Micro- and Nanopatterning Polymers - American Chemical Society

Chapter 26

Post-Exposure Bake Kinetics in Epoxy Novolac-Based Chemically Amplified Resists 1

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Downloaded by UNIV OF OKLAHOMA on October 27, 2014 | http://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch026

P. Argitis , S. Boyatzis , I. Raptis , N. Glezos , and M . Hatzakis 1

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Institute of Microelectronics and Institute of Physical Chemistry, NCSR Demokritos, 15310 Aghia Paraskevi, Attiki, Greece

The chemical mechanism and the kinetics of photoacid initiated crosslinking reactions in epoxy novolac based chemically amplified resists are consistent with the strong cage effect observed during post exposure bake (PEB) in these resists. FTIR has confirmed that the acid initiating the crosslinking reaction is bound to the polymer and suggests that the bulky carbonium intermediate is the actually moving species. Single pixel e-beam exposures used to measure the influence of acid diffusion in lithographic feature dimensions show limited diffusion with increasing PEB time. Microlithographic results from deep UV and e -beam contrast curve experiments reveal that the resist crosslinking is diffusion controlled at the PEB conditions of lithographic interest: the conditions that favour reactant diffusion facilitate reaction completion and increase resist contrast but can restrict resolution. Thermomechanical analysis and differential scanning calorimetry results are in agreement with the above observation.

The chemical amplification concept (I) is still the dominant strategy followed i n the design o f new resists for microlithographic applications including not only deep U V and 193 nm optical lithography but also X U V , x-ray, e-beam and lately micromachining. Since the chemical reactions inducing the solubility change i n the chemically amplified resists occur during the post exposure bake the understanding and control o f events occurring at this step is o f great importance for the optimization of their formulations and processes.

© 1 9 9 8 American Chemical Society In Micro- and Nanopatterning Polymers; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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A c i d diffusion i n particular determines to a significant extent the resist process windows, dose requirements, contrast and high resolution properties, as it controls the local reactant concentration and thus the chemical reaction inducing the solubility change i n the resist. Thus the study o f the phenomena related to the extent o f diffusion has drawn great research effort. Since the acid diffusion related effects are independent from the source o f radiation used for its generation there are opportunities for using different experimental conditions for their study (2). In this context there has been developed by Raptis et al. a methodology based on single pixel e-beam exposures aiming at the study o f post apply bake ( P A B ) and post exposure bake ( P E B ) effects o f chemically amplified resists (3). Based on this experimental technique diffusion coefficients can be calculated for different resists and processing conditions to be used in proximity correction. O n the other hand an insight into the specific resist chemistry should give further information on the expected behavior, rationalize the results obtained from microlithograpic experiments and help the resist process and formulation optimization. These goals motivated the study on the chemistry o f epoxy novolac based chemically amplified resists presented here. The concept o f chemical amplification has been fruitfully applied to systems containing epoxy functionality giving resists with good performance in a number o f applications: deep and near U V lithography (4), micromachining (5,6) and fast high resolution e-beam lithography (7,8). In this last case the resist formulations are mainly characterized by high sensitivity and capability for high aspect ratio microlithography down to the 0.1 | i m resolution whereas they show very good process latitude regarding P E B conditions. In the present study, the chemical mechanism and the kinetics o f acid initiated crosslinking reactions o f epoxy novolac based chemically amplified resists are examined. F T I R and thermal analysis have been used as the basic methods for elucidating chemical mechanism. Lithographic results obtained i n a number o f different processing conditions are interpreted in the context of the proposed mechanism.

Experimental Materials. The epoxy novolac polymers used in this study were purchased from D o w (Quatrex 3710) and from Shell (Epikote 164, Epon). They were used either as received or after fractionation. Details on the fractionation procedure and material characterization have been published elsewhere (8). Triphenylsulfonium hexafluoroantimonate was used as photo sensitizer in most formulations and propylene glycol methyl ether acetate, as solvent. Concentration o f resin in the formulation was varied in the 20%-35% range for the formation o f films of desired thickness (thicker films are required in thermal analysis). L i t h o g r a p h i c Processing / E v a l u a t i o n . Standard thermal processing and development were used i n the microlithographic experiments (8), unless otherwise specified. A n E B M F 10cs/120 e-beam machine operated at 40 K V , and a prototype vector scan ebeam operated at 20-30 K V were used for e-beam exposures. A n Oriel illuminator was used for deep U V exposures. The standard film thickness measurement and S E M techniques were employed for lithographic evaluation.

In Micro- and Nanopatterning Polymers; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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F T I R Study. The study was done on S i wafers o f 500 p m thickness that are positioned in the F T I R instrument (Nicolet) at a specific angle with respect to the IR beam to minimize the noise produced by reflection. T h e r m a l analysis. A DuPont 943 T M A instrument was used for the measurement o f mechanical properties o f resist films o f 2.0-0.5 p m thickness. A DuPont 910 D S C instrument was used for differential scanning calorimetry.


Results and Discussion Resist C h e m i s t r y . The basic chemistry o f epoxy novolac based chemically amplified resists has been proposed in the past by Stewart et al. (9). According to this the Br0nsted acid generated either photochemically or through electron beam exposure from the onium salt induces acid catalysed polymerization o f the epoxy functionality. This mechanism implies that the proton generated by the exposure is actually bound to the polymer. Since the lithography consequences o f this mechanism are obvious we decided to seek possible experimental evidence for the proton binding in the resist film under conditions o f lithographic interest. F T I R Results. F T I R spectroscopy has been applied to follow the chemical reactions occurring during P E B . The epoxy ring gives quite a few characteristic bands in the IR spectrum. In the first experiments the epoxy ring consumption has been followed through the bands assigned to epoxy ring vibrations (915.2 and 864.0 cm' ) and the band assigned to epoxy ring C ~ O stretching (1265.5 cm' ). In F i g 1 spectra recorded after exposure ( U V exposure was used i n these experiments for practical reasons) and P E B corresponding to different exposure times are presented. A s it can be seen all the epoxy related bands are reduced as the reaction proceeds but the degree o f reduction o f the two bands assigned to ring vibrations is distinctly higher than the corresponding one o f the C - O stretching band. Based on literature suggestions for other epoxy systems (10) the reaction has been also followed through a 3000 cm" band assigned to the epoxy ring CH2 stretchnig. The results presented i n Fig. 1 are i n good agreement with the ones obtained with the bending bands, suggesting that at the 1265.5 cm" a separate band must be superimposed to the epoxy C - O stretching. In F i g . 2 the degree o f epoxy ring consumption vs the exposure time is presented as calculated from the 3000 cm" band decrease. It must be noticed that in the process conditions used in this specific case the exposure time to obtain the lithographically useful dose is 4 sec corresponding to an epoxy ring consumption o f ~ 10%. 1

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In the same IR spectrum series it is clearly shown that the O H band increases as the reaction proceeds. In Fig. 3 this increase is plotted vs the exposure time showing a kinetic behaviour i n good correspondence with the consumption o f epoxy ring presented in F i g . l . The increase can be attributed to the attachment o f the photogenerated H to the epoxy ring confirming that the H in this specific reaction acts as a crosslinking initiator and not as a catalyst as is the case with most other chemically amplified resists. Thus, amplification in this case results from the diffusion of the intermediate o f the epoxy ring opening reaction, ie the carbocation, and not o f the H . This picture (elucidated i n the mechanism at Scheme I) is i n agreement with the slow reactant diffusion behaviour during P E B that has been reported before ^77^ : the diffusion o f the bulky carbocation, which is part o f a polymer chain, is expected to be much more difficult than the diffusion of the small H * . +

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Wavenumbers (cm" ) Figure 1. F T I R spectra o f epoxy novolac resist on S i wafer recorded after P E B . Each spectrum corresponds to different exposure time: 1 to 0s, 2 to 4s, 3 to 30s, 4 to 120s and 5 to 240s. The lithographically useful dose is close to 4s i n this case.


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Figure 2. Decrease in the epoxy CH2 band intensity during processing o f the epoxy novolac resist monitored with FTIR. Absorbance values calculated from the areas under the 3000 cm" band o f Fig. 1 spectra. 1

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CO -Q

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C/) -Q CO

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exposure time, s Figure 3. O H formation during processing o f epoxy novolac resist. Absorbance values calculated from the areas under the 3450 cm" band o f F i g . 1 spectra. 1



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Scheme I: Proposed mechanism o f crosslinking showing the H attachment to the epoxy oxygen and the carbocation transfer. Arrows indicate the movement o f the reacting species. T h e r m a l analysis studies. Thermomechanical analysis ( T M A ) has been used for the in situ determination o f the resist thermal properties after processing steps (12). In the present study, T M A showed the influence o f the sample preparation, the prebake conditions and formulation parameters ( M W distribution o f the polymer) to the resist glass transition temperature (T ) (Table 1). g

T a b l e 1. T g values for epoxy novolac polymer and resist samples. Sample Polymer Fractionated Photoresist Polymer Processing P A B 90 °C P A B 90 °C 20 m i n 4 min 40 (32) 70 (54) 70 (53) Tg(°C) " 45 (35) (

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PAB 110°C 4 min 70 (53) ( 2 )

Measured i n films with T M A (onset T M A values i n parentheses) In this case T g was 74 °C with D S C

Similar T values were obtained by differential scanning calorimetry (DSC) i n samples prepared after scraping the resist films off the wafer. The last technique has also been used in the investigation o f the resist chemical changes induced by light or temperature. It has been shown (in accordance with results presented in ref. 12) that the unexposed resist is stable up to - 1 7 0 °C, where an exothermic reaction leading to resist crosslinking starts. O n the other hand, in exposed samples, where acid has been photogenerated, an exothermic reaction starts practically at temperatures above T (at scan rates 5 - 2 0 deg/min). During the P E B step this reaction proceeds up to a certain degree depending on the amount o f acid produced, P E B temperature and time. If a limited amount o f acid is produced (ie. at a lithographically useful dose or below), the g

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available epoxy reaction sites are not fully consumed (cf. IR results) even at elevetated temperatures, and a separate thermal crosslinking reaction is observed at higher temperatures. This behaviour is reflected i n the D S C curves presented i n Fig. 4.


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temperature, C Figure 4. D S C curve o f epoxy novolac resist after prebake and exposure at a low dose (0.2sec exposure time vs. 2 sec for the lithographically useful dose in the same conditions). M i c r o l i t h o g r a p h i c Results: P E B Dependence of L i t h o g r a p h i c Features. Single pixel electron beam exposures have been introduced for the study o f P E B acid diffusion and diffusion coefficient measurements. In F i g . 5 a representative column (single pixel exposure) and a representative isolated line (three pixel exposure) o f comparable dimensions are shown. In F i g . 6 the dependence o f column diameter on square root of P E B time is presented. This diagram shows limited acid diffusion effects. The diffusion coefficient concluded from a reaction-diffusion modeling scheme (11) based on single pixel exposure information is 10" pm /sec whereas for S A L - 6 0 1 the corresponding value is 10" pm /sec. It is reasonable to attribute this difference to the different nature o f the chemical mechanism presented above. 6

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Figure 5. Representative lithographic features o f epoxy novolac resist obtained with beam lithography (40 K V ) . a) Resist column o f 78 nm diameter and aspect ratio over Exposure charge = 6 10" C b , b) Isolated line o f linewidth = 71 nm. Exposure dose 4.25 uX7cm 16

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Figure 6. Column radius dependence on P E B time for 3 different column sizes.


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Diffusion vs. Reaction C o n t r o l l e d Kinetics Contrast curves obtained from e-beam experiments at the lithographically useful doses at 90 °C and 110 °C for different P E B times show a very characteristic behavior for this epoxy chemistry based chemically amplified resist. The gel dose does not change i n this P E B temperature and time range whereas the contrast increases i n the higher temperature and time regime (Figures 7 and 8). This behavior has been further examined i n a series o f similar simple lithographic experiments with U V exposure at different P E B times but at lower P E B temperatures (Figs. 9-12). These experiments show that the gel dose decreases constantly with P E B time at temperatures below T and practically levels at P E B temperatures above T which is approximately 70 °C for this material. Above T longer P E B times and higher P E B temperatures result in contrast increase i n accordance with the e-beam experiments. These experimental results can be rationalized on the basis o f the complex reaction-diffusion behaviour encountered in chemically amplified resists (see also 75). They are controlled by the relative rates o f the chemical reaction resulting i n crosslinking and o f reactant diffusion at the different temperatures and crosslink densities. The rate o f crosslink formation is characterized by diffusion controlled kinetics at temperatures below T g and by reaction controlled kinetics above this temperature. When the system is above T g the reaction proceeds too fast to be followed by lithographic experiments: gelation takes place at times shorter than 30 sees (compare with results on epoxy monomer polymerization in ref. 14). A t temperatures below T the crosslink formation is controlled by the diffusion o f reactants and thus there is a clear influence o f the temperature and acid concentration on the initial reaction rate. A t P E B temperatures o f lithographic interest (above T ) the parameter that changes with P E B conditions is contrast, because the rate o f the reaction decreases as the crosslink density increases (cage effect). In other words the system enters again in diffusion controlled kinetics since the T g o f the material locally approaches or reaches the P E B temperature (vitrification). Therefore, the reaction proceeds to high conversion degrees at low doses (increased contrast) only under P E B conditions (temperature, time) that favour reactant diffusion. Nevertheless, increased diffusion can degrade lithographic performance and this is the reason for the fact that increased contrast is not always accompanied by better lithography in the E P R resist (8). g


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Conclusions The study o f the epoxy novolac resist chemistry confirms that the photogenerated acid is consumed during the P E B . According to the proposed mechanism the propagating species is the bulky carbonium intermediate in compliance with the relatively small diffusion coefficient measured. The microlithographic results also suggest diffusion controlled kinetics i n the lithographically useful P E B time and temperature regime. Optimized lithography is obtained under processing conditions resulting i n optimized diffusion length. Acknowledgements. The above work was supported by the European Union E S P R I T #2284 project " N A N C A R " (Nanofabrication with Chemically Amplified Resists). The


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Figure 8: Contrast curves at 110 °C P E B temperature obtained for different P E B times. E beam exposure at 40 K e V .



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Dose, mJ cm-2 Figure 12: Contrast curves o f epoxy novolac resists exposed to D U V and postexposure-baked at 90 °C for the times shown.


357 help of M r . R.Maggiora, M r . L.Scopa and Dr. M . Gentili of I E S S - C N R Rome, Italy, with single pixel e-beam experiments is greatly acknowledged.

References


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Ito,H.;Willson, C. G. Polym. Eng. Sci. 1983, 23, 1012. a) Fedynyshyn, T.; Thakeray, J.; Georger, J.; Denison, M. J.Vac.Sci.Technol.B1994, 12(6), 3888, b) J. Nakamura,H.Ban, andA.Tanaka,Polym.Mat.Sci.Eng. 1995, 72, 155. 3. Raptis, I.; Grella, L.; Argitis, P.; Gentili, M.; Glezos, N.; Petrocco G. Microelecton Engineering 1996, 30, 295-9. 4. Allen, R. D.;Conley,W.; Gelorme, J. D. SPIE Proceedings 1992, 1672, 513. 5. Lee, K. Y.; LaBianca, N.; Rishton, S.A.; Zolgharnain, S.; Gelorme, J.D.; Shaw J.;Chang,,T.H.P. J. Vac.Sci.Technol.B 1995, 13(6), 3012. 6. Despont, M.; Lorenz,H.;Fahmi, N.; Brugger, J.; Renaud, P.; Vettiger, P. Proc. of 10thIEEEInt'l, Workshop on Micro Electro Mechanical Systems (MEMS '97) 1997. 7. Hatzakis, M.; Stewart, K. J.; Shaw, J. M.; Rishton,S.A.J.Electrochem.Soc. 1992,138,1076. 8. a) Argitis, P.; Raptis, I.; Aidinis, C.J.; Glezos, N.; Baciocchi, M.; EverettJ.;Hatzakis,M.J.Vac. Sci. Technol.. B 1995, 13(6), 3030, b) Miller Tate, P. C.; Jones, R.G.; Murphy, J.; Everett, J. Microelecton. Engineering 1995, 27, 409. 9. a) Stewart, K.J.; Hatzakis, M.; Shaw, J. M.; Seeger, D. E. J. Vac.Sci.Technol..B., 7, 1734 (1991), b) Haller,I.;Stewart K.J.J.Vac.Sci.Technol.B1991, 9(6), 3370. 10. Crivello J. V.; Narayan,R.Macromolecules 1996, 29, 439. 11. Glezos, N.; Patsis, G.; Raptis, I.; Argitis, P.; Gentili M.; Grella L. J. Vac.Sci.Technol.B 1996, 14(6), 4252. 12. Tegou, E; Gogolides E.; Hatzakis, M.Microelectron.Engineering 1997, 35, 141. 13. Patsis, G. ; Raptis, I.; Glezos, N.; Argitis, P.; Hatzakis, M.; Aidinis, C. J.; Gentili,M.;Maggoira,R. Microelectron Engineering 1997,35,157. 14. a) Decker, C.; Moussa, K.J. Polym. Sci. Polym.Chem.Ed.1990, 28, 3429, b) Decker C. Progr. Polym. Sci. 1996, 21, 593. 15. Mack, C. A. in "Microelectronics Technology", E. Reichmanis, C.K. Ober, S.A. McDonald,T. Iwayanagi and T. Nishikubo eds, ACS Symp. Ser. 1983, 614, 57.


Micro- and Nanopatterning Polymers - American Chemical Society

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