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ESD events: Breakdown characterization, dielectric charging, and realistic cures
Journal of Electrostatics 69 (2011) 547e553
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
A comprehensive study of MEMS behavior under EOS/ESD events: Breakdown characterization, dielectric charging, and realistic cures Augusto Tazzoli a,1, *, Marco Barbato a, Vincenzo Ritrovato a, Gaudenzio Meneghesso a, b a b
DEI, University of Padova, Via Gradenigo 6/B, 35131, Padova, Italy IU.NET, Inter-University Nano-Electronics Team
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2011 Accepted 21 July 2011 Available online 11 August 2011
The breakdown characterization of both out- and in-plane electrostatically actuated RFeMEMS switches with air-gaps from 1.0 to 6.7 mm was studied. The emitted electromagnetic field during the testing was analyzed, in order to have an indication of the air-breakdown occurrence. Furthermore, we studied the effect of TLP and HBM events on the dielectric charging of tested MEMS, furnishing the experimental evidence that ESD events should not be responsible of this important reliability problem for MEMS, and that ESD tester parasitic elements can influence the MEMS electromechanical behavior characterization. Finally, a simple, but effective, varistor based protection structure was explored. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: MEMS EOS/ESD Breakdown Dielectric charging
1. Introduction Micro-Electro-Mechanical-Systems (MEMS) are acquiring an increasing number of applications because of their proven better performance compared to currently available state of art solid state devices. As a result, several topologies of micro-machined sensors and actuators are flooding the market [1]. RFMEMS devices, that are micro-machined devices specialized for radio frequency (RF) applications, have been considered to show great promise for at least the last decade [2]. However, the reliability assessment of RFMEMS has not yet completely fulfilled, because of their relative novelty, lack of standardized processes, and almost absence of accelerated lifetime tests. EOS/ESD events partially contribute to the limitation of the success of such devices. In fact, RFMEMS have input/output pins exposed to the world (e.g. antenna connection) that must confront with signals in the GHz range, and without any chance to integrate EOS/ESD protection structure because of the simplicity of the technological process. Our first work on the sensitivity of electrostatically actuated RFMEMS switches to EOS/ESD faced up the problem in 2006 [3], and other works followed confirming the poor robustness of such devices, as well as of micro-mirrors [4e7].
* Corresponding author. Tel.: þ1 215 573 3276; fax: þ1 215 573 2068. E-mail address: [email protected] (A. Tazzoli). 1 He is now with University of Pennsylvania, Department of Electrical and Systems Engineering. 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.07.007
In this work we have studied the breakdown occurrence as a function of the air-gap thickness, also measuring the electromagnetic field emitted by tested devices before and after the failure point, with the aim of proposing an easy methodology to easily distinguish the air-breakdown occurrence from other failure modes. In the last years some interesting works appeared in the literature theorizing about dielectric charging issues caused by ESD events [8e10]. The proposed theories consider only ideal, perfectly insulating dielectrics, with no leakage current. In reality, the very short duration of ESD events, combined with the typical low quality of MEMS oxides (especially if compared to dielectric layers of state of the art CMOS technology), would be expected to mitigate this problem in real devices. However, since the actuation structure of electrostatically actuated MEMS switches can be essentially modeled as a very low value capacitor (in the range of hundreds of fF in the OFF-state), the measurement setup can easily influence the behavior of tested devices. In order to better analyze the problem, we have experimentally verified that a great number of fast ESD events like 100 ns long TLP pulses (up to 59,400,000 pulses were applied to MEMS switches) do not cause evident dielectric charging issues on fully SiO2 covered MEMS switches, both when applied to the actuation pad vs. ground, and between the RFeoutput port and ground. Regarding dielectric charging phenomena, similar results were obtained also with HBM pulses (i.e. no charging). On the contrary to what happens with TLP systems, we found that the parasitic elements of HBM testers and their switching timing can heavily influence the behavior of tested devices leading
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to, if not correctly considered, to measurement artifacts. The effect of the not-infinite impedance of a voltage probe on the transient behavior of the suspended membrane of RFeMEMS switches was then analyzed by means of electromechanical characterizations and simplified PSpice simulations, with the main result that dielectric charging should not really impair this kind of devices. Finally, we propose an inexpensive but effective way to protect MEMS devices using commercially available varistors. 2. Device description and measurement setups This work was based on the characterization of several topologies of RFMEMS devices and ad-hoc designed test structures. Pictures and schematic design of tested devices are shown in Fig. 1. Tested devices were gold-based capacitive (a) and ohmic (bed) RFeMEMS switches, out-of-plane (a, b, d) and in-plane (c) actuated, manufactured by FBK-IRST (Trento, Italy). The technology utilized for the fabrication of studied devices consists of an eight mask surface micromachining process [11], and the air-gap (sacrificial layer thickness) varies from 1.0 to 4.5 mm for the vertical devices, and 6.7 mm for the lateral ones. A novel “circular” structure (d) was also studied, with four different percentages of dielectric layer over the polysilicon actuator: 0% (no dielectric, (e)), 33% (f), 66% (g), 100% (h). ESD events were generated by means of a 100 ns TLP-TDR system [12], and an Hp 8114A solid state pulser, always 100 ns long, but at a repetition rate of 1 kHz. A homemade HBM pulser [12] was also developed and mounted directly on the micromanipulators, in order to reduce to a minimum value the parasitic effects. In fact, it is well known that ESD testers parasitic can have a negative impact on the correct evaluation of ESD effects on traditional solid state devices and, as described in [6], such impact can be more severe with the testing of micro-machined systems, that can be modeled as very low capacitance devices (typically hundreds of fF, some pF). Being interested in the study of dielectric charging issues induced by HBM events, once the setup was fully checked and characterized, we preferred to not use any probes on our HBM system. The electromagnetic field emitted by the device under test was acquired by means of a simple wire antenna hooked to the microscope revolver. This solution was adopted in order to have the antenna placed in a repeatable condition, always at the same distance from the MEMS switch (about 1 cm).
Fig. 2. Breakdown voltage as a function of the air-gap spacing of capacitive, ohmic, and lateral devices.
A Polytec MSA-500 optical profilometer mounted on a Cascade R4800 probe-station, and isolated from environmental noise by means of a Newport anti-vibrating table, was used to characterize the effective spacing of the air-gap of tested devices. The integrated Laser-Doppler Vibrometer (LDV) was used to characterize the motion of suspended structures during ESD events. Finally, a Keithley 6517B electrometer was adopted to characterize the leakage current of the developed HBM tester and tested switches. 3. Breakdown characterization Our work started with the characterization of the breakdown voltage of structures with different air-gaps, nominally from 1.0 up to 6.7 um, in order to verify the validity of the (modified) Paschen’s law for MEMS devices. 100 ns TLP pulses were applied to the RFoutput port vs. ground, in order to recreate a possible critical situation for MEMS in a real environment (antenna connection). All others configurations were tested in our previous works [3,4]. Fig. 2 shows the evolution of the measured breakdown voltage as a function of the air-gap spacing. An interesting result is the good linear dependence of the breakdown voltage with the air-gap spacing for all the devices. The slope of the linear fitting curve of such results is slightly higher than the modified Paschen’s law as shown in [9], and this difference could be due to the different materials used for the experiments (different
Fig. 1. Pictures of tested devices and layout of the dielectric area of circular devices (e ¼ 0%, h ¼ 100%). The ligther area on the central circular actuator (red) correspond to the mask for the removal of SiO2 layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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material work functions) [13]. The SiO2 layer of capacitive switch was about 120 nm thick, and it slight lowers the device breakdown voltage. The interesting thing is that also lateral devices fit very well the linear trend, rather than following the modified Paschen’s curve, predicting about 700 V for an air-gap of 6.7 mm at same ambient pressure. The low number of available samples did not permit us to study in detail what happens in lateral devices, but this behavior will be studied more in detail in future works. It is not always straightforward understanding where the breakdown occurs in MEMS devices. With the goal of better investigating this problem, we have developed a new setup, in order to acquire the electromagnetic field emitted by tested devices during ESD events, in particular during the failure. This is not a really new invention: ironically, spark discharge was how radio began. Guglielmo Marconi used “spark transmitters” to establish the first commercial wireless telegraphy service between Europe and America in 1904. In its simplest form, a spark-gap transmitter consists of two conducting electrodes separated by a gap. When a large enough voltage is applied to the two terminals, a spark jumps between them, ionizing the air and drastically reducing its electrical resistance. An electric current then flows until the spark is broken by lowering the voltage, cooling the ionized air, or moving the electrodes further apart. The emitted signal, sensed by the antenna, and measured by the DSO, is then the time derivative of the transient electric field generated by the contacting tips and the device. In a first approximation, considering an antenna made with a single wire, the charge measured is proportional to E L W, where E is the electric field, L is the length (1 m), and W is the width of the wire. An example of acquired signals during the testing of an open circuit (signal vs. gnd of a CPW line), a short circuit, and before and after the breakdown occurrence of a 2 um air-gap (device (b)) is shown in Fig. 3. The main result is the very good similitude between the open circuit and “before breakdown” waveforms, and short circuit and “after breakdown” ones. The comparison of acquired waveforms leads then to the result that studying the derivative over time of the electric field, in particular from 30 ns to 60 ns it is possible to understand where the failure is occurring (air-gap or not). Another interesting result is obtained plotting the averaging of the acquired electric field derivative over time versus the voltage applied to the device during the 100 ns TLP testing, as shown in Fig. 4. In fact, it is possible to identify three zones in the graph: open circuit (before the failure), air-breakdown, short circuit (hard failure). As already demonstrated in previous works, leakage current does not always offer a clear indication on the upcoming failure [3,4]. On the contrary, using this simple method (just a single wire connected to the DSO), it is possible to analyze with a better precision what is happening to the device under test.
Fig. 3. Emitted electric field before and after breakdown of a 2 mm air-gap and comparison with ref. open and short circuits.
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Fig. 4. Evolution of the averaging of the d(Efield)/dt and leakage current during 100 ns TLP test between RF-out and gnd.
4. Dielelectric charging and ESD tester influence In order to understand if ESD events can bring MEMS switches to dielectric charging issues, we have submitted circular devices (deh) with different area of dielectric materials to a large number of 100 ns TLPelike and HBM pulses, applied between the actuation pad and ground, and between RFeoutput pad and ground. Even if it is not really common that thousands or millions of ESD events happen in a very short time to devices used in real applications, we performed such experiments on purpose in order to shed a better light on this topic, up to now theorized in the literature [8e10], but with no experimental results shown. Here we show the result of the effect of stressing devices fully covered with a SiO2 dielectric layer (type h), ideally more sensitive to dielectric charging issues than dielectric-less ones. All tested devices showed a very similar behavior. In this work we were not interested in studying the failure values of tested devices (already characterized in our previous works [3,4]), but only in the analysis of dielectric charging issues. For this reason, and to avoid structural damages to the switches under test, we limited the TLP and HBM voltage to 100 V. 4.1. TLP stress First of all, we verified that a very short but high voltage pulse, like a 100 ns TLP pulse, can move the suspended membrane of a MEMS, thus verifying simulation results previously presented in [4]. Fig. 5 shows the vibration velocity over frequency of a circular device (d) during a single 100 V, 100 ns long square pulse applied between the actuation pad and ground. The presence of lines in the
Fig. 5. Vibration velocity over frequency of a circular device during a 100 V, 100 ns square pulse.
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frequency response is a good indicator that the membrane structure moves itself during the TLP pulses (even if it does not reach the full actuation). We have then stressed with repeated 100 V, 100 ns long square pulses at 1 kHz for 12.8 s the same device, acquiring with the LaserDoppler Vibrometer the displacement over the time of the central part of the suspended membrane. Another reference laser was pointed on the CPW line (fixed structure) in order to reduce the noise during the measurement. In total, the device was stressed with 12,800 pulses, but, as shown in Fig. 6, the displacement, that can be related to the entrapped charge, is lower than some tens of nanometers: negligible, if considered the number of pulses applied and that the air-gap is 3 mm. Such test was repeated for all other device typologies, obtaining similar results. In order to verify whether a large number of short (100 ns long or less) ESD events can really cause dielectric charging problems, the experiment was repeated stressing in the same configuration the device for 16.5 h, e.g. for a total of 59,400,000 pulses. Because of the intrinsic limitation of the LDV setup when used to acquire displacement measurements for very long time (the system performs the integral of the acquired velocity signal and some drifts can occur influencing the displacement measurement during long time characterizations), we acquired the whole topography of the device before, and right after the end of the stress. The comparison of the profile along the central part of the suspended membrane before and after the stress is shown in Fig. 7. The result of this stress is similar to the previous one: just a difference of some tens of nanometers between the fresh and after the stress profiles (mainly on the left part of the device). This difference could be explained with an almost negligible charge entrapment, even if, considering the very high number of cycles (about 60 millions), it could be also explained thinking to some plastic deformation of the suspending springs leading to a deformed up-state position of the suspended membrane. The negligible difference between the fresh device topography and right after the stress one was quite expected because, despite the very big number of pulses, the TLP stress can be, in the worst case, correlated to a DC bias signal applied to the MEMS actuator for about 6 s, too short to expect dielectric charging issues. In fact, such devices start to show some deviations of their electrical characteristics (actuation and release voltages shifts due to dielectric charging) after some hours continuously spent in the on-state (DC bias). An example of this behavior is shown in Fig. 8. Here we compared the shift of the positive actuation voltage (þVACT) of a fully SiO2 covered and of a dielectric-less (over the actuator electrode) MEMS switch. The devices were left continuously biased for 4 h at VBIAS ¼ þ60 V, and then at VBIAS ¼ 0 V, in order to evaluate both the charging and the discharging phases. VACT values were
Fig. 6. Displacement vs. time of a stressed circular devices with full SiO2 dielectric layer with 100 V, 100 ns square pulses.
Fig. 7. Displacement profile of the central part of a stressed circular devices with full SiO2 dielectric layer before and after 59,400,000 100 V, 100 ns long, square pulses.
measured at the end of the 4 h long DC stress and at selected intervals during the discharging phase. Measured values were then normalized to the initial VACT values (51 and 54 V, respectively), in order to easily compare the two behaviors. More details on the adopted procedure can be found in [14]. As expected, the SiO2 covered device was more prone to charging than the dielectric-less one, but the important difference with respect of previously described TLP stress is the very different time range. At firstglance, it may appear strange that a dielectricless device could suffer from dielectric charging, but it must be considered that only over the actuation electrode the dielectric layer was removed. On the contrary, the dielectric layer over the substrate and, in particular, the zone close to the periphery of the electrode, plays an important role in this phenomenon as demonstrated in [14]. The first conclusion coming from this study was that “short” ESD events (<100 ns) should not induce dielectric charging problems to MEMS switches, at least with comparable sizes and dielectric properties. 4.2. HBM stress We have then moved our interest in analyzing what can happen, from the dielectric charging point of view, when other kind of ESD events, like HBM ones, are applied to electrostatically actuated micro-machined RF-switches. It is well known that parasitic elements of the ESD tester can heavily influence the characterization of virtually any kind of devices. The situation can be worse when the devices under test
Fig. 8. Comparison of dielectric charging evolution of a SiO2 fully covered switch and a dielectric-less one during a 4 h long DC stress @ þ60 V and the following discharging phase (@ 0 V). Please, note that the dashed line does not necessarily indicate the real charging evolution since no measurements were taken during the 4 h long stress.
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Fig. 11. Electrical model of the HBM tester, the MEMS switch, and their parasitic elements. Fig. 9. Variation of the height of the central point of the suspended membrane of tested switch as a function of the number of 100 V HBM pulses applied between the actuator (SiO2 covered) and ground.
can be modeled essentially as very low value capacitors, as in the case of MEMS switches (typically just some hundreds of fF). We have developed a homemade HBM tester, and it was mounted directly on the wafer probe micromanipulators, in order to minimize the parasitic given by interconnections and unwanted ground planes. Once the HBM tester was fully characterized, we disconnected the voltage probe in order to reduce to a minimum value the parasitic elements of the circuit. This was very important for the correct evaluation of the MEMS response to HBM pulses, as it will be demonstrated later. We started the analysis of the influence of HBM pulses on (if any) dielectric charging problems stressing the actuation pad vs. ground. Even if this pad is the easiest to be protected (with external components) because it is not an RF pad, this condition should be the most favorable for charge entrapment. We have then applied an increasing number of 100 V HBM pulses (up to 1000 at a repetition rate of 1 Hz) to fully SiO2 covered circular switches (type h), measuring the device topography at the end of each stress in order to investigate any bending of the suspended membrane. A typical result is shown in Fig. 9. From the analysis of the graph, two results are straightforward: (1) HBM pulses can bend downward the suspended membrane, and (2) the number of applied HBM pulses does not have a direct influence on the bending of the membrane, being the displacement almost constant (an average of 750 nm with respect to the initial value). From the literature [9] it seemed that the influence of HBM pulses on electrostatically actuated switches should have a shorter lifetime (in the milliseconds range).
Fig. 10. Comparison of the measurement of the decay time of 100 V HBM pulses with 10 and 100 MU voltage probes (continuous lines) and simulation of the circuit shown in Fig. 10 (dotted line).
In order to better investigate the problem, we carefully characterized the influence of the voltage probe impedance on the HBM waveform decay, as shown in Fig. 9 for a 100 V pre-charging. As expected, higher the impedance, higher the discharge time. Using a 100 MU probe the decay time is about 20 ms, but it must be considered that the equivalent resistance of tested MEMS switches (virtually open circuits) is very higher than 100 MU, and this can lead to an obvious longer decay of the HBM pulse. So, we performed a detailed characterization of the parasitics of the HBM tester and of the tested switches with a high resolution electrometer (Keithly 6517B). The extracted model of the HBM tester, its parasitic elements, and the tested switch is shown in Fig. 11. The interesting thing to note is the value of the leakage resistors, in the range of hundreds of GU for the HBM tester, and in the TU range for the MEMS switch, very higher than voltage probes impedance. We have then simulated with PSpice the simple circuit shown in Fig. 11, setting the initial condition of the HBM capacitor to 100 V (and no voltage probe). The result is shown in Figs. 10 and 12. Being the resistors used to model the leakage current paths very high, the decay of the HBM pulse is in the tens of seconds range. This long discharge time can easily explain the bending of the suspended membrane after HBM stresses, as previously shown in Fig. 9. To verify this assumption, we acquired the velocity of the central part of the suspended membrane with the LDV setup during a 100 V HBM pulse, monitoring with a good time resolution (390 ns) the membrane behavior for about 25 s. The result is shown in Fig. 13. In the left part of the graph it is possible to see the increase of the membrane velocity, i.e. the actuation of the switch, and, in the right part, the switch release, occurring after about 21 s. Lower HBM pre-charge voltages showed, as expected, a lower release time.
Fig. 12. Simulation of the time evolution of the discharge of the MEMS switch actuator during an HBM event. The release voltage of the switch (about 31 V) is reached after 21 s.
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Fig. 13. Velocity measurements of the central part of the suspended membrane of a switch (f) during a 100 V HBM pulse (the graph was split for clarity purpose).
All these results can be correlated very well together considering the traditional hysteresis curve of tested switches, shown in Fig. 14 (more details on the characterization procedure can be found in [15]). From this measurement it is possible to extract a positive actuation voltage (þVACT) of about 51 V, and a positive release voltage (þVREL) of about 31 V. Comparing the results from Figs. 12 and 13, it is possible to conclude that the switch, after one 100 V HBM pulse remains in the actuated state for about 21 s, exactly the time the circuit needs to discharge reaching the release voltage of the switch, about 31 V. This conclusion helps to explain the bending of the suspended membrane shown in Fig. 9, due to the fact that the topography measurements were carried out immediately after the HBM stress, when the suspended membrane did not yet reached its nominal off-state position. Furthermore, considering the very low discharge time (as previously demonstrated), the dielectric of tested switches remained stressed for a total equivalent time of about 20 min, again too short to lead dielectric charging issues. For the sake of completeness, we characterized the electromechanical behavior of the same RFeMEMS switch when a lower voltage HBM pulse (about 60 V) is applied. As shown in Fig. 15, it is possible to note that the switch was actuated, but it suddenly release itself (please note the oscillating behavior typical of a free moving plate). We have also tested the influence on the switch dielectric charging of HBM pulses applied to the RFeoutput port vs. ground. In this configuration almost no bending (and then actuation) of the suspended membrane was observed, since the area of the
Fig. 14. Insertion loss (S21 parameter) evolution as a function of the switch bias voltage, used to extract the actuation and release voltage values of the device under test (type f).
Fig. 15. Velocity measurement of the central part of the suspended membrane of a switch (f) during a 60 V HBM pulse, showing the actuation and the sudden release transients.
equivalent actuation electrode (the small half-moon at the right of the circular central actuator in Fig. 1(eeh)) is too small to have an influence on the electromechanical behavior of the switch. This result is further evidence that it is not correct to consider a partial actuation of the switch as caused by dielectric charging. On the contrary, the behavior is simply governed by the transfer of the charge of the HBM capacitor to the MEMS switch and its slow discharge time. We expect a similar behavior also considering other kind of ESD stress models, always based on the discharge of a charged capacitor, even if of different capacitance and time ranges. Furthermore, it is important to note that a great care must be taken in consideration when developing setup for ESD testing of MEMS devices to avoid introducing parasitic elements that could seriously change the device behavior. 5. Varistor ESD protection As a last point of this work we wanted to briefly introduce a simple, but effective, way to protect RFeMEMS switches from ESD events. In fact, the robustness of such devices to ESD phenomena is, as well known, critical, because of the difficulty, or impossibility, of integrating effective protection structures with the same technological process (on the contrary of traditional state of the art technologies). By the way, typical failure voltages of MEMS are in the range of some hundreds of Volts, relatively low considering typical ESD voltage levels, but very high if compared to traditional
Fig. 16. IeV 100 ns TLP and leakage curves of the PCB with RFMEMS andvaristor mounted as shown in the inset.
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was that parasitic elements of HBM testers (but extendible to other ESD tester) can play a detrimental role on the correct evaluation of the electromechanical behavior of RFeMEMS switches stressed with ESD events, lowering the stress time from tens of seconds to just some milliseconds. As a last point, we proposed a very cheap and easy solution to protect both DC and RF ports of such switches based on a commercially available varistor, showing good ESD robustness under 100 ns TLP regime, and RF performances. Clearly, all these results can easily be applied for the testing and protection of other families of micro-machined systems.
Acknowledgements Fig. 17. Insertion losses of the test PCB with and without the varistor mounted between RFoutput and ground.
solid state devices values. We had then experimentally verified that a commercially available (for less than 1 cent) varistor could be used as a starting point to protect such devices between the RFeoutput and gnd. An RFMEMS switch (type (b)) was mounted on a 50 U coplanar waveguide (CPW) line designed on a PCB, together with a varistor placed between RFeoutput and ground, as shown in the inset of Fig. 16. 100 TLP stresses were then applied to the right SMA connector, measuring the IeV and leakage curves shown in the same Figure. As it is possible to see, the varistor turns on at about 300 V, lowering the transient voltage down to about 50 V, and showing a very high robustness (ITLP > 10 A, with no leakage variations). Mounted varistor was rated for VDC ¼ 12 V, clamping voltage of max. 50 V, and a capacitance of 0.5 pF. This value leads to the insertion loss (S21) vs. frequency behavior shown in Fig. 17, not bad considering this value comprehends the losses of the PCB, the SMA connector, and the wires bonding for MEMS connection. Clearly, the same technique can be used to protect also the actuation pad of the device in real application, even if such pad is usually not exposed, and it must not be treated as an RF pad. 6. Conclusion RFeMEMS switches are relatively simple devices based on the movement of a suspended membrane. Being the typical air-gap thickness of MEMS devices in the range of mm, it is important from the ESD robustness point of view studying the breakdown of such air layer. The first aim of this work was to study the breakdown occurrence of devices with air-gap from 1.0 to 6.7 mm, under 100 ns TLP regime. A good linear trend as a function of the air-gap thickness was obtained, even if slightly higher than the traditional modifiedPaschen’s law curve. Furthermore, the breakdown occurrence was studied also measuring the emitted electromagnetic field, showing how it is possible to separate air-breakdown events from other kind of failures. The second aim of this work was then showing experimental results on the previously theorized problem of dielectric charging issues induced by ESD events. Here we demonstrated that even under a great number of TLP and HBM pulses, such devices were insensitive to this reliability issue. A second result from this analysis
Authors would like to thank Albert Wallash (Hitachi Global Storage Technologies) for useful discussions of the results and for reviewing the paper. This work was partially supported by the European Space Agency under Contract No. ITT AO/1-5288/06/NL/GLC., and by ENIAC project END “Models, Solutions, Methods and Tools for Energy-Aware Design”. The END project has received funding from the ENIAC Joint Undertaking under grant agreement n 120214 and from the national programs/funding authorities of Belgium, Greece, Italy, and Slovakia.
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
A comprehensive study of MEMS behavior under EOS/ESD events: Breakdown characterization, dielectric charging, and realistic cures Augusto Tazzoli a,1, *, Marco Barbato a, Vincenzo Ritrovato a, Gaudenzio Meneghesso a, b a b
DEI, University of Padova, Via Gradenigo 6/B, 35131, Padova, Italy IU.NET, Inter-University Nano-Electronics Team
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2011 Accepted 21 July 2011 Available online 11 August 2011
The breakdown characterization of both out- and in-plane electrostatically actuated RFeMEMS switches with air-gaps from 1.0 to 6.7 mm was studied. The emitted electromagnetic field during the testing was analyzed, in order to have an indication of the air-breakdown occurrence. Furthermore, we studied the effect of TLP and HBM events on the dielectric charging of tested MEMS, furnishing the experimental evidence that ESD events should not be responsible of this important reliability problem for MEMS, and that ESD tester parasitic elements can influence the MEMS electromechanical behavior characterization. Finally, a simple, but effective, varistor based protection structure was explored. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: MEMS EOS/ESD Breakdown Dielectric charging
1. Introduction Micro-Electro-Mechanical-Systems (MEMS) are acquiring an increasing number of applications because of their proven better performance compared to currently available state of art solid state devices. As a result, several topologies of micro-machined sensors and actuators are flooding the market [1]. RFMEMS devices, that are micro-machined devices specialized for radio frequency (RF) applications, have been considered to show great promise for at least the last decade [2]. However, the reliability assessment of RFMEMS has not yet completely fulfilled, because of their relative novelty, lack of standardized processes, and almost absence of accelerated lifetime tests. EOS/ESD events partially contribute to the limitation of the success of such devices. In fact, RFMEMS have input/output pins exposed to the world (e.g. antenna connection) that must confront with signals in the GHz range, and without any chance to integrate EOS/ESD protection structure because of the simplicity of the technological process. Our first work on the sensitivity of electrostatically actuated RFMEMS switches to EOS/ESD faced up the problem in 2006 [3], and other works followed confirming the poor robustness of such devices, as well as of micro-mirrors [4e7].
* Corresponding author. Tel.: þ1 215 573 3276; fax: þ1 215 573 2068. E-mail address: [email protected] (A. Tazzoli). 1 He is now with University of Pennsylvania, Department of Electrical and Systems Engineering. 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.07.007
In this work we have studied the breakdown occurrence as a function of the air-gap thickness, also measuring the electromagnetic field emitted by tested devices before and after the failure point, with the aim of proposing an easy methodology to easily distinguish the air-breakdown occurrence from other failure modes. In the last years some interesting works appeared in the literature theorizing about dielectric charging issues caused by ESD events [8e10]. The proposed theories consider only ideal, perfectly insulating dielectrics, with no leakage current. In reality, the very short duration of ESD events, combined with the typical low quality of MEMS oxides (especially if compared to dielectric layers of state of the art CMOS technology), would be expected to mitigate this problem in real devices. However, since the actuation structure of electrostatically actuated MEMS switches can be essentially modeled as a very low value capacitor (in the range of hundreds of fF in the OFF-state), the measurement setup can easily influence the behavior of tested devices. In order to better analyze the problem, we have experimentally verified that a great number of fast ESD events like 100 ns long TLP pulses (up to 59,400,000 pulses were applied to MEMS switches) do not cause evident dielectric charging issues on fully SiO2 covered MEMS switches, both when applied to the actuation pad vs. ground, and between the RFeoutput port and ground. Regarding dielectric charging phenomena, similar results were obtained also with HBM pulses (i.e. no charging). On the contrary to what happens with TLP systems, we found that the parasitic elements of HBM testers and their switching timing can heavily influence the behavior of tested devices leading
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to, if not correctly considered, to measurement artifacts. The effect of the not-infinite impedance of a voltage probe on the transient behavior of the suspended membrane of RFeMEMS switches was then analyzed by means of electromechanical characterizations and simplified PSpice simulations, with the main result that dielectric charging should not really impair this kind of devices. Finally, we propose an inexpensive but effective way to protect MEMS devices using commercially available varistors. 2. Device description and measurement setups This work was based on the characterization of several topologies of RFMEMS devices and ad-hoc designed test structures. Pictures and schematic design of tested devices are shown in Fig. 1. Tested devices were gold-based capacitive (a) and ohmic (bed) RFeMEMS switches, out-of-plane (a, b, d) and in-plane (c) actuated, manufactured by FBK-IRST (Trento, Italy). The technology utilized for the fabrication of studied devices consists of an eight mask surface micromachining process [11], and the air-gap (sacrificial layer thickness) varies from 1.0 to 4.5 mm for the vertical devices, and 6.7 mm for the lateral ones. A novel “circular” structure (d) was also studied, with four different percentages of dielectric layer over the polysilicon actuator: 0% (no dielectric, (e)), 33% (f), 66% (g), 100% (h). ESD events were generated by means of a 100 ns TLP-TDR system [12], and an Hp 8114A solid state pulser, always 100 ns long, but at a repetition rate of 1 kHz. A homemade HBM pulser [12] was also developed and mounted directly on the micromanipulators, in order to reduce to a minimum value the parasitic effects. In fact, it is well known that ESD testers parasitic can have a negative impact on the correct evaluation of ESD effects on traditional solid state devices and, as described in [6], such impact can be more severe with the testing of micro-machined systems, that can be modeled as very low capacitance devices (typically hundreds of fF, some pF). Being interested in the study of dielectric charging issues induced by HBM events, once the setup was fully checked and characterized, we preferred to not use any probes on our HBM system. The electromagnetic field emitted by the device under test was acquired by means of a simple wire antenna hooked to the microscope revolver. This solution was adopted in order to have the antenna placed in a repeatable condition, always at the same distance from the MEMS switch (about 1 cm).
Fig. 2. Breakdown voltage as a function of the air-gap spacing of capacitive, ohmic, and lateral devices.
A Polytec MSA-500 optical profilometer mounted on a Cascade R4800 probe-station, and isolated from environmental noise by means of a Newport anti-vibrating table, was used to characterize the effective spacing of the air-gap of tested devices. The integrated Laser-Doppler Vibrometer (LDV) was used to characterize the motion of suspended structures during ESD events. Finally, a Keithley 6517B electrometer was adopted to characterize the leakage current of the developed HBM tester and tested switches. 3. Breakdown characterization Our work started with the characterization of the breakdown voltage of structures with different air-gaps, nominally from 1.0 up to 6.7 um, in order to verify the validity of the (modified) Paschen’s law for MEMS devices. 100 ns TLP pulses were applied to the RFoutput port vs. ground, in order to recreate a possible critical situation for MEMS in a real environment (antenna connection). All others configurations were tested in our previous works [3,4]. Fig. 2 shows the evolution of the measured breakdown voltage as a function of the air-gap spacing. An interesting result is the good linear dependence of the breakdown voltage with the air-gap spacing for all the devices. The slope of the linear fitting curve of such results is slightly higher than the modified Paschen’s law as shown in [9], and this difference could be due to the different materials used for the experiments (different
Fig. 1. Pictures of tested devices and layout of the dielectric area of circular devices (e ¼ 0%, h ¼ 100%). The ligther area on the central circular actuator (red) correspond to the mask for the removal of SiO2 layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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material work functions) [13]. The SiO2 layer of capacitive switch was about 120 nm thick, and it slight lowers the device breakdown voltage. The interesting thing is that also lateral devices fit very well the linear trend, rather than following the modified Paschen’s curve, predicting about 700 V for an air-gap of 6.7 mm at same ambient pressure. The low number of available samples did not permit us to study in detail what happens in lateral devices, but this behavior will be studied more in detail in future works. It is not always straightforward understanding where the breakdown occurs in MEMS devices. With the goal of better investigating this problem, we have developed a new setup, in order to acquire the electromagnetic field emitted by tested devices during ESD events, in particular during the failure. This is not a really new invention: ironically, spark discharge was how radio began. Guglielmo Marconi used “spark transmitters” to establish the first commercial wireless telegraphy service between Europe and America in 1904. In its simplest form, a spark-gap transmitter consists of two conducting electrodes separated by a gap. When a large enough voltage is applied to the two terminals, a spark jumps between them, ionizing the air and drastically reducing its electrical resistance. An electric current then flows until the spark is broken by lowering the voltage, cooling the ionized air, or moving the electrodes further apart. The emitted signal, sensed by the antenna, and measured by the DSO, is then the time derivative of the transient electric field generated by the contacting tips and the device. In a first approximation, considering an antenna made with a single wire, the charge measured is proportional to E L W, where E is the electric field, L is the length (1 m), and W is the width of the wire. An example of acquired signals during the testing of an open circuit (signal vs. gnd of a CPW line), a short circuit, and before and after the breakdown occurrence of a 2 um air-gap (device (b)) is shown in Fig. 3. The main result is the very good similitude between the open circuit and “before breakdown” waveforms, and short circuit and “after breakdown” ones. The comparison of acquired waveforms leads then to the result that studying the derivative over time of the electric field, in particular from 30 ns to 60 ns it is possible to understand where the failure is occurring (air-gap or not). Another interesting result is obtained plotting the averaging of the acquired electric field derivative over time versus the voltage applied to the device during the 100 ns TLP testing, as shown in Fig. 4. In fact, it is possible to identify three zones in the graph: open circuit (before the failure), air-breakdown, short circuit (hard failure). As already demonstrated in previous works, leakage current does not always offer a clear indication on the upcoming failure [3,4]. On the contrary, using this simple method (just a single wire connected to the DSO), it is possible to analyze with a better precision what is happening to the device under test.
Fig. 3. Emitted electric field before and after breakdown of a 2 mm air-gap and comparison with ref. open and short circuits.
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Fig. 4. Evolution of the averaging of the d(Efield)/dt and leakage current during 100 ns TLP test between RF-out and gnd.
4. Dielelectric charging and ESD tester influence In order to understand if ESD events can bring MEMS switches to dielectric charging issues, we have submitted circular devices (deh) with different area of dielectric materials to a large number of 100 ns TLPelike and HBM pulses, applied between the actuation pad and ground, and between RFeoutput pad and ground. Even if it is not really common that thousands or millions of ESD events happen in a very short time to devices used in real applications, we performed such experiments on purpose in order to shed a better light on this topic, up to now theorized in the literature [8e10], but with no experimental results shown. Here we show the result of the effect of stressing devices fully covered with a SiO2 dielectric layer (type h), ideally more sensitive to dielectric charging issues than dielectric-less ones. All tested devices showed a very similar behavior. In this work we were not interested in studying the failure values of tested devices (already characterized in our previous works [3,4]), but only in the analysis of dielectric charging issues. For this reason, and to avoid structural damages to the switches under test, we limited the TLP and HBM voltage to 100 V. 4.1. TLP stress First of all, we verified that a very short but high voltage pulse, like a 100 ns TLP pulse, can move the suspended membrane of a MEMS, thus verifying simulation results previously presented in [4]. Fig. 5 shows the vibration velocity over frequency of a circular device (d) during a single 100 V, 100 ns long square pulse applied between the actuation pad and ground. The presence of lines in the
Fig. 5. Vibration velocity over frequency of a circular device during a 100 V, 100 ns square pulse.
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frequency response is a good indicator that the membrane structure moves itself during the TLP pulses (even if it does not reach the full actuation). We have then stressed with repeated 100 V, 100 ns long square pulses at 1 kHz for 12.8 s the same device, acquiring with the LaserDoppler Vibrometer the displacement over the time of the central part of the suspended membrane. Another reference laser was pointed on the CPW line (fixed structure) in order to reduce the noise during the measurement. In total, the device was stressed with 12,800 pulses, but, as shown in Fig. 6, the displacement, that can be related to the entrapped charge, is lower than some tens of nanometers: negligible, if considered the number of pulses applied and that the air-gap is 3 mm. Such test was repeated for all other device typologies, obtaining similar results. In order to verify whether a large number of short (100 ns long or less) ESD events can really cause dielectric charging problems, the experiment was repeated stressing in the same configuration the device for 16.5 h, e.g. for a total of 59,400,000 pulses. Because of the intrinsic limitation of the LDV setup when used to acquire displacement measurements for very long time (the system performs the integral of the acquired velocity signal and some drifts can occur influencing the displacement measurement during long time characterizations), we acquired the whole topography of the device before, and right after the end of the stress. The comparison of the profile along the central part of the suspended membrane before and after the stress is shown in Fig. 7. The result of this stress is similar to the previous one: just a difference of some tens of nanometers between the fresh and after the stress profiles (mainly on the left part of the device). This difference could be explained with an almost negligible charge entrapment, even if, considering the very high number of cycles (about 60 millions), it could be also explained thinking to some plastic deformation of the suspending springs leading to a deformed up-state position of the suspended membrane. The negligible difference between the fresh device topography and right after the stress one was quite expected because, despite the very big number of pulses, the TLP stress can be, in the worst case, correlated to a DC bias signal applied to the MEMS actuator for about 6 s, too short to expect dielectric charging issues. In fact, such devices start to show some deviations of their electrical characteristics (actuation and release voltages shifts due to dielectric charging) after some hours continuously spent in the on-state (DC bias). An example of this behavior is shown in Fig. 8. Here we compared the shift of the positive actuation voltage (þVACT) of a fully SiO2 covered and of a dielectric-less (over the actuator electrode) MEMS switch. The devices were left continuously biased for 4 h at VBIAS ¼ þ60 V, and then at VBIAS ¼ 0 V, in order to evaluate both the charging and the discharging phases. VACT values were
Fig. 6. Displacement vs. time of a stressed circular devices with full SiO2 dielectric layer with 100 V, 100 ns square pulses.
Fig. 7. Displacement profile of the central part of a stressed circular devices with full SiO2 dielectric layer before and after 59,400,000 100 V, 100 ns long, square pulses.
measured at the end of the 4 h long DC stress and at selected intervals during the discharging phase. Measured values were then normalized to the initial VACT values (51 and 54 V, respectively), in order to easily compare the two behaviors. More details on the adopted procedure can be found in [14]. As expected, the SiO2 covered device was more prone to charging than the dielectric-less one, but the important difference with respect of previously described TLP stress is the very different time range. At firstglance, it may appear strange that a dielectricless device could suffer from dielectric charging, but it must be considered that only over the actuation electrode the dielectric layer was removed. On the contrary, the dielectric layer over the substrate and, in particular, the zone close to the periphery of the electrode, plays an important role in this phenomenon as demonstrated in [14]. The first conclusion coming from this study was that “short” ESD events (<100 ns) should not induce dielectric charging problems to MEMS switches, at least with comparable sizes and dielectric properties. 4.2. HBM stress We have then moved our interest in analyzing what can happen, from the dielectric charging point of view, when other kind of ESD events, like HBM ones, are applied to electrostatically actuated micro-machined RF-switches. It is well known that parasitic elements of the ESD tester can heavily influence the characterization of virtually any kind of devices. The situation can be worse when the devices under test
Fig. 8. Comparison of dielectric charging evolution of a SiO2 fully covered switch and a dielectric-less one during a 4 h long DC stress @ þ60 V and the following discharging phase (@ 0 V). Please, note that the dashed line does not necessarily indicate the real charging evolution since no measurements were taken during the 4 h long stress.
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Fig. 11. Electrical model of the HBM tester, the MEMS switch, and their parasitic elements. Fig. 9. Variation of the height of the central point of the suspended membrane of tested switch as a function of the number of 100 V HBM pulses applied between the actuator (SiO2 covered) and ground.
can be modeled essentially as very low value capacitors, as in the case of MEMS switches (typically just some hundreds of fF). We have developed a homemade HBM tester, and it was mounted directly on the wafer probe micromanipulators, in order to minimize the parasitic given by interconnections and unwanted ground planes. Once the HBM tester was fully characterized, we disconnected the voltage probe in order to reduce to a minimum value the parasitic elements of the circuit. This was very important for the correct evaluation of the MEMS response to HBM pulses, as it will be demonstrated later. We started the analysis of the influence of HBM pulses on (if any) dielectric charging problems stressing the actuation pad vs. ground. Even if this pad is the easiest to be protected (with external components) because it is not an RF pad, this condition should be the most favorable for charge entrapment. We have then applied an increasing number of 100 V HBM pulses (up to 1000 at a repetition rate of 1 Hz) to fully SiO2 covered circular switches (type h), measuring the device topography at the end of each stress in order to investigate any bending of the suspended membrane. A typical result is shown in Fig. 9. From the analysis of the graph, two results are straightforward: (1) HBM pulses can bend downward the suspended membrane, and (2) the number of applied HBM pulses does not have a direct influence on the bending of the membrane, being the displacement almost constant (an average of 750 nm with respect to the initial value). From the literature [9] it seemed that the influence of HBM pulses on electrostatically actuated switches should have a shorter lifetime (in the milliseconds range).
Fig. 10. Comparison of the measurement of the decay time of 100 V HBM pulses with 10 and 100 MU voltage probes (continuous lines) and simulation of the circuit shown in Fig. 10 (dotted line).
In order to better investigate the problem, we carefully characterized the influence of the voltage probe impedance on the HBM waveform decay, as shown in Fig. 9 for a 100 V pre-charging. As expected, higher the impedance, higher the discharge time. Using a 100 MU probe the decay time is about 20 ms, but it must be considered that the equivalent resistance of tested MEMS switches (virtually open circuits) is very higher than 100 MU, and this can lead to an obvious longer decay of the HBM pulse. So, we performed a detailed characterization of the parasitics of the HBM tester and of the tested switches with a high resolution electrometer (Keithly 6517B). The extracted model of the HBM tester, its parasitic elements, and the tested switch is shown in Fig. 11. The interesting thing to note is the value of the leakage resistors, in the range of hundreds of GU for the HBM tester, and in the TU range for the MEMS switch, very higher than voltage probes impedance. We have then simulated with PSpice the simple circuit shown in Fig. 11, setting the initial condition of the HBM capacitor to 100 V (and no voltage probe). The result is shown in Figs. 10 and 12. Being the resistors used to model the leakage current paths very high, the decay of the HBM pulse is in the tens of seconds range. This long discharge time can easily explain the bending of the suspended membrane after HBM stresses, as previously shown in Fig. 9. To verify this assumption, we acquired the velocity of the central part of the suspended membrane with the LDV setup during a 100 V HBM pulse, monitoring with a good time resolution (390 ns) the membrane behavior for about 25 s. The result is shown in Fig. 13. In the left part of the graph it is possible to see the increase of the membrane velocity, i.e. the actuation of the switch, and, in the right part, the switch release, occurring after about 21 s. Lower HBM pre-charge voltages showed, as expected, a lower release time.
Fig. 12. Simulation of the time evolution of the discharge of the MEMS switch actuator during an HBM event. The release voltage of the switch (about 31 V) is reached after 21 s.
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Fig. 13. Velocity measurements of the central part of the suspended membrane of a switch (f) during a 100 V HBM pulse (the graph was split for clarity purpose).
All these results can be correlated very well together considering the traditional hysteresis curve of tested switches, shown in Fig. 14 (more details on the characterization procedure can be found in [15]). From this measurement it is possible to extract a positive actuation voltage (þVACT) of about 51 V, and a positive release voltage (þVREL) of about 31 V. Comparing the results from Figs. 12 and 13, it is possible to conclude that the switch, after one 100 V HBM pulse remains in the actuated state for about 21 s, exactly the time the circuit needs to discharge reaching the release voltage of the switch, about 31 V. This conclusion helps to explain the bending of the suspended membrane shown in Fig. 9, due to the fact that the topography measurements were carried out immediately after the HBM stress, when the suspended membrane did not yet reached its nominal off-state position. Furthermore, considering the very low discharge time (as previously demonstrated), the dielectric of tested switches remained stressed for a total equivalent time of about 20 min, again too short to lead dielectric charging issues. For the sake of completeness, we characterized the electromechanical behavior of the same RFeMEMS switch when a lower voltage HBM pulse (about 60 V) is applied. As shown in Fig. 15, it is possible to note that the switch was actuated, but it suddenly release itself (please note the oscillating behavior typical of a free moving plate). We have also tested the influence on the switch dielectric charging of HBM pulses applied to the RFeoutput port vs. ground. In this configuration almost no bending (and then actuation) of the suspended membrane was observed, since the area of the
Fig. 14. Insertion loss (S21 parameter) evolution as a function of the switch bias voltage, used to extract the actuation and release voltage values of the device under test (type f).
Fig. 15. Velocity measurement of the central part of the suspended membrane of a switch (f) during a 60 V HBM pulse, showing the actuation and the sudden release transients.
equivalent actuation electrode (the small half-moon at the right of the circular central actuator in Fig. 1(eeh)) is too small to have an influence on the electromechanical behavior of the switch. This result is further evidence that it is not correct to consider a partial actuation of the switch as caused by dielectric charging. On the contrary, the behavior is simply governed by the transfer of the charge of the HBM capacitor to the MEMS switch and its slow discharge time. We expect a similar behavior also considering other kind of ESD stress models, always based on the discharge of a charged capacitor, even if of different capacitance and time ranges. Furthermore, it is important to note that a great care must be taken in consideration when developing setup for ESD testing of MEMS devices to avoid introducing parasitic elements that could seriously change the device behavior. 5. Varistor ESD protection As a last point of this work we wanted to briefly introduce a simple, but effective, way to protect RFeMEMS switches from ESD events. In fact, the robustness of such devices to ESD phenomena is, as well known, critical, because of the difficulty, or impossibility, of integrating effective protection structures with the same technological process (on the contrary of traditional state of the art technologies). By the way, typical failure voltages of MEMS are in the range of some hundreds of Volts, relatively low considering typical ESD voltage levels, but very high if compared to traditional
Fig. 16. IeV 100 ns TLP and leakage curves of the PCB with RFMEMS andvaristor mounted as shown in the inset.
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was that parasitic elements of HBM testers (but extendible to other ESD tester) can play a detrimental role on the correct evaluation of the electromechanical behavior of RFeMEMS switches stressed with ESD events, lowering the stress time from tens of seconds to just some milliseconds. As a last point, we proposed a very cheap and easy solution to protect both DC and RF ports of such switches based on a commercially available varistor, showing good ESD robustness under 100 ns TLP regime, and RF performances. Clearly, all these results can easily be applied for the testing and protection of other families of micro-machined systems.
Acknowledgements Fig. 17. Insertion losses of the test PCB with and without the varistor mounted between RFoutput and ground.
solid state devices values. We had then experimentally verified that a commercially available (for less than 1 cent) varistor could be used as a starting point to protect such devices between the RFeoutput and gnd. An RFMEMS switch (type (b)) was mounted on a 50 U coplanar waveguide (CPW) line designed on a PCB, together with a varistor placed between RFeoutput and ground, as shown in the inset of Fig. 16. 100 TLP stresses were then applied to the right SMA connector, measuring the IeV and leakage curves shown in the same Figure. As it is possible to see, the varistor turns on at about 300 V, lowering the transient voltage down to about 50 V, and showing a very high robustness (ITLP > 10 A, with no leakage variations). Mounted varistor was rated for VDC ¼ 12 V, clamping voltage of max. 50 V, and a capacitance of 0.5 pF. This value leads to the insertion loss (S21) vs. frequency behavior shown in Fig. 17, not bad considering this value comprehends the losses of the PCB, the SMA connector, and the wires bonding for MEMS connection. Clearly, the same technique can be used to protect also the actuation pad of the device in real application, even if such pad is usually not exposed, and it must not be treated as an RF pad. 6. Conclusion RFeMEMS switches are relatively simple devices based on the movement of a suspended membrane. Being the typical air-gap thickness of MEMS devices in the range of mm, it is important from the ESD robustness point of view studying the breakdown of such air layer. The first aim of this work was to study the breakdown occurrence of devices with air-gap from 1.0 to 6.7 mm, under 100 ns TLP regime. A good linear trend as a function of the air-gap thickness was obtained, even if slightly higher than the traditional modifiedPaschen’s law curve. Furthermore, the breakdown occurrence was studied also measuring the emitted electromagnetic field, showing how it is possible to separate air-breakdown events from other kind of failures. The second aim of this work was then showing experimental results on the previously theorized problem of dielectric charging issues induced by ESD events. Here we demonstrated that even under a great number of TLP and HBM pulses, such devices were insensitive to this reliability issue. A second result from this analysis
Authors would like to thank Albert Wallash (Hitachi Global Storage Technologies) for useful discussions of the results and for reviewing the paper. This work was partially supported by the European Space Agency under Contract No. ITT AO/1-5288/06/NL/GLC., and by ENIAC project END “Models, Solutions, Methods and Tools for Energy-Aware Design”. The END project has received funding from the ENIAC Joint Undertaking under grant agreement n 120214 and from the national programs/funding authorities of Belgium, Greece, Italy, and Slovakia.
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