Laser induced plasma plume imaging and surface morphology of silicon

Nuclear Instruments and Methods in Physics Research B 267 (2009) 1085–1088

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Laser induced plasma plume imaging and surface morphology of silicon M. Khaleeq-ur-Rahman, K. Siraj *, M.S. Rafique, K.A. Bhatti, A. Latif, H. Jamil, M. Basit Advanced Physics Laboratory, Department of Physics, University of Engineering and Technology, G.T. Road, Lahore 54890, Pakistan

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Article history: Received 5 September 2008 Received in revised form 23 December 2008 Available online 23 February 2009 PACS: 52.50.Jm Keywords: Nd: YAG laser Silicon CCD plasma plume images SEM micrographs Ripple formation

a b s t r a c t Shot-to-shot variation in the characteristics of laser produced plasma plume and surface profile of N-type silicon (1 1 1) are investigated. In order to produce plasma, a Q-switched Nd: YAG laser (1064 nm, 10 mJ, 9–14 ns) is tightly focused on silicon target in air at room temperature. Target was exposed in such a way that number of laser shots was increased from point to point in ascending order starting from single shot at first point. Target was moved 2 mm after each exposure. In order to investigate shot-to-shot variation in the time integrated emission intensity regions within the plasma plume, a computer controlled CCD based image capture system was employed. Various intensity regimes were found depending strongly on the number of incident laser pulses. Plasma plume length was also found to vary with the number of pulses. The topographic analysis of the irradiated Si was performed by Scanning Electron Microscope (SEM) which shows the primary mechanisms like thermal or non-thermal ablation depend on the number of shots. Surface morphological changes were also studied in terms of ripple formation, ejection, debris and re-deposition of material caused by laser beam at sample surface. The micrographs show ripples spacing versus wavelength dependence rule [K  k/(1 sin h)]. Intensity variations with number of shots are correlated with the surface morphology of the irradiated sample. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction When strongly focused high power laser interacts with the solid target, the plasma formation takes place. Laser induced plasmas are transient in nature and their properties depend upon laser parameters, target composition, ambient atmosphere and surface morphology of target material [1]. Laser induced plasma exploits many applications such as pulsed laser deposition [2,3], laser induced breakdown spectroscopy [4–6], ions generation [7,8], soft and hard X-ray emission [9–11] etc. In many applications like pulsed laser deposition of high quality thin films or nano-cluster formation, the studies of plasma plume dynamics and expansion are of extreme importance [12]. Time resolved imaging of excimer laser produced silicon plasma in high O2/Ar ambient is done by Trusso et al. [13] which show the production of shock wave by the supersonic expansion of the silicon plasma. Various components of propagating silicon plasma at different time delays is also reported [14]. Different physical properties of the silicon expanding plasma plume like vapour pressure, temperature, velocity and stopping distance of the plume, etc. can also be measured with the help of the time resolved images of expanding plasma plume [15]. The lateral expansion of the vapour/plasma plume, motion

of liquid layer due to recoil pressure and surface tension effects can also excites periodic structures on the irradiated semiconductor surface [16]. Multiple pulse irradiation increases the motion of the liquid from the valleys of the capillary waves resulting in more surface corrugation [17]. The ripple formation on silicon is a subject of study since many years and has been reported by many authors [16,17–20]. Ripples can be produced on silicon by nano-, pico- and femto-second laser pulses [17,21]. The characteristics of ripples are also wavelength dependant [19,22]. It is often desirable to suppress the ripple formation in many material processing applications [16] but sometimes ripple formation can be helpful in the fabrication of gratings [23], to improve the adhesion of other materials [24], biocompatible materials [25], replacement of antireflection coatings [26] and in many applications related to electronic industry [27]. This paper reports the studies of time integrated laser produced silicon plasma plume images captured by CCD and analyzed by image-J software. Shot-to-shot variation in optical emission intensities and length of plasma plume are investigated. Moreover, surface morphology of silicon is also studied using Scanning Electron Microscope. 2. Experimental setup

* Corresponding author. Tel.: +92 42 902904 (office); +92 42 6682383 (home); +92 322 4166143 (mobile). E-mail addresses: [email protected], [email protected] (K. Siraj). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.02.053

A passive Q-switched pulsed Nd: YAG laser (k = 1064 nm, E = 10 mJ, 9–14 ns) is irradiated on the n-type crystalline silicon

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sample of size 12  12  0.1 cm3. Laser is focused using IR transmitting lens of focal length 10 cm to an experimentally measured spot diameter 400 lm and the laser intensity is 108 W/cm2. The experiment was performed in air (1 atm) at room temperature. A CCD Camera is placed at 90° from the incident laser beam which is at 45° from the plume axis (Fig. 1). Silicon sample is exposed at 50 different points each with increasing number of shots keeping a distance of 2 mm from each other. Total 1275 shots were taken at 50 different points. The laser induced silicon plasma plume images from each shot were captured using computer controlled image capture system. Surface morphology of irradiated N-type silicon was also studied using Scanning Electron Microscope (Hitachi S 2000H). 3. Results and discussion 3.1. Plasma plume imaging In this work, the intensity profiles of silicon plasma plume images were analyzed using image-J software. Fig. 2(a) is an original image of a plasma plume induced by a single laser shot captured by CCD. Inset is the pseudo-coloured image showing various intensity regions within the plume. The intensity profile of the image along the line in (2a) is shown in (2b) where one pixel represents the distance of 0.29 mm. The captured images clearly describe the overall view of plasma plume. Variation in the intensities signifies different zones subjected to different density and temperature gradients. In general, the plume intensity is uniform at the center and decreases abruptly at the edges. High intensity laser irradiation on target surface creates a large population of excited non-equilibrium electrons leading to bond breaking of the sample and subsequently causes atomic size particulates ejection via non-thermal ablation process. In semiconductors, if the photon energy is above the band gap energy, the electron-hole pairs are created. If this excitation is local, no electron–phonons conversion occurs, and after the distortion of electronic states of the component, particles are ejected [28]. Another channel of material ablation is thermal process where laser excites the electrons which transfer energy to phonons during electron–phonon relaxation through lattice vibrations and consequently heat is conducted through the sample. This heating leads to local melting and then vaporization takes place. These vapours exert a recoil pressure on the melt surface and molten mass is pushed outwards forming plasma plume. The plasma plume con-

Nd-YAG Laser CCD based computer controlled image capture system Plume axis

IR lens

Silicon Target Fig. 1. A schematic of experimental setup for the plasma plume imaging.

sists of ions, electrons, neutrals, clusters, micron sized particles, molten globules and electromagnetic radiations. Plasma near the surface has maximum density of ions, electrons and atoms, etc. forming Knudsen layer. The species produce more ionization within this layer due to more collision. As a result bright luminous plasma plume is observed due to photo-ionization, Bremsstrahlung, recombination and de-excitation processes. The high intensity plasma plume of silicon is produced by tightly focused laser that is strongly forward directed due to the strong temperature and pressure gradients in the axial direction as compare to corresponding gradients in the radial direction. The shot-to-shot variation in length of the plasma plume is also measured from the images and is shown in Fig. 3. The plume length is 67 mm in the beginning, decreases almost 10 mm after 15 shots at the same point and stagnate upto 50 shots. The shorter length of the plume with the increase in the number of shots can be explained by considering that after multiple exposures the surface of the target becomes shallow or a crater is formed. Due to this crater, the direction of plume ejection is no more along the normal to the surface but a bit tilted and it appears shorter to CCD [29]. The variation in the maximum intensity and relative integrated intensity of the plasma plume with number of laser shots is shown in Fig. 4. It is evident from the figure that the emission intensity and relative integrated intensity both sharply increase upto 15 pulses. However, there is no significant change observed in the intensities after 15 pulses. This is attributed to the fact that the initial laser pulses induce significant incubation centers, thermal stresses, cracks, surface roughness, etc. which in turn reduce the ablation threshold and most of the laser pulse energy is utilized to energize the plasma plume resulting in more emission intensity. Afterwards, the intensity starts to decrease because of the crater formation at the surface of the target due to the multiple exposures. Due to this crater, the incoming laser energy is not fully utilized for plasma formation but a major portion is contributing towards the conduction of heat to the surroundings. 3.2. Surface morphology Laser induced structures on solids and liquids are generally classified into coherent and non-coherent structures. Coherent structures are dependent on certain range of laser parameters such as wavelength, polarization, fluence or angle of incidence, etc. whereas non-coherent structure formation is independent to these laser parameters. Ripples are such spatially periodic coherent structures that are most frequently observed in laser surface interaction. In present work, shot-to-shot variation of ripple formation on silicon surface has been studied. Fig. 5 represents the SEM micrographs of laser irradiated silicon. The surface profiles of silicon irradiated by 10, 20, 30 and 50 laser shots are shown in Fig. 5(a)–(d), respectively. All figures show ripple formation which is the consequence of inhomogeneous absorbed energy distribution over the silicon surface. The figures show more damage in the center as compare to outer edges. As the number of shots increases, the damage within that spot area also increases. The ripple density at the periphery of the spot also decreases because of limited energy transfer. Fig. 5(d), where maximum shots are taken, shows more damage area at the center and significant heat affected zone (HAZ) at the boundary of irradiated surface. Figures show the small crater formation upon laser irradiation which is the indication of mass ejection from the surface. The ripple spacing 3– 5 lm is observed which shows K  k/(1 sin h) dependence where K is ripple spacing, k is wavelength and h is angle of incidence of laser beam. This relation holds well in case of the incident beam focused on the target at 0° with respect to the target normal. Under this condition, we expect the spacing of the structures to be almost same as that of incident wave length. But, in our case the incident

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Fig. 2. (a) Original image of plasma (bright spot) induced on Si target (black background) by Nd: YAG laser incident at 45° to the target normal. Inset is the pseudo-colored image exhibiting various intensity regions within the plume, (b) intensity profile of the image along the line in (a). (one pixel = 0.29 mm.)

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Fig. 4. Maximum and integrated intensities of the plasma plume as a function of number of laser shots.

beam is at 45°. Therefore, the periodicity of the grown structures is calculated to be 3–5 lm which agrees with our experimentally measured values. In literature, the periodicity of such structures in some cases is reported to exhibit a direct correlation with the incident wavelength [30]. However, evidences have been reported that no correlation with the incident wavelength exists [31]. Generally, these

periodic structures are attributed to an inhomogeneous distribution of energy on the surface due to interaction between the incident light and surface scattered waves or radiation remnant field structures. Thus, in response to spatial modulation of energy, the modulated modifications on the surface of the materials are formed. The question is how the modulated irradiance is physically transformed into a persisting variation of surface geometry. The basic sequence is that the material melts, undergoes deformation, and finally after irradiation re-solidifies making the deformation permanent. This surface deformations or irregularities act to scatter a small amount of light from the incident laser beam. This scattered light may propagate as a surface wave above or within the irradiated material, and interferes with the incident beam producing an intensity distribution across the surface. The intensity distribution acts as a diffraction grating, scattering additional light into the surface wave, therefore creating a positive feedback effect [32]. The rippling can also be attributed to scattering from the surface roughness and to re-radiation from surface defect sizes [33]. Ripples can also be formed by Surface Electromagnetic Waves (SEW) which could be excited if the material under consideration posses an optically active mode near laser frequency. Surface phonon – polaritons in semiconductors and insulators, surface plasmons – polaritons in metals are the examples of such SEW. Enhanced ripple formation is observed if one of the scattered waves is in resonance with a SEW. Theoretical details of coherent, non-coherent structure formation and SEW can be found in the literature [16]. At the irradiated spot, the droplet formation is also observed due to hydrodynamic instabilities on the material surface which grows in size and material ejection increases. Black spots may also be due to sputtering phenomenon. Some re-deposited material can also be seen at the irradiated spots. As the number of shots increases, the continuous circular ripple is distorted into grain or smaller cone like structures which increases with number of laser shots. The grains or cone like structures are formed by the shielding effects related to material impurities, increased concentration of photo-fragments or by debris formation. The condensation of silicon vapours on the cones tip also lead to the formation of column like structures which could also be seen in SEM micrographs. Surface melting is frequently observed upon laser irradiation on semiconductors. The ripples on semiconductor surface appear at much lower intensities as compare to metals due to lower melting threshold. 4. Conclusion It is concluded from the plume images that there is shot-to-shot variation in both maximum and integrated intensities. The emission intensities increase up to 15 number of pulses and becomes approximately constant. Various intensity regions are present

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Fig. 5. SEM micrographs of laser irradiated silicon. (a) With 10 laser shots, (b) with 20 laser shots, (c) with 30 laser shots and (d) with 50 laser shots.

within the plume, regardless of the number of shots. Central portion of the plume is more intense whereas the intensity drastically decreases at the edges. The plume length appears larger for first few shots but decreases with subsequent laser pulses due to crater formation on the target surface. Surface morphology exhibits that mechanism like thermal or non-thermal ablation depending on the number of shots lead towards the formation of ripples on silicon surface. Inhomogeneous distribution of energy on the surface due to interaction between the incident light and surface scattered waves is responsible for the growth of these ripples. The observed ripple spacing is 3–5 lm which shows K  k/(1 sin h) dependence (when laser is incident at 45° at the target surface). References [1] S.S. Harilal, C.V. Bindhu, M.S. Tillack, F. Najmabadi, A.C. Gaeris, J. Appl. Phys. 93 (5) (2003) 2380. [2] Douglas B. Chrisey, Graham K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, John Wiley and Sons Inc., New York, 1994. [3] K. Siraj, Pulsed Laser Deposition, Planarization and Ion Beam Nano-patterning of High-Tc Superconducting Thin Films, Ph.D. Dissertation, Institute of Applied Physics, Johannes Kepler University Linz, Austria, 2007. [4] J.D. Pedarnig, J. Heitz, T. Stehrer, B. Praher, R. Viskup, K. Siraj, A. Moser, A. Vlad, M.A. Bodea, D. Bäuerle, N. Hari Babu, D.A. Cardwell, Spectrochim. Acta Part B 63 (2008) 1117. [5] Y. Godwal, M.T. Taschuk, S.L. Lui, Y.Y. Tsui, R. Fedosejevs, Laser Part. Beams 26 (2008) 95. [6] G. Cristoforetti, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, Spectrochim. Acta Part B 63 (2008) 312. [7] M.S. Rafique, M. Khaleeq-ur-Rahman, M.S. Anwar, F. Mahmood, A. Ashfaq, K. Siraj, Laser Part. Beams 23 (2005) 131. [8] F. Belloni, D. Doria, A. Lorusso, V. Nassisi, Nucl. Instr. and Meth. B 240 (2005) 40. [9] M.S. Rafique, M. Khaleeq-ur-Rahman, I. Riaz, R. Jalil, N. Farid, Laser Part. Beams 26 (2008) 217.

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