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This paper presents the influence of Bragg reflector sur- face roughness on the resonance characteristics of an SMR. Originally, an AlN/Al multilayer is used as ...
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ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 54, no. 4, april 2007

Influence of Surface Roughness of Bragg Reflectors on Resonance Characteristics of Solidly-Mounted Resonators Chung-Jen Chung, Ying-Chung Chen, Chien-Chuan Cheng, Ching-Liang Wei, and Kuo-Sheng Kao Abstract—The solidly mounted resonator (SMR) is fabricated using planar processes from a piezoelectric layer sandwiched between two electrodes upon Bragg reflectors, which then are attached to a substrate. To transform the effective acoustic impedance of the substrate to a near zero value, the Bragg reflectors are composed of alternating high and low acoustic impedance layers of quarter-wavelength thickness. This paper presents the influence of Bragg reflector surface roughness on the resonance characteristics of an SMR. Originally, an AlN/Al multilayer is used as the Bragg reflector. The poor surface roughness of this Bragg reflector results in a poor SMR frequency response. To improve the surface roughness of Bragg reflectors, a molybdenum (Mo)/titanium (Ti) multilayer with a similar coefficient of thermal expansion is adopted. By controlling deposition parameters, the surface roughness of the Bragg reflector is improved, and better resonance characteristics of SMR are obtained.

I. Introduction ecently, extensive research has been done on thin film bulk acoustic wave (BAW) resonators in the RF microelectronics field. BAW resonators require free or clamped interfaces that confine waves in the piezoelectric layer and resonate as standing waves [1]–[10]. This can be achieved using the microelectromechanical systems (MEMS) technique to etch the substrate or the sacrificial layers to form an air groove, which is known as a thin film bulk acoustic resonator (FBAR). Alternatively, it can be accomplished by stacking quarter wavelength thick thin films, named a solidly mounted resonator (SMR). The concept of the SMR was introduced by Newell [11] in 1965, and further promoted by Lakin in 1995 [12], [13]. Newell and Lakin mention a method that can transform the impedance of the substrate to a near zero value in order to avoid wave energy dissipating into the substrate. In this method, the SMR is fabricated from a piezoelectric layer formed between two electrodes and attached to

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Manuscript received July 3, 2006; accepted November 24, 2006. This study was supported by the National Science Council, Taiwan, R.O.C. under contract nos. NSC 94-2216-E-110-021 and NSC 942216-E-237-001. C.-J. Chung, Y.-C. Chen, and C.-L. Wei are with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. (e-mail: [email protected]). C.-C. Cheng is with the Department of Electronic Engineering, De Lin Institute of Technology, Taipei, Taiwan, R.O.C. K.-S. Kao is with the Department of Computer and Communication, SHU-TE University, Kaohsiung, Taiwan, R.O.C. Digital Object Identifier 10.1109/TUFFC.2007.313

stacked thin films. These thin films, with different acoustic impedance, are stacked onto the substrate in sequence. The formation of multilayer thin films is similar to the Bragg reflector used in the semiconductor laser devices [14]–[16]. Therefore, it also was named as a Bragg reflector [17]. Moreover, two critical factors for the Bragg reflector were proposed in Newell and Lakin’s methods, which were the specific thickness of each thin film and the necessity for a large impedance ratio [8], [9], [12]. The specific thickness is designed to be one quarter wavelength at fundamental resonant frequency. The large impedance ratio can result in good acoustic reflection. Another important factor in the Bragg reflector is the roughness of the thin films, which contributes to the scattering of acoustic waves in the SMR [18]. This paper examines the influences of surface roughness of Bragg reflectors on the resonance characteristics of the SMR. The multilayer Bragg reflector is mainly deposited by RF/DC magnetron sputtering, chemical vapor deposition (CVD), or electron cyclotron resonance (ECR) electronbeam evaporation, the thicknesses of thin films being controlled by various kinds of deposition monitors [19]–[21]. In this study, the Bragg reflector is fabricated by magnetron sputtering. In brief, if the fabrication factors for Bragg reflectors can be controlled well in order to maximize performance, it will improve the SMR’s potential as a technique for system on chip (SOC) fabrication [22].

II. Experimental Procedure The SMR device is composed of an AlN piezoelectric layer sandwiched between two electrodes on alternating high/low acoustic impedance layers attached to one-side polished p-type (100) Si substrate, as shown in Fig. 1. RF/DC magnetron sputtering and photolithography were used to construct the SMR devices. The structure starts with the deposition of the multilayer Bragg reflector, performed with two combinations of thin films. One is the combination of AlN/Al thin films deposited by RF magnetron sputtering; another is the combination of Mo/Ti, deposited by DC magnetron sputtering. The RF magnetron sputtering is a single-gun system, and the DC magnetron sputtering is a twin-gun system with a rotational substrate holder. Al and AlN thin films are deposited using 3 in. in diameter Al (99.9995%) target in the atmosphere of pure argon and an argon and

c 2007 IEEE 0885–3010/$25.00 

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TABLE I Sputtering Parameters of All Materials Used in SMR Structure.

Target size parameters

RF magnetron sputtering 3 inches AlN Al

DC magnetron sputtering 2 inches Mo Ti

Base pressure (Torr) Power (W) Deposition pressure (mTorr) Substrate temp. (◦ C) Ar/N2 flow rate (sccm)

< 6 × 10−6 370 60∼300 1∼15 1∼45 400 RT 1.5∼6/1.5∼6 8∼10/0

< 10−5 50∼200 5∼30 RT∼400 10/0

WI), was adopted to identify the crystalline structure of thin films. The roughness of the thin films was analyzed by Digital Instrument NanoMan NS4+D3100 atomic force microscopy (AFM) (Veeco, Woodbury, NY), and the crosssection of the structure was observed using a field emission scanning electron microscope (Philips XL40 FE-SEM, Eindhoven, The Netherlands). Manufacture parameters of thin films in the sputtering system, such as deposition temperature and pressure, were adjusted in accordance with the results of XRD, AFM, and SEM. After the formation of the multilayer Bragg reflector, the Al/AlN/Mo sandwiched structure is patterned by the photolithography process using three masking processes. The bottom electrode is first patterned using the lift-off method. Then the highly c-axis oriented AlN thin film is deposited by RF magnetron sputtering as the piezoelectric layer [23], [24]. The thickness of AlN is designed to be one-half wavelength of the resonance frequency. To expose the bottom electrode for electrical contact, AlN is wet-etched in phosphoric acid (H3 PO4 ) in the second mask process. The top electrodes patterned by the third masking process exhibit G-S-G type in order to adapt the Cascade Microtech’s Air Coplanar Probes (Cascade Microtech Taiwan, Hsin Chu City, Taiwan). The fabricated SMR devices are examined by microscope to confirm the pattern is aligned accurately, as in Fig. 1(c). To investigate the frequency response, devices are measured with the HP 8720ET network analyzer (Agilent, Palo Alto, CA). The electrical admittance Y is calculated from the following relationship: Y = Fig. 1. The schematic structure of an SMR device. (a) Side-view. (b) Top-view. (c) The fabricated SMR device.

nitrogen mixture, respectively. The process of fabricating a Bragg reflector is performed in one run in the sputtering system to prevent oxidation at the interface. The detailed RF/DC sputtering parameters are listed in Table I. For applications at a frequency of 2.4 GHz, the specific thicknesses of AlN, Al, Mo, and Ti were considered individually in the deposition process. The thickness of each Bragg reflector layer is in accordance with onequarter wavelength. X-ray diffraction (XRD), carried out on a Siemens D5000 diffractometer (Siemens, Madison,

1 1 − S11 . Z0 1 + S11

(1)

The quality factor Q was defined from the full width at half maximum (FWHM) of conductance curve G at resonance frequency fr :     fr  . Q =  (2) FWHMG fr

III. Results and Discussion Selection of the materials for multilayer Bragg reflector is an important issue that has been discussed broadly in existing research [20], [25]. The preferred construction

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ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 54, no. 4, april 2007 TABLE II Physical Parameters of All Materials Used in SMR Structure.

Parameters (10−6 /◦ C)

Coefficient of thermal expansion Mass density (kg/m3 ) Acoustic impedance (106 kg/m2 s) Bulk acoustic velocity (m/s) Designed thickness (µm) Measured thickness (µm)

Piezoelectric layer AlN

AlN

4.2 3270 34 10400 2.08 1.82

4.2 3270 34 10400 1.04 0.9 ± 0.1

Fig. 2. Cross-sectional SEM image of an AlN/Al Bragg reflector.

of the Bragg reflector is with component materials having a high acoustic impedance ratio. The major reason is that less reflector layers are needed. The higher acoustic impedance ratio, the less reflector layers needed to obtain efficient acoustic wave reflecting. The benefit of reduction reflector layers is a decrease in surface roughness. Acoustic impedance is determined from the following relationship: Acoustic Impedance = V × ρ,

(3)

where V is the acoustic velocity of bulk material, and ρ is the mass density of the material. The physical parameters of materials used in this paper are listed in Table II. In this paper, two combinations of materials for the multilayer Bragg reflector are studied. A first combination of AlN and Al is chosen to compose the quarter-wave layers. Taking into account the fact that Al is a general material in the IC process, and AlN, its nitrifier, has higher acoustic impedance of 2.5 times than Al, AlN and Al are particularly suitable for combining as the multilayer Bragg reflector. In this combination, only one Al target is used so that the constituent materials and deposition processes are simplified. By controlling the flow of N2 gas in the sputtering system, AlN and Al can be deposited alternately in one chamber. Although various deposition parameters were used to deposit the AlN/Al Bragg reflector, the surface still remains rough, as shown in Fig. 2. The rough surface is due to the divergent coefficient of thermal expansion (CTE) between AlN and Al, as is shown in Table II. The thermal expansion mismatch between AlN and Al films produces a large stress upon heating, and results in the formation of hillocks or whiskers. There are several

Bragg reflector Al Mo 23.1 2695 13.7 5100 0.51 0.5 ± 0.1

4.8 10200 64.2 6290 0.63 0.59 ± 0.06

Ti 8.5 4500 27.3 6071 0.61 0.56 ± 0.04

mechanisms for hillock or whisker growth; their formation can be the result of thermal cycling and CTE mismatch between materials. After thermal cycling in the deposition process, the Al/AlN Bragg reflector displays a significant amount of roughness. If the CTE differs by a factor of more than five, whiskers or hillocks will form [26]. Furthermore, recrystallization of the Al may occur when depositing AlN at 400◦C on top of the Al. As the result of recrystallization, Al thickness is reduced and the surface morphology is rough. Thus, if AlN/Al are used to form the Bragg reflector, the rugged interface between AlN and Al thin film causes serious acoustic scattering at the interface and reduces the acoustic isolation from the substrate. The clamped and free interfaces in the SMR structure cannot be obtained, and the BAW is unable to resonate well in the piezoelectric layer [11], [12]. According to these results, relatively close CTE values is a required criterion for fabricating a quarter wavelength Bragg reflector. In order to improve the acoustic resonance of Bragg reflectors, materials that have similar coefficients of thermal expansion, Mo and Ti, are adopted. The acoustic impedance ratio of Mo/Ti is 2.4, similar to that of AlN/Al. First, Mo and Ti are deposited and deposition parameters are adjusted in accordance with the XRD results. The optimal X-ray diffraction intensity is obtained for (110) Mo under the sputtering pressure of 30 mTorr, at DC power of 150 W and substrate temperature of 300◦ C, and for (002) Ti it is obtained under the sputtering pressure of 5 mTorr, at DC power of 200 W and substrate temperature of 200◦ C. As shown in Fig. 3(a), the cross-sectional scanning electron microscope (SEM) image of a Mo/Ti Bragg reflector with four paired Mo/Ti layers exhibits better interface morphology than the AlN/Al Bragg reflector, thus revealing that materials with a similar coefficient of thermal expansion are indeed more suitable for the multilayer Bragg reflector. However, as observed from the SEM top view and three-dimensional (3-D) AFM image [Figs. 3(b) and (c)] the surface—with a surface roughness of 43.7 nm—is not smooth enough so that the acoustic wave would be scattered and the BAW resonance becomes weakened. Next, the deposition parameters were tuned in order to improve the roughness of Mo and Ti thin films, with the relationship between surface roughness and deposition parameters analyzed by AFM. We were primarily concerned

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(a)

(b) Fig. 4. The relationships between surface roughness and (a) sputtering pressure, and (b) substrate temperature.

Fig. 3. (a) Cross-sectional SEM image. (b) Top view SEM image. (c) The AFM image of four-pair Mo/Ti Bragg reflector based on optimal XRD intensity.

with the effects of changes in sputtering pressure and substrate temperature. As shown in Fig. 4(a), the surface roughness decreases with decreased sputtering pressure. Owing to the lower quantity of gas molecules in the chamber, the average distance between collisions of molecule increases, and the probability of collision between particles decreases—thus, sputtered particles possess enough energy to arrange in order on the substrate. In addition, the surface roughness decreases with an increase in substrate temperature [Fig. 4(b)]. The sputtered particles hold more kinetic energy under proper high temperature, which benefits the nucleation and grain growth. Therefore, the atoms arrange compactly to form a smooth, thin, film surface. With the optimal deposition temperature and pressure of 400◦C and 5 mTorr, the cross-sectional image and surface morphology of four paired Mo/Ti layers is smooth, with the roughness reduced to 13.9 nm, as shown in Fig. 5. The piezoelectric layer plays an important role on SMR devices. In order to obtain a suitable electromechanical

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Fig. 6. Θ-2Θ x-ray scans of AlN on Mo electrode.

Fig. 7. The frequency responses, S11 , of SMR with multilayer Mo/Ti Bragg reflectors, (a) deposited according to optimal XRD intensity, and (b) after surface roughness improvement.

Fig. 5. (a) Cross-sectional SEM image. (b) Top view SEM image. (c) The AFM image of four-pair Mo/Ti Bragg reflector displaying optimal surface roughness.

coupling coefficient, AlN with a crystalline formation oriented highly on the c-axis is deposited on the Mo electrode. The AlN (002) diffraction peak corresponding to the crystal plane, which occurs at 2Θ = 36◦ , was observed by Θ-2Θ x-ray scans, as shown in Fig. 6. The sandwiched structure of an Al/AlN/Mo piezoelectric layer is constructed on a Bragg reflector with four paired Mo/Ti layers, and the frequency responses of the SMR devices are measured by the HP 8720ET network analyzer. The results of frequency responses, S11 , are shown in Fig. 7. The SMR device, whose Bragg reflector is fabricated in accordance with the optimal XRD intensity, does not show particularly good resonant characteristics. But with surface roughness improved, the device shows a distinct resonant phenomenon at 2.31 GHz and excellent noise restraint. The admittance curve and Smith chart are shown in Fig. 8. The calculated quality factor is 140, and the coupling coefficient k 2 is 2.1%

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(a)

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are responsible for roughness, a more optimal combination of Mo/Ti, whose coefficients of thermal expansion are more closely matched, was adopted for study. However, this study found that the devices fabricated using parameters of optimal XRD intensity did not perform as well as expected due to the roughness of the Bragg reflector. As the surface roughness is improved through a series of adjustments in deposition factors, the SMR begins to show the distinct resonant characteristics at 2.31 GHz and an excellent noise restraint. In conclusion, it is not only the impedance ratio but also the roughness of each consisting layer that have great influence on the frequency response of SMR devices. They are critical and essential issues with regard to the selection and fabrication of thin films for the fabrication of a Bragg reflector.

References

(b) Fig. 8. (a) Admittance curve. (b) Smith chart of SMR with smoother surface Mo/Ti Bragg reflectors.

calculated from absolute admittance curve. Owing to the results, the roughness of a Bragg reflector exhibits great influence on the frequency response characteristics of such SMR devices.

IV. Conclusions We experimentally investigated the properties of Bragg reflectors with two combinations of materials. The first combination, AlN/Al, exhibited a rugged interface unsuitable for optimal acoustic resonance as well as for fabrication of the reflector. Due to the fact that the recrystallization and CTE mismatch accompanying thermal cycles

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Chung-Jen Chung was born in Tainan City, Taiwan, R.O.C., on April 24, 1979. He received the B.S. and M.S. degrees in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2001 and 2003, respectively. Currently, he is working toward the Ph.D. degree at National Sun Yat-Sen University. His current research interests are in the areas of thin film technology, surface acoustic wave devices, and bulk acoustic wave devices.

Ying-Chung Chen was born in Tainan, Taiwan, R.O.C., on November 4, 1956. He received the M.S. and Ph.D. degrees in electrical engineering from National Cheng Kung University, Tainan, Taiwan, in 1981 and 1985, respectively. Since 1983, he has been at National Sun Yat-Sen University, Kaohsiung, Taiwan. He is a professor of electrical engineering at National Sun Yat-Sen University. His current research interests are in the areas of electronic devices, surface acoustic wave devices, thin-film technology, and electronic ceramics. He is a member of the Chinese Society for Materials Science and a registered electrical engineer at Taiwan.

Chien-Chuan Cheng was born in Keelung, Taiwan, R.O.C., on March 26, 1964. He received the M.S. and Ph.D. degrees in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 1988 and 1995, respectively. Since 1990, he has been at De Lin Institute of Technology, Taipei, Taiwan. Currently, he is a professor of electronic engineering at De Lin Institute of Technology. His current research interests are in the areas of surface acoustic wave devices, electronic ceramics, and thin film technology.

Ching-Liang Wei was born in Hsinchu City, Taiwan, R.O.C., on March 18, 1980. He received the M.S. degree in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2005. Currently, he is working toward the Ph.D. degree at National Sun Yat-Sen University. His current research interests are in the areas of thin film technology, surface acoustic wave devices, and bulk acoustic wave devices.

Kuo-Sheng Kao was born in Chia-Yi City, Taiwan, R.O.C., on September 11, 1973. He received the M.S. and Ph.D. degrees in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 1999 and 2004, respectively. Currently, he is an assistant professor of computer and communication at SHU-TE University, Kaohsiung, Taiwan. His current research interests are in the field of thin-film-based acoustic wave devices.