Download Microelectronics Technology

Microelectronics Technology
Microelectronics Technology
Mustafa Arikan
University of Iceland
Contact info
Mustafa Arıkan (Musti)
 [email protected] ;
[email protected]
 Tel : 525-4751 (Ingvarsson Lab., VR-III)
 Office hours ???
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In this course…
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Two parts:
Semiconductor processing (from raw
material to microelectronic components)
 Semicondcutor characterization methods
(physical & electrical-optical)
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Lectures & Labs
Two lectures on 07.02.2008 and 07.03.2008
 Two labs in two groups on 14.02, 21.02 and
14.03, 21.03.2008
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Goal of this lectures…
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Overview of the fundamentals of
microelectronics technology
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Fast & quick
The tools we employ to produce and characterize
electronic components
Complexity and beauty of the technology
Desired outcome
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Understanding of whole process
Big picture
Different approaches
What is microelectronics? What is it
about?
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Microelectronics is a subfield of electronics
study and manufacture of electronic
components which are very small (i.e.
transistors, diodes…)
Semiconductors , metals, organic & plastic
Real small…and impressive…
But very complex sometime…
What takes to achieve it?
What takes to achieve it?
Different approaches
The basics of semiconductor device
fabrication
Proper material for the purpose
 Geometry
 Material growth and removal (over and
over again) by the help of lithography
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Simple example : MESFET
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Metal-Semiconductor Field Effect Transistor
MESFET fabrication & The idea of
lithography
A real device from substrate to final form
 MESFET is relatively simple but not all
the devices can be fabricated this easily
 Inverter fabrication
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CMOS Inverter
Fabrication of a cmos inverter :
Silicon technology
Includes many steps
 Many different tools & technologies
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Crystal (substrate) growth
 Oxidation
 Diffusion & implantation
 Material growth (metal evaporation,
sputtering, vapor deposition, epitaxy)
 Lithography & etching
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We need a substrate !
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How do we get single crystalline Si?
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Czochralski
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Majority of the wafers
Floating zone (high purity)
High purity – low oxygen & carbon impurity
 More complex w.r.t. Czochralski
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Bridgman
Easy (melting & cooling)
 Low quality
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Drip melting, strain annealing and others
Czochralski growth
Ingot by Czochralski method
Czochralski growth
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Typically used for Silicon but also
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Single crystal semiconductors (Si, Ge, GaAs)
Metals (Pd, Pt, Ag, Au)
Salts etc…
Requires seed crystal
Fast (1-2 mm/min)
Oxygen contamination from crucible
Uniformity of axial resistivity is poor
Segregation problems for dopants
We have Si substrate… Next…
Let’s focus on individual steps and technologies from now on
Oxidation
CVD – LPCVD (chemical vapor deposition (film growth)
 Thermally grown oxide (Oxidation)
 Photoresist (Lithography & etching)
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Oxidation
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One of the two main advantages of Si
Ge is superior to Si (mobility, power
consumption)
SiGe (MOSFET channel), Gd2O3
Dry oxidation : Si + O2  SiO2
Wet oxidation : Si + 2H2O  SiO2 + 2H2
oxygen must diffuse through the oxide to react
at the Si/SiO2 interface, so rate depends on
the thickness of the oxide and reduces as the
oxidation progresses.
Oxidation
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thermal oxidation is performed in furnaces at temperatures
between 800 and 1200°C
 Many wafers on the boat (a quartz rack) at the same time
 Variants : RTO
Oxidation : dry vs. wet
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Dry (molecular oxygen) : better oxide but slow (gate
oxide)
Wet (steam – water vapor) : fast but porous (isolation)
Deal-Grove model : thickness vs. time - theory
Oxidation
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Thickness vs. time – practice : Charts !
Oxidation
Lithography & Pattern Transfer
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Used for pattern transfer into metals, oxides and semiconductors
Thin film deposition and lithography (including photo and e-beam,
wet etching and lift-off) are the most frequently used method in
our labs
2 types of resists:
 Positive : PR pattern is same as mask. On exposure to light,
light degrades the polymers resulting in the photoresist being
more soluble in developers. The PR can be removed in
inexpensive solvents such as acetone.
 Negative : PR pattern is the inverse of the mask. On exposure
to light, light polymerizes the rubbers in the photoresist to
strengthen it’s resistance to dissolution in the developer
Lithography & Pattern Transfer
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Black areas (PR) are the openings after
development of PR
Lithography & Pattern Transfer
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How do we perform this “lithography” thing?
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Dehydration bake or pre-bake
Adhesion promoter (i.e. HMDS)
Apply resist – spinner
Soft bake
UV-exposure with mask
Post-bake
Post processing such as development & etching &
lift-off
Other processes required by specific needs
(MEMS)
Lithography & Pattern Transfer
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Baking
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spinner
Lithography & Pattern Transfer
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Expose
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Develop
Lithography & Pattern Transfer :
Uses of lithography
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Etching Processes: open windows in oxides for diffusion,
masks for ion implantation, etching, metal contact to the
semiconductor, or interconnect.
Lithography & Pattern Transfer
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Lift off Processes: Metalization
Lithography & Pattern Transfer
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Issues with photolithography
Resolution : feature size (~0.5 micron
usually)
 Shorter wavelength = better resolution
 Registration : alignment of different layers
on the same wafer (~ 1/3 of the resolution
or 0.06 micron)
 Throughput : effective cost and time
 Resist thickness ~ 1/spin speed
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Lithography & Pattern Transfer
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Photolithography systems
Lithography & Pattern Transfer
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Contact  Resist is in contact with the mask: 1:1 magnification
 Inepensive, relatively high resolution (~ 0.5 micron), contact
with the mask (scratches, particles and dirt are imaged in the
wafer)
Proximity  Resist is almost but not in contact with the mask:
1:1 magnification
 Inexpensive, low resolution (~ 1-2micron), diffraction effects
limit accuracy of pattern transfer. Less repeatable than contact
methods,
Projection  Mask image is projected a distance from the mask
and de-magnified to a smaller image: 1:4 -1:10magnification
 Can be very high resolution (~0.07 um or slightly better), No
mask contact results in almost no mask wear (high production
compatible), mask defects or particles on mask are reduced in
size on the wafer. Extremely expensive and complicated
equipment, Diffraction effects limit accuracy of pattern transfer
Lithography & Pattern Transfer
Lithography & Pattern Transfer :
Light sources
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Typically mercury (Hg)- Xenon (Xe) vapor bulbs are
used as a light source in visible (>420 nm) and
ultraviolet (>250-300 nm and <420 nm) lithography
equipment
Lasers are used to increase resolution, and decrease
the optical complexity for deep ultraviolet (DUV)
lithography systems. Excited dimer (Excimer or
Exiplex) pulsed lasers are typically used. These are
powerful, extremely expensive to purchase and
maintain, optically noisy lasers.
Alternative approaches such as: Nano-imprint, soft,
dip-pen, e-beam, FIB, x-ray lithography : Very active
research field!
Lithography & Pattern Transfer :
some examples
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Pictures for good and bad lithography
Oxidation
Chemical vapor deposition CVD – LPCVD (film growth)
 Thermally grown oxide (Oxidation)
 Photoresist (Lithography & etching)
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Diffusion & Implantation
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Dopants for N+ and P+ regions (implantation & diffusion)
Diffusion & Implantation
Diffusion & Implantation
Diffusion & Implantation
Diffusion & Implantation
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What is diffusion?
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Commonly used for
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Bipolar technology (base, emitters)
FET (source, drain)
Use when
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Diffusion is the spontaneous net movement of particles from
an area of high concentration to an area of low concentration
(particle penetration from surface into the wafer)
Ion implantation damage is not acceptable
Deep junctions are needed
Cheap & easy solutions are seeked
Don’t use for
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Ultra-shallow junctions
Forming channel in MOSFET
Diffusion & Implantation :
Types of diffusion
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Instertital
Vacancy
Interstitialcy
Kick-out
Dissociative
Diffusion & Implantation
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Diffusion equation (derived from Fick’s Law):
Different solution for different approximations
 Best solution for an experimentalist: Charts
(again!)
Diffusion & Implantation
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Diffusion depends on:
 Diffusion time
 Diffusion constant (diffusivity)
 Material density
 Temperature
Diffusion & Implantation
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Ion implantation :
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Ions (charged atoms or molecules) are created via
an enormous electric field stripping away an
electron.
These ions are filtered and accelerated toward a
target wafer, where they are buried in the wafer.
The depth of the implantation depends on the
acceleration energy (voltage).
The dose is very carefully controlled by integrating
the measured ion current.
Diffusion & Implantation
Diffusion & Implantation
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Advantages
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Very precise control of the dose and position
Independent control of impurity depth and dose
Very fast (just few seconds)
Complex profiles can be achieved by multiple & sequential
implantations
Disadvantages
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Very deep and very shallow profiles are difficult
Not all the damage can be corrected by annealing.
Typically has higher impurity content than diffusion.
Often uses extremely toxic gas sources such as arsine
(AsH3), and phosphine (PH3).
expensive
Diffusion & Implantation
Diffusion & Implantation
Fabrication of a CMOS Inverter
Fabrication of a CMOS Inverter
Fabrication of a CMOS Inverter
Poly-Si deposition (LPCVD)
 Let’s look at film deposition
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Film deposition & growth
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Physical deposition
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Chemical vapor deposition
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Thermal evaporation
E-beam evaporation
Sputtering
CVD
LPCVD
PECVD
Epitaxial growth
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MBE
MOCVD
CBE
Thermal & E-beam Evaporation
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The source material is evaporated in a vacuum. The vapors other
than the source material are almost entirely removed before the
process begins.
The vacuum allows vapor particles to travel directly to the target
object (substrate), where they condense back to a solid state.
Advantages
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Disadvantages
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High purity (good for Schottky contacts), simple, easy & cheap, fast,
low vacuum (10-4)
Poor alloy formation, step coverage problems, low throughput (low
vacuum), relatively non-uniform deposition, non-smooth surfaces,
short mean free path (~60m), high temperatures.
Two basic forms:
 Thermally assisted
 E-beam (electron beam by thermionic, field emission or arc)
Thermal & E-beam Evaporation
Thermal
E-beam
Sputtering
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A "target" made of the material to be deposited is bombarded by
energetic ions which will dislodge atomes of the target, i.e.,
"sputter them off".
The dislodged atoms will have substantial kinetic energies, and
some will fly to the substrate to be coated and stick there.
Sputtering
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Advantages
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The target atoms hit the substrate with an energy large enough so they "get
stuck", but not so large as to liberate substrate atoms. Sputtered layers
therefore usually stick well to the substrate (in contrast to other techniques,
most notably evaporation
All atoms of the target will become deposited, in pretty much the same
composition as in the target. It is thus possible, e.g., to deposit a silicide
slightly off the stoichiometric composition
The target atoms hit the substrate coming from all directions.
Homogeneous coverage of the substrate is relatively easy to achieve- just
make the substrate holder and the target big enough. The process is also
relatively easily scaled to larger size substrates.
Disadvantages
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Sputtered layers usually have a very bad crystallinity - very small grains full of
defects or even amorphous layers result. Usually some kind of annealing of
the layers is necessary to restore acceptable crystal quality.
Sputtering works well for metals or other somewhat conducting materials. It is
not easy or simply impossible for insulators. Sputtering SiO2 layers, e.g., has
been tried often, but never made it to production (Zn-oxide, tin-oxide etc. are
easily achieved however)
Chemical Vapor Deposition
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the substrate is placed
inside a reactor to which
a number of gases are
supplied.
a chemical reaction
takes place between the
source gases.
The product of that
reaction is a solid
material with condenses
on all surfaces inside
the reactor.
Chemical Vapor Deposition
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Various different types of CVD but mainly 4 categories
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Atmospheric Pressure (APCVD)
 Advantages: High deposition rates, simple, high throughput
 Disadvantages: Poor uniformity, purity is less than LPCVD
 Thick oxides
Low Pressure (LPCVD, 0.2 – 20 Torr)
 Poly-silicon deposition, dielectric layer and doped dielectric deposition.
 Advantages: Excellent uniformity, purity
 Disadvantages: Lower (but reasonable) deposition rates than APCVD
Metal Organic (MOCVD)  alternative for MBE
 Advantages.: Highly flexible (semiconductors, metals, dielectrics)
 Disadvantages: Highly toxic, very expensive source material,
environmental disposal costs are high.
Plasma Enhanced (PECVD)
 dielectric coating such as silicon nitride
 Advantages.: Uses low temperatures necessary for rear end processing.
 Disadvantages: Plasma damage typically results
Epitaxy
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We can grow* crystalline semiconductors by raising the
temperature to allow more surface migration and by using a
crystalline substrate (Si, GaAs, InP wafer, etc…)
Growth, not deposition !
The lattice constant of the epitaxially grown layer needs to
be close to the lattice constant of the substrate wafer.
Otherwise the bonds can not stretch far enough and
dislocations will result.
Advantages : Very high quality, extremely clean
samples,crystallinity, very long mean free path (few
hundred meters), precise atomic layer deposition
Disatvantages : UHV system, low deposition rate, very
expensive equipment, not suitable for mass production
Different versions: LPE, VPE, MBE,CBE etc…
Expitaxy
Vacuum
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A vacuum is a volume of space that is essentially empty of
matter such that its gaseous pressure is much less than
standard atmospheric pressure.
A perfect vacuum with a gaseous pressure of absolute zero
is a philosophical concept that is never observed in practice
quantum theory predicts that no volume of space can be
perfectly empty in this way.
The quality of a vacuum is measured in relation to how
closely it approaches a perfect vacuum. The residual gas
pressure is the primary indicator of quality, and is most
commonly measured in units called torr
The average distance between collisions (mean free path)
Vacuum
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Vacuum quality is subdivided into ranges according to the
technology required to achieve it or measure it. These ranges do
not have universally agreed definitions (hence the gaps below),
but a typical distribution is as follows:
Atmospheric
Low vacuum
Medium vacuum
High vacuum
Ultra high vacuum
Extremely high vacuum
Outer Space 1×10-6 to
Perfect vacuum
760 Torr
760 to 25 Torr
25 to 1×10-3 Torr
1×10-3 to 1×10-9 Torr
1×10-9 to 1×10-12 Torr
<1×10-12 Torr
<3×10-17 Torr
0 Torr
Vacuum pumps
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Rough & medium vacuum
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Piston pumps (particle problems)
Rotary vane pumps (cheap)
Dry pumps
High vacuum & UHV
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Diffusion (oil contamination)
Turbo
Cryo
Ion (low pumping speed & capacity)
Transfer pumps
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Rotary pump (mechanical)
Vacuum pumps
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Turbomolecular pumps
Fabrication of a CMOS inverter
Fabrication of a CMOS inverter
Fabrication of a CMOS inverter
Inverter – After few steps