micromachines Article
Separated Type Atmospheric Pressure Plasma Microjets Array for Maskless Microscale Etching Yichuan Dai 1 , Man Zhang 1 , Qiang Li 1 , Li Wen 1, *, Hai Wang 2 and Jiaru Chu 1 1
2
*
Department of Precision Machinery and Instrumentation, University of Science and Technology of China, Hefei 230026, China;
[email protected] (Y.D.);
[email protected] (M.Z.);
[email protected] (Q.L.);
[email protected] (J.C.) School of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu 241000, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-0551-6360-0214
Academic Editor: Massood Tabib-Azar Received: 26 March 2017; Accepted: 10 May 2017; Published: 1 June 2017
Abstract: Maskless etching approaches such as microdischarges and atmospheric pressure plasma jets (APPJs) have been studied recently. Nonetheless, a simple, long lifetime, and efficient maskless etching method is still a challenge. In this work, a separated type maskless etching system based on atmospheric pressure He/O2 plasma jet and microfabricated Micro Electro Mechanical Systems (MEMS) nozzle have been developed with advantages of simple-structure, flexibility, and parallel processing capacity. The plasma was generated in the glass tube, forming the micron level plasma jet between the nozzle and the surface of polymer. The plasma microjet was capable of removing photoresist without masks since it contains oxygen reactive species verified by spectra measurement. The experimental results illustrated that different features of microholes etched by plasma microjet could be achieved by controlling the distance between the nozzle and the substrate, additive oxygen ratio, and etch time, the result of which is consistent with the analysis result of plasma spectra. In addition, a parallel etching process was also realized by plasma microjets array. Keywords: atmospheric pressure plasma microjets array; maskless etching; photoresist; MEMS; separation design; parallel processing
1. Introduction In recent years, Micro Electro Mechanical Systems (MEMS) devices, especially micro-fluidic devices and bio-MEMS have attracted the attentions of researchers due to the significant potential in fields of biological processing and chemical reactions [1,2]. As one of the key processes for MEMS fabrication, plasma treatments have been widely used in various applications including etching [3], functionalization [4], and surface modification [5–7] owing to its cleanliness, reactivity, and high fabrication resolution based on photolithography [8]. Nonetheless, plasma processes have high operating costs and are time-consuming owing to their requirements of vacuum components and several steps of preparing the mask. Hence, an affordable, efficient method is urgently required of the localized plasma treatment. To solve these concerns, a large amount of effort has been made by researchers in recent decades. Microplasma generated by various types of microdischarges have been developed for the localized surface modification of materials [9–13]. For instance, microscale porous N2 /C2 H2 and He microdischarges array have been developed for the localized coating and functionalization of polymeric surface. This approach is known as “plasma printing” [10]; a 500 µm foldable paper-based microdischarges array has also been designed for flexible maskless patterning on the curved or structured surface [12,13]. These modification processes both require the substrate to be in contact with Micromachines 2017, 8, 173; doi:10.3390/mi8060173
www.mdpi.com/journal/micromachines
Micromachines 2017, 8, 173
2 of 12
the microdischarges, which results in the resolution of tens to hundreds micrometers corresponding to the size of these devices. An agreement on microplasma devices is that they are suitable for modification of materials since their high electron density of 1013 –1016 cm−3 [14] and large-scale localized treatment with dozens of microdischarges in array. However, the complex fabrication process of microdischarges and requirement of vacuum components makes these approaches time-consuming and expensive. On the other hand, non-thermal atmospheric pressure plasma microjet (APPµJs) have attracted a lot of attention due to their widespread applications in maskless materials processing. In the last decade, various types of He-O2 /air APPµJs driven by radio frequency or low-frequency power have been developed for modification and removal of materials [15,16]. Compared with low-pressure microplasma devices such as microdischarges, APPµJs generate room-temperature plasma containing highly reactive species to downstream in open air, which gives them several advantages in direct treatment of hard and soft materials with outstanding flexibility and simplicity. For instance, a tapered air APPµJs produced by pulling quartz microcapillary has been developed for the removal of Parylene-C films [16,17]; in addition, Masaaki Nagatsu and co-workers have developed an APPµJ system based on ultrafine nanopipette nozzle for sub-micron modification and removal of carbon nanotubes and photoresists [18–20]. Their results are considered as the minimum line width of current APPµJs. Although APPµJs are a promising technology for fabrication polymer microstructures, careful consideration of the gas flow dynamics in plasma treatment is required because of their fragile thin tube wall and small tip [21]. Besides, continual collisions between energetic species and a tapered tube-wall [22] result in a requirement of higher ignition voltage in APPµJ systems. More importantly, parallel maskless processing remains a challenge for single microjet configurations of APPµJs. Therefore, improvement of the present maskless plasma fabrication approach is still needed. Here, we propose a novel maskless plasma process approach. In our system, a microfabricated micro/nano nozzle was attached to the outlet of millimeter scale capillary tube. When polymer films were exposed to the device at atmospheric pressure, downstream reactive species generated by plasma in the tube injected from micro/nano nozzle holes reacted with polymer films and led to the localized fabrication. Several advantages of our system were as follows: (1) ease and relatively lowcost MEMS technology were applied for fabrication of nozzle. Hence, dimensions of nozzle array could be micro/nanoscale based on precise microfabrication capacity, which satisfies the requirement of localized etching without mask; (2) plasma microjet array emanating from nozzle array makes high efficiency, large-area maskless etching realizable; (3) since we separate injection components from plasma generation components in our system, it is convenient to replace the nozzle if different size or number of nozzles in the array is needed, which would be conducive to the maintaining and updating the system; (4) since the system operated in ambient air, flexible scanning fabrication can be achieved while integrated with roll-to-roll systems. In this paper, the separated type atmospheric pressure plasma microjets array system was utilized for microscale polymer etching using a maskless and non-contact approach. Two main issues were investigated: (1) the electrical and spectra characteristic of the system which related to the capability for maskless etching; (2) the effect of the distance, additive oxygen ratio, and etch time between the nozzle and substrate on features of the microholes on polymer films. Finally, plasma microjets array etching process has also been studied. 2. Materials and Methods 2.1. Main Components of Separated Type Atmospheric Pressure Plasma Microjets Array System The schematic diagram of atmospheric pressure plasma microjets array system and its maskless etching process is shown in Figure 1a. This system included an APPJ based on the dielectric barrier discharge (DBD) principle, a micro nozzle holes array, a Z-axis sliding platform, an optical apparatus, and an analytical balance. A glass tube with 4 mm inner diameter (ID) and 6 mm outer diameter (OD) was used as the dielectric material in APPJ. The 9 mm long ring-shape copper ground electrode
Micromachines 2017, 8, 173 Micromachines 2017, 8, 173
3 of 12 3 of 12
was placed around the glass tube 3 cm from the outlet. The high-voltage copper electrode was and 7 cm which which was machined to fit to tight around the glass tube.tube. A 3diameter mm in diameter andin7 length, cm in length, was machined fit tight around the glass polytetrafluoroethylene (PTFE) hollow cylinder 7 cm in length was wrapped around the glass tube. A polytetrafluoroethylene (PTFE) hollow cylinder 7 cm in length was wrapped around the glass tube. Theatmospheric atmospheric pressure pressure plasma plasma microjet microjet was driven The driven by by aa low low frequency frequency (around (around11 11kHz) kHz) sinusoidalhigh highvoltage voltagesource. source. Besides, Besides, aa Tektronix Tektronix P6015A P6015A (Tektronix, sinusoidal (Tektronix, Beaverton, Beaverton, OR, OR,USA) USA)high high voltage probe was applied to measure the voltage. The discharge current of plasma in the glass tube voltage probe was applied to measure the voltage. The discharge current of plasma in the glass and and plasma microjet was calculated by theby voltage across aacross 50 Ω non-inductive resistor R1 and R2, tube plasma microjet was calculated the voltage a 50 Ω non-inductive resistor R1 respectively. The voltage and current waveforms were recorded by a Tektronix digital oscilloscope and R2, respectively. The voltage and current waveforms were recorded by a Tektronix digital DPO-3014. DPO-3014. oscilloscope
Figure pressure plasma plasma microjets microjets array array system system Figure1.1. (a) (a) The The schematic schematic diagram diagram of of the the atmospheric atmospheric pressure maskless etching process; (b) fabrication sequence of a Micro Electro Mechanical Systems (MEMS) maskless etching process; (b) fabrication sequence of a Micro Electro Mechanical Systems (MEMS) nozzle; electron microscope (SEM) image of single holehole on the the upper and lower nozzle;(c)(c)scanning scanning electron microscope (SEM) image of single on nozzle, the nozzle, the upper and diameter of the hole were 262 µm262 andμm 50 and µm,50 respectively. lower diameter of the holearound were around μm, respectively.
Moreover,a single-hole a single-hole nozzle was when usedmeasuring when measuring spectra characterization of Moreover, nozzle was used spectra characterization of atmospheric atmospheric pressure plasma microjet, which avoids the measuring plasma microjets array interfered pressure plasma microjet, which avoids the measuring plasma microjets array interfered by other by other plasma microjets. To clarify the reactive species of plasma microjet in the air, the plasma microjets. To clarify the reactive species of plasma microjet in the open air, open the emission emission spectra of the atmospheric pressure plasma microjet was measured using a fiber optical spectra of the atmospheric pressure plasma microjet was measured using a fiber optical spectrometer spectrometer (AvaSpec-ULS2048-2-USB2, Avantes, Apeldoorn, The The fiber (AvaSpec-ULS2048-2-USB2, Avantes, Apeldoorn, The Netherlands). TheNetherlands). fiber integrated with the integrated with the collimating lens was placed at the side of the microjet and collected optical collimating lens was placed at the side of the microjet and collected optical emission from plasma. emission from plasma.component of our system, the micro nozzle holes array was fabricated on As a preliminary As10 a preliminary component ofMEMS our system, the micro nozzle array was fabricated onsquare a 150 a 150 ± µm thick silicon wafer by fabrication process, as holes shown in Figure 1b. Nozzle ± 10 μm thick silicon wafer by MEMS fabrication process, as shown in Figure 1b. Nozzle square holes holes for plasma injection were fabricated by wet etching based on a SiO2 layer mask formed by wet for plasma injection were fabricated by wet etching based on a SiO2 layer mask formed by wet oxidation. As the result of anisotropic wet etching property of silicon, the pyramidal-structure cavities oxidation. As the result of anisotropic wet etching property of silicon, the pyramidal-structure array with their upper and lower dimensions of 262 µm and 50 µm was formed, Figure 1c shows cavities array with their upper and lower dimensions of 262 μm and 50 μm was formed, Figure 1c the SEM image of the micro nozzle array. Smaller dimensions of square nozzle arrays from several shows the SEM image of the micro nozzle array. Smaller dimensions of square nozzle arrays from micrometers to hundreds of nanometers could also be fabricated by controlling fabrication parameters several micrometers to hundreds of nanometers could also be fabricated by controlling fabrication in future work. parameters in future work. Adjusting the distance and parallelism between the surface of nozzle and polymer sample was Adjusting the distance and parallelism between the surface of nozzle and polymer sample was a must since the consistency of microstructures fabricated by the microjet should be guaranteed in a must since the consistency of microstructures fabricated by the microjet should be guaranteed in parallel processing. An optical apparatus was built based on the equal-thickness interference principle. parallel processing. An optical apparatus was built based on the equal-thickness interference Two transparent and insulated polymethylmethacrylate (PMMA) plates applied as platforms of nozzle principle. Two transparent and insulated polymethylmethacrylate (PMMA) plates applied as and polymer. The nozzle adhered to the lower surface of the nozzle platform while the substrate was platforms of nozzle and polymer. The nozzle adhered to the lower surface of the nozzle platform placed on the Z-axis sliding platform. Interference fringes were formed as a result of laser beam being while the substrate was placed on the Z-axis sliding platform. Interference fringes were formed as a focused twobeam platforms. was obvious the wedge angle formed bythe twowedge platesangle correlated to by the result ofonlaser beingItfocused on twothat platforms. It was obvious that formed number of interference fringes based on the equal-thickness interference principle. The wedge angle two plates correlated to the number of interference fringes based on the equal-thickness interference −3 degree after adjustment of micrometer screws on the nozzle plate, which satisfied the was about 1.5 principle. The wedge angle was about 1.5−3 degree after adjustment of micrometer screws on the requirement for parallel plasma processing. nozzle plate, which satisfied themaskless requirement for parallel plasma maskless processing.
Micromachines 2017, 8, 173 Micromachines 2017, 8, 173
4 of 12 4 of 12
“Force feedback” distance control. Operating procedures werewere that kept “Force feedback” method methodwas wasapplied appliedforfor distance control. Operating procedures that raising the Zthe sliding platform untiluntil contact occurs between nozzle and and platform surface firstly, the kept raising Z sliding platform contact occurs between nozzle platform surface firstly, digital readout from the balance vary from zero to certain value in that the nozzle gives a push force the digital readout from the balance vary from zero to certain value in that the nozzle gives a push to thetoplatform. ThenThen we adjusted platform surface to the appropriate height which corresponds to force the platform. we adjusted platform surface to the appropriate height which corresponds expected distance between nozzle shown in in to expected distance between nozzleand andplatform. platform.The Theschematic schematicofofthe theleveling leveling device device is is shown Figure 2a, the plasma microjet system is exhibited in Figure 2b. Figure 2a, the plasma microjet system is exhibited in Figure 2b.
Figure schematic of leveling device for thefor maskless parallel parallel etching process. apparatus Figure 2.2.(a) (a)The The schematic of leveling device the maskless etchingOptical process. Optical was not shown in this schematic; (b) the separated type atmospheric pressure plasma apparatus was not shown in this schematic; (b) the separated type atmospheric pressuremicrojets plasma array system. microjets array system.
2.2. Etching and Characterization Photoresist a light-sensitive, soft polymer material that used inthat processes as photolithography. Photoresistis is a light-sensitive, soft polymer material used such in processes such as Photoresist etchingPhotoresist by oxygenetching plasmabyisoxygen one ofplasma the most widely MEMS and fabrication photolithography. is one of thein most widely in LSI MEMS and LSI technology [23], in that[23], a mass of reactive oxygen species (ROS) such(ROS) as oxygen generated fabrication technology in that a mass of reactive oxygen species such atoms as oxygen atoms by plasma could react with polymer films, then form volatile small molecules and thus remove of generated by plasma could react with polymer films, then form volatile small molecules and thus material [24,25]. To [24,25]. investigate the plasma the maskless capacity of our device remove of material To investigate plasmaetching maskless etching capacity of on ourphotoresist, device on the 7.5 µm thickness AR-3210 positive photoresist was used inwas thisused study waswhich spin-coated on photoresist, the 7.5 μm thickness AR-3210 positive photoresist in which this study was spinacoated slide-glass substrate. The etching time varied from 1 min to 5 min. The effect of distance between the on a slide-glass substrate. The etching time varied from 1 min to 5 min. The effect of distance nozzle and thenozzle substrate, oxygen ratio was investigated. Furthermore, the Furthermore, 2D morphologies between the and the the additive substrate, the additive oxygen ratio was investigated. the of on photoresist etched by single plasma were measured bywere a scanning electron 2Dmicroholes morphologies of microholes on photoresist etchedmicrojet by single plasma microjet measured by a microscope (SEM) and an optical microscope. the stylus profiler wasthe applied measuring scanning electron microscope (SEM) and an Additionally, optical microscope. Additionally, stylusfor profiler was the irregular depth shape the etched microholes. applied for measuring theofirregular depth shape of the etched microholes. 3. 3. Results Results and and Discussions Discussions 3.1. The Electrical Characteristics of the Atmospheric Pressure Plasma Microjet 3.1. The Electrical Characteristics of the Atmospheric Pressure Plasma Microjet Helium/oxygen plasma was easily generated in the glass tube when the voltage raised to 3.5 kVpp . Helium/oxygen plasma was easily generated in the glass tube when the voltage raised to After raising applied voltage up to about 4.5 kVpp , plasma microjet expanded out of the glass tube 3.5 kVpp. After raising applied voltage up to about 4.5 kVpp, plasma microjet expanded out of the glass was still too short to be visible to the naked eyes, as shown in Figure 3a. A brighter and longer plasma tube was still too short to be visible to the naked eyes, as shown in Figure 3a. A brighter and longer microjet was observed when placed the Z sliding platform connected to electrical ground and the plasma microjet was observed when placed the Z sliding platform connected to electrical ground and substrate under the nozzle, as shown in Figure 3b. This was due to the substrate acting as a floating the substrate under the nozzle, as shown in Figure 3b. This was due to the substrate acting as a electrode, the plasma microjet was greatly enhanced when contact with the platform. The plasma in the floating electrode, the plasma microjet was greatly enhanced when contact with the platform. The glass tube increased the density of the seed electrons to lower the ignition voltage of plasma microjet, plasma in the glass tube increased the density of the seed electrons to lower the ignition voltage of which resulted in a non-thermal and stable plasma microjet [26]. The plasma microjet array was plasma microjet, which resulted in a non-thermal and stable plasma microjet [26]. The plasma formed successfully in Figure 3c. In addition, silicon micro-nozzles get charged from the bombardment microjet array was formed successfully in Figure 3c. In addition, silicon micro-nozzles get charged from the bombardment of plasma in the glass tube, the unexpected plasma was generated by the
Micromachines 2017, 8, 173 Micromachines 2017, 2017, 8, 8, 173 173 Micromachines
5 of 12 of 12 12 55 of
strong electric field which exceeds the threshold for ignition in the whole space under the nozzle, as strong electric field which exceeds the threshold for ignition in the whole space under the nozzle, as shown in Figure 3d. tube, the unexpected plasma was generated by the strong electric field which of plasma in the glass shown in Figure 3d. exceeds the threshold for ignition in the whole space under the nozzle, as shown in Figure 3d.
Figure 3. image of single plasma microjet without substrate and sliding platform no plasma Figure 3. (a) (a)Optical Optical image of single plasma microjet without substrate and sliding platform no Figure 3. (a) Optical image of single plasma microjet without substrate and sliding platform no plasma plumes was seen by naked eyes; (b,c) optical image of single plasma microjet and 1 × 3 ×2 plasma plumes was seen by naked eyes; (b,c) optical image of single plasma microjet and 1and × 32and plumes was seen by naked eyes; (b,c) optical image of single plasma microjet and 1 × 3 and 2 × 2 microjets arrayarray in the dark condition; (d)(d) discharge × 22 plasma plasma 2plasma × 2 plasma microjets in the dark condition; dischargegenerated generatedsurround surroundthe the 22 × plasma microjets array in the dark condition; (d) discharge generated surround the 2 × 2 plasma microjets when when the the distance distance was was close closeto to 11 mm. mm. Experiment Experiment conditions: conditions: applied applied voltage voltage was was 4.5 4.5kV kVpp,, microjets microjets when the distance was close to 1 mm. Experiment conditions: applied voltage was 4.5 kVpp pp, 11 kHz; 700 sccm helium mixed with 7 sccm oxygen. 11 kHz; 700 sccm helium mixed with 7 sccm oxygen. 11 kHz; 700 sccm helium mixed with 7 sccm oxygen.
As shown shown in in Figure Figure 4a,b, the the discharge current current waveform was was measured by by subtracting the the As As shown in Figure 4a,b, 4a,b, the discharge discharge current waveform waveform was measured measured by subtracting subtracting the displacementcurrent current from the total current, corresponding the situation in Figure 3a,b. Plasma displacement total current, corresponding the situation in Figure Plasma displacement currentfrom fromthethe total current, corresponding the situation in 3a,b. Figure 3a,b. microjet Plasma microjet could hardly be seen in Figure 3a, thus, discharge waveforms in Figure 4a mainly represent could hardly seen in 3a, thus, 3a, discharge waveforms in Figurein 4aFigure mainly4arepresent the DBD microjet couldbehardly beFigure seen in Figure thus, discharge waveforms mainly represent the DBD discharge current in the glass tube. The peak value of discharge current was around 50 mA, discharge current in the glass tube. The peak value discharge currentcurrent was around 50 mA,50 which the DBD discharge current in the glass tube. The peakofvalue of discharge was around mA, which suggested that the plasma was active. According to the literature [27], two discharge current suggested that thethat plasma was active. According to the literature [27], two discharge currentcurrent pulses which suggested the plasma was active. According to the literature [27], two discharge pulses were commonly observed in one DBD discharge cycle. It is speculated that other herringbonewere commonly observed in one DBD discharge cycle. It is speculated that other herringbone-like pulses were commonly observed in one DBD discharge cycle. It is speculated that other herringbonelike current peaks in Figure 4a may represent the discharge betweencharged charged siliconnozzle nozzle and the the current peaks in Figure 4a 4a may represent thethe discharge like current peaks in Figure may represent dischargebetween between chargedsilicon silicon nozzle and and the high-voltage electrode. high-voltage electrode. electrode. high-voltage
Figure Figure 4. 4. (a,b) Measured waveforms of applied voltage and discharge current current of of dielectric dielectric barrier barrier Figure 4. (a,b) Measured waveforms ofsingle applied voltage and discharge current of dielectric barrier discharge plasma in the glass tube and plasma microjet, respectively. Experiment discharge plasma in the glass tube and single plasma microjet, respectively. Experiment conditions: conditions: discharge plasma in theflow glass tube andsccm/7 single plasma microjet, respectively. Experiment helium/oxygen mixed rate of 700 700 sccm,the the frequency wasaround around 11kHz. kHz. conditions: helium/oxygen mixed flow rate of sccm/7 sccm, frequency was 11 helium/oxygen mixed flow rate of 700 sccm/7 sccm, the frequency was around 11 kHz.
Besides,the thecurrent current in Figure 4b represents the discharge plasmaAnmicrojet. An Besides, in Figure 4b represents the discharge current ofcurrent plasmaof microjet. asymmetric Besides, the current in Figure 4b represents the discharge current of plasma microjet. An asymmetric characteristic of discharge currentinwas observed in amplitude Figure 4b,ofas the amplitude of characteristic of discharge current was observed Figure 4b, as the discharge current in asymmetric characteristic of discharge current was observed in Figure 4b, as the amplitude of
Micromachines 2017, 8, 173
6 of 12
Micromachines 2017, 8, 173
6 of 12
discharge current in positive polarity was relatively larger than in negative polarity. Some speculations for the formation of plasma jet may explain this phenomenon. It is known that plasma positive was relatively thanmechanism, in negative in polarity. speculations foristhe formation of jet of thispolarity kind propagates by a larger streamer which Some a source of electrons needed to form plasma jet may explain this phenomenon. It is known that plasma jet of this kind propagates by a streamer the ionization front [28]. The electrons in the negative plasma streamer are scattered, in that an mechanism, in which a source electrons is needed to form the ionization front [28]. The electrons in enhanced electric field couldofnot be created in the plasma channel to maintain the streamer the negative plasma streamer are scattered, in that an enhanced electric field could not be created in the propagation, as compared to the positive plasma streamer [29]. plasma channel to maintain the streamer propagation, as compared to the positive plasma streamer [29]. 3.2. The Spectral Characteristics of Atmospheric Pressure Plasma Microjet 3.2. The Spectral Characteristics of Atmospheric Pressure Plasma Microjet Since different additive oxygen ratio of working gas affected the O-containing radicals in Since different additive oxygen ratio of working gas affected the O-containing radicals in plasma, the effect of additive oxygen gas on the emission intensities from plasma microjet should be plasma, the effect of additive oxygen gas on the emission intensities from plasma microjet should be investigated. The helium gas was maintained at a flow rate of 700 sccm, the oxygen gas flow rates investigated. The helium gas was maintained at a flow rate of 700 sccm, the oxygen gas flow rates varied from 0 sccm to 115 sccm. varied from 0 sccm to 115 sccm. Figure 5a shows a comparison of the optical emission spectrum (OES) observed in pure helium Figure 5a shows a comparison of the optical emission spectrum (OES) observed in pure helium and He/O2 mixed plasma microjet. It was illustrated that ultraviolet (UV) radiation attributed to Nand He/O2 mixed plasma microjet. It was illustrated that ultraviolet (UV) radiation attributed to containing radicals at wavelengths of 300–410 nm in OES, OH band, and atomic oxygen lines were N-containing radicals at wavelengths of 300–410 nm in OES, OH band, and atomic oxygen lines were detected at the wavelength of 309, 777, and 844 nm, respectively. N-containing radicals such as N2 detected at the wavelength of 309, 777, and 844 nm, respectively. N-containing radicals such as N2 band band and OH band was generated by the electrons collide with helium radical species, nitrogen and OH band was generated by the electrons collide with helium radical species, nitrogen components, components, and vapor in ambient air [30,31]. It was noted that the addition of oxygen to the He and vapor in ambient air [30,31]. It was noted that the addition of oxygen to the He plasma caused plasma caused a decrease in the optical intensity of N-containing radicals while the intensity of a decrease in the optical intensity of N-containing radicals while the intensity of reactive oxygen reactive oxygen species (ROS) increased obviously. This suggested that some electrons are consumed species (ROS) increased obviously. This suggested that some electrons are consumed to produce O to produce O radicals. Thus, collisions with air molecules decreased [32], the intensity of N2 band and radicals. Thus, collisions with air molecules decreased [32], the intensity of N2 band and OH band OH band decreased slightly. decreased slightly.
Figure 5. (a) Optical emission spectrum (OES) of the plasma microjet from 200 nm to 900 nm measured Figure 5. (a) Optical emission spectrum (OES) of the plasma microjet from 200 nm to 900 nm measured in the pure helium and helium/oxygen plasma, respectively, the applied voltage was 8.5 kVpp , 11 kHz. in the pure helium and helium/oxygen plasma, respectively, the applied voltage was 8.5 kVpp, 11 kHz. The inner picture was the magnified details of OES from 300 nm to 450 nm; (b) comparisons of the The inner picture was the magnified details of OES from 300 nm to 450 nm; (b) comparisons of the change of the emission intensities from He (587 nm and 706 nm) and O radicals (777 nm and 844 nm) change of the emission intensities from He (587 nm and 706 nm) and O radicals (777 nm and 844 nm) in different additive oxygen ratio increasing from 0 to 15%. in different additive oxygen ratio increasing from 0 to 15%.
As shown in Figure 5b, different additive oxygen ratio affected the intensity of species in the range from 0 to 15%. The intensities of He were decreased with additive oxygen flow. Furthermore, highest intensity intensity of ofROS ROScould couldbe beobtained obtainedatat the additive oxygen ratio of 4%. phenomenon the additive oxygen ratio of 4%. ThisThis phenomenon was was consistent reduction of ROS at the ratio above 4%may maynot notonly onlybe be caused caused by decreased consistent withwith the the reduction of ROS at the ratio above 4% plasma intensity intensity but but also also the the reaction reactionof ofOO++OO22 ++ He He → →O O33 + He that producing producing ozone ozone [33]. [33]. The The O O33 production thus ofof ROS may also decrease at productionbecame becamemore moreeffective effectiveatatthe thehigher higherOO 2 ratio, thusthe theintensity intensity ROS may also decrease 2 ratio,
at high O2 concentrations. In addition, the plasma microjet quenched gradually when oxygen flow
Micromachines 2017, 8, 173
7 of 12
high O2 concentrations. In addition, the plasma microjet quenched gradually when oxygen flow raised Micromachines 2017, 8, 173 7 of 12 to 126 sccm (18%). Correspondingly, the intensity of all species significantly decreased to almost 0. raised to 126 sccm (18%). Correspondingly, the intensity of all species significantly decreased to
3.3. The Maskless Etching of Photoresist almost 0.
3.3.1. 3.3. Maskless Etching of Photoresist The Maskless Etching of PhotoresistUsing Single-Hole Plasma Microjet A single-hole nozzle was used to quantify the result of maskless etching of photoresist film 3.3.1. Maskless Etching of Photoresist Using Single-Hole Plasma Microjet initially. Plasma was ignited by He/O2 mixed gas with different additive oxygen ratio and 700 sccm A single-hole nozzlewas was maintained used to quantify of maskless etching of photoresist film helium. The applied voltage at 8.5the kVresult pp after the generation of plasma microjet. initially. Plasma was ignited by He/O 2 mixed gas with different additive oxygen ratio and 700 sccm The effect of the distance between the nozzle and the polymer surface was examined firstly. helium. The applied voltage was maintained at 8.5 kVpp after the generation of plasma microjet. Figure 6a shows SEM images of the microholes on the photoresist etched by plasma microjet. The effect of the distance between the nozzle and the polymer surface was examined firstly. Additionally, Figure 6b illustrates magnified SEM image on the side wall of the microhole fabricated by Figure 6a shows SEM images of the microholes on the photoresist etched by plasma microjet. plasma microjet. some residual photoresist particles theofetched region I.fabricated Furthermore, Additionally,Still, Figure 6b illustrates magnified SEM imageremained on the sidein wall the microhole the surface on the outside of microholes was decorated with the debris, of which the size was I.varied by plasma microjet. Still, some residual photoresist particles remained in the etched region from nanometers shownofin Figure 6d. The nonhomogeneous etched region Furthermore, to themicrometers, surface on theasoutside microholes was decorated with the debris, of which theII was wasexpansion varied from nanometers to micrometers, shown into in Figure 6d.with The ambient nonhomogeneous owingsize to the effect of the plasma microjetas coming contact air. The data etched II was owing the expansion effect of plasma microjet comingatinto in Figure 6c region indicated that the to maximum diameter of the microholes was etched thecontact closestwith distance ambient air. The data in Figure 6c indicated that the maximum diameter of microholes was etched of 0.5 mm, smaller microholes were etched as distance increased. Possible explanations are at drawn the closest distance of 0.5 mm, smaller microholes were etched as distance increased. Possible that when plasma microjet comes into contact with the substrate, ROS will diffuse and move along explanations are drawn that when plasma microjet comes into contact with the substrate, ROS will the surface via gas pressure. Hence, the closer the distance was, the bigger diffusion area it formed, diffuse and move along the surface via gas pressure. Hence, the closer the distance was, the bigger whichdiffusion leads toarea a bigger etched Moreover, highly applied voltage closer distance it formed, whichmicrohole. leads to a bigger etched microhole. Moreover, highlyand applied voltage enhanced the electric field between the silicon and the substrate. Thus, an active plasma region and closer distance enhanced the electric field between the silicon and the substrate. Thus, an active was generated surrounding the surrounding plasma microjet, which resulted in an enlarged Hence, plasma region was generated the plasma microjet, which resulted in an etched enlargedarea. etched Hence, much bigger werethe etched when was the distance was closer than 1 mm. mucharea. bigger microholes weremicroholes etched when distance closer than 1 mm.
(a) SEM image microholesetched etched on by plasma microjet. The diameter of FigureFigure 6. (a)6.SEM image of of microholes onthe thephotoresist photoresist by plasma microjet. The diameter holes was 266.7 μm. The etch parameters were: distance between nozzle and polymer surface was of thethe holes was 266.7 µm. The etch parameters were: distance between nozzle and polymer surface 1.5 mm, the etch time was 3 min; (b) magnified details of side wall of the microhole in the green box; was 1.5 mm, the etch time was 3 min; (b) magnified details of side wall of the microhole in the green (c) the maximum diameter of the microholes on polymer at different distance between nozzle and box; (c) the maximum diameter of the microholes on polymer at different distance between nozzle substrate; Experimental conditions: 700 sccm helium gas and 28 sccm additive oxygen at applied and substrate; conditions: 70011 sccm gas and 28 image sccm additive oxygen at applied voltage ofExperimental 8.5 kVpp, frequency was around kHz;helium (d) magnified SEM of small area formed by voltage of 8.5 kV , frequency was around 11 kHz; (d) magnified SEM image of small area formed by pp of plasma microjet in the blue box. sputtering effect sputtering effect of plasma microjet in the blue box.
Micromachines 2017, 8, 173 Micromachines 2017, 8, 173
8 of 12 8 of 12
In addition, the experiments show that circle-shaped microholes of several hundred micrometers were etched by plasma microjet emanating from 50 μm square nozzle. This might be In addition, the experiments show that circle-shaped microholes of several hundred micrometers caused by the enlarged plasma zone while the plasma microjet reached the polymer and formed a were etched by plasma microjet emanating from 50 µm square nozzle. This might be caused by the larger disk-like area on its surface compared with the dimension of plasma microjet. This plasma enlarged plasma zone while the plasma microjet reached the polymer and formed a larger disk-like expansion typically existed in DBDs due to the spreading of the accumulated discharge [34] and area on its surface compared with the dimension of plasma microjet. This plasma expansion typically movement of reactive species above the polymer surface. We note here that an understanding of the existed in DBDs due to the spreading of the accumulated discharge [34] and movement of reactive exact dimension of the plasma microjet would be greatly enhanced by imaging of plasma bullet. species above the polymer surface. We note here that an understanding of the exact dimension of However, at present, such facilities are unavailable and will form part of our future experimental the plasma microjet would be greatly enhanced by imaging of plasma bullet. However, at present, work on the system. such facilities are unavailable and will form part of our future experimental work on the system. Another important process parameter affecting the diameter of microholes was additive oxygen Another important process parameter affecting the diameter of microholes was additive oxygen ratio. Figure 7 shows optical images of etching results in different additive oxygen ratio. It was ratio. Figure 7 shows optical images of etching results in different additive oxygen ratio. It was illustrated that the maximum diameter of microholes was fabricated at the additive oxygen ratio of illustrated that the maximum diameter of microholes was fabricated at the additive oxygen ratio 4% since the highest intensity of ROS acted as a key reactive species of removing photoresist. It was of 4% since the highest intensity of ROS acted as a key reactive species of removing photoresist. seen that with the decrease of oxygen flow rate, high temperature, and large amount of high energetic It was seen that with the decrease of oxygen flow rate, high temperature, and large amount of high reactive species in the center of the plasma jet accelerated the carbonization and formed a energetic reactive species in the center of the plasma jet accelerated the carbonization and formed nonhomogeneous and rough zone in the center of microholes [35]. Additionally, as the additive a nonhomogeneous and rough zone in the center of microholes [35]. Additionally, as the additive oxygen ratio increased from 0 to 8%, the diminution of rough and nonhomogeneous etching oxygen ratio increased from 0 to 8%, the diminution of rough and nonhomogeneous etching boundary boundary of etched microholes was seen in Figure 7. This could be explained by that while oxygen of etched microholes was seen in Figure 7. This could be explained by that while oxygen admixture admixture produced chemically active species, the physical sputtering delivered nearly no produced chemically active species, the physical sputtering delivered nearly no contribution to etching contribution to etching process subsequently [36]. The additive oxygen acted as electronegative gas, process subsequently [36]. The additive oxygen acted as electronegative gas, causing the reduction causing the reduction of metastable He, thus eventually weakening the physical sputtering and of metastable He, thus eventually weakening the physical sputtering and decreasing the amounts of decreasing the amounts of sputtering fragments at the boundary of microholes [37]. sputtering fragments at the boundary of microholes [37].
Figure 7. Optical microscopy images of photoresist etched by plasma microjets with different additive Figure 7. Optical microscopy images of photoresist etched by plasma microjets with different additive oxygen ratio from of 0, 1, 2, 4, 6, and 8%, the diameters from 2 to 8% were 322, 438, 429, and 310 µm, oxygen ratio from of 0, 1, 2, 4, 6, and 8%, the diameters from 2 to 8% were 322, 438, 429, and 310 μm, respectively. Other etch parameters were: applied voltage 8.5 kVpp with a frequency of 11 kHz, helium respectively. Other etch parameters were: applied voltage 8.5 kVpp with a frequency of 11 kHz, helium flow rate fixed at 700 sccm, etch time of 3 min and a distance of 1 mm. flow rate fixed at 700 sccm, etch time of 3 min and a distance of 1 mm.
Different byby adjusting thethe etching time. AsAs shown in Figure 8a, Differentetch etchfeatures featurescan canalso alsobebefabricated fabricated adjusting etching time. shown in Figure the of the of microholes varied by etching time under different additive oxygen ratio 8a, data the data of diameter the diameter of microholes varied by etching time under different additive oxygen was in Figure 8a. Figure 8b indicated that as theasetch increase, the horizontal etching rate ratioshown was shown in Figure 8a. Figure 8b indicated that the time etch time increase, the horizontal etching of photoresist increased slightly in the first 3 min, and decreased significantly in subsequent. Indeed, rate of photoresist increased slightly in the first 3 min, and decreased significantly in subsequent. this notable attributed to the weaker of plasma away from thefrom central Indeed, this feature notablewas feature was attributed to the reactivity weaker reactivity of microjet plasma microjet away the axis. Moreover, high intensity of ROS resulted in a stronger etching capability of the plasma microjet, central axis. Moreover, high intensity of ROS resulted in a stronger etching capability of the plasma which caused higher etching rate in horizontal of microholes compared compared the etch result of 2 to 4% and microjet, which caused higher etching rate in horizontal of microholes the etch result of 2 8%. It and is expected increasing of the etching couldrate alsocould be achieved by using by higher to 4% 8%. It isthat expected that increasing of therate etching also be achieved usingapplied higher voltage higher rateflow [35].rate [35]. appliedand voltage andoxygen higherflow oxygen
Micromachines 2017, 8, 173 Micromachines 2017, 8, 173
9 of 12 9 of 12
Figure8.8.(a)(a) The maximum diameter of microholes the microholes the polymer at the different Figure The maximum diameter of the on theonpolymer at the different distancedistance between between and substrate; (b) time-dependent maximum of the microholes at different nozzle andnozzle substrate; (b) time-dependent maximum diameterdiameter of the microholes at different additive additive oxygen ratios; profiles (c) depth of microholes etched by plasma Experimental microjets. Experimental oxygen ratios; (c) depth of profiles microholes etched by plasma microjets. conditions: conditions: 700 sccm helium gas additive and 28 sccm additive oxygen voltage at an applied voltage of 8.5 kVpp, the 700 sccm helium gas and 28 sccm oxygen at an applied of 8.5 kV , the frequency was pp frequency was around 11 kHz. around 11 kHz.
For the purpose of evaluating the morphologies of etching results, the depth profiles were For the purpose of evaluating the morphologies of etching results, the depth profiles were measured by stylus profile crossing the center of the microholes. Figure 8c shows the depth profile measured by stylus profile crossing the center of the microholes. Figure 8c shows the depth profile of microholes etched in photoresist films with etching times from 1 min to 5 min. It was founded that of microholes etched in photoresist films with etching times from 1 min to 5 min. It was founded a V-shape depth profile was measured before 2 min and platform was formed after treated 3 min. that a V-shape depth profile was measured before 2 min and platform was formed after treated 3 min. This indicated that all photoresist was removed in the treatment area. It could be calculated that the This indicated that all photoresist was removed in the treatment area. It could be calculated that the highest etching rate was approximately 4 μm/min. As the etch time increased after 3 min, the profile highest etching rate was approximately 4 µm/min. As the etch time increased after 3 min, the profile changed slightly. The asymmetries of these profiles may originate from the glass tubes was not changed slightly. The asymmetries of these profiles may originate from the glass tubes was not exactly exactly perpendicular to the polymer surface. perpendicular to the polymer surface. 3.3.2. Maskless Etching of Photoresist Using Array-Holes Plasma Microjet 3.3.2. Maskless Etching of Photoresist Using Array-Holes Plasma Microjet Based on the above fundamental etch characteristics of the plasma microjet formed by the singleBased on the above fundamental etch characteristics of the plasma microjet formed by the hole nozzle, parallel etching process could be achieved by controlling the distance, additive oxygen single-hole nozzle, parallel etching process could be achieved by controlling the distance, additive ratio, and the etch time using the micro nozzle array, as shown in Figure 3c. oxygen ratio, and the etch time using the micro nozzle array, as shown in Figure 3c. As a key issue for large-scale applications of the plasma jet array, the spatial uniformity of As a key issue for large-scale applications of the plasma jet array, the spatial uniformity of plasma plasma microjets array caused deviation in diameter of each microholes, which reduces the quality microjets array caused deviation in diameter of each microholes, which reduces the quality of maskless of maskless fabrication. The spatial nonuniformity of the plasma microjet array has been investigated fabrication. The spatial nonuniformity of the plasma microjet array has been investigated and it is and it is caused by the inhomogeneous distribution of the electric and flow field [30] and the repulsion caused by the inhomogeneous distributioncoupling of the electric and flow field [30] and the repulsion of of adjacent microjet caused by momentum and Coulomb force [36]. adjacent microjet causedan byarray momentum coupling was and generated Coulomb force [36]. In our experiment, of 1 × 3 microjets by corresponding nozzle hole array, In our experiment, an array of 1 × 3 microjets was generated by corresponding hole the distance between adjacent nozzle was 1 mm, as shown in Figure 9a. Nonuniformity of 2nozzle × 2 etched array, the distance between adjacent nozzle was 1 mm, as shown in Figure 9a. Nonuniformity microholes array was also obtained when using nozzle holes array with their spacing of 0.5 mm andof 20.8 × 2mm. etched microholes array also obtained nozzle holes array with However, similar sizewas of microholes was when etchedusing if increasing the spacing to 1.5their mm,spacing as can ofbe0.5 mm and 0.8 mm. However, similar size of microholes was etched if increasing the seen the 2 × 2 array in Figure 9b. With the increase of the spacing and symmetric positionspacing of nozzleto 1.5 mm,along as canthe be seen the 2 × 2electrode, array in Figure 9b. With theofincrease of the spacing and symmetric holes high-voltage the electric field the adjacent nozzle was relatively position of nozzle holes along the high-voltage electrode, the electric field of the adjacent nozzle was homogeneous, which suggests that the consistent excitation of the adjacent plasma streamer. relatively homogeneous, which suggests that the consistent excitation of the adjacent plasma streamer. Moreover, higher applied voltage was considered as an increasing of the electric field in each plasma Moreover, higher voltage was considered asplasma an increasing of array the electric field in each plasma plume of the 2 × 2applied array. The inhomogeneity of the microjets was less once the electric plume of the 2 × 2 array. The inhomogeneity of the plasma microjets array was less once the field exceeds the threshold for igniting the plasma microjet [38]. As a result, the diameter ofelectric each field exceeds igniting plasma microjet a result, the diameter each microhole in the 2threshold × 2 array for is almost thethe same. However, the [38]. shapeAs of each microhole in both of arrays microhole in the 2circle-shape, × 2 array iswhich almostsuggests the same. However, the shape of each microhole in both arrays was a defective the inhomogeneous distribution of reactive species in was a defective circle-shape, which suggests the inhomogeneous distribution of reactive species the etched region. There is a speculation that, as the distance between the nozzle and the center axisin the etched region.electrode There isincreased, a speculation that, as field the distance betweenrapidly the nozzle and theposition center axis of high-voltage the electric was weakened at the edge of ofthe high-voltage electrode increased, the electric field was rapidly at the edge position nozzle. According to the literature [39], the structure ofweakened multi-electrodes structure could improveof the According to the literature structure of multi-electrodes structure could thenozzle. uniformity of plasma microjet array[39], via the homogeneous electric field distribution. In theimprove future,
Micromachines 2017, 8, 173
10 of 12
Micromachines 2017, 8, 173
10 of 12
the uniformity of plasma microjet array via the homogeneous electric field distribution. In the future, the electrode structure of an atmospheric pressure plasma microjet should be optimized based on the electrode structure of an atmospheric pressure plasma microjet should be optimized based on theory and simulation. theory and simulation.
Figure 9. 9. (a,b) (a,b) Optical Optical microscopy microscopy images and Figure images of of photoresist photoresist etched etchedby byplasma plasmamicrojets microjetsarray arrayof of11×× 33 and 2 × 2. The diameter of the microholes from left to right in (a) was 196, 251, and 266 µm; diameters in 2 × 2. The diameter of the microholes from left to right in (a) was 196, 251, and 266 μm; diameters in (b) were 251, 268, 253, and 258 µm. Experiment conditions: 700 sccm helium gas and 28 sccm additive (b) were 251, 268, 253, and 258 μm. Experiment conditions: 700 sccm helium gas and 28 sccm additive oxygen at an applied voltage of 7.5 kVpp and 8.5 kVpp , the frequency was around 11 kHz, the distance oxygen at an applied voltage of 7.5 kVpp and 8.5 kVpp, the frequency was around 11 kHz, the distance was 1 mm. was 1 mm.
4. Conclusions Conclusions 4. In this In this study, study, aa novel novel and and simple simple system system based based on on the the atmospheric atmospheric pressure pressure plasma plasma jet jet and and microfabricated MEMS nozzle has been investigated for maskless etching of photoresist microfabricated MEMS nozzle has been investigated for maskless etching of photoresist films.films. The The plasma microjet was emanated from the nozzle, which could etch a single microhole on plasma microjet was emanated from the nozzle, which could etch a single microhole on the the photoresist photoresist successfully. successfully. The The highest highest etching etching rate rate of of the the AR-3210 AR-3210 photoresist photoresist film film reached reached 44 µm/min. μm/min. The The morphology morphology of of microholes microholes affected affected by by the the distance distance between between the the nozzle nozzle and and substrate, substrate, additive additive oxygen ratio, and etching time were investigated. Spectra characteristics of plasma microjet verified oxygen ratio, and etching time were investigated. Spectra characteristics of plasma microjet verified the etching results that the maximum diameter was obtained at an O admixture of 4%, lower additive the etching results that the maximum diameter was obtained at an O22 admixture of 4%, lower additive oxygen flow flow rates Additionally, parallel has oxygen rates led led to to aa carbonization carbonization phenomenon. phenomenon. Additionally, parallel plasma plasma processing processing has been realized via nozzle array. However, an improvement in the consistency of etched microholes been realized via nozzle array. However, an improvement in the consistency of etched microholes array is is still still required, required, comprehensive of the the parallel parallel etching etching properties properties affected affected by array comprehensive investigation investigation of by gas gas flow rate and distribution of nozzle holes array is also needed. Further research will be aimed at flow rate and distribution of nozzle holes array is also needed. Further research will be aimed at these these targets. targets. Based on preliminary experiment, it coulditbecould expected large-scale manufacturing Based onthisthis preliminary experiment, be that expected thatflexible large-scale flexible could be realized using the system integrated with roll-to-roll system soon. This maskless fabrication manufacturing could be realized using the system integrated with roll-to-roll system soon. This method using the atmospheric pressure jet andpressure MEMS nozzle applications in maskless fabrication method using the plasma atmospheric plasmahas jet potential and MEMS nozzle has localized fabrication micropatterns and localized surface modification of several kinds of materials. potential applicationsofin localized fabrication of micropatterns and localized surface modification of
several kinds of materials.
Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 51375469 and No. 51275001). Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. Author Contributions: Y.D. conceived and designed the experiments; Y.D. and M.Z. performed the experiments; 51375469 and No. 51275001). Y.D. and Q.L. analyzed the data; J.C., H.W., and L.W. contributed reagents/materials/analysis tools; Y.D. wrote the paper.Contributions: Y.D. conceived and designed the experiments; Y.D. and M.Z. performed the Author experiments; Y.D. andThe Q.L.authors analyzed the data; J.C., H.W., and L.W. contributed reagents/materials/analysis tools; Conflicts of Interest: declare no conflict of interest. Y.D. wrote the paper.
References Conflicts of Interest: The authors declare no conflict of interest. 1.
Bange, A.; Halsall, H.B.; Heineman, W.R. Microfluidic immune sensor systems. Biosens. Bioelectron. 2005, 20,
1.
Bange, A.; Halsall, H.B.; Heineman, W.R. Microfluidic immune sensor systems. Biosens. Bioelectron. 2005, 20, 2488–2503. Li, J.; Wang, X.; Cheng, C.; Wang, L.; Zhao, E.; Wang, X.; Wen, W. Selective modification for polydimethylsiloxane chip by micro-plasma. J. Mater. Sci. 2012, 48, 1310–1314.
References 2488–2503. [CrossRef] [PubMed]
2.
Micromachines 2017, 8, 173
2. 3.
4.
5.
6. 7.
8. 9. 10. 11.
12. 13. 14. 15. 16. 17.
18. 19.
20.
21. 22. 23.
11 of 12
Li, J.; Wang, X.; Cheng, C.; Wang, L.; Zhao, E.; Wang, X.; Wen, W. Selective modification for polydimethylsiloxane chip by micro-plasma. J. Mater. Sci. 2012, 48, 1310–1314. [CrossRef] Penache, C.; Gessner, C.; Betker, T.; Bartels, V.; Hollaender, A.; Klages, C.P. Plasma printing: Patterned surface functionalisation and coating at atmospheric pressure. IEE Proc. Nanobiotechnol. 2004, 151, 139–144. [CrossRef] [PubMed] Gandhiraman, R.P.; Nordlund, D.; Jayan, V.; Meyyappan, M.; Koehne, J.E. Scalable low-cost fabrication of disposable paper sensors for DNA detection. ACS Appl. Mater. Interfaces 2014, 6, 22751–22760. [CrossRef] [PubMed] Priest, C.; Gruner, P.J.; Szili, E.J.; Al-Bataineh, S.A.; Bradley, J.W.; Ralston, J.; Steele, D.A.; Short, R.D. Microplasma patterning of bonded microchannels using high-precision “injected” electrodes. Lab Chip 2011, 11, 541–544. [CrossRef] [PubMed] Hu, S.W.; Xu, B.Y.; Ye, W.K.; Xia, X.H.; Chen, H.Y.; Xu, J.J. Versatile microfluidic droplets array for bioanalysis. ACS Appl. Mater. Interfaces 2015, 7, 935–940. [CrossRef] [PubMed] Huang, S.; Song, J.; Lu, Y.; Chen, F.; Zheng, H.; Yang, X.; Liu, X.; Sun, J.; Carmalt, C.J.; Parkin, I.P.; et al. Underwater spontaneous pumpless transportation of nonpolar organic liquids on extreme wettability patterns. ACS Appl. Mater. Interfaces 2016, 8, 2942–2949. [CrossRef] [PubMed] Poulsen, R.G. Plasma etching in integrated circuit manufacture—A review. J. Vac. Sci. Technol. 1977, 14, 266–274. [CrossRef] Sankaran, R.M.; Giapis, K.P. Maskless etching of silicon using patterned microdischarges. Appl. Phys. Lett. 2001, 79, 593–595. [CrossRef] Uwe, S.; Dohse, A.; Hoppe, P.; Gehringer, A.; Thomas, M.; Klages, C.-P.; Reinecke, H. Porous Photoresist Stamps for Selective Plasma Treatment. Plasma Process. Polym. 2010, 7, 9–15. Al-Bataineh, S.A.; Szili, E.J.; Gruner, P.J.; Priest, C.; Griesser, H.J.; Voelcker, N.H.; Short, R.D.; Steele, D.A. Fabrication and operation of a microcavity plasma array device for microscale surface modification. Plasma Process. Polym. 2012, 9, 638–646. [CrossRef] Yang, Y.-J.; Hsu, C.-C. A flexible paper-based microdischarge array device: A novel route to cost-effective and simple setup microplasma generation devices. IEEE Trans. Plasma Sci. 2014, 42, 3756–3759. [CrossRef] Hsu, C.-C.; Tsai, J.-H.; Yang, Y.-J.; Liao, Y.-C.; Lu, Y.-W. A foldable microplasma-generation device on a paper substrate. J. Microelectromech. Syst. 2012, 21, 1013–1015. [CrossRef] Sakaguchi, T.; Sakai, O.; Tachibana, K. Photonic bands in two-dimensional microplasma arrays. II. Band gaps observed in millimeter and subterahertz ranges. J. Appl. Phys. 2007, 101, 073305. [CrossRef] Yoshiki, H.; Taniguchi, K.; Horiike, Y. Localized removal of a photoresist by atmospheric pressure micro-plasma jet using RF corona discharge. Jpn. J. Appl. Phys. 2002, 41, 5797–5798. [CrossRef] Guo, H.; Liu, J.; Yang, B.; Chen, X.; Yang, C. Localized etching of polymer films using an atmospheric pressure air microplasma jet. J. Micromech. Microeng. 2015, 25, 015010. [CrossRef] Wang, T.; Liu, J.; Yang, B.; Chen, X.; Wang, X.; Yang, C. Optimization of micropipette fabrication by laser micromachining for application in an ultrafine atmospheric pressure plasma jet using response surface methodology. J. Micromech. Microeng. 2016, 26, 065001. [CrossRef] Kakei, R.; Ogino, A.; Iwata, F.; Nagatsu, M. Production of ultrafine atmospheric pressure plasma jet with nano-capillary. Thin Solid Films 2010, 518, 3457–3460. [CrossRef] Morimatsu, D.; Sugimoto, H.; Nakamura, A.; Ogino, A.; Nagatsu, M.; Iwata, F. Development of a scanning nanopipette probe microscope for fine processing using atmospheric pressure plasma jet. Jpn. J. Appl. Phys. 2016, 55, 08NB15. [CrossRef] Abuzairi, T.; Okada, M.; Mochizuki, Y.; Poespawati, N.R.; Purnamaningsih, R.W.; Nagatsu, M. Maskless functionalization of a carbon nanotube dot array biosensor using an ultrafine atmospheric pressure plasma jet. Carbon 2015, 89, 208–216. [CrossRef] Shimane, R.; Kumagai, S.; Hashizume, H.; Ohta, T.; Ito, M.; Hori, M.; Sasaki, M. Localized plasma irradiation through a micronozzle for individual cell treatment. Jpn. J. Appl. Phys. 2014, 53, 11RB03. [CrossRef] Jõgi, I.; Talviste, R.; Raud, J.; Piip, K.; Paris, P. The influence of the tube diameter on the properties of an atmospheric pressure He micro-plasma jet. J. Phys. D Appl. Phys. 2014, 47, 415202. [CrossRef] Inomata, K.; Koinuma, H.; Oikawa, Y.; Shiraishi, T. Open air photoresist ashing by a cold plasma torch: Catalytic effect of cathode material. Appl. Phys. Lett. 1995, 66, 2188–2190. [CrossRef]
Micromachines 2017, 8, 173
24. 25. 26. 27. 28. 29. 30.
31.
32.
33. 34. 35.
36. 37.
38. 39.
12 of 12
Hopf, C.; Schlüter, M.; Schwarz-Selinger, T.; von Toussaint, U.; Jacob, W. Chemical sputtering of carbon films by simultaneous irradiation with argon ions and molecular oxygen. New J. Phys. 2008, 10, 093022. [CrossRef] Wang, L.; Zheng, Y.; Wu, C.; Jia, S. Experimental investigation of photoresist etching by kHz AC atmospheric pressure plasma jet. Appl. Surf. Sci. 2016, 385, 191–198. [CrossRef] Li, X.; Yuan, N.; Jia, P.; Chen, J. A plasma needle for generating homogeneous discharge in atmospheric pressure air. Phys. Plasmas 2010, 17, 093504. [CrossRef] Lu, X.; Laroussi, M. Dynamics of an atmospheric pressure plasma plume generated by submicrosecond voltage pulses. J. Appl. Phys. 2006, 100, 063302. [CrossRef] Walsh, J.L.; Olszewski, P.; Bradley, J.W. The manipulation of atmospheric pressure dielectric barrier plasma jets. Plasma Sources Sci. Technol. 2012, 21, 034007. [CrossRef] Lu, X.; Laroussi, M.; Puech, V. On atmospheric-pressure non-equilibrium plasma jets and plasma bullets. Plasma Sources Sci. Technol. 2012, 21, 034005. [CrossRef] Zhang, C.; Shao, T.; Zhou, Y.; Fang, Z.; Yan, P.; Yang, W. Effect of O2 additive on spatial uniformity of atmospheric-pressure helium plasma jet array driven by microsecond-duration pulses. Appl. Phys. Lett. 2014, 105, 044102. Joh, H.M.; Choi, J.Y.; Kim, S.J.; Chung, T.H.; Kang, T.H. Effect of additive oxygen gas on cellular response of lung cancer cells induced by atmospheric pressure helium plasma jet. Sci. Rep. 2014, 4, 6638. [CrossRef] [PubMed] Ellerweg, D.; Benedikt, J.; von Keudell, A.; Knake, N.; Schulz-von der Gathen, V. Characterization of the effluent of a He/O2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry. New J. Phys. 2010, 12, 013021. [CrossRef] Park, G.; Lee, H.; Kim, G.; Lee, J.K. Global model of He/O2 and Ar/O2 atmospheric pressure glow discharges. Plasma Process. Polym. 2008, 5, 569–576. [CrossRef] Sun, L.; Huang, X.; Zhang, J.; Zhang, J.; Shi, J.J. Discharge dynamics of pin-to-plate dielectric barrier discharge at atmospheric pressure. Phys. Plasmas 2010, 17, 113507. [CrossRef] Wang, T.; Yang, B.; Chen, X.; Wang, X.; Yang, C.; Liu, J. Nonhomogeneous surface properties of parylene-C film etched by an atmospheric pressure He/O2 micro-plasma jet in ambient air. Appl. Surf. Sci. 2016, 383, 261–267. [CrossRef] Fricke, K.; Steffen, H.; von Woedtke, T.; Schröder, K.; Weltmann, K.-T. High Rate Etching of Polymers by Means of an Atmospheric Pressure Plasma Jet. Plasma Process. Polym. 2011, 8, 51–58. [CrossRef] Weltmann, K.D.; Brandenburg, R.; von Woedtke, T.; Ehlbeck, J.; Foest, R.; Stieber, M.; Kindel, E. Antimicrobial treatment of heat sensitive products by miniaturized atmospheric pressure plasma jets (APPJs). J. Phys. D Appl. Phys. 2008, 41, 194008. [CrossRef] Ghasemi, M.; Olszewski, P.; Bradley, J.W.; Walsh, J.L. Interaction of multiple plasma plumes in an atmospheric pressure plasma jet array. J. Phys. D Appl. Phys. 2013, 46, 052001. [CrossRef] Sun, P.P.; Cho, J.H.; Park, C.H.; Park, S.J.; Eden, J.G. Close-Packed Arrays of Plasma Jets Emanating from Microchannels in a Transparent Polymer. IEEE Trans. Plasma Sci. 2012, 40, 2946–2950. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
