Porous Silicon Nanowires: Evaluation of Thermal Properties By a Vamas Interlaboratory Comparison on Scanning Thermal Microscopy

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The Versailles Project on Advanced Materials and Standards (VAMAS) has been established by G7 in 1982 and supports world trade in products dependent on advanced materials technologies, through International collaborative projects aimed at providing the technical basis for harmonized measurements, testing, specifications, and standards. It is composed by more than 15 Technical Working Areas devoted to the standardization and measurements of advanced materials. The participants are selected on a volunteering base and must represent at least the three international areas, Europe, Americas and East. The aim of this international interlaboratory comparison is to determine quantitatively the thermal properties of arrays of silicon nanowires measuring periodicity, diameter and height, effusivity and thermal conductance using scanning electron microscopy (SEM), atomic force microscopy (AFM) and scanning thermal microscopy (SThM). The samples are fabricated by INRiM and will be circulated among the interested partners starting from November 2024 for the SThM measurements. Data will be evaluated by CMI and INRiM and included in the VAMAS final report of the project. 1. Introduction The use of nanowires is diffused in several applications, ranging from photovoltaics to hydrogen production by photo catalysis, and the evaluation of the thermal properties is a critical assessment for their use. Scanning Thermal Microscopy is a promising technique for this purpose, but needs accurate study and calibration to measure values of thermal and conductivity. University, industry and calibration laboratories need reference samples and standards at the nanometric level for resolution certification of a variety of measuring instruments, such as Scanning Thermal Microscopes. There are no currently available nanowires arrays standard for thermal properties on the market and the measurements of nanowires thermal properties are typically qualitative and with too low uncertainty levels for the increasing needs of metrology. This VAMAS interlaboratory comparison will try to approach the thermal measurements of thermal properties of silicon and porous silicon nanowires with standardized samples and procedures. The main goals of the interlaboratory comparison will be:- Develop a standard based on silicon nanowires with different thermal conductance values, ranging from bulk silicon to silicon dioxide.- Develop a calibration procedure, combining experimental data from different characterization techniques, like Scanning Probe Thermal Microscopy and the 3ω method.- Evaluate measurement uncertainty.- Development of standardized procedures to define the thermal properties of the nanowires. 2. Samples fabrication and Interlaboratory comparison The samples to be measured will be fabricated by INRiM by MACE and Deep Reactive Ion Etching, aside of the Region of Interest (RoI) containing one or more arrays of porous silicon nanowires, three SiO2 layers with different thickness will allow a calibration close to the porous silicon thermal conductivity values [1] (Fig.1a). The MACE etching will be performed after nanosphere lithography and Au metallization [2] or by patterning the gold layer by FIB. Another set of samples will be fabricated by Electron Beam Lithography and Deep Reactive Ion Etching (Fig.1b). The SThM calibration and measurements will be performed following the works in reference [3,4]. The thermal conductivity of the initial substrates, both crystalline silicon and SiO2 layers, will be measured in parallel with the 3ω approach to obtain real values with a complementary and quantitative method. 3. Funding Participants fund their own involvement in the project. Samples for the interlaboratory comparison will be supplied by INRiM, the calibration procedure will be developed and diffused by CMI. 4. Deliverables and Dissemination Report will evaluate the variance observed in the associated measurement protocol, to guide further development. Results will be published in a peer-reviewed journal and will be proposed in ISO/IEC standards. 5. Conclusions The VAMAS project on the Thermal properties of Silicon and Porous Silicon Nanowires has been presented illustrating the fabrication methodology, the calibration approach, and the measurement protocol. The participation to the interlaboratory comparison is free and the measurement activity will start on November 2024.