This invention relates to forming a semiconductor tin oxide thin film element on a selected surface region by thermally decomposing a tin carboxylate compound. More particularly, this invention relates to such method that employs a photoresist mask to define the element and further comprises a pre-mask partial decomposition treatment of the tin compound to allow removal of unwanted tin compound in the same step and with the same solvent as nonmasking photoresist material, without disturbing the masked tin compound, to permit film definition without a separate etching operation.
U.S. patent application Ser. No. 808,546 to Chang et al describes a gas sensor comprising a thin film gas-sensing element composed of a semiconductor tin oxide. The element covers a region of the sensor that is less than the entire surface. The tin oxide film is characterized by a nonstoichiometric, oxygen-deficient composition having a measurable electrical resistivity, the value of which is dependent upon the gas composition. One method for forming a semiconductor tin oxide thin film is by sputter deposition using a tin oxide target. A more convenient method is by metallo-organic deposition (MOD), wherein an ink comprising an organic tin compound, such as tin(II) 2-ethylhexanoate, is applied and fired to form a tin oxide film having the desired composition and gas-sensitive resistivity. Heretofore, the tin oxide is applied to a large surface and etched using a photoresist mask to define the element. A typical mask is formed by applying a photoresist layer and selectively exposing the photoresist layer to light in regions other than the region of the element. The photoresist material is insoluble in aqueous alkaline solution as applied, but becomes soluble upon exposure to light. Thus, following irradiation, nonmask photoresist is washed from the surface using alkaline solvent, leaving the mask. Tin oxide only dissolves slowly in aqueous alkaline solution and thus is removed from unwanted regions in a separate etching operation. After etching, the mask is removed to expose the element.
It is helpful to understanding this invention to recognize that, in a conventional MOD process, the ink is fired prior to applying the mask. The organic tin ink is readily soluble in aqueous alkaline solution. When a photoresist mask is applied to unfired ink, the alkaline solution that removes the nonmask photoresist also washes the ink from the entire surface, including from under the mask.
It has now been found that a partial decomposition treatment of the organic tin compound prior to masking reduces the rate at which the tin material is dissolved by alkaline solution so as to suitably inhibit washing away of masked material, while still allowing removal of nonmasked material.
It is an object of this invention to provide an improved MOD process for producing a semiconductor tin oxide thin film element by thermal decomposition of an organic tin compound, which process comprises a partial decomposition of the tin compound prior to applying and developing a photoresist layer, and which further comprises removal of unwanted tin compound by dissolution in alkaline solvent used to remove nonmask photoresist. Masked regions of the partially decomposed tin compound remain substantially intact despite the solvent. In this manner, the unwanted tin compound is removed in the same step as the nonmask photoresist, thereby eliminating a separate etching step that would otherwise be required to pattern the film.