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Glow-discharge ion source for MEMS mass spectrometer
Journal Pre-proof Glow-discharge ion source for mems mass spectrometer
Tomasz Grzebyk, Tomasz Szmajda, Piotr Szyszka, Anna Górecka-Drzazga, Jan Dziuban PII:
S0042-207X(19)32262-6
DOI:
https://doi.org/10.1016/j.vacuum.2019.109008
Reference:
VAC 109008
To appear in:
Vacuum
Received Date: 26 June 2019 Revised Date:
11 October 2019
Accepted Date: 14 October 2019
Please cite this article as: Grzebyk T, Szmajda T, Szyszka P, Górecka-Drzazga A, Dziuban J, Glowdischarge ion source for mems mass spectrometer, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109008. 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. © 2019 Elsevier Ltd. All rights reserved.
GLOW-DISCHARGE ION SOURCE FOR MEMS MASS SPECTROMETER
Tomasz Grzebyk1, Tomasz Szmajda, Piotr Szyszka, Anna Górecka-Drzazga, Jan Dziuban
Wroclaw University of Science and Technology, Faculty of Microsystem Electronics and Photonics, Department of Microengineering and Photovoltaics, 11/17 Janiszewskiego St., 50-372 Wroclaw, Poland
Abstract This work describes construction, technology, working principle and properties of an ion source dedicated to a miniature MEMS (Micro-Electro-Mechanical System) mass spectrometer. Conditions allowing efficient ionization of a gas sample, and obtaining a properly directed ion beam were determined. An influence of parameters such as shape, dimension and distance between the electrodes, as well as applied magnetic and electric field and pressure level, on the operation of this device were investigated. The experiment allowed choosing the optimal parameters, thus it may be concluded that the ion source will satisfy all the requirements necessary for the MEMS mass spectrometer. 1. Introduction Mass spectrometers are analytical instruments which allow for the identification of a gas sample composition. First, the examined gas sample introduced into the spectrometer is ionized and later individual gas species are separated according to their mass to charge ratio. Usually, these processes take place in high vacuum conditions, therefore mass spectrometer must be continuously connected to a pumping system, which makes the whole device bulky, expensive and stationary. A strong research trend towards miniaturization of mass spectrometers, often using MEMS technology [1–5], can be noticed. However, in most cases it applies only to its certain parts, mostly ion sources, analyzers or ion traps. Seldom, few components are integrated on the same chip. And even then, vacuum is generated by an external, conventional pumping systems. Thus, the complete instrument is still, in best case, suitcase-size [6–8]. Recently, our research group demonstrated the first silicon-glass MEMS-type ionsorption vacuum pump, which enables generating high vacuum (10−7 mbar) on a chip level [9]. This invention opened the possibility of creating more complex chip-scale high-vacuum analytical instruments, including mass microspectrometer. Currently, our goal is to integrate 1
Corresponding author: tel. +48 71 320 49 74 E-mail address: [email protected]
all the components of a spectrometer on a single chip: a sample injection system, an ion source, a mass analyzer and the high vacuum micropump. These all need to be produced in a consistent technology and co-work properly. Therefore, we have designed a completely new, uniform construction of a MEMS spectrometer. 2. Design of a MEMS mass spectrometer According to our concept, the miniature, on-chip integrated spectrometer, consists of two major blocks – an ion column including: sample injector, ion source, drift zone, collector; and the above mentioned ion-sorption micropump, both connected by a microchannel etched in one of the glass wafers (Fig. 1). Electrodes are made of monocrystalline silicon and are insulated with borosilicate glass plates. All elements are connected together using anodic bonding. This process ensures protection from unwanted gas leakages and when performed in vacuum conditions, it allows obtaining initial vacuum necessary to turn on the micropump [10–12]. The ion-sorption micropump enables further reduction of pressure down to high vacuum (<10−5 mbar). A gas sample is introduced by a nanochannel made in the silicon wafer at the bottom of the ion column, until the pressure reaches a desired level of about 10−2 – 10−4 mbar. At that moment gas particles are ionized inside the ion source and formed as a beam that is attracted to the collector placed a few centimeters above it. The mass spectrum is obtained by recording a collector ion current after applying an impulse voltage to one of the electrodes (so called Time-of-Flight method).
Fig. 1. Schematic diagram of the miniature MEMS mass spectrometer: cross section of the complete device (left) and the ion source (right). 3. MEMS glow-discharge ion source The ion source is one of the most important parts of a mass spectrometer. It should efficiently ionize gas particles, but at the same time it should also form a vertically oriented ion beam, which is later separated on the way to the collector. There are number of ion sources presented in literature [13,14]. However, it is hard to find the one which fulfills requirements necessary for the proposed mass spectrometer: operation on the aforementioned
pressure level, stable and efficient ionization and consistent technology. Commonly used electron impact ion sources based on field emitters suffer from poor stability and degradation when working in medium vacuum [15–18]. Glow-discharge ion sources are free from these drawbacks [19,20], however it is not possible to apply any of the already existing ones, because its construction has to be compatible with the rest of the spectrometer. The ion source presented in this paper consists of three electrodes placed one above another and a permanent magnet located below them. There is a larger through-hole in the central electrode (anode) and a smaller one in the top electrode (anti-cathode). These holes can be either circular (obtained by deep reactive ion etching) or square (formed by an anisotropic silicon etching). In the presence of the magnetic field this set of electrodes forms an electron trap (Fig. 2).
Fig. 2. Distribution of the electric (left) and magnetic field (right) inside the ion source. The gas molecules are ionized in a glow discharge which is initiated by spontaneous electrons. No other electron source is needed, neither thermal nor cold, and this improves the robustness of the device and increases its lifetime. In this trap paths of the electrons are elongated (Fig. 3 left), electric and magnetic field distribution prevents them from reaching the electrodes. Therefore, they finally collide with gas molecules and create ions and additional electrons, what leads to a maintenance of a glow discharge. This process occurs even in high vacuum, because of the very long path of electrons.
Fig. 3. Simulations of electrons (left) and ions (right) trajectories in the MEMS glowdischarge ion source (COMSOL software).
The most preferred conditions for ionization of gas particles are ensured when the trap is symmetric. However, to create a consistent, focused beam of ions (Fig. 3 right), the full symmetry has to be disrupted. First of all, a via-hole needs to be formed in the anti-cathode to allow the ions to leave the trap. Secondly, the distances between ion source electrodes and their potentials should be precisely selected to direct them in vertical direction and prevent from spreading. 4. Experiment In the experimental stage, the authors focused on determining the optimal conditions and constructional parameters to ensure both: efficient gas ionization and formation of a vertically oriented ion beam. During investigation, a specially designed 3D printed holder was utilized (Fig. 4). It allowed a simple replacement of electrodes differing in shapes (rectangular and circular) and dimensions of the anode and anticathode through-holes. Three different combinations of hole size were examined: ϕA/ϕAC = 3/1; 2/1 and 3/2 mm. The electrodes could be mounted on one of several “shelves” separated from each other by 0.55 mm. In all the configurations the cathode was placed on shelf no. 1. In configuration I – the anode was placed on shelf no. 2, the anti-cathode on shelf no. 3; in configuration II – it was shelf no. 2 and shelf no. 4, respectively; in configuration III – shelf no. 2 and shelf no. 5; and in configuration IV – shelf no. 3 and shelf no. 5. The measurements were performed inside a reference vacuum chamber. Electronically controlled valve allowed to introduce a desired amount of atmospheric gases and precisely adjust the internal pressure. Although the 3D printed holder was made of a polymer (VisiJet M3 Crystal2), residual gas analyzer mounted in the vacuum set-up did not show any serious outgassing. During the measurements the cathode was kept at the ground potential, a positive voltage was applied to the anode and a small negative potential was applied to the anticathode to attract the positive ions.
5 4 3 2 1
Fig. 4. The test structure of a glow-discharge ion source placed in a special 3D printed holder
2
https://www.3dsystems.com/sites/default/files/2018-11/3d-systems-proJet-mjp-3600-plastic-tech-specs-a4-us2018-11-08-web.pdf
In the first experiment an influence of shape and size of the through-holes etched in the anode on the properties of ion source was investigated. Although it is possible to obtain a discharge for the structures with square holes, the process is unstable and the ion beam splits into four directions after passing the hole in the anti-cathode. The structures with circular holes exhibit much better properties, the discharge is stable and the ion beam is consistent. The size of the holes and their spacing have also a big impact on the obtained results. More important is the diameter of the anode hole then the anti-cathode hole. In case it is 2 mm wide, discharge can be obtained only for configurations II and III and the current values measured at the anode and at the anti-cathode hardly depend on the anode voltage (Fig. 5a). In case the anode hole is 3 mm wide, it is possible to obtain a discharge in all the examined configurations, both for 1 and 2 mm wide anti-cathode hole. This time the current strongly depends on the anti-cathode voltage (Fig. 5b). Results change in different conditions (pressure, anode voltage), but usually up to 200–300 V the currents increase, and above this level they slightly decrease, stabilize or in some cases increase further (Fig. 6).
a) b) Fig. 5. The anode and anti-cathode current as a function of anti-cathode voltage: a) for the structure with anode hole diameter ϕA = 2 mm, and anti-cathode ϕAC = 1 mm, configuration II, b) ϕA = 3 mm, ϕAC = 2 mm; UA = 1200 V, p = 10−3 mbar, configuration IV.
Fig. 6. Anti-cathode currents as a function of anti-cathode voltage for different electrode configurations, UA = 1200 V, p = 10−3 mbar.
The other important parameter affecting an operation of the ion source is the strength of the magnetic field. It is important that its value in the middle of the ion source excides 0.25 T. For the weaker field it is impossible to maintain a discharge below 0.01 mbar. Such a field can be ensured even with usage of a small neodymium magnets (ϕ = 8 mm; h = 6 mm), which induction just above the surface is equal to 0.5 T. Further increase of the field does not increase the discharge current significantly (< 20%) and it requires a use of much bigger magnets. After investigating geometrical and structural parameters, the proper operation conditions were established. The most preferable pressure for stable ionization of atmospheric gases is in a range between 10−4 and 10−3 mbar. Below that range the anode voltage needs to be increased above 1500 V and ion/electron currents are relatively small (about a few microamperes). In lower vacuum – the discharge is often noticed near the edges of the electrodes, not in the middle; moreover high pressure is also not preferred for the operation of the spectrometer itself – after obtaining a mass spectrum, the entire volume must be evacuated to high vacuum before the next measurement, and the capacity of the micropump is limited. In the mentioned pressure range the voltage applied to the anode should usually be equal to at least 800 V, preferably 1000−1200 V. Always, with increasing pressure and increasing anode voltage, the anode and anti-cathode currents rise. Basing on the fig. 5 and 6 one can state that in case of 2 mm hole in the anode, the anti-cathode voltage can be chosen arbitrarily and in case of 3 mm hole it should be kept on small negative potential (about 200 V). In these conditions the anti-cathode current is the highest. Moreover, this potential helps to attract the ions towards the upper electrode and in the same time prevents from spreading of the ion beam after passing the anti-cathode, as it happens in case of too high negative potential. The observation of the ion beam shape (Fig. 7) and measurement of the ion current value enabled to choose the optimal conditions for the ion source operation. They were obtained in configuration II, with ϕA = 3 mm, and ϕAC = 2 mm at p = 10−3 mbar; UA = 1200 V and UAC = −200 V.
a)
b)
Fig. 7. Image of the working device: a) vertical beam of ions expands above the structure, electrode voltages: UA = 1200 V, UAC = -200 V, b) ions spread after passing the anti-cathode, electrode voltages: UA = 1200 V, UAC = -500 V; p = 10−3 mbar, spectral analysis indicates the presence mostly of nitrogen ions in the plasma.
After choosing the proper parameters, the forth electrode (the collector) was placed 2 cm above the ion source on a separate holder. It was investigated what part of ions can leave through the hole in the anti-cathode – only these ions give a signal to the mass spectrum. The anode potential was set to 1200 V, and anti-cathode to −200 V. Collector voltage was changed between 0 and −600 V, and the anti-cathode and collector currents were measured at the same time (Fig. 8).
Fig. 8. Anti-cathode and collector currents measured in the function of a negative collector voltage, UA = 1200 V, UAC = −200 V, p = 10−3 mbar. It appeared, that both currents changed with increasing collector voltage. For UCOL = 0 V, 10% of the ions reaching the anti-cathode passed to the collector, for –500 V the ratio exceeded 25%. This increase might be an effect of a change in potential distribution, but also it could be caused by a higher secondary electron emission. Nevertheless, a collector current at the level above 10 µA should be sufficient to detect a mass spectra. In the next step, to obtain mass spectra, DC voltage should be replaced with a very short impulse voltage applied to the collector or to the anti-cathode (rising time below 100 ns) and the ion current will be measured in the function of time. Up to now, computer simulations have been performed to examine this issue (Fig. 9, 10). It was assumed that ions are created in the cylindrical volume between the cathode and anticathode (φ = 3 mm). The proportion of different gases was the same as in the atmosphere (total number of ions – 1 000 000). Potentials at the electrodes of the ion source were equal to UC = 0 V, UA = 1200 V, UAC = −200 V, and an impulse of -500 V was applied to the collector. The influence of the presence of plasma and associated space charge, as well as secondary emission was omitted. The potential distribution and the trajectories of the ions were the same as in the absolute vacuum.
Fig. 9. Separation of ions having different atomic masses (marked on scale with different colors) in a MEMS mass spectrometer (COMSOL Multiphysics simulations), starting area – cylinder between the cathode and anti-cathode, ϕ = 3 mm, starting energy – 0 eV.
Fig. 10. Simulation result of mass spectrum of atmospheric gases. Since different ions have different masses, the time necessary to reach the collector varies – lighter ions (as N+) need shorter times (0.22 µs) and heavier (as carbon dioxide) need longer times (0.48 µs). One can notice that the peaks representing most dominant gases can be easily separated from each other. However, peaks coming from molecules that are not so numerous in the atmosphere are far less visible. It is believed that the resolution could be improved, when a space charge was taking into account. The energy distribution of the extracted particles should be in that case narrower. 5. Conclusion The article presents the design and parameters of the glow-discharge miniature ionsource. This device consists of set of 3 electrodes and a permanent magnet, it does not require any fragile electron source – gas particles are ionized in a glow discharge obtained in a properly formed electric and magnetic field. A stabile, well oriented and focused ion beam leaves the MEMS ion source and directs towards the collector. This source fulfils all the
requirements necessary to be applied in the on-chip integrated MEMS mass spectrometer. Application of an impulse voltage on the collector electrode should allow to separate the ions of individual gases and to obtain a mass spectrum. In the near future, the test structure of a complete MEMS mass spectrometer will be manufactured and measured. It is possible that it will have lower resolution and sensitivity than the conventional spectrometers, but for sure it will be more versatile, portable, cheaper and will have wide range of applications. Maybe, it will not be capable of separating all the ions in an unknown atmosphere, but for a detection of certain gases, for example in automotive, chemical or space industry, it should give satisfactory results. Acknowledgements This work was supported by the Polish National Center for Research and Development, project no POL-SINIV/2/2018. References [1]
S. Vigne, T. Alava, H. Videlier, R. Mahieu, C.-M. Tassetti, L. Duraffourg, F. Progent, Gas analysis using a MEMS linear time-of-flight mass spectrometer, Int. J. Mass Spectrom. (2017). doi:10.1016/j.ijms.2017.03.011.
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[10] Z. Gan, D. Huang, X. Wang, D. Lin, S. Liu, Getter free vacuum packaging for MEMS, Sensors Actuators A Phys. 149 (2009) 159–164. doi:10.1016/J.SNA.2008.10.014. [11] T. Rogers, Considerations of anodic bonding for capacitive type silicon/glass sensor fabrication, J. Micromechanics Microengineering. 2 (1992) 164–166. doi:10.1088/0960-1317/2/3/008. [12] H. Henmi, S. Shoji, Y. Shoji, K. Yoshimi, M. Esashi, Vacuum packaging for microsensors by glass-silicon anodic bonding, Sensors Actuators A Phys. 43 (1994) 243–248. doi:10.1016/0924-4247(94)80003-0. [13] B. Wolf, Handbook of ion sources, CRC Press, 1995. https://www.crcpress.com/Handbook-of-Ion-Sources/Wolf/p/book/9780849325021 (accessed July 28, 2017). [14] I.G. Brown, John Wiley & Sons., The physics and technology of ion sources, WileyVCH, 2004. https://books.google.pl/books?hl=pl&lr=&id=jOlKdGPBpD0C&oi=fnd&pg=PR7&dq =ion+sources&ots=ReFnpNz3hj&sig=ih5CAgPCrFuhXZjgST5Bc2Raow4&redir_esc= y#v=onepage&q=ion sources&f=false (accessed January 31, 2019). [15] G. Petzold, P. Siebert, J. Mulle, J.M. Muller¨, A micromachined electron beam ion source, 2000. www.elsevier.nlrlocatersensorb (accessed May 24, 2018). [16] C.A. Bower, K.H. Gilchrist, J.R. Piascik, B.R. Stoner, S. Natarajan, C.B. Parker, S.D. Wolter, J.T. Glass, On-chip electron-impact ion source using carbon nanotube field emitters, Appl. Phys. Lett. 90 (2007) 124102. doi:10.1063/1.2715457. [17] E.J. Radauscher, C.B. Parker, K.H. Gilchrist, S. Di Dona, Z.E. Russell, S.D. Hall, J.B. Carlson, S. Grego, S.J. Edwards, R.P. Sperline, M.B. Denton, B.R. Stoner, J.T. Glass, J.J. Amsden, A miniature electron ionization source fabricated using microelectromechanical systems (MEMS) with integrated carbon nanotube (CNT) field emission cathodes and low-temperature co-fired ceramics (LTCC), Int. J. Mass Spectrom. 422 (2017) 162–169. doi:10.1016/J.IJMS.2016.10.021. [18] L.F. Velaásquez-Garcia, B.L.P. Gassend, A.I. Akinwande, CNT-based MEMS/NEMS gas ionizers for portable mass spectrometry applications, J. Microelectromechanical Syst. 19 (2010) 484–493. doi:10.1109/JMEMS.2010.2045639. [19] H. Li, H. Yun, X. Du, C. Guo, R. Zeng, Y. Jiang, Z. Chen, Design, Fabrication and Mass-spectrometric Studies of a Micro Ion Source for High-Field Asymmetric Waveform Ion Mobility Spectrometry, Micromachines. 10 (2019) 286. doi:10.3390/mi10050286. [20] L. Gao, Q. Song, R.J. Noll, J. Duncan, R.G. Cooks, Z. Ouyang, Glow discharge electron impact ionization source for miniature mass spectrometers, J. Mass Spectrom. 42 (2007) 675–680. doi:10.1002/jms.1201.
Highlights - Miniature ion source dedicated for a MEMS mass spectrometer has been developed - It generates coherent, vertically oriented ion beam of several microamperes - It is technologically compatible and can be integrated on the same chip with the rest of the components of the mass spectrometer.
Highlights - Miniature ion source dedicated for a MEMS mass spectrometer has been developed - It generates coherent, vertically oriented ion beam of several microamperes - It is technologically compatible and can be integrated on the same chip with the rest of the components of the mass spectrometer.
There is no conflict of interest.
Tomasz Grzebyk, Tomasz Szmajda, Piotr Szyszka, Anna Górecka-Drzazga, Jan Dziuban PII:
S0042-207X(19)32262-6
DOI:
https://doi.org/10.1016/j.vacuum.2019.109008
Reference:
VAC 109008
To appear in:
Vacuum
Received Date: 26 June 2019 Revised Date:
11 October 2019
Accepted Date: 14 October 2019
Please cite this article as: Grzebyk T, Szmajda T, Szyszka P, Górecka-Drzazga A, Dziuban J, Glowdischarge ion source for mems mass spectrometer, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109008. 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. © 2019 Elsevier Ltd. All rights reserved.
GLOW-DISCHARGE ION SOURCE FOR MEMS MASS SPECTROMETER
Tomasz Grzebyk1, Tomasz Szmajda, Piotr Szyszka, Anna Górecka-Drzazga, Jan Dziuban
Wroclaw University of Science and Technology, Faculty of Microsystem Electronics and Photonics, Department of Microengineering and Photovoltaics, 11/17 Janiszewskiego St., 50-372 Wroclaw, Poland
Abstract This work describes construction, technology, working principle and properties of an ion source dedicated to a miniature MEMS (Micro-Electro-Mechanical System) mass spectrometer. Conditions allowing efficient ionization of a gas sample, and obtaining a properly directed ion beam were determined. An influence of parameters such as shape, dimension and distance between the electrodes, as well as applied magnetic and electric field and pressure level, on the operation of this device were investigated. The experiment allowed choosing the optimal parameters, thus it may be concluded that the ion source will satisfy all the requirements necessary for the MEMS mass spectrometer. 1. Introduction Mass spectrometers are analytical instruments which allow for the identification of a gas sample composition. First, the examined gas sample introduced into the spectrometer is ionized and later individual gas species are separated according to their mass to charge ratio. Usually, these processes take place in high vacuum conditions, therefore mass spectrometer must be continuously connected to a pumping system, which makes the whole device bulky, expensive and stationary. A strong research trend towards miniaturization of mass spectrometers, often using MEMS technology [1–5], can be noticed. However, in most cases it applies only to its certain parts, mostly ion sources, analyzers or ion traps. Seldom, few components are integrated on the same chip. And even then, vacuum is generated by an external, conventional pumping systems. Thus, the complete instrument is still, in best case, suitcase-size [6–8]. Recently, our research group demonstrated the first silicon-glass MEMS-type ionsorption vacuum pump, which enables generating high vacuum (10−7 mbar) on a chip level [9]. This invention opened the possibility of creating more complex chip-scale high-vacuum analytical instruments, including mass microspectrometer. Currently, our goal is to integrate 1
Corresponding author: tel. +48 71 320 49 74 E-mail address: [email protected]
all the components of a spectrometer on a single chip: a sample injection system, an ion source, a mass analyzer and the high vacuum micropump. These all need to be produced in a consistent technology and co-work properly. Therefore, we have designed a completely new, uniform construction of a MEMS spectrometer. 2. Design of a MEMS mass spectrometer According to our concept, the miniature, on-chip integrated spectrometer, consists of two major blocks – an ion column including: sample injector, ion source, drift zone, collector; and the above mentioned ion-sorption micropump, both connected by a microchannel etched in one of the glass wafers (Fig. 1). Electrodes are made of monocrystalline silicon and are insulated with borosilicate glass plates. All elements are connected together using anodic bonding. This process ensures protection from unwanted gas leakages and when performed in vacuum conditions, it allows obtaining initial vacuum necessary to turn on the micropump [10–12]. The ion-sorption micropump enables further reduction of pressure down to high vacuum (<10−5 mbar). A gas sample is introduced by a nanochannel made in the silicon wafer at the bottom of the ion column, until the pressure reaches a desired level of about 10−2 – 10−4 mbar. At that moment gas particles are ionized inside the ion source and formed as a beam that is attracted to the collector placed a few centimeters above it. The mass spectrum is obtained by recording a collector ion current after applying an impulse voltage to one of the electrodes (so called Time-of-Flight method).
Fig. 1. Schematic diagram of the miniature MEMS mass spectrometer: cross section of the complete device (left) and the ion source (right). 3. MEMS glow-discharge ion source The ion source is one of the most important parts of a mass spectrometer. It should efficiently ionize gas particles, but at the same time it should also form a vertically oriented ion beam, which is later separated on the way to the collector. There are number of ion sources presented in literature [13,14]. However, it is hard to find the one which fulfills requirements necessary for the proposed mass spectrometer: operation on the aforementioned
pressure level, stable and efficient ionization and consistent technology. Commonly used electron impact ion sources based on field emitters suffer from poor stability and degradation when working in medium vacuum [15–18]. Glow-discharge ion sources are free from these drawbacks [19,20], however it is not possible to apply any of the already existing ones, because its construction has to be compatible with the rest of the spectrometer. The ion source presented in this paper consists of three electrodes placed one above another and a permanent magnet located below them. There is a larger through-hole in the central electrode (anode) and a smaller one in the top electrode (anti-cathode). These holes can be either circular (obtained by deep reactive ion etching) or square (formed by an anisotropic silicon etching). In the presence of the magnetic field this set of electrodes forms an electron trap (Fig. 2).
Fig. 2. Distribution of the electric (left) and magnetic field (right) inside the ion source. The gas molecules are ionized in a glow discharge which is initiated by spontaneous electrons. No other electron source is needed, neither thermal nor cold, and this improves the robustness of the device and increases its lifetime. In this trap paths of the electrons are elongated (Fig. 3 left), electric and magnetic field distribution prevents them from reaching the electrodes. Therefore, they finally collide with gas molecules and create ions and additional electrons, what leads to a maintenance of a glow discharge. This process occurs even in high vacuum, because of the very long path of electrons.
Fig. 3. Simulations of electrons (left) and ions (right) trajectories in the MEMS glowdischarge ion source (COMSOL software).
The most preferred conditions for ionization of gas particles are ensured when the trap is symmetric. However, to create a consistent, focused beam of ions (Fig. 3 right), the full symmetry has to be disrupted. First of all, a via-hole needs to be formed in the anti-cathode to allow the ions to leave the trap. Secondly, the distances between ion source electrodes and their potentials should be precisely selected to direct them in vertical direction and prevent from spreading. 4. Experiment In the experimental stage, the authors focused on determining the optimal conditions and constructional parameters to ensure both: efficient gas ionization and formation of a vertically oriented ion beam. During investigation, a specially designed 3D printed holder was utilized (Fig. 4). It allowed a simple replacement of electrodes differing in shapes (rectangular and circular) and dimensions of the anode and anticathode through-holes. Three different combinations of hole size were examined: ϕA/ϕAC = 3/1; 2/1 and 3/2 mm. The electrodes could be mounted on one of several “shelves” separated from each other by 0.55 mm. In all the configurations the cathode was placed on shelf no. 1. In configuration I – the anode was placed on shelf no. 2, the anti-cathode on shelf no. 3; in configuration II – it was shelf no. 2 and shelf no. 4, respectively; in configuration III – shelf no. 2 and shelf no. 5; and in configuration IV – shelf no. 3 and shelf no. 5. The measurements were performed inside a reference vacuum chamber. Electronically controlled valve allowed to introduce a desired amount of atmospheric gases and precisely adjust the internal pressure. Although the 3D printed holder was made of a polymer (VisiJet M3 Crystal2), residual gas analyzer mounted in the vacuum set-up did not show any serious outgassing. During the measurements the cathode was kept at the ground potential, a positive voltage was applied to the anode and a small negative potential was applied to the anticathode to attract the positive ions.
5 4 3 2 1
Fig. 4. The test structure of a glow-discharge ion source placed in a special 3D printed holder
2
https://www.3dsystems.com/sites/default/files/2018-11/3d-systems-proJet-mjp-3600-plastic-tech-specs-a4-us2018-11-08-web.pdf
In the first experiment an influence of shape and size of the through-holes etched in the anode on the properties of ion source was investigated. Although it is possible to obtain a discharge for the structures with square holes, the process is unstable and the ion beam splits into four directions after passing the hole in the anti-cathode. The structures with circular holes exhibit much better properties, the discharge is stable and the ion beam is consistent. The size of the holes and their spacing have also a big impact on the obtained results. More important is the diameter of the anode hole then the anti-cathode hole. In case it is 2 mm wide, discharge can be obtained only for configurations II and III and the current values measured at the anode and at the anti-cathode hardly depend on the anode voltage (Fig. 5a). In case the anode hole is 3 mm wide, it is possible to obtain a discharge in all the examined configurations, both for 1 and 2 mm wide anti-cathode hole. This time the current strongly depends on the anti-cathode voltage (Fig. 5b). Results change in different conditions (pressure, anode voltage), but usually up to 200–300 V the currents increase, and above this level they slightly decrease, stabilize or in some cases increase further (Fig. 6).
a) b) Fig. 5. The anode and anti-cathode current as a function of anti-cathode voltage: a) for the structure with anode hole diameter ϕA = 2 mm, and anti-cathode ϕAC = 1 mm, configuration II, b) ϕA = 3 mm, ϕAC = 2 mm; UA = 1200 V, p = 10−3 mbar, configuration IV.
Fig. 6. Anti-cathode currents as a function of anti-cathode voltage for different electrode configurations, UA = 1200 V, p = 10−3 mbar.
The other important parameter affecting an operation of the ion source is the strength of the magnetic field. It is important that its value in the middle of the ion source excides 0.25 T. For the weaker field it is impossible to maintain a discharge below 0.01 mbar. Such a field can be ensured even with usage of a small neodymium magnets (ϕ = 8 mm; h = 6 mm), which induction just above the surface is equal to 0.5 T. Further increase of the field does not increase the discharge current significantly (< 20%) and it requires a use of much bigger magnets. After investigating geometrical and structural parameters, the proper operation conditions were established. The most preferable pressure for stable ionization of atmospheric gases is in a range between 10−4 and 10−3 mbar. Below that range the anode voltage needs to be increased above 1500 V and ion/electron currents are relatively small (about a few microamperes). In lower vacuum – the discharge is often noticed near the edges of the electrodes, not in the middle; moreover high pressure is also not preferred for the operation of the spectrometer itself – after obtaining a mass spectrum, the entire volume must be evacuated to high vacuum before the next measurement, and the capacity of the micropump is limited. In the mentioned pressure range the voltage applied to the anode should usually be equal to at least 800 V, preferably 1000−1200 V. Always, with increasing pressure and increasing anode voltage, the anode and anti-cathode currents rise. Basing on the fig. 5 and 6 one can state that in case of 2 mm hole in the anode, the anti-cathode voltage can be chosen arbitrarily and in case of 3 mm hole it should be kept on small negative potential (about 200 V). In these conditions the anti-cathode current is the highest. Moreover, this potential helps to attract the ions towards the upper electrode and in the same time prevents from spreading of the ion beam after passing the anti-cathode, as it happens in case of too high negative potential. The observation of the ion beam shape (Fig. 7) and measurement of the ion current value enabled to choose the optimal conditions for the ion source operation. They were obtained in configuration II, with ϕA = 3 mm, and ϕAC = 2 mm at p = 10−3 mbar; UA = 1200 V and UAC = −200 V.
a)
b)
Fig. 7. Image of the working device: a) vertical beam of ions expands above the structure, electrode voltages: UA = 1200 V, UAC = -200 V, b) ions spread after passing the anti-cathode, electrode voltages: UA = 1200 V, UAC = -500 V; p = 10−3 mbar, spectral analysis indicates the presence mostly of nitrogen ions in the plasma.
After choosing the proper parameters, the forth electrode (the collector) was placed 2 cm above the ion source on a separate holder. It was investigated what part of ions can leave through the hole in the anti-cathode – only these ions give a signal to the mass spectrum. The anode potential was set to 1200 V, and anti-cathode to −200 V. Collector voltage was changed between 0 and −600 V, and the anti-cathode and collector currents were measured at the same time (Fig. 8).
Fig. 8. Anti-cathode and collector currents measured in the function of a negative collector voltage, UA = 1200 V, UAC = −200 V, p = 10−3 mbar. It appeared, that both currents changed with increasing collector voltage. For UCOL = 0 V, 10% of the ions reaching the anti-cathode passed to the collector, for –500 V the ratio exceeded 25%. This increase might be an effect of a change in potential distribution, but also it could be caused by a higher secondary electron emission. Nevertheless, a collector current at the level above 10 µA should be sufficient to detect a mass spectra. In the next step, to obtain mass spectra, DC voltage should be replaced with a very short impulse voltage applied to the collector or to the anti-cathode (rising time below 100 ns) and the ion current will be measured in the function of time. Up to now, computer simulations have been performed to examine this issue (Fig. 9, 10). It was assumed that ions are created in the cylindrical volume between the cathode and anticathode (φ = 3 mm). The proportion of different gases was the same as in the atmosphere (total number of ions – 1 000 000). Potentials at the electrodes of the ion source were equal to UC = 0 V, UA = 1200 V, UAC = −200 V, and an impulse of -500 V was applied to the collector. The influence of the presence of plasma and associated space charge, as well as secondary emission was omitted. The potential distribution and the trajectories of the ions were the same as in the absolute vacuum.
Fig. 9. Separation of ions having different atomic masses (marked on scale with different colors) in a MEMS mass spectrometer (COMSOL Multiphysics simulations), starting area – cylinder between the cathode and anti-cathode, ϕ = 3 mm, starting energy – 0 eV.
Fig. 10. Simulation result of mass spectrum of atmospheric gases. Since different ions have different masses, the time necessary to reach the collector varies – lighter ions (as N+) need shorter times (0.22 µs) and heavier (as carbon dioxide) need longer times (0.48 µs). One can notice that the peaks representing most dominant gases can be easily separated from each other. However, peaks coming from molecules that are not so numerous in the atmosphere are far less visible. It is believed that the resolution could be improved, when a space charge was taking into account. The energy distribution of the extracted particles should be in that case narrower. 5. Conclusion The article presents the design and parameters of the glow-discharge miniature ionsource. This device consists of set of 3 electrodes and a permanent magnet, it does not require any fragile electron source – gas particles are ionized in a glow discharge obtained in a properly formed electric and magnetic field. A stabile, well oriented and focused ion beam leaves the MEMS ion source and directs towards the collector. This source fulfils all the
requirements necessary to be applied in the on-chip integrated MEMS mass spectrometer. Application of an impulse voltage on the collector electrode should allow to separate the ions of individual gases and to obtain a mass spectrum. In the near future, the test structure of a complete MEMS mass spectrometer will be manufactured and measured. It is possible that it will have lower resolution and sensitivity than the conventional spectrometers, but for sure it will be more versatile, portable, cheaper and will have wide range of applications. Maybe, it will not be capable of separating all the ions in an unknown atmosphere, but for a detection of certain gases, for example in automotive, chemical or space industry, it should give satisfactory results. Acknowledgements This work was supported by the Polish National Center for Research and Development, project no POL-SINIV/2/2018. References [1]
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Highlights - Miniature ion source dedicated for a MEMS mass spectrometer has been developed - It generates coherent, vertically oriented ion beam of several microamperes - It is technologically compatible and can be integrated on the same chip with the rest of the components of the mass spectrometer.
Highlights - Miniature ion source dedicated for a MEMS mass spectrometer has been developed - It generates coherent, vertically oriented ion beam of several microamperes - It is technologically compatible and can be integrated on the same chip with the rest of the components of the mass spectrometer.
There is no conflict of interest.