The rapid expansion of the wireless market has led to a huge growth of more advanced mobile communication systems. Especially, the miniaturized mobile phones have been developed that have multi-functions with higher operating frequencies. Complying with the recent trends, there has been a great demand particularly for ultra-miniaturization and monolithic integration of RF filters as one of core components in mobile communication systems. Typical filters used in RF front-end for commercial wireless handsets are ceramic or surface acoustic wave (SAW) resonators. However, neither of them is compatible fully with the standard IC-technology.
Film bulk acoustic wave resonator (FBAR) devices and their related fundamentals can play an important role for the fabrication of the next generation radio-frequency (RF) filters. The FBAR devices basically utilize the acoustic resonant characteristics of piezoelectric materials such as AlN or ZnO. Compared with the so-called Surface Acoustic Wave (SAW) filters, FBAR device filters can also be realized to have smaller size and higher performance especially in power handling capability.
The typical FBAR device is composed of a thin piezoelectric film sandwiched between top and bottom conductor plates (electrodes). The devices must have two acoustically reflecting surfaces in order to trap energy and produce a resonating characteristic. As the reflecting surfaces for FBAR devices, the solidly mounted-type has a Bragg reflector part which is made up of alternating thin-film layers of both low and high acoustic impedance materials.
The ZnO-based FBAR devices are made up of a piezoelectric ZnO film sandwiched between top and bottom electrodes (e.g., aluminum) deposited on 5-layer W/$SiO_2$ Bragg reflectors. The 5-layer W/$SiO_2$ Bragg reflectors were fabricated by alternately depositing the tungsten (0.57 $\mum$-thick) of high acoustic impedance material and $SiO_2$ films (0.6 $\mum$-thick) of low acoustic impedance material on a 4-inch Si wafer. After depositing five layers ($SiO_2$/W/$SiO_2$/W/$SiO_2$) of Bragg reflectors, the Al bottom electrodes ($1.2\microm$-thick) were deposited on the 5-layer Bragg reflectors. Then, 1.2 \mum$-thick ZnO piezoelectric films were deposited on the bottom electrodes. Next, the top electrodes were patterned on the piezoelectric film using a conventional photolithography technique and then, aluminum top electrodes (0.2 $\mum$-thick) were deposited. The three different top electrode patterns were completed by the lift-off processing to strip off the remaining PR layers. The return losses ($S_11$) of three resonators were measured by using the Network Analyzer-System Agilent HP 8510C and a probe station.
In this work, very effective methods to improve the resonance characteristics of FBAR devices as well as an approach for inductor fabrication based on Bragg reflectors were proposed.
First, Cr films ($300\Aring$ - thick) between $SiO_2$ film and W film were formed by deposition in a metal sputter in order to enhance the adherence at their interfaces. As a result, the addition of Cr adhesion layers seems to enhance the adhesion quality between $SiO_2$ and W layers in the Bragg reflectors, eventually leading to improvements of resonance characteristics.
Second, the use of the thick bottom electrodes (1.2 $\mum$) in FBAR devices appears to further improve the resonance characteristic ($S_{11}$) and increase the resonance frequency. The measured S-parameters indicate that the FBARs can be used for the application of 2.7~3 GHz broadband WiMAX.
Third, to investigate the annealing effects on resonance characteristics of FBAR devices, four different thermal annealing samples were performed. In sample A, no thermal annealing treatment was done in the whole steps. The first thermal annealing process, called inter-fab annealing as in sample B, is used to anneal the sample in an electronic dehydrate furnace at 200 $\degC$ for one hour before the deposition of top electrodes. After the top electrodes deposition, the sample C is annealed at 200 $\degC$ for 2 hours, called post-annealing process. The last process, the combination of inter-fab annealing (200 $\degC$/1 hour) $\amp$ postannealing(200 $\degC$/2 hours) was performed on the sample D. According to the measurement results, the addition of the post-annealing for the sample D, already treated by inter-fab annealing (200 $\degC$/1 hour), is shown to further improve return loss ($S_11$). Regardless of the annealing processes, the resonance frequency of three different FBAR devices was ~ 1.71 GHz. As a result, the resonance characteristics ($S_11$) are observed to be improved by the thermal annealing in the ZnO-based FBAR devices with 5-layered Bragg reflectors. Also, this approach will be very promising for the future FBAR device applications.
On the other hands, in this work, a novel approach to realize inductor based on Bragg reflectors is proposed. By using Bragg reflectors, the parasitic effects of the inductor can be reduced in terms of substrate losses because the multilayer Bragg reflectors of FBAR have the special characteristics by acting as a mirror to prevent the losses into the substrate. The effects of the multi-layer Bragg reflectors and inductor patterns on the characteristics of inductors were investigated. The measurement results showed that the inductors fabricated on the Bragg reflector result in a significant improvement in terms of the $S_11$ parameter. This approach seems highly feasible and promising for future Sibased RF IC applications.
좀 더 발전된 무선 통신 시스템의 거대한 성장으로 무선 시장에 빠른 팽창과 더불어, 높은 동작 주파수에서 다양한 기능을 가지는 소형화된 이동전화가 발전하고 있다. 따라서 모바일 통신 시스템에서 초소형 크기와 집적화 될 수 있는 RF 필터에 대한 관심이 집중되고 있다. 지금까지 주로 사용된 세라믹 필터와 SAW 필터는 기존의 IC 기술과 호환이 될 수 없기 때문에, FBAR 필터가 주목을 받기 시작했다. FBAR 소자는 AlN나 ZnO의 압전체 물질을 사용해서 공진 특성을 가지며, SAW 필터와 비교해 봤을 때 훨씬 작은 크기와 높은 성능을 가진다. 또한, FBAR 소자를 구현할 때 반사 표면에 따라 세 가지로 나눌 수 있는데, SMR type의 Bragg reflector를 기반으로 FBAR 소자를 제작하였다.
본 논문에서는 Bragg reflector를 기반으로 한 FBAR 소자에 대한 향상된 공진 특성을 얻고, 이에 대한 응용으로 RF Inductor를 구현 및 공정을 수행했다. FBAR 소자의 공진 특성을 향상시키기 위해서 Bragg reflector의 다중 층 사이에 Cr adhesion 층을 삽입했을 때의 공진 특성, 두꺼운 하부 전극을 사용했을 때의 공진 특성, thermal annealing을 가했을 때의 공진 특성에 대해 분석하였다.
첫째, Bragg reflector는 $SiO_2$막과 텅스텐막이 교대로 다중 층으로 증착한다. 이때 층 사이에 Cr adhesion 막을 삽입하면, $SiO_2$ 와 텅스텐 사이의 접착성이 향상되어서 공진 특성이 향상되는 것을 알 수 있다.
둘째, 기존의 얇은 하부 전극(bottom electrodes)이 아니라 두꺼운 하부 전극을 증착 했을 때, 공진 특성이 훨씬 향상되는 것을 알 수 있었다. 또한 2.7 GHz 에서 공진 특성이 -25 dB이므로 WiMAX용으로 사용할 수 있다.
셋째, FBAR 소자에 적절한 thermal annealing을 가했다. 본 논문에서는 열처리를 하지 않은 경우, 상부 전극을 증착하기 전에 열처리를 한 경우, 상부 전극을 증착한 후 열처리를 한 경우, 상부 전극을 증착하기 전과 후에 열처리를 한 경우의 4가지의 샘플을 통해 상부 전극을 증착하기 전과 후에 열처리를 했을 때, 가장 공진 특성이 향상되는 것을 확인할 수 있다.
또한 기존의 RF Inductor를 구현할 때, Si 기판위에 Oxide 층을 두껍게 증착한 후 Inductor를 구현한다. 이 때 RF Inductor 소자를 반도체 기판 위에 바로 형성한 경우 신호가 기판으로 흘러나가 소자의 성능이 떨어지는 문제점을 해결하기 위해, 본 논문에서는 Bragg reflector를 이용한 RF용 소자를 구현한 것으로, Bragg reflector를 이용하여 반도체 기판으로부터 acoustic 분리시킴으로서, 기판의 존재로 인하여 발생하는 여러 가지 손실, 노이즈, 커플링 등의 효과를 최소화할 수 있다.
결론적으로 SMR type의 Bragg reflector을 기반으로 RF Inductor를 구현을 구현하고, FBAR 소자의 성능을 향상시키는 방법을 제안하였다. 이를 통해 매우 우수한 공진특성을 얻을 수 있고, 이러한 효율적 방법은 차세대 FBAR 필터 개발에 충분히 사용 가능하다. 또한 RF Inductor 뿐만 아니라 다른 RF용 소자에도 적용시켜서 Bragg reflector를 기반으로 향상된 특성을 가지는 소자를 구현할 수 있다고 사료된다.