Resist materials photoresists

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). 

This technique is used to transfer a computer-generated pattern onto a substrate. Here, a film of photoresist is spin-coated onto the substrate and exposed to UV light through a photolithographic mask the light exposure transfers the desired pattern to the photoresist. Depending on whether the resist material is positive or negative , the photoresist... 

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. 

The incorporation of PDMSX into conventional novolac resins has produced potential bilevel resist materials. Adequate silicon contents necessary for O2 RIE resistance can be achieved without sacrificing aqueous TMAH solubility. Positive resist formulations using an o-cresol novolac-PDMSX (510 g/mole) copolymer with a diazonaphthoquinone dissolution inhibitor have demonstrated a resolution of coded 0.5 pm L/S patterns at a dose of 156 mJ/cm2 upon deep-UV irradiation. A 1 18 O2 etching selectivity versus hard-baked photoresist allows dry pattern transfer into the bilevel structure. 

Preliminary Examination of PATE Imaging Characteristics. The data herein have shown that PATE resins are easily photolyzed in the deep UV region to form crosslinked films which are of sufficient integrity for photoresists. However, in addition to these film performance properties, a potential resist material must meet other equally important criteria. For example, the masked (unphotolyzed) portion of the resist film must be removed prior to etching, without damage to the cured film. Also, the cured films must withstand an etchant bath. Therefore, since PATE resins seem to meet the necessary requirements of solubility and solvent resistance, investigation of performance under crude simulated processing conditions was undertaken. 

The most familiar negative photoresists are examples of two-component, resist materials. These include Kodak s KTFR, Merck s Selectilux N, Hunt s HNR, etc., all of which consist of a cyclized synthetic rubber matrix resin which is radiation insensitive but forms excellent films. This resin is combined with a bis-arylazide sensitizer. 

The positive resist materials evolved from discoveries made by the Kalle Corporation in Germany who developed the first positive-acting photoresist based on the use of a novolac matrix resin and a diazoquinone photoactive compound or sensitizer. The original materials were designed to produce photoplates used in the printing industry. These same materials have been adopted by semi-conductor fabrication engineers and continue to function effectively in that more demanding application. 

The latest addition to this list of dry developing resist materials is a contribution from IBM s San Jose Research Laboratory (66-67) that evolved from efforts to design positive-tone resist materials that incorporate chemical amplification. These efforts were stimulated by the fact that the quantum yield of typical diazoquinones of the sort used in the formulation of positive photoresists is 0.2 to 0.3 thus, three or four photons are required to transform a single molecule of sensitizer. This places a fundamental limit on the photo-sensitivity of such systems. 

Note 2 A resist material that is optimized for use with ultraviolet or visible light, an electron beam, an ion beam, or X-rays is called a photoresist (see [2], p. 307), electron-beam resist, ion-beam resist, or X-ray resist, respectively. 

Trilevel Schemes. Trilevel processing (6, 7) requires planarization of device topography with a thick layer of an organic polymer, such as polyimide or a positive photoresist that has been baked at elevated temperatures ( hard baked ) or otherwise treated to render it insoluble in most organic solvents. An intermediate RIE barrier, such as a silicon dioxide, is deposited, and finally, this structure is coated with the desired resist material. A pattern is delineated in the top resist layer and subsequently transferred to the substrate by dry-etching techniques (Figure 3). 

Photosensitizers can also be used to great advantage in multilevel photoresists. A typical system incorporates a long wavelength uv or visible light photosensitized resist on the top level and a deep uv sensitive resist on the bottom. Both resists may be based on cationic photoinitiators or one may be of this type while the other may be a conventional resist material. 

The incorporation of cyclic olefins into the photoresist polymer backbone, it must be noted, represents a radical departure from the conventional 193-nm photoresist design concept, as typified by acrylate polymers or acrylate polymers with pendant cyclic olefins, or cyclic olefin/acrylate hybrid polymers. It is the unique architectures of the polymer backbones of these resist materials that impart properties (such as high etch resistance, low UV absorption at 193 nm, etc.) that make them very good candidates for 193-nm resist applications. 

The earhest photoresists used in integrated circuit manufacture consisted of polymers that were rendered insoluble by photo-cross-linking and thus operated in the negative tone mode. For instance, partially cycUzed poly(cis-isoprene) containing a bisazide as additive served for a long time as the workhorse resist material in photohthography applications [15]. This system has already been described in Section 7.2.3. Subsequently, Novolak-based positively functioning sys-... 

There is a variety of photoresist materials on the market. Some of these photosensitive materials are negative types and some are positive. Both wet and dry resist materials may be used. It is beyond the scope of this section to discuss the advantages and disadvantages of each. The reader is directed to the references for additional information on the subject [20-22]. 

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