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Integration of a MEMS based safe arm and fire device
Sensors and Actuators A 159 (2010) 157–167
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Integration of a MEMS based safe arm and fire device Hélène Pezous a,b , Carole Rossi a,b,∗ , Marjorie Sanchez a,b , Fabrice Mathieu a,b , Xavier Dollat a,b , Samuel Charlot a,b , Ludovic Salvagnac a,b , Véronique Conédéra a,b a b
CNRS, LAAS, 7 Avenue du Colonel Roche, F-31077, Toulouse, France Université deToulouse, UPS, INSA, INP, ISAE, LAAS, F-31077, Toulouse, France
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 18 January 2010 Accepted 12 March 2010 Available online 18 March 2010 Keywords: MEMS Micro initiator, Pyrotechnical microactuator, Safe arm and fire
a b s t r a c t The paper describes a new architecture of a Safe Arm and Fire device (SAF) that could constitute a real breakthrough for safe miniature fuzing device. On the one hand, it takes all the functions embodied in a conventional mechanical arm and fire system and integrates them in a single 1 cm3 package made of assembly of different parts. On the other hand, for the first time, it combines a mechanical arming unit with electrical safety functionalities on the same silicon initiator’s chip. It respects the STANAG 4187 norm (1 A/W during 5 min of not fire) and requires only 635 mW for ignition. The paper presents the design, fabrication and test of one miniature SAF device integrating a micropyrotechnical actuation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An innovative concept and technology called micropyrotechnics was proposed by LAAS-CNRS in 1995. Since then, this concept has been applied into many fields by many teams: micropropulsion [1–10], microinitiation [11,13–15] (in both military and civil domains), gases for actuation (including injection or moving fluid) [12,16–19], gases for chemical reaction [21–23], welding [24] and switching [25]. In this paper, we propose to explore micropyrotechnics to fabricate a miniature Safe Arm and Fire device (SAF). The main functions of a SAF device are to keep the detonator safe (a screen interrupts the explosive train), to arm it (the screen is mechanically removed from the safe position) and to initiate one primary explosive necessary for initiating the munition (secondary explosive). A SAF is therefore not a “sensor” or a pyrotechnical initiator, but it combines both sensing and actuation functions in a very tiny volume and must operate with a high reliability level. Different architectures of miniature SAF have been proposed in the opened literature or patented. Most of realizations use conventional technologies but recently some teams have explored MEMS technologies to fabricate some parts (initiator [26–34], screen actuation element [37–40,43,46,47]) or to integrate the whole SAF device [32,35,36,40,41,42,44,45,48,49]. They faced 2 main difficulties:
∗ Corresponding author at: CNRS, LAAS, 7 Avenue du Colonel Roche, F-31077, Toulouse, France. E-mail address: [email protected] (C. Rossi). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.03.017
(1) the integration of sensing elements, actuators, pyrotechnical elements and safety functions in a very tiny volume with a sufficient reliability level is not impossible by still requires nettlesome technological efforts. (2) the use of electrical micro actuator to move the metallic screen from the safe position to arm position requires high amount of electrical energy and therefore makes the device difficult to be integrated within a few cm3 . None of published work, at this stage has proposed convincing solutions for both issues. In this paper, we present the design, fabrication and characterization of a new MEMS SAF architecture, integrating initiator, electrical and mechanical protections within a 1 cm3 , and based on micropyrotechnical actuation. 2. Architecture and principle of operation of the SAF device As illustrated in Fig. 1, the architecture of the SAF MEMS device consists in an assembly of different wafers. The bottom layer of Fig. 1 constitutes the mechanical arming function. The intermediary layer is a silicon chip, called Si-based safe initiator, on which are integrated 2 electrical resistances (one to initiate an explosive and one for the pyrotechnical actuator) and microswitches to realize the electrical arming and disarming functions. The top layer, thicker one, is the electronic circuitry and power supply integrating if required one supercapacity. To prevent that a shock unlocks the mechanical screen, an inertial pin can be inserted into the device to block the screen. Even if this piece has not been integrated into the demonstrator detailed in this paper, its place
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Fig. 1. View of the MEMS SAF device made of three stacked parts: electronic circuitry, Si-based safe initiator and the mechanical arming function.
has been reserved and it will present no difficulty for its integration. The SAF device operations procedure is illustrated in Fig. 2: The MEMS SAF is stored in safe mode: the screen is locked and the initiator pads are both connected to the electrical ground (cf. Fig. 2a). The first order is for the mechanical arming (cf. Fig. 2b): the inertial pin is removed by the acceleration; then the microcontroller sends an
electrical order to the microactuator resistance. The gas generated by the pyrotechnical actuator moves the screen in armed position. Then, the SAF is electrically armed, that is to say the microinitiator electrical short-circuit to electrical ground is cut (cf. Fig. 2c). At this stage, if the operator does not send an order to stop the procedure, the microcontroller sends an electrical order to the initiator to ignite the primary explosive located in its cavity (cf. Fig. 2d), and
Fig. 2. Operations procedure of the MEMS SAF device (a) in safe mode, (b) mechanically armed, (c) electrically armed and (d) secondary explosive initiation by initiator.
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Fig. 3. Layout of the Si-based safe initiator layer (total area: 7.4 mm × 8.4m).
the secondary explosive can be initiated. If there is a failure during the sequences, it is possible to disconnect definitively the microinitiator from its power supply by breaking its electrical connections.
3. Design of the Si-based safe initiator The main component of the silicon Si-based safe initiator chip is 1 resistive microinitiator with a primary explosive and three bistable electro-thermal MEMS switches (2 ON–OFF switches and 1 OFF–ON switch). The Si-based safe initiator layer contains also a second resistance with a gas generator energetic material (called microactuator) used to move the ceramic screen from safe to arm position. Its principle of operation is simple: after ignition by joule effect, the energetic material decomposition generates hot gases that increase the pressure into the micro actuator cavity that moves the mechanical barrier from safe to arm position (see Fig. 2b). The silicon chip layout is detailed in Fig. 3.
3.1. The microinitiator and microactuator Both consist of a heating resistance (polysilicon resistive element) on a 1 mm2 dielectric membrane. Inside the cavity, underneath the membrane, a thin layer of energetic material is deposited (see Fig. 4). The heating resistance is of 120 . A highly energetic material (explosive for example) is loaded in the microinitiator cavity built in the silicon substrate whereas the gas generator material (propellant for example) is loaded in the microactuator cavity. The volume of primary explosive is not chosen at this stage of the project because it depends on the application. However, the volume of the propellant for the actuation has been calculated using an in house 1D computational model capable to predict the pressure, temperature of the gas inside the cavity as a function of the propellant properties and cavity volume. This model is based on gas flow conservation of mass, momentum and energy [50]. Using a composite propellant, to generate 6 Bar, the energetic material volume-over-cavity volume ratio must be of 50. The
Fig. 4. Schematic 3D view of the microactuator or microinitiator.
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3.2. The ON–OFF switch The concept is based on the rupture of an aluminum electrical connection when the ON–OFF switching is required. For that, an aluminum line is placed on top of a heating resistance deposited on a SiO2 /SiNx membrane, as seen in Fig. 6. The heating resistance is a polysilicon resistance of 140 deposited and the membrane has a diameter of 500 m. When ON–OFF switching is required, the resistance heats the aluminium until its evaporation.
3.3. The OFF–ON switch The OFF–ON switch (Fig. 7) is complementary to the ON–OFF switch. Its principle is based on a microbrazing by locally heating up to 183 ◦ C a Sn/Pb solder microball deposited between two copper tracks to connect. Before welding, the two electric tracks are electrically insulated by a thin layer of rosin which evaporates at a lower temperature than the brazing one (∼83 ◦ C). Rosin, formerly called colophony is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. Rosin is the precursor to the flux used in soldering. The switch structure is the same one as the ON–OFF one and consists in a resistive element of 50 deposited on a dielectric membrane with a diameter of 1.5 mm.
4. Fabrication and assembly Fig. 5. Schema of the micro actuator cavity (a) before the gases are released (b) after the gases are released.
4.1. Fabrication of the Si-based safe initiator
propellant volume has been calculated at 0.08 mm3 . We choose a pastille of Ø1 mm and 100 m thick. Fig. 5 gives the geometry and dimensions of actuator cavity and propellant volume used for the modelling.
The manufacturing process, set up in LAAS-CNRS microfabrication center, for the Si-based safe initiator is a collective process allowing the integration of all the functions (ON–OFF and OFF–ON switches, the microactuator and the microinitiator) in the same silicon chip. The process starts with a 4 (100) double face polished silicon wafer and requires seven mask levels.
Fig. 6. Schematic 3D view of the ON–OFF switch (a) before switching (b) after switching.
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Fig. 7. Schematic 3D view of the OFF–ON switch (a) before switching (b) after switching.
4.1.1. Front side processing The wafer is initially cleaned in two successive baths: Piranha (H2 O2 + H2 SO4 ) followed by a diluted 50% HF bath. First step is the silicon thermal oxidation (SiO2 ) on 1.4 m at 1150 ◦ C. Then 0.6 m of SiNx is deposited by LPCVD (Low Pressure Chemical Vapor Deposition) at 705 ◦ C. The SiO2 /SiNx bilayer forms a thin low-stressed dielectric membrane (2 m, 0.1 MPa) for suspended heating platforms. Third step is the deposition of a 0.5 m thick polysilicon by LPCVD at 605 ◦ C. It is then doped by phosphorus diffusion. The resistivity of this polysilicon layer that will constitute the resistive layer is about 8.9.10−4 cm−1 . A RIE (Reactive Ion Etching) step through a photo resist mask reveals the resistance shape. Then, 0.7 m of low-stressed PECVD (Plasma Enhanced Chemical Vapor Deposition) oxide is deposited and then etched everywhere except on the switches polysilicon resistances. This 0.7 m of PECVD oxide isolates electrically the metal layer (next processing steps) from the polysilicon resistance. The following steps are the metallization: first, 3 m thick of copper is electroplated through a resist mask. The electroplating bath is referred as MicrofabCU200 (25 g (Cu)/L (H2 SO4 )) Enthone Omi. The copper is then covered by 500 Å of gold to prevent its oxidation. Then, 3 m of aluminum is evaporated and etched away through a photoresist mask to leave Al only on the ON–OFF switches polysilicon resistance. Metallization steps end with a thermal annealing at 270 ◦ C during 30 min to get ohmic contact. 4.1.2. Back-side processing The three back-side SiO2 , SiNx and polysilicon layers are etched away by RIE. Afterwards, the 400 m thick silicon is etched by DRIE (Deep Reactive Ion Etching) through a photoresist mask to release the micro heater membranes. 4.1.3. Post-processing It remains three post-processing steps before the Si-based safe initiator could be operational. (1) A rosin (called colophane) thin layer is deposited between the two OFF–ON switches Cu tracks by stamping. The chip and rosin reservoir are heated to 140 ◦ C to obtain a good viscosity of the
rosin. Then a Ø350 m Sn/Pb solder ball made by the Indium Corporation is deposited on the rosin. The rosin evaporation temperature is 83 ◦ C. Finally, the chip is cooled down to solidify the rosin that traps the solder ball. (2) The microactuator cavity (Ø 1 mm) is filled with a propellant type energetic material. (3) To validate its functioning, the icroinitiator cavity (Ø 1 mm) is filled with a second propellant type energetic material (which is not the final one). The process is summarized in Fig. 8. Fig. 9 shows one photo of a fabricated Si-based safe initiator with zooms on the ON–OFF and OFF–ON switches, the microactuator and the microinitiator. 4.2. Assembly of the arming function The aluminum has been chosen for the arming function case because it presents good mechanical properties and a good longevity so it is used in assemblies subjected to high thermal gradients and pressure. The MACOR [51] has been chosen for the movable mechanical screen: it is a ceramic which can support high thermal gradients (800–1000 ◦ C). It is a rigid material featuring high resistance to wear effect like climatic or chemical aggressions and a very low thermal conductivity. The thermal dilation coefficient ˛ like its physical properties as Young modulus approaches those of silicon. Both materials, aluminum and MACOR, can be machined precisely by traditional way. The mechanical screen geometrical parameters are given in Fig. 10. The MACOR piece corner radius of curvature is slightly larger than the one of the slide corners in which it moves. It is thus able to achieve its end of course in force in the slider and to remain thus blocked in armed position. The aluminum case is divided in two parts: the cap and the base, as it can be seen of Fig. 10. These two parts are machined in a complementary way. Indeed, the base has a 400 m height step in periphery of the cavity. And the cap has the complementary print as a crenel shape. The objective of this geometry takes all its direction during the assembly of the base and the cap by gluing.
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Fig. 8. Summary of the main process steps for the Si-based safe initiator fabrication.
It prevents any glue flow inside the cavity. External dimensions of mechanical safety are the same ones as those of the Si-based safe initiator chip (8.4 mm × 7.4 mm). 4.3. SAF final assembly We work with the Epotek H70E glue to assemble the arming function parts. It has good mechanical properties and in particular a rigidity modulus of = 1.35·107 Pa and a very good thermal dissipation [52]. Surfaces to be stuck are cleaned in an ethanol bath. And a 99% ethanol and 1% promoter of adherence Epotek AP100 solution is deposited on surfaces to be assembled. Then the glue is
dispensed on the part to be assembled using a calibrated syringe (Ø 250 m) and then the assembly is realized with a calibrated force (5000 g) and a reticulation is done at 150 ◦ C during 300 s. After the arming function is glued, the silicon chip is glued with the same epoxy on the arming function part (see Fig. 11) and is reticulated at 80 ◦ C during 90 min. The temperature of reticulation has been reduced to prevent rosin to evaporate. 5. Characterizations Each Si chip elements has been tested to determine the electrical power and time of response.
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Fig. 9. Photo of the fabricated Si-based safe initiator: zooms on (a) and (b) ON–OFF switches, (c) OFF–ON switch with its Sn/Pb solder ball, (d) microinitiator and (e) microactuator with propellant.
5.1. ON–OFF switch The ON–OFF characterization aims to determine time and power before is meafor ON–OFF actuation. The ON–OFF switch resistance Rswitch
sured first. Then an electrical power (see Table 1), is applied to the heater (Rheater ) to switch from ON to OFF position. The switchbefore , Rafter ing is recorded via an oscilloscope to record Rswitch (the switch final state) and the switching time. Results are given in Table 1.
Fig. 10. Schematic view and photos of the different parts of the mechanical arming function (a) aluminum base, (b) aluminum cap and (c) MACOR screen.
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Fig. 13. Photos of OFF–ON microswitch (a) the solder ball is deposited on top of the heater and rosin spread on the whole membrane, (b) the commutation is done in the ON state. Table 3 Experimental results of the microactuator and microinitiator ignition.
Fig. 11. Photo of the final assembly of the Si-based safe initiator stuck on the mechanical arming function. Table 1 Experimental results of the ON–OFF switches. Rheater ()
Iin (mA)
Vin1 (V)
Pin (mW)
tin (s)
before Rswitch ()
after Rswitch ()
% of success
280
53
12
508.8
1.51
1.4
∞
25
Rheater ()
Vin (V)
Iin (mA)
Pin (mW)
tin (ms)
% of success
226
12
53
635
36.49
100
and 90% of successful OFF–ON commutations have been obtained. It requires about 72.07 mJ (162.7 mW during 443 ms). Photos of OFF–ON microswitch before and after actuation are given in Fig. 13. After the soldering is done, currents of 100 mA are applied to the switch to measure the contact resistance which is of 1.2 and remains stable. 5.3. Microinitiator/microactuator ignition The polysilicon heater resistance (Rheater ) of about 220 is powered with 53 mA and under 12 V. And the propellant ignition requires roughly 40 ms. 20 samples have been tested and 100% of ignition success has been reached. Results are summarized in Table 3. Photos of microinitiator/microactuator before and after actuation are given in Fig. 14.
Fig. 12. Photos of thermal ON–OFF microswitch: the polysilicon resistance on the membrane with the aluminium line on top (a) before actuation and (b) after actuation.
It requires about 760 mJ (500 mW during 1500 ms) to switch. It has to be noticed that only 25% of the ON–OFF switches were successful (20 samples have been characterized). Main causes of failure are the rupture of the membrane before heating up the aluminum track. To reduce this problem, we will reduce the thickness of the aluminum track to 1 m. Photos of ON–OFF microswitch before and after actuation are given in Fig. 12. 5.2. OFF–ON switch
5.4. Screen actuation The test bench carried out is the same one as for the electric characterizations of the microactuator on glass leaf. A constant current pulse Iin of 53 mA powers the micro actuator heater resistance (Rheater ). The assembly is hermetically sealed to confine gases resulting from the decomposition of the propellant contained in the microactuator cavity. After ignition, the screen moves from the safe position to armed position on 1.41 mm. The screen actuation functionality has been validated. However, we get only 5 successful actuation over 10 tests. This result should be improved by choosing a better energetic material to generate gas. Photos of the MACOR screen in the arming function before and after actuation of the microactuator are given in Fig. 15.
As for ON–OFF microswitch, the characterization aims to determine time and power necessary for OFF–ON actuation. First, voltage up to 100 V has been applied to the switch in OFF state validating before is measured first. Then an its opened state. The resistance Rswitch electrical power (see Table 2) is applied to the heater (Rheater ) to switch from OFF to ON position. The switching time is recorded after . via an oscilloscope and the final state is also monitored Rswitch Results are given in Table 2. 20 samples have been characterized Table 2 Experimental results of the OFF–ON switches. Rheater ()
Iin1 (mA)
Vin1 (V)
Pin (mW)
tin (ms)
before Rswitch ()
after Rswitch ()
% of success
45.2
60
3.4
162.7
443
∞
1.2
90
Fig. 14. Photos of microinitiator/microactuator (a) before actuation, (b) after actuation.
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Fig. 15. Photos of the back-side arming function (a) before actuation :the MACOR screen is in safe position and (b) after actuation : the MACOR screen is in armed position.
6. Electronic circuitry and power supply device In the embedded and autonomous system area, we developed a very low-power consumption electronic scheme to drive a thermal initiator microdevice (cf. Fig. 16). Usually, autonomous electronic systems are used to communicate or to measure physical signals, which need low energetic quantity. In this application, electronic has to provide less than 650 mW to each of the five polysilicon resistances of the Si-based safe initiator, that corresponds to a hundred to thousand factor in comparison with traditional energetic consumption in autonomous systems. There are five resistances to be controlled by the electronic circuitry. Furthermore, to supply this power for the device, we need to work at an important voltage (10 V) which does not match with standards in low-power world. The selected solution for energy storage is a pre-loaded supercapacitor (Cooperbussmann Powerstor PA-5R0V474-R) kept on load by button cell. To maximize the lifetime, a low-consumption electronic is used and set on sleep mode until its use. The waking is made by an external order. The system’s lifetime reaches 30 000 h. It is also planned to keep the embedded electronic on voltage during the stocking. To provide the required power, the power supply voltage is raised by a DC/DC converter. This power sending is driven by anaFig. 17. Electronic circuitry with three PCB layers interconnected on their sides.
log switches. Two kinds of fast switching (less than 200 ns) and low on resistance components are used: a CMOS analog switch and MOSFET. A microcontroller (Microchip PIC18F1220) supervises all required stages for the SAF operation described in previous section. Small components and multilayer circuit fulfil size specification (1 cm3 ). The embedded electronic is composed of three double-side stages connected each other by vias, and connected with the silicon chip by Sn/Pb soldering. The power supply is placed above the system and is the bulkiest element (not shown on Fig. 17). 2 Entire assemblies as shown on Fig. 17 have been tested with electronic and all the functional steps (mechanical arming, electrical arming and simulation of initiation) have been successful. 7. Conclusions and perspectives
Fig. 16. Cut view of the electronic circuitry and its power supply made of stacked PCB layers.
Very innovative MEMS based safe, arm and fire device has been proposed, designed, fabricated. It is a multilayer stackedwafers: the bottom layer is the mechanical arming function made of Aluminum and MACOR. The intermediary layer is a silicon chip integrating electrical resistances and microswitches to realize the electrical arming and disarming functions. The top layer is the electronic and power circuitry with one supercapacitor.
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The technology (silicon chip fabrication and whole device assembling) and its operation have been validated by experimentations. Microactuator, ON–OFF and OFF–ON microswitches characterizations gave encouraging results: • The microactuator functioning principle is validated. The MACOR screen could move from safe to armed position in the aluminium slide (displacement of 1.41 mm). • OFF–ON microswitches require 72.07 mJ (162.7 mW during 443 ms) to commute from OFF to ON state. 90% of success has been obtained. • ON–OFF microswitches require 772 mJ (508 mW during 1.51 s) to commute from ON to OFF state. However, only 25% of success has been obtained which will be improved by reducing the aluminium thickness. In perspective, we need to integrate the good energetic material to improve the actuation reliability, and also improve mechanical arming hermiticity. Reliability tests have not been done at this stage of project. In a second period, we have to check the extreme temperature operation and mechanical shock tests. References [1] C. Rossi, D. Briand, M. Dumonteuil, T. Camps, P.Q. Pham, N.F. de Rooij, Matrix of 10 × 10 addressed solid propellant microthrusters: review of the technologies, Sensors and Actuators A: Physical 126 (1) (2006) 241–252. [2] D.W. Youngner, S.T. Lu, E. Choueiri, J.B. Neidert, R.E. Black III, K.J. Graham, D. Fahey, R. Lucus, and X. Zhu, MEMS Mega-pixel Micro-thruster Arrays for Small Satellite Stationkeeping, Proceedings of the 14th Annual AIAA/USU Conference on Small Satellites, AIAA Paper SSC00-X-2, August 21–24, 2000. [3] D.H. Lewis Jr., S.W. Janson, R.B. Cohen, E.K. Antonsson, Digital micropropulsion, Sensors and Actuators A: Physical 80 (2000) 143–154. [4] D. Teasdale, V. Milanovic, P. Chang, K. Pister, Microrockets for smart dust, Smart Materials and Structure 10 (2001) 1145–1155. [5] W. Lindsay, D. Teasdale, V. Milanovic, K. Pister, C.F. Pello, Thrust and electrical power from solid propellant microrockets, in: Technical Digest of the 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001), Piscataway, USA, 2001, pp. 606–610. [6] K. Takahashi, H. Ebisuzaki, H. Kajiwara, T. Achiwa, and K. Nagayama, Design and Testing of Mega-bit Microthruster Arrays, presented at the Nanotech, Houston, TX, Sep. 9-12, 2002, Paper AIAA 2002-5758. [7] S. Tanaka, R. Hosokawa, S. Tokudome, K. Hori, H. Saito, M. Watanabe, M. Esashi, MEMS-based solid propellant rocket array thruster with electrical feedthroughs, Transactions of the Japan Society for Aeronautical and Space Science 46 (2003) 47–51. [8] P.Q. Pham, D. Briand, C. Rossi, N.F. De Rooij, Downscaling of solid propellant pyrotechnical microsystems, in: Proceedings of the 12th International Conference on Solid-State Sensors and Actuators (Transducers’03), June 8–12, Boston, USA, 2003, pp. 1423–1426. [9] Y. Zhao, B.A. English, Y. Choi, H. DiBiaso, G. Yuan, M.G. Allen, Polymeric microcombustors for solid-phase conductive fuels, in: Proceedings of the 17th IEEE International Conference on Micro Electro Mechanical Systems, (MEMS 2004), January 25–29, Maastricht, Netherlands, 2004, pp. 498–501. [10] K.L. Zhang, S.K. Chou, S.S. Ang, X.S. Tang, A MEMS-based solid propellant microthruster with Au/Ti igniter, Sensors and Actuators A: Physical 122 (1) (2005) 113–123. [11] T. Troianello, Precision foil resistors used as electro-pyrotechnic initiators, in: Proceedings of the 51st Electronic Components and Technology Conference, May 29–June 1, Florida, USA, 2001. [12] H. DiBiaso, B.A. English, M.G. Allen, Solid-phase conductive fuels for chemical microactuators, Sensors and Actuators A: Physical 111 (2–3) (2004) 260– 266. [13] H. Laucht, H. Bartuch, D. Kovalev, Silicon initiator, from the idea to functional tests, in: Proceedings of the 7th International Symposium and Exhibition on Sophisticated Car Occupant Safety Systems, 2004. [14] A. Hofmann, H. Laucht, D. Kovalev, V.Yu. Timoshenko, J. Diener, N. Kunzner, E. Gross, Explosive composition and its use, US Patent, Patent No.: 0072502 A1, 2005. [15] T.W. Barbee, R.L. Simpson, A.E. Gash, and J.H. Satcher, Nano-laminate-based ignitors, US Patent, Patent No: WO 2005016850 A2, 2005. [16] J.C. Hinshaw, Thermite compositions for use as gas generants, International Patent, Patent No: WO 95/04672, 1995. [17] C. Rossi, D. Estève, C. Mingués, Pyrotechnic actuator: a new generation of Si integrated actuator, Sensors and Actuators A: Physical 74 (1–3) (1999) 211–215. [18] C.C. Hong, S. Murugesan, G. Beaucage, J.W. Choi, C.H. Ahn, A Functioning on-chip pressure generator using solid chemical propellant for disposable lab-on-achip, Lab-on-a-Chip 3 (4) (2003) 281–286.
[19] A.A. Norton, M.A. Minor, Pneumatic microactuator powered by the deflagration of sodium azide, Journal of Microelectromechanical Systems 15 (2) (2006) 344–354. [21] Patent FR2 818 808, Pile à combustible pour l’alimentation d’appareils électroniques, notamment portables. (in French). [22] O. Vasylkiv, Y. Sakka, Nanoexplosion synthesis of multimetal oxide ceramic nanopowders, Nano Letters 5 (12) (2005) 2598–2604. [23] O. Vasylkiv, Y. Sakka, V.V. Skorokhod, Nano-blast synthesis of nano-size CeO2 -Gd2 O3 powders, Journal of the American Ceramic Society 89 (6) (2006) 1822–1826. [24] D.S. Stewart, Miniaturization of explosive technology and microdetonics, in: Proceedings of 21st the International Conference of Theoretical and Applied Mechanics (ICTAM04), August 15–21, Varsaw, Poland, 2004. [25] P. Pennarun, C. Rossi, D. Estève, D. Bourrier, Design, fabrication and characterization of a MEMS safe pyrotechnical igniter integrating arming, disarming and sterilization functions, Journal of Micromechanics and Microengineering 16 (1) (2006) 92–100. [26] S. Phelep, T. Nguyen, Method of producing an electro-pyrotechnic interface between an electrothermal bridge and a primary composition inside an initiator, WO Patent number 2006/051199A1, 2006. [27] F.N. Mandigo, G.C. Mei, J.C. Fister, Semiconductor bridge (SCB) packaging system, US Patent number 5,029,529, 1991. [28] P. Wilson, K.L. Gray, Non-explosive two component initiator, EP Patent number 1 662 224 A1, 2005. [29] C. Rivière, E. Poulard, Initiateur électro-pyrotechnique sécurisé à haute énergie, EP Patent number 1 172 628 A2, 2001. [30] W. Smetana, R. Reicher, M. Mundlein, J. Nicolics, H. Homolka, Thick film initiator elements—an alternative to resistive wire initiators for automotive safety units, in: 26th International Spring Seminar on Electronics Technology, May 8–11,2003, Star& Lesni, Slovak Republic, 2003. [31] M. Park, H. Choo, S. Yoo, C.O. Park, Comparison of plasma generation behaviors between a single crystal semiconductor bridge and a polysilicon semiconductor bridge, Sensors and Actuators A 115 (2004) 104–108. [32] R. L. Allen, Safe arm initiator, US Patent number 3,658,009, 1972. [33] H.L. Grant, A. Djidics, Electric detonator, US Patent number 1,516,009, 1924. [34] L.H. Johnston, Electric initiator with exploding bridge wire, Patent number 3,040,660, 1962. [35] K. F. Hodge, D. H. Lewis Jr, S. D. Nelson, V.W. Ruwe, MEMS arm fire and safe and arm devices, US Patent number 6,431,071, 2002. [36] W.H. Maurer, GH. Soto, D.R. Hollingsworth, MEMS Safe Arm device for microdetonation, US Patent number 7,040,234, 2006. [37] M. Belgerian, J. C. Depeigne, M. Vincent, Dispositif de sécurité et d’armement de charges militaires, European Patent number 0 463 974 A1, 1991. [38] P. Magnan, R. Lafont, Pyrotechnic safety device of reduced dimensions, US Patent number 7,490,553, 2009. [39] W.O. Maruska, M.A. Bayerkohler, Safing and arming mechanism, US Patent number 4,770,096, 1988. [40] M. Worthington, Safety and arming apparatus and method for a munition, US Patent number 7,258,068, 2007. [41] C.H. Robinson, R.H. Wood, T.Q. Hoang, Miniature MEMS-based electromechanical safety and arming device, US Patent number 6,964,231, 2005. [42] C.H. Robinson, R.H. Wood, M.R. Gelak, D. Hollingsworth, Micro-scale firetrain for ultra-miniature electro-mechanical safety and arming device, US Patent number 7,055,437, 2006. [43] D.L. Jean, MEMS resettable timer, US Patent number 7,398,734, 2008. [44] J.D. Titus, P.E. Hall, Electro-explosive safety and arming device, US Patent number 4,691,634, 1987. [45] D.S. Lenko, T.P. Frederick, Miniature, low power, electromechanical safety and arming device, US Patent number 5,594,195, 1997. [46] R. Buxton, Multi-circuit safing and arming switch, US Patent number 3,771,457, 1973. [47] W.J. Donahue, D.L. Blanchard, J.M. Wack, Fuze arming device, US Patent number 3,670,656, 1972. [48] R.E. Edminster, Safe and arm device, US Patent number 5,016,532, 1991. [49] D.L. Ramstad, Safe and arm device, US Patent number 3,951,036, 1976. [50] C. Rossi, D. Estève, Micropyrotechnics, a new technology for making energetic microsystems: review and prospective, Sensors and Actuators A120/2 (2005) 297–310. [51] www.sceram.com/macor.html. [52] http://www.epotek.com/SSCDocs/msds/H70E-175 msds.PDF.
Biographies Hélène Pezous was born in Agen, France, in 1983. She received the M.S. degree in electronic engineering from Montpellier University, Montpellier, France, in 2006. She is currently working toward the Ph.D. degree in the Laboratory for Analysis and Architecture of the Systems (LAAS), MEMS Department, Toulouse, France. LAAS is a national laboratory under the French National Center for Scientific Research (CNRS). Her Ph.D. topic concerns the micropyrotechnical systems via MEMS. Carole Rossi received the engineer degree in physics and the Ph.D. degree in electrical engineering from the National Institute for Applied Science, Toulouse, France, in 1994 and 1997, respectively. After her postdoctoral research at the University of California, Berkeley, under the supervision of Prof. Pister, she joined the French National
H. Pezous et al. / Sensors and Actuators A 159 (2010) 157–167 Center for Scientific Research (CNRS), Toulouse, to develop her research at the Laboratory for Analysis and Architecture of the Systems (LAAS). She is currently leading the power MEMS research area at LAAS, with her team proposing new concepts for actuation and energy on a chip. Her research interests include nanoenergetics, micropyrotechnical systems, and power MEMS for electrical generation. Fabrice Mathieu was born in Orléans, France, in 1972. He received the Engineer degree in communication systems and electronics from the CNAM (Conservatoire National des Arts et Métiers), Toulouse, France, in 2003. He joined the Laboratoire d’Analyse et d’Architecture des Systèmes (LAAS), Centre National de la Recherche Scientifique (CNRS), Toulouse, in 2001, where he is currently in charge of the development and design of very low signal detection systems applied to the micro(nano)electromechanical systems area and its complete electronic treatment and control for automation. Veronique Conedera, 49 years old, engineer of research, worked in the field of the hybrid circuits of 1983–1990, and integrated LAAS-CNRS in 1990. She develops on the one hand the techniques of implementation of MEMS by inkjet and micro machining of silicon surface and on the other hand, the implementation of layers containing organic materials and polymers: resin moulds strong thickness, technology of sol gel for example. Xavier Dollat was born the 10th of December 1966. He joined the Laboratoire d’Architecture et d’Analyse des Systèmes from the French Centre National de la
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Recherche Scientifique (LAAS–CNRS) as an technical engineer in 2001. He is responsible for the technological development of mechanical structures and elements applied systems and microsystems. Marjorie Sanchez was born in Narbonne, France, in 1987. She is currently working toward the M.S. degree in electronic engineering from engineering school CESI, Labège, France, by apprenticeship in the Laboratory for Analysis and Architecture of the Systems (LAAS), II Department, Toulouse, France. LAAS is a national laboratory under the French National Center for Scientific Research (CNRS). One of her apprenticeship projects is the electronic design for micropyrotechnical systems via MEMS. Samuel Charlot is born in 1979, he received the master degree in Materials and Micro Technology from the Var Institute of technology, Toulon, France in 2004. From 2004 to 2006, he worked on the development of microfluidics systems in LAASCNRS, Toulouse, France. In 2006, he worked in the packaging of oxygen sensor at Néosens, Labège, France. From 2007 to 2008, he worked on a low cost technology for the integration of sensors silicon in polymer microfluidic systems in LAAS-CNRS, Toulouse, France. Since 2008, he works on packaging area and more specifically in screen printing technology and flip chip interconnect technology in the team packaging in LAAS-CNRS, Toulouse, France.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Integration of a MEMS based safe arm and fire device Hélène Pezous a,b , Carole Rossi a,b,∗ , Marjorie Sanchez a,b , Fabrice Mathieu a,b , Xavier Dollat a,b , Samuel Charlot a,b , Ludovic Salvagnac a,b , Véronique Conédéra a,b a b
CNRS, LAAS, 7 Avenue du Colonel Roche, F-31077, Toulouse, France Université deToulouse, UPS, INSA, INP, ISAE, LAAS, F-31077, Toulouse, France
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 18 January 2010 Accepted 12 March 2010 Available online 18 March 2010 Keywords: MEMS Micro initiator, Pyrotechnical microactuator, Safe arm and fire
a b s t r a c t The paper describes a new architecture of a Safe Arm and Fire device (SAF) that could constitute a real breakthrough for safe miniature fuzing device. On the one hand, it takes all the functions embodied in a conventional mechanical arm and fire system and integrates them in a single 1 cm3 package made of assembly of different parts. On the other hand, for the first time, it combines a mechanical arming unit with electrical safety functionalities on the same silicon initiator’s chip. It respects the STANAG 4187 norm (1 A/W during 5 min of not fire) and requires only 635 mW for ignition. The paper presents the design, fabrication and test of one miniature SAF device integrating a micropyrotechnical actuation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction An innovative concept and technology called micropyrotechnics was proposed by LAAS-CNRS in 1995. Since then, this concept has been applied into many fields by many teams: micropropulsion [1–10], microinitiation [11,13–15] (in both military and civil domains), gases for actuation (including injection or moving fluid) [12,16–19], gases for chemical reaction [21–23], welding [24] and switching [25]. In this paper, we propose to explore micropyrotechnics to fabricate a miniature Safe Arm and Fire device (SAF). The main functions of a SAF device are to keep the detonator safe (a screen interrupts the explosive train), to arm it (the screen is mechanically removed from the safe position) and to initiate one primary explosive necessary for initiating the munition (secondary explosive). A SAF is therefore not a “sensor” or a pyrotechnical initiator, but it combines both sensing and actuation functions in a very tiny volume and must operate with a high reliability level. Different architectures of miniature SAF have been proposed in the opened literature or patented. Most of realizations use conventional technologies but recently some teams have explored MEMS technologies to fabricate some parts (initiator [26–34], screen actuation element [37–40,43,46,47]) or to integrate the whole SAF device [32,35,36,40,41,42,44,45,48,49]. They faced 2 main difficulties:
∗ Corresponding author at: CNRS, LAAS, 7 Avenue du Colonel Roche, F-31077, Toulouse, France. E-mail address: [email protected] (C. Rossi). 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2010.03.017
(1) the integration of sensing elements, actuators, pyrotechnical elements and safety functions in a very tiny volume with a sufficient reliability level is not impossible by still requires nettlesome technological efforts. (2) the use of electrical micro actuator to move the metallic screen from the safe position to arm position requires high amount of electrical energy and therefore makes the device difficult to be integrated within a few cm3 . None of published work, at this stage has proposed convincing solutions for both issues. In this paper, we present the design, fabrication and characterization of a new MEMS SAF architecture, integrating initiator, electrical and mechanical protections within a 1 cm3 , and based on micropyrotechnical actuation. 2. Architecture and principle of operation of the SAF device As illustrated in Fig. 1, the architecture of the SAF MEMS device consists in an assembly of different wafers. The bottom layer of Fig. 1 constitutes the mechanical arming function. The intermediary layer is a silicon chip, called Si-based safe initiator, on which are integrated 2 electrical resistances (one to initiate an explosive and one for the pyrotechnical actuator) and microswitches to realize the electrical arming and disarming functions. The top layer, thicker one, is the electronic circuitry and power supply integrating if required one supercapacity. To prevent that a shock unlocks the mechanical screen, an inertial pin can be inserted into the device to block the screen. Even if this piece has not been integrated into the demonstrator detailed in this paper, its place
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Fig. 1. View of the MEMS SAF device made of three stacked parts: electronic circuitry, Si-based safe initiator and the mechanical arming function.
has been reserved and it will present no difficulty for its integration. The SAF device operations procedure is illustrated in Fig. 2: The MEMS SAF is stored in safe mode: the screen is locked and the initiator pads are both connected to the electrical ground (cf. Fig. 2a). The first order is for the mechanical arming (cf. Fig. 2b): the inertial pin is removed by the acceleration; then the microcontroller sends an
electrical order to the microactuator resistance. The gas generated by the pyrotechnical actuator moves the screen in armed position. Then, the SAF is electrically armed, that is to say the microinitiator electrical short-circuit to electrical ground is cut (cf. Fig. 2c). At this stage, if the operator does not send an order to stop the procedure, the microcontroller sends an electrical order to the initiator to ignite the primary explosive located in its cavity (cf. Fig. 2d), and
Fig. 2. Operations procedure of the MEMS SAF device (a) in safe mode, (b) mechanically armed, (c) electrically armed and (d) secondary explosive initiation by initiator.
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Fig. 3. Layout of the Si-based safe initiator layer (total area: 7.4 mm × 8.4m).
the secondary explosive can be initiated. If there is a failure during the sequences, it is possible to disconnect definitively the microinitiator from its power supply by breaking its electrical connections.
3. Design of the Si-based safe initiator The main component of the silicon Si-based safe initiator chip is 1 resistive microinitiator with a primary explosive and three bistable electro-thermal MEMS switches (2 ON–OFF switches and 1 OFF–ON switch). The Si-based safe initiator layer contains also a second resistance with a gas generator energetic material (called microactuator) used to move the ceramic screen from safe to arm position. Its principle of operation is simple: after ignition by joule effect, the energetic material decomposition generates hot gases that increase the pressure into the micro actuator cavity that moves the mechanical barrier from safe to arm position (see Fig. 2b). The silicon chip layout is detailed in Fig. 3.
3.1. The microinitiator and microactuator Both consist of a heating resistance (polysilicon resistive element) on a 1 mm2 dielectric membrane. Inside the cavity, underneath the membrane, a thin layer of energetic material is deposited (see Fig. 4). The heating resistance is of 120 . A highly energetic material (explosive for example) is loaded in the microinitiator cavity built in the silicon substrate whereas the gas generator material (propellant for example) is loaded in the microactuator cavity. The volume of primary explosive is not chosen at this stage of the project because it depends on the application. However, the volume of the propellant for the actuation has been calculated using an in house 1D computational model capable to predict the pressure, temperature of the gas inside the cavity as a function of the propellant properties and cavity volume. This model is based on gas flow conservation of mass, momentum and energy [50]. Using a composite propellant, to generate 6 Bar, the energetic material volume-over-cavity volume ratio must be of 50. The
Fig. 4. Schematic 3D view of the microactuator or microinitiator.
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3.2. The ON–OFF switch The concept is based on the rupture of an aluminum electrical connection when the ON–OFF switching is required. For that, an aluminum line is placed on top of a heating resistance deposited on a SiO2 /SiNx membrane, as seen in Fig. 6. The heating resistance is a polysilicon resistance of 140 deposited and the membrane has a diameter of 500 m. When ON–OFF switching is required, the resistance heats the aluminium until its evaporation.
3.3. The OFF–ON switch The OFF–ON switch (Fig. 7) is complementary to the ON–OFF switch. Its principle is based on a microbrazing by locally heating up to 183 ◦ C a Sn/Pb solder microball deposited between two copper tracks to connect. Before welding, the two electric tracks are electrically insulated by a thin layer of rosin which evaporates at a lower temperature than the brazing one (∼83 ◦ C). Rosin, formerly called colophony is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. Rosin is the precursor to the flux used in soldering. The switch structure is the same one as the ON–OFF one and consists in a resistive element of 50 deposited on a dielectric membrane with a diameter of 1.5 mm.
4. Fabrication and assembly Fig. 5. Schema of the micro actuator cavity (a) before the gases are released (b) after the gases are released.
4.1. Fabrication of the Si-based safe initiator
propellant volume has been calculated at 0.08 mm3 . We choose a pastille of Ø1 mm and 100 m thick. Fig. 5 gives the geometry and dimensions of actuator cavity and propellant volume used for the modelling.
The manufacturing process, set up in LAAS-CNRS microfabrication center, for the Si-based safe initiator is a collective process allowing the integration of all the functions (ON–OFF and OFF–ON switches, the microactuator and the microinitiator) in the same silicon chip. The process starts with a 4 (100) double face polished silicon wafer and requires seven mask levels.
Fig. 6. Schematic 3D view of the ON–OFF switch (a) before switching (b) after switching.
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Fig. 7. Schematic 3D view of the OFF–ON switch (a) before switching (b) after switching.
4.1.1. Front side processing The wafer is initially cleaned in two successive baths: Piranha (H2 O2 + H2 SO4 ) followed by a diluted 50% HF bath. First step is the silicon thermal oxidation (SiO2 ) on 1.4 m at 1150 ◦ C. Then 0.6 m of SiNx is deposited by LPCVD (Low Pressure Chemical Vapor Deposition) at 705 ◦ C. The SiO2 /SiNx bilayer forms a thin low-stressed dielectric membrane (2 m, 0.1 MPa) for suspended heating platforms. Third step is the deposition of a 0.5 m thick polysilicon by LPCVD at 605 ◦ C. It is then doped by phosphorus diffusion. The resistivity of this polysilicon layer that will constitute the resistive layer is about 8.9.10−4 cm−1 . A RIE (Reactive Ion Etching) step through a photo resist mask reveals the resistance shape. Then, 0.7 m of low-stressed PECVD (Plasma Enhanced Chemical Vapor Deposition) oxide is deposited and then etched everywhere except on the switches polysilicon resistances. This 0.7 m of PECVD oxide isolates electrically the metal layer (next processing steps) from the polysilicon resistance. The following steps are the metallization: first, 3 m thick of copper is electroplated through a resist mask. The electroplating bath is referred as MicrofabCU200 (25 g (Cu)/L (H2 SO4 )) Enthone Omi. The copper is then covered by 500 Å of gold to prevent its oxidation. Then, 3 m of aluminum is evaporated and etched away through a photoresist mask to leave Al only on the ON–OFF switches polysilicon resistance. Metallization steps end with a thermal annealing at 270 ◦ C during 30 min to get ohmic contact. 4.1.2. Back-side processing The three back-side SiO2 , SiNx and polysilicon layers are etched away by RIE. Afterwards, the 400 m thick silicon is etched by DRIE (Deep Reactive Ion Etching) through a photoresist mask to release the micro heater membranes. 4.1.3. Post-processing It remains three post-processing steps before the Si-based safe initiator could be operational. (1) A rosin (called colophane) thin layer is deposited between the two OFF–ON switches Cu tracks by stamping. The chip and rosin reservoir are heated to 140 ◦ C to obtain a good viscosity of the
rosin. Then a Ø350 m Sn/Pb solder ball made by the Indium Corporation is deposited on the rosin. The rosin evaporation temperature is 83 ◦ C. Finally, the chip is cooled down to solidify the rosin that traps the solder ball. (2) The microactuator cavity (Ø 1 mm) is filled with a propellant type energetic material. (3) To validate its functioning, the icroinitiator cavity (Ø 1 mm) is filled with a second propellant type energetic material (which is not the final one). The process is summarized in Fig. 8. Fig. 9 shows one photo of a fabricated Si-based safe initiator with zooms on the ON–OFF and OFF–ON switches, the microactuator and the microinitiator. 4.2. Assembly of the arming function The aluminum has been chosen for the arming function case because it presents good mechanical properties and a good longevity so it is used in assemblies subjected to high thermal gradients and pressure. The MACOR [51] has been chosen for the movable mechanical screen: it is a ceramic which can support high thermal gradients (800–1000 ◦ C). It is a rigid material featuring high resistance to wear effect like climatic or chemical aggressions and a very low thermal conductivity. The thermal dilation coefficient ˛ like its physical properties as Young modulus approaches those of silicon. Both materials, aluminum and MACOR, can be machined precisely by traditional way. The mechanical screen geometrical parameters are given in Fig. 10. The MACOR piece corner radius of curvature is slightly larger than the one of the slide corners in which it moves. It is thus able to achieve its end of course in force in the slider and to remain thus blocked in armed position. The aluminum case is divided in two parts: the cap and the base, as it can be seen of Fig. 10. These two parts are machined in a complementary way. Indeed, the base has a 400 m height step in periphery of the cavity. And the cap has the complementary print as a crenel shape. The objective of this geometry takes all its direction during the assembly of the base and the cap by gluing.
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Fig. 8. Summary of the main process steps for the Si-based safe initiator fabrication.
It prevents any glue flow inside the cavity. External dimensions of mechanical safety are the same ones as those of the Si-based safe initiator chip (8.4 mm × 7.4 mm). 4.3. SAF final assembly We work with the Epotek H70E glue to assemble the arming function parts. It has good mechanical properties and in particular a rigidity modulus of = 1.35·107 Pa and a very good thermal dissipation [52]. Surfaces to be stuck are cleaned in an ethanol bath. And a 99% ethanol and 1% promoter of adherence Epotek AP100 solution is deposited on surfaces to be assembled. Then the glue is
dispensed on the part to be assembled using a calibrated syringe (Ø 250 m) and then the assembly is realized with a calibrated force (5000 g) and a reticulation is done at 150 ◦ C during 300 s. After the arming function is glued, the silicon chip is glued with the same epoxy on the arming function part (see Fig. 11) and is reticulated at 80 ◦ C during 90 min. The temperature of reticulation has been reduced to prevent rosin to evaporate. 5. Characterizations Each Si chip elements has been tested to determine the electrical power and time of response.
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Fig. 9. Photo of the fabricated Si-based safe initiator: zooms on (a) and (b) ON–OFF switches, (c) OFF–ON switch with its Sn/Pb solder ball, (d) microinitiator and (e) microactuator with propellant.
5.1. ON–OFF switch The ON–OFF characterization aims to determine time and power before is meafor ON–OFF actuation. The ON–OFF switch resistance Rswitch
sured first. Then an electrical power (see Table 1), is applied to the heater (Rheater ) to switch from ON to OFF position. The switchbefore , Rafter ing is recorded via an oscilloscope to record Rswitch (the switch final state) and the switching time. Results are given in Table 1.
Fig. 10. Schematic view and photos of the different parts of the mechanical arming function (a) aluminum base, (b) aluminum cap and (c) MACOR screen.
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Fig. 13. Photos of OFF–ON microswitch (a) the solder ball is deposited on top of the heater and rosin spread on the whole membrane, (b) the commutation is done in the ON state. Table 3 Experimental results of the microactuator and microinitiator ignition.
Fig. 11. Photo of the final assembly of the Si-based safe initiator stuck on the mechanical arming function. Table 1 Experimental results of the ON–OFF switches. Rheater ()
Iin (mA)
Vin1 (V)
Pin (mW)
tin (s)
before Rswitch ()
after Rswitch ()
% of success
280
53
12
508.8
1.51
1.4
∞
25
Rheater ()
Vin (V)
Iin (mA)
Pin (mW)
tin (ms)
% of success
226
12
53
635
36.49
100
and 90% of successful OFF–ON commutations have been obtained. It requires about 72.07 mJ (162.7 mW during 443 ms). Photos of OFF–ON microswitch before and after actuation are given in Fig. 13. After the soldering is done, currents of 100 mA are applied to the switch to measure the contact resistance which is of 1.2 and remains stable. 5.3. Microinitiator/microactuator ignition The polysilicon heater resistance (Rheater ) of about 220 is powered with 53 mA and under 12 V. And the propellant ignition requires roughly 40 ms. 20 samples have been tested and 100% of ignition success has been reached. Results are summarized in Table 3. Photos of microinitiator/microactuator before and after actuation are given in Fig. 14.
Fig. 12. Photos of thermal ON–OFF microswitch: the polysilicon resistance on the membrane with the aluminium line on top (a) before actuation and (b) after actuation.
It requires about 760 mJ (500 mW during 1500 ms) to switch. It has to be noticed that only 25% of the ON–OFF switches were successful (20 samples have been characterized). Main causes of failure are the rupture of the membrane before heating up the aluminum track. To reduce this problem, we will reduce the thickness of the aluminum track to 1 m. Photos of ON–OFF microswitch before and after actuation are given in Fig. 12. 5.2. OFF–ON switch
5.4. Screen actuation The test bench carried out is the same one as for the electric characterizations of the microactuator on glass leaf. A constant current pulse Iin of 53 mA powers the micro actuator heater resistance (Rheater ). The assembly is hermetically sealed to confine gases resulting from the decomposition of the propellant contained in the microactuator cavity. After ignition, the screen moves from the safe position to armed position on 1.41 mm. The screen actuation functionality has been validated. However, we get only 5 successful actuation over 10 tests. This result should be improved by choosing a better energetic material to generate gas. Photos of the MACOR screen in the arming function before and after actuation of the microactuator are given in Fig. 15.
As for ON–OFF microswitch, the characterization aims to determine time and power necessary for OFF–ON actuation. First, voltage up to 100 V has been applied to the switch in OFF state validating before is measured first. Then an its opened state. The resistance Rswitch electrical power (see Table 2) is applied to the heater (Rheater ) to switch from OFF to ON position. The switching time is recorded after . via an oscilloscope and the final state is also monitored Rswitch Results are given in Table 2. 20 samples have been characterized Table 2 Experimental results of the OFF–ON switches. Rheater ()
Iin1 (mA)
Vin1 (V)
Pin (mW)
tin (ms)
before Rswitch ()
after Rswitch ()
% of success
45.2
60
3.4
162.7
443
∞
1.2
90
Fig. 14. Photos of microinitiator/microactuator (a) before actuation, (b) after actuation.
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Fig. 15. Photos of the back-side arming function (a) before actuation :the MACOR screen is in safe position and (b) after actuation : the MACOR screen is in armed position.
6. Electronic circuitry and power supply device In the embedded and autonomous system area, we developed a very low-power consumption electronic scheme to drive a thermal initiator microdevice (cf. Fig. 16). Usually, autonomous electronic systems are used to communicate or to measure physical signals, which need low energetic quantity. In this application, electronic has to provide less than 650 mW to each of the five polysilicon resistances of the Si-based safe initiator, that corresponds to a hundred to thousand factor in comparison with traditional energetic consumption in autonomous systems. There are five resistances to be controlled by the electronic circuitry. Furthermore, to supply this power for the device, we need to work at an important voltage (10 V) which does not match with standards in low-power world. The selected solution for energy storage is a pre-loaded supercapacitor (Cooperbussmann Powerstor PA-5R0V474-R) kept on load by button cell. To maximize the lifetime, a low-consumption electronic is used and set on sleep mode until its use. The waking is made by an external order. The system’s lifetime reaches 30 000 h. It is also planned to keep the embedded electronic on voltage during the stocking. To provide the required power, the power supply voltage is raised by a DC/DC converter. This power sending is driven by anaFig. 17. Electronic circuitry with three PCB layers interconnected on their sides.
log switches. Two kinds of fast switching (less than 200 ns) and low on resistance components are used: a CMOS analog switch and MOSFET. A microcontroller (Microchip PIC18F1220) supervises all required stages for the SAF operation described in previous section. Small components and multilayer circuit fulfil size specification (1 cm3 ). The embedded electronic is composed of three double-side stages connected each other by vias, and connected with the silicon chip by Sn/Pb soldering. The power supply is placed above the system and is the bulkiest element (not shown on Fig. 17). 2 Entire assemblies as shown on Fig. 17 have been tested with electronic and all the functional steps (mechanical arming, electrical arming and simulation of initiation) have been successful. 7. Conclusions and perspectives
Fig. 16. Cut view of the electronic circuitry and its power supply made of stacked PCB layers.
Very innovative MEMS based safe, arm and fire device has been proposed, designed, fabricated. It is a multilayer stackedwafers: the bottom layer is the mechanical arming function made of Aluminum and MACOR. The intermediary layer is a silicon chip integrating electrical resistances and microswitches to realize the electrical arming and disarming functions. The top layer is the electronic and power circuitry with one supercapacitor.
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The technology (silicon chip fabrication and whole device assembling) and its operation have been validated by experimentations. Microactuator, ON–OFF and OFF–ON microswitches characterizations gave encouraging results: • The microactuator functioning principle is validated. The MACOR screen could move from safe to armed position in the aluminium slide (displacement of 1.41 mm). • OFF–ON microswitches require 72.07 mJ (162.7 mW during 443 ms) to commute from OFF to ON state. 90% of success has been obtained. • ON–OFF microswitches require 772 mJ (508 mW during 1.51 s) to commute from ON to OFF state. However, only 25% of success has been obtained which will be improved by reducing the aluminium thickness. In perspective, we need to integrate the good energetic material to improve the actuation reliability, and also improve mechanical arming hermiticity. Reliability tests have not been done at this stage of project. In a second period, we have to check the extreme temperature operation and mechanical shock tests. References [1] C. Rossi, D. Briand, M. Dumonteuil, T. Camps, P.Q. Pham, N.F. de Rooij, Matrix of 10 × 10 addressed solid propellant microthrusters: review of the technologies, Sensors and Actuators A: Physical 126 (1) (2006) 241–252. [2] D.W. Youngner, S.T. Lu, E. Choueiri, J.B. Neidert, R.E. Black III, K.J. Graham, D. Fahey, R. Lucus, and X. Zhu, MEMS Mega-pixel Micro-thruster Arrays for Small Satellite Stationkeeping, Proceedings of the 14th Annual AIAA/USU Conference on Small Satellites, AIAA Paper SSC00-X-2, August 21–24, 2000. [3] D.H. Lewis Jr., S.W. Janson, R.B. Cohen, E.K. Antonsson, Digital micropropulsion, Sensors and Actuators A: Physical 80 (2000) 143–154. [4] D. Teasdale, V. Milanovic, P. Chang, K. Pister, Microrockets for smart dust, Smart Materials and Structure 10 (2001) 1145–1155. [5] W. Lindsay, D. Teasdale, V. Milanovic, K. Pister, C.F. Pello, Thrust and electrical power from solid propellant microrockets, in: Technical Digest of the 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2001), Piscataway, USA, 2001, pp. 606–610. [6] K. Takahashi, H. Ebisuzaki, H. Kajiwara, T. Achiwa, and K. Nagayama, Design and Testing of Mega-bit Microthruster Arrays, presented at the Nanotech, Houston, TX, Sep. 9-12, 2002, Paper AIAA 2002-5758. [7] S. Tanaka, R. Hosokawa, S. Tokudome, K. Hori, H. Saito, M. Watanabe, M. Esashi, MEMS-based solid propellant rocket array thruster with electrical feedthroughs, Transactions of the Japan Society for Aeronautical and Space Science 46 (2003) 47–51. [8] P.Q. Pham, D. Briand, C. Rossi, N.F. De Rooij, Downscaling of solid propellant pyrotechnical microsystems, in: Proceedings of the 12th International Conference on Solid-State Sensors and Actuators (Transducers’03), June 8–12, Boston, USA, 2003, pp. 1423–1426. [9] Y. Zhao, B.A. English, Y. Choi, H. DiBiaso, G. Yuan, M.G. Allen, Polymeric microcombustors for solid-phase conductive fuels, in: Proceedings of the 17th IEEE International Conference on Micro Electro Mechanical Systems, (MEMS 2004), January 25–29, Maastricht, Netherlands, 2004, pp. 498–501. [10] K.L. Zhang, S.K. Chou, S.S. Ang, X.S. Tang, A MEMS-based solid propellant microthruster with Au/Ti igniter, Sensors and Actuators A: Physical 122 (1) (2005) 113–123. [11] T. Troianello, Precision foil resistors used as electro-pyrotechnic initiators, in: Proceedings of the 51st Electronic Components and Technology Conference, May 29–June 1, Florida, USA, 2001. [12] H. DiBiaso, B.A. English, M.G. Allen, Solid-phase conductive fuels for chemical microactuators, Sensors and Actuators A: Physical 111 (2–3) (2004) 260– 266. [13] H. Laucht, H. Bartuch, D. Kovalev, Silicon initiator, from the idea to functional tests, in: Proceedings of the 7th International Symposium and Exhibition on Sophisticated Car Occupant Safety Systems, 2004. [14] A. Hofmann, H. Laucht, D. Kovalev, V.Yu. Timoshenko, J. Diener, N. Kunzner, E. Gross, Explosive composition and its use, US Patent, Patent No.: 0072502 A1, 2005. [15] T.W. Barbee, R.L. Simpson, A.E. Gash, and J.H. Satcher, Nano-laminate-based ignitors, US Patent, Patent No: WO 2005016850 A2, 2005. [16] J.C. Hinshaw, Thermite compositions for use as gas generants, International Patent, Patent No: WO 95/04672, 1995. [17] C. Rossi, D. Estève, C. Mingués, Pyrotechnic actuator: a new generation of Si integrated actuator, Sensors and Actuators A: Physical 74 (1–3) (1999) 211–215. [18] C.C. Hong, S. Murugesan, G. Beaucage, J.W. Choi, C.H. Ahn, A Functioning on-chip pressure generator using solid chemical propellant for disposable lab-on-achip, Lab-on-a-Chip 3 (4) (2003) 281–286.
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Biographies Hélène Pezous was born in Agen, France, in 1983. She received the M.S. degree in electronic engineering from Montpellier University, Montpellier, France, in 2006. She is currently working toward the Ph.D. degree in the Laboratory for Analysis and Architecture of the Systems (LAAS), MEMS Department, Toulouse, France. LAAS is a national laboratory under the French National Center for Scientific Research (CNRS). Her Ph.D. topic concerns the micropyrotechnical systems via MEMS. Carole Rossi received the engineer degree in physics and the Ph.D. degree in electrical engineering from the National Institute for Applied Science, Toulouse, France, in 1994 and 1997, respectively. After her postdoctoral research at the University of California, Berkeley, under the supervision of Prof. Pister, she joined the French National
H. Pezous et al. / Sensors and Actuators A 159 (2010) 157–167 Center for Scientific Research (CNRS), Toulouse, to develop her research at the Laboratory for Analysis and Architecture of the Systems (LAAS). She is currently leading the power MEMS research area at LAAS, with her team proposing new concepts for actuation and energy on a chip. Her research interests include nanoenergetics, micropyrotechnical systems, and power MEMS for electrical generation. Fabrice Mathieu was born in Orléans, France, in 1972. He received the Engineer degree in communication systems and electronics from the CNAM (Conservatoire National des Arts et Métiers), Toulouse, France, in 2003. He joined the Laboratoire d’Analyse et d’Architecture des Systèmes (LAAS), Centre National de la Recherche Scientifique (CNRS), Toulouse, in 2001, where he is currently in charge of the development and design of very low signal detection systems applied to the micro(nano)electromechanical systems area and its complete electronic treatment and control for automation. Veronique Conedera, 49 years old, engineer of research, worked in the field of the hybrid circuits of 1983–1990, and integrated LAAS-CNRS in 1990. She develops on the one hand the techniques of implementation of MEMS by inkjet and micro machining of silicon surface and on the other hand, the implementation of layers containing organic materials and polymers: resin moulds strong thickness, technology of sol gel for example. Xavier Dollat was born the 10th of December 1966. He joined the Laboratoire d’Architecture et d’Analyse des Systèmes from the French Centre National de la
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Recherche Scientifique (LAAS–CNRS) as an technical engineer in 2001. He is responsible for the technological development of mechanical structures and elements applied systems and microsystems. Marjorie Sanchez was born in Narbonne, France, in 1987. She is currently working toward the M.S. degree in electronic engineering from engineering school CESI, Labège, France, by apprenticeship in the Laboratory for Analysis and Architecture of the Systems (LAAS), II Department, Toulouse, France. LAAS is a national laboratory under the French National Center for Scientific Research (CNRS). One of her apprenticeship projects is the electronic design for micropyrotechnical systems via MEMS. Samuel Charlot is born in 1979, he received the master degree in Materials and Micro Technology from the Var Institute of technology, Toulon, France in 2004. From 2004 to 2006, he worked on the development of microfluidics systems in LAASCNRS, Toulouse, France. In 2006, he worked in the packaging of oxygen sensor at Néosens, Labège, France. From 2007 to 2008, he worked on a low cost technology for the integration of sensors silicon in polymer microfluidic systems in LAAS-CNRS, Toulouse, France. Since 2008, he works on packaging area and more specifically in screen printing technology and flip chip interconnect technology in the team packaging in LAAS-CNRS, Toulouse, France.