Transcript Transistors and Layout 1

Topics
Basic fabrication steps.
 Transistor structures.
 Basic transistor behavior.
 Latch up.
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Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
Fabrication services
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Educational services:
–
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–
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U.S.: MOSIS
EC: EuroPractice
Taiwan: CIC
Japan: VDEC
Foundry = fabrication line for hire.
(A building equipped for the casting of metal or
glass, microsoft office dictionary)
– Foundries are major source of fab capacity today.
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
Wafers
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A wafer is a thin slice of semiconducting material, such
as a silicon crystal, upon which microcircuits are
constructed by doping (for example, diffusion or ion
implantation, etching, and deposition of various materials.
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Wafers are cut out of silicon boules
A boule is a single crystal silicone from which
wafers are cut using diamond saws.
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http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
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Fabrication Process
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Once the wafers are prepared, many process steps are necessary to
produce the desired semiconductor integrated circuit. In general, the
steps can be grouped into four areas:
•Front end processing (formation of transistors on silicon wafers)
•Back end processing (interconnection of transistors by metal wires)
•Test
•Packaging
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In semiconductor device fabrication, the various processing steps fall into four
general categories: deposition, removal, patterning, and modification of
electrical properties.
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http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
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Deposition
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Deposition is any process that grows, coats, or otherwise transfers a material
onto the wafer. Available technologies consist of physical vapor deposition
(PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD),
molecular beam epitaxy (MBE) and more recently, atomic layer deposition
(ALD) among others.
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http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
Removal or Etching Process
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Removal processes are any that remove material from the wafer either
in bulk or selective form and consist primarily of etch processes, both
wet etching and dry etching such as reactive ion etch (RIE). Chemical
mechanical planarization (CMP) is also a removal process used
between levels.
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http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
Modern VLSI Design 3e: Chapter 2
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Masking and Patterning
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Patterning covers the series of processes that shape or alter the existing shape
of the deposited materials and is generally referred to as lithography. For
example, in conventional lithography, the wafer is coated with a chemical
called a photoresist. The photoresist is exposed by a stepper, a machine that
focuses, aligns, and moves the mask, exposing select portions of the wafer to
short wavelength light. The unexposed regions are washed away by a
developer solution. After etching or other processing, the remaining
photoresist is removed by plasma ashing.
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Many modern chips have eight or more levels produced in over 300 sequenced
processing steps.
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http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
Modern VLSI Design 3e: Chapter 2
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Fabrication processes
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IC built on silicon substrate (mono crystal silicone):
– some structures diffused into substrate;
– other structures built on top of substrate.
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Substrate regions are doped with n-type and p-type impurities. (n+,p+
= heavily doped)
When silicon is doped, n-type impurities (5-valence electron elements
such as arsenic) charge silicon atoms with electrons, p-type impurities
(3- valence electrons such as boron) charge them with holes
Wires made of polycrystalline silicon (poly), and/or multiple layers of
aluminum (metal).
Silicon dioxide (SiO2) is insulator. (is grown over Si by heating Si in a
pure oxygen or water vapor atmosphere)
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Simple cross section
SiO2(insulator)
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:
A typical wafer is made out of extremely pure silicon that is
grown into mono-crystalline cylindrical ingots (boules) up to 12
in (300 mm) in diameter using the Czochralski process. These
ingots are then sliced into wafers about 0.75 mm thick and
polished to obtain a very regular and flat surface.
http://en.wikipedia.org/wiki/Fabrication_%28semiconductor%29
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Process steps
First place tubs to provide properly-doped
substrate for n-type, p-type transistors:
(Front-end processing)
p-tub
n-tub
substrate
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Process steps, cont’d.
Pattern polysilicon before diffusion regions:
poly
p-tub
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gate oxide
poly
p-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+
p-tub
p+
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Process steps, cont’d
Start adding metal layers: (Backend processing)
metal 1
metal 1
vias
poly
n+
p-tub
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n+
poly
p+
n-tub
p+
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Transistor structure
n-type transistor:
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0.25 micron transistor (Bell Labs)
gate oxide
silicide
source/drain
poly
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Transistor layout
n-type (tubs may vary):
L
w
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Electrical Transistor Model
Vgs: gate to source voltage
 Vds: drain to source voltage
 Ids: current flowing between drain and
source
 k’: transconductance > 0
 Vt: threshold voltage > 0 for n-type <0 for
p-type transistor.
 W/L: width to length ratio

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Drain current characteristics
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Drain current
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Linear region (Vds < Vgs - Vt):
– Id = k’ (W/L)[(Vgs - Vt)Vds - 0.5 Vds2]
– Not quite a linear relation between Id and Vds but the
quadratic term becomes more negligible than the linear
term as Vds approaches 0. This is typically the case with
the absolute value of the threshold Vt voltage remaining
close to 0.
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Saturation region (Vds >= Vgs - Vt):
– Id = 0.5k’ (W/L)(Vgs - Vt) 2
– Id remains constant over changes in Vds
– Increases with transconductance, channel width, and
decreases with channel length.
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
0.5 m transconductances
From a MOSIS process:
 n-type:
– kn’ = 73 A/V2
– Vtn = 0.7 V
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p-type:
– kp’ = 21 A/V2
– Vtp = -0.8 V
Modern VLSI Design 3e: Chapter 2
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Current through a transistor
(At saturation)
Example: Using 0.5 m transconductance parameter of 73
A/V2, threshold voltage of 0.7 volts, and SCMOS rules
(http://www.mosis.com/Technical/Designrules/scmos/scmosmain.html)
with W 3, L = 2 :
 Saturation current at Vgs = 2V:
Id = 0.5k’(W/L)(Vgs-Vt)2= 93 A
 Saturation current at Vgs = 5V:
Id = 1012 A ~ 1 mA
Modern VLSI Design 3e: Chapter 2
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Basic transistor parasitics
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There are myriad parasitics and parasitics models. The ones considered
here are the most widely-encountered parasitics.
Gate to substrate, also gate to source/drain.
Source/drain capacitance, resistance.
Cg
substrate
Modern VLSI Design 3e: 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.
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– Cgs = Col W (Col is the unit overlap capacitance
per m2, For small channel length, Col might
indirectly depend on L.)
– Gate/bulk overlap capacitance.
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
Latch-up
CMOS ICs have built-in undesirable parasitic
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.

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Silicon Controlled Rectifier(SCR)
Tyristor Circuit
anode
p
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
FB
n
RB
gate
FB
p
n cathode
http://en.wikipedia.org/wiki/Thyristor
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In normal mode, no current flows over the pnpn path when the middle
pn junction is reverse-biased. With the help of a gate pulse voltage, this
pn junction can be forced into its breakdown region, making it conduct
current. At that point, there will be a path of current from the anode to
the cathode with no resistance even after the gate voltage is withdrawn.
This is the basis for a high current from VDD to the ground (substrate)
in MOS transistors, called the latch up.
Modern VLSI Design 3e: Chapter 2
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Parasitic SCR
FB
Breakdown
V
Reverse voltage
breakdown
FB
circuit
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Rs and Rw control the bias
voltage on the green diodes
I-V behavior
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Parasitic SCR structure
When transistor on the right conducts, it turns on the
transistor on the left, and this in turn forces the first
transistor to draw more current, establishing a positive
feedback loop.
n
p
p
p
n
n
Solution: connect the n-tub to the VDD
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Solution to latch-up
Use tub ties to connect tub to power rail. Use
enough to create low-voltage connection.
Doping the tub at the
point of contact reduces
the resistance of contact,
and this makes it more
difficult for bipolar
transistor to turn on.
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR
Tub tie layout
n+
metal (VDD)
n-tub
You can learn more about latch up by downloading the article at
http://www.fairchildsemi.com/an/AN/AN-339.pdf#search=%22latch%20up%20problem%22
Modern VLSI Design 3e: Chapter 2
Copyright  1998, 2002 Prentice Hall PTR