SEM confirmed the formation of a Ti suspended-gate, 200 μm in length and 50 μm in width, over the channel, as shown in Fig. 2. In the process, the NPR was a supporter to prop the Ti gate and was used to determine the air gap thickness. The thickness of the NPR was controlled by adjusting the speed of the spin coating step. In this device, the air gap was found to be approximately 5 μm. After the vacuum drying step, there was no photoresist or water observed under the Ti gate, proving that the process can release the thin metal film as a suspended-gate for TFT devices.
Figure 3 presents the transfer curves that measure from the back gate structure with a SiO2 insulator and top gate structure with an Al2O3 insulator. The bottom gate was used to confirm that the channel is working after the entire process. For the back gate structure, the device had the W/L = 400/50 and the drain current (Ids) was measured with the gate voltage (Vg) scanned from -60 to 60 V at a drain voltage Vd = 1 V and 10 V. The field-effect mobility of the back gate device was approximately 8.7 cm2V−1s−1. The back-gate structure was fabricated to calculate the saturation mobility of IGZO at high Vd. For the top gate structure, the Id was swept with Vg from − 60 to 60 V at Vd = 1 V with the W/L = 50/50 based on the Ti gate size. The pressure was placed on the surface of the Ti suspended-gate using an insulated probe tip at the top to increase the contact area and prevent the current to the tip. Without the application of stress to the top gate, there was no drain current passing through the channel, even though the gate voltage was increased up to 60 V. The drain current increased gradually by applying pressure to the tip. The threshold voltages were also shifted from 42 to 3.9 V. The entire enhancement in the electrical properties shows that the suspended-gate TFT with the IGZO channel can detect the diverse pressure. After pushing the gate with the tip and releasing it, the drain current did not return to the value before stressing due to damage from pressing the gate.
In this structure, the insulator layer for oxide TFTs involves the alumina protective dielectric. This prevents the suspended gate from coming into contact with the channel directly as excessive pressure is applied. Direct contact of the suspended gate and channel leads to the collapse of the gate and damage to the channel. The other insulator layer is the air gap, which has the rubber sensitivity role for the tactile force sensor. The pressure was applied using an insulated probe tip. The drain current in the saturation region of the suspended-gate TFT can be calculated from the following equation [17]
$$I_{DS} = \frac{{W\mu_{sat} C_{t} }}{2L}\left( {V_{g} - V_{t} } \right)^{2}$$
(1)
where W and L are the channel width and length of TFT, \(V_{g}\) and \(V_{t}\) are the gate voltage and threshold voltage, respectively; \(\mu_{sat}\) is the saturation mobility, and \(C_{t}\) is the total capacitance of the insulator layers, which is expressed as [17]:
$$\frac{1}{{C_{t} }} = \frac{1}{{C_{pd} }} + \frac{1}{{C_{gap} }}$$
(2)
where \(C_{pd}\) and \(C_{gap}\) are the protective dielectric (alumina) and air gap capacitance, respectively. \(C_{gap}\) can be expressed as
$$C_{gap} = \frac{{\varepsilon_{air} \varepsilon_{0} A}}{{d_{gap} }}$$
(3)
where \(\varepsilon_{air}\), \(\varepsilon_{0}\), A, and dgap are the relative dielectric constant of air, absolute dielectric constant, area of the gate of TFT, and air gap thickness, respectively. By changing dgap with the pressure on the suspended gate, Cgap can be modulated, and Ids would increase or decrease accordingly. The accurate displacements according to the air gap thicknesses from the transfer curves were calculated.
Figure 4 shows the displacement of the Ti gate simulated from the COMSOL simulation when the stress to the gate was varied from 75 to 100 kPa. In the simulation, the Ti gate has a matching size with a movable part in the practical device structure, and Young’s modulus, density, and Poisson’s ratio were 116 GPa, 4506 kg.m−3, and 0.3, respectively. The displacements in the middle of the gate increased from 4.5 to 5 μm as the pressure was increased. When the stress was higher than 100 kPa, the displacement became larger than the air gap thickness, 5 μm.
The applied stress corresponding to the displacement was extracted by matching the displacement from the simulation and calculation based on (3), as shown in Fig. 5a. Figure 5b shows the difference in the drain current ∆Ids/I0 (∆Ids is the relative change in the drain current corresponding to the change in pressure loading ∆P and I0 is the initial current without pressure [11]) under various pressures. The sensitivity of the devices was also calculated using the sensitivity, which was defined as S = (∆Ids/I0)/∆P. The highest sensitivity was observed at the largest applied pressure of 180 Pa−1.
The sensitivity acquired from the measured current curve showed high practicability for a range of applications, which overcomes the difficulties of the polymer dielectric. Because the air gap is used as the dielectric layer in a field-effect transistor, a higher sensitivity target was reached with the microsensor device. Despite the great sensitivity, this structure may have problems in the recovery of the suspended gate after removing the pressure because of the thicker Ti film thickness and the large gate size. This problem can be alleviated by covering one PDMS layer as a passivation layer and improving the resilience of the metal gate [18].