bow – a measure of concave or convex deformation of the median surface of a wafer, independent of any thickness variation which may be present. Bow is determined at the center point of the wafer with respect to a reference plane determined by three points equally spaced on a circle whose diameter is 6.35 mm less than the nominal wafer diameter. Bow is a bulk property of the wafer, not a property of an exposed surface. Generally, bow is determined with the wafer in a free, unclamped position. (Not to be confused with warp.)
diameter – the linear distance across a circular silicon wafer which includes the wafer center and excludes any flats or other peripheral fiducial areas.
edge contour – the cross sectional profile of a wafer edge shaped by grinding or etching. Edges may be either rounded or bevelled.
flatness – for wafer surfaces, the deviation of the front surface, expressed as TIR or maximum FPD, relative to a specified reference plane when the back surface of the wafer is ideally flat, as when pulled down by vacuum onto an ideally clean flat chuck. The flatness of a wafer may be described as;
1. the global flatness, or
2. the maximum value of site flatness as measured on all sites, or
3. the percentage of sites which have a site flatness equal to or less than a specified value.
flatness qualitiy area – that portion of the surface of a wafer over which the specified flatness values apply. The flatness qualitiy area is most frequently defined with an edge exclusion area, a peripheral annulus usually 3 mm wide.
focal plane – the plane perpendicular to the optical axis of an imaging system which contains the focal point of the imaging system.
focal plane deviation (FPD) – the distance parallel to the optical axis from a point on the wafer surface to the focal plane.
global flatness – the TIR or maximum FPD within the flatness quality area relative to a specified reference plane.
maximum FPD – the largest of the absolute values of the focal plane deviations.
primary flat – the flat of longest length which is oriented with respect to a specific crystallographic plane. Also known as major flat.
reference plane – a plane specified by one of the following:
1. three points at specified locations on the front surface of the wafer, or
2. the least squares fit to the front surface of the wafer using all points within the flasness quality area, or
3. the least squares fit to the front surface of the wafer using all points within a site, or
4. an ideal back surface (equivalent to the ideally flat chuck surface that contacts the wafer).
secondary flat(s) – the flat or flats of a length shorter than that of the primary flat whose angular position with respect to the primary flat identifies the conductivity type and orientation of the wafer. Also known as minor flat.
site – a rectangular area, on the front surface of the wafer, whose sides are parallel with and perpendicular to the primary flat and whose center falls within the flatness quality area.
site flatness – the TIR or maximum FPD of the portion of a site which falls within the flatness quality area.
thickness – the distance through the wafer between corresponding points on the front and back surface.
total indicator reading (TIR) – the smallest perpendicular distance between two planes, both parallel with the reference plane, which enclose all points within a specified flatness quality area or site on the front surface of a wafer.
total thickness variation (TTV) – the difference between the maximum and minimum thickness values encountered during a scan pattern or a series of point measurements on a wafer.
warp – the difference between the maximum and minimum distances of the median surface of the wafer from a reference plane encountered during a scan pattern. Warp is a bulk property of the wafer, not a property of an exposed surface. The median surface may contain regions with upward or downward curvature or both. Generally, warp is determined with the wafer in a free, umclamped position. (Not to be confused with bow.)