The Effects of Iron Contamination on Thin Oxide Breakdown - Experimental and Modeling
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THE EFFECTS OF IRON CONTAMINATION ON THIN OXIDE BREAKDOWN EXPERIMENTAL AND MODELING Worth B. Henley*, Lubek Jastrzebski*, and Nadim F. Haddad** *Center for Microelectronics Research, University of South Florida **IBM Corporation, Manassas, VA
Abstrac The effect of iron contamination in silicon on the properties of thermally grown thin oxides is studied through electrical modelling and experimental MOSDOT testing. Iron concentration is measured using a surface photovoltage / diffusion length technique. Failure mechanisms related to iron contamination are proposed. Contamination limits for various gate oxide thicknesses are defined. Experimental results show that reduction of oxide thickness from 20nm to 10nm requires a reduction in iron conntamination by 100 times.
Introuci Heavy metal (Fe, Cu, Ni, Cr) impurities in silicon can have adverse effects on electrical properties and yield of integrated circuits'. Heavy metal contaminates may be present in the starting silicon wafers or may be introduced at any of the various stages of IC processing through contaminated chemicals, corroded supply lines, wafer handling, etc. Typically, the most prevelant heavy metal contaminate is iron. Iron concentration can be determined non-destructively using diffusion length measurement procedures2. The effect of heavy metal impurities on silicon electrical properties depends on the form the impurity takes in the silicon crystal matrix3. Metallic impurities which remain dissolved within the silicon matrix create mid-bandgap trap centers which increase generation rate thereby leading to increased diffused junction leakage. Heavy metal impurities may also agglomerate and form distinct, separate phase, metal-silicide precipitates. For advanced VLSI process technologies, the problem of heavy metal silicide precipitation is the most critical yield aspect of heavy metal contamination. The problem is exacerbated as technology scaling reduces feature size and film thickness such that the precipitate defects are more pronounced in effect. Heavy metals introduced during semiconductor processing become dissolved and uniformly distributed in a silicon wafer during high temperature processing (i.e. gate oxidation) due to the inherit high diffusivity of transition metal impurities4. The solubility of heavy metals in silicon is very temperature dependent and relatively high at elevated temperatures. During cooling following a high temperature step, the heavy metal concentration will exceed the solubility limit and some amount of the heavy metals will precipitate. The ratio between the amount of precipitated heavy metals and the amount of dissolved heavy metals is characteristic for each type of heavy metal and process conditions5.e.
Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
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Iron impurities have been shown to form rod-like FeSi 2 precipitates during thermal processing7. Such precipitates occurring near the wafer surface can penetrate the Si-Si0 2 interfaces. This results in a local thinning of the oxide in the reg
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