Aluminum Nitride Thin Film Pyroelectric Detector Based on Metamaterial Absorber

Highly integrated pyroelectric detectors have been widely used in infrared spectrometers and gas detection. Aluminum nitride(AlN) has excellent compatibility with CMOS processes and is often used to fabricate piezoelectric and optoelectronic devices with superior performance. This study proposes a pyroelectric detector with AlN thin film as the sensitive element and integrated metamaterial absorber. Magnetron sputtering technology was used to prepare the AlN thin film as the sensitive element. To achieve narrowband specific absorption subsequently, a three-layer metamaterial absorber structure was designed and simulated. Preliminary processing of AlN pyroelectric detectors was performed based on MEMS technology. The designed narrowband absorbing structure exhibits near unity specific absorption, with a quality factor of 17.75. This study preliminarily verifies the application potential of AlN thin films in mid-infrared pyroelectric sensors, and realizes narrowband absorption through metamaterial structures, laying the foundation for the development of mid-infrared gas detection sensors.


Introduction
Pyroelectric detectors, as the core components of nondispersive infrared spectroscopic sensors, directly affect the overall performance of the sensors.The key parameters of pyroelectric detectors like sensitivity, response speed, and operating temperature range directly affect the overall sensor performance metrics such as detection sensitivity, response time, and adaptability.Compared with traditional catalytic and electrochemical gas sensors, non-dispersive infrared sensors based on the pyroelectric effect have advantages like better stability, anti-poisoning, and faster response, thus becoming a more popular choice.To achieve high-performance sensors, the optimization of pyroelectric sensitive element design and fabrication is crucial.Currently, single crystal lithium tantalate, polymers like polyvinyl fluoride (PVF), and thin film materials like aluminum nitride(AlN) and Pb(Zrx,Ti1-x)O3 (PZT) have been extensively studied.Among them, AlN features low dielectric constant, small dielectric loss, and CMOS process compatibility, making it a very promising pyroelectric material.Metamaterial absorbers, as sub-wavelength periodic microstructures, are essentially antennas.Through structural resonance with the incident electromagnetic waves, selective and perfect absorption can be achieved.By carefully optimizing the structural parameters, the absorption peak can be precisely controlled, enabling selective absorption for specific frequency bands.This opens up new possibilities for electromagnetic spectrum control and sensing technologies.
Suen [1] et al. fabricated lithium tantalate thin film detectors with optical antennas, achieving 86% absorbance and 560nm FWHM in 2017.Gaur et al. [2] studied multi-oriented AlN films and fabricated pyroelectric detectors using 160nm thick gold films, achieving 32μC/(m 2 K) in 2019.In 2020, Wu Guoqiang et al. [3] deposited AlN films on 8-inch wafers using SiO2-Si3N4-SiO2 absorption layers.Also in 2020, Zhang T T et al. [4] in Singapore proposed monolithic integrated AlN pyroelectric gas sensors using SiO2-Si3N4-SiO2 absorption.Tan et al. [5] integrated metamaterial absorbers on lithium tantalate, achieving spectrally tunable photoresponse.Researchers are exploring monolithic integration for higher integration.However, traditional detectors need optical filters to match gas absorption peaks.This limits further improvement of integration degree.With the vigorous development of metamaterial design theory and preparation technology [6], We can now directly integrate metamaterial absorbers on pyroelectric devices, thereby achieving narrowband absorption at the target band for infrared gas detection.
This study completed the structural design of the AlN pyroelectric detector.AlN thin films were prepared using magnetron sputtering technology.To realize narrowband specific absorption, a MIM trilayer metamaterial absorber structure was designed and simulated in this study.It exhibits near-unity specific absorption at 70.3 THz with a quality factor of 17.75.This provides a feasible solution for subsequent integration of the metamaterial absorber with the AlN pyroelectric sensor.This new integration approach brings significant improvements in sensor performance and integration level.

The Structure Design and Simulation of Metamaterial Absorbers
The AlN pyroelectric detector and metamaterial absorber are modeled and analyzed using the multiphysics simulation software COMSOL Multiphysics based on finite element method, to further optimize the sensor performance.The optical absorption characteristics and electromagnetic field distributions of the metamaterial absorber are studied.The steady-state heat transfer of the AlN pyroelectric sensitive element is simulated, and the pyroelectric current generated under periodic thermal radiation is calculated.The detectivity D* of the detector is then obtained.

The Structure Design and Simulation
Since Landy et al. proposed the metamaterial perfect absorber in 2008 [7], related research has emerged successively.The typical metamaterial absorber structure usually adopts a metal-insulator-metal (MIM) sandwich structure.The top layer is a patterned periodic unit structure, with common unit types including discs [8], crosses [9], rings, etc.The disc structure is relatively simple.The designed MIM absorber is based on an Au-SiO2-Au disc structure.The main absorption characteristics of metamaterial absorbers include: absorption peak position, full width at half maximum (FWHM), and absorbance.The main structural parameters affecting absorption characteristics are: dielectric layer thickness, periodic array period, and geometric parameters of the unit itself .The resonance peak position of the MIM absorber can be calculated using the LC circuit model shown in Figure 2, where Cg represents the capacitance between the periodic gold discs, Cm represents the capacitive effect of the dielectric layer between the gold discs and gold backplane, Lm represents the mutual inductance, indicating the magnetic field coupling between the gold discs and gold backplane, Le represents the dynamic inductance of the metal itself.μ0 is the magnetic permeability in vacuum, d is the dielectric layer thickness, p is the period of the disc array, h is the height of the disc， w is the diameter of the disc, ε0, εr are the vacuum dielectric constant and relative dielectric constant of the SiO2 dielectric layer, ε1, ε2 are the real and imaginary parts of the metal dielectric constant, ω is the angular frequency of the incident light, and c1 is a numerical factor that takes into account the fringe effect or non-uniform charge distribution along the surface of the capacitor.0 0.5 From equations ( 1)-( 4), the total impedance of the equivalent circuit Zall(ω) can be derived.When resonance occurs, Zall(ω)=377Ω, and the corresponding resonant frequency can be solved, which is obtained as 39.3 THz.
When simulating the MIM absorber in COMSOL, some parameters are first fixed, and then the incident light frequency and other parameters are simulated.The array period affects the FWHM parameter of the MIM absorber.A larger period can achieve better narrowband absorption characteristics, but an overly large period will affect the absorptance.When the array period p=3 μm, The absorption rate is close to 1, FWHM<300nm.Next, fix the disc radius and array period, and simulate for different incident light frequencies, SiO2 dielectric layer thicknesses and disc radii: According to the simulation results, within a certain range, the characteristic absorption peak wavelength is positively correlated with the radius of the disc structure.The dielectric layer thickness should be around 0.06 μm and the disc radius should be around 1.2 μm.The structural parameters of the MIM absorber are determined, which are: 3 μm array period, 0.06 μm dielectric layer thickness, and 1.2 μm disc radius.COMSOL is further used to simulate the absorbance of the MIM absorber with the given structural parameters.The MIM absorber with the given structural parameters produces near-unity specific absorption at 70.3THz, with a FWHM of about 240 nm and a quality factor of 17.75.To further verify the energy dissipation process of the incident light when resonance occurs in the MIM absorber, further numerical simulation of the electromagnetic field distribution is performed when specific absorption occurs in the absorber.The simulated electromagnetic field distribution shows that the incident light couples with the metallic discs to excite surface electromagnetic waves.The electromagnetic waves are confined in the near-field region between the dielectric and metal to excite magnetic and electric resonances, and induced currents are formed on the metal backplane.The magnetic and electric fields are coupled between the two resonant cavities, eventually converting the light energy into ohmic loss in the metal and dielectric loss in the dielectric, achieving complete absorption of the incident light.This process verifies the working principle of the MIM absorber.The above figure depicts the relationship between the absorption frequency and incident angle of the disc array absorber under TE polarization mode.It can be observed from the figure that the designed metamaterial absorber exhibits good angular isotropy.Within the 45°incident angle range, it maintains relatively high absorption coefficients, and the center frequency remains largely unchanged.In addition, owing to the inherent in-plane structural symmetry of the discs, the absorber also demonstrates good angular polarization independence.By simulating the pyroelectric current of AlN thin films with different areas, the variation of their peak values was observed (Figure 9a).The research results indicate that increasing the sensitive element area of the pyroelectric chip prolongs the infrared thermal response time and increases the pyroelectric current.Although increasing the current enhances the responsivity, larger area also introduces more noise.Overall, there exists an optimal area that achieves the highest detection efficiency.To investigate the effect of sensitive area on the performance of pyroelectric detectors, the detectivity D * of different area AlN pyroelectric detectors was calculated through simulations.The results demonstrate that as the area is increased from 0.3×0.3mm 2 to 1×1 mm 2 , D * increases from 3.63×10 7 cm•√Hz•W -1 to 2.25×10 8 cm•√Hz•W -1 .This is due to the enhancement of pyroelectric response with the increase in area.However, this also leads to increased device heat capacity and noise.Additionally, larger area introduces challenges in etching and significantly influences thin film stress effects.Considering factors such as detection performance, fabrication processes, and thin film integrity, this study determines that the optimal detection area is 1.25×1.25 mm 2 .

Preparation and Characterization of AlN Thin Films
AlN has compatibility with CMOS processes, but its pyroelectric coefficient is not particularly outstanding compared to other pyroelectric materials.Doping scandium into AlN is an effective method to further improve its pyroelectric performance.Scandium doping will change the c/a lattice constant ratio of AlN, exacerbating the non-symmetry of the AlN lattice.When the c/a lattice parameter ratio changes, the spontaneous polarization effect of the crystal will be enhanced, thus improving the overall pyroelectric performance of AlN.This strategy of optimizing pyroelectric performance by controlling the lattice parameter ratio provides an effective way for AlN materials to be applied in pyroelectric detectors.Scandium-doped aluminum nitride (ScAlN) thin films with a thickness of 500nm were prepared using magnetron sputtering.By studying the characteristics of AlN thin films deposited at different substrate temperatures, as the XRD spectra of AlN thin films shown in Figure 10, with increasing temperature, the diffraction intensity of the (002) peak continues to increase and reaches a maximum at 400°C.The (100) diffraction peak begins to appear at 300°C and shows a slight red shift, indicating lattice distortion occurs during growth.The activation energy required for (100) oriented film growth is lower than that of (002) orientation, so the change in deposition temperature leads to the crystallite reorientation.At relatively low temperatures, the (100) orientation predominates, corresponding to a larger diffraction peak intensity; at relatively high temperatures, the (002) orientation dominates, the diffraction peak intensity increases and the peak narrows, indicating improved crystallinity of the (002) plane.The morphological characteristics of the cross-section of AlN thin films can be analyzed by scanning electron microscopy (SEM).As shown in Figure 11a, ScAlN exhibits a columnar crystal structure.Under high magnification in Figure 11b, it can be seen that the film surface has tiny nanostructures and a few very fine depressions.These structures may be caused by nonuniformity during thin film growth.Overall, the ScAlN thin film has good morphology and surface characteristics.It can be seen that the crystal structure is neatly arranged with good orientation.The fabrication process for the AlN pyroelectric detector is illustrated in Figure 12.First, 4-inch silicon nitride wafers were prepared and cleaned.Next, 150 nm thick Mo bottom electrodes were deposited on the silicon nitride wafers by magnetron sputtering and patterned.This was followed by the growth of 500 nm thick ScAlN thin films, also using magnetron sputtering.The ScAlN films were subsequently dry etched via ion beam etching (IBE).To prevent electrical conduction caused by contact between the top and bottom electrodes, the ScAlN layer area was designed to be slightly larger than that of the bottom Mo electrode.Mo top electrodes were then deposited and patterned by a lift-off process, identical to the steps for the bottom electrodes.Because Mo is incompatible with subsequent wire bonding processes, gold (Au) pads were deposited onto the electrodes.Finally, reactive ion etching (RIE) was utilized to deeply etch the silicon substrate down to the silicon nitride layer, thereby releasing the backside of the detector.An optical micrograph of the completed AlN pyroelectric detector fabricated by this MEMS process is presented in Figure 13.detector.AlN thin films were prepared using magnetron sputtering technology.Preliminary processing of AlN pyroelectric detectors was performed based on MEMS technology.To realize narrowband specific absorption, this study designed and simulated a MIM three-layer metamaterial absorber, which exhibited near-unity specific absorption at 70.3 THz with a Q-factor of 17.75.This provides a feasible scheme to integrate the metamaterial absorber with the AlN pyroelectric sensor.This new integration method brings significant improvements to the performance and integration of the sensor.

Figure 3 .
Figure 3.Under different incident light frequencies: (a)The relationship between dielectric layer thickness and absorbance; (b) The relationship between disc radius and absorbance.

Figure 4 .
Figure 4. Absorption characteristics curve of the given parameters

Figure 5 .Figure 6 .
Figure 5. Electromagnetic characteristics simulation of the MIM absorber: (a) and (d) are the electric and magnetic field distributions at f0; (b) and (e) are the electric and magnetic field distributions at f1; (c) and (f) are the electric and magnetic field distributions at f2. f0, f1, and f2 correspond to the absorption peaks at 39.3 THz, 70.3 THz, and 95 THz from left to right in Figure 4.

Figure 7 .
Figure 7. Schematic of AlN pyroelectric detectorBy simulating the device, establish a thermal steady-state model and apply constant infrared thermal radiation as the boundary condition to simulate the temperature response on the sensitive element surface and obtain the steady-state temperature distribution.Then, establish a transient heat conduction model, add periodic infrared laser pulses at a given frequency as a heat source, and record the dynamic response process of the temperature at the center point of the sensitive element over time.Extract the thermal time constant through data fitting and substitute it into the pyroelectric effect formula to calculate the pyroelectric current response under infrared pulsed excitation.

Figure 8 .
Figure 8. (a)The highest temperature values of the detector at different areas; (b)The relationship between the pyroelectric current and time variation.

Figure 9 .
Figure 9. (a) Simulated relationship between pyroelectric current and AlN size area variation;(b) Relationship between detectivity D* and AlN size area variation.

Figure 10 .
Figure 10.XRD diffraction spectra of AlN thin films deposited at different substrate temperatures.