ATOMIC LITHOGRAPHY & SOL GEL PROCESSING Dhruvi Mehta SAP ID: Batch: A2

Lithography  Lithography (lithos, meaning "stone", and graphein, meaning "to write") is a method of printing.  Nanolithography is concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm.

 Nanolithography is used during the fabrication of leading-edge semiconductor integrated circuits or nanoelectromechanical systems.

Atomic Force Microscopy  Atomic force microscopy (AFM) is a high resolution type of scanning probe microscopy that allows us to see and measure surface structure in length scale 10nm-100μm with unprecedented resolution and accuracy.  Unlike an imaging traditional microscope, AFM provides height information of the sample.  Almost any sample can be imaged, be it very hard (ceramic material) or very soft (human cells, individual molecules of DNA).  We can generate images which look at the sample from any conceivable angle with simple analysis software.  Currently AFM is the most common form of scanning probe microscopy and is used in all fields of science as chemistry, biology, physics, materials science, nanotechnology, astronomy, medicine and more.

Construction Of AFM  AFM provides a 3D profile of the surface on a Nano scale, by measuring forces and surface at very short distance.  The probe is supported on a flexible cantilever.  The AFM tip gently touches the surface and records the small force between the probe and the surface.  Forces between the tip and the sample lead to a deflection of the cantilever

How Are The Forces Measured?  The probe is placed on the end of a cantilever (which one can think of as a spring).  The amount of force between the probe and sample is dependent on the spring constant (stiffness of the cantilever) and the distance between the probe and the sample.  This force can be described using Hooke’s Law: F=-k.x

Schematic Diagram Of AFM

Working Of AFM  The cantilever is a bendable structure used to hold the tip.  The piezoelectric materials are used for controlling the motion of the probe as it is scanned across the sample surface.  A laser beam is reflected by the back side of a reflective cantilever onto the photo detector.  The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.  The feedback loop includes all of the structural elements that are required to hold the probe at a fixed distance from the sample.

AFM Tip  The tip of the AFM is used: 1. for imaging 2. for measuring forces (and mechanical properties) on the Nano scale 3. as a Nano scale tool, i.e. for bending, cutting and extracting soft materials 4. for high-resolution image control  In AFM all what is “seen", is seen by the tip, so everything depends on its shape.

Comparison Advantages  Minimal sample preparation  Does not require a conductive sample  Provides a three-dimensional surface profile (ability to magnify in the X,Y,Z axes)  Works perfectly well in ambient air or even a liquid environment  Possible to study biological macromolecules and even living organisms  Does not require expensive equipment Disadvantages  Not practical to make measurements on areas greater than 100μm  Limited scanning speed, requiring several minutes for a typical scan  Images can be affected by nonlinearity, hysterisis and creep of the piezoelectric material  An AFM image does not reflect the true sample topography, but rather represents the interaction of the probe with the sample surface

Sol-Gel  A sol is a type of colloid in which a dispersed solid phase is mixed in a homogeneous liquid medium. An example of a naturally occurring sol is blood.  A gel is an interconnected, rigid network with pores of sub micrometer dimensions and polymeric chains, whose average length is greater than a micrometer.  Hence, the sol-gel process involves the evolution of networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel).

Basics Of Sol-Gel Processing  Sol-gel processing is a wet chemical route for the synthesis of colloidal dispersions of inorganic and organic-inorganic hybrid materials.  This method is performed in the liquid phase.  It is a useful self-assembly process for fabricating nanoparticles as well as nanostructured surfaces and three-dimensional nanostructured materials such as aerogels.  Sol-gel processing takes place in two steps: hydrolysis and condensation-polymerization.

Hydrolysis  In the this first hydrolysis reaction, the -OR group is replaced with an -OH group. The hydrolysis reaction can occur without a catalyst but is more rapid and complete when they are used. The catalyst can be a base or an acid.  Reaction: M(H 2 O) b Z+ ↔ [M(H 2 O) b-1 OH] (Z-1)+ + H +

Condensation-Polymerization  After hydrolysis, the sol starts to condense and polymerize. This leads to a growth of particles which, depending on various conditions such as pH, reach dimensions of a few nanometers. This reaction is quite complex and involves many intermediate products. The particles then agglomerate; A network starts to form throughout the liquid medium, resulting in thickening, which forms a gel.  Reaction: M(H 2 O) b Z+ ↔ [(H 2 O) b-1 M(OH) 2 M(H 2 O) b-1 ] (2Z-2)+ + 2H +

Formation Of Silica Gels  A silica gel may be formed by network growth from an array of discrete colloidal particles or by formation of an interconnected 3-D network by the simultaneous hydrolysis and polycondensation of an organometallic precursor.  When the pore liquid is removed as a gas phase from the interconnected solid gel network under hypercritical conditions, the network does not collapse and a low density aeroge l is produced.  When the pore liquid is removed at or near ambient pressure by thermal evaporation and shrinkage occurs, the monolith is termed a xerogel.  If the pore liquid is primarily alcohol based, the monolith is often termed an alcoge l.

Schematic Overview Of Different Materials That Can Be Obtained Through A Sol-Gel Process

Sol-Gel Coating Process Steps  The desired colloidal particles once dispersed in a liquid form a sol.  The deposition of sol solution produces the coatings on the substrates by spraying, dipping or spinning.  The particles in sol are polymerized through the removal of the stabilizing components and produce a gel in a state of a continuous network.  The final heat treatments pyrolyze the remaining organic or inorganic components and form an amorphous or crystalline coating.

Advantages  Can produce thin bond-coating to provide excellent adhesion between the metallic substrate and the top coat.  Can produce thick coating to provide corrosion protection performance.  Can easily shape materials into complex geometries in a gel state.  Can produce high purity products because the organo- metallic precursor of the desired ceramic oxides can be mixed, dissolved in a specified solvent and hydrolyzed into a sol, and subsequently a gel, the composition can be highly controllable.  Can have low temperature sintering capability, usually °C.  Can provide a simple, economic and effective method to produce high quality coatings.

Applications  It can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes.  Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation technology. One of the more important applications of sol-gel processing is to carry out zeolite synthesis.  Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.  Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis.