MoS₂, a two-dimensional material, has garnered significant attention due to its unique properties that make it an attractive candidate for FET (BEOL) applications and optoelectronic applications such as photodetectors, solar cells and light-emitting devices. However, traditional methods of synthesizing high-quality MoS₂ often involve high-temperature chemical vapor deposition (CVD) on substrates like sapphire with temperatures exceeding 600°C. These conditions are incompatible with standard silicon-based manufacturing lines and pose challenges for scalability.
Improving yield and integration into standard manufacturing processes
The innovative technique involves directly bonding MoS₂ onto a silicon substrate. The process begins with the growth of MoS₂ on a donor wafer. This donor wafer is then bonded to a target silicon wafer through molecular direct bonding, a method that forms bonds at the atomic level without the need for adhesives or intermediate layers. Subsequently, the bonded wafers are separated using a precision blade insertion technique, during which the wafers are immersed in water. Due to the different hydrophobic levels of the donor and target wafers, the separation occurs at the right interface.
Lucie Le Van-Jodin, a CEA-Leti research engineer, further explains: "We were able to create a process that is applicable to industry. Up until now, high-quality growth required processes with higher temperatures and special growth substrates such as sapphire. This approach required a transfer step that generated defects and was mainly developed at the lab scale."
One of the standout features of this new technique is its compatibility with existing cleanroom environments. By operating within standard manufacturing settings, the process ensures high yield and minimal defects. The team achieved a transfer yield exceeding 95%, a significant improvement over previous methods. This high yield is attributed to meticulous control over the bonding and debonding processes, particularly the blade insertion speed during wafer separation.
Characterization confirms excellent results
"One of the key aspects of our work is that we carried out an important characterization and analysis of these 2D surfaces. We analyzed the entire wafer surface, not just a single area," explains CEA-Leti research engineer Paul Brunet.
They employed various analytical techniques such as Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) to confirm the preservation of the MoS₂'s chemical structure. In addition, atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses revealed that there were no mechanical defects, such as cracks or wrinkles, and confirmed the absence of residues.
This extensive characterization underscores the robustness of the process and its potential for integration into industrial applications. The ability to transfer high-quality MoS₂ onto silicon wafers overcomes a key challenge and opens the way for many applications, including memory devices, switch technology and photodetectors. Moreover, the process's adaptability suggests potential for extending this technique to other 2D materials, further broadening its applicability.
Cutting-edge expertise and equipment
The success of this project is deeply rooted in CEA-Leti's extensive expertise and state-of-the-art facilities.
"We were able to achieve this result because of CEA's long-standing expertise in bonding technologies combined with access to an advanced 200 mm 2D manufacturing cleanroom setup," highlights Lucie Le Van-Jodin.
Looking ahead, CEA-Leti aims to refine the process further, with goals such as eliminating the use of water during wafer separation and adapting the technique for substrates beyond silicon. These advancements could further streamline the integration of 2D materials into semiconductor manufacturing, paving the way for new electronic and optoelectronic devices with enhanced performance and reduced dimensions