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

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

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

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

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 + + + +

+ + +

- - - - - - -

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

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

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

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

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

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

+ + +

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

diffusion barrier conductor

Si oxide

diffusion barrier conductor

Si

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

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

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

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

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

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

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

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

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{

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

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

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

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

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

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 + - + -

+ -

- +

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

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

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

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

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

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

+

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

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

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)

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

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

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

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