Lecture #42
OUTLINE
• IC technology
• MOSFET fabrication process
• CMOS latch-up
Cross-sectional SEM view of
AMD Athlon 64 x2 processor
Reading: Chapter 4
courtesy of Chipworks
Spring 2007
EE130 Lecture 42, Slide 1
Die photo of
Intel Penryn processor
(Intel®CoreTM2 family)
Integrated Circuit Technology
“Planar” fabrication process:
Simultaneous fabrication of many “chips” on a wafer, each
comprising an integrated circuit (e.g. a microprocessor or
memory chip) containing millions or billions of transistors
300mm Si wafer
Method:
Sequentially lay down and pattern thin films of
semiconductors, metals and insulators.
Materials used in a basic CMOS integrated circuit:
• Si substrate – selectively doped in various regions
• SiO2 insulator
• Polycrystalline silicon – used for the gate electrodes
• Metal contacts and wiring
Spring 2007
EE130 Lecture 42, Slide 2
Formation of Insulating Films
•
The favored insulator is pure silicon dioxide (SiO2).
•
A SiO2 film can be formed by one of two methods:
1. Oxidation of Si at high temperature in O2 or steam ambient
2. Deposition of a silicon dioxide film
Applied Materials lowpressure chemical-vapor
deposition (CVD) chamber
ASM A412
batch
oxidation
furnace
Spring 2007
EE130 Lecture 42, Slide 3
Patterning the Layers
Planar processing consists of a sequence of
additive and subtractive steps with lateral patterning
oxidation
deposition
ion implantation
etching
lithography
Lithography refers to the process of transferring a pattern
to the surface of the wafer
Equipment, materials, and processes needed:
• A mask (for each layer to be patterned) with the desired pattern
• A light-sensitive material (called photoresist) covering the wafer so as
to receive the pattern
• A light source and method of projecting the image of the mask onto the
photoresist (“printer” or “projection stepper” or “projection scanner”)
• A method of “developing” the photoresist, that is selectively removing it
from the regions where it was exposed
Spring 2007
EE130 Lecture 42, Slide 4
Pattern Transfer by Etching
In order to transfer the photoresist pattern to an underlying film, we need a
“subtractive” process that removes the film, ideally with minimal change in
the pattern and with minimal removal of the underlying material(s)
 Selective etch processes (using plasma or aqueous chemistry)
have been developed for most IC materials
photoresist
First: pattern
photoresist
Si
We have exposed mask pattern,
and developed the resist
SiO2
Next: Etch oxide
oxide etchant …
photoresist is resistant.
etch stops on silicon
(“selective etchant”)
Last: strip
resist
only resist is attacked
Jargon for this entire sequence of process steps: “pattern using XX mask”
Spring 2007
EE130 Lecture 42, Slide 5
The Photo-Lithographic Process
optical
mask
Oxidation or thin-film deposition
photoresist
exposure
photoresist
removal (ashing)
optional
additional
process
step(s)
Spring 2007
photoresist coating
spin, rinse, dry
etch
EE130 Lecture 42, Slide 6
photoresist
develop
Adding Dopants into Si
Suppose we have a wafer of Si which is p-type and we want to
change the surface to n-type. The way in which this is done is by
ion implantation. Dopant ions are shot out of an “ion gun” called
an ion implanter, into the surface of the wafer.
As+ or P+ or B+ ions
+
Eaton HE3
High-Energy
Implanter,
showing the
ion beam
hitting the
end-station
+
+
+
+
+
SiO2
x
Si
Typical implant energies are in the range 1-200 keV. After the ion
implantation, the wafers are heated to a high temperature (>1000oC).
This “annealing” step heals the damage and causes the implanted
dopant atoms to move into substitutional lattice sites.
Spring 2007
EE130 Lecture 42, Slide 7
N-channel MOSFET
Schematic Cross-Sectional View
Layout (Top View)
4 lithography steps
are required:
1. active area
2. gate electrode
3. contacts
4. metal interconnects
channel
width, W
Spring 2007
EE130 Lecture
42, Slide
8
gate
length,
Lg
CMOS Technology
Both n-channel and p-channel MOSFETs are
fabricated on the same chip (VTp = -VTn )
• Primary advantage:
– Lower average power dissipation
• Ideally, in steady state either the NMOS or PMOS device
is off, so there is no DC current path between VDD & GND
• Disadvantages:
– More complex (expensive) process
– Latch-up problem
Spring 2007
EE130 Lecture 42, Slide 9
Need p-regions (for NMOS) and n-regions (for PMOS)
on the wafer surface, e.g.:
Single-well technology
• n-well must be deep enough
to avoid vertical punch-through
Twin-well technology
• Wells must be deep enough
to avoid vertical punch-through
Spring 2007
ND
n-well
NA
EE130 Lecture 42, Slide 10
p-substrate
NA
p-well
ND
n-well
p- or n-substrate
(lightly doped)
Modern CMOS Fabrication Process
p-type Silicon Substrate
Shallow Trench Isolation (STI) - oxide
p-type Silicon Substrate
p-type Silicon Substrate
Spring 2007
• A series of lithography, etch,
and fill steps are used to create
silicon mesas isolated by
silicon-dioxide
• Lithography and implant steps
are used to form the NMOS
and PMOS wells and to set the
channel doping levels
EE130 Lecture 42, Slide 11
• The thin gate dielectric layer
is formed
p-type Silicon Substrate
• Poly-Si is deposited and
patterned to form gate
electrodes
p-type Silicon Substrate
p-type Silicon Substrate
Spring 2007
• Lithography and implantation
are used to form NLDD and
PLDD regions
EE130 Lecture 42, Slide 12
p-type Silicon Substrate
• A series of steps is used to
form the deep source / drain
regions as well as body
contacts
• A series of steps is used to
encapsulate the devices and
form metal interconnections
between them.
p-type Silicon Substrate
Spring 2007
EE130 Lecture 42, Slide 13
Intel’s 65 nm CMOS Technology
NMOSFET
• Lg = 35 nm
• Tox = 1.2 nm
• Strained Si channel
NMOS: tensile capping layer
PMOS: epitaxial Si1-xGex embedded in S/D
PMOSFET
Spring 2007
EE130 Lecture 42, Slide 14
CMOS Inverter
Vin
VDD
VSS
Vout
n+
p+
p+
n+
n+
p+
SiO2
n-well
p-Si
VDD
Equivalent circuit:
Spring 2007
Vin
EE130 Lecture 42, Slide 15
Vout
CMOS Latchup
Coupled parasitic npn and pnp bipolar transistors:
If either BJT enters the active mode,
the SCR will enter into the forward
conducting mode (large current
flowing between VDD and GND) if
bnpnbpnp > 1
=> circuit burnout!
Latch-up is triggered by a transient increase in current, caused by
• transient currents (ionizing radiation, impact ionization, etc.)
• voltage transients
• e.g. negative voltage spikes which forward-bias the pn junction momentarily
Spring 2007
EE130 Lecture 42, Slide 16
How to Prevent CMOS Latchup
1. Reduce minority-carrier lifetimes in well/substrate
2. Use highly doped substrate or wells:
(a)
n-well
p epitaxial layer
p+-substrate
(b)
Spring 2007
Rsub
b npn
p-sub
Rwel
lb
“retrograde well”
pnp
n
n+
EE130 Lecture 42, Slide 17
IC Technology Trends
• Increasing # of levels of wiring (Cu interconnects)
Up to 8 levels of metal are used in ICs today.
Photo from IBM Microelectronics Gallery:
Colorized scanning-electron micrograph of
the copper interconnect layers, after removal
of the insulating layers by a chemical etch
• Increasing variety of materials
– high-k gate dielectric, metal gate, low-k intermetal dielectrics, etc.
• Continued scaling of MOSFETs toward 10 nm Lg:
– CMOSFETs with gate lengths below 20 nm have already been
demonstrated by leading semiconductor manufacturers.
 The most advanced transistor designs are based on UC-Berkeley
research (Prof’s Hu, King Liu, Bokor).
Spring 2007
EE130 Lecture 42, Slide 18