Ultra-High Vacuum Fabrication of Ordered Nanoparticles and Their Device Applications
Authors: Das Biswajit , Wilkins Ginger-Marie , Das Sanjana 
Published: February 2020

Nanoparticles are important for the implementation of a variety of photonic and electronic devices with new functionalities and improved performances. While multiple photonic devices using nanoparticles have been successfully implemented, the use of nanoparticles in electronic devices have significantly lagged; one of the primary reasons being the more stringent material and surface purity requirements for such devices. While the feasibility of a variety of electronic devices based on nanoparticles have been demonstrated in the laboratory, such as nanoparticle flash memory and single electron transistors, the fabrication technique employed is electron-beam lithography, which unfortunately is not suitable for large scale manufacturing. For most electronic devices, it is typically required that the nanoparticle dimensions be in the 1 nm to 10 nm range with size variations of 10% or less. Current photolithographic techniques are not suitable for the implementation of such nanoparticles, and nonlithographic techniques are typically used for their fabrication. However, most nonlithographic techniques are based on natural self-organization processes and suffer from a lack of flexibility or a lack of engineering control. Among the various nonlithographic techniques, the predominant ones are solution based, examples of which include sol-gel synthesis, chemical synthesis, and electrochemical synthesis inside self-organized nanoporous templates. While such solution based techniques can produce nanoparticles with the required dimensions and size distributions, they also need complex surface passivation involving capping molecules to prevent aggregation. These capping molecules modify the electrical surface properties of the nanoparticles making charge injection/extraction difficult. In addition, the solution based synthesis techniques are not compatible with the silicon integrated circuit (IC) process, in particular the Complementary Metal Oxide Silicon (CMOS) process, which is the predominant manufacturing process for electronic devices. It is widely believed that the availability of a CMOS compatible nanoparticle fabrication technique can greatly increase the viability of nanoparticle based electronic devices.
We have developed a CMOS compatible ultra-high vacuum system for the implementation of nanoparticle based electronic devices that addresses the above issues. The system consists of (i) a nanoparticle unit that provides the capability to deposit nanoparticles of any metal, semiconductor or insulator of diameters as low as 1 nm with less than 5% size variation on an arbitrary substrate, (ii) a 4-pocket electron-beam evaporation unit that allows the in-situ deposition of four different materials (metals or insulators) with less than 5% thickness uniformity, and (iii) a UHV pulsed DC sputter source that allows the in-situ deposition of thick layers of insulators (semiconductors or metals) including isolation dielectrics. The nanoparticle deposition unit, the ohmic contact metallization unit and the isolation dielectric deposition unit are all housed inside an ultra-high vacuum (10-10 torr) chamber to ensure high purity and good surface properties of the nanoparticles. The nanoparticle deposition system is based on sputtering, thus making it possible to deposit nanoparticles of any material that can be sputtered. In addition, nanoparticle deposition can be carried out at room temperature, making it possible to deposit the nanoparticles on a variety of substrates including flexible substrates. Size selection of nanoparticles is achieved by using a quadrupole mass filter with the capability to provide as low as 2% size control. One advantage of the system is that ohmic contacts and isolation dielectrics can be formed in an integrated manner without breaking vacuum, which is important for electrical continuity. In addition, the deposited nanoparticles can be embedded in, or coated with, metallic, semiconducting or insulating layers without breaking vacuum, thus making complex multi-layered structures possible that can have applications in a variety of fields including nanoscale detectors, nano-optics, nano-sensors, field-emitters etc. For nanoparticle based electronic devices to function properly, it is essential that the nanoparticles are physically separated from each other. In addition, if the nanoparticles can be ordered in a periodic manner, it can significantly improve device reliability and performance. To address these issues, we have developed a technique for the electrical ordering of the nanoparticles that can be carried out in-situ without breaking vacuum. The ordering process is rapid, does not require physical contact with nanoparticles and thus prevents contamination.
We have used the system and technique for the deposition of a variety of nanoparticles of metals and semiconductors including Ni, Fe, Co, Au, Ag, Cu, Si, CdTe, CdSe, ZnS on a variety of substrates including silicon, silicon dioxide coated Si, metal coated silicon, quartz, sapphire. We have also fabricated complex structures containing nanoparticles for various applications In this presentation, we will provide details of the apparatus, fabrication process, experimental results as well some of the nanoparticle based electronic devices currently being developed.