In 1982, Lakin et al.[1
] demonstrated the potential for thin-film bulk acoustic wave (BAW) devices for filters and resonators. Acoustic filters such as surface acoustic wave (SAW) filters and BAW filters are a cornerstone of modern wireless communication as they enable radio frequency (RF) filters with very low loss, excellent frequency selectivity and very small size. The frequency spectrum in the range between 400 MHz and 3 GHz is packed with bands allocated to various broadcasting and wireless communication systems. Avoiding interference between various systems is becoming increasingly difficult. Effective use of this spectrum requires RF filters to meet demanding specifications with regard to steepness of the transition between passband and adjacent rejection bands. The market for RF filters has grown to more than 3 billion units per year, while unit price is in a steady decline and averaged less than $0.25 in 2011. BAW filters are used for the most demanding applications - as they outperform SAW filters - but they require a complex manufacturing process which is more expensive. 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 distributed Bragg reflector (ADBR) 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 FBAR structures that they are less delicate to manufacture, more rugged when produced and have better power handling as they have thermal contact with the substrate[2
]. A schematic cross-section of an SMR is shown in Figure .
Schematic cross-section of solidly mounted resonator (SMR). A piezo resonator is sandwiched between two electrodes and mounted on an acoustic Bragg reflector.
For commercial acoustic mirrors which are components of SMRs and filters, a low-acoustic impedance material such as SiO2
is layered with high-impedance materials such as tungsten or molybdenum [2
]. Recently, it has been suggested that metal-oxide ADBRs could be replaced by porous silicon (PSi) ADBR [4
]. This has several advantages: the potential for integrated devices on a Si substrate, the elimination of several processing steps, the impedance mismatch (defining the bandwidth) can be easily and continuously adjusted, advanced filter design including apodization, impedance matching and rugatization become possible [5
], and no lattice mismatch exist between the layers of the ADBR; thus, the interfaces are smoother leading to lower losses and better device performance.
For an ADBR, the center frequency of stop bands
for different orders m, for normal incidence of the wave, can be expressed as:
are the thickness and the acoustic velocity of the two alternating layers, respectively. The corresponding gap widths Δf
for odd and Δf
for even m
can be expressed following reference [6
where M=(ρ1v1−ρ2v2)/(ρ1v1 + ρ2v2), ζ=d1/v1−d2/v2, and ρ
is the mass density of the layers. It can be seen that the gap width depends on M which is the acoustic impedance mismatch between the layers and on ζwhich is the difference in the time required for an acoustic wave to cross the layers in the structure. When d1/v1=d2/v2, as follows from Equations 1 and 2, all gaps with even m disappear, and widths of all gaps with odd m become equal.
Mass density ρ
and acoustic velocity v
in PSi are functions of porosity, i.e., the volume fraction of voids ϕ
and can be expressed as ρ
, where ρ0
are mass density and acoustic velocity of bulk Si, respectively, and κ
is a parameter which depends on PSi morphology. To design ADBRs, the results of the recently published comprehensive study of porosity dependence of acoustic velocity in PSi[7
] have been used. For the type of a Si wafer used in this work, κ
Here, we demonstrate that AlN transducer can be successfully integrated, in a commercial BAW-SMR production line, with an electrochemically etched PSi acoustic mirror consisting of a single layer or a multilayered stack. We present results on individual devices defined on a Si wafer by lithography.