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Investigations on etching resistance of undoped and boron doped polycrystalline diamond films by oxygen plasma etching
Vacuum 128 (2016) 80e84
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Investigations on etching resistance of undoped and boron doped polycrystalline diamond films by oxygen plasma etching Dan Liu a, Li Gou a, b, *, Junjie Xu a, Kangning Gao a, Xing Kang a a b
College of Materials Science and Engineering, Sichuan University, Chengdu, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 3 March 2016 Accepted 14 March 2016 Available online 15 March 2016
The reactive ion etching (RIE) technique was used to etch the undoped and boron-doped diamond (BDD) polycrystalline films using oxygen plasma. The effect of boron within the BDD coatings on the morphology was investigated. BDD films exhibited much superior etching resistance than the undoped diamond films, wherein the (111) planes of BDD films were more etching resistant than (100) planes due to much higher boron concentration. However, this is in contradiction to undoped diamond films whose (111) planes were etched more quickly. The results would help to better design a particular and efficient etching method for undoped and BDD films to get a well-patterned microstructures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Diamond film Boron-doped diamond Reactive ion etching Oxygen plasma
Patterning of diamond surfaces by dry etching is a critical step for the fabrication of many diamond-based electronic devices and sensors [1e3]. Reactive ion etching (RIE) of CVD diamond film using pure O2-based plasma has the inherent advantages of compatibility with lithographic techniques and the ability to produce moderately high etching rate, therefore, it was reported to be the most suitable technique for CVD diamond patterning [4e6]. Diamond doped with boron results in electrically conductive boron-doped diamond (BDD), which is widely used as implantable microelectrode arrays for collecting and monitoring electrical signals [7e10], and wherein microfabrication technology such as RIE has also drawn considerable interest in the field of BDD film patterning. However, in previous RIE works [3,11e15], most studies pay close attention to etching mechanism of undoped diamond with oxygen plasma or the influence of gas pressure and microwave excitation power on the etching rate. O. Dorsch [16] studied reactive ion etching of undoped and boron doped polycrystalline diamond films to form columnar structures, and he has addressed the influence of doping concentration on the formation of columns. Foundational studies of the different etching characteristics of undoped and BDD films would benefit us to understand the
* Corresponding author. College of Materials Science and Engineering, Sichuan University, Chengdu, China. E-mail address: [email protected] (L. Gou). http://dx.doi.org/10.1016/j.vacuum.2016.03.012 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
significant role of boron in the RIE process, and better design a particular and efficient etching method for these films to get a wellpatterned microstructures. In the present study, the effect of boron on the etching performances and the etching mechanisms for undoped and BDD films were proposed. The undoped and BDD films were grown on intrinsic Si substrates using the microwave plasma chemical vapor deposition (MPCVD) technique. The film deposition was performed at the following parameters: total gas pressure was 4.6 kPa, microwave power 1400 W, 0.56% of methane diluted in hydrogen, temperature ~950 C and growth time 6 h. In the preparation, boron-doping was achieved by adding trimethylboron (TMB) to the gas mixture and the B/C ratio was 8600 ppm in the gas phase. The prepared BDD films had a metallic-like resistivity of 2.993 103 U cm measured by Hall effect measurements. After film deposition, these CVD films were simultaneously etched in a bell-jar type microwave plasma assisted system for periods of 40, 60, and 90 min. The oxygen contained was excited by a 2.45 GHz microwave plasma. The etching plasma was sustained with a microwave power of 700 W, a process pressure of 800e900 Pa, and the O2 flow rate of 50 sccm. The diamond character of BDD film was confirmed by Raman spectroscopy (LabRAM HR, France). Thermogravimetric analysis (TGA) together with differential scanning calorimetry (DSC) (NETZSCH STA 449C, Germany) under flowing air was performed on the undoped and BDD samples (before plasma etching) to investigate and compare the oxidation resistant properties,
D. Liu et al. / Vacuum 128 (2016) 80e84
samples of ~0.09 cm2 were oxidized at temperature rate of 10 K/ min to 1200 C. The etched surface morphology changes were observed by scanning electron microscopy (SEM, Hitachi S-4800, Japan). Fig. 1 displays the Raman spectrum of the undoped and BDD films. Compared to undoped diamond films whose classical diamond peak was around 1333 cm1, the characteristic peak for diamond (sp3 carbon) exhibits an asymmetric Fano-like lineshape, downshifts to lower wavenumbers (1301 cm1) and attenuates. Meanwhile, two broad Raman peaks at 469.1 and 1226.2 cm1 become noticeable for BDD films, and these two peaks are contributed to the locally disordered structures induced by the heavily boron doping [17,18]. Fig. 2 shows TG-DSC curves of undoped and BDD films which were simultaneously oxidized in the same condition. As shown in Fig. 2a, the undoped diamond films begin to loss mass when temperature is below 800 C, and the mass loss platform between 780 C and 880 C is extremely steep. During the mass loss process, chemically activated oxygen particles would firstly convert the diamond into graphitic structure, and then the graphite-like carbons reacted with oxygen in air to release the volatile CO or CO2 gases. The DSC curve of the undoped diamond film has a sharp peak at 885.2 C, which is corresponded to the point of maximum slope of mass loss region in the TG curve. When temperature goes up to 1200 C, the percentage of total mass loss is 4.86%. Compared with Fig. 2a, the onset temperature of BDD film is approximately 830 C. The mass loss platform is extremely flat and the percentage of mass loss is just 0.94%. Results above demonstrate that the thermal stability or oxidation resistance of boron-doped diamond is much superior to that of undoped diamond. Fig. 3aed shows SEM images of undoped diamond film etched by oxygen plasma with increasing time. As shown in Fig. 3a and the magnified image in Fig. 3e, after being etched for 20 min, some shallow trenches appear on (111) faces of diamond grains and grain boundaries, however, (100) faces are not eroded. The sides of the shallow trenches within (111) faces are found to be parallel to their crystal edges. As the etching time increased to 40 min, numerous deepening etch pits spread over the entire (111) faces, along with the etched crystal edges of (100) textured surfaces, as we can seen from Fig. 3b. When etching for 60 and 90 min, as shown in Fig. 3c and d, {111} faceted large crystallites are severely damaged and collapsed into a multitude of wreckage of small grains. Meanwhile, some deeper etch pits gradually appear at the corner or center of the (100) faces. From the observation of surface morphology of etched (111) and (100) faces, we can find that the oxidation
81
resistance of (100) faces is far beyond the (111) faces. As is well known [19,20], the etching characteristics of diamond films in oxygen plasma had found to proceed via a step mechanism, that is, the desorption of original terminated hydrogen from the diamond surface and oxygen atoms saturate the dangling bonds instead, then the highly reactive oxygen atoms chemically react with the diamond film, and finally the oxidative products (CO or CO2 gases) would escape from the system. In this point, it could be suggested that the different etching behaviors of different diamond crystallographic planes in one film is a result of their different surface conditions, namely the number of dangling bonds on the surfaces which is proportional to the plane atomic density of some certain crystallographic planes. Considering diamond is the facecentered cubic crystal structure and its lattice parameter (a) as 0.3567 nm, the plane density of some certain crystallographic planes can be computed through the relationship: plane density ¼ n/A, where n is the number of atoms associated with the planes, A is the area of the planes. Thus, the plane density of {111} and {100} facets can be calculated as fellows:
.pffiffiffi. 3 2 a2 pffiffiffi. 2 ¼2 3 2 0:3567 109 ¼ 1:8152 1019 m2
f111g facets : 2
. 2 f100g facets : 2 a2 ¼ 2 0:3567 1019 ¼ 1:5720 1019 m2
(1)
(2)
Apparently, {111} facets are more reactive since more dangling bonds are revealed on their surfaces. Etching will, therefore, preferentially occur at (111) faces and the higher etching rate would be expected, while etching rate is considerably slower at (100) faces. Similar results was achieved by oxidative etching of (100) textured diamond by P. John [21] and F.K. de Theije [22], who showed that (111) faces etched in dry oxygen are morphologically rough and oxygen would destabilize the surfaces, while (100) faces are strongly stabilized by the adsorption of oxygen. Taking account of this mechanism, the sides of trenches should parallel to the 〈110〉 direction which has the highest linear atomic density as shown in Fig. 3e. The surface morphology of BDD films etched by oxygen plasma
Fig. 1. Raman spectrum of undoped (a) and boron-doped (b) diamond films, lex ¼ 632 nm.
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D. Liu et al. / Vacuum 128 (2016) 80e84
Fig. 2. TG-DSC curves of undoped (a) and boron-doped (b) diamond film.
Fig. 3. SEM images of undoped diamond surfaces etched by different times, (a) 20 min, (b) 40 min, (c) 60 min, (d) 90 min, (e) the magnified image of (a).
with increasing time are shown in Fig. 4. Untreated BDD film shows well-defined crystals with (100) preferred facets as seen in Fig. 4a. After being etched for 40, 60 and 90 min respectively, several big but shallow etch pits appear on the {100} facets while the other crystallographic planes are slightly etched and their crystal features remain relatively complete. In contradiction to the etching performance of undoped diamond film, BDD film has much less crystal damages even after a long etching period. This indicates that BDD film show much superior etching-resistant properties. We propose that the remarkable enhancement of the etching resistance of BDD films could be ascribed to the doped boron. As discussed above, the etching rate is directly determined by the capacity of oxygen absorption which is associated with the surface and internal structure of diamond films (i.e. the number of dangling bonds, interatomic bonding energy) to a large extent. The ideal surface structures of undoped and BDD film are illustrated schematically in Fig. 5. For the undoped diamond film (Fig. 5a), the
surface carbon atoms have four valence electrons, whereas only three of them participate in the covalent bonding with the inner carbon atoms, leaving one spare valence electron for each carbon atom which results in the appearance of dangling bonds on the outermost surface. However, the oxygen plasma contains a number of chemically reactive oxygen particles whose outermost valence shells are unfilled, in this condition, oxygen atoms are fairly easy to occupy the dangling bonds of carbon atoms and eventually react with them to produce CO or CO2. As a result, the undoped diamond film tends to be etched more easily. As far as BDD film concerned (Fig. 5b), once surface carbon atoms are replaced by boron atoms with three valence electrons, wherein each boron atom covalently bonds with three inner carbon atoms, that is, the states within the outermost or valence electron shell of boron are completely filled. In this point, the surface boron atoms will have no additional dangling bonds, therefore, the surface of BDD film is quite stable and virtually chemically unreactive. This implies that, at the early
D. Liu et al. / Vacuum 128 (2016) 80e84
83
Fig. 4. SEM images of boron doped diamond surface etched by different times, (a) 0 min, (b) 40 min, (c) 60 min, (d) 90 min.
Fig. 5. Schematic diagrams of the ideal surface structure of undoped (a) and boron-doped (b) diamond film.
stage of etching, removal of diamond from the BDD surface is much more difficult than that of undoped diamond film. Oxygen plasma etching of diamond film not only undergoes the absorption of oxygen on the surface but also the breakage of the internal covalent bonds to form CO or CO2 gases. The absorption of oxygen atom may determine the ease or complexity of the removal of surface carbon atoms, but nonetheless the total etching rate depends on the inner structural carbon atoms. In pure diamond films, the predominate bonding are CeC bonds whose bond strength is 349 kJ/mol [22], however, for BDD films, much higher bond strength for BeC bonds leads to an increase of activation energy needed for the breakage of surface structures. Consequently, after the surface carbon atoms are etched, the etching rate of the revealed inner carbon atoms for BDD film will be much slower than that of pure diamond as shown in Fig. 4. It is clear that the different surface morphologies of the etched {100} and {111} facets in Fig. 4 present the formation of shallow
etch pits on {100} facets while only rounded edges and corners on {111} facets. The difference of etching rate between {100} and {111} facets could be closely associated with the boron concentration within the grains. T. Kolber [23] reported that, for BDD films, the boron concentration in {111} growth sectors was about 5 times higher than that in {100} growth sectors, and this would provide an explanation of the superior etching resistance of (111) planes. In conclusion, the difference of surface damages generated by pure O2 RIE of undoped and BDD film was analyzed, and the effects of boron within the BDD layers on the etch performances were investigated. The much superior etching resistance of BDD film can be ascribed to the existence of boron within the film, wherein the (111) planes were more etching resistant than (100) planes due to much higher boron concentration. However, this is in contradiction to undoped diamond films whose (111) planes can be etched more quickly. Thus, due to the remarkable effect of boron on the etching resistance of BDD films, high conductive BDD films which have
84
D. Liu et al. / Vacuum 128 (2016) 80e84
enormous applications in electronic devices would require a more appropriate etching method to get an effective patterning. Acknowledgments The research work was supported by the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices of the University of Electronic Science and Technology of China. No. KFJJ201313. References [1] C. Pietzka, et al., Surface modification of single-crystal boron-doped diamond electrodes for low background current, Diam. Relat. Mater. 18 (2009) 816e819. [2] Anna L. Colley, et al., Examination of the spatially heterogeneous electroactivity of boron-doped diamond microarray electrodes, Anal. Chem. 78 (2006) 2539e2548. [3] Tibor Izak, et al., Comparative study on dry etching of polycrystalline diamond thin films, Vacuum 86 (2012) 799e802. [4] S. Sandhu, et al., Reactive ion etching of diamond, Appl. Phys. Lett. 55 (1989) 437e438. [5] W.J. Zhang, et al., Structuring single- and nano-crystalline diamond cones, Diam. Relat. Mater. 13 (2004) 1037e1043. [6] Y.S. Zou, et al., Fabrication of diamond nanocones and nanowhiskers by biasassisted plasma etching, Diam. Relat. Mater. 16 (2007) 1208e1212. bert, et al., Boron doped diamond biotechnology: from sensors to neu[7] C. He rointerfaces, Faraday Discuss. 172 (2014) 47e59. [8] V. Maybeck, et al., Boron-doped nanocrystalline diamond microelectrode arrays monitor cardiac action potentials, Adv. Healthc. Mater. 3 (2014) 283e289. ment He bert, et al., Microfabrication, characterization and in vivo MRI [9] Cle compatibility of diamond microelectrodes array for neural interfacing, Mater. Sci. Eng. C 46 (2015) 25e31.
[10] Raphael Kiran, et al., Multichannel boron doped nanocrystalline diamond ultramicroelectrode arrays: design, fabrication and characterization, Sensors 12 (2012) 7669e7681. [11] I. Bello, et al., Effects at reactive ion etching of CVD diamond, Thin Solid Films 368 (2000) 222e226. [12] X.F. Zheng, et al., Investigation on the etching of thick diamond film and etching as a pretreatment for mechanical polishing, Diam. Relat. Mater. 16 (2007) 1500e1509. [13] Srinivasu Kunuku, et al., Investigations on diamond nanostructuring of different morphologies by the reactive-ion etching process and their potential applications, ACS Appl. Mater. Inter. 5 (2013) 7439e7449. [14] G.F. Ding, et al., Micromachining of CVD diamond by RIE for MEMS applications, Diam. Relat. Mater. 14 (2005) 1543e1548. [15] A. Denisenko, et al., Surface damages in diamond by Ar/O2 plasma and their effect on the electrical and electrochemical characteristics of boron-doped layers, J. Appl. Phys. 108 (2010), 074901e1-7. [16] O. Dorsch, et al., Dry etching of undoped and boron doped polycrystalline diamond films, Diam. Relat. Mater. 4 (1995) 456e459. Zivcov , et al., Electrochemistry and in situ Raman spec[17] Zuzana Vl ckova a troelectrochemistry of low and high quality boron doped diamond layers in aqueous electrolyte solution, Electrochim. Acta 87 (2013) 518e525. [18] M. Bernard, et al., Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy, Diam. Relat. Mater. 13 (2004) 282e286. [19] P.L. Walker Jr., et al., Gas reactions of carbon, Adv. Catal. 11 (1959) 133e221. [20] Toshihiro Ando, et al., Vapor-phase oxidation of diamond surfaces in O2 studied by diffuse reflectance Fourier-transform infrared and temperatureprogrammed desorption spectroscopy, J. Chem. Soc. Faraday Trans. 89 (1993) 3635e3640. [21] P. John, et al., The oxidation of (100) textured diamond, Diam. Relat. Mater. 11 (2002) 861e866. [22] F.K. de Theije, et al., Oxidative etching of diamond, Diam. Relat. Mater. 9 (2000) 929e934. [23] T. Kolber, et al., Quantitative investigation of boron incorporation in polycrystalline CVD diamond films by SIMS, Fresen. J. Anal. Chem. 365 (1999) 636e641.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Investigations on etching resistance of undoped and boron doped polycrystalline diamond films by oxygen plasma etching Dan Liu a, Li Gou a, b, *, Junjie Xu a, Kangning Gao a, Xing Kang a a b
College of Materials Science and Engineering, Sichuan University, Chengdu, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2016 Received in revised form 3 March 2016 Accepted 14 March 2016 Available online 15 March 2016
The reactive ion etching (RIE) technique was used to etch the undoped and boron-doped diamond (BDD) polycrystalline films using oxygen plasma. The effect of boron within the BDD coatings on the morphology was investigated. BDD films exhibited much superior etching resistance than the undoped diamond films, wherein the (111) planes of BDD films were more etching resistant than (100) planes due to much higher boron concentration. However, this is in contradiction to undoped diamond films whose (111) planes were etched more quickly. The results would help to better design a particular and efficient etching method for undoped and BDD films to get a well-patterned microstructures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Diamond film Boron-doped diamond Reactive ion etching Oxygen plasma
Patterning of diamond surfaces by dry etching is a critical step for the fabrication of many diamond-based electronic devices and sensors [1e3]. Reactive ion etching (RIE) of CVD diamond film using pure O2-based plasma has the inherent advantages of compatibility with lithographic techniques and the ability to produce moderately high etching rate, therefore, it was reported to be the most suitable technique for CVD diamond patterning [4e6]. Diamond doped with boron results in electrically conductive boron-doped diamond (BDD), which is widely used as implantable microelectrode arrays for collecting and monitoring electrical signals [7e10], and wherein microfabrication technology such as RIE has also drawn considerable interest in the field of BDD film patterning. However, in previous RIE works [3,11e15], most studies pay close attention to etching mechanism of undoped diamond with oxygen plasma or the influence of gas pressure and microwave excitation power on the etching rate. O. Dorsch [16] studied reactive ion etching of undoped and boron doped polycrystalline diamond films to form columnar structures, and he has addressed the influence of doping concentration on the formation of columns. Foundational studies of the different etching characteristics of undoped and BDD films would benefit us to understand the
* Corresponding author. College of Materials Science and Engineering, Sichuan University, Chengdu, China. E-mail address: [email protected] (L. Gou). http://dx.doi.org/10.1016/j.vacuum.2016.03.012 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
significant role of boron in the RIE process, and better design a particular and efficient etching method for these films to get a wellpatterned microstructures. In the present study, the effect of boron on the etching performances and the etching mechanisms for undoped and BDD films were proposed. The undoped and BDD films were grown on intrinsic Si substrates using the microwave plasma chemical vapor deposition (MPCVD) technique. The film deposition was performed at the following parameters: total gas pressure was 4.6 kPa, microwave power 1400 W, 0.56% of methane diluted in hydrogen, temperature ~950 C and growth time 6 h. In the preparation, boron-doping was achieved by adding trimethylboron (TMB) to the gas mixture and the B/C ratio was 8600 ppm in the gas phase. The prepared BDD films had a metallic-like resistivity of 2.993 103 U cm measured by Hall effect measurements. After film deposition, these CVD films were simultaneously etched in a bell-jar type microwave plasma assisted system for periods of 40, 60, and 90 min. The oxygen contained was excited by a 2.45 GHz microwave plasma. The etching plasma was sustained with a microwave power of 700 W, a process pressure of 800e900 Pa, and the O2 flow rate of 50 sccm. The diamond character of BDD film was confirmed by Raman spectroscopy (LabRAM HR, France). Thermogravimetric analysis (TGA) together with differential scanning calorimetry (DSC) (NETZSCH STA 449C, Germany) under flowing air was performed on the undoped and BDD samples (before plasma etching) to investigate and compare the oxidation resistant properties,
D. Liu et al. / Vacuum 128 (2016) 80e84
samples of ~0.09 cm2 were oxidized at temperature rate of 10 K/ min to 1200 C. The etched surface morphology changes were observed by scanning electron microscopy (SEM, Hitachi S-4800, Japan). Fig. 1 displays the Raman spectrum of the undoped and BDD films. Compared to undoped diamond films whose classical diamond peak was around 1333 cm1, the characteristic peak for diamond (sp3 carbon) exhibits an asymmetric Fano-like lineshape, downshifts to lower wavenumbers (1301 cm1) and attenuates. Meanwhile, two broad Raman peaks at 469.1 and 1226.2 cm1 become noticeable for BDD films, and these two peaks are contributed to the locally disordered structures induced by the heavily boron doping [17,18]. Fig. 2 shows TG-DSC curves of undoped and BDD films which were simultaneously oxidized in the same condition. As shown in Fig. 2a, the undoped diamond films begin to loss mass when temperature is below 800 C, and the mass loss platform between 780 C and 880 C is extremely steep. During the mass loss process, chemically activated oxygen particles would firstly convert the diamond into graphitic structure, and then the graphite-like carbons reacted with oxygen in air to release the volatile CO or CO2 gases. The DSC curve of the undoped diamond film has a sharp peak at 885.2 C, which is corresponded to the point of maximum slope of mass loss region in the TG curve. When temperature goes up to 1200 C, the percentage of total mass loss is 4.86%. Compared with Fig. 2a, the onset temperature of BDD film is approximately 830 C. The mass loss platform is extremely flat and the percentage of mass loss is just 0.94%. Results above demonstrate that the thermal stability or oxidation resistance of boron-doped diamond is much superior to that of undoped diamond. Fig. 3aed shows SEM images of undoped diamond film etched by oxygen plasma with increasing time. As shown in Fig. 3a and the magnified image in Fig. 3e, after being etched for 20 min, some shallow trenches appear on (111) faces of diamond grains and grain boundaries, however, (100) faces are not eroded. The sides of the shallow trenches within (111) faces are found to be parallel to their crystal edges. As the etching time increased to 40 min, numerous deepening etch pits spread over the entire (111) faces, along with the etched crystal edges of (100) textured surfaces, as we can seen from Fig. 3b. When etching for 60 and 90 min, as shown in Fig. 3c and d, {111} faceted large crystallites are severely damaged and collapsed into a multitude of wreckage of small grains. Meanwhile, some deeper etch pits gradually appear at the corner or center of the (100) faces. From the observation of surface morphology of etched (111) and (100) faces, we can find that the oxidation
81
resistance of (100) faces is far beyond the (111) faces. As is well known [19,20], the etching characteristics of diamond films in oxygen plasma had found to proceed via a step mechanism, that is, the desorption of original terminated hydrogen from the diamond surface and oxygen atoms saturate the dangling bonds instead, then the highly reactive oxygen atoms chemically react with the diamond film, and finally the oxidative products (CO or CO2 gases) would escape from the system. In this point, it could be suggested that the different etching behaviors of different diamond crystallographic planes in one film is a result of their different surface conditions, namely the number of dangling bonds on the surfaces which is proportional to the plane atomic density of some certain crystallographic planes. Considering diamond is the facecentered cubic crystal structure and its lattice parameter (a) as 0.3567 nm, the plane density of some certain crystallographic planes can be computed through the relationship: plane density ¼ n/A, where n is the number of atoms associated with the planes, A is the area of the planes. Thus, the plane density of {111} and {100} facets can be calculated as fellows:
.pffiffiffi. 3 2 a2 pffiffiffi. 2 ¼2 3 2 0:3567 109 ¼ 1:8152 1019 m2
f111g facets : 2
. 2 f100g facets : 2 a2 ¼ 2 0:3567 1019 ¼ 1:5720 1019 m2
(1)
(2)
Apparently, {111} facets are more reactive since more dangling bonds are revealed on their surfaces. Etching will, therefore, preferentially occur at (111) faces and the higher etching rate would be expected, while etching rate is considerably slower at (100) faces. Similar results was achieved by oxidative etching of (100) textured diamond by P. John [21] and F.K. de Theije [22], who showed that (111) faces etched in dry oxygen are morphologically rough and oxygen would destabilize the surfaces, while (100) faces are strongly stabilized by the adsorption of oxygen. Taking account of this mechanism, the sides of trenches should parallel to the 〈110〉 direction which has the highest linear atomic density as shown in Fig. 3e. The surface morphology of BDD films etched by oxygen plasma
Fig. 1. Raman spectrum of undoped (a) and boron-doped (b) diamond films, lex ¼ 632 nm.
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D. Liu et al. / Vacuum 128 (2016) 80e84
Fig. 2. TG-DSC curves of undoped (a) and boron-doped (b) diamond film.
Fig. 3. SEM images of undoped diamond surfaces etched by different times, (a) 20 min, (b) 40 min, (c) 60 min, (d) 90 min, (e) the magnified image of (a).
with increasing time are shown in Fig. 4. Untreated BDD film shows well-defined crystals with (100) preferred facets as seen in Fig. 4a. After being etched for 40, 60 and 90 min respectively, several big but shallow etch pits appear on the {100} facets while the other crystallographic planes are slightly etched and their crystal features remain relatively complete. In contradiction to the etching performance of undoped diamond film, BDD film has much less crystal damages even after a long etching period. This indicates that BDD film show much superior etching-resistant properties. We propose that the remarkable enhancement of the etching resistance of BDD films could be ascribed to the doped boron. As discussed above, the etching rate is directly determined by the capacity of oxygen absorption which is associated with the surface and internal structure of diamond films (i.e. the number of dangling bonds, interatomic bonding energy) to a large extent. The ideal surface structures of undoped and BDD film are illustrated schematically in Fig. 5. For the undoped diamond film (Fig. 5a), the
surface carbon atoms have four valence electrons, whereas only three of them participate in the covalent bonding with the inner carbon atoms, leaving one spare valence electron for each carbon atom which results in the appearance of dangling bonds on the outermost surface. However, the oxygen plasma contains a number of chemically reactive oxygen particles whose outermost valence shells are unfilled, in this condition, oxygen atoms are fairly easy to occupy the dangling bonds of carbon atoms and eventually react with them to produce CO or CO2. As a result, the undoped diamond film tends to be etched more easily. As far as BDD film concerned (Fig. 5b), once surface carbon atoms are replaced by boron atoms with three valence electrons, wherein each boron atom covalently bonds with three inner carbon atoms, that is, the states within the outermost or valence electron shell of boron are completely filled. In this point, the surface boron atoms will have no additional dangling bonds, therefore, the surface of BDD film is quite stable and virtually chemically unreactive. This implies that, at the early
D. Liu et al. / Vacuum 128 (2016) 80e84
83
Fig. 4. SEM images of boron doped diamond surface etched by different times, (a) 0 min, (b) 40 min, (c) 60 min, (d) 90 min.
Fig. 5. Schematic diagrams of the ideal surface structure of undoped (a) and boron-doped (b) diamond film.
stage of etching, removal of diamond from the BDD surface is much more difficult than that of undoped diamond film. Oxygen plasma etching of diamond film not only undergoes the absorption of oxygen on the surface but also the breakage of the internal covalent bonds to form CO or CO2 gases. The absorption of oxygen atom may determine the ease or complexity of the removal of surface carbon atoms, but nonetheless the total etching rate depends on the inner structural carbon atoms. In pure diamond films, the predominate bonding are CeC bonds whose bond strength is 349 kJ/mol [22], however, for BDD films, much higher bond strength for BeC bonds leads to an increase of activation energy needed for the breakage of surface structures. Consequently, after the surface carbon atoms are etched, the etching rate of the revealed inner carbon atoms for BDD film will be much slower than that of pure diamond as shown in Fig. 4. It is clear that the different surface morphologies of the etched {100} and {111} facets in Fig. 4 present the formation of shallow
etch pits on {100} facets while only rounded edges and corners on {111} facets. The difference of etching rate between {100} and {111} facets could be closely associated with the boron concentration within the grains. T. Kolber [23] reported that, for BDD films, the boron concentration in {111} growth sectors was about 5 times higher than that in {100} growth sectors, and this would provide an explanation of the superior etching resistance of (111) planes. In conclusion, the difference of surface damages generated by pure O2 RIE of undoped and BDD film was analyzed, and the effects of boron within the BDD layers on the etch performances were investigated. The much superior etching resistance of BDD film can be ascribed to the existence of boron within the film, wherein the (111) planes were more etching resistant than (100) planes due to much higher boron concentration. However, this is in contradiction to undoped diamond films whose (111) planes can be etched more quickly. Thus, due to the remarkable effect of boron on the etching resistance of BDD films, high conductive BDD films which have
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