WO2008078952A1

WO2008078952A1 - System and method for deliverying laser beam and laser lift-off method using the same

Info

Publication number: WO2008078952A1
Application number: PCT/KR2007/006817
Authority: WO
Inventors: Beng So Ryu; Seong Hun Lee
Original assignee: QMC Co Ltd
Current assignee: QMC Co Ltd
Priority date: 2006-12-26
Filing date: 2007-12-26
Publication date: 2008-07-03
Anticipated expiration: 2009-06-26
Also published as: TW200827770A; KR100724540B1; CN101595572B; TWI331228B; CN101595572A

Abstract

The present invention provides a system and method for delivering laser beam, and a laser lift-off (LLO) method, one of the inevitable processes for fabricating a vertical type LED. A laser beam delivery system of the present invention comprises a laser beam source for emitting laser beam; a beam homogenizer for improving uniformity of energy intensity of the laser beam, the beam homogenizer comprising a microlens type fly-eye lens; a mask for masking a peripheral area of a cross section of the laser beam having penetrated the beam homogenizer at a focal plane; and an imaging lens for applying the laser beam to a unit irradiation area of a target. According to the present invention, the uniformity of energy intensity all over the beam spot is improved and thus the process yield is also remarkably increased. Besides, the beam transmittance is improved and thus the production per unit time is also raised. Further, the manufacturing process is simplified, the manufacturing cost is reduced, and thus the competitive power in LED market is improved.

Description

SYSTEM AND METHOD FOR DELIVERYING LASER BEAM AND LASER LIFT-OFF METHOD USING THE SAME

Technical Field

[1] The present invention relates to a system and method for delievering laser beam, particularly, to a system and method for delievering laser beam to separate a thin film from a substrate, and more particularly, a system and method for delievering laser beam applicable to LLO (Laser Lift-Off) process which is one of the inevitable processes for manufacturing a vertical type LED. Background Art

[2] Generally, the excimer laser has various uses in processing materials, for example, precision processing and separation of two different materials coupled to each other. Recently, as the stability and power of the excimer laser beam has improved, the range of the use thereof is getting wider to include processing of the semiconductor materials, and especially, separating a thin film from a wafer substrate for manufacturing a device. The kind of the separated thin film is so various to include compound semiconductor, copper, aluminum, gold, polymer and so on. To separate such various kinds of thin films, the laser beam has material factors such as target energy density, target energy uniformity, and target exposing area.

[3] Hereinafter will be explained a related art and the present invention from the viewpoint of the LLO(Laser Lift-Off) process which is one of the inevitable processes for manufacturing a vertical type LED while the present invention is not limited to the LLO process.

[4] A LED is a well-known semiconductor device for converting a current into light. The

LED emits light when electrons of an active layer excited from the valence band to the conduction band of the semiconductor across the corresponding band gap falls back to the valence band. Accordingly, the wavelength and color of the emitted light depend on the band gap energy, and thus depend on the semiconductor material since the band gap energy is one of the characteristics pertinent to the material.

[5] A LED is used for emitting light of various range of color such as red, green, blue, and yellow. The LED has a limit, however, in that it is a monochromatic light source. In certain cases, it is required to emit white light which includes all of the red, green, and blue lights. For example, the backlight unit of a LCD monitor is required to emit white light. Usually, white light is provided by an incandescent bulb or a fluorescent lamp. While being cheap, the incandescent bulb has very short lifetime and low light- emitting efficiency. The fluorescent lamp has a demerit in that its life time is limited while it has higher efficiency than the incandescent bulb. Further, the fluorescent lamp requires a relatively large, heavy, and expensive additional component such as a stabilizer.

[6] A light source of white LED may be manufactured by locating red, green, and blue

LEDs closely with each other and making each of them emit light at an appropriate ratio. However, the blue LED is not easy to manufacture since it is difficult to make an appropriate crystal having the corresponding band gap. Particularly, it is difficult to embody a blue LED of good quality with such compound semiconductors as InP, GaAs, and GaP.

[7] In spite of such difficulties as above, a blue LED based on GaN has been commercially used since it was introduced to the market in 1994. Technology of the GaN- based blue LED is developing rapidly so as to exceed, in the field of illumination, an incandescent bulb and a fluorescent lamp in terms of light-emitting efficiency now.

[8] Meanwhile, in case of InP-based, GaAs-based, or GaP-based LED, it is not difficult to manufacture a vertical type LED having p-n junction since those kinds of semiconductor layer may grow on a conductive substrate. In case of GaN-based LED, however, a non-conductive sapphire (Al₂O₃) substrate is used to reduce crystal defect that might otherwise occur during the epitaxial growth of GaN, and thus horizontal type having both first and second electrodes on the top surface of the epi layer has been generally adopted since sapphire is non-conductive.

[9] Figures 1 and 2 are schematic diagrams showing the structure of a vertical type LED of the related art.

[10] Referring to FIG. 1 which is a cross sectional view of the vertical type LED of the related art, a n-GaN layer 11, a active layer 12 having multiple quantum wells, p-GaN layer 13, and a transparent conductive layer 14 are formed sequentially on a sapphire substrate 10. Then, a first electrode 15 is formed on the specific part of the transparent conductive layer 14.

[11] Then, photoresist patterns (not shown) are formed on the transparent conductive layer 14 including the first electrode 15 in such a way that a portion of the other part of the transparent conductive layer 14 on which the first electrode 15 is not formed is not covered by the photoresist patterns. The transparent conductive layer 14, p-GaN layer 13, and active layer 12 are selectively etched using the photoresist patterns as a mask. At this time, a portion of the n-GaN layer 11 is slightly etched. Wet etch is preferred to dry etch since GaN layer is difficult to etch.

[12] Subsequently, the photoresist patterns are removed through a strip process and a second electrode 16 is formed on the revealed portion of the n-GaN layer 11.

[13] As shown in FIG. 2 which is a top view of the LED of the related art, since both of the first and second electrodes 15 and 16 need to be bonded with wire, the chip size of the LED should be large enough to ensure the electrode area, which acts as an obstacle to improvement of the output per unit area of a wafer. Additionally, the complexity in the wire bonding during the packaging process increases the manufacturing cost.

[14] Further, since the sapphire substrate is used which is non-conductive, it is hard to emit the static electricity which increases the possibility of inferior devices and thus decreases the credibility of the devices. In addition to that, since sapphire has low thermal conductivity, it is hard to emit the heat occurring when the LED works, which acts as a limit in applying high electric current for high output power of the LED.

[15] To overcome the problems due to limitations and disadvantages of the horizontal type LED, a vertical type LED, especially a vertical type LED whose final product does not have a sapphire substrate, has been widely studied.

[16] In case of the vertical type LED whose final product does not have a sapphire substrate, GaN -based epi layer is formed on a sapphire substrate and then a metal support layer is formed on the epi layer. Since the epi layer may be supported by this metal support layer after the sapphire layer is separated from the epi layer, it is feasible to separate the sapphire layer from the epi layer. Generally, a laser lift-off (LLO) method is used to separate the sapphire layer from the epi layer.

[17] The laser lift-off method is based on the principle that a material having a band gap is permeable to the light of energy lower than the band gap but absorbs the light of energy higher than the band gap. For instance, since KrF excimer laser beam of 248 nm wavelength and ArF excimer laser beam of 193 nm wavelength have energy between about 3.3 eV band gap of GaN and about 10.0 eV band gap of sapphire, those excimer laser beams penetrate the sapphire substrate but are absorbed in the GaN- based epi layer. Accordingly, the excimer laser beam penetrating the sapphire substrate heats and dissolves the epi layer at the interface, thereby separating the sapphire substrate from the epi layer. Disclosure of Invention Technical Problem

[18] The laser lift-off is roughly classified into two groups, scan method and pulse method, based on how to irradiate a wafer on which a plurality of LED devices are formed.

[19] In case of the scan method, there will inevitably be a part which is repeatedly irradiated. Fracture or crack might occur at the repeatedly irradiated part. To avoid this kind of problem, it is preferable to adopt the pulse method. That is, it is desirable to instantaneously apply a pulse of laser beam to a unit irradiation area, move to next irradiation area, apply a pulse of laser beam thereto, and repeat those steps until the whole target area of the wafer is irradiated.

[20] Although the pulse method would be adopted, however, it is still required that a beam spot should be exactly correspondent to the unit irradiation area in terms of shape and size. If the beam spot irradiates a part other than the unit irradiation area, it would cause the same problem as that of the scan method, that is, fracture or crack at the part. On the other hand, if the beam spot does not completely cover the unit irradiation area, it would cause the problem that the sapphire substrate cannot be completely separated from the GaN-based epi layer.

[21] Even if a beam spot exactly correspondent to the unit irradiation area in terms of shape and size is applied, such problem as above might still occur if the energy intensity is not uniform over the whole area of the beam spot. That is, as shown in FIG. 3, since the energy intensity at the cross section of the original laser beam follows Gaussian distribution, the original laser beam has relatively high energy intensity at the central part and relatively low energy at the peripheral part. Thus, if the energy intensity of the laser beam is so high to ensure the separation of the sapphire substrate from the GaN-based epi layer at the peripheral part of the unit irradiation area, defects may occur at the central part of the unit irradiation area. On the other hand, if the energy intensity of the laser beam is so low to prevent defects at the central part of the unit irradiation area, the sapphire substrate cannot be separated from the GaN-based epi layer at the peripheral part of the unit irradiation area. To sum, using the original laser beam without any treatment has bad influence upon the yield which means the number of the LED devices of good quality compared to the number of all LED devices that might be made out of one wafer.

[22] Accordingly, as shown in FIG. 4 to 6, a beam homogenizer 100 is used to improve the uniformity of energy intensity of the beam spot. The beam homogenizer 100 of the related art comprises the first and second fly-eye lenses 110 and 120 for dividing the laser beam from a laser beam source (not shown) into a plurality of beamlets and adjusting the divergent angle of the beamlets, and a condensing lens 130 for overlapping the plurality of beamlets. The fly-eye lenses 110 and 120 of the related art, however, are cylindrical type. The cylindrical type fly-eye lens 110 is made with two plates 111 and 112 bonded with each other, wherein each of the plates 111 and 112 is made with a plurality of cylindrical lenses aligned in parallel with each other, and wherein the cylindrical lenses of one plate 111 is perpendicular to those of the other plate 112 thereby forming a plurality of lenslets.

[23] In case of the cylindrical type fly-eye lenses 110 and 120, the pitch which represents the size of the lenslet of a fly-eye lens is about 5 mm and is not easy to reduce under a certain length due to its structure. Accordingly, as shown in FIG. 7, there is a limit in increasing the number of the effective lenslets, the lenslets which the laser beam actually passes through, and thus there is also a limit in increasing the number of beamlets into which the laser beam is divided. As a result, satisfactory uniformity of energy intensity of the beam spot is hard to obtain since there is a limit in increasing the number of beamlets in case of the cylindrical type fly-eye lens, and thus the number of the LED devices of good quality compared to the number of all LED devices that might be made out of one wafer, i.e., yield, would be adversely affected.

[24] Meanwhile, to increase the number of the effective lenslets, the size of the cross section of the laser beam may be enlarged by a beam expansion telescope (BET) (not shown) located between the laser beam source and the beam homogenizer 100, and cylindrical type fly-eye lenses large enough to receive the enlarged laser beam may be adopted. However, there will still be the limitation of the uniformity of energy intensity due to the limitation of the size of the whole system. Further, since the beam expansion telescope should be used additionally, the complexity increases in manufacturing the whole laser beam delivery system and the manufacturing cost also increases. Besides, the beam transmittance of the system decreases since the laser beam must pass through an additional optical element, the beam expansion telescope.

[25] Furthermore, the cylindrical type fly-eye lenses 110 and 120 have a fundamental problem in terms of the beam transmittance since the laser beam cannot penetrate the cylindrical type fly-eye lens at the interfaces between the cylindrical lenses and a cylindrical type fly-eye lens comprises two layers which are optical elements for the laser beam to penetrate. Consequently, since the beam transmittance which represents how much of the original beam penetrates the whole system and reaches the wafer is low, LED production per unit time is remarkably limited. Technical Solution

[26] Accordingly, the present invention is directed to a system and method for delivering laser beam and a laser lift-off method using the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.

[27] In accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a laser beam delivery system, comprising: a laser beam source for emitting laser beam; a beam homogenizer for improving uniformity of energy intensity of the laser beam, the beam homogenizer comprising a microlens type fly-eye lens; a mask for masking a peripheral area of a cross section of the laser beam having penetrated the beam homogenizer at a focal plane; and an imaging lens for applying the laser beam to a unit irradiation area of a target.

[28] In another aspect of the present invention, there is provided a method for delivering laser beam, comprising: emitting excimer laser beam; dividing the emitted excimer laser beam into a plurality of beamlets using a microlens type fly-eye lens; overlapping the plurality of beamlets, thereby creating homogenized laser beam; masking a peripheral area of the homogenized laser beam; and applying the masked homogenized laser beam to a target.

[29] In still another aspect of the present invention, there is provided a laser lift-off method comprising: forming a GaN-based epi layer on a sapphire substrate; emitting excimer laser beam; dividing the emitted excimer laser beam into a plurality of beamlets using a microlens type fly-eye lens; overlapping the plurality of beamlets, thereby creating homogenized laser beam; masking a peripheral area of the homogenized laser beam; applying the masked homogenized laser beam to a unit irradiation area of the sapphire substrate; and separating the sapphire substrate from the GaN-based epi layer.

[30] It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Advantageous Effects

[31] An advantage of the present invention is to provide a system and method for delivering laser beam and a laser lift-off method using the same which are configured in such a way that the uniformity of energy intensity all over the beam spot is improved and thus the process yield is also remarkably increased.

[32] Another advantage of the present invention is to provide a system and method for delivering laser beam and a laser lift-off method using the same which are configured in such a way that the beam transmittance is improved and thus the production per unit time is also raised. [33] Still another advantage of the present invention is to provide a system and method for delivering laser beam and a laser lift-off method using the same which are configured in such a way that the manufacturing process is simplified, the manufacturing cost is reduced, and thus the competitive power in LED market is improved.

[34] Additional advantages and features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. Brief Description of the Drawings

[35] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[36] In the drawings:

[37] FIG. 1 is a cross section of a horizontal type LED of the related art;

[38] FIG. 2 is a top view of a horizontal type LED of the related art;

[39] FIG. 3 shows the energy intensity profile of the cross section of an original laser beam;

[40] FIG. 4 to 6 are a perspective view, a top view and a side view of a beam ho- mogenizer of the related art, respectively;

[41] FIG. 7 shows effective lenslets of a cylindrical type fly-eye lens used in a beam ho- mogenizer of the related art;

[42] FIG. 8 to 14 are cross sections showing a method for fabricating a vertical type LED;

[43] FIG. 15 is a schematic diagram of a laser beam delivery system of the present invention;

[44] FIG. 16 to 18 are a perspective view, a top view and a side view of a beam ho- mogenizer of the present invention;

[45] FIG. 19 shows effective lenslets of a microlens type fly-eye lens used in a beam ho- mogenizer of the present invention;

[46] FIG. 20 is a picture and graph showing the energy intensity profile of the cross section of the laser beam of the related art which has passed through a mask at the focal plane; and

[47] FIG. 21 is a picture and graph showing the energy intensity profile of the cross section of the laser beam of the present invention which has passed through a mask at the focal plane.

Best Mode for Carrying Out the Invention

[48] Reference will now be made in detail to an embodiment of the present invention, example of which is illustrated in the accompanying drawings.

[49] Reference will now be made in detail to an embodiment of the present invention, example of which is illustrated in the accompanying drawings.

[50] It should be understood that a laser beam delivery system further including optional optical elements, e.g., a reflecting mirror, in addition to those described and explained below is within the scope of the present invention.

[51] Figures 8 to 14 are cross sections showing a method for fabricating a vertical type

LED according to the present invention.

[52] Referring to FIG. 8, a series of layers 30 comprising a GaN buffer layer 31, a N-type

GaN layer 32, a InGaN/GaN/ AlGaInN active layer 33 having a multiple quantum well, and a P-type GaN layer 34 are formed on a sapphire substrate 20 sequentially using a conventional semiconductor technology such as MOCVD (Metal Oxide Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy). If a thin film of GaN is formed directly on a sapphire (A12O3) (001) substrate, the surface uniformity of the thin film might be adversely affected due to the lattice incoherency. Thus, it is desirable to form a buffer layer 31 on a sapphire substrate 20 first, and then to form GaN-based layers on the buffer layer 31. Usually, the sapphire substrate 20 has thickness of about 330 to 430 /M. The entire thickness of the series of GaN-based layers 30 is less than about 5 /M.

[53] Then, as shown in FIG. 9, a plurality of trenches 40 are formed through the series of the GaN-based layers 30a. The trenches 40 may extend into the sapphire substrate 20a at a predetermined depth to avoid any defects that might otherwise occur at the subsequent process for separating the sapphire substrate 20a from the GaN-based layers 30a. The trenches 40 are to define the individual LED devices to be formed and assist a subsequent chip separation process. Each individual LED semiconductor is beneficially a square about 200 μm wide. The trenches 40 are beneficially narrower than about 10 μm and extend deeper than about 5 μm into the sapphire substrate 20a.

[54] Because of the hardness of the sapphire substrate 20 and GaN-based layers 30, the trenches 40 are beneficially formed using reactive ion etching, preferably inductive coupled plasma reactive ion etching (ICP RIE). As the first step to form the trenches 40, a photoresist (not shown) is spin coated on the GaN-based layers 30 and then patterned using lithographic techniques and development. After development, the ICP RIE process is performed to selectively etch the GaN-based layers 30 and sapphire substrate 20 by using the photoresist pattern (not shown) as a mask, thereby forming the trenches 40.

[55] Referring now to FIG. 10, after the trenches 40 are formed, a conductive support layer 50 is formed over the entire upper surface of the GaN-based layers 30a and sapphire substrate 20a. As a result, the trenches 40 are filled with the conductive support layer 50. Although the conductive support layer 50 may be formed of any non- metallic material, e.g., Si, as far as it has good conductivity, it is preferably formed of a metal having good electrical and thermal conductivity such as Cu, Au, and Al, by using physical vapor deposition, chemical vapor deposition, or electroplating.

[56] A layer (now shown) including Cr or Au may further be formed between the GaN- based layer 30a and the conductive support layer 50 to strengthen the adhesive strength therebetween.

[57] After the conductive support layer 50 is formed, referring to FIG. 11, the sapphire substrate 20a is separated from the GaN-based layers 30a by applying laser beam to the sapphire substrate 20a using the laser beam delivery system of the present invention, while the sapphire substrate 20a is biased away from the GaN-based layers 30a using vacuum chucks. Detailed description about this process will be followed.

[58] Then, as shown in FIG. 12, the lower surface of the GaN-based layers 30a opposite to the conductive support layer 50 is cleaned with HCl and then polished to smooth the surface.

[59] Turning now to FIG. 13, a plurality of contact layers 60 are formed on the exposed surfaces of the GaN-based layers 30a. Each contact layer 60 comprises an interface layer 61 directly contacting with the GaN-based layers 30a and a contact pad 62 on the interface layer 61. It is desirable that the interface layer 61 contains Ti or Al and the contact pad contains Cr or Au.

[60] After the plurality of the contact layers 60 are formed, a dicing process is performed to divide the structure of FIG. 13 into individual LED devices. The dicing process may be performed using various mechanical or chemical methods. FIG. 14 shows a final product of a LED device.

[61] Among the above-described processes, the process for separating the sapphire substrate 20a from the GaN-based layers 30a can be effectively carried out by the laser beam delivery system of the present invention, detailed description of which will be followed hereinafter referring to FIG. 15 to FIG. 21.

[62] FIG. 15 is a schematic diagram of a laser beam delivery system of the present invention.

[63] Referring to FIG. 15, the laser beam delivery system 200 of the present invention comprises a laser beam source 210. Since KrF excimer laser beam of 248 nm wavelength and ArF excimer laser beam of 193 nm wavelength have energy between about 3.3 eV band gap of the GaN and about 10.0 eV band gap of the sapphire, those excimer laser beams penetrate the sapphire substrate 20a but are absorbed in the GaN- based layers 30a. Accordingly, both of them may be used as a laser beam source 210 of the present invention. Nevertheless, KrF excimer laser beam is preferable to ArF excimer laser beam in that the ArF excimer laser beam might be more or less absorbed in the sapphire substrate 20a.

[64] The laser beam source 210 emits laser beam in a pulse form. The pulse energy of the laser beam may be precisely adjusted by a variable attenuator (not shown).

[65] Generally, it is necessary to improve the uniformity of energy intensity at the beam spot since the energy intensity at the cross section of the laser beam emitted from the laser beam source 210 follows Gaussian distribution. The cross section of the laser beam mentioned herein is defined as a cross section of the laser beam that appears when the laser beam is cut in a direction perpendicular to the laser beam's progressing direction. The laser beam delivery system 200 of the present invention improves the energy density uniformity using a beam homogenizer 220 such that the energy intensity profile might be uniform throughout the cross section of the laser beam having passed through the beam homogenizer 220 at the focal plane. Detailed structure and function thereof will be followed below.

[66] As shown in FIG. 15, to adjust the distance between the beam homogenizer 220 and the focal plane at which the laser beam having penetrated the beam homogenizer 220 is focused, the laser beam delivery system 200 of the present invention may further comprise a field lens 230 between the beam homogenizer 220 and the focal plane.

[67] The laser beam delivery system 200 of the present invention further comprises a mask 240 at the focal plane whose location is adjusted by the field lens 230 so that the peripheral area of the cross section of the laser beam having penetrated the beam homogenizer 22 is masked at the focal plane. Consequently, the masked laser beam may have completely uniform energy intensity throughout the cross section thereof.

[68] The masked laser beam is applied to a unit irradiation area of a wafer 300 through an imaging lens 250. Once the entire surface of the wafer 300 is irradiated sequentially, the sapphire substrate 20a is separated from the GaN-based layer 30a.

[69] FIG. 16 to FIG. 18 are individually a perspective view, a top view and a side view of a beam homogenizer 220 of the present invention.

[70] The beam homogenizer 220 according to an embodiment of the present invention comprises a first fly-eye lens 221 of microlens type for dividing the laser beam emitted from the laser beam source 210 into a plurality of beamlets, a second fly-eye lens 222 of microlens type for adjusting divergent angles of the plurality of beamlets, and a condensing lens 223 for overlapping the plurality of beamlets divergent angles of which are adjusted such that the cross section of the laser beam has uniform energy intensity profile at the focal plane.

[71] That is, the beam homogenizer 220 of the present invention uses microlens type fly- eye lenses 221 and 222. A microlens type fly-eye lens means a monolithic lens having a plurality of lenslets and is fabricated by forming the lenslets aligned in two- dimensional way on a lens plate by use of semiconductor etching process.

[72] Accordingly, the microlens type fly-eye lenses 221 and 222 have no interface between the lenslets, and thus have beam transmittance higher than that of the cylindrical type fly-eye lenses 110 and 120 of the related art since there is no loss of laser beam that otherwise occurs at the interface. The entire beam transmittance of the laser beam delivery system of the present invention may be further improved in comparison to the related art since less optical elements is used.

[73] According to the present invention, the size of the lenslet represented by pitch may be reduced into hundreds of μm since the microlens type fly-eye lenses 221 and 222 are fabricated using the semiconductor etching process. Thus, as shown in FIG. 19, the fly-eye lenses 221 and 222 of the present invention have more effective lenslets, the lenslets which the laser beam actually passes through, than cylindrical type fly-eye lenses 110 and 120 of the related art, and thus can divide the laser beam into much more beamlets.

[74] Consequently, as shown in FIG. 20 and FIG. 21, the beam homogenizer 220 of the present invention may improve the uniformity of the energy intensity of the cross section of the laser beam much more than the beam homogenizer 100 of the related art.

[75] While the reduction of the pitch of the fly-eye lenses 221 and 222 of the present invention may improve the energy intensity uniformity of laser beam, if a lenslet has too small pitch, the focal distance of the lenslet becomes too short to adjust the beam size at the focal plane where a plurality of beamlets are overlapped. In detail, referring to FIG 18, the formula below should be satisfied for the plurality of beamlets to be overlapped and have a certain size of cross section at the focal plane:

[76] f_LAi < a < f_LAi + f_LA2 [77] wherein f_LA1 and f_LA2 are the focal distances of the first and second fly-eye lenses 221 and 222 respectively, and a is the distance between the fist and second fly-eye lenses 221 and 222.

[78] The size of the cross section is proportional to

(wherein fp_L is the focal distance of the condensing lens 223). Since each focal distance of the lenses 221, 222 and 223 is constant, the size of the cross section of the laser beam at the focal plane is determined by 'α,' the distance between the fist and second fly-eye lenses 221 and 222.

[79] If the focal distance of the lenslet is too short, however, the adjustable range of the 'α' is limited since the 'α' should satisfy the formula above, and thus the size of the cross section of the laser beam may be adjusted only in a limited range. Hence, it is necessary to optimize the pitch of the first and second fly-eye lenses 221 and 222 considering the energy intensity uniformity and size of the laser beam at the focal plane. According to the preferred embodiment of the present invention, each of the first and second fly-eye lenses 221 and 222 has a pitch in the range of 0.5 to 2.0 mm.

[80] As mentioned above, the cylindrical type fly-eye lens 110 of the related art is required to be much bigger than the microlens type fly-eye lens 221 to divide the laser beam into the same number of beamlets as the micro type fly-eye lens 221. Furthermore, for every lenslets of the cylindrical type fly-eye lens 110 to be used, it is required to add an additional optical element, a beam expansion telescope, between the laser beam source and the cylindrical type fly-eye lens 110. Since the beam ho- mogenizer 220 of the present invention does not require such a beam expansion telescope, the process for manufacturing the system is simplified, the manufacturing cost is reduced, and thus the competitive power in LED market is improved. Additionally, since the system of the present invention uses less optical elements for the laser beam to pass through than the related art, the entire beam transmittance is improved and thus the production per unit time is also raised.

[81] According to an embodiment of the present invention, the cross section of the original KrF excimer laser beam is a rectangular shape 10 mm long and 23 mm wide. Since each lenslet of the first fly-eye lens 221 has a pitch of 1.015 mm, the laser beam is divided into about 230 beamlets by the first fly-eye lens 221.

[82] According to an embodiment of the present invention, each of the first and second fly-eye lenses 221 and 222 is a rectangular shape 15 mm long and 30 mm wide. On the other hand, according to another embodiment of the present invention, each of them is a rectangular shape having a horizontal length and a vertical length, wherein a ratio of the horizontal length to the vertical length is substantially identical to that of the cross section of the laser beam emitted from the laser beam source.

[83] Meanwhile, although the distance between the condensing lens 223 and the focal plane may be adjusted by the field lens 230 as shown in FIG. 15, the field lens 230 is omitted in FIG. 18 just for convenience of the explanation.

[84] The laser beam having penetrated the beam homogenizer 220 of the present invention has an almost square cross section at the focal plane and has much improved uniformity of the energy intensity throughout the cross section. The peripheral area of the cross section, however, has relatively low energy intensity in comparison to other areas. Thus, a mask 240 is disposed at the focal plane to mask the peripheral area so that only about 80 % of the entire laser beam might be effective beam.

[85] The masked laser beam is applied to a unit irradiation area of the wafer 300 through an imaging lens 250. Once the entire surface of the wafer 300 is irradiated sequentially, the sapphire substrate 20a is separated from the GaN-based layer 30a.

[86] Although the related art and the present invention are explained above from the viewpoint of laser lift-off (LLO) process, an inevitable process for fabricating a vertical type LED, the system and method for delivering laser beam of the present invention is not limited to the LLO process and may also be applied to other semiconductor fabricating process, particularly, a process for separating a thin film on a wafer substrate to fabricate a device. A variety of the thin film such as a compound semiconductor, copper, aluminum, gold, polymer and so on may be separated according to the present invention.

[87] As described above, according to the present invention, the uniformity of energy intensity all over the beam spot is improved and thus the process yield is also remarkably increased. Besides, the beam transmittance is improved and thus the production per unit time is also raised. Further, the manufacturing process is simplified, the manufacturing cost is reduced, and thus the competitive power in LED market is improved.

[88] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

[1] A laser beam delivery system, comprising: a laser beam source for emitting laser beam; a beam homogenizer for improving uniformity of energy intensity of the laser beam, the beam homogenizer comprising a microlens type fly-eye lens; a mask for masking a peripheral area of a cross section of the laser beam having penetrated the beam homogenizer at a focal plane; and an imaging lens for applying the laser beam to a unit irradiation area of a target. [2] The laser beam delivery system of claim 1, wherein the beam homogenizer comprises: a first fly-eye lens for dividing the laser beam emitted from the laser beam source into a plurality of beamlets; a second fly-eye lens for adjusting divergent angles of the plurality of beamlets; and a condensing lens for overlapping the plurality of beamlets divergent angles of which are adjusted. [3] The laser beam delivery system of claim 2, wherein the first and second fly-eye lenses are microlens type. [4] The laser beam delivery system of claim 2, wherein the first and second fly-eye lenses have a pitch of 0.5 to 2.0 mm. [5] The laser beam delivery system of claim 2, wherein each of the first and second fly-eye lenses is a rectangle shape having a horizontal length and a vertical length, wherein a ratio of the horizontal length to the vertical length is substantially identical to that of the cross section of the laser beam. [6] The laser beam delivery system of claim 2, wherein, when focal distances of the first and second fly-eye lenses are f _LA1 and f_LA2 respectively, a distance between the first and second fly-eye lenses is greater than f _LAI but less than f_LAi + ϊua- [7] The laser beam delivery system of claim 1, wherein the laser beam is KrF excimer laser beam or ArF excimer laser beam. [8] The laser beam delivery system of claim 1, further comprising an attenuator of adjusting power of the laser beam emitted from the laser beam source. [9] The laser beam delivery system of claim 1, further comprising a field lens between the beam homogenizer and the mask for adjusting a distance between the beam homogenizer and the mask. [10] A method for delivering laser beam, comprising: emitting excimer laser beam; dividing the emitted excimer laser beam into a plurality of beamlets using a microlens type fly-eye lens; overlapping the plurality of beamlets, thereby creating homogenized laser beam; masking a peripheral area of the homogenized laser beam; and applying the masked homogenized laser beam to a target. [11] The method of claim 10, further comprising adjusting divergent angles of the plurality of beamlets before overlapping the plurality of beamlets. [12] The method of claim 10, wherein the overlapping of the plurality of beamlets comprises adjusting a place where the plurality of beamlets are overlapped. [13] The method of claim 10, wherein the laser beam is KrF excimer laser beam or

ArF excimer laser beam. [14] The method of claim 10, wherein the emitted laser beam is divided into at least

230 beamlets. [15] The method of claim 10, wherein the applying of the masked homogenized laser beam comprises condensing the masked homogenized laser beam so as to applying the laser beam to a unit irradiation area of the target exactly. [16] The method of claim 10, wherein the excimer laser beam is emitted in pulse form. [17] A laser lift-off method, comprising: forming a GaN-based epi layer on a sapphire substrate; emitting excimer laser beam; dividing the emitted excimer laser beam into a plurality of beamlets using a microlens type fly-eye lens; overlapping the plurality of beamlets, thereby creating homogenized laser beam; masking a peripheral area of the homogenized laser beam; applying the masked homogenized laser beam to a unit irradiation area of the sapphire substrate; and separating the sapphire substrate from the GaN-based epi layer. [18] The method of claim 17, further comprising physically separating the sapphire layer from the GaN-based epi layer. [19] The method of claim 17, further comprising adjusting divergent angles of the plurality of beamlets before overlapping the plurality of beamlets. [20] The method of claim 17, wherein the excimer laser beam is emitted in pulse form.