Fabrication of ZnO nanorods based gas sensor pattern by photolithography and lift off techniques

2.1 Gas sensor component

The gas sensors typically consist of substrate, electrodes, heaters, and sensing materials. To simplify the process, two photomasks were used such as first for Pt/Ti deposition and second for the deposition of ZnO sensing materials. The photolithography and lift off techniques are used for the fabrication. The sensor photograph after completion of the device is shown in Fig. 1.

Silicon had an excellent oxide film (SiO₂), abundantly exists on earth, stability as elemental semiconductor and ease of production, and excellent temperature characteristics have led to the dominance of semiconductor market. The SiO₂ layer was used as thermal and electrical insulator. Temperature control is essential for gas sensors to work well. Because it has the optimum operating temperature according to sensing material. The two electrodes of the sensor play an important role in measuring the change of resistance when exposed to target gas. Platinum is used for heater and electrodes because of its high thermal and electrical stability at high temperature. In this work, a platinum heater and electrodes were designed on SiO₂ layer like Fig. 1(c). That means electrode and heater patterns of one plane designs simply the process of sensor fabrication.

When the sensor is exposed to target gas, the sensing layer is a part where interacts with the gas on the sensor surface. At first, the ZnO seed layer was deposited by spin coater or RF sputtering device. Afterward the growth of ZnO nanorods carried out by chemical bath deposition and hydrothermal method.

2.2 Fabrication process

Fig. 2 (a-j) shows the fabrication processes of ZnO nanorods gas sensor. Fig. 2(a) shows a silicon wafer (4 in., 500 µm thickness, single sided polished, 〈1 0 0〉 oriented, p-type), which was sonicated with acetone for 10 min then washed with D.I. water and dried. To get 500 nm SiO₂ layer, the Si wafer was oxidized by thermal oxidation as shown in Fig. 2 (b). Consequently, positive photoresist was deposited with 1.6 µm thickness by spin coating as shown in Fig. 2 (c) then pre-patterned electrodes and heater were deposited by using photolithography method in Fig. 2(d). Pt/Ti layer of 200 nm/10 nm thickness was deposited by DC sputtering system in Fig. 2(e). Titanium (Ti) and Platinum(Pt) layer means the adhesion and the electrodes and heater materials respectively. Then by using lift off method, pre-patterned electrodes and heater were obtained like Fig. 2 (f).

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Fig. 2

Fabrication processes of ZnO nanorods gas sensor; (a, b) Si wafer and thermal oxidized SiO₂ layer, (c, d) deposition and pattern of photoresist, (e) Pt/Ti layer deposition, (f) electrode and heater pattern, (g) deposition and patterns of photoresist layer, (h) ZnO seed layer deposition, (i) ZnO sensing area and (j) ZnO nanorods growth.

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Afterward, using second photomask which patterned sensing area, schemes of Fig. 2 (g) – (i) were carried out by photolithography and lift off techniques. ZnO seed layer of 30 nm thickness was deposited by RF sputtering system like Fig. 2 (i). Finally, vertical growth of ZnO nanorods was progressed selectively on ZnO seed layers by wet chemical process with a solution of zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] and hexamethylenetetramine [HMTA: C₆H₁₂N₄] as shown in the Fig. 2(j). The solutions were made with mixing ratio 1:1 between Zn(NO₃)₂·6H₂O and C₆H₁₂N₄. The growth of ZnO nanorods was processed as below two types of wet chemical method namely, chemical bath deposition(CBD) method and hydrothermal(HD) method as shown in Fig. 3.

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Fig. 3

SEM images (Top-view) of vertically aligned ZnO nanorods in sensing area by chemical bath deposition (a-c) EDX spectra of ZnO nanorods (d).

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2.3 Growth of ZnO nanorods by wet chemical method

Zinc acetate dehydrate((CH₃COO)₂Zn·2H₂O), sodium hydroxide beads (NaOH), zincnitrate hexahydrate (Zn(NO₃)₂·6H₂O), hexamethylenetetramine (C₆H₁₂N₄) and ethyl alcohol(C₂H₅OH) were purchased from DAEJEUNG. All materials were used without any other treatments and purifications.

The chemical bath deposition (CBD) synthesis is to proceed the condensation and precipitation of the precursor solution compound on a substrate by dipping. Chemical reaction in bath results in precipitation on solid surface by saturation. Deposition thickness can be controlled by controlling the chemical reaction time. The CBD synthesis is used as growth of ZnO nanorods because of its merits such as low operating temperature, good compatibility with other devices and low cost (Li et al., 2010). The schematic diagram of ZnO nanorods synthesis process shown in Fig. SI. 1. Fig. SI. 1 (a) shows the schematic diagram of chemical bath deposition of ZnO nanorods. The pre-patterned ZnO seed layer substrate was dipped in freshly prepared growth solution at 80 °C for 1 h. Then the samples were taken out from the solution and thoroughly rinsed with DI water, and then dried in the oven at 80 °C for 4 h. In hydrothermal (HD) process, the growth solution and pre-patterned ZnO seed layer substrate was placed in the Teflon lined autoclave. Fig. SI. 1 (b) shows the schematic diagram of hydrothermal synthesis of ZnO nanrods. The hydrothermal method has some advantage such low cost, size controllability and reproducibility to make nanostructure. This method was done in an oven at 90 °C for 6 h. Next this sample was cleaned with DI water, and dried on the hot plate at 60 °C for 2 min. To obtain the stabilized material structure, annealing method was conducted for 5 h at 600 °C (Khoang et al., 2013).

2.4 Characteristic analysis

To analyze structural morphology and characteristics of sensing film, field emission scanning electron microscopy (FE-SEM, HITACHI SU8230) was used. The phase purity and crystallinity of synthesized ZnO nanorods were characterized by and X-ray diffraction (XRD, RIGAKU D/MAX-2500) and ATR-FTIR (Thermo Scientific USA). In addition, the chemical bonding was analyzed by X-ray & UV Photoelectron spectrometer (Thermo Scientific USA).

3.1 Structural characteristic of ZnO nanorods

Fig. 3 shows SEM images and EDS spectra of grown ZnO nanorods by chemical bath deposition method. It can be seen that the ZnO nanorods with hexagonal top morphology uniformly grown at right angles on the pre-patterned electrodes. The average diameter of the formed ZnO nanorods are about 250 nm. Furthermore, the EDS spectra Fig. 4d confirmed the existence of the Zn and O elements.

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Fig. 4

SEM images (Top-view) of vertically aligned ZnO nanorods in sensing area by hydrothermal deposition (a-c) EDX spectra of ZnO nanorods (d).

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To explore the effect of synthesis route on the morphology and diameter of ZnO nanorods, the hydrothermally synthesized ZnO nanorods morphologies were analyzed by SEM and obtained results are shown in Fig. 4. The hydrothermally synthesized ZnO nanorods were uniformly grown on the patterned electrodes. The formed ZnO nanorods shown uniform length and diameter. The average diameter of the hydrothermally formed ZnO nanorods about 55 nm. The ZnO nanorods density increased in hydrothermal deposition than chemical bath deposition. In addition, the EDS spectra confirmed the presence of Zn and O elements in the nanorods.

The phase purity and crystalline structure of the synthesized ZnO nanorods were inspected by powder XRD. Fig. 5(a) shows the XRD pattern of ZnO nanorods. The XRD spectrum indicated the wrutzite hexagonal ZnO phase peaks with 2θ values of 31.77, 34.44, 36.26, 47.56, 56.60 and 62.89° belonging to the reflection of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) and (1 0 3), respectively. The strongest peak (0 0 2) indicated growth along the c-axis of ZnO nanorods (Khoang et al., 2013, Li et al., 2010, Vanalakar et al., 2015, Wang et al., 2006). That is due to the fact that the (0 0 2) plane has the lowest surface energy with their c-axis perpendicular to the substrate, so it tends to grow fastest in that direction. In addition, the average crystallite size (D) of ZnO target material was calculated to be 55.02 nm using the first diffraction peak by Debye-Scherrer equation (Kadhim and Hassan, 2015)

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Fig. 5

(a) XRD spectrum of ZnO nanorods (b) FTIR spectrum of ZnO nanorods.

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Furthermore, the XPS pattern was obtained for ZnO nanorods and the XPS study results are shown in Fig. 6. The Fig. 6a shows the survey spectrum of the ZnO nanorods, confirmed the presence of Zn, O elements and trace amount of C in the sample. The deconvoluted Zn system (Fig. 6b) shows two major bonds Zn 2P _3/2 and Zn 2P _½ at binding energy 1021.4 eV and 1044.5 eV respectively. Which is well agreement with reported articles. The O1S spectrum (Fig. 6c) confirmed the ZnO, and C-O, C = O bonds at 530.6 eV and 532.7 eV respectively (Ramu et al., 2020). In addition, the XPS atomic percentage of ZnO nanrods shown in Fig. SI. 2. The obtained XPS results validate the FTIR results.

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Fig. 6

XPS spectrum (a) Survey spectrum (b) Zn 2P (c) O 1 s (d) C1s of ZnO nanorods.

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3.2 Growth mechanism of ZnO nanorods

This mechanism is explained with chemical reaction of wet chemical process and theory of ZnO crystal nucleation and growth (Li et al., 2010, Wang et al., 2014). Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ in growth solution provided Zn²⁺ and OH⁻ ions, respectively. The chemical reactions of ZnO nucleation were progressed as follows (Li et al., 2010, Wang et al., 2014)

The polar surface of ZnO is so stable that it is easy to form various nanostructures. ZnO nanostructures followed the shape of wrutzite hexagonal ZnO structure. Fig. 7 shows scheme of growth mechanism of ZnO nanorods and ZnO structure. The top side and the bottom side are polar Zn²⁺ (0001) and polar O²⁻ (000 $\stackrel{-}{1}$ ), respectively. Typically, the morphology of ZnO crystals is explained by the growth velocity of each planes. According to [28], the fastest plane of growth was (0001) as followed $v\left(000\stackrel{-}{1}\right)<v\left(10\stackrel{-}{1}1\right)<v\left(10\stackrel{-}{1}0\right)<v\left(10\stackrel{-}{11}\right)<v\left(0001\right)$ via hydrothermal process, which was corresponded with XRD result as shown in Fig. 5(a). Thus, ZnO nanorods were glowed along c-axis with direction (0001).

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Fig. 7

Scheme of growth mechanism of ZnO nanorods and ZnO structure.

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3.3 ZnO nanorods interaction

The well aligned ZnO nanorods have been focused to sense target gas for a long time (Khoang et al., 2013). The grown ZnO nanorods on patterned sensor electrodes were not all vertically aligned but some ones inclined or having junctions. When gas sensor based on ZnO nanorods is exposed to target gas, the electrical path or flow between nanorods decide the sensor performance. Overlapping of nanorods network was divided into three forms which are point, cross and block junctions (Khoang et al., 2013). Table 1 explains three junctions of ZnO nanorods overlapping via growth mechanism. First, the point junctions were denoted as meeting by a point between growing nanorods. After point contact, two nanorods grow in their own direction. Second, the cross junctions mean to cross between two nanorods. It is also to grow their own directions after crossing. Third, block junctions were denoted to block one nanorod growth by another nanorod. After crossing, it is possible just one nanorod growth. Among these junctions, the point junctions haven large change of the potential barriers before and after exposure to target gas have the most important effects on sensor (Khoang et al., 2013). Generally, electron flow is efficiently going on dense nanorods networks. However, the dense nanorods network sometimes interrupt the diffusion path of target gas into nanorods. So, the optimal growth conditions for well aligned nanorods should be requested because of contradicting as mentioned upper.

Table 1 Three junctions of ZnO nanorods overlapping via growth mechanism.

Junctions type	Diagram of junctions	Potential barriers	Diagram of grain boundaries	Growing process after junction
Point junction				Two direction maintain each other
Cross junction				Two direction maintain each other
Block junction				One nanorod is maintained own direction but another one is stopped