the electromagnetic properties of materials. morris, jr . university of california, berkeley mse...

J.W. Morris, Jr. University of California, Berkeley

MSE 200A Fall, 2008

The Electromagnetic Properties of Materials

•  Electrical conduction –  Metals –  Semiconductors –  Insulators (dielectrics) –  Superconductors

•  Magnetic materials –  Ferromagnetic materials –  Others

•  Photonic Materials (optical) –  Transmission of light –  Photoactive materials

•  Photodetectors and photoconductors •  Light emitters: LED, lasers


MSE 200A Fall, 2008

Semiconductor Junctions

•  Join n- and p-type regions to create a junction –  Junctions have asymmetric electrical properties

•  Can be done by doping adjacent regions –  Write junction devices onto a single crystal (chip) –  This is the basis of all microelectronics


MSE 200A Fall, 2008

The Band Structure at an n|p Junction

•  Join n and p regions –  Just prior to join, EF high on n-side –  Electrons flow from n to p (holes flow p to n) –  Charges at interface create potential, Δφ, across interface –  Potential raises E on p-side (ΔE = -eΔφ )

•  Equilibrium (current stops) when EF(n)=EF(p) –  The electron and hole occupancies are constant across the interface at every E

E

x

E F e e e e e e e

E E F

x

e e e e e e

n p + + + +

+ + +

- - - - - - -


MSE 200A Fall, 2008

Current-Voltage Characteristic: n|p Junction

•  Electron current density

•  Hole current density

•  Total current

- I

V

Ie=

€

je = je+ + je

− = je0 exp eV

kT

−1

€

j = jp − je = −2 je0 exp eV

kT

−1

€

jp = − je


MSE 200A Fall, 2008

The Bipolar Transistor as a Switch Controlled by Vb

•  Vb > 0 opens switch –  So long as Vb > Ve, electrons flow into the base –  To achieve equilibrium, electrons recombine with holes in

base –  Given small size of base, holes are exhausted by

recombination –  Holes cannot be replenished

•  Collector in reverse bias •  Emitter voltage attracts holes

•  Base becomes transparent to electrons –  Current controlled by Vb-Ve

+ - n p n +

The image cannot be displayed. Your computer may not have enough memory

e e e e e e e e e e e e E

E F

e e e


MSE 200A Fall, 2008

The Field Effect Transistor: Metal-Oxide-Semiconductor Junctions (MOS)

•  MOS can invert semiconductor type –  Positive potential lowers EC, attracts electrons –  When EC-EF < EG/2, semiconductor “inverts” (p→n)

•  Potential creates “field effect” - –  n-region near surface –  p-region in depth –  p- (depleted zone) acts as insulator

n metal oxide + + + + + + + +

V +

p - p

metal oxide

p valence band

conduction band

E F e e e e

E F

metal o x i d e E

inversion depletion normal

+ + +


MSE 200A Fall, 2008

MOSFET: Metal-Oxide-Semiconductor Field Effect

Transistor

•  Construct n|p|n junction at MOS as shown –  In this case n|p|n called source|gate|drain (Vd > Vs)

•  When Vg = 0, gate|drain in reverse bias –  Switch is off

•  When Vg > VI, gate is n-type and current flows –  Switch is on

p p -

V - V + n + n +

n metal oxide + + + + + + + +

V +

gate source drain

channel

I

V

V g = 0

V g >VI


MSE 200A Fall, 2008

•  Semiconductor type and conductivity –  Conductivity dominated by carrier density –  Intrinsic semiconductors (excitation across band gap) –  Extrinsic semiconductors

•  n-type (donors) or p-type (acceptors) •  Permit precise control over σ and type of carrier

•  Semiconductor junctions –  n|p diode –  n|p|n bipolar transistor –  Field effect transistor (mosfet)

•  Manufacturing semiconductor devices –  Lithography –  Doping –  Packaging

Semiconductors


MSE 200A Fall, 2008

Semiconductor Device Processing

•  Manufacture millions of devices simultaneously on a “chip”

•  Steps in manufacture (simplified) –  Crystal growth and dicing to “chip” –  Photolithography to locate regions for doping –  Doping to create n-type regions –  Overlay to create junctions –  Metallization to interconnect devices –  Passivation to insulate and isolate devices –  Higher level “packaging” to interconnect chips

active devices (transistors, etc.)

metallic conductors oxide passivation

silicon chip


MSE 200A Fall, 2008

Photolithography

•  Minimum feature size depends on wavelength of “light” –  Visible light: ~ 1 µm –  Ultraviolet light: ~ 0.1 µm –  Electrons, x-rays 0.1-1 nm –  New and exotic methods

•  Must have photoresist suitable to the “light” –  Or use “light” to cut through oxide directly

siliconoxidecoating

mask

light

siliconoxidecoating


MSE 200A Fall, 2008

Doping

•  Add electrically active species

•  Simple method: Coat surface and diffuse –  Diffusion field is electrically active

•  More precise:Ion implantation: –  Accelerate ions of the electrically active species toward surface –  Ions embed to produce doped region

dopant distribution dopant

ions


MSE 200A Fall, 2008

Doping: The Chemical Distribution

•  Initial distribution is inhomogeneous –  Diffusion produces gradient from surface –  Ion implantation produces concentration at depth

beneath surface

•  Can homogenize by “laser annealing” –  Use a laser to melt rapidly, locally –  Rapid homogenization n melted region –  Rapid re-solidification since rest of body is heat sink

diffusion

ion implantation laser anneal

c

x

dopant distribution

laser light


MSE 200A Fall, 2008

Overlay to Create Junctions

•  Once primary doping is complete –  Re-coat –  Re-mask –  Re-pattern –  Dope second specie to create desired distribution of

junctions

p n n p n n


MSE 200A Fall, 2008

Metallization

•  After devices are made –  Coat with oxide for insulation –  Etch for conductor pattern

•  Coat and etch (Al) –  Coat surface with Al(Cu) –  Pattern and etch to create desired pattern of conductors

•  Damascene process (Cu, which is difficult to pattern-etch) –  Pattern oxide with trenches for Cu lines –  Coat with Cu, polish off to leave filled trenches

devices oxide

diffusion barrier conductor

Si

oxide


Si oxide


Si


MSE 200A Fall, 2008

Passivation and Packaging

•  Coat with insulator to isolate device –  Oxide to isolate metallic conductors –  “Hermetic seal”, usually polymer, to insulate form

environment –  Sealing is difficult since electrical contacts must penetrate

•  Interconnect devices –  Wire and solder chips to “boards” –  Boards to one another to make electronic device

= oxide = metal = devices = semiconductor


MSE 200A Fall, 2008







MSE 200A Fall, 2008

Insulators (Dielectrics)

•  Characteristics: –  Large band gap (> 2 eV) –  Very low conductivity

•  Engineering uses –  Separate conductors

•  No leakage current •  No interference

–  Support electric fields •  Store energy (capacitors) •  Induce charge (MOSFET)

EFE EG

x

valence band

conduction band


MSE 200A Fall, 2008

Insulators: Material Properties

•  Ability to insulate ⇒ critical field (Ec) –  Insulator separates conductors until E reaches Ec

•  Support internal field ⇒ dielectric constant (ε) –  High ε ⇒ high induced charge for given voltage

•  Capacitors: high ε ⇒ efficient energy storage •  Oxide in MOSFET: high ε ⇒ low switching voltage

–  Low ε ⇒ small induced charges •  “low-k” insulators essential for microelectronic packaging

•  Energy dissipation from current ⇒ loss tangent (δ) –  Low δ ⇒ low rate of energy loss from propagating e-m fields


MSE 200A Fall, 2008

Insulators: Breakdown Voltage

•  Insulator protects until –  E reaches Ec “breakdown” –  Catastrophic increase in j at Ec –  Example: lightning

•  Common “cascade mechanism” –  Electron accelerated in field –  Excites new carriers by collision –  These accelerate in chain reaction

•  Material and microstructure variables –  Band gap: Ec increases with EG –  Purity: Ec usually increases with purity –  Temperature: minimum at intermediate T

•  Few carriers at low T •  Low mobility at high T

j

E Ec

E

x

e ee{


MSE 200A Fall, 2008

Dielectrics

•  Dielectrics (insulators) support internal fields –  The “dielectric constant” relates field to charge –  Sometimes use “susceptibility” χ = ε - 1 (χ = 0 in free space)

Q = CV C = capacitance

σA = C(Ed)σ = D = εε0E

D = electric displacement

ε ≥ 1 (= 1 in free space)

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - -

d

+ Q

- Q

V dielectric


MSE 200A Fall, 2008

Source of the Dielectric Constant

•  Internal polarization –  Dipoles align in applied field –  Create reverse field (EI)

ε0E = ε0E0 − ε0EI = σ − P

D = σ = ε0E + P = εε0E

P = pii∑ = χE

ε = 1+ Pε0E

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - d

+ Q

- Q

V + -

+ - + -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

p i P


MSE 200A Fall, 2008

Polarization Mechanisms

•  Space charges –  Porous materials (large pores) –  Slow response in insulators

•  Molecular dipoles –  Large polar organics have big ε –  Relatively slow response (like diffusion)

•  Ionic displacements –  Ionic crystals have moderate ε –  Fast response (like optical phonon)

•  Atomic dipole –  Small ε –  Very fast response (plasmon frequency)

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - d

+ Q

- Q

V + -

+ - + -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

+ -

p i P

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - d

+ Q

- Q

V + - + -

+ -

- +


MSE 200A Fall, 2008

Influence of the Dielectric Constant

•  For given σ (Q) increasing ε decreases field (E)

•  For given voltage drop (E), increasing ε increases Q (σ) –  Energy stored in a capacitor increases with ε –  Induced charge between adjacent conductors increases with ε

•  MOSFET oxides need maximum ε •  Insulators in microelectronic packaging need minimum ε •  Both are major objectives in modern microelectronics

–  (many jobs, much money)

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - -

d

+ Q

- Q

V dielectric U =12DE =

12εε0E

2


MSE 200A Fall, 2008

Ultra-low Dielectric Constant

•  “Low-k” materials –  Critical for applications in electronic packaging

•  Materials design –  Organics based on non-polar molecules –  Dense array of nanopores (ε = 1)

•  Materials issues –  Mechanical integrity - must support device

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - -

d

+ Q

- Q

V dielectric

•  For a given voltage drop (E), increasing ε increases Q (σ) ⇒ Induced charge increases with ε


MSE 200A Fall, 2008

High Dielectric Constant - Ferroelectricity

•  Ferroelectric materials –  BaTiO3 (for example) –  Effective CsCl

•  At high T (T > Tc) –  Central ion centered –  No dipole moment

•  At low T (T < Tc) –  Central ion displaces to create dipole –  All neighboring cells displace parallel ⇒ Large net dipole moment

P

T

α ’

α

+


MSE 200A Fall, 2008







MSE 200A Fall, 2008

The Optical Properties of Materials: Photonic Materials

•  Beauty: one-half of the earliest materials science –  Pottery glazes(the origin of metals), paints and cosmetics –  Jewelry - the development of metals and metalworking

•  Information –  Window glass –  Optical fibers (rapidly replacing copper wire)

•  Light –  The electric light –  LEDs and Lasers –  Photodetectors and photoconductors

•  Power –  Photovoltaics (solar cells) –  Laser power transmission (welding, surface treatments)


MSE 200A Fall, 2008

The Optical Properties of Materials: Photonic Materials

•  “Optical” means the whole electromagnetic spectrum –  From radio waves to γ-rays –  Can be regarded as

•  Waves in space •  Particles with quantized energies

•  Light as waves –  Refraction and reflection at an interface (windows, light pipes, solarium) –  Absorption and scattering (optical fibers) –  Diffraction (x-ray and electron crystallography)

•  Light as particles –  Transmission and absorption –  Photodetectors and photoconductors: switches, photocopiers –  Photoemitters: LEDs and lasers


MSE 200A Fall, 2008

The Electromagnetic Spectrum

-10

-8

-6

-4

-2

0

2

4

6

8

1 km

1 m

1 mm

1 µm

1 nm1 Å

4

2

0

-2

-4

-6

-8

-10

-12

-14

4

6

8

10

12

14

16

18

20

22

radio

microwave

infrared

ultraviolet

x-ray

©- ray

visible

0.4 µm

0.5 µm

0.6 µm

0.7 µm

violet

blue

green

yellow

orange

red

log[fr

eque

ncy(

Hz)]

log[en

ergy(

eV)]

log[w

avele

ngth(

m)]

•  Visible light: –  λ ~ 0.4-1 µm –  E ~ 1.2-3 eV

the electromagnetic properties of materials. morris, jr . university of california, berkeley mse...

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