Recently, Professor sunlitao of Southeast University, in cooperation with Professor zhenghaimei of Lawrence Berkeley National Laboratory and Professor fanghaiping of East China University of technology, combined with experiments and molecular simulation, revealed the complete solid-liquid-gas three-phase reaction mechanism in the etching process from the atomic scale for the first time.
On May 26, 2022, relevant research was published on nature materials under the title”solid – liquid – gas reaction accelerated by gas molecular tunneling like effect”. The research team also presented the 120th anniversary of Southeast University with this article.
The corresponding authors of this work are professor sunlitao of Southeast University, Professor zhenghaimei of Lawrence Berkeley National Laboratory and Professor fanghaiping of East China University of technology. Dr. WangWen (now working in Zhengzhou University), associate researcher xutao and associate researcher chenjige are the co first authors.
Based on the in-situ electron microscopy system, the research team observed the whole process of nano bubble accelerated (~20 times) wet etching in real time, and revealed the complete solid-liquid-gas three-phase reaction mechanism in the etching process for the first time from the atomic scale, providing a new implementation means and manufacturing principle for the development of efficient and high-precision manufacturing processes and methods.
Wet etching is widely used in semiconductor manufacturing and other important fields, but the direction selectivity of wet etching is limited, so it is difficult to obtain micro nano structures with accurate and controllable size. Micro nano scale solid liquid gas reaction is the basic physical and chemical process in integrated circuit manufacturing, and also involves the key processes such as cleaning and polishing in transistor processing. At present, 7Nm, 5nm and other advanced transistor devices have sub nanometer precision requirements for the geometric dimensions of internal metal, semiconductor and dielectric layers. Limited by characterization means, the above process development can only be characterized by off-line detection means. The research results have a basic supporting role in establishing the process parameter structure size model and accelerating the process research and development.
The solid-liquid-gas three-phase reaction involved in this study exists widely in nature and industry. Besides dehumidification etching, there are also atmospheric corrosion, biological aerobic respiration, photocatalysis, fuel cell, etc. Because it is very difficult to track the evolution of single particle and three-phase interface at the nano scale, there has been a lack of quantitative analysis of reaction kinetics and accurate understanding of gas transport mechanism at the three-phase interface. Professor sunlitao’s team used the electron beam to irradiate water to generate oxygen bubbles, and constructed and observed in real time the solid liquid gas three-phase reaction of oxygen bubbles etching gold nanorods in hydrogen bromide aqueous solution (as shown in Figure 1).
Figure 1:schematic diagram of solid liquid gas reaction established in the liquid pool.
It was found that when there were no nano bubbles around the gold nanorods, the gold nanorods were gradually oxidized and etched into ellipsoids with smooth surface and finally disappeared; However, when there are nano bubbles around the gold nanorods, the nanorods near the nano bubbles will be accelerated to etch and evolve into local concave structures. It is worth noting that when local depression occurs, the nanorods and nanobubbles are not in direct contact, and there is an ultra-thin liquid film between them (as shown in Figure 2). Quantitative analysis of a large number of experimental results shows that only when the distance between the nano bubble and the solid is less than the critical size (~1 nm), the etching rate increases significantly (more than one order of magnitude); Otherwise, the etching rate is almost constant. The discovery that there is a critical distance for nano bubbles to participate in the etching reaction has overturned the traditional understanding that the closer the bubble is to the solid reactant, the faster the reaction is.
Figure 2:etching process of gold nanorods in the presence of oxygen nano bubbles.
Figure 3:etching process when there are oxygen nano bubbles on the top of the nanorods.
Professor fanghaiping of East China University of science and technology and chenjige, associate researcher of Shanghai Institute of higher studies, Chinese Academy of Sciences, etc., used classical molecular dynamics and first principles molecular dynamics simulation to point out that the existence of nano bubbles did not affect the adsorption position of bromine ions on the surface of gold nanorods, and the adsorption of oxygen molecules released from nano bubbles on the surface of gold nanorods was the key to accelerating the reaction. When the thickness of the liquid layer between the nano bubble and the gold nanorod surface is greater than ~1 nm, the oxygen molecules released by the nano bubble pass through the liquid layer through the concentration gradient dominated diffusion to reach the gold nanorod surface, and the process speed is slow. However, when the thickness of the liquid layer between the nano bubble and the surface of the gold nanorods is reduced to less than ~1 nm, the transport process of oxygen molecules has a”like penetration” effect. Oxygen molecules pass through the liquid layer at a very high speed and are adsorbed to the surface of the gold nanorods, which greatly accelerates the etching reaction.
This study reveals for the first time a complete solid liquid gas reaction path at the atomic scale:(1) when the liquid layer thickness is greater than the critical value, oxygen molecules undergo concentration gradient dominated diffusion in the liquid layer; (2) When the liquid layer thickness is less than the critical value, oxygen molecules are rapidly adsorbed on the solid surface under the action of van der Waals force; (3) Oxygen molecules participate in chemical reactions on the solid surface (as shown in Figure 4). This achievement makes it possible for the wet etching technology to greatly improve the controllability of etching direction and size, and it is also very likely to develop into a new technology in the future micro nano processing field. In addition, researchers have proposed several methods suitable for different scenarios to enhance the three-phase reaction, which is of great significance for the future regulation of micro nano processing and heterogeneous catalysis involving solid liquid gas three-phase.
Figure 4:solid liquid gas etching mechanism of gold nanorods.
In order to verify the universality of this mechanism, Professor sunlitao’s team also studied the etching of palladium nano cubes by oxygen bubbles in hydrogen bromide aqueous solution, and reached consistent conclusions.
This work has been supported by the national fund for Distinguished Young Scholars, the national special project for the development of major scientific research instruments and equipment, the international cooperation project of the National Natural Science Foundation, the general project of the National Natural Science Foundation, the Shanghai Natural Science Foundation and other projects.
Relevant paper information:https://doi.org/10.1038/s41563-022-01261-x