New etching method to enhance smartphone circuitry performance
In circuitry, etching is used to remove the deformed layer created during the grinding and polishing of metal components by selective chemical attach. A research group at Nagoya University in Japan has developed a new method called “wet-like plasma etching” that combines the selectivity of wet etching with the controllability of dry etching. The technique makes it possible to etch new and hard-to-etch materials, enabling higher performance and lower power consumption of the silicon semiconductor integrated circuits used in smartphones and data centres.
In the race to create the fastest and most energy-efficient circuits for computing devices, scientists are looking for new transistor designs. There has been a shift from FinFET-type transistors, so-called because the gate is raised above the silicon plane like a shark’s fin, to gate-all-around transistors, in which the fin is replaced by a stack of horizontal sheets. In this type, the sheets surround the channel to reduce leakage and increase the drive current.
To fabricate these complex structures, metal carbides consisting of titanium (Ti) and aluminium (Al) such as TiC or TiAlC, are used as metal gates where the voltage is applied. TiAlC is a ternary material with high hardness, high wear resistance, high melting point, and excellent electrochemical performance. There are two ways to etch such materials — wet etching uses chemical solutions, while dry etching uses gases. Conventionally, TiAlC films used in semiconductor devices are etched by wet etching using hydrogen peroxide liquid mixtures. However, this process requires a long etching time to completely remove the target materials. It also runs the risk of chemically damaging the metal gate. Additionally, the liquids used can create surface tension at the atomic level, destroying important features.
In order to develop an advanced etching process for the selective removal of TiAlC over other Ti compounds, non-halogen etching has been tested as a solution. Currently, there is no non-halogen dry etching for metal carbides made of the three elements. Now, a research group led by Professors Masaru Hori, Kenji Ishikawa and Thi-Thuy-Nga Nguyen from Nagoya University, in collaboration with the companies Hitachi Ltd. and Hitachi-Tech Corp., has developed a new dry etching method for metal carbides. This method uses a floating wire-assisted vapour plasma of argon gas mixed with vapour sources of ammonium hydroxide-based mixtures at medium pressure. In the circuit, the plasma is generated by adding energy to the gas, so the additional floating wire can enhance the generation of high-density plasma.
Since this process generates active radicals of H, NH and OH from the ammonium hydroxide gas (NH4OH), the treated surface of TiAlC can be removed after surface modifications of the TiAlC film. According to Ishikawa, atmospheric pressure plasma and medium-pressure plasma techniques are used to miniaturise equipment size, fabrication cost and energy consumption.
“It is difficult to etch off compounds involving multiple elements. Therefore, control of surface modification plays a key role. Our group investigated the use of various radicals for surface modification and developed a method for generating such radicals using floating wire plasma and a vapour supplement. This provides a rich radical source of NH, H and OH, which react with the TiAlC surface to form volatile products and etch the TiAlC surface,” Ishikawa said.
The floating wire-assisted plasma technique is expected to be available for highly selective etching of metals and metal compounds used in semiconductor device fabrication.
“Metal carbides are promising gate electrode materials for advanced silicon semiconductors, and our joint research group was the first in the world to succeed in chemical dry etching of non-silicon semiconductor materials. This achievement is important for the development of atomic layer-level etching technology, which has been difficult to achieve so far. Our results represent an important milestone, and a dramatic technological leap forward, in microfabrication technology,” Ishikawa said.
The researchers’ findings were published in the journal Scientific Reports.
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