Thermal Stability of A1-Pt Thin Films/GaAs for Self-Aligning Gate Contacts
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Thermal Stability of Al-Pt Thin Films/GaAs for SelfAligning Gate Contacts
G.D. Wilk*, B. Blanpain*, J.O. Olowolafe*, J.W. Mayer* and L.R. Zheng** *Departmentof MaterialsScience and Engineering, Bard Hall, Cornell University, Ithaca,NY 14853 "**CorporateResearch Laboratories, Eastman Kodak Company, Rochester, NY 14650-2011
Abstract Several compositions of Al-Pt thin films have been co-evaporated on GaAs substrates to study the stability of the alloys at high-temperature anneals. The Al concentration in the alloys ranges from 45 at.% to 70 at.%, and we show that the films meet thermal stability requirements imposed by GaAs selfaligning gate technology for compositions between AIPt and A12Pt. Introduction Self-aligning gates offer many advantages in the manufacturing process of MESFETs (metal-semiconductor field effect transistors), and much research has been done on different metallizations to find a suitable gate material. The major difficulty with self-aligning gate contacts on GaAs substrates is the necessity for the metal to withstand a post-implantation anneal ol7 900oC for 20 seconds, and adequate adhesion between the metal and substrate must also be maintained during the process. Metallizations thus far researched have been based on refractory metals, alloys of refractory metals or alloys of refractory metals with Si. Although some of these materials show desirable qualities, they do not exhibit reproducible thermal stability at the required temperature, and the search is still open for a better gate material [1]. Sands et al. [2] has shown AlNi to be epitaxial on GaAs, and found it to be significantly more stable than noble metals, which react at very low temperatures [3,4]. Although the A1Ni films exhibit increased stability over other materials, they cannot endure the postimplantation anneal. We selected the Al-Pt system because it is thermodynamically more stable than Al-Ni alloys (AHj(AINi) = -59 kJ/gmatom, AHf(A1Pt) = -100 kJ/gmatom) [5]. We have determined a composition region where the Al-Pt alloys meet the thermal stability requirements imposed by selfaligning gate technology on GaAs MESFETs.
Mat. Res. Soc. Symp. Proc. Vol. 184. @1990 Materials Research Society
232
Exrerimental Samples were prepared on semi-insulating GaAs substrates, coated on the back with reactive sputtered Si 3N 4 (50 nm thick). We simultaneously coevaporated the films on to Si0 2 and GaAs substrates, using Si0 2 as control samples and to facilitate the Rutherford Backscattering (RBS) analysis of the alloy composition. Before the deposition, the GaAs substrates were cleaned in a HCI:H 20 (1:1) for 5 minutes, rinsed in H 20 for 1 minute, etched in NH4 OH:H 20 (1:10) for 15 seconds and finally dried with nitrogen. The Al-Pt thin films were co-evaporated in a cryo-pumped dual electron-gun deposition system with a base pressure below 1 x 10-7 Torr. The deposition rate for Pt was fixed at 0.15 nm/sec and the Al deposition rate was adjusted according to the desired composition. Four different compositions were deposited ranging fr
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