Si-Hyung Lee

Growth of highly c-axis textured AlN films on Mo electrodes for film bulk acoustic wave resonators

Highly c-axis textured AlN films on Mo could be obtained using the structural modification of Mo thin films by the reactive rf magnetron sputtering at a low temperature. The correlation of structural properties of Mo and the degree of c-axis texturing in AlN films was studied as a function of sputtering pressure during the dc sputter deposition of Mo. The microstructure and residual stress of Mo films were found to be very dependent on the sputtering pressure. As the pressure decreases and the stress changes from tension to compression, the surface morphology and roughness of Mo films became gradually denser and smoother. It is found that the controlled smooth surface of Mo electrodes plays a key role in the growth of highly c-axis textured AlN films deposited onto them. A full width at half maximum of the x-ray rocking curve of the best AlN film on the surface-controlled Mo electrode was 2.30°. Film bulk acoustic wave resonators with an effective coupling coefficient of 5.6% could be achieved using the i...

Influence of electrode configurations on the quality factor and piezoelectric coupling constant of solidly mounted bulk acoustic wave resonators

This article presents an electrode effect on the quality factor and effective coupling coefficient empirically and theoretically. The possible quality factor detractors in film bulk acoustic wave resonator devices are the acoustic wave attenuation in the metal and piezoelectric films, the wave scattering loss due to the surface roughness, and the electrode electrical loss. Due to the small acoustic wave attenuation of the Mo electrode, the composite Mo–AlN–Mo resonator with the thick electrode provides higher Q values than the resonator with the good quality AlN film and the thin Mo electrode. In such Mo electroded resonators, the effective coupling coefficient is kept relatively high. It is found that the Q value of solidly mounted resonators (SMRs) is not determined by only the piezoelectric film quality, but by a weighted average of the electrode and piezoelectric film quality. Of Q value detractors, the scattering loss originating from surface roughness is thought to be very important factor in SMR qu...

(200)-predominant growth of radio-frequency sputtered SrBi2Ta2O9 thin films

SrBi 2 Ta 2 O 9 (SBT) thin films were prepared by the radio-frequency (rf) magnetron sputtering method on Pt/Ti/SiO 2 /Si substrates. The composition and orientation of SBT thin films were changed by the control of sputtering parameters such as pressure and rf power. As the sputtering pressure increased from 2.5 to 300 mtorr, the film was changed from Sr- and Bi-deficient SBT film to stoichiometric film. The SBT thin films with stoichiometric composition showed good electrical properties. As the rf power increased from 25 to 40 W, the Sr content decreased. However, the Bi content was maximized in the power of 30 W, where the (200)-predominant SBT thin films were fabricated. In lower power of 25 W, typical polycrystalline SBT films were obtained. The Sr and Bi contents in both films were not deficient. However, at the higher power of 35 and 40 W, the secondary phase appeared due to the Sr deficiency. The Bi content of (200)-predominant SBT film was higher than that of polycrystalline films. The degree of the (200) orientation depended on the magnitude of excess Bi content. It is also suggested that the (200)-predominant SBT films were formed by the decomposition of SBT phase to the Bi and Sr atoms caused by rf power control and the lower atomic migration energy along the a axis.

Bragg reflector thin film resonator using aluminium nitride deposited by RF sputtering

Bragg reflector thin film resonators (TFRs) using AlN reactively sputtered at room temperature were investigated. The Bragg reflector TFR is composed of a piezoelectric aluminium nitride thin film, electrodes of Al (1500 /spl Aring/) on both sides and reflector layers of an Al-AlN pair. The resonator has a fundamental resonance at 2.81 GHz with a return loss of 3.07 dB.

Ex situ growth of the c-axis preferred oriented SrBi2Ta2O9 thin films on Pt/Ti/SiO2/Si substrates

SrBi2Ta2O9 (SBT) thin films were deposited on (111) oriented Pt bottom electrodes using the rf magnetron sputtering method and then postannealed at 650–800 °C for 30 min in different oxygen atmospheres. The effect of Bi content on the c-axis preferred oriented growth was confirmed by the control of the Bi2O3 loss during the postannealing. With increasing Bi content in SBT thin films, the degree of the c-axis preferred orientation was enhanced. In addition, a bimodal grain size distribution due to the Sr deficiency in the SBT film was observed. It is suggested that the c-axis preferred oriented growth on Pt(111) bottom electrodes can be attributed to growth controlled by surface energy minimization.

Influence of electrodes and Bragg reflector on the quality of thin film bulk acoustic wave resonators

It is considered that the acoustic wave attenuation in the metal and piezoelectric films, the wave scattering loss, and the electrode electrical loss are the possible quality factor detractors in FBAR devices. Due to the small acoustic wave attenuation of the Mo electrode, the composite Mo-AlN-Mo resonator with thick electrodes provides higher Q than that with thin Mo electrodes. In such Mo electroded resonators, a relatively high effective coupling coefficient is retained. In Q value detractors, the acoustic wave scattering loss originating from surface roughness is thought to be fatal in terms of FBAR quality. The thickness of the low impedance SiO/sub 2/ layer of the Bragg reflector also affects the Q value and effective coupling constant due to the low material Q of the SiO/sub 2/ film.

Role of abnormal grain growth on the ferroelectric properties of SrBi2Ta2O9 thin films fabricated by R.F. Magnetron sputtering

Abstract With increasing sputtering pressure and post-annealing temperature, uniform rodlike grains were obtained through bimodal grain size distribution. But, (00l) peaks of SrBi2Ta2O9 thin films were increased. Improvement of electrical properties is expected by removing the abnormal grain growth and c-axis oriented SrBi2Ta2O9.

Role of AlN Piezoelectric Crystal Orientation in Solidly Mounted Film Bulk Acoustic Wave Resonators

To investigate the effect of AlN c-axis orientation on the resonance performance of film bulk acoustic wave resonators, solidly mounted resonators with crystallographically different AlN piezoelectric films were prepared by changing only the bottom electrode surface conditions. As increasing the degree of c-axis texturing, the effective electromechanical coupling coefficient (k eff )² in resonators increased gradually. The least 4 degree of full width at half maximum in an AlN (002) rocking curve, which corresponds to k² eff of above 5%, was measured to be necessary for band pass filter applications in wireless communication system. The longitudinal acoustic wave velocity of AㅣN films varied with the degree of c-axis texturing. The velocity of highly c-axis textured AㅣN film was extracted to be about 10200 m/s by mathematical analysis using Matlab.

High frequency response of Bragg reflector type film bulk acoustic wave resonator

Abstract We fabricated a Bragg reflector type FBAR using AlN piezoelectric with quarter wavelength thickness, where the Bragg reflector was composed of W-SiO2 pairs. By numerical simulation considering actual acoustic losses of each layer, we analyzed the frequency response of the resonator and could explain it using an equivalent circuit with parasitic elements. The Effective electromechanical coupling constant (K 2 eff ) and the Quality factor (Qs ), figures of merit of the resonator, were about 1.1% and 307, respectively.

Acoustic characterization of AlN films using a High Overtone Bulk acoustic wave resonator

Abstract Using the composite resonator with additional thick AlN film with the FWHM of rocking curve of 2.53° under investigation, the bulk acoustic wave attenuation constant and sound velocity along c-axis were directly measured. The method of High Overtone Bulk acoustic wave Resonator (HBAR) spectroscopy based on frequency, measurements of the positions of the resonator peculiarities of phase and amplitude of electromagnetic wave reflection coefficient from composite resonator structure was applied. The method provides attenuation and velocity measurement in a wide frequency band, using the same sample. For the measurements, standard and modified HBAR spectroscopy based on the analysis of only parallel resonance properties of composite resonator structures were used.