Method for measuring concentration of dopant within a semiconductor substrate

A method for deriving a concentration density of dopant within a dopant region as a function of etching depth into said dopant region, comprising the steps of:

• providing a wafer having an upper surface with an oxide formed thereon and a dopant region extending from said upper surface to a depth below said upper surface;
• cleaving said wafer perpendicular to said upper surface to expose a cross-section to said oxide and said dopant region;
• exposing said wafer to an oxide etchant and then to a dopant region etchant to remove along said cross-section a portion of the oxide and the dopant region, wherein said dopant region is removed at a rate proportional to a concentration of dopant atoms within said dopant region, and said oxide and a portion of the dopant region abutting said oxide are removed to a topological height disparity greater than 10 nm and less than 40 nm;
• scanning a probe of an atomic force microscope across said cross-section, wherein said topological height disparity signals to said atomic force microscope the uppermost boundary of said dopant region;
• and
• computing concentration of dopant within said dopant region from said uppermost boundary to depths below said uppermost boundary as a function of the amount of dopant region removed by the exposing step.

A method for deriving a calibration curve of concentration density versus etch depth on a calibration wafer and for relating etch depth on a target wafer to target wafer concentration density:

• forming cross-sections through calibration and target wafers having respective calibration and target oxides formed on one surface of said wafers and respective calibration and target doping regions formed directly beneath said calibration and target oxides;
• exposing the cross-sections of said calibration and target wafers to identical etchant conditions to remove a portion of the calibration and target oxides separate from the removal of a portion of the calibration and target regions, wherein a measurable upper surface disparity exists between said calibration oxide and said calibration region and well as between said target oxide and said target region, and wherein the calibration and target regions are removed at a rate proportional to a concentration density of dopants therein;
• scanning a probe of an atomic force microscope across the cross-sections of said calibration and target wafers to determine respective etch depth;
• deriving a calibration curve of concentration density versus etch depth across the cross-section of said calibration wafer;
• and
• from etch depth of said target wafer, relating comparable etch depth on said calibration curve to concentration density of said target wafer.