Photoresist dissolution, importance

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

Postexposure bake of the wafer. A postexposure bake (PEB) improves contrast of the photoresist before its development. The PEB process causes three effects 1) diffusion of the PAC 2) solvent evaporation and 3) thermally induced chemical reactions. In general, the dissolution rate of a resist decreases as a function of a PEB temperature. PEB becomes more important for the photoresists with a chemical amplification (CA) feature. The photoresists need the PEB to complete chemical reactions initiated by exposure. 

P5-13n Micmelecimnic devices are formed by first forming SiO- on a silicon wafer by chemical vapor deposition (Figure P5-I3). This procedure is followed by coating the SiO, with a polymer called a photoresist. The pattern of the electronic circuit is then placed on the polymer and the. sample is irradiated with ultraviolet light. If the polymer is a positive photoresist, the sections that were irradiated will dissolve in the appropriate solvent, and those sections not irradiated wtU protect the SiO, from further treatment. The wafer is then exposed to strong acids, such as HF. which eich (i.e.. dissolve the exposed SiO,. It is extremely important to know the kinetics of the reaction so that the proper depth of the channel can be achieved. The dissolution reaction is... 

Most AFM studies of dissolution and corrosion processes have focused on technologically relevant materials such as aluminum alloys [25-27], copper [28], and steel [29-32]. For example, in situ electrochemical-AFM was used to investigate the corrosion activity of Al-6061 -T6 (alloyed with Fe) immersed in 0.6 mol dm NaCl, near iron-rich intermetallic (AI3 Fe) inclusion sites [26]. Inclusion sites are thought to be favorable areas for pit initiation and represent an important area of corrosion research. By marking the sample using a photoresist grid, it was... 

The dissolution behavior of copolymers in aqueous base is of primary importance in photoresist performance. As microlithography continues to decrease in exposure wavelength and the minimum feature size required from photoresist compositions decreases below 0.20 pm, control of macromolecular architecture will play an increasingly significant role. That is, how does the random, alternating or "blocky nature of the matrix polymer effect its solubility in aqueous base. It has been well documented for novolac based photoresists that the molecular weight distribution... 

As the resist film thickness shrinks, the interactions of photoresist polymers and substrates become increasingly important. Dissolution rates of photoresist polymers were found to change as the film thickness decreases. Figure 57.8 shows variation of the dissolution rates of poly(4-hydroxystyrene) and polyfnorbomene-methylene-hexafluoroisopropanol) as a functimi film thickness. The dissolution rates of both polymers increase with decreasing initial film thickness [41]. 

A very important property of novolak is that the resolution of photoresists based on this resin is not limited by swelling during development in aqueous base. Recently, it was argued that some swelling, at least superficial, may occur This enables penetration of ions into the layer which can explain the observation that the rate of dissolution decreases, when the cation of the aqueous base developer increases in size. 

With respect to single layer systems (conventional, ImRe, Promote and dry developable resists), we expect that computer programs that can simulate latent image formation and development processes will become of great importance in the near future. A better understanding of the mechanism which underlies the dissolution or etching of resist layers in a developer or plasma respectively is also crucial in order to achieve the ultimate resolution of single layer photoresists. In this respect, well chosen and perfectly defined polymers are required. 

Additives are used in the developer solution to prevent foaming. The solution is filtered to remove resist particles and either replenished with fresh solution to maintain a consistent dissolved resist content and solution concentration or operated continuously for a certain amount of product and then replaced. Waste-developer solution is treated (aqueous and semi-aqueous) or distilled and reused (solvent). Rinsing is important in stopping the dissolution and, for aqueous photoresists, water with a high-mineral content often improves the resist image and the conductor yield. Tank systems can also be used with photoresists that have a wide-process latitude. Ultrasonic agitation is often used to aid in the dissolution. 

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