SCR with Change Side of P+ and N+ Diffusions in the Cathode

The cross-sectional views of traditional SCR with NBL and modified SCR with NBL are shown in Fig. 4.15. The difference between the traditional SCR and modified SCR structure is that the locations of P+ and N+ diffusions in the cathode. The widths and lengths of anode to cathode are 100µm and 7.1µm, respectively. The base widths of the PNP and NPN transistors are kept constant, which are 3µm and 4.1µm, respectively. The base resistance of the NPN transistor in the modified SCR structure can be greatly reduced by changing the positions of P+ and N+.

(a)

(b)

Figure 4.15 The cross-sectional views of (a) traditional SCR and (a) modified NBL.

(a)

(b)

Figure 4.16 Comparisons of the TLP-measured I-V characteristics of (a) modified SCR with and without NBL and (b) modified SCR with NBL and traditional SCR with NBL.

(a)

(b)

Figure 4.17 The DC-measured I-V characteristics of (a) traditional SCR with NBL and (b) modified SCR with NBL.

Fig. 4.16 shows the comparisons of the TLP-measured I-V characteristics of the modified SCR with and without NBL and the modified SCR with NBL and traditional SCR with NBL. In Fig. 4.16 (a), the I_t2 current of the modified SCR without NBL is severely degraded, which can be attributed to the parasitic PNP transistor takes the most current after the device goes into the high current region. Hence, the inefficient beta gain of PNP transistor causes the poor ESD robustness. However, when applying NBL into the modified SCR, the I_t2 current is significantly increased [16]. The parasitic vertical SCR device is triggered on to take the most current after the device goes into the high current region. In the structure of modified SCR with NBL, not only the high It2 current is developed but also the high holding voltage can be realized due to the vertical current path through the NBL, as shown in Fig. 4.16.

In Fig. 4.17, the DC-measured I-V characteristics of the traditional SCR with NBL and modified SCR with NBL are quite different. Compared to the measured results in Fig. 4.16 and Fig. 4.17, the DC-measured holding voltage of the modified SCR with NBL is higher than the TLP-measured holding voltage. For the holding voltages discussed above, the DC-measured holding voltages greatly decrease due to the joule effect. Another explanation is provided in this work to investigate the mechanism of the extremely low holding voltage under normal circuit operation condition. The huge DC power occurring at the revered junction leads to large generation of electron-hole pairs. Such the large numbers of electron-hole pairs can easily promote the turn-on of the lateral SCR and greatly increase the avalanche multiplication factor. Therefore, the holding voltage is drastically decreased in the DC measurement results. In this work, the lateral conduction path of the modified SCR with NBL structure is blocked because the P+ diffusion is located in front of the N+

diffusion. In other words, the vertical conduction path is dominated under normal

easily triggered on than the lateral NPN transistor. However, it is necessary to further investigate the validity of the DC-measured holding voltage of 22V.

The TLU-measured results of the traditional SCR with NBL and the modified SCR with NBL are shown in Figs. 4.18-4.21. The latch-up immunity is further investigated by the TLU test. From the results, the positive or negative charging voltages of 195V or -215V stored in the capacitor in the modified SCR device with NBL are significantly higher than that of the traditional SCR device with NBL.

Therefore, the immunity against to the transient latch-up can be increased

(a)

(b)

Table 4.4 The measurement results of traditional and modified SCR devices.

(a)

(b)

Figure 4.18 The measured I-V waveforms (a) before transient trigger and (b) after transient trigger on the traditional SCR under TLU test with positive charging voltage.

(a)

(b)

Figure 4.19 The measured I-V waveforms (a) before transient trigger and (b) after transient trigger on the traditional SCR under TLU test with negative charging voltage.

(a)

(b)

Figure 4.20 The measured I-V waveforms (a) before transient trigger and (b) after transient trigger on the modified SCR under TLU test with positive charging voltage.

(a)

(b)

Figure 4.21 The measured I-V waveforms (a) before transient trigger and (b) after transient trigger on the modified SCR under TLU test with negative charging voltage.

4.2 Brief Summary

In this work, several single SCR structures with NBL implantation are investigated and verified in the 0.5-µm 16-V BCD process. The parasitic vertical conduction path can be developed by the NBL implantation. From the measured results, the DC-measured holding voltages of the single SCR structures with NBL implantation are measured to about 2V to 5V. Compared to the power supply voltage of 16V, such a low holding voltage of ESD protection devices can cause the latch-up issue under normal circuit operation condition. However, there are other advantages can be realized by inserting the NBL implantation. One is to increase the DC-measured holding current and the other is to increase the It2 current.

Under normal circuit operation condition, the current tends to flow in the lateral direction instead of the vertical direction in the SCR structure. In other words, the conduction of the lateral current path is dominated at the absence of the vertical conduction path. However, adding the NBL implantation on the SCR structure can develop the vertical path to divide some current away the dominated current path.

This implies that more current is needed to maintain the turn-on of the lateral conduction path and thus increase the DC-measured holding current.

Besides, adding the NBL implantation on the SCR structures can further improve the ESD performance significantly. This is because the NBL implantation can switch the current passage from the surface region to the bulk region during an ESD zapping, thus, avoiding the local heating and damage in the surface region.

When applying the NBL, the lateral and vertical current paths can be formed within the SCR device. However, it can not ensure the turn-on of the vertical SCR even though the NBL has been implanted. In this work, several methods to hinder the conduction of the lateral SCR structures and further promote the turn-on of the

extended anode and the modified SCR structure have been developed. In a summary, both the lateral SCR and the vertical SCR are competitive with each other. From the TLP measurement result, the lateral or vertical path within the parasitic SCR structure can be selected by controlling the turn-on capability of the both lateral SCR and the vertical SCR devices.

Chapter 5

Proposed Designs of Stacked SCR Devices with Efficient Trigger Circuits

5.1 Diode-Breakdown-Trigger SCR (DBTSCR) with Current Mirror