PHOTOLITHOGRAPHY

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PHOTOLITHOGRAPHY Photolithography is the heart of
the whole integrated circuit technology. It is
the process whereby patterns such as diffusion
regions, metal tracks, contact holes etc are
transferred onto the silicon wafer. The transfer
can be direct such as etching it on a layer of
metal. Or it can be indirect such as etching the
pattern first onto a layer of sacrificial silicon
dioxide and using the etched silicon dioxide
window (i.e. where oxide is removed) to carry out
a diffusion of the original pattern onto the
substrate silicon. Diagram on right shows the
same pattern transferred directly onto a metal
layer and indirectly on the silicon as an n-type
diffusion.
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The term photolithography correctly refers only
to the transfer of pattern from a master plate,
known as a mask, onto a light sensitive emulsion,
known as photoresist, uniformly coated on the
surface of the wafer. By transferring we mean
that the pattern on the mask is reproduced on the
photoresist by it being dissolved away
selectively as shown in this diagram (this
process is very similar to developing photograph
negatives). Note that the pattern on the
photoresist remaining can be the same as the
opaque pattern on the mask or the opposite
depending on the photoresist type used. When the
photoresist remaining has the same pattern as the
dark region(s) on the mask, it is known as a
positive photoresist. Photoresist retaining the
clear pattern on the mask (or having the opaque
pattern removed) is known as negative resist.
Positive resists are more popular currently
higher resolution. The transfer of pattern from
the mask to the photoresist is through exposure
to ultra violet light.
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The basic idea of photolithography is very
simple. However actual implementation for modern
day submicron technology is very demanding and
very costly operation. Resolution of the very
small features (lt0.1 micron in some cases) and
accurate alignment of these fine features on one
mask with existing features on the wafer
accurately (to less than about 1/3 the smallest
feature size) require extremely fine control of
the equipment and their operation high optical
resolution in the optical system and positional
resolution in alignment of mask pattern with
existing patterns on wafer. It is worth pointing
out that a complete modern integrated circuit
fabrication technology requires over twenty
separate photolithography steps i.e. involving
over twenty mask patterns to be transferred onto
the same chip on a wafer and for all chips across
the wafer. After each photolithography process,
the wafer is subjected to one or more fabrication
process steps (oxidation, diffusion,
metallization etc). Mask making is considered as
a separate operation and often carried out
outside the wafer fabrication plant with the
patterns for each mask (in what is known as cif
files) derived from the output of computer- aided
circuit design/layout of the integrated circuit
itself. We will not cover this side of
semiconductor industry. The following diagram
shows up clearly design and fabrication as two
distinct aspects of the integrated circuit
industry and how the design results are
transferred through the making of mask sets.
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Part of the wafer fab sequence
Information transfer is actually not one-sided as
shown the fabrication plant has to supply to
the designers the necessary design rules (such as
minimum feature size for each process, minimum
separation between features) and the information
needed convert the circuit layout to the masks
needed and patterns on each mask.
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The complete photolithography consists of a large
number of wafer processing steps summarised by
the flow chart on the right (for positive resists
but is also true for negative resists). After
these steps the patterns are transferred to the
relevant layer on the wafer (typically silicon
dioxide, silicon nitride or the metal/polysilicon
layer) by Pattern etch Photoresist removal and
clean The flow chart is demonstrated with the
diagram on the next slide in which the align and
exposure is by contact printing.
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Wafer cleaning Cleaning the wafer surface is
important prior to any fabrication step. The
cleaning is typically a chemical clean in a
number of chemical solutions followed by rinsing
high purity water (deionised water). In the case
of photolithography the critical process is
drying the surface to removal of any water
molecules on the surface by heating in an inert
ambient (100 to 120 C). A well known cleaning
recipe in the industry is the RCA cleaning
procedure which was developed in the 60s but is
still used today. The major steps for this
procedure is shown in the flowchart on the right.
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Coating of Adhesion Promoter (HMDS
Application) This is the application of a
chemical that would promote the adhesion of the
photoresist to the silicon surface. The chemical
used is hexamethyldisilane. It is applied to the
wafer by spinning coating the wafer is spun at
high speed (3000 to 6000 rpm, 30 to 60 seconds)
on a spinner and drops of the chemical is
applied to the surface. It spreads evenly on the
wafer. Control of spin speed and viscosity of the
adhesion promoter (hence also temperature as
viscosity changes with temperature) are essential
to ensure correct film thickness. Photoresist
Coating Photoresist is next applied to the wafer
in the same way. Again control of viscosity, spin
speed and even spin acceleration are important
parameters to control the thickness and
uniformity of photoresist film. The photoresist
can be applied while the wafer is stationary or
after the wafer reached its desired speed. Spin
speed of 3000 to 6000 rpm is used. The following
photos show the coating station and the spinner.
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There are various chemicals making up photoresist
(positive or negative). We will not cover this.
It is sufficient to say that in the part of the
photoresist that is eventually left behind as the
protective layer on the wafer is sensitive to
ultra-violet light. In the operation of the
negative photoresist, exposure to UV light
induces polymerization of this chemical. This
makes the exposed photoresist less soluble in the
solvent (referred to as the developer) used to
wash the resist after exposure. In the unexposed
region, no polymerization occurs and the chemical
is removed by the solvent. For positive resist,
there is a sensitizer in the photoresist which
inhibits the dissolution of the main chemical by
the developer. However exposure to UV light
causes the sensitizer to lose its ability to
inhibit the reaction between the developer and
the main chemical. Therefore during development,
the exposed photoresist is removed. UV intensity
and exposure time are two adjusting parameters
available for to achieve optimal exposure of the
photoresist. Pre-Bake The active components of
the photoresist are dissolved in a solvent to
allow the resist to be spin coated. After the
photoresist film is formed, the wafer is heated
up typically in an inert ambient (120 C) for a
controlled period to remove the solvent, harden
the resist and to improve adhesion to the wafer.
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Mask pattern transfer alignment and photoresist
exposure The process of pattern transfer from
the mask to the photoresist is carried out in a
optical tool known as a projection aligner or
wafer stepper. The pattern on the mask (also
referred to as the reticle) to be transferred to
the photoresist is optically (UV light) projected
on wafer and aligned with to the existing
patterns on the wafer and exposing the
photoresist on the wafer. In such a machine, the
pattern on the mask is typically at 10 times the
actual size to be exposed on the wafer. The chip
or die locations on the wafer are aligned and
exposed (for controlled length of time) one at
time by stepping the wafer in the x and y
direction (hence the term wafer stepper) until
all chips are exposed.
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In such as machine, the alignment is done
automatically by monitoring alignment of special
patterns known as alignment marks on the wafer
and on the mask pattern. Various optical
techniques are used to enhance the resolution of
the pattern projected on the image and going to
shorter wavelength (going from a wavelength of
365nm to 248nm). Prior to the use of wafer
steppers, the exposure is done with a mask
aligner. This uses a mask in which the patterns
to be transferred are at the final size and there
are as many such patterns as there are dies on
the wafer. This is shown below to contrast with
the projection system used in the stepper. The
diagram also shows two slightly different version
of mask aligners contact printer and proximity
printer. In the contact printer the mask is in
physical contact with the wafer whereas the
proximity printer, the mask and wafer are
separated by a small separation (to eliminate the
wear and tear on the mask). Notice that with a
mask aligner all dies on the wafer are exposed at
the same time but there is a special requirement
of stepping and repeating the same pattern on
the mask.
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