Notes: For safety reasons, the reaction cell and the aerosol generator are placed inside a fume hood. Employ tweezers to handle the samples, wear gloves, a lab-coat, and goggles, and follow common laboratory safety practices.
1 . Preparation of Substrates and Set-up of Deposition Temperature
- Cut 10 mm x 10 mm silicon substrates using a diamond tip scribe (the substrate dimensions have been adapted to the size of our reaction cell). For this experiment, use a home-made stainless steel cylindrical reaction cell with an internal volume of ~7,000 mm3 (diameter: 30 mm, height: 10 mm) adapted to the dimensions of the silicon-based micromachined platforms employed for the fabrication of gas sensors.
- Clean the substrates in isopropanol, rinse with deionized water, and blow-dry the substrates with nitrogen to ensure good adherence of the films and uniform covering of the substrate.
- Place the substrate into the reaction cell. When using silicon-based micromachined platforms, instead of bare silicon substrates for the fabrication of gas sensors, place the micromachined platforms into the reaction cell and then align with a shadow mask to confine the growth of material to the area of interest.
- Close the reaction cell. Make sure that the lid of the reaction cell is properly sealed to avoid the leakage of reactive species.
- Switch-on the temperature control system, consisting of resistive heaters integrated with the reaction cell, a thermocouple to sense the temperature of the substrate and a proportional-integral-derivative (PID) controller.
- Set the temperature to 400 °C and let it to stabilize (this process takes approximately 30 min, but it may change depending on the reaction cell dimensions and the characteristics of the temperature control system).
2 . Preparation of Solution for Aerosol Generation
- Add 50 mg of ZnCl2 to a 100 mL glass vial equipped with a vacuum trap (29/32 joint, 200 mm length, 5 mm hose barbs).
- Dissolve the ZnCl2 in 5 mL of ethanol and then cap the vial with the vacuum trap. Ensure the down-pipe end sits 60 mm above the bottom of the vial and without submerging in the solution. If needed, employ glass joint clips to secure the vial and the vacuum trap together during the CVD process.
- Clamp the vial to a universal support. Adjust the height to meet the bottom of the vial and the optimal focal point of the ultrasonic atomizer that operates at 1.6 MHz and delivers an average size of the aerosol droplets of ∼3 µm.
- Connect the inlet and the exhaust of the vacuum trap to the nitrogen pipe and the reaction cell, respectively, as shown in the simplified scheme of the aerosol-assisted CVD system in Figure 1.
- Use a fresh solution of reactants for each deposition.
3 . CVD Process
- Before starting the CVD process, verify that the temperature in the reaction cell has reached the steady state.
- Adjust the nitrogen flow to 200 cm3/min and allow it to flow through the system (the flow rate has been tuned according to the dimensions of the reaction cell used in our experiments). The use of a mass-flow controller is recommended to ensure a constant flow during the deposition.
- Switch-on the aerosol generator and keep the aerosol constant during the process until the solution containing the zinc precursor is completely delivered to the reaction cell (this process takes approximately 120 min considering a solution volume of 5 mL and a flow rate of 200 cm3/min).
- As soon as the solution has been fully delivered to the reaction cell, switch-off the aerosol generator and the temperature system to cool down the reaction cell. Meanwhile keep the nitrogen flowing throughout the system.
- When the temperature has dropped to room temperature, close the nitrogen flow, open the reaction cell, and remove the samples. The substrate will show a greyish matte color on the surface, different from the shiny bare silicon wafer (the silicon-based micromachined platforms display a similar appearance after the CVD step). This matte color is associated with the presence of columnar ZnO structures in the form of rods like those observed by scanning electron microscopy (Figure 2).
Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods
Learning Objectives
The aerosol-assisted CVD of ZnCl2 dissolved in ethanol leads to the formation of greyish uniform and adherent films on bare silicon wafers (relatively easily abraded mechanically). Characterization of the films using scanning electron microscopy (SEM) above 8,000X magnification displays quasi-aligned hexagonal shaped ZnO rods with lengths of ∼1,600 and diameters of ∼380 nm (Figure 2). Large errors in the set-point temperature or the presence of temperature gradients along the substrate during the CVD may cause the deposition of other ZnO morphologies (Figure 3) or films with non-uniform structures. In addition, uneven or non-adherent coatings may be related in part to poor temperature control, incorrect adjustment of the flow, and/or the use of a different carrier solvent than that specified in this protocol.
X-ray diffraction (XRD) analysis of the rods shows diffraction patterns associated with a hexagonal ZnO phase (P63mc space group, a = 3.2490 Å, b = 3.2490 Å, and c = 5.2050 Å; ICCD Card No. 5-0664). These patterns display a high intensity diffraction peak at 34.34° 2θ, corresponding to the (002) plane of the hexagonal ZnO phase, along with other seven low intensity diffraction peaks at 31.75, 36.25, 47.54, 56.55, 62.87, 67.92, and 72,61° 2θ, corresponding to the (100) (101) (102) (110) (103) (201) and (004) planes of the hexagonal ZnO phase, respectively. Characterization of the rods by high-resolution transmission electron microscopy (TEM) shows marked planar spacing (0.26 nm) consistent with the internal lattice of the (002) plane (d = 0.26025 nm) of the hexagonal ZnO phase identified by XRD. Energy-dispersive X-ray (EDX) spectroscopy shows the presence of Zn with relatively low chlorine contamination (found for Cl:Zn 0.05 at.%).
The estimation of the optical bandgap of the rods by means of diffuse reflectance measurements of films indicates an optical bandgap of 3.2 eV, consistent with the literature values for ZnO10. The analysis of the films using X-ray photoelectron spectroscopy (XPS) is characterized by Zn 2p1/2 and Zn 2p3/2 core level peaks spectra at 1,045 and 1,022 eV, respectively, consistent with those observed previously for ZnO11,12.
The use of this protocol on silicon-based micromachined platforms intended for gas sensing lead to the direct integration of columnar ZnO rods confined on the sensing-active area (400 x 400 µm2), which is defined by a shadow mask. The electrical resistance of the films is in the order of kΩ (∼ 100 kΩ) measured at room temperature by using the interdigitated electrodes integrated into the silicon-based micromachined platforms. Figure 4 displays the picture of an array of four micromachined gas sensors based on aerosol-assisted CVD rods. The characteristics and fabrication process for the micromachined platforms have been described previously13. These microsystems are sensitive to relative low concentrations of carbon monoxide, with the maximum responses recorded (using a continuous gas flow test chamber13) when the sensors were operated at 360 °C using the resistive microheaters integrated in the system (Figure 5).
Figure 1: Schematic View of the Aerosol-assisted CVD System.
Figure 2: Top (A) and Cross-sectional (B) SEM Images of the ZnO Rods Deposited via Aerosol-Assisted CVD. Please click here to view a larger version of this figure.
Figure 3: Cross-sectional SEM Images of ZnO Deposited via Aerosol-assisted CVD at 300 (A), 400 (B), 500 (C), and 600 °C (D). Please click here to view a larger version of this figure.
Figure 4: Silicon-based Micromachined Platform with 4 Microsensors Mounted on a TO8-package (A), and Detailed View of a Microsensor (B) and the ZnO Rods Deposited on the Edge of an Electrode (C). Please click here to view a larger version of this figure.
Figure 5: Electrical Resistance Changes of the ZnO Rods Towards Various Concentrations (25, 20, 10 and 5 ppm) of Carbon Monoxide. Please click here to view a larger version of this figure.
List of Materials
| ZnCl2 99,999 % trace metal basis | Sigma-Aldrich | 229997 | used as purchased from manufacturer |
| Ethanol ≥96% | Penta | 71430 | used as purchased from manufacturer |
| Reaction cell | home-made | stainless steel cylindrical reaction cell (7000 mm3, diameter: 30 mm, height: 10 mm) with integrated heaters to reach the temperature of deposition and provided with a PID controller | |
| Ultrasonic liquid atomizer | Johnson Matthey | Operating frequency ∼1,6 MHz | |
| Flowmeter | To have a better control of this step the use of a mass flow controller is recommended. | ||
| Nitrogen | Linde Gas A.S. | ||
| Silicon wafers | MicroChemicals | <100>, p-type, 525 µm thick, cut into pieces (10 mm × 10 mm ) | |
| Glass vial – 100 ml | 29/32 joint, 200 mm lenght | ||
| Vacuum trap | 29/32 joint, 5 mm hose barbs | ||
| Graduated cylinder – 10 ml | |||
| Universal support | |||
| Balance | |||
| Scanning Electron Microscopy (SEM) | Tescan | Mira II LMU | |
| X-ray diffraction (XRD) | Rigaku | Smart Lab 3kW | Cu Kα radiation |
| X-ray Photoelectron spectroscopy (XPS) | Kratos | AXIS Supra | Monochromatic Kα radiatio, 300 W emission power, magnetic lens, and charge compensation |
| Transmission Electron Microscopy (TEM) | Jeol | JEM 2100F | operated at 200kV using Schottky cathode and equiped with EDX |
Lab Prep
Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 °C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 °C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.
Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 °C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 °C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.
Procedure
Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 °C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 °C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.