Design of a surface acoustic wave mass sensor in the 100 GHz range

Abstract

A design for photoacoustic mass sensors operating above 100 GHz is proposed. The design is based on impulsive optical excitation of a pseudosurface acoustic wave in a surface phononic crystal with nanometric periodic grating, and on time-resolved extreme ultraviolet detection of the pseudosurface acoustic wave frequency shift upon mass loading the device. The present design opens the path to sensors operating in a frequency range currently unaccessible to electro-acoustical transducers, providing enhanced sensitivity, miniaturization and incorporating time-resolving capability while forgoing the piezoelectric substrate requirement.The field of microscale mass sensors has been booming recently, driven by the advances in nanopatterning techniques and the increasing request for devices capable of minute amounts of matter detection, most notably for biological and environmental interests. A variety of sensors have been implemented, among them surface acoustic waves (SAW)-based devices. 1,2A SAW is an elastic wave that propagates confined to the surface of a semi-infinite medium, the penetration depth being a fraction of its wavelength λ = v SAW /ν, with v SAW and ν as the SAW velocity and frequency, respectively. The interaction with any medium in contact with the surface affects frequency and lifetime of the wave itself. As the SAW frequency ν increases, so do its surface confinement (∼λ −1 ) and the device's sensitivity to mass loading.In typical SAW-based devices, SAWs are launched and detected via two interdigital transducers (IDT) patterned on a piezoelectric substrate. Upon mass loading the free surface between IDTs, the unperturbed SAW frequency ν 0 shifts downward of an amount ∆ν. Measurement of ∆ν/ν 0 enables quantification of the bound mass. 3 These devices perform well in terms of ∆ν, but the drawbacks are the maximum operating frequency bounded to the GHz range due to speed limits in the electronics, the lack of fast temporal resolution, and the piezoelectric substrate requirement.A strategy is here outlined to overcome these limitations. It relies on all-optical generation of a pseudosurface acoustic wave (pseudo-SAW) in a hypersonic surface phononic crystal (SPC), 4-6 and on time-resolved extreme ultraviolet (EUV) detection of the pseudo-SAW frequency in a diffractive scheme. 7,8 The device we propose relies on a SPC made of periodic Al stripes (Ni stripes are also investigated) deposited on sapphire (stripes' periodicity p = 50 nm, height h = 2 nm, filling fraction f = d/p = 0.2, mass density ρ Al = 2700 Kg/m 3 ). The stripes' width d ∼ 10 nm is within reach of state of the art e-beam lithography. The pseudo-SAW frequency ν 0 , set by v SAW /p, is expected to shift of ∆ν upon mass loading the SPC, 9 enabling mass detection. The present scheme gives access to operating acoustic frequencies beyond 100 GHz, enhancing the device's sensitivity, incorporates ultrafast time-resolving capabilities, forgoes the piezoelectric substrate requirement, and allows for increased miniaturization with respect to standa...

Impulsively excited Surface Phononic Crystals: A route towards novel sensing schemes

“…for small Δm s For the present SPC, calculations give S f ~ 10 5 cm 2 /g and Δm s ~ 10 ng/cm 2 , the latter value being dictated by the Rayleigh criterion. This value of S f is three orders of magnitude greater than currently achieved by traditional mass sensing technologies [11]. In 1 Δfint depends on the acoustic problem, not on optical detection issues.…”

“…A case study is here reviewed involving an SPC made of Al stripes -P = 50 nm, h = 2 nm, w = 10 nm -deposited on a Sapphire substrate [11]. The pseudo-SAW frequency of the bare SPC, f res,0 = 127.78 GHz, is down-shifted of Δf res = 0.61 GHz.…”

A route towards surface-sensitive ultrafast spectroscopy

“…This technique has developed into a useful tool for material science and biotechnology. In particular, Surface Phononic Crystals (SPCs), as composite materials with periodic modulation of the surface elastic properties, are expected to play a key role in future nanometrology and sensing applications [4]. SPCs can sustain surface acoustic waves (SAWs) -mechanical oscillations confined within a wavelength from the surface -at the base of a variety of sensing schemes.…”

Enhanced sensitivity of a GHz surface acoustic wave humidity sensor based on Ni(SO4)0.3(OH)1.4 nanobelts and NiO nanoparticles

“…[11][12][13] In order to increase the working frequency, a layered nanocrystalline diamond (NCD)/AlN structure was used to obtain a higher acoustic phase velocity. The SAW could be dramatically affected by the interaction with the external medium.…”