Novel MEMS Dipole/Monopole Antenna for Wireless Systems Operating at 77 GHz

Abstract

In this paper, we present a novel MEMS antenna designed for operation at 77GHz. The antenna consists of two right-angle metal arms which are supported by two vertical silicon walls. These walls are fabricated using bulk micromachining through Deep Reactive Ion Etching (DRIE) process. The two right-angle arms are fed via a microstrip ring coupler which has two input ports. A new process ∞ow is introduced in this paper, which allows the realization of the proposed antenna on a single silicon wafer. The antenna is analyzed and optimized thoroughly using Ansoft/HFSS. The results show that based on the selection of port of excitation, the proposed antenna can radiate either dipole or monopole patterns. In the dipole (monopole) mode, the antenna has impedance bandwidth of 4.3% (4.2%), gain of 8.9dBi (5.5dBi), radiation e-ciency of 94% (95%). Micromachining technology is very attractive for integrated antennas as it ofiers e-cient packaging, high radiation e-ciency, wide impedance bandwidth, and less mutual coupling between antenna elements. These advantages are more di-cult to be achieved using the conventional planar technol- ogy especially at high frequencies. Research carried out on MEMS antennas can be classifled into two main categories. The flrst category features a ∞at antenna, such as patch, realized on a thin membrane surrounded by air. The membrane can be fabricated via either bulk (1) or surface (2) micromachining technology. This results in reducing the efiective dielectric constant of the medium around the antenna and consequently increases the bandwidth and radiation e-ciency. The second category features a 3D antenna, such as horn or waveguide, realized by etching grooves in a number of silicon wafers via bulk micromachining (3). The walls of these grooves are covered with metal. Each groove represents part of the desired 3D structure. The wafers are bonded together to form the complete 3D antenna. In this paper, a novel MEMS antenna design is presented. The proposed antenna is 3D in shape and it can be fabricated on a single silicon wafer without any need for wafer bonding or hybrid integration. The proposed antenna is capable of operating as either dipole or monopole based on the selection of the port of excitation. 2. ANTENNA STRUCTURE AND FABRICATION TECHNOLOGY The proposed MEMS antenna is shown in Fig. 1. It has two vertical silicon walls that can be fabricated using bulk micromachining through 0.675mm thick high resistivity silicon wafer with dielectric constant of 11.9 and conductivity of 0.05S/m. The width, length, and depth of each wall are 70m, 953m, and 475m, respectively. The last two dimensions correspond to ‚g=2 and ‚g=4 at the operating frequency of 77GHz. The thickness of the remaining silicon substrate is 200m. Two horizontal metal arms are covering the top surfaces of the walls. Two vertical pillars with square cross section of 55m £ 55m are drilled through the entire wafer. The inner surfaces of these pillars are covered with vertical metal arms, as shown in Fig. 1. It can be seen in this flgure that there are gaps between the vertical and horizontal arms. The top surface of the substrate is covered with slotted ground metal plane, through which the silicon walls are going up. This plane serves as a re∞ector that increases the directivity of the antenna. Moreover, it isolates between the antenna and the bulk silicon substrate, which reduces the surface wave losses and increases the radiation e-ciency. All metallic parts of this structure are made of copper with thickness of 3m. From the bottom side of the substrate, the vertical arms are connected to the two output ports of a ring coupler made of microstrip lines. Two feeding microstrip lines are connected to the input ports of the ring coupler. The width of each line is 200m which corresponds to 50› at 77GHz. The ring is made of a microstrip line whose width and characteristic impedance are 88m