19 Photoresist Materials and Processing Ce ´sar M. Garza Will Conley Freescale Semiconductor, Inc. Jeff Byers KLA-Tencor 19.1 Formation of the Relief Image ........................................ 19-1 Overview † Description of the Lithographic Process 19.2 Formation of a Relief Image in Novolac-Based Photoresists ....................................................................... 19-5 Overview † Elements of the Dissolution Mechanism of Novolac-Based Photoresists † Development Mechanisms in Novolac-Based Photoresists 19.3 Formation of the Relief Image in Chemically Amplified Resists............................................................. 19-23 Overview † Exposure Step 19.4 ArF Materials, Immersion Lithography and Extension of ArF ............................................................. 19-40 ArF Materials † ArF Transparent Polymer Systems † Extending ArF † Topcoats for Immersion Lithography † New Immersion Fluids † High Refractive Index (RI) Polymers † Post-ArF-Material Requirements References .................................................................................... 19-53 19.1 Formation of the Relief Image Optical microlithography is the technology that determines, in practical terms, the smallest transistor dimensions that can be manufactured on a semiconductor chip. As such it has been the primary driver for the remarkable improvements in performance and reduction in cost per function, the hallmark of the microelectronics industry. Optical microlithography involves the practice of multiple disciplines: physics, chemistry, and engineering specialties. Physics is used to form the aerial image; and it has been covered in the previous chapter. Chemistry is involved in the formation of the latent and relief images on the recording medium, know as photoresist, and it is the subject matter of the present chapter. 19.1.1 Overview As it was covered in the previous chapter, the smallest dimension that be printed is given by the Rayleigh criteria: Resolution Z k 1 l=NA ð19:1Þ where l is the actinic wavelength used in the formation of the aerial image, k 1 is a proportionality 19-1 q 2007 by Taylor & Francis Group, LLC
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19
q 2007 by Taylor & Francis Group, LLC
Photoresist Materialsand Processing
Cesar M. Garza
Will ConleyFreescale Semiconductor, Inc.
Jeff Byers
KLA-Tencor
19.1 Formation of the Relief Image ........................................ 19-1
Overview † Description of the Lithographic Process
19.2 Formation of a Relief Image in Novolac-BasedPhotoresists ....................................................................... 19-5
Overview † Elements of the Dissolution Mechanism of
Novolac-Based Photoresists † Development Mechanisms in
Novolac-Based Photoresists
19.3 Formation of the Relief Image in ChemicallyAmplified Resists............................................................. 19-23
Overview † Exposure Step
19.4 ArF Materials, Immersion Lithography andExtension of ArF............................................................. 19-40
ArF Materials † ArF Transparent Polymer Systems †
Extending ArF † Topcoats for Immersion
Lithography † New Immersion Fluids † High Refractive
Index (RI) Polymers † Post-ArF-Material Requirements
19-44 Handbook of Semiconductor Manufacturing Technology
19.4.2.5 New Classes of ArF Polymers
The development of polymers to meet the transparency requirements for 157-nm lithography created a
class of polymers that consisted of high contents of fluorine. These systems126 incorporated nearly 50%
fluorine to achieve transparency goals. The unfortunate demise of 157-nm technology did create a vast
library of knowledge in new systems that not only have nearly 99% transmission at 193 nm, but also had
unique properties that improved the performance of various types of polymers. In this section, the
authors will discuss these new classes of polymers along with new systems that have been created
as topcoat or protective layer materials for immersion lithography.
The Willson Research Group at the University of Texas127 explored the selective incorporation of
fluorine in a norbornane system. The plot in Figure 19.46 demonstrates the improvement in absorbance
at 157 nm of norbornane dependant on the location of the flouro group. In this plot we also see a
significant improvement in absorbance at longer wavelengths.
This activity yielded several interesting polymers with low absorbance initially at 157 nm and later at
193 nm. The polymer shown in Figure 19.47 is copolymer of NBHFA and NBHFA t-BOC. Trinque
et al.128 discuss the synthesis and application of this polymer for imaging at 157 nm. Further,
investigation into the optical properties of this system and imaging capability has also been
investigated129 that this copolymer is 99% transmissive at 193 nm.
Recently, Varanasi et al.130 published variations of polymers shown in Figure 19.48, which takes
advantage of simple free radical polymerization of acrylate systems that have incorporated norbornane
for etch resistance. Up to now, we have discussed the incorporation of fluorine for improvements in
transparency, which is still true, however, in this work not only is there an improvement, but Varanasi
et al. discovered that the incorporation of a monomer containing fluorine assists in reducing swelling in
acrylate polymer systems during development. Varanasi reported that since the pKa of HFA is similar to
that of phenol, that HFA incorporated methacrylate resists would behave similar to ESCAP-based KrF
resists in terms of resist dissolution kinetics. For the purpose of a comparison study, Varanasi prepared a
simple copolymer of t-butylmethacrylate and NB-HFA-MA (40/60) using free radical polymerization
method. This composition was chosen, primarily, to mimic well-known ESCAP copolymer of
t-butylacrylate and p-hydroxystyrene (40/60). The corresponding resist formulation was prepared
using industry standard PAG and quencher combinations. Dissolution rate vs. exposure dose curves
were obtained by flood exposing (254-nm wavelength, obtained from Hg–Xe lamp) cast resist films at
various exposures doses, processed and, then obtained dissolution rate information using quartz-crystal
microbalance (QCM) method. The comparison of data shown in Figure 19.49 reveals that HFA-based
ArF methacrylate resist behaves similar to ESCAP KrF resist, and do not show any swelling behavior even
at the onset of dissolution contrast, unlike typical ArF methacrylate resists.
0
0.0004
0.0008
0.0012
155 160 165 170 175 180
Wavelength (nm)
Abs
orba
nce
per
mT
orr
F
F
CF3OH
0.0016
FIGURE 19.46 Absorbance data of norbornane and fluoronorbornane derivatives.
q 2007 by Taylor & Francis Group, LLC
92 8
CF3CF3
F3CF3C OH O
O O
FIGURE 19.47 Fluoropolymer of NBHFA and NBHF t-boc.
Photoresist Materials and Processing 19-45
Another interesting aspect of these systems is the improvement in PEB sensitivity. Typically high etch
resistant methyl acrylates resists are based on multi-cyclic bulky protecting groups such as methyl
adamantyl group. Resists derived from methyl adamantyl protecting group-based polymers often suffer
from higher PEB sensitivity (5–10 nm/8C) with these systems reporting PEB sensitivities approximately
1 nm/8C.
19.4.3 Extending ArF
ArF immersion lithography has emerged as a promising candidate for 65-nm node technology.131 The
basic idea of immersion lithography is filling the gap between the final lens element and the photoresist
with a fluid, which has a higher refractive index (n) than air (nZ1) so that resolution and (depth of
focus) DOF can be increased.132 Figure 19.50 depicts the two advantages of immersion technology. One is
500
400
300
200
Thi
ckne
ss (
nm)
Time in 0.26 N TMAH (s)
5 mJ/cm2
7 mJ/cm2
8 mJ/cm2
10 mJ/cm2
20 mJ/cm2100
00 10 20 30 40 50 60 70 80 90
40 60
O O O O
O
O
FIGURE 19.48 Dissolution rate vs. exposure dose curves generated for state-of-the-art KrF ester capped (ESCAP)
and ArF (Methacrylate Resists).
q 2007 by Taylor & Francis Group, LLC
500
400
300
200
100
00
@254 nm exposure:10 mJ/cm2
14 mJ/cm2
15 mJ/cm2
20 mJ/cm2
10 20 30 40 50 60 70 80 90
Time in 0.26 N TMAH (s)
Typical ArF resist formulation
Thi
ckne
ss (
nm)
40 60
O
OHCF3
F3C
O
HFA-Methacrylate polymer platform
OO
FIGURE 19.49 Dissolution rate vs. exposure dose curve obtained with an ArF resist formulation containing
copolymer of t-butylmethacrylate and NB-HFA-MA.
19-46 Handbook of Semiconductor Manufacturing Technology
to increase DOF of an exposure system, while maintaining same resolution of a dry system at equal NA.
The image-forming angle of the deflected light in the photoresist does not change, but the incident angle
in the fluid above the resist surface does change. Because the incident angle in the fluid becomes smaller,
the available DOF is increased. Existing dry scanner lenses need little modification on the shape and
position of the lens elements to preserve the incident angle in the resist. For NA beyond one, the
advantage is to enhance the resolution beyond the limit of a dry system using the same vacuum
wavelength. The optical system is re-designed to preserve the physical angle in the coupling medium. The
incident angle of the exposure light in the resist can then be enlarged to resolve features in smaller half
α
FIGURE 19.50 The two advantages of immersion lithographic system, (a) increase depth of focus by decreasing the
incident angle in water, and (b) enhance resolution by enabling hyper NA lens design.
q 2007 by Taylor & Francis Group, LLC
W (hp) = k1l
NA
2nd gen.
1st gen.
32nmhp
45nmhp
k1=0.30
k1=0.25
Dry(n=1.0)
Wet(n=1.44)
Wet(n=1.64)
90
80
70
60
50
40
30
200.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
NA
Cal
cula
ted
reso
lutio
nhp
(nm
)
FIGURE 19.51 Calculated resolution vs. NA.
Photoresist Materials and Processing 19-47
pitch. Of course, the incident angle does not have to be confined to only these two specific cases.
Resolution and DOF can be traded off against each other by selecting the incident angle properly.
The success of ArF water immersion lithography is inspiring many engineers and scientists to think, if
ArF immersion lithography could be put forward further. Could CD smaller than 45 nm, for example,
32 nm, be achieved by ArF immersion lithography with a high refractive index fluid? Figure 19.51 shows
the calculated resolution (W) based on the Rayleigh equation; (Equation 19.1) where k1 is process
constant and is related to the difficulty of lithography process and has the lower theoretical limit of 0.25, l
is the wavelength, and NA is numerical aperture of the optical system.
19.4.4 Topcoats for Immersion Lithography
During the initial introduction of water immersion lithography, photoresist companies quickly
discovered that existing ArF photoresists produced reasonably good lithography. SEMATECH sponsored
an Immersion Task Force, which quickly investigated a number of aspects of photoresist chemistry. A
series of surface experiments were performed ranging from contact angle, to investigate any surface
energy changes, to XPS and TOF-SIMS to understand the contents of the film.133 These investigations
quickly pointed the industry in the direction to understand the surface interactions and components
from the photoresist that leach into the water. These studies investigated the use of model resist systems
based on copolymer of methyl-adamantyl methacyralate and g-butryl lactone methacyralate along with
three commons PAGs shown in Figure 19.52.134 Data presented in Figure 19.53 was a clear indication on
the amount of PAG that was leaching from the resist surface, however, there was a surprise that the
perfluoro-octanoic sulfonic acid (PFOS) system had higher concentrations of PAG in the water and that
the triflate system was less.
TOK developed a “cover material” called TSP-3A, which was a fluoropolymer that was cast over the
ArF photoresist. The purpose of this cover material was to prevent any leaching and improvements in
image quality. This material was insoluble in developer and required a separate solvent for removal. Due
to the high fluorine content of the polymer the contact angle was extremely high, which lead to a number
of other problems.135 The industry quickly developed “top coats” that are developer soluble with lower
q 2007 by Taylor & Francis Group, LLC
S+ −OSO2CF3 S+ −OSO2C4F9 S+H3C−OSO2C8F17
TPS-Nf TPS-Tf PFOS (TPS-Of)
O
O
O
O
O
O
50 50
MA dMA GBLMA
FIGURE 19.52 Photoacid generators and polymer for leaching studies.
19-48 Handbook of Semiconductor Manufacturing Technology
contact angle.126 This ability to quickly develop these systems is a benefit of the vast amount of material
that occurred during the 157-nm development programs. As previously discussed, the highly fluorinated
materials were used to gain the necessary transparency needed at 157 nm and the benefit was virtually
99.5% transmissive materials. These cover coats are excellent in the reduction of leaching, but not the
total prevention.136
0
20
40
60
80
100
120
10
Anion chain length
PA
G le
vel (
ppb)
PAG loading 0.7PAG loading 1PAG loading 1.5
86420
FIGURE 19.53 PAG leaching vs. anion chain length.
q 2007 by Taylor & Francis Group, LLC
Photoresist Materials and Processing 19-49
19.4.5 New Immersion Fluids
The further extension of ArF immersion can, in principle, continue if a fluid exists with physical
properties similar to water, but maintains a higher refractive index at 193 nm. This increase in
refractive index allows lens designers to build a larger lens system of greater NA and thus higher
resolution.137,138
Water, as an immersion fluid, has a theoretical limit in NA equal to the index of water.133 The
practical limit for lens design is even less and estimated to be approximately 1.3 NA. With k1 of 0.27,
this would result in 40-nm half pitch resolution. The latest experimental data on high index fluids is
presented in the paper of Sewell.127 Burnett125 pointed out that next to high index fluids also high
index glass materials are required to enable the super high NA lens designs. Regarding lens designs,
immersion lenses may follow two different approaches. The first one is the approach with a flat surface
near the image side, the second one with a curved surface near the imaging side. With the flat surface
approach, the refractive power is dominated by the glass material and the fluid index should be
matched as good as possible to the index of the glass. The advantage of this approach is that the fluid
film can be relatively thin. This relaxes the absorption requirements on the fluid. With the approach of
a curved last lens surface, only the fluid index determines the maximum NA. However, in this case, the
optical path through the fluid cannot be small, and thus, the requirements on the fluid absorption
become very tight. Besides absorption, there are additional requirements on the fluid, like viscosity,
thermal dependency, and cost. If we compare the basic requirements with the published experimental
data123–125,127 we conclude that the current fluids are too high in absorption, too high in dn/dT, and
too expensive. If we assume the condition nZn(fluid)Zn(glass) and assume maximum NAZ0.9n and
minimum k1Z0.27, we can plot the resolution limit of ArF immersion lithography. The result is shown
in Figure 19.54. With the currently published index number of fluids and glass materials, ArF resolution
is limited to 36 nm. In order to reach 32 nm, new fluid and glass materials are required with refractive
index numbers exceeding 1.8.
From this calculation, 32 nm or below resolution can be achieved with high refractive index fluid
(nZ1.64). Although extreme-ultraviolet (EUV) (13 nm) lithography has been suggested to be used in
32-nm node or below, the development of exposure tools for EUV is still in early stage and much time
and effort is thought to be needed because of the technical hurdle. By making use of existing water
Water80
70
60
50
40
30
20
10
01 1.2 1.4 1.6 1.8
Immersion fluid refractive index (n)
2 2.2 2.4
Hal
f pitc
h re
solu
tion
(nm
)
Sinθ = 0.90, k = 0.25Sinθ = 0.90, k = 0.27Sinθ = 0.90, k = 0.30Sinθ = 0.90, k = 0.35
2nd Gen.fluid
3rd Generationimmersion
fluid?
FIGURE 19.54 Resolution vs. immersion fluid refractive index.
q 2007 by Taylor & Francis Group, LLC
19-50 Handbook of Semiconductor Manufacturing Technology
immersion technology, ArF immersion with high index fluids has apparently the advantage of lower cost
and risk for tool development. This is why ArF immersion is now gaining more and more spotlight as a
candidate for the next generation lithography (NGL) technology. Initial attempts to develop high
refractive index (RI) fluids for ArF immersion has been carried out aiming at increasing refractive index
by addition of inorganic materials. Smith et al. reported various refractive indexes at 193-nm wavelength
with doped waters. They utilized “charge-transfer-to-solvent” (CTTS) transition to induce the small
absorption near the 193-nm wavelength with inorganic ions, and therefore, heightened the refractive
index of water. They presented the result of 68 nm L–S imaging by an aqueous solution of 85%
phosphoric acid with refractive index of 1.55 at 193 nm wavelength.133 A unique approach is also
reported by applying nano-sized metal oxide. Researchers at SEMATECH and Clemson University
reported that refractive index of water dispersed with aluminum oxide nano-particles could be as high as
1.6.134 Although this kind of an approach can take advantage of some favorable properties of water, they
appear to sacrifice others. For example, although CTTS can increase the refractive index of water, it also
reduces the transmittance of water. Inorganic ions of metal oxides can damage lens and or leave
photoresist defects. Furthermore, mixed aqueous compositions have another disadvantage, the difficulty
to precisely control the accuracy of their refractive indexes, as small amounts of variation in
concentration would cause enough fluctuation in refractive index. The ideal solution would be a
single component fluid.139 Recently, researchers from JSR and Dupont disclosed organic fluids with a
refractive index of 1.65 at 193 nm. Imaging studies have been completed through the use of
interferometric lithography demonstrating 32 nm 1/2 pitch imaging. This demonstration is a great
step forward in the further extension of immersion ArF lithography; however, there are still numerous
challenges not only in fluids, but resist materials and the optical system of the exposure tool.
19.4.6 High Refractive Index (RI) Polymers
The idea of increasing the refractive index is not a relativity new concept; however, understanding the
impact is.139 Recent studies at SEMATECH and the University of Queensland140 have focused on the
incorporation of sulfur into the polymer. The results have demonstrated increases in refractive with
relatively small amounts of sulfur incorporation. Presently the vast majority of ArF polymers have a
refractive index of approximately 1.7. Figure 19.55 is the structure of a typical ArF acrylate polymer
system.141 Figure 19.56 is the structure of a sulfur containing copolymer,142 and Figure 19.57 is a UV
spectrum of each polymer demonstration, the increase in refractive index. But, why increase the
refractive index? Figure 19.58 is a plot of exposure latitude vs. refractive index. This plot demonstrates
with increase refractive index improvements in exposure latitude can be achieved. The theory has been
previously discussed143 and Figure 19.59 is the individual process capability plots for polymers with
O
O
O
O
n
O
Om
O
O
O
O
o
FIGURE 19.55 Structure of standard ArF polymer.
q 2007 by Taylor & Francis Group, LLC
*
OO O O
n m
O
O
S
SS
FIGURE 19.56 Structure of sulfur containing copolymer.
Photoresist Materials and Processing 19-51
increasing refractive from 1.69 (current polymers) up to 2.29 demonstrating the increased exposure
latitude for a 50 nm-image on a 130-nm pitch using water as the immersion fluid and an NA of 1.35.
19.4.7 Post-ArF-Material Requirements
At this time (mid-2005), 90-nm device fabrication is continuing to ramp up. The International Technical
Roadmap for Semiconductors112 (ITRS), which outlines target device and materials requirements for
future generations of semiconductor devices, calls for device dimensions to shrink to approximately
20 nm minimum size by the year 2016. It is anticipated that the NGL exposure technologies115 using
EUV144 radiation or electron beam projection145 (EBP) will be necessary to achieve adequate resolution.
It is not surprising that resist functional requirements become increasingly stringent as dimensions of the
target devices shrink. For the ITRS 22-nm technology node (dynamic random access memory half-
pitch), which is the most stringent metric for resist resolution rather than the less reliable measurement
Wavelength (nm)
180 190 200 210 220 230 240 250 260
Ref
ract
ive
inde
x
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
FIGURE 19.57 UV spectrograph of polymers from Figure 19.55 and Figure 19.56.
q 2007 by Taylor & Francis Group, LLC
Max EL for 50 nm on 130 nm pitch1.35 NA
20
21
22
23
24
25
26
27
28
29
30
1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.3 2.4 2.5
Refractive index at 193 nm
%E
L
Max EL for 50 nm on 120 nm pitch
2
FIGURE 19.58 Exposure latitude vs. photoresist refractive index.
19-52 Handbook of Semiconductor Manufacturing Technology
of isolated features,146 requirements are that the resist will be used at a film thickness between 40 and
80 nm, will exhibit a LER of not greater than 1 nm per edge (3s) and will support overall control of CDs
to 1 nm (3s).112,145 These tolerances are smaller than the dimensions of the polymer molecules that
constitute today’s resists,146 and given a typical carbon–carbon bond length of 0.13–0.15 nm,147 it is clear
that this specification is a call for atomic-scale control. To find practical use, a resist material must satisfy
an extensive, comprehensive list of functional properties. Any viable resist must simultaneously achieve
the target resolution, adequate sensitivity and acceptable imaging precision. These attributes ultimately
Exposure latitude vs. DOF
2.2972 − 1.35 NA− 50 on 130 p Quas
1.6972 − 1.35 NA− 50 on 130 p Quas
1.9972 − 1.35 NA− 50 on 130 p Quas
Depth of focus
30
20
00.0 0.1 0.2 0.3
10
Exp
osur
e la
titud
e (%
)
FIGURE 19.59 Process capability vs. refractive index.
q 2007 by Taylor & Francis Group, LLC
Photoresist Materials and Processing 19-53
are dictated by economics: the need to produce a product that the market wants at acceptable cost. Resist
resolution determines the number of devices per circuit, device speed, and the number of devices per
wafer; resist sensitivity governs wafer throughput per tool; and imaging precision affects device
performance, and yield. Advanced research, largely carried out at academic laboratories active in
nanoscience and nanofabrication, has sought to identify and extend the limits of nanoscale lithography.
Among more conventional organic resist materials, the consensus is that PMMA is capable of imaging
line-space arrays (formed by electron beam lithography) at the 15–20 nm scale (30–40 nm pitch)147–153
without excessive LER,124 currently the record for a polymer-based resist. Other nonpolymeric organic
resist materials have been reported to exhibit similar resolution and low LER.154,155 While these studies
provide proof that resolution and LER consistent with the 2016-roadmap requirements is, in principle,
attainable by currently known means, the radiation sensitivity of the materials used for these
demonstrations is inadequate by orders of magnitude. The anticipated low brightness of NGL radiation
sources is such that resists with very high radiation sensitivity will be required. Resist resolution criterion
for 20-nm scale lithography development of EUV and EBP prototype tools.144,156 The expectation that
CA resists will be used with NGL is signaled by an ITRS specification of allowable change in image size
with PEB temperature.157 A key issue, then, and still unproven, is whether CA resists can simultaneously
satisfy resolution and image precision specifications, while maintaining adequate radiation sensitivity.
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