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738 Polymers, Photoresponsive

FIGURE 22 SEM images depicting the manifestation of line-
width slimming and T-topping associated with chemically amplified
resists.

also is aggravated upon delays in baking. Figure 22 de-
picts examples of each of these issues. Base additives are
knowntoreducelinewidthslimming,possiblybyreducing
FIGURE 24 SEM images depicting nominal equal 60-nm
the diffusion of acid 126 or, more probably, by scavenging line/space features printed in a norbornene–maleic anhydride
small amounts of acid formed in nominally unexposed based photoresist similar to that described in Fig. 21.
areas. 127 In the case of T-topping, base additives have
been shown to alleviate this issue through either reduced
volatilization of acid via decreased diffusion or, alterna- inert compound was far less sensitive (50 mJ/cm ).
2 128
tively, creation of a low, uniform concentration of amine Other groups have also applied the concept of photode-
in the resist film which acts to overwhelm airborne basic composable base to 193-nm resins by using the more
contaminants depositing at the surface of the resist. 126 transparent trialkylammonium hydroxyde. 129
The possibility of utilizing photodecomposable Figure 24 depicts representative images that can be
aminosulfonate moieties capable of affording free amino- achievedinalicyclic193-nmlithographicmaterials.Inthis
sulfonic acids has been investigated. 128 In ester form, particular case, the polymer was a multicomponent mate-
these materials are inherently basic, yet upon exposure to rial comprised of norbornene, maleic anhydride, t-butyl
light they generate an acid, so they have been called acrylate, and acrylic acid. A t-butyl cholate-based disso-
photodefinable bases (PDB). It has been shown that the lution inhibitor was used in conjunction with an onium
use of such materials leads to enhanced resist sensitivity salt photoacid generator.
because the aminosulfonate moiety is partially removed
in the exposed resist film but remains unchanged in the
unexposed areas where its basic properties act to limit VI. 157-NM RESIST DESIGN
diffusion. For instance, Fig. 23 shows the structure of two
different cyclamate materials employed as additives in a
As device design rules continue to shrink, research direc-
193-nm single-layer resist based on a norbornene–maleic
tions transition towards future lithographic alternatives.
anhydride resist platform: The photodecomposable The next logical extension of optical lithography involves
2
moiety gave a resolution dose of 22 mJ/cm , while the
the continued progression to still shorter wavelengths. As
optical lithography has evolved over the past few decades,
first from tools based on the 248-nm excimer laser (KrF),
followed by 193-nm (ArF) systems, the next frontier uti-
lizesfluorine,157-nmUVsources. 130 Whereopticaltrans-
parency and etching resistance were the key concerns for
the development of 193-nm lithographic materials, the
overriding issue for 157-nm advanced optical lithography
isresistmaterialstransparency.Traditionalresistmaterials
platforms are too opaque to allow imaging in sufficiently
thick films to address defect density concerns. For an op-
tical density of 0.4, which is considered optimum for most
resist applications, acrylic, phenolic, and cycloolefin poly-
mer platforms would require resist thicknesses less than
FIGURE 23 Structural representation of two cyclamates that 131
could be employed as base additives in chemically amplified 100 nm (Table III). This value needs to be compared to
resists. the anticipated required film thickness for 50- to 100-nm