16 Sci. Technol. Energ. Mater., Vol. 82, No. 1, 2021

© Copyright Japan Explosives Society. All rights reserved.

Ignition and safety characteristics of semiconductor bridge with NTC thermistor

Jun Wang*, Bin Zhou *†, Shu-qin Ye**, and Hou-he Chen*

* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, CHINA* *North Special Energy Group Co. LTD., Xi’an 710000, Shanxi, CHINA Phone: +86 18061601916

† Corresponding address: zhoubinnust@126.com

Received: April 2, 2020, Accepted: September 24, 2020

Abstract　The ignition and safety characteristics of a radio frequency (RF) insensitive semiconductor bridge (SCB) was presented. Firstly, the influence of resistance of negative temperature coefficient (NTC) thermistor on SCB ignition performance was studied by a capacitor discharge unit. Secondly, the constant current firing set and an infrared temperature measurement system were utilized to study the temperature change of the SCB with different NTC thermistor. The results showed that as the resistance of the NTC thermistor increased, its effect on the ignition and safety characteristics of SCB became weaker. NTC with a resistance of 10 Ω succeeded in protecting SCB initiators against electromagnetic environment without affecting their performance.

Keywords: semiconductor bridge initiator, NTC thermistor, ignition characteristic, RF insensitive

1. Introduction

　Semiconductor bridge (SCB) chip is a small, heavily doped, polysilicon device which is formed out of a polysilicon-on-silicon wafer 1),2). Since its invention in 1968 3), it has been used in weapons, thrusters and arm-and-safe devices. With a small input energy (3 mJ), the polysilicon bridge is heated and generates high temperature plasma. The temperature of plasma is up to 4000 K and rapidly ignites the primer charge which presses against the bridge, thereby initiating the device 4),5). It provides advantages over bridge-wire igniter, including low all-fire energy, high reliability and rapid response time.　Although SCB already has such advantages, a problem of SCB is sensitive to electromagnetic radiation, which results from military communications and radar systems. Therefore, the electromagnetic compatibility design for SCB is an important part of safety design 6)-9). Techniques exist to withstand harsh radio frequency (RF), including the use of filter 10), ferrite beads 11), capacitor 12) and NTC thermistor 13). When the above-mentioned devices are used, their influences on SCB ignition and safety performance remain to be studied. In this paper, the influence of NTC thermistor resistance on the ignition performance of SCB was first studied with a capacitor discharge unit.

Subsequently, the difference in RF reinforcement with different NTC thermistor was studied by using infrared temperature measurement technology.

2. Experiment

2.1 Experiment sample　By taking advantage of micro-electromechanical systems technology, the SCB chip was fabricated by following steps and Figure 1 is a schematic diagram of SCB chip. Starting material was silicon substrate which was polished on one side. Then, the silicon substrate was thermally oxidized to form a layer of silicon dioxide that was about 1 μm in thickness. The wafer was then heavily doped to form a layer of poly-silicon that was about 2 μm in thickness. The doping element was phosphorus, and doping concentration was about 10 20 P atom cm －3. The wafer was then coated with photoresist and the poly-silicon layer was etched to form a “H”-shaped region shown by the dashed lines in Figure 1. Finally, a layer of metal was selectively deposited over the surface and provide the contact pads. The width (W, 250 μm) and length (L, 72 μm) of the bridge area were determined by the area of metal layer which contribute to a final resistance of 1 Ω.　For subsequent experiment, the SCB chip was mounted on a ceramic plug. The ceramic plug used in the test had

Researchpaper

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two grooves and two-spaced lead wires passing through. The chip was first mounted onto the upper groove and the NTC thermistor was placed in the lower groove. Via an ultrasonic wire bonding process, the SCB chip was connected to the two lead wires with bonding wires and the silver-filled conductive epoxy was coated onto the bonding wires to provide protection. Meanwhile, the pins of NTC thermistor were also connected to two lead wires with conductive epoxy. Thus, the NTC thermistor was in parallel with SCB which is shown in Figure 2. In subsequent tests, NTC thermistors with different resistance were used to investigate their effects on SCB. Their resistance values at normal temperature (298 K) were 5 Ω, 10 Ω, 16 Ω, 22 Ω and 30 Ω, respectively.

2.2 Experiment setup and measurement method　The ignition characteristic of SCB is tested by a firing set which is shown in Figure 3. It consists of a low voltage capacitor discharge unit (CDU, ALG-CN1) with a 22 μF capacitor and an oscilloscope. The capacitive discharge unit

is the excitation source which provide certain energy to actuate SCB. During the firing process, the oscilloscope (Lecroy 604Zi) is used to monitor the electrical behavior of the SCB and the data were recorded for analysis of the energy dissipated by the SCB.　To gain a better understanding of protection capabilities of different NTC thermistors, an infrared temperature measurement system is utilized which is shown in Figure 4. It consists of a constant current power supply (ALG-HL-15A), an oscilloscope and an infrared thermal imaging camera (ThermoVision A40-M; temperature range from 233 K to 773 K; manufactured by FLIR Systems, Inc.). The test principle is as follows. A constant current was first applied to the SCB with a duration of 5 min. The current that flows through the SCB and NTC thermistor is recorded by the oscilloscope. At the same time, the current that flows through the SCB chip will generate heat. Thus, infrared signal is generated on the sample surface. The infrared signal is detected by the infrared thermal imaging camera, and it is converted into electrical signal. Through the

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Figure 1　Schematic diagram of SCB chip.

Figure 2　Fabrication of SCB initiator.

Figure 4　The infrared temperature measurement system.

oscilloscope

Figure 3　Capacitor discharge firing set.

18 Jun Wang et al.

computer processing system, the electrical signal is converted into temperature value, so the change in the surface temperature of the SCB chip over time is obtained.

3. Result and discussion

3.1 Ignition characteristics of SCB　In order to test the current distribution of SCB and NTC on the sample, one of each of the samples was connected to the NTC and SCB by external wires. Figure 5 is the electric discharge characteristic curve of SCB and SCB with different NTC thermistors. In general, the firing process included heating, melting, vaporization and plasma generation. Here, t 0 is the starting time. t 1 is the first peak of voltage curve which corresponded to the melting of

silicon. t 2 is the second peak of voltage which corresponded to the vaporization of the silicon, and it was denoted as burst time. t 4 is the moment that current dropped to 0 A which indicated the end of the firing process. Comparing the curves in Figure 5, when 5 Ω NTC was connected in parallel with SCB, the first peak of the voltage curve becomes flat. The current curve did not change and the peak value has decreased. At the same time, it was found that a current signal passed through the NTC branch. When the NTC resistance was increased to 30 Ω, the above effects had basically disappeared. Almost no current flowed through the NTC branch.　In this study, the characteristic times t 1, t 2, and t 4 of different samples, as shown in Figure 6 and Table 1 were

Figure 5　 The electric discharge characteristic curve of testing samples. (a) SCB. (b) SCB with 5 Ω NTC. (c) SCB with 30 Ω NTC.

Figure 6　The characteristic time and energy of test samples.

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compared. Meanwhile, the energy dissipated in SCB is obtained by integrating the power versus time, and the energy corresponding to the above time is shown in Figure 6 (b) and Table 1. The results showed that after connecting with different types of NTC thermistors, the melting time (t 1) and energy (E 1) of SCB had not changed significantly. Comparing the burst time (t 2) and the burst energy (E 2), it could be seen that the burst time of SCB with 5 Ω NTC thermistor had a delay of about 1 μs, and the burst energy remained unchanged. However, after the 10 Ω thermistor was connected, the burst time of SCB did not change, the burst energy was reduced by 0.05 mJ. When the resistance of the NTC thermistor exceeded 10 Ω, the thermistor had little effect on the burst time and the energy of the SCB. The above test results showed that even if the SCB burst time was short (microsecond level), the thermistor with small resistance would still have an impact on the ignition performance of the SCB.

3.2 Temperature measurement of SCB with 1 A DC-current

　Figure 7 is the thermal image of samples at different points in time. As could be seen from the images, the brightness of the bridge area first became brighter which means that the temperature of bridge area rose fastest after the input of current. At 20 s, the brightness of the bridge area was significantly higher than the surrounding area, indicating that the temperature of the bridge area was always the highest. As the electrical energy was

continuously converted into Joule energy, the heat generated in the bridge area diffused from the chip to the surrounding ceramic plug area. After 60 s, the brightness of the bridge area did not change, and the temperature of the conductive silver and the lower surface of the chip kept rising. After 100 s, there was no obvious change in temperature from the thermal image. When the SCB was connected in parallel with 10 Ω and 22 Ω NTC thermistors, the thermal image at 20 s was compared. The thermal image showed that the brightness of the SCB chip and its surroundings became significantly lower, which indicated that the NTC thermistor had a shunt effect that reduced the current through the SCB chip. Also, at the same time, the larger the NTC resistance, the brighter the brightness of the thermal image. After 60 s, the obvious changes in several samples could not be seen from the thermal image.　The software was used to obtain the maximum temperature of the central bridge area of the above samples, and the temperature-time curve was obtained. As can be seen from Curve 1 in Figure 8 (a), the moment the power was applied, the temperature of the SCB bridge area rose rapidly. It reached a temperature of about 380 K and then the temperature gradually increased. The maximum temperature at 300 s was 555 K. When a 10 Ω NTC thermistor was connected in parallel with SCB (Curve 2), the temperature of the SCB decreased significantly (335 K) at the initial moment. Then the temperature rose slowly and the maximum temperature during the testing was 397 K. Meanwhile, temperature of the bridge area decreased significantly. When the resistance of NTC rose to 22 Ω (Curve 3), the temperature rapidly rose to 369 K after the power was turned on. The rising trend of temperature in the bridge area was similar to Curve 1 and the maximum temperature was 509 K.　The variation in voltage during the test was further compared in Figure 8 (b). Curve 1 showed that the SCB voltage slowly rose from 1.23 V to 1.38 V. However, after connecting with 10 Ω NTC (Curve 2), the voltage on the SCB decreased rapidly from 1 V to 0.88 V within 20 s, and it remained the same until the end of the test. With 22 Ω NTC in parallel (Curve 3), the SCB voltage was maintained

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Table 1　 Electro-explosive performance parameters of SCB with and without NTC.

SampleAverage of burst

time [μs]Average of burst

energe [mJ]

SCB 5.337 0.496SCB-5 ΩNTC 6.229 0.508SCB-10 ΩNTC 5.283 0.450SCB-16 ΩNTC 5.495 0.502SCB-22 ΩNTC 5.346 0.507SCB-30 ΩNTC 5.115 0.486

Figure 7　Thermal images of samples at different points in time.

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at about 1.15 V.　From the above test results, it could be found that when SCB was connected to NTC and under the same input current condition, NTC with small resistance value responded faster. At the same time, the resistance of NTC decreased faster and the shunting ability was stronger, which would make SCB safer.

4. Conclusion

　The ignition and safety characteristics of SCB with NTC thermistor has been demonstrated by using a capacitor-driven discharge and infrared temperature measurement system. The test results show that the resistance of NTC will have a great impact on the ignition performance and RF protection capability of SCB. Firstly, with the increase of NTC resistance, its impact on SCB gradually weakens, and NTC above 10 Ω has no effect on SCB burst performance. Secondly, under the same current input condition, as the NTC resistance increases, its shunting effect will weaken, which will cause the temperature of the SCB chip to become higher. This means that SCB is more susceptible to electromagnetic environments. Therefore, NTC with proper resistance is very important for the electromagnetic compatibility design of SCB.

References

1) D. A. Benson, M. Larsen, A. Renlund, W. Trott, and R. Bickes, J. Appl. Phys., 62, 1622-1632 (1987).

2) R. Bickes Jr, Propellants. Explos. Pyrotech., 21, 146-149 (1996).

3) L. E. Hollander, U.S. Patent 3366055 (1968). 4) J.-U. Kim, C.-O. Park, and M.-I. Park, Phys. Lett. A, 305,

413-418 (2002). 5) J. Lee and T. Kim, Sensor Actuat. A-Phys, 190 (2013). 6) Department of Defense, Electromagnetic environmental

effects requirements for systems, MIL-STD-464C (2010). 7) J. J. Pantoja, N. Pena, F. Rachidi, and F. Vega, IEEE Trans.

Electromagn. Compat., 56, 393-403 (2014). 8) Z. Lv, N. Yan, and B. Bao, Microelectron Reliab., 75, 37-42

(2017). 9) D. B. Novotney, B. M. Welch, and D. W. Ewick, 35th AIAA/

ASME/SAE/ASEE Joint Propul. Conf. and Exhibit., AIAA 99-2417, 1-6 (1999).

10) J. H. Henderson and T. A. Baginski, IEEE T. Ind. Appl., 32, 465-470 (1996).

11) W. Ren, Y.-W. Bai, and E.-Y. Chu, Acta Armamentarii, 35, 1375-1380 (2014)

12) T. L. King and W. W. Tarbell, SAND2002-2213 (2002). 13) F. Chen, B. Zhou, and Z.-C. Qin, IEEE T. Electromagn.

Compat., 54, 1216-1221 (2012).

Figure 8　Temperature curves over time.