Method of forming multiple wells in a semiconductor integrated circuit using fewer photolithography steps

A method for forming a triple well of a semiconductor device, comprising:

• forming a first ion-implantation mask over a semiconductor substrate of a first conductivity type to expose a first region of a semiconductor substrate;
• while said first ion implantation mask is disposed over the semiconductor substrate, implanting an impurity of the first conductivity type into the first region of the semiconductor substrate left exposed by the first ion-implantation mask, to thereby form a pocket well region of the first conductivity type;
• while said first ion implantation mask is disposed over the semiconductor substrate, implanting an impurity of the second conductivity type deeper into said first region of the semiconductor substrate than the impurities of the first conductivity type are implanted, to thereby form a first deep well region of a second conductivity type under the pocket well region;
• removing the first ion-implantation mask;
• forming a second ion-implantation mask over the semiconductor substrate to expose a second region of the semiconductor substrate spaced laterally apart from said first region;
• while said second ion-implantation mask is disposed over said semiconductor substrate, implanting an impurity of the first conductivity type into the second region of the semiconductor substrate left exposed by the second ion-implantation mask, to thereby form a first peripheral well region of the first conductivity type;
• removing the second ion-implantation mask;
• subsequently forming a third ion-implantation mask covering the pocket well region and the first peripheral well region of the first conductivity type;
• while said third ion-implantation mask covers the pocket well region and the peripheral well region, implanting an impurity of the second conductivity type into a region of the semiconductor substrate left exposed by the third ion-implantation mask and to a depth equal to that of the pocket well region, to thereby form a second peripheral well region of the second conductivity type extending laterally of and between the pocket well region and the first peripheral well region;
• and
• while said third ion-implantation mask covers the pocket well region and the peripheral well region, implanting an impurity of the second conductivity type into region of the semiconductor substrate left exposed by the third ion-implantation mask and to a depth equal to that of the first deep well region, to thereby form a second deep well region of the second conductivity type under the peripheral well region and extending laterally of and between the first deep well region and the first peripheral well region.