Research on pyroelectric enhancement mechanism of PZT thin films and optimization design of infrared detectors

In this paper, the PZT10/90 thin film was successfully deposited on the Pt/TiO2/SiO2/Si substrate using RF magnetron sputtering, and the dielectric and pyroelectric properties of the thin film were measured and characterized. The measurements show that the dielectric constant of the PZT10/90 thin film is further reduced compared to the previously reported PZT30/70 thin films, which is beneficial for improving the figure of merit of the PZT. The dielectric constant and loss tangent of the PZT10/90 thin film are 173 and 0.016 at 1kHz and the pyroelectric coefficient of the thin film is 20 nC·cm-2·K-1 at room temperature. In addition, a pyroelectric infrared detector based on the PZT thin film was designed, a feasible detector fabrication process was proposed, and the structure of the pyroelectric infrared detector was optimized using simulation software based on the finite element method. The developed infrared detector has promising applications in the fields of spectroscopic instruments and smart seekers.


Introduction
Infrared detectors, as an important tool for human beings to perceive the spectrum of infrared wavelengths and transfer information, have been successfully applied in the fields of environmental  [1][2][3], medical diagnostics [4,5], industrial process control [6] and others.Infrared detectors can be classified into two types based on their energy conversion method: thermal detectors and photon detectors [7].Cryogenic cooling is necessary for photon detectors to achieve optimal signal-to-noise performance and rapid response.The presence of cooling devices makes semiconductor photodetectors bulky, heavy, expensive and inconvenient to use, limiting their development in the civil applications.In addition, the response of photon detectors per unit incident radiation depends on the wavelength of the radiation.[8].In contrast, thermal detectors do not require cooling devices and have advantages such as a wide response wavelength range, low cost, and simple preparation methods.Pyroelectric infrared (PIR) detectors are a typical and widely used type of infrared thermal detector.Pyroelectric detectors have gained considerable attention for their notable features, including low power consumption, rapid response time, and high sensitivity across a wide temperature range and broad spectral bandwidth.[9].Therefore, the development of pyroelectric infrared detectors with excellent performance has become one of the focal points of current research and application of uncooled infrared detection technology.
Due to its high Curie temperature and pyroelectric coefficient, Pb(ZrxTi1-x)O3 (PZT) has become a suitable material for used in PIR detectors.In addition, the pyroelectric and dielectric properties of PZT can be modified over a wide range by adjusting the ratio of Zr and Ti, controlling the crystallographic orientation of the films and doping other elements.Since high-performance pyroelectric infrared detectors require sensitive materials with high pyroelectric coefficient and low permittivity, titanic-rich PZT has been proven to be a rational choice for pyroelectric infrared detectors [10].In addition, the pyroelectric film used for infrared detectors also has many advantages, such as the low heat capacity, reducing heat loss to the surrounding environment, thereby increasing the pyroelectric response, shortening the response time and so on [11].
In this study, the researchers successfully fabricated the PZT10/90 (PbZr0.1Ti0.9O3)thin film on the Pt/TiO2/SiO2/Si substrate using the RF magnetron sputtering.The morphology, pyroelectric and dielectric properties of the PZT10/90 were characterized.Furthermore, the finite element method based simulation software was used to optimize the pyroelectric detector structure, providing guidance for the pyroelectric infrared detector fabricating.

Figures of merit
The performance of pyroelectric materials directly affects the detector.When evaluating the response of pyroelectric detectors, the figures of merit are often used as an important index, such as the voltage responsivity (FV), the current responsivity (Fi), and detectivity (Fd), where Fd is an important index for evaluating the overall performance of the material.The figure of merit of the detectivity is defined by equation (1), = 0 1/2 # 1 where p is the pyroelectric coefficient, c is the specific heat, ε0 is the vacuum permittivity, εr is the relative permittivity and tanδ is the dielectric loss.To achieve high detectivity in single-point detectors, it is desirable to use pyroelectric thin films characterized by low permittivity, low dielectric loss and high pyroelectric coefficient.

Device design and optimization
The structure of the PIR detector is shown in Figure 1.The substrate of the detector is double polished (100) silicon with 1 μm SiO2 layer.To reduce thermal losses, the Si substrate is intentionally etched from the back, creating an air cavity beneath the pyroelectric sensing region.This design strategy exploits the low thermal conductivity of air, as opposed to the relatively high thermal conductivity of bulk silicon.In addition, the presence of the SiO2 layer serves to insulate the thermal energy absorbed by the sensing layer of the pyroelectric detector, effectively reducing the rate of thermal conduction to the underlying medium.Pt films are used as the electrodes of the device.The inclusion of a TiO2 layer increases the adhesion strength between the bottom electrode and the SiO2 layer.The PZT thin film acted as the sensing layer.For pyroelectric infrared detectors, the temperature change of pyroelectric materials is closely related to the signal output of the detector.Therefore, in order to ensure that the pyroelectric infrared detector can produce as large an output signal as possible, the thermal insulation structure on the back of the pyroelectric detector is particularly important.Therefore, the influence of the backside cavity structure on the temperature variation of pyroelectric material is investigated by using the simulation software based on the finite element method.Table 1 presents the simulation parameters utilized in the study.A schematic of the constructed model is shown in Fig. 2, where the area of the PZT was set to be 1×1 mm 2 .Under the condition of 2 mW incident optical power, the area of the back cavity was varied to observe the temperature change of the PZT layer.It is assumed that the incident optical power is completely absorbed and the incident light is uniformly distributed over the surface of the top electrode.The model's initial temperature was set to 293.15 K and the depth of the back cavity was set to 500 μm.If the area of the backside thermal insulation structure is smaller than the area of the PZT layer, the pyroelectric layer cannot achieve a sufficient temperature rise, resulting in a weakened output signal from the detector.Therefore, in order to maximize the output signal strength of the detector, it is necessary to ensure that the area of the backside thermal insulation structure is larger than the area of the pyroelectric layer.According to this, under the condition that the back cavity area was larger than the PZT layer area, the relationship between the average temperature of the upper surface of the PZT layer and the back cavity area was investigated by simulation, and the results are shown in Fig. 3.
According to the simulation results in Fig. 3, the average temperature of the upper surface of the PZT layer gradually increases with the increase of the thermal insulation area, but the increase of the average temperature of the upper surface of the PZT layer begins to gradually decrease when the thermal insulation area is larger than 1.2×1.2mm 2 , and the efficiency of the method of increasing the area of the thermal insulation structure in order to increase the temperature of the pyroelectric layer decreases.In addition, considering the mechanical stability and long-term reliability of the device structure, the area of the thermal insulation structure was ultimately controlled at 1.2×1.2mm 2 .

Figure 3.
Average temperature and temperature growth rate of the upper surface of the PZT thin film under different thermal insulation structures.The simulation starts with the 1×1mm 2 thermal insulation structure.

Deposition of the PZT10/90 thin film
The PbZr0.1Ti0.9O3thin film was fabricated on the Pt/TiO2/SiO2/Si substrate through the RF magnetron sputtering technique, with a 100 nm LaNiO3 (LNO) seed layer deposited before sputtering.As for the PZT10/90 thin film deposition, the substrate was maintained at a temperature of 600 ℃.The gas mixture used for the deposition process consisted of Ar and O2 in a ratio of 9:1.The deposition was carried out at a working pressure of 1.3 Pa and an RF power of 160 W. After the deposition, a postannealing process was carried out in air at a temperature of 650 °C for the PZT thin film.

Characterizations and analysis
The crystal structures and orientations of the PZT10/90 thin film were investigated by XRD (D8 ADVANCE, Bruker), and Fig. 5 shows the XRD pattern of the PZT10/90 thin film.The thin film contains a mixed crystalline orientation of (001) and (l00).The XRD pattern confirms the presence of a well-crystallized PZT with a perovskite structure and there is no secondary phase.The scanning electron microscope (JSM-7900F, JEOL) was used to observe the surface and cross-sectional morphologies of the PZT10/90 thin film.Fig. 6 shows the morphologies of the PZT10/90 thin film prepared by RF magnetron sputtering.As shown in Fig. 6(a), the surface of the thin film has a very compact structure with no noticeable defects.The PZT thin film has a densely packed columnar structure and no obvious holes were observed within the thin film, as shown in Fig. 6(b).The PZT thin film has an approximate thickness of 385nm.The X-ray photoelectron emission spectra of the PZT thin film were obtained using an XPS spectrometer (Nexsa, Thermo Scientific).Fig. 7 shows the survey spectrum of the PZT thin film, revealing distinct peaks corresponding to the elements Pb, Zr, Ti and O, as well as the presence of adventitious carbon (C).
Signal peaks of Pb 4f, Zr 3d, O 1s, Ti 2p and C 1s are observed in the XPS full spectrum of the PZT thin film respectively, with the C 1s peak as the reference peak.Based on the XPS characterization results, the relative content of each element in the film was quantitatively analyzed and the results are shown in Table 2.The relative content of each element is 20% Pb, 2% Zr, 18% Ti and 60% O for the ideal PZT10/90 thin film.The Pb content in the prepared PZT thin film is lower than the theoretical content, which may be due to the volatilization of Pb elements near the film surface during the annealing process.The pyroelectric and dielectric properties of the PZT10/90 thin film were measured using a pyroelectric current measurement system (TSC6520, Partulab) with an impedance analyzer (6630, MICROTEST).Fig. 8(a) shows the measured pyroelectric coefficient result of the PZT10/90 thin film.The Byer-Roundy method was employed to determine the pyroelectric coefficient.[14].Using equation ( 2), the electric current gives the pyroelectric coefficient as where Ip is the pyroelectric current, A is the area of electrode and dT/dt is slope of the temperature versus time curve.Assuming that the generated current Ip comes only from pyroelectric contribution.Pyroelectric coefficient of the PZT10/90 thin film at room temperature is about 20 nC•cm -2 •K -1 .Fig. 8(b) illustrates the frequency-dependent behavior of the dielectric properties of the PZT thin film.In the frequency range from 100 Hz to 10 kHz, there is a slight decrease in both the permittivity and the dielectric loss with increasing frequency.At 1 kHz the permittivity is 173 and the loss tangent is 0.016.Table 3 summarizes the pyroelectric and dielectric properties as well as the figure of merit of the detectivity of PZT thin films with a Ti-rich composition reported in the literature.The measurements show that the dielectric constant of the PZT10/90 thin film is further reduced compared to the previously reported PZT30/70 films.The increase in permittivity can be attributed to the expansion of the unit cell size and the improved mobility of the central ions (Zr 4+ /Ti 4+ ).For PZT thin films with a Ti-rich composition, the presence of higher Zr content leads to an increase in unit cell volume, which can be attributed to the larger ion radius of the Zr ions.The radius of Ti ions is only 61 pm, while Zr ions have a larger radius of 87 pm.In addition, the increase in cell volume will be more favorable for the central ion mobility.The reason for the higher dielectric loss compared to the same material reported in the literature may be due to the more disordered crystal orientation of the PZT thin film, the details of which require further investigation.In addition to this, the dielectric losses caused by the leakage currents in the film should not be neglected.In summary, the measurement results obtained confirm that the fabricated PZT thin film exhibits excellent sensing properties, making it a promising material for high performance pyroelectric infrared detectors.

Figure 1 .
Figure 1.Schematic structure of the PZT pyroelectric infrared detector.The layers from top to bottom are: patterned Pt top electrode, patterned PZT/LNO, Pt bottom electrode, TiO2 adhesion layer, SiO2 insulation layer and Si substrate.

Figure 2 .
Figure 2. Schematic of the model used to solve the steady state temperature distribution of the PZT: (a) front view of the structure and the area of the PZT film is 1×1 mm 2 ; (b) back view of the structure, and the area of the thermal insulation structure is larger than the PZT area.

Fig. 4
shows the manufacturing process of the PZT pyroelectric infrared detector.(1) First, a 20 nm TiO2 adhesion layer was deposited on the front of the wafer by RF magnetron sputtering, followed by the deposition of 200 nm Pt as the bottom electrode; (2) The 400 nm thick PZT thin film was deposited onto the Pt bottom electrode using RF magnetron sputtering.Prior to the deposition of the PZT thin film, a seed layer of 100 nm LaNiO3 (LNO) was first deposited.;(3) PZT/LNO film was patterned by lithography and ion beam etching; (4) The top electrode was created by depositing and patterning a 200 nm Pt film using lithography, magnetron sputtering, and lift-off processes; (5) Finally, the silicon was removed by using lithography and deep reactive ion etching.

Figure 4 .
Figure 4. Fabrication process of the PZT pyroelectric infrared detector

Figure 8 .
Figure 8. Pyroelectric and dielectric properties of the PZT10/90 thin film.The pyroelectric coefficients were determined from the measured pyroelectric current.The PZT thin film was heated at a constant rate of 10 ℃/min.And the permittivity and dielectric loss were measured between 100 Hz and 10 kHz.

Table 2
Relative contents of Pb, Zr, Ti and O elements in the PZT thin film obtained from the XPS