2.2.3 Selective Epitaxial Growth Technique


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2.2.3 Selective Epitaxial Growth Technique

Uniaxial stress can be introduced into the channel of a MOSFET by selectively growing a local epitaxial film in the source and drain regions of the transistor. Large uniaxial stress can be created using this technique, originating from the difference of the lattice constants in the channel region and source/drain region. For this purpose, first the source and drain regions are etched, creating a recess area. Afterwards, this recess is filled by epitaxy. Alternatively, the epitaxial film can be directly grown on top of source and drain without previously etching these regions [Chidambaram06].

The level of stress depends on the thickness of the epitaxial layer and the lattice constant mismatch. Epitaxially grown SiGe is used to induce uniaxial compressive stress in p-channel MOSFETs [Thompson04,Eneman05,Horstmann05,Ouyang05,Bai04,Ouyang05,Zhang05]. In the same manner, tensile stress can be induced in the n-channel MOSFET by using Si $_{1-x}$ C $_{x}$ stressors with molefractions around 1 % [Ang04]. Si $_{0.99}$ C $_{0.01}$ was found to induce the same amount of stress as Si $_{0.75}$ Ge $_{0.25}$ [Ang05]. It was recently reported that for n-channel MOSFETs the implementation of the Si $_{0.987}$ C $_{0.013}$ source/drain regions provide significant drive current enhancement of up to 50% at a gate length of 50 nm [Chui07].

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E. Ungersboeck: Advanced Modelling Aspects of Modern Strained CMOS Technology