2017 Volume 58 Issue 1 Pages 91-94
Chemical vapor deposition (CVD) diamond micro-components are attracting considerable interest due to their exceptional properties and potential applications. Micro-gears are an important actuating component in micro-machines or micro-electromechanical systems. In this study, nano-crystalline diamond duplex micro-gears for micro-machine applications have been fabricated by combining hot filament CVD (HFCVD) with inductively coupled plasma etching. Based on scanning electron microscopy, X-ray diffraction, and micro-Raman spectroscopy observations, the nano-crystalline diamond duplex micro-gears produced by HFCVD are found to be faithful replicas of microstructure silicon molds produced by time-multiplexed deep etching. The fabricated duplex micro-gear consists of two gears. A gear with 14 teeth constitutes the top layer and has root and tip diameters of 1.14 mm and 1.52 mm, respectively, while the second 20-tooth gear constitutes the bottom layer and has root and tip diameters of 1.75 mm and 2.20 mm, respectively. The total thickness of the micro-gear is about 20 μm.
Micro-machines or micro-electromechanical systems (MEMS), such as sensors (e.g., accelerometers, gyroscopes), actuators (optical and RF switches, micro-grippers), electronic devices (e.g., RF oscillators and filters) or integrated microfluidic systems1,2), involve complex design, manufacturing, and packaging processes. It is therefore very important to maximize the reliability of micro-machines or MEMS devices. Due to the high friction coefficient and low thermal conductivity of silicon materials, micro-machines formed from silicon-based materials cannot be used in harsh conditions requiring them to operate at high speed while being corrosion and wear resistant and also bio-compatible. In such harsh environments, diamond would be an excellent material for micro-machines or MEMS because of its unique properties of low friction coefficient, high thermal conductivity, corrosion resistance, and bio-compatibility3–5). Therefore, diamond patterning has become a key technology for fabricating diamond-based micro-components for micro-machine or MEMS applications7–11). However, it is very difficult to fabricate a micron or submicron-scale diamond micro-machine due to the high hardness and chemical inertness of the material6).
Micro-gears are an important actuating component that are widely used in micro-machines. Diamond micro-gears have been receiving increased attention for application to micro-machines. Recently, several methods have been developed for fabricating diamond micro-gears, such as selective deposition, chemical vapor deposition (CVD) of diamond films etched by reactive ions with masks or laser projection patterning, and CVD of diamond film on a mask by inductively coupled plasma (ICP) etching12–19). Among these technologies, dry etching is one of the most important as it is more compatible with traditional IC processes6,7,14). To date, a single diamond micro-gear with a diameter of several microns and a very fine microstructure has been achieved. In general, gear shafts, gear trains, and complex gear systems are used to transmit motion and power in micro-machine applications. The fabrication of the duplex gears of a gear train is key to achieving complex gear motion and the fabrication of power transmission systems. However, fabrication of the complex double-layer structure of diamond micro-gears remains a challenge. Researchers have mainly focused on the fabrication of single diamond gears with a high aspect ratio, which is a very small and fine microstructure, and to the best of our knowledge, there have been no reports on the preparation of multilayer diamond gears.
In the present study, nano-crystalline diamond duplex micro-gears were fabricated by combining hot filament chemical vapor deposition (HFCVD) and ICP etching. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and micro-Raman spectroscopy were used to characterize the surface morphologies and microstructures of the samples.
Figure 1 is a schematic diagram of the procedure for the fabrication of duplex micro-gears. This method involves three steps for fabricating the diamond micro-gears. Firstly, a silicon fine duplex micro-gear structure array is etched by time-multiplexed deep etching (TMDE). Then, nano-crystalline diamond films are deposited on micro-structured silicon molds by HFCVD. Finally, the silicon micro-molds are removed to reveal the nano-crystalline diamond duplex micro-gears.
Schematic of fabrication of duplex micro-gear.
As the first step, a silicon micro-gear mask is formed by TMDE (Bosch). Prior to the etching, the silicon substrate is ultrasonically cleaned in acetone, de-ionized water, and 5% HF to remove the oxide layer. Then, the silicon substrate is placed in the chamber of an ICP system (A601E) in which SF6 and C4F8 are used as the etching and protective gases, respectively. First, the patterned mask layer is fabricated by applying a standard photolithography process to the silicon substrate. Due to the protection provided by the mask layer, the SF6 only reacts with the Si in the exposed area. Then, a (CF2)n polymer film is formed on the etched region by introducing C4F8 protective gas into the chamber. Subsequently, the polymer film is etched by SF6 plasma and the Si substrate under the polymer film is also etched. During this process, the polymer film on the sidewalls moves down the side walls as a result of the collision of non-vertical ions, leading to the anisotropic etching of the local area. Then, C4F8 gas is introduced such that a (CF2)n polymer film is deposited on the etched area. Finally, the etching and deposition process are repeated until the etching reaches the design depth. The etching process parameters of the gas flow rate (in standard cubic centimeters per minute (SCCM)) and the times of the initial, intermediate, and end stages are listed in Table 1.
In the next step, silicon micro-gear molds are used as substrates for the diamond film deposition in HFCVD. Prior to the deposition, the silicon micro-gear molds are ultrasonically etched in an acetone solution with a nano-crystalline diamond powder suspension. Subsequently, the substrates are rinsed again, sequentially in de-ionized water, 5% HF, and then de-ionized water before being loaded into the reaction chamber. The deposition chamber is evacuated to a pressure of 2 × 10−3 Pa. Then, methane (CH4) and hydrogen (H2) are introduced. The working pressure is stabilized typically at 4.0 kPa. The H2 gas is introduced into the chamber at a constant flow rate of 388 SCCM, followed by CH4 gas at a flow rate of 11.2 SCCM in the HFCVD system for 45 min in the nucleation stage. Then, the CH4 flow rate is changed to 5.5 SCCM, the hydrogen flow rate is changed to 294 SCCM, and Ar is introduced into the chamber at a constant flow rate of 40 SCCM for 6 h in the growth stage. In the final step, the diamond micro-gear deposited on the silicon mask is rinsed with a mixed solution of HNO3 and HF. The silicon mask is then removed to reveal the diamond micro-gear.
The morphology of the sample was observed through SEM (Hitachi-S4800). The characteristics of the sample were investigated by XRD (Bruker AXS D8 Advanced) and micro-Raman spectroscopy (Dilor LabRam-1B) at room temperature.
A SEM image of the duplex silicon micro-gear molds produced by TMDE is shown in Fig. 2. The gears constituting the top and bottom layers are composed of arc segments having different curvature radii and straight lines, which is consistent with the design pattern of a duplex micro-gear mold. There are clear signs of the layer-by-layer etching in the cross-sectional image.
SEM image of duplex silicon micro-gear molds produced by TMDE: (a) full morphology of duplex layer; (b) side image of gear mold.
SEM images of the duplex micro-gear produced by CVD are shown in Fig. 3. Figure 3(a) shows the morphology of the duplex micro-gears. The micro-gear is composed of two gears of different sizes. It can be seen that the gear constituting the top layer has 14 teeth, while the root and tip diameters of the top layer are about 1.14 mm and 1.52 mm, respectively. There are 20 teeth on the gear constituting the second layer, for which the root and tip diameters are about 1.75 mm and 2.20 mm, respectively. Figure 3(b) shows an enlarged SEM image of the local areas around the duplex micro-gear. Figure 3(c) is a high-resolution SEM image of the duplex micro-gear. Figure 3(d) is a cross-sectional image of a single tooth. The teeth surfaces of the micro-gear are smooth. The high-resolution SEM image shows that the film is a continuous dense film and that there are no particles with a well-defined crystal shape such as facetted pyramidal structures, which could be formed in polycrystalline diamond films. The cross-sectional image shows that there is a layer-by-layer structure across the width of the micro-gear, with 16 layers, which indicates that the diamond film is faithfully copied from the silicon fine microstructure by TMDE. The total thickness of the micro-gear is about 20 μm. The thickness of the layer-by-layer structure in the thickness direction is about 0.5 μm.
SEM images of duplex micro-gear produced by HFCVD: (a) morphology of duplex micro-gear; (b) enlarged SEM image of duplex micro-gear. (c) high-resolution SEM image of duplex micro-gear. (d) cross-sectional image of single tooth.
The XRD spectrum of the nano-crystalline diamond micro-gear is shown in Fig. 4. The spectrum indicates the diamond (111) and (220) diffraction peaks at 44.00° and 75.50°, respectively. The intensity ratio of I (220)/I (111) is about 0.25, consistent with that of randomly oriented diamond film. The XRD spectrum indicates that the carbon film produced by HFCVD is a diamond film.
XRD spectrum of nano-crystalline diamond micro-gear.
Raman spectroscopy was used to characterize the quality of the deposited films. A typical micro-Raman spectrum of a nano-crystalline diamond micro-gear produced by HFCVD is shown in Fig. 5. There are five bands between 1000 and 2000 cm−1. The sharp peak at 1332.8 cm−1 is the diamond characteristic peak. Meanwhile, weak shoulder peaks at 1143 cm−1 and 1496 cm−1 can be observed, which are related to the transpolyacetylene (TPA) segments at the grain boundaries of the nano-crystalline diamond. These can be a convenient probe for nano-crystalline diamond thin films20–22). The peak at 1568 cm−1 (G band) is related to the vibration of the sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, corresponding to the E2g mode of graphite. The peak at 1378 cm−1 (D band) is associated with the amorphous carbon, which represents the non-diamond phases23,24).
Typical micro-Raman spectrum of nano-crystalline diamond micro-gear by HFCVD.
The SEM and XRD observations indicate that the carbon film deposited on the silicon micro-gear structure by ICP etching technology is diamond film. This diamond film is a continuous dense smooth film, with no particles with a well-defined crystal shape, such as facetted pyramidal structures. In the Raman spectrum, there is not only the sharp diamond characteristic peak, but also weak shoulder peaks associated with the TPA segments at the grain boundaries of the nano-crystalline diamond. Hence, it is believed that a diamond film produced by HFCVD is a nano-crystalline diamond film. The nano-crystalline diamond duplex micro-gear was fabricated successfully by combining HFCVD and ICP etching.
The formation of a nano-crystalline diamond duplex micro-gear involves a three-step process. First, a silicon fine duplex micro-gear structure is formed by TMDE. It is believed that the micro structure and morphology could be adjusted by adjusting the experimental conditions, such as the morphology of the gears, the number of teeth, the root diameter, and the tip diameter. Then, a nano-crystalline diamond film is deposited on the micro-structured silicon molds by HFCVD. The characteristics of the diamond could again be controlled by adjusting the experimental parameters, such as the substrate temperature, the gas volume ratio of CH4 to H2, and the work pressure. In general, methane and hydrogen should be ionized and activated at high temperature in the HFCVD system. The hydrocarbon active groups can arrive at the surface of the Si substrate and form carbon clusters consisting of a mixture of Sp3 and Sp2 carbon structures. At the same time, the hydrogen atoms have a significant etching effect on the Sp2 carbon structure. The carbon atoms of the Sp3 structures should continue to aggregate and form a diamond crystal nucleus. Then, the diamond crystal nucleus grows as a result of the heterogeneous epitaxial growth on the silicon substrate. However, the nucleation density of diamond on the surface of the silicon substrate is relatively low and it is difficult to form a continuous diamond film. In the ultrasonic pre-grinding process using the nano-diamond suspension in acetone, scratches and defects may be formed on the silicon substrate surface, and some diamond nanoparticles in suspension could also adhere to the surface of the substrate. These scratches, defects, and diamond nanoparticles should increase the diamond nucleation density and form a continuous thin film. In our experiment, a continuous dense nano-crystalline diamond thin film was fabricated by adjusting the pre-grinding process with the nano-diamond suspension in acetone, the substrate temperature, and the mixture of gases. The resulting duplex micro-gear is thus formed from a continuous dense nano-crystalline diamond thin film.
Finally, the silicon mold is removed to reveal the nano-crystalline diamond duplex micro-gear. Compared with polycrystalline diamond films with a large grain size of several μm, the nano-crystalline diamond film has a very smooth surface, low friction coefficient, and superior transparency due to its lower Sp2 content. Therefore, the nano-crystalline diamond film could be applied to micro-machines, tribology, and biological applications. The characterization of nano-crystalline diamond is affected by the initial nucleation densities, re-nucleation rates, grain sizes, and the Sp2 carbon and hydrogen contents. In the future, the controlled fabrication of nano-crystalline diamond duplex micro-gears should be studied and micro-machine devices or bio-MEMS based on nano-crystalline diamond micro-gears should be developed.
A process has been developed for fabricating nano-crystalline diamond duplex micro-gears by applying HFCVD to a micro-structured silicon mold using ICP etching. The micro-gear consists of two gears. The top gear has 14 teeth with root and tip diameters of about 1.14 mm and 1.52 mm, respectively. The bottom gear has 20 teeth with root and tip diameters of about 1.75 mm and 2.20 mm, respectively. The total thickness of the micro-gear is about 20 μm. A layer-by-layer structure can be observed across the thickness of the micro-gear. The nano-crystalline diamond duplex micro-gear is produced by HFCVD and replicates the silicon microstructure etched by TMDE. This method involves a three-step process. First, the silicon fine duplex micro-gear structure is etched by TMDE. Then, a nano-crystalline diamond film is deposited on the silicon mold by HFCVD. Finally, the silicon mold is removed to obtain the nano-crystalline diamond duplex micro-gear. This CVD diamond micro-gear could be used in micro-machines, MEMS, tribology, and biological applications. In the future, a controlled fabrication process for nano-crystalline diamond duplex micro-gears should be developed.
This research is sponsored by the National Natural Science Foundation of China (No. 51005117, 51075211), Natural Science Fund project in JiangSu, China (No.BK2012530) and Qing Lan Project of JiangSu colleges and universities.