4-1 Summary
This thesis studies the ρc extraction of the CBKR and mTLM methods by both simulation and experiment. 3-D simulation of device characteristics was performed to evaluate the extraction accuracy of the ρc, and several parameters were discussed.
Then, test structures were realized by using the STI process, and the experimental data were shown and compared with the simulation.
First, 3-D simulation is employed. For the CBKR method, the accuracy of the ρc
extraction could be enhanced by scaling down the dimensions of the test structures, while would be degraded due to the process limitation. Several parameters are investigated: the contact size (Ac), the process tolerance (δ), the corner-rounded contact hole, and the recessed contact interface. As the Ac decreases, the parasitic resistance (Rp) decreases, and hence the extraction accuracy increases. It is noticed that only 10 % error exists as ρc = 1×10-8 Ω-cm2 as the Ac reduces to 10 nm×10 nm.
The δ has less influence on the extraction accuracy, which is different from the fact presented in 2-D simulation, since the relative smaller contact area makes the parasitic resistance have no influence on the parasitic term in the ρce. Besides, as the Ac is shrunk, it is expected that corner rounding would affect the accuracy of the ρc
extraction. The influence of corner rounding is investigated by using circular contacts, and it is observed that corner rounding makes a large difference, i.e., increasing 30%
error, to the ρc extraction, which means that the CBKR method would become erroneous as the contact areas decrease. The recessed contact is another issue. The ρce
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would decrease with the recession depth for higher ρc, while would increase for lower ρc. Also, the Ac dependence is observed in the recessed contact cases. As the Ac
reduces, the recession has more influence on extracting the ρc, which means that the recessed contact would become more critical to the ρc extraction as devices scale down.
Next, the mTLM method is also analyzed by simulation. The dimensions of test structures is designed and optimized at first. Then the extraction accuracy of the mTLM method is compared with that of the CBKR method. Moreover, several parameters are discussed. Since the mTLM method is designed for a self-aligned process, it would be immune from the δ and the corner-rounding contact. In addition, due to its sensitivity to the process, the variation of the dopant concentration in semiconductors and the variation of the tapered sidewall angle of the active region were both analyzed by statistic. The recessed contact was also investigated. In this thesis, with a micro-process and Lc = 2 μm the mTLM structures could provide an accurate ρc extraction, and the extraction error is only few percent. Compared with the CBKR structure with Ac = 50×50 nm2, the mTLM method shows a better accuracy at lower ρc regime, because the requirement of a larger ratio of Lc and LT would be reached as lower ρc and thus an accurate extraction would be achieved for the mTLM method. Then, this thesis studied the dependence on the variation of the dopant concentration and the variation of tapered sidewall angles of the active region according to the statistic. It is observed that there is a strong dependence on the variation of the dopant concentration, i.e., the ρce would vary four orders of magnitude when the surface concentration has only 5 % variation. On the other hand, there is a relative slight dependence on the variation of tapered sidewall angles of the active region, which shows 50 % variation when a 2 % variation exists for the tapered sidewall angle. Finally, as the recessed contact is considered, the ρc is overestimated
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with higher ρc because the current tends to flow into the bottom contact, but underestimated with lower ρc because the side contact dominated the current flow and hence the ρc extraction.
Then, test structures of the CBKR and the mTLM methods were fabricated in an identical process flow and then their extraction results were discussed with the simulation. In order to define the active region explicitly, shallow trench isolation (STI) is utilized. The CBKR method is easier to extract the ρc, but the parasitic resistance is about to result in inevitable extraction error. It could be observed that the noticeable Ac dependence that the ρce would increase with the Ac and the less δ dependence is shown in this thesis, which is consistent with the simulation results.
The lowest ρce obtained by the CBKR method is 6.2×10-8 Ω-cm2. As for the mTLM method, the primitive data were disordered. However, for the mTLM method, its sensitivity to the process variation could be reduced and its extraction accuracy could be enhanced by means of averaging data according to the statistic. The ρce obtained by the mTLM method are 2.9×10-7 Ω-cm2 with Lc = 2 μm, and 9.8×10-7 Ω-cm2 with Lc = 1 μm. Unfortunately, for both methods, the recessed contact interface would exist and affect the ρce. Thus, it is difficult to quantify its influence since not only its contribution to the ρce is complicated contribution but also there are too many parameters affect the extraction as well.
In conclusion, this thesis discussed the CBKR and the mTLM method to extract the ρc by simulation and experiment. The CBKR structure with a smaller contact area due to the devices scale down would obtain more accurate results in theory, while the parasitic resistance would still limit the accuracy. On the other hand, the self-aligned mTLM structure proposed in this thesis is realized by using the STI process. Its sensitivity to the process variation could be diminished by averaging data. Therefore, the TLM method could be more accurate and is promising for ρc extraction.
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Nevertheless, too many parameters would affect the ρc extraction, and within those factors, the recessed contact interface would cause a complicated ρc extraction for both the CBKR and mTLM methods. It would still be a critical issue to the accuracy of the ρc extraction.