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Mask membrane distortions due to pattern transfer for electron-beam lithography (SCALPEL) masks
IVIICRO~klg~/'ilO~C ELSEVIER
Microelectronic Engineering 46 (1999) 259-262
Mask membrane distortions due lithography (SCALPEL) masks
to pattern transfer
for
electron-beam
G. Dicks, a R. Engelstad, a E. Lovell, a and J. Liddle b aComputational Mechanics Center, University of Wisconsin-Madison 1513 University Avenue, Madison, WI 53706 USA bLucent Technologies 600 Mountain Avenue, Murray Hill, NJ 07974 USA in order to successfully employ Scattering with Angular Limitation Projection Electron-Beam Lithography (SCALPEL) to produce integrated circuits with features below 0.13 ~tm, mask membrane distortions (which lead to pattern placement errors) must not exceed the error budget. When designing a mask, finite element (FE) models are created to identify sources of distortion and quantify the resulting errors. Distortions arise during fabrication, mounting, and in situ exposure of the mask. The focus of this study was to determine the mask membrane distortions induced during the pattern transfer process for a large format SCALPEL mask. Three cases were investigated: the IBM Talon pattern, 100% removal of the scatterer layer, and 50% removal of the scatterer layer. The IBM Talon pattern was chosen to quantify typical pattern specific-distortions while the other two cases, 100% and 50% removal of the scatterer layer, were investigated to determine distortions corresponding to worst case situations. 1.
Introduction
10X 100 MEMBRANES /
12.98
Important to the success of projection electron-beam lithography (SCALPEL) in the sub-0.13 ~tm regime is the development of a low distortion mask. Finite element (FE) models have been developed in order to simulate the response of the mask membrane to both internal and external loading [1, 2, 3, 4, 5]. In this way, we can identify the sources of distortion and characterize the resulting error components. The models are then used as predictive tools to optimize the individual steps involved in fabrication, mounting, and in situ exposure. The purpose of this study was to investigate the SCALPEL mask membrane distortions induced during pattern transfer for a large format SCALPEL mask (see Fig. 1). Or more specifically, distortions due to transfer of the IBM Talon pattern and distortions due to the removal of both 100% and 50% of the scatterer layer were determined. The IBM Talon pattern was chosen as a test case to quantify typical pattern-specific distortions while the other two cases, 100% and 50% removal of the scatterer layer, were investigated to determine distortions during worst case scenarios. (Note: the IBM Talon pattern is a 32 Mbit SRAM layout at 0.25 ktm [6].)
0 11 I ~
~ ~ 0.072s TYPICAL STRUT SECTIONSFOR MASK USING <100> or <110> SILICON WAFER ALL DIMENSIONSIN CM
Fig. 1. Schematic of the SCALPEL mask. 0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0167-93 17(99)00076-3
260
2.
G. Dicks et al. / Microelectronic Engineering 46 (1999) 259-262
SCALPEL Mask Geometry
A large format SCALPEL mask is currently under development at Lucent Technologies. Figure 1 shows a schematic of the wafer geometry and pattern area support structure, comprised of an orthogonal strut system. It has been designed to accept a support ring if necessary. The wafer is made of either <100> or <110> Si, which when appropriately etched, results in the struts having a rectangular cross section. The number and dimensions of the struts may vary in future designs, however the current pattern area can be as large as 130 mm x 123 mm. The struts are in place to support a silicon nitride membrane (0.1 ~tm thick, 100 MPa prestress), a chrome etch stop (0.005 ~tm thick, 25 MPa prestress), and a tungsten scatterer layer (0.025 Ixm thick, 25 MPa prestress).
3.
Finite Element Models
allows for modeling the deposition and removal of layers which occurs during the fabrication process and pattem transfer. Boundary conditions consist of fixing the translational and rotational degrees of freedom at the center of the mask, allowing it to displace in- and out-of-plane.
4.
IBM Talon Test Case
Because it is impractical to model the small circuit features on a patterned mask, an equivalent equal volume criterion is employed to simulate their behavior. Figure 3 shows the IBM Talon pattern geometry with percent scatterer coverage identified in each field. The Talon design has been modified slightly to accommodate the SCALPEL mask and the fact that the SCALPEL technique provides 4x reduction. For this case, 532 of the 1000 membranes have been patterned giving a membrane area of 84 mm x 85 mm and covering a pattern area of 100 mm x 86 mm on the mask. .~
Structural FE models of the SCALPEL mask have been generated using the commercial FE code ANSYS ®. Figure 2 shows a model of the wafer, strut support system, and membrane. The Si wafer with rectangular struts is generated with eight-node isoparametric solid elements, while the SiNx membrane, Cr etch stop, W scatterer, and quasiPMMA resist layers are modeled with four-node elastic shell elements. Layers can be activated/deactivated within the FE model by employing the element birth/death option. This
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Fig. 3. IBM Talon pattern geometry adapted for the SCALPEL mask. All dimensions in mm.
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Fig. 2. Finite element model of the SCALPEL mask (complete model used in analysis).
Simulating Pattern Transfer
A contour plot of the out-of-plane distortions (OPD) due to transfer of the IBM Talon pattem is shown in Fig. 4. The maximum OPD is 2.6 ~m. The nonaxisymmetric contour results are primarily due to the orthogonal strut system of the SCALPEL mask design.
G. Dicks et al. I Microelectronic Engineering 46 (1999) 259-262
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OPD contour plot of the wafer due to transfer of the IBM Talon pattern. Max OPD is 2.6 gm. FE scale in cm.
Fig. 6. IPD vector plot of the pattern area due to transfer of the IBM Talon pattern. Max IPD (for patterned membranes only) is 27.2 nm. FE scale in cm.
A contour plot of the in-plane distortions (IPD) within the pattern area is shown in Fig. 5 and a vector plot of these same distortions is shown in Fig. 6 (for clarity, not all vectors are shown). When considering patterned membranes only, the maximum IPD is 27.2 nm.
Since the distortions are nearly radial, an anisotropic mag correction can be applied which causes most of the distortions to become negligible. Figure 7 shows the IPD contour plot after applying the mag correction and Fig. 8 shows a vector plot of these same distortions (again, for clarity, not all vectors are shown).
Fig. 4.
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Fig. 5. IPD contour plot of the pattern area due to transfer of the IBM Talon pattern. Max IPD (for patterned membranes only) is 27.2 nm. FE scale in cm.
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.460E-o6 .gzxz-0~ .138z-o5 .1B4z-o5 .23oz-os .~z-os .322z-os .36BE-05 ,aXlE os
(lucent)
Fig. 7. IPD contour plot of the pattern area due to transfer of the IBM Talon pattern with anisotropic mag correction. Max IPD (for patterned membranes only) is 5.1 nm. FE scale in cm.
G. Dicks et al. / Microelectronic Engineering 46 (1999) 259-262
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Although the case in which 50% of the scatterer layer was removed appears to be the most severe, results from the IBM Talon pattern indicate that distortion gradients can exist in pattern-specific layouts (see Fig. 5). Additional layouts with more severe nonuniformities are being evaluated to better characterize such effects. ACKNOWLEDGMENTS
. . . . . . . .
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This research was supported by International SEMATECH and the Semiconductor Research Corporation (SRC).
I
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REFERENCES Fig. 8. IPD vector plot of the pattern area due to transfer of the IBM Talon pattern with anisotropic mag correction. Max IPD (for patterned membranes only) is 5.1 nm. FE scale in cm.
1.
2.
6. Summary Simulation of IPD due to pattern transfer for a large format SCALPEL mask (100 mm x 86 mm pattern area for the IBM Talon design and 130 mm x 123 mm pattern area for the other cases) show the following maximum values: Pattern Transfer Layout
IBM Talon design (patterned membranes onl),) 100% removal of the scatterer la),er 50% removal of the scatterer layer
Max IPD
Max IPD with mag correction
Max IPD with mag correction and 4x reduction
27.2 nm
5.1 nm
1.3 nm
33.8 nm
19.2 nm
4.8 nm
46.7 nm
41.0 nm
10.3 nm
3.
4.
5.
6.
G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, Jap. J. of Appl. Phys., Vol. 36, Part 1, No. 12B, pp. 7564-7569, 1997. G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, Proc. of SPIE Emerging Lithographic Technologies H, Vol. 3331, pp. 612-620, 1998. G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, to appear in the Proc. of TECHCON '98, 1998. W. Semke, R. Engelstad, E. Lovell, and J. Liddle, to appear in the J. Vac. Sci. Technol. B, 1998. W. Semke, R. Engelstad, E. Lovell, and J. Liddle, to appear in the Proc. of TECHCON '98, 1998. D. Puisto, IBM, private communication.
Microelectronic Engineering 46 (1999) 259-262
Mask membrane distortions due lithography (SCALPEL) masks
to pattern transfer
for
electron-beam
G. Dicks, a R. Engelstad, a E. Lovell, a and J. Liddle b aComputational Mechanics Center, University of Wisconsin-Madison 1513 University Avenue, Madison, WI 53706 USA bLucent Technologies 600 Mountain Avenue, Murray Hill, NJ 07974 USA in order to successfully employ Scattering with Angular Limitation Projection Electron-Beam Lithography (SCALPEL) to produce integrated circuits with features below 0.13 ~tm, mask membrane distortions (which lead to pattern placement errors) must not exceed the error budget. When designing a mask, finite element (FE) models are created to identify sources of distortion and quantify the resulting errors. Distortions arise during fabrication, mounting, and in situ exposure of the mask. The focus of this study was to determine the mask membrane distortions induced during the pattern transfer process for a large format SCALPEL mask. Three cases were investigated: the IBM Talon pattern, 100% removal of the scatterer layer, and 50% removal of the scatterer layer. The IBM Talon pattern was chosen to quantify typical pattern specific-distortions while the other two cases, 100% and 50% removal of the scatterer layer, were investigated to determine distortions corresponding to worst case situations. 1.
Introduction
10X 100 MEMBRANES /
12.98
Important to the success of projection electron-beam lithography (SCALPEL) in the sub-0.13 ~tm regime is the development of a low distortion mask. Finite element (FE) models have been developed in order to simulate the response of the mask membrane to both internal and external loading [1, 2, 3, 4, 5]. In this way, we can identify the sources of distortion and characterize the resulting error components. The models are then used as predictive tools to optimize the individual steps involved in fabrication, mounting, and in situ exposure. The purpose of this study was to investigate the SCALPEL mask membrane distortions induced during pattern transfer for a large format SCALPEL mask (see Fig. 1). Or more specifically, distortions due to transfer of the IBM Talon pattern and distortions due to the removal of both 100% and 50% of the scatterer layer were determined. The IBM Talon pattern was chosen as a test case to quantify typical pattern-specific distortions while the other two cases, 100% and 50% removal of the scatterer layer, were investigated to determine distortions during worst case scenarios. (Note: the IBM Talon pattern is a 32 Mbit SRAM layout at 0.25 ktm [6].)
0 11 I ~
~ ~ 0.072s TYPICAL STRUT SECTIONSFOR MASK USING <100> or <110> SILICON WAFER ALL DIMENSIONSIN CM
Fig. 1. Schematic of the SCALPEL mask. 0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0167-93 17(99)00076-3
260
2.
G. Dicks et al. / Microelectronic Engineering 46 (1999) 259-262
SCALPEL Mask Geometry
A large format SCALPEL mask is currently under development at Lucent Technologies. Figure 1 shows a schematic of the wafer geometry and pattern area support structure, comprised of an orthogonal strut system. It has been designed to accept a support ring if necessary. The wafer is made of either <100> or <110> Si, which when appropriately etched, results in the struts having a rectangular cross section. The number and dimensions of the struts may vary in future designs, however the current pattern area can be as large as 130 mm x 123 mm. The struts are in place to support a silicon nitride membrane (0.1 ~tm thick, 100 MPa prestress), a chrome etch stop (0.005 ~tm thick, 25 MPa prestress), and a tungsten scatterer layer (0.025 Ixm thick, 25 MPa prestress).
3.
Finite Element Models
allows for modeling the deposition and removal of layers which occurs during the fabrication process and pattem transfer. Boundary conditions consist of fixing the translational and rotational degrees of freedom at the center of the mask, allowing it to displace in- and out-of-plane.
4.
IBM Talon Test Case
Because it is impractical to model the small circuit features on a patterned mask, an equivalent equal volume criterion is employed to simulate their behavior. Figure 3 shows the IBM Talon pattern geometry with percent scatterer coverage identified in each field. The Talon design has been modified slightly to accommodate the SCALPEL mask and the fact that the SCALPEL technique provides 4x reduction. For this case, 532 of the 1000 membranes have been patterned giving a membrane area of 84 mm x 85 mm and covering a pattern area of 100 mm x 86 mm on the mask. .~
Structural FE models of the SCALPEL mask have been generated using the commercial FE code ANSYS ®. Figure 2 shows a model of the wafer, strut support system, and membrane. The Si wafer with rectangular struts is generated with eight-node isoparametric solid elements, while the SiNx membrane, Cr etch stop, W scatterer, and quasiPMMA resist layers are modeled with four-node elastic shell elements. Layers can be activated/deactivated within the FE model by employing the element birth/death option. This
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Fig. 3. IBM Talon pattern geometry adapted for the SCALPEL mask. All dimensions in mm.
5.
200 ~
scalpel ~sk
(lucent)
Fig. 2. Finite element model of the SCALPEL mask (complete model used in analysis).
Simulating Pattern Transfer
A contour plot of the out-of-plane distortions (OPD) due to transfer of the IBM Talon pattem is shown in Fig. 4. The maximum OPD is 2.6 ~m. The nonaxisymmetric contour results are primarily due to the orthogonal strut system of the SCALPEL mask design.
G. Dicks et al. I Microelectronic Engineering 46 (1999) 259-262
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a U G Z~ xsss
261
AU~ Z7 ISSS
ls:o5:~2 WCrOR
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l x ~ =.257z-o~ S~.~STZ-03
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.es~z-ol .I14E-03
.~7~S-05
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.379~-05
200 ~
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{lucre)
OPD contour plot of the wafer due to transfer of the IBM Talon pattern. Max OPD is 2.6 gm. FE scale in cm.
Fig. 6. IPD vector plot of the pattern area due to transfer of the IBM Talon pattern. Max IPD (for patterned membranes only) is 27.2 nm. FE scale in cm.
A contour plot of the in-plane distortions (IPD) within the pattern area is shown in Fig. 5 and a vector plot of these same distortions is shown in Fig. 6 (for clarity, not all vectors are shown). When considering patterned membranes only, the maximum IPD is 27.2 nm.
Since the distortions are nearly radial, an anisotropic mag correction can be applied which causes most of the distortions to become negligible. Figure 7 shows the IPD contour plot after applying the mag correction and Fig. 8 shows a vector plot of these same distortions (again, for clarity, not all vectors are shown).
Fig. 4.
~sxs s,4 AUG 27 1998 oo:so:os pLC~r NO.
~sys
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AUG 28 1998
la~06:19
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1 l~om'J, s o L . i o n
1
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sr~=9999 usuM
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o
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.22xE-es .294E-os
m~
.441E-05 .stss-os .ssB~-os .66zE-os
mm
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:00~scalpel
Fig. 5. IPD contour plot of the pattern area due to transfer of the IBM Talon pattern. Max IPD (for patterned membranes only) is 27.2 nm. FE scale in cm.
~k
.460E-o6 .gzxz-0~ .138z-o5 .1B4z-o5 .23oz-os .~z-os .322z-os .36BE-05 ,aXlE os
(lucent)
Fig. 7. IPD contour plot of the pattern area due to transfer of the IBM Talon pattern with anisotropic mag correction. Max IPD (for patterned membranes only) is 5.1 nm. FE scale in cm.
G. Dicks et al. / Microelectronic Engineering 46 (1999) 259-262
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Although the case in which 50% of the scatterer layer was removed appears to be the most severe, results from the IBM Talon pattern indicate that distortion gradients can exist in pattern-specific layouts (see Fig. 5). Additional layouts with more severe nonuniformities are being evaluated to better characterize such effects. ACKNOWLEDGMENTS
. . . . . . . .
200 ~
.eal~l
~ k
-
This research was supported by International SEMATECH and the Semiconductor Research Corporation (SRC).
I
(Zue.nc~
REFERENCES Fig. 8. IPD vector plot of the pattern area due to transfer of the IBM Talon pattern with anisotropic mag correction. Max IPD (for patterned membranes only) is 5.1 nm. FE scale in cm.
1.
2.
6. Summary Simulation of IPD due to pattern transfer for a large format SCALPEL mask (100 mm x 86 mm pattern area for the IBM Talon design and 130 mm x 123 mm pattern area for the other cases) show the following maximum values: Pattern Transfer Layout
IBM Talon design (patterned membranes onl),) 100% removal of the scatterer la),er 50% removal of the scatterer layer
Max IPD
Max IPD with mag correction
Max IPD with mag correction and 4x reduction
27.2 nm
5.1 nm
1.3 nm
33.8 nm
19.2 nm
4.8 nm
46.7 nm
41.0 nm
10.3 nm
3.
4.
5.
6.
G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, Jap. J. of Appl. Phys., Vol. 36, Part 1, No. 12B, pp. 7564-7569, 1997. G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, Proc. of SPIE Emerging Lithographic Technologies H, Vol. 3331, pp. 612-620, 1998. G. Dicks, R. Engelstad, E. Lovell, and J. Liddle, to appear in the Proc. of TECHCON '98, 1998. W. Semke, R. Engelstad, E. Lovell, and J. Liddle, to appear in the J. Vac. Sci. Technol. B, 1998. W. Semke, R. Engelstad, E. Lovell, and J. Liddle, to appear in the Proc. of TECHCON '98, 1998. D. Puisto, IBM, private communication.