Problems bubble up to the top

One problem with future metal interconnect systems is that they may be too fragile to be reliable. The problem is the stress caused by thermal mismatch between a package and a chip's surface that occurs when the two are attached to each other by reflowing the solder bumps. This stress can cause cracking at the surface interface and can be transmitted into the interconnect stack, causing cracks at other interfaces, such as between the barrier layers protecting the copper lines and the dielectric material. Their research used finite-element analysis to derive the electric-field distribution and intensity in a dielectric film, given various pore sizes, shapes, and permittivities, and various interconnections of the pores. Once the field had been calculated, it was used to drive a Monte Carlo analysis of charge flow. The work showed that, when the permittivity of a pore is less than that of the matrix it is in, the electric field tends to be enhanced within it, accelerating local charge carriers. If pores join, they can create channels of high electric field that provide a conduction route that promotes dielectric breakdown. So porous dielectrics can sometimes be worse insulators than non-porous ones.