Imaging Exposure

Figure 17. Schematic cross sections of a photopolymer material in which a grating pattern is being written, (a) The image exposure forms polymer and depletes the monomer concentration, (b) Additional monomer diffuses into the exposed areas, (c) An overall development exposure completes the polymerization to give a polymer with a modulated density.

An overall exposure of relatively low intensity light, before the imaging exposure, can be used to bring the sample to this state. 

The results obtained with these mixtures were rationalized on the following basis. Polymerization of the most reactive species is initiated first, raising the density in the irradiated areas. At the same time some of the less reactive species is expelled from the polymer, presumably a consequence of insolubility (or incompatibility). The diffusion rates decrease during exposure as the polymer concentration increases. After the imaging exposure a second uniform exposure can be used to polymerize all remaining monomer. The result is a sample whose chemical composition is spatially modulated in correspondence to the pattern of the optical exposure. 

The absorption of an image exposure by the photoreceptor creates electron-hole pairs. Under the influence of a field, a fraction of the pairs separate and are displaced to the free surface and the substrate electrode. The surface charge is thus dissipated in the exposed regions and an electrostatic charge pattern is created. 

Latent image fonnation involves the creation of free electron-hole pairs by the absorption of an image exposure and the displacement of these carriers by a field. Experimental techniques for characterizing these phenomena are of considerable importance to xerography. This section reviews experimental methods for measuring parameters that describe charge generation and transport phenomena. 

The above requirements have been discussed in detail by Pai and Yanus (1983). The key requirement is that the charge created in the image exposure must transit the thickness in the absence of trapping in a time that is short compared to the time between the exposure and development steps. For most applications, mobilities in excess of a few multiples of 10-6 cm2/Vs are required. Traps may be present as a result of insufficient purification, chemical instability of the oxidized or reduced transport material, or instability such as induced by light or the exposure to chemicals associated with corona charging. 

Labelled probe hybridized to target sequences on the membrane may be detected by autoradiography, p radiation emitted from 32p labelled nucleotides incorporated in the probe will expose X-ray film. Intensifying screens are used for initial exposures to amplify the image, exposures without amplification offer improved resolution but require much longer exposures even when high specific activity probes are used. 

In the wet conditions of the developer solution however the trap sites on the surface became more effective. This meant that when exposed during development, the unexposed crystals, instead of producing internal silver, produced surface silver at sites previously uncompetitive with the internal sites. On the other hand, crystals which already had internal silver produced from the image exposure, continued to trap photoelectrons internally because the accumulation of silver at the trap sites made them more effective traps than those on the surface despite the change in exposure conditions. They therefore did not develop. A direct-positive system was thus discovered [36]. 

Figure 14 shows the process flow of the Resolution Enhanced Lithography (REL) process (10). In this process, after the image exposure the resist is baked at around 100 0 and exposed to deep-UV light to make the resist crosslink. This temperature is too low to decompose NQD rapidly, so the reaction needs to be induced by light exposure to take place. And at this temperature, there is less water in the resist and the novolak resin is in the rubbery state, which means the resin has higher reactivity than in the glassy state at room temperature. Therefore, upon deep-UV exposure, NQD may tend to react with the novolak resin to crosslink. 

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