Download Introduction to FinFet

Introduction to FinFet
Haiying Zhao
What does FinFet look like
Gate
Source
Drain
3D view of FinFET
3D view of multi-fin
FinFET
What does FinFet look like
Moore’s law and scaling theory
Ideal scaling:
Reduce W,L by a factor of a
Reduce the threshold voltage and supply voltage by a factor of a
Increasing all of the doping levels by a
(W,L,tox,VDD,VTH, etc, are scaled down by a factor a)
For a ideal square-law device, Id is reduced by a, but gm and intrinsic gain
Gm* ro remain the same.
As scaling into submicron region, Short Channel effects prevent further
scaling.
Short Channel Effects:DIBL
DIBL: drain induced barrier lowering. DIBL = d(Vth)/d(Vds)
Barrier lowering increases as channel length is reduced, even at zero applied drain bias,
because the source and drain form pn junction with the body, and so have associated built-in
depletion layers associated with them that become significant partners in charge balance at
short channel lengths, even with no reverse bias applied to increase depletion width
Short Channel Effects: Subthreshold swing
Biasing a nmos in subthreshold resgion, Vgs < Vth, Vds is large enough.
Id = u Cd W/L (KT/q)^2 (exp(Vgs – Vth)/M))
Cd is capacitance of the depletion layer under gate.
M = (1 + Cd/Cox)*KT/q
Id = f(Vgs-Vth)
To turn off the transistor, How much reduction of (Vgs-Vth) could lead to a small
enough Id.
Subthreshold Swing S = d(Vgs)/d(log(Id)) = 2.3 KT/q( 1+ Cd/Cox)
The smaller S is , the better it is.
Bad Subthreshold Swing will result in higher off-state current if the Vgs applied
to turn off the transistor is the same.
Short Channel Effects: Velocity Saturation
V = u E ( E is small enough)
V = Vsat ( E is strong enough)
As Vgs increases , the drain current saturates well before pinch-off occurs.
Short Channel Effects:DIBL
DIBL: drain induced barrier lowering. DIBL = d(Vth)/d(Vds)
Barrier lowering increases as channel length is reduced, even at zero applied drain bias,
because the source and drain form pn junction with the body, and so have associated built-in
depletion layers associated with them that become significant partners in charge balance at
short channel lengths, even with no reverse bias applied to increase depletion width
Conclusion
To reduce short channel effects, we need to reduce Xd(channel depletion
layer thickness), Xj( Junction depletion width),Xox (oxide layer
thickness under gate).
The scale length of bulk MOSFET is an indication of Lg. Lg>
Dealing with Short Channel Effects in bulk MOSFET
1.Increasing body doping concentration
2.Using halo implant
High doping density results in:
Lower carrier mobility;
high tunneling effect which increases off-state currents;
Larger depletion capacitors leading to high subthreshold swing which
increases off-state currents;
Larger parasitic capacitance, Cgd, Cds.
Dealing with Short Channel Effects in Fully depleted
Silicon on Insulator (SOI)
Use ultra-thin film (tsi is small) as the conducting body, depletion layer
is confined in the film.( Xd<= tsi).
Eliminate the junction parasitic capacitors.
Cuff off the leakage current path from drain to substrate.
From FD/SOI to FinFET
Bend up the gate and narrow the gate. Fin width = 2* film thickness
The effect body thickness is reduced by 2. Xd can be regarded as
Fin width /2. To obtain good control of SCE, Leff > 1.5*Wfin ( Fin
width).
Finfet can operate at two mode, single gate and double gate.
FinFet characteristics
Lg = 15nm
Some values:
Threshold Voltage = 0.196 V
Subthreshold Slope = 72 mV/decade
Off Current = 70 A/m
Lg = 30nm
DIBL = 64.67 mV/V
Approximate dc I-V equations?
Square law?
One way is using nth power law to computer the FinFet current.
FinFet: Challenges or Opportunities
Carrier mobility:
Lightly doped or undoped fin body increases carrier mobility.
Short channel length enables velocity overshoot, which increases
mobility.
Low Vth decreases the vertical electric field ,which increases carrier
mobility.
FinFet: Challenges or Opportunities
Tunneling effects:
Gate to channel tunneling,
Band to band tunneling at
PN junction
FinFet: Challenges or Opportunities
Parasitic resistance: a raised source/drain structure can be used to reduce
the parasitic resistance.
However, the overlap capacitance is increased.
Prasitic resistance is the main adverse factor which prevents finfets’
application, which leads to lower speed and high noise.
rg
rd
Cgd
Gate
Cgs
ri
+
vgs’
Cds
rds
gm*vgs’
rs
Source
Drain
FinFet: big advantages
1.
2.
3.
Having excellent control of short channel effects in submicron
regime and making transistors still scalable. Due to this reason, the
small- length transistor can have a larger intrinsic gain compared to
the bulk counterpart.
Much Lower off-state current compared to bulk counterpart.
Promising matching behavior.
Applications
1. Low power design in digital circuit, such as
RAM, because of its low off-state current.
2. Power amplifier or other application in analog
area which requires good linearity.