Characterization and degradation of ZEP520 resist film by TOF-PSID and NEXAFS
Abstract
Degradation process of ZEP520 resist induced by chlorine atom-selective excitation was analysed in situ by photon-stimulated ion desorption (PSID) using time-of-flight (TOF) mass spectrometry. Above the chlorine K-edge, ionic products generated by main-chain scissions were observed. This observation confirms that the ZEP520 resist can achieve high sensitivity by chlorine 1s core excitation. Surface and bulk electronic structures of the ZEP520 were also probed by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy monitoring the total electron yield (TEY) and PSID, respectively. It was found that Cl+ PSID yield was strongly enhanced at the Cl 1s → σ*(Cl
C) resonance and peak positions of all ions were different from that in the TEY. This indicates that the desorbing ions observed are formed by a direct photon excitation. It was suggested that most ions are produced by photoionization of neutral precursors lacking volatile products that are generated by radiolysis on the surface.
Introduction
For ultra-large scale integration (ULSI) technology with a minimum dimension, the development of resist with higher resolution is one of the key issues. Polymethylmethacrylate (PMMA) is the most common high-resolution positive-tone electron-beam resist. Many electron-beam resists are based on PMMA. ZEP520 (Nippon Zeon Co.) is a positive-tone electron-beam resist with comparable resolution to PMMA. Molecular structure of this resist is alternating copolymer of α-chloromethacrylate and α-methylstyrene [1] as drawn in Scheme 1. The replacement of α-methyl groups in PMMA by α-chlorine atoms is expected to have higher resist sensitivity and the introduction of α-phenyl groups into PMMA produces high dry-etching durability. Thus, ZEP520 improves sensitivity and etch resistance compared with PMMA and then has been widely used for devise fabrication including zone plate optics. In higher voltage electron-beam exposure system, however, ZEP520 is needed to be further improved both in resist contrast and sensitivity, and enhanced for reactive ion etching (RIE) resistance [2]. Line-edge roughness (LER) is also a serious problem to reduce pattern size, which is caused by polymer aggregates in the size of 20–30 nm [3]. To improve the performance of the existing resist and design new resist, understanding of characteristics of ZEP520 is profitable. In this work, the characterization and the degradation process of ZEP520 were studied by time-of-flight mass spectrometry for photon-stimulated ion desorption (TOF-PSID) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. To investigate the effect of the α-chlorine atom for sensitization on the degradation of ZEP520, the α-chlorine atom was selectively excited using tunable synchrotron radiation (SR).
Section snippets
Experimental
ZEP520 resist was diluted with o-dichlorobenzene, spin-coated on a silicon substrate to a thickness of 100 nm, and prebaked on a hot plate at 200 °C for 4 min. The molecular weight was 57,000.
The TOF-PSID and NEXAFS measurements were performed at the Photon Factory using an InSb(1 1 1) double-crystal monochromator beamline (BL-27A) with an energy width of 1 eV around Cl K-edge. TOF-PSID mass spectra were measured using a rotatable TOF spectrometer [4] combined with pulsed SR. PSID-NEXAFS spectra were
Results and discussion
Fig. 1 shows TOF-PSID spectra, observed at 2814, 2823, 2830 and 2849 eV photon energy in an X-ray incident angle of 45°, together with a calculated mass scale. These energies correspond to off resonance, Cl 1s → σ*(ClC), Cl 1s → σ* shape resonance and continuum, respectively. The time reproducibility of the SR pulses is extremely good, heavy ions whose flight times were longer than 624 ns could also be observed. The flight time of a heavy ion is given by its position in the spectrum plus an integer
Conclusions
The atom-selective excitation of the side-chain Cl produces degradation of ZEP520 resist by main-chain scissions. This finding implies an unusual phenomenon that localized excitation forms ionic fragments produced by bond breaking of a remote site. Considering that the observed ions were induced by direct photon excitation process, the phenomenon can be explained by photoionization of neutral precursors lacking volatile products that are generated by radiolysis on the surface. The energy
Acknowledgments
The authors would like to thank the staff of the Photon Factory for their support. This work was conducted with the approval of the Photon Factory Program Advisory Committee. Sample preparation in this work was conducted in AIST Nano-Processing Facility, supported by “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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