Tunable Inductors Using Integrated Vanadium Dioxide Phase Change Thin Films

This paper presents tunable inductors designed with vanadium dioxide (VO2) thin film to implement tunability. Two types of configurations, a single line coil inductor and a dual line coil inductor, are proposed. Tunable inductance is realized by thermally controlling VO2 thin film stub to be an insulator or a conductor.Themeasurements of inductors are taken in the frequencies range from 0.01 GHz to 10GHz. The electrical model has been developed to derive the corresponding inductance. The results show that the tuning range is about 69% when the temperature changes from room temperature to above the critical temperature of 68C.


Introduction
Tunable inductor has been a research focus for years to achieve more versatility in modern radio frequency circuits.Tunable inductors with good  values can lead to reconfigurable communication systems.Tunable inductors are building blocks of reconfiguration resonators, filters, impedance matching networks, voltage-controlled oscillators (VCOs), or power amplifiers [1].Due to demand for high performance and small size, RF tunable inductors have a major role in RF circuit design.Although the MEMS structures were widely used in many applications, they require complex fabrication and structures [2,3].To achieve high quality and low loss requirements, the size of the inductor is usually large, and the fabrication process is normally complicated compared to the device proposed in this article [4][5][6].The proposed research makes it possible for a novel tunable inductive structure with smaller size and straight forward manufacturing.The structure of the tunable inductor is a planar design with simple fabrication process, and the vanadium dioxide (VO 2 ) thin film stubs are used as switches instead of having any movable parts such as in a MEMS switch [7,8].
This paper highlights the latest results of a continuous effort in tunable inductor research [9].In this study, VO 2 thin films were fabricated on a sapphire substrate.The VO 2 thin film is one of the insulator-to-metal (ITM) phase transition materials at a transition temperature of 68 ∘ C with excellent electrical properties, which is utilized as a switch in the circuit [9].These tunable inductors are thermally controlled to accomplish variable inductance [9].Due to a large change in electrical resistance, as well as electromagnetic properties, VO 2 thin films are attractive for a wide variety of applications from RF switches to tunable antennas [9][10][11][12][13][14][15][16][17][18][19].The VO 2 thin films also have memristive properties, which can be demonstrated as an adaptive ("learning") filter by using a VO 2 based memristor [20].To expand applications of VO 2 thin films, many methods of changing the transition temperature of VO 2 have been verified.The VO 2 thin films synthesized on aluminum nitride (AIN)/Si (111) substrates revealed ITM transition at 350 K (76.8 ∘ C) [21].The tungsten-(W-) doped VO 2 thin films revealed a decrease of the transition temperature to 321 K (49 ∘ C) [14].In this study, all the inductor structures and corresponding equivalent electrical models are designed and simulated in AWR design environment.The design method of structures and fabrication are presented in Section 2. Experimental results are analyzed in Section 3. The summary and conclusions are provided in Section 4.

Tunable Inductor Designs
The tunable inductors structures shown in Figure 1 are inductor designs studied in this paper.Figure 1(a) is the single line coil inductor without any VO 2 .It is used as a reference design to assess other designs with VO 2 .In this design, Au is used as the material for the inductor.Figures 1(b) and 1(c) are dual line designs with various numbers of turn parallel with the top Au only inductor.The bottom coil inductor consists of two short portions of VO 2 thin film at both ends and Au thin film in between.Therefore, when VO 2 portion is turned on by the temperature, the VO 2 controlled bottom coil inductor will be turned on as well to form parallelly connected inductors.To use network analyzer to study the inductor performance, all designs use single layer coplanar structures (1568 m × 1180 m) with ground/signal/ground coplanar waveguide lines.The width of the inductor coil is 25 m.The VO 2 thin films are deposited and patterned in the dark portion labeled in Figure 1.
At room temperature, the VO 2 thin film is in the high resistivity state (insulator); the input signal of the dual line coil structures can only pass through the top metal coil but not the bottom VO 2 coil, which resembles the behavior of the single line inductor.When the temperature is above 68 ∘ C, VO 2 thin film is in the low resistance state (conductive), and part of the input signal will be able to pass through the VO 2 side of coil and thus the equivalent inductance of the whole structure varies.
The tunable inductor test structures were fabricated on 3 inch diameter sapphire substrate wafer.First, the 150 nm VO 2 thin films were deposited by pulsed laser deposition system at 500 ∘ C and etched in RIE.Secondly, the 350 nm metal layer (20 nm Ti, 30 nm Pt, and 300 nm Au) was deposited using electron beam evaporation technique followed by standard positive photoresist liftoff process.Lastly, a layer of photoresist is used to cover the VO 2 thin film to prevent oxidation with only probing regions exposed.The details of fabrication process can also be found in our previous publication [9].The devices were measured using an HP 8720 Vector Network Analyzer and a thermoelectric temperature controller with a frequency sweep from 0.01 GHz to 10 GHz.

Results and Discussion
Figure 2 shows the resistivity measurement results of a 2 mm by 0.1 mm VO 2 thin film connect with two 500 m by 500 m Au conductor deposited on sapphire substrate.The resistivity curves show the insulator-to-metal (ITM) phase transition around 68 ∘ C. The resistivity variation is approximately four orders of magnitude.The measured results exhibit that VO 2 thin film has high quality.
The measured results of insertion loss for all inductor designs at 25 ∘ C are shown in Figure 3.It can be seen that all curves are almost coincident.This means that the dual line coil inductors structures behave the same as single line coil structure at room temperature.The VO 2 pads act as open switches and the signal cannot be passed through the bottom inductor coil.
Measurements of scattering parameters (-parameters) for all devices at 80 ∘ C are given in  The tunable inductor structures can be electrically modeled by using AWR design environment software.The equivalent electrical model for single line coil inductor (Figure 1(a)) and the equivalent electrical model for dual line coil inductors (Figures 1(b) and 1(c)) are shown in Figures 5(a) and 5(b), respectively.The effective inductance of single line coil inductor is assumed to be .At room temperature, the effective inductance of dual line coil inductor is equal to the inductance of single line coil due to the signal being blocked by the insulating VO 2 thin film.At high temperature, the effective inductance of dual line coil inductor can be calculated by  in parallel with   .The value of   is controlled by the various numbers of turns of the bottom coil.Inductance variation of each structure can be compared with that of the reference structure.Equivalent inductance can be extracted from this equivalent electrical model by matching the scattering parameter of the experimental results and the electrical model.Figure 6 shows the matched results.The   total inductance values at high temperature are matched and calculated as shown in Table 1.The effective inductance of dual line coil inductor decreases at high temperature due to the signal can pass through the bottom coil inductor.
Tunability is defined by using [22] Tunability =  single −    single × 100%, where  single is the total inductance of single line coil structure and   is the total inductance of dual line coil structure, with  being the number of turns in the bottom coil.Tunability of inductors structures is summarized in Table 1.The inductance turning is above 69% when the VO 2 thin film changes from insulating at room temperature to conducting above the critical temperature of 68 ∘ C.

Figure 1 :
Figure 1: Top views of fabricated tunable inductors structures with (a) single line coil inductor, (b) dual line coil inductor with two turns in the bottom coil, and (c) dual line coil inductor with four turns in the bottom coil.Yellow areas are gold.Black areas on the yellow wires are the VO 2 stubs.The dimension of VO 2 is 23.1 m × 26.3 m shown in (d).All units are in m.

Figure 4 .Figure 2 :
Figure2shows the resistivity measurement results of a 2 mm by 0.1 mm VO 2 thin film connect with two 500 m by 500 m Au conductor deposited on sapphire substrate.The resistivity curves show the insulator-to-metal (ITM) phase transition around 68 ∘ C. The resistivity variation is approximately four orders of magnitude.The measured results exhibit that VO 2 thin film has high quality.The measured results of insertion loss for all inductor designs at 25 ∘ C are shown in Figure3.It can be seen that all curves are almost coincident.This means that the dual line coil inductors structures behave the same as single line coil structure at room temperature.The VO 2 pads act as open switches and the signal cannot be passed through the bottom inductor coil.Measurements of scattering parameters (-parameters) for all devices at 80 ∘ C are given in Figure4.Measured insertion loss ( 21 ) is in the range from −0.7 dB to −2.5 dB when the VO 2 patches change from room temperature to

Figure 3 :
Figure 3: Measured results of  21 magnitudes for single line coil inductor structure and dual line coil inductor structure at room temperature.

Figure 4 :
Figure 4: Measured results of  21 magnitudes for single line coil inductor structure and dual line coil inductor structure at high temperature (80 ∘ C).

Figure 5 :Figure 6 :
Figure 5: The (a) equivalent model for single line coil inductor structure and (b) equivalent electrical model for dual line coil inductor structure.

Table 1 :
Inductance and tunability of electrical model.