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Abstract
The infrared absorption efficiency is essential for an infrared sensor. We propose a quartz bulk acoustic wave (BAW) uncooled infrared sensor coated with MXene quantum dot film. The infrared detection is realized by measuring the resonant frequency of a Y-cut quartz BAW sensitive unit. An infrared sensor is fabricated by MEMS process, then the MXene quantum dot film is coated through the spin coating technology. The mechanism of infrared absorption enhancement is analyzed. Test results show that after coating the film, the responsivity (R) of the sensor increased by nearly 41% at a wavelength of 830nm, from 10.88MHz/W to 15.28 MHz/W. The quartz BAW infrared sensor combined with MXene quantum dots film has the potential of high-performance infrared detection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Due to the advantages of no refrigeration, operating at room temperature, low cost, small size, and low power consumption, uncooled infrared detectors have been widely studied and applied [1]. After absorbing infrared radiation, the temperature of the thermal detector changes. Based on the thermal resistance characteristics of materials such as vanadium oxide [2] and amorphous silicon(a-Si) [3], the temperature is changed by the variation of resistance. Typical heat detectors include microbolometers [4], pyroelectric infrared detectors [5], thermopile infrared detectors [6], etc. With the development of micro-nano electromechanical systems, high-sensitivity infrared detectors with frequency characteristics have become one of the research hotspots, which greatly improves the performance of thermal detectors, reduces the cost and has good application prospects. New resonant uncooled infrared detectors of different materials have been reported, such as aluminum nitride [7], Y-cut quartz [8], gallium nitride (GaN) [9], zinc oxide [10], and shape memory polymer (SMP) [11], etc.
Resonant infrared detectors focus on obtaining high-Q (quality factor) resonant sensing elements combined with surface coating materials to improve sensitivity. In terms of materials, the temperature coefficient of frequency (TCF) of piezoelectric materials reflects the ability of piezoelectric materials to sense temperature. The TCF of different piezoelectric materials are aluminum nitride approximately −25 ppm/K [7], gallium nitride approximately −18 ppm/K [9], lithium niobate approximately −80 ppm/K [12], Y-cut quartz crystal 90ppm/K [8], and Y-cut quartz provides the basis for high-sensitivity quartz bulk acoustic resonators. In terms of coating research, to further improve the detection capability of resonant uncooled infrared detectors, metasurfaces have been generally used to improve infrared absorption efficiency, but the large-scale manufacturing process is complex and expensive [13,14]. On the other hand, the method of using absorbing material film can also improve the detection ability of its infrared detector, such as gold nanorod film combined with aluminum nitride [15,16] can greatly enhance the sensitivity up to 598%; carbon nanotube combined with gallium nitride [17] enables noise equivalent temperature difference (NETD) below 5mK at room temperature, silicon nitride film combined with shape memory polymer materials [11] enables NETD to reach 6mK in vacuum; graphene combined with aluminum nitride [7] implement ultrathin (460 nm) piezoelectric nanomechanical resonant structures with improved over 50% electromechanical performance (frequency $\times$ Q) and infrared detection capabilities compared with metal-electrode counterparts.
Because the features of high specific surface area, excellent electrical conductivity, abundant hydrophilic functional groups, and remarkable optical saturable absorption, MXenes have applied in various fields like ultra-narrow photonics [18], ultrafast photonics [19,20], optical communication nanosystem [21], supercapacitor [22], and solar cell [23]. Due to the remarkable electromagnetic wave absorption properties, much research interest over MXenes has been aroused, especially using its prominent feature of photothermal conversion.
The infrared potential of Y-cut quartz has been reported in the literature [8], and to further improve its detection ability, we investigate the improvement of infrared absorption efficiency by MXene quantum dot film. The Y-cut quartz BAW uncooled infrared detector is used as the detection carrier, and a layer of MXene quantum dot film is coated on its surface through the spin coating technology, which improves the infrared absorption efficiency of the infrared detector. The high electromechanical performance (frequency $\times$ Q) of the infrared detector improves the responsivity (R) of the infrared detector.
2. Materials and method
2.1 Fabrication of quartz BAW resonators
A cross-sectional overview of the fabrication process is schematically shown in Fig. 1. We observe that the preparation of resonator is summarized as silicon wafer, cavity, definition, bonding, debonding, thinning and cross section.
1) A 500-nm-thick silicon dioxide film is thermally grown on a silicon wafer to electrically insulate the bottom electrode of the quartz BAW resonator.
2) The support structure region of the resonator is obtained by photolithography using a positive photoresist. A 10-nm-thick Cr and a 0.5-to 1-$\mathrm {\mu }$m-thick Au film are evaporated on the wafer. A 10-to 30-$\mathrm {\mu }$m-deep cavity is etched in silicon by wet etching.
3) A 500-$\mathrm {\mu }$m-thick silicon wafer and a 100-$\mathrm {\mu }$m-thick Y-cut 4-inch-diameter quartz wafer are temporarily bonded using a bonding system. The bonded quartz wafer is thinned by physical grinding and polishing, and then thinned and polished by chemical mechanical polishing (CMP) to reach the required thickness.
4) The bonding area and the bottom electrode are prepared by evaporation, photolithography, and etching of 0.5-to 1-$\mathrm {\mu }$m-thick Au film and 100-to 200-nm-thick Au film.
5) Align and bond the two wafers from step 2) and step 4). The two wafers are bonded using a bonder at temperature of 300$^{\circ }\textrm {C}$ to 350$^{\circ }\textrm {C}$ and a pressure of 1000N to 3000N for 1 to 2 hours.
6) Use a debonding system to release the temporary bonding in step 3). Preparation of the top electrode on the quartz wafer after de-bonding. A 10-nm-thick Cr film and a 100-nm-thick Au film for the top electrode are deposited by evaporation. Thus, the overall structure of the resonator is released. The photo of the fabricated quartz BAW resonator taken under the microscope is shown in Fig. 2(a).
2.2 Preparation of MXene quantum dot solutions
1g of LiF is added to the 9M of HCl solution and stirred at room temperature for 5 min. 1g of $\textrm {Ti}_{3}\textrm {AlC}_{2}$ is added slowly to avoid local overheating. The reaction is left to stir at 35 $^{\circ }\textrm {C}$ for 24h. After the reaction, the solution is centrifuged and washed to pH 6 or more, dried at 50 $^{\circ }\textrm {C}$ for 3 h, and collected the powder. Dissolving 0.2g of the dried powder in 30 ml of water, transfering the mixed solution to a reaction kettle, and reacting at 120 $^{\circ }\textrm {C}$ for 6 h. After the reaction, the supernatant is taken to obtain the MXene quantum dot solution. The absorption spectrum of the MXene quantum dot solution is tested by UV-NIR spectrophotometer. The MXene $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}$ quantum dot solution is photographed by using a transmission electron microscope and examined by using X-ray photoelectron spectroscopy (XPS).
2.3 Performance measurement of MXene quantum dot thin film detectors
We measure the Q value of the 50MHz quartz BAW infrared detectors, and select 3 quartz BAW infrared detectors with close Q values. The process of spin coating method is shown in Fig. 3. We suck a certain amount of MXene quantum dot solution with a pipette and coat the same amount of MXene quantum dots, respectively. Two quartz BAW infrared detectors are coated with the solution and the third piece is used as a reference quartz BAW infrared detector which is not coated. The drip-coated quartz BAW infrared detectors are fixed on the spin coater, and the spin speeds are set to 4000r/min and 5000r/min, respectively. After the spin coater runs for about 5 minutes, a quartz BAW infrared detector with uniform dispersed coating is obtained. The Q value of the final quartz BAW infrared detector is measured, and the Q value loss of the quartz BAW infrared detectors at different speeds of the spin coater are compared. Meanwhile, the 830nm laser (NLM-830-IMF (S)-500) is used to irradiate the quartz BAW infrared detector at 2cm, and the 830nm lasers of different intensities are adjusted. After stabilization, the network analyzer (S$\&$A 250B) is used to test the admittance versus frequency over different laser intensities. By extracting the maximum admittance, moving the frequency point of the value can find the frequency change corresponding to the change of the laser intensity, so as to obtain the R of the quartz BAW infrared detector. The detailed test setup for R is shown in Fig. 4.
3. Results and discussion
3.1 Structure of the infrared detector
The physical picture taken under the microscope and structural schematic diagram of the prepared quartz BAW infrared detector are shown in Fig. 2. The frequency of quartz crystal vibration in thickness mode can be expressed as
where $\upsilon$ is the rate of propagation of elastic waves in a quartz plate, $d$ is the thickness of the plate, $c_{ij}$ is temperature coefficient and $\rho$ is quartz density. According to Eq. (1), it can be known that the thickness of Y-cut quartz is inversely proportional to the resonant frequency. The size of the quartz piezoelectric layer of the final quartz BAW infrared detector is 2 mm$\times$1.3 mm$\times$35$\mathrm {\mu }$m. The structure of the infrared detection unit is, from top to bottom, the top gold electrode, the quartz piezoelectric layer, the bottom gold electrode and the bottom silicon substrate. When coating, a certain amount of MXene quantum dot solution is drawn with a pipette and dropped on the top center of the infrared sensitive unit, which is convenient for spin coating.3.2 MXene quantum dot analysis
MXene materials combine the properties of ceramics and metals, exhibiting semi-metallic properties with ultra-high metallic conductivity due to the incorporation of carbon atoms into the metallic lattice. Experiments show that MXene $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}$ has a strong absorption ability in the near-infrared region, similar to some noble metal nanoparticles with localized surface plasmon resonance (LSPR) effect, such as Au NPs [24]. The strong LSPR effect of MXene $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}$ enhances the optical absorption performance of $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}-\textrm {H}_{2}\textrm {O}$ nanofluids. LSPR is defined as the coherent oscillation of photon-induced charge resonance at the metal-dielectric interface when the photon frequency coincides with the natural frequency of electrons on the surface of metal nanoparticles. There are two competing paths for the decay of surface plasmons. One is a radiative decay process in which light scattering is generated by re-emitted photons, and the other is a non-radiative decay process in which hot electrons formed by excitation are converted from occupied states to unoccupied states into thermal energy. The LSRP effect induces the generation of photothermal electrons, which is beneficial for light absorption, especially in the near-infrared region. The absorbed infrared radiation is converted into heat, which is dissipated into the surrounding medium by vibrations scattered by the lattice, thereby increasing the surrounding temperature. In addition, the coupling effect and the shape of $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}$ particles also play important roles in photothermal absorption and conversion. The infrared absorption efficiency can be improved by controlling the uniformity of the MXene quantum dot film. Figure 5(a) is the transmission electron microscope image of the MXene quantum dot solution. It can be seen that the size of the MXene quantum dots is about 5nm, the clear lattice fringes with an interval of 0.17nm and the MXene quantum dots have high crystallinity from the high-resolution transmission electron microscope (HRTEM) figure and selected area electron diffraction (SAED) pattern. Moreover, the distribution is uniform, which is very helpful for the performance improvement of the quartz BAW resonator. The lattice constant of $\textrm {Ti}_{3}\textrm {C}_{2}\textrm {T}_{\textrm {x}}$ MXene is 3.071 Å [25]. Figure 5(b) shows the absorption spectrum of the MXene quantum dot solution, which is consistent with the description in the [26,27]. There is a strong absorption ability in the near-infrared region of 750nm-850nm and a near-infrared absorption peak near 808nm, which are the reasons why the 830nm laser light source is used for testing in the experiment. The MXene quantum dot solution exhibits luminescent state under UV light irradiation. The surface composition and chemical structure of the as-prepared MXene quantum dot is examined by using XPS. As shown in Fig. 5(c), four peaks at 285, 458, 533 and 686 eV are attributed to C 1s, Ti 2p, O 1s, and F 1s, respectively. Fig. 5(d) shows the high-resolution spectra of C 1s. The C 1s region is fitted by four peaks. These are $\textrm {Ti-C}$ (283.07eV), $\textrm {C-C}$ (284.47eV), $\textrm {C-O}$ (285.27eV) and $\textrm {C=O}$ (287.87eV).
Fig. 5. (a) The TEM picture of MXene quantum dots (top left inset:HRTEM image and SAED pattern of MXene quantum dot ; top right inset:size distribution histogram). (b) The UV-NIR absorption spectrum of MXene quantum dots (inset: MXene quantum dot solution under UV light irradiation). (c) Survey XPS spectrum for the MXene quantum dots. (d) High-resolution of C 1s for the MXene quantum dots.
Fig. 6. Schematic of quartz BAW infrared detector response to NIR irradiation before and after coating.
3.3 Performance evaluation of infrared detectors
The frequency change of the Y-cut based quartz BAW infrared detector is changed according to the temperature change of the quartz part. The inverse piezoelectric effect of the quartz piezoelectric material converts the electrical signal into a sound wave, and then resonates over the conditions of extreme acoustic wave boundary. When infrared radiation is irradiated on the quartz piezoelectric layer, the temperature of which changes resulting the variations of the resonant frequency. After coating the MXene quantum dot film, more infrared radiation is absorbed and converted into heat energy, resulting increasing variations of both frequency and temperature of the quartz piezoelectric layer, as shown in Fig. 6.
Quality factor (Q) is an important parameter to measure the noise performance of resonant system. The frequency resolution can be improved by improving the Q value of the resonator. We use spin coating method to reduce the loading effect of MXene quantum dots on quartz BAW resonators, thereby ensuring high Q value. Using S$\&$A 250B to test the quartz BAW infrared detector whose resonant frequency is about 50MHz and the Q value before coating is 51.6k. For the Q value of the uncoated quartz BAW infrared detector is unchanged. While, for the coated quartz BAW infrared detector, the Q values at the spin speeds of 4000r/min and 5000r/min are 43.2k and 51.1k, respectively. From Fig. 7, a graph of the admittance characteristics of the quartz BAW infrared detector measured by the network analyzer in the S$\&$A 250B. We measure the frequency value of the maximum admittance point to characterize the frequency of the sensor for the reason that the error between the frequency value of the maximum admittance point and the accurate value of the frequency is small [28]. During the test, the R of the quartz BAW infrared detector can be reflected by tracking the change of the frequency value of the maximum admittance point.
Table 1 shows the electromechanical performances of three classical coated resonators and that of our proposed quartz BAW resonator. Compared with other resonant infrared detectors after coating, the resonant frequency of the infrared detector combined with graphene and aluminum nitride is 307MHz, and the Q value is 1709; the resonant frequency of the infrared detector combining carbon nanotube and gallium nitride resonates is 120MHz, and the Q value is 12185; the resonant frequency of the infrared detector combining silicon nitride and shape memory composite material is 0.145MHz, and the Q value is 1050; the resonant frequency of the infrared detector combining MXene quantum dots and Y-cut quartz is 50MHz and the Q value is 51100. It can be seen that the electromechanical performance of the infrared detector combined with MXene quantum dots and Y-cut quartz is better than that of other resonant infrared detectors after coating. Therefore, we know that applying a certain rotating spin coater to coat the film will enhance the electromechanical performance of the quartz BAW resonator, and even the electromechanical performance of the uncoated infrared detector after coating is unchanged.
Table 1. Comparison of electromechanical performance of different devices
Figure 8 shows a summary of the admittance-frequency graphs of the quartz BAW infrared detector under different laser intensities measured by network analyzer in the S$\&$A 250B. With the equal increasing of the laser intensity, the maximum impedance frequency point also increases in an equal amount, extract the frequency value of each maximum admittance frequency point, and do linear fitting to obtain the R of the quartz BAW infrared detector. The R (in hertz per watt) of the detector is given by
where $\Delta f_0$ is the change in the resonance frequency in response to the incident radiation, $Q_0$ is the incident radiation received by the detector, $\zeta$ is the absorption coefficient of the detector, $f_c$ is modulation frequency, $f$ is the resonant frequency of the detector, $G$ is thermal conductivity of the detection unit and $H$ is the heat capacity of the detection unit.Fig. 8. Admittance and frequency shift at different intensities. (a) Admittance versus frequency without coating. (b) Admittance versus frequency over coated detector (rotational speed: 4000r/min). (c) Admittance versus frequency over coated detector (rotational speed: 5000r/min). (d) Linear fitting of uncoated quartz BAW detector over different laser intensities. (e) Linear fitting of coated quartz BAW detector (rotational speed: 4000r/min). (f) Linear fitting of coated quartz BAW detector (rotational speed: 5000r/min).
The experimental results in Fig. 8 are the admittance and frequency shift of the uncoated quartz BAW infrared detector, and the coated quartz BAW infrared detector at a rotational speed of 4000r/min and 5000r/min, respectively. The frequency of the detector rises with increasing the received laser intensity. By linear fitting, the responsivities of the quartz BAW infrared detector are obtained as 10.88 MHz/W, 14.32 MHz/W and 15.28 MHz/W. From Fig. 8, we observe that over the rotating speed of 5000r/min, the coated quartz BAW infrared detector not only keeps its Q value slightly lower compared with that before coating, but also has a 41% increasing in the R of the quartz BAW infrared detector. Table 2 shows the detection performance of different sensors. Applying a coater at a certain spin speed could form the film with more evenly dispersed MXene quantum dots, reduce the load interference, and improve the stability of the quartz BAW resonator. The R of the combined device at only 50MHz is measured to be 15.28MHz/W, which is higher than what has been recently reported for quartz (11.4MHz/W) at 241MHz, and gallium nitride (1.7MHz/W) at 101MHz based piezoelectric IR detectors. The combined device at a lower resonant frequency not only has higher R, but also has higher Q value.
Table 2. Comparison of R of different devices
4. Conclusion
First, we fabricate the MXene quantum dots and the quartz BAW resonator (50MHz). Then, the MXene quantum dots are coated on the quartz BAW resonators by a spin coater at an optimized speed. The electromechanical performance of the infrared detector combined with MXene quantum dots and Y-cut quartz is better than that of other resonant infrared detectors after coating. The R of the quartz BAW infrared detector (50MHz) has been improved from 10.88MHz/W to 15.28 MHz/W (an increase of nearly 41% at a spin speed of 5000 r/min) while slowly decreasing the Q value. This method of coating MXene quantum dots film to improve the performance of infrared detectors with great potential near-infrared detection. Further, combining MEMS technology, a quartz BAW resonator detection array could be fabricated either to provide more detection for the recognition of near-infrared optical imaging field.
Funding
National Natural Science Foundation of China (61675025); Beijing Chaoyang District Collaborative Project (CYXC2109).
Acknowledgments
We are grateful for the teachers of the Analysis Testing Center in Beijing Institute of Technology for their kindly help. Some charts in this paper are obtained by the instruments of the center.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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