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Strained Silicon

Strained Silicon. Aaron Prager EE 666 April 21, 2005. Outline. Introduction CMOS Strained Silicon Why is it good? How does it work? How do we make it? Conclusions. CMOS. Complimentary Metal-Oxide-Semiconductor Basis for most computer chips We’re good at making CMOS!. Moore’s Law….

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Strained Silicon

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  1. Strained Silicon Aaron Prager EE 666 April 21, 2005

  2. Outline • Introduction • CMOS • Strained Silicon • Why is it good? • How does it work? • How do we make it? • Conclusions

  3. CMOS • Complimentary Metal-Oxide-Semiconductor • Basis for most computer chips • We’re good at making CMOS!

  4. Moore’s Law… • More and more transistors every year • Higher and higher speeds • CMOS can’t get much smaller • How can we continue innovating and moving faster?

  5. Strained Silicon • As gate length shrinks, mobility decreases • Increased doping to combat short channel effects • Method to increase mobility of electrons and holes in the channel of an FET • Tensile and compressive strain applied to channel

  6. Strained silicon • How does it work? • Basic idea: Change the lattice constant of material • Changes energy band structure!

  7. Strained Silicon So, what is actually happening?

  8. Strained Silicon: Electrons So, what is actually happening? • Electrical symmetry destroyed by strain • Four energy valleys go down in energy, two go up (in biaxal strain) • Vice versa in unaxial • INTERVALLY SCATTERING REDUCED! • Change in curvature • Reduction in effective mass!

  9. Strained Silicon: Holes How about the holes? Unaxial strain reduces effective mass Biaxial Strain splits LH and HH bands, reduces scattering

  10. Fabrication • Effect was noticed in early 1950’s • Generally an effect to be avoided • Why bother? • In recent years scaling has become more difficult • Idea revived at MIT in early 1990s

  11. Fabrication • Two types of strain • Compressive • Tensile • Two “directions” of strain • Unaxial • Biaxial

  12. Biaxial Fabrication • Biaxial techniques pioneered first • Method preferred by IBM though examined by all major semiconductor firms • Graded SixGe1-x layer grown on silicon substrate • Si Lattice constant = 5.4309Å • Ge Lattice constant = 5.6575Å • X~.9

  13. Biaxial Fabrication • Additional SixGe1-x grown on top of graded layer • Wafer is polished • Thin layer of silicon epitaxially grown on layer of SiGe • SiGe has larger lattice constant than Si (1% larger) • Strains the “x” and “y” directions • Layer must be thin to avoid relaxation • Pseudomorphic

  14. Biaxial Fabrication • Increases electron mobility • No great change in hole mobility • Has problems • Diffusion of Ge into Si • Dislocations • Cost (?)

  15. Biaxial Fabrication: IBM Gets Clever • First cleverness: SiGe on insulator, with silicon epitaxially grown on SiGe • Positive benefits of SOI (lower capacitances, faster switching, lower leakage) as well as benefits of strain • But wouldn’t it be good if we could do strained silicon on insulator without SiGe underlayer?

  16. Biaxial Fabrication: IBM Gets Clever

  17. Biaxial Strain • IBM proposing strained Germanium channel • Excellent electrical properties • Hard to grow insulator • Epitaxial growth makes it possible

  18. Unaxial Strain: Intel’s Baby • Problems with Biaxial strain • Reexamine the problem • Need strain in the channel of the MOSFET • Tensile strain in NMOS, Compressive in PMOS • Strain needed in direction from source to drain

  19. Unaxial Strain: Intel’s Baby • NMOS • Standard fabrication • Cap with SiNx • High temperature deposition, thermal expansion “pulls” channel • Unaxial Strain – Only one direction • 10% increase in Idsat

  20. Unaxial Strain: Intel’s Baby • PMOS • Remove source and drain • Selectively grow epi-SiGe in source and drain • Nickel Silicide grown on source, drain, gate • Improvements in Idsat of 25%-30%

  21. Unaxial Strain: Intel’s Baby • PMOS • In Intel fabrication method FET also covered by the SiNx layer • No major loss! • In general, strain showing compatibility with future technology

  22. Quantum Effects • Strain always a problem • Unwanted strain changes wave functions • Work always done to remove strain • Proposals to actually increase strain instead • Remove all energy valleys except one, meaning no other changes could occur

  23. Conclusions • Strain in Silicon can increase mobility in NMOS and PMOS FETs • Biaxial and Unaxial strain techniques developed • In use by major players

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