Exposure system, electron beam

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

These are systems in which multilayer stmctures are fonned from molecules containing one or more double bonds and in which polymerization is subsequently initiated by appropriate means such as electron beam or UV light exposure. 

Silicone acrylates (Fig. 5) are again lower molecular weight base polymers that contain multiple functional groups. As in epoxy systems, the ratio of PDMS to functional material governs properties of release, anchorage, transfer, cure speed, etc. Radiation induced radical cure can be initiated with either exposure of photo initiators and sensitizers to UV light [22,46,71 ] or by electron beam irradiation of the sample. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

Electron beam systems can be conveniently considered in two broad categories those using scanned, focused electron beams which expose the wafer in serial fashion, and those projecting an entire pattern simultaneously, onto a wafer. Electron beam projection systems have been investigated extensively since they offer the potential of higher exposure rates as a... 

Figure 38. Schematic of an electron beam exposure system.

As in all processing steps, cleanliness of the exposure hardware is of paramount importance. Any particle that lands on the resist prior to exposure, will shield the film underneath the particle from the exposing radiation and give rise to opaque spots in the case of positive resist, or pinholes in the case of negative resists. Particulate contamination is especially troublesome with electron beam and ion beam systems where the probability of a particle landing on a substrate is increased relative to other techniques because of the much longer exposure times involved. 

Methods 1 and 3 have been utilized in dry developed resist systems. To our knowledge, there are no resist systems commercially available that depend on post-exposure treatment other than the post-curing effect in negative electron beam resists mentioned earlier. Since such systems are still largely in the research phase we will not discuss them here but rather refer the reader to the literature for more detailed descriptions (44-50). 

This problem leads to two undesirable phenomena first, since an electron beam exposure system requires 10 to 60 minutes in order to serially expose a wafer or mask, sections of the surface exposed last will show a degree of crosslinking less than sections exposed earlier. Since the linewidth of an image is proportional to the exposure dose, it is imperative that we allow the reaction to proceed to the same extent across the entire wafer if we are to control the linewidth across a wafer or mask. 

Onium salts have been widely used as an acid generator for photo-, EB, and x-ray resist. In addition, aromatic polymers such as novolak and polyhydroxystyrene have been often used as a base polymer for EB and x-ray resist. The reaction mechanisms in a typical resist system have been investigated by pulse radiolysis [43,52,77-88], SR exposure [79,80,83-85], and product analysis [88]. Figure 6 shows the acid-generation mechanisms induced by ionizing radiation in triphenylsulfonium triflate solution in acetonitrile. The yields of products from electron beam and KrF excimer laser irradiation of 10 mM triphenylsulfonium triflate solution in acetonitrile are shown in Fig. 7 to clarify the... 

The saturated hydrocarbons are very susceptible to electron beam damage, both in the monolayer and multilayer forms. While aromatic hydrocarbons and other conjugated systems exhibit minimal or no beam damage effects during the time necessary to carry out the LEED experiments, the ordered structures of paraffins disappear after 5 sec of electron beam exposure as a result of desorption or partial dissociation of the organic adsorbates. 

For resist exposure, the resolution limit will be set by the range over which the ions interact with the resist. As with electron beam exposure, ions create secondary electrons up to several nanometers away from the beam, and these electrons can travel further before their energy is absorbed. Ultimate resolution will probably be about 10-20 nm, as it is with electrons. At present this limit is beyond the capabilities of the ion optical systems. 

Clearly, the sensitivity of a resist should be commensurate with machine design parameters to allow optimized throughput. For example, an electron beam exposure system writing at a modulation rate of 20 MHz (dwell time of 50 nsec), a beam current of 5 x 10-8 amps at 10 kV, and an address structure (spot size) of 0.25 2 would require a resist with a sensitivity of 10 6 C/cm2 (1 / 2) or better in order to write the maximum number of wafers per hour of which it is capable. The same argument also applies to other exposure tools. 

For further enhancement of electron beam sensitivity, the chlorinated Novolak resin was studied using poly (2-methyl-1-pentene sulfone) as a dissolution inhibitor. The chlorinated Novolak resin mixed well with the polysulfone, and there was no phase separation observed when the films were spin-coated. With 13 wt% of the polysulfone, the chlorinated Novolak resist cast from a cellosolve acetate solution yielded fully developed images with R/Ra = 9.2 after exposure to 2 / 2. It gave fully developed images with R/R0 = 3.2 at a dose of 1 / 2, as shown in Figure 3. There are some problems with this resist system some cracking of the developed resist images... 

Lithographic Evaluation. Films were spin-coated onto silicon substrates from 10% solutions in chlorobenzene and prebaked at temperatures between 90 °C and 100°C for 1 hour to ensure solvent removal. The thickness of each film was about 5000 A. Electron beam exposures were performed on the AT T Bell Laboratories electron beam exposure system (EBES-I) operating at 20 kV with a beam adress and spot size both equal to 0.25 . A minimal cure time was required since there is no post-exposure reaction (4,16). 

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