Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process

Article Journal of Nanoscience and Nanotechnology

Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America

Vol. 14, 8237–8241, 2014 www.aspbs.com/jnn

Blue Inorganic Light Emitting Diode on Flexible Polyimide Substrate Using Laser Lift-Off Process Nilesh Barange1 2 , Young Dong Kim2 , Hyungduk Ko1 , Joon-Suh Park1 , Byoungnam Park3 , Doo-Hyun Ko1 ∗ , and Il Ki Han1 ∗ 1

Center for Opto-Electronic Convergence Systems, Korea Institute of Science and Technology (KIST), l Hwarangno 14-Gil 5, Seongbuk-Gu, Seoul 136-791, South Korea 2 Nano-Optical Property Laboratory and Department of Physics, Kyung Hee University, Seoul 130-701, South Korea 3 Department of Materials Science and Engineering, Hongik University 72-1, Sangsu-Dong, Mapo-Gu, Seoul 121-791, South Korea

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Devicebyfabrication was carried out on rigid silicon and flexible polyimide substrate, and Delivered Publishing Technology to: both Korea Institute of Science & Technology (KIST) On: Thu, 18optimized Sep 2014LLO 08:00:53 I–V performance for IP: both161.122.32.122 devices was measured. The process for the whole GaN Scientific Publishers LED film transfer would beCopyright: applicable American in flexible LED applications without compromising electrical properties.

Keywords: Laser Lift Off, Inorganic Light Emitting Diode, Flexible Substrate, Blue Light Emitting Diode.

1. INTRODUCTION Gallium nitride (GaN) has been widely used in the fabrication of optoelectronic devices such as light emitting diode (LED), laser diode (LD) and especially blue LEDs. Fabrication of GaN LEDs on sapphire wafer is becoming popular due to its relatively low cost of fabrication.1–3 In particular, patterned sapphire wafer is most commonly used to fabricate GaN LED structure for better surface contact and less defects. However, the sapphire wafer provides poor thermal and electrical characteristics. Also, the complex process for device fabrication includes wafer dicing, robotic manipulation to separate LED from the sapphire wafer and its transfer process on lead frame. Besides, LEDs fabricated using above process give rise to resolution limitation due to physical dimensions and are not suitable for flexible substrate. Laser lift-off (LLO) is one of the most popular techniques to separate and etch GaN LED from sapphire wafer.4–10 In this case, the individual LED separated by LLO technique finally transfer to foreign ∗

Authors to whom correspondence should be addressed.

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substrate.11–13 However these processes exert high thermal stress on fabricated LEDs, thereby leading to cracks propagated in thin layers of the device. Although combination of SU-8, Indium (In), Chromium (Cr) and Palladium (Pd) for the passivation of GaN LED are incorporated to reduce the thermal stress, the extra process for passivation layer demands associated fabrication steps, high temperature process, and cost. Therefore, instead of individual LEDs, whole GaN LED film transferring technique would be preferable to overcome these above mentioned issues. In this paper, we report detail investigation of facile process by LLO to transfer the whole film of GaN LED layer onto flexible PI and rigid silicon substrate. The complete process for LLO, GaN film transfer, and LED chip fabrication on PI is studied. Flexibility for the fabricated LED on PI substrate is well demonstrated by showing comparable device performance with LED on hard silicon (Si).

2. EXPERIMENTAL DETAILS Figure 1(a) shows the fabricated GaN LED film structure on ‘mother’ sapphire wafer. The sapphire wafer cut into

1533-4880/2014/14/8237/005

doi:10.1166/jnn.2014.9898

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Blue Inorganic Light Emitting Diode on Flexible Polyimide Substrate Using Laser Lift-Off Process

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Delivered by Publishing Technology to: Korea Institute of Science & Technology (KIST) IP: 161.122.32.122 On: Thu, 18 Sep 2014 08:00:53 Copyright: American Scientific Publishers

Figure 1. The process scheme for transferring the whole GaN LED film on flexible substrate using LLO process (a) GaN LED structure was prepared on sapphire wafer (b) Ni metal was deposited on the GaN LED sample by e-beam evaporator (c) Ni metal deposited on PI/epoxy/glass substrate by e-beam evaporator (d) The Ni/PI/epoxy/glass sample was spin coated with flexible conducting epoxy (e) The previously prepared Ni/GaN LED was bonded to this flexible conducting epoxy/PI/epoxy/glass at 170  C for 10 min. (f) LLO process was carried out by irradiating the sample with Krf excimer laser from back side of transparent sapphire wafer, (g) The sample was kept on hot plate to melt gallium, (h) The sapphire wafer was separated from whole GaN LED film to give GaN LED film/Ni metal/flexible conducting epoxy/Ni metal/PI/epoxy/glass.

the size of 1 cm × 1 cm by laser die cutting and dicing machine (Exitech M2000). The back side of the sapphire wafer was grinded and polished by various sizes of diamond paste, and its final thickness was ∼ 60 m. The KrF 8238

excimer laser (ELMS-1000) was calibrated for the wavelength of 248 nm with pulse time of 25 ns and an energy range of 600–900 mJ/cm2 . The beam was focused on the sample having an active area of 1 cm × 1 cm. The machine J. Nanosci. Nanotechnol. 14, 8237–8241, 2014

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Blue Inorganic Light Emitting Diode on Flexible Polyimide Substrate Using Laser Lift-Off Process

with laser beam size of 300 m × 300 m provides step by step irradiation of laser beam on the sample with the help of a stepper motor arrangement. The whole irradiation was monitored by charge coupled device (CCD) camera mounted on the moving part of laser beam.

3. RESULTS AND DISCUSSION Figure 1 shows the schematic representation of LLO process. After cleaning the sample (Fig. 1(a)), nickel (Ni) ∼1500 Å was deposited on top side of the GaN sample by e-beam evaporator (Fig. 1(b)). PI substrate was firmly pasted on glass sample holder with epoxy and then Ni (∼1500 Å) was deposited on it by e-beam evaporator (Fig. 1(c)). Successively, flexible and electrically conducting epoxy 117-26 (Creative Materials, Inc.) was spin coated on Ni/PI substrate (Fig. 1(d)). After that, GaN LED sample and PI substrate were bonded together at 170  C for 10 min (Fig. 1(e)). Later, aligned and focused laser beam was irradiated from the back side of the GaN LED sample (Fig. 1(f)). By melting Ga at 40  C on hot plate, GaN LED film was separated from the sapphire (Figs. 1(g)–(h)). Figure 3. The schematic diagram of the GaN LED islands on flexible Figure 2 highlights importance of optimization for PI substrate (a) Separated GaN LED film after LLO process was prepared the LLO process where decomposition of the samples as previously discussed, (b) 100 m × 100 m mesa was patterned by arose due to high energy pulse, large pulse time and photolithography process (c) The GaN film was etched by ICP RIE power defocussing of the beam. While the laser irradiation 800 W, RF power 200 W, gases Cl2 and BCL3 with 7.5 sccm for 20 min byatPublishing Technology Koreaand Institute of Science & Technology (KIST) caused high Delivered temperature GaN/sapphire interface,to:the the photoresist was cleaned. (d) 25 m × 25 m area of metal was IP: 161.122.32.122 On: Thu,prepared 18 Sep 08:00:53 with2014 the help of e-beam evaporator followed by lift off process film decomposed to gaseous nitrogen (N2 ) and gallium Copyright: American Scientific in acetone. Publishers (Ga) droplets. Figure 2(a) shows the remaining layers on the ‘mother’ sapphire wafer after laser irradiation, In order to fabricate flexible GaN blue LED for the and presence of metallic dust-like particles from the film on PI, a standard fabrication process was performed back side of the sapphire wafer. Moreover, the sepaas shown in Figure 3. The process includes LLO on rated film from sapphire wafer onto the foreign subGaN LED film, followed by the fabrication of LEDs. strate (Fig. 2(b)) was not uniform. Thus, optimization for Square mesa 100 m × 100 m were patterned using LLO process required exact laser power and exposure photolithography (Fig. 3(b)) where photoresist (PR) AZ time. Sets of experiments were carried out, and finally P4620 was used, and thereby array of LEDs on the subthe laser power 600 mJ/cm2 and 25 nm pulse time for strate was prepared. Individual LEDs were isolated by the process was accomplished as the optimized value for inductive couple plasma reactive ion etching (ICP-RIE) uniform film separation. Figure 2(c) shows successful uni(Oxford Plasmalab system 100) at ICP power 800 W, form separation of GaN LED film on PI substrate after RF power 200 W, gases Cl2 and BCL3 with 7.5 sccm optimization.

Figure 2. Microscope images of samples after LLO process (a) Residual layer of metallic particles remained on the sapphire substrate after separating GaN LED film (b) Decomposed GaN LED film was observed on foreign substrate after separation. (c) Uniform separation of the whole GaN film was achieved by optimized LLO process at laser power 600 mJ/cm2 and 25 ns pulse time.

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Figure 4. Microscope images of individual GaN LED on (a) sapphire and (b) PI substrate (a) LED was fabricated on sapphire wafer and bonded to the PI substrate for LLO process. (b) Removal of the sapphire substrate after LLO process yields decomposed devices on PI substrate.

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Figure 6. Flexibility test of Blue ILEDs on PI substrate shows (a) bending of substrate approximately 50–60 times at angle 40 and (b) performance after bending.

The result shown in Figure 4 proves that our approach to transfer whole film is much beneficial for the device fabrication without any defects. Figure 5(a) shows the I–V characteristics for the fabrifor 20 min (Fig. 3(c)). The PR was cleaned after ICPcated blue GaN LED on PI (blue line) and similarly on siliRIE using microwave plasma asher for 10 min followed con substrate (red line) via the whole film transfer method. by acetone, methanol and DI water cleaning. The area The device performance was characterized by precision of 25 m × 25 m was patterned by photolithography parameter analyzer (HP 4156A). The devices on hard Si for top contact metal deposition. Ti/Au (20 nm/30 nm) and flexible PI substrate show typical diode characteristics, deposited by e-beam evaporator and sequentially lift-off however the flexible device shows a little higher operatwas done (Fig. 3(d)), and then PI substrate was firmly seping voltage. This is because we employed relatively thin arated from glass support. Similar procedure was carried layer of conductive epoxy to achieve flexibility for the PI out for silicon substrate. substrate, and it caused resistive contact although fabricaFor meaningful comparison with proposed method, we tion process was identical for both substrates. Figures 5(b) have also fabricated the individual LEDs on sapphire and (c) show the working LED on flexible PI and rigid wafer. LED sample on sapphire (Fig. 1(a)) etched into silicon substrate respectively. Delivered by Publishing Technology Korea Institute Science & Technology the square islands of 100 m × 100 m (Fig. to: 4(a)). Figure of 6 shows flexibility test for (KIST) the fabricated blue IP:to161.122.32.122 18 Sep 08:00:53 These LED islands were bonded Ni metal with On: flex- Thu,ILED. We2014 investigated flexibility by bending the fabricated Copyright: American Scientific Publishers ible conducting epoxy sample (Fig. 1(d)), and then they devices several times by 40 (Fig. 6(a)). It is intrigued were separated after LLO process. However, it is clear that functionality of multiple blue ILEDs on the substrate from the Figure 4(b) that devices are damaged after sepadoes not change before and after bending of the PI subration. Therefore, implementation of LLO process on LED strate as shown in Figure 6(b). These results indicate that islands was not possible due to high thermal stress as the LLO method for whole GaN film separation provides we previously discussed. This damage would be prevented much suitable fabrication process for flexible LED application compared with individual device transfer process. using passivation layer with unwanted fabrication steps.14

Figure 5. (a) Electrical performance of ILED (100 m × 100 m) devices on silicon and on PI substrate. Blue ILED fairly works on (b) rigid silicon and (c) on flexible PI substrate.

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Blue Inorganic Light Emitting Diode on Flexible Polyimide Substrate Using Laser Lift-Off Process

4. CONCLUSION

References and Notes

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In summary, we proposed facile process for uniform separation of the whole GaN LED film from sapphire wafer to PI substrate. Also, the device fabrication of blue GaN LEDs was carried out on flexible PI as well as on rigid silicon substrate. This method provides fast transfer of GaN LED film on foreign substrate, and the whole GaN LED thin film separation using LLO from sapphire wafer shows more promising results than individual separation from the prefabricated LED chips. I–V characteristic of blue GaN LED on flexible and rigid substrate shows comparable property, and performance on flexible substrate would be improved by using high quality conducting flexible epoxy. Matrix electrode patterning to address individual LED would provide further improvement. We believe that proposed method is apparently useful for flexible LED applications.

IP: 161.122.32.122 On: Thu, 18 Sep 2014 08:00:53 Copyright: American Scientific Publishers Received: 9 April 2013. Accepted: 9 January 2014.

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