157-nm Resist Materials Using Main-Chain Fluorinated Polymers

Chapter 6

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157-nm Resist Materials Using Main-Chain Fluorinated Polymers Toshiro Itani, Hiroyuki Watanabe, Tamio Yamazaki, Seiichi Ishikawa, Naomi Shida, and Minoru Toriumi Semiconductor Leading Edge Technologies, Inc. (Selete), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

157-nm lithography is one of the most promising technologies for next-generation lithography. Due to the high absorbance at 157 nm, existing photoresist materials can not be used for 157-nm lithography. Fluoropolymers show good optical transparency and fluorinated resist materials have been intensively studied. Side-chain-fluorinated polymers have been shown a possibility of use for multi-layer resists. However they can not be applied to a single-layer resist, which is the most favored

approach among

potential industry. We have studied the main-chain-fluorinated polymers that meet the transparency requirement for use as single layer resists. They are two families composed of tetrafluoroethylene and monocyclic fluorocarbons. They showed good absorption coefficients of less than 1.5 μm-1 and fine dry etching durability. Positive-working

resists

were

developed

using

these 2

fluoropolymers and showed good sensitivity less than 10 mJ/cm . The resolution of the 80-nm dense pattern was determined at the film thickness thicker than 200 nm.

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© 2004 American Chemical Society

Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction 157-nm lithography is one of the most promising technologies for next-generation lithography. Conventional organic materials used in the resist have fatal disadvantages of opaqueness at the exposure wavelength. Resist Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 14, 2016 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch006

materials must meet the transparency requirement for use as single layer resists. High fluorine or silicon content can lead to a substantial increase in transparency at

157 nm. Therefore fluoropolymers are extensively studied. (1-6)

Side-chain-fluorinated polymers based on alicyclic and aromatic structures have been studied. (1-4) These polymers inherited good dry-etching resistance in addition to good spinnability, adhesion, developability and so on. Their 1

absorption coefficients are ca. 2 to 3 μπι" . They are better than conventional materials and may be used as multi-layer resists. However they can not be applied to a single-layer resist. Main-chain-fluorinated polymers were reported to have an absorption 1

coefficient lower than 1.5 μπι" . (5,6) They can be potentially used in a single-layer resist. In this paper we focus on the characteristics of the resist materials for 157-nm lithography: main-chain-fluorinated base resins containing tetrafluoroethylene (TFE) and monocyclic fluorocarbons.

Experimental

Materials Fluoropolymers studied here were synthesized either at Asahi Glass Co., Ltd. or Daikin Industries, Ltd. through procedures described in the literatures (7,8). They are many kinds of fluoropolymersfluorinatedat main-chain and side chain. As mentioned in the following sections, main-chain-fluorinated polymers show the better results. Thefluoropolymerscan be classified into two groups


74 as shown in Scheme. One group is characterized by α-fluorination of polymer backbones and the presence of dry-etching-resistant norbornene (NB) as poly(TFE/NB/a-fluoroolefin) fluoropolymers (FP1). The other group includes new monocyclic fluoropolymers (FP2). The monocyclicringsare considered as


resistant to dry etching. The synthesis procedures and the details of the preparation and chemistry are described.

Measurements

Polymer samples were dissolved in casting solvent of propylene glycol methyl ether acetate (PGMEA), or ethyl lactate (EL) with a solids content of 5-10%. Sample processing was carried out on a Si, calcium fluoride or quartz crystal at a film thickness of 100-300 nm with softbake of 130-150 °C. The vacuum ultra-violet (VUV) absorption properties of the sample films were recorded on a calcium fluoride substrate by using a single-beam VUV spectrophotometer, VU-201, of Bunkoh-Keiki Co., Ltd. Dry-etch measurements were done using an IEM etcher of Tokyo Electron, Ltd. The process parameters were under contact-hole etching conditions with a total pressure of 30 mTorr, a C4F flux of 11 seem, an O2 flux of 8 seem, and an Ar flux of 400 seem. Quartz 8

crystal microbalance (QCM) method was used to study the dissolution characteristics of fluoropolymers in an aqueous tetramethylammonium hydroxide (TMAH) solution in a fashion similar to the reported procedure (P). Acidities of alkaline soluble functional groups influoropolymerswere measured by potentiometric titration in dimethylsulfoxide non-aqueous solution (P). Resists used triphenylsulfonium triflate as sensitizer at approx. 5% of solids. Resist processing was carried out on organic bottom antireflection coating on Si at a film thickness of 100-300 nm with softbake of 130-150 °C. Exposures to resist films were performed in an open-frame F laser exposure system, 2

2

VUVES-4500 of Litho Tech Japan Co. on 1-cm areas with doses ranging from 2

0.1 to 100 mJ/cm to obtain the photospeed and contrast curve. To evaluate the resolution, the resists were patterned by using a Levenson-type strong


75 phase-shift mask on a 157-nm laser microstepper (NA=0.6 and 0.85, σ =0.3). For lithographic evaluation, micrographs offinepatterns were obtained using a


Hitachi S-5000 scanning electron microscope.

Results and Discussions

Characterization of Fundamental Properties of Fluoropolymers

We have measured the absorption coefficients at 157 nm of many polymers including more than 400 fluoropolymers. Figure 1 shows the histograms of fluoropolymers and non-fluoropolymers. The average absorption coefficient of 1

1

fluoropolymers is ca. 2.3 μπι" , which is smaller than 6.3 μπι" of non-fluoropolymers. Thefluorinationmakes polymers more transparent by 4 1

μπι" . Figure 2 shows the details offluoropolymerbar chat in Figure 1. They are classified intofluoropolymersincorporating fluorine at their backbones and side-chains. The main-chain-fluorinated polymers have the good optical property transparent than side-chain-fluorinated polymers. Somefluoropolymershave the 1

good absorption coefficient less than 1 μπι" , which can not be achieved by fluorination at side-chains. Tetrafluoroethylene (TFE) is one of good monomers for backbone-fluorinated polymers. Figure 3 shows the TFE-content dependence of absorption coefficient of TFE-norbornene (NB) copolymers. The more TFE-content increases, the more transparent the copolymer becomes. Note that the TFE polymers have the potential of the absorption coefficient less than 1 1

μπΓ . Figure 4 shows the dependence of optical absorption coefficients of the fluoropolymers on thefluorineconcentration. Fluoropolymers generally become more transparent with increasingfluorinecontent. The second ordinate, or the "50%T thickness", shows the film thickness at the bottom of which the light intensity decreases to half the incident intensity of 157-nm light. Some FP1



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Scheme. Molecular structures of FP1 and FP2.

Figure 1. Absorption coefficient histogram of polymers.



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Figure 2. Absorption coefficient histogram of fluoropolymers.

Figure 3. The dependence of absorption coefficient on TFE contents in TFE-NB-polymers.



78

Ο

20 40 60 Fluorine content [wt%]

80

Figure 4. The dependence of absorption coefficient on fluorine contents influoropolymers.FP1: open dots, FP2: closed dots, Others: x.

140

150

160

170

180

190

200

Wavelength [nm]

Figure 5. VUV spectrum of a perfluoropolymer, Cytop®.


79 polymers achieve good transparency even at a thickness of more than 200 nm. The dashed line was determined by a least-squared fit using all the data. This figure shows thatfluorinecontent of more than 40% is needed for 200-nm-thick films. Data are widely scattered in thefigure.Figure 2 indicates also that some 1


fluoropolymers have the larger absorption coefficients larger than 3 μηι" in spite of its backbonefluorination.Note that main-chainfluorinationnot always make sufficiently polymers more transparent at 157 nm. Typical example is shown in figure 5, which is the Vacuum ultraviolet (VUV) spectrum of Cytop®. It is a fluoropolymer of perfluoroether with highfluorinecontent of 68 %. However it 1

has the absorption coefficient of 1.7 μηι" , which is not so good as the estimated value. The absorption spectrum has the maximum at 157nm and it increases the absorption coefficient at 157nm. In order to obtain the transparent fluoropolymers we have to optimize the molecular structure such as fluorinated positions in addition to the highfluorinecontent. We have studied two platforms of main-chain-fluorinated polymers for resist base resins. One family is a TFE-type polymer, FP1. The other family is a non-TFE-type polymer, FP2 as shown in Scheme. Open dots indicate FP1, closed dots FP2 and χ others in Figure 4. FP1 shows the wide range of absorption coefficients, which is depending upon the molecular structures. FP2 shows the narrower range and has absorption coefficient smaller than thefittingline. Figure 6 shows relative etching rates to the acrylate-base ArF resist. The etching rates were measured under the conditions of the fluorocarbon-based plasma oxide etching. FP1 polymers contain NB derivatives and some FP1 polymers achieve etching resistance higher than that of ArF resists. Poly(TFE/NB)s have a dry-etching rate 0.8-1.1 times larger than that of ArF resists, and some FP1 polymers are comparable in this respect to KrF resists. The etching rates of monocyclic FP2 polymers are scattered more widely than


80 those of FP1 polymers. It indicates the weak dry-etching durability of the main structure in itself. Some FP2s have slightly lower etching selectivity but it is still comparable to that of acrylate-based ArF resists. Thefluorineincorporation to the backbones such as TFE increases the durability (8). This is another


advantage of main-chain fluorination. Figure 7 shows dissolution behavior of FP1 and FP2, both of which dissolved away in the standard alkaline developer of 0.26N aqueous tetramethylammonium hydoxide (TMAH). The FP1 polymer dissolves as fast due to the larger acidic functional group than that of FP2. However FP1 shows the interface layer where it dissolves slower than FP2. This is one of the causes of difference of their resist resolving powers. Most of monomers of FP2 polymer contain the functional group soluble in the alkaline solution. The high contents of alkaline soluble groups may enable its good solubility in the standard developer.

Lithography Performances To formulate a positive tone resist, fluoropolymers with protective functional groups were evaluated. These polymers underwent a chemically amplified change in solubility through a deprotection reaction. Figure 8 shows the contrast curves of the positive-working resists using FP1 and FP2. The photo-acid generator (PAG) was triphenylsulfonium (TPSTf). They showed no film-thickness loss in the unexposed areas and good clearing doses of 1.7 and 2

3.2 mJ/cm for FP1 and FP2, respectively. They also had good contrast curves over two dose ranges and did not display negative tone behavior at an exposure 2

dose of 100 mJ/cm .



Figure 6.

Dependence of dry-etching rate of fluoropolymers

onfluorineatom content. FP1: open dots, FP2: closed dots, Others:

-0.2

Figure 7.

0.0

0.2 0.4 0.6 0.8 Development time [s]

1.0

QCM traces of (a) FP1 and (b) FP2 .


82 Imaging Performances Figure 9 shows lithographic results obtained using an FP1- and FP2-based resists. FP-1 has tert-butyl group as the protection group of 65%, and FP-2 has


methoxymethyl group as the protection group of 15%. This imaging experiment was performed on a 0.6-NA microstepper, using a phase-shift mask at a 150-nm film thickness. The resist showed good imaging performance for dense patterns with 1-to-l lines and spaces. Projection imaging yielded 90- and 80-nm patterns respectively. FPl-based resists lose resolutions. This footing in FP-1 patterns may be the result of the dissolution-rate retardation in the substrate region as shown in Figure 7. We are now improving the dissolution behavior or FP1 polymers to accomplish the higher resolution. The FP2-based resist shows the delineated patterns with a round top. To improve the resist profile we increased the content of the protective functional groupfrom20% to 30%. The 30%-protected FP2-based resist achieves the better rectangular shape offinepatterns as shown in Figure 10. These main-chain fluorinated have high transmittance at the exposure wavelength and can be used to obtain resist films thicker than 200 nm. For instance, Figure 11 shows the 95-nm 1:1 patterns delineated by a 0.6-NA microstepper using a phase-shift mask at a 300-nm film thickness. This a great advantage of main-chain-fluorinated polymers with high transmittance.

Conclusions

Main-chain-fluorinated polymers were investigated and these polymers have the great advantages of high transmittance with absorption coefficients of less than 1 1

μηι" at 157 nm and good solubility without swelling during the development and fine

dry-etching

resistant

comparable

to

ArF

resist.

These

main-chain-fluorinated polymers can thus be used as single-layer resists to obtainfilmslarger than 200 nm.



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Figure 8.

Figure 9.

Sensitivity curves of (a) FP1 and (b) FP2-TPSTf systems.

(a) 90-nm patterns in FP1 resist and (b) 80-nm in FP2 resist after

development on a F laser microstepper, NA 0.6, using a phase-shift mask. Film 2

thickness is 150 nm.



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Figure 10.

Figure 11.

85-nm features in FP2 resist with high content protective-group.

95-nm patterns in FP2 resist with the film thickness of 300 nm.


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Watanabe, M.; Itani, T., Proc. SPIE 2001, 4345, p.371 and references therein. 7. Shun-ichi Kodama, S.; Kaneko, I.; Takebe, Y.; Okada, S.; Kawaguchi, Y,; Shida, N.; Ishikawa, S.; Toriumi, M.;

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p.76. 8. Koh, M.; Ishikawa, T.; Araki, T.; Aoyama, H.; Yamashita, T.; Yamazaki, T.; Watanabe, H.; Toriumi, M.; Itani, T., Proc. SPIE 2002, 4690, p486. 9. Toriumi, M.; Itani, T.; Yamashita, J.; Sekine, T.; Nakatani, K., Proc. SPIE 2002, 4690, p191 and references therein.


157-nm Resist Materials Using Main-Chain Fluorinated Polymers

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