# Fabrication and Testing of Thermoelectric CMOS-MEMS Microgenerators with CNCs Film
## 2 Design of Microgenerator  
The TMG structure is illustrated in Figure 1. The microgenerator is constructed by thermopiles, thermometers, and CNCs film. There are 129 thermopiles in series. Each thermopile includes n-type and p-type polysilicon. Figure 2 illustrates the schematic thermoelectric microgenerator cross-sectional structure. P-type and n-type polysilicon junctions in thermopiles are designed as hot and cold parts. According to the Seebeck effect [22], the greater the temperature difference of HP and CP in thermopiles is, the more  
Figure 1: Thermoelectric microgenerator structure. increased the output voltage of the TMG becomes. To enhance the temperature difference of HP and CP, the hot part is designed as a suspension structure to isolate heat dissipation. A stacked metal layer with tungsten and aluminum, which covers the top of the hot part, increases heat conducting to the hot part because metal has a higher thermal conductivity than silicon oxide. The cold part anchors on the silicon substrate to increase heat dissipation, and a low thermal conductivity silicon oxide layer covers the cold part to reduce heat absorption. The CNCs layer is deposited on the surface of the microgenerator, and the layer is used to enhance heat absorption of the microgenerator. The thermometers are arranged at HP and CP in thermopiles, and they are employed to detect the temperature at HP and CP. The microgenerator area is 1.28 \(\times\) 1.2 mm\({}^{2}\). The length, thickness, and width of each thermopile are 180 \(\upmu\)m, 0.2 \(\upmu\)m, and 20 \(\upmu\)m, respectively. The operation mechanism of the TMG is demonstrated in Figure 2. Heat radiates to the CNCs film and is absorbed by the film. The heat absorbed by CNCs has two conducting ways. One conducts to the hot part through the metal layer, and the other conducts to the cold part through the thick silicon oxide layer. The thermal conductivity of metal is better than that of silicon oxide, so the metal layer is good for heat conducting to the hot part. The hot part that is part of the suspension structure can keep heat well. The heat at the cold part is dissipated through the silicon substrate. Thereby, the temperature at the hot part is higher than the temperature at the cold part, resulting in a temperature difference between HP and CP. The output voltage of the TMG is given by [23],  
\[V_{out}=n\big{(}\alpha_{n}-\alpha_{p}\big{)}(T_{h}-T_{c}) \tag{1}\]  
where \(n\) is the thermopile number, \(\alpha_{n}\) represents the n-type polysilicon Seebeck coefficients, \(\alpha_{p}\) represents the p-type polysilicon Seebeck coefficients, \(T_{h}\) is the HP temperature, and \(T_{c}\) is the CP temperature. \(T_{h}-T_{c}\) represents TD between HP and CP. \(\alpha_{n}-\alpha_{p}\) is the Seebeck coefficient difference of n-type and p-type polysilicon. The higher the thermopile number and TD between the HP and CP are, the more increased the microgenerator OV becomes. In this evaluation, there are 129 thermopiles and \(\alpha_{n}-\alpha_{p}\) is 8.14 \(\upmu\)V/K. The values are substituted into Equation (1), and we obtain the microgenerator OV. Figure 3 shows the evaluated OV of the microgenerator. The results show that the microgenerator OV is 3.1 mV at TD 3 K.  
Figure 2: Schematic thermoelectric microgenerator cross-sectional structure. The output power of the TMG is given by [24],  
\[P_{out}=\frac{V_{out}^{2}}{4R_{g}} \tag{2}\]  
where \(R_{g}\) is the microgenerator internal resistance. The output power is inversely proportional to microgenerator internal resistance and is related to the square of its output voltage. As the microgenerator internal resistance reduces, the microgenerator OP enhances.