The surface quality of indium phosphide single wafer polishing has an important effect on the epitaxial and device performance

Source: Electronic process Technology
Indium phosphide crystal material has the advantages of high electric field drift speed, good thermal conductivity and strong radiation resistance, which is suitable for the manufacture of high frequency, high speed and low power microwave devices and circuits. In the field of optical fiber communication, indium phosphide is also the preferred communication material. The semi-insulated indium phosphide substrate is mainly used in optoelectronic integrated circuits (OEics), high electron mobility transistors (HEMTs) and heterojunction devices (HBT) [1]. These devices or circuits are usually achieved by growing thin films on the surface of indium phosphide polishing by molecular beam epitaxial (MBE) or metal-organic chemical vapor deposition (MOCVD). The surface roughness and lattice integrity of the polishing film directly affect the atomic arrangement of the film layer, requiring the surface of the polishing film to have very high lattice integrity and low roughness. In order to meet the needs of device performance and epitaxial technology, the surface roughness of the polishing is generally required to be less than 0.5nm.
Indium phosphide and gallium arsenide belong to the same Ⅲ-Ⅴ group of compounds, but their properties are very different. At present, the polishing process of gallium arsenide has been very mature, but the research on indium phosphide materials mostly focuses on crystal growth and surface element analysis [2-8], and there are almost no reports on polishing and cleaning. This shows that the polishing and cleaning technology of indium phosphide single wafer is still in the secret stage. With the same polishing process, the coarse polishing rate of gallium arsenide wafers can reach (0.7 ~ 1.0) μm/min, and the polished surface can reach the "clean free" level. However, the coarse polishing rate of indium phosphide wafer is only 0.10m /min ~ 0.15m /min, and the surface condition of polished indium phosphide wafer is not up to the level of gallium arsenide coarse polishing wafer, and there are many scratches. Therefore, it is of great significance to study the polishing process of indium phosphide single wafer.
Indium phosphide polishing is usually double-sided and requires high surface quality and consistency on both surfaces of the polishing. A double-sided polishing machine is adopted. The star wheel drives the wafer to operate in planetary mode between the footwall of the polishing machine. The upper and lower surfaces of the wafer can be polished at the same time. In this paper, the polishing mechanism of indium phosphide single wafer is analyzed, and the SPEEDFAM series double-sided polishing machine is used to carry out polishing process experiments. By comparing the surface roughness of the wafer after different polishing processes, it is found that oxidant has an important effect on the polishing rate, and the three-step polishing process is conducive to improving the surface quality of the polished wafer. The surface structure of the polished cloth has an important effect on the surface quality of the polished surface. By re-designing the surface groove structure of the polished cloth, high quality indium phosphide double-sided polishing with surface roughness < 0.5nm was fabricated.
1. Analysis of polishing mechanism
The polishing of indium phosphide wafer is a process combining chemistry and machinery. During the polishing process, the surface of wafer reacts with the oxidant in the polishing liquid to produce soluble salt, and the SiO2 colloids in the polishing liquid and the rapidly rotating polishing cloth produce mechanical friction with the surface of wafer, wiping the surface reactants into the flowing polishing liquid and then being discharged. The newly exposed surface continues to interact with the polishing fluid. The chemical mechanical action is carried out in an alternating cycle to remove the micro-defects and damage layers left by the previous process on the surface, so as to obtain a bright "mirror" with very low local surface smoothness and surface roughness to meet the needs of the subsequent process.
1.1 Influence of mechanical action
The mechanical action in the polishing process comes from the friction between the surface of the wafer and the surface of the polishing cloth and the silicone particles in the polishing liquid, which is related to the polishing pressure, the rotation speed of the polishing disc, the properties of the polishing cloth and the polishing liquid. It is generally believed that the mechanical grinding rate is determined by Formula (1) [9], from which it can be seen that, for the same kind of wafer, the mechanical removal rate is directly proportional to the polishing pressure and the hardness of the polishing cloth, and inversely proportional to the hardness of the wafer itself, under the condition that the polishing liquid and the polishing speed are fixed.
Where, HV p is the hardness of the polishing cloth; HV w is the hardness of the wafer; E s is the elastic coefficient of particles in polishing liquid; E w is the elastic coefficient of the wafer; P loc is the pressure in a local area; V rel is relative speed; C is a constant.
Indium phosphide material is soft, and its hardness is obviously lower than that of silicon, germanium and gallium arsenide materials, as shown in Table 1. In the polishing process, the surface of indium phosphide wafer is more prone to mechanical damage and scratches, so the polishing cloth with less hardness should be selected. In order to improve the removal rate of polishing and ensure the change of the total thickness of the polishing sheet, we should choose a polishing cloth with greater hardness and less compression. Therefore, we must greatly improve the chemical action of polishing process to ensure the balance of chemical mechanical action.
Table 1 Properties of semiconductor materials
Material name silicon germanium gallium arsenide indium phosphide
Kirschner hardness HK 1 150.0 780.0 750.0 537.0
1.2 Effects of chemical action
The chemical action in the polishing process is mainly the REDOX reaction between the oxidant and indium phosphide in the polishing liquid and the dissolution of the reaction product in the polishing liquid. The chemical reaction rate is greatly affected by the oxidant concentration and reaction temperature. As a semiconducting material of group Ⅲ-Ⅴ compounds, compared with gallium arsenide, indium phosphide has a special property. At room temperature, indium phosphide can react with hydrochloric acid, react very slowly with alkali solution [10], react with hydrogen peroxide, sodium hypochlorite and other oxidants, but the reaction coefficient is very low. Taking hypochlorite as an example, the reaction mechanism is as follows
(2) ~ Equation (4) is shown [11].
When hydrogen peroxide and sodium hypochlorite are used as oxidants to polish indium phosphide single crystal, the polishing rate can only reach 0.10μm /min ~ 0.15μm /min, and the surface fog is serious after polishing, which is easy to appear scratches. Increasing the polishing temperature and oxidant concentration has little effect on the polishing rate. It can be seen that since the reaction coefficient between indium phosphide and the oxidizer is very small, it is difficult to match the chemical action with the mechanical action. The polishing rate mainly depends on the strength of the chemical action in the polishing process. To improve the chemical action in the polishing process, oxidant with stronger oxidation must be used.
1.3 Selection of polishing fluid
Polishing fluid is an important part of the whole polishing process, which directly affects the polishing speed and surface effect. Indium phosphide, which reacts with hydrochloric acid, is superficially more suitable for acid polishing. Hydrochloric acid has a strong volatile and irritating smell, the use of hydrochloric acid solution polishing, process operability and safety is very poor, and dilute acid polishing rate is very low, acidic polishing liquid types are very few, when the configuration of fine polishing and finishing liquid, with the reduction of solution acidity, silica sol in polishing liquid is very easy to gelatinate, polishing process is difficult to control. After polishing, the surface effect is poor and there are more lines. The use of alkaline silica sol polishing liquid, the polishing process is relatively stable, and can learn from the polishing experience of silicon, germanium and gallium arsenide materials, the only deficiency is that we need to solve the problem of indium phosphide and hydrogen peroxide, sodium hypochlorite and other conventional oxidants reaction rate is too slow, choose a more suitable oxidant.
Based on the above analysis, we decided to use alkaline silica sol polishing solution to polish indium phosphide single crystal sheet to ensure the stability of the polishing process. Perchlorate with better solubility was selected as oxidant to improve the chemical action in polishing process and ensure the matching of chemical mechanical action. The polishing process adopts three steps of rough polishing, fine polishing and fine polishing, and obtains high quality polished surface by gradually reducing the chemical mechanical action on the surface of the wafer. In the rough polishing stage, hard polishing cloth, oxidant and silicon dioxide content of polishing liquid are used to ensure a high polishing rate and small change in the total thickness; In the fine polishing stage, soft polishing cloth is used to reduce the content of oxidant and silica in the polishing liquid, which further reduces the local flatness and surface roughness of the polishing sheet. In the polishing stage, softer polishing cloth is used to further reduce the oxidant and silica content in the polishing liquid, and the fog defect on the surface of the wafer is removed to ensure that the wafer has a very high surface nano-morphology.
For indium phosphide single crystal polishing, it is necessary to use a stronger oxidizing agent. Perchlorate as an oxidizing agent can improve the polishing rate from 0.1μm /min ~ 0.15μm /min to 0.5μm /min ~ 0.6μm /min. The three-step polishing process is beneficial to improve the surface quality of the polished sheet. The surface condition of the polished cloth has an important effect on the surface roughness of indium phosphide single crystal polishing. Using the curved channel structure to finish polishing cloth can ensure the consistency of the top and bottom surface quality of the polishing sheet. We have fabricated indium phosphide single crystal double-sided polishing with surface roughness < 0.3nm by using a three-step polishing process and improved finishing cloth.