**3.4 Nickel-doped thermoelectric material**

Although the thick-film thermoelectric materials have been investigated successfully, as described in Section 3.2, further investigations are still required to enhance their thermoelectric characteristics. Moreover, in order to open an opportunity for mass production, highly scalable synthesis electrodeposition on a large wafer size for thermoelectric materials should be conducted. In this section, a novel process technology for the ultra-thick film as well as high-performance characteristics (high Seebeck coefficient, large electrical conductivity, and low thermal conductivity) is investigated. Both electrodeposited films, including pure Bi2Te3 and Ni-doped Bi2Te3, reaching in mm-order thickness, have been synthesized, evaluated, and compared. Moreover, a highly scalable electrodeposition process for large wafer size has been performed and proven.

#### **Figure 6.**

*SEM image of pure Bi2Te3. (b) SEM image of Ni doped Bi2Te3. (c) Selected area electron diffraction pattern of pure Bi2Te3. (c) Selected area electron diffraction pattern of pure Ni-doped Bi2Te3.*

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

**Figure 6(a)** and **(b)** show the surface crystal structure of the electrodeposited pure Bi2Te3 and Ni-doped Bi2Te3, respectively. As can be seen that the crystal grain size of pure Bi2Te3 is much larger than that of Ni-doped Bi2Te3. The selected area electrode diffraction patterns for pure Bi2Te3 and Ni-doped Bi2Te3 are shown in **Figure 6(c)** and **(d)**, respectively. Diffraction spots in **Figure 6(c)** and **(d)** indicate that both electrodeposited films pose polycrystalline structures. In quantitative comparison, the spots in **Figure 6(d)** are much more than those in **Figure 6(c)**. One possible cause is the grain size effects. Decreasing the grain size results in an increase of the boundary scattering and lattice defects, as discussed in Section 3.3. Thereby, not only the trade-off between Seebeck coefficient and electrical conductivity could be adjusted (changing the carrier concentration), but also the thermal conductivity gets lower due to photon scattering.

**Figure 7(a)** shows the experimental result of the highly scalable synthesis process, which is performed on a 4-inch wafer size. The deposited film reaches 2 mm thickness with a high uniform surface, as shown in **Figure 7(b)**. The success of the highly scalable electrodeposition could open up the opportunity for mass production to reduce the fabrication cost.

Summary characteristics of the electrodeposited thermoelectric materials can be found in **Table 4**. Experimental results show that 0.7 at% Ni-doped Bi2Te3 has the highest Seebeck coefficient as well as largest electrical conductivity compared with others, including pure Bi2Te3, 0.3 at% Ni-doped Bi2Te3, 1.0 at% Ni-doped Bi2Te3, and 1.5 at% Ni-doped Bi2Te3. Although the thermal conductivity of 0.7 at% Ni-doped Bi2Te3 is not the smallest one, its thermal conductivity is two times smaller than that of the pure Bi2Te3. The ZT of Ni-doped Bi2Te3 is estimated as 0.78, which is five times larger than that of the pure Bi2Te3. The details of evaluation setup, and measurement results, and other discussions can be found in [38, 39].

### **4. Device fabrication**

#### **4.1 Micro-thermoelectric-generator-based on micro/nano fabrication technology**

One of the challenges for micro-TEG is the small harvested temperature difference across the module, thus resulting in low output power. In the conventional design of micro-TEG, the heat flows in the vertical direction (thermoelectric elements such as column structure); therefore, ultra-height thermoelectric elements are typically


#### **Table 4.**

*Summary characteristics of the electrodeposited thermoelectric materials.*

#### **Figure 8.**

*(a) Proposed micro-thermoelectric generator structure. (b) Heat flow in lateral direction.*

needed. However, to fabricate micro-TEG based on micro/nano technologies, the height of thermoelectric elements is limited to a hundred micrometers due to the limitation of the photoresist thickness and a patterning aspect ratio. To overcome this issue, thermoelectric elements are proposed to be laid in a lateral direction instead of a vertical one. The proposed structure for micro-TEG is shown in **Figure 8(a)**, which consists of n- and p-types thermoelectric elements (Bi2Te3 and Sb2Te3), copper heat guide, and PDMS (polydimethylsiloxane) as a base material. This microthermoelectric generator possesses a flexible characteristic that can be utilized in wearable electronic applications. The heat flow direction is shown in **Figure 8(b)**.

**Figure 9** shows the fabrication process for micro-TEG, which begins with a silicon wafer. The SiO2 with 500 nm thickness and Cr-Au layers with 10 nm thickness and 150 nm thickness, respectively, are deposited on the top of the silicon wafer, respectively, by PECVD and sputtering methods (**Figure 9(a)**). The thermoelectric materials are selectively deposited on the Au surface by electrodeposition technique via the patterned photoresist with a thickness of 100 μm (**Figure 9(b)**). Next, Ti-TiN-Au-Cu layer as a barrier contact layer is formed by sputter via a stencil mask, as shown in **Figure 9(c)**–**(e)**. The copper heat guides are subsequently grown on the barrier contact layer by the electroplating method (**Figure 9(f)**). The front side of micro-TEG is then filled by PDMS (**Figure 9(g)**). To create the heat guide from backside, a deep reactive ion etching (RIE) is conducted (**Figure 9(g)**). A thermal glue with high thermal conductivity is refilled into the molds by a screen printing technique (**Figure 9(h)**). The remaining silicon layer is etched out by plasma etching, and SiO2

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

#### **Figure 9.**

*Fabrication process. (a) SiO2-Cr-Au deposition. (b) Thermoelectric material synthesis. (c) Photolithography process. (d, e) Multilayers of barrier metal contacts of Ti-TiN-Au-Cu. (f) Copper heat guides. (g) PDMS refilling and Si-SiO2 removing processes; (h) screen printing process of thermal conductive glue. (i) Backside etching process; (k) PDMS refilling process.*

and Cr-Au layers are removed by the ion beam milling technique (**Figure 9(i)**). Finally, PDMS is filled into the backside cavities (**Figure 9(k)**).

**Figure 10(a)** shows the fabricated micro-TEG based on micro/nano fabrication technologies. The micro-TEG contains 24 pairs of electrodeposited n- and p-type thermoelectric materials integrated on 1 cm<sup>2</sup> . The output power density of the fabricated micro-TEG is displayed in **Figure 10(b)**, which reaches 3 μW/cm<sup>2</sup> under a temperature difference caused by human body (37°C) and ambient environment (15°C) using natural convection. The details of evaluation setup, measurement results, and other discussions can be found in [40].

In summary, a novel design and fabrication process for the micro-TEG have been proposed and investigated. Micro-TEG has been fabricated successfully by micro/ nano fabrication technologies. Also, its performance has been evaluated. Although the power density of the fabricated micro-TEG is small, it could be improved by increasing the density of n- and p-types thermoelectric elements. The idea and experimental results in this work may be useful for applications in wearable electronic devices.

**Figure 10.**

*(a) Fabricated micro-TEG. (b) Applied temperature and output power.*

#### **4.2 Micro-thermoelectric generator based on assembling technology**

To improve the performance of the micro-TEG, enhancing the performance of the thermoelectric materials is a critical point. Another important point is an increase in the number of thermoelectric elements, which can significantly enhance output voltage and output power, as discussed by Eqs. (12) and (13). Thus, the power density can be significantly increased. High-density n- and p-type thermoelectric elements could be formed on a small foot print by utilizing the micro/nano fabrication technologies, as discussed in Section 4.1 and in Refs. [41, 42]. However, some issues need to be addressed, as follows. Complex processes, including photolithography, etching, deposition, and lift-off processes, are needed to construct the air bridge between thermoelectric elements. Therefore, the fabrication time is long, and the cost is high. Moreover, the bonding strength between thermoelectric elements and substrate is weak; thereby, the internal resistance of the fabricated micro-TEG is high, caused by the large contact resistance. Such issues make the performance of the micro-TEG low, which is against it for realistic applications. In this section, a novel method to produce the micro-TEG based on ultra-thick and dense electrodeposited thermoelectric material (presented in Section 3.4) and assembly technique is proposed and investigated.

To fabricate a high-density micro-TEG, small thermoelectric elements are needed, which are prepared as follows. The 4-inch electrodeposited wafer (**Figure 11(a)**) is diced into many small elements (**Figure 11(b)**). It is noted that before cutting, Ni-Au

#### **Figure 11.**

*(a) Four-inch electrodeposited thermoelectric material wafer. (b) Thermoelectric elements with dimensions of 0.4 mm 0.4 mm 2 mm. (e) Close-up image of thermoelectric elements.*

*Micro-Thermoelectric Generators: Material Synthesis, Device Fabrication, and Application… DOI: http://dx.doi.org/10.5772/intechopen.102649*

#### **Figure 12.**

*Fabrication process and fabricated micro-TEG. (a) Silicon substrate with SiO2 and Cr-Au layers on top. (b) Cr-Au patterning. (c) TEG schematic. (d) Device fabrication setup including holders, substrate, and stencil wafer. (e) After the first alignment and bonding. (f) Completely fabricated device.*

layers as barrier contact layers are formed on both sides of the wafer by electroplating method [43, 44] to decrease the ohmic contact resistance between thermoelectric elements and substrate. **Figure 11(c)** shows the magnified image of the diced thermoelectric elements with dimensions of 0.4 mm 0.4 mm 2 mm.

The fabrication process for the micro-TEG based on the assembly technique is shown in **Figure 12(a)**–**(c)**. The SiO2 layer as an insulator layer is formed on a silicon wafer by PECVD, and Cr-Au layers are deposited on the SiO2 layer by the sputtering method, as given in **Figure 12(a)**. Cr-Au layers are patterned to form the bottom interconnection by a wet etching method [45, 46], as shown in **Figure 12(b)**. Next, thermoelectric elements are aligned and bonded on the substrate by conductive glue. Finally, a top wafer cover is aligned and bonded on top of the thermoelectric elements (**Figure 12(c)**). Because the thermoelectric elements are pretty small, the process for vertical alignment becomes difficult. To overcome this issue, a stencil silicon wafer with patterned through holes is proposed, and a simple metal holder tool is employed to fix and align the stencil wafer and substrate, as shown in **Figure 12(d)**. Thermoelectric elements are inserted into holes of the stencil wafer. **Figure 12(e)** shows the experimental image after the thermoelectric elements are bonded on the substrate. The completely fabricated micro-TEG is shown in **Figure 12(f)**. In total, 127 pairs, including n- and p-type thermoelectric elements, are formed successfully on a small footprint of 15 mm<sup>2</sup> . Thus, although a simple assembly technique is employed, the integration density of thermoelectric elements could be comparable to the micro-fabrication of the micro-TEG.

The fabricated micro-TEG shows a high output power of 33.9 mW and a large power density of 15.1 mW/cm<sup>2</sup> under a temperature difference across the micro-TEG of 75 °C, which is much higher performance than those of other published works [42, 47–51]. More comparisons to other works are shown in **Table 5**. The details of evaluation setup, measurement results, and other discussions can be found in [52].


**Table 5.** *Comparison of TEG performance.*

In summary, the high integration density of the micro-TEG has been demonstrated by utilizing a simple assembly technique. Micro-TEG consisting of 127 pairs is successfully fabricated on 15 mm<sup>2</sup> . The fabricated micro-TEG possesses a high performance, which may satisfy the demand for being a reliable power source for electronic devices.
