Head-up display using an inclined Al2O3 column array Wen-Hao Cho,1 Chao-Te Lee,1 Chi-Chung Kei,1 Bo-Huei Liao,1 Donyau Chiang,1 and Cheng-Chung Lee2,* 1
Instrument Technology Research Center, National Applied Research Laboratories, 300 Hsin-Chu, Taiwan 2
Thin Film Technology Center, National Central University, 320 Chung-Li, Taiwan *Corresponding author: [email protected]
Received 4 September 2013; revised 30 October 2013; accepted 30 October 2013; posted 31 October 2013 (Doc. ID 196895); published 3 December 2013
An orderly inclined Al2 O3 column array was fabricated by atomic layer deposition and sequential electron beam evaporation using a hollow nanosphere template. The transmittance spectra at various angles of incidence were obtained through the use of a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer. The inclined column array could display the image information through a scattering mechanism and was transparent at high viewing angles along the deposition plane. This characteristic of the inclined column array gives it potential for applications in head-up displays in the automotive industry. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (230.4000) Microstructure fabrication. http://dx.doi.org/10.1364/AO.53.00A121
1. Introduction
Head-up displays (HUDs) have been used in the aircraft industry since 1960, when they were first introduced in the military aircraft Hawker-Siddeley Buccaneer (Enderby and Wood 1992) [1]. In 1970, HUDs were introduced to commercial aviation. They have also been used in the automotive industry since 1988 [2] and have recently become more popular in vehicles due to driver safety concerns. HUDs can reduce the frequency and duration of the driver’s eyes being off the road by projecting the required information in front of the driver. This should aid drivers by making them able to react more quickly to changes in driving conditions [3]. Most HUD screens display information by reflecting light to the user’s eyes. However, the reflection method has the disadvantage of producing a ghost image due to multiple reflections [4,5]. In this study, we develop a thin film with a novel microstructure that can display information via the scattering mechanism and is transparent at a high viewing angle. This special characteristic of the
1559-128X/14/04A121-04$15.00/0 © 2014 Optical Society of America
thin film gives it potential for application in HUDs in the automotive industry. 2. Experimental Procedures
As shown in Fig. 1, polystyrene (PS) nanospheres of 590 nm diameter received as suspensions in water (10.0 wt. %, Bangs Laboratories Inc.) were spin coated onto substrates without dilution. Reactive ion etching (RIE) using oxygen plasma was applied to reduce the diameter of the PS nanospheres. The size of the PS nanospheres can be controlled by changing the duration of ion bombardment. An Al2 O3 coating was deposited on the surface of the nanospheres through atomic layer deposition (ALD) to prevent melting of the PS due to the high substrate temperature during electron beam evaporation. ALD is a unique process that produces highly conformal films and allows atomic-scale thickness control. ALD is based on the self-limiting reaction. In the ALD process, Al2 O3 was coated by using H2 O and trimethylaluminum (TMA) as precursors at room temperature, and nitrogen was the purge gas. This process has been described in detail by Wang et al. [6]. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 3. Cross-sectional SEM images of Al2 O3 along direction c.
Fig. 1. Flow chart for preparation of ordered arrays of Al2 O3 columns: (a) nanospheres were spin coated on the substrate; (b) the diameter of the nanospheres was reduced after a 130 s RIE treatment; (c) after the ALD process, Al2 O3 was coated on the surface of the nanospheres; (d) nanosphere shells were formed by heating at 350°C; (e) after oblique deposition, inclined Al2 O3 columns were grown.
deposition and the shape of the top of the column, we know that the vapor flows along direction c in Fig. 2(b) during evaporation [7]. Furthermore, the
The PS nanospheres were removed by heating to 350°C in air to form an ordered array of hollow nanospheres. The samples were then transferred to the electron beam deposition system for the preparation of inclined column arrays. The oblique deposition angle between the substrate normal and the direction of vapor flow was 75.5°. Specular transmittance spectra at various angles of incidence were obtained by a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer equipped a rotatable sample holder. The beam size is 2 mm (width) × 5 mm (height). The surface morphologies and dimensions of the nanospheres and inclined column array were observed by scanning electron microscopy (SEM) (Hitachi S-4300). 3. Results and Discussion
As shown in Fig. 2(a), the diameter of the hexagonally close-packed PS nanospheres was reduced to 410 nm after the RIE size-reduction process. After deposition of the 950 nm thick Al2 O3 film by electron beam evaporation, an inclined column array was formed on the nanosphere shell template, as shown in Fig. 2(b). From the fan-out phenomenon of oblique
Fig. 2. Plan-view SEM images of (a) nanosphere array and (b) Al2 O3 inclined columns. A122
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Fig. 4. Specular transmittance spectra at various angles of incidence.
Fig. 5. Specular and diffuse spectra of the inclined Al2 O3 column array at a normal angle of incidence.
Fig. 6. Effect on a real object viewed through the inclined column array at (a) normal and (b) a tilted angle (about 45°).
surface of the top of the column facing the vapor flow, the yellow line in Fig. 3, was smoother than that of the perpendicular one. The specular transmittance spectra at the various angles of incidence are shown in Fig. 4. The incident light was on the deposition plane. The incidence angle was defined as the angle between the surface normal and the direction of incidence, with positive angles being toward the direction of the deposition direction as shown in the inset of Fig. 4. As shown in Fig. 4, the inclined column array had a lower specular transmittance at around the normal incidence angle, and the transmittance increased as the incidence angle increased. Figure 5 shows the scattering measurement at the normal incidence angle. As shown in Fig. 5, the lower specular transmittance resulted from light scattering caused by rough morphology. According to the SEM image discussed above, the high transmittance at a high angle of incidence resulted from the lower amount of scattering caused by the stair-like surface at a high angle of view and the smooth surface of the top of the column facing the vapor flow. We found that scattering made up a major portion of the total reflection and was the reason for lower specular transmittance. Therefore, the inclined column array provided a real image such as a projection screen at the normal direction by scattering. However, the troughs in the transmittance
spectra were a result of the photonic crystalline behavior of the orderly inclined column array [8]. Figure 6 shows the effect on the real object through the inclined column array at the normal and tilt angles along the deposition plane. It can be seen that the words on the pen are hidden in a mist at the normal viewing angle but appear when the film sample is tilted. Figure 7 shows the scattering and specular transmittance spectrum at a 45° incidence angle extracted from Fig. 4. The scattering and transmittance spectra of the sample in Fig. 7 correspond to those in Figs. 6(a) and 6(b), respectively. The average specular transmittance at 45° was 68.23% in the visible wavelength. The characteristics of the inclined column array could have potential applications as HUDs in the automotive industry. Figure 8 shows a schematic drawing of HUDs for a projection screen fabricated using inclined column arrays. The information light propagates through a condensed lens and is incident on the inclined column array at the normal direction. Therefore, a driver can see the information on the projection screen due to the scattering characteristics of inclined column arrays. Furthermore, the driver also sees the road conditions through the inclined column array at a high viewing angle. The view angles from 40° to 60° are appropriate application ranges as shown in Fig. 4. According to Fermat’s principle, the information reflected by a conventional display can only be seen from a few narrow viewing angles. On the other hand, the inclined column array
Fig. 7. Scattering spectrum at normal and specular transmittance spectrum at 45°.
Fig. 8. Schematic representation of HUDs using an inclined column array as a projection screen. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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provides a real image that can be seen from every angle of view. However, the change of focus point is still a disadvantage of this display method during the drive as well as conventional methods. 4. Conclusion
In this study, an orderly inclined Al2 O3 column array was fabricated by ALD and sequential electron beam evaporation using a hollow nanosphere template. The inclined Al2 O3 column array deposited on a rigid or flexible transparent object as a novel display method was proposed for application in the automotive industry. Through scattering on the surface, the inclined Al2 O3 column array provided a real image that was different from the conventional display methods. References 1. J. Crawford and A. Neal, “A review of the perceptual and cognitive issues associated with the use of head-up displays in commercial aviation,” Int. J. Aviat. Psychol. 16, 1–19 (2006).
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2. S. Okabayashi, N. Sugie, M. Imaizumi, M. Furukawa, and T. Hatada, “Visual perception of HUD (head-up display) image with small angle of depression in practical automotive use,” Electron Comm. Jpn. 82, 1–9 (1999). 3. S. Smith and S.-H. Fu, “The relationships between automobile head-up display presentation images and drivers’ Kansei,” Displays 32, 58–68 (2011). 4. K. Makita, “Combiner for head-up display (HUD) system,” J. Sol-Gel Sci. Technol. 47, 209–211 (2008). 5. E. Tolstik, A. Winkler, V. Matusevich, R. Kowarschik, U. V. Mahilny, D. N. Marmysh, Y. I. Matusevich, and L. P. Krul, “PMMA-PQ photopolymers for head-up displays,” IEEE Photon. Technol. Lett. 21, 784–786 (2009). 6. C. C. Wang, C. C. Kei, Y. W. Yu, and T. P. Perng, “Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition,” Nano Lett. 7, 1566–1569 (2007). 7. D.-X. Ye, T. Karabacak, B. K. Lim, G.-C. Wang, and T.-M. Lu, “Growth of uniformly aligned nanorod arrays by oblique angle deposition with two-phase substrate rotation,” Nanotechnology 15, 817–821 (2004). 8. W.-H. Cho, C.-T. Lee, C.-C. Yu, C.-C. Kei, D.-R. Liu, and C.-C. Lee, “Microstructure and optical properties of Al2O3 prepared by oblique deposition using microsphere shell templates,” Appl. Opt. 50, C246–C249 (2011).
Instrument Technology Research Center, National Applied Research Laboratories, 300 Hsin-Chu, Taiwan 2
Thin Film Technology Center, National Central University, 320 Chung-Li, Taiwan *Corresponding author: [email protected]
Received 4 September 2013; revised 30 October 2013; accepted 30 October 2013; posted 31 October 2013 (Doc. ID 196895); published 3 December 2013
An orderly inclined Al2 O3 column array was fabricated by atomic layer deposition and sequential electron beam evaporation using a hollow nanosphere template. The transmittance spectra at various angles of incidence were obtained through the use of a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer. The inclined column array could display the image information through a scattering mechanism and was transparent at high viewing angles along the deposition plane. This characteristic of the inclined column array gives it potential for applications in head-up displays in the automotive industry. © 2013 Optical Society of America OCIS codes: (310.1860) Deposition and fabrication; (230.4000) Microstructure fabrication. http://dx.doi.org/10.1364/AO.53.00A121
1. Introduction
Head-up displays (HUDs) have been used in the aircraft industry since 1960, when they were first introduced in the military aircraft Hawker-Siddeley Buccaneer (Enderby and Wood 1992) [1]. In 1970, HUDs were introduced to commercial aviation. They have also been used in the automotive industry since 1988 [2] and have recently become more popular in vehicles due to driver safety concerns. HUDs can reduce the frequency and duration of the driver’s eyes being off the road by projecting the required information in front of the driver. This should aid drivers by making them able to react more quickly to changes in driving conditions [3]. Most HUD screens display information by reflecting light to the user’s eyes. However, the reflection method has the disadvantage of producing a ghost image due to multiple reflections [4,5]. In this study, we develop a thin film with a novel microstructure that can display information via the scattering mechanism and is transparent at a high viewing angle. This special characteristic of the
1559-128X/14/04A121-04$15.00/0 © 2014 Optical Society of America
thin film gives it potential for application in HUDs in the automotive industry. 2. Experimental Procedures
As shown in Fig. 1, polystyrene (PS) nanospheres of 590 nm diameter received as suspensions in water (10.0 wt. %, Bangs Laboratories Inc.) were spin coated onto substrates without dilution. Reactive ion etching (RIE) using oxygen plasma was applied to reduce the diameter of the PS nanospheres. The size of the PS nanospheres can be controlled by changing the duration of ion bombardment. An Al2 O3 coating was deposited on the surface of the nanospheres through atomic layer deposition (ALD) to prevent melting of the PS due to the high substrate temperature during electron beam evaporation. ALD is a unique process that produces highly conformal films and allows atomic-scale thickness control. ALD is based on the self-limiting reaction. In the ALD process, Al2 O3 was coated by using H2 O and trimethylaluminum (TMA) as precursors at room temperature, and nitrogen was the purge gas. This process has been described in detail by Wang et al. [6]. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A121
Fig. 3. Cross-sectional SEM images of Al2 O3 along direction c.
Fig. 1. Flow chart for preparation of ordered arrays of Al2 O3 columns: (a) nanospheres were spin coated on the substrate; (b) the diameter of the nanospheres was reduced after a 130 s RIE treatment; (c) after the ALD process, Al2 O3 was coated on the surface of the nanospheres; (d) nanosphere shells were formed by heating at 350°C; (e) after oblique deposition, inclined Al2 O3 columns were grown.
deposition and the shape of the top of the column, we know that the vapor flows along direction c in Fig. 2(b) during evaporation [7]. Furthermore, the
The PS nanospheres were removed by heating to 350°C in air to form an ordered array of hollow nanospheres. The samples were then transferred to the electron beam deposition system for the preparation of inclined column arrays. The oblique deposition angle between the substrate normal and the direction of vapor flow was 75.5°. Specular transmittance spectra at various angles of incidence were obtained by a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer equipped a rotatable sample holder. The beam size is 2 mm (width) × 5 mm (height). The surface morphologies and dimensions of the nanospheres and inclined column array were observed by scanning electron microscopy (SEM) (Hitachi S-4300). 3. Results and Discussion
As shown in Fig. 2(a), the diameter of the hexagonally close-packed PS nanospheres was reduced to 410 nm after the RIE size-reduction process. After deposition of the 950 nm thick Al2 O3 film by electron beam evaporation, an inclined column array was formed on the nanosphere shell template, as shown in Fig. 2(b). From the fan-out phenomenon of oblique
Fig. 2. Plan-view SEM images of (a) nanosphere array and (b) Al2 O3 inclined columns. A122
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Fig. 4. Specular transmittance spectra at various angles of incidence.
Fig. 5. Specular and diffuse spectra of the inclined Al2 O3 column array at a normal angle of incidence.
Fig. 6. Effect on a real object viewed through the inclined column array at (a) normal and (b) a tilted angle (about 45°).
surface of the top of the column facing the vapor flow, the yellow line in Fig. 3, was smoother than that of the perpendicular one. The specular transmittance spectra at the various angles of incidence are shown in Fig. 4. The incident light was on the deposition plane. The incidence angle was defined as the angle between the surface normal and the direction of incidence, with positive angles being toward the direction of the deposition direction as shown in the inset of Fig. 4. As shown in Fig. 4, the inclined column array had a lower specular transmittance at around the normal incidence angle, and the transmittance increased as the incidence angle increased. Figure 5 shows the scattering measurement at the normal incidence angle. As shown in Fig. 5, the lower specular transmittance resulted from light scattering caused by rough morphology. According to the SEM image discussed above, the high transmittance at a high angle of incidence resulted from the lower amount of scattering caused by the stair-like surface at a high angle of view and the smooth surface of the top of the column facing the vapor flow. We found that scattering made up a major portion of the total reflection and was the reason for lower specular transmittance. Therefore, the inclined column array provided a real image such as a projection screen at the normal direction by scattering. However, the troughs in the transmittance
spectra were a result of the photonic crystalline behavior of the orderly inclined column array [8]. Figure 6 shows the effect on the real object through the inclined column array at the normal and tilt angles along the deposition plane. It can be seen that the words on the pen are hidden in a mist at the normal viewing angle but appear when the film sample is tilted. Figure 7 shows the scattering and specular transmittance spectrum at a 45° incidence angle extracted from Fig. 4. The scattering and transmittance spectra of the sample in Fig. 7 correspond to those in Figs. 6(a) and 6(b), respectively. The average specular transmittance at 45° was 68.23% in the visible wavelength. The characteristics of the inclined column array could have potential applications as HUDs in the automotive industry. Figure 8 shows a schematic drawing of HUDs for a projection screen fabricated using inclined column arrays. The information light propagates through a condensed lens and is incident on the inclined column array at the normal direction. Therefore, a driver can see the information on the projection screen due to the scattering characteristics of inclined column arrays. Furthermore, the driver also sees the road conditions through the inclined column array at a high viewing angle. The view angles from 40° to 60° are appropriate application ranges as shown in Fig. 4. According to Fermat’s principle, the information reflected by a conventional display can only be seen from a few narrow viewing angles. On the other hand, the inclined column array
Fig. 7. Scattering spectrum at normal and specular transmittance spectrum at 45°.
Fig. 8. Schematic representation of HUDs using an inclined column array as a projection screen. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A123
provides a real image that can be seen from every angle of view. However, the change of focus point is still a disadvantage of this display method during the drive as well as conventional methods. 4. Conclusion
In this study, an orderly inclined Al2 O3 column array was fabricated by ALD and sequential electron beam evaporation using a hollow nanosphere template. The inclined Al2 O3 column array deposited on a rigid or flexible transparent object as a novel display method was proposed for application in the automotive industry. Through scattering on the surface, the inclined Al2 O3 column array provided a real image that was different from the conventional display methods. References 1. J. Crawford and A. Neal, “A review of the perceptual and cognitive issues associated with the use of head-up displays in commercial aviation,” Int. J. Aviat. Psychol. 16, 1–19 (2006).
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2. S. Okabayashi, N. Sugie, M. Imaizumi, M. Furukawa, and T. Hatada, “Visual perception of HUD (head-up display) image with small angle of depression in practical automotive use,” Electron Comm. Jpn. 82, 1–9 (1999). 3. S. Smith and S.-H. Fu, “The relationships between automobile head-up display presentation images and drivers’ Kansei,” Displays 32, 58–68 (2011). 4. K. Makita, “Combiner for head-up display (HUD) system,” J. Sol-Gel Sci. Technol. 47, 209–211 (2008). 5. E. Tolstik, A. Winkler, V. Matusevich, R. Kowarschik, U. V. Mahilny, D. N. Marmysh, Y. I. Matusevich, and L. P. Krul, “PMMA-PQ photopolymers for head-up displays,” IEEE Photon. Technol. Lett. 21, 784–786 (2009). 6. C. C. Wang, C. C. Kei, Y. W. Yu, and T. P. Perng, “Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition,” Nano Lett. 7, 1566–1569 (2007). 7. D.-X. Ye, T. Karabacak, B. K. Lim, G.-C. Wang, and T.-M. Lu, “Growth of uniformly aligned nanorod arrays by oblique angle deposition with two-phase substrate rotation,” Nanotechnology 15, 817–821 (2004). 8. W.-H. Cho, C.-T. Lee, C.-C. Yu, C.-C. Kei, D.-R. Liu, and C.-C. Lee, “Microstructure and optical properties of Al2O3 prepared by oblique deposition using microsphere shell templates,” Appl. Opt. 50, C246–C249 (2011).
