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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.

The step by step fabrication of the slot iris and waveguide process is shown in Figure 2.23. Thermally grown SiO₂ is used as a hard mask in the front and back of the SOI wafer. Since the small size of the double slot iris, an i‐line Canon stepper is used to illuminate the SiO₂. And for the rest of the waveguide and alignment features, regular UV photolithography can be employed. The pre‐etching of the features, i.e. the slot iris and the waveguide, on the front and back of the SiO₂ layer are performed using photoresist and an inductively coupled plasma (ICP) reactive ion etcher. Then, the silicon on the front backside is etched using a PlasmaTherm DRIE system. While the selectivity of the silicon to photoresist is around 70 : 1, the selectivity of the silicon to SiO₂ can be optimized to reach 130 : 1, which allows to etch high depth features with high control. The SiO₂ was removed using a hydrogen fluoride HF solution.

Figure 2.23 (a) Sketch of 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.