Introduction

Since the 1950s, Scanning Electron Microscopy (SEM) has been commercially available and used to measure feature sizes below 1 micron. Modified SEMs have been employed since the 1960s to perform sub-micron lithography, which then made rapid advances in the 1990s to a process, known as electron beam lithography (EBL). Since the 1980s, Surface Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have ushered the era of nanotechnology where it is possible to measure and control the manipulation of matter on the 100nm scale and below. These techniques are broadly classified as "Scanning Probe Microscopy (SPM)". The earliest forms of nanofabrication using STM based approaches were used to pattern "hard" materials (such as silicon-dioxide; as opposed to "soft" materials such as polymers or biological materials) and restricted to single layer processing. These methods were initially motivated by applications in the semi-conductor industry.

The nanofabrication processes pioneered by EBL and STM were followed by development of diverse nanofabrication approaches in the 1990s, which included micro contact printing (which is derived from soft lithography), step and flash imprint lithography (SFIL), nano imprint lithography (NIL), etc. However, these methods are only capable of single layer registration and the feasibility of registering multi-layers in alignment with the previously fabricated features are yet to be demonstrated. Also, these methods are not capable of spatially addressing biological materials in chemically distinct arrays. Of these methods, micro contact printing is the most flexible in its ability to pattern different materials while

AFM tip as a point source

AFM Tip

AFM tip as a point source

AFM Tip

FIGURE 10.1. (a) Schematic representation of the dip-pen nanolithography process. (Reprinted with the permission from Piner etal., Science 283:661-63, @ 1999, AAAS). (b) Proposed transport mechanism of ink molecules from the tip to the substrate. The incoming molecular flux from the tip creates a concentration gradient around the tip, and ink molecules subsequently diffuse over the region already occupied by other ink molecules (drawn as filled circles) to be finally trapped by the bare surface of the substrate. (Reprinted with the permission from Jang et al., Journal of Chemical Physics 115 (6):661-63, @ 2001, American Institute of Physics).

FIGURE 10.1. (a) Schematic representation of the dip-pen nanolithography process. (Reprinted with the permission from Piner etal., Science 283:661-63, @ 1999, AAAS). (b) Proposed transport mechanism of ink molecules from the tip to the substrate. The incoming molecular flux from the tip creates a concentration gradient around the tip, and ink molecules subsequently diffuse over the region already occupied by other ink molecules (drawn as filled circles) to be finally trapped by the bare surface of the substrate. (Reprinted with the permission from Jang et al., Journal of Chemical Physics 115 (6):661-63, @ 2001, American Institute of Physics).

others are fairly restrictive in patterning a narrow range of materials. So far, NIL has been demonstrated to have the highest precision of close to 20 nm. The speed of fabrication for NIL, SFIL and micro contact printing is quite high compared to EBL at the expense of accuracy. Currently, the equipment cost and operating cost of these methods are estimated to range from approximately $500,000 (NIL) to about $20 million for EBL.

Dip Pen Nanolithography™ (DPNTM) was invented by Chad Mirkin, Richard Piner and Seunghun Hong at North Western University in 1999 (US Patent 6635311). DPN uses chemically coated scanning probe microscopy tips to deposit nano-scale chemical patterns on a substrate in a direct write process. Feature sizes as small as 15 nm have been demonstrated with 5 nm spatial resolution for small molecules [30]. The genesis of the invention is the investigation of small capillary meniscus ("capillary bridge") that was found to nucleate on scanning probe tips when brought in close proximity to a substrate [32]. Experimental pictures showing the existence of these capillary bridges have been reported by Schenk et al, 1998. Molecular dynamic simulations have also indicated the existence of such a nucleating process [17]. Also, a thin aqueous film (of the order of 1-2 molecule thickness) is believed to adhere on all surfaces under ambient conditions, which can also aid in the formation of a capillary bridge.

Initial experiments using DPN were demonstrated by forming self assembled mono-layer (SAM) of different small molecules primarily on gold and oxidized silicon surfaces. Typically ODT (octa-decane-thiol) and MHA (mercapto-hexadecanoic-acid) were used in these initial experiments. These deposited chemistries were used as etch masks to pattern oxidized silicon and metals to demonstrate their efficacy for semi-conductor applications [43].

In the DPN terminology—the chemically coated scanning probe tip is called the "pen", the chemical used to coat the tip is known as the "ink" and the substrate for writing is known as the "paper". The DPN process is similar to writing using quill pens. Incidentally, quill pens were used as late as the 18th century until the invention of fountain pen, which were followed by invention of the ball point pen in 1940. Prior to quill pens, pens made out of reeds were used as early as 2000 B.C. for writing on parchments (or papyrus) in the ancient Egyptian civilization, the Roman civilization and the civilizations in South-East Asia. In a close resemblance to this development, the development of DPN has been followed by the development of Fountain Pen Lithography (FPN), which was proposed by Kim et al., 2003 (Espinosa group, Northwestern University). The desired mode of FPN operation would require a continuous supply of ink to the pen tip from a reservoir that would obviate the need for loading the pen with ink by dipping. Espinosa's group has fabricated a FPN device that consists of an ink supply reservoir connected by micro-channels to a hollow scanning probe tip. Successful demonstration of an FPN device that does not need dipping for loading ink and is able to write patterns with feature size less than 100 nm is yet to be demonstrated. Similar approaches to using fountain pen nano-patterning are also described by Lewis et al. [22] and Shalom et al. [37].

A number of experiments with small molecules (e.g., MHA, ODT) show that the deposition process is diffusion limited [39,42]. By dragging the pen tip on a surface, larger feature sizes were obtained when dragged at a lower velocity or at elevated temperatures. Similarly, by touching the tip on a substrate larger spot sizes were obtained when the dwell times were increased for a particular temperature or when the experiments were performed for a fixed dwell time and at an elevated temperature. These are evidences that prove that the deposition process is diffusion controlled. Also, the relative humidity of the ambient conditions around the pen tip has a significant effect on the writing process. At higher relative humidity larger feature sizes are obtained, keeping all other factors constant. Within the bounds of experimental error it was observed that the deposited features were independent of the force applied on the tips [30]. This behavior enables an array of scanning probes employed for DPN to have a lower variability in spot sizes due to different contact forces being applied to the tips. This is of great significance for a multi-pen DPN architecture targeted for a reliable industrial scale instrument since it enables a more repeatable process with lower variability.

Thus, the DPN process is independent of the contact forces and is affected by the dwell time (or speed of writing), ambient humidity, the surface diffusivity of the deposited chemical species (which is a function of the molecular weight of the chemical species, activation energy for detachment from scanning probe tip and temperature), surface roughness of the substrate, material properties of the substrate, material properties of the scanning probe tip and the tip radius of the scanning probe tip.

DPN has significant advantages over other nano-lithographic processes mentioned earlier. DPN can be used to deposit multi-layer patterns in close registration with the different features fabricated in the previous steps. The features deposited in the prior fabrication steps can be registered by scanning the chemically coated probe tips on the substrate at faster scan speeds that prevents diffusional deposition of the ink. After registering the location of the previous features new features can then be aligned and deposited using different sets of inks. Thus it has been demonstrated as a direct write tool with high precision, accuracy and resolution. DPN can be used to deposit biological and soft polymeric materials at room temperature and under ambient conditions without exposing them to harsh physical environments (e.g., vacuum) and harsh chemical environments (e.g., etching solutions, photoresist, etc.). The fabrication steps are performed at the desired locations with minimal risk for cross contamination. It is also possible to perform these fabrication steps under a water droplet to prevent denaturing of sensitive proteins. DPN process has been demonstrated for a variety of inks and substrates. The inks demonstrated range from small molecules (ODT, MHA) to polymers to biological materials (proteins, peptides, oligo-nucleotides). The substrates used for performing DPN range from silicon, glass, germanium, gallium-arsenide to metals (such as gold). Using DPN it is possible to integrate small molecule chemistry and large bio-molecules on a single substrate. The lithography process does not need a costly clean room infrastructure for operation. The whole operation can be performed on a tabletop instrument using a humidity control chamber. The DPN platform is flexible, digitally programmable and reconfigurable. The DPN process has enabled a rapid proto-typing platform with a fast turn around. However, a single pen platform can be quite slow and currently has limited small-scale application only in research laboratories. The process can be scaled to a high throughput industrial tool. This can be achieved by a multi-pen (or "plotter") configuration where an array of probe tips is used with each probe tip coated with unique ink chemistry. NanoInk Inc. has licensed the core DPN technology for commercializing it as an industrial scale process. Zhang et al. (2002) and research groups at NanoInk [36] have developed various multi-pen DPN plotters. The development of high throughput DPN platform is a separate subject area and can be found in a separate reference. Software called DPNWrite™ is also available from NanoInk for the automated operation of the DPN instrument (commercially available as NScriptorTM). The DPN technology commercially available from NanoInk requires a relatively low investment for infrastructure and operation, which is currently at a fraction of the cost required for other nano-lithographic platforms of comparable resolution, accuracy and precision.

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