A novel silicon containing chemical amplification resist for electron beam lithography

Microelectronic Engineering 13 (1991) 69-72 Elsevier

69

A NOVEL SILICON CONTAINING CHEMICAL AMPLIFICATION RESIST FOR ELECTRON BEAM LITHOGRAPHY H.Watanabe, Y.Todokoro, and M.Inoue Kyoto Research Laboratory Matsushita Electronics Corporation Kyoto, 601 JAPAN

A new negative working electron beam resist which is applicable to a bi-layer resist system has been developed. This resist consists of an onium salt cationic initiator and poly(methyl silsesquioxane). The resist can be developed in aqueous base solutions and shows very high sensitivity of 0.2 IxC/cm 2 on an electron beam exposure and high resistance to etching in an oxygen plasma.

1. INTRODUCTION With increasing topographic complexity of VLSI, Multi-layer resist systems become indispensable. Since a tri-layer resist system is complicated, the development of a bi-layer resist system has been required. Although conventional resists can be used in a tri-layer resist system, it is necessary to develop new resists for a bi-layer resist system. In most cases, top imaged-layers for a bi-layer resist system contain silicon for oxygen plasma resistance, so that images can be transferred to bottom layers [11,[2]. Several types of resists have been proposed for silicon-containing top layers : (1) mono-component organosilicon polymers which are mainly negative resists using radiation induced crosslinking[1]-[4], (2) two component resist systems consisting of diazonaphthoquinone and alkali-soluble organosilicon polymers as the matrix polymers [51-[6], (3) chemical amplification silicon-containing resist systems [71-[8]. The present study belongs to the third category. A few chemical amplification resists containing silicon atoms have been reported. In these resists, silicon atoms were introduced in the side chain of matrix polymers. The present study reports a novel chemical amplification resist containing silicon atoms in the matrix polymer main chain, which brings high silicon contents and good oxygen plasma resistance. 2.RESIST DESIGN The requirements for top imaged-layers for a bi-layer resist system are high sensitivity, high resolution, and high resistance to an oxygen plasma. Furthermore, it is desirable that the top imaging resists can be developed in alkaline aqueous solutions. Chemical amplification resist systems are most promising techniques to satisfy both requirements of high sensitivity and high resolution [9],110]. Since the higher silicon content in resist materials bring higher etching durability to an oxygen plasma, we have chosen a siloxane polymer as the matrix polymer which includes silicon atoms in the polymer main chain. To achieve an alkaline developable resist, we have chosen a small molecular weight siloxane polymer with polar units as end groups.

0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.

70

H. Watanabe et al. / A chemical amplification resist

3. EXPERIMENTAL 3.1. Materials Chemical structures of the resist components are shown in Fig.1. We have selected triphenyl sulfonium triflate (Ph3S+-CF3SO4) as a sensitizer because it has high quantum efficiency and no potential sources of contamination. Poly(methyl silsesquioxane), PMSQ containing hydroxyl and ethoxy groups as end groups have been selected as a siloxane matrix polymer because of its high silicon content of 42 wt% and its solubility property in alkaline aqueous solutions. To compare the oxygen reactive ion etching (RIE) resistances, a commercially available siliconcontaining negative resist, SNR (Tosoh) [4] and inorganic spin-on-glass, OCD Type-1 (Tokyo Ohka) were used. 3.2. Lithography Spin-coated resist films were baked at 75 °C for 2 min and exposed to a 25 kV electron beam. Exposed resist films were developed in a 5 % aqueous tetra-methylammonium hydroxide (TMAH) solution for 30 s. Oxygen RIE was carried out under the following conditions. The oxygen pressure was 2 Pa, the gas flow was 7.5 seem, the substrate bias was -520 V, and power density was 0.2 W/cm 2. 4. RESULTS AND DISCUSSION 4.1 Mechanism of imaging Onium salt generates Bronsted acid during electron beam irradiation and the formed acid induces the condensation of PMSQ. The changes occurring in the resist film during electron irradiation have been studied by infrared (IR) spectroscopy. Figure 2 shows the IR spectra of this resist film comprising PMSQ and 1 wt% Ph3S+-CF3SO4 before and after electron beam exposure. The film thickness was 0.9 ktm on silicon wafers. Strong bands observed in Fig.2 are attributed to Si-CI-I3 and Si-O-Si modes. The Si-CH3 associated modes are observed at 2990 cm 1 (stretch), 1275 cm -1 (bend) and 780 cm -1 (bend). The Si-O-Si bands are observed at 1130 and 1035 cm -1 (stretch) and 800 cm -1 (bend). The change of IR spectra is not noticeable in Fig.2. The difference of IR spectra before and after exposure is shown in Fig.3. The intensity of Si-OH bonds at 3500 and 900 cm -1 and Si-OC2I-B bond at 1080 cm -1 decreases while the intensity of strong Si-O-Si bands at 1035 and 800 cm -1 increases. These changes in spectra suggest that the condensation reaction occurs mainly through hydroxyl and ethoxy groups as shown in Fig.4. The peaks due to the Si-CH3 bonds at 2990 and

( CH3 qH3% H 1 0 - S i - O - Si "1" OC2Hs

/

CF3S03

?

°/

C2Hs 1 - O - S i - O - Si -1" OH \ CH3 CH~/n Triphenyl sulfonium triflate

Poly(methyl silsesquioxane)

Fig.l. Chemical structure of the new resist.

Dose Si-OH (izC/cm 2) Si CH3 0

3000

Si-O-Si '

s,-o-sij

1800 1000 Wavenumber (cm-1)

Fig. 2. IR spectra of the resist comprising PMSQ and 1 wt% Ph3S+-CF3SO4before and after an e-beam exposure.

11. Watanabe et aL / A chemical amplification resist

1275 cm 1 also slightly decrease with exposures. These changes indicates that the cross-linking occurs to a lesser extent through methyl groups [3]. During these electron beam exposures, the acid catalyzed condensation occurs, which causes the resist films to become insoluble in solvents.

71

Si-OH 1 Si-CH3 ('uO/cm D°se2 ) o~c

Si-OC2Hs I SI-OH

1

I--

4.2. Lithographic properties Si-O-Si Figure 5 shows the sensitivity curves for 0.5 ~tm r e s i s t comprising poly(methyl silsesquioxane) and 0, 1, 2, 3 wt% Ph3S +CF3SO4. The sensitivities of resists containing onium salt are about fifty times higher than that of PMSQ. This enhancement in sensitivity due to the presence of the onium salt confirms that the condensation reaction is catalyzed by radiochemically generated acid from the onium salt. The effect of onium salt contents from 1 to 3 wt% is not remarkable. Figure 6 shows the developed resist profile of 0.3 I.tm lines and spaces. The images were delineated by an electron beam at a dose of 0.2 g C / c m 2. High resolution has been obtained because of the development in an aqueous solution of TMAH. The important property which distinguishes the aqueous developed resists from solvent developed negative-tone resists is that they do not swell during development [11]. Extremely high sensitivity of this resist is offered by not only chemical amplified reaction but also solubility characteristics. The solubility differentiation is attributed to the polarity change in end groups in addition to the molecular weight increase because of acid catalyzed condensation.

I

I

I

I

I

I

3000 1800 1000 Wavenumber (cml)

I

Fig. 3. Difference of IR spectra of the resist comprising PMSQ and 1 wt% Ph~S+-CF3SO4 before and after an e-beam exposure.

I -Si--OC2H5 I

I

+ HO--Si--

H +

I

I

I

I

,~ --Si--O--Si--

t

+ C2HsOH

Fig.4. Acid catalyzed condensation reaction

1.0

3 ~/f1%

z

0%

0.5

c~ O

4.3 Dry etch resistance Several materials with various silicon contents were etched under the same oxygen RIE conditions. Table 1 shows the silicon contents, etch rates and the selectivity ratio of silicon containing materials to the hard-baked photoresist, OFPR-800. With increasing silicon content, the resistance to oxygen RIE is increased[12].

z

0 .

I

I

1 0 Dose (p.C/cnt2)

100

Fig 5. Sensitivity curves for the resist comprising PMSQ and 0, 1,2, 3 wt% Ph3S~- CF3SO4

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H. Watanabe et aL / A chemical amplification resist

Table 1. Oxygen RIE resistance Sample

Si content Etch rate Selectivit;'1) (wt%) (nm/min)

SNR OCD(Type-1) PMSQ/onium salt Fig. 6. SEM micrograph of 0.3 t.tm lines and spaces in the resist comprising PMSQ and 2 wt% Ph3S+'CF3SO4. Exposure dose : 0.2 ~C/cr~ at 25kV Development : 30 s in TMAH 5% solution

13 47 42

5.7 2.0 2.9

25 73 50

"1) Selectivity ratio of silicon containing materials to hardbaked OFPR-800 (145nm/min)

5. CONCLUSION A novel silicon-containing chemical amplification resist has been developed. The resist can be developed in aqueous base solutions and shows very high sensitivity of 0.2 I.tC/cm2 on an electron beam exposure, high resolution in deep sub-micron region and high resistance to etching in an oxygen plasma. Thus the resist can be applied to a bi-layer resist system. Besides its sensitivity and plasma etch durability, it has good thermal stability, durability to most acids and bases, high transparency beyond the deep UV region and good dielectric properties. The resist can be applied widely for semiconductor technology. REFERENCES [1] Hatzakis, M., Paraszczak, J., Shaw, J., Proc. Microcurcuit Engineering, Lausanne, Switzerland (1981) 386. [2] Ohnisi, Y., Suzuki, M., Saigo, K., Saotomo, Y. and Gokan, H., Proc. SPIE, 539 (1985) 62. [3] Roberts E.D., J.Electrochem.Soc., 120, (1973) 1716. [4] Morita M., Tanaka A., Imamura S., Tamamura T., and Kogure O., Jpn. J. Appl.Phys. 22 (1983) 1659. [5] Saotome Y., Gokan H., Saigo K., Suzuku M., and Ohnishi Y., J.Electrochem. Soc., 132 (1985) 909. [6] Imamura S., Tanaka A., Onose K., Proc. SPIE, 920, (1988) 291. [7] Buiguez, F., Guibert J.C., Schue F., Sagnes R., Serres B., Giral L., Abou-Madi W., Montginoul C., Proc. Microcurcuit Engineering 1984, pp-471-481. [8] Steinmann A., Proc. SPIE, 920 (1988) 13. [9] Ito, H. and Willson, C.G., Polym.Eng.Sci., 23(1983) 1012. [10] Ito, H. and Geant, C., in "Polymers in Electronics," ACS Symp. Series, No.242, Davidson,T.,ed., American Chemical Society, Washington D.C., 1984, p. 11 [11] Lui, H., deGrandepre, M.P., Feely, W.E., J. Vac. Sci. Technol., B6 (1988) 379. [12] G.N.Taylor, M.Y.Hellman and T.M.Wolf, Proc. SPIE, 920 (1988) 279.