Lecture 04 wafer technology & basics of cmos

Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
ICE 4010: MICRO ELECTRO
MECHANICAL SYSTEMS (MEMS)
Lecture #04
Wafer Technology & Basics of CMOS
Dr. S. Meenatchi Sundaram
Email: meenasundar@gmail.com
1

Wafer Technology
2Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Making wafers is a closely guarded secret and it is possibly
even more difficult to see a wafer production than a single Si
crystal production.
First, wafers must all be made to exceedingly tight geometric
specifications. Not only must the diameter and the thickness be
precisely what they ought to be, but the flatness is constrained
to about 1 m.
This means that the polished surface deviates at most about 1
m from an ideally flat reference plane.
And this is not just true for one wafer, but for all 10,000 or so
produced daily in one factory. The number of Si wafers sold in
2001 is about 100,000,000 or roughly 300,000 a day! Only
tightly controlled processes with plenty of know-how and
expensive equipment will assure these specifications. The
following picture gives an impression of the first step of a
many-step polishing procedure.

Wafer Technology

Wafer Technology
Diameter
(mm/in)
Thickness
(mm)
100/4 0.5
150/6 0.75
200/8 1
300/12 0.75

Wafer Orientation
Wafers are grown on crystals that have a regular crystal
structures.
When they are sliced from the crystal, the surface is aligned in
one of several relative directions, known as the orientationorientationorientationorientation.
This is also referred to as the growth plane of the crystalline
silicon.
The orientation is important for the electronic properties of the
wafer. Ion implantation depths depend on the orientation and
it will affect the paths for transport.
The different planes have different arrangements of atoms and
lattices so it will affect the way the electricity travels in the
circuit.
The orientations of silicon wafers are classified using Miller
indices. These indices include such descriptions as (100), (111),
and (110).

Wafer Orientation
x
y
z
(100) x
y
z
(110) x
y
z
(111)

Wafer Flats
Wafers had flats, and the flats tell you two things:
o The doping type of the wafer (n- or p-type)
o The orientation of the wafer: {100} or {111}
While this is trivial information, consider: All wafers, whatever
doping type or crystal orientation, look exactly the same!

CMOS Transistors
8
• CMOS is also sometimes referred to as complementary-
symmetry metal–oxide–semiconductor (or COS-MOS).
• The words "complementary-symmetry" refer to the fact that the
typical design style with CMOS uses complementary and
symmetrical pairs of p-type and n-type metal oxide
semiconductor field effect transistors (MOSFETs) for logic
functions.
• Two important characteristics of CMOS devices are high noise
immunity and low static power consumption.
• Since one transistor of the pair is always off, the series
combination draws significant power only momentarily during
switching between on and off states.

CMOS Transistors
9
• Consequently, CMOS devices do not produce as much waste
heat as other forms of logic, for example transistor–transistor
logic (TTL) or NMOS logic, which normally have some standing
current even when not changing state.
• CMOS also allows a high density of logic functions on a chip.
• It was primarily for this reason that CMOS became the most used
technology to be implemented in VLSI chips.
• The phrase "metal–oxide–semiconductor" is a reference to the
physical structure of certain field-effect transistors, having a
metal gate electrode placed on top of an oxide insulator, which in
turn is on top of a semiconductor material.
• Aluminium was once used but now the material is polysilicon.
Other metal gates have made a comeback with the advent of
high-k dielectric materials in the CMOS process.

nMOS Transistors
10
• Four terminals: gate, source, drain, body
• Gate – oxide – body stack looks like a capacitor
– Gate and body are conductors
– SiO2 (oxide) is a very good insulator
– Called metal – oxide – semiconductor (MOS) capacitor
n+
p
GateSource Drain
bulk Si
SiO2
Polysilicon
n+

nMOS Operation
11
• Body is commonly tied to ground (0 V)
• When the gate is at a low voltage:
– P-type body is at low voltage
– Source-body and drain-body diodes are OFF
– No current flows, transistor is OFF
Dr. S. Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n+
p
GateSource Drain
bulk Si
SiO2
Polysilicon
n+
D
0
S

nMOS Operation Contd…
12
• When the gate is at a high voltage:
– Positive charge on gate of MOS capacitor
– Negative charge attracted to body
– Inserts a channel under gate to n-type
– Now current can flow through n-type silicon from source
through channel to drain, transistor is ON
n+
p
GateSource Drain
bulk Si
SiO2
Polysilicon
n+
D
1
S

pMOS Operation
13
• Similar, but doping and voltages reversed
– Body tied to high voltage (VDD)
– Gate low: transistor ON
– Gate high: transistor OFF
– Bubble indicates inverted behavior
SiO2
n
GateSource Drain
bulk Si
Polysilicon
p+ p+

Power Supply Voltage
14
• GND = 0 V
• In 1980’s, VDD = 5V
• VDD has decreased in modern processes due to scaling
– High VDD would damage modern tiny transistors
– Lower VDD saves power (Dynamic power is proportional to
C.VDD
2.f.a)
• VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, …

Transistors as Switches
15
• We can view MOS transistors as electrically controlled
switches
• Voltage at gate controls path from source to drain
g
s
d
g = 0
s
d
g = 1
s
d
g
s
d
s
d
s
d
nMOS
pMOS
OFF
ON
ON
OFF

CMOS Inverter
16Dr. S. Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
A Y
0
1
VDD
A Y
GND
A Y

CMOS Inverter
A Y
0 1
1 0
VDD
A=0 Y=1
GND
OFF
ON
A Y

CMOS Inverter
A Y
0 1
1 0
VDD
A=1 Y=0
GND
ON
OFF
A Y

Lecture 04 wafer technology & basics of cmos

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