Crystalline characteristics of annealed AlN films by pulsed laser treatment for solidly mounted resonator applications
- 147 Downloads
AlN films were deposited on Si substrates using a reactive RF magnetron sputtering process and then the films were annealed by using different laser powers and wavelengths (355 nm, 532 nm and 1064 nm). For all three laser systems, the (002) peak intensity was obviously improved following laser irradiation. The improvement in the crystalline property was particularly obtained in the AlN film processed at 355 nm. In particular, given the use of the optimal laser power (0.025 W), the (002) peak intensity was 58.7% higher than that of the as-deposited film. The resonant frequency and 3 dB bandwidth of a SMR filter with an unprocessed AlN film were found to be 2850 MHz and 227.81 MHz, respectively. Following laser treatment with a wavelength of 1064 nm and a power of 0.25 W, the resonant frequency changed from 2850 to 2858 MHz. Moreover, 3 dB bandwidth changed from 227.81 to 202.49 MHz and the return loss of the filter reduced from 17.28 to 16.48 dB. Overall, the results thus show that the frequency response of the SMR filter can be adjusted and the return loss reduced by means of laser treatment with an appropriate wavelength.
KeywordsLaser Acoustic wave device AlN Annealing
Acoustic wave devices are widely used in the wireless communication field. Surface acoustic wave filters (SAWFs) [1, 2, 3] and film bulk acoustic resonators (FBARs) [4, 5, 6] have attracted particular attention as a means of achieving high operating frequencies (exceeding gigahertz) in radio frequency (RF) communications. In realizing such devices, the acoustic wave is confined to resonate as a standing wave using either air gap isolated resonators or solidly mounted resonators (SMRs) [7, 8, 9]. SMRs typically consist of a Bragg reflector and a piezoelectric film sandwiched between two electrodes.
The performance of acoustic wave filters is highly dependent on the crystalline quality of the piezoelectric layer. Of the various piezoelectric materials available, single crystal aluminum nitride (AlN) is one of the most commonly used in the optoelectronic, sensor and wireless communication fields due to its wide bandgap (6.2 eV), favorable thermal conductivity (> 100 W/mk) and high dielectric constant (~ 8.5) [10, 11, 12]. AlN piezoelectric films can be deposited using various methods, including chemical vapor deposition (CVD) , reactive sputtering , molecular beam epitaxy (MBE) , and pulsed laser deposition (PLD) . However, irrespective of the method used, the quality of the AlN film (and hence the device performance) is strongly dependent on the nitrogen concentration and the processing parameters . Various studies have shown that the crystalline structure of AlN films can be improved through post-deposition plasma, laser, or rapid thermal annealing (RTA) treatment [12, 17, 18]. Lee also reported that an excellent return loss of the solidly mounted resonator-type film bulk acoustic wave resonator devices were observed after the post annealing process . High crystallinity AlN films were obtained by modulating the growth temperature and thermal annealing conditions. High thermal annealing temperature and short annealing time further improve the crystallinity and also preserve the smooth surface . For (10-1-3) and (11-22) AlN layers grown on m-plane sapphire, the crystal quality improved with increasing annealing temperature up to 1700 °C because the density of basal plane stacking faults reduced. These results indicate that the thermal annealing technique offers a new way of fabricating highly efficient semipolar UV LEDs on sapphire substrates . Moreover, laser annealing has been applied in various fields, including active-matrix organic light-emitting diode (AMOLED), metallic glass thin films, complementary metal oxide semiconductor (CMOS), and thin film bulk acoustic wave (TFBAR) [22, 23]. Compared to these traditional annealing techniques, laser annealing has advantages including faster actuation response, rapidly cooling rate and controllable penetration depth into the substrate. It is beneficial for a uniform concentration, lower defect density and local heating area . Cheng  reported that laser treatment results in an effective improvement in the c-axis preferred orientation of ZnO films and is thus beneficial in reducing the return loss of ZnO-based longitudinal mode FBARs.
In the present study, the SMR device is consisted by Bragg reflector, AlN thin films and electrode. Firstly, the Bragg reflector is fabricated on Si substrate, and then the bottom electrode is fabricated on Bragg reflector through DC sputter combined with lithography process technology. Secondly, the AlN thin films are deposited on the bottom electrode using reactive RF magnetron sputter. Finally, top electrode is constructed on the AlN thin films by lithography process technology; the SMR device is then completed. The AlN films are then treated by laser irradiation using various laser powers and wavelengths. The effects of the laser processing parameters on the crystalline quality, optical transmittance, resonant frequency and return loss properties of the AlN films are then examined and compared.
The surface morphologies of the various samples were observed using a field emission scanning electron microscope (FESEM, JSM-7600F). Moreover, the microstructures of the samples were examined using an X-ray Diffractometer (Bruker D8) with Cu-Ka radiation. The optical transmittance was measured using a UV–VIS-IR spectrophotometer (Lambda 35). Finally, the frequency response of the SMR filters was measured using a network analyzer (E5071C) and a CASCADE high-frequency probe (RHM-06/V + GSG 150).
Results and discussions
AlN films have been deposited on Si substrates using a reactive sputtering process. The AlN films have been laser annealed with various laser powers and wavelengths. The XRD results have shown that the laser annealing process yields an effective improvement in the (002) peak intensity of the AlN film. For irradiation wavelengths of 355, 532 and 1064 nm, the optimal laser power has been found to be 0.025, 0.15 and 0.25 W, respectively. The corresponding enhancement in the (002) peak intensity is equal to 58.7%, 16.8% and 36.8%, respectively. For a SMR filter with an as-deposited AlN film, the resonant frequency is equal to 2850 MHz and the 3 dB bandwidth is 227.81. Following laser treatment with a wavelength of 1064 nm and a power of 0.25 W, the resonant frequency increases to 2858 MHz. In addition, the 3 dB bandwidth reduces to 202.49 MHz and the return loss reduces from 17.28 to 16.48 dB. In other words, the resonant frequency of the SMR filter can be tuned and the return loss reduced through the use of a laser irradiation process with appropriate wavelength and power settings.
HKL conceived of the study, and participated in its design and coordination and drafted the manuscript. YJH carried out the laser and film studies. WCS prepared the AlN film and solidly mounted resonators. YCC participated in the design of the resonator and helped to draft the manuscript. WTC did some microstructure experiments. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan under Grant Nos. MOST 105-2221-E-020-007 and 104-2221-E-020-007.
The authors declare that they have no competing interests.
Availability of data and materials
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
- 13.Claudel A, Blanquet E, Chaussende D, Boichot R, Doisneau B, Berthomé G, Crisci A, Mank H, Moisson C, Pique D, Pons M (2011) Investigation on AlN epitaxial growth and related etching phenomenon at high temperature using high temperature chemical vapor deposition process. J Cryst Growth 335(1):17–24CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.