## 4 Results and Discussion  
For the pyroelectric sensor considered in the present study, the output voltage scales proportionally with the temperature variation within the ZnO layer. Hence, the study commenced by performing COMSOL FLUENT simulations to determine the temperature variation at each of the node points shown in Figure 3c. Figure 5 shows the corresponding results obtained for the temperature variation at node point A1 given five different thicknesses of substrate (i.e., 100, 200, 300, 400 and 500 \(\upmu\)m, respectively). Note that the boundary condition for the temperature is set as 36.5 \({}^{\circ}\)C in every case. The results show that the rate of change of the temperature at point A1 increases with a decreasing substrate thickness. In other words, as the thickness of the substrate reduces, the time required to reach steady-state temperature conditions also decreases. Figure 4: Schematic illustration showing main steps in fabrication process of pyroelectric infrared sensor: (**a**) LPCVD deposition of Si\({}_{3}\)N\({}_{4}\), (**b**) E-beam deposition of Au/Cr bottom electrode, (**c**) photolithography, (**d**) patterning of bottom electrode by wet etching, (**e**) RF sputter deposition of ZnO thin film, (**f**) photolithography, (**g**) patterning of sensing layer by wet etching, (**h**) photolithography, (**i**) E-beam deposition of Au/Cr top electrode, (**j**) lift-off, (**k**) dry etching of back Si\({}_{3}\)N\({}_{4}\) layer, and (**l**) KOH wet etching of silicon substrate. An experimental responsivity measurement system was established to further evaluate the performance of the ZnO pyroelectric sensors (see Figure 6). The radiation source had the form of a calibrated infrared (IR) laser with a maximum power of 10 mW and a wavelength of 765 nm. The laser beam was chopped and molded as a wave with a modulated frequency of \(w\) by an optical chopper and was then expanded by a beam expander such that the beam spot diameter had a size of 3 mm and thus covered the entire region of the patterned top electrode of the sensor. In performing the experiments, the distance between the infrared source and the sensing layer was set as 15 cm unless specified otherwise, and the output voltage of the sensor was amplified using an SR570 low-noise voltage amplifier. Finally, the output signal of the sensor was displayed and recorded using a digital oscilloscope. For ZnO pyroelectric material, there exists a strong relation between the intensity of the (002) crystal peak and the responsivity of the pyroelectric sensor, where a higher value of the crystal peak indicates a higher quality of the crystal structure. In order to investigate the crystal orientation of the present ZnO sensing layer, ZnO films with a thickness of 889 nm were deposited on silicon substrates in an argon atmosphere with an RF sputtering power of 150 W and a working pressure of 5 \(\times\) 10\({}^{-3}\) Torr. The films were then annealed at  
Figure 5: Simulated temperature variation at point A1 of the sensing layer for different substrate thicknesses (100, 200, 300, 400 and 500 \(\upmu\)m) and a temperature boundary condition of 36.5 \({}^{\circ}\)C.  
Figure 6: Experimental setup for evaluating voltage response of ZnO pyroelectric sensors. various temperatures in the range of 300 to 600 \({}^{\circ}\)C for 4 h. The structures of the annealed films were characterized by X-ray power diffraction with a scanning angle of 30-60\({}^{\circ}\); an operating voltage and current of 40 kV and 40 mA, respectively, and Cu K\(\alpha_{1}\) radiation (\(\lambda\) = 0.154 nm). For each film, the sampling interval angle was set as 0.03\({}^{\circ}\) and the sampling time was 0.45 s.  
Figure 7 shows that XRD patterns of the ZnO films annealed at temperatures of 300, 400, 500 and 600 \({}^{\circ}\)C, respectively. For reference purposes, the figure also shows the XRD pattern of an as-deposited (non-annealed) film.