Local Thinning Induced Less Oxide Breakdown in MOS Structures Due to Lateral Non-Uniformity Effect
Authors: Lin Huang-Hsuan , Hwu Jenn-Gwo 
Published: February 2020

The downscaling of gate oxide thickness leads to increasing importance of issues on oxide degradation characteristics due to larger operating oxide field and direct tunneling current. For gate oxide thinner than 4.0 nm, the direct tunneling becomes dominant mechanism for carrier transport in MOS capacitors. In this thickness regime, the voltage drop and electric field in oxide layer can be significantly altered by the magnitude of direct tunneling current, and the oxide field does not increase monotonically with decreasing oxide thickness. In addition, the lateral non-uniformity (LNU) effect is a great concern for MOS with ultrathin gate oxide, which may lead to unexpected electrical behavior. In this work, we demonstrate that in certain oxide thickness range (2.5－3.5 nm), the degradation behavior can be altered by introducing LNU in oxide thickness to reduce the effective oxide field in p-type MOSCAP.

Two Al/SiO2/p-Si structured MOSCAPs are fabricated for constant voltage stress (CVS) tests. One of the samples (i.e., sample w/ LNU) has partially roughened silicon surface to form non-uniform SiO2 layer as shown in Fig. 1, while another sample (i.e., sample w/o LNU) has not. Fig. 2 and 3 show the I-V characteristics measured after various CVS ranging from +1 V to +4 V for 1000 seconds sequentially for samples with 2.9 and 3.2 nm oxide thickness, respectively. For 2.9 nm-oxide samples, slight increase in inversion current is observed with increasing stress voltage, but the accumulation current maintains even after +4 V CVS. For 3.2 nm-oxide samples, on the contrary, an apparent increase in accumulation current is observed in the sample w/o LNU after +1 V CVS as shown in Fig. 3 (a). Fig. 4 (a) and (b) plot the current-time (I-t) measurements for various stress voltages. It is clearly shown that the breakdown occurs during +1 V stress in the sample w/o LNU. The results are non-intuitive since that the oxide electric field is expected to decrease with thicker oxide, which results in smaller stress field. In addition, the sample w/ LNU appears to possess better oxide reliability compared to the sample w/o LNU in this experiment. For further analysis, the TCAD device simulation tools are applied to obtain the oxide electric field in MOSCAP with various oxide thicknesses as shown in Fig. 5 (a). Two different cases are considered in the simulations, the black square stands for the case including the direct tunneling model (DT) and the red circle stands for the case without DT model. For the case without DT, the voltage drop in oxide layer is approximately fixed for every oxide thickness, which results in inverse-proportionality of oxide electric field to oxide thickness since Eox = Vox/dox
. In realistic device, the leakage current by direct tunneling significantly affects the electrical characteristics of the device. For thinner oxide, the high tunneling probability results in more expansion in depletion region and larger silicon band bending, and the oxide voltage drop varies accordingly. As shown in Fig. 5 (b), the depletion width expands in thinner oxide layer, and the expansion is most pronounced for oxide thickness between 3.5 nm and 2.5 nm, leading to the drastic reduction in oxide electric field as shown in Fig. 5 (a). This explains the breakdown behavior in 3.2 nm (w/o LNU) since there exists almost twofold increase in oxide electric field in 3.2 nm-oxide sample compared to 2.9 nm-oxide sample. The strong field transition region (2.5 nm－3.5 nm) also gives an explanation to the reduced degradation behavior in the sample w/ LNU. Fig. 6 illustrates MOSCAP with LNU in oxide layer. For non-uniform oxide, the depletion region expands non-uniformly due to various local tunneling probability. From macroscopic viewpoint, the device can be regarded as a sample with thinner effective oxide thickness. Therefore, the effective electric field shifts toward the lower field region [Fig. 5 (a)].

 In summary, the field transition region gives rise to unique breakdown behavior for p-type MOSCAP with oxide thickness between 2.5 nm to 3.5 nm. It is demonstrated that by introducing LNU in oxide thickness, the degradation behavior can be reduced due to lower sustaining field in local region. It is worth noticing that the LNU brings up undesired interface traps and leakage current increase, which must also be carefully considered.

Figure 1