- Email: [email protected]
Correction of a coherent image during KrF excimer laser ablation using a mask projection
ARTICLE IN PRESS
Optics and Lasers in Engineering 44 (2006) 615–622
Correction of a coherent image during KrF excimer laser ablation using a mask projection Dong Sig Shina,, Jae Hoon Leea, Jeong Suha, To Hoon Kimb a
Laser Application Group, KIMM (Korea Institute of Machinery and Materials) 171 Jang-dong, Yuseong-gu, Dae-Jeon, South Korea b Department of Metallurgical System Engineering, Yonsei University, Seoul, South Korea Received 26 June 2004; received in revised form 20 April 2005; accepted 4 July 2005 Available online 22 August 2005
Abstract Using masks for laser ablation has proven useful in the fabrication of prototypes for the manufacturing of micro-fluidic devices. In this work, an excimer laser was used to engrave microscopic channels on the surface of polyethylene terephthalate (PET), which showed a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. When 50 mm wide rectangular microscopic channels were made using a 500 500 mm square mask and a magnification ratio of 1/10, ditch-shaped defects were found at both corners. The calculation of the laser beam intensity showed that a coherent image in the PET specimen caused the defects. An analysis based on the Fourier diffraction theory enabled the prediction of a coherent shape at the image plane, as well as a diffracted beam between the mask and the image plane. The analysis also showed that the diameter of the aperture was a predominant factor toward the elimination of ditch-shaped defects in the rectangular microscopic channels on the PET produced by an excimer laser ablation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Excimer laser ablation; Polyethylene terephthalate (PET); Mask projection; Fourier diffraction theory; Coherent image; Ditch-shaped defects; Micro-fluidic device
Corresponding author. Tel.: +82 72 868 7484; fax +82 42 868 7431.
E-mail address: [email protected] (D.S. Shin). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.07.001
ARTICLE IN PRESS 616
D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
1. Introduction The laser fabrication technique is a clean, safe, and convenient process for manufacturing MEMS devices in comparison with other technologies, such as chemical etching, deposition process, and lithography [1]. In particular, the advantage of the excimer laser application is that its non-thermal effects make it suitable for machining plastic, glass, and ceramic crystal [2]. Recently, mask patterning and direct laser writing techniques have been used for making various types of micro-fluidic channels and reservoirs [3]. However, this technology cannot be applied in mass production. Thus, the requirement of a micromold insert appeared on manufacturing micro-fluidic channels using excimer laser ablation. A micro-mold insert is produced by means of an electroforming process, after the platinum sputtering on a specific polymer, which ablated by means of an excimer laser. When this is done, micro-fluidic devices can be manufactured by using the micro-mold insert and the injection molding process. This implies that the excimer laser ablation method can be an alternate method for the X-ray lithography of Lithographie Galvanoformung Abformtechnik (LIGA) [4]. In contrast with lithography, excimer laser ablation using mask projection has possibilities available for three-dimensional pattern, varying an in-depth dimension across the lateral shape, by controlling the mask shape applied to each step [5]. However, the excimer laser beam should pass through a long distance compared to the lithography method. Thus, the diffraction theory should be considered when using a mask projection. A typical defect of laser ablation, as compared with lithography, is generated by a coherent image arising from the diffraction effect. The perturbation of energy is usually detected on the edge of a laser beam. This phenomenon affects any fabrication process using excimer laser mask projection. The micro-channels ablated by a coherent laser beam have ditch-shaped defects at the corners. As a result, these defects are transferred to an electroformed mold insert on an ablated polymer. Therefore, the coherent image should be corrected to prevent ditch-shaped defects in micro-channels. However, few studies have reported on the effects of a diffracted beam and the coherent image used in mask projected excimer laser ablation. In this study, the origin of a coherent image was clarified by applying Fourier optic theory. Then, the optimal factor was found to eliminate these defects, through a comparison between a simulated value and a real cross sectional view.
2. Method In this study, the effect of a coherent image was investigated. A KrF excimer laser (wavelength 248 nm) was used as a laser source to ablate polymers. An optical imaging system (LightDeck OPTEC S.A.) with a three-element processing lens (focal length: 86.9 mm) was used on the polymer surface. A laser beam passes through a mask, with a prefabricated pattern, and irradiates on the polymer surface by an imaging lens set.
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Fig. 1 shows the geometrical layout of the mask projection system. As shown in Fig. 1, the optical elements are composed of a square mask (500 500 mm), which is composed of a circular aperture and a converging lens. 1 1 1 þ ¼ . z1 z2 f
(1)
Eq. (1) shows that each point in the object plane is imaged onto a corresponding point in the plane along with a magnification factor. Therefore, the focal length (f) of a lens determines completely its effect on the paraxial rays [6]. This experiment was conducted using the following conditions: z1 ¼ 955, z2 ¼ 95:59, f ¼ 86:9 mm, and the overall magnification factor (B/A) was 1/10. The specimen material was polyethylene terephthalate (PET), which has a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. Therefore, a photochemical reaction [7] is the main process between the PET and the UV laser beam (wavelength: 248 nm). Thus, PET is a useful material to analyze the correspondence between a beam shape and an ablated channel shape. Z Z
1
U i ðu; vÞ ¼
hðu; v; x; ZÞU 0 ðx; ZÞ dx dZ,
(2)
1
hðu; v; x; ZÞ ¼
1 k 2 k 2 exp j ðu þ v2 Þ exp j ðx þ Z2 Þ jlz1 z2 2z2 2z1 Z Z 1 k 1 1 1 Pðx; yÞ exp j þ ðx2 þ y2 Þ 2 z1 z2 f 1 x u Z v exp jk þ þ xþ y dx dy z1 z2 z1 z2
Fig. 1. Geometrical layout of mask projection.
ð3Þ
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618
Z Z 1 A u v hðu; v; x; ZÞ ¼ Pðx; yÞ exp j2p x þy dx dy lz2 lz2 lz2 1 A F xy fPðx; yÞgkx ¼ðu=lz2 Þ ky ¼v=lz2 : ¼ lz2
ð4Þ
Eqs. (2) and (3) now provide a formal solution specifying the relationship that exists between the object and the image. Eq. (3) represents the field amplitude produced by a unit amplitude point source applied at the object coordinates [8]. U0 and Ui are field distribution in the object and image planes. Goodman [8] simplified Eqs. (3)–(4). This implies that the aperture function, P(x,y), is a primary reaction factor. Hðf x ; f y Þ Z Z 1 1 2p ¼F P ðx; yÞ exp j ðux þ vyÞ dx dy , lz2 lz2 1 ¼ ðlz2 ÞPðlz2 f x ; lz2 f y Þ, Hðf x ; f y Þ ¼ Pðlz2 f x ; lz2 f y Þ,
f x;y ¼
w . lzi
ð5Þ (6)
(7)
The central matter is that a coherent imaging system is linear in complex amplitude. If a frequency analysis is to be applied in its usual form, it must be applied to the linear amplitude mapping. To do so, the amplitude transfer function, H(fx,fy), is induced [8]. Eq. (5) defines an amplitude transfer function as the Fourier transform of the amplitude point spread function, h(u,n;x,Z). Eq. (6) supplies very revealing information regarding the behavior of diffractionlimited coherent imaging systems in the frequency domain. If the aperture function, P(x,y), is indeed unity within some region, then there exists a finite pass-band in the frequency domain within the diffraction limited imaging system. At the boundary of this pass-band, the frequency response suddenly drops to zero, implying that frequency components outside the pass-band are completely eliminated. Eq. (7) indicates the cutoff frequency, where w is the radius of the aperture and zi is the distance to the image plane. Thus, the aperture can be defined as a low-pass filter, because high spatial frequency is blocked. As emphasized previously, the aperture is the most important factor for the definition of a coherent image. Thus, the experiment involved varying the diameter of circular aperture. Moreover, the beam shape was simulated in the image plane using Matlab 5.3 on the basis of Eqs. (2)–(7). Finally, the optimal value was determined after a comparison between a simulated beam shape and an ablated pattern.
ARTICLE IN PRESS D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
619
3. Results and discussion The characteristics of the mask projection method can be summarized that complex patterns can be machined with the use of a mask projection [9–11]. This implies that a three-dimensional ablated shape is dependent upon the shape of a projected beam. For instance, a rectangular-shaped channel could be fabricated with a mask projection using a square mask. However, ditch-shaped defects arise from a coherent image due to the miniaturization tendency of the pattern. Fig. 2 shows a coherent image on the PET surface after laser ablation under the following conditions: energy density per pulse 3.1 J/cm2, repetition ratio 200 Hz, and pulse number 10. It is evident that the spatial distribution of the laser beam is not ideal in the image plane. Therefore, the experiment was carried out by controlling the diameter of the aperture to correct the coherent image. Diffraction is variation in the directions and intensities of a group of waves after each passes through a mask. The Fresnel number, an index of diffraction, is a dimensionless parameter, F
w2 , lz
(8)
where w is the characteristic size (‘‘radius’’) of the aperture, l is the wavelength, and z is the distance from the aperture. The regime of F51 is called a Fraunhofer pattern in the far field, while FX1 produces a Fresnel pattern in the near field [12]. Fig. 3 shows the diffracted energy distribution simulated by Fourier optic theory, when the laser beam reaches the aperture. At that time, the Fresnel number [12] is 0.264, which implies that the beam shape in the aperture plane is close to the Fraunhofer diffraction pattern. According to Fourier optic theory, this Fraunhofer diffraction pattern is associated with the intensity of each spatial frequency of the
Fig. 2. Coherent image on the surface of a PET specimen.
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D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
Fig. 3. Diffracted energy distribution on a square mask in the aperture plane.
image plane. From these facts, it can be concluded that the distribution of the spatial frequency is influenced by diffraction in this optic system. Fig. 4 shows comparisons between the simulated beam shape and the crosssectional shape after PET ablation with various diameters of aperture. The laserbeam irradiation was conducted under the following conditions: scanning speed 100 mm/s, energy density per pulse 3.1 J/cm2, and a 200 Hz repetition ratio. In the image plane, the energy density value is 100, because the energy density of the source beam is converged with the magnification ratio (1/10) in the two-dimensional plane. By minimizing the diameter of the aperture, ditch-shaped defects were eliminated, because high spatial frequencies were blocked. At that point, the cutoff frequencies (fx,y) were as follows: (a) 116 cycles/mm, (b) 69.6 cycles/mm, (c) 46.4 cycles/mm, (d) 23.2 cycles/mm. Thus, it can be speculated that the coherent image originated from a high spatial frequency and can be controlled by the selection of the proper aperture. When the diameter of the aperture was minimized, the aspect ratio of the ablated channel became low while ditch-shaped defects were diminished. For the manufacture of a micro-mold insert, a low aspect ratio provides an advantage in separating between mold insert and product. The simulated beam shape, which was composed of both two- and threedimensional views, coincided approximately with the morphology of the ablated PET. However, there was a slight discrepancy, as illustrated in Fig. 4(b). We presumed that this difference was caused by the limitation of mesh size (5 5 mm). More exact results will be pursued in future studies.
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Fig. 4. Comparison between simulated energy density and a cross section of an ablated PET with various aperture diameters: (a) 5 mm, (b) 3 mm, (c) 2 mm and (d) 1 mm.
Fig. 5. Cross-sectional view of a nickel replica after electroforming on an ablated PET, with aperture diameters of (a) 5 mm, (b) 3 mm and (c) 2 mm.
Fig. 5 shows a cross-sectional view of a nickel replica after electroforming on an ablated PET using various aperture diameters. As shown in Fig. 5(a), protrusive edges (height: 5 mm) appeared at both corners. This was caused by the ditch-shaped defects of the prototype, which was manufactured by laser ablation. As shown in Figs. 5(b) and (c), the protrusive edges disappeared when the diameters of aperture were 3 and 2 mm, respectively. It is evident that a minimized aperture blocks high spatial frequencies in the aperture plane. Our experiment has revealed that aperture diameter is a primary factor for the prevention of ditch-shaped defects caused by a coherent image.
ARTICLE IN PRESS 622
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4. Conclusions This work is concerned about problems originating from a coherent beam in the image plane when a polymer prototype is fabricated by KrF excimer laser ablation using a mask projection. From a manufacturing perspective, this is a serious problem, because the ditch-shaped defects of the prototype are transferred to the protrusive edges of a mold insert and then to the final product. In order to eliminate these defects, Fourier optic theory was applied to our optic system. From the simplification of this theory and our experimental results, it was found that aperture diameter was the primary factor to correct such defects. It is possible that Fourier optic theory could calculate a beam shape by varying the aperture diameter in a mask projection system. Using an aperture with a diameter of less than 3 mm helped to eliminate ditch-shaped defects in the ablated rectangular microscopic channels on the PET. The analysis based on Fourier diffraction theory allows the prediction of the beam shape at the image surface when varying the aperture diameter. References [1] Lapezyna M, Stuke M. Rapid prototype fabrication of smooth microreactor channel systems in PMMA by VUV laser ablation at 157 nm for applications in genome analysis and biotechnology. Mater Res Soc Symp Proc 1998;526:143–8. [2] Kincade K. European firms take the lead in high-precision micromachining applications. Laser Focus World 2003;39(9):111–4. [3] Kim J, Xu X. Excimer laser fabrication of polymer microfluidic devices. J Laser Appl 2003;15(4):255–60. [4] Macdou M. Fundamentals of microfabrication. CRC Press; 1997. pp. 275–323. [5] Srinvansan R, Braren B. Excimer laser induced ablation has frequently been dealt with in Lambda Physik Highlights. J Polym Sci: Polym Chem Ed 22 1984:2601. [6] Hecht E. Optics, 4th ed. San Francisco: Addison Wesley; 2002. pp. 160–165. [7] Rabek JF. Photodegradation of polymers. Berlin: Springer; 1996. pp. 146–160. [8] Goodman JW. Introduction to Fourier optics, 2nd ed. McGraw-Hill; 1996. pp. 83–138. [9] Rumsby PT, Harvey EC, Thomas DW. Laser microprojection for micromechanical device fabrication. Proc SPIE 1996;2921:684–92. [10] Harvey EC, Rumsby PT. Fabrication techniques and their application to produce novel micromachined structures and devices using excimer laser projection. Proc SPIE 1997;3223:26–33. [11] Rizvi NH, Rumsby PT, Gower MC. New developments and applications in the production of 3D micro-structures by laser micro-machining. Proc SPIE 1999;3898:240–9. [12] Saleh BEA, Teich MC. Fundamentals of photonics. A Wiley-Interscience Publication; 1991. pp. 49–50.
Optics and Lasers in Engineering 44 (2006) 615–622
Correction of a coherent image during KrF excimer laser ablation using a mask projection Dong Sig Shina,, Jae Hoon Leea, Jeong Suha, To Hoon Kimb a
Laser Application Group, KIMM (Korea Institute of Machinery and Materials) 171 Jang-dong, Yuseong-gu, Dae-Jeon, South Korea b Department of Metallurgical System Engineering, Yonsei University, Seoul, South Korea Received 26 June 2004; received in revised form 20 April 2005; accepted 4 July 2005 Available online 22 August 2005
Abstract Using masks for laser ablation has proven useful in the fabrication of prototypes for the manufacturing of micro-fluidic devices. In this work, an excimer laser was used to engrave microscopic channels on the surface of polyethylene terephthalate (PET), which showed a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. When 50 mm wide rectangular microscopic channels were made using a 500 500 mm square mask and a magnification ratio of 1/10, ditch-shaped defects were found at both corners. The calculation of the laser beam intensity showed that a coherent image in the PET specimen caused the defects. An analysis based on the Fourier diffraction theory enabled the prediction of a coherent shape at the image plane, as well as a diffracted beam between the mask and the image plane. The analysis also showed that the diameter of the aperture was a predominant factor toward the elimination of ditch-shaped defects in the rectangular microscopic channels on the PET produced by an excimer laser ablation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Excimer laser ablation; Polyethylene terephthalate (PET); Mask projection; Fourier diffraction theory; Coherent image; Ditch-shaped defects; Micro-fluidic device
Corresponding author. Tel.: +82 72 868 7484; fax +82 42 868 7431.
E-mail address: [email protected] (D.S. Shin). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.07.001
ARTICLE IN PRESS 616
D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
1. Introduction The laser fabrication technique is a clean, safe, and convenient process for manufacturing MEMS devices in comparison with other technologies, such as chemical etching, deposition process, and lithography [1]. In particular, the advantage of the excimer laser application is that its non-thermal effects make it suitable for machining plastic, glass, and ceramic crystal [2]. Recently, mask patterning and direct laser writing techniques have been used for making various types of micro-fluidic channels and reservoirs [3]. However, this technology cannot be applied in mass production. Thus, the requirement of a micromold insert appeared on manufacturing micro-fluidic channels using excimer laser ablation. A micro-mold insert is produced by means of an electroforming process, after the platinum sputtering on a specific polymer, which ablated by means of an excimer laser. When this is done, micro-fluidic devices can be manufactured by using the micro-mold insert and the injection molding process. This implies that the excimer laser ablation method can be an alternate method for the X-ray lithography of Lithographie Galvanoformung Abformtechnik (LIGA) [4]. In contrast with lithography, excimer laser ablation using mask projection has possibilities available for three-dimensional pattern, varying an in-depth dimension across the lateral shape, by controlling the mask shape applied to each step [5]. However, the excimer laser beam should pass through a long distance compared to the lithography method. Thus, the diffraction theory should be considered when using a mask projection. A typical defect of laser ablation, as compared with lithography, is generated by a coherent image arising from the diffraction effect. The perturbation of energy is usually detected on the edge of a laser beam. This phenomenon affects any fabrication process using excimer laser mask projection. The micro-channels ablated by a coherent laser beam have ditch-shaped defects at the corners. As a result, these defects are transferred to an electroformed mold insert on an ablated polymer. Therefore, the coherent image should be corrected to prevent ditch-shaped defects in micro-channels. However, few studies have reported on the effects of a diffracted beam and the coherent image used in mask projected excimer laser ablation. In this study, the origin of a coherent image was clarified by applying Fourier optic theory. Then, the optimal factor was found to eliminate these defects, through a comparison between a simulated value and a real cross sectional view.
2. Method In this study, the effect of a coherent image was investigated. A KrF excimer laser (wavelength 248 nm) was used as a laser source to ablate polymers. An optical imaging system (LightDeck OPTEC S.A.) with a three-element processing lens (focal length: 86.9 mm) was used on the polymer surface. A laser beam passes through a mask, with a prefabricated pattern, and irradiates on the polymer surface by an imaging lens set.
ARTICLE IN PRESS D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
617
Fig. 1 shows the geometrical layout of the mask projection system. As shown in Fig. 1, the optical elements are composed of a square mask (500 500 mm), which is composed of a circular aperture and a converging lens. 1 1 1 þ ¼ . z1 z2 f
(1)
Eq. (1) shows that each point in the object plane is imaged onto a corresponding point in the plane along with a magnification factor. Therefore, the focal length (f) of a lens determines completely its effect on the paraxial rays [6]. This experiment was conducted using the following conditions: z1 ¼ 955, z2 ¼ 95:59, f ¼ 86:9 mm, and the overall magnification factor (B/A) was 1/10. The specimen material was polyethylene terephthalate (PET), which has a high absorption ratio for an excimer laser beam with a wavelength of 248 nm. Therefore, a photochemical reaction [7] is the main process between the PET and the UV laser beam (wavelength: 248 nm). Thus, PET is a useful material to analyze the correspondence between a beam shape and an ablated channel shape. Z Z
1
U i ðu; vÞ ¼
hðu; v; x; ZÞU 0 ðx; ZÞ dx dZ,
(2)
1
hðu; v; x; ZÞ ¼
1 k 2 k 2 exp j ðu þ v2 Þ exp j ðx þ Z2 Þ jlz1 z2 2z2 2z1 Z Z 1 k 1 1 1 Pðx; yÞ exp j þ ðx2 þ y2 Þ 2 z1 z2 f 1 x u Z v exp jk þ þ xþ y dx dy z1 z2 z1 z2
Fig. 1. Geometrical layout of mask projection.
ð3Þ
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618
Z Z 1 A u v hðu; v; x; ZÞ ¼ Pðx; yÞ exp j2p x þy dx dy lz2 lz2 lz2 1 A F xy fPðx; yÞgkx ¼ðu=lz2 Þ ky ¼v=lz2 : ¼ lz2
ð4Þ
Eqs. (2) and (3) now provide a formal solution specifying the relationship that exists between the object and the image. Eq. (3) represents the field amplitude produced by a unit amplitude point source applied at the object coordinates [8]. U0 and Ui are field distribution in the object and image planes. Goodman [8] simplified Eqs. (3)–(4). This implies that the aperture function, P(x,y), is a primary reaction factor. Hðf x ; f y Þ Z Z 1 1 2p ¼F P ðx; yÞ exp j ðux þ vyÞ dx dy , lz2 lz2 1 ¼ ðlz2 ÞPðlz2 f x ; lz2 f y Þ, Hðf x ; f y Þ ¼ Pðlz2 f x ; lz2 f y Þ,
f x;y ¼
w . lzi
ð5Þ (6)
(7)
The central matter is that a coherent imaging system is linear in complex amplitude. If a frequency analysis is to be applied in its usual form, it must be applied to the linear amplitude mapping. To do so, the amplitude transfer function, H(fx,fy), is induced [8]. Eq. (5) defines an amplitude transfer function as the Fourier transform of the amplitude point spread function, h(u,n;x,Z). Eq. (6) supplies very revealing information regarding the behavior of diffractionlimited coherent imaging systems in the frequency domain. If the aperture function, P(x,y), is indeed unity within some region, then there exists a finite pass-band in the frequency domain within the diffraction limited imaging system. At the boundary of this pass-band, the frequency response suddenly drops to zero, implying that frequency components outside the pass-band are completely eliminated. Eq. (7) indicates the cutoff frequency, where w is the radius of the aperture and zi is the distance to the image plane. Thus, the aperture can be defined as a low-pass filter, because high spatial frequency is blocked. As emphasized previously, the aperture is the most important factor for the definition of a coherent image. Thus, the experiment involved varying the diameter of circular aperture. Moreover, the beam shape was simulated in the image plane using Matlab 5.3 on the basis of Eqs. (2)–(7). Finally, the optimal value was determined after a comparison between a simulated beam shape and an ablated pattern.
ARTICLE IN PRESS D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
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3. Results and discussion The characteristics of the mask projection method can be summarized that complex patterns can be machined with the use of a mask projection [9–11]. This implies that a three-dimensional ablated shape is dependent upon the shape of a projected beam. For instance, a rectangular-shaped channel could be fabricated with a mask projection using a square mask. However, ditch-shaped defects arise from a coherent image due to the miniaturization tendency of the pattern. Fig. 2 shows a coherent image on the PET surface after laser ablation under the following conditions: energy density per pulse 3.1 J/cm2, repetition ratio 200 Hz, and pulse number 10. It is evident that the spatial distribution of the laser beam is not ideal in the image plane. Therefore, the experiment was carried out by controlling the diameter of the aperture to correct the coherent image. Diffraction is variation in the directions and intensities of a group of waves after each passes through a mask. The Fresnel number, an index of diffraction, is a dimensionless parameter, F
w2 , lz
(8)
where w is the characteristic size (‘‘radius’’) of the aperture, l is the wavelength, and z is the distance from the aperture. The regime of F51 is called a Fraunhofer pattern in the far field, while FX1 produces a Fresnel pattern in the near field [12]. Fig. 3 shows the diffracted energy distribution simulated by Fourier optic theory, when the laser beam reaches the aperture. At that time, the Fresnel number [12] is 0.264, which implies that the beam shape in the aperture plane is close to the Fraunhofer diffraction pattern. According to Fourier optic theory, this Fraunhofer diffraction pattern is associated with the intensity of each spatial frequency of the
Fig. 2. Coherent image on the surface of a PET specimen.
ARTICLE IN PRESS 620
D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
Fig. 3. Diffracted energy distribution on a square mask in the aperture plane.
image plane. From these facts, it can be concluded that the distribution of the spatial frequency is influenced by diffraction in this optic system. Fig. 4 shows comparisons between the simulated beam shape and the crosssectional shape after PET ablation with various diameters of aperture. The laserbeam irradiation was conducted under the following conditions: scanning speed 100 mm/s, energy density per pulse 3.1 J/cm2, and a 200 Hz repetition ratio. In the image plane, the energy density value is 100, because the energy density of the source beam is converged with the magnification ratio (1/10) in the two-dimensional plane. By minimizing the diameter of the aperture, ditch-shaped defects were eliminated, because high spatial frequencies were blocked. At that point, the cutoff frequencies (fx,y) were as follows: (a) 116 cycles/mm, (b) 69.6 cycles/mm, (c) 46.4 cycles/mm, (d) 23.2 cycles/mm. Thus, it can be speculated that the coherent image originated from a high spatial frequency and can be controlled by the selection of the proper aperture. When the diameter of the aperture was minimized, the aspect ratio of the ablated channel became low while ditch-shaped defects were diminished. For the manufacture of a micro-mold insert, a low aspect ratio provides an advantage in separating between mold insert and product. The simulated beam shape, which was composed of both two- and threedimensional views, coincided approximately with the morphology of the ablated PET. However, there was a slight discrepancy, as illustrated in Fig. 4(b). We presumed that this difference was caused by the limitation of mesh size (5 5 mm). More exact results will be pursued in future studies.
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621
Fig. 4. Comparison between simulated energy density and a cross section of an ablated PET with various aperture diameters: (a) 5 mm, (b) 3 mm, (c) 2 mm and (d) 1 mm.
Fig. 5. Cross-sectional view of a nickel replica after electroforming on an ablated PET, with aperture diameters of (a) 5 mm, (b) 3 mm and (c) 2 mm.
Fig. 5 shows a cross-sectional view of a nickel replica after electroforming on an ablated PET using various aperture diameters. As shown in Fig. 5(a), protrusive edges (height: 5 mm) appeared at both corners. This was caused by the ditch-shaped defects of the prototype, which was manufactured by laser ablation. As shown in Figs. 5(b) and (c), the protrusive edges disappeared when the diameters of aperture were 3 and 2 mm, respectively. It is evident that a minimized aperture blocks high spatial frequencies in the aperture plane. Our experiment has revealed that aperture diameter is a primary factor for the prevention of ditch-shaped defects caused by a coherent image.
ARTICLE IN PRESS 622
D.S. Shin et al. / Optics and Lasers in Engineering 44 (2006) 615–622
4. Conclusions This work is concerned about problems originating from a coherent beam in the image plane when a polymer prototype is fabricated by KrF excimer laser ablation using a mask projection. From a manufacturing perspective, this is a serious problem, because the ditch-shaped defects of the prototype are transferred to the protrusive edges of a mold insert and then to the final product. In order to eliminate these defects, Fourier optic theory was applied to our optic system. From the simplification of this theory and our experimental results, it was found that aperture diameter was the primary factor to correct such defects. It is possible that Fourier optic theory could calculate a beam shape by varying the aperture diameter in a mask projection system. Using an aperture with a diameter of less than 3 mm helped to eliminate ditch-shaped defects in the ablated rectangular microscopic channels on the PET. The analysis based on Fourier diffraction theory allows the prediction of the beam shape at the image surface when varying the aperture diameter. References [1] Lapezyna M, Stuke M. Rapid prototype fabrication of smooth microreactor channel systems in PMMA by VUV laser ablation at 157 nm for applications in genome analysis and biotechnology. Mater Res Soc Symp Proc 1998;526:143–8. [2] Kincade K. European firms take the lead in high-precision micromachining applications. Laser Focus World 2003;39(9):111–4. [3] Kim J, Xu X. Excimer laser fabrication of polymer microfluidic devices. J Laser Appl 2003;15(4):255–60. [4] Macdou M. Fundamentals of microfabrication. CRC Press; 1997. pp. 275–323. [5] Srinvansan R, Braren B. Excimer laser induced ablation has frequently been dealt with in Lambda Physik Highlights. J Polym Sci: Polym Chem Ed 22 1984:2601. [6] Hecht E. Optics, 4th ed. San Francisco: Addison Wesley; 2002. pp. 160–165. [7] Rabek JF. Photodegradation of polymers. Berlin: Springer; 1996. pp. 146–160. [8] Goodman JW. Introduction to Fourier optics, 2nd ed. McGraw-Hill; 1996. pp. 83–138. [9] Rumsby PT, Harvey EC, Thomas DW. Laser microprojection for micromechanical device fabrication. Proc SPIE 1996;2921:684–92. [10] Harvey EC, Rumsby PT. Fabrication techniques and their application to produce novel micromachined structures and devices using excimer laser projection. Proc SPIE 1997;3223:26–33. [11] Rizvi NH, Rumsby PT, Gower MC. New developments and applications in the production of 3D micro-structures by laser micro-machining. Proc SPIE 1999;3898:240–9. [12] Saleh BEA, Teich MC. Fundamentals of photonics. A Wiley-Interscience Publication; 1991. pp. 49–50.