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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