Reactance-loaded Sequential-phase Feed Network for a Highly Compact Wideband On-chip Circularly Polarized Antenna

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Opportunity

The advancement of complementary metal-oxide semiconductor (CMOS) technology has enabled the integration of terahertz (THz) components, driving interest in system-on-chip (SoC) and antenna-on-chip (AoC) solutions. Circularly polarized (CP) antennas are crucial for applications like high-resolution radar imaging, short-range communication, automotive systems, biomedicine, and inter-satellite communications due to their ability to mitigate multipath interference, maintain consistent transmission regardless of antenna orientation, and reduce susceptibility to ghost targets and jamming. However, designing on-chip CP antennas faces significant challenges. The extremely thin dioxide layer in the Back-End-of-Line (BEOL) process limits the axial-ratio (AR) bandwidth of conventional on-chip CP antennas to typically within 5–6%. While some designs utilize the silicon base in CMOS processes to enhance bandwidth by up to 16%, this approach introduces issues such as reduced radiation efficiency, surface waves, distorted radiation, and potential interference with active devices in the Front-End-of-Line (FEOL) process. Sequential-phase feed techniques offer an alternative to expand bandwidth without using the silicon base, but they often require large antenna arrays and complex feed networks, leading to bulky sizes. Phase differences between adjacent outputs are typically achieved through quarter-wavelength transmission lines, which are not cost-effective and hinder miniaturization and integration. Additionally, the thin SiO₂ layer constrains impedance bandwidth, often resulting in an overlapped bandwidth between impedance and AR below 10%. These limitations have restricted the development of on-chip CP antennas, creating a significant demand for compact, wideband designs that can be seamlessly integrated with RF and digital circuits on a single chip.

Technology

This patent introduces a highly compact, wideband on-chip circularly polarized antenna that addresses existing challenges through innovative refinements in both the radiating elements and the feed network. The antenna features a plurality of radiating elements arranged in a rotationally symmetric pattern, each comprising a patch element and a T-shaped shorting wall that shorts the patch to a ground layer. The radiating elements are designed to resonate in unique half TM₀₁ and quarter TE₂₀₁ modes by utilizing equivalent electric and magnetic walls, which expand the impedance bandwidth while maintaining a compact size. Key to the design is a reactance-loaded sequential-phase feed network that replaces traditional delay lines with equivalent capacitors and inductors, allowing for phase shifting. The feed network includes a core portion with a square-ring shape and multiple output arms connected to the radiating elements. Parallel-plate capacitors are strategically integrated into the feed network by exploiting the thin interlayer separation in CMOS technology, such as the 0.8μm gap between metal layers, to create tunable capacitances that control power distribution and phase differences. An equivalent inductor is formed using an end-shorted transmission line. This approach eliminates the need for long delay lines, enabling a stable 90° phase difference between adjacent radiating elements over a wide bandwidth. The entire antenna is fabricated using a standard 65-nm CMOS process, with dimensions as small as 0.55λ₀ × 0.55λ₀ and a low profile of 0.012λ₀, making it highly suitable for integration into on-chip systems. The design methodology can be extended to cover applications at microwave and millimeter-wave frequencies.

Advantages

Achieves a wide overlapped impedance-AR bandwidth of 19%, significantly exceeding conventional on-chip CP antenna limits.
Enables extreme miniaturization with overall dimensions of 0.55λ₀ × 0.55λ₀ and a low profile of 0.012λ₀.
Utilizes a reactance-loaded feed network that replaces bulky delay lines with compact capacitors and inductors, reducing size and cost.
Maintains stable circular polarization over a broad frequency range (e.g., 360–440 GHz) with low axial ratio.
Avoids the use of silicon substrate, preventing issues like reduced radiation efficiency and interference with active devices.
Compatible with standard CMOS processes, facilitating easy integration with RF and digital circuits on a single chip.
Offers versatile phase-shifting capabilities applicable beyond antennas, such as in hybrids and couplers.