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Soft X-ray projection imaging with multilayer reflection masks
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 27 (1995) 285-290
Soft x-ray projection imaging with multilayer reflection masks Masaaki Ito, Hiroaki Oizumi*, Takashi Soga, Hiromasa Yamanashi, Taro Ogawa, Soichi Katagifi, Eiiehi Seya, and Eiji Takeda Central Research Laboratory, Hitachi, Ltd., 1-280 Higashi-Koigakubo, Kokubunji, Tokyo 185, Japan *present address: SORTEC Corporation, 16-1 Wadai, Tsukuba, Ibaraki 300-42, Japan Patterning methods for an etched multilayer mask and an absorber overlayer mask are investigated for use at a 13-rim wavelength. RIE in SF6 is used to etch a Mo/Si multilayer and a W absorber overlayer with a SiO2 etch-stop layer. Fine patterns as small as 0.25 lain are clearly formed. In particular, pattern sidewalls of the absorber overlayer mask are extremely steep. The reflectivity measurement using large reflective-area samples indicates that neither method causes significant damage to the multilayer. The mask patterns are imaged onto a resist-coated wafer using a 20:1 Schwarzschild optic, confirming that 0.07-1am line-and-space patterns can be printed using either mask. 1. I N T R O D U C T I O N Soft x-ray projection lithography (SXPL), also called extreme ultraviolet (EUV) lithography, is a promising candidate for the fabrication of microcircuit devices with minimum feature size below 0.1 p.m. The feasibility of SXPL has so far been investigated using multilayer-coated reflective optics at a wavelength of 13 nm, at which a reflectivity of about 60% can be obtained at nearnormal incidence. In these experiments, a reduced pattern of either a transmission or multilayer reflection mask was imaged onto a resist-coated wafer. The transmission mask, comprising a patterned absorber overlayer atop a thin membrane, is simple in structure, and hence, is useful in evaluating the performance of optics and resist characteristics. Fine patterns as small as 0.1-0.05 pan have already been printed using transmission masks and Schwarzschild optics, although the dimensions of the field are 25-50 lain [1-3]. One of the great advantages of SXPL is the use of robust reflection masks, which are immune to stress-induced pattern distortion. The reflection masks should provide a minimum feature size from 0.5 to 0.25 lain (for use in a 5: I reduction system) with a clearly defined pattern as well as a high reflectivity contrast. Furthermore, the patterning processes should not deteriorate the multilayer reflectivity so as not to degrade the system throughput. From a practical point of view, the defect density should be tolerably low and defects must be readily repairable. Tennant, et al. systematically investigated a variety of reflection mask technologies and
i ~ ~ ~ Multiayer Substrate !!!!!!!!!!!!!!!E!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!E!!!!!!!!!!!!E!!
(a) Etched multilayer mask ¢-..-.-.-.-, ~.-.-.-.~ L........~........~ Absorber Substrate (b) Absorber ovedayer mask ~=:=t~,~, .}===:~._,~,,.~:==~,~,. ~ ; ; ~ ; ; ~ Multilayer ..~ Substrate
(c) Ion-damaged multilayer mask Figure 1. Schematic of multilayer reflection masks.
demonstrated 0.1-1xm resolution [4]. Figure 1 schematically shows the structures of various reflection ma.~k.~: a multilayer etched by reactive ion etching (RIE), an absorber overlayer atop a
0167-9317/95/$09.50 © 1995- Elsevier Science B.V. All rights reserved. SSDI 0167-9317(94)00108-1
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M. Ito et al. / Microelectronic Engineering 27 (1995) 285-290
multilayer, and a multilayer damaged by ion implantation. In the patterning process for the etched multilayer mask, an etch mask protects the reflective portion from being etched; hence damage to the multilayer is eased [5,6]. However, it is difficult to repair an opaque defect in which the reflective portion is missing. The absorber overlayer mask, of which the pattern thickness can be very small, is favorable for obtaining fine patterns. The absorber overlayer is patterned by either lift-off [7] or RIE [8]. Although the lift-off process causes no significant damage to the multilayer, it is likely that particles in a solvent adhere to the reflective portion, which then suffers from opaque defects. This problem can be avoided by using RIE. However, protection of the reflective portion is vital for the success of this method. The ion damaged multilayer mask, of which the reflectivity contrast is relatively low, seems to be less practical than the other masks. In this paper, we present patterning methods for the etched multilayer mask and the absorber overlayer mask with Mo/Si multilayers for use at a 13-nm wavelength. RIE of both the multilayer and the absorber overlayer are discussed with an emphasis on the resulting pattern profiles and multilayer reflectivities. Results of a resolution test using a 20:1 Schwarzschild optic are also given. 2. M A S K P A T T E R N I N G M E T H O D S
To evaluate the pattern profiles and changes in reflectivity caused by the patterning processes, we prepared several mask samples. They were fabricated on silicon wafers coated with Mo/Si multilayers, containing 40 layer pairs, each with a period of 6.7 rim. The ratio of the Mo layer thickness to the period was 0.44. The multilayer was deposited using a magnetron sputtering system (ANELVA, SPF-530) ata Ar gas pressure of 0.3 Pa. To average spatial variation in the sputtering rate, the wafer was spun throughout the deposition. The temperature of the wafer was kept at 25 °C to obtain a smooth Mo/Si interface. Fine feature patterns were generated using a direct-write e-beam system (Hitachi, HL-750) at an acceleration voltage of 50 kV. Etching was performed in a reactive ion etcher (Plasma Therm, PK-1441) and resulting pattern profiles were observed with an SEM (Hitachi, S-800). Reflectivity measurement samples, of which the entire surface was reflective, were prepared by the same methods used for the fine-pattern samples. The
reflectivity was measured using a soft x-ray reflectometer with a laser-produced plasma source at Tohoku University [9]. In the measurement the incident beam was fixed at an angle of 15° from the normal to the sample and the wavelength was scanned. 2.1. Etched muitilayer m a s k
Anisotropy and linewidth control are important for RIE of the Mo/Si multilayer. To form fine patterns with good linewidth control, we chose SiO2 for the etch mask. A 300-nm-thick negative-tone resist and a 300-nm-thick SiO2 layer were stacked on the multilayer. After e-beam lithography, RIE in CHF3 was applied to etch the SiO2 layer. The 270nm-thick multilayer was then etched in SF6 at a rf power of 100 W and a gas flow rate of 25 seem. Examination of the etch profiles showed good anisotropy at a pressure of 0.3 Pa, at which the etch selectivity (etch rate ratio) of the multilayer to SiO2 was about 2. Finally, the SiO2 layer was removed in a mixture of HF and NH4F with a 1:6 ratio. The highest temperature during the patterning process was 210 °C. Figure 2 shows an SEM photograph of a 0.25gm line-and-space pattern. Although the pattern edge is smooth, the sidewalls are slightly tapered with a lateral broadening of 0.06 gm. Since the number of layers in the multilayer varies in this region, so does the reflectivity contrast. This might degrade the resolution and linewidth accuracy when the pattern is imaged into low-contrast resist.
~./Mo/Si ~/Mo 0.25 I.u'n Figure 2. SEM photograph of the etched multilayer mask with a feature size of 0.25gin. The total thickness of the multilayer is 270 nm.
287
M. Ito et al. / Microelectronic Engineering 27 (1995) 285-290
Figure 3 shows the reflectivity of the multilayer portion of the etched-multilayer mask. The monitor multilayer deposited at the same time provides a peak reflectivity of about 60% at 12.8 nm and no significant change in reflectivity is observed after the patterning process.
80
6O
after P
before
..g,
RIE in 0 2. In this study, for the sake of process simplicity, the etch-stop layer was not removed. However, it is so thin that the transmission loss in the etch-stop layer is expected to be small. Figure 4 shows an SEM photograph of a 0.25~tm line-and-space pattern. The sidewalls are extremely steep with a lateral broadening of less than 0.02 pan. Hence, a sharp contrast can be expected at the pattern edges. As shown in Fig. 5, the reflectivity of the multilayer after the patterning process is almost identical to that of the monitor multilayer. This indicates that the multilayer was successfully protected during RIE.
._> t3 40 m n" 20 0 12.0
~,W 12.5 13.0 Wavelength (nm)
13.5
~,/Mo/Si
j/s
Figure 3. Measured reflectivity of the multilayer before and after patterning process for the etched multilayer mask. 2.2. Absorber overlayer mask As a way to prevent the multilayer from being etched, previous researchers have proposed a process in which a Ge absorber overlayer and a polyimide etch-stop layer are stacked on a multilayer, and after RIE of the absorber, the etch-stop layer is removed [4]. To put this process into a practical use, however, long-term stability of the polyimide layer, which lies beneath the absorber, should be thoroughly evaluated. We chose W as the absorber overlayer because it is highly absorbing at 13 nm and suitable for finepattern formation by RIE. In our process, SiO2 was used as the etch-stop layer instead of polyimide. A 300-nm-thick negative-tone resist, a 50-nm-thick W absorber overlayer, and a 10-nm-thick SiO2 etchstop layer were stacked on the multilayer. After resist development, RIE in SF6 was applied to etch the absorber overlayer at a rf power of 100 W, a flow rate of 25 seem, and a pressure of 2.1 Pa. In this case the etch selectivity of W to SiO2 was about 4. Thus, the etch-stop layer was resistant to 80% overetch. The resist was subsequently removed by
0.25 Ixl'n Figure 4. SEM photograph of the absorber overlayer mask with a feature size of 0.25 lim. The absorber thickness is 50 nm.
80
after
before
6O •-> 40 m n- 20 0 12
i
13 Wavelength (nm)
14
Figure 5. Measured reflectivity of the multilayer before and after patterning process for the absorber overlayer mask.
288
M. lto et al. / Microelectronic Engineering 27 (1995) 285-290
3. IMAGING EXPERIMENTS The printing capability of the reflection masks was evaluated by imaging fine patterns. A schematic diagram of the experimental setup is shown in Fig. 6. As the x-ray source we used the 2.5-GeV storage ring (Photon Factory) at the National Laboratory for High Energy Physics. Synchrotron radiation (SR) is reflected by a goldcoated plane mirror to cut off hard x-ray radiation. The beam passes through a Be filter that cuts off VUV and UV radiation. The beam then hits the mask, and the mask pattern is imaged onto a resistcoated wafer at a magnification of 20:1 using a Schwarzschild optic. The numerical aperture (NA) of the optic is 0.08 and the field size is limited to 60 × 60 tun due to off-axis aberration.
Radiation ~
.
.
.
.
.
.
.
.
.
.
Reflectionmask
.
Optical _axi_s__ ~
~~,,f
Wafer
k , > Schwarzschild optic Figure 6. Schematic diagram of the experimental setup for projection imaging.
The Schwarzschild optic has convex and concave spherical mirrors with curvature radii of 15 mm and 43 ram, respectively. To reflect 13-rim radiation at near-normal incidence, Mo/Si multilayers were deposited on the surfaces by the Ovonic Synthetic Materials Company. The figure errors (rms) were measured to be 8 nm and 6 nm over the convex and concave surfaces, respectively. The mask patterns were imaged into a 0.13-1amthick film of chemically amplified negative-tone resist AZ-PN100. The resist absorbance is estimated to be 0.6, assuming the linear absorption coefficient of 0.44 lain-l[10]. The typical exposure time was 60 s for a beam current of 300 mA. Post-exposure baking (PEB) was performed at 105 °C for 120 s followed by developing with 0.18-N MF-312 developer for 30 s. Figures 7 (a) and Co) show SEM photographs of O.l-lxm line-and-space patterns obtained using the etched multilayer and absorber overlayer masks, respectively. Both patterns are well resolved to the same extent. Figures 8 (a) and Co) show that 0.07[tm line-and-space patterns are also delineated. In this case, however, the edge roughness for the absorbing overlayer mask is slightly larger than that for the etched multilayer mask. This difference cannot necessarily be attributed to mask quality because these images were printed onto different wafers and unfortunately reproducibility was not good in our experiments.
0.2~m (a)
(b)
Figure 7. SEM photographs of 0.i-lain line-and-space resist pattern obtained using (a) the etched multilayer mask and CO)the absorber overlayer mask.
M. lto et al. /Microelectronic Engineering 27 (1995) 285-290
289
0.1 l~m
(a)
(b)
Figure 8. SEM photographs of 0.07-p.m line-and-space resist pattern obtained using (a) the etched multilayer mask and (b) the absorber overlayer mask. In our experiments, the illumination was nearly coherent since the SR source size is small and the beam simply expands toward the mask. The cut-off frequency for coherent illumination, at which the image contrast becomes zero, is NA/k = 6150 lines/mm (0.08-gm line-and-space). On the other hand, the resolved pattern is slightly finer than that corresponding to the cut-off frequency. Moreover, a weak modulation was observed even for 0.05-pro features. These results suggest that the two beams (0-order and + lst-order) diffracted by the mask pattern pass through the optic and contribute to form an image [11]. 4. CONCLUSIONS We have investigated patterning methods for an etched multilayer mask and an absorber overlayer mask for use at a 13-nm wavelength. RIE in SF6 is applied to etch a Mo/Si multilayer with a SiO2 etch mask. A W absorber overlayer was etched by RIE in SF 6 with a SiO2 etch-stop layer. Fine patterns with a minimum feature size of 0.25 gm were clearly formed by either method. In particular, pattern sidewalls of the absorber overlayer mask were extremely steep. The reflectivity measurement using large reflective-area samples indicated that neither method causes significant damage to the multilayer. The mask patterns were imaged onto a resist-coated
wafer using a 20:1 Schwarzschild optic and 0.07-pro line-and-space patterns were printed using either mask. These results support the feasibility of SXPL using reflection masks in the sub-0.1-1ma regime. ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Masaki Yamamoto and Dr. Mihiro Yanagihara at Tohoku University for their valuable discussions on multilayer reflectors. Thanks also to Mr. Tsuneo Terasawa at Central Research Laboratory, Hitachi, Ltd. for his helpful discussions on soft x-ray projection imaging. They further scknowledge Mr. Kunio Harada and Mr. Minoru Hidaka for their assistance with constructing the optic. This work has been performed under the approval of the National Laboratory for High Energy Physics (Acceptance No. 93Y003). REFERENCES 1. J. E. Bjorkholm, J. Boker, L. Eichner, R. R. Freeman, J. Gregus, T. E. Jewell, W. M. Mansfield, A. A. MacDowell, E. L. Raab, W. T. Silfvast, L. H. Szeto, D. M. Tennant, W. K. Waskiewicz, D. L. White, D. L. Windt, and O. R. Wood, II: J. Vac. Sci. Technol. B8 (1990) 1509.
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M. Ito et al. / Microelectronic Engineering 27 (1995) 285-290
2. H. Oizumi, Y. Maejima, T. Watanabe, T. Taguchi, Y. Yamashita, N. Atoda, K. Murakami, M. Ohtani, and H. Nagata: Jpn. J. Appl. Phys. 32 (1993) 5914. 3. H. Nagata, M. Ohtani, K. Murakami, T. Oshino, H. Oizumi, Y. Maejima, T. Watanabe, T. Taguchi, Y. Yamashita, and N. Atoda: Jpn. J. Appl. Phys. 33 (1994) 360. 4. D. M. Tennant, J. E. Bjorkholm, R. M. D'Souza, L. Eichner, R. R. Freeman, J. Z. Pastalan, L. H. Szeto, O. R. Wood, II, T. E. Jewell, W. M. Mansfield, W. K. Waskiewicz, D. L. White, D. L. Windt, and A. A. MacDowell: J. Vac. Sci. Technol. B9 (1991) 3176. 5. H. Kinoshita, K. Kurihara, and H. Takenaka: Jpn. J. Appl. Phys 30 (1991) 3048. 6. H. Oizumi, T. Soga, H. Yamanashi, T. Ogawa, M. Itoh, M. Yamamoto, M. Yanagihara, K. Mayama, and E. Takeda: Digest of Papers MicroProcess 93 (1993) 234. 7. A. M. Hawryluk, N. M. Ceglio, and D. P. Gaines: J. Vac. Sci. Technol. B7 (1989) 1702. 8. H. Kinoshita, K. Kurihara, Y. Ishii, and Y. Torii: J. Vac. Sci. Technol. B7 (1989) 1648. 9. S. Nakayama, M. Yanagihara, M. Yamamoto, H. Kimura, and T. Namioka: Phys. Scr. 41 (1990) 754. 10. G. D. Kubiak, E. M. Kneedler, R. Q. Hwang, M. T. Schulberg, K. W. Berger, J. E. Bjorkholm, and W. M. Mansfield: J. Vac. Sci. Technol. B10 (1992) 2593. 11. N. Shiraishi, S. Hirukawa, Y. Takeuchi, and N. Magome: Proc. SPIE 1674 (1992) 741.
ELSEVIER
Microelectronic Engineering 27 (1995) 285-290
Soft x-ray projection imaging with multilayer reflection masks Masaaki Ito, Hiroaki Oizumi*, Takashi Soga, Hiromasa Yamanashi, Taro Ogawa, Soichi Katagifi, Eiiehi Seya, and Eiji Takeda Central Research Laboratory, Hitachi, Ltd., 1-280 Higashi-Koigakubo, Kokubunji, Tokyo 185, Japan *present address: SORTEC Corporation, 16-1 Wadai, Tsukuba, Ibaraki 300-42, Japan Patterning methods for an etched multilayer mask and an absorber overlayer mask are investigated for use at a 13-rim wavelength. RIE in SF6 is used to etch a Mo/Si multilayer and a W absorber overlayer with a SiO2 etch-stop layer. Fine patterns as small as 0.25 lain are clearly formed. In particular, pattern sidewalls of the absorber overlayer mask are extremely steep. The reflectivity measurement using large reflective-area samples indicates that neither method causes significant damage to the multilayer. The mask patterns are imaged onto a resist-coated wafer using a 20:1 Schwarzschild optic, confirming that 0.07-1am line-and-space patterns can be printed using either mask. 1. I N T R O D U C T I O N Soft x-ray projection lithography (SXPL), also called extreme ultraviolet (EUV) lithography, is a promising candidate for the fabrication of microcircuit devices with minimum feature size below 0.1 p.m. The feasibility of SXPL has so far been investigated using multilayer-coated reflective optics at a wavelength of 13 nm, at which a reflectivity of about 60% can be obtained at nearnormal incidence. In these experiments, a reduced pattern of either a transmission or multilayer reflection mask was imaged onto a resist-coated wafer. The transmission mask, comprising a patterned absorber overlayer atop a thin membrane, is simple in structure, and hence, is useful in evaluating the performance of optics and resist characteristics. Fine patterns as small as 0.1-0.05 pan have already been printed using transmission masks and Schwarzschild optics, although the dimensions of the field are 25-50 lain [1-3]. One of the great advantages of SXPL is the use of robust reflection masks, which are immune to stress-induced pattern distortion. The reflection masks should provide a minimum feature size from 0.5 to 0.25 lain (for use in a 5: I reduction system) with a clearly defined pattern as well as a high reflectivity contrast. Furthermore, the patterning processes should not deteriorate the multilayer reflectivity so as not to degrade the system throughput. From a practical point of view, the defect density should be tolerably low and defects must be readily repairable. Tennant, et al. systematically investigated a variety of reflection mask technologies and
i ~ ~ ~ Multiayer Substrate !!!!!!!!!!!!!!!E!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!E!!!!!!!!!!!!E!!
(a) Etched multilayer mask ¢-..-.-.-.-, ~.-.-.-.~ L........~........~ Absorber Substrate (b) Absorber ovedayer mask ~=:=t~,~, .}===:~._,~,,.~:==~,~,. ~ ; ; ~ ; ; ~ Multilayer ..~ Substrate
(c) Ion-damaged multilayer mask Figure 1. Schematic of multilayer reflection masks.
demonstrated 0.1-1xm resolution [4]. Figure 1 schematically shows the structures of various reflection ma.~k.~: a multilayer etched by reactive ion etching (RIE), an absorber overlayer atop a
0167-9317/95/$09.50 © 1995- Elsevier Science B.V. All rights reserved. SSDI 0167-9317(94)00108-1
286
M. Ito et al. / Microelectronic Engineering 27 (1995) 285-290
multilayer, and a multilayer damaged by ion implantation. In the patterning process for the etched multilayer mask, an etch mask protects the reflective portion from being etched; hence damage to the multilayer is eased [5,6]. However, it is difficult to repair an opaque defect in which the reflective portion is missing. The absorber overlayer mask, of which the pattern thickness can be very small, is favorable for obtaining fine patterns. The absorber overlayer is patterned by either lift-off [7] or RIE [8]. Although the lift-off process causes no significant damage to the multilayer, it is likely that particles in a solvent adhere to the reflective portion, which then suffers from opaque defects. This problem can be avoided by using RIE. However, protection of the reflective portion is vital for the success of this method. The ion damaged multilayer mask, of which the reflectivity contrast is relatively low, seems to be less practical than the other masks. In this paper, we present patterning methods for the etched multilayer mask and the absorber overlayer mask with Mo/Si multilayers for use at a 13-nm wavelength. RIE of both the multilayer and the absorber overlayer are discussed with an emphasis on the resulting pattern profiles and multilayer reflectivities. Results of a resolution test using a 20:1 Schwarzschild optic are also given. 2. M A S K P A T T E R N I N G M E T H O D S
To evaluate the pattern profiles and changes in reflectivity caused by the patterning processes, we prepared several mask samples. They were fabricated on silicon wafers coated with Mo/Si multilayers, containing 40 layer pairs, each with a period of 6.7 rim. The ratio of the Mo layer thickness to the period was 0.44. The multilayer was deposited using a magnetron sputtering system (ANELVA, SPF-530) ata Ar gas pressure of 0.3 Pa. To average spatial variation in the sputtering rate, the wafer was spun throughout the deposition. The temperature of the wafer was kept at 25 °C to obtain a smooth Mo/Si interface. Fine feature patterns were generated using a direct-write e-beam system (Hitachi, HL-750) at an acceleration voltage of 50 kV. Etching was performed in a reactive ion etcher (Plasma Therm, PK-1441) and resulting pattern profiles were observed with an SEM (Hitachi, S-800). Reflectivity measurement samples, of which the entire surface was reflective, were prepared by the same methods used for the fine-pattern samples. The
reflectivity was measured using a soft x-ray reflectometer with a laser-produced plasma source at Tohoku University [9]. In the measurement the incident beam was fixed at an angle of 15° from the normal to the sample and the wavelength was scanned. 2.1. Etched muitilayer m a s k
Anisotropy and linewidth control are important for RIE of the Mo/Si multilayer. To form fine patterns with good linewidth control, we chose SiO2 for the etch mask. A 300-nm-thick negative-tone resist and a 300-nm-thick SiO2 layer were stacked on the multilayer. After e-beam lithography, RIE in CHF3 was applied to etch the SiO2 layer. The 270nm-thick multilayer was then etched in SF6 at a rf power of 100 W and a gas flow rate of 25 seem. Examination of the etch profiles showed good anisotropy at a pressure of 0.3 Pa, at which the etch selectivity (etch rate ratio) of the multilayer to SiO2 was about 2. Finally, the SiO2 layer was removed in a mixture of HF and NH4F with a 1:6 ratio. The highest temperature during the patterning process was 210 °C. Figure 2 shows an SEM photograph of a 0.25gm line-and-space pattern. Although the pattern edge is smooth, the sidewalls are slightly tapered with a lateral broadening of 0.06 gm. Since the number of layers in the multilayer varies in this region, so does the reflectivity contrast. This might degrade the resolution and linewidth accuracy when the pattern is imaged into low-contrast resist.
~./Mo/Si ~/Mo 0.25 I.u'n Figure 2. SEM photograph of the etched multilayer mask with a feature size of 0.25gin. The total thickness of the multilayer is 270 nm.
287
M. Ito et al. / Microelectronic Engineering 27 (1995) 285-290
Figure 3 shows the reflectivity of the multilayer portion of the etched-multilayer mask. The monitor multilayer deposited at the same time provides a peak reflectivity of about 60% at 12.8 nm and no significant change in reflectivity is observed after the patterning process.
80
6O
after P
before
..g,
RIE in 0 2. In this study, for the sake of process simplicity, the etch-stop layer was not removed. However, it is so thin that the transmission loss in the etch-stop layer is expected to be small. Figure 4 shows an SEM photograph of a 0.25~tm line-and-space pattern. The sidewalls are extremely steep with a lateral broadening of less than 0.02 pan. Hence, a sharp contrast can be expected at the pattern edges. As shown in Fig. 5, the reflectivity of the multilayer after the patterning process is almost identical to that of the monitor multilayer. This indicates that the multilayer was successfully protected during RIE.
._> t3 40 m n" 20 0 12.0
~,W 12.5 13.0 Wavelength (nm)
13.5
~,/Mo/Si
j/s
Figure 3. Measured reflectivity of the multilayer before and after patterning process for the etched multilayer mask. 2.2. Absorber overlayer mask As a way to prevent the multilayer from being etched, previous researchers have proposed a process in which a Ge absorber overlayer and a polyimide etch-stop layer are stacked on a multilayer, and after RIE of the absorber, the etch-stop layer is removed [4]. To put this process into a practical use, however, long-term stability of the polyimide layer, which lies beneath the absorber, should be thoroughly evaluated. We chose W as the absorber overlayer because it is highly absorbing at 13 nm and suitable for finepattern formation by RIE. In our process, SiO2 was used as the etch-stop layer instead of polyimide. A 300-nm-thick negative-tone resist, a 50-nm-thick W absorber overlayer, and a 10-nm-thick SiO2 etchstop layer were stacked on the multilayer. After resist development, RIE in SF6 was applied to etch the absorber overlayer at a rf power of 100 W, a flow rate of 25 seem, and a pressure of 2.1 Pa. In this case the etch selectivity of W to SiO2 was about 4. Thus, the etch-stop layer was resistant to 80% overetch. The resist was subsequently removed by
0.25 Ixl'n Figure 4. SEM photograph of the absorber overlayer mask with a feature size of 0.25 lim. The absorber thickness is 50 nm.
80
after
before
6O •-> 40 m n- 20 0 12
i
13 Wavelength (nm)
14
Figure 5. Measured reflectivity of the multilayer before and after patterning process for the absorber overlayer mask.
288
M. lto et al. / Microelectronic Engineering 27 (1995) 285-290
3. IMAGING EXPERIMENTS The printing capability of the reflection masks was evaluated by imaging fine patterns. A schematic diagram of the experimental setup is shown in Fig. 6. As the x-ray source we used the 2.5-GeV storage ring (Photon Factory) at the National Laboratory for High Energy Physics. Synchrotron radiation (SR) is reflected by a goldcoated plane mirror to cut off hard x-ray radiation. The beam passes through a Be filter that cuts off VUV and UV radiation. The beam then hits the mask, and the mask pattern is imaged onto a resistcoated wafer at a magnification of 20:1 using a Schwarzschild optic. The numerical aperture (NA) of the optic is 0.08 and the field size is limited to 60 × 60 tun due to off-axis aberration.
Radiation ~
.
.
.
.
.
.
.
.
.
.
Reflectionmask
.
Optical _axi_s__ ~
~~,,f
Wafer
k , > Schwarzschild optic Figure 6. Schematic diagram of the experimental setup for projection imaging.
The Schwarzschild optic has convex and concave spherical mirrors with curvature radii of 15 mm and 43 ram, respectively. To reflect 13-rim radiation at near-normal incidence, Mo/Si multilayers were deposited on the surfaces by the Ovonic Synthetic Materials Company. The figure errors (rms) were measured to be 8 nm and 6 nm over the convex and concave surfaces, respectively. The mask patterns were imaged into a 0.13-1amthick film of chemically amplified negative-tone resist AZ-PN100. The resist absorbance is estimated to be 0.6, assuming the linear absorption coefficient of 0.44 lain-l[10]. The typical exposure time was 60 s for a beam current of 300 mA. Post-exposure baking (PEB) was performed at 105 °C for 120 s followed by developing with 0.18-N MF-312 developer for 30 s. Figures 7 (a) and Co) show SEM photographs of O.l-lxm line-and-space patterns obtained using the etched multilayer and absorber overlayer masks, respectively. Both patterns are well resolved to the same extent. Figures 8 (a) and Co) show that 0.07[tm line-and-space patterns are also delineated. In this case, however, the edge roughness for the absorbing overlayer mask is slightly larger than that for the etched multilayer mask. This difference cannot necessarily be attributed to mask quality because these images were printed onto different wafers and unfortunately reproducibility was not good in our experiments.
0.2~m (a)
(b)
Figure 7. SEM photographs of 0.i-lain line-and-space resist pattern obtained using (a) the etched multilayer mask and CO)the absorber overlayer mask.
M. lto et al. /Microelectronic Engineering 27 (1995) 285-290
289
0.1 l~m
(a)
(b)
Figure 8. SEM photographs of 0.07-p.m line-and-space resist pattern obtained using (a) the etched multilayer mask and (b) the absorber overlayer mask. In our experiments, the illumination was nearly coherent since the SR source size is small and the beam simply expands toward the mask. The cut-off frequency for coherent illumination, at which the image contrast becomes zero, is NA/k = 6150 lines/mm (0.08-gm line-and-space). On the other hand, the resolved pattern is slightly finer than that corresponding to the cut-off frequency. Moreover, a weak modulation was observed even for 0.05-pro features. These results suggest that the two beams (0-order and + lst-order) diffracted by the mask pattern pass through the optic and contribute to form an image [11]. 4. CONCLUSIONS We have investigated patterning methods for an etched multilayer mask and an absorber overlayer mask for use at a 13-nm wavelength. RIE in SF6 is applied to etch a Mo/Si multilayer with a SiO2 etch mask. A W absorber overlayer was etched by RIE in SF 6 with a SiO2 etch-stop layer. Fine patterns with a minimum feature size of 0.25 gm were clearly formed by either method. In particular, pattern sidewalls of the absorber overlayer mask were extremely steep. The reflectivity measurement using large reflective-area samples indicated that neither method causes significant damage to the multilayer. The mask patterns were imaged onto a resist-coated
wafer using a 20:1 Schwarzschild optic and 0.07-pro line-and-space patterns were printed using either mask. These results support the feasibility of SXPL using reflection masks in the sub-0.1-1ma regime. ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Masaki Yamamoto and Dr. Mihiro Yanagihara at Tohoku University for their valuable discussions on multilayer reflectors. Thanks also to Mr. Tsuneo Terasawa at Central Research Laboratory, Hitachi, Ltd. for his helpful discussions on soft x-ray projection imaging. They further scknowledge Mr. Kunio Harada and Mr. Minoru Hidaka for their assistance with constructing the optic. This work has been performed under the approval of the National Laboratory for High Energy Physics (Acceptance No. 93Y003). REFERENCES 1. J. E. Bjorkholm, J. Boker, L. Eichner, R. R. Freeman, J. Gregus, T. E. Jewell, W. M. Mansfield, A. A. MacDowell, E. L. Raab, W. T. Silfvast, L. H. Szeto, D. M. Tennant, W. K. Waskiewicz, D. L. White, D. L. Windt, and O. R. Wood, II: J. Vac. Sci. Technol. B8 (1990) 1509.
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