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Article

Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect

by
Ya Liu
1,2,*,
Ke Sun
1,*,
Jinyi Ma
2,
Zhong Yu
1 and
Zhongwen Lan
1
1
School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 610054, China
2
26th Institute of China Electronics Technology Group Corporation, Chongqing 400060, China
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(8), 1126; https://doi.org/10.3390/coatings12081126
Submission received: 23 June 2022 / Revised: 30 July 2022 / Accepted: 2 August 2022 / Published: 5 August 2022
(This article belongs to the Special Issue Functional Magnetic and Dielectric Composites: Fabrication, Properties and Applications)
Browse Figures
Versions Notes

Abstract

:
To ensure that the performance of filters matches the continuous development in communication frequency bands, the influence of temperature on filter performance must be considered during the fabrication of filters. In this study, a cavity-type temperature-compensated film bulk acoustic resonator (TC-FBAR) device was prepared with an SiO2 temperature filter between the bottom electrode and the piezoelectric layer. A one-dimensional Mason model of the TC-FBAR was established. An advanced design system, a high-frequency structure simulator, and COMSOL software were used to optimize the design of the TC-FBAR. After the optimization, the out-of-band rejection was improved by 10 dB. To address the compensation effect of the tensile and compressive stresses, a multilayer film was implemented for low-stress control and a reduction in stress to |P| ≤ 150 MPa, thereby improving the orientation of the piezoelectric film. Moreover, the influence of the thickness of the SiO2 temperature-compensated layers on the temperature-compensated characteristics was studied. When the SiO2 thickness was 50 nm, the temperature coefficient of frequency (TCF) was ±1 ppm/°C. The center frequency and 3 dB bandwidth of TC-FBAR were 2.492 GHz and 15.02 MHz, respectively, and the center insertion loss was −3.1 dB. Moreover, the out-of-band rejection was greater than 40 dBc, and TCF was 0.8 ppm/°C.

1. Introduction

With the rapid development of electronic equipment systems and mobile communication technology, communication frequency bands (S and C bands) are further extended to higher frequencies. Consequently, phased-array radars, electronic countermeasures, and communication systems have increasingly high requirements for the temperature coefficient of frequency (TCF), performance, and volume of high-frequency filters [1,2,3]. Further, the increased scarcity of communication spectrum resources complicates the frequency band allocation and narrows the protection frequency band with more stringent market requirements on filter performance.
The film bulk acoustic resonator (FBAR) filter is a key component in radio frequency (RF) systems. In particular, every RF signal and communication frequency band uses filters to suppress interference signals. Thus, filters play a vital role in the control of electromagnetic power. As FBARs have a large TCF [4,5,6,7,8], the device performance in the entire temperature range should be considered in the filter design. As the TCF increases, the filter bandwidth widens to more than the required bandwidth, resulting in a more difficult design. The resonant frequency (f) of the FBAR is determined by the thickness (d) and the longitudinal wave velocity (v) of the volume acoustic wave propagating in the resonator, v as follows:
f = v/d
v = E / ρ  
where E is the Young’s modulus of the material, and ρ is the density of the material [9,10].
For AlN and Mo, the Young’s modulus decreases with increasing temperature [11,12,13,14]. The TCF of conventional Mo–AlN–Mo-structured FBAR is approximately −30 ppm/°C [15]. As a commonly used semiconductor material, the Young’s modulus of SiO2 increases with the increase of temperature, achieving a TCF of +85 ppm/°C [16,17]; thus, SiO2 can be used as the temperature-compensated material for FBARs. By increasing the thickness of the SiO2 temperature-compensated layer, which has a positive TCF [18], and using the principle of mutually compensated materials with positive and negative TCFs, the TCF of FBARs can be controlled at a low value; however, the insertion loss increases with the addition of SiO2. In particular, using a ZnO micromachined cantilever as the piezoelectric layer and SiO2 as the temperature-compensated layer, a TCF of −0.45 ppm/°C can be achieved [19]. However, the micromachined cantilever has low reliability and is prone to fracture. Moreover, ZnO is not compatible with semiconductor processing. When a small modular reactor (SMR) filter adopts SiO2 as the temperature-compensated layer, the TCF can reach 1 ppm/°C [20,21]. However, SMRs require a highly precise film thickness. After doping fluorine in SiO2 as the temperature-compensated layer, the electromechanical coupling coefficient (k2t) can be increased to 6.26%, and a TCF of −11.1 ppm/°C can be achieved [22]. Meanwhile, after doping boron in SiO2 as the temperature-compensated layer, a TCF of up to −1.5 ppm/°C was obtained with an improved quality factor (Q) value [23]. Thus, the doped SiO2 temperature-compensated layer is suitable for the production of broadband filters. However, this process is relatively complex, and requires the use of expensive equipment.
Thus, in this study, based on the conventional Mason model, a Mason model of the temperature-compensated FBAR (TC-FBAR) was established, and the resonator and filter were co-simulated and optimized by COMSOL, a high-frequency structure simulator (HFSS), and an advanced design system (ADS). Further, a method for preparing the cavity structure of a low-stress multilayer composite film was adopted to incorporate an SiO2 temperature-compensated layer between the bottom electrode and the piezoelectric layer. The proposed method can obtain piezoelectric thin films with the preferred orientation as well as fabricate filters with a low insertion loss, a narrow bandwidth, a high out-of-band rejection, and a low TCF that meet the requirements of a low-temperature drift narrowband filter.

2. Materials and Methods

2.1. Design of the FBAR Filter

2.1.1. FBAR Model

According to Mason’s equivalent model in the traditional piezoelectric theory [24], the equivalent circuit of the piezoelectric and acoustic layers was cascaded to obtain the traditional FBAR equivalent circuit. From the traditional FBAR Mason model, the equivalent circuits of the adhesion and temperature-compensated layers were added to obtain a Mason model of the TC-FBAR, as shown in Figure 1a. A finite element analysis (FEA) based on COMSOL Multiphysics software was performed using the TC-FBAR Mason model structure, as shown in Figure 1b.

2.1.2. Design of TC-FBAR

A three-dimensional electromagnetic model of the package, a die pad, and a wire bonding line was established in a HFSS. The size, position, and thickness of the pad, and the number and position of the wire bonding lines were optimized, thereby optimizing the near and outer ends of the out-of-band filter. From the FEA, the peripheral electromagnetic environmental parameters of TC-FBAR were obtained, as shown in Figure 2a.
The peripheral electromagnetic parameters and resonator model obtained by the HFSS and, subsequently, the lumped parameter matching circuit were imported into the ADS. The optimized design of the device was realized through the optimization algorithm in ADS. The topological structure adopted six series and six shunts, as shown in Figure 2b. This structure can address the problems of narrow bandwidth, high out-of-band rejection, and low insertion loss filters. In COMSOL, the resonator film thickness structure was optimized to ensure that it was in its optimal matching state, thereby reducing the insertion loss and improving the Q and k2eff. Qs and Qp are computed as follows:
Qs = fs/Δfs
Qp = fp/Δfp
where Δfs and Δfp are the −3 dB frequency widths of the impedance response |Z| at fs and fp, respectively. The effective coupling coefficient k2eff is derived from the series resonant frequency fs and parallel resonance frequency fp [22] as follows:
k2eff = (π2/4)(fs/fp)[(fp − fs)/fp]
In the HFSS, the layout of the resonators and the number and position of wire bonding were optimized to reduce the electromagnetic parasitic effect and inductance as well as improve the out-of-band rejection. In ADS, filters with low loss and high out-of-band suppression were obtained by optimizing the topology, number, and size of the resonators, as well as the series–parallel mode. The simulation performance diagram after optimization is shown in Figure 2c. After optimization, the out-of-band rejection was 50 dB, which was 10 dB higher than that before optimization.

2.2. Film Fabrication

In this study, a high-resistance silicon wafer (resistivity of >5000 Ω·cm) and SiO2 films were used as the temperature-compensated layer to produce the TC-FBAR filter. The fabrication process is shown in Figure 3. Owing to the introduction of the SiO2 temperature-compensated layer, the upper and lower film layers were prepared using different methods and physical parameters. This resulted in difficulties in controlling the stress, easy separation of the components [25], widened full width at half-maximum (FWHM), and other phenomena that seriously affect the device performance or even cause failure of the composite film. Therefore, the preparation of the temperature-compensated composite film is a key technology in TC-FBAR.

2.2.1. SiO2 Film Fabrication

Plasma-enhanced chemical vapor deposition method was used to prepare the SiO2 films [26,27]. The film thickness was adjusted using the growth time. The growth of a substrate layer, such as Ti or AlN, before SiO2 deposition can improve adhesion during SiO2 growth, which can prevent the fall off of the SiO2 film.

2.2.2. Mo Electrode Film Fabrication

The Mo electrode thin film was prepared by direct current (DC) magnetron sputtering. By adjusting the sputtering power and argon flow rate [28], the stress of the Mo film in the electrode layer can be adjusted to 0 and 100 MPa, as shown in Figure 4a. The process parameters of the Mo film growth are as follows: alternating current (AC) power, DC power, RF power, and Ar flow rate of 0.5 kW, 3 kW, 35 W, and 60 sccm, respectively. In addition to optimizing the process parameters during Mo sputtering, an AlN seed layer was grown before the deposition of the Mo electrode, which greatly improved its orientation. To improve the induction effect of the seed layer, the substrate support layer was pretreated before growing the seed layer, and the support layer on the substrate was dusted with argon plasma to activate it and improve the preferred orientation of the Mo film. The FWHM of Mo’s (110) swing curve was reduced from 2.0197° to 1.6164° after optimization, as shown in Figure 4b. The surface, as evidenced by the increased intensity of the peak, indicated a higher degree of crystallization. The FWHM of the Mo films largely depends on the orientation and surface roughness of the substrate. The roughness of the Mo films prepared by DC magnetron sputtering was 0.294 nm, as shown in Figure 4c, which satisfies the process requirements. Moreover, the film surface is dense, smooth, and evenly distributed with few protrusions.
To avoid defects, such as poor coverage of the upper film, poor orientation, and excessive stress at the step, due to the excessive angle after etching the bottom electrode, a bottom electrode step with a smaller angle was obtained by adjusting the process parameters [29], as shown in Figure 5. An angle of 13.23° was used to ensure the growth quality of the upper film.

2.2.3. AlN Piezoelectric Film Fabrication

The AlN films were prepared on composite films, i.e., SiO2 film passivation layer, sacrificial polysilicon film layer, AlN-supporting layer, Mo bottom electrode, and SiO2 film temperature-compensated layer, by growing on a silicon substrate. Owing to the influence of stress accumulation and surface roughness, the preferred-orientation growth of the AlN film was more difficult than the direct growth of the AlN film on a silicon substrate. Thus, AlN film is mainly produced by magnetron sputtering [30,31]. In this study, the AlN film was prepared by intermediate frequency AC magnetron sputtering. The stress of the AlN film was adjusted by controlling the flow rate of the argon gas and through a stress-regulating resistance. The thin film stress measurement system was used to measure the stress of the thin film on the wafer before the growth of the AlN film. The stress of the AlN film was adjusted by preparing the multilayer film. If the composite film exhibited compressive stress, the AlN film was adjusted to exhibit tensile stress to achieve low stress for the film. The process growth parameters of the AlN film are as follows: AC power, DC power, RF power, Ar flow rate, and N2 flow rate of 9 kW, 10 kW, 50 W, 60 sccm, and 45 sccm, respectively. The orientation of the AlN thin films was improved by RF plasma cleaning before AlN film plating. During the RF plasma cleaning, argon ions bombarded the substrate to remove water vapor and other adjacent crops on the substrate surface, which increased the mobility of the particles on the substrate surface and, consequently, improved the crystallization orientation of the AlN film. For the tensile and compressive stress compensation effects, the bottom electrode, temperature-compensated, and piezoelectric layers under stress, and roughness of the monolayer film were adjusted to decrease the stress of the multilayer piezoelectric composite film.

3. Results and Discussion

The roughness of the film was tested by atomic force microscopy (AFM). The roughness of the SiO2 films with different thicknesses is shown in Figure 6. With the increase in the SiO2 thickness, the roughness of the films gradually increases, which affects their quality. In particular, an excessively high roughness causes energy loss of the film surface, thereby increasing device loss [32]. Moreover, roughness affects the internal stress of the film, which is controlled by adjusting the process parameters, such as gas flow rate, power, and temperature.
The stress of the film was tested by the stress tester. The stress of the SiO2 films with different thicknesses is shown in Figure 7. As the SiO2 film thickness was increased, the stress gradually decreased. Moreover, a transformation from compressive to tensile stress was noted. This is ascribed to the lower deposition temperature in the films with a smaller thickness, which allows the escape of the reaction by-products as disordered substances in the film [33,34]. Thus, with the increase of the film thickness, the temperature of the film increased gradually due to the heat energy accumulation effect, it decreased the number of residual by-products in the film with accelerated escape, and transformed the stress from compressive to tensile stress. In addition, the (002) orientation of AlN is affected by the high stress, which can result in device film cracking or collapse after release. Thus, to improve the quality of the thin film and AlN (002) preferred orientation, a single-layer film was adopted to address the tensile- and compressive-stress-compensated effects, thereby realizing the low-stress control of the multilayer composite film. A schematic of the stress-compensated effect is shown in Figure 8. When the lower film exhibited compressive stress, the stress of the upper film was adjusted to the tensile stress to eliminate the stress of the composite film.
The X-ray diffraction (XRD) (002) orientation swing curve of the AlN film is shown in Figure 9. The FWHM of the AlN (002) swing curve was reduced from 2.361° to 1.885° after optimization, and the surface had a high peak strength, indicating the c-axis orientation and good crystallization of the AlN film [35,36,37].
The stress of the composite film was tested by the stress tester. The stress in the multilayer composite film is shown in Figure 10. By comparing the stress of the composite film before and after optimization, the stress (|P|) generated by the multilayer composite film was reduced from 800 to 150 MPa after optimization, thereby achieving the low-stress growth of the composite film.
A high-resistance silicon wafer and a cavity-type structure were used to fabricate the TC-FBAR filter. The appearance of the device was observed by a microscope, and the microstructure and membrane structure of the device were analyzed with transmission electron microscopy (TEM). The top view and TEM diagram of the TC-FBAR structure are shown in Figure 11. The layers exhibit good growth quality and adhesion, as shown in Figure 11b. The temperature-compensated characteristics of the temperature-compensated layers of different thicknesses were tested, as shown in Figure 12. As the thickness of the SiO2 temperature-compensated layer gradually increased, the TCF changed from negative to positive, and gradually increased because of the positive TCF of SiO2 and the negative TCF of the FBAR, as shown in Figure 12a. When the thickness of the temperature-compensated layer was approximately 50 nm, the TCF was ±1 ppm/°C, k2eff gradually decreased because the difference between fp and fs decreased gradually [38,39,40], and the corresponding bandwidth of the filter became narrower. As SiO2 was added between the piezoelectric layer and bottom electrode, the piezoelectric performance of the piezoelectric layer was affected when the piezoelectric property was converted into electrical energy; thus, the piezoelectric property cannot be completely converted into electrical energy, thereby reducing the effective coupling coefficient. The Q decreased gradually with an increase in the thickness of the temperature-compensated layer, because the acoustic energy was lost after adding SiO2, and −3 dB frequency widths were increased.
A vector network analyzer (VNA) was used to test the electrical performance of the TC-FBAR, as shown in Figure 13. As shown in Figure 13a, the measurements achieved were f0 = 2.492 GHz, BW3dB = 15.02 MHz, IL = −3.1 dB, and out-of-band rejection of more than 40 dBc. The test and simulation results have consistent trends. However, compared to the simulation results, the measured insertion loss and the out-of-band suppression were slightly larger and smaller, respectively. During the fabrication, the Mo films may be oxidized to increase the insertion loss. The differences in the out-of-band rejection can be attributed to the electromagnetic parasitism that was not considered during encapsulation. Furthermore, the measured frequency was lower than the simulated frequency because of the deviations between the actual and designed film thicknesses. Figure 13b shows the temperature characteristic test curve of the TC-FBAR, which has a TCF of 0.8 ppm/°C at −55–125 °C. The test results were compared with different references, as shown in Table 1 [12,13,14,41,42,43,44]. From the comparison results, the lowest TCF and narrowest bandwidth were obtained in this work; thus, this resonator is suitable for fabricating a narrowband filter with a low TCF.

4. Conclusions

In this study, a structural model of a TC-FBAR and its one-dimensional Mason model were established. ADS, HFSS, and COMSOL software were used to optimize the design of the TC-FBAR. Thus, a preparation method for the low-stress composite film was investigated. The principle of compressive- and tensile-stress-compensated effect was adopted to ensure that the stress of the composite film was within the required range. Moreover, the influence of the SiO2 thickness on the temperature-compensated characteristics was studied. At a suitable SiO2 thickness, the TCF could reach 0 ppm/°C. The TC-FBAR prepared by the cavity and composite film structures exhibited a low TCF.

Author Contributions

Y.L. provided the idea, designed the filter, implemented the simulation, and wrote the manuscript. K.S. and J.M. optimized the fabrication process, tested the devices, performed the theoretical analysis, and contributed to the manuscript preparation. Z.Y. provided theoretical guidance and revised the manuscript. Z.L. formulated the research issues and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Core Electronic Devices, high-end universal chips and basic software products of China, 2017ZX01001201-002, and the Innovation Group Project of Sichuan Province (2022JDTD0018).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable to this article.

Acknowledgments

The authors would like to thank the editors and anonymous reviewers for their special effort.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Mason model of the TC-FBAR. (b) Simulated displacement field at the resonant frequency of TC-FBAR.
Figure 1. (a) Mason model of the TC-FBAR. (b) Simulated displacement field at the resonant frequency of TC-FBAR.
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Figure 2. (a) HFSS model. (b) TC-FBAR full-wave simulation diagram. (c) Performance comparison after the simulation optimization.
Figure 2. (a) HFSS model. (b) TC-FBAR full-wave simulation diagram. (c) Performance comparison after the simulation optimization.
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Figure 3. Fabrication process of the TC-FBAR: (a) silicon etching; (b) thermal oxidation and poly-Si deposition and chemical mechanical polishing; (c) AlN seed layer and Mo deposition and etching; (d) adhesion layer AlN and SiO2 temperature-compensated layer deposition and etching; (e) piezoelectric layer and AlN deposition; (f) top Mo and AlN protective layer deposition and etching; (g) Al pad formation; (h) AlN etching and polysilicon release.
Figure 3. Fabrication process of the TC-FBAR: (a) silicon etching; (b) thermal oxidation and poly-Si deposition and chemical mechanical polishing; (c) AlN seed layer and Mo deposition and etching; (d) adhesion layer AlN and SiO2 temperature-compensated layer deposition and etching; (e) piezoelectric layer and AlN deposition; (f) top Mo and AlN protective layer deposition and etching; (g) Al pad formation; (h) AlN etching and polysilicon release.
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Figure 4. (a) Stress diagram of the bottom electrode of Mo. (b) Swing curve of Mo (110). (c) Surface roughness of the Mo film.
Figure 4. (a) Stress diagram of the bottom electrode of Mo. (b) Swing curve of Mo (110). (c) Surface roughness of the Mo film.
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Figure 5. Small-angle diagram after etching the bottom electrode.
Figure 5. Small-angle diagram after etching the bottom electrode.
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Figure 6. Roughness of the SiO2 films of different thicknesses: (a) 80, (b) 160, (c) 240, and (d) 320 nm. (e) Surface roughness with different SiO2 thicknesses.
Figure 6. Roughness of the SiO2 films of different thicknesses: (a) 80, (b) 160, (c) 240, and (d) 320 nm. (e) Surface roughness with different SiO2 thicknesses.
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Figure 7. Stress of the SiO2 films with different thicknesses: (a) 80, (b) 160, (c) 240, and (d) 320 nm. (e) Stress with different SiO2 thicknesses.
Figure 7. Stress of the SiO2 films with different thicknesses: (a) 80, (b) 160, (c) 240, and (d) 320 nm. (e) Stress with different SiO2 thicknesses.
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Figure 8. Schematic of the stress-compensated effect.
Figure 8. Schematic of the stress-compensated effect.
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Figure 9. Swing curve of the AlN thin film.
Figure 9. Swing curve of the AlN thin film.
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Figure 10. Three-dimensional stress diagram of the composite film: (a) before optimization, (b) after optimization.
Figure 10. Three-dimensional stress diagram of the composite film: (a) before optimization, (b) after optimization.
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Figure 11. (a) Top view of the TC-FBAR. (b) TEM diagram of the TC-FBAR.
Figure 11. (a) Top view of the TC-FBAR. (b) TEM diagram of the TC-FBAR.
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Figure 12. (a) Relationship between TCF, k2eff, and thickness of the SiO2 film. (b) Q with different thicknesses.
Figure 12. (a) Relationship between TCF, k2eff, and thickness of the SiO2 film. (b) Q with different thicknesses.
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Figure 13. (a) Test and simulation comparison diagram of the TC-FBAR. (b) Frequency response diagram of the temperature characteristics of the TC-FBAR.
Figure 13. (a) Test and simulation comparison diagram of the TC-FBAR. (b) Frequency response diagram of the temperature characteristics of the TC-FBAR.
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Table
Table 1. Performance comparison.
Table 1. Performance comparison.
Referencef0 (GHz)IL (dB)
(Within the Passband)		BW/3 dB (MHz)TCF (ppm/°C)
(−55–85 °C)
[12]2.080––+1.5
[13]1.320––±2.0
[14]1.500––−6.0
[41]3.7232.40 (at f0)840.00–
[42]2.4422.2082.00–
[43]2.5952.00120.00–
[44]2.4923.7417.00+5.0
This work2.4923.1015.02+0.8 (−55–125 °C)
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Liu, Y.; Sun, K.; Ma, J.; Yu, Z.; Lan, Z. Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect. Coatings 2022, 12, 1126. https://doi.org/10.3390/coatings12081126

AMA Style

Liu Y, Sun K, Ma J, Yu Z, Lan Z. Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect. Coatings. 2022; 12(8):1126. https://doi.org/10.3390/coatings12081126

Chicago/Turabian Style

Liu, Ya, Ke Sun, Jinyi Ma, Zhong Yu, and Zhongwen Lan. 2022. "Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect" Coatings 12, no. 8: 1126. https://doi.org/10.3390/coatings12081126

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