*3.5.3.1. Dip Pen Nanolithography (DPN)*

Dip Pen Nanolithography (DPN) is known as a soft-lithography technique that uses an AFM scanning probe tip to draw nanostructures. In this process, a probe tip is coated with liquid ink, which then flows onto the surface to make patterns wherever the tip makes contact. This kind of directwrite technique provides high-resolution patterning capabilities for a number of molecular and biomolecular "inks" on a variety of substrates. Substrates are the base material that the images are printed on. Some of the applications of the DPN technique include sol gel templates that are used to prepare nanotubes and nanowires, and protein nanoarrays to detect the amount of proteins in biological samples such as blood. (Ref. Fig. 12).

This process was first developed by Professor Chad Mirkin at Northwestern University Nanotechnology Institute for depositing thin organic films in patterns with feature sizes of around 10 nm ( about 20 times better than the best optical lithography) (Mirkin, 1999).

In DPN technology, the ink on a sharp object is transported to a paper substrate via capillary forces. The capillary transport of molecules from the AFM tip to the solid substrate is used in DPN to directly "write" pattern consisting of a relatively small collection of molecules in nanometer dimesions. An AFM tip is used to write alkanethiols with 30-nm line width resolution on a gold thin film in a manner analogous to that of a dip pen. Molecules are delivered from the AFM tip to a solid substrate of interest via capillary transport, making DPN a potentially useful tool for creating and functionalizing nanoscale devices (Mirkin, 1999).

Several factors decide the resolution of DPN:


**Figure 12.** Schematic representation of DPN. A water meniscus forms between the AFM tip coated with ODT and the Au substrate. The size of the meniscus, which is controlled by relative humidity, affects the ODT transport rate, the effective tip-substrate contact area and DPN resolution. [Courtesy: C.A. Mirkin, et al, Science 283, 661, 1999]

### *3.5.3.2. DPN application on semiconductor surfaces*

**4.** Reliability

*3.5.3.1. Dip Pen Nanolithography (DPN)*

Several factors decide the resolution of DPN:

the resolution of conventional writing.

the molecules after deposition.

Scanning probe lithography involves a set of lithographic techniques, in which a microscopic or nanoscopic stylus is moved mechanically across a surface to form a pattern. In this method, another technique describes a SPL technique which is known as Dip Pen Nanolithography.

Dip Pen Nanolithography - in this process, the patterning is done by directly transferring

Dip Pen Nanolithography (DPN) is known as a soft-lithography technique that uses an AFM scanning probe tip to draw nanostructures. In this process, a probe tip is coated with liquid ink, which then flows onto the surface to make patterns wherever the tip makes contact. This kind of directwrite technique provides high-resolution patterning capabilities for a number of molecular and biomolecular "inks" on a variety of substrates. Substrates are the base material that the images are printed on. Some of the applications of the DPN technique include sol gel templates that are used to prepare nanotubes and nanowires, and protein nanoarrays to detect

This process was first developed by Professor Chad Mirkin at Northwestern University Nanotechnology Institute for depositing thin organic films in patterns with feature sizes of around 10 nm ( about 20 times better than the best optical lithography) (Mirkin, 1999).

In DPN technology, the ink on a sharp object is transported to a paper substrate via capillary forces. The capillary transport of molecules from the AFM tip to the solid substrate is used in DPN to directly "write" pattern consisting of a relatively small collection of molecules in nanometer dimesions. An AFM tip is used to write alkanethiols with 30-nm line width resolution on a gold thin film in a manner analogous to that of a dip pen. Molecules are delivered from the AFM tip to a solid substrate of interest via capillary transport, making DPN a potentially useful tool for creating and functionalizing nanoscale devices (Mirkin, 1999).

**1.** The grain size of the substrate affects DPN resolution much as the texture of paper controls

**3.** Chemisorption and self-assembly of the molecules can be used to limit the diffusion of

**4.** Relative humidity seems to affect the resolution of the lithographic process by controlling the rate of ODT transport from the tip to the substrate. The size of the water meniscus that bridges the tip and substrate depends on relative humidity. For example, the 30-nm wide line required 5 min to generate in a 34% relative humidity environment, whereas the 100-

nm line required 1.5 min to generate in a 42% relative humidity environment.

**2.** The tip-substrate contact time and thus the scan speed influence DPN resolution.

chemical species to the surface. We can call this process as constructive process.

202 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

the amount of proteins in biological samples such as blood. (Ref. Fig. 12).

Dip-Pen Nanolithography can not only apply to gold surface using alkyl or aryl thiols as inks, but also to semiconductor surfaces, such as silicon and gallium arsenide. Hexamethyldisila‐ zane (HMDS) is used as the ink to pattern and modify (polarity) the surface of semiconductors. Lateral force microscopy (LFM) can be used to differentiate between oxidized semiconductor surfaces and patterned areas with the deposited monolayers of HMDS. The choice of the silazane ink is a critical component of the process since the traditional adsorbates such as trichlorosilanes are incompatible with the water meniscus and polymerize during ink depo‐ sition. This work provides insight into additional factors, such as temperature and adsorbate reactivity, that control the rate of the DPN process and paves the way for researchers to interface organic and biological structures generated via DPN with electronically important semiconductor substrates (Ivanisevic, 2001).

## *3.5.3.3. DPN application on magnetic materials: Approach to high density recording and storage*

Over the past decade, there has been considerable interest in methods for synthesizing and patterning nanoscale magnetic materials. These nanomaterials show novel size-dependent properties, are potentially useful for high-density recording. Two of the main challenges in this field are: (a) site-and shape-specific patterning of hard magnetic nanostructure on the sub-100 nm scale; (b) ability to reliably and reproducibly read/write such minute features. The conventional top-down approach in recording media is plagued by the difficulties of etching and patterning novel hard magnetic systems, especially as the individual recording elements approach the super paramagnetic limit at room temperature operations. DPN can be used as a direct-write method for fabricating "hard" magnetic barium hexaferrite, BaFe12O19 (BaFe), nanostructures. This method utilizes a conventional atomic force microscope tip, coated with the BaFe precursor solution, to generate patterns that can be post-treated at elevated temper‐ ature to generate magnetic features consisting of barium ferrite in its hexagonal magneto‐ plumbite (M-type) structure. Features ranging from several hundred nm down to below 100 nm can be generated. (Fu, 2003).
