JP5103054B2 - Laser processing method and laser processing apparatus - Google Patents

Description

    The present invention provides a method and apparatus for processing a transparent body with respect to visible light or near-infrared light efficiently and with high quality using ultrashort pulses. The energy of a part of the ultrashort pulsed laser beam is converted into a UV (ultraviolet) wavelength, and the workpiece is irradiated with the laser beam having the UV wavelength temporally ahead of the original fundamental laser beam. To do. The processing efficiency of the transparent body, which is the object to be processed, is improved by irradiation with the total energy of the UV laser light and the long-wavelength laser light at the same place, and the transmission of the original fundamental laser light A transparent body processing method and a processing apparatus with high processing quality are provided by reducing the effect of the preceding irradiation.

  In the electronic industry, miniaturization of semiconductor devices such as CPUs, DRAMs, and SRAMs has progressed year by year, and the integration of internal circuits has been increased accordingly. The structure of these devices is often composed of a low dielectric constant insulating film structure including a part of metal wiring called a low-k film on a semiconductor substrate such as a silicon wafer. In the semiconductor device manufacturing process, it is necessary to remove only a part of the insulating film under conditions that do not cause thermal damage or mechanical damage to the surroundings. For this reason, nanosecond lasers are currently used for this purpose. It is used.

  In contrast to conventional laser processing using a laser with a pulse width longer than the femtosecond region, such as a nanosecond laser, the processing using an ultrashort pulse laser can reduce the generation of a thermally damaged layer and reduce the wavelength. It is a well-known technique that even a transparent material can be processed by nonlinear light absorption typified by multiphoton absorption. However, since ultra-short pulses are generated using a laser medium that can obtain a laser gain in a broad spectrum in the near-infrared wavelength region, ultra-short pulses with high oscillation efficiency are in the near-infrared to mid-infrared wavelength region. .

  When it is intended to selectively remove only the low-k film composition of a device component of an insulating film such as a low-k film formed on a semiconductor substrate using an ultrashort pulse laser beam having a wavelength in the mid-infrared region, When laser processing is performed on the insulating film from the surface, part of the laser light that is not absorbed by multiphoton absorption is transmitted and reaches the semiconductor substrate. Since the semiconductor substrate material has a laser processing threshold much lower than that of the insulating film, mechanical damage such as delamination (chip delamination) and chipping (chip) occurs in the substrate material. For this reason, when processing a transparent body, it is considered to use a UV ultrashort pulse laser beam having a laser wavelength having a large absorption rate with respect to the transparent body. UV ultra-short pulse laser light is generated by oscillating a solid-state laser to generate near-infrared laser light and converting the output laser light into a harmonic using a nonlinear optical crystal. However, since the conversion efficiency into harmonics is low, the use efficiency of laser energy is extremely reduced when UV ultrashort pulse laser light is used.

  Conventionally, there has been a proposal for processing to proceed from the surface by irradiating near infrared laser light while irradiating the processing surface with ultraviolet rays such as a mercury lamp. However, the irradiation area of mercury lamp light is large, and UV light is irradiated even on unnecessary parts to be processed. Therefore, it is not suitable as a processing method for fine functional devices, and a conventional UV light source such as a mercury lamp is used. The power density applied to the surface of the workpiece from is small, and it was practically difficult to actually observe the effect.

US Reissue Patent No. 37585 Specification

  The problem to be solved by the present invention is to reduce the amount of light transmitted through the transparent body when processing the transparent body on the semiconductor substrate or the like from the surface with high accuracy using ultrashort pulse laser light. An object of the present invention is to provide a processing method and a processing apparatus that improve the processing efficiency with high efficiency.

In order to solve the above-mentioned problems, the present invention is a processing method using a laser, the step of generating a first laser which is an ultrashort pulse having a first wavelength, and a part of the energy of the first laser is a first. Converting the second laser to a second laser that is an ultrashort pulse having a second wavelength that is a harmonic of the first wavelength, providing a time delay to the second laser, the first laser, and The method includes a step of condensing the second laser on the same axis, and a step of irradiating the object to be processed with the first and second condensed laser beams.
The time delay is within 100 ps.
Further, the first wavelength is a wavelength exceeding 500 nm, and the second wavelength is a wavelength of 500 nm or less .
Further, the object to be processed is transparent at least with respect to the first wavelength.
Further, the object to be processed is the transparent layer portion of an object comprising a substrate and a transparent layer that is formed at least on the substrate surface and is transparent to at least the first wavelength.
The transparent layer is an insulator.
Furthermore, the substrate is a semiconductor substrate.

On the other hand, the present invention is a laser processing apparatus, laser generating means for generating a first laser that is an ultrashort pulse having a first wavelength, and a part of the energy of the first laser having a first wavelength. Wavelength converting means for converting to a second laser that is an ultrashort pulse having a second wavelength that is a harmonic, a delay generating means for giving a time delay to the second laser, and a first It is characterized by comprising condensing means for condensing the laser and the second laser on the same axis.
Further, the time delay by the delay generation means is within 100 ps.
Further, the first wavelength is a wavelength exceeding 500 nm, and the second wavelength is a wavelength of 500 nm or less .
Further, the delay generating means is characterized in that an optical path length longer than that of the second laser is given to the first laser.

A part of the first pulse laser beam is converted into a harmonic to generate a second pulse laser beam, and a time delay is given to the first pulse laser beam to condense the two laser beams on the same axis. Then, by irradiating the processed object, the processing efficiency obtained by the ratio of the removal processing amount with respect to the irradiation energy can be increased.
In addition, since the processing object is transparent to the first pulse laser beam, the amount of light transmitted by the first laser light through the processing object can be reduced, and the processing object that is particularly transparent is placed on the substrate. When formed, damage to the substrate can be reduced.

  Hereinafter, the present invention will be described in detail with reference to FIG. FIG. 1 shows a configuration example for generating a multiwavelength pulse laser. Using a nonlinear optical crystal from the mode-locked output of a mode-locked laser in the near-infrared wavelength range from 700 to 980 nm, such as Ti-doped sapphire (center wavelength 780 nm), or a fiber laser of erbium or ytterbium-doped as a laser amplification medium The laser oscillation unit 1 that multiplies the oscillation frequency, amplifies the oscillation frequency with a Ti sapphire amplification medium, and oscillates an ultrashort pulse near-infrared laser is used as a laser source. The ultrashort pulse is assumed to have a pulse width of 100 ps or less.

  The near-infrared laser output beam 2 from the laser oscillation unit 1 is rotated in the direction of the polarization plane using a wave plate (for example, a half-wave plate) 3, and this is converted into a wavelength conversion unit 5 made of a nonlinear optical crystal. , UV (ultraviolet) laser light having a half, a third, or a quarter of the wavelength of the fundamental wave is generated. Since this conversion technique can generate UV laser light by using a known wavelength conversion technique, the details are omitted here. Since the relative relationship between the crystal axis direction of the nonlinear optical crystal and the linear polarization plane of the laser light before conversion affects the conversion efficiency of harmonic generation, the wave plate 3 is provided for adjusting the conversion efficiency. When the near-infrared laser output beam 2 passes through the wavelength converter 5, it becomes the first laser light 8 of the fundamental wave component that has passed without being wavelength-converted and the second laser light 9 that has been converted into UV light. The ratio between the two is determined by the conversion efficiency, and the two-wavelength laser light 6 mixed at an appropriate ratio can be obtained in a coaxial arrangement.

  This two-wavelength laser light 6 is branched into components of a second laser light 9 and a first laser light 8 by a wavelength division filter 7. The second laser light 9 is reflected by the wavelength division filter 7 and directed to the wavelength synthesis filter 17, while the near-infrared first laser light 8 passes through the filter 7 and has total reflection mirrors 11 and 12. It is directed to the wavelength synthesis filter 17 via the optical path detour unit 19. Since the first laser beam 8 travels on an optical path longer than the second laser beam 9 by the optical path detour unit 19, a delay time corresponding to the optical path difference traveling time is given. A time delay can be realized with a simple configuration of giving an optical path difference, and a delay time can be changed by changing the optical path difference. The first laser beam 8 and the second laser beam 9 are synthesized by the wavelength synthesizing filter 17 and arranged again on the same axis as the irradiation laser beam 18.

  At the time of the output of the wavelength converter 5, the first laser pulse that is near infrared of the two-wavelength laser light 6 and the second laser pulse that is UV are sufficiently overlapped in time. In 18, the pulse of the second laser beam 9 reaches the total reflection mirror 15 first, and the pulse of the first laser beam 8 arrives at the total reflection mirror 15 with a delay. The two-wavelength laser light is spatially overlapped and enters the condenser lens 16. Then, it is irradiated to the condensing point 13 generated on the surface or inside of the processed object 20, and the interaction between the UV second laser light 9, the near-infrared first laser light 8 and the processed object 20 is efficient. Fine processing is carried out precisely.

  The processed object 20 is preferably an object that is at least transparent to the first laser beam. Alternatively, as shown in FIG. 1, an object is formed of a substrate 22 and a transparent body 14 formed on at least a part of the surface of the substrate 22 and transparent to at least the first laser beam. Here, the term “transparent” does not necessarily mean that 100% light is transmitted, and includes a case where light can be transmitted to some extent. The transparent body 14 is an insulator such as glass, and the substrate 22 is a semiconductor substrate such as silicon. A condensing point 13 is determined on the surface or inside of the transparent body 14 and irradiated with laser light.

When only the near-infrared long-wavelength laser light is irradiated onto the processed object 20 having the above-described structure, processing is performed on the surface 31 of the transparent body 14 when the power density of the processing threshold is exceeded. On the other hand, when a condensing point is formed inside the transparent body 14 and the electric field at the condensing point is sufficiently strong, dielectric breakdown occurs inside the transparent body, and density changes and defects are formed. In any case, when only near-infrared laser light is condensed on a transparent body, all laser power is not absorbed by nonlinear absorption action such as multiphoton absorption. To penetrate.
On the other hand, when the UV laser light is irradiated together with the near-infrared laser light, in many transparent bodies, the laser light does not enter inside and is absorbed near the surface.

  Irradiation with laser light in which UV and near infrared are superimposed with a time delay improves processing accuracy and increases processing removal compared with laser irradiation with only a single wavelength. This is because free electron plasma is generated on the surface of the transparent body by the UV laser light, and the subsequently irradiated near-infrared laser light is absorbed by the reverse bremsstrahlung. At this time, near-infrared laser light is absorbed by reverse bremsstrahlung in addition to multiphoton absorption, so that the amount of energy absorption is increased compared with the case where only near-infrared laser light is irradiated, resulting in removal processing efficiency. (Removal processing volume / irradiation energy) increases. The time delay at which the removal processing efficiency increases is determined by the electron-lattice relaxation time and is generally 100 ps or less.

  By changing the ratio of the power of the near-infrared first laser beam and the power of the second UV laser beam, the characteristics of the desired processed portion can be optimized. When changing the ratio of the power, the energy ratio of the two wavelengths can be controlled by rotating the polarizing plate 3 to control the conversion efficiency of the nonlinear optical crystal unit.

Using a titanium sapphire crystal as a laser medium, a first laser beam having a pulse width of 100 ps or less having a wavelength of 780 nm, which is in the near infrared region, is generated, and further, the frequency is tripled using a nonlinear crystal BBO. A second laser beam having a wavelength of 260 nm was generated. By passing the detour optical path through the first laser beam, a time delay was given to the second laser beam irradiation. Furthermore, it condensed with the lens and irradiated to the soda-lime glass. The first laser pulse was 15 μJ, and the second laser pulse was 10 μJ.
The laser medium of the first laser beam generating means uses titanium sapphire crystal (center wavelength 780 nm), erbium-doped fiber, ytterbium-doped fiber, Nd: YAG crystal, Nd: YVO ₄ crystal, Nd: YLF crystal, and the like. be able to.

  FIG. 2 shows the delay time dependency of the removal processing volume of the transparent body. In this figure, the horizontal axis represents the irradiation time delay of the first laser beam (wavelength 780 nm) with respect to the second laser beam (wavelength 260 nm). The vertical axis represents the removal processing volume per pulse of the laser beam. However, 260 nm + 780 nm is the removal processing volume of this example. What is 260 nm is a case where the second laser beam (wavelength 260 nm) is irradiated alone, and what is 780 nm is a removal processing volume when the first laser beam (wavelength 780 nm) is irradiated alone. is there. When the time delay was given, the removal processing volume per pulse of the laser light increased. That is, when the delay was about 1 ps or more, the removal processing volume per pulse energy increased to 3 times or more of the removal volume for simultaneous irradiation. That is, a processing efficiency three times higher than that obtained when each laser beam was individually irradiated onto the transparent body was obtained. Further, it shows that there is an optimum delay time in order to obtain the maximum removal processing efficiency.

  FIG. 3 shows the delay time dependence of the first laser light transmittance. In this figure, the horizontal axis represents the irradiation time delay of the first laser beam (wavelength 780 nm) with respect to the second laser beam (wavelength 260 nm). The vertical axis represents the transmitted light amount of the first laser light, which is a relative value with respect to the case where the first laser light is irradiated alone. When the time delay was given, the transmittance decreased. The time delay for decreasing the transmittance was the same as the time delay for increasing the removal processing efficiency. This reduction in the transmittance of the first laser beam is important when processing a semiconductor device having a thin film-like transparent body, and reduces damage to a substrate such as an underlying semiconductor. That is, by using the present invention, it is possible to realize high-quality processing that reduces damage to a substrate such as an underlying semiconductor while improving processing efficiency when thin film removing processing of various devices including a semiconductor is performed.

  Examples of utilization of the present invention include removal processing of an insulating film such as a low-k film used in a semiconductor device, processing of a transparent electrode film used in a display device such as a liquid crystal, and a redundancy circuit of a semiconductor memory of a silicon wafer. This is effective for the processing of removing the transparent protective film of the circuit element of the conductive link, and other processing that is fine and has little thermal influence when the processing of removing the inside of the layer of the multilayer structure electronic device is advanced from the surface. Trimming capacitors, resistors, inductances, etc. with an inert layer formed on the upper surface of the circuit element as a surface protective layer, LCD display panel correction processing, PDP display device correction processing, circuit board function trimming, and other semiconductor substrate lasers Applicable to precision machining. In the manufacture of highly integrated circuits, it is possible to reduce the manufacturing cost of electronic components by improving the product yield by reducing the processing width and reducing the amount of processed removal. Furthermore, the effectiveness can be obtained for hole processing of a substrate of a semiconductor device such as quartz or sapphire.

It is explanatory drawing of the method and apparatus structure which implement multi-wavelength laser beam irradiation regarding this invention. The removal processing amount at the time of changing the delay time in multiwavelength superimposition irradiation is shown. The transmittance of the first laser beam when the delay time in multi-wavelength superimposed irradiation is changed is shown.

Explanation of symbols

1: Laser oscillation unit 2: Near-infrared laser output beam 3: Wave plate 5: Wavelength conversion unit
6: Two-wavelength laser light 7: Wavelength division filter 8: Near-infrared laser light (first laser light)
9: UV laser light (second laser light)
DESCRIPTION OF SYMBOLS 11, 12, 15: Total reflection mirror 19: Optical path detour unit 13: Condensing point 14: Transparent body 16: Condensing lens 17: Wavelength synthesis filter 18: Irradiation laser beam 20: Processing object 22: Substrate 31: Transparent body surface

Claims (7)

A method of processing a workpiece by irradiating a first laser having a first wavelength and a second laser having a second wavelength,
The second wavelength has a harmonic relationship with the first wavelength,
The object to be processed is the transparent layer portion of an object composed of a substrate and a transparent layer that is transparent to at least a first wavelength formed on at least a part of the substrate surface;
Generating a first laser that is an ultrashort pulse;
The step of converting a portion of the first laser energy in the second laser is a ultra-short pulse,
Providing the first laser with a time delay of 1 ps to 100 ps with respect to the second laser;
A step of concentrating the first laser and the second laser on the same axis, and focusing the focused first and second lasers on the surface or inside of the transparent layer as a workpiece A laser processing method comprising a step of irradiating, having higher processing efficiency than irradiating each laser beam individually, and reducing damage to the substrate . The laser processing method according to claim 1, wherein the first wavelength is a wavelength exceeding 500 nm, and the second wavelength is a wavelength of 500 nm or less. The laser processing method according to claim 1 , wherein the transparent layer is an insulator. The laser processing method according to claim 1 , wherein the substrate is a semiconductor substrate. A laser processing apparatus for processing the transparent layer portion of an object comprising a substrate and a transparent layer transparent to at least a first wavelength formed on at least a part of the substrate surface,
Laser generating means for generating a first laser that is an ultrashort pulse having a first wavelength;
Wavelength converting means for converting a part of the energy of the first laser into a second laser that is an ultrashort pulse having a second wavelength that is a harmonic of the first wavelength;
A first laser second delay generating means for laser give 100ps less time delay than 1ps respect, focusing means for focusing the first laser and the second laser coaxially, and
Comprising means for adjusting the focal point to the surface or inside of the transparent layer, which is an object to be processed, having higher processing efficiency than individually irradiating each laser beam, and reducing damage to the substrate; Laser processing equipment. The laser processing apparatus according to claim 5 , wherein the first wavelength is a wavelength exceeding 500 nm, and the second wavelength is a wavelength of 500 nm or less. 7. The laser processing apparatus according to claim 5 , wherein the delay generation unit gives the first laser a longer optical path length than the second laser. 8.