Simple fabrication of micro time-of-flight mass spectrometer using a carbon nanotube ionizer

Sensors and Actuators B 243 (2017) 394–402

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Simple fabrication of micro time-of-flight mass spectrometer using a carbon nanotube ionizer Ki Jung Lee a , Nguyen Tuan Hong b , Soonil Lee c , Dong-Wook You d , Kwang-Woo Jung d , Sang Sik Yang a,∗ a

Department of Electrical and Computer Engineering, Ajou University, Suwon, Gyeonggi 443-749, Republic of Korea Center for High Technology Development (HTD), 18 Hoang Quoc Viet St., Cau Giay Dist., Ha Noi 122-121, Vietnam c Division of Energy Systems Research, Ajou University, Suwon, Gyeonggi 443-749, Republic of Korea d Department of Chemistry, Wonkwang University, Iksan, Jeonbuk 570-749, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 7 July 2016 Received in revised form 8 November 2016 Accepted 2 December 2016 Available online 5 December 2016 Keywords: Carbon nanotube-based ionizer ␮-TOFMS Finite element simulation

a b s t r a c t This paper demonstrates a time-of-flight mass spectrometer (TOFMS) fabricated using simple micro electromechanical system technologies. It consists of two components: a triode-type field emitter for electron-impact ionization and ion separator with a repeller, an acceleration electrode, and a flight tube. As a cold-cathode, the field emitter with carbon nanotube is a promising electron source and offers several advantages over conventional thermionic electron sources, such as compact size, low power consumption because of the absence of heating elements, excellent durability, and high electron emission density. Micromachined TOFMS successfully detects signals of Ar and CH3 I. This is practically proved with preliminary experiments on ionization and mass spectrum tests with Ar and CH3 I, ions, including finite element modeling and theoretical analysis. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The mass spectrometer (MS) system is a powerful analytical instrument for professionals from a wide range of fields. However, it is still bulky, expensive, requires high vacuum, and consumes high power for in situ analysis or monitoring. In the last three decades, miniaturized or micromachined MS with various mass-filtering methods such as quadrupoles [1–4], ion traps [5–9], Wien filters [10–13], and time-of-flight (TOF) [14–19] have been developed. A miniature or micromachined MS system considerably reduces these disadvantages, thus enhancing the portability and diversity of applications, e.g., environmental analysis, processing or pollutant emission monitoring, space exploration, explosives and drugs detection, etc. [20]. Carbon nanotube (CNT), known as a cold cathode, is adopted as the electron source. CNT-based field electron emitters– have been widely studied for two decades [21–23]. CNT-based field emitters are still suffering from a maximum emission current density and emission current stability compared to Spindt-type metal microtips. Nevertheless, they stay attractive due to characteristics of

∗ Corresponding author. E-mail address: [email protected] (S.S. Yang). http://dx.doi.org/10.1016/j.snb.2016.12.007 0925-4005/© 2016 Elsevier B.V. All rights reserved.

CNT emitter such as a low power consumption and simple structure. Moreover, the absence of heating elements prevents thermal cracking of delicate molecules in the residual gas [24–28]. There are several researches on a micromachined time-of-flight mass spectrometer (␮-TOFMS) with various types of carbon based field emitters such as CNT tips [29], CNT forests [30], CNT bundles [31] or pillars [32], and CNPs (Carbon Nano Particles) [33]. But they showed the performance of the carbon-based electron sources as ionizers only. A mass analyzer is the largest and most important component of MS instruments. There are several advantages offered by the TOF analyzing method over others, such as freedom from strict geometric parameters, the fastest analysis in a matter of several microseconds, and no limitation to the dynamic mass range. However, there are inherent limitations of low mass resolution and low ion intensity because of the working principle of TOF analyzers. TOFMS systems are highly dependent on electronics with fast time response, which was a significant reason for their past failure, and they demand ion sources with low duty cycle pulses, inducing low intensities of ions [34–37]. Thus, improving electronic systems in order to enable them to process data within nanoand picoseconds and enhancing their digital storage capacities will aid the development of TOFMS. The demand for ion sources with low duty cycle pulses has been met by deploying the orthogonal

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acceleration structure attempted by O’Halloran et al. and reexamined by the groups of Guilhaus and Dodonov, which is more suitable to continuous ion sources compared with the pulsed ionization methods such as matrix-assisted laser desorption/ionization (MALDI) [38–42]. Therefore, to fulfill these requirements, the ␮TOFMS with triode-type field emitter aligned orthogonally with the plane of the mass analyzer is proposed. This paper demonstrates the ␮-TOFMS with a CNT-based triodetype field emitter as an ionizer. In [43], it is successfully used to obtain the mass spectrum of air, but there are some issues such as the requirement of high voltage to extract electrons, which induces high fragmentations during ionization, and low fabrication yield of the mass analyzer resulting from bonding problems between substrates. The proposed ␮-TOFMS is fabricated by simple, reliable, and reproducible techniques such as silicon wet etching and metal deposition via shadow masking. The field emission and ionization characteristics of the triode-type field emitter with CNT are examined. The microchannel plate (MCP) is coupled to the ␮-TOFMS for ion detection.

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has three aluminum electrodes expected to repel, accelerate, and drift ions. A two stage acceleration method has been proposed to enhance the performance of TOFMS and well-defined by Wiley and McLaren [34]. With Newtonian physics, calculations in the mass filtering mechanics are done easily as shown by the equations that follow. After ionization, ions are accelerated by two acceleration region, s and d with electric field, Es and Ed , respectively. Then, the total energy of ion, U, could be described as follows: U = Us + Ud = qsEs + qdEd

(1)

which implies the initial energy of ions is zero and ions position at the half of the ionization region, q is the ion charge, respectively. Then, the total flight time of ions, Tt , can be shown to be Tt =



2sm/qEs +



1 1 (2m)1/2 (Us + Ud ) 2 − (Us ) 2 qEd

  m  12 +

2U

D.

(2)

where, D is the length of the drift region. 3. Fabrication

2. Design and theoretical analysis The schematic structure and operating principle of the ␮-TOFMS are shown in Fig. 1(a) [43]. It consists of the ionization source, described in [44], and the mass analyzer, which has five pairs of two stage acceleration regions and a drift region. Design parameters of the analyzer layer as shown in Fig. 1(b) and (c). The mass analyzer

The process used to fabricate the extraction-gate layer with a nickel mesh grid is almost the same as that described in [44], except that the holes for electrical connections are fabricated simultaneously with the Si supporting frames. As illustrated in Fig. 2, the top and the bottom layers of the mass analyzer are fabricated simultaneously in one wafer because the same steps are involved in the fabrication of both layers. A mask for the Si wet-etching

Fig. 1. Schematics of (a) a cross sectional view and design parameter values (in mm) of the ␮-TOFMS; (b) the bottom substrate and (c) the top substrate of the analyzer layer.

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Fig. 2. Fabrication process of the mass analyzer, composed of bottom and top layers, along A–A .

process is patterned on one side of the Si substrate. After the Si wet etching process, the substrate has 150-␮m-deep cavities. Then, the chemical-mechanical planarization (CMP) process is used to smooth the other side and make the 400-␮m-thick Si wafer with 150-␮m-deep cavities. An SiO2 layer is thermally grown again and patterned on the other side of the plane with cavities. With this pattern, the second Si wet etching is carried out until there are sufficient holes for an electron beam path, electric connections, and assembly. Then, the last oxidation process grows a 1-␮m-thick SiO2 film for electric insulation. After that, 1-␮m-thick aluminum electrode patterns are deposited by the evaporation process with a shadow mask. Silicon spacers 280-␮m-thick with 1-␮m-thick SiO2 films are fabricated by using the Si wet etching, CMP, and oxidation processes to act as electrical insulation between layers. Fig. 3 shows optical images of the fabricated layers. Each layer has four 3 × 3 mm2 via holes for assembly, four 1 × 1 mm2 via holes for alignment in silicon spacers (Fig. 3(a)), and two layers of the mass analyzer (Fig. 3(c) and (d)). The extraction-gate (Fig. 3(b)) and top layers of the mass analyzer (Fig. 3(c)) have one and three via holes, respectively, for electrical connections.

in Fig. 4(e)–(g). It is expected that these results induce ions to travel toward the center of the cross section in the xz-plane. Ion trajectories are simulated using Newtonian physics. Ions are initially positioned on the y-axis at the center of the first acceleration region with an area of 3.04 × 0.4 mm2 . The released ions travel according to the electrostatic potential distributions as shown in Fig. 5. Fig. 5(a) shows the Ar+ ion trajectory with a repelling voltage of 100 V and ions injected every 50 ns for a period of 500 ns. Ion beams become narrower along the x- and z-axis as the ions travel. This is because of the curvature of the isopotential map due to the asymmetric electrostatic potentials. The inset of Fig. 5(a) predicts that most Ar+ becomes extinct after colliding with the flight tube during the trip to the detector, after which the detector will show low ion transparency. The trajectory of CH3 I+ ions with a repelling voltage of 150 V and ions injected every 50 ns for a period of 600 ns is depicted in Fig. 5(b). The ion transparency of CH3 I+ ions is slightly higher than that of Ar+ ions since CH3 I+ ion has a higher mass and a lower velocity than Ar+ ion. Simulation results imply that a small amount of ions reaches the detector, and the detector signal is small because of the low ion transparency of the detector induced by the curvature of the electrostatic field distribution.

4. Ion trajectory simulation 5. Experiments A finite element (FE) simulation is carried out using COMSOL and the proposed analyzer geometry to investigate several properties of the ␮-TOFMS design, including electric potential distribution and ion trajectories, as depicted in Figs. 4 and 5. Simulation is operated in 3D mode with approximately 1.3 million elements. The geometry for the simulations is simplified to have one pair of the basic layout and avoid huge amounts of data caused by structural duplication. It is supposed the detector to be closed with the end of the flight tube 2 mm apart. It is assumed that a specific voltage is applied to the repeller, and that the acceleration electrode and flight tube are grounded for examining the electric potential distribution. Fig. 4 shows the electrostatic potential distribution and accompanying normal vectors with a repelling voltage of 100 V. It shows that a linear relationship exists between the electric potential and the distance from the center of the repelling electrode. On the other hand, the curvatures are identified along the edges of the electrodes, resulting in curved normal vector distributions as depicted

As shown in Fig. 6(a), the ion source and mass analyzer are assembled by employing the jig set and 8 mm parallel pins with −10 ␮m tolerance for aligning the layers. Fig. 6(b) shows the assembled test jig mounted on the chamber plate as well as the connections between the ␮-TOFMS and electric ports. However, details of the CNT ionizer designs, operation, and characteristics can be found in [44], thus there are only brief descriptions of the ionizer here. 5.1. Ionization test An examination of the ionization efficiency is carried out with electronic instruments with Ar gas at a pressure of approximately 4 × 10−6 Torr. The extraction gate (Ni mesh grid) is grounded, and the cathode is connected to a high voltage power supply (Stanford research systems PS300) for negative biasing. The approximate

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Fig. 3. Optical images of fabricated layers: (a) silicon spacer, (b) extraction-gate layer, (c) top, and (d) bottom layers of the mass analyzer.

value of field emission current can be measured with a PS300 connected to a cathode. The repeller used as ion collector is biased at −40 V to attract positive ions and repel electrons. The experiment parameters are described in Table 1. The emission current, ion current, and pressure are measured by varying the reverse bias voltage of the cathode with respect to the extraction-gate electrode from −250 to −350 V in steps of −25 V. As the bias voltage increases, the ion current also increases, and the maximum value of ion current produced is approximately 31.5 nA when the bias voltage and pressure are −350 V and 4.6 × 10−6 Torr, respectively, as described in Fig. 7(a). The number of electrons emitted from the cathode and ions collected from the repeller can be estimated from the measured emission and ion currents, respectively. Fig. 7(b) shows the ionization efficiency, or the ratio of number of ions formed to the number of electrons used. The ionization efficiency corresponding to the maximum cathode bias voltage of −350 V is close to 1‰. This shows that the triode-type field emitter is comparable with a conventional ionization source, and generally produces one ion for 1000 electrons introduced to the ionization region [45]. 5.2. Mass spectrum test A pulse generator (Stanford research systems DG535) and PS300 are linked to a high voltage pulse generator (IXYS Colorado GRX3K-H) for periodic repelling pulse generation. The other PS300 is connected to the MCP, placed about 3 mm away from the test jig for ion detection, and the detector signal is recorded on the

Table 1 Experimental conditions for the ionization test. Experiments

Parameter

Value

Ionization test

Chamber Pressure Cathode voltage Extraction-gate voltage Repelling voltage (electrons)

4.0 × 10−6 to 4.7 × 10−6 Torr −350 to −250 V 0V −40 V

oscilloscope (Lecroy Wavesurfer 422) after a pulse voltage is applied to the repeller. Signal averaging technique is adopted as a method to increase the relative strength of ion detection signals to noise. Ion detection signals are recorded as average values over 1000 consecutive pulses at a repetition rate of 10 Hz. The performance of the ␮-TOFMS is estimated with Ar and CH3 I, which have masses of 39.948 amu and 141.94 amu, respectively, and the experiment parameters and values are listed in Table 2. Ar ions are injected into the vacuum chamber through a needle valve, and the pressure of the chamber is maintained under 9 × 10−5 Torr. In Fig. 8(a), ion signals corresponding to the presence of Ar+ ions are shown approximately 2 ␮s after the rising edge of the repelling pulse. Consequently, it is difficult to characterize the performance of the ␮-TOFMS because it is difficult to identify the arrival time at the detector because the magnitudes of ion signals are too low. Additionally, there is no observable ion signal in experiments of mass spectrum with a two stage acceleration due to some issues like an electrical insulation of the mass analyzer layer and too low ion signals of detector. Therefore, followings are dealing with an experimental results of the device with one stage acceleration. Fig. 8(b) shows the mass spectrum of CH3 I at a repelling voltage of 150 V. Ion signals of CH3 I are shown more clearly compared with those of Ar because of its large mass, resulting in a time lag in ion detection. In Fig. 8, they are observed the saturated MCP signals in all the conditions. Those signals are noise resulting from an imperfect electrical isolation in the test setup including the device and

Table 2 Experimental conditions for the mass spectrum test. Component

Parameter

Value

Ion source

Chamber Pressure Cathode voltage Gate voltage Repelling voltage Acceleration/Flight tube voltage Pulse duration of the repelling MCP bias voltage

∼ 10−5 Torr ∼ −350 V 0V ∼ 270 V 0V 500 (Ar)/600 (CH3 I) ns −2200 V

Electron source Separator MCP detector

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Fig. 4. Simulation results of the electric potential distribution (a–d) and the normal vectors to the isosurfaces of electrostatic potentials (e–g) with the repelling voltage of 100 V when the acceleration electrode and flight tube are grounded; (a) results in full scale model, (b) magnification of the ionization region, and equipotential lines (c) in xy-plane, (d) in yz-plane, (e) results of the normal vectors in the ionization region, and (f) top view and (g) side view in xy-plane and yz-plane, respectively.

instruments, and they are observed in both situations whether gas sample introduced or not. The mass spectrum test is repeated with CH3 I by maintaining the repeller voltage in the range from 100 to 270 V. In Fig. 9, as the repelling voltage increases, ion signals are detected faster and show

sharper peaks. According to the theoretical analysis by Wiley and McLaren, the flight time of TOFMS, Tt , with one acceleration region can be written as Tt =



m/qEs



√  2s0 + D/ 2s

(3)

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Fig. 5. Ion trajectory simulation results for (a) Ar+ with repelling voltage of 100 V (inset shows Ar+ ion trajectory without preserving the aspect ratio of the geometry) and (b) CH3 I+ ions with repelling voltage of 150 V.

Fig. 6. Photographs of (a) assembled test jig and (b) test jig on the chamber plate.

Fig. 7. (a) Measured emission (

), ion currents (

) and (b) plot of the ratio of ion current to emission current versus cathode bias voltage at pressure (in Torr).

where s0 , m, and D are the half of the length of the ionization region, ion mass (in kg), and drift distance, respectively. Fig. 9 shows the measured values from mass spectrums and simulation results of CH3 I ions, and it is observed that there are differences between the travel times of the measurement and simulated results. The arrival time of ions at the detector is governed by several factors, such as the mass, applied electric field in the acceleration region and its distance, and length of the flight tube as described in Eq. (3). For the analysis of the ␮-TOFMS, it is assumed that the repelling pulse is ideal and instantly transitions from the minimum to maximum, and the travel length in the ionization region, denoted by s, is half of the length of the ionization region. Fig. 10(a) shows the measured repelling voltage versus time when a voltage of 150 V and period 600 ns is applied to the repeller. Even though a repelling pulse with a rise time of several nanoseconds is typically

applied by the commercial pulse generator, it shows the repelling signal as an exponential function of time with a time constant of hundreds of nanoseconds as depicted in Fig. 10(b), resulting from imperfect electric insulation of the device. Moreover, the measured repelling voltage is 90% of the applied voltage. The deviation of the initial positions of ions, caused by the open area with longitudinal distance of 400 ␮m for the electron path in the ionization region, induces the distance distribution of s. It is expected that the time-varying repelling pulse and spatial distribution of ions cause variations in the arrival time and velocity of ions. The effects of the shape of the repelling pulse signal and initial space distribution of ions The repelling signal can be  are evaluated.  expressed as V (t) = Vr 1 − ae−bt where Vr , is the repelling voltage and a and b are constants that depend on the pulse shape. In the ionization region, ions generated at time T0 exit the ionization

400

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region after time Ts . They enter the flight tube with the velocity vs , which can be written as

Ts vs =

Ts adt =

T0

T0

qE dt = m

Ts

  q Vr 1 − ae−bt dt s0 m

(4)

T0

The travel distance ls in the ionization region is given as

Ts Ts ls =

  q Vr 1 − ae−bt dtdt s0 m

(5)

T0 T0

The equations described above can be solved by the Newton–Raphson method to obtain the values of Ts and vs , and the drift time in the flight tube can also be calculated by using the value of vs . It is supposed that the values of ls and T0 range from 400 to 800 ␮m in steps of 40 ␮m and 100–550 ns with a 50 ns step, respectively. Fig. 10(c) shows histograms of the calculated arrival time considering the time dependent repelling signal and initial space distribution of ions with the measured spectrum of CH3 I when the repelling voltage is 150 V. The sum of the histograms of two mass has a shape and time of arrival at detector similar to those observed in experimental results. It is also explained that the time spread of the mass peak is wide because of the shape of the repelling pulse and initial position distribution of ions.

6. Conclusions

Fig. 8. Time dependent voltage signals of the MCP with the micro TOFMS (a) with Ar gas and (b) CH3 I gas inserted.

It is demonstrated the ␮-TOFMS with the CNT-based triodeype field emitter. A proof of the device is obtained through the experimental results, theoretical analysis, and FEM simulations. Moreover, the proposed design allows simple fabrication and easy integration of each component at low cost. The ionization test results show that the triode field emitter using CNT films, which has a high ionization efficiency of nearly 1‰ at 4.6 × 10−6 Torr, can compete with conventional ionization sources. It successfully detects ions of Ar and CH3 I signals, but it did not show a typical mass spectrum consisting of strong peaks, especially while working with Ar. The main issues affecting the ion arrival time at the detector are the low level of ion detection signals in spite of sufficient ion generation and electrical insulation property revealed by the repelling pulse shape. They degrade the performance of the ␮-TOFMS because the sensitivity and resolution is reduced. To improve its performance, the following improvements are required: (a) In order to increase the ion detection level, it is necessary to modify the geometry of electrodes for higher ion transparency, which means that more ions reach the detector. More flat and symmetric electrostatic potential distribution, especially along the flight tube that has a relatively long distance, makes fewer ions collide with the electrodes. (b) To enhance the resolution, electrodes should be perfectly insulated. As shown in the mass spectrum test, imperfect electrical insulation of the repeller causes the repelling pulse to become time-dependent and spreads the arrival time of ions. This degrades the overall resolution and resolvable mass range.

Fig. 9. Simulated and measured arrival time of CH3 I+ ions as a function of repelling voltage.

These will help overcome the difficulties of the proposed ␮TOFMS and obtain a mass spectrum that can be used to analyze the substance both qualitatively and quantitatively.

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Fig. 10. (a) Measured repelling voltage of 150 V, (b) Curve fit, and (c) Calculated arrival time of ions with ion masses of 127 amu and 142 amu with the time dependent repelling signal.

Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2008-0061182).

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Biographies Ki Jung Lee is a researcher in Korea Advanced Nano Fab Center (KANC). He received the M.S. and Ph.D. degrees in electrical engineering from Ajou University, Suwon, Korea, in 2009 and 2016, respectively. His graduate research interests include SAW sensors and micro TOF mass spectrometry. Nguyen Tuan Hong is researcher in center for High technology development (HTD), Vietnam academy of science and tech. He obtained Ph.D. degree in applied physics from Ajou University, Suwon, Korea, in 2010. His current research interests include space instrumentation, carbon-based nanomaterials, and nanocomposites. Soonil Lee is a professor in Department of Physics, Ajou University, Suwon, Korea. He obtained PhD degree in Physics from Michigan State University, USA, in 1989. His current research interests include nanomaterials and devices, plastic optoelectronics (solar cells, LEDs, flexible transparent electrodes), and CNT-based field electron emitters. Dong-Wook You received the M.S. degree in chemistry from the Wonkwang University, Korea, in 2013, where he is currently pursuing the Ph.D. degree in chemistry. His current research interests are synthesis of CNT-nanocomposites and chemical analysis. Kwang-Woo Jung is a Professor in the Department of Chemistry, Wonkwang University, Korea. He received his Ph.D. degree in chemistry from the Korea Advanced Institute of Science and Technology in 1992. His current research interests include MEMS based chemical sensors, synthesis of CNT-nanocomposites, and chemical instrumentation. Sang Sik Yang has been a professor in the Department of Electronics Engineering at Ajou University since 1989. In 1988, he received his Ph.D. degree in mechanical engineering from the University of California, Berkeley. He was then a research assistant professor at New Jersey Institute of Technology. His research interests include the mechanism and actuation of microelectromechanical devices, SAW sensors and micro plasma devices.