The film bulk acoustic resonator (FBAR) has attracted a great attention as a promising technology to fabricate the next-Generation radio frequency (RF) filters. This is mainly because the FBAR technology can be integrated fully with the current silicon process technology, eventually realizing the microwave monolithic integrated circuits. FBAR filter can be designed by connecting FBARs in series and parallel. To realize an FBAR filter of high performance, the resonance characteristic of each FBAR needs to be improved. Consequently, in this thesis, we present three new approaches to more effectively improve the resonance characteristic of the solidly mounted resonator (SMR)-type FBAR for RF FBAR filter application.
The SMR, one of the FBAR structures, has a Bragg reflector, consisting of alternately deposited multi-layer films. Three critical factors that determine the resonance characteristics of the SMR-type FBAR are the performance of the piezoelectric property of piezoelectric material (ZnO) and Bragg reflector layer, and the selection of bottom electrode matched with ZnO film because the quality of the ZnO film depends highly on the bottom electrode film quality.
Firstly, to improve the performance of the piezoelectric property of ZnO film, we suggest the new approach, which is about the effects of the deposition temperature on the preferred orientation or grain growth behaviors of the ZnO deposited on an Al bottom electrode The ZnO films were deposited using an RF sputtering method by varying the deposition temperature from room temperature to 350$\deg$C. The growth characteristics of the deposited ZnO films are shown to have a strong dependence on the deposition temperature. Overall, the ZnO films deposited at/below 200$\deg$C exhibited reasonably good columnar grain structures with a highly preferred c-axis orientation while those above 200$\deg$C showed very poor columnar grain structures with a mixed-axis orientation. This temperature dependence study will be very useful for improving the performance of the piezoelectric property of ZnO film, resulting in the resonance characteristic improvement of FBAR devices.
Secondly, to improve the performance of Bragg reflector layer, we suggest the new approach, which is about the thermal annealing of Bragg reflector for improving the resonance characteristics of SMR-type FBAR devices. The SMR has a Bragg reflector acting as a mirror to prevent possible energy loss into the substrate from the resonating piezoelectric region. Therefore, in addition to the improvement of the piezoelectric film itself, a high-quality Bragg reflector fabrication also seems to be important to further improve the resonance characteristics in the SMR-type FBAR devices. Usually, the Bragg reflectors for the SMR-type FBAR devices have been fabricated by alternately depositing two different materials with high and low acoustic impedances, respectively. In order to further improve the resonance characteristics of the FBAR devices, a thermal annealing process was additionally employed for the W/$SiO_2$ multi-layered Bragg reflectors immediately after they were deposited on a silicon substrate using an RF sputtering technique. As a result, the resonance characteristics of the FBAR devices were observed to strongly depend on the annealing conditions applied to the Brags reflectors. In addition, the FBAR devices with the Bragg reflectors annealed at 400$\deg$C/30 min have shown excellent resonance characteristics In terms of return loss and Q-factor. Based on these findings, the optimum thermal annealing condition seems to be around 400$\degC$/30 min and this approach seems very useful for improving the resonance characteristics of SMR-type FBAR devices with the multi-layer Bragg reflectors.
Finally, the effects of the thermal annealing of the Bragg reflectors on the resonance characteristics of the FBAR devices particularly with Co electrodes were investigated and compared with those with Al electrodes. We fabricate the two devices, which are Al-FBARs (FBAR devices with Al electrode) and Co-FBARs (FBAR devices with Co electrode) for comparing the resonance characteristics. Compared with Al-FBARs, the resonance characteristics are more improved in CO-FBARs, indicating that the ZnO film deposited on Co electrode has more highly preferred orientation towards c-axis than that deposited on Al electrode. Consequently, the resonance characteristics could be further improved by using Co electrodes, instead of using Al electrodes far FBAR devices. The combined use of both the thermal annealing and Co electrodes seems very useful to more effectively improve the resonance characteristics of the SMR-type FBAR devices with the W/ $SiO_2$ multi-layer reflectors.
The study for the high-quality film growth and the improvement of FBAR resonance characteristics is very important because the filter is composed of FBARs and its performance depends highly on each FBAR device characteristic. The FBAR filter considering the optimal ZnO film deposition temperature condition (200$\deg$C) and the thermal annealing at 400$\deg$C/30min was fabricated by combining several FBARs.
차세대 RF 무선이동통신 필터를 개발하기 위한 가장 근접된 필터 기술로 인정받고 있는 것이 FBAR 기술이다. FBAR 필터는 직렬과 병렬로 여러개의 FBAR 공진기들을 연결하여 구현된다. 따라서, 높은 성능을 갖는 필터를 위해서는 FBAR 공진기의 공진특성을 향상시킬 필요가 있다. 따라서, 본 논문에서는 RF 필터 응용을 위한 SMR 구조를 가지는 FBAR 의 공진특성을 향상시키는 세가지의 새로운 방법들을 제안한다.
첫번째로, ZnO 박막의 압전특성을 향상시키기 위해, Al 하부전극 상에서 ZnO 박막 증착 및 온도가 ZnO 결정의 C 축 성장에 미치는 영향에 관한 새로운 접근방법을 제안한다. ZnO 박막의 성장특성은 상온에서부터 350$\degC$의 온도조건에서 C 축 우선배향성의 정도에 따라 3 개의 온도구간 (temperature region)으로 구분할 수 있었다. 결과적으로 200$\deg$C 이하의 공정온도에서 주상형 결정립 (columnar grain)을 가지고 C축 우선배향된 양질의 ZnO 박막을 얻을 수 있었다. 이러한 온도의존성에 관한 연구는 ZnO 박막의 압전특성을 향상시키며, 결국 FBAR 의 공진특성을 향상시키는데 기여한다.
두번째로, Bragg reflector 의 성능을 향상시킴으로써 SMR 구조를 가지는 FBAR 의 공진특성을 향상시키는 새로운 접근을 하였다. 같은 증착조건으로 형성된 Bragg reflector 를 갖는 FBAR 소자를 서로 다른 온도에서 어닐링하여 S-parameter 를 추출하고, 어닐링하지 않은 소자와의 비교를 통해 공진특성의 변화를 고찰한다. 400$\deg$C/30min 의 조건에서 어닐링된 SMR-type FBAR 소자가 가장 좋은 공진특성을 보였다. 새롭게 제안된 어닐링공정은 W/ $SiO_2$ 다층 박막 Bragg reflector 를 갖는 SMR-type FBAR 소자의 공진특성을 효과적으로 개선시키는데 매우 유용할 것으로 보인다.
FBAR 의 공진특성을 향상시키기 위해 세번째로 제안한 것은 ZnO 박막의 C 축성장을 보다 더 향상시킬 수 하부전극의 올바른 선택이다. 기존에 가장 일반적으로 사용되어온 Al전극을 갖는 FBAR(Al-FBAR)와 온도안정성이 뛰어난 Co 전극을 갖는 FBAR (Co-FBAR) 제작하였다. Al-FBAR 에 비해 공진특성은 Co-FBAR 에서 크게 향상되었다. 결과적으로, 200$\deg$C에서 증착된 ZnO, 400$\deg$C/30min 에서 어닐링된 Bragg reflector와 Co 전극의 사용은 SMR-type FBAR의 공진특성을 향상시키는데 크게 기여할 것이다.
