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fabrication of high efficiency gan based high voltage led chip and cob package

HV-LED chip package is usually used in ordinary high-power LED chip COB (Chip-on-board) package. Because the COB package is directly attached to the LED chip on the substrate, so the substrate thermal conductivity and reflected light performance is particularly important.

I. Introduction
High-voltage light-emitting diode (HV-LED) is in the chip preparation process will be a large-size chip is divided into a number of small light-emitting unit, and then light-emitting units connected through the bridge electrode in series to achieve a small current, LED. The luminescent efficiency can be improved by optimizing the size and shape of the luminescent unit, isolating the trench width and sidewall microstructure, and preparing the backside mirror. The HVLED chip with the size of 370 μm × 221 μm and 228 μm × 190 μm, respectively, was fabricated with the same total chip size. The results showed that the current spreading of the chip was uniform, Luminous efficiency is higher. The HV-LED chip with rectangular cross section, inverted trapezoidal shape and positive trapezium shape was fabricated. The results show that the structure of the positive trapezoid luminescence unit can obtain high luminous efficiency and yield. In order to facilitate the connection of the electrodes, an HV-LED chip with 20,40,60,80 μm width of isolation trench was fabricated. The results showed that with the isolation of trench, As the width of the trench increases, the luminous efficiency increases. In this study, we prepared columnar microstructures on the sidewall of the isolation trench and optimized the back reflector under the sapphire substrate to improve the light extraction efficiency and enhance the light output.

HV-LED chip package usually used in ordinary high-power LED chip COB (Chip-on-board) package. Because the COB package is directly attached to the LED chip on the substrate, so the substrate thermal conductivity and reflected light performance is particularly important. The ceramic substrate has good corrosion resistance, high thermal conductivity (about 24 W / (m · K)) and the thermal expansion coefficient (about 7 × 10 – 6 mm / ℃) of the sapphire substrate. The stability of the package after the high, better solution to the LED heat dissipation problems and life issues; and mirror aluminum substrate has a higher thermal conductivity (about 137 W / (m K)) and reflectivity (about 98 %), Can effectively improve the saturation of high-power LED chip output power.

In this paper, HVLED chips with 16 luminescent units were fabricated in series, and the sidewalls of the trench were etched by using photoresist and SiO2 double-layer masks to facilitate the electrode connection. The effect of isolation trench width on the electrical properties and Optical properties. The HVLEDs were encapsulated with a commercial mirror aluminum substrate and a ceramic substrate. The effects of implantation current, temperature and substrate on the electrical and optical properties of the device were investigated.

Second, HV-LED chip
2.1 chip preparation

The 5.08 cm (2 in) epitaxial wafer used in the experiment was obtained by metalorganic chemical vapor deposition (MOCVD) using a patterned sapphire substrate (PSS). The structure of the epitaxial wafer is 0.025μm buffer layer, 3.5μm undoped GaN layer, 2.7μm silicon doped n-GaN, 0.24μm multi-quantum well active region and 0.25μm magnesium doped p-GaN. The finished high-voltage LED chip is made up of 16 light-emitting units connected in series. As shown in Fig. 1, the arrangement of the light-emitting units is [3,5,5,3]. The number of cells in the first and last columns is 3, The number of cells in the second and third columns is five, and the dimensions of the cells are matched to each other so that the chip is rectangular in shape to facilitate the subsequent laser cutting process.

Fig.1 Metallographic micrograph of HV-LED chip

The specific preparation process shown in Figure 2. First, photoresist was used as mask, Cl2 and BCl3 as etching gas, and 1.2μm step was etched by inductively coupled plasma (ICP) to expose n-GaN layer. Then, photoresist and SiO2 double layer And the ITO thin film of 100 nm is deposited on the p-GaN by using electron beam evaporation as the current spreading layer; and then, by using the ICP-etched substrate, an ITO thin film is deposited on the p- The dielectric barrier layer was formed by plasma-enhanced chemical vapor deposition (PECVD). The layer was also fabricated with a 700 nm thick SiO2 passivation layer, which also insulates the sidewall of the isolation trench to prevent leakage. Finally, wet etching of HF solution exposed the n- and p- The light emitting cells were connected in series by electron beam evaporation deposition of 50 nm / 50 nm / 1 200 nm Cr / Pt / Au metal electrodes. On the basis of the above process, a single HV-LED chip is prepared through subsequent grinding, polishing, back plating, cutting and cracking process.

The high-voltage LED chips with 10,20,30,40 μm isolation trench width were prepared by the above-mentioned process. In order to keep the shape of the chip rectangular, the dimensions of the light emitting cells of the second and third rows of the four kinds of samples are kept constant while the lengths of the light emitting cells of the first and fifth rows are required to be slightly adjusted. In addition, the sidewall of the isolation trench of the luminescence unit is shown in Fig. 3 when etching the isolation trench using the lateral microstructure technique.

2.2 chip test and results analysis

The optical and electrical parameters of the four samples were tested using a fully automatic wafer spot tester (Model LEDA-8F P7202) equipped with a semi-integrating sphere. The number of chips tested in each sample was over 200 and the injection current range was 10 ~ 50 mA, typical current – voltage curve, current – optical power and current – electro – optical conversion efficiency curve shown in Figure 4. As the injection current increases, both the forward voltage and the output power increase gradually, while the electro-optical conversion efficiency decreases. Four kinds of samples showed similar trend, and the chip with 20μm width had lower forward voltage, higher output optical power and electro – optical conversion efficiency. In which the forward voltage is saturated when the injection current is large. In particular, the saturation current of the chip sample with an isolation trench width of 40μm is the smallest, which may be due to the maximum total resistance of the chip sample with an isolation trench width of 40μm. More heat and makes the minority carrier concentration is higher, the reverse saturation current is larger, leading to a decrease in forward voltage.

When the injection current is 20 mA, the average of the test results is shown in Table 1. As the width of the isolation trench increases from 10μm to 40μm, the forward voltage of HV-LED chip first decreases and then increases, and the minimum value is 50.72V when the isolation trench width is 20μm. The output power increases first and then decreases to a maximum value of 373.64 mW when the isolation trench width is 20μm. This makes the electro-optic conversion efficiency increase and then decrease, and the maximum value is 36.83% when the isolation trench width is 20μm. The forward voltage appears to rise first and then rise, due to the isolation trench at the electrode connection bridge resistance caused by different. When the width of the isolation trench is 10μm, the quality of metal deposition is poor, the resistivity is high, so the resistance of electrode bridge is high, so the voltage of the chip is larger; when the width of isolation trench is increased to 20μm, The resistance of the electrode connection bridge decreases, so the voltage of the chip decreases. With the further increase of the isolation trench width, the resistance of the electrode bridge increases with the increase of the length, So the voltage of the chip rises. The output optical power showed a trend of first rising and then falling, because the adjacent light-emitting units of different optical coupling capacity. When the width of the isolation trench is 10 m, the light emitted from the side wall of a certain light emitting unit will be more coupled into the adjacent light emitting unit, and the light due to the absorption of GaN material, metal electrode, quantum well, The light emitted from the side wall of the light emitting unit will be less coupled into the adjacent light emitting unit, so that the light extraction efficiency is increased, so that the light extraction efficiency is small, so that the emitted light power is small; when the isolation groove width is 20 μm, The optical power of the exit decreases with the increase of the isolation trench width. This may be because the total resistance increases, so that the chip generates more heat and the internal quantum efficiency decreases.

In the above experiment, the maximum value of the output optical power and the electro-optical conversion efficiency appeared in the width of 20 m for several cases of the isolation trench width of 10, 20, 30, 40 m, which was different from the results of the above-mentioned literature. The above document fills the isolation trench with a polymer to achieve planarization of the trench to facilitate electrode connection. When the width of the isolation trench is small, holes will appear in the polymer, which will reduce the light extraction efficiency and reduce the output optical power and electro-optical conversion efficiency. When the width of the isolation trench is 20, 40, 60, 80μm, the filling quality of the polymer is improved with the increase of the groove width, the hole is reduced, the light extraction efficiency is increased, and the output optical power and electro-optical conversion efficiency are increased. The groove width is optimized at a maximum of 80 μm. In this paper, photoresist and SiO2 double-layer masks are used to make the sidewall gradient of the isolation trench after ICP etching flat to facilitate the electrode connection. At this point, the isolation between the trench is the air, the main factor affecting the efficiency of light extraction is no longer isolated between the trenches inside the polymer hole, but the adjacent light-emitting units between the optical coupling capability.

Three, four-chip package of HV-LED devices

3.1 Device Packaging

A chip sample with an isolation trench width of 20 μm was selected for encapsulation. To get close to the mains 220 V, the four chip samples in series. Figure 5 and Figure 6, respectively, is the use of mirror aluminum substrate and ceramic substrate package of micrographs, four chips in a “one” -shaped arrangement. The HV-LED chip was fixed on the package substrate with an insulating adhesive and baked at 150 ° C for 2 h. Then, gold wire bonding was performed using a wire bonding machine. Four HV-LEDs The chip is connected in series, and the first chip p electrode, the fourth chip n electrodes are connected with the substrate positive and negative; then the use of plastic dam encircle the appropriate dispensing range, in order to facilitate the comparison of test results, mirror aluminum plate and (A / B glue) and phosphor (yellow powder / red powder) are mixed uniformly according to the requirement of color temperature of 6 000 K, and the points are put into the embankment dam. .

3.2 Device testing and results analysis

The optical parameters and electrical parameters of the samples were tested by ZWL-3900, the number of devices tested in each sample were more than 5, the injection current range was 10 ~ 100 mA, the test system was heated The temperature of the substrate was 20, 120, 150 ° C. The typical current-voltage, current-optical power and luminous efficiency curves are shown in Fig. With the increase of injection current, the forward voltage gradually increased; the output power increased first and then decreased, and the luminous efficiency was decreased. This shows that four-chip package of HV-LED devices only suitable for small current density situation. With the substrate temperature rise, the forward voltage, output optical power, saturation optical power, saturation current and luminous efficiency showed a downward trend. This is because the temperature of the chip makes minority carrier concentration increases, the internal quantum efficiency and the quantum efficiency of the phosphor decline. The output power, saturation power, saturation current and luminous efficiency of the samples were all better than those of the ceramic substrates under the same substrate temperature and the same injection current, and after the samples reached the saturation power , With the injection current increases, the output optical power attenuation is slow. This is because the thermal conductivity and the reflectivity of the mirror aluminum substrate are higher than those of the ceramic substrate. In the case of the same substrate temperature and the same injection current, the forward voltage of the mirror aluminum substrate sample is higher. This is because the experiment used in the mirror aluminum substrate LED bracket circuit metal materials for copper, and ceramic substrate LED bracket circuit metal material for the silver. On the one hand, the resistivity of metallic copper is lower than that of metallic silver; on the other hand, when the metal copper is exposed to air, it is easy to generate high-resistance copper oxide, and the contact resistance is larger when gold wire is welded; Material is a good conductor, in the gold wire welding contact resistance is small.

fabrication of high efficiency gan based high voltage led chip and cob package

When the injection current is 20 mA, the test data of HV-LED devices with mirror aluminum substrate and ceramic substrate package are shown in Table 2. At room temperature, the forward voltage of the sample is 195.4 V and the luminous efficiency is 118.8 lm / W. The forward voltage of the sample is 198.9 V and the luminous efficiency is 122.2 lm / W. Compared with the use of ceramic substrate package HV-LED devices, compared to the mirror aluminum substrate packaging device luminous efficiency is higher. However, the ceramic substrate and the LED chip sapphire substrate thermal expansion coefficient is similar to the automated packaging process which will not produce high-temperature chip skew problem, and the ceramic substrate insulation, high pressure, so the high-power flip LED Chip packaging applications more and more widely.

The typical spectra are shown in Fig. 8 at different injection currents and substrate temperatures, and the peak wavelengths are shown in Table 3. When the injection current is 20 mA and the substrate temperature is 20 ° C, the HV-LED devices encapsulated on the mirror aluminum substrate and the ceramic substrate have the same blue light peak wavelength of 457 nm. With the increase of injection current and the increase of substrate temperature, the peak wavelength of blue light is red-shifted. This is because the injection current increases and the substrate temperature rise makes the chip junction temperature rise, the quantum well band gap. Since the thermal conductivity of the mirror aluminum substrate is higher than that of the ceramic substrate, the red-shift of the blue peak wavelength of the HV-LED device encapsulated in the mirror aluminum substrate is small.

 Fourth, the conclusion

A high-voltage LED chip with 10,20,30,40μm isolation trench width was fabricated. When the trench width is 20μm, the chip voltage is lower, the output optical power is higher, the electro-optical conversion efficiency is the best. When the injection current is 20 mA, the forward voltage of the chip is 50.72 V, the output optical power is 373.64 mW, and the electro – optical conversion efficiency is 36.83%. The use of mirror aluminum substrate and ceramic substrate for four chip in series form of COB package. Because the thermal conductivity and reflectivity of the mirror aluminum substrate are higher than those of the ceramic substrate, the peak wavelength of the blue light is red-shifted due to the output optical power, saturated optical power, saturation current, luminous efficiency, light attenuation, and temperature of the encapsulated sample Better than the ceramic substrate test results. When the injection current is 20 mA, the forward voltage of the chip is 50.72 V, the output optical power is 373.64 mW, and the electro – optical conversion efficiency is 36.83%. The use of mirror aluminum substrate and ceramic substrate for four chip in series form of COB package. Because the thermal conductivity and reflectivity of the mirror aluminum substrate are higher than those of the ceramic substrate, the peak wavelength of the blue light is red-shifted due to the output optical power, saturated optical power, saturation current, luminous efficiency, light attenuation, and temperature of the encapsulated sample Better than the ceramic substrate test results. When the injection current was 20 mA and the substrate temperature was 20 ℃, the forward voltage of the HV-LED device was 198.9 V and the luminous efficiency was 122.2 lm / W. The luminous efficiency of LED devices can be further improved by using high-efficiency epitaxial wafers, developing high-light-extraction-efficiency chip processes, coating high-efficiency phosphors, and optimizing circuit metal materials for mirror-mounted aluminum-based LED stents.

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