Acid Generation in Chemically Amplified Resist Films - ACS Publications

Chapter 8

Acid Generation in Chemically Amplified Resist Films 1

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Takeo Watanabe , Yoshio Yamashita , Takahiro Kozawa , Yoichi Yoshida , and Seiichi Tagawa 3

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SORTEC Corporation, 16-1 Wadai, Tsukuba-shi, Ibaraki 300-42, Japan Nuclear Engineering Research Laboratory, Faculty of Engineering, University of Tokyo, 2-22 Sirakata-Sirane, Tokai-mura, Ibaraki 319-11, Japan Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan

Downloaded by FUDAN UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch008

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A new interpretation of radiation-induced acid generation processes in polymer films is reported. For the investigation of the acid generation in chemically amplified resist films we employed a model system. In order to analyze the acid generation process, we utilized the visible absorption characteristics from a conventional spectrophotometer and a nanosecond pulse radiolysis system. The acid generation mechanisms of triphenylsulfonium triflate in m-cresol novolac and p-cresol novolac systems are discussed on the basis of absorption spectra. The acid-catalyzed reaction during post-exposure bake is also discussed in terms of the absorption spectra of the m-cresol novolac system. Chemically amplifiedresists(1) for X-ray and electron beam lithography can provide the high performance required for fabrication of LSI's of quarter-micron feature size and below. Many papers have reported the effects in chemically amplified resists such as the processing of prebake, development, post-exposure bake (PEB), and delay time between exposure and PEB (1-8). As their sensitivity and resolution strongly depend on acid generation upon exposure and subsequent acid diffusion during PEB, it is important to understand the behavior of acid in theresistfilms. The acid photogeneration mechanisms of onium salts have been investigated (9,10). However, they have not been fully verified yet. Acid generation by photoinduced and radiation-inducedreactionswas studied in solution with several acid indicators. Quantum yields for acid generation have been determinedfromphotolysis of onium salts in acetonitrile solution (11). For the analysis of acid generated by photoinduced and radiation-inducedreactionsin chemically amplified resists, an IBM group (12,13) employed the merocyanine dye technique (14). For the analysis of the radiation-induced mechanisms in chemically amplified resists, a University of Tokyo group used pyrene as a model compound for crosslinkers and dissolution inhibitors with the experiment on nanosecond pulse radiolysis (15,16). The present paper describes a new interpretation of acid generation process by radiation-inducedreactionin polymerfilmsin the absence of acid indicators. In order to investigate the acid generation process, we utilized the visible absorption characteristics from a conventional spectrophotometer and a nanosecond pulse radiolysis system. 0097-^156/94/0579-0110$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

8. WATANABE ET AL.



Experimental Two kinds of model systems which were examined consisted of m-cresol or p-cresol novolac as the base resin. M-cresol novolac and p-cresol novolac, which were provided by Sumitomo Durez Co., have a weight average molecular weight (Mw) of 2,000 and that of 700, respectively. Each model system consisted of triphenylsulfonium triflate (03SCF3SO3) as the acid generator (PAG) and methyl 3methoxypropionate (MMP) as the solvent. Triphenylsulfonium triflate was purchased from Midori Kagaku Co.. Methyl 3-methoxypropionate was purchased from Tokyo Ohka Co.. The content of the novolac resin was 40 wt% of MMP, and the content of 03SCF3SO3 was 5 wt.% of the novolac resin. Two^m-thickfilmswere coated on 3inch-diameter quartz wafers. Prebake and PEB were carried out on a hot plate for 120 sec at temperatures of 120 °C and 110 °C, respectively. A SORTEC synchrotron radiation (SR) ring was used as the exposure source (17). Energy and critical wavelength are 1 GeV and 1.55 nm,respectively.The beamline has an oscillating Pt mirror and a 40^m-thick Be window. The peak wavelength of SR irradiation through the mask membrane was 0.7 nm and the wavelength range was 0.2-1.5 nm. The exposures were carried out in helium at 1 atm. For the films with and without PAG, absorption spectra before exposure, after exposure, and after PEB were recorded on a spectrophotometer, Hitachi model U3410. It took about 1 min to set a wafer in spectrophotometer after exposure. Data collection time was about 5 min per absorption spectrum. In order to investigate the irradiation effect in the time range of nanoseconds, an experiment on nanosecond pulse radiolysis was carried out The nanosecond pulse radiolysis system for optical absorption spectroscopy is composed of a 28 MeV linac as an irradiation source, a Xe lamp as analyzing light, a monochromator, a Si photodiode, a Ge photodiode, and a transient digitizer. The width of an electron pulse was 2 nsec. The solid samples were exposed by an electron pulse with a current of 2 nC. The details of the nanosecond pulse radiolysis system werereportedpreviously (18). Results and discussion For the m-cresol novolac system, absorption spectra of the films were measured before exposure, after exposure, and after PEB, in the time range of 1 min by the conventional spectrophotometer. For films without PAG, the spectra before exposure, after exposure with a dose of 540 mJ/cm , and after PEB are shown in Figure 1. The weak absorption peaks at wavelengths less than 400 nm may be products of decomposition of the m-cresol novolac by SR irradiation. The weak absorption became weaker after PEB. For films with PAG, the spectra before exposure, after exposure with a dose of 540 mJ/cm , and after PEB are shown in Figure 2. There are no characteristic absorption peaks before exposure. After exposure there is a strong absorption peak at 542 nm. The exposure dose dependence of the absorbance difference at 542 nm before exposure and after exposure is shown in Figure 3. We can fit the curve with the equation 2

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ΔΕ = 4.29 x 10-1 ( 1 - exp( -7.77 x 10^ X ))


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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS


0.50

300

400

500 600 700 Wavelength (nm)

800

Figure 1: Absorption spectra before exposure, after exposure with a dose of 540 mJ/cm , and after PEB, for m-cresol novolac without PAG. 2

0.50 0.40

r

ι 'Ur

1

\

\

after PEB

A

/ \

Κ

\ before exposure *-»

300

after exposure

ι I ι

400

..I,

\ !

...I

500 600 700 Wavelength (nm)

...1

800

Figure 2: Absorption spectra before exposure, after exposure with a dose of 540 mJ/cm , and after PEB, for m-cresol novolac with PAG. 2


8. WATANABE ET AL.



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for 98 % confidence level (C.L.), where ΔΕ and X (mJ/cm ) are absorbance difference and exposure dose, respectively. The equation indicates that a first-order reaction occurred by exposure. Two 1.5-mm-thickfilmsof m-cresol novolac without and with 30 wt.% PAG were prepared for the experiment on nanosecond pulse radiolysis. Absorption-timedependent behaviors at 540 nm for the film with and without PAG are shown in Figure 4. Absorption at 540 nm for the film without PAG is weak and includes a large amount of noise, while absorption at 540 nm for the film with PAG is strong. Absorption-time-dependent behaviors at 540 nm and 700 nm for the film with PAG arc shown in Figure 5. The decay at 540 nm is very slow, while the decay at 700 nm is very fast. Hie absorption at 700 nm may be due to the existence of cationic species of m-cresol novolac. For m-cresol novolac with PAG, the strong absorption near 542 nm may be due to the existence of a protonated intermediate. The protonated intermediate may be a productfromthe m-cresol novolac and acid (proton). The spectra after exposure and after PEB, as shown in Figure 2, indicate that after PEB the protonated intermediate decomposed and then an absorption peak at 394 nm appeared. The exposure dose dependence of the absorbance difference at 542 nm after exposure and after PEB is shown in Figure 6. We canfitthe curve with the equation 4

ΔΕ = -2.83 Χ ΙΟ-» ( 1 - exp( -6.49 Χ ΙΟ" X )) for 99 % C.L.. The equation indicates that the decomposition by PEB of the protonated intermediate is afirst-orderreaction.The exposure dose dependence of the absorbance difference at 394 nm after exposure and after PEB is shown in Figure 7. We canfitthe curve with the equation 2

ΔΕ = 7.04 χ 10- ( 1 - exp( -1.85 x 10-3 X )) for 99 % C.L.. The equation indicates that the decompositionreactionby PEB of the product species is afirst-orderreaction. The protonated intermediate seemed to be decomposed by PEB and the absorption peak at 542 nm became weaker, while the absorption peak at 394 nm became stronger. The absorption spectrum of m-cresol novolac with 5 wt.% p-toluenesulfonic acid is shown in Figure 8. PEB was carried out on a hot plate for 120 sec at a temperature of 120 °C. An absorption peak near 394 nm appeared. Before PEB absorption peaks were not observed. Therefore during PEB, it could be assumed that protonated adducts of m-cresol novolac was formed by the attachment of a proton. Though it was previously considered that the acid is generated during exposure, comparing the absorption spectrum after PEB for m-cresol novolac which contains PAG with that after PEB for m-cresol novolac which contains p-toluenesulfonic acid, it can be assumed that the acid may be generated not only during exposure but also during PEB in the m-cresol novolac with PAG. For the p-cresol novolac system, absorption spectra of the films were measured before exposure and after exposure, in the time range of 1 min by the conventional spectrophotometer. For the film without PAG, the spectra before exposure, and after exposure with a dose of 2160 mJ/cm are shown in Figure 9. There are no strong absorption peaks. For the film with PAG, the spectra before exposure and after exposure with a dose of 2160 mJ/cm are shown in Figure 10. There are no strong absorption peaks. Two 2-mm-thick films of p-cresol novolac without and with 15 wt.% PAG were prepared for the experiment on nanosecond pulse radiolysis. Transient absorption spectra are shown in Figure 11. For the film without PAG, there are no strong absorption peaks in thetimerange of 450 nsec. The film with PAG was exposed at a 2

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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

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Acid Generation in Chemically Amplified Resist Films - ACS Publications

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