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EPL electron optics performance on test stand.
Microelectronic Engineering 67–68 (2003) 109–121 www.elsevier.com / locate / mee
EPL electron optics performance on test stand. 1. Resolution results M. Hamashima*, S. Kojima, T. Umemoto, H. Shimizu, J. Ikeda, A. Yamada, S. Takahashi, T. Yahiro, S. Shimizu, K. Okamoto, T. Yamaguchi, S. Miura, S. Kawata, K. Suzuki 2 nd Development Department, IC Equipment Division, Nikon Corporation; 201 -9, Miizugahara, Kumagaya City, Saitama Prefecture 360 -8559, Japan
Abstract The performance of the electron optical (EO) column of Nikon’s EB-stepper mounted on its test stand is reported, mainly focusing on the subject of resolution as preliminary results. Stitching data will be presented in the next paper at MNE2002. Resolution performance data are presented with SEM photos of line / space patterns and contact hole features, printed in both positive and negative resists. EO adjustment techniques were established to obtain the maximum resolution with best trajectory optimisation so that both resolution and distortion can meet their specifications. It is essential to have a metrology capability to measure distortion as well as resolution. Good resolution results have been achieved over the large deflection (5 mm) field of the EO column using the curvilinear variable axis lens (CVAL) concept, which has been demonstrated by IBM theoretically and experimentally. Current beam blur, depth of focus (DOF) and some characteristic features of the EO conditions are also demonstrated. Image blur at small-deflected positions has also been determined by a direct measurement method. The minimum feature size of 50 nm and a large depth of focus over 5 mm, have been obtained. Overall performance over the whole 5 mm deflected range has not been accomplished yet due to stability limitations of the test stand stage, but good 80-nm line and space resist images have been obtained for the whole deflected range. 2003 Elsevier Science B.V. All rights reserved. Keywords: Electron Projection Lithography (EPL); Resolution; Distortion; Image blur; Illumination uniformity; Trajectory; EO adjustment; EO column
1. Introduction Electron Projection Lithography (EPL) is one of the promising technologies for lithography below * Corresponding author. Tel.: 181-48-533-4728; fax: 181-48-533-7458. E-mail address: [email protected] (M. Hamashima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00180-1
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Fig. 1. PREVAIL concept for electron optical system.
the 65-nm feature size. The basic study of this technology has been done under the name of SCALPEL by AT&T [1] (Lucent Technologies, now Agere Systems) from the beginning of the 1990s and with the name of PREVAIL for the electron optic system (Fig. 1) by the joint work of IBM [2] and Nikon [3]. PREVAIL will provide very high throughput capability to customers because of its wide deflection optics. Total integration and adjustments to the full system known as EB stepper are going well [4,5]. Some recent results for the stage and body sub-system of EB stepper have already been reported [6]. Nikon has just started to integrate the electron optics (EO) sub-system on the main body and stage system. Basic EO performance has been confirmed by EO adjustment of the EO column mounted on a test stand [7]. A final EO adjustment process for the EB stepper can be accomplished practically by combining the stage scanning control with the electron beam deflection control, thereby integrating the whole system.
2. Electron optics aberrations By employing the CVAL concept in the EO column design for the EB stepper, it has become possible for the electron optics to drastically reduce aberrations over a large deflection range, thereby enabling the EO column to achieve small image blur, i.e. good resolution with a wide (5 mm) deflection field at the wafer. Image blur can be used as a metric for resolution capability of EPL. The image blur is defined as the full width at half maximum (FWHM) of the point spread function, which corresponds to the width of the 12–88% edge slope height. The total blur is defined as the square root of the sums of the squares of the following contributions.
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2.1. Diffraction aberration Diffraction limited resolution is expressed as: Bd 5 Cd l /a where l is the de Broglie wavelength of the electron, Cd is the diffraction coefficient and a is the electron beam semi-angle at the wafer. As discussed below, assuming an electron beam voltage of E5100 kV and a ¯6–8 mrad: Bd , 1 nm
2.2. Chromatic aberration ( CA) Chromatic aberration (CA) blur is expressed as Bcc 5 Cc a DE /E where Cc is the chromatic aberration coefficient, E is the electron energy, and DE the electron energy dispersion. Energy dispersion can be neglected when using a PREVAIL stencil mask compared to a SCALPEL membrane type mask. Bcc is estimated in our current system as: Bcc , 5 nm where V5100 kV and DE52 eV due to high voltage ripple. In addition, CA at deflection can be almost cancelled out in the magnetic doublet lens configuration.
2.3. Coulomb interaction ( CI) The image blur Bc by Coulomb interaction (CI) between electrons is given approximately by: Bc 5 k(I 5 / 6 L 5 / 4 M) /(a 3 / 5 SF 1 / 2 V 3 / 2 ) where k is a coefficient depending on the EO design [3,8]. For the current EPL system, the other parameters and their values are: L, length between reticle and wafer is optimized; M, magnification of projection lens is 1 / 4; a, beam semi-angle is 6–8 mrad on the wafer; this will be discussed in detail below; SF, sub-field size is 0.25 mm30.25 mm on wafer; V, Acceleration voltage is 100 kV; I, beam current is 100 mA maximum on the reticle or 25 mA maximum on the wafer. In the current experiments so far, I is less than 1 mA, then Bc,5 nm.
2.4. Geometrical aberrations ( GA) In principle, CVAL optics mainly consists of lenses and deflection yokes. Geometrical aberrations in the lens system of the EO column involve the conventional optical geometrical aberrations: spherical, coma, astigmatism, field curvature, and distortion. However, distortion does not affect image blur. Additional parameters such as deflection length and telecentricity affect total GA. Specific correctors, for example stigmator and focus correctors, can compensate several aberration terms. Reduction of the beam semi-angle significantly decreases GA but increases Bc. There is a
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competition between these antagonistic effects, leading to optimal beam semi-angles in the range of 5–10 mrad for EO systems with sufficient beam current for required wafer exposure throughput. An optimized value of a 56–8 mrad was selected. Total blur from both CI and GA is analyzed and evaluated from the point spread function model including the vector sum of both aberrations. Each GA term is expressed as a function of a, b, and c, where b is the distance of an electron from the SF center, and c is the distance of a SF center from the center of the main deflection field (MF). Below are summarized the functional dependences of the geometrical aberrations on a, b, and c.
2.4.1. Lens aberrations (on-axis, i.e. undeflected) • • • •
Spherical [a 3 ] Coma [a 2 b] Astigmatism [a b 2 ] a Field curvature [a b 2 ] fc
Image blur due to these aberration terms is minimized when all the lens conditions and electron beam alignments are optimized.
2.4.2. Lens 1 yoke hybrid aberrations (off-axis, i.e. deflected: additional terms to on-axis aberrations) • Coma [a 2 c] • Astigmatism [a bc] a • Field curvature [a bc] fc GA is estimated as the square root of the sum of the squares of all the elements above, and depends on powers of a, b, and c. Here only the third-order aberrations are described, but higher order terms can possibly contribute at large values of b and c. A field curvature term of a c 2 and distortion terms of bc 2 and c 3 are excluded here, because dynamic focus coils can correct them. GA changes depending on the position in SF and MF as illustrated by the following relations: • • • •
SF10.5 SF10.5 SF1-20 SF1-20
(On-axis) center: [a 3 ] (On-axis) corner: [a 3 ] 1 [a 2 b] 1 [a b 2 ] a 1 [a b 2 ] fc (Off-axis) center: [a 3 ] 1 [a 2 c] (Off-axis) corner: [a 3 ] 1 [a 2 b] 1 [a b 2 ] a 1 [a b 2 ] fc 1 [a 2 c] 1 [a bc] a 1 [a bc] fc 2
At off-axis positions, hybrid aberrations such as [a c] or [a bc] increase proportional to the deflection distance c (Fig. 2). However, the actual increase in blur at large field deflection is greatly suppressed by the CVAL design, which establishes a near-cancellation of the c-dependent terms in the above expressions. In the current EO design, the theoretical total blur including GA and CI at full deflection is less than 40 nm, and almost the same as the on-axis blur.
3. Electron optics adjustment (method and metrology) As described in Section 2, only some of the third-order aberrations can be corrected. All other aberration terms are minimized by optimization of lens and CVAL settings. To achieve best resolution
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Fig. 2. Schematic diagram of electron optics column and CVAL trajectory.
and distortion values, the lens and CVAL should be adjusted initially according to design. However, as shown in a schematic example of CVAL trajectory in Fig. 3, it is difficult to trace the beam three-dimensionally through the EO optics to confirm the adjustment. The EO adjustment procedure is briefly described in Fig. 4, where the means for adjustment of each step, i.e. lens, aligner, yoke, corrector, are shown in sequence. Completion of adjustment can be judged by measuring the output parameters described in Fig. 4. To confirm whether appropriate lens and CVAL adjustment is correctly done, the following EO parameters have been measured or verified at each step.
3.1. Illumination uniformity After optimization by lens setting and beam pivot alignment in the condenser section, the illumination uniformity within the SF is improved. Fig. 5 shows the best data with 3.5% (3s ) uniformity.
Fig. 3. Schematic diagram of geometric aberration in full deflection field.
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Fig. 4. Adjustment procedure of electron optics.
3.2. a ( Beam truncation angle and beam semi-angle) Numerical aperture (NA) or a is one of the basic parameters for resolution performance in EPL, as is also the case with optical steppers. As mentioned earlier, some of the geometrical aberration terms involve powers of a, which is generally defined as the beam semi-angle. However, more precisely, the aberrations should be integrated over the electron beam angular distribution, which approximates a truncated Gaussian distribution defined by a 1 /e Gaussian half angle and the beam truncation angle. Fig. 6a shows the truncated Gaussian beam profile at a crossover. The truncation angle determined by the cut-off of the beam by the aperture at the 1st beam crossover position and the 1 /e half angle of the Gaussian beam distribution have been measured by fitting a truncated Gaussian function to the signal
Fig. 5. Illumination uniformity within on-axis sub-field.
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Fig. 6. Schematic image of truncated Gaussian beam is shown in the left part, and the calculated and experimental signal waveform of aperture current at crossover position on the right.
waveform of the aperture current at the 2nd crossover position in the projection optics as shown in Fig. 6b. The beam semi-angle is 5–6 mrad and the beam truncation angle is 7 mrad. The beam semi-angle was used to evaluate the aberration expression.
3.3. Shape of SF After the lens adjustments, linear distortions of the SF shape—magnification, rotation, orthogonality, and anisotropic magnification—are measured. Iso- and aniso-magnification are adjusted to within 20 ppm from the target value and rotation and orthogonality to within 20 mrad.
3.4. Telecentricity of SF and main-field ( MF) Telecentricity within the sub-field is determined by the EO design. The definition of lens telecentricity is shown schematically in Fig. 7a. Telecentricity is determined by measuring SF magnification change during height (Z) changes in reticle and wafer. Launching and landing angles are shown in Fig. 7b. These are measured by measuring SF position change during height (Z) change in reticle and wafer. A telecentric lens condition is required in order to achieve focus independent of image magnification. It is also required to keep telecentricity of the main-field, which is determined by CVAL design concept, if deflection or sub-field position is to be maintained constant despite focus changes. SF telecentricity is ,0.5 mrad at the reticle and ,3 mrad at the wafer. MF telecentricity is ,0.5 mrad at the reticle and ,0.3 mrad at the wafer.
3.5. Convergence of resolution and non-linear distortion The initial adjustment criterion is to confirm that the position and angle of launching, and landing rays and aperture pivot (where the aperture image stops moving during beam deflection) agree with the model (designed) trajectory. However, due to mechanical errors of parts or assembly, there are possibly small changes in the optimum trajectory from the design trajectory. Therefore it is necessary
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Fig. 7. (a) Telecentrity of sub-field (lens telecentricity), and (b) telecentrity of main-field (CVAL telecentricity).
to perturb the initial set and check SF distortion and resolution, and to repeat until we have reached convergence to within the allowable error budget conditions.
4. Results of resolution performance
4.1. Resist resolution image Some results of the EO imaging performance are presented. The current condition is: Nikon Test reticle of less than 5% transmittance and beam current of 3 mA at the reticle, using negative type resist of 200 nm thickness. SEM images of resist patterns with optimum electron dose at SF10 (near on-axis) are shown in Fig. 8, and at the largest deflected positions (SF1: 22.5 mm and SF20: 12.5 mm) in Fig. 9. Figs. 8 and 9 exhibit resolution uniformity at five points (center and four corners) within the subfield. There is no significant difference in resolution within the SF area or between SF1, SF10, and SF20. 1:1 lines and spaces of nominal 100 nm pattern and isolated lines of 70 nm pattern (Fig. 10) are well resolved over the whole area. This means that the CVAL trajectory has been properly adjusted. Fig. 11 shows cross-sections of 1:1 holes and spaces (positive resist) and various pitches of L / S down to 50-nm patterns in negative resist of 350 nm thickness. Although line structures collapsed, they are well resolved down to 60 nm at SF10.
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Fig. 8. SEM image of 1:1100-nm lines and spaces at SF0 (near on-axis) shown in Fig. 8.
Fig. 9. (a) Resolution of the same pattern as Fig. 8 at SF1 in the left part, and (b) at SF20 in the right part; both are at the largest deflected positions.
4.2. Blur estimate Beam blur has been estimated independently by two methods. One estimate is from the resolution of resist patterns by SEM measurement. Empirically blur can be estimated from resist resolution versus blur data. We have also measured image blur directly using an aerial image sensor (AIS) which
Fig. 10. Resolution of 70 nm isolated lines at SF1 and SF20 (largest deflections).
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Fig. 11. SEM image of cross-section of 1:1 holes and spaces and various pitches of L / S down to 50-nm patterns in resist of 350 nm thickness.
had been introduced in Refs. [9,10]. Fig. 12 shows the signal waveform and measurement from the calculation of blur defined as the 12–88% width of the beam edge slope. Our results are: Blur (Resolution of resist exposure image),50 nm Blur (Direct beam edge measurement)557 nm. Beam current I has been set to 3–5 mA on the test stand. Therefore, in this blur estimate so far the Coulomb interaction blur Bc has almost no effect on aberration.
Fig. 12. Waveform of direct image blur measurement with aerial image sensor (AIS).
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4.3. DBlur estimate DBlur originates from aberration variations within the subfield, as shown in Section 2.4. DBlur affects resist CD uniformity within the SF. CD uniformity is determined not only by Dblur but illumination uniformity, etc. From the direct beam on edge (BOE) measurement, which is preferable to CD uniformity measurement, Dblur is estimated to be approximately 5 nm.
4.4. DOF estimate To evaluate depth of focus (DOF), cross-sectional SEM photographs of resist images of 100–70 nm contact holes have been taken by changing focus. Fig. 13 shows DOF of 1:180 nm contact holes is over 8 mm. The resist is 350 nm thick and was exposed at 9 mC / cm 2 dose. A focus value of 0 mm is around the best focus, and the opposite defocus side is out of exposed defocus range. This demonstrates EPL has a much larger focus margin than an optical stepper [11].
Fig. 13. DOF of 1:180 nm contact holes in resist of 350 nm thickness.
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Table 1 Analysis and summary of all the factors affecting resolution Class
Factor
Checking method
Aberration
Geometrical Chromatic Coulomb Higher order
NA dependence HVPS ripple Beam current Mechanical error
Vibration
Electric or magnetic Mechanical
Beam on edge method
Adjustment
Alignment Lens1CVAL Focus1Stig
Various trajectory checks Stigmator adjustment
Material or process
SEM
Resist
Result
20–30 nm
5. Discussion and summary Nikon is developing an EPL exposure tool, the EB stepper. At present, the EO column has proved its basic resolution performance, and established the process how to achieve it, as described. Actual resolution is influenced not only by EO design but by various other factors as described above. Table 1 shows the analysis and present summary of all the factors affecting resolution. Some of them are quantitatively measured or analyzed by original metrology or other unique methods. Beam fluctuations due to electronics noise, stage vibration, and external magnetic field were measured by states-of-the art beam-on-edge method on the test stand. One of the remaining issues is higher order aberrations caused by mechanical errors of parts or assembly. The EO column has now been integrated on the main body. The various factors affecting resolution will be checked and eliminated soon using high accuracy of stage system, and some of them will be improved further in the course of EPL tool development. Higher performance of overall specification including field distortion and stitching results [12] is expected after all the remaining ambiguity is excluded.
Acknowledgements The authors would like to thank all members of the EPL team of Nikon / IBM / NRCA (Nikon Research Corporation of America) for their technical contributions.
References [1] [2] [3] [4]
S.D. Berger, J.M. Gibson, Appl. Phys. Lett. 57 (1990) 153. H.C. Pfeiffer, Jpn. J. Appl. Phys. 34 (1995) 6658. H.C. Pfeiffer et al., J. Vac. Sci. Technol. B17 (1999) 2840. K. Suzuki et al., Proc. SPIE 3997 (2000) 214.
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K. Suzuki et al., Proc. SPIE 4343 (2001) 80. T. Miura et al., Proc. SPIE 4688 (2002) to be published. S. Golladay et al., J. Vac. Sci. Technol. B19 (2001) 2459. W. Stickel, J. Vac. Sci. Technol. B16 (1998) 3211. K. Suzuki, Proc. SPIE 4754 (2002) 775. T. Okino, K. Suzuki, Proc. SPIE 4343 (2001) 483–490. T. Yahiro, S. Suzuki, T. Irita, N. Hirayanagi, H. Shimizu, S. Kojima, K. Morita, S. Kawata, T. Okino, K. Suzuki, J. Vac. Sci. Technol. B19 (2001) 2468. [12] S. Kojima et al., Microelectron. Eng. 67–68 (2003) 122.
EPL electron optics performance on test stand. 1. Resolution results M. Hamashima*, S. Kojima, T. Umemoto, H. Shimizu, J. Ikeda, A. Yamada, S. Takahashi, T. Yahiro, S. Shimizu, K. Okamoto, T. Yamaguchi, S. Miura, S. Kawata, K. Suzuki 2 nd Development Department, IC Equipment Division, Nikon Corporation; 201 -9, Miizugahara, Kumagaya City, Saitama Prefecture 360 -8559, Japan
Abstract The performance of the electron optical (EO) column of Nikon’s EB-stepper mounted on its test stand is reported, mainly focusing on the subject of resolution as preliminary results. Stitching data will be presented in the next paper at MNE2002. Resolution performance data are presented with SEM photos of line / space patterns and contact hole features, printed in both positive and negative resists. EO adjustment techniques were established to obtain the maximum resolution with best trajectory optimisation so that both resolution and distortion can meet their specifications. It is essential to have a metrology capability to measure distortion as well as resolution. Good resolution results have been achieved over the large deflection (5 mm) field of the EO column using the curvilinear variable axis lens (CVAL) concept, which has been demonstrated by IBM theoretically and experimentally. Current beam blur, depth of focus (DOF) and some characteristic features of the EO conditions are also demonstrated. Image blur at small-deflected positions has also been determined by a direct measurement method. The minimum feature size of 50 nm and a large depth of focus over 5 mm, have been obtained. Overall performance over the whole 5 mm deflected range has not been accomplished yet due to stability limitations of the test stand stage, but good 80-nm line and space resist images have been obtained for the whole deflected range. 2003 Elsevier Science B.V. All rights reserved. Keywords: Electron Projection Lithography (EPL); Resolution; Distortion; Image blur; Illumination uniformity; Trajectory; EO adjustment; EO column
1. Introduction Electron Projection Lithography (EPL) is one of the promising technologies for lithography below * Corresponding author. Tel.: 181-48-533-4728; fax: 181-48-533-7458. E-mail address: [email protected] (M. Hamashima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00180-1
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Fig. 1. PREVAIL concept for electron optical system.
the 65-nm feature size. The basic study of this technology has been done under the name of SCALPEL by AT&T [1] (Lucent Technologies, now Agere Systems) from the beginning of the 1990s and with the name of PREVAIL for the electron optic system (Fig. 1) by the joint work of IBM [2] and Nikon [3]. PREVAIL will provide very high throughput capability to customers because of its wide deflection optics. Total integration and adjustments to the full system known as EB stepper are going well [4,5]. Some recent results for the stage and body sub-system of EB stepper have already been reported [6]. Nikon has just started to integrate the electron optics (EO) sub-system on the main body and stage system. Basic EO performance has been confirmed by EO adjustment of the EO column mounted on a test stand [7]. A final EO adjustment process for the EB stepper can be accomplished practically by combining the stage scanning control with the electron beam deflection control, thereby integrating the whole system.
2. Electron optics aberrations By employing the CVAL concept in the EO column design for the EB stepper, it has become possible for the electron optics to drastically reduce aberrations over a large deflection range, thereby enabling the EO column to achieve small image blur, i.e. good resolution with a wide (5 mm) deflection field at the wafer. Image blur can be used as a metric for resolution capability of EPL. The image blur is defined as the full width at half maximum (FWHM) of the point spread function, which corresponds to the width of the 12–88% edge slope height. The total blur is defined as the square root of the sums of the squares of the following contributions.
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2.1. Diffraction aberration Diffraction limited resolution is expressed as: Bd 5 Cd l /a where l is the de Broglie wavelength of the electron, Cd is the diffraction coefficient and a is the electron beam semi-angle at the wafer. As discussed below, assuming an electron beam voltage of E5100 kV and a ¯6–8 mrad: Bd , 1 nm
2.2. Chromatic aberration ( CA) Chromatic aberration (CA) blur is expressed as Bcc 5 Cc a DE /E where Cc is the chromatic aberration coefficient, E is the electron energy, and DE the electron energy dispersion. Energy dispersion can be neglected when using a PREVAIL stencil mask compared to a SCALPEL membrane type mask. Bcc is estimated in our current system as: Bcc , 5 nm where V5100 kV and DE52 eV due to high voltage ripple. In addition, CA at deflection can be almost cancelled out in the magnetic doublet lens configuration.
2.3. Coulomb interaction ( CI) The image blur Bc by Coulomb interaction (CI) between electrons is given approximately by: Bc 5 k(I 5 / 6 L 5 / 4 M) /(a 3 / 5 SF 1 / 2 V 3 / 2 ) where k is a coefficient depending on the EO design [3,8]. For the current EPL system, the other parameters and their values are: L, length between reticle and wafer is optimized; M, magnification of projection lens is 1 / 4; a, beam semi-angle is 6–8 mrad on the wafer; this will be discussed in detail below; SF, sub-field size is 0.25 mm30.25 mm on wafer; V, Acceleration voltage is 100 kV; I, beam current is 100 mA maximum on the reticle or 25 mA maximum on the wafer. In the current experiments so far, I is less than 1 mA, then Bc,5 nm.
2.4. Geometrical aberrations ( GA) In principle, CVAL optics mainly consists of lenses and deflection yokes. Geometrical aberrations in the lens system of the EO column involve the conventional optical geometrical aberrations: spherical, coma, astigmatism, field curvature, and distortion. However, distortion does not affect image blur. Additional parameters such as deflection length and telecentricity affect total GA. Specific correctors, for example stigmator and focus correctors, can compensate several aberration terms. Reduction of the beam semi-angle significantly decreases GA but increases Bc. There is a
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competition between these antagonistic effects, leading to optimal beam semi-angles in the range of 5–10 mrad for EO systems with sufficient beam current for required wafer exposure throughput. An optimized value of a 56–8 mrad was selected. Total blur from both CI and GA is analyzed and evaluated from the point spread function model including the vector sum of both aberrations. Each GA term is expressed as a function of a, b, and c, where b is the distance of an electron from the SF center, and c is the distance of a SF center from the center of the main deflection field (MF). Below are summarized the functional dependences of the geometrical aberrations on a, b, and c.
2.4.1. Lens aberrations (on-axis, i.e. undeflected) • • • •
Spherical [a 3 ] Coma [a 2 b] Astigmatism [a b 2 ] a Field curvature [a b 2 ] fc
Image blur due to these aberration terms is minimized when all the lens conditions and electron beam alignments are optimized.
2.4.2. Lens 1 yoke hybrid aberrations (off-axis, i.e. deflected: additional terms to on-axis aberrations) • Coma [a 2 c] • Astigmatism [a bc] a • Field curvature [a bc] fc GA is estimated as the square root of the sum of the squares of all the elements above, and depends on powers of a, b, and c. Here only the third-order aberrations are described, but higher order terms can possibly contribute at large values of b and c. A field curvature term of a c 2 and distortion terms of bc 2 and c 3 are excluded here, because dynamic focus coils can correct them. GA changes depending on the position in SF and MF as illustrated by the following relations: • • • •
SF10.5 SF10.5 SF1-20 SF1-20
(On-axis) center: [a 3 ] (On-axis) corner: [a 3 ] 1 [a 2 b] 1 [a b 2 ] a 1 [a b 2 ] fc (Off-axis) center: [a 3 ] 1 [a 2 c] (Off-axis) corner: [a 3 ] 1 [a 2 b] 1 [a b 2 ] a 1 [a b 2 ] fc 1 [a 2 c] 1 [a bc] a 1 [a bc] fc 2
At off-axis positions, hybrid aberrations such as [a c] or [a bc] increase proportional to the deflection distance c (Fig. 2). However, the actual increase in blur at large field deflection is greatly suppressed by the CVAL design, which establishes a near-cancellation of the c-dependent terms in the above expressions. In the current EO design, the theoretical total blur including GA and CI at full deflection is less than 40 nm, and almost the same as the on-axis blur.
3. Electron optics adjustment (method and metrology) As described in Section 2, only some of the third-order aberrations can be corrected. All other aberration terms are minimized by optimization of lens and CVAL settings. To achieve best resolution
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Fig. 2. Schematic diagram of electron optics column and CVAL trajectory.
and distortion values, the lens and CVAL should be adjusted initially according to design. However, as shown in a schematic example of CVAL trajectory in Fig. 3, it is difficult to trace the beam three-dimensionally through the EO optics to confirm the adjustment. The EO adjustment procedure is briefly described in Fig. 4, where the means for adjustment of each step, i.e. lens, aligner, yoke, corrector, are shown in sequence. Completion of adjustment can be judged by measuring the output parameters described in Fig. 4. To confirm whether appropriate lens and CVAL adjustment is correctly done, the following EO parameters have been measured or verified at each step.
3.1. Illumination uniformity After optimization by lens setting and beam pivot alignment in the condenser section, the illumination uniformity within the SF is improved. Fig. 5 shows the best data with 3.5% (3s ) uniformity.
Fig. 3. Schematic diagram of geometric aberration in full deflection field.
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Fig. 4. Adjustment procedure of electron optics.
3.2. a ( Beam truncation angle and beam semi-angle) Numerical aperture (NA) or a is one of the basic parameters for resolution performance in EPL, as is also the case with optical steppers. As mentioned earlier, some of the geometrical aberration terms involve powers of a, which is generally defined as the beam semi-angle. However, more precisely, the aberrations should be integrated over the electron beam angular distribution, which approximates a truncated Gaussian distribution defined by a 1 /e Gaussian half angle and the beam truncation angle. Fig. 6a shows the truncated Gaussian beam profile at a crossover. The truncation angle determined by the cut-off of the beam by the aperture at the 1st beam crossover position and the 1 /e half angle of the Gaussian beam distribution have been measured by fitting a truncated Gaussian function to the signal
Fig. 5. Illumination uniformity within on-axis sub-field.
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Fig. 6. Schematic image of truncated Gaussian beam is shown in the left part, and the calculated and experimental signal waveform of aperture current at crossover position on the right.
waveform of the aperture current at the 2nd crossover position in the projection optics as shown in Fig. 6b. The beam semi-angle is 5–6 mrad and the beam truncation angle is 7 mrad. The beam semi-angle was used to evaluate the aberration expression.
3.3. Shape of SF After the lens adjustments, linear distortions of the SF shape—magnification, rotation, orthogonality, and anisotropic magnification—are measured. Iso- and aniso-magnification are adjusted to within 20 ppm from the target value and rotation and orthogonality to within 20 mrad.
3.4. Telecentricity of SF and main-field ( MF) Telecentricity within the sub-field is determined by the EO design. The definition of lens telecentricity is shown schematically in Fig. 7a. Telecentricity is determined by measuring SF magnification change during height (Z) changes in reticle and wafer. Launching and landing angles are shown in Fig. 7b. These are measured by measuring SF position change during height (Z) change in reticle and wafer. A telecentric lens condition is required in order to achieve focus independent of image magnification. It is also required to keep telecentricity of the main-field, which is determined by CVAL design concept, if deflection or sub-field position is to be maintained constant despite focus changes. SF telecentricity is ,0.5 mrad at the reticle and ,3 mrad at the wafer. MF telecentricity is ,0.5 mrad at the reticle and ,0.3 mrad at the wafer.
3.5. Convergence of resolution and non-linear distortion The initial adjustment criterion is to confirm that the position and angle of launching, and landing rays and aperture pivot (where the aperture image stops moving during beam deflection) agree with the model (designed) trajectory. However, due to mechanical errors of parts or assembly, there are possibly small changes in the optimum trajectory from the design trajectory. Therefore it is necessary
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Fig. 7. (a) Telecentrity of sub-field (lens telecentricity), and (b) telecentrity of main-field (CVAL telecentricity).
to perturb the initial set and check SF distortion and resolution, and to repeat until we have reached convergence to within the allowable error budget conditions.
4. Results of resolution performance
4.1. Resist resolution image Some results of the EO imaging performance are presented. The current condition is: Nikon Test reticle of less than 5% transmittance and beam current of 3 mA at the reticle, using negative type resist of 200 nm thickness. SEM images of resist patterns with optimum electron dose at SF10 (near on-axis) are shown in Fig. 8, and at the largest deflected positions (SF1: 22.5 mm and SF20: 12.5 mm) in Fig. 9. Figs. 8 and 9 exhibit resolution uniformity at five points (center and four corners) within the subfield. There is no significant difference in resolution within the SF area or between SF1, SF10, and SF20. 1:1 lines and spaces of nominal 100 nm pattern and isolated lines of 70 nm pattern (Fig. 10) are well resolved over the whole area. This means that the CVAL trajectory has been properly adjusted. Fig. 11 shows cross-sections of 1:1 holes and spaces (positive resist) and various pitches of L / S down to 50-nm patterns in negative resist of 350 nm thickness. Although line structures collapsed, they are well resolved down to 60 nm at SF10.
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Fig. 8. SEM image of 1:1100-nm lines and spaces at SF0 (near on-axis) shown in Fig. 8.
Fig. 9. (a) Resolution of the same pattern as Fig. 8 at SF1 in the left part, and (b) at SF20 in the right part; both are at the largest deflected positions.
4.2. Blur estimate Beam blur has been estimated independently by two methods. One estimate is from the resolution of resist patterns by SEM measurement. Empirically blur can be estimated from resist resolution versus blur data. We have also measured image blur directly using an aerial image sensor (AIS) which
Fig. 10. Resolution of 70 nm isolated lines at SF1 and SF20 (largest deflections).
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Fig. 11. SEM image of cross-section of 1:1 holes and spaces and various pitches of L / S down to 50-nm patterns in resist of 350 nm thickness.
had been introduced in Refs. [9,10]. Fig. 12 shows the signal waveform and measurement from the calculation of blur defined as the 12–88% width of the beam edge slope. Our results are: Blur (Resolution of resist exposure image),50 nm Blur (Direct beam edge measurement)557 nm. Beam current I has been set to 3–5 mA on the test stand. Therefore, in this blur estimate so far the Coulomb interaction blur Bc has almost no effect on aberration.
Fig. 12. Waveform of direct image blur measurement with aerial image sensor (AIS).
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4.3. DBlur estimate DBlur originates from aberration variations within the subfield, as shown in Section 2.4. DBlur affects resist CD uniformity within the SF. CD uniformity is determined not only by Dblur but illumination uniformity, etc. From the direct beam on edge (BOE) measurement, which is preferable to CD uniformity measurement, Dblur is estimated to be approximately 5 nm.
4.4. DOF estimate To evaluate depth of focus (DOF), cross-sectional SEM photographs of resist images of 100–70 nm contact holes have been taken by changing focus. Fig. 13 shows DOF of 1:180 nm contact holes is over 8 mm. The resist is 350 nm thick and was exposed at 9 mC / cm 2 dose. A focus value of 0 mm is around the best focus, and the opposite defocus side is out of exposed defocus range. This demonstrates EPL has a much larger focus margin than an optical stepper [11].
Fig. 13. DOF of 1:180 nm contact holes in resist of 350 nm thickness.
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Table 1 Analysis and summary of all the factors affecting resolution Class
Factor
Checking method
Aberration
Geometrical Chromatic Coulomb Higher order
NA dependence HVPS ripple Beam current Mechanical error
Vibration
Electric or magnetic Mechanical
Beam on edge method
Adjustment
Alignment Lens1CVAL Focus1Stig
Various trajectory checks Stigmator adjustment
Material or process
SEM
Resist
Result
20–30 nm
5. Discussion and summary Nikon is developing an EPL exposure tool, the EB stepper. At present, the EO column has proved its basic resolution performance, and established the process how to achieve it, as described. Actual resolution is influenced not only by EO design but by various other factors as described above. Table 1 shows the analysis and present summary of all the factors affecting resolution. Some of them are quantitatively measured or analyzed by original metrology or other unique methods. Beam fluctuations due to electronics noise, stage vibration, and external magnetic field were measured by states-of-the art beam-on-edge method on the test stand. One of the remaining issues is higher order aberrations caused by mechanical errors of parts or assembly. The EO column has now been integrated on the main body. The various factors affecting resolution will be checked and eliminated soon using high accuracy of stage system, and some of them will be improved further in the course of EPL tool development. Higher performance of overall specification including field distortion and stitching results [12] is expected after all the remaining ambiguity is excluded.
Acknowledgements The authors would like to thank all members of the EPL team of Nikon / IBM / NRCA (Nikon Research Corporation of America) for their technical contributions.
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