Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

27A/2004G127

Ionic degradation products of ZEP520 resist film
Hiromi IKEURA-SEKIGUCHI*1, Tetsuhiro SEKIGUCHI2, Masaki KOIKE1
1AIST, Tsukuba, Ibaraki 305-8568, Japan
2
JAEA, Tokai, Ibaraki 319-1195, Japan
Introduction
ZEP520 (Nippon Zeon Co.) is a positive-tone electronbeam
resistwithcomparable
resolution
topolymethylmethacrylate (PMMA). Chemical structure ofthis resist is alternating copolymer of αchloromethacrylate and α-methylstyrene. This resist
improves sensitivity and etch resistance compared with
PMMA and thus has been widely used for devise
fabrication including zone plate optics. In higher-voltage
e-beam exposure system, however, ZEP520 is needed tobe improved contrast, enhanced reactive ion etching(RIE) resistance and improved sensitivity [1]. Line-edge
roughness (LER) is also serious problem to reduce patternsize, which is caused by polymer aggregates in the size of
20-30nm [2]. To improve the performance of resist anddesign new resist, understanding of characteristics of
ZEP520 is needed. In this study, ionic degradation
products from ZEP520 thin film is measured using timeof-flight mass spectrometry for photon-stimulated ion
desorption (TOF-PSID) near Cl K-edges.

methylstyrene unit. These desorbing ions are almost
similar to those produced by laser ionization of polyαmethylstyrene [5]. In fact, only small amounts of ionicfragments larger than m/z > 50 were observed in PMMA[4]. Although the assignment of larger fragment ionsremains obscure, it is clear that Cl atom-selective
excitation produces main chain scissions. This finding
implies a phenomenon that localized excitation forms
ionic fragments produced by bond breaking of a remotesite. The phenomenon is not a common reaction processof PSID since generally PSID occurs prior to energy
redistribution over molecules of localized excitation (i.e.
fast reaction) and Auger decay following core excitation
also produces localized holes to the excited site, resulting
in site-selective ionic fragmentations.

Experimental
Measurements were performed at BL-27A with anenergy width of 1 eV around Cl K-edge. TOF-PSID mass
spectra were measured using a rotatable TOF
spectrometer [3] combined with pulsed SR.

ClCH3CCH2CO

CCH2nO
CH3Results and discussion
Figure 1 shows TOF-PSID spectra, observed at 2814
eV, 2823 eV, 2830 eV and 2849 eV photon energy in anx-ray incident angle of 45˚, together with a calculated
mass scale. These energies correspond to off resonance,
Cl 1s → σ*(Cl-C), Cl 1s → σ* shape resonance and++continuum, respectively. H and CH3 ions were observed
as major products above and below the Cl K-edge, whileother product ions were observed only above the Cl Kedge. TOF-PSID spectra are not different greatly above
the Cl K-edge. The features at m/z = 1, 2, 13, 14, 15, 27,29, 35(37), 39, 41, and 43 were observed, which++++++correspond to H , H2 , CH , CH2 , CH3 , C2H3 ,++++++Cl ,C3H3 ,COCH /C3H5 ,and
OCH /C2H5 ,++CH3OC /C3H7 , respectively. These ions are mostly
cracking fragments of the α-chloromethacrylate unit and+++
OCH , COCH , and CH3OC ions are common products
resulting from PMMA [4]. For heavier desorbing ions+with m/z > 50 amu, m/z = 51(C4H3 ) and aromatic++fragments such as m/z = 77(C6H5 ) and/or 78(C6H6 ),+++91(C7H7 ) and/or 92(C7H8 ), 104(C8H8 , monomer less++CH2), 116(C9H8 , monomer less H2) and 129 (C10H9 ) canreadily be interpreted based on structure of the α-Fig. 1 Time of flight mass spectra for ion desorption
from ZEP520 resist film.
References[1] D.R. Medeiros et al., IBM J. RES & DEV. 45, 639(2001).
[2] H. Namatsu et al., J. Vac. Sci. Technol. B16, 3315
(1998).
[3] T. Sekiguchi et al., Surf. Sci. 482/485, 279 (2001).
[4] M.L.M. Rocco et. al., Polym. Degrad. Stabil. 80, 263(2003).
[5] S.G. Hansen, J. Appl. Phys. 66, 1411 (1986).
*ikeura-sekiguchi@aist.go.jp

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