Sub-50 Nm Patterning by Immersion Interference Lithography Using a Littrow Prism As a Lloyd’S Interferometer

3450 OPTICS LETTERS / Vol. 35, No. 20 / October 15, 2010

Sub-50 nm patterning by immersion interference lithography using a Littrow prism as a Lloyd’s interferometer

Johannes de Boor,* Dong Sik Kim, and Volker Schmidt Max-Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany *Corresponding author: deboor@mpi‑halle.mpg.de Received July 27, 2010; revised September 7, 2010; accepted September 7, 2010; posted September 20, 2010 (Doc. ID 132405); published October 12, 2010 We present a simple setup that combines immersion lithography with a Lloyd’s mirror interferometer. Aiming for smaller structure sizes, we have replaced the usual Lloyd’s interferometer by a triangular Littrow prism with one metal-coated side, which acts as a mirror. Because of the higher refractive index of the prism, the wavelength and, thus, the attainable structure sizes, are decreased significantly. Using a laser with a wavelength of 244 nm, we could produce line patterns with a period of less than 100 nm and a width of 45 nm. The introduced setup retains all the advantages of a Lloyd’s mirror interferometer, in particular the flexibility in periodicity. © 2010 Optical Society of America OCIS codes: 120.3180, 120.4610, 120.4640, 220.4241, 220.4610.

Laser interference lithography (LIL) is a powerful tool that sity distribution across the sample. The distance between is frequently used for the fabrication of nanostructures [1]. the intensity maxima, the pitch or periodicity pLIL,is Simple line gratings, but also more complex two- or even given by [14] three-dimensional photoresist patterns, can be created λ [2,3]. Laser interference lithography is an attractive p ¼ ; ð Þ LIL 2n θ 1 method to reproducibly and efficiently produce large-area air sin air periodic patterns [4]. These can then be used, e.g., as tem- where nair is the refractive index of air and θair is the plates for nanowire growth [5], nanowire etching [6,7], or angle (in air) between the incoming laser light and the solar cell optimization [8]. sample normal. Because of Snell’s law (n sin θ ¼ Smaller structures offer the opportunity to investigate PR PR nair sin θair), refraction at the air/photoresist interface new physical phenomena and allow for the fabrication of does not change the periodicity. For evaluation of Eq. (1), devices with a higher packing density [1]. The structure one can therefore use the refractive index of air and the size obtainable by laser interference lithography is pro- angle (in air) between the laser and the substrate normal. portional to the wavelength of the laser but also depends Both are experimentally easily accessible. Experimen- on the refractive index of the surrounding medium. Inter- tally, pLIL can easily be varied by rotating the Lloyd’s ference in a prism instead of in air reduces the obtainable interferometer before exposure. structure size by the refractive index of the prism. In Fig. 1(b) we present an immersion Lloyd’s mirror in- Using different setups [9–13], immersion interference terferometer, composed of a Littrow prism and water as lithography has been used to produce patterns with the immersion liquid. The expanded laser beam is directed periodicities below 50 nm. However, such setups are de- onto the prism, which has a right-angled base and is posi- signed to produce patterns with a certain, fixed periodi- tioned with its short side parallel to the sample. The side city. Thus, to vary the periodicity, optical components perpendicular to the sample has a metal coating and re- have to be exchanged and/or the optical setup has to places the mirror of the standard setup. To ensure optimal be realigned, which can be delicate and time consuming. contrast [1,14], the beam should be centered at the angle In contrast, changing the pattern periodicity in a Lloyd’s between the mirror and the short side of the prism. The mirror interferometer setup is fast and simple and requires resulting periodicity in the immersion LIL (ILIL) setup no realignment. In this Letter, we combine the principle of shown in Fig. 1(b) is reduced due to the refraction of a Lloyd’s mirror interferometer with immersion lithogra- the light in the prism and given by phy by using a Littrow prism as a Lloyd’s interferometer λ and water as the immersion liquid to prevent total internal p ¼ ; ð Þ ’ ILIL 2 reflection. As for a standard Lloyd s interferometer, the 2nprism sin θ periodicity of the obtained patterns can be varied by ad- justing the angle between incident laser light and the sam- and, thus, it is a factor nprism=nair smaller than in the stan- ple. Using a laser with a wavelength of 244 nm and a prism dard setup shown in Fig. 1(a). 30°–60°–90° prisms, so- with a refractive index of ∼1:5, patterns with a periodicity called Littrow prisms (22 mm × 37:9 mm × 22 mm), made below 100 nm and a width of 45 nm were fabricated. from fused silica with an Al coating (λ=10 quality) on the A standard Lloyd’s mirror interferometer is shown in long leg were purchased from Altechna Co. Ltd., Lithua- Fig. 1(a). It consists of a sample holder and a mirror per- nia. These Littrow prisms, used, e.g., for wavelength selec- pendicular to the sample. The direct incident light and tion in laser cavities, are standard components and are, the light reflected from the mirror reach the sample at therefore, inexpensive (~$100). Note that the prism does the same angle, interfere, and create a sinusoidal inten- not have to be a 30°–60°–90° prism; it just has to be 0146-9592/10/203450-03$15.00/0 © 2010 Optical Society of America October 15, 2010 / Vol. 35, No. 20 / OPTICS LETTERS 3451

because the reduction in periodicity is only very moder- ate for angles θ larger than 65°. The angle θ in Eq. (2) is the angle between substrate normal and laser light inside the prism, which is not directly experimentally accessible. For our geometry, Snell’s law can be rewritten as

sinð60° − θ0Þ n ¼ Prism ; ð3Þ sinð60° − θÞ nair

where θ0 is the angle in air, which is easily controlled experimentally. A negative tone diazonaphthoquinone-based resist (AR-N 4240, mixed with diluter AR 300-12 at ratio 1:4) was spin coated onto cleaned silicon wafers at 4000 rpm (30 s), with adhesion improved by primer AR 300-80 ’ Fig. 1. (Color online) (a) Standard Lloyd s mirror interfero- (2000 rpm, 30 s). This was followed by a prebake at meter: a fraction of the light from an expanded laser beam 85 °C for 2 min. After these steps, the photoresist thick- reaches the sample directly, another fraction after reflection 100 from a mirror (yellow), creating a sinusoidal intensity pattern ness was nm. After exposure, the sample was post- across the sample. The substrate (dark gray) is coated with baked at 85 °C for 30 min and developed in AR 300-475 photoresist (red). (b) Immersion Lloyd’s mirror interferometer: for 30 s. All photochemicals were obtained from the laser light is directed onto a triangular prism at an angle θ0 to Allresist GmbH. the sample normal. Instead of by a separate mirror, the light is Figure 2(a) shows a large-area SEM image of the photo- reflected by the metal coating (yellow) of the prism. The per- resist after exposure and development. The line pattern is iodicity in (b) is smaller than that in (a) because of the refrac- clearly visible and is regular and uniform. From the higher tive index of the prism. Total reflection at the bottom side of the magnification image shown in Fig. 2(b), a periodicity of prism is avoided by using an immersion liquid (blue). 90 2 nm can be deduced. This fits well to the expected 0 result of Eq. (2): pILIL ¼ 93 4 nm at θ ¼ θ ≈ 60°. The rectangular. The chosen prism has the advantage, how- width of the resist stripes is 43 4 nm and shows some ever, that it minimizes undesired reflection at the roughness, presumably because a standard photoresist ∼50 prism/air interface for large θ and θ0, and, thus, for small with a spatial resolution of nm was used. Depending on optical constants and angles, a fraction of the laser light periodicities. – Total internal reflection occurs at the prism/air inter- is reflected from the resist substrate interface. Using an face for sin θ ≥ n =n . Because we are interested in antireflection coating beneath the resist layer could sup- air prism press these reflections and, thereby, further improve the small periodicities, i.e., large θ, this is highly undesirable, quality of the resist profile [14]. Coherence loss could, in but can potentially be circumvented if a substance with a principle, also be a reason for poor pattern quality, as one higher refractive index is situated between photoresist requirement for good imaging in a Lloyd’s mirror interfe- and prism. We used deionized water as the immersion rometer is good temporal and spatial coherence. How- liquid to decrease reflection at the short side of the prism. ever, the temporal coherence length of the employed While the short side of the prism was rinsed with deionized water, the samples were carefully pressed against the prism. Because of surface tension, a water film wets the prism and the sample, separating them. The laser used for illumination is a frequency-doubled argon-ion laser with λ ¼ 244 nm (85-SHG, Cambridge Lasers, USA) and a typical output power of 3 mW. The laser is used in the TEM00 mode and the light is TE po- larized. TE polarization is desired for most applications, as it ensures optimal contrast [10,15]. The light is directed into a spatial filter, consisting of a focusing lens and a 10 μm diameter pinhole. The distance between the spatial filter and the sample holder is around 1 m. The resist requires an exposure dose of ∼50 mJ cm−2 and, with typical exposure times of 2 to 3 min, the intensity at the resist is of the order of 2 Wm−2. The refractive index of the prism at 244 nm is calculated from the Sellmeier equation [16]tonprism ¼ 1:51, while the refractive index n ≈ 1:38 of water is H2O [17]; thus the critical angle for total θ ≈ 66 Fig. 2. Photoresist patterns after exposure and development. internal reflection is crit °. It is possible to use liquids (a) Low magnification and (b) high magnification images of the with higher refractive index [12,17] to increase the criti- pattern; the width of the resist lines is 43 4 nm. (c) Silver lines cal angle further. We did not pursue this approach mainly after evaporation of 15 nm Ag and lift-off. 3452 OPTICS LETTERS / Vol. 35, No. 20 / October 15, 2010

different, in particular, smaller wavelengths, provided the laser source meets the coherence requirements for the targeted field sizes and throughputs. In summary, we have shown a simple and inexpensive modification of a Lloyd’s mirror interferometer. Using a laser with λ ¼ 244 nm, a fused silica coupling prism, and water as the immersion liquid to avoid total reflection, line patterns with sub-100 nm periodicity and 45 nm width were fabricated. The proposed setup extends the range of periodicities that are accessible with a Lloyd’s mirror interferometer significantly, while retain- ing its advantages like ease of alignment and flexibility in periodicity.

References Fig. 3. (Color online) Periodicity as a function of the angle between substrate normal and laser beam in air; λ ¼ 244 nm. Solid 1. S. R. J. Brueck, Proc. IEEE 93, 1704 (2005). curve, result of Eq. (2). Diamonds, experimental data. Dashed 2. J. Moon, J. Ford, and S. Yang, Polym. Adv. Technol. 17, curve, periodicity of a standard Lloyd’s interferometer. The 83 (2006). smallest periodicity for a standard Lloyd’s interferometer is 3. J. de Boor, N. Geyer, U. Gösele, and V. Schmidt, Opt. Lett. λ=2 ¼ 122 nm, while, with prism and immersion liquid, the 34, 1783 (2009). working range is expanded well below 100 nm. 4. C. G. Chen, R. K. Heilmann, C. Joo, P. T. Konkola, G. S. Pati, and M. L. Schattenburg, J. Vac. Sci. Technol. B 20, laser is ∼10 m and, thus, orders of magnitude larger than 3071 (2002). typical beam path differences in the setup. Regarding the 5. D. S. Kim, R. Ji, H. J. Fan, F. Bertram, R. Scholz, A. Dadgar, K. Nielsch, A. Krost, J. Christen, U. Gösele, and M. spatial coherence, we can conservatively estimate from 3 – Zacharias, Small , 76 (2007). the Van Cittert Zernike theorem [18] that the pattern qual- 6. J. de Boor, N. Geyer, J. V. Wittemann, U. Gosele, and V. ity should not be affected significantly for a patterned area Schmidt, Nanotechnology 21, 095302 (2010). width below 5 mm. 7. W. K. Choi, T. H. Liew, M. K. Dawood, H. I. Smith, C. V. Resist patterns obtained by lithography are usually Thompson, and M. H. Hong, Nano Lett. 8, 3799 (2008). transferred into other materials, e.g., by etching or by 8. Y. M. Song, J. S. Yu, and Y. T. Lee, Opt. Lett. 35, 276 (2010). evaporation and lift-off. Figure 2(c) shows a metal stripe 9. T. M. Bloomstein, M. F. Marchant, S. Deneault, D. E. Hardy, pattern after evaporation of 15 nm Ag and subsequent and M. Rothschild, Opt. Express 14, 6434 (2006). lift-off with acetone. This yielded a metal stripe pattern 10. J. A. Hoffnagle, W. D. Hinsberg, M. Sanchez, and F. A. 17 with a stripe width of 47 6 nm. Houle, J. Vac. Sci. Technol. , 3306 (1999). To prove the versatility of the proposed setup, we have 11. A. K. Raub, A. Frauenglass, S. R. J. Brueck, W. Conley, R. R. θ0 Dammel, A. Romano, M. Sato, and W. Hinsberg, Proc. SPIE conducted exposures at different . The resulting periodi- 5377, 306 (2004). cities, shown in Fig. 3, were deduced from SEM images. 12. R. H. French, H. Sewell, M. K. Yang, S. Peng, D. McCafferty, The solid curve represents the theoretical curve accord- W. Qiu, R. C. Wheland, M. F. Lemon, L. Markoya, and M. K. ing to Eq. (2). This can be compared to the periodicity of a Crawford, J. Microlith. Microfab. Microsyst. 4, 031103 standard Lloyd’s mirror interferometer (dashed curve). (2005). Structures with ð86 4Þ nm ≤ p ≤ ð132 5Þ nm have 13. A. Bourov, Y. Fan, F. C. Cropanese, N. V. Lafferty, L. V. been successfully fabricated. The measured periodicities Zavyalova, H. Kang, and B. W. Smith, Proc. SPIE 5377, follow the theoretical curve, albeit being systematically 1573 (2004). 14. M. E. Walsh, “On the design of lithographic interferometers about 5% too small. Possible error sources include incor- ” rect measurement of θ0, inaccuracies in the analysis of the and their application, Ph.D. dissertation (MIT, 2004). 15. B. Smith, J. Zhou, and P. Xie, Proc. SPIE 6924, 69240J SEM images, or a refractive index of the prism differing n ¼ 1:51 (2008). from the assumed prism . 16. I. H. Malitson, J. Opt. Soc. Am. 55, 1205 (1965). The presented results have been obtained with a laser 17. S. G. Kaplan and J. H. Burnett, Appl. Opt. 45, 1721 (2006). with λ ¼ 244 nm. The presented immersion Lloyd’s inter- 18. H. H. Solak, D. He, W. Li, and F. Cerrina, J. Vac. Sci. ferometer concept is, of course, applicable to lasers with Technol. B 17, 3052 (1999).