**1. Introduction**

334 Recent Advances in Nanofabrication Techniques and Applications

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2004

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The development of lithographic technology that has been used in semiconductor electronics has led to systems that put a premium on spatial resolution, throughput and reliability, regardless of cost and flexibility. According to Abbe's theory, the spatial resolution can be improved by using either shorter wavelength or higher numerical aperture (*NA*). Although the semiconductor industry has made significant progress in increasing the lithography resolution in the past decades, further improvement of the resolution by accessing shorter wavelengths is facing critical challenges due to the availability of optical materials with suitable refractive index. The expansion of nanoscale science and engineering will require flexible, high spatial resolution, and low-cost nanolithographic techniques and systems other than those employed in the semiconductor industry, for reasons of both cost and limited flexibility. The research on Emerging Maskless Nanolithography Based on Novel Diffraction Gratings presented in this chapter is a step in the direction of providing affordable, highly flexible nanolithography.

In this chapter, we present two cases for maskless nanolithography employed novel diffraction optics elements as objective lens to focus illumination light. The diffractive objective lens (DOL) operates by the principle of diffraction other than conventional objective lens functioning by refraction. DOL can be designed to operate at any wavelength while refractive elements are constrained at short wavelengths by material transmission properties. DOL are thinner, and can be fabricated by planar techniques that are reliable and low cost.

DOL, in a form of photon sieves originally used in x-ray microscopy and achieved 6nm resolution, is possible to extend the use of diffractive elements down to the limits of nanolithography. Recently, we present the scheme of photon sieve array X-ray maskless nanolithography (PSAL) to fabricate novel nanometer devices (Cheng et al., 2006, 2007a, 2007b, 2008). The lithographic principle is shown in figure 1. Firstly, each of the photon sieve

Emerging Maskless Nanolithography Based on Novel Diffraction Gratings 337

can be resolved by using I-line illumination exposure. Compared with the model of original superlens, we separated the superlens 100nm away from the substrate, under the illumination of I-line light, the initial simulation shows that the sub-diffraction-limited feature as small as 60nm line width with 120nm pitch can be clearly resolved without hard contact between the substrate and the superlens. This is shown in figure 2. By proper design of the materials and the parameters of nano-filmed layers, better resolution can be realized.

Fig. 2. Electric eld distribution of dual-layered heterostructure for CD=30nm.

Lithography has been the key technology in the semiconductor industry. Beyond the semiconductor industry, lithography has also been widely employed in such technological fields as microoptics, nanophotonics, MOEMS, nanotechnology, etc. In order to achieve the minimum feature size, the development of lithography has resulted in setups with high

For nanoscale science and engineering, however, the lower cost and higher flexibility of lithography must be considered. Fortunately, maskless lithography can meet the requirements of nanotechnology. There are various forms of maskless lithography that include scanning electron-beam lithography, focused ion-beam lithography, multiaxis electron-beam lithography, interference lithography, maskless optical-projection lithography, scanning probe lithography, zone plate array lithography (ZPAL) (Menon et al., 2005), etc. It is a very complex process of selecting an optimum lithography tool that requires knowledge and experience in several disciplines including physics, chemistry, electronics, device design, manufacturing, processing, cost and marketing. Although the selection strategy consists of many aspects, the technical aspect is the dominant item because the tool has to be technically performable. Menon et al. showed the feasibility of ZPAL operating at a wavelength of 400nm and its potential for the fabrication of novel

Photon sieve is a novel diffractive optical element which consists of a great number of pinholes distributed appropriately over the Fresnel zones for the focusing and imaging of soft X-rays (Kipp et al., 2001). Photon sieve has advantages of the diameter of pinholes beyond the limitation of the corresponding Fresnel zone width and the minimum background in the focal

**2. Photon sieve array X-ray maskless nanolithography** 

throughput and reliability regardless of cost and flexibility.

devices.

array focuses incident X-ray into a diffraction-limited on-axis spot on the surface of a photoresist-coated substrate, the X-ray intensity of each spot is modulated by means of a spatial light modulator. Then, patterns of arbitrary geometry are written in a dot matrix fashion while the photoresist-coated substrate on a precision stepping stage is exposed to the properly modulated X-ray.

Fig. 1. Schematic of PSAL. An array of photon sieves focuses incident X-ray into a matrix of spots on the substrate coated photoresist. Patterns of arbitrary geometry are recorded while the stepping stage is driven.

In combination with the synchrotron light sources, PSAL can offer a new lithographic tool for high-resolution X-ray nanolithography in physical and nanoscale sciences. The PSAL lithographic system will be further discussed in detail on the synchrotron radiation light, resolution limits, depth of focus, etc. The design, fabrication, and experimental characteristic of X-ray photon sieve will also be illustrated by numerical analyse and experimental results. According to Fresnel-Kirchhoff diffraction theory, the diffractive field of photon sieve is described by means of the discrete fast Fourier transform algorithm. The approaches to enhancing imaging resolution of photon sieve are presented in detail. The related contents include the calculation of point spread function, the suppression of side lobes, the imaging bandwidth, the physical limit of resolution, and the diffraction efficiency. Imaging properties of photon sieve are analyzed on the basis of precise test and shown that photon sieve is a kind of diffractive optical element modulating either amplitude or phase and thus suffers from chromatic aberration or low diffraction efficiency. Hybrid lens consisting of both refractive optical surfaces and photon sieve are suggested to correct the chromatic aberration. Phase-photon sieve technology and surface plasmon polaritons technology are promising approaches to improve the diffraction efficiency and spatial resolution.

DOL, in another form of superlens consisted of nano-filmed noble metals on which the evanescent field is strongly enhanced using the resonant excitation of surface plasmons that can be excited at given conditions, is also possible to extend the use of diffractive elements down to the limits of nanolithography (Yang et al., 2007a, 2007b, 2009). The high-resolution plasmonic nanolithography has been investigated by using optical proximity exposure in the evanescent near field in nano-filmed noble metals. Sub-diffraction-limited feature size

array focuses incident X-ray into a diffraction-limited on-axis spot on the surface of a photoresist-coated substrate, the X-ray intensity of each spot is modulated by means of a spatial light modulator. Then, patterns of arbitrary geometry are written in a dot matrix fashion while the photoresist-coated substrate on a precision stepping stage is exposed to

Photon sieve array

Focused X-ray

Stepping stage

Substrate

Fig. 1. Schematic of PSAL. An array of photon sieves focuses incident X-ray into a matrix of spots on the substrate coated photoresist. Patterns of arbitrary geometry are recorded while

Y

In combination with the synchrotron light sources, PSAL can offer a new lithographic tool for high-resolution X-ray nanolithography in physical and nanoscale sciences. The PSAL lithographic system will be further discussed in detail on the synchrotron radiation light, resolution limits, depth of focus, etc. The design, fabrication, and experimental characteristic of X-ray photon sieve will also be illustrated by numerical analyse and experimental results. According to Fresnel-Kirchhoff diffraction theory, the diffractive field of photon sieve is described by means of the discrete fast Fourier transform algorithm. The approaches to enhancing imaging resolution of photon sieve are presented in detail. The related contents include the calculation of point spread function, the suppression of side lobes, the imaging bandwidth, the physical limit of resolution, and the diffraction efficiency. Imaging properties of photon sieve are analyzed on the basis of precise test and shown that photon sieve is a kind of diffractive optical element modulating either amplitude or phase and thus suffers from chromatic aberration or low diffraction efficiency. Hybrid lens consisting of both refractive optical surfaces and photon sieve are suggested to correct the chromatic aberration. Phase-photon sieve technology and surface plasmon polaritons technology are

promising approaches to improve the diffraction efficiency and spatial resolution.

DOL, in another form of superlens consisted of nano-filmed noble metals on which the evanescent field is strongly enhanced using the resonant excitation of surface plasmons that can be excited at given conditions, is also possible to extend the use of diffractive elements down to the limits of nanolithography (Yang et al., 2007a, 2007b, 2009). The high-resolution plasmonic nanolithography has been investigated by using optical proximity exposure in the evanescent near field in nano-filmed noble metals. Sub-diffraction-limited feature size

the properly modulated X-ray.

the stepping stage is driven.

X

can be resolved by using I-line illumination exposure. Compared with the model of original superlens, we separated the superlens 100nm away from the substrate, under the illumination of I-line light, the initial simulation shows that the sub-diffraction-limited feature as small as 60nm line width with 120nm pitch can be clearly resolved without hard contact between the substrate and the superlens. This is shown in figure 2. By proper design of the materials and the parameters of nano-filmed layers, better resolution can be realized.
