- Email: [email protected]
High-power laser interference lithography process on photoresist: Effect of laser fluence and polarisation
Applied Surface Science 255 (2009) 5537–5541
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
High-power laser interference lithography process on photoresist: Effect of laser fluence and polarisation M. Ellman a,*, A. Rodrı´guez a, N. Pe´rez a, M. Echeverria a, Y.K. Verevkin b, C.S. Peng c, T. Berthou d, Z. Wang e, S.M. Olaizola a, I. Ayerdi a a
CEIT and Tecnun (University of Navarra), Manuel de Lardiza´bal 15, 20018, San Sebastia´n, Spain Institute of Applied Physics, 46 Ul’yanova Street, 603600 Nizhny Novgorod, Russia ORC (Tampere University of Technology), Korkeakoulunkatu 3, 33720 Tampere, Finland d SILIOS Technologies SA, Rue Gaston Imbert prolonge´e 13790 Peynier, France e MEC (Cardiff University), Queen’s Buildings, The Parade, Newport Road, Cardiff CF24 3AA, UK b c
A R T I C L E I N F O
A B S T R A C T
Article history:
High throughput and low cost fabrication techniques in the sub-micrometer scale are attractive for the industry. Laser interference lithography (LIL) is a promising technique that can produce one, two and three-dimensional periodical patterns over large areas. In this work, two- and four-beam laser interference lithography systems are implemented to produce respectively one- and two-dimensional periodical patterns. A high-power single pulse of 8 ns is used as exposure process. The optimum exposure dose for a good feature patterning in a 600 nm layer of AZ-1505 photoresist deposited on silicon wafers is studied. The best aspect ratio is found for a laser fluence of 20 mJ/cm2. A method to control the width of the sub-micrometer structures based on controlling the resist thickness and the laser fluence is proposed. ß 2008 Elsevier B.V. All rights reserved.
Available online 12 August 2008 Keywords: Laser interference lithography LIL Photoresist Surface nanostructuring Pulsed lithography
1. Introduction Periodic and quasi-periodic sub-micrometer structures attract great interest for applications such as photonic crystals, magnetic arrays or sub-micrometer patterned sensitive layers. However, the main limitation for the industrial exploitation of devices with sub-micrometer dimensions is the absence of a cost-effective fabrication technology [1]. Electron beam lithography (EBL) and ion beam lithography (IBL) are powerful technologies for prototyping, but both slow and expensive [2–4]. Several structuring techniques such as self-assembly [5,6], nano-imprint lithography [7], two-photon polymerisation [8] or laser interference lithography (LIL) [8–10] have been considered to overcome these serious drawbacks. Among them, LIL is the only maskless technique that provides high throughput and low cost for high volume processing. The working principle of LIL is based on the interference of two or more coherent laser beams that produces a periodic pattern with a minimum period down to half the laser wavelength. The
* Corresponding author. E-mail address: [email protected] (M. Ellman). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.201
definition of the interference pattern on the substrate has been usually performed by two different methods: direct writing with high-energy pulses [11] or a full lithographic cycle with a continuous wave laser in a Lloyd’s mirror configuration [12]. The response of photoresists to high-power pulses in LIL remains largely unexplored and, therefore, the technology has not yet demonstrated its full potential. In this work, LIL is used to pattern 1D and 2D periodic submicrometer structures in a photoresist in order to determine the optimum parameters for a full lithographic cycle with a single high-power laser pulse. The influence of the number of beams, the angle of incidence of the beams and their polarisation on the obtained structures is theoretically analysed. Two-beam and four-beam LIL configurations are implemented and different angles of incidence and fluences (energy per unit area) are tested. 2. Principles of laser interference lithography Laser interference lithography takes advantage of the periodic or quasi-periodic sub-micrometer interference patterns generated by the superimposition of two or more coherent light beams to define 1D, 2D or even 3D nanostructures.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5538
Fig. 1. Four-beam laser interference lithography system (a) schematic and (b) developed setup.
The electric field resulting from the superimposition of N laser beams can be expressed as: ~ E¼
N X
~ Em ¼
m¼1
N X
Am ~ nm~ pm cos ðk~ r 2pyt þ fm Þ
(1)
m¼1
where A is the amplitude, ~ p is the polarisation vector, k = 2p/l is the wave number, ~ r is the position vector, ~ n is the unit vector in the propagation direction, f is the initial phase and y is the frequency. It can be seen from the analytical expression (1) that the electric field is a function of the amplitude, phase, polarisation plane, angle of incidence and wavelength of the beams and so will be the interference pattern. In a practical LIL system, the wavelength of the light source is defined by the laser; any difference in phase will lead to a spatial translation of the interference pattern and the signal amplitude of all the beams is the same as their optical paths are identical. In consequence, there are three parameters that will determine the interference pattern. Firstly, the number of beams, which is responsible of the geometry of the sub-micrometer structures. Secondly, the angles of incidence of the beams, responsible of the period of the structures. Finally, the polarisation of the beams, which conditions the intensity profile of the interference pattern. 3. Experimental The initial substrates for the LIL process are thermally oxidised silicon wafers with a 1.5 mm SiO2 layer. The substrates are first cleaned in two sequential ultrasonic baths in acetone and isopropanol. It is essential to avoid water condensation on the substrate surface before applying the photoresist as this would lead to a bad adherence of the resist. The complete lithography process starts with the deposition of a 600 nm layer of AZ-1505, a positive i-line photoresist, by spin coating,
followed by a prebaking for 30 s at 100 8C on a hotplate. Then, the photoresist is exposed with a single laser pulse followed by a postbake of 2 min at 110 8C on a hotplate. Finally, the resist is developed in a 1:5 dissolution of AZ-315B developer and deionised water for 30 s followed by a hardbake of 30 s at 115 8C on a hotplate. The four-beam laser interference system used as sub-micrometer structuring tool is shown in Fig. 1. A high-energy pulsed frequency-tripled TM polarised Nd:YAG laser (355 nm) with a coherent length of 3 m, energy power up to 320 mJ/cm2, pulse duration of 8 ns and 10 Hz repetition rate is used as light source. Three optical beamsplitters divide the input beam into four noncoplanar coherent beams. A set of mirrors steer the beams towards the substrate with the same angle of incidence. In order to ensure the maximum signal at the interference plane, the optical paths of the four beams are identical. Periodical 1D (gratings) and 2D (holes) sub-micrometer patterns are defined in photoresist with a single pulse over an area of 70 mm2 using a two- and four-beam configuration, respectively. The samples are patterned in the photoresist layer with angles of incidence of 208 and 608. Different exposure doses, ranging from 6 to 24 mJ/cm2 are used to determine the optimum dose for a good feature patterning. As the irradiated area of the substrate depends on the angle of incidence, the exposure dose is calculated from Eq. (2). F¼
E E cos u ¼ Asubstrate Abeam
(2)
where E is the energy of a laser pulse, Asubstrate is the irradiated area on the substrate, Abeam is the area of the laser beam and u is the angle of incidence of the beam. The characterisation of the samples is made by atomic force microscopy (AFM) using 10:1 aspect ratio probes.
Fig. 2. Theoretical intensity values of the maxima and minima of a two-beam interference with angles of incidence of 208 and 608 against the beam polarisation in the case of (a) two beams with the same polarisation; (b) one beam with a fixed polarisation at 908 and the other one varying. In this case 08 means TM polarisation.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5539
Fig. 3. Cross-sectional views of samples fabricated with a two-beam interference system and (a) angle of incidence of 208 (550 nm of period), showing a processed depth of 380 nm; (b) angle of incidence of 608 (200 nm of period), showing a processed depth of 85 nm.
4. Results and discussion Software simulations have determined the main importance of the polarisation of the beams in the interference pattern generated [13]. For a two-beam configuration, the polarisation only affects the relative intensity of the maxima and minima of the interference patterns. The maximum contrast is obtained when the two beams are TE polarised, independently of the angle of incidence of the beams. The behaviour of the interference intensity vs. beam polarisation can be seen in Fig. 2 for angles of incidence of 208 and 608. The figure shows that the interference contrast (difference between the maximum and minimum intensities) has only a small dependence on the polarisation of the beams when the angle of incidence is 208. However, the polarisation has a major effect when the angle of incidence increases to 608 to the point that the interference fringes disappear for a polarisation angle of 358. Thus, when the angle of incidence of the beams increases (the period of the interference decreases) special care has to be taken to assure a high contrast for an effective process of the resist. For a four-beam configuration, the polarisation of the beams affects not only the interference contrast but also the pattern itself. The maximum contrast is obtained when two opposite beams are TE polarised and the other two beams are TM polarised for any angle of incidence. Any other polarisation configuration will lead to different patterns with less contrast. The pattern generated for a given polarisation is not maintained when the angle of incidence changes. Two-beam interference samples have been fabricated for angles of incidence of 208 and 608, TM polarisation and same fluence to analyse the polarisation influence in the generated interference patterns (Fig. 3). The processed depths in the samples with an angle of incidence of 608 show smaller values than the expected analysing simulations and previous experimental results. The AFM images taken show that the resist is processed at the minima of the interference, suggesting that a light intensity value higher than the polymerisation threshold of the resist may be reached at the minima of the fringes. For this reason, the results presented in this work can be generalised to angles of incidence greater than 208 only in the case that both beams are TE polarised. 4.1. Two-beam interference patterns The AFM sections of a processed photoresist with fluences of 13.2 mJ/cm2; 15.8 mJ/cm2 and 20 mJ/cm2 are observed in Fig. 4. The cross-sections of the fabricated structures show that when the fluence of the laser is increased, the depth of the structures
Fig. 4. Cross-sectional views of 1D samples made by two-beam interference with an angle of incidence of 208 and a fluence of (a) 13.2 mJ/cm2; (b) 15.8 mJ/cm2; (c) 20 mJ/cm2.
5540
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
Fig. 5. Depth of the structures defined in the photoresist as function of the applied fluence.
increases. A flat plain at the bottom of the structure indicates that the SiO2 layer has been reached. Fig. 5 illustrates the processed depth of the photoresist for different laser fluences. As fluence increases, the processed depth increases linearly up to 16 mJ/cm2. At this point, the SiO2 layer is reached. However, the feature shape is not well defined yet as seen in Fig. 4b. More energy is still needed to completely remove the resist from the bottom of the gratings and to obtain a linear shape at the bottom and the walls of the structures. A fluence value of 20 mJ/cm2 has been considered as an optimum dose for well defined features in a photoresist layer with a thickness of 600 nm (Figs. 4c and 6). There are two parameters intrinsic to the resist that influence the fabrication process: the damage threshold and the polymerisation threshold. The thicker the resist layer is, the higher the fluence required to process it. As a consequence, the photoresist damage threshold, which must not be surpassed, determines the maximum depth that can be reached. The higher the applied fluence is, the higher the light intensity at the interference maxima (Fig. 7a). Since the interference period and the resist polymerisation threshold are constant, the width of the processed structures in the resist increases with the fluence applied, as is shown in Fig. 7. This result suggests a method to control the linewidth of the sub-micrometer structures: by depositing a resist of the right thickness and applying the right fluence, the width at the bottom of the resist can be controlled. A further step to process the substrate surface
Fig. 6. Three-dimensional view of a 550 nm period grating sample, fabricated by a two-beam LIL system with an angle of incidence of 208 and a fluence of 20 mJ/cm2 polymerised by a single pulse.
would result in a surface sub-micrometer structure with the desired width. 4.2. Four-beam interference patterns 2D periodic arrays have been fabricated with a four-beam interference system using the same laser energies as for the fabrication of the grating samples. The analysis of the processed photoresist depth vs. the fluence applied has shown the same behaviour as in the grating samples (Fig. 5). Although the deposited resist layers are 30–40 nm thicker than in the grating samples, a fluence of 20 mJ/cm2 produces well defined features (Fig. 8). The fabricated dot samples, as the one in the Fig. 8, have shown a spatial modulation of the interference pattern. This modulation is mainly caused by slight differences in the angles of incidence of the beams, which are due to the inaccurate placement of the last four mirrors. Hence, a difference of 18 in a beam causes a modulation with a period of 22 mm. This period decreases exponentially as the angle mismatch increases. In Fig. 8 a mismatch of 3–48 in the angle of incidence explains the 8– 9 mm modulation period.
Fig. 7. (a) Interference intensity distribution for smaller (doted line) and greater (thin line) fluences and the photoresist polymerisation threshold (thick line). (b) Aspect ratio in 1D samples fabricated with an angle of incidence of 208.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5541
respectively, for an angle of incidence of 208. Areas about 1 cm in diameter have been processed with a single pulse of 8 ns. An optimum fluence of 20 mJ/cm2 has been proposed for the deposited photoresist. In addition, a method to control the width of the sub-micrometer structures based on the control of the resist thickness and the laser fluence is proposed. In a two-beam interference the results discussed are valid for laser angles of incidence smaller than 208 or for any angle if the beams are TE polarised. For a four-beam interference, the results are valid for angles of incidence smaller than 208 and two opposite beams TE polarised and the other two TM polarised. References
Fig. 8. AFM image of a dot sample of 850 nm period, fabricated with a four-beam LIL system with an angle of incidence of 208 and a fluence of 20 mJ/cm2.
5. Conclusions One- and two-dimensional periodical patterns have been recorded in a 600 nm layer of AZ-1505 positive i-line photoresist using high-power two- and four-beam interference systems,
[1] M. Salau¨n, A.L. Joudrier, M. Audier, S. Pignard, Appl. Surf. Sci. 253 (19) (2007) 7947. [2] S. Iwamitsu, M. Nagao, S.A. Pahlovy, K. Nishimura, M. Kashihara, S. Momota, Y. Nojiri, J. Taniguchi, I. Miyamoto, T. Nakao, N. Morita, N. Kawasegi, Colloids Surf. A: Physicochem. Eng. Aspects 313/314 (2008) 407. [3] M. Ha¨ffner, A. Haug, R.T. Weitz, M. Fleischer, M. Burghard, H. Peisert, T. Chasse´, D.P. Kern, Microelectron. Eng. 85 (2008) 768. [4] Y. Cheng, A. Pe´pin, Electrophoresis 22 (2001) 187. [5] N. Pe´rez, A. Hu¨ls, D. Puente, W. Gonza´lez-Vin˜as, E. Castan˜o, S.M. Olaizola, Sens. Actuators B 126 (2007) 86. [6] A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S.W. Leonard, C. Lopez, F. Meseguer, H. Miguez, J.P. Mondla, G.A. Ozin, O. Toader, H.M. Van Driel, Nature 405 (6785) (2000) 437. [7] T. DiBiase, M. Ahamdian, I. Malik, Microelectron. Eng. 84 (2007) 989. [8] H.B. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1994) 786. [9] J.M. Cees, van Rijn, J. Microlith. Microfab. Microsyst. 5 (1) (2006) 011012. [10] T. Kondo, S. Juodkazis, V. Mizeikis, H. Misawa, Opt. Expr. 14 (17) (2006) 7943. [11] A. Ferrer, V. Diez-Blanco, A. Ruiz, J. Siegel, J. Solis, Appl. Surf. Sci. 254 (4) (2007) 1121. [12] Q. Xie, M.H. Hong, H.L. Tan, G.X. Chen, L.P. Shi, T.C. Chong, J. Alloy. Compd. 449 (2008) 261. [13] A. Fernandez, D.W. Phillion, Appl. Opt. 37 (1998) 473.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
High-power laser interference lithography process on photoresist: Effect of laser fluence and polarisation M. Ellman a,*, A. Rodrı´guez a, N. Pe´rez a, M. Echeverria a, Y.K. Verevkin b, C.S. Peng c, T. Berthou d, Z. Wang e, S.M. Olaizola a, I. Ayerdi a a
CEIT and Tecnun (University of Navarra), Manuel de Lardiza´bal 15, 20018, San Sebastia´n, Spain Institute of Applied Physics, 46 Ul’yanova Street, 603600 Nizhny Novgorod, Russia ORC (Tampere University of Technology), Korkeakoulunkatu 3, 33720 Tampere, Finland d SILIOS Technologies SA, Rue Gaston Imbert prolonge´e 13790 Peynier, France e MEC (Cardiff University), Queen’s Buildings, The Parade, Newport Road, Cardiff CF24 3AA, UK b c
A R T I C L E I N F O
A B S T R A C T
Article history:
High throughput and low cost fabrication techniques in the sub-micrometer scale are attractive for the industry. Laser interference lithography (LIL) is a promising technique that can produce one, two and three-dimensional periodical patterns over large areas. In this work, two- and four-beam laser interference lithography systems are implemented to produce respectively one- and two-dimensional periodical patterns. A high-power single pulse of 8 ns is used as exposure process. The optimum exposure dose for a good feature patterning in a 600 nm layer of AZ-1505 photoresist deposited on silicon wafers is studied. The best aspect ratio is found for a laser fluence of 20 mJ/cm2. A method to control the width of the sub-micrometer structures based on controlling the resist thickness and the laser fluence is proposed. ß 2008 Elsevier B.V. All rights reserved.
Available online 12 August 2008 Keywords: Laser interference lithography LIL Photoresist Surface nanostructuring Pulsed lithography
1. Introduction Periodic and quasi-periodic sub-micrometer structures attract great interest for applications such as photonic crystals, magnetic arrays or sub-micrometer patterned sensitive layers. However, the main limitation for the industrial exploitation of devices with sub-micrometer dimensions is the absence of a cost-effective fabrication technology [1]. Electron beam lithography (EBL) and ion beam lithography (IBL) are powerful technologies for prototyping, but both slow and expensive [2–4]. Several structuring techniques such as self-assembly [5,6], nano-imprint lithography [7], two-photon polymerisation [8] or laser interference lithography (LIL) [8–10] have been considered to overcome these serious drawbacks. Among them, LIL is the only maskless technique that provides high throughput and low cost for high volume processing. The working principle of LIL is based on the interference of two or more coherent laser beams that produces a periodic pattern with a minimum period down to half the laser wavelength. The
* Corresponding author. E-mail address: [email protected] (M. Ellman). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.201
definition of the interference pattern on the substrate has been usually performed by two different methods: direct writing with high-energy pulses [11] or a full lithographic cycle with a continuous wave laser in a Lloyd’s mirror configuration [12]. The response of photoresists to high-power pulses in LIL remains largely unexplored and, therefore, the technology has not yet demonstrated its full potential. In this work, LIL is used to pattern 1D and 2D periodic submicrometer structures in a photoresist in order to determine the optimum parameters for a full lithographic cycle with a single high-power laser pulse. The influence of the number of beams, the angle of incidence of the beams and their polarisation on the obtained structures is theoretically analysed. Two-beam and four-beam LIL configurations are implemented and different angles of incidence and fluences (energy per unit area) are tested. 2. Principles of laser interference lithography Laser interference lithography takes advantage of the periodic or quasi-periodic sub-micrometer interference patterns generated by the superimposition of two or more coherent light beams to define 1D, 2D or even 3D nanostructures.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5538
Fig. 1. Four-beam laser interference lithography system (a) schematic and (b) developed setup.
The electric field resulting from the superimposition of N laser beams can be expressed as: ~ E¼
N X
~ Em ¼
m¼1
N X
Am ~ nm~ pm cos ðk~ r 2pyt þ fm Þ
(1)
m¼1
where A is the amplitude, ~ p is the polarisation vector, k = 2p/l is the wave number, ~ r is the position vector, ~ n is the unit vector in the propagation direction, f is the initial phase and y is the frequency. It can be seen from the analytical expression (1) that the electric field is a function of the amplitude, phase, polarisation plane, angle of incidence and wavelength of the beams and so will be the interference pattern. In a practical LIL system, the wavelength of the light source is defined by the laser; any difference in phase will lead to a spatial translation of the interference pattern and the signal amplitude of all the beams is the same as their optical paths are identical. In consequence, there are three parameters that will determine the interference pattern. Firstly, the number of beams, which is responsible of the geometry of the sub-micrometer structures. Secondly, the angles of incidence of the beams, responsible of the period of the structures. Finally, the polarisation of the beams, which conditions the intensity profile of the interference pattern. 3. Experimental The initial substrates for the LIL process are thermally oxidised silicon wafers with a 1.5 mm SiO2 layer. The substrates are first cleaned in two sequential ultrasonic baths in acetone and isopropanol. It is essential to avoid water condensation on the substrate surface before applying the photoresist as this would lead to a bad adherence of the resist. The complete lithography process starts with the deposition of a 600 nm layer of AZ-1505, a positive i-line photoresist, by spin coating,
followed by a prebaking for 30 s at 100 8C on a hotplate. Then, the photoresist is exposed with a single laser pulse followed by a postbake of 2 min at 110 8C on a hotplate. Finally, the resist is developed in a 1:5 dissolution of AZ-315B developer and deionised water for 30 s followed by a hardbake of 30 s at 115 8C on a hotplate. The four-beam laser interference system used as sub-micrometer structuring tool is shown in Fig. 1. A high-energy pulsed frequency-tripled TM polarised Nd:YAG laser (355 nm) with a coherent length of 3 m, energy power up to 320 mJ/cm2, pulse duration of 8 ns and 10 Hz repetition rate is used as light source. Three optical beamsplitters divide the input beam into four noncoplanar coherent beams. A set of mirrors steer the beams towards the substrate with the same angle of incidence. In order to ensure the maximum signal at the interference plane, the optical paths of the four beams are identical. Periodical 1D (gratings) and 2D (holes) sub-micrometer patterns are defined in photoresist with a single pulse over an area of 70 mm2 using a two- and four-beam configuration, respectively. The samples are patterned in the photoresist layer with angles of incidence of 208 and 608. Different exposure doses, ranging from 6 to 24 mJ/cm2 are used to determine the optimum dose for a good feature patterning. As the irradiated area of the substrate depends on the angle of incidence, the exposure dose is calculated from Eq. (2). F¼
E E cos u ¼ Asubstrate Abeam
(2)
where E is the energy of a laser pulse, Asubstrate is the irradiated area on the substrate, Abeam is the area of the laser beam and u is the angle of incidence of the beam. The characterisation of the samples is made by atomic force microscopy (AFM) using 10:1 aspect ratio probes.
Fig. 2. Theoretical intensity values of the maxima and minima of a two-beam interference with angles of incidence of 208 and 608 against the beam polarisation in the case of (a) two beams with the same polarisation; (b) one beam with a fixed polarisation at 908 and the other one varying. In this case 08 means TM polarisation.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5539
Fig. 3. Cross-sectional views of samples fabricated with a two-beam interference system and (a) angle of incidence of 208 (550 nm of period), showing a processed depth of 380 nm; (b) angle of incidence of 608 (200 nm of period), showing a processed depth of 85 nm.
4. Results and discussion Software simulations have determined the main importance of the polarisation of the beams in the interference pattern generated [13]. For a two-beam configuration, the polarisation only affects the relative intensity of the maxima and minima of the interference patterns. The maximum contrast is obtained when the two beams are TE polarised, independently of the angle of incidence of the beams. The behaviour of the interference intensity vs. beam polarisation can be seen in Fig. 2 for angles of incidence of 208 and 608. The figure shows that the interference contrast (difference between the maximum and minimum intensities) has only a small dependence on the polarisation of the beams when the angle of incidence is 208. However, the polarisation has a major effect when the angle of incidence increases to 608 to the point that the interference fringes disappear for a polarisation angle of 358. Thus, when the angle of incidence of the beams increases (the period of the interference decreases) special care has to be taken to assure a high contrast for an effective process of the resist. For a four-beam configuration, the polarisation of the beams affects not only the interference contrast but also the pattern itself. The maximum contrast is obtained when two opposite beams are TE polarised and the other two beams are TM polarised for any angle of incidence. Any other polarisation configuration will lead to different patterns with less contrast. The pattern generated for a given polarisation is not maintained when the angle of incidence changes. Two-beam interference samples have been fabricated for angles of incidence of 208 and 608, TM polarisation and same fluence to analyse the polarisation influence in the generated interference patterns (Fig. 3). The processed depths in the samples with an angle of incidence of 608 show smaller values than the expected analysing simulations and previous experimental results. The AFM images taken show that the resist is processed at the minima of the interference, suggesting that a light intensity value higher than the polymerisation threshold of the resist may be reached at the minima of the fringes. For this reason, the results presented in this work can be generalised to angles of incidence greater than 208 only in the case that both beams are TE polarised. 4.1. Two-beam interference patterns The AFM sections of a processed photoresist with fluences of 13.2 mJ/cm2; 15.8 mJ/cm2 and 20 mJ/cm2 are observed in Fig. 4. The cross-sections of the fabricated structures show that when the fluence of the laser is increased, the depth of the structures
Fig. 4. Cross-sectional views of 1D samples made by two-beam interference with an angle of incidence of 208 and a fluence of (a) 13.2 mJ/cm2; (b) 15.8 mJ/cm2; (c) 20 mJ/cm2.
5540
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
Fig. 5. Depth of the structures defined in the photoresist as function of the applied fluence.
increases. A flat plain at the bottom of the structure indicates that the SiO2 layer has been reached. Fig. 5 illustrates the processed depth of the photoresist for different laser fluences. As fluence increases, the processed depth increases linearly up to 16 mJ/cm2. At this point, the SiO2 layer is reached. However, the feature shape is not well defined yet as seen in Fig. 4b. More energy is still needed to completely remove the resist from the bottom of the gratings and to obtain a linear shape at the bottom and the walls of the structures. A fluence value of 20 mJ/cm2 has been considered as an optimum dose for well defined features in a photoresist layer with a thickness of 600 nm (Figs. 4c and 6). There are two parameters intrinsic to the resist that influence the fabrication process: the damage threshold and the polymerisation threshold. The thicker the resist layer is, the higher the fluence required to process it. As a consequence, the photoresist damage threshold, which must not be surpassed, determines the maximum depth that can be reached. The higher the applied fluence is, the higher the light intensity at the interference maxima (Fig. 7a). Since the interference period and the resist polymerisation threshold are constant, the width of the processed structures in the resist increases with the fluence applied, as is shown in Fig. 7. This result suggests a method to control the linewidth of the sub-micrometer structures: by depositing a resist of the right thickness and applying the right fluence, the width at the bottom of the resist can be controlled. A further step to process the substrate surface
Fig. 6. Three-dimensional view of a 550 nm period grating sample, fabricated by a two-beam LIL system with an angle of incidence of 208 and a fluence of 20 mJ/cm2 polymerised by a single pulse.
would result in a surface sub-micrometer structure with the desired width. 4.2. Four-beam interference patterns 2D periodic arrays have been fabricated with a four-beam interference system using the same laser energies as for the fabrication of the grating samples. The analysis of the processed photoresist depth vs. the fluence applied has shown the same behaviour as in the grating samples (Fig. 5). Although the deposited resist layers are 30–40 nm thicker than in the grating samples, a fluence of 20 mJ/cm2 produces well defined features (Fig. 8). The fabricated dot samples, as the one in the Fig. 8, have shown a spatial modulation of the interference pattern. This modulation is mainly caused by slight differences in the angles of incidence of the beams, which are due to the inaccurate placement of the last four mirrors. Hence, a difference of 18 in a beam causes a modulation with a period of 22 mm. This period decreases exponentially as the angle mismatch increases. In Fig. 8 a mismatch of 3–48 in the angle of incidence explains the 8– 9 mm modulation period.
Fig. 7. (a) Interference intensity distribution for smaller (doted line) and greater (thin line) fluences and the photoresist polymerisation threshold (thick line). (b) Aspect ratio in 1D samples fabricated with an angle of incidence of 208.
M. Ellman et al. / Applied Surface Science 255 (2009) 5537–5541
5541
respectively, for an angle of incidence of 208. Areas about 1 cm in diameter have been processed with a single pulse of 8 ns. An optimum fluence of 20 mJ/cm2 has been proposed for the deposited photoresist. In addition, a method to control the width of the sub-micrometer structures based on the control of the resist thickness and the laser fluence is proposed. In a two-beam interference the results discussed are valid for laser angles of incidence smaller than 208 or for any angle if the beams are TE polarised. For a four-beam interference, the results are valid for angles of incidence smaller than 208 and two opposite beams TE polarised and the other two TM polarised. References
Fig. 8. AFM image of a dot sample of 850 nm period, fabricated with a four-beam LIL system with an angle of incidence of 208 and a fluence of 20 mJ/cm2.
5. Conclusions One- and two-dimensional periodical patterns have been recorded in a 600 nm layer of AZ-1505 positive i-line photoresist using high-power two- and four-beam interference systems,
[1] M. Salau¨n, A.L. Joudrier, M. Audier, S. Pignard, Appl. Surf. Sci. 253 (19) (2007) 7947. [2] S. Iwamitsu, M. Nagao, S.A. Pahlovy, K. Nishimura, M. Kashihara, S. Momota, Y. Nojiri, J. Taniguchi, I. Miyamoto, T. Nakao, N. Morita, N. Kawasegi, Colloids Surf. A: Physicochem. Eng. Aspects 313/314 (2008) 407. [3] M. Ha¨ffner, A. Haug, R.T. Weitz, M. Fleischer, M. Burghard, H. Peisert, T. Chasse´, D.P. Kern, Microelectron. Eng. 85 (2008) 768. [4] Y. Cheng, A. Pe´pin, Electrophoresis 22 (2001) 187. [5] N. Pe´rez, A. Hu¨ls, D. Puente, W. Gonza´lez-Vin˜as, E. Castan˜o, S.M. Olaizola, Sens. Actuators B 126 (2007) 86. [6] A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S.W. Leonard, C. Lopez, F. Meseguer, H. Miguez, J.P. Mondla, G.A. Ozin, O. Toader, H.M. Van Driel, Nature 405 (6785) (2000) 437. [7] T. DiBiase, M. Ahamdian, I. Malik, Microelectron. Eng. 84 (2007) 989. [8] H.B. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1994) 786. [9] J.M. Cees, van Rijn, J. Microlith. Microfab. Microsyst. 5 (1) (2006) 011012. [10] T. Kondo, S. Juodkazis, V. Mizeikis, H. Misawa, Opt. Expr. 14 (17) (2006) 7943. [11] A. Ferrer, V. Diez-Blanco, A. Ruiz, J. Siegel, J. Solis, Appl. Surf. Sci. 254 (4) (2007) 1121. [12] Q. Xie, M.H. Hong, H.L. Tan, G.X. Chen, L.P. Shi, T.C. Chong, J. Alloy. Compd. 449 (2008) 261. [13] A. Fernandez, D.W. Phillion, Appl. Opt. 37 (1998) 473.