Epoxy-based negative resists

Although the silicon micromachining process is well developed, extensive use of costly clean-room instrumentation is required. Thus, alternative and simpler template construction approaches are being pursued. One such method utilizes SU-8, an epoxy-based negative photoresist, which has excellent chemical resistance and mechanical properties. Patterned SU-8 is being applied increasingly in making microstructures for templates in microchip production... 

In many cases the use of epoxy materials in so-called field conditions (for industrial, construction sites, etc.) demand an increase in the reaction velocity, which is usually achieved by adding accelerators. At present, the widely used accelerators include alkyl-substituted phenols, benzyl alcohol, carboxylic acids (in particular, salicylic acid), and others. A major disadvantage of these accelerators is their tendency to migrate from the cured epoxy matrix during the exploitation, which could lead to a change in the physical properties of the polymer. They also act as a plasticizer of epoxy-based polymers, and as a result reduce the polymer s chemical resistance. Thus, there is a need for new accelerator-modifiers that can provide faster curing of epoxy-amine compositions without negative side effects, and also improve the properties of the finished product. 

A popular resist for MEMS processing is SU-8 [US Patent No. 4882245 (1989)]. This is a negative tone resist and is commonly applied as a layer, tens of micrometres in thickness. SU-8 was developed by Shell Chemicals and uses epoxy-based chemistry. Microchem [www.microchem.com] is one of the companies that now holds the licence for production and sale of SU-8 photoresists. 

Reactive diluents are low viscosity mono- or difunctionai epoxies based on aiiphatic aicohols, diols, alkylated phenols or carboxylic acid that are used to reduce the viscosity of standard epoxy resins and react with curatives during the curing process (Figure 2.42). They tend to reduce chemicai resistance, heat resistance and hardness of the coatings. The difunctionai diluents have fewer negative effects than monofunctional diluents. Higher vapor pressure (volatility) of some of the reactive diluents increases their toxicity, heaith risks and problems with skin irritancy. 

Fig. 24. Representative cross-linking systems employed in negative tone CA resists, (a) Epoxy polymers requiring organic solvent development, (b) PHOST-based cross-linking systems requiring aqueous development, (c) Monomeric cross-linking agents used in PHOST matrix polymers.

C The Epoxy Resists. The first negative tone electron beam resist materials with useful sensitivity were based on utilizing the radiation chemistry of the oxirane or epoxy moiety. The most widely used of these materials, COP (Figure 32) is a copolymer of glycidyl methacrylate and ethyl acrylate and was developed at Bell Laboratories (43,44). COP has found wide applicability in the manufacturing of photomasks. The active element... 

From this result on MRS, we expected that a combination of phenolic-resin-based resist and aqueous alkaline developer would lead to etching-type dissolution and non-swelling resist patterns. In this paper, we report on a new non-swelling negative electron beam resist consisting of an epoxy novolac, azide compound and phenolic resin matrix (EAP) and discuss the radiation chemistry of this resist. 

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