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MEMS solid propellant thruster array with micro membrane igniter
Sensors and Actuators A 190 (2013) 52–60
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
MEMS solid propellant thruster array with micro membrane igniter Jongkwang Lee a , Taegyu Kim b,∗ a b
Department of Mechanical Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daegeon 305-719, Republic of Korea Department of Aerospace Engineering, College of Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 501-759, Republic of Korea
a r t i c l e
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Article history: Received 12 July 2012 Received in revised form 6 November 2012 Accepted 6 November 2012 Available online 16 November 2012 Keywords: MEMS thruster Microigniter Glass membrane Solid propellant
a b s t r a c t A microigniter with improved membrane for MEMS solid propellant thruster array is proposed. Although existing igniters using dielectric SiNx membranes consume low power, they cannot withstand shocks during the propellant charging process. A glass wafer is selected as the material for the igniter. A glass membrane is fabricated by the anisotropic wet etching of photosensitive glass. Platinum is used for the ignition coil. A 30–40-m thick glass membrane exhibited sufficient strength to withstand accidental impacts during the propellant charging process. Further, the microigniter using the glass membrane had an appropriate power consumption to ignite the solid propellant. The thermal, electrical, and mechanical characteristics of the fabricated microigniter were measured. The proposed microigniter provided the sufficient heat for the propellant to be ignited with the given electric power. The fracture pressure of the glass membrane was thrice higher than the conventional microigniter. A solid propellant lead styphnate was filled into the microigniter without any additional processes. Ignition tests with fully assembly MEMS thrusters were performed successfully. Thus, it is demonstrated that a simpler, robust, and low-cost fabrication process of an igniter that performs well when assembled into a micro solid propellant thruster (MSPT) array is possible. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent advances in microelectromechanical system (MEMS) and micromachining technology have encouraged miniaturization in the space industry [1,2]. Microsystems are especially beneficial for satellites. Microsatellites and nanosatellites reduce costs due to their low weight and size and shorten the development time. Moreover, reliability and flexibility is also enhanced since a cluster of satellites are employed in these systems. Substantial developmental efforts are required to achieve the miniaturization of components in microsatellites and nanosatellites. Not only does the size and weight have to be reduced, but it must also be possible to obtain the requisite power. In these satellites, the microthrusters providing the micropropulsion are key components because a very small and accurate force is required to attain orbit transfer, attitude control, and station keeping [3,4]. The development of microthrusters has been an active research area in recent years, and the performance of various microthrusters has been investigated, for example, microthrusters employing monopropellants [5], hybrid cold gases [6], bipropellants [7], and solid propellants [8–12]. Among these microthrusters, the development of micro solid propellant thrusters (MSPTs) is the most feasible with the
∗ Corresponding author. Tel.: +82 62 230 7123; fax: +82 62 230 7729. E-mail address: [email protected] (T. Kim). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.11.013
current MEMS technology. MSPTs have no moving parts, no liquid fuel leakage, and fabrication is relatively simple. However, MSPTs also have a disadvantage: they have one-shot characteristics, but this can be compensated by employing a suitable array configuration. Typically, MSPTs consist of a micronozzle, microigniter, propellant chamber, and propellant. The microigniter, which initiates an MSPT, is the most important component for realizing the thruster. Further, microigniters also have diverse applications such as in micro heat sources and microsensors [13–16]. The major requirements of a microigniter are low power consumption, good confinement of heat, fast response, structural stability, and low fabrication cost. Macroscale solid propellant thrusters are triggered by hot gas fluxes or strong electrical sparks, but these cannot be applied to MSPTs because of their large size and power consumption. A different approach is required for the development of microigniters [26–30]. As a result, various microigniters based on Joule heating have been widely investigated. Rossi et al. proposed polysilicon microigniters that consisted of a thin dielectric membrane from a silicon substrate on which a polysilicon was patterned [17]. Tanaka et al. attempted noncontact ignition to avoid the adhesion problem by using a reactive boron (B)/titanium (Ti) multilayer on a thin dielectric membrane igniter [18]. Thin dielectric membranes decrease power consumption, but they are not reliable in harsh environments; moreover, filling the propellant is very difficult if a few micron-thick membranes are used due to the possible
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rupture caused by the impact of the propellant. Briand et al. realized a freestanding microigniter keeping in mind power consumption [19]. However, the mechanical properties of the microheater were not good because the freestanding resistor had a thickness less than 1 m. Thus, filling the propellant is still problematic. Furthermore, its fabrication cost was relatively expensive compared to other fabrication processes such as glass materials. Zhang et al. suggested a microigniter on a glass substrate to enhance the structural stability [20]. However, this microigniter exhibited high power consumption and did not have a fast response time since the thick substrate of several hundred microns had a very large thermal mass. Thus, the microigniter cannot provide the sufficient ignition energy to the propellant in the configuration of vertical-type MSPTs. In this study, we improved the mechanical stability of a microigniter by using a glass membrane having a thickness of tens of microns and developed 3 × 3 MSPT array without compromising its performance. In such a microigniter, filling the propellant is easy and low power consumption can be maintained; moreover, the fabrication method has low fabrication cost and the integration process with other components of the MSPT is relatively simple. We then evaluated the performance of the microigniter. After the development of the microigniter, other components of the MSPT were fabricated from glass and integrated with the microigniter. Further, the ignition characteristics of MSPT were determined. 2. Design MSPTs can be classified into two types on the basis of their geometry. In horizontal-type MSPTs, which have a two-dimensional shape, most of the thruster components are present in one layer and various geometries can be realized. In vertical-type MSPTs, a stack of several wafers is present, and the fabrication process is more complicated. However, vertical-type MSPTs are more suitable for the arrays of a thruster, and hence, the vertical type was selected for this study. The schematic of a 3 × 3 MSPT array is illustrated in Fig. 1. The MSPT consists of four layers and a solid propellant. The micronozzle is located at the first layer. The third layer contains the microigniter and propellant chamber. The fourth layer seals the propellant chamber. An intermediary layer is added between the microigniter and micronozzle in order to prevent nozzle plugging. The operation principle is as follows: the microigniter is supplied with electric power. The temperature of the microigniter is then increased by Joule heating. The solid propellant is ignited by the microigniter when the ignition temperature of the propellant is achieved. High-temperature and high-pressure combustion gases are thereby generated. The membrane is subsequently broken by the gases, and the thrust is generated through the micronozzle. The important design parameters of the microigniter were the material of the resistance and membrane, and the geometry of the resistance. Pt (platinum) was adopted as the material of the resistance because of its high stability at high temperature and resistance to oxidation and corrosion [21]. A glass membrane having thickness of tens of microns was selected for maintaining stability, even if it has a lower thermal conductivity than the dielectric membrane. The fabrication cost of the glass membrane is cheaper than that of the dielectric membrane. The resistance was designed to have a meandering shape since this shape has a relatively high ratio of pattern length to area, which enables good thermal performance. 3. Fabrication The microigniter on the glass membrane was fabricated on a 300-m thick photosensitive glass (Hoya-PEG3). Photosensitive
Fig. 1. Schematic of MEMS solid propellant thruster array.
glass is a useful material in the manufacturing of the components of many microsystems because etching with a high aspect ratio is relatively easy. Moreover, the heat conductivity is lower (1.35 W/mK at 20 ◦ C), which ensures good thermal insulation between the chambers [22]. Initially, photosensitive glass has no specific orientation, which implies that the glass is etched isotropically in all directions. Hence, it is difficult to achieve a precise structure with high aspect ratio. However, photosensitive glass can induce an anisotropic property. Photosensitive glass can be made anisotropic by exposing it to UV (ultraviolet) light. The exposed portion of the glass becomes crystallized with heat treatment while the non-exposed portion remains in the vitreous form. Moreover, the crystallized parts can be etched 20 times faster than the glass parts. The fabrication process of the microigniter on the glass membrane is shown in Fig. 2. A quartz wafer with a chromium absorber pattern was prepared for transmission at the required wavelength of 310 nm (Fig. 2(a)). Further, the substrate was exposed to UV light at an approximate wavelength of 310 nm (Fig. 2(b)). The photosensitive glass was illuminated by UV light with an energy density of approximately 2.5 J/cm2 . During the UV exposure step, irradiation releases free electrons from a donor ion, such as Ce3+ . These electrons then reduce silver ions (Ag+ ) to silver atoms (Ag0 ). Heat treatment (Fig. 2(c)) was achieved using a programmable furnace. The photosensitive glass was heated to 500 ◦ C at a ramp rate of 3 ◦ C/min. Subsequently, it was reheated to 585 ◦ C at a ramp rate of 1 ◦ C/min and then maintained constant at 585 ◦ C for 1 h. Finally, the glass was cooled at a cooling rate of −3 ◦ C/min. During the
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Fig. 2. Method for fabricating microigniter on glass membrane.
heat treatment, the photosensitive glass crystallized around the silver atoms. The physical properties of the crystallized sections (glass-ceramic) were different from those of the glass itself. The heat treatments led to poor adhesion of the two glass wafers with the surface roughness and flatness. Hence, the surface was polished by chemical and mechanical polishing for the photolithography process. The photoresist was spin-coated onto the glass and patterned using photolithography (Fig. 2(d)). A Ti layer of thickness 200 A˚ was deposited by sputtering in order to improve the Pt adhesion to the glass. A Pt layer of thickness 2000 A˚ was then deposited onto the Ti layer by sputtering (Fig. 2(e)). The photoresist above the glass and above Pt/Ti were removed by using acetone for 2 min. After this process, only the Pt/Ti pattern of the microigniter remained (Fig. 2(f)). The surface on which the Pt/Ti layer was patterned had to be protected from the hydrofluoric (HF) acid solution. Photoresist was spin-coated onto the surface, and the surface was then taped by Kapton® tape (Fig. 2(g) and (h)). The final step in the fabrication was to etch the glassceramic (Fig. 2(i)). The etching was performed in a diluted 10% HF solution. The etching time was monitored in order to control the thickness of the glass membrane. During the etching process, the membrane thickness and etching rate were measured by using Alpha-step surface profiler to obtain the precise membrane thickness. After the etching process, the Kapton® tape and photoresist were carefully removed in acetone. The microigniter fabricated on the glass membrane is shown in Fig. 3. The yellow part is the glass membrane. The average thickness and diameter of the membrane were 35 m and 1 mm, respectively. The width of the Pt/Ti pattern was 40 m, and the heating area was 600 m × 440 m. The ratio of pattern length to the heating area was 0.545.
We also fabricated the microigniter on a 1.5-m thick Si3 N4 membrane and 500-m thick photosensitive glass substrate to determine the effects of the membrane. The schematic of the microigniters is shown in Fig. 4. These microigniters had the same material and geometry as the resistance. The fabrication process of the microigniter on a Si3 N4 membrane is illustrated in Fig. 5. First, a 600-m thick silicon wafer was taken. SiO2 and Si3 N4 were then deposited by thermal oxidation and low pressure chemical vapor deposition (LPCVD) (Fig. 5(a)). A ˚ ˚ coil was fabricated using a lift-off technique Pt (2000 A)/Ti (200 A) (Fig. 5(b) and (c)). A 1 mm circular window pattern was opened in the dielectric layer on the back side of the wafer by using conventional photolithography and reactive ion etching (RIE) (Fig. 5(d) and (e)). Deep RIE (DRIE) was performed to etch the silicon. SiO2 was etched by RIE, and the photoresist was then removed (Fig. 5(f)). The microigniter on the glass substrate was fabricated by using the same method as that for the proposed microigniter except that HF etching was not employed. 4. Characteristics of microigniter The thermal, electrical, and mechanical characteristics of the microigniter were measured. The thermal and electrical characteristics are the temperature responses to the given electric power, while the mechanical characteristics are represented by the fracture pressure of the membrane. 4.1. Heating characteristics The temperature response to the given electric power was evaluated by an indirect method that is commonly used in the
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Fig. 4. Three types of microigniters.
Fig. 3. Microigniter fabricated on glass membrane.
performance estimation of microigniters. The relationship of the temperature with the given power was induced from the relationship of the resistance variation with the temperature and given electric power [17]. In order to determine the effects of membranes, the microigniter performance was estimated with different membranes: the proposed glass membrane, a 1.5-m thick Si3 N4 membrane, and a 500-m thick photosensitive glass substrate. The relationships of the variations in the resistance with the temperature and the given electric power for the microigniter on the glass membrane are shown in Fig. 6. The solid and dashed lines represent linear fits to the experimental results, which had good accuracy. The temperature response to the given electric power was then estimated from Fig. 6. Other cases were also estimated by using the same method. The temperature responses to the given electric power in all cases are shown in Fig. 7. The ignition temperature of lead styphnate is 260 ◦ C [23]. The glass membrane required approximately 340 mW, the Si3 N4 membrane approximately 285 mW, while the glass substrate required 1150 mW to obtain a
Fig. 5. Method for fabricating microigniter on dielectric membrane.
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Fig. 8. Fracture pressure as a function of membrane thickness.
Fig. 6. Variations in resistance as a function of temperature and power.
temperature of 260 ◦ C. The heating property of the microigniter on the glass membrane was approximately 20% lower than that on the dielectric membrane and four times higher than that on the glass substrate. 4.2. Mechanical characteristics The fracture pressure of the membrane is also an important characteristic because the membrane is closely related to the filling propellant. The membrane can withstand the filling up process of the solid propellant and is sufficiently thin to be able to break when gases are generated. The microigniter on the dielectric membrane requires an additional process because of the thin membrane [16,24]. In the present study, the fracture pressure of the membranes was measured. An apparatus that pressured only the membrane was prepared. Pure nitrogen gas was used for pressurizing a side of the membrane, whereas the opposite side maintained the atmospheric pressure. Teflon holder was used to fix the membrane and to connect the pressurizing gas line. Glass membranes in a range of thicknesses (27–66 m), and a 1.5-m thick Si3 N4 membrane were prepared for the experiments. The fracture pressure is presented in Fig. 8. A 35-m thick glass membrane could withstand 1531 kPa;
this is thrice the pressure that the 1.5-m thick Si3 N4 membrane could withstand. 5. Ignition test 5.1. Fabrication and assembly of MSPT array An MSPT array was fabricated for performing the ignition test on the microigniter. The micronozzle layer, intermediary layer, and seal layer were also fabricated with anisotropic etching of the photosensitive glass after taking workability into consideration. The divergence angle of the micronozzle could be realized by controlling the etching rate because the crystallized part was etched 20 times faster than the other glass part. The average diameter of the nozzle throat was 416 m, the average area ratio of the micronozzle was 1.85, and the average divergence angle was approximately 4.29◦ . The thickness of the micronozzle layer, intermediary layer, and seal layer were 1 mm, 0.3 mm, and 0.5 mm, respectively. The integration process of the MSPT array is shown in Fig. 9. The first step of the assembly process is applicable to the nozzle part. The fabricated micronozzle layer and intermediary layer were polished and cleaned in a piranha solution (H2 SO4 :H2 O2 = 3:1) for 10 min. The layers were pressed together at 500 ◦ C at 2 kPa for 12 h at a ramp rate of 2 ◦ C/min and a cooling rate of approximately −3 ◦ C/min [25]. In the second step, the propellant lead styphnate was filled in the microigniter. Lead styphnate is one of the most widely used initiating compounds and in its original form is a powder. Lead styphnate was dissolved in a solution comprising the solvent and binder. The solution was then poured into the microchamber and dried to remove the solvent. Fig. 10 shows the microchamber filled with lead styphnate. After the filling process, UV curable glue (OP-24 REV B, Dymax Corp.) was spincoated onto the seal layer. The bottom side of the microigniter and seal layer was then carefully aligned. The glue was cured by UV exposure. Finally, the nozzle and microigniter were also bonded by the UV curable glue. We used Kapton® tape at the UV bonding to prevent the nozzle from being plugged and the propellant from being wetted by the glue. Fig. 11 shows the fabricated MSPT array. 5.2. Ignition characteristics
Fig. 7. Exothermic properties of microigniters.
Firing test was performed in order to estimate the ignition characteristics and thrust performance of the MSPT array. Fig. 12
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Fig. 9. Process for integrating MSPT array.
illustrates the experimental setup. The ignition characteristics were obtained by using the electric probes and oscilloscope. In order to estimate the ignition delay and ignition energy, the time between the beginning of ignition and membrane rupture was measured. A quartz force sensor was used to measure the thrust and total impulse. The sensor was a piezoelectric sensor (Kistler, 9205), which produced an electric charge. The charge meter (Kistler, 5015A) converted the electric charge into a voltage. A high-speed digital video camera was used to capture the combustion of the MSPT.
Fig. 10. (a) Fabricated microchamber and (b) filling of lead styphnate.
Fig. 11. MSPT array.
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Fig. 12. Experimental setup for obtaining ignition characteristics.
Fig. 13 shows the combustion process of lead styphnate. The images were acquired at 2000 frame/s. Combustion was successfully observed after igniting the MSPT with a microigniter on the glass membrane. Only those gases that were released from the chamber are shown in Fig. 13. This phenomenon implies that combustion completely occurred in the propellant chamber. The ignition delay, ignition energy, and ignition power are presented in Figs. 14 and 15. The ignition failed below 6 V even when the time of electric supply was sufficient. The ignition delay and ignition energy exponentially decreased as the voltage increased. The ignition also failed above 14.4 V because the Pt coil was
damaged. The minimum ignition delay was 27.5 ms with ignition energy of 19.3 mJ. The four cases of thrusters were fired in order to measure the thrust. The ignition can be failed if the solid propellant does not contact the glass membrane well because the heat from the microigniter does not be transferred to the propellant efficiently. Nevertheless, all cases of thrusters were ignited successfully. From the combustion test, we found that the averages of the maximum thrust and total impulse were 3619 mN and 0.381 mN s, respectively. The average specific impulse was 62.3 s.
Fig. 13. Combustion test of lead styphnate.
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average of the maximum thrust and total impulse in the case of lead styphnate was 3619 mN and 0.381 mN s, respectively. The present study has demonstrated that a simpler, robust, and low cost fabrication process of the igniter is possible, and moreover, the proposed igniter performs well when assembled into an MSPT array. References
Fig. 14. Ignition delay as a function of voltage.
Fig. 15. Ignition energy as a function of voltage.
6. Conclusion In this paper, the fabrication and performance evaluation of a microigniter with improved membrane for MEMS solid propellant thruster array have been described. We improved the stability of the microigniter by using a glass membrane having a thickness of tens of microns. We developed a process to fabricate the microigniter on a photosensitive glass wafer instead of on a dielectric membrane, which is the conventional method. The membrane was fabricated by the anisotropic etching of photosensitive glass. Pt was used as the heat coil. The average thickness of the membrane was 35 m. The heating characteristic with the given electric power was estimated. The electric power required to obtain temperatures up to 260 ◦ C, which is the ignition temperature of lead styphnate, was approximately 340 mW. The proposed microigniter provided the sufficient heat for the propellant to be ignited with the given electric power. The pressure at which the membrane yielded was measured. The glass membrane withstood 1531 kPa; this is approximately 3 times higher than the pressure withstood by the dielectric membrane. The solid propellant could be filled into the microchamber without the need for a special technique due to the high structural stability of the glass membrane. The MSPT array was fabricated for performing the ignition test of the proposed microigniter. Lead styphnate was used as the propellant. The microigniter operated in a reliable manner and delivered the required power to achieve a successful ignition. The minimum ignition energy was 19.3 mJ with an ignition delay of 27.5 ms. The
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Biographies Jongkwang Lee received the B.S. degree from Korea Aerospace University (KAU), Goyang, Korea, in 2002 and the M.S. and Ph.D. degrees in Aerospace Engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea,
in 2004 and 2009, respectively. From 2009 to 2012, he worked in the LCD R&D Center, Samsung Electronics, Yongin, Korea, as a Senior Researcher. In 2012, he joined the Department of Mechanical Engineering at Hanbat National University, Daejeon, Korea, where he is currently an Assistant Professor. His current research focuses micro-propulsion devices, micro-reaction technology, and micro-fluidics. Taegyu Kim received the B.S. degree from Korea Aerospace University (KAU), Goyang, Korea, in 2003 and the M.S. and Ph.D. degrees in Aerospace Engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2005 and 2008, respectively. In 2009, he joined the Department of Aerospace Engineering at Chosun University, Gwangju, Korea, where he is currently an Assistant Professor. His research interests include various kinds of micro energy conversion systems, such as micro fuel cell, micro reformer and micro plasma reactor.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
MEMS solid propellant thruster array with micro membrane igniter Jongkwang Lee a , Taegyu Kim b,∗ a b
Department of Mechanical Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daegeon 305-719, Republic of Korea Department of Aerospace Engineering, College of Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 501-759, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 July 2012 Received in revised form 6 November 2012 Accepted 6 November 2012 Available online 16 November 2012 Keywords: MEMS thruster Microigniter Glass membrane Solid propellant
a b s t r a c t A microigniter with improved membrane for MEMS solid propellant thruster array is proposed. Although existing igniters using dielectric SiNx membranes consume low power, they cannot withstand shocks during the propellant charging process. A glass wafer is selected as the material for the igniter. A glass membrane is fabricated by the anisotropic wet etching of photosensitive glass. Platinum is used for the ignition coil. A 30–40-m thick glass membrane exhibited sufficient strength to withstand accidental impacts during the propellant charging process. Further, the microigniter using the glass membrane had an appropriate power consumption to ignite the solid propellant. The thermal, electrical, and mechanical characteristics of the fabricated microigniter were measured. The proposed microigniter provided the sufficient heat for the propellant to be ignited with the given electric power. The fracture pressure of the glass membrane was thrice higher than the conventional microigniter. A solid propellant lead styphnate was filled into the microigniter without any additional processes. Ignition tests with fully assembly MEMS thrusters were performed successfully. Thus, it is demonstrated that a simpler, robust, and low-cost fabrication process of an igniter that performs well when assembled into a micro solid propellant thruster (MSPT) array is possible. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent advances in microelectromechanical system (MEMS) and micromachining technology have encouraged miniaturization in the space industry [1,2]. Microsystems are especially beneficial for satellites. Microsatellites and nanosatellites reduce costs due to their low weight and size and shorten the development time. Moreover, reliability and flexibility is also enhanced since a cluster of satellites are employed in these systems. Substantial developmental efforts are required to achieve the miniaturization of components in microsatellites and nanosatellites. Not only does the size and weight have to be reduced, but it must also be possible to obtain the requisite power. In these satellites, the microthrusters providing the micropropulsion are key components because a very small and accurate force is required to attain orbit transfer, attitude control, and station keeping [3,4]. The development of microthrusters has been an active research area in recent years, and the performance of various microthrusters has been investigated, for example, microthrusters employing monopropellants [5], hybrid cold gases [6], bipropellants [7], and solid propellants [8–12]. Among these microthrusters, the development of micro solid propellant thrusters (MSPTs) is the most feasible with the
∗ Corresponding author. Tel.: +82 62 230 7123; fax: +82 62 230 7729. E-mail address: [email protected] (T. Kim). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.11.013
current MEMS technology. MSPTs have no moving parts, no liquid fuel leakage, and fabrication is relatively simple. However, MSPTs also have a disadvantage: they have one-shot characteristics, but this can be compensated by employing a suitable array configuration. Typically, MSPTs consist of a micronozzle, microigniter, propellant chamber, and propellant. The microigniter, which initiates an MSPT, is the most important component for realizing the thruster. Further, microigniters also have diverse applications such as in micro heat sources and microsensors [13–16]. The major requirements of a microigniter are low power consumption, good confinement of heat, fast response, structural stability, and low fabrication cost. Macroscale solid propellant thrusters are triggered by hot gas fluxes or strong electrical sparks, but these cannot be applied to MSPTs because of their large size and power consumption. A different approach is required for the development of microigniters [26–30]. As a result, various microigniters based on Joule heating have been widely investigated. Rossi et al. proposed polysilicon microigniters that consisted of a thin dielectric membrane from a silicon substrate on which a polysilicon was patterned [17]. Tanaka et al. attempted noncontact ignition to avoid the adhesion problem by using a reactive boron (B)/titanium (Ti) multilayer on a thin dielectric membrane igniter [18]. Thin dielectric membranes decrease power consumption, but they are not reliable in harsh environments; moreover, filling the propellant is very difficult if a few micron-thick membranes are used due to the possible
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rupture caused by the impact of the propellant. Briand et al. realized a freestanding microigniter keeping in mind power consumption [19]. However, the mechanical properties of the microheater were not good because the freestanding resistor had a thickness less than 1 m. Thus, filling the propellant is still problematic. Furthermore, its fabrication cost was relatively expensive compared to other fabrication processes such as glass materials. Zhang et al. suggested a microigniter on a glass substrate to enhance the structural stability [20]. However, this microigniter exhibited high power consumption and did not have a fast response time since the thick substrate of several hundred microns had a very large thermal mass. Thus, the microigniter cannot provide the sufficient ignition energy to the propellant in the configuration of vertical-type MSPTs. In this study, we improved the mechanical stability of a microigniter by using a glass membrane having a thickness of tens of microns and developed 3 × 3 MSPT array without compromising its performance. In such a microigniter, filling the propellant is easy and low power consumption can be maintained; moreover, the fabrication method has low fabrication cost and the integration process with other components of the MSPT is relatively simple. We then evaluated the performance of the microigniter. After the development of the microigniter, other components of the MSPT were fabricated from glass and integrated with the microigniter. Further, the ignition characteristics of MSPT were determined. 2. Design MSPTs can be classified into two types on the basis of their geometry. In horizontal-type MSPTs, which have a two-dimensional shape, most of the thruster components are present in one layer and various geometries can be realized. In vertical-type MSPTs, a stack of several wafers is present, and the fabrication process is more complicated. However, vertical-type MSPTs are more suitable for the arrays of a thruster, and hence, the vertical type was selected for this study. The schematic of a 3 × 3 MSPT array is illustrated in Fig. 1. The MSPT consists of four layers and a solid propellant. The micronozzle is located at the first layer. The third layer contains the microigniter and propellant chamber. The fourth layer seals the propellant chamber. An intermediary layer is added between the microigniter and micronozzle in order to prevent nozzle plugging. The operation principle is as follows: the microigniter is supplied with electric power. The temperature of the microigniter is then increased by Joule heating. The solid propellant is ignited by the microigniter when the ignition temperature of the propellant is achieved. High-temperature and high-pressure combustion gases are thereby generated. The membrane is subsequently broken by the gases, and the thrust is generated through the micronozzle. The important design parameters of the microigniter were the material of the resistance and membrane, and the geometry of the resistance. Pt (platinum) was adopted as the material of the resistance because of its high stability at high temperature and resistance to oxidation and corrosion [21]. A glass membrane having thickness of tens of microns was selected for maintaining stability, even if it has a lower thermal conductivity than the dielectric membrane. The fabrication cost of the glass membrane is cheaper than that of the dielectric membrane. The resistance was designed to have a meandering shape since this shape has a relatively high ratio of pattern length to area, which enables good thermal performance. 3. Fabrication The microigniter on the glass membrane was fabricated on a 300-m thick photosensitive glass (Hoya-PEG3). Photosensitive
Fig. 1. Schematic of MEMS solid propellant thruster array.
glass is a useful material in the manufacturing of the components of many microsystems because etching with a high aspect ratio is relatively easy. Moreover, the heat conductivity is lower (1.35 W/mK at 20 ◦ C), which ensures good thermal insulation between the chambers [22]. Initially, photosensitive glass has no specific orientation, which implies that the glass is etched isotropically in all directions. Hence, it is difficult to achieve a precise structure with high aspect ratio. However, photosensitive glass can induce an anisotropic property. Photosensitive glass can be made anisotropic by exposing it to UV (ultraviolet) light. The exposed portion of the glass becomes crystallized with heat treatment while the non-exposed portion remains in the vitreous form. Moreover, the crystallized parts can be etched 20 times faster than the glass parts. The fabrication process of the microigniter on the glass membrane is shown in Fig. 2. A quartz wafer with a chromium absorber pattern was prepared for transmission at the required wavelength of 310 nm (Fig. 2(a)). Further, the substrate was exposed to UV light at an approximate wavelength of 310 nm (Fig. 2(b)). The photosensitive glass was illuminated by UV light with an energy density of approximately 2.5 J/cm2 . During the UV exposure step, irradiation releases free electrons from a donor ion, such as Ce3+ . These electrons then reduce silver ions (Ag+ ) to silver atoms (Ag0 ). Heat treatment (Fig. 2(c)) was achieved using a programmable furnace. The photosensitive glass was heated to 500 ◦ C at a ramp rate of 3 ◦ C/min. Subsequently, it was reheated to 585 ◦ C at a ramp rate of 1 ◦ C/min and then maintained constant at 585 ◦ C for 1 h. Finally, the glass was cooled at a cooling rate of −3 ◦ C/min. During the
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Fig. 2. Method for fabricating microigniter on glass membrane.
heat treatment, the photosensitive glass crystallized around the silver atoms. The physical properties of the crystallized sections (glass-ceramic) were different from those of the glass itself. The heat treatments led to poor adhesion of the two glass wafers with the surface roughness and flatness. Hence, the surface was polished by chemical and mechanical polishing for the photolithography process. The photoresist was spin-coated onto the glass and patterned using photolithography (Fig. 2(d)). A Ti layer of thickness 200 A˚ was deposited by sputtering in order to improve the Pt adhesion to the glass. A Pt layer of thickness 2000 A˚ was then deposited onto the Ti layer by sputtering (Fig. 2(e)). The photoresist above the glass and above Pt/Ti were removed by using acetone for 2 min. After this process, only the Pt/Ti pattern of the microigniter remained (Fig. 2(f)). The surface on which the Pt/Ti layer was patterned had to be protected from the hydrofluoric (HF) acid solution. Photoresist was spin-coated onto the surface, and the surface was then taped by Kapton® tape (Fig. 2(g) and (h)). The final step in the fabrication was to etch the glassceramic (Fig. 2(i)). The etching was performed in a diluted 10% HF solution. The etching time was monitored in order to control the thickness of the glass membrane. During the etching process, the membrane thickness and etching rate were measured by using Alpha-step surface profiler to obtain the precise membrane thickness. After the etching process, the Kapton® tape and photoresist were carefully removed in acetone. The microigniter fabricated on the glass membrane is shown in Fig. 3. The yellow part is the glass membrane. The average thickness and diameter of the membrane were 35 m and 1 mm, respectively. The width of the Pt/Ti pattern was 40 m, and the heating area was 600 m × 440 m. The ratio of pattern length to the heating area was 0.545.
We also fabricated the microigniter on a 1.5-m thick Si3 N4 membrane and 500-m thick photosensitive glass substrate to determine the effects of the membrane. The schematic of the microigniters is shown in Fig. 4. These microigniters had the same material and geometry as the resistance. The fabrication process of the microigniter on a Si3 N4 membrane is illustrated in Fig. 5. First, a 600-m thick silicon wafer was taken. SiO2 and Si3 N4 were then deposited by thermal oxidation and low pressure chemical vapor deposition (LPCVD) (Fig. 5(a)). A ˚ ˚ coil was fabricated using a lift-off technique Pt (2000 A)/Ti (200 A) (Fig. 5(b) and (c)). A 1 mm circular window pattern was opened in the dielectric layer on the back side of the wafer by using conventional photolithography and reactive ion etching (RIE) (Fig. 5(d) and (e)). Deep RIE (DRIE) was performed to etch the silicon. SiO2 was etched by RIE, and the photoresist was then removed (Fig. 5(f)). The microigniter on the glass substrate was fabricated by using the same method as that for the proposed microigniter except that HF etching was not employed. 4. Characteristics of microigniter The thermal, electrical, and mechanical characteristics of the microigniter were measured. The thermal and electrical characteristics are the temperature responses to the given electric power, while the mechanical characteristics are represented by the fracture pressure of the membrane. 4.1. Heating characteristics The temperature response to the given electric power was evaluated by an indirect method that is commonly used in the
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Fig. 4. Three types of microigniters.
Fig. 3. Microigniter fabricated on glass membrane.
performance estimation of microigniters. The relationship of the temperature with the given power was induced from the relationship of the resistance variation with the temperature and given electric power [17]. In order to determine the effects of membranes, the microigniter performance was estimated with different membranes: the proposed glass membrane, a 1.5-m thick Si3 N4 membrane, and a 500-m thick photosensitive glass substrate. The relationships of the variations in the resistance with the temperature and the given electric power for the microigniter on the glass membrane are shown in Fig. 6. The solid and dashed lines represent linear fits to the experimental results, which had good accuracy. The temperature response to the given electric power was then estimated from Fig. 6. Other cases were also estimated by using the same method. The temperature responses to the given electric power in all cases are shown in Fig. 7. The ignition temperature of lead styphnate is 260 ◦ C [23]. The glass membrane required approximately 340 mW, the Si3 N4 membrane approximately 285 mW, while the glass substrate required 1150 mW to obtain a
Fig. 5. Method for fabricating microigniter on dielectric membrane.
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Fig. 8. Fracture pressure as a function of membrane thickness.
Fig. 6. Variations in resistance as a function of temperature and power.
temperature of 260 ◦ C. The heating property of the microigniter on the glass membrane was approximately 20% lower than that on the dielectric membrane and four times higher than that on the glass substrate. 4.2. Mechanical characteristics The fracture pressure of the membrane is also an important characteristic because the membrane is closely related to the filling propellant. The membrane can withstand the filling up process of the solid propellant and is sufficiently thin to be able to break when gases are generated. The microigniter on the dielectric membrane requires an additional process because of the thin membrane [16,24]. In the present study, the fracture pressure of the membranes was measured. An apparatus that pressured only the membrane was prepared. Pure nitrogen gas was used for pressurizing a side of the membrane, whereas the opposite side maintained the atmospheric pressure. Teflon holder was used to fix the membrane and to connect the pressurizing gas line. Glass membranes in a range of thicknesses (27–66 m), and a 1.5-m thick Si3 N4 membrane were prepared for the experiments. The fracture pressure is presented in Fig. 8. A 35-m thick glass membrane could withstand 1531 kPa;
this is thrice the pressure that the 1.5-m thick Si3 N4 membrane could withstand. 5. Ignition test 5.1. Fabrication and assembly of MSPT array An MSPT array was fabricated for performing the ignition test on the microigniter. The micronozzle layer, intermediary layer, and seal layer were also fabricated with anisotropic etching of the photosensitive glass after taking workability into consideration. The divergence angle of the micronozzle could be realized by controlling the etching rate because the crystallized part was etched 20 times faster than the other glass part. The average diameter of the nozzle throat was 416 m, the average area ratio of the micronozzle was 1.85, and the average divergence angle was approximately 4.29◦ . The thickness of the micronozzle layer, intermediary layer, and seal layer were 1 mm, 0.3 mm, and 0.5 mm, respectively. The integration process of the MSPT array is shown in Fig. 9. The first step of the assembly process is applicable to the nozzle part. The fabricated micronozzle layer and intermediary layer were polished and cleaned in a piranha solution (H2 SO4 :H2 O2 = 3:1) for 10 min. The layers were pressed together at 500 ◦ C at 2 kPa for 12 h at a ramp rate of 2 ◦ C/min and a cooling rate of approximately −3 ◦ C/min [25]. In the second step, the propellant lead styphnate was filled in the microigniter. Lead styphnate is one of the most widely used initiating compounds and in its original form is a powder. Lead styphnate was dissolved in a solution comprising the solvent and binder. The solution was then poured into the microchamber and dried to remove the solvent. Fig. 10 shows the microchamber filled with lead styphnate. After the filling process, UV curable glue (OP-24 REV B, Dymax Corp.) was spincoated onto the seal layer. The bottom side of the microigniter and seal layer was then carefully aligned. The glue was cured by UV exposure. Finally, the nozzle and microigniter were also bonded by the UV curable glue. We used Kapton® tape at the UV bonding to prevent the nozzle from being plugged and the propellant from being wetted by the glue. Fig. 11 shows the fabricated MSPT array. 5.2. Ignition characteristics
Fig. 7. Exothermic properties of microigniters.
Firing test was performed in order to estimate the ignition characteristics and thrust performance of the MSPT array. Fig. 12
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Fig. 9. Process for integrating MSPT array.
illustrates the experimental setup. The ignition characteristics were obtained by using the electric probes and oscilloscope. In order to estimate the ignition delay and ignition energy, the time between the beginning of ignition and membrane rupture was measured. A quartz force sensor was used to measure the thrust and total impulse. The sensor was a piezoelectric sensor (Kistler, 9205), which produced an electric charge. The charge meter (Kistler, 5015A) converted the electric charge into a voltage. A high-speed digital video camera was used to capture the combustion of the MSPT.
Fig. 10. (a) Fabricated microchamber and (b) filling of lead styphnate.
Fig. 11. MSPT array.
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Fig. 12. Experimental setup for obtaining ignition characteristics.
Fig. 13 shows the combustion process of lead styphnate. The images were acquired at 2000 frame/s. Combustion was successfully observed after igniting the MSPT with a microigniter on the glass membrane. Only those gases that were released from the chamber are shown in Fig. 13. This phenomenon implies that combustion completely occurred in the propellant chamber. The ignition delay, ignition energy, and ignition power are presented in Figs. 14 and 15. The ignition failed below 6 V even when the time of electric supply was sufficient. The ignition delay and ignition energy exponentially decreased as the voltage increased. The ignition also failed above 14.4 V because the Pt coil was
damaged. The minimum ignition delay was 27.5 ms with ignition energy of 19.3 mJ. The four cases of thrusters were fired in order to measure the thrust. The ignition can be failed if the solid propellant does not contact the glass membrane well because the heat from the microigniter does not be transferred to the propellant efficiently. Nevertheless, all cases of thrusters were ignited successfully. From the combustion test, we found that the averages of the maximum thrust and total impulse were 3619 mN and 0.381 mN s, respectively. The average specific impulse was 62.3 s.
Fig. 13. Combustion test of lead styphnate.
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average of the maximum thrust and total impulse in the case of lead styphnate was 3619 mN and 0.381 mN s, respectively. The present study has demonstrated that a simpler, robust, and low cost fabrication process of the igniter is possible, and moreover, the proposed igniter performs well when assembled into an MSPT array. References
Fig. 14. Ignition delay as a function of voltage.
Fig. 15. Ignition energy as a function of voltage.
6. Conclusion In this paper, the fabrication and performance evaluation of a microigniter with improved membrane for MEMS solid propellant thruster array have been described. We improved the stability of the microigniter by using a glass membrane having a thickness of tens of microns. We developed a process to fabricate the microigniter on a photosensitive glass wafer instead of on a dielectric membrane, which is the conventional method. The membrane was fabricated by the anisotropic etching of photosensitive glass. Pt was used as the heat coil. The average thickness of the membrane was 35 m. The heating characteristic with the given electric power was estimated. The electric power required to obtain temperatures up to 260 ◦ C, which is the ignition temperature of lead styphnate, was approximately 340 mW. The proposed microigniter provided the sufficient heat for the propellant to be ignited with the given electric power. The pressure at which the membrane yielded was measured. The glass membrane withstood 1531 kPa; this is approximately 3 times higher than the pressure withstood by the dielectric membrane. The solid propellant could be filled into the microchamber without the need for a special technique due to the high structural stability of the glass membrane. The MSPT array was fabricated for performing the ignition test of the proposed microigniter. Lead styphnate was used as the propellant. The microigniter operated in a reliable manner and delivered the required power to achieve a successful ignition. The minimum ignition energy was 19.3 mJ with an ignition delay of 27.5 ms. The
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Biographies Jongkwang Lee received the B.S. degree from Korea Aerospace University (KAU), Goyang, Korea, in 2002 and the M.S. and Ph.D. degrees in Aerospace Engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea,
in 2004 and 2009, respectively. From 2009 to 2012, he worked in the LCD R&D Center, Samsung Electronics, Yongin, Korea, as a Senior Researcher. In 2012, he joined the Department of Mechanical Engineering at Hanbat National University, Daejeon, Korea, where he is currently an Assistant Professor. His current research focuses micro-propulsion devices, micro-reaction technology, and micro-fluidics. Taegyu Kim received the B.S. degree from Korea Aerospace University (KAU), Goyang, Korea, in 2003 and the M.S. and Ph.D. degrees in Aerospace Engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2005 and 2008, respectively. In 2009, he joined the Department of Aerospace Engineering at Chosun University, Gwangju, Korea, where he is currently an Assistant Professor. His research interests include various kinds of micro energy conversion systems, such as micro fuel cell, micro reformer and micro plasma reactor.