Resist materials contrast

The pursuit of further miniaturization of electronic circuits has made submicrometer resolution Hthography a cmcial element in future computer engineering. LB films have long been considered potential candidates for resist appHcations, because conventional spin-coated photoresist materials have large pinhole densities and variations of thickness. In contrast, LB films are two-dimensional, layered, crystalline soHds that provide high control of film thickness and are impermeable to plasma down to a thickness of 40 nm (46). The electron beam polymerization of CO-tricosenoic acid monolayers has been mentioned. Another monomeric amphiphile used in an attempt to develop electron-beam-resist materials is a-octadecylacryUc acid (8). 

To date, we have exercised these materials in basically three types of multilayer lithographic applications (1) as short wavelength contrast enhancing layers, (2) as imagable 02-RIE resistant materials in bilayer processes and (3) as radiation sensitive materials for multilayer, e-beam processes. 

Most conducting materials, even good conductors such as copper, give off wasted heat as electric current passes though them. In other words, they have resistance. By contrast, superconductors are perfect electrical conductors. 

Wilkins and coworkers have redesigned both the sensitizer and the matrix resin (78-79). They have tested a variety of o-nitrobenzyl esters of cholic acid as sensitizers. These substances, like the diazoquinones, are insoluble in aqueous base but undergo a photo-reaction that yields base soluble products. The matrix resin chosen for the new sensitizer materials is a copolymer of methyl methacrylate and methacrylic acid that is far more transparent than novolac resins in the DUV. The new resist materials are reported to have useful sensitivity (ca. 00mJ/cm ) and extremely high contrast. The resist formulation is essentially aliphatic in nature and would be expected to be less stable to dry etching environments than the aromatic-based novolac resin materials (24). 

Although PS is largely commercially produced using free radical polymerization, it can be produced by all four major techniques—anionic, cationic, free radical, and coordination-type systems. All of the tactic forms can be formed employing these systems. The most important of the tactic forms is syndiotactic polystyrene (sPS). Metallocene-produced sPS is a semicrystalline material with a of 270°C. It was initially produced by Dow in 1997 under the trade name Questra. It has good chemical and solvent resistance in contrast to regular PS that has generally poor chemical and solvent resistance because of the presence of voids that are exploited by the solvents and chemicals. 

Mass production poses strict requirements on resist materials, most important of which are sensitivity, spatial resolution, contrast, and etch resistance. The sensitivity of the next-generation resists is required to be less than 10 pC/cm (EB), 100 mJ/cm (x-ray), and 25 mJ/cm (EUV) [89]. It should be noted that the resist sensitivity is traditionally expressed not by absorbed dose but exposure charge or energy per unit area. As for the spatial resolution, 45 nm is needed for the production of dynamic random access memory (DRAM) in 2010 [89]. Although resist patterns below 10 nm are presently fabricated by some kinds of resists, they do not have enough sensitivity required for the mass production [90,91]. 

These results confirm the potential of poly (p-formyloxystyrene) as a useful resist material combining such interesting properties as ability to be imaged in both positive or negative tone, ease of preparation, and activity in the deep UV, with a moderate sensitivity and good contrast. 

Although DNQ-novolac systems were first used more that 40 years ago in the printing-plate industry, they continue to be the focus of significant interest. For example, several recent studies have addressed the optimization of novolac properties through manipulation of molecular weight, isomeric structure of the phenolic starting materials, and methylene bond position (26-28). These changes are reported to influence resist sensitivity, contrast, and process latitude. 

In addition to good sensitivity, issues for X-ray resist materials are analogous to those of optical and e-beam resists resolution, contrast, etch resistance, thermal stability, and adhesion. To stay competitive with e-beam and even optical lithography, X-ray lithography must have a resolution performance better than 0.5 p,m. An extensive list of X-ray resist properties has been collected in the literature (83, 116, 121). 

After PVC, polyolefin copolymers, predominantly polyethylene copolymers, are the next most widely used material for FR applications in wire and cable. Polyethylenes have very good dielectric strength, volume resistivity, mechanical strength, low temperature flexibility, and water resistance. In contrast to PVC, polyolefins are not inherently FR and thus are more highly formulated, requiring the addition of FRs to meet market requirements for flame retardancy. For this reason, and because of the steady global trend toward halogen-free materials for wire and cable applications, more space will be devoted to this section on FR polyolefins compared with the above discussion of PVC. 

High density charring ablators such as carbon-phenolic contain high density reinforcements to improve shear resistance. In contrast, lower density charring ablators as a rule are used for low shear environments. Hie Apollo mission reentry conditions are typical of a relatively low shear environment, so low density ablators consisting of epoxy—novolac resin containing phenolic microballoons and silica fiber reinforcement have been used. In order to improve the shear resistance and safety factor of the material for this mission, the ablator was injected into the cavities of a fiberglass -reinforced phenolic honeycomb that was bonded to the substructure of the craft (48). 

Because of their desirable properties and radiation sensitivity, polysilanes have been used in a variety of microlithographic applications as (1) mid-UV contrast-enhancing materials, (2) imaging layers in a variety of bilayer lithographic processes, and (3) new resist materials for ionizing radiation. 

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