Design of tunable slow-wave coplanar waveguides (TS-CPWs) based on distributed MEMS

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In this section, we present the state-of-the-art of passive phase shifters at mm-waves. Four technologies available at mm-waves are described:
• Ferroelectrics,
• Liquid crystals,
• Silicon (CMOS/BiCMOS),
Ferroelectrics are non-linear materials, but their main characteristic is the high and tunable relative permittivity. Several materials are available: Barium-Strontium-Titanate (BST), Potassium-Niobium-Titanate (KTN) or Lead-Zirconium-Titanate (PZT). The BST is the most popular ferroelectric material used in various microwave devices, such as phase shifters, filters, etc., because it has low dielectric losses, high tunability and low coercive field, thus reducing the hysteresis effect and simplifying the electric command [19].
A 360° phase shifter in the Ka-band with a moderate bias voltage using BST material was proposed in [20]. The phase shifter consists of a CPW for (CoPlanar Waveguide) periodically loaded by interdigitates BST capacitors (Figure 1-8). This design is also used by [21].
Figure 1-8: Schematic and optical views of the phase shifter proposed in [20].
The measured insertion loss of the proposed phase shifter is around 0.8 dB at low frequencies (1 to 5 GHz) and 3.6 dB at 30 GHz. The return loss is better than 12 dB in the entire frequency range which shows a good matching of this circuit. The maximum value of phase shift is 310° with a DC voltage of 35 V, leading to a figure of merit of 85°/dB.
The phase shifter proposed in [22], based on [23] and [24], is based on a high impedance CPW loaded with 12 identical MIM capacitors. It represents the first tunable phase shifter based on BST MIM capacitors working at 60 GHz. The schematic of the phase shifter and the design of a MIM capacitor are given in Figure 1-9.
Figure 1-9: (a) Circuit schematic photograph of the fabricated phase shifter [24], (b) example of MIM capacitor [22] and (c) photograph of the BST tunable phase shifter, and of the MIM capacitor [23].
The measured electrical characteristics of the phase shifter are 12 dB and 5.9 dB for return loss and insertion loss, respectively. When varying the applied voltage from 0 to 90 V, the maximum value of phase shift is 150° at 60 GHz, which gives a FoM of 25°/dB.
Few instances of phase shifter based on BST MIM capacitor working at 60 GHz are available in the literature. Table 1-1 presents the state-of-the-art of passive phase shifters in BST technology working at 30 GHz and 60 GHz.
Due to the high dielectric losses (in literature tan δ is 0.07, 0.13 and 0.2 in [27], [28] and [23], respectively) in the mm-waves, the FoM remains quite low at 60 GHz.
Liquid crystal (LC) is an anisotropic material, which exhibits two different permittivities depending on molecule direction. In between their solid and liquid states it has, depending on the LC mixture used, different mesophases such as the smectic and the nematic phase [26]. The nematic liquid crystal (LC) can be employed as a tunable dielectric in the microwave devices.
In [29], two LC-based phase shifters working in the 60 GHz band were presented. The first one is a meandered line phase shifter (MLPS) and is illustrated in Figure 1-10(a). It is based on a microstrip line with CPW transitions, as detailed in [30]. These transitions are proposed in order to provide low reflection as well as DC connectivity to the conductor pattern exposed to LC.
Figure 1-10: Structure of (a) LC-based MLPS for 60 GHz band, (b) reflection load and (c) LC-based RTPS for 60 GHz band [29].
The second phase shifter is a RTPS (Figure 1-10(c)) composed of a 3 dB hybrid coupler and two reflective loads (Figure 1-10(b)) exposed to the LC layer. When a bias voltage is applied through the probe pads to the conductor, the LC molecules change direction beneath the electrode. This changes the effective dielectric constant, and hence a variation of the reactance of the reflective loads and a differential phase shift occur.
The measured phase shift of the MLPS and the RTPS for an applied bias voltage of 11 V is 240° and 150°, with a return loss of 14 dB and 10 dB, respectively. The figure of merit is 20°/dB for the MLPS and 11°/dB for the RTPS. However, the advantage of the RTPS is its size. It occupies an area nearly 4 times lower than the MLPS.
Another phase shifter based on liquid crystal was proposed in [31]. The topology of a two layers microstrip meander line LC-based phase shifter is shown in Figure 1-11. This phase shifter is interesting owing to its planar structure and compact design. The total length of the phase shifter exposed to the LC is around 7 mm. Three phase shifters in series are implemented to design the four-element array.
Figure 1-11: (a) Meander line phase shifter and (b) four-elements array including meander line phase shifter [31].
An insertion loss of 11 dB and a return loss of 12 dB were observed. The maximum value of phase shift was 47°, leading to a figure of merit around 4°/dB.
In [32], a LC phase shifter with a stripline configuration is integrated in a LTCC technology (for low temperature cofired ceramic), a low loss multilayer technology. The LC is an available commercial solution, as compared to the special LC solution used by the same team in [33], together with a standard inverted microstrip in a TMM3 technology (a low permittivity composite ceramic substrate). In [32][33], the figure of merit at 24 GHz was as high as 110°/dB, but the used LC solution might suffer from badly aging which does not seem the case in [32]. What’s more, the topology suffered from large response time, which was in the order of minutes depending on the LC material and layer thickness. These are the two reasons why this work has not been referred to in Table 1-2.
Figure 1-12(a) describes the stripline topology used in [32], where the ground plane has the same width as the signal line. A photograph and 3D-model of the LTCC based LC stripline phase shifter are shown in Figure 1-12(b) and Figure 1-12(c). Experimental results between 30 and 34 GHz give a return loss equal to 10 dB, and a worst insertion loss of 6 dB and 8 dB, at 30 GHz and 34 GHz, respectively. The phase shift is 61°, leading to a FoM of 10°/dB at 30 GHz. De-embedding of these results highlights that the FoM of the phase shifter increases to 13°/dB at 30 GHz.
In [34], the same team associates with IMEP-LaHC on a commercially available LC solution. The phase shift effect is high but conjugated with a distributed-MEMS effect which leads to a very interesting FoM of 51°/dB at 45 GHz. In [35], in 2005, the aim was to test a specific solution, with similar benefits and disadvantages than in [33]. The FoM is thus very interesting but the PS presents an off-time still superior to 10 s.
Figure 1-12: (a) Schematic cross section of the stripline LC phase shifter, (b) photograph of the LTCC-based LC phase shifter device and (c) 3D-model of the phase shifter realized in LTCC technology [32].
Main results for phase shifters using liquid crystal technology are presented and compared in Table 1-2.
In recent years, devices based on silicon in CMOS/BiCMOS technologies have dominated the integrated circuits applications. Thus, in this sub-section we describe the main results carried out in these technologies.
In [36], three different passive and active phase shifters were designed and fabricated in a standard 1P7M 90 nm CMOS process. Here, we present only the passive phase shifter used in the frequency range from 50 to 56 GHz for a passive quadrature generator. This phase shifter is of RTPS type. It consists in a coupled-line coupler loaded by shunts varactors. The schematic diagrams and the photographs of the phase shifter and the passive quadrature generator are shown in Figure 1-13 (a), Figure 1-13(b), Figure 1-13(c) and Figure 1-13(d), respectively.
Figure 1-13: (a) Schematic of the phase shifter, (b) chip microphotograph of the passive phase shifter, (c) schematic of the quadrature generator with VM and (d) chip microphotograph of the quadrature generator based phase shifter [36].
Insertion loss of 16.75 dB and phase shift of 141° were obtained, leading to a FoM of 8.4°/dB.
A 4-bit passive phase shifter with phase resolution of 22.5° for automotive radar applications operating in the frequency range from 67 to 78 GHz was designed in a 0.12 µm BiCMOS process (IBM 8HP) in [37]. It consists of a low-pass π-network and CMOS passive switches. All inductors that form the low-pass π-network were implemented with a grounded CPW as shown in Figure 1-14 (a). Figure 1-14 (b) presents the schematic of the 4-bit digital phase shifter made of two 90° phase shifters tied together to form 180° phase shifting in series with 22.5°, 45° and 90° phase shifters. Figure 1-14: (a) The grounded-CPW, (b) schematic of the 4-bit digital phase shifter, (c) chip photograph of the 90°phase shifter cell and (d) chip photograph of the 4-bit digital phase shifter [37].
This digital phase shifter was measured up to 110 GHz with return loss of 10 dB for all 16 different phase states from 68 to 81 GHz. The highest value of phase shift was 337° with a maximum insertion loss of 19 dB, leading to a FoM equal to 17.7°/dB.
A summary of the main characteristics of phase shifters implemented in CMOS/BiCMOS technologies is carried out in Table 1-3.
The main advantages of MEMS phase shifters are: low loss, good linearity, broadband frequency operation, large tuning ratio and good power handling capability. In addition, in recent years, MEMS devices combined with CMOS/BiCMOS technologies have paved the way of a great revolution for mm-wave systems. In this section, state-of-the-art of phase shifters in MEMS technology with pure MEMS or CMOS-MEMS technology is described.
The design, fabrication and characterization of a 2–bit switched line phase shifter using dc-contact single-pole-quadruple-throw (SP4T) RF-MEMS switches for 60 GHz applications were investigated in [41]. The SEM of the SP4T switches and the 2-bit phase shifter are shown in Figure 1-15(a) and Figure 1-15(b), respectively.
Figure 1-15: (a) SEM of the fabricated SP4T RF-MEMS switch, (b) SEM images of the fabricated V-band 2-bit phase shifter [41].
An average insertion loss of 2.5 dB in the frequency range from 55 to 65 GHz was achieved for the 2–bit phase shifter, with a return loss better than 12 dB for all 4 states (0°, 90°, 180°and 270°), leading to a FoM equal to 108°/dB when considering the maximum phase shift of 270°.
A V-band CMOS-MEMS switchable phase shifter based on reflection-type topology was proposed in [42]. The schematic of the proposed phase shifter and its images of fabrication are shown in Figure 1-16(a) and Figure 1-16 (b, c), respectively.
After the fabrication of the phase shifter in a 0.18 µm CMOS technology, a post-CMOS process is necessary to release the MEMS structure. When an actuation voltage of 46 V is applied, three discrete phase states of 0°, 89° and 144° are achieved at 65 GHz. The measured return loss is better than 14 dB and the insertion loss is around 2.2 dB over the 55-65 GHz range. This performance is attributed to a specifically developed technology with an optimization of the design methodology.
Table 1-4 shows a comparison of the state-of-the-art for the phase shifters based on MEMS at mm-waves.

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Table of contents :

Chapter 1  Mm-wave phase shifters: overview and state-of-the-art
1.1. Overview
1.1.1. Principle
1.1.2. Applications
1.1.3. Topologies
1.2. State-of-the-art
1.2.1. Ferroelectrics
1.2.2. Liquid crystal
1.2.3. CMOS/BiCMOS
1.2.4. MEMS
1.2.5. Discussion
1.3. Solution based on slow wave transmission lines
1.3.1. Transmission line characteristics
1.3.2. Slow-wave coplanar waveguide concept
1.4. Tunable slow-wave transmission lines
1.4.1. Principle
1.4.2. Varactor based
1.4.3. Distributed MEMS based
1.5. Integrated technologies description for distributed MEMS
1.5.1. Previous work
1.5.2. IHP technology
1.5.3. Discussion
1.6. Conclusion
Chapter 2 Design of tunable slow-wave coplanar waveguides (TS-CPWs) based on distributed MEMS
2.1. RF-MEMS switches: types and operating principle
2.1.1. Overview
2.1.2. RF-MEMS ohmic switches
2.1.3. RF-MEMS capacitive switches
2.1.4. Electrostatic actuator
2.1.5. Reliability issues
2.2. Tunable Slow Wave CoPlanar Waveguide (TS-CPW) with distributed MEMS in IHP 0.25 μm BiCMOS technology
2.2.1. S-CPW in the 0.25 μm BiCMOS technology
2.2.2. Design topology of TS-CPW
2.2.3. Pull-in voltage calculation
2.2.4. TS-CPW simulations
2.3. Previous works
2.3.1. Clean room technology: TS-CPW topology
2.3.2. CEA technology: TS-CPW topology
2.3.3. AMS standard CMOS technology with post-process
2.4. Conclusion
Chapter 3 Design of digital phase shifters
3.1. Previous Work
3.1.1. Clean room phase shifter design: PTA
3.1.2. CEA phase shifter design
3.1.3. AMS phase shifter design
3.2. TS-CPW as a 1-bit phase shifter
3.1.4. Principle
3.1.5. Need to combine the moveable ribbons
3.3. Phase shifter (N bit/N commands): design1
3.1.6. Principle
3.1.7. Design methodology: 3-bit phase shifter design 1
3.1.8. 315° phase shifter (3 bits/ 3 commands)
3.1.9. HFSS simulation
3.4. Optimized Phase shifter: design 2
3.1.10. Proposed solution
3.1.1. Design issues
3.1.2. Validation of the solution
3.5. Conclusion
Chapter 4 TS-CPWs and phase shifters measurements
4.1. Previous Work measurements
4.1.1. Clean room (PTA) phase shifter
4.1.2. CEA phase shifter
4.1.3. AMS phase shifter
4.2. IHP phase shifter design 1 measurements
4.2.1. Optical and electrical characterization
4.2.2. RF measurements: S-parameters
4.2.3. Observed limitations
4.3. IHP phase shifter design 2 measurements
4.4. Discussion on the parasitic effect of the substrate
4.4.1. Analytical calculation of capacitances
4.4.2. Solution to improve Con/Coff
4.5. Comparison with state-of-the art


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