Millimeter Wave on Chip Antenna Using Dogbone Shape Artificial Magnetic Conductor

An artificial magnetic conductor (AMC) applied in millimeter wave on chip antenna design based on a standard 0.18 μm CMOS technology is studied.The AMC consisting of two-dimensional periodic dogbone shape elements is constructed at one metal layer of the CMOS structure. After its performance has been completely investigated, it has been used in an on chip dipole antenna design as an artificial background to enhance efficiency of the dipole antenna.The result shows that 0.72 dB gain has been achieved at 75GHz when the AMC is constructed by a 4∗6 dogbone array.


Introduction
In recent years, applications of millimeter-wave systems have great significance in communications, radar, guidance system, remote sensing technology, radio astronomy, and so on.Millimeter-wave band is defined as a spectrum from 30 GHz to 300 GHz, whose corresponding wavelength is from 10 mm to 1 mm.Millimeter-wave system researches were firstly started in the United States, Japan, Europe, and other advanced countries.In the past, millimeter-wave systems were used in the military and the scientific community.Now, they are widely used in commercial applications, such as automotive collision avoidance radar, local multipoint distribution services (LMDS), and local area networks (WLANs) [1][2][3][4].Because of the short wave length in millimeter-wave band, millimeter-wave devices have small size, strong antiinterference, and other advantages.
Reflector antennas, horn antennas and lens antennas, are typically used for millimeter-wave antenna of the radio system [5][6][7].Reflector antenna and horn antenna are mechanical structures and occupy large volumes.With the development of wireless communication and semiconductor technology, antenna on chip (AoC) was presented to meet the requirement of high integration and compact size.In this paper, a standard 0.18 m CMOS technology with six metal layers is used in AoC design.In order to avoid the low radiation efficiency, the bottom metal layer cannot be used as the shield between the silicon dioxide layer and the silicon substrate.But most of the power will be dissipated in the silicon substrate as heat for its low resistivity and high permittivity.For instance, a 140 GHz on chip antenna implemented in 65 nm CMOS process by [8] has a gain of only −25 dB.Therefore, gain enhanced methods of AoC should be investigated.
Artificial magnetic conductor (AMC) inserted between antenna and the silicon substrate is presented in [9][10][11][12].AMC can be adopted to redirect one-half of the radiation back to the opposite direction so that antenna gain can be enhanced to some extent.In this paper, an AMC composed of periodic dogbone shape conductors above a ground plane is placed underneath a dipole to improve the proposed on chip antenna performance.

Dogbone Shape AMC
The dogbone shape AMC, as a sort of the texture presented in [13], provides a good shielding and reflecting capability.Its shape and dimension are shown in Figure 1.Its equivalent circuit model provides an approximate method to calculate the magnetic resonant frequency of a ground-backed dogbone element.According to the equations, we can easily International Journal of Antennas and Propagation   and B1, less determined by geometrical parameters of A, B, and B2.
As we can see, the influence of the structure parameters on  11 can be divided into three cases.For the magnitude of  11 , the cell size A and size B have similar influence.With increment of size A or B, the AMC operation frequency increases.Conversely, with increment of length A1, A2, or B1, the AMC operation frequency decreases.For the phase of  11 , with increment of size A, A1, or A2, the center operating frequency of the AMC increases.Conversely, with increment of length B or B1, the center operating frequency of the AMC decreases.However, the variation of width B2 has little influence on  11 , both its magnitude and phase.

On Chip Dipole Antenna
Figure 9 shows the phases of the reflection coefficient by implanting AMC into CMOS when its location is differed from M1 to M5.From the figure, it can be found that the AMC has the largest bandwidth when it is constructed in the M5 metal layer.
Figure 10 shows the configuration of a dipole antenna backed by the proposed AMC.The geometrical dimensions of the proposed AMC are shown in Figure 2. The dipole antenna is located on the top metal layer M6, while the AMC layer is located on M5.A metal ground plane is located on the bottom surface of the silicon substrate.The dipole length is 696 m.The antenna input reflection coefficient is plotted in Figure 11.Its matched input impedance bandwidth ( 11 < −10 dB) is more than 20 GHz.Radiation patterns of the proposed antenna at 75 GHz are shown in Figure 12.Due to the perfect symmetry of the proposed antenna, its radiation patterns are symmetrical in both the  and  planes.

International Journal of Antennas and Propagation
The proposed antennas whose AMC is constructed by different numbers of the dogbone shape element have been investigated.Their realized gains are listed in Table 1 when the dogbone shape array varies between 4 * 6, 4 * 8, and 6 * 6.

Conclusion
Artificial magnetic conductor (AMC) constructed by twodimensional periodic dogbone shape element is applied in millimeter-wave CMOS on chip dipole antenna design.The detailed frequency response of the proposed AMC has been completely studied.A gain of 0.72 dBi is realized at 75 GHz when the dipole antenna is backed by the proposed AMC with a 4 * 6 dogbone shape array.

Figure 7 :
Figure 7: Magnitude and phase of reflection coefficient versus geometrical parameter B1.

Figure 9 :
Figure 9: Variations of  11 phase when the AMC is constructed in CMOS different metal layers (from M1 to M5).

Figure 12 :
Figure 12: Radiation patterns of the proposed antenna.

Table 1 :
Realized gains of the proposed antenna backed by AMC with different element numbers.