**1. Introduction**

It is quite evident that extending or improving human senses has enabled human societies to prosper by acquiring information from their surroundings and gaining knowledge from it. Internet of Things (IoT) embody this trend today combining distributed sensing with high connectivity so that wise decisions and actions follow information gathering and analysis [1, 2]. Trillion Sensors is another paradigm onto which IoT is further exploited on the basis that the more extensive or intensive the deployment of sensor networks is, the more fruitful the knowledge that can be derived from them would be [3, 4].

Small dimensions (nanometers to micrometers) are appropriate in the sensitive part of sensors when they need to interact with phenomena or entities equally characterized by such small dimensions (light, molecules, living cells … ). An overall small size for the sensors themselves is not devoid of interest either. The smaller they are, the more sustainable their fabrication is in terms of materials and energy, and the more cost-effective they become. Small size is also enabling in itself, e.g. medical implants, as well as convenient, e.g. payloads.

**2.1 Micro thermal device architecture**

*DOI: http://dx.doi.org/10.5772/intechopen.96246*

*Managing Heat Transfer Issues in Thermoelectric Microgenerators*

power (*VI)*.

thermocouples.

cold(er) part of it.

**351**

technologically convenient.

defined.

The traditional thermoelectric generators mentioned above feature a π-architecture, where the π symbol gives a visual clue about how each thermocouple is built assembling vertically two semiconductor pellets (*aka* legs) of different polarity (to add-up the contribution of both electrons and holes) and connecting them electrically with a horizontal conductive strip. Several of those thermocouples are then connected in 1D or 2D arrangements [8]. Such disposition is well adapted to exploit vertically occurring gradients: the bottom part is placed in contact with the heat source while the top part contacts the heat sink and the thermoelectric material in between translates the heat flowing through it (or the temperature difference spanning across it) into magnitudes of electrical relevance, *V* and *I*, and therefore

Silicon technologies are of planar nature. They enable massive parallelism at *x* and *y* directions for shaping *laterally* devices made from the superposition of several active thin films. Such shaping also involves patterning in the *z* direction, but the accumulated depth of the films is much lower than the lateral dimensions at play, leading to aspect ratios that are opposite to those that characterize π –shape

The main objective when defining the architecture and the technological route for a *micro* thermoelectric device is to obtain two areas of contrasted temperature in the *surface* of the chip since the thermoelectric materials will be arranged *laterally*. An architecture that translates an external vertical gradient into an internal lateral one is called *transversal,* and to make it possible a *thermal isolated platform* is

The platform consists of a thin silicon area fabricated by eliminating the silicon beneath it. In order to preserve its thermal isolation from the surrounding bulk silicon, the physical connections between them should be minimized. Such connections are the mechanical supports that keep the platform in place (e.g. ancillary silicon bridges) and the thermoelectric materials themselves (and whatever supports they may need). In order to minimize the thermal conduction of these elements, they must be produced with *low thermal conductance eith*er by resourcing to low thermal *conductivity* materials, when available and technologically feasible, or

**Figure 1** shows the schematics for such a device. Any hot surface in which this device is placed will act as a heat source. The top surface will be exposed to air acting as heat sink and will exchange heat with it. Due to their different thermal mass, the bulk rim area will hardly cool down, thus being the hot part of the device, while the platform will experience a larger decrease of temperature becoming the

With respect to the thermoelectric material, the depicted device follows a unileg approach. Two thermoelectric materials are still at play, but a metal one replaces one of the semiconductor legs in order to close the circuit. Some thermoelectric performance is sacrificed because metals behave poorly thermoelectrically (they have higher thermal conductivities and close to zero Seebeck coefficients), but for the architecture presented and to keep processing simple, the use of a metal leg is

Regarding the semiconductor thermoelectric material, one distinct feature of our approach is resourcing to silicon materials, namely arrays of silicon nanowires (Si NWs). The rationale behind this option is to attempt the fabrication of *all-silicon* microgenerators, thus leveraging the full potential of silicon technologies. Thermoelectric performance of bulk silicon at ambient or moderate temperatures is bad because of its high thermal conductivity. Incidentally, this is the reason why it is

by acting on their *geometrical dimensions* making them long and thin.

Sensing requires energy. A certain provision of energy autonomy is needed for sensors to be deployed in remote locations, harsh environments, or where they need to remain temporary unattended. Batteries is a common way to provide such autonomy, but their charge is finite impeding long-term autonomy scenarios. Moreover, their recharge, replacement and disposal imply a logistic and environmental burden that will not be affordable when IoT gets to its full extent mobilizing tens of billions of devices and an even larger number of sensors.

Secondary batteries can be kept recharged by coupling them with energy harvesters able to draw energy present in the environment [5]. Heat is abundant in natural scenarios, and waste heat is also abundant in human-made scenarios due to laws of thermodynamics and our profuse use of thermal machines. When such heat gives rise to temperature gradients (a situation as simple as a hot surface exposed to air), thermoelectricity is a convenient way to extract electric energy from them [6, 7].

For that extraction to be optimum, the external thermal gradient needs to be fully transposed into the thermoelectric generator itself. Physical interaction of small devices with their environment may exploit profitably some scale factors when going down in dimensions, but, sometimes, small sizes pose a handicap or challenge for such interaction, too. This is the case when trying to cool down locally a part of a small device by exchanging heat with the surrounding air. This chapter tries to illustrate this point by sharing the issues and strategies the authors have dealt, and are dealing with, in their quest for silicon-based miniaturized thermoelectric generators.

#### **2. Silicon-based thermoelectric generators**

Silicon technology has been developed around an enabling and highly abundant semiconductor material. It is a mature technology apt to mass-production of devices with economy of scale and it is the champion technology of miniaturization. Not surprisingly, it boosted microelectronics in the XX century and nanoelectronics in the XXI century. In addition to the set of techniques that allow the fabrication of integrated circuits by depositing and patterning thin films on a silicon wafer, silicon technologies also developed micromachining techniques that allow carving and shaping the silicon wafers into structures that are able to interact with the environment. Sensors and actuators belong to the latter category. Since energy harvesters are environmental interacting devices and, application-wise, they should not be much larger than the sensors they will feed, it is only logical that their fabrication will similarly benefit from the silicon technologies toolbox. These technologies do not only excel in miniaturization but also in *integration* capabilities. This is an important aspect as well. Traditional thermoelectric generators are *assembled* from couples of semiconductor pellets, various millimeters in side, that are arranged electrically in series and thermally in parallel together with additional connecting strips and appropriate thermal elements. When going down in dimensions, assembly becomes harder and offers much less latitude for process automation. In this way, resourcing to technologies that inherently offer integration capabilities is convenient, if not a must.
