Research on design and firing performance of Si-based detonator

Rui-zhen XIE*,Xiao-ming REN,Lan LIU,Yan XUE,Dong-xiao FU,Rui ZHANG

Science and Technology on Applied Physical Chemistry Laboratory,Shaanxi Applied Physics-Chemistry Research Institute,Xi’an 710061,China Received 25 November 2013;revised 30 December 2013;accepted 6 January 2014

Available online 24 January 2014

1.Introduction

With the development of ammunition technology,the developing trends of fuze technology should be miniaturization,smart,high reliability and low cost[1].Traditional safety and arming device can accomplish the safety and arming functions of fuze,but it is not available for small-caliber ammunition because of its large size,many parts and poor anti-overload capability[2].MEMS safety and arming device has small size,light weight and good anti-overload capability,which makes conventional fuze has more space to accommodate the multi-sensor detection circuit and the main charge to improve the precision and lethality of ammunition[3,4].In recent years,the U.S.fuze annual conferences reported that the performance of on-chip-based safety and arming device is tested using the missile warhead at present[5-8],and the development trend of fuze technology is on-chip integration of machinery,electronics and pyrotechnics in the future.The micro-detonator has been paid more and more attention as one key technology of MEMS fuze safety and arming device.In this paper,Si-based micro-detonator was designed and fabricated,its firing sensitivity,function time, firing energy were tested,and the test results were analyzed.

2.Design and fabrication of Si-based micro-detonator

The design idea of Si-based micro-detonator is to breakthrough the traditional concept of the minimum primer charge mass for achieving sub-milligram charge.Fabrication of micro-detonator is to use MEMS fabrication process supplemented by other technical means to achieve the integration manufacture of igniter resistor and charge chamber.The charge chamber is made of silicon and glass,etc.

Si-based micro-detonator was designed according to the design idea and MEMS fabrication process.Fig.1 shows the principle structure of micro-detonator,which contains charge layer 1,igniter resistor 2(Ni-Cr film bridge),substrate 3,and primer charge.The charge layer 1 has a chamber which is fi lled with the energetic material,and the igniter resistor 2 is fabricated on the Pyrex7740 glass substrate 3.Pyrex7740 glass substrate has much lower thermal conductivity compared to that of silicon substrate,and is easily integrated with charge layer.

Fig.1.Schematic diagram of Si-based detonator.

The igniter resistor is fabricated on the double-polished Pyrex7740 glass substrate[9,10].In the first step,the substrate is cleaned using a special solvent,thoroughly rinsed by deionized(DI)water,and blow-dried by nitrogen.And then the glass substrate is placed into a hotplate at 120°C for 10 min for further drying.In the second step,a metal layer of Ni-Cr is deposited on the glass substrate by sputtering.The Ni-Cr film acts as the bridge area of igniter resistor.In the third step,a positive photoresist is spincoated onto the Pyrex7740 glass substrate and patterned using photolithography,and then the exposed photoresist is removed by resist developer.After the substrate is put into Ni-Cr etchant(HCLO4+Ce2(SO4)3+H2O),the designed Ni-Cr film bridge area comes into being.

A metal layer of 1.5 μm-thick Al film is deposited onto the Ni-Cr film layer by sputtering after Ni-Cr film bridge area is finished.The Al film acts as the bonding pad.A positive photoresist is also spincoated onto Al film layer and patterned,andthe exposedphotoresististhen removed.Afterthe substrate is put into Al etchant(HPO4+HNO3+CH3COOH+H2O)at 60°C,thedesignedbondingpadisformed.Theglasswaferwith the Ni-Cr film heating resistor is blown-dried with nitrogen after cleaning and then put into an oven with nitrogen fl ux at 250°Cfor20minFig.2illustratesthephotographofNi-Cr film bridge igniter resistor.

Fig.2.Photograph of Ni-Cr film bridge igniter resistor.

The fabrication process of charge chamber starts with a 0.5 mm-thick double-polished silicon chip.In the first step,the substrate is cleaned by RCA1(NH4ON+H2O2+H2O)at 80°C for 20 min.In the second step,a positive photoresist is spincoated onto a top of silicon chip and patterned using photolithography,and then the exposed photoresist is removed by resist developer.After the silicon chip is put into etchant,the designed groove for embedding igniter resistor comes into being.In the third step,Al film is deposited on the other surface of silicon substrate chip by sputtering.Then Al mask of chamber etching is fabricated by photolithography and wet etching.In the fourth step,the charge chamber is formed by Induction Couple Plasma(ICP)etching.

The assembly of micro-detonator is accomplished after the glass substrate and silicon chip is bonded together by chip--chip anodic bonding method.Fig.3 illustrates the process flow chart of micro-detonator.Fig.4 illustrates the photograph of Si-based micro-detonator without energetic material.

Fig.3.Process flow chart of Si-based initiator.

Fig.4.Photograph of Si-based detonator.

The micro-detonator is loaded with nano-porous copper azide.The copper azide contains a certain amount of cuprous azide because of the limit of energetic material preparation process.The size of charge chamber is 1.0 mm(D)×0.5 mm(H),and the average charge quantity is 0.93 mg.Fig.5 illustrates the photograph of detonator loading with energetic material.

Fig.5.The photograph of Si-based detonator loading with energetic material.

3.Performance characterization of Si-based microdetonator

3.1.Firing sensitivity test of Si-based micro-detonator

Three sensitivity test methods,such as up-and-down method,Langlie method and D-optimization method,are provided in GJB/Z377A-1994“sensitivity test mathematical statistics methods”.The ideal parameter estimates can be got using Langlie method because the initial parameters have a little effect on sensitivity testing results.But the process of using Langlie method to calculate the total parameter estimation is complex.Therefore,a portable personal digital assistant(PDA)developed by ShaanxiApplied Physics-Chemistry Research Institute[11]is used for test data processing.Its functions are mainly data entry,data processing,and data transfer,etc in sensitivity test of initiators and pyrotechnics.

The firing sensitivity of micro-detonator was tested according to GJB/z377A-94 sensitivity test method:Langlie.The resolution of the power supply is 0.01 V,the minimal and maximum stimuli are selected to be 5 Vand 18 V,respectively,the data distribution is assumed to be normal distribution,and a mercury switch is used as a discharge switch.Standard deviation of the test results is not altered.The firing circuit is illustrated in Fig.6,and the capacitance of ignition capacitor is 33 μF.

Fig.6.Firing circuit of Si-based detonator.

The firing sensitivity test results of micro-detonator are listed in Table 1.The calculated 50% firing voltage is 6.40 V,and the standard deviation is 0.12 V.The 99.9% firing voltage is 6.78 V,the firing energy calculated by the following equation is 0.76 mJ

3.2.Function time test of Si-based micro-detonator

The test principle of function time is that the power supply energy is provided to the igniter resistor of detonator,at the same time,an activation signal is given by the startup circuit,and the chronometer begins timing.The terminating circuit gives a stop signal to the chronometer when a detonation output exists at the output end of detonator.This time period is the function time of detonator.An activation signal is also given simultaneously when the micro-detonator initiation(start)circuit is switched on.The principle block diagram of test is shown in Fig.7.

Fig.7.The testing sketch of Si-based detonator’s function time.

The function time of micro-detonator was tested using wire probe and photoelectric transducer.If the wire probe is used for test,two juxtaposed enameled copper wires are adhered to the output end of micro-detonator.When a detonation output exists at the output end of detonator,the stop circuit is switched on to give a stop signal because of plasmas formed between two wire probes.The stop circuit consisting of photoelectric transducer gives a stop signal using photoelectric method.

The function time at different excitation voltages was tested using wire probe.The test results are listed in Table 2,and the oscilloscope graphs are shown in Fig.8,(a)6.26 V,(b)6.72 V,(c)7.23 V,(d)10.5 V.The function times at 15 V excitation voltage are listed in Table 3,and the oscilloscope graphs are shown in Fig.9.

Table 2 The function times of detonator at different excitation voltages.

Table 1 The test results of firing sensitivity of Si-based detonator.

Fig.8.Oscilloscope graphs of function times at different excitation voltages.

Table 3 The function times of detonator at 15 V.

The function time was tested using photoelectric transducer.The test results are listed in Table 3,and the oscilloscope graphs are shown in Fig.10.

3.3.Firing energy test of Si-based micro-detonator

Fig.9.Oscilloscope graphs of function time tested using wire probe.

Fig.10.Oscilloscope graphs of function time tested using photoelectric transducer.

In order to monitor the IV characteristics of microdetonator and further explore the actually utilized firing energy,the current loop is connected with other test apparatus in series.The input current(I0)and voltage(U0)are monitored and recorded in an oscilloscope in order to get the firing curves for each test of micro-detonator.The action of micro-detonator requires energy based on the following integral equation.

When the firing voltage is 15 V,the IV test results of microdetonator are shown in Fig.11,where(a)shows the test results for 6.4 μs of function time,and(b)shows the test results for 5.2 μs of function time.The integrate results of microdetonator firing energies are shown in Fig.12,which is 0.24 mJ and 0.20 mJ respectively.On the other hand,the firing energies are also calculated using the following integral equation.The results of two calculating method are accordant.

Fig.11.The IV curve of micro-detonator(1).

The IV characteristics of micro-detonator are tested when the firing voltage is 6.26 V and 6.72 V,the test results are shown in Fig.13 where(a)shows the test results for 6.26 V,and(b)shows the test results for 6.72 V.The integrate results of micro-detonator firing energies are shown in Fig.14,which is 0.23 mJ and 0.21 mJ respectively.The result is accordant with 15 V.

Fig.12.Firing energy of Si-based detonator(1).

Fig.13.The IV curve of micro-detonator(2).

Fig.14.Firing energy of Si-based detonator(2).

4.Results and discussions

Firing sensitivity of micro-detonator was tested according to GJB/z377A-94,the average firing voltage is 6.4 V when the capacitor of firing circuit is 33 μF.The result shows that the micro-detonator is sensitive.Further research will be focused on optimizing the design and reducing the firing energy to meet the low-energy ignition requirement.

The test results show that the function time reduces linearly with the increase in firing voltage.Both the test results obtained by wire probe are in agreement with those obtained by photoelectric transducer.The average function time is 5.48 μs under 15 V,33 μF.

The IV characteristics of micro-detonator were tested by using current coils,and the obtained actual firing energy is 0.22 mJ.The firing energy is 0.76 mJ,which is calculated based on 1/2CU2.The actually utilized firing energy is only 29%of calculated firing energy,which is consistent with the theoretical result in Ref.[12].The reason is that the firing circuit uses a part of energy,on the other hand,the initiation time is only a small part of the RC discharge time.

5.Conclusions

A Si-based micro-detonator was designed and fabricated.Detailed design and fabrication process of each part of microdetonator were presented,and the assembly of the two layers(Si-layer and glass layer)was described.Its operation was validated by experimentation.The firing performance is excellent for low-energy ignition and fl eetly action requirement.This paper also proposed a feasible method for testing the actual firing energy.

In future,we plan to make further research into the input performanceofSi-basedmicro-detonatortoprovideareference for its optimization design;test its output performance and optimizeitsfunctionalitydesigntomeetthelowenergyignition and ef ficient output requirements of miniature munitions.

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