- Email: [email protected]
Nanosecond pulse laser melting investigation by IR radiometry and reflection-based methods
Applied Surface Science 253 (2006) 1170–1177 www.elsevier.com/locate/apsusc
Nanosecond pulse laser melting investigation by IR radiometry and reflection-based methods J. Martan a,b,*, O. Cibulka b, N. Semmar a a
GREMI, CNRS/Universite´ d’Orle´ans, 14, rue d’Issoudun, BP 6744, 45067 Orle´ans Cedex 2, France Department of Physics, University of West Bohemia, Univerzitnı´ 22, 306 14 Plzenˇ, Czech Republic
b
Received 3 January 2006; received in revised form 25 January 2006; accepted 25 January 2006 Available online 13 March 2006
Abstract Experimental system for nanosecond laser melting investigation was developed containing three independent noncontact methods: infrared radiometry, time-resolved reflectivity of He–Ne laser and sample surface reflected KrF heating laser pulse. The system was applied to the investigation of laser melting of Cu, Mo, Ni, Si, Sn, Ti, steel CˇSN 15330 and stainless steel CˇSN 17246 samples. For metallic samples the IR radiometry signal was transformed to temperature. Obtained surface temperature and reflectivity spectra in nanosecond time scale (10–1000 ns) for wide range of energy densities (100–5500 mJ cm 2) are presented. Interesting evolutions were found. Melting thresholds and melting durations were determined from the measured curves. The applicability of the methods is evaluated. # 2006 Elsevier B.V. All rights reserved. PACS: 42.62. b; 06.60.Jn; 78.47.+p; 64.70.Dv; 44.40.+a; 42.55.Lt; 07.57.Kp; 07.07.Df; 07.20.Ka; 07.60.Dq Keywords: Nanosecond pulsed laser melting; Melting duration; Melting threshold; Infrared radiometry; Surface temperature measurement; Time-resolved reflectivity; Experimental
1. Introduction Nanosecond pulse lasers are often used in surface treatment of material by heating, melting or ablation with plasma formation. Because of short pulse duration, only thin subsurface part of the material is affected by the absorbed energy. This is important when the inner part of the material should not be affected. A lot of work has been done concerning laser heating and melting of semiconductors, especially silicon. Melting durations, depths, thresholds, surface temperatures and solidification velocities were determined for excitation by nanosecond pulse lasers [1–3]. Concerning metals, nanosecond time-resolved reflectivity measurements were done for the melting duration and threshold determination for metals (Cu, Au and Ni thin films) upon pulsed laser annealing [4]. The surface temperature and melting depth during laser nitriding of metals were predicted by a numerical
* Corresponding author. Tel.: +420 377632288; fax: +420 377632202. E-mail address: [email protected] (J. Martan). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.077
model in [5]. Other numerical modeling works focused on laser melting of metals are for example in [6,7]. On the other hand experimental determination of surface temperature during and after the nanosecond laser pulse is rare, especially for metals. Surface temperature evolution for a- and polysilicon thin films irradiated by an excimer laser were measured by InGaAs thermal radiation detector in [8]. A timeresolved spectroscopic measurements of surface temperature were made during very intensive laser irradiation of metals in vacuum which revealed peak black-body temperatures of 8500 1500 K while the electronic temperatures were in the order of 3 105 K [9]. A surface temperature measurement system based on field emission technique was described in [10]. A Pt sensor deposited under the layer exposed to nanosecond pulsed laser irradiation was used to measure temperatures during laser melting of the surface layer in [11]. Normalized temperature of the electron gas in a metallic thin film after the femtosecond laser irradiation was determined by a transient thermoreflectance in [12]. The presented work is focused on the application of developed surface temperature measurement system [13], in combination with two reflection-based methods, to the
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
1171
Fig. 1. Schematic representation of the experimental system.
investigation of melting process induced by a nanosecond pulsed laser on different materials especially metals. 2. Experimental system The experimental system (Fig. 1) consists of a heating laser, sample holder and equipment for three independent melting investigation methods: infrared (IR) radiometry, time-resolved reflectivity (TRR) and sample surface reflection of the heating laser pulse. The system and its calibration are described in detail in [14]. The heating laser is KrF excimer laser at wavelength 248 nm with the pulse duration 27 ns (FWHM) and maximum repetition rate 50 Hz. A mirror directs KrF laser beam to an optical axis of a beam homogenizer and a variable attenuator. The KrF laser beam is further directed by two mirrors and focused by a field lens to a spot 2.1 mm 1.8 mm on the sample surface. The laser beam incidence angle to the sample surface normal is 308. A beam splitter is placed into the laser beam for energy measurement of each pulse. It reflects 5 0.2% to an energy meter with a pyroelectric head. Scattered KrF light from the sample surface is detected by a fast phototube detector which triggers the oscilloscope and records the KrF laser pulse temporal shape. The infrared (IR) light emitted from the surface is focused on the IR radiation detector using two off-axis paraboloid mirrors. The detector is a liquid-nitrogen-cooled HgCdTe photovoltaic photodiode with a diameter of 0.25 mm and sensitivity in the spectral range 2–12 mm. The obtained signal is amplified with an internal preamplifier with the frequency range DC-100 MHz, and is recorded by a digital oscilloscope. The detector is battery-powered. In front of the detector there is a germanium filter which cuts off shorter wavelengths than 1.8 mm. In order to obtain absolute surface temperature evolution the IR detector output is calibrated for each sample by steady state heating in the temperature range from 25 to 350 8C. The obtained calibration curve is fitted with a theoretical calibration curve which is then used for the whole temperature range. The fitting is done for the sample emissivity in IR wavelengths [13]. Time-resolved reflectivity method (TRR) is used to probe phase transitions on the sample surface during and after the KrF laser irradiation. He–Ne CW laser beam (l = 632.8 nm) is precisely directed and focused by a beam steerer including lens
into the central part of the heated zone on the sample surface to form a spot one order smaller than the heated zone. The He–Ne laser beam incident angle to the sample surface normal is 378. Specularly reflected He–Ne light propagates through a narrow band interference filter to a photodiode. The obtained signal is amplified with a variable gain high-speed voltage amplifier. Changes in the reflectivity are also probed by a measurement of the laser pulse diffuse reflection using the phototube detector. The first intensive part of the pulse heats and melts the surface and the second part – the tail – is used for reflectivity change detection. The investigated samples were pure metals, metallic alloys and silicon in the bulk form: Cu, Mo, Ni, Si, Sn, Ti, steel CˇSN ˇ SN 17246. Chemical composition of 15330 and stainless steel C the samples is in the Table 1. The Si sample was a monocrystalline wafer with orientation (1 0 0) and thickness 575 mm doped with Sb with a 50 mm thick surface layer doped with P. The metallic samples were mechanically polished. 3. Results and discussion 3.1. Si sample Specular reflectivity and thermal emission transient traces induced by the KrF laser with different incident energy densities for monocrystalline silicon sample are shown in Figs. 2 and 3. These waveforms have a number of specific features which constitute a characteristic signature for melting of the material. Table 1 Chemical composition of the measured samples Material
Composition (wt.%)
Copper Molybdenum Nickel
Cu, 99.99; S, <0.01 Mo, 99.5; W, 0.4; Cu, 0.07 Ni, 99.7; Co, 0.062; Cu, 0.060; Fe, 0.048; Al, 0.043; Mn, 0.038; Si, 0.025 C, 0.29; Cr, 2.5; Mn, 0.6; Si, 0.27; Mo, 0.25; V, 0.22; P, <0.035; S, <0.035 C, 0.26; Cr, 18.06; Ni, 9.68; Mn, 1.22; Ti, 0.663; Si, 0.65; Mo, 0.28; P, 0.03; S, 0.021 Sn, 99.5; Pb, 0.275; Ba, 0.097; Fe, 0.038; Cu, 0.022 Ti, pure
Steel 15330 Steel 17246
Tin Titanium
1172
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Transient thermal emission has three important parts: first high peak, middle plateau and second low peak. Phase changes, melting and resolidification, induce emissivity changes and consequently form both peaks. Their positions are in an agreement with rapid reflectivity changes during melting and resolidification. The middle plateau represents thermal emission of the liquid silicon at almost stabilized temperature close to the melting temperature. For higher energy densities heating of the liquid phase forms the third peak between the first peak and the plateau. The thermal emission spectra evolve with increasing laser pulse energy density in following stages: Fig. 2. Si sample IR signal forms for different incident laser energy densities.
The reflectivity spectrum is a typical spectrum for silicon. The specular reflectivity at 632.8 nm at 300 K on monocrystalline silicon is 35%. Melted silicon has the properties of a metal, i.e. high reflectivity, in general 73% [15]. Melted silicon forms straight and smooth plateau at approximately 73% of reflectivity. This result indicates correctly constructed experimental set-up and promises correct results in other measurements. The reflectivity spectra evolve with increasing laser pulse energy density in following stages: In the energy density range 700–950 mJ cm 2 the maximum reflectivity of Si increases slowly with increase of temperature of the solid state. After that in the energy density range 950–1500 mJ cm 2 the maximum reflectivity increases rapidly from the value corresponding to the melting point of Si to the value of liquid Si. In this region there are two contributions to the measured maximum reflectivity. The sample surface is considered to be covered by a homogeneous thin molten layer which is thinner than the penetration depth of the He–Ne light. The thicker the molten layer, the higher the reflectivity. In the energy density range 1500–4300 mJ cm 2 measurements show approximately constant value of maximum reflectivity corresponding to the reflectivity of liquid Si. The molten layer is thicker than the penetration depth of He–Ne light.
Fig. 3. Si sample TRR signal forms for different incident laser energy densities.
The first thermal peak grows gradually from 700 mJ cm 2 up to the melt threshold 1100 mJ cm 2. This growth represents heating of the sample surface up to the melting point. In the measured energy density range 1100–1500 mJ cm 2 the molten layer changes from inhomogeneous to homogeneous and the material melts up to maximum depth from which the thermal radiation is detected. Then in the energy density range 1500–1900 mJ cm 2, the height of the first thermal peak very slowly decreases and for higher energy densities the first peak is covered by the third peak. In the largest energy density range 1500–4300 mJ cm 2 the height of the third thermal peak slowly grows up to the saturation of the HgCdTe detector. This growth is caused by the increase of the temperature of the melt during the laser irradiation. Similar behaviour of the IR signal was observed by an InGaAs radiation detector in [8] but the energy densities of the specific points were found in lower values. The second thermal peak appears when silicon starts to melt and its height is very similar to the height of the first peak. This second peak represents a phase change from the liquid (or partially liquid) to the solid phase. The height of the second peak first decreases (from 1100 to 1400 mJ cm 2), then increases and after 1500 mJ cm 2 stabilizes and represents the phase change from the homogeneous liquid phase to the solid phase. When the second thermal peak starts to appear and starts to fall, a valley occurs between the two peaks. The valley falls and extends in the energy density range 1150–1900 mJ cm 2 and changes to the straight and smooth plateau in the energy density range 1900–4300 mJ cm 2. The Si thermal emission data are not transformed to the temperature because silicon is semi-transparent in the solid state for the IR wavelengths. The thermal radiation is emitted from the surface and also from interior of the sample and during very fast processes, as laser heating in our case, the temperature distribution in the depth under the surface is not homogeneous and thus the steady state calibration cannot be used for voltage to temperature transformation. On the other hand metallic samples are opaque for the IR wavelengths and the thermal radiation is emitted only from very thin surface layer (10– 20 nm). In the liquid state silicon material behave like metals— is opaque for the IR wavelengths. The thermal emission signal
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
is thus possible to recalculate to the surface temperature evolution but knowledge of the liquid phase emissivity is necessary. The observed thermal emission signal shapes for different laser pulse energy densities are caused by the silicon optical properties. The silicon material has high emissivity in solid phase and low emissivity in the liquid phase and it is semi-transparent in solid phase and opaque in liquid phase for IR wavelengths. During laser heating the solid phase emits radiation with high emissivity from the surface and from the interior of the sample. When the melting starts during the laser irradiation a very thin liquid layer forms on the surface. The liquid layer has low emissivity but it is not thick enough to stop all the radiation coming from the interior of the sample and thus the obtained signal is a mixture of radiation from the liquid film and solid material under it. When the material is melted deeper under the surface the thermal radiation is emitted only from the liquid layer with low emissivity and thus a smaller signal is collected despite the fact that the liquid layer has higher temperature than the solid material. The same effect is observed during solidification. The peak of thermal emission is observed during solidification because more and more thermal radiation come from solid material with higher emissivity and less from the liquid layer with low emissivity and higher temperature. When the material is completely solidified the temperature of the solid material decreases and thus the thermal emission also decreases. 3.2. Mo sample Melting temperature of molybdenum is very high— 2623 8C, so the observation of melting by IR radiometry was perturbed by saturation of the IR detector for the higher energy density laser pulses. A neutral density filter with attenuation factor 100 (QNDS2, Polytech PI) was used for decreasing IR radiation flux onto the detector. As the filter transmission spectra for far-IR is not known, the curve shapes can be deformed. The IR curves observed with the attenuation filter were multiplied by 100 before transformation to the temperature. In the Fig. 4 both saturated and attenuated temperature evolutions including melting for the Mo sample induced by the KrF laser pulses are shown. The temperature increase level of the phase transformation observed in the curves is about 4000 K which is much higher than the mentioned melting temperature. This is probably caused by strong emissivity temperature dependence. The temperatures were calculated using calibration curve up to 300 8C. The phase transformation threshold is found at 3450 mJ cm 2. The reflectivity curves (TRR) induced on molybdenum sample changed during first laser pulses on the same spot at the sample surface at one energy density level due to cleaning of the surface, oxidation etc. After several pulses the TRR spectrum shape becomes steady. The steady spectra at four different energy density levels are shown in the Fig. 5. At low energy densities a simple peak of increased reflectivity appear during laser irradiation. At the energy densities bigger than 3900 mJ cm 2 the laser pulse induces a widening of the peak
1173
Fig. 4. Mo sample surface temperature change evolutions for different incident laser pulse energy densities. First two curves – IR detector saturation; other four curves – detected using attenuation filter. Molybdenum equilibrium melting temperature is 2623 8C.
approximately at the half of the peak height. With further increase of the energy density to 5600 mJ cm 2 the laser pulse induces a different TRR spectra shape. Widening of the peak changes its shape to a valley and forms a second peak. During the first three pulses the valley decreases its depth and stabilizes on the initial level. The first peak is narrower than peaks induced by low energy density pulses. This reflectivity behaviour can be explained by increase of the reflectivity with temperature but decrease with the phase change to the liquid phase. The thin liquid layer with lower reflectivity then forms the valley in the reflection spectra. The material is probably not melted down to the penetration depth of the He– Ne light, because no plateau is observed. The sample surface reflected KrF laser pulse profiles show similar behaviour as TRR spectra. The stabilized profiles are shown in the Fig. 6. The curves are normalized because of different amplitudes of the signal for different positions of the laser spot on the sample. The reflected temporal laser pulse shapes are the same up to a certain energy density value. For higher energy density the second part of the laser pulse is significantly more reflected. The enhanced reflection is caused by the liquid phase. Melting of the molybdenum sample starts at about 17 and 10 ns from the beginning of the laser pulse for the energy densities
Fig. 5. Mo sample TRR evolutions induced by the laser pulses with different incident energy densities.
1174
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 6. Normalized Mo sample surface reflected KrF laser light evolutions for different incident energy densities.
4010 and 5780 mJ cm 2. For the last case, when the melting occurs, the reflectivity grows during the laser pulse of about 30% of the reflection from the solid phase only during the main part of the pulse and of 200% during the tail part of the pulse. The three investigation methods indicated melting threshold laser energy density for molybdenum sample in the range from 3450 to 4010 mJ cm 2. The TRR at wavelength 632.8 nm observed reflectivity decrease during melting and KrF laser reflection method at wavelength 193 nm observed reflectivity increase during melting.
Fig. 8. Sn sample laser induced surface temperature evolution. Tin equilibrium melting temperature is 232 8C.
Investigation of laser melting process for Ni, Cu, Sn and stainless steel 17246 samples revealed behaviour similar for all the four samples. A plateau of stable temperature was observed before the end of solidification by the IR radiometry. The reflectivity detected by the TRR method decreased under irradiation and the TRR profiles widened during melting. Cu sample showed an increase of KrF laser light reflectivity when the melting occurred. The Ni sample IR signals induced by the KrF laser are shown in the Fig. 7. The figure shows a liquid material plateau with the duration increasing with the incident laser energy density. The solidification takes part in the time from 60 to 120 ns for energy density range 1500–2000 mJ cm 2. The peak of the highest
temperature evolution is saturated. Ni melting threshold determined from IR measurement is 1540 mJ cm 2. The surface temperature evolution during melting of Sn sample is shown in Fig. 8. It was obtained by composition of three laser pulses with the same laser energy density. Each of the pulses was recorded with a different oscilloscope voltage range in order to obtain the evolution with a low oscilloscope noise. The plateau of almost stabilized temperature before solidification at time about 600 ns is very well seen. The maximum temperature increase is in this case 1650 K. Melting of the stainless steel 17246 induced similar IR curves as described in the previous paragraphs. The plateaus are observed in the temperature evolutions when melting is attained during the laser pulse (Fig. 9). The two highest IR curves were obtained by a composition of three laser pulses with similar energy densities. Different signals were obtained by covering a part (up to 99%) of the paraboloid mirror in order to reduce the radiation reaching the detector and thus reducing or eliminating its saturation. The recalculated peak IR voltages for the energy densities 1200–2150 mJ cm 2 were in the range 10–100 V which cannot be recalculated to the temperature using the low temperature calibration and which indicates plasma formation by the laser pulse. The stainless steel 17246 sample TRR signals are shown in Fig. 10. The minimum reflectivity decreases down to 33% and the TRR curves widen with the laser energy density for the case of melting.
Fig. 7. Ni sample laser induced temperature evolutions for different incident energy densities. Nickel equilibrium melting temperature is 1453 8C.
Fig. 9. Stainless steel 17246 sample laser induced surface temperature evolutions for different incident energy densities. Stainless steel 17246 equilibrium melting temperature is about 1500 8C.
3.3. Ni, Cu, Sn and stainless steel 17246 samples
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 10. TRR evolutions during laser heating and melting of the stainless steel 17246.
Minimum reflectivities obtained on Ni, Cu, Sn and stainless steel 17246 samples decreased linearly with the increase of the energy density up to the melting threshold. After the threshold there was a change in the slope of the line. For the steel 17246 sample the reflectivity slightly increased for low energy densities and decreased for higher energy densities, showing a linear decrease, as for other samples. The change of the reflectivity behaviour was at energy density 240 mJ cm 2. From the discussion about changes of the thermoreflectance coefficient with temperature and wavelength in [16] comes out that the coefficient can change sign with temperature. The observed behaviour in the present work can be thus explained by positive thermoreflectance coefficient for lower temperatures and negative for higher temperatures for the steel 17246 sample and the probing wavelength 632.8 nm. 3.4. Ti and steel 15330 samples Melting and phase transformations in solid state of Ti and steel 15330 samples were investigated by IR and TRR methods. The IR radiometry showed solidification of the liquid phase and also a phase change in the solid phase. The TRR method revealed specific behaviour of the two samples for different incident laser energy densities. In the Fig. 11 there are shown surface temperature evolutions induced on the Ti sample by different energy density laser pulses. With the increase of the energy density the temperature rises. When the melting is attained, the IR curve widens and a plateau appears in the cooling phase as described before for other materials. On the other hand, the TRR evolutions evolve in a very interesting way (Fig. 12). The low energy density laser pulses induce decrease of the reflectivity— usual form of the TRR spectra for metals. With an increase of the energy density the decrease of the reflectivity shortens and after the laser pulse the reflectivity rises above the initial value rapidly. With further energy density increase the melting is attained and the reflectivity rises directly from the beginning of the laser pulse and at the end it suddenly starts to decrease— indicating a solidification of the melt. The cooling part of the temperature evolutions for high energy density laser pulses shows a second plateau in much
1175
Fig. 11. Surface temperature evolutions for Ti sample showing solidification and solid phase transformation. Titanium equilibrium melting temperature is 1620 8C and b to a transformation temperature is 882 8C.
lower temperature values than melting temperature (Fig. 11). This plateau indicates a phase change in the solid material—a transformation from the high temperature body-centered cubic b phase to the low temperature hexagonal close-packed a phase of the titanium. The transition temperature is 882 8C [17]. Temperature evolutions induced by the KrF laser on steel 15330 sample were similar to ones obtained on Ti sample. There were observed processes ranging from only heating to melting with phase change in the solid phase. The curve for the highest energy density 968 mJ cm 2 contained the signs of solidification and the g to a phase transformation. The reflectivity increases under laser irradiation for the steel 15330 sample (Fig. 13). For the pulses, where the g phase exists, the TRR curves show a rise in the beginning and then a constant reflectivity level up to the end of the laser pulse. Then the reflectivity increases again. This behaviour is similar to the Ti sample behaviour when the phase transformation in the solid phase occurred. 3.5. Melting threshold and duration Melting threshold of the investigated materials was determined by the three investigation methods: IR radiometry, TRR and sample surface reflected KrF laser pulse. The measured melt thresholds are presented in Table 2. The lowest
Fig. 12. Ti sample TRR curves induced by the laser pulses with different incident energy densities.
1176
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 13. Steel 15330 sample TRR curves induced by the laser pulses with different incident energy densities.
Fig. 14. Melting duration dependence on incident laser pulse energy density for Si sample obtained by IR radiometry and TRR methods.
threshold was observed for Sn sample and the highest for Mo sample. The melting threshold value of 950 mJ cm 2 obtained for steel 15330 is comparable to the calculated value of 1000 mJ cm 2 [5] for iron irradiated with a pulsed laser (l = 308 nm and tp = 55 ns). The melting thresholds in the present work are assigned to the lowest energy density value of the laser pulse when a trace of existence of the molten state was observed. The peak surface temperature observed for the laser pulse with melting threshold energy density is much higher than the equilibrium melting temperature. In many cases the peak temperature was almost twice as high as the melting temperature (in 8C) when the melting was first observed. The melting duration induced by the KrF laser pulse was determined by use of the IR radiometry and TRR methods. The melting duration dependencies on the incident laser energy density for Si, Ni, Cu, Mo, Sn, Ti, steel 15330 and stainless steel 17246 samples are shown in Figs. 14–16. Both the TRR and IR measurements reveal the same melting duration over the whole range of the energy density. The melting duration grows faster than linearly with the energy density. The fastest rise of the melting duration with the energy density was found for the tin sample and the slowest for the molybdenum sample.
In the figures the durations of existence of the Ti-b phase and the 15330-g phases are shown as well. They were determined using the phase transformation traces in the temperature evolution. In the case of attainment of the molten state during the laser pulse the durations of existence of the different solid phases are not exact because of the liquid phase existence during this period, but the values indicate well the time between the transformations from and to the stable solid phase. 3.6. Melting temperature and emissivity Comparison of the measured melting temperature levels of different metallic samples shown in the figures and equilibrium melting temperatures, obtained from literature [17–19] and mentioned in the figure captions, indicates that measured values are in all cases higher than literature values. From the material science we would expect the measured values to be slightly lower than equilibrium ones because of undercooling during solidification. This discrepancy can be explained by increase of the material emissivity with temperature and with phase change to the liquid state, which is usual behaviour of metals [19]. On the other hand the difference is relatively small (5–18%) except for Mo (53%) and Ti (36%) samples. This small difference can justify our use of low temperature calibration to high
Table 2 Melting thresholds for pulsed KrF laser irradiation for different samples identified by IR radiometry, TRR and sample surface reflected heating KrF laser pulse Sample
Si Mo Ni Cu Sn Ti Ti-b phase Steel 15330 Steel 15330-g phase Stainless steel 17246
Melting threshold (mJ cm 2) IR
TRR
1100 3450 1540 2275 250 845 495 950 695 610
1045 4010 1540 2065 135
600 610
KrF 4010 3250
Mean 1073 3820 1540 2530 193 845 495 950 648 610
Fig. 15. Melting duration dependence on incident laser pulse energy density for Ni, Cu and Mo samples obtained by IR radiometry and TRR methods.
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 16. Melting duration dependence on incident laser pulse energy density obtained by IR radiometry and TRR methods for Sn, Ti, steel 15330 and stainless steel 17246 samples and duration of existence of Ti-b phase and steel 15330-g phase.
1177
for all the samples: solidification traces in IR curves, a linear decrease of the reflectivity with an increase of the laser energy density for laser heating and widening of the TRR profiles during melting. For Ti and steel 15330 samples the IR radiometry showed solidification of the liquid phase and also phase change in the solid phase. Specific evolutions of reflectivity with phase changes were observed by the TRR method. The IR radiometry and TRR methods are well suited for the observation of solidification of the liquid phase. The IR evolutions have the same shape for all metals. The reflectivity (TRR) changes under irradiation in different way (increase/ decrease and different shapes) for different metals. The sample surface reflected KrF laser pulse method is suitable for detection of melting start. However the corresponding signals were detected only for two samples.
temperatures and even to liquid phase temperature recalculation from IR signal during and after the laser pulse. Using the voltage level of the solidification plateau in the IR signal and knowledge of the equilibrium melting temperature the liquid phase emissivity can be calculated. This problematic will be treated in another paper.
Acknowledgement
4. Conclusion
References
Three independent investigation methods were developed for laser induced thermal processes investigation: infrared radiometry, time-resolved reflectivity and sample surface reflection of the heating laser pulse. Combination of these three methods enhances the accuracy in identification of the process characteristics. The KrF pulsed laser melting was investigated for eight samples. The melting threshold and melting duration were determined. For the IR radiometry in the case of only heating a smooth surface temperature evolution is obtained. In the case of melting, during the cooling phase a plateau of almost stable temperature (or inflection point) forms at the end of the solidification of the liquid phase. The temperature level of the plateau is close to the equilibrium melting temperature. The TRR evolution shows a rapid change during melting of the material surface. Investigation of Si sample revealed a standard TRR response indicating correctly constructed experimental set-up. The IR radiation evolution during different stages of melting showed changes caused by very different emissivity of solid and liquid phase. The Mo sample showed a reflectivity increase during laser heating and decrease during melting at the He–Ne laser wavelength and increase during melting at the KrF laser wavelength. The sample surface reflected KrF laser pulse method indicated the start of melting at 17 and 10 ns from the beginning of the laser pulse for the energy densities 4 and 5.8 J cm 2. Investigation of laser melting process for Ni, Cu, Sn and stainless steel 17246 samples revealed similar behaviour
[1] R.B. James, G.A. Geist, R.T. Young, W.H. Christie, F.A. Greulich, J. Appl. Phys. 62 (1987) 2981–2988. [2] S. De Unamuno, E. Fogarassy, Appl. Surf. Sci. 36 (1989) 1–11. [3] R.K. Singh, D.R. Gilbert, J. Viatella, Mater. Sci. Eng. B40 (1996) 89– 95. [4] J. Boneberg, J. Bischof, P. Leiderer, Opt. Commun. 174 (2000) 145–149. [5] P. Shaaf, Prog. Mater. Sci. 47 (2002) 1–161. [6] N. Semmar, C. Georges, C. Leborgne, Microelectron. J. 33 (2002) 705– 710. [7] G. Nicolas, A. Yanez, A. Ramil, J.C. Alvarez, E. Saavedra, A. GarciaBeltran, C. Molpeceres, M. Autric, J.L. Ocana, Appl. Surf. Sci. 138/139 (1999) 169–173. [8] M. Hatano, S. Moon, M. Lee, J. Suzuki, C.P. Grigoropoulos, J. Appl. Phys. 87 (2000) 36–43. [9] F.A. Richardst, D. Walsh, Br. J. Appl. Phys. (J. Phys. D) 2 (1969) 663–668. [10] F.F. Kormendi, J. Phys. E: Sci. Instrum. 7 (1974) 1004–1006. [11] J.A. Kittl, P.G. Sanders, M.J. Aziz, D.P. Brunco, M.O. Thompson, Acta Mater. 48 (2000) 4797–4811. [12] D.Y. Tzou, K.S. Chiu, Int. J. Heat Mass Transfer 44 (2001) 1725–1734. [13] J. Martan, N. Semmar, C. Leborgne, E. Le Menn, J. Mathias, Appl. Surf. Sci. 247 (2005) 57–63. [14] O. E. Cibulka, N. Semmar, J. Martan, E. Le Menn, Combination of two optical methods for monitoring of the thermal processes in the material induced by pulsed laser irradiation, Opt. Lasers Eng., in press. [15] I. Lukesˇ, Modifikace povrchu monokrystalicke´ho krˇemı´ku za´rˇenı´m ArF excimerove´ho laseru, Ph.D. thesis, FZU-CSAV, Praha, 1989. [16] M.G. Burzo, P.L. Komarov, P.E. Raad, IEEE Trans. Compon. Pack. Technol. 28 (2005) 39–44. [17] Hugh Baker (Ed.), ASM Handbook, vol. 3: Alloy Phase Diagrams, ASM International, Materials Park, Ohio, USA, 1999. [18] WebElements material properties database, http://www.webelements.com. [19] M. von Almen, Laser-beam Interactions with Materials, Springer Series in Material Sciences, Springer Verlag, Berlin, 1987.
The work has been supported by research project MSM4977751302 of the Ministry of Education of the Czech Republic.
Nanosecond pulse laser melting investigation by IR radiometry and reflection-based methods J. Martan a,b,*, O. Cibulka b, N. Semmar a a
GREMI, CNRS/Universite´ d’Orle´ans, 14, rue d’Issoudun, BP 6744, 45067 Orle´ans Cedex 2, France Department of Physics, University of West Bohemia, Univerzitnı´ 22, 306 14 Plzenˇ, Czech Republic
b
Received 3 January 2006; received in revised form 25 January 2006; accepted 25 January 2006 Available online 13 March 2006
Abstract Experimental system for nanosecond laser melting investigation was developed containing three independent noncontact methods: infrared radiometry, time-resolved reflectivity of He–Ne laser and sample surface reflected KrF heating laser pulse. The system was applied to the investigation of laser melting of Cu, Mo, Ni, Si, Sn, Ti, steel CˇSN 15330 and stainless steel CˇSN 17246 samples. For metallic samples the IR radiometry signal was transformed to temperature. Obtained surface temperature and reflectivity spectra in nanosecond time scale (10–1000 ns) for wide range of energy densities (100–5500 mJ cm 2) are presented. Interesting evolutions were found. Melting thresholds and melting durations were determined from the measured curves. The applicability of the methods is evaluated. # 2006 Elsevier B.V. All rights reserved. PACS: 42.62. b; 06.60.Jn; 78.47.+p; 64.70.Dv; 44.40.+a; 42.55.Lt; 07.57.Kp; 07.07.Df; 07.20.Ka; 07.60.Dq Keywords: Nanosecond pulsed laser melting; Melting duration; Melting threshold; Infrared radiometry; Surface temperature measurement; Time-resolved reflectivity; Experimental
1. Introduction Nanosecond pulse lasers are often used in surface treatment of material by heating, melting or ablation with plasma formation. Because of short pulse duration, only thin subsurface part of the material is affected by the absorbed energy. This is important when the inner part of the material should not be affected. A lot of work has been done concerning laser heating and melting of semiconductors, especially silicon. Melting durations, depths, thresholds, surface temperatures and solidification velocities were determined for excitation by nanosecond pulse lasers [1–3]. Concerning metals, nanosecond time-resolved reflectivity measurements were done for the melting duration and threshold determination for metals (Cu, Au and Ni thin films) upon pulsed laser annealing [4]. The surface temperature and melting depth during laser nitriding of metals were predicted by a numerical
* Corresponding author. Tel.: +420 377632288; fax: +420 377632202. E-mail address: [email protected] (J. Martan). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.077
model in [5]. Other numerical modeling works focused on laser melting of metals are for example in [6,7]. On the other hand experimental determination of surface temperature during and after the nanosecond laser pulse is rare, especially for metals. Surface temperature evolution for a- and polysilicon thin films irradiated by an excimer laser were measured by InGaAs thermal radiation detector in [8]. A timeresolved spectroscopic measurements of surface temperature were made during very intensive laser irradiation of metals in vacuum which revealed peak black-body temperatures of 8500 1500 K while the electronic temperatures were in the order of 3 105 K [9]. A surface temperature measurement system based on field emission technique was described in [10]. A Pt sensor deposited under the layer exposed to nanosecond pulsed laser irradiation was used to measure temperatures during laser melting of the surface layer in [11]. Normalized temperature of the electron gas in a metallic thin film after the femtosecond laser irradiation was determined by a transient thermoreflectance in [12]. The presented work is focused on the application of developed surface temperature measurement system [13], in combination with two reflection-based methods, to the
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
1171
Fig. 1. Schematic representation of the experimental system.
investigation of melting process induced by a nanosecond pulsed laser on different materials especially metals. 2. Experimental system The experimental system (Fig. 1) consists of a heating laser, sample holder and equipment for three independent melting investigation methods: infrared (IR) radiometry, time-resolved reflectivity (TRR) and sample surface reflection of the heating laser pulse. The system and its calibration are described in detail in [14]. The heating laser is KrF excimer laser at wavelength 248 nm with the pulse duration 27 ns (FWHM) and maximum repetition rate 50 Hz. A mirror directs KrF laser beam to an optical axis of a beam homogenizer and a variable attenuator. The KrF laser beam is further directed by two mirrors and focused by a field lens to a spot 2.1 mm 1.8 mm on the sample surface. The laser beam incidence angle to the sample surface normal is 308. A beam splitter is placed into the laser beam for energy measurement of each pulse. It reflects 5 0.2% to an energy meter with a pyroelectric head. Scattered KrF light from the sample surface is detected by a fast phototube detector which triggers the oscilloscope and records the KrF laser pulse temporal shape. The infrared (IR) light emitted from the surface is focused on the IR radiation detector using two off-axis paraboloid mirrors. The detector is a liquid-nitrogen-cooled HgCdTe photovoltaic photodiode with a diameter of 0.25 mm and sensitivity in the spectral range 2–12 mm. The obtained signal is amplified with an internal preamplifier with the frequency range DC-100 MHz, and is recorded by a digital oscilloscope. The detector is battery-powered. In front of the detector there is a germanium filter which cuts off shorter wavelengths than 1.8 mm. In order to obtain absolute surface temperature evolution the IR detector output is calibrated for each sample by steady state heating in the temperature range from 25 to 350 8C. The obtained calibration curve is fitted with a theoretical calibration curve which is then used for the whole temperature range. The fitting is done for the sample emissivity in IR wavelengths [13]. Time-resolved reflectivity method (TRR) is used to probe phase transitions on the sample surface during and after the KrF laser irradiation. He–Ne CW laser beam (l = 632.8 nm) is precisely directed and focused by a beam steerer including lens
into the central part of the heated zone on the sample surface to form a spot one order smaller than the heated zone. The He–Ne laser beam incident angle to the sample surface normal is 378. Specularly reflected He–Ne light propagates through a narrow band interference filter to a photodiode. The obtained signal is amplified with a variable gain high-speed voltage amplifier. Changes in the reflectivity are also probed by a measurement of the laser pulse diffuse reflection using the phototube detector. The first intensive part of the pulse heats and melts the surface and the second part – the tail – is used for reflectivity change detection. The investigated samples were pure metals, metallic alloys and silicon in the bulk form: Cu, Mo, Ni, Si, Sn, Ti, steel CˇSN ˇ SN 17246. Chemical composition of 15330 and stainless steel C the samples is in the Table 1. The Si sample was a monocrystalline wafer with orientation (1 0 0) and thickness 575 mm doped with Sb with a 50 mm thick surface layer doped with P. The metallic samples were mechanically polished. 3. Results and discussion 3.1. Si sample Specular reflectivity and thermal emission transient traces induced by the KrF laser with different incident energy densities for monocrystalline silicon sample are shown in Figs. 2 and 3. These waveforms have a number of specific features which constitute a characteristic signature for melting of the material. Table 1 Chemical composition of the measured samples Material
Composition (wt.%)
Copper Molybdenum Nickel
Cu, 99.99; S, <0.01 Mo, 99.5; W, 0.4; Cu, 0.07 Ni, 99.7; Co, 0.062; Cu, 0.060; Fe, 0.048; Al, 0.043; Mn, 0.038; Si, 0.025 C, 0.29; Cr, 2.5; Mn, 0.6; Si, 0.27; Mo, 0.25; V, 0.22; P, <0.035; S, <0.035 C, 0.26; Cr, 18.06; Ni, 9.68; Mn, 1.22; Ti, 0.663; Si, 0.65; Mo, 0.28; P, 0.03; S, 0.021 Sn, 99.5; Pb, 0.275; Ba, 0.097; Fe, 0.038; Cu, 0.022 Ti, pure
Steel 15330 Steel 17246
Tin Titanium
1172
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Transient thermal emission has three important parts: first high peak, middle plateau and second low peak. Phase changes, melting and resolidification, induce emissivity changes and consequently form both peaks. Their positions are in an agreement with rapid reflectivity changes during melting and resolidification. The middle plateau represents thermal emission of the liquid silicon at almost stabilized temperature close to the melting temperature. For higher energy densities heating of the liquid phase forms the third peak between the first peak and the plateau. The thermal emission spectra evolve with increasing laser pulse energy density in following stages: Fig. 2. Si sample IR signal forms for different incident laser energy densities.
The reflectivity spectrum is a typical spectrum for silicon. The specular reflectivity at 632.8 nm at 300 K on monocrystalline silicon is 35%. Melted silicon has the properties of a metal, i.e. high reflectivity, in general 73% [15]. Melted silicon forms straight and smooth plateau at approximately 73% of reflectivity. This result indicates correctly constructed experimental set-up and promises correct results in other measurements. The reflectivity spectra evolve with increasing laser pulse energy density in following stages: In the energy density range 700–950 mJ cm 2 the maximum reflectivity of Si increases slowly with increase of temperature of the solid state. After that in the energy density range 950–1500 mJ cm 2 the maximum reflectivity increases rapidly from the value corresponding to the melting point of Si to the value of liquid Si. In this region there are two contributions to the measured maximum reflectivity. The sample surface is considered to be covered by a homogeneous thin molten layer which is thinner than the penetration depth of the He–Ne light. The thicker the molten layer, the higher the reflectivity. In the energy density range 1500–4300 mJ cm 2 measurements show approximately constant value of maximum reflectivity corresponding to the reflectivity of liquid Si. The molten layer is thicker than the penetration depth of He–Ne light.
Fig. 3. Si sample TRR signal forms for different incident laser energy densities.
The first thermal peak grows gradually from 700 mJ cm 2 up to the melt threshold 1100 mJ cm 2. This growth represents heating of the sample surface up to the melting point. In the measured energy density range 1100–1500 mJ cm 2 the molten layer changes from inhomogeneous to homogeneous and the material melts up to maximum depth from which the thermal radiation is detected. Then in the energy density range 1500–1900 mJ cm 2, the height of the first thermal peak very slowly decreases and for higher energy densities the first peak is covered by the third peak. In the largest energy density range 1500–4300 mJ cm 2 the height of the third thermal peak slowly grows up to the saturation of the HgCdTe detector. This growth is caused by the increase of the temperature of the melt during the laser irradiation. Similar behaviour of the IR signal was observed by an InGaAs radiation detector in [8] but the energy densities of the specific points were found in lower values. The second thermal peak appears when silicon starts to melt and its height is very similar to the height of the first peak. This second peak represents a phase change from the liquid (or partially liquid) to the solid phase. The height of the second peak first decreases (from 1100 to 1400 mJ cm 2), then increases and after 1500 mJ cm 2 stabilizes and represents the phase change from the homogeneous liquid phase to the solid phase. When the second thermal peak starts to appear and starts to fall, a valley occurs between the two peaks. The valley falls and extends in the energy density range 1150–1900 mJ cm 2 and changes to the straight and smooth plateau in the energy density range 1900–4300 mJ cm 2. The Si thermal emission data are not transformed to the temperature because silicon is semi-transparent in the solid state for the IR wavelengths. The thermal radiation is emitted from the surface and also from interior of the sample and during very fast processes, as laser heating in our case, the temperature distribution in the depth under the surface is not homogeneous and thus the steady state calibration cannot be used for voltage to temperature transformation. On the other hand metallic samples are opaque for the IR wavelengths and the thermal radiation is emitted only from very thin surface layer (10– 20 nm). In the liquid state silicon material behave like metals— is opaque for the IR wavelengths. The thermal emission signal
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
is thus possible to recalculate to the surface temperature evolution but knowledge of the liquid phase emissivity is necessary. The observed thermal emission signal shapes for different laser pulse energy densities are caused by the silicon optical properties. The silicon material has high emissivity in solid phase and low emissivity in the liquid phase and it is semi-transparent in solid phase and opaque in liquid phase for IR wavelengths. During laser heating the solid phase emits radiation with high emissivity from the surface and from the interior of the sample. When the melting starts during the laser irradiation a very thin liquid layer forms on the surface. The liquid layer has low emissivity but it is not thick enough to stop all the radiation coming from the interior of the sample and thus the obtained signal is a mixture of radiation from the liquid film and solid material under it. When the material is melted deeper under the surface the thermal radiation is emitted only from the liquid layer with low emissivity and thus a smaller signal is collected despite the fact that the liquid layer has higher temperature than the solid material. The same effect is observed during solidification. The peak of thermal emission is observed during solidification because more and more thermal radiation come from solid material with higher emissivity and less from the liquid layer with low emissivity and higher temperature. When the material is completely solidified the temperature of the solid material decreases and thus the thermal emission also decreases. 3.2. Mo sample Melting temperature of molybdenum is very high— 2623 8C, so the observation of melting by IR radiometry was perturbed by saturation of the IR detector for the higher energy density laser pulses. A neutral density filter with attenuation factor 100 (QNDS2, Polytech PI) was used for decreasing IR radiation flux onto the detector. As the filter transmission spectra for far-IR is not known, the curve shapes can be deformed. The IR curves observed with the attenuation filter were multiplied by 100 before transformation to the temperature. In the Fig. 4 both saturated and attenuated temperature evolutions including melting for the Mo sample induced by the KrF laser pulses are shown. The temperature increase level of the phase transformation observed in the curves is about 4000 K which is much higher than the mentioned melting temperature. This is probably caused by strong emissivity temperature dependence. The temperatures were calculated using calibration curve up to 300 8C. The phase transformation threshold is found at 3450 mJ cm 2. The reflectivity curves (TRR) induced on molybdenum sample changed during first laser pulses on the same spot at the sample surface at one energy density level due to cleaning of the surface, oxidation etc. After several pulses the TRR spectrum shape becomes steady. The steady spectra at four different energy density levels are shown in the Fig. 5. At low energy densities a simple peak of increased reflectivity appear during laser irradiation. At the energy densities bigger than 3900 mJ cm 2 the laser pulse induces a widening of the peak
1173
Fig. 4. Mo sample surface temperature change evolutions for different incident laser pulse energy densities. First two curves – IR detector saturation; other four curves – detected using attenuation filter. Molybdenum equilibrium melting temperature is 2623 8C.
approximately at the half of the peak height. With further increase of the energy density to 5600 mJ cm 2 the laser pulse induces a different TRR spectra shape. Widening of the peak changes its shape to a valley and forms a second peak. During the first three pulses the valley decreases its depth and stabilizes on the initial level. The first peak is narrower than peaks induced by low energy density pulses. This reflectivity behaviour can be explained by increase of the reflectivity with temperature but decrease with the phase change to the liquid phase. The thin liquid layer with lower reflectivity then forms the valley in the reflection spectra. The material is probably not melted down to the penetration depth of the He– Ne light, because no plateau is observed. The sample surface reflected KrF laser pulse profiles show similar behaviour as TRR spectra. The stabilized profiles are shown in the Fig. 6. The curves are normalized because of different amplitudes of the signal for different positions of the laser spot on the sample. The reflected temporal laser pulse shapes are the same up to a certain energy density value. For higher energy density the second part of the laser pulse is significantly more reflected. The enhanced reflection is caused by the liquid phase. Melting of the molybdenum sample starts at about 17 and 10 ns from the beginning of the laser pulse for the energy densities
Fig. 5. Mo sample TRR evolutions induced by the laser pulses with different incident energy densities.
1174
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 6. Normalized Mo sample surface reflected KrF laser light evolutions for different incident energy densities.
4010 and 5780 mJ cm 2. For the last case, when the melting occurs, the reflectivity grows during the laser pulse of about 30% of the reflection from the solid phase only during the main part of the pulse and of 200% during the tail part of the pulse. The three investigation methods indicated melting threshold laser energy density for molybdenum sample in the range from 3450 to 4010 mJ cm 2. The TRR at wavelength 632.8 nm observed reflectivity decrease during melting and KrF laser reflection method at wavelength 193 nm observed reflectivity increase during melting.
Fig. 8. Sn sample laser induced surface temperature evolution. Tin equilibrium melting temperature is 232 8C.
Investigation of laser melting process for Ni, Cu, Sn and stainless steel 17246 samples revealed behaviour similar for all the four samples. A plateau of stable temperature was observed before the end of solidification by the IR radiometry. The reflectivity detected by the TRR method decreased under irradiation and the TRR profiles widened during melting. Cu sample showed an increase of KrF laser light reflectivity when the melting occurred. The Ni sample IR signals induced by the KrF laser are shown in the Fig. 7. The figure shows a liquid material plateau with the duration increasing with the incident laser energy density. The solidification takes part in the time from 60 to 120 ns for energy density range 1500–2000 mJ cm 2. The peak of the highest
temperature evolution is saturated. Ni melting threshold determined from IR measurement is 1540 mJ cm 2. The surface temperature evolution during melting of Sn sample is shown in Fig. 8. It was obtained by composition of three laser pulses with the same laser energy density. Each of the pulses was recorded with a different oscilloscope voltage range in order to obtain the evolution with a low oscilloscope noise. The plateau of almost stabilized temperature before solidification at time about 600 ns is very well seen. The maximum temperature increase is in this case 1650 K. Melting of the stainless steel 17246 induced similar IR curves as described in the previous paragraphs. The plateaus are observed in the temperature evolutions when melting is attained during the laser pulse (Fig. 9). The two highest IR curves were obtained by a composition of three laser pulses with similar energy densities. Different signals were obtained by covering a part (up to 99%) of the paraboloid mirror in order to reduce the radiation reaching the detector and thus reducing or eliminating its saturation. The recalculated peak IR voltages for the energy densities 1200–2150 mJ cm 2 were in the range 10–100 V which cannot be recalculated to the temperature using the low temperature calibration and which indicates plasma formation by the laser pulse. The stainless steel 17246 sample TRR signals are shown in Fig. 10. The minimum reflectivity decreases down to 33% and the TRR curves widen with the laser energy density for the case of melting.
Fig. 7. Ni sample laser induced temperature evolutions for different incident energy densities. Nickel equilibrium melting temperature is 1453 8C.
Fig. 9. Stainless steel 17246 sample laser induced surface temperature evolutions for different incident energy densities. Stainless steel 17246 equilibrium melting temperature is about 1500 8C.
3.3. Ni, Cu, Sn and stainless steel 17246 samples
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 10. TRR evolutions during laser heating and melting of the stainless steel 17246.
Minimum reflectivities obtained on Ni, Cu, Sn and stainless steel 17246 samples decreased linearly with the increase of the energy density up to the melting threshold. After the threshold there was a change in the slope of the line. For the steel 17246 sample the reflectivity slightly increased for low energy densities and decreased for higher energy densities, showing a linear decrease, as for other samples. The change of the reflectivity behaviour was at energy density 240 mJ cm 2. From the discussion about changes of the thermoreflectance coefficient with temperature and wavelength in [16] comes out that the coefficient can change sign with temperature. The observed behaviour in the present work can be thus explained by positive thermoreflectance coefficient for lower temperatures and negative for higher temperatures for the steel 17246 sample and the probing wavelength 632.8 nm. 3.4. Ti and steel 15330 samples Melting and phase transformations in solid state of Ti and steel 15330 samples were investigated by IR and TRR methods. The IR radiometry showed solidification of the liquid phase and also a phase change in the solid phase. The TRR method revealed specific behaviour of the two samples for different incident laser energy densities. In the Fig. 11 there are shown surface temperature evolutions induced on the Ti sample by different energy density laser pulses. With the increase of the energy density the temperature rises. When the melting is attained, the IR curve widens and a plateau appears in the cooling phase as described before for other materials. On the other hand, the TRR evolutions evolve in a very interesting way (Fig. 12). The low energy density laser pulses induce decrease of the reflectivity— usual form of the TRR spectra for metals. With an increase of the energy density the decrease of the reflectivity shortens and after the laser pulse the reflectivity rises above the initial value rapidly. With further energy density increase the melting is attained and the reflectivity rises directly from the beginning of the laser pulse and at the end it suddenly starts to decrease— indicating a solidification of the melt. The cooling part of the temperature evolutions for high energy density laser pulses shows a second plateau in much
1175
Fig. 11. Surface temperature evolutions for Ti sample showing solidification and solid phase transformation. Titanium equilibrium melting temperature is 1620 8C and b to a transformation temperature is 882 8C.
lower temperature values than melting temperature (Fig. 11). This plateau indicates a phase change in the solid material—a transformation from the high temperature body-centered cubic b phase to the low temperature hexagonal close-packed a phase of the titanium. The transition temperature is 882 8C [17]. Temperature evolutions induced by the KrF laser on steel 15330 sample were similar to ones obtained on Ti sample. There were observed processes ranging from only heating to melting with phase change in the solid phase. The curve for the highest energy density 968 mJ cm 2 contained the signs of solidification and the g to a phase transformation. The reflectivity increases under laser irradiation for the steel 15330 sample (Fig. 13). For the pulses, where the g phase exists, the TRR curves show a rise in the beginning and then a constant reflectivity level up to the end of the laser pulse. Then the reflectivity increases again. This behaviour is similar to the Ti sample behaviour when the phase transformation in the solid phase occurred. 3.5. Melting threshold and duration Melting threshold of the investigated materials was determined by the three investigation methods: IR radiometry, TRR and sample surface reflected KrF laser pulse. The measured melt thresholds are presented in Table 2. The lowest
Fig. 12. Ti sample TRR curves induced by the laser pulses with different incident energy densities.
1176
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 13. Steel 15330 sample TRR curves induced by the laser pulses with different incident energy densities.
Fig. 14. Melting duration dependence on incident laser pulse energy density for Si sample obtained by IR radiometry and TRR methods.
threshold was observed for Sn sample and the highest for Mo sample. The melting threshold value of 950 mJ cm 2 obtained for steel 15330 is comparable to the calculated value of 1000 mJ cm 2 [5] for iron irradiated with a pulsed laser (l = 308 nm and tp = 55 ns). The melting thresholds in the present work are assigned to the lowest energy density value of the laser pulse when a trace of existence of the molten state was observed. The peak surface temperature observed for the laser pulse with melting threshold energy density is much higher than the equilibrium melting temperature. In many cases the peak temperature was almost twice as high as the melting temperature (in 8C) when the melting was first observed. The melting duration induced by the KrF laser pulse was determined by use of the IR radiometry and TRR methods. The melting duration dependencies on the incident laser energy density for Si, Ni, Cu, Mo, Sn, Ti, steel 15330 and stainless steel 17246 samples are shown in Figs. 14–16. Both the TRR and IR measurements reveal the same melting duration over the whole range of the energy density. The melting duration grows faster than linearly with the energy density. The fastest rise of the melting duration with the energy density was found for the tin sample and the slowest for the molybdenum sample.
In the figures the durations of existence of the Ti-b phase and the 15330-g phases are shown as well. They were determined using the phase transformation traces in the temperature evolution. In the case of attainment of the molten state during the laser pulse the durations of existence of the different solid phases are not exact because of the liquid phase existence during this period, but the values indicate well the time between the transformations from and to the stable solid phase. 3.6. Melting temperature and emissivity Comparison of the measured melting temperature levels of different metallic samples shown in the figures and equilibrium melting temperatures, obtained from literature [17–19] and mentioned in the figure captions, indicates that measured values are in all cases higher than literature values. From the material science we would expect the measured values to be slightly lower than equilibrium ones because of undercooling during solidification. This discrepancy can be explained by increase of the material emissivity with temperature and with phase change to the liquid state, which is usual behaviour of metals [19]. On the other hand the difference is relatively small (5–18%) except for Mo (53%) and Ti (36%) samples. This small difference can justify our use of low temperature calibration to high
Table 2 Melting thresholds for pulsed KrF laser irradiation for different samples identified by IR radiometry, TRR and sample surface reflected heating KrF laser pulse Sample
Si Mo Ni Cu Sn Ti Ti-b phase Steel 15330 Steel 15330-g phase Stainless steel 17246
Melting threshold (mJ cm 2) IR
TRR
1100 3450 1540 2275 250 845 495 950 695 610
1045 4010 1540 2065 135
600 610
KrF 4010 3250
Mean 1073 3820 1540 2530 193 845 495 950 648 610
Fig. 15. Melting duration dependence on incident laser pulse energy density for Ni, Cu and Mo samples obtained by IR radiometry and TRR methods.
J. Martan et al. / Applied Surface Science 253 (2006) 1170–1177
Fig. 16. Melting duration dependence on incident laser pulse energy density obtained by IR radiometry and TRR methods for Sn, Ti, steel 15330 and stainless steel 17246 samples and duration of existence of Ti-b phase and steel 15330-g phase.
1177
for all the samples: solidification traces in IR curves, a linear decrease of the reflectivity with an increase of the laser energy density for laser heating and widening of the TRR profiles during melting. For Ti and steel 15330 samples the IR radiometry showed solidification of the liquid phase and also phase change in the solid phase. Specific evolutions of reflectivity with phase changes were observed by the TRR method. The IR radiometry and TRR methods are well suited for the observation of solidification of the liquid phase. The IR evolutions have the same shape for all metals. The reflectivity (TRR) changes under irradiation in different way (increase/ decrease and different shapes) for different metals. The sample surface reflected KrF laser pulse method is suitable for detection of melting start. However the corresponding signals were detected only for two samples.
temperatures and even to liquid phase temperature recalculation from IR signal during and after the laser pulse. Using the voltage level of the solidification plateau in the IR signal and knowledge of the equilibrium melting temperature the liquid phase emissivity can be calculated. This problematic will be treated in another paper.
Acknowledgement
4. Conclusion
References
Three independent investigation methods were developed for laser induced thermal processes investigation: infrared radiometry, time-resolved reflectivity and sample surface reflection of the heating laser pulse. Combination of these three methods enhances the accuracy in identification of the process characteristics. The KrF pulsed laser melting was investigated for eight samples. The melting threshold and melting duration were determined. For the IR radiometry in the case of only heating a smooth surface temperature evolution is obtained. In the case of melting, during the cooling phase a plateau of almost stable temperature (or inflection point) forms at the end of the solidification of the liquid phase. The temperature level of the plateau is close to the equilibrium melting temperature. The TRR evolution shows a rapid change during melting of the material surface. Investigation of Si sample revealed a standard TRR response indicating correctly constructed experimental set-up. The IR radiation evolution during different stages of melting showed changes caused by very different emissivity of solid and liquid phase. The Mo sample showed a reflectivity increase during laser heating and decrease during melting at the He–Ne laser wavelength and increase during melting at the KrF laser wavelength. The sample surface reflected KrF laser pulse method indicated the start of melting at 17 and 10 ns from the beginning of the laser pulse for the energy densities 4 and 5.8 J cm 2. Investigation of laser melting process for Ni, Cu, Sn and stainless steel 17246 samples revealed similar behaviour
[1] R.B. James, G.A. Geist, R.T. Young, W.H. Christie, F.A. Greulich, J. Appl. Phys. 62 (1987) 2981–2988. [2] S. De Unamuno, E. Fogarassy, Appl. Surf. Sci. 36 (1989) 1–11. [3] R.K. Singh, D.R. Gilbert, J. Viatella, Mater. Sci. Eng. B40 (1996) 89– 95. [4] J. Boneberg, J. Bischof, P. Leiderer, Opt. Commun. 174 (2000) 145–149. [5] P. Shaaf, Prog. Mater. Sci. 47 (2002) 1–161. [6] N. Semmar, C. Georges, C. Leborgne, Microelectron. J. 33 (2002) 705– 710. [7] G. Nicolas, A. Yanez, A. Ramil, J.C. Alvarez, E. Saavedra, A. GarciaBeltran, C. Molpeceres, M. Autric, J.L. Ocana, Appl. Surf. Sci. 138/139 (1999) 169–173. [8] M. Hatano, S. Moon, M. Lee, J. Suzuki, C.P. Grigoropoulos, J. Appl. Phys. 87 (2000) 36–43. [9] F.A. Richardst, D. Walsh, Br. J. Appl. Phys. (J. Phys. D) 2 (1969) 663–668. [10] F.F. Kormendi, J. Phys. E: Sci. Instrum. 7 (1974) 1004–1006. [11] J.A. Kittl, P.G. Sanders, M.J. Aziz, D.P. Brunco, M.O. Thompson, Acta Mater. 48 (2000) 4797–4811. [12] D.Y. Tzou, K.S. Chiu, Int. J. Heat Mass Transfer 44 (2001) 1725–1734. [13] J. Martan, N. Semmar, C. Leborgne, E. Le Menn, J. Mathias, Appl. Surf. Sci. 247 (2005) 57–63. [14] O. E. Cibulka, N. Semmar, J. Martan, E. Le Menn, Combination of two optical methods for monitoring of the thermal processes in the material induced by pulsed laser irradiation, Opt. Lasers Eng., in press. [15] I. Lukesˇ, Modifikace povrchu monokrystalicke´ho krˇemı´ku za´rˇenı´m ArF excimerove´ho laseru, Ph.D. thesis, FZU-CSAV, Praha, 1989. [16] M.G. Burzo, P.L. Komarov, P.E. Raad, IEEE Trans. Compon. Pack. Technol. 28 (2005) 39–44. [17] Hugh Baker (Ed.), ASM Handbook, vol. 3: Alloy Phase Diagrams, ASM International, Materials Park, Ohio, USA, 1999. [18] WebElements material properties database, http://www.webelements.com. [19] M. von Almen, Laser-beam Interactions with Materials, Springer Series in Material Sciences, Springer Verlag, Berlin, 1987.
The work has been supported by research project MSM4977751302 of the Ministry of Education of the Czech Republic.