Incident dose

A quantum yield was determined for acid generation based on the total absorbance of the film and the reflectivity from single crystal silicon at 254 nm. Total film absorbance at 254 nm is 0.185 which corresponds to 35% absorption by the film. Reflectivity at 254 nm is 0.66 (17) and so an additional 15% of the incident light is absorbed after reflection. Total absorption was taken to be 50% of incident dose. It should be noted that the resin absorption without sulfonium salt is 0.155 and thus constitutes about 84% of the total film absorbance with sulfonium salt. 

Incident Dose (mJ/cm2)a Absorbed Dose (mJ/cm2)b Acid Production (mmol)c Percent Conversion (%) Quantum Yieldd... 

The resist films were irradiated with the same incident doses used for the determination of acid content. A postbake was performed at 100°C for one minute. The carbonyl absorption was measured before and after irradiation and postbake. The difference was used to determine the number of t-BOC groups removed for a given weight of resist film. By dividing the number of t-BOC groups by the amount of photogenerated acid, the catalytic chain length was determined (Table II). 

For the incident doses studied, the catalytic chain length varies from about 800 to 1100. The lowest numbers were observed for the lowest and highest doses. At 2.0 mJ/cm2 incident dose the catalytic chain begins to drop off as t-BOC group removal starts to approach completion. 

Contrast curves were obtained for each resist by measuring the thickness after development of a series of 1 mm by 5 mm exposed areas the exposure dose typically varied from approximately 1 mJ/ cm2 to several J/cm2 for the slowest resists. The majority of the resists were developed in ethyl acetate for 30 to 60 sec followed by a 20-sec rinse in 2-propanol. Initially, THF or a THF/2-propanol mixture was used as the developer they were replaced by ethyl acetate because it provided superior contrast. Resist sensitivity was taken to be the incident dose which resulted in 50% exposed thickness remaining after development, Dg 5. This is the standard convention for a negative resist. 

The value in units of incident dose per unit area for either a positive or negative resist system is of little value unless accompanied by a detailed description of the conditions under which it was measured. This description should include, at the minimum, the initial film thickness, the characteristics of the substrate, the temperature and time of the post- and pre-bake, the characteristics of the exposing radiation, and the developer composition, time and temperature. The structure, copolymer ratio, sequence distribution, molecular weight, and dispersity of polymers included in the formulation should also be provided. 

Fig. 26. The absorbance of poly T at 265 nm as a function of incident doses of radiation. The curve beginning in the upper left represents initial irradiation at 280 nm, followed by irradiation at 240 nm. The curve beginning in the upper right represents the effect of irradiation only with light of 240 nm wavelength (Deering and Setlow6e).

The soluble fraction for a given incident dose can be determined from Figure 2 provided that (l) all the soluble fraction was removed from the film during the development step, and (2) there was no volume contraction due to gel formation upon irradiation. If soluble material remained in the film, the measured soluble fraction will be systematically lower than the actual value. However, a volume contraction would result in a systematically higher than actual value for the soluble fraction. 

Calculation of G -values from the results shown in Figures 2 and 3 requires determining the amount of dose absorbed in the film for a given incident dose. Bowden (7) has shown, using the depth-dose model of Heidenreich, (9) an accurate measure of the energy absorbed in a polymer film can be determined. An example of such a calculation for polystyrene is given in the appendix. 

Convert the incident dose (coulombs/cm2) to absorbed dose (Mrads) as follows The sheet density (ps) of a polystyrene film = p x Z = 5.25 x -5 g/cm2, the energy E absorbed in a volume element comprised of one cm2 and Z cm thick is then given by... 

Expressing the incident dose in coulombs/cm2/Mrad, the conversion factor C becomes... 

Absorbed Dose (Mrad) Incident Dose (Coulombs/cm2) x 7 ... 

Figure 2. Contrast curve for the copolymer of Figure 1. Films were cast from methyl cellosolve acetate, prebaked at 120°C (in vacuum), and developed using a mixture of ethyl and methyl cellosolve acetates. Incident doses of 20 keV electrons were imposed on a film about one micrometer thick.

Classification of chemically induced hepatotoxicity is primarily based upon pattern of incidence and histopathological morphology. Intrinsic hepatotoxic drugs demonstrate a broad incidence, dose-response relationship and will usually give similar results in humans and experimental animals. The incidence of liver damage from idiosyncratic hepatotoxicants is limited to susceptible individuals and results from hypersensitivity reactions or unusual metabolic conversions that can occur due to polymorphisms in drug metabolism genes (see Chapters 11 and 13). 

No significant increase in tumour incidence Dose-re laied increase in hepatocyiomegaly or eosinophilic foci Dose-reLaled increase in lymphomas in female rats, but incidence in controls was also high... 

In order to have a uniform exposure to electrons over the entire wafer, a standard transmission microscope (RCA EMV-3) was used. The apertures were opened and the 40 kv beam spread out over a circle larger than the wafer itself. The coated wafer was introduced into the microscope via the film cassette drawer (below the fluorescent screen). Charge density was measured with a Faraday Cup. Each polymer was irradiated at doses of 2, 5, 10, and 20 yC/cm. The radiation chemical yield was measured from plots of 1/Mjj versus the incident dose. A sample plot is shown in Figure 1, in this case for itaconic acid-MMA copolymers. The slope of such a plot is given by ... 

Sensitivity Measurements Resist films were spun onto silicon wafers. The films were then prebaked and exposed in a modified Hitachi electron microscope to an undeflected, nearly collimated electron beam at an acceleration voltage of 15kV. After exposure the resist films were developed in an aqueous solution of tetramethylammonium hydroxide. Sensitivity curves were obtained by plotting film thickness against incident dose. 

Thickness required to reduce the incident dose or dose rate ... 

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