Dennis R. McKean

Acid generation and acid diffusion in photoresist films

Triarylsulfonium salts have been widely used as acid photogenerators in chemically amplified photoresists. A technique was developed for measuring the amount of acid which is generated on irradiation of polymer films containing sulfonium salts and the results of these measurements were used to obtain an understanding of the importance of acid chemistry on the lithographic performance of acid-catalyzed resists. The results from both electron beam and ultraviolet irradiations suggest that polarity effects are very important factors in determining the efficiency of acid generation. The effect of sulfonium salt loading percentage on acid generating efficiency was also studied. Thermal analysis of polymer films containing sulfonium salts has shown greater plasticization effects for sulfonium salts in polar polymer films indicating that the interaction between sulfonium salts and polymers is better for polar polymers. ESR experiments have determined the nature of the radicals which are produced following irradiation and these results suggest a mechanistic understanding of the polymer structural effects. In the case of acid-sensitive polymers, it has been possible to determine the extent of acid-catalyzed chemistry which, in turn, provides a means of estimating the acid diffusion range in this polymer film.

New Diazoketone Dissolution Inhibitors For Deep U.V. Photolithography

The results of studies directed toward the design of diazoketone dissolution inhibitors for deep U.V. photolithography are described. This work has identified a useful chromophore, the 1,3-diacy1-2-diazo linkage which has the requisite spectral characteristics for use in the deep UV. Incorporation of appropriate hetero-atomes into the structure has allowed synthesis of analogs that survive common processing sequences. A detailed study of the photochemistry of these analogs led to an understanding of the importance of the stability of the ketene photoproduct on resist contrast. Imaging results are presented for a trilayer application and for excimer laser projection printing.

Modified polyhydroxystyrenes as matrix resins for dissolution inhibition type photoresists

It is generally accepted that the production of shrink versions of the 16 MB DRAM and the 64 MB DRAM generations will be patterned using deep UV radiation. This provides a new challenge to the photoresist suppliers, as the standard photoresist formulations are not suitable for this technology, mainly because the presently used novolak resins are highly opaque in the 200 - 300 nm region. This is especially true for the 248 nm wavelength of KrF eximer lasers. Poly 4- hydroxystyrene [PHS] has several advantages in transmission and thermal stability; however, its dissolution rate in commercial grade developers is unacceptably high. We report some recent results on modified, alkyl-substituted PHS derivatives. These polymers combine reduced alkaline solubiity with adequate optical and thermal properties, making them acceptable for future deep UV based production processes. Selected data of these new (co)polymers are discussed.

Novolac Based Deep-UV Resists

This paper describes the results obtained from studies of deep-UV lithographic properties of resists which utilize chemical amplification in novolac resins. These studies have shown that three-component novolac-based systems are surprisingly effective as deep-UV resists despite the high opacity of novolac at 254 nm. Analytical results are described which have measured the amount of Bronsted acid produced in chemical amplification resist films. The minimum acid concentration necessary for acceptable dissolution rates is far less than the corresponding active species content necessary for the proper functioning of diazoketone-novolac deep-UV resists.

Thick film positive photoresist: Development and resolution enhancement technique

The development of a positive, thick film photoresist consisting of a diazonaphthoquinone sensitizer and a novolac resin is described which has the capability of meeting a wide range of thick film requirements necessary for magnetic recording head fabrication. This photoresist system can coat 4–15‐μm‐thick films over various types of topography on a variety of different metal and dielectric surfaces (copper, Permalloy, and alumina). Moreover, this resist is compatible with copper, permalloy, and gold plating baths as well as different metal etchants. The photoresist can be used to produce images with wall profiles greater than 80° with a resolution capability approaching 2 μm lines and spaces in a 7.5‐μm‐thick film. Process latitude is demonstrated over a wide range of exposure doses and focus positions. Enhancement of thick film photoresist resolution has been achieved using a method which involves the treatment of photoresist films with dilute surfactant solutions. A number of factors have been found to...

Highly etch resistant, negative resist for deep-UV and electron beam lithography

I lighly sensitive resists have been developed from a combination of silsesquioxanes, acid photogenerators, and phenolic resins. The resist forms negative images and is developable with aqueous base. Both electron beam and deep-uv optical sensitivity was observed. The films displayed exceptionally high oxygen reactive ion etch resistance. Bilayer systems were developed with this resist as the imaging layer with either polyimide or hard baked novolac as the planarizing layer. Improved optical resists for single layer application have been obtained by using poly(4-hydroxystyrene) derivatives as the matrix resin.

Base-catalyzed photosensitive polyimide

A scheme for imaging of polyimide films is described which is based on the amine-catalyzed imidization of poly(amic alkyl ester) precursor polymers. Films containing amine photogenerators along with poly(amic alkyl esters) are patterned by exposure followed by heating to partially imidize the exposed portion of the film. Negative images are developed by taking advantage of the greater solubility of the precursor polymer which is dissolved in an appropriate solvent mixture. A final cure is carried out to complete the imidization of the patterned film.

Development of bilayer resists for deep‐ultraviolet and i‐line application

Bilayer photoresist schemes have the potential of satisfying the many stringent demands associated with the lithographic exposure step. Because the film thickness for this process is quite small, the aspect ratios are increased relative to single layer lithography and hence increased resolution may be obtained. The planarizing layer used in bilayer schemes can also be dyed to avoid reflectivity problems. In this paper the development of a resist for use as the imaging layer in a bilayer resist scheme incorporating oxygen reactive ion etch for image transfer is described. The resist chemistry is based on the coupling reaction(s) of cyclic polyfunctional silanols. The resist is composed of a cyclic silanol, an acid photogenerator, and a phenolic resin. The catalytic nature of the chemistry involved in the resist resulted in high sensitivity during the exposure process. The observed contrast was greater than four for several of the formulations due in part to the unique ‘‘bimodal’’ characteristics of this resist. With sufficient silanol content, high etch resistance was observed and bilayer resist methodology was carried out without loss of film thickness in the exposed regions. Submicron resolution was demonstrated.

Methylated poly(4‐hydroxystyrene): A new resin for deep‐ultraviolet resist application

Positive photoresists which function on the basis of dissolution inhibition are the mainstay of optical lithographic technology at long wavelength. Described in this paper is a new type of material which permits the dissolution inhibition mechanism to be extended to deep‐UV wavelengths. Methylated poly(4‐hydroxystyrene) resins are prepared by a copolymerization reaction which results in a final product whose dissolution properties can be defined by the ratio of the two monomers used in the reaction. The deep‐UV optical absorbance of the methylated resin is about one‐half the absorbance of novolac at 248 nm which makes these materials attractive for application in deep‐UV resist formulations. Binary photoresists using diazo compounds as dissolution inhibitors have been prepared with the methylated poly(4‐hydroxystyrene) as the host resin. These materials function very similarly to the familiar diazonaphthoquinone/novolac photoresists at long wavelength. For deep‐UV application, both binary and three component resists have been evaluated. The three component system has high sensitivity ( 3). The high contrast is due in part to the induction effect observed from dissolution data. Projection printed images have steep wall profiles indicative of high contrast photoresists.

Rational design of bleachable nonchemically amplified DUV photoactive compounds

Photoactive compounds have been designed, synthesized and characterized for deep ultraviolet non-chemically amplified resist applications. These resist materials may have potential use in next generation 257nm mask fabrication. Mask fabrication requires stringent linewidth specifications over long post-coat and post-exposure bake delays. Lithography simulation and imaging experiments have been done to determine the lithographic performance of resists formulated with these new photoactive compounds. Previously studied chromophores, 7 substituted 3-diazo 4- hydroxycoumarin and N-substituted 3-diazo-2, 4-piperidione, both have the transparency, bleaching and exposure rate kinetics in the DUV that are analogous to those exhibited by the diazonaphthoquinone chromophore at 365nm. The sulfonate linkages attached to these photoactive compounds provide dissolution rate inhibition of novolak that is very similar to the diazonaphthoquinone-sulfonates. The trifunctional diazopiperidione that incorporates three sulfonate linkages provides more efficient inhibition per chromophore than the corresponding bisfunctional photoactive compound. The diazocoumarin based novolak resist demonstrates image reversal (negative tone) with the use of a post-exposure bake. The post-exposure bake causes the exposed photoactive compound to decarboxylate, which dramatically reduces its solubility in aqueous base. The trifunctional diazopiperidione provides the best overall imaging results due to almost complete bleaching and high contrast.