Download Undergraduate Admissions & College of Engineering

Course Overview
ECE/ChE 4752: Microelectronics
Processing Laboratory
Gary S. May
January 8, 2004
Outline








Introduction
Silicon Processing
History of ICs
Review of Semiconductor Devices
Conductivity and Resistivity
MOS Transistors
Hot-Point Probe
4-Point Probe
Growth of Electronics Industry

Electronics industry is fundamentally dependent on semiconductor
integrated circuits (ICs).
What do you learn in 4752?




This course deals with the fabrication of
semiconductor devices and ICs.
ICs today have over 107 components per chip, and
this number is growing.
Fabricating these circuits requires a sophisticated
process sequence which consists of hundreds of
process steps.
In this course, we’ll go through a process
sequence to make complementary metal-oxidesemiconductor (CMOS) transistors.
Outline
Introduction
 Silicon Processing
 History of ICs
 Review of Semiconductor Devices
 Conductivity and Resistivity
 MOS Transistors
 Hot-Point Probe
 4-Point Probe

Types of Semiconductors
Elemental
Compound
Si
GaAs, InP (III-V)
Ge
CdS, CdTe (II-VI)
Silicon vs. Germanium
Ge was used for transistors initially, but silicon took over in the late
1960s; WHY?
(1) Large variety of process steps possible without the problem of
decomposition (as in the case of compound semiconductors)
(2) Si has a wider bandgap than Ge
=> higher operating temperature (125-175 oC vs. ~85 oC)
(3) Si readily forms a native oxide (SiO2)

high-quality insulator

protects and “passivates” underlying circuitry

helps in patterning

useful for dopant masking
(4) Si is cheap and abundant
Silicon Disadvantages

Low carrier mobility (m) =>
slower circuits (compared to GaAs)
Material
Si
Ge
GaAs

Mobility (cm2/V-s)
mn = 1500, mp = 460
mn = 3900, mp = 1900
mn = 8000, mp = 380
Indirect bandgap:
 Weak absorption and emission of light
 Most optoelectronic applications not possible
Outline
Introduction
 Silicon Processing
 History of ICs
 Review of Semiconductor Devices
 Conductivity and Resistivity
 MOS Transistors
 Hot-Point Probe
 4-Point Probe

The Transistor



Bell Labs invented the transistor in 1947, but
didn’t believe ICs were a viable technology
REASON: Yield
 For a 20 transistor circuit to work 50% of the
time, the probability of each device functioning
must be:
(0.5)1/20 = 96.6%
 Thought to be unrealistic at the time
1st transistor => 1 mm x 1 mm Ge
ICs and Levels of Integration

1st IC: TI and Fairchild (late 50s)
A few transistors and resistors => “RTL”

Levels of integration have doubled every 34 years since the 1960s)
Moore’s Law
Complexity Acronyms






SSI = small scale integration (~100 components)
MSI = medium scale integration (~1000
components)
LSI = large scale integration (~105 components)
VLSI = very large scale integration (~105 - 106
components)
ULSI = ultra large scale integration (~106 - 109
components)
GSI = giga-scale integration (> 109 components)
State of the Art
1 GB DRAM
 90 nm features
 12” diameter wafers
 Factory cost: ~ $3-4B
=> Only a few companies can afford to
be in this business!

Outline
Introduction
 Silicon Processing
 History of ICs
 Review of Semiconductor Devices
 Conductivity and Resistivity
 MOS Transistors
 Hot-Point Probe
 4-Point Probe

Diamond Lattice

Tetrahedral
structure

4 nearest
neighbors
Covalent Bonding


Each valence electron
shared with a nearest
neighbor
Total of 8 shared valence
electrons => stable
configuration
Doping
Intentional addition of impurities
 Adds either electrons (e-) or holes (h+) =>
varies the conductivity (s) of the material
 Adding more e-: n-type material
 Adding more h+: p-type material

Donor Doping



Impurity “donates”
extra e- to the material
Example: Column V
elements with 5
valence e-s (i.e., As, P)
Result: one extra
loosely bound e-
eP
Acceptor Doping



Impurity “accepts”
extra e- from the
material
Example: Column III
elements with 3
valence e-s (i.e., B)
Result: one extra
loosely bound h+
h+
B
Outline
Introduction
 Silicon Processing
 History of ICs
 Review of Semiconductor Devices
 Conductivity and Resistivity
 MOS Transistors
 Hot-Point Probe
 4-Point Probe

Ohm’s Law

J = sE = E/r
where: s = conductivity, r = resistivity,
and E = electric field

s = 1/r = q(mnn+ mpp)
where: q = electron charge, n = electron concentration,
and p = hole concentration


For n-type samples: s ≈ qmnND
For p-type samples: s ≈ qmpNA
Resistance and Resistivity
length = L
area = A
R = rL/A
Outline
Introduction
 Silicon Processing
 History of ICs
 Review of Semiconductor Devices
 Conductivity and Resistivity
 MOS Transistors
 Hot-Point Probe
 4-Point Probe

MOSFET

Metal-oxide-semiconductor field-effect transistor
IDn
D
+
VSG
-
+
G
B
+
VGS
-
+
VB S
S
n-channel device
VDS
G
S
+
VS B
B
-
+
VS D
-
-IDp
D
p-channel device
G = gate, D = drain, S = source, B = body (substrate)
MOSFET Cross-Section
S
VG
VD > 0
ID
G
oxide
n+
ID
n+
L
D
S
p-type Si
cross-sectional view (not to scale)
top view (not to scale)
Basic Operation
1) Source and substrate grounded (zero voltage)
2) (+) voltage on the gate
 Attracts e-s to Si/SiO2 interface; forms channel
3) (+) voltage on the drain
 e-s in the channel drift from source to drain
 current flows from drain to source
valve (gate)
pipe (channel)
drain
source
Hot-Point Probe


Determines whether a semiconductor is n- or p-type
Requires:
 Hot probe tip (soldering iron)
 Cold probe tip
 Ammeter
Hot-Point Probe
1) Heated probe creates high-energy “majority” carriers
 holes if p-type
 electrons if n-type
2) High-energy carriers diffuse away
3) Net effect:
a) deficit of holes (net negative charge for p-type); OR
b) deficit of electrons (net positive charge for n-type)
4) Ammeter deflects (+) or (-)
4-Point Probe

Used to determine
resistivity
4-Point Probe
1) Known current (I) passed through outer probes
2) Potential (V) developed across inner probes
r = (V/I)tF
where: t = wafer thickness
F = correction factor (accounts for probe geometry)
OR:
Rs = (V/I)F
where: Rs = sheet resistance (W/
)
=> r = Rst
Virtual Cleanroom
http://www.ece.gatech.edu/research/labs/vc/
Web site that describes entire ECE/ChE 4752
CMOS Fabrication Process!