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Integration design of a MEMS based fuze
Accepted Manuscript Title: Integration design of a MEMS based fuze Authors: Tengjiang Hu, Yulong Zhao, You Zhao, Wei Ren PII: DOI: Reference:
S0924-4247(17)31572-8 https://doi.org/10.1016/j.sna.2017.09.051 SNA 10359
To appear in:
Sensors and Actuators A
Received date: Accepted date:
31-8-2017 27-9-2017
Please cite this article as: Tengjiang Hu, Yulong Zhao, You Zhao, Wei Ren, Integration design of a MEMS based fuze, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.09.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Integration design of a MEMS based fuze Tengjiang Hu1, Yulong Zhao1,*, You Zhao1, Wei Ren2 1 2
State Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, China
Science and Technology on Applied Physical Chemistry Laboratory, Shaanxi Applied Physical Chemistry Research Institute, China *
Corresponding author, E-mail: [email protected]
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Highlights
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1. A new architecture of MEMS fuze is presented. 2. Based on the modularized method, the basic components (MEMS detonator, SA device and MEMS encapsulation) can be fabricated separately and then assembled to meet different requirements. 3. The fabrication process and experiments of MEMS fuze (static/dynamic performance, safety/arming function) are reported.
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Abstract-The design, fabrication and test of a novel MEMS fuze are presented in this paper. It is a multilayer stacked device:
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the bottom layer is the MEMS detonator, which can generate a high speed slapper (4088 m/s) when initiated by 2500 V surge voltage. The middle layer is the SA (safety and arming) device. With proper driven voltages, the SA device can generate
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508.98 m output displacement and realize changing safety mode to arming mode in 16 ms. The top layer is the MEMS encapsulation, which is integrated with the position limiting structure and energetic grain. 3D printing process and laser
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process are introduced during the whole fabrication process, and the total size of the device can be minimized into 15 mm17 mm5 mm successfully.
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Keywords: MEMS fuze; MEMS detonator; SA device; MEMS encapsulation
Introduction
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Fuze is one of the most critical components in munitions (keeping safe when they are transported and arming when they meet the target) and its miniaturization contributes a lot to the weapon supporting systems [1~4]. Smaller size can provide additional space within the weapon for sensors, power arrangement and guidance electronics. Traditional fabrication process
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can hardly meet the demand of fuze miniaturization, while MEMS technology shows a tremendous potential and has been explored in this field recently. The biggest difficulty they meet is the integration of the detonator, safety-and-arming (SA) device and energetic structure. All these components should be fabricated and assembled in a very tiny volume with a sufficient reliability level [5~12]. Robinson [6] has done some early researches on MEMS SA device. He fabricated two MEMS springs, and set them
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perpendicular to each other to form interlock mechanism. It can function well when driven by proper set back and rotation acceleration. However, springs fabricated by LIGA separately can raise the cost and make it difficult to assemble. Steven S. Mink [7] and Robert A. Lake [5,8] used electro-thermal principle to actuate the SA device, and successfully minimized its size into millimeter utilizing surface micromachining technology. However, confined by the fabrication limitation, thickness of the SA device was no more than 3.5 m which made the whole system too fragile to integrate with detonator and energetic structure. Helene Pezous [1] had reported the integration of MEMS SA and firing device, and the total size (control circuit included) was less than 10 mm10 mm10 mm. Because of the fuze they designed was based on the micropyrotechnical principle, high contact quality between heater and energetic material was required. In order to achieve that, they have to be kept together during the whole fabrication process, and this may bring the unsafe factor. Moreover, the heater was fabricated
on the thin silica membrane (2 m), when filled with the energetic material, it could be fragile to overcome the high overload acceleration. Herein, in this paper, we will represent a novel MEMS fuze, which can realize fabricating the detonator, SA device and energetic structure separately. These basic components then will be bonded together to compose the ultimate MEMS fuze. Based on the modularize method, size and even materials of these devices can be changed and fabricated more easily and efficiently, and a series of devices can be made. Moreover, by combining these different devices together, MEMS fuze can be applied in various environments. 3D printing process and laser process are introduced in the conventional MEMS fabrication process, and the ultimate device is successfully minimized into 15 mm17 mm5 mm.
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Modeling
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2.1. Working principle The MEMS fuze mentioned in this paper is composed of MEMS detonator, SA device and MEMS encapsulation. All of the components will be assembled in the sequence shown in Fig. 1. When certain conditions such as acceleration, altitude or velocity have been met, the detonator will generate a high speed slapper to strike the energetic material and trigger the
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explosion. The SA device should act as a barrier to block and unblock the acceleration barrel during safety mode and arming
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mode.
Fig. 1. Illustration of MEMS fuze. (a). In safety mode, the barrier will block the path to prevent the flyer from striking the energetic material. (b). In
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arming mode, the barrier will be moved away to let the flyer trigger the explosion.
2.2. Design of MEMS detonator
The detonator is a device which can generate a high speed slapper to trigger the explosion [13]. It is composed of insulating substrate, metal foil, polyimide (PI) membrane and acceleration barrel, shown in Fig. 2. The materials of metal
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foil can be Cu, Al or NiCr alloy, which is fabricated on the insulating substrate by sputtering. Stimulated by the surge voltage (1500 V~2500 V), the metal foil can be changed from solid into plasma with high temperature and pressure, then the PI membrane which is covered on the metal foil will be cut out and accelerated. The material of the acceleration barrel
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should be hard enough to cut the PI membrane out, and the ceramic and sapphire are satisfied. In this paper, we explored the possibility of using silicon as the material of acceleration barrel. The diameter and thickness of the barrel are 500 m and
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520 m respectively.
Fig. 2. The structure of MEMS detonator.
According to the principle of energy conservation, the speed of the flyer can be expressed as in (1). mg and mf are the quality of the metal gas and the flyer; E(t) is the internal energy of the metal gas per unit mass; S is the area of the
acceleration barrel; (r,t) is the density of the metal gas; rf(t) is the flying distance of the flyer; u(r,t) and uf(t) are the speed of the metal gas and the flyer; Ee,g is the Gurney energy of the metal gas per unit mass, which is measured by experiment.
1 r f (t ) 1 mg E (t ) S (r , t )u 2 (r , t )dr m f u 2f (t ) mg Ee, g 0 2 2
(1)
Two hypotheses are introduced here: 1. the internal energy, pressure and density of the metal gas are evenly distributed in the space; 2. the velocity of the metal gas is linear distributed. R=mf/mg, which represents the ratio of the mass of the flyer and the metal gas, and the velocity of the flyer can be expressed as in (2). 0.5
(2)
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f 2 Ee, g uf ) 1 ( 1 rf (t ) R 3
Where =2/3+2/(9R). The parameter of the Gurney energy Ee,g can be calculated by (3). Where Jb is the burst current density; k and n of several common metals are listed in Table 1. When the surge voltage was applied at 2500 V,
Ee, g kJbn , (n 2)
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the burst current density was obtained 2305.8 GA/m2, and the flyer velocity can be calculated as 4487 m/s.
(3)
Table. 1. The Gurney constant of several metals k
n
Al
6.5810-3
1.41
Mg
6.810-3
Cu
4.210-3
0.85
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2.3. Design of SA device
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Ee,g (MJ/kg)，Jb (GA/m2)
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Metal
Electro-thermal principle is investigated in the MEMS SA device and the typical structure is shown in Fig. 3. It is
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mainly composed of backside cavity (which is used to be assembled with MEMS detonator) and movable structure. Considering the small thermal expansion coefficient of silicon, 4 V-shape electro-thermal actuators with micro levers are set
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axisymmetrically to the cavity. This arrangement not only can enhance the device output performance, but also facilitate to form interlock mechanism. Four sliders, which to form the barrier, with the same features of teeth and groove are designed at the end of each micro lever respectively. By engaging with the neighbor actuator, the function of interlock can be realized. When the device is working under high acceleration circumstance, the interlock mechanism will constrict the deformation
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and keep sliders covering the cavity. Moreover, the drive signal should be applied on the 4 electro-thermal actuators
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simultaneously to switch the device from safety mode into arming mode.
Fig. 3. The structure of SA device.
Joule heat generated by current flow can realize electro-thermal actuation. Since the length of the actuator is much larger than its cross section, the thermal analysis can be simplified as one-dimensional heat diffusion problem [14~16], and the temperature distribution of the V-shape beam can be obtained by (4): ks
d 2T ( x) S T ( x) Tr J 2 dx 2 h RT
(4)
With the thermal boundary conditions:
T (0) T ( L) Tr
(5)
Where ks is the thermal conductivity of silicon. J=I/(wh)=(Vcos)/(L), it represents the density of the electrical current. V is the voltage that applied on the actuator. θ is the angle of the V-shape beam. is the electrical resistivity of silicon. L means the length of the chevron beam. RT=tv/kv, it is the thermal resistivity between the bottom of the structure and the
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surface of the substrate. It reflects the amount of heat dissipation to substrate. tv and kv are the thickness and the thermal conductivity coefficient of the air gap respectively. S=(2tv+h+w)/w, it is the shape factor which accounts for the heat transfer through all sides of a beam. w and h are the width and height of V-shape beam respectively. Tr is the reference temperature,
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and usually it equals to the room temperature.
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Fig. 4. The structure of V-shape electro-thermal actuator.
The thermal expansion generated by the V-shape electro-thermal actuator, shown in Fig. 4, can be expressed as in (6): L (1 (T Tr ))2 ( 1 tan ) 2 cos 2
(6)
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d
L
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Where, α is the thermal expansion coefficient of silicon, T ( T ( x)dx) / L is the average temperature of the beam. 0
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Table 2. Basic parameters of the material Data
Unit
W
38
m
L
2130
m
h
50
m
5
°
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Following an optimization study of actuator dimensions, we list the geometry parameters in Table 2. The V-shape
electro-thermal actuator in these dimensions can generate at least 11 m displacement, 64 mN output force while keeping the applied voltage as low as 17 V. The maximum temperature occuring in the middle of the V-shape beam is 907.349 K, less than the melting point of silicon 1670 K. Considering the insufficient displacement generated by thermal expansion, micro
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lever with 20 magnification coefficient is introduced, and the displacement can be enlarged into 220 m. 2.4. Design of MEMS encapsulation
Encapsulation is really a challenging work in MEMS field. It will affect the device performance severely. In this paper,
the basic requirements of encapsulation are: 1. Isolate and protect the whole device from the outside environment damage. 2. Constrain the out-off-plane movement of the SA device. MEMS fuze should work under some extreme circumstances, such as high acceleration, thus, position limiting structure should be introduced. 3. Fix the energetic material. A chamber should be designed to fill the energetic grain and align to the barrier. Shown in Fig. 5 is the structure of the MEMS encapsulation.
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Fig. 5.The structure of MEMS encapsulation.
Fabrication
The MEMS detonator and SA device are fabricated on silicon wafer and SOI (silicon-on-insulator) separately. All of the wafers are double-side polished and detailed parameters are shown in Table 3 and the main fabrication steps are illustrated in
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Fig. 6. Table 3. Parameters of silicon wafer and SOI
Diameter (mm)
100
Orientation
(100)
Resistivity
Device layer
0.01~0.02
(cm)
Handle layer
1~20
Device layer
Specification
Diameter (mm)
100
Orientation
(100)
Resistivity (cm)
1~20
Buried oxide layer
3
Thickness (m)
520
Handle layer
400
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(m)
Parameters
50
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Thickness
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Specification
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Parameters
Si
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SOI
The fabrication process of MEMS encapsulation: a1. 3D printing. Considering the complex structure that the encapsulation requires, MEMS process can hardly meet the needs. In this paper, 3D printing is the alternative method; b1.
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Energetic material assembly. Ø3 mm×3 mm energetic grain was filled and aligned in the chamber. The fabrication process of SA device: a2. Bond pad layer. Bond pad was placed on device layer using lift-off. In order to have a good ohmic contact and stable pad surface for wire bonding, two-layer structures were introduced—Cr/Au in 50 nm/300 nm; b2. Structure mask layer. 400 nm layer of Al was sputtered and patterned as the structure mask; c2. Protection mask layer. 400 nm layer of SiO2 and 200 nm layer of Si3N4 were deposited on handle layer by chemical vapor deposition,
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while the device layer was covered by PDMS (protection mask); d2. Backside cavity. The layers of SiO2 and Si3N4 were patterned, and then the whole SOI wafer was etched by 33% KOH 80℃ for 7 hours to form 400 m depth backside cavity; e2. ICP (inductively-coupled-plasma) etching. The PDMS layer was stripped off, and the device layer was etched to the SiO2 layer by ICP; f2. Releasing. The whole wafer was diced into separated chips and released by HF. The fabrication process of detonator: a3. Insulating layer. 400 nm layer of SiO2 was deposited by chemical vapor deposition; b3. Metal foil layer. 4 m layer of Cu was sputtered and etched into 400 m×400 m bridge shape; c3. PI membrane. 25 m PI membrane was covered on the metal bridge; d3. Acceleration barrel. 400 m depth silicon island was fabricated by wet etching (33% KOH, 80℃), then the Ø500 m through-hole (barrel) was formed by laser processing. e3. Bonding. The acceleration barrel, PI membrane and the metal foil were bonded together to form the MEMS detonator. The MEMS detonator, SA device and encapsulation are assembled by the micro operation workbench and then glued
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together through the high temperature adhesive, shown in Fig. 7.
Fig. 6. The fabrication process of MEMS fuze. (a1). 3D printing. (b1). Energetic material assembly.
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(a2). Bond pad layer. (b2). Structure mask layer. (c2). Protection mask layer. (d2). Backside cavity. (e2). ICP etching. (f2). Releasing.
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(a3). Insulating layer. (b3). Metal foil layer. (c3). PI membrane. (d3). Acceleration barrel. (e3). Bonding.
Fig. 7. MEMS fuze after assembly.
Test and discussion
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4.1. Tests of MEMS detonator
The 45 mΩ metal bridge was powered with 2500 V shock voltage, and the speed of slapper was obtained by VISAR (velocity-interferometer-system-for-any-reflector). The test results showed that the PI slapper can be accelerated into 4088 m/s in 800 ns, as shown in Fig. 8 (b). Compared the result with the calculation one (4487 m/s), the deviation is 9.7%. We disassembled the MEMS detonator after it has been initiated, shown in Fig. 8 (a). The Cu metal bridge did no longer exist
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and left the trails of electric explosion. The Ø500 m×25 m PI slapper has been cut out by the silicon barrel successfully, which means silicon can be used as the substitute material of the acceleration barrel.
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Fig. 8. The test results of MEMS detonator. (a). Disassembled detonator after initiation. (2). The velocity of the flyer.
4.2. Tests of SA device
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The experiment stage for measuring output displacement of SA device has been set up, which contains power supplier, microscope, resistor and the SA device. The 4 sliders can be moved away successfully when voltage applied on the 4 actuators simultaneously, and the results shown in Fig. 9 (a) indicated that higher voltage can lead to larger displacement. The SA device got its maximum displacement 508.98 m with 6.12 W (19 V applied voltages), shown in Fig. 9 (c).
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Compared to the results of simulation and calculation, which are 559.68 m and 579.83 m, the deviations are 9.96% and 13.9% respectively. Considering the micro lever could consume the energy, the average temperature of the device will be
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lower than the simulation result. Moreover, some parts of SiO 2 may still remain beneath the movable structure, which will
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lead to the nonuniformity of the air gap and affect the thermal dissipation.
Fig. 9. The test results of the SA device. (a). The output displacement vs the applied voltage. (b). 0 V applied voltage. (c). 17 V applied voltage.
Interlock system can protect the SA device from damage and unexpected malfunction. Fig. 10 showed the test results. The voltage was only applied on one actuator, and the relative slider was still confined by the actuator nearby. The barrier
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can be moved away when the voltage is applied on the 4 actuators simultaneously, just as predicted in the previous section.
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Fig. 10. The test results of interlock function. (a). The deformation of sliders with 0 V applied voltage. (b). The deformation of sliders when voltage is applied on one actuator. The teeth can not move away from the groove, and the SA device stays in safety mode.
High speed camera was used for measuring the step response time of electro-thermal actuator. The high speed camera is
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set 1000 FPS and triggered by the signal generator. Once the signal is applied, the computer will analyze all of the image data captured from the camera. The actuation lasts for 1.5 s, which is sufficient for the actuator to reach its steady state, and
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the measurement result shows that the response time of electro-thermal actuator is about 16 ms, shown in Fig. 11.
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Fig. 11. Experimental setup for response time measurement.
4.3. Tests of MEMS fuze
Based on the basic function of MEMS fuze, the firing tests were divided into two parts: safety tests and arming tests. In
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the safety tests, the acceleration barrel was blocked by the SA barrier, and the surge voltages were applied from 1500V to
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2500V. The results showed that the energetic grain can not explode in the safety mode.
Fig. 12. Results of the safety tests with 1800 V surge voltage.
4 groups of MEMS fuzes were disassembled once been tested. Both of the SA barrier and the energetic grain can stay intact when the surge voltage was lower than 1800V, shown in Fig. 12. When the surge voltage was higher than 2000V, the SA barrier was broken into pieces and the debris hit the energetic grain, which left a shallow pit (600 m in diameter) on its
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surface (still cannot trigger the explosion), shown in Fig. 13 (a) and Fig. 13 (b).
Fig. 13. Results of the safety tests with 2000 V surge voltage. (a). The missing barrier of SA device. (b). The shallow pit on the surface of the energetic grain.
In the arming tests, the SA barrier was kept open during the whole experiments. The surge voltage was applied from
Arming mode
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1500V to 2500V, and the PI slapper hit the energetic grain directly. The test results were shown in Table 4.
Number
Surge voltage (V)
Result
1
1500
unexploded
4
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unexploded
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2 3
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Table 4. Firing test results of MEMS fuze
1800
exploded
2000
exploded
2000
exploded
2500
exploded
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5 6
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The results showed that low surge voltage might cause failure explosion even under the arming mode, and the 2000 V was the proper one. Combining the results of the safety tests with the firing tests, 2000 V surge voltage can satisfy the basic requirements. In the safety mode, the barrier of SA device was hit by the PI slapper directly and absorbed the most of the
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kinetic energy. The barrier structure may break up, but the energy left was not enough to make the energetic material explode. While in the arming mode, there was no obstacle in the acceleration barrel, and the PI slapper could hit the energetic material with high speed and caused the explosion.
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Conclusion and perspectives
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The design, fabrication and test of a novel MEMS fuze are presented. It is a multilayer stacked device: the bottom layer
is the MEMS detonator, which can generate a high speed slapper (4088m/s) when initiated by 2500 V surge voltage. The middle layer is the SA device. With proper driven voltages, the SA device can generate 508.98 m output displacement and realize changing safety mode to arming mode in 16 ms. The top layer is the MEMS encapsulation, which is integrated with the position limiting structure and energetic grain. 3D printing process and laser process are introduced during the whole fabrication process, and the total size of the device can be minimized into 15 mm17 mm5 mm successfully. Safety function and arming function are the main characteristics of MEMS fuze, and they have been validated by experiments in the previous section. The use of MEMS technology in the weapon system is still a new concept, so there is no standard for MEMS fuze to compare the output performance quantitatively. Some typical devices of the previous work are listed in Table 4, and we can
find out that there were more researches on the SA device than on the whole fuze system. Herein, in this paper, a complete form of MEMS fuze is introduced. Since the MEMS detonator, SA device and MEMS encapsulation are fabricated separately, materials and even structures can be changed and investigated easily. For example, the size and material property of the slapper, the thickness of the barrier, the chemical composition and compound method of the energetic material, all these parameters are worthwhile to be investigated in the future work. Table 4. Comparison results of previous work Group
Barrier Material (thickness)
Mechanism
Displacement
Size
Fan [4]
Ni (175 m)
Electro-magnetic
300 m
20 mm20 mm0.175 mm (SA device only)
Robinson [6]
Ni (100 m)
Inertia force
>1 mm
10 mm10 mm0.1 mm (SA device only)
Mink [7]
Si (2 m)
Electro-thermal
32 m
1.9 mm1.9 mm0.5 mm (SA device only)
Lake [5]
Si (3.5 m)
Electro-thermal
785 m
Not report
Pezous [1]
Ceramic (400 m)
Pyrotechnics
1.41 mm
10 mm10 mm10 mm (Fuze system)
This work
Si (50 m)
Electro-thermal
508 m
15 mm17 mm5 mm (Fuze system)
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(SA device only)
Acknowledgement
The work was supported by Changjiang Scholars and Innovative Research Team in University of China (No.
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IRT_14R45) and The National Science Fund for Distinguished Young Scholars (No. 51325503).
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Reference
H. Pezous, et al. Integration of a MEMS based safe arm and fire device. Sensors & Actuators A. 159 (2010) 157-167.
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J.L. Zunino, et al. Reliability testing and analysis of safing and arming devices for army fuzes. Proceedings of SPIE. 2008, 6884:0C1-0C12.
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X. Li, Y. Zhao, T. Hu, et al. Design of a large displacement thermal actuator with a cascaded V‑beam amplification for MEMS safety‑and‑arming
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L. Fan, H. Last, R. Wood, et al. SLIGA Based underwater weapon safety and arming system. Microsystem Technologies 4.4 (1998) 168-171.
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C.H. Robinson, R.H. Wood, T.Q. Hoang, Miniature MEMS-based electromechanical safety and arming device, US Patent number 6964231, 2005.
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S.A. Ostrow, R.A. Lake, et al. Fabrication Process Comparison and Dynamics Evaluation of Electrothermal Actuators for a Prototype MEMS Safe
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and Arming Devices. Experimental Mechanics 52.8 (2012) 1229–1238. W.H. Maurer, G.H. Soto, D.R. Hollingsworth. Method for utilizing a MEMS safe arm device for microdetonation. US Patent number 7007606, 2006.
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[10] T. Hu, Y. Zhao, X. Li., et al. Integration design of MEMS electro-thermal safety-and-arming devices. Microsystem Technologies (2016) 1-6. doi:10.1007/s00542-016-2901-8. [11] J. Zhang, D. Li. Failure analysis of set-back arming process of MEMS S&A device. Sensors & Transducers, 166.3 (2014) 66-72. [12] B. Bao, N. Yan, W. Geng, et al. Simulation and experiment investigation on structural design and reinforcement of pyrotechnical sliding micro-actuators. Analog Integrated Circuits and Signal Processing, 3 (2016) 1-11. [13] R. Xix, X. Ren, L. Liu, et al. Research on design and firing performance of Si-based detonator. Defence Technology. 10 (2014) 34~39.
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[14] E.T. Eniko, S.S. Shantanu, K.V. Lazarov. Analytical Model for Analysis and Design of V-Shaped Thermal Microactuators. Journal of Microelectromechanical System.14.4 (2005) 788-798.
[15] Y.S. Yang, Y.H. Lin, Y.C. Hu, et al. A large-displacement thermal actuator designed for MEMS pitch-tunable grating. Journal of Micromechanics & Microengineering. 19.1 (2009) 15001-15012. [16] Y. Zhang, Q.A. Huang, R.G. Li, et al. Macro-modeling for polysilicon cascaded bent beam electrothermal microactuators. Sensors and Actuators A. 128.1 (2006) 165–175.
Author Biographies Tengjiang Hu received the B.S. and M.S. degrees in mechanical engineering from Xi’an Jiaotong University, China, in 2012 and 2014, respectively. Now, he is pursuing his Ph.D. degree in the same university. His research interest includes MEMS sensors, actuators and integration of MEMS. Yulong Zhao received his B.S., M.S. and Ph.D. degrees in 1991, 1999 and 2003, respectively. Dr. Zhao is currently a professor of Xi’an Jiaotong University, China. His main research fields include MEMS sensors, biosensors, precise instrument and micro/nano manufacturing technology. You Zhao received his B.S. degree in School of Electro-Mechanical Engineering, Xidian University in 2012. Since then, he has been studying in the State Key Laboratory for Mechanical Manufacturing Systems Engineering, Xi’an Jiaotong Universit y, China, to pursue his Ph.D. degree. His current research interest includes triaxial cutting force sensor and cutting force mea surement technology. Wei Ren is the principal staff member of Shaanxi Applied Physical Chemistry Research Institute, China. His research is in
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chemistry and MEMS explosive technology.
S0924-4247(17)31572-8 https://doi.org/10.1016/j.sna.2017.09.051 SNA 10359
To appear in:
Sensors and Actuators A
Received date: Accepted date:
31-8-2017 27-9-2017
Please cite this article as: Tengjiang Hu, Yulong Zhao, You Zhao, Wei Ren, Integration design of a MEMS based fuze, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.09.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Integration design of a MEMS based fuze Tengjiang Hu1, Yulong Zhao1,*, You Zhao1, Wei Ren2 1 2
State Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, China
Science and Technology on Applied Physical Chemistry Laboratory, Shaanxi Applied Physical Chemistry Research Institute, China *
Corresponding author, E-mail: [email protected]
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Highlights
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1. A new architecture of MEMS fuze is presented. 2. Based on the modularized method, the basic components (MEMS detonator, SA device and MEMS encapsulation) can be fabricated separately and then assembled to meet different requirements. 3. The fabrication process and experiments of MEMS fuze (static/dynamic performance, safety/arming function) are reported.
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Abstract-The design, fabrication and test of a novel MEMS fuze are presented in this paper. It is a multilayer stacked device:
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the bottom layer is the MEMS detonator, which can generate a high speed slapper (4088 m/s) when initiated by 2500 V surge voltage. The middle layer is the SA (safety and arming) device. With proper driven voltages, the SA device can generate
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508.98 m output displacement and realize changing safety mode to arming mode in 16 ms. The top layer is the MEMS encapsulation, which is integrated with the position limiting structure and energetic grain. 3D printing process and laser
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process are introduced during the whole fabrication process, and the total size of the device can be minimized into 15 mm17 mm5 mm successfully.
1.
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Keywords: MEMS fuze; MEMS detonator; SA device; MEMS encapsulation
Introduction
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Fuze is one of the most critical components in munitions (keeping safe when they are transported and arming when they meet the target) and its miniaturization contributes a lot to the weapon supporting systems [1~4]. Smaller size can provide additional space within the weapon for sensors, power arrangement and guidance electronics. Traditional fabrication process
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can hardly meet the demand of fuze miniaturization, while MEMS technology shows a tremendous potential and has been explored in this field recently. The biggest difficulty they meet is the integration of the detonator, safety-and-arming (SA) device and energetic structure. All these components should be fabricated and assembled in a very tiny volume with a sufficient reliability level [5~12]. Robinson [6] has done some early researches on MEMS SA device. He fabricated two MEMS springs, and set them
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perpendicular to each other to form interlock mechanism. It can function well when driven by proper set back and rotation acceleration. However, springs fabricated by LIGA separately can raise the cost and make it difficult to assemble. Steven S. Mink [7] and Robert A. Lake [5,8] used electro-thermal principle to actuate the SA device, and successfully minimized its size into millimeter utilizing surface micromachining technology. However, confined by the fabrication limitation, thickness of the SA device was no more than 3.5 m which made the whole system too fragile to integrate with detonator and energetic structure. Helene Pezous [1] had reported the integration of MEMS SA and firing device, and the total size (control circuit included) was less than 10 mm10 mm10 mm. Because of the fuze they designed was based on the micropyrotechnical principle, high contact quality between heater and energetic material was required. In order to achieve that, they have to be kept together during the whole fabrication process, and this may bring the unsafe factor. Moreover, the heater was fabricated
on the thin silica membrane (2 m), when filled with the energetic material, it could be fragile to overcome the high overload acceleration. Herein, in this paper, we will represent a novel MEMS fuze, which can realize fabricating the detonator, SA device and energetic structure separately. These basic components then will be bonded together to compose the ultimate MEMS fuze. Based on the modularize method, size and even materials of these devices can be changed and fabricated more easily and efficiently, and a series of devices can be made. Moreover, by combining these different devices together, MEMS fuze can be applied in various environments. 3D printing process and laser process are introduced in the conventional MEMS fabrication process, and the ultimate device is successfully minimized into 15 mm17 mm5 mm.
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Modeling
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2.1. Working principle The MEMS fuze mentioned in this paper is composed of MEMS detonator, SA device and MEMS encapsulation. All of the components will be assembled in the sequence shown in Fig. 1. When certain conditions such as acceleration, altitude or velocity have been met, the detonator will generate a high speed slapper to strike the energetic material and trigger the
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explosion. The SA device should act as a barrier to block and unblock the acceleration barrel during safety mode and arming
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mode.
Fig. 1. Illustration of MEMS fuze. (a). In safety mode, the barrier will block the path to prevent the flyer from striking the energetic material. (b). In
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arming mode, the barrier will be moved away to let the flyer trigger the explosion.
2.2. Design of MEMS detonator
The detonator is a device which can generate a high speed slapper to trigger the explosion [13]. It is composed of insulating substrate, metal foil, polyimide (PI) membrane and acceleration barrel, shown in Fig. 2. The materials of metal
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foil can be Cu, Al or NiCr alloy, which is fabricated on the insulating substrate by sputtering. Stimulated by the surge voltage (1500 V~2500 V), the metal foil can be changed from solid into plasma with high temperature and pressure, then the PI membrane which is covered on the metal foil will be cut out and accelerated. The material of the acceleration barrel
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should be hard enough to cut the PI membrane out, and the ceramic and sapphire are satisfied. In this paper, we explored the possibility of using silicon as the material of acceleration barrel. The diameter and thickness of the barrel are 500 m and
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520 m respectively.
Fig. 2. The structure of MEMS detonator.
According to the principle of energy conservation, the speed of the flyer can be expressed as in (1). mg and mf are the quality of the metal gas and the flyer; E(t) is the internal energy of the metal gas per unit mass; S is the area of the
acceleration barrel; (r,t) is the density of the metal gas; rf(t) is the flying distance of the flyer; u(r,t) and uf(t) are the speed of the metal gas and the flyer; Ee,g is the Gurney energy of the metal gas per unit mass, which is measured by experiment.
1 r f (t ) 1 mg E (t ) S (r , t )u 2 (r , t )dr m f u 2f (t ) mg Ee, g 0 2 2
(1)
Two hypotheses are introduced here: 1. the internal energy, pressure and density of the metal gas are evenly distributed in the space; 2. the velocity of the metal gas is linear distributed. R=mf/mg, which represents the ratio of the mass of the flyer and the metal gas, and the velocity of the flyer can be expressed as in (2). 0.5
(2)
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f 2 Ee, g uf ) 1 ( 1 rf (t ) R 3
Where =2/3+2/(9R). The parameter of the Gurney energy Ee,g can be calculated by (3). Where Jb is the burst current density; k and n of several common metals are listed in Table 1. When the surge voltage was applied at 2500 V,
Ee, g kJbn , (n 2)
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the burst current density was obtained 2305.8 GA/m2, and the flyer velocity can be calculated as 4487 m/s.
(3)
Table. 1. The Gurney constant of several metals k
n
Al
6.5810-3
1.41
Mg
6.810-3
Cu
4.210-3
0.85
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2.3. Design of SA device
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Ee,g (MJ/kg)，Jb (GA/m2)
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Metal
Electro-thermal principle is investigated in the MEMS SA device and the typical structure is shown in Fig. 3. It is
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mainly composed of backside cavity (which is used to be assembled with MEMS detonator) and movable structure. Considering the small thermal expansion coefficient of silicon, 4 V-shape electro-thermal actuators with micro levers are set
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axisymmetrically to the cavity. This arrangement not only can enhance the device output performance, but also facilitate to form interlock mechanism. Four sliders, which to form the barrier, with the same features of teeth and groove are designed at the end of each micro lever respectively. By engaging with the neighbor actuator, the function of interlock can be realized. When the device is working under high acceleration circumstance, the interlock mechanism will constrict the deformation
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and keep sliders covering the cavity. Moreover, the drive signal should be applied on the 4 electro-thermal actuators
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simultaneously to switch the device from safety mode into arming mode.
Fig. 3. The structure of SA device.
Joule heat generated by current flow can realize electro-thermal actuation. Since the length of the actuator is much larger than its cross section, the thermal analysis can be simplified as one-dimensional heat diffusion problem [14~16], and the temperature distribution of the V-shape beam can be obtained by (4): ks
d 2T ( x) S T ( x) Tr J 2 dx 2 h RT
(4)
With the thermal boundary conditions:
T (0) T ( L) Tr
(5)
Where ks is the thermal conductivity of silicon. J=I/(wh)=(Vcos)/(L), it represents the density of the electrical current. V is the voltage that applied on the actuator. θ is the angle of the V-shape beam. is the electrical resistivity of silicon. L means the length of the chevron beam. RT=tv/kv, it is the thermal resistivity between the bottom of the structure and the
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surface of the substrate. It reflects the amount of heat dissipation to substrate. tv and kv are the thickness and the thermal conductivity coefficient of the air gap respectively. S=(2tv+h+w)/w, it is the shape factor which accounts for the heat transfer through all sides of a beam. w and h are the width and height of V-shape beam respectively. Tr is the reference temperature,
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and usually it equals to the room temperature.
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Fig. 4. The structure of V-shape electro-thermal actuator.
The thermal expansion generated by the V-shape electro-thermal actuator, shown in Fig. 4, can be expressed as in (6): L (1 (T Tr ))2 ( 1 tan ) 2 cos 2
(6)
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d
L
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Where, α is the thermal expansion coefficient of silicon, T ( T ( x)dx) / L is the average temperature of the beam. 0
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Table 2. Basic parameters of the material Data
Unit
W
38
m
L
2130
m
h
50
m
5
°
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Following an optimization study of actuator dimensions, we list the geometry parameters in Table 2. The V-shape
electro-thermal actuator in these dimensions can generate at least 11 m displacement, 64 mN output force while keeping the applied voltage as low as 17 V. The maximum temperature occuring in the middle of the V-shape beam is 907.349 K, less than the melting point of silicon 1670 K. Considering the insufficient displacement generated by thermal expansion, micro
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lever with 20 magnification coefficient is introduced, and the displacement can be enlarged into 220 m. 2.4. Design of MEMS encapsulation
Encapsulation is really a challenging work in MEMS field. It will affect the device performance severely. In this paper,
the basic requirements of encapsulation are: 1. Isolate and protect the whole device from the outside environment damage. 2. Constrain the out-off-plane movement of the SA device. MEMS fuze should work under some extreme circumstances, such as high acceleration, thus, position limiting structure should be introduced. 3. Fix the energetic material. A chamber should be designed to fill the energetic grain and align to the barrier. Shown in Fig. 5 is the structure of the MEMS encapsulation.
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Fig. 5.The structure of MEMS encapsulation.
Fabrication
The MEMS detonator and SA device are fabricated on silicon wafer and SOI (silicon-on-insulator) separately. All of the wafers are double-side polished and detailed parameters are shown in Table 3 and the main fabrication steps are illustrated in
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Fig. 6. Table 3. Parameters of silicon wafer and SOI
Diameter (mm)
100
Orientation
(100)
Resistivity
Device layer
0.01~0.02
(cm)
Handle layer
1~20
Device layer
Specification
Diameter (mm)
100
Orientation
(100)
Resistivity (cm)
1~20
Buried oxide layer
3
Thickness (m)
520
Handle layer
400
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(m)
Parameters
50
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Thickness
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Specification
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Parameters
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SOI
The fabrication process of MEMS encapsulation: a1. 3D printing. Considering the complex structure that the encapsulation requires, MEMS process can hardly meet the needs. In this paper, 3D printing is the alternative method; b1.
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Energetic material assembly. Ø3 mm×3 mm energetic grain was filled and aligned in the chamber. The fabrication process of SA device: a2. Bond pad layer. Bond pad was placed on device layer using lift-off. In order to have a good ohmic contact and stable pad surface for wire bonding, two-layer structures were introduced—Cr/Au in 50 nm/300 nm; b2. Structure mask layer. 400 nm layer of Al was sputtered and patterned as the structure mask; c2. Protection mask layer. 400 nm layer of SiO2 and 200 nm layer of Si3N4 were deposited on handle layer by chemical vapor deposition,
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while the device layer was covered by PDMS (protection mask); d2. Backside cavity. The layers of SiO2 and Si3N4 were patterned, and then the whole SOI wafer was etched by 33% KOH 80℃ for 7 hours to form 400 m depth backside cavity; e2. ICP (inductively-coupled-plasma) etching. The PDMS layer was stripped off, and the device layer was etched to the SiO2 layer by ICP; f2. Releasing. The whole wafer was diced into separated chips and released by HF. The fabrication process of detonator: a3. Insulating layer. 400 nm layer of SiO2 was deposited by chemical vapor deposition; b3. Metal foil layer. 4 m layer of Cu was sputtered and etched into 400 m×400 m bridge shape; c3. PI membrane. 25 m PI membrane was covered on the metal bridge; d3. Acceleration barrel. 400 m depth silicon island was fabricated by wet etching (33% KOH, 80℃), then the Ø500 m through-hole (barrel) was formed by laser processing. e3. Bonding. The acceleration barrel, PI membrane and the metal foil were bonded together to form the MEMS detonator. The MEMS detonator, SA device and encapsulation are assembled by the micro operation workbench and then glued
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together through the high temperature adhesive, shown in Fig. 7.
Fig. 6. The fabrication process of MEMS fuze. (a1). 3D printing. (b1). Energetic material assembly.
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(a2). Bond pad layer. (b2). Structure mask layer. (c2). Protection mask layer. (d2). Backside cavity. (e2). ICP etching. (f2). Releasing.
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(a3). Insulating layer. (b3). Metal foil layer. (c3). PI membrane. (d3). Acceleration barrel. (e3). Bonding.
Fig. 7. MEMS fuze after assembly.
Test and discussion
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4.1. Tests of MEMS detonator
The 45 mΩ metal bridge was powered with 2500 V shock voltage, and the speed of slapper was obtained by VISAR (velocity-interferometer-system-for-any-reflector). The test results showed that the PI slapper can be accelerated into 4088 m/s in 800 ns, as shown in Fig. 8 (b). Compared the result with the calculation one (4487 m/s), the deviation is 9.7%. We disassembled the MEMS detonator after it has been initiated, shown in Fig. 8 (a). The Cu metal bridge did no longer exist
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and left the trails of electric explosion. The Ø500 m×25 m PI slapper has been cut out by the silicon barrel successfully, which means silicon can be used as the substitute material of the acceleration barrel.
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Fig. 8. The test results of MEMS detonator. (a). Disassembled detonator after initiation. (2). The velocity of the flyer.
4.2. Tests of SA device
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The experiment stage for measuring output displacement of SA device has been set up, which contains power supplier, microscope, resistor and the SA device. The 4 sliders can be moved away successfully when voltage applied on the 4 actuators simultaneously, and the results shown in Fig. 9 (a) indicated that higher voltage can lead to larger displacement. The SA device got its maximum displacement 508.98 m with 6.12 W (19 V applied voltages), shown in Fig. 9 (c).
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Compared to the results of simulation and calculation, which are 559.68 m and 579.83 m, the deviations are 9.96% and 13.9% respectively. Considering the micro lever could consume the energy, the average temperature of the device will be
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lower than the simulation result. Moreover, some parts of SiO 2 may still remain beneath the movable structure, which will
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lead to the nonuniformity of the air gap and affect the thermal dissipation.
Fig. 9. The test results of the SA device. (a). The output displacement vs the applied voltage. (b). 0 V applied voltage. (c). 17 V applied voltage.
Interlock system can protect the SA device from damage and unexpected malfunction. Fig. 10 showed the test results. The voltage was only applied on one actuator, and the relative slider was still confined by the actuator nearby. The barrier
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can be moved away when the voltage is applied on the 4 actuators simultaneously, just as predicted in the previous section.
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Fig. 10. The test results of interlock function. (a). The deformation of sliders with 0 V applied voltage. (b). The deformation of sliders when voltage is applied on one actuator. The teeth can not move away from the groove, and the SA device stays in safety mode.
High speed camera was used for measuring the step response time of electro-thermal actuator. The high speed camera is
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set 1000 FPS and triggered by the signal generator. Once the signal is applied, the computer will analyze all of the image data captured from the camera. The actuation lasts for 1.5 s, which is sufficient for the actuator to reach its steady state, and
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the measurement result shows that the response time of electro-thermal actuator is about 16 ms, shown in Fig. 11.
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Fig. 11. Experimental setup for response time measurement.
4.3. Tests of MEMS fuze
Based on the basic function of MEMS fuze, the firing tests were divided into two parts: safety tests and arming tests. In
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the safety tests, the acceleration barrel was blocked by the SA barrier, and the surge voltages were applied from 1500V to
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2500V. The results showed that the energetic grain can not explode in the safety mode.
Fig. 12. Results of the safety tests with 1800 V surge voltage.
4 groups of MEMS fuzes were disassembled once been tested. Both of the SA barrier and the energetic grain can stay intact when the surge voltage was lower than 1800V, shown in Fig. 12. When the surge voltage was higher than 2000V, the SA barrier was broken into pieces and the debris hit the energetic grain, which left a shallow pit (600 m in diameter) on its
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surface (still cannot trigger the explosion), shown in Fig. 13 (a) and Fig. 13 (b).
Fig. 13. Results of the safety tests with 2000 V surge voltage. (a). The missing barrier of SA device. (b). The shallow pit on the surface of the energetic grain.
In the arming tests, the SA barrier was kept open during the whole experiments. The surge voltage was applied from
Arming mode
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1500V to 2500V, and the PI slapper hit the energetic grain directly. The test results were shown in Table 4.
Number
Surge voltage (V)
Result
1
1500
unexploded
4
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unexploded
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Table 4. Firing test results of MEMS fuze
1800
exploded
2000
exploded
2000
exploded
2500
exploded
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The results showed that low surge voltage might cause failure explosion even under the arming mode, and the 2000 V was the proper one. Combining the results of the safety tests with the firing tests, 2000 V surge voltage can satisfy the basic requirements. In the safety mode, the barrier of SA device was hit by the PI slapper directly and absorbed the most of the
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kinetic energy. The barrier structure may break up, but the energy left was not enough to make the energetic material explode. While in the arming mode, there was no obstacle in the acceleration barrel, and the PI slapper could hit the energetic material with high speed and caused the explosion.
5.
Conclusion and perspectives
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The design, fabrication and test of a novel MEMS fuze are presented. It is a multilayer stacked device: the bottom layer
is the MEMS detonator, which can generate a high speed slapper (4088m/s) when initiated by 2500 V surge voltage. The middle layer is the SA device. With proper driven voltages, the SA device can generate 508.98 m output displacement and realize changing safety mode to arming mode in 16 ms. The top layer is the MEMS encapsulation, which is integrated with the position limiting structure and energetic grain. 3D printing process and laser process are introduced during the whole fabrication process, and the total size of the device can be minimized into 15 mm17 mm5 mm successfully. Safety function and arming function are the main characteristics of MEMS fuze, and they have been validated by experiments in the previous section. The use of MEMS technology in the weapon system is still a new concept, so there is no standard for MEMS fuze to compare the output performance quantitatively. Some typical devices of the previous work are listed in Table 4, and we can
find out that there were more researches on the SA device than on the whole fuze system. Herein, in this paper, a complete form of MEMS fuze is introduced. Since the MEMS detonator, SA device and MEMS encapsulation are fabricated separately, materials and even structures can be changed and investigated easily. For example, the size and material property of the slapper, the thickness of the barrier, the chemical composition and compound method of the energetic material, all these parameters are worthwhile to be investigated in the future work. Table 4. Comparison results of previous work Group
Barrier Material (thickness)
Mechanism
Displacement
Size
Fan [4]
Ni (175 m)
Electro-magnetic
300 m
20 mm20 mm0.175 mm (SA device only)
Robinson [6]
Ni (100 m)
Inertia force
>1 mm
10 mm10 mm0.1 mm (SA device only)
Mink [7]
Si (2 m)
Electro-thermal
32 m
1.9 mm1.9 mm0.5 mm (SA device only)
Lake [5]
Si (3.5 m)
Electro-thermal
785 m
Not report
Pezous [1]
Ceramic (400 m)
Pyrotechnics
1.41 mm
10 mm10 mm10 mm (Fuze system)
This work
Si (50 m)
Electro-thermal
508 m
15 mm17 mm5 mm (Fuze system)
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(SA device only)
Acknowledgement
The work was supported by Changjiang Scholars and Innovative Research Team in University of China (No.
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IRT_14R45) and The National Science Fund for Distinguished Young Scholars (No. 51325503).
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Reference
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[10] T. Hu, Y. Zhao, X. Li., et al. Integration design of MEMS electro-thermal safety-and-arming devices. Microsystem Technologies (2016) 1-6. doi:10.1007/s00542-016-2901-8. [11] J. Zhang, D. Li. Failure analysis of set-back arming process of MEMS S&A device. Sensors & Transducers, 166.3 (2014) 66-72. [12] B. Bao, N. Yan, W. Geng, et al. Simulation and experiment investigation on structural design and reinforcement of pyrotechnical sliding micro-actuators. Analog Integrated Circuits and Signal Processing, 3 (2016) 1-11. [13] R. Xix, X. Ren, L. Liu, et al. Research on design and firing performance of Si-based detonator. Defence Technology. 10 (2014) 34~39.
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[14] E.T. Eniko, S.S. Shantanu, K.V. Lazarov. Analytical Model for Analysis and Design of V-Shaped Thermal Microactuators. Journal of Microelectromechanical System.14.4 (2005) 788-798.
[15] Y.S. Yang, Y.H. Lin, Y.C. Hu, et al. A large-displacement thermal actuator designed for MEMS pitch-tunable grating. Journal of Micromechanics & Microengineering. 19.1 (2009) 15001-15012. [16] Y. Zhang, Q.A. Huang, R.G. Li, et al. Macro-modeling for polysilicon cascaded bent beam electrothermal microactuators. Sensors and Actuators A. 128.1 (2006) 165–175.
Author Biographies Tengjiang Hu received the B.S. and M.S. degrees in mechanical engineering from Xi’an Jiaotong University, China, in 2012 and 2014, respectively. Now, he is pursuing his Ph.D. degree in the same university. His research interest includes MEMS sensors, actuators and integration of MEMS. Yulong Zhao received his B.S., M.S. and Ph.D. degrees in 1991, 1999 and 2003, respectively. Dr. Zhao is currently a professor of Xi’an Jiaotong University, China. His main research fields include MEMS sensors, biosensors, precise instrument and micro/nano manufacturing technology. You Zhao received his B.S. degree in School of Electro-Mechanical Engineering, Xidian University in 2012. Since then, he has been studying in the State Key Laboratory for Mechanical Manufacturing Systems Engineering, Xi’an Jiaotong Universit y, China, to pursue his Ph.D. degree. His current research interest includes triaxial cutting force sensor and cutting force mea surement technology. Wei Ren is the principal staff member of Shaanxi Applied Physical Chemistry Research Institute, China. His research is in
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chemistry and MEMS explosive technology.