MÜGE BAŞARAN 030090704 WEEK-7

PHOTOLITHOGRAPHY (OPTICAL LITHOGRAPHY)

group: material process

Old definition:

Optical Lithography(also called photolithography) uses light as the writing material. The drawn(greased) and undrawn(moistened) areas on the limestone in lithography become bright and dark regions on a reticle or phptmask, the template of optical lithography. Just as greased ink discriminately deposits on the limestone, light passes only through the clear opening the mask. The transmitted energy is recorded on a light-sensitive medium called the photoresist.

(Alfred Kwok- Kit Wong, Resolution Enhancement Techniques In Optical Lithograpy,p.2)

New definition:

An often-used technique for device fabrication is photolithographic masking. Here, a film of photoresist is applied to the substrate, and the photoresist is exposed to light through a photolithographic mask. After exposure, the photoresist is developed, which transfers the desired pattern to the photoresist (Fig. 6.3). When the substrate is subjected to a chemical treatment, the photoresist protects the surface, and thus the pattern on the mask is transferred to the substrate. The photoresist is removed by stripping, which is essentially dissolution of the photoresist in a nonuniform but fast way. Acetone is usually used to strip resist, but in some cases a special stripper must be used. The photoresist manufacturer generally makes such a stripper available. Photoresists can be any of various photosensitive polymers. These polymers can be applied by different techniques such as spinning or spraying. In spinning, the thickness of the film can be expressed by this empirical expression:

Where t _coating is the thickness of the coating, K is a proportionality constant, r is the kinematic viscosity (mm² s^-1), and w is the number of revolutions per minute. The polymers are sensitive to light of particular wavelengths, and when exposed to these the chemical structure of the photoresist changes. Usually they are sensitive to UV light, with the i-line of mercury (365 nm) being particularly popular. After exposure to light, the photoresist is developed by using a particular chemical solution. Some photoresists become more soluble in a developer after exposure, some become less soluble. The photoresists that become more soluble are called positive tone photoresists (Fig. (6.3); an example is AZ4562 photoresist. The other type of photoresist becomes less soluble after light exposure, due to cross linking of the polymer, and is called negative tone photoresist; an example is SU-8.

(Oliver Geschke,Henning Klank,Pieter Telleman, Microsystem Engineering of Lab-On-A-Chip Devices, pg. 122)

My definition makes it easy to understand about this method by given formulations and layout of process.

BLACK ‘ORLON’

group: material

Old definition:

Black Orlon is a ladder polymer obtained by regulated pyrolysis of polyacrylonitrile. The structure of black orlon contains H,C and N atoms. Fibres of black orlon are highly tough and find applications in fibre reinforced plastics.

(A.K. Bhargava, Engineering Materials: Polymers Ceramics And Composites, 200, page 118)

New definition:

For some time it has been known that PAN fibres blacken on heating in air at 220 ⁰C and that this 'black Orion' is remarkably flame-proof and stable. Such stabilization by oxidation permits the formation of oriented ladder polymer whilst reducing the intensity of the exothermic peak, which would otherwise build up if the oxidation process were to be omitted. After stabilization, the modified ladder molecules have a high enough glass transition temperature to retain any preferred orientation imposed in the original polymer, even subsequent to a carbonization process.

The proposed mechanism of carbonization is illustrated also in fig. 9. The ladder molecules coalesce progressively to form the ribbons characteristic of all polymeric carbons. It should be stressed that the structures drawn in fig. 9 are essentially related to isotactic homopolymers, whereas the fibres used to make high stiffness, high tenacity carbon fibres are atactic copolymers. This makes the true mechanism difficult to interpret.

(G. M. Jenkins,K. Kawamura, Polymeric Carbons, pg.31-33)

Old definition is also good because of being simple and easy to understand but against my definition is more satisfactory to observe the structure.

VGCF (VAPOR GROWN CARBON FIBERS)

group: material

Old definition:

VGCFs comprise a large family of filamentous nanocarbons.Thwy can be distinguished in terms of arrangement of the graphane layers in their molecular scale structure.They can be plate-like with near paralell graphane layers that are approximately perpendicular to the fiber axis or they can have the 'fish-bone' microstructure with stacked cones of graphane planes.(Functional Fillers For Plastics,Marino Xanhtos,p:194)

New definition:

Carbon fibers produced by direct growth from the vapor phase, i.e., vapor grown carbon fibers (VGCF)is an another variety of reinforcement with high mechanical and thermal properties and significant commercial potential. These are also called catalytic chemical vapor deposition (CCVD) carbon fibers. These fibers arc produced by catalytic decomposition of a hydrocarbon such as benzene, methane, or propane at 1000-I 500^o C in the presence of a transition metal (Fe, Ni, or Co) or metallo-organics such as ferrocene. (C ₅H₅)₂,Fe (Tibbetts ei at 1987). The catalyst plays a vital role in the growth of VGCFs. The carbon fiber yield is enhanced if a catalyst with a small but broader distribution of particle sizes is used. VGCFs are produced in short lengths (50-70 mm) and small diameter (0.5-2 pm). Benzene, or other hydrocarbons which generate benzene during decomposition, are preferred as a precursor for a higher rate of production. These fibers have also been produced from low cost sources such as linz-donawitz converter gas and coal-derived hydrocarbons. These fibers possess very high mechanical properties but have a large scatter in the values.

The structure, and thus the mechanical properties of VGCF, are independent of the precursor gas source employed, but are extremely dependent on processing parameters such as temperature of growth, type and distribution of catalyst, etc. The aspect ratio of VGCF is found to be dependent on catalyst to hydrocarbon flow ratio. Low temperature deposition (<900⁰C) results in vermicular filaments, while high temperatures (1500-2500⁰C) favor the growth of long, straight filaments. Since VGCF are produced in a one-step process, these possess significant commercial potential as low cost carbon fibers.

Based on electron microscopic studies on the structure of the fibers, the growth process has been proposed consisting of two parts (Endo et al. 1977). In the primary process a thin tube of carbon is formed by catalytic growth on a catalyst particle. This is followed by secondary growth onto the first tube. VGCF consist of turbostratic carbon layers parallel to the fiber axis arranged in annular concentric sheets like rings in a tree trunk. The core is more perfectly ordered while the secondary sheets may contain some defects. VGCF are graphitizable carbons, and when heat treated to more than 2500 ⁰C, develop a well-orianted graphite structure and increase in density and Young’s modulus.

(Andreas Mortensen, Concise Encyclopedia of Composite Materials, pg. 93 )

My definition is so good against the older definition because the previous one is so poor to explain the process.

CHEMICAL SYNTHESIS

group: material chemistry

Old definition:

Chemical synthesis is uniquelly positioned at the heart of chemistry, the central science, and its impact on our lives and society is all pervasive. For instance, many of todays medicines are synthetic and many of tomorrows will be conceived and produced by synthetic chemist. To the field of synthetic chemistry belongs an array of responsibilities which are crucial for the future of mankind, not only with regard to the health, material and economic needs of our society, but also for the attainment of understanding of matter, chemical change and life at the highest level of which the human minds is capable.

(Chemistry 1981-1990, B.G. Malmström, p.686)

New definition:                               

A century and a half later Crnforth defined chemical synthesis as "international construction of molecules by chemical means". The period from the second half of the 19th century through the first half of the 20th century witnessed three primary pursuits in the field of organic chemistry: (I) the elucidation of the structures of natural products, (2) the investigation of basic reactions, and (3) the preparation of new chemical substances (Fig. 1.1-1). These activities continue to this day simplified by modern technology. Organic chemists pursue new activities as %sell, and synthetic organic chemistry is now associated with biology, medicinal chemistry, and materials sciences.

Any chemical synthesis can be resolved into three basic processes as shown in Table I. The information associated with each process must be captured and stored in an automated procedure. The design process is a melding of target selection with synthetic methodology. For a traditional synthesis, the potential targets are only limited by the chemist's skill. Because the glassware used for traditional syntheses is of modular design and can be built into a large number of configurations, the reactor design generally does not impose any restrictions on the reactions that can be run. Thus the chemist can select any method for synthesis of the desired targets. The commercial availability of chemical building blocks becomes a major factor in library design. Due to the long lead times for custom-synthesized building blocks, the synthesis of compound libraries for the drug discovery are often restricted to commercially available starting materials. Availability of building blocks also impact the selection of the chemistry used to synthesize the final products. The synthetic methodologies are restricted to those chemical reactions that use the available building blocks. The synthetic transformations must also be selected that provide the best chance of success (e.g., high yield limited reaction steps, solution or solid phase techniques, easy to handle reagents, common solvents, etc.).

(Tomás Hudlicky,Josephine W. Reed, The Way of Synthesis: Evolution of Design and Methods for Natural Products, pg. 3-4)

(By Swartz, Analytical Techniques in Combinatorial Chemistry, pg.177)

My definition is richer for an engineer to understand how and why chemical synthesis used in our lifes.

CHEMICAL VAPOR DEPOSITION

group: material process

Old definition:

Chemical vapor deposition (CVD) is a widely used materials- processing technology. The majority of its applications involve applying solid thin-film coating to surfaces, but it is also used to produce high-purity bulk materials and powders, as well as fabrikating composite materials via infiltration techniques. It has been used to deposit a very wide range of materials. The majority of the elements in the periodic table have been deposited by CVD techniques, some in the form of the pure elements, but more often combined to form compounds.

(Jong-Hee Park,T. S. Sudarshan, Chemical vapor deposition, p. 1)

 New definition:

Thermal CVD (or vapor plating) is the deposition of atoms or molecules by the high temperature reduction or decomposition of a chemical vapor precursor species, which contains the material to be deposited. Reduction is normally accomplished by hydrogen at an elevated temperature. Decomposition is accomplished by thermal activation. The deposited material may react with other gaseous species in the system to give compounds (e.g. oxides, nitrides). Chemical vapor deposition processing is generally accompanied by volatile reaction byproducts and unused precursor species. Chemical vapor deposition has numerous other names and adjectives associated with it such as vapor phase epitaxy (VPE) when CVD is used to deposit single crystal films, metalorganic CVD (MOCVD) when the precursor gas is a metalorganic species, plasma-enhanced CVD (PECVD) when a plasma is used to induce or enhance decomposition and reaction, and low pressure CVD (LPCVD) when the pressure is less than ambient. Plasmas may be used in CVD reactors to "activate" and partially decompose the precursor species. This allows deposition at a temperature lower than thermal CVD and the process is called plasma-enhanced CVD (PECVD) or plasma-assisted CVD (PACVD). The plasmas are typically generated by radio frequency (rf) techniques. Figure 1.2 shows a parallel plate CVD reactor that uses rf power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit silicon nitride (Si3N4) and phosphosilicate glass (PSG), encapsulating layers a few microns thick with deposition rates of 5-100 nm/min. at low pressures, concurrent energetic particle bombardment during deposition can affect the properties of filns deposited by PECVD.

CVD Diamond Cutting Tools

Chemical vapor deposition processes now permit the economical manufacture of large sizes and commercial quantities of synthetic diamond at temperatures less than 1000  ⁰C(1830 °F) at relatively low pressures (<100 kPa, or 1 atm). CVD diamond has as low a coefficient of friction as Teflon, is as hard as natural diamond, and exhibits thermal conductivity four to five times that of copper. Tools coated with diamond can machine a wide variety of nonferrous materials. The coating exhibits high lubricity, generates low cutting forces. wears slowly, and does not heat-distort the workpiece. The properties of single-crystal diamond, CVD diamond, and PCD arc compared in Table 9. 

The CVD process can he defined as the deposition of a solid on a heated surface via a chemical reaction from the vapor or gas phase. It belongs to the class of vapor-transport processes that arc atomistic in nature, that is, the deposition species are atoms or molecules, or a combination thereof.

The CVD of diamond requires the presence of atomic hydrogen, which selectively removes graphite and activates and stabilities the diamond structure. To dissociate hydrogen requites a high-energy source. In addition to the need for atomic hydrogen, other factors, such as energy output and the presence of oxygen, have been shown to be important as well. Although the deposition mechanism associated with CVD processing of diamond is complex, the basic reaction involves the decomposition of a hydrocarbon, such as methane:

CH₄→C(diamond)+2H₂(g)

The reaction can be activated by microwave plasma or direct current (dc) plasma arc.

Applications

CVD and PCD diamond can be used in many of the same applications, but PCD is more suited to roughing and to machining applications and materials that require high fracture toughness of the tool. CVD diamond excels at finishing, semi-finishing, and continuous turning applications because of its superior wear resistance, and its hardness allows it to produce more precisely machined parts. Materials commonly machined by CVD diamond include high-silicon content aluminum casting alloys, aluminum matrix composites, graphite, fiber reinforced plastics and other nonhomogeneous materials, carbon-carbon composites, polyvinyl fluoride, fiberglass, Kevlar, honeycomb materials(such as Nomex), Inconel, and copper alloys.

(ASM International. Handbook Committee, Tool Materials, pg. 98)

(Donald M. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, pg.6)

My definition is so long but it gives more detailed example for why it is used and why it is important for machining processes.