Ultra-Thick Metal Ohmic Contact Fabrication Using Surface Activated Bonding
Authors: Liang Jianbo , Furuna Katuya , Matsubara Moeko , Dhamrin Marwan , Nishio Yositaka , Shigekawa Naoteru 
Published: February 2020

Abstract:
We successfully bonded aluminum foils to p+-Si substrates to fabricate Al-foil/p+-Si junctions by surface activated bonding (SAB). The electrical properties of the junctions were investigated by measuring their current voltage (I-V) characteristics. The I-V characteristics of the junctions showed excellent linearity properties. The interface resistance of the junctions was found to be 7.8 Ω·cm2. Furthermore, the resistance decreased with increasing annealing temperature and decreased to 2.4 Ω·cm2 after the junction annealing at 300 °C. These results demonstrated that thick metal ohmic contact in devices could be realized by SAB.

Introduction:
It is well known that parasitic resistance substantially degrades the overall performance of the electronic and optical devices in the form of contact resistance [1]. The major loss of performance is usually due to high resistance metal-semiconductor “ohmic” contacts. Therefore, in order to attain optimum device performance, the minimization of the contact resistance is absolutely necessary. The primary factors determining contact resistance are carrier concentration, semiconductor surface preparation and cleaning, and contact metal work functions hence Schottky barrier height [2]. In addition, it was reported that the ohmic contact resistance decreased with increasing the metal thickness [3]. However, the deposition of thick metal is difficult to obtain because it would take a large quantity of time and the production cost by the conventional coating method such as Electron-Beam-Evaporation and Sputter-Deposition. Junctions with different materials in lattice constants, thermal expansion coefficients, and crystal structures could be fabricated by SAB [4]. Furthermore, we previously fabricated Al-foil/p-Si Schottky diodes and investigated their electrical properties [5]. It was found that the reverse leakage currents of those diodes were decreased and their ideality factors were improved by annealing them at high temperatures. In this work, we demonstrated the potential application of metal foils as thick ohmic contacts on semiconductor substrates by applying the SAB for bonding Al foils to Si substrates.

Experiments:
B-doped (100) p-Si substrates and aluminum foils (Al) with a thickness of 38 μm (it is commercially available) were used for the bonding experiment. The resistivity and carrier concentrations were estimated to be 0.0034 Ω·cm and 2.64 × 1019 cm-3 respectively, by the hall measurements at room temperature. The Al/Ni/Au multilayers were evaporated on the back surfaces of p+-Si substrates prior to the bonding, and the ohmic contacts of p+-Si substrates were achieved by rapid thermal annealing at 400 °C for 1 min in N2 gas ambient. Al foils were attached to the p+-Si substrates by using SAB [4], so that Al-foil/p+-Si junctions were obtained. After the bonding, a 4-by-6 mesa array was fabricated on the Al foils by using Al wet etching for 12 h. The wide and length of the mesas were all 1.4 mm. I-V measurements were performed using an Agilent B2902A Precision Measurement Unit at room temperature.

Results:
Figure 1 shows the I-V characteristics of Al-foil/p+-Si junctions without and with annealing at 100, 200, 300, 400 °C measured at room temperature. We found that, in the bias voltage range for measurements, the I-V characteristics of all the junctions showed linearity. The interface resistance were found to be 7.8 Ω·cm2 for the junctions without annealing, by least-square fitting at approximately 0 V. Furthermore, the interface resistance decreases with increasing annealing temperature. It was found that the interface resistance was obtained to be 2.4 Ω·cm2 after the junction annealing at 300 °C. The interface resistance should be further decreased by optimizing the condition of SAB process and increasing the annealing temperature. These results indicate that thick electrodes with a thickness of several-tem micrometers are obtained by apply SAB.
Fig. 1. I-V characteristics of Al-foil/p+-Si junctions without and with annealing at 100, 200, 300, and 400 °C measured at room temperature.

References

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[3] R. S. Chen, C. C. Tang, W. C. Shen, and Y. S. Huang, Nanotechnology, 25, 415706 (2014)
[4] J. Liang, T. Miyazaki, M. Morimoto, S. Nishida, N. Watanabe, and N. Shigekawa, Appl. Phys. Express 6, 021801 (2013).
[5] K. Furuna, J. Liang, M. Matsubara, M. Dhamrin, Y. Nishio, and N. Shigekawa, be presented at IMFEDK 2016

Figure 1