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Lithography
Published in Andrew Sarangan, Nanofabrication, 2016
Buffered oxide etch (BOE) clean is a very popular method used for removing SiO2 from silicon sur-faces, including native oxides and grown oxide films. It is a mixture of hydrofluoric acid and ammonium fluoride [10]. This is a self-terminating reaction, that is, the reaction comes to a stop when all of the oxides are consumed, leaving the bare unoxidized silicon surface. The termination point is identified by the characteristic change in the surface property from hydrophilic to hydrophobic. Even with a very thin oxide layer, the silicon surface will be hydrophilic. It will turn to hydrophobic when all of the oxides have been etched away, a condition easily identified by the liquid simply rolling off the substrate without wetting it. Buffering is the process of mixing an acid (HF, in this case) with its conjugate base (NH4F) so that it maintains a constant pH value throughout its use. The buffered HF provides a stable etch rate and a longer shelf life compared to HF alone. When used in conjunction with photoresists, BOE will not penetrate the photoresist through microscopic pinholes and cracks as much as HF. Nevertheless, BOE and HF should be used with extreme caution due to their health hazard. HF is a very small molecule, so it is able to easily penetrate skin and tissue and cause internal organ damage.
Serially connected tantalum and amorphous indium tin oxide for sensing the temperature increase in IGZO thin-film transistor backplanes
Published in Journal of Information Display, 2023
EunSeong Yu, SeoungGyun Kim, SeoJin Kang, HyuckSu Lee, SeungJae Moon, JongMo Lee, SeungBae An, ByungSeong Bae
The IGZO layer was patterned by wet etching using a buffered oxide etch (BOE) solution diluted with water at a ratio of 3000:1. The IGZO layer was post-annealed in O2 at 250 °C for 1 h. An interlayer dielectric (ILD) was spin-coated using a negative photoresist (SU-8 2000.5) that measures 1-µm-thick. A 200-nm-thick Ta layer was deposited by DC magnetron sputtering and patterned by RIE with SF6. A 150-nm-thick ITO electrode was deposited by RF magnetron sputtering and patterned by wet etching with photolithography.
Effect of the spin-on-glass curing atmosphere on In–Ga–Zn–O thin-film transistors
Published in Journal of Information Display, 2020
Yeong Jo Baek, Eui-Jung Yun, Byung Seong Bae
The oxide TFT was developed with siloxane-based SOG, which was used for both the gate insulator and the interlayer dielectric in the experiment. The gate electrode was Cr, and the source/drain was Al. The TFTs were fabricated on a glass substrate via photolithography, as shown in Figure 1. After cleaning the glass substrate, a 50-nm-thick IGZO active layer was deposited thereon at 250°C via RF magnetron sputtering using a target with a 1:1:1 In:Ga:Zn atomic ratio, under the following conditions: 50 W RF power, 0.7 Pa working pressure, and Ar:O2=25:7.5 flow ratio. The IGZO layer was patterned via wet etching with a buffered-oxide-etch (BOE) solution diluted with DI water at a 500:1 ratio. After the IGZO patterning, the IGZO layer was annealed at 250°C for 1 h under an O2 atmosphere to reduce the oxygen vacancies and the process-related defects. For the gate insulator on the IGZO layer, the SOG solution (Cospeen-1225G from NEPES) without dilution was spin-coated at 3000 rpm and soft-baked at 80°C for 1 min, followed by baking at 180°C for 1 min. After the soft baking, final curing was done to obtain a 360-nm-thick SOG gate insulator by increasing the temperature from 150°C to 450°C in 100°C intervals (the curing was performed for 15 min at 150, 250, 350, and 450°C, respectively). A 75-nm-thick Cr layer was deposited for the gate electrode via DC magnetron sputtering, and was patterned via wet etching. The gate insulator was etched via reactive ion etching (RIE), with the gate electrode used as an etching mask. The RF power, etching time, and working pressure were 180 W, 270 s, and 13.3 Pa, respectively. CF4 and O2 were used as the etching gases, and the CF4:O2 gas mixing ratio was 60:20. After etching the gate insulator, IGZO was doped via O2 plasma treatment at room temperature for 30 s in an RIE chamber, under the following process conditions: 180 W RF power, 60 sccm O2 flow, and 13.3 Pa working pressure. After doping, a 360-nm-thick SOG interlayer dielectric layer was formed via spin coating and curing, under the same conditions as those for the gate insulator. For the source/drain contact, a contact hole was formed in the interlayer dielectric. Before the contact hole patterning, oxygen plasma treatment under the same conditions as those for the doping was carried out to make the SOG surface hydrophilic. After performing contact hole dry etching for 170 s under the same conditions as those for the gate insulator etching, 75-nm-thick source and drain Al was deposited via DC magnetron sputtering, and was patterned via wet etching. The channel width and length of the fabricated TFTs were 40 and 20 μm, respectively.
Phase retrieval for studying the structure of vitreous floaters simulated in a model eye
Published in Journal of Modern Optics, 2021
Varis Karitans, Sergejs Fomins, Maris Ozolinsh
The optical setup used in the study is shown in Figure 1. Depending on whether the vitreous floaters were imaged, or their phase was retrieved the optical system was slightly modified. First, the optical system was adjusted to image the vitreous floaters simulated in a model eye. A positive lens of focal power +45 D simulating the cornea was placed in front of the intraocular lens. The intraocular lens was simulated by a manually tuneable lens (EL-10-30-TC; Optotune). A charge-coupled device (CCD) simulating the retina was placed at the back focal plane of the model eye by adjusting the optical power of the tuneable lens. A glass plate with etched microstructures simulating the vitreous floaters was placed in front of the retina to observe the diffraction patterns caused by the vitreous floaters. The microstructures were designed using direct write lithography and etching the glass plate in a buffered oxide etch (BOE). The model eye was followed by a lens L2 (fL2 = 60 mm). The model eye and the lens L2 were placed in a 4f configuration. The lens L2 was followed by a pair of lenses L3 (fL3 = 60 mm) and L4 (fL4 = 150 mm) also placed in a 4f configuration like lenses L2 and L3. The lens L4 was followed by lenses L5 (fL5 = 150 mm) and L6 (fL6 = 150 mm) acting as a telescope of unit magnification. The lenses L4 and L5 were again placed in a 4f configuration. Behind the lens L6, the beam passed through a linear polarizer LP and split into two parts by a beam-splitter (BS). One part was directed towards a spatial light modulator (SLM) placed at the secondary focal plane of the lens L6 and optically conjugated to the plane of microstructures. The model of the SLM was PLUTO-2-VIS-014 (HoloEye). The size of the pixel was 8 µm, however, the binning of pixels 2 × 2 was used to match the geometry of the optical system and reduce the angle of diffraction. The maximum phase shift at the wavelength of a He–Ne laser (λ = 0.6328 µm) was 1.76·π radians. The linear polarizer LP ensured that the light incident on the SLM was polarized along its longest axis. A circular aperture CA1 of diameter about 3 mm was placed in front of the SLM to limit the area of the object under study, however, the CA1 was used only when retrieving the structure of the vitreous floaters. The SLM was placed at the primary focal plane of the lens L7 (fL7 = 125 mm). Light coming from an infinitely distant object and travelling through the optical system starting from the lens L7 was focused on the retina, and a scene with the diffraction patterns superimposed on it was captured. The kinematic mirror KM was removed to unblock the optical path of the light coming from the distant object. Another circular aperture CA2 of diameter about 3 mm was placed in front of the lens L7 to improve the visibility of the vitreous floaters.