Novel body contact structure for partially depleted SOI devices

1. Proposal of new structure
As shown in Figure 2, this paper proposes a new body contact technology. This method uses local SIMOX technology to form a thin oxide layer close to the Si surface under the source and drain of the transistor, and uses shallow junction diffusion of the source and drain to form a lateral body lead structure. On this basis, the Si film thickness is appropriately increased to reduce the body lead resistance. Compared with the previous method, this method has the advantages of smaller body-source and body-drain parasitic capacitance, completely eliminating the back gate effect, the body lead resistance decreases with the increase of the device width, and the body resistance can be reduced with the increase of the Si film thickness without increasing the parasitic capacitance. Therefore, the device can more effectively suppress the floating body effect. Moreover, in order to form a local buried oxide layer, this method only adds a mask to the process, and the other process flows are consistent with the standard SOI CMOS process, so this method has good process compatibility.

This structure can be realized by using low-energy, low-dose local SIMOX technology. In order to prevent the formation of a BOX layer under the channel of the device, a SiO2 mask is used to cover the device during oxygen ion implantation. The mask uses RIE (reactive ion etching). The energy and dosage of the implantation are determined according to the depth and thickness of the local buried oxide layer formed. After the implantation is completed, a high-temperature annealing is performed in an Ar+0.5% O2 atmosphere for several hours to form a local buried oxide layer. The experimental results of Y. M. Dong and P. He et al. verified the feasibility of the local SIMOX technology in the process. The microstructure of the sample was observed using a transmission electron microscope. The BOX layer under the source and drain was very complete. The port of the BOX layer was aligned with the polycrystalline Si gate, and the spacing was slightly greater than the length of the gate. The surface of the entire single-crystalline Si is very flat. The source and drain regions are not raised due to the formation of the BOX layer, nor are they reduced by oxidation during the annealing process. Table 1 shows the main process parameters of the new structure device, where: Tox is the gate oxide thickness; TSi is the Si film thickness; Tbox is the buried oxide layer thickness; Tsdbox is the buried oxide layer thickness under the source and drain; Nch is the channel doping concentration; Nsub is the substrate doping concentration; Ldrawn is the channel length; Wdrawn is the channel width; Xj is the source and drain junction depth.

2 Simulation results and discussion
The device was simulated using the ISE-TCAD simulator and the simulation results were discussed. Body contact can suppress the floating body effect to a certain extent. The effect of body contact is also related to the contact position, device size and process. If the body contact effect is not good, the holes generated by the impact ionization of the drain junction will still accumulate in the body region, which will increase the hole concentration in the body region, increase the potential in the body region, reduce the threshold voltage, and thus increase the leakage current. Figure 3 shows the output characteristics, hole concentration at the tangent and transfer characteristics of the device for the floating body device, T-type gate-body contact structure and the new body contact structure proposed in this paper. The three structures have the same process conditions. As can be seen from Figure 3, the structure proposed in this paper has the lowest hole concentration in the body region, the highest threshold voltage, and no kink effect, which successfully suppresses the generation of the floating body effect.

[page] Figure 4 is a comparison of the output characteristics of the T-type gate contact and the new body contact structure as the device width changes. It can be clearly seen from the figure that when the device width is large, the leakage current characteristics of the T-type gate change more dramatically, and the kink effect is obvious, while the new structure does not have the kink effect. This is because as the device width increases, the H-type gate body lead resistance increases, and the trigger voltage of the kink effect gradually decreases. The new structure adopts a lateral body lead structure, and the body resistance of this structure decreases as the device width increases. Therefore, when the device width is large, the structure has a significant effect in suppressing the floating body effect.

It can be seen that the size of the device's body lead resistance is crucial to the floating body effect. In order to effectively suppress the floating body effect, a smaller device body resistance is necessary. C. F. Edwards et al. reported the first-order approximate calculation formula for body contact resistance:

Where: Weff is the effective channel width; Leff is the effective channel length; NA is the channel doping concentration; up is the carrier mobility; TSi is the Si film thickness; ε0 and εSi are the vacuum dielectric constant and relative dielectric constant, respectively. It can be seen from formula (1) that the body resistance Rb is inversely proportional to the Si film thickness TSi. Increasing the Si film thickness can reduce the body resistance. However, the source and drain ends of SOI devices are usually diffused into the buried oxide layer. Increasing the Si film thickness will increase the contact area between the source and drain ends of the device and the body region, resulting in an increase in the body parasitic capacitance, which will affect the device performance. The increase in parasitic capacitance will also prolong the body discharge time, which is not conducive to suppressing the floating body effect. Moreover, a larger source-drain junction depth may cause a punch-through effect.
The body contact structure proposed in this paper can solve this contradiction. The structure uses a local oxide layer generated by low-energy, low-dose oxygen injection annealing under the source and drain, and uses source-drain shallow junction diffusion. The source-drain area is small, the body parasitic capacitance is relatively small, and the parasitic capacitance does not increase with the increase of Si film thickness. Figure 5 shows the effect of film thickness on the hole extraction speed RbCb in the body region. As can be seen from the figure, as the thickness of the device increases, the RbCb delay of the H-type gate tends to saturation, while the delay of the new structure decreases as the thickness of the Si film increases. This is consistent with the results of the previous analysis. It shows that as the width of the device increases, the body resistance Rb of the H-type gate structure decreases, but at the same time, the body capacitance Cb increases, and the increase in Cb is consistent with the decrease in wind, so that RbCb tends to saturation. For the new body contact structure, while the resistance Rb decreases with the increase in Si film thickness, the body capacitance Cb does not change. Therefore, RbCb gradually decreases as the Si film increases. The above discussion results show that the structure can reduce the body extraction resistance by appropriately increasing the Si film thickness without increasing the parasitic capacitance, thereby better suppressing the floating body effect. It should be noted that if the Si film of this device is relatively thin, due to the use of a lateral body lead-out structure, the junction depth and the space occupied by the local buried oxide layer will result in a narrow body lead-out channel, resulting in a larger body resistance, which is not desirable. Therefore, in practical applications, the Si film thickness of the new structure device must be large enough. Experimental data show that the body resistance of the new structure device with a Si film thickness of 250 nm is comparable to that of the conventional device with a Si film thickness of 200 nm. This shows that under the same other process parameters, the advantage of the new structure device will only be obvious when the Si film thickness is greater than 250 nm. In addition, in the case of a thicker Si film, it is possible to consider using reverse doping technology to increase the impurity concentration in the body region and further reduce the body resistance. This requires considering the impact of the process on the floating body effect, which is beyond the scope of this article. The impact of the process on the floating body effect will be further studied in the future.

3 Conclusions
This paper proposes a body contact structure. Compared with other body contact technologies, this method has a small body lead resistance and parasitic capacitance, and the body lead effect is not affected by the device width. And without increasing the parasitic capacitance, the body lead resistance can be reduced by appropriately increasing the Si film thickness, thereby better suppressing the floating body effect. In addition, since the shallow junction diffusion of the source and drain does not reach the BOX layer of SOI, a back-gate-opened channel will not be formed. Therefore, this structure does not have a back-gate effect.