Strained Silicon

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
• Were good at making CMOS!

4
Moores Law

• More and more transistors every year
• Higher and higher speeds
• CMOS cant 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 1950s
• 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 wouldnt 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 Intels 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 Intels 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 Intels 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 Intels 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