Stacking die permits designers to pack a great deal of capacity and increase processing speed in a much smaller volume. In the process the die stack also creates many internal interfaces where critical defects, such as voids and delaminations, can lurk.
Makers of die stacks are very interested in using non-destructive acoustic microscopy, such as C-SAM®, to locate and image these critical defects, but until recently the die stacks have been far more difficult to image clearly than other IC devices.
The problem is that when ultrasound is pulsed into a die stack, the numerous internal interfaces create multiple return echoes in the A-Scan. When the pulse strikes the interface between die 1 and 2, for example, part of the pulse is reflected as an echo and part of it travels deeper. At the interface between die 2 and 3, ultrasound is again both reflected and transmitted. On its way back to the transducer the return echo from the 2 and 3 interface strikes the die 1 and 2 interface again, repeating the transmission/reflection pattern over and over until the pulse energy is sufficiently damped to be insignificant.
When a die stack has many more die, the A-Scan from the sample has so many return echoes that a technician who wants to look specifically at a deeper interface will have a hard time knowing which of the arriving echoes actually came from the interface of interest. The technician may be able to work it out, but it will take a while and it requires some expertise.
To speed up and simplify the task of assigning each echo to its proper interface within stacked die, Sonoscan® has developed over the last several years, in collaboration with T.U. Dresden, a software program called SonoSimulator™ that creates a virtual model of the echo returns of the die stack. The user of the program enters data about the dimensions and materials of the stack and then the software creates the virtual model for simulation of the A-Scan.