process of defining the regions or patterns on the wafer where material
is to be deposited or removed, or where dopants are to be introduced. One important
aspect of lithography is photoresist processing, which is the process of
covering areas that either need to be subsequently removed or retained
with a light sensitive film known as
The process of material removal following a photolithographic process is
known as etching.
layers have two basic functions: 1) precise
of the substrate from chemical attack during the etch process. Typical
resists consist of three components: 1) the
which serves as the binder of the film; 2) the
or sensitizer, which is the photoactive ingredient; and 3) the
which keeps the resist in liquid state until it is processed. The
pattern formed by the resist layer actually results only after the
unwanted portions of the erstwhile uniformly distributed resist
have been removed, as
explained in the following discussion on photoresist processing.
Photoresist processing, or simply resist processing, basically consists of
six steps: 1)
dehydration and priming; 2) resist coating; 3) soft baking; 4) exposure;
5) development; and 6) post-development inspection.
Prior to the
application of resist to a wafer, the wafer must be free of moisture and
contaminants, both of which cause a multitude of resist processing
Dehydration baking is performed to eliminate any
moisture adsorbed by substrate surfaces, since hydrated substrates
result in adhesion failures. The bake is usually performed between 400
deg C to 800 deg C. Convection ovens may be used for baking up to
400 deg C, while furnace tubes are used for 800 deg C baking. After
dehydration baking, the wafer is coated with a
pre-resist priming layer
designed to enhance the adhesion properties of the wafer even further.
One of the most common primers used for this purpose is hexamethyldisilazane (HMDS). Resist coating must follow as soon as
possible after priming (within an hour after priming).
or the process itself of producing a uniform, adherent, and defect-free
resist film of correct thickness over the wafer, is usually performed by
consists of dispensing the resist solution over the wafer surface and
rapidly spinning the wafer until it becomes dry. Most spin-coating
processes are conducted at final spin speeds of 3000-7000 rpm for a
duration of 20-30 seconds.
Resist coating is followed
which is done to: 1) drive away the solvent from the spun-on resist; 2)
improve the adhesion of the resist to the wafer; and 3) anneal the
shear stresses introduced during the spin-coating. Soft baking may be
performed using one of several types of ovens (e.g., convection, IR, hot
plate). Soft-bake ovens must provide well-controlled and uniformly
distributed temperatures and a bake environment that possesses a high
degree of cleanliness. The recommended temperature range for soft
baking is between 90-100 deg C, while the exposure time needs to be
established based on the heating method used and the resulting
properties of the soft-baked resist.
After a wafer has been
coated with photoresist and subjected to soft baking, it has to undergo
to some form of radiation
that will produce the pattern image on the resist. The pattern is
formed on the wafer using a mask, which defines which areas of the
resist surface will be exposed to radiation and those that will be
covered. The chemical properties of the resist regions struck by
radiation change in a manner that depends on the type of resist used.
Irradiated regions of
will become more soluble in
the developer, so positive resists form a positive image of the mask on
a negative image of the mask on the wafer because the exposed regions
become less soluble in the developer.
which is the process step that follows resist exposure, is done to leave
behind the correct resist pattern on the wafer which will serve as the
physical mask that covers areas on the wafer that need to be protected
from chemical attack during subsequent etching, implantation, lift-off,
and the like. The development process involves chemical reactions
wherein unprotected parts of the resist get dissolved in the developer.
A good development process has a short duration (less than a minute),
results in minimum pattern distortion or swelling, keeps the original
film thickness of protected areas intact, and recreates the intended
is carried out either by immersion developing, spray developing, or
puddle developing. Regardless of method used, it should always be
followed by thorough
that the development action will not continue after the developer has
been removed from the wafer surface.
as the name implies, is an inspection conducted after development to
ensure that the resist processing steps conducted earlier have produced
the desired results. This is typically done using an optical
microscope, although SEM and laser-based systems are also used in some
post-development inspection tasks. Items that this inspection step
checks for include the following: 1) use of the correct mask; 2) resist
film quality; 3) adequate image definition; 4) dimensions of critical
features; 5) defects and their densities; and 6) pattern registration.
S. Wolf and R. N. Tauber, "Silicon Processing for the VLSI Era Vol. 1",
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Silicon Processing for the VLSI Era, Vol. 1: Process Technology
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