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Development of an in-situ nanofabrication instrument for ice lithography
Journal Pre-proof Development of an in-situ nanofabrication instrument for ice lithography
Yu Hong, Ding Zhao, Dongli Liu, Guangnan Yao, Min Qiu PII:
S0167-9317(20)30039-3
DOI:
https://doi.org/10.1016/j.mee.2020.111251
Reference:
MEE 111251
To appear in:
Microelectronic Engineering
Received date:
9 December 2019
Revised date:
29 January 2020
Accepted date:
4 February 2020
Please cite this article as: Y. Hong, D. Zhao, D. Liu, et al., Development of an insitu nanofabrication instrument for ice lithography, Microelectronic Engineering (2020), https://doi.org/10.1016/j.mee.2020.111251
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© 2020 Published by Elsevier.
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Development of an in-situ nanofabrication instrument for ice lithography Yu Hong1, Ding Zhao2,3*, Dongli Liu2,3, Guangnan Yao2,3, and Min Qiu2,3* 1 State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China. 2
Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310024, China 3
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Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310024, China *
Email: [email protected] (D.Z.), [email protected] (M.Q.)
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KEYWORDS: nanofabrication, electron beam lithography, ice lithography, in-situ alignment, three-dimensional nanostructures
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ABSTRACT: Ice lithography (IL) enables in-situ nanofabrication by electron-beam patterning and subsequent pattern transfer of water ice deposited on cryogenic samples. Here we report the design and operation of an IL instrument consisting of a scanning electron microscope, a gas injection system, cryogenic components, a metal deposition chamber, and a sample transfer assembly. The steps required and the amount of equipments involved are significantly reduced compared with ordinary electron beam lithography methods. Different from the previous apparatus, thermal evaporation and internal cooling were implemented and evaluated in this instrument. The in-situ nanofabrication is demonstrated by decorating nanoparticles on or close to a single nanowire and constructing three-dimensional layered structures. Finally, we present an outlook on the further improvement of our instrument. 1. Introduction As key tools of nanotechnology, methods to in-situ construct nanostructures are in high demand [1-8]. Especially for fabricating low-dimensional material-based nanodevices[1-3] and hybrid nanodevices[4-7, 9], high-precision positioning of desired structures is required. Examples include the formation of electrode contacts on two-dimensional materials[10] and the decoration of nanostructures on waveguides[11] or nanowires[12]. Electron beam lithography (EBL) is a well-established technique for routine fabrication of nanostructures with high resolution and reliability. Typical EBL processes include spin-coating, e-beam exposure, chemical development, metal deposition, and lift-off. All steps take place in different scenes, thus involving plenty of individual equipments. During EBL overlay exposure, alignment marks should be fabricated around the working area beforehand, leading to additional processing steps.
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Recently, an emerging e-beam patterning method, also called ice lithography (IL), has shown a great advance over the EBL technique[13-17]. In IL, water molecules are introduced into a scanning electron microscope (SEM) from a nozzle directed toward a cryogenic substrate, to form a layer of ice resist, which is subsequently patterned in-situ by a focused e-beam. The water ice resist has a lower sensitivity to e-beam than conventional EBL resists, enabling in-situ imaging of nanostructures covered by the ice. Therefore, overlay exposure can be performed without additional alignment marks. In this paper, we develop an in-situ nanofabrication instrument for IL and focus on its advantage in overlay fabrication. Here, all processes except the last lift-off step are conducted in an integrated vacuum system, and thus extreme cleanliness is maintained coupled with streamlined processing. Also, liquid phase resists and developers are avoided, enabling patterning on three-dimensional (3D) delicate nanostructures. Different from the magnetic sputtering and external cooling system used in the previous instrument[18], our instrument demonstrates that thermal evaporation can also be applied for metal deposition at cryogenic temperatures, benefiting from the efficient cooling of internal liquid nitrogen (LN2) Dewar. At the end of this paper, we discuss and evaluate the quality of deposited metal films in our instrument.
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2. Instrument design As the schematic diagram shows in Fig.1, our instrument consists of five subsystems: 1) a ZEISS SIGMA scanning electron microscope (SEM), in which water ice is formed and patterned, 2) a gas injection system (GIS) for storing and supplying water vapor precursor, 3) cryogenic components, providing cryogenic conditions for forming and maintaining the water ice resists, 4) a metal deposition chamber (MDC), in which ice patterns are metalized, 5) a sample transfer assembly, transferring samples among SEM, MDC and ambient.
Journal Pre-proof Fig.1. A schematic diagram of our IL instrument. The number before each part name indicates which subsystem they belong to: 1) SEM, 2) GIS, 3) cryogenic components, 4) MDC, 5) sample transfer assembly.
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The SEM with a maximum accelerating voltage of 30 kV is equipped with an EBL system (ELPHY Quantum, Raith, Germany) so that high-resolution e-beam patterning can be performed. However, the actual EBL performance is affected by vibrations and magnetic fields in our working environment. The minimum pattern line width and alignment error when using PMMA resist are typically 50 nm and 500 nm, respectively. The GIS is installed on the side port of the SEM. A four-way cross tube in the GIS acts as a vapor chamber, which is connected to a glass bottle and a nozzle extending into the SEM with a direct valve and a leak valve (VAT, Switzerland), respectively. The gas pressure in the vapor chamber is monitored by a capacitance diaphragm gauge (INFICON, Switzerland).
Fig.2. Cryogenic components installed (a) in the SEM and (b) in the MDC. Cryogenic components based on LN2 Dewar are installed in both SEM and MDC. In SEM, the outer wall of the Dewar is integrated with the SEM flange (Fig.2a). An inner container with a capacity of 1.9 L is suspended from the top flange by the support of three neck tubes, which are also channels for LN2 flowing in and evaporated nitrogen out. The slender thin-walled neck tubes and the evacuated space between the inner container and the outer wall prevent excessive heat leakage from the Dewar’s surroundings to the inner container. The bottom of the inner container is made of oxygen-free copper (OFC), on which an OFC cold finger that protrudes into the SEM chamber is fixed. The e-beam hole on the cold finger is aligned with the electron gun and secondary electron detector to avoid blocking the path of electrons.
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A copper braid is welded to the lower surface of the cold finger, and the other end is connected to the cryostage. The copper braid undergoes annealing to release internal stress. A heat conduction path of LN2 Dewar – cold finger – copper braid – cryostage, therefore forms, and the temperature rises sequentially. The lower temperature of the cold finger allows it to trap undesired gas molecules and particles to prevent their deposition on samples. In MDC, the same principle is adopted to design the cryogenic components, but the structure is more compact than that in SEM. An LN2 Dewar with a 1.7 L-capacity inner container is mounted on the top of the MDC (Fig.2b). The cold finger and the copper braid are not needed here. A cryostage is directly fixed on the bottom of the inner container. The cryostage in SEM is mounted on a dovetail stage, which can be installed onto the SEM navigation stage (Fig.3a). Four nylon hex studs support the cryostage and also serve as thermal isolations to reduce thermal leakage from the dovetail stage. A copper sheet welded with the copper braid is fixed on the side of the cryostage. The sample holder with trapezoid cross-section matches the groove on the cryostage. Their contact surfaces are polished to ensure close contacting and smooth embedding. Up to four 1cm×1cm samples can be fixed on the top surface of the sample holder with screws at the same time. This design keeps the cryostage and sample holder stable. The typical vibration amplitude of the sample holder measured with a nanowire sample is about 10 nm. The cryostage in MDC is mounted downwards to face the thermal evaporator (Fig.3.b). When the sample holders are embedded in the cryostage, their bevels are in close contact due to gravity. All cryostages and sample holders are made of OFC.
Fig.3. Cryostages and sample holders (a) in the SEM (inset shows the cross-section view of cryostage assembly) and (b) in the MDC. Temperatures across the cryogenic components are measured by platinum resistance temperature detectors and a multimeter (model 2700, Keithley, US). About 90 min after LN2 added into the Dewar, the cold finger and cryostage in SEM are cooled to 80 K and 130 K, respectively, which meet the requirements for IL. The cryostage in SEM could be further cooled to the lowest temperature of 108 K, which is helpful for suppressing the sublimation or crystallization of amorphous water ice during processing. Meanwhile, the temperature of the dovetail stage remains above 273 K, avoiding affecting the function of the SEM navigation stage. As for the
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cryostage in MDC, it is cooled to 79 K within 10 min after LN2 added into the Dewar, and kept at this temperature during the whole metal deposition process. The MDC is a cylindrical chamber with multiple flange ports to connect with other equipments (Fig.4). It is pumped by an independent pump system, which includes a rotary vane pump (DS102, Agilent, US) and a turbomolecular pump (Turbo-V 1K-G, Agilent, US). Air pressure in the MDC is monitored by an ionization vacuum gauge (DL-7, Peking University, China). We choose thermal evaporation as the method for metal deposition due to the simplicity of devices and the ease of maintenance. A thermal evaporator (CHI-VAC, China) is installed on the bottom port of MDC, and its distance to the cryostage in MDC is 25 cm. An alternative thermal evaporator (Dr. Eberl MBE-Komponenten GmbH, Germany) is also used to deposit materials with higher melting points.
Fig.4. MDC and sample transfer assembly. Some flange ports on MDC are omitted for clarity. Yellow arrows indicate the movement of the sample holder during sample transfer. A linear-rotary magnetic coupling transfer rod (CHI-VAC, China) with a PTFE stud fixed on its end is installed on the side port of the MDC (Fig.4). The PTFE stud can be screwed into or out of the screw hole in the sample holder by rotating the transfer rod. The sample hold thus can be loaded to or unloaded from cryostages and transferred along the pipeline between the SEM and MDC. A manual XY-stage (CHI-VAC, China) can adjust the position of the pivot of the transfer rod to fit the height difference between two cryostages in SEM and MDC. The temperature of samples can be kept during transfer due to the thermal isolation of PTFE stud and the cold thermal storage of the sample holder. This is essential to prevent amorphous ice from heating above 130K and transforming into crystalline ice[18]. A cubic airlock
Journal Pre-proof chamber with an independent connection to the pump system is installed between the SEM and MDC. And the air pressure of the airlock chamber is measured by a Pirani vacuum gauge (Kurt J. Lesker, USA). A PTFE plate is placed in the airlock chamber to stand by the sample holder. Unless specified, all custom-made parts in our instrument are made of stainless steel. All chambers are connected to the pump system via bellows to isolate vibrations, as is the connection between the SEM and the airlock chamber.
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3. Instrument operation The process of IL in our instrument is shown in Fig.5. We describe the instrument operation in detail as follows.
Fig.5. The process flow of IL in our instrument. Typical IL steps are connected with solid arrows. Dashed arrows indicate the cycle steps to fabricate overlayered structures. Step 0: GIS preparation. This step is not a routine step and only performed when water source needs to be replenished. We choose MgSO4·7H2O (Sinopharm Chemical Reagent Co., Ltd, China) as the water source since it can supply pure water vapor with stable gas pressure. MgSO4·7H2O is filled in a glass vial and sealed with a PTFE filter, which only allows water vapor to come out. After placing the glass vial into the glass bottle, open all the valves in GIS and evacuate it to remove the internal air. When the leak valve is closed, the pressure in the vapor chamber starts to rise until it stabilizes after a few hours. At this time, the vapor chamber is filled with pure water vapor. Step 1: Sample cooling. After the sample holder fixed with samples loaded in the
Journal Pre-proof cyrostage, the SEM chamber is pumped down. LN2 is then poured into the Dewar. It takes 90 min to cool the cryostage to 130 K. The pressure in the SEM chamber drops from 1×10-6 mbar to 2×10-7 mbar due to the cryo-absorption of the cold assemblies. To prevent the sample from being contaminated by condensation of residual gas in the vacuum, the sample holder is sheltered 1 mm under the cold finger during the cooling process. Step 2: Ice resist deposition. The valve to the water bottle is first closed to cut off the vapor supply to the vapor chamber. Then the sample holder is moved 3 mm under the GIS nozzle. As the leak valve opens, water vapor passes through the nozzle and deposits on the sample, forming an amorphous ice resist. We can deposit ice resist locally at specific locations, or a large area of uniform ice resist by meander movement of the stage during deposition.
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The thickness of ice resist ( ti ) is proportional to the amount of water vapor ( nH 2O )
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Thus, we can derive that ti is proportional to the pressure drop in the vapor
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i is the density of amorphous ice, Ai is the coverage area of ice resist, R is the gas constant, T is the room temperature ,and k is the geometrical factor, which might be related to the proportion of vapor gas molecule deposited on the sample, distance from the nozzle to the sample and the shape of nozzles. Our test results roughly fit this linear relationship (Fig.6). The thickness of ice resists is obtained by measuring the cross-section of ice resist formed by e-beam exposure[13].
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Fig.6. Calibration curve for ice resist thickness in our instrument. The thickness of deposited ice resist is proportional to the pressure drop in the vapor chamber.
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Step 3: E-beam patterning. This step is similar to the conventional EBL procedure. Materials that are recognizable through ice resist (e.g., nanowires, carbon nanotubes, MoS2 flakes) or pre-fabricated structures can be used as alignment marks. Step 4: Metal deposition. The thermal evaporator is heated to a standby temperature, which is lower than the melting point of the target material. LN2 is poured into the MDC’s Dewar to cool the cryostage to 79 K in 10 min. Before opening the gate valve to MDC, the column chamber valve in SEM should be closed to prevent free metal atoms from contaminating the SEM gun column. Then we transfer the sample holder onto the cryostage in MDC and continue to raise the temperature of the thermal evaporator. When it reaches the working temperature, the shutter of the thermal evaporator opens to perform the deposition. The usual deposition pressure is about 5×10-5 Pa to 1×10-3 Pa, depending on different target materials and working temperatures. We used to concern that the heat radiated from the evaporator and the heat of evaporated materials could cause the ice resists to melt. However, no recognizable impact on the ice pattern is observed even after metal deposition at 1300 ℃. We attribute this to the efficient dissipation of heat deposited on the sample realized by the cryogenic components. Step 5: Sample holder unloading. After the metal deposition, the evaporator is cooled to the standby temperature. In a typical IL process, the sample holder is then transferred onto the PTFE plate in the airlock chamber, which is individually vented then. Because the sample is kept cold through the sample holder, the ice resist does not melt immediately. Step 6: Lift-off. We quickly immerse the sample in isopropanol so that the ice resist can be melted to reveal underlying metallic nanostructures. Additional ultrasonic treatment is usually unnecessary. For fabricating stacking layered nanostructures, in Step 5, the sample holder may optionally be transferred to the SEM. Afterwards, a layer of structure overlaid on the previous layer can be formed through a new round step of ice resist deposition, e-beam patterning, and metal deposition. Each processing cycle from Step 2 to Step 5 can add one more layer of structures until proceeding to Step 6 when needed.
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4. Instrument performance Through this instrument, we obtained patterned lines with the minimum linewidth of 20 nm, in a 340 nm thick ice resist, using an e-beam dose of 0.75 μC/cm at 10 keV[13]. Although it is difficult to transfer the ice pattern with such a high aspect ratio of 17 into a metal structure by deposition, we used a thinner ice resist and obtained a Ag line structure with a width of 27 nm and a height of 56 nm (Fig.7a). We also varied the line width for comparison. The 24-nm-wide line structure maintains continuity but has larger line-width roughness (LWR) and line-edge roughness (LER) (Fig.7b), while both LWR and LER in the 35-nm-wide line structure are improved (Fig.7c). The area dose for totally removing a 600 nm thick ice resist at 5 kV and 20 kV is 0.35 C/cm2 and 0.67 C/cm2, respectively[13]. This dose is one to three orders of magnitude greater than other cryogenic nanolithography technologies (e.g., cryogenic beam-induced deposition[19] and organic ice lithography[16]). The relatively higher required dose increases the exposure time, which is a major factor limiting the throughput of the current instrument. All tests were performed on cleaned Si substrates.
Fig. 7. Annotated SEM images of Ag line structures fabricated by IL. (a) A 27-nm-wide line. Its height is 56nm, which is obtained by measuring a collapsed line aside. (b) A 24-nm-wide line. (c) A 35-nm-wide line. All scale bars are 200 nm.
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Fig. 8. SEM images of nanostructures fabricated by IL. (a) Top view and (b) tilted view (30°) of Ag nanoparticle arrays on and close to a single Ag nanowire. (c, d) Ag nanoparticle arrays fabricated on different nanowires in the same batch as those in (a). (e) A stacking layered Ag structure. (f) Tilted view (30°) of a bent layered Ag structure. The thickness of each layer is 100 nm. All scale bars are 500 nm. The in-situ alignment error for IL in our instrument is below 100 nm. We demonstrate this feature by fabricating nanoparticle array on a single nanowire (Fig.8a-d). The nanoparticles on the same nanowire have a good consistency. Their shapes are approximate to parabolic cones or water droplets, varying with the relative size of the diameter of the nanowires and nanoparticles. 3D layered structures are also fabricated for demonstration (Fig.8e). In rare cases, it is observed that thick multilayered structures would bend after lift-off (Fig.8f). We suppose it was caused by the poor adhesion between structures and substrates, and the release of internal stress in structures due to the change of temperature. This phenomenon opens a new way for IL fabricating 3D nanostructures with curved features. The surface roughness of deposited metal structures could be reduced due to the low adatom surface mobility at cryogenic substrate temperature[20]. We compared Ag films deposited at different substrate temperatures in our IL instrument. On the Ag film deposited on a Si substrate at room temperature, we observed crystal grain nanostructures (Fig.9a), which did not appear on that deposited on the substrate with
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LN2 cooling (Fig.9b). The root-mean-square (RMS) roughness of two Ag films was 1.30 nm and 0.66 nm, respectively (Fig.9). On the other hand, the surface roughness of deposited structures is also affected by the substrate roughness[20]. To avoid increasing the substrate roughness, we usually use a moderately excessive e-beam dose for keeping the substrate with no residual ice particles and suppress the recrystallization of amorphous ice resist during metal deposition by lowering the temperature of the sample holder[13].
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Fig. 9. Ag films deposited at different substrate temperatures of (a) room temperature and (b) LN2 temperature. The Ag films are 100 nm thick and grown on cleaned Si chips with a deposition rate of 10 nm/min. The backgrounds are SEM images, and insets are AFM images of 2.5 μm × 2.5 μm areas, from which the root-mean-square (RMS) roughness values are calculated. All scale bars are 300 nm.
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5. Conclusion and outlook We have developed an IL instrument integrated with thermal evaporation devices. Newly designed cryogenic components and sample transfer mechanisms are adopted to ensure the stability and convenience of instrument operation. In-situ nanofabrication with simplified e-beam patterning processes using water ice is realized in this instrument. The minimum line width of ice patterns and metal structures obtained by this instrument is 20 nm and 24 nm, respectively. Registration error is under 100 nm with in-situ alignment, which has been demonstrated by decorating nanoparticle arrays on a nanowire and fabricating layered structures. For instrumentation perspectives, cryocoolers can be utilized to reduce the required cooling time significantly. Automated GIS and sample transfer schemes can improve the smoothness of instrument operation. All of these will ultimately increase the throughput of IL instruments. Secondly, compared to thermal evaporators, e-beam evaporators can be used in MDC to deposit a wider variety of materials. Finally, it is beneficial to develop lift-off methods in vacuum so that all steps of IL can be performed in a single vacuum instrument. This will simplify processing further and help make devices based on materials (e.g., perovskite) that are sensitive to the solvents used in common lift-off steps. Acknowledgements The authors thank Prof. Kehui Wu (Institute of Physics, Chinese Academy of Science) and Prof. Zhihua Gan (Zhejiang University) for their help on vacuum system
Journal Pre-proof design and cryogenic solutions. The authors also gratefully acknowledge the support from the National Natural Science Foundation of China (61927820, 51806199) and the China Postdoctoral Science Foundation (2017M621921). The authors declare no competing interests.
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Journal Pre-proof Design, implementation and evaluation of advanced ice lithography instrument In-situ alignment and high-resolution e-beam lithography with water ice
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3D nanofabrication through a streamlined process
Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Yu Hong: Conceptualization, Resources, Investigation, WritingOriginal Draft, Writing- Reviewing and Editing Ding Zhao: Conceptualization, Resources, Writing- Reviewing and Editing, Project administration, Funding acquisition Dongli Liu: Resources Guangnan Yao: Resources Min Qiu: Conceptualization, Supervision, Funding acquisition
Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Yu Hong, Ding Zhao, Dongli Liu, Guangnan Yao, Min Qiu PII:
S0167-9317(20)30039-3
DOI:
https://doi.org/10.1016/j.mee.2020.111251
Reference:
MEE 111251
To appear in:
Microelectronic Engineering
Received date:
9 December 2019
Revised date:
29 January 2020
Accepted date:
4 February 2020
Please cite this article as: Y. Hong, D. Zhao, D. Liu, et al., Development of an insitu nanofabrication instrument for ice lithography, Microelectronic Engineering (2020), https://doi.org/10.1016/j.mee.2020.111251
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier.
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Development of an in-situ nanofabrication instrument for ice lithography Yu Hong1, Ding Zhao2,3*, Dongli Liu2,3, Guangnan Yao2,3, and Min Qiu2,3* 1 State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China. 2
Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310024, China 3
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Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310024, China *
Email: [email protected] (D.Z.), [email protected] (M.Q.)
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KEYWORDS: nanofabrication, electron beam lithography, ice lithography, in-situ alignment, three-dimensional nanostructures
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ABSTRACT: Ice lithography (IL) enables in-situ nanofabrication by electron-beam patterning and subsequent pattern transfer of water ice deposited on cryogenic samples. Here we report the design and operation of an IL instrument consisting of a scanning electron microscope, a gas injection system, cryogenic components, a metal deposition chamber, and a sample transfer assembly. The steps required and the amount of equipments involved are significantly reduced compared with ordinary electron beam lithography methods. Different from the previous apparatus, thermal evaporation and internal cooling were implemented and evaluated in this instrument. The in-situ nanofabrication is demonstrated by decorating nanoparticles on or close to a single nanowire and constructing three-dimensional layered structures. Finally, we present an outlook on the further improvement of our instrument. 1. Introduction As key tools of nanotechnology, methods to in-situ construct nanostructures are in high demand [1-8]. Especially for fabricating low-dimensional material-based nanodevices[1-3] and hybrid nanodevices[4-7, 9], high-precision positioning of desired structures is required. Examples include the formation of electrode contacts on two-dimensional materials[10] and the decoration of nanostructures on waveguides[11] or nanowires[12]. Electron beam lithography (EBL) is a well-established technique for routine fabrication of nanostructures with high resolution and reliability. Typical EBL processes include spin-coating, e-beam exposure, chemical development, metal deposition, and lift-off. All steps take place in different scenes, thus involving plenty of individual equipments. During EBL overlay exposure, alignment marks should be fabricated around the working area beforehand, leading to additional processing steps.
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Recently, an emerging e-beam patterning method, also called ice lithography (IL), has shown a great advance over the EBL technique[13-17]. In IL, water molecules are introduced into a scanning electron microscope (SEM) from a nozzle directed toward a cryogenic substrate, to form a layer of ice resist, which is subsequently patterned in-situ by a focused e-beam. The water ice resist has a lower sensitivity to e-beam than conventional EBL resists, enabling in-situ imaging of nanostructures covered by the ice. Therefore, overlay exposure can be performed without additional alignment marks. In this paper, we develop an in-situ nanofabrication instrument for IL and focus on its advantage in overlay fabrication. Here, all processes except the last lift-off step are conducted in an integrated vacuum system, and thus extreme cleanliness is maintained coupled with streamlined processing. Also, liquid phase resists and developers are avoided, enabling patterning on three-dimensional (3D) delicate nanostructures. Different from the magnetic sputtering and external cooling system used in the previous instrument[18], our instrument demonstrates that thermal evaporation can also be applied for metal deposition at cryogenic temperatures, benefiting from the efficient cooling of internal liquid nitrogen (LN2) Dewar. At the end of this paper, we discuss and evaluate the quality of deposited metal films in our instrument.
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2. Instrument design As the schematic diagram shows in Fig.1, our instrument consists of five subsystems: 1) a ZEISS SIGMA scanning electron microscope (SEM), in which water ice is formed and patterned, 2) a gas injection system (GIS) for storing and supplying water vapor precursor, 3) cryogenic components, providing cryogenic conditions for forming and maintaining the water ice resists, 4) a metal deposition chamber (MDC), in which ice patterns are metalized, 5) a sample transfer assembly, transferring samples among SEM, MDC and ambient.
Journal Pre-proof Fig.1. A schematic diagram of our IL instrument. The number before each part name indicates which subsystem they belong to: 1) SEM, 2) GIS, 3) cryogenic components, 4) MDC, 5) sample transfer assembly.
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The SEM with a maximum accelerating voltage of 30 kV is equipped with an EBL system (ELPHY Quantum, Raith, Germany) so that high-resolution e-beam patterning can be performed. However, the actual EBL performance is affected by vibrations and magnetic fields in our working environment. The minimum pattern line width and alignment error when using PMMA resist are typically 50 nm and 500 nm, respectively. The GIS is installed on the side port of the SEM. A four-way cross tube in the GIS acts as a vapor chamber, which is connected to a glass bottle and a nozzle extending into the SEM with a direct valve and a leak valve (VAT, Switzerland), respectively. The gas pressure in the vapor chamber is monitored by a capacitance diaphragm gauge (INFICON, Switzerland).
Fig.2. Cryogenic components installed (a) in the SEM and (b) in the MDC. Cryogenic components based on LN2 Dewar are installed in both SEM and MDC. In SEM, the outer wall of the Dewar is integrated with the SEM flange (Fig.2a). An inner container with a capacity of 1.9 L is suspended from the top flange by the support of three neck tubes, which are also channels for LN2 flowing in and evaporated nitrogen out. The slender thin-walled neck tubes and the evacuated space between the inner container and the outer wall prevent excessive heat leakage from the Dewar’s surroundings to the inner container. The bottom of the inner container is made of oxygen-free copper (OFC), on which an OFC cold finger that protrudes into the SEM chamber is fixed. The e-beam hole on the cold finger is aligned with the electron gun and secondary electron detector to avoid blocking the path of electrons.
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A copper braid is welded to the lower surface of the cold finger, and the other end is connected to the cryostage. The copper braid undergoes annealing to release internal stress. A heat conduction path of LN2 Dewar – cold finger – copper braid – cryostage, therefore forms, and the temperature rises sequentially. The lower temperature of the cold finger allows it to trap undesired gas molecules and particles to prevent their deposition on samples. In MDC, the same principle is adopted to design the cryogenic components, but the structure is more compact than that in SEM. An LN2 Dewar with a 1.7 L-capacity inner container is mounted on the top of the MDC (Fig.2b). The cold finger and the copper braid are not needed here. A cryostage is directly fixed on the bottom of the inner container. The cryostage in SEM is mounted on a dovetail stage, which can be installed onto the SEM navigation stage (Fig.3a). Four nylon hex studs support the cryostage and also serve as thermal isolations to reduce thermal leakage from the dovetail stage. A copper sheet welded with the copper braid is fixed on the side of the cryostage. The sample holder with trapezoid cross-section matches the groove on the cryostage. Their contact surfaces are polished to ensure close contacting and smooth embedding. Up to four 1cm×1cm samples can be fixed on the top surface of the sample holder with screws at the same time. This design keeps the cryostage and sample holder stable. The typical vibration amplitude of the sample holder measured with a nanowire sample is about 10 nm. The cryostage in MDC is mounted downwards to face the thermal evaporator (Fig.3.b). When the sample holders are embedded in the cryostage, their bevels are in close contact due to gravity. All cryostages and sample holders are made of OFC.
Fig.3. Cryostages and sample holders (a) in the SEM (inset shows the cross-section view of cryostage assembly) and (b) in the MDC. Temperatures across the cryogenic components are measured by platinum resistance temperature detectors and a multimeter (model 2700, Keithley, US). About 90 min after LN2 added into the Dewar, the cold finger and cryostage in SEM are cooled to 80 K and 130 K, respectively, which meet the requirements for IL. The cryostage in SEM could be further cooled to the lowest temperature of 108 K, which is helpful for suppressing the sublimation or crystallization of amorphous water ice during processing. Meanwhile, the temperature of the dovetail stage remains above 273 K, avoiding affecting the function of the SEM navigation stage. As for the
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cryostage in MDC, it is cooled to 79 K within 10 min after LN2 added into the Dewar, and kept at this temperature during the whole metal deposition process. The MDC is a cylindrical chamber with multiple flange ports to connect with other equipments (Fig.4). It is pumped by an independent pump system, which includes a rotary vane pump (DS102, Agilent, US) and a turbomolecular pump (Turbo-V 1K-G, Agilent, US). Air pressure in the MDC is monitored by an ionization vacuum gauge (DL-7, Peking University, China). We choose thermal evaporation as the method for metal deposition due to the simplicity of devices and the ease of maintenance. A thermal evaporator (CHI-VAC, China) is installed on the bottom port of MDC, and its distance to the cryostage in MDC is 25 cm. An alternative thermal evaporator (Dr. Eberl MBE-Komponenten GmbH, Germany) is also used to deposit materials with higher melting points.
Fig.4. MDC and sample transfer assembly. Some flange ports on MDC are omitted for clarity. Yellow arrows indicate the movement of the sample holder during sample transfer. A linear-rotary magnetic coupling transfer rod (CHI-VAC, China) with a PTFE stud fixed on its end is installed on the side port of the MDC (Fig.4). The PTFE stud can be screwed into or out of the screw hole in the sample holder by rotating the transfer rod. The sample hold thus can be loaded to or unloaded from cryostages and transferred along the pipeline between the SEM and MDC. A manual XY-stage (CHI-VAC, China) can adjust the position of the pivot of the transfer rod to fit the height difference between two cryostages in SEM and MDC. The temperature of samples can be kept during transfer due to the thermal isolation of PTFE stud and the cold thermal storage of the sample holder. This is essential to prevent amorphous ice from heating above 130K and transforming into crystalline ice[18]. A cubic airlock
Journal Pre-proof chamber with an independent connection to the pump system is installed between the SEM and MDC. And the air pressure of the airlock chamber is measured by a Pirani vacuum gauge (Kurt J. Lesker, USA). A PTFE plate is placed in the airlock chamber to stand by the sample holder. Unless specified, all custom-made parts in our instrument are made of stainless steel. All chambers are connected to the pump system via bellows to isolate vibrations, as is the connection between the SEM and the airlock chamber.
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3. Instrument operation The process of IL in our instrument is shown in Fig.5. We describe the instrument operation in detail as follows.
Fig.5. The process flow of IL in our instrument. Typical IL steps are connected with solid arrows. Dashed arrows indicate the cycle steps to fabricate overlayered structures. Step 0: GIS preparation. This step is not a routine step and only performed when water source needs to be replenished. We choose MgSO4·7H2O (Sinopharm Chemical Reagent Co., Ltd, China) as the water source since it can supply pure water vapor with stable gas pressure. MgSO4·7H2O is filled in a glass vial and sealed with a PTFE filter, which only allows water vapor to come out. After placing the glass vial into the glass bottle, open all the valves in GIS and evacuate it to remove the internal air. When the leak valve is closed, the pressure in the vapor chamber starts to rise until it stabilizes after a few hours. At this time, the vapor chamber is filled with pure water vapor. Step 1: Sample cooling. After the sample holder fixed with samples loaded in the
Journal Pre-proof cyrostage, the SEM chamber is pumped down. LN2 is then poured into the Dewar. It takes 90 min to cool the cryostage to 130 K. The pressure in the SEM chamber drops from 1×10-6 mbar to 2×10-7 mbar due to the cryo-absorption of the cold assemblies. To prevent the sample from being contaminated by condensation of residual gas in the vacuum, the sample holder is sheltered 1 mm under the cold finger during the cooling process. Step 2: Ice resist deposition. The valve to the water bottle is first closed to cut off the vapor supply to the vapor chamber. Then the sample holder is moved 3 mm under the GIS nozzle. As the leak valve opens, water vapor passes through the nozzle and deposits on the sample, forming an amorphous ice resist. We can deposit ice resist locally at specific locations, or a large area of uniform ice resist by meander movement of the stage during deposition.
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i is the density of amorphous ice, Ai is the coverage area of ice resist, R is the gas constant, T is the room temperature ,and k is the geometrical factor, which might be related to the proportion of vapor gas molecule deposited on the sample, distance from the nozzle to the sample and the shape of nozzles. Our test results roughly fit this linear relationship (Fig.6). The thickness of ice resists is obtained by measuring the cross-section of ice resist formed by e-beam exposure[13].
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Fig.6. Calibration curve for ice resist thickness in our instrument. The thickness of deposited ice resist is proportional to the pressure drop in the vapor chamber.
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Step 3: E-beam patterning. This step is similar to the conventional EBL procedure. Materials that are recognizable through ice resist (e.g., nanowires, carbon nanotubes, MoS2 flakes) or pre-fabricated structures can be used as alignment marks. Step 4: Metal deposition. The thermal evaporator is heated to a standby temperature, which is lower than the melting point of the target material. LN2 is poured into the MDC’s Dewar to cool the cryostage to 79 K in 10 min. Before opening the gate valve to MDC, the column chamber valve in SEM should be closed to prevent free metal atoms from contaminating the SEM gun column. Then we transfer the sample holder onto the cryostage in MDC and continue to raise the temperature of the thermal evaporator. When it reaches the working temperature, the shutter of the thermal evaporator opens to perform the deposition. The usual deposition pressure is about 5×10-5 Pa to 1×10-3 Pa, depending on different target materials and working temperatures. We used to concern that the heat radiated from the evaporator and the heat of evaporated materials could cause the ice resists to melt. However, no recognizable impact on the ice pattern is observed even after metal deposition at 1300 ℃. We attribute this to the efficient dissipation of heat deposited on the sample realized by the cryogenic components. Step 5: Sample holder unloading. After the metal deposition, the evaporator is cooled to the standby temperature. In a typical IL process, the sample holder is then transferred onto the PTFE plate in the airlock chamber, which is individually vented then. Because the sample is kept cold through the sample holder, the ice resist does not melt immediately. Step 6: Lift-off. We quickly immerse the sample in isopropanol so that the ice resist can be melted to reveal underlying metallic nanostructures. Additional ultrasonic treatment is usually unnecessary. For fabricating stacking layered nanostructures, in Step 5, the sample holder may optionally be transferred to the SEM. Afterwards, a layer of structure overlaid on the previous layer can be formed through a new round step of ice resist deposition, e-beam patterning, and metal deposition. Each processing cycle from Step 2 to Step 5 can add one more layer of structures until proceeding to Step 6 when needed.
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4. Instrument performance Through this instrument, we obtained patterned lines with the minimum linewidth of 20 nm, in a 340 nm thick ice resist, using an e-beam dose of 0.75 μC/cm at 10 keV[13]. Although it is difficult to transfer the ice pattern with such a high aspect ratio of 17 into a metal structure by deposition, we used a thinner ice resist and obtained a Ag line structure with a width of 27 nm and a height of 56 nm (Fig.7a). We also varied the line width for comparison. The 24-nm-wide line structure maintains continuity but has larger line-width roughness (LWR) and line-edge roughness (LER) (Fig.7b), while both LWR and LER in the 35-nm-wide line structure are improved (Fig.7c). The area dose for totally removing a 600 nm thick ice resist at 5 kV and 20 kV is 0.35 C/cm2 and 0.67 C/cm2, respectively[13]. This dose is one to three orders of magnitude greater than other cryogenic nanolithography technologies (e.g., cryogenic beam-induced deposition[19] and organic ice lithography[16]). The relatively higher required dose increases the exposure time, which is a major factor limiting the throughput of the current instrument. All tests were performed on cleaned Si substrates.
Fig. 7. Annotated SEM images of Ag line structures fabricated by IL. (a) A 27-nm-wide line. Its height is 56nm, which is obtained by measuring a collapsed line aside. (b) A 24-nm-wide line. (c) A 35-nm-wide line. All scale bars are 200 nm.
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Fig. 8. SEM images of nanostructures fabricated by IL. (a) Top view and (b) tilted view (30°) of Ag nanoparticle arrays on and close to a single Ag nanowire. (c, d) Ag nanoparticle arrays fabricated on different nanowires in the same batch as those in (a). (e) A stacking layered Ag structure. (f) Tilted view (30°) of a bent layered Ag structure. The thickness of each layer is 100 nm. All scale bars are 500 nm. The in-situ alignment error for IL in our instrument is below 100 nm. We demonstrate this feature by fabricating nanoparticle array on a single nanowire (Fig.8a-d). The nanoparticles on the same nanowire have a good consistency. Their shapes are approximate to parabolic cones or water droplets, varying with the relative size of the diameter of the nanowires and nanoparticles. 3D layered structures are also fabricated for demonstration (Fig.8e). In rare cases, it is observed that thick multilayered structures would bend after lift-off (Fig.8f). We suppose it was caused by the poor adhesion between structures and substrates, and the release of internal stress in structures due to the change of temperature. This phenomenon opens a new way for IL fabricating 3D nanostructures with curved features. The surface roughness of deposited metal structures could be reduced due to the low adatom surface mobility at cryogenic substrate temperature[20]. We compared Ag films deposited at different substrate temperatures in our IL instrument. On the Ag film deposited on a Si substrate at room temperature, we observed crystal grain nanostructures (Fig.9a), which did not appear on that deposited on the substrate with
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LN2 cooling (Fig.9b). The root-mean-square (RMS) roughness of two Ag films was 1.30 nm and 0.66 nm, respectively (Fig.9). On the other hand, the surface roughness of deposited structures is also affected by the substrate roughness[20]. To avoid increasing the substrate roughness, we usually use a moderately excessive e-beam dose for keeping the substrate with no residual ice particles and suppress the recrystallization of amorphous ice resist during metal deposition by lowering the temperature of the sample holder[13].
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Fig. 9. Ag films deposited at different substrate temperatures of (a) room temperature and (b) LN2 temperature. The Ag films are 100 nm thick and grown on cleaned Si chips with a deposition rate of 10 nm/min. The backgrounds are SEM images, and insets are AFM images of 2.5 μm × 2.5 μm areas, from which the root-mean-square (RMS) roughness values are calculated. All scale bars are 300 nm.
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5. Conclusion and outlook We have developed an IL instrument integrated with thermal evaporation devices. Newly designed cryogenic components and sample transfer mechanisms are adopted to ensure the stability and convenience of instrument operation. In-situ nanofabrication with simplified e-beam patterning processes using water ice is realized in this instrument. The minimum line width of ice patterns and metal structures obtained by this instrument is 20 nm and 24 nm, respectively. Registration error is under 100 nm with in-situ alignment, which has been demonstrated by decorating nanoparticle arrays on a nanowire and fabricating layered structures. For instrumentation perspectives, cryocoolers can be utilized to reduce the required cooling time significantly. Automated GIS and sample transfer schemes can improve the smoothness of instrument operation. All of these will ultimately increase the throughput of IL instruments. Secondly, compared to thermal evaporators, e-beam evaporators can be used in MDC to deposit a wider variety of materials. Finally, it is beneficial to develop lift-off methods in vacuum so that all steps of IL can be performed in a single vacuum instrument. This will simplify processing further and help make devices based on materials (e.g., perovskite) that are sensitive to the solvents used in common lift-off steps. Acknowledgements The authors thank Prof. Kehui Wu (Institute of Physics, Chinese Academy of Science) and Prof. Zhihua Gan (Zhejiang University) for their help on vacuum system
Journal Pre-proof design and cryogenic solutions. The authors also gratefully acknowledge the support from the National Natural Science Foundation of China (61927820, 51806199) and the China Postdoctoral Science Foundation (2017M621921). The authors declare no competing interests.
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Journal Pre-proof Design, implementation and evaluation of advanced ice lithography instrument In-situ alignment and high-resolution e-beam lithography with water ice
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3D nanofabrication through a streamlined process
Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Yu Hong: Conceptualization, Resources, Investigation, WritingOriginal Draft, Writing- Reviewing and Editing Ding Zhao: Conceptualization, Resources, Writing- Reviewing and Editing, Project administration, Funding acquisition Dongli Liu: Resources Guangnan Yao: Resources Min Qiu: Conceptualization, Supervision, Funding acquisition
Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: