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Boron ion beam exposure characteristics of metacrylate-type layers
869
Nuclear Instruments and Methods in Physics Research B7/8 (1985) 869-871 North-Holland, Amsterdam
BORON ION BEAM EXPOSURE C~~~~S~~ Maria
KISZA
I) and Ewa OLESZKIEWICZ
OF METACRY~~-APE
LAYERS
2,
It Institute of Electron Technologv, Technrca! Unrversi@ Wroclaw, Poland ” institute of Ph&s,
Technical University
Wroclaw, Poland
A modification of PMMA with di-butyl maleate was treated with B+ ions with energies 30 and 50 keV. In comparison with PMMA it has a better contrast - up to a factor of 4, is more resistant to crushing with ions and seems to have a better screening ability. Exposure characteristics and layer thickness plots vs the B+ dose are presented
Application of ion beams to lithography came about through the need for better methods of reproducing submicron patterns. Using electrons and ions instead of hght as information carriers results in the possibility of obtaining submicrometer resolution at a very high depth of focus - more than 100 pm [l]. Electron beam exposure systems enabled numerous achievements, but the proximity effect caused by scattering of electrons in the “ registration layer” limits its practical applicability [2,3]. The scattering of secondary electrons caused by ion bombardment is smaller 141and can be neglected in the submicron range, while the organic resist sensitivity to ions is often higher than to electrons [5]. Though the idea of using ions resembles electron or X-ray lithography in numerous respects, some specific processes such as implantation and amorphisation of layers or substrates only occur during ion bombardment. In the first place, the particles cause the conversion of the resist, but they may also reach the substrate and modify it. A change of the resist layer parameters bombarded with ions may alter the ion range which is a function of material constants. The layer thickness itself may also change. In order to prevent the ions from implanting the substrate, but to copy the patterns correctly, resists of high contrast and stable parameters are needed. We proposed PMMA modified with di-butyl maleate for this purpose. The poIymer has not only better plasticity, but makes the layer contrast higher and stab&es its inner parameters. The organic resist layers were bombarded with ions and the changes in the layers, especially their protective ability was tested. A pure PMMA, M, = 230000, M,/M, = 2 was used as a standard resist. It was compared with the modified resist (PMMAm) of the same M, and M,/M,,. Impiantation parameters were chosen according to the figures published by I. Adesida [6] and to the LSS theory. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Adesida’s experiments involved the exposure of PMMA films to various doses of boron ions at several incident energies followed by development in a solution of 1: 1 methyl isobutyl ketone and isopropyl alcohol. ft was assumed, that the saturated developed depth (for light ions) can be taken as the mean path length of the ions in PMMA. Monte Carlo methods were then used to obtain the best fit to experimental data and with this fit calculate the projected range of boron ions R,(B+) in PMMA. In agreement with this work R, of boron ions implanted with the energy 30 keV into PMMA is about 1180 A; at the energy 50 keV it increases to 2900 A. On the basis of the second work, assuming that the effective atomic number Z = 367 [7], we can estimate R, as 2670 A and e as 340 A.
2. Experimental Resist layers of both types were spin-coated on Si substrates. Patterns were produced with B” ions with energies 30 and 50 keV. The resists were developed in methyl-ethyl ketone + isopropyl alcohol 1: 2 vol. Layer thickness diminishing as a result of ion beam bombardment was observed for both materials. The relative resist thickness depends on the dose and on the primary layer thickness. For PMMA a “crushing” of the layer is apparent: Ad/d, increases with d, and reaches 15% at the dose 6 x 1014 cm-*, 30 kV (fig. 1). The same measurements of PMMAm show a different character of the process. The highest Ad/d, is 11%. Diminishing role of the primary layer thickness is also visible. Modification of the refractive index n accompanies implantation, Wada et al. (41 published plots of refractive index vs ions dose. They indicated, that the increase of n from 1.48 to 1.8 is connected with creating an amorphous structure and carbonisation of the layer. XI. PINE LINE STR./D~SITIDN/ADH~ION
870
M. Ki~za, E. Oleszkieuxz / Churacteristrcs of meracryiate - rype layers
dose Pig. f. Relative undeveloped resist thickness vs B’ dase for two resists: PMMA and PMMA modified with di-butyl maleate (PMMAm). Energy: 30 keV. Three layer thicknesses: lSO0 A, 2500 A, 3500 A.
Fig, 3. Exposure characteristics of B + + PMlHA and PMMAm for different layer thicknesses: IS00 A, 2500 A, 3500 A. Energy: 30 keV.
3. Exposure &ivaete&ties Such a process is irreversible. The modified resist changes its refractive index (fig. 2), but in another way. The primary value is I.63 and this increases during the implantation to 1.83, but it diminishes after being put into the developer. The absorption rate k of the layer does not change. To estimate the role of the developer, some tests on the PMMAm layers without implantation were made. it was found, that the refractive index slightly decreases (l-2%), but the layer thickness increases by 5%.
Pig. 2. Refractive index n and residual resist thickness change during the implantakion and developing process.
Exposure characteristics of PMMAm compared with pure PMMA are presented in fig. 3. Both resists have the same molecular parameters: lU,, it&,. 3+ ions with energy 30 keV were used. For this energy the projected range R, is not greater than the layer thickness. The better contrast of the modified layer is visible. For higher energies the projected range is greater than the polymer layer thickness. Bombarding ions B f enter the Si substrate. They create an active dope after the annealing. Fig. 4 shows the surface substrate resistivity for two
Fig. 4. Substrate surface resistivity vs B+ dose for two different ion energies. Layer thickness 2500 A.
M. Kisza, E. Olesrkiewicz
/ Characteristics
of metacrylate - type layets
871
Fig. 5. Profile of an etched Si substrate. The silicon wafer had been treated with R + ions through polymer masks of both types. Then the polymer layers were removed and the substrate was etched.
different energies. The modified resist layer thickness is 2500 A. This layer is thick enough to screen the substrate when the energy of ions is 30 keV, but for the energy 50 keV a thicker resist is needed. This is in a good agreement with figures published [6] and obtained from the LSS theory for PMMA mentioned above. This fact indicates, that the energy loss of ions passing through the layer does not change when compared with the standard polymer. But while the absorption rate increases to 0.32 (A = 550 nm) a difference in ion etching rate of the substrate is observed (fig. 5). The region having been under the modified mask has a lower average etching rate. This seems to be caused by a change in the ions range in the substrate. Energy loss in the modified layer is higher than in the standard one. The ions reaching the polymer-substrate interface have lower energy. In consequence the etching rate of the substrate decreases by 2.
4. Conclusions The proposed modification of the popular PMMA makes it more useful for ion lithography. The contrast increases up to 4, and the screening ability (observed as twice as low etching rate of the Si substrate) and elasticity of the resist offer advantages for ion lithography. References [l] [2] [3] [4]
G. StengI et al., Solid State Technol. August (1982) 104. T.H.P. Chang, J. Vat. Sci. Technol. 12 (1975) 1271. D.C. Joy, Microelectronic Engineering 1 (1983) 103. Y. Wada, M. Magitaka, K. Mochiji and H. Obayashi, J. Electrochem. Sot. 5 (1983) 1127. [S] I. Adesida, L. Karapiperis, C.A. Lee and E.D. Wolf, Proc. Microcircuit Engineering (Lausanne, 1981) (1981). [6] I. Adesida, Nucl. Instr. and Meth. 209/210 (1983) 79. [7] M. Kissa, Doctor Thesis (unpublished).
XI. FINE LINE STR./DEPOSITION/ADHESION
Nuclear Instruments and Methods in Physics Research B7/8 (1985) 869-871 North-Holland, Amsterdam
BORON ION BEAM EXPOSURE C~~~~S~~ Maria
KISZA
I) and Ewa OLESZKIEWICZ
OF METACRY~~-APE
LAYERS
2,
It Institute of Electron Technologv, Technrca! Unrversi@ Wroclaw, Poland ” institute of Ph&s,
Technical University
Wroclaw, Poland
A modification of PMMA with di-butyl maleate was treated with B+ ions with energies 30 and 50 keV. In comparison with PMMA it has a better contrast - up to a factor of 4, is more resistant to crushing with ions and seems to have a better screening ability. Exposure characteristics and layer thickness plots vs the B+ dose are presented
Application of ion beams to lithography came about through the need for better methods of reproducing submicron patterns. Using electrons and ions instead of hght as information carriers results in the possibility of obtaining submicrometer resolution at a very high depth of focus - more than 100 pm [l]. Electron beam exposure systems enabled numerous achievements, but the proximity effect caused by scattering of electrons in the “ registration layer” limits its practical applicability [2,3]. The scattering of secondary electrons caused by ion bombardment is smaller 141and can be neglected in the submicron range, while the organic resist sensitivity to ions is often higher than to electrons [5]. Though the idea of using ions resembles electron or X-ray lithography in numerous respects, some specific processes such as implantation and amorphisation of layers or substrates only occur during ion bombardment. In the first place, the particles cause the conversion of the resist, but they may also reach the substrate and modify it. A change of the resist layer parameters bombarded with ions may alter the ion range which is a function of material constants. The layer thickness itself may also change. In order to prevent the ions from implanting the substrate, but to copy the patterns correctly, resists of high contrast and stable parameters are needed. We proposed PMMA modified with di-butyl maleate for this purpose. The poIymer has not only better plasticity, but makes the layer contrast higher and stab&es its inner parameters. The organic resist layers were bombarded with ions and the changes in the layers, especially their protective ability was tested. A pure PMMA, M, = 230000, M,/M, = 2 was used as a standard resist. It was compared with the modified resist (PMMAm) of the same M, and M,/M,,. Impiantation parameters were chosen according to the figures published by I. Adesida [6] and to the LSS theory. 0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Adesida’s experiments involved the exposure of PMMA films to various doses of boron ions at several incident energies followed by development in a solution of 1: 1 methyl isobutyl ketone and isopropyl alcohol. ft was assumed, that the saturated developed depth (for light ions) can be taken as the mean path length of the ions in PMMA. Monte Carlo methods were then used to obtain the best fit to experimental data and with this fit calculate the projected range of boron ions R,(B+) in PMMA. In agreement with this work R, of boron ions implanted with the energy 30 keV into PMMA is about 1180 A; at the energy 50 keV it increases to 2900 A. On the basis of the second work, assuming that the effective atomic number Z = 367 [7], we can estimate R, as 2670 A and e as 340 A.
2. Experimental Resist layers of both types were spin-coated on Si substrates. Patterns were produced with B” ions with energies 30 and 50 keV. The resists were developed in methyl-ethyl ketone + isopropyl alcohol 1: 2 vol. Layer thickness diminishing as a result of ion beam bombardment was observed for both materials. The relative resist thickness depends on the dose and on the primary layer thickness. For PMMA a “crushing” of the layer is apparent: Ad/d, increases with d, and reaches 15% at the dose 6 x 1014 cm-*, 30 kV (fig. 1). The same measurements of PMMAm show a different character of the process. The highest Ad/d, is 11%. Diminishing role of the primary layer thickness is also visible. Modification of the refractive index n accompanies implantation, Wada et al. (41 published plots of refractive index vs ions dose. They indicated, that the increase of n from 1.48 to 1.8 is connected with creating an amorphous structure and carbonisation of the layer. XI. PINE LINE STR./D~SITIDN/ADH~ION
870
M. Ki~za, E. Oleszkieuxz / Churacteristrcs of meracryiate - rype layers
dose Pig. f. Relative undeveloped resist thickness vs B’ dase for two resists: PMMA and PMMA modified with di-butyl maleate (PMMAm). Energy: 30 keV. Three layer thicknesses: lSO0 A, 2500 A, 3500 A.
Fig, 3. Exposure characteristics of B + + PMlHA and PMMAm for different layer thicknesses: IS00 A, 2500 A, 3500 A. Energy: 30 keV.
3. Exposure &ivaete&ties Such a process is irreversible. The modified resist changes its refractive index (fig. 2), but in another way. The primary value is I.63 and this increases during the implantation to 1.83, but it diminishes after being put into the developer. The absorption rate k of the layer does not change. To estimate the role of the developer, some tests on the PMMAm layers without implantation were made. it was found, that the refractive index slightly decreases (l-2%), but the layer thickness increases by 5%.
Pig. 2. Refractive index n and residual resist thickness change during the implantakion and developing process.
Exposure characteristics of PMMAm compared with pure PMMA are presented in fig. 3. Both resists have the same molecular parameters: lU,, it&,. 3+ ions with energy 30 keV were used. For this energy the projected range R, is not greater than the layer thickness. The better contrast of the modified layer is visible. For higher energies the projected range is greater than the polymer layer thickness. Bombarding ions B f enter the Si substrate. They create an active dope after the annealing. Fig. 4 shows the surface substrate resistivity for two
Fig. 4. Substrate surface resistivity vs B+ dose for two different ion energies. Layer thickness 2500 A.
M. Kisza, E. Olesrkiewicz
/ Characteristics
of metacrylate - type layets
871
Fig. 5. Profile of an etched Si substrate. The silicon wafer had been treated with R + ions through polymer masks of both types. Then the polymer layers were removed and the substrate was etched.
different energies. The modified resist layer thickness is 2500 A. This layer is thick enough to screen the substrate when the energy of ions is 30 keV, but for the energy 50 keV a thicker resist is needed. This is in a good agreement with figures published [6] and obtained from the LSS theory for PMMA mentioned above. This fact indicates, that the energy loss of ions passing through the layer does not change when compared with the standard polymer. But while the absorption rate increases to 0.32 (A = 550 nm) a difference in ion etching rate of the substrate is observed (fig. 5). The region having been under the modified mask has a lower average etching rate. This seems to be caused by a change in the ions range in the substrate. Energy loss in the modified layer is higher than in the standard one. The ions reaching the polymer-substrate interface have lower energy. In consequence the etching rate of the substrate decreases by 2.
4. Conclusions The proposed modification of the popular PMMA makes it more useful for ion lithography. The contrast increases up to 4, and the screening ability (observed as twice as low etching rate of the Si substrate) and elasticity of the resist offer advantages for ion lithography. References [l] [2] [3] [4]
G. StengI et al., Solid State Technol. August (1982) 104. T.H.P. Chang, J. Vat. Sci. Technol. 12 (1975) 1271. D.C. Joy, Microelectronic Engineering 1 (1983) 103. Y. Wada, M. Magitaka, K. Mochiji and H. Obayashi, J. Electrochem. Sot. 5 (1983) 1127. [S] I. Adesida, L. Karapiperis, C.A. Lee and E.D. Wolf, Proc. Microcircuit Engineering (Lausanne, 1981) (1981). [6] I. Adesida, Nucl. Instr. and Meth. 209/210 (1983) 79. [7] M. Kissa, Doctor Thesis (unpublished).
XI. FINE LINE STR./DEPOSITION/ADHESION