I. INTRODUCTION

The digital dividend band (698-862 MHz), as a frequency range previously used for analog television broadcasting, has recently been in high demand for mobile communications applications mainly because of its excellent in-building penetration and propagation properties that allow for more extensive coverage with fewer base stations as compared to other bands at higher frequencies ^(1,2). Thus, the digital dividend band has emerged as an important potential solution to resolve the increasingly more challenging mobile data issues that have drawn considerable attention from telecommuni-cations regulators who have tried to find an additional spectrum as part of their efforts to deploy new mobile broadband networks, particularly Long Term Evolution (LTE) services. To enable the target mobile data rates for these mobile services, the most critical issue to be considered is widening of the transmission bandwidths and channels ^(3,4). This is why the World Radio Conference of 2007 (WRC-07) agreed to allocate the digital dividend band to LTE mobile services ^(1,5,6). To more efficiently utilize wider bandwidths available for LTE services, the transceiver architectures suitable for wideband communications must be used ⁽⁵⁾. In a single wideband receiver case, as shown in Fig. 1, most of its radio-frequency (RF) components including the band pass filter (BPF) should be able to be operated in the wideband mode because of its intrinsic wideband nature ⁽⁵⁾.

The LTE BPFs for wideband communications below 1 GHz have been proposed by some companies ^(7-9). Reactel, Anatech, and Mini-Circuits have especially proposed the LTE BPFs with bandwidths of 698-806 MHz, 600-800 MHz, and 500-700 MHz, respectively. However, these filters are usually available only in cavity, interdigital or discrete component configurations of off-chip type with relatively large sizes of ~ cm³ unit. In the

Fig. 1. Schematic block diagram of single wideband receiver composed of RF bandpass filter, RF front-end, analog-to-digital converter, and digital signal processing blocks.

past, many studies on the small-sized filters using acoustic wave devices, $\textit{i.e.}$, surface acoustic wave (SAW) and film bulk acoustic wave resonators (FBARs), have been implemented ^(10-12).

Moreover, FBAR devices have attracted much attention as a promising technology due to their high device performance and strong potential for realization of the microwave monolithic integrated circuits (MMICs) ⁽¹⁰⁾. Table 1 shows the characteristics of the FBAR filters presented by several research groups for the frequency range above 1 GHz ^(13-19). These filters have an appropriate center frequency and bandwidth in accordance with target applications, such as wireless local area network (WLAN), satellite, multimedia, mobile communications, and so on.

In addition, for the 700 MHz band below 1 GHz, FBAR filters with bandwidth of 30-40 MHz have been reported ⁽²⁰⁾. However, to our knowledge, few studies have been reported on the FBAR-based LTE BPFs for wideband communications that could cover the entire digital dividend band (698-862 MHz).

In this work, we present a feasibility study on the small-sized FBAR-based BPF device with the multilayer Bragg reflector obtained at optimum deposition conditions. The fabricated BPF filter could cover the digital dividend LTE band (698-862 MHz) for wideband communications.

II. EXPERIMENTS

The stacked crystal filter (SCF)-type bandpass filter (BPF), as one of FBAR filter structures, has been fabricated by combining several unit-FBAR devices ⁽¹¹⁾. This SCF-type BPF consists largely of two SCF sections that are connected electrically by bottom electrodes, as shown in Fig. 2(a) ⁽¹¹⁾. The two unit-FBAR devices

Fig. 2. (a) Equivalent circuit diagram of SCF-type BPF considered in this work and the schematics of its fabrication process, including the formation of (b) seven-layer Bragg reflector on 4-inch Si wafer, (c) bottom Al electrode and lower ZnO piezoelectric layer, (d) middle Al electrode, upper ZnO layer, and top Al electrode.

acoustically coupled in one SCF section are in close contact with each other and also share a common electrode that is usually grounded in the circuit (Fig. 2(a)). And the entire device configuration is shown in Fig. 2(d).

The SCF-type BPF has been fabricated by the deposition of thin films using an RF/DC magnetron sputtering technique. The SCF-type BPF fabrication process can be divided largely into three stages as follows.

The first stage (Fig. 2(b)) involves the deposition and thermal annealing of the multilayer Bragg reflector (BR) formed on silicon (Si) substrates. The BR is used to acoustically isolate the resonating piezoelectric region from the substrate. We have used two well-known materials, tungsten (W) as a high acoustic impedance material and silicon dioxide (SiO$_{2}$) as a low acoustic impedance material, because their combination enables a relatively large acoustic impedance ratio (approximately 8:1).

In this work, a seven-layer BR block was formed on a 4-inch Si wafer by alternately depositing W (0.6 µm) and SiO$_{2}$ (0.6 µm) thin films, both of which correspond to one-quarter wavelength thicknesses at the resonance frequency. Immediately after the BR block was formed, the wafers with the BR block were annealed at 400°C for 30 minutes in a thermal annealing furnace ⁽²¹⁾.

The second stage (Fig. 2(c)) includes the fabrication of the lower ZnO components. The 0.3-µm-thick bottom aluminium (Al) electrodes were first deposited and then patterned using a lift-off method to connect each of the SCF sections, followed by the deposition of the 1.2-µm-thick lower ZnO films.

In the final stage (Fig. 2(d)) of the upper ZnO component fabrication process, the 0.3-µm-thick middle Al electrodes, which act as the common ground electrodes for each SCF section, were deposited on the lower ZnO parts and patterned using a lift-off technique. Then, the 1.2-µm-thick upper ZnO films were deposited and also patterned by wet etching to allow access to the common ground electrodes. The upper ZnO films were etched in a solution of CH$_{3}$COOH:H$_{3}$PO$_{4}$:H$_{2}$O (1:1:50) without having any undesirable etching impact on the middle Al electrodes. Then, the deposition and patterning of the 0.3-µm-thick top Al electrodes on the upper ZnO films completed the fabrication of the SCF-type BPFs. Fig. 2(d) shows the top and cross-sectional schematic structures of SCF-type BPFs designed in this work. The return loss (S$_{11}$) and insertion loss (S$_{21}$) characteristics of these devices were then extracted using a measurement system combined with a probe station and a network analyzer.

III. RESULTS AND DISCUSSION

1. Deposition Condition for Multilayer Bragg Reflector

The multilayer Bragg reflector (BR) physically supports the resonating piezoelectric region, and also it acts as a mirror to prevent possible energy loss into the substrate from the resonating piezoelectric region. Therefore, the high-quality BR fabrication is critical to improve the resonance characteristics of the FBAR device.

Considering the intrinsic amorphous property of the SiO$_{2}$ film, it is necessary to deposit the high-quality W films using optimized deposition rates. When the W films were deposited for 40 min with the DC power of 125 Watt on the SiO$_{2}$/Si substrates at chamber pressures

Fig. 3. Cross-sectional SEM images ((a)-(c)), AFM images ((d)-(f)), surface SEM images ((g)-(h)) of W films deposited on (SiO$_{2}$)/p-Si substrate at various chamber pressure: 5 mTorr ((a), (d), (g)), 15 mTorr ((b), (e), (h)), and 25 mTorr ((c) and (f)).

of 5, 15, and 25 mTorr, their deposition rates were measured in 116, 175, and 163 Å/min from SEM images (Fig. 3(a)-(c)), respectively, and also their surface roughness values were measured to be 53.7, 24.3, and 70.9 Å from AFM images (Fig. 3(d)-(f)), respectively, showing the fastest W deposition rate and its excellent morphology at 15 mTorr. Moreover, it seems that the W films with fine and excellent crystalline morphologies are deposited at 15 mTorr, as shown in the surface SEM images (Fig. 3(g)-(h)).

Fig. 4. SCF-type bandpass filter fabricated using FBAR devices (a) Schematic diagram and cross-sectional SEM images, (b) top-view SEM image.

2. FBAR-based Bandpass Filter for LTE Services

Fig. 4 shows the cross-sectional and top-view SEM images of the SCF-type bandpass filter (BPF) fabricated employing the multilayer Bragg reflector. The SEM images show that the SCF-type BPF device was well fabricated as designed and the device size is 590×340 µm². As shown in Fig. 5 and Table 2, the fabricated SCF-type BPF device was measured to have a wide bandwidth of 227 MHz (680-907 MHz), which includes the digital dividend band (698-862 MHz), and stop-band attenuation (rejection) of >32 dB from 1.2 up to beyond 8 GHz. This filter shows a center frequency ($\textit{f}_{c}$) of 785 MHz, insertion

Fig. 5. Return loss and insertion loss characteristics of the fabricated SCF-type BPF.

Table 2. Characteristics of the fabricated SCF-type BPF

Parameters	Values
Center frequency (MHz)	785
Bandwidth (MHz)	227
Insertion loss (dB)	11
Return loss (dB)	33
Selectivity	3.5
Attenuation rate (dB/GHz)	34

loss (S$_{21}$) of 11 dB, return loss (S$_{11}$) of 33 dB, a selectivity of 3.5, and a skirt attenuation rate of 34 dB/GHz on its lower and upper sides

In spite of this effort, to more effectively select the desired ranges of the digital dividend band frequencies as well as the attenuation of undesirable pass bands, the SCF-type BPF device presented in this work may have to show even the higher selectivity and skirt attenuation rates, in addition to the more improved insertion loss characteristics.

While the design and processes used to fabricate the filters must be refined further to produce better filter characteristics, the fabricated SCF-type BPFs and their characteristics appear to be very significant from the perspective of small-sized FBAR-based BPF applications in single wideband transceivers, mainly because their wideband characteristics enable filtering of the digital dividend LTE band (698-862 MHz).

IV. CONCLUSIONS

We have presented a feasibility study on the FBAR-based BPF with a small device size of 590×340 µm² and a wide operating bandwidth of 227 MHz (680-907 MHz).

This FBAR-based BPF can cover the entire digital dividend LTE full band (698-862 MHz) and will foster the further improvement of LTE mobile services. Above all, to fabricate the FBAR-based BPF, we have achieved the high-quality W/SiO$_{2}$ multilayer Bragg reflector for FBAR device applications through the optimization of the W film deposition process to attain its excellent crystalline characteristics. The fabricated SCF-type BPF device has been demonstrated to have a center frequency ($\textit{f}_{c}$) of 785 MHz, insertion loss (S$_{21}$) of 11 dB, return loss (S$_{11}$) of 33 dB, a selectivity of 3.5, and a skirt attenuation rate of 34 dB/GHz. Although our experimental results for FBAR-based LTE full-band BPFs seem to be far from perfect in many respects and thus a further study needs to be done in the future, we believe that the device fabrication technology presented in this work is useful particularly for wideband communications applications below 1 GHz.