PORTLAND, Ore. Low-k dielectric materials are super-insulators that help designers scale down chip size, but at the expense of shortening the average lifetimes of their chips. The problem is that low-k dielectrics add porosity—vacancy defects—which can act as electron traps, letting current leak through the insulator and eventually short-circuiting the chip.
Now, by using photo-induced current and laser-based second-harmonic generation, researchers at Columbia University are perfecting a testing regime for Semiconductor Research Corp. (SRC; Durham, N.C.) that can spot potential failures while the chips are still on the wafer.
The ultimate low-k dielectrics are air gaps, but those can introduce mechanical strength problems. The next best thing is to add pockets of air—pores—to a material, thereby decreasing its "k" and making it a better insulator. To date, however, no quick and easy testing methods have been available to determine whether porous insulators are also introducing problems that may cause chips to fail prematurely.
"Low-k dielectric materials usually have a lot of porosity and sometimes include organic materials like carbon, both of which can generate defects that trap electrons," said Scott List, director of interconnect and packaging sciences at SRC. The trapped electrons can hop from one trap to another, generating a leakage through the insulator that eventually destroys the chip.
Usually, engineers evaluate low-k dielectrics by performing a time-dependent dielectric breakdown (TDDB) test, in which they look at leakage as a function of time, mapping out how long it takes the dielectric to fail but not revealing the underlying mechanism. Columbia's regime instead measures the average number of density traps in a low-k material, enabling the breakdown mechanism to be identified more easily.
"Columbia's ability to measure the density of traps and similar parameters enables them to predict the underlying mechanism causing the failure—in particular, whether it's the materials or the processing conditions," said List.
SRC members such as Freescale, IBM and Intel have supplied Columbia with samples of their low-k dielectrics. Columbia is using the samples to perfect a test regime that not only will help characterize new dielectrics but also will be applicable in production facilities for testing low-k materials after they have been fabricated on wafers.