VO2‐Based Reconfigurable Antenna Platform with …serious challenges for their use in...

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VO2-Based Reconfigurable Antenna Platform with Addressable Microheater Matrix

Burak Gerislioglu,* Arash Ahmadivand, Mustafa Karabiyik, Raju Sinha, and Nezih Pala

DOI: 10.1002/aelm.201700170

the return loss have been increasingly studied owing to the demand in advanced communication, radar, and imaging sys-tems, particularly for emerging ultra wide-band (UWB) applications.[23–27] Varactors, p-i-n diodes,[28–31] GaAs FET switches,[22] and micro-electro-mechanical systems (MEMS)[32] have been proposed to recon-figure antennas. However, many of them require wired biasing, which could easily restrict their working bandwidth.[33] For instance, tunability of the radio frequency (RF)-MEMS solutions[34] are not sufficient for RF applications in terms of switching speed and reliability. They are expensive and require power-up converters to pro-duce high voltages of 30–80 V.[22] On the other hand, p-i-n diodes present too high power consumption and low isolation on millimeter-wave frequencies.[35] Recently, RF switches based on PCMs (e.g., vana-

dium oxide (VxOy), germanium antimony telluride (GexSbyTez), and germanium telluride (GexTey)) have received growing atten-tion as an alternative solution due to their high amorphous to crystalline resistivity ratio, fast switching characteristics, and small form factor.[17,33,36] Particularly, PCM-based switches have lower ON resistances and easier integration with complemen-tary metal-oxide semiconductor in comparison to MEMS-based switches.[36] The most commonly used PCMs however require quite high temperatures for phase transition. Ge2Sb2Te5, for example, needs to be heated up to ≈750 K[37] to be crystal-lized, which is highly dependent on the total volume of the PCM structure and the crystallization time.[14,38,39] Returning to amorphous state requires further heating to ≈873 K for melting and quenching in a short period of time (≈ns).[14,40,41] These transition temperatures are ≈460[42] and ≈1000 K,[43] respectively, for GexTey. Such high transition temperatures pose serious challenges for their use in reconfigurable antenna and RF switching applications. Thus, VO2 with its relatively lower transition temperature, high resistivity change ratio, and easier processing emerges as a strong candidate for reconfigurable and multifunctional antennas in RF.[33,44] Also, in comparison to RF-MEMS or semiconductor based switches, VO2 can modu-late the properties of a large surface are at once.[2]

In this article, we propose a VO2-based reconfigurable antenna platform with individually controlled microheater pixels. We use an indirect excitation mechanism to change the electrical resistivity of VO2 through metal-to-insulator phase transition by transferring the heat generated within the resistive heater to the VO2 layer. In this scheme, electrical stimuli are only applied to

This study reports on a reconfigurable antenna platform based on vanadium dioxide (VO2), a phase-change material (PCM) with low transition tempera-ture, integrated with an addressable microheater matrix. For the first time, it is shown that an entire planar antenna can be thermally reconfigured virtually to any pattern by switching the phase of the selected regions of the VO2 layer from insulator to metallic state by indirect thermal stimuli generated by individually addressable nichrome resistive microheaters. Careful selection of the individual pixels to start the phase transition from insulator (OFF) to metallic (ON) state allows creating different antenna patterns with unique resonance properties. Reconfiguring the antenna into different patterns, oper-ating frequency can be tuned from C-band (4–8 GHz) to X-band (8–12 GHz), and S-band (2–4 GHz) frequencies which cover the entire ultra wideband spectrum (3.1–10.6 GHz). The proposed approach provides a promising plat-form for designing monolithically integrated reconfigurable antennas, radio frequency devices, and circuits for various applications.

B. Gerislioglu, A. Ahmadivand, Dr. M. Karabiyik, Dr. R. Sinha, Prof. N. PalaDepartment of Electrical and Computer EngineeringFlorida International UniversityMiami, FL 33174, USAE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.201700170.

Phase-Change Materials

1. Introduction

Vanadium dioxide (VO2) is a phase-change material (PCM) (also referred as metal-to-insulator transition material) which behaves as an insulator at room temperature, but undergoes a phase transition to metallic state when heated above ≈343 K due to reorganization of its molecular structure.[1–10] In such phase transitions (switching), its electrical resistivity can be varied from 0.1 to 3 × 10−6 Ω m in a few nanoseconds by using either electrical stimulation or optical excitation.[11] To achieve the targeted metal-to-insulator transition, direct excitation methods are employed such as conductive heating,[12] photo-thermal heating,[13] Joule heating,[14–16] and ultrafast optical stimuli.[11,17–20] Drastic changes in its resistivity and permittivity based on solid-to-solid phase transition led VO2 to be consid-ered for many electronic applications such as switches and antennas.[18,21]

Reconfigurable antennas with wideband tunability of radia-tion pattern, frequency, and polarization[22] without affecting

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the microheaters, not directly to the PCM. By taking the advan-tage of this technique and calculating the corresponding elec-trical properties including voltage standing wave ratio (VSWR), as well as return loss (|S11|) parameters, we show that a planar antenna placed on a 9 × 9 microheater matrix can be configured into different patterns with the operating frequencies in S-band (2–4 GHz), C-band (4–8 GHz), and X-band (8–12 GHz) which cover the entire UWB spectrum (3.1–10.6 GHz). We believe that our results pave new approaches to tailor reconfigurable antenna for various practical circuit applications.

2. Proposed Device Design

The proposed device structure is shown in Figure 1. The entire device considered in our study has the dimensions of 18.4 × 18.4 mm2. The active microheater matrix consists of 9 × 9 pixels each of which is 2 × 2 mm in size and separated by a gap of 0.05 mm.

In Figure 1a, the VO2 layer is rendered semitransparent to show one of the proposed antenna structure clearly. The details of a single microheater pixel are shown in Figure 1b, and the cross-sectional details of the layers are indicated in Figure 1c. In this figure, the entire structure is built on sapphire (Al2O3) substrate with the thickness of 254 µm. The nickel (Ni) elec-trodes with the thickness of 3 µm and width of 190 µm, which serve as the ground and DC bias contacts to 0.75 µm thick ser-pentine nichrome (Ni-Cr) resistive heater that is located on top of 3 µm thick silicon dioxide (SiO2) layer. This layer provides good electrical and thermal insulation between the neighboring layers and Ni electrodes. It should be noted that the outer Ni electrodes of each pixel is connected to the heater layer through via like structures as shown in Figure 1c. Next, a 50 µm hex-agonal silicon carbide (6H-SiC) barrier is inserted between the heater layer and the top 10 µm thick VO2 active layer. All of the geometrical parameters are optimized using analytical and numerical techniques for the selected antenna structures. The thickness of the VO2 layer is determined by considering two


Figure 1. a) Schematic overview of the proposed VO2 based reconfigurable antenna. b) Magnified view of one pixel with highlighted serpentine Ni-Cr heater regions (not to scale). c) Detailed cross-sectional view of the layers of two pixels (not to scale). d) The equivalent circuit model for the control matrix consisting of a 9 × 9 active microheaters. Squares illustrate each of the microheater pixels and resistors symbolize the heating elements. For a clear view, not all pixels are shown.




main factors: to obtain the maximum heat transition across the layer and to transmit and receive RF signals efficiently.

The operating principle of the antenna platform can be ana-lyzed in two parts. First, generation and transfer of the heat required for transition between OFF and ON states of VO2 is studied. Second, the antenna characteristics depending on the selection of the pixels are presented.

Applying proper potential to the electrodes of each intended pixel for a sufficient time allows its Ni-Cr layer to generate heat through Joule heating, which would eventually propagate to the VO2 layer. For instance, to heat up the bottom left pixel (red) in Figure 1d, a specific potential and ground must be applied to its outermost terminals. A biasing network can be easily designed on the Ni Layer to select all (and only) the desired pixels without affecting the others which allows controlling their temperature and configure the desired antenna pattern in the VO2 layer. In the presented 9 × 9 case, we assumed that the DC bias lines of the five bottom rows are connected to the power regulator at the bottom, while the ones of the four upper rows are con-nected to a power regulator at the top. This scheme could be implemented using only a single Ni layer. The proposed active microheater matrix-based tunable antenna platform can be fab-ricated using standard nanofabrication tools and techniques such as electron beam evaporation, sputtering, photolithog-raphy patterning, RF-sputtering, reactive-ion etching, pulsed-laser deposition, and eutectic bonding or flip chip bonding.

Most of the broadband telecommunication systems require compact, low radiation losses, and wideband antenna struc-tures.[45–49] More specifically, UWB applications have attracted a significant amount of research activity due to their unprec-edented potential for low-energy, high-bandwidth communi-cations. U.S. Federal Communications Commission (FCC) authorized the unlicensed use of UWB in the frequency range from 3.1 to 10.6 GHz.[46] Requirements for an antenna design strongly depends on the intended application, however, for wireless applications, especially for mobile handheld devices, the small size and the omni-directional pattern are highly demanded.[47]

Coplanar waveguide (CPW)-fed planar antenna platforms have received increasing attention due to simple fabrication process, enhancement of the bandwidth, easier integration to monolithic microwave integrated circuits, and less dispersion characteristics in comparison to microstrip-fed antennas.[23,48–51] Also, utilizing CPW-fed based patterning for the antenna plat-forms is an advantageous approach to facilitate formation of the antenna structure for mobile applications,[52] and to match the antenna with the feed line. To demonstrate the feasibility of our approach we selected planar monopole antenna with a CPW feedline, a radiator plane and two identical and finite ground planes, which are orientated in a different fashion in comparison to the conventional UWB antennas’ ground planes. Usually, bandwidth of the microstrip antennas is not wide enough to support multiple resonances in one antenna pattern. The major difference here is the position of the ground planes that are located at the same plane with the other antenna com-ponents. More precisely, these ground planes are horizontally located on each side of the radiator element to ensure the CPW-fed. In addition, the feeding linewidth (B) is set to 2.05 mm for a good impedance matching at the operating bandwidth.[53] To

realize capacitive coupling between the feed line and each one of the ground planes, we introduced a 0.12 mm spacings (D), as shown in Figure 2. These spacings are attained by slightly modifying the microheater pixels, at the edges of the ground planes.

Figure 2 exhibits a top-view schematic for one of the inves-tigated antenna patterns to introduce the geometrical para-meters for each antenna part. This antenna design comprises nearly T-shaped antenna pattern, coplanar feed line, and two ground planes. For this particular design, the significant parameters that can directly influence the performance of the antenna are width (W = 18.4 mm) and length (L = 18.4 mm) of the VO2 layer, the length of the ground plane (A = 2.025 mm), the gap between the antenna pattern and the ground planes (C = 4.1 mm), the space between the edge of the VO2 layer and the radiation plane (h = 2.025 mm). To estimate these antenna parameters, we utilized Equation (1)[53]

W Lc

f2 effε= =

(1)

where c is the speed of light, f is the operating frequency, εeff can be defined as (εr + 1)/2, and εr is the dielectric constant of the 6H-SiC layer between the VO2 layer and the matrix configu-ration. The 6H-SiC layer, besides serving as a heat bridge with high thermal conductivity, electrically isolate the antenna pat-tern induced in the VO2 from the metallic components of the underlying microheater matrix configuration to prevent pos-sible destructive effects.

3. Numerical Studies

Thermal studies of the proposed antenna platform were per-formed using commercial finite element method software,[54]


Figure 2. Top-view plot of one of the proposed CPW-fed planar monopole antenna pattern with corresponding geometrical parameters and phase status of the selected VO2 regions.




while characteristics of different antenna configurations were simulated using a commercial electromagnetic (EM) field solver.[55] In the thermal simulations, the generation and trans-ferring of heat is modeled by applying a necessary DC bias to the appropriate electrodes. By flowing current through the microheaters, the electric power is converted to heat. Heat transfer in solids and electric currents are coupled through the time-dependent thermal transport equation (Equation (2)) to monitor temperature changes in the studied system[14]

dCdT

dtk T V JP ( )− ∇ ⋅ ∇ = − ∇ ⋅

(2)

where d is the mass density, CP is the heat capacity, T is the temperature, k is the thermal conductivity, V is the applied elec-trical potential, and J is the current density. To accurately model the electrical and the thermal characteristics of the structure, mesh with minimum lateral grid size ≈0.1 µm were applied. Material related parameters for each layer used in the electro-thermal simulations are presented in Table 1.

In the electromagnetic wave propagation analysis, dielectric permittivity (εr) and electrical conductivity (σ) values used for both states of the VO2 were taken from the experimental data obtained by Huitema et al.[35] For Ni and Ni-Cr, we used given constants by Giancoli et al.[62]

4. Results and Discussion

The temperature distribution plots of the two neighboring pixels are shown in Figure 3. The critical point of interest is the

midpoint between two neighboring pixels on the upper surface of the VO2 layer which must be converted to metallic phase to obtain a desired antenna configuration. During the heating pro-cess, 6H-SiC acts as a heat transfer bridge channeling most of the generated heat to the VO2 layer due to its high thermal con-ductivity[57] compared to the other components of the structure. Therefore, the need for reaching very high temperatures within the Ni-Cr is not required for the phase transition.

While applying bias to selected pixels we monitored the time evolution of the temperature distribution on the proposed antenna structure with paying special attention to the critical point of interest. At the beginning, VO2 is assumed to be in the OFF state, and the temperature at the bottom of the substrate is set to 300 K, which could be realized by thermoelectric cooling if necessary. By applying appropriate electrical pulse (0.08 mA for 60 ms) to the selected Ni electrodes, the Ni-Cr is heated up to 358 K, which increased the temperature of the critical point on the VO2 up to the TC to initiate the phase transition (Figure 3a).


Table 1. Thermal conductivity, heat capacity, and density of the antenna platform.

Material k [W m−1 K] Cp [J kg−1 K] d [kg m−3]

Ni-Cr 15[54] 20[54] 9000[54]

Ni 90[54] 445[54] 8900[54]

Al2O3 35[56] 729[56] 3970[56]

6H-SiC 490[57] 690[58] 3216[57]

VO2 6[59] 690[19] 4540[60]

SiO2 1.1[61] 8375[61] 2500[61]

Figure 3. a) Time-dependent temperatures at the Ni-Cr heater and at the middle point between two neighboring pixels on the VO2 layer for heating cycle. b) Temperature plot at the top surface of the VO2 layer between three neighboring pixels. c) Time-dependent temperatures at the Ni-Cr heater and at the middle point between two neighboring pixels on the VO2 layer. d) Temperature map across the structure at the ON (metallic) state of VO2.




Temperature profile on the VO2 layer across three pixels two of which are in the ON state and the one on the rightmost is in the OFF state is shown in Figure 3b. It should be noted that the temperature on the surface of the VO2 layer is extremely uni-form with only ≈3 K difference between the one at the critical point and the one at center of the pixels (see also Figure 3d). Moreover, the temperature abruptly drops at the edge of the active pixel to the level at which the VO2 has very high resis-tivity. The generated antenna pattern is preserved as long as the power compensating the heat dissipation to the ambient is provided to keep the temperature of the critical point above the insulator-to-metal transient temperature of the VO2. To maintain this thermal balance and keep the temperature of the selected pixel constant for a desired duration the applied bias is reduced to 0.075 mA. Once the DC bias is removed (cooling cycle), temperature of the critical point drops back to 300 K in 55 ms (Figure 3c). It should be noted that, even with the most severe hysteresis cases, VO2 loses its metallic phase ≈315 K in a cooling cycle.[1,2,17] Hence, a new antenna pattern can be gen-erated as soon as after 35 ms. Thermal crosstalk due to lateral heating between the Ni electrodes, which causes a modest tem-perature increment (≈30 K) under the Ni-Cr heater section is also taken into account in the analysis.

By following the indirect heating procedure, the required TC for the phase transition of the selected VO2 regions (i.e., CPW-feed, radiator, and ground patterns) is achieved. After com-pleting this process, the metallic regions of the VO2 layer can be employed as antenna to transmit and receive high frequency EM signals with low transmission losses. The remaining VO2 regions act as high resistive (dielectric) material since they did not exceed the critical phase transition temperature. Achieving a complete control on the operating frequency of the antenna is challenging because when one parameter is modified, the antenna should be reoptimized. To demonstrate the feasibility of our approach, we designed several antenna patterns to work

at different frequency bands. The corresponding |S11|, VSWR and far-field radiation patterns of the proposed antenna pat-terns are investigated using numerical analysis. The |S11| char-acteristics show pronounced resonant frequencies to operate in the S, C, and X-bands, thereby, the versatility of the antenna platform. It should be noted that the goal of the following anal-ysis is to show how the proposed platform allows reconfiguring different antenna structures and their characteristics in wide ranges rather than presenting optimized antennas for specific applications. Obviously, reducing the size and increasing the number of the pixels in the matrix along with possible altera-tions in their shapes would allow reconfiguring antennas with better performances for intended applications.

The antenna structures shown in Figure 4a,b are designed for the C-band spectrum. The antenna patterns in Figure 4c–e are studied to operate in the S-band, while the last pattern in Figure 4f is formed to operate in the X-band. The |S11| para-meters of the proposed antenna structures with and without the recesses are demonstrated in Figure 5a. During the geo-metrical variations from antenna “a” to “b,” “c,” “d,” “e,” and “f,” we observed notable resonance shifts from 7.4 to 5.2, 2.8, 2, 2.7, and 10 GHz, respectively. Moreover, antenna perfor-mance can be substantially improved as it is reconfigured to new patterns. The initial antenna “a” pattern presented the lowest impedance matching in the operating frequency band. Inclusion of the specific recesses enhances its impedance matching performance in patterns “b,” “f,” and “d” where “d” providing the best transmission behavior compared to the other designs. Hence, we can control the operating fre-quencies of the antenna platform to switch between different bands by actively altering the radiator pattern. The demon-strated frequency range covers the UWB range (3.1–10.6 GHz) defined by the FCC, and gain values of proposed antennas comparable to previously studied works over all the frequency band.[47,63–69]


Figure 4. a–f) Layouts of the investigated antenna patterns obtained by reconfiguring the metallic regions of the VO2 layer.




The simulated far-field radiation patterns in the xz plane (E-plane) of the investigated antennas are exhibited for the frequencies at 7.4, 5.2, 2.8, 2, 2.7, and 10 GHz, respectively, in Figure 6. These results indicate that antenna “a” radiates quasi-bidirectional and dipolar-type radiation pattern. With the help of the appropriate recesses that are introduced to the antenna “a,” radiation patterns lose their bidirectional property (see Figure 6b–f), and become relatively unidirectional toward the z-direction. In addition, it is observed that these far-field radia-tion patterns are smooth for the intended frequencies and stable along the operating band.

5. Conclusion

To conclude, we have presented a VO2-based reconfigurable planar antenna platform with a microheater matrix that con-trols entire antenna pattern. The demonstrated platform pro-vides the capability of quickly generating any desired antenna patterns by changing the phase of the selected VO2 regions between the metallic and dielectric phases through thermal stimuli. The heat for the indirect thermal stimulation is gen-erated by Joule heating in the Ni-Cr resistive layer and trans-ferred to the VO2 through SiC spacer layer with high thermal


Figure 5. a) Calculated |S11| parameters and b) calculated VSWR versus frequency for different antenna structures which are shown in Figure 4.

Figure 6. a–f) Simulated radiation patterns in the xz-plane at 7.4, 5.2, 2.8, 2, 2.7, and 10 GHz, respectively.





conductivity. Using numerical analyses for the thermal and electrical properties of the VO2 and a configuration composed of semiconductor and metallic components, we have shown that the proposed antenna can efficiently operate at S, C, and X-band covering the entire UWB range for the next genera-tion radio technologies. The proposed platform could be used in various reconfigurable antenna and circuit applications including but not limited to low-energy, high-bandwidth com-munications, non-cooperative radar imaging, target sensor data collection, precision locating, and tracking.

AcknowledgementsThis work was supported by Army Research Laboratory (ARL) Multiscale Multidisciplinary Modeling of Electronic Materials (MSME) Collaborative Research Alliance (CRA) (Grant No. W911NF-12-2-0023, Program Manager: Dr. Meredith L. Reed). A.A. gratefully acknowledges the financial support provided through doctoral evidence acquisition year fellowship by the University Graduate School (UGS) at Florida International University.

Conflict of InterestThe authors declare no conflict of interest.

Keywordscoplanar waveguides, microheater matrix, phase change materials, reconfigurable antennae, vanadium dioxide

Received: April 19, 2017Revised: May 12, 2017

Published online:

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