The increasing demand for various flexible electronic devices has encouraged the development of flexible electronic materials. The fabrication of flexible metal oxides, without the aid of polymer substrates, as support structures is still a challenging issue. The fabrication of bendable freestanding SiO2/Si films has been demonstrated for a capacitive humidity sensing device. Figure 1 is a schematic illustration of the fabrication procedures for a bendable SiO2/Si humidity sensor. First, a Ni/Ti adhesive layer was deposited on a bare Si wafer using an e-beam evaporator. An additional Ni layer was electrodeposited on the e-beam evaporated Ni/Ti adhesive layer. The thin Si layer, with the deposited Ni/Ti layers, was exfoliated by the high tensile stress that was induced in the electrochemical deposition process of the Ni layer. The freestanding flexible single crystalline Si thin film was fabricated by the removal of the deposited Ni/Ti layers on the exfoliated Si layer using chemical etchants. The porous structure of the surface of the exfoliated flexible Si film was formed by a metal-assisted chemical etching process.[18] The surface of the Si film, including the porous Si layer, was oxidized by a thermal oxidation process, resulting in the fabrication of the porous SiO2/Si thin film as a freestanding bendable structure. Au/Ti layers on both the top and bottom sides of the SiO2/Si film were deposited by an e-beam evaporator as electrodes for a humidity-sensing device.
The flexible Si thin films prior to the fabrication of the porous SiO2/Si thin film were prepared by the exfoliation of the Si wafer due to the tensile stress of the electrodeposited Ni layer. Figure 2a illustrates the thicknesses of the exfoliated Si films as a function of the applied current density to electrodeposit the Ni layers. The thicknesses of the exfoliated layers linearly decreased with the increase in the applied current density for the electrodeposition. The thickness was reduced from approximately 88 µm to 48 µm, as the applied current density was increased from 10 mAcm− 2 to 50 mAcm− 2. The previous works have shown that the internal stress of the electrodeposited Ni films increased with the increase in the applied current density.[17] The enhanced film stress was attributed to the increase of the over-potential, the rate of nucleation, and the rate of hydrogen evolution. The enhanced strength of the applied tensile stress decreased the thickness of the spalled Si thin film. The initial spalling time for crack propagation from the edge of the bulk Si wafer might be decreased with an increase in the internal stress intensity of the electrodeposited Ni layer. Therefore, the thickness of the exfoliated Si thin film was reduced by the reduced initial spalling time of the Si thin film. The exfoliated freestanding Si thin film, with well-controlled thickness, can have flexibility, as shown in Fig. 2b.
The porous structure on the surface of the flexible Si thin film was formed by a metal-assisted chemical etching process.18 The selective etching of Si was performed at the interface between Si and noble metals. The noble metals act as catalytic cathodes for hole injection in order to accelerate the etching of Si in a HF etchant. Therefore, the morphology of the porous Si structure can be determined by the distribution, size, and shape of noble metals on the Si. Ag nanoparticles were deposited on a Si thin film as the metal catalysts for a Si etching process, as described in Fig. 3a. The good distribution of Ag nanoparticles on the Si thin film was achieved by the controllable nucleation from tin sensitization.19 Sn2+ ions immobilized on the Si thin film can be utilized as nucleation sites for Ag nanoparticles and as reducing agents for the Ag electroless plating. Figure 3b shows the top-view and cross-sectional view (inset) of the porous Si on the Si film prepared by the Ag-assisted chemical etching process. The porous Si layer was etched for 6 hours had a thickness of approximately 2.3 µm. The morphologies and the thicknesses of the porous Si layers as a function of the metal-assisted chemical etching times are described in Figure S1. The thickness of the porous Si layers linearly increased with an increase in the etching time from 30 minutes to 6 hours.
For the fabrication of freestanding bendable SiO2/Si films as capacitive humidity sensors, the Si thin film, including the porous Si layer, was annealed at 1000 ℃ for 6 hours in the air gas including 21% O2. Figure 4a shows the morphology of the oxidized SiO2 on the surface of the bendable SiO2/Si film. The oxidized SiO2 layer was formed at the surface of the porous Si structures. The thickness of the SiO2 layers on the fabricated Si thin film, with a porous surface, can be assumed by the thicknesses of SiO2 layers oxidized in bare Si wafers under the same condition at 1000 ℃ in air. The thicknesses of the SiO2 layers formed on Si wafers as a function of annealing time were measured by an ellipsometer, as described in Figure S2. The SiO2 layer formed in the porous Si structure might be expected to be thicker than the thickness of 145.6 nm in the SiO2 layer oxidized on a bare Si wafer for 6 hours, due to the increased surface area for the oxidation. The cross section of the SiO2/Si film was analyzed by high-resolution TEM (HRTEM), as shown in Fig. 4b. The interface of the two layers, consisting of SiO2 in the amorphous phase and Si in the single crystalline phase, was clearly distinguished in the HRTEM image. The Si layer prepared by the exfoliation of a single crystal Si (1 0 0) wafer clearly maintained the well-oriented lattices. The HRTEM image of the oxidized SiO2 layers unable to observe any structure in the amorphous phase. Additionally, the amorphous SiO2 structure and the single crystal Si structure were confirmed with the fast Fourier transform converted from selected area electron diffraction patterns (FFT SAED), as shown in the insets (i and ii) of Fig. 4b, respectively. The image of a freestanding bendable SiO2/Si film including a metal oxide layer is shown in Fig. 4c.
The capacitive humidity sensing performance of the fabricated porous SiO2/Si film was characterized by controlling the frequency and RH. To measuring the variation of the capacitance of the SiO2/Si film, Cu wires were connected with Au electrodes and were deposited on the top and bottom sides of the fabricated SiO2/Si film. The variation of the capacitance between the top and bottom of electrodes was measured to characterize the humidity sensing properties. The SiO2/Si film was transferred to a humidity sensing chamber. Figure 5 is a schematic illustration of the humidity sensing environment. The humidity of the chamber was controlled by changing the ratio of dry gas to wet gas. The capacitance of the porous SiO2/Si sensing material with leak conduction can be described by Eq. (1), where C0, εr, γ, ω, and ε0 are the capacitance, and relative dielectric constant of and ideal capacitor, conductance, angular frequency, and permittivity of free space, respectively.20 The capacitance of the sensing materials is proportional to the reciprocal of the angular frequency, 1/ω, and the conductance, γ. The sensitivity for the humidity sensing response was calculated by the following Eq. (2), where CRH and Cdry were the capacitance under the varied RH and the capacitance of the lowest RH of 1.63%, respectively. Figure 6a shows the sensitivity of the fabricated porous SiO2/Si film as a function of frequency and RH. The frequency was varied as 100k Hz, 500k Hz, and 1M Hz, and the RH ranged from 13.8–79.0%. As described in Eq. (1), the increase in the applied frequency shows a reduced sensitivity in the range of high RH. No sensing properties were demonstrated for frequencies higher than 1M Hz. The adsorbed water molecules on the sensing material under high frequencies, with a quickly changing electrical field, have insensitive polarization and a reduced dielectric constant.21 The humidity sensitivity of the porous SiO2/Si film linearly increases with an increase in RH. The variation of capacities for the humidity sensing response in the porous SiO2/Si film is attributed to the varied conductance from water molecules adsorbed on the surface of porous SiO2. Therefore, the enhanced sensitivity in high RH conditions can be explained by the increased polarization of the adsorbed water molecules on the surface of the porous SiO2 in the electrical field.
$$C=\left({\epsilon }_{r}-i\frac{\gamma }{\omega {\epsilon }_{0}}\right){C}_{0}$$
1
$$\text{S}\text{e}\text{n}\text{s}\text{i}\text{t}\text{i}\text{v}\text{i}\text{t}\text{y}=\frac{({C}_{RH}-{C}_{dry})}{{C}_{dry}}$$
2
Figure 6b shows the hysteresis characteristics for the capacitive humidity sensing response of the fabricated porous SiO2/Si film. The solid square line and solid circle line in Fig. 6b stand for the adsorption of water with increasing RH and for the desorption with decreasing RH, respectively. The hysteresis properties were calculated using Eq. (3), where He, ∆Hmax, and FFS are the hysteresis error, a maximum difference of sensitivity between adsorption and desorption at the same RH, and a full-scale output (Sensitivity at 13.8% - Sensitivity at 78.4%).22,23 The hysteresis error is 5.0% for a RH of 50.1%. A single sensing curve at 100k Hz at a RH of 71.4% is shown in Fig. 6c. The response time and recovery time for humidity sensing, defined as the time to reach 90% of the total variation, were 18 seconds and 30 seconds, respectively.
$${H}_{e}=\pm \frac{{\varDelta H}_{max}}{2{F}_{FS}}\times 100$$
3
The humidity sensing behavior of the porous SiO2/Si film at various RH levels of 1.8%, 53.4%, and 84.5% was investigated with Nyquist plots utilizing EIS analysis, as shown in Fig. 6d. The frequency varied from 1k Hz to 1M Hz with a testing voltage of 2 V. The semicircles were observed in the complex impedance plots regardless of the controlled RH levels. The unaffected semicircular shapes imply similar sensing behaviors of the porous SiO2/Si film on the various RH levels. The semicircles represent the behavior for the intrinsic impedance of the porous SiO2/Si film, indicating an equivalent circuit of R (CR), as described in the inset of Fig. 6d. The impedance spectra of the fabricated porous SiO2/Si film didn’t show the inclined lines representing a Warburg impedance due to the ion diffusion, even at high RH. Therefore, the sensing behavior of the porous SiO2/Si film in the RH range of 1.8–84.5% is dominantly determined by the hopping transfer of protons between adjacent hydroxyl groups in the chemisorbed water molecular layer.20,21,23 With the increase in RH levels, the resistance, Rct, of the porous SiO2/Si film decreases, and the capacitance, CSiO2, gradually increases, displaying the decrease in the radius and curvature of the semicircle. The sensing behavior from the intrinsic impedance of the bulk SiO2/Si film at even high RH might be attributed to the highly porous and rough surface structure of SiO2. In contrast with the enhanced sensing properties in the porous SiO2/Si film, a SiO2 layer formed on a bare Si wafer shows no sensing response to the tailored RH, as shown in Figure S3.
The reliability of the bendable porous SiO2/Si film for a capacitive humidity sensing material was investigated, as shown in Fig. 7. The reproducibility for the capacitive humidity sensing properties of the porous SiO2/Si films was tested with the sensing of 10 cycles at the frequency of 100k Hz for the RH of 71.4%. The repeated specific sensing signals displayed a reproducible sensitivity with a standard deviation of 0.048. Additionally, the humidity sensing signals at the frequency of 100k Hz for a RH of 53.4% for 15 weeks demonstrate the stable sensing performance for the long-term test.