Silicon dioxide thin film removal using high-power nanosecond lasers

Transcription

1 Applied Surface Science 207 (2003) Silicon dioxide thin film removal using high-power nanosecond lasers J. Magyar a,b, A. Sklyarov c, K. Mikaylichenko d, V. Yakovlev a,* a Department of Physics, University of Wisconsin Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA b Department of Electrical Engineering, University of Wisconsin, Milwaukee, WI, USA c Advanced Analysis Facility, University of Wisconsin Milwaukee, Milwaukee, WI, USA d Lam Research Corporation Inc., Milwaukee, WI, USA Received 19 August 2002; received in revised form 13 December 2002; accepted 13 December 2002 Abstract High intensity nanosecond laser pulses of different wavelengths are used to remove thin film silicon dioxide coatings from the silicon surface. Films as thick as 0.7 mm can be removed from the surface in one shot with very minor or no damage to the bare silicon surface. The accumulated effect of multiple-pulse irradiation is studied. It is found that multiple-pulse irradiation results in some reduction of the threshold for coating removal. However, the surface quality gets significantly worse with the increasing number of shots. # 2003 Elsevier Science B.V. All rights reserved. PACS: Cf; a; j; w; Ds Keywords: Silicon dioxide; Thin film; High-power nanosecond lasers 1. Introduction There is an urgent need to develop a novel, powerful, and environmentally-friendly technique capable of removing coatings from solid surfaces. This need arises from diverse technologies. For example, surface contamination is one of the most critical problems in semiconductor fabrication technology [1,2]. Traditional cleaning techniques based on ultrasonics and megasonics are inefficient for sub-micron particles, for which a stronger removing force is required. In nuclear technology, it is frequently required to remove * Corresponding author. Tel.: þ ; fax: þ address: yakovlev@uwm.edu (V. Yakovlev). the contaminated protective oxide layer from the reactor vessel without damaging the underlying steel. There is no practical way to remove this oxide, since it is typically strongly bonded to the solid surface, and wet chemistry methods are too complicated and are not environmentally friendly. Recently, a practical laser-based removal system has been successfully demonstrated for some particular cases of oxidized metal surfaces [3]. Within the last decade, the use of lasers for surface cleaning and thin film removal has emerged as a very promising technology that meets the above requirements [4 8]. Furthermore, the technique can be applied remotely and locally (i.e. the area affected by laser cleaning can be potentially as small as the size of the focal spot of the laser beam). During the last /03/$ see front matter # 2003 Elsevier Science B.V. All rights reserved. doi: /s (02)

2 J. Magyar et al. / Applied Surface Science 207 (2003) several years this technique has been rapidly moving into commercial applications [9 11]. Most of these applications, however, require removing particles and coatings that are not strongly adherent to the surface. Less progress has been made in commercializing laser technology for removal of strongly adhering coatings, such as oxide films. In that case, the major obstacle is controlling significant damage to the underlying surface [12 17]. This problem comes in part from a lack of understanding of the exact physical mechanism involved in coating or particle removal which may include ablation, explosive vaporization, and acoustic excitation due to rapid thermal expansion, ablative photodecomposition, or photospallation. It is always difficult, but practically very important, to choose the parameters of the laser radiation for which the optimal or/and sufficient removal is achieved. In this work, we study the removal of silicon oxide films from the surface of silicon using nanosecond pulsed Nd:YAG laser. We employ different diagnostic tools to study post-irradiated effect to determine the favorable conditions for thin film removal. 2. Experimental procedures 2.1. Samples We use two types of samples during the course of these studies. The first one is a commercial Si(0 0 1) wafer with 0.7-mm-thick film of SiO 2 produced on its surface using a conventional chemical vapor deposition technology (sample #1) and supplied by Lam Research Corporation Inc. This material will be further referred as SiO 2 /Si. As reference (sample #2), we use Si(0 0 1) wafer with a native oxide film of a negligible thickness. The later material will be further referred as Si Laser system We use a commercial Q-switched Nd:YAG laser with a fundamental wavelength of 1064 nm (Continuum Inc.). This laser produces short (6 7 ns) laser pulses at a repetition rate of up to 10 Hz. Non-linear crystals are used to convert this radiation into secondand third-harmonic (at 532 and 355 nm, respectively). As much as 800 mj per pulse is available at 1064 nm, 310 mj at 532 nm and 150 mj at 355 nm. For all these wavelength energy pulse-to-pulse fluctuations typically do not exceed 5%. The repetition rate of these pulses can be varied from a single shot to 10 Hz. Both regimes allow us to change the position of the sample to expose a fresh spot to the incoming radiation. The output beam has a smooth distribution of the energy within a beam diameter of about 6-mm. At the maximum available energy per pulse the beam has an almost flat-top profile on the exit of the laser, and gradually transforms into the Gaussian shape profile in the far field. It allows us to study the effect of the energy distribution on the film removal process. The output energy can be varied in a broad range by changing the delay of Q-switching with respect to the flash-light firing. For all these experiments pulse duration does not change and remains 6 7 ns for all the wavelengths. The later fact is confirmed with a fast Si diode (ThorLabs Inc.) and 400 MHz oscilloscope (Tektronix Inc.). However, we observe some changes in the energy distribution on the beam, especially significant for 355 nm and the lowest energy densities. Typically, these modulation of energy distribution do not exceed 15 20%, but do result is a modulated structure on the surface Ablation procedure All experiments with SiO 2 /Si sample are done using two distinct wavelengths: 355 and 532 nm. Both of these wavelengths are transparent for silicon dioxide films, however, the absorption depths in silicon are different for these wavelengths almost by an order of magnitude. The sample is positioned with respect to incoming laser radiation to ensure a close to a normal incidence. This way at a maximum energy density we create an ablation region of almost a circular shape with a diameter 6-mm. For each wavelength we start at the maximum energy density of the incident radiation and reduce it until we see no changes induced on the sample surface. This way the threshold for laserinduced film removal is determined. For each energy value, the wafer is treated with a single pulse and then multiple overlapping shots (10, 100, and 1000). Through this, the dependence of both coating removal and surface damage can be examined as a function of

3 308 J. Magyar et al. / Applied Surface Science 207 (2003) number of shots. Finally, we perform mass-loss measurements to determine the amount of material removed from the wafer after 300 shots. Si sample is also treated with single- and multiple-shot exposure, but no mass-loss measurements are done. All the above experiments are done at room temperature in air. After irradiation experiments the samples are examined using X-ray photoelectron spectroscopy (ESCA) to confirm the chemical composition on the surface, infrared and Raman spectroscopy, and optical and scanning electron microscopy (SEM). In some cases, X-ray diffraction is used to verify the crystal structure of the remaining surface. In the following sections, we will present and discuss our experimental results. 3. Experimental results 3.1. Mass loss Mass-loss experiments are taken to estimate how much film is removed. We measure the mass of the wafer before and after laser irradiation (300 shots are used). For these experiments SiO 2 /Si wafers of a large area are used to ensure that there is enough space on the sample for irradiated spots not to be overlapped. However, we cannot exclude that film, removed from a sample is not re-deposited on the surface. That is why we consider the above measurements as a lower limit for the mass loss. Fig. 1 represents the experimental dependences of the mass loss on the incident energy. We note that the reflective coefficient of SiO 2 /Si surface does not change dramatically for all the range of energy density used for these measurements, and, thus, these dependences can be considered as the function of deposited energy as well. All the above experiments are done for a single shot exposure, i.e. the sample is moved from point to point and there is no accumulation effect to be taken into account. Despite of a significant change of absorption properties for 532- and 355-nm light, the mass-loss curves are very similar one to each other. They start from about the same energy 90 mj per pulse and gradually increase with the increase of the incident energy. We attribute this increase to the change of the size of the ablated area. In fact, when the mass loss is plotted against the visually modified area of the film for each of the irradiation wavelength, these curves are within the accuracy of our measurements straight lines with approximately the same slope. We try to compare the slope of these lines with the film mass per unit area. However, the later number appears to be approximately two times larger, than the experimentally measured one. It seems to be quite reasonable, assuming that some of the ablated material is back deposited on the surface since all the described above experiments are performed at atmospheric pressure. For comparison, we measure the threshold for laser ablation of silicon wafer with a native oxide (sample #2) to be 130 mj per pulse for 355-nm radiation and 180 mj per pulse for 532-nm radiation). Since we do not see a significant difference in terms of the threshold for coating removal and the amount of the mass loss for samples irradiated with 532 and 355 nm laser beams, we suggest that the wavelength, as far as it lies in the range of significant absorption, does not play a significant role in the coating removal process. In fact a simple p estimation of a thermal diffusion length, L ffiffiffiffiffiffi Dt, gives a value of 4 mm, when the pulse duration, t ¼ 6 ns, and the thermal diffusion coefficient for Si, D 0.3 cm/s 2, are taken into account. From these estimations we should expect the heating of the surface layer much thicker than the absorption depths at both 532 nm (500 nm) and 355 nm (100 nm) within the time of laser interaction with the surface. Since the depth of the Si layer affected by the heat is the same for both irradiation wavelengths, we should expect the same thresholds for coating removal, as well as the energy dependences of the mass loss, which we observe in Fig Single shot and multiple-shots experiments We study the effect of multiple-shot exposures on the threshold for coating removal and on the surface re-structuring after laser irradiation. For each wavelength and each energy density we perform irradiation experiments using 1, 10, 100, and 1000 pulses without moving the sample between each individual pulses. In order to exclude any thermal accumulation effects we reduce the repetition rate of the laser to approximately 0.1 Hz. We perform optical microscopy, Raman microscopy, as well as SEM to evaluate the changes of the morphological structure of ablated region.

4 J. Magyar et al. / Applied Surface Science 207 (2003) Fig. 1. Mass-loss measurements for (a) 532-nm radiation; (b) 355-nm radiation. An arrow indicates a pulse energy at which the visible damage to bare silicon surface is observed.

5 310 J. Magyar et al. / Applied Surface Science 207 (2003) Our first finding is that the threshold for the coating removal is lower for a multiple-shot irradiation. This fact is not surprising for short-pulse laser interactions with solids (see, for example [13]). The first pulse interacting with the surface does not have enough energy to remove the thin film, but it can modify the layerofsiliconbycreatingthedefectsorchangingthe mechanical properties of the interface layer. The later pulses can remove the coating at the energy density below the threshold for a single-pulse irradiation. In our particular case, we find threshold for multiplepulse (10, 100, and 1000) irradiation 10 15% lower than that for a single shot. We also compare our results on the threshold measurements for coating removal with the ablation threshold for the silicon wafer (sample #2). We find that thresholds for laser ablation in the case of singleand multiple-shot irradiation of the silicon sample are almost identical. However, this threshold depends on the excitation wavelength. We find that for 532-nm excitation wavelength the ablation (visible damage) threshold is different from one for 355-nm excitation wavelength by almost 30%. It leads to conclusion that coating removal from Si surface is different from the process of just simple ablation of Si surface. Most probably, it is due to laser modification of a thin interface layer between silicon oxide and silicon. After several shots with the energy density just below the single-pulse threshold for coating removal, this layer is modified and coating removal takes place. It suggests that the mechanism for the coating removal is not assisted by the ablation process. Finally, we evaluate the surface of silicon after the coating is removed. Both the border and the central area of the irradiated spot are analyzed using scanning electron microscope (SEM). Due to a significant difference in electrical conductivity of silicon and silicon dioxide both materials can be clearly distinguished under proper imaging conditions. We find that for both irradiation wavelengths used for these studies the features look very much alike. All samples have cracking, abandoned regions where silicon oxide has been removed, and debris scattered around the irradiated area. By comparing the samples irradiated with different energy density we find essentially no significant differences. However, as the number of consecutive shots increases larger pieces of film Fig. 2. SEM image (1000 magnification) of the film edge for a single shot (a) 532-nm radiation, 0.42 J/cm 2 ; (b) 355-nm radiation, 0.39 J/cm 2. are broken away, giving the edge a more jagged appearance. Fig. 2 shows a rather clean edge of the film for both 532-nm (a) and 355-nm (b) irradiation. If we increase the number of shots (see Fig. 3), the cracks within the material start to show up and the border appears to be constructed by the breaking away of individual pieces. Although this sample, exposed to 1000 consecutive shots, is severely damaged, we find that the thickness of the removed material is approximately equal to the film thickness. It once again confirms our initial hypothesis that coating removal is happening only due to interaction of high-power radiation with thin silicon/silicon oxide interface. We also analyze in details the irradiated area of the surface, from which the coating is removed. We find

6 J. Magyar et al. / Applied Surface Science 207 (2003) Fig. 3. SEM image (1000 magnification) of the film edge for 1000 shots (a) 532-nm radiation, 0.42 J/cm 2 ; (b) 355-nm radiation, 0.39 J/cm 2. that the surface exhibits a rather complicated structure, which depends not only on the energy density and number of shots, but also on the position on the sample with respect to the center of the laser beam. Fig. 4 shows the changes of the surface structure for the increasing number of shots for 355-nm radiation. Very similar tendency is observed for 532-nm radiation (Fig. 5). We can clearly see that the surface exposed to a multiple-shot radiation develops a complex microstructure. Similar micro-structures have been observed earlier on the surface of silicon irradiated by significantly shorter pulses in the presence of different gaseous environment [18]. However, the surface of silicon exposed to a single shot is typically rather smooth (see Figs. 4 and 5), although some residuals of silicon dioxide do present on the surface. The craters with and without ripples extending from them, occur for all energy values within the range tested at single shot exposure. The largest crater at the highest energy density level, however, is still microscopic in area. Microscopic debris located at the center of these formations suggests that the craters are a result of ejected particles during melting and vaporization of the surface. No traces of such formations are observed at the lowest energies used to remove the coating. This fact also suggests that at the lower energy densities no melting takes place on the silicon surface. There is also a possibility that by irradiating the samples with multiple shots the cleaned area heated by laser radiation can re-oxidize. In fact, we do observe microscopic non-conductive (i.e. silicon oxide) particles on the samples exposed to more than one shot. Chemical analysis of the damaged surface also reveals substantial presence of silicon oxide on the surfaces exposed to the highest energy with multiple shots compared to single shot exposure. From all the above observation, we may conclude that single shot exposure just near the threshold for coating removal with 532- or 355-nm light is the most favorable for cleaning the surface of silicon from its oxide. Fig. 4. SEM images (1000 magnification) of the surface of silicon inside the irradiated area for increasing number of shots (355-nm irradiation, 0.39 J/cm 2 ).

7 312 J. Magyar et al. / Applied Surface Science 207 (2003) Fig. 5. SEM images (1000 magnification) of the surface of silicon inside the irradiated area for increasing number of shots (532-nm irradiation, 0.42 J/cm 2 ) Discussion There are many competing mechanisms for the coating removal [1,2], and the exact mechanism may include a combination of ablation, explosive vaporization, acoustic excitation due to rapid thermal expansion, ablative photodecomposition, and photospallation. However, we believe our results support the hypothesis that for nanosecond pulses photospallation plays a dominant role in the coating removal. Photospallation involves surface acceleration due to rapid heating and expanding of the surface just after the laser radiation. Removal of a ceramic coating on a surface (substrate) can be described by the following sequence of events [19]. The substrate is irradiated by a short (nanosecond) laser pulse. Since the pulse duration is much shorter than a typical relaxation time in a solid, the substrate will heat rapidly, rapidly expand, and then rapidly compress, giving rise to a stress wave. A bipolar wavefront is formed when this stress wave reflects off a free surface, e.g. the coating air interface. If the stress accompanying the bipolar wavefront is greater than the tensile stress of the coating, the coating will split. We find that the threshold for coating removal is lower than the one for ablation of bare silicon surface. Near the threshold for the coating removal the remaining substrate surface exhibits no damage and X-ray diffraction pattern of the cleaned area is undistinguishable from one of bare silicon. At higher fluencies, however, we see significant degradation of the surface, which is believed, is the result of consecutive melting and ablation, and/or explosive vaporization of silicon. 4. Conclusion We have demonstrated that nanosecond laser pulses can be used for successful coating removing of thick (up to 0.7-mm-thick) coatings of silicon dioxide from a silicon surface. It is found that the cleanest results are achieved by using laser pulses just above the threshold for laser coating removal. Multiple-pulse exposure reduces the threshold for coating removal but the quality of the silicon surface significantly degrades with the increasing number of pulses. We have found no significant differences between 532 and 355 nm irradiation and attribute it to the heat-wave mediated photospallation mechanism for the coating removal. Acknowledgements This research is supported by Petroleum Research Foundation Grant ACS-PRF G5, NASA Wisconsin Space Grant Consortium and UWM Graduate School Research Award. Jenny Magyar gratefully acknowledges AAF-Graduate School UWM Summer Fellowship. References [1] D. Bauerle, Laser Processing and Chemistry, third ed., Springer-Verlag, Berlin, [2] B. Luk yanchuk (Ed.), Laser Cleaning, World Scientific, Singapore, [3] M.L. Sentis, P. Delaporte, W. Marine, O. Uteza, Quant. Electron. 30 (2000) 495. [4] S.M. Bedair, H.P. Smith, J. Appl. Phys. 40 (1969) [5] A.C. Tam, W.P. Leung, W. Zapka, W. Ziemlich, J. Appl. Phys. 71 (1992) 3515;

8 J. Magyar et al. / Applied Surface Science 207 (2003) A.C. Tam, H.K. Park, C.P. Grigoropoulos, Appl. Surf. Sci. 129 (1998) [6] D.A. Wesner, M. Mertin, F. Lupp, E.W. Kreutz, Appl. Surf. Sci (1996) 479. [7] Y.-F. Lu, Y. Aoyagi, Jpn. J. Appl. Phys. 33 (1994) L430. [8] W. Kautek, S. Pentzien, P. Rudolph, J. Kruger, E. Konig, Appl. Surf. Sci (1998) 746. [9] In the News : Laser Cleaning, Deep-UV Process Offers Reduced Cleaning Cost, Laser FocusWorld, March 1996, p. 33. [10] In the News : Laser-based paint removal system, Ind. Laser Rev. April (1996) 6. [11] Accent on Application : Photonic cleaning process moves to heavy industry, Photon. Spectr. March (1997) 22. [12] W. Aihua, Z. Beidi, T. Zengyi, M. Xianyao, D. Shijun, C. Zudong, Surf. Coat. Technol. 57 (1993) 169. [13] P. Psyllaki, R. Oltra, Mater. Sci. Eng. A282 (1 2) (2000) ; R. Oltra, O. Yavas, F. Cruz, J.P. Boquillon, C. Satori, Appl. Surf. Sci (1996) ; P. Meja, M. Autric, P. Alloncle, P. Pasquet, R. Oltra, J.P. Boquillon, Appl. Phys. A69 (1999) S687 S690. [14] V. Dobler, R. Oltra, J.P. Boquillon, M. Mosbacher, J. Boneberg, P. Leiderder, Appl. Phys. A69 (1999) S335. [15] P. Leiderer, J. Boneberg, V. Dobler, M. Mosbacher, H.-J. Munzer, N. Chaoui, J. Siegel, J. Solis, C.N. Afonso, T. Fourier, G. Schrems, D. Bauerle, Proc. SPIE 4065 (2000) 249. [16] A. Kearns, C. Fischer, K.G. Watkins, M. Glasmacher, H. Kheyrandish, A. Brown, W.M. Steen, P. Beachan, Appl. Surf. Sci. 129 (1998) [17] D. Ashkenasi, M. Lorenz, R. Stoian, A. Rosendfeld, Appl. Surf. Sci. 150 (1 4) (1999) [18] T.H. Her, R.J. Finlay, C. Wu, E. Mazur, Appl. Phys. A70 (2000) 383. [19] H.J. Hoffman, W.B. Telfair, J. Refract. Surg. 16 (2000) 90.