Optics & Laser Technology 34 (2002) 37–43www.elsevier.com/locate/optlastec

Laser damage studies of silicon surfaces using ultra-short laser pulses

Amit Pratap Singha, Avinashi Kapoora ; ∗, K.N. Tripathia, G. Ravindra KumarbaDepartment of Electronics Science, Delhi University, South Campus, Benito Juarez Road, New Delhi-21, India

bDepartment of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005, India

Received 14 May 2001; accepted 11 September 2001

Abstract

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablationof material. A magni1ed view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these rippleswas between 0:5 and 2 �m and increased, on increasing the power density or number of pulses, and 1nally broke into parts, leavingwell-ordered grains of approximate diameter 5 �m. Also for 100 or larger number of pulses, an amorphous ring in the periphery wasformed. The diameter of this ring increased, on increasing either the laser 5uence or the number of pulses. The formation of ripples hasbeen explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285).c© 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Si; Semiconductor; Superconductor; BCS; Femtosecond laser pulses; Plasma annealing; Thermalization process; Ablation

1. Introduction

There has been a widespread interest in laser damagestudies motivated both by investigation of the mechanismsinvolved and the practical application of damage thresholdlevels to laser system design [1–4, 6–12]. Silicon surfaceshave attracted particular interest, in connection with laserannealing phenomenon in this material. A number of studieshave been performed in this material at various pulse du-rations. Grinberg et al. [1] experimentally investigated thedamage mechanism on polished Si samples using 0:5 msduration pulses. It was inferred that the damage mecha-nism is semiconducting in nature for carrier concentration∼1017 cm−3 andmetallic in nature for higher concentrations.Similar 1ndings were made by Screckovic et al. [2] using100 �s laser pulses. MacDonald et al. [3] presented damagestudies using 100 ns and 15 ns laser pulses. For 100 ns-pulseduration at 10.6 wavelength the results show that the surfaceabsorption governs the laser damage threshold. The polished1nish of the material surface in5uences the damage mor-phology. For 15 ns pulse duration at 1.06 wavelength the

∗ Corresponding author. Tel.: +91-11-688-9750x201.E-mail address: avinashi kapoor@yahoo.com (A. Kapoor).

damage morphology was explained in terms of bulk opticalproperties. Surface eEects have negligible role. The dam-age morphology is independent of carrier concentration. Liuet al. [4] reported ultrafast melting and resolidi1cation us-ing the 30 ps laser pulses. However, using the intense ul-trashort laser pulses, energy is deposited in the materialin such a short time that the lattice remains cool [10,11].Therefore, a diEerent type of damage morphology may beexpected.Here, in this paper, a thorough study on the damage mor-

phology of Si surfaces (thickness 39 mm) induced by fem-tosecond laser pulses is presented. Surface damage is de1nedas any change in the Si surface for which the re5ectivity ofHe–Ne laser beam, which has been 1xed on the spot to bedamaged, changes by more than 10% [5].

2. Experimental details

The laser con1gurations and the experimental setup wereas follows.The laser used in the present experiments was a

titanium : sapphire (Ti : S) system (wavelength 806 nm),based on the principle of chirped pulse ampli1cation (Strick-land and Mourou, Opt. Comm. 56, pp. 219, 1985). The

0030-3992/02/$ - see front matter c© 2002 Elsevier Science Ltd. All rights reserved.PII: S 0030 -3992(01)00090 -1

38 A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 37–43

Fig. 1. (a) The damaged spot at threshold 5uence for 10 pulses; (b) The amorphous behavior of the ablated material; (c) The periphery of the damagedspot of 1(a). Some redeposited particles are present.

average mode locked output power was 450 mW at 90 fswith a bandwidth of 20 nm. The single shot experimentsused pulses after ampli1cation in a multipass ampli1er.The 1nal output obtained from this laser system was 50–55 mJ=pulse with a pulse duration of 110 fs. The resultingmaximum output power was 0:5 TW. The prf of laser is10 Hz. The energy incident on the sample could be adjustedusing neutral density 1lters as well as by placing the sampleat diEerent positions between the lens and its focal plane.A He–Ne laser beam was used as a probe beam to detectthe damage. The probe beam, which was smaller than thespot size, was focused onto the site to be damaged, whichafter re5ection from the sample fell on the photodetec-tor. The damage morphology has been studied using SEMmicrographs.

3. Results

The results obtained from the micrographs are summa-rized as follows.

Damage was initiated in our experiments at a powerdensity of 1:6×10−14 W=cm2 for 10 pulses revealing theablation of material (Figs. 1(a)–(c)). After ablation thematerial became amorphous (Fig. 1(b)). Some redeposits ofmicron-sized particles after ablation is shown in Fig. 1(c).By increasing the number of pulses to 100, a combinationof bright and dark ring was formed in the periphery ofthe spot (Fig. 2(a)). The bright ring was amorphous Siand the dark ring was the boundary of the ablated area[10,11] (Fig. 2(d)). This ablation (surface removal) andredeposition of small clusters is clearer in Fig. 2(c) whichshows the peripheral view of the damaged spot near thedark ring. By magnifying the central portion of the spotwe found that ripples of long range order were formed(Fig. 2(b)). The spacing of these ripples was not constantand varied in the range of 0:5–2 �m. By increasing thenumber of pulses to 500 (Figs. 3(a) and (b)), the longrange ordering was broken in parts. On breaking in parts,ripples took the form of bubbles (Figs. 3(b)). Figs. 3(a)and (b) show the complete damaged spot and a magni1ed

A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 37–43 39

Fig. 2. (a) The damaged spot at threshold 5uence (of 10 pulses) for 100 pulses; (b) The magni1ed view of the central portion of the spot showing longrange ordering of the ripples; (c) The periphery of the damaged spot of 2(a). Ablation and redeposited particles are shown; (d) The magni1ed view ofthe rings formation in the spot of 2(a).

view of the central portion of the damaged spot, respec-tively.By increasing the power density either to two times

or three times the threshold 5uence, the morphology for10-pulse damage was approximately the same as for thethreshold 5uence, in the form of amorphous ablation (Figs.4(a) and (b), 7(a) and (b)). For 100 pulses, at two timesthe threshold 5uence, the spacing of the ripples and thearea of the black ring between the ablated area and theouter bright ring increased (Figs. 5(a) and (b)). For 100pulses, at three times the threshold 5uence (3×Fth), theripples were still formed but started to expand in the per-pendicular direction (Figs. 8(a) and (b)). Figs. 8(a) and(b) show the complete damaged spot and a magni1ed viewof the central portion of the damaged spot. Lastly, for500 pulses, for two times the threshold 5uence (2×Fth),only the broken ripples with large spacing, in comparisonto ripples which were formed at the threshold 5uence for500 pulses, were formed (Figs. 6(a) and (b)). At three

times the threshold 5uence (3×Fth) the ripples took theform of well-ordered spherical grains of diameter 5 �m(Figs. 9(a) and (b)). Figs. 9(a) and (b) show the dam-aged spot at three times the threshold 5uence (3×Fth) for500 pulses and a magni1ed view of the central portion ofthe damaged spot. It is a notable point that, either for 10or 100 or 500 pulses, there is a trend that whenever weincreased the power 5uence, at 1rst on shifting from dam-age threshold 5uence to two times the threshold 5uence(2×Fth) (Figs. 1(a), 2(a), 3(a), 4(a), 5(a) and 6(a)), thebrightness (optical re5ectivity) [6] of the spot increasedand then on shifting from two times the threshold (2×Fth)to three times the threshold (3×Fth) (Figs. 4(a), 5(a), 6(a),7(a), 8(a) and 9(a)), it decreased. Also at 1rst, there is noring formation for 10 pulses (Fig. 1(a)). Then, when a ringwas formed for 100-pulses (Fig. 2(a)), the thickness of theouter bright ring decreased, on increasing the number ofpulses from 100 to 500 pulses (Figs. 2(a) and 3(a)) forthreshold 5uence and from 500 pulses of threshold 5uence

40 A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 37–43

Fig. 3. (a) The damaged spot at threshold 5uence (of 10 pulse) for 500pulses; (b) The ripples in the form of broken parts in the damaged spotof 3(a).

to 100 pulses of two times the threshold 5uence (2×Fth)(Figs. 3(a) and 5(a)). On further increasing either power5uence or the number of pulses it remained approximatelyconstant.

4. Discussion

The discussion for our results can be divided intothree parts. Firstly, we have shown that damage ini-tiates in the form of surface removal. Figs. 1(a) and(b) and the formation of rings for 100 pulses [6] con-1rm this view. Therefore, we can say that this dam-age threshold is indeed the ablation threshold in thecontext of our laser parameters. It is, however, a no-table point that in our experiments material ablates be-fore the formation of rings. Secondly, the formationof bright and dark rings and expansion of the areaof the dark ring are noticed on increasing the power

Fig. 4. (a) The damaged spot for 2×Fth and 10 pulses; (b) The magni1edview of the central portion of the spot of 4(a).

5uence. Sokolowski-Tinten et al. have shown [6] thatthese rings display optical interference patterns (New-ton rings), which are formed during free expansion ofstrongly heated, pressurized matter. The third and im-portant result of our experiment is the formation ofripples, which gives information about the ablationprocess.The nature of these ripples has been the subject of a

great discussion in the past [7–12]. Guosheng et al. [7]gave a detailed theoretical model which predicted the spac-ing, polarization and growth properties of these ripples.According to this theory, sinusoidal corrugation at themolten surface after the interference between the diEractedoptical wave with the incident beam, is the main reasonfor ripple growth. Driel et al. [8] presented a universalphenomenon for this periodic surface damage structurewith respect to materials, wavelength, beam polarizationand details of surface roughness. However, at the sametime there were some results [11,12], that contradict their

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Fig. 5. (a) The damaged spot for 2×Fth and 100 pulses; (b) The rippleformation for 2×Fth and 100 pulses.

universal theory. Preston et al. [9] also studied the pat-tern formation using cw laser beams. According to theirobservation for ordered pattern formation, the laser pa-rameters like incident laser intensity, the spot size ofthe laser beam, and the amount of lateral heat 5ow areresponsible.In the present experiment, the results do not agree with

the above-discussed theories. Guosheng related the ripplespacing with the laser wavelength. They said that at normalincidence the ripple spacing, in nearly all cases, is ap-proximately equal to the laser wavelength in vacuum.Preston et al. [9] extended the view by saying that forsmall spot size, highly correlated, long-range periodicstructure with spatial period of n� are observed. Here,n is any integer which increases on increasing the laserintensity. � is the laser wavelength. Now it is obvi-ous in the present observations that neither the spacingof these ripples is constant and equal to laser wave-length nor does it increase discretely (n� dependency),but it increases smoothly on increasing the power den-

Fig. 6. (a) The damaged spot for 2×Fth and 500 pulses; (b) The rippleformation for 2×Fth and 500 pulses.

sity or the number of pulses and 1nally breaks intoparts and forms well-de1ned grains of approximate di-ameter of 5 �m. The formation of grains shows thatthe material cools down without the surrounding mate-rial being involved, which further shows that ultrafastchanges occurred through electronic excitation when thelattice remains cool. To explain all these results, we sug-gest that a possible mechanism may be Boson conden-sation hypothesis, given by Van Vechten [12]. This isbecause, according to the Van Vechten hypothesis, thiscondensation requires a high carrier density (¿1023) onthe antibonding state, for which the plasmon energy isgreater than the band gap energy of the material. Re-cent experiments [13,14] show that when intense pulsesof femtosecond duration interact with material, a highcarrier density (¿ 1023) is induced, in such a short timethat the lattice remains cool. According to the Bosoncondensation hypothesis, for such a density of carriers,the polarization waves of the plasmon modes begin tocreate and destroy electron–hole pairs in a coherent

42 A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 37–43

Fig. 7. (a) The damaged spot for 3×Fth and 10 pulses; (b) The magni1edview of the central portion of the spot of 7(a).

fashion so that they are no longer independent freecarriers but paired carriers. These paired carriers behaveas bosons, which condense according to the BCS mech-anism. In the same paper they propose some tests forprediction of the hypothesis. They said that if the presenthypothesis is true, then a low intensity linearly polarizedlaser, irradiating the surface at the time the high re5ec-tivity phase appears in the sample could induce linearripples.Lastly, it may be noted that the formation of an amorphous

ring in the damaged spot with high re5ectivity, which is notseen for longer pulses may 1nd application in high densityoptical memory system.

5. Conclusion

It has been shown that, using femtosecond-laserpulses, damage initiates in the Si surface in the formof ablation. The formation of grains indicates that be-

Fig. 8. (a) The damaged spot for 3×Fth and 100 pulses; (b) The expansionof the ripples in a perpendicular direction for 3×Fth and 100 pulses.

cause of the short pulse length the surface materialheats up and cools down without the bulk material be-ing involved. Therefore, the excitations are electronicexcitations where the lattice remains cool. Further-more, the formation of ripples in the damaged sur-face of Si crystal and the behavior of these ripples,on changing the power 5uence and the number ofpulses, indicate that ultrafast changes in Si surfaces af-ter exposure to femtosecond-laser pulses should occurthrough boson condensation which is based on electronicexcitation.

Acknowledgements

We are grateful to Dr. R.B. Singh, Dr. B.S. Patel,Dr. R.K. Jain, Dr. R.K. Bagai, and Dr. S.K. Aggrawalfor their cooperation in the preliminary stage of thework.

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Fig. 9. (a) The damaged spot for 3×Fth and 500 pulses; (b) Thewell-de1ned grains for 3×Fth and 500 pulses.

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