In a process using a hot phosphoric acid etchant to etch silicon nitride on a semiconductor wafer submerged in a tank of the etchant, a recirculating path is established for the etchant. Control value of the tone color effect parameter is determined in response to variation in relationship between the first and second display objects, and tone control is performed on the basis of the determined control value. A dielectric layer is also provided to form one boundary of the base layer material gap.