An Integrated Microfabricated Chip with Double Functions as ... - MDPI

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An Integrated Microfabricated Chip with Double Functions as an Ion Source and Air Pump Based on LIGA Technology Hua Li 1,2, *, Linxiu Jiang 1 , Chaoqun Guo 1 , Jianmin Zhu 1 , Yongrong Jiang 1 and Zhencheng Chen 1, * 1

2

*

School of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin 541004, China; [email protected] (L.J.); [email protected] (C.G.); [email protected] (J.Z.); [email protected] (Y.J.) Guangxi Experiment Center of Information Science, Guilin 541004, China Correspondence: [email protected] (H.L.); [email protected] (Z.C.)

Academic Editor: Vittorio M. N. Passaro Received: 2 October 2016; Accepted: 22 December 2016; Published: 4 January 2017

Abstract: The injection and ionization of volatile organic compounds (VOA) by an integrated chip is experimentally analyzed in this paper. The integrated chip consists of a needle-to-cylinder electrode mounting on the Polymethyl Methacrylate (PMMA) substrate. The needle-to-cylinder electrode is designed and fabricated by Lithographie, Galvanoformung and Abformung (LIGA) technology. In this paper, the needle is connected to a negative power supply of −5 kV and used as the cathode; the cylinder electrodes are composed of two arrays of cylinders and serve as the anode. The ionic wind is produced based on corona and glow discharges of needle-to-cylinder electrodes. The experimental setup is designed to observe the properties of the needle-to-cylinder discharge and prove its functions as an ion source and air pump. In summary, the main results are as follows: (1) the ionic wind velocity produced by the chip is about 0.79 m/s at an applied voltage of −3300 V; (2) acetic acid and ammonia water can be injected through the chip, which is proved by pH test paper; and (3) the current measured by a Faraday cup is about 10 pA for acetic acid and ammonia with an applied voltage of −3185 V. The integrated chip is promising for portable analytical instruments, such as ion mobility spectrometry (IMS), field asymmetric ion mobility spectrometry (FAIMS), and mass spectrometry (MS). Keywords: needle-to-cylinder; ionic wind; ion source; air pump; LIGA

1. Introduction For an analytical instrument, the ion source is a very important component. It can ionize neutral sample molecules and transform them into positive or negative ions, which can be detected by the following detectors [1–3]. In previous studies, the ion source was usually used under the condition of vacuum, for which a large gas device was needed but was not suitable for portable instruments. In 2004, desorption electro-spray ionization (DESI) was first reported by Cooks et al., which is operated at atmospheric pressure and with little or no pretreatment of the sample [4]. After that, many studies on ambient ion sources have been successfully achieved, such as dielectric barrier desorption ionization (DBDI), direct analysis in real-time (DART), atmospheric solids analysis probe (ASAP), low temperature plasma (LTP), desorption corona beam ionization (DCBI), microwave-induced plasma desorption/ionization source (MIPDI), and so on [5–10]. In previous studies on the ambient ion source, the asymmetric electrode for the ion source has attracted much attention for its simple structure and high ionization efficiency. Kun Liu et al. adopt a line-cylinder electrode to form corona and glow discharges and ionize the sample, and the mass Sensors 2017, 17, 87; doi:10.3390/s17010087

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spectrometry (MS) and Faraday cup experiments show that the line-cylinder ion source has a high efficiency and stable propertiess [11]. Abdel Rahman et al. designed and fabricated a spherical configuration ion source, where the cathode fall is used to accelerate ions [12]. Ding et al. reported a novel ambient ionization technique for mass spectrometry. It is a microfabricated glow discharge plasma (MFGDP), where He or Ar plasma can be generated by a direct current voltage [13]. Although many attempts have been made to realize the ambient ion source, there is still an important problem that needs to be resolved. In the above-mentioned studies, nitrogen, air, or argon are employed as the carrier gas for cooling the air discharge region and carry ions through the next detecting part. The external air supply equipment needs to supply the gas, which is not appropriate for the portable instruments [14–16]. The work presented herein describes a micro-integrated system chip with integrated functions of the ion source and air pump, and the ionic wind is used to produce ionization and supply the gas. 2. Device Design and Fabrication 2.1. Design of the Integrated System In the former studies of our group, a needle-to-cylinder electrode was designed and the ionic wind was obtained. The needle is parallel to the cylinder in this structure [17–19]. However, it is not favorable to fabricate the chip by micro-electro-mechanical system (MEMS) technology. We then designed a new configuration in which the needle is vertical to the cylinder. The ionic wind induced by corona discharge is described by the Equation (1), named as Poisson’s equation, current density equation, and the Navier-Stokes equations, respectively [20]:  ρ 2   ∇ V = − ε0 J = µ E Eρ + Uρ − D ∇ρ   ρ U ·∇U = −∇ p + µ∇2 U − ρ∇V air

(1)

where V is discharge voltage between the needle and cylinder electrode, ρ is the space charge density, ε 0 is dielectric permittivity of free space, J is current density, µ E is ion mobility coefficient, E is electric field intensity, U is velocity vector of airflow, D is diffusivity coefficient of ions, ρ air is the air density, p is the air pressure, and µ is the air dynamic viscosity. According to the Equation (1), three modules named as Electrostatics (es), Incompressible Navier-Stokes (ns), PDE, and Coefficient Form (c) are set up in the COMSOL software (The COMSOL Group, Stockholm, Sweden). The other parameters are shown below. The total dimensions of the computation domain are 20 mm × 16 mm. The needle with a radius of curvature r = 50 µm and the cylinder with radius R = 2 mm are regarded as the cathode and anode, respectively. The physical parameters used in the simulation are ε 0 = 8.85 × 10−12 C/(V·m), µ E = 2.1 × 10−4 m2 /(V·s), D = 5.3 × 10−5 m2 /s, ρ air = 1.23 kg/m3 , and µ = 1.8 × 10−5 N·s/m2 . The voltage applied between the needle and cylinder is 9100 V. Four types of configuration, including single needle-to-single cylinder, single bottom needle-to-double cylinders, single central needle-to-double cylinders, and double needles-to-double cylinders are simulated. The simulation results are shown in Figure 1. The ionic wind velocity for the single needle-to-single cylinder is about 1.55 m/s in the outlet. The wind velocity in the outlet for the single bottom needle-to-double cylinders and single central needle-to-double cylinders are almost the same at 2.49 m/s. The difference between them is that the ionic wind velocity for the single central needle-to-double cylinders is more parallel and uniform. However, the smallest velocity is 1.44 m/s for the double needles-to-double cylinders. The reason is that the ionic wind produced by the two needles will disturb each other. The similar phenomenon was also observed by the former studies [21]. According to the simulation results, it shows that the single central needle-to-double cylinder is the best structure in the four types of electrodes to get the maximum ionic wind velocity under the same conditions.

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to the simulation results, it shows that the single central needle-to-double cylinder is the best to the2017, simulation it electrodes shows thattothe central needle-to-double cylinder best structure in17,the types of get single the maximum ionic wind velocity under isthethe same Sensors 87 fourresults, 3 of 14 structure in the four types of electrodes to get the maximum ionic wind velocity under the same conditions. conditions.

(a) (a)

(b) (b)

(c) (c)

(d) (d)

Figure 1. The Finite Element Method (FEM) simulation results for four different needle-to-cylinder Figure 1. The Finite Element Method (FEM) simulation results for four different needle-to-cylinder Figure 1. (a) Thesingle Finiteneedle-to-single Element Method (FEM) (b) simulation resultsneedle-to-double for four different needle-to-cylinder structures: cylinder; single bottom cylinders; (c) single structures: (a) single needle-to-single cylinder; (b) single bottom needle-to-double cylinders; (c) single structures: (a) single needle-to-single (b) single bottom cylinders. needle-to-double cylinders; (c) single central needle-to-double cylinders; (d) cylinder; double needles-to-double central needle-to-double cylinders; (d) double needles-to-double cylinders. central needle-to-double cylinders; (d) double needles-to-double cylinders.

Based on the simulation results, the experimental setup shown in Figure 2 was designed and Based the results, setup shown in 22was designed Based on thesimulation simulation results,the theexperimental experimental setup shownand inFigure Figure was designed and fabricated toon observe the relationship between the cylinder number ionic wind velocity of and the fabricated to observe the relationship between the cylinder number and ionic wind velocity of the fabricated to observe theproduced relationship between the cylinder number and ionic wind velocity of the systems. Discharge was between a needle and one or several cylinder electrodes. The systems. Discharge produced between a needle andthe one orone several cylinder electrodes. The systems. Discharge produced between a needle and or several cylinder electrodes. The needle had a sharp was tipwas with a diameter of 27 μm, and copper cylinder had a diameter of needle 4 mm had a sharp withtip a diameter of 27 µm, and theand copper cylinder had a had diameter of the 4 mm needle had with a diameter ofthe 27 μm, the copper cylinder a diameter of 4and mm and length ofatip 60sharp mm. The distance between needle and cylinder was adjusted by fixing needle length of 60 mm. The distance between the needle and cylinder was adjusted by fixing the needle in length holes of 60 mm. The distance the velocity needle and waswith adjusted by fixing the(Testo needle inand different in the frame. Thebetween ionic wind wascylinder measured an anemometer different holes in the frame. The ionic wind velocity was measured with an anemometer (Testo 405-V1, in different holes Germany). in the frame. The ionic wind velocity was measured with an anemometer (Testo 405-V1, Lenzkirch, Lenzkirch, Germany). 405-V1, Lenzkirch, Germany).

(b) (b)

(a) (a) cylinder

hole cylinder hole cylinder

cylinder

needle needle

needle needle

circuit board circuit board Figure 2. The schematic diagram of the experimental setup: (a) schematic diagram of the experimental Figure 2.2.The of setup; and (b) schematic photograph of the experimental setup. setup: Figure The schematicdiagram diagram ofthe theexperimental experimental setup:(a) (a)schematic schematicdiagram diagramof ofthe theexperimental experimental setup; and (b) photograph of the experimental setup. setup; and (b) photograph of the experimental setup.

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We conducted experiments using groups of different cylinder numbers with the applied voltage of −12 kV.We Figure 3 shows that oneusing group of cylinders with a gap numbers distance with l = 10the mm gavevoltage a small  x conducted experiments groups of different cylinder applied of m/s. −12 The kV. Figure 3 shows cylinders with a gap distance l = 10 mm gave a small of 1.96 combination ofthat twoone andgroup three of groups of cylinders with l = 10 mm + 16 mm and l = 10 mm νx of 1.96 m/s. The combination of two and three groups of cylinders with l = 10 mm + 16 mm and + 16 mm + 22 mm obtained  x of 2.75 m/s and 3.08 m/s, respectively. The maximum  x of 3.22 m/s l = 10 mm + 16 mm + 22 mm obtained νx of 2.75 m/s and 3.08 m/s, respectively. The maximum νx

was realized using combination four cylinders with l = 10 with mm +l =1610mm 28 mm mm.+ The of 3.22 m/s wasarealized using aof combination of four cylinders mm++22 16 mm mm ++ 22 experiment shows that several groups of cylinders can obtain larger ionic wind velocities than 28 mm. The experiment shows that several groups of cylinders can obtain larger ionic wind velocities that of one group of one cylinders. reason The is that there exists airexists discharge between the needle and each than that of group ofThe reason is that there air discharge between the Sensors 2017, 17, 87cylinders. 4 of 13needle and group of cylinders. The velocity ionic wind is theeffect mutual of each of group of cylinders. group ofeach cylinders. The ionic wind is velocity the mutual of effect each group cylinders. Then the We conducted experiments using groups of different cylinder numbers with the applied voltage Then the whole velocity is larger than thatgroups of fewerofgroups of cylinders. whole velocity is of larger than that of fewer cylinders. −12 kV. Figure 3 shows that one group of cylinders with a gap distance l = 10 mm gave a small  x

of 1.96 m/s. The combination of two and three groups of cylinders with l = 10 mm + 16 mm and l = 10 mm 4.0

10mm  x of 2.75 m/s and 3.08 m/s, respectively. The maximum  x of 3.22 m/s + 16 mm + 22 mm obtained 10mm+16mm

Velocity(m/s)

10mm+16mm+22mm 3.5 a combination was realized using of four cylinders with l = 10 mm + 16 mm + 22 mm + 28 mm. The 10mm+16mm+22mm+28mm experiment shows that several groups of cylinders can obtain larger ionic wind velocities than that of one group of3.0cylinders. The reason is that there exists air discharge between the needle and each group of cylinders. The ionic wind velocity is the mutual effect of each group of cylinders. Then the 2.5 whole velocity is larger than that of fewer groups of cylinders. 2.0

4.0 3.5

1.5

10mm 10mm+16mm 10mm+16mm+22mm 10mm+16mm+22mm+28mm

3.0

0.5 0.0

Velocity(m/s)

1.0 2.5 2.0 1.5 1.0

Different cylinder combination

0.5

Figure 3. Dependence onthe thegroups groups cylinders. Figure 3. Dependenceofofionic ionicwind wind velocity velocity on of of cylinders. 0.0

Different cylinder combination

above theoretical and experimental results show that central needle-to-double cylinders The The above theoretical and experimental show that the central needle-to-double cylinders Figure 3. Dependence results of ionic wind velocity on the the groups of cylinders. structure and four groups of cylinder combinations are all favorable to increase the ionic wind velocity. structure and four groups of cylinder combinations are all favorable to increase the ionic wind The above theoretical and experimental results show that the central needle-to-double cylinders To fabricate the group cylinders easily by Lithographie, Galvanoformung and Abformung (LIGA) velocity. To fabricate theand group cylinders easily by Lithographie, Galvanoformung and Abformung structure four groups of cylinder combinations are all favorable to increase the ionic wind technology, the cylinders are connected together. The schematic of the needle-to-double cylinders is velocity. fabricate theare group cylinders easily by Lithographie, Galvanoformung (LIGA) technology, the Tocylinders connected together. The schematic of and theAbformung needle-to-double (LIGA) technology, the cylinders are connected together. Thethickness schematic of shown in Figure 4. A needle with a tip diameter of 20 µm and ofthe 20 needle-to-double µm is employed as cylinders is shown in Figure 4. inAFigure needle with awith tipa diameter of 20 μm and thickness of 20 μm is shown 4. A air needle tip diameter of 20 and thickness of 20 μmcylinder. is the discharge cylinders cathode.isTo generate stable discharges, the anode is μm designed as a smooth employed as the employed discharge cathode. generate stable airairdischarges, anode is designed as a as the discharge To cathode. To generate stable discharges, the the anode is designed as a Compared with the anode of plate and mesh, the cylinder electrode more easily generates stable corona smooth cylinder.with Compared with the anode of plate andmesh, mesh, the cylinder electrode more easily smooth cylinder. Compared the anode of plate and the cylinder electrode more easily and glow discharges. cylinder 1 mm in thickness 0.4 of mm in diameter isand used theinanode. generatesThen stableacorona and of glow discharges. Then a and cylinder 1 mm in thickness 0.4as mm generates stable corona and glow discharges. Then a cylinder of 1 mm in thickness and mm in is used as wind the anode. To achieve the maximum ionic wind velocity and form an airflow To achieve the diameter maximum ionic velocity and form an airflow channel, four cylinders with the0.4 same cylinders the same dimension areThe connected together, as shown in Figure 4. The diameter is used as the four anode. Towith achieve the maximum ionic wind velocity and form an airflow dimension arechannel, connected together, as shown in Figure 4. gap distance between the upper cylinder gap distance between the upper cylinder electrode and the bottom cylinder electrode is 3 mm. The channel, four cylinders with the same dimension are connected together, as shown in Figure 4. The electrode and the bottom cylinder electrode is 3 mm. The solder joints of the needle electrode and the solder joints of the needle electrode and the cylinder electrode are designed in a circular shape with cylinder electrode are designed circular shape a diameter of 0.5cylinder mm for the weldingiswire. gap distance between the cylinder electrode and the bottom electrode 3 mm. The a diameter ofupper 0.5 mm in forathe welding wire. with solder joints of the needle electrode and the cylinder electrode are designed in a circular shape with a diameter of 0.5 mm for the welding wire. Upper cylinder electrode

Solder joint

Solder joint

Needle electrode

Upper cylinder electrode

Solder joint

Solder joint

Needle electrode

Bottom cylinder electrode

4. Design of the needle-to-cylinder electrode. Figure 4. Figure Design of the needle-to-cylinder electrode.

Bottom cylinder electrode

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17, 87 5 of 13 The Sensors steps2017, of assembling the integrated system are shown in Figure 5. Firstly, mounting holes are arranged on The thesteps lower substrate (Figure 5a). Secondly, the cylinder electrodes and the supports are of assembling the integrated system are shown in Figure 5. Firstly, mounting holes are the lower substrate (Figure 5a). cylinder electrodes andisthe supports to arethe support mountedarranged into theonmounting holes (Figure 5b).Secondly, Thirdly,the the needle electrode attached mounted into the the mounting holes (Figureis5b). Thirdly, the needle electrode is attached to theelectrodes support (Figure 5c). Lastly, upper substrate manually assembled on the cylinder and the (Figure 5c). Lastly, the upper substrate is manually assembled on the cylinder electrodes and the supports, and then the airflow channel is formed (Figure 5d).

supports, and then the airflow channel is formed (Figure 5d).

(a)

(c)

(b)

(d)

Figure 5. The steps of assembling the integrated chip: (a) the lower substrate; (b) mount the cylinders

Figure 5.and Thethesteps of assembling integrated chip: the onto lower mount the cylinders supports onto the lowerthe substrate; (c) mount the(a) needle thesubstrate; support; (d) (b) place and glue the upper substrate. and the supports onto the lower substrate; (c) mount the needle onto the support; (d) place and glue the upper substrate. The geometric dimensions of the chip are given in Table 1.

Table The geometric dimensions of the1.chip. The geometric dimensions of the1. chip are given in Table Features Dimensions Curvature radius of pin Table10 1.μm The Radius of cylinder 0.2 mm Center distance between 0.9 mm the cylinders Features Dimensions Radius of injection hole 0.75 mm CurvatureHeight radius of pin 10 µm of top substrate 0.5 mm Height of bottom substrate 0.2 mm 0.8 mm Radius of cylinder

Features of needleof the chip. geometricThickness dimensions Thickness of cylinder Inter-electrode spacing between two array cylinders Features Thickness of substrate of pin electrode Thickness of needle Length of pin electrode The whole dimension of the chip Thickness of cylinder

Dimensions 20 μm 1 mm 3 mm

Dimensions

0.49 mm 20 µm 3 mm 11 mm × 10 mm × 2.3 mm 1 mm

Center distance between Inter-electrode spacing between mm 3 mmare When a high negative0.9 DC voltage is applied to the needle electrode and the two cylinders the cylinders two array cylinders connected to the ground, there will be an air discharge between the needle and the cylinders. of substrate of pin DC discharge [22]. Then Comparatively, the negative discharge isThickness more stable than the positive Radius of injection hole 0.75DC mm 0.49 mm electrode the discharge is powered by a 0 to −5000 V, 500 W DC power supply. The discharge circuit is shown Height top substrate 0.5 mmof 12 MΩ is connected Length ofto pin electrode mm in of Figure 6. A discharge resistor the power supply to release the3electric charge when the power supply voltage decreases. The needle is connected to the negative high Height of bottom mm of 6 MΩ, The whole the chip current. 11 mm 10 mm × 2.3 mm voltage by way of a ballast0.8 resistor which candimension restrict theofdischarge The×upper and substrate bottom cylinder electrodes are connected to the ground of the power supply by a test resistor of 1 kΩ. On the both sides of the test resistor, an oscilloscope (TektronixTDS1002B-SC, Beaverton, OR, USA) and a digital multimeter are usedis to applied record theto actual voltage and the voltage effective When a high negative DC voltage the AC needle electrode and the two cylinders value, respectively. The discharge images recorded by using a high-resolutiondigital camera are connected to the ground, there will bewere an air discharge between the needle and the cylinders. (Nikon D810, Tokyo, Japan) with a special micro lens. A Faraday cup is fitted into the chip outlet to Comparatively, the negative DC discharge is more stable than the positive DC discharge [22]. Then the catch ions ionized by the needle-to-cylinder electrodes. To eliminate interfere signal, the Faraday cup

discharge is powered by a 0 to −5000 V, 500 W DC power supply. The discharge circuit is shown in Figure 6. A discharge resistor of 12 MΩ is connected to the power supply to release the electric charge when the power supply voltage decreases. The needle is connected to the negative high voltage by way of a ballast resistor of 6 MΩ, which can restrict the discharge current. The upper and bottom cylinder electrodes are connected to the ground of the power supply by a test resistor of 1 kΩ. On the both sides of the test resistor, an oscilloscope (TektronixTDS1002B-SC, Beaverton, OR, USA) and a digital multimeter are used to record the actual AC voltage and the voltage effective value, respectively. The discharge images were recorded by using a high-resolutiondigital camera (Nikon D810, Tokyo,

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Japan) with a special micro lens. A Faraday cup is fitted into the chip outlet to catch ions ionized Sensors 2017, 17, 87 6 of 13 by the needle-to-cylinder electrodes. To eliminate interfere signal, the Faraday cup is enclosed in the metal shield. that, the detected micro currentmicro is measured a Keithley electrometer. is enclosed in After the metal shield. After that, the detected current isby measured by a6487 Keithley 6487 The corresponding current data is stored in the computer by using LabVIEW software. electrometer. The corresponding current data is stored in the computer by using LabVIEW software.

888

Figure Theschematic schematic of the setup. Figure 6. 6.The theexperimental experimental setup.

2.2. Material Choice

2.2. Material Choice

The integrated system is composed of three components bonded together: the needle and

The integrated system is composed of three components bonded together: the needle and cylinder cylinder electrodes, the bottom substrate, and the upper cover. For the above three components, the electrodes, the bottomare substrate, the upper cover. For the above three components, the main main requirements shown inand the following: requirements are shown in the following: (1) (2) (3)

(1) The needle-to-cylinder electrodes must be conductive materials and strong enough to resist the of air discharge; The erosion needle-to-cylinder electrodes must be conductive materials and strong enough to resist the (2) The bottom and upper substrates should be an electrical insulator so as to accomplish the erosion of air discharge; electrical isolation from the needle and the cylinders; and The bottom and upper substrates should be an electrical insulator so as to accomplish the electrical (3) The upper substrate should be transparent to observe the discharging phenomenon.

isolation from the needle and the cylinders; and According to the above requirements, electroplated Cu was selected as the material for the The upper substrate should be transparent to observe the discharging phenomenon.

needle-to-cylinder electrodes. Although silicon is also conductive, its brittleness limits its use as the discharge electrode. Comparatively, electroplated Cu is not only conductive, but also has favorable According to the above requirements, electroplated Cu was selected as the material for the mechanical properties and can resist the discharging erosion. Polymethyl Methacrylate (PMMA) was needle-to-cylinder electrodes. Although silicon is also conductive, its brittleness limits its use as the selected as the material of upper and bottom substrates for its good electrical insulation and discharge electrode. Comparatively, electroplated Cu is not only conductive, but also has favorable transparent properties.

mechanical properties and can resist the discharging erosion. Polymethyl Methacrylate (PMMA) was selected as theProcess material of upper and bottom substrates for its good electrical insulation and 2.3. Fabrication transparent Theproperties. fabrication process for the micro cylinder electrode is shown in Figure 7. The chip is fabricated by LIGA technology. The main steps of the fabrication process are as follows:

2.3. Fabrication Process

(a) Photoresist coating: The photoresist is chosen as PMMA of AR-P 6510 style from ALLRESIST in

The fabrication process fortemperature the micro cylinder is shown in Figure 7. The chip is fabricated Germany. The baking is 160 °C electrode lasting 50 min. by LIGA main steps of exposure the fabrication (b) technology. Exposure: TheThe time for the X-ray is 2.2 h.process are as follows: (a) (b) (c) (d)

(c) Developing: The developing agent is R600-56, and the developing time is eight hours. Photoresist coating: is chosen of the AR-P 6510ofstyle ALLRESIST (d) Sputtering copper:The Thephotoresist copper sulfate solutionasisPMMA used with density 200 from g/L. The density in ◦ C lasting 50 min. Germany. The temperature is 160 current of sulfate is baking 100 mL/L. The electroplating is 200 mA at room temperature, and the lasting time under electroplating current is 24 h. of electroplated Cu for the cylinder and Exposure: The the time for the X-ray exposure isThe 2.2 height h. the needleThe is 1 developing mm and 20 μm, respectively. Developing: agent is R600-56, and the developing time is eight hours. (e) Lapping: The grinding process is finished by a UNIPOL-802 grinding machine from MIT Sputtering copper: The copper sulfate solution is used with the density of 200 g/L. The density Company. The rotational speed is 50 r/min until the grinding process is finished. of sulfate is 100 mL/L. The electroplating current is 200 mA at room temperature, and the lasting (f) Removing the photoresist: The photoresist is removed by the Remover AR600-70.

time under the electroplating current is 24 h. The height of electroplated Cu for the cylinder and the needle is 1 mm and 20 µm, respectively.

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(e)

Lapping: The grinding process is finished by a UNIPOL-802 grinding machine from MIT Company. The rotational speed is 50 r/min until the grinding process is finished. (f) Removing the photoresist: The photoresist is removed by the Remover AR600-70. Sensors 2017, 17, 87 7 of 13 (g) Removal of the Ti substrate: The Ti substrate is removed by 30% hydrofluoric acid with adensity 20 mL/Loffor (g)ofRemoval the8Tih.substrate: The Ti substrate is removed by 30% hydrofluoric acid with adensity of 20 mL/L for 8 h.

The fabrication process of the needle is similar to that of the copper cylinder, but there are still The fabrication process of the needle is similar toisthat of the from copper cylinder, but there are stillat 90 ◦ C some differences for the substance. The photoresist AZ5620 AZ Company, prebaking some differences for the substance. The photoresist is AZ5620 from AZ Company, prebaking at 90 °C for 40 min. The lithography process is realized by the BG-401A lithography machine, and the exposure for 40 min. The lithography process is realized by the BG-401A lithography machine, and the time is five minutes. The development solution is 7% NaOH, lasting 10 min. The photoresist is exposure time is five minutes. The development solution is 7% NaOH, lasting 10 min. The photoresist removed by 20% NaOH. The electroplated solution is copper sulfate with adensity of 200 g/L, and the is removed by 20% NaOH. The electroplated solution is copper sulfate with adensity of 200 g/L, and density of sulfate is 100 mL/L. current is 200 at room temperature for 30 min. the density of sulfate is 100 mL/L.The Theelectroplated electroplated current is 200 mAmA at room temperature for 30 min. TheThe thickness of the electroplated needle electrode is 20 µm. Finally, the Ti substrate is removed by thickness of the electroplated needle electrode is 20 μm. Finally, the Ti substrate is removed by 30% HFHF acid with mL/Lforfor four hours. 30% acid withadensity adensity of of 20 mL/L four hours.

(a)

(b)

(c)

(d)

(e)

(f) (g) Figure 7. Fabrication details of the needle and cylinder electrode; (a) photoresist coating; (b) exposure;

Figure 7. Fabrication details ofcopper; the needle and cylinder electrode; (a) photoresist coating; (b) exposure; (c) developing; (d) sputtering (e) lapping; (f)removing photoresist; (g) removing Ti substrate (c) developing; (d) sputtering copper; (e) lapping; (f)removing photoresist; (g) removing Ti substrate Figure 8 presents the needle-to-cylinder electrodes fabricated by LIGA technology. It shows that theFigure needle8ispresents sharp-tipped. To increase the strength, the needle bottombyis LIGA wider technology. than the needle tip, the needle-to-cylinder electrodes fabricated It shows that and the needle tip is placed over the centre of the injection hole, which is designed to inject the sample the needle is sharp-tipped. To increase the strength, the needle bottom is wider than the needle tip, substance. The cylinders are connected with each other by a 20 μm wide plate. Once assembled, the and the needle tip is placed over the centre of the injection hole, which is designed to inject the sample needle points to the center of the gap between the two cylinders (Figure 8c).

substance. The cylinders are connected with each other by a 20 µm wide plate. Once assembled, the needle points to the center of the gap between the two cylinders (Figure 8c).

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(a)

(b)

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(a)

(b)

0.4 mm

1 mm (c)

0.4 mm

1 mm

1 mm

(c) 1 mm

Figure 8. The needle and cylinder electrodes fabricated by Lithographie, Galvanoformung and Figure 8. The needle and cylinder electrodes fabricated by Lithographie, Galvanoformung and Abformung (LIGA) technology: (a) needle electrode; (b) cylinder electrode; and (c) the assembled Abformung (LIGA) technology: (a) needle electrode; (b) cylinder electrode; and (c) the assembled needle and the cylinder. Figure 8. The needle and cylinder electrodes fabricated by Lithographie, Galvanoformung and needle and the cylinder. Abformung (LIGA) technology: (a) needle electrode; (b) cylinder electrode; and (c) the assembled

and the cylinder. 3. Resultsneedle and Discussion 3. Results and Discussion

3. Results andand Discussion 3.1. The Discharge Ionic Wind Velocity 3.1. The Discharge and Ionic Wind Velocity The discharge waveform of the needle-to-cylinder electrodes was measured by the oscilloscope. 3.1. The Discharge and Ionic Wind Velocity The discharge waveform of the needle-to-cylinder electrodes was measured by the oscilloscope. At the beginning of discharge (with an applied voltage of −3050was V), there is abytypical Trichel pulse The discharge waveform of thean needle-to-cylinder the oscilloscope. At the beginning of discharge (with applied voltageelectrodes of −3050 V),measured there is a typical Trichel pulse (Figure 9a). As the Trichel pulse is the characteristic for corona discharge, this proves that the stable At the beginning of discharge (with an applied voltage of −3050 V), there is a typical Trichel (Figure 9a). As the Trichel pulse is the characteristic for corona discharge, this proves that pulse the stable corona discharge achieved by the needle-to-cylinder electrode without external airflow (Figure 9a). Asis Trichel pulse is the characteristic for corona discharge, this an proves that the stable [23]. corona discharge isthe achieved by the needle-to-cylinder electrode without an external airflow [23]. With corona an increase in the appliedby voltage, the pulse frequency correspondingly, and the discharge is achieved the needle-to-cylinder electrodedecreases without an external airflow [23]. With an increase in the applied voltage, the pulse frequency decreases correspondingly, and the With an in the applied voltage, the pulseThen frequency correspondingly, the amplitude of increase the pulse waveform also decreases. a DCdecreases component arises with and an applied amplitude of the pulse waveform also decreases. Then a DC component arises with an applied voltage amplitude of the pulse waveform also decreases. Then a DC component arises with an applied voltage of −3500 V (Figure 9e). At last, the Trichel turns into a DC line when the applied voltage is of −3500 V (Figure 9e). At last, the into a DC line when applied voltage is −3563 V of −3500 V (Figure 9e). AtTrichel last, theturns Trichel turns into a DC linethe when the applied voltage is −3563voltage V (Figure 9f). This means that corona discharge transforms into glow discharge. (Figure 9f). This means that corona discharge transforms into glow discharge. −3563 V (Figure 9f). This means that corona discharge transforms into glow discharge. 0.4

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Figure The chip with (d)9.−3400 V; (e)discharge −3500 V; (f)waveform −3563 V; and (g)spacing −3600 V.of 2 mm at (a) −3050 V; (b) −3200 V; (c) −3300 V; (d) −3400 V; (e) −3500 V; (f) −3563 V; and (g) −3600 V. Figure 10 shows the chip discharge images recorded by a Nikon D810 camera. At the beginning of corona discharge (U = −3050 V, Figure 10A), the discharge light is mainly concentrated on the Figure 10 shows the chip discharge images recorded by a Nikon D810 camera. At the beginning of needle tip. Additionally, there is discharge plasma on the cylinder which faces the needle directly. corona discharge (U = −3050 V, Figure 10A), the discharge light is mainly concentrated on the needle However, on the other three cylinders, no discharge plasma is observed. With an increase in applied tip. Additionally, is discharge dischargeplasma plasma on the larger cylinder faces the 10B). needle directly. However, voltage (−3500there V), the becomes andwhich brighter (Figure This corresponds on the three cylinders, nocorona discharge plasma is observed. an increase applied voltage to other the transition stage from discharge to glow discharge.With In previous work,inthe discharge (−3500 V), the discharge plasma on becomes larger andthere brighter (Figure 10B). This corresponds plasma is mainly concentrated the needle tip, and was no, or little, discharge plasma on theto the cylinder surface luminous to plasma appearsInbetween thework, electrodes. When the plasma glow transition stage from[17–19]. coronaAdischarge glowsheet discharge. previous the discharge is discharge is ignited (Figure 10C), there are familiar bright and dark regions shown in the low pressure mainly concentrated on the needle tip, and there was no, or little, discharge plasma on the cylinder glow discharge. The discharge space between the between needle and the cylinder can be divided intodischarge four surface [17–19]. A luminous plasma sheet appears the electrodes. When the glow regions: negative glow, Faraday dark space, positive column, and anode glow. Near the needle tip,glow is ignited (Figure 10C), there are familiar bright and dark regions shown in the low pressure there exists the negative glow region which is a hierarchical structure. Following the negative glow, discharge. The discharge space between the needle and the cylinder can be divided into four regions: the Faraday dark space appears. In this region, there is no apparent brightness. It can be found that negative glow, Faraday dark space, positive column, and anode glow. Near the needle tip, there exists moving from the ‘Faraday dark space’ there is a ‘positive column’ region. In this region, the bright the negative glow region which is aThe hierarchical structure. the negative thethan Faraday discharge plasma appears again. ‘positive column’ hasFollowing a longer length and largerglow, volume dark the space appears. In this region, there is no apparent brightness. It can be found that moving ‘negative glow’. Finally, the ‘anode spot’ is located on the cylinder surface, which is like a from lightened the ‘Faraday dark space’ there is a ‘positive column’ region. In this region, the bright discharge blue flame. The luminous and nonluminous regions of the discharge image are similar to those observed in The [24,25]. Therefore, the has discharge is an atmospheric pressure plasma appears again. ‘positive column’ a longer length and larger volumeglow thandischarge the ‘negative glow’.(APGD). Finally, the ‘anode spot’ is located on the cylinder surface, which is like a lightened blue flame.

The luminous and nonluminous regions of the discharge image are similar to those observed in [24,25]. Therefore, the discharge is an atmospheric pressure glow discharge (APGD).

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Figure10. 10.Images Images of of the air discharge distance d =d2=mm): (A) Figure dischargewith withdifferent differentapplied appliedvoltages voltages(the (thegap gap distance 2 mm): Figure 10. Images of the air discharge with different applied voltages (the gap distance d = 2 mm): (A) U =UU −3050 V; (B) U = −3500 V; and (C) U = −3600 V, (a) negative glow; (b) Faraday dark space; (c) (A) = =−−3050 3050 V; (B) U = − 3500 V; and (C) U = − 3600 V, (a) negative glow; (b) Faraday dark space; V; (B) U = −3500 V; and (C) U = −3600 V, (a) negative glow; (b) Faraday dark space; (c) positive column; (d) anode spot. (c) positive column; (d) anode spot. positive column; (d) anode spot.

Figure 11a11a shows the volt-ampere the chip. As can be seen the figure, Figure shows volt-amperecharacteristics characteristics ofofthe be seen fromfrom the figure, the the Figure 11a shows thethe volt-ampere characteristics of thechip. chip.As Ascan can be seen from the figure, the discharge current is about2 2μA–15 μA–15μA μA in in the the corona With thethe applied voltage beyond discharge current is about coronadischarge. discharge. With applied voltage beyond discharge current is about 2 µA–15 µA in the corona discharge. With the applied voltage beyond −3500 V, the corona dischargetransitions transitions to to the and thethe discharge current increases V, the corona discharge the glow glowdischarge discharge and discharge current increases −−3500 3500significantly. V, the corona discharge transitions to the glow discharge and the discharge current increases dischargeisis harmful harmful to and testtest instruments, it does not not significantly. As As thethe arcarc discharge to the themicroneedle microneedle and instruments, it does significantly. As the arcthe discharge is harmful to the microneedle and test instruments, it does not increase again when glow discharge appears. increase again when the glow discharge appears. increase again when the glow discharge appears. 120

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Figure 11. The discharge properties of the chip. (a) The volt-ampere characteristics; and (b) ionic wind Figure 11. The discharge properties of the chip. (a) The volt-ampere characteristics; and (b) ionic wind velocities as a function of applied voltage. velocities as a function of applied voltage.

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From the above analysis, it can be seen that there are corona discharge, glow discharge, and arc discharge phases. To compare the ionic wind velocities at different discharge phases, an experiment Sensors 2017, 17,The 87 11 of 13 was conducted. ionic wind velocity is measured by using an anemometer (Testo 405-V1) with range and sensitivity of 0–10 m/s and 0.01 m/s, respectively. The velocities were measured at a From the above analysis, it can be seen that there are corona discharge, glow discharge, and arc distance of 1 mm fromTothe outletthe of the discharge phases. compare ionicchip. wind velocities at different discharge phases, an experiment Figure 11b shows the ionic wind is velocity is up 0.14anm/s when the(Testo applied voltage was conducted. Thethat ionic wind velocity measured by to using anemometer 405-V1) with reaches −2940range V. Atand thissensitivity stage, the discharge ignited and the ionic wind were appears. With at increasing of corona 0–10 m/s and 0.01 is m/s, respectively. The velocities measured a distance of 1 mm from the outlet of the chip. applied voltage, the ionic wind velocity increases correspondingly until reaching 0.79 m/s when the Figure appears. 11b showsAfter that the ionic wind velocity up to 0.14decreases m/s when with the applied voltage reaches voltage glow discharge that, the ionic windisvelocity increasing applied −2940 V. At this stage, the corona discharge is ignited and the ionic wind appears. With increasing in the glow discharge phase. At the end of the corona discharge, a maximum ionic wind velocity of applied voltage, the ionic wind velocity increases correspondingly until reaching 0.79 m/s when the 0.79 m/s is obtained at a voltage of −3300 V. When the applied voltage reaches −3600 V, the discharge glow discharge appears. After that, the ionic wind velocity decreases with increasing applied voltage current is and the phase. ionic wind almost close toa zero. Thisionic result is also observed in in thelarger glow discharge At the velocity end of theiscorona discharge, maximum wind velocity of our previous research [17]. This shows that the maximum ionic wind velocity appears during the 0.79 m/s is obtained at a voltage of −3300 V. When the applied voltage reaches −3600 V, the discharge current is larger anddischarge the ionic wind velocity is almostTo close to zero. This resultvelocity, is also observed in our transition from corona to glow discharge. achieve maximum the glow discharge previous research [17]. This shows that the maximum ionic wind velocity appears during the should be avoided. transition from corona discharge to glow discharge. To achieve maximum velocity, the glow discharge be avoided. 3.2. The Sampleshould Injection and Ionization

According to the above 3.2. The Sample Injection andanalysis, Ionizationthe maximum ionic wind velocity is achieved when the applied voltage reaches − 3300 V. The other experimental conditions the same. According to the above analysis, the maximum ionic wind are velocity is achieved when the applied Avoltage 0.5 mL amount acid and 0.5 mL of ammonia water are poured into a 3 mL box reaches −3300of V. acetic The other experimental conditions are the same. separately.A In the hole on the over the water smallare box. The pH 0.5the mLexperiment, amount of acetic acid and 0.5 chip mL ofisammonia poured intopaper a 3 mLwetted box with separately. In the experiment, the chip is overthe theapplied small box. The pHispaper wettedoff, with clear water is placed in front of the thehole chiponoutlet. When voltage switched the color clear water is placed in front of the chip outlet. When the applied voltage is switched off, the color of of the test paper will not change. From Figure 12 it appears that pH paper turns red and blue in the the test paper will not change. From Figure 12 it appears that pH paper turns red and blue in the presence of acetic acid and ammonia water, respectively. Thus, this proves that the samples have been presence of acetic acid and ammonia water, respectively. Thus, this proves that the samples have injected through the chip. been injected through the chip.

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Figure 12. The sample injection andcurrent; ionization experimental results: (a) acid injection; (b) ammonia water injection; (c) acid ionization and (d) ammonia water ionization current. water injection; (c) acid ionization current; and (d) ammonia water ionization current. The ionization experiment was conductedby using a Faraday cup, as shown in Figure 6. When the sample is ionized by the chip, the ions collide with the Faraday cup and are converted into current.

The ionization experiment was conductedby using a Faraday cup, as shown in Figure 6. When the sample is ionized by the chip, the ions collide with the Faraday cup and are converted into current.

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When the Keithley 6487 unit was connected to the computer, the measured current data is stored and handled by the LabVIEW software. With a sample frequency of 0.5 Hz, the current data for acid and ammonia water is shown in Figure 12c,d. The sample points in Figure 12 represent the experimental point numbers during the experiment. When the applied voltage is 0 V, there is no current detected, and no discharge phenomenon. As the applied voltage increases to −3185 V, there is a current of approximately −10 pA detected. With an increase in applied voltage, the current also increases correspondingly. When the applied voltage increases to −3300 V, the current increases to −150 pA. A similar situationarose with ammonia water. In this condition, the cuurent is about −10 pA at an applied voltage of −3186 V. 4. Conclusions In this paper, we have developed a needle-to-cylinder electrode chip based on LIGA technology. The needle tip is 20 µm in diameter and 10 µm in thickness. The whole dimension of the chip is 11 mm × 10 mm × 2.3 mm. Without an air supply and at room temperature, corona discharge is realized when a voltage of −3050 V is applied between the needle and the cylinder electrode. The Trichel pulse appearing on the oscilloscope proves the existance of corona discharge. With an increase in the voltage, corona discharge transits to the glow discharge. The Trichel turns into a DC line when the applied voltage is −3563 V, which means that glow discharge appears. The images recorded by a Nikon camera also show that there is stable corona and glow discharge. We have experimentally demonstrated that the chip yields an ionic wind based on the air discharge. The interesting result is that the ionic wind velocity increases from 0.14 m/s to 0.79 m/s in the process of corona discharge, increasing with the applied voltage. However, the ionic wind decreases when the corona discharge transitions to glow discharge. The ionic wind velocity is almost zero when the arc discharge appears. The acetic acid and ammonia water sample injection are proved by the pH paper turning red and blue. At the same time, the ionization current reaches several pA. This shows that the chip functions doubly as the air pump and ion source, which can be well applied to analytical instruments, such as IMS, FAIMS, and MS. Acknowledgments: This work is supported by the National Natural Science Foundation of China under Grant No. 61204120, the Natural Science Foundation of Guangxi under Grant No. 2013GXNSFBA019273 and No. 2015GXNSFAA139304, the Foundation of Guangxi Key Laboratory of Automatic Detecting Technology and Instruments (Guilin University of Electronic Technology) under Grant No. YQ15114, the Foundation from Guangxi Experiment Center of Information Science under Grant No. YB1420 and the National Innovation Project for the college student of China under Grant No. XY290406. Author Contributions: Hua Li, Linxiu Jiang, and Chaoqun Guo designed the chip and wrote the paper; Jianmin Zhu and Yongrong Jiang performed the experiments; Zhencheng Chen revised the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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