**2. DUV interferometry**

#### **2.1 Introduction**

The needs for simple, fast and versatile techniques for micro- and nanomachining have accounted for many works during the last years. The most active sector in this field has been the microelectronics industry. Researches in this area were essentially motivated by finding new solutions to follow the trend towards a constant decrease of the size of the transistors as stated in the "Moore's law" (Moore, 1965). To reach these objectives, the recourse to deeper wavelengths has been proposed. Today, the most used wavelength for chips manufacturing is 193 nm provided by ArF excimer lasers. In this case, the use of short wavelength allows decreasing the limitations linked to diffraction. Patterns with dimensions as small as 22 nm can be produced by industrial machines (steppers) on 300 mm diameter Silicon wafers. However, these high performance lithographic tools are restricted to production use and complex patterns due to their very high cost.

For these reasons, there is a parallel need of less sophisticated setups but able to produce sub-100 nm structures on relatively wide surfaces. For some applications, the structures do not need to be complex and can be limited to a regular replication of simple shapes like dots or lines. For the fabrication of such periodical nanostructures, interferometric techniques fulfil most of the needed requirements.

Interferometry consists in combining two or more monochromatic, coherent and polarized beams to produce an interference pattern. This technique is sometimes also called holography. The spatially controlled irradiation is used to induce a local modification of the material that can be either a photocrosslinking or an ablation due to different molecular mechanisms.

The period p is given by the following relation:

$$p = \frac{\lambda}{2 \cdot n \cdot \sin \theta} \tag{1}$$

Where λ is the wavelength, and n is the refractive index of the medium in which the two beams are recombined and θ is the half-angle between the two beams.

The recourse to short wavelengths (248 nm, 193 nm) allows producing periods as low as 100 nm in air (Charley et al., 2006), meaning that this route is competitive in terms of resolution with most advanced photolithography industrial tools. In comparison, the alternative to mask projection imaging is limited to gratings with periods > 1 μm (Mihailov & Gower, 1994). Interferometric techniques are much more adapted for sub-micron structuring.

to λ<300 nm) allows producing periodic nanostructures with typical dimension down to

The aim of this chapter is to review some recent works about DUV interferometric lithography nanofabrication. In the first part, a brief introduction to interferometric lithography will allow illustrating its interest and main applications. The second part will be dedicated to applications with organic materials (polymers) that have been widely used for micro and nanopatterning with such a technique. However, organic materials present some inherent limitations that have justified many efforts during the last years to developed inorganic materials prepared by sol-gel technique. This will be the topic of the third part of

The needs for simple, fast and versatile techniques for micro- and nanomachining have accounted for many works during the last years. The most active sector in this field has been the microelectronics industry. Researches in this area were essentially motivated by finding new solutions to follow the trend towards a constant decrease of the size of the transistors as stated in the "Moore's law" (Moore, 1965). To reach these objectives, the recourse to deeper wavelengths has been proposed. Today, the most used wavelength for chips manufacturing is 193 nm provided by ArF excimer lasers. In this case, the use of short wavelength allows decreasing the limitations linked to diffraction. Patterns with dimensions as small as 22 nm can be produced by industrial machines (steppers) on 300 mm diameter Silicon wafers. However, these high performance lithographic tools are restricted to production use and

For these reasons, there is a parallel need of less sophisticated setups but able to produce sub-100 nm structures on relatively wide surfaces. For some applications, the structures do not need to be complex and can be limited to a regular replication of simple shapes like dots or lines. For the fabrication of such periodical nanostructures, interferometric techniques

Interferometry consists in combining two or more monochromatic, coherent and polarized beams to produce an interference pattern. This technique is sometimes also called holography. The spatially controlled irradiation is used to induce a local modification of the material that can be either a photocrosslinking or an ablation due to different molecular

(1)

2 *n* sin

Where λ is the wavelength, and n is the refractive index of the medium in which the two

The recourse to short wavelengths (248 nm, 193 nm) allows producing periods as low as 100 nm in air (Charley et al., 2006), meaning that this route is competitive in terms of resolution with most advanced photolithography industrial tools. In comparison, the alternative to mask projection imaging is limited to gratings with periods > 1 μm (Mihailov & Gower,

*p*

1994). Interferometric techniques are much more adapted for sub-micron structuring.

beams are recombined and θ is the half-angle between the two beams.

several tens of nm.

this chapter.

**2.1 Introduction** 

mechanisms.

**2. DUV interferometry** 

complex patterns due to their very high cost.

fulfil most of the needed requirements.

The period p is given by the following relation:

Interferometry in the DUV range has been enabled by the development of excimer lasers that have two main advantages: first, the use of short wavelengths is an effective way to provide the requested resolution since the period is directly proportional to the wavelength. Secondly, DUV wavelength permits direct writing via photoinduced processes provoked by high-energy photons. Examples of suitable materials for such wavelengths will be given in the following sections.

On the experimental point of view, one of the difficulties for interferometry in the DUV range is due to the low coherence of available DUV lasers. As an example, typical coherence of ArF lasers is limited to a few hundreds of microns, which justifies efforts to develop specific experimental setup for short wavelengths (Figure 1):

Fig. 1. Example of interferometric lithography configuration: a) Talbot prism (Bourov et al., 2004), b) Lloyd setup (Raub & Brueck, 2003) and c) achromatic holographic configuration (Yen et al., 1992)


DUV Interferometry for Micro and Nanopatterned Surfaces 247

Finally, the development of photoresists for microelectronics applications has also accounted largely for the success of DUV interferometric lithography. This point will be developed below.

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

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

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).

was demonstrated using DUV wavelengths (Yao et al., 2008).

**2.4 Wide surface micro and nanopatterning** 

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

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

**2.3 3D micro and nanostructures** 

interference lithography (Jeon et al., 2004).

shaping.


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 same substrate, which simplifies the alignments.

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

### **2.2 Applications of periodical structures**

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 light exhibit strong interaction with light with specific effects.

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

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...

Finally, the development of photoresists for microelectronics applications has also accounted largely for the success of DUV interferometric lithography. This point will be developed below.
