Physical mechanisms of short pulse laser ablation

Transcription

1 Physical mechanisms of short pulse laser ablation D. von der Linde and K. Sokolowski-Tinten Institut für Laser- und Plasmaphysik, Universität Essen, D Essen, Germany ABSTRACT Removal of material from the surface of metals and semiconductors following irradiation with pico- or femtosecond laser pulses is a thermal process involving states of matter having unusual thermodynamic, hydrodynamic and optical properties. Keywords: Laser ablation, femtosecond pulses 1. INTRODUCTION Laser processing of materials has become an important tool in many areas of technology. Laser radiation is used for welding, cutting, and drilling, and also for more exigent application such as production of complex surface patterns or modification of the physical or chemical microstructure of materials. In many applications removal of material from the surface following laser exposure, commonly called laser ablation or laser sputtering, plays an important role. The ablation process depends on thermal and optical properties of the materials and on laser parameters such as wavelength, laser intensity, pulse duration. Comparison of ablation structures produced by laser pulses of different pulse widths suggests that a higher structuring precision can be obtained with the use of very short laser pulses 1. However, a convincing explanation in terms of fundamental physical processes is still lacking. Therefore, clarification of the physical mechanisms of short pulse laser ablation should be of great current interest, both from the point of view of an assessment of the potential of ultrashort pulses in laser materials processing and of understanding the underlying fundamental physics. 2. FUNDAMENTALS To remove macroscopic amounts of matter from the surface of a solid, the material must usually undergo some changes of the fundamental state of aggregation and transform into some volatile phase, e.g. a gas or a plasma. The transition from the solid to the gas phase can occur stepwise by melting of the solid followed by evaporation of the liquid. Boiling can occur when the vapor pressure of the liquid phase exceeds the ambient pressure. Boiling rates become very fast when the temperatures of the liquid approaches the critical temperature. The latter case is sometimes referred to as phase explosion 2. Under certain conditions sublimation may occur, i.e. a direct transition between the solid phase and the gas phase. Otherwise, a direct transition to a gaseous or a fluid phase can be accomplished by very rapid heating of solid matter to temperatures higher than the critical point. Concerning the transition to the plasma state, it is well known that an intense laser pulse can directly ionize materials and transform a solid into a dense plasma 3. Concerning the advantages of short pulse laser ablation, the reduction or apparent absence of heat-effected zones has been attributed to a suppression of thermal diffusion resulting from the short pulse duration. Non-thermal mechanisms have also been invoked to explain the differences between short and long pulse laser ablation. In order to understand clearly the significance of short pulse width, it is instructive to consider some of the characteristic time constants in the laser ablation process. Laser radiation primarily couples with the electronic states of the valence bands and the conduction bands. The laser pulse width mainly determines the duration of the energy deposition into these states. The deposited optical energy is subsequently redistributed over the various energy levels of the system, mostly by carrier-carrier, carrierphonon and phonon-phonon interaction. The energy relaxation time is the characteristic length of time required to approach thermal energy distributions, i.e. Fermi-Dirac for electrons and Bose-Einstein for phonons. The energy thermalization times in metals and semiconductors are typically of the order of to s. Other important time constants include the times for the transport of energy and mass. Energy transport can have a significant effect on the ablation threshold, which is the minimum energy fluence that must be supplied in order to remove material. The

2 actual time required to carry away the material, the ablation time, is determined by the speed of mass transport. Because ablation involves transport of heavy particles, the ablation times are generally rather long, being determined by hydrodynamic and acoustic processes. When the ablation time is much longer than the thermalization time, it is justified to speak of a thermal ablation mechanism. The consideration of the different time constants suggests that generally ablation is expected to be a thermal process. 3. EXPERIMENTAL We have studied single pulse laser ablation in metals and semiconductors using well-defined pico- and femtosecond laser pulses. The laser fluence was kept low enough to avoid plasma formation. The structural changes of the material were recorded using time-resolved optical microscopy 4. A schematic of the experimental setup is shown in Fig. 1. To start ablation the surface of the sample under study was excited by a p-polarized pump pulse at approximately 55 angle of incidence. The excited surface area was illuminated by a weak, time-delayed probe pulse at normal incidence and observed with a high-resolution optical microscope. The reflected probe light picked up by the microscope was recorded with a CCD-camera. Picture frames representing a map of the optical reflectivity could thus be recorded at arbitrary instants after the excitation pulse. The spatial resolution corresponded to a few micrometers, and the temporal resolution was determined by the duration of the probe pulses. This technique permitted to monitor all stages of the evolution of the ablation process, from the deposition of the optical energy at early times all the way to the formation of the final surface morphology. In most cases laser pulses of 120 fs duration at a wavelength of 620 nm were used, both for pump and probe. A limited number of experiments were performed with laser pulses of 5 ps duration in order to study the effect of different pulse widths on ablation. probe pulse sample microscope CCD-camera pump pulse Fig. 1: Experimental setup Femtosecond snapshot pictures 4. RESULTS Examples of characteristic stages of ablation on a silicon wafer are shown in Fig. 2. These pictures represent time-resolved optical micrographs of the surface viewed in normal direction. The pump pulse was incident from the left. In the upper left corner (0.2 ps) the pump pulse has swept about half way across the surface from left to right, leaving behind a bright area. The brightening is due to an increase of the optical reflectivity caused by the photoexcited electron-hole plasma 5. The very bright, oval-shaped area at slightly later time (1 ps) represents liquid metallic Si (top right). Melting is brought about in less than a picosecond 6 by very strong electronic excitation of the semiconductor 7. At much later time (900 ps), a system of distinct dark rings appears in the center of the molten area. The final picture at 75 ns of the resolidified Si surface shows a bright ring at the periphery and a dark ring inside. It can be shown that these features represent, respectively, amorphous Si 8 and the boundary of the ablated area.

3 The striking feature in Fig. 2 is the system of dark rings observed after a delay of 900 ps. The measured reflectivity profile shown in Fig. 3 demonstrates the high contrast of the ring structure. It will be shown in the following that these ring patterns are a distinct signature of the ablation process and reveal useful information about the physical mechanisms. Reflectivity (%) R of liquid Si Fig. 2: Snapshot of a Si surface showing characteristic stages of the evolution. Pump energy: 0.47 J/cm Position (µm) Fig. 3: Reflectivity profile of the 900 ps frame from Fig. 2 Nanosecond ring structures An example of the temporal evolution of the ring structure in Si is shown in Fig. 4. The number of rings increases with time, while the area covered by the rings remains constant, i.e. the spacing between the rings decreases. Short pulse laser ablation has been studied in a variety of other materials: GaAs, Au, Ti, Al, and Hg. Figure 5 shows three examples of metal films on glass substrates. The data demonstrate that in all cases a system of dark rings has developed on the metal film a few nanoseconds after laser excitation. In fact, such surface patterns have been observed in all semiconductors and metals investigated so far. The picture on the right at the bottom of Fig. 5 shows the final surface morphology of the Al film. Comparison with the corresponding time-resolved pictures (bottom, left) shows that a thin dark line is left at the circumference of the area covered by the rings. An independently measured interferogram of the same surface area is superimposed on the picture of the final structure for comparison. Note the shift of the interference fringes across the thin line. This shift represents clear evidence of a Fig. 4: Temporal evolution of the ring structureon a Si (111) surface. Pump energy 0.43 J/cm 2. Fig. 5: Example of ring structures on metal films. Thickness: 200 nm

4 step in the depth profile of the surface, indicating that material has been removed from the area enclosed by the thin line. In the particular case of Al, the ablated layer was approximately 50 nm near the boundary, and somewhat larger in the center. Thus, the perimeter of the dark ring area is identical with the boundary of the ablated area. The local laser fluence at the boundary corresponds to the threshold fluence of ablation. The main observations concerning the nanosecond ring structures can be summarized as follows: 1. There is a sharp threshold in laser fluence for the formation of the ring structure. 2. The area covered by the dark rings coincides precisely with the ablated area. 3. Very similar behavior is observed in all materials. The experiments demonstrate that the formation of dark rings and laser ablation are closely related and that the underlying physics must be of general nature. Interpretation of the ring patterns To determine whether the ring patterns represent an actual transient surface structure or possibly, as the appearance might suggest, some interference fringes, series of pictures were recorded using different probe wavelengths. The pump pulse energy and the probe delay time were kept constant. These experiments showed that the feature sizes of the ring patterns are in fact dependent on the wavelength. Figure 6 shows the spacings between equivalent pairs of annuli measured in horizontal and vertical direction. A linear increase with wavelength is observed, as expected for an interference pattern. Thus, an actual material surface profile can be ruled out. Rather, it must be concluded that the ablation process leads to the formation of two optical interfaces which give rise to Newton-ring-type interference fringes. If the dark rings are interpreted as Newton rings, one can determine the local optical thickness between the interfering interfaces. From these data, the temporal evolution of the thickness profile and thus the velocity of the relative motion of the interfaces can be obtained. Figure 7 shows examples of spatial profiles of the ablation layer for Si. In this case, the velocity in the center of the ablation layer is approximately 1000 m/s. It will be shown that this velocity is the hydrodynamic flow velocity of the ablation front moving into vacuum. Ring spacing (µm) horizontal vertical Wavelength (nm) Fig. 6: Example of the measured spacing between rings in vertical and horizontal direction. Ablation front ( µm) ns 0.7 ns 0.9 ns 1.2 ns 2.6 ns Position ( µm) Fig. 7: Example of the spatial profiles of the ablation front for different delay times. The corresponding front velocity in the center is about 1000 m/s 5. DISCUSSION Implications of the observation of Newton fringes The fact that interference fringes occur in the early stage of ablation is compelling evidence that two optically sharp interfaces must be created during the expansion of the material. It is plausible to assume that the fringe patterns are formed as a result of interference between light reflected from the front side and the backside of the ablating layer, as illustrated in Fig. 8. The backside boundary separates non-ablating, dense liquid material from ablating, volatile material of low density. The rear boundary of the ablating layer probably represents one of the required interfaces. Concerning the second interface and the

5 ablation front transparent fluid Fig. 8: Schematic of the structure of the ablation layer illustrating interference of reflected light rays. Dotted area: Ablating material. Area shown in gray: Sub-threshold, non-ablating material. optical properties of the layer, the following conditions must be fulfilled, if high contrast interference fringes are to be produced: (1) The optical density across the ablation front must drop towards vacuum over a distance much smaller than the wavelength. (2) The ablating material must be optically transparent and possess a high index of refraction. For example, to produce the measured fringe contrast in Si (see Fig. 3), a refractive index n > 2 and an absorption coefficient k < 0.1 are required. Both requirements are not readily satisfied. Firstly, a gaseous ablating layer is unlikely to have a high refractive index. Secondly, sudden expansion of a gas into vacuum generally is not expected to lead to a step-like density profile 9. Nevertheless, it will be shown in the following that during the expansion of a hot, pressurized fluid conditions (1) and (2) are met upon entering the liquid-gas coexistence regime. Pathways of unsteady expansion Consider the case of Al as an example. Figure 9 shows pressure versus density from equation-of-state (EOS) data 10 of Al. The gray area is the two-phase regime, where the liquid phase and the gas phase coexist. Let us pursue the path of the material during heating, cooling and expansion. Because the energy relaxation time is so short, the expansion during the thermalization of the laser energy is negligible. Thus, laser excitation results isochoric heating along a vertical line corresponding to solid density. Marker (A) in Fig. 9 represents the thermodynamic state of the material reached after laser excitation and thermalization of the optical energy. The actual temperature at (A) is determined by the amount of deposited energy. Close to the ablation threshold, this temperature is typically several thousand degrees Kelvin. The pressure corresponds to tens of Gigapascal. The subsequent evolution of the material can be considered as an isentropic expansion. The dashed and the dotted line in Fig. 9 show two isentropes of Al corresponding to starting temperatures of 7000 K and 5000 K, respectively. The isentropes cross the boundary of the two-phase regime in the neighborhood of marker (B). Nucleation of gas bubbles sets in, and a heterogeneous phase of liquid and gas develops. Pressure (GPa) Aluminium 3529 K C K 6339 K 5476 K 4730 K B 1965 K A Fig. 9: Equation of state diagram of aluminium: Thin lines: Isotherms. Dashed and dotted lines: Isentropes. Gray area: Two-phase regime Density (g/cm 3 ) solid density ρ 0

6 The further evolution of the system is governed by the properties of a heterogeneous liquid-gas mixture. Concerning the dynamics of hydrodynamic expansion, the most significant change of properties in the two-phase regime is a drastic decrease of the velocity of sound 11 2, csound = ( p/ ρ ). This is can be readily recognized from the change of slope of the isentropes at point S (B). The regime of low sound velocity is indicated by marker (C) in Fig. 9. The change of sound speed is very important, because it explains the formation of a sharp ablation front which has the properties required by condition (1). The hydrodynamic consideration leading to this conclusion can be summarized as follows: The initial phase of the expansion can be described by a simple one dimensional self-similar rarefaction wave 9. For an isentropic expansion, the sound velocity is dependent on the density only. The rarefaction wave develops a step-like density profile towards the vacuum side with an upper shelf density given by the liquid density ρ 1 at the two-phase boundary. The flow velocity of the expansion front is approximately given by u c ln ( ρ / ρ ), where ρ 0 is the solid density. = Structure of the ablating layer Figure 10 shows a sequence of schematic density profiles along the z-direction (normal to the surface). The purpose of Fig. 10 is to illustrates the structure and the evolution of the ablating layer, as implied by the thermodynamic and hydrodynamic properties, and to explain the formation of two interfaces giving rise to optical interference. The top panel shows the situation just after energy relaxation, before significant expansion has occurred. The two area in different shades of gray represent, respectively, a layer of material of thickness d in above-threshold condition, corresponding to marker (A) in the EOS diagram, and sub-threshold bulk material, which will not undergo ablation. The dashed curve in the center panel indicates the profile of the rarefaction wave. The head of the wave travels towards the bulk with the unperturbed sound velocity c 0. The area in light gray represents two-phase material corresponding to marker (C) in the EOS diagram (ρ < ρ 1 ). As explained above, there is a sharp drop of the density of the rarefaction wave towards vacuum. This feature accounts for density discontinuity on the front side. The subsequent evolution can be understood by regarding the rear boundary of the ablation layer as a rigid wall where the flow velocity must vanish. It follows that a second density discontinuity develops after t 0 = d/c 0, when the head of the rarefaction wave reaches the boundary. The reflection of the rarefaction wave will then result in a drop of the density at the boundary and create a density discontinuity on the backside. The layer thickness d can be estimated from the measured ablation depth, typically 50 to 100 nm. A typical value of the sound velocity is 2500 m/s. Thus, the time for the formation of a pair of interfaces is estimated to t 0 40 ps. fluid layer ρ 0 t = 0 d surface Density ρ 1 ρ 1 c 0 rarefaction wave u inhomogeneous phase z t < d/c 0 t > d/c 0 Fig. 10: Structure and evolution of the ablation layer. Top: Unperturbed density profile immediately after excitation; d is the thickness of the surface layer above threhold. Center: Density profile modified by the rarefaction wave. Bottom: Density profile after the reflection of the rarefaction wave at the backside. rear interface front interface

7 The situation for t > t 0 can be characterized as follows (bottom of Fig. 10). A layer of expanding material exists which is confined by two sharp interfaces. The front interface moves with the flow velocity u. Given sufficient time for expansion, the density of the layer drops below ρ 1 and the entire ablating material consists of an inhomogeneous two-phases-mixture of low average density. Optical properties of the inhomogeneous phase To conclude the explanation of the observed interference patterns, it must be shown how condition (2) can be satisfied. Neither a homogeneous liquid phase nor a homogeneous gas phase possesses the necessary optical properties. On the other side, to be consistent with our model of the structure of the ablation layer, one should seek for an explanation in terms of the optical properties of an inhomogeneous phase. A proper analysis would require knowledge of the detailed structure of the two-phase mixture. However, if it is assumed that the inhomogeneous phase consists of small liquid droplets surrounded by the vapor phase, the Maxwell-Garnett formula 12 can be applied. The Maxwell-Garnett model describes the optical properties of particles immersed in a dielectric medium, if the size is much smaller than the wavelength. By way of example, the Maxwell-Garnett effective dielectric function for metallic droplets of liquid Si is plotted in Fig. 11 as a function of the filling factor. It can be seen that for filling factors around fifty percent the two-phase medium would be optically transparent and exhibit a refractive index of approximately two. Thus, this simple model can qualitatively account for a transformation of the ablating material into a transparent optical medium with high refractive index. Dielectric function ε Fig. 11: 1 10 Real and imaginary part of the effective dielectric function of droplets of liquid silicon as a function of the ratio of the volume 0 occupied by the droplets to the total available volume (filling -10 factor), according to the Maxwell-Garnett model Filling factor ε 2 6. SUMMARY Femtosecond laser pulses were used to study laser ablation of metals and semiconductors in the laser fluence regime below the onset of plasma formation. Because the pulse duration is much shorter than the energy relaxation time, a description of laser ablation in terms of thermal processes is adequate. Laser excitation leads to isochoric heating of the material to a fluid state, followed by isentropic expansion of the hot, pressurized material into vacuum. During expansion and cooling, the system runs into the liquid-gas coexistence regime and decomposition into an inhomogeneous mixture of gas and liquid takes place. The two-phase mixture exhibits particular hydrodynamic and optical properties, which affect the evolution of ablation primarily in two ways: (i) upon entering the two-phase regime, a drastic decrease of the sound velocity occurs. This effect is responsible for the formation of a steep ablation front in which the refractive index drops sharply over a distance much smaller than the wavelength. (ii) The opaque, metallic homogeneous fluid material is transformed into an inhomogeneous highly transparent medium with a high refractive index. The combined effect of (i) and (ii) result in conspicuous transient interference phenomena which represent a characteristic signature of the ablation process in metals and semiconductors. ACKNOWLEDGEMENT The authors are particularly indebted to Jörg Bialkowski for his important contributions to this work and to A. Cavalleri for a very enjoyable cooperation. The collaboration with S. A. Anisimov, J. Meyer-ter-Vehn and A. Oparin on theoretical issues is gratefully acknowledged. The work was partially supported by the Deutsche Forschungsgemeinschaft, the European Union under Human Capital and Mobility, and the Frankfurter Flughafen Gesellschaft.

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