The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Study on Reaction Mechanisms in Electron Beam and X-Ray Resists
Using Time-Resolved Absorption Spectroscopy
Seiji Nagahara, Takahiro Kozawa, Seiichi Tagawa
The Institute of Scientific and Industrial Research, Osaka University,
8-1 Miho-ga-oka, Jbaraki-shi, Osaka 567, Japan

Abstract

2. Experimental

Acid generation from onium salt acid generators in EBand X-ray chemically amplified (CA) resists was investigatedusing nanosecond time-resolved absorption spectroscopy(pulse radiolysis). The results indicate that an acid may generate through the one electron reduction of the onium salts.
The behavior of protic acid generated in the CA resists wasalso analyzed using both conventional steady-state optical absorption spectroscopy and the pulse radiolysis. The results
indicate that the protons are transferred from the proton adducts (acid precursors) of matrix polymers to other composition of CA resists. The proton transfer to a melaminecrosslinker in a CA negative resist was much faster than thatto a dissolution inhibitor in a CA positive resist.

The nanosecond pulse radiolysis experiments were carriedout using the 28 MeV L-band linear accelerator (LINAC)
located at the Institute of Scientific and Industrial Research,
Osaka University. The system is described in the previousliterature[?]. The width of an electron beam pulse from theLINAC is 8 ns and the absorption dose per pulse is 70 Gy (JI kg). The sample solutions in quartz cells with 10-mm optical
path length were bubbled with Ar gas before they were set inthe system. These experiments were carried out at room
teiJlperature.
X-ray exposure to the resist film on quartz substrates wasdone using a SORTEC SR ring[9] in I atm He ambient. Thepeak wavelength of the X-ray was about 0. 7 nm and the exposure dose was 540 mJ/cm 2 • UV-visible absorption spectraof the resist films were measured before and 5 min after exposure. The spectra were normalized for a tilm thickness of I
J.lm. These experiments were carried out in the clean room
(CR) at SORTEC.

1. Introduction
Electron Beam (EB) lithography and X-ray lithographyusing synchrotron radiation (SR) can be used for productionof ultra-large-scale integrated circuits (ULS!) with very highintegration. In such lithography, chemically amplified (CA)resists can provide high performance with high sensitivity.
TheCA resist employs the photogeneration of an acid whichcatalyzes many subsequent chemical event such as deblocking
of a protecting group or cross-linking of a matrix resin.
Onium salts play an important role as the acid generator(AG) in the CA resists. Photochemistry of the onium salts
has been widely investigated using product analysis, flash photolysis techniques, and so on[l-5]. It was reported that
iodobenzene and diphenylsulfide cation radicals (Pht · andPhzS+") are formed in the photolysis of diphenyliodonium salt(PhztX) and triphenylsulfonium salt (Ph3S+X) according toreactions (I) and (2)[5]. On the other hand, the radiation-in-

3. Results and Discussion

3.1 Acid Generation Reaction Mechanism in EB and X-ray
CA resists
Figure 1 shows the optical absorption spectra obtained
immediately after the electron pulse irradiation of m-cresol
solution with and without onium salt (Phzi+CF3So3· or
Ph3S+CF3So3·). m-Cresol was used as a model of phenolic~ 0.

Ph3s+x· __hv_ _monitored at A.= 400 nm~~---~i"'hv

Ph 2 1+x---- Phlt + Ph· + x·

2

(1)

~

0.2

Time (ns):I.e.!!!.

(2)

duced acid generation mechanism via onium salt decomposition has not been clarified yet. To improve capabilities of AGs
for EB or X-ray CA resists, we investigated the radiationinduced acid generation mechanism of iodonium and sulfonium salts using EB pulse radiolysis experiments[6].
In most CA systems, deformation of resist pattern easily
occurs and the effect is most readily seen by increasing thetime between the exposure and post-exposure bake process
steps (post-exposure delay; PED). To solve the problems andto design a new CA resist system, it is es~ential to understandthe behavior of acids in the resist films. Another objective of
the present work is to investigate the behavior of protic acidin EB and X-ray CA resists[7,8].

j0.1~
0.0 l_____J__...J.__L__.....L_...J.____JL_--l
300 400 500 600 700 800 900 1000
Wavelength (nm)Fig.l Optical absorption spectra of m-cresol solution obtained
immediately after 8 ns electron pulses; (a) without
onium salt (b) with Ph2r+CF3so,- (100 mM) and (c)with Ph,S+CF,so3· (1 00 mM). The insert shows the timedependent behavior of phenoxy radicals of m-cresol (A.
= 400nm).

-320-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

.of a matrix polymer (ArOH), and a cation radical of a matrix
polym~r (ArOH+") is formed as shown in reaction (5). In Fig.
ArOH

EB or X-ray~ ArOHt

ArO· + Ar0H 2 +ArOHt + ArOH -Wavelength (nm)Fig.2 Optical absorption spectra immediately after an 8 ns
electron pulse irradiation of (a) Phi (10 mM) and (b)Ph2S (I 0 mM) in dichloromethane so!ution.

,T_~.e"'(7)1 insert, phenoxy radicals (A.=400 n~) of m-cresols increasedby the addition of onium salts. This inay indicate that the ac~
ids are formed from hydroxyl groups of phenolic compounds(ArOH) as the reaction (6) (or (7)).
3.2 Behavior ofAcids in EB and X-Ray CA Positive Resists
00Phs~'suo

120.06JlMe.

o-ofO-o()For 1-f..!.m-thick films of CA resists, optical absorption
spectra of the films were measured before and after X-ray
exposure. Figure 4 shows the absorption spectra of a novolac(1) film which contains 10 wt% onium salt AG (2). Before

0.D"' 0.02<(0.00 '---..L.----'---'---.......1.-~

600

700

800

900

0.2.--------------.,

Wavelength (nm)

5 min after exposure

Fig.3 Optical absorption spectra immediately after an 8 ns
electron pulse irradiation of (a) Ph2rcF,so3· (200 mM)
and (b) PhJS+CF3So.1· (200 mM) in acetonitrile solution.
matrix resins in CA resists. Figure 2 shows the transient absorption spectra obtained immediately after the electron pulse
irradiation of I 0 mM iodobenzene (Phi) and 10 mM
diphenylsulfide (PhzS) in dichloromethane solution. The absorption peaks of Pht · and PhzS+ · are observed at 690 nm
and 770 nm, respectively. In Fig.l, there were no absorption
peaks of Pht· and Phzs+· in the presence of onium salts. One
reason of the result is that the contribution of the cleavage ofC-·I orC-S bond via excited states of the onium salts (reactions (1) and (2)) is small in the case of electron beam irradiation. And another reason may be that electrons transferedfrom m-cresols to the cation radicals (Phr · and Ph2S+ ·) during the electron pulse width. Figure 3 shows the optical absorption spectra obtained immediately after an 8 ns electronpulse irradiation of the onium salts in acetonitrile solution
where the electron transfer from the solution does not occur.
When the onium salt concentration is high in the acetonitrile
solution, the absorption peak of the cation radicals was observed. However, the yield of the cation radicals is low.
The main reaction pathway of acid generation in EB andX-ray CA resists is considered to be one-electron reductionof the salts as reported earlier[ 10]. The decomposition of theonium salts induced by electron reduction is considered to bereactions (3) and (4)[11,12]. The electron in reactions (3) andPh 2 1+x· + e· __,._ Phi + Ph·+ x·
Ph 3 S+x· + e· -Ph 2 S + Ph·+ X"

OBu'32"'c 0.04-e"'500JlMe~400(6)ArO· + WArOHt 0.0 '----'---'---L----''----'----L--=u300 400 500 600 700 BOO 900 1000(5)+ e·(3)(4)(4) is mainly generated by the radiation-induced ionization..
c:~'" 0.1.

.<><400500600

700800

Wavelength (nm)Fig.4 Optical absorption spectra of a resist film; novolac resin(1) I AG (2) = 10 I 90 (wt ratio).
exposure, there was no absorption peak in the wavelength
range 300 nm to 800 nm. However, after exposure, anabsorption peak appeared at around 540 nm. The absorption
peak at 540 nm is attributed to a proton adduct of m-cresol
novolac resin (1) [12]. The proton adduct may be a trappingsite of proton and is considered to be a precursor of an acid
which catalyzes subsequent chain reactions to changesolubility to the alkaline developer. As shown in Fig. 4, thevalue of absorbance at 540 nm decreases as the PED timeincreases and reduced 29 % in CR I during the period from 5
min to 120 min after exposure. This experiment was carriedout in CR 1 where the concentration of NH/ is 5-10 ppb. On
the contrary, the value of absorbance reduced 22% during thesame period in CR2 where the concentration of NH/ is lessthan 1 ppb. From the above results, it is considered that theproton adducts were captured by airborne basic contaminants
in some extent.
In order to investigate the effect of a dissolution inhibitor
on proton adducts in CA positive resists, optical absorption
spectra of the films containing a dissolution inhibitor weremeasured before and after X-ray exposure. The composition-321-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

0.08of the resist film was novolac resin (I) I AG (2) I dissolution
inhibitor (3) =60/1 0130 (wt ratio). The absorption peak ofproton adducts also appeared at 540 nm in the film. The valueof absorbance reduced 38% in CRl during the period from 5min to 130 min after exposure. This decrease is faster thanthat in the absence of a dissolution inhibitor. Therefore, thedecrease might be partly due to migration of a proton to adissolution inhibitor.

E"

(a)

c::0

€

..
..

.!!-.."

0.04(b)c-e0.0<(0.00.L___0100200300Time (ns)3.3 Behavior of Acids in EB and X-Ray CA Negative Resists
OMe OMel__N)
NhNMeO........_N)LNAN./""-OMelOMeMeO)

45Figure 5 (a) shows the optical absorption spectra of thefilm which consists ofnO\',Olac resin (I) I AG (4) = 60 I 10 (wtratio) and does not contain any crosslinker. After X-ray ex0.3 . - - - - - - - - - - - - - ,after exposure., 0.2.

Fig.6 The time-dependent behavior of intermediates obtainedin the pulse radiolysis of novolac resin (I) (3 base M)
with an AG (200 mM Ph 2 IPF6 ) in acetone solutions;(a) without a crosslinker (b) with a crosslinker (5) (300mM).
initially formed in those solutions by the irradiation. However, the intensity of the absorption of the proton adducts decreased thereafter. This is considered to have been due to thetransfer of protons from proton adducts of novo lac resin (I)to crosslinker (5). This proton transfer is apparently fast in
the solution containing crosslinker (5), compared to that in
the case for theCA positive resists containing dissolution inhibitor (3). This fast proton transfer may be due to high proton affinity of crosslinker (5) and may contribute to the environmental stability of this type of CA negative resist.

()c.00(b)Cl).0<(0.1

1+4+5400

500

600

700800

Wavelength (nm)Fig.5 Optical absorption spectra of resist films; (a) novolac
resin (1) I AG (4) = 60 I 10 (wt ratio) (b) novo lac resin(I) I AG (4) I cross linker (5) = 60 I I0 I 30 (wt ratio).
posure, an absorption peak of the proton adduct of novolac
resin (1) appears at 540 nm. Figure 5 (b) shows the opticalabsorption spectra of the film of which the composition isnovo lac resin (1) I AG (4) I crosslinker (5) = 60 I 10 I 30 (wtratio). This resist formulation provides excellent resolution
in EB and X-ray lithographies. For this film, there is no strong
absorption peak at around 540 nm before X-ray exposure nor
after X-ray exposure. These results suggests that the protontransfer from the proton adducts of matrix polymers tocrosslinker (5) is fast, and completed until the absorption
measurement.
We analyzed the proton transfer using pulse radiolysis because the transfer is very fast. Figure 6 shows the time-dependent behavior of the absorption peak at 540 nm obtainedby electron beam pulse radiolysis of novolac resin (1) (3 basemolll) with an AG (200m moll/ Ph}PF6) in acetone solution.
The absorption of proton adducts initially produced by theelectron pulse has differs little between the cases of acetone
solution with and that without crosslinker (5). This meansthat approximately the same amounts of proton adducts were4. Conclusion
From the results of pulse radio lysis experiments, we canconclude that the decomposition of an onium salt via excitedstates is not the main reaction path of acid generation in EBand X-ray CA resists. The main path is considered to be theonium salt decomposition via electron transfer from a matrix
polymer to an onium salt.
We observed the transfer of protons from the proton adducts of matrix polymers to other resist composition in CA
resists using the combination of conventional steady state optical absorption spectroscopy and nanosecond pulse radiolysis. The rate of proton transfer to the crosslinker (5) in CAnegative resist was much higher than that to the dissolution
inhibitor (3) in CA positive resist.
Acknowledgments
We thank Dr. Yoshio Yamashita, Dr. Takao Taguchi ofSORTEC Co. for experimental assistance and discussing theresults, and other staff of the SORTEC Co. for the synchrotron and storage ring operation. We also thank the staff of theradiation laboratory at Osaka University for operating LIN AC.

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et aL, Microprocesses and Nanotechnology '97 228 7) S. Nagahara, S. Tagawa, et al.,Jpn. J. AppL Phys. 35 (1996}6491 8) S. Nagahara, S. Tagawa. et aL. J.Photopolym.
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T. Kozawa, S. Tagawa, et aLJpn.l Appl. Phys. 31 (1992) 4301 II) H. E. Bachofneret.
et al., J. Am. Chern. Soc. 80 (1958) 4269 12) X. H. Ma, et aL. Macromolecules 20(1987) 2703 13) T Watanabe, S. Tagawa, et al., J. Vac. Sci. Techno!. Bl2 ( 1994) 387-322-