**1. Introduction**

The high-voltage switch is a core key component in exploding foil initiation systems (EFIs). It can not only control the on-off of the current in the initiation discharge circuit, but also has a high turn-off impedance to reduce the power consumption of the exploding foil initiation system: it is more important to have lower on-resistance and inductive reactance to improve the output characteristics of narrow pulse current. Technical indicators and performance parameters directly affect the overall performance of exploding foil initiation

**Citation:** Han, K.; Zhao, W.; Deng, P.; Chu, E.; Jiao, Q. Research on Characteristics of Copper Foil Three-Electrode Planar Spark Gap High Voltage Switch Integrated with EFI. *Appl. Sci.* **2022**, *12*, 1989. https://doi.org/10.3390/ app12041989

Academic Editors: Pingjuan Niu, Li Pei, Yunhui Mei, Hua Bai and Jia Shi

Received: 7 January 2022 Accepted: 6 February 2022 Published: 14 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

systems (EFIs) [1–3]. With the continuous development of low-energy exploding foil initiators (LEEFI), high-voltage discharge switches are also constantly updated. Reynolds Company reported the use of a spark-gap gas discharge switch at the 43rd Annual Meeting of Fuzes in 1999 [4], and the use of an MCT (MOS controlled thyristor) switch was reported at the 45th Annual Meeting of Fuzes in 2001 [5], Xu Cong, Zhu Peng and Chen Kai et al. [6] made Schottky barrier diodes into single-trigger high-voltage switches through MEMS technology in 2017. In 2021, Yang Zhi, Zhu Peng et al. [7] reported the spark-gap switch triggered by a parallel plane sealed on a PCB base. The high-voltage discharge switches reported in the data mainly include three-dimensional cold-cathode-triggered gas-sparkgap high-voltage switches and vacuum-triggered high-voltage switches based on metal ceramic packages, based on IGBT (insulated gate bipolar transistor, insulated gate bipolar transistor) and MCT (metal- oxide-semiconductor controlled thyristor MOS control thyristor) semiconductor high-voltage switches, based on micro-electro-mechanical system (MEMS) and other processes, including the plane electric explosion high-voltage switch and the plane spark gap high-voltage switch.

The cold cathode-triggered gas spark gap high-voltage switch and the vacuumtriggered high-voltage switch based on the three-dimensional structure of the metal-ceramic package have fast closing speed, high operating temperature, small current leakage, and are little influence by radiation. Because this system has good performance and is favored, it has always dominated. However, because it is a ceramic metal package structure, it has the disadvantages of poor mechanical overload resistance, large volume, and high cost. With the continuous development of semiconductor technology, Tom Nickolin [8] proposed a metal oxide semiconductor field effect transistor switch (MOSFET) and N-channel MOS control in LEEFI (low energy exploding foil initiator) at the 45th Fuse Annual Conference in 2001. In 2002, Hanks RL [9] published a design scheme using MCT to control the discharge of high voltage capacitors in a patent. In 2008, Kluge Design Inc (KDI) [10] announced the multiple launch rocket system (MLRS) using LEEFI, indicating that the MCT high voltage switch has achieved engineering applications. In 2016, the U.S. Navy [11] announced a high-voltage switch composed of stacked IGBTs, implemented a discharge test study under short-circuit and load conditions, and planned to use it for live-fire testing at the end of the year. As a typical representative of high-power MOS-controlled semiconductor devices, MCT high-voltage switches have the advantage that conventional MOS gate control signals can be used to control the conduction of the switch, and the pulse current can reach thousands of amperes within 100 nanoseconds, which is suitable for explosion foil ignition with detonation. The on-current of the semiconductor high-voltage switch is large and the working life is long. However, due to its large size, low operating voltage and large leakage current, it is difficult to achieve derating design, resulting in low working reliability and safety of the exploding foil initiation system. In addition, its performance is greatly affected by environmental factors (temperature, electromagnetic, etc.). The above two switches belong to the discrete structure, their volume and the comprehensive performance of the ability to withstand high voltage are low, and the impedance and inductance introduced into the discharge circuit are relatively large, which seriously affects the reliability of the explosion foil initiator system andthe development of miniaturization and low energy of exploding foil initiator.

The planar electric explosion switch is a high-voltage switch that uses the bridge foil of the trigger electrode to generate an electric explosion to generate plasma with conductive properties, thereby making the main electrodes on both sides perpendicular to the trigger electrode conduct. As early as the 1980s, Graham et al. [12] studied the conductive properties of polymer films induced by explosive shock. In 1986, Richardson et al. [13] invented a planar dielectric high-voltage switch suitable for exploding foil initiators (EFI), when the switching dielectric layer is impacted at high speed by the RDX-driven flyer, the upper and lower electrodes of the switch are turned on. In 1989, Nerheim E et al. invented a silicon-based planar electrical explosion high-voltage switch [14], the structure of which is shown in Figure 1. Its manufacturing process is to sequentially deposit switch high-voltage

electrodes on a silicon substrate, the trigger electrode is made of amorphous silicon or polysilicon of an electric explosion bridge, and an insulating gap is arranged between the high-voltage electrode and the trigger electrode. Before switching, the two ends of the high-voltage electrode are charged with high voltage, and the trigger electrode is excited by a constant current source, so that when the polysilicon or amorphous silicon bridge foil of the trigger electrode is electrically exploded, a conductive plasma cloud is generated. Under the action of the electric field, the insulating gap between the high-voltage electrodes is broken down and turned on, so as to realize the output of short pulse and large current, and complete the conduction and closing function of the high-voltage switch.

**Figure 1.** Schematic representation of planar-electric explosion switch based on silicon substrate.

In 2009, Baginski T A et al. [15,16] designed and manufactured a planar dielectric explosion high-voltage switch triggered by Schottky diode and a micro-bridge explosion planar switch with a series structure, and proposed the idea of integrating the switch with EFI. The switch structure is shown in Figure 2.

**Figure 2.** Schematic representation of planar high-voltage switch with SBD trigger and planarelectric explosion switch with series structure. (**a**) Planar high-voltage switch with SBD trigger. (**b**) Planar-electric explosion switch with series structure.

The planar dielectric explosion high voltage switch based on Schottky diode is composed of substrate, lower electrode, dielectric layer, upper electrode, and Schottky diode. By applying a reverse voltage to the Schottky diode to cause reverse breakdown, and then under the thermal effect of pulse current, an electrical explosion is caused, resulting in dielectric breakdown; the upper and lower electrodes are connected, and the switch is turned on. The series-structured micro-bridge explosive planar switch is based on the silicon-based planar electro-explosive high-voltage switch, and adopts the series connection of multiple planar electro-explosive high-voltage switches to improve the electrical performance and reliability of the planar electro-explosive switch. In 2011, Baginski T A et al. [17] proposed a planar trigger switch (PTS) containing a polyimide film insulating layer. The current test and simulation study of the switch discharge circuit are completed, and the test results are good. Combined with LEEFI, the HNS-IV was successfully detonated, verifying the practicability of the switch. In 2012, Zhou Mi, Han Kehua et al. [18] prepared a single-bridge planar electric explosion switch based on copper film by ion etching method, and studied the effect of gap distance on the switch performance. The results show that as the gap distance decreases, the action time of the switch decreases. In 2018, Wang Runyu et al. [19] used an improved manufacturing process to replace the wet etching process to prepare a miniature metal bridge foil explosive planar switch, and studied the influence of the thickness of the adhesive layer and the insulation method on the insulation effect in the switch insulation treatment. In 2020, Xu Cong et al. designed three trigger modes: the Schottky diode [20], pn junction diode [21], and micro bridge foil [22,23] based on the planar dielectric explosion high voltage switch of the Schottky diode, such as the Baginski TA flat dielectric high voltage switch. The electrical characteristics of the three switches are preliminarily studied, and the results show that, among the three trigger mode switches, the micro-foil planar dielectric switch can obtain the highest peak current and the shortest rise time at a lower operating voltage. When the planar electric explosion high-voltage switch is turned on and closed, a trigger voltage needs to be added to make the core part of the switch generate an electric explosion instantaneously and complete the closing function of the switch, indicating that this type of switch can only perform a single action. Because the characteristics of the planar electric explosion high-voltage switch is a one-time function, the conduction of the switch is not reversible, so the switch cannot complete the testability before the system is used in the exploding foil initiation system, which seriously affects the reliability and safety of the system.

The use of micro-electromechanical machining technology to planarize the spark gap high-voltage switch can effectively solve the above problems. The planar spark gap highvoltage switch can not only complete multiple discharge functions, but also improve the switch's resistance to mechanical overload, reduce costs, and reduce system volume. It can also realize the integration function of the high-voltage switch and the explosion foil, reduce the parasitic impedance and inductive reactance in the discharge circuit, reduce the energy consumption of the system, and improve the integration degree of the system. In this paper, a copper foil-based three-electrode planar spark-gap high-voltage switch is designed and fabricated by using a magnetron sputtering coater to sputter copper film on the surface of the substrate. The static self-breakdown characteristics, dynamic operating characteristics, and discharge life characteristics of the three-electrode planar spark gap high voltage switch based on copper foil are studied.
