- Email: [email protected]
Low roughness laser etching of fused silica using an adsorbed layer
Applied Surface Science 239 (2004) 109–116
Low roughness laser etching of fused silica using an adsorbed layer R. Bo¨hme, K. Zimmer* Leibniz-Institut fu¨r Oberfla¨chenmodifizierung e.V., Permoserstrasse 15, D-04318 Leipzig, Germany Received in revised form 14 May 2004; accepted 15 May 2004 Available online 25 June 2004
Abstract High quality etching of transparent materials for applications in microstructuring and precision engineering is still a challenge for current laser processing techniques. Laser etching at a surface adsorbed layer called LESAL allows precision etching of fused silica at low laser fluences with small etch rates. The etch rate and the surface quality depend on the applied laser fluence and can be divided into three regions with different etch behavior. The poor surface quality in the low and the high fluence regions are caused by incubation processes and thermal ablation processes, respectively. In the fluence range from 2.0 to 5.0 J/cm2 a constant etch rate was observed that increases from 1 to 2 nm/pulse with raising the temperature from 60 to 105 8C. In this region of etch rate saturation a very low surface roughness of less than 1 nm rms was measured. Both the etch rate saturation and the low roughness of the LESAL process enable new applications of laser processing transparent materials. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.C; 81.05.K; 79.20.D; 61.80.B; 42.70.C; 42.55.L Keywords: Excimer laser; Laser etching; Low roughness; Adsorbed layer
1. Introduction The processing of surfaces for the fabrication of high-end components requires methods of etching materials with high accuracy. In particular, applications in micro-optics and micro-fluidic call for high quality etching of different dielectric materials with lateral dimensions in the micron and sub-micron range as well as an accurate etch depth control and a low roughness of the etched surface. For those applications
*
Corresponding author. Tel.: þ49 341 235 3287; fax: þ49 341 235 2584. E-mail address: [email protected] (K. Zimmer).
transparent materials such as fused silica, sapphire and different types of glass have to be etched. The etching of materials can be achieved by standard techniques, e.g., dry etching, but these processes require photolithographic masking of the material surface [1]. In the case of etching multi-stepped micro-optics the cycle of repeated photolithographic masking and dry etching has to run several times with special sets of photolithographic masks [2]. Therefore, these processing schemes do not feature a great flexibility for small-scale production and do not permit rapid prototyping. Laser processing allows both the direct writing of surface features and the fabrication of complex topographies by different mask projection techniques [3].
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.095
110
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
Inorganic materials such as fused silica, sapphire, different types of glass and calcium fluoride can be machined by laser radiation with various laser wavelengths and pulse lengths. Carbon dioxide lasers etch glass materials very quickly by evaporation of the laser heated surface but generate frequently microcracks [4]. VUV lasers at 157 nm with photon energies exceeding the band-gap of the material to be processed are used to ablate transparent materials like fused silica [5] with rates of about 100 nm per pulse. The ablation process causes a growth of the surface roughness also at this very short wavelength. Additional to nanosecond laser pulses ultrashort pulse lasers (e.g. fslaser) can be exploited for transparent materials processing [6,7]. Here multi-photon absorption and other nonlinear processes are responsible for the energy deposition and the subsequent laser etching. Unfortunately, the etched surfaces are often uneven and rough. Some laser techniques make use of indirect interaction processes of the laser radiation for the etching of materials. The LIBWE technique uses an absorbing liquid that is in contact with the back side of the material to cause laser etching of the transparent material [8,9]. Here the etch process of the material is based on the rapid heating of the absorbing liquid by the laser beam to a supercritical state. The hot liquid heats the interface of the solid material to high temperatures and the softened material is removed by high forces from the thermoelastic stress of the heated material and the shock wave generated in the liquid. Zang et al. reports on a laser ablation technique for glass machining named LIPAA [10]. This method exploits a laser-induced plasma to ablate fused silica and Pyrex glass at the back side of the sample. The plasma needed for ablation is generated by the same laser pulse after hitting a metal target beneath the transparent sample to be etched. Recently, a new technique called LESAL (laser etching at a surface adsorbed layer) was demonstrated [11]. This new method makes use of a thin layer adsorbed onto the back side of a transparent material that absorbs the laser radiation and causes the etching of the material surface. In contrast to the mentioned techniques (LIBWE, LIPAA) the LESAL process does not need neither an absorbing liquid nor a thin metal layer for the etching process. The laser beam only interacts with the adsorbed film at the back side that results in smooth etching with low etch rates. The
usage of adsorbed layers formed by the vapor phase of, e.g., organic solvent, for laser etching by means of physical processes is not known yet. This method was applied to fused silica and shows unusual attributes of the laser etching. In particular, compared to laser ablation in air the threshold fluence and the etch rates for laser etching are very low and account about 0.7 J/ cm2 and 1 nm/pulse, respectively.
2. Experimental The experimental set-up for the laser etching at a surface adsorbed layer is schematically shown in Fig. 1. Similar to experiments on LIBWE and LIPAA the laser interaction takes place at the backside of the sample where the laser beam leaves the transparent material [8–10]. In the case of LESAL the laser beam penetrates the transparent sample first but is absorbed thereafter by a thin layer of hydrocarbons, which is continuous formed by adsorption from the vapor at the backside of the transparent sample. However, the laser beam does not interact with the liquid toluene because the stainless steel heater blocks the laser beam. To create this thin adsorbed layer the sample is fit into a laser processing chamber equipped with an electric heater that evaporates liquid toluene. In the initial state the chamber is filled with air under atmospheric pressure and the liquid toluene phase. In dependence of the heating power the temperature equilibrium of both the liquid and the sample surface can be adjusted. At the temperature equilibrium no condensation has
Fig. 1. Experimental set-up for the LESAL technique. The thin absorbing layer is formed by adsorption from the vapour phase of evaporated toluene.
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
been observed at the sample surface. For instance, with a heating power of 1350 W the temperatures of the toluene at the bottom of the chamber, the furnace surface, and the chamber measured with a thermocouple are 94, 119 and 75 8C, respectively. Due to the higher temperature of the upper part of the furnace the sample surface is heated by the vapor phase as well as by the thermal radiation of the heater. Therefore, the sample temperature is slightly higher than the temperature of the liquid at the bottom of the chamber. Due to the thermal equilibrium of the whole set-up the chamber temperature determined by a thermocouple was used as a measure for the adsorbed layer formation conditions. At the bottom of the chamber toluene has been evaporated at temperatures below the boiling point of 110 8C. The evaporated toluene forms the adsorbed layer at the back side of the transparent material. First ellipsometric measurements of the adsorbed toluene layer at the back side of the fused silica sample reveals, that the thickness of the adsorbed layer is less than 5 nm, that is the detection limit of the used ellipsometer in this optical set-up. To avoid excess pressures due to the toluene evaporation the chamber was equipped with a valve to assure a constant pressure of one atmosphere inside. Laser etching has been accomplished using a commercial excimer laser workstation. The laser workstation is equipped with an LPX 220i excimer laser, beam shaping and homogenizing optics, a dielectric attenuator, and provides approximately 30 ns pulses at 248 nm with an overall energy deviation in the mask plane of below 5% rms. The laser processing chamber was attached to the x–y–z stage of the workstation. The stage enables program controlled positioning and scanning of the laser beam across the sample surface with a resolution of 1 mm and a maximum velocity of 100 mm/s. A Schwarzschild objective (15 demagnification) with an optical resolution of 1.5 mm was used for projecting a variable aperture onto the sample back side. In all investigations fused silica samples cut from double side polished wafers with a thickness of about 380 mm have been used. The samples that feature a low surface roughness of less than 0.3 nm (rms) on both sides were used as received without additional cleaning. A scanning electron microscope (SEM) was employed for examination of the etched structures and the surface quality. The etch depth and the rough-
111
ness on the bottom of the etched pits were measured with white light interference microscope. Due to the wavelength of 550 nm used in these optical measurements the lateral resolution is slightly less than 1 mm. Hence the cut-off wavelength of the measured surface roughness is in the same range and gives no information on the sub-micron surface features. Therefore, selected samples were investigated by atomic force microscopy using the tapping mode. Additional to the different lateral resolutions of the used methods the measured area is dissimilar and accounts for the AFM 4 mm2 and for the optical measurements 120 mm2.
3. Results and discussion The etch rate dependence of the presented etch technique is shown in Fig. 2. The etch rates were calculated from the final depths of etch pits with an area of 100 mm 100 mm after 300 laser pulses. The threshold fluence for etching of fused silica at a chamber temperature of 75 8C was determined to about 0.7 J/cm2 [11]. This threshold fluence is one magnitude lower than the ablation threshold of fused silica in air of more than 12.0 J/cm2 [12]. First the etch rate rises with increasing laser fluence up to 2.0 J/cm2, than shows a saturation and keeps constant between 2.0 and 5.0 J/cm2. In the rate saturation fluence range a
Fig. 2. Etch rates on fused silica in dependence on the laser fluence at a chamber temperature of 75 8C using an adsorbed toluene layer (the line is used for guide the eyes). The splitting into three fluence regions is pointed out.
112
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
low etch rate of about 1 nm/pulse has been observed. At laser fluences higher than 5.0 J/cm2 the etch rate grows again and reaches values as high as 100 nm/pulse. Therefore, a splitting of the investigated fluence range into three regions with different etch behavior is apparent. These sections are the low fluence, the saturation and the high fluence region as depicted in Fig. 2. The mechanism of etching at the solid/adsorbate interface is very complex, but some processes involved in etching can be discussed. The laser beam absorption of the adsorbed hydrocarbon layer results in fast heating of this layer and the near surface region of the transparent sample. The high temperatures achieved by this fast laser heating cause probably stress and structural transitions of the glass material near the surface and result finally in materials etching. Additional to the fast heating of the surface other processes can be involved in LESAL. The most probable processes are the desorption of the adsorbed layer [13], the generation of shock waves [14], and the decomposition of the adsorbed hydrocarbons [15]. In addition, the toluene vapor in the chamber can contribute only little to the etching due to the low density of the vapor phase. In the low fluence region the etch rate averaged over 300 laser pulses increases nonlinearly over two orders of magnitude to about 1 nm/pulse at 2.0 J/cm2. Here, incubation effects, i.e., the material etching starts after a certain number of laser pulses, were observed. Similar to other incubation processes, e.g., laser ablation of fused silica in air [12], the etching process is strongly influenced by the formation of local surface defects during the onward laser irradiation. After generating a sufficient number of defects, that can be material defects into the fused silica or decomposition products at the surface, the etching occurs. Due to the nonuniform defect formation the surface is as, shown in Fig. 5b) uneven etched. The nonlinear, almost exponential increase of the etch rate in the low fluence region can be attributed to typical thermal processes such as laser-induced thermal decomposition of the hydrocarbons or desorption of the adsorbed layer. The activation energy from this thermal process can be estimated from the slope of the etch rate in this region. If the temperature grows linear with the laser fluence the thermal activation energy can be estimated to 50 kJ/mol. This high value is characteristic for chemical reactions rather than for physical desorption processes.
The most interesting feature of LESAL is the saturation of the etch rate in a wide fluence range. This rate saturation is exceptional for laser processing but some examples are known. The most saturation processes are caused by mass transport limitations, i.e., of reactive molecules, to the surface area to be etched [16]. Some processes make use of an adsorbed film of a certain thickness [17] that limits the etch rate. As soon as all adsorbed molecules have been used up by the etching process the process stops and causes the rate saturation. Another example of rate limiting is the digital etching of GaAs by chlorine [18], where the chlorine forms exactly one monolayer of chlorides that are removed by an UV-laser pulse. The etching of fused silica by means of an adsorbed hydrocarbon layer is determined by the properties of the adsorbed hydrocarbon film. In difference to the chemical reaction of GaAs with chlorine, hydrocarbons do not react with fused silica to form reaction products and a physically adsorbed toluene layer is expected. Hence, physical processes must be responsible for the rate saturation. It is known that multi-molecular layers can be physisorbed at surfaces according to different adsorption theories [19]. For thermal driven processes at high laser fluences the increase of the laser fluence results in higher heating rates and temperatures that typically are the reason for higher etch rates. However, this is inconsistent with the observed rate saturation. Additional to growing interface temperatures higher desorption rates of the adsorbed layer are the consequence of raising the laser fluence. Laser desorption studies of hydrocarbons show the possibility of the very fast desorption at rapid heating by laser pulses [13]. Hence, the laser heating of the adsorbed layer results in quick desorption processes that cause the decrease of the adsorbed layer thickness during the laser pulse. If the adsorbed layer is removed within the pulse duration at certain fluence, the further increase of the laser fluence reduces the time for the film desorption without significant changes of the maximum surface temperature. In this case the rate saturation is caused by limiting the absorbed laser energy that is responsible for the limitation of the surface temperature even though the laser fluence increases. In the high fluence region the etch rates are much higher compared to the saturated rate value and resemble the rates of typical ablation processes. Additional to the high etch rates both the fluence values and
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
113
Fig. 3. Fluence dependence of the etch rate for different chamber temperatures.
the rough surfaces that are observed in this fluence region are an indication for a changed etch mechanism. Hence, the etching is probably dominated by ablation processes similar to laser ablation in air. The lower threshold of the adsorbed layer ablation compared to laser ablation in air is caused by the adsorbed hydrocarbon layer that provides additional absorption at the sample surface where the ablation process starts. The substantial reduction of the ablation threshold of transparent samples was already observed for a layer consisting of aluminum particles covering the transparent substrates [20]. The fluence dependence of the etch rate for different chamber temperatures in the range from 60 to 105 8C is shown in Fig. 3. The etch rate behavior with the typical three fluence regions is almost identical for the investigated temperatures. However, the temperature affects the etch rate in the saturation fluence region as represented in Fig. 4 for two different laser fluences. The saturated etch rate increases almost linear with the temperature from 0.9 nm/pulse at 60 8C to 2 nm/pulse at 105 8C. It is supposed that the raise of the etch rate with increasing temperature associates with the increasing of the adsorbed layer thickness due to the higher partial pressure of toluene. The absorbed laser energy increases with growing adsorbed layer thickness and causes higher laser-induced surface temperatures and consequently higher etch rates.
Furthermore, the temperature affects the laser fluences delimiting the regions of different etch behavior, too. The upper limit of the rate saturation region is shifted down to smaller fluence values with raising temperature (Fig. 3). The reason for the fluence shifting is also the expected increase of the thickness of the adsorbed toluene layer at higher temperatures. Optical investigations of the etched surface show surface alterations taking place at LESAL processing. Similar to high-intensity irradiation of liquid toluene with a copper vapor laser [15] and KrF excimer laser
Fig. 4. The etch rate increases with the chamber temperature in the saturation fluence region.
114
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
[21] the LESAL processing results in surface modifications such as the deposition of a thin layer. Hence, it can be assumed that additional to a laser-induced rapid heating of the adsorbed layer the decomposition of the adsorbed toluene layer by the laser heating is involved in the etch process. The roughness of the fused silica surfaces etched with LESAL is remarkably low. First results revealed a measured roughness of well below 1 nm rms [11]. However, the roughness changes considerably with the processing parameters, e.g., the laser fluence. Fig. 5 shows AFM images of laser etched surfaces processed in the saturation region (F ¼ 5.0 J/cm2 and 3.5 J/cm2) and low fluence region (F ¼ 1.9 J/cm2), respectively. Although the etch depth is nearly identical the etched surface morphology and the roughness differ significantly. At the higher fluence of 5.0 J/cm2 the etched surface is extremely plane and smooth as shown in Fig 5a. It is remarkable that scratches probably originating from the polishing process are still visible and are partially responsible for the rms roughness value of the fused silica surface of 0.23 nm. For comparison the roughness of the virgin surface was measured with 0.23 nm, too. The observed scratches at the bottom of the etched grooves are either transferred from already existing scratches at the polished surface or the scratches are developed by the laser etching from a modified near surface region. Both, scratches at the surface as well as subsurface damage are well known from mechanical polishing. However, the occurrence of such scratches after etching indicates to a very homogenous etch behavior and confirms the inherent low roughness of the LESAL process. In contrast to the smooth surface in the rate saturation region the surface etched at low fluences, e.g., 1.9 J/cm2, shows a granular microstructure as depicted in Fig. 5c, which is responsible for the roughness, that amounts 1.6 nm rms. The surface etched at a laser fluence of 3.5 J/cm2 shows a fine granular surface topography without any visible scratch and a rms value of 0.36 nm. Fig. 6 shows the roughness (rms) of typical etched surfaces sides measured with interference microscopy and AFM in dependence on laser fluence for a single temperature and a given pulse number. For comparison the etch rate is shown, too. Similar to the etch rate also the roughness evolution with the laser fluence can be divided in the three regions. In the low fluence
Fig. 5. AFM images of 400 nm deep grooves etched with 300 pulses in fused silica by LESAL method at fluences of (a) 5.0 J/ cm2, (b) 3.5 J/cm2 and (c) 1.9 J/cm2.
region a higher surface roughness has been observed than in the rate saturation region. This comparatively high roughness has been measured at a low etch depth caused by the low etch rate. The origin of the high roughness are the mentioned incubation effects and
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
115
Fig. 6. Measured microroughness for different laser fluences at 75 8C chamber temperature. For comparison the etch rate is shown, too.
the very strong fluence dependence of the etch rate, so that fluence variation cause different etch depths and consequently a high roughness. In the high fluence region a dramatic increase of the roughness is observed due to the thermal ablation-like processes responsible for the etching. In the saturation range the measured roughness (AFM) is below 1 nm (rms) at an etch depth of 400 nm for all investigated sites. In addition, a further decrease of the roughness with increasing laser fluence is noticed. At a fluence of 5.0 J/cm2, which is slightly below the high fluence region, the lowest roughness with 0.23 nm (rms) has been measured by AFM in an area of 2 mm 2 mm. The slight increase of the roughness in the saturated region at lower laser fluences can be explained with an enhanced influence of incubation effects at the early stage of laser etching. In the saturation region the quality of the laser etched surface is similar to that of a well polished surface as the morphology and the roughness demonstrate. Moreover, almost no debris deposition is visible in and around the etched area. The main differences of the LESAL process compared to other laser etch techniques are the use of an adsorbed hydrocarbon layer, the low etch rates and the exceptional etch rate behavior, the low roughness of the etched surfaces and the absence of secondary effects such as bubble formation. Especially, exceptional attributes such as the remarkable low roughness,
the low etch rate, and the etch rate saturation are not known from other physical laser etching processes, i.e., laser ablation. These outstanding characteristics allow the very precise machining of surfaces and enable new applications of the LESAL method for microfabrication. For this purpose different laser processing techniques like mask projection, gray scale, and contour mask technique can be exploited for applications in micro-optics and micro-fluidic.
4. Conclusions The demonstrated method for laser etching of surfaces by means of adsorbed layers allows the etching of transparent materials, e.g., fused silica, by pulsed excimer laser radiation of 248 nm. This technique makes use of the absorption of the laser energy by a thin toluene film that is adsorbed on the back side of the transparent material to be etched. The LESAL etching of fused silica is characterized by a low threshold fluence of 0.7 J/cm2 and a low etch rate of some nanometers per laser pulse. The three fluence regions of LESAL characterized by different etch rates and surface roughness are the result of different processes dominating the etching. In the low fluence range the etching is characterized by a low etch rate of less than 1 nm/pulse, incubation processes and a granular, rough surface. In the fluence
116
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
range 2.0–5.0 J/cm2 the etch rate shows a saturation and the surface features a very good quality with a very low roughness. The etch rate saturation is the result of the competition processes of laser heating of the material interface and the thermal desorption of the heated adsorbed layer. Hence, the etching of the LESAL process is significantly affected by the adsorbed hydrocarbon layer. In this saturation region the etch rate grows with the temperature of the toluene from 0.6 to 2 nm/pulse. At well chosen etch conditions a surface roughness of 0.23 nm rms has been measured at an etch depth of 400 nm. This rms roughness value is nearly equal the roughness of a virgin fused silica sample. The high fluence region is characterized by high etch rates and roughness due to the thermal ablation dominated etching. The laser etching of surfaces by an adsorbed layer is a new promising technique for laser processing of transparent materials that features a low etch rate and a very good quality of the etched surface and enables new applications in micron technology and precision engineering.
Acknowledgements The authors wish to acknowledge Mr. D. Hirsch for his skilled AFM investigations. References [1] Rai-Choudhury, Handbook of Microlithography, Micromachining and Microfabrication, SPIE PM39, 1997, p. 768.
[2] St. Sinzinger, J. Jahns, Microoptics, Wiley-VCH, Weinheim, 1999. [3] K. Zimmer, A. Braun, Excimer laser machining for 3Dsurface structuring, in: A. Peled (Ed.) Photo-Excited Processes, Diagnostics and Applications, Kluwer, 2003, pp. 301–335. [4] G. Allcock, P.E. Dyer, G. Elliner, H.V. Snelling, J. Appl. Phys. 78 (1995) 7295. [5] P.R. Herman, R.S. Marjoribanks, A. Oettl, K.P. Chen, Appl. Surf. Sci. 154–155 (2000) 577. [6] M. Lenzner, J. Kru¨ ger, W. Kautek, F. Krausz, Appl. Phys. A 68 (1999) 369. [7] H. Varel, D. Ashkenasi, A. Rosenfeld, M. Wa¨ hmer, E.E.B. Campbell, Appl. Phys. A 65 (1997) 367. [8] J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68 (1999) 111. [9] R. Bo¨ hme, A. Braun, K. Zimmer, Appl. Surf. Sci. 186 (2002) 276. [10] J. Zang, K. Sugioka, T. Takahashi, K. Toyoda, K. Midorikawa, Appl. Phys. A 71 (2000) 23. [11] K. Zimmer, R. Bo¨ hme, B. Rauschenbach, Laser etching of fused silica by the use of adsorbed layers, Appl. Phys. A, submitted for publication. [12] J. Ihlemann, B. Wolf-Rottke, Appl. Surf. Sci. 106 (1996) 282. [13] K. Onda, A. Wada, K. Domen, C. Hirose, Surf. Sci. 502 (2002) 319. [14] A.A. Kolomenskii, A.M. Lomonosov, R. Kuschnereit, P. Hess, V.E. Gusev, Phys. Rev. Lett. 29 (7) (1997) 1325. [15] A.V. Simakin, E.N. Loubnin, G.A. Shafeev, Appl. Phys. A 69 (1999) 267. [16] G.P. Vakanas, A.A. Tseng, P. Winer, J. Laser Appl. 14 (2002) 185. [17] B.-J. Kim, S. Chung, S.M. Cho, Appl. Surf. Sci. 187 (2002) 124. [18] T. Meguro, Y. Aoyagi, Appl. Surf. Sci. 112 (1997) 55. [19] P. Atkins, Physical Chemistry, VCH, Weinheim, 1996. [20] D.M. Kane, D.R. Halfpenny, J. Appl. Phys. 87 (2000) 4548. [21] R. Bo¨ hme, D. Spemann, K. Zimmer, Thin Solid Films 453–454 (2004) 127.
Low roughness laser etching of fused silica using an adsorbed layer R. Bo¨hme, K. Zimmer* Leibniz-Institut fu¨r Oberfla¨chenmodifizierung e.V., Permoserstrasse 15, D-04318 Leipzig, Germany Received in revised form 14 May 2004; accepted 15 May 2004 Available online 25 June 2004
Abstract High quality etching of transparent materials for applications in microstructuring and precision engineering is still a challenge for current laser processing techniques. Laser etching at a surface adsorbed layer called LESAL allows precision etching of fused silica at low laser fluences with small etch rates. The etch rate and the surface quality depend on the applied laser fluence and can be divided into three regions with different etch behavior. The poor surface quality in the low and the high fluence regions are caused by incubation processes and thermal ablation processes, respectively. In the fluence range from 2.0 to 5.0 J/cm2 a constant etch rate was observed that increases from 1 to 2 nm/pulse with raising the temperature from 60 to 105 8C. In this region of etch rate saturation a very low surface roughness of less than 1 nm rms was measured. Both the etch rate saturation and the low roughness of the LESAL process enable new applications of laser processing transparent materials. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.C; 81.05.K; 79.20.D; 61.80.B; 42.70.C; 42.55.L Keywords: Excimer laser; Laser etching; Low roughness; Adsorbed layer
1. Introduction The processing of surfaces for the fabrication of high-end components requires methods of etching materials with high accuracy. In particular, applications in micro-optics and micro-fluidic call for high quality etching of different dielectric materials with lateral dimensions in the micron and sub-micron range as well as an accurate etch depth control and a low roughness of the etched surface. For those applications
*
Corresponding author. Tel.: þ49 341 235 3287; fax: þ49 341 235 2584. E-mail address: [email protected] (K. Zimmer).
transparent materials such as fused silica, sapphire and different types of glass have to be etched. The etching of materials can be achieved by standard techniques, e.g., dry etching, but these processes require photolithographic masking of the material surface [1]. In the case of etching multi-stepped micro-optics the cycle of repeated photolithographic masking and dry etching has to run several times with special sets of photolithographic masks [2]. Therefore, these processing schemes do not feature a great flexibility for small-scale production and do not permit rapid prototyping. Laser processing allows both the direct writing of surface features and the fabrication of complex topographies by different mask projection techniques [3].
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.095
110
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
Inorganic materials such as fused silica, sapphire, different types of glass and calcium fluoride can be machined by laser radiation with various laser wavelengths and pulse lengths. Carbon dioxide lasers etch glass materials very quickly by evaporation of the laser heated surface but generate frequently microcracks [4]. VUV lasers at 157 nm with photon energies exceeding the band-gap of the material to be processed are used to ablate transparent materials like fused silica [5] with rates of about 100 nm per pulse. The ablation process causes a growth of the surface roughness also at this very short wavelength. Additional to nanosecond laser pulses ultrashort pulse lasers (e.g. fslaser) can be exploited for transparent materials processing [6,7]. Here multi-photon absorption and other nonlinear processes are responsible for the energy deposition and the subsequent laser etching. Unfortunately, the etched surfaces are often uneven and rough. Some laser techniques make use of indirect interaction processes of the laser radiation for the etching of materials. The LIBWE technique uses an absorbing liquid that is in contact with the back side of the material to cause laser etching of the transparent material [8,9]. Here the etch process of the material is based on the rapid heating of the absorbing liquid by the laser beam to a supercritical state. The hot liquid heats the interface of the solid material to high temperatures and the softened material is removed by high forces from the thermoelastic stress of the heated material and the shock wave generated in the liquid. Zang et al. reports on a laser ablation technique for glass machining named LIPAA [10]. This method exploits a laser-induced plasma to ablate fused silica and Pyrex glass at the back side of the sample. The plasma needed for ablation is generated by the same laser pulse after hitting a metal target beneath the transparent sample to be etched. Recently, a new technique called LESAL (laser etching at a surface adsorbed layer) was demonstrated [11]. This new method makes use of a thin layer adsorbed onto the back side of a transparent material that absorbs the laser radiation and causes the etching of the material surface. In contrast to the mentioned techniques (LIBWE, LIPAA) the LESAL process does not need neither an absorbing liquid nor a thin metal layer for the etching process. The laser beam only interacts with the adsorbed film at the back side that results in smooth etching with low etch rates. The
usage of adsorbed layers formed by the vapor phase of, e.g., organic solvent, for laser etching by means of physical processes is not known yet. This method was applied to fused silica and shows unusual attributes of the laser etching. In particular, compared to laser ablation in air the threshold fluence and the etch rates for laser etching are very low and account about 0.7 J/ cm2 and 1 nm/pulse, respectively.
2. Experimental The experimental set-up for the laser etching at a surface adsorbed layer is schematically shown in Fig. 1. Similar to experiments on LIBWE and LIPAA the laser interaction takes place at the backside of the sample where the laser beam leaves the transparent material [8–10]. In the case of LESAL the laser beam penetrates the transparent sample first but is absorbed thereafter by a thin layer of hydrocarbons, which is continuous formed by adsorption from the vapor at the backside of the transparent sample. However, the laser beam does not interact with the liquid toluene because the stainless steel heater blocks the laser beam. To create this thin adsorbed layer the sample is fit into a laser processing chamber equipped with an electric heater that evaporates liquid toluene. In the initial state the chamber is filled with air under atmospheric pressure and the liquid toluene phase. In dependence of the heating power the temperature equilibrium of both the liquid and the sample surface can be adjusted. At the temperature equilibrium no condensation has
Fig. 1. Experimental set-up for the LESAL technique. The thin absorbing layer is formed by adsorption from the vapour phase of evaporated toluene.
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
been observed at the sample surface. For instance, with a heating power of 1350 W the temperatures of the toluene at the bottom of the chamber, the furnace surface, and the chamber measured with a thermocouple are 94, 119 and 75 8C, respectively. Due to the higher temperature of the upper part of the furnace the sample surface is heated by the vapor phase as well as by the thermal radiation of the heater. Therefore, the sample temperature is slightly higher than the temperature of the liquid at the bottom of the chamber. Due to the thermal equilibrium of the whole set-up the chamber temperature determined by a thermocouple was used as a measure for the adsorbed layer formation conditions. At the bottom of the chamber toluene has been evaporated at temperatures below the boiling point of 110 8C. The evaporated toluene forms the adsorbed layer at the back side of the transparent material. First ellipsometric measurements of the adsorbed toluene layer at the back side of the fused silica sample reveals, that the thickness of the adsorbed layer is less than 5 nm, that is the detection limit of the used ellipsometer in this optical set-up. To avoid excess pressures due to the toluene evaporation the chamber was equipped with a valve to assure a constant pressure of one atmosphere inside. Laser etching has been accomplished using a commercial excimer laser workstation. The laser workstation is equipped with an LPX 220i excimer laser, beam shaping and homogenizing optics, a dielectric attenuator, and provides approximately 30 ns pulses at 248 nm with an overall energy deviation in the mask plane of below 5% rms. The laser processing chamber was attached to the x–y–z stage of the workstation. The stage enables program controlled positioning and scanning of the laser beam across the sample surface with a resolution of 1 mm and a maximum velocity of 100 mm/s. A Schwarzschild objective (15 demagnification) with an optical resolution of 1.5 mm was used for projecting a variable aperture onto the sample back side. In all investigations fused silica samples cut from double side polished wafers with a thickness of about 380 mm have been used. The samples that feature a low surface roughness of less than 0.3 nm (rms) on both sides were used as received without additional cleaning. A scanning electron microscope (SEM) was employed for examination of the etched structures and the surface quality. The etch depth and the rough-
111
ness on the bottom of the etched pits were measured with white light interference microscope. Due to the wavelength of 550 nm used in these optical measurements the lateral resolution is slightly less than 1 mm. Hence the cut-off wavelength of the measured surface roughness is in the same range and gives no information on the sub-micron surface features. Therefore, selected samples were investigated by atomic force microscopy using the tapping mode. Additional to the different lateral resolutions of the used methods the measured area is dissimilar and accounts for the AFM 4 mm2 and for the optical measurements 120 mm2.
3. Results and discussion The etch rate dependence of the presented etch technique is shown in Fig. 2. The etch rates were calculated from the final depths of etch pits with an area of 100 mm 100 mm after 300 laser pulses. The threshold fluence for etching of fused silica at a chamber temperature of 75 8C was determined to about 0.7 J/cm2 [11]. This threshold fluence is one magnitude lower than the ablation threshold of fused silica in air of more than 12.0 J/cm2 [12]. First the etch rate rises with increasing laser fluence up to 2.0 J/cm2, than shows a saturation and keeps constant between 2.0 and 5.0 J/cm2. In the rate saturation fluence range a
Fig. 2. Etch rates on fused silica in dependence on the laser fluence at a chamber temperature of 75 8C using an adsorbed toluene layer (the line is used for guide the eyes). The splitting into three fluence regions is pointed out.
112
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
low etch rate of about 1 nm/pulse has been observed. At laser fluences higher than 5.0 J/cm2 the etch rate grows again and reaches values as high as 100 nm/pulse. Therefore, a splitting of the investigated fluence range into three regions with different etch behavior is apparent. These sections are the low fluence, the saturation and the high fluence region as depicted in Fig. 2. The mechanism of etching at the solid/adsorbate interface is very complex, but some processes involved in etching can be discussed. The laser beam absorption of the adsorbed hydrocarbon layer results in fast heating of this layer and the near surface region of the transparent sample. The high temperatures achieved by this fast laser heating cause probably stress and structural transitions of the glass material near the surface and result finally in materials etching. Additional to the fast heating of the surface other processes can be involved in LESAL. The most probable processes are the desorption of the adsorbed layer [13], the generation of shock waves [14], and the decomposition of the adsorbed hydrocarbons [15]. In addition, the toluene vapor in the chamber can contribute only little to the etching due to the low density of the vapor phase. In the low fluence region the etch rate averaged over 300 laser pulses increases nonlinearly over two orders of magnitude to about 1 nm/pulse at 2.0 J/cm2. Here, incubation effects, i.e., the material etching starts after a certain number of laser pulses, were observed. Similar to other incubation processes, e.g., laser ablation of fused silica in air [12], the etching process is strongly influenced by the formation of local surface defects during the onward laser irradiation. After generating a sufficient number of defects, that can be material defects into the fused silica or decomposition products at the surface, the etching occurs. Due to the nonuniform defect formation the surface is as, shown in Fig. 5b) uneven etched. The nonlinear, almost exponential increase of the etch rate in the low fluence region can be attributed to typical thermal processes such as laser-induced thermal decomposition of the hydrocarbons or desorption of the adsorbed layer. The activation energy from this thermal process can be estimated from the slope of the etch rate in this region. If the temperature grows linear with the laser fluence the thermal activation energy can be estimated to 50 kJ/mol. This high value is characteristic for chemical reactions rather than for physical desorption processes.
The most interesting feature of LESAL is the saturation of the etch rate in a wide fluence range. This rate saturation is exceptional for laser processing but some examples are known. The most saturation processes are caused by mass transport limitations, i.e., of reactive molecules, to the surface area to be etched [16]. Some processes make use of an adsorbed film of a certain thickness [17] that limits the etch rate. As soon as all adsorbed molecules have been used up by the etching process the process stops and causes the rate saturation. Another example of rate limiting is the digital etching of GaAs by chlorine [18], where the chlorine forms exactly one monolayer of chlorides that are removed by an UV-laser pulse. The etching of fused silica by means of an adsorbed hydrocarbon layer is determined by the properties of the adsorbed hydrocarbon film. In difference to the chemical reaction of GaAs with chlorine, hydrocarbons do not react with fused silica to form reaction products and a physically adsorbed toluene layer is expected. Hence, physical processes must be responsible for the rate saturation. It is known that multi-molecular layers can be physisorbed at surfaces according to different adsorption theories [19]. For thermal driven processes at high laser fluences the increase of the laser fluence results in higher heating rates and temperatures that typically are the reason for higher etch rates. However, this is inconsistent with the observed rate saturation. Additional to growing interface temperatures higher desorption rates of the adsorbed layer are the consequence of raising the laser fluence. Laser desorption studies of hydrocarbons show the possibility of the very fast desorption at rapid heating by laser pulses [13]. Hence, the laser heating of the adsorbed layer results in quick desorption processes that cause the decrease of the adsorbed layer thickness during the laser pulse. If the adsorbed layer is removed within the pulse duration at certain fluence, the further increase of the laser fluence reduces the time for the film desorption without significant changes of the maximum surface temperature. In this case the rate saturation is caused by limiting the absorbed laser energy that is responsible for the limitation of the surface temperature even though the laser fluence increases. In the high fluence region the etch rates are much higher compared to the saturated rate value and resemble the rates of typical ablation processes. Additional to the high etch rates both the fluence values and
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
113
Fig. 3. Fluence dependence of the etch rate for different chamber temperatures.
the rough surfaces that are observed in this fluence region are an indication for a changed etch mechanism. Hence, the etching is probably dominated by ablation processes similar to laser ablation in air. The lower threshold of the adsorbed layer ablation compared to laser ablation in air is caused by the adsorbed hydrocarbon layer that provides additional absorption at the sample surface where the ablation process starts. The substantial reduction of the ablation threshold of transparent samples was already observed for a layer consisting of aluminum particles covering the transparent substrates [20]. The fluence dependence of the etch rate for different chamber temperatures in the range from 60 to 105 8C is shown in Fig. 3. The etch rate behavior with the typical three fluence regions is almost identical for the investigated temperatures. However, the temperature affects the etch rate in the saturation fluence region as represented in Fig. 4 for two different laser fluences. The saturated etch rate increases almost linear with the temperature from 0.9 nm/pulse at 60 8C to 2 nm/pulse at 105 8C. It is supposed that the raise of the etch rate with increasing temperature associates with the increasing of the adsorbed layer thickness due to the higher partial pressure of toluene. The absorbed laser energy increases with growing adsorbed layer thickness and causes higher laser-induced surface temperatures and consequently higher etch rates.
Furthermore, the temperature affects the laser fluences delimiting the regions of different etch behavior, too. The upper limit of the rate saturation region is shifted down to smaller fluence values with raising temperature (Fig. 3). The reason for the fluence shifting is also the expected increase of the thickness of the adsorbed toluene layer at higher temperatures. Optical investigations of the etched surface show surface alterations taking place at LESAL processing. Similar to high-intensity irradiation of liquid toluene with a copper vapor laser [15] and KrF excimer laser
Fig. 4. The etch rate increases with the chamber temperature in the saturation fluence region.
114
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
[21] the LESAL processing results in surface modifications such as the deposition of a thin layer. Hence, it can be assumed that additional to a laser-induced rapid heating of the adsorbed layer the decomposition of the adsorbed toluene layer by the laser heating is involved in the etch process. The roughness of the fused silica surfaces etched with LESAL is remarkably low. First results revealed a measured roughness of well below 1 nm rms [11]. However, the roughness changes considerably with the processing parameters, e.g., the laser fluence. Fig. 5 shows AFM images of laser etched surfaces processed in the saturation region (F ¼ 5.0 J/cm2 and 3.5 J/cm2) and low fluence region (F ¼ 1.9 J/cm2), respectively. Although the etch depth is nearly identical the etched surface morphology and the roughness differ significantly. At the higher fluence of 5.0 J/cm2 the etched surface is extremely plane and smooth as shown in Fig 5a. It is remarkable that scratches probably originating from the polishing process are still visible and are partially responsible for the rms roughness value of the fused silica surface of 0.23 nm. For comparison the roughness of the virgin surface was measured with 0.23 nm, too. The observed scratches at the bottom of the etched grooves are either transferred from already existing scratches at the polished surface or the scratches are developed by the laser etching from a modified near surface region. Both, scratches at the surface as well as subsurface damage are well known from mechanical polishing. However, the occurrence of such scratches after etching indicates to a very homogenous etch behavior and confirms the inherent low roughness of the LESAL process. In contrast to the smooth surface in the rate saturation region the surface etched at low fluences, e.g., 1.9 J/cm2, shows a granular microstructure as depicted in Fig. 5c, which is responsible for the roughness, that amounts 1.6 nm rms. The surface etched at a laser fluence of 3.5 J/cm2 shows a fine granular surface topography without any visible scratch and a rms value of 0.36 nm. Fig. 6 shows the roughness (rms) of typical etched surfaces sides measured with interference microscopy and AFM in dependence on laser fluence for a single temperature and a given pulse number. For comparison the etch rate is shown, too. Similar to the etch rate also the roughness evolution with the laser fluence can be divided in the three regions. In the low fluence
Fig. 5. AFM images of 400 nm deep grooves etched with 300 pulses in fused silica by LESAL method at fluences of (a) 5.0 J/ cm2, (b) 3.5 J/cm2 and (c) 1.9 J/cm2.
region a higher surface roughness has been observed than in the rate saturation region. This comparatively high roughness has been measured at a low etch depth caused by the low etch rate. The origin of the high roughness are the mentioned incubation effects and
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
115
Fig. 6. Measured microroughness for different laser fluences at 75 8C chamber temperature. For comparison the etch rate is shown, too.
the very strong fluence dependence of the etch rate, so that fluence variation cause different etch depths and consequently a high roughness. In the high fluence region a dramatic increase of the roughness is observed due to the thermal ablation-like processes responsible for the etching. In the saturation range the measured roughness (AFM) is below 1 nm (rms) at an etch depth of 400 nm for all investigated sites. In addition, a further decrease of the roughness with increasing laser fluence is noticed. At a fluence of 5.0 J/cm2, which is slightly below the high fluence region, the lowest roughness with 0.23 nm (rms) has been measured by AFM in an area of 2 mm 2 mm. The slight increase of the roughness in the saturated region at lower laser fluences can be explained with an enhanced influence of incubation effects at the early stage of laser etching. In the saturation region the quality of the laser etched surface is similar to that of a well polished surface as the morphology and the roughness demonstrate. Moreover, almost no debris deposition is visible in and around the etched area. The main differences of the LESAL process compared to other laser etch techniques are the use of an adsorbed hydrocarbon layer, the low etch rates and the exceptional etch rate behavior, the low roughness of the etched surfaces and the absence of secondary effects such as bubble formation. Especially, exceptional attributes such as the remarkable low roughness,
the low etch rate, and the etch rate saturation are not known from other physical laser etching processes, i.e., laser ablation. These outstanding characteristics allow the very precise machining of surfaces and enable new applications of the LESAL method for microfabrication. For this purpose different laser processing techniques like mask projection, gray scale, and contour mask technique can be exploited for applications in micro-optics and micro-fluidic.
4. Conclusions The demonstrated method for laser etching of surfaces by means of adsorbed layers allows the etching of transparent materials, e.g., fused silica, by pulsed excimer laser radiation of 248 nm. This technique makes use of the absorption of the laser energy by a thin toluene film that is adsorbed on the back side of the transparent material to be etched. The LESAL etching of fused silica is characterized by a low threshold fluence of 0.7 J/cm2 and a low etch rate of some nanometers per laser pulse. The three fluence regions of LESAL characterized by different etch rates and surface roughness are the result of different processes dominating the etching. In the low fluence range the etching is characterized by a low etch rate of less than 1 nm/pulse, incubation processes and a granular, rough surface. In the fluence
116
R. Bo¨ hme, K. Zimmer / Applied Surface Science 239 (2004) 109–116
range 2.0–5.0 J/cm2 the etch rate shows a saturation and the surface features a very good quality with a very low roughness. The etch rate saturation is the result of the competition processes of laser heating of the material interface and the thermal desorption of the heated adsorbed layer. Hence, the etching of the LESAL process is significantly affected by the adsorbed hydrocarbon layer. In this saturation region the etch rate grows with the temperature of the toluene from 0.6 to 2 nm/pulse. At well chosen etch conditions a surface roughness of 0.23 nm rms has been measured at an etch depth of 400 nm. This rms roughness value is nearly equal the roughness of a virgin fused silica sample. The high fluence region is characterized by high etch rates and roughness due to the thermal ablation dominated etching. The laser etching of surfaces by an adsorbed layer is a new promising technique for laser processing of transparent materials that features a low etch rate and a very good quality of the etched surface and enables new applications in micron technology and precision engineering.
Acknowledgements The authors wish to acknowledge Mr. D. Hirsch for his skilled AFM investigations. References [1] Rai-Choudhury, Handbook of Microlithography, Micromachining and Microfabrication, SPIE PM39, 1997, p. 768.
[2] St. Sinzinger, J. Jahns, Microoptics, Wiley-VCH, Weinheim, 1999. [3] K. Zimmer, A. Braun, Excimer laser machining for 3Dsurface structuring, in: A. Peled (Ed.) Photo-Excited Processes, Diagnostics and Applications, Kluwer, 2003, pp. 301–335. [4] G. Allcock, P.E. Dyer, G. Elliner, H.V. Snelling, J. Appl. Phys. 78 (1995) 7295. [5] P.R. Herman, R.S. Marjoribanks, A. Oettl, K.P. Chen, Appl. Surf. Sci. 154–155 (2000) 577. [6] M. Lenzner, J. Kru¨ ger, W. Kautek, F. Krausz, Appl. Phys. A 68 (1999) 369. [7] H. Varel, D. Ashkenasi, A. Rosenfeld, M. Wa¨ hmer, E.E.B. Campbell, Appl. Phys. A 65 (1997) 367. [8] J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68 (1999) 111. [9] R. Bo¨ hme, A. Braun, K. Zimmer, Appl. Surf. Sci. 186 (2002) 276. [10] J. Zang, K. Sugioka, T. Takahashi, K. Toyoda, K. Midorikawa, Appl. Phys. A 71 (2000) 23. [11] K. Zimmer, R. Bo¨ hme, B. Rauschenbach, Laser etching of fused silica by the use of adsorbed layers, Appl. Phys. A, submitted for publication. [12] J. Ihlemann, B. Wolf-Rottke, Appl. Surf. Sci. 106 (1996) 282. [13] K. Onda, A. Wada, K. Domen, C. Hirose, Surf. Sci. 502 (2002) 319. [14] A.A. Kolomenskii, A.M. Lomonosov, R. Kuschnereit, P. Hess, V.E. Gusev, Phys. Rev. Lett. 29 (7) (1997) 1325. [15] A.V. Simakin, E.N. Loubnin, G.A. Shafeev, Appl. Phys. A 69 (1999) 267. [16] G.P. Vakanas, A.A. Tseng, P. Winer, J. Laser Appl. 14 (2002) 185. [17] B.-J. Kim, S. Chung, S.M. Cho, Appl. Surf. Sci. 187 (2002) 124. [18] T. Meguro, Y. Aoyagi, Appl. Surf. Sci. 112 (1997) 55. [19] P. Atkins, Physical Chemistry, VCH, Weinheim, 1996. [20] D.M. Kane, D.R. Halfpenny, J. Appl. Phys. 87 (2000) 4548. [21] R. Bo¨ hme, D. Spemann, K. Zimmer, Thin Solid Films 453–454 (2004) 127.