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3.3 Diffusion in Oxide and Silicides

  Besides monocrystalline and polycrystalline silicon, other materials like silicon dioxide and silicides are currently of interest for diffusion investigations [Lip89] [Hov89] [Lis95] .

Silicon dioxide is used as masking material or as diffusion barrier in multilayer device structures. Compared to single crystal silicon the diffusivities of impurities in oxide is extremely low for almost all common dopants. Chemical elements like He, Na, tex2html_wrap_inline5573 and tex2html_wrap_inline5575 are fast diffusers in oxides and hence are influencing the dopant diffusivity. Due to the void silicon dioxide structure and the variety of affecting parameters it is not possible to define exact diffusion coefficients for the different species. The diffusion coefficients depend on local process conditions and have to be investigated for specific problems. The diffusion problem itself is captured by the simplest approach, assuming Fickian diffusion without field enhancement and constant diffusion coefficients. Recently, List et al. published data for the diffusion coefficient of boron for multilayer experiments [Lis95] . They investigated the diffusion of boron across the gate oxide initiated by the excessively high doped tex2html_wrap_inline5577 -poly gate. A transient diffusivity effect for boron was observed, which could not be simulated with a constant diffusion coefficient. This suggests a step function for the oxide diffusivity tex2html_wrap_inline5579 as given by (3.3-1), where tex2html_wrap_inline5581 denotes the start value for the diffusivity and tex2html_wrap_inline5583 the end value.

  equation1261

Values for the diffusion coefficients of dopants within oxides can be found in [Won86] [Mat93a] . They postulated a diffusivity enhancement at early stages of diffusion coming from hydrogen. Hydrogen is able to influence the dopant diffusivity significantly, but it is not clear where the hydrogen is coming from. Unfortunately, short time annealing experiments are not useful to investigate the boron outdiffusion. Only a small amount of dopants is able to diffuse across the oxide because of the the low dopant diffusivity. The resulting dopant profiles would be to steep for SIMS measurements. But even this annoying outdiffusion affects the threshold voltage in conventional CMOS device technology.

For two decades metal silicides such as tex2html_wrap_inline5589 or tex2html_wrap_inline5591 are used to reduce sheet and contact resistance of polysilicon and diffusion regions. For thick polysilicon layers and deep junctions the silicidation process does not impact the active area of the devices. Indeed silicidation technology gained rapid acceptance because of its reliability. With decreasing device dimensions effects like dopant redistribution, anomalous silicon consumption, and enhanced diffusivity of dopants caused by the silicidation became important [Hov87] [Mae89] . Therefore, the dimensions of the silicide layers have been also reduced. Unfortunately, the thermal stability of thinner layers is rather poor compared to earlier technologies.

One of the most significant changes in silicide technology was to use silicides as dopant diffusion sources. Silicidation is performed before the junction formation. After fabrication of the metal silicide dopants were implanted into the silicide layer, followed by a thermal cycle, usually RTA, to diffuse out the dopants into the substrate [Hov89] . Processes like silicide grain growth, silicon consumption, silicon agglomeration, and diffusion of dopants are involved during junction formation. The transport of dopants within the silicide material is assumed to be based on grain boundary and bulk diffusion. The dopants can either diffuse towards the surface, where they accumulate and evaporate or towards the underlying substrate, where they form shallow junctions. The diffusivities of the dopants within silicide materials are in the same order of polysilicon diffusivities or even higher for arsenic and phosphorus. Boron however, shows quite contrary diffusion properties. While it is fast diffusing in tex2html_wrap_inline5593 , it is nearly immobile within tex2html_wrap_inline5589 , which is attributed to a stable alloy between the dopant and the metal silicide ( tex2html_wrap_inline5597 ). Due to the lack of understanding the details of diffusion within silicides, the diffusion of dopants is modeled by the Fickian approach neglecting more sophisticated processes like activation within the silicide. The influence of the silicidation on the diffusivity of the dopants on the underlying substrate material is modeled by inclusion of an enhancement factor, as given for arsenic in (3.3-2) and (3.3-3).

   eqnarray1274

tex2html_wrap_inline5599 denotes the well known arsenic diffusion coefficient for silicon, and tex2html_wrap_inline5601 is the enhanced diffusivity factor, which is applied for a limited time period onto the dopants diffusion [Osb93] .

As a drawback of the silicidation technology the roughness of the layer during thermal treatment is known. Due to agglomeration the interface between the substrate and the silicide as well as the surface of the silicide is very rough. The roughness depends on the silicide thickness and the applied thermal budget and has to be carefully optimized during processing to obtain low sheet resistances, without degrading junction performance. The high segregation coefficients or in the case of phosphorus the high surface evaporation can lead to degradation of dopants within the substrate caused by the silicide layer. Generally, silicide layers are fast diffusion pathes for silicon point defects and other chemical elements.

Due to the different linear thermal expansion coefficients of silicide materials compared to other semiconductor materials stress will be induced during thermal treatment. In the worst case the silicide layer looses its adhesion and bends off the underlying material layers, or the induced stress affects the electrical properties of the substrate.


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Next: 4 Simulation of Diffusion Up: 3 The Diffusion Process Previous: 3.2.4 Epitaxial Realignment of

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