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Highly sensitive resist material for deep X-ray lithography
MICROELECTiONIC ENGINEEBING
ELSEVIER
Microeleetronic Engineering 35 (1997) 105-108
HIGHLY SENSITIVE RESIST MATERIAL FOR DEEP X-RAY LITHOGRAPHY
R. Schenk a, O. Halle a, K. Mtillen b, W. Ehrfeld a, M. Schmidt a alnstitut f'tir Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18-20, D-55129 Mainz, Germany bMax-Planck-Institut Ft~r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany
The present paper describes the first chemically amplified negative-tone resist for deep X-ray lithography (DXRL). The choice of the resist material for this new resist has been oriented on the experience of the photo, electron beam and X-ray lithography (XRL) for microelectronic applications. In this work a negative tone resist containing a novolak, a crosslinker and an acid generator was developed by variing the different components. It was found that only few components, which proved to be good in thin films, were suitable for DXRL. The new resist fulfills all technological requirements and shows an increased sensitivity by a factor 15 as compared to the standard resist material, poly(methyl methacrylate). This tremendous increase in sensitivity leads to a huge cost reduction of the DXRL process. Furthermore, an excellent adhesion of this new resist to metallic substrates has been achieved which allows to fabricate free standing columns with an aspect ratio of 80. 1. I N T R O D U C T I O N Threedimensional microcomponents and microsysterns are manufactured by means of the LIGA technique from a wide variety of materials [1]. The major process steps are deep lithography, electroforming and replication processes (German acronym for LIGA: Lithographie, Galvanoformung, Abformung). Structurization of a radiation sensitive polymer is the first step in the LIGA process sequence. This can be achieved by use of lasers, electron or ion beams as well as by optical or X-ray lithography, the latter of which has been shown to give the best results. Compared to standard X-ray lithography (XRL) in VLSI fabrication where resist film thicknesses are in the order of one micrometer DXRL imposes completely different requirements which restrict the choice of resist materials. Film thicknesses up to 1 mm must be producible. Furthermore, the concentration of atoms with an atomic number Z > 10 has to be kept low. During the long development process required for thick layers, leaching and swelling of the structures have to be avoided. The microstructures need to exhibit extremely smooth side walls, strong adhesion to the metallic substrates and remain stable in the 0167-9317(97)/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0167-9317(96)00165-7
electroplating bath towards aqueous acids and bases at temperatures up to 60 °C. The standard resist material for deep X-ray lithography, poly(methyl methacrylate) (PMMA), fulfills all technical requirements mentioned above. However, PMMA has a low sensitivity and, as a result, long irradiation times are required. Due to this fact direct deep lithographical structurization without use of replication techniques is at present an unefficient and rarely applied process. Resist materials that use chemical amplification have been described to offer a higher sensitivity. They have, however, up to now not been used in DXRL. The present paper describes the first application of the principle of chemical amplification in the field of DXRL to obtain a highly sensitive resist. 2. EXPERIMENTAL Hexa(methoxymethyl)melamine, 2,4,6-triphenylpyrylium tetrafluoroborate and 2,2-bis-(3,5-dichlorobydroxyphenyl)propane are commercially available. 1,4-Bis-(acetoxymethyl)benzene [2], 1,4-bis-(fluoroacetoxymetbyl)benzene [2], triphenylsulphonium tetrafluoroborate [3], and bis-(4-tert-butylphenylsul-
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108
106
SbF6e
OCH3
U
(
hv
~,-
HSbF6
OCH3
OCH3 S +H e
H3CO
® CH2 I
_~
N..~N
H3CO
H3COvNvOCH3
N..~N
+ CH3 OH
H3COwNwOCH3
OCH3 OH
e CH2 I
_H ~ H3CO
NyN
H3COvNwOCH3
N.fN
H3COvNwOCH3
Figure 1. X-ray induced crosslinking reactions of the negative resist. Upon irradiation the acid generator supplies a proton (top reaction) which catalyses an electrophilic aromatic substitution and leads to bonding of the crosslinker to novolak (middle and bottom reaction). phonyl)diazomethane [4] were synthesized according to the literature. The DXRL process has been performed at the DCI storage ring (beamline D 45, Xc = 0.33 nm, 500 ~tm Be as prefilter in the beamline) with an X-ray scanner (Jenoptik GmbH). 500 ~tm thick Be mask blanks covered with 15 - 18 ~tm thick gold absorber structures were used as X-ray masks. Immediatly after exposure the resist was baked for 30 min at 80 °C in a laboratory drying oven. 3. THE M E C H A N I S M OF CROSSLINKING In order to obtain a resist with high sensitivity, the principle of chemical amplification [5] which has been described for photolithography by lto and Willson has been applied to deep X-ray lithography for the first time. Chemical amplification is achieved by one single photoreaction creating a cascade of chemical reactions which is independent of further
irradiation, and yields the desired functions, e. g. crosslinking reactions in the case of negative resists. In X-ray lithography (structural heights 1 - 2 ~tm) a number of three component negative tone resists with chemical amplification containing a novolak, a crosslinker and an acid generator have been described earlier [6]. Figure 1 shows a reaction sequence published in [7]. In the first step the acid generator (e. g. a triaryl sulfonium salt) supplies a proton (fig. 1, top reaction). Subsequent reaction of the proton with a latent electrophile, such as hexa(methoxymethyl)melamine, produces a carbocationic intermediate while methanol is released (fig. 1, middle reaction). The carbocationic intermediate reacts with an electron rich aromatic moiety in an electrophilic aromatic substitution reaction (fig. 1, bottom reaction) which also liberates a proton. This substitution is a thermal reaction and independent of further irradiation. So
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108 the overall process of the electrophilic aromatic substitution has a catalytic mechanism and results in chemical amplification. 4. RESULTS AND DISCUSSION For application of these three component systems in deep X-ray lithography the novolak component has been left unchanged, while the crosslinker and the acid generator have been varied. The experiments were carried out with resist systems containing 75 % m-cresol novolak, 20 % crosslinker and 5 % acid generator. The crosslinkers tested were 1,4-Bis(acetoxymethyl)benzene, 1,4-Bis-(fluoroacetoxymethyl)benzene, and hexa-(methoxymethyl)melamine. The best results were obtained with hexa(methoxymethyl)melamine. The acid generators investigated were onium salts (triphenylsulphonium tetrafluoroborate, 2,4,6-triphenylpyrylium tetrafluoroborate), bis-(4-tertbutylphenylsulfonyl)diazomethane, and 2,2-bis-(3,5dichloro-hydroxyphenyl)propane. Only the usage of 2,2-bis-(3,5-dichloro-hydroxyphenyl)propane results in reproducible and stable microstructures. Thus, it can be concluded that only two of seven compounds which have been successfully applied in thin films, were suitable for DXRL. Indeed, the choice of resist material is restricted to thick film applications.
Figure 2. Scanning electron micrograph of columns showing T-top formation. Even if the two optimal compounds are used in combination with the novolak resin, the quality of
107
the microstructures depends significantly on their relative content of the three components. Figure 2 shows an example of microstructures with a non optimized resist. The columns exhibit an extreme "T-top" formation, where only the size of the top corresponds to the size of the holes in the mask. The part of the column which is beneath the top is much smaller due to leaching. The leaching at the bottom, however, is considerably lower compared to the top.
Figure 3. Scanning force micrograph of a surface of a resist sidewall. The measured average surface roughness was beyond 20 nm. Films of the optimized resist have been produced with thicknesses up to 600 ~tm. Furthermore, the concentration of elements with a atomic number larger than 10 can be kept low, since the chlorine containing acid generator is the minor component in the resist. The resist shows no swelling and dark erosion during the development step. The resulting microstructures had smooth side walls (roughness < 20 nm) which was measured by scanning force microscopy (fig. 3) and were stable in the electroforming step which is demonstrated by the deposition of gold, nickel and nickel-iron alloy. An impressive example of the possibilities of this new resist is the production of real threedimensional microstructures as shown in figure 4. This structure has been produced by a triple irradiation with an angle of 35 ° between the beam and the resist. The substrate has been rotated at an angle of 120° between each irradiation. The detailed description of the preparation and the application of such microstructures is given elsewhere [8].
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108
108
5. CONCLUSION
Figure 4. Scanning electron micrograph of a real threedimensional microstructure, which is produced by a triple irradiation. The most remarkable feature of the new resist is the increase in sensitivity by a factor of 15 as compared to the standard DXRL resist material poly(methyl methacrylate). This tremendous increase of sensitivity leads to a huge cost reduction of the DXRL process. Furthermore, the adhesion of this new resist material on metallic substrates has been improved as compared to PMMA so that free standing columns (fig. 5) with an aspect ratio of 80 can be obtained. Similar structures with such a high aspect ratio on metallic substrates have to our knowledge not been reached with other resists and/or other lithographic techniques.
ooolo~51
:
loo ..1
144-5~
Figure 5. Scanning electron micrograph of columns with a high aspect ratio (diameter = 7 ~tm, heigths = 570 ~tm, aspect ratio = 80).
The manufacturing of microcomponents by direct deep lithographic structurization without use of replication techniques was until now an unefficient and seldomly applied process, since long irradiation times were necessary. The long irradiation time results from the low sensitivity of the standard DXRL resist material poly(methyl methacrylate). The sensitivity of the new negative resist described here is 15 times higher compared to PMMA. This reduction of the irradiation time leads to a tremendous increase of productivity. The new resist allows a fast and cost effective fabrication of microstructures and shows the potential of direct manufacturing. Furthermore, an excellent adhesion of this new resist to metallic substrates has been achieved resulting in a higher structural variability. 6. REFERENCES
1. W. Ehrfeld, H. Lehr, Radiat. Phys. Chem., 3 (1995) 349. 2. J. T. Fahey, K. Shimuzu, J. M. J. Frechet, N. Clecak, C. G. Willson, J. Polym. Sci., Part A, 31 (1993) 1. 3. J.V. Crivello, Adv. Polym. Sci., 62 (1984) 1. 4. G. Pawlowski, R. Dammel, H. R6schert, W. SpieB, W. Meier, German Patent, DE 40 06 190 A1. 5. H. Ito, C. G. Willson, Polym. Eng. Sci., 23 (1983) 1002. 6. W. M. Moreau, "Semiconductor Lithography: Principles, Practices and Materials", Plenum Press, New York 1988. J. Lingnau, R. Dammel, J. Theis, Solid State Technol., (Sept. 1989) 105; (Oct. 1989) 107. 7. W. E. Feely, J. C. lmhof, C. M. Stein, Polym. Eng. Sci., 26 (1986) 1101; D. R. McKean, U. P. Schaedeli, P. H. Kasai, S. A. MacDonald, J. Polym. Sci., Part A, 29 (1991) 309. 8. G. Feiertag, W. Ehrfeld, H. Freimuth, G. Kiriakidis, H. Lehr, T. Pederson, M. Schmidt, C. Soukoulis, R. Weiel, NATO ASI Series E, 315 (1996) 63.
ELSEVIER
Microeleetronic Engineering 35 (1997) 105-108
HIGHLY SENSITIVE RESIST MATERIAL FOR DEEP X-RAY LITHOGRAPHY
R. Schenk a, O. Halle a, K. Mtillen b, W. Ehrfeld a, M. Schmidt a alnstitut f'tir Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18-20, D-55129 Mainz, Germany bMax-Planck-Institut Ft~r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany
The present paper describes the first chemically amplified negative-tone resist for deep X-ray lithography (DXRL). The choice of the resist material for this new resist has been oriented on the experience of the photo, electron beam and X-ray lithography (XRL) for microelectronic applications. In this work a negative tone resist containing a novolak, a crosslinker and an acid generator was developed by variing the different components. It was found that only few components, which proved to be good in thin films, were suitable for DXRL. The new resist fulfills all technological requirements and shows an increased sensitivity by a factor 15 as compared to the standard resist material, poly(methyl methacrylate). This tremendous increase in sensitivity leads to a huge cost reduction of the DXRL process. Furthermore, an excellent adhesion of this new resist to metallic substrates has been achieved which allows to fabricate free standing columns with an aspect ratio of 80. 1. I N T R O D U C T I O N Threedimensional microcomponents and microsysterns are manufactured by means of the LIGA technique from a wide variety of materials [1]. The major process steps are deep lithography, electroforming and replication processes (German acronym for LIGA: Lithographie, Galvanoformung, Abformung). Structurization of a radiation sensitive polymer is the first step in the LIGA process sequence. This can be achieved by use of lasers, electron or ion beams as well as by optical or X-ray lithography, the latter of which has been shown to give the best results. Compared to standard X-ray lithography (XRL) in VLSI fabrication where resist film thicknesses are in the order of one micrometer DXRL imposes completely different requirements which restrict the choice of resist materials. Film thicknesses up to 1 mm must be producible. Furthermore, the concentration of atoms with an atomic number Z > 10 has to be kept low. During the long development process required for thick layers, leaching and swelling of the structures have to be avoided. The microstructures need to exhibit extremely smooth side walls, strong adhesion to the metallic substrates and remain stable in the 0167-9317(97)/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0167-9317(96)00165-7
electroplating bath towards aqueous acids and bases at temperatures up to 60 °C. The standard resist material for deep X-ray lithography, poly(methyl methacrylate) (PMMA), fulfills all technical requirements mentioned above. However, PMMA has a low sensitivity and, as a result, long irradiation times are required. Due to this fact direct deep lithographical structurization without use of replication techniques is at present an unefficient and rarely applied process. Resist materials that use chemical amplification have been described to offer a higher sensitivity. They have, however, up to now not been used in DXRL. The present paper describes the first application of the principle of chemical amplification in the field of DXRL to obtain a highly sensitive resist. 2. EXPERIMENTAL Hexa(methoxymethyl)melamine, 2,4,6-triphenylpyrylium tetrafluoroborate and 2,2-bis-(3,5-dichlorobydroxyphenyl)propane are commercially available. 1,4-Bis-(acetoxymethyl)benzene [2], 1,4-bis-(fluoroacetoxymetbyl)benzene [2], triphenylsulphonium tetrafluoroborate [3], and bis-(4-tert-butylphenylsul-
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108
106
SbF6e
OCH3
U
(
hv
~,-
HSbF6
OCH3
OCH3 S +H e
H3CO
® CH2 I
_~
N..~N
H3CO
H3COvNvOCH3
N..~N
+ CH3 OH
H3COwNwOCH3
OCH3 OH
e CH2 I
_H ~ H3CO
NyN
H3COvNwOCH3
N.fN
H3COvNwOCH3
Figure 1. X-ray induced crosslinking reactions of the negative resist. Upon irradiation the acid generator supplies a proton (top reaction) which catalyses an electrophilic aromatic substitution and leads to bonding of the crosslinker to novolak (middle and bottom reaction). phonyl)diazomethane [4] were synthesized according to the literature. The DXRL process has been performed at the DCI storage ring (beamline D 45, Xc = 0.33 nm, 500 ~tm Be as prefilter in the beamline) with an X-ray scanner (Jenoptik GmbH). 500 ~tm thick Be mask blanks covered with 15 - 18 ~tm thick gold absorber structures were used as X-ray masks. Immediatly after exposure the resist was baked for 30 min at 80 °C in a laboratory drying oven. 3. THE M E C H A N I S M OF CROSSLINKING In order to obtain a resist with high sensitivity, the principle of chemical amplification [5] which has been described for photolithography by lto and Willson has been applied to deep X-ray lithography for the first time. Chemical amplification is achieved by one single photoreaction creating a cascade of chemical reactions which is independent of further
irradiation, and yields the desired functions, e. g. crosslinking reactions in the case of negative resists. In X-ray lithography (structural heights 1 - 2 ~tm) a number of three component negative tone resists with chemical amplification containing a novolak, a crosslinker and an acid generator have been described earlier [6]. Figure 1 shows a reaction sequence published in [7]. In the first step the acid generator (e. g. a triaryl sulfonium salt) supplies a proton (fig. 1, top reaction). Subsequent reaction of the proton with a latent electrophile, such as hexa(methoxymethyl)melamine, produces a carbocationic intermediate while methanol is released (fig. 1, middle reaction). The carbocationic intermediate reacts with an electron rich aromatic moiety in an electrophilic aromatic substitution reaction (fig. 1, bottom reaction) which also liberates a proton. This substitution is a thermal reaction and independent of further irradiation. So
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108 the overall process of the electrophilic aromatic substitution has a catalytic mechanism and results in chemical amplification. 4. RESULTS AND DISCUSSION For application of these three component systems in deep X-ray lithography the novolak component has been left unchanged, while the crosslinker and the acid generator have been varied. The experiments were carried out with resist systems containing 75 % m-cresol novolak, 20 % crosslinker and 5 % acid generator. The crosslinkers tested were 1,4-Bis(acetoxymethyl)benzene, 1,4-Bis-(fluoroacetoxymethyl)benzene, and hexa-(methoxymethyl)melamine. The best results were obtained with hexa(methoxymethyl)melamine. The acid generators investigated were onium salts (triphenylsulphonium tetrafluoroborate, 2,4,6-triphenylpyrylium tetrafluoroborate), bis-(4-tertbutylphenylsulfonyl)diazomethane, and 2,2-bis-(3,5dichloro-hydroxyphenyl)propane. Only the usage of 2,2-bis-(3,5-dichloro-hydroxyphenyl)propane results in reproducible and stable microstructures. Thus, it can be concluded that only two of seven compounds which have been successfully applied in thin films, were suitable for DXRL. Indeed, the choice of resist material is restricted to thick film applications.
Figure 2. Scanning electron micrograph of columns showing T-top formation. Even if the two optimal compounds are used in combination with the novolak resin, the quality of
107
the microstructures depends significantly on their relative content of the three components. Figure 2 shows an example of microstructures with a non optimized resist. The columns exhibit an extreme "T-top" formation, where only the size of the top corresponds to the size of the holes in the mask. The part of the column which is beneath the top is much smaller due to leaching. The leaching at the bottom, however, is considerably lower compared to the top.
Figure 3. Scanning force micrograph of a surface of a resist sidewall. The measured average surface roughness was beyond 20 nm. Films of the optimized resist have been produced with thicknesses up to 600 ~tm. Furthermore, the concentration of elements with a atomic number larger than 10 can be kept low, since the chlorine containing acid generator is the minor component in the resist. The resist shows no swelling and dark erosion during the development step. The resulting microstructures had smooth side walls (roughness < 20 nm) which was measured by scanning force microscopy (fig. 3) and were stable in the electroforming step which is demonstrated by the deposition of gold, nickel and nickel-iron alloy. An impressive example of the possibilities of this new resist is the production of real threedimensional microstructures as shown in figure 4. This structure has been produced by a triple irradiation with an angle of 35 ° between the beam and the resist. The substrate has been rotated at an angle of 120° between each irradiation. The detailed description of the preparation and the application of such microstructures is given elsewhere [8].
R. Schenk et al. / Microelectronic Engineering 35 (1997) 105-108
108
5. CONCLUSION
Figure 4. Scanning electron micrograph of a real threedimensional microstructure, which is produced by a triple irradiation. The most remarkable feature of the new resist is the increase in sensitivity by a factor of 15 as compared to the standard DXRL resist material poly(methyl methacrylate). This tremendous increase of sensitivity leads to a huge cost reduction of the DXRL process. Furthermore, the adhesion of this new resist material on metallic substrates has been improved as compared to PMMA so that free standing columns (fig. 5) with an aspect ratio of 80 can be obtained. Similar structures with such a high aspect ratio on metallic substrates have to our knowledge not been reached with other resists and/or other lithographic techniques.
ooolo~51
:
loo ..1
144-5~
Figure 5. Scanning electron micrograph of columns with a high aspect ratio (diameter = 7 ~tm, heigths = 570 ~tm, aspect ratio = 80).
The manufacturing of microcomponents by direct deep lithographic structurization without use of replication techniques was until now an unefficient and seldomly applied process, since long irradiation times were necessary. The long irradiation time results from the low sensitivity of the standard DXRL resist material poly(methyl methacrylate). The sensitivity of the new negative resist described here is 15 times higher compared to PMMA. This reduction of the irradiation time leads to a tremendous increase of productivity. The new resist allows a fast and cost effective fabrication of microstructures and shows the potential of direct manufacturing. Furthermore, an excellent adhesion of this new resist to metallic substrates has been achieved resulting in a higher structural variability. 6. REFERENCES
1. W. Ehrfeld, H. Lehr, Radiat. Phys. Chem., 3 (1995) 349. 2. J. T. Fahey, K. Shimuzu, J. M. J. Frechet, N. Clecak, C. G. Willson, J. Polym. Sci., Part A, 31 (1993) 1. 3. J.V. Crivello, Adv. Polym. Sci., 62 (1984) 1. 4. G. Pawlowski, R. Dammel, H. R6schert, W. SpieB, W. Meier, German Patent, DE 40 06 190 A1. 5. H. Ito, C. G. Willson, Polym. Eng. Sci., 23 (1983) 1002. 6. W. M. Moreau, "Semiconductor Lithography: Principles, Practices and Materials", Plenum Press, New York 1988. J. Lingnau, R. Dammel, J. Theis, Solid State Technol., (Sept. 1989) 105; (Oct. 1989) 107. 7. W. E. Feely, J. C. lmhof, C. M. Stein, Polym. Eng. Sci., 26 (1986) 1101; D. R. McKean, U. P. Schaedeli, P. H. Kasai, S. A. MacDonald, J. Polym. Sci., Part A, 29 (1991) 309. 8. G. Feiertag, W. Ehrfeld, H. Freimuth, G. Kiriakidis, H. Lehr, T. Pederson, M. Schmidt, C. Soukoulis, R. Weiel, NATO ASI Series E, 315 (1996) 63.