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Lecture 1 Designing CMOS gates
Lecture outline
• CMOS design overview
• Skills developed and knowledge gained from the course • Tool handling skills: Cadence Electronic Design Automation (EDA) • Theoretical insights – estimations with pencil and paper
• The design flow – textbook MIPS example
• CMOS Fabrication
• Designing CMOS logic gates using MOSFET switches
• Iterative Logic Arrays (ILAs)
• Adder designs – custom design vs. synthesis from VHDL
• Summary
2016 Integrated Circuit Design 2
Design flow
Simulation
Extraction
DRC
Schematic
Layout
LVS
Post Lay Mod
DRC = Design Rule Checker
LVS = Layout Versus Schematic
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Gajski Y-chart
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Block Diagram (Textbook MIPS example)
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Floor planning
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MIPS Layout
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Datapath layout
Bitslice 7
Bitslice 6
Bitslice 5
Bitslice 4
Bitslice 3
Bitslice 2
Bitslice 1
Flo
p w
ord
slic
e
add
er w
ord
slic
e
mu
x w
ord
slic
e
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Custom design vs. synthesis from VHDL • 8-bit implementations of MIPS from Patterson & Hennessy
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Standard cells, 130 nm CMOS, 7 metal layers Custom design, 0.6 um CMOS, 1.5x1.5 mm die
Example: synthesized 32-bit ALU layout
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CMOS Fabrication
• CMOS transistors are fabricated on silicon wafer
• Lithography process similar to printing press
• On each step, different materials are deposited or etched
• Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process
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Inverter Cross-section
• Typically use p-type substrate for nMOS transistors
• Requires n-well for body of pMOS transistors
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n+
p substrate
p+
n well
A
YGND VDD
n+ p+
SiO2
n+ diffusion
p+ diffusion
polysilicon
metal1
nMOS transistor pMOS transistor
Well and substrate taps
• Substrate must be tied to GND and n-well to VDD
• Metal to lightly-doped semiconductor forms poor connection called Shottky Diode
• Use heavily doped well and substrate contacts/taps
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n+
p substrate
p+
n well
A
YGND V
DD
n+p+
substrate tapwell
tap
n+ p+
Inverter Mask Set
• Transistors and wires are defined by masks
• Cross-section taken along dashed line
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GND VDD
Y
A
substrate tap well tap
nMOS transistor pMOS transistor
Inverter mask set and fabrication
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P-type silicon substrate
N-well
Active areas P+ select
Poly gate
Contact cuts
Metal wires
Fabrication
• Chips are built in huge factories called fabs
• Contain clean rooms as large as football fields
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Courtesy of IBM Corporation. Unauthorized use not permitted.
Back to the Y-chart
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Transistors
Designing gates with switches
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N-switch
A A
P-switch
A A
nMOSFET pMOSFET
ON when input A is HIGH ON when input A is LOW
Designing gates with switches
2016 Integrated Circuit Design 18:2
N-switch
A A
P-switch
A A
nMOSFET pMOSFET
ON when input A is HIGH OFF when input A is HIGH
Designing gates with switches
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N-switch
A A
P-switch
A A
nMOSFET pMOSFET
OFF when input A is LOW ON when input A is LOW
Designing gates: AOI22
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Inputs: A, B, C, D
pMOS pull-up
network
nMOS pull-down network
VDD
VSS
Y
Y = AB + CD
&
&
≥1
AB
C D
Y = AB + CD
Designing gates: AOI22
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Y = AB + CDB
A
D
C
pMOS pull-up
network
Inputs: A, B, C, D
VDD
VSS
Y
&
&
≥1
AB
C D
Designing gates: AOI22
2016 Integrated Circuit Design 19:3
B
A
D
C
pMOS pull-up
network
Inputs: A, B, C, D
VDD
VSS
Y
According to de Morgan´s theorem
Y = AB + CD
Y = A + B C + D
&
&
≥1
AB
C D
Designing gates: AOI22
Y = A + B C + D
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B
A
D
C
A
C
B
D
VDD
VSS
Y
Y = AB + CD
&
&
≥1
AB
C D
Designing gates: AOI22
2016 Integrated Circuit Design 19:5
B
A
D
C
A
C
B
D
VDD
VSS
Y
Y = AB + CD
Y = A + B C + D
&
&
≥1
AB
C D
Simple CMOS gates
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Y A B A B Y AB A B
Exercise: AOI31, OAI22
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End of CMOS design lecture!
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Q & A?