Electrical Characteristics Of Amorphous Ni 36 W 64 Contacts On Silicon

and M -A. NicoletDivision of Engineering and Applied Science, California Institute of TechnologyMail Stop 116 -81, Pasadena, California 91125AbstractThe Schottky barrier heights of amorphous Ni36W64 film contacts on n -type and p -typesilicon are 0.65 eV and 0.45 eV, respectively. The barrier heights stay constant up to550 °C for a 30 min annealing in vacuum, but the interface, the leakage current, and theideality factors degrade at 450 °C. The contact resistivities of amorphous Ni -W films is(1.4 ± 0.3) 10 -6 Qcm2 on n +Si and (8.9 ± 0.2) 10 -7 Qcm2 on p +Si. The values remain stableafter vacuum annealing for 30 min up to 600 °C.IntroductionStable and reliable metallic contacts are of critical importance in the field of micro-electronics. The failure of devices during high- temperature processing or operation isusually caused by the deterioration of the contacts. Atomic interdiffusion and compoundformation are the main modes of degradation. Diffusion barriers can limit this degradation.Elemental metallic layers such as W2, compound metal layers such as Ti -W3, and interstitialalloy films of TiN and TaN, etc.4,5 have been investigated or applied widely as diffusionbarriers. A new class of films made of amorphous metallic alloys has recently become asubject of interesto -8 as diffusion barriers for metallization systems. The reason for thisinterest is that amorphous layers do not have grain boundaries or other extended crystallinedefects9 which are the main path of rapid atomic diffusion at moderate temperatures. Aninvestigation of multilayer metallizations on silicon indeed indicates that an amorphousFe37W63 layer prevents the direct interaction between the Si substrate and a 1000 A thickmetal overlayer during thermal annealing for 30 min at 650 °C10. The crystallizationtemperature (Tc) is the parameter that characterizes this metastability. A high Tc is anessential criterion for the selection of an amorphous metallic film as a diffusion barrier.But interfacial reactions can limit the lifetime of these diffusion barriers even below Tc.For example, it was found that amorphous Mo -Ni films have a high Tc (% 700 °C). But thisfact does not prevent the film from reacting with a Si substrate at 450 °C11.The purpose of this study is to examine the electrical properties and the thermal stabil-ity of co- sputtered amorphous Ni -W film contacts with a Si substrate. The crystal structureof Ni (f.c.c.) differs from that of W °(b.c.c.), and there is a large difference in atomicsize between Ni (1.24 A) and W (1.44 A). Both features aid the formation of an amorphousphase12. Because W reacts with Si at a higher temperature than Mo does, a higher stabilityand reaction temperature might be expected for Ni -W than for Mo- Ni7,8.Experimental ProcedureAmorphous Ni -W films were deposited in a radiofrequency sputtering system equipped witha planar magnetron cathode. A composite target was made of a pure W disk covered withnarrow strips of high purity Ni. To obtain a uniform composition and thickness, thesubstrate table was rotated perpendicularly to the Ni stripes in continuous fashion.Sputtering was carried out at 1°0 mTorr argon pressure and 400 W RF power, resulting in anaverage deposition rate of 45 A /min on a substrate. The composition of the amorphous Ni -Wfilms was established by backscattering spectrometry as being Ni 36W64. To determine thefilm thickness from the backscattering results, the density of the amorphous layer wasassumed to be a linear combination of the element atomic densities. The resulting filmthickness was in the order of 1000 R. The amorphous structure was confirmed by x -raydiffraction (Read camera).The barrier height of the amorphous Ni -W films on Si was determined by analyzingthe I(V) characteristics of circular Schottky diodes on 1 - 10 Qcm for n -type and 3 -8 Qcm