used to perform alignment between the substrate 10 and the focusing means 52 and also to detect the position of each electrode pad 12a formed on each component 12.

[0023] The plasma light detection device 70 has an optical system provided in the horizontal section 4b of the support element 4. As shown in Fig. As shown in Figure 2, a plasma light receiving means 71 is provided as a component of the plasma light detection means 70 on the underside of the front end section of the horizontal section 4b at a position adjacent to the focusing means 52 in the X direction and opposite the imaging unit 60. Fig. As shown in Figure 3, the plasma light detection means 70 includes the plasma light receiving means 71, as mentioned above. The plasma light receiving means 71 serves to receive plasma light generated by the substrate 10 by the application of the laser beam LB from the focusing means 52 of the laser beam application unit 50 to the substrate 10, which is held on the vacuum chuck 40 of the receiving stage 28. The plasma light detection means 70 further includes a beam splitter 72 for dividing the plasma light received by the plasma light receiving means 71 into light with a first optical path 72a and light with a second optical path 72b, and a first bandpass filter 73, which is provided on the first optical path 72a for transmitting (or allowing) only the light with a first predetermined wavelength (i.e.,, the wavelength of the first plasma light generated by the lithium tantalate forming the substrate 10), a first photodetector 74 for detecting the light passed through the first bandpass filter 73 and for outputting a light intensity signal, a direction-changing mirror 75 provided on the second optical path 72b, a second bandpass filter 76 for receiving the light whose direction of travel has been changed by the direction-changing mirror 75 and for passing through only the light with a second predetermined wavelength (i.e., the wavelength of the second plasma light generated by the copper forming each electrode pad 12a)), and a second photodetector 77 for detecting the light passed through the second bandpass filter 76 and for outputting a light intensity signal.The plasma light receiving device 71 consists of a focusing lens and a lens housing for accommodating the focusing lens (both not shown).

[0024] The first bandpass filter 73 has a passband of 600 to 680 nm with respect to wavelength, so that in the present embodiment only the first plasma light with a specific wavelength (670 nm), generated by the lithium tantalate, is transmitted. Conversely, the second bandpass filter 76 has a passband of 510 to 520 nm wavelength, so that in the present embodiment only the second plasma light with a specific wavelength (515 nm), generated by the copper, is transmitted. Both the first photodetector 74 and the second photodetector 77 output a voltage signal to the control unit 100, corresponding to the detected light intensity.

[0025] Using the laser processing device 1 with the above configuration in the present embodiment, a laser processing operation is carried out to form a fine hole in the substrate 10 at a position corresponding to each electrode pad 12a formed on each component 12 formed on the front 10a of the substrate 10, wherein the fine hole has a depth from the back 10b of the substrate 10b to the corresponding electrode pad 12a.

[0026] As described above, the substrate 10 is held by the protective band T on the ring frame F in the position in which the back side 10b of the substrate 10 is facing upwards. First, the substrate 10 is held by the protective band T on the vacuum chuck 40 of the receiving table 28 in the Fig. The laser processing device 1 shown in Figure 2 is placed in the state in which the back side 10b of the substrate 10 is facing upwards. Then, the suction device (not shown) connected to the vacuum chuck 40 is actuated so that the substrate 10 is held under suction by the protective band T on the vacuum chuck 40. Furthermore, the ring frame F is secured by the clamps 42.

[0027] The receiving table 28, which holds the substrate 10 under suction, is then moved into a position directly below the imaging unit 60 by operating the X-movement mechanism 31. The imaging unit 60 is then operated to determine whether the division lines 14 formed on the substrate 10 held on the receiving table 28 are parallel to the X and Y directions. Based on this determination, the orientation of the substrate 10 is adjusted by rotating the receiving table 28 accordingly. The coordinate position of each electrode pad 12a formed on each component 12 is then detected, and the alignment is performed to set the laser application position at which the laser beam LB is to be applied to the substrate 10.

[0028] After performing the alignment, a laser beam application step is carried out so that the laser beam LB is applied to the back 10b of the substrate 10 at the position corresponding to each electrode pad 12a. (Laser beam application step)

[0029] As described above, the laser beam application step is performed after alignment. The coordinate position of each component 12 formed on the substrate 10, which is held on the receiving table 28, and the coordinate position of each electrode pad 12a formed on each component 12, are stored and managed in the control unit 100. Accordingly, by performing the alignment of each electrode pad 12a on the substrate 10, it can be precisely set to a specific position.

[0030] For example, the laser processing process in this preferred embodiment can be carried out under the following conditions. Wavelength of the laser beam: 343 nm Repetition rate: 50 kHz Average power consumption: 2 W Pulse energy: 40 µJ Pulse width: 10 ps Spot diameter: 50 µm

[0031] When performing the laser processing operation under the conditions mentioned above, the laser beam LB is applied to the substrate 10 at time intervals of 0.1 ms or more by the following procedure. As already mentioned, the repetition frequency of the laser beam LB to be generated by the pulsed laser oscillator 51 is set to 50 kHz. With this setting, the laser beam LB is generated at time intervals of 0.02 ms. By appropriately controlling the first acousto-optic deflection device 54 and the second acousto-optic deflection device 55 in this setting, the laser beam LB is dispersively applied to the back side 10b of the substrate 10 in such a way that the application positions of five pulses LB1 to LB5 of the laser beam LB are changed so that they each correspond to five electrode pads 12a1 to 12a5 arranged side by side in a line, as shown in Fig. Figure 4 illustrates this process. This process is repeated to form five fine holes 16, resulting in the laser beam LB, generated at intervals of 0.02 ms, being applied to the same fine hole 16 at intervals of 0.1 ms. According to this preferred embodiment, the time intervals of the laser beam LB applied to the substrate 10 are increased, thus eliminating the need to generate a wasted laser beam LB' that would be absorbed by the laser beam absorber 57. Furthermore, the multiple fine holes 16 can be formed simultaneously, ensuring excellent processing efficiency. (Capture step)

[0032] During the laser beam application step, a detection step is performed to detect the first plasma light generated by the lithium tantalate forming substrate 10 and the second plasma light generated by the copper forming each electrode pad 12a. This detection step will now be described.

[0033] In the detection step, light intensity signals are output as voltage values ​​from the first photodetector 74 and the second photodetector 77 of the plasma light detection device 70 to the control unit 100 in the state in which the laser beam application step is carried out. Fig. Figure 5 represents a change in the voltage value V(LT) and a change in the voltage value V(Cu) over time, where the voltage value V(LT) is a voltage value output by the first photodetector 74 to detect the light intensity of the first plasma light, and the voltage value V(Cu) is a voltage value output by the second photodetector 77 to detect the light intensity of the second plasma light. Fig. In figure 5, the horizontal axis represents time (T) and the vertical axis represents the voltage value (V) according to the light intensity.

[0034] When the laser beam LB is applied to the back side 10b of the substrate 10 at the position corresponding to each electrode pad 12a, the first plasma light is generated from the substrate 10 by the application of the laser beam LB. Accordingly, as in Fig. As shown in Figure 5, the voltage value V(LT) output by the first photodetector 74 increases until a predetermined voltage value (e.g., 2.5 V) is reached. Thereafter, the voltage value V(LT) is maintained at a substantially constant value until the laser beam LB reaches each electrode pad 12a. When the laser beam LB reaches each electrode pad 12a, the voltage value V(LT) output by the first photodetector 74 begins to decrease. (Laser application completion step)

[0035] By performing the detection step, the state of the generation of the first and second plasma lights can be detected. That is, when performing the detection step, a laser application termination step is carried out to detect the second plasma light, thereby ending the application of the laser beam LB. This laser application termination step will now be described in more detail.

[0036] When the laser beam LB reaches the electrode pads 12a1 to 12a5, the voltage value V(Cu) output by the second photodetector 77 begins to rise, as shown in Fig. Figure 5 illustrates this. Shortly after the voltage value V(Cu) increases, the fine holes 16 may not be able to sufficiently reach the electrode pads 12a1 to 12a5, so there is a possibility that poor conductivity may occur after the fine holes 16 are filled with a conductive material. To solve this problem, a threshold value S (e.g., 1.0 V) is set for the voltage value V(Cu) output by the second photodetector 77, so that it is detected when the fine holes 16 have sufficiently reached the electrode pads 12a1 to 12a5. The voltage value V(Cu) output by the second photodetector 77 is compared with this threshold value S. When the voltage value V(Cu) has become greater than the threshold value S, it is determined that the fine holes 16 have sufficiently reached the electrode pads 12a1 to 12a5, i.e., it is determined that the fine holes 16 in the substrate 10 are properly formed.Consequently, the control unit 100 sends a stop signal to the laser beam application unit 50, thus terminating the application of the laser beam LB. If the above determination using the threshold value S is not performed and the laser beam application step continues to apply the laser beam even after the voltage value V(Cu) is greater than the threshold value S, the voltage value V(Cu) continues to increase, as shown by a dashed line V(Cu)' in . Fig. Figure 5 is shown until a substantially constant electrode value (e.g., 2.5 V) is reached. However, when the voltage value V(Cu) reaches such a constant electrode value, there is a possibility that a through-hole could form in each of the electrode pads 12a1 to 12a5. Accordingly, the threshold value S is set to a value that is smaller than the constant value mentioned above.

[0037] As described above, the laser beam application step, the detection step, and the laser application termination step are performed while the receiving stage 28 is moved in the X-direction by operating the X-movement mechanism 31, thereby forming the correct fine holes 16, each extending from the rear 10b of the substrate 10 to the five electrode pads 12a1 to 12a5. The receiving stage 28 is then moved in the X-direction so that the five electrode pads 12a are positioned on the next component 12 adjacent to the current component 12 in the X-direction, directly below the focusing means 52. Once the five electrode pads 12a are positioned on the next component 12 directly below the focusing means 52, the laser beam application step, the detection step, and the laser application termination step are performed in the same manner on the five electrode pads 12a of this next component 12.This process is repeated to create a multitude of fine holes 16, each corresponding to one of the electrode pads 12a arranged in a line in the X direction. The Y-movement mechanism 32 is then actuated to move (index) the substrate 10 in the Y direction around the division lines 14, so that the five electrode pads 12a on the next component 12 adjacent to the current component 12 in the Y direction are positioned directly under the focusing means 52. The laser processing is then carried out in the same manner on the five electrode pads 12a on the next component 12, and this process is repeated to create a multitude of fine holes 16, each corresponding to one of the electrode pads 12a of all components 12 on the front surface 10a of the substrate 10.

[0038] As described above, in the present embodiment, the repetition frequency of the laser beam LB generated by the pulsed laser oscillator 51 is set to 50 kHz, i.e., the pulsed laser beam LB is generated in time intervals of 0.02 ms. This laser beam LB is then dispersively applied to the substrate 10, such that the laser beam LB is applied to the same fine hole 16 in time intervals of 0.1 ms. This is based on the understanding that the time intervals of the laser beam applied to the same fine hole 16 must be set to 0.1 ms or more, and this understanding constitutes the technical concept of the present invention. The reason for setting the aforementioned time intervals as conditions for laser processing is described below.

[0039] The present inventors conducted tests to investigate the time intervals of the laser beam LB, which is to be applied to the same fine hole 16, when applying the laser beam LB to the back 10b of the substrate 10 at the positions corresponding to the electrode pads 12a, in order to form the correct fine holes 16 extending to the electrode pads 12a. This test is now described with reference to Fig. 3 and Fig. Sections 6A to 6H are described. Fig. Figures 6A to 6H show various examples of the signal pulses of the laser beam LB applied to the back side 10b of the substrate 10b at the position corresponding to one of the electrode pads 12a by operating the laser beam application unit 50. More precisely, the pulses of the laser beam LB actually applied to the substrate 10 are shown by solid lines in the Fig. 6A to 6H are shown, while the pulses of the laser beam LB' (see Fig. 3), which is deflected from the optical path by the second acousto-optic deflection device 55 and absorbed by the laser beam absorption device 57 to be omitted, by broken lines in the Fig. Figures 6A to 6H are shown. In the following examples of the test, the other processing conditions, as a parameter to be changed, are similar to those of the preferred embodiment mentioned above, and the description of these similar conditions is omitted here.

[0040] The laser processing conditions in the following examples of the test are essentially as follows: Wavelength of the pulsed laser beam: 343 nm Repetition rate: 50 kHz (reference repetition rate) Average power consumption: 2 W Pulse energy: 40 µJ Pulse width: 10 ps Spot diameter: 50 µm (Example 1)

[0041] As in Fig. As shown in Figure 6A, the reference repetition frequency (50 kHz: 0.02 ms as time intervals of the laser beam LB) was used unchanged in the laser processing conditions described above to perform the laser processing and thus form a fine hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a hole was formed through the electrode pad 12a. (Example 2)

[0042] As in Fig. As shown in Figure 6B, under the laser processing conditions described above, the even-numbered pulses of the laser beam LB with the reference repetition frequency (50 kHz) were omitted; that is, one pulse was omitted at a time. In other words, the repetition frequency was essentially changed to 25 kHz (0.04 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was carried out to form a fine hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a hole was formed through the electrode pad 12a. (Example 3)

[0043] As in Fig. As shown in Figure 6C, two pulses were always omitted from the laser beam LB with the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 16.7 kHz (0.06 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was carried out to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was captured to stop the application of the laser beam LB. As a result, a hole was formed through the electrode pad 12a. (Example 4)

[0044] As in Fig. As shown in Figure 6D, three pulses were always omitted from the laser beam LB at the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 12.5 kHz (0.08 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was carried out to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a small hole was formed in the electrode pad 12a. (Example 5)

[0045] As in Fig. As shown in Figure 6E, four pulses were always omitted from the laser beam LB at the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 10 kHz (0.1 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was carried out to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a depression was recorded on the back of the electrode pad 12a, but no hole was formed in the electrode pad 12a. (Example 6)

[0046] As in Fig. As shown in Figure 6F, five pulses were always omitted from the laser beam LB at the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 8.3 kHz (0.12 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was performed to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a depression was recorded on the back of the electrode pad 12a, but no hole was formed in the electrode pad 12a. (Example 7)

[0047] As in Fig. As shown in Figure 6G, six pulses were always omitted from the laser beam LB at the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 7.1 kHz (0.14 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was performed to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, a slight indentation was observed on the back of the electrode pad 12a, but no hole was formed in the electrode pad 12a. (Example 8)

[0048] As in Fig.As shown in Figure 6H, seven pulses were always omitted from the laser beam LB at the reference repetition frequency (50 kHz) under the laser processing conditions described above. In other words, the repetition frequency was essentially changed to 6.25 kHz (0.16 ms as the time intervals of the laser beam LB). Under the conditions described above, the laser processing was carried out to create a small hole 16 at the position corresponding to one of the electrode pads 12a. During this laser processing, the second plasma light was detected to stop the application of the laser beam LB. As a result, no hole was created in the electrode pad 12a, and no indentation was recorded on the back of the electrode pad 12a. (Conclusion)

[0049] From the results of the aforementioned tests, the present inventors found that if the essential repetition frequency of the laser beam LB to be generated by the pulsed laser oscillator 51 is set to 10 kHz or less, i.e., if the time intervals of the laser beam LB to be applied to the same fine hole 16 are set to 0.1 ms or more, the second plasma light can be sufficiently detected at the time when the fine hole 16 has reached the electrode pad 12a, thus solving the problem of a hole being formed in the electrode pad 12a. Furthermore, the present inventors also found that by setting the time intervals of the laser beam LB to 0.15 ms or more, the detection of the second plasma light can be achieved without forming a large opening in the electrode pad 12a, so that it can be correctly (or appropriately) determined that the fine hole 16 has reached the electrode pad 12a.This means that in the laser beam application step, it is preferred to set the time intervals of the pulsed laser beam LB to 0.15 ms or more.

[0050] In the preferred embodiment described above, the repetition rate of the pulsed laser beam LB generated by the pulsed laser oscillator 51 is set to 50 kHz, and this pulsed laser beam LB is dispersively applied to the five electrode pads 12a1 to 12a5 arranged in a line, thereby forming five fine holes 16, each reaching one of the five electrode pads 12a1 to 12a5. Accordingly, the time intervals of the laser beam LB applied to each fine hole 16 are set to 0.1 ms. However, this configuration is only illustrative. As a modification, the number of electrode pads to be dispersively irradiated with the laser beam LB can be adjusted according to the repetition rate.As a further modification, the repetition rate of the pulsed laser beam LB, which is to be generated by the pulsed laser oscillator 51, can be set to 10 kHz or less, and the dispersive application of the laser beam LB can be omitted. In this case as well, the time intervals of the laser beam LB, which is to be applied to each electrode pad 12a, can be set to 0.1 ms or more.

[0051] As a further modification, the repetition rate of the pulsed laser beam LB, which is to be generated by the pulsed laser oscillator 51, can be set to 50 kHz, and the second acousto-optic deflection device 55 can be appropriately controlled to absorb (omit) the unneeded laser beam LB' at the laser beam absorption device 57, as in the tests above. In this case, the time intervals of the laser beam LB to be applied to the same fine hole 16 can be set to 0.1 ms or more without performing the dispersive application of the laser beam LB described above.

[0052] While the substrate 10 in the preferred embodiment described above is formed from lithium tantalate, the present invention is not limited to this configuration, and the substrate 10 can be formed from any other material, such as silicon, lithium niobate (LN), and glass. In this case, the wavelength of the first plasma light changes according to the material forming the substrate 10. Accordingly, the transmission wavelength of the beam splitter 72 and the passband of the first bandpass filter 73 are adjusted according to the wavelength change of the first plasma light. Furthermore, while the preferred embodiment described above generally uses an electrode pad 12a formed from copper, the present invention is not limited to this configuration, and each electrode pad 12a can be formed from any other material, such as gold.In this case, the passband of the second bandpass filter 76 can be adjusted according to the wavelength change of the second plasma light.

[0053] The present invention is not limited to the details of the preferred embodiment described above. The scope of the invention is defined by the appended claims, and all changes and modifications that fall within the scope of the claims are therefore included in the invention.

Claims

[1] Laser processing method for processing a substrate (10) using a pulsed laser beam (LB), wherein the substrate (10) has a front (10a) and a back (10b), wherein a component (12) with an electrode pad (12a) is formed on the front (10a) of the substrate, wherein the pulsed laser beam is applied to the back (10b) of the substrate to form a fine hole (16) extending to the electrode pad (12a), wherein the laser processing method comprises: a laser beam application step to apply the pulsed laser beam (LB) to the back (10b) of the substrate at a position corresponding to the electrode pad (12a), thereby forming the fine hole (16) in the substrate; a detection step for detecting a first plasma light generated by the substrate (10) by applying the pulsed laser beam (LB) to the substrate (10), and also for detecting a second plasma light generated by the electrode pad (12a) by applying the pulsed laser beam (LB) to the electrode pad (12a); and a laser application termination step to terminate the application of the pulsed laser beam (LB) when the second plasma light is detected in the detection step; wherein Time intervals of the pulsed laser beam (LB) to be repeatedly applied to the same fine hole (16) in the laser beam application step are set to 0.1 ms or more, so that after the first plasma light generated by applying the pulsed laser beam (LB) to the same fine hole (16) disappears, the next pulsed laser beam (LB) generating the second plasma light is applied to the same fine hole (16). [2] Laser processing method according to claim 1, wherein the time intervals in the laser beam application step are set to 0.15 ms or more. [3] Laser processing method according to claim 1 or 2, wherein in the detection step the first plasma light generated by the substrate (10) by applying the pulsed laser beam (LB) to the substrate (10) is detected by a first photodetector (74) and the second plasma light generated by the electrode pad (12a) by applying the pulsed laser beam (LB) to the electrode pad (12a) is detected by a second photodetector (77), wherein in the laser application termination step an output voltage value (V(Cu)) from the second photodetector (77) is compared with a threshold value (S) to determine whether the fine hole (16) is correctly formed in the substrate (10).

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