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Design and analysis of the single-step nanoimprinting lithography equipment for sub-100 nm linewidth
Current Applied Physics 6 (2006) 1007–1011 www.elsevier.com/locate/cap www.kps.or.kr
Design and analysis of the single-step nanoimprinting lithography equipment for sub-100 nm linewidth JaeJong Lee *, Kee-Bong Choi, Gee-Hong Kim Intelligence and Precision Machine Department, Korea Institute of Machinery and Materials, P.O. Box 101 Yusung-ku, Taejon 305-600, Republic of Korea Received 4 September 2004; received in revised form 28 March 2005 Available online 15 August 2005
Abstract Nanoimprint lithography (NIL) is the cutting-edge technology to produce sub-100 nm scale features on substrates. The fundamental procedure of nanoimprint lithography is replicating the patterns defined in the stamp to any deformable materials such as photoresist spun on substrates by pressing and the physical shape of the resist is deformed during the imprinting process. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes, and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer are proposed. Using the designed nanoimprinting equipment, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate. Finally, the nanoimprinting results show the validity of the developed equipment. Ó 2005 Elsevier B.V. All rights reserved. PACS: 07.10.Cm Keywords: Nanoimprinting lithography; Compliant flexure stage; Overlay and alignment
1. Introduction Nanoimprint lithography (NIL) is a promising technology to produce sub-100 nm scale features on silicon chips. The fundamental procedure of nanoimprint lithography is replicating the patterns defined in the stamp to any deformable materials such as photoresist spun on substrates by pressing and the physical shape of the resist is deformed during the imprinting process [1,4]. The nanoimprint lithography, such as thermal-imprint and UV-imprint, is also well known as the next generation lithography. This lithography has advantages of simple process, low cost, high replication fidelity *
Corresponding author. Tel.: +82 42 868 7145; fax: +82 42 868 7721. E-mail address: [email protected] (J. Lee). 1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.07.007
and relatively high throughput [1]. Key issues in the nanoimprint lithography are how to make non-defective nanopatterns on a large area with high throughput, and how to align a mold and a wafer for fabrication of multi-layered patterns. To accomplish these issues, an imprinting head, a tilting stage for surface parallelism, an alignment system for multi-layer process, mold/wafer chucking units, an overlay measurement system, a releasing unit, a clearance control system, and an antivibration unit are necessarily required. In order to replicate the patterns defined in the stamp to the substrate, the surface contact between stamp and wafer is required during the nanoimprint lithography process [2]. While the contact process, especially, the surface parallelization between the mold and the wafer is important. A parallel misalignment between the stamp and the wafer causes incomplete replicas to the transferred patterns on the wafer. In a mechanical system, a
1008
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
pose misalignment due to the geometric and the assembly errors of sub-micron order is inevitable. To replicate the patterns with complete fidelity, the wafer stage must have the function of the pose alignment. The function of the pose alignment is accomplished by an active nanomotion stage or a passive compliant stage. For the active nanomotion, actuators for nanomotion, high-resolution sensors, amplifiers, and a control system, which cause the complicated structure of the stage, are required. On the other hand, a passive compliant stage is possible to have a compact structure. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes, and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer are proposed. The compliant mechanism consists of two rigid bodies and two types of flexures. Leaf-type outer flexures are for out-of-plane motion and L-type inner flexures are for in-plane motion. The designed and analyzed systems are adapted to the single-step nanoimprinting lithography equipment.
2. Structure of compliant flexure stage Fig. 1 shows the behavior of the compliant flexure stage with the passive compliance mechanism during imprinting process. In initial state, the stamp is misaligned with tilt angle Dh on the substrate because of the geometrical clearance in the sliding unit and the set-up errors of the stamp and the wafer. The center A on the wafer was misaligned from the rotational center of the wafer stage O. These misalignment errors are cause of the trouble in the nanoscale imprinting. When some imprinting forces are act on the stamp for imprinting process in the z-axis, the wafer stage translated along z-axis and rotated with respect to the rotational center O 0 . Thus, the center of wafer A translates to A 0 by Dx in the in-plane of the wafer surface as well as Dz and Dh in the out-of-plane. Enlarging this concept to 3dimension, the wafer stage requires the passive compliance mechanism with two translations in the in-plane motion, and one translation and two rotations in the out-of-plane motion. Therefore, to follow the motion
Fig. 1. Behavior of wafer stage with passive compliance mechanism.
of the template, the mobility of the wafer stage must have, at least, five degree-of-freedom. The passive compliant mechanism for the wafer stage can be designed using the flexure hinge-type stage. For the in-plane motion, three pairs of flexures are used. A pair of flexure consists of an L and its mirror L-type leaf springs. For the out-of-plane motion, three leaf springs with large width and small height are also used. Fig. 2 shows the schematic configuration of the compliant flexure stage and its elements. The flexure stage has monolithic flexures on a plane, and symmetric structure to cope with thermal deformation. Owing to the flexures, the motions of the wafer stage is decoupled through in-plane motion with two translations and one rotation, and out-of-plane motion with one translation and two tilts. Therefore, the flexure stage has total six degree-of-freedom, which satisfies the mobility requirement.
3. Design and analysis of compliant flexure stage The compliant flexure stage consists of an inner mechanism for the in-plane motion and an outer mechanism for the out-of-plane motion due to the decoupled motions. The dimensions of the inner circular plate and the outer ring plate of the compliant flexure stage are decided using the Min–Max algorithm [3]. In the outer mechanism of the compliant flexure stage, three of outer flexure has the same dimensions, and the shape of outer flexure is as shown in Fig. 2. The design parameters are chosen by a width bo, a height to and a length Lo of the outer flexure. The subscript o means the outer flexure. The constrain condition of the outer flexures is as follows: ro;max 6 rY =S f ;
ð1Þ
where ro,max and rY mean the maximum and the yielding stress of the flexure material, and Sf is the safety factor, respectively. When the maximum imprinting force acts on the stamp along the z-axis, the deflection of zaxis is constrained by do;dsr D 6 do;z 6 do;dsr þ D;
ð2Þ
where do,dsr and D are a desired z-directional deflection and allowable range. The flexures are also constrained by the geometric constraints formula (3)–(5): bo;min 6 bo 6 bo;max ;
ð3Þ
to;min 6 to 6 to;max ;
ð4Þ
Lo;min 6 Lo 6 Lo;max ;
ð5Þ
where the subscripts min and max are the minimum and the maximum values of the allowable geometric dimensions, respectively. Design variables bo, to and Lo are
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
1009
Fig. 2. Configuration of compliant flexure stage and its elements.
designed optimally using a Min–Max algorithm. For the optimal design of the outer flexures, design indices wo1 and wo2 can be represented as follows: wo1 ¼ rY S f ro;max ;
ð6Þ
wo2 ¼ fz ;
ð7Þ
where fz is the resonance frequency in z-axis. The design indices wo1 and wo2 are in favor of the maximum values in the constraint conditions. After the design indices are normalized, they are weighted. Then, a composite design index Woc is given by ~ o1 ^ w ~ o2 ; W oc ¼ w
ð8Þ
~ o1 and w ~ o2 are the weighted normalized design where w index of wo1 and wo2, and ^ is a fuzzy intersection. The optimal design parameters are obtained by the maximal position of the composite design index. The three pairs of inner flexures have the same dimensions. The shape and the dimensions of the inner flexures are as shown in Fig. 2. The inner mechanism is also designed by the similar method of the outer mechanism. The constraints of the in-plane motion mechanism can be represented as follows: ri;max 6 rY =S f ; bi;min 6 bi 6 bi;max ;
Fig. 3. Modal analysis results.
ð9Þ ð10Þ
where the subscript i means inner flexure. Design indices for the optimal design of the inner flexures are given by wi1 ¼ jdi;x di;dsr j;
ð11Þ
wi2 ¼ rY S f ri;max ;
ð12Þ
wi3 ¼ fx ;
ð13Þ
where di,x and di, dsr are a x-directional deflection and a desired x-directional deflection when maximum force is applied along the x-axis, and fx is a resonance frequency of x-axis. The design index wi1 is in favor of the minimum value, whereas the design indices wi2 and wi3 are in favor of the maximum values, in the constraint conditions.
Fig. 4. Static analysis results.
After the design indices are normalized, they are weighted. Then the composite design index Wic is represented as follows: ~ i1 ^ w ~ i2 ^ w ~ i3 ; W ic ¼ w
ð14Þ
~ i1 , w ~ i2 and w ~ i3 are the weighted normalized dewhere w sign index of wi1, wi2 and wi3, and ^ is a fuzzy intersection. The optimal design parameters are obtained by the maximal position of the composite design index. The designed wafer stage is analyzed using the FEM. Fig. 3 shows the results of modal analyses. Fig. 4 shows the results of static analysis. The stiffnesses can be calculated by these results and they accord with the analytic results. In addition, it is observed that the maximum stresses are generated in the ends of
1010
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
Fig. 5. Configurations of the dual grating method and alignment system.
flexures and they are lower than the yield stress of the material.
4. Design of overlay and alignment system The nanoimprinting lithography that can make sub100 nm patterns effectively and economically is recently classified it as next generation lithography. However, it should solve some problems to expand its applications. The one of the most urgent problems is the overlay and the alignment. The overlay requirement depends on the design-rule and the overlay budget is usually approximately 30% of the design rule CD [1]. Therefore, in order to do nanoimprinting under 50 nm linewidth, the overlay and alignment system with under 15 nm alignment accuracy should be required. There are two kinds of method, which have high alignment accuracy, Moire´ technology and optical diffraction method with the dual-grating pattern. Optical diffraction method uses the high order diffracted and the interfered lights from grating patterns that are patterned on a mask and a substrate, and it was developed to apply to proximity lithography like X-ray lithography. The principle of this method is that the intensity variations of diffracted lights are depend on the lateral relative translations of these two gratings, and it can make overlay and alignment up to the order of 10 nm [2]. In the designed dual grating alignment system, the pitch size of align mark on the stamp is equal to half-pitch of align mark on the prior layer in order to minimize the background noise and to enhance the sensitivity of
interference signals as shown in Fig. 5. In this alignment system, the incident beam has transmitted from the top of the quartz stamp. The diffraction light signal of the transmitted beam can be used for alignment. Using the summation and difference values of the ±1st order diffraction light signals, alignment position can be determined. The final goal is to obtain the alignment accuracy <20 nm overlay. The designed alignment system is as shown in Fig. 5.
5. Nanoimprinting results Fig. 6 shows the developed compliant flexure stage and the manufactured nanoimprinting equipment. Using this nanoimprinting equipment, several nanoimprinting processes were performed under 4-in. Si wafer
Fig. 6. Developed the nanoimprinting lithography equipment and the flexure stage.
Fig. 7. SEM images of resist patterns obtained by the developed NIL equipment with quartz stamp.
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
with trialkoxy siliane, UV resin with less than 6 cps viscosity and 15 mW/cm2 curing intensity, and quartz stamp. After the nanoimprinting processes, the hydrophobic self-assembled monolayer is coated on the stamp in order to easy release the stamp from the wafer. The UV resin for nanoimprinting is coated on the substrate by drop and spin-coating methods. The quartz stamp for nanoimprinting has been fabricated using E-beam lithography and laser interference lithography. The quartz stamps using the E-beam lithography are fabricated with 56–100 nm linewidth. Applied imprinting forces on the stamp are less than 20 kgf. As results of the nanoimprinting process, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate as shown in Fig. 7.
6. Conclusions Key issues in the nanoimprint lithography are how to make non-defective nanopatterns on a large area with high throughput, and how to align a mold and a wafer for fabrication of multi-layer patterns. To accomplish these issues, an imprinting head, a tilting stage for surface parallelism, an alignment system for multi-layer process, mold/wafer chucking units, an overlay measurement system, a releasing unit, a clearance control system, and an anti-vibration unit are necessarily required. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes,
1011
and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer were proposed. The proposed compliant stage has a monolithic, symmetric and planar six degree-of-freedom structure. For a passive compliant motion, flexure hinge is implemented. Using the developed nanoimprinting equipment, several nanoimprinting processes are performed on the 4-in. Si wafer with trialkoxy siliane using the quartz stamp. Using the designed nanoimprinting equipment, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate.
Acknowledgment This work is supported by Center for Nanoscale Mechatronics and Manufacturing, Korea.
References [1] S.Y. Chou, P.R. Krauss, Imprint lithography with sub-10 nm feature size and high throughput, Microelectronic Engineering 35 (1997) 237–240. [2] B.J. Choi, S.V. Sreenivasan, S. Johnson, M. Colburn, C.G. Wilson, Design of orientation stage for step and flash imprint lithography, Precision Engineering 25 (2001) 192–199. [3] K.-B. Choi, Kinematic analysis and optimal design of 3-PPR planar parallel manipulator, KSME International Journal 17 (4) (2003) 528–537. [4] M.M. Alkaisi et al., Multilevel nanoimprint lithography, Current Applied Physics (4) (2004).
Design and analysis of the single-step nanoimprinting lithography equipment for sub-100 nm linewidth JaeJong Lee *, Kee-Bong Choi, Gee-Hong Kim Intelligence and Precision Machine Department, Korea Institute of Machinery and Materials, P.O. Box 101 Yusung-ku, Taejon 305-600, Republic of Korea Received 4 September 2004; received in revised form 28 March 2005 Available online 15 August 2005
Abstract Nanoimprint lithography (NIL) is the cutting-edge technology to produce sub-100 nm scale features on substrates. The fundamental procedure of nanoimprint lithography is replicating the patterns defined in the stamp to any deformable materials such as photoresist spun on substrates by pressing and the physical shape of the resist is deformed during the imprinting process. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes, and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer are proposed. Using the designed nanoimprinting equipment, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate. Finally, the nanoimprinting results show the validity of the developed equipment. Ó 2005 Elsevier B.V. All rights reserved. PACS: 07.10.Cm Keywords: Nanoimprinting lithography; Compliant flexure stage; Overlay and alignment
1. Introduction Nanoimprint lithography (NIL) is a promising technology to produce sub-100 nm scale features on silicon chips. The fundamental procedure of nanoimprint lithography is replicating the patterns defined in the stamp to any deformable materials such as photoresist spun on substrates by pressing and the physical shape of the resist is deformed during the imprinting process [1,4]. The nanoimprint lithography, such as thermal-imprint and UV-imprint, is also well known as the next generation lithography. This lithography has advantages of simple process, low cost, high replication fidelity *
Corresponding author. Tel.: +82 42 868 7145; fax: +82 42 868 7721. E-mail address: [email protected] (J. Lee). 1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.07.007
and relatively high throughput [1]. Key issues in the nanoimprint lithography are how to make non-defective nanopatterns on a large area with high throughput, and how to align a mold and a wafer for fabrication of multi-layered patterns. To accomplish these issues, an imprinting head, a tilting stage for surface parallelism, an alignment system for multi-layer process, mold/wafer chucking units, an overlay measurement system, a releasing unit, a clearance control system, and an antivibration unit are necessarily required. In order to replicate the patterns defined in the stamp to the substrate, the surface contact between stamp and wafer is required during the nanoimprint lithography process [2]. While the contact process, especially, the surface parallelization between the mold and the wafer is important. A parallel misalignment between the stamp and the wafer causes incomplete replicas to the transferred patterns on the wafer. In a mechanical system, a
1008
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
pose misalignment due to the geometric and the assembly errors of sub-micron order is inevitable. To replicate the patterns with complete fidelity, the wafer stage must have the function of the pose alignment. The function of the pose alignment is accomplished by an active nanomotion stage or a passive compliant stage. For the active nanomotion, actuators for nanomotion, high-resolution sensors, amplifiers, and a control system, which cause the complicated structure of the stage, are required. On the other hand, a passive compliant stage is possible to have a compact structure. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes, and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer are proposed. The compliant mechanism consists of two rigid bodies and two types of flexures. Leaf-type outer flexures are for out-of-plane motion and L-type inner flexures are for in-plane motion. The designed and analyzed systems are adapted to the single-step nanoimprinting lithography equipment.
2. Structure of compliant flexure stage Fig. 1 shows the behavior of the compliant flexure stage with the passive compliance mechanism during imprinting process. In initial state, the stamp is misaligned with tilt angle Dh on the substrate because of the geometrical clearance in the sliding unit and the set-up errors of the stamp and the wafer. The center A on the wafer was misaligned from the rotational center of the wafer stage O. These misalignment errors are cause of the trouble in the nanoscale imprinting. When some imprinting forces are act on the stamp for imprinting process in the z-axis, the wafer stage translated along z-axis and rotated with respect to the rotational center O 0 . Thus, the center of wafer A translates to A 0 by Dx in the in-plane of the wafer surface as well as Dz and Dh in the out-of-plane. Enlarging this concept to 3dimension, the wafer stage requires the passive compliance mechanism with two translations in the in-plane motion, and one translation and two rotations in the out-of-plane motion. Therefore, to follow the motion
Fig. 1. Behavior of wafer stage with passive compliance mechanism.
of the template, the mobility of the wafer stage must have, at least, five degree-of-freedom. The passive compliant mechanism for the wafer stage can be designed using the flexure hinge-type stage. For the in-plane motion, three pairs of flexures are used. A pair of flexure consists of an L and its mirror L-type leaf springs. For the out-of-plane motion, three leaf springs with large width and small height are also used. Fig. 2 shows the schematic configuration of the compliant flexure stage and its elements. The flexure stage has monolithic flexures on a plane, and symmetric structure to cope with thermal deformation. Owing to the flexures, the motions of the wafer stage is decoupled through in-plane motion with two translations and one rotation, and out-of-plane motion with one translation and two tilts. Therefore, the flexure stage has total six degree-of-freedom, which satisfies the mobility requirement.
3. Design and analysis of compliant flexure stage The compliant flexure stage consists of an inner mechanism for the in-plane motion and an outer mechanism for the out-of-plane motion due to the decoupled motions. The dimensions of the inner circular plate and the outer ring plate of the compliant flexure stage are decided using the Min–Max algorithm [3]. In the outer mechanism of the compliant flexure stage, three of outer flexure has the same dimensions, and the shape of outer flexure is as shown in Fig. 2. The design parameters are chosen by a width bo, a height to and a length Lo of the outer flexure. The subscript o means the outer flexure. The constrain condition of the outer flexures is as follows: ro;max 6 rY =S f ;
ð1Þ
where ro,max and rY mean the maximum and the yielding stress of the flexure material, and Sf is the safety factor, respectively. When the maximum imprinting force acts on the stamp along the z-axis, the deflection of zaxis is constrained by do;dsr D 6 do;z 6 do;dsr þ D;
ð2Þ
where do,dsr and D are a desired z-directional deflection and allowable range. The flexures are also constrained by the geometric constraints formula (3)–(5): bo;min 6 bo 6 bo;max ;
ð3Þ
to;min 6 to 6 to;max ;
ð4Þ
Lo;min 6 Lo 6 Lo;max ;
ð5Þ
where the subscripts min and max are the minimum and the maximum values of the allowable geometric dimensions, respectively. Design variables bo, to and Lo are
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
1009
Fig. 2. Configuration of compliant flexure stage and its elements.
designed optimally using a Min–Max algorithm. For the optimal design of the outer flexures, design indices wo1 and wo2 can be represented as follows: wo1 ¼ rY S f ro;max ;
ð6Þ
wo2 ¼ fz ;
ð7Þ
where fz is the resonance frequency in z-axis. The design indices wo1 and wo2 are in favor of the maximum values in the constraint conditions. After the design indices are normalized, they are weighted. Then, a composite design index Woc is given by ~ o1 ^ w ~ o2 ; W oc ¼ w
ð8Þ
~ o1 and w ~ o2 are the weighted normalized design where w index of wo1 and wo2, and ^ is a fuzzy intersection. The optimal design parameters are obtained by the maximal position of the composite design index. The three pairs of inner flexures have the same dimensions. The shape and the dimensions of the inner flexures are as shown in Fig. 2. The inner mechanism is also designed by the similar method of the outer mechanism. The constraints of the in-plane motion mechanism can be represented as follows: ri;max 6 rY =S f ; bi;min 6 bi 6 bi;max ;
Fig. 3. Modal analysis results.
ð9Þ ð10Þ
where the subscript i means inner flexure. Design indices for the optimal design of the inner flexures are given by wi1 ¼ jdi;x di;dsr j;
ð11Þ
wi2 ¼ rY S f ri;max ;
ð12Þ
wi3 ¼ fx ;
ð13Þ
where di,x and di, dsr are a x-directional deflection and a desired x-directional deflection when maximum force is applied along the x-axis, and fx is a resonance frequency of x-axis. The design index wi1 is in favor of the minimum value, whereas the design indices wi2 and wi3 are in favor of the maximum values, in the constraint conditions.
Fig. 4. Static analysis results.
After the design indices are normalized, they are weighted. Then the composite design index Wic is represented as follows: ~ i1 ^ w ~ i2 ^ w ~ i3 ; W ic ¼ w
ð14Þ
~ i1 , w ~ i2 and w ~ i3 are the weighted normalized dewhere w sign index of wi1, wi2 and wi3, and ^ is a fuzzy intersection. The optimal design parameters are obtained by the maximal position of the composite design index. The designed wafer stage is analyzed using the FEM. Fig. 3 shows the results of modal analyses. Fig. 4 shows the results of static analysis. The stiffnesses can be calculated by these results and they accord with the analytic results. In addition, it is observed that the maximum stresses are generated in the ends of
1010
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
Fig. 5. Configurations of the dual grating method and alignment system.
flexures and they are lower than the yield stress of the material.
4. Design of overlay and alignment system The nanoimprinting lithography that can make sub100 nm patterns effectively and economically is recently classified it as next generation lithography. However, it should solve some problems to expand its applications. The one of the most urgent problems is the overlay and the alignment. The overlay requirement depends on the design-rule and the overlay budget is usually approximately 30% of the design rule CD [1]. Therefore, in order to do nanoimprinting under 50 nm linewidth, the overlay and alignment system with under 15 nm alignment accuracy should be required. There are two kinds of method, which have high alignment accuracy, Moire´ technology and optical diffraction method with the dual-grating pattern. Optical diffraction method uses the high order diffracted and the interfered lights from grating patterns that are patterned on a mask and a substrate, and it was developed to apply to proximity lithography like X-ray lithography. The principle of this method is that the intensity variations of diffracted lights are depend on the lateral relative translations of these two gratings, and it can make overlay and alignment up to the order of 10 nm [2]. In the designed dual grating alignment system, the pitch size of align mark on the stamp is equal to half-pitch of align mark on the prior layer in order to minimize the background noise and to enhance the sensitivity of
interference signals as shown in Fig. 5. In this alignment system, the incident beam has transmitted from the top of the quartz stamp. The diffraction light signal of the transmitted beam can be used for alignment. Using the summation and difference values of the ±1st order diffraction light signals, alignment position can be determined. The final goal is to obtain the alignment accuracy <20 nm overlay. The designed alignment system is as shown in Fig. 5.
5. Nanoimprinting results Fig. 6 shows the developed compliant flexure stage and the manufactured nanoimprinting equipment. Using this nanoimprinting equipment, several nanoimprinting processes were performed under 4-in. Si wafer
Fig. 6. Developed the nanoimprinting lithography equipment and the flexure stage.
Fig. 7. SEM images of resist patterns obtained by the developed NIL equipment with quartz stamp.
J. Lee et al. / Current Applied Physics 6 (2006) 1007–1011
with trialkoxy siliane, UV resin with less than 6 cps viscosity and 15 mW/cm2 curing intensity, and quartz stamp. After the nanoimprinting processes, the hydrophobic self-assembled monolayer is coated on the stamp in order to easy release the stamp from the wafer. The UV resin for nanoimprinting is coated on the substrate by drop and spin-coating methods. The quartz stamp for nanoimprinting has been fabricated using E-beam lithography and laser interference lithography. The quartz stamps using the E-beam lithography are fabricated with 56–100 nm linewidth. Applied imprinting forces on the stamp are less than 20 kgf. As results of the nanoimprinting process, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate as shown in Fig. 7.
6. Conclusions Key issues in the nanoimprint lithography are how to make non-defective nanopatterns on a large area with high throughput, and how to align a mold and a wafer for fabrication of multi-layer patterns. To accomplish these issues, an imprinting head, a tilting stage for surface parallelism, an alignment system for multi-layer process, mold/wafer chucking units, an overlay measurement system, a releasing unit, a clearance control system, and an anti-vibration unit are necessarily required. In this study, for the single-step nanoimprinting process, the 4-in. imprinting head, the fabricated 4-in. mask, the alignment system for multi-layer processes,
1011
and the six-DOF compliant mechanism of a wafer stage for single-step nanoimprinting on a 4-in. wafer were proposed. The proposed compliant stage has a monolithic, symmetric and planar six degree-of-freedom structure. For a passive compliant motion, flexure hinge is implemented. Using the developed nanoimprinting equipment, several nanoimprinting processes are performed on the 4-in. Si wafer with trialkoxy siliane using the quartz stamp. Using the designed nanoimprinting equipment, the nanoscale patterns with 100 nm linewidth and 150 nm height were clearly patterned on the substrate.
Acknowledgment This work is supported by Center for Nanoscale Mechatronics and Manufacturing, Korea.
References [1] S.Y. Chou, P.R. Krauss, Imprint lithography with sub-10 nm feature size and high throughput, Microelectronic Engineering 35 (1997) 237–240. [2] B.J. Choi, S.V. Sreenivasan, S. Johnson, M. Colburn, C.G. Wilson, Design of orientation stage for step and flash imprint lithography, Precision Engineering 25 (2001) 192–199. [3] K.-B. Choi, Kinematic analysis and optimal design of 3-PPR planar parallel manipulator, KSME International Journal 17 (4) (2003) 528–537. [4] M.M. Alkaisi et al., Multilevel nanoimprint lithography, Current Applied Physics (4) (2004).