Light sources of nanolithography

Historically, photolithography has used ultraviolet light from gas-discharge lamps using mercury, sometimes in combination with noble gases such as xenon. These lamps produce light across a broad spectrum with several strong peaks in the ultraviolet range. This spectrum is filtered to select a single spectral line, usually the "g-line" (436 nm) or "i-line" (365 nm).
More recently, lithography has moved to "deep ultraviolet", produced by excimer lasers. (In lithography, wavelengths below 300 nm are called "deep UV".) Krypton fluoride produces a 248-nm spectral line, and argon fluoride a 193-nm line. Generally, changing wavelength is not a trivial matter, as the method of generating the new wavelength is completely different, and the absorption characteristics of materials change. For example, air begins to absorb significantly around the 193 nm wavelength; moving to sub-193 nm wavelengths would require installing vacuum pump and purge equipment on the lithography tools (a significant challenge). Furthermore, insulating materials such as silicon dioxide(SiO2), when exposed to photons with energy greater than the band gap, release free electrons and holes which subsequently cause adverse charging.
Optical lithography has been extended to feature sizes below 50 nm using 193 nm and liquid immersion techniques. Also termed immersion lithography, this enables the use of optics with numerical apertures exceeding 1.0. The liquid used is typically ultra-pure, deionised water, which provides for a refractive index above that of the usual air gap between the lens and the wafer surface. The water is continually circulated to eliminate thermally-induced distortions. Water will only allow NA's of up to ~1.4, but materials with higher refractive indices will allow the effective NA to be increased further.

Changing the lithography wavelength is significantly limited by absorption. Air absorbs below ~ 185 nm.
Experimental tools using 157 nm wavelength DUV in a manner similar to current exposure systems have been built. These were once targeted to succeed 193 nm at the 65 nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157 nm technology and economic considerations that provided strong incentives for the continued use of 193 nm technology. High-index immersion lithography is the newest extension of 193 nm lithography to be considered. In 2006, features less than 30 nm were demonstrated by IBM using this technique.

Basic procedure photolithography

A single iteration of photolithography combines several steps in sequence. Modern cleanrooms use automated, robotic wafer track systems to coordinate the process. The procedure described here omits some advanced treatments, such as thinning agents or edge-bead removal.
Cleaning
If organic or inorganic contaminations are present on the wafer surface, they are usually removed by wet chemical treatment, e.g. the RCA clean procedure based on solutions containing hydrogen peroxide.
Preparation
The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Wafers that have been in storage must be chemically cleaned to remove contamination. A liquid or gaseous "adhesion promoter", such as Bis(trimethylsilyl)amine ("hexamethyldisilazane", HMDS), is applied to promote adhesion of the photoresist to the wafer. The phrase "adhesion promoter" is in fact incorrect, as the surface layer of Silicondioxide on the wafer reacts with the agent to form Methylated Silicon-hydroxide, a highly water repellent layer not unlike the layer of wax on a car's paint. This water repellent layer prevents the aqueous developer from penetrating between the photoresist layer and the wafer's surface, thus preventing so-called lifting of small photoresist structures in the (developing) pattern.
Photoresist application
The wafer is covered with photo resist by spin coating. A viscous, liquid solution of photo resist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The spin coating typically runs at 1200 to 4800 rpm for 30 to 60 seconds, and produces a layer between 0.5 and 2.5 micrometres thick. The spin coating process results in a uniform thin layer, usually with uniformity of within 5 to 10 nanometres. This uniformity can be explained by detailed fluid-mechanical modelling, which shows that essentially the resist moves much faster at the top of the layer than at the bottom, where viscous forces bind the resist to the wafer surface. Thus, the top layer of resist is quickly ejected from the wafer's edge while the bottom layer still creeps slowly radially along the wafer. In this way, any 'bump' or 'ridge' of resist is removed, leaving a very flat layer. Final thickness is also determined by the evaporation of liquid solvents from the resist. For very small, dense features (<125>Exposure and developing
After prebaking, the photoresist is exposed to a pattern of intense light. Optical lithography typically uses ultraviolet light (see below). Positive photoresist, the most common type, becomes soluble in the basic developer when exposed; exposed negative photoresist becomes insoluble in the (organic) developer. This chemical change allows some of the photoresist to be removed by a special solution, called "developer" by analogy with photographic developer. To learn more about the process of exposure and development of positive resist, see: Ralph Dammel, "Diazonaphtoquinone-based resists", SPIE Optical Engineering Press, Vol TT11 (1993).
A PEB (post-exposure bake) is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. In DUV (deep ultraviolet, or 248 nm exposure wavelength) lithography, CAR (chemically amplified resist) chemistry is used. This process is much more sensitive to PEB time, temperature, and delay, as most of the "exposure" reaction (creating acid, making the polymer soluble in the basic developer) actually occurs in the PEB.The develop chemistry is delivered on a spinner, much like photoresist. Developers originally often contained sodium hydroxide(NaOH). However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides (specifically, sodium ions can migrate in and out of the gate, changing the threshold voltage of the transistor and making it harder or easier to turn the transistor on over time). Metal-ion-free developers such as tetramethylammonium hydroxide (TMAH) are now used.
The resulting wafer is then "hard-baked" if a non-chemically amplified resist was used, typically at 120 to 180 °C[citation needed] for 20 to 30 minutes. The hard bake solidifies the remaining photoresist, to make a more durable protecting layer in future ion implantation, wet chemical etching, or plasma etching.
Etching
Main article: Etching (microfabrication)
In etching, a liquid ("wet") or plasma ("dry") chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. In semiconductor fabrication, dry etching techniques are generally used, as they can be made anisotropic, in order to avoid significant undercutting of the photoresist pattern. This is essential when the width of the features to be defined is similar to or less than the thickness of the material being etched (ie when the aspect ratio approaches unity). Wet etch processes are generally isotropic in nature, which is often indispensable for microelectromechanical systems, where suspended structures must be "released" from the underlying layer.
The development of low-defectivity anisotropic dry-etch process has enabled the ever-smaller features defined photolithographically in the resist to be transferred to the substrate material.
Photoresist removal
After a photoresist is no longer needed, it must be removed from the substrate. This usually requires a liquid "resist stripper", which chemically alters the resist so that it no longer adheres to the substrate. Alternatively, photoresist may be removed by a plasma containing oxygen, which oxidizes it. This process is called ashing, and resembles dry etching.

Photolithography

Optical lithography, is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photo mask to a light-sensitive chemical photo resist, or simply "resist," on the substrate. A series of chemical treatments then engraves the exposure pattern into the material underneath the photo resist. In complex integrated circuits, for example a modern CMOS, a wafer will go through the photolithographic cycle up to 50 times.
Optical lithography shares some fundamental principles with photography in that, the pattern in the etching resist is created by exposing it to light, either using a projected image or an optical mask. This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than to lithographic printing. It is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
Contents
1 Basic procedure
1.1 Cleaning
1.2 Preparation
1.3 Photoresist application
1.4 Exposure and developing
1.5 Etching
1.6 Photoresist remova

Bottom-up methods nanolithography

Nanosphere lithography uses self-assembled monolayers of spheres (typically made of polystyrene) as evaporation masks. This method has been used to fabricate arrays of gold nanodots with precisely controlled spacings.
It is possible that molecular self-assembly methods will take over as the primary nanolithography approach, due to ever-increasing complexity of the top-down approaches listed above. Self-assembly of dense lines less than 20 nm wide in large pre-pattearned trenches has been demonstrated. The degree of dimension and orientation control as well as prevention of lamella merging still need to be addressed for this to be an effective patterning technique. The important issue of line edge roughness is also highlighted by this technique.
Self-assembled ripple patterns and dot arrays formed by low-energy ion-beam sputtering are another emerging form of bottom-up lithography. Aligned arrays of plasmonic and magnetic wires and nanoparticles are deposited on these templates via oblique evaporation. The templates are easily produced over large areas with periods down to 25 nm

Bottom-up methods nanolithography

Nanolithography

Optical lithography
Main article: Photolithography
Optical lithography, which has been the predominant
patterning technique since the advent of the semiconductor age, is capable of producing sub-100-nm patterns with the use of very short wavelengths (currently 193 nm). Optical lithography will require the use of liquid immersion and a host of resolution enhancement technologies (phase-shift masks (PSM), optical proximity correction (OPC)) at the 32 nm node. Most experts feel that traditional optical lithography techniques will not be cost effective below 22 nm. At that point, it may be replaced by a next-generation lithography (NGL) technique.
Other nanolithography techniques
X-ray lithography can be extended to an optical resolution of 15 nm by using the short wavelengths of 1 nm for the illumination. This is implemented by the proximity printing approach. The technique is developed to the extent of batch processing. The extension of the method relies on Near Field X-rays in Fresnel diffraction: a clear mask feature is "demagnified" by proximity to a wafer that is set near to a "Critical Condition". This Condition determines the mask-to-wafer Gap and depends on both the size of the clear mask feature and on the wavelength. The method is simple because it requires no lenses.
A method of pitch resolution enhancement which is gaining acceptance is double patterning. This technique increases feature density by printing new features in between pre-printed features on the same layer. It is flexible because it can be adapted for any exposure or patterning technique. The feature size is reduced by non-lithographic techniques such as etching or sidewall spacers.
Work is in progress on an optical maskless lithography tool. This uses a digital micro-mirror array to directly manipulate reflected light without the need for an intervening mask. Throughput is inherently low, but the elimination of mask-related production costs - which are rising exponentially with every technology generation - means that such a system might be more cost effective in the case of small production runs of state of the art circuits, such as in a research lab, where tool throughput is not a concern.
The most common nanolithographic technique is Electron-Beam Direct-Write Lithography (EBDW), the use of a beam of electrons to produce a pattern — typically in a polymeric resist such as PMMA.
Extreme ultraviolet lithography (EUV) is a form of optical lithography using ultrashort wavelengths (13.5 nm). It is the most popularly considered NGL technique.
Charged-particle lithography, such as ion- or electron-projection lithographies (PREVAIL, SCALPEL, LEEPL), are also capable of very-high-resolution patterning. Ion beam lithography uses a focused or broad beam of energetic lightweight ions (like He+) for transferring pattern to a surface. Using Ion Beam Proximity Lithography (IBL) nano-scale features can be transferred on non-planar surfaces.
Neutral Particle Lithography(NPL) uses a broad beam of energetic neutral particle for pattern transfer on a surface.
Nanoimprint lithography (NIL), and its variants, such as Step-and-Flash Imprint Lithography, LISA and LADI are promising nanopattern replication technologies. This technique can be combined with contact printing.
Scanning probe lithography (SPL) is a promising tool for patterning at the deep nanometer-scale. For example, individual atoms may be manipulated using the tip of a scanning tunneling microscope (STM). Dip-Pen Nanolithography (DPN) is the first commercially available SPL technology based on atomic force microscopy.
Atomic Force Microscopic Nanolithography (AFM) is a chemomechanical surface patterning technique that uses an atomic force microscope.
Magnetolithography (ML) based on applying a magnetic field on the substrate using paramagnetic metal masks call "magnetic mask". Magnetic mask which is analog to photomask define the spatial distribution and shape of the applied magnetic field. The second component is ferromagnetic nanoparticles (analog to the photoresist) that are assembled onto the substrate according to the field induced by the magnetic mask

Antibody-Nanoparticle Computational Modeling

The conjugation of antibodies and nanoparticles with high affinity & specificity through receptor-ligand recognition modes is of paramount importance in the development of vehicles which can be used for diagnosis, treatment of cancer and various other diseases, application of immunodiagnostic nano-biosensors etc. The bio-nanocomplex formed by an artificial nanomaterial (nanoliposomes , nanoparticles ) and a biological entity such as an antibody is brought about by the formation of covalent bonds based on their specific chemical and structural properties such as water solubility, biocompatibility, and biodegradability. [5]. There is a requirement of a comprehensive understanding of the relationship of the thermodynamic & kinetic aspects of antibody-membrane association, translational , rotational mobilities of membrane bound antibodies, interactions with the diverse cell surface , circulating molecules and various artificial nanomolecules as well as the conformation. These details are of great importance in the development, application of various nanoscale immunodiagnostic devices. The association of antibodies with cell surfaces is a key molecular event in antibody-mediated immune mechanisms such as phagocytosis, antibody mediated immune dependent cell-mediated cytotoxicity.
Recently it has been noted that there exists certain natural proteins, antibodies, that can recognize specific nanoparticles . For example, a specific antibody from the mouse immune system can specifically recognize derivatized C60 fullerenes with a binding affinity of about 25 nM . From the studies carried out by Noon et al., it is hypothized that the fullerene-binding site is formed at the interface of the light and heavy chains lined with a cluster of shape-complementary hydrophobic amino acid residues. As the covalent modifications of the functionalized fullerenes, occupy only a small fraction of the particle surface area , the largely unoccupied surface would be free to interact with the antibody. Therefore, in order to gain in-depth understanding of the detailed interactions of the nps and the antibody, molecular dynamics simulation is carried out using molecular dynamics simulation; the purpose of our theoretical modeling studies is to be able to identify the energetically favorable binding modes. .
For the modeling study, the initial coordinates of the antibody can be made available from the Protein Data Bank (PDB). .
The basic assumptions, as a first approximation, during the modeling study would be that the hydrophilic derivatizations do not play a critical role in the predominantly hydrophobic nanomaterial-antibody interactions and that the electronic structure remains undisturbed during the conjugation. The nanoparticle is docked into a suggested binding site from the previously done literature studies. Polar-hydrogen potential function (PARAM19) and a modified TIP3P water solvent model for the protein is used.
The simulation involves approximately about 300 steps of minimization, using the Steepest Descent and the Newton Raphson method. To reduce the necessary simulation time, a highly efficient method for simulating the localized interactions in the active site of a protein, the stochastic boundary molecular dynamics (SBMD) is used. The reference point for partitioning the system in SBMD was chosen to be near the center of the nanomaterials, which is assumed to be a uniform sphere. The complex nano-bio system can be assumed to be separated into spherical reservoir and reaction zones; the latter is further sub-divided into a reaction region and a buffer region. The atoms in the reaction region are propagated by molecular dynamics, whereas those in the buffer region involve Langevin dynamics are retained using harmonic restoring forces. 'At the highly recognized Indian Institute Of Sciences (IISc),Antibody-Nanoparticle recognition technique will be duly modified to develop new vaccines against cancer,hepatitis and AIDS by the team under.