Читать книгу Fundamentals of Terahertz Devices and Applications - Группа авторов - Страница 31

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.

Figure 2.24 (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 process to fabricate the silicon shallow lens consists of four steps, illustrated in Figure 2.24a. The first step consists of the patterning of the photoresist on a high resistivity wafer with the desired lens aperture diameter. The thickness and aperture diameter of the photoresist applied onto the silicon wafer defines the thickness and curvature of the lens surface. Multiple photoresist coatings can be applied to achieve the desired thickness. Next, the photoresist reflows by applying heat, i.e. around 110 °C on a hot plate, to the wafer. The surface tension applied to the photoresist by its coating above the glass transition temperature, makes the surface reflow into a spherical shape. Last, the photoresist shape is transferred into the silicon wafer using a DRIE process. The photoresist and silicon etching selectivity, controlled by the CF₄ and O₂ gas ratio and the DC and RF power applied in the process, defines the curvature of the overall lens. The examples shown below a selectivity of 1 : 1.3 was used to achieve a total height of 475 μm for a 360 μm of photoresist. The surface roughness achieved with the process in the order of hundreds of nanometers.

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.