0018-8646/97/$5.00  (C)  1997 IBM

Chemical amplification resists: History and development within IBM

by H. Ito

The chemical amplification concept was invented at IBM Research and
quickly brought into use in the production of dynamic random access
memory devices in the company.  It has remained as an important
foundation for the design of advanced resist systems for use in
short-wavelength (<300-nm) lithographic technologies.

Introduction

We are perhaps in the most exciting era of microelectronics technology.
The home computer market is growing rapidly in quantity and quality.
Furthermore, the microlithographic technology, the core technology of
semiconductor device manufacture, is drastically changing as the minimum
feature size of electronic devices is shrinking to less than 0.25 micro-m.

Since the resolution (R) is proportional to the exposing wavelength
(lambda) and inversely proportional to the numerical aperture (NA) of the
lens (R = k lambda/NA), higher resolutions are achieved by increasing the
numerical aperture or by reducing the exposing wavelength.  The most
dominant approach to resolution enhancement has been to shift from the
G-line (436 nm) to I-line (365 nm) and then to increase the NA of the
I-line step-and-repeat exposure tools.  Another approach is to move to
much shorter wavelengths.

The shift from the near-UV (436-365 nm) to the mid-UV region (300-350 nm)
required modification of the diazonaphthoquinone/novolac resist to
improve its absorption characteristics at the shorter wavelength [1].
Further reduction of the wavelength to the deep-UV region (254 nm) was
sought at IBM in the late 1970s, which necessitated the development of a
revolutionary resist system.  The classical near-UV positive resist
consisting of a novolac resin and a photoactive diazonaphthoquinone
dissolution inhibitor does not perform adequately because of its
excessive unbleachable (i.e., inability to become more transparent during
exposure) absorption in the deep-UV region.  Several attempts to overcome
the problem were only partially successful [2].  Furthermore, phenolic
resins were believed to be too absorbing for use in the deep-UV region,
prompting serious research activities to utilize deep-UV-transparent
methacrylate polymers in the new lithographic technology [3].  However,
low resist sensitivity and poor dry-etch resistance precluded the use of
methacrylate resists in semiconductor manufacture.  In fact, sensitivity
enhancement was a major research subject for many years, but the
achievement was only incremental and too marginal to support the new
high-resolution technologies.  The resist systems based on photochemical
events that require several photons to generate one useful product have
inherently limited sensitivities.

Chemical amplification concept

In order to circumvent this intrinsic sensitivity limitation and to
dramatically increase the resist sensitivity, the concept of chemical
amplification was proposed in 1980 and reported in 1982 [4].  In
chemically amplified resist systems, a catalytic species generated by
irradiation induces a cascade of subsequent chemical transformations,
providing a gain mechanism.  The original chemical amplification scheme
included

o Cross-linking through ring-opening polymerization of pendant epoxide
  groups for negative imaging.
o Deprotection (cleavage) of pendant groups to induce a polarity change for
  dual-tone (positive/negative) imaging.
o Depolymerization for self-developing positive imaging.

These three systems are all acid-catalyzed [4, 5].  The chemical
amplification concept1 was considered as a laboratory curiosity when
reported.  However, as the value of this totally new system became
apparent, it was used in production of 1Mb dynamic random access memory
(DRAM) chips by deep-UV lithography in the mid-1980s [6].  Although the
use of acid as a catalyst has eventually become the major foundation for
the entire family of advanced resist systems, and the semiconductor
industry is currently moving steadily toward deep-UV lithography based on
chemical amplification resists, IBM already had a long history of DRAM
production by deep-UV lithography, made possible by the availability of
chemical amplification resists [6, 7] (see the paper by Holmes et al. in
this issue [8]).

The above three imaging systems were subsequently refined.  The epoxy
cross-linking chemistry was developed into the design of high-performance
negative resists [9] and also combined with aqueous-base development
(Scheme I) [10] for semiconductor lithography.  However, acid-catalyzed
epoxy cross-linking has been most successfully applied to
high-aspect-ratio imaging of thick resists [11] (described by Shaw et al.
in this issue [12]).  The acid-catalyzed depolymerization of
polyphthalaldehyde, which was later modified from self-development to
thermal development in order to eliminate tool contamination [13, 14], is
utilized in all dry bilayer lithography based on thermal development of
Si-containing polyphthalaldehyde [14, 15] and also employed in the design
of a polymeric dissolution inhibitor [16, 17].  The deprotection
mechanism (Scheme II) has attracted the most attention and was
successfully employed in the manufacture of DRAMs in IBM.

tBOC resist

The diazoquinone/novolac resist was unlikely to be capable of supporting
deep-UV lithography.  A new base-soluble etch-resistant matrix resin was
sought.  However, poly(4-hydroxystyrene), or PHOST, commercially
available in the early 1980s and employed in a negative deep-UV resist
MRS [18], was too absorbing, which gave an incorrect impression that
phenolic polymers would not be useful in the deep-UV lithography.

The tBOC resist design provided a breakthrough [4, 5, 19].  First of all,
this work has demonstrated that "pure" PHOST has a very low absorption
in the 250-nm region [20].  PHOST dissolves so fast in aqueous base that
the classical dissolution inhibition mechanism is rather incompatible
with this phenolic polymer.  The problem has been solved by protection of
the phenolic OH group with an acid-labile functionality such as
t-butoxycarbonyl (tBOC).

As shown in Scheme II, the IBM tBOC resist consists of
poly(4-t-butoxycarbonyloxystyrene) (PBOCST), PHOST fully protected with
tBOC.  The lipophilic PBOCST is converted to hydrophilic PHOST by
reaction with a photochemically generated acid.  This change of the
polarity from a nonpolar to a polar state allows dual-tone imaging simply
by changing a developer solvent.  The use of a polar solvent such as
alcohol or aqueous base results in the generation of positive-tone
images, while development with a nonpolar organic solvent such as anisole
provides negative-tone images.  Although this polarity change concept has
become the basis for the design of aqueous-base-developable
positive-resist systems, the tBOC resist containing triphenylsulfonium
hexafluoroantimonate was used in its negative mode in manufacture of 1Mb
DRAMs on Perkin-Elmer Micralign 500 mirror projection scanners operating
in the deep-UV mode (Figure 1) [6].  This internal availability of
sensitive resist systems based on chemical amplification motivated IBM to
further deep-UV lithography by developing new exposure tools (Micrascan)
with Perkin-Elmer and then with Silicon Valley Group Lithography.
Positive imaging of the tBOC resist was very problematic because of skin
or postexposure bake (PEB) delay phenomena [21].

Chemically amplified resist family

The use of acid as a catalyst provides design versatility.  Varieties of
new imaging mechanisms based on acid catalysis and new acid generators
for use with chemical amplification resists have been reported [22-24].
The chemical amplification resists can be classified most conveniently
according to their imaging mechanisms:

o Deprotection.
o Depolymerization.
o Rearrangement.
o Intramolecular dehydration.
o Condensation.
o Cationic polymerization.

Among the imaging mechanisms listed above, the deprotection (Scheme II)
and condensation (Scheme III) systems have been studied most extensively
by many research groups [22-24], especially for the design of
aqueous-base-developable positive and negative resists, respectively.  In
addition to the intended sensitivity enhancement, the chemically
amplified imaging mechanisms provide high contrasts and high resolutions,
which have played a decisive role in acceptance of the totally new
imaging materials.

Negative resists

The first commercial chemical amplification resist was a three-component
negative system consisting of a novolac resin, a photochemical acid
generator, and a melamine cross-linker which undergoes acid-catalyzed
condensation with the phenolic resin (Scheme III) [25].  This
commercialization allowed the lithographic community to experience the
high-resolution capability of chemical amplification resists and promoted
further interest in the new imaging concept.  The acid-catalyzed
condensation involving a phenolic resin is the most dominant imaging
mechanism for advanced negative resist systems today [26, 27].  A
three-component negative CGR resist based on acid-catalyzed condensation
for deep-UV lithography [26] has been developed at IBM and is now
marketed through the IBM/Shipley Deep-UV Resist Alliance.
Aqueous-base-developed 0.25-micro-m line/space patterns produced in the CGR
resist on a Micrascan II (NA = 0.50) are shown in Figure 2. In addition
to the three-component design, two-component negative resists based on
hydroxystyrene (HOST) copolymers and an acid generator (Scheme IV) were
extensively investigated [27].

In the above negative-imaging systems, the base-solubilizing
electron-rich phenolic group functions as the reaction site (C- or
O-alkylation).  Aqueous-base-developable negative-resist systems were
also formulated utilizing acid-catalyzed intermolecular dehydration of
pendant secondary alcohol (Scheme V) [28].

Negative-resist systems that do not involve cross-linking have been
extensively developed at IBM.  The first example of such resist systems
was the tBOC resist mentioned earlier.  The polarity change from a
nonpolar to a polar state induced by acid-catalyzed deprotection of the
tBOC resist allows swelling-free negative imaging with a nonpolar organic
solvent.  The concept has been extended to a reverse polarity change from
a polar to a nonpolar state in the design of negative systems which can
be developed with a polar solvent such as alcohol or aqueous base
[28-30].  The chemistries employed to induce the reverse polarity change
were acid-catalyzed intramolecular dehydration and pinacol rearrangement
(Schemes VI and VII) [28-30].

Intramolecular dehydration of pendant tertiary alcohol converts a
hydrophilic functionality to a lipophilic olefin, allowing negative
imaging with alcohol as a developer.  Similarly, the pinacol
rearrangement, which involves acid-catalyzed dehydration, of vic-diol
results in the transformation of polar alcohol to less polar ketone or
aldehyde, providing a negative system that can be developed with alcohol.
Aqueous-base development has been achieved by synthesizing HOST
copolymers containing pendant vic-diol [28] and also by blending a small
vic-diol with a phenolic matrix resin [29].  The latter system functions
on the basis of conversion of hydrophilic alcohol to
dissolution-inhibiting ketone through pinacol rearrangement.

Positive resists

As mentioned earlier, the chemical amplification concept based on
photochemically induced acid catalysis has had a significant impact on
the design of positive resists.  In fact, all modern advanced positive
resists are based on acid-catalyzed deprotection of partially protected
PHOST (Scheme VIII) [4, 5, 19].

The representative example is the APEX resist [31].  This resist was
employed in the manufacture of 16Mb DRAMs, and is currently marketed
through the IBM/Shipley Deep-UV Resist Alliance.

The acid-catalyzed deprotection mechanism, which is the foundation for
high-contrast positive resists, has allowed the semiconductor industry to
extend photolithography to the deep-UV region for higher resolution.
However, a major problem recognized in the early 1980s that is particular
to chemical amplification resists has appeared: Positive images exhibit
T-top (T-shaped profile) or skin formation upon standing after coating,
especially after exposure (Figure 3) [21].  After numerous attempts to
identify the cause, IBM researchers, by using activated carbon filtration
[21] and a 14C labeling technique [32] (Figure 4), have successfully
ascribed the formation of a surface insoluble layer to contamination by a
trace amount (of the order of 10 ppb) of airborne basic substances such
as N-methylpyrolidone (NMP) absorbed by the resist film [21].  Because of
the catalytic nature of the imaging mechanisms, a trace amount of
airborne basic substances absorbed by the resist film interferes with the
desired acid-catalyzed reaction.  This contamination study was later
extended to many other airborne bases originating from, for example, wall
paints [21].

This finding was very pivotal in solving the delay problem of chemical
amplification resists.  In fact, the activated carbon filtration to
purify the enclosing atmosphere has been installed in IBM ever since 1Mb
DRAMs were manufactured using the negative tBOC resist [6].  Air
filtration to remove airborne bases is now becoming a standard practice
in the industry.

Alleviation of the delay problem has been sought, and some engineering
solutions have been employed at IBM:

o Purification of the enclosing atmosphere by activated carbon filtration
  [21].
o Application of a protective overcoat [33-35].
o Incorporation of additives in resist formulation [36, 37].

Two important and more fundamental approaches to the contamination
problem have recently been proposed: reduction of the activation energy
of deprotection [38] and reduction of the free volume by annealing
[39-41].  In the first case, the acid-catalyzed reaction occurs
spontaneously at room temperature (without PEB) upon generation of acid
by irradiation, while the majority of the chemical amplification resists
require PEB and therefore are susceptible to the PEB delay problem.

The annealing concept [42] for environmental stabilization of chemical
amplification resists is based on a systematic study on the propensity of
thin polymer films to absorb NMP [43].  Polymer films with lower glass
transition temperatures (Tg) absorb smaller amounts of NMP because of
better annealing and reduced free volume.  The validity of the annealing
concept has been proven by lowering the Tg of the tBOC-related resist
resins through use of meta-isomers [39, 40].  Since the diffusivity of
small molecules in polymer films is an exponential function of the free
volume, a small difference in the free volume is translated to an
extremely large difference in the diffusivity.

A production-worthy environmentally stable chemically amplified positive
(ESCAP) resist has been designed on the basis of the annealing concept,
with its contamination resistance achieved by carrying out the bake
processes at unconventionally high temperatures [41, 44, 45].  The resist
consists of a copolymer of HOST with t-butyl acrylate (Scheme IX) and is
characterized by its exceptional thermal stability, permitting the film
to be baked at temperatures higher than its high Tg of 150 degrees C.  Other
chemically amplified positive-resist films (Scheme VIII) cannot be
annealed because premature thermal deprotection occurs at temperatures
below their Tg.  ESCAP has demonstrated a superb PEB delay stability in
comparison with APEX.  Figure 5 demonstrates the ESCAP overnight delay
stability of 0.35-micro-m line/space patterns exposed on a Micrascan II and
developed with 0.255 N tetramethylammonium hydroxide (TMAH) aqueous
solution.  ESCAP has been selected for commercialization for 256Mb DRAM
production by 0.25-micro-m imaging through the IBM/Shipley Deep-UV Resist
Alliance, which has refined the resist to exhibit extraordinary
resolution (200 nm on 0.53 NA at 248 nm) (Figure 6).

Another important advance in the design of positive chemical
amplification resists is their extension to the ArF excimer laser
wavelength at 193 nm, which is expected to play a key role in manufacture
of 1Gb devices.  Although the chemistry of 193-nm lithography is likely
to be based on chemical amplification schemes such as acid-catalyzed
deprotection which are already available, there are many materials
challenges.  PHOST cannot be used to formulate single-layer 193-nm
resists because of its excessive absorption.  Polymethacrylates are
highly transparent at 193 nm and are thus used in the design of ArF
excimer laser positive resists, with their aqueous-base development
achieved by acid-catalyzed conversion of t-butyl ester (t-butyl
methacrylate) to carboxylic acid (methacrylic acid) [46, 47].  The IBM
resist, a terpolymer of t-butyl methacrylate, methacrylic acid, and
methyl methacrylate (x = 0, Scheme X) was first developed for dry-film
laser direct writing [48], but has subsequently been found to function as
a high-resolution single-layer positive resist at 193 nm.  Since the
polymer lacks dry-etch resistance, the resist has been used primarily to
qualify prototype ArF excimer laser steppers.

Incorporation of alicyclic structures such as adamantane and norbornane
has been shown to enhance dry-etch resistance [47, 49-52].  IBM has
employed isobornyl methacrylate as an etch-resistant component (Scheme X)
and utilized a dissolution-inhibiting steroid to further increase etch
durability [50].  IBM's 193-nm-resist activities are described in detail
by Allen et al. in this issue [53].

Bilayer resist systems

The bilayer lithographic scheme utilizing silicon-containing resists was
first reported in 1983 by IBM [54] and studied extensively by many
research groups until a few years ago (Figure 7).  Although the
single-layer technology is favored, the 1Gb generation demands
consideration of all alternative technologies including the bilayer
scheme.  The chemical amplification concept has also been applied to the
bilayer lithography based on the use of silicon-containing resists and
oxygen reactive ion etching (RIE) for pattern transfer.

Aqueous-base-soluble polysilsesquioxane was protected with the t-BOC
group to formulate a positive resist based on acid-catalyzed deprotection
[55].  Wet development with an aqueous base followed by O2 RIE pattern
transfer results in the generation of high-aspect-ratio images.

A drawback of bilayer lithography is its need for extra processing steps.
In an attempt to simplify bilayer imaging, an all-dry process was
proposed; it is based on thermal development of
poly(4-trimethylsilylphthalaldehyde) through acid-catalyzed
depolymerization followed by an O2 RIE pattern transfer to the underlying
layer (Scheme XI, Figure 8) [14, 15].

All-dry lithographic processes

All-dry lithography has been another important area based on selective
silylation of organic resists and subsequent O2 RIE development, which is
a single-layer process that may play an important role in the generation
of 1Gb chips.

The deprotection chemistry is uniquely suited to the silylation process.
The reactive phenolic functionality unmasked by acid-catalyzed
deprotection is selectively and covalently reacted with a gaseous
silylating agent such as hexamethyldisilazane.  This renders the exposed
region impervious to O2 plasma, while the organic polymer film in the
unexposed regions is rapidly etched by O2 RIE (Scheme XII) [56, 57].
This process provides negative-tone images, and the silylation
selectivity is based on the high-contrast reactivity difference.

An alternative silylation process first reported by UCB [58] is based on
diffusion-controlled silylation of diazonaphthoquinone/novolac resists;
thermally cross-linked unexposed areas are less susceptible to silylation
because of limited diffusion, while the un-cross-linked novolac resin in
the exposed region is silylated.  This silylation technique of
cross-linking phenolic resists has been successfully applied to chemical
amplification resists based on acid-catalyzed condensation, which provide
positive images upon O2 RIE (Figure 9) [59].

Summary

The chemical amplification concept was invented by IBM and quickly
applied to the manufacture of DRAMs by deep-UV lithography.  After more
than fifteen years since its invention, the entire lithography community
is moving toward deep-UV lithography, a technology made possible by
chemical amplification.  In addition to enhanced sensitivity, high
contrast and high resolution are the major reasons for the industry-wide
acceptance of chemical amplification.  Materials and processes of
chemical amplification resists will continue to be refined to support
highly demanding future lithographic technologies.

Micralign and Micrascan are registered trademarks of SVG Lithography
Systems, Inc.

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Received February 9, 1996; accepted for publication October 31, 1996

Hiroshi Ito

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose,
California 95120 (HIROSHI at ALMADEN, hiroshi@almaden.ibm.com).  Dr. Ito
is a research staff member in the Science and Technology Department at
the Almaden Research Center.  He received his B.S., M.S., and Ph.D.
degrees in chemistry from the University of Tokyo in 1970, 1972, and
1976, respectively.  From 1976 to 1980, he worked as a research associate
at the Chemistry Department of the State University of New York,
Syracuse, on the synthesis of stereoregular polysaccharides for
biological and medical applications.  Dr. Ito joined IBM at the San Jose
Research Laboratory in 1980 and has since been working on the development
of new advanced resist materials.  In 1986 and 1989, respectively, he
received IBM Outstanding Innovation Awards for his work on a dry
development process involving selective silylation and for his scientific
achievements in developing chemical amplification resists.  In 1988 and
1993 he also received IBM Outstanding Innovation Awards for inventing a
bilayer lift-off process for magnetoresistive head fabrication.  Dr. Ito
is a recipient of the Arthur K. Doolittle Award (1990, American Chemical
Society, Division of Polymeric Materials Science and Engineering), the
Award of the Society of Polymer Science, Japan (1990), and the
Cooperative Research Award (1994, American Chemical Society, Division of
Polymeric Materials Science and Engineering).  He is a member of the
American Chemical Society, the Society of Polymer Science, Japan, and the
Chemical Society of Japan.


1 The tBOC resist patent disclosure, which is the basis of all current
  chemical amplification resists, was initially rated "publish" in the
  IBM Technical Disclosure Bulletin.

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