Volume 6 Issue 7 - November 21, 2008
Tin-based Metal Substrate Technology for the Fabrication of Vertical-Structure Gallium Nitride-Based Light-Emitting Diodes
Hon-Yi Kuo, Shui-Jinn Wang*, Pei-Ren Wang, Kai-Ming Uang, Tron-Min Chen, and Hon Kuan

*Professor of Institute of Microelectronics, College of electrical engineering and computer science, National Cheng Kung University
* sjwang@mail.ncku.edu.tw

APPLIED PHYSICS LETTERS 92, 021105 (2008)

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I. INTRODUCTION
Worldwide changing climate from greenhouse effect and the reduction in fossil fuel together drive the advance of environmental energy-saving technology. In lighting field, GaN-based White LED is hot-spot because of its small volume, long lifetime, and being less mercury-pollutant. Great efforts have been devoted to further boost its applications into flashlight, backlight source for LCD, and even solid-state lighting [1-2]. Conventionally, GaN-based LEDs were fabricated on insulating sapphire substrates, this resulted in severe current crowding due to lateral conducting and thermal accumulation for the poor thermal conductivity provided by sapphire substrates.

To release this situation, substrate transfer techniques by means of laser lift-off (LLO) with wafer bonding or electroplating have been reported for the fabrication of vertical-structured GaN-based LEDs [2-5]. In 2007, Wang and Chen et al. demonstrated the use of selective electroplating nickel (Ni) substrates with patterned LLO for the fabrication of Vertical-structured Metal-substrate GaN-based light-emitting diodes (VM-LEDs) [4]. Metal-cutting was avoided and superior electrical and optical characteristics in comparison with regular LEDs have been presented.

For the packaging of these high performance LED chips, conventional epoxy or conductive glue used in the die-attaching process has been less favored. Instead, tin-based solder material is becoming the mainstream for its superior thermal and electrical characteristics. In this paper, to simplify the device fabrication processes of VM-LEDs with high throughput and cost effectiveness, a metal substrate technology using tin (Sn) based solder balls with a nickel barrier layer and an Au wetting layer was proposed and demonstrated. Electrical and optical characteristics of the fabricated VM-LEDs were reported and compared to those of regular LEDs as well.

Fig. 1 The schematic device structure of VM-LEDs using the proposed Sn-based metal substrate technology at some specific processing stages. (a) Sample at the patterned LLO processing stage. (b) Sample at the n-GaN ohmic contact processing stage. (c) The schematic cross section of a regular GaN-based LED. Note that device structures shown in the present figure were not in scale.
II. EXPERIMENTS

The schematic drawings for the key fabrication processes of the proposed Sn-based metal substrate technology were shown in Figure 1. Here, lead-free Sn-based solder-balls were employed for the implementation of dicing free metal substrates. Samples prepared in this work were epitaxially grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) [6]. Oxidized Ni(2.5 nm)/Au(3.5 nm), Ti(15 nm)/Al(400 nm)/Ti(100 nm)/Au(200 nm), and Ni(200 nm) Au(200 nm) metal systems were deposited sequentially on p-GaN layer by e-beam evaporator to serve as ohmic contact, adhesive/mirror layer, and barrier/wetting layer, respectively.

SU8-2035 thick photoresist was employed here to defined the device region (750×750 μm2) with a cutting-way width of 90 μm, followed by a selective electroplating process to form 10-μm-thick Ni metal frames over the entire cutting-way region under a constant current of 1.7 A within 10 min. After that, Sn-based solder balls (350 μm in diameter) were placed within each device region, a rapid-thermal (RT) reflow treatment at 280~300˚C for 90 s was conducted. The Ni metal frames from last step acted as molds to confine melting Sn-based alloy during reflow for the formation of patterned Sn-based metal substrates. Particularly, they were self-removed after the RT reflow treatment because of the difference in thermal expansion coefficient with respect to the beneath metal layers. Note that, for higher melting points and better thermal conductivity of the Sn-based metal substrate, its alloy composition could be suitably changed to accommodate surface mount technology (SMT) used for LED applications [7]. Prior to the patterned LLO process, the sample was glued to a temporal silicon (Si) substrate. Through the use of a mask to define both size (620×620 μm2) and shape of excimer laser beam (248 nm) and an alignment to the patterned Sn-based metal substrate, the patterned LLO process (Fig. 1(a)) was performed at a reactive energy of 850 mJ/cm2 [4, 6]. It should be noted that the area ratio (AR) of the laser beam size (also the size of the device emission area) to the chip size (i.e., device region) was kept to be less than 1 (0.82 in the present work) to prevent the edge of epitaxial structure from irregular breach or flaw during the fabrication of device [4]. The sample was then heated to about 40˚C for 10 min to remove the sapphire substrate.

An ICP dry-etching process was used to remove the u-GaN. For better light extraction and the contact characteristics, a surface treatment with 6-mol KOH solution at 60˚C for 90 s was also made [6]. After the surface of the exposed n-GaN was cleaned with HF and diluted HCl:H2O (1:1) solution for 1 min, a metal contact pad comprised of Ti-Al-Ti-Au was deposited by e-beam evaporator (Fig. 1(b)). Finally, the fabrication of the proposed n-side up VM-LED chips with Sn-based metal substrates was completed. Note that regular LEDs, as shown in Fig. 1(c), of the same chip size with two electrodes on the same side of the device were also fabricated with the same wafer for comparison.
Fig. 2 SEM and OM images of samples at some processing states. (a) Top view of the patterned Sn-based metal substrate before the removal of nickel metal frames. (b) SEM image of the cross-sectional structure of the sample after the reflow process. (c) Top-view of the exposed epi-GaN layers after patterned LLO process. (d) OM image of the exposed n-GaN after the auto-removal of the nickel metal frames.

III. RESULTS AND DISCUSSION

Figure 2(a) shows that top view of the patterned Sn-based metal substrates. They were oval-like due to internal cohesion. Figure 2(b) shows the cross-sectional structure of the sample before patterned LLO. Flat and smooth interface between GaN epilayer and metal layers was seen. Figure 2(c) shows the top-view of the exposed n-GaN. An even GaN surface was obtained. Figure 2(d) reveals that the Ni metal frames were completely self-removed after the RT reflow treatment.

Fig. 3 Comparison of typical forward current–voltage (I-V) characteristics of VM-LEDs and regular LEDs. The inset shows the reverse current characteristic of VM- and regular LEDs
Figure 3 showed the comparison of current-voltage (I-V) characteristics of the fabricated VM-LEDs and regular LEDs with the same chip size. At an injection current of 350 mA, the forward voltage (VF) of the proposed VM-LEDs are 3.46 V. This is comparably good with previous works reported by author’s and Lin’s groups (Refs. 4 and 5), while that of the regular one is 4.51 V. As compared with regular LEDs, the reduction in VF of the n-side up VM-LEDs should be attributed to the considerable improvement in current spreading and the realization of a much shorter vertical conduction path between the two electrodes (~4 μm in this work) [4, 6]. It is noted that the incremental series resistance ((∂i/∂v)-1) of the VM-LEDs is about 1.0 Ω at 350 mA, which is about 1/3 that of regular LEDs at the same current. The typical reverse characteristics of VM- and regular LEDs were shown in the inset of Fig. 3. It is seen that the VM-LED has a relatively inferior reverse characteristic. This might be attributed to the LLO process increases the density of screw dislocations, which penetrated through the MQW region, and/or causes damages to the periphery of the device [8]. Optimizing the LLO process to minimize the damage to the epilayer and employing suitable passivation to VM-LEDs are now underway.
Fig. 4 (a) Comparison of typical Lop-I characteristics of VM-LEDs and regular LEDs (The insets show the light emission images at 350 mA of the VM- and regular LED). (b) Comparison of the peak wavelength dependence on injection current.

The typical light output power-current (Lop-I) characteristics of VM-LEDs and regular LEDs were shown in Fig. 4(a). The proposed VM-LED was found having an increase in light output power (i.e., ΔLop/Lop) of about 145.36% over that of regular LEDs at 350 mA. The power conversion efficiency (λ=Po/Pe, i.e., the ratio of optical output power Po to input electrical power Pe) of the VM-LEDs at 350 mA is about 3.20× that of regular LEDs. These improvements should be mainly attributed to the fact that the n-side up vertical structure enables better current spreading, less series resistance, larger light extraction area benefiting from single n-electrode, roughened n-GaN layer, and higher light reflection [4]. Further, a curve fitting technique was used (blue solid lines in Fig. 4(a)). Under high injection current, Lop-I curves of VM- and regular LEDs can be expressed as Lop∝I0.71 and Lop∝I0.56 respectively. Obviously, the light output power of the VM-LED exhibited a better response under high injection currents. Note that the improvement in ΔLop/Lop and λ are comparably good with previous works, indicating that the present Sn-based substrate technology would be attractive for thin GaN LED fabrication [4, 5].

The dependence of peak wavelength (WP) on injection current of both VM- and regular LEDs was also plotted in Fig. 4(b). Under low injection currents, the band-filling effects dominated and led to the blue shift in WP for both the VM- and regular LEDs [9-11]. As the injection current went up, thermal accumulation from Joule heating due to parasitic series resistance and non-radiative recombination of carriers resulted in a strong red shift and the blue shift from the band filling effect was overridden [10]. One observes that the prevailing of the red shift occurred at 280 and 170 mA for the VM- and regular-LEDs, respectively. This revealed that a relatively slight accumulation of Joule heating of the VM-LEDs was achieved.

VI. CONCLUSION

A metal substrate technology utilizing Sn-based solder balls and patterned laser lift-off (LLO) technique was proposed for the fabrication of VM-LEDs. As compared to regular LEDs at an injection current of 350 mA, the proposed VM-LEDs have been shown to have an enhancement in Lop about 145.36% and a VF drop of 1.05 V, which provides an improvement in the power conversion efficiency by about 3.20× that of regular LEDs. It thus could be expected that the proposed Sn-based metal substrate technology would serve to boost high performance GaN-based LEDs into solid-state lighting in the foreseeable future.

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