the electromagnetic properties of materials. morris, jr . university of california, berkeley mse...
TRANSCRIPT
![Page 1: The Electromagnetic Properties of Materials. Morris, Jr . University of California, Berkeley MSE 200A Fall, 2008 The Electromagnetic Properties of Materials • Electrical conduction](https://reader033.vdocuments.net/reader033/viewer/2022051601/5ae384937f8b9ad47c8e60d6/html5/thumbnails/1.jpg)
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
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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
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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 + + + +
+ + +
- - - - - - -
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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
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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
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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
+ + +
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
![Page 17: The Electromagnetic Properties of Materials. Morris, Jr . University of California, Berkeley MSE 200A Fall, 2008 The Electromagnetic Properties of Materials • Electrical conduction](https://reader033.vdocuments.net/reader033/viewer/2022051601/5ae384937f8b9ad47c8e60d6/html5/thumbnails/17.jpg)
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
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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
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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{
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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
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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
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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 + - + -
+ -
- +
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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
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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
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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
+
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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
![Page 27: The Electromagnetic Properties of Materials. Morris, Jr . University of California, Berkeley MSE 200A Fall, 2008 The Electromagnetic Properties of Materials • Electrical conduction](https://reader033.vdocuments.net/reader033/viewer/2022051601/5ae384937f8b9ad47c8e60d6/html5/thumbnails/27.jpg)
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)
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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
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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