Well formation For CMOS devices integrated circuit structures
 A multiple well formation is provided in a CMOS region of a semiconductor substrate to provide enhanced latchup protection for one or more CMOS transistors formed in the wells. The structure comprises an N well extending from the substrate surface down into the substrate, a buried P well formed in the substrate beneath the N well, a second P well extending from the substrate surface down into the substrate, and an isolation region formed in the substrate between the N well and the second P well. The buried P well may extend beneath both the N well and the second P well in the substrate. In a preferred embodiment of the invention, the N well and the second P well are each implanted in the substrate at an energy level sufficient to provide a dopant concentration peak in the substrate below the depth of the isolation region to provide punch through protection and to provide a channel stop beneath the isolation region by proving a P-N junction between the N well and P well beneath the isolation region. The dopant concentration level peak of the dopants forming the buried P well in the substrate will be located below the dopant concentration level peak of the N well a minimum distance sufficient to inhibit reduction of the effective depth of the N well, and a maximum distance not exceeding the maximum distance which will still provide enhanced latchup protection to one or more transistors formed in the CMOS region. 
LSI Logic Corporation
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inventional inventional inventional
Ruggero Shih-Fen Helmut
Castagnetti Huang Puchner