EFFECT OF RF SIGNALS ON TVS DIODE TRIGGER VOLTAGE FOR ESD PROTECTION
Abstract— Discrete transient voltage suppressor are used in addtion to the on-chIp ESD protection to protect the ICs from ESD damage. In applications involving snapback TVS devices, the trigger voltage is selected to be higher than the desired signals on the net. The presence of RF on the net affects the TVS behavior, even the RF levels are less than the snapback trigger voltage. Known affects are RF intermodulation and harmonic generation, which diminish HNR. This work describes the other effect on the TVS diodes, the observed reduction of the snapback trigger voltage, monotonically dependent on the amplitude of the RF signal. This translates into snapback triggering at low stress levels than specified in the datasheet or expected by the design engineer. TLP testing was performed using a 100 ns pulse with RF signal from 100 MHz to 2 GHz present at the diode terminals. The result show that the higher frequencies have a weaker impact on the reduction of snapback trigger voltage, and the phase of the RF signal impact the amount of reduction in the trigger voltage. It is also observed that certain TVS diodes recover from snapback even when the RFamplitude is higher than the holding voltage of the diode.
Keywords— Electrostatic discharge(ESD); radio frequency(RF) signals; snapback devices; transmission line pulse(TLP); TVS.
ICs are designed with internal protection elements to protect them for component level electrostatic discharge (ESD) stress events. In addition to the component level protection circuits, external transient voltage suppressor (TVS) devices are necessary at system-level to provide sufficient ESD robustness to a device . Based on the signal interface type, appropriate TVS device are selected based on various TVS parameters and the signal interface design requirements. In , various TVS devices such as diodes, varistors, spark gaps, and polymers were analyzed using a custom designed board and the pros and cons were discussed for different use cases. The PIN limiter diodes which are typically used for power limiting applications, were analyzed in  to quantify their ESD robustness on RF paths and compared against the protection capability of a TVS diode. In -, methods for characterization of protection devices were evaluated. ESD simulator, 100 ns TLP and VF-TLP are common measurement approaches to evaluate various devices for component-level ESD characterization. VNA measurement is used to evaluate impact of the protection device on signal integrity.
In , the harmonic generation of the diodes is characterized to understand their RF large signal behavior. RF was applied, but no TLP pulses have been applied at the same time. The authors of  propose a system that combines TLP and RF, but not simultaneously. It subjects the device under test (DUT) to TLP pulses, then followed by RF measurement (S-parameters, IIP3, noise figure, etc.) to characterize the DUT. The work presented in  characterizes the RF devices before and after ESD event. It was shown that the performance of RF circuits and devices can degrade at ESD stress levels below the failure levels proposed by the commercial human body model (HBM) testers. However, none of these works address the possibility of RF signals being present at the time of the ESD pulse.
In this paper, the ESD protection capability of the TVS diodes was experimentally characterized during the presence of RF signals at the diode. The goal is to quantify the change in the TVS behavior. Two types of TVS diodes were investigated: the non-snapback and the snapback type. The snapback type diodes are designed to have a holding voltage Vh lower than the trigger voltage Vt1. An example of snapback behavior in the I-V curve is illustrated in Fig. 1.
It is shown that non-snapback diodes are largely unaffected, but snapback device Vt1 is changed by the RF. At 100 MHz it is observed that Vt1 reduces proportionally to RF amplitude. The effect decreases with increasing RF frequency. A new measurement method was proposed which applies both the RF and the TLP pulse at the DUT simultaneously. The transient voltage and current, and the quasi-static I-V curve are analyzed. Based on the experimental results, an additional consideration is proposed for the TVS diode selection on RF interfaces.
A. Traditional TLP characterization process
The industry-defined standard - 100 ns TLP setup consists of five main parts: a pulse generator, a voltage probe, a current probe, an oscilloscope, and a DUT – e.g., an ESD protection TVS diode. The simplified block diagram is shown in Fig. 2. The quasi-static I-V characteristic of the TVS diode under test is extracted from 70-90% window of the transient V(t) and I(t) waveforms captured by two channels of an oscilloscope.
The typical testing approach is to perform a sweep of the TLP charge voltage, and establish several characteristics: a) diode breakdown voltage Vbr, b) diode dynamic resistance Rdyn; in case of snapback TVS diodes: c) holding voltage Vh, d) snapback trigger voltage Vt1. In addition, device self-heating effect and physical failure levels can be observed.
The very fast TLP (VF-TLP) variation  of the characterization setup is often used for evaluating transient behavior of the protection devices (e.g. switching speed) for shorter pulses (1-10 ns) and shorter rise times (100-600 ps). This work, however, focuses on the 100 ns TLP test configuration.
B. RF+TLP characterization process
The 100 ns TLP setup allows for a wide variety of test results, but assumes no RF signal present across the diode terminals. A novel testing method is proposed; it allows to force RF signal on the DUT during characterization. ESD protection devices are evaluated in this scenario and the effect of RF on their performance is observed.
The RF+TLP test setup modifies the 100 ns TLP measurement system by ESDEMC . The simplified block diagram is given in Fig. 2. The signal generator RF Source output is amplified to 15 W using AMP1. The 20 dB attenuator ATTEN1 reduces the stress seen by AMP1 output terminal, as TLP pulse is injected via a T-junction TEE1. As the pulse is injected, part of it propagates towards the amplifier and is subsequently dissipated and absorbed, while the other part is forced on the DUT.
Oscilloscope Channel 3 with a CT1 probe is used to determine the current that is forced into the DUT. Oscilloscope channel 4 with attenuator ATTEN2 measures the voltage at the DUT terminal. The original voltage pick-off tee and the CT1 probe on oscilloscope channels 1 and 2 respectively measure total injected current and voltage close to the injection point. Captured VDUT(t) and IDUT(t) are processed in the same way as the standard TLP in order to produce quasi-static I-V curves.
Four TVS devices are evaluated in this work, as mentioned in Table I.
A. Effect of RF on non-snapback TVS diode I-V characteristic
TVS1 is a non-snapback diode with Vbr = 9 V and Rdyn = 2.1 Ω. The effect of RF at 100 MHz, 1 GHz, and 2 GHz on this non-snapback diode is negligible. As an example, the effect of 100 MHz on the I-V curve is depicted in Fig. 3.
Figs. 4 and 5 illustrate the time-domain voltage and current waveforms during the presence of RF signals. To obtain the quasi-static I-V curve, the 70-90% window is applied to the time-domain waveforms. The voltage and current values during the 70 ns to 90 ns time are averaged to obtain the I-V curve plot. For a 100 MHz RF signal, the period is about 10 ns, which covers 2 cycles within the 20 ns long averaging window. Similarly, the period for 1 GHz RF signal is 1 ns and about 20 cycles are superimposed on the TLP signal in the averaging window. Lastly for a 2 GHz RF signal, the period is about 0.5 ns and about 40 cycles are overlapping in the I-V curve averaging window. Therefore, for each RF signal a full number of periods fall in the averaging window, thus the effect of added RF voltage is zero in average. In case of other frequencies, incomplete cycles within the 70-90% averaging window may affect the I-V curve values depending upon the phase of the RF signal and the TLP pulse. Furthermore, it is expected that lower frequencies may introduce more change in the I-V curve. In this case, the cycle is wider, resulting in stronger contribution to the average value calculation, as opposed to a narrower incomplete cycle (i.e. of a higher frequency signal).
B. Effect of RF on snapback TVS diode I-V characteristic
TVS2 is a deep snapback diode with Vt1 = 18.1 V and Vh = 2.6 V. The effect of RF with varied Vpp at 100 MHz on this snapback diode is depicted in Fig. 6. The effect on I-V curve is not strongly observed at 1 GHz and 2 GHz RF. Contrary to the non-snapback TVS diodes, the snapback diodes do exhibit reduction in their snapback trigger voltage Vt1 in the presence of RF signals. The reduction in the Vt1 also reduces with the frequency of the RF signal. At 3 Vp 100 MHz RF signal, a maximum reduction of 2.8 V is measured on the Vt1 of TVS2. TABLE II gives a summary of the effect for TVS2 snapback trigger voltage.
A deep snapback device showed reduction in the snapback trigger voltage, though a non-snapback TVS diode did not exhibit any change in its clamping behavior. To investigate this unique snapback TVS behavior, additional snapback TVS diodes are measured. TVS3 is a shallow snapback diode with Vt1 = 4.6 V and Vh = 2 V. The effect of RF with varied Vp at 100 MHz, 1 GHz, and 2 GHz on this snapback diode is depicted in Fig. 7. TABLE III gives a summary of the effect for TVS3 snapback trigger voltage.
The time-domain voltage and current waveforms of TVS3 during the presence of 100 MHz RF signal are plotted with and without RF. Fig. 8 illustrates the voltage and current waveforms prior to the snapback trigger. Fig. 9 illustrates the waveforms after snapback occurs. Contrary to the voltage time-domain waveforms in Figs. 4 and 5 for a non-snapback device, the voltage waveforms in a snapback device exhibit two key differences in their response. Consider the TLP waveform from 0 to 100 ns as the reference. For non-snapback TVS, the Vpp cycles are superimposed equally above and below the TLP pulse. However, for snapback type devices, the RF signal is not superimposed equally on the TLP pulse. Secondly, the Vpp swing on the non-snapback during the TLP pulse is not the same as the Vpp prior to the application of the TLP pulse. These two observations suggest why the two types of devices respond differently to the same RF signal.
The effect at lower frequencies is due to superposition of the voltage waveforms, so that the diode snapback triggers as , thus snapback occurs at lower TLP stress than nominally. At higher frequencies, the effect is weaker and does not follow this rule. A negative monotonic relationship between the RF frequency and reduction in Vt1 is observed.
When each measurement is repeated multiple times, another effect on Vt1 is observed: the voltage varies with the phase of the RF at the moment of the rising edge of the TLP pulse. The effect is illustrated in Fig. 10, and summarized for TVS4 for 100 MHz RF and swept RF amplitude. The figure shows positive monotonic relationship between reduction in Vt1 and RF amplitude. However, the trend does not follow the expected linear reduction of Vt1 with amplitude increase, caused by superposition of RF and the TLP pulse.
C. TVS device appears to not recover from snapback
TVS4 is a shallow snapback diode with Vt1 = 4 V and Vh = 1 V. The effect of RF with 1-7 Vpp at 100 MHz on the diode is depicted in Fig. 11. In this case, the TVS4 was only measured at 100 MHz, as maximal reduction in other snapback TVS diodes was observed at this frequency. The Vpp was increased to observe the trend in the reduction of the snapback trigger voltage. At 5 Vpp and higher, the TVS diode appears to not recover from snapback, but rather appears to stay in that regime for all consecutive pulses as long as RF is present on the net. Normally, this happens if Vdc > Vh and leads to diode damage. While a detailed understanding requires knowledge on the type and implementation of the snapback structure within the TVS a preliminary explanation is suggested. The RF voltage causes currents for two reasons:
Conduction current: if the peak amplitude of the RF current is large enough before diode turn-on, carriers are injected into the base regions (assuming SCR snapback TVS). Thus, even a weak excitation can trigger the snapback. Fig. 12 (b) compares transient voltages and describes the diode triggering and switching to low-impedance regime (i.e. snapback). It is evident from the curve corresponding to 5 Vpp RF, that triggering happens at a much lower voltage than in nominal conditions (i.e. “No RF”). Although not fully understood, it is reasonable to assume that the time needed to flush out charge carriers will influence the severity of this effect.
Displacement current: the RF voltage will cause a current in the parasitic capacitive paths within the TVS. This current may also inhibit the return from snapback.
TVS2 and TVS3 were tested for similar effect, but only reduction in Vt1 was observed. This suggests that specificity of die layout defines whether the appearance of snapback latch-up will occur.
In the process of interface protection design, it is important to account for the reduction in the trigger voltage of snapback TVS diode due to the presence of the desired RF signals. The following difficulties can be anticipated and avoided if the diode is well characterized:
As observed in Fig. 12 (a) the current through the diode at RF Vpp = 5V is much higher than Vpp = 4 V prior to the arrival of the TLP pulse. Thus, the RF signal voltage will drop across the diode and reduce SNR at the receiver.
The increased current at higher RF Vpp = 5 V is only observed after the snapback device is triggered once by the transient event. The device remains in this state (conducting current) at large RF Vpp as long as the RF signal stays across the device.
Various non-linear effects of a TVS diode (e.g. voltage- dependent capacitance, etc.) cause intermodulation distortion (IMD) and harmonic generation , which degrade receiver performance. It is likely that a diode that behaves like TVS4 will cause stronger harmonics, leading to antenna desense. This behavior is an avenue for further investigation.
As RF Vpp increases, the snapback diode behaves effectively like a non-snapback TVS as observed in the I-V curve in Fig. 11. The figure shows an effective trigger voltage of Vbr < Vh. From the quasi-static behavior of the device, it appears that the TVS does not recover from snapback for the cases where Vpp = 5 V, 6 V, and 7 V. However, the time-domain waveforms in Fig. 12 depicts that the TVS goes into snapback at Vdiode << Vt1. It should be noted that the RF signal was constantly applied at the TVS device throughout the testing. This behavior was only observed for consecutive TLP pulses.
The root cause of the RF frequency dependence on device behavior is not well understood and needs further investigation. Device transient behavior simulation using SPICE-based models ,  and TCAD are a possible tools to further analyze the reduction in trigger voltage due to presence of RF signals. Particularly for the lower frequency RF signal such as 100 MHz, the phase alignment of the RF pulse with the TLP pulse affects the reduction in the trigger voltage.
While selecting a snapback TVS diode for providing ESD protection on RF interfaces, the effect of RF signals on the trigger voltage of the diodes must be taken into consideration. Based on the proposed TLP + RF characterization setup, it was observed that in presence of RF signals, the TLP voltage that is needed to trigger snapback is reduced. The sample size of 4 diodes is limited and no generalizations can be made, however the trends show the following: 1) the effect of Vt1 reduction is positively dependent on RF amplitude, 2) reduction in Vt1 shows negative dependency on RF frequency, 3) non-snapback TVS diodes are largely unaffected by the RF signal.
For one of the investigated TVS diodes a sufficiently large RF signal makes the TVS appear to not recover from snapback. Due to the presence of RF, even weaker ESD stress can easily trigger the TVS diode into snapback. This effect is expected to take place when Vp > Vh, but the results suggest that this is not necessarily the case for all diodes. Each protection device should be evaluated separately. The system designer must account for a lowered trigger voltage for the snapback TVS diodes. A typical data sheet of a protection device does not provide information about the change of protection characteristics under various operating conditions. Including the effects observed in this work would lead to a more accurate understanding of the devices.
This paper is based upon work supported partially by the National Science Foundation under Grant No. IIP-1916535.