**2. Force lithography**

458 Recent Advances in Nanofabrication Techniques and Applications

or nanoelectromechanical systems (NEMS). This method is not restricted to conductive materials (Fonseca Filho et al., 2004). The advantages of this technique are high resolution and alignment accuracy, which could not be achieved by conventional lithographic techniques (Sheehan & Whitman, 2002; Martin et al., 2005). Moreover, the AFM nanolithography technique takes advantages of the ability to move a probe over the sample in a controllable way (Samori, 2005). Nanolithography using AFM can be done in various modes (Jones et al., 2006): chemical and molecular patterning (DPN), mechanical patterning by scratching or nanoindentation, local heating, voltage bias application and manipulation of nanostructures. Most popular AFM lithographic techniques are resist film lithography (Li et al., 1997) and lithography by oxidation (Sheglov et al., 2005; Sugimura et al., 1993; Sadegh Hassani et al., 2008a; Dubois & Bubbendroff, 1999; Avouris et al., 1998; Snow et al., 1999; Lemeshko et al., 2005; Avouris et al., 1997). The atomic force microscope has also become an increasing popular tool for manipulating thin films of many different types of materials. Lithography techniques can be carried out on the film of polymers such as polymethylmethaacrylate (PMMA), chloromethyl phenyltrichlorosilan (CMPTS), polyethylene (PE) and others (Lee et al., 1997; Sadegh Hassani et al., 2008b, Yoshimura et al., 1993; Chen et al., 1999; Huang et al., 2001). This capability can potentially be extended to evaluate nano-scale material response to indentation and would be ideal for evaluation of mechanical characteristic of surfaces (Burnham & Colton, 1989; Hues et al., 1994; Sadegh

To apply force optimally for making nano scratches, we require to understand the underlying behavior and parameter that control it, a tip which is optimized for applying force under the experimental conditions and scanning techniques which allows one to use these tips and retain desired properties (Yasin et al., 2005; Sadegh Hassani et al., 2008a; 2010). Some factors such as resolution, accuracy of alignment and reproducibility are important in this way. By reducing wear of AFM tip and controlling variables such as applying force, scan speed and environment, it can be systematically calibrated the size of features that is written by AFM tip. So, the reproducibility of issues can be

In this chapter, it is focused on the use of lithography process to build the desired nanostructures and nanolithography on surface of different substrates by AFM. Creating the scratches on various surfaces by silicon nitride and diamond tips using contact mode is discussed. For scratching, the mechanical action of the tip as a sharp pointed tool in order to produce fine scratches is used (Notargiacomo et al., 1999; Sadegh Hassani et al., 2010). The direct scratching is possible with high precision but low quality results are obtained due to

Silicon nitride cantilever tip with average spring constant is used to investigate soft surfaces including poly methyl methacrylate (PMMA) (LG-IH 830) thin film coated on the silicon and glass substrates (Sadegh Hassani et al., 2008a). A diamond cantilever tip with high spring constant is used for hard surfaces including highly-oriented pyrolytic graphite (HOPG) and polyethylene substrate (Sadegh Hassani et al., 2010). Since its hardness is much more than

Effects of applied normal force, time of applying pressure, speed and number of scratching cycles on the geometry and depth of scratches are studied. This study shows that there is a critical tip force to remove material from various surfaces (Sadegh Hassani et al., 2008a;

silicon nitride, the direct formation of nanoscratches could easily be achieved.

Hassani et al., 2008b).

controlled.

2008b; 2010).

probe wear during lithographic process.

An interesting way of performing nanometer pattern is force lithography which based on direct mechanical impact produced by a sharp probe on the sample surface (Lyuksyutov et al., 2003; Park et al., 2000; Sadegh Hassani et al., 2008a). The probe tip pressure on the surface is sufficient to cause plastic deformation of the substrate surface. This type of modification has been used in nanoelectronics, nanotechnology, material science, etc. It enables the fabrication of electronic components with active areas of nanometer scale, super dense information recording and study of mechanical properties of material.

In force lithography no bias voltage is required to produce nanostructures. The nanostructure formation normally occurs as a result of AFM tip motion above the polymer surface with set point magnitude constraining the tip to come closer to the surface (Lyuksyutov et al., 2004; Sadegh Hassani et al., 2008a; 2008b; 2010). In order to apply sufficient normal load to reach plastic deformation of surface, a three-side pyramidal single crystalline diamond tip or another tip with high spring constant is used and pressed against a desired surface (Santinacci et al., 2003; Sadegh Hassani et al., 2010). Much higher forces are achieved by accordingly increasing the applied voltage to piezo-scanner. By scanning the sample in the X or Y direction at various conditions (such as different scanning velocity and number of cycles) grooves are created. However, the protrusions along the edges are formed, which indicates clearly stress deformation during the scratching process (Santinacci et al., 2005).

It is shown that by applying a little force (severalN), removing an amount of material from a metal or polymer film is possible (Bruckl et al., 1997).

Use of cantilevers with high spring constant could apply the desired amount of force without large bending. When tip move toward the substrate or reverse direction, up or down bending of cantilever occurs, respectively. Since an angle of about 10° is typically set between the cantilever and the substrate (see Fig.1. b), this bending influence the tip– substrate interaction, so the geometry and size of scratches are affected in this way. However, increase of applied force cause cumulating of material along or at the end of the grooves. This deformity is occurred because of cantilever bending at the start point of moving tip through the surface. In this way cantilever reach the desired force to create scratch. (Notargiacomo et al., 1999; Sadegh Hassani & Sobat, 2011).

Fig. 1. Typical silicon cantilever with pyramidal tip: (a) upper view; (b) lateral view showing the 10° angle formed with the substrate surface; (c) cantilever bending and (d) torsion (Notargiacomo et al., 1999).

Nanolithography Study Using Scanning Probe Microscope 461

this value into a force F, using Hook's law (F = kZ) (Heimberg & Zandbergen, 2004; Ebrahimpoor Ziaie et al., 2005; Carpick & Salmern, 1997; Sadegh Hassani & Ebrahimpoor

In some reported experiments, a commercial scanning probe microscope (Solver P47H, NT-MDT Company), operated in AFM contact and noncontact modes, equipped with (NSG11) and (DCP20) cantilevers were used to perform the lithography of desired surfaces (Sadegh Hassani et al., 2008a; 2008b; 2010). The NSG11 cantilever made of silicon nitride had a rectangular shape, and its length, width and thickness were 100 ± 15 m, 35 ± 3 µm and 1.7– 2.3 m, respectively. Its normal bending constant measured by supplier was 11.5 nN/nm. Another cantilever which was used in this process was DCP20 Cantilever made of diamond with the length, width and thickness of 90±5m, 60±3m and 1.7-2.3m, respectively. The

These two types of cantilever were selected to reach deformation of different types of surfaces and also for obtaining good images of scratches. These experiments were designed

The lithography process was executed with the use of lithography menu supported by the microscope software. The AFM tip was brought into contact with the sample surface using the smallest force possible to minimize any undesired surface modification. An image of surface was prepared in order to choose a suitable surface free of defects for lithography; then the nanolithography process was executed under various specific and controlled conditions to analyze the effect of lithography important factors on the shape

For studying force effect, the force was increased by applying a higher voltage to the piezoscanner in order to reach the cantilever deflection (Z) corresponding to the force (F) range where plastic deformation of polymeric surface occurred. (Santinacci et al., 2003; Notargiacomo et al., 1999; Sadegh Hassani et al., 2008 b). Scratches were made in Y direction on various substrates in different conditions (Sadegh Hassani et al., 2008a; 2008b; 2010), so in this way the influences of applied normal force, scanning velocity, time of applying pressure and number of scratching cycles were investigated. Finally surface was scanned by atomic force microscope in non-contact mode to observe and evaluate the shape and depths of scratches. If the contact mode had been chosen to image the scratches, the surface of

Sadegh Hassani et al. (2008a; 2008b; 2010) reported lithography performance on PMMA thin films. In this regard, soft thin films of PMMA polymer on the silicon and glass substrates were prepared. For making PMMA (LG-IH830) thin film on silicon and glass substrates, these substrates were washed and sonicated in acetone/ethanol (50-50 % vol.) for 15 minutes at room temperature. Then a very small amount of diluted PMMA/CHCl3 solution was coated over the silicon and glass surfaces using spin coater with 6000 rpm for 30 seconds. The coated substrates were dried in an oven at 130 °C for 30 minutes. The thickness

of these coated layers was ~150 nm, measured by atomic force microscope.

Ziaie, 2006).

of scratches.

**4. Nanolithography on various substrates** 

substrates would have probably been damaged.

**4.1 Nanolithography on PMMA thin films** 

normal bending constant measured by supplier was 48 nN/nm.

to fabricate scratches on the various surfaces with the different rigidity.

Resolution will be major challenge in lithographic fabrication and the limiting factor for resolution is the tip quality. Sharp silicon tips deliver brilliant and reproducible results. To even further achieve the fine lithographic structure, electron beam deposited tips (EBD tips) can be additionally sharpened in oxygen plasma (Wendel et al., 1995).

Wearing of probe led to low-quality results and reduced the repeatability of produced scratches. Indeed, by using the same tip at another experiment, the sample surface could experience two completely different values of pressure, because the amount of produced pressure depends on the shape of tip (Hu et al., 1998). To decrease wearing of probe, a soft resist polymer film (usually PMMA film) is coated on the surface. On the other hand, the roughness of surface is very important to take high quality scratches. Observations show that surface roughness is strongly influenced by its thickness as while; the surface roughness increases with the increase of the thickness. So, to perform the lithography process, the smoothest surface has to be chosen (Yasin et al., 2005; Fonseca et al., 2004).

Studies show that in the case of AFM, the possibility of directly machining a surface has been explored in two ways, i.e. by either using a static approach in which the microscope is operated in conventional contact mode (Magno & Bennett, 1997; Sumomogi et al., 1995) or using a dynamic approach in which the microscope is operated in the tapping mode (Heyde et al., 2001; Davis et al., 2003). Usually the lithography developed using both static and dynamic approaches are employed to pattern a resist layer, subsequently the patterned layer is used as an etch mark. Both techniques are giving lithography resolution of the order of tens of nanometer (Wendel et al., 1994; Quate, 1997).

An advantage of the vibration in the tapping mode is that very small lateral forces stress the tips, resulting in very slow tip degradation (Wendel et al., 1996).
