Dielectric and Metal Layer Fabrication in Wet Etching : Chemical Control from SiO₂ to Metal Interconnects

etching
is widely used not only for processing silicon materials themselves, but more importantly, for selectively patterning dielectric and metal thin films. Whether it's the gate dielectric, isolation layer, passivation layer, or the metal wiring in subsequent interconnect structures, the surface finish directly determines the electrical performance and long-term reliability of the device. Therefore, establishing wet etching systems with high selectivity, high uniformity, and low damage characteristics for different material systems has always been an important aspect of microelectronics process development.

Dielectric Material-Silicon Dioxide:
Among dielectric materials, silicon dioxide is one of the most typical and widely used insulating layer materials. Since SiO₂ can dissolve in hydrofluoric acid, HF becomes the core chemical system for silicon dioxide wet etching. The essential mechanism is that fluoride ions react with the silicon-oxygen bonds in the silicon dioxide network, ultimately forming soluble fluorosilicates, thereby achieving material removal. However, the pure hydrofluoric acid system is unstable in actual processes. As the reaction continues, the effective fluoride ion concentration in the solution gradually decreases, and the photoresist mask is also easily corroded. Therefore, ammonium fluoride is usually introduced in industry to form a buffer system, namely buffered oxide etchant (BOE) or buffered hydrofluoric acid (BHF).
The core function of the buffer system is to maintain a stable supply of fluoride ions in the etchant while reducing the impact of pH fluctuations on etching uniformity during the reaction. More importantly, the addition of NH₄F can effectively slow down the degradation rate of the photoresist in the etchant, allowing the photoresist to be used as a mask material for short-time silicon oxide etching, thereby improving pattern transfer capability. In thermal oxidation of SiO₂, the BHF system usually has a relatively stable etching rate. However, for PECVD or LPCVD deposited oxides, due to differences in internal structural density, hydrogen content, and defect density, their etching behavior is often more sensitive, and the etching rate is usually significantly higher than that of thermally oxidized layers. This difference actually reflects the influence of different silicon oxide formation mechanisms on the stability of the network structure and is also one of the key issues in process window control. Compared to silicon oxide, silicon nitride (SiN₄) is significantly more difficult to wet etch
as a dielectric material
. Si₃N₄ possesses higher chemical stability and stronger bond energy, resulting in a much lower reaction rate in the HF system compared to SiO₂. To improve etching efficiency, higher temperatures are typically required, but these high temperatures accelerate photoresist aging and desorption, making traditional photoresists unsuitable as effective mask materials. In practical processes, hot phosphoric acid systems are more commonly used for selective etching of SiN₄. High-temperature phosphoric acid effectively breaks the Si-N bonds in SiN₄ while maintaining a low etching rate for both SiO₂ and the silicon substrate, making it valuable for applications such as STI isolation structures, spacer layer formation, and MEMS sacrificial layer release.
It is noteworthy that wet etching of SiN₄ is extremely temperature-sensitive. The etching rate fluctuates significantly with changes in phosphoric acid temperature; therefore, modern process platforms typically employ isothermal circulation systems to maintain solution stability and ensure batch-to-batch consistency. Furthermore, as the size of advanced node devices continues to shrink, the stress release, interface roughening, and localized corrosion problems caused by traditional high-temperature phosphoric acid etching are gradually attracting attention. Therefore, low-temperature selective wet etching and atomic-level surface modification techniques are becoming new research directions. Besides dielectric layer processing, wet etching of
metal thin films
is also a key process step in semiconductor interconnect manufacturing. In early integrated circuits, aluminum was the mainstream interconnect material for a long time due to its excellent conductivity, low cost, and ease of deposition. Aluminum wet etching typically uses a mixed acid system containing phosphoric acid, nitric acid, and acetic acid. Phosphoric acid is used to dissolve the alumina layer, nitric acid provides oxidation, and acetic acid is mainly used to improve solution wettability and buffer the reaction rate. By adjusting the proportions of different components, a balance can be achieved between etching rate, edge morphology, and linewidth retention.

With the development of high-frequency devices, MEMS, and advanced packaging, functional metals such as gold, silver, and chromium have also been widely introduced into micro-nano fabrication systems. Among them, gold, due to its extremely high chemical stability, is difficult to directly etch using traditional acid systems, and therefore usually requires the use of aqua regia, a highly oxidizing solution, for dissolution. However, aqua regia has a strong destructive effect on photoresist. Therefore, in fine pattern processing, the iodine-potassium iodide system is preferred for low-damage etching. This system removes material by forming a soluble gold-iodine complex while preserving the integrity of the photoresist mask, making it highly valuable in microelectrodes, radio frequency devices, and biochip fabrication.
Wet etching of silver thin films relies more on the synergistic effect of oxidants and complexing agents. In a typical system, hydrogen peroxide provides oxidation, while ammonia promotes the dissolution process by forming silver-ammonia complex ions. As silver's application in microelectronic packaging and transparent conductive electrodes increases, the uniformity and residue control of its wet etching have received increasing attention in recent years. In contrast, chromium is more often used as an adhesion layer or mask material. Its etching system typically uses a combination of strong oxidizing cerium salts and perchloric acid, achieving material removal through an oxidation-complexation synergistic mechanism.
Currently, with the development of advanced interconnect technologies towards new metal systems such as copper, cobalt, and ruthenium, traditional wet etching processes are facing new challenges. On the one hand, novel metallic materials exhibit higher chemical stability, making it difficult for traditional acid-base systems to meet the demands of highly selective processing. On the other hand, nanoscale interconnect structures place higher demands on sidewall roughness, residual contamination, and electromigration reliability. Therefore, in recent years, selective wet etching, low-damage surface treatment, and atomic-layer wet etching have developed rapidly, gradually forming a synergistic integrated process system with CMP, ALD, and dry etching.
Overall, although wet etching is an earlier-developed microfabrication technology, its technological value in dielectric layer processing, metal patterning, and surface modification remains irreplaceable. With the continuous development of advanced packaging, heterogeneous integration, and three-dimensional device structures, wet etching is gradually evolving from traditional "large-scale material removal" to "precise interface control," and will continue to play a crucial role in future micro/nano manufacturing systems.