Poly resists, sensitivity

Figure 2. Experimental data on copolymer resist sensitivities in relation to constituent monomer mole fraction. The following references and abbreviations are used P(ST-CMS), chloromethylated polystyrene (8) P(ST-CS), chlorinated polystyrene ( ) P(ST-IS), iodi-nated polystyrene (100 P(ET-CE), chlorinated polyethylene ( 6)a P(ET-BE), brominated polyethylene C165a P(ST-GMA), poly (styrene glycidyl methacrylate)...

As a thermally driven process, the photoacid diffusion length and rate in CARs is temperature dependent, as illustrated in Fig. 17.24 for the diffusion of perfluoro-octane sulfonate photoacid in partially protected poly(4-t-butyloxycarbonyloxy-strene)-based resist. Within normal processing conditions, the higher the PEB temperature, the faster is the rate of the photoacid diffusion and the longer is the diffusion length. Although the PEB temperature can be used to modulate photoacid diffusion and consequently resist sensitivity, it nevertheless involves a trade-off... 

The highest X-ray resist sensitivity has been obtained by using an elegant resist technique referred to as photo-locking [47]. The resist consists of a plasma-degradable acrylic polymer (poly(2,3-dichloropropyl acrylate)) and a volatile silicon-containing acrylate (bis-acryloxybutyl tetramethyl disiloxane, Figure 6.22) which can be readily polymerised by the absorbed radiation. 

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). 

Poly(vinyl cinnamate) Resists. Dichromated resists exhibit numerous shortcomings which include lot-to-lot variabiUty of the components, aging of the formulated resists in solution and in coated form, poor process stabiUty (due to a sensitivity to variations in temperature and humidity), and intrinsically low photosensitivity requiring long exposure times for adequate insolubilization. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

In order to be successful as part of a medical device a polymer has to resist both biological rejection by the patient s body and degradation. The human body is an enviromnent which is simultaneously hostile and sensitive, so that materials for application in medicine must be carefully selected. The essential requirement is that these materials are biocompafible with the particular part of the body in which they are placed. The extent to which polymers fulfil this requirement of biocompafibility depends partly on the properties of the polymer and partly on the location in which they are expected to perform. For example the requirements for blood biocompafibility are stringent since blood coagulation may be triggered by a variety of materials. By contrast, the requirements for materials to be used in replacement joints in orthopaedic surgery are less severe and materials as diverse as poly (methyl methacrylate) and stainless steel can be used with minimal adverse reaction from the body. 

Whereas the tensile strength was not a sensitive function of the monomer structure, the tensile modulus (Young s Modulus) was clearly related to the monomer structure. This is expected since the tensile modulus is a measure of the polymer s resistance to deformation and is related to the "stiffness" of a polymeric material. The highest tensile modulus (22,000 kg/cm2,2.2 GPa) was measured for poly(BPA iminocarbonate). Replacement of BPA by Dat-Tyr-Hex reduced the tensile modulus significantly. This observation can possibly be attributed to the presence of the long hexyl ester pendent chain in Dat-Tyr-Hex. Generally, the polyiminocarbonates were somewhat "stiffer" than the corresponding polycarbonates. Thus, the tensile moduli of poly(Dat-Tyr-Hex iminocarbonate) and poly(Dat-Tyr-Hex carbonate) were 16,300 kg/cm2 (1.6 GPa) and 13,900 kg/cm2 (1.3 GPa) respectively. 

Table IV compares the X-ray exposure characteristics (at 8.3 X, Al Kai,2 emission line) of the halogenated resists and of PMMA Elvacite 20U1. It can be seen that poly(2-ehloroethyl methacrylates) and poly(2-bromoethyl methacrylates) exhibit a low sensitivity unlike poly(2-fluoroethyl methacrylates) and poly(2-, 2-,2-trifluoroethyl methacrylates) which are more sensitive than PMMA as shown in Figures 2a, 2b, 2c, 2d where the dose-thickness curves of these resists are plotted. The low sensitivity of the PC1EMA and PBrEMA samples may be explained by some competing crosslinking reactions which could occur during exposure as a result of C-Cl and C-Br homolytic bond scissions as noted by Tada...

Neanthes arenaceodentata is the most sensitive marine organism yet tested. In worms exposed to sublethal concentrations of CC6, feeding was disrupted after 14 days at 79 pg/L (USEPA 1980), reproduction ceased after 440 days (three generations) at 100 pg/L (Oshida et al. 1981), brood size was reduced after 309 to 440 days at 12.5 to 16.0 pg/L (Oshida et al. 1981 Oshida and Word 1982), and abnormalities in larval development increased after 5 months at 25 pg/L (Reish 1977). On the other hand, exposure for 293 days (two generations) in 50,400 pg Cr+3/L caused no adverse effects on survival, maturation time required for spawning, or brood size (Oshida et al. 1981). The poly-chaete Capitella capitata was more resistant than Neanthes, a decrease in brood size was noted only after exposure for 5 months to 50 and 100 pg Cr+6/L (USEPA 1980). 

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