DS 7-7R 17-12R Semiconductor Fabrication Facilities ... - FM Global

7-7R
REFERENCE DOCUMENT
17-12R SEMICONDUCTOR FABRICATION FACILITIES
Page 20
with the mask plate, and then exposed through the action of the shutter of the machine opening to allow ultraviolet
light to hit the unmasked portion of the wafer.
After the wafer has been aligned and exposed, the next step is developing. In developing a wafer, a machine
similar to a ‘‘spinner’’ is used. The developing is done by chemicals which are sprayed down onto the wafer.
This spray washes away the nonexposed resist (areas where the light was not allowed to pass through the
mask plate) while the exposed or ‘‘polymerized’’ resist remains. The preferred developing chemical for negative
photoresist is xylene. A Stoddard solvent may also be used in certain cases. Positive photoresist is developed
in an alkaline solution, such as potassium hydroxide or sodium hydroxide.
Other flammable solvents are also used in the wafer fabrication process. Butyl acetate and isopropyl alcohol
will be used as washes for wafers after they have been developed with negative resist. D.I. water is more
commonly used with positive photoresist as a post develop wash.
Etching removes layers of silicon dioxide, metals and polysilicon as well as resists, according to desired
patterns delineated by the resist. The two major categories of etching are wet and dry chemical. Wet etching
is predominantly used and involves solutions containing the etchants (usually an acid mixture) at the
desired strengths, which react with the materials to be removed. Plastic wet benches and plastic fume exhaust
ductwork are typically used in wet etching operations (Figures 17 and 18 of Data Sheet 7-7/17-12). Dry etching
involves the use of reactive gases (hydrogen chloride, ammonia, etc.) under vacuum in a highly energized
chamber, which also removes the desired layers not protected by resist.
To form the junctions where current will flow, a controlled number of impurities or dopants must be introduced
into a selected region of the wafer either by diffusion or ion implantation. Diffusion is a high temperature
(1652°F to 2372°F [900°C to 1300°C]) process in which certain chemicals (dopants) are introduced into
the surface layer of the semiconductor material to change its electrical characteristics. Diffusion is the most
established method of applying dopant material. Ion implantation is a technique for doping impurity atoms into
an underlying substrate by accelerating the selected dopant ion towards the silicon target through an electrical
field. Ion implantation is often preferred over standard diffusion methods because it is more precise,
faster and less expensive. Annealing usually is required following ion implantation because of the structural
damage caused by bombardment of the substrate by the accelerated ions.
The need for annealing after ion implantation led to the development of a technology called Rapid Thermal
Processing (RTP). This process, which takes place in seconds, eliminated the need for a minutes-long
process in a tube furnace, which had undesirable side effects of migration of dopant atoms within the wafer.
Also, every time a wafer is heated near diffusion temperatures and then cooled down, crystal dislocation
forms, which can result in circuit failures. In the single wafer RTP tool, radiation heating (usually from tungsten
halogen lamps) is very rapid and the body of the wafer never comes up to temperature. Annealing can
take place without undesirable side effects. The trend to small feature sizes on wafers has also lead to thinner
layers. Thermally grown gate oxide layers now may be less than 100 Angstroms thick. RTO ( Rapid Thermal
Oxidation) tools are similar to the RTP annealing tools but have an oxygen atmosphere in the chamber
rather than an inert gas. RTP technology is now used in various oxide, nitride and silicon layer processes.
Deposition is the process of placing additional layers onto the wafer surface, either by epitaxial or chemical
vapor deposition (CVD). Chemical vapor deposition is the process of forming a thin film on a substrate by
the chemical reaction of various gases. CVD is usually promoted by heating the substrate, either at atmospheric
pressure, or low pressure (LPCVD). Epitaxy is the process of depositing a crystalline layer having the
same structure as the substrate. Epitaxy represents a special form of chemical vapor deposition. Often,
epitaxial layers are grown with intentionally added impurities such as boron or phosphorus. These change
the electrical conductivity of the crystalline silicon. Some of the more common process reactions can be found
in Table 6 of Data Sheet 7-7/17-12.
The photoresist, masking, etching, doping and deposition processes are repeated many times until the complete
circuit is produced.
After the final diffusion step, the devices which have been fabricated into the silicon wafer must be connected
together to perform circuit functions. This process is known as metalization. Metalization provides a
means of wiring or interconnecting the uppermost layers of integrated circuits by depositing complex patterns
of conductive material, which route electrical energy within the circuits. To do this, a conductive metal is either
sputtered or evaporated over the front of the wafer. A photoresist pattern is then aligned over the metal and
some of it is etched away, leaving the desired metal coverage. The most common metals used for metalization
are: aluminum, nickel, chromium, gold, copper, silver, titanium, tungsten and platinum.
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