Transcript Transistors and Layout 2

Topics
SCMOS scalable design rules.
 Reliability.
 Stick diagrams.

Modern VLSI Design 4e: Chapter 2
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MOSIS SCMOS design rules
Designed to scale across a wide range of
technologies.
 Designed to support multiple vendors.
 Designed for educational use.
 Ergo, fairly conservative.

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 and design rules
 is the size of a minimum feature.
 Specifying  particularizes the scalable
rules.
 Parasitics are generally not specified in
units

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Wires
6
metal 3
3
metal 2
3
metal 1
3
pdiff/ndiff
2
poly
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Transistors
2
3
2
3
1
5
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Vias

Types of via: metal1/diff, metal1/poly,
metal1/metal2.
4
4
1
2
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Metal 3 via
Type: metal3/metal2.
 Rules:

–
–
–
–
cut: 3 x 3
overlap by metal2: 1
minimum spacing: 3
minimum spacing to via1: 2
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Tub tie
4
1
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Spacings
Diffusion/diffusion: 3
 Poly/poly: 2
 Poly/diffusion: 1
 Via/via: 2
 Metal1/metal1: 3
 Metal2/metal2: 4
 Metal3/metal3: 4

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Overglass
Cut in passivation layer.
 Minimum bonding pad: 100 m.
 Pad overlap of glass opening: 6
 Minimum pad spacing to unrelated
metal2/3: 30
 Minimum pad spacing to unrelated metal1,
poly, active: 15

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Scmos VARIATIONS
SCMOS
SCMOS
submicron
SCMOS deep
Poly space
2
3
3
Active extension
beyond poly
3
3
4
Contact space
2
3
4
Via width
2
2
3
Metal 1 space
2
3
3
Metal 2 space
3
3
4
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Lithography for nanometer
processes


Interference causes
drawn features to be
distorted during
lithography.
Optical proximity
correction pre-distorts
masks so they create
the proper features
during lithography.
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3-D integration
3-D technology stacks multiple levels of
transistors and interconnect.
 Through-silicon-via (TSV) with die
stacking uses special via to connect between
separately fabricated chips.
 Multilayer buried structures build several
layers of devices on a substrate.

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Reliability



Failures happen early,
late in chip’s life.
Infant mortality is
caused by marginal
components.
Late failures are
caused by wear-out
(metal migration,
thermal, etc.).
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Mean-time-to-failure
MTF for metal wires = time required for
50% of wires to fail.
 Depends on current density:

–
–
–
–

proportional to j-n e Q/kT
j is current density
n is constant between 1 and 3
Q is diffusion activation energy
Can determine lifetime from MTTF.
Modern VLSI Design 4e: Chapter 2
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Traditional sources of
unreliability
Diffusions and junctions: crystal defects,
impurity precipitation, mask misalignment,
surface contamination.
 Oxides: Mobile ions, pinholes, interface
states, hot carriers, time-dependent
dielectric breakdown.
 Metalization: scratches/voids, mechanical
damage, non-ohmic contacts, step coverage.

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TDDB
Time-dependent dielectric breakdown: gate
voltages cause stress in gate oxides.
 More common as oxides become thinner.
 TDDB failure rate:

– MTTF = A 10 bE eEs/kt
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Hot carriers
Hot carrier has enough energy to jump from
silicon to oxide.
 Accumulated hot carriers create a space
charge that affects threshold voltage.

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NTBI
Negative bias temperature instability is
particular to pMOS devices.
 Threshold voltage, transconductance change
due to stresses.
 Can be reversed by applying a reverse bias
to the transistor.

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Electromigration and stress
migration
Degenerative failure for wires.
 Grains in metal have defects at grain surface
that cause electromigration.
 Stress migration caused by mechanical
stress.

– Can occur even with zero current.
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Soft errors
Caused by alpha particles.
 Packages contain small amounts of uranium
and thorium, which generate error-inducing
radiation.

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PVT

Borkar et al.:
variations in process,
supply voltage,
temperature are key
design challenges in
nanometer technology.
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PVT challenges
Process variations: channel length and
threshold significantly in nanometer
technologies.
 Supply voltage: non-ideal wires introduce
variations in supply across chip.
 Temperature: higher chip operating
temperatures degrade both transistors and
interconnect.

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On-chip temperature sensors
Temperature sensors are used to shut off
part or all of the chip to stop thermal
runaway.
 Use a pn junction from a parasitic bipolar
transistor.

– Can also use MOS transistor.
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Stick diagrams
A stick diagram is a cartoon of a layout.
 Does show all components/vias (except
possibly tub ties), relative placement.
 Does not show exact placement, transistor
sizes, wire lengths, wire widths, tub
boundaries.

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Stick layers
metal 3
metal 2
metal 1
poly
ndiff
pdiff
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Dynamic latch stick diagram
VDD
in
out
VSS
phi’
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phi
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Sticks design of multiplexer

Start with NAND gate:
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NAND sticks
VDD
a
out
b
VSS
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One-bit mux sticks
VDD
ai
bi
a
out
N1
(NAND)
b
a
out
select
select’
a
N1
(NAND)
b
out
N1
(NAND)
b
VSS
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3-bit mux sticks
select’
a2
b2
a1
b1
ai
bi
ai
bi
select
select’
m2(one-bit-mux)
select’
select
m2(one-bit-mux)
select’
a0
b0
select
ai
bi
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select
m2(one-bit-mux)
VDD
oi
VSS
o2
VDD
oi
VSS
o1
VDD
oi
VSS
o0
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Layout design and analysis tools
Layout editors are interactive tools.
 Design rule checkers are generally batch--identify DRC errors on the layout.
 Circuit extractors extract the netlist from the
layout.
 Connectivity verification systems (CVS)
compare extracted and original netlists.

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Automatic layout
Cell generators (macrocell generators)
create optimized layouts for ALUs, etc.
 Standard cell/sea-of-gates layout creates
layout from predesigned cells + custom
routing.

– Sea-of-gates allows routing over the cell.
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Standard cell layout
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routing area
routing area
routing area
routing area
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