Surname1, Surname2 and Surname3
POROUS SILICON BULK-ACOUSTIC-WAVE RESONATOR WITH INTEGRATED
TRANSDUCER
G. N. ALIEV1, B. GOLLER1, P. A. SNOW1, H. HEINRICH2, B. YUAN2 AND R. AIGNER3
1
Department of Physics, University of Bath, Claverton Down BA2 7AY Bath UK; E-mail: g.aliev@bath.ac.uk;
Tel: (44) 1225 38 45 32
2
Physics Department, Advanced Materials Processing and Analysis Center (AMPAC),
University of Central Florida, Orlando, Florida, USA
3
TriQuint Semiconductor, Apopka, Florida, USA
SUMMARY
We report that porous silicon Bragg reflector and AlN-based transducer can be successfully combined, in a commercial solidly mounted resonators production line, to form a monolithically integrated bulk acoustic wave resonator widely used in applications for wireless communications.
1. INTRODUCTION
In 1982, Lakin et al. demonstrated the potential for thin film bulk acoustic wave (BAW) devices for filters and
resonatorsi. In BAW devices the acoustic wave, generated in a thin film piezoelectric layer sandwiched between
electrodes, propagates in a vertical direction towards the substrate. The frequency of the acoustic resonance is determined by the thickness of the piezolayer and the mass of the electrodes but must be confined by structuring the
substrate. In a film-bulk acoustic resonator (FBAR) a cavity is etched below the active structure to create a suspended membrane. Alternatively, an acoustic Bragg reflector can be used to stop the acoustic wave penetrating the
substrate. These devices are called solidly mounted resonators (SMR). SMR-BAW devices have the advantages
over the FBAR structure that they are less delicate to manufacture, more rugged when produced and have better
power handling as they have good thermal contact with the substrate. Here we demonstrate that distributed Bragg
reflector (DBR) consisting of electrochemically etched porous silicon (PSi) can be successfully integrated with
AlN transducers.
2. EXPERIMENTAL RESULTS AND DISCUSSIONS
The wafer material used was highly-boron-doped CZ silicon with maximum resistivity of 25 mΩ-cm. Room temperature anodization was performed in a 1:1 solution of 49% aqueous-HF and hydrous ethanol. Porosity is controlled by variation of the current (50-150 mA/cm^2), layer thickness by etch duration.
Figure 1 shows the sketch of the structure fabricated.
Piezo
film
Electrode
Figure 1. Schematics of SMR.
Reflector
stack
Substrate
After the PSi etching process the wafers were coated with thin reactively RF magnetron sputtered SiO 2. The BAW
device layers were sputtered in a commercial BAW production line with thicknesses aimed at an operating frequency of 1.9GHz. A 1.3 m AlN piezo layer was sandwiched between AlCu/W electrodes. Surface of PSi DBR
was sealed by 20 nm amorphous Si (aSi) and coupled to the transducer via 0.5 m SiO2 layer. Layer thicknesses
for the processed PSi layer were obtained from TEM measurements on sample devices. Figure 2(a) shows micrographs of a deposited transducer. It can be seen that the SiO2 layer has a clean interface with the PSi. Fig 2(b)
shows a map of oxygen concentration for a device obtained from energy-filtered TEM microscopy. It is clear that
there is a consistent oxygen level across the PSi layer with no gradients apparent after processing. However, this
does suggest that some oxidation of the PSi has occurred.
Surname1, Surname2 and Surname3
(a)
W
(b)
AlCu
AlN
AlCu
W
SiO2
SiO2
(c)
PSi
(d)
SiO2
Figure 2 (a) Cross section of transducer deposited
on single PSi layer. (b) A map of oxygen concentration The layers with higher oxygen concentration
appear brighter. (c) PSi/aSi/SiO2 interface. (d)
PSi/Si-substrate interface.
PSi
PSi
aSi
Si-substrate
Figs 2 (c-d) show examples of interfaces of PSi with the unetched substrate layers and a top interface with deposited Si cap. The scale bar shown is 0.2 μm. The typical directional mesopores of PSi etched from highly p-doped
material are clear with a slightly rough interface at the pSi/Si-substrate interface. The acoustic wavelength is
~3 μm so the roughness is approximately on the scale of 1 % of a wavelength and not a significant source of loss
from interface roughness scattering. Layer thicknesses for the processed PSi layer were obtained from TEM measurements on sample devices. The backside of the wafer was polished to a nearly optical mirror surface finish to
reduce scattering at that surface. The bottom electrode is un-patterned and grounded by capacitive coupling to a
ground plane formed by the top electrode. The top electrode defines the driven resonator and had an area of 23.5 x
10-9 m2. The devices were tested by RF-probing for a 1-port S parameter measurement of the electrical impedance
with sweeps performed from 0.2-4.2 GHz with a resolution of 0.5MHz. Fig 3(a) shows acoustic band gap measured for the PSi DBR before deposition of the actuator stack using external transducers (see Ref 2 for details). Fig
3(b)shows typical raw data of the S11 response (i.e. reflection) for SMR structure with integrated transducer. Reference data from the structure without DBR are shown in background by gray color. The position of the acoustic
gap in Fig.3b is slightly shifted to higher frequencies compared with the gap position in Fig3a., however the gap
width is the same.
(a)
Figure 3. (a) Measured acoustic longitudinal wave
transmission in PSi DBR (not corrected for transducer response; note semi-logarithmic scale) (b)
Data from electrical measurement of the SMR structure with a similar DBR as in (a) , linear scale (black
thick line), and reference data from the spot of the
structure without DBR (grey thin line).
(b)
5. CONCLUSIONS
We have demonstrated that PSi DBR and transducers can be successfully combined, in a commercial
SMR production line, to form a monolithically integrated BAW resonator.
REFERENCES
1.
K.M. Lakin , J. Wang, G. Kline, A. Landin, Y. Chen, J. Hunt, “Thin Film Resonators and Filters”, Ultrasonics
Symp., 1982, pp. 466- 475
2.
G.N. Aliev, B. Goller, D. Kovalev and P.A. Snow, Appl. Phys. Lett. 96 124101 (2010), Wang, E.M. Scherr,
A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B, 45, 4190 (1992).