**1. Introduction**

138 Advances in Unconventional Lithography

[25] Masato Okano et al (2004). Optimization of Diffraction Grating Profiles in Fabrication by Electron-Beam Lithography. *Appl. Opt.,* Vol. 43, pp. 5137-5142, ISSN 0003-6935 [26] M. T. Gale (1997). Replication, In: *Binary optics fabrication in Micro-optics: Elements,* 

[27] J. M. Miller et al (1993). Multilevel grating array generators: fabrication error analysis and experiments. *Applied Optics*, Vol.32, pp.2519-2525, ISSN 0003-6935 [28] C. Zheng (2005). *Micro-Nanofabricaton Technologies and Applications*, Springer*,* ISBN 7-04-

London, England

017663-7, China

*Systems and Applications,* H. P. Herzig (Ed.), Taylor and Francis, ISBN 0748404813,

Two types of lenses can focus light: an optical lens using refraction phenomenon and a diffractive lens using diffraction phenomena. Table 1 shows the characteristics of each lens. The focal length of the diffractive lens is controlled by the structures of the lens, as mentioned in detail in Section 2.2. This suggests that the focal length of the diffractive lens is independent of refractive index and curvature. Thus, application of diffractive lenses to UV optical elements or thin optical elements is possible.


Table 1. Comparison of characters between refractive lens and diffractive lens

Fabrication of Binary Diffractive Lens on Optical Films by Electron Beam Lithography 141

low cost and over a large area. However, EBL does not use any molds or masks. Therefore,

In this study, we carry out the design and fabrication of the binary diffractive lens with 2 mm focal lengths for controlling the luminosity distribution and the binary diffractive lens with the 100-μm-order focal length. Furthermore, to improve the diffraction efficiency, we characterize the detailed relationship between the lens structure and the light intensity.

In this section, the methods of design, fabrication, and characterization of the binary

The binary diffractive lenses, on which this study is focused, were fabricated on the poly(ethylene terephthalate) (PET) films. The PET films are often used as optical sheets for liquid crystal displays. There are many types of optical films such as polycarbonate (PC) and poly(methyl methacrylate) (PMMA). In this study, the EBL process was used for fabricating the binary diffractive lenses; this process required the optical films to endure high

In this study, the binary diffractive lens was fabricated by developing the resist for EBL (ZEP-520A, ZEON Co.) on the PET films (Teijin® Tetoron® Film, Teijin DuPont Films, Japan). If the refractive indexes of both materials are almost same, the binary diffractive lens can be fabricated by developing the resist instead of etching the PET films. Therefore, the refractive indexes of the PET films and the resist are evaluated by ellipsometry (M-2000DI, J.A. Woollam Co., Inc.). Fig. 2 shows the wavelength dispersion of the PET film and the resist on the PET film, including the data from the catalog of ZEP-520A. D2 and halogen lamps were used for this measurement. The refractive index of the PET film is relatively

> ZEP-520A resist (Data from catalog) ZEP-520A resist on PET film (Elipsometry)

PET film (Elipsometry)

300 400 500 600 700 800 900 1000

Wavelength (nm)

Fig. 2. Wavelength dispersion of the PET film and the resist on the PET film, including the

**2.1 Basic optical characteristics of materials related to binary diffractive lens** 

temperature and chemicals, making them more suitable than PC or PMMA.

it is convenient to examine EBL in detail to obtain optimum structures.

**2. Experimental procedure** 

diffractive lens are described.

1.5

data from the catalog of ZEP-520A

1.6

Refractive index

1.7

1.8

Recently, the emitting efficiency of light emitting diodes (LEDs) has improved; thus, they are used in lighting devices. To this end, miniaturizing the LEDs for smaller lighting devices and controlling the luminosity of LEDs are required. The conventional oval lamp-type LEDs cannot realize these requirements because the lens height of such LEDs is approximately 5 mm and its distribution of luminosity is determined by its shape. In this study, instead of the oval lamp-type lens, we used the diffractive lens on the optical films, as shown in Fig. 1. If the diffractive lens with short focal length (order of micrometer) can be fabricated, miniaturization of the lens system and consequently the LED lighting devices can be achieved. In order to realize the refractive lenses with the short focal length, large curvature radius is needed, thus making it difficult to realize it easily. Therefore, the diffractive lenses are suitable for realizing the short focal length lenses. Furthermore, by modifying the structure of the diffractive lens, it is easy to control the luminosity of the LEDs and the farfield pattern. Therefore, in this study, we focused on the diffractive lens because it enabled us to reduce the thickness of the lens, control the luminosity distribution of LEDs, and facilitate the realization of the binary structure.

Fig. 1. Schematic representation of diffractive lens on the optical film

The zone plate was the first diffractive lens invented by I. L. Solet in 1875. To improve light efficiency, kinoform was invented by J. A. Jordan (Jordan et al., 1970). Recently, binary optics technology was developed using CAD design and VLSI technology (Swason and Veldkamp, 1989). The diffractive optical elements with multi-level grating having step-like cross-section have been developed. By controlling thestructure of the multi-level gratings, an optical effect almost same as that of the kinoform can be obtained (Orihara et al., 2001 & Yamada et al., 2004).

On the other hand, subwavelength structures (SWSs), which are equivalent to a blazed structure, were suggested by P. Lalanne (Lalanne et al., 1999 & Mait et al., 1999). These structures are fabricated binary SWSs converted from Fresnel lenses. These structures are fabricated easily than those of the multi-level gratings because they can be fabricated by electron beam lithography (EBL) or nanoimprint lithography (NIL). Furthermore, in the case of photolithography, combining some masks is not necessary. By using these structures, achromatized diffractive lenses were reported (Kleemann et al., 2008).

We aim to realize a highly effective short focal length diffractive lens using the binary diffractive lens fabricated by EBL, and expect the equivalent effect with the diffractive lens of the saw-like structure. NIL or photolithography can easily fabricate these structures at low cost and over a large area. However, EBL does not use any molds or masks. Therefore, it is convenient to examine EBL in detail to obtain optimum structures.

In this study, we carry out the design and fabrication of the binary diffractive lens with 2 mm focal lengths for controlling the luminosity distribution and the binary diffractive lens with the 100-μm-order focal length. Furthermore, to improve the diffraction efficiency, we characterize the detailed relationship between the lens structure and the light intensity.
