Periodic Reporting for period 1 - MAGNIFIC (Materials for a next-generation (nano-)opto-electro-mechanical systems)
Reporting period: 2022-12-01 to 2024-05-31
Information and communication technology (ICT) relies on CMOS microelectronics for processing and optical telecom networks for distribution. Introducing phonons as a 3D variable can expand ICT applications by enhancing interactions among electrons, photons, and phonons.This,combined with nano-opto-electro-mechanical systems (NOEMS), can significantly improve low-power information processing and transmission, heralding the next generation of low-power ICT.The technology uses miniaturized NOEMS components based on CMOS-compatible nanocrystalline silicon and aluminum nitride, operating at telecom light wavelengths and GHz microwave frequencies to convert THz & GHz signals for internet and mobile applications with high power efficiency.
WP1
1.Properties of nc-Si
To study the morphology of nanocrystalline silicon (nc-Si), we use 380-µm-thick double-side polished Si wafers with a 500-nm-thick layer of silicon dioxide as substrate.The 220-nm-thick nc-Si films, deposited by low pressure chemical vapour deposition at 535°C, were initially amorphous, and they were annealed for 60 minutes at a temperature ranging from 650℃-1050℃.During the annealing, the films transformed to nanocrystalline.Circular windows with a diameter of 340 µm were opened through deep etching from the backside of the wafers;these windows are employed for optical absorption measurements and TEM characterisation.
2.Theoretical model of nc-Si
Theoretical models of grain boundaries with around 200 atoms were constructed using different grain orientations.These structures were relaxed using machine learning interatomic potentials validated with DFT phonon calculations.We identified possible electronic in-gap states localized at the GB, as well as local vibrational modes. The supercell preparation approach has been adapted to the computational workflow based on LAMMPS and ML potentials.Simultaneously the computation of electron-phonon coupling terms has been implemented within the SIESTA ecosystem.
3.AlN AlN/nc-Si interface
In the BAW resonators and FBAR filters, the piezoelectric AlN film is deposited by sputtering directly on the bottom electrode, usually molybdenum.The resulting film structure is columnar and supports well the generation of bulk acoustic waves (BAW) in the vertical direction and to some extend surface acoustic waves (SAW) in plane.Nc-Si has different crystallographic orientations on the surface of the nc-Si film, potentially affecting the nucleation and growth of the AlN layer.However the X-ray rocking curve measurements have shown that the structure of the AlN films sputtered on nc-Si is similar to those deposited on metal electrodes, posing no risk for further developments
WP2
1.Optomechanical cavity design and modeling for 5G & SATCOM
The simultaneous confinement of elastic and electromagnetic waves inside the optomechanical (OM) nanobeam was performed by using 2 distinct OM cavity designs.The first one is based on the concept of topological interface modes, where the design of the OM cavity is obtained by using the so-called Su-Schrieffer-Heeger model, while the second one is based on a specific phononic resonance mode of the OM nanobeam.Both OM cavity designs operate in the phononic frequency range above 4GHz, and they have been optimized to give rise to photonic cavity modes with high Q-factor.Comparative analysis demonstrates superior OM coupling rates with the 2nd design.
2.SAW metasurface for improved multi-modal coupling
To improve the electromechanical coupling between the IDT system and the OM nanobeam, we propose the design of a meta-surface that acts as a lens for the elastic waves generated by the IDT system.The elastic wave focusing effect of the meta-surface is based on resonance modes of a pillared silicon membrane.The performance of the meta-surface has been examined using a silicon membrane.
3.Bulk acoustic wave electromechanical excitation
Electromechanical excitation of NEMS and optomechanical structures is performed using surface acoustic waves generated by interdigitated transducers.To optimise the conversion efficiency from electrical power fed into the device to mechanical energy flow in nanostructures is challenging and can be in the real devices utilising SAW and IDTs only of the order of a percent. In MAGNIFIC we have developed a concept of integrating micro/nanoscale bulk acoustic wave transducers directly in the optomechanical nanobeam.This approach provides a technology for very compact devices and efficient conversion of signals.
4.Test structures
VTT has designed the process flow and masks to fabricate the test devices utilising the integrated BAW excitation of nanobeams in electromechanical excitation.UPV has designed and modelled dedicated optomechanical nanobeams to be used in the test devices.
WP3
1.Impact of fabrication process variability on NEOMS performance
Variations in the dimensions of NOEMS affect their performance by altering band gaps and dispersion relations.We use FEM via COMSOL Multiphysics software to investigate these effects on 1D hypersonic phononic and photonic crystals at telecom wavelengths and a 2 GHz mechanical frequency.The effect of air hole diameters, stub lengths, stub widths, off-centre air hole positions, and sidewall angles on the band structures for photonic and phononic modes in nanobeam cavities are computed.
2.Role of thermal fluctuations
To drive the cavity into the lasing regime a close analysis of the mechanical properties and fluctuations is being performed.The OM coupling rate is calculated to elucidate the driving mechanism enabling or preventing the nc-Si to oscillate.A characterization of the wavelength and frequency shift from the designed cavities as well as a quantification of the photoelastic effect is being performed and phase noise measurements on MHz lasing modes have been obtained to compare the origin of factors affecting the noise in the material.
3.Model verification
Characterization of optomechanical cavities across various designs, including those emphasizing photoelastic or moving boundary effects on optomechanical coupling is performed.Optical and mechanical response of nc-Si versus SOI wafers is underway for material assessment.The electrical integration has been explored via the numerical design and tested of reference structures of IDTs via a Vector Network Analyzer in the 4 GHz frequency range tunable via the pitch and width of fabricated transducer.
1.Properties of nc-Si
To study the morphology of nanocrystalline silicon (nc-Si), we use 380-µm-thick double-side polished Si wafers with a 500-nm-thick layer of silicon dioxide as substrate.The 220-nm-thick nc-Si films, deposited by low pressure chemical vapour deposition at 535°C, were initially amorphous, and they were annealed for 60 minutes at a temperature ranging from 650℃-1050℃.During the annealing, the films transformed to nanocrystalline.Circular windows with a diameter of 340 µm were opened through deep etching from the backside of the wafers;these windows are employed for optical absorption measurements and TEM characterisation.
2.Theoretical model of nc-Si
Theoretical models of grain boundaries with around 200 atoms were constructed using different grain orientations.These structures were relaxed using machine learning interatomic potentials validated with DFT phonon calculations.We identified possible electronic in-gap states localized at the GB, as well as local vibrational modes. The supercell preparation approach has been adapted to the computational workflow based on LAMMPS and ML potentials.Simultaneously the computation of electron-phonon coupling terms has been implemented within the SIESTA ecosystem.
3.AlN AlN/nc-Si interface
In the BAW resonators and FBAR filters, the piezoelectric AlN film is deposited by sputtering directly on the bottom electrode, usually molybdenum.The resulting film structure is columnar and supports well the generation of bulk acoustic waves (BAW) in the vertical direction and to some extend surface acoustic waves (SAW) in plane.Nc-Si has different crystallographic orientations on the surface of the nc-Si film, potentially affecting the nucleation and growth of the AlN layer.However the X-ray rocking curve measurements have shown that the structure of the AlN films sputtered on nc-Si is similar to those deposited on metal electrodes, posing no risk for further developments
WP2
1.Optomechanical cavity design and modeling for 5G & SATCOM
The simultaneous confinement of elastic and electromagnetic waves inside the optomechanical (OM) nanobeam was performed by using 2 distinct OM cavity designs.The first one is based on the concept of topological interface modes, where the design of the OM cavity is obtained by using the so-called Su-Schrieffer-Heeger model, while the second one is based on a specific phononic resonance mode of the OM nanobeam.Both OM cavity designs operate in the phononic frequency range above 4GHz, and they have been optimized to give rise to photonic cavity modes with high Q-factor.Comparative analysis demonstrates superior OM coupling rates with the 2nd design.
2.SAW metasurface for improved multi-modal coupling
To improve the electromechanical coupling between the IDT system and the OM nanobeam, we propose the design of a meta-surface that acts as a lens for the elastic waves generated by the IDT system.The elastic wave focusing effect of the meta-surface is based on resonance modes of a pillared silicon membrane.The performance of the meta-surface has been examined using a silicon membrane.
3.Bulk acoustic wave electromechanical excitation
Electromechanical excitation of NEMS and optomechanical structures is performed using surface acoustic waves generated by interdigitated transducers.To optimise the conversion efficiency from electrical power fed into the device to mechanical energy flow in nanostructures is challenging and can be in the real devices utilising SAW and IDTs only of the order of a percent. In MAGNIFIC we have developed a concept of integrating micro/nanoscale bulk acoustic wave transducers directly in the optomechanical nanobeam.This approach provides a technology for very compact devices and efficient conversion of signals.
4.Test structures
VTT has designed the process flow and masks to fabricate the test devices utilising the integrated BAW excitation of nanobeams in electromechanical excitation.UPV has designed and modelled dedicated optomechanical nanobeams to be used in the test devices.
WP3
1.Impact of fabrication process variability on NEOMS performance
Variations in the dimensions of NOEMS affect their performance by altering band gaps and dispersion relations.We use FEM via COMSOL Multiphysics software to investigate these effects on 1D hypersonic phononic and photonic crystals at telecom wavelengths and a 2 GHz mechanical frequency.The effect of air hole diameters, stub lengths, stub widths, off-centre air hole positions, and sidewall angles on the band structures for photonic and phononic modes in nanobeam cavities are computed.
2.Role of thermal fluctuations
To drive the cavity into the lasing regime a close analysis of the mechanical properties and fluctuations is being performed.The OM coupling rate is calculated to elucidate the driving mechanism enabling or preventing the nc-Si to oscillate.A characterization of the wavelength and frequency shift from the designed cavities as well as a quantification of the photoelastic effect is being performed and phase noise measurements on MHz lasing modes have been obtained to compare the origin of factors affecting the noise in the material.
3.Model verification
Characterization of optomechanical cavities across various designs, including those emphasizing photoelastic or moving boundary effects on optomechanical coupling is performed.Optical and mechanical response of nc-Si versus SOI wafers is underway for material assessment.The electrical integration has been explored via the numerical design and tested of reference structures of IDTs via a Vector Network Analyzer in the 4 GHz frequency range tunable via the pitch and width of fabricated transducer.
-Develop an atomistic model for grain boundaries, focusing on electronic and vibrational properties using first-principles DFT, enabling coupled electron and phonon dynamics
-Enhance electromechanical conversion efficiency with a GHz-range acoustic metasurface to optimize the conversion of mechanical signals from IDTs to confined modes in nanobeams
-Improve understanding of intrinsic dissipation mechanisms at the nanoscale to devise strategies for blocking higher-order modes in NEMS and NOEMS designs
-Enhance electromechanical conversion efficiency with a GHz-range acoustic metasurface to optimize the conversion of mechanical signals from IDTs to confined modes in nanobeams
-Improve understanding of intrinsic dissipation mechanisms at the nanoscale to devise strategies for blocking higher-order modes in NEMS and NOEMS designs