Types of developers
There are two main groups of developers: solvent-based and aqueous-alkaline developers. The solvent-based ones are intended mainly for PMMA e-beam resists (→ see process procedure e-beam resists), for CSAR 62 (→ CSAR 62 developers) and also for special polymer based products (e.g. SX AR-N 4800/16 (PMMA)) and accordingly SX AR-PC 5000/40 (KOH and HF protective coating). In this process, solvents such as methylisobutylketone (MIBK), IPA (partly with water) for PMMA, amyl acetate, MIBK, DEK, DEM, xylol for CSAR 62 and ethylbenzene as well as hexane are used for the protective coating.
Aqueous-alkaline developers are divided into metal ion free (MIF) and anorganic metal salts. MIF developers contain tetramethylammonium hydroxide (TMAH) as active component. The commercial 25 % solution is diluted to 2,38 % for the production of a 0,26 n solution (n = normality) verdünnt. This is the standard concentration used for developing most resists worldwide. For some resists, however, particularly Allresist resists, developers with lower concentration are necessary. In this case, the normality of less than 0,26 n is adjusted down to 0,1 n. The 0,1 n developers are the thinnest recommended solutions, because due to the low TMAH concentration, small amounts of CO2 from the air are sufficient to change the normality in a few hours. For a better wetting, MIF developers contain tensides which optimize the surface tension.
The developers based on metal salts consist of metasilicates, phosphates, borates, caustic soda (NaOH) and caustic potash (KOH). The monovalent salts NaOH and KOH are very strong developers, they develop speedily and are used for high coating thickness. The disadvantages are a possible attack on the unexposed areas (dark erosion) and a low contrast.
Due to their polyvalency, the polyvalent silicates, phosphates and borates act as buffer systems. Thus, a gentler development is possible, there is virtually no dark erosion at the recommended concentrations. With regard to service life, they are more efficient than die MIF developers and can be diluted more easily and accordingly more precisely. Unlike TMAH developers, their development strength barely reacts to slight deviations of the concentration. Nonetheless, the use of a scale is recommended for all diluted developers if possible. Due to the higher accuracy when subject to a scale or beaker (volume measurements are, in addition, dependent on temperature) an optimum development result is ensured.