Enhancing Thermal Stability of Nickel-Germanide Alloy up to 600 °C By Using Metal Passivation
Authors: Jau Yu-Hau , Chou Chen-Han , Tsai Yi-He , Lin Yu-Hsien , Chien Chao-Hsin 
Published: February 2020

I. Background/ Objectives

For continuous MOSFET scaling, Ge is one of the most promising candidates due to its higher mobility than Si. But, Ge has lower activated carrier concentration that will induce larger source/drain resistance because of lower sloid solubility and density of state. Hence, metal source/drain with remarkably low resistance is attractive for high performance Ge MOSFET. Even so, poor thermal stability of alloy is a critical problem in process integration. In the previous studies [1]-[3], Ni Germanide (NiGe) shows an enough low resistivity, low formation temperature and feasibility for self-aligned fabrication. However, NiGe through high temperature (>500 °C) show a fatal agglomeration phenomenon that badly degrades the junction performance. In this study, we successfully demonstrate that NiGe with Ti or TiN passivation can suppress the degradation of NiGe alloy and depict excellent thermal stability up to 600 °C. An on/off current ratio of approximately 4x105and low leakage current are shown for the NiGe contact with covering Ti and/or TiN are obtained by 600 °C alloy formation.

II.
Experiment Details

Fig. 1 shows the process flow and device structure. n-Ge (100) substrates with a resistivity of 0.04 ~ 0.6 Ω-cm were cleaned by cyclic diluted HF (1:20) and DI water to remove the native oxide. After the clean process, a 420 nm SiO2 film was first deposited by plasma-enhanced chemical vapor deposition (PECVD) for isolation. The contact window was defined by using photolithography, and the subsequent oxide etch was accomplished by using a buffer oxide etchant (BOE). Multi-layer metal deposition was accomplished without reaking the vacuum in sputter chamber, and metal lift-off process was performed. Post-deposition annealing (PDA) conditions at 200/300/400/500/600 °C were employed for 60 s in N2ambient by rapid thermal annealing (RTA). Finally, the metal electrodes were carried out by metal sputter.

III.
Experiment Results

Fig. 2(a) shows the I-V characteristics for all junctions formed 600 °C. We find that NiGe without cover shows serious agglomeration, which can be clearly observed by the Atomic Force Microscopy (AFM) image shown in the insert of Fig. 2(a). Compared to the Ni case, Ni/TiN or Ni/Ti/TiN cases show no degradation in off-current and enhanced on-current up to near one order of magnitude.
Fig. 2(b) shows the electrical characteristics of on-/off-current for varied PDA conditions. Ni case is obviously degraded as the temperature increases. Compared to Ni case, Ni/TiN and Ni/Ti/TiN cases can successfully suppress the junction leakage and improve the thermal stability of NiGe. The electrical data reveal that Ni/Ti/TiN contact with 600 °C PDA shows the best on-current (approximately 4.4x102 A/cm2) and off-current (approximately 1x10-3 A/cm2). On-off ratio of Ni case is only of approximately 3x102, but Ni/TiN and Ni/Ti/TiN cases are of approximately 4x105. The current ratio can get the three orders enhancement on high temperature annealing at 600 °C. Fig. 3 shows the X-ray Photoelectron Spectroscopy (XPS) spectra of the Ni/Ti/TiN case, and we can see Ni has already diffused into Ge but Ti show much less diffusion. So, the junction is still a NiGe/Ge one.

IV.
Conclusions

In summary, we have successfully eased the high temperature degradation for NiGe by using TiN or Ti metal passivation. The high on-off current ratio (approximately 4x105) was arrived by performing Ti/TiN on Ni under high temperature. It is very straightforward to extend the NiGe application and enlarge the process window for high performance Ge device.

References

[1] Qingchun ZHANG et al., “Formation and Thermal Stability of Nickel Germanide on Germanium Substrate,” J.J. Appl. Phys., Vol. 44, No. 45, 2005, pp. L1389–L1391.
[2] Mengrao Tang, Wei Huang et al., “Thermal Stability of Nickel Germanide Formed on Tensile-Strained Ge Epilayer on Si Substrate,” IEEE Elec. Dev. Lett., Vol. 31, No. 8, Aug. 2010.
[3] Chawanda et al., “Thermal annealing behavior of platinum, nickel and titanium Schottky barrier diodes on n-Ge (100),” J. Alloys Compd., 492 (2010) 649-655.

Figure 1