Electrically-reconfigurable metasurface with fully printed varactors

Abstract

Electrically-Reconfigurable Metasurface with fully printed vertical varactors with a substrate (1), a plurality of unit cells (10), each cell comprising resonators and a vertical varactor (30) coupling the resonators, the varactor having at least a lower electrode (2), at least an upper electrode (4), a tunable dielectric material (3) between the lower and upper electrodes, the electrodes and tunable dielectric material being manufactured solely by an additive process.

Description

BACKGROUND OF THE INVENTIONField of the Invention

[0001] The present invention relates to the field of electronic devices for microwave technology. More in particular, the invention refers to a reconfigurable metasurface.Description of the Related Art

[0002] A metasurface is a two-dimensional, ultra-thin material that controls the propagation of electromagnetic waves by adjusting the phase, amplitude, or polarization of light. Metasurfaces are made up of sub-wavelength structures, called meta-atoms, that are arranged normally periodically. These periodic structures are formed by unit cells. The extraordinary ability of metasurfaces to control electromagnetic waves has drawn considerable attention in recent times. However, metasurfaces are usually made of passive components, confining their functionality to a fixed state once fabricated. This limitation results in devices incapable of adapting to changing requirements. To address this, active elements must be integrated to enable dynamic tuning, which is critical for many applications. Consequently, reconfigurable metasurfaces as the ones disclosed in US20240195084-A1 have gained traction as a promising solution, offering adjustable control over wave propagation and reflection properties. Electrically tunable metasurfaces are particularly appealing due to the simplicity of their tuning mechanisms. In the microwave range (1 to 100 GHz), creating electrically reconfigurable metasurfaces typically involves incorporating discrete lumped components such as PIN diodes or varactors. These components are often soldered onto the metasurface in a secondary fabrication step, a process that tends to be slow and imperfect.SUMMARY OF THE INVENTION

[0003] The present invention addresses the challenges of existing metasurface designs by introducing a metasurface with MIM (metal-insulator-metal) varactors that is manufactured entirely through additive processes, such as inkjet or screen printing.

[0004] By adopting this vertical configuration, it becomes possible to achieve the close gap between electrodes necessary for tunability, overcoming the limitations of in-plane printing and expanding the scope of reconfigurable metasurface technology. Preferably, the vertical varactors have a barium strontium titanate (BST) as the tunable dielectric material, as the BST's voltage-dependent permittivity brings continuous states over a voltage range, thus allowing a dynamic range instead of a simple binary switch between state 1 and state 2 as in the prior art.

[0005] Positioning this tunable material (e.g. BST) between conductive electrodes, it is obtained a variable capacitor, commonly known as a varactor. Considering the capacitor's formulation, C=Aε / d, where ε represents the permittivity of the BST responsible for capacitance tuning, A is the area of the electrodes, and d is the distance between the electrodes. The parameter d, which corresponds to the thickness of the BST layer, plays a critical role in the design. This thickness directly determines the nominal capacitance, which serves as the baseline for tuning.

[0006] The thickness of the BST (barium strontium titanate) layer is defined relative to the operating wavelength λ in the microwave spectrum. For effective performance, the layer thickness is typically a small fraction of λ, with a range optimized to balance the dielectric properties and mechanical stability required for the specific application.

[0007] For metasurface applications in the microwave frequency range for the proposed absorber, the thickness of the barium strontium titanate (BST) dielectric layer is specified within a range of 5-30 µm, with 10 µm being identified as an optimal value for specific implementations. The thickness of the BST layer is determined by the selected manufacturing technique and its associated parameters, including but not limited to the viscosity of the ink, the mesh size employed in screen printing, the printing speed, and the dispenser size utilized in inkjet printing. Additive manufacturing techniques such as screen printing and inkjet printing are capable of producing BST layers with thicknesses within this defined range.

[0008] For applications requiring broader frequency ranges, a BST layer thickness closer to the 5-10 µm range may be necessary. However, nowadays, such reduced thicknesses are associated with increased risks of mechanical failure, including dielectric cracking and silver intrusion, which could result in electrical short circuits between the top and bottom electrodes. To mitigate these risks, and to ensure high repeatability and mechanical stability, particularly in periodic configurations of printed varactors, the BST layer thickness is preferably maintained within the 10-30 µm range. This specified range ensures a balance between structural reliability, optimal electrical performance, and manufacturing consistency, thereby facilitating the efficient fabrication of vertically integrated varactors.

[0009] More in particular, the invention provides an Electrically-Reconfigurable Metasurface with fully printed vertical varactors comprising a substrate, a plurality of unit cells, each cell comprising resonators coupled by a vertical varactor, where the varactors comprise at least a lower electrode, at least an upper electrode, a tunable dielectric material between the lower and upper electrodes, the electrodes and tunable dielectric material is manufactured solely by an additive process. The substrate is made of aluminium oxide Al 2 O 3 , sapphire, quartz, polyimide or PET, and the electrodes are made of Ag, Au-based inks and or conductive polymer-doped inks. Preferably, the substrate is made of Al 2 O 3 and has a thickness of 0.6 mm, a permittivity of 9.4, and a loss tangent of 0.0004.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] To complete the description and provide for better understanding of the invention, a set of drawings is provided. Said drawings illustrate a preferred embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. Figure 1 shows the difference between a static element as in-plane capacitors and active elements as in-plane varactors and vertical varactors. Figure 2 shows a printed vertical varactor suitable for the present invention. Figure 3 shows a print unit cell design of the metasurface of the invention. The zoom shows the printed vertical varactor layer by layer. The corresponding dimensions are: Pa = 19.57, Pb = 38. Figures 4a and 4b are the equivalent circuits of the unit cell and the varactor of the invention. Figures 5 and 6 are graphs showing the absorption of the metasurface of the invention shown in Figure 7. Figure 7 shows a periodic arrangement of unit cell illustrated in Figure 3, presenting a metasurface structure. It is also presented the biasing network and the effect in the corresponding electromagnetic performance switching the frequency band. The term "bias" refers to the application of a controlled voltage to the varactors to adjust their capacitance and, consequently, the electromagnetic response of the metasurface. Figure 8 shows a schematic of a feeding network example for biasing the reconfigurable metasurface with printed varactors. In this example, V1 represents the bias voltage applied to each varactor. DESCRIPTION OF THE INVENTION

[0011] The invention integrates vertical varactors (Figure 1) comprising a tunable dielectric material, such as barium strontium titanate (BST), using only additive manufacturing techniques like inkjet and screen printing. These varactors exhibit higher integrability in a fast and more efficient process as well as reduced complexity and low power consumption, surpassing the versatility of commercial discrete varactor. In addition, it offers high switching speeds, higher frequency of operation and linearity competing with another switching technologies such as electrochemical transistors. With reference to figures 2 and 7, the system comprises an electrically reconfigurable metasurface featuring unit cells (10) on a substrate (1), the unit cells comprising resonators and vertical varactors that are constructed using a vertical configuration of electrodes (2) enclosing a tunable dielectric material, for example a BST layer (3). In specific applications, such as the proposed metasurface absorber shown in Figure 7, though not limited to this use, a metallic ground plane (GND) is incorporated at the bottom of the structure. The configuration allows for the application of a voltage across the varactor, dynamically altering its permittivity and, consequently, the electromagnetic response of the metasurface.

[0012] The invention provides, in a particular embodiment, a substrate, such as Al 2 O 3 (1) and Ag electrodes (2). However, this approach is not limited to these materials. Thanks to the versatility of the printing process, a wide variety of inks can be utilized for the electrodes and resonators (metallic grid) including gold-based inks and conductive polymer-doped inks, offering flexibility in material selection to meet specific design and application requirements. A BST layer (3) is provided therebetween as the tunable dielectric material. Al 2 O 3 is a suitable material for the substrate because of its resistance to high temperatures (other options are also valid such as sapphire or quartz), since the BST layer is sintered at very high temperature after printing. If the BST formulation has a low sintering temperature as 200°C, other substrates can be employed, such as polyimide or PET.

[0013] By applying varying voltages to the varactors (30), the permittivity of the BST layer (3) changes, enabling dynamic control over functionalities such as frequency tuning and beam steering. For example, the metasurface absorber of the invention demonstrated a 5% tunability in central frequency, adjusting from 4.4 GHz to 4.2 GHz. The equivalent circuits of both the unit-cell and the varactor are shown in Figures 4a and 4b.

[0014] Preferably, the aluminum oxide Al 2 O 3 substrate has a thickness of 0.6 mm, a permittivity of 9.4, and a loss tangent of 0.0004. We aim for a substrate with low loss tangent because excessive losses in the substrate of an absorber reduce its efficiency by dissipating a significant portion of the incident energy as heat rather than allowing it to be absorbed by the designed resonant structure. This can lead to non-selective absorption, reduced resonance quality (Q-factor), and challenges in impedance matching at the target frequency, which are critical for achieving high absorption at specific frequencies. A permittivity of 9.4 is good for designing a metasurface perfect absorber in the GHz frequency range because it enables compact designs while maintaining strong electromagnetic coupling. High permittivity materials allow for smaller unit cells and thinner substrates, enhancing the ability of the absorber to confine the electromagnetic fields and achieve resonance at the target frequencies.

[0015] The location and dimensions of the varactor within the resonator are optimized to maximize the tuning of the resonance frequency while ensuring compatibility with the printing process. Ideally, minimizing the size of the varactor enhances tunability, however, practical considerations during fabrication impose constraints on the electrode dimensions. Specifically, the width of the electrodes cannot be smaller than the printing resolution, which is approximately 100 µm for the screen-printing process. In addition, the alignment resolution between layers is approximately twice as large as the printing resolution. This low resolution increases the potential for errors, raising the likelihood of short circuits. To mitigate these risks, the electrode size should exceed the resolution limit while incorporating an error margin to account for potential inaccuracies during fabrication. This approach ensures reliable performance and structural integrity in the final design.

[0016] Other reconfigurable electronic devices using commercial diodes typically require a feeding network divided across two planes and connected through vias to route the bias voltage. In contrast, a fully printed reconfigurable device inherently simplifies the feeding network (Figure 8) by integrating the biasing connections directly into the top plane unit cell design (10). By embedding these connections within the same layer as the resonant structure, the design eliminates the need for vias and additional alignment steps, streamlining the fabrication process. This intrinsic implementation ensures connectivity between cells, avoid weldings and facilitates the application of bias voltage while maintaining the compactness and functionality of the metasurface. Examples of feeding the reconfigurable metasurface are illustrated in Figure 7 and Figure 8 (though not limited to these configurations). These configurations involve applying a positive voltage to one of the bottom electrodes (bias) and a negative voltage (Figure 8) or ground (Figure 7) to the other bottom electrode.

[0017] The schematic in the inset of Figure 7 illustrates the band tuning concept by applying different bias voltages (Bias 1 and Bias 2). In this case is represented the S11 parameter which represents the reflection coefficient. This frequency band switching is demonstrated in Figure 5 through electromagnetic field simulations, where the permittivity of the BST is changed according to the applied bias voltages to evaluate each state absorption.

[0018] This feeding method is designed to uniformly tune all the printed vertical MIM varactors of the metasurface by applying the same voltage (6) to each varactor electrode (5). However, alternative feeding methods may be employed for other applications, such as beam steering, where different voltages can be applied to individual varactor diodes to achieve the desired functionality.

[0019] The functionality of the absorber is based on matching the surface impedance of the metasurface with the free space impedance. This is achieved through careful engineering of the metallic grid layer printed on top of the dielectric. The periodicity and geometry of the grid define the coupling between neighbouring unit cells and establish the resonance conditions that dictate impedance matching. In this absorber unit cell specific case aiming to work in the 3GHz band (wavelength (λ) equal to approximately 0.1mm), which gives subwavelength periodicity sizes. In the horizontal direction is Pa = 0.195λ while in the vertical direction is Pb = 0.38λ. When an electromagnetic wave impinges on the metasurface at the resonant frequency, it excites distinct current distributions depending on the polarization.

[0020] The metasurface is designed to support tunable absorption for transverse magnetic (TM) polarization due to the resonance response in the gap where the varactor is placed. The varactor modulates the resonance by altering the effective capacitance of the gap between the top and bottom ELCs, which directly influences the impedance matching for that polarization. The varactor is positioned exclusively in the vertical gap, enabling tunability for the vertical polarization while leaving the horizontal polarization unaffected. This specific placement avoids the complexity of introducing additional feeding networks that would be required if tunability were extended to the horizontal direction as well. This design choice simplifies implementation while achieving tunability in the desired polarization.

[0021] With reference to Figure 3, a unit cell (10) according to the invention comprises two single polarized electric-LC resonators (ELCs) (20) coupled by the printed varactor. It has a bottom ground layer of PEC (Perfect Electric Conductor) and an Al203 substrate with thickness 0.6 mm, permittivity equal to 9.4 and loss tangent with a value of 0.0004. In Figure 3 we observe the layer-by-layer decomposition of the varactor architecture: the top figure shows the printed bottom electrodes (2), then the BST layer (3) which is tuned by the potential applied between the bottom (2) and top electrodes (4). Figure 4a shows the equivalent circuit of a perfect metasurface absorber. In this equivalent circuit, the impedance grid designed is dependent of the permittivity change in the BST layer. This permittivity change is governed by the architecture of the varactor, which can be analytically represented by its equivalent circuit. In this model, the inductance corresponds to the current flow through the electrodes, while the capacitance is determined by the BST layer gap between the bottom and top electrodes. In this way, the reconfigurability can be controlled positioning the varactors in key positions of the current distribution to make a substantial change using this capacitance change. In Figure 5, the absorption switch for TE polarization is observed. This absorption is calculated following the well-known absorption formula A = 1 - |S1,1| 2< . A tuning of the resonance from 4.4GHz (ε = 50) to 4.2GHz (ε = 300) is observed. In the inset figure is represented the surface currents in the unit cell for the tm polarization and can be observed how the area with higher amplitude is in the varactor zone. On the other hand, Figure 6 illustrates the absorption for TM polarization. In this case, as shown in the inset depicting the current distribution for this polarization, the currents are primarily localized in the top and bottom regions of the ELC, rather than in the coupling gap as seen previously. Since the resonance is not dependent on the coupling varactor of the ELC, the tunability is minimal, as evidenced by the negligible resonance variation observed with changes in BST permittivity.

[0022] The invention has the following advantages: Mass-Manufacturability. - by leveraging additive manufacturing, the process eliminates the need for manual assembly, significantly enhancing scalability. Fast prototyping. - since it is a process that can be performed by printers, it is not needed a complex factory to manufacture advanced devices. Performance. - the invention achieves a high frequency of operation (up to 30 GHz) with precise electrical tuning, allowing for applications in advanced wireless communication and radar systems. Thermal Stability. - unlike thermal switching mechanisms, the BST varactors operate through electrical tuning, enabling rapid and energy-efficient reconfiguration.

[0023] Applications for the invention are, as way of example: Smart Radio Environments (enhances dynamic control over wireless communication systems); Stealth Technology (tunable absorbers for low observability coating); Energy-Efficient Systems (adaptive filtering and beam steering for loT applications).

[0024] The manufacturing method uses a composite barium strontium titanate (BST) thick films and printed silver electrodes. For the conductor electrodes a commercial silver paste can be used, preferably with high resistance to elevated temperatures and remarkable conductivity (in the proposed absorber case the silver inks have a sheet resistance values as low as 9 mΩ / sq / mil).

[0025] The screen-printing process for fabricating the MIM varactor involves sequentially depositing each layer onto an alumina substrate using a mesh screen. First, the bottom electrode pattern is printed with a conductive silver paste through a 90-tpcm mesh screen, ensuring precise alignment and uniform layer thickness. After printing, the layer is dried at approximately 150°C to stabilize it. Next, the BST dielectric layer is screenprinted over the bottom electrode using a separate mesh screen aligned to ensure proper overlap. This layer is dried and co-fired in purified air at 850°C for 1 hour. Finally, the top electrode is printed over the BST layer, maintaining precise alignment with the bottom electrode to form the MIM architecture. Each printed layer undergoes drying to remove residual solvents and ensure stability before co-firing. This sequential process ensures accurate layer positioning and uniformity, which are critical for the performance of the MIM varactors in microwave applications where the resolutions requirements are the order of micrometers.

[0026] As it is used herein, the term "comprises" and derivations thereof (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc. On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.) to be within the general scope of the invention as defined in the claims.

Claims

1. Electrically-Reconfigurable Metasurface with vertical varactors characterized in that it comprises: - a substrate (1); - a plurality of unit cells (10), each cell comprising resonators (20) coupled by a vertical varactor (30) with - at least a lower electrode (2), - at least an upper electrode (4), - a tunable dielectric material (3) between the lower and upper electrodes, the electrodes and tunable dielectric material being manufactured solely by an additive process.

2. The Electrically-Reconfigurable Metasurface of claim 1, wherein the tunable dielectric material (3) is a layer of barium strontium titanate (BST).

3. The Electrically-Reconfigurable Metasurface of claims 1 or 2, wherein the substrate is made of aluminium oxide Al2O3, sapphire, quartz, polyimide or PET, and the electrodes are made of Ag, Au-based inks and or conductive polymer-doped inks.

4. The Electrically-Reconfigurable Metasurface of claim 3, wherein the substrate is made of Al2O3 and has a thickness of 0.6 mm, a permittivity of 9.4, and a loss tangent of 0.0004.

5. The Electrically-Reconfigurable Metasurface of any claims 1- 4 for microwave absorber applications.

6. The Electrically Reconfigurable Metasurface of any claims 1-5 further provided with a biasing feeding network fully printed and integrated in the unit cell of the Electrically-Reconfigurable Metasurface.

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