Silicon dioxide sacrificial layer etching has become a major surface micromachining method to fabricate microsensors and microactuators often made of polycrystalline silicon. An overview of the SiO 2 materials available in integrated circuit manufacturing is given, and the SiO 2 etch mechanism and sacrificial layer etch kinetics are reviewed. Selectivity issues important for the proper choice of layers and etchants are addressed discussing the chemical attack of aluminum during long sacrificial SiO 2 layer etching, as an example. Various etchants known from other studies are compared: concentrated and dilute HF, buffered HF (BHF), nitric acid (HNO 3 ) based etchants known as P-etch, R-etch, S-etch, as well as mixtures of HF and HCl, and vapor HF. 'Pad-etch', an acetic acid/ammonium fluoride/ethyleneglycole solution is shown to have an enhanced selectivity against aluminum. Some device examples such as arrays of deflectable micromirrors demonstrate the versatile application of sacrificial oxide etching in surface micromachining.
The application of metal oxide gas sensors in Internet of Things (IoT) devices and mobile platforms like wearables and mobile phones offers new opportunities for sensing applications. Metal-oxide (MOx) sensors are promising candidates for such applications, thanks to the scientific progresses achieved in recent years. For the widespread application of MOx sensors, viable commercial offerings are required. In this publication, the authors show that with the new Sensirion Gas Platform (SGP) a milestone in the commercial application of MOx technology has been reached. The architecture of the new platform and its performance in selected applications are presented.
We directly trace the near-and mid-infrared transmission change of a VO 2 thin film during an ultrafast insulator-to-metal transition triggered by high-field multi-terahertz transients. Non-thermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analogue of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1 Å.
We demonstrate sub-cycle manipulation of mid-infrared optical waveforms in the time domain. This goal is accomplished via efficient reflection at a semiconductor surface induced by femtosecond interband excitation. The ultrafast response of this process allows slicing of high-field multi-terahertz transients down to the single optical cycle. Ultrabroadband and phase-stable transients with peak amplitudes beyond 10 MV cm −1 are obtained, paving the way for efficient coherent control of light-matter interaction in the non-perturbative regime. The microscopic analysis of electron-hole plasma generation in germanium reveals a decisive role of two-photon absorption allowing efficient slicing up to midinfrared frequencies.
Many properties of solids result from the fact that in a periodic crystal structure, electronic wave functions are delocalized over many lattice sites. Electrons should become increasingly localized when a strong electric field is applied. So far, this Wannier–Stark regime has been reached only in artificial superlattices. Here we show that extremely transient bias over the few-femtosecond period of phase-stable mid-infrared pulses may localize electrons even in a bulk semiconductor like GaAs. The complicated band structure of a three-dimensional crystal leads to a strong blurring of field-dependent steps in the Wannier–Stark ladder. Only the central step emerges strongly in interband electro-absorption because its energetic position is dictated by the electronic structure at an atomic level and therefore insensitive to the external bias. In this way, we demonstrate an extreme state of matter with potential applications due to e.g., its giant optical non-linearity or extremely high chemical reactivity.
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