Preparation and Ignition Properties of Tantalum Nitride Thin-Film Energy Transducers


 Tantalum nitride (TaN) has excellent electrical properties that can be used as an energy transducer in the ignition field. In this study, TaN film transducers with different bridge parameters were designed and fabricated in an attempt to reduce its energy consumption. The ignition sensitivity of the film transducers was tested using the Langley method. The results revealed that the ignition voltage is the lowest when the thickness of the film is 0.9 µm. If the thickness and length of the bridge film are fixed, the ignition voltage of the transducer first decreases and then increases with the width of the bridge film increases. When the thickness and width of the bridge film are fixed, the ignition voltage of the transducer is first decrease and then increase with the length of the bridge film increases. We also evaluated the ignition mechanism of TaN film transducers. By comparing the performance of TaN, semiconductor bridge (SCB), and nickel–chromium (Ni–Cr) film transducers, the TaN and SCB transducers are proven to have similar ignition performances, which are better than the Ni–Cr transducer. The negative temperature coefficient of TaN and the positive feedback after the initial electrothermal ignition promoted the growth and strengthening of plasma in the bridge film, allowing the medicament to ignite quickly. When the feasibility of the process and the influence of the bridge film parameters on ignition sensitivity are considered, the preferred design parameters of the transducer are a thickness of 0.9 µm and a bridge film size of 0.3 mm×0.3 mm. This study shows that TaN can be utilized as a high-performance transducer.


Introduction
Microelectromechanical system (MEMS) transducers have the characteristics of structural miniaturization, energy exchange information, and sequence integration that are developmentally important to the new generation of pyrotechnics. Microstructure transducers are a core part of MEMS pyrotechnics, and determine their ignition performance, safety, and reliability, all of which affect the combat effectiveness of weapons and ammunition [1][2][3]. Microstructure transducers are made mainly of metal or semiconductor materials deposited on insulating substrates. Currently, most ignition transducers are made from nickel-chromium (Ni-Cr) and semiconductor bridge (SCB) materials [4][5]. The Ni-Cr bridge lm is consistent, reliable, and antistatic, but its temperature coe cient of resistance (TCR) is positive. Therefore, the resistance and ignition voltage increase as temperature increase, so the energy conversion e ciency is not high [6]. In addition, Ni-Cr bridge lms dissolve easily in the presence of moisture, which causes the failure of the microstructure energy exchanger [7].
The TCR of the SCB is negative, and the resistance decreases with the increase in temperature, so the SCB requires less ignition energy. The SCB also has the advantage of small ignition time delay, providing safety and compatibility when compared with the traditional integrated circuit process. However, the SCB cannot carry a large amount of power and is affected greatly by the environment, resulting in possible ignition failure. Tantalum nitride (TaN), on the other hand, has excellent electrical properties. It has a high melting point, high hardness, stable chemical and thermal properties, good oxidation resistance, and corrosion resistance, and has some important applications in aerospace, microelectronics, biomedicine, power machinery, and other elds [8][9].
The TCR of TaN is negative and the resistance decreases with the increase in temperature, leading to a low energy conversion of the microstructure transducer. TaN has self-passivating characteristics and oxidizes in air to form a dense Ta 2 O 5 lm, with a thickness of approximately 12 nm, which resists erosion from water and gas when working in an unsealed state, ensuring excellent stability and reliability. In addition, TaN has excellent blocking performance, preventing the mutual diffusion between the functional lms, which made the working e ciency and service life of the microstructure transducers can be improved [10][11].
This article discussed how a TaN lm transducer is designed and prepared, the in uence of bridge lm parameters ( lm thickness, size, length, and shape) on the ignition sensitivity of TaN lm transducers, and their ignition mechanism. We also provided the basic parameters for the low energy conversion of microstructural transducers.

Design and Preparation
The overall structure of a TaN lm transducer consists of a substrate (glass 7740), two intermediate layer [titanium (Ti)], an ignition layer [TaN], and a welding layer [copper (Cu)]. The schematic diagram is shown on the left in Fig. 1.
The support carrier of the substrate is glass 7740. The function of the intermediate layer (Ti) is to increase the adhesion between the materials. The ignition layer material is TaN, and the welding layer (Cu) is used to improve conductivity. The shape of the TaN bridge lm is rectangular, as shown on the right side in Fig. 1.
A MEMS process was used to prepare the TaN lm transducer, as follows: substrate ultrasonic cleaning →homogenization → pre-drying → etching → post-drying → Ti lm sputtering → TaN lm sputtering → stripping → ultrasonic cleaning → blow-drying → homogenization → pre-drying → set engraving → post-drying → exposure → hard mask → blow-drying → Ti lm sputtering → Cu lm sputtering → stripping → ultrasonic cleaning → blowdrying. The related process parameters are shown in Table 1. Among them, all three lms were prepared by magnetron sputtering (model: KS60VR, KENOSISTEC Company, Italy). The common deposition parameters of the TaN, Ti, and Cu lms were a background vacuum of 5×10−6 Pa, an Ar ow rate of 60 sccm, and a substrate temperature of 70°C. The other deposition parameters were as follows: (1) the target purity of TaN was 99.9%, sputtering power was 200W, sputtering time was 50 min; (2) the target purity of Ti was 99.9%, sputtering power was 100 W, sputtering time was 50s; and (3) the target purity of Cu was 99.95%, sputtering power was 200W, and sputtering time was 40 min. The photoresist used in the stripping process was the RN-246. Note: (1) tetramethylammonium hydroxide (TMAH); (2) N-Methyl pyrrolldone (NMP).
The thickness of the TaN lm was changed by adjusting the magnetron sputtering time. The shape of the bridge lm was controlled by lithography. This process resulted in the error between the design size of each part of the transducer and the actual measurement size of each part being very small. Therefore, the thickness and size of the TaN lms were calculated according to the design values. The actual measurement with the copper wire indicates the resistance of TaN lm transducers.
The appearance of a TaN lm transducer and the microstructure of its bridge lm are shown in Fig. 2. The threedimensional surface of the TaN lm transducer was observed by an Agilent5500 atomic force microscope (AFM; gilent Technologies, Inc. USA), as shown in Fig. 3. What the root-mean-square-roughness of the TaN lm was 3.05nm indicated that the surface of the TaN bridge lm was smooth and dense.

Ignition Sensitivity Testing
The ignition sensitivity of TaN lm energy transducers with different parameters of bridge lms were tested by the Langley method and the GJB5309.9-2004 explosive test method. The instrument resolution was 0.1V, and the test data were assumed to be normally distributed. The test initiation circuit is shown in Fig. 4. Meanwhile, the ignition capacitance was 33 µF and the energetic material coated on the bridge lm was lead styphnate.

Ignition Energy Testing
According to the formula of electrical energy: is the total thermal energy (J), U is the charging voltage (V), I is the current (A), and t is the charging time (s). As electrical energy is a function of voltage, current, and time, the ignition energy of a TaN lm transducer was obtained by integrating power curve. Therefore, this study demonstrated that a comprehensive evaluation of ignition energy can be carried out by using electrical energy [12][13][14]. The ignition energy test circuit used in this experiment is shown in Fig. 5.

Effects of Different Thickness on Ignition Sensitivity and Energy
When the shape and size of the bridge lm (0.1 mm × 0.1 mm) were xed, the ignition sensitivity was tested with different bridge lm thicknesses of 0.3, 0.6, 0.9, 1.2, and 1.5 µm. The results are shown in Table 2. As the thickness of the bridge lm increased, the resistance of the converter decreased, and the ignition voltage of the converter rst decreased and thenincreased. When the thickness of the bridge lm was increased to 0.9 µm, the ignition voltage was at itssmallest. Since thevacuum sputtering of the lm resulted in different degrees of defects, including grain stacking mismatch dislocations, lmpores, impurity intrusion, and reactive gas inclusions,these defects had a direct effect on the resistance, thermal resistance,internal stress, and adhesion strength of the lm. Therefore, when the 0.3µm-thick TaN lm transducer used in the process of ignition, the current caused a change in the inside composition of the lm or the smaller rearrangement of the grain,which madethe ignition voltage be larger.

Effects of Different Widths on Ignition Sensitivity and Energy
With a thickness of 0.9 µm and a bridge length of 0.1 mm, the ignition sensitivity of TaN lm transducers was tested with bridge widths of 0.1, 0.2, 0.3, 0.4, and 0.5 mm. The results are shown in Table 3. The results showed that when the thickness and length of the bridge lm were xed, the resi ergy all increased as the bridge width increased.

Effects of Different Lengths on Ignition Sensitivity and Energy
With a thickness of 0.9 µm and a width of 0.1 mm, the ignition sensitivity of TaN lm transducers was tested with the bridge lengths of 0.1, 0.2, 0.3, 0.4, and 0.5 mm. The results are shown in Table 4. The results demonstrated that when the thickness and the width of the bridge area were xed and the bridge length was increased, the resistance decreased, but the ignition voltage and the ignition energy both increased.

Effects of the type of shape on Ignition Sensitivity and Energy
The shape of TaN bridge lm with thickness of 0.9 µm and size of 0.1 mm × 0.1 mm when θ = 30°, is shown in Fig.  6. The test results of ignition sensitivity of this structure are shown in Table 5. The results showed that the average ignition voltage of the bridge lm was 4.8V. However, in Fig. 2, the rectangle shape of the TaN lm shown had a minimum average ignition voltage of 5.8V and a maximum average ignition voltage of 11V. These results indicate that the bridge lm shape has an obvious in uence on the converter thermal power pressure.

IV Testing
In general, the action mode of a transducer is divided into electro-thermal conversion and electro-induced plasmaexplosion [15]. Nano-energetic materials, such as metal foil, metal wire, and nanoscale alternating multilayers, undergo an electrical explosion under the action of a large pulse current, producing plasma that generates shock waves and optical radiation into the surrounding medium [16]. IV testing of TaN, SCB, and Ni-Cr bridge lms was completed by setting the voltage to 8 V and the capacitance to 33 µF. The results of these tests are shown in Fig. 7.
Under these conditions, the three kinds of bridge lms demonstrated an electro-thermal conversion action process. Additional IV testing on the three bridge lms was completed by setting the voltage to 16 V, and the results are shown in Fig. 8. As demonstrated in Fig. 8(a) and Fig. 8(b), the voltage and current of the two transducers rise by transition when the TaN and SCB discharge at the beginning of the capacitor. The voltage and current lines decrease again after 5 and 10µs, respectively, followed by a horizontal constant (original value). These results demonstrate that during this process, the bridge region rapidly fused and gasi ed into luminescent plasma, and that the action process of both is electro-induced plasma explosion [17][18][19]. Fig. 8(c) shows that the initial voltage and current appear to transition during the combustion of the Ni-Cr bridge lm, then become a continuous and stable decline curve, and nally trend to a constant value (original value). This result shows that the Ni-Cr bridge area does not fuse and that it uses an electric heat transfer process during the power-up excitation process.

Ignition of Lead Styphnate
IV and ignition testing of TaN lm transducers containing lead styphnate were also completed. The voltages were set to 8 and 16 V, respectively. The test results are shown in Fig. 9, which show that lead styphnate was ignited successfully in both cases. The voltage-current curve of the TaN lms can rise rapidly at low voltage, as shown in Fig. 9(a). At about 20 µs, the current rises to the maximum value, and then begins to decrease slowly. During this process, the TaN lm transducer converts electrical energy into thermal energy, and when the heat rises to a certain level (at approximately 120 µs) leads styphnate ignition. The bridge lm is not broken until the current and voltage reach 400 µs, where it trends to the constant value (initial value). According to Fig. 9(b), the TaN lm transducers are vaporized rapidly under the excitation of 16 V, where a strong glow is emitted by the action of the electrical eld, forming a plasma discharge of thousands of degrees. This uid, at high temperature and high pressure, diffuses rapidly into the charge, causing the lead styphnate to heat upto ignition temperature. The total ignition time is short, and the response speed is fast.
Grundmann et al. [20] found that plasma can be formed only when a delay in discharge is formed, which time of the ignition can be realized. Fig. 9(b) shows that two voltage peaks are generated when the TaN lm transducer ignites, which are located near the times of 4 and 15 µs. When the current ows through the bridge area, the discharge luminescence is rst generated along the bridge edge with the largest potential gradient. As the impedance of TaN is a negative temperature coe cient, the resistance value decreases with the increase in temperature, forming a positive feedback in the increased temperature. The decrease of resistance and the increase of current lead to the rapid temperature increase that ultimately reaches the TaN melting point. It will produce a melt and gasi cation process that drives continuously to the center of the bridge, and eventually produces a strong ionized vapor layer on the surface of the bridge lm. The resistance of TaN for an ion state is larger than its resistance to melting and gasi cation. Thus, a large partial voltage is obtained, and a second voltage spike is formed. At this moment, the current enters this region and heats enough to form a delay in discharge. The TaN lm transducer goes through four stages: heating, melting, vaporizing, and plasma generation. The negative temperature coe cient of TaN and the positive feedback produced after the initial electro-thermal ignition that promotes the growth and strengthening of the plasma in the bridge area. Finally, it caused the medicament to ignite quickly.

Conclusions
TaN is a new energy-exchange material that is corrosion resistant, suitable for harsh environments, and has a simple manufacturing process. (1) When the size and shape of the bridge area are xed, the resistance increases with the increase thickness of the bridge lm, and the thermal power pressure rst decreases then increases. To realize the low energy conversion of TaN lm transducers, the optimal thickness of the transducer was approximately 0.9 µm that based the factors of a MEMS preparation process, lm adhesion, and ignition energy. (2) When the thickness and length of the bridge lm are xed, the resistance, ignition voltage of the transducer, and ignition energy all increase with the width increases. The low energy conversion of TaN lm transducers is realized by reducing the size or changing the shape of the bridge lm. (3) Through IV testing of TaN, SCB, and Ni-Cr lm transducers, TaN lm transducers were found to successfully ignite lead styphnate at voltages of 8V and 16V. When the charging voltage is 8V, an electro-thermal conversion phenomenon is demonstrated. When the charging voltage is 16V, an electro-induced plasma explosion phenomenon is appeared. The ring performance of TaN lm transducers can reach the same level as mature SCB lm transducers, but they have better performances than metal bridge lm transducers. Therefore, TaN can be utilized successfully in high-performance transducers.
Declarations Figure 1 Structural diagram of a TaN lm transducer (left: section structure; right: surface structure) Figure 2 The TaN lm transducer and its lm bridge Page 11/14 Figure 3 The AFM photograph of a TaN thin lm Figure 4 The ignition sensitivity testing circuit of a TaN lm transducer Page 12/14

Figure 5
The diagram of an ignition energy test circuit Diagram of the bridge shape and the actual sample