Abstract: Aqueous mixed permonosulphuric acid/sulphuric acid solutions, such as semiconductor surface photoresist stripping solutions may be stabilised for storage by the addition of one or more of the metals gallium, germanium, indium, tin in the 4-valent form, antimony, thallium, bismuth and lead suitably in the oxide or hydrous oxide form. Mixtures of metals, for example of bismuth, tin in the 4-valent form, gallium and germanium may be particularly effective.

Abstract: Perhalogenated compounds, such as perfluorinated surfactants, for inclusion in aqueous systems, are provided in a cosolvent system consisting essentially of one or more acidic phosphorous-containing compounds selected from phosphoric or phosphorous acids, water-soluble di- or polyphosphonic or phosphinic acids and water-soluble salts of any of the aforesaid, for example 1-hydroxyethylidene 1,1-diphosphonic acid, and sufficient water, if required, to form a solution. Such solutions may be included with strongly acidic and/or peroxygen compositions, for example sulphuric acid or Caro's acid and avoid the need for contamination with organic co-solvents.

Abstract: A photoresist stripping solution is prepared by adding hydrogen peroxide to concentrated sulphuric acid under controlled temperature conditions, preferably at below 20.degree. C. The concentration of the sulphuric acid in the solution is preferably maintained at at least 80% wt to maximize the concentration of the active stripping species permonosulphuric acid. Stripping solutions having extended storage life have a content of soluble tim compound preferably sodium stannate and preferalby also a phosphonate sequestrant which shows a synergism with the tin compound. Such extended life solutions also preferably have a low content of transition metals and of particles. The stripping solution may be utilized at ambient temperature.

Abstract: A storable semiconductor cleaning solution consists essentially of sulphuric acid at a concentration of from 30% to 70% and hydrogen peroxide at a concentration of from 1% to 20% preferably also containing a pentavalent phosphorus sequestrant and tin ions. The solution is produced by diluting concentrated sulphuric acid, aging the diluted acid until it reaches a temperature below 30.degree. C. and introducing the hydrogen peroxide, preferably having a concentration of at least 70% by weight, into the aged acid and equilibrating the resulting solution. The cleaning solutions can be stored for an extended period at ambient temperature and can be used to clean semiconductors, and with suitable precautions aluminized semiconductors, at temperatures below 100.degree. C.