Wen-zhong Lou,Heng-zhen Feng,Jin-kui Wang,Yi Sun,Yue-cen Zhao

The School of Mechatronical Engineering,Beijing Institute of Technology,100081,Beijing,China

Keywords: Corona discharge Peek’s law Optimal ratioε MEMS switch

ABSTRACT Abnormal voltages such as electrostatic,constant current,and strong electromagnetic signals can erroneously trigger operation of MEMS pyrotechnics and control systems in a fuze,which may result in casualties.This study designs a solid-state micro-scale switch by combining the corona gas discharge theory of asymmetric electric fields and Peek’s Law.The MEMS switch can be transferred from “off” to “on” through the gas breakdown between the corona electrodes.In the model,one of the two electrodes is spherical and the other flat,so a non-uniform electric field is formed around the electrodes.The theoretical work is as follows.First,the relation among the radius of curvature of the spherical electrode,the discharge gap,and the air breakdown voltage is obtained;to meet the low voltage(30-60 V)required to drive the MEMS switch,the radius of curvature of the spherical electrode needs to be 10-50μm and the discharge gap between the two electrodes needs to be 9-11μm.Second,the optimal ratioεis introduced to parameterize the model.Finally,the corona discharge structural parameters are determined by comparing the theoretical and electric field simulation results.The switch is then fabricated via MEMS processing.A hardware test platform is built and the performing chip tested.It is found that when the electrode gap is 9μm,the electrostatic voltage is at least 37.3 V,with an error of 2.6%between the actual and theoretical air breakdown voltages.When the electrode gap is 11μm,the electrostatic voltage is at least 42.3 V,with an error of 10.5%between the actual and theoretical air breakdown voltages.Both cases meet the design requirements.

1.Introduction

Electrostatic discharge is the most common source of electromagnetic radiation in nature[1,2].When the field strength of a charged body exceeds the dielectric breakdown field strength of the surrounding medium,a strong electric field and a high instantaneous current occur between the dielectrics,and strong electrical radiation is generated.This forms a broadband electromagnetic pulse that irreversibly affects the control system[3-7],including the fuze and radar equipment.Many researchers have integrated microelectromechanical systems(MEMS)to develop MEMS switches that can guide abnormal signals[8-10].These structures use metal bridge wires to minimize electrical explosions while achieving signal switching,which can cause performance degradation.Therefore,extensive research has been performed on gas breakdown switches[11-13].Corona discharges between sharp and blunt electrodes are among the most common lowtemperature,non-equilibrium plasma processes because they are easy to generate,are stable at atmospheric pressure,and operate at low currents(order of mA)and with low power consumption.

In 2017,Kamaljeet Singh et al.developed a gas breakdown discharge switch[14].This solution is based on corona discharges and uses ring electrodes to achieve switching on and off,but the gap between the electrodes of this switch reaches 800μm.When the switch breaks down,the driving voltage goes up to 2.8 kV,and the breakdown voltage is too high to conduct switching at low voltage(below 100 V).

Many devices,including electrostatic precipitators[15],ion sources[13],and ion blowers[14]of mass spectrometers use the above scheme,but all theoretical studies have been on the order of thousands of microns in terms of device size.The switches therefore had high working thresholds.To achieve reliable grooming of abnormal energy levels and reduce the threshold voltage for switching,development of these MEMS switches now focuses mainly on the response times for sensitive excitation signals.This paper addresses these problems using the technical foundation mentioned above.Research is carried out on MEMS switches on the basis of the corona discharge effect,and device processing is realized using a combination of theoretical calculations,finite element analysis,and MEMS processing techniques.

2.Model design theory

The three characteristics of radio frequency interference are high potential,a strong electric field,and a high instantaneous current,which cause charge accumulation and abnormal discharge in an electric system.During the discharge,the current pulse rises extremely fast and releases strong electromagnetic radiation.An electrostatic discharge forms as an electromagnetic pulse this way.Although the duration is extremely short,the electromagnetic energy is often strong enough to damage sensitive devices in electronic systems.The mechanisms developed by various researchers have reduced the electrostatic and radio energy thresholds to 130 V,but this has not improved system safety.

Because most of the current working voltages of control systems and pyrotechnic products are below 30 V,the focus of this study is reducing the grooming threshold voltage.

We design a silicon-based MEMS corona switch on the basis of the corona discharge effect and the voltage-current coupling energy.A structural model of the switch is shown in Fig.1.The switch mainly includes:a spherical corona plate,a flat plate,and a substrate.The radius of curvature of the spherical corona plate isr0,and the gap between the two electrodes isd.

Fig.1.The model of microswitch.

The switch produces an instantaneous breakdown of the air via the asymmetric electric field distribution between the electrodes,which causes the switch to close.The breakdown voltage during a corona discharge(VC)is the applied potential required to attain the critical electric field that ionizes the air molecules near the wire surface[16].In this paper,the spherical corona structure is designed according to the actual needs for its application:Peek’s law relates this onset potential to the gap distancedas a function of the wire radiusr0,the interstitial gas pressurep,and the temperatureT[16].For a spherical wire-to-plane corona,the relationship is given by:

Wheremvis an irregularity factor that accounts for the condition of the electrode,r0is the electrode radius,and d is the distance between the electrode and the parallel electrode in cm.The parameterδis the gas density factor based on the pressure and temperature,which is given by

wherepis the pressure in centimeters andTis the temperature.In practice,mv varies from 0.85 to 0.98 and is dependent on the condition of the wires.The factorg0is the “disruptive critical potential gradient” and is of the order of 30 kV/cm for air[16].

Fig.2 shows that,for the air breakdown voltage amplitudeVCto be close to(±)30 V,the radius of curvature of the spherical plate should be 10-50μm and the gap between the two electrodes should be 9-11μm.However,the error(1-2μm)caused by the actual processing technology cannot be ignored.

Considering the relation betweenr0,d,and the air breakdown voltageVc combined with inevitable problems such as machining accuracy and alignment errors that cannot be ignored during actual processing,we introduce the optimal ratioεbetween the air breakdown voltage and actual processing error:

whereVCis the electrode breakdown voltage obtained through theoretical calculation,r0is the radius of curvature of the spherical electrode,Δris the processing error of the radius of curvature of the spherical electrode due to the actual conditions,d is the gap between the spherical electrode and the planar electrode,andΔdis the processing error between the two pairs of electrode gaps caused by the actual conditions.

According to Eq.(3),when the driving voltage of the switch is constant,the radius of curvature of the spherical electrode ranges from 10 to 50μm,and the gap between the two electrodes ranges from 9 to 11μm.Therefore,when the radius of curvature of the spherical electrode range is close to the range of the electrode gap(r0≈0.8d/1.2d),r0anddjointly determine the optimal ratioε.The smaller the error caused by the machining accuracy,the largerεis.If the range ofr0is greater thand,andr0≥1.2d,thenεis mainly related tor0.The lower the processing accuracy,the greater the optimization coef ficient.

Fig.3(a)and(b)show the calculated ranges ofεwhen the processing errors are 1μm and 2μm,respectively.

In Fig.3,Xrepresents the electrode gapd,andYrepresents the radius of curvature of the spherical electrode,r0.When the processing error equals the radius of curvature,the optimal ratioε decreases as the electrode gap increases.Whendandr0are constant,εincreases as the machining error increases.The structural design of this paper will give rise to dimensional error owing to the large span of adjacent structures.For this reason,the ratio of the radius of the spherical corona plate to the width of the electrode gap is set at 2:1.This paper uses a structural model with a value forε around 0.2.The theoretical values of the structural parameters are shown in Table 1.

Table 1 The theoretical parameters of corona discharge model.

To guide the voltage pulse signal generated by the abnormal signal reliably,we select the two electrode gap values of 9μm and 11μm for the structural simulation and design processes.

A discharge model is designed according to the principle of the corona discharge.Among the model elements,this discharge mechanism mainly includes the underlying substrate,a metal discharge structure,and a fluid medium designed for the discharge environment.

3.Model simulation

The theoretical calculations described above are used to develop an electrostatic simulation based on the corona model.The bottom material is selected to be silicon oxide,and the metal structure is composed of aluminum and gold(Al-Au).

Fig.2.(a)Relation among electrode radius r0(10-50μm),electrode gap d(5-15μm)and air breakdown voltage V c in an ideal environment,(b)Relation among electrode radius r0(10-50μm),electrode gap d(9-11μm)and air breakdown voltage V c in an ideal environment.

Fig.3.(a)Distribution of optimal ratioεunder processing accuracy error is 1μm,(b)Distribution of optimal ratioεunder processing accuracy error is 2μm.

The structural simulation was carried out using the software COMSOL Multiphysics to obtain the energies at voltages of 30 V,40 V,50 V,and 60 V with discharge electrode gaps of 9μm and 11μm,and the metal electrodes were combined with the Al-Au.The breakdown voltage and the electric field strength are shown in Figs.4 and 5.

According to Fig.6.The simulation reveals that for constantr0and constantd,the field strength generated by the switch breakdown increases with increasing driving voltage.However,at constantr0and constant breakdown voltage,the electric field distribution caused by the electrode breakdown decreases as the electrode gap increases.When the electrode gap ranges from 5 to 11μm,the field strength tends to be stable,and the air can be reliably broken down.Because of the switch preparation error,the larger the electrode gap,the smaller the effect of the processing error on the device function.Table 2 shows the structural parameters obtained from the simulation and theoretical calculations.

4.Process design

Following the theoretical calculations and the analysis above,the design of the process flow shown in Fig.7:

A.The SiO2substrate was prepared;

B.Al(1μm thick)was sputtered on the metal substrates to graphically form discharge electrodes;

C.Spin coating.Select AZ6130,speed 3000 rpm/min,thickness 3μm;

D.Developing.Photoresist(AZ6130)was developed,where the graphic photoresist had a development time of 45 s;

E.ICP etching of Al.Improve the accuracy of metal patterning and avoid the phenomenon of horizontal drilling caused by wet corrosion.Etching rate:100 nm/min.Remove Photoresist,cleaning.

F.Spin coating.Select AZ6130,speed 3000 rpm/min,thickness 3μm;

G.Developing.Photoresist(AZ6130)was developed,where the graphic photoresist had a development time of 45 s;

Fig.4.Electric field strength distribution under 9μm.

Fig.5.Electric field strength distribution under 11μm.

H.Evaporate metal Au with an evaporation thickness of 1μm;I.Stripping the photoresist and heating in acetone water bath,temperature(70°C),release time 15 min,Photoresist removal, cleaning, and energy grooming structure processing.

5.Test

A micro-electrostatic grooming solid-state protection test was performed using a DC high-voltage generator developed by our group.

The test system is shown schematically in Fig.8.The testing was divided into the two stages of charging and discharging.According to the theoretical calculation and simulation analysis in Chapters 2 and 3,the microswitch based on corona discharge state changes can be realized at the driving voltage of 30-60 V respectively.Therefore,the following speci fications of charging and discharging circuits are selected in this paper:

Fig.6.Relation between electrode gap,electrostatic excitation voltage and field strength density.

The charging circuit included a DC high-voltage generator,a 500Ωcharging resistor,an energy storage capacitor,and a charging switch.The DC high-voltage generator could generate a 100 V DC pulsed voltage,while the charging resistor was used to limit current to prevent the DC high-voltage generator and the energy storage capacitor from being burned by the charging current.

The discharging circuit included a 220μF capacitor(as the Energy storage capacitor),diode(Prevent current reverse),a currentlimiting resistor,a discharging switch,and the electrostatic grooming solid-state protection structure.The schematic of the system test is shown in Fig.9.Through the MEMS process,the device wafer map and device structure diagram by scanning electron microscope are shown in Figs.10 and 11.

First,the system charge switch was closed,the discharge switch was opened,the DC power supply charged the capacitor,and the capacitor voltage was detected.

Second,after the capacitor was charged,the charging switch was disconnected.

The discharge switch was again closed to complete the solidstate switch driving capability test.After the test,a microscope was used to determine whether the switch was broken.

To verify the action threshold of the electrostatically-conducting structure,the electrostatic voltage was set from 30 V and the boost gradient used for testing was 5 V.When the electrode gap is 9μm,the electrostatic voltage is at least 37.3 V,and the electrostatic conduction structure based on the corona principle can achieve an air breakdown.When the electrode gap is 11μm,the electrostatic voltage is at least 42.7 V,and the electrostatic conduction structure based on the corona principle can achieve an air breakdown.Table 1 shows the threshold voltages at which air ionization breakdown occurred when the electrode gaps were 9μm and 11μm.

On the basis of the above tests,we analyze the error between the air breakdown voltage and the theoretical calculation for different electrode gaps.The average breakdown voltage(Vtest)for different electrode gaps can be calculated from Table 3.The calculation is as follows:

Table 2 The structure parameters of corona discharge model.

Table 3 Threshold voltage at which air ionization breakdown occurs.

Fig.7.Process flow for corona structure.

Fig.8.Test platform schematic of switch based on corona discharge effect.

Fig.9.Testing schematic diagram of switch based on corona discharge effect.

The errorηbetween the test and theoretical values is

The calculation shows that the structure meets the design standards.

The above test analysis reveals the following.When the radius of curvature is 20μm,the electrode gap is 9μm;the error between the actual test air breakdown voltage and the theoretical air breakdown voltage is 2.6%.When the radius of curvature is 20μm,the electrode gap is 11μm;the error between the actual test air breakdown voltage and the theoretical air breakdown voltage is 10.5%.

Fig.10.Optical images of the electrostatic grooming structure:(a)after dicing(b)chip-on-board assembly.

Fig.11.Test veri fication:(a)Before test(b)After test(9μm).

The main reason for the electrode breakdown error is that the method of preparing the Au electrode in this solution included evaporative peeling.After the peeling process,the metal electrode structure is prone to adhesion and the surface structure is incomplete.Therefore,in the future device design,we plan to use an optimized structural electrode material and etching process to achieve metal patterning.This is to avoid structural adhesion or structural defects of the electrode at the micro-nano scale,which result in errors between the theoretical and actual test results.

6.Conclusion

This paper has presented the design of a corona-based electrostatic conduction structure using MEMS processing technology.A combination of theoretical calculations and simulation showed that the system has good discharge characteristics when the discharge gaps are set to be 9μm and 11μm and the implementation period is combined with the MEMS processing platform.Electrostatic excitation testing showed that when the electrode gap was 9μm,the electrostatic voltage was at least 37.3 V,and the electrostatic conduction structure achieved air breakdown according to the corona principle.When the electrode gap was 11μm,the electrostatic voltage was at least 42.3 V.The error between the actual test air breakdown voltage and the theoretical air breakdown voltage was 10.5%,which again meets the design requirements.

Declaration of competing interest

We would like to submit the enclosed manuscript entitled“Research on fuze microswitch based on corona discharge effect” ,which we wish to be considered for publication in “Defence Technology” .No con flict of interest exits in the submission of this manuscript,and manuscript is approved by all authors for publication.I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously,and not under consideration for publication elsewhere,in whole or in part.All the authors listed have approved the manuscript that is enclosed.