1.
Introduction

ESD is one of the most important research issues about IC reliability. ESD brings transient high voltage and high current, which will cause damage to the chip including thermal damage and gate oxide breakdown. The main reasons of chip failure caused by ESD are electrical failure and thermal failure^{[1, 2]}.



The continuing advancement of Si MOS technology has placed a high demand on effective solutions for electrostatic discharge (ESD) protection. Although many ESD protection devices have been developed to address this prevalent reliability issue, the trade-off between both the robustness and the area efficiency are still hard to come by.



The lateral-insulated-gate-bipolar-transistor (LIGBT) is commonly used as power device in power management system, automotive electronics, and display drive circuit. The application of ultra-high voltage and large current needs to design ESD protection devices with good performance^[3]. In LIGBT structure, the internal zone forms a PNPN path to bleed high ESD current. In terms of the ESD protection, the LIGBT itself has parasitic PNPN structure, which has the same turn-on conditions as a conventional SCR structure and can help handle large current^[4]. Therefore, the LIGBT structure can be a good application for high-voltage ESD protection. However, its relatively high trigger voltage and low holding voltage make it not suitable for some applications. As a result, it is necessary to study the working principle of the conventional LIGBT structure and propose a novel LIGBT structure.



At present, the researches on ESD reliability of LIGBT mainly include the following structural optimization schemes. In 2011, a composite MOS-IGBT-SCR device with high ESD robustness was proposed^[5]. It is a good choice for power clamp circuit; In 2013, a new SOI-LIGBT device with anode/cathode segmental injection technology was proposed, which can effectively improve the holding voltage and reduce the risk of latch-up under ESD stress^[6]; In 2013, a SOI-LIGBT structure with no risk of latch-up was proposed^[7]. The device has high robustness and can replace the conventional diode as a new ESD protection device in high-voltage integrated circuits; In 2014, a novel LIGBT structure with w-shaped buffer window was proposed^[8]. The W-type window can help achieve high current discharge capability and high robustness. In 2018, several anti-latch-up structures based on SOI-LIGBT device were proposed and the relationship between anti-latch-up capability and robustness was analyzed^{[9, 10]}.



In this article, the ESD protection performance of the conventional LIGBT structure and the proposed novel NMOS triggered LIGBT (NTLIGBT) structure is compared by simulation and analysis. The results of simulation show that the performance of the proposed novel NTLIGBT structure is obviously better than the conventional one.

2.
Device structure and working mechanism

2.1
Design of device structure

The LIGBT can be used as ESD protection device with strong robustness, but it still has the disadvantage of the large ESD design window. In this paper, an improved LIGBT structure with embedded NMOS for ESD protection is proposed. Figs. 1(a) and 1(b) show cross-sections of the conventional LIGBT and the proposed novel NTLIGBT structure.






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Figure1.
Cross-sectional view of (a) conventional LIGBT structure, (b) NTLIGBT structure.




Compared with the conventional LIGBT structure, the main difference between two structures is a heavily doped P⁺-region and N⁺-region1 in the N-epi region and the N⁺-region2 in the P-body zone. Otherwise, the other parameters will remain unchanged.

2.2
Analysis of working mechanism

The working principle of two structures under ESD stress is analyzed. Unlike the conventional structure, the proposed structure adds an additional parasitic P⁺-N-buffer-N-epi-P-body-newly inserted P⁺ (PNP1) transistor. From the analysis of the ESD characteristics of the MOS tube under ESD stress, we can know that the working principle of the MOS tube under the ESD stress is to use the triggering of the internal NPN tube to turn on the device. Under a forward ESD pulse, a reverse PN junction formed by N⁺/P-body junction in the NMOS structure will breakdown firstly to provide currents. Therefore, when a forward ESD pulse is applied to the anode of the novel structure, the NMOS structure will breakdown before the reverse PN junction originally formed by N-epi/P-body inside the device, which means that the novel structure changes the position of the breakdown point inside the device. The reason is that the doping concentration of the N⁺ implant region is relatively high compared to the N-epi and newly added P⁺ can also raise the potential of the N⁺ terminal 2, which result in the priority breakdown of the reverse PN junction formed by N⁺/P-body in the NMOS structure. The trigger voltage of the novel NTLIGBT structure is significantly lower than that of the conventional LIGBT structure. At the same time, the gate electrode is introduced to reduce the trigger voltage and obtain different ESD design windows by the variation of the gate length in simulation. The newly added parasitic P⁺-N-buffer-N-epi-P-body-newly inserted P⁺ (PNP1) transistor plays a role in diversion to weaken the internal positive feedback mechanism, which improves the holding voltage of the device and reduce the risk of latch-up. As a result, the low trigger voltage and high holding voltage are achieved simultaneously.

3.
Comparison and analysis of simulation results

3.1
ESD protection characteristics

The proposed NTLIGBT structure adopts the conditions of simulation as same as the conventional LIGBT structure. Human body model (HBM) is used to simulate the transmission line pulsing (TLP) pulses and distinguish the improved performance of the NTLIGBT device through the trigger voltage and holding voltage. When the same positive ESD pulse is added to the drain electrodes of the novel NTLIGBT structure and the conventional structure, the comparison of their I–V curve is shown in Fig. 2.






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Figure2.
(Color online) I–V curves of LIGBT under ESD stress.




From the Fig. 2, we can see that the trend of the curve is consistent. The three typical processes of triggering, holding and secondary breakdown zones will be experienced. The trigger voltage $ {V_{{
m{t}}1}}$ of the proposed structure is greatly reduced and the holding voltage $ {V_{{
m{h}}}}$ is also increased. So, the ESD design window of proposed NTLIGBT structure is narrower than the conventional LIGBT structure. Obviously, the ESD protection performance of the proposed structure has been greatly improved.



The trigger voltage of the conventional LIGBT structure is 95.1 V, while that of the proposed one is 27.1 V. This is due to the prioritized avalanche breakdown of internal NMOS structure. Then the parasitic transistor in the proposed LIGBT structure can be turned on more quickly. So, it changes the position of avalanche breakdown of the conventional structure. As shown in Fig. 3, the relationship between voltage and time can be verified. From the figure, it can be concluded that the proposed NTLIGBT structure can discharge the ESD current faster comparing with the conventional structure. And the overshoot is so small, so that when it is under ESD positive stress, the device can be turned on in time and provide a discharge channel to discharge ESD current, and the protected circuit will not be damaged by excessive overshoot.






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Figure3.
(Color online) Comparison of voltage and time under ESD stress.




Furthermore, it can also be seen from the I–V curve shown in Fig. 2. that the holding voltage ($ {V_{
m{h}}}$) of the proposed NTLIGBT structure is numerically increased by weakening the positive feedback effect of the device. This is because the base current of the original PNP transistor does not completely flow to the collector of the NPN and a part of the base current becomes the base current of the newly added parasitic PNP transistor. So, the newly added parasitic PNP transistor can help weaken the positive feedback to clamp the internal voltage of device and reduce the risk of latch-up effect.



Actually, the ESD signal is very fast. The device is too late to establish thermal balance in such a short period of time, so the second breakdown current ($ {I_{{
m{t}}2}}$) of the device can get though transient simulation. The method in this paper is by adding a series of TLP current pulses with rising and failing edges of 10 ns and pulse width of 100 ns to drain electrode. The temperature of the device will increase with the increasing of current. So, the value of the $ {I_{{
m{t}}2}}$ can be achieved when the highest temperature in the device exceeds the melting point of silicon (the melting point of bulk silicon is 1687 K). Fig. 4. gives the relationship between the time and the temperature at different ESD currents. As can be seen from the figure, the secondary breakdown current of the proposed NTLIGBT structure is slightly decreased. This is because the parasitic resistance of the base region is increased. So, the secondary breakdown current is decreased. Although the secondary breakdown current of the proposed structure is reduced, the ESD robustness of the device remains at a high level based on the advantages of the low trigger voltage and high holding voltage.






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Figure4.
The relationship between the time and the temperature with different ESD currents.




Fig. 5 shows the current flow lines of the proposed NTLIGBT structure after it is fully turned on. The current distribution and the analysis of the operating principle are consistent.






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Figure5.
(Color online) Current distribution after the novel NTLIGBT is fully turned on.

3.2
Optimization and selection of parameters

This paper will optimize these parameters such as the position S where the P⁺-region is inserted (S represents the distance between the inserted P⁺-region and the P⁺ drain region), the length L of the inserted P⁺-region, the gate length of the NMOS structure. For effective comparison, it is necessary to use the same condition of simulation for both structure and consistent parameters.



When optimizing the position of the heavily doped P⁺-region, the position S of the inserted P⁺-region is taken to be 0.5, 1, 1.5, and 2 μm on the premise of the length of the inserted P⁺-region to be 1 μm. Moreover, other parameters remain unchanged.



As shown in Fig. 6, it can be seen from the I–V graph that the larger the S, the higher the trigger voltage and the lower the holding voltage of the novel NTLIGBT structure. It can be found from the cross-sectional view of the conventional structure and the novel structure that the smaller the value of the parameter S is, the closer the P⁺-region is to the drain. It is not hard to see that the length of the drift region of the device is reduced and the width of the depletion layer is also reduced at the side of the drift region, which leads to a reduction in the trigger voltage. At the same time, the reduction in the length of the drift region also causes the decrease of on-resistance. Simultaneously, the holding voltage is also decreased. In term of the ESD protection, the ESD design window should be as narrow as possible. As a result, the best position S for P⁺-region is equal to 1 μm.






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Figure6.
(Color online) I–V curve of LIGBT structure with different P⁺ locations.




Similarly, the length L₁ of P⁺ is optimized based on the position S of the P⁺-region being 1 μm. Based on the requirements of ESD design window of ESD protection devices, L₁ = 1 μm is selected as the optimal length under compromise consideration.



The Fig. 7 shows the variation of the value of the trigger voltage $ {V_{{
m{t}}1}}$ and the holding voltage $ {V_{{
m{h}}}}$ when the embedded NMOS structure is at different gate lengths. It can be seen intuitively in the Fig. 7 that the value of $ {V_{{
m{t}}1}}$ and $ {V_{{
m{h}}}}$ of the proposed structure increases simultaneously as the gate length of the NMOS structure increases. Increasing the length of gate means that the base width of the internal parasitic NPN transistor is also increasing. That is to say the current needs to flow through a longer path to reach the cathode of the device, which makes it difficult for the parasitic transistor to turn on. So, the trigger voltage is increased. In addition, when the gate length of NMOS structure increases, the increase of the base width of the original parasitic NPN transistor will also cause the increase of the parasitic resistance of the base region. Furthermore, the holding voltage ($ {V_{{
m{h}}}}$) can be expressed as^[11]:






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Figure7.
(Color online) The variation of the $ {V_{{
m{t}}1}}$ and the $ {V_{{
m{h}}}}$ with different gate lengths.

${V_{ m{h}}} = {V_{{ m{pbe}}}} + {V_{{ m{nbe}}}} + {V_{{ m{potential}}}} + I {R_{{ m{on}}}}.$

(1)

In the above formula: $ {V_{{
m{nbe}}}}$ is the voltage between the emitter-base junction of the NPN transistor, $ {V_{{
m{pbe}}}}$ is the voltage between the emitter-base junction of the PNP transistor, $ {V_{{
m{potential}}}}$ is the applied voltage. $ {R_{{
m{on}}}}$ is the parasitic resistance of the base region. I is the current flowing through the bleed path.



According to the Eq. (1), the holding voltage will increase. As a result, based on the trade-off between the trigger voltage and the holding voltage, the ESD protection performance of the device is optimal when the gate length L₂ of the NMOS structure is equal to 0.5 μm.



In addition to the length parameters analyzed above, the difference in concentration also has a certain impact on the ESD protection performance. It can be seen from the Fig. 8 that the lower the concentration of N-buffer layer, the relatively lower the trigger voltage. This is because the lower the concentration of N-buffer layer, the fewer the electrons in the N-buffer layer. Therefore, the number of holes neutralized will be decreased. That is to say more holes can reach the cathode to turn on parasitic transistor more quickly. At the same time, the variation of holding voltage is not obvious because the N-buffer layer is only a small part of the drift zone, which has no significant influence on the holding voltage. From the analyses made above, the concentration of N-buffer should be as low as possible. As a result, the concentration of N-buffer layer in this paper is on the order of 10¹⁶.






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Figure8.
(Color online) The variation of the $ {V_{
m{t1}}}$ and the $ {V_{
m{h}}}$ with different concentration of the N-buffer layer.

4.
Conclusion

In this paper, the working principle of the conventional LIGBT structure and the proposed NTLIGBT structure under ESD stress is compared and analyzed. The NTLIGBT structure with narrow ESD working window for ESD protection is designed. Compared with the position of avalanche breakdown in the conventional structure, the MOS transistor in the proposed structure breaks down before the PN junction in the conventional structure. Then the parasitic transistor inside the device can be turned on prior to conventional structure. So, the trigger voltage is significantly reduced. At the same time, an additional parasitic PNP transistor is introduced in the structure, which can be used to clamp the internal voltage of the device and weaken the internal positive feedback effect. The holding voltage is greatly increased. As a result, the proposed NTLIGBT structure has a narrow ESD design window.

Acknowledgement

This work is supported by National Natural Science Foundation of China (Grant No. 61274080).