Interference lithography processes with high-power laser pulses

Transcription

1 TP PT P Interference lithography processes with high-power laser pulses A Rodriguez TP * PTPa, b c M Ellan Pa, I Ayerdi Pa, N Perez Pa, S M Olaizola Pa, J Zhang, Z Ji Pb, T Berthou, C S Peng d, Y K Verekin e b and Z Wang P Pa CEIT & Tecnun (Uniersity of Naarra), San Sebastian, Spain b Manufacturing Engineering Centre, Cardiff Uniersity, Cardiff, UK c Optoelectronics Research Centre, Tapere Uniersity of Technology, Tapere, Finland d SILIOS Technologies SA, Peynier, France e Institute of Applied Physics, Russian Acadey of Sciences, Nizhny Nogorod, Russia ABSTRACT Laser interference lithography (LIL) is concerned with the use of interference patterns generated fro two or seeral coherent beas of laser radiation for the structuring of aterials. This paper presents the work on the processes based on resists and direct writing with laser interference lithography. In the work, a four-bea laser interference syste was used as a subicroeter structuring tool in which a high-energy pulsed, frequency-tripled and TM polarized Nd:YAG laser (355 n) with a coherent length of 3, energy power up to 320 J/c 2, pulse duration of 8 ns and 10 Hz repetition rate was used as a light source. The experiental results were achieed with 2-bea and 4-bea interference patterning. The processes can be used to define subicron surface reliees in large areas for use in the field of MEMS. Keywords: Laser interference lithography, patterning, interference patterns. 1. INTRODUCTION Laser interference lithography (LIL) has been used for icro and nano anufacturing of regular surface patterns in recent years [1-3]. This technique has found soe applications due to the low-cost approach and relatie siplicity of the process. For exaple, one of these LIL applications was the deelopent of an interference lithography tool called Nanoruler for patterning large gratings in 2004 [4-5]. In this work, a 400n-period grating that was a one-diensional regular fringe pattern coering 300-diaeter silicon wafer was written in around 20 inutes. LIL is a askless technology to produce periodic and quasi-periodic structures that are spatially coherent oer large areas. LIL is highly innoatie in lithography due to its high efficiency, large patterning areas (up to hundreds of illietres in diaeter) and low cost in icro and nano scale structuring of aterials as copared to the IBL (ion bea lithography) or EBL (electron bea lithography) technology. Other nanodefinition techniques such as SPL (scanning probe lithography) are also either slow or based in contact projection which results in inefficient fabrication and the need of expensie expendables which are two decisie disadantages with respect to eerging nanotechnology production requireents. On the other hand, the ain disadantage of the LIL technology is that only spatially periodic patterns can be defined. This liits LIL to be used as a general patterning technique. Howeer, there are niche applications in fields such as icrofluidcs, photonic aterials, and icro(bio)sensors were this technique is the right tool to process subicroeter features in a cost-effectie. Preious approaches to LIL ade by MIT and others hae focused on a ulti-step process that includes recording an interference pattern on photoresist followed by a series of process to transfer the pattern into the underlying layer. Eery step should be perfored carefully since an error in any step could lead to the failure of final results. In this point of iew, the industrial needs to lower the fabrication cost, fewer steps are desired. Thus, direct writing can be an adantage * phone ; fax ; ceit.es Laser Applications in Microelectronic and Optoelectronic Manufacturing VII, edited by Michel Meunier, Andrew S. Holes, Hiroyuki Niino, Bo Gu, Proc. of SPIE Vol. 7201, 72010R 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol R-1

2 for soe applications. Also, there has been a liited research effort to deelop a technology to change the interference pattern in a siple way. This paper presents the work on a LIL based syste that can generate a range of 2D interferences and soe processes based on resists and direct writing with laser interference lithography. In this work, we hae used as a four-bea laser interference syste as a subicroeter structuring tool. We hae used as a light source a high-energy pulsed, frequencytripled and TM polarised Nd:YAG laser (355 n) with a coherent length of 3, energy power up to 320 J/c 2, pulse duration of 8 ns and 10 Hz repetition rate. The experiental results were achieed with 2-bea and 4-bea interference patterning. In the following sections, the principle, syste and experient will be introduced. 2. PRINCIPLE AND SYSTEM The interference patterns, generated fro ultiple coherent beas of laser radiation, can be arrays or atrices of laser bea lines or dots. The intensity distribution of the interference patterns exposes aterials with a pitch of subwaelength of the interfering light. When using such radiation to interact with aterials, feature sizes down to a fraction of the laser waelength can be created. This technology proides a way for icro and nano patterning periodic and quasi-periodic patterns that are spatially coherent oer large areas. Figure 1 shows the principle of two-bea laser interference and laser bea lines are fored. The pattern period is basically goerned by λ / 2sinθ as shown in Figure 1, where λ is the waelength and θ is the incidence angle of the beas. Figure 2 shows a configuration of four-bea laser interference and laser bea dots are fored. It can be seen, fro the two figures, that arrays or atrices of laser bea lines or dots can be fored by ulti-bea laser interference. Laser Bea 1 Laser / Bea 2 e Period=X/(2sine) Substrate Figure 1 Two-bea laser interference. Figure 2 Four-bea laser interference. In the case of N-bea interference, the superposition of the electric field ectors of N laser beas ( E, 1 E, 2 E, and 3 E ) can be expressed as [6-8] N E = N = 1 E = N = 1 A p cos ( kn r ± 2πt + φ ) (1) where A (=1, 2, 3,, N) is the aplitude. p (=1, 2, 3,, N) is the unit polarization ector. k = 2π λ is the wae nuber ( λ = waelength). n (=1, 2, 3,, N) is the unit ector in the propagation direction. r is the position ector and is the frequency. φ (=1, 2, 3,, N) is the phase constant. Proc. of SPIE Vol R-2

3 It can be seen fro Equation (1) that the analytical expression is a function of aplitudes, phases, angles of incidence, polarization states and the waelength. These paraeters are related to the syste requireents and they need to be selected and onitored in a laser interference nanolithography syste [9]. Figure 3 shows a schee of a ulti-bea laser interference lithography syste for foration of laser interference patterns. The syste consists of nine parts [10]: Laser radiation The role of laser radiation is to supply coherent light with an appropriate waelength, power and coherence length. Bea shaping Gaussian intensity distribution is transfored into a flat-top distribution to obtain unifor patterns. Bea splitting Bea splitting is needed to obtain seeral coherent beas for a laser interference nanolithography syste. Phase control Phase control is related to pattern orientation or localization of a pattern. Interference control Interference control is concerned with the arrangeent of the coherent beas to for required interference patterns. Polarization control Polarization control is related to the arrangeent of the polarization states of interfering beas to for required interference patterns. Bea onitoring Bea onitoring is concerned with the onitoring of a ulti-bea interference nanolithography process. Saple positioning Positioning of saples (x, y, z), repeated patterning of large saples (x, y), changing of the incident angles of the laser beas and the periods to keep the saple in a position for patterning (z), rotation of the angle between the saple and the incoing beas to hae ore flexibility to the shape of patterns that can be defined. Syste control Syste control is designed to control all the syste coponents and eet the requireents. LASER RADIATION BEAM SHAPING BEAM SPLITTING PHASE CONTROL INTERFERENCE CONTROL SYSTEM CONTROL POLARIZATION CONTROL BEAM MONITORING SAMPLE POSITIONING Figure 3 Schee of a ulti-bea laser interference lithography syste. The four-bea laser interference syste used as a subicroeter structuring tool is shown in Figure 4. A high-energy pulsed, frequency-tripled Nd:YAG laser (355 n) with a coherent length of 3, energy power up to 320 J/c 2, pulse Proc. of SPIE Vol R-3

4 duration of 8 ns and 10 Hz repetition rate was used as a light source. Three optical beasplitters diided the input bea into four non-coplanar coherent beas. A set of irrors steered the beas towards the substrate with the sae angle of incidence. In order to ensure the axiu signal at the interference plane, the optical paths of the four beas were identical. Figure 5 shows the configuration of four interfering beas. The last four irrors that before the interference point are placed in a irtual circuference. Two of the irrors are placed in fixed positions and the two other irrors can oe in along the circuference to change the interference pattern. At the sae tie, the interference point can be displaced on the axis of the centre of the circuference (see figure 5) so that the scale of the pattern can be easily adjusted. The bea path defined by the placeent of the irrors is aintained constant independently of the position of the rotating irrors and the interference point. Figure 4 Four-bea laser interference syste. Figure 5 Configuration of four interfering beas. Proc. of SPIE Vol R-4

5 3. EXPERIMENT This syste was used to process aterials with a lithography cycle and direct writing with laser interference lithography. 3.1 Processes based on resists The substrates for the LIL process were therally oxidized silicon wafers with a 1.5 μ SiO 2 layer. The lithographic process started with the deposition of a 600 n-thick layer of AZ-1505, a positie i-line photoresist, by spin coating. This was followed by a pre-baking step. Then, the photoresist was exposed with a single laser pulse followed by a postbake. Finally, the photoresist was deeloped in a 1:5 dissolution of AZ-315B deeloper and subsequently hard-baked. Figure 6 shows the experiental results with 2-bea and 4-bea interference patterning. Figure 6 Experiental results with 2-bea and 4-bea interference patterning. Periodical 1D (gratings) and 2D (holes) subicroeter patterns were defined in photoresist with a single pulse of the laser described aboe oer an area of 70 2 using a two- and four-bea configuration, respectiely. The saples were patterned in the photoresist layer with angles of incidence of 20 and 60. After deeloping, the resist showed well defined subicroeter features as obsered in figure 6. The exposure dose was optiized to process the resist. Different exposure doses, ranging fro 6 to 24 J/c 2 were used to deterine the optiu dose for an optiu feature patterning. As the irradiated area of the substrate depends on the angle of incidence, the exposure dose is therefore calculated by Equation (2). E E cosθ F = = (2) A sub A bea where E is the energy of a laser pulse, A sub is the irradiated area on the substrate, A bea is the area of the laser bea and θ is the angle of incidence of the bea. Proc. of SPIE Vol R-5

6 Figure 7 illustrates the processed depths of the photoresist with different laser fluences. As fluence increases, the processed depth increases linearly up to 16 J/c 2. At this point, the SiO 2 layer is reached. Howeer, the feature shape is not well defined yet. More energy is still needed to copletely reoe the resist fro the botto of the gratings and to obtain a linear shape at the botto and the walls of the structures. It was concluded that a fluence of 20J/c2 was the optiu dose to process the fil for the gien conditions / -,.. / / A 5, 5 'A A S S A , / I, S 2 beas A4 beas Fluence (Jfc2) Figure 7 processed depths of the photoresist with different laser fluences. 3.2 Direct writing processes with laser interference lithography Direct processing of aterials is possible if the higher fluence is applied. In this case the processing of the aterials is quite siple as there is no need of the iddle steps to process the resists. Howeer, the quality of the surface is not as good as with the lithographic approach. Figure 8 shows an AFM iage of a 2-bea interference lithography on BaTiO 3 :CuO with a period of 480n. The fluence of this saple was around 100 J/c 2. Subicroeter line structures of 50 n of height were well defined. This surface relief can be used to functionalise fils to enhance the response of the aterial to a particular type of olecules. Proc. of SPIE Vol R-6

7 Figure 8 An AFM iage of a 2-bea interference lithography on BaTiO 3 :CuO with a period of 480n. 4. CONCLUSION The work on the processes based on resists and direct writing with laser interference lithography hae been presented. A four-bea laser interference syste was used as a subicroeter structuring tool in which a high-energy pulsed, frequency-tripled Nd:YAG laser (355 n) with a coherent length of 3, energy power up to 320 J/c 2, pulse duration of 8 ns and 10 Hz repetition rate was used as the light source. The experiental results were achieed with 2-bea and 4-bea interference patterning in accordance with the theoretical expressions. The lithography processes and the direct writing process can be used to define subicron surface reliees in large areas for use in the field of MEMS. ACKNOWLEDGEMENTS This work was part of the Deelopent of lithography technology for nanoscale structuring of aterials using laser bea interference (DELILA) project funded by the European Coission under the 6 th Fraework Prograe (FP6) Priority 2: Inforation Society Technologies (IST). The authors would like to thank the European Counity for supporting the project. REFERENCES 1. Deelopent of lithography technology for nanoscale structuring of aterials using laser bea interference (DELILA), 20 Deceber S. R. J. Brueck, Optical and interferoetric lithography nanotechnology enablers, Proc. IEEE, Vol.93, No.10, pp , October H. H. Solak, C. Daid, J. Gobrecht, L. Wang, and F. Cerrina, Four-wae EUV interference lithography, Microelectronic Engineering, Vol.61-62, pp77-82, C. G. Chen and M. L. Schattenburg, A brief history of gratings and the aking of the MIT nanoruler, Space Nanotechnology Laboratory, MIT, 22 January Proc. of SPIE Vol R-7

8 5. M. Zheng, M. Yu, Y. Liu, R Skoski, S. H. Liou, D. J. Sellyer, V. N. Petryako, Y. K. Verekin, N. I. Polushkin and N. N. Salashchenko, Magnetic nanodot arrays produced by direct laser interference lithography, Applied Physics Letters, Vol.79, No.16, pp , K. J. Gåsik, Optical Metrology, John Wiley & Sons Ltd., Chichester, E. Hecht, Optics, 3rd Ed, Addison Wesley, Reading, J. Sladkoa, Interference of Light, Iliffe Books Ltd., London, Z. Wang, J. Zhang, C. S. Peng, C. Tan, I. Ayerdi, A. Rodríguez, Y. Verekin, T. Berthou and S. Tisserand, Syste Requireent Analysis of Laser Interference Nanolithography, Proc. IEEE International Conference on Mechatronics and Autoation, pp , J. Zhang, Z. Wang, Y. Verekin, S. Olaizola, C. Peng, C. Tan, A. Rodríguez, E. Daue, T. Berthou, S. Tisserand and Z. Ji, Foration of 4-bea laser interference patterns for nanolithography, Proc. SPIE, Vol. 6593, pp65930i, May Proc. of SPIE Vol R-8