Читать книгу Fundamentals of Terahertz Devices and Applications - Группа авторов - Страница 29

The main advantage of having a lens antenna with a waveguide feed is its seamless integration in an array configuration for waveguide‐based receivers at submillimeter‐wave frequencies. As explained in previous sections, the leaky‐wave feed illuminates a small aperture of diameter D of the lens, making the resulting effective lens surface a very shallow lens. These shallow lenses can be packed together forming an array and it can be fabricated using silicon micromachining techniques. The feed and lens array can be entirely fabricated over a few silicon wafers and stacked to the rest of the instrument (see Figure 2.22).

The micromachining techniques used for the fabrication of heterodyne receivers and sources at submillimeter‐wave frequencies are based on CNC metal block machining, as well as wafer‐level processing using photolithographic techniques. Compared with metal block machining, wafer‐level processing allows a higher integration of the whole receiver, which reduces the volume, mass, and losses. The different waver level processing solutions in literature, from the use of SU‐8 [36] machining to lithography, electroplating, and molding (LIGA)‐based processes [37] that use thick resist and electroplating processes to form the waveguide walls, suffer from limitations in terms of multi‐etching capabilities and non‐uniformity problems. Silicon micromachining based on DRIE allows the fabrication of high aspect ratio features while maintaining straight sidewalls and smooth multi‐depth surfaces. This technology is very attractive for the development of heterodyne receivers and sources as the technology allows the fabrication of complex circuit features that require multiple waveguides of various depths, especially as the wavelength decreases, when machining tolerances become a concern.

Figure 2.20 (a) Radius R and (b) height H of the lens as a function of the diameter D for different field tapers.

Figure 2.21 (a) Directivity and (b) Gaussicity achieved for the shallow lens antenna Reflection coefficient centered at a central frequency f for the dimensions shown in the table.

As previously stated, the presented antenna composed by a leaky‐wave feed and the shallow lens can be fabricated in four silicon wafers using DRIE processes. The first wafer consists of the iris and the waveguide feature. The second wafer contains the air cavity. The third wafer contains dummy silicon wafer to achieve the correct lens thickness. The fourth wafer contains the lens surface. The four different wafers (i.e. iris air gap, bulk wafers, and lens) will be assembled together on a single wafer stack. Note that high resistivity silicon wafers, i.e. 10 kΩ cm, are required to reduce the dielectric absorption loss. And, the wafers need to be double‐sided polished in order to have good surface contact between all the wafers in the stack, avoiding air gaps. The alignment, which is a challenge when working at such high frequencies, has been solved by using a silicon compressive pin that is slightly larger than the pocket and can be compressed slightly when put into place [26].

Figure 2.22 Reflection coefficient centered at a central frequency f for the dimensions shown in the table.