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Printing via hot embossing of optically variable images in thermoplastic acrylic lacquer
Microelectronic Engineering 83 (2006) 1961–1965 www.elsevier.com/locate/mee
Printing via hot embossing of optically variable images in thermoplastic acrylic lacquer Patrick W. Leech *, Robert A. Lee, Tim J. Davis CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South MDC, 3169 Vict., Australia Received 8 November 2005; accepted 5 February 2006 Available online 9 March 2006
Abstract A novel printing process via hot embossing of either grating or micro-mirror microstructures has been demonstrated in thermoplastic acrylic lacquer. Embossing experiments were performed in the temperature range 100–150 C and at 80 kN force. The range of microstructures has included a dot-matrix hologram, grating-based optically variable devices (OVDs) and a micro-mirror based OVD. High quality replicas of each type of device have been fabricated using this process. Embossed replicas of grating-based OVDs have shown optical effects including image switching and color movement. For devices based on micro-mirror arrays, the embossed replicas have shown an optically variable switch between a portrait and a non-portrait image. Printing via an embossing process offers the possibility of incorporating optically variable devices into documents without the use of hot stamping foil. This is particularly relevant for documents based on polymeric substrates such as credit cards and polymer banknotes. 2006 Elsevier B.V. All rights reserved. Keywords: Hot embossing; Thermoplastic acrylic lacquer; Optically variable devices
1. Introduction Hot embossing has become a key technique in the replication of microstructures [1]. In particular, the hot embossing of diffractive gratings into metallised foil has been widely used in the mass production of holograms [2]. In many of these foil-based holograms, the microstructure has been based on a dot-matrix array which was generated by laser interference methods [2]. Each dot-shaped diffractive element within the matrix comprised a grating pattern with a single frequency and orientation. In recent years, the introduction of optically variable devices (OVDs) has provided a range of kinematic and optical switching effects for the enhanced security of documents [3,4]. These pixelated grating structures have been designed to give a variation in the diffractive image with change in the angle of viewing [3]. The use of electron beam lithography in the fabrication of OVD grating structures has produced images with the *
Corresponding author. Tel.: +61 3 9545 2791; fax: +61 3 9545 2844. E-mail address: [email protected] (P.W. Leech).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.02.001
sharpest definition of shapes and maximum brightness, together with a range of optical effects not able to be originated by traditional holographic techniques [3–5]. Another advantage of an electron beam has been the ability to specify the diffractive color within each pixel in the OVD [3,4]. However, the requirement of using metallised foil as a base for these diffractive microstructures has imposed significant additional cost in the fabrication process. In this paper, we examine a novel printing process in which security microstructures are embossed directly into a thermoplastic acrylic lacquer, thereby eliminating the requirement for metallised foil. This printing process is analogous to intaglio printing used in traditional security printing of banknotes. Both processes involve the transfer of a surface relief structure from a plate to a substrate by high pressure applied to an intermediate ink or lacquer layer. Advantages of acrylic lacquer are the low cost and ease of processing. We have examined the hot embossing of three types of microstructure: (a) a dot-matrix pattern, (b) optically variable devices based on arrays of diffraction gratings and (c) an optically variable device based on arrays of micro-mir-
1962
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
rors. Micro-mirror arrays have been produced by graytone lithography as an alternative architecture to gratings [6,7]. 2. Experimental details The substrates used in these experiments consisted of a layer of acrylic lacquer (100 lm thick) deposited on a sheet of polyvinyl chloride (PVC). The coating comprised a standard thermoplastic acrylic lacquer or external paint used in automotive applications [8]. The formulation was based on polymethyl methacrylate with a hydrocarbon solvent (toluene and xylene). The lacquer was applied by spray coating at room temperature directly onto rectangular samples (8 cm · 10 cm · 3 mm) of the PVC sheet. The coated substrates were immediately transferred to a convection oven in which they were baked at 70 C for 15 min. For the patterns used in embossing, a dot-matrix hologram (25 mm · 25 mm) was sourced from Shanghai Henglei Hologram Co. This hologram was fabricated by laser interferometry and comprised a portrait image equivalent to one of the images in the grating-based OVDs. The second type of pattern comprised optically variable devices (OVDs) based on diffraction gratings which were generated by electron beam lithography. Within each OVD were encoded two separate images by the interleaving of alternate tracks of pixels of width 30 lm. The alternate tracks had different arrangements of the groove angles and/or spacings (0.7–1.3 lm) [3,4]. The grating array was fabricated in an EBR-9 resist layer at a dose of 24 lC/cm2 using a Leica EBMF10.5 electron beam lithography system [3,4]. After exposure, the plate was developed by immersion in 1:1 MIBK:IPA solution. The pattern was then replicated as a nickel shim. Initially, a thin film (100 nm) of Ni was dc sputtered onto the patterned surface of the resist followed by electrodeposition of a thicker Ni layer (150 lm) in a nickel sulphamate bath. The dissolution of the resist layer by immersion in acetone then allowed the release of the Ni shim which contained a reverse copy of the original image. The third type of pattern comprised optically variable devices based on micro-mirror arrays which were fabricated by graytone lithography [6,7]. The
graytone mask was written using e-beam lithography and consisted of a periodic array of apertures. A dithering technique was used to smooth out the discrete nature of the apertures during UV exposure in AZ P4620 resist. An optical switch effect between a portrait and a non-portrait image was encoded into the OVD by the arrangement of the micro-mirrors as a separate interleaved channel for each image. During the embossing step, the Ni shim of hardness 265 HV and thickness 150 lm was pressed against the acrylic layer between the two heated platens. A force of 80 KN was applied between two planar platens at 100–150 ± 1 C for 60 s. The heating elements and a thermocouple were located within holes in the lower platen of the press. 3. Results and discussion Figs. 1(a) and (b) show scanning electron micrographs of the embossed microstructures for the dot-matrix hologram. The matrix of dots which defined the image was evident in Fig. 1(a) with a higher magnification view of the groove structure in Fig. 1(b). The grating structure had a uniform spacing with an undulating relief profile. As seen in Fig. 1(b), there was no evidence of defects in the replicated structure in the embossed lacquer. Figs. 2(a) and (b) show an embossed replica of the grating-based OVD. In Fig. 2(a), the lower magnification view shows the columns of vertical channels of width 30 lm with alternate grating structures. One series of alternating channels contained gratings of a wider spacing and corresponded to the first diffractive image. The alternate channels contained gratings of a finer pitch and corresponded to the second diffractive image. The higher magnification view in Fig. 2(b) shows the interface between a channel of coarser gratings (left) and finer gratings (right). Fig. 2(b) shows that the grating structures have been embossed into the coating with the generation of few defects. Minor regions of delamination were evident in some gratings adjacent to the interface. This defect has been attributed to a localized adhesion of lacquer to the shim during the de-molding stage. Measurements of the dimensional parameters of
Fig. 1. Scanning electron micrographs of dot-matrix hologram embossed into acrylic lacquer showing (a) the dot pattern and (b) the grating structure.
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
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Fig. 2. Scanning electron micrographs of a grating-based optically variable device embossed into acrylic lacquer showing (a) the pattern of vertical channels and (b) the grating structure at an interface between two channels.
Table 1 Dimensional measurements of the embossed microstructures in cured acrylic lacquer Microstructure
Depth of structure
Pitch of structure
Side-wall angle ()
Dot-matrix hologram Grating-based OVD Micro-mirror based OVD
275 nm 278 nm 15–20 lm
1150 nm 1250 nm 60 lm
38 ± 2 55 ± 3 30
the embossed microstructures have been summarized in Table 1. The depth and pitch of the grooves were equivalent for both the hologram and the grating-based OVD replicas. However, the average side-wall angle of the grooves in the OVD (55 to the horizontal) was significantly greater in inclination than for the side-walls of the hologram (38 to the horizontal). Figs. 3 and 4 show lower magnification views of the grating-based OVDs embossed into acrylic lacquer. These images were obtained under illumination using a standard fluorescent source. Fig. 3 shows a single embossing with outer dimensions of 80 mm · 80 mm and comprising examples of four grating-based OVDs. The individual OVDs shown in Fig. 3 were in the size range 15 cm–25 mm. Each of these OVDs has demonstrated an optical switch in the embossed image. Another feature of the embossed OVDs was the ability to reproduce a full range of iridescent diffractive colors. The level of intensity of the embossed iridescent images was dependent on both the color and texture of the cured layer of lacquer. Embossing trials were conducted in several standard colors of acrylic lacquer coating including blue, red, white, brown and black. The highest intensity in the images embossed in the acrylic lacquer was displayed with the use of black gloss coatings. Figs. 4(a) and (b) show an example of an image switch obtained by tilting of the grating-based OVD in relation to the fixed light source. The main switch in this OVD comprised the portrait at the centre of the image and the numeric ‘‘50’’. Also visible in Fig. 4(a) was a movement in the color within the concentric background rings as well as across the radially oriented segments. In the design of
the OVD, a different grating spacing was assigned in each of the segments as selected from a 10 element palette. In Fig. 4(a) and (b), the color movement and image switch were visible as a change in the gray level in the individual areas. The grating arrays were uniformly embossed over the entire area of a nickel shim which was typically 80 · 80 mm. Within the embossed area were typically included several OVDs. The temperature of embossing was a critical factor in attaining a precise replication of the gratings over this area. In the present experiments, a temperature range of 120–140 C was established as optimal for embossing in the acrylic lacquer. Embossing at temperatures below 120 C resulted in areas of incomplete reproduction of the image. At these temperatures, the pattern was replicated into the acrylic coating only across the central region of the surface with an incomplete patterning
Fig. 3. A single embossing of four grating-based OVDs into black acrylic lacquer over dimensions of 80 mm · 80 mm.
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P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
Fig. 4. Optical micrographs of an OVD with dimensions of 25 · 25 mm embossed into black acrylic lacquer. The images in a) and b) were photographed at two different angles of tilt.
of the outer regions of the test sample. In comparison, the embossing of the acrylic lacquer at temperatures in excess of 140 C produced an excessive level of compressive deformation of the PVC substrate at the standard force of 80 KN. These results were consistent with the reflow temperature of 130–150 C typically used in the processing of thermoplastic acrylic [8]. Figs. 5(a) and (b) show scanning electron micrographs of the surface relief formed by hot embossing of a micromirror based OVD. In order to produce two images comprised of micro-mirrors which reflected light at different angles, but overlapping each other within the same OVD area, the entire structure was divided into two alternating rows. One image was encoded into the structure on the first row while the other image was encoded as micro-mirrors of different slope on the alternate rows. The lower magnification micrograph in Fig. 5(a) shows alternate rows of micromirrors corresponding to the non-portrait (continuous micro-mirrors) and portrait (individual micro-mirrors with variable-width) elements. For the portrait image, the intensity of the reflected light from an individual pixel as a function of local position within the portrait was modulated by
varying the area of the micro-mirror surface within a pixel. A variation in the width of the micro-mirror regions from 10–60 lm was implemented across the area of the portrait. The level of grayscale in each pixel region of the portrait then corresponded to the distribution of brightness within the artwork. The non-portrait image was designed in the form of a graphic element or logo. The outline of the shape of the logo was defined by micro-mirrors of a specific slope (30) and orientation. In this image, we have used a dithered pattern of dot exposures in the mask in order to achieve the smoothest possible gradation in graytone density across a pixel. A single micro-mirror was encoded into the mask pixel as a gradation in transparency. The slope was controlled by variation in the graded density of the apertures. In the micrograph of the embossed acrylic lacquer shown in Fig. 5(b), both the variable-width micro-mirrors and the continuous micro-mirrors were reproduced with high precision. Fig. 5(b) has demonstrated the ability of the embossing process to replicate micro-mirror structures which were 15 lm deep. The pre-curing step was evidently effective in removing most of the solvents in the
Fig. 5. Scanning electron micrographs of micro-mirror based optically variable device embossed in acrylic lacquer and shown at (a) low magnification and (b) higher magnification.
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
1965
Fig. 6. Micro-mirror based optically variable device embossed in black acrylic lacquer. The two images (a) and (b) were obtained at different angles of tilt to the fixed light source. Magnification3.5 ·.
coating, since there were few signs of bubbles due to gas escape during embossing. Also, the absence of other defects in these structures has indicated that the sloped side-walls facilitated the separation of the replica from the shim during de-molding. In general, the deeper micro-mirror structures (15 lm) were more difficult to emboss than the shallower grating structures (0.25 lm depth). The main defect encountered was the attachment of small areas of acrylic layer to the recessed regions of the shim. Separation of the shim then resulted in the transfer of segments of the acrylic layer to the shim. The application of a very thin layer of lubricant at the commencement of use of the shim has effectively eliminated any evidence of adhesion. A lower magnification photograph of the micro-mirror array produced in these experiments is shown in Fig. 6. The optical switch between the portrait and non-portrait (logo) image was clearly evident without signs of interference effects from one image to the other. The optical switch between the images was based on the different angles of reflection of the two groups of micro-mirrors. The switch effect was visible by varying either the angle of illumination or the angle of viewing of the device. The entire array of micro-mirrors shown in Fig. 6 was comprised of individual pixels of 60 lm · 60 lm. An array of the dimensions shown in Fig. 6 contained 50,000 micro-mirrors. These results have demonstrated the ability of acrylic lacquer to function as a workpiece for the hot embossing of both grating and micro-mirror arrays. This process has several advantages compared with embossing of standard foil including a significantly lower cost and a simpler method of fabrication. Furthermore, the embossing of a hologram or OVD in thermoplastic lacquer produced a continuity of the feature with the surrounding coating instead of application as a separate foil patch. In this way, the process provides the potential to integrate the embossed structure with the surrounding coating.
4. Conclusions A novel process has been demonstrated for the hot embossing of security microstructures in thermoplastic acrylic lacquer. High quality images of both grating and micro-mirror devices have been successfully embossed over areas up to 80 mm · 80 mm. The optimum temperature for embossing was 120–140 C, corresponding to the thermal conditions for reflow of the acrylic lacquer. Grating-based OVDs which were embossed in acrylic layer have displayed a full range of optically variable effects in the diffractive images. Deeper structures (15 lm) comprised of micromirror arrays have been embossed without defects provided that a thin lubricant layer was initially applied to the Ni shim. The embossed micro-mirror arrays have displayed an optical switch between a portrait and a non-portrait image. Acknowledgements E-beam lithography was performed by R. Marnock and growth of nickel shims by B. Sexton and F. Smith. References [1] M. Hechele, W.K. Schomburg, J. Micromech. Microeng. 14 (2004) R1. [2] R.L. van Renesse, Optical Document Security, third ed., Artech House, Boston, 2005. [3] R.A. Lee, Microelectron. Eng. 53 (2000) 513. [4] R.A. Lee, Proceedings of the 9th Interpol Conference on Currency Counterfeiting, Helsinki, 9–13 June 1997. [5] P.W. Leech, B.A. Sexton, R.J. Marnock, Microelectron. Eng. 60 (2002) 339. [6] P.W. Leech, H. Zeidler, Microelectron. Eng. 65 (2003) 439. [7] P.W. Leech, R.A. Lee, Microelectron. Eng. 83 (2006) 351. [8] D.A. Ansdell, in: R. Lambourne, T.A. Strivens (Eds.), Paint and Surface Coatings – Theory and Practice, Woodhead Publishing, 1999.
Printing via hot embossing of optically variable images in thermoplastic acrylic lacquer Patrick W. Leech *, Robert A. Lee, Tim J. Davis CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South MDC, 3169 Vict., Australia Received 8 November 2005; accepted 5 February 2006 Available online 9 March 2006
Abstract A novel printing process via hot embossing of either grating or micro-mirror microstructures has been demonstrated in thermoplastic acrylic lacquer. Embossing experiments were performed in the temperature range 100–150 C and at 80 kN force. The range of microstructures has included a dot-matrix hologram, grating-based optically variable devices (OVDs) and a micro-mirror based OVD. High quality replicas of each type of device have been fabricated using this process. Embossed replicas of grating-based OVDs have shown optical effects including image switching and color movement. For devices based on micro-mirror arrays, the embossed replicas have shown an optically variable switch between a portrait and a non-portrait image. Printing via an embossing process offers the possibility of incorporating optically variable devices into documents without the use of hot stamping foil. This is particularly relevant for documents based on polymeric substrates such as credit cards and polymer banknotes. 2006 Elsevier B.V. All rights reserved. Keywords: Hot embossing; Thermoplastic acrylic lacquer; Optically variable devices
1. Introduction Hot embossing has become a key technique in the replication of microstructures [1]. In particular, the hot embossing of diffractive gratings into metallised foil has been widely used in the mass production of holograms [2]. In many of these foil-based holograms, the microstructure has been based on a dot-matrix array which was generated by laser interference methods [2]. Each dot-shaped diffractive element within the matrix comprised a grating pattern with a single frequency and orientation. In recent years, the introduction of optically variable devices (OVDs) has provided a range of kinematic and optical switching effects for the enhanced security of documents [3,4]. These pixelated grating structures have been designed to give a variation in the diffractive image with change in the angle of viewing [3]. The use of electron beam lithography in the fabrication of OVD grating structures has produced images with the *
Corresponding author. Tel.: +61 3 9545 2791; fax: +61 3 9545 2844. E-mail address: [email protected] (P.W. Leech).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.02.001
sharpest definition of shapes and maximum brightness, together with a range of optical effects not able to be originated by traditional holographic techniques [3–5]. Another advantage of an electron beam has been the ability to specify the diffractive color within each pixel in the OVD [3,4]. However, the requirement of using metallised foil as a base for these diffractive microstructures has imposed significant additional cost in the fabrication process. In this paper, we examine a novel printing process in which security microstructures are embossed directly into a thermoplastic acrylic lacquer, thereby eliminating the requirement for metallised foil. This printing process is analogous to intaglio printing used in traditional security printing of banknotes. Both processes involve the transfer of a surface relief structure from a plate to a substrate by high pressure applied to an intermediate ink or lacquer layer. Advantages of acrylic lacquer are the low cost and ease of processing. We have examined the hot embossing of three types of microstructure: (a) a dot-matrix pattern, (b) optically variable devices based on arrays of diffraction gratings and (c) an optically variable device based on arrays of micro-mir-
1962
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
rors. Micro-mirror arrays have been produced by graytone lithography as an alternative architecture to gratings [6,7]. 2. Experimental details The substrates used in these experiments consisted of a layer of acrylic lacquer (100 lm thick) deposited on a sheet of polyvinyl chloride (PVC). The coating comprised a standard thermoplastic acrylic lacquer or external paint used in automotive applications [8]. The formulation was based on polymethyl methacrylate with a hydrocarbon solvent (toluene and xylene). The lacquer was applied by spray coating at room temperature directly onto rectangular samples (8 cm · 10 cm · 3 mm) of the PVC sheet. The coated substrates were immediately transferred to a convection oven in which they were baked at 70 C for 15 min. For the patterns used in embossing, a dot-matrix hologram (25 mm · 25 mm) was sourced from Shanghai Henglei Hologram Co. This hologram was fabricated by laser interferometry and comprised a portrait image equivalent to one of the images in the grating-based OVDs. The second type of pattern comprised optically variable devices (OVDs) based on diffraction gratings which were generated by electron beam lithography. Within each OVD were encoded two separate images by the interleaving of alternate tracks of pixels of width 30 lm. The alternate tracks had different arrangements of the groove angles and/or spacings (0.7–1.3 lm) [3,4]. The grating array was fabricated in an EBR-9 resist layer at a dose of 24 lC/cm2 using a Leica EBMF10.5 electron beam lithography system [3,4]. After exposure, the plate was developed by immersion in 1:1 MIBK:IPA solution. The pattern was then replicated as a nickel shim. Initially, a thin film (100 nm) of Ni was dc sputtered onto the patterned surface of the resist followed by electrodeposition of a thicker Ni layer (150 lm) in a nickel sulphamate bath. The dissolution of the resist layer by immersion in acetone then allowed the release of the Ni shim which contained a reverse copy of the original image. The third type of pattern comprised optically variable devices based on micro-mirror arrays which were fabricated by graytone lithography [6,7]. The
graytone mask was written using e-beam lithography and consisted of a periodic array of apertures. A dithering technique was used to smooth out the discrete nature of the apertures during UV exposure in AZ P4620 resist. An optical switch effect between a portrait and a non-portrait image was encoded into the OVD by the arrangement of the micro-mirrors as a separate interleaved channel for each image. During the embossing step, the Ni shim of hardness 265 HV and thickness 150 lm was pressed against the acrylic layer between the two heated platens. A force of 80 KN was applied between two planar platens at 100–150 ± 1 C for 60 s. The heating elements and a thermocouple were located within holes in the lower platen of the press. 3. Results and discussion Figs. 1(a) and (b) show scanning electron micrographs of the embossed microstructures for the dot-matrix hologram. The matrix of dots which defined the image was evident in Fig. 1(a) with a higher magnification view of the groove structure in Fig. 1(b). The grating structure had a uniform spacing with an undulating relief profile. As seen in Fig. 1(b), there was no evidence of defects in the replicated structure in the embossed lacquer. Figs. 2(a) and (b) show an embossed replica of the grating-based OVD. In Fig. 2(a), the lower magnification view shows the columns of vertical channels of width 30 lm with alternate grating structures. One series of alternating channels contained gratings of a wider spacing and corresponded to the first diffractive image. The alternate channels contained gratings of a finer pitch and corresponded to the second diffractive image. The higher magnification view in Fig. 2(b) shows the interface between a channel of coarser gratings (left) and finer gratings (right). Fig. 2(b) shows that the grating structures have been embossed into the coating with the generation of few defects. Minor regions of delamination were evident in some gratings adjacent to the interface. This defect has been attributed to a localized adhesion of lacquer to the shim during the de-molding stage. Measurements of the dimensional parameters of
Fig. 1. Scanning electron micrographs of dot-matrix hologram embossed into acrylic lacquer showing (a) the dot pattern and (b) the grating structure.
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
1963
Fig. 2. Scanning electron micrographs of a grating-based optically variable device embossed into acrylic lacquer showing (a) the pattern of vertical channels and (b) the grating structure at an interface between two channels.
Table 1 Dimensional measurements of the embossed microstructures in cured acrylic lacquer Microstructure
Depth of structure
Pitch of structure
Side-wall angle ()
Dot-matrix hologram Grating-based OVD Micro-mirror based OVD
275 nm 278 nm 15–20 lm
1150 nm 1250 nm 60 lm
38 ± 2 55 ± 3 30
the embossed microstructures have been summarized in Table 1. The depth and pitch of the grooves were equivalent for both the hologram and the grating-based OVD replicas. However, the average side-wall angle of the grooves in the OVD (55 to the horizontal) was significantly greater in inclination than for the side-walls of the hologram (38 to the horizontal). Figs. 3 and 4 show lower magnification views of the grating-based OVDs embossed into acrylic lacquer. These images were obtained under illumination using a standard fluorescent source. Fig. 3 shows a single embossing with outer dimensions of 80 mm · 80 mm and comprising examples of four grating-based OVDs. The individual OVDs shown in Fig. 3 were in the size range 15 cm–25 mm. Each of these OVDs has demonstrated an optical switch in the embossed image. Another feature of the embossed OVDs was the ability to reproduce a full range of iridescent diffractive colors. The level of intensity of the embossed iridescent images was dependent on both the color and texture of the cured layer of lacquer. Embossing trials were conducted in several standard colors of acrylic lacquer coating including blue, red, white, brown and black. The highest intensity in the images embossed in the acrylic lacquer was displayed with the use of black gloss coatings. Figs. 4(a) and (b) show an example of an image switch obtained by tilting of the grating-based OVD in relation to the fixed light source. The main switch in this OVD comprised the portrait at the centre of the image and the numeric ‘‘50’’. Also visible in Fig. 4(a) was a movement in the color within the concentric background rings as well as across the radially oriented segments. In the design of
the OVD, a different grating spacing was assigned in each of the segments as selected from a 10 element palette. In Fig. 4(a) and (b), the color movement and image switch were visible as a change in the gray level in the individual areas. The grating arrays were uniformly embossed over the entire area of a nickel shim which was typically 80 · 80 mm. Within the embossed area were typically included several OVDs. The temperature of embossing was a critical factor in attaining a precise replication of the gratings over this area. In the present experiments, a temperature range of 120–140 C was established as optimal for embossing in the acrylic lacquer. Embossing at temperatures below 120 C resulted in areas of incomplete reproduction of the image. At these temperatures, the pattern was replicated into the acrylic coating only across the central region of the surface with an incomplete patterning
Fig. 3. A single embossing of four grating-based OVDs into black acrylic lacquer over dimensions of 80 mm · 80 mm.
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P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
Fig. 4. Optical micrographs of an OVD with dimensions of 25 · 25 mm embossed into black acrylic lacquer. The images in a) and b) were photographed at two different angles of tilt.
of the outer regions of the test sample. In comparison, the embossing of the acrylic lacquer at temperatures in excess of 140 C produced an excessive level of compressive deformation of the PVC substrate at the standard force of 80 KN. These results were consistent with the reflow temperature of 130–150 C typically used in the processing of thermoplastic acrylic [8]. Figs. 5(a) and (b) show scanning electron micrographs of the surface relief formed by hot embossing of a micromirror based OVD. In order to produce two images comprised of micro-mirrors which reflected light at different angles, but overlapping each other within the same OVD area, the entire structure was divided into two alternating rows. One image was encoded into the structure on the first row while the other image was encoded as micro-mirrors of different slope on the alternate rows. The lower magnification micrograph in Fig. 5(a) shows alternate rows of micromirrors corresponding to the non-portrait (continuous micro-mirrors) and portrait (individual micro-mirrors with variable-width) elements. For the portrait image, the intensity of the reflected light from an individual pixel as a function of local position within the portrait was modulated by
varying the area of the micro-mirror surface within a pixel. A variation in the width of the micro-mirror regions from 10–60 lm was implemented across the area of the portrait. The level of grayscale in each pixel region of the portrait then corresponded to the distribution of brightness within the artwork. The non-portrait image was designed in the form of a graphic element or logo. The outline of the shape of the logo was defined by micro-mirrors of a specific slope (30) and orientation. In this image, we have used a dithered pattern of dot exposures in the mask in order to achieve the smoothest possible gradation in graytone density across a pixel. A single micro-mirror was encoded into the mask pixel as a gradation in transparency. The slope was controlled by variation in the graded density of the apertures. In the micrograph of the embossed acrylic lacquer shown in Fig. 5(b), both the variable-width micro-mirrors and the continuous micro-mirrors were reproduced with high precision. Fig. 5(b) has demonstrated the ability of the embossing process to replicate micro-mirror structures which were 15 lm deep. The pre-curing step was evidently effective in removing most of the solvents in the
Fig. 5. Scanning electron micrographs of micro-mirror based optically variable device embossed in acrylic lacquer and shown at (a) low magnification and (b) higher magnification.
P.W. Leech et al. / Microelectronic Engineering 83 (2006) 1961–1965
1965
Fig. 6. Micro-mirror based optically variable device embossed in black acrylic lacquer. The two images (a) and (b) were obtained at different angles of tilt to the fixed light source. Magnification3.5 ·.
coating, since there were few signs of bubbles due to gas escape during embossing. Also, the absence of other defects in these structures has indicated that the sloped side-walls facilitated the separation of the replica from the shim during de-molding. In general, the deeper micro-mirror structures (15 lm) were more difficult to emboss than the shallower grating structures (0.25 lm depth). The main defect encountered was the attachment of small areas of acrylic layer to the recessed regions of the shim. Separation of the shim then resulted in the transfer of segments of the acrylic layer to the shim. The application of a very thin layer of lubricant at the commencement of use of the shim has effectively eliminated any evidence of adhesion. A lower magnification photograph of the micro-mirror array produced in these experiments is shown in Fig. 6. The optical switch between the portrait and non-portrait (logo) image was clearly evident without signs of interference effects from one image to the other. The optical switch between the images was based on the different angles of reflection of the two groups of micro-mirrors. The switch effect was visible by varying either the angle of illumination or the angle of viewing of the device. The entire array of micro-mirrors shown in Fig. 6 was comprised of individual pixels of 60 lm · 60 lm. An array of the dimensions shown in Fig. 6 contained 50,000 micro-mirrors. These results have demonstrated the ability of acrylic lacquer to function as a workpiece for the hot embossing of both grating and micro-mirror arrays. This process has several advantages compared with embossing of standard foil including a significantly lower cost and a simpler method of fabrication. Furthermore, the embossing of a hologram or OVD in thermoplastic lacquer produced a continuity of the feature with the surrounding coating instead of application as a separate foil patch. In this way, the process provides the potential to integrate the embossed structure with the surrounding coating.
4. Conclusions A novel process has been demonstrated for the hot embossing of security microstructures in thermoplastic acrylic lacquer. High quality images of both grating and micro-mirror devices have been successfully embossed over areas up to 80 mm · 80 mm. The optimum temperature for embossing was 120–140 C, corresponding to the thermal conditions for reflow of the acrylic lacquer. Grating-based OVDs which were embossed in acrylic layer have displayed a full range of optically variable effects in the diffractive images. Deeper structures (15 lm) comprised of micromirror arrays have been embossed without defects provided that a thin lubricant layer was initially applied to the Ni shim. The embossed micro-mirror arrays have displayed an optical switch between a portrait and a non-portrait image. Acknowledgements E-beam lithography was performed by R. Marnock and growth of nickel shims by B. Sexton and F. Smith. References [1] M. Hechele, W.K. Schomburg, J. Micromech. Microeng. 14 (2004) R1. [2] R.L. van Renesse, Optical Document Security, third ed., Artech House, Boston, 2005. [3] R.A. Lee, Microelectron. Eng. 53 (2000) 513. [4] R.A. Lee, Proceedings of the 9th Interpol Conference on Currency Counterfeiting, Helsinki, 9–13 June 1997. [5] P.W. Leech, B.A. Sexton, R.J. Marnock, Microelectron. Eng. 60 (2002) 339. [6] P.W. Leech, H. Zeidler, Microelectron. Eng. 65 (2003) 439. [7] P.W. Leech, R.A. Lee, Microelectron. Eng. 83 (2006) 351. [8] D.A. Ansdell, in: R. Lambourne, T.A. Strivens (Eds.), Paint and Surface Coatings – Theory and Practice, Woodhead Publishing, 1999.