Method of producing solid electrolyte oxygen-sensing element of laminated structure

A method of producing an oxygen sensing element of the concentration cell type, the method comprising the steps of:

• preparing a lamination of a shield layer made of a ceramic material, a reference electrode layer laid on a major surface of said shield layer, an oxygen ion conductive solid electrolyte layer formed on said surface of said shield layer such that said reference electrode layer is tightly sandwiched between said shield layer and said solid electrolyte layer and a first measurement electrode layer which has a microscopically porous and gas-permeable structure and is formed on and in close contact with the outer surface of said solid electrolyte layer by applying a wet composition containing fine particles of a metal onto said surface of said solid electrolyte layer, drying the applied composition to evaporate the liquid component of the applied composition and firing the uncompleted lamination to sinter the metal particles contained in the applied composition;
• and
• forming a second measurement electrode layer having a microscopically porous and gas-permeable structure on the outer surface of said first measurement electrode layer by physical vapor deposition of a metal on said surface of said first measurement electrode layer, wherein said forming of said second measurement electrode layer comprises a first sub-step of performing said physical vapor deposition for a limited time such that the thickness of a metal film on said lamination during said first sub-step does not exceed 0.5 microns, a second sub-step of subjecting said lamination with said metal film deposited thereon from said first sub-step to a heat treatment performed in air at a temperature in the range from 900.degree. to 1100.degree. C. and a third sub-step of resuming said physical vapor deposition wherein said metal film subjected to said heat treatment becomes an innermost part of said second measurement electrode layer.