2.1. The FBAR Sensor Chip

The FBAR sensor chip designed for gas detection was shown in Figure 1. The FBAR multilayer-film structure, consisting of two electrodes and a piezoelectric film between them, was made on a silicon nitride support film and a silicon substrate. Center of the substrate was removed to release a suspending area for the resonator. A micro resistor temperature sensor and a micro resistor heater were placed around the resonator in the suspending area for temperature monitoring and control.

Cross-section of the film bulk acoustic resonator (FBAR) chip with detail of the micro through-holes in top electrode. 1—Silicon substrate; 2—Silicon nitride support film; 3—Bottom electrode; 4—ZnO piezoelectric film; 5—Top electrode; 6—Resistor heater; 7—Resistor temperature sensor; 8—Micro through hole.

Performance of the FBAR sensor is decided by the material and the structure. ZnO is semiconductor material with piezoelectric properties [31]. For wurtzite phase ZnO, the piezoelectric constant of the c-axis, namely (002) orientation, is the largest among all the crystal orientation. So c-axis-oriented polycrystalline ZnO film was chosen as the piezoelectric layer in the presented FBAR sensor for best piezoelectric properties. Furthermore, there are sensitive chemical adsorption and strong physical adsorption between crystalline ZnO and some kinds of molecules, such as water [32,33], ozone [34], hydrogen [35] and ethanol [36]. ZnO film in the sensor acted as both piezoelectric layer and sensitive layer.

For sensitivity enhancement, micro through holes with size of 10 μm × 10 μm were made in the top electrode. As shown in Figure 2, when the top electrode is complete, most of the molecules adsorb on surface of the electrode and only a few ones adsorb on the ZnO crystal at the edges. With micro through holes in the top electrode, more and stronger adsorption occurs on the exposed ZnO surface, and the molecules diffuse into the polycrystalline film through grain boundaries. By comparison, the presented FBAR sensor with micro through-holes in top electrode obtains more mass loading and higher sensitivity than the existing one.

Schematic diagrams of adsorption on surface of the FBAR sensors: (a) model with complete electrode; (b) model with micro through-holes in top electrode.

The newly designed FBAR sensor chip was fabricated with the MEMS process in Figure 3.

Fabrication process for the sensor chip.

First, 1.5 μm silicon nitride film was deposited on the silicon substrate by low pressure chemical vapor deposition (LPCVD).

Pt film for bottom electrode and resistor heater was deposited on top surface of the silicon nitride film by physical vapor deposition (PVD) and patterned.

Then 1.2 μm ZnO film was sputtered on the top surface of the chip and patterned.

Pt film for top electrode and resistor temperature sensor was deposited on surface of the ZnO film and patterned.

On the back of the chip, silicon nitride film in suspending area was etched by reactive ion etching (RIE).

With the patterned silicon nitride film as mask, silicon was etched from the back by deep reactive ion etch (DRIE), until it reached the top silicon nitride film.

As shown in Figure 4a,b, the ZnO film deposited on this chip obtained porous surface and highly c-axis-oriented polycrystalline structure. So it had strong adsorption as a sensitive layer and good piezoelectric properties as a piezoelectric layer. The fabricated chip was shown in Figure 4c. The films were smooth and the edges of the patterns were clear, especially of the through holes.

Scanning electron microscope (SEM) photos of the ZnO film: (a) top view; (b) cross-section; (c) the microscopic photo of the fabricated FBAR chip.