Semiconductor Wet Cleaning Process

Abstract: As the size of transistors continues to shrink, the wafer manufacturing process becomes increasingly complex, and the requirements for semiconductor wet cleaning technology are becoming higher and higher. Based on traditional semiconductor cleaning technology, this paper introduces the wafer cleaning technology in advanced semiconductor manufacturing and the cleaning principles of various cleaning processes. From the perspective of economy and environmental protection, improving wafer cleaning process technology can better meet the needs of advanced wafer manufacturing.

0 Introduction The cleaning process is an important link throughout the entire semiconductor manufacturing process and is one of the important factors affecting the performance and yield of semiconductor devices. In the chip manufacturing process, any contamination may affect the performance of semiconductor devices and even cause failure [1-2]. Therefore, a cleaning process is required before and after almost every process in chip manufacturing to remove surface contaminants and ensure the cleanliness of the wafer surface, as shown in Figure 1. The cleaning process is the process with the highest proportion in the chip manufacturing process, accounting for about 30% of all chip manufacturing processes.

With the development of ultra-large-scale integrated circuits, chip process nodes have entered 28nm, 14nm and even more advanced nodes, the integration has continued to increase, the line width has continued to decrease, and the process flow has become more complex [3]. Advanced node chip manufacturing is more sensitive to contamination, and contamination cleaning under small size conditions is more difficult, which leads to an increase in cleaning process steps, making the cleaning process more complex, more important and more challenging [4-5]. The cleaning process for 90nm chips is about 90 steps, and the cleaning process for 20nm chips has reached 215 steps. As chip manufacturing enters 14nm, 10nm and even higher nodes, the number of cleaning processes will continue to increase, as shown in Figure 2.

1 Introduction to semiconductor cleaning process

Cleaning process refers to the process of removing impurities on the surface of the wafer through chemical treatment, gas and physical methods. In the semiconductor manufacturing process, impurities such as particles, metals, organic matter, and natural oxide layer on the surface of the wafer may affect the performance, reliability and even yield of the semiconductor device [6-8].

The cleaning process can be said to be a bridge between the various wafer manufacturing processes. For example, the cleaning process is used before the coating process, before the photolithography process, after the etching process, after the mechanical grinding process, and even after the ion implantation process. The cleaning process can be roughly divided into two types, namely wet cleaning and dry cleaning.

1.1 Wet cleaning

Wet cleaning is to use chemical solvents or deionized water to clean the wafer. According to the process method, wet cleaning can be divided into two types: immersion method and spraying method, as shown in Figure 3. The immersion method is to immerse the wafer in a container tank filled with chemical solvents or deionized water. The immersion method is a widely used method, especially for some relatively mature nodes. The spraying method is to spray chemical solvents or deionized water onto the rotating wafer to remove impurities. The immersion method can process multiple wafers at the same time, while the spraying method can only process one wafer at a time in one operating chamber. With the development of technology, the requirements for cleaning technology are getting higher and higher, and the use of the spraying method is becoming more and more widespread.

1.2 Dry cleaning

As the name implies, dry cleaning is a process that does not use chemical solvents or deionized water, but uses gas or plasma for cleaning. With the continuous advancement of technology nodes, the requirements for cleaning processes are getting higher and higher [9-10], and the proportion of use is also increasing. The waste liquid generated by wet cleaning is also increasing. Compared with wet cleaning, dry cleaning has high investment costs, complex equipment operation, and more stringent cleaning conditions. However, for the removal of some organic matter and nitrides and oxides, dry cleaning has higher precision and excellent results.

2 Wet cleaning technology in semiconductor manufacturing According to the different components of the cleaning liquid, the commonly used wet cleaning technology in semiconductor manufacturing is shown in Table 1.

2.1 DIW cleaning technology

In the wet cleaning process of semiconductor manufacturing, the most commonly used cleaning liquid is deionized water (DIW). Water contains conductive anions and cations. Deionized water removes the conductive ions in water, making the water basically non-conductive. In semiconductor manufacturing, it is absolutely not allowed to use raw water directly. On the one hand, the cations and ions in the raw water will contaminate the device structure of the wafer, and on the other hand, it may cause the performance of the device to deviate. For example, the raw water may react with the material on the surface of the wafer to corrode, or form battery corrosion with some metals on the wafer, and may also cause a direct change in the surface resistivity of the wafer, resulting in a significant decrease in the yield of the wafer or even direct scrapping. In the wet cleaning process of semiconductor manufacturing, there are two main applications of DIW.

(1) Only use DIW to clean the wafer surface. There are different forms such as rollers, brushes or nozzles, and the main purpose is to clean some impurities on the wafer surface. In the advanced semiconductor manufacturing process, the cleaning method is almost always a single wafer method, that is, only one wafer can be cleaned in a chamber at the same time. The method of cleaning a single wafer is also introduced above. The cleaning method used is the spin spray method. During the rotation of the wafer, the surface of the wafer is cleaned by rollers, brushes, nozzles, etc. In this process, the wafer will rub against the air, thereby generating static electricity. Static electricity may cause defects on the wafer surface or directly cause device failure. The higher the semiconductor technology node, the higher the requirements for handling defects. Therefore, in the DIW wet cleaning process of advanced semiconductor manufacturing, its process requirements are higher. DIW is basically non-conductive, and the static electricity generated during the cleaning process cannot be well released. Therefore, in advanced semiconductor manufacturing process nodes, in order to increase conductivity without contaminating the wafer, carbon dioxide gas (CO2) is usually mixed into DIW. Due to different process requirements, ammonia gas (NH3) is mixed into DIW in a few cases.

(2) Clean the residual cleaning liquid on the wafer surface. When using other cleaning liquids to clean the wafer surface, after the cleaning liquid is used, as the wafer rotates, although most of the cleaning liquid has been thrown out, there will still be a small amount of cleaning liquid remaining on the wafer surface, and DIW is needed to clean the wafer surface. The main function of DIW here is to clean the residual cleaning liquid on the wafer surface. Using cleaning liquid to clean the wafer surface does not mean that these cleaning liquids will never corrode the wafer, but their etching rate is quite low, and short-term cleaning will not affect the wafer. However, if the residual cleaning liquid cannot be effectively removed and the residual cleaning liquid is allowed to stay on the wafer surface for a long time, it will still corrode the wafer surface. In addition, even if the cleaning solution corrodes very little, the residual cleaning solution in the wafer is still redundant, which is likely to affect the final performance of the device. Therefore, after cleaning the wafer with the cleaning solution, be sure to use DIW to clean the residual cleaning solution in time.

2.2 HF cleaning technology

As we all know, sand is refined into a core. The chip is formed by countless carvings on a single crystal silicon wafer. The main component on the chip is single crystal silicon. The most direct and effective way to clean the natural oxide layer (SiO2) formed on the surface of single crystal silicon is to use HF (hydrofluoric acid) to clean. Therefore, it can be said that HF ​​cleaning is the cleaning technology second only to DIW. HF cleaning can effectively remove the natural oxide layer on the surface of single crystal silicon, and the metal attached to the surface of the natural oxide layer will also dissolve into the cleaning solution. At the same time, HF can also effectively inhibit the formation of natural oxide film. Therefore, HF cleaning technology can remove some metal ions, natural oxide layer and some impurity particles. However, HF cleaning technology also has some unavoidable problems. For example, while removing the natural oxide layer on the surface of the silicon wafer, some small pits will be left on the surface of the silicon wafer after being corroded, which directly affects the roughness of the wafer surface. In addition, while removing the surface oxide film, HF will also remove some metals, but some metals do not want to be corroded by HF. With the continuous advancement of semiconductor technology nodes, the requirements for these metals not to be corroded by HF are getting higher and higher, resulting in the HF cleaning technology being unable to be used in places where it could have been used. At the same time, some metals that enter the cleaning solution and adhere to the surface of the silicon wafer as the natural oxide film dissolves are not easily removed by HF, resulting in them remaining on the surface of the silicon wafer. In response to the above problems, some improved methods have been proposed. For example, dilute HF as much as possible to reduce the concentration of HF; add oxidant to HF, this method can effectively remove the metal attached to the surface of the natural oxide layer, and the oxidant will oxidize the metal on the surface to form oxides, which are easier to remove under acidic conditions. At the same time, HF will remove the previous natural oxide layer, and the oxidant will oxidize the single crystal silicon on the surface to form a new oxide layer to prevent the metal from attaching to the surface of the single crystal silicon; add anionic surfactant to HF, so that the surface of the single crystal silicon in the HF cleaning solution is negative potential, and the surface of the particle is positive potential. Adding anionic surfactant can make the potential of the silicon surface and the particle surface have the same sign, that is, the surface potential of the particle changes from positive to negative, which is the same sign as the negative potential of the silicon wafer surface, so that the electrical repulsion is generated between the silicon wafer surface and the particle surface, thereby preventing the attachment of particles; add complexing agent to the HF cleaning solution to form a complex with impurities, which is directly dissolved in the cleaning solution and will not attach to the silicon wafer surface.

2.3 SC1 cleaning technology

SC1 cleaning technology is the most common, low-cost and high-efficiency cleaning method for removing contamination from the wafer surface. SC1 cleaning technology can remove organic matter, some metal ions and some surface particles at the same time. The principle of SC1 to remove organic matter is to use the oxidizing effect of hydrogen peroxide and the dissolving effect of NH4OH to turn organic contamination into water-soluble compounds, and then discharge them with the solution. Due to its oxidizing and complexing properties, SC1 solution can oxidize some metal ions, turning these metal ions into high-valent ions, and then further react with alkali to form soluble complexes that are discharged with the solution. However, some metals have high free energy of oxides generated after oxidation, which are easy to adhere to the oxide film on the wafer surface (because SC1 solution has certain oxidizing properties and will form an oxide film on the wafer surface), so they are not easy to be removed, such as metals such as Al and Fe. When removing metal ions, the rate of metal adsorption and desorption on the wafer surface will eventually reach a balance. Therefore, in advanced manufacturing processes, the cleaning fluid is used once for processes that have high requirements for metal ions. It is directly discharged after use and will not be used again. The purpose is to reduce the metal content in the cleaning fluid to wash away the metal on the wafer surface as much as possible. SC1 cleaning technology can also effectively remove surface particle contamination, and the main mechanism is electrical repulsion. In this process, ultrasonic and megasonic cleaning can be carried out to obtain better cleaning effects. SC1 cleaning technology will have a significant impact on the surface roughness of the wafer. In order to reduce the impact of SC1 cleaning technology on the surface roughness of the wafer, it is necessary to formulate a suitable cleaning fluid component ratio. At the same time, the use of cleaning fluid with low surface tension can stabilize the particle removal rate, maintain a high removal efficiency, and reduce the impact on the surface roughness of the wafer. Adding surfactants to SC1 cleaning fluid can reduce the surface tension of the cleaning fluid. In addition, adding chelating agents to SC1 cleaning fluid can cause the metal in the cleaning fluid to continuously form chelates, which is beneficial to inhibit the surface adhesion of metals.

2.4 SC2 cleaning technology

SC2 cleaning technology is also a low-cost wet cleaning technology with good contamination removal ability. SC2 has extremely strong complexing properties and can react with metals before oxidation to form salts, which are removed with the cleaning solution. The soluble complexes formed by the reaction of oxidized metal ions with chloride ions will also be removed with the cleaning solution. It can be said that under the condition of not affecting the wafer, SC1 cleaning technology and SC2 cleaning technology complement each other. The metal adhesion phenomenon in the cleaning solution is easy to occur in alkaline cleaning solution (that is, SC1 cleaning solution), and it is not easy to occur in acidic solution (SC2 cleaning solution), and it has a strong ability to remove metals on the wafer surface. However, although metals such as Cu can be removed after SC1 cleaning, some metal adhesion problems of the natural oxide film formed on the wafer surface have not been solved, and it is not suitable for SC2 cleaning technology.

2.5 O3 cleaning technology

In the chip manufacturing process, O3 cleaning technology is mainly used to remove organic matter and disinfect DIW. O3 cleaning always involves oxidation. Generally speaking, O3 can be used to remove some organic matter, but because of the oxidation of O3, redeposition will occur on the wafer surface. Therefore, HF is generally used in the process of using O3. In addition, the process of using HF with O3 can also remove some metal ions. It should be noted that, in general, higher temperatures are beneficial for removing organic matter, particles and even metal ions. However, when using O3 cleaning technology, the amount of O3 dissolved in DIW will decrease as the temperature increases. In other words, the concentration of O3 dissolved in DIW will decrease as the temperature increases. Therefore, it is necessary to optimize the O3 process details to maximize the cleaning efficiency. In semiconductor manufacturing, O3 can also be used to disinfect DIW, mainly because the substances used to purify drinking water generally contain chlorine, which is unacceptable in the field of chip manufacturing. Another reason is that O3 will decompose into oxygen and will not pollute the DIW system. However, it is necessary to control the oxygen content in DIW, which cannot be higher than the requirements for use in semiconductor manufacturing. 2.6 Organic solvent cleaning technology In the semiconductor manufacturing process, some special processes are often involved. In many cases, the methods introduced above cannot be used because the cleaning efficiency is not enough, some components that cannot be washed away are etched, and oxide films cannot be generated. Therefore, some organic solvents are also used to achieve the purpose of cleaning.

3 Conclusion

In the semiconductor manufacturing process, the cleaning process is the process with the most repetitions. The use of appropriate cleaning technology can greatly improve the yield of chip manufacturing. With the large size of silicon wafers and the miniaturization of device structures, the stacking density index increases, and the requirements for wafer cleaning technology are getting higher and higher. There are more stringent requirements for the cleanliness of the wafer surface, the chemical state of the surface, the roughness and the thickness of the oxide film. Based on mature process technology, this article introduces the wafer cleaning technology in advanced wafer manufacturing and the cleaning principles of various cleaning processes. From the perspective of economy and environmental protection, improving wafer cleaning process technology can better meet the needs of advanced wafer manufacturing.