Yield and yield improvement of LED extraction by wet etching process

1, preface

In recent years III nitride (III-Nitride) high brightness light-emitting diode (High Brightness Light Emission Diode; HB-led) won wide attention, has been widely used in traffic lights, LCD backlight and lighting use. Basically, GaN LED is epitaxial (Epitaxial) growth on sapphire substrate (Sapphire Substrate), the lattice constant of epitaxial GaN and the bottom of the sapphire substrate (Lattice Constant) and the coefficient of thermal expansion (Coefficient of Thermo Expansion; CTE) is very large, it will produce a high density dislocation line (Thread Dislocation) 108~1010 / cm2, the high density dislocation line will limit the luminous efficiency of GaN LED.

In addition, in HB-LED, in addition to the active layer (Active Region) and the other layer absorbs light, high refractive index also must pay attention to is the semiconductor (High Refractive Index), which will make the LED generated by the light Limited (Trapped Light). Figure 1 to illustrate, the light emitted from the active region in the semiconductor and the surrounding air reaches the interface, if light incident angle is greater than the pyramid (Escape Cone) the escape of the critical angle (Critical Angle; alpha C), will have a total internal reflection (Total Internal Reflection) for semiconductor high refractive index; in terms of the critical angle is very small, when the refractive index is 3.3, the total internal reflection angle is only 17O, so most of the light rays emitted from the active region, will be limited (Trapped) in the semiconductor, absorption of substrate is limited this light may be thicker. In addition, due to the substrate of the electron and hole pairs, due to poor substrate quality or low efficiency, resulting in a greater probability of non radiative Non-RadiativELy (Recombine), thereby reducing the efficiency of LED. So how to extract the light source from the active region of the semiconductor, and then increase the efficiency of LED (Light Extraction Efficiency), is becoming the most important target of each manufacturer.

Currently has two kinds of methods can increase the extraction efficiency of LED light: (1) the first method in LED epitaxial sapphire substrate, etching process (Pattern Sapphire Substrate; PSS); (2) the second methods in LED epitaxy, the side etching of sapphire substrate (Sapphire Sidewall Etching; SSE), and the back of the substrate roughness (Sapphire Backside Roughing; SBR). In this paper, we will discuss how to use high temperature phosphoric acid wet chemical etching technology to achieve the purpose of increasing the efficiency of LED light extraction by referring to the relevant literature [1~6]. In addition, in order to achieve the purpose of increasing the efficiency of LED light extraction, it is necessary to take into account the factors which should be taken into account in the design and manufacture of the LED system when the production capacity and the yield ratio are high.

Figure 1, when the light emitted from the active region is at the interface between the semiconductor and the surrounding air, the total internal reflection will occur if the incident angle is greater than the critical angle (C).

2, etching and patterning (PPS) process of the pre crystal sapphire substrate

Sapphire substrate etching process (PPS) can effectively increase the extraction efficiency of light, due to the substrate surface geometry changes, scattering mechanism can change the LED, or will lead to light scattering inside the LED, and then run out by pyramid. The use of single step maskless dry etching technology (Maskless Dry Etching) to the processing of sapphire (Sapphire) substrate, although can improve the internal quantum efficiency (Internal Quantum Efficiency) and extraction rate (Light Extraction Efficiency), but because Blaupunkt stone substrate surface very hard, dry etching will damage the sapphire surface, making the line difference row (Thread Dislocation) by the substrate and gradually extended to the top of the GaN epitaxial layer, thus affecting the LED crystalline quality, so they generally tend to use wet chemical etching method. The wet chemical etching of sapphire substrate, as well as the front of the LED process flow, as follows:

A. first use photolithography process to produce a desired pattern on sapphire substrates. B. uses plasma assisted chemical vapor deposition (Plasma Enhanced Vapor Deposition PE-CVD) system to deposit SiO2 on the sapphire substrate, and then set up the array pattern with interval of 3 m when the light is removed. C. uses the SiO2 as the etching mask layer, and etching the sapphire substrate in the mixture of high temperature phosphoric acid and sulfuric acid at 280 DEG C to form a patterned structure. Figure 2 is the schematic diagram of the cross section of the wet chemical etching of the sapphire substrate (PSS). The optical microscope image is shown in figure 3. The use of D. MO-CVD GaN-LED on the growth of C etching of patterned sapphire substrate (0001) on the surface of GaN-LED structure, from the bottom up, including: GaN nucleation layer, an undoped GaN layer and silicon doped N-type GaN layer, MQW layer and P-type GaN layer. E. uses standard photolithography and dry etching to etch a portion of the P-type GaN layer to expose the N-type GaN layer, thereby defining the light-emitting region and the electrode. ITO transparent conductive layer was deposited by F. and then deposited with Cr/Au metal layer, and the P electrode and N electrode were fabricated under the condition of nitrogen gas at 200 DEG C. Figure 4 is the front of the GaN LED process flow diagram; Figure 5 is a chemical wet etching patterned sapphire substrate (PSS), followed by the growth of GaN epitaxial layer LED structure.

Fig. 2 Schematic diagram of cross section of wet chemical etching sapphire substrate (PSS).

Figure 3. Optical microscope images of wet chemically etched sapphire substrates (PSS).

Figure 4, GaN LED process flow diagram [3, 4, 5].

Figure 5. The LED structure of the GaN epitaxial layer after the wet etching of the patterned sapphire substrate, [2].

As shown in Figure 6, by wet chemical etching