Positive photoresist, reactions

The photoactive compounds, or sensitizers, that are used in the formulation of positive photoresists, are substituted diazonaphthoquinones shown in Figure 17. The substituent, shown as R in Figure 17, is generally an aryl sulfonate. The nature of the substituent influences the solubility characteristics of the sensitizer molecule and also influences the absorption characteristics of the chromophor (79). The diazonaphthoquinone sulfonates are soluble in common organic solvents but are insoluble in aqueous base. Upon exposure to light, these substances undergo a series of reactions that culminate in the formation of an indene carboxylic acid as depicted in Figure 17. The photoproduct, unlike its precursor, is extremely soluble in aqueous base by virtue of the carboxylic acid functionality. 

At the present time, most of the positive photoresists used in the manufacture of microcircuits consist of a low molecular weight phenolic resin and a photoactive dissolution inhibitor. This composite system is not readily soluble in aqueous base but becomes so upon irradiation with ultraviolet light. When this resist is exposed, the dissolution inhibitor, a diazoketone, undergoes a Wolff rearrangement followed by reaction with ambient water to produce a substituted indene carboxylic acid. This photoinduced transformation of the photoactive compound from a hydrophobic molecule to a hydrophillic carboxylic acid allows the resin to be rapidly dissolved by the developer. (L2,3)... 

Positive Photoresists. Positive resists are entirely different from negative resists. For the purposes of this discussion we restrict ourselves to visible-light-sensitive materials. Typically, these materials are mixtures of low-molecular-weight phenol-formaldehyde polymers and derivatives of naphtho-1,2-quinone diazide, the photosensitive component. The former is soluble in aqueous alkali, but the presence of the latter, a hydrophobic species, inhibits attack of this developer on the film. On irradiation the "sensitizer" is converted to a ketene, which, after reaction with water, forms a base-soluble carboxylic acid. Thus the irradiated part of the film is rendered soluble in the developer and it can be removed selectively. The important feature of this system is that the unirradiated areas are not swollen by the developer and the resolution of this material is quite high. It is possible to prepare gratings having several... 

The difference of the amount of scum among the samples is clearly explained assuming the scum to be silicon oxide produced by the reaction of oxygen and silicon atom. The difference of the amount of scum in Figures 6a and 6c comes from the different concentration of silicon contained in X-8000K2 and conventional positive photoresist. The amount of scum observed in Figure 5 is the... 

Conventional positive photoresists consist of a matrix resin and a photoactive compound. The matrix resin is a cresol-formaldehyde novolac resin (structure 3.1) that is soluble in aqueous base solution, and the photoactive compound is a substituted diazonaphthoquinone (structure 3.2) that functions as a dissolution inhibitor for the matrix resin. As outlined in Scheme 3.1 (20), the photoactive compound undergoes a structural transformation upon UV radiation, known as WolflFrearrangement, foUowed by reaction with water... 

Sometimes, the last reaction is used to obtain a negative-tone resist mask by means of the DNQ-novo-lac positive photoresist.Imidazole, triethanol amine, and other bases can be used as tone-modifiers. 

P5-13n Micmelecimnic devices are formed by first forming SiO- on a silicon wafer by chemical vapor deposition (Figure P5-I3). This procedure is followed by coating the SiO, with a polymer called a photoresist. The pattern of the electronic circuit is then placed on the polymer and the. sample is irradiated with ultraviolet light. If the polymer is a positive photoresist, the sections that were irradiated will dissolve in the appropriate solvent, and those sections not irradiated wtU protect the SiO, from further treatment. The wafer is then exposed to strong acids, such as HF. which eich (i.e.. dissolve the exposed SiO,. It is extremely important to know the kinetics of the reaction so that the proper depth of the channel can be achieved. The dissolution reaction is... 

The classical example of radiation induced polarity change is the photodecomposition of diazoqui none into an ionizable compound, the indene carboxylic acid. This reaction is the basis of one of the important positive photoresists discussed in Chapter 7. Attaching diazoquinone units to a polymer backbone, for example, via acrylic side groups, makes the unexposed polymer soluble in organic sol vents and the exposed polymer soluble in dilute aqueous basic solutions. 

The main components of most commercially available positive photoresists are novolak as a binder and naphthoquinone-diazdde as a light-sensitive component. This light-sensitive compound is not base soluble and acts as a dissolution inhibitor for the novolak, which results in a very low dissolution rate of unexposed resist in aqueous base developer. Upon exposure a reaction is induced to yield indene carboxylic acid via a ketene ... 

Although thermal and base catalyzed reactions of NQD are becoming important, there is limited information on these reactions which take place in the positive photoresist. The authors carried out model experiments using 1,2-naphthoquinone diazide-5-sulfonyloxybenzene (DAM) and p-cresol. DAM was selected as a model for the PAC and p-cresol was a model for the novolak resin. These two compounds were mixed together and reacted either by heating or in an alkaline developer which is 2.38wt% tetramethylammonium hydroxide (TMAH) aqueous solution. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

The sulfonic acid moiety has been iacorporated iato a variety of nonfluofinated polymeric materials (111). Chain-end sulfonated polymers are produced by the reaction of sultones with polymeric organolithiums (112). Polymeric sulfonic acids such as these are iacorporated ia positive-working photoresist compositions (113). 

The object of this study is to develop new photoresists for deep-UV lithography, by using the reversible photoreaction of pyrimidine bases (17-19). Applicability of pyrimidine containing polymers to both negative and positive type photoresists is due to this photoreversible reaction in which cyclobutane dimers are either formed or cleaved depending on the exposure wavelength (Scheme 2). 

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