1. Introduction In recent years, surface acoustic wave (SAW) sensors have received widespread attention in biodetection research due to their advantages such as small size, light weight, stable performance, and low cost. Among the sensors reported for biochemical detection, piezoelectric sensors include quartz crystal microbalances, bulk shear waves, compliant plate waves, Rayleigh waves, surface transverse waves, and Love waves. Quartz crystal microbalances and bulk shear wave sensors have low sensitivity, while compliant plate wave sensors are difficult to fabricate. Rayleigh wave sensors suffer from significant energy loss due to the presence of a normal displacement component perpendicular to the crystal surface, causing sound wave energy to radiate into the adjacent liquid. Surface transverse waves and Love waves, which only have a horizontal shear displacement component, are commonly used for biochemical detection. Surface transverse waves have the advantage of strong acoustic wave confinement, which translates to high sensitivity. For surface transverse wave sensors, the type and thickness of the metal material in the grating are crucial factors determining the device's sensitivity. This paper will derive the influence of these factors on the sensitivity of surface transverse wave sensors and conduct relevant analyses. 2. Basic Theory This paper analyzes the sensitivity of the grating array portion of the STW sensor to small mass loads. The sensor structure, as shown in Figure 1, includes an input/output transducer (IDT) and a central grating array. Based on an AT-cut quartz substrate, when there is no grating array between the two transducers, the acoustic wave operates as a surface skimming bulk wave (SSBW). If a grating array is placed along the propagation path, the SSBW is constrained by the grating array and propagates on the crystal surface, transforming into a surface transverse wave. Like the SSBW, the surface transverse wave is also an SH-type surface acoustic wave, exhibiting only a single particle vibration direction parallel to the crystal surface. Therefore, it can be assumed that the surface transverse wave propagates along the Z-direction on the crystal surface, with the particles vibrating in the X-direction. The Christoff equation and the boundary conditions of the crystal surface can be expressed as follows: where c55, c56 and c66 represent the elastic stiffness coefficients of the substrate material; ρ is the density of the substrate material; Tiy and Tsiy (i=1, 2, 3) represent the stresses of the grating and the substrate, respectively; vx is the particle vibration velocity. According to the Floquet theorem, the solution vx of the equation is expressed as a superposition of a series of spatial harmonics. Here, only the 0th and -1st orders are taken, and other orders are ignored. Where: an, αn, βn represent the amplitude, attenuation coefficient and propagation coefficient of the nth harmonic, respectively. Substituting the vx expression (3) into equations (1) and (2), the parameters in the expression can be obtained. After calculating the particle vibration velocity, the sensitivity Sβ of the sensor can be solved based on Auld perturbation theory. For a surface transverse wave with only X-direction vibration displacement, the sensitivity can be expressed as: where ρ′, μ′, and vn are the density of the grating material, Lamé coefficient, and acoustic phase velocity, respectively; Pn is the acoustic energy density of the surface transverse wave. According to equation (4), the design parameters of the sensor can be substituted to obtain Sβ. From the expression of Sβ, it can be seen that Sβ is a function of the acoustic wave operating frequency, substrate material parameters, and grating structure and material. Usually, the development of STW sensors is based on a given operating frequency and substrate material. At this time, the parameter design of the grating (grating material, thickness) determines Sβ. 3 Calculation Example According to the derivation of equation (2), the sensitivity analysis is carried out using an STW sensor with an AT-cut quartz substrate as an example. The grating period λ is 20μm, and the metallization ratio is 0.5. The calculation is carried out according to equations (1) to (4). When the metal material is aluminum and the relative film thickness is 0.5%, 1%, 1.5%, and 2%, respectively, the sensor sensitivity changes with frequency as shown in Figure 2. The horizontal axis represents the normalized frequency of the stopband center frequency. As shown in the figure, at the same operating frequency, the sensitivity increases with increasing thickness. Furthermore, the sensitivity increases rapidly when the normalized frequency approaches 1. This is because the grating array's ability to confine sound waves is enhanced when the excitation frequency is close to the stopband edge, concentrating more and more sound wave energy on the crystal surface. If the normalized frequency point of 0.97 at the stopband edge is used as the observation point, the sensitivity of different grating materials—gold, silver, and aluminum—is compared. The calculation results are shown in Figure 3. The sensor sensitivity increases with increasing thickness. At the same thickness, the sensitivity of the gold grating array is more than twice that of silver, and significantly higher than that of aluminum. 4. Conclusion In summary, for STW sensors, increasing the grating array thickness is beneficial to improving sensitivity; the larger the grating array thickness, the higher the sensitivity. However, at the same operating frequency, increasing the grating array thickness also causes changes in loss. In practical device design, both factors should be considered simultaneously to select an optimal thickness. When the grating array thickness is determined, a higher sensitivity can be obtained when the excitation frequency of the IDT is close to the stopband edge frequency. Regarding the selection of grating materials, the above analysis shows that gold has the highest sensitivity, more than twice that of silver, and far superior to aluminum.