How to clean the mask?

In the process of countless exposures and transfers, the mask will inevitably encounter pollution, and its cleanliness and integrity are directly related to the quality and performance of the final product. 

  First, the importance of mask 

 As a key tool of graphic transfer, mask is self-evident. It is not only a bridge connecting design and manufacturing, but also the core to ensure the accurate control of the feature size of semiconductor devices. In the process of chip manufacturing, the patterns on the mask are accurately copied to the surface of the silicon wafer through the exposure process to form the required circuit patterns. Therefore, any tiny defects of the mask, such as pollution, scratches or defects, may have a far-reaching impact on the performance of the final product, and even lead to the scrapping of the whole batch of products. 

Second, the source and influence of mask pollution Mask plates may suffer from various forms of pollution during use, including but not limited to airborne particles, organic residues, metal ions, moisture and chemical reagent residues. These pollutants will not only affect the transparency and graphic accuracy of the mask, but also cause defects in the lithography process, such as line width change, graphic distortion or loss. Especially oil pollution, because of its strong viscosity, it is difficult to completely remove, which often becomes a major problem in mask cleaning. 

Third, the traditional cleaning method-potassium hydroxide solution For a long time, potassium hydroxide (KOH) solution has been widely used in the preliminary cleaning of mask plates because of its good solubility in most organic substances. KOH solution can effectively decompose and remove organic pollutants on the mask surface through its strong alkaline environment. However, as mentioned at the beginning of this paper, this method is inadequate in the face of oil pollution and needs repeated cleaning, which not only increases the cleaning cost, but also may cause mechanical damage to the mask, such as increased surface roughness and blurred graphics edges. 

Fourth, the new cleaning technology-tetramethylammonium hydroxide solution In order to overcome the limitation of KOH solution, the industry began to explore a more efficient and gentle cleaning scheme. Among them, tetramethylammonium hydroxide (TMAH) solution stands out for its excellent solubility and low corrosiveness. TMAH solution shows excellent cleaning effect on oil pollution, which can more thoroughly remove stubborn stains on the mask surface and reduce physical damage to the mask. However, the high cost is a big obstacle to its popularization and application. Therefore, how to reduce the cost while ensuring the cleaning effect has become one of the hot spots in current research.

  V. Clean room In addition to efficient cleaning technology, the importance of clean room as a key environment for mask storage and use can not be ignored. The clean room provides a dust-free and particle-free working space for the mask plate through strict air filtration system, temperature and humidity control, electrostatic protection and other measures. This not only reduces the damage of external pollutants to the mask, but also improves the cleaning efficiency and quality.

 In the clean room, the operator should strictly abide by the operating rules and handle the mask lightly to avoid any form of physical damage. With the continuous progress of semiconductor technology, mask cleaning technology is also moving towards intelligence and automation. By integrating sensor, machine vision and big data analysis technology, the intelligent cleaning system can monitor various parameters in the cleaning process in real time, such as solution concentration, temperature, cleaning time, etc., and automatically adjust the cleaning strategy according to the feedback results to achieve accurate cleaning. At the same time, the on-line monitoring technology can evaluate the cleaning effect immediately, avoiding the inefficient cycle of repeated cleaning and testing in traditional methods, and greatly improving the production efficiency and product quality. Although the existing cleaning technology meets the requirements of semiconductor manufacturing to a certain extent, it still faces some problems. First of all, with the continuous shrinking of chip feature size, the accuracy requirements of mask are getting higher and higher, which puts forward higher requirements for the accuracy and stability of cleaning technology. Secondly, masks made of different materials (such as Shi Ying and metal) have different adaptability to cleaning agents, so it is necessary to develop more compatible cleaning schemes.