- Email: [email protected]
Characteristics of an AlN-based bulk acoustic wave resonator in the super high frequency range
Vacuum 83 (2009) 672–674
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characteristics of an AlN-based bulk acoustic wave resonator in the super high frequency range Keiichi Umeda*, Hideki Kawamura, Masaki Takeuchi, Yukio Yoshino Murata Manufacturing. Co., Ltd., Nagaokakyo-shi, Kyoto 617-8555, Japan
a b s t r a c t Keywords: AlN Off-axis sputtering Bulk acoustic wave BAW FBAR
We have evaluated the electrical performance of bulk acoustic wave (BAW) resonators in the range of resonance frequency from 5 to 20 GHz. The BAW resonators, consisting of a piezoelectric AlN film and top and bottom electrodes of Al/Ti films, were designed and prepared in the fundamental thicknessextensional mode. The value of effective coupling coefficient (k2eff ) up to 6% and constant QF factors were achieved in these frequency ranges, which closely followed the theoretical calculation. It was found that AlN films were suitable for high frequency application because of their superior piezoelectricity even in the ultra thin region of 200 nm thickness. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Bulk acoustic wave (BAW) resonators and filters are suitable for mobile wireless communications due to their high resonance frequencies and high quality factor (Q) value. A BAW resonator consists of a piezoelectric film arranged between top and bottom metal electrodes. The acoustical isolation from the substrate can be realized by two different methods. One is the use of a micro machined air gap to create an air interface on both sides of the resonator. The other is the use of a Bragg reflector between the resonator and the substrate. AlN and ZnO are commonly used as a piezoelectric material for BAW resonators [1]. AlN has several advantages over ZnO and are as follows: AlN films with excellent crystallinity can be reproducibly deposited on various kinds of substrates and metals. Also AlN exhibits a moderate electro mechanical coupling factor, higher acoustic velocity and higher Q value than those of ZnO [2]. AlN is suitable for the high frequency resonator in the point where acoustic velocity is fast and Q is large. The resonance frequency of BAW resonators depends on the film thickness. To control resonance frequency with layer thickness may have some advantages compared to surface acoustic wave (SAW) resonators whose resonance frequency depends on the pitch of IDT electrode patterns. One advantage of a BAW resonator is that it is easy to provide resonators with a resonance frequency higher than 3 GHz. In the early studies, BAW resonators have been studied in the frequency range from 0.8 to 10 GHz [3–7]. We are interested how
* Corresponding author. E-mail address: [email protected] (K. Umeda). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.044
much is the limit of higher frequency of BAW resonator and what does determine the limit. In this paper, electrical performance of BAW resonators using piezoelectric AlN films has been evaluated in the frequency range from 5 to 20 GHz. 2. Experiment The BAW resonators consisting of a piezoelectric AlN film and top and bottom electrodes of Al/Ti films were designed to achieve the maximum effective coupling coefficient (k2eff ) on the fundamental thickness-extensional mode. The top and bottom electrodes had the same thickness. Ti layer of thickness 10 nm was used as an adhesion layer. The k2eff can be expressed as: k2eff ¼
p2 fa fr 4
fa
(1)
where the fa and fr are the anti-resonance frequency and the resonance frequency, respectively. The values of k2eff were theoretically calculated by the Mason model [8] at the resonance frequencies of 5, 10, 15, and 20 GHz as a function of Al thickness as shown in Fig. 1. The k2eff decreases as the Al thickness goes up. So the thickness of Al electrode is designed from 20 to 50 nm as listed in Table 1 for the highest performance in which BAW resonators should have a k2eff around 6%. The thickness of AlN is determined according to each resonance frequency. The AlN thin films were prepared by the off-axis sputtering system, which can achieve good thickness uniformity and high crystallinity of AlN films [9]. The Al/Ti thin films were fabricated by a vacuum evaporation. Deposition conditions of the AlN thin films were optimized to obtain c-axis orientation. A via in a Si substrate was formed below the resonator to create an air interface.
K. Umeda et al. / Vacuum 83 (2009) 672–674
673
1000
7.0
90
6.0
5.0
30 0
10
-30
Phase (deg)
Impedance ( )
60 100
-60 5GHz
1 4800
10GHz
4.0
20GHz
100
150
90
200
60
Fig. 1. Theoretically calculated k2eff as a function of Al thickness.
3. Results
30 0
10
-30 -60
1 8500
9500
-90 10500
Frequency (MHz) 1000
90 60
100
30 0
10
-30
3.1. Characteristics of resonators
Phase (deg)
Two kinds of test patterns were also fabricated on the same wafer of the BAW resonators to estimate parasitic components such as electrode resistance, inductance and capacitance in order to evaluate the k2eff and Q value of resonators. One of the test patterns had a short circuit and the other had an open circuit. Both test patterns consisted of a lower and upper electrode without AlN film. The resistivity of Al/Ti films was determined by a four-point probe measurement. The crystal structure of AlN film was analyzed by the X-ray diffraction (XRD) rocking curve method and the transmission electron microscopy (TEM).
100
Phase (deg)
Al thickness (nm)
Impedance ( )
50
1000
Impedance ( )
0
-90 5800
Frequency (MHz)
15GHz 3.0
5300
-60 1 13000
The electrical characteristics of resonators at each resonance frequency are shown in Fig. 2. A bulk acoustic resonance could be observed in the profiles of 5, 10, 15 GHz and even in the 20 GHz ranged resonator with ultra thin AlN film of 200 nm.
14500
-90 16000
Frequency (MHz) 1000
90
100
30 0
10
-30
Phase (deg)
Parasitic components with the BAW resonators were estimated by two kinds of test patterns. The BVD model [10] is usually used as an equivalent circuit of a BAW resonator. We modified the BVD model by including an electrode inductance (Lx), resistance (Rx) and a parasitic capacitance through a substrate (Cx) as shown in Fig. 3. Typical values of the parasitic components and calculated electrical characteristics of BAW resonators excluding the parasitic components from the measurement data in Fig. 2 are listed in Table 2. The Q values were calculated by fitting the resonator response to the BVD model. The Q values were decreased with increasing the resonance frequency. Meanwhile, the k2eff values were almost constant.
Impedance ( )
60
3.2. Equivalent circuit including parasitic components and electrical characteristics of BAW resonators
-60 1 17000
19000
-90 21000
Frequency (MHz) Fig. 2. Measured electrical characteristics of resonators at the resonance frequency of 5, 10, 15, and 20 GHz (solid line: impedance, dotted line: phase).
4. Discussion Table 1 Designed layer thickness in each frequency range Frequency range (GHz)
5 10 15 20
4.1. Crystal structure of AlN films
Thickness (nm) Al
Ti
AlN
50 50 30 20
10 10 10 10
930 445 290 215
The full widths at the half maximums (FWHMs) of rocking curve at AlN (0002) are shown in Fig. 4. AlN films were deposited on an Al/Ti/Si substrate. The FWHM values decreased as the AlN thickness went up, for example, 2.3 was obtained at the thickness of 200 nm. The tendency that the FWHM value decreases as the film thickness increases agrees well with the previous work by Martin et al. [11].
674
K. Umeda et al. / Vacuum 83 (2009) 672–674
L1
C1
1E+14
R1 Rx Q F (Hz)
Lx
C0
Cx
1E+13
1E+12
1
10
100
Frequency (GHz)
Fig. 3. Lumped element model of a BAW resonator with parasitic components.
Fig. 6. QF value as a function of resonance frequency.
Table 2 Electrical characteristics of BAW resonators in which the parasitic components are excluded from the equivalent circuits Frequency range (GHz)
C0 (pF)
Rx (U)
Lx (nH)
Cx (pF)
Q
QF (Hz)
k2eff (%)
5 10 15 20
0.90 0.50 0.12 0.09
0.8 0.8 1.5 7.0
0 0.05 0.11 0.11
0.03 0.03 0.03 0.03
900 330 300 280
4.5E þ 12 3.3E þ 12 4.5E þ 12 5.6E þ 12
6.2 6.3 6.0 6.1
Fig. 5 shows a TEM photograph of the cross-section around the interface between Al and AlN. There exists an amorphous layer of approximately 5 nm in thickness at the interface. The TEM image indicated a well-aligned AlN structure on Al electrode. AlN was the c-axis crystallinity, which was enough for piezoelectricity from the initial growth layer, and it is considered that piezoelectricity was provided in the layer of thickness 200 nm enough and the k2eff of resonators were around 6% in the range from 5 to 20 GHz. 4.2. QF value as a function of resonance frequency QF values as a function of resonance frequency are plotted in Fig. 6. The QF values did not extremely depend on resonant frequency in the range from 5 to 20 GHz when the parasitic components were excluded. It is considered that scattering of acoustic waves at the grain boundary and defects increased and Q value was deteriorated as the resonance frequency rose and the acoustic wavelength shortened. Tsubouchi and Mikoshiba measured the propagation loss of AlN films by SAW devices [12]. They reported that the propagation loss of AlN was proportional to the 1.7th power of the frequency. The loss mechanism of BAW might be different from that of SAW because a BAW resonator contains only 1/2 wavelength between top and bottom electrodes.
XRD RC FWHM (deg)
4.0
3.0
2.0
1.0
5. Conclusion 0.0
0
200
400
600
800
1000
AlN thickness (nm) Fig. 4. AlN (0002) rocking curve FWHM as a function of AlN thickness.
5nm
c-axis
AlN
Amorphous
Al
Fig. 5. A TEM photograph of the cross-section at the interface between Al and AlN.
It was found that AlN films were suitable for high frequency application up to 20 GHz because of their superior piezoelectricity even in the ultra thin region of 200 nm thickness. AlN film was a promising material for high frequency BAW devices if we could solve the issue of the parasitic components. References [1] Lakin KM, Kline GR, McCarron KT. IEEE Trans Microwave Theory Tech 1995;43: 2933. [2] Kawabata A. Jpn J Appl Phys 1984;23:17. [3] Lakin KM, Belsick J, McDonald JF, McCarron KT. Proc IEEE Ultrason Symp 2001:827. [4] Lanz R, Carazzetti P, Muralt P. Proc IEEE Ultrason Symp 2002:981. [5] Lanz R, Muralt P. Proc IEEE Ultrason Symp 2003:178. [6] Loebl HP, Metzmacher C, Peligrad DN, Mauczok R, Klee M, Brand W, et al. Proc IEEE Ultrason Symp 2002:919. [7] Kubo R, Fujii H, Kawamura H, Takeuchi M, Inoue K, Yoshino Y, et al. Proc IEEE Ultrason Symp 2003:166. [8] Mason WP. Electromechanical transducers and wave filters. Van Nostrand; 1948. p. 201. [9] Umeda K, Takeuchi M, Yamada H, Kubo R, Yoshino Y. Vacuum 2006;80:658. [10] Larson JD, Bradley P, Wartenberg S, Ruby RC. Proc IEEE Ultrason Symp 2000:863. [11] Martin F, Muralt P, Dubois MA, Pezous A. J Vac Sci Technol 2004;A22:361. [12] Tsubouchi K, Mikoshiba N. Proc IEEE Ultrason Symp 1989:299.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characteristics of an AlN-based bulk acoustic wave resonator in the super high frequency range Keiichi Umeda*, Hideki Kawamura, Masaki Takeuchi, Yukio Yoshino Murata Manufacturing. Co., Ltd., Nagaokakyo-shi, Kyoto 617-8555, Japan
a b s t r a c t Keywords: AlN Off-axis sputtering Bulk acoustic wave BAW FBAR
We have evaluated the electrical performance of bulk acoustic wave (BAW) resonators in the range of resonance frequency from 5 to 20 GHz. The BAW resonators, consisting of a piezoelectric AlN film and top and bottom electrodes of Al/Ti films, were designed and prepared in the fundamental thicknessextensional mode. The value of effective coupling coefficient (k2eff ) up to 6% and constant QF factors were achieved in these frequency ranges, which closely followed the theoretical calculation. It was found that AlN films were suitable for high frequency application because of their superior piezoelectricity even in the ultra thin region of 200 nm thickness. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Bulk acoustic wave (BAW) resonators and filters are suitable for mobile wireless communications due to their high resonance frequencies and high quality factor (Q) value. A BAW resonator consists of a piezoelectric film arranged between top and bottom metal electrodes. The acoustical isolation from the substrate can be realized by two different methods. One is the use of a micro machined air gap to create an air interface on both sides of the resonator. The other is the use of a Bragg reflector between the resonator and the substrate. AlN and ZnO are commonly used as a piezoelectric material for BAW resonators [1]. AlN has several advantages over ZnO and are as follows: AlN films with excellent crystallinity can be reproducibly deposited on various kinds of substrates and metals. Also AlN exhibits a moderate electro mechanical coupling factor, higher acoustic velocity and higher Q value than those of ZnO [2]. AlN is suitable for the high frequency resonator in the point where acoustic velocity is fast and Q is large. The resonance frequency of BAW resonators depends on the film thickness. To control resonance frequency with layer thickness may have some advantages compared to surface acoustic wave (SAW) resonators whose resonance frequency depends on the pitch of IDT electrode patterns. One advantage of a BAW resonator is that it is easy to provide resonators with a resonance frequency higher than 3 GHz. In the early studies, BAW resonators have been studied in the frequency range from 0.8 to 10 GHz [3–7]. We are interested how
* Corresponding author. E-mail address: [email protected] (K. Umeda). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.044
much is the limit of higher frequency of BAW resonator and what does determine the limit. In this paper, electrical performance of BAW resonators using piezoelectric AlN films has been evaluated in the frequency range from 5 to 20 GHz. 2. Experiment The BAW resonators consisting of a piezoelectric AlN film and top and bottom electrodes of Al/Ti films were designed to achieve the maximum effective coupling coefficient (k2eff ) on the fundamental thickness-extensional mode. The top and bottom electrodes had the same thickness. Ti layer of thickness 10 nm was used as an adhesion layer. The k2eff can be expressed as: k2eff ¼
p2 fa fr 4
fa
(1)
where the fa and fr are the anti-resonance frequency and the resonance frequency, respectively. The values of k2eff were theoretically calculated by the Mason model [8] at the resonance frequencies of 5, 10, 15, and 20 GHz as a function of Al thickness as shown in Fig. 1. The k2eff decreases as the Al thickness goes up. So the thickness of Al electrode is designed from 20 to 50 nm as listed in Table 1 for the highest performance in which BAW resonators should have a k2eff around 6%. The thickness of AlN is determined according to each resonance frequency. The AlN thin films were prepared by the off-axis sputtering system, which can achieve good thickness uniformity and high crystallinity of AlN films [9]. The Al/Ti thin films were fabricated by a vacuum evaporation. Deposition conditions of the AlN thin films were optimized to obtain c-axis orientation. A via in a Si substrate was formed below the resonator to create an air interface.
K. Umeda et al. / Vacuum 83 (2009) 672–674
673
1000
7.0
90
6.0
5.0
30 0
10
-30
Phase (deg)
Impedance ( )
60 100
-60 5GHz
1 4800
10GHz
4.0
20GHz
100
150
90
200
60
Fig. 1. Theoretically calculated k2eff as a function of Al thickness.
3. Results
30 0
10
-30 -60
1 8500
9500
-90 10500
Frequency (MHz) 1000
90 60
100
30 0
10
-30
3.1. Characteristics of resonators
Phase (deg)
Two kinds of test patterns were also fabricated on the same wafer of the BAW resonators to estimate parasitic components such as electrode resistance, inductance and capacitance in order to evaluate the k2eff and Q value of resonators. One of the test patterns had a short circuit and the other had an open circuit. Both test patterns consisted of a lower and upper electrode without AlN film. The resistivity of Al/Ti films was determined by a four-point probe measurement. The crystal structure of AlN film was analyzed by the X-ray diffraction (XRD) rocking curve method and the transmission electron microscopy (TEM).
100
Phase (deg)
Al thickness (nm)
Impedance ( )
50
1000
Impedance ( )
0
-90 5800
Frequency (MHz)
15GHz 3.0
5300
-60 1 13000
The electrical characteristics of resonators at each resonance frequency are shown in Fig. 2. A bulk acoustic resonance could be observed in the profiles of 5, 10, 15 GHz and even in the 20 GHz ranged resonator with ultra thin AlN film of 200 nm.
14500
-90 16000
Frequency (MHz) 1000
90
100
30 0
10
-30
Phase (deg)
Parasitic components with the BAW resonators were estimated by two kinds of test patterns. The BVD model [10] is usually used as an equivalent circuit of a BAW resonator. We modified the BVD model by including an electrode inductance (Lx), resistance (Rx) and a parasitic capacitance through a substrate (Cx) as shown in Fig. 3. Typical values of the parasitic components and calculated electrical characteristics of BAW resonators excluding the parasitic components from the measurement data in Fig. 2 are listed in Table 2. The Q values were calculated by fitting the resonator response to the BVD model. The Q values were decreased with increasing the resonance frequency. Meanwhile, the k2eff values were almost constant.
Impedance ( )
60
3.2. Equivalent circuit including parasitic components and electrical characteristics of BAW resonators
-60 1 17000
19000
-90 21000
Frequency (MHz) Fig. 2. Measured electrical characteristics of resonators at the resonance frequency of 5, 10, 15, and 20 GHz (solid line: impedance, dotted line: phase).
4. Discussion Table 1 Designed layer thickness in each frequency range Frequency range (GHz)
5 10 15 20
4.1. Crystal structure of AlN films
Thickness (nm) Al
Ti
AlN
50 50 30 20
10 10 10 10
930 445 290 215
The full widths at the half maximums (FWHMs) of rocking curve at AlN (0002) are shown in Fig. 4. AlN films were deposited on an Al/Ti/Si substrate. The FWHM values decreased as the AlN thickness went up, for example, 2.3 was obtained at the thickness of 200 nm. The tendency that the FWHM value decreases as the film thickness increases agrees well with the previous work by Martin et al. [11].
674
K. Umeda et al. / Vacuum 83 (2009) 672–674
L1
C1
1E+14
R1 Rx Q F (Hz)
Lx
C0
Cx
1E+13
1E+12
1
10
100
Frequency (GHz)
Fig. 3. Lumped element model of a BAW resonator with parasitic components.
Fig. 6. QF value as a function of resonance frequency.
Table 2 Electrical characteristics of BAW resonators in which the parasitic components are excluded from the equivalent circuits Frequency range (GHz)
C0 (pF)
Rx (U)
Lx (nH)
Cx (pF)
Q
QF (Hz)
k2eff (%)
5 10 15 20
0.90 0.50 0.12 0.09
0.8 0.8 1.5 7.0
0 0.05 0.11 0.11
0.03 0.03 0.03 0.03
900 330 300 280
4.5E þ 12 3.3E þ 12 4.5E þ 12 5.6E þ 12
6.2 6.3 6.0 6.1
Fig. 5 shows a TEM photograph of the cross-section around the interface between Al and AlN. There exists an amorphous layer of approximately 5 nm in thickness at the interface. The TEM image indicated a well-aligned AlN structure on Al electrode. AlN was the c-axis crystallinity, which was enough for piezoelectricity from the initial growth layer, and it is considered that piezoelectricity was provided in the layer of thickness 200 nm enough and the k2eff of resonators were around 6% in the range from 5 to 20 GHz. 4.2. QF value as a function of resonance frequency QF values as a function of resonance frequency are plotted in Fig. 6. The QF values did not extremely depend on resonant frequency in the range from 5 to 20 GHz when the parasitic components were excluded. It is considered that scattering of acoustic waves at the grain boundary and defects increased and Q value was deteriorated as the resonance frequency rose and the acoustic wavelength shortened. Tsubouchi and Mikoshiba measured the propagation loss of AlN films by SAW devices [12]. They reported that the propagation loss of AlN was proportional to the 1.7th power of the frequency. The loss mechanism of BAW might be different from that of SAW because a BAW resonator contains only 1/2 wavelength between top and bottom electrodes.
XRD RC FWHM (deg)
4.0
3.0
2.0
1.0
5. Conclusion 0.0
0
200
400
600
800
1000
AlN thickness (nm) Fig. 4. AlN (0002) rocking curve FWHM as a function of AlN thickness.
5nm
c-axis
AlN
Amorphous
Al
Fig. 5. A TEM photograph of the cross-section at the interface between Al and AlN.
It was found that AlN films were suitable for high frequency application up to 20 GHz because of their superior piezoelectricity even in the ultra thin region of 200 nm thickness. AlN film was a promising material for high frequency BAW devices if we could solve the issue of the parasitic components. References [1] Lakin KM, Kline GR, McCarron KT. IEEE Trans Microwave Theory Tech 1995;43: 2933. [2] Kawabata A. Jpn J Appl Phys 1984;23:17. [3] Lakin KM, Belsick J, McDonald JF, McCarron KT. Proc IEEE Ultrason Symp 2001:827. [4] Lanz R, Carazzetti P, Muralt P. Proc IEEE Ultrason Symp 2002:981. [5] Lanz R, Muralt P. Proc IEEE Ultrason Symp 2003:178. [6] Loebl HP, Metzmacher C, Peligrad DN, Mauczok R, Klee M, Brand W, et al. Proc IEEE Ultrason Symp 2002:919. [7] Kubo R, Fujii H, Kawamura H, Takeuchi M, Inoue K, Yoshino Y, et al. Proc IEEE Ultrason Symp 2003:166. [8] Mason WP. Electromechanical transducers and wave filters. Van Nostrand; 1948. p. 201. [9] Umeda K, Takeuchi M, Yamada H, Kubo R, Yoshino Y. Vacuum 2006;80:658. [10] Larson JD, Bradley P, Wartenberg S, Ruby RC. Proc IEEE Ultrason Symp 2000:863. [11] Martin F, Muralt P, Dubois MA, Pezous A. J Vac Sci Technol 2004;A22:361. [12] Tsubouchi K, Mikoshiba N. Proc IEEE Ultrason Symp 1989:299.