Process for making a dual implanted drain extension for bucket brigade device tetrode structure

A process for making a serially connected plurality of bucket brigade devices in a silicon semiconductor substrate of P-type conductivity and a dopant concentration of from 7 to 10.times.10.sup.15 /cm.sup.3, each of said devices occupying a device area bounded on opposed ends by a first and a second diffusion of N-type conductivity, the first diffusion of a first device serving as the second diffusion of a next serially connected device, with a thick insulating layer overlying said semiconductor substrate having a plurality of windows therethrough, each said window juxtaposed over and exposing a corresponding one of said device areas, comprising the steps of:

• growing a thin insulating layer of silicon dioxide on the surface of said substrate in each of said device areas, to a thickness of from 500 A to 700 A;
• depositing a first ion implantation blocking mask over a first portion of each of said device areas, masking a channel region adjacent to said first diffusion and exposing a capacitor plate region adjacent to said second diffusion, said first mask having an edge defining the boundary between said channel region and said capacitor plate region;
• ion implanting N-type conductivity dopant ions at an energy of 65 Kev and a first dose of from 1 to 5.times.10.sup.13 /cm.sup.2 through said thin insulating layer and into said substrate in said capacitor plate region of each of said device areas, forming an N-type conductivity capacitor plate in said substrate which has a terminal edge at said boundary between said channel region and said plate region and which electrically contacts said second diffusion;
• removing said first ion implantation blocking mask and depositing a second ion implantation blocking mask over a second portion of each of said device areas, masking a first, high threshold voltage portion of said channel region adjacent to said first diffusion and exposing a second, low threshold voltage portion of said channel region adjacent to said terminal edge of said capacitor plate;
• ion implanting said N-type conductivity ions at a second dose of from 0.5 to 1.times.10.sup.12 /cm.sup.2, which is lower than said first dose, through said thin insulating layer and into said substrate in said low threshold voltage portion of said channel region, forming an N-type conductivity doped low threshold voltage region having a threshold voltage of between 0 and -1 volts;
• said high threshold voltage portion of said channel region having a threshold voltage of approximately +1 volts;
• removing said second mask and depositing a gate electrode on said thin insulator layer over said channel region and said capacitor plate region in each of said device areas;
• whereby an improved, serially connected plurality of bucket brigade devices is formed.