We have seen that many identical microelectronic devices
can be realized on a single wafer
which is a technology that is at the basis of many
high performance applications in nearly every domain
of our daily life.
The reproducibility of microfabrication indeed
is a key factor for economic realization
of any consumer product.
Therefore, these devices are most of the time realized
in a clean room environment where experimental
and manufacturing conditions are reproducible
and very strictly controlled.
In this lesson, we will introduce a few of the basic aspects
of the design of and work in a clean room.
We will start by discussing different contamination problems
that may arise when one wants to develop
a microfabrication process.
We will the different sources that are at the basis
of contamination and that can result in failure of devices.
We will introduce different options for creating an environment
with clean air and for clean room construction.
Finally, we will give an example of a clean room
by introducing the center of micro and nano technology at EPFL.
Different types of contamination exist which all can compromise
successful realization of a microfabricated device.
The first problem is posed by the presence particles in the air
which, when they have the size of the features
of the microfabricated structure, can result in failure
of the device.
This drawing shows as an example two micron wide metal lines
which can be realized using a thin film deposition technique.
When during deposition, micro particles are situated
on the substrate and when they have this size,
they can give rise to these connected metal structure
like shown here.
Another contamination problem is that of iron impurities.
If one works with semiconductor circuits,
the operation of device can be severely disturbed
when unknown metal impurities are present in the semi conductor.
Especially so-called Mobile Ion Contaminants
or MICs, most problems.
These contaminants are present in most of the chemicals
that are used for microfabrication of a device.
And when incorporated in the silicon,
they can migrate to a sensitive area where they can cause devise failure
even a long time after fabrication.
This implies that chemicals should be as pure as possible.
The table here shows metal impurities
that are present in a typical wet chemical
that is used in the clean room.
More generally, the presence of trace chemicals,
unwanted chemicals and process water of insufficient purity,
can result in unwanted surface etching effects
or deposition effects which are at origin
of non-uniformities in microfabrication processes.
If these contamination sources are present,
they affect the devices in different ways.
First of all, there is the devise processing yield.
Like shown in the drawing on the left,
a change in dimension or an interruption of device parts
results in non-functional device.
Such failure can be measured immediately
after microfabrication step by optical or microscopic inspection
or microscopic inspection.
Some defects are not directly visible but can be measured
by evaluation of the devise performance
after completion of the full microfabrication process.
Usually, electrical testing allows eliminating devices from the wafers
or chips from the wafer that are not functional.
Contamination also affects device reliability
and can cause failure of device a long time
after it's microfabrication.
An example of such failure can be the diffusion
of a mobile ion contaminant in an electronic circuit.
The impurity can migrate to a sensitive region
on the wafer, a long time after microfabrication
leading to disfunctioning of a microelectronic circuit.
An important source of possible contamination
in a clean room is the air.
Small particles can float in the air and remain there for a long time.
They're called also aerosols.
The fewer and the smaller particles are in the air,
the better is the quality of the clean room.
The quality of a clean room is therefore characterized
by the clean room class number
which is the number of particles above a specified diameter
in a cubic foot of air.
For example in this graph, we see that the class
thousand clean room has not more than
thousand particles of half a micron in size per cubic foot.
A class 10 clean room has not more than 10
of these particles in a cubic foot of air.
The lower the class number, the better the quality
of the clean room is.
As a rule of thumb, particles that are present in the air
should not exceed half of the feature size
of the microstructure to be fabricated.
As examples of air quality, a normal house room
has a class of 100,000, while a very large scale integration area
in a cleaning room should have class 10
for particles of 0.3 micrometer.
A person can produce in between 100,000
and 1 million particles per minute which are flakes
of dead hair and skin,
while normal clothing adds more millions of particles.
It is clear that the person has to be completely covered
when he is in the clean room.
And he needs to be covered with a special clean
and dust free tissue.
The photograph on the left shows the person
wearing special overshoes
which are above the normal shoes.
The person has gloves, and he wears an integral clean room suit,
a mask of the mouth and the nose
and gloves, and glasses to protect the eyes.
A particular dressing protocol has to be followed
when someone enters the clean room to minimize particle contamination.
Such protocol is shown in the following video.
A person who in a clean room, first, takes care of not entering dust
or contamination via shoes.
There's a particular way of entering.
Before the barrier, the environment is not clean
and going in by this type of motion during which the shoes are covered
with a clean protective coating,
ensures that no contamination is released from the shoes
into the clean room.
Next, follows putting on a coat where by one,
assures that the sleeves do not touch the ground.
Then follows putting on this type of overshoes to further
protect the clean room from contamination
originating from the feet.
Then follows putting on a mask for covering the heads and the hair
and a mask that covers nose and mouth.
Glasses are put on for eye protection and security.
The last stage is that the operator puts on gloves
for protection of the hands.
Now finally, he's ready to enter into the clean room.
A second source of contamination in a clean room is the water used
for processing the wafers.
All water that is used has to be treated,
as normal city water contains dissolved minerals,
particles and bacteria, which can introduce impurities
or non-uniformities during microfabrication.
Dissolved ions like sodium and chlorine ions
can compromise quality of microfabricated circuits
and can originate from salts dissolved in the water.
Therefore, the water has to be deionized
and the degree of ion concentration in the water
can be measured by measuring the resistivity of the water.
Indeed, if there are very few dissolved impurities,
the resistivity of the water is very large.
Eighty million Ohm centimeters.
If there are more impurities, resistivity is much lower.
For water used in a very large scale integration area in a clean room,
the resistivity of minimum 18 M Ohm cm is required.
Deionisation is a chemical process
that uses ion exchange resins that exchange protons
and hydroxyl ions for dissolved ions in the water.
After which, protons and hydroxyl ions recombine
to form a water molecule.
Besides process water, all process chemicals and gases
that are used in a clean room should be of high purity.
The most serious impurities are the metallic
mobile ion contaminants in chemicals.
And in a chemical concentration smaller than
one part per million are desired.
Also common gases that are used in a clean room such as oxygen,
nitrogen and hydrogen, and special gases like arsine
and carbon tetrafluoride, should be very pure.
The gas quality can be classified according to four criteria.
The percentage of purity of the gas.
It's water vapor content.
The number of particulates in the gas,
and the number of metallic ions in the gas.
The photographs on the left show the typical infrastructure
for water and gas handling which is present in proximity
of the clean room.
How does one obtain clean air with a minimum number of particles?
The first possibility is to do the processing of the wafers
in individual work stations with filtered air.
These work stations or hoods are arranged in rows
and are provided with high efficiency
particulate arrest or HEPA filters.
which contains small holes and a large surface.
Air is blown through such a filter
and a vertical laminar flow exposes the wafers
that are manipulated here in the hood.
So the wafers are only exposed to highly purified air
with the minimum number of particles.
If chemicals are operated in the hood,
this is not the right construction as the toxic fumes of the chemical
would go the operator.
If one needs to manipulate chemicals,
the hood is of this type.
Part of laminar flow is evacuated via this line
so that there is no risk for the operator who is working here
to be in contact with these chemical vapors
because basically, he's doing his manipulation here
in this area.
The picture on the left shows a hood
for manipulation of chemicals.
A drawback of such laminar flow hood is that the environmental conditions
can change due to the people that are present in the room
and who can be the origin of particles underneath the hood.
Therefore, a better but also a more costly solution,
is to build this HEPA filters in the ceiling of the room
and provide everywhere in the room clean conditions.
That is when one makes really clean room.
This brings us to concept of the total clean room
where the complete clean room has purified air
due to the presence of a continuous laminar flow
from the ceiling to the floor.
Also, through table tops with perforations, and so on.
Purified air will expose the wafers and the people,
and any generated particles will be transported away
through the floor and be purified before being re-injected
to HEPA filters in a clean room.
The clean room not only has good purity of the air,
but there is also a tight control of the temperature
which is essential for operate and comfort
and for reproducibility of chemical processes.
Also the humidity has to be controlled
as the presence of moisture on wafers
can influence many processes
like at adherence of a photoresist for example.
Also, smoke needs to be avoided.
As a reactive gas, it's like ozone,
it can interfere with many chemical processes
that are running in the clean room.
These two photographs show part of the air-handling infrastructure
of a clean room that is present above and below the clean room area.
A high quality clean room is essential for high quality research
in microfabrication related areas.
At EPFL, we have such a clean room which really is a corner stone
of many of our research and educational activities.
The following video provides a glimpse
of the EPFL center of micro and nano technology.
In this lesson I explained that the clean room
is a necessary infrastructure for the reproducible microfabrication
of miniaturized devices.
Air, process chemicals, water and gases
should all be of high purity.
We have discussed two approaches of creating
a clean room environment.
First there was the laminar flow hood for creating
locally clean air conditions
and next, there was a total clean room strategy
Finally, I introduce to you
the center of micro and nano technology at EPFL.