Course Overview

ECE/ChE 4752: Microelectronics ECE/ChE 4752: Microelectronics Processing LaboratoryProcessing Laboratory

Gary S. May

January 8, 2004

Outline

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-Point Probe

Growth of Electronics Industry

Electronics industry is fundamentally dependent on semiconductor Electronics industry is fundamentally dependent on semiconductor integrated circuits (ICs).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-oxide-semiconductor (CMOS) transistors.

Outline

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-Point Probe

Types of Semiconductors

ElementalElemental CompoundCompound

SiSi GaAs, InP (III-V)GaAs, InP (III-V)

GeGe CdS, CdTe (II-VI)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 (Low carrier mobility () => ) =>

slower circuits (compared to GaAs)slower circuits (compared to GaAs)

Indirect bandgap:Indirect bandgap: Weak absorption and emission of lightWeak absorption and emission of light Most optoelectronic applications not possibleMost optoelectronic applications not possible

MaterialMaterial Mobility (cmMobility (cm22/V-s)/V-s)

SiSi nn = 1500, = 1500, pp = 460 = 460

GeGe nn = 3900, = 3900, pp = 1900 = 1900

GaAsGaAs nn = 8000, = 8000, pp = 380 = 380

Outline

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-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 3-4 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

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-Point Probe

Diamond Lattice

Tetrahedral structure Tetrahedral structure

4 nearest neighbors4 nearest neighbors

Covalent Bonding

Each valence electron Each valence electron shared with a nearest shared with a nearest neighborneighbor

Total of 8 shared valence Total of 8 shared valence electrons => stable electrons => stable configurationconfiguration

Doping

Intentional addition of impurities Adds either electrons (e-) or holes (h+) =>

varies the conductivity () 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-

P

e-

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+

B

h+

Outline

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-Point Probe

Ohm’s Law

J = E = E/where: = conductivity, = resistivity,

and E = electric field

= 1/ = q(nn+ pp)

where: q = electron charge, n = electron concentration,

and p = hole concentration

For n-type samples: ≈ qnND

For p-type samples: ≈ qpNA

Resistance and Resistivity

area = A

length = L

R = L/A

Outline

IntroductionIntroduction Silicon ProcessingSilicon Processing History of ICsHistory of ICs Review of Semiconductor DevicesReview of Semiconductor Devices Conductivity and ResistivityConductivity and Resistivity MOS TransistorsMOS Transistors Hot-Point ProbeHot-Point Probe 4-Point Probe4-Point Probe

MOSFET

Metal-oxide-semiconductor field-effect transistor

D

G B

S

IDn

VGS

VDS+

-

VBS

+

-

+

-

S

G B

D

-IDp

VSD

+

-

VSG

+

-

VSB

+

-

n-channel devicep-channel device

G = gate, D = drain, S = source, B = body (substrate)

MOSFET Cross-Section

S VG

oxide

VD > 0

n+ n+ID

ID

L

p-type Si

cross-sectional view (not to scale)

G

D S

top view (not to scale)

Basic Operation1) Source and substrate grounded (zero voltage)2) (+) voltage on the gate

Attracts e-s to Si/SiO2 interface; forms channel3) (+) 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 away3) Net effect:

a) deficit of holes (net negative charge for p-type); ORb) deficit of electrons (net positive charge for n-type)

4) Ammeter deflects (+) or (-)

4-Point Probe

Used to determine resistivity

4-Point Probe1) Known current (I) passed through outer probes2) Potential (V) developed across inner probes

= (V/I)tF

where: t = wafer thicknessF = correction factor (accounts for probe geometry)

OR: Rs = (V/I)F

where: Rs = sheet resistance (/)

=> = Rst

Virtual Cleanroom

http://www.ece.gatech.edu/research/labs/vc/http://www.ece.gatech.edu/research/labs/vc/

Web site that describes entire ECE/ChE 4752 Web site that describes entire ECE/ChE 4752 CMOS Fabrication Process!CMOS Fabrication Process!