Cost-Effective Formation of Ti/Ni/Ti/Au Ohmic Contacts to n-type SiC Using SiO2 Encapsulation Layer during Phosphorous Implant Activation
Authors: Shimbori Atsushi , Huang Alex Q. 
Published: November 2022

The purpose of this research is to develop a cost-effective formation of ohmic contacts to n-type SiC through ion implantation and activation. Although Nitrogen is commonly used as an n-type dopant, especially for in-situ doping during epitaxial growth, Phosphorus was chosen as dopant for this work due to the following reasons:

Phosphorus has higher solubility than Nitrogen. Thus, it is easier to achieve higher concentration locally through ion implantation, which is needed for quality ohmic contact.

Phosphorous has a higher activation rate at lower activation temperature, which could be beneficial if the availability of furnace equipment and cap layer material is limited.

Phosphorus-implanted doping profile can be accurately detected through SIMS analysis against a constant in-situ doped nitrogen background doping of the n-type epi layer.

Phosphorus was implanted using the implantation series shown in Table 1. As indicated, the three multiple energy implants, with the highest energy of 200keV and the lowest energy of 50keV, will form a 200-nm-deep box profile against a n- epitaxial layer (0.4μm, 2×1017cm-3) used as a RESURF layer for device fabrication (see Figure 1 and 2). Obtaining a good box profile with high doping concentration is necessary in achieving quality ohmic contact with low contact resistance.
After ion implantation, the dopants must be thermally activated, and crystal damage must be recovered through annealing at high temperature between 1400~1700°C. The process of ion implantation and activation is shown in Figure 1. Traditionally, cap layers, such as pyrolyzed negative photoresist and AlN, have been used during implant activation to prevent surface deterioration. But using pyrolyzed negative photoresist as a cap layer requires a complex furnace setup, where inert gasses, such as Ar or N2, are needed to prevent the cap layer from oxidizing and evaporating. The deposition of a thick AlN layer requires a DC sputtering system where a 3-inch AlN sputtering target could cost over $500~. The AlN cap layer also requires KOH etch for removal after activation, which can also etch the SiC layer underneath. Because of the above reasons, we propose the use of 1µm thick SiO2 to encapsulate the front and back of implanted sample using PECVD deposition. The SiO2 encapsulation layer withstands temperature up to ~1500°C in air ambient, and the activation temperature and the ambient are set to these values for evaluation and analysis. As shown in Figure 3, if there is sufficient thickness (>1µm) of SiO2, the surface is protected from the deterioration caused by high temperature, and surface roughness is suppressed to the minimum as summarized in Table 2.
Ti/Ni/Ti/Au (20nm/90nm/5nm/120nm) were deposited using e-beam evaporation system and annealed at 1050℃ for 2min using Heatpulse 610 RTA (Rapid Thermal Annealing) system in N2 ambient. This unique metal contact stacks are effective in forming quality ohmic contacts even when oxidation during annealing is inevitable. It has been widely understood that only Ni layer is necessary to form nickel silicide during thermal formation of ohmic contacts, but when oxygen is present from incomplete inert gas ambient or poor vacuum conditions, contact layer surface starts to oxides, forming a green NiO (nickel oxide) layer. This unwanted layer significantly deteriorates current conduction. The Ti layer (5nm) over the Ni layer in the proposed metal stacks reacts with oxygen easily, preventing further oxidation inward and achieving quality ohmic contact through effective nickel silicide formation. Even when state-of-the-art semiconductor fabrication equipment is unavailable, the proposed ohmic metal contact stacks will still be effective.
Two-contact two-terminal patterns using the above ohmic metal stacks were initially fabricated for the confirmation of activation process. The plots in Figure 5 show the current conduction capability of evaluation pattern with an activation temperature of 1300°C and 1500°C, compared with a non-implanted epitaxial layer. As shown in the figure, with 1300°C annealing, the activation ratio is only ~20% and negative effects from the epi layer degradation dominates over carrier activation. Whereas, with 1500°C annealing, the activation ratio is roughly 70% and current conduction increases by a factor of 4 through the dopant activation process.
The specific contact resistance ρC is significantly improved through N+ implant with an obtained value of 2.461×10-5 Ω·cm2 from the TLM evaluation plot in Figure 6. While the N+ implant is effective in reducing contact resistance, the high temperature activation annealing still slightly deteriorates the conductivity of the drift layer, where increase in sheet resistance RS is observed.
The proposed ohmic metal stacks and SiO2 encapsulation enable cost-effective formation of ohmic contacts to n-type SiC where complex fabrication equipment setup is unavailable.

Figure 1