A SAW delay line with NCIDT is simulated and the results are compared with an identical conventional SAW delay line. The x displacement and y displacement of NCIDT SAW delay line are 2 orders less than the conventional delay line device. The delay times of the conventional SAW delay line device and NCIDT SAW delay line are close to the delay time calculated from the free surface velocity.
The following aspects specific to the proposed NCIDT SAW devices are investigated in this chapter. The first parameter analyzed is effects of air gap between NCIDT and piezo-substrate on device parameters. The electric field coupling decreases exponentially with increase in the air gap, thus the gap has to be less than λ/4 for the device to be practically operational. The effects of the substrate holding NCIDT are analyzed by varying the thickness and by changing the holding substrate materials. The SAW phase velocity decreases as the thickness of holding substrate increases, however the change in velocity is negligible for the thickness of the holding substrate greater than 0.5 λ. By employing various holding substrate materials such as Si, FR4, poly Si, and SiO2 it is found that the permittivity of the holding substrate is the only dominant property that affects the SAW phase velocity. The simulations of NCIDT SAW resonator with and without the holding substrate and the simulations with varying the metallization ratio show that the effect of short-circuiting of electric field by the metallic IDTs is significantly greater than the effect of permittivity of the holding substrate. Since the mass loading effect of the IDTs is completely eliminated in NCIDT, the NCIDT fingers can be made thicker for obtaining greater electromechanical coupling coefficient and reduced resistive losses in the IDT. The BAW displacement in NCIDT SAW devices is found to be less than the conventional SAW devices. In conclusion the following guidelines are recommended for designing a typical NCIDT SAW device. Keep the air gap less than λ/4, choose the substrate holding NCIDT of low permittivity and thickness greater than λ/2, implement the metallization ratio of 0.3 and use thick IDT fingers.
Two types of SAW sensors are simulated, direct mass load sensors and sensors with sensing film. In case of direct mass load sensor the shift in resonance frequency of NCIDT SAW sensors is found to be about 6.6 times greater than that in conventional SAW sensor. The significant increase in sensitivity in the NCIDT SAW sensor is due to the absence of static mass of IDT on the substrate surface. In the simulation of the second type of sensors, PIB sensing film is coated on the device substrate and used to sense TCE and DCE vapors. The presence of IDT over the piezo-substrate in conventional SAW sensors subdues the mass
loading effect by the sensing film. Hence the NCIDT SAW sensor provides greater shift in resonance frequency than the conventional SAW sensor.
4
SAW Devices with Electric Field Coupled
Bond Pads
The previous chapter discussed about the two types of proposed SAW devices: 1. SAW devices with non-contact interdigital transducer (NCIDT) and 2. SAW devices with electric field coupled bond pads. The first type of SAW devices have been simulated in chapter 3. The results of simulation are compared with identical conventional SAW devices. Various aspects of SAW devices with NCIDT and secondary effects due to IDTs are discussed. Advantages of proposed SAW devices over conventional SAW devices are presented. NCIDT SAW sensors are simulated and their sensitivities are compared with conventional SAW devices.
This chapter deals with the study of the second type of SAW devices proposed viz., SAW devices with electric field coupled bond pads. It comprises of a piezo-substrate and IDTs fabricated on the piezo-substrate, and a pair of metal plates for external electrodes placed over the bond pads with or without an air gap. The pictorial representation of one periodic section of a conventional SAW device and the proposed SAW device with electric field coupled bond pads are shown in Figures 4.1 (a) and (b), respectively. The unique feature of the proposed device is the absence of the bonding wires. Normally, the electrical connections to the SAW device are realized through bonding wires connected to the bond pads on the SAW device. The bonding wires are fragile and offer inconvenience in some applications. In SAW
Piezo-substrate IDT electrodes on piezo-substrate
External metal plates
Bond pads on piezo-substrate
λ Piezo-substrate
IDT electrodes on piezo-substrate
λ
(a) (b)
Figure 4.1 Pictorial representation of one period of (a) conventional SAW device, (b) proposed SAW device with electric field coupled bond pads.
sensors the presence of bonding wires interfere with the coating procedures of the sensing films and limit the operating temperature of the device.
An attempt to get rid of bonding wires has been reported by Beck et al. [16] by demonstrating a SAW sensor in which the RF energy is coupled inductively. The IDTs in the device are connected to a large loop that communicates signals by transformer action.
In the proposed SAW device with electric field coupled bond pads, the IDTs are fabricated on the piezo-substrate conventionally and the signal is coupled to the IDT bond pads from the external electrodes using electric field coupling. The metal plates of the external electrodes can be fabricated on a suitable substrate like Si and FR4. The pictorial representation of proposed new SAW device with electric field coupled bond pads through external electrodes is shown in Figure 4.1. In the following section, a one port SAW resonator with bond pads using electric field coupling is simulated by FEM in COMSOL Multiphysics.