Course Overview

1
Course Overview

• ECE/ChE 4752 Microelectronics Processing
  Laboratory

Gary S. May January 8, 2004
2
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

3
Growth of Electronics Industry

• Electronics industry is fundamentally dependent
  on semiconductor integrated circuits (ICs).

4
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, well go through a process
  sequence to make complementary metal-oxide-semicon
  ductor (CMOS) transistors.

5
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

6
Types of Semiconductors
Elemental Compound
Si GaAs, InP (III-V)
Ge CdS, CdTe (II-VI)
7
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
• gt 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

8
Silicon Disadvantages

• Low carrier mobility (m) gt
• slower circuits (compared to GaAs)

Material Mobility (cm2/V-s)
Si mn 1500, mp 460
Ge mn 3900, mp 1900
GaAs mn 8000, mp 380

• Indirect bandgap
• Weak absorption and emission of light
• Most optoelectronic applications not possible

9
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

10
The Transistor

• Bell Labs invented the transistor in 1947, but
  didnt 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 gt 1 mm x 1 mm Ge

11
ICs and Levels of Integration

• 1st IC TI and Fairchild (late 50s)
• A few transistors and resistors gt RTL
• Levels of integration have doubled every 3-4
  years since the 1960s)

12
Moores Law
13
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 (gt 109 components)

14
State of the Art

• 1 GB DRAM
• 90 nm features
• 12 diameter wafers
• Factory cost 3-4B
• gt Only a few companies can afford to be in this
  business!

15
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

16
Diamond Lattice

• Tetrahedral structure
• 4 nearest neighbors

17
Covalent Bonding

• Each valence electron shared with a nearest
  neighbor
• Total of 8 shared valence electrons gt stable
  configuration

18
Doping

• Intentional addition of impurities
• Adds either electrons (e-) or holes (h) gt
  varies the conductivity (s) of the material
• Adding more e- n-type material
• Adding more h p-type material

19
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-

20
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

21
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

22
Ohms 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

23
Resistance and Resistivity
R rL/A
24
Outline

• Introduction
• Silicon Processing
• History of ICs
• Review of Semiconductor Devices
• Conductivity and Resistivity
• MOS Transistors
• Hot-Point Probe
• 4-Point Probe

25
MOSFET

• Metal-oxide-semiconductor field-effect transistor

G gate, D drain, S source, B body
(substrate)
26
MOSFET Cross-Section
27
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

28
Hot-Point Probe

• Determines whether a semiconductor is n- or
  p-type
• Requires
• Hot probe tip (soldering iron)
• Cold probe tip
• Ammeter

29
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 (-)

30
4-Point Probe

• Used to determine resistivity

31
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/?)
• gt r Rst

32
Virtual Cleanroom

• http//www.ece.gatech.edu/research/labs/vc/
• Web site that describes entire ECE/ChE 4752 CMOS
  Fabrication Process!