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TiB2 and ZrB2 are members of the larger group of interstitial compounds which the transition metals form with boron, carbon, and nitrogen. Such materials are of significant interest for microelectronic application owing to a combination of low resistivity and outstanding diffusion barrier properties. We report here studies on the application of these diborides as diffusion barriers for both Si and GaAs based device technology. rf diode sputtered thin film ZrB2 has an annealed resistivity of 25 μΩ cm, rivaling that of its most widely employed silicide competitor, TiSi2 (18 μΩ cm). In addition, in experiments with bilevel structures consisting of first-level ZrB2 and second-level Al, no evidence of reaction has been observed in RBS spectra for samples heat treated in N2 at 625 °C for 2 h. These compounds thus appear to offer the prospect of serving the dual role of low resistivity contact metal as well as a self-diffusion barrier in multilevel contact structures. Recent work on the application of these materials for GaAs contact technology is even more significant. Alloyed Ohmic contacts to n-GaAs, a traditionally difficult technological problem, have been fabricated in which the Ohmic metallization (Ni/Ge/Au) in contact with the GaAs substrate is separated from the overlying thick Au upper contact by an intervening e-beam deposited TiB2 layer. The resultant structure shows improved electrical stability on aging at 350 °C to those of a more conventional nature prepared similarly, but employing a Ni film in place of the TiB2. This improved stability is attributed to the effectiveness of the diboride as a diffusion barrier to the in-migration of Au, thus preventing further modification of the Ohmic contact metallurgy obtained on alloying.

A study to elucidate the role of processing-induced changes in Si, subjected to ion-beam etching has been made. It is shown that these changes can be related to the primary ion beam used in ion-beam etching. Using ESR, trivalently bonded Si has been shown to be present. Fe and Cr have been found to be the main contaminants. An annealing study revealed that the damage can be annealed out at relatively high temperatures.

In order to estimate the depth of penetration of Ohmic metals into n-GaAs during the alloying process, marker layer samples fabricated by molecular-beam epitaxy were utilized. The samples consisted of n-GaAs substrates onto which were grown a 500-nm undoped GaAs buffer layer, followed by 20 layers, each 50 nm thick, alternately doped with Si at levels of 1×1016 and 1×1018 atoms/cm3. Ohmic metals consisting of Ni/Ge/Au/Ni/Au or Ni/Ge/Au/TiB2/Au were evaporated onto the marker layer samples and alloyed on a strip heater. Secondary ion mass spectroscopy (SIMS) depth profiling showed that metal penetration extended as much as 0.5 μm into the GaAs. In addition, significant redistribution of the marker layer Si was observed, and is assumed to result from a vacancy enhanced diffusion process. In a SIMS depth profile performed from the back side of the wafer no dispersion of the metal depth penetration was observed, suggesting that differences in penetration depth of the metals observed in front side profiles are the result of ion beam mixing, surface roughness, and/or inhomogeneities within the alloyed region.

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