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J. Phys. Chem. C 2007, 111, 7058-7062
Maskless Plasma Etching of Diamond Cones: The Role of CH4 Gas and Enhanced Field Emission Property Q. Wang,† J. J. Li,*,† Y. L. Li,† Z. L. Wang,† C. Z. Gu,† and Z. Cui‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China, and Rutherford Appleton Laboratory, Chilton, Didcot, Oxon Ox11 0QX, United Kingdom ReceiVed: December 30, 2006; In Final Form: March 6, 2007
Diamond cone arrays were formed by plasma etching of diamond films in a hot filament chemical vapor deposition (HFCVD) system. The role of CH4 in the formation of diamond cone arrays was investigated. It was found that addition of CH4 to H2 plasma could enhance the plasma intensity and hence the etching efficiency to improve the formation of diamond cones with high aspect ratio and controllable density. The microscopic measurement of the as-formed diamond cones shows that they have an inner core of polycrystalline structure and an outer layer of amorphous carbon. These as-formed diamond cones have exhibited field emission current density of 1 µA/cm2 at threshold field of 5 V/µm. For an applied field of 12 V/µm, the emission current density can rapidly reach as high as 560 µA/cm2. The property of high field electron emission is attributed to the high aspect ratio of as-formed diamond cones and the appropriate cone density as well as the outer amorphous carbon layer which acts as a pathway for electron-hopping conduction during field electron emission.
1. Introduction A field electron emitter with high aspect ratio is known to be highly advantageous because of its high field enhancement factor. Compared with the cylindrical structure, the conical structure with high aspect ratio may be a more ideal geometric configuration for electron field emission because of its good mechanical and thermal stability. Meanwhile, it has been reported that the conical field emitter can be made with high aspect ratio while still maintaining good mechanical and thermal stability.1 Conical field emitters made from materials such as Mo, Si, and C have been widely reported,2-4 in particular the conical emitters made of diamond which has much lower work function for field electron emission. However, diamond field emitters often suffer from poor electrical conductivity and lack of sharpness, which results in low field enhancement.5,6 It has been a challenge to make sharp diamond field emitters with good conductivity. Some fabrication techniques have been reported to form diamond cone arrays in recent years. One of the techniques is to grow diamond into conical holes that are formed by focused ion beam sputter etching in silicon.7 The sharpness and aspect ratio of diamond cones solely depend on the geometry of the holes, which can be accurately controlled in the silicon etching process. However, most of the techniques reported so far are of high fabrication cost and low throughput. Recently, a maskless hydrogen plasma etching method has attracted considerable interest because uniform diamond cones over a large area can be formed easily on diamond film at low cost,8,9 though the formation mechanism is still not fully understood. In this paper, a maskless plasma etching method is reported for the formation of high aspect ratio diamond cones, and in * To whom correspondence should be addressed. Tel: +86-10-82648198. Fax: +86-10-82648198. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Rutherford Appleton Laboratory.
particular, CH4 is added to the etching gas H2 to improve the plasma etching intensity and efficiency. The role of CH4 in the formation of diamond cones is discussed. Also, the surface morphology, microstructure, and chemical bonding state of asformed diamond cones are characterized, and then field emission properties of as-formed diamond cones with different cone densities are investigated in detail. 2. Experimental Section The polycrystalline diamond film of over 10 µm thickness was grown on an n-type silicon wafer by hot filament chemical vapor deposition (HFCVD) method with the following experimental conditions: substrate temperature of 900 °C, CH4/H2 flow ratio of 1.5:98.5, and growth duration of 10 h, just as discussed elsewhere.7 Etching of diamond cone arrays was also done in the same HFCVD system, where CH4/H2 plasma was generated. The diamond-coated silicon substrate was biased at -350 V so that a negative electrical field was generated for ion sputtering of the diamond film. The substrate was heated by a Ta filament, and substrate temperature was kept at about 900 °C and was monitored by a thermocouple. The chamber pressure was maintained at 25 Torr whereas CH4 and H2 gas mixture (with flow ratio of CH4/H2 ) 1.5:100) and pure H2 were used to investigate the effect of CH4 during plasma etching process. For CH4/H2 plasma etching, the etching time was varied from 0.5 to 2 h to control the density of diamond cones. The morphology, microstructure, component, and chemical bonding state of as-formed cone arrays were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy, respectively. The electron field emission of as-formed diamond cones was measured in a parallel-plate configuration with an anode-tosample spacing of 300 µm (using glass fibers as spacers for all testing). The diamond cone array sample of 0.4 cm2 was the
10.1021/jp069042l CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007
Maskless Plasma Etching of Diamond Cones
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7059
Figure 1. SEM images of before and after plasma etching diamond film: (a) unetched diamond films; (b) diamond cone arrays formed by H2 plasma etching; diamond cone arrays formed by CH4/H2 plasma etching with different cone densities (c) 2 × 108, (d) 1 × 107, and (e) 2 × 106 cones/cm2.
TABLE 1: Cone Height, Angle, and Aspect Ratio of As-Etched Samples Corresponding to Figure 1b-1e sample label
cone height
cone angle
aspect ratio
Figure 1b Figure 1c Figure 1d Figure 1e
∼250 nm ∼1.5 µm ∼6 µm ∼3 µm
∼25° ∼27° ∼23° ∼30°
∼50 ∼300 ∼1200 ∼600
cathode and an indium-tin-oxide (ITO) plate was the anode to collect the emitted electrons. The pressure in the measuring chamber was maintained at 10-6 Pa during measurement. The sample was heated to degas its surface prior to field emission measurements. While increasing the bias voltage between the electrodes, the current flowing to the ITO electrode was measured using an ammeter. The electrode configuration has been tested to ensure that the measured current is the field emission current rather than Frenkel-Poole leakage current. 3. Results and Discussion 3.1. Surface Morphologies and Microstructures of Diamond Cones. Figure 1a∼1e shows the surface morphologies of unetched diamond, H2 plasma-etched diamond films, and CH4/H2 plasma-etched diamond films under different etching durations. As seen in Figure 1, there are significant changes of surface morphology before and after plasma etching of the diamond films. In all the cases, these as-formed diamond cone arrays show good uniformity in the height, cone angle, and aspect ratio. Further, the cone height, angle, and aspect ratio for the samples shown in Figure 1b-1e are given in Table 1. It is found that the size of diamond cone formed by CH4/H2 plasma etching is much bigger than that formed by pure H2 plasma etching, which implies that the addition of CH4 to H2 plasma has caused much intensified etching of diamond film under the same etching condition. The density of diamond cone formed by pure H2 plasma etching is estimated to be larger than 1 × 109 cones/cm2. Furthermore, prolonged etching in CH4/H2 plasma from 0.5 to 1.5 h and up to 2 h has significantly reduced the density of as-formed diamond cone, which is estimated to be about 2 × 108, 1 × 107, and 2 × 106 cones/cm2, respectively. The aspect ratio of as-formed diamond cones is clearly related
Figure 2. TEM image (a), HRTEM image (b), and EDX spectrum (c) of a single diamond cone.
to the cone density. The diamond cones with the highest aspect ratio are found in the medium density of 1 × 107 cones/cm2. The diamond cone formed by CH4/H2 plasma etching is not pure diamond but has a very thin amorphous carbon coating as the outer layer and an inner core of polycrystalline diamond, as shown by the TEM image in Figure 2. Figure 2b shows that the main lattice space is about 0.22 nm, which is related to the diamond (111).10 The fine tip radius r of this as-formed diamond cone is about 5 nm as shown in Figure 2a. EDX characterization
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Figure 3. Raman spectra of unetched diamond film (a), diamond cone formed by H plasma etching (b), and diamond cone formed by CH4/ H2 plasma etching (c).
is conducted during the SEM characterization, and the spectrum of the as-formed diamond cone is shown in Figure 2c, from which only C, O, and Si elements are detected, verifying that no metal or other foreign particles are dwelt on the cone top as masking. The existence of amorphous carbon outer layer is further confirmed by comparison of Raman spectra of unetched diamond film, etched diamond cone in H2 plasma, and etched diamond cone in CH4/H2 plasma, as shown in Figure 3a, 3b, and 3c, respectively. It can be seen that Raman spectra of unetched and H2 plasma-etched diamond films both have a strong peak at ∼1332 cm-1 with a weak broad peak centered at ∼1420 cm-1, indicting diamond with a few amorphous phases. When CH4 is added to H2 plasma, Raman spectrum displays a remarkable change, as shown in Figure 3c. Apart from the sharp peak at ∼1332 cm-1 with a reduced peak intensity, there are other peaks at ∼1140, ∼1350, ∼1480, and ∼1586 cm-1. The peaks at ∼1350 and ∼1580 cm-1 are the D band and G band of amorphous carbon phase.11 The peaks at ∼1140 and ∼1480 cm-1, combined with the peak at ∼1332 cm-1, indicate the formation of nanodiamond crystalline after CH4/H2 plasma etching.12 The amorphous carbon phases are believed to originate from the bombardment of energetic ions on the original diamond surface, and the formation of nanodiamond phase may be due to the dimensional reduction of original diamond crystallites into nanometer scale. Therefore, the addition of CH4 not only influences significantly the morphologies such as the size, density, and aspect ratio of diamond cones but also changes the chemical bonding state and component phases of diamond cone arrays. 3.2. Formation Mechanism of Diamond Cones. The formation of diamond cones in our experiment is a maskless plasma etching process. Such a unique feature has been found from the formation of silicon cones in the HFCVD plasma etching.13 Uniformly distributed conelike nanoscale silicon tips were formed on a smooth silicon surface without any forms of masks. The mechanism of cone formation was speculated as the combination of silicon atom redeposition and ion-sputtering etching.13 Later experiments also found that much sharper silicon cones could be formed in the same plasma on a porous silicon surface.14 For the formation of diamond cones in the HFCVD plasma, a self-organized selective sputtering mechanism has
Figure 4. Schematic image illustrating the formation of diamond cone because of selective etching process.
been proposed.15 It can be briefly illustrated in Figure 4. For the initiation of cone formation, the surface morphology of asformed diamond film has played a key role. Figure 4a shows the initial rough surface of diamond film. There are randomly distributed hillocks all over the surface, as the SEM image shows in Figure 1a. Under the ion bombardment in the plasma, the removal of material is faster at the side of a hillock than at the top, because the ion-sputtering yield is always higher at an oblique incident angle.16 As a result, the initial surface morphology is enhanced as shown in Figure 4b because of the difference in sputtering rate (υ). With the advance of further ion sputtering, cone geometry is gradually formed and sharpened. Longer sputtering will cause part removal of the diamond film, resulting in reduced cone density, as those SEM images show in Figure 1d and 1e. The addition of methane gas (CH4) in the plasma can greatly enhance the sputter-etching process because the methylic ions have larger mean free path length, and therefore, higher mean energy for sputtering. Figure 5 shows the dependence of ion mean free path (λ) and mean ion energy (E h ) on the gas pressure at a discharge current j of 100 mA, calculated from the previously reported model.15 It indicates that methylic ions always have a longer ion mean free path and higher mean ion energy than that of H+ ions at different gas pressures. Therefore, methylic ions will be dominant in the etching of diamond even though their densities are much less than that of H+ ions in the plasma. Meanwhile, the deposition of amorphous carbon occurs simultaneously with the etching process, and these processes finally form a very thin coating on the outer layer of diamond cone. 3.3. Field Emission Property of Diamond Cones. Field emission properties of unetched diamond films and as-formed diamond cone array with different densities were measured. The current density versus electric field (J-E) and FowlerNordheim (F-N) plots of all samples are shown in Figure 6a and 6b, respectively. In Figure 6a, it is obvious that as-formed diamond cone arrays formed by CH4 and H2 plasma etching have much enhanced field emission compared with unetched diamond films, because of the high field enhancement factor
Maskless Plasma Etching of Diamond Cones
Figure 5. The dependence of mean ion free path (a) and mean ion energy (b) on the gas pressure at the discharge current of 100 mA.
Figure 6. J-E characteristics (a) and F-N characteristics (b) for unetched diamond film, diamond film etched in H2 plasma, and asformed diamond cone arrays etched in CH4/H2 plasma with different cone densities.
of sharp diamond cones and amorphous carbon outer layer in the diamond cones. The cone arrays formed by pure H2 plasma etching also show enhanced emission compared with the unetched diamond film, but they show a poorer field emission than the cones formed by CH4/H2 plasma etching, owing to the lower aspect ratio structure, higher cone density, and lack of outer amorphous carbon coating. The amorphous carbon layer can increase the surface electrical conductivity and provide a pathway for electron-hopping conduction through the defect
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7061 bands within the wide band gap of local sp3 sites so as to enhance the field emission ability of as-formed diamond cone. In general, the FN plots are near linear, indicating that electron field emission from these as-formed diamond cone arrays basically follows the FN law. However, some FN plots of asformed diamond cone arrays are somewhat nonlinear, as shown in Figure 6b. This nonlinear FN plot is often observed in some carbon-based materials. Usually, this plot has to be fitted with two straight lines instead of one, each of which can derive its own field enhancement factor (β-value), suggesting that two different types of emission sites are contributing to the total emission current. In Figure 6b, the lowest density (2 × 106 cones/cm2) plot shows evident differences from others, which is due to a slower increase in emission current than others at an applied field of less than 10 V/µm and then a quicker increase in emission current than others after 10 V/µm. In addition, the plot of H plasma etching shows a flat trend at a low applied field, which can also be attributed to a rather slow increase of emission current with applied field at the first stage of field emission. It can be seen from Figure 6a that the field emission of diamond cones depends on its cone density. For the diamond cone array with medium density (about 1 × 107 cones/cm2), the threshold field is about 5 V/µm at the emission current density of 1 µA/cm2. The emission current density can reach about 560 µA/cm2 at applied field of 12.5 V/µm. Diamond cone arrays with higher density (about 2 × 108 cones/cm2) or lower density (about 2 × 106 cones/cm2) have the threshold field of about 6 and 8 V/µm, respectively. Both are higher than that of the cone arrays with medium density (about 1 × 107 cones/ cm2). When an electric field of 12.5 V/µm is applied, both have emission current densities of only 140 and 180 µA/cm2, respectively, which are much less than that of the cone arrays with medium density (about 1 × 107 cones/cm2). For the diamond cone arrays formed by pure H2 plasma etching, the threshold field is about 9.2 V/µm while the current density reaches 112 µA/cm2 at applied field of 12.5 V/µm. For unetched diamond film, the threshold field is about 11 V/µm, and the emission current density reaches 60 µA/cm2 at 12.5 V/µm. The slower current increasing trend of diamond film before etching is similar to cone arrays with density of 2 × 108 cones/cm and the cone arrays formed by pure H2 plasma etching with cone density larger than 1 × 109 cones/cm2, and the quicker current increasing trend of cone arrays with density of 2 × 106 cones/ cm2 is similar to that of 1 × 107 cones/cm. The above trends are closely related to the field-shielding effect existing in the emitters with a larger emitting site density and field enhancement factor determined by cone geometrical shape. We found that these phenomena can be mainly attributed to the appropriate balance between field-shielding effect and number of field emission sites. Too high density of cones will lead to a strong field-shielding effect,17 which reduces the surface field of each individual cone, resulting in low emission current. Too low density of cones will have less number of emission sites. The total emission current will reduce, despite the less field-shielding effect. There is an optimum cone density at which the maximum emission current can be obtained. In addition, when the cone density of one sample is close to that of another, the field enhancement factor will determine the field emission property of the sample, that is, a higher field enhancement factor leads to a better field emission. The as-formed diamond cones also demonstrated stable field emission. Figure 7 shows the variation of emission current density over 40 min period for the sample with cone density of
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Wang et al. than that formed by pure H2 plasma etching because of a more proper cone density and a higher field enhancement factor. The threshold field for electron emission depends on the density of diamond cones because of the field-shielding effect. The optimum cone density is found to be 1 × 107 cones/cm2, at which the as-formed diamond cones exhibited field emission current density of 1 µA/cm2 at threshold field of 5 V/µm, and it rapidly reached as high as 560 µA/cm2 for an applied field of 12 V/µm. Acknowledgment. This work was supported by the National High Technology Development program of China (Grant No.2002AA325090), National Natural Science Foundation of China (Grant No. 50472071 and 60671048), and National Center for Nanoscience and Technology, China.
Figure 7. Emission stability of as-formed diamond cone arrays.
1 × 107 cones/cm2 at a constant electric field of 12.5 V/µm. No significant degradation in the current density was observed over the measurement duration. 4. Conclusions Diamond cones with different aspect ratios and cone densities were formed by HFCVD plasma etching with CH4/H2 gas mixture. The role of the methane gas (CH4) in the formation of diamond cones and in the enhancement of field electron emission from the as-formed diamond cones was investigated. It is found that the addition of CH4 can effectively enhance plasma etching, which is mainly attributed to higher mean energy of methylic ions than that of H+ ions. The initial surface morphology of diamond film plays a key role in subsequent formation of cone shapes during ion-sputtering etching. The aspect ratio and density of diamond cone can be controlled by the duration of ion sputtering. Another advantage of adding methane gas to the plasma is the formation of a thin amorphous carbon coating over the diamond cones, which is favorable to the enhancement of field emission by providing a pathway for electron-hopping conduction through the defect bands. Compared to unetched diamond film, as-formed diamond cones have demonstrated enhanced field electron emission with good emission stability, and moreover, the diamond cones formed by CH4/H2 plasma etching have a better field emission property
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