Porous silicon bulk acoustic wave resonator with integrated transducer
© Aliev et al.; licensee BioMed Central Ltd. 2012
Received: 30 April 2012
Accepted: 9 July 2012
Published: 9 July 2012
We report that porous silicon acoustic Bragg reflectors and AlN-based transducers can be successfully combined and processed in a commercial solidly mounted resonator production line. The resulting device takes advantage of the unique acoustic properties of porous silicon in order to form a monolithically integrated bulk acoustic wave resonator.
In 1982, Lakin et al. 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, 3]. A schematic cross-section of an SMR is shown in Figure 1.
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 . Recently, it has been suggested that metal-oxide ADBRs could be replaced by porous silicon (PSi) ADBR . 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 , and no lattice mismatch exist between the layers of the ADBR; thus, the interfaces are smoother leading to lower losses and better device performance.
where M=(ρ 1v 1−ρ 2v 2)/(ρ 1v 1 + ρ 2v 2), ζ=d 1/v 1−d 2/v 2, and ρ i 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 ζ which is the difference in the time required for an acoustic wave to cross the layers in the structure. When d 1/v 1=d 2/v 2, 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=ρ 0(1−ϕ) and , where ρ 0 and v 0 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 have been used. For the type of a Si wafer used in this work, κ=0.77.
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.
PSi ADBRs and single layers were fabricated using electrochemical etching. The wafer material used was highly-boron-doped CZ silicon with maximum resistivity of 25 mΩ cm, with 150-mm diameter and 675±20-μ m thickness. Room temperature anodization was performed in a 1:1 solution of 49% aqueous HF and hydrous ethanol. High- and low-porosity layers were obtained by alternating the current density (50 to 150 mA/cm2). The thickness of the layers was controlled by the etch duration. Subareas of a wafer, of diameter 40 mm, were etched separately to allow investigation of different PSi structures. A set of single-layer mirrors of thickness of λ at 1.9 GHz was etched with porosity from 30% to 75%. Also, a set of ADBRs was etched with the number of repeats of the alternating layers varying from 5 to 30. The full analysis of these characterization results will be presented elsewhere.
The spectrum has been modelled using a generalized matrix method given by Mitsas and Siapkas  for optical wave propagation through multilayered structure. The method is convenient to include wave scattering on interface roughness. At normal incidence for acoustic waves, this method can be used for both optical and acoustic waves by replacing optical admittance, i.e., refractive index n with the acoustic admittance 1/Z, where Z=ρv. The BAW device actuator layers were deposited in commercial BAW production line using fully automated production equipment and process control (TriQuint Semiconductor, 1818 S HW441, Apopka, FL, 32703, USA). The thicknesses were monitored by laser-acoustic measurements, optical interferometry and precision mass metrology.
Layer thicknesses for the processed PSi layers were obtained from TEM measurements on sample devices. A map of oxygen concentration for a device has been obtained from energy-filtered TEM microscopy.
The final BAW-SMR devices were tested by RF-probing for a 1-port S-parameter (S 11) measurement of the electrical impedance with sweeps performed from 0.2 to 4.2 GHz with a resolution of 0.5 MHz.
Results and discussion
Acoustic distributed Bragg reflector: acoustic transmittance measurement with simulation
BAW resonator: design and fabrication
Thickness and composition of deposited layers
velocity (ms− 1)
Seal to PSi
We have demonstrated that porous silicon processing can be successfully combined with thin-film deposition methods needed to create high-performance BAW resonators. Adjustment of acoustic properties by changing a single process parameter is a unique feature of porous silicon and highly attractive for future BAW devices.
This work was partly supported by the Engineering and Physical Sciences Research Council (UK) under grant EP/J007552/1. Financial support for TEM measurements was provided in part by the Florida High Tech Corridor Industry Matching Research Program of the University of Florida.
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