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A novel 3D nanolens for sub-wavelength focusing by self-aligned nanolithography
Microelectronic Engineering 87 (2010) 1506–1508
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel 3D nanolens for sub-wavelength focusing by self-aligned nanolithography Bing-Rui Lu a,b, Yifang Chen a,*, Shao-Wei Wang a,c, Ejaz Huq a, Edward Rogers d, Tsung Sheng Kao d, Xin-Ping Qu b, Ran Liu b, Nikolay I. Zheludev d a
Micro and Nanotechnology Centre, Rutherford Appleton Laboratory, STFC, Didcot, Oxon, OX11 0QX, UK State Key Lab of Asic and System, Department of Microelectronics, Fudan University, Shanghai 200433, China c National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 17 November 2009 Keywords: Nanolens Nanofabrication Electron beam lithography Self-aligned nanolithography Wet/dry etching
a b s t r a c t In this work, a novel type of nanolens based on super oscillation theory has been developed and fabricated. A self-aligned nanolithography process is developed to achieve error-free structured nanolens without the need of complex registration which always carries intrinsic errors. This fabrication technique significantly simplifies the process and reduces the production costs consequently. The success of this process will enable us to achieve focusing and imagining beyond diffraction limit, which will be presented in other communications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. The process
Motivated by broad applications in bioscience and nanolithography, various types of nanolens are being attempted at many laboratories around the world. It has been reported that nanolens structures include quasi-crystals with nanohole or metal nanosphere arrays, nanostructured metamaterials, surface Plasmon polariton assisted enhancement and nanosized slits in metal slabs [1–5]. Among them, two mainstreams of nanolens have been anticipated to possess unique capability of imagining and focusing with sub-wavelength resolution. One is built on metamaterial with negative refractive index (Pendry’s perfect lens [6]). It has been proved that such kind of lens is extremely daunting in fabrication, involving many levels of processing. The other is based on super oscillation theory [7] with a much simpler configuration: a binary annular ring structure consisting of both dielectric and metallic structure in one layer. The challenge is to fabricate both metallic part and dielectric part on one plane without geometry offset. In this work, we have developed a single step lithography process for the nanofabrication of such a specially designed nanolens. Due to the scope of this paper, the physical performance of the novel nanolens will be presented elsewhere.
Fig. 1 illustrates the novel nanolens with binary annular ring structure, which consists of a series of concentric dielectric ring structures (grey area in Fig. 1a) with air gap in between (white area) on quartz substrate surrounded by an opaque area. For high quality focusing performance, it is critical for the precision of the structure in diameter and height of the rings as well as the concentricity between rings. Conventional registration technique in EBL is not able to meet this requirement, especially in concentricity. To overcome this difficulty, we have developed a novel self-aligned nanolithography using one-step EBL and multi-step wet/dry etching to achieve the structure with high precision. In our work, SiNx is used for dielectric ring structure. Firstly on a quartz substrate, a layer of 320 nm thick SiNx is deposited using PECVD technique. The thickness of SiNx has been carefully calculated according to its refractive index of 2.02. A bi-layer of 200 nm PMMA and 500 nm UVIII is spin coated on top of SiNx layer. The two resist layers are separated by either a 20 nm thick Al film or a thin lift-off-resist (LOR) supplied by MicroChem Ltd. to prevent the cross-link between PMMA and UVIII. The designed process flow is described in Fig. 2. One step E-beam exposure is carried out to expose PMMA and UVIII at two different levels of dose, respectively as shown in Fig. 2a. A high resolution vector beam writer, VB6-HR with 100 kV capability supplied by Vistec is used for the EBL exposure. The exposure of UVIII forms the area 4 in Fig. 1a with a radius
* Corresponding author. Tel.: +44 1235 445159. E-mail address: [email protected] (Y. Chen). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.064
B.-R. Lu et al. / Microelectronic Engineering 87 (2010) 1506–1508
1507
Fig. 2. The flow chart of the self-aligned nanolithography process with one-step EBL and multi-step dry/wet etching for the nanofabrication of such the designed nanolens.
Fig. 1. (a) Structural design of binary phase annular ring mask for a novel nanolens; (b) 3D demonstration of the ring mask design.
of 1 lm while PMMA defines the shapes of the rings. After development of the resists, 20 nm Cr and 220 nm Al are deposited onto the pattern subsequently, followed by a lift-off process using acetone as shown in Fig. 2b and c. With the Al cap as an etching mask, SiNx outside the nanolens area is etched down and a total of 120 nm Cr/Au layer is deposited for the opaque area (Fig. 2d) which is formed by a self-aligned shadow evaporation process. The Al cap is then removed by alkaline solution (Fig. 2e) and the remaining PMMA with Cr top is removed by soaking in acetone (Fig. 2f). The rest of the Cr is used as mask for SiNx dry etching to form the annular rings as shown in Fig. 2g. The deposition of the metal was carried out by an electron beam thermal evaporator SVS V2000 and the dry etching process was done using Oxford Plasma System 90.
3. Experimental results and discussion 3.1. EBL results To achieve high precision of the ring sizes, careful EBL tests of different e-beam dose have been carried out. An e-beam exposure with dosage of 100 lC/cm2 has been carried out to fully expose the top UVIII resist. This dose is not high enough to expose the bottom PMMA resist, which therefore remains intact at this stage. Since the size of the annular ring structure is mainly controlled by the exposure of the PMMA, an exposing dosage range between 1000 lC/cm2 and 2500 lC/cm2 is selected and lithography results are carefully measured by a high resolution Hitachi S4000 SEM. Fig. 3 presents the size against the dose. The designed sizes for the structure are r1 = 298 nm; r2 = 613 nm; r3 = 855 nm; r4 = 1000 nm, where r1 to r4 correspond to the same symbol in Fig. 1a. Fig. 3 shows that the exposure dosage window between 1220 lC/cm2 and 1270 lC/cm2 (red tinted area)1 should give rise to the best result in defining the particular size of the ring structures. Fig. 4 demonstrates the SEM picture of the exposed bi-layer resists after development. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Fig. 3. The radium of the developed PMMA ring structures for: (a) r1 (b) r2 (c) r3 in the design diagram (Fig. 1) against the e-beam exposure dose between 1000 lC/cm2 and 2500 lC/cm2. Red tinted area suggests the best dosage window between 1220 lC/cm2 and 1270 lC/cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. The sacrificial layer To prevent cross-link between PMMA and UVIII in the bi-layer stack, a sacrificial layer is necessary to sandwich the two layers. This layer has to be thin, dissolvable in developer but not hinder the EBL of both resist layers. Initially, a thin layer of 20 nm Al is chosen as the sacrificial layer. Although the results are quite promising, the preparation for the bi-layer resists involves a thermal deposition which is complicated. Also, the high temperature during evaporation of Al may cause a potential problem in the stability of the process for repeated fabrication. Therefore, a thin layer of LOR is applied as a substitution of Al. The same quality of resist profile after EBL is achieved. We found that the 20 nm thickness should be the minimum. Below 20 nm, the LOR is less functional than thick ones. But, if the LOR is too thick, shift between top layer UVIII and PMMA may happen during the development process of UVIII.
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B.-R. Lu et al. / Microelectronic Engineering 87 (2010) 1506–1508
wall etching and low etching rate of 17.8 nm/min are achieved at the same time. Fig. 4 presents the final fine binary phase ring structure for the nanolens. The outermost ring is a thin wall of Au, which was deposited onto the side wall of the SiNx during the process in Fig. 1d. However, this part also belongs to the opaque area, and would not affect the performance of the nanolens. 4. Conclusion
Fig. 4. SEM picture of the final fine binary phase annular ring structure as a nanolens.
Table 1 Comparison of different SiNx dry etching recipes and corresponding etching rate. Recipe
A
B
C
D
E
C2F6 (sccm) CHF3 (sccm) Ar (sccm) O2 (sccm) Etching rate (nm/min)
30
15
15 15
15 15
15 15
10 44
2 42.5
17.8
15 32.5
28
3.3. Dry etching results In order to achieve high verticality of the SiNx ring structure, comprehensive dry etching tests were carried out with a number of recipes. For each etching recipe, pressure is fixed at 30 mT and power at 250 W. Several combinations of CHF3, C2F6, O2 and Ar etching gas were employed and systematically investigated. Different combination of etching gases and corresponding etching rates are listed in Table 1. For the single gas C2F6 with 30 sccm flow rate as recipe A in Table 1, the rate is at 32.5 nm/min, which is slightly high. When half of C2F6 is changed to Ar, (recipe B) the etching rate slightly drops to 28 nm/min, while the surface smoothness of the substrate is severely sacrificed. On the other hand, when O2 is added to the C2F6 + CHF3 recipe (recipes C and D), not only the etching rate rises significantly, but also rapid side wall etching is observed. Finally the optimized dry etching process for SiNx is found as recipe E in Table 1, where smooth surface, minimum side
This paper reports our recent technical development of a selfaligned nanolithography process for the fabrication of nanolens array. The structure contains both dielectric ring for phase shift and metallic opaque zone for blocking light on the same surface of a quartz wafer, which requires multi-level lithography with critical demand of high precision registration. The self-aligned nanolithography process developed in this work only needs a single level lithography, successfully avoiding the registration for multi-level patterning. Thus, intrinsic error which may cause displacements between different patterns is eliminated. Furthermore, this process is much simpler than multiple level process, hence has the prospect of applying nanoimprint without registration problem in the future, leading to economic patterning for nanometer scale lens in high volume. Acknowledgements This work is partially supported by EPSRC Basic Technology grant (Ref. 3339) of Nanoscope project. The authors would like to thank financial support from Shanghai Municipal of Science and Technology (08QH14002), National Basic Research Program of China (2006CB302703), Shanghai Municipal of Science and Technology (08QH14002) and the Seed Funds for Key Science and Technology Innovation Projects of MOE (708020). This work is also supported by the ‘‘985” Micro/Nanoelectronics Science and Technology Platform. Bingrui Lu’s study at STFC UK is financially supported by a Chinese CSC Scholarship. References [1] Fu Min Huang, Tsung Sheng Kao, Vassili A. Fedotov, Yifang Chen, Nikolay I. Zheludev, Nano Lett. 8 (2008) 2469. [2] M.V. Bashevoy, F. Jonsson, K.F. MacDonald, Y. Chen, N.I. Zheludev, Opt. Express 15 (2007) 11313. [3] Ting Xu, Cunlei Du, Changtao Wang, Xiangang Luo, Appl. Phys. Lett. 91 (2007) 201501. [4] Fu Min Huang, Yifang Chen, F. Javier Garcia de Abajo, Nikolay I. Zheludev, J. Opt. A: Pure Appl. Opt. 9 (2007) S285. [5] N. Papasimakis, V.A. Fedotov, A.S. Schwanecke, N.I. Zheludev, Appl. Phys. Lett. 91 (2007) 081503. [6] J.B. Pendry, Phys. Rev. Lett. 85 (2000) 3966. [7] M.V. Berry, S. Popescu, J. Phys. A: Math. Gen. 39 (2006) 6965.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel 3D nanolens for sub-wavelength focusing by self-aligned nanolithography Bing-Rui Lu a,b, Yifang Chen a,*, Shao-Wei Wang a,c, Ejaz Huq a, Edward Rogers d, Tsung Sheng Kao d, Xin-Ping Qu b, Ran Liu b, Nikolay I. Zheludev d a
Micro and Nanotechnology Centre, Rutherford Appleton Laboratory, STFC, Didcot, Oxon, OX11 0QX, UK State Key Lab of Asic and System, Department of Microelectronics, Fudan University, Shanghai 200433, China c National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 17 November 2009 Keywords: Nanolens Nanofabrication Electron beam lithography Self-aligned nanolithography Wet/dry etching
a b s t r a c t In this work, a novel type of nanolens based on super oscillation theory has been developed and fabricated. A self-aligned nanolithography process is developed to achieve error-free structured nanolens without the need of complex registration which always carries intrinsic errors. This fabrication technique significantly simplifies the process and reduces the production costs consequently. The success of this process will enable us to achieve focusing and imagining beyond diffraction limit, which will be presented in other communications. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. The process
Motivated by broad applications in bioscience and nanolithography, various types of nanolens are being attempted at many laboratories around the world. It has been reported that nanolens structures include quasi-crystals with nanohole or metal nanosphere arrays, nanostructured metamaterials, surface Plasmon polariton assisted enhancement and nanosized slits in metal slabs [1–5]. Among them, two mainstreams of nanolens have been anticipated to possess unique capability of imagining and focusing with sub-wavelength resolution. One is built on metamaterial with negative refractive index (Pendry’s perfect lens [6]). It has been proved that such kind of lens is extremely daunting in fabrication, involving many levels of processing. The other is based on super oscillation theory [7] with a much simpler configuration: a binary annular ring structure consisting of both dielectric and metallic structure in one layer. The challenge is to fabricate both metallic part and dielectric part on one plane without geometry offset. In this work, we have developed a single step lithography process for the nanofabrication of such a specially designed nanolens. Due to the scope of this paper, the physical performance of the novel nanolens will be presented elsewhere.
Fig. 1 illustrates the novel nanolens with binary annular ring structure, which consists of a series of concentric dielectric ring structures (grey area in Fig. 1a) with air gap in between (white area) on quartz substrate surrounded by an opaque area. For high quality focusing performance, it is critical for the precision of the structure in diameter and height of the rings as well as the concentricity between rings. Conventional registration technique in EBL is not able to meet this requirement, especially in concentricity. To overcome this difficulty, we have developed a novel self-aligned nanolithography using one-step EBL and multi-step wet/dry etching to achieve the structure with high precision. In our work, SiNx is used for dielectric ring structure. Firstly on a quartz substrate, a layer of 320 nm thick SiNx is deposited using PECVD technique. The thickness of SiNx has been carefully calculated according to its refractive index of 2.02. A bi-layer of 200 nm PMMA and 500 nm UVIII is spin coated on top of SiNx layer. The two resist layers are separated by either a 20 nm thick Al film or a thin lift-off-resist (LOR) supplied by MicroChem Ltd. to prevent the cross-link between PMMA and UVIII. The designed process flow is described in Fig. 2. One step E-beam exposure is carried out to expose PMMA and UVIII at two different levels of dose, respectively as shown in Fig. 2a. A high resolution vector beam writer, VB6-HR with 100 kV capability supplied by Vistec is used for the EBL exposure. The exposure of UVIII forms the area 4 in Fig. 1a with a radius
* Corresponding author. Tel.: +44 1235 445159. E-mail address: [email protected] (Y. Chen). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.064
B.-R. Lu et al. / Microelectronic Engineering 87 (2010) 1506–1508
1507
Fig. 2. The flow chart of the self-aligned nanolithography process with one-step EBL and multi-step dry/wet etching for the nanofabrication of such the designed nanolens.
Fig. 1. (a) Structural design of binary phase annular ring mask for a novel nanolens; (b) 3D demonstration of the ring mask design.
of 1 lm while PMMA defines the shapes of the rings. After development of the resists, 20 nm Cr and 220 nm Al are deposited onto the pattern subsequently, followed by a lift-off process using acetone as shown in Fig. 2b and c. With the Al cap as an etching mask, SiNx outside the nanolens area is etched down and a total of 120 nm Cr/Au layer is deposited for the opaque area (Fig. 2d) which is formed by a self-aligned shadow evaporation process. The Al cap is then removed by alkaline solution (Fig. 2e) and the remaining PMMA with Cr top is removed by soaking in acetone (Fig. 2f). The rest of the Cr is used as mask for SiNx dry etching to form the annular rings as shown in Fig. 2g. The deposition of the metal was carried out by an electron beam thermal evaporator SVS V2000 and the dry etching process was done using Oxford Plasma System 90.
3. Experimental results and discussion 3.1. EBL results To achieve high precision of the ring sizes, careful EBL tests of different e-beam dose have been carried out. An e-beam exposure with dosage of 100 lC/cm2 has been carried out to fully expose the top UVIII resist. This dose is not high enough to expose the bottom PMMA resist, which therefore remains intact at this stage. Since the size of the annular ring structure is mainly controlled by the exposure of the PMMA, an exposing dosage range between 1000 lC/cm2 and 2500 lC/cm2 is selected and lithography results are carefully measured by a high resolution Hitachi S4000 SEM. Fig. 3 presents the size against the dose. The designed sizes for the structure are r1 = 298 nm; r2 = 613 nm; r3 = 855 nm; r4 = 1000 nm, where r1 to r4 correspond to the same symbol in Fig. 1a. Fig. 3 shows that the exposure dosage window between 1220 lC/cm2 and 1270 lC/cm2 (red tinted area)1 should give rise to the best result in defining the particular size of the ring structures. Fig. 4 demonstrates the SEM picture of the exposed bi-layer resists after development. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Fig. 3. The radium of the developed PMMA ring structures for: (a) r1 (b) r2 (c) r3 in the design diagram (Fig. 1) against the e-beam exposure dose between 1000 lC/cm2 and 2500 lC/cm2. Red tinted area suggests the best dosage window between 1220 lC/cm2 and 1270 lC/cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. The sacrificial layer To prevent cross-link between PMMA and UVIII in the bi-layer stack, a sacrificial layer is necessary to sandwich the two layers. This layer has to be thin, dissolvable in developer but not hinder the EBL of both resist layers. Initially, a thin layer of 20 nm Al is chosen as the sacrificial layer. Although the results are quite promising, the preparation for the bi-layer resists involves a thermal deposition which is complicated. Also, the high temperature during evaporation of Al may cause a potential problem in the stability of the process for repeated fabrication. Therefore, a thin layer of LOR is applied as a substitution of Al. The same quality of resist profile after EBL is achieved. We found that the 20 nm thickness should be the minimum. Below 20 nm, the LOR is less functional than thick ones. But, if the LOR is too thick, shift between top layer UVIII and PMMA may happen during the development process of UVIII.
1508
B.-R. Lu et al. / Microelectronic Engineering 87 (2010) 1506–1508
wall etching and low etching rate of 17.8 nm/min are achieved at the same time. Fig. 4 presents the final fine binary phase ring structure for the nanolens. The outermost ring is a thin wall of Au, which was deposited onto the side wall of the SiNx during the process in Fig. 1d. However, this part also belongs to the opaque area, and would not affect the performance of the nanolens. 4. Conclusion
Fig. 4. SEM picture of the final fine binary phase annular ring structure as a nanolens.
Table 1 Comparison of different SiNx dry etching recipes and corresponding etching rate. Recipe
A
B
C
D
E
C2F6 (sccm) CHF3 (sccm) Ar (sccm) O2 (sccm) Etching rate (nm/min)
30
15
15 15
15 15
15 15
10 44
2 42.5
17.8
15 32.5
28
3.3. Dry etching results In order to achieve high verticality of the SiNx ring structure, comprehensive dry etching tests were carried out with a number of recipes. For each etching recipe, pressure is fixed at 30 mT and power at 250 W. Several combinations of CHF3, C2F6, O2 and Ar etching gas were employed and systematically investigated. Different combination of etching gases and corresponding etching rates are listed in Table 1. For the single gas C2F6 with 30 sccm flow rate as recipe A in Table 1, the rate is at 32.5 nm/min, which is slightly high. When half of C2F6 is changed to Ar, (recipe B) the etching rate slightly drops to 28 nm/min, while the surface smoothness of the substrate is severely sacrificed. On the other hand, when O2 is added to the C2F6 + CHF3 recipe (recipes C and D), not only the etching rate rises significantly, but also rapid side wall etching is observed. Finally the optimized dry etching process for SiNx is found as recipe E in Table 1, where smooth surface, minimum side
This paper reports our recent technical development of a selfaligned nanolithography process for the fabrication of nanolens array. The structure contains both dielectric ring for phase shift and metallic opaque zone for blocking light on the same surface of a quartz wafer, which requires multi-level lithography with critical demand of high precision registration. The self-aligned nanolithography process developed in this work only needs a single level lithography, successfully avoiding the registration for multi-level patterning. Thus, intrinsic error which may cause displacements between different patterns is eliminated. Furthermore, this process is much simpler than multiple level process, hence has the prospect of applying nanoimprint without registration problem in the future, leading to economic patterning for nanometer scale lens in high volume. Acknowledgements This work is partially supported by EPSRC Basic Technology grant (Ref. 3339) of Nanoscope project. The authors would like to thank financial support from Shanghai Municipal of Science and Technology (08QH14002), National Basic Research Program of China (2006CB302703), Shanghai Municipal of Science and Technology (08QH14002) and the Seed Funds for Key Science and Technology Innovation Projects of MOE (708020). This work is also supported by the ‘‘985” Micro/Nanoelectronics Science and Technology Platform. Bingrui Lu’s study at STFC UK is financially supported by a Chinese CSC Scholarship. References [1] Fu Min Huang, Tsung Sheng Kao, Vassili A. Fedotov, Yifang Chen, Nikolay I. Zheludev, Nano Lett. 8 (2008) 2469. [2] M.V. Bashevoy, F. Jonsson, K.F. MacDonald, Y. Chen, N.I. Zheludev, Opt. Express 15 (2007) 11313. [3] Ting Xu, Cunlei Du, Changtao Wang, Xiangang Luo, Appl. Phys. Lett. 91 (2007) 201501. [4] Fu Min Huang, Yifang Chen, F. Javier Garcia de Abajo, Nikolay I. Zheludev, J. Opt. A: Pure Appl. Opt. 9 (2007) S285. [5] N. Papasimakis, V.A. Fedotov, A.S. Schwanecke, N.I. Zheludev, Appl. Phys. Lett. 91 (2007) 081503. [6] J.B. Pendry, Phys. Rev. Lett. 85 (2000) 3966. [7] M.V. Berry, S. Popescu, J. Phys. A: Math. Gen. 39 (2006) 6965.