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Radiation chemistry of polymeric materials:novel chemistry and applications formicrolithographyElsa Reichmanis,* Omkaram Nalamasu, Francis M Houlihan andAnthony E NovembreBell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA
Abstract: In the last two decades, major advances in fabricating very large scale integration (VLSI)
electronic devices have placed increasing demands on microlithography, the technology used to
generate today's integrated circuits. In 1970, state-of-the-art devices contained several thousand
transistors with minimum features of 10±12mm. Today, they have several million transistors and
minimum features of less than 0.3mm. Within the next 10±15 years, a new form of lithography will be
required that routinely produces features of less than 0.2mm. Short-wavelength (deep-UV)
photolithography and scanning and projection electron-beam and X-ray lithography are the possible
alternatives to conventional photolithography. The consensus candidate for the next generation of
lithography tools is photolithography using 193nm light. At this wavelength, the opacity of traditional
materials precludes their use, and major research efforts to develop alternative materials are currently
underway. Notably, the materials being developed for these short UV wavelengths are demonstrating
compatibility with the more advanced electron-beam technologies. Materials properties must be
carefully tailored to maximize lithographic performance with minimal sacri®ce of other performance
attributes, eg adhesion, solubility and RF plasma etching stability.
# 1999 Society of Chemical Industry
Keywords: lithographic materials; resists; photopolymers; imaging
INTRODUCTIONMicrolithography has frequently been called the
lynchpin technology used to generate today's inte-
grated circuits. These devices are complex three-
dimensional structures of alternating, patterned layers
of conductors, dielectrics and semiconductor ®lms
which are fabricated on an ultrahigh purity wafer
substrate of a semiconducting material such as silicon.
The structure is produced by a series of steps (Fig 1)
used to precisely pattern each layer by lithographic
processes that consist of two stages: (i) delineation of
the patterns in a radiation sensitive thin-polymer ®lm
called the resist, and (ii) transfer of that pattern using
an appropriate etching technique.1 The performance
of the device is, to a large degree, governed by the size
of the individual elements and as a general rule, the
smaller the elements, the higher the device perfor-
mance will be.
The very signi®cant advances in the design and
fabrication of very large scale integration (VLSI)
electronic devices that have occurred over the past
few decades, have placed increasing demands on
microlithographic technology. Put in perspective, in
1970 state-of-the-art devices contained several thou-
sand transistors with minimum features of 10±12mm,
while today's advanced commercial products have
several million transistors and minimum features of
less than 0.3mm. Continued advances will require
implementation of a new form of lithography that
routinely produces features of less than 0.15mm within
the next 5±10 years. Alternative candidates to con-
ventional photolithography employing 265±425nm
light include short-wavelength (deep-UV) photolitho-
graphy, scanning and projection electron-beam, and
X-ray lithography. Photolithography employing
193nm light is the consensus candidate for the next
generation of lithography tools.2 At this wavelength
however, the opacity of traditional materials precludes
their use, and major research efforts to develop
alternative materials are currently underway. Materials
properties must be carefully tailored to maximize
lithographic imaging performance with minimal sacri-
®ce of other performance attributes, eg adhesion,
solubility and radio-frequency (RF) plasma etching
stability. Notably, the materials being developed for
these short UV wavelengths are demonstrating com-
patibility with the more advanced electron-beam
technologies.
Polymer International Polym Int 48:1053±1059 (1999)
* Correspondence to: Elsa Reichmanis, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA(Received 19 November 1998; accepted 23 April 1999)
# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50 1053
The radiation chemistry of polymeric and organic
materials plays a signi®cant role in today's device
fabrication technologies. Poly(ole®n-sulphones), a
class of materials pioneered by O'Donnell and co-
workers,3 are the workhorse materials for the fabrica-
tion of the photomasks used to image a circuit pattern
onto a device substrate.4,5 While materials such as
poly(1-butene sulphone) (Fig 2) continue as the
dominant imaging resists for photomask fabrication,
issues such as dry-etching resistance and aqueous
solubility must be addressed for both advanced mask
and device applications. In the device arena, an
overwhelming preponderance of devices continues to
be fabricated via `conventional photolithography'
employing 350±450nm light.6 Incremental improve-
ments in tool design and performance with concomi-
tant re®nements in the `workhorse resist', a material
based on an aqueous base soluble novolac resin
formulated with a substituted diazonaphthoquinone
dissolution inhibitor (Fig 3), have allowed the con-
tinued use of this technology to produce ever smaller
features. The cost of introducing a new technology,
which includes the cost associated with the develop-
ment and implementation of new hardware and resist
materials, is a strong driving force pushing photo-
lithography to its absolute resolution limit and
extending its commercial viability. As device feature
sizes approached 0.25mm and the industry moved
towards using 248nm excimer laser UV light as the
exposing wavelength for advanced lithographic appli-
cations, the materials community saw the ®rst revolu-
tionary change in resist materials chemistry to be
adopted.
The absorbance of conventional novolac/diazo-
naphthoquinone photoresists is too high to allow
uniform imaging through a practical ®lm thickness
(0.5±1mm) at exposing wavelengths less than about
300nm. In addition, the available light at the exposure
plane of commercial exposure tools using light sources
below 300nm is insuf®cient to provide for manufac-
turable processes when the quantum ef®ciency of a
resist in less than unity.7,8 The materials breakthrough
that ultimately led to the adoption of 248nm
lithography as the technology of choice for advanced
device fabrication was the announcement of what has
been termed the `chemically ampli®ed' resist mechan-
ism. The pioneering work relating to the development
of chemically ampli®ed resists based on deprotection
mechanisms was carried out by Ito, Willson and
Frechet.9,10 The initial studies dealt with the catalytic
deprotection of poly(tert-butoxycarbonyloxystyrene)
(TBS) in which the thermally stable, acid-labile tert-butoxycarbonyl group is used to mask the hydroxyl
functionality of poly(vinylphenol). The mechanism of
resist action is shown in Fig 4.
Since the initial reports regarding chemically ampli-
®ed resist mechanisms in the early 1980s, numerous
research groups have expanded on the concept.7,8
Thermally stable, acid-labile substituents are desirable
as protective groups for aqueous base soluble parent
polymers. Examples of chemistries that have been
employed include tert-butyl ethers and esters, tetra-
hydropyranyl ethers, a,a-dimethylbenzyl esters, and
ketals and acetals. Alternative polymer backbones
include poly(hydroxystyrene), poly(vinylbenzoic
acid), and poly(methacrylic acid). Additionally, high
glass transition temperature (Tg) polymers based on
N-blocked maleimide/styrene resins and substituted
styrene±sulphone copolymers have been explored. In
this latter case, tert-butoxycarbonyloxystyrene was
copolymerized with sulphur dioxide to afford a
random copolymer of the two monomers.11±13 As in
the case of TBS, the tert-butoxycarbonyl moiety was
used as the acid labile protective group. The inclusion
of sulphur dioxide into the backbone of the polymer
affords a high Tg that gives greater ¯exibility for
processing. Additionally, introduction of sulphur
dioxide into similar polymers effectively improved
Figure 1. Schematic representation of the lithographic process.
Figure 2. Structural representation of poly(butene-1-sulphone).
1054 Polym Int 48:1053±1059 (1999)
E Reichmanis et al
their sensitivity to electron beam radiation due to
CÐS bond scission. While negligible difference
in sensitivity between TBS and the sulphone analogue
were observed when these polymers were used in
conjunction with onium salt photoacid generator
materials, the resist exposure dose was reduced by as
much as a factor of 2.5 when a nitrobenzyl ester
photoacid generator was employed. In fact, when
exposed to X-ray irradiation, the copolymer is an
effective single component chemically ampli®ed re-
sist.14 Presumably, radiation induced CÐS bond
scission leads to generation of either sulphinic or
sulphonic acid end-groups that subsequently induce
the deprotection reaction (Fig 5).
193 nm LITHOGRAPHIC MATERIALSClearly, materials structure plays the key role in
de®ning performance. As the drive to still smaller
features to accommodate increased circuit densities
continues, alternative lithographic exposure strategies
are evolving. The leading candidate for the manufac-
ture of 0.18 ±0.13mm design rule devices is photo-
lithography using 193nm radiation.2,15,16 The ®rst
experiments demonstrating the feasibility of UV light
as an imaging source for lithography occurred at Bell
Laboratories in 1975.17,18 Bowden and Chandross
demonstrated the concept using poly(butene-1-sul-
phone) which, upon exposure to 185nm light,
exhibited a sensitivity of 5mJcmÿ2. The drive towards
increased integration, fuelled by a desire to maintain
the availability of optical lithography in the device
production environment, led to research and develop-
ment efforts aimed at developing a production-worthy
193nm lithographic technology.
The intense absorption of aromatic molecules at
193nm severely limits the use of conventional matrix
resins such as novolacs and polyvinylphenols for
193nm lithography. This has both necessitated a
paradigm shift in the approach to lithographic
materials and process design, and spawned the design
of new resist schemes.2 Processes under consideration
for use with the 193nm technology include the
traditional solution developed methodologies in addi-
tion to dry-developed techniques. The latter span the
range of silicon containing bilevel approaches that
were of signi®cant interest approximately one decade
ago, to silylation processes, to the more recently
described all-dry, plasma deposit/plasma develop
systems. In recognition of the strong motivation of
Figure 3. Schematic representation of conventional novolac/diazonaphthoquinone photoresist chemistry.
Figure 4. Acid-catalysed deprotection of poly(tert-butoxycarbonyloxystyrene) affording the aqueous base solublepolyvinylphenol.
Polym Int 48:1053±1059 (1999) 1055
Radiation chemistry of polymeric materials
device manufacturing engineers to retain as much of
the acquired knowledge base regarding solution
developed resists as possible in the design of materials
for advanced lithographic applications, many of the
current research efforts related to 193nm materials
involve the design of new chemistries that provide for
aqueous base solubility, etching resistance, resolution,
photospeed and process latitude.
Avenues that can lead to transparent, etching
resistant polymer include the incorporation of alicyclic
and/or silicon bearing substituents.19±22 To date, most
efforts have focused on derivatized acrylate and
methacrylate copolymers.15,16,22±26 The fundamental
design challenge that has emerged appears to be the
necessary trade-off between plasma-etching resistance
and requisite materials properties for lithographic
performance. On the whole, high carbon content
copolymers functionalized with pendant alicyclic
moieties possess adequate etching resistance, but tend
to be brittle, display poor adhesion and have sub-
optimal imaging characteristics due to poor aqueous
base solubility. Decreased alicyclic carbon content
results in improved lithographic performance at the
cost of lower etching resistance. Recent approaches for
addressing this fundamental design challenge include
both careful tailoring of polymer properties to max-
imize lithographic performance with minimal sacri®ce
in etching resistance and development of three-
component systems in which high carbon content
alicyclic additives serve not only as dissolution
inhibitors but also enhance the etching resistance of
the matrix as a whole.16,22,27±30
While methacrylate-based resist platforms are at-
tractive from an economic perspective, they suffer
from the fundamental drawback of possessing a linear,
oxygen rich scaffold whose poor plasma-etching
stability can be offset only partially by functionaliza-
tion with more stable pendant groups. In a more ideal
resist platform, greater intrinsic plasma-etching stabi-
lity might be imparted through incorporation of
alicyclic moieties directly into the polymer backbone
and by minimizing oxygen content, preferably by
designing oxygenated functionalities to play only
necessary imaging and solubilizing roles rather than
incidental structural roles.
In pursuing alternate 193nm single-layer resist
platforms, we have developed a new class of matrix
resins that more closely approximates the structural
ideal. Unlike methacrylate-based systems, these ma-
terials, based on cycloole®n±maleic anhydride alter-
nating copolymers, contain large quantities of alicyclic
structures directly in the polymer backbone.31,32
While these copolymers retain some oxygenated
functionalities in structural roles, the oxygen content
is decreased relative to methacrylates. Compelling
features of such copolymers include: (i) facile synthesis
via standard radical polymerization techniques, (ii) a
potentially large pool of cycloole®n feedstocks, and
(iii) a generic structural motif that incorporates
alicyclic structures directly into the polymer backbone
Figure 5. Deprotection mechanismassociated with the radiation inducedchain scission of poly(tert-butoxycarbonyloxystyrene-sulphone).
1056 Polym Int 48:1053±1059 (1999)
E Reichmanis et al
and provides a latent water-solubilizing group that
may also be useful for further structural elaboration.
While a large number of cycloole®ns are known to
copolymerize with maleic anhydride, to date, our
efforts have concentrated on norbornene (Fig 6).
Alternating copolymerization of norbornene (NB) and
maleic anhydride (MA) occurs readily at 65°C in a
variety of solvents including THF, dioxane, acetone
and cyclohexanone, using 2,2'-azobisisobutyronitrile
(AIBN) as an initiator.31 P(NB/MA) is a colourless
powder with Tg>300°C and an onset temperature of
decomposition (under argon) of 370°C. Thin ®lms
cast on quartz display excellent transparency at 248
and 193nm. The polymer is soluble in ketones,
including cyclohexanone and methyl isobutyl ketone,
but is insoluble in esters such as ethyl-3-ethoxypro-
pionate and propylene glycol methyl ether acetate.
P(NB/MA) is hydrolytically robust: resist formulations
prepared in cyclohexanone may be stored for extended
periods without signi®cant change in development
behaviour if minimal precautions are taken to prevent
hydrolysis. A less fortunate consequence of hydrolytic
stability is that P(NB/MA) ®lms do not dissolve at
useful rates in standard (0.262N tetramethylammo-
nium hydroxide) developing solutions. Terpolymer-
ization with acrylic acid provides a controllable
method of synthesizing aqueous base soluble resins:
systematic variation of acrylic acid feed ratios from 5 to
20% yielded progressively more base-developable
Figure 6. Structural representation of norbornene–maleic anhydridecopolymer.
Figure 7. Structural representation of the chemistry associated with a 193nm resist formulation during selected process steps.
Polym Int 48:1053±1059 (1999) 1057
Radiation chemistry of polymeric materials
formulations. For acrylate loadings up to about 20%,
no signi®cant deviation from a 1:1 ratio of the
norbornene to maleic anhydride `repeat unit' was
observed. Additionally, there is a simple, apparently
linear relationship between the feed of acrylate
monomers and their incorporation into the ®nal
polymer.31 Gel permeation chromatographic analysis
of these materials gives clean monomodal peaks with a
polydispersity of less than 2.5. The polystyrene
equivalent Mw ranged from 4000 to 8000. Composi-
tions resulting from 15 and 17.5% acrylic acid feeds
display the most useful development behaviour and
have been studied in some detail.31±36 The acrylate
terpolymers possess thermal and optical properties
and organic solubilities indistinguishable from those of
P(NB/MA). At 248 and 193nm, the absorbances per
micron of poly(norbornene-alt-maleic anhydride-co-
acrylic acid) where the acrylate unit constitutes 15%
were 0.05 and 0.27, respectively. Adhesion to silicon
device substrates is improved upon incorporation of
the acrylate unit.
In addition to its base solubilizing attributes, acrylic
acid provides a template to further functionalize the
polymer. Notably, tert-butyl acrylate is readily incor-
porated into the polymer chain via free radical
copolymerization.31 In the presence of acid and mild
heating, the ester is cleaved to liberate isobutylene and
the parent acid. This mechanism can be used in the
design of sensitive, high-resolution, chemically-ampli-
®ed resist formulations. In one example, a quaternary
polymer of NB, MA, acrylic acid and tert-butyl acrylate
is formulated with a substituted cholate ester that acts
as a dissolution inhibitor, and an onium salt-based
photoacid generator. Structures of each of the resist
components and associated relevant process steps are
shown in Fig 7. Exposure to 193nm light generates an
acid which then reacts with the ester appendages on
both the polymer and inhibitor. Development of the
resultant latent image in aqueous base affords high
resolution patterns. Figure 8 depicts SEM micro-
graphs of typical patterns obtained with these materi-
als upon 193nm exposure. Similar results are obtained
Figure 8. Scanning electron micrographs of nominal 0.15mm equal line/space patterns and 0.11mm isolated line features printed in a norbornene–maleicanhydride based photoresist.
Figure 9. Scanning electron micrographs depicting 60nm images printed at 193nm using phase shift mask technology.
1058 Polym Int 48:1053±1059 (1999)
E Reichmanis et al
upon electron-beam irradiation. Additionally, images
as small as 60nm have been demonstrated using
advanced phase-shift mask techniques (Fig 9).
CONCLUSIONSThe radiation chemistry of polymeric and organic
materials plays a signi®cant role in today's device
fabrication technologies. O'Donnell's work in the
synthesis and radiation degradation of poly(ole®n-
sulphones) led to the development of poly(butene-1-
sulphone) as a high resolution, positive electron-beam
resist. While the poly(ole®n-sulphone) continues as
the dominant imaging resist for photomask fabrica-
tion, issues such as dry-etching resistance and aqueous
solubility must be addressed for both advanced mask
and device applications. Clearly, materials structure
plays the key role in de®ning performance. Recently, it
has been demonstrated that alicyclic units can be
effectively incorporated into polymer architectures to
afford radiation sensitive resist compositions that are
sensitive to both short-wavelength UV and electron-
beam irradiation. Fundamental studies into the
materials properties and interactions between resist
components and materials radiation chemistry enable
formulation of high-performance resists that display
extremely reproducible lithographic properties. This
requires careful manipulation of a large number of
physical properties that govern solubility, sensitivity,
image ®delity, etc. In designing new lithographic
materials chemistry, it is imperative to remember that
the principles governing different properties may not
always work in concert, and that in the end, a
functional resist design will re¯ect multiple compro-
mises and trade-offs.
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Radiation chemistry of polymeric materials