Patent Application
20130017640
1. Field of the Invention
The present invention relates to a method of processing an optical device wafer in which an optical device layer including an n-type gallium nitride semiconductor layer and a p-type gallium nitride semiconductor layer or the like is stacked over a surface of a sapphire substrate, with a buffer layer therebetween, so as to peel the sapphire substrate from the optical device wafer.
2. Description of the Related Art
In an optical device manufacturing process, an optical device wafer is configured by forming optical devices such as light emitting diodes, laser diodes, etc. in a plurality of regions demarcated by a plurality of streets formed in a grid pattern on an optical device layer which includes an n-type semiconductor layer and a p-type semiconductor layer and which is stacked over a surface of a substantially circular disc-shaped sapphire substrate, with a buffer layer therebetween. Subsequently, the optical device wafer is divided along the streets to thereby manufacture the individual optical devices (see, for example, Japanese Patent Laid-open No. Hei 10-305420).
In addition, as a technology for enhancing a cooling effect on optical devices and the luminance of the optical devices, a manufacturing process called lift-off process is disclosed in JP-T-2004-72052. In the disclosed process, a transfer substrate of molybdenum (Mo), copper (Cu), silicon (Si) or the like is joined to an optical device layer, which is stacked over a surface of a sapphire substrate constituting an optical device wafer with a buffer layer therebetween and which includes an n-type semiconductor layer and a p-type semiconductor layer, with a joint metal layer of gold (Au), platinum (Pt), chromium (Cr), indium (In), palladium (Pd) or the like therebetween. Then, the buffer layer is irradiated with a laser beam from the back side of the sapphire substrate, to thereby peel the sapphire substrate. In this manner, the optical device layer is transferred onto the transfer substrate.
In this connection, since the buffer layer is as thin as around 1 μm and is formed of the same kind of semiconductor as that forming the optical device layer including the n-type semiconductor layer and the p-type semiconductor layer, it is difficult to break only the buffer layer by irradiation with the laser beam. In addition, since the buffer layer after the peeling of the sapphire substrate has a surface roughness of not less than 250 nm, it may be necessary to polish the surface of the buffer layer. Besides, there is a problem that when a metallic substrate is mounted to the buffer layer side, warpage may be generated in the resulting assembly as a whole, making it difficult to accurately position the condensing point of the laser beam to the buffer layer.
Accordingly, it is an object of the present invention to provide a method of processing an optical device wafer wherein a transfer substrate is joined to an optical device layer stacked over a surface of a sapphire substrate constituting an optical device wafer with a buffer layer therebetween, and thereafter the buffer layer is irradiated with a laser beam from the back side of the sapphire substrate, whereby the buffer layer can be assuredly broken without damaging the optical device layer and the sapphire substrate can be peeled off reliably.
In accordance with an aspect of the present invention, there is provided a method of processing an optical device wafer in which an optical device layer including an n-type semiconductor layer and a p-type semiconductor layer is stacked over a surface of a sapphire substrate, with a buffer layer therebetween, so as to peel the sapphire substrate. The method includes a transfer substrate joining step of joining a transfer substrate to a surface of the optical device layer, a buffer layer breaking step of breaking the buffer layer by irradiation with a pulsed laser beam from the sapphire substrate side of the optical device wafer with the transfer substrate joined to the surface of the optical device layer, and a sapphire substrate peeling step of peeling off the sapphire substrate from the optical device wafer with the buffer layer broken, so as to transfer the optical device layer onto the transfer substrate. The pulsed laser beam for irradiation therewith in the buffer layer breaking step has a wavelength set to be longer than an absorption edge of the sapphire substrate and shorter than an absorption edge of the buffer layer, and a pulse width so set that a thermal diffusion length will be not more than 200 nm.
The buffer layer is preferably formed of gallium nitride (GaN), and the pulse width of the pulsed laser beam for irradiation therewith in the buffer layer breaking step is preferably set to be not more than 200 ps, more preferably not more than 100 ps. In addition, the wavelength of the pulsed laser beam for irradiation therewith in the buffer layer breaking step is preferably set in the range of 150 to 355 nm, more preferably 150 to 250 nm.
In the method of processing an optical device wafer according to the present invention, the pulsed laser beam for irradiation therewith in the buffer layer breaking step has a wavelength set to be longer than the absorption edge of the sapphire substrate and shorter than the absorption edge of the buffer layer, and a pulse width so set that the thermal diffusion length will be not more than 200 nm. This ensures that the energy of the pulsed laser beam is consumed in the buffer layer, and would not damage the optical device layer. In addition, since the thermal diffusion length is as short as 200 nm or below, the energy of the pulsed laser beam is absorbed along the boundary surface with the sapphire substrate in the range of the thermal diffusion length; accordingly, even if the energy distribution is Gaussian distribution, equivalent processing to that in the case of a top-hat shape can be achieved. Furthermore, since the thermal diffusion length is as short as 200 nm or below, the pulsed laser beam is absorbed instantaneously on reaching the buffer layer, in the range of the thermal diffusion length. Therefore, only the buffer layer can be securely broken, even if the sapphire substrate has warpage and the condensing point of the pulsed laser beam is thereby deviated from the buffer layer. Besides, the surface roughness of the buffer layer after the peeling of the sapphire substrate is at a permissible level of 100 nm or below, so that there is no need for an after-treatment such as polishing.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.
Now, preferred embodiments of the method of processing an optical device wafer according to the present invention will be described in detail below, referring to the attached drawings.
In order to peel the sapphire substrate 20 of the optical device wafer 2 from the optical device layer 21 and transfer the optical device layer 21 onto a transfer substrate as above-mentioned, a transfer substrate joining step of joining the transfer substrate to the surface 21a of the optical device layer 21 is carried out. Specifically, as shown in
After the above-mentioned transfer substrate joining step is conducted, a transfer substrate adhering step is carried out wherein the transfer substrate 3 composed of copper substrate and joined to the surface 21a of the optical device layer 21 formed over the surface 20a of the sapphire substrate 20 constituting the optical device wafer 2 is adhered to a surface of a pressure sensitive adhesive tape mounted to an annular frame. More specifically, as shown in
After the above-mentioned transfer substrate adhering step is performed, a buffer layer breaking step is carried out wherein the buffer layer 22 is broken by irradiating with a pulsed laser beam from the side of the sapphire substrate 20 of the optical device wafer 2 with the transfer layer 3 adhered to the surface of the optical device layer 21. The buffer layer breaking step is, in the embodiment shown in the drawings, carried out by use of a laser beam processing apparatus 5 shown in
The chuck table 51 holds the work on a holding surface (which is an upper surface) thereof by suction. The chuck table 51 is put to processing feed in the direction of arrow X in
The laser beam irradiation means 52 includes a cylindrical casing 521 disposed substantially horizontally. In the casing 521 is housed pulsed laser beam oscillation means having a pulsed laser beam oscillator and repetition frequency setting means, which are not shown. A condenser 522 for condensing a pulsed laser beam oscillated from the pulsed laser beam oscillation means is mounted to a tip portion of the casing 521.
The buffer layer breaking step carried out by use of the laser beam processing apparatus 5 as above-mentioned will be described referring to
After the optical device wafer 2 with the transfer substrate 3 joined thereto is held on the chuck table 51 by suction as above-mentioned, the chuck table 51 is moved into a laser beam irradiation region in which the condenser 522 of the laser beam irradiation means 52 is located, as shown in
The processing conditions in the above-mentioned buffer layer breaking step are set, for example, as follows.
Light source: YAG laser
Wavelength: 257 nm
Repetition frequency: 50 kHz
Mean output: 0.12 W
Pulse width: 100 ps
Spot diameter: φ70 μm
Defocus: 1.0 mm (the condenser is moved by 1 mm toward the sapphire substrate, in the condition where the laser beam is positioned on the surface of the sapphire substrate)
Processing feed velocity: 600 mm/sec
When the buffer layer breaking step is carried out under the above-mentioned processing conditions, the pulsed laser beam with a spot diameter of φ70 μm has a spot interval of 12 μm and a spot overlapping rate of 83%, in irradiating the optical device layer 21 therewith. Incidentally, while an example wherein the chuck table 51 suction-holding thereon the optical device wafer 2 with the transfer substrate 3 joined thereto is moved rectilinearly in the processing feed direction while radiating the pulsed laser beam from the condenser 522 by operating the laser beam irradiation means 52 has been shown in the above-mentioned buffer layer breaking step, the chuck table 51 may be moved in the processing feed direction or the indexing feed direction while being rotated so that irradiation with the pulsed laser beam takes place in a spiral pattern.
After the above-mentioned buffer layer breaking step is performed, a sapphire substrate peeling step is carried out wherein the sapphire substrate 20 is peeled from the optical device layer 21. Specifically, since the buffer layer 22 bonding the sapphire substrate 20 and the optical device layer 21 to each other has been broken, and its bonding function lost, by the buffer layer breaking step, the sapphire substrate 20 can be easily peeled from the optical device layer 21, as shown in FIG. 6.
Now, the wavelength of the pulsed laser beam for irradiation therewith in the above-mentioned buffer layer breaking step will be described. It is important for the wavelength of the pulsed laser beam for irradiation therewith in the buffer layer breaking step to be set longer than an absorption edge of the sapphire substrate and shorter than an absorption edge of the buffer layer. In other words, it is necessary for the wavelength of the pulsed laser beam for irradiation therewith in the buffer layer breaking step to be such that the laser beam is transmitted through the sapphire substrate to reach the buffer layer and be absorbed by the buffer layer, whereby the buffer layer can be broken.
In the next place, the pulse width of the pulsed laser beam for irradiation therewith in the buffer layer breaking step will now be described. It is important for the pulse width of the pulsed laser beam for irradiation therewith in the buffer layer breaking step to be so set that thermal diffusion length will be not more than 200 nm. With the pulse width so set that the thermal diffusion length will be not more than 200 nm, it is ensured that the energy of the pulsed laser beam is consumed in the buffer layer and would not damage the optical device layer. In other words, when the pulse width is so set that the thermal diffusion length will be more than 200 nm, the energy of the pulsed laser beam would not only break the buffer layer but also damage the optical device layer. Incidentally, the short thermal diffusion length of 200 nm or below ensures that the energy of the pulsed laser beam is absorbed along the boundary surface with the sapphire substrate in the range of the thermal diffusion length, so that even if the energy distribution is Gaussian distribution, equivalent processing to that in the case of a top-hat shape can be achieved. Furthermore, the short thermal diffusion length of 200 nm or below ensures that the pulsed laser beam is absorbed instantaneously on reaching the buffer layer in the range of the thermal diffusion range, so that only the buffer layer can be securely broken even if the sapphire substrate has warpage and the condensing point of the pulsed laser beam is thereby deviated from the buffer layer. In addition, the surface roughness of the buffer layer after the peeling of the sapphire substrate is at a permissible level of 100 nm or below, and, therefore, there is no need for an after-treatment such as polishing.
Now, experimental results obtained by the present inventors will be described below.
(1) When the buffer layer is irradiated with a pulsed laser beam having a wavelength longer than 355 nm, which is the absorption edge of gallium nitride (GaN), the pulsed laser beam is transmitted through the buffer layer to damage the optical device layer, and undergoes an increased energy loss.
(2) When the buffer layer is irradiated with a pulsed laser beam having a wavelength shorter than 150 nm, which is the absorption edge of the sapphire substrate, the energy of the pulsed laser beam is absorbed by the sapphire substrate, damaging the sapphire substrate, and the energy of the pulsed laser beam reaching the buffer layer is lost more.
(3) When the buffer layer is irradiated with a pulsed laser beam having a wavelength (250 nm) corresponding to the highest absorbance of gallium nitride (GaN), processing efficiency is good, and the surface roughness of the buffer layer is not more than 50 nm.
(4) When irradiation with the pulsed laser beam is conducted with a pulse width set to 1 ns, the buffer layer can be broken assuredly, but cracks would extend to the optical device layer, damaging the optical devices.
(5) When irradiation with the pulsed laser beam is performed with a pulse width set to 500 ps, the buffer layer can be broken reliably, but the surface roughness of the buffer layer would be 500 nm, necessitating removal of the roughness by polishing. Besides, some cracks would extend to the optical device layer, damaging the optical devices.
(6) When irradiation with the pulsed laser beam is conducted with a pulse width set to 300 ps, the buffer layer can be broken assuredly, but the surface roughness of the buffer layer would be 300 nm, necessitating removal of the roughness by polishing.
(7) When irradiation with the pulsed laser beam is carried out with a pulse width set to 200 ps, the buffer layer can be broken securely. In addition, the surface roughness of the buffer layer is 100 nm, which is within the permissible range, so that polishing is not needed.
(8) When irradiation with the pulsed laser beam is performed with a pulse width set to 100 ps, the buffer layer can be broken assuredly. Besides, the surface roughness of the buffer layer is 50 nm, which is within the permissible range, so that polishing is not needed at all.
The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-154906 | Jul 2011 | JP | national |
