**2.3 3D micro and nanostructures**

246 Recent Advances in Nanofabrication Techniques and Applications



One very elegant way to cope with this problem has been proposed by Yen et al (Yen et al., 1992). It consists in using an achromatic configuration. In this case, two matched fused silica phase gratings were employed and, the demonstration of 100 nm period grating was achieved using an ArF excimer laser. High contrast fringes were obtained with a depth-offocus compatible with practical applications. Here, the 0th order can be physically blocked providing a perfect sinusoidal light pattern on the photosensitive resin. The advantages of the achromatic configuration are obvious for laser sources with limited coherence. Since then, this technique has been used by several research teams. Recently, Bourgin et al. (Bourgin et al., 2009) have proposed an integrated solution providing the 2 gratings on the

Significant progresses in resolution have also been achieved using immersion technique. Using an immersion fluid between the phase mask and the sample, it is possible to increase the numerical aperture and thus decrease the period (see Eq. 1). The most widely used fluid for immersion is water since water is transparent at 193 nm. High refractive index fluids have demonstrated their interest to reach resolutions as low as 32 nm HP corresponding to

Most of the applications of periodical micro- or nanostructures are in the field of optics and photonics. Indeed, such structures with periods in the range, or under, the wavelength of

It is not possible to mention here all the applications of gratings that could be provided by means of DUV laser interferometry. The most important ones are probably linked to spectroscopy, especially for high resolution spectrometers for astronomy (Heilmann et al., 2004), low-loss polarisers, grating for laser pulses shortening, motion sensors, displays (Braun, 2002), microlasers (Wegmann, 1998; Schon, 2000), white light processing like antireflective (Gombert et al., 2004) or diffusing surfaces (Menez et al., 2008), and in the solar cell technology (light concentrators (Karp et al., 2010), and Sub-Wavelength Gratings SWG

Beside optical applications, new applications have emerged from the spectacular properties of patterned surface when the size of patterns is reaching the nanometre scale. These properties can be superhydrophobicity, interaction with biofilms, nanotribology, etc...

lasers (Raub &Brueck, 2003).

is thus to be considered with particular care.

same substrate, which simplifies the alignments.

**2.2 Applications of periodical structures** 

(Y. Kanamori et al., 2005)).

the 65 nm node in microelectronics (Santillan et al., 2006).

light exhibit strong interaction with light with specific effects.

There are specific interests in developing 3D periodical structures. One of the most important applications is the fabrication of photonics crystals (PC). PCs are crystalline materials where the refractive index is periodically modulated on a length scale comparable to the light wavelength of interest. Interference of the light waves scattered from the dielectric lattice (i.e., Bragg scattering) leads to omnidirectional stop bands or photonic band gaps (PBGs), which are analogous to the electronic energy band gaps in a semiconductor. (Joannopoulos et al., 1995; Lin et al., 1998). PCs potentially offer revolutionary advances in the next-generation microphotonic devices and the integration of existing optoelectronic devices, including integrated optical circuits, lasers, sensing, spectroscopy, and pulse shaping.

In recent years there has been a considerable effort to develop novel methods for mass production of 3D PCs with controlled size, symmetry, and defect(s) on a large-scale basis (Moon & Yang, 2009). Among others techniques, interferometric techniques appear very interesting since they are massively parallel techniques of microfabrication (unlike 2-photon fabrication), and they are potentially free of random defects (unlike self-assembly techniques). Several experimental configurations have been developed, including multibeam interference (Figure 2. Campbell et al., 2000; Yang et al., 2002) and mask interference lithography (Jeon et al., 2004).

Fig. 2. Left) Holographic lithography process using an umbrella-like four beam setup, forming diamond- like interference patterns (Moon & Yang, 2009); Right) Structure and optical reflectance of 3D hydrogel PCs via holographic lithography (Kang et al., 2008).

Many different materials have been proposed for the fabrication of 3D structures. Most of these materials are sensitive in the UV or visible range of wavelengths (Moon & Yang, 2009) since the requirements in terms of period are not targeting the highest resolutions. Recently, a 3D "woodpile" structure with 1.55μm lattice constant and a 2mm-by-2mm pattern area was demonstrated using DUV wavelengths (Yao et al., 2008).

#### **2.4 Wide surface micro and nanopatterning**

Applications of nanopatterned substrates in practical application in optics or biology require the generation of nanopatterns over wide surfaces. Such requirements are specially needed

DUV Interferometry for Micro and Nanopatterned Surfaces 249

process parameters have an impact on the photolithographic performances of the photoresist. Among those parameters, those linked to the resist materials are considered as the most critical. Three factors are essential to consider: resolution limit, sensitivity and line-width roughness (LWR). Resolution limit is the most important criterion since the new platforms of resists should be able to address the challenging next lithographic nodes under 45 nm. However, sensitivity is a parameter of importance for practical applications due to the necessity to achieve short exposure times. Line width roughness (LWR) and line edge roughness (LER), measuring the deviation from an atomically smooth surface have also become new parameters preventing the feasibility of smaller feature sizes

Fig. 4. a) Molecular mechanism of photoinduced modification of CAR photoresists (positive

These material requirements for the next generations of DUV lithography have justified the recent efforts to develop innovative photoresists. The cost of industrial lithography tools destined at microelectronics applications (few tens of M\$) is hardly compatible with timeconsuming and potentially contaminant resist development experiments. This is one of the reasons explaining the success of DUV interferometric lithography for developing new photoresists since this tool is relatively easy to install with a reasonable cost, and however, it provides resolutions in the range of the most advanced DUV industrial lithographic tools (few tens of nm). Moreover, immersion lithography or double patterning can be proceeded. The nature of the polymer significantly contributes to all aspects of resist characteristics and performance. Most of the polymers used as resists are linear copolymers or terpolymers synthesized by the free radical polymerization technique (Ito, 2005; Kang et al., 2006). Such technique has the advantage to be relatively simple but the main drawback is a limited

Advanced polymer synthesis strategies like Atom Transfer Radical Polymerization (ATRP) were recently proposed to achieve a better control of the polymer structure, with both linear and hyperbranched structure and for a large variety of monomers (Xia & Matyjaszewski, 2001;

tone resist). b) SEM images of typical samples prepared by DUV interferometric

(Yoshimura et al., 1993).

lithography.

control of the polymer chain structure.

for applications in displays, light concentrators for solar cells, displacement sensors or compression of high power laser pulses (Figure 3). The extension of nanostructures over meter square area has generated many efforts. The Fraunhofer Institute fur Solare Energiesysteme (ISE, Freiburg en Brisgau) developed a holographic tool (Holotool) based on a Mach-Zender configuration with an irradiation surface greater than 1 m2. The Lawrence Livermore National Laboratory, in Livermore (Califormia, USA), has developed also an interference lithographic tool compatible with substrates as wide as 80 cm. In both case, the requirements of environmental conditions stability (temperature, mechanical vibrations, etc...) are extremely severe and despite sophisticated monitoring and correctives devices, the resolutions are limited to periods greater than 200 nm.

A very interesting alternative has been proposed at Massachusetts Institute of Technology, named Scanning Beam Interference Lithography (SBIL). The principle consists in generating a small area interferometric pattern and then, scans the surface to cover a wide substrate. The main difficulty relies on insuring a controlled displacement at the nanoscale of the writing interferometric head over 1 m2. This is achieved thanks to the development of a sophisticated interferometric displacement sensor. This technique allowed producing 900 mm x 500 mm gratings.

Fig. 3. Left) Multilayer dielectric diffraction gratings produced for NIF's Advanced Radiographic Capability petawatt laser have record size, damage resistance and efficieny (www.lasers.llnl.gov) and right) a 300 mm-diameter silicon wafer patterned with a 400 nmperiod grating by the Nanoruler (http://snl.mit.edu/).
