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Analysis of the line pattern width and exposure efficiency in maskless lithography using a digital micromirror device
Microelectronic Engineering 88 (2011) 3145–3149
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Analysis of the line pattern width and exposure efficiency in maskless lithography using a digital micromirror device Hoonchul Ryoo, Dong Won Kang, Jae W. Hahn ⇑ Nano Photonics Laboratory, School of Mechanical Engineering, Yonsei University, 134, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 December 2010 Received in revised form 25 May 2011 Accepted 21 June 2011 Available online 30 June 2011 Keywords: Maskless lithography DMD (digital micromirror device) Point array method Line/space pattern Spot overlap Exposure efficiency
a b s t r a c t In this study, we calculate the exposure intensity of line/space patterns recorded with a DMD (digital micromirror device) digital maskless lithography system using the point array method. With a diffracted beam spot with a radius of 4 lm, we simulate the line space patterns over a spot overlap section ranging from 85% to 95%. From the results of the simulation, we analyze the relationships among the exposure intensity, the width of the line pattern, and the exposure efficiency, which are process parameters used in maskless lithography. From a numerical analysis of the relationship between the line pattern width and the exposure efficiency, it is estimated that the practical acceptable minimum width of the line pattern recorded with a maskless lithography system using a 4-lm radius diffracted beam spot array is approximately 4.5 lm, which is 11% larger than the spot radius with the exposure efficiency of 73%. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction As the fabrication and the maintenance costs of photomasks continue to escalate, rapid progress has been made in the area of maskless lithography over the past decade due to the strong motivation to reduce production costs through the use of digital photomasks that use TFT (thin film transistor)-LCD and DMDs (digital micromirror devices) [1–5]. However, there are distinct advantages and disadvantages associated with the use of these new methods, in terms of cost efficiency, digital transferability, and throughput. Nonetheless, DMD-based optical maskless lithography is receiving considerable attention in applications to flat-panel-displays (FPDs) and in the printed circuit board (PCB) industry, both of which require a mass production system [6]. In DMD-based optical maskless lithography, the elements of a deformable micromirror device selectively reflect the incident light to act as a programmable mask. The DMD pixel images are focused into tiny dots on the substrate by a microlens array (MLA), a spatial filter array (SFA), and a projection lens system while the stage is translating along the scanning direction. To expose a desired pattern with inherently disconnected spots, a point array method has been developed which requires the DMD to be rotated at a small angle in the scanning direction so that the overlapping of the spots can create lines and other patterns [2,7–11].
In this scanner-type exposure method, the accumulated exposure intensity due to spot overlapping has a great influence on the pattern resolution and certain aspects of the integrity of the line pattern, such as the roughness. In particularly, pattern resolution is critical in the determination of the performance of a lithography system, and it is determined by the spot size. Experimental results of previous studies have shown discrepancies between the spot size and the minimum pattern size. Chan, Feng, Yang, Ishikawa, and Mei performed exposure simulations and experiments to evaluate a DMD optical maskless lithography system using a spot size of 1.0 lm. From the simulation results, they found that a line/space (L/S) pattern with a half period of 1.0 lm may not be achievable owing to the reduced overlaying intensity. In their experiments, they recorded L/S patterns with half periods of 1.5, 1.8 and 2.0 lm, showing sparsely located patterns in the case of 1.5 lm and clear patterns in the other cases [2]. Simulating L/S patterns recorded by a maskless lithography system using a point array method, we obtained the relationship of the line width of the pattern and the exposure intensity accumulated by the overlapping of spot beams. We also analyzed the major process parameters of the maskless lithography system, such as the practical acceptable value of the pattern resolution and the exposure efficiency. 2. Simulation of L/S patterns
⇑ Corresponding author. E-mail address: [email protected] (J.W. Hahn). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.06.016
Following the point array method with a scanner-type exposure technique, we simulated L/S patterns under a given spot size while varying the spot overlap.
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H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
creates a magnified image of spatially modulated light by the DMD on first image plane to fit the size of the microlens array (MLA). The first image acts as the second illuminator for the MLA and the spatial filter array (SFA). The MLA and the SFA eliminate stray light and enlarge the exposure field without decreasing the resolution. Beams from the spatial filter array behave as a second object, which is an array of point sources. The point sources are projected with demagnification on the substrate by projection lens 2 (the second image plane). Fig. 2(a) depicts the operating principle of the point array method. When we rotate the DMD at a certain small angle of a few degrees, we can arrange the spots in the same lines both normal and parallel to the scanning direction. The amounts of spot overlap in the scanning direction and its normal direction are determined by the stage speed and the rotation angle of the DMD, respectively. The spot overlap in the scanning direction can be simply written as
Osp;scan ¼
1
Vs 100; F r dsp
ð1Þ
where Osp,scan is the spot overlap in the scanning direction, Fr is the frame rate of the DMD, dsp is the spot diameter, and Vs is the stage speed. The rotation angle of the DMD also satisfies the equation, Fig. 1. Schematic description of the DMD based optical lithography system. M1 is mirror 1, MLA is a microlens array, and SFA is a spatial filter array. Projection lens 1 is used for magnification of modulated light by DMD and projection lens 2 is used for demagnification of the point source from the spatial filter array.
A conceptual design of the maskless lithography system used for the simulation is shown schematically in Fig. 1. In the DMDbased optical maskless lithography system, the light source is generally a high-pressure mercury lamp or a diode laser. The beam from the light source is homogenized using a fly’s eye lens and is positioned as incident on the DMD using a condenser lens. Mirror, or total internal reflection prism is used to create an incident angle of DMD 24°. With a DMD tilt angle of ±12°, the DMD will reflect the illumination beam in the normal direction. Projection lens 1
x ¼ d sin h;
ð2Þ
where x denotes the horizontal spot separation, d is the pitch size of the point array, and h is the rotation angle of the DMD [2]. From Eq. (2), horizontal spot separation can be induced, and the spot overlap in the normal direction of the scan can be easily estimated as
x 100; Osp;norm ¼ 1 dsp
ð3Þ
where Osp,norm is the spot overlap in the normal to the scanning direction. For the simulation, we assume that the spot is a 4-lm radius airy disk which is a diffraction pattern of a circular aperture, and the peak intensity value is normalized as 1, as depicted in Fig. 2(b). The spot overlap in the scanning direction is given in
Fig. 2. (a) Pattern generation fundamentals of point array method. h is the rotation angle of DMD, x is horizontal spot separation, d is the pitch size of DMD, and dsp is the spot diameter. (b) Spot definition on the substrate (airy disk), and the spot radius is 4 lm.
H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
the range from 85% to 95% in steps of 2% adjusted by translation speed of the stage. The amount of overlap in the normal direction of the scan is given as 95%, which is determined by the rotation angle of the DMD (2.86°). The DMD used in the calculation is made by Texas Instruments. It has a pitch size of 13.68 lm, and the frame rate of the DMD is fixed at 16,000 frames per second in all cases. The calculations were carried out using the mathematical software MATLABÒ by MathWorks™, and the grid size in the calculation is given as 20 nm.
width, as the spot overlap increases, the peak exposure intensity grows exponentially. Contrary to the mask-based projection type lithography system, the results of the simulations shown in Fig. 4 clearly indicate that the exposure intensity of a line pattern recorded by the maskless lithography system strongly depends on the width of the line pattern. This tendency results from the accumulation of the exposure intensity by the scanning of the twodimensional spot array. For a quantitative analysis of the variation of the peak exposure intensity with the width of the line pattern, we define a new parameter of exposure efficiency simply as
3. Result and discussion The calculated L/S patterns with periods of 6 and 8 lm are plotted in Fig. 3(a), while the cross-section profiles of exposure intensities are shown in Fig. 3(b). We determined the line width with the full width of half maximum (FWHM) of the exposure intensity profile. In both cases, elevation of the baseline is observed because the exposure intensity accumulated due to the two-dimensional overlapping of the spots in the parallel and normal directions of the scanning. For various spot overlaps in the range from 85% to 95%, the peak exposure intensity variation with respect to the line width was analyzed with the simulated patterns as shown in Fig. 4. As the line width of the pattern increases, the peak exposure intensity rapidly increases to the section adjacent to the spot size; soon after the region, the increasing rate gradually decreases, showing a step-like change. We also found that the peak exposure intensity reaches a limited value of saturation intensity, as shown in Fig. 4. In a previous work, it was reported that under a fixed line
3147
gexp ¼
Iexp 100; Isat
ð4Þ
where gexp is the exposure efficiency, Iexp is the peak exposure intensity, and Isat is the saturation intensity introduced in Fig. 4. Here we note that the exposure efficiency is an important parameter to be considered in the patterning process with the maskless lithography system using the point array method. The variations of the line width with the spot overlap for various exposure efficiencies are depicted in Fig. 5. Although the exposure efficiency strongly depends on the width of the line pattern, the variation of the spot overlap has little effect on the line width given a fixed level of exposure efficiency. At exposure efficiencies of 100%, and 80%, the widths of the line patterns are 17.6 and 7.8 lm with standard deviations of 0.18 and 0.78, respectively. From these results, the width of the line pattern can be averaged over the spot overlap for a given exposure efficiency.
Fig. 3. Typical simulated result of L/S patterns. (a) L/S = 6 lm, (b) L/S = 8 lm. In both cases, the spot overlap is 95%, which yields the stage speed of 3.2 mm/s in the scanning direction, and the scan step is 0.2 lm.
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Fig. 4. Peak exposure intensity variation with respect to the width of the line pattern in relation to the change of the spot overlap.
Fig. 5. Width of the line pattern variation with respect to the spot overlap in the relation to the exposure efficiency.
To find the correlation between the exposure efficiency and the width of the line pattern, we plot the exposure efficiency as a function of the averaged width of the line pattern. This is shown in Fig. 6. While the averaged width of the line pattern increases until about 4.5 lm, the exposure efficiency rapidly increases. Beyond the average width of 5 lm, the exposure efficiency gradually increases at a much smaller increasing rate. As shown in Fig. 6, we plot two linear regression line fits of the first three data points and the last three data points. By extending the line fits, we find a crossing point at the exposure efficiency of 73% and an average width of 4.5 lm. As shown in this graph, the exposure efficiency and the average width have a trade-off relationship, i.e., to obtain the possible minimum width of the line pattern, the loss of the exposure efficiency needs to be considered. By analyzing the correlation between the exposure efficiency and the width of the line pattern, we theoretically estimate the practical acceptable tradeoff value of the minimum width of the line pattern to be 4.5 lm, which is 11% larger than the spot radius of diffracted light with an exposure efficiency of about 73%.
4. Conclusion We calculated the exposure intensity of line/space patterns recorded with a DMD digital maskless lithography system using the point array method. Using the simulation results of the L/S patterns recorded with the maskless lithography system using the point array method, we analyzed the relationships among the width of the line patterns, the peak exposure intensity, and the exposure efficiency. We found that the exposure efficiency, which is a key parameter in the operation of a maskless lithography system, strongly depends on the width of the line pattern. We also estimated that the practical acceptable trade-off value of the minimum width of the line pattern is approximately 11% larger than the spot radius of diffracted light with an exposure efficiency of about 73%. The result of this study is expected to be used as an effective analytic tool for the determination of the design parameters of a maskless lithography system using a DMD. These parameters can include the pattern resolution, the scanning speed and the power of the light source. It will also be useful in the management of the patterning process when using a maskless lithography system.
H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
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Fig. 6. Exposure efficiency variation with respect to the average width of the line pattern. The dashed dotted lines are linear fits of the two increasing region of exposure efficiency, and the resulting crossing point is 4.54 lm of average width of the line pattern and 73.3% of exposure efficiency.
Further work will cover the experimental verification of the simulation results as regards L/S patterning and evaluations of the exposure efficiency. Acknowledgment This work was supported by the Ministry of Knowledge Economy of Korea (Project No. 10031688). References [1] D. Smith, D. Klenk, Pros. SPIE 7210 (2009) 72100K. [2] K.F. Chan, Z. Feng, R. Yang, A. Ishikawa, W. Mei, J. Microlith. Microfab. Microsyst. 2 (4) (2003) 331–339.
[3] T.H.P. Chang, Marian Mankos, Kim Y. Lee, Larry P. Muray, Microelectron. Eng. 57–58 (2001) 117–135. [4] P. Vettiger, M. Despont, U. Drechsler, U. Durig, W. Haberie, M. Lutwyche, H. Rothuizen, R. Stutz, R. Widmer, G. Binnig, IBM J. Res. Develop. 44 (2000) 323– 340. [5] T. Wang, M. Quaglio, F. Pirri, Y. Cheng, D. Busacker, F. Cerrina, Pros. SPIE 7274 (2009) 727420. [6] K. Kim, J. Yi, S. Cho, N. Kang, M. Cho, B. Shin, B. Choi, Appl. Surf. Sci. 255 (2009) 7835–7840. [7] W. Mei, T. Kanatake, K. Powell, U.S. Patent No. 6425,669 B1, 2002. [8] W. Mei, U.S. Patent No. 6473,237 B2, 2002. [9] W. Mei, T. Kanatake, A. Ishikawa, U.S. Patent No. 6379,867 B1, 2002. [10] H. Ryoo, D.W. Kang, J.W. Hahn, Microelectron. Eng. 88 (2011) 235–239. [11] O.D. Negrete, F. Cerrina, Microelectron. Eng. 85 (2008) 834–837.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Analysis of the line pattern width and exposure efficiency in maskless lithography using a digital micromirror device Hoonchul Ryoo, Dong Won Kang, Jae W. Hahn ⇑ Nano Photonics Laboratory, School of Mechanical Engineering, Yonsei University, 134, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 December 2010 Received in revised form 25 May 2011 Accepted 21 June 2011 Available online 30 June 2011 Keywords: Maskless lithography DMD (digital micromirror device) Point array method Line/space pattern Spot overlap Exposure efficiency
a b s t r a c t In this study, we calculate the exposure intensity of line/space patterns recorded with a DMD (digital micromirror device) digital maskless lithography system using the point array method. With a diffracted beam spot with a radius of 4 lm, we simulate the line space patterns over a spot overlap section ranging from 85% to 95%. From the results of the simulation, we analyze the relationships among the exposure intensity, the width of the line pattern, and the exposure efficiency, which are process parameters used in maskless lithography. From a numerical analysis of the relationship between the line pattern width and the exposure efficiency, it is estimated that the practical acceptable minimum width of the line pattern recorded with a maskless lithography system using a 4-lm radius diffracted beam spot array is approximately 4.5 lm, which is 11% larger than the spot radius with the exposure efficiency of 73%. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction As the fabrication and the maintenance costs of photomasks continue to escalate, rapid progress has been made in the area of maskless lithography over the past decade due to the strong motivation to reduce production costs through the use of digital photomasks that use TFT (thin film transistor)-LCD and DMDs (digital micromirror devices) [1–5]. However, there are distinct advantages and disadvantages associated with the use of these new methods, in terms of cost efficiency, digital transferability, and throughput. Nonetheless, DMD-based optical maskless lithography is receiving considerable attention in applications to flat-panel-displays (FPDs) and in the printed circuit board (PCB) industry, both of which require a mass production system [6]. In DMD-based optical maskless lithography, the elements of a deformable micromirror device selectively reflect the incident light to act as a programmable mask. The DMD pixel images are focused into tiny dots on the substrate by a microlens array (MLA), a spatial filter array (SFA), and a projection lens system while the stage is translating along the scanning direction. To expose a desired pattern with inherently disconnected spots, a point array method has been developed which requires the DMD to be rotated at a small angle in the scanning direction so that the overlapping of the spots can create lines and other patterns [2,7–11].
In this scanner-type exposure method, the accumulated exposure intensity due to spot overlapping has a great influence on the pattern resolution and certain aspects of the integrity of the line pattern, such as the roughness. In particularly, pattern resolution is critical in the determination of the performance of a lithography system, and it is determined by the spot size. Experimental results of previous studies have shown discrepancies between the spot size and the minimum pattern size. Chan, Feng, Yang, Ishikawa, and Mei performed exposure simulations and experiments to evaluate a DMD optical maskless lithography system using a spot size of 1.0 lm. From the simulation results, they found that a line/space (L/S) pattern with a half period of 1.0 lm may not be achievable owing to the reduced overlaying intensity. In their experiments, they recorded L/S patterns with half periods of 1.5, 1.8 and 2.0 lm, showing sparsely located patterns in the case of 1.5 lm and clear patterns in the other cases [2]. Simulating L/S patterns recorded by a maskless lithography system using a point array method, we obtained the relationship of the line width of the pattern and the exposure intensity accumulated by the overlapping of spot beams. We also analyzed the major process parameters of the maskless lithography system, such as the practical acceptable value of the pattern resolution and the exposure efficiency. 2. Simulation of L/S patterns
⇑ Corresponding author. E-mail address: [email protected] (J.W. Hahn). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.06.016
Following the point array method with a scanner-type exposure technique, we simulated L/S patterns under a given spot size while varying the spot overlap.
3146
H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
creates a magnified image of spatially modulated light by the DMD on first image plane to fit the size of the microlens array (MLA). The first image acts as the second illuminator for the MLA and the spatial filter array (SFA). The MLA and the SFA eliminate stray light and enlarge the exposure field without decreasing the resolution. Beams from the spatial filter array behave as a second object, which is an array of point sources. The point sources are projected with demagnification on the substrate by projection lens 2 (the second image plane). Fig. 2(a) depicts the operating principle of the point array method. When we rotate the DMD at a certain small angle of a few degrees, we can arrange the spots in the same lines both normal and parallel to the scanning direction. The amounts of spot overlap in the scanning direction and its normal direction are determined by the stage speed and the rotation angle of the DMD, respectively. The spot overlap in the scanning direction can be simply written as
Osp;scan ¼
1
Vs 100; F r dsp
ð1Þ
where Osp,scan is the spot overlap in the scanning direction, Fr is the frame rate of the DMD, dsp is the spot diameter, and Vs is the stage speed. The rotation angle of the DMD also satisfies the equation, Fig. 1. Schematic description of the DMD based optical lithography system. M1 is mirror 1, MLA is a microlens array, and SFA is a spatial filter array. Projection lens 1 is used for magnification of modulated light by DMD and projection lens 2 is used for demagnification of the point source from the spatial filter array.
A conceptual design of the maskless lithography system used for the simulation is shown schematically in Fig. 1. In the DMDbased optical maskless lithography system, the light source is generally a high-pressure mercury lamp or a diode laser. The beam from the light source is homogenized using a fly’s eye lens and is positioned as incident on the DMD using a condenser lens. Mirror, or total internal reflection prism is used to create an incident angle of DMD 24°. With a DMD tilt angle of ±12°, the DMD will reflect the illumination beam in the normal direction. Projection lens 1
x ¼ d sin h;
ð2Þ
where x denotes the horizontal spot separation, d is the pitch size of the point array, and h is the rotation angle of the DMD [2]. From Eq. (2), horizontal spot separation can be induced, and the spot overlap in the normal direction of the scan can be easily estimated as
x 100; Osp;norm ¼ 1 dsp
ð3Þ
where Osp,norm is the spot overlap in the normal to the scanning direction. For the simulation, we assume that the spot is a 4-lm radius airy disk which is a diffraction pattern of a circular aperture, and the peak intensity value is normalized as 1, as depicted in Fig. 2(b). The spot overlap in the scanning direction is given in
Fig. 2. (a) Pattern generation fundamentals of point array method. h is the rotation angle of DMD, x is horizontal spot separation, d is the pitch size of DMD, and dsp is the spot diameter. (b) Spot definition on the substrate (airy disk), and the spot radius is 4 lm.
H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
the range from 85% to 95% in steps of 2% adjusted by translation speed of the stage. The amount of overlap in the normal direction of the scan is given as 95%, which is determined by the rotation angle of the DMD (2.86°). The DMD used in the calculation is made by Texas Instruments. It has a pitch size of 13.68 lm, and the frame rate of the DMD is fixed at 16,000 frames per second in all cases. The calculations were carried out using the mathematical software MATLABÒ by MathWorks™, and the grid size in the calculation is given as 20 nm.
width, as the spot overlap increases, the peak exposure intensity grows exponentially. Contrary to the mask-based projection type lithography system, the results of the simulations shown in Fig. 4 clearly indicate that the exposure intensity of a line pattern recorded by the maskless lithography system strongly depends on the width of the line pattern. This tendency results from the accumulation of the exposure intensity by the scanning of the twodimensional spot array. For a quantitative analysis of the variation of the peak exposure intensity with the width of the line pattern, we define a new parameter of exposure efficiency simply as
3. Result and discussion The calculated L/S patterns with periods of 6 and 8 lm are plotted in Fig. 3(a), while the cross-section profiles of exposure intensities are shown in Fig. 3(b). We determined the line width with the full width of half maximum (FWHM) of the exposure intensity profile. In both cases, elevation of the baseline is observed because the exposure intensity accumulated due to the two-dimensional overlapping of the spots in the parallel and normal directions of the scanning. For various spot overlaps in the range from 85% to 95%, the peak exposure intensity variation with respect to the line width was analyzed with the simulated patterns as shown in Fig. 4. As the line width of the pattern increases, the peak exposure intensity rapidly increases to the section adjacent to the spot size; soon after the region, the increasing rate gradually decreases, showing a step-like change. We also found that the peak exposure intensity reaches a limited value of saturation intensity, as shown in Fig. 4. In a previous work, it was reported that under a fixed line
3147
gexp ¼
Iexp 100; Isat
ð4Þ
where gexp is the exposure efficiency, Iexp is the peak exposure intensity, and Isat is the saturation intensity introduced in Fig. 4. Here we note that the exposure efficiency is an important parameter to be considered in the patterning process with the maskless lithography system using the point array method. The variations of the line width with the spot overlap for various exposure efficiencies are depicted in Fig. 5. Although the exposure efficiency strongly depends on the width of the line pattern, the variation of the spot overlap has little effect on the line width given a fixed level of exposure efficiency. At exposure efficiencies of 100%, and 80%, the widths of the line patterns are 17.6 and 7.8 lm with standard deviations of 0.18 and 0.78, respectively. From these results, the width of the line pattern can be averaged over the spot overlap for a given exposure efficiency.
Fig. 3. Typical simulated result of L/S patterns. (a) L/S = 6 lm, (b) L/S = 8 lm. In both cases, the spot overlap is 95%, which yields the stage speed of 3.2 mm/s in the scanning direction, and the scan step is 0.2 lm.
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H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
Fig. 4. Peak exposure intensity variation with respect to the width of the line pattern in relation to the change of the spot overlap.
Fig. 5. Width of the line pattern variation with respect to the spot overlap in the relation to the exposure efficiency.
To find the correlation between the exposure efficiency and the width of the line pattern, we plot the exposure efficiency as a function of the averaged width of the line pattern. This is shown in Fig. 6. While the averaged width of the line pattern increases until about 4.5 lm, the exposure efficiency rapidly increases. Beyond the average width of 5 lm, the exposure efficiency gradually increases at a much smaller increasing rate. As shown in Fig. 6, we plot two linear regression line fits of the first three data points and the last three data points. By extending the line fits, we find a crossing point at the exposure efficiency of 73% and an average width of 4.5 lm. As shown in this graph, the exposure efficiency and the average width have a trade-off relationship, i.e., to obtain the possible minimum width of the line pattern, the loss of the exposure efficiency needs to be considered. By analyzing the correlation between the exposure efficiency and the width of the line pattern, we theoretically estimate the practical acceptable tradeoff value of the minimum width of the line pattern to be 4.5 lm, which is 11% larger than the spot radius of diffracted light with an exposure efficiency of about 73%.
4. Conclusion We calculated the exposure intensity of line/space patterns recorded with a DMD digital maskless lithography system using the point array method. Using the simulation results of the L/S patterns recorded with the maskless lithography system using the point array method, we analyzed the relationships among the width of the line patterns, the peak exposure intensity, and the exposure efficiency. We found that the exposure efficiency, which is a key parameter in the operation of a maskless lithography system, strongly depends on the width of the line pattern. We also estimated that the practical acceptable trade-off value of the minimum width of the line pattern is approximately 11% larger than the spot radius of diffracted light with an exposure efficiency of about 73%. The result of this study is expected to be used as an effective analytic tool for the determination of the design parameters of a maskless lithography system using a DMD. These parameters can include the pattern resolution, the scanning speed and the power of the light source. It will also be useful in the management of the patterning process when using a maskless lithography system.
H. Ryoo et al. / Microelectronic Engineering 88 (2011) 3145–3149
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Fig. 6. Exposure efficiency variation with respect to the average width of the line pattern. The dashed dotted lines are linear fits of the two increasing region of exposure efficiency, and the resulting crossing point is 4.54 lm of average width of the line pattern and 73.3% of exposure efficiency.
Further work will cover the experimental verification of the simulation results as regards L/S patterning and evaluations of the exposure efficiency. Acknowledgment This work was supported by the Ministry of Knowledge Economy of Korea (Project No. 10031688). References [1] D. Smith, D. Klenk, Pros. SPIE 7210 (2009) 72100K. [2] K.F. Chan, Z. Feng, R. Yang, A. Ishikawa, W. Mei, J. Microlith. Microfab. Microsyst. 2 (4) (2003) 331–339.
[3] T.H.P. Chang, Marian Mankos, Kim Y. Lee, Larry P. Muray, Microelectron. Eng. 57–58 (2001) 117–135. [4] P. Vettiger, M. Despont, U. Drechsler, U. Durig, W. Haberie, M. Lutwyche, H. Rothuizen, R. Stutz, R. Widmer, G. Binnig, IBM J. Res. Develop. 44 (2000) 323– 340. [5] T. Wang, M. Quaglio, F. Pirri, Y. Cheng, D. Busacker, F. Cerrina, Pros. SPIE 7274 (2009) 727420. [6] K. Kim, J. Yi, S. Cho, N. Kang, M. Cho, B. Shin, B. Choi, Appl. Surf. Sci. 255 (2009) 7835–7840. [7] W. Mei, T. Kanatake, K. Powell, U.S. Patent No. 6425,669 B1, 2002. [8] W. Mei, U.S. Patent No. 6473,237 B2, 2002. [9] W. Mei, T. Kanatake, A. Ishikawa, U.S. Patent No. 6379,867 B1, 2002. [10] H. Ryoo, D.W. Kang, J.W. Hahn, Microelectron. Eng. 88 (2011) 235–239. [11] O.D. Negrete, F. Cerrina, Microelectron. Eng. 85 (2008) 834–837.