**4.2 Manual assembly and impact on measurements**

The significant increase in the generated power when applying a forced convection or a cold finger and the results from the simulations including a heat exchanger directed our efforts to the construction of the previously described heat exchanger assembly on our μTEGs (see **Figure 10**).

The preparation sequence of the required components is given in **Figure 14a**. A heat exchanger adapter is made from four Cu wires (one per on-chip a device), with diameter similar to the size of the suspended platform (which they will contact after the assembly) inserted in a square brass piece and machined to the appropriate length. The tips of the wires are dipped with thermal paste (**Figure 14b**) to fill the gap between the Cu wires and the suspended platforms to guarantee good thermal contact (**Figure 14c**). A PMMA spacer with a thickness appropriately matching the length of the protruding Cu wires is then assembled between the heat exchanger adapter and the μTEG, and finally, the aluminum heat exchanger is placed on top of adapter using a thermal paste (**Figure 14d**). Further details can be found in [27] from which **Figures 14**–**17** have been adapted.

#### **Figure 14.**

*(a) Steps of the construction of the heat sink adapter. Optical microscope images of (b) the Cu wire dipped in thermal paste and (c) the footprint left on the platform of the test device. (d) An image of the final assembly.*

Through the described integration scheme, a first evaluation of the performance improvement brought by a heat exchanger to the μTEG is enabled. In this study harvesting measurements with and without heat exchanger were performed by placing the assembled devices on a Linkam THMS 350 V heating stage at various temperatures in a natural convection environment. Three different cases were measured: without heat sink, with heat sink and with heat sink + pressing, where for the latter a force is applied on top of the assembly to reduce the thermal resistance of the thermal paste. Chips with different thermoelectric materials were measured: Si NWs, Si-Ge NWs and Si microbeams. At the current stage of technology maturity, a rather low number of devices has been measured, but the results shown in the next subsections correspond to significant devices of each category. In terms of measurement uncertainties, the most important source are thermal fluctuations that due to the thermoelectric nature of the device introduce variations in the measured V and I, which have been estimated to be below 10 μV and 1 μA, respectively.

As expected, a tremendous increase in power density was observed after the integration of the heat exchanger + pressing, and values in the range 7–42 μWcm<sup>2</sup> were observed. No clear trends were observed with the number of trenches.

*Seebeck voltage vs. hot plate temperature for SiGe NWs-based μTEGs with different number of trenches (T1-*

*Managing Heat Transfer Issues in Thermoelectric Microgenerators*

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

For devices with SiGe NWs, considerable higher Seebeck voltages were observed

, respectively, considering a T3

. This result evidences again that once the heat exchanger

when compared to Si NWs (**Figure 16**), due to the higher thermal resistance resulting from the lower intrinsic thermal conductivity of the former. With and without heat exchanger, the devices performed better with increasing number of trenches. Also, power densities rose considerably with the integration of the heat exchanger. As already observed for the Si NWs, the voltage and generated power improved further when a slight pressure was applied to the heat exchanger, It is worth noticing that the maximum power thus obtained does not differ much for Si

device on a hotplate at 100 °C. This points to the dilution of the effect of better

Si microbeams devices were fabricated to compare the performance of bulk Si with Si NWs. After the integration of the heat exchanger + pressing, the results presented in **Figure 17** show a remarkable three orders of magnitude increase in the

generated power from 650 pW to 690 nW for a T1 device, i.e. from 32.5

*I-V and power curves for the Si microbeams based μTEGs without heat exchanger (left) and with heat*

*exchanger and pressing (right) for a hot plate temperature of 100 °C.*

starting thermal properties when the heat exchanger is present.

*4.2.2 Measurements with SiGe NWs*

*T3) with and without heat exchanger.*

**Figure 16.**

and SiGe NWs: 41.6 μWcm<sup>2</sup> vs. 45.2 μWcm<sup>2</sup>

*4.2.3 Measurements with Si microbeams*

nWcm<sup>2</sup> to 34.5 μWcm<sup>2</sup>

**Figure 17.**

**365**

#### *4.2.1 Measurements with Si NWs*

The Seebeck voltage vs. hotplate temperature curves for the Si NWs-based μTEGs with different number of trenches are shown in **Figure 15**. As anticipated, all the devices presented output voltages that scaled with the number of trenches (i.e. length of NWs). However, the reduction of the thermal resistance between the cold side (suspended platform) and the ambient when a heat exchanger is integrated resulted in a large increase of ΔT across the NWs and hence higher overall voltages.

In terms of power, the maximum power densities obtained at hot plate temperatures of 100 °C without the heat exchanger were in the range of 0.05–0.1 μWcm<sup>2</sup> .

**Figure 15.** *Seebeck voltage vs. hot plate temperature for Si NWs-based μTEGs with different number of trenches (T1-T4) with and without heat exchanger.*

*Managing Heat Transfer Issues in Thermoelectric Microgenerators DOI: http://dx.doi.org/10.5772/intechopen.96246*

**Figure 16.**

Through the described integration scheme, a first evaluation of the performance improvement brought by a heat exchanger to the μTEG is enabled. In this study harvesting measurements with and without heat exchanger were performed by placing the assembled devices on a Linkam THMS 350 V heating stage at various temperatures in a natural convection environment. Three different cases were measured: without heat sink, with heat sink and with heat sink + pressing, where for the latter a force is applied on top of the assembly to reduce the thermal resistance of the thermal paste. Chips with different thermoelectric materials were measured: Si NWs, Si-Ge NWs and Si microbeams. At the current stage of technology maturity, a rather low number of devices has been measured, but the results shown in the next subsections

correspond to significant devices of each category. In terms of measurement uncertainties, the most important source are thermal fluctuations that due to the thermoelectric nature of the device introduce variations in the measured V and I,

The Seebeck voltage vs. hotplate temperature curves for the Si NWs-based μTEGs with different number of trenches are shown in **Figure 15**. As anticipated, all the devices presented output voltages that scaled with the number of trenches (i.e. length of NWs). However, the reduction of the thermal resistance between the cold side (suspended platform) and the ambient when a heat exchanger is integrated resulted in a large increase of ΔT across the NWs and hence higher overall voltages. In terms of power, the maximum power densities obtained at hot plate temperatures of 100 °C without the heat exchanger were in the range of 0.05–0.1 μWcm<sup>2</sup>

*Seebeck voltage vs. hot plate temperature for Si NWs-based μTEGs with different number of trenches (T1-T4)*

.

which have been estimated to be below 10 μV and 1 μA, respectively.

*Heat Transfer - Design, Experimentation and Applications*

*4.2.1 Measurements with Si NWs*

**Figure 15.**

**364**

*with and without heat exchanger.*

*Seebeck voltage vs. hot plate temperature for SiGe NWs-based μTEGs with different number of trenches (T1- T3) with and without heat exchanger.*

As expected, a tremendous increase in power density was observed after the integration of the heat exchanger + pressing, and values in the range 7–42 μWcm<sup>2</sup> were observed. No clear trends were observed with the number of trenches.

#### *4.2.2 Measurements with SiGe NWs*

For devices with SiGe NWs, considerable higher Seebeck voltages were observed when compared to Si NWs (**Figure 16**), due to the higher thermal resistance resulting from the lower intrinsic thermal conductivity of the former. With and without heat exchanger, the devices performed better with increasing number of trenches. Also, power densities rose considerably with the integration of the heat exchanger. As already observed for the Si NWs, the voltage and generated power improved further when a slight pressure was applied to the heat exchanger, It is worth noticing that the maximum power thus obtained does not differ much for Si and SiGe NWs: 41.6 μWcm<sup>2</sup> vs. 45.2 μWcm<sup>2</sup> , respectively, considering a T3 device on a hotplate at 100 °C. This points to the dilution of the effect of better starting thermal properties when the heat exchanger is present.

#### *4.2.3 Measurements with Si microbeams*

Si microbeams devices were fabricated to compare the performance of bulk Si with Si NWs. After the integration of the heat exchanger + pressing, the results presented in **Figure 17** show a remarkable three orders of magnitude increase in the generated power from 650 pW to 690 nW for a T1 device, i.e. from 32.5 nWcm<sup>2</sup> to 34.5 μWcm<sup>2</sup> . This result evidences again that once the heat exchanger

#### **Figure 17.**

*I-V and power curves for the Si microbeams based μTEGs without heat exchanger (left) and with heat exchanger and pressing (right) for a hot plate temperature of 100 °C.*

is in place, the thermal properties of the thermoelectric material become second order. Hence, by optimizing their electrical properties and ensuring a good ΔT with the aid of a heat exchanger, it is possible to obtain high power densities even with high thermal conductivity thermoelectric materials such as Si microbeams.

### **4.3 Semiautomatic assembly with integration density**

In order to translate the promising power densities of a single structure into useful absolute power levels, a certain number of thermocouples needs to be integrated and connected. The μTEGs design was modified to attain a higher integration density by reducing the number of active sides. The new thermocouple has a rectangular shape with one side featuring the membrane providing mechanical support and metallic connection, and the opposite side composed of the trenches to be filled with NWs. In **Figure 18**, a 3D schematic of the new unitary thermocouple is shown. The same cross-section profile of **Figure 1** still applies. With this new design, many elements can be integrated in the same chip: up to fifty thermocouples (each with an approximate size of 5 x 0.6 mm<sup>2</sup> ) fit in series or series–parallel configuration in a 49 mm<sup>2</sup> chip, as shown in **Figure 19**. Both configurations would lead to the same harvested power, but the series one will scale up voltage while the parallel one will scale up current.

This compact design requires new components and a novel and more efficient approach for the integration of the heat exchanger in order to boost their thermal performance. A micromachined Si adapter (substituting the Cu wires, brass plate and PMMA spacer of the previous section) is necessary for the distribution of the force exerted on the platform by the heat exchanger, and different designs featuring the corresponding serial or parallel arrangements were fabricated. **Figure 20** (left) depicts the Si adapter designs where the central columns contact the platform of each thermocouple in the chip and the ones at the corners act as force distributers. Similar to the previous section, a commercial Al mini heat exchanger will be placed on top of the Si adapter to help the circulation of heat from the Si rim of the μTEG (warm side) through the thermoelectric material to the platform (cold side) to achieve the desired larger ΔT. The heat flux representation through the assembly is shown in **Figure 20** (right).

To achieve a good thermal contact, which is key for a maximum generated power, a thermal interface material (TIM) needs to be placed between the thermocouple and the adapter. To this aim, an inkjet printer (Dimatix) was chosen to deposit a controlled amount of TIM only on the columns of the adapter.

After dispensing the TIM, the adapter is placed face-down onto the thermocouple chip already wirebonded on a PCB. This is done with the help of a pick & place machine (Finetech) that enables proper chip alignment and attachment with a controlled gentle force (0.1 N). A holder with a removable lid for the adequate handling of the Si adapter during the process has been designed and 3D-printed. It allows accessing the corresponding side of the adapter, first to the inkjet printer, and then to the pick & place machine. The whole assembly process is depicted in

*Schematic of the different designs of the micromachined Si adapter (left). Heat flux through a parallel type*

*Layout of the new compact design featuring 50 thermocouples in serial connection (left) and serial connection of*

*10 arrangements of 5 thermocouples in parallel configuration (right).*

*Managing Heat Transfer Issues in Thermoelectric Microgenerators*

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

This is a still ongoing process. Two different inks are being tested to act as TIM between the adapter and the suspended platforms: a conductive silver nanoparticle ink (Agfa Orgacon SI-J20X) and a SU8 based polymeric ink (Micro Chem Prielex). The tests involve the assessment of the adequacy of the viscosity and adhesion of the TIM and the evaluation of the endurance of the μTEGs platforms during the assembly. Other TIM materials already used for the mainstream attachment of heat exchangers onto microprocessors can be also evaluated, as well as other ways of locally dispensing them, such as stamping. In any case, the goal is to obtain an integration route for the heat exchanger, without which no workable ΔT would be possible in such miniaturized devices, that is prone to the automatic handling of the

**Figure 21**.

**367**

**Figure 20.**

*μTEG, the adapter and the heat exchanger (right).*

**Figure 19.**

#### **Figure 18.**

*3D schematic of the new thermocouple design (left) and an optical microscope image of the fabricated micromachined platform.*

*Managing Heat Transfer Issues in Thermoelectric Microgenerators DOI: http://dx.doi.org/10.5772/intechopen.96246*

**Figure 19.**

is in place, the thermal properties of the thermoelectric material become second order. Hence, by optimizing their electrical properties and ensuring a good ΔT with the aid of a heat exchanger, it is possible to obtain high power densities even with high thermal conductivity thermoelectric materials such as Si microbeams.

In order to translate the promising power densities of a single structure into useful absolute power levels, a certain number of thermocouples needs to be integrated and connected. The μTEGs design was modified to attain a higher integration density by reducing the number of active sides. The new thermocouple has a rectangular shape with one side featuring the membrane providing mechanical support and metallic connection, and the opposite side composed of the trenches to be filled with NWs. In **Figure 18**, a 3D schematic of the new unitary thermocouple is shown. The same cross-section profile of **Figure 1** still applies. With this new design, many elements can be integrated in the same chip: up to fifty thermocouples

configuration in a 49 mm<sup>2</sup> chip, as shown in **Figure 19**. Both configurations would lead to the same harvested power, but the series one will scale up voltage while the

This compact design requires new components and a novel and more efficient approach for the integration of the heat exchanger in order to boost their thermal performance. A micromachined Si adapter (substituting the Cu wires, brass plate and PMMA spacer of the previous section) is necessary for the distribution of the force exerted on the platform by the heat exchanger, and different designs featuring the corresponding serial or parallel arrangements were fabricated. **Figure 20** (left) depicts the Si adapter designs where the central columns contact the platform of each thermocouple in the chip and the ones at the corners act as force distributers. Similar to the previous section, a commercial Al mini heat exchanger will be placed on top of the Si adapter to help the circulation of heat from the Si rim of the μTEG (warm side) through the thermoelectric material to the platform (cold side) to achieve the desired larger ΔT. The heat flux representation through the assembly is

To achieve a good thermal contact, which is key for a maximum generated power, a thermal interface material (TIM) needs to be placed between the thermocouple and the adapter. To this aim, an inkjet printer (Dimatix) was chosen to deposit a controlled amount of TIM only on the columns of the adapter.

*3D schematic of the new thermocouple design (left) and an optical microscope image of the fabricated*

) fit in series or series–parallel

**4.3 Semiautomatic assembly with integration density**

*Heat Transfer - Design, Experimentation and Applications*

(each with an approximate size of 5 x 0.6 mm<sup>2</sup>

parallel one will scale up current.

shown in **Figure 20** (right).

**Figure 18.**

**366**

*micromachined platform.*

*Layout of the new compact design featuring 50 thermocouples in serial connection (left) and serial connection of 10 arrangements of 5 thermocouples in parallel configuration (right).*

**Figure 20.**

*Schematic of the different designs of the micromachined Si adapter (left). Heat flux through a parallel type μTEG, the adapter and the heat exchanger (right).*

After dispensing the TIM, the adapter is placed face-down onto the thermocouple chip already wirebonded on a PCB. This is done with the help of a pick & place machine (Finetech) that enables proper chip alignment and attachment with a controlled gentle force (0.1 N). A holder with a removable lid for the adequate handling of the Si adapter during the process has been designed and 3D-printed. It allows accessing the corresponding side of the adapter, first to the inkjet printer, and then to the pick & place machine. The whole assembly process is depicted in **Figure 21**.

This is a still ongoing process. Two different inks are being tested to act as TIM between the adapter and the suspended platforms: a conductive silver nanoparticle ink (Agfa Orgacon SI-J20X) and a SU8 based polymeric ink (Micro Chem Prielex). The tests involve the assessment of the adequacy of the viscosity and adhesion of the TIM and the evaluation of the endurance of the μTEGs platforms during the assembly. Other TIM materials already used for the mainstream attachment of heat exchangers onto microprocessors can be also evaluated, as well as other ways of locally dispensing them, such as stamping. In any case, the goal is to obtain an integration route for the heat exchanger, without which no workable ΔT would be possible in such miniaturized devices, that is prone to the automatic handling of the

but gentle attachment of an intermediate adapter that needs to be designed ad hoc for proper heat flow handling. The presence of the heat exchanger also affects the tilting point of the previously mentioned thermal/electrical trade-off, and thus on the final choice of materials. In the examples given, silicon-based materials have been used (silicon microbeams, silicon and silicon germanium nanowires), but similar structures could be devised for instance for any thermoelectric material in

*Managing Heat Transfer Issues in Thermoelectric Microgenerators*

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

This manuscript contains work supported by projects FP7-NMP-2013-SMALL-7

, Marc Dolcet<sup>1</sup>

, Gerard Gadea<sup>2</sup>

, Joaquin Santander<sup>1</sup>

, Alex Morata<sup>2</sup>

,

,

(Contract n. 604169) SiNERGY, TEC2016-78284-C3-1-R (AEI/FEDER, EU) MINAUTO and TEC2016-78284-C3-2-R (AEI/FEDER, EU) SIGGNAL. This research has made use of the infrastructure of the Spanish ICTS Network MICRONANOFABS (CNM site) partially supported by MINECO. I. Donmez-Noyan thanks the 'Programa de Doctorat en Ciència de Materials de la UAB' for the

thin film form.

**Author details**

Denise Estrada-Wiese<sup>1</sup>

Albert Tarancon2,3 and Luis Fonseca<sup>1</sup>

provided the original work is properly cited.

Marc Salleras<sup>1</sup>

**369**

**Acknowledgements**

support in her formative activities.

, Inci Donmez-Noyan<sup>1</sup>

, Jose-Manuel Sojo2

\*Address all correspondence to: luis.fonseca@imb-cnm.csic.es

\*

2 Institut de Recerca de l'Energia de Catalunya, IREC, Barcelona, Spain

3 Institució Catalana de Recerca i Estudis Avançats, ICREA, Barcelona, Spain

1 Instituto de Microelectrónica de Barcelona, IMB-CNM (CSIC), Bellaterra, Spain

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

involved chips and it is compatible with their dimensional and mechanical endurance constraints.

### **5. Conclusions**

With this chapter, the authors have tried to show the challenges to sort out when fabricating microgenerators (μTEGs) with planar silicon technologies. Such technologies offer a cost effective way of mass-production of miniaturized devices. However, the very nature of such technologies, the high thermal conductivity of bulk silicon and the typical thickness of the layers involved advises using silicon micromachining to enable areas of lateral thermal contrast. Such transversal architectures helps to translate naturally occurring vertical thermal gradients into internal lateral ones. In this way, a temperature difference will develop across a horizontally and self-standing laid thermoelectric material whose length is a design parameter. A material properties trade-off ensues: the longer the material, the higher its thermal resistance, increasing the attainable ΔT and the obtained Seebeck voltage, but the larger will be its electrical resistance, reducing the power obtainable from that voltage. In addition, it has been shown that the overall attainable ΔT is heavily influenced by the very poor heat exchange capabilities with the environment of small bare surfaces. Simulations and experiments show that the presence of a heat exchanger largely increase the effective ΔT, but brings into play interesting heterogeneous integration challenges still to be fully solved in terms of an effective

*Managing Heat Transfer Issues in Thermoelectric Microgenerators DOI: http://dx.doi.org/10.5772/intechopen.96246*

but gentle attachment of an intermediate adapter that needs to be designed ad hoc for proper heat flow handling. The presence of the heat exchanger also affects the tilting point of the previously mentioned thermal/electrical trade-off, and thus on the final choice of materials. In the examples given, silicon-based materials have been used (silicon microbeams, silicon and silicon germanium nanowires), but similar structures could be devised for instance for any thermoelectric material in thin film form.
