L. Norder
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1
Plasma Underetching of Suspended Silicon Nitride Membranes
Process Optimisation of SF6 ICP-RIE for Suspended Photonic Crystal Membranes
Suspended silicon nitride membranes are promising structures for ultra-light photonic applications, including photonic crystal membranes for future lightsail concepts. These membranes require a combination of low mass, optical functionality and mechanical stability. A major fabrication challenge is the final release step: the silicon substrate must be removed sufficiently to suspend the membrane, while the thin silicon nitride layer must remain intact. This thesis investigated (cryogenic) SF6 inductively coupled plasma reactive ion etching (ICP-RIE) as a dry-release method for silicon underetching beneath patterned silicon nitride membranes.
...
Suspended silicon nitride membranes are promising structures for ultra-light photonic applications, including photonic crystal membranes for future lightsail concepts. These membranes require a combination of low mass, optical functionality and mechanical stability. A major fabrication challenge is the final release step: the silicon substrate must be removed sufficiently to suspend the membrane, while the thin silicon nitride layer must remain intact. This thesis investigated (cryogenic) SF6 inductively coupled plasma reactive ion etching (ICP-RIE) as a dry-release method for silicon underetching beneath patterned silicon nitride membranes.
Master thesis
(2026)
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A. Noschese, G.J. Verbiest, R.H. Guis, R.A. Norte, L. Norder, Silvana Pereira
The continued scaling of integrated circuits has driven overlay tolerances in semiconductor lithography to the single-nanometre level. Conventional optical overlay metrology, however, becomes unreliable when the layers above the target structure are optically opaque, as is increasingly the case in advanced back-end-of-line stacks. This thesis investigates the use of photonic crystals (PtCs) as optical transducers in a pump–probe photoacoustic measurement scheme, with the long-term objective of enabling overlay sensing through optically opaque substrates.
The working principle relies on the steep reflectance resonance of a PtC fabricated on the metallic surface layer: an acoustic wave generated by the absorption of a femtosecond pump pulse deforms the crystal lattice, shifting the resonance and producing a large change in probe reflectance. A square lattice PtC with circular holes milled into an aluminum film on a silicon substrate is used throughout.
The thesis makes three main contributions. First, a simulation framework based on the Stanford Stratified Structure Solver (S4) is extended to incorporate the realistic illumination conditions of both experimental setups used in this work: the angular spread and Gaussian weighting of the focused probe beam, and the fiber-tip geometry of the reflectance spectroscopy setup. Second, photonic crystal structures are fabricated by focused-ion-beam milling and electron-beam lithography on aluminum films, and their geometry is verified by atomic force microscopy. Third, the fabricated structures are characterized on a pump–probe photoacoustic setup and on a broadband reflectance spectroscopy setup.
The experimental results confirm that the photonic crystal enhances the photoacoustic signal amplitude by approximately 282% relative to a plain aluminum film, corresponding to a reduction in measurement averaging time by a factor of approximately 14. The FFT spectra of the photoacoustic signal exhibit two peaks whose frequencies are governed by the lattice period and are consistent with substrate-dominated surface acoustic waves propagating at the Rayleigh velocity of silicon (v ≈ 4900 ms−1), in close analogy with the phononic crystal framework established in prior literature. The reflectance spectra show geometry-dependent resonance features that shift horizontally with the lattice period and vertically with the hole radius, in qualitative agreement with the S4 simulations. Incorporating the angular averaging model into the simulations significantly improves the agreement with the measured spectra, confirming the importance of accounting for realistic illumination conditions in any future geometry optimization. ...
The working principle relies on the steep reflectance resonance of a PtC fabricated on the metallic surface layer: an acoustic wave generated by the absorption of a femtosecond pump pulse deforms the crystal lattice, shifting the resonance and producing a large change in probe reflectance. A square lattice PtC with circular holes milled into an aluminum film on a silicon substrate is used throughout.
The thesis makes three main contributions. First, a simulation framework based on the Stanford Stratified Structure Solver (S4) is extended to incorporate the realistic illumination conditions of both experimental setups used in this work: the angular spread and Gaussian weighting of the focused probe beam, and the fiber-tip geometry of the reflectance spectroscopy setup. Second, photonic crystal structures are fabricated by focused-ion-beam milling and electron-beam lithography on aluminum films, and their geometry is verified by atomic force microscopy. Third, the fabricated structures are characterized on a pump–probe photoacoustic setup and on a broadband reflectance spectroscopy setup.
The experimental results confirm that the photonic crystal enhances the photoacoustic signal amplitude by approximately 282% relative to a plain aluminum film, corresponding to a reduction in measurement averaging time by a factor of approximately 14. The FFT spectra of the photoacoustic signal exhibit two peaks whose frequencies are governed by the lattice period and are consistent with substrate-dominated surface acoustic waves propagating at the Rayleigh velocity of silicon (v ≈ 4900 ms−1), in close analogy with the phononic crystal framework established in prior literature. The reflectance spectra show geometry-dependent resonance features that shift horizontally with the lattice period and vertically with the hole radius, in qualitative agreement with the S4 simulations. Incorporating the angular averaging model into the simulations significantly improves the agreement with the measured spectra, confirming the importance of accounting for realistic illumination conditions in any future geometry optimization. ...
The continued scaling of integrated circuits has driven overlay tolerances in semiconductor lithography to the single-nanometre level. Conventional optical overlay metrology, however, becomes unreliable when the layers above the target structure are optically opaque, as is increasingly the case in advanced back-end-of-line stacks. This thesis investigates the use of photonic crystals (PtCs) as optical transducers in a pump–probe photoacoustic measurement scheme, with the long-term objective of enabling overlay sensing through optically opaque substrates.
The working principle relies on the steep reflectance resonance of a PtC fabricated on the metallic surface layer: an acoustic wave generated by the absorption of a femtosecond pump pulse deforms the crystal lattice, shifting the resonance and producing a large change in probe reflectance. A square lattice PtC with circular holes milled into an aluminum film on a silicon substrate is used throughout.
The thesis makes three main contributions. First, a simulation framework based on the Stanford Stratified Structure Solver (S4) is extended to incorporate the realistic illumination conditions of both experimental setups used in this work: the angular spread and Gaussian weighting of the focused probe beam, and the fiber-tip geometry of the reflectance spectroscopy setup. Second, photonic crystal structures are fabricated by focused-ion-beam milling and electron-beam lithography on aluminum films, and their geometry is verified by atomic force microscopy. Third, the fabricated structures are characterized on a pump–probe photoacoustic setup and on a broadband reflectance spectroscopy setup.
The experimental results confirm that the photonic crystal enhances the photoacoustic signal amplitude by approximately 282% relative to a plain aluminum film, corresponding to a reduction in measurement averaging time by a factor of approximately 14. The FFT spectra of the photoacoustic signal exhibit two peaks whose frequencies are governed by the lattice period and are consistent with substrate-dominated surface acoustic waves propagating at the Rayleigh velocity of silicon (v ≈ 4900 ms−1), in close analogy with the phononic crystal framework established in prior literature. The reflectance spectra show geometry-dependent resonance features that shift horizontally with the lattice period and vertically with the hole radius, in qualitative agreement with the S4 simulations. Incorporating the angular averaging model into the simulations significantly improves the agreement with the measured spectra, confirming the importance of accounting for realistic illumination conditions in any future geometry optimization.
The working principle relies on the steep reflectance resonance of a PtC fabricated on the metallic surface layer: an acoustic wave generated by the absorption of a femtosecond pump pulse deforms the crystal lattice, shifting the resonance and producing a large change in probe reflectance. A square lattice PtC with circular holes milled into an aluminum film on a silicon substrate is used throughout.
The thesis makes three main contributions. First, a simulation framework based on the Stanford Stratified Structure Solver (S4) is extended to incorporate the realistic illumination conditions of both experimental setups used in this work: the angular spread and Gaussian weighting of the focused probe beam, and the fiber-tip geometry of the reflectance spectroscopy setup. Second, photonic crystal structures are fabricated by focused-ion-beam milling and electron-beam lithography on aluminum films, and their geometry is verified by atomic force microscopy. Third, the fabricated structures are characterized on a pump–probe photoacoustic setup and on a broadband reflectance spectroscopy setup.
The experimental results confirm that the photonic crystal enhances the photoacoustic signal amplitude by approximately 282% relative to a plain aluminum film, corresponding to a reduction in measurement averaging time by a factor of approximately 14. The FFT spectra of the photoacoustic signal exhibit two peaks whose frequencies are governed by the lattice period and are consistent with substrate-dominated surface acoustic waves propagating at the Rayleigh velocity of silicon (v ≈ 4900 ms−1), in close analogy with the phononic crystal framework established in prior literature. The reflectance spectra show geometry-dependent resonance features that shift horizontally with the lattice period and vertically with the hole radius, in qualitative agreement with the S4 simulations. Incorporating the angular averaging model into the simulations significantly improves the agreement with the measured spectra, confirming the importance of accounting for realistic illumination conditions in any future geometry optimization.