Intro to Semiconductors

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Intro to Semiconductors
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This is a picture of a silicon wafer after has
finished fabrication in the fab (factory).
Silicon Wafer
Single Die
Most semiconductor circuits are manufactured on
silicon based wafers. Modern wafers are 8-12 in
diameter, and only a few millimeters deep.
A Die refers to one instance of a silicon chip
that is fabricated. Designers make their chip
design as small as possible to squeeze more die
onto the wafer, which increases the number of
silicon chips they can sell.
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This is a circuit diagram. It shows how different
devices are connected.
These are transistors.
Dont worry about the other components in the
circuit diagram. For our purposes, theyre not
important.
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• Transistors have multiple terminals designers
  hook up to create a circuit
• 2 Types of Transistors
• Bipolar Junction Transistor (BJT) we wont talk
  about this one
• Metal Oxide Semicondutor (MOS)

The terminals of this MOS transistor are the
Gate, Bulk, Source, and Drain
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Weve looked at a schematic representation of a
MOS transistor so far. Weve seen transistors in
a circuit diagram. But what does a transistor
really look like?
Silicon substrate
Bulk
Channel
By applying the right voltage to the gate, we can
create a channel. The channel allows current
(flow of electrons) in the substrate therefore
electrically connecting the source and the drain.
If we apply a completely different voltage, the
channel disappears and no current flows between
the source and drain.
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There are many ways to characterize the
performance of a transistor. Usually this
involves doing some measurements in the lab and
looking at graphs.
Dont dwell on this slide you dont need to
understand the graphs!
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Now you understand how a transistor works and how
it looks on a wafer. A silicon chip contains
millions of these transistors. Each transistor
has to be electrically connected with many other
transistors. How does this happen?
Metal Interconnect! 2 layers are shown here, but
some processes have as many as 9-12 layers of
metal!
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We have millions of transistors on a single die.
If there is a manufacturing defect anywhere on a
die, the product may no longer function. How do
we manufacture a silicon wafer?
Many processes! Ill give you a general overview.
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Thermal Oxidation
Put a bare silicon wafer into a high temperature
(1000C) furnace with dry oxygen (O2) or steam
(H2O) and it will grow a silicon dioxide layer.
Silicon dioxide is used as an insulating material
separating the gate from the substrate.
Silicon Dioxide
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Lithography
The lithography process transfers a pattern from
a photomask to a pattern on the wafer. Think of
it like a stencil. You create a stencil, put it
over the wafer, shine light through the
photomask, and it will create a pattern on the
wafer.
Photoresist is the layer of material that is
sensitive to light. Photoresist exposed to the
light will dissolve away.
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Etching
Dry Etching
Wet Etching
We can use lithography to define areas that we
dont want to etch. Now we can use chemicals to
etch the areas to create the geometry we desire.
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Ion Implantation
We typically introduce atoms like Boron,
Phosphorus, and Arsenic, into the silicon to
create our source, drain, and substrate regions.
These atoms are called dopants and this process
is called doping. Again, we can use
lithography to cover the areas where we dont
want these atoms. Then we use an ion beam to
accelerate these atoms towards the wafer. These
atoms collide and implant themselves in the wafer.
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Deposition
We also deposit layers of material onto the
surface. One such example is photoresist
(liquid), which is used in the lithography step.
For more complex materials, we use a process
called Chemical Vapor Deposition (CVD). We flow a
special gas mixture with the wafers in a furnace.
The gas reacts with the wafer and a layer of
material is formed. We use CVD to create the
insulating layers between interconnect as well as
the metal interconnect layers.
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By now you should have an idea of how this
transistor is made. We have many tools at our
disposal we use lithography to define
geometries, we can etch through different layers,
we can grow oxide, we can use ion implantation to
bombard atoms to create the source and drain, and
we can deposit layers of material onto the
surface.
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Its important to understand a few trends in
silicon based technologies
Were squeezing more transistors onto a chip! How?
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We make our transistors smaller and smaller. We
improve our manufacturing processes. What are the
advantages of this scaling?
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Lower cost!
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Polymer Electronics use organic compounds instead
of silicon to create transistors. Organic
anything with a carbon-hydrogen bond
Primary advantages over silicon based
technologies

• Cheaper to manufacture! We can print (using
  inkjet cartridges) these transistors on plastic.
• Lightweight and flexible

Infineon's plastic test chip could theoretically
be integrated on a potato chip bag.
http//www.eet.com/news/latest/showArticle.jhtml?a
rticleID10800776
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How do we create these circuits?
Inline Manufacturing Think printing press!
Current research done at Berkeley Nozzle system
to print transistors, like an inkjet printer
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Droplet on Demand Research at Berkeley
http//organics.eecs.berkeley.edu
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Applications of Polymer Electronics
Special LCD Screens developed by Phillips
http//www.research.philips.com/newscenter/archive
/2004/lcd-technology.html
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Applications of Polymer Electronics
RFID Sensors on Plastic to replace UPC Codes
http//organics.eecs.berkeley.edu
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Applications of Polymer Electronics
Look here!
A transistor for a fully-integrated low-power,
low-cost, low-speed, low-resolution display
http//organics.eecs.berkeley.edu
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Applications of Polymer Electronics
DNA Sensors
http//organics.eecs.berkeley.edu
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Please visit all of these links! http//en.wikiped
ia.org/wiki/Organic_semiconductor http//www.eet.c
om/news/latest/showArticle.jhtml?articleID1771056
78pgno1 http//www.eet.com/news/latest/showArtic
le.jhtml?articleID10800776 http//www.polyapply.o
rg/home.html http//www.ee.calpoly.edu/dbraun/pol
yelec/moreinfo.html http//organics.eecs.berkeley.
edu http//www.research.philips.com/newscenter/pic
tures/ldm-polelec.html http//peumans-pc.stanford.
edu/research http//mctp.chem.ucla.edu/yang/ourres
earch.html http//www.nsf.gov/pubs/2004/nsf04554/n
sf04554.htm NSF estimates the 2007 market for
polymer electronics, photonics, magenetic
devices at 7 billion. http//www.organicid.com/ind
ex3.html