Lawrence Berkeley National Laboratory
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Title
Resist requirements and limitations for nanoscale electron-beam patterning
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https://escholarship.org/uc/item/7rh1h6sf
Authors
Liddle, J. Alexander
Gallatin, Gregg M.
Ocola, Leonidas E.
Publication Date
2002-11-25
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Resist Requirements and Limitations for Nanoscale Electron-Beam Patterning
J. Alexander Liddle,
1
Gregg M. Gallatin
2
and Leonidas E. Ocola
3
1
Materials Sciences Division, Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
2
IBM T.J. Watson Research Center
Yorktown Heights, N.Y. 10598, USA
3
Advanced Photon Source, Argonne National Laboratory
Argonne, IL 60439, USA
ABSTRACT
Electron beam lithography still represents the most effective way to pattern materials
at the nanoscale, especially in the case of structures, which are not indefinitely repeating
a simple motif. The success of e-beam lithography depends on the availability of suitable
resists. There is a substantial variety of resist materials, from PMMA to calixarenes, to
choose from to achieve high resolution in electron-beam lithography. However, these
materials suffer from the limitation of poor sensitivity and poor contrast.
In both direct-write and projection e-beam systems the maximum beam current for a
given resolution is limited by space-charge effects. In order to make the most efficient
use of the available current, the resist must be as sensitive as possible. This leads,
naturally, to the use of chemically amplified (CA) systems. Unfortunately, in the quest
for ever smaller feature sizes and higher throughputs, even chemically amplified
materials are limited: ultimately, sensitivity and resolution are not independent. Current
resists already operate in the regime of < 1 electron/nm
2
. In this situation detailed models
are the only way to understand material performance and limits.
Resist requirements, including sensitivity, etch selectivity, environmental stability,
outgassing, and line-edge roughness as they pertain to, high-voltage (100 kV) direct write
and projection electron-beam exposure systems are described. Experimental results
obtained on CA resists in the SCALPEL
®
exposure system are presented and the
fundamental sensitivity limits of CA and conventional materials in terms of shot-noise
and resolution limits in terms of electron-beam solid interactions are discussed.
INTRODUCTION
Some years ago it was still possible to treat the resist requirements for high sensitivity
and high resolution as two separate cases, divided between commercial applications, such
as mask making, and small scale patterning for research. More recently, however, the
separation between these two domains has all but disappeared. This has been driven
primarily by the continuing reduction in feature size accompanying the progress of the
semiconductor industry (Figure 1). The concomitant demands on mask fabrication have
become rigorous, particularly with the introduction of sub-resolution elements for optical
proximity effect correction [1]. In addition, as optical lithography becomes more difficult
and costly, there is a strong possibility that electron-beam systems will be used directly
[2, 3] for the mass production of integrated circuits (IC’s). These factors mean that
electron-beam resists are now required to provide high resolution and high speed
simultaneously.
1975 1980 1985 1990 1995 2000 2005
50
100
500
1000
5000
10000
Projected
Feature Size (nm)
Year
1975 1980 1985 1990 1995 2000 2005
50
100
500
1000
5000
10000
Projected
Feature Size (nm)
Year
Figure 1. Minimum integrated circuit feature size as a function of time.
In this paper we will discuss the detailed requirements for advanced electron-beam
resists, review the basic mechanisms governing resist exposure and development with
particular attention to the statistics of the processes involved, and, finally, consider what
the fundamental limitations to the resolution of electron-beam lithography might be.
RESIST REQUIREMENTS
Process stability
Process stability covers a number of specific factors related to variations in the printed
feature size or critical dimension (CD). The overall CD tolerance is usually given as
±10%, with process related effects allowed to contribute only ±3%.
Table I. Values of process sensitive parameters for electron-beam resists.
Process Sensitive Parameter Value
Post Exposure Delay (PED) Time > 3 hrs
Development Time < 5% CD/minute
Etch Resistance = Polyhydroxystyrene
Post Exposure Bake (PEB) < 1% CD/°C
Developer 0.26N TMAH
Vacuum Compatibility Zero outgassing
The requirements in Table I are generic for all lithographic technologies, apart from
the need for vacuum compatibility. The evolution of volatile organic compounds from
the resist while it is in the electron-beam system is highly undesirable because of the
potential for contamination of the electron-optical column with material that can charge
and degrade the system performance. In addition, the loss of material from the resist can
adversely affect the resist performance.
In a single component material, such as PMMA, solvent can escape from the film, but
this can be addressed using a suitable pre-exposure bake protocol. However, particularly
in view of the indiscriminate nature of the radiolysis induced by electron-beam exposure,
there is always the potential for the production of volatile moieties – in the case of
PMMA substantial amounts of MMA monomer are evolved.
Chemically amplified materials contain a base resin with pendant protecting groups
(positive tone), or a cross-linker (negative tone), in addition to a base, a photoacid
generator (PAG), residual solvent, as well as additives to promote adhesion and film
formation. During exposure a latent image of acid is formed in the film. The acid then
goes on to catalyze a number of deprotection or cross-linking reactions (Figure 2).
Protected site
Deprotected site
Acid
Cross-linker
Backbone
PositiveNegative
Post-Exposure Bake
Soluble Insoluble
SolubleInsoluble
Protected site
Deprotected site
Acid
Cross-linker
Backbone
Protected site
Deprotected site
Acid
Cross-linker
Backbone
PositiveNegative
Post-Exposure Bake
Soluble Insoluble
SolubleInsoluble
Figure 2. a) Schematic of the exposure mechanism for a CA resist [7]. Note that the crosslinker functions
to protect soluble sites, rather than link polymer chains.
Loss of the PAG from the film, which could be exacerbated by the vacuum
environment, will obviously degrade the lithographic performance of the resist, which
suggests that large molecules that can associate with the polar functionalities of the resist
are required. Ideally the acid formation process should not occur through a reaction
involving leaving groups, and the acid molecule itself should be large enough to be non-
volatile. It is also important that either the kinetics of the acid catalyzed reaction is
minimal at the maximum temperature the resist experiences in the exposure system,
which requires that it be a high activation energy (E
a
) process, or that the volatility of the
products is zero. Current experience suggests that the high E
a
materials are most suitable.
O
R
(
)
)
(
R
O
O
Protected hydroxystyrene Protected Methacrylic Acid
(Resin in 248 nm resists)
(Resin in 193 nm resists)
E
a
High
Moderate
Low
Protecting Group(R)
t-butyl ester, isopropyl
ester,
Carbonates: t-BOC, BOCME
Acetals, silyl ethers
a) b)
O
R
(
)
)
(
R
O
O
Protected hydroxystyrene Protected Methacrylic Acid
(Resin in 248 nm resists)
(Resin in 193 nm resists)
E
a
High
Moderate
Low
Protecting Group(R)
t-butyl ester, isopropyl
ester,
Carbonates: t-BOC, BOCME
Acetals, silyl ethers
O
R
(
)
)
(
R
O
O
Protected hydroxystyrene Protected Methacrylic Acid
(Resin in 248 nm resists)
(Resin in 193 nm resists)
O
R
(
)
)
(
R
O
O
Protected hydroxystyrene Protected Methacrylic Acid
(Resin in 248 nm resists)
(Resin in 193 nm resists)
E
a
High
Moderate
Low
Protecting Group(R)
t-butyl ester, isopropyl
ester,
Carbonates: t-BOC, BOCME
Acetals, silyl ethers
E
a
High
Moderate
Low
Protecting Group(R)
t-butyl ester, isopropyl
ester,
Carbonates: t-BOC, BOCME
Acetals, silyl ethers
a) b)
Figure 3. a) Typical resist structures for 248 nm and 193 nm resists, which are also suitable for electron-
beam systems. b) Influence of protecting group, R, on activation energy for deprotection reaction.
It is important to note that the absorption or optical density is not an issue for electron-
beam resists. This imposes a major constraint on the design of optical resists because the
transparency of the materials needs to be adjusted so that the absorption through the film
thickness does not lead to excessive resist sidewall angles due to depth variation of the
deposited energy profile [4]. This is becoming harder to realize as the exposure
wavelengths used decrease from 248 nm to 193 nm and below as fewer and fewer
polymer systems are available with acceptable absorption characteristics. Limitations on
the choice of polymer components can also affect the etch resistance of the material,
which is influenced by its fractional carbon content, and by the relative proportion of
carbon atoms contained within a ring structure [5,6].
Contrast
The resist contrast is a measure of how non-linear the response of the development
process is to the chemical contrast produced in the material after exposure and is essential
in determining how small an image modulation can be successfully converted into an
actual developed resist image. A typical requirement for conventional IC manufacture is
that the contrast should exceed 5, but there is no upper limit, greater values leading to
steeper sidewall angles. Note, however, that if topographic control of a resist surface is
desired, as in the production of diffractive optical elements, then low contrast values are
useful.
-
15
-
10
-
5 0 5 10 15 20
0.2
0.4
0.6
0.8
1
0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
γ=1
2
3
5
10
100
γ=1
2
3
5
10
100
Dose
Profile
Normalized Dose Position (nm)
Normalized Thickness
Normalized Thickness
CD = 20 nm
a)
b)
-
15
-
10
-
5 0 5 10 15 20
0.2
0.4
0.6
0.8
1
0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
γ=1
2
3
5
10
100
γ=1
2
3
5
10
100
Dose
Profile
Normalized Dose Position (nm)
Normalized Thickness
Normalized Thickness
CD = 20 nm
a)
b)
Figure 4. a) Normalized film-thickness remaining versus normalized dose for a positive resist material for
different values of the dissoOXWLRQUDWHFRQWUDVW
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d
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ln
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=
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, where r is the dissolution rate
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uniform energy deposition through the film thickness.
The contrast is determined by the nature of the development process. In a non-
chemically amplified material, such as PMMA, the effect of exposure is to cause
scissioning of the main polymer chain. The developer used is a minimal solvent for the
material, and it extracts the lower molecular weight polymer chains from the exposed
regions more rapidly than the high molecular weight chains from the unexposed regions
leading to a variation in dissolution rate with dose. This is a process that is strongly
