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DUV resists in negative tone high resolution electron beam lithography
IqIICRO~-~ ELSEVIER
Microelectronic Engineering 46 (1999) 383-387
DUV resists in negative tone high resolution electron beam lithography Falco C.M.J.M. van Delft and Frans G. Holthuysen
Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Phone: +31 40 27 43124, Fax: +31 40 27 42346, e-mail: (internet) vandelf@natlab, research.philips.com Shipley's chemically amplified DUV resists UVN-2 (negative tone) and UV-5 (positive tone) have been studied for their high resolution capabilities in electron beam lithography. UV-5 is also capable of negative tone behaviour in case of e-beam overexposure. This effect is shown to be due to direct electron beam cross linking superimposed on the normal catalytically induced positive tone behaviour. The ultimate resolution for 100 nm thick UVN-2 is below 50 nm for single lines, when using the shortest possible developing time in order to prevent swelling. The best obtained positive tone resolution of UV-5 is 50 nm for single trenches and 120 nm for 1:1 lines and spaces. The best obtained negative tone resolution of overexposed UV-5 is below 90 nm. This negative tone behaviour is accompanied by a 60% swelling of a 100 nm thick layer, which swelling is most probably related to the relatively long developing time. This swelling is not observed for a 485 nm thick UV-5 layer.
1 INTRODUCTION
2
EXPERIMENTAL
Reliable commercially available electron beam resists are hard to get. As an alternative, chemically amplified DUV resists can be used [1]. Negative tone resists are commercially not important in DUV lithography, because mask tone reversal and the projection illumination allow a coverage independent throughput for any desired pattern. In electron beam lithography, however, the sequential writing requires negative tone resists for high resolution single lines in order to maintain a reasonable throughput. In this work, Shipley's negative tone chemically amplified DUV resist UVN-2 has been studied for its high resolution capabilities. This resist is the successor of UVN, which allowed 140 nm lines and spaces and 70 nm single lines to be made [1]. Another way for obtaining negative tone behaviour in e-beam writing is the overexposure of positive tone resists. Although this results in poor resolution for most (novolac) systems, in a few cases resolutions below 100 nm have been reported [2]. In this work, also Shipley's positive tone chemically amplified DUV resist UV-5 has been studied for its (positive and) negative tone high resolution capabilities.
Silicon wafers (4 inch) were coated with TriChloroPhenylSilane primer and baked for 120 s at 200 °C on a hotplate. UVN-2 and UV-5 resists solutions were, if necessary, diluted with ethyllactate in order to obtain the desired layer thickness by means of spin coating on a CONVAC spinner (during 45 s at 5000 rpm) [1]. UV-5 layers were soft baked for 60 s at 130 °C, exposed in an Electron Beam Pattern Generator (EBPG Philips-Leica 4V-HR) at 50 kV, directly followed by a Post Exposure Bake (PEB) for 60 s at 135 °C (unless otherwise stated), directly developed by hand in 0.26N PD523 at 20 °C during 60 s, rinsed in demineralized water and blown dry in N 2 . UVN-2 layers were softbaked for 60 s at 110 °C, exposed in the same EBPG at 50 kV, directly followed by a PEB for 60 s at 95 °C (unless otherwise stated), directly developed by hand at 20 °C in 0.26N PD523 during 5 s or in 0.17N 2:1 PD523:H20 during 15 s, rinsed in 1:9 PD523:H20 during 5 s, rinsed in demineralized water and blown dry. Test structures of crossing lines and spaces were written at various exposure doses. Scanning Electron Micrographs were made using a Philips XI.A0 FEG SEM.
0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0167-9317(99)0011 2-4
384
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
3 RESULTS AND DISCUSSION In figs. 1 and 2 contrast curves are shown for 100 and 485 nm thick UV-5 respectively after PEB at various temperatures. The 100 nm thick UV-5 layer shows a positive tone behaviour with a Dose-to-Clear of 12 ~tC/cm2 (for PEB=135 °C), a contrast "/= 5 and an apparent activation energy of Eact / m = 72 IO/mol (m reaction order). The temperature dependence order of the negative tone behaviour (at 500 ~tC/cm2 ) is consistent with a temperature independent electron beam induced cross linking, with the positive tone temperature behaviour of the catalyst superimposed. At very high doses (at 5000 ~tC/cm2 ) electron beam induzed evaporation seems to break down the layer. A peculiar phenomenon is the 60% thickness increase in the negative tone behaviour, which effect is not observed in 485 nm thick UV-5. The observed swelling may be related to the relatively long developing time (as compared to UVN-2, see below) and the use of ethyllactate diluter (for 100 nm thickness). 2
Figure 3 SEM graph of dot exposure in 100 nm thick UV-5, with 70 nm spotsize and 250 nm beam step size and a nominal dose of 20 ~tC/cm2.
• PEB130C
,~, 1.5
° PEB135C o PEB140C
,
~
~
O
-
0.5 O
100
y ~
101
_
_
~
-1()2 ~" D (~C/cm 2)
10 3
10 4
Figure 1 Contrast curves for 100 nm thick UV-5 written at 50 kV (spot size 280 nm, beam step size 250 nm) and various PEB temperatures.
1 0.8
~
• PEB130C
~'
* PEB135C • PEB140C
"-" 0.6 O "u 0.4
0.2 0 100
] 10":" ~
102
103
104
D (I.tC/cm 2)
Figure 2 Contrast curves for 485 nm thick UV-5 written at 50 kV (spot size 280 nm, beam step size 250 nm) and various PEB temperatures.
Figure 4 SEM graph of dot exposure in 100 nm thick UV-5, with 70 nm spotsize and 250 nm beam step size and a nominal dose of 500 ~tC/cm2.
In figs. 3 and 4, SEM graphs are shown of dots written in 100 nm thick UV-5 with nominal doses of 20 and 500 l.tC/cm2 respectively. The negative tone swelling can be observed by comparing both figures. Note that on the overexposed wafer the surroundings of the dots are sufficiently exposed by the proximity effect in order to be opened by the positive tone behaviour. The observed ultimate negative tone resolution of overexposed UV-5 is below 90 nm. The observed ultimate positive tone resolution of UV-5 is 50 nm for single trenches (at 100 ~tC/cm2) and 120 nm for 1:1 lines and spaces (at 50 p.C/cm2), both in a 100 nm thick resist layer and using a PEB of 135 °C. For comparison, the ultimate positive tone resolution in UVIII resist is reported to be 60 nm for 1:1 lines and spaces [1]. This seems to be in accordance with the fact that UVIII was designed for imaging dense lines and UV-5 for isolated features.
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387 The negative tone resist UVN-2 shows its best resolution for a 100 nm thick layer when using the shortest possible developing time in order to prevent swelling. In the standard 0.26 N developer (PD523) the optimum time is 5 s, which is impracticable. With a 0.17N 2:1 PD523:H20 dilution (c.f.[1]), the optimum development time appears to be 15 s. When rinsing with demineralized water after development by hand, redeposition of globules of the dissolved polymer may occur. This is provoked by the slightly acidic nature of demineralized water, due to carbonic acid (from airborn carbon dioxide). In order to prevent this pH shock at the interface between developer residues on the wafer and water, the developer residues are first washed away with a very diluted developer; this solution is too weak to perform further development, but it removes the dissolved polymer from the wafer without the pH shock, that will provoke redeposition. Once the dissolved polymer is removed from the wafer, the final water rinse can be performed [3].
Figure 5 Test structure of vertically & horizontally crossing bars of lines & spaces with 1:4, 1:3, 1:2, 1:1, 2:1, 3:1 and 4:1 dutycycles. The number at the bottom left is the minimum feature size (nm), which is an integer multiple of the beam step size.
385
Figure 6 Test structure of fig. 5 (detail): At the top of the central 1:1 dutycycle bar, five lines extend 5 ktm out, of which the central three extend another 5 ~.m out, of which the middle one extends out 5 Ixm once again as a single line.
Figure 7 Test structure of fig. 5 (detail): The crossing of the 1:3 and 1:4 dutycycle vertically lines and spaces with the 1:3 and 1:4 dutycycle horizontally lines and spaces.
386
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
User friendly test structures of crossing lines and spaces have been designed in order to facilitate high resolution (re)search in a Scanning Electron Microscope (figs. 5-7). These structures were written for several exposure doses with 10, 20 and 30 nm beam step size (20 nm spot size), 40, 50 and 60 nm beam step size (40 nm spot size) and 70, 80, 90 and 100 nm beam step size (80 nm spot size). In fig. 8, measured single line widths for UVN-2 are shown as a function of beam step size at a 30 l~C/cm2 nominal dose for various PEB temperatures and spot sizes. Note that this is equivalent to the line width as a function of line dose (in e.g. nC/m). Single line widths of below 50 nm can be obtained using a 20 nm spot size at a 30 lxC/cm2 nominal dose. The resist thickness, however, is reduced to between 50 and 70 nm, despite the mild developer conditions. The PEB influence is qualitatively as expected, but a quantification is difficult, because of the 'room temperature PEB' in the time between exposure and actual PEB and the time between actual PEB and development. Also, during exposure, a modest warming up on a nanometer scale at the writing spot may contribute to the thermal activation of the catalytic reaction. Note that a high PEB is still needed for stable line thickness definition (and etch resistance).
Figure 9 Crossing lines and spaces (1:4 design) writtten in 100 nm thick UVN-2 at 20 lxC/cm2 nominal dose and 50 nm beam step size.
In figs. 9 and 10, crossing lines and spaces (1:4 design) in (initially) 100 nm thick UVN-2 are shown with 60 nm line widths.
120
spotsize E
80
20
nm
"lO
.~
•
40
•
• . I
0
20
L
L
40 60 BSS (nm)
no PEB PEB45C PEB70C PEB95C I
80
1 O0
Figure 8 UVN-2 single line width versus beam step size at 100 nm thickness and 30 ~tC/cm2 nominal dose for various PEB temperatures and spotsizes.
Figure 10 Crossing lines and spaces (1.'4 design) writtten in 100 nm thick UVN-2 at 30 lxC/cm2 nominal dose and 60 nm beam step size.
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
In fig. 11, a 50 nm wide single line end is shown as written in (initially) 100 nm thick UVN-2.
387
4 CONCLUSIONS High resolution can be achieved in negative tone electron beam lithography using Shipley's chemically amplified DUV resists. Image tone reversal of positive tone UV-5 resist by overexposure, allows sub 90 nm resolution. The negative tone UVN-2 resist allows 50 nm resolution, provided that the development load is critically minimized.
ACKNOWLEDGEMENTS Els Alexander-Moonen is gratefully acknowledged for her helpful discussions concerning the gradual rinsing of the developer and Casper Juffermans for his remarks concerning the resist swelling. Michel Bruyninckx is gratefully acknowledged for his help with designing the test structures.
REFERENCES
Figure 11 Single line end in 100 nm thick UVN-2 at 20 gC/cm 2 nominal dose, 30 nm beam step size.
[1]
D.S. Macintyre and S. Thorns, Proc. MNE'96 (Glasgow), Microelectr. Eng. 35 (1997) 213.
[2]
H. de Koning, P. Zandbergen, M.J. Verheijen and U.K.P. Biermann, Proceedings MNE'94 (Davos), Microelectr. Eng. 27 (1995) 421.
[3]
E.M.L. Alexander-Moonen(private communication).
Microelectronic Engineering 46 (1999) 383-387
DUV resists in negative tone high resolution electron beam lithography Falco C.M.J.M. van Delft and Frans G. Holthuysen
Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Phone: +31 40 27 43124, Fax: +31 40 27 42346, e-mail: (internet) vandelf@natlab, research.philips.com Shipley's chemically amplified DUV resists UVN-2 (negative tone) and UV-5 (positive tone) have been studied for their high resolution capabilities in electron beam lithography. UV-5 is also capable of negative tone behaviour in case of e-beam overexposure. This effect is shown to be due to direct electron beam cross linking superimposed on the normal catalytically induced positive tone behaviour. The ultimate resolution for 100 nm thick UVN-2 is below 50 nm for single lines, when using the shortest possible developing time in order to prevent swelling. The best obtained positive tone resolution of UV-5 is 50 nm for single trenches and 120 nm for 1:1 lines and spaces. The best obtained negative tone resolution of overexposed UV-5 is below 90 nm. This negative tone behaviour is accompanied by a 60% swelling of a 100 nm thick layer, which swelling is most probably related to the relatively long developing time. This swelling is not observed for a 485 nm thick UV-5 layer.
1 INTRODUCTION
2
EXPERIMENTAL
Reliable commercially available electron beam resists are hard to get. As an alternative, chemically amplified DUV resists can be used [1]. Negative tone resists are commercially not important in DUV lithography, because mask tone reversal and the projection illumination allow a coverage independent throughput for any desired pattern. In electron beam lithography, however, the sequential writing requires negative tone resists for high resolution single lines in order to maintain a reasonable throughput. In this work, Shipley's negative tone chemically amplified DUV resist UVN-2 has been studied for its high resolution capabilities. This resist is the successor of UVN, which allowed 140 nm lines and spaces and 70 nm single lines to be made [1]. Another way for obtaining negative tone behaviour in e-beam writing is the overexposure of positive tone resists. Although this results in poor resolution for most (novolac) systems, in a few cases resolutions below 100 nm have been reported [2]. In this work, also Shipley's positive tone chemically amplified DUV resist UV-5 has been studied for its (positive and) negative tone high resolution capabilities.
Silicon wafers (4 inch) were coated with TriChloroPhenylSilane primer and baked for 120 s at 200 °C on a hotplate. UVN-2 and UV-5 resists solutions were, if necessary, diluted with ethyllactate in order to obtain the desired layer thickness by means of spin coating on a CONVAC spinner (during 45 s at 5000 rpm) [1]. UV-5 layers were soft baked for 60 s at 130 °C, exposed in an Electron Beam Pattern Generator (EBPG Philips-Leica 4V-HR) at 50 kV, directly followed by a Post Exposure Bake (PEB) for 60 s at 135 °C (unless otherwise stated), directly developed by hand in 0.26N PD523 at 20 °C during 60 s, rinsed in demineralized water and blown dry in N 2 . UVN-2 layers were softbaked for 60 s at 110 °C, exposed in the same EBPG at 50 kV, directly followed by a PEB for 60 s at 95 °C (unless otherwise stated), directly developed by hand at 20 °C in 0.26N PD523 during 5 s or in 0.17N 2:1 PD523:H20 during 15 s, rinsed in 1:9 PD523:H20 during 5 s, rinsed in demineralized water and blown dry. Test structures of crossing lines and spaces were written at various exposure doses. Scanning Electron Micrographs were made using a Philips XI.A0 FEG SEM.
0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0167-9317(99)0011 2-4
384
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
3 RESULTS AND DISCUSSION In figs. 1 and 2 contrast curves are shown for 100 and 485 nm thick UV-5 respectively after PEB at various temperatures. The 100 nm thick UV-5 layer shows a positive tone behaviour with a Dose-to-Clear of 12 ~tC/cm2 (for PEB=135 °C), a contrast "/= 5 and an apparent activation energy of Eact / m = 72 IO/mol (m reaction order). The temperature dependence order of the negative tone behaviour (at 500 ~tC/cm2 ) is consistent with a temperature independent electron beam induced cross linking, with the positive tone temperature behaviour of the catalyst superimposed. At very high doses (at 5000 ~tC/cm2 ) electron beam induzed evaporation seems to break down the layer. A peculiar phenomenon is the 60% thickness increase in the negative tone behaviour, which effect is not observed in 485 nm thick UV-5. The observed swelling may be related to the relatively long developing time (as compared to UVN-2, see below) and the use of ethyllactate diluter (for 100 nm thickness). 2
Figure 3 SEM graph of dot exposure in 100 nm thick UV-5, with 70 nm spotsize and 250 nm beam step size and a nominal dose of 20 ~tC/cm2.
• PEB130C
,~, 1.5
° PEB135C o PEB140C
,
~
~
O
-
0.5 O
100
y ~
101
_
_
~
-1()2 ~" D (~C/cm 2)
10 3
10 4
Figure 1 Contrast curves for 100 nm thick UV-5 written at 50 kV (spot size 280 nm, beam step size 250 nm) and various PEB temperatures.
1 0.8
~
• PEB130C
~'
* PEB135C • PEB140C
"-" 0.6 O "u 0.4
0.2 0 100
] 10":" ~
102
103
104
D (I.tC/cm 2)
Figure 2 Contrast curves for 485 nm thick UV-5 written at 50 kV (spot size 280 nm, beam step size 250 nm) and various PEB temperatures.
Figure 4 SEM graph of dot exposure in 100 nm thick UV-5, with 70 nm spotsize and 250 nm beam step size and a nominal dose of 500 ~tC/cm2.
In figs. 3 and 4, SEM graphs are shown of dots written in 100 nm thick UV-5 with nominal doses of 20 and 500 l.tC/cm2 respectively. The negative tone swelling can be observed by comparing both figures. Note that on the overexposed wafer the surroundings of the dots are sufficiently exposed by the proximity effect in order to be opened by the positive tone behaviour. The observed ultimate negative tone resolution of overexposed UV-5 is below 90 nm. The observed ultimate positive tone resolution of UV-5 is 50 nm for single trenches (at 100 ~tC/cm2) and 120 nm for 1:1 lines and spaces (at 50 p.C/cm2), both in a 100 nm thick resist layer and using a PEB of 135 °C. For comparison, the ultimate positive tone resolution in UVIII resist is reported to be 60 nm for 1:1 lines and spaces [1]. This seems to be in accordance with the fact that UVIII was designed for imaging dense lines and UV-5 for isolated features.
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387 The negative tone resist UVN-2 shows its best resolution for a 100 nm thick layer when using the shortest possible developing time in order to prevent swelling. In the standard 0.26 N developer (PD523) the optimum time is 5 s, which is impracticable. With a 0.17N 2:1 PD523:H20 dilution (c.f.[1]), the optimum development time appears to be 15 s. When rinsing with demineralized water after development by hand, redeposition of globules of the dissolved polymer may occur. This is provoked by the slightly acidic nature of demineralized water, due to carbonic acid (from airborn carbon dioxide). In order to prevent this pH shock at the interface between developer residues on the wafer and water, the developer residues are first washed away with a very diluted developer; this solution is too weak to perform further development, but it removes the dissolved polymer from the wafer without the pH shock, that will provoke redeposition. Once the dissolved polymer is removed from the wafer, the final water rinse can be performed [3].
Figure 5 Test structure of vertically & horizontally crossing bars of lines & spaces with 1:4, 1:3, 1:2, 1:1, 2:1, 3:1 and 4:1 dutycycles. The number at the bottom left is the minimum feature size (nm), which is an integer multiple of the beam step size.
385
Figure 6 Test structure of fig. 5 (detail): At the top of the central 1:1 dutycycle bar, five lines extend 5 ktm out, of which the central three extend another 5 ~.m out, of which the middle one extends out 5 Ixm once again as a single line.
Figure 7 Test structure of fig. 5 (detail): The crossing of the 1:3 and 1:4 dutycycle vertically lines and spaces with the 1:3 and 1:4 dutycycle horizontally lines and spaces.
386
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
User friendly test structures of crossing lines and spaces have been designed in order to facilitate high resolution (re)search in a Scanning Electron Microscope (figs. 5-7). These structures were written for several exposure doses with 10, 20 and 30 nm beam step size (20 nm spot size), 40, 50 and 60 nm beam step size (40 nm spot size) and 70, 80, 90 and 100 nm beam step size (80 nm spot size). In fig. 8, measured single line widths for UVN-2 are shown as a function of beam step size at a 30 l~C/cm2 nominal dose for various PEB temperatures and spot sizes. Note that this is equivalent to the line width as a function of line dose (in e.g. nC/m). Single line widths of below 50 nm can be obtained using a 20 nm spot size at a 30 lxC/cm2 nominal dose. The resist thickness, however, is reduced to between 50 and 70 nm, despite the mild developer conditions. The PEB influence is qualitatively as expected, but a quantification is difficult, because of the 'room temperature PEB' in the time between exposure and actual PEB and the time between actual PEB and development. Also, during exposure, a modest warming up on a nanometer scale at the writing spot may contribute to the thermal activation of the catalytic reaction. Note that a high PEB is still needed for stable line thickness definition (and etch resistance).
Figure 9 Crossing lines and spaces (1:4 design) writtten in 100 nm thick UVN-2 at 20 lxC/cm2 nominal dose and 50 nm beam step size.
In figs. 9 and 10, crossing lines and spaces (1:4 design) in (initially) 100 nm thick UVN-2 are shown with 60 nm line widths.
120
spotsize E
80
20
nm
"lO
.~
•
40
•
• . I
0
20
L
L
40 60 BSS (nm)
no PEB PEB45C PEB70C PEB95C I
80
1 O0
Figure 8 UVN-2 single line width versus beam step size at 100 nm thickness and 30 ~tC/cm2 nominal dose for various PEB temperatures and spotsizes.
Figure 10 Crossing lines and spaces (1.'4 design) writtten in 100 nm thick UVN-2 at 30 lxC/cm2 nominal dose and 60 nm beam step size.
F.C.M.J.M. van Delft, F.G. Holthuysen / Microelectronic Engineering 46 (1999) 383-387
In fig. 11, a 50 nm wide single line end is shown as written in (initially) 100 nm thick UVN-2.
387
4 CONCLUSIONS High resolution can be achieved in negative tone electron beam lithography using Shipley's chemically amplified DUV resists. Image tone reversal of positive tone UV-5 resist by overexposure, allows sub 90 nm resolution. The negative tone UVN-2 resist allows 50 nm resolution, provided that the development load is critically minimized.
ACKNOWLEDGEMENTS Els Alexander-Moonen is gratefully acknowledged for her helpful discussions concerning the gradual rinsing of the developer and Casper Juffermans for his remarks concerning the resist swelling. Michel Bruyninckx is gratefully acknowledged for his help with designing the test structures.
REFERENCES
Figure 11 Single line end in 100 nm thick UVN-2 at 20 gC/cm 2 nominal dose, 30 nm beam step size.
[1]
D.S. Macintyre and S. Thorns, Proc. MNE'96 (Glasgow), Microelectr. Eng. 35 (1997) 213.
[2]
H. de Koning, P. Zandbergen, M.J. Verheijen and U.K.P. Biermann, Proceedings MNE'94 (Davos), Microelectr. Eng. 27 (1995) 421.
[3]
E.M.L. Alexander-Moonen(private communication).