Surface texturing of silicon substrates is performed by femtosecond laser irradiation at high repetition rates. Various fabrication parameters are optimized, in order to achieve very high absorptance in the visible region from the micro-structured silicon wafers as compared to the unstructured ones. A 35-fold reduction of the processing time is demonstrated by increasing the laser repetition rate from 1 kHz to 200 kHz. Further scaling up to 1 MHz is proved with potential reduction of the processing time by a factor of 65. A figure of merit ξ is introduced for a quantitative guidance in the choice of fabrication parameters.
© 2013 OSA
Efficient light harvesting is a key aspect in several fields like photovoltaics and photodetectors. In such fields the material of choice is currently silicon, due to its abundance on Earth and to its high optical-electrical conversion efficiency in the visible spectral range. The quest for improved conversion efficiency is extremely important to increase the production of sustainable energy while reducing size and cost of a solar panel, or, in the case of photodetectors, to improve their sensitivity. A first step in increasing the device efficiency is achieved by improving the material absorption. Surface texturing is commonly used to enhance the light harvesting capability of silicon. This technique consists in the creation of micrometer-sized structures on the surface of silicon wafers. These structures are able to trap incident light until it is absorbed, reducing the optical power lost by reflection at the silicon – air interface [1,2]. Due to its very low reflectivity, surface-structured silicon is named black silicon; it has proved to increase the performance of photovoltaic cells and photodetectors [3,4] and it is already being commercialized .
Surface texturing techniques [1,6–10] can be roughly divided into two categories: parallel micro-structuring techniques and serial ones. Parallel techniques, like (a) photolithography, (b) wet chemical etching and (c) reactive ion etching, are characterized by a fabrication time that is not dependent on the dimension of the area to be micro-structured. They are therefore suitable for large area processing. On the other hand in serial techniques, like (d) mechanical texturing and (e) laser texturing, the fabrication time is linearly dependent on the dimension of the area to be processed, but they typically provide high conversion efficiencies. For these reason serial techniques are more appropriate for small devices requiring high performance (photodetectors, solar cells for portable devices or solar panels combined with solar concentrators).
- (a) Photolithographic texturing allows creating precise and well-defined textures on the surface of silicon, making it possible to achieve high efficiency in solar cells produced in the laboratory. Though a well-established technique, several fabrication steps are needed for this kind of processing, thus resulting in a certain complexity (mask fabrication, spin-coating, irradiation and development of photoresist, use of etchants to transfer the desired pattern onto the substrate) .
- (b) Wet chemical etching produces randomly distributed pyramids on the surface of the material by anisotropic etching. Results strongly depend on the crystallographic orientation of the substrate. Chemical texturing makes it difficult to reduce reflection under 10-15%  and it is moreover characterized by low reproducibility. Precise control of temperature and composition of the chemicals is required .
- (c) In reactive ion etching, wafers are placed inside a vacuum chamber and a gas, usually a mixture of SF6 and O2, is ionized by means of a strong electromagnetic RF field. The surface of the wafer is subjected to SF2/O2 plasma etching for a certain duration creating needle-like structures (diameter in the order of 100 nm and depth in the order of 500 nm) on the surface of the material, reducing reflectance with average values around 10%. With respect to chemical etching the loss of silicon material connected to texturing process is much lower .
- (d) Mechanical texturing could be performed by using a dicing saw and carving V-shaped grooves on the surface of silicon. This kind of micro-structuring cannot be applied to brittle and thin substrates .
- (e) Femtosecond laser micro-structuring produces self-assembled micro/nano-texturing on the silicon surface upon laser irradiation. A series of parallel and equally-spaced irradiation lines are scanned on the surface of a silicon substrate yielding an ordered grid of micro-structures of roughly conical shape and consistent size and spacing. No additional fabrication steps are needed. Laser texturing can be performed by using nano, pico and femtosecond laser pulses, but femtosecond laser texturing has advantages over the former as it avoids heat affected zones around the micromachined area , and it benefits from the fact that femtosecond laser systems are already in use for some production steps of solar cell manufacturing.
- Femtosecond laser processing can also be carried out in different background gases. In particular, when the fabrication is performed in the presence of a gas containing sulfur, the black silicon absorption spectrum is extended in the near infrared region .
Fabrication of black silicon by femtosecond laser micromachining has been demonstrated mainly using 1 kHz repetition rate pulses. It enables almost unitary absorption of the incident light but it requires long processing times. Very little work has been done on the fabrication of black silicon with high repetition rate laser pulses [19,20]. In this work we study the effect of high repetition rate femtosecond laser pulses on silicon surface-texturing in order to reduce the sample processing time. The scaling is non-trivial since high repetition rates may trigger thermal cumulative phenomena that could wash-out the surface structuring. The work is focused on the optimization of the morphological aspect, which is dependent on the several correlated irradiation parameters. To better understand the combined effect of these parameters an empirical figure of merit ξ is introduced, which helps to gain further insight in the laser-matter interaction and serves as a simple tool to quantitatively guide the irradiation parameter choice.
The possibility of extending the absorption spectral range of silicon through sulfur doping is not considered here, since the focus in on the texturing morphology. All the fabrications are indeed performed in ambient air conditions. Several parameters, such as laser fluence, sample processing speed, and distance between two scanned lines, were optimized for various repetition rates of the laser, ranging from 1 kHz to 1 MHz. The textured samples show an increase in absorptance from 60% (unstructured) to 95% (optimized micro-structured samples). The processing time is reduced by a factor of 35 by using 200 kHz instead of the typical 1 kHz repetition rate, and further scaling down can be foreseen by using pulses at 1 MHz.
A femtosecond laser micromachining system ‘FemtoLab’ (Altechna, Lithuania) was employed for the experiments. The custom made system consisted of a femtosecond laser based on Yb:KGW active medium (Pharos Laser, Light Conversion, Lithuania). The laser generates femtosecond laser pulses with a pulse duration of 250 fs and a maximum average power of 10 W at 1030 nm with variable pulse repetition rate up to 1 MHz. After passing through attenuators and polarization controls, laser light is focused by a microscope objective (20 × , 0.4 N.A.) and is delivered onto the sample substrate placed on a translation stage, Fiberglide - 3D stage (Aerotech, USA) with a resolution of 2 nm (see Fig. 1(a)). The laser beam was linearly polarized orthogonally with respect to the sample translation direction.
Single crystalline double polished silicon wafers were used and the sample processing was done in ambient conditions in the laboratory with low vacuum suction close to the sample (ULT, Germany). Square shaped regions of 7 × 7 mm2 were micro-structured by femtosecond laser irradiation. Morphological characterization of the micro-structured samples was carried out using scanning electron microscope (SEM) (JEOL, TouchScope JSM-6010LA). The SEM images were acquired with a sample holder tilted at 45° (in order to observe the shape and the height of the structures).
The total absorptance of the micro-structured samples was measured using a spectrophotometer (Perkin Elmer LAMBDA 1050 UV/Vis/NIR) and compared to unstructured silicon wafers. Total reflectance (R) and transmittance (T) measurements were obtained using an integrating sphere (the precision of the instrument is 2%). The total absorptance (A) was calculated using the formula A = 1 – (T + R). Since the measured T for our samples is close to zero in the visible spectral region, absorptance is calculated by the approximated formula A = 1 - R.
Since fabrication is performed in air, no significant changes in infrared absorption are produced , therefore the spectral range of the analysis is limited between 350 nm and 1000 nm.
2.1 Fabrication parameters
In femtosecond laser micro-structuring the main irradiation parameters which need to be optimized are: laser fluence and number of pulses interacting with the silicon surface in a given area. The laser fluence impinging on the sample surface was varied by changing the distance L between the focal plane of the microscope objective and the surface of the sample (see Fig. 1(b)). The optimal value of L was found by optically inspecting the ablation lines (see Figs. 1(c)-1(e)) and choosing the value that yields the best-defined micro-structures (Fig. 1(d)); the corresponding spot size on the sample surface was calculated by the knife-edge method (see Table 1). The number of pulses interacting with the silicon surface in a given area, for a fixed spot size, can be controlled by varying the laser repetition rate, by changing the translation speed along the single ablation line, and by regulating the overlap between two successive laser line scans.
Initially, the repetition rate was chosen to be 1 kHz (as most commonly reported in the literature) . A good processing window has been found at that repetition rate and then the process has been scaled at higher repetition rates up to 1 MHz. To achieve the desired black silicon performances at all repetition rates we had to vary almost all relevant parameters. In particular, the sample translation speed was varied from 0.1 mm/s to 150 mm/s; the fluence from 0.29 J/cm2 to 1.64 J/cm2; the interline spacing from 2 µm to 80 µm.
3. Results and discussion
3.1 Micromachining at 1 kHz repetition rate
Samples were micro-structured at a repetition rate of 1 kHz, using a pulse energy of 100 µJ on a beam waist radius of 44 µm (resulting in a fluence of 1.64 J/cm2) and interline spacing of 35 µm. Sample scanning speeds were varied from 0.1 mm/s to 150 mm/s.
The results obtained, in terms of morphology and absorptance, are consistent with the ones found in literature [1,23]. At very low sample translation speeds, (0.1 mm/s; see Fig. 2(a)) high material removal is predominant resulting in highly irregular structures. Higher values of scanning speed lead to the creation of well-developed micro-structures (sample processing speed between 0.35 mm/s and 1.4mm/s; see Figs. 2(b) and 2(c)). On the other hand, when sample translation speed is faster (see Fig. 2(d)) minimal interaction between laser pulses and irradiated region of the silicon substrate takes place.
The absorption spectra for the processed samples are measured and reported in Fig. 2(e). Absorptance increases from about 60% for unstructured silicon to 90-95% in the laser textured silicon. An increase in absorption is one of the preliminary steps in the quest for higher efficiency. Setting an acceptable absorptance threshold at 90% (the value achieved by the wet etching technique used in commercial solar cells ) we can identify the range of speeds providing an above threshold performance (0.35 mm/s - 1.4mm/s) for our set of irradiation parameter (see Table 1). We do not include in the optimal range the slowest translation speed (0.1 mm/s), although it provides a comparably high absorptance, since it leads to the removal of a large amount of silicon due to strong ablation, thus wasting a large quantity of material.
Morphologically the samples processed in the optimal range have micro-structures with peak heights ranging from 10 – 15 µm (at 0.35 mm/s - 0.7 mm/s) to 2 – 5 µm (at 1.4 mm/s). We can thus observe that good performance black silicon can be obtained for a rather large interval of texture amplitude. However, the average absorptance of textured samples rapidly drops as the dimensions of the micro-structures further reduce.
3.2 Micromachining at high repetition rate
In this section the effect of repetition rates higher than 1 kHz for femtosecond laser micro-structuring is discussed. At 10 kHz and 20 kHz the same values of fluence and interline spacing, identified at 1 kHz, were kept constant. In this condition we observed that if the repetition rate and the sample processing speed are increased by the same factor, the obtained samples show similar morphology (Figs. 2(b) and 2(c) and Figs. 3(a) and 3(b)) and absorption spectra (Fig. 3(c)).
In order to further up-scale the repetition rate (from 100 kHz to 1 MHz) we had to reduce the pulse energy. In fact, the average power was limited to an upper bound of 5 W by the damage threshold of the microscope objective. The progressive reduction of the pulse energy from 100 µJ to 5 µJ (at 1 MHz) caused all the other parameters to vary. At each repetition rate the fabrication parameters were empirically tuned in order to keep the same optimal absorptance obtained at 1 kHz. The combinations of irradiation parameters that were considered optimal at the different repetition rates are reported in Table 1.
SEM images of the micro-structures fabricated at repetition rates ranging from 100 kHz to 1 MHz are shown in Figs. 4(a)-4(d). The morphology of the structures varies appreciably. Nevertheless, the absorptance spectra are comparable (within a range of 2%) as shown in Fig. 4(e).
It is worth noting that heat accumulation is not observed even at the highest repetition rates of 500 kHz and 1 MHz. In fact, the observed morphology is comparable to that produced at low repetition rates (e.g. compare Fig. 3(b) – 20 kHz, 28 mm/s – with Figs. 4(c) and 4(d)), and no washing out of the micro-structures occurs due to thermal diffusion effects. We attribute the absence of heat accumulation, even at repetition rates where it is observed in glass , to the higher thermal conductivity of silicon.
3.3 Figure of merit of the micro-structuring process
Given the large amount of data gathered, we try to identify a figure of merit for the black silicon fabrication process with femtosecond lasers, capable of well-representing the observed features and to provide further insight and prediction capabilities.
It is widely accepted that a key parameter in ablation processes is the laser fluence, defined as the laser pulse energy divided by the spot area. Experience with black silicon evidences that the creation of the characteristic surface texturing is also dependent on the amount of pulses delivered in the same spot area . Following this idea we can define a figure of merit ξ [J/cm2]Table 1 and K, discussed in the following, is a further factor that has been introduced to better fit this formula to the experimental results. The ξ parameter is therefore the product between the laser fluence of the single pulse (E/πw2), the number of pulses delivered in the spot area in a single line irradiation (2wR/v), and the number of adjacent lines hitting the same spot area (2w/s). The figure of merit can therefore be considered as the cumulative fluence delivered to the sample. The present formula is capable of representing the behavior we have observed up to 20 kHz: keeping all other parameters constant, we achieve the same results (and thus the same figure of merit) if repetition rate R and translation speed are scaled proportionally. However, applying this formula at higher repetition rates we observe an increasing discrepancy between the figure of merit and the observed morphology: it seems that, at higher repetition rates, increasingly higher fluences are required to obtain the same result. A possible explanation of this phenomenon is the plume shielding effect . In fact, at increasing repetition rates the temporal distance between subsequent pulses reduces and the debris plume, generated by ablation from a pulse, is capable of progressively attenuating the impinging energy of the subsequent pulse. Hence, the K factor, introduced in Eq. (1), should not be a constant, but a function implementing the above described effect in the figure of merit.
Since, as discussed in Sect. 3.1, the morphology of the texturing has an important role in the black silicon performance, the chosen function K = K(R) should improve the correlation between the figure of merit and the observed morphology at the different repetition rates. To this aim we measured the periodicity of the texturing in all our experiments (which can be considered linearly proportional with respect to the peak heights of the created spikes ) and found that a good function well-representing our results isEq. (1) becomesFigure 5 shows the linear dependence of the figure of merit defined in Eq. (3) with the measured texturing periodicity at all the explored repetition rates.
In order to define the optimal range of the figure of merit we used the data at 1 kHz. Figure 6 shows the figure of merit calculated at all the translation speeds explored at that repetition rate and it correlates this parameter with the corresponding measured absorption (averaged over the 350-1000 nm spectrum).
Setting the desired performance of black silicon above 90% absorptance (red shaded area in Fig. 6) defines the optimal range for the figure of merit ξ (blue shaded area in Fig. 6) – the point at 0.1 mm/s is excluded from the optimal range for the reasons discussed in Sect. 3.1.
Having set the optimal range for ξ at 1 kHz we verified that also at higher repetition rates all the experimental conditions, yielding absorptances above 90%, provided values of ξ fitting in the same optimal range (Fig. 7). We can observe that, although irradiation parameters may vary significantly (even orders of magnitude – see Table 1), the value of the identified figure of merit ξ is a good indication of the morphology and hence absorptance of the produced black silicon. Given the complexity of the fabrication process and the large number of parameters involved, we believe that this is an important result that introduces a quantitative tool in the otherwise difficult quest for the optimal processing window. In particular, a figure of merit is useful when exploring new regions of the irradiation parameter space, as for example when scaling the laser repetition rate as in the present case.
Other parameters, that have not been explicitly included in the present discussion, may influence the micro-structuring process; an example is the pulse duration that has been kept constant at 250 fs in this work. However, femtosecond laser sources may have different pulse durations and black silicon may even be produced with pico and nanosecond pulses. A more general figure of merit should also include this parameter and work is in progress to obtain data at different fabrication conditions to further refine the figure of merit definition.
3.4 Scalability of micro-fabrication processing time
Our motivation for scaling the repetition rate in the manufacturing process was to investigate the possibility of reducing the processing time of black silicon. In the previous sections we have shown that it is possible to increase the repetition rate up to 1 MHz while keeping the absorptance above 90%. In this section we will discuss how this reflects in the processing time. As a test bench we considered the time needed to surface structure silicon wafer areas of 10 × 10 cm2. Figure 8 shows the fabrication time as a function of repetition rate.
In order to provide a fair comparison, among the combinations of irradiation parameters in the optimal range (see Fig. 7) we have chosen those providing a comparable average absorptance of about 95%.
The fabrication time significantly decreases from 110 hours at 1 kHz to 3 hours at 200 kHz and then increases for 500 kHz and 1 MHz as shown in Fig. 8. This increase is due to our specific fabrication conditions. In fact, as discussed in Sect. 3.2, we are limited by the focusing objective to an average power of 5 W and this forced us to reduce the pulse energy at increasing repetition rates. As can be inferred from Eq. (3), to achieve the same figure of merit we had to vary different parameters and in particular reduce the translation speed and the interline spacing (see also Table 1), thus significantly undermining the overall processing time. However, there are different focusing optics that can withstand much higher average powers. Therefore it is of interest to investigate what we would have obtained if we did not have this limitation. The empty triangles in Fig. 8 represent the extrapolation of the fabrication time at 500 kHz and 1 MHz assuming that the pulse energy and interline spacing are the same as those used at 200 kHz. The processing speed is scaled with the repetition rate in order to keep the figure of merit within the identified optimal range, according to Eq. (3); a sample translation speed of 110 mm/s and an average power of 25 W would be required at 1 MHz, which are within the reach of state of the art equipment. In this way we estimate that the fabrication time can be further reduced to about 1 hour and 40 minutes with a potential reduction in processing time by a factor of 65 with respect to 1 kHz processing.
It should be noted that, according to Eq. (3), further up-scaling of the repetition rate will provide increasingly smaller advantages in terms of processing speed. In fact, the figure of merit ξ becomes almost independent of R at high repetition rates and thus no gain in processing speed can be achieved if ξ has to be kept in the optimal range.
Femtosecond laser micro-structuring of silicon surfaces is performed at varying repetition rates, from 1 kHz up to 1 MHz. Morphological characterization of the micro-structured samples is carried out using optical and scanning electron microscopy. The total absorptance of the micro-structured samples is measured and compared to unstructured silicon wafers. The textured samples show an improved absorptance from 60% (unstructured) to 95% due to a strong reduction in surface reflection of light. A figure of merit ξ for the microstructuring process is introduced, which strongly correlates with the surface morphology and the absorptance value. An optimal range of this parameter is identified, which will be useful as a quantitative guidance in the choice of the irradiation parameters for the manufacturing of high performance black silicon.
Up-scaling the repetition rate of the femtosecond laser proved to be a viable approach to down-scale the processing time of black silicon, while keeping the same level of absorptance. A reduction of a factor of 35 working at 200 kHz and a further potential reduction up to a factor of 65 at 1 MHz can be foreseen with respect to 1 kHz processing. A step-improvement in down scaling the fabrication time is envisaged with the next generation of high-energy femtosecond lasers by employing techniques like cylindrical lens focusing and process parallelization .
We thank Dr. Stefano Perissinotto for the help with absorptance measurements.
References and links
1. C. H. Crouch, J. E. Carey, M. Shen, E. Mazur, and F. Y. Génin, “Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation,” Appl. Phys., A Mater. Sci. Process. 79(7), 1635–1641 (2004). [CrossRef]
2. M. S. Kang, S. J. Joo, W. Bahng, J. H. Lee, N. K. Kim, and S. M. Koo, “Anti-reflective nano- and micro-structures on 4H-SiC for photodiodes,” Nanoscale Res. Lett. 6(1), 236 (2011). [CrossRef] [PubMed]
3. J. Oh, H. C. Yuan, and H. M. Branz, “An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures,” Nat. Nanotechnol. 7(11), 743–748 (2012). [CrossRef] [PubMed]
4. Z. Li, B. K. Nayak, V. V. Iyengar, D. McIntosh, Q. Zhou, M. C. Gupta, and J. C. Campbell, “Laser-textured silicon photodiode with broadband spectral response,” Appl. Opt. 50(17), 2508–2511 (2011). [CrossRef] [PubMed]
6. H. Hauser, B. Michl, V. Kübler, S. Schwarzkopf, C. Müller, M. Hermle, and B. Bläsi, “Nanoimprint lithography for honeycomb texturing of multicrystalline silicon,” Energy Procedia 8, 648–653 (2011). [CrossRef]
7. P. Papet, O. Nichiporuk, A. Kaminski, Y. Rozier, J. Kraiem, J. F. Lelievre, A. Chaumartin, A. Fave, and M. Lemiti, “Pyramidal texturing of silicon solar cell with TMAH chemical anisotropic etching,” Sol. Energy Mater. Sol. Cells 90(15), 2319–2328 (2006). [CrossRef]
8. K. Kim, S. K. Dhungel, S. Jung, D. Mangalaraj, and J. Yi, “Texturing of large area multi-crystalline silicon wafers through different chemical approaches for solar cell fabrication,” Sol. Energy Mater. Sol. Cells 92(8), 960–968 (2008). [CrossRef]
9. D. S. Ruby, S. H. Zaidi, S. Narayanan, B. M. Damiani, and A. Rohatgi, “Rie-texturing of multicrystalline silicon solar cells,” Sol. Energy Mater. Sol. Cells 74(1-4), 133–137 (2002). [CrossRef]
10. C. Zechner, G. Hahn, W. Jooss, M. Wibral, B. Bitnar, S. Keller, M. Spiegel, P. Fath, G. Willeke, and E. Bucher, “Systematic study towards high efficiency multicrystalline silicon solar cells with mechanical surface texturization,” Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference (1997), pp. 243–246. [CrossRef]
11. E. Manea, E. Budianu, M. Purica, D. Cristea, I. Cernica, R. Muller, and V. Moagar Poladian, “Optimization of front surface texturing processes for high-efficiency silicon solar cells,” Sol. Energy Mater. Sol. Cells 87(1-4), 423–431 (2005). [CrossRef]
12. B. K. Nayak, V. V. Iyengar, and M. C. Gupta, “Efficient light trapping in silicon solar cells by ultrafast-laser-induced self-assembled micro/nano structures,” Prog. Photovolt. Res. Appl. 19(6), 631–639 (2011). [CrossRef]
13. L. A. Dobrzanski and A. Drygala, “Surface texturing of multicrystalline silicon solar cells,” J. Achiev. Mater. Manuf. Eng. 31, 77–82 (2008).
14. J. Yoo, G. Yu, and J. Yi, “Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE),” Sol. Energy Mater. Sol. Cells 95(1), 2–6 (2011). [CrossRef]
15. J. S. Yoo, I. O. Parm, U. Gangopadhyay, K. Kim, S. K. Dhungel, D. Mangalaraj, and J. Yi, “Black silicon layer formation for application in solar cells,” Sol. Energy Mater. Sol. Cells 90(18-19), 3085–3093 (2006). [CrossRef]
16. J. Yoo, G. Yu, and J. Yi, “Black surface structures for crystalline silicon solar cells,” Mater. Sci. Eng. B 159–160, 333–337 (2009). [CrossRef]
17. J. T. Zhu, G. Yin, M. Zhao, D. Y. Chen, and L. Zhao, “Evolution of silicon surface microstructures by picosecond and femtosecond laser irradiations,” Appl. Surf. Sci. 245(1-4), 102–108 (2005). [CrossRef]
18. M. A. Sheehy, L. Winston, J. E. Carey, C. M. Friend, and E. Mazur, “Role of the background gas in the morphology and optical properties of laser-microstructured silicon,” Chem. Mater. 17(14), 3582–3586 (2005). [CrossRef]
19. T. Chen, J. Si, X. Hou, S. Kanehira, K. Miura, and K. Hirao, “Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses,” J. Appl. Phys. 110(7), 073106 (2011). [CrossRef]
20. V. Schütz, A. Horn, and U. Stute, “High-throughput process parallelization for laser surface modification on Si-Solar cells: determination of the process window,” Proc. SPIE 8244, 82440X, 82440X-7 (2012). [CrossRef]
21. R. Younkin, J. E. Carey, E. Mazur, J. A. Levinson, and C. M. Friend, “Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses,” J. Appl. Phys. 93(5), 2626–2629 (2003). [CrossRef]
22. M. Halbwax, T. Sarnet, Ph. Delaporte, M. Sentis, H. Etienne, F. Torregrosa, V. Vervisch, I. Perichaud, and S. Martinuzzi, “Micro and nano-structuration of silicon by femtosecond laser: application to silicon photovoltaic cells fabrication,” Thin Solid Films 516(20), 6791–6795 (2008). [CrossRef]
23. B. K. Nayak and M. C. Gupta, “Ultrafast laser-induced self-organized conical micro/nano surface structures and their origin,” Opt. Lasers Eng. 48(10), 966–973 (2010). [CrossRef]
24. S. M. Eaton, H. Zhang, M. L. Ng, J. Li, W. J. Chen, S. Ho, and P. R. Herman, “Transition from thermal diffusion to heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides,” Opt. Express 16(13), 9443–9458 (2008). [CrossRef] [PubMed]
25. J. Schille, R. Ebert, U. Loeschner, P. Regenfuss, T. Suess, and H. Exner, “Micro structuring with highly repetitive ultra short laser pulses,” in Proceedings of LPM, The 9th International Symposium on Laser Precision Microfabrication (2008).
26. T. H. Her, R. J. Finlay, C. Wu, and E. Mazur, “Femtosecond laser-induced formation of spikes on silicon,” Appl. Phys., A Mater. Sci. Process. 70(4), 383–385 (2000). [CrossRef]