Fluoropolymer Outgassing in Micro- and Nanofabrication - ACS

Chapter 21

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch021

Fluoropolymer Outgassing in Microand Nanofabrication 1,

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Yoshinori Matsui *, Shu Seki , Seiichi Tagawa *, Shou Tsuji , and Toshiro Itani 2

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The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Semiconductor Leading Edge Technologies, Inc., 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Current address: Matsuisita Electric Industrial Company, Ltd., 19 Nishikujo-Kasugacho, Minami-ku, Kyoto, Japan *Corresponding authors

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For improved micro- and nano-fabrication, we investigated the outgassing characteristics of fluoropolymers for the semiconductor industry on exposure to 157 nm, and for poly(tetrafluoroethylene) (PTFE) on exposure to a focused ion beam (FIB). The volatiles from thesefluoropolymerswere observed by in-situ quadrupole mass spectrometry. On exposure to 157 nm, side chain scission and the C-F bond play an important role in the outgassing process. Some fluoropolymers produce harmful HF which can damage exposure tools. HF evolution strongly depends on the polymer structure rather than the fluorine content, and can be reduced by optimizing the polymer structure. In contrast, main chain scission plays an important role in the outgassing characteristics of PTFE during FIB exposure. Fluorocarbons close to the upper detection limit of 298 amu were observed, and there is a possibility that some species with masses greater than 298 amu may evolve from the PTFE.

© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Fluoropolymers have received much attention for applications in various fields because of their excellent properties (1) such as high thermal stability, high chemical stability, low adhesion, biological suitability, low frictional resistance, and transparency at vacuum ultraviolet (VUV) region (2). Because of the sufficient transparency at 157 nm, fluoropolymers are used as a polymer for F and F immersion lithography for semiconductor industry (2). Polymethacrylates containing bi- and/or tri-alicyclic structures, which are major polymers in ArF resist (3), are too absorptive at 157 nm and cannot be used as a polymer in F and F immersion resist (2). Shortening of exposure wavelength from deep ultraviolet to VUV causes polymer photolysis by direct irradiation, leading to significant amount of resist outgassing. Outgassed species leaving exposed resist film deposit on system lenses and cause optical contamination. Fluorolymer outgassing is therefore a serious problem in F lithography (4 - 8). One of the most important outgassed species is HF which damages exposure tools (4 - 6). On the other hand, it was reported that fluorocarbons does not affect the optical contamination (7). Hydrocarbons such as isobutene is important from the viewpoint of the optical contamination (8). In the case of Polymethacrylates containing bi- and/or trialicyclic structures on exposure to 157 nm, outgassed species except for H originate from side chains (4). On exposure to 157nm, scission of side chain plays an important roll in outgassing characteristics of these polymers (4). Besides the use in F and F immersion lithography, fluoropolymers are used in variousfields.Fluoropolymer is one of the candidate for a polymer for MEMS, and fabrication of fluoropolymers by ultra-short pulsed lasers (9), and synchrotron radiations (10) has been studied. Polymer outgassing as well as the micro- and nano-fabrication depends on the polymer radiolysis. It is important for the micro- and nano-fabrication to investigate outgassing characteristics which is obvious outcome in the polymer radiolysis. In the present study, we investigated outgassing characteristics of fluoropolymers for semiconductor industry on exposure to 157 nm, and also investigated outgassing characteristics of PTFE on exposure to focused ion beam (FIB). Outgassed species from these fluoropolymer were detedted by in-situ quadrupole mass spectrometry. 2

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Experimental Figure la shows the experimental set-up for measurement of outgassed species from fluoropolymers on exposure to 157 nm. The exposure source is a LAMBDA PHYSIK OPTex UV excimer laser (Wavelength: 157 nm, Power: 0. 8 mJ/pulse, Rep. Rate: 30 Hz, Pulse duration: 5-10 ns FWHM, laser intensity: 2 MW/cm ) where a vacuum chamber was connected via CaF lens. A PFEIRRER 2

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Figure 1. Experimental set-up for measuring outgassed species from (a) fluoropolymers for a resist on exposure to 157 nm, and (b) PTFE on exposure to FIB.


256 VACUUM Prisma™ QMS 200 M3 mass spectrometer with a 100 mA, 300 V ionizing source is integrated into the vacuum chamber to examine outgassed species. The QMS is operational at the pressure of ~1 χ 10" torr and scanned over 1-299 amu. The mass spectra of outgassed species were obtained by subtraction of the background mass spectra from the spectra taken with the exposure. The details of the experimental set-up has been described elsewhere (4). Figure lb shows experimental set-up for measuring outgassed species from PTFE on exposure to FIB, using in-situ quadrupole mass spectrometry. Exposure of FIB was carried out by Seiko Instruments Inc. SMI2050 where a PFEIRRER VACUUM Prisma™ QMS 200 M3 mass spectrometer with a 100 mA, 300 V ionizing source is integrated. The details of the experimental set-up has been described elsewhere (11).


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Results and Discussion Outgassed Species from Fluoropolymers for F lithography on Exposure to 157 nm 2

Outgassed species during 157 nm exposure were observed for two classes of fluorinated backbone-alicyclic-polymers. The former is main-chain fluorinated polymers (TFE-a, TFE-b) and the latter is side-chain fluorinated polymers (FNb) as shown in Figure 2. These polymers were supplied by Semiconductor Leading Edge Technologies, Inc. Fluorine contents of these polymers are 38, 29, and 40 wt%, respectively. Thickness of films of these polymers is -100 nm. The absorbance of these films is ca. 0.66 ~ 2.7 μιη" . UV and film thickness measurements were performed using a HITACHI VUV-VASE spectroscopic ellipsometer. Figure 3a shows mass spectra of outgassed species from a film of TFE-a during 157 nm exposure. Outgassed species from TFE-a, which has no side chains, include HF (20 amu) that may be one of the most harmful outgassed species to optical elements in exposure tools. As for main-chain fluorinated polymers, HF as well as H evolve from their main chain. H 0 , N , and 0 might be mainly the adsorbates of the vacuum chamber. Figure 3b shows mass spectra of outgassed species from a film of TFE-b containing terf-butyl afluoroacrylate group. Outgassed species observed in the exposure of the film of TFE-b were H (2 amu), HF (20 amu), N (28 amu), 0 (32 amu), C 0 (44 amu), isobutene (56 amu). HF also evolve from this polymer. Some outgassed species such as isobutene evolvefromteri-butylα-fluoroacrylate group. Figure 5c shows mass spectra of outgassed species from a film of FNb. In comparison with TFE-a and TFE-b, very small amount of HF was detected. Side-chain fluorinated polymer of FNb produce fluorine-containing outgassed species with high molecular weight instead of HF. It was reported that fluorocarbon does not cause optical contamination (7). In view of outgassing issue, side-chain fluorinated polymer FNb is superior to main-chain fluorinated polymer TFE-b. 1

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\

/soV^/k,

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''3f

'--

.'30-

§°'39 CH3~Ç CH3 CH

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(C)

v-CF

VCF

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F3C OH

F C 3

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- ° CH

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Figure 2. Thefluoropolymersemployed: (a) TFE-a, (b) TFE-b, (c) FNb.

Outgassed Species from PTFE on Exposure to FIB Figure 4 shows the mass spectra of outgassed species from FIB irradiated PTFE. There are many mass spectral peaks from 1 amu to 298 amu. The peak located at 69 amu is assigned to CF . Other significant peaks are due to fragments of C F . Outgassed species with the mass-number ~ 298 amu were observed, and it is predicted that outgassed species with mass-number greater than 298 amu evolve from PTFE. Scission of main chain and low boiling point of resulting fluorocarbons (Table 1) are responsible for the outgassed fluorocarbons with high molecular weight. 3

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Table 1. Molecular Weights and Boiling Points of Fluorocarbons. CF 3

g

C4F10 C5F12

CeF|4

Molecular weight

Boiling point [°C] (12)

188 238 288 338

-36.6 -1.9 29.2 56.6

Conclusions We focused on outgassing characteristics of fluoropolymers for semiconductor industry on exposure to 157 nm, and also focused on outgassing characteristics of PTFE on exposure to FIB. Outgassed species from these fluoropolymers were observed by in-situ quadrupole mass spectrometry. On exposure to 157 nm, scission of side chain and C-F bond plays an important roll in outgassing characteristics. Some fluoropolymers for semiconductor industry produce harmful outgassed species of HF which


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Mass number (amu) H0 N (18) (28) Η HF,' (2) [20) o 2

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CH| to (15) !> / CO, (44) ( >

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Fluoropolymer Outgassing in Micro- and Nanofabrication - ACS

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