## 1 Introduction  
Pyroelectric infrared sensors, consisting of a thin film of ferroelectric crystal material sandwiched between two electrodes, produce an electrical voltage (or current) in response to temperature variation and have found widespread use in recent years for many applications, including thermal comfort monitoring, infrared light detection and imaging, fire detection, intruder alarms, body temperature monitoring, and so on [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Many studies have reported that the sensitivity of pyroelectric sensors can be improved by reducing heat losses from the sensing elements to the supporting substrate or environment [11, 12, 13, 14, 15]. It has been shown that this can be readily achieved by using bulk micromachining techniques to realize freestanding sensing structures, such as floating membranes, cantilever beams and bridges, which are effectively suspended above the supporting silicon substrate and shielded from the environment [16]. Moreover, pyroelectric sensors can be easily integrated with modern IC circuits on the same chip to realize sophisticated system-on-chip (SoC) devices for a wide variety of sensing and detection applications [17, 18]. [PERSON] et al. [19] constructed a people-counting system based on a pyroelectric array detector for detecting the moving direction and number of passersby at a door. The pyroelectric detector incorporated a thin PbTiO\({}_{3}\) bulk ceramic film with a thickness of approximately 40 \(\upmu\)m and was driven by a supply voltage of 5 V. The detector showed a maximum voltage sensitivity of 23 mV/mW at a chopping frequency of 10 Hz. [PERSON] and [PERSON] [20] fabricated an integrated pyroelectric sensor consisting of a PZT thin film and a Si JFET. The device was characterized over a modulation frequency range of 0.2 to 10 Hz and was found to have a voltage sensitivity of 191 mV/mW at a modulation frequency of 1 Hz when implemented with a 500 \(\upmu\)m thick silicon substrate. Some other ferroelectric ceramics (e.g., (Bi, Na)TiO\({}_{3}\), (Sr, Ba)NbO\({}_{3}\), Pb(Sc, Ta)O\({}_{3}\) (Ba, Sr)TiO\({}_{3}\) and Pb(Zr, Sn, Ti)O\({}_{3}\)) were investigated to be suitable for pyroelectric detection applications, recently [21]. [PERSON] and [PERSON] [22] investigated the transmittance of thin nickel (Ni) films deposited on pyroelectric polyvinylidene fluoride (PVDF) films over the infrared spectral range of 2-50 \(\upmu\)m. It was shown that the absorbance characteristics of the pyroelectric sensor could be effectively controlled through an appropriate specification of the Ni film thickness. [PERSON] et al. [23] fabricated a thin-film pyroelectric sensor array consisting of 16 sensing elements made of metal-ZnO-metal sandwiched layers with dimensions of 200 \(\upmu\)m \(\times\) 200 \(\upmu\)m. The sensor response was enhanced by depositing the sensing elements on a thermally isolated and freestanding Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) membrane created by removing the underside of the SiO\({}_{2}\) substrate using a conventional back-etching technique. The experimental results showed that the device achieved a maximum voltage sensitivity of 110 mV/mW at a cutoff frequency of 20 Hz. [PERSON] et al. [24] used a two-step radio frequency (RF) sputtering technique to fabricate a ZnO pyroelectric sensor on a back-etched silicon wafer. The use of the two-step deposition process was found to produce a ZnO film with a preferred c-axis orientation and an enhanced voltage response. It was shown that the sensor achieved a maximum voltage sensitivity of 8.6 mV/mW at a cutoff frequency of 500 Hz when implemented with a Ni coating on the uncovered part of the ZnO film in order to enhance the absorption of the incident energy. [PERSON] et al. [25] fabricated ZnO pyroelectric sensors with partially covered and fully covered electrodes, respectively. The experimental results showed that the responsivity of the partially covered device was around four times higher than that of the fully covered device for incident light frequencies in the range of 10-1000 Hz.