Casting photoresist

Metal and polysilicon films are formed by a chemical-vapor deposition process using organometallic gases that react at the surface of the IC structure. Various metal silicide films may also be deposited in this manner by reaction with the surface of the silicon wafer to form metal silicides. Glass and pol3uner films are deposited or spin cast or both, as are photoresist films (those of a photosensitive material). This process is accomplished by applying a liquid polymer onto a rapidly rotating wafer. The exact method used varies from manufacturer to manufacturer and usually remains proprietary. 

The development of new classes of cationic photoinitiators has played a critical role in the production of highly sensitive, acid-catalyzed deep-uv photoresists. Sulfonium salts have been widely used in this respect (4). These materials are relatively easy to prepare and structural modifications can be used to produce desired wavelength sensitivity. Triphenylsulfonium salts are particularly well suited for deep-uv application and in addition can be photosensitized for longer wavelength. These salts are quite stable thermally and certain ones such as the hexafluoroantimonate salt are soluble in casting solvents and thus easily incorporated within resist materials. 

FIGURE 2.14 Electron micrograph of a turn in the channel fabricated in a PDMS chip, created by casting the polymer against a positive relief, which is made of photoresist patterned on a glass substrate. The roughness in the side wall arises from the limited resolution of the transparency used as a photomask in photolithography [1033]. Reprinted with permission from the American Chemical Society. 

PDMS chips have also been cast from a glass (not Si) master containing photoresist as the positive relief stmcture [556,960]. PDMS was also cast against... 

A two-mask process has been used to create a glass master containing two levels of positive photoresist relief structures. The master was used to cast a PDMS chip consisting of the channel/chamber (25-30 pm deep) and weir (7-12 pm clearance) [960]. Another two-mask process was used to create a Si molding master consisting of 3-pm-high Si relief stmctures and 25-pm-high photoresist relief structures [364]. 

Whitesides and coworkers describe the use of an elastomeric membrane to pattern proteins and cells on bacteriological polystyrene (PS), glass, and poly(dimethyl-siloxane) (PDMS) substrates [92], A patterned PDMS membrane was casted from lithographically structured photoresists and brought into close contact with the substrates (Fig. 6). When incubated with a solution of fibronectin (FN), adsorption of the cell-adhesion-mediating protein to the surface was restricted to the exposed areas. The membrane was peeled off and cells were seeded from a serum-free medium. Passivation to cell attachment of the untreated portions of the surface was achieved by adding 1% bovine serum albumin (BSA) to the cell-seeding medium, which... 

Epoxy resins have great and wide commercial applications and are used in many industries. About 45% of production is used in protective coatings and the remainder in various applications such as composites, laminates, casting, molding, tooling, construction, adhesives, photoresists, and others. 

An effective contrast-enhancing layer must possess certain essential characteristics It must be thermally stable, soluble in solvents that permit casting on photoresist layers with no mixing at the interface, strongly absorbing in the desired spectral region, and bleachable upon exposure. In addition, removal of this layer from the exposed photoresist prior to image development is desirable. 

PDMS is a popular and frequently used elastomer not only because of its cost-effectiveness, but also because of the ease of use and its specific molding characteristics. For instance, PDMS can form conformal contact with a substrate and it can be released again easily from complex or fragile substrates. In addition, due to its low interfacial free energy (ca. 21.6 dyn cm ) and its low reactivity, it does not adhere irreversibly to other substrates such as polymers, silicon, etc. [14,15]. It does, however, bind to photoresist residues on the silicon template. Thus, careful photoresist removal is advised before casting the elastomer on its template. 

PDMS prepolymer is cast on the photoresist master and heat cured. 

Photoresist Processing for Fabricating the Master Wafer and Casting PDMS against the Patterned... 

Figure 4.4 Photomask images for the photoresist patterns of the PDMS emitter device produced by the resin casting method. (Reprinted with permission from ref. 22).

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