Some coatings, such as solder or plating resists and photoresists, are used temporarily during processing. These coatings are usually used to shield critical portions of the circuitry during subsequent processing, such as soldering or laser ablation, or to selectively mask portions of a surface in the photofabrication processing of microcircuits and thin-film circuits.
Permanent maskants, previously described, are generally used for PWBs as solder maskants but, in some cases, removable maskants are more appropriate. For example, in assembling hybrid microcircuits, in which both wire bonding and solder attachment processes are used, the soldering processes are performed first, but the remainder of the circuit must first be protected from solder splatter and flux residues. Polyvinyl alcohol and polyvinylacetate coatings provide this protection and are subsequently removed by dissolving in water or alcohol. Urethane/acrylate formulations that cure on exposure to 365 nm wavelength radiation may subsequently be removed either by peeling or dissolving in water.
In the formation of microvias in polyimide film or in other substrates by laser ablation, carbonaceous, non-volatile residues deposit on adjacent surfaces. Here too, water-soluble coatings such as PVA have been used to shield adjacent surfaces, for example, in the processing of Tape Automated Bonding (TAB) devices.[92]
Photoresists are a major class of temporary organic coatings essential in the fabrication of all microelectronic devices and thin-film circuits. Advances in photoresists together with optical tools have enabled the continued shrinking that has been occurring in device dimensions from
5 µm in the late 1960s to approximately 0.1 µm today. During this period, chemists have synthesized unique photosensitive polymers to satisfy the resolution, light sensitivity, and processing requirements for each successive generation of semiconductor and integrated circuit chips. Concurrently, physicists have optimized the optics and optical tools. The process by which photoresists are used to etch intricate and precise lines, spacings, and vias in metals and dielectrics has been a key factor in the rapid development of microelectronics from the early 1960s until today.
Photoresists are organic compositions consisting of radiation-sensitive
polymers or polymer precursors together with additives such as
photoinitiators dissolved in one or more non-aqueous or aqueous
solvents. They are of two types: those that on exposure to uv light or
other radiation source polymerize or crosslink to form a hardened
coating that is resistant to etching solutions (negative types) and
those that on exposure to light or other radiation source decompose
into constituents that are easily washed away with a developer solution
while the remaining coating becomes hardened and further polymerized by
the same developer (positive types).
Negative photoresists were the first compositions used to pattern
semiconductor devices and thin-film microcircuits and still comprise
the largest segment of the photoresist industry because they are also
widely used in the fabrication of PCBs. Early formulations from Eastman
Kodak consisted of cinnamic acid esters that polymerized by a
free-radical addition mechanism on exposure to uv light.[93]–[96]
Positive photoresists, first introduced by Azoplate in the 1970s, are
based on an entirely different chemistry than negative resists.
Positive photoresists have replaced negative types for high resolution
circuits, but advances are still being made in the chemistry of the
negative resists to enhance their ability to produce very fine circuit
features.[97] In general, uv light in the 200 to 450 nm wavelength is
used, with 300–450 nm used for most applications while 200–300 nm deep
uv is used to obtain higher resolutions. Deep-uv negative resists have
been reported to produce 150 nm lines and
230 nm spacings.[98]
Generally, the photoresist is first applied over the entire surface of
a substrate (for example, by spin coating), baked at a low temperature
(soft bake) to remove solvents, then exposed to the radiation source
through a separate mask which may consist of either a Mylar film or a
glass plate having opaque and transparent regions corresponding to the
image to be produced. This mask is referred to as the artwork,
photo-tool, or hard mask. After exposure to light, the photoresist is
developed, a process by which it is immersed in or sprayed with a
chemical solution that dissolves the unexposed portions of the
photoresist (in the case of negative resists) or dissolves the exposed
portions (in the case of positive resists). The remaining photoresist
may then be hard-baked to render it more resistant to the subsequently
used etching solutions. In using a negative resist, the mask must have
a negative image of the pattern to be produced. Figure 4.42 illustrates
the steps in etching a thin film of gold on an alumina substrate using
a negative photoresist and a negative image mask. The pattern produced
is the opposite of the image on the mask.
In a positive photoresist, the areas exposed to uv light are degraded
or decomposed and consequently readily dissolved and removed. The
chemistry is such that the remaining (unexposed) areas are hardened and
rendered resistant to etching solutions simultaneously by the same
solution (developer) that is used to dissolve the exposed areas. Hence,
a positive image on the mask results in the same pattern on the
substrate. The steps in etching metallization using a positive resist
are shown in Fig. 4.43. In all, four combinations are possible,
depending on whether a positive or negative resist is used with a
positive or negative mask (Fig. 4.44). The chemistry of photoresists is
quite varied and complex and has been extensively described in numerous
articles and books.[8][99]–[101]
Besides uv and deep uv, other radiation sources and lithographic
processes are being used to produce even finer dimensions.[96]–[98]
Among these are: electron beam lithography (<1 µm resolution), x-ray
lithography (0.02 µm resolution), and ion-beam lithography (<1 µm
resolution). The resolution of features is ultimately limited by the
wavelength of the exposing radiation and the chemistries of the resist
materials.[102] These factors are critical in fabricating semiconductor
devices where submicron dimensions are required. However, for PCBs
whose geometries are on the order of mils, uv photolithography is the
most commonly used process. These resists may also be negative or
positive.[103]
Photoresists are generally liquid compositions that are applied by spin
coating onto circular wafers or, by roller-coating or screen printing
on large panels for printed wiring boards. The introduction of dry-film
Riston® photoresists by DuPont in 1968 was a great advance in uniformly
coating large PWBs, producing finer lines and spacings, and improving
yields. Pinholes and touch-up, previously required with the liquid
photoresists, were virtually eliminated since the dry-film was
processed under clean room conditions and protected by a polyester
support film and a separator sheet that are removed just prior to use.
The dry-film resist is then laminated to the circuit board, exposed and
developed. Riston photoresists are now available as both negative and
positive types having various resolution capabilities from several mils
to below 0.1 mil.
Figure 4.44. Mask/photoresist combinations.
A recent development is the deposition of liquid photoresist by an
electrophoretic process. This process also permits the coating of large
flat panels, but has the added advantage of being able to coat
irregular shapes and three-dimensional surfaces. A positive-acting
photoresist is electro-deposited from an electrolytic aqueous bath in a
fashion similar to the electroplating of metals. The photoresist
consists of positively charged particles that are attracted to a
metallized surface (such as a copper-clad PWB laminate) that is
rendered cathodic through an applied potential of 60 to 100 volts. It
is reported that thin uniform coatings of 0.2 mils can be deposited and
can conformally coat all metal surfaces.[104]