In this blog post, we’ll examine the causes of pattern collapse—a phenomenon that occurs as semiconductor patterns become increasingly refined—as well as the principles and advantages of supercritical drying technology for preventing it.
With the rapid and continuous advancement of science and technology in recent years, we have entered the “Smart Era”—a time when, without even realizing it, we can’t put down various IT products such as smartphones, digital cameras, laptops, and tablets. In the 21st century, which has evolved into a digital age, the development of semiconductors—which play a central role in the IT industry—has become a matter of great interest not only to scientists, companies, and policymakers but also to many people around the world. Semiconductors are now ubiquitous in our daily lives, and the technology continues to advance through ongoing research by companies and researchers. In this article, I will explain one such technology: supercritical drying. In addition, I will examine the basic knowledge of semiconductor technology necessary to understand this technique, explore the causes of pattern collapse, and compare the differences between supercritical drying and conventional drying methods. Through this, we aim to explain why supercritical drying technology is crucial not only for semiconductor manufacturing but also for modern society, which relies on various electronic devices, including smartphones.
In semiconductor manufacturing, wafers made of silicon serve as the basic substrate. Through various processes applied to these wafers, microscopic structures such as transistors and interconnects are formed to create integrated circuits (ICs). The electrical properties of semiconductors vary depending on their materials and structure, and these properties can be controlled through processes such as doping, which involves adding specific impurities to silicon. In particular, as the line widths of integrated circuits have been miniaturized to the tens-of-nanometers level, the mechanical stability of the patterns that make up the circuits has become increasingly important. Because semiconductor circuit structures are so fine, it is crucial to maintain the stability of these patterns throughout the manufacturing process. To understand the fine patterns formed on a wafer, it is necessary to examine the concept of aspect ratio. The aspect ratio is generally defined as the height of a pattern divided by its width; the greater the height relative to the width, the larger the aspect ratio.
Today, users of electronic devices such as smartphones demand higher-resolution photos and videos, as well as high-capacity memory capable of storing more applications and data. To produce smartphones with higher-resolution cameras and a wide range of features, memory semiconductors capable of storing larger amounts of data are required. To achieve this, semiconductor companies must create patterns—composed of silicon and various other materials—that are smaller and more precise in order to integrate more devices and memory cells onto a limited wafer area. However, as the line width of the patterns decreases and their width becomes narrower relative to their height, the patterns become more susceptible to “pattern collapse”—a phenomenon where the patterns collapse due to external forces. If pattern collapse occurs, it becomes difficult to form the desired integrated circuit, and it can also affect semiconductor manufacturing yield. Therefore, when manufacturing integrated circuits, it is crucial to maintain the stability of fine patterns to prevent them from collapsing. To create more patterns on a single wafer, the pattern width must be reduced while ensuring the necessary structural height; however, as the aspect ratio increases during this process, the likelihood of pattern collapse during the cleaning and drying stages also increases. For example, while a pattern with a width of 67 nm and an aspect ratio of 4.18 remains relatively stable, pattern collapse may occur if the width is 64 nm and the aspect ratio increases to 4.38, or if the width is 60 nm and the aspect ratio rises to 4.67. Thus, while pattern miniaturization and high aspect ratios are advantageous for integrating more devices, they also present a new challenge: ensuring the mechanical stability of the patterns during the manufacturing process.
Pattern collapse is particularly likely to occur during the drying process following wet cleaning of the wafer. During the wet cleaning process, which uses water, rinse solution remains trapped between the fine patterns; as this liquid evaporates during drying, surface tension and capillary forces exert forces on adjacent patterns. If the mechanical strength of the fine patterns is insufficient to withstand these forces, the patterns may tilt, stick together, or collapse. Simply put, pattern collapse occurs when forces generated during the evaporation of water—left between the patterns after water-based cleaning—pull on the extremely fine patterns. In particular, nanoscale patterns formed on silicon wafers are often much taller than they are wide, making them even more vulnerable to this phenomenon. Therefore, preventing pattern collapse is crucial when cleaning and drying microstructures in semiconductor manufacturing processes. Semiconductor companies and researchers have conducted various studies to improve cleaning and drying processes in order to stably maintain the microstructures formed on wafers. One method to address this issue is supercritical drying technology. Supercritical drying technology has been studied as a key technique for fabricating microstructures, as it can reduce pattern collapse caused by the surface tension of liquids during the drying process.
A supercritical fluid is a fluid in which a substance has exceeded both its critical temperature and critical pressure. In this state, the distinction between a liquid and a gas disappears, allowing the properties of both to be utilized simultaneously. Supercritical carbon dioxide (supercritical CO₂) has been the focus of research for drying semiconductor micro-patterns. Carbon dioxide has relatively low critical temperatures and pressures, making it easy to induce a supercritical state; in this state, surface tension virtually disappears, and it exhibits diffusion characteristics similar to those of a gas. Therefore, it can penetrate into fine, narrow structures and be used to replace conventional rinse solutions or organic solvents. Subsequently, removing the fluid while maintaining the supercritical state significantly reduces the influence of surface tension that occurs during the conventional evaporation process, which involves the transition between liquid and gas phases. As a result, this helps prevent pattern collapse by reducing the phenomenon where liquid remaining between fine patterns pulls on the patterns as it evaporates. Previous studies have explored the potential applications of processes using supercritical carbon dioxide in various areas of semiconductor and nano-device manufacturing, including photoresist pattern drying, wafer cleaning, etch residue removal, and particle removal. In particular, research has shown that, for fine patterns with high aspect ratios, the supercritical drying method is more advantageous than conventional nitrogen drying in enhancing the structural stability of the patterns. Therefore, drying technology using supercritical fluids is being researched as a promising solution for micro-semiconductor processes where pattern collapse due to surface tension is a significant issue.
In typical semiconductor drying processes, various methods have been used, including drying with nitrogen gas, spin drying, drying with isopropyl alcohol (IPA), and Marangoni drying. While these methods can effectively remove liquid from the wafer surface depending on process conditions, pattern collapse caused by surface tension and capillary forces during the drying process can become a more significant issue as pattern sizes decrease and aspect ratios increase. In contrast, supercritical drying technology uses supercritical carbon dioxide to displace liquid or solvent remaining between microstructures, thereby avoiding the conventional evaporation process in which a liquid-vapor interface forms. Actual research has confirmed that while high-aspect-ratio silicon or resist patterns may collapse when dried using nitrogen gas, drying with supercritical carbon dioxide can significantly reduce pattern collapse. However, supercritical drying technology does not universally replace conventional drying methods in all semiconductor manufacturing processes; the appropriate drying method may vary depending on the type of material, pattern structure, and process conditions. Therefore, the advantage of supercritical drying technology should be understood specifically in terms of its ability to reduce damage caused by surface tension, particularly in fine structures with high aspect ratios.
In conclusion, supercritical drying technology plays a crucial role in reliably maintaining the fine patterns formed on wafers during the semiconductor manufacturing process and in reducing structural damage caused by pattern collapse. As semiconductor miniaturization continues, technologies to reduce pattern width and increase aspect ratio are necessary to integrate more devices and memory cells onto a single wafer; however, technologies to prevent these fine patterns from collapsing during cleaning and drying processes are also becoming increasingly important. To prevent pattern collapse, factors such as pattern width, height, aspect ratio, the mechanical properties of the material, and cleaning and drying conditions must be comprehensively considered. Among these, supercritical drying technology offers the advantage of effectively removing liquid from between microstructures and reducing the impact of capillary forces generated during the drying process by utilizing the low surface tension and high diffusivity of supercritical carbon dioxide. In particular, as semiconductor structures continue to shrink and nanostructures with high aspect ratios become increasingly important, research on drying and cleaning technologies using supercritical carbon dioxide is ongoing. In short, supercritical drying technology is one of the key process technologies that supports the advancement of semiconductor technology—which requires the integration of smaller and more numerous devices onto a single wafer—and it is a technology directly linked to the improved performance of various electronic devices we use, such as smartphones and computers.
