Research On Etching of Distributed Bragg Reflector

This study investigated the effect of etching process parameters on the sidewall morphology and bottom metal etching damage of Distributed Bragg Reflector (DBR), and analyzed the underlying mechanisms. By comparing the etching morphology under different RF power and pressure conditions, it was found that increasing RF power and reducing pressure can solve the problem of sidewall fracture and obtain a smooth sidewall morphology. By comparing the effect of different process gases on the sidewall angle, it was found that adding O2 can reduce the DBR/Photoresit selectivity and sidewall angle while adding BCl3 can increase the DBR/Photoresit selectivity and sidewall angle. Therefore, the sidewall angle can be adjusted by controlling the type and flow rate of the etching gas. By comparing the DBR/metal selectivity under different RF power, it was found that as the RF power decreases, the DBR/metal selectivity increases, which can prevent metal splashing caused by over-etching of metal. Therefore, in DBR etching, high RF power is used for main etching to obtain a smooth sidewall morphology and the sidewall angle can be adjusted by varying the gas type and flow rate, while low RF power is used for over-etching to improve the DBR/metal selectivity and prevent metal over-etching. This study has reference significance for the development of the inverted chip DBR process.


Introduction
Etching is one of the key processes in the fabrication of semiconductor optoelectronic devices.Among them, Inductively Coupled Plasma (ICP) etching technology is a high-density plasma etching process that combines physical and chemical effects, with advantages such as high plasma density, high control accuracy, and good uniformity [1][2][3] .The etching effect of ICP is affected by process parameters such as gas composition and ratio, chamber pressure, ICP power, and RF power [4][5] .The extensive use of ICP etching in device fabrication has greatly improved device performance.The exploration of the optimal conditions of this process can increase the efficiency of device fabrication.
Figure 1 shows the layered structure of the inverted LED, with the epitaxial layer grown on the substrate forming the light-emitting layer.On top of the light-emitting layer, it is the DBR layer, which comprises a series of alternating layers with different refractive indices.The DBR based on SiO 2 /TiO 2 film can provide high reflectivity because of the relatively large contrast of refractive index between SiO 2 and TiO 2 [6][7] , with the thickness of each layer finely tuned to reflect the targeted wavelength of light into the light-emitting layer.On top of the DBR layer, it sits a p-type electrode, which is typically made of transparent conductive oxide (TCO) and serves as the reflective contact for the LED.Finally, a metal electrode is placed on the backside of the substrate to complete the electrical connection.
To effectively use the light emitted from the front of the emitting layer, the inverted LED chip has its light-emitting surface on the sapphire side, requiring a reflective layer to be prepared on the front side [8] .Typically, a Ni/Ag/Au composite electrode metal reflector is used, which has serious light absorption and complex process, with reflectivity below 90% in the blue wavelength range.Compared with metal reflectors, Distributed Bragg Reflector (DBR) can change the material's refractive index or adjust the bandgap position by adjusting the thickness, while avoiding the absorption problem of metal reflectors.DBR is a periodic structure composed of two materials with different refractive indices arranged in an ABAB manner, with the optical thickness of each layer being 1/4 of the central reflection wavelength, and the reflectivity can reach more than 99%.The typical DBR is a film alternation structure composed of SiO 2 /TiO 2 , which needs to be etched to expose the metal electrode, requiring smooth sidewalls and no metal damage at the bottom.Several papers have discussed the growth of high-quality DBR film structures [9][10] , while there is limited research on the system study of devices formed by etching the DBR structure.This article uses ICP technology for dry etching of DBR and studies the effect of ICP power, RF power, chamber pressure, and gas type on etching morphology and bottom metal damage by controlling these process parameters.

Experiment
In this study, the etching samples used were sapphire substrates, with the DBR structure consisting of a film alternation structure composed of SiO 2 /TiO 2 and a Cr-Pt metal layer at the bottom.The etching gases used were CF 4 /Ar (BCl 3 /O 2 ).The samples were subjected to ICP etching experiments under different ICP power (600 -1200 W), RF power (80 -500 W), process pressure (4 -8 mT), and gas composition.After etching, the etching depth and sidewall morphology were observed by using a scanning electron microscope (SEM).

The Influence of ICP Etching Parameters on Sidewall Morphology
Figure 2 shows the SEM images of the DBR sample etched using different RF powers and pressures, with an ICP power of 600 W. The results indicate that under high pressure and low RF power, the etching results show the fracture.With increasing RF power, the sidewall fracture phenomenon is improved, but a slight fracture still exists at the bottom of the etch.By further reducing the process pressure based on raising the RF power, the sidewall fracture problem can be solved, and a smooth sidewall morphology can be obtained.It can be concluded that if the reaction conditions are selected improperly and the chemical reactions are imbalanced, it will cause one material to be etched too fast while another material is etched too slowly, resulting in sidewall fracture morphology.To solve this problem, it is necessary to increase the bombardment and improve the etching rate of the latter material so that the etching rates of the two materials are close and a smooth sidewall morphology can be obtained.Figure 3 shows the SEM images of DBR samples etched using different etching gases (100sccm CF 4 +10sccm Ar/O 2 /BCl 3 ) in ICP etching.During the experiment, the ICP power was 1200 W, the RF power was 500 W, and the pressure was 5 mT.The gases used were 100sccm CF 4 +10sccm Ar, 100sccm CF 4 +10sccm O 2 , and 100sccm CF 4 +10sccm BCl 3 .It can be seen from Figure 3 and Table 1 that the addition of O 2 resulted in a significant increase in the photoresist (PR) etching rate, while the etching rate of DBR was comparable to that of Ar, leading to a decrease in the DBR/PR selectivity.On the other hand, when BCl 3 was added, the etching rate of DBR was enhanced, and the DBR/PR selectivity was significantly increased relative to Ar.Therefore, by controlling the type and flow rate of the secondary etching gases, it is possible to adjust the sidewall angle and meet the requirements for subsequent metal coverage.
Table 1.Etching results under different gas types.

The effect of ICP etching parameters on bottom metal etching damage
As the DBR etching depth increases, the metal layer gradually becomes exposed, which can cause etching defects caused by the bottom metal.Due to the thin metal layer, when the DBR/metal selectivity is insufficient, over-etching of the metal can occur, leading to metal spattering morphology (as shown in Figure 4).To avoid damage to the bottom metal, ICP etching parameters should be optimized to improve the DBR/metal selectivity.As shown in Figure 5 and Table 2, with an ICP power of 700 W, a pressure of 5 mT, and 100CF4+10Ar gas, the etching rates of DBR and metal under different RF powers were compared.As the RF power decreased, both the DBR and metal etching rates showed a decreasing trend, with the metal etch rate decreasing faster and the DBR/metal selectivity increasing.To obtain a smooth sidewall morphology and reduce over-etching of the bottom metal, a stepwise etching process can be used.The main etching uses high RF power to make the etching rates of SiO 2 and TiO 2 materials close to each other, obtain a smooth sidewall morphology, and adjust the sidewall angle by using different etching gases and flow rates.The over-etching uses low RF power to improve the DBR/metal selectivity and reduce the etching damage to the bottom metal.This stepwise etching process can achieve a smooth sidewall and bottom metal without over-etching, as shown in Figure 6.
(a) Main etching; (b) Main etching + over etching Figure 6.SEM images of main etching and over-etching.

Figure 1 .
Figure 1.Schematic of DBR application in inverted LED.

Figure 2 .
Figure 2. SEM images under different RF powers and pressures.

Figure 3 .
Figure 3. SEM images under different gas types.

Table 2 .
DBR and metal etching rates under different RF powers.
Figure 5. DBR and metal etching rates under different RF powers.