Materials Science Forum Vols. 645-648

Abstract: In this work, the surface, lattice and electrical properties of implanted 4H-SiC, GaN and ZnO, annealed by a novel ultra-fast microwave heating method, are compared to that of conventional annealing methods. In this new method, amplified and variable frequency microwaves from a signal generator are directly coupled to the semiconductor sample through a microwave head. Since, the microwaves are only absorbed by the sample, without heating of the ambient, ultra-high heating (> 2000°C/s) and cooling rates and very high (2100°C) annealing temperatures can be reached. For Al and P species implants into 4H-SiC, record low resistivity values were achieved with a lattice quality better than that of the virgin crystal. This annealing method improved the lattice quality of un-implanted region below the surface implanted region as well. Improved material characteristics were also obtained for GaN and ZnO.

Abstract: This paper reports on the electrical activation and structural analysis of Al implanted 4H-SiC. The evolution of the implant damage during high temperature (1650 – 1700 °C) annealing results in the presence of extended defects and precipitates, whose density and depth distribution in the implanted sheet was accurately studied for two different ion fluences (1.31014 and 1.31015 cm-2) by transmission electron microscopy. Furthermore, the profiles of electrically active Al were determined by scanning capacitance microscopy. Only a limited electrical activation (10%) was measured for both fluences in the samples annealed without a capping layer. The use of a graphite capping layer to protect the surface during annealing showed a beneficial effect, yielding both a reduced surface roughness and an increased electrical activation (20% for the highest fluence and 30% for the lowest one) with respect to samples annealed without the capping layer.

Abstract: We report on topographical, structural and electrical measurements of aluminum-implanted and annealed 4H-SiC epitaxial samples. The influence of heating-up and cooling-down temperature rates on the SiC surface roughness, the crystal volume reordering and the dopant electrical activation was particularly studied. A higher heating-rate was found to preserve the rms roughness for annealing temperatures lower than 1700°C, and to improve the sheet resistance whatever the annealing temperature due to a better dopant activation (except for 1600°C process, which induced a dark zone in the sample volume). A complete activation was calculated for an annealing at 1700°C during 30 minutes, with a ramp-up at 20°C/s. Rising the cooling-down rate appeared to increase the sheet resistance, probably due to a higher concentration of point defects in the implanted layer.

Abstract: The evolution of mechanical properties of helium-implanted 4H-SiC at room temperature has been mainly studied by nanoindentation tests. The curves of hardness and elastic modulus present a maximum at low levels of damage while a degradation of the mechanical properties is observed for high levels of damage. However, when the concentration of implanted ions exceeds 0.5 %, complex defects (helium-vacancy defects) become predominant which results in the increase of both the hardness and the modulus. Under high fluence of helium implantation tiny bubbles form and the amorphous transition is observed above a critical level of damage.

Abstract: A high dose impurity doping process for 4H-SiC crystals has been developed using electron cyclotron resonance (ECR) sputtered carbon cap film and ECR plasma ashing. ECR-sputtered carbon films are newly found crystalline carbon films of which the hardness is comparable to that of diamonds. Since this carbon film showed such a high thermal tolerance that the hardness did not change after 1900oC annealing, this carbon cap film worked well for suppressing roughening during annealing for aluminum-ion implanted 4H-SiC. Cap carbon film can be removed successfully by using high density ECR plasma ashing.

Abstract: Improved AlNi-based ohmic contacts to p-type 4H-SiC have been achieved using low energy ion (Al+)implantation, the addition of a thin Ti layer, and a novel two-step implant activation anneal process. AlNi/Au contacts with and without Ti were studied, which resulted in contact resistivities around 1.8x10-4 -cm2 and 2.0x10-3 -cm2 respectively. Even though these values were higher than those of the Ti/AlNi/W system, which was the focus of previous studies, the reduced anneal temperature (650 to 700°C) implies that Ti/AlNi/Au is a promising composite configuration. Cross-sectional TEM and EDX were used to investigate the interfacial structure of the contacts. One possible mechanism for the improved ohmic contact behavior is that the addition of Au and Ti resulted in a reduction barrier height.

Abstract: Nanocrystalline diamond (NCD) films were deposited using plasma-enhanced chemical vapor deposition. The NCD films were Boron-doped for p-type conductivity, yielding sheet resistances from 6.17x1011 to 522.5 /. Four different metals were deposited as Ohmic contacts and investigated for contact resistance and thermal stability. Contact and film annealing was performed under different atmospheric conditions with variable N2 content.

Abstract: The reliability of Ni2Si/n-SiC ohmic contacts with Au overlayer either without or with Ta-Si-N diffusion barrier was investigated after long-time aging in air at 400oC and rapid thermal annealing in Ar up to 800oC. It is shown that aging of the Au/Ni2Si/n-SiC contacts in air at 400oC resulted in complete degradation due to both oxygen penetration and interdiffusion/reaction processes. In contrast, only a small change in properties was detected on the contacts annealed in Ar at 800°C. The stability of both electrical and structural properties of Au/TaSiN/Ni2Si/n-SiC thermally stressed contacts at different conditions points out their superior thermal stability.

Abstract: In this paper a die-attachment technology for high temperature applications based on the Low Temperature Joining Technique (LTJT) is presented. The present challenge is to fit the thermal expansion as well as the mechanical properties of the die-attach layer to the characteristics of chip and substrate. While the classic LTJT is based on sintering a sub-micron silver paste at temperatures between 150°C and 300°C to bond an electronic device to a substrate, the modified procedure employs a powder mixture consisting of silver powder and special filling powder material. Type and amount of the filling material is dependent on the application and the used substrates. Considering a low thermal expansion and high electrical as well as thermal conductivity we chose SiC, TiC, and BN as filling materials in this work.

Abstract: The stability of Au wire connections to n-SiC/Ti ohmic contacts and to n-SiC/Ni ohmic contacts with top Au or Pt layers has been investigated. Long-term tests of the connections are performed in air at 400oC. Evaluation of electrical parameters, morphology and structure of the metallization as well as the strength of Au joint show stable Au wire bonds to the metallization with Ti-ohmic contacts.