Femtosecond laser-induced formation of spikes on silicon

Transcription

1 Femtosecond laser-induced formation of spikes on silicon Tsing-Hua Her, * Richard J. Finlay, Claudia Wu, and Eric Mazur Division of Engineering and Applied Sciences and Department of Physics, Harvard University 9 Oxford St., Cambridge, MA (Electronic mail: mazur@physics.harvard.edu, Fax: 1-617/ , Tel: 1-617/ ) * Current address: Lawrence Berkeley National Laboratory, ms 2-300, 1 Cyclotron Rd., Berkeley, CA (Electronic mail: thher@lbl.gov, Fax: 1-5/ , Tel: 1-5/ ) Current address: Northern Telecom, PO Box 3511 Stn. C Ottawa, ON, K1Y 4H7 Canada 1

2 Abstract We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500 femtosecond laser pulses in SF 6. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. PACS:61.80.B;79.20.D;82.65.J 2

3 Various micron-sized surface features have been observed on silicon surfaces after ion beam or pulsed laser irradiation. For example, mounds and columns are formed when silicon is sputtered by energetic Ar ions.[1,2] When irradiated by nanosecond laser pulses with fluence close to the melting threshold, silicon surfaces develop ripples with submicron periodicity.[3] Laser pulses with even higher fluence can induce laser ablation of silicon, leaving a crater on the surface surrounded by irregular cones protruding above the surface.[4] Previously we reported that silicon surfaces spontaneously develop arrays of conical spikes when repeatedly irradiated with high fluence femtosecond laser pulses in SF 6 or Cl 2.[5] Similar structures have recently been reported with nanosecond laser irradiation.[6] The spikes we observe have a high aspect ratio, a quasi-periodic spatial distribution, and are only formed in an atmosphere containing a halogen. Besides their scientific interest, the silicon spikes are of interest because of their potential applications as light absorbers[7] for solar cells and photodetectors, and as microneedles for transdermal drug delivery.[8] In this paper, we characterize the surface morphology and study the effect of laser fluence and pulse duration on the spikes. 1 Experimental We carried out our experiments on n-type (arsenic doped) Si(0) wafers with resistivity less than 5 5 Ωm. Each wafer is cleaned with trichloroethylene, rinsed in acetone, and then rinsed in methanol. The wafer is mounted on a three-axis translation stage in a vacuum chamber with a base pressure of less than 4 torr. During the experiments, the chamber is backfilled with 500-torr SF 6. The laser system, consisting of a Ti:Sapphire oscillator and a chirped-pulseregenerative amplifier, produces a 1-kHz train of 0-fs, 0.5-mJ pulses at 800 nm. 3

4 Longer pulses are obtained by adjusting the pulse compressor. The laser pulses are focused with a 0.1-m focal-length lens and, except where noted, incident normal to the sample. The spatial profile of the laser pulse is nearly Gaussian, with a fixed beam waist of 200 µm at the sample. The fluence (energy per unit area) varies over the laser spatial profile; values quoted below refer to the fluence at the center of the spatial profile. The fluence is controlled by changing the total incident energy with a half-wave plate and a polarizer. The polarization of the laser pulse is controlled by a second half-wave plate. Each spot on the sample surface is exposed to 500 laser pulses. We translate the sample by approximately 1 mm between runs. Following irradiation, the sample is analyzed with a scanning electron microscope (SEM). 2 Results and discussion Figure 1 shows planar and cross-sectional views of the sharp conical spikes on silicon produced with 0-fs pulses at a fluence of kj/m 2 in 500-torr SF 6. The spike size varies across the irradiated region, reflecting the variation of the fluence across the laser spatial profile. The spikes are up to 40-µm tall and have a subtended angle of approximately 20. The base of the spikes has an asymmetric shape. When we rotate the laser polarization, the orientation of the base rotates accordingly. We find that the short axis of the base is always parallel to the laser polarization. We determined the atomic structure of each individual spike by performing back scattering Kikuchi diffraction.[9] The diffraction pattern obtained from the spike shows the same Kikuchi lines as those taken from a flat undamaged surface next to the irradiated areas. This indicates that the spike is crystalline and has the same crystallographic orientation as bulk silicon. 4

5 We also observed similar sharp spikes on Si(111) with the laser at normal incidence and Si(0) with the laser incident at 20 off the surface normal. In all cases, the spikes point along the incident laser direction, independent of the crystallographic orientation of the substrate. Figure 1 shows that the distribution of the spikes reflects the intensity variation over the spatial profile of the laser beam. At the center of the irradiated area, the spikes are wider, taller, and more sparsely spaced, while at the edges they are narrower, shorter, and more closely packed. We determined the relation between spike height and spike separation from Fig.1 by measuring the spike height h as a function of the distance from the center of the irradiated region Fig. 1 (b) and the average separation between spikes d from Fig. 1 (a). The average spike separation is obtained from the number density of the spikes within a µm 2 square area. Figure 2 shows that the spike height scales nonlinearly with the spike separation. The solid line in Fig. 2 is a least-squared fit of a power law, h d p to the data, yielding a power law exponent p = 2.4 ± 0.1. When the spike separation drops below 1.5 µm, the surface is corrugated but no sharp spikes are observed. Figure 3 shows the dependence of the spike separation on laser fluence with 0-fs pulses. To avoid any problems associated with fluence variations over the beam profile, we obtained these data by measuring the average spike separation at the center of the laser profile for different fluences. The spike separation increases sharply with increasing laser fluence: as the fluence increases from 5 to kj/m 2, the spike separation increases by a factor of three. Using the power law fit from Figure 2, this yields a twelve-fold increase in spike height. Below 2 kj/m 2, the surface shows ripples of submicrometer periodicity and no spikes are observed. At 1 kj/m 2, the surface remains intact even after 500 laser pulses. Figure 4 shows the dependence of the spike separation on pulse duration for laser pulses at a fixed fluence of kj/m 2. The spike separation decreases sharply as the pulse 5

6 duration increases from 0.1 to 1 ps, and remains approximately constant between 1 and ps. At 250 ps, the surface is corrugated but no sharp spikes are observed. Cones with a morphology similar to the ones we observe on silicon have been seen on metals, dielectrics, and oxides after ion or nanosecond laser sputtering.[,11] These cones also have the same crystallographic orientation as the bulk and point along the incident beam direction. Their formation has been attributed to shielding of the underlying substrate by sputtering-resistant impurities on the surface, leaving the cones behind as the surrounding material is removed by the sputtering particles. A similar argument can be used to explain the formation of the spikes on silicon reported here. Initial fluctuations[12] give rise to preferential removal of material at certain locations, explaining the observed crystallographic orientation and pointing of the spikes we observe on silicon. Because more silicon is removed in areas where more energy is deposited, a difference in light absorption on different sides of the spikes would explain the asymmetric shape of their bases visible in Fig. 1 (a). Indeed, the laser pulses in Fig. 1 (a) are polarized along the x axis; therefore, they are p-polarized on the sides of spikes oriented along the yz plane, and s-polarized on the sides of spikes oriented along the xz plane. Because absorption of p-polarized light is larger than that of s-polarized light, the sides of spikes oriented along the yz plane absorb more energy, causing the spikes to become narrower in the x direction. Our observations are therefore consistent with a mechanism involving the removal of silicon substrate that is dependent on the local energy absorption. In a previous paper we reported the effect of ambient gases on spike formation.[5] The observation of blunt spikes in vacuum and sharp spikes in halogen containing gases suggests that both laser ablation and laser-induced chemical etching are involved in the removal of substrate material. Both of these processes could explain the strong dependence of the spike separation on laser fluence and pulse duration we observe. On 6

7 one hand, studies of the subpicosecond laser ablation of materials have shown that ablation depends strongly on the laser fluence and pulse duration.[13,14] On the other hand, multiphoton ionization caused by the femtosecond laser pulses can produce reactive fluorine radicals or SF 6 plasmas,[15] which are known to etch silicon.[16,17] Recent studies have also shown that the cross-sections of femtosecond laser-induced surface reactions increase dramatically as a function of fluence and pulse duration.[18,19] Additional experiments will be needed to determine which of these two processes is responsible for the observed phenomena. Acknowledgements We thank Professor Mike Aziz for valuable discussions and Dr. Daniel Mumm for help with the back scattering Kikuchi diffraction. This work was supported by the Army Research Office. 7

8 Figure 1 Sharp conical spikes produced on Si(0) by 500 laser pulses of 0-fs duration and -kj/m 2 fluence in SF 6 at a pressure of 500 torr viewed (a) from the surface normal, and (b) parallel to the surface. Figure 2 Dependence of average spike height on average spike separation, determined from Fig.1 (a) and (b). The solid straight line is a least-squared fit to a power law dependence. Figure 3 Dependence of average spike separation on laser fluence for laser pulses of a fixed pulse duration of 0 fs. Figure 4 Dependence of average spike separation on laser pulse duration for laser pulses of a fixed fluence of kj/m 2. 8

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11 0 average spike height (µm) average spike separation (µm) Fig. 1 Fig average spike separation (µm) 5 average spike separation (µm) incident fluence ( kj/m 2 ) pulse duration (ps) Fig. 3 Fig. 4