Chapter 5
Acid Diffusion in Chemically Amplified Resists The Effect of Prebaking and Post-Exposure Baking Temperature
Downloaded by UNIV OF QUEENSLAND on March 29, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch005
Jiro Nakamura, Hiroshi Ban, and Akinobu Tanaka NTT LSI Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-01, Japan
Evaluations of acid diffusion in chemically amplified resists using three different methods have revealed that higher prebaking and lower post-exposure baking (PEB) temperatures are effective in reducing the diffusion distance. Investigations into the influence of diffusion on resist characteristics show that there is a reciprocal relationship between sensitivity andresolutiondue to the effects of acid diffusion. This relationship is able to be formulated using the terms of activation energies for overall catalytic reactions and for acid diffusion.
Reactions in the formation of latent images in chemically amplifiedresistsfi)take place in two consecutive steps: acid generation during exposure, and acid-catalyzed reactions during post-exposure baking (PEB). This mechanism makes the lithographic performance more sensitive to the process conditions than that of conventional resist systems. In particular,resistcharacteristics are greatly influenced by the acid diffusion during PEB(2-6). Acid diffusion is essential to induce long chains of catalytic reactions. Appropriate diffusion also suppresses edge roughness caused by the nonuniform exposure that results from the effects of multi-interference in photolithography and from electron scattering in e-beam and X-ray lithographies. Excess acid diffusion, on the other hand, degrades latent image quality and thus, the replicated pattern profile. When developing high-performance resist materials and optimizing process conditions, it is therefore important to know how far catalytic acids diffuse inresistfilms, and to understand therelationshipbetween acid diffusion and resist characteristics. We have developed three ways to evaluate acid diffusion inresistfilmsduring PEB: the mask-contact method, the electrochemical method, and the ion conductivity method. This paper describes these method and presents the associatedresults.In addition, it discusses the influence of acid diffusion on lithographic performance based on experimental and calculated results. 0097-6156/95/0614-0069$12.00/0 © 1995 American Chemical Society
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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Evaluation of Acid Diffusion Experiment. a) Mask-Contact Replication Method. A mask contact replication method was used to determine acid diffusion in a positiveresist.The EXP (Ebeam/X-ray Positive resist) (7) was spin-coated onto a 4-inch Si wafer, followed by sprinkling small pieces (about 100-1000 μπι ) of Ta, as an X-ray absorber mask, onto the wafer before drying. X-ray exposure, PEB, and development were carried out while keeping the absorber on theresist.After development, the undercut depth of the resists for various exposure dosesfromthe absorber pattern edge was measured with scanning electron microscopy (SEM). Downloaded by UNIV OF QUEENSLAND on March 29, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch005
2
b) Electrochemical Method. An electrochemical method was used for the negativeresist.An interdigitated array (IDA) electrode was fabricated using Pton a S1O2 substrate by photolithography. The width of each electrode and of the gaps between the electrodes was 2 μπι. Chemically amplified negativeresistSAL 601 (Shipley. Co.) was spincoated onto the IDA electrode and prebaked on a hot plate at 100°C for 120 s. All areas of the resist film on the IDA electrode were irradiated with an e-beam and then post-exposure baked in an oven at 100°C for 30 min while direct current of 10 V was applied. After theresistfilm on the electrode was cut orthogonal to the electrodes to obtain a cross section, it was developed. c) Ion Conductivity Method. The diffusion coefficient of protons in a novolac resin was evaluated by measuring ion conductivity. Novolac resin with 5 wt% p-toluenesulfonic acid was coated on an IC sensor and baked in an oven for 30 min at various temperatures and the dielectric loss factor of films was measured with an Eumetric System II Microdielectrometer (Micromet Instruments). The dielectric loss factor varies with the ion conductivity, the dipoles of polar groups inresists,and the electrode polarization. The effect of the electrode polarization can be corrected with appropriate software (8) in the microdielectrometer. The dielectric loss factor ε" is given by
ωε
ο
1+
2
(ωτ ) ά
where ais the ion conductivity, e is the "unrelaxed" permittivity, e is the "relaxed" permittivity, to is the permittivity in a vacuum, i»is thefrequencyof alternating current applied, and τ0
AE=0kJ/mol
1.0
—-5 ^—-10
Q
0.5
Ν
""^30
ι
1 5 0 ^ ^ ^ Illrrr^—• 60
70
50
too
80 90 100 110 120 130 PEB Temperature (°C)
Fig. 8. Dependence of diffusion length on PEB temperature. ΔΕ is the difference in activation energies between overall acid-catalyzed reactions and diffusion of acids.
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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5. NAKAMURA ET AL.
Acid Diffusion in Chemically Amplified Resists 81
The slope obtained from Eq. (14) is shown by the dotted line as a comparison. Judging from the fairly good correlation of calculated results with experimental ones, the proposed model seems reasonable in explaining the role of acid diffusion in catalytic reactions. Next, the relationship between resolution and sensitivity is discussed when PEB temperature is changed. The relationship between the two in Eq. (14) is applicable only when the reaction probability is constant for various temperatures. Constant reaction probability means that the activation energy for the overall acidcatalyzed reaction is equal to that for the acid diffusion, or in other words, acid diffusion controls the rate of acid-catalyzed reactions. To evaluate thisrelationshipmore generally, the term for the activation energy is introduced into equations forreactionrate and diffusion length:
Û^lDoe-lért
9
where Ear and E d are the activation energies for overall acid-catalyzed reaction and diffusion, ko and D are the pre-exponential factors, and R and Τ are the gas constant and the absolute temperature. In the solutions, the activation energies for the diffusion are much smaller than for the overall reactions. On the other hand, the activation energies for the diffusion in films are usually much larger than those in solutions. By eliminating the term for PEB time from Eqs. (15) and (16), the relationship between diffusion length and concentration of acids becomes a
0
K
°
^
A
(17)
In this case, the PEB time is changed to induce a fixed extent of acid-catalyzed reactions, depending on the PEB temperature applied. For a given resist material, the ratio of decomposed dissolution inhibitors needed for fixed development is constant. Equation (17) can therefore berearrangedinto
w
w
L=Κe
αr
u
2ΚΓ
(18) where Κ is constant for a given material. Here, C[ is the decomposition ratio of dissolution inhibitors needed for development. The diffusion length needed for fixed amounts of acids and catalytic reactions depends on PEB temperature, as shown in Fig. 8. This is for various values of difference ΔΕ between the activation energies of the overall acid-catalyzedreactionand D
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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MICROELECTRONICS TECHNOLOGY
acid diffusion. The diffusion length values plotted here are normalized by the value given at PEB of 55°C. This figure, therefore, shows what diffusion length is needed to induce a reaction of a given extent in the presence of acids of fixed concentration at various PEB temperatures. The diffusion length at higher temperatures for shorter PEB times is smaller than that at lower temperatures for longer periods when compared under the same catalytic chain length, while the diffusion length at higher temperatures is naturally larger that at lower temperatures when compared over the same PEBtime.When the difference between both energies is 150 kJ/mol, diffusion length at 65°C is only about one-half that at 55°C. In this case, a higher PEB temperature will be preferable to enhance the resolution. Therelationshipbetween acid concentration and diffusion length for various PEB temperatures is shown in Fig. 9. The graphs show diffusion length normalized by that for PEB at 55°C and for a normalized acid concentration of 10. When both activation energies are the same, the diffusion length is independent of PEB temperature and is inversely proportional to the square root of acid concentration. When activation energy for overallreactionis 30 kJ/mol larger than that for diffusion, required acid concentration increases at a lower temperature even at the same diffusion length.
loo.
i
ι
ι
ι—ι—ι—ι
80-
j|
I
ι
3
2.5
2
1.5
70-
H
I
60Acid Concentration (a. u.)
(:jÉ^Kgg4^3Ô:^bQ
Acid Concentration (a. u.)
Fig. 9. Relationship between acid concentration and diffusion length needed to induce a given amount of catalytic reactions.
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
5. NAKAMURA ET AL.
Acid Diffusion in Chemically Amplified Resists
83
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Conclusion Acid diffusion in aresistfilm was evaluated using three different techniques: the mask contact, the electrochemical, and ion conductivity methods. Mask contact replication is useful for positive resists and the electrochemical method for negativeresists.Ion conductivity method can be used to determine acid diffusion in both positive and negative resists and is a simpler and more direct way. It was found that higher prebaking temperatures and/or lower post-exposure baking temperatures are effective inreducingthe acid diffusion distance that greatly influencesreplicatedpattern width. An evaluation of the influence of acid diffusion on lithographic performance revealed that there is an optimum diffusion length to produce the maximum sensitivity without deterioration of the resolution and that there is a reciprocal relationship between sensitivity and resolution that can be attributed to acid diffusion. By formulating a relationship between the activation energies of diffusion and overall acid-catalyzed reactions and the diffusion length needed to decompose a given degree of acid-catalyzed reactions, it was found that when these two activation energies are the same, the diffusion length is inversely proportional to the square root of the amount of generated acid. When the activation energy for the overall acid-catalyzed reaction is larger than that for the acid diffusion, a shorter diffusion length is required for higher PEB temperatures. Higher resolution should therefore be obtained by increasing the PEB temperature forfixedresistsensitivity. Acknowledgments The authors wish to thank Tetsushi Sakai, Yutaka Sakakibara and Tadahito Matsuda for their advice and encouragement. Literature Cited 1) Ito, H.; Willson, C. G. Polym. Eng. Sci. 1983, 23, 1012. 2) McKean, D. R.; Schaedeli, U.; MacDonald, S. A. In Polymers in Microlithography; Reichmanis, E.; MacDonald, S. Α.; Iwayanagi, T., Eds.; ACS Symp. Ser. 412; ACS: Washington, D. C. 1989, 27-38. 3) Schlegel, L.; Ueno, T.; Hayashi, N.; Iwayanagi, T. J. Vac. Sci. & Technol. 1991, B9, 278. 4) Fedynyshyn, T.; Cronin, M.; Szmanda, C. J. Vac. Sci. & Technol. 1991, B9, 3380. 5) Nakamura, J.; Ban, H.; Deguchi, K.; Tanaka, A. Jpn. J. Appl. Phys. 1991, 30, 2619. 6) Yoshimura, T.; Nakayama, Y.; Okazaki. S, J. Vac. Sci. & Technol. 1991, B10, 2615. 7) Ban, H.; Nakamura, J.; Deguchi, K.; Tanaka, A. J. Vac. Sci. & Technol. 1991, B9, 3387. 8) Day, D.; Lewis, J.; Lee, H.; Senturia, S. J. Adhesion 1985, 18 , 73. 9) Everhart, T.; Hoff, P. J. Appl. Phys. 1971, 42 , 5837. 10) Bard, Α.; Faulkner, L. ElectrochemicalMethods;John Wiley & Sons : NY, 1980. 11) Yoshino, H.; Matsumoto, H. Jpn. J. Appl. Phys. 1992, 31, 4283. 12) Nakamura, J.; Ban, H.; Tanaka, A. Jpn. J. Appl. Phys. 1992, 31, 4294. 13) Tsuboi, S.: Genzaikagaku 1992, No 3. 12. RECEIVED July 17,
1995
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
