**1. Introduction**

From sensing motion to identifying a gas and measuring temperature, sensors are a key element in our daily lives for analytical, monitoring, and diagnostic applications [1–3]. Following the progress of technology and current concerns for the protection of the environment and people, the development of these devices is expanding significantly, to transform chemical, mechanical, and thermal phenomena into a measurable quantity: electrical signal. Nowadays, we are facing an explosion in the sensor market, and the number of applications is expanding in parallel with advances in electronics and wireless communication technologies.

Temperature detection is currently one of the most expected needs, as it is generally not well controlled at a low cost. Temperature sensors have been one of the first fields of application of micro-systems, and they now represent a very important part of this market due to the increasing demand in the consumer and domestic application sectors but also in production, aeronautics, and health. The main characteristics currently required of these components are most often to be miniature, efficient, and economical and can be integrated into complex electronic systems.

Several research projects focus on optimizing the energy consumption of sensors by using innovative conservation techniques to improve the network's performance, including maximizing its lifespan. The sensor proposed in this chapter presents an interesting technological solution for temperature detection, thus allowing an extremely low consumption compared to conventional techniques. This new, highly integrated device requires no onboard power supply and uses electromagnetic transduction for temperature measurement.

## **2. Passive and wireless temperature sensor design**

For decades, dielectric resonators (RDs) have been very important in the microwave field for many applications, such as oscillators and filtering devices [4]. The significant progress in the development of dielectric materials, both in terms of reliability and in improving the loss tangent at microwave frequencies, makes it possible to use them from microwave frequencies to millimetric frequencies. As the dimensions of these resonators can nowadays be small, they can be integrated into many telecommunication systems, in particular filtering systems where dielectric resonators have made it possible to maintain very good characteristics while reducing their size. These fields have led to a mature technology that allows the realization of reliable devices.

We have included the device proposed by Guillon et al. [5], composed of a silicon platform with coplanar lines on membrane. The second part is a dielectric resonator mounted on a support between the two coplanar lines. The entire device was coupled to an Monolithic Microwave Integrated Circuit (MMIC) amplifier, which subsequently made it possible to design a millimetric oscillator [6]. We can see that the device designed by Guillon was intended for the realization of an oscillator, which is not the case for us. The objective was to explore the interest that this device can provide in measuring small fluctuations in the dielectric permittivity of a material sensitive to temperature change. Therefore, the proposed structure is designed with more powerful simulation tools than those that existed in the 1990s, since they allow us to simulate the entire structure: coplanar lines, sensitive material, and dielectric resonator. In addition, we have resized the device to have the best possible transmission signal.

The temperature sensor shown in **Figure 1** consists of two parts: the micromachined coplanar lines and the dielectric resonator covered with the sensitive material maintained by a support between and above these two lines. In the rest of this chapter, we will detail the design of this device before presenting the simulation results of the optimized structure.

#### **2.1 Presentation of the gallery modes**

Dielectric resonators operating in conventional electric (TE) or magnetic (TM) transverse modes radiate a significant portion of energy at millimeter wavelength frequencies [7]. In order to avoid dimensional problems and radiation losses at millimeter frequencies, we have chosen to use an excited dielectric resonator on gallery modes (whispering gallery modes).

From an electromagnetic point of view, one of the essential characteristics of WGMs is the distribution of energy in the resonator. The energy of the gallery modes has the particularity of being confined in a region close to the air—dielectric interface. Moreover, one of the main advantages offered by this type of mode is the possibility of exciting a dielectric resonator with an oversized geometry while remaining at the millimetric frequencies [1, 2]. The dimensions of a resonator

**159**

*Optimal Temperature Sensor Based on a Sensitive Material*

excited in a gallery mode are much larger than those of the resonators used in conventional TE or TM modes. This makes it easier to use them at high frequencies

since, given the dimensions of the resonator, it can be handled more easily. The gallery modes of dielectric resonators are classified into two families: WGHn,m,l (magnetic field gallery modes) and WGEn,m,l (electric field gallery modes). This nomenclature makes it possible to identify each mode by taking into account the state of polarization and the importance of the transverse components of the electromagnetic field [3]. Thus, we are able to distinguish, on the one hand, WGE modes where the axial component of the field is essentially magnetic, while the transverse components are mainly electric. On the other hand, we distinguish the WGH modes, which correspond to the dual modes of the WGE. The three integers n, m, and l indicate the spatial configuration of the electromagnetic field inside the resonator (number of variations of the field in the three directions of the

• n: number of variations along the azimuthal direction

• m: number of variations according to radial direction

• l: number of variations according to the axial direction

**2.2 Advantages of gallery modes for temperature detection**

It is important to mention that the azimuth number has an influence on the caustic radius. Indeed, a high azimuth number results in a higher caustic radius and therefore in a confinement of electromagnetic energy closer to the lateral surface of

Among the main advantages of gallery modes, we will note here the most important for our study. First of all, the dimensions of the resonator excited on a gallery mode are much larger than in the case of conventional TE or TM modes. This oversizing makes it possible to consider the use of this resonator at millimeter frequencies by facilitating temperature detection. On the other hand, thanks to the high-energy confinement in the dielectric, the vacuum quality factors are practically limited only to the loss tangent of the material used [3]. The latter will thus be very sensitive to the presence of a variation in the dielectric properties of the sensitive material, which will improve its detection thanks to an offset in the resonance frequency of a gallery mode.

*DOI: http://dx.doi.org/10.5772/intechopen.90733*

*Design of the temperature sensor: cross-section view.*

cylindrical reference mark):

the resonator.

**Figure 1.**

*Perovskite and Piezoelectric Materials*

of reliable devices.

possible transmission signal.

results of the optimized structure.

**2.1 Presentation of the gallery modes**

modes (whispering gallery modes).

transduction for temperature measurement.

**2. Passive and wireless temperature sensor design**

Several research projects focus on optimizing the energy consumption of sensors by using innovative conservation techniques to improve the network's performance, including maximizing its lifespan. The sensor proposed in this chapter presents an interesting technological solution for temperature detection, thus allowing an extremely low consumption compared to conventional techniques. This new, highly integrated device requires no onboard power supply and uses electromagnetic

For decades, dielectric resonators (RDs) have been very important in the microwave field for many applications, such as oscillators and filtering devices [4]. The significant progress in the development of dielectric materials, both in terms of reliability and in improving the loss tangent at microwave frequencies, makes it possible to use them from microwave frequencies to millimetric frequencies. As the dimensions of these resonators can nowadays be small, they can be integrated into many telecommunication systems, in particular filtering systems where dielectric resonators have made it possible to maintain very good characteristics while reducing their size. These fields have led to a mature technology that allows the realization

We have included the device proposed by Guillon et al. [5], composed of a silicon platform with coplanar lines on membrane. The second part is a dielectric resonator mounted on a support between the two coplanar lines. The entire device was coupled to an Monolithic Microwave Integrated Circuit (MMIC) amplifier, which subsequently made it possible to design a millimetric oscillator [6]. We can see that the device designed by Guillon was intended for the realization of an oscillator, which is not the case for us. The objective was to explore the interest that this device can provide in measuring small fluctuations in the dielectric permittivity of a material sensitive to temperature change. Therefore, the proposed structure is designed with more powerful simulation tools than those that existed in the 1990s, since they allow us to simulate the entire structure: coplanar lines, sensitive material, and dielectric resonator. In addition, we have resized the device to have the best

The temperature sensor shown in **Figure 1** consists of two parts: the micromachined coplanar lines and the dielectric resonator covered with the sensitive material maintained by a support between and above these two lines. In the rest of this chapter, we will detail the design of this device before presenting the simulation

Dielectric resonators operating in conventional electric (TE) or magnetic (TM) transverse modes radiate a significant portion of energy at millimeter wavelength frequencies [7]. In order to avoid dimensional problems and radiation losses at millimeter frequencies, we have chosen to use an excited dielectric resonator on gallery

From an electromagnetic point of view, one of the essential characteristics of WGMs is the distribution of energy in the resonator. The energy of the gallery modes has the particularity of being confined in a region close to the air—dielectric interface. Moreover, one of the main advantages offered by this type of mode is the possibility of exciting a dielectric resonator with an oversized geometry while remaining at the millimetric frequencies [1, 2]. The dimensions of a resonator

**158**

**Figure 1.** *Design of the temperature sensor: cross-section view.*

excited in a gallery mode are much larger than those of the resonators used in conventional TE or TM modes. This makes it easier to use them at high frequencies since, given the dimensions of the resonator, it can be handled more easily.

The gallery modes of dielectric resonators are classified into two families: WGHn,m,l (magnetic field gallery modes) and WGEn,m,l (electric field gallery modes). This nomenclature makes it possible to identify each mode by taking into account the state of polarization and the importance of the transverse components of the electromagnetic field [3]. Thus, we are able to distinguish, on the one hand, WGE modes where the axial component of the field is essentially magnetic, while the transverse components are mainly electric. On the other hand, we distinguish the WGH modes, which correspond to the dual modes of the WGE. The three integers n, m, and l indicate the spatial configuration of the electromagnetic field inside the resonator (number of variations of the field in the three directions of the cylindrical reference mark):


It is important to mention that the azimuth number has an influence on the caustic radius. Indeed, a high azimuth number results in a higher caustic radius and therefore in a confinement of electromagnetic energy closer to the lateral surface of the resonator.
