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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 14 — Jul. 6, 2009
  • pp: 11309–11314
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193nm Superlens Imaging Structure for 20nm Lithography Node

Zhong Shi, Vladimir Kochergin, and Fei Wang  »View Author Affiliations


Optics Express, Vol. 17, Issue 14, pp. 11309-11314 (2009)
http://dx.doi.org/10.1364/OE.17.011309


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Abstract

We are showing that a 20nm lithography resolution is theoretically feasible at a 193nm illumination wavelength if employing aluminum (Al) superlens structure with index matching layer. It is illustrated that transmissivity of evanescent waves for certain wavevector bands can be enhanced by an index matching layer. It is further shown a minimal resolution of ~λ/10 can be achieved by appropriately engineering mask material and superlens structure. A depth of focus of several nanometers is predicted to be possible for a periodic structure with 20nm half pitch. Assistant features were adopted in superlens structure to successfully suppress the side lobes and resolve a 20nm two-slit structure.

© 2009 Optical Society of America

1. Introduction

Photolithography is a key process in the fabrication of microelectronic and photonic components and integrated circuits (ICs). In order to comply with Moore’s law, a continual improvement of resolution for photolithography systems is necessary. While the resolution of 193nm photolithography systems can be improved by using high index immersion fluids to increase numerical aperture (NA) or shorter illumination wavelength (e.g., 157nm), such techniques are still unlikely to resolve feature sizes below 30nm without using a pitch doubling technique. Several other lithographic techniques, such as extreme ultraviolet (EUV), imprint and interference lithography, have been suggested for technology nodes at or below 30nm [1

1. P. J. Silverman, “Extreme ultraviolet lithography: overview and development status,” J. Microlith., Microfab. Microsyst. 4, 011006 (2005). [CrossRef]

3

3. S. R. J. Brueck, “Optical and interferometric lithography - nanotechnology enablers,” Proc. of IEEE 93, 1704–1721 (2005). [CrossRef]

]. However these techniques impose a large number of technical hurdles (materials, cost, throughput, limited pattern printing capability, etc.) which prohibit their immediate utilization in IC manufacturing. Contact lithography, another photolithographic approach, is questionable for the targeted application due to its severe evanescent wave decay away from the mask [4

4. M. M. Alkaisi, R. J. Blaikie, and S. J. Mcnab, “Nanolithography in the evanescent near field,” Adv. Mater. 13, 877–887 (2001). [CrossRef]

]. Superlens imaging was recently proposed as a potential alternative for photolithography due to its sub-wavelength resolution achievable with a single exposure process. The sub-wavelength resolution of superlens relies on the negative refractive index material (NRIM) [5

5. V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of ε and µ,” Sov. Phys. USP 10, 509–514 (1968). [CrossRef]

], which was predicted to exhibit diffraction free optical imaging [6

6. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85, 3966–3969 (2000). [CrossRef] [PubMed]

] by collecting and enhancing near-field evanescent waves through surface plasmon excitation. A “perfect” NRIM (ε=-1 and µ=-1), required to achieve the superlens effect with arbitrary polarization/angle of incidence, is difficult to realize experimentally for UV wavelengths. However, as was suggested by Pendry [6

6. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85, 3966–3969 (2000). [CrossRef] [PubMed]

], for normal incidence illumination and TM polarization, negative dielectric permittivity of superlens layer is sufficient for imaging with sub-wavelength resolution. The 40nm resolution imaging of an arbitrary two-dimensional object with a 365nm illumination was experimentally demonstrated [7

7. N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308, 534–537 (2005). [CrossRef] [PubMed]

,8

8. H. Lee, Y. Xiong, N. Fang, W. Srituravanich, S. Durant, M. Ambati, C. Sun, and X. Zhang, “Realization of optical superlens imaging below the diffraction limit,” New J. Phys. 7, 255 (2005). [CrossRef]

]. However, this resolution is on the order of what is already achieved with state of the art 193nm immersion lithography tools with a NA of 1.35. Therefore, more efforts are needed for superlens imaging in order to scale the resolution down below 40nm.

In this paper we propose a 193nm superlens structure to resolve 20nm feature sizes. The main challenge of efficient plasmon excitation (or permittivity matching) is solved by the introduction of an “index matching layer” between the metal (Al) and dielectric material layer. To evaluate the resolution limit of the proposed structure, the full wave simulations were performed and a 193nm superlens structure was optimized. Simulations indicate that utilization of Al as a mask material improves the resolution at a 193nm wavelength compared to using chromium (Cr) mask material. The simulations further demonstrate that a minimal feature size of 20nm (~λ/10) can be resolved with the designed 193nm superlens structure. A DoF of several nanometers is predicted for the periodic pattern with a 20nm half pitch (HP). Simulations also verify that assistant features (AFs) help to suppress side lobes and improve resolution of a 20nm two-slit imaging structure.

2. Material selection

Previously, silver (Ag) was proven to be a material of choice for a superlens at a 365nm wavelength [9

9. W. Srituravanich, N. Fang, S. Durant, M. Ambati, C. Sun, and X. Zhang, “Sub-100 nm lithography using ultrashort wavelength of surface plasmons,” J. Vac. Sci. Technol. B 22, 3475–3478 (2004). [CrossRef]

]. In the far UV simple calculation of surface plasmon wavevectors (ksp=k0εdεmεd+εm, where εd and εm are the permittivities of the dielectric material and metal material respectively [10

10. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1985).

].) indicated Al is an optimal material for the 193nm superlens structure. Furthermore, we propose to utilize the index matching layer between the Al layer and the dielectric layer to maximize surface plasmon excitation efficiency. Although exact permittivity matching is not obtainable, Magnesium oxide (MgO) with Re(εMgO)=4.08 (compared to Re(εAl)~-4.43) at 193nm [11

11. D. M. Roessler and D. R. Huffman, “Magnesium oxide (MgO),” in Handbook of Optical Constants of Solid II, E. D. Palik, ed. (Academic Press, 1991).

] was identified as the most promising material candidate. Fig. 1 right shows schematics of the proposed 193nm superlens structure. An Al superlens layer (3) of thickness d is “sandwiched” between the index matching layer (2) and the spacer layer (4). An Al mask layer serves as an imaging object. The Al mask material is surrounded by a SiO2 dielectric layer (1). The bottom of the device is a quartz substrate. The spacer layer is used to facilitate the pattern transfer which will be explained in a later section of this paper. The photoresist layer (5) is to record the image pattern transferred from the superlens layer. The transmissivity of the multilayer thin film structure can be solved by using the characteristic matrix method for a stratified medium [12

12. C. C. Katsidis and D. I. Siapkas, “General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference,” Appl. Opt. 41, 3978–3987 (2002). [CrossRef] [PubMed]

]. The calculated transmissivity as a function of normalized wavevector kx/k0 is shown in Fig. 1 for TM polarization (H along Y direction). For the structure without a super lens layer (1-4-5), the transmissivity for the propagating waves is close to 1 at ~kx/k0<1.7 (except a sharp drop at kx/k0=1.55), and the evanescent waves decay rapidly at ~kx/k0>1.7, as expected for diffraction-limited imaging. The other two structures, both with superlens layers (without index matching layer: 1-3-4-5, with index matching layer: 1-2-3-4-5), have very close transmissivities for the propagating waves at ~kx/k0<1.55. However, the superlens structure with the index matching layer has enhanced evanescent wave vector transmissivity especially for kx/k0~2-3, leading to improved optical transfer as confirmed by our simulation results which will be discussed in the later sections of the paper. It should be noted that, for ~kx/k0>3, the superlens structure (1-3-4-5) has stronger evanescent wave vector transmissivity compared with the structure with the index matching layer (1-2-3-4-5). Since the transmissivity at ~kx/k0>3 is generally weak, it does not affect the imaging quality significantly. An optimal Al (εAl=-4.43+0.42i [13

13. A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewaki, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 22, 5271–5283 (1998). [CrossRef]

]) thickness was determined to be 13nm through the transmissivity calculation. SiO2, spacer layer and photo resist were assumed to have refractive index of 1.55, 1.7 and 1.7 respectively. The thicknesses for SiO2 layer, index matching layer and spacer layer are determined to be 10nm, 10nm and 8nm, respectively.

Fig. 1. Calculated transmissivity of the multilayer superlens structure. Circle: w/index matching layer. Star: w/o index matching. Dot: diffraction limited. Right: schematics of a 193nm superlens imaging structure, where material stacked, from bottom to top, respectively: quartz substrate, Al mask layer, dielectrics layer (1), index matching layer (2), Al superlens layer (3), spacer layer (4) and photo resist layer (5).

Fig. 2. Total energy density distribution immediately after the Cr and Al mask materials.

3. Resolution and depth-of-focus (DoF)

Fig. 3. (a) Calculated power distribution of a periodic grating structure with and without an index matching layer. (b): Ex cross-section distribution in the Z direction where 1, 2, 3, 4 and 5 represent mask, SiO2, MgO for top curve/SiO2 for bottom curve, Al and photo resist layer respectively.
Fig. 4. Calculated power distribution of a periodic grating structure with 20nm HP at different image location where legend numbers represent imaging distances from the superlens layer.
Fig. 5. (a). Calculated energy distribution for a 20nm two-slit structure without assistant feature placement. (b). Calculated energy density for a 20nm two-slit structure with assistant feature placement. AFs are 80nm away from the mask center. The slit opening size is 20nm and the spacing between them is 40nm.

For practical applications, the inter-wafer pattern transfer or bond-detach lithographic techniques [16

16. M. A. Meitl, Z. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, and J. A. Rogers, “Transfer printing by kinetic control of adhesion to an elastomeric stamp,” Nat. Mater. 5, 33–38 (2006). [CrossRef]

] can possibly be combined with a single superlens exposure step to form a complete superlens pattern transfer process. In such a realization, the thin spacer layer and photoresist layer can be spin-coated and then exposed on the superlens layer, but not developed. The “device” wafer can then be “bonded” to the exposed and then debonded from the superlens structure by a combination of wet chemistry, mechanical force and possibly heating. The superlens structure can then be cleaned while the photoresist layer will be developed (i.e. morphological features will be formed). Our team is working on an experimental evaluation of such an approach.

4. Conclusion

In conclusion, a 193nm Al superlens imaging structure with index matching layer was proposed and designed. Simulations demonstrated that utilization of the index matching layer around the Al superlens layer together with the use of Al mask layer results in a 20nm resolution. A DoF of several nanometers is predicted to be possible for the 20nm HP periodic structure. A 20nm two-slit structure is resolvable by placing AFs to suppress side lobes. However, further research work is needed to get a common process window among different patterns. The proposed 193nm superlens structure together with the suggested novel patterning process may provide a viable way to reach the 20nm lithography node with a single exposure step without the reduction of the illumination wavelength.

Acknowledgements

The authors acknowledge K. Flanagan for preparing this manuscript and support from Dr. D. Shenoy of DARPA/MTO under SBIR Grant No. W31P4Q-08-C-0204.

References and links

1.

P. J. Silverman, “Extreme ultraviolet lithography: overview and development status,” J. Microlith., Microfab. Microsyst. 4, 011006 (2005). [CrossRef]

2.

M. D. Stewart, S. C. Johnson, S. V. Sreenivasan, D. J. Resnick, and C. G. Willson, “Nanofabrication with step and flash imprint lithography,” J. Microlith., Microfab. Microsyst. 4, 011002 (2005). [CrossRef]

3.

S. R. J. Brueck, “Optical and interferometric lithography - nanotechnology enablers,” Proc. of IEEE 93, 1704–1721 (2005). [CrossRef]

4.

M. M. Alkaisi, R. J. Blaikie, and S. J. Mcnab, “Nanolithography in the evanescent near field,” Adv. Mater. 13, 877–887 (2001). [CrossRef]

5.

V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of ε and µ,” Sov. Phys. USP 10, 509–514 (1968). [CrossRef]

6.

J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85, 3966–3969 (2000). [CrossRef] [PubMed]

7.

N. Fang, H. Lee, C. Sun, and X. Zhang, “Sub-diffraction-limited optical imaging with a silver superlens,” Science 308, 534–537 (2005). [CrossRef] [PubMed]

8.

H. Lee, Y. Xiong, N. Fang, W. Srituravanich, S. Durant, M. Ambati, C. Sun, and X. Zhang, “Realization of optical superlens imaging below the diffraction limit,” New J. Phys. 7, 255 (2005). [CrossRef]

9.

W. Srituravanich, N. Fang, S. Durant, M. Ambati, C. Sun, and X. Zhang, “Sub-100 nm lithography using ultrashort wavelength of surface plasmons,” J. Vac. Sci. Technol. B 22, 3475–3478 (2004). [CrossRef]

10.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1985).

11.

D. M. Roessler and D. R. Huffman, “Magnesium oxide (MgO),” in Handbook of Optical Constants of Solid II, E. D. Palik, ed. (Academic Press, 1991).

12.

C. C. Katsidis and D. I. Siapkas, “General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference,” Appl. Opt. 41, 3978–3987 (2002). [CrossRef] [PubMed]

13.

A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewaki, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 22, 5271–5283 (1998). [CrossRef]

14.

S. A. Ramakrishna, “Physics of negative refractive index materials,” Rep. Prog. Phys. 68, 449–521 (2005). [CrossRef]

15.

H. J. Levinson, Principles of lithography, (SPIE Press, 2004).

16.

M. A. Meitl, Z. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, and J. A. Rogers, “Transfer printing by kinetic control of adhesion to an elastomeric stamp,” Nat. Mater. 5, 33–38 (2006). [CrossRef]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(110.4235) Imaging systems : Nanolithography

ToC Category:
Imaging Systems

History
Original Manuscript: March 16, 2009
Revised Manuscript: May 15, 2009
Manuscript Accepted: June 15, 2009
Published: June 22, 2009

Citation
Zhong Shi, Vladimir Kochergin, and Fei Wang, "193nm Superlens Imaging Structure for 20nm Lithography Node," Opt. Express 17, 11309-11314 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-14-11309


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References

  1. P. J. Silverman, "Extreme ultraviolet lithography: overview and development status," J. Microlith., Microfab. Microsyst. 4, 011006 (2005). [CrossRef]
  2. M. D. Stewart, S. C. Johnson, S. V. Sreenivasan, D. J. Resnick, and C. G. Willson, "Nanofabrication with step and flash imprint lithography," J. Microlith., Microfab. Microsyst. 4, 011002 (2005). [CrossRef]
  3. S. R. J. Brueck, "Optical and interferometric lithography - nanotechnology enablers," Proc. of IEEE 93, 1704-1721 (2005). [CrossRef]
  4. M. M. Alkaisi, R. J. Blaikie, and S. J. Mcnab, "Nanolithography in the evanescent near field," Adv. Mater. 13, 877-887 (2001). [CrossRef]
  5. V. G. Veselago, "The electrodynamics of substances with simultaneously negative values of ε and μ," Sov. Phys. USP 10, 509-514 (1968). [CrossRef]
  6. J. B. Pendry, "Negative refraction makes a perfect lens," Phys. Rev. Lett. 85, 3966-3969 (2000). [CrossRef] [PubMed]
  7. N. Fang, H. Lee, C. Sun, and X. Zhang, "Sub-diffraction-limited optical imaging with a silver superlens," Science 308, 534-537 (2005). [CrossRef] [PubMed]
  8. H. Lee, Y. Xiong, N. Fang, W. Srituravanich, S. Durant, M. Ambati, C. Sun, and X. Zhang, "Realization of optical superlens imaging below the diffraction limit," New J. Phys. 7, 255 (2005). [CrossRef]
  9. W. Srituravanich, N. Fang, S. Durant, M. Ambati, C. Sun, and X. Zhang, "Sub-100 nm lithography using ultrashort wavelength of surface plasmons," J. Vac. Sci. Technol. B 22, 3475-3478 (2004). [CrossRef]
  10. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, 1985).
  11. D. M. Roessler, D. R. Huffman, "Magnesium oxide (MgO)," in Handbook of Optical Constants of Solid II, E. D. Palik, ed. (Academic Press, 1991).
  12. C. C. Katsidis and D. I. Siapkas, "General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference," Appl. Opt. 41, 3978-3987 (2002). [CrossRef] [PubMed]
  13. A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewaki, "Optical properties of metallic films for vertical-cavity optoelectronic devices," Appl. Opt. 22, 5271-5283 (1998). [CrossRef]
  14. S. A. Ramakrishna, "Physics of negative refractive index materials," Rep. Prog. Phys. 68, 449-521 (2005). [CrossRef]
  15. H. J. Levinson, Principles of lithography, (SPIE Press, 2004).
  16. M. A. Meitl, Z. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, and J. A. Rogers, "Transfer printing by kinetic control of adhesion to an elastomeric stamp," Nat. Mater. 5, 33-38 (2006). [CrossRef]

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