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Selective etching of Si3N4 to SiO2 is essential in the semiconductor fabrication process. In particular, as the number of alternating Si3N4/SiO2 multi-layered stacks increases, selective removal of Si3N4 without loss of SiO2 becomes difficult. In this study, the dissolution of Si3N4 was demonstrated in superheated water without addition of H3PO4, which has been widely used to etch Si3N4. The dissolution rates of Si3N4 and SiO2 in the superheated water depended strongly on the concentration of OH−, and the activation energy obtained for the dissolution of Si3N4 was 72.65 ± 0.95 kJ/mol. It is believed that the attack of the partially δ+ charged Si atoms in the Si3N4 by nucleophilic OH− was the key step in the dissolution of Si3N4 in the superheated water. Because a tradeoff between the dissolution rate of Si3N4 and the Si3N4-to-SiO2 etching selectivity was observed, H2SiO3 and HF were added to HCl-based superheated water for optimization. The HCl-based superheated water with the addition of 0.005 vol% HF and 0.01 M H2SiO3 allowed successful fabrication of a horizontal SiO2 trench structure on a patterned Si3N4/SiO2 15 pair-layered stack through selective etching of Si3N4 without thinning of the SiO2 layer.

We present the plasmonic fiber based optical glucometer. A thin gold layer is coated on clad-free core of multimode optical fiber along 3 cm length to excite surface plasmons at 632.8 nm wavelength. Glucose oxidase is immobilized on the metal surface for glucose sensing. The effective surface refractive index increases by gluconic acid and hydrogen peroxide that are generated upon glucose injection, leading to plasmonic condition change with a consequence of optical power change at the fiber output. We obtain limit of detection of glucose concentration of 6.75 mg/dL, indicating higher sensitivity than the wavelength interrogating SPR glucometer that uses a spectrometer of 1nm spectral resolution. The coefficient of variation is 8.6% at a glucose concentration of 80 mg/dL at room temperature. We also examine the effects of ambient temperature variations from −10 °C to 40 °C on the performance of the presented sensor and compared them with those on commercially available glucometers that are based on enzyme electrodes. We find that the presented fiber sensor produced standard deviation of 12.1 mg/dL at a glucose concentration of 80 mg/dL under such varying temperature, which is, even without additional temperature correction function, comparable to the commercialized ones.

3D NAND has a vertically stacked semiconductor structure to increase the memory density of semiconductor devices. 3D NAND devices are based on the multiply stacked silicon nitride (Si3N4) and silicon dioxide (SiO2) structure, and it is essential to high selectively etch Si3N4-to-SiO2 in the fabrication process. Hot phosphoric acid (H3PO4) has been typically used in the Si3N4-to-SiO2 selective etching process and the etch selectivity can increase by adding SiO2 etching inhibitors in H3PO4. In order to increase etching rate of Si3N4 and Si3N4-to-SiO2 etch selectivity, additives can be added in H3PO4. However, SiO2 etching inhibitors to increase Si3N4-to-SiO2 etch selectivity may occur by-product redeposition issue around the SiO2-layered trenches of Si3N4/SiO2 multi-stack structure. In this study, the etching behavior of the Si3N4/SiO2 multi-stack structures in SiO2 etching inhibitor-added H3PO4 was investigated. In addition, high selective Si3N4 etching with the control of redeposition was performed on the Si3N4/SiO2 multi-stack structure by adding proper SiO2 etching inhibitor in H3PO4.

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