by Elmer Epistola
everywhere. From appliances to space ships,
have pervaded every fabric of our society.
allow machines to talk to us, and probably even understand us. They do
our jobs, go where man has never gone before, and help us explore and
utilize the universe around us. So overwhelming is the power of
computing and signal processing today that it's difficult to believe
how these can come from sand.
world has indeed been reinvented simply by
purifying sand, making
it flat, and adding materials to it. This magical process of
building integrated circuits from sand is now referred to as
consists of the following steps:
1) production of silicon wafers from very pure silicon
2) fabrication of integrated circuits onto these
3) assembly of every integrated circuit on the wafer into
a finished product; and
4) testing and back-end processing of the finished
Wafer fabrication generally refers to the process of
building integrated circuits on silicon wafers. Prior to wafer
fabrication, the raw silicon wafers to be used for this purpose are
first produced from very pure silicon ingots, through either the
Czochralski (CZ) or
Float Zone (FZ)
method. The ingots are shaped then sliced into thin
wafers through a process called
The semiconductor industry has already advanced
tremendously that there now exist so many distinct wafer fab processes,
allowing the device designer to optimize his design by selecting the
best fab process for his device. Nonetheless, all existing fab
processes today simply consist of a series of steps to deposit special
material layers on the wafers one at a time in precise amounts and
patterns. Below is an example of what fabricating a simple CMOS integrated
circuit on a wafer may entail.
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The first step might be to grow a p-type epitaxial layer on the silicon substrate
through chemical vapor deposition. A
nitride layer may then be deposited over the epi-layer, then
etched according to specific patterns, leaving behind exposed areas on
the epi-layer, i.e., areas no longer covered by the nitride layer.
These exposed areas may then be masked again in specific patterns before
being subjected to diffusion or
ion implantation to receive dopants such
as phosphorus, forming n-wells.
Silicon dioxide may then be grown thermally to form field oxides that
isolate the n-wells from other parts of the circuit. This may be
followed by another masking/oxidation cycle to grow gate oxide layers
over the n-wells intended for p-channel MOS transistors later on.
This gate oxide layer will serve as isolation between the channel and
the gate of each of these transistors. Another
mask and diffusion/implant cycle may then follow to adjust threshold
voltages on other parts of the epi, intended for n-channel transistors
Deposition of a
polysilicon layer over the wafer may
then be done, to be followed by a masking/etching cycle to remove
unwanted polysilicon areas, defining the polysilicon gates over the gate
oxide of the p-channel transistors. At the same time, openings for the
source and drain drive-ins are made on the n-wells by etching away oxide
at the right locations.
Another round of mask/implant cycle may then follow, this
time driving in boron dopants into new openings of the n-wells, forming
the p-type sources and drains. This may then be followed by
a mask/implant cycle to form the n-type sources and drains of the
n-channel transistors in the p-type epi.
The wafer may then be covered with phospho-silica glass,
which is then subjected to reactive ion etching in specific patterns to
expose the contact areas for metallization. Aluminum is then
sputtered on the wafer, after which it is subjected to reactive ion
etching, also in specific patterns, forming connections between the
various components of the circuit.
The wafer may then be covered with
glassivation as its
top protective layer, after which a mask/etch process removes the glass
over the bond pads.
Such is the process of wafer fabrication, consisting of a
long series of mask/etch and mask/deposition steps until the circuit is
The process of putting the integrated circuit inside a
package to make it reliable and convenient to use is known as
semiconductor package assembly, or simply 'assembly'. Over the
years, the direction of assembly technology is to develop smaller,
cheaper, more reliable, and more environment-friendly packages. Just
like wafer fabrication technology, assembly technology has advanced
tremendously that there are now a multitude of packages to choose from.
Despite glaring differences between the various packages
available in the industry today, all packages share some things in
common. To name a few, all of them: 1) provide the integrated circuit
with a structure to operate in; 2) protect the integrated circuit from
the environment; 3) connect the integrated circuit to the outside world;
and 4) help optimize the operation of the device.
In general, an assembly process would consist of the
following steps: 1) die preparation, which cuts the wafer into
individual integrated circuits or dice; 2) die attach, which attaches the die to the support structure (e.g., the leadframe) of the package; 3)
which connects the circuit to the electrical extremities of the package,
thereby allowing the circuit to be connected to the outside world; and
4) encapsulation (usually by plastic molding), which provides 'body' to
the package of the circuit for physical and chemical protection.
Subsequent steps that give the package its final form and
appearance (e.g., DTFS) vary from package to package. Steps like marking
and lead finish give the product its own identity, improve reliability,
and add an extra shine at that.
Once assembled, the integrated circuit is ready to use.
However, owing to the imperfection of this world, assembled devices
don't always work. Many things can go wrong to make a device fail, e.g.,
the die has wafer fab-related defects, or the die cracked during
assembly, or the bonds were poorly connected or not connected at all.
Thus, prior to shipment to the customer, assembled devices must first be
Electrical testing of devices in big volumes must be done
fast and inexpensively. Mass-production electrical testing
therefore requires an automated system for doing the test.
Equipment used to test devices are called, well, testers, and equipment
used to handle the devices while undergoing testing are called, well,
handlers. Tester/handler systems are also known as automatic test
Different products require different levels of
sophistication in ATE requirements. Electrical testing of voltage
reference circuits certainly don't require high-end ATE such as those
used to test state-of-the-art microprocessors or digital signal
processors. One area of electrical testing that continuously challenge
engineers is building an ATE that can test the speed of new IC's that
are much faster than what they can use in building their ATE's.
Software written for testing a device with an ATE is
known as a test program. Test programs consist of a series of
subroutines known as test blocks. Generally, each test block has a
corresponding device parameter to test under specific conditions.
This is accomplished by subjecting the device under test (DUT) to
specific excitation and measuring the response of the device. The
measurement is then compared to the pass/fail limits set in the
test program. After the device is tested, the handler bins it out
either as a reject or as a good unit.
After a lot is tested, it is subjected to other back-end
processes prior to shipment to the customer.
Tape and reel is the
process of packing surface mount devices in tapes with pockets while
this tape is being wound around a reel. Boxing and labeling is the
process of putting the reels or tubes in shipment boxes, and labeling
these shipment boxes in accordance with customer requirements.
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