Method of forming alignment marks in a semiconductor body

Claim 2

Method of forming alignment marks (26) in a semiconductor body (12) having a lightly doped first layer (24) and an heavily doped second layer (22), said first layer having an optically exposed surface (16), characterized by the steps of: directing a laser beam (20) onto said exposed surface in a direction perpendicular thereto, - thereby forming in said exposed surface a scribed first alignment mark (26); and choosing the wavelength of the laser beam to be beyond the absorption edge of the material of said semiconductor body so that the beam passes through said first layer without absorption but is absorbed by said second layer to produce therein a defect (29) representing a second alignment mark positioned identically to the scribed first alignment mark.

Claim 3

The method according to claim 1, characterized in that said heavily doped layer (22) is covered by a lightly doped epitaxial layer (30), and further characterized by comprising the step of heating the semiconductor body until the defect propagates through said epitaxial layer to the external surface thereof to form therein a visible image of the scribed first alignment mark, said image being a second alignment mark positioned identically to the first alignment mark.

Claim 4

The method according to claim 2, characterized in that the material of the semiconductor body is silicon, and wherein the heating step comprises heating the body at a temperature of 700°C for one hour.

Claim 5

The method according to claim 2, characterized in that said first layer is a substrate (24), said second alignment mark is concealed by a structure (34) on said epitaxial layer, and said second layer is heavily doped with boron, further characterized by the step of etching away said first layer with an etchant which is stopped by the heavily boron-doped second layer but which etches into said second and epitaxial layers only in the region of the defect to form an etched third alignment mark positioned identically to the scribed first alignment mark.

Claim 6

The method according to claim 1, characterized in that said second layer (40) is a heavily doped substrate, said first layer (42) is an epitaxial layer on said second layer, and further characterized by the steps of: heating the semiconductor body until the defect (50) migrates through said first layer (42) to the exposed surface thereof to form therein a visible image (52) of the scribed first alignment mark and also through said first layer into said second layer (40), said image being a second alignment mark positioned identically to the scribed first alignment mark; forming on said first layer a structure (56) which conceals said first and second alignment marks; and etching away said substrate with an etchant which is stopped by said lightly doped first layer but which etches only into the defect region therein to form therein an etched third alignment mark (62) identically positioned to said first and second alignment marks.