| The growing demand for wireless communication has led to the development of communication technology,thereby raising the performance requirements for filters.To address these issues,the development of high-frequency surface acoustic wave(SAW)filters is currently underway.However,it should be noted that the preparation of highfrequency SAW devices is challenging,and their power tolerance is limited,which restricts their applicability in the high frequency field.On the contrary,film bulk acoustic resonator(FBAR)exhibits the advantages of both high frequency and high power capacity.Nonetheless,the requirements of FBAR on material and the precision of structure are very strict,and there are technological challenges in the design and fabrication.In addition to the device structure and fabrication process,the properties of piezoelectric materials are also the key factors that determine the performance of acoustic resonators.AlN piezoelectric films exhibit excellent physical and chemical properties,including high phase velocity,high thermal conductivity,high dielectric constant,low high-frequency loss,good piezoelectric properties,good chemical stability,and so on,which are widely used in surface acoustic wave devices and bulk acoustic wave devices.However,the deposited AlN films typically have high stress and high-density threading dislocations,which hinder the improvement of device performance.Therefore,it is crucial to obtain high-quality and low stress AlN films for developing high-performance resonators.We have conducted in-depth research on AlN deposition,stress relaxation,and high-performance resonator fabrication.The following are the main research contents and results:1.Deposition of c-axis oriented AlN filmsThe impact of process parameters on the crystallization of AlN films were discussed in detail using RF magnetron sputtering.AlN films with(100)orientation were prepared by magnetron sputtering with 16.7%N2 concentration.Furthermore,the ionization rate of N2 was significantly improved through reactive ion assisted sputtering,leading to the successful preparation of AlN thin films with a thickness of 200 nm and high c-axis orientation on the sapphire substrate.The full width at half maximum(FWHM)value of the rocking curve of the(002)diffraction peak was 0.8° and the roughness was only 0.39 nm.The same method was used to prepare AlScN films with high electromechanical coupling coefficient(keff2),laying the foundation for the realization of high-performance resonators.2.Study on stress relaxation of AlN filmsThe stress relaxation process of AlN thin films was studied systematically.1000 nm AlN films with high c-axis orientation were deposited at room temperature,and the FWHM of the rocking curve of the(002)diffraction peak was only 0.63°.In response to problems with film peeling following conventional thermal annealing(CTA)and stress in the deposition films,rapid thermal annealing(RTA)was proposed to release film stress.The results showed a linear relationship between crack growth rate and dwell time,and RTA can shorten the dwell time above the critical cracking temperature while avoiding film peeling.Furthermore,RTA can increase the critical stress intensity factor of the material,which reduces the probability of film peeling by increasing the critical stress of cracking.When AlN films were annealed by RTA at 900℃,the E2 Raman peak of the AlN films shifted by 4.4 cm-1 compared to the as-deposited AlN films,and the tensile stress decreased from 1.97 GPa to 1.17 GPa.The mechanism of stress relaxation by RTA can be attributed to the grain coalescence and recrystallization,resulting in the reduction of grain boundaries and dislocation.To avoid film peeling and ensure sufficient time for stress release,a multiple-cycle rapid thermal annealing method was proposed to accumulate effective annealing time.3.Construction of AIScN/Diamond Surface Acoustic Wave resonators based on substrate removal structureThe AlScN/Diamond structure SAW resonator was prepared by growing diamond layer on AlScN thin film on Si substrate,followed by etching the Si substrate.The device exhibited operating frequencies up to 2.77 GHz with an insertion loss of 15.46 dB.effective electromechanical coupling coefficients keff2 of 4.31%,and a quality factor Q of 11 8 under the conditions of Sc concentration of 20%and a wavelength of 1200 nm.The effects of Sc concentration and the wavelength on SAW resonators were thoroughly studied.The introduction of Sc softens the AlN structure,enhances the piezoelectric properties of the material,improves the electromechanical coupling coefficient,and leads to a decreased material phase velocity,resulting in a lower phase velocity of the device.In addition,the phase velocity of the device is inversely proportional to the period,therefore the longer the period of the interdigital transducer,the lower the device’s phase velocity.The impact of film-thickness-to-wavelength ratio on the phase velocity was studied by analyzing the phase velocity dispersion curves for SAW propagation,and the cause of low phase velocity was identified.The performance of Al0.8Sc0.2N/Diamond and Al0.8Sc0.2N/Si SAW resonators was contrasted based on the same device structure parameters.Al0.8Sc0.2N/diamond SAW devices exhibited a reduction in insertion loss of roughly 1～5 dB and an increase in the effective electromechanical coupling coefficients of about 1.8%in comparison to Al0.8Sc0.2N/Si devices.4.Study on AlN cavity structure film bulk acoustic resonatorThe influence of electrode material,electrode thickness and electrode shape on the resonance performance of FBAR was studied by finite element model(FEM).Based on the simulation results,an irregular pentagonal Mo electrode with a thickness of 200 nm was selected as the upper electrode.Subsequently,the fabrication process of FBAR was explored and optimized through a series of experiments.The influence of chlorinebased gases on the etching rate of AlN and the selectivity was studied,and two etching rate parameters were used to achieve precise etching of AlN.Additionally,the effect of HF gas on the etching rate of SiO2 was investigated,and the air-gap type FBAR with resonant frequency of 2.35 GHz and insertion loss of about 30 dB was successfully prepared.The resonance curve revealed an apparent parasitic resonance,which was related to the transverse vibration and acoustic transmission loss caused by the warping of the functional layer after the release of the sacrificial layer,incomplete release in the cavity and the roughness of the film.Finally,parasitic resonance suppression and sacrificial-release process improvement were discussed.The frame structure can be employed to suppress the parasitic resonance,and the sacrificial release window position and release time should be arranged reasonably to minimize damage to the device.The dissertation was divided into four parts.The first part was chapter 1,which introduced the current research status of piezoelectric films and AlN thin film,as well as the basic principle and research progress of resonator.The second part was chapter 2,which introduced the preparation and characterization methods of AlN films and the theoretical basis of resonators.The third part,chapters 3 to 5,systematically investigated the stress relaxation of AlN as well as the SAW resonators and FBAR resonators.In chapter 6,a summary of the research content and innovation point was provided,and the prospect of AlScN/Diamond structure SAW resonators with high frequency and high quality factors and high performance air-gap type FBAR resonators were presented. |
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