Seed-Free Growth of Diamond Patterns on Silicon Predefined by

Article pubs.acs.org/crystal

Seed-Free Growth of Diamond Patterns on Silicon Predefined by Femtosecond Laser Direct Writing Mengmeng Wang,†,‡ Yun Shen Zhou,† Zhi Qiang Xie,† Yang Gao,† Xiang Nan He,† Lan Jiang,‡ and Yong Feng Lu*,† †

Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, United States School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China

‡

ABSTRACT: Seed-free growth of diamond patterns on Si surfaces was realized by surface patterning via femtosecond laser direct writing followed by laser-assisted combustion synthesis in open air. Prepatterned Si surfaces provide active nucleation sites for diamond growth. The dimension and shape of the diamond patterns can be precisely controlled at micrometer scales. Field-enhanced thermionic emission current was measured to monitor diamond nucleation and growth process in real time. The diamond nucleation density and quality depend strongly on the surface roughness of the prepatterned Si patterns. A higher surface roughness leads to more nucleation sites and higher nucleation density. However, high local temperature at peaks due to a large exposed surface to the flame on an excessively rough surface degrades the diamond quality. The easy fabrication of diamond patterns on nondiamond surfaces suggests potential applications such as cutting tools, protective coatings, heat sinks, etc.

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INTRODUCTION

In this study, we achieved seed-free growth of diamond patterns on prepatterned Si substrates using laser-assisted combustion synthesis. Fs laser direct-writing (FsLDW) was used for patterning Si substrates. Diamond patterns of microand macroscales were deposited on the prepatterned Si substrates without diamond seeding. The influence of the surface roughness after fs laser processing on diamond growth was investigated in terms of nucleation density and diamond quality. Diamond patterns of arbitrary geometries and dimensions can be easily programmed and fabricated using this technique. Successful application of FsLDW in seed-free diamond growth provides a convenient approach for fabricating diamond patterns on nondiamond substrates.

Because of its outstanding physical and chemical properties, diamond is considered as an ideal material for mechanical and electric applications under harsh environments, such as high temperature, high voltage, radiation, etc.1−3 In particular, diamond-based microelectromechanical system (MEMS) applications such as cantilevers,4 packaging,5 probes,6 nanomechanical resonators,7 and radio frequency (RF) devices,8 have been extensively investigated and demonstrated. Realization of various diamond-based devices depends on the precise fabrication of diamond patterns. However, the ultrahigh hardness and brittleness of diamond make the diamond patterning a tough task. Currently available diamond patterning techniques include dry etching of single diamond crystals using high energy beams,9 area selective diamond deposition,10−20 patterning synthetic diamond nanogrit onto polished substrates using the inkjet printer technology,21 and microcontact printing (μCP) of monodiamond nanoparticles.22 One key challenge to diamond patterning is selective diamond seeding on nondiamond substrates for the fabrication of diamond-based devices. Nevertheless, current techniques generally require expensive masks and diamond seeds. Compared with long-pulsed lasers, femtosecond (fs) lasers exhibit obvious advantages in extremely high precision, minimized thermal affected zone, nonlinear effects for materials processing due to their ultrashort pulse duration and ultrahigh peak power.23,24 The nonthermal mechanism of material removal results in a rough morphology composed of microstructures, which can provide active nucleation sites for diamond deposition. © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Substrate Preparation. P-type Si(100) substrates with a dimension of 10 × 10 × 0.6 mm3 were rinsed using acetone, alcohol, and deionized water sequentially, and then mounted on a programmable motorized stage. A commercial 1 kHz amplified Ti:sapphire laser system (Legend F, Coherent Inc., central wavelength of 800 nm, pulse duration of 120 fs) was used as the irradiation source for patterning the Si substrates, as shown in Figure 1a. The fs laser beam passed through an optical path consisting of an attenuator, a half-wave plate, and a polarizer in order to vary the incident energy. After that, the laser beam was focused on the Si surface with a beam spot size of 25 μm in diameter. The laser output power was fixed at 20 mW, and the fluence was calculated to be 4.1 J/cm2, much higher than the ablation threshold of Si.25 Different patterns were fabricated by Received: October 2, 2012 Revised: December 12, 2012 Published: January 2, 2013 716

dx.doi.org/10.1021/cg301440k | Cryst. Growth Des. 2013, 13, 716−722

Crystal Growth & Design

Article

Figure 1. Schematic experimental setups of (a) FsLDW and (b) CO2 laser-assisted combustion diamond synthesis.

Figure 2. SEM images of (a) an array of holes on a Si substrate fabricated by FsLDW, (b) a zoomed view of a hole, (c) the morphology of the roughed edges of the hole shown in (b), (d) a diamond ring after 1 h deposition, (e) an array of diamond grains after 3 h deposition, and (f) a zoomed view of a diamond grain shown in (e). maintained at 760−780 °C. The distance between the substrate and inner flame tip was kept to be around 0.5 mm. Characterizations. Surface morphology of the patterned Si substrates and deposited diamond patterns were characterized using a scanning electron microscope (SEM; XL-30, Philips Electronics). Diamond quality was evaluated using a Raman spectrometer (inVia H 18415, Renishaw) with a 514.5 nm argon ion laser as the excitation source. A 3000 line/mm grating was used to obtain the highest spectral resolution of 0.31 cm−1. Surface roughness of the prepatterned Si substrates was studied using a stylus profiler (XP-2, Ambios Technology).

programing the movement of the motorized stage. A scanning speed of 1.00 mm/s was used for patterning Si substrates. To investigate the diamond deposition on surfaces of different roughness, scanning speeds of 0.25, 1.00, 2.00, and 3.00 mm/s were used. The laser patterned Si substrates were ultrasonically cleaned to remove Si debris. Prior to diamond deposition, surface morphology and roughness of the patterns were characterized using a scanning electron microscope (SEM) and a stylus profiler. Laser-Assisted Combustion Synthesis of Diamond. A laserassisted combustion synthetic method was used for diamond deposition.26−28 As shown in Figure 1b, a gas mixture containing O2, C2H2, and C2H4 with a volume ratio of 2:1:1 was used as the precursor for diamond deposition. The wavelength of the CO2 laser was fixed at 10.532 μm to resonantly excite the CH2 wagging mode of the C2H4 molecules.26 Substrate temperature was monitored using a noncontact pyrometer (Omega Engineering Inc., OS3752) and

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RESULTS AND DISCUSSION Formation of Diamond Patterns. To investigate the flexibility in diamond pattern formation, Si substrates were prepatterned with structures of different geometries and dimensions via FsLDW and used for diamond deposition.

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Figure 3. SEM images of (a) a grid pattern on a Si substrate fabricated by FsLDW, and (b) polycrystalline diamond grids after 55 min deposition.

Figure 4. SEM images of (a) square patterns on a Si substrate fabricated by FsLDW (inset: the zoomed view of fs laser processed Si surface), and (b) diamond square patterns after 1 h deposition (inset: the zoomed view of the diamond film).

promoted nucleation and diamond growth on the roughened surfaces are ascribed to following reasons: (a) presence of abundant dangling bonds aiding the chemisorption of reactive species and formation of diamond nuclei;30 (b) increased reactant flux at the protrusions and edges;31 and (c) minimized interfacial energy by forming diamond nuclei at the prominent tips.32 Typical Raman spectra of the diamond patterns are shown in Figure 5. A sharp and strong diamond peak at 1332 cm−1 in the

Figure 2a shows an array of holes on a Si substrate fabricated via FsLDW. Figure 2b shows a zoomed image of a single hole, and Figure 2c shows the rough edges exhibiting microstructures such as pits, edges, and protrusions compared with the primitive Si surface. Figure 2d shows an annular diamond ring after 1 h diamond deposition, illustrating that the diamond nucleation starts at the microstructures along the rough edges of the holes. Figure 2e shows an array of diamond grains after 3 h deposition corresponding to the hole array shown in Figure 2a. Figure 2f is a zoomed view of a diamond grain, which is formed by piling up deposited diamond crystals along the hole edges. Figure 3a shows a grid pattern with a line pitch of 150 μm fabricated by FsLDW. Figure 3b shows the polycrystalline diamond grid patterns after 55 min deposition. Figure 4a shows 500 × 500 μm2 square patterns on a Si substrate fabricated by FsLDW with a line pitch of 25 μm. The inset shows a zoomed view of the fs laser roughened Si surface. Figure 4b shows square diamond patterns after 1 h deposition. The inset shows a zoomed image of the diamond patterns, suggesting the formation of dense diamond films on the laser processed regions. In Figures 2−4, it is evidently illustrated that diamond was selectively deposited on the patterned areas fabricated by FsLDW. For diamond nucleation on nondiamond substrates, both sufficient carbon saturation on substrate surfaces and the presence of high-energy reactive sites are required.29 Compared with primitive Si surfaces, the laser processed regions show roughened surface morphology containing small pits, protrusions, and sharp edges, which serve as the reactive sites for diamond nucleation and subsequent crystal growth. The

Figure 5. Raman spectra of the diamond patterns. 718



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Figure 6. SEM images (45° viewing angle) of fs laser processed samples I−IV in (a−d), and (e−h) show corresponding zoomed images of the samples I−IV, respectively.

Raman spectra is observed. The diamond quality was quantified by calculating the diamond quality parameter33,34 Q [514 nm] =

ID

(ID + 233I )

100(%)

C

(1)

where ID is the intensity of the diamond peak, and IC is the total intensity of nondiamond carbon peaks. The calculated diamond quality parameters are all above 99.9%, demonstrating high quality of the diamond patterns. Dependence of Diamond Nucleation Density on Surface Roughness. Since the laser processed Si surfaces significantly stimulate the formation of diamond nuclei, the dependence of the nucleation density on the surface roughness was investigated. The same 1 × 1 mm2 square patterns with a line pitch of 25 μm were fabricated on four Si substrates at different laser scanning speeds of 0.25, 1.00, 2.00, and 3.00 mm/s, respectively, corresponding to samples I, II, III, and IV, as shown in Figure 6. The low-magnification images of Figure 6a−d at 45° viewing angle show obvious peaks and valleys. The high-magnification images of Figure 6e−h show surface microstructures such as protrusions, edges, voids, pits, and ripples. From sample I to IV, the processed surfaces demonstrate reduced surface roughness with the increased fs laser processing speed. The surface cross-section profiles of the samples were measured using a stylus profiler as shown in Figure 7, demonstrating different surface roughness. Corresponding two-dimensional roughness parameters, Ra (arithmetic average of absolute value), are 1.08849, 0.47646, 0.24814, and 0.17601 μm for samples I, II, III, and IV, respectively. Field-enhanced thermionic emission current from the deposited diamond was measured to monitor the diamond nucleation and subsequent growth processes on the fs laser processed Si substrates in real time.35 The bias voltage on the Si substrates was −137 V with respect to the flame torch. The current between the substrates and the torch was measured by an ampere meter and recorded every 0.5 min. Figure 8 shows the current evolutions with respect to the deposition time. The zero emission current periods (the flat segments of the curves) indicate diamond-free surfaces, representing incubation periods. The incubation periods are 4.5, 5.5, 6.5, and 7.5 min for samples I−IV, respectively, indicating an inverse relationship between surface roughness and diamond nucleation time. This

Figure 7. Surface cross-section profiles of four samples fabricated at different fs laser scanning speeds, showing different surface roughness.

Figure 8. Current evolutions of samples I−IV regarding deposition time.

is in agreement with the theoretical calculations by Louchev and co-workers, who reported that diamond nuclei first appeared on the tips of 3-D protrusions (points where three 719



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Figure 9. Panels (a−d) show low-magnification images of the deposited diamond after 15 min growth on samples I−IV, respectively. Panels (e−h) show corresponding high-magnification images.

that the nucleation density is proportional to the surface roughness (Ra). Quality parameters of the diamond grains deposited on the laser processed regions were calculated based on the Raman spectra, as shown in Figure 10, and compared in Table 1. A

or more surfaces intersect), then 2-D edges (locations where two surfaces intersect), and finally on plane surfaces.31 The nucleation site preference is ascribed to a superposition of diffusion fluxes of carbon feedstock that is able to decrease the carbonization time on the prominent tips. Therefore, the pretreated rough surfaces of sharper protrusions result in a faster diamond nuclei formation and shorter incubation period.31 Surface roughness shows a statistical result of the protrusions and edges. Accordingly, the processed Si surfaces with of higher roughness suggest sharper protrusions (shown in Figure 6), which can provoke a shorter incubation period. Subsequent current increase is ascribed to the newly nucleated diamond particles and increased surface coverage by diamond. The emission currents reach a maximum value of around 2.3 mA after 36.5 and 50 min deposition for samples I and II, respectively, suggesting the formation of dense diamond films with a whole coverage on the prepatterned surfaces. However, the emission currents of samples III and IV increase steadily at much lower increase rates compared with samples I and II, even after 60 min deposition. The low increase rate is ascribed to the low nucleation densities, resulting in scattered diamond islands. Diamond nucleation density was calculated by counting the number of diamond grains per square millimeter after a fixed deposition time. To ensure sufficient formation of diamond nuclei, avoid overlapped large diamond grains, and prevent secondary crystallization of the diamond crystals, samples after 15 min diamond deposition were collected for studying diamond nucleation densities on substrates of different roughness. Figure 9a−d shows the SEM images of deposited diamond on four different substrates after 15 min deposition for nucleation density counting and calculation. Compared with Figure 6a−d, diamond was randomly deposited on the processed Si surface including the valleys and peaks. However, most of diamond crystals were deposited at peaks on sample I. Figure 9e−h shows zoomed views of the deposited diamond. Sample I shows obvious secondary growth and ball-like diamond grains, while samples II−IV show well-faceted diamond crystals. For the sample I (Ra of 1.08849 μm), a high nucleation density around 6.3 × 103 mm−2 was obtained with an almost full surface coverage and obvious particle overlaps. The nucleation densities for the samples II, III, and IV (Ra of 0.47646, 0.24814, and 0.17601 μm, respectively) are 3.1 × 103, 2.0 × 103, and 1.3 × 103 mm−2, respectively, illustrating

Figure 10. Raman spectra of diamond deposited on samples I−IV.

broad G-band and diamond quality parameter of 99.745% are observed for the diamond grains deposited on sample I (Ra of 1.08849 μm). For the diamond grains deposited on the other three samples, no G-band is observed in Raman spectra. The degraded diamond quality is ascribed to the high local temperature at peaks due to large surface exposures to the flame. The rough surface in sample-I poses obvious peak-tovalley height distance shown in Figure 7. Therefore, the exposure surfaces to the flame at peaks are much larger than those at valleys, leading to a higher local temperature at the peaks. As reported in previous studies, high-quality diamond is obtained within a narrow temperature window of 760−780 °C, and graphitic carbon is deposited at a temperature above the window.26,36 The higher local temperature at peaks also promotes fast secondary growth and ball-like diamond formation, which occupies surrounding space shown in Figure 9e. As a result, the carbon flux supply for the continuous growth of diamond deposited at valleys is suppressed. That is 720



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Table 1. The Investigated Samples’ Parameters Calculated from Raman Spectra and Eq 1 sample

fs laser processing speed v (mm/s)

surface roughness Ra (μm)

nucleation density Nd (103 mm−2)

diamond peak position (cm−1)

diamond peak fwhm (cm−1)

intensity ratio ID/IG

diamond quality parameter Q (%)

I II III IV

0.25 1.00 2.00 3.00

1.08849 0.47646 0.24814 0.17601

6.3 3.1 2.0 1.3

1333.5 1333.4 1334.2 1333.4

5.0 2.8 3.1 2.5

1.6 +∞ +∞ +∞

99.745 99.991 99.981 99.964

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why most of the deposited diamond grains are located at the peaks shown in Figure 9a. With a low surface roughness, height differences for samples II−IV between peaks and valleys are reduced, resulting in similar exposed surfaces and carbon flux supplies. Therefore, the local temperatures at peaks and valleys are almost the same and diamond nuclei grow simultaneously. Consequently, high quality diamond crystals are deposited. However, low diamond nucleation densities on smoother surfaces, such as samples III and IV (Ra of 0.24814 and 0.17601 μm separately), result in incomplete diamond coverage even after long time deposition due to insufficient diamond nucleation sites. Therefore, laser processed regions with proper roughness, for example, Ra of 0.47646 μm, leads to a full coverage of high-quality diamond films.

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CONCLUSIONS Seed-free growth of diamond patterns on Si substrates was realized by patterning Si surfaces using FsLDW followed by laser-assisted combustion synthesis in open air. The roughened Si surfaces caused by laser processing provide active sites for the formation of diamond nuclei and crystal growth. The diamond nucleation and subsequent growth processes were investigated by observing field-enhanced thermionic emission current from the deposited diamond. The diamond nucleation density is proportional to the surface roughness of the laser processed regions. A rougher surface leads to a higher diamond nucleation density and a larger surface coverage. However, excessive surface roughness also results in the increased graphitic carbon deposition and degraded diamond quality due to the temperature increase caused by large exposed surface to the flame at peaks. The seed-free formation of diamond patterns on nondiamond surfaces provides a convenient approach for fabricating diamond-based structures.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 402-472-4732. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. D. R. Alexander in the Department of Electrical Engineering at the University of Nebraska-Lincoln (UNL) for his technical support in SEM characterization. This work was financially supported by National Science Foundation (CMMI 0852729 and CMMI 1129613), Office of Naval Research (ONR) (Grant N0001409-1-0943), National Basic Research Program of China (973 Program) (Grant 2011CB013000), and the National Natural Science Foundation of China (NSFC) (Grants 90923039 and 51025521).

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Seed-Free Growth of Diamond Patterns on Silicon Predefined by

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