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RET simulations for SLM-based maskless lithography
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Microelectronic Engineering 85 (2008) 929–933 www.elsevier.com/locate/mee
RET simulations for SLM-based maskless lithography XiaoWei Guo a,b,*, Jinglei Du b, Xiangang Luo c, Qiling Deng c, Chunlei Du c a
School of Optoelectronic Information, University of Electronic Science and Technology, Chengdu 610054, PR China b Physics Department, Sichuan University, Chengdu 610064, PR China c Institute of Optics and Electronics, CAS, Chengdu, 610209, PR China Received 29 September 2007; received in revised form 20 January 2008; accepted 21 January 2008 Available online 2 February 2008
Abstract In this paper, we have preformed some simulations of reticle enhancement techniques (RET) for SLM-based maskless lithography. Our results show that, on the one hand, increased slope and larger process window can be simultaneously obtained by overtilting some mirrors around the printed structure; On the other hand, fully making use of the grayscaling generated by tilting a mirror across it, a maskless tool can use the same OPC model as a mask-based scanner, such as gray level serifs, sub-resolution scatter bars, transmittance controlled mask, and so on. The results also imply that optical maskless lithography (OML) provides the ability in attaining high resolution comparable to the mask-based lithography. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Optical maskless lithography; Spatial light modulator; Reticle enhancement technique
1. Introduction Largely owing to the high cost of masks for lithography, there is a strong interest today to develop a high throughput maskless lithography. OML is an extension of a conventional photolithography and offers the potential for not only costs saving but also significant time saving, which is particularly true for ASIC devices and small volume fabrications. In the OML technology concept [1], light is used to illuminate spatial light modulators (SLM)-MEMS device-consisting of millions of pixels as a dynamic pattern generator instead of the stationary photomask. The MEMS device may utilize one of several geometrical actuation types [2– 5](e.g. tilt, phase-step, piston, etc.) to create a section of the desired circuit pattern. The pattern is imaged onto the substrate through a high demagnification image lens. RET such as ATT-PSM and OPC are restricted in their effectiveness with conventional masks because of very lim-
ited number of phase and amplitude values [6]. Use of tilt and piston mirrors allows finer control over amplitude and phase that can be gainfully used to create sophisticated images pre-compensated for complex lithographic effects [7–9]. Especially tilt micro-mirrors enable single mirror pixel rasterization in an SLM-based lithography system. Adding a phase step to the surface of the micro-mirrors extends the available grayscale addressing range to include full phase shifting [7]. This makes it possible to take advantage of the same resolution enhancement techniques in an SLM-based lithography system as used in steppers with phase shift mask technology. If coupled with some optimized algorithms [10–12] although they are evolving, OML can utilize the pixels of the SLM in the most efficient way to obtain the image with desired properties. In this paper, we simulate some examples in OML for the goal to provide an understanding of its same RET effect as the mask-based lithography. 2. Finer control of intensity and phase in OML
*
Corresponding author. Address: School of Optoelectronic Information, University of Electronic Science and Technology, Chengdu 610054, PR China. Tel.: +86 28 83204363. E-mail address: [email protected] (X. Guo). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.055
Now commercial pattern generator for high resolution, the SigmaÓ Tool by Micronic Inc., is electricity-addressed
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X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
tilt-mirrors which are actuated to cause continually bidirectional deflection [13]. So the work in this paper is based on tilt-mirror architecture. The simulations were done by a combination of matlab and a lithography program based on partial coherence imaging theory we made for aerial image forming. It is essential for the performance of the tool to be able to control phase arrangement of each mirror to generate amplitude or intensity needed. In Fig. 1a, light reflected from a mirror in the fully-‘ON’ state goes through the pupil and essentially lights up a spot as it is focused at the wafer plane. Tilting the mirror attenuates the light intensity at that spot, eventually resulting in a null-intensity at the corresponding spot on the wafer. The tilt of the mirrors is specified in terms of the optical path difference (measured in units of wavelength, or k) which is twice the displacement from the center to the edge of the mirror. It is clearly that the tilting mirror design does allow limited negative amplitude up to approximately 20% at 0.75k OPD corresponding to a negative intensity of about 5%. This is enough to achieve some feature resolution enhancement similar to attenuated phase shifting in steppers which is useful for the improvement of the log slope of a line edge. Fig. 1b shows an edge is formed between a group of ‘on’ mirrors next to a group of ‘off ’ mirrors. This edge can be moved one physical address grid by simply turning on the next row of mirrors. The edge can be moved a smaller amount or a subgrid, however, by tilting a row of mirrors. The subgrid does not reduce the minimum feature size that is decided by the system but allows the placement of these features anywhere on the subgrid. The subgrids in a given imaging system can be used as gray level serifs [14,15], subresolution scatter bars [16], and transmittance controlled mask [17] as in the traditional OPC mask, etc. TM
3. RET simulations In simulation, the wavelength used is 193 nm, the numerical aperture is 0.7 and the partial coherence factor is 0.6. The spot size in wafer plane (or the pixel size in image plane) is 55 nm. By correctly reversing the direction of mirror modulation down a feature, the mirrors balanced phase locally to eliminate defocus-related image shift and improve the process window [10,18]. Thus checkerboard design is adopted for the arrangement of mirror array. In addition, 0.3 is adopted as the light intensity threshold of image aerial. 3.1. ‘ATT-PSM’ simulations An isolated line with 100 nm width is taken for example. The line with adjacent mirrors fully off at 0.5k OPD is our nominal tilt case. If all the ‘off ’ mirrors are overtilted to 0.75k OPD, we get undesired uniform background intensity as expected. However, keeping only few nearest to edge ‘off’ mirrors overtilted gives us the dual advantage of increased slope and larger process window. Fig. 2 a presents the cross section of aerial image under three conditions above. It must be noticed that the ‘overtilted’ method somewhat increases the light intensity of background. Fig. 2b shows the EL-DOF curves with nominal tilting and overtilting mirrors. The CD deviation tolerance is set to 10%. It is found that patterns generated with dark pixels overtilted clearly provide a larger process window. This means it has some ‘ATT-PSM’ effects. As said in the introduction, the mirror can obtain arbitrary phase-shifting ability if the tilt-mirror architecture is modified into phase-step type.
Fig. 1. (a) Upper: Cross-section of a tilted mirror of sizes; Bottom: Intensity and amplitude at the wafer plane as a function of OPD. (b) The edge is moved a smaller amount or subgrid by tilting a row of mirrors.
X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
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Fig. 2. (a) The cross section of the aerial images with nominal tilt, all the ‘off ’ mirrors overtilted and part of the ‘off’ mirrors overtilted; (b) EL-DOF process window under the former two conditions.
Fig. 3. (a) Subgrids as gray level serifs at four corners; (b, c) are the contour plots of aerial image before and after correction, separately.
3.2. OPC simulations 3.2.1. Subgrids as gray level serifs Fig. 3 gives out the simulation results of a two-spot wide isolated line in which 50% subgrids as gray level ser-
ifs are added at the four corners. The phase arrangement of each mirror is given in Fig. 3a. For clarification, the added subgrids are presented in a given color(the below is alike). The contour plots of aerial image before and after correction are shown in Fig. 3b and c, separately.
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X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
Fig. 4. (a) Phase arrangement for printing a ‘‘T” gate structure and (b) is the aerial image; (c) is the phase arrangement with 37.5% subgrids as transmittance controlled mask and gray level serifs, and (d) is the imaging result; (e) is the phase arrangement with 12.5% subgrids as sub-resolution scatter bars except for transmittance controlled mask and gray level serifs, and (f) is the imaging result.
Compare Fig. 3c with b, it is easily observed that the lineend shortening and corner distortion are compensated to a
high extent, and the intensity uniformity of aerial image is also improved.
X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
3.2.2. Subgrids as transmittance controlled mask and subresolution scatter bars Fig. 4. shows an OPC example for a ‘T’ gate structure in SLM-based maskless lithography. The printed effect of the structure usually results in two inner corners’ protuberance except for line-end shortening and outside corner distortion (see Figs. 4a and b). In order to obtain a perfect result, the similar pattern can be broken into two subsections for lithography purpose [11]. Most popular way is finer control of the deflection position of each mirror to generate grayscaling so as to control the flux into pupil, which is similar to the transmittance controlled mask, as shown in Fig. 4c in which 37.5% subgrids are used. Fig. 4d gives out the corresponding OPC result (note: the 0.9 contour line is not visibility). As above, the line-end shortening and corner distortion are greatly mended. In order to further increase the image intensity and uniformity of the vertical line, subgrids below the resolution limit of the imaging system as sub-resolution scatter bars can be used around the line. Fig. 4e and f present the related phase arrangement and imaging result (12.5% grids are used). Obviously, the intensity is superior to that without scatter bars. 4. Conclusion Because of programmable operation of mirror array, it is very convenience to control over amplitude and phase in a SLM-based maskless lithography so that RETs such as ATT-PSM and OPC can be easily realized compared with the tradition mask. The RET examples given in this paper preliminarily show a maskless tool can use the same OPC model as a mask-based scanner, including phaseshifting. Our OPC results are not optimal, it is necessary to develop more elaborate rasterization algorithm which is our further work.
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Acknowledgments This work was supported by 973 Program of China (No. 2006CD302900-2), National Natural Science Foundation of China (No. 60376021, No. 60676024), and the Specialized Research Fund of China for the Doctoral Program of Higher Education (No. 20060610006). References [1] Elizabeth M. stone, Jason D. Hintersteiner, Wenceslao A. Cebuhar, et al., Proc. SPIE 6515 (2006) 651512E-1. [2] U. Ljungblad, Solid State Technol. 48 (2005) 8. [3] Dauderstadt, Ulrike, Durr, Peter, Karlin, Tord, et al., Proc. SPIE 5348 (2004) 119. [4] H. Lakner, P. Durr, U. Dauderstaedt, et al., Proc. SPIE 4561 (2001) 255. [5] Yijian Chen, Chi Hui Chu, Y. Shroff, et al., Proc. SPIE 5751 (2005) 1023. [6] Y.C. Pati, T. Kailath, J. Opt. Soc. Am. A 11 (1994) 2438. [7] Y. Shroff, Y. Chen, W. Oldham, Proc. SPIE 5037 (2003) 550. [8] Tor Sandstrom, Ulric Ljungblad, Proc. SPIE 5567 (2004) 529. [9] Ulric Ljungblad, Hans Martinsson, Torbjo¨rn Sandstro¨m, Microelectron. Eng. 78–79 (2005) 398. [10] Yashesh A. Shroffu, Yijian Chen, William G. Oldham, Proc. SPIE 5374 (2004) 637. [11] Tor Sandstrom, Hans Martinson, Proc. SPIE 5377 (2004) 1570. [12] Hans Martinsson, Tor Sandstrom, Proc. SPIE 5567 (2004) 557. [13] T. Sandstrom, T. Fillion, U. Ljungblad, M. Rosling, Proc. SPIE 4409 (2001) 270. [14] Chris A. Mack, Patricia M. Kaufman, J. Vac. Sci. Technol., B 6 (1988) 2206. [15] Jinglei Du, Qizhong Huang, Yongkang Guo, et al., Proc. SPIE 3334 (1998) 932. [16] J. Fung Chen, Tom Laidig, Kurt E. Wampler, et al., J. Vac. Sci. Technol., B 15 (1997) 2426. [17] W.S. Han, C.J. Sohn, Y.B. Kim, et al., Proc. SPIE 2440 (1995) 494. [18] Ebo Croffie1a, Nick Eiba, Neal Callana, Proc. SPIE 5256 (2003) 842.
Microelectronic Engineering 85 (2008) 929–933 www.elsevier.com/locate/mee
RET simulations for SLM-based maskless lithography XiaoWei Guo a,b,*, Jinglei Du b, Xiangang Luo c, Qiling Deng c, Chunlei Du c a
School of Optoelectronic Information, University of Electronic Science and Technology, Chengdu 610054, PR China b Physics Department, Sichuan University, Chengdu 610064, PR China c Institute of Optics and Electronics, CAS, Chengdu, 610209, PR China Received 29 September 2007; received in revised form 20 January 2008; accepted 21 January 2008 Available online 2 February 2008
Abstract In this paper, we have preformed some simulations of reticle enhancement techniques (RET) for SLM-based maskless lithography. Our results show that, on the one hand, increased slope and larger process window can be simultaneously obtained by overtilting some mirrors around the printed structure; On the other hand, fully making use of the grayscaling generated by tilting a mirror across it, a maskless tool can use the same OPC model as a mask-based scanner, such as gray level serifs, sub-resolution scatter bars, transmittance controlled mask, and so on. The results also imply that optical maskless lithography (OML) provides the ability in attaining high resolution comparable to the mask-based lithography. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Optical maskless lithography; Spatial light modulator; Reticle enhancement technique
1. Introduction Largely owing to the high cost of masks for lithography, there is a strong interest today to develop a high throughput maskless lithography. OML is an extension of a conventional photolithography and offers the potential for not only costs saving but also significant time saving, which is particularly true for ASIC devices and small volume fabrications. In the OML technology concept [1], light is used to illuminate spatial light modulators (SLM)-MEMS device-consisting of millions of pixels as a dynamic pattern generator instead of the stationary photomask. The MEMS device may utilize one of several geometrical actuation types [2– 5](e.g. tilt, phase-step, piston, etc.) to create a section of the desired circuit pattern. The pattern is imaged onto the substrate through a high demagnification image lens. RET such as ATT-PSM and OPC are restricted in their effectiveness with conventional masks because of very lim-
ited number of phase and amplitude values [6]. Use of tilt and piston mirrors allows finer control over amplitude and phase that can be gainfully used to create sophisticated images pre-compensated for complex lithographic effects [7–9]. Especially tilt micro-mirrors enable single mirror pixel rasterization in an SLM-based lithography system. Adding a phase step to the surface of the micro-mirrors extends the available grayscale addressing range to include full phase shifting [7]. This makes it possible to take advantage of the same resolution enhancement techniques in an SLM-based lithography system as used in steppers with phase shift mask technology. If coupled with some optimized algorithms [10–12] although they are evolving, OML can utilize the pixels of the SLM in the most efficient way to obtain the image with desired properties. In this paper, we simulate some examples in OML for the goal to provide an understanding of its same RET effect as the mask-based lithography. 2. Finer control of intensity and phase in OML
*
Corresponding author. Address: School of Optoelectronic Information, University of Electronic Science and Technology, Chengdu 610054, PR China. Tel.: +86 28 83204363. E-mail address: [email protected] (X. Guo). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.055
Now commercial pattern generator for high resolution, the SigmaÓ Tool by Micronic Inc., is electricity-addressed
930
X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
tilt-mirrors which are actuated to cause continually bidirectional deflection [13]. So the work in this paper is based on tilt-mirror architecture. The simulations were done by a combination of matlab and a lithography program based on partial coherence imaging theory we made for aerial image forming. It is essential for the performance of the tool to be able to control phase arrangement of each mirror to generate amplitude or intensity needed. In Fig. 1a, light reflected from a mirror in the fully-‘ON’ state goes through the pupil and essentially lights up a spot as it is focused at the wafer plane. Tilting the mirror attenuates the light intensity at that spot, eventually resulting in a null-intensity at the corresponding spot on the wafer. The tilt of the mirrors is specified in terms of the optical path difference (measured in units of wavelength, or k) which is twice the displacement from the center to the edge of the mirror. It is clearly that the tilting mirror design does allow limited negative amplitude up to approximately 20% at 0.75k OPD corresponding to a negative intensity of about 5%. This is enough to achieve some feature resolution enhancement similar to attenuated phase shifting in steppers which is useful for the improvement of the log slope of a line edge. Fig. 1b shows an edge is formed between a group of ‘on’ mirrors next to a group of ‘off ’ mirrors. This edge can be moved one physical address grid by simply turning on the next row of mirrors. The edge can be moved a smaller amount or a subgrid, however, by tilting a row of mirrors. The subgrid does not reduce the minimum feature size that is decided by the system but allows the placement of these features anywhere on the subgrid. The subgrids in a given imaging system can be used as gray level serifs [14,15], subresolution scatter bars [16], and transmittance controlled mask [17] as in the traditional OPC mask, etc. TM
3. RET simulations In simulation, the wavelength used is 193 nm, the numerical aperture is 0.7 and the partial coherence factor is 0.6. The spot size in wafer plane (or the pixel size in image plane) is 55 nm. By correctly reversing the direction of mirror modulation down a feature, the mirrors balanced phase locally to eliminate defocus-related image shift and improve the process window [10,18]. Thus checkerboard design is adopted for the arrangement of mirror array. In addition, 0.3 is adopted as the light intensity threshold of image aerial. 3.1. ‘ATT-PSM’ simulations An isolated line with 100 nm width is taken for example. The line with adjacent mirrors fully off at 0.5k OPD is our nominal tilt case. If all the ‘off ’ mirrors are overtilted to 0.75k OPD, we get undesired uniform background intensity as expected. However, keeping only few nearest to edge ‘off’ mirrors overtilted gives us the dual advantage of increased slope and larger process window. Fig. 2 a presents the cross section of aerial image under three conditions above. It must be noticed that the ‘overtilted’ method somewhat increases the light intensity of background. Fig. 2b shows the EL-DOF curves with nominal tilting and overtilting mirrors. The CD deviation tolerance is set to 10%. It is found that patterns generated with dark pixels overtilted clearly provide a larger process window. This means it has some ‘ATT-PSM’ effects. As said in the introduction, the mirror can obtain arbitrary phase-shifting ability if the tilt-mirror architecture is modified into phase-step type.
Fig. 1. (a) Upper: Cross-section of a tilted mirror of sizes; Bottom: Intensity and amplitude at the wafer plane as a function of OPD. (b) The edge is moved a smaller amount or subgrid by tilting a row of mirrors.
X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
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Fig. 2. (a) The cross section of the aerial images with nominal tilt, all the ‘off ’ mirrors overtilted and part of the ‘off’ mirrors overtilted; (b) EL-DOF process window under the former two conditions.
Fig. 3. (a) Subgrids as gray level serifs at four corners; (b, c) are the contour plots of aerial image before and after correction, separately.
3.2. OPC simulations 3.2.1. Subgrids as gray level serifs Fig. 3 gives out the simulation results of a two-spot wide isolated line in which 50% subgrids as gray level ser-
ifs are added at the four corners. The phase arrangement of each mirror is given in Fig. 3a. For clarification, the added subgrids are presented in a given color(the below is alike). The contour plots of aerial image before and after correction are shown in Fig. 3b and c, separately.
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X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
Fig. 4. (a) Phase arrangement for printing a ‘‘T” gate structure and (b) is the aerial image; (c) is the phase arrangement with 37.5% subgrids as transmittance controlled mask and gray level serifs, and (d) is the imaging result; (e) is the phase arrangement with 12.5% subgrids as sub-resolution scatter bars except for transmittance controlled mask and gray level serifs, and (f) is the imaging result.
Compare Fig. 3c with b, it is easily observed that the lineend shortening and corner distortion are compensated to a
high extent, and the intensity uniformity of aerial image is also improved.
X. Guo et al. / Microelectronic Engineering 85 (2008) 929–933
3.2.2. Subgrids as transmittance controlled mask and subresolution scatter bars Fig. 4. shows an OPC example for a ‘T’ gate structure in SLM-based maskless lithography. The printed effect of the structure usually results in two inner corners’ protuberance except for line-end shortening and outside corner distortion (see Figs. 4a and b). In order to obtain a perfect result, the similar pattern can be broken into two subsections for lithography purpose [11]. Most popular way is finer control of the deflection position of each mirror to generate grayscaling so as to control the flux into pupil, which is similar to the transmittance controlled mask, as shown in Fig. 4c in which 37.5% subgrids are used. Fig. 4d gives out the corresponding OPC result (note: the 0.9 contour line is not visibility). As above, the line-end shortening and corner distortion are greatly mended. In order to further increase the image intensity and uniformity of the vertical line, subgrids below the resolution limit of the imaging system as sub-resolution scatter bars can be used around the line. Fig. 4e and f present the related phase arrangement and imaging result (12.5% grids are used). Obviously, the intensity is superior to that without scatter bars. 4. Conclusion Because of programmable operation of mirror array, it is very convenience to control over amplitude and phase in a SLM-based maskless lithography so that RETs such as ATT-PSM and OPC can be easily realized compared with the tradition mask. The RET examples given in this paper preliminarily show a maskless tool can use the same OPC model as a mask-based scanner, including phaseshifting. Our OPC results are not optimal, it is necessary to develop more elaborate rasterization algorithm which is our further work.
933
Acknowledgments This work was supported by 973 Program of China (No. 2006CD302900-2), National Natural Science Foundation of China (No. 60376021, No. 60676024), and the Specialized Research Fund of China for the Doctoral Program of Higher Education (No. 20060610006). References [1] Elizabeth M. stone, Jason D. Hintersteiner, Wenceslao A. Cebuhar, et al., Proc. SPIE 6515 (2006) 651512E-1. [2] U. Ljungblad, Solid State Technol. 48 (2005) 8. [3] Dauderstadt, Ulrike, Durr, Peter, Karlin, Tord, et al., Proc. SPIE 5348 (2004) 119. [4] H. Lakner, P. Durr, U. Dauderstaedt, et al., Proc. SPIE 4561 (2001) 255. [5] Yijian Chen, Chi Hui Chu, Y. Shroff, et al., Proc. SPIE 5751 (2005) 1023. [6] Y.C. Pati, T. Kailath, J. Opt. Soc. Am. A 11 (1994) 2438. [7] Y. Shroff, Y. Chen, W. Oldham, Proc. SPIE 5037 (2003) 550. [8] Tor Sandstrom, Ulric Ljungblad, Proc. SPIE 5567 (2004) 529. [9] Ulric Ljungblad, Hans Martinsson, Torbjo¨rn Sandstro¨m, Microelectron. Eng. 78–79 (2005) 398. [10] Yashesh A. Shroffu, Yijian Chen, William G. Oldham, Proc. SPIE 5374 (2004) 637. [11] Tor Sandstrom, Hans Martinson, Proc. SPIE 5377 (2004) 1570. [12] Hans Martinsson, Tor Sandstrom, Proc. SPIE 5567 (2004) 557. [13] T. Sandstrom, T. Fillion, U. Ljungblad, M. Rosling, Proc. SPIE 4409 (2001) 270. [14] Chris A. Mack, Patricia M. Kaufman, J. Vac. Sci. Technol., B 6 (1988) 2206. [15] Jinglei Du, Qizhong Huang, Yongkang Guo, et al., Proc. SPIE 3334 (1998) 932. [16] J. Fung Chen, Tom Laidig, Kurt E. Wampler, et al., J. Vac. Sci. Technol., B 15 (1997) 2426. [17] W.S. Han, C.J. Sohn, Y.B. Kim, et al., Proc. SPIE 2440 (1995) 494. [18] Ebo Croffie1a, Nick Eiba, Neal Callana, Proc. SPIE 5256 (2003) 842.