Photoresist vinyl polymers

Dry-etch selectlvlties for several negative e-beam resists are also listed in Table V. They are more resistant than the positive e-beam resists of the Table except PMCN and the positive photoresists, AZ2400 and PC 129. The positive-behaving vinyl polymer resists tested are generally less resistant than the negative-behaving systems. This generality, however, does not hold for the photoresists tested, as the data of Table VII verifies. 

In general, the photoresists exhibit greater dry-process resistance than the vinyl polymers of Table II. The greater dry-etch resistances of photoresists is attributed to the aromatic nature of the crosslinking agents, photoactive components, and novolac resins (positive photoresists only). In addition, the... 

A.L.R. Williams and D.G. Borden, The preparation and properties of photoreactive polymers I. 2 (arylvinyl) N vinylpyridinium arylsulfonate polymers, Makromol. Chem. 73, 203 (1964) D.G. Borden and J.L.R. Williams, Photopolymer design Photocrosslinkable styrylpyridinium substituted vinyl polymers with absorption maxima from 270 nm to 540 nm, Makromol. Chem. 178, 3035 (1977) K. Ichimura and N. Oohara, J. Polym. Set, Polym. Chem. Ed. 25, 3063 (1987) K. Ichimura and S. Watanabe, Immobilization of enzymes with use of photosensitive polymers having the stilbazolium group, J. Polym. Sci. Polym. Chem., Ed. 18, 891 (1980) K. Ichimura, Prep aration of water soluble photoresist derived from poly(vinyl alcohol), J. Polym. Sci. Polym. Chem., Ed. 20, 1411 (1982) Preparation and characteristics of photocross linkable poly(vinyl alcohol), 20, 1419 (1982). 

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

PE, RIE and IM resistances for an extensive list of commercial photoresists are included as well for comparison with the vinyl systems and amongst themselves. Although the exact com-osition of these systems is not public information, the generic type of base resin or polymer binder is generally known. In addition, the photoactive components are all known to be aromatic azides or azo-compounds. 

If one wishes to prepare a positive photoresist it is important to obtain polymers vdiich undergo efficient chain scission in the solid phase. Recently we reported studies on a series of copolymers of styrene with a variety of ketone functional groups which were introduced by copolymerization with substituted vinyl ketone monomers. The copolymer structures are shown schematically in Table V. Two processes are responsible for the reduction in molecular weight in these polymers when irradiated with either UV light or electron beams. These are shown schematically below. 

It is neccessary that the binder polymers (1) be compatible with diazoniun salts, (2) be uniform in film thickness when the layer is coated on the photoresist film, (3) show a high gas permeability to allow nitrogen photogenerated during the exposure of diazonaphtoquinone resist to diffuse. Many water soluble polymers were examined, and it has been found that poly(N-vinyl pyrrolidone) and its copolymers satisfy the above requirements. 

Conventional Photoresists. PE rate ratio values for several positive photoresists are also included in this study (see Table II), because several of these novolac resin containing formulations also function as positive e-beam and x-ray resists. Generally speaking, these formulations are more dry-process compatible than most of the vinyl systems (see also ref.2). This is due primarily to the aromatic nature of the novolak resins in the photoresists. Thus, the photoresist PE rate ratio data is close in value to those of the aromatic vinyl and negative behaving polymers. 

Other studies on ketone containing polymers include copolymers of unsaturated ketones with styrene and methyl methacrylate and poly(methylisoprenyl ketones) for use as photoresists. A review has appeared on the laser flash photolysis of vinyl ketones. ... 

Poly(vinyl cinnamate) is conveniently made by the Schotten-Baumann reaction using poly(vinyl alcohol) in sodium or potassium hydroxide solution and cinnamoyl chloride in methyl ethyl ketone. The product is, in effect, a copolymer of vinyl alcohol and vinyl cinnamate, as shown. The polymer has the ability to cross-link on exposure to light, which has led to its important applications in photography, lithography, and related fields as a photoresist (see also Chapter 5). 

A silicon wafer that has one surface oxidized to a controlled depth is coated (on the oxide surface) with a photoresist, such as poly(vinyl cinnamate), to produce a thin and uniform coating several micrometers thick when dry. Exposure to UV light through a mask insolubilizes part of the polymer. The uncross-linked polymer is washed off solvents. The bare substrate parts that thus reappear are etched through the oxide layer down to the silicon layer by a fluoride solution in water or by a plasma that contains reactive ions. 

Uses Crosslinking agent for actylic/vinyl resins PVC plastisol comonomer (reduces initial vise, and oil extractability. and improves ultimate hardness, heat distort., hot tear strength, and stain resistance) adhesion promoter hardener used in cast acrylic sheet and rod, contact lens, elastomers, ion exchange resins, dental polymers, adhesives, coatings (paper, plastic), cosmetics, paints, sealants, photopolymers, electronics (photoresists, solder masks)... 

Intermixing at the interface of the CEL layer and the photoresist can give problems with stripping and, therefore, a barrier layer in between is sometimes applied. This layer is a approximately 1000 A thick poly(vinyl alcohol) layer which is spincoated from a solution in water As this polymer exhibits no intermixing with the photoresist, an alternative CEL system was based on this polymer mixed with a water-soluble diazonium salt... 

After having discussed the photochemistry involved in a number of photoresists and their technological applications, we will consider in some more detail the role of polymers in photoresist systems. The most familiar polymers in photoresist are poly(vinyl alcohol), poly(vinyl cinnamate), poly(isoprene) and novolak. These resins are manufactured on an industrial scale according to general procedures, but the exact conditions, applied in the production process are mainly proprietary to the manufacturers. 

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