Download Transistors and Layout 1

Topics
Basic fabrication steps.
 Transistor structures.
 Basic transistor behavior.
 Latch up.

FPGA-Based System Design: Chapter 2
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Fabrication processes

IC built on silicon substrate:
– some structures diffused into substrate;
– other structures built on top of substrate.



Substrate regions are doped with n-type and ptype impurities. (n+ = heavily doped)
Wires made of polycrystalline silicon (poly),
multiple layers of aluminum/copper (metal).
Silicon dioxide (SiO2) is insulator.
FPGA-Based System Design: Chapter 2
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Simple cross section
SiO2
metal3
metal2
metal1
transistor
via
poly
n+
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p+
n+
substrate
substrate
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Photolithography
Mask patterns are put on wafer using photosensitive material:
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Process steps
First place tubs to provide properly-doped
substrate for n-type, p-type transistors:
p-tub
n-tub
substrate
FPGA-Based System Design: Chapter 2
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Process steps, cont’d.
Pattern polysilicon before diffusion regions:
poly
p-tub
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gate oxide
poly
n-tub
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Process steps, cont’d
Add diffusions, performing self-masking:
poly
n+
p-tub
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poly
n+
p+
n-tub
p+
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Process steps, cont’d
Start adding metal layers:
metal 1
metal 1
vias
poly
n+
p-tub
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n+
poly
p+
n-tub
p+
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Level 2 metal

Polish SiO2 before adding metal 2:
metal 2
metal 1
metal 1
vias
poly
n+
p-tub
FPGA-Based System Design: Chapter 2
n+
poly
p+
p-tub
p+
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Transistor structure
n-type transistor:
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Transistor layout
n-type (tubs may vary):
L
w
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Drain current characteristics
FPGA-Based System Design: Chapter 2
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Drain current

Linear region (Vds < Vgs - Vt):
– Id = k’ (W/L)(Vgs - Vt)(Vds - 0.5 Vds2)

Saturation region (Vds >= Vgs - Vt):
– Id = 0.5k’ (W/L)(Vgs - Vt) 2
FPGA-Based System Design: Chapter 2
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90 nm transconductances
Typical parameters:
 n-type:
– kn’ = 13 A/V2
– Vtn = 0.14 V

p-type:
– kp’ = 7 A/V2
– Vtp = -0.21 V
FPGA-Based System Design: Chapter 2
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Current through a transistor
Use 90 nm parameters. Let W/L = 3/2.
Measure at boundary between linear and
saturation regions.
 Vgs = 0.25V:
Id = 0.5k’(W/L)(Vgs-Vt)2= 0.12 A

Vgs = 1V:
Id = 7.2 A
FPGA-Based System Design: Chapter 2
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Basic transistor parasitics
Gate to substrate, also gate to source/drain.
 Source/drain capacitance, resistance.

FPGA-Based System Design: Chapter 2
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Basic transistor parasitics, cont’d
Gate capacitance Cg. Determined by active
area.
 Source/drain overlap capacitances Cgs, Cgd.
Determined by source/gate and drain/gate
overlaps. Independent of transistor L.

– Cgs = Col W

Gate/bulk overlap capacitance.
FPGA-Based System Design: Chapter 2
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Latch-up
CMOS ICs have parastic silicon-controlled
rectifiers (SCRs).
 When powered up, SCRs can turn on,
creating low-resistance path from power to
ground. Current can destroy chip.
 Early CMOS problem. Can be solved with
proper circuit/layout structures.

FPGA-Based System Design: Chapter 2
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Parasitic SCR
circuit
FPGA-Based System Design: Chapter 2
I-V behavior
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Parasitic SCR structure
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Solution to latch-up
Use tub ties to connect tub to power rail. Use
enough to create low-voltage connection.
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