Low-carbon steel ultra-high-vacuum Schottky emitter electron gun with double O-rings for axis adjustment In-Yong Park and Boklae Cho Citation: J Vac Sci Technol A 35, 020604 (2017); doi: 10.1116/1.4971413 View online: http://dx.doi.org/10.1116/1.4971413 View Table of Contents: http://avs.scitation.org/toc/jva/35/2 Published by the American Vacuum Society Articles you may be interested in Metal-vapor integration/transportation based on metal-atom desorption from polymer surfaces with a low glasstransition temperature J Vac Sci Technol A 35, 020603020603 (2016); 10.1116/1.4971415 Patterning optically clear films: Coplanar transparent and color-contrasted thin films from interdiffused electrodeposited and solution-processed metal oxides J Vac Sci Technol A 35, 020602020602 (2016); 10.1116/1.4968549 Evolution of microstructure and macrostress in sputtered hard Ti(Al,V)N films with increasing energy delivered during their growth by bombarding ions J Vac Sci Technol A 35, 020601020601 (2016); 10.1116/1.4967935 Atomic fluorine densities in electron beam generated plasmas: A high ion to radical ratio source for etching with atomic level precision J Vac Sci Technol A 35, 01A10401A104 (2016); 10.1116/1.4971416 Low-carbon steel ultra-high-vacuum Schottky emitter electron gun with double O-rings for axis adjustment In-Yong Park and Boklae Choa) Division of Industrial Metrology, Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, South Korea (Received 26 September 2016; accepted 14 November 2016; published December 2016) With the aim to create a simpler structure and reduce the production cost of an existing Schottky emitter-scanning electron microscope (SE-SEM), the authors have built and tested a double-O-ring electron gun which is also compatible with ultrahigh vacuum (UHV) Specifically, the gun and column of the SEM consist of low-carbon steel, of which the magnetic shielding effect is greater than that of stainless steel, allowing magnification of Â200 000 in the adapted SEM base without additional magnetic shielding material, such as permalloy or mu-metal The position of the electron gun can be adjusted along the horizontal axis while maintaining the UHV condition Excellent beam current stability with less than 1% variation for more than h was noted Therefore, the authors anticipate that the double-O-ring electron gun and column of low-carbon steel together represent an C 2016 Author(s) All article inexpensive and uncomplicated SE-SEM compared to existing types V content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1116/1.4971413] I INTRODUCTION Since Ruska initially developed the first electron microscope (EM) that exceeded the resolution of an optical microscope in 1933,1 EMs have played a vital role in basic research and in many industrial applications by providing shape, structural, and compositional analyses of materials at a high spatial resolution In cutting-edge research and industries, EMs capable of atomic-scale imaging resolution have seen strong levels of demand A current goal has been to improve the imaging resolutions of EMs In this regard, better resolutions are realized by increasing the acceleration voltage up to mega-electron-volt level and developing an aberration corrector.2,3 Another method by which to reach the ultimate resolution involves the electron source, such as a Schottky emission (SE) or a cold field emission (CFE), with an even smaller source size and higher angular current density.4–6 Devising new methods to generate the beam is very important, as the intrinsic properties of the source can restrict the resolution fundamentally in spite of a high acceleration voltage and the removal of aberrations Tip-based SE and CFE utilize not only a nanoscale source size but also a bright electron beam such that a high resolution is possible despite a low demagnification level as compared to a tungsten filament source However, a tip-based electron source should operate in a UHV; otherwise, the generation of the electron beam can fluctuate because the surface of the tip would be contaminated by gas adsorption Therefore, it is very important to maintain the UHV in the vicinity of the tip to generate a stable electron beam Generally, a level in the low 10À10 Torr range allows for the stable operation of CFE, and a SE requires the low 10À9 Torr range for the generation of a stable electron beam ConFlat (CF) knife-edge flanges are the most common type a) Electronic mail: blcho@kriss.re.kr 020604-1 J Vac Sci Technol A 35(2), Mar/Apr 2017 for UHVs; they use usually a copper gasket between the two CF flanges for sealing, allowing the CF flange to be adopted for use with most gun chambers of SE and CFE devices However, opposing knife-edges should impress grooves into the copper gasket, meaning that the bolts between the mating CF flanges are strongly tightened Therefore, when flanges or chambers are assembled and disassembled, space is necessary for a tool, such as a wrench Furthermore, reusing the copper gasket runs the risk of leaks Thus, the installation of a new copper gasket is strongly recommended whenever UHV components based on a knife-edge sealing are assembled and combined In addition, a mechanical alignment is required in an electron gun when changing its position.7,8 In this regard, for CF-type electron guns, flexible bellows and related supports between the flanges are added This can make the structure complicated and can increase the production cost Furthermore, it is very important to protect a SEM from the external stray magnetic fields, as the electron beam is easily affected by magnetic fields.9,10 Most UHV SEM columns consist of stainless steel, of which the outgassing rate is low However, stainless steel cannot act as a shield against stray magnetic fields Therefore, permalloy, a highpermeability magnetic material, is inserted inside the stainless column From the point of construction, a double-wall structure of an electron gun may lead to a more complicated design and a higher production cost Recently, Park et al showed that low-carbon steel (C 0.2 wt %) has a sufficiently low outgassing rate such that it is possible to construct a UHV chamber of this material.11 Low-carbon steel is a less expensive and soft magnetic material with high magnetic permeability This allows it in itself to act as a shield against stray magnetic fields In this study, we present a more compact and less expensive structure as compared to that of existing SE-SEM devices We applied differentially pumped double-O-rings to an adjustable electron gun structure along the x and y axes 0734-2101/2017/35(2)/020604/6 C Author(s) 2016 V 020604-1 020604-2 I.-Y Park and B Cho: Low-carbon steel ultra-high-vacuum SE electron gun while maintaining a proper UHV condition This structure does not use a CF flange at the electron gun, making assembly and disassembly relatively easy and quick compared to a typical electron gun Furthermore, we show that Â200 000 imaging is possible without permalloy inside the gun chamber to act as a shield against external stray magnetic fields, as low-carbon steel is used as the gun chamber material II EXPERIMENTAL APPARATUS Currently, SE is widely used as an electron source because it can emit highly stable electron beam with a narrow energy spread and high brightness.12,13 Therefore, we used SE in our system as an electron source Figure illustrates a cross-sectional view of the designed SE-SEM, which consists of two condenser lenses, a stigmator, a scanner, and an objective lens, as in a typical SE-SEM In order to measure the stability of the generated electron beam and the SE-SEM image quality, we replaced the electron gun and condenser lenses of a commercial SM300 (Topcon) device with the aforementioned developed parts The SE-SEM column is divided into three parts: an electron gun chamber, an intermediate chamber, and a sample chamber Molybdenum apertures (A 500 lm) are inserted between the chambers to restrict the flow through the column The electron gun chamber is pumped out and maintained at $10À10 Torr using a NEXTorrV device (SAES Getters, D 200-5), which combines an ion pump and a nonevaporable getter (NEG) pump The first condenser lens and variable apertures (A 30, 50, 100, and 200 lm) are in the intermediate chamber, which is maintained at $10À8 Torr by the ion pump (pumping speed: 10 l/s) Lastly, the sample chamber, which R 020604-2 has a second condenser lens, a scanner, and an objective lens, is held to 10À6 Torr with a combination of a diffusion pump and a rotary pump during the imaging process Although vacuum valves and metal hoses are not shown in Fig 1, the three pumps are connected to each other such that the valves are opened for rough pumping and then closed after the baking process The design described here incorporates double-Orings in the geometry of the electron gun, making it quite compact and enabling it to slide transversely An O-ring is usually suitable not for UHV applications but the high vacuum range and rough conditions due to the permeation and the outgassing of materials Therefore, although the chamber sealed with a single O-ring is baked out at the temperature limit of the O-ring, the approximate ultimate pressure reaches only a Torr range of 10À7–10À8 In order to decrease the ultimate pressure of the O-ring-based vacuum sealing, two O-rings are used in a conventional UHV feedthrough as sliding seals, and a pumping port between the O-rings is installed to extract any permeated and/or outgassed gases.14,15 Similarly, we applied double-O-rings to the electron gun structure for UHV compatibility and to enable sliding motions Four through holes (A mm) connected to the intermediate chamber and positioned so as not to interfere with other ports constitute differential pumping ports and a gauge, as shown in Fig A SE (DENKA TFE) is attached onto the bottom side of a plate which is insulated to more than 30 kV The DENKA TFE is a ZrO/W Schottky electron emitter of which the work function is reduced by the coating of the tungsten surface with a layer of zirconium oxide The alumina plate is in contact with the double-O-rings and slides along the horizontal axis by means of four clamping screws We used an alumina content of more than 99% and did not apply a glaze substance onto the surface of the alumina For a good seal with the O-ring, we processed the alumina surface and ensured peak-to-valley roughness measurements of less than 3.2 lm To control the electron beam current and shape, an extractor and a grounded circular hole plate are positioned under the SE tip The gun valve is closed and maintained at the UHV when the sample chamber is opened to load the sample III RESULTS AND DISCUSSION A Vacuum level of the electron gun chamber FIG (Color online) Schematic diagram of a designed SEM and a picture of double O-rings electron gun chamber Pump is NEXTorrV, pump is ion pump, and pump consists of diffusion pump and rotary pump Pump 1, pump 2, and pump are connected with metal hoses and vacuum valve for rough pumping process at first CL1: first condenser lens, CL2: second condenser lens, OL: objective lens, HVC: high voltage cable, EG: electron gun, GV: gun valve, and VA: variable aperture R J Vac Sci Technol A, Vol 35, No 2, Mar/Apr 2017 As a high-temperature pretreatment of the raw materials, the low-carbon steel was baked at 850 C for h in a vacuum furnace to degas the adsorbed gas molecules from the inside of the material After assembling with machined parts, only the electron gun of the SE-SEM was baked at 150 C for approximately 120 h to remove any adsorbed water molecules from the inner walls, as water molecules become adsorbed onto the surface after being exposed to the atmosphere Both the heating temperature and time are important in a baking process If possible, a higher temperature and a longer time are better, but when this is not feasible, the two conditions should be controlled to complement each other We set and maintained the baking temperature to 150 C because the recommended maximum operating temperature of a fluoroelastomer is 150 C This temperature is lower 020604-3 I.-Y Park and B Cho: Low-carbon steel ultra-high-vacuum SE electron gun FIG (Color online) Comparison of gun chamber pressure with double fluoroelastomers and double perfluoroelastomers before, during, and after baking process The pressure is measured by an ion gauge than the temperature of a normal baking process for UHV; therefore, we extended the time as compared to the normal baking time to compensate for the lower baking temperature Activation of the NEG pump was accomplished at 450 C for h immediately before the end of the baking step After the entire baking process, all of the ion pumps were turned on and the heated SEM was cooled to room temperature naturally Perfluoroelastomer (Kalrez) and fluoroelastomer (Viton) are widely used as O-ring seals Their characteristics were compared according to applications.16,17 As shown in Fig 2, in order to compare both fluorinated elastomers in our system, the vacuum pressure was carefully examined with double fluoroelastomers and perfluoroelastomers before, during, and after the baking process The pressure of the double fluoroelastomers gun chamber was higher than that of the double perfluoroelastomers gun chamber during the baking process This phenomenon appears to be related to the outgassing rates of the elastomers because the fluoroelastomer normally have a higher outgassing rate than the perfluoroelastomer types However, there were no differences between them during the NEG activation process due to the large amount of degassed gases from the NEG The pressure tendencies are similar until the temperature dropped to room temperature; however, the pressure of the fluoroelastomer type h after the end of the baking process was order of magnitude lower than that of the perfluoroelastomer These remarkable results can be attributed to the relatively high permeation rate of the perfluoroelastomer type as compared to the fluoroelastomers type Hence, the fluoroelastomer is appropriate for our double-O-ring type of electron gun because the generated electron beam is more stable at a low pressure level B Electron beam current stability Deterioration of the vacuum inside of the SE gun increases the level of instability of the beam current, which causes streaks on SEM images That is to say, it is practically impossible to obtain an ultra-high-resolution image with an unstable electron beam We determined whether the JVST A - Vacuum, Surfaces, and Films 020604-3 beam stability of the double-O-ring electron gun was reliable or not by measuring the beam current at an extractor and a Faraday cup which was placed under the objective lens The current of the extractor was nearly identical to that of the total emission current from the tip The current of the Faraday cup indicates that the probe current is focused on the target The extractor current was measured using the power supply input current of the extractor (Spellman, EBM30N6/718), and the probe current was measured using a picoammeter (Keithley, 6485) Figure 3(a) shows a SEM image of the aperture of the Faraday cup To confirm that the entire probe beam goes inside of the Faraday cup, we increased the magnification of the SEM such that the boundary of the aperture disappeared in the image, indicating that the scanning area of the probe beam is smaller than the size of the aperture of the Faraday cup Moreover, the back plate of the Faraday cup is conical such that incident electrons are not reflected directly back toward the aperture This guaranteed that the total current of the probe beam was measured accurately Figure 3(b) presents the current stability levels of the extractor and the probe beam over a time of h at an acceleration voltage of 20 kV, an extractor voltage of ỵ2.6 kV, and a suppressor voltage of À0.3 kV The condenser lens and variable aperture settings were identical to the conditions of the image at a magnification of Â100 000 For stabilization of the SE tip, the electron beam was generated for 48 h before the measurement Although the extractor current was nearly 100 lA, the probe current, actually used for imaging, is only 17 pA, approximately, as most of the beam current is blocked out when passing through the aperture in the beam path The stability was estimated with Eq (1), in which S is the beam stability (%), r is the standard deviation, and Iave refers to the average of the measured current S¼ 2r  100: Iave (1) We compared the long-term stability (1 h) and short-term stability (2 min) of the probe beam, as short-term stability is sometimes very important when attempting to capture scanned images The measured and estimated values are summarized in Table I The long-term stability of the SE is known to be approximately 1% for h under normal operating conditions.4,12,18 Our designed double-O-ring electron gun also provides stability of less than 1% for h, indicating that the proposed method to generate an electron beam can support similar circumstances compared to those of the conventional method In addition, the short-term fluctuation of probe beam was observed to be less than 0.2% for min, as shown in Fig 3(c) and in Table I Therefore, it appears that the stability levels of the long- and short-term probe beam are sufficient for application to a common SE-SEM system C Electron gun alignment and gold particle SE-SEM imaging The optical components of a SE-SEM device should be aligned along the optical axis in order to minimize distortion 020604-4 I.-Y Park and B Cho: Low-carbon steel ultra-high-vacuum SE electron gun 020604-4 TABLE I Characteristics of generated electron beam current a EC LPCb SPCc r Iave S(%) 0.0375 lA 0.0569 pA 0.0154 pA 99.574 lA 17.0176 pA 17.0101 pA 0.075 0.669 0.182 a EC is the current measured at extractor LPC is the probe current measured by Faraday cup for h as long-term c SPS is the probe current measured by Faraday cup for as short-term b designed the mechanical translation range of electron gun to be approximately 6500 lm because this value can correct the error range that occurs when components are manufactured and assembled We intentionally misaligned the electron gun position by about 500 lm in Fig 4(a) Subsequently, we shifted the electron gun in the horizontal direction using four screw-thread adjustments, thereby moving the beam image to the middle of the image, as shown in Fig 4(b) We also checked whether the afterimages of the beam were concentric or not when the magnification was changed continuously If misalignment arose, the afterimages could not be concentric FIG (Color online) (a) SEM image of an aperture of Faraday cup (b) Current curve of probe beam and extractor for h Solid line indicates the probe beam current and dot line means extractor current (c) Short term plot of probe beam current for and aberrations of the beam shape With these considerations, mechanical alignment of SE-SEM components can be achieved precisely by physically shifting To ascertain the position of the gun on the optical axis, we acquired a beam image at the aperture positioned between the first condenser lens and the second lens by scanning the electron beam at the aperture The beam position in the image is determined by the relative position depending on the SE tip and the aperture Therefore, if the beam direction generated from the SE tip is not aligned with the aperture along the optical axis, the image of the beam does not appear in the middle of the scanning image For the alignment of electron gun by screws, we J Vac Sci Technol A, Vol 35, No 2, Mar/Apr 2017 FIG Beam images according to the scanning aperture which is positioned at the between first condenser lens and second condenser lens at 10 kV acceleration voltages, (a) when the electron gun is misaligned with aperture at optical axis, (b) electron gun position is adjusted mechanically approximately 500 lm along the horizontal axis by screw, thereby electron gun and aperture are aligned well at the optical axis 020604-5 I.-Y Park and B Cho: Low-carbon steel ultra-high-vacuum SE electron gun 020604-5 When the SE gun was aligned mechanically by screws, the alumina plates at which the SE was fixed moved at the contact surface with the double-O-rings while keeping the vacuum state Generally, the pressure could change slightly when sliding motion occurred on the surface of the O-rings Thus, in order to apply the double-O-ring structure to an electron gun system, it is very important to ensure maintenance of the vacuum pressure in the UHV range during the mechanical movement of the alumina plate for proper beam alignment Therefore, we measured the pressure-time trace experimentally while moving the alumina plate using a screw, as shown in Fig Figure 5(a) presents the pressure change when the alumina plate was shifted without the generation of an electron beam There were small fluctuations, but the range of 10À10 Torr was still preserved When the electron beam was generated from a SE tip which was heated to a temperature of 1800 K, the pressure of the gun chamber was increased to the 10À9 Torr range because gas molecules are desorbed from the anode by electron bombardment However, the pressure still remained in the 10À9 Torr range during the movement of the alumina plate as shown in Fig 5(b), which supports the contention of the need for a vacuum condition FIG SEM images of evaporated gold particles at 20 kV acceleration voltages: (a) Â100 000 magnification and (b) Â200 000 magnification FIG (Color online) Variations of the gun chamber pressure trace over time when the SE gun was moved mechanically by a screw (a) Mechanical moving without the generation of an electron beam, and (b) mechanical moving when the electron beam was generated from the SE tip JVST A - Vacuum, Surfaces, and Films to generate an electron beam from the SE tip In our system, although small-scale pressure fluctuations were observed in the UHV range during the mechanical movement step, the double-O-ring design can provide a UHV condition and mechanical alignment simultaneously for an electron gun system To assess the quality and resolution of the resulting SESEM images, we observed evaporated gold particles with an acceleration voltage of 20 kV, as shown in Fig The image quality at Â100 000 is similar when compared to images of a commercial SE-SEM; however, the resolving power at Â200 000 provides less visibility The image resolution in SEM is determined not only by the diameter of the probe beam but also by aberration effects A series of electromagnetic lenses, usually composed of condenser lenses and an objective lens, is used to create a smaller probe size However, the size of the probe beam generally becomes larger than expected due to the aberration effect which arises while the electron beam passes through the series of lenses In order to reduce these aberration effects, the optimum divergence and convergence angles of the cone of the electron beam should be determined by varying the lens power, which affects the beam angle, path and cross-over point in the column We attempted to find optimized divergence and convergence angles to lessen the aberration influence at 200 000 magnification, but the aberration was not satisfactorily eliminated In addition, we renovated not a SE-SEM but a tungsten-filament-based SEM with the designed SE electron gun It appeared that the assembly of a platform for a tungsten-filament-based SEM and SE electron 020604-6 I.-Y Park and B Cho: Low-carbon steel ultra-high-vacuum SE electron gun gun is not sufficient for high magnification Although we did not obtain a high-magnification image such as that produced by a commercial SE-SEM, we verified that the SE can generate a stable electron beam and that it can create images with the double-O-ring gun (National Research Council of Science and Technology) of Republic of Korea (Grant No CAP-14-3-KRISS) The authors appreciate B You and J Lim for comment and support in assembly and experimental result analysis IV SUMMARY AND CONCLUSIONS We developed a simple and inexpensive electron gun for SE-SEM with double-O-rings and a column consisting of a low-carbon steel material This simple structure allows a UHV condition for electron beam generation from SE and provides mechanical movement for an adjustment of the electron beam while maintaining the pressure, with beam stability of less than 1% for h By mounting the designed gun on a commercial tungsten-filament-based SEM, we showed an image of Â200 000 magnification without permalloy to shield against stray magnetic fields It is expected that the developed double-O-ring system can be used not only with existing SE-SEM systems to decrease the production cost but also with UHV devices in cases where the movement of components is strongly required ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from the R&D Convergence Program of NST J Vac Sci Technol A, Vol 35, No 2, Mar/Apr 2017 020604-6 E Ruska, Biosci Rep 7, 607 (1987) M A O’Keefe, Ultramicroscopy 108, 196 (2008) T Akash et al., Appl Phys Lett 106, 074101 (2015) L Swanson and N A Martin, J Appl Phys 46, 2029 (1975) G A Schwind, G Magera, and L W Swanson, J Vac Sci Technol., B 24, 2897 (2006) B Cho, K Shigeru, and C Oshima, Rev Sci Instrum 84, 013305 (2013) E N Beebe and V O Kostroun, Rev Sci Instrum 63, 3399 (1992) I.-Y Park, B Cho, C Han, S Shin, D Lee, and S Ahn, Rev Sci Instrum 86, 016110 (2015) I M€ ullerova and J Fiser, Rev Sci Instrum 65, 1968 (1994) 10 H Hilbrecht, M Knappertsbusch, and H R Thierstein, Mar Micropaleontol 24, 201 (1995) 11 C Park, T Ha, and B Cho, J Vac Sci Technol., A 34, 021601 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perfluoroelastomers... to the relatively high permeation rate of the perfluoroelastomer type as compared to the fluoroelastomers type Hence, the fluoroelastomer is appropriate for our double- O- ring type of electron