Highly Sensitive SAW Pressure Sensor Based on Simply Supported Beam

In this work, a SAW pressure sensor based on a simply supported beam was designed and fabricated. The SAW resonator deposited on 128° YX-lithium niobate (LNO) substrate was used as the sensing element. The LNO substrate was used as a simply supported beam, therefore strain transfer loss was avoided and uniaxial stress was directly applied to the SAW resonator. An indenter with two ledges was used to indent the LNO beam and then uniform strain was applied to the SAW resonator. The sensitivity was experimentally measured in a pressure range of 330 kPa. The results demonstrated that the sensitivity of the SAW pressure sensor in this work was as high as 2514 ppm/MPa. The high pressure sensitivity of the proposed SAW pressure sensor was analyzed. The device structure of the proposed SAW pressure sensor may help to design highly sensitive SAW pressure sensors.


Introduction
Surface acoustic wave (SAW) technology has the advantages of wireless transmission [1] and easy integration [2].SAW sensors were applied to measurements of acceleration, temperature, pressure, and other parameters.The SAW pressure sensor was reported as early as 1975.In recent years, along with the progress of automobile, aviation and petroleum industries, SAW pressure sensor has gradually attracted the attention of researchers all over the world.For example, Quintero et al. [3] fabricated a passive SAW pressure sensor consisting of a SAW resonator pasted on a metal diaphragm.The sensitivity was about 191 ppm/MPa.Memon et al. [4] put forth a distinctive approach to further enhance pressure sensitivity by applying strain along the SAW propagation direction.The sensitivity was improved to 228 ppm/MPa.Xue et al. [5] developed a SAW pressure sensor using Langasite (LGS).The pressure sensitivity was up to 998 ppm/MPa by reducing diaphragm thickness to 30 μm.Li et al. [6] prepared a SAW pressure sensor on Z-cut LGS utilizing the reinforced structure, which let pressure force be applied at one point to enlarge deformation.And their experimental pressure sensitivity was about 178 ppm/MPa.These earlier reports show that the sensitivity of SAW pressure sensors still needs to be further improved to avoid the influences of temperature, humidity, vibration et al. on the signals of SAW devices in practical applications.
Lithium niobate (LNO) is piezoelectric material with excellent acoustic properties.128º YX-LNO has the advantages of a large electromechanical coupling coefficient (K 2 ) and low propagation loss [7] and is commonly utilized for the preparation of SAW devices.
In this paper, a SAW resonator was fabricated on the 128º YX-LNO substrate.Then the SAW resonator was used as a sensing element to prepare the pressure sensor.To avoid the strain transfer loss induced by adhesives, the LNO substrate was directly used as a beam to sense strain.According to previous work [8,9], it was found that the biaxial stress would have a canceling effect on the frequency shift of the Rayleigh wave, therefore, decreasing the sensitivity.Therefore, to improve pressure sensitivity, we proposed a novel SAW pressure sensor based on the simply supported beam that makes the SAW resonator operate under uniaxial stress without glue.At the same time, an indenter with two ledges was used to decrease the influences of non-uniform strain.The frequency responses of the pressure sensor under different applied pressures were investigated.

Experiments and Methods
In this paper, one-port SAW resonators were fabricated on 128º YX-LNO substrate, which was purchased from Deqing Huaying Electronics Co., Ltd.The size of the LNO substrate was 23 × 6 × 0.5 mm 3 .The SAW resonator was deposited in the middle of the LNO substrate.The structure of the SAW resonator consisted of two parts.One was the interdigital transducer (IDT) in the middle.The other was the reflectors on both sides.The number of finger pairs of both IDT and reflectors was 200.The wavelength λ was 12 μm.The aperture of the resonator was 200 λ.The space between the IDT and reflector was 7λ/4. 10 nm Ti was first deposited onto the piezoelectric substrate.120 nm thick Au was then deposited as a conductive layer.Standard lithography and lift-off processes were used to pattern film electrodes.The prepared SAW resonator was used as the sensing element.Figure 1 presents the schematic of the SAW pressure sensor.The LNO substrate was used as a simply supported beam.Both ends of the LNO beam were held.An indenter with two ledges was mounted on the bottom of the LNO beam.When the pressure was applied, the stainless steel diaphragm deformed and pushed the indenter upward.Then SAW resonator was strained and its resonance frequency shifted.
The SAW resonator was connected to a vector network analyzer (Agilent E5071C).As pressure is applied, resonance frequency was continuously recorded for 2 min.The average value was used as the resonance frequency at the applied pressure.

Results and Discussions
Figure 2 (a) presents the S11 parameter of the SAW resonator.It can be observed that one resonance peak excited by the IDT is at 313.4 MHz.Since the wavelength is 12 Pm, the acoustic velocity corresponding to this resonance can be calculated as 3761 m/s, which comes near the velocity of the Rayleigh wave reported by [10].So we think that the resonance at 313.4 MHz is the Rayleigh resonance.To further explore the mode shape of the acoustic wave, we conducted 3D simulations.Since the IDT is a periodic structure, we utilized one pair of fingers as the structural unit in the simulation.The size of the IDT structural unit was 40 × 12 × 2 μm 3 .Figure 2 (b) presents the calculation results in the frequency domain.The inset presents the mode shape of the resonance peak.The simulation resonance frequency is 316.05MHz, which is close to our experimental result.It can be seen from the mode shape that elliptic displacement occurs on the substrate surface, which is similar to the simulation result of the Rayleigh wave in [11].Moreover, according to Hashimoto's description of the Rayleigh wave [12], it can be inferred that the resonance at 313.4 MHz is the Rayleigh wave. Figure 3 (a) shows the S11 parameters of the SAW pressure sensor from 0 kPa to 330 kPa.It can be found that as pressure increases, the resonance frequency shifts towards a lower frequency.When pressure increases to 330 kPa, the resonance frequency decreases to 312 MHz.
Figure 3 (b) is the relative resonance frequency shifts of the SAW pressure sensor with applied pressure.It could be observed from the figure that relative resonance frequency shifts decrease with the increase of pressure.When pressure increases to 330 kPa, the relative resonance frequency shifts decrease to -819 ppm.It can also be found that there is a strong linear relationship between the relative resonance frequency shifts and applied pressure.According to linear fitting results, the pressure sensitivity is about 2514 ppm/MPa.    1 lists the pressure sensitivities of SAW pressure sensors reported in some typical works.It can be found that the sensitivity in this work is higher than that reported in [3][4][5][6] and [13,14].Then it is interesting to investigate the reasons for the high pressure sensitivity of our SAW pressure sensor.The average strain in the IDT region can be obtained to be about 176.46 PH.According to the data in Figure 3, the strain sensitivity of the SAW resonator can be calculated to be 4.27 ppm/PH, which is larger than some previous work [15].This may be due to the fact that the previous strain sensors are installed by attaching SAW resonators on the substrate surface with adhesives such as cyanoacrylate [16].Then the adhesives would release parts of applied stress [17], which cause a decrease in strain sensitivity.However, for the simply supported beam structure in this work, no adhesives were used and the force was directly applied on the LNO beam (SAW resonator), which avoided the loss [16] caused by the strain transfer and thus increased the sensitivity.
On the other hand, the SAW pressure sensors reported in [3][4][5][6] and [13,14] are diaphragm-type SAW pressure sensors, which means that the SAW resonator was fabricated or pasted on the diaphragm.According to Sinha's research [8], the pressure subjected to the diaphragm could be regarded as biaxial stress (strain).One axis is parallel to the acoustic wave propagation direction.The other axis is perpendicular to the acoustic wave propagation direction.So, biaxial stresses are applied on the SAW resonator for diaphragm-type SAW pressure sensors.However, according to Donohoe's report [9], when strain is parallel/perpendicular to the Rayleigh wave propagating direction, the resonance frequency decreases/increases with the increase of applied strain.Therefore, the resonance frequency shifts will decrease due to the canceling effect of biaxial stress when the diaphragm is subjected to pressure.This canceling effect results in the relatively low sensitivity of diaphragm-type pressure sensors reported in [3][4][5][6] and [13,14].In our SAW pressure sensor, due to the simply supported beam structure, uniaxial stress is applied along the long axis of the LNO substrate.Then the SAW pressure sensor can obtain a high pressure sensitivity.
Thirdly, Kalinin [18] studied the influences of non-uniform strain on the characteristics of one-port SAW resonators.They found that strain non-uniformity may reduce the sensitivity by 21%.This may interpret the low sensitivity of the SAW pressure sensor reported in [6].Kalinin's results suggest that the strain applied on the SAW resonator should be uniform to improve sensitivity.It can be found in Figure 4 that the difference in strain at A, B and C is very small, i.e., we can think the strain is uniform in the IDT region.Then the high pressure sensitivity may benefit from the uniform applied strain.In short, by using a simply supported beam and indenter with two ledges, a SAW pressure sensor with high sensitivity was proposed in this work.This approach avoids the strain transfer loss induced by adhesives.Meanwhile, it avoids the canceling effect caused by biaxial strain and improves strain uniformity.

Conclusions
In this work, a SAW pressure sensor was designed and fabricated with a SAW resonator deposited on an LNO substrate.The experimental results show that the pressure sensitivity was up to 2514 ppm/MPa.The influences of the biaxial stress and uniaxial stress on the SAW resonator were analyzed.Then the high pressure sensitivity was mainly attributed to the large uniaxial stress directly applied on the LNO beam in the absence of adhesives.The high pressure sensitivity also benefits from the uniform strain generated by the indenter with two ledges.This work may provide references for the design of the SAW pressure sensor.

Figure 1 .
Figure 1.Schematic of SAW pressure sensor based on simply supported beam.

3 Figure 2 .
Figure 2. (a) Experimental results and (b) simulation results of SAW resonator.

Figure 3 .
Figure 3. (a) Measured S11 parameters of SAW resonator at different pressures.(b) Relative resonance frequency shifts of SAW resonator vs pressure.

Figure 4 .
Figure 4. Strain distribution of simply supported beam along x directions (εxx).

Figure 4
Figure 4 demonstrates the strain distribution of the LNO beam along the x direction under 300 kPa pressure.The strains at the left, middle and right of the IDT (marked as A, B and C in Figure 4) are 176.45PH, 176.51 PH and 176.41 PH respectively.The average strain in the IDT region can be obtained to be about 176.46 PH.According to the data in Figure3, the strain sensitivity of the SAW resonator can be calculated to be 4.27 ppm/PH, which is larger than some previous work[15].This may be due to the fact that the previous strain sensors are installed by attaching SAW resonators on the substrate surface with adhesives such as cyanoacrylate[16].Then the adhesives would release parts of applied stress[17], which cause a decrease in strain sensitivity.However, for the simply supported beam structure in this work, no adhesives were used and the force was directly applied on the LNO beam (SAW resonator), which avoided the loss[16] caused by the strain transfer and thus increased the sensitivity.On the other hand, the SAW pressure sensors reported in[3][4][5][6] and[13,14] are diaphragm-type SAW pressure sensors, which means that the SAW resonator was fabricated or pasted on the diaphragm.According to Sinha's research[8], the pressure subjected to the diaphragm could be regarded as biaxial stress (strain).One axis is parallel to the acoustic wave propagation direction.The other axis is perpendicular to the acoustic wave propagation direction.So, biaxial stresses are applied on the SAW resonator for diaphragm-type SAW pressure sensors.However, according to Donohoe's report[9], when strain is parallel/perpendicular to the Rayleigh wave propagating direction, the resonance frequency decreases/increases with the increase of applied strain.Therefore, the resonance frequency shifts will decrease due to the canceling effect of biaxial stress when the diaphragm is subjected to pressure.This canceling effect results in the relatively low sensitivity of diaphragm-type pressure sensors reported in[3][4][5][6] and[13,14].In our SAW pressure sensor, due to the simply supported beam structure, uniaxial stress is applied along the long axis of the LNO substrate.Then the SAW pressure sensor can obtain a high pressure sensitivity.Thirdly, Kalinin[18] studied the influences of non-uniform strain on the characteristics of one-port SAW resonators.They found that strain non-uniformity may reduce the sensitivity by 21%.This may interpret the low sensitivity of the SAW pressure sensor reported in[6].Kalinin's results suggest that the strain applied on the SAW resonator should be uniform to improve sensitivity.It can be found in Figure4that the difference in strain at A, B and C is very small, i.e., we can think the strain is uniform in the IDT region.Then the high pressure sensitivity may benefit from the uniform applied strain.

Table 1 .
Summary of the main characteristics of different SAW pressure sensors.