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Fundamentals of Terahertz Devices and Applications


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Frequencies

      In order to achieve the depth accuracy and smoothness of the features on silicon, Silicon‐on‐insulator (SOI) wafers are commonly employed, since thin layers of SiO2 act as etch stops of the silicon. For example, [26] presented an iris of the leaky‐wave antenna at 1.9 THz fabricated on a 2.5 μm silicon membrane. Compared with a thermal oxide membrane, the crystalline silicon provides more robustness to compressive stress. In that case, a SOI wafer with a 2 μm device layer was used because, together with the metallization, the membrane would reach the desired 2.5 μm. And a handling wafer of 400 μm defined the waveguide on the back of the membrane.

Schematic illustration of (a) the membrane fabrication process that contains the iris and waveguide of the leaky-wave feed. (b) SEM of the iris developed at 1.9 THz in [26] using the explained process.

      Source: Alonso‐delPino et al. [26]; IEEE.

      At the end of the process, the overall wafer was sputtered with gold, used due to its high conductivity, immunity to oxidation, and ease of deposition. The overall results of this process can be observed in the scanning electron microscope (SEM) image in Figure 2.23b, showing a very clean and well‐defined pair of double slots.

      The rest of the wafers that define the lens are processed similarly as the procedure described but, because the radiation is going through the wafers, it is necessary the use of high resistivity silicon wafers (the resistivity around 10 kΩ cm) to avoid the introduction of absorption losses.

      2.5.1.1 Fabrication of Silicon Lenses Using DRIE

      Silicon shallow lens arrays have been fabricated either by laser micro‐machining process or using photolithographic processes based on DRIE. Laser micro‐machining allows the fabrication of 3‐D geometries with accuracy as presented in [33], however, it is a linear process where the cost depends on the laser time, which might not be the most cost‐efficient method for large lens arrays. A novel DRIE silicon process presented in [47] allows the fabrication of arrays of lenses on a single wafer and in parallel. This section will provide an overview of the process and a fast method to estimate the overall fabrication accuracy without needing to test the lens antenna.

Schematic illustration of (a) Sketch of the fabrication process of the shallow silicon lens. (b) Photograph of shallow lens antennas at 1.9 THz of diameter 2.6 and 6.3 mm presented in [26].

      Source: Alonso‐delPino et al. [26]; IEEE.

      The last step of the process consists of applying an antireflective coating to the lens which is essential to reduce the high reflection losses that occur by using a dielectric with high permittivity. A coating with the polymer Parylene is usually employed as matching layer at submillimeter‐wave frequencies.It has an index of refraction around 1.64, which is not the ideal for silicon, but it is close enough to considerably reduce the reflections. The coating conformal is deposited using vapor deposition which allows high control of the thickness and uniformity.

      2.5.1.2 Surface Accuracy

Schematic illustration of (a) Surface measured of the fabricated lens of D = 2.6 mm. The error surface defined as the difference between the measured surface and a perfect sphere is shown underneath. (b) Computed radiation pattern of the measured lens surface and the measured radiation pattern of the whole lens antenna at 1.9 THz from [26].