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EL-3+ electron beam direct write system
Microelectronic Engineering 17 (1992) 7-10 Elsevier
EL-3-1- ELECTRON BEAM DIRECT WRITE SYSTEM H.C. Pfeiffer, R. Butsch and T.R. Groves
IBM General Technology Division Advanced Technology Center East Fishkill, New York 12533-0999, U.S.A.
Abstract The EL-3 + is the most advanced of IBM's EL series shaped electron beam lithography tools. The first direct write version of EL-3 + has been installed in IBM's Advanced Semiconductor Technology Center (ASTC) in East Fishkill, New York. This paper describes the novel features of this tool together with early results obtained in the ASTC.
t.
INTRODUCTION
IBM's electron beam direct write strategy for the production of bipolar logic chips has been based on the concept of large field deflection and step-and-repeat stage moves. Large deflection enables detection of registration marks in the four corners of the chip and subsequent exposure of the entire chip without intermediate stage movement and without field stitching. This concept provides the most reliable overlay but was increasingly difficult to implement. Figure ! illustrates the
1.0 50,000
Z
2.0
0.5 g-
u_
20,000
~ 1.0 E
:_e
0.5
1980
DeflectionField= ChipSize
1985 Year
1990
Fig. 1 EL-3 performance evolution; minimum feature size, deflection field size and resolved lines per field.
10,000
~"~---'~
(mm)[ Planar "^5 3"~
o Q.
~ J
Stacked~ Crossed Roller 0.2 1980
1985 Year
1990
Fig. 2 EL-3 wafer handling capability and overlay accuracy evolution.
performance improvements achieved on EL-3 during the past decade to meet the increasing requirements of ASIC manufacturing. During the early 1980's, 2.5 /am design rule.~ were used with 5 x 5 mm 2 chips; by 1990 the requirements had reached 0.7/~m minimum features and 10 x 10 mm 2 chips. Figure 2 shows the evolution in 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
8
H.C. Pfeiffer et al. / EL-3+
wafer sizes from 82 mm in 1980 to 200 mm in 1990 and the reduction of overlay errors during the same period. Significant tool enhancements were needed to meet these increasingly stringent demands. 2.
KEY EL-3-1- FEATURES
EL-3 + combines several novel features to meet the lithography requirements for the early 1990's as outlined in Figures I and 2. 2.1 Variable Axis Immersion Lens
To maintain the writing strategy of chip size deflection, it was necessary to advance the state-of-the-art of imaging and deflection techniques. Beam deflection is usually limited, because the resolution deteriorates when the beam leaves the eleclron optical axis of the imaging system. Figure 3 shows schematically our Z
/ Object Plane
Dynamic Stigmator
Collimato:" Lens
Pra-Oe,a[°,~,
Telecentric ~-Lens System
Yokes
xa:i:b'e-.S ,
... ~ . . ' ~ / " 1 " ~
"~ ~
Stage Plate
Projection
Lens
V I./~afar -
er-=-n
[ ~ .
I
Stage
FrictionDrive Surface Prate / & Gauge Bars
Fig. 3
Schematic of Variable Axis Immersion Lens. Target wafer is immersed in the lens field and is accessed through
Drive
Fig. 4 Schematicof the Servo Guided Planar Stage.
a narrow pole piece gap. solution to the problem: The Variable Axis Immersion Lens (VAIL) [I]. The variable axis yoke in VAIL shifts the electron optical axis in synchronism with the pre-deflected beam, eliminating off-axis aberrations since the beam effectively remains "on-axis" throughout the scan field. The two-lens, double deflection system provides telecentricity, achieving close to normal beam landing at all points of the scan field [2]. High speed writing is accomplished through dual channel magnetic and electrostatic deflection, both in telecentric mode. Deflection distortions of both channels are measured and corrected through an automated calibration scheme [3]. There are approximately 20,000 subfields in a 10.35 x 10.35 mm 2 field and 80,000 points are being measured to calibrate the major and minor deflections.
H.C. Pfeiffer et al. / EL-3+
9
In addition to off-axis aberrations, on-axis aberrations are reduced, because the wafer location within the lens pole piece gap provides for a short focal length and consequently for low aberrations of the projection lens; in contrast to more conventional deflection systems, the V A I L focal length can be chosen largely independent of the deflection field size. Through this technique, it was possible to improve field size and resolution simultaneously. For a variable shaped beam of maximal 2 x 2 ~tm2 size and a maximum current of I #a at 50 keV energy, the VAIL system has achieved 100,000 resolved lines over a 10 x 10 mm 2 field [4].
2.2 Planar Stage For a stage to be compatible with VAIL, its structure has to fit inside the narrow pole piece gap; its moving parts must be non-magnetic to avoid interference with the lens and yoke fields and largely non-conductive (no bulk conductors) to suppress eddy currents otherwise generated by the dynamically changing deflection fields and by the stage movement within the static lens field. Figure 4 shows schematically the servo guided planar stage [5] , which was specifically developed for operation inside VAIL. All six parameters of freedom of the precision stage are controlled without the use of stacked guide rails. Stage position and movement are exclusively controlled by three servo-based friction drives, determining x, y and yaw and the precision surface place, determining pitch, roll and z; the surface plate contains the lower VAIL pole piece. The stage plate, which glides on the precision surface plate, has integrated mirrors for the 3-axis laser and consists of gold-coated zero expansion glass. The wafer is held by an electrostatic chuck mounted to the stage plate. The friction drive bars and c a p s t a n s are made from silicon nitride ceramic; ceramic ball bearings on a high resistive titanium alloy shaft attach the bars to the stage plate. The drive motors are located outside the vacuum and are coupled to the capstans through ferrofluidic seals. The stage provides more than 200 mm travel in x and y directions and can be operated at lg acceleration and a velocity of up to 250 mm/sec. A 10.35 mm move is completed in approximately 120 msec without exhibiting overshoots or ringing during acceleration and settling.
3.
RESULTS
EL-3 + is an e-beam direct write tool for the exposure of 200 mm wafers. In the ASTC, it is being used in a mix and match mode with optical steppers to expose 10.35 x 10.35 mm 2 chips without stitching at 0.7 /~m lithography ground rules and 200 nm overlay. With a variable shaped spot of maximal 4 x 4 ~tm2 and 4 pA beam current at 50 keV beam energy, the edge resolution is about 0.2 gm resulting in approximately 2.5 x 10 9 pixels per field. Figure 5 shows resist images of 0.5 gm lines and spaces in the center and the four corners of the 10.35 x 10.35 mm 2 deflection field. The images have been written with a 4 x 0.5 pm 2 shaped beam in negative resist; a slight spot overlap identifies the individual flashes. The micrographs show, however that the image quality achieved at the center of the field is maintained throughout an no deterioration or distortion of the images due to deflection aberrations can be detected. Equally important to resolution is the beam placement accuracy over the field of deflection. The auto-calibration scheme which measures and corrects deflection
10
H.C. Pfeiffer et al. / EL-3+
errors via precision calibration grids results in typical mean plus 3a values of 75nm. Figure 6 illustrates the overlay performance of the tool in a mix and match mode with an optical stepper. The vector plot reflects 242 measurements made at 9 chips per wafer. The mean plus 3a overlay error is 179 nm in x and 190 nm in y direction, well within the 200 nm specification.
..~,
.....................................................................................
I"
10.35mm
•] 250nm
Fig. 5 Test pattern; resist images of 0.5/~m lines and spaces in the center and the 4 comers of a 10.35 x 10. 35 mm 2 deflection field.
Fig. 6
Overlay
result;
vector plot
of
EL-3+ overlay errors to an optical stepper level exposed on 200 mm wafer; mean plus 3 sigma x = 179 rim, y = 190 rim, 242 data
points per wafer. 4.
ACKNOWLEDGEMENT
The development of the EL-3 + System has been a large team effort and the authors wish to acknowledge the many contributions made by their colleagues of the Electron Beam Systems Group at IBM's Advanced Technology Center in East Fishkill, New York.
,
I 2 3 4 5
REFERENCES
P.W. Hawkes and E. Kasper, Principles of Electron Optics, 2 Voi. 2. Applied Geometrical Optics (Academic, New York) 1989 pp 839-~54. W.Stickel, G.O. Langner and P.F. Petric, "Telecentric Beam Positioning for Advanced E-Beam Lithography", these proceedings E.V. Weber, Fine Line Lithography, R. Newman ed. (North-Holland Publishing Company, Amsterdam, New York, Oxford) 1980, p 417. M.A. Sturans et. al., J. Vac. Sci. Technol. B 8 (6), Nov/Dec (1990) 1682. R. Kendall, S. Doran and E. Weissmann, to be published in J. Vac. Sci. Technical Nov/Dec 1991.
EL-3-1- ELECTRON BEAM DIRECT WRITE SYSTEM H.C. Pfeiffer, R. Butsch and T.R. Groves
IBM General Technology Division Advanced Technology Center East Fishkill, New York 12533-0999, U.S.A.
Abstract The EL-3 + is the most advanced of IBM's EL series shaped electron beam lithography tools. The first direct write version of EL-3 + has been installed in IBM's Advanced Semiconductor Technology Center (ASTC) in East Fishkill, New York. This paper describes the novel features of this tool together with early results obtained in the ASTC.
t.
INTRODUCTION
IBM's electron beam direct write strategy for the production of bipolar logic chips has been based on the concept of large field deflection and step-and-repeat stage moves. Large deflection enables detection of registration marks in the four corners of the chip and subsequent exposure of the entire chip without intermediate stage movement and without field stitching. This concept provides the most reliable overlay but was increasingly difficult to implement. Figure ! illustrates the
1.0 50,000
Z
2.0
0.5 g-
u_
20,000
~ 1.0 E
:_e
0.5
1980
DeflectionField= ChipSize
1985 Year
1990
Fig. 1 EL-3 performance evolution; minimum feature size, deflection field size and resolved lines per field.
10,000
~"~---'~
(mm)[ Planar "^5 3"~
o Q.
~ J
Stacked~ Crossed Roller 0.2 1980
1985 Year
1990
Fig. 2 EL-3 wafer handling capability and overlay accuracy evolution.
performance improvements achieved on EL-3 during the past decade to meet the increasing requirements of ASIC manufacturing. During the early 1980's, 2.5 /am design rule.~ were used with 5 x 5 mm 2 chips; by 1990 the requirements had reached 0.7/~m minimum features and 10 x 10 mm 2 chips. Figure 2 shows the evolution in 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
8
H.C. Pfeiffer et al. / EL-3+
wafer sizes from 82 mm in 1980 to 200 mm in 1990 and the reduction of overlay errors during the same period. Significant tool enhancements were needed to meet these increasingly stringent demands. 2.
KEY EL-3-1- FEATURES
EL-3 + combines several novel features to meet the lithography requirements for the early 1990's as outlined in Figures I and 2. 2.1 Variable Axis Immersion Lens
To maintain the writing strategy of chip size deflection, it was necessary to advance the state-of-the-art of imaging and deflection techniques. Beam deflection is usually limited, because the resolution deteriorates when the beam leaves the eleclron optical axis of the imaging system. Figure 3 shows schematically our Z
/ Object Plane
Dynamic Stigmator
Collimato:" Lens
Pra-Oe,a[°,~,
Telecentric ~-Lens System
Yokes
xa:i:b'e-.S ,
... ~ . . ' ~ / " 1 " ~
"~ ~
Stage Plate
Projection
Lens
V I./~afar -
er-=-n
[ ~ .
I
Stage
FrictionDrive Surface Prate / & Gauge Bars
Fig. 3
Schematic of Variable Axis Immersion Lens. Target wafer is immersed in the lens field and is accessed through
Drive
Fig. 4 Schematicof the Servo Guided Planar Stage.
a narrow pole piece gap. solution to the problem: The Variable Axis Immersion Lens (VAIL) [I]. The variable axis yoke in VAIL shifts the electron optical axis in synchronism with the pre-deflected beam, eliminating off-axis aberrations since the beam effectively remains "on-axis" throughout the scan field. The two-lens, double deflection system provides telecentricity, achieving close to normal beam landing at all points of the scan field [2]. High speed writing is accomplished through dual channel magnetic and electrostatic deflection, both in telecentric mode. Deflection distortions of both channels are measured and corrected through an automated calibration scheme [3]. There are approximately 20,000 subfields in a 10.35 x 10.35 mm 2 field and 80,000 points are being measured to calibrate the major and minor deflections.
H.C. Pfeiffer et al. / EL-3+
9
In addition to off-axis aberrations, on-axis aberrations are reduced, because the wafer location within the lens pole piece gap provides for a short focal length and consequently for low aberrations of the projection lens; in contrast to more conventional deflection systems, the V A I L focal length can be chosen largely independent of the deflection field size. Through this technique, it was possible to improve field size and resolution simultaneously. For a variable shaped beam of maximal 2 x 2 ~tm2 size and a maximum current of I #a at 50 keV energy, the VAIL system has achieved 100,000 resolved lines over a 10 x 10 mm 2 field [4].
2.2 Planar Stage For a stage to be compatible with VAIL, its structure has to fit inside the narrow pole piece gap; its moving parts must be non-magnetic to avoid interference with the lens and yoke fields and largely non-conductive (no bulk conductors) to suppress eddy currents otherwise generated by the dynamically changing deflection fields and by the stage movement within the static lens field. Figure 4 shows schematically the servo guided planar stage [5] , which was specifically developed for operation inside VAIL. All six parameters of freedom of the precision stage are controlled without the use of stacked guide rails. Stage position and movement are exclusively controlled by three servo-based friction drives, determining x, y and yaw and the precision surface place, determining pitch, roll and z; the surface plate contains the lower VAIL pole piece. The stage plate, which glides on the precision surface plate, has integrated mirrors for the 3-axis laser and consists of gold-coated zero expansion glass. The wafer is held by an electrostatic chuck mounted to the stage plate. The friction drive bars and c a p s t a n s are made from silicon nitride ceramic; ceramic ball bearings on a high resistive titanium alloy shaft attach the bars to the stage plate. The drive motors are located outside the vacuum and are coupled to the capstans through ferrofluidic seals. The stage provides more than 200 mm travel in x and y directions and can be operated at lg acceleration and a velocity of up to 250 mm/sec. A 10.35 mm move is completed in approximately 120 msec without exhibiting overshoots or ringing during acceleration and settling.
3.
RESULTS
EL-3 + is an e-beam direct write tool for the exposure of 200 mm wafers. In the ASTC, it is being used in a mix and match mode with optical steppers to expose 10.35 x 10.35 mm 2 chips without stitching at 0.7 /~m lithography ground rules and 200 nm overlay. With a variable shaped spot of maximal 4 x 4 ~tm2 and 4 pA beam current at 50 keV beam energy, the edge resolution is about 0.2 gm resulting in approximately 2.5 x 10 9 pixels per field. Figure 5 shows resist images of 0.5 gm lines and spaces in the center and the four corners of the 10.35 x 10.35 mm 2 deflection field. The images have been written with a 4 x 0.5 pm 2 shaped beam in negative resist; a slight spot overlap identifies the individual flashes. The micrographs show, however that the image quality achieved at the center of the field is maintained throughout an no deterioration or distortion of the images due to deflection aberrations can be detected. Equally important to resolution is the beam placement accuracy over the field of deflection. The auto-calibration scheme which measures and corrects deflection
10
H.C. Pfeiffer et al. / EL-3+
errors via precision calibration grids results in typical mean plus 3a values of 75nm. Figure 6 illustrates the overlay performance of the tool in a mix and match mode with an optical stepper. The vector plot reflects 242 measurements made at 9 chips per wafer. The mean plus 3a overlay error is 179 nm in x and 190 nm in y direction, well within the 200 nm specification.
..~,
.....................................................................................
I"
10.35mm
•] 250nm
Fig. 5 Test pattern; resist images of 0.5/~m lines and spaces in the center and the 4 comers of a 10.35 x 10. 35 mm 2 deflection field.
Fig. 6
Overlay
result;
vector plot
of
EL-3+ overlay errors to an optical stepper level exposed on 200 mm wafer; mean plus 3 sigma x = 179 rim, y = 190 rim, 242 data
points per wafer. 4.
ACKNOWLEDGEMENT
The development of the EL-3 + System has been a large team effort and the authors wish to acknowledge the many contributions made by their colleagues of the Electron Beam Systems Group at IBM's Advanced Technology Center in East Fishkill, New York.
,
I 2 3 4 5
REFERENCES
P.W. Hawkes and E. Kasper, Principles of Electron Optics, 2 Voi. 2. Applied Geometrical Optics (Academic, New York) 1989 pp 839-~54. W.Stickel, G.O. Langner and P.F. Petric, "Telecentric Beam Positioning for Advanced E-Beam Lithography", these proceedings E.V. Weber, Fine Line Lithography, R. Newman ed. (North-Holland Publishing Company, Amsterdam, New York, Oxford) 1980, p 417. M.A. Sturans et. al., J. Vac. Sci. Technol. B 8 (6), Nov/Dec (1990) 1682. R. Kendall, S. Doran and E. Weissmann, to be published in J. Vac. Sci. Technical Nov/Dec 1991.