Improvement of insertion loss and quality factor of flexural plate-wave-based alpha-fetoprotein biosensor using groove-type reflective grating structures

Abstract

Abstract. Conventional flexural plate-wave (FPW) transducers have limited applications in biomedical sensing due to their disadvantages such as high insertion loss and low quality factor. To overcome these shortcomings, we propose a FPW transducer on a low phase velocity insulator membrane (5-μm-thick SiO 2 ) with a novel groove-type reflective grating structure design. Additionally, a cystamine self-assembly monolayer and a glutaraldehyde cross-linking layer are implemented on the backside of the FPW device to immobilize alpha-fetoprotein (AFP) antibody. A FPWbased AFP biosensor with low detection limit (5 ng∕mL) can be achieved and used to measure the extreme low concentration of AFP antigen in human serum for early detection of hepatocellular carcinoma. The proposed FPW-based AFP biosensor also demonstrates a very high quality factor (206), low insertion loss (−40.854 dB), low operating frequency (6.388 MHz), and high sensing linearity (90.7%). © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Development of an FPW Biosensor with Low Insertion Loss and High Fabrication Yield for Detection of Carcinoembryonic Antigen

“…A 200-Å-thick Cr layer and a 1500-Å-thick Au layer were continually deposited onto the Si/SiO 2 /Si 3 N 4 layers by an e-beam evaporator to form a ground plane of the FPW device. The (111) plane of the gold (Au) metal layers well matched with the (002) plane of the piezoelectric layer (ZnO) [8,11,12]. The Au and Cr thin layers were patterned by the Au etchant (3%I 2 : 40%KI: 57%H 2 O) and the Cr etchant (Cr-7T), respectively (Figure 3b).…”

“…Furthermore, to construct the IDTs of the proposed FPW devices, a 200-Å-thick Cr and an 1800-Å-thick Au layer were deposited by an e-beam evaporator and patterned by a lift-off photolithographic method, as shown in Figure 3d. These processes are similar to those in our previous studies [7,8,12], but to accurately control the thickness of the silicon thin-plate and substantially improve the fabrication yield of the FPW transducer, we replaced the original 80 °C [8] or 90 °C [12] one-step high-temperature anisotropic wet etching process with a 60 °C/27 °C two-step low-temperature anisotropic wet etching process. First, most of the backside silicon substrate (approximately 475 μm thick) of the four-inch Si-wafer (525 μm thick) was quickly removed in 30 wt %, 60 °C KOH anisotropic etching solution (Figure 3e).…”

“…In an FPW device, the acoustic wave propagates in a floating thin-plate (usually constructed by a silicon/isolation/metal/piezoelectric multilayer) whose thickness (approximately 3–20 μm) is smaller than the wavelength (approximately 75–120 μm) [7,8,11]. When the ratio of thin-plate thickness to wavelength is smaller than 1, the phase velocity of the FPW device is lower than the sound velocity in most liquids, so only a minor amount of energy dissipates from the thin solid plate to the testing liquid.…”

“…Based on FPW theory [1,7,8,12], the operating frequency shift caused by mass-loading of the silicon/isolation/metal/piezoelectric floating thin-plate can be expressed by the following equation [1,13,14]: $\frac{\Delta f}{f_0}{=S}_m{\Delta }_m$ where S m denotes the mass-sensitivity of the FPW device in mass per unit area ( Δ m ). Since the mass of the floating thin-plate of the FPW device is much smaller than that of the bulky substrates adopted in other acoustic microsensors, any small changes in the mass can affect phase velocity and operating frequency shift (>5 kHz).…”

“…Some of the wave energy dissipates to the silicon substrate or the outside area of the output IDTs, increasing the insertion loss of the FPW device. Previously [7,8], we adopted a parallel-type silicon-grooved reflective grating structure (RGS), as shown in Figure 1b, to reduce the wave energy loss, which improved the insertion loss from −51.08 dB to −40.85 dB. To further enhance the propagation and reflection efficiency of the launched flexural plate-wave, reduce the insertion energy loss, and increase the signal-to-noise (S/N) ratio, we first introduced focus-type metal-thin-film IDTs and a focus-type silicon-grooved RGS into the geometric designs of the FPW device.…”