Page 220 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GOJ/GOI/LCC/HAR P2: GOJ Final Pages
Encyclopedia of Physical Science and Technology en012K-946 July 26, 2001 11:14

Polymers, Photoresponsive 729

a hydrophobic, substituted diazonaphthoquinone (DNQ)
dissolution inhibitor (DI). Addition of this component
to the novolac renders the polymer matrix insoluble in
aqueous-base developers. Upon irradiation, the DNQ un-
dergoes a Wolff rearrangement followed by hydrolysis,
yielding indene carboxylic acid. The photogenerated acid
is hydrophilic and allows the exposed regions of the film to
be dissolved in aqueous alkaline solution. The remaining,
nonexposed regions are unaffected and do not swell in the
developer. The dissolution mechanism has long attracted
57
the attention of many workers. Most recently, Reiser has
postulated a mechanism based on percolation theory. In
this model, the diffusion of base is regulated by the density
of hydrophilic percolation sites in the solvent penetration
zone. This density can be changed by the introduction of
additives (i.e., the inhibitor) which block some of the hy-
drophilic sites, and it can be changed by irradiation (i.e.,
conversion of inhibitor to indene carboxylic acid). The
novolac–DNQ chemistry affords high resolution and, as
a consequence of the aromatic nature of the resin, good
dry-etching resistance for pattern-transfer processes.
The performance of conventional resists depends on the
precise structure of the photosensitive DI and the novolac
resin. 58,59 Most photoresists designed for 365- to 436-nm
exposure utilize a 1,2-diazonaphthoquinone-5-sulfonate
ester that exhibits absorbance maxima at ∼340, 400, and
430 nm. Changes in the position of the aryl substituent can
lead to variations in sensitivity and light absorption. For
example, the 4-aryl sulfonate analogs exhibit absorption
FIGURE 7 Structural representation of the chemistry and pro-
cesses associated with plasma-polymerized methylsilane pho- characteristics that are more appropriate for shorter wave-
58
toresists with an SEM image depicting 0.25-µm images obtained length exposure (i.e., 313 and 365 nm). This substitution
in this resist. pattern leads to the appearance of a bleachable absorbance
at ∼315 and 385 nm, extending the sensitivity of conven-
tionalresiststoshorterwavelengths.Optimizationofresist
D. Dissolution-Inhibition Resists
sensitivity for a particular exposure tool requires an under-
Photolithography represents the workhorse technology for standing of the effect of substituents on the absorportion
device manufacture and has traditionally used a Hg or Hg– characteristics of the materials. This approach was used
Xe discharge lamp as the radiation source. Resist systems effectively by Miller and coworkers, 52 who coupled such
that have been developed to respond favorable to this en- studies with semi-empirical calculations to facilitate the
ergy spectrum (250–450 nm) are often called “conven- design of diazonaphthoquinone dissolution inhibitors for
tional photoresists” (Fig. 8) and are typically comprised mid-UV applications.
of two components: an aqueous alkali-soluble resin and The high absorbance of conventional photoresists pre-
a photosensitive dissolution inhibitor. 2,55 The alkaline- vents their application to shorter wavelength (<280 nm)
soluble resin, a novolac, is prepared via condensation lithographies. As a result, alternative resins and dis-
polymerization of a substituted phenol and formaldehyde. solution inhibitors have been proposed. Examples in-
These resins, and modifications thereof, are formulated to clude dissolution inhibitors based on 5-diazo-Meldrums’s
exhibit low absorbance in the near- and mid-UV region, Acid chemistry for novolac resins 60 and 2-nitrobenzyl
61
are glassy amorphous materials at room temperature, and carboxylates (Fig. 9). In the case of the nitrobenzyl car-
can be dissolved in a variety of organic solvents useful boxylates, optimum results were obtained for ester deriva-
for spin-coating applications. In addition to alkyl sub- tives of large-molecule organic acids such as cholic acid.
stituents, silicon-bearing moieties have been examined for These esters are nonvolatile and allow conversion of a
enhancement of oxygen-reactive ion-etching resistance. 56 relatively large volume fraction of resist from an alkali-
The second component of conventional photoresists is insoluble to an alkali-soluble state. While these resists