Fracture Tracing and Analysis in Silicon and Glass Wafers
Authors: Dyer Lawrence D. 
Published: January 2022

Silicon wafers and chips are subject to brittle fracture. Microscale devices have unique problems of fracture owing to the thinness of the wafers, small dimensions of active device elements and to chemical or mechanical processing on a small scale.

The first step in analyzing fractures is to look through a microscope at the fracture surfaces of the broken pieces. The fracture is traced back to an origin by use of markings that appear on the fracture surfaces [Dyer, 1984; van Kessel & McGee, 1985; Perrin, 1989,1990; and Pote, 1990]. This step is more difficult for the microscale because more magnification is required and because tools to make the step as easy as for the macroscale have not been developed. The second step is to look at markings near the origin and at the other device surfaces near the origin for clues to what caused the material to exceed the critical stress. It is important to know what processes the wafer or device has undergone to assess the possibilities for identifying the stage that introduced the critical weakness.

Markings that are convenient to trace the fracture back to an origin are: rib lines, hackle (tear) marks, river lines, lance marks, and Wallner lines. Clues to stresses involved can come from the size of the mirror region, from hesitation lines, faceted Wallner areas, and from the shape of the fracture surface near the origin. Clues from the front and back surfaces can be scratches, Hertzian partial cone cracks (horseshoe-shaped) [Dyer, 1987a, 1989], rubbing marks, or chips and other damage near the origin.

Rib lines are essentially the instantaneous locations of the crack front. They are made visible because the crack sometimes does not proceed smoothly on a fixed plane, but oscillates back and forth around this plane as the crack front moves to relieve the applied stress. They are sometimes difficult to see in a particular area. Interference contrast or oblique lighting help.

Hackle marks are essentially the places where the material tears because the advancing crack is taking two different levels and must tear the step between them for the crack to advance. They are generally normal to the advancing crack front Lance markings are hackle marks where the two levels have interpenetrated before separation occurs. River lines are hackle marks where smaller steps coalesce into taller ones. They are the most reliable indicator of the direction of crack travel.

Wallner lines are fine, rib-like markings where the running crack front is intercepted by transverse shear waves that are produced by the crack snagging on obstacles, usually at the surface. Since they are bowed in a consistent manner, they may be used to locate the origin.

Faceted Wallner areas [Dyer, 1987b] occur around the origin when the first fracture is near a {110} plane. They occur when some part of the crack meets an obstacle and creates a transverse shear wave which interferes with the crack front enough to make the Wallner lines form into large peaks and valleys with crystalline facets. Faceted Wallner areas make the job of finding the origin much simpler in complicated cases because they are so visible under bright or dark field. Their appearance also gives an indication of the stress level encountered. This marking does not appear in glass, but all the other markings do. Table I summarizes the various fracture markings and their uses.

Several examples of fracture problems that have been solved by the foregoing approach will be given.