With the decrease of the process size, the thickness of the oxide layer of the bulk silicon MOS device is also thinner. Therefore, the threshold voltage drift caused by the total dose effect can be ignored. The leakage of the source / leakage region caused by the total dose effect and the leakage of the field oxygen can only be reinforced by the layout design. Some specific single particle effects can also be reinforced by the means of layout design.
1.1 Strengthening method of total dose effect layout
First, the structure of the selectable device layout is the structure of a ring gate, as an example of the NMOS tube, as shown in Figure 7. In Figure 7 (a), the D terminal represents the drain end area of the device. The S terminal represents the source end area of the device, G is the gate of the NMOS tube, the black block is the contact hole, and the periphery is the protection ring injected by P+. The layout structure, eliminates the parasitic original MOS devices at the edges of the tube, the MOS device is not endogenous / drain leakage path between end and joined the P+ protection ring; after the NMOS between different electronic devices due to leakage caused by the total dose effect of field oxide layer under the inverted result, can to the role of absorption. The longitudinal section of the section is shown in Figure 7 (b). From the profile, we can see that because of the isolation of the grid, the side parasitic tube is eliminated between the source / drain of the device, and the leakage path caused by the total dose effect is eliminated.
Though the ring gate structure can improve the leakage of MOS tube under the total dose irradiation condition, the W/L ratio of MOS tube is greatly restricted and the area is very expensive after adopting the ring gate. The minimum W/L ratio of a MOS device in a ring gate is 4:1, and it is almost impossible to use this structure to achieve a small proportion or an inverted MOS tube. When the inverse MOS tube is encountered in the design of anti radiation layout, the layout structure, such as figure 8, can be used. In this structure, the gate and gate oxide are also used to isolate the source and drain ends of the MOS tube, eliminating the existing edge parasitic tube, thus eliminating the leakage between the source and drain ends of the device. The P+ ring is also used to isolate the devices from the surrounding devices, ensuring that there is no leakage between the different devices under the total dose radiation. Figure 9 is a MOS tube reinforcement structure similar to an inverted proportional pipe. In the strengthened cell structure, in order to avoid the leakage in the field caused by the total dose effect, a similar PMOS tube structure is adopted to isolate the leakage path between the units. The principle is shown in Figure 10. This structure adds gate control structure in the presence of oxygen. When grid negative voltage is applied, positive charge is absorbed from the substrate, thus absorbing the electrons in the leakage channel caused by radiation, so that the leakage channel is isolated from the region with positive charges. Compared with the P+ active region surrounding ring isolation structure is traditional, the design not only eliminates the active region between N+ and P+ active region minimum spacing required by process size restrictions, save the unit area, negative charge pump series can also produce negative voltage by adjusting the output voltage, and thus more negative, in response to the amount of leakage due to the different radiation dose caused by different.
1.2 Layout reinforcement of single particle Kun effect (Quenching)
With the shrinking process size, the effect of single event effect on devices will not only be limited to a single node, but will also share charge between neighboring nodes. The single particle effect charge sharing mechanism is the Kun tilting effect (Quenching). For example, in the design of NAND gate, or gate logic, often two series MOS version of the picture tube structure as shown in Figure 11. The circuit made by this layout is affected by the single particle effect and affects the active region of the two MOS tubes, as shown in Figure 12.
In order to reduce the existence of this sharing mechanism, two MOS layout structures in series can be replaced by the structure of Figure 13. Under the influence of single event effect, the layout structure isolates the shared active region of two MOS pipes, thus eliminating the existence of charge sharing mechanism. As shown in Figure 14, the reliability of the device is improved.