Novel 3-D Multilayer Terahertz Packaging Technology for Integrating Photodiodes Arrays and Rectangular Waveguide-Power Combiners

A novel 3-D multilayer packaging technology for integrating an array of indium phosphide (InP)-based terahertz photodiodes (THz-PDs) with a rectangular waveguide power combiner (WR-PC) is proposed. The packaging concept is based on a vertical integration of an InP THz-PDs array with a multilayered WR-PC made of metallized glass-reinforced epoxy FR4 laminates using direct wafer bonding. The key motivation of this work is to develop a low-cost packaging technology for coherent power combining in the THz regime. The proposed multilayered packaging technology is generic, i.e., in principle, it would allow the integration of different and multiple planar arrays featuring photonic or electronic devices. To our knowledge, this is the first 3-D packaging concept for the THz frequency range that enables the integration of 2-D arrays of photonic or electronic devices. As a proof of concept, we here report on the design, fabrication, and experimental characterization of a straight hollow rectangular waveguide (WR-waveguide) and a <inline-formula> <tex-math notation="LaTeX">$2 \times 1$ </tex-math></inline-formula> WR-PC for the WR3-band (220–320 GHz). Both packages feature standard cross-sectional hollow WR3-waveguides (<inline-formula> <tex-math notation="LaTeX">$862\,\,\mu \text{m}\,\,\times 431\,\,\mu \text{m}$ </tex-math></inline-formula>) and corresponding standard UG-387/U-M flanges. They are fabricated using a stack of vertically bonded unit cells, each consisting of a 50-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>-thick glass-reinforced epoxy FR4 laminate with a 23-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>-thick top and bottom metallization. The size of a single quadratic FR4 unit cell is 24 mm <inline-formula> <tex-math notation="LaTeX">$\times24$ </tex-math></inline-formula> mm. The fabricated straight WR3-waveguide consists of a stack of 57 FR4 unit cells of a total length of 5.4 mm. The measured transmission loss is less than 0.3 dB/mm and the return loss (RL) is less than 10 dB, within the entire WR3-band. Next, a multilayered FR<inline-formula> <tex-math notation="LaTeX">$4\,\,2\,\,\times1$ </tex-math></inline-formula> WR3-PC is reported. Its design is based on that of a T-junction in <inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula>-plane where the inputs are modified to be on the same plane for facilitating subsequent integration with the planar InP chips of THz-PDs. The length of the WR3-PC is intentionally reduced for compactness and its impedance is gradually matched to achieve a low insertion loss (IL) over the entire WR3-band. The fabricated multilayered WR3-PC consists of 41 FR4 unit-cells resulting in a total thickness of only 3.9 mm. For the frequency range from 240 to 320 GHz, the simulated IL, minimum isolation, and RL are 0.16, 4.2, and 6.5 dB, respectively. For the experimental characterization using a WR3-band vector network analyzer (VNA), two multilayered FR<inline-formula> <tex-math notation="LaTeX">$4\,\,2\times 1$ </tex-math></inline-formula> WR3-PCs are connected back-to-back (B2B). The measured average IL and RL within the frequency range from 240 to 320 GHz are found to be 3.6 dB and below 5.2 dB, respectively. The measured average IL for a single WR3-PC is 1.8 dB, which is higher than expected because of imperfect fabrication and misalignment losses during the integration. An improved fabrication and packaging process will allow mitigating the losses in future runs.